Reviewer #2 (Public Review):

Summary:

This study uses transcriptome sequence from a dioecious plant to compare evolutionary rates between genes with male- and female-biased expression and distinguish between relaxed selection and positive selection as causes for more rapid evolution. These questions have been explored in animals and algae, but few studies have investigated this in dioecious angiosperms, and none have so far identified faster rates of evolution in male-biased genes (though see Hough et al. 2014 https://doi.org/10.1073/pnas.1319227111).

Strengths:

The methods are appropriate to the questions asked. Both the sample size and the depth of sequencing are sufficient, and the methods used to estimate evolutionary rates and the strength of selection are appropriate. The data presented are consistent with faster evolution of genes with male-biased expression, due to both positive and relaxed selection.

This is a useful contribution to understanding the effect of sex-biased expression in genetic evolution in plants. It demonstrates the range of variation in evolutionary rates and selective mechanisms, and provides further context to connect these patterns to potential explanatory factors in plant diversity such as the age of sex chromosomes and the developmental trajectories of male and female flowers.

Weaknesses:

The presence of sex chromosomes is a potential confounding factor, since there are different evolutionary expectations for X-linked, Y-linked, and autosomal genes. Attempting to distinguish transcripts on the sex chromosomes from autosomal transcripts could provide additional insight into the relative contributions of positive and relaxed selection.

Reviewer #3 (Public Review):

The potential for sexual selection and the extent of sexual dimorphism in gene expression have been studied in great detail in animals, but hardly examined in plants so far. In this context, the study by Zhao, Zhou et al. al represents a welcome addition to the literature.

Relative to the previous studies in Angiosperms, the dataset is interesting in that it focuses on reproductive rather than somatic tissues (which makes sense to investigate sexual selection), and includes more than a single developmental stage (buds + mature flowers).

eLife assessment

The authors present useful findings on the use of a Drosophila behavioral paradigm for assessing different fly genetic models of neurodegeneration. The experimental design and analyses are solid and can be used for quick behavioral assessment in fly models of various neurodegenerative diseases, especially those having an impact on locomotion. The work will be of interest to Drosophila biologists using behavior as a readout for their studies.

Reviewer #1 (Public Review):

Summary: Translating discoveries from model organisms to humans is often challenging, especially in neuropsychiatric diseases, due to the vast gaps in the circuit complexities and cognitive capabilities. Kajtor et al. propose to bridge this gap in the fly models of Parkinson's disease (PD) by developing a new behavioral assay where flies respond to a moving shadow by modifying their locomotor activities. The authors believe the flies' response to the shadow approximates their escape response to an approaching predator. To validate this argument, they tested several PD-relevant transgenic fly lines and showed that some of them indeed have altered responses in their assay.

Strengths: This single-fly-based assay is easy and inexpensive to set up, scalable, and provides sensitive, quantitative estimates to probe flies' optomotor acuity. The behavioral data is detailed, and the analysis parameters are well-explained.

Weaknesses: While the abstract promises to give us an assay to accelerate fly-to-human translation, the authors need to provide evidence to show that this is indeed the case. They have used PD lines extensively characterized by other groups, often with cheaper and easier-to-setup assays like negative geotaxis, and do not offer any new insights into them. The conceptual leap from a low-level behavioral phenotype, e.g. changes in walking speed, to recapitulating human PD progression is enormous, and the paper does not make any attempt to bridge it. It needs to be clarified how this assay provides a new understanding of the fly PD models, as the authors do not explore the cellular/circuit basis of the phenotypes. Similarly, they have assumed that the behavior they are looking at is an escape-from-predator response modulated by the central complex- is there any evidence to support these assumptions? Because of their rather superficial approach, the paper does not go beyond providing us with a collection of interesting but preliminary observations.

Reviewer #2 (Public Review):

In this study, Kajtor et al investigated the use of a single-animal trial-based behavioral assay for the assessment of subtle changes in the locomotor behavior of different genetic models of Parkinson's disease of Drosophila. Different genotypes used in this study were Ddc-GAL4>UAS-Parkin-275W and UAS- α-Syn-A53T. The authors measured Drosophila's response to predator-mimicking passing shadow as a threatening stimulus. Along with these, various dopamine (DA) receptor mutants, Dop1R1, Dop1R2 and DopEcR were also tested.<br /> The behavior was measured in a custom-designed apparatus that allows simultaneous testing of 13 individual flies in a plexiglass arena. The inter-trial intervals were randomized for 40 trials within 40 minutes duration and fly responses were defined into freezing, slowing down, and running by hierarchical clustering. Most of the mutant flies showed decreased reactivity to threatening stimuli, but the speed-response behavior was genotype invariant.<br /> These data nicely show that measuring responses to the predator-mimicking passing shadows could be used to assess the subtle differences in the locomotion parameters in various genetic models of Drosophila.

The understanding of the manifestation of various neuronal disorders is a topic of active research. Many of the neuronal disorders start by presenting subtle changes in neuronal circuits and quantification and measurement of these subtle behavior responses could help one delineate the mechanisms involved. The data from the present study nicely uses the behavioral response to predator-mimicking passing shadows to measure subtle changes in behavior. However, there are a few important points that would help establish the robustness of this study.<br /> 1) The visual threat stimulus for measuring response behavior in Drosophila is previously established for both single and multiple flies in an arena. A comparative analysis of data and the pros and cons of the previously established techniques (for example, Gibson et al., 2015) with the technique presented in this study would be important to establish the current assay as an important advancement.<br /> 2) Parkinson's disease mutants should be validated with other GAL-4 drivers along with Ddc-GAL4, such as NP6510-Gal4 (Riemensperger et al., 2013). This would be important to delineate the behavioral differences due to dopaminergic neurons and serotonergic neurons and establish the Parkinson's disease phenotype robustly.<br /> 3) The DopEcR mutant genotype used for behavior analysis is w1118; PBac{PB}DopEcRc02142/TM6B, Tb1. Balancer chromosomes, such as TM6B,Tb can have undesirable and uncharacterised behavioral effects. This could be addressed by removing the balancer and testing the DopEcR mutant in homozygous (if viable) or heterozygous conditions.<br /> 4) The height of the arena is restricted to 1mm. However, for the wild-type flies (Canton-S) and many other mutants, the height is usually more than 1mm. Also, a 1 mm height could restrict the fly movement. For example, it might not allow the flies to flip upside down in the arena easily. This could introduce some unwanted behavioral changes. A simple experiment with an arena of height at least 2.5mm could be used to verify the effect of 1mm height.<br /> 5) The detailed model for Monte Carlo simulation for speed-response simulation is not described. The simulation model and its hyperparameters need to be described in more depth and with proper justification.<br /> 6) The statistical analysis in different experiments needs revisiting. It wasn't clear to me if the authors checked if the data is normally distributed. A simple remedy to this would be to check the normality of data using the Shapiro-Wilk test or Kolmogorov-Smirnov test. Based on the normality check, data should be further analyzed using either parametric or non-parametric statistical tests. Further, the statistical test for the age-dependent behavior response needs revisiting as well. Using two-way ANOVA is not justified given the complexity of the experimental design. Again, after checking for the normality of data, a more rigorous statistical test, such as split-plot ANOVA or a generalized linear model could be used.<br /> 7) The dopamine receptor mutants used in this study are well characterized for learning and memory deficits. In the Parkinson's disease model of Drosophila, there is a loss of DA neurons in specific pockets in the central brain. Hence, it would be apt to use whole animal DA receptor mutants as general DA mutants rather than the Parkinson's disease model. The authors may want to rework the title to reflect the same.

eLife assessment

This valuable study reports the induction of supernumerary inner hair cells in the mouse cochlea upon reducing the expression level of a tight-junction protein (claudin-9). However, the evidence supporting the claims is incomplete and the work would be strengthened by adding several control experiments, resolving inconsistencies and imprecisions in the presentation of the results, and providing more mechanistic insight. The work will be of interest to scientists working in the development and regeneration of hair cells in the inner ear.

Reviewer #1 (Public Review):

Summary:<br /> The focus of this manuscript was to investigate the role of Cldn9 in the development of the mammalian cochlea. The main rationale of the study is the fact that cochlear hair cells do not regenerate, so when damaged they are lost forever, causing irreparable hearing loss. The authors have attempted to address this problem by inducing the ectopic production of additional hair cells and testing whether they acquire the morphological and functional characteristics of native hair cells. They show that downregulation of Cldn9 using a well-established genetic manipulation of transgenic mice led to the production of extra numerary inner hair cells, which were able to survive for several months. By performing a large battery of experiments, the authors were able to determine that the native and ectopic inner hair cells have comparable morphological and physiological characteristics. There are several conclusions highlighted by the authors in different parts of the manuscript, including the key role of Cldn9 in coordinating embryonic and postnatal development, the differentiation of supporting cells into inner hair cells, and the possible use of Cldn9 to induce inner hair cell differentiation following deafness induced by hair cell loss.

Strengths:<br /> Several of the conclusions in this study are well supported by the experimental work.

Weaknesses:<br /> Some aspects of the data and its interpretation needs better explanation and requires further investigation.

1) The Results section is the most difficult part to read and understand. It contains a very limited, and in some places confusing and repetitive, description of the data. Statistical analysis is missing for some of the key data (e.g., ABRs), and in some places the text contradicts the data presented in the figures (e.g., Figure 8). I am sure a careful revision of the text would clarify some of these issues.

2) One puzzling finding that is not addressed in the manuscript is the lack of functional benefit from these additional inner hair cells. In fact, it appears to be detrimental based on the increased ABR thresholds. Maybe it would be useful to analyze the wave 1 characteristics.

3) It is not clear what direct evidence there is, apart from some immunostaining, indicating that the ectopic inner hair cells derive from the supporting cells. This part would benefit from a more careful consideration and maybe an attempt at a more direct experimental approach.

4) One point that should be made clear throughout the manuscript is that the ectopic inner hair cells are generated in a cochlea that is undergoing normal maturation. Thus, there is no guarantee that modulating the expression levels of Cldn9 in a deaf mouse lacking hair cells would produce the same result as that shown in this study. My guess is that it probably won't, but I am sure this could be tested (maybe in the future) using the excellent experimental approach applied in this study.

Reviewer #2 (Public Review):

Summary:

The generation of functional extranumerary inner hair cells (IHCs) in postnatal mice, particularly with virus-mediated knockdown of Cldn9 mRNA expression in the neonatal cochlear duct, is an important observation. It is significant because not many studies exist that report molecular manipulations of the neonatal organ of Corti that result in the generation of new hair cells that remain functional and appear to be intact for an extended time, here more than one year. Overall, this is a carefully conducted study; the observations are clear, and the methods are solid. Two independent methods for reducing the expression of Cldn9 mRNA were used: a conditional transgenic model and AAV-mediated knockdown with shRNA. The lack of a functional explanation of how the reduced expression of Cldn9 specifically leads to the formation of extranumerary IHCs leaves open questions. For example, it is not clear whether there is indeed a fate change happening and whether Cldn9 reduction affects developmental processes. The discussion of how Cldn9 reduction potentially affects Notch signaling, without hard evidence, is handwaving.

Strengths:

It is a very interesting observation and somewhat unexpected in its specificity for inner hair cells. Using two different approaches to manipulate Cldn9 expression provides a strong experimental foundation. The study is conducted quantitatively and with care.

Weaknesses:

The lack of mechanistic insight results in an open-ended story where at least the potential interaction of Cldn9 reduction with known and well-characterized signaling pathway components should have been investigated. This missed opportunity limits the scope of the study and should be addressed: How does Cldn9 downregulation affect the expression levels of other known genes linked to hair cell production and cell fate decisions? Quantitative RT-PCR is working well for the authors, and a comparison of the expression of Notch or other known pathway components could provide mechanistic insight.

It is unclear how P21 inner hair cells were identified for the patch clamp experiments shown in Fig 4E-H. This is a challenging endeavor without the possibility of using specific markers.

Please also address the numerous minor points outlined below; it will improve the paper's readability.

Please include page numbers and line numbers in a revised manuscript.

Reviewer #3 (Public Review):

This important study by Chen et al help in advancing our knowledge about the regulation of inner hair cell (IHC) development and revealed the role of Cldn9 in IHC embryonic and postnatal induction by transdifferentiation from the supporting cells. The authors developed an inducible doxycycline (dox)-tet-OFF-Cldn9 transgenic mice to regulate expression levels of Cldn9 and show that downregulation of Cldn9 resulted in additional, although incomplete row of IHCs immediately adjacent to the original IHC row. These induced extra IHCs had similar well developed hair bundles, able to mechanotransduce and were innervated by auditory neurons resembling wild-type IHCs. In addition, the authors knock down Cldn9 postnatally using shRNA injections in P1-7 mice with similar induction of extranumerary IHC next to the original row of IHCs. The conclusions of this paper are mostly well supported by the data, but some data analysis needed to be clarified and some crucial controls should be provided to improve the confidence in the presented results. There is a great potential for practical use of these valuable findings and new knowledge on IHC developmental regulation to design Cldn9 gene therapy in the future.<br /> The described by Chen et al mechanisms of extra hair cell generation by suppression of the tight junction protein Cldn9 expression level are very interesting and previously unknown. In particular, the generation of extra IHCs postnatally using downregulation of Cldn9 by shRNA could potentially be very useful as a replacement of HCs lost after noise-induced trauma, ototoxic agents, or other environmental trauma. On the other hand, the replacement of lost hair cells due to various genetic mutations by inducing a supernumerary IHCs with the same abnormalities would not be reasonable.<br /> The authors show that postnatally generated ectopic IHCs are viable and mechanotransducive, but it would be nice to show the maturation steps of ectopic IHC during this postnatal period. For example, stereocilia bundles of the ectopic hair cells should mature later than the original IHCs. A few days after viral delivery of shRNA, you should be able to observe immature IHC bundles that unequivocally will define newly generated IHCs. Unfortunately, the authors show only examples of already mature ectopic IHCs at P21 and in 5-6 weeks old mice and at relatively low resolution. Also, during maturation, IHCs usually have transient axo-somatic synapses that are not present in mature IHCs. It would be great to see if, in 5-6 weeks old mouse, the ectopic IHCs still have axo-somatic synapses or not, and if the majority of the ectopic IHCs have innervation. Some of the data in this study would benefit from showing corresponding controls and some - from higher resolution imaging.<br /> In the mammalian cochlea, each HC is separated from the next by intervening supporting cells, forming an invariant and alternating mosaic along the cochlea's length. Cochlear supporting cells in some conditions can divide and trans-differentiate into HCs, serving as a potential resource for HC differentiation, using transcription and other developmental signaling factors.<br /> However, when ectopic hair cells are generated from supporting cell trans-differentiation, the intricate mosaic of the organ of Corti is altered, which could by itself lead to hearing issues. In case of downregulation of Cldn9, the extra row of IHCs seems to be positioned immediately adjacent to the original IHC row. It is not clear if the newly formed unusual junctions between the ectopic and original IHCs are sufficiently tight to prevent leakage of the endolymph to the basolateral surface of IHCs. Also, it is not clear if the other organ of Corti tight junctions could lose their tightness due to the downregulation of Cldn9, which could over time affect the endocochlear potential as shown by this study and hearing abilities.<br /> Importantly, CLDN9 immunofluorescence staining data that show cytoplasmic staining of supporting cells should be revisited and the organ of Corti schematics showing CLDN9 expression should be corrected, considering that CLDN9 localizes to the tight junctions of the reticular lamina as was shown by immunoEM in this study and described in previous publications (Kitajiri et al., 2004; Nakano et al., 2009, Ramzan et al., 2021).<br /> While the current version of the manuscript will be of interest to scientists working in the inner ear development and regeneration field, it could be more valuable to the hearing researchers outside this immediate field and perhaps developmental biologists and cell biologists after proper revision.

eLife assessment

This important study used a new double knockout mouse model to investigate the role of two neuropeptides, substance P and CGRPa, in pain signaling. There is convincing evidence that double knockout of these two molecules, both of which have historically been associated with pain, does not affect nociception or acute pain behaviors in males and females. The conclusions would further benefit from additional validation of the approach, consideration of potential outliers and statistical approach in cases with smaller sample sizes, and consideration of the potential for opposing effects across region or peptide. This paper will be of interest to those interested in the neurobiology of pain and/or neuropeptide function.

Reviewer #1 (Public Review):

MacDonald et al., investigated the consequence of double knockout of substance P and CGRPα on pain behaviors using a newly created mouse model. The investigators used two methods to confirm knockout of these neuropeptides: traditional immunolabeling and a neat in vitro assay where sensory neurons from either wildtype or double knock are co-cultured with substance P "sniffer cells", HEK cells stably expressing NKR1 (a substance P receptor), GCaMP6s and Gα15. It should be noted that functional assays confirming CGRPα knockout were not performed. Subsequently, the authors assayed double knockout mice (DKO) and wildtype (WT) mice in numerous behavioral assays using different pain models, including acute pain and itch stimuli, intraplanar injection of Complete Freund's Adjuvant, prostaglandin E2, capsaicin, AITC, oxaliplatin, as well as the spared nerve injury model. Surprisingly, the authors found that pain behaviors did not differ between DKO and WT mice in any of the behavioral assays or pain paradigms. Importantly, female and male mice were included in all analyses. These data are important and significant, as both substance P and CGRPα have been implicated in pain signaling, though the magnitude of the effect of a single knockout of either gene has been variable and/or small between studies.

The conclusions of the study are largely supported by the data; however, additional experimental controls and analyses would strengthen the authors claims.

1) The authors note that single knockout models of either substance P or CGRPα have produced variable effects on pain behaviors that are study-dependent. Therefore, it would have strengthened the study if the authors included these single knockout strains in a side-by-side analysis (in at least some of the behavioral assays), as has been done in prior studies in the field when using double- or triple-knockout mouse models (for example, see PMID: 33771873). If in the authors hands, single knockouts of either peptide also show no significant differences in pain behaviors, then the finding that double knockouts also do not show significant differences would be less surprising.

2) It is unclear why the authors only show functional validation of substance P knockout using "sniffer" cells, but not CGRPα. Inclusion of this experiment would have added an additional layer of rigor to the study.

3) The authors should be a bit more reserved in the claims made in the manuscript. The main claim of the study is that "CGRPα and substance P are not required for pain transmission." However, the authors also note that neuropeptides can have opposing effects that may produce a net effect of no change. In my view, the data presented show that double knockout of substance P and CGRPα do not affect somatic pain behaviors, but do not preclude a role for either of these molecules in pain signaling more generally. Indeed, the authors also note that these neuropeptides could be involved in nociceptor crosstalk with the immune or vascular systems to promote headache. The authors only assayed pain responses to glabrous skin stimulation. How the DKO mice would behave in orofacial pain assays, migraine assays, visceral pain assays, or bone/joint pain assays, for example, was not tested. I do not suggest the authors include these experiments, only that they address the limitations/weaknesses of their study more thoroughly.

4) A more minor but important point, the authors do not describe the nature of the WT animals used. Are the littermates or a separately maintained colony of WT animals? The WT strain background should be included in the methods section.

Reviewer #2 (Public Review):

Summary,<br /> The paper aimed to examine the effect of co-ablating Substance P and CGRPα peptides on pain using Tac1 and Calca double knockout (DKO) mice. The authors observed no significant changes in acute, inflammatory, and neuropathic pain. These results suggest that Substance P and CGRPα peptides do not play a major role in mediating pain in mice. Moreover, they reveal that the lack of behavioral phenotype cannot be explained by the redundancy between the two peptides, which are often co-expressed in the same neuron

Strengths,<br /> The paper uses a straightforward approach to address a significant question in the field. The authors confirm the absence of Substance P and CGRPα peptides at the levels of DRG, spinal cord, and midbrain. Subsequently, they employ a comprehensive battery of behavioral tests to examine pain phenotypes, including acute, inflammatory, and neuropathic pain. Additionally, they evaluate neurogenic inflammation by measuring edema and extravasation, revealing no changes in DKO mice. The data are compelling, and the study's conclusions are well-supported by the results. The manuscript is succinct and well-presented.

Reviewer #3 (Public Review):

In this study, the authors were assessing the role of double global knockout of substance P and CGPRα on the transmission of acute and chronic pain. The authors first generated the double knockout (DKO) mice and validated their animal model. This is then followed by a series of acute and chronic pain assessments to evaluate if the global DKO of these neuropeptides are important in modulating acute and chronic pain behaviors. Authors found that these DKO mice Substance P and CGRPα are not required for the transmission of acute and chronic pain although both neuropeptides are strongly implicated in chronic pain. This study does provide more insight into the role of these neuropeptides on chronic pain processing, however, more work still needs to be done. (see the comments below).

1. In assessing the double KO (result #1), why are different regions of the brains shown for substance P and CGRPα (for example, midbrain for substance P and amygdala for CGRPα)? Since the authors mentioned that these peptides co-expressed in the brain (as in the introduction), shouldn't the same brain regions be shown for both IHC? It would be ideal if the authors could show both regions (midbrain and amygdala) in addition to the DRG and spinal cord for both peptides in their findings.<br /> In addition, since this is double KO, the authors should show more representative IHC-stained brain regions (spanning from the anterior to posterior).<br /> 2. It is also unclear as to why the authors only assessed the loss of substance P signaling in the double KO mice. Shouldn't the same be done for CGRPα signaling? Either the authors assess this, or the authors have to provide clear explanations as to why only substance P signaling was assessed.<br /> 3. Has these animal's naturalistic behavior been assessed after the double KO (food intake, sleep, locomotion for example)? I think this is important as changes to these naturalistic behaviors can affect pain processes or outcomes.<br /> 4. Figure 2H: The authors acknowledge that there is a trend to decrease with capsaicin-evoked coping-like responses. However, a close look at the graph suggests that the lack of significance could be driven by 1 mouse. Have the authors run an outlier test? Alternatively, the authors should consider adding more n to these experiments to verify their conclusions.<br /> 5. Similarly, the values for WT in the evoked cFos activity (Figure 2- Suppl Figure 1) are pretty variable. Considering that the n number is low (n = 5), authors should consider adding more n.<br /> Also, since the n number is low in this experiment (eg. 5 vs 4), does this pass the normality test to run a parametric unpaired t-test? Either the authors increase their n numbers or run the appropriate statistical test.<br /> 6. In most of the results, authors ran a parametric test despite the low n number. Authors have to ensure that they are carrying out the appropriate statistical test for their dataset and n number.<br /> 7. Along the same line of comment with the previous, authors should increase the n number for DKO for staining (Figure 4) as n number is only 3 and there is variability in the cFos quantification in the ipsilateral side.<br /> 8. Authors should provide references for statement made in Line 319-321 as authors mentioned that there are accumulating evidence indicating that secretion of these neuropeptides from nociceptor peripheral terminals modulates immune cells and the vasculature in diverse tissues.<br /> 9. Authors state that the sample size used was similar to those from previous studies, but no references were provided. Also, even though the sample sizes used were similar, I believe that the right statistic test should be used to analyze the data.<br /> 10. In the discussion, the authors noted that knocking out of a gene remains the strongest test of whether the molecule is essential for a biological phenomenon. At the same time, it was acknowledged that Substance P infusion into the spinal cord elicits pain, but it is analgesic in the brain. The authors might want to expand more on this discussion, including how we can selectively assess the role of these neuropeptides in areas of interest. For example, knocking out both Substance P and CGRPα in selected areas instead of the global KO since there are reported compensatory effects.

eLife assessment

Yang et al. investigate whether distinct sources of conflict are represented in a common cognitive space. The study uses an interesting task that mixes different sources of difficulty and reports that the brain appears to represent these sources as a mixture on a continuum in prefrontal areas. While the findings could be valuable to theory in this area, there are concerns with the analysis, design and results, that raise uncertainty regarding the main conclusion of a shared cognitive space. Thus, the evidence reported here ranges from solid to incomplete.

Reviewer #1 (Public Review):

People can perform a wide variety of different tasks, and a long-standing question in cognitive neuroscience is how the properties of different tasks are represented in the brain. The authors develop an interesting task that mixes two different sources of difficulty, and find that the brain appears to represent this mixture on a continuum, in the prefrontal areas involved in resolving task difficulty. While these results are interesting and in several ways compelling, they overlap with previous findings and rely on novel statistical analyses that may require further validation.

Strengths<br /> 1. The authors present an interesting and novel task for combining the contributions of stimulus-stimulus and stimulus-response conflict. While this mixture has been measured in the multi-source interference task (MSIT), this task provides a more graded mixture between these two sources of difficulty.

2. The authors do a good job triangulating regions that encoding conflict similarity, looking for the conjunction across several different measures of conflict encoding. These conflict measures use several best-practice approaches towards estimating representational similarity.

3. The authors quantify several salient alternative hypothesis, and systematically distinguish their core results from these alternatives.

4. The question that the authors tackle is important to cognitive control, and they make a solid contribution.

Concerns<br /> 1. The framing of 'infinite possible types of conflict' feels like a strawman. While they might be true across stimuli (which may motivate a feature-based account of control), the authors explore the interpolation between two stimuli. Instead, this work provides confirmatory evidence that task difficulty is represented parametrically (e.g., consistent with literatures like n-back, multiple object tracking, and random dot motion). This parametric encoding is standard in feature-based attention, and it's not clear what the cognitive map framing is contributing.

2. The representations within DLPFC appear to treat 100% Stoop and (to a lesser extent) 100% Simon differently than mixed trials. Within mixed trials, the RDM within this region don't strongly match the predictions of the conflict similarity model. It appears that there may be a more complex relationship encoded in this region.

3. To orthogonalized their variables, the authors need to employ a complex linear mixed effects analysis, with a potential influence of implementation details (e.g., high-level interactions and inflated degrees of freedom).

Reviewer #2 (Public Review):

Summary<br /> This study examines the construct of "cognitive spaces" as they relate to neural coding schemes present in response conflict tasks. The authors use a novel experimental design in which different types of response conflict (spatial Stroop, Simon) are parametrically manipulated. These conflict types are hypothesized to be encoded jointly, within an abstract "cognitive space", in which distances between task conditions depend only on the similarity of conflict types (i.e., where conditions with similar relative proportions of spatial-Stroop versus Simon conflicts are represented with similar activity patterns). Authors contrast such a representational scheme for conflict with several other conceptually distinct schemes, including a domain-general, domain-specific, and two task-specific schemes. The authors conduct a behavioral and fMRI study to test whether prefrontal cortex activity is correlated to one of these coding schemes. Replicating the authors' prior work, this study demonstrates that sequential behavioral adjustments (the congruency sequence effect) are modulated as a function of the similarity between conflict types. In fMRI data, univariate analyses identified activation in left prefrontal and dorsomedial frontal cortex that was modulated by the amount of Stroop or Simon conflict present, and representational similarity analyses that identified coding of conflict similarity, as predicted under the cognitive space model, in right lateral prefrontal cortex.

Strengths

This study addresses an important question regarding how conflict or difficulty might be encoded in the brain within a computationally efficient representational format. Relative to the other models reported in the paper, the evidence in support of the cognitive space model is solid. The ideas postulated by the authors are interesting and valuable ones, worthy of follow-up work that provides additional necessary scrutiny of the cognitive-space account.

Weaknesses

Future, within-subject experiments will be necessary to disentangle the cognitive space model from confounded task variables. A between-subjects manipulation of stimulus orientation/location renders the results difficult to interpret, as the source and spatial scale of the conflict encoding on cortex may differ from more rigorous (and more typical) within-subject manipulations.

Results are also difficult to interpret because Stroop and Simon conflict are confounded with each other. For interpretability, these two sources of conflict need to be manipulated orthogonally, so that each source of conflict (as well as their interaction) could be separately estimated and compared in terms of neural encoding. For example, it is therefore not clear whether the RSA results are due to encoding of only one type of conflict (Stroop or Simon), to a combination of both, and/or to interactive effects.

Finally, the motivation for the use of the term "cognitive space" to describe results is unclear. Evidence for the mere presence of a graded/parametric neural encoding (i.e., the reported conflict RSA effects) would not seem to be sufficient. Indeed, it is discussed in the manuscript that cognitive spaces/maps allow for flexibility through inference and generalization. Future work should therefore focus on linking neural conflict encoding to inference and generalization more directly.

eLife assessment

This work is of fundamental significance to the field of nervous system development as it advances our mechanistic understanding of axon guidance. The rigorous biochemical and genetic approaches are compelling, experiments are well-controlled, and the major claims are supported by convincing data. The study should be of general interest to the developmental neurobiology community.

Reviewer #1 (Public Review):

Summary:<br /> This study is focused on an important aspect of axon guidance at the central nervous system (CNS) midline: how neurons extend axons that either do or do not cross the CNS midline. The authors here address contradictory work in the field relating to how cell surface expression of the slit receptor Robo1 is regulated to generate crossed and non-crossed axon trajectories during Drosophila neural development. They use fly genetics, cell lines, and biochemical assessments to define a complex consisting of the commissureless, Nedd4 and Robo1 proteins necessary for regulating Robo1 protein expression. This work resolves certain remaining questions in the field regarding midline axon guidance, with strengths outweighing weaknesses; however, addressing some of these weaknesses would strengthen this study.

Strengths:<br /> Strengths include:<br /> -The use of well-controlled genetic gain-of-function (overexpression) approaches in vivo in Drosophila to show that phosphorylation sites (there are 2, and this study allows for assessment of the contributions made by each) in the commissureless (Comm) protein are indeed required for Comm function with respect to regulating axon midline guidance via their role in directing Comm-mediated Robo1 ubiquitination and degradation in the lysosome.<br /> -The demonstration that in vitro, and in a sensitized genetic background in vivo, the Nedd4 ubiquitin ligase regulates Robo1 protein cell surface distribution and also midline axon crossing in vivo.<br /> -Important evidence here that serves to resolve many questions raised by previous studies (not from these authors) regarding how Robo1 is regulated by Comm and Nedd4 family ubiquitin ligases. Further, these results are likely to have implications for thinking about the regulation of midline guidance in more complex nervous systems.

Weaknesses:<br /> -The authors in part rely on GOF genetic approaches to infer roles for Comm and Nedd4, and this is understood in light of the lack of phenotypes in certain mutant backgrounds, providing evidence for their capabilities in these GOF paridigms. However, there are a few missed opportunities in some experiments that would allow for conclusions to be drawn regarding endogenous Comm function, some involving relatively simple inclusion of null mutants in the sensitized genetic backgrounds used here.<br /> -A weakness beyond the purview of revision but important to mention is that the authors chose not to complement their GOF experiments with gene editing approaches to generate endogenous PY mutant alleles of Comm that might have been useful in genetic interaction experiments directed toward revealing roles for endogenous Comm in the regulation of Robo1.<br /> -There are very minor concerns regarding protein expression levels in various experiments that should be easy to address.

Reviewer #2 (Public Review):

Summary:<br /> Sullivan and Bashaw delve into the mechanisms that drive neural circuit assembly, and specifically, into the regulation of cell surface proteins that mediate axon pathfinding. During nervous system development, axons must traverse a molecularly and physically complex extracellular milieu to reach their synaptic targets. A fundamental, conserved repulsive signaling pathway is initiated by the Slit-Robo ligand-receptor pair. Robo, expressed on axon growth cones, binds Slit, secreted by midline cells, to prevent "pre-crossing" and "re-crossing" of axons at the midline. To control this repulsion, Robo surface levels are tightly regulated. In Drosophila, Commissureless (Comm) downregulates Robo surface levels and is required for axon crossing at the midline. Several studies suggest that PY motifs in Comm are required to localize Robo to endosomes. PY motifs have been shown to bind WW-domain containing proteins including the ubiquitin ligase Nedd4 family, so the authors propose that Comm may regulate Robo through Nedd4 interactions. Previous studies have hinted at a role for Nedd4-mediated ubiquitination of Comm in the regulation of Robo localization, but there have also been conflicting data. For example, Comm mutants that are unable to be ubiquitinated mimic wild-type Comm, suggesting that ubiquitination of Comm is not required for regulation of Robo. The current study utilizes a suite of genetic analyses in Drosophila to resolve discrepancies pertaining to the mode of Comm-dependent regulation of Robo1 and proposes that Comm acts as an adapter for the Nedd4 ubiquitin ligase to recognize Robo1 as a substrate. The authors also demonstrate that Nedd4 is indeed required for midline crossing.

Strengths:<br /> While this work is more incremental rather than field-shifting, it is nonetheless an excellent example of a rigorous, thorough analysis that culminates in enriching our mechanistic understanding of how neurons regulate cell-surface receptors in a spatiotemporal manner to control fundamental steps of circuit wiring. The experimental approach is thorough, and the manuscript is extremely well-written.

Weaknesses:<br /> Some key experiments (eg. complex formation) were performed in cell culture in an overexpression background. Also, there was a missed opportunity to bolster the model proposed by using Comm PY mutants in several experiments. Finally, Comm PY domains are required for proper Comm localization in neurons, but corresponding Robo localization was not analyzed.

eLife assessment

These ingenious and thoughtful studies present important findings concerning how people represent and generalise abstract patterns of sensory data. The issue of generalisation is a core topic in neuroscience and psychology, relevant across a wide range of areas, and the findings will be of interest to researchers across areas in perception, learning, and cognitive science. The findings have the potential to provide compelling support for the outlined account, but there appear other possible explanations, too, that may affect the scope of the findings but could be considered in a revision.

Reviewer #1 (Public Review):

Summary:<br /> This manuscript reports a series of experiments examining category learning and subsequent generalization of stimulus representations across spatial and nonspatial domains. In Experiment 1, participants were first trained to make category judgments about sequences of stimuli presented either in nonspatial auditory or visual modalities (with feature values drawn from a two-dimensional feature manifold, e.g., pitch vs timbre), or in a spatial modality (with feature values defined by positions in physical space, e.g., Cartesian x and y coordinates). A subsequent test phase assessed category judgments for 'rotated' exemplars of these stimuli: i.e., versions in which the transition vectors are rotated in the same feature space used during training (near transfer) or in a different feature space belonging to the same domain (far transfer). Findings demonstrate clearly that representations developed for the spatial domain allow for representational generalization, whereas this pattern is not observed for the nonspatial domains that are tested. Subsequent experiments demonstrate that if participants are first pre-trained to map nonspatial auditory/visual features to spatial locations, then rotational generalization is facilitated even for these nonspatial domains. It is argued that these findings are consistent with the idea that spatial representations form a generalized substrate for cognition: that space can act as a scaffold for learning abstract nonspatial concepts.

Strengths:<br /> I enjoyed reading this manuscript, which is extremely well-written and well-presented. The writing is clear and concise throughout, and the figures do a great job of highlighting the key concepts. The issue of generalization is a core topic in neuroscience and psychology, relevant across a wide range of areas, and the findings will be of interest to researchers across areas in perception and cognitive science. It's also excellent to see that the hypotheses, methods, and analyses were pre-registered.

The experiments that have been run are ingenious and thoughtful; I particularly liked the use of stimulus structures that allow for disentangling of one-dimensional and two-dimensional response patterns. The studies are also well-powered for detecting the effects of interest. The model-based statistical analyses are thorough and appropriate throughout (and it's good to see model recovery analysis too). The findings themselves are clear-cut: I have little doubt about the robustness and replicability of these data.

Weaknesses:<br /> I have only one significant concern regarding this manuscript, which relates to the interpretation of the findings. The findings are taken to suggest that "space may serve as a 'scaffold', allowing people to visualize and manipulate nonspatial concepts" (p13). However, I think the data may be amenable to an alternative possibility. I wonder if it's possible that, for the visual and auditory stimuli, participants naturally tended to attend to one feature dimension and ignore the other - i.e., there may have been a (potentially idiosyncratic) difference in salience between the feature dimensions that led to participants learning the feature sequence in a one-dimensional way (akin to the 'overshadowing' effect in associative learning: e.g., see Mackintosh, 1976, "Overshadowing and stimulus intensity", Animal Learning and Behaviour). By contrast, we are very used to thinking about space as a multidimensional domain, in particular with regard to two-dimensional vertical and horizontal displacements. As a result, one would naturally expect to see more evidence of two-dimensional representation (allowing for rotational generalization) for spatial than nonspatial domains.

In this view, the impact of spatial pre-training and (particularly) mapping is simply to highlight to participants that the auditory/visual stimuli comprise two separable (and independent) dimensions. Once they understand this, during subsequent training, they can learn about sequences on both dimensions, which will allow for a 2D representation and hence rotational generalization - as observed in Experiments 2 and 3. This account also anticipates that mapping alone (as in Experiment 4) could be sufficient to promote a 2D strategy for auditory and visual domains.

This "attention to dimensions" account has some similarities to the "spatial scaffolding" idea put forward in the article, in arguing that experience of how auditory/visual feature manifolds can be translated into a spatial representation helps people to see those domains in a way that allows for rotational generalization. Where it differs is that it does not propose that space provides a *scaffold* for the development of the nonspatial representations, i.e., that people represent/learn the nonspatial information in a spatial format, and this is what allows them to manipulate nonspatial concepts. Instead, the "attention to dimensions" account anticipates that ANY manipulation that highlights to participants the separable-dimension nature of auditory/visual stimuli could facilitate 2D representation and hence rotational generalization. For example, explicit instruction on how the stimuli are constructed may be sufficient, or pre-training of some form with each dimension separately, before they are combined to form the 2D stimuli.

I'd be interested to hear the authors' thoughts on this account - whether they see it as an alternative to their own interpretation, and whether it can be ruled out on the basis of their existing data.

Reviewer #2 (Public Review):

Summary:<br /> In this manuscript, L&S investigates the important general question of how humans achieve invariant behavior over stimuli belonging to one category given the widely varying input representation of those stimuli and more specifically, how they do that in arbitrary abstract domains. The authors start with the hypothesis that this is achieved by invariance transformations that observers use for interpreting different entries and furthermore, that these transformations in an arbitrary domain emerge with the help of the transformations (e.g. translation, rotation) within the spatial domain by using those as "scaffolding" during transformation learning. To provide the missing evidence for this hypothesis, L&S used behavioral category learning studies within and across the spatial, auditory, and visual domains, where rotated and translated 4-element token sequences had to be learned to categorize and then the learned transformation had to be applied in new feature dimensions within the given domain. Through single- and multiple-day supervised training and unsupervised tests, L&S demonstrated by standard computational analyses that in such setups, space and spatial transformations can, indeed, help with developing and using appropriate rotational mapping whereas the visual domain cannot fulfill such a scaffolding role.

Strengths:<br /> The overall problem definition and the context of spatial mapping-driven solution to the problem is timely. The general design of testing the scaffolding effect across different domains is more advanced than any previous attempts clarifying the relevance of spatial coding to any other type of representational codes. Once the formulation of the general problem in a specific scientific framework is done, the following steps are clearly and logically defined and executed. The obtained results are well interpretable, and they could serve as a good stepping stone for deeper investigations. The analytical tools used for the interpretations are adequate. The paper is relatively clearly written.

Weaknesses:<br /> Some additional effort to clarify the exact contribution of the paper, the link between analyses and the claims of the paper, and its link to previous proposals would be necessary to better assess the significance of the results and the true nature of the proposed mechanism of abstract generalization.

1) Insufficient conceptual setup: The original theoretical proposal (the Tolman-Eichenbaum-Machine, Whittington et al., Cell 2020) that L&S relate their work to proposes that just as in the case of memory for spatial navigation, humans and animals create their flexible relational memory system of any abstract representation by a conjunction code that combines on the one hand, sensory representation and on the other hand, a general structural representation or relational transformation. The TEM also suggests that the structural representation could contain any graph-interpretable spatial relations, albeit in their demonstration 2D neighbor relations were used. The goal of L&S's paper is to provide behavioral evidence for this suggestion by showing that humans use representational codes that are invariant to relational transformations of non-spatial abstract stimuli and moreover, that humans obtain these invariances by developing invariance transformers with the help of available spatial transformers. To obtain such evidence, L&S use the rotational transformation. However, the actual procedure they use actually solved an alternative task: instead of interrogating how humans develop generalizations in abstract spaces, they demonstrated that if one defines rotation in an abstract feature space embedded in a visual or auditory modality that is similar to the 2D space (i.e. has two independent dimensions that are clearly segregable and continuous), humans cannot learn to apply rotation of 4-piece temporal sequences in those spaces while they can do it in 2D space, and with co-associating a one-to-one mapping between locations in those feature spaces with locations in the 2D space an appropriate shaping mapping training will lead to the successful application of rotation in the given task (and in some other feature spaces in the given domain). While this is an interesting and challenging demonstration, it does not shed light on how humans learn and generalize, only that humans CAN do learning and generalization in this, highly constrained scenario. This result is a demonstration of how a stepwise learning regiment can make use of one structure for mapping a complex input into a desired output. The results neither clarify how generalizations would develop in abstract spaces nor the question of whether this generalization uses transformations developed in the abstract space. The specific training procedure ensures success in the presented experiments but the availability and feasibility of an equivalent procedure in a natural setting is a crucial part of validating the original claim and that has not been done in the paper.

2) Missing controls: The asymptotic performance in experiment 1 after training in the three tasks was quite different in the three tasks (intercepts 2.9, 1.9, 1.6 for spatial, visual, and auditory, respectively; p. 5. para. 1, Fig 2BFJ). It seems that the statement "However, our main question was how participants would generalise learning to novel, rotated exemplars of the same concept." assumes that learning and generalization are independent. Wouldn't it be possible, though, that the level of generalization depends on the level of acquiring a good representation of the "concept" and after obtaining an adequate level of this knowledge, generalization would kick in without scaffolding? If so, a missing control is to equate the levels of asymptotic learning and see whether there is a significant difference in generalization. A related issue is that we have no information on what kind of learning in the three different domains was performed, albeit we probably suspect that in space the 2D representation was dominant while in the auditory and visual domains not so much. Thus, a second missing piece of evidence is the model-fitting results of the ⦰ condition that would show which way the original sequences were encoded (similar to Fig 2 CGK and DHL). If the reason for lower performance is not individual stimulus difficulty but the natural tendency to encode the given stimulus type by a combo of random + 1D strategy that would clarify that the result of the cross-training is, indeed, transferring the 2D-mapping strategy.

Reviewer #3 (Public Review):

Summary:<br /> Pesnot Lerousseau and Summerfield aimed to explore how humans generalize abstract patterns of sensory data (concepts), focusing on whether and how spatial representations may facilitate the generalization of abstract concepts (rotational invariance). Specifically, the authors investigated whether people can recognize rotated sequences of stimuli in both spatial and nonspatial domains and whether spatial pre-training and multi-modal mapping aid in this process.

Strengths:<br /> The study innovatively examines a relatively underexplored but interesting area of cognitive science, the potential role of spatial scaffolding in generalizing sequences. The experimental design is clever and covers different modalities (auditory, visual, spatial), utilizing a two-dimensional feature manifold. The findings are backed by strong empirical data, good data analysis, and excellent transparency (including preregistration) adding weight to the proposition that spatial cognition can aid abstract concept generalization.

Weaknesses:<br /> The examples used to motivate the study (such as "tree" = oak tree, family tree, taxonomic tree) may not effectively represent the phenomena being studied, possibly confusing linguistic labels with abstract concepts. This potential confusion may also extend to doubts about the real-life applicability of the generalizations observed in the study and raises questions about the nature of the underlying mechanism being proposed.

Next, the study does not explore whether scaffolding effects could be observed with other well-learned domains, leaving open the question of whether spatial representations are uniquely effective or simply one instance of a familiar 2D space, again questioning the underlying mechanism.

Further doubt on the underlying mechanism is cast by the possibility that the observed correlation between mapping task performance and the adoption of a 2D strategy may reflect general cognitive engagement rather than the spatial nature of the task. Similarly, the surprising finding that a significant number of participants benefited from spatial scaffolding without seeing spatial modalities may further raise questions about the interpretation of the scaffolding effect, pointing towards potential alternative interpretations, such as shifts in attention during learning induced by pre-training without changing underlying abstract conceptual representations.

Conclusions:<br /> The authors successfully demonstrate that spatial training can enhance the ability to generalize in nonspatial domains, particularly in recognizing rotated sequences. The results for the most part support their conclusions, showing that spatial representations can act as a scaffold for learning more abstract conceptual invariances. However, the study leaves room for further investigation into whether the observed effects are unique to spatial cognition or could be replicated with other forms of well-established knowledge, as well as further clarifications of the underlying mechanisms.

Impact:<br /> The study's findings are likely to have a valuable impact on cognitive science, particularly in understanding how abstract concepts are learned and generalized. The methods and data can be useful for further research, especially in exploring the relationship between spatial cognition and abstract conceptualization. The insights could also be valuable for AI research, particularly in improving models that involve abstract pattern recognition and conceptual generalization.

In summary, the paper contributes valuable insights into the role of spatial cognition in learning abstract concepts, though it invites further research to explore the boundaries and specifics of this scaffolding effect.

eLife assessment

This valuable study links the "taste" of botanicals to their application as medicines used by the ancient Greco-Roman society. The authors used phylogenetic linear mixed models in a Bayesian framework to test the relationships between taste qualities, intensities, complexities, and therapeutic use. The evidence supporting the conclusions is solid, although there is a minor weakness concerning the somewhat inconsistent method of botanical preparation and presentation to the taster panelists; subjective bias and robustness of the participants' responses might have been overlooked. The study may be of broad interest to pharmacologists and scientists working on drug discovery, particularly those interested in natural products.

Reviewer #1 (Public Review):

Summary: The authors explored correlations between taste features of botanical drugs used in ancient times and therapeutic uses, finding some potentially interesting associations between intensity and complexity of flavors and therapeutic potential, plus some more specific associations described in the discussion section. I believe the results could be of potential benefit for the drug discovery community, especially for those scientists working in the field of natural products.

Strengths:

Owing to its eclectic and somehow heterodox nature, I believe the article might be of interest for a general audience. In fact, I have enjoyed reading it and my curiosity was raised by the extensive discussion.

The idea of revisiting a classical vademecum with new scientific perspectives is quite stimulating.

The authors have undertaken a significant amount of work, collecting 700 botanical drugs and exploring their taste and association with known uses via eleven trained panellists.

Weaknesses:

I have some methodological concerns. Robustness in the panelists' perceptions has not been addressed, and not every panellist tasted every drug because of time constrains. The breaks between tasting different samples was not standardized, and depended on the persistence of chemosensory perception, possibly also due to time constraints.

Reviewer #2 (Public Review):

Summary:<br /> This is an unusual, but interesting approach to link the "taste" of plants and plant extracts to their therapeutic use in ancient Graeco-Roman culture. The authors used a panel of 11 trained tasters to test ~700 different medicinal plants and describe them in terms of 22 "taste" descriptors. They correlated these descriptors with the plant's medical use as reported in the De Materia Medica (DMM 1st Century, CE). Correcting for some of the plants' evolutionary phylogenetic relationships, the authors found that taste descriptors along with intensity measures were correlated with the "versatility" and/or a specific therapeutic use of the medicine. For example, simple but intense tastes were correlated with versatility of a medicine. Specific intense tastes were linked to versatility while others were not; intense bitter, starchy, musky, sweet, cooling and soapy were associated with versatility, but sour and woody were negatively associated. Also some specific tastes could be associated with specific uses - both positive and negative associations. Some of these findings make sense immediately, but others are somewhat surprising, and the authors propose some links between taste and medicinal use (both historical and modern use) in the discussion. The authors state that this study allows for a re-evaluation of pre-scientific knowledge, pointing toward a central role for taste in medicine.

Strengths:<br /> The real strength of this study is the novelty of this approach - using modern day tasters to evaluate ancient medicinal plants to understand the potential relationships between taste and therapeutic use, lending some support to the idea that the "taste" of a medicine is linked to its effectiveness as a treatment.

Weaknesses:<br /> Because of the limitations of time and the type of botanicals being tested, there is an inherent difficulty in assessing taste intensity. However, because these botanicals are tested by multiple panelists and sometimes tested repeatedly by individual panelists, this helps support the author's analyses.

eLife assessment

This fundamental study provides compelling evidence to explain how chemical variations within a set of kinase inhibitors drive the selection of specific Erk2 conformations. Conformational selection plays a critical role in targeting medically relevant kinases such as Erk2 and the findings reported here open new avenues for designing small molecule inhibitors that block the active site while also steering the population of the enzyme into active or inactive conformations. Since protein dynamics and conformational ensembles are essential for enzyme function, this work will be of broad interest to those working in drug development, signal transduction, and enzymology.

Reviewer #1 (Public Review):

Summary:<br /> The authors set out to determine how chemical variation on kinase inhibitors determines selection of Erk2 conformations and how inhibitor binding affects ERk2 structure and dynamics.

Strengths:<br /> The study is beautifully presented both verbally and visually. The NMR experiments and the HDX experiments complement each other for the study of Erk2 solution dynamics. X-ray crystallography of Erk2 complexes with inhibitors show small but distinct structural changes that support the proposed model for the impact of inhibitor binding.

Reviewer #2 (Public Review):

Erk2 is an essential element of the MAP kinase signaling cascade and directly controls cell proliferation, migration, and survival. Therefore, it is one of the most important drug targets for cancer therapy. The catalytic subunit of Erk2 has a bilobal architecture, with the small lobe harboring the nucleotide-binding pocket and the large lobe harboring the substrate-binding cleft. Several studies by the Ahn group revealed that the catalytic domain hops between (at least) two conformational states: active (R) and inactive (L), which exchange in the millisecond time scale based on the chemical shift mapping. The R state is a signature of the double phosphorylated Erk2 (2P-Erk2), while the L state has been associated with the unphosphorylated kinase (0P-Erk2). Interestingly, the X-ray structures reveal only minimal differences between these two states, a feature that led to the conclusion that active and inactive states are structurally similar but dynamically very different. The Ahn group also found that ATP-competitive inhibitors can steer the populations of Erk2 either toward the R or the L state, depending on their chemical nature. The latter opens up the possibility of modulating the activity of this kinase by changing the chemistry of the ATP-competitive inhibitor. To prove this point, the authors present a set of nineteen compounds with diverse chemical substituents. From their combined NMR and HDX-Mass Spec analyses, fourteen inhibitors drive the kinase toward the R state, while four compounds keep the kinase hopping between the R and L states. Based on these data, the authors rationalize the effects of these inhibitors and the importance of the nature of the substituents on the central scaffold to steer the kinase activity. While all these inhibitors target the ATP binding pocket, they display diverse structural and dynamic effects on the kinase, selecting a specific structural state. Although the inhibited kinase is no longer able to phosphorylate substrates, it can initiate signaling events functioning as scaffolds for other proteins. Therefore, by changing the chemistry of the inhibitors it may be possible to affect the MAP cascade in a predictable manner. This concept, recently introduced as proof of principle, finds here its significance and practical implications. The design of the next-generation inhibitors must be taken into account for these design principles.<br /> The research is well executed, and the data support the author's conclusions.

Reviewer #3 (Public Review):

Summary:<br /> Anderson at al utilize an array of orthogonal techniques to highlight the important of protein dynamics for the function and inhibition of the kinase ERK2. ERK2 is important for a large variety of biological functions.

Strengths:<br /> This is a thorough and detailed study that uses a variety of techniques to identify critical molecular/chemical parameters that drive ERK2 in specific states.

Weaknesses:<br /> No details rules were identified so that novel inhibitors could be designed. Nevertheless, the mode of action of these existing inhibitors are much better defined.

eLife assessment

This valuable study reports on a new tool that allows for light-controlled protein degradation in Escherichia coli. With the improved light-responsive protein tag, endogenous protein levels can be reduced severalfold. The methodology is convincing and will be of interest to the fields of gene expression regulation in bacteria and, more generally to synthetic biologists.

Reviewer #1 (Public Review):

Specifically controlling the level of proteins in bacteria is an important tool for many aspects of microbiology, from basic research to protein production. While there are several established methods for regulating transcription or translation of proteins with light, optogenetic protein degradation has so far not been established in bacteria. In this paper, the authors present a degradation sequence, which they name "LOVdeg", based on iLID, a modified version of the blue-light-responsive LOV2 domain of Avena sativa phototropin I (AsLOV2). The authors reasoned that by removing the three C-terminal amino acids of iLID, the modified protein ends in "-E-A-A", similar to the "-L-A-A" C-terminus of the widely used SsrA degradation tag. The authors further speculated that, given the light-induced unfolding of the C-terminal domain of iLID and similar proteins, the "-E-A-A" C-terminus would become more accessible and, in turn, the protein would be more efficiently degraded in blue light than in the dark.

Indeed, several tested LOVdeg-tagged proteins show clearly lower cellular levels in blue light than in the dark. Depending on the nature and expression level of the target protein, protein levels are reduced modestly to strongly (2 to 20x lower levels upon illumination). Accordingly, the authors propose to use their system in combination with other light-controlled expression systems and provide data validating this approach. The LOVdeg system allows to modulate protein levels to a similar degree and with comparable kinetics as optogenetic systems controlling transcription or translation of protein, and can be combined with such systems.

The manuscript and the figures are generally very well-composed and follow a clear structure. The schematics nicely explain the underlying principles. Besides the advantages of the LOVdeg approach, including its complementarity to controlled expression of proteins, the revised version of the manuscript also highlights the limitations of the method more clearly, e.g., (i) the need to attach a C-terminal tag of considerable size to the protein of interest, (ii) the limited efficiency (slightly less efficient and slower than EL222, a light-dependent transcriptional control mechanism), and (iii) the incompletely understood prerequisites for its application. Taken together, this manuscripts describes the LOVdeg system as a valuable addition to the tool box for controlling protein levels in prokaryotic cells.

Reviewer #2 (Public Review):

In this manuscript the authors present and characterize LOVdeg, a modified version of the blue-light sensitive AsLOV2 protein, which functions as a light-inducible degron in Escherichia coli. Light has been shown to be a powerful inducer in biological systems as it is often orthogonal and can be controlled in both space and time. Many optogenetic systems target regulation of transcription, however in this manuscript the authors target protein degradation to control protein levels in bacteria. This is an important advance in bacteria, as inducible protein degradation systems in bacteria have lagged behind eukaryotic systems due to protein targeting in bacteria being primarily dependent on primary amino acid sequence and thus more difficult to engineer. In this manuscript, the authors exploit the fact that the J-alpha helix of AsLOV2, which unwinds into a disordered domain in response to blue light, contains an E-A-A amino acid sequence which is very similar to the C-terminal L-A-A sequence in the SsrA tag which is targeted by the unfoldases ClpA and ClpX. They truncate AsLOV2 to create AsLOV2(543) and combine this truncation with a mutation that stabilizes the dark state to generate AsLOV2*(543) which, when fused to the C-terminus of mCherry, confers light-induced degradation. The authors do not verify the mechanism of degradation due to LOVdeg, but evidence from deletion mutants contained in the supplemental material hints that there is a ClpA dominated mechanism. The LOVdeg is able to target mCherry for protein degradation across different phases of bacterial growth, which is important for regulating processes at stationary phase and a potential additional advantage over transcriptional repression systems. They demonstrate modularity of this LOVdeg by using it to degrade the LacI repressor, CRISPRa activation through degradation of MCP-SoxS, and the AcrB protein which is part of the AcrAB-TolC multidrug efflux pump. In all cases, measurement of the effect of the LOVdeg is indirect as the authors measure reduction in LacI repression, reduction in CRISPRa activation, and drug resistance rather than directly measuring protein levels. Nevertheless the evidence is convincing, although seemingly less effective than in the case of mCherry degradation, although it is hard to compare due to the different endpoints being measured. The authors further modify LOVdeg to contain a known photocycle mutation that slows its reversion time in the dark, so that LOVdeg is more sensitive to short pulses of light which could be useful in low light conditions or for very light sensitive organisms. They also demonstrate that combining LOVdeg with a blue-light transcriptional repression system (EL222) can decrease protein levels an additional 23-fold (relative to 7-fold with LOVdeg alone). Finally, the authors apply LOVdeg to a metabolic engineering task, namely reducing expression of octanoic acid by regulating the enzyme CpFatB1, an acyl-ACP thioesterase. The authors show that tagging CpFatB1 with LOVdeg allows light induced reduction in octanoic acid titer over a 24 hour fermentation. In particular, by comparing control of CpFatB1 with EL222 transcriptional repression alone, LOVdeg, or both the authors show that light-induced protein degradation is more effective than light-induced transcriptional repression. The authors suggest that this is because transcriptional repression is not effective when cells are at stationary phase (and thus there is no protein dilution due to cell division). Overall, the authors have generated a modular, light-activated degron tag for use in Escherichia coli that is likely to be a useful tool in the synthetic biology and metabolic engineering toolkit.

Reviewer #3 (Public Review):

The authors present the mechanism, validation, and modular application of LOVtag, a light-responsive protein degradation tag that is processed by the native degradosome of Escherichia coli. Upon exposure to blue light, the c-terminal alpha helix unfolds, essentially marking the protein for degradation. The authors demonstrate the engineered tag is modular across multiple complex regulatory systems, which shows its potential widespread use throughout the synthetic biology field. The step-by-step rational design of identifying the protein that was most dark-stabilized as well as most light-responsive for degradation, was useful in terms of understanding the key components of this system. The most compelling data shows that the engineered LOVTag can be fused to multiple proteins and achieve light-based degradation, without affecting the original function of the fused protein.

eLife assessment

This work shows for the first time that the balance of mTOR (mTORC1 and mTORC2) in Sertoli cells regulates the rate of sperm epigenetic aging. The manuscript presents valuable findings that have some theoretical and practical implications. The strength of the evidence is however incomplete: they are limited in some places and the aims are not always fully supported by the results.

Reviewer #1 (Public Review):

In the manuscript "Mechanistic target of rapamycin (mTOR) pathway in Sertoli cells regulates age-dependent changes in sperm DNA methylation", the authors proposed to test if the balance of mTOR complexes in Sertoli cells may play a significant role in age-dependent changes in the sperm epigenome. The paper could be of interest and has a good scientific aim but there are too many drawbacks that hamper the initial enthusiasm. All sections need extensive revision. The paper is mostly descriptive without a mechanistic-orientated explanation for the observed results.

Specific comments:

1. The abstract is poorly written. There is a lot of unnecessary introduction that does not provide a rationale for the work. It is not possible to understand the experimental approach or the major data just by reading the abstract. It does not clearly represent the work.

2. The introduction is somewhat vague and does not provide a clear rationale for the hypothesis. There should be more focus more on the role of mTOR in Sertoli cells that goes far beyond BTB. That will give more focus on mTOR. Then it is important to focus on BTB and mTOR: what is known? What is the gap and how can it be solved? Several relevant references are missed concerning mTOR and Sertoli cells.

3. The Material and Methods section needs improvement. There is much important information missing. For instance: how many animals were used per group and how was the breeding done? At what age? Statistical analysis should be explained in detail.

4. The results description could be improved. It is vague without highlighting how much difference was detected. The results should be numerically described when possible and the differences should be highlighted. A 10% difference may be significant but not biologically relevant. To correctly evaluate the differences it is important to describe them with some degree of detail.

5. There is no discussion of the data. The authors just summarize their findings without a comprehensive analysis of the literature and how the effects can be mediated. mTOR interacts with different pathways (mTORC1 and mTORC2 are even mediators of distinct pathways). This would be very relevant to discuss. In addition, there are many study limitations not discussed. There is no clear mechanistic explanation of the way by which the mTOR pathway in Sertoli cells regulates age-dependent changes in sperm DNA methylation. The paper seems preliminary.

6. Figure 1 is too simple and does not provide any schematic support for the text.

7. Figure 2 lacks some detail. For instance, how many animals were used for each step?

8. Taking into consideration the roles of mTOR on sperm, particularly mTORC1, it is not clear whether there were any differences in sperm motility.

Reviewer #2 (Public Review):

In this study, the authors hypothesized that the balance of mTOR complexes in Sertoli cells may also play a significant role in age-dependent changes in the sperm epigenome. To test this hypothesis, the authors use transgenic mice with manipulated activity of mTOR complexes in Sertoli cells. These results suggest that the mTOR pathway in Sertoli cells may be used as a novel target of therapeutic interventions to rejuvenate the sperm epigenome in advanced-age fathers.

The authors attempt to demonstrate that the balance of mTOR complexes in Sertoli cells regulates the rate of sperm epigenetic aging. The authors have effectively met their research objectives, and their conclusions are supported by the data presented.

Reviewer #3 (Public Review):

Summary and Strength:

The manuscript by Amir et al. describes that Sertoli-specific inactivation of the mTORC1 and mTORC2 complex by KO of either Raptor or Rictor, respectively, resulted in progressive changes in blood-testis-barrier (BTB) function, testis weight, and sperm parameters, including counts, morphology, mtDNA content and sperm DNA methylation.

The described studies are based on the hypothesis that a decline of BTB function with increasing chronological age of a male contributes to the DNA methylation changes that are known to occur in sperm DNA of old males when compared to sperm DNA from isogenic young males. In order to demonstrate the relevance of a functioning BTB for the maintenance of sperm methylation patterns, the authors generated mice with genetically disrupted mTORC2 complex or mTORC1 complex in Sertoli cells and determined sperm methylation patterns in comparison to isogenic wild-type males. In line with previously published scientific literature (e.g. Mok et al., 2013; Dong et al, 2015; and others), the manuscript corroborates that a Sertoli-cell specific deletion of mTORC2 caused a loss of BTB function and a progressive spermatogenic defect. The authors further show that sperm DNA is differentially methylated (DMRs) as a consequence of either a mTORC2 disruption (associated with a loss of BTB function) or following a mTORC1 disruption (BTB function either increased or not leaky) when compared to their isogenic age-matched wt controls. Those DMRs overlap partially with changes in sperm DNA methylation that were found when comparing sperm from 8-week males with sperm isolated from 22-week-old male mice.

The authors interpret the observed changes as representative of the sperm DNA methylation changes that occur during normal chronological aging of the male. For an aged control group, the authors use sperm DNA of 22-week-old wild-type mates from the mTORC2 and mTORC2 KO breeding and compare the sperm methylation patterns found in sperm from those 22-week males to 8-week young males, that are intended to represent an old and a young cohort, respectively. DNA methylation analysis indicates that a disruption of mTORC2 (& decrease of BTB function) results in increased DNA methylation of sperm DNA, while a disruption of mTORC1 (and proposed increase of BTB tightness, not shown in the manuscript, though) resulted in increased hypomethylation.

Weaknesses:

While the hypothesis and experimental system are interesting and the data demonstrating the relevance of the mTORC2 complex for BTB function is convincing, several open questions limit the evidence that supports the hypothesis that the sperm DNA methylation changes seen in old males are caused by BTB failure following an imbalance of mTOR signaling complexes. The major critique points are the lack of a chronologically old group and the choice of 8 weeks & 22 weeks age of age:

- Data illustrating the degree of BTB decline and sperm DNA methylation changes from chronologically "old" male mice is missing. 22-week-old mice are not considered old but are of good and mature breeding age, equivalent to humans in their mid-late twenties. (In the manuscript, the 22-week-old wildtype mice show no evidence of BTB breakdown (Figure 3), so why are their sperm used to represent "aged" sperm?

- Adding a group of "old" wild-type mice of 12-14 months of age, which is closer to the end of effective reproduction in mice, more equivalent to 45-59 year-old humans) could be used to illustrate that (a) aging causes a marked decrease in BTB function at this time in mouse life, and that this BTB breakdown chronologically aligns with the age-associated DNA hypermethylation seen in old sperm. Age-matched "old" mTORC1 KO, with a (supposedly) tighter BTB barrier, could then be expected to have a sperm DMA methylation profile closer to that of younger wild-type animals. Such data are currently missing. While the progressive testicular decline observed in the mTORC1 KO (Fig.5) could make it difficult to obtain the appropriately aged mTORC1 KO tissues, it is completely feasible to obtain data from chronologically old wild-type males. (The progressive testicular decline further raises the question of what additional defects the KO causes, and how such additional defects would influence the sperm DNA methylation profile.) The addition of data from an old group to the currently included groups could strengthen the interpretation that the observations in the BTB-defective mTORC2 KO mice are modelling an age-related testicular decline, provided that the DMRs seen in the chronologically old group significantly overlap with the BTB-defective changes.

- In the current form, the described differences in sperm DNA methylation are based on comparisons between pubertal mice (8 weeks) and mature but not old adult males (22 weeks), while a chronologically "old" group is missing from the data sets and comparisons. Thus, it appears that the described sperm methylation changes reflect developmental changes associated with normal maturation and not necessarily declining sperm quality due to aging. (Sperm obtained from 8-week-old mice likely were generated, at least in part, during the 1st wave of spermatogenesis, which is known to differ from the continuously proceeding spermatogenesis during the remained of the mature life. During the 1st wave of spermatogenesis, Sertoli cells are known to undergo gene expression changes which could contribute to varying degrees of BTB function, and thus have effects on the sperm DNA methylation profiles of such 1st wave sperm.)

- It is unclear why the aging-related DMRs between the 8 and 22-week-old wild-type mice vary so dramatically between the two wild-type groups derived from the mTORC1 and the mTORC2 breeding (Fig. S4). If the main difference was due to mTORC1 or mTORC2 activity, both wildtype groups should behave very similarly. Changes seen in a truly "old" mouse (e.g. 20 weeks to 56 weeks), changes in "young mTORC1" and in "old mTORC2" are missing. How do those numbers and profiles compare to the shown samples?

Some general comments regarding the chosen age of animals:

- As mentioned, sperm from 8-week-old mice represent many sperm that were produced in the 1st wave of spermatogenesis; 22-week-old mice are not considered chronologically old mice, but mature and "relatively" young animals. 18-24 month-old mice are considered to be equivalent to 56-69 year-old humans, and might be more suitable to detect aging effects. "Old mice" for study purposes should be at least 12-14 months of age, ideally >18 months of age. 22 weeks (5 months of age) are mice at good breeding age, but still considered mature adults, not old males, and therefore are not expected to show typical aging health problems (like declining fertility).

Even the cited reference (Flurkey et al. 2007) defines that "... mice used a reference group for "young mice" should be at least 3 months of age (~ 13 weeks), i.e. fully sexually mature. The authors specifically state: " The young adult group should be at least 3 months old because, although mice are sexually mature by 35 days, relatively rapid maturational growth continues for most biologic processes and structures until about 3 months. The upper age range for the young adult group is typically about 6 months. ... For the middle-aged group, 10 months is typically the lower limit.... The upper age limit for the middle-aged group is typically 14-15 months, because at this age, most biomarkers still have not changed to their full extent, and some have not yet started changing. For the old group, the lower age limit is 18 months because age-related change for almost all biomarkers of aging can be detected by then. The upper limit is 22-26 months, depending on the genotype." According to this reference, mice up to 6 months of age are generally considered "mature adults" (equivalent to humans 20-30 yrs), mice of 10-14 month are "middle-aged adults" (equivalent to ~38-47 human years) and 18-24 month mice are "old" (equivalent to human of 56-69 yrs.).

Going on these commonly used age ranges, it is unclear why the authors used 8-week-old mice (generally considered pubertal to late adolescent age) as young mice and 5-month-old mice as "old mice".

Differences seen between these cohorts most likely do not reflect aging, but more likely reflect changes associated with normal developmental maturation, since testis and epididymides continue to grow until about 10-11 weeks of age.

- The DMRs identified between 8 and 22-week-old animals could represent DMRs that are dependent on developmental maturation more than being changed in an "age-dependent" manner (in the sense of increased chronological age). This interpretation is congruent with the fact that those DMRs are enriched for developmental categories.

eLife assessment

The main idea tested in this work is that host galectin-9 inhibits Mycobacterium tuberculosis (Mtb) growth by recognizing the Mtb cell wall component arabinogalactan (AG) and, as a result, disrupting mycobacterial cell wall structure. Moreover, a similar effect is achieved by anti-AG antibodies. While the hypothesis is intriguing and the work has the potential to make a valuable contribution to Mtb therapy, the evidence presented is incomplete and does not explain several critical points including the dose-independent effect of galectin-9 on Mtb growth and how anti-AG antibodies and galectin-9 access the AG layer of intact Mtb.

Reviewer #1 (Public Review):

The molecular interactions that determine infection (and disease) trajectory following human exposure to Mycobacterium tuberculosis (Mtb) are critical to understanding mycobacterial pathogenicity and tuberculosis (TB), a global public health threat that disproportionately impacts a number of high-burden countries and, owing to the emergence of multidrug-resistant Mtb strains, is a major contributor to antimicrobial resistance (AMR). In this submission, Qin and colleagues extend their own previous work which identified a potential role for host galectin-9 in recognizing the major Mtb cell wall component, arabinogalactan (AG). First, the authors present data indicating that galectin-9 inhibits mycobacterial growth during in vitro culture in liquid and on solid media and that the inhibition depends on carbohydrate recognition by galectin-9. Next, the authors identify anti-AG antibodies in sera of TB patients and use this observation to inform isolation of monoclonal anti-AG antibodies (mAbs) via an in vitro screen. Finally, they apply the identified anti-AG mAbs to inhibit Mtb growth in vitro via a mechanism that proteomic and microscopic analyses suggest is dependent on the disruption of the cell wall structure. In summary, the dual observation of (i) the apparent role of naturally arising host anti-AG antibodies to control infection and (ii) the potential utility of anti-AG monoclonal antibodies as novel anti-Mtb therapeutics is compelling; however, as noted in the comments below, the evidence presented to support these insights is inadequate and the authors should address the following:

1. The experiment that utilizes lactose or glucose supplementation to infer the importance of carbohydrate recognition by galectin-9 cannot be interpreted unequivocally owing to the growth-enhancing effect of lactose supplementation on Mtb during liquid culture in vitro.

2. Similar to the comment above, the apparent dose-independent effect of galectin-9 on Mtb growth in vitro is difficult to reconcile with the interpretation that galectin is functioning as claimed.

3. The claimed differences in galectin-9 concentration in sera from tuberculin skin test (TST)-negative or TST-positive non-TB cases versus active TB patients are not immediately apparent from the data presented.

4. Neither fluorescence microscopy nor electron microscopy analyses are supported by high-quality, interpretable images which, in the absence of supporting quantitative data, renders any claims of anti-AG mAb specificity (fluorescence microscopy) or putative mAb-mediated cell wall swelling (electron microscopy) highly speculative.

5. Finally, the absence of any discussion of how anti-AG antibodies (similarly, galectin-9) gain access to the AG layer in the outer membrane of intact Mtb bacilli (which may additionally possess an extracellular capsule/coat) is a critical omission - situating these results in the context of current knowledge about Mtb cellular structure (especially the mycobacterial outer membrane) is essential for plausibility of the inferred galectin-9 and anti-AG mAb activities.

Reviewer #2 (Public Review):

Summary:

In this manuscript, the authors work to extend their previous observation that galectin-9 interacts with arabinogalactans of Mtb in their EMBO reports 2021 manuscript. Here they provide evidence that the CARD2 domain of galectin-9 can inhibit the growth of Mtb in culture. In addition, antibodies that also bind to AG appear to inhibit Mtb growth in culture. These data indicate that independent of the common cell-associated responses to galectin-9 and antibodies, the interaction of these proteins with AG of mycobacteria may have consequences for bacterial growth.

Strengths:

The authors provided several lines of evidence in culture media that the introduction of galectin-9 proteins and antibodies inhibits the growth rate of Mtb.

Weaknesses:

In light of other observations that cleaved galectin-9 levels in the plasma is a biomarker for severe infection (Padilla A et al Biomolecules 2021 and Iwasaki-Hozumi H et al. Biomoleucles 2021) it is difficult to reconcile the author's interpretation that the elevated gal-9 in Active TB patients (Figure 1E) contributes to the maintenance of latent infection in humans. The authors should consider incorporating these observations in the interpretation of their own results.

The anti-AG titers were measured only in individuals with active TB (Figure 3C), generally thought to be a less protective immunological state. The speculation that individuals with anti-AG titers have some protection is not founded. Further only 2 mAbs were tested to demonstrate restriction of Mtb in culture. It is possible that clones of different affinities for AG present within a patient's polyclonal AG-antibody responses may or may not display a direct growth restriction pressure on Mtb in culture. The authors should soften the claims about the presence of AG-titers in TB patients being indicative of protection.

eLife assessment

This study offers a useful advance by introducing a cord blood DNA methylation score for maternal smoking effects, with the inclusion of diverse cohorts. However, the overall strength of evidence is deemed incomplete, due to concerns regarding low exposure levels, low statistical power, potential overfitting, and the need for clearer descriptions of statistical methods. Building more directly from the existing evidence base, exploring differences between ancestries, and considering additional health outcomes would help to enhance the study.

Reviewer #1 (Public Review):

Summary:

The authors report on the development of the first cord blood DNA methylation score to capture the epigenetic effects of maternal smoking. The score was built in a White European cohort and tested in White European and South Asian ancestry cohorts. Additionally, epigenome-wide association studies were conducted to quantify the impact of maternal smoking on newborn health.

Strengths:

The main strengths include the use of multiple cohorts of different ancestries. This is also the first study to build a cord blood predictor of maternal smoking.

Weaknesses:

The manuscript could benefit from a more detailed description of methods, especially those used to derive MRS for maternal smoking, which appears to involve overfitting. In particular, the addition of a flow chart would be very helpful to guide the reader through the data and analyses. The FDR correction in the EWAS corresponds to a fairly liberal p-value threshold.

Reviewer #2 (Public Review):

Summary:

The authors generated a DNA methylation score in cord blood for detecting exposure to cigarette smoke during pregnancy. They then asked if it could be used to predict height, weight, BMI, adiposity, and WHR throughout early childhood.

Strengths:

The study included two cohorts of European ancestry and one of South Asian ancestry.

Weaknesses:

1. The number of mothers who self-reported any smoking was very low, much lower than in the general population and practically non-existent in the South Asian population. As a result, all analyses appeared to have been underpowered. It is possibly for this reason that the authors chose to generate their DNA methylation model using previously published summary statistics. The resulting score is not of great value in itself due to the low-powered dataset used to estimate covariance between CpG sites. In fact, a score was generated for a much larger, better-powered dataset several years ago (Reese, EHP, 2017, PMID 27323799).

2. The conclusion that "even minimal smoking exposure in South Asian mothers who were not active smokers showed a DNAm signature of small body size and low birthweight in newborns" is not warranted because no analyses were performed to show that the association between DNA methylation and birth size/weight was driven by maternal smoking.

3. Although it was likely that some mothers were exposed to second-hand smoke and/or pollution, data on this was either non-existent or not included in this study. Including this would have allowed a more novel investigation of the effects of smoke exposure on the pregnancies of non-smoking mothers.

4. One of the European cohorts and half of the South Asian cohort had DNA methylation measured on only 2500 CpG sites. This set of sites included only 125 sites previously linked to prenatal smoking. The resulting model of prenatal smoking was small (only 11 CpG sites). It is possible that a large model may have been more powerful.

5. The health outcomes investigated are potentially interesting but there are other possibly more important outcomes of interest such as birth complications, asthma, and intellectual impairment which are known to be associated with prenatal smoking.

eLife assessment

This study presents a valuable finding on the impact of metformin-induced shifts in gut microbial community structure and metabolite levels for drug efficacy in a mouse model of liver injury. The current evidence supporting the claims of the authors is incomplete, although inclusion of additional controls and a revision to clarify the reviewer's methodological concerns could strengthen the study. With revision, this paper could be of broad interest to researchers across multiple disciplines, including the microbiome, liver disease, and pharmacology.

Reviewer #1 (Public Review):

The major strength of this work is its scope, including detailed mouse phenotyping, inter-disciplinary methods, and numerous complementary experiments. The antibiotic depletion and FMT experiments provide support for a role of the gut microbiota in this mouse model.

A major limitation is the lack of studies narrowing down which microbes are responsible. Sequencing data is shown, but no follow-up studies are done with bacterial isolates or defined communities.

The link to GABA is also somewhat tenuous. While it does match the phenotypic data, there are no targeted experiments in which GABA producing microbial communities/strains are compared to a control community/strain. As such, it seems difficult to know how much of the effects in this model are due to GABA vs. other metabolites.

My major recommendation would be to revise the title, abstract, and discussion to provide more qualification and to consider alternative interpretations.

Some key controls are also missing, which could be addressed by repeat experiments in the mouse model. The antibiotic depletion experiment would be improved by testing the effect of antibiotics in the absence of metformin, to see if the effect is just driven by the model itself as opposed to an interaction between metformin and antibiotics. The FMT experiment lacks a control group and suffers from pseudoreplication: multiple donors from metformin treated and untreated mice could be used to colonize separate groups of recipient mice.

eLife assessment

In this important manuscript, the authors used unbiased approaches to identify somatic mutations in publicly available databases that would disrupt clinically approved antibodies targeting HER2. Using a solid combination of both computational and experimental approaches they identify mutations that could restore therapeutic antibody sensitivity in a series of disease-relevant model systems. Additional cell-based and in vivo assays would strengthen the work and increase the translational and potential clinical relevance of the proposed work.

Reviewer #1 (Public Review):

Summary:<br /> Starting from an unbiased search for somatic mutations (from COSMIC) likely disrupting binding of clinically approved antibodies the authors focus on mutations known to disrupt binding between two ERBB2 mutations and Pertuzamab. They use a combined computational and experimental strategy to nominate a position that when mutated could result in restoring the therapeutic activity of the antibody. Using in vitro assays the authors confirm that the engineered antibody binds to the mutant ERBB2 and prevents ERBB3 phosphorylation

Strengths:<br /> 1. In my assessment, the data sufficiently demonstrates that a modified version of Pertuzamab can bind both the wild-type and S310 mutant forms of ERBB2.

2. The engineering strategy employed is rational and effectively combines computational and experimental techniques.

3. Given the clinical activity of HER2-targeting ADCs, antibodies unaffected by ERBB2 mutations would be desired.

Weaknesses:<br /> 1. There is no data showing that the engineered antibody is equally specific as Pertuzamab i.e. that it does not bind to other (non-ERBB2) proteins.

2. There is no data showing that the engineered antibody has the desired pharmacokinetics/pharmacodynamics properties or efficacy in vivo.

3. Computational approaches are only used to design a phage-screen library, but not used to prioritize mutations that are likely to improve binding (e.g. based on predicted impact on the stability of the interaction). A demonstration of how computational pre-screening or lead optimization can improve the time-intensive process would be a welcome advance.

Context:<br /> The conflict of interest statement is inadequate. Most authors of the study (but not the first author) are employees of Biolojic, a company developing multi-specific antibodies, but the statements do not clarify whether the presented antibodies represent Biolojic IP, whether the company sponsored the research, and whether the company is further developing the specific antibodies presented.

Reviewer #2 (Public Review):

Summary:<br /> Peled et al identified HER2 mutations in connection with resistance to the anti-HER2 antibody Pertuzumab-mediated therapy. After constructing a yeast display library of Pertuzumab variants with 3.86×1011 sequences for targeted screening of variant combinations in chosen 6 out of 14 residues, the authors performed experimental screening to obtain the clones that bind to HER2 WT and/or mutants (S310Y and S310F), and then combined new variations to obtain antibodies with a broad spectrum binding to both WT and two HER2 mutants. These are interesting studies of clinical impact and translational potential.

Strengths:<br /> 1. Deep computational analyses of large datasets of clinical data provide useful information about HER2 mutations and their potential relevance to antibody therapy resistance.

2. There is valuable information analyzing the residues within or near the interface between the antigen HER2 and the Pertuzumab antibody (heavy chain). The experimental antibody library screening obtained 90+ clones from 3.86×1011 sequences for further functional validation.

Weaknesses:<br /> 1. There is a lack of assessment for antibody variant functions in cancer cell phenotypes in vitro (proliferation, cell death, motility) or in vivo (tumor growth and animal survival). The only assay was the western blotting of phosphopho-HER3 in Figure 4. However, HER2 levels and phosphor-HER2 were not analyzed.

2. There is a misleading impression from the title of computational engineering of a therapeutic antibody and the statement in the abstract "we designed a multi-specific version of Pertuzumab that retains original function while also bindings these HER2 variants" for a few reasons:<br /> a. The primary method used for variant antibody identification for HER2 mutant binding is rather traditional experimental screening based on yeast display instead of the computational design of a multi-specific version of Pertuzumab.<br /> b. There is insufficient or lack of computational power in the antibody design or prioritization in choosing variant residues for the library construction of 3.86×1011 sequences. It seems random combinations from 6 residues out of 4 groups with 20 amino acid options.<br /> c. The final version of the tri-binding variant is a combination of screened antibody clones instead of computation design from scratch.<br /> d. There is incomplete experimental evidence about the therapeutic values of newly obtained antibody clones.

3. Figures can be improved with better labeling and organization. Some essential pieces of data such as Supplementary Figure 1B on HER2 mutations in S310 that abrogated its binding to Pertuzumab should be placed in the main figures.

4. It is recommended to provide a clear rationale or flowchart overview into the main Figure 1. Figure 2A can be combined with Figure 1 to the list of targeted residues.

5. The quality of Figures such as Figure 2B-C flow data needs to be improved.

eLife assessment

This important study presents the validation of an oral delta9-tetrahydrocannabinol (THC) consumption mouse model utilizing highly palatable e-capsule gelatin. The results convincingly demonstrate that oral consumption produced THC behavioral and physiological effects, as well as measurable brain levels. The utility of the model for chronic consumption remains to be determined. The authors have clearly acknowledged limitations of their model and areas for future study and development. As the field of cannabinoid research moves toward application of routes of administration that mimic human use, these model systems will be pivotal in assessing the effects of cannabinoid-based drugs.

eLife assessment

Focusing mainly on var genes, the investigators performed comprehensive computational analyses of gene expression in malaria parasites isolated from patients and assessed changes that occur as these parasites adapt to in vitro culture conditions. The study provides an improved computational pipeline for monitoring var gene expression, and importantly, the study documents changes in expression of the core genome and thus provides insights into metabolic adaptations that parasites undergo while transitioning to culture conditions. The findings are important for their technical advances that are more rigorous than the current state-of-the-art. The solid data analyses, broadly support the claims with only minor weaknesses, tell us to be cautious when interpreting results obtained only from cultured parasites.

Author Response

The following is the authors’ response to the original reviews.

Reviewer #1 (Recommendations For The Authors):

This reviewer found the paper of very high interest, well supported, and well written. I have only a few suggestions to the authors for further improvement:

1. TRAIL mutants carrying individual mutations of basic residues R119, R122 and K125 were tested, but a TRAIL mutant lacking all three residues was not. This combined mutant protein would have allowed to test whether all heparin binding is abolished (e.g. that no other residues contribute to HS binding) and could have also been used as an independent control replacing heparin and heparinase treatment in binding/apoptosis studies. Given that the DR4/5 and heparin binding sites of TRAIL do not overlap, this form would be useful in determining the extent to which HS contributes to, or serves as a prerequisite for TRAIL binding to its receptor and cell death. Moreover, if bound to the receptor, this mutant TRAIL is expected to completely prevent HS-mediated receptor internalization. The added value of this experiment therefore is that it may provide an answer to the controversial debate on whether DR receptor internalization promotes or inhibits apoptosis.

In Fig. 5C, we provided data showing that the binding of R115A mutant of hTRAIL (equivalent to murine R199A mutant) to MB-453 cells was very similar to the binding of WT hTRAIL to heparin lyase treated cells. This finding suggests that nearly all HS-dependent binding to cell surface HS was abolished by mutating R115. Since a single mutant is sufficient, we felt there is little point in combining multiple mutations. We also used R115A mutant as an independent control replacing heparin and heparinase treatment in apoptosis assay in Fig. 7E. With regard to using the mutant in the internalization assay, we thank the reviewer for this excellent suggestion and will incorporate it into our future study as we intend to perform more in-depth investigation on the exact mechanism of internalization.

1. The domain data is interesting, but its physiological significance remains obscure and it also somewhat distracts from the main theme of the study. It may be removed from a revised manuscript.

We partially agree with the reviewer’s assessment, but we felt that this discovery is of sufficient novelty and should be made known to the whole community.

1. TUNEL data is shown as a picture in Figure 6, but quantification is lacking.

We have included the statistics of the TUNEL data in the final version as Fig. 6D.

1. Is the HS20 antibody a well-suited pan-anti-HS antibody? Why was this antibody used instead of heparinase digestion followed by the use of HS "stub" antibodies that were previously used as a reliable readout for overall sulfation?

The HS20 mAb has been very well characterized by Dr. Mitchell Ho group (Gao et al., 2016). We have also done side-by-side comparison of HS20 and the most commonly used anti-HS mAb 10E4 by immunostaining and FACS. In nearly all tissues and cells tested, HS20 gave better sensitivity and lower background (after heparin lyase treatment) compared to 10E4. The staining pattern of the two mAbs are usually identical, but the signal/noise ratio of HS20 is much better than 10E4. The HS ”stub” antibody can be useful in certain applications, but it is used mainly as an indicator of the distribution/abundance of HSPGs, rather than a readout of overall sulfation.

1. The discussion should be stripped from expressions such as interestingly, curiously, unexpectedly, certainly, undoubtedly and the like to improve readability. The manuscript should be checked for typos (for example surface plasma resonance line 473, was served line 481).

We thank the reviewer for the suggestions and many of these expressions were removed in the final version.

1. Last but not least: to test the physiological relevance of these findings, it would be of the highest interest to use a mouse model harboring a tumor cell line of choice and derived lines with impaired or increased HS expression, as outlined in my public comments, and to test tumor responsiveness to TRAIL treatment. If already planned, I wish you Good Luck with the experiments!

We thank the reviewer for this excellent suggestion and we have indeed planned to do exactly that!

Reviewer #2 (Recommendations For The Authors):

1. The authors showed in Fig.2 that 12mer HS forms complex with TRAIL homotrimer. Please clarify if 12mer HS binding leads to the formation of the TRAIL homotrimer or TRAIL can form homotrimer in the absence of HS binding. Do the TRAIL mutations that affect HS binding, such as R115A, also impact the homotrimer formation?

TRAIL automatically forms a homotrimer independent of HS. It is known that formation of the homotrimer critically depends on a zinc ion, which is located on the threefold axis of the trimer and is bound by cysteine 240. We have also verified that all TRAIL mutants remain homotrimeric by size exclusion chromatography.

1. Does 12mer HS also suppress TRAIL-mediated apoptosis in MDA-MB-453 cells?

We thank the reviewer for this question but felt performing this experiment will not add any more insight to the main conclusion. Most likely, the result will be similar to what we saw in Fig. 7D, where we found 12mer significantly inhibits TRAIL-induced apoptosis, but inhibits less efficiently compared to heparin.

1. The authors nicely showed the correlation between surface HS level and sensitivity to TRAIL-induced apoptosis in MM cell lines and implicated that such correlation could be related with the difference in the expression level of SDC1. This is an interesting point worth further validation. Does ectopic SDC1 expression in IM-9 cells lead to increase cell surface HS and sensitivity to TRAIL treatment? On the other hand, will depletion of SDC1 expression in U266 or RPMI8226 cells decrease their sensitivity to TRAIL treatment?

We agree that this would be an excellent experiment to try and have actually attempted to overexpress SDC1 in IM-9 cells. But we found IM-9 cells are very difficult to transfect and we only managed to convert a small percentage of SDC1 negative cells to positive cells. Also, the level of SDC1 expression on the SDC1-positive cells was not changed after overexpression. We have not tried depleting SDC1 expression in U266 and RPMI8226 cells because such an experiment might change the property of these cells in unexpected ways, which would make result interpretation impossible. A previous report has shown that knocking down SDC1 could enhance clustering of TRAIL receptors in H929 cells (Wu et al., J Immunol 2012;), which actually led to slightly increased apoptosis.

Reviewer #1 (Public Review):

Summary: TRAIL (Tumor necrosis factor (TNF)-related apoptosis-inducing ligand) is a potent inducer of apoptosis in tumor cells. Initially, this finding raised high expectations on the possibility to induce tumor-specific apoptosis by activation of TRAIL-receptors DR4 and DR5. However, attempted TRAIL-based anti-tumor therapies failed so far, and several tumor types were found to resist TRAIL-induced apoptosis. Yin Luo and colleagues provide an explanation for these observations with the potential to provide a new important biomarker for future TRAIL-based anti-tumor therapies and to reduce resistance. The authors reveal that sensitivity towards TRAIL correlates inversely with heparan sulfate (HS) expression levels at the surface of tumor cells, suggesting that HS functions as a tumor suppressor. These observations are explained by two two mechanisms: First, HS induces the assembly of higher-order oligomers from soluble TRAIL trimers, and second, TRAIL and HS form a ternary complex with DR5 to promote its cellular internalization. Therefore, this timely and important work provides a better mechanistic understanding of TRAIL-induced apoptosis and TRAIL resistance of some tumor types, with the potential to improve therapy.

Strengths: The major novel finding of this study is that extracellular heparan sulfate (HS) acts as a positive regulator of TRAIL-induced tumor cell apoptosis, and that HS expression of different tumor cell lines correlates with their capacity to induce cell death. The authors first show by affinity chromatography and SPR that murine and human TRAIL bind strongly to heparin (heparin is a highly sulfated, and thus strongly negatively charged form of HS that is derived from connective tissue type mast cells), and identify three basic amino acids on the TRAIL N-terminus that are required for the interaction. Size exclusion chromatography (SEC) and multiangle light scattering (MALS) revealed that TRAIL exists as a trimer that requires a minimum heparin length of 8 sugar residues for binding, and small angle X-ray scattering (SAXS) showed that TRAIL interaction with longer oligosaccharides induced higher order multimerization of TRAIL. Consistent with these biochemical and biophysical analyses, HS on tumor cells contributes to TRAIL-binding to their cell surface and subsequent apoptosis. The study also describes domain swapping observed by TRAIL trimer crystallization, and demonstrates different degrees of HS core protein and DR receptor expression in different tumor cell types. These findings are well supported and together with the advanced and established methodology used by the authors are the strengths of this paper. The paper will be of great interest to medical biologists studying TRAIL-resistance of tumors, to biologists interested in DR4 and DR5 receptor function and the effects of receptor internalization, and to glycobiologists aiming to understand the multiple important roles that HS plays in development and disease. The authors also raise the important point (and support it well) that routine heparin treatment of cancer patients potentially interferes with TRAIL-based therapies, providing one possible reason for their failure.

Weaknesses: Despite the importance and the clear strengths of the paper, some of its aspects could have been developed further. First, the authors findings that HS at the tumor surface promotes TRAIL binding, and that HS promotes TRAIL-induced breast cancer and myeloma cell apoptosis, are based on pre-treatment of cells with heparinase to remove surface HS prior to TRAIL-treatment, or on the addition of soluble heparin to compete with cell-surface HS for TRAIL binding. A more direct way to establish such new HS function could have been the genetic manipulation of cancer cells to overexpress HS or to express less or undersulfated HS. Changed susceptibility of these cells to TRAIL-induced apoptosis would have greatly underlined the physiological significance of the authors findings. Second, the mechanistics of TRAIL-induced, HS-modulated tumor cell apoptosis could have been more clearly defined. For example, the authors demonstrate convincingly that cell surface HS is essential for TRAIL-induced apoptosis in MDA-MB-453 breast cancer cells, and show that a tumor cell line (IM-9 cells) that expresses HS and the core protein to which HS is attached to only limited degrees is the most resistant to TRAIL-induced apoptosis. However, Indeed, the authors later also report that cell surface HS promotes TRAIL-induced myeloma cell apoptosis regardless of the sensitivity levels, and that other factors - the degree of TRAIL multimerization or DR4/DR5 receptor internalization - are also important. Therefore, HS levels do not play a sole determining role in TRAIL-induced apoptosis. Along the same line, the authors show that RPMI-8226 cell-surface HS promotes DR5 internalization despite the absence of direct DR5/heparin interactions. This suggests that HS at the cell surface may also affect apoptosis indirectly. To test this hypothesis, it would have been worthwhile to include the binding characteristics and HS-dependent internalization of DR4 into the study.

Reviewer #2 (Public Review):

Summary:<br /> In the manuscript by Luo et al, the authors investigated the nature and function of TRAIL-HS binding for the regulation of TRAIL-mediated apoptosis in cancer cells. The authors discovered that TRAIL binds to 12mer HS and identified the amino acid residues critical for the binding. The authors further nicely showed that 12mer HS binds to TRAIL homotrimer and larger HS can further promote the formation of larger TRAIL oligomers. Structural analyses were conducted to characterize the binding of TRAIL/HS complexes. At functional level, the authors demonstrated that HS promotes the cell surface binding of TRAIL to enhance TRAIL-mediated apoptosis in a variety of cancer cells. Moreover, the ability of TRAIL to induce apoptosis is correlated with cell surface HS level. Lastly, the authors showed that HS forms complex with TRAIL and its receptor DR5 and promotes DR5 internalization.

Strengths:<br /> Overall, this is a nicely executed study providing both mechanistic and functional insight for TRAIL-mediated apoptosis. It conducted detailed characterization on the direct binding between HS and TRAIL and provided solid evidence supporting the role of such interaction for the regulation of TRAIL-induced apoptosis. The experiments were well-designed with proper controls included. The data interpretation is accurate. The manuscript was clearly written and easy to follow by general readers.

Weaknesses:<br /> There is no major weakness identified from this study. As the authors pointed out, the current relationship between cell surface HS level and sensitivity to TRAIL-mediated apoptosis is still correlative and will need further investigation in the future.

Author Response

The following is the authors’ response to the original reviews.

eLife assessment

This important study extends insights on NAFLD and NASH regarding the role of plasma lactate levels using mice haplo-insufficient for the gene encoding lactate transporter MCT-1. While the evidence is largely convincing and the work significantly advances our understanding of the roles of distinct hepatic cell types in steatosis, a number of issues require attention and would best be solved by further experimentation.

RESPONSE: We agree with this assessment by eLife, and appreciate the reviewers’ view that the study is important and extends insights into liver disease.

Public Reviews:

Reviewer #1 (Public Review):

The authors put forth the hypothesis that hepatocyte and/or non-parenchymal liver MCT1 may be responsible for physiologic effects (lower body weight gain and less hepatic steatosis) in MCT1 global heterozygote mice. They generate multiple tools to test this hypothesis, which they combine with mouse diets that induce fatty liver, steatohepatitis and fibrosis. Novel findings include that deletion of hepatocyte MCT1 does not change liver lipid content, but increases liver fibrosis. Deletion of hepatic stellate cell (HSC) MCT1 does not substantially affect any liver parameter, but concomitant HSC MCT1 deletion does reverse fibrosis seen with hepatocyte MCT1 knockout or knockdown. In both models, plasma lactate levels do not change, suggesting that liver MCT1 does not substantially affect systemic lactate. In general, the data match the conclusions of the manuscript, and the studies are well-conducted and well-described. Further work would be necessary to dissect mechanism of fibrosis with hepatocyte MCT1, and whether this is due to changes in local lactate (as speculated by the authors) or another MCT1 substrate. This would be important to understand this novel potential cross-talk between hepatocytes and HSCs.

A parallel and perhaps more important advance is the generation of new methodology to target HSC in mice, using modified siRNA and by transduction of AAV9-Lrat-Cre. Both methods would reduce the need to cross floxed mice with the Lrat-Cre allele, saving time and resources. These tools were validated to an extent by the authors, but not sufficiently to ensure that there is no cross-reactivity with other liver cell types. For example, AAV9-LratCre-transduced MCT1 floxed mice show compelling HSC but not hepatocyte Mct1 knockdown, but other liver cell types should be assessed to ensure specificity. This is particularly important as overall liver Mct1 decreased by ~30% in AAV9-Lrat-Cre-transduced mice, which may exceed HSC content of these mice, especially when considering a 60-70% knockdown efficiency. This same issue also affects Chol-MCT1-siRNA, which the authors demonstrate to affect hepatocytes and HSC, but likely affects other cell types not tested. As this is a new and potentially valuable tool, it would be important to assess Mct1 expression across more non-parenchymal cells (i.e. endothelial, cholangiocytes, immune cells) to determine penetration and efficacy.

RESPONSE: We appreciate the reviewer’s view that the new methods we describe represent an important advance. To ensure the specificity of our novel AAV-Lrat-Cre construct, it would be fair to test its distribution among all possible hepatic cell types, including endothelial cells, cholangiocytes, and other immune cells, as suggested. Our efforts in this study were primarily focused on the major cell types thought to contribute to NASH, namely hepatocytes, Kupffer cells, and in particular hepatic stellate cells. The reasons for this focus were:

1) Our primary goal was to investigate the role of MCT1 in hepatic fibrogenesis. According to Manderacke et al. (2013, Nature Comm), hepatic stellate cells account for the dominant proportion (82-96%) of myofibroblast progenitors, which produce collagen fibers. While there may be interesting roles of MCT1 in those other cell types, to elucidate MCT1's role in fibrogenesis, focusing on the dominant fibrogenic cell type, hepatic stellate cells, was the most appropriate approach for this goal.

2) Considering the proportion of each hepatic cell type in the liver, hepatocytes constitute the majority (60-70%), followed by endothelial cells (15%), immune cells (10%), and stellate cells (5%), among others.

3) The AAV-Cre system is highly specific to its promoter, in this case, Lrat, which has been well established in multiple previous studies to exhibit high specificity for hepatic stellate cells in the liver. We will certainly conduct more comprehensive biodistribution studies in the future, as we believe that our AAV-Lrat-Cre system could be a valuable tool in this field.

Reviewer #2 (Public Review):

In this study, the authors seek to answer two main questions: 1) Whether interfering with lactate availability in hepatocytes through depletion of hepatocyte specific MCT-1 depletion would reduce steatosis, and 2) Whether MCT-1 in stellate cells promote fibrogenesis. While the first question is based on the observation that haploinsufficiency of MCT-1 makes mice resistant to steatosis, the rationale behind how MCT-1 could impact fibrogenesis in stellate cells is not clear. A more detailed discussion regarding how lactate availability would regulate two different processes in two different cell types would be helpful. The authors employ several mouse models and in vitro systems to show that MCT1 inhibition in hepatic stellate cells reduces the expression of COL-1. The significance of the findings is moderately impacted due to the following considerations:

RESPONSE: We have included additional in vitro data in order to provide a more comprehensive discussion of MCT1's potential role in regulating collagen production. Please refer to the new Figure 8, Supplementary Figure 6, and the results section (Potential Mechanism). Also note that our original hypothesis was that depleting MCT1 specifically in hepatocytes would protect mice with MCT1 haploinsufficiency from liver lactate overload and NAFLD. Furthermore, we postulated that this protection might prevent NASH progression since lipotoxicity-driven hepatocyte damage is a central factor in NASH pathogenesis. However, our findings did not support this hypothesis. We found only one brief article (2015, Z Gastroenterol et al., "Functional effects of monocarboxylate transporter 1 expression in activated hepatic stellate cells") that discussed the potential role of MCT1 depletion in hepatic stellate cells in regulating collagen production or fibrosis, as mentioned in their abstract. Unfortunately, the DOI for this article is not functional, and the data cannot be located. Moreover, when we attempted to replicate their results, we were unable to do so, leading us to report our own findings in the current paper.

a. Fibrosis in human NAFLD is a significant problem as a predictor of liver related mortality and is associated with type 1 and type 3 collagen. However, the reduction in COL1 in stellate cells did not amount to a reduction in liver fibrosis even in cell specific KO (in Fig 7E, there is no indication of whether Sirius red staining was different between HSC KO and control mice- the authors mention a downward trend in the text). The authors postulate that type 1 COL may not be the more predominant form of fibrosis in the model. This does not seem likely, since the same ob/ob mouse model was used to determine that fibrosis was enhanced with hepatocyte specific MCT-1 KO and decreased with Chol MCT-1KO. Measurements of different types of collagens in their model and the effect of MCT-1 on different types could be more informative. In particular, although collagens are the structural building blocks for hepatic fibrosis, fibrosis can also be controlled by matrix remodeling factors such as Timp1, Serpine 1, PAI-1 and Lox.

RESPONSE: We monitored the expression levels of matrix remodeling factors, such as Timp1 (Figure 5C, 5F). There was no change in expression upon Chol-MCT1-siRNA treatment, while a significant increase was observed upon GN-MCT1-siRNA treatment. This trend was similar to collagen expression in both cases. Regarding the different types of collagen, instead of measuring each individual type of collagen, we conducted Sirius red and trichrome staining, which enabled us to detect multiple types of collagen simultaneously (Figure 5G, Figure 7D).

b. The authors use multiple animal models including cell specific KO to conclude that stellate cell MCT-1 inhibition decreases COL-1. However, the mechanisms behind this reduced expression of COL-1 are not discussed or explored, making it descriptive.

RESPONSE: We agree that the mechanisms involved are not fully defined but have added new data (Figure 8, Supplement Figure 6) and text to discuss possibilities.

c. Different types of diets are used in this study which could impact lactate availability. Choline deficiency diets are reported to cause weight loss, and importantly have none of the metabolic features of human NASH. Therefore, their utility is doubtful, especially for this study which proposes to investigate if metabolic dysregulation and substrate availability could be a tool for therapy.

RESPONSE: Unfortunately, none of the rodent models used to study NASH completely replicate the condition in human patients, each having its own set of advantages and drawbacks. In line with the concern raised by reviewer #2, there has been a shift away from the use of severely detrimental methionine and choline-deficient diets in contemporary NASH research. Instead, diets that combine methionine and other amino acids with cholinedeficient diets, in conjunction with high-fat diets, have become more popular. The diet we employed in our study consists of high-fat diet combined with choline-deficient diets. We believe that our findings, which are consistent and established across two distinct NASH pathogenesis models and genetic backgrounds, lend additional robustness to our results.

d. Hepatocyte specific MCT-1 KO mice seem to have increased COL-1 production, despite no noticeable difference in hepatocyte steatosis. The reasons for this are not discussed. Fibrosis in NASH is thought to be from stellate cell activation secondary to signals from hepatocellular damage. There is no evidence that there was a difference in either of these parameters in the mouse models used.

RESPONSE: While lipotoxicity-driven liver damage remains a central aspect of NASH pathogenesis, the traditional two-hit theory has become less tractable, giving way to the multi-hit theory in the NASH field. The current debate revolves around whether steatosis is a decisive factor and requirement for NASH fibrogenesis. Our previous publication (Yenilmez et al., 2022, Mol Ther) demonstrated that nearly complete resolution of steatosis did not prevent other NASH features like inflammation and fibrosis, indicating the existence of multiple factors beyond steatosis in NASH pathogenesis. We believe that steatosis and fibrosis influence each other but can also develop independently.

e. The authors report that serum lactate levels did not rise after MCT-1 silencing, but the reasons behind this are unclear. There is insufficient data about lactate production and utilization in this model, which would be useful to interpret data regarding steatosis and fibrosis development. For example, does the MCT-1 KO prevent hepatocyte and stellate cell net import or export of lactate? What is the downstream metabolic consequence in terms of pyruvate, acetylCoA and the NAD/NADH levels. Does the KO have downstream effects on mitochondrial TCA cycling?

RESPONSE: Due to both biological and technical challenges (which are described in the new draft), conducting a comprehensive metabolomics study comparing hepatocyte MCT1 KO to hepatic stellate cell MCT1 KO was not feasible. It is important to note that MCT1 can also transport other substrates that are often overlooked, including pyruvate, short-chain fatty acids, and ketone bodies. Also, in addition to MCT1, there are at least two other functional isoforms of MCT: MCT2 and MCT4. Regrettably, due to these biological and technical complications, conducting a comprehensive metabolomic analysis is extremely complicated and difficult to interpret. Nevertheless, some insights are gained from a study involving MCT1 chaperone protein Basigin/CD147 knockout (KO) mice in a high-fat diet- induced hepatic steatosis model. Basigin acts as an auxiliary protein for MCT1, and its absence leads to improper localization and stabilization of MCT1, effectively simulating a state of MCT1 deficiency. In this context, hepatic lactate levels were reduced by half, and other metabolites such as pyruvate, citrate, α-ketoglutarate, fumarate, and malate were significantly decreased. While we must exercise caution when extrapolating these findings to our MCT1 study, they suggest that multiple metabolites, particularly pyruvate, may play a crucial role in the context of MCT1 deficiency.

f. MCT-1 protein expression is measured only in the in vitro assay. Similar quantitation through western blot is not shown in the animal models.

RESPONSE: We monitored MCT1 protein expression with either Western blot (Fig 2D, 2E (in vitro)) or immune-histology (Fig 4B, 4C (in vivo, ob/ob + GAN diet NASH model), Sup Fig 5F, 5G (in vivo, MCT1 f/f + CDHFD model)).

Reviewer #3 (Public Review):

A major finding of this work is that loss of monocarboxylate transporter 1 (MCT1), specifically in stellate cells, can decrease fibrosis in the liver. However, the underlying mechanism whereby MCT1 influences stellate cells is not addressed. It is unclear if upstream/downstream metabolic flux within different cell types leads to fibrotic outcomes. Ultimately, the paper opens more questions than it answers: why does decreasing MCT1 expression in hepatocytes exacerbate disease, while silencing MCT1 in fibroblasts seems to alleviate collagen deposition? Mechanistic studies in isolated hepatocytes and stellate cells could enhance the work further to show the disparate pathways that mediate these opposing effects. The work highlights the complexity of cellular behavior and metabolism within a disease environment but does little to mechanistically explain it.

RESPONSE: Described above to Reviewer #2

The observations presented are compelling and rigorous, but their impact is limited by the nearly complete lack of mechanistic insight presented in the manuscript. As also mentioned elsewhere, it is important to know whether lactate import or export (or the transport of another molecule-like ketone bodies, for example) is the decisive role of MCT1 for this phenotype. Beyond that, it would be interesting, albeit more difficult, to determine how that metabolic change leads to these fibrotic effects.

RESPONSE: Described above to Reviewer #2

Kuppfer cells are initially analyzed and targeted. These cells may play a major role in fibrotic response. It will be interesting to determine the effects of lactate metabolism in other cells within the microenvironment, like Kuppfer cells, to gain a complete understanding of how metabolism is altered during fibrotic change.

RESPONSE: To address the potential involvement of inflammatory cells, we added new data to the manuscript (Supplement Figure 4). Given the distinct hepatic cellular distribution of Chol-MCT1-siRNA and GN-MCT1-siRNA, the opposite fibrogenic phenotype observed may be attributed to MCT1’s role in non-hepatocyte cell types such as the inflammatory Kupffer cells and the fibrogenic hepatic stellate cells. To determine which hepatic cell type drives the opposite fibrotic phenotypes, we first hypothesized that GN-MCT1-siRNA activates M2 pro-fibrogenic macrophages more than Chol-MCT1-siRNA does. The representative M1/ M2 macrophage polarization gene markers were monitored in Kupffer cells. However, GN-MCT1-siRNA treatment caused comparable M1/M2 macrophage activation levels to Chol-MCT1-siRNA treatment (Supplement Figure 4A, 4B). These data suggest that the opposite fibrotic phenotypes caused by the different siRNA constructs are not due to M1/M2 macrophage polarization.

The timing of MCT1 depletion raises concern, as this is a largely prophylactic experiment, and it remains unclear if altering MCT1 would aid in the regression of established fibrosis. Given the proposal for translation to clinical practice, this will be an important question to answer.

RESPONSE: Agree these are important experiments for future evaluation.

Reviewer #1 (Recommendations For The Authors):

As above, in general, the conclusions match the data presented. The one exception is the authors discussion point that these data show the importance of lactate flux in fibrosis. As MCT1 has other substrates, it does not seem this is definitively due to lactate flux. It would be helpful to have additional experiments to clarify mechanism by which loss of hepatocyte MCT1 leads to increased fibrosis, while loss of HSC MCT1 reverses this finding. This may aid in concluding that altered fibrosis is in fact due to lactate flux in these cell types.

RESPONSE: Described above to Reviewer #2

In addition, it is unclear why the authors switched NASH models for the two tools generated (GAN diet for siRNA, CDHFD for AAV). Similarly, methodology to assess fibrosis switched between these two experiments - i.e. Sirius Red staining for siRNA-treated GAN diet-fed mice vs. Trichrome staining for AAV-transduced CDHFD-fed mice. These changes make it difficult to perform cross-comparisons of the data, to explain (for example), why GN-siRNA to Mct1 reduced body weight but AAV8-TBG-Cre did not. Similarly, GN-siRNA increased liver Col1a1 protein but AAV8-TBG-Cre did not. These differences could be explained by model system, or tool efficacy/off-target effects.

RESPONSE: We agree that different model systems can explain difference in results, but there is also an advantage of using different models and various methodologies as preclinical tests of consistency of data on NASH under different conditions. There are no perfect mouse models for human NASH.

• Phenotyping is also incomplete for the latter experiment, in particular amount of liver lipid content –

RESPONSE: We estimated lipid content by H&E (Fig 6E, F). In some experiments, we focused mostly on COL1 protein expression, as this rather than mRNA is the functional aspect of fibrosis.

Reviewer #2 (Recommendations For The Authors):

This study could benefit from standardization of the types of diet used across all animal models and a more comprehensive focus on the metabolic/substrate availability and utilization aspects of NAFLD and NASH affected in the mouse models with MCT-1 dependent lactate transport deficiency. Since hepatic fibrogenesis in NASH is impacted by signals following hepatocyte damage, the extent of cell death in these models could also be better characterized.

RESPONSE: Our ALT data provides indirect insight into hepatocyte damage. Our histology images did not reveal significant changes in cell morphology or integrity and there were no notable changes in caspase protein levels.

Other comments:

In Fig 4G, there is an increase in the number of lipid droplets with Chol- MCT-1 siRNA compared to GN-MCT1-sirRNA, suggesting that the stellate cell component might be responsible for this finding. The possible reasons for this are not discussed.

RESPONSE: The effects in Fig 4G were exceedingly small and there is no difference in total TG in these experiments, so it is hard to interpret these data and provide logical explanations.

In Fig 5A. A western Blot for aSMA and COL 1 is shown but the sample labeling is unclear i.e, do the lanes belong to different mice of the same condition? HFD mice vs Ctr mice?

RESPONSE: Both groups of ob/ob mice were fed a GAN diet. The graph in Fig 5 is a direct comparison between NTC-siRNA and MCT1-siRNA. To enhance clarity, this is indicated in the figure legends, and the data in Fig 5 is a continuation of the data presented in Fig 4

In Fig 5E, COL1 densitometry data should also be provided for non-silenced mice on HFD and Chow diet for appropriate comparison

RES\PONSE: Both groups of ob/ob mice were fed a GAN diet. The graph in Fig 5 represents a comparison between NTC-siRNA and MCT1-siRNA. It's important to note that, typically, ob/ob mice fed either a chow diet or a high-fat diet do not exhibit fibrogenic phenotypes within this time frame (3 weeks of dietary intervention).

There are many mis-statements throughout the text.Page 6 - "MCT1 silencing significantly inhibited Tgf1β-stimulated ACTA2 mRNA expression as well as collagen 1 protein production" but it is not stated that CO1A1 mRNA is unchanged in Fig 1C.

RESPONSE: We observed no change in CO1A1 mRNA levels (Fig 1C), so we focused on collagen 1 protein production (Fig 1B) on page 6. Given the consistent trend observed in Chol-MCT1-siRNA (Fig 5C), we proposed the possibility of MCT1's influence on collagen translation or protein turnover on page 11.

Page 7- ".......our Chol-MCT1-siRNA does not require transfection reagents as it is fully chemically modified". What does fully chemically modified mean and why does this mean in terms of transfection efficiency.

RESPONSE: One of the primary challenges in utilizing RNAi as a therapeutic approach has been the effective in vivo delivery strategy, particularly concerning stability and longevity against systemic nucleases. Recent developments in siRNA duplex chemical modification strategies, such as 2-Fluoro and 2-O-Methyl ribose substitutions, as well as phosphorothioate backbone replacements, have addressed these challenges (Please see Figure 3. In our current study, we employed 'chemically fully modified' siRNA, featuring several key modifications: (1) every single ribose is chemically modified to 2-F or 2-OMeribose, (2) phosphorothioate backbone replacement, (3) 5'-end of the antisense strand modification to (E)-Vinyl-phosphonate, and (4) 3'-end of the sense strand linkers such as Cholesterol or Tri-N-Acetyl-galactosamine. These chemical enhancements significantly improve transfection efficiency, longevity, and selectivity, setting it apart from traditional siRNA lacking such chemical modifications. A prior study from the Khvorova lab has demonstrated substantial efficiency differences between partially and fully modified siRNA in vivo.

Page 7- the results present for Fig 2 ignores Fig, 2C, if this is important it needs to be described if not, please delete.

RESPONSE: The dose-response potency results, crucial for identifying the most potent Chol-MCT1-siRNA compound, are depicted in Figure 2C. The wording "(Figure 2C)" has been inserted in the sentence as follows. “The silencing effect on Mct1 mRNA was monitored after 72 hours (Figure 2B). Several compounds elicited a silencing effect greater than 80% compared to the NTC-siRNA. The two most potent Chol-MCT1-siRNA, Chol- MCT1-2060 (IC50: 59.6nM, KD%: 87.2), and Chol-MCT1-3160 (IC50: 32.4nM, KD%: 87.7) (Figure 2C) were evaluated for their inhibitory effect on MCT1 protein levels (Figure 2D, 2E). Based on its IC50 value and silencing potency, Chol-MCT1-3160 construct was chosen for further studies in vivo (Table 2).”

Supplement Fig 1A-F should be analyzed by multiple comparisons not by paired t-tests.

RESPONSE: We performed t-tests for every comparison between two groups. However, for Sup Fig 1A-F, which involved a comparison among three different groups, we applied oneway ANOVA.

The x-axis in supplement Fig 2A and B are not labeled, and I assume are in weeks. The Fig 2B x-axis numbers also mis-labeled and should also be 0-3 and not 10-13.

RESPONSE: The x-axis is now appropriately labeled.

Page 10 - the description of supplement Fig 4A is not accurate. Srebf1 mRNA is unchanged by the GN-MCT1-siRNA treatment and Mlxipl mRNA is unchanged by Chol-MCT1-siRNA treatment. Is this total Mlxipl mRNA or can you distinguish between the alpha and beta variants.

RESPONSE: We adhered to NCBI nomenclature, where 'SREBP1' and 'ChREBP' represent proteins, not mRNA. The Mlxipl mRNA we tested pertains to total Mlxipl mRNA. Original draft shown below.

“To investigate the underlying mechanism by which lipid droplet morphological dynamics change, we monitored the effect of hepatic MCT1 depletion on DNL-related gene expression. Both GN-MCT1-siRNA and Chol-MCT1-siRNA strongly decreased the mRNA and protein levels related to representative DNL genes (Supplement Figure 4A-4D). Intriguingly, both modes of hepatic MCT1 depletion also inhibited expression of the upstream regulatory transcription factors SREBP1 and ChREBP.”

There are no molecular weight markers in supplement Fig 4C and D. Is the Srebp1c blot for the nuclear or precursor form?

RESPONSE: The Srebp1c blot presented represents the precursor form. I have edited the figure legend accordingly. It's worth noting that the cleaved form of Srebp1c either exhibited significantly lower expression compared to its precursor form or displayed comparable expression between the control group and the MCT1 depletion group.

Changes in mRNA and protein do not always reflect changes in activity (allosteric regulation). If you want to draw any conclusions about de novo lipogenesis you need to directly measure fatty acid synthesis rates from a carbohydrate precursor.

RESPONSE: We completely agree. Therefore, in the current study, we emphasized two key points: (1) hepatic MCT1 depletion affects the expression levels of representative DNL genes, and (2) however, this regulation was insufficient to resolve the steatosis phenotypes in our NASH model. We have added the text “while recognizing that the decreased expression of DNL genes does not necessarily indicate inhibited fatty acid synthesis rate” on page 15.

Reviewer #3 (Recommendations For The Authors):

Figure 1 - Are there changes to fibroblast phenotype with TGF-beta stimulation and are these changes reversed with MCT1 siRNA-mediated silencing, or is this purely an expression phenomenon?

RESPONSE: This study was designed to assess the preventative effect of MCT1 silencing on Tgf1β-induced fibrosis, rather than a reversal study. As detailed in the methods section, LX2 cells were initially cultured in DMEM/high glucose media with 2% FBS. The following day, we transfected the cells with either NTC-siRNA or MCT1-siRNA (IDT, cat 308915476) using Lipofectamine RNAi Max (ThermoFisher, cat 13778075) for 6 hours in serum-reduced Opti-MEM media (ThermoFisher, cat 31985062). Subsequently, the cells were maintained in serum-starved media, with or without 10ng/ml of recombinant human Tgf1β (R&D Systems, cat 240-B/CF), for 48 hours before harvesting.

Is lactate import/export itself responsible for this phenotype? It is presumed that MCT1 depletion alters import/export of lactate and subsequently modulates this phenotype, but this is never shown experimentally. Does lactate accumulate in these cells or in the medium in culture? The foundation of the paper rests on this hypothesis, so we believe that this is critical to establish. This is particularly relevant as MCT1 has been proposed to function primarily as a lactate importer, so the availability of medium lactate could be easily modulated to determine whether that mimics MCT1 loss.

RESPONSE: To address the underlying mechanism of MCT1/Lactate in stellate cells, we added a new figure to the manuscript (Figure 8). We had previously conducted an experiment to determine whether MCT1 depletion in LX2 cells in vitro influences extracellular lactate concentrations in DMEM/high glucose (25mM glucose) media supplemented with 1mM sodium pyruvate but without sodium lactate. Interestingly, we found no significant difference in extracellular glucose and lactate concentrations, which remained at 25mM and 5mM, respectively. These concentrations were comparable between groups, regardless of MCT1 loss. Additionally, we investigated the effects of MCT1 silencing in the presence of potent fibrogenic inducer TGF-β1. Intriguingly, MCT1 depletion effectively prevented TGF-β1-induced collagen production, irrespective of lactate (+/- pyruvate) supply in the media. LX2 cells with MCT1 depletion exhibited reduced collagen 1 production when lactate was solely generated by endogenous glycolysis (Figure 8F) and when exogenous lactate was supplied (Figure 8G).

Figure 2 - It is compelling that the Chol-MCT1-siRNA compounds are effective at targeting MCT1. However, is it clear how specific the siRNA target is? Are other MCT genes affected as well (if the siRNAs target areas of homology, for example)? Given that this siRNA strategy is used going forward and proposed as a therapeutic, it would be important to discuss and perhaps characterize off-target effects. A simple BLAST search for homology for the chosen siRNAs could help answer this question.

RESPONSE:

1) We designed the siRNA to specifically avoid any potential off-target effects on MCT1's 14 isoforms, and this approach aligns with the results obtained from the NCBI-BLAST analysis.

2) While there are 14 isoforms of MCTs, only the first four are functional. To assess the off-target effect of Chol-MCT1-siRNA on MCT2 and MCT4 (MCT3 was excluded due to its limited expression in retinal pigment epithelium), we conducted in vivo experiments in ob/ob mice, which demonstrated a highly selective MCT1 silencing effect. We have also included MCT1, MCT2, and MCT4 rt-qPCR data in the manuscript (Supplement Figure 2A, 2B).

3) We plan to further optimize and validate the human MCT1-targeting siRNA sequence for use in humanized mouse studies. It's important to note that the MCT1-siRNA used in this study was designed for mice.

Supplemental Figure 1 - brain would be one other highly metabolic tissue wherein it would be important to show lack of activity/accumulation.

RESPONSE: Undoubtedly, the brain is one of the most metabolically active tissues, playing a pivotal role in regulating signaling pathways and metabolism in other tissues. However, it poses a significant challenge in terms of targeting due to the presence of the blood-brain barrier (BBB). Overcoming BBB penetration remains one of the foremost challenges in the field of therapeutic siRNA delivery. For many therapeutic oligonucleotides, including Cholesterol-conjugated siRNAs, systemic administration alone is normally insufficient to achieve BBB penetration. Direct local injection or transient disruption of the BBB is normally required.

Figure 4 - The image shown for chol-MCT1-siRNA seems to show variation in lipid droplet size. Is this just this single image? The authors quantify smaller lipid droplets in this group, so the image may not be representative as there are many large droplets. Ultimately, additional mechanisms as to how alterations in lactate metabolism could mediate this phenotype are missing. This hypothesis also rests upon the assumption that MCT1 is modulating lactate, which is not shown experimentally, as discussed above.

RESPONSE: We changed the representative images (Fig 4B). We agree this aspect of the study is not resolved, and we have related text in the manuscript on this point: “neither GNMCT1-siRNA nor Chol-MCT1-siRNA decreased total hepatic TG levels (Figure 4H), although quantitative analysis of H&E images showed a small decrease in mean lipid droplet size and increased number of lipid droplets upon MCT1 silencing (Figure 4F, 4G). These data suggest the possibility that hepatic MCT1 depletion either 1) inhibits formation or fusion of lipid droplets, or 2) enhances lipolysis to diminish lipid droplet size.”

Figure 5 provides evidence that Chol-MCT1-siRNA expression decreases fibrosis but this is attributed to the effects on stellate cells. While GN-MCT1-siRNA and subsequent MCT1 silencing in hepatocytes has an opposite effect. The cell population that is not discussed, however, is the Kupffer cell. Could MCT1 silencing in this cell population be mediating part of the phenotype observed? How does MCT1 silencing affect Kupffer cell phenotype and activity?

This extends into Figure 6 where Kupffer cells are not given consideration in targeted experiments.

RESPONSE: Described above to Reviewer #3

Figure 6 and 7 use a different model to show that stellate cell depletion of MCT1, specifically, decreases collagen 1 protein levels in NASH, which reinforces the authors claims. Given the cell specificity of this experiment, it is more compelling data. It would be nice to show that Kupffer cell depletion of MCT1 does not have any affect (or perhaps show that it does.

RESPONSE: We agree, but Kupffer selective depletion is not possible to do with this siRNA technology. Please see the response above as our most recent attempt to address this question.

Figure 7 shows that even with decreased collagen deposition, there is no effect on liver stiffness or chronic liver injury as measure by ALT. This may suggest that the decreased level of fibrosis is either not significant to overall clinical outcome or that there are other fibroinflammatory mechanisms compensating for lack of COL1 deposition. Is there increased reticulin fibrosis when MCT1 is knocked down? This could be assessed with IHC or monitoring type 3 collogen (COL3A1).

RESPONSE: Reticulin fibrosis results from the excessive deposition of reticular fibers, primarily composed of type 3 collagen. However, based on our observation of trichrome staining in whole liver histology data (Fig 7D-E), which exhibited nearly identical trends to collagen type 1 expression (Fig 7A-C), it seems unlikely that type 3 collagen compensated for the decrease in type 1 collagen protein expression upon hepatic stellate cell MCT1 KO. We plan to perform detailed analysis of a more comprehensive list of ECM proteins including type 3 collagen in our humanized mouse model with engrafted human liver cells in future experiments.

Additional considerations:

It may be useful to know if inhibition of fibrosis affects survival/progression in these NASH models over a longer timeframe, although this may understandably be beyond the scope of the current work. The timing of MCT1 depletion is prophylactic and given the proposal to translate this research, it would be important to determine whether MCT1 inhibition reversed fibrosis, and if so, by what metabolic mechanism?

RESPONSE: We have observed that extending the duration of the NASH model increases the likelihood of hepatocarcinoma development. Exploring the aim to include survival and disease progression as well as reversal of fibrosis would be important in future experiments.

Summary of new Figures and Figures modified:

• Fig 1B: added "and" (significance) between the first and the third group, and the second and the last group.

• Fig 4B: replaced images with more representative ones as the mean lipid size was questioned by the reviewer.

• Fig 7D: made the images bigger (original images cropped and enlarged → 5X)

• Fig 8: newly created to explain the underlying pathway of lactate, and MCT1 regulating collagen production. Please find the results sections.

• Sup fig 2A, B: newly added to show our compounds’ selective silencing effect. - Sup Fig 2C-D: Added missing x-axis (moved from previous Figure 2A, 2B) - Sup Fig 2E-F: moved from sup Fig 3 not to have too many sup figures.

• Sup Fig 3C-D: showed both precursor and cleaved form of SREBP1 bands as requested (moved from previous sup Figure 4)

• Sup Fig 4: newly created, as questioned many times for the effect on Kupffer cells or other inflammatory cells.

• Sup Fig 6: newly created to explain the potential underlying mechanism of MCT1 depletion on collagen production.

• Sup Fig 7: moved from previous sup Fig 6.

• Sup Fig 8: moved from previous sup Fig 7.

Reviewer #1 (Public Review):

The authors put forth the hypothesis that hepatocyte and/or non-parenchymal liver MCT1 may be responsible for physiologic effects (lower body weight gain and less hepatic steatosis) in MCT1 global heterozygote mice. They generate multiple tools to test this hypothesis, which they combine with mouse diets that induce fatty liver, steatohepatitis and fibrosis. Novel findings include that deletion of hepatocyte MCT1 does not change liver lipid content, but increases liver fibrosis. Deletion of hepatic stellate cell (HSC) MCT1 does not substantially affect any liver parameter, but concomitant HSC MCT1 deletion does reverse fibrosis seen with hepatocyte MCT1 knockout or knockdown. In both models, plasma lactate levels do not change, suggesting that liver MCT1 does not substantially affect systemic lactate. In general, the data match conclusions of the manuscript, and the studies are well-conducted and well-described. Further work would be necessary to dissect mechanism of fibrosis with hepatocyte MCT1, and whether this is due to changes in local lactate (as speculated by the authors) or another MCT1 substrate. This would be important to understand this novel potential cross-talk between hepatocytes and HSCs.

A parallel and perhaps more important advance is the generation of new methodology to target HSC in mice, using modified siRNA and by transduction of AAV9-Lrat-Cre. Both methods would reduce the need to cross floxed mice with the Lrat-Cre allele, saving time and resources. These tools were validated to an extent by the authors, but not sufficiently to ensure that there is no cross-reactivity with other liver cell types. For example, AAV9-Lrat-Cre-transduced MCT1 floxed mice show compelling HSC but not hepatocyte Mct1 knockdown, but other liver cell types should be assessed to ensure specificity. This is particularly important as overall liver Mct1 decreased by ~30% in AAV9-Lrat-Cre-transduced mice, which may exceed HSC content of these mice, especially when considering a 60-70% knockdown efficiency. This same issue also affects Chol-MCT1-siRNA, which the authors demonstrate to affect hepatocytes and HSC, but likely affects other cell types not tested. As this is a new and potentially valuable tool, it would be important to assess Mct1 expression across more non-parenchymal cells (i.e. endothelial, cholangiocytes, immune cells) to determine penetration and efficacy.

Reviewer #2 (Public Review):

In this study, the authors seek to answer two main questions: 1) Whether interfering with lactate availability in hepatocytes through depletion of hepatocyte specific MCT-1 depletion would reduce steatosis, and 2) Whether MCT-1 in stellate cells promote fibrogenesis. While the first question is based on the observation that haploinsufficiency of MCT-1 makes mice resistant to steatosis, the rationale behind how MCT-1 could impact fibrogenesis in stellate cells is not clear. A more detailed discussion regarding how lactate availability would regulate two different processes in two different cell types would be helpful. The authors employ several mouse models and in vitro systems to show that MCT1 inhibition in hepatic stellate cells reduces the expression of COL-1.

The authors have sufficiently addressed prior comments and added new experiments to provide details on possible mechanisms.

Reviewer #3 (Public Review):

We commend the authors on addressing our points and do believe that the manuscript is much improved. Even with the added in vitro data (Figure 8, Supplementary Figure 6), however, a clear mechanistic explanation for how MCT1 is modulating/inhibiting fibrosis in hepatic stellate cells is lacking and this represents a key area for future exploration. The authors provide interesting follow up experiments that suggest lactate can potentiate TGF-β1 signaling, a phenomenon that has previously been described in pulmonary fibrosis. Additionally, MCT1 depletion decreased the pSMAD3/SMAD ratio, but this was overcome with higher doses of TGB- β1 ligand. It remains unclear how intracellular versus extracellular lactate is signaling to exert the observed effects, and how the altered metabolism/metabolic flux in NAFLD is contributing to organ level metabolic dysregulation. These will be keys questions to answer going forward potentially using the novel in vivo models that the authors have contributed here.

A major finding of this work is that loss of monocarboxylate transporter 1 (MCT1), specifically in stellate cells, can decrease fibrosis in the liver. However, the underlying mechanism whereby MCT1 influences stellate cells is not addressed. It is unclear if upstream/downstream metabolic flux within different cell types leads to fibrotic outcomes. Ultimately, the paper opens more questions than it answers: why does decreasing MCT1 expression in hepatocytes exacerbate disease, while silencing MCT1 in fibroblasts seems to alleviate collagen deposition? Mechanistic studies in isolated hepatocytes and stellate cells could enhance the work further to show the disparate pathways that mediate these opposing effects. The work highlights the complexity of cellular behavior and metabolism within a disease environment but does little to mechanistically explain it.

Author Response

The following is the authors’ response to the previous reviews.

Public Reviews:

Reviewer #1 (Public Review):

The authors have previously employed micrococcal nuclease tethered to various Mcm subunits to the cut DNA to which the Mcm2-7 double hexamers (DH) bind. Using this assay, they found that Mcm2-7 DH are located on many more sites in the S. cerevisiae genome than previously shown. They then demonstrated that these sites have characteristics consistent with origins of DNA replication, including the presence of ARS consensus sequences, the location of very inefficient sites of initiation of DNA replication in vivo, and for the most part are free of nucleosomes. They contain a G-C skew and they locate to intergenic regions of the genome. The authors suggest, consistent with published single molecule results, that there are many more potential origins in the S. cerevisiae genome than previously annotated, but also conclude that many of the newly discovered Mcm2-7 DH are very infrequently used as active origins of DNA replication.

The results are convincing and are consistent with prior observations. The analysis of the origin associated features is informative.

Specific Comments:

1. Page 8. The addition of an estimate of the most active origins using Southern blotting is fine for highly active origins, but how was Southern blotting used to calculate that 1-2% of cells in the eight cohort have an Mcm complex loaded.

We used a combination of Southern blotting and qPCR to measure licensing at the most active origins and then used our abundance curve to extrapolate these values to the less abundant cohorts. We expand on this point below, and we have changed the text to clarify this issue.

Reviewer #3 (Public Review):

By mapping the sites of the Mcm2-7 replicative helicase loading across the budding yeast genome using highresolution chromatin endogenous cleavage or ChEC, Bedalov and colleagues find that these markers for origins of DNA replication are much more broadly distributed than previously appreciated. Interestingly, this is consistent with early reconstituted biochemical studies that showed that the ACS was not essential for helicase loading in vitro (e.g. Remus et al., 2009, PMID: 19896182). To accomplish this, they combined the results of 12 independent assays to gain exceptionally deep coverage of Mcm2-7 binding sites. By comparing these sites to previous studies mapping ssDNA generated during replication initiation, they provide evidence that at least a fraction of the 1600 most robustly Mcm2-7-bound sequences act as origins. A weakness of the paper is that the group-based (as opposed to analyzing individual Mcm2-7 binding sites) nature of the analysis prevents the authors from concluding that all of the 1,600 sites mentioned in the title act as origins. The authors also show that the location of Mcm2-7 location after loading are highly similar in the top 500 binding sites, although the mobile nature of loaded Mcm2-7 double hexamers prevents any conclusions about the location of initial loading. Interestingly, by comparing subsets of the Mcm2-7 binding sites, they find that there is a propensity of at least a subset of these sites to be nucleosome depleted, to overlap with at least a partial match to the ACS sequence (found at all of the most well-characterized budding yeast origins), and a GC-skew centered around the site of Mcm loading. Each of these characteristics is related to previously characterized S. cerevisiae origins of replication.

Overall, this manuscript greatly broadens the number of sites that are capable of loading Mcm2-7 in budding yeast cells and shows that a subset of these additional sites act as replication origins. Although these studies show that the sequence specificity of S. cerevisiae replication origins still sets it apart from metazoan origins, the ability to license and initiate replication from sites with increasing sequence divergence suggests a previously unappreciated versatility.

Specific points:

1. The authors need to come up with a consistent name for loaded Mcms at an origin. In the manuscript they variously use 'MCM'(page 3), 'Mcm complexes' (page 4), 'MCM double hexamer' (page 6), and 'double-helicase' (page 8) to describe the Mcm2-7 complexes detected in their ChEC experiments. They should pick one name (Mcm2-7 double hexamer or MCM double hexamer would be the most accurate and clear) and stick with it throughout the manuscript.

We appreciate the criticism and agree that consistency is important for clarity, thus we tried using the term "Mcm2-7 double hexamer" in every instance in which we refer to Mcm loaded at an origin. However, upon reading the resulting manuscript, we felt that these changes hurt readability more than they helped with clarity, so we left the manuscript in its original form.

1. The authors state that "It is notable that, when Mcm is present, it is present predominantly as a single doublehexamer (right panel of Figure 3A), and that this remains true across the entire range of abundance shown in Figure 3A." This statement would be improved by prefacing it with "Based on the size of the protected regions" or some other clarifying statement that lets the reader know what they should be looking for in the data in 3A.


We thank the reviewer for the helpful suggestion. We have added the underlined words to the text to clarify this point.

It is notable that, when Mcm is present, it is present predominantly as a single doublehexamer (based on the size of the protected region in the right panel of Figure 3A), and that this remains true across the enAre range of abundance shown in Figure 3A.

1. The revised statements that "We have previously used Southern blotting to demonstrate that approximately 90% of the DNA at one of the most active known origins (ARS1103) is cut by Mcm-MNase (Foss et al., 2021), and to thereby infer that 90% of cells have a double- helicase loaded at this origin. Using this as a benchmark, we estimate that ~1-2% cells have an Mcm complex loaded at the Mcm binding sites in the eighth cohort (ranks 1401- 1600)." partially clarifies how the authors came to the 1-2% number, however, the calculation is still unclear. Based on Figure 1A, there are at least three logs (1,00 fold) difference in the number of CBMSs between the best origins (which is what they state the 90% comes from) to anywhere close to the 1400-1600 rank. Seems like the number should be at best 0.1% and probably less. Either way, the authors need to explain this calculation either in the text or in the text. This sort of number tends to get thrown around later and without a clear explanation readers cannot evaluate its credibility. 
<br /> We apologize for insufficiently clarifying how we arrived at our estimate of licensing. We believe that we have now remedied this, both by incorporating more measurements of licensing to improve our accuracy and by expanding the text to make our calculation unambiguous. We have added a supplemental figure showing the linear regression, based on 7 qPCR-based measurements of licensing, that we used to determine the median level of licensing of the first cohort of 200, and the altered text in the main text reads as follows:

We have previously used Southern blotting to demonstrate that approximately 90% of the DNA at one of the most active known origins (ARS1103) is cut by Mcm-MNase (Foss et al. 2021), and to thereby infer that 90% of cells have a double-helicase loaded at this origin. Combining this measurement with 6 additional measurements of licensing in cohort 1, we used linear regression (r2=0.7) to infer a median value of 69% for cohort 1. Because the median abundance in the 8th cohort is 1.5% of that in the first cohort, we estimate that CMBSs in the 8th cohort are typically licensed in 1% of cells in the population (69% x 0.015 = 1.0%).

1. The authors make the point in the introduction and discussion that recent single-molecule studies of replication origins indicate that as many as 20% of the origins identified are outside of known origins. This is very interesting but there seems to be a missed opportunity of comparing the location of these origins with the CBMSs. It would improve the manuscript to include some sort of comparison rather than using only the much older and less accurate ssDNA analysis.

Unfortunately, coverage and resolution with nanopore-based single-molecule precludes such an analysis.

1. The authors state at the end of the first paragraph on page 6 that the ChEC data is "very reproducible" which does seem to be the case but it is a little confusing for the knowledgeable reader since one would expect quite different results for an HU arrested strain versus a asynchronous or G1 arrested strain. This is hidden in the analysis in Figure S1 since 13 experiments are compared against one in each plot, however, if one x one comparisons were done there would certainly be substantial differences (or if there are not, there is a problem with the data - e.g. HU arrested cells should lack licensing at early firing origins).

It may appear counterintuiAve that one could obtain high r2 values when comparing G1 and HU-arrested samples. However, HU arrest was performed by transferring log phase cultures to 200 mM HU and harvesting cells after just 50 minutes. In this situation, most cells will be in G1 or very early S phase. Presumably, increasing times of incubation in HU would cause r2 values to decline.

1. On page 8 the authors state, "First, clear peaks of ssDNA extend down to the eighth cohort..." This seems to be stretching the data. There are clear peaks for the first five cohorts and then there is a notable change with any peak being much broader, extending over at least 10,000 bp. The authors should reconsider their statement here as it is not well supported by the data.

We have softened our language to the following: First, peaks of ssDNA signal, as judged by higher signal at the midpoints than the edges, extend down to the eighth cohort (brown line), which corresponds to CMBSs ranked 1401-1600.

1. There is one last missing reference. Wherever Eaton et al, 2010 is referenced Berbenetz, et al, 2010 (full ref below) should also be referenced as they come to very similar conclusions.

Berbenetz, N. M., Nislow, C. & Brown, G. W. Diversity of eukaryotic DNA replication origins revealed by genome-wide analysis of chromatin structure. PLoS Genet 6, (2010).

We have added this reference at all 4 instances in which we reference Eaton et al., 2010.

Recommendations for the authors:

Reviewer #3 (Recommendations For The Authors):

There are missing references in several places:

All references are included, and the references in point 3 have been split according to the reviewer's suggestion.

1. "For example, 15 of the 56 genes that contained a high abundance site have been implicated in meiosis and sporulation and are not expressed during vegetative growth (~5 out of 56 expected from random sampling), consistent with previous observations (Mori and Shirahige, 2007)." Should include Blitzblau et al., 2012 (PMC3355065) which showed that Mcm2-7 loading was impacted by differences in meiotic and mitotic transcription.

2. "In contrast to the low abundance sites, the most abundant 500 sites showed a preference for convergent over divergent transcription (left of vertical dotted line in Figure 4B), in agreement with a previous report (Li et al., 2014)." This preference was first pointed out in MacAlpine and Bell, 2005 (PMID: 15868424).

3. "This sequence is recognized by the Origin Recognition Complex (Orc), a 6-protein complex that loads MCM (Broach et al., 1983; Deshpande and Newlon, 1992; Eaton et al., 2010; Kearsey, 1984; Newlon and Theis, 1993; Singh and Krishnamachari, 2016; Srienc et al., 1985)." This list should include a reference to Bell and Stillman, 1992 (PMID: 1579162), which first described ORC and showed that it recognized the ACS. It would also be more helpful to the reviewer to distinguish the references that identified that ACS from those concerning ORC binding to it.

eLife assessment

This study represents a valuable addition to the understanding of the DNA replication origin selection process in the budding yeast. The authors provide convincing evidence that the number of possible origins of replication is much higher than previously appreciated, although many of the newly identified origins are likely to only direct replication initiation rarely. This work will be of interest to those studying DNA replication and investigating protein-DNA interactions across the genome.

Reviewer #1 (Public Review):

The authors have previously employed micrococcal nuclease tethered to various Mcm subunits to the cut DNA to which the Mcm2-7 double hexamers (DH) bind. Using this assay, they found that Mcm2-7 DH are located on many more sites in the S. cerevisiae genome than previously shown. They then demonstrated that these sites have characteristics consistent with origins of DNA replication, including the presence of ARS consensus sequences, the location of very inefficient sites of initiation of DNA replication in vivo, and for the most part are free of nucleosomes. They contain a G-C skew and they locate to intergenic regions of the genome. The authors suggest, consistent with published single molecule results, that there are many more potential origins in the S. cerevisiae genome than previously annotated, but also conclude that many of the newly discovered Mcm2-7 DH are very infrequently used as active origins of DNA replication.

The results are convincing and are consistent with prior observations. The analysis of the origin associated features is informative.

Reviewer #2 (Public Review):

By mapping the sites of the Mcm2-7 replicative helicase loading across the budding yeast genome using high-resolution chromatin endogenous cleavage or ChEC, Bedalov and colleagues find that these markers for origins of DNA replication are much more broadly distributed than previously appreciated. Interestingly, this is consistent with early reconstituted biochemical studies that showed that the ACS was not essential for helicase loading in vitro (e.g. Remus et al., 2009, PMID: 19896182). To accomplish this, they combined the results of 12 independent assays to gain exceptionally deep coverage of Mcm2-7 binding sites. By comparing these sites to previous studies mapping ssDNA generated during replication initiation, they provide evidence that at least a fraction of the 1600 most robustly Mcm2-7-bound sequences act as origins. A weakness of the paper is that the group-based (as opposed to analyzing individual Mcm2-7 binding sites) nature of the analysis prevents the authors from concluding that all of the 1,600 sites mentioned in the title act as origins. The authors also show that the location of Mcm2-7 location after loading are highly similar in the top 500 binding sites, although the mobile nature of loaded Mcm2-7 double hexamers prevents any conclusions about the location of initial loading. Interestingly, by comparing subsets of the Mcm2-7 binding sites, they find that there is a propensity of at least a subset of these sites to be nucleosome depleted, to overlap with at least a partial match to the ACS sequence (found at all of the most well-characterized budding yeast origins), and a GC-skew centered around the site of Mcm loading. Each of these characteristics is related to previously characterized S. cerevisiae origins of replication.

Overall, this manuscript greatly broadens the number of sites that are capable of loading Mcm2-7 in budding yeast cells and shows that a subset of these additional sites act as replication origins. Although these studies show that the sequence specificity of S. cerevisiae replication origins still sets it apart from metazoan origins, the ability to license and initiate replication from sites with increasing sequence divergence suggests a previously unappreciated versatility.

Author Response

The following is the authors’ response to the previous reviews.

On behalf of all the authors, I'd like to thank you for your insightful comments and valuable suggestions, which fully reflect your high level of scientific thinking and point the direction of our research and help us and other future researchers in the field to more comprehensively study and interpret the toxic effects of imidacloprid on honey bee larvae and its potential mechanisms, as well as the mechanisms of larval resistance and adaptations to imidacloprid. We have addressed each of the questions and revised the manuscript point-by-point in response to your comments. Below are detailed point-by-point responses to each question.

Public Review:

This study provides evidence of the ability of sublethal imidacloprid doses to affect growth and development of honeybee larva. While checking the effect of doses that do not impact survival or food intake, the authors found changes in the expression of genes related to energy metabolism, antioxidant response, and metabolism of xenobiotics. The authors also identified cell death in the alimentary canal, and disturbances in levels of ROS markers, molting hormones, weight and growth ratio. The study strengths come from exploring different aspects and impacts of imidacloprid exposure on honeybee juvenile stages and for that it demonstrates potential for assessing the risks posed by pesticides. The study weaknesses come from the lack of in depth investigation and an incomplete methodological design. For instance, many of the study conclusions are based on RT-qPCR, which show only a partial snapshot of gene expression, which was performed at a single time point and using whole larvae. There is no understanding of how different organs/tissues might respond to exposure and how they change over time. That creates a problem to understand the mechanisms of damage caused by the pesticide in the situation studied here. There is no investigation of what happens after pupation. The authors show that the doses tested have no impact on survival, food consumption and time to pupation, and the growth index drops from ~0.96 to ~0.92 in exposed larvae, raising the question of its biological significance. The origin of ROS are not investigated, nor do the authors investigate if the larvae recover from the damage observed in the gut after pupation. That is important as it could affect the adult workers' health. One of the study's central claims is that the reduced growth index is due to the extra energy used to overexpress P450s and antioxidant enzymes, but that is based on RT-qPCR only. Other options are not well explored and whether the gut damage could be causing nutrient absorption problems, or the oxidative stress could be impairing mitochondrial energy production is not investigated. These alternatives may also affect the growth index. The authors also state that the honeybee larvae has 7 instars, which is an incorrect as Apis mellifera have 5 larval instars. It is not clear from methods which precise stage of larval development was used for gut preparations. That information is important because prior to pupation larvae defecate and undergo shedding of gut lining. That could profoundly affect some of the results in case gut preparations for microscopy were made close to this stage. A more in-depth investigation and more complete methodological design that investigates the mechanisms of damage and whether the exposures tested could affect adult bees may demonstrate the damage of low insecticide doses to a vital pollinator insect species.

Recommendations for the authors:

This study presents a useful investigation on changes in gene expression by real time PCR and some of the physiological consequences of sublethal exposures to the neonicotinoid insecticide imidacloprid in honeybee larvae. It offers preliminary evidence of imidacloprid impacts on the development of bee larvae by interfering with molting and metabolism. Whereas the study provides evidence that small doses of imidacloprid affect larval growth rate, there is no investigation on whether that could affect the overall colony health, and some of the results open the possibility that the larvae may overcome some of the impacts of the exposure. As the authors state, the doses tested show no impact on larvae survival, food consumption or time to pupation. The investigation and methodological design lack in depth to explain the findings and provide incomplete evidence to support the authors conclusions. The study would benefit from a more thorough mechanistic characterization to better sustain the findings and demonstrate their biological relevance.

Response: I would like to express, on behalf of all the authors, our sincere appreciation for your insightful and insightful comments and suggestions, which significantly enhanced the quality of the manuscript. Your incisive insights point the way for future research in the field of bee biology on the mechanisms underlying imidacloprid-induced delays in larval development.

In this study, we investigated the effects of imidacloprid on honey bee larval development, including macro and micro changes and possible causes. This is the first of its kind in the field of honeybee biology research. However, we found that the underlying mechanism is extremely complex. The effects of toxic substances on animals and their interactions with larval development are complex and far-reaching. They include oxidative stress and damage; disruption of nutrient metabolic homeostasis; inhibition of detoxification and immunity; adverse effects on the nervous, circulatory, and digestive systems; inflammation, disease, and even organ failure; and subsequent effects on physiological activities such as development, reproduction, and behavior, and even death. These toxic effects interact in complex ways with the development of young animals, with some effects directly or indirectly affecting development while others do not.

Addressing this complex mechanistic issue based solely on the results of this study is a formidable challenge, which leads to some limitations of our study as pointed out by the reviewers. Although our study is not comprehensive enough in terms of mechanistic analysis and does not fully elucidate the mechanism, we believe it is an important and valuable first step in this area.

In the future, we will follow the reviewers' suggestions and deliberately redesign the experiments to focus on further research on the issues they raised. These include examining the effects of larval developmental delay on adult and colony health, investigating the post-pupal situation, identifying the source of ROS, and determining whether the larval gut damage observed after pupalization recovers.

In accordance with the reviewers' comments and suggestions, we have revised the manuscript to improve its rigor and scientific quality. We sincerely ask the reviewers to understand and accept this modification from us!

Next is our response to each of the questions and valuable suggestions provided by reviewers:

Recommendations For The Authors:

1. The authors found a reduction in growth index and body mass, but document no impact on survival, food consumption or time to pupariation. How much exactly is the reduction in growth index? It seems to be from ~0.96 to ~0.93. Is this biologically relevant? Would that be enough to impact the colony health?

Response: Thank you for your comments. In this study, we observed a gradual decrease in larval growth index from day 4, which stabilized by day 6. At the 4th, 5th and 6th instars, the growth index of the imidacloprid-treated groups were significantly lower than those of the control group by an average of 1.35%, 4.49% and 2.76%, respectively (Figure 1, source data 8). Statistical analysis confirmed the significance of the difference in these results. We have incorporated the above description into the red text on lines 148-152 of the Results section. Regarding the reviewer's inquiry on colony health, including imidacloprid-induced delayed larval development and some reduction in growth index and body weight with no effect on survival, food consumption, or time develop to pupation, because we do not currently have the technical capabilities to culture larvae to adulthood in laboratory incubators, this has resulted in a failure to further investigate the effects of imidacloprid-induced delayed larval development on adult colony health. However, this is a very important scientific question for future colony health. We will design experiments to address this issue in a follow-up study.

1. The authors find that P450s can help in detoxifying mechanisms to mitigate imidacloprid impacts. That however is a well-known fact. What is new about this claim?

Response: The point at which the ability to detoxify toxic substances is acquired during early development varies widely among animals. Although many studies have reported that the detoxification function of P450s helps mitigate the effects of imidacloprid in adult honey bees, there is no conclusive evidence as to whether or not honey bee larvae have acquired this ability at early stages of development. This ability is critical to the defense and health of honey bee larvae. Therefore, it is incumbent upon this study to clarify this issue, which is important in explaining the effects of imidacloprid on honey bee larvae.

1. Some references are cited incorrectly. The first and last name are swapped, for instance Charles et al.

Response: Thank you very much for pointing out this error, which we have corrected. Please see lines 92 and 889 in our revised version.

1. I still encounter important methodological flaws. The authors acknowledge my previous suggestions but only address a small fraction of them. The most relevant points regarding the understanding of the mechanisms behind the delayed growth rate remain unexplored. The expression levels of other nAChRs target of imidacloprid in honeybees were not investigated. The expression analyses are still based on a single time point and using whole larvae, which only superficially explore the problem and may lead to misinterpretations. I do not understand the authors claim that a technological breakthrough is required to address these issues, when performing more PCRs and doing dissections should cover the matter.

Response: Thank you very much for your important comment. You point out several unexplored issues related to understanding the mechanisms behind delayed growth rates. For example, The most relevant points regarding the understanding of the mechanisms behind the delayed growth rate remain unexplored. The expression levels of other nAChRs target of imidacloprid in honeybees were not investigated. The expression analyses are still based on a single time point and using whole larvae. Please allow me to explain. Honeybees (Apis mellifera) have nine different α-subunits, Amelα1-9, and two β-subunits, Amelβ1-2. Amelα5, Amelα7, and Amelα8 are expressed in MB Kenyon cells and AL neurons, and the Amelβ2 subunit is present in Kenyon cells. Amelα2, Amelα3, and Amelα7-2 are expressed in the optic lobes. The aim of this study was to investigate whether imidacloprid induces larval neurotoxicity. Based on the above information, we selected the two most representative nAChRs (Alph1 and Alph2) for analysis. The results showed that exposure to imidacloprid increased the expression of the Alph2 gene and inhibited AChE activity, indicating that imidacloprid is neurotoxic to larvae. This result answered our question of whether imidacloprid induces neurotoxicity in larvae. Therefore, we did not further analyze the expression levels of other nAChRs. We believe that this does not affect the understanding of the mechanism behind the delayed growth rate and that it is not necessarily necessary to analyze all 11 nAChRs to find an answer. We sincerely hope that the reviewers will understand and agree with this.

Furthermore, regarding the expression analysis based on a single time point and whole larvae. In this study, 72 h after imidacloprid exposure Fig. 1J, 5 days of age) was chosen for sampling because this is when imidacloprid has the greatest and most representative effect on larval development. Therefore, analyzing samples at this time point did not interfere with our exploration of the mechanisms by which imidacloprid causes larval developmental retardation. We used whole larvae rather than individual tissues for sample selection, which is a shortcoming for us. This was mainly due to technical challenges where we were unable to obtain pure single tissues through dissection. Nevertheless, we will make technical breakthroughs in the future so that we can sample and compare different tissues and developmental stages to obtain more comprehensive and accurate data. Thank you again for raising this important issue and for your valuable suggestions.

1. The authors could in many different ways explore what are the origin of ROS is. That is important to further develop their hypothesis on reduced energy levels.

Response: Thank you very much for your insightful comment and suggestion, it gives us great insight. Mitochondria are the main producers of ATP for cellular metabolism, accounting for approximately 90% of the total. However, mitochondria are also involved in the generation of reactive oxygen species (ROS). Excessive accumulation of ROS in mitochondria leads to oxidative stress, which in turn damages mitochondria and further increases ROS levels, creating a vicious cycle (Boovarahan and Kurian, 2018). In the present study, it was found that imidacloprid exposure led to increased ROS and MDA levels in larvae (Figure 5A and Figure 5-source data 14), indicating that imidacloprid induced severe oxidative stress and lipid damage, which may damage mitochondria and in turn affect mitochondrial ATP production, resulting in insufficient energy supply for larval development. This factor may also be an important explanation for the larval developmental delay caused by imidacloprid. We have included the above text in our revised manuscript. Please see the lines 432-442 in the revised manuscript.

1. If there is gut damage, is it restored in the adults? It is not clear from the methods which precise stage of larval development was used for gut preparations. That information is important because prior to pupation larvae defecate for the first time and undergo shedding of the gut lining. That could profoundly affect some of the results in case gut preparations for microscopy were made close to this stage. If no food residues are found in the gut of control larvae, does it mean that they are close to pupation? Could the apoptosis found in gut of exposed larvae be the natural shedding of gut lining prior to pupation? All these possibilities have to be discussed and authors should clarify the precise larval stage used in every assay.

Response: Thank you for your important comments. In this study, all samples used for the assay were larvae that had developed to 5-day-old after oral administration imidacloprid at 2-day-old. This is described in detail in the Materials and Methods. See lines 507, 517-521 in the revised manuscript. In general, 6-day-old bee larvae cease feeding and begin their first defecation at approximately 7-day-old. However, in our study, intestinal sections were prepared from 5-day-old larvae that had not fasted or defecated, when the intestinal mucosa was normal and not undergoing shedding. In this case, we found that imidacloprid caused damage to intestinal structures, apoptosis of intestinal cells, incomplete formation of the peritrophic membrane, and undigested food residues in the intestine. We believe that these results are objective and reliable.

1. Honeybee have 5 larval instars, not 7 (Figure 1). That creates confusion about which larval stage the authors used.

Response: Thank you very much for pointing out this editorial error, which we have corrected, please see Figure 1.

1. The Results section does not state the numbers by which parameters measures have changed, neither the values of significance. How much is the impact in growth index, body mass, gene fold change, etc?

Response: Thank you very much for pointing out this important problem. We have revised the Results section according to your suggestions. Please see the revised manuscript.

1. Mention figures in order (5c comes before 5b in the text)

Response: Thank you very much for the comment. We have revised according to your suggestions. Please see the lines 208-212 in the revised manuscript.

1. Paraquat is a herbicide not a pesticide

Response: Thank you for pointing out the loose wording. We have revised according to your suggestions. Please see the lines 316-319 in the revised manuscript.

1. What is the evidence that imidacloprid reduces growth index by inhibiting 20E? The authors provide real time data and discuss the data in terms of correlation. But correlation does not mean causation. Reduction in growth index could come from multitude of factors such as ROS affecting mitochondrial energy metabolism.

Response: We deeply appreciate your insightful comments and valuable suggestions. In this study, although we conducted an in-depth analysis of ecdysone regulation, which is crucial for insect larval development, and found some clues, as you pointed out, this is not the sole reason for larval developmental delay. In fact, animal growth and development are collectively regulated by numerous physiological, biochemical, and genetic factors. The the decline in the growth index may be due to other factors as you mentioned, such as oxidative stress impairing mitochondria, dysregulated neuro-endocrine axis caused by imidacloprid targeting neurons, poor nutrient absorption, impaired movement, etc, as animal growth and development are collectively regulated by numerous physiological, biochemical, and genetic factors. We have incorporated this understanding into the revised manuscript. Please see the lines 389-394 in the revised manuscript.

1. The authors state that "digestion and breakdown of nutrients is impaired by imidacloprid", the evidence discussed in the paragraph however supports only that imidacloprid impairs some of the genes involved in these processes.

Response: Thank you for your comments and valuable insights. In this paragraph, a lack of clarity and completeness in our writing may have led to the misconception that the evidence discussed only demonstrates the effects of imidacloprid on specific genes in these processes. In fact, our intent in this paragraph was to analyze and discuss the effects of imidacloprid on nutrient digestion and breakdown in larvae and to explore the causes of larval developmental delay. We demonstrated this using tissue sections, qRT-PCR and correlation analysis, which showed that the intestinal structure was disrupted and the expression of genes involved in nutrient digestion and catabolism was suppressed, resulting in defects in the catabolic utilization of food and consequently the presence of many food residues. In addition, there was a positive correlation between these genes and larval developmental delay. All this may be another important factor contributing to imidacloprid-induced larval developmental delay. We have revised and incorporate the above logic into the revised manuscript. Please see the lines 407-431 in the revised manuscript.

1. There is no evidence for the claim that overexpressing P450s and antioxidant enzymes cause a reduction in growth index. No transcriptome analysis was performed so it is unknown under the circumstances presented here how all the other P450s, antioxidant genes and overall gene profiles are responding. Surely, some genes will be repressed. Reduction in growth index could stem from, oxidative stress impairing mitochondria, dysregulated neuro-endocrine axis caused by imidacloprid targeting neurons, poor nutrient absorption, impaired movement, etc.

Response: Thank you for your comments and valuable insights. Indeed, as you have pointed out, drawing the conclusion that antioxidants and detoxification are significant contributors to larval developmental retardation solely based on correlation analysis is inherently flawed and lacks critical support, especially in the absence of P450 and antioxidant enzyme overexpression and comprehensive transcriptome analysis of other P450s, antioxidant genes, and the entire gene map. We have revised and included in the revised manuscript. Please see lines 461-467 in the red text in the revised manuscript. We have revised and incorporate the above logic into the revised manuscript. Please see the lines 407-431 in the revised manuscript.

1. How come the decreased ATP and glycogen levels have no effect on time to pupation? Extra time points for gene expression, measurements of gut damage, ATP levels, ROS, etc, are vital to answer how the exposed larvae eventually catch up with the unexposed group. Also, it is vital to understand whether these larval impacts translate to impacts on adults.

Response: We sincerely thank you for your insightful comments and suggestions! These important scientific issues you've raised are a good example of your high-level scientific thinking, and they will help us and other future researchers in the field to more comprehensively study and interpret the toxic effects of imidacloprid on honey bee larvae and their potential mechanisms, as well as the mechanisms of larval resistance and adaptation to imidacloprid. According to your comments, we will adapt our experiments and conduct more thorough research in the future to address the above issues.

1. I am confused about the author's definition of developmental rate; rate gives the notion of speed to achieve something. But the authors use developmental rate as a measure of viability (number of larvae that successfully pupated). There seems to be a significant decrease in their developmental rate plot (Fig 1i), but at the same time the authors show in Figure 1c (and mention throughout the manuscript) that there is no difference in probability of survival. This is quite confusing and the method section regarding these data is too concise and does little to help explain what the authors were trying to measure. The whole section on developmental traits would benefit of more details on how experiments were conducted and equipment used.

Response: Thank you so much for your valuable comments. Yes, as you can see, there appears to be a significant decrease in developmental rate but no difference in survival probability, which is an intriguing finding of this study. This finding suggests that the 377 ppb imidacloprid dose is not as harmful to the larvae as previously thought. Imidacloprid appeared to limit the larval ability to molt and develop only to a certain extent, but had no effect on the developmental process, let alone survival. It's worth investigating the underlying mechanism. As a result, we have included this question in the design of future studies. In addition, following your suggestion, we have revised the description of the material and methods in this section, including the experimental method in more detail. For more information, please see the revised manuscript, lines 530-541.

1. The authors should try to make it clear what percentage of exposed larvae become adults? I am confused because the plot called developmental rate might be trying to convey this message, but developmental rate and viability are very distinct traits. What is the difference, if any, in the time it takes for exposed larvae to become adults in comparison to non-exposed ones? Is there a difference in adult body weight? The answers to these last two questions are important to start understanding if the impacts of imidacloprid on larvae alimentation would still impact these same individuals once they become adults, i.e., would there be impacts for the colony and workers activity?

Response: Thank you very much for your insightful comments. Unfortunately, this is where the research falls short. Culturing larvae to adulthood in 24-well cell culture plates is a significant technical challenge that we have yet to overcome. As a result, the important questions you raise, such as what percentage of exposed larvae become adults? How does the time to adulthood differ (if at all) for exposed larvae versus non-exposed larvae? Is there a difference in adult weight? Do the effects of imidacloprid on larval feeding persist after these individuals reach adulthood? Does imidacloprid damage to larvae affect colony and adult activity? We do not have answers at this time. We are aware that answers to the above questions will help people better understand how serious the effects of imidacloprid environmental residues on honey bee larvae and adults, as well as bee colonies as a whole, are, and will draw sufficient attention to them. We intend to break through this technological bottleneck of culture larvae to adulthood in future studies and incorporate the above scientific questions into our next research design. Thank you again for your insightful comments! This gives us new research ideas.

eLife assessment

This investigation of the changes in gene expression and some of the physiological consequences of sublethal exposures to the neonicotinoid insecticide imidacloprid in honeybee larvae is useful, although numerous experiments were not considered based on technical issues. The methodological design leads to concerns and it is therefore not obvious that all conclusions are justified. The study adds to our understanding of how this insecticide impacts development and growth of honeybees, but the evidence supporting the major claims is incomplete.

Joint Public Review:

This study provides evidence on the ability of sublethal imidacloprid doses to affect growth and development of honeybee larva. While checking the effect of doses that do not impact survival or food intake, the authors found changes in the expression of genes related to energy metabolism, antioxidant response, and P450 metabolism. The authors also identified cell death in the alimentary canal, and disturbances in levels of ROS markers, molting hormones, weight, and growth ratio. The study strengths come from employing these different approaches to investigate the impacts of imidacloprid exposure. The study weaknesses come from the lack of a in depth investigation and drawing many conclusions solely from punctual gene expression, that are not representative of complete biological processes. Though relevant to understand the impacts on neonicotinoid contamination on insect pollinators, the study conclusions should be carefully weighted as they are often not fully substantiated. Follow up studies using in-depth investigation and more robust methodological design testing whether the impacts observed lead to post-metamorphosis effects and impacts in the colony would have a significant impact.

Author Response

The following is the authors’ response to the original reviews.

eLife assessment

This important paper builds on a method, previously conceptualized and validated, of genetic control for insect populations. The method, called pgSIT, uses integrated CRISPR-Cas9 based constructs to generate, in certain combinations of genotypes, mutations that cause both male sterility and female inviability. Release of such genotypes in sufficiently large numbers can lead to an inundation of a local insect population with sterile males and this can lead to localised population suppression, which represents an important method of control for problematic insect populations. The data are convincing and will be valuable to anyone working on vector control strategies.

Public Reviews:

Reviewer #1 (Public Review):

Precision guided sterile insect technology (pgSIT) is a means of mosquito vector control that aims to simultaneously kill females while generating sterile males for field release. These sterile males are expected to mate with 'wild' females resulting in very few eggs being laid or low hatching rates. Repeated releases are expected to result in the suppression of the mosquito population. This method avoids cumbersome sex-sorting while generating the sterile males. Importantly, until release, the two genetic elements that bring about female lethality and male sterility - the Cas9 and the gRNA carrying mosquitoes - are maintained as separate lines. They are crossed only prior to release, and therefore, this approach is considered to be more safe than gene drives.

The authors had made a version of this pgSIT in their 2021 paper where they targeted β-Tubulin 85D, which is only expressed in the male testes and its loss-of-function results in male sterility. In that pgSIT, they did not have female lethality, but generated flightless females by simultaneously targeted myosin heavy chain, which is expressed only in the female wings. Here the authors argue, that the survival of females is not ideal, and so modify their 2021 approach to achieve female lethality/sterility.

To do this, they target two genes - the female specific isoform of Dsx and intersex. They use multiple gRNAs against these genes and validate their ability to cause female lethality/sterility. Having verified that these do indeed affect female fertility, they combine gRNAs against Dsx and ix to generate female lethality/sterility and use β-Tubulin 85D to generate male sterility (previously validated). When these gRNA mosquitoes are crossed to Cas9 and the progeny crossed to WT (the set-up for pgSIT), they find that very few eggs are laid, larval death is high, and what emerges are males or intersex progeny that are sterile.

As this is the requirement for pgSIT, the authors then test if it is able to induce population suppression. To do this, they conduct cage trials and find that only when they use 20:1 or 40:1 ratio of pgSIT:WT cages, does the population crash in 4-5 generations. They model this pgSIT's ability to suppress a population in the wild. Unfortunately, I was not able to assess what parameters from their pgSIT were used in the model and therefore the predicted efficacy of their pgSIT, (though the range of 0-.1 is not great, given that the assessment is between 0-0.15).

We express our sincere appreciation for the valuable comments received. A wide range of ♀ viability and ♂ fertility values were explored in the model. The results determined that: “Achieving a ≥90% probability of elimination places slightly tighter restrictions on ♀ viability and ♂ fertility - a safe ballpark being ♀ viability and ♂ fertility both in the range 0-0.10, given a release scheme of ~26 releases of 250 pgSIT eggs per wild adult (Fig. 4B). These results suggest a target product profile for pgSIT to be ♀ viability and ♂ fertility both in the range 0-0.10.” A subsequent sentence has been added pointing out how the described pgSIT strain falls within this range: “The pgSIT strain described here falls well within these bounds, with ♀ viability of 0 and ♂ fertility of ~0.01.” The parameters of the described pgSIT strain are also listed throughout the paper and quoted here: “Cas9 in combination with gRNAdsx,ix,βTub induces either the lethality or transformation of pgSIT ♀’s into sterile unfit ⚥’s.” And: “Firstly, we determined that pgSIT males were not 100% sterile, with an estimated ~1% still producing some progeny.”

Finally, they also develop a SENSR with a rapid fluorescence read-out for detecting the transgene in the field. They show that this sensor is specific and sensitive, detecting low copy numbers of the transgene. This would be important for monitoring any release.

Overall, the data are clear and well presented. The manuscript is well written (albeit likely dense for the uninitiated!). I had concerns about the efficacy of generating the pgSIT animals - the overall number of eggs hatched from the gRNA (X) Cas9 cross appears to be low, therefore, very large numbers of parental animals would have to be reared and crossed to obtain enough sterile males for the SIT. In addition to this, I was concerned about the intersex progeny that can blood-feed. These could potentially contribute to the population and it would be useful to see the data that suggest that these numbers are low and the animals will not be competent in the field.

Reviewer #2 (Public Review):

This is a thorough and convincing body of work that represents an incremental but significant improvement on iterations of this method of CRISPR-based Sterile Insect Technique ('pgSIT'). In this version, compared to previous, the authors target more genes than previously, in order to induce both female inviability (targeting the genes intersex and doublesex, compared to fem-myo previously) and male sterility (targeting a beta-tubulin, as previously in the release generation. The characterization of the lines is extensive and this data will be useful to the field. However, what is lacking is some context as to how this formulation compares to the previous iteration. Mention is made of the possible advantage of removing most females, compared to just making them flightless (as previously) but there is no direct comparison, either experimental, or theoretical i.e. imputing the life history traits into a model. For me this is a weakness, yet easily addressed. In a similar vein, much is made in alluding to the 'safety concerns of gene drive' and how this is a more palatable half-way house, just because it has CRISPR component within it; it is not. It would be much more sensible, and more informative, to compare this pgSIT technology to RIDL (release of insects carrying a dominant lethal), which is essentially a transgene-based version of the Sterile Insect Technique, as is the work presented here.

We express our sincere appreciation for the valuable comments received. A wide range of ♀ viability and ♂ fertility values were explored in the model. Given the intricate nature of this study and taking into account the recommendations provided by multiple reviewers and the editor, we have eliminated superfluous comparisons among various methodologies.

The authors achieve impressive results and show that these strains, under a scenario of high levels of release ratios compared to WT, could achieve significant local suppression of mosquito populations. The sensitivity analysis that examines the effect of changing different biological or release parameters is well performed and very informative.

The authors are honest in acknowledging that there are still challenges in bringing this to field release, namely in developing sexing strains and optimizing release strategies - a question I have here is how to actually release eggs, and could variability in the efficiency of this aspect be modelled in the sensitivity analysis? It seems to me like this could be a challenge and inherently very variable.

We really appreciate comments. Several approaches are available to release eggs - either in pre-existing breeding sites in the field, or in artificial breeding sites (e.g., cups). We have added a sentence in the Discussion section to highlight that this is an area requiring further research: “Secondly, studies are required to determine the survival and mating competitiveness of released pgSIT males under field conditions, and to optimize their release protocol.” Regarding the efficiency of egg releases, the following sentence in the modeling results section has been added: “We assume released eggs have the same survival probability as wild-laid eggs; however if released eggs do have higher mortality, this would be equivalent to considering a smaller release.” As stated in the modeling results (and depicted in Figure 4 and Supplementary Figure 5): “Suppression outcomes were found to be most sensitive to release schedule parameters (number, size and interval of releases), ♂ fertility and ♀ viability.” It follows that suppression outcomes are equivalently sensitive to the efficiency of an egg release.

Reviewer #3 (Public Review):

Summary and Strengths:

The manuscript by Li et al. presents an elegant application of sterile insect technology (pgSIT) utilizing a CRISPR-Cas9 system to suppress mosquito vector populations. The pgSIT technique outlined in this paper employs a binary system where Cas9 and gRNA are conjoined in experimental crosses to yield sterile male mosquitoes. Employing a multiplexed strategy, the authors combine multiple gRNA to concurrently target various genes within a single locus. This approach successfully showcases the disruption of three distinct genes at different genomic positions, resulting in the creation of highly effective sterile mosquitoes for population control. The pioneering work of the Akbari lab has been instrumental in developing this technology, previously demonstrating its efficacy in Drosophila and Aedes aegypti. By targeting the female-specific splice isoform (exon-5) of doublesex in conjunction with intersex and β-tubulin, the researchers induce female lethality, leading to a predominance of sterile male mosquitoes. This innovation is particularly noteworthy as the deployment of sterile mosquitoes on a large scale typically requires substantial investment in sex sorting. However, this study circumvents this challenge through genetic manipulation.

Weaknesses:

One notable concern arising from this manuscript pertains to the absence of data regarding the potential off-target effects of the gRNA. Given the utilization of multiple gRNA, the risk of unintended mutations in non-target areas of the genome increases. With around 1% of males still capable of producing fertile offspring, understanding the frequency of unintended genome targeting becomes crucial. Such mutations could potentially become fixed within the natural population.

We express our sincere appreciation for the valuable comments received and fully agree with the reviewer regarding the importance of understanding the frequency of unintended genome targeting. However, the likelihood of off-target effects becoming fixed within the population is exceedingly low. To mitigate potential negative impacts, we employed CHOPCHOP V3.0.0 (https://chopchop.cbu.uib.no) for the selection of gRNAs, which will specifically tminimize the occurrence of genomic off-target cleavage events. Furthermore, our releasing process will be carried out in multiple rounds. In the event that an undesired mutant is introduced into the local population, the mutated gene will either be quickly eradicated through subsequent rounds of releases or be naturally eliminated through the process of natural selection over time.

The experiments are well-conceived, featuring suitable controls and repeated trials to yield statistically significant data. However, a primary issue with the manuscript lies in its data presentation. The authors' graphical representations are intricate and demand considerable attention to discern the nuances, especially due to the striking similarity between the symbols representing different genotypes. As it stands, the manuscript primarily caters to experts within the field, thereby warranting improvements in data visualization for broader comprehension.

We appreciate the comment. However, as this work is indeed complex and intricate and as there is limitations imposed by the publisher on data visualizations (i.e. number of figures in the main text, etc.) we have tried our best for presenting our data in full.

All three reviewers were appreciative of the work presented in this manuscript. There were some common concerns that we shared, that the authors could consider revising. They are listed below.

Essential revisions:

1. Formal comparison with the previous/other methods: The authors make many statements that compare this pgSIT with their previous method, gene drives, or with RIDL. We suggest that they focus their comparisons within the scope of data and avoid comparisons between RIDL, gene drive, and pgSIT that are based on perceptions of these methods. It would be useful if, for example, they could impute life history traits and demonstrate this pgSIT's efficacy over their previous versions.

We express our sincere appreciation for the valuable comments received. We have removed the unnecessary comparisons between different methods, please review the revised version.

1. Writing and presentation of figures: The authors should please take advantage of the eLife format and unpack each sentence/figure so that it's accessible to readers outside this field.

We appreciate your comment, and we have implemented some necessary changes based on your suggestions.

1. Data to support claims made in passing: There are many instances, such as detailed in the reviews (and the entire second paragraph in the discussion) that are not supported by data. The authors should either provide that data or not make these claims.

Thank you for the comment. We have removed these claims.

1. Off target effects: There is the formal possibility that off target effects that might get fixed in the population. Could the authors please address this in the discussion.

We appreciate the comment and fully agree with the reviewer regarding the importance of understanding the frequency of unintended genome targeting. However, the likelihood of off-target effects becoming fixed within the population is exceedingly low. We have address this in the discussion.

“Even though mutations could potentially become fixed within the natural population, the likelihood of off-target effects becoming fixed within the population is exceedingly low. To mitigate potential negative impacts, we employed CHOPCHOP V3.0.0 (https://chopchop.cbu.uib.no) for the selection of gRNAs, specifically to minimize the occurrence of genomic off-target cleavage events. Furthermore, our releasing process will be carried out in multiple rounds. Even in the event that an undesired mutant is introduced into the local population, it will either be completely eradicated through subsequent rounds of releases or be naturally eliminated through the process of natural selection over time.”

Aside from this, we ask that the authors please pay attention to the detailed reviews.

Reviewer #1 (Recommendations For The Authors):

The writing: Each sentence is packed with information and while this is fine for those immersed in the field, it might be dense for those who are not. There are a lot of nuances in such an approach and clearly laying it out for the reader is important. The authors should unpack some of these sentences to make their work more accessible.

Thank you for the comment. We have unpacked some of sentences, please review the revised version.

It will help to have a schematic linked to the introduction about how these mosquitoes are designed to be used. Which strains would be scaled up in the lab, which ones (and what stage) could be released, and in which animal/generation they expect sterility or lethality. This would be useful while interpreting the schematics of the genetic crosses in the rest of the figures (1B, 2B). Li et al 2021 has something to this effect. I say this particularly because in the text, 'pgSIT' is used to refer to both the lab stocks and the F1s.

We really appreciate the suggestion to incorporate a schematic into the introduction to clarify the intended use of these mosquitoes. Taking into account all the suggestions, we would like to keep textual descriptions and context provided within the manuscript, which, together with Figures 1B and 2B, illustrate our intentions. Nevertheless, we value your input and have taken other feedback into account to improve the overall quality of the content.

Because Figure 1A depicts all the gRNAs I thought that's what they were testing in the first results section. But the legends seems to suggest that the individual gRNAs have been tested. Such issues will be sorted with attention to the writing. It would also be nice to have Figure 2A here.

We apologize for any misunderstanding. Figure 1A displays two gRNA constructs: one for dsx (comprising 4 gRNAs) and another for ix (with 2 gRNAs). All of these gRNAs were tested in the initial results section. Subsequently, we engineered the final gRNA construct, denoted as gRNAdsx,ix,βTub, which combines the effective gRNAs described earlier (3 targeting dsx and 1 targeting ix, as illustrated in Supplementary Figure 2).

It wasn't clear to me how egg laying percentages were calculated or what it means.

We appreciate your comment. Female fecundity depends on the egg output (egg laying percentage) and the egg hatching rate, since insect female can lay unfertalized eggs that does not hatch. Egg laying percentages were calculated by dividing the numbers of laid eggs by a test female group by that of the control female group that laid the highest egg number. This procedure is called normalization and enable relative comparison of laid egg number.

How is hatching at times more than laying?

When a female group laid a small egg number but the high percentage of those eggs hatched.

Calling something 'intersex': The authors are assessing intersex by malformed genitalia, maxillary palps and ovaries. But the genitalia defects in Fig1D were not clear to me. Can the authors show better images? While the MP snd ovary phenotypes were clear, it would be nice to see these quantified - what proportion of the females have each/some/all of these phenotypes? It would be nice to see this quantified. (They have some of this in the supplementary table).

We express our gratitude for the comment received and acknowledge the issue regarding the clarity of the images. It is important to note that these photographs represent the highest level of clarity achieved thus far. We value your interest in the quantification of the observed phenotypes. However, due to certain constraints, we were unable to quantify the proportions for all the females, and we did not retain all the samples needed for this specific quantification.

It's interesting that 50% of the intersex don't blood-feed - is this because they do not have appropriately formed stylets? It would be important to quantify the number of hatch-able eggs. This is particularly important in the context of field application and should ideally be included in the mathematical modelling. In the discussion, the authors mention that they are not able to host-seek and a variety of other behaviours - these data should be presented as it would be important for assessing the efficacy of the pgSIT.

Thank you for the comment. We did not find the mutant stylets from these intersex mosquitoes. We agree with the reviewer that the number of hatchable eggs is particularly important in the context of field application. Indeed, the number of hatchable eggs is what was considered in the mathematical modeling. We did a blood feed assay (small cage and big cage) for host seeking behavior. Data were presented in Supplementary Table 5.

At the end of the first results section, the authors state, "Taken together, these findings reveal that ♀-specific lethality and/or ⚥..." But I don't see data that show female-specific lethality until Figure 2C.

Thank you for pointing out this. In order to describe our results clearly, we have deleted “♀-specific lethality and/or”

In the combined gRNA mosquito (the pgSIT), they find that the cross between the gRNA and Cas9 results in very few eggs being laid, high larval death, and what emerges are males. This suggests that it would be a poor pgSIT, right? You'd have to set up huge crosses to get enough males emerging in the wild to mate with WT females to bring about population suppression. Could the authors comment on this?

We appreciate the comment. Even in the presence of imperfections, such as reduced egg production resulting from the gRNA and Cas9 cross and the necessity of extensive mating to obtain an adequate number of males, population suppression is very promising with the pgSIT, both in terms of the potential to eliminate a mosquito population, or to suppress it to an extent that would largely interrupt disease transmission. It's worth noting that our current efforts serve as a validation of the system before its potential large-scale application, because we have demonstrated that removing females by disrupting sex determinate genes is possible with pgSIT, which can inform the development of such systems in other species in the future.

If I'm reading Figure 2C right, the authors have combined the results from two types of crosses in the last two plots: 1) the Cas9 (X) gRNA mosquitoes and 2) the progeny from these crossed to WTs. This is not ideal. I would suggest the authors unpack the text around this data and plot it separately.

We really appreciate the comment here, the panel 2C depicts the phenotypic data of the F1 progeny generated by the cross of the parents indicated below the X axis: egg-to-adult survival, larval death, sex ratios, and fertility. The fertility of F1 progeny is the major phenotypic feature for the project. To assess the fertility of the surviving F1 progeny, we had to cross the F1 females and males to WT males and females, respectively and assess the hatching rate of produced eggs before sacrificing emerged larvae and unhatched eggs. It's important to note that mosquito females can lay unfertilized eggs that fail to hatch.

The text around 2F needs to be more explanatory. There are lots of labels in the figure that are not referred to, making it difficult to follow the data.

We have gone through and expanded many of the figure legends and modified some figures to help make them more understandable.

The supplementary figure numbering is off.

We really appreciate the comment. The supplementary figure numbering have been fixed.

I cannot comment on Figure 4 as this is outside my expertise. However, I do feel that some attention to the writing might help make the approach more accessible to the invested advanced lay-person.

We appreciate the comment, and we re-wrote some of the sentences describing Figure 4.

Reviewer #2 (Recommendations For The Authors):

Line 49 'resistances' is a strange plural.

Corrected. Thank you so much!

the genitive, used with the sex symbols throughout, looks very weird e.eg line 60, 66 etc. Also the intersex symbol, on my copy at least, just prints as a square

These have been fixed in the revised version. Thank you so much!

Line 74 syntax (...: the spread of...") seems off

Corrected. Thank you for pointing out this.

Line 80-81 " to address some of the challenges with gene drives, pgSIT also leverages....." this is a straw man/red herring argument, and simply does not follow. It is this element that I raised above in the public review. See also line 84 'gene drive safety concerns'.

Thank you, we have re-wrote the paragraph.

Line 128 "the induced phenotypes were especially strong in intersex individuals" - this is a curious statement since, if intersex, they are by definition already showing a strongly induced phenotype

We apologize for the lack of clarity and have updated the text, we have deleted “the induced phenotypes were especially strong in intersex individuals”, to be more explicit, now stating “These gRNAdsx/+; Cas9/+ ⚥ exhibited multiple malformed morphological features, such as mutant maxillary palps, abnormal genitalia, and malformed ovaries”

The extent and completeness of the supplementary data is appreciated but there needs to be some statistical tests applied to back up statements like 'showed normal fertility' (line 138) or wind lengths 'were a bit larger'. None seem to have been applied.

We appreciate the comment. We've removed these sentences in the new version.

Supp Fig 4 - on left of panel C there is a small blue square at dsx locus that is unexplained. What is this?

Thank you for pointing this. It was a mistake, we have removed the small blue square from Sup Fig4.

Line 182 the reduction in flight activity in release genotype of pgSIT males - is it only those coming with the maternal source of Cas9 that are plotted (only pink dots)?

We appreciate the comment. pgSIT males, regardless of whether they originate from a maternal or paternal source of Cas9, exhibit a similar reduction in flight activity compared to wild-type (WT) males.

Figure 3A legend - I think there is a typo that says males were fed

Corrected. Thank you for pointing this out.

“♂’s” to “♀’s”

On the window of protection (WOP) plots (e.g. supp fig 12) what is the unit on Y-axis for WOP? It goes from 0-1, as if it were probability, but I was expecting some duration.

Thanks for the comment. The y-axis for WOP in Supp Fig 12 had been normalized unnecessarily. It has now been corrected to span from 0 to 5 years.

Fig 4B blue (line) on blue(shading) is impossible to decipher on my copy

Thank you for pointing this out. We have changed the colors of the traces (population dynamics), made the window of protection line thicker, and have made the shading less opaque to make the population dynamics in this figure clearer.

Line 250 and 252: supp Fig 13 (not 12)

Corrected. Thank you for pointing this out.

Line 279 "potentially a more widespread effect of sex determination genes than previously expected" - I simply don't see how this is so, or why there is the need to make such a claim. Dsx is known to underpin almost of somatic determination of sex-specific morphologies, in a range of insects.

We appreciate the comment. We have delete the sentence:

“Taken together, these observations indicate a potentially more widespread effect of sex determination genes than previously expected, though regardless.”

Line 320 "We would expect pgSIT to be regulated similarly to Oxitec's RIDL" because they are similar, which goes to my main point above about more appropriate context, and this warrants some direct attention to a comparison of the efficacy.

We appreciate the comment. We have delete these sentences:

“We would expect pgSIT to be regulated similarly to Oxitec's RIDL technology (Spinner et al., 2022), which has already been successfully deployed in numerous locations, including the United States.”

Was there a minimal performance advantage with strain #1 with the triple locus g-RNA suite, over the other two strains? Am just curious as to why one was chosen over the other

We appreciate the comment. There was no performance advantage with the strain #1 over the other two strains.

Reviewer #1 (Public Review):

Precision guided sterile insect technology (pgSIT) is a means of mosquito vector control that aims to simultaneously kill females while generating sterile males for field release. These sterile males are expected to mate with 'wild' females resulting in very few eggs being laid or low hatching rates. Repeated releases are expected to result in the suppression of the mosquito population. This method avoids cumbersome sex-sorting while generating the sterile males. Importantly, until release, the two genetic elements that bring about female lethality and male sterility - the Cas9 and the gRNA carrying mosquitoes - are maintained as separate lines. They are crossed only prior to release, and therefore, this approach is considered to be more safe than gene drives.

The authors had made a version of this pgSIT in their 2021 paper where they targeted *β-Tubulin 85D*, which is only expressed in the male testes and its loss-of-function results in male sterility. In that pgSIT, they did not have female lethality, but generated flightless females by simultaneously targeted *myosin heavy chain,* which is expressed only in the female wings. Here the authors argue, that the survival of females is not ideal, and so modify their 2021 approach to achieve female lethality/sterility.

To do this, they target two genes - the female specific isoform of Dsx and intersex. They use multiple gRNAs against these genes and validate their ability to cause female lethality/sterility. Having verified that these do indeed affect female fertility, they combine gRNAs against Dsx and ix to generate female lethality/sterility and use *β-Tubulin 85D* to generate male sterility (previously validated). When these gRNA mosquitoes are crossed to Cas9 and the progeny crossed to WT (the set-up for pgSIT), they find that very few eggs are laid, larval death is high, and what emerges are males or intersex progeny that are sterile.

As this is the requirement for pgSIT, the authors then test if it is able to induce population suppression. To do this, they conduct cage trials and find that only when they use 20:1 or 40:1 ratio of pgSIT:WT cages, does the population crash in 4-5 generations. They model this pgSIT's ability to suppress a population in the wild. Unfortunately, I was not able to assess what parameters from their pgSIT were used in the model and therefore the predicted efficacy of their pgSIT, (though the range of 0-.1 is not great, given that the assessment is between 0-0.15).

Finally, they also develop a SENSR with a rapid fluorescence read-out for detecting the transgene in the field. They show that this sensor is specific and sensitive, detecting low copy numbers of the transgene. This would be important for monitoring any release.

Overall, the data are clear and well presented.

Comments on revised version:

The authors have addressed the major issues raised by reviewers related to off target effects, writing and figures, and comparisons with other vector control methods and claims made in passing.

Reviewer #2 (Public Review):

This is a thorough and convincing body of work that represents an incremental but significant improvement on iterations of this method of CRISPR-based Sterile Insect Technique ('pgSIT'). In this version, compared to previous, the authors target more genes than previously, in order to induce both female inviability (targeting the genes intersex and doublesex, compared to fem-myo previously) and male sterility (targeting a beta-tubulin, as previously in the release generation.

The characterization of the lines is extensive and this data will be useful to the field. However, what is lacking is some context as to how this formulation compares to the previous iteration. Mention is made of the possible advantage of removing most females, compared to just making them flightless (as previously) but there is no direct comparison, either experimental, or theoretical i.e. imputing the life history traits into a model. For me this is a weakness, yet easily addressed. In a similar vein, much is made in alluding to the 'safety concerns of gene drive' and how this is a more palatable half-way house, just because it has CRISPR component within it; it is not. It would be much more sensible, and more informative, to compare this pgSIT technology to RIDL (release of insects carrying a dominant lethal), which is essentially a transgene-based version of the Sterile Insect Technique, as is the work presented here.

The authors achieve impressive results and show that these strains, under a scenario of high levels of release ratios compared to WT, could achieve significant local suppression of mosquito populations. The sensitivity analysis that examines the effect of changing different biological or release parameters is well performed and very informative.

The authors are honest in acknowledging that there are still challenges in bringing this to field release, namely in developing sexing strains and optimizing release strategies.

Reviewer #3 (Public Review):

The manuscript by Li et al. presents an elegant application of sterile insect technology (pgSIT) utilizing a CRISPR-Cas9 system to suppress mosquito vector populations. The pgSIT technique outlined in this paper employs a binary system where Cas9 and gRNA are conjoined in experimental crosses to yield sterile male mosquitoes. Employing a multiplexed strategy, the authors combine multiple gRNA to concurrently target various genes within a single locus. This approach successfully showcases the disruption of three distinct genes at different genomic positions, resulting in the creation of highly effective sterile mosquitoes for population control. The pioneering work of the Akbari lab has been instrumental in developing this technology, previously demonstrating its efficacy in Drosophila and Aedes aegypti.

By targeting the female-specific splice isoform (exon-5) of doublesex in conjunction with intersex and β-tubulin, the researchers induce female lethality, leading to a predominance of sterile male mosquitoes. This innovation is particularly noteworthy as the deployment of sterile mosquitoes on a large scale typically requires substantial investment in sex sorting. However, this study circumvents this challenge through genetic manipulation.

Author Response

The following is the authors’ response to the original reviews.

Public Reviews:

Reviewer #1 (Public Review):

Summary

In this manuscript, Hagihara et al. characterized the relationship between the changes in lactate and pH and the behavioral phenotypes in different animal models of neuropsychiatric disorders at a large-scale level. The authors have previously reported that increased lactate levels and decreased pH are commonly observed in the brains of five genetic mouse models of schizophrenia (SZ), bipolar disorder (BD), and autism spectrum disorder (ASD). In this study, they expanded the detection range to 109 strains or conditions of animal models, covering neuropsychiatric disorders and neurodegenerative disorders. Through statistical analysis of the first 65 strains/conditions of animal models which were set as exploratory cohort, the authors found that most strains showed decreased pH and increased lactate levels in the brains. There was a significant negative correlation between pH and lactate levels both at the strain/condition level and the individual animal level. Besides, only working memory was negatively correlated with brain lactate levels. These results were successfully duplicated by studying the confirmative cohort, including 44 strains/conditions of animal models. In all strains/conditions, the lactate levels were not correlated with age, sex, or storage duration of brain samples.

Strengths

1. The manuscript is well-written and structured. In particular, the discussion is really nice, covering many potential mechanisms for the altered lactate levels in these disease models.

2. Tremendous efforts were made to recruit a huge number of various animal models, giving the conclusions sufficient power.

We are grateful to Reviewer #1 for the positive evaluation of our manuscript. As indicated in the responses that follow, we have taken all the comments and suggestions made by the reviewer into account in the revised version of our paper.

Weaknesses

1. The biggest concern of this study is the limited novelty. The point of "altered pH and/or lactate levels in the brains from human and rodent animals of neuropsychiatric disorders" has been reported by the same lab and other groups in many previous papers.

The previous study mentioned by the reviewer evaluated a small number of animal models of psychiatric disorders. The novelty of this study is underscored by two key findings: 1) the generality of changes in brain pH and lactate levels across a diverse range of disease models, and 2) the association of these phenomenon with specific behaviors. First, this large-scale animal model study revealed that alterations in brain pH/lactate levels can be found in approximately 30% of the animal models examined. This generality suggests a common basis in the neuropathophysiology of not only schizophrenia, bipolar disorder, and ASD, but also of Alzheimer’s disease (APP-J20 Tg mice), Down’s syndrome (Ts1Cje mice), Mowat–Wilson syndrome (Zeb2 KO mice), Dravet syndrome (Scn1a-A1783V KI mice), tuberous sclerosis complex (Tsc2 KO mice), Ehlers-Danlos syndrome (Tnxb KO mice), and comorbid depression in diabetes (streptozotocin-treated mice) and colitis (dextran sulfate sodium-treated mice). Secondly, this study demonstrated that these phenomenon in the brain are primarily associated with working memory impairment over depression- and anxiety-related behaviors. Importantly, developing these hypotheses in an exploratory cohort of animals and confirming them in an independent cohort within this study enhances the robustness and reliability of our hypotheses, which we believe are equally crucial as their novelty. Accordingly, we have revised the discussion section as follows (page 31, line 7):

Original text

"We performed a large-scale analysis of brain pH and lactate levels in 109 animal models of neuropsychiatric disorders, which revealed the diversity of brain energy metabolism among these animal models. Some strains of mice that were considered models of different diseases showed similar patterns of changes in pH and lactate levels. Specifically, the SZ/ID models (Ppp3r1 KO, Nrgn KO mice, and Hivep2 KO mice), BD/ID model (Camk2a KO mice), ASD model (Chd8 KO mice), depression models (mice exposed to social defeat stress, corticosterone-treated mice, and Sert KO mice), AD model (APP-J20 Tg mice), and DM model (Il18 KO and STZ-treated mice) commonly exhibited decreased brain pH and increased lactate levels."

Revised text

"We performed a large-scale analysis of brain pH and lactate levels in 109 animal models of neuropsychiatric disorders, which revealed the diversity of brain energy metabolism among these animal models. The key findings of this study are as follows: 1) the generality of changes in brain pH and lactate levels across a diverse range of disease models, and 2) the association of these phenomenon with specific behaviors. First, this large-scale animal model study revealed that alterations in brain pH/lactate levels can be found in approximately 30% of the animal models examined. This generality suggests a common basis in the neuropathophysiology of not only schizophrenia, bipolar disorder, and ASD, but also of Alzheimer’s disease (APP-J20 Tg mice), Down’s syndrome (Ts1Cje mice), Mowat–Wilson syndrome (Zeb2 KO mice), Dravet syndrome (Scn1a-A1783V KI mice), tuberous sclerosis complex (Tsc2 KO mice), Ehlers-Danlos syndrome (Tnxb KO mice), and comorbid depression in diabetes (streptozotocin-treated mice) and colitis (dextran sulfate sodium-treated mice). Secondly, this study demonstrated that these phenomenon in the brain are primarily associated with working memory impairment over depression- and anxiety-related behaviors. Importantly, developing these hypotheses in an exploratory cohort of animals and confirming them in an independent cohort within this study enhances the robustness and reliability of our hypotheses."

1. This study is mostly descriptive, lacking functional investigations. Although a larger cohort of animal models were studied which makes the conclusion more solid, limited conceptual advance is contributed to the relevant field, as we are still not clear about what the altered levels of pH and lactate mean for the pathogenesis of neuropsychiatric disorders.

We agree with the reviewer’s comment. To address this issue, it is necessary to comprehensively identify brain regions and cell types responsible for pH and lactate changes in each strain/condition of animals, as these may differ among them. Subsequently, based on such findings, we can then proceed with functional investigations that specifically target the identified brain regions/cell types. However, conducting such investigations would require a significant amount of time to complete, approximately 2–3 years, and is beyond the scope of this study. Therefore, we would like to conduct such studies in the future. We have mentioned this limitation by revising the discussion section of this study as follows (page 43, line 5):

Original text

"Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). The brain region-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Further studies are needed to address this issue by measuring microdissected brain samples and performing in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011) and MRS (Davidovic et al., 2011)."

Revised text:

"The major limitations of this study include the absence of analyses specific to brain regions or cell types and the lack of functional investigations. Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. It is known that certain molecular expression profiles and signaling pathways display brain region-specific alterations, and in some cases, even exhibit opposing changes in neuropsychiatric disease models (Hosp et al., 2017; Floriou-Servou et al. 2018; Reim et al., 2017). Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). Additionally, it has been reported that the basal intracellular pH differs between neurons and astrocytes (lower in astrocytes than in neurons), and their responsiveness to conditions simulating neural hyperexcitation and the metabolic acidosis in terms of intracellular pH also varies (Raimondo et al., 2016; Salameh et al., 2017). It would also be possible that the brain region/cell type-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Given the assumption that the brain regions and cell types responsible for pH and lactate changes vary across different strains/conditions, comprehensive studies are needed to thoroughly examine this issue for each animal model individually. This can be achieved through techniques such as evaluating microdissected brain samples, conducting in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011), and MRS (Davidovic et al., 2011). Subsequently, based on such findings, it is also necessary to conduct functional analyses for each model animal by manipulating pH or lactate levels in specific brain regions/cell types and evaluating behavioral phenotypes relevant to neuropsychiatric disorders."

1. The experiment procedure is also a concern. The brains from animal models were acutely collected without cardiac perfusion in this study, which suggests that resident blood may contaminate the brain samples. The lactate is enriched in the blood, making it a potential confounded factor to affect the lactate levels as well as pH in the brain samples.

We thank the reviewer for pointing this out. We have discussed this issue as follows (page 45, line 4):

We also note that there are several potential confounding factors in this study. The brain samples analyzed in this study contained cerebral blood. The cerebral blood volume is estimated to be approximately 20–50 μl/g in human and feline brains (Leenders et al., 1990; van Zijl et al., 1998). When we extrapolate these values to murine brains, it would imply that the proportion of blood contamination in the brain homogenates analyzed is 0.2–0.6%. Additionally, lactate concentrations in the blood are two to three times higher than those in the brains of mice (Béland-Millar et al., 2017). Therefore, even if there were differences in the amount of resident blood in the brains between control and experimental animals, the impact of such differences on the lactate measurements would likely be minimal.

1. The lactate and pH levels may also be affected by other confounded factors, such as circadian period, and locomotor activity before the mice were sacrificed. This should also be discussed in the paper.

Following the reviewer’s suggestion, we have discussed the matter as follows (page 45, line 12): Other confounding factors include circadian variation and locomotor activity before the brain sampling. Lactate levels are known to exhibit circadian rhythm in the rodent cortex, transitioning gradually from lower levels during the light period to higher levels during the dark period (Dash et al., 2012; Shram et al., 2002; Wallace et al., 2022). The variation in the times of sample collection during the day was basically kept minimized within each strain/condition of animals. However, the sample collection times were not explicitly matched across the different laboratories, which may contribute to variations in the baseline control levels of pH and lactate among different strains/conditions of animals (Table S3). In addition, motor activity and wake/sleep status immediately before brain sampling can also influence brain lactate levels (Neylor et al., 2012; Shram et al., 2002). These factors have the potential to act as confounding variables in the measurement of brain lactate and pH in animals.

1. Another concern is the animal models. Although previous studies have demonstrated that dysfunctions of these genes could cause related phenotypes for certain disorders, many of them are not acknowledged by the field as reliable disease models. Besides, gene deficiency could also cause many known or unknown unrelated phenotypes, which may contribute to the altered levels of lactate and pH, too. In this circumstance, the conclusion "pH and lactate levels are transdiagnostic endophenotype of neuropsychiatric disorders" is somewhat overstated.

We thank the reviewer for pointing this out. We should have taken this issue into consideration. Accordingly, we have discussed this issue as the limitation of this study in the discussion section as follows (page 34, line 14):

"While we analyzed 109 strains/conditions of animals, we included both those that are widely recognized as animal models for specific neuropsychiatric disorders and those that are not. For example, while interleukin 18 (Il18) KO mice and mitofusin 2 (hMfn2-D210V) Tg mice exhibited changes in pH and lactate levels, the evidence that these genes are associated with specific neuropsychiatric disorders is limited. However, these strains of mice exhibited behavioral abnormalities related to neuropsychiatric disorders, such as depressive-like behaviors and impaired working memory (Ishikawa et al., 2019, 2021; Yamanishi et al., 2019). Furthermore, these mice showed maturation abnormality in the hippocampal dentate gyrus and neuronal degeneration due to mitochondrial dysfunction, respectively, suggesting conceptual validity for utilization as animal models for neuropsychiatric and neurodegenerative disorders (Cunnane, et al., 2021; Burté et al., 2015; Hagihara et al., 2013, 2019). In contrast, mice with heterozygous KO of the synaptic Ras GTPase-activating protein 1 (syngap1), whose mutations have been identified in human patients with ID and ASD, showed an array of behavioral abnormalities relevant to the disorders (Komiyama et al., 2002; Nakajima et al., 2019), but did not show changes in brain pH or lactate levels. Therefore, while changes in brain pH and lactate levels could be transdiagnostic endophenotypes of neuropsychiatric disorders, they might occur depending on the subpopulation due to the distinct genetic and environmental causes or specific disease states in certain disorders."

Regarding the latter point suggested by the reviewer, we consider that alterations in brain pH and lactate levels occur, whether they are a direct and known consequence or indirect and unknown ones of genetic modifications. We have proposed that genetic modifications, along with environmental stimulations, may induce various changes, which subsequently converge toward specific endophenotypes in the brain, such as neuronal hyperexcitation, inflammation, and maturational abnormalities (Hagihara et al., 2013; Yamasaki et al., 2008). The findings of this study, demonstrating the commonality of alteration of brain pH and lactate levels, align with this concept, suggesting that these alterations could serve as brain endophenotypes in multiple neuropsychiatric disorders. We have revised the discussion section as follows (page 42, line 8):

Original text

"These findings suggest that the observed increase in lactate production and subsequent decrease in pH in whole-brain samples may be attributed to the hyperactivity of specific neural circuits in a subset of the examined animal models."

Revised text

"These findings suggest that neuronal hyperexcitation may be one of the common factors leading to increased lactate production and decreased pH in the brain. We consider that alterations in brain pH and lactate levels occur, whether they are a direct and known consequence or indirect and unknown ones of genetic modifications. We have proposed that genetic modifications, along with environmental stimulations, may induce various changes, which subsequently converge toward specific endophenotypes in the brain, such as neuronal hyperexcitation, inflammation, and maturational abnormalities (Hagihara et al., 2013; Yamasaki et al., 2008). The findings of this study, demonstrating the commonality of alterations in brain pH and lactate levels, align with this concept and suggest that these alterations could serve as brain endophenotypes in multiple neuropsychiatric disorders."

1. The negative correlationship between pH and lactate is rather convincing. However, how much the contribution of lactate to pH is not tested. In addition, regarding pH and lactate, which factor contributes most to the pathogenesis of neuropsychiatric disorders is also unclear. These questions may need to be addressed in the future study.

To estimate the degree of contribution of lactate to pH, we determined the contribution ratio using the regression coefficient within a linear regression model applied to a combined cohort. The results showed that 33.2% of changes in pH may be explained by changes in lactate level. We have added the following text in the Results section (page 28, line 7).

The contribution ratio of lactate to pH, calculated based on the regression coefficient in a linear regression model, was 33.2% at the individual level, suggesting a moderate level of contribution.

Regarding the latter suggestion, we would like to address the issue in the future study. Accordingly, we have added the following sentence in the discussion section (page 40, line 11):

Original text

"Further studies are needed to address these hypotheses by chronically inducing deficits in mitochondrial function to manipulate endogenous lactate levels in a brain region-specific manner and to analyze their effects on working memory."

Revised text

"Further studies are needed to address these hypotheses by chronically inducing deficits in mitochondrial function to manipulate endogenous lactate levels in a brain region-specific manner and to analyze their effects on working memory. It is also important to consider whether pH or lactate contributes more significantly to the observed behavioral abnormalities."

1. The authorship is open to question. Most authors listed in this paper may only provide mice strains or brain samples. Maybe it is better just to acknowledge them in the acknowledgments section.

In the light of the current circumstances, wherein there is no universally agreed definition of authorship (the Committee on Publication Ethics1), we acknowledge the reviewer’s concern. Collecting a comprehensive range of mouse strains and brain samples is a fundamental principle of this study. Maintaining mouse lines, breeding mice, genotyping, drug administration, and preparation of brain samples each require specialized expertise. Therefore, the scientific and technical contributions of individuals who only provided mouse strains or brain samples was also crucial for obtaining the data essential to this study. In accordance with the authorship guidelines outlined by the journal, which stipulate that “We recommend that all researchers who made substantial or important contributions to the design of a work, or the acquisition, analysis or interpretation of the data used in the paper, be included as authors.”, we would like to retain their authorship status. Furthermore, we ensured that all authors had read and approved the manuscript before submission, using Google Forms.

1. GUIDELINES ON GOOD PUBLICATION PRACTICE, Committee on Publication Ethics (COPE), https://publicationethics.org/files/u7141/1999pdf13.pdf

1. The last concern is about the significance of this study. Although the majority of strains showed increased lactate, some still showed decreased lactate levels in the brains. These results suggested that lactate or pH is an endophenotype for neuropsychiatric disorders, but it is hard to serve as a good diagnostic index as the change is not unidirectional in different disorders. In other words, the relationship between lactate level and neuropsychiatric disorders is not exclusive.

As pointed out by the reviewer, whether brain pH and lactate levels increase or decrease could vary among animal models. Such variation may represent subpopulations of patients or specific disease states. Considering both increases and decreases in changes in pH and lactate levels could be important to achieve that goal. Accordingly, we have revised the text as follows:

Added text (page 33, line 12)

"Detecting changes in brain pH and lactate levels, whether resulting in an increase or decrease due to their potential bidirectional alterations, using techniques such as MRS may help the diagnosis, subcategorization, and identification of specific disease states of these biologically heterogeneous and spectrum disorders, as has been shown for mitochondrial diseases (Lin et al., 2003)."

Added text (page 35, line 14)

"Therefore, while changes in brain pH and lactate levels could be transdiagnostic endophenotypes of neuropsychiatric disorders, they might occur depending on the subpopulation due to the distinct genetic and environmental causes or specific disease states in certain disorders."

Reviewer #2 (Public Review):

Hagihara et al. conducted a study investigating the correlation between decreased brain pH, increased brain lactate, and poor working memory. They found altered brain pH and lactate levels in animal models of neuropsychiatric and neurodegenerative disorders. Their study suggests that poor working memory performance may predict higher brain lactate levels.

However, the study has some significant limitations. One major concern is that the authors examined whole-brain pH and lactate levels, which might not fully represent the complexity of disease states. Different brain regions and cell types may have distinct protein and metabolite profiles, leading to diverse disease outcomes. For instance, certain brain regions like the hippocampus and nucleus accumbens exhibit opposite protein/signaling pathways in neuropsychiatric disease models.

We want to thank the reviewer for the valuable suggestions. To address this issue, it is necessary to comprehensively identify brain regions and cell types responsible for pH and lactate changes in each strain/condition of animals, as these may differ among them. Subsequently, based on such findings, we can then proceed with functional investigations that specifically target the identified brain regions/cell types. However, conducting such investigations would require a significant amount of time to complete, approximately 2–3 years, and is beyond the scope of this study. Therefore, we would like to conduct such studies in the future. We have mentioned this limitation by revising the discussion section of this study as follows (page 43, line 5):

Original text

"Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). The brain region-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Further studies are needed to address this issue by measuring microdissected brain samples and performing in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011) and MRS (Davidovic et al., 2011)."

Revised text

"The major limitations of this study include the absence of analyses specific to brain regions or cell types and the lack of functional investigations. Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. It is known that certain molecular expression profiles and signaling pathways display brain region-specific alterations, and in some cases, even exhibit opposing changes in neuropsychiatric disease models (Hosp et al., 2017; Floriou-Servou et al. 2018; Reim et al., 2017). Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). Additionally, it has been reported that the basal intracellular pH differs between neurons and astrocytes (lower in astrocytes than in neurons), and their responsiveness to conditions simulating neural hyperexcitation and the metabolic acidosis in terms of intracellular pH also varies (Raimondo et al., 2016; Salameh et al., 2017). It would also be possible that the brain region/cell type-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Given the assumption that the brain regions and cell types responsible for pH and lactate changes vary across different strains/conditions, comprehensive studies are needed to thoroughly examine this issue for each animal model individually. This can be achieved through techniques such as evaluating microdissected brain samples, conducting in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011), and MRS (Davidovic et al., 2011). Subsequently, based on such findings, it is also necessary to conduct functional analyses for each model animal by manipulating pH or lactate levels in specific brain regions/cell types and evaluating behavioral phenotypes relevant to neuropsychiatric disorders."

Moreover, the memory tests used in the study are specific to certain brain regions, but the authors did not measure lactate levels in those regions. Without making lactate measurements in brain-regions and cell types involved in these diseases, any conclusions regarding the role of lactate in CNS diseases is premature.

Regarding the point about “lactate measurements in brain-regions and cell types involved in these diseases,” please refer our responses provided above.

Additionally, evidence suggests that exogenous treatment with lactate has positive effects, such as antidepressant effects in multiple disease models (Carrard et al., 2018, Carrard et al., 2021, Karnib et al., 2019, Shaif et al., 2018). It also promotes learning, memory formation, neurogenesis, and synaptic plasticity (Suzuki et al., 2011, Yang et al., 2014, Weitian et al., 2015, Dong et al., 2017, El Hayek et al. 2019, Wang et al., 2019, Lu et al., 2019, Lev-Vachnish et a.l, 2019, Descalzi G et al., 2019, Herrera-López et al., 2020, Ikeda et al., 2021, Zhou et al., 2021,Roumes et al., 2021, Frame et al., 2023, Akter et al., 2023).

We thank the reviewer for pointing out many references regarding the effects of lactate that were not cited in our paper. We have since included these studies and discussed in more detail the effect of lactate at molecular, cellular, and behavioral levels (page 39, line 11).

Original text

"Moreover, increased lactate may have a positive or beneficial effect on memory function to compensate for its impairment, as lactate administration with an associated increase in brain lactate levels attenuates cognitive deficits in human patients (Bisri et al., 2016) and rodent models (Rice et al., 2002) of traumatic brain injury. In addition, lactate administration exerts antidepressant effects in a mouse model of depression (Carrard et al., 2016)."

Revised text

"Moreover, increased lactate may have a positive or beneficial effect on memory function to compensate for its impairment, as lactate administration with an associated increase in brain lactate levels attenuates cognitive deficits in human patients (Bisri et al., 2016) and rodent models (Rice et al., 2002) of traumatic brain injury. In addition, lactate administration exerts antidepressant effects in a mouse model of depression (Carrard et al., 2021, 2016; Karnib et al., 2019; Shaif et al., 2018). Lactate has also shown to promote learning and memory (Descalzi G et al., 2019; Dong et al., 2017; Hayek et al. 2019; Lu et al., 2019; Roumes et al., 2021; Suzuki et al., 2011), synaptic plasticity (Herrera-López et al., 2020; Yang et al., 2014; Zhou et al., 2021), adult hippocampal neurogenesis (Lev-Vachnish et al., 2019), and mitochondrial biogenesis and antioxidant defense (Akter et al., 2023), while its effects on adult hippocampal neurogenesis and learning and memory are controversial (Ikeda et al., 2021; Lev-Vachnish et al., 2019; Wang et al., 2019)."

In conclusion, the relevance of total brain pH and lactate levels as indicators of the observed correlations is controversial, and evidence points towards lactate having more positive rather than negative effects. It is important that the authors perform studies looking at brain-region-specific concentrations of lactate and that they modulate lactate levels (decrease) in animal models of disease to validate their conclusions. it is also important to consider the above-mentioned studies before concluding that "altered brain pH and lactate levels are rather involved in the underlying pathophysiology of some patients with neuropsychiatric disorders" and that "lactate can serve as a potential therapeutic target for neuropsychiatric disorders".

Regarding the points about positive effects of lactate, measurement of brain-region-specific lactate concentrations, and modulation of lactate levels, please refer to our responses provided earlier. The points raised by the reviewer are important and should be addressed in future studies.

Reviewer #2 (Recommendations For The Authors):

• Measure lactate in specific brain regions. The whole brain measurements are not relevant to the disease states.

We thank the reviewer for pointing this out. We totally agree with the reviewer’s comment and recognize that the lack of investigations in specific brain regions is one of the major limitations of this study. To address this issue, it is necessary to comprehensively identify brain regions and cell types responsible for pH and lactate changes in each strain/condition of animals, as these may differ among them. Subsequently, based on such findings, we can then proceed with functional investigations that specifically target the identified brain regions/cell types. However, conducting such investigations would require a significant amount of time to complete, approximately 2–3 years, and is beyond the scope of this study. Therefore, we would like to conduct such studies in the future. We have mentioned this limitation by revising the discussion section of this study as follows (page 43, line 5):

Original text

"Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). The brain region-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Further studies are needed to address this issue by measuring microdissected brain samples and performing in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011) and MRS (Davidovic et al., 2011)."

Revised text:

"The major limitations of this study include the absence of analyses specific to brain regions or cell types and the lack of functional investigations. Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. It is known that certain molecular expression profiles and signaling pathways display brain region-specific alterations, and in some cases, even exhibit opposing changes in neuropsychiatric disease models (Hosp et al., 2017; Floriou-Servou et al. 2018; Reim et al., 2017). Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). Additionally, it has been reported that the basal intracellular pH differs between neurons and astrocytes (lower in astrocytes than in neurons), and their responsiveness to conditions simulating neural hyperexcitation and the metabolic acidosis in terms of intracellular pH also varies (Raimondo et al., 2016; Salameh et al., 2017). It would also be possible that the brain region/cell type-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Given the assumption that the brain regions and cell types responsible for pH and lactate changes vary across different strains/conditions, comprehensive studies are needed to thoroughly examine this issue for each animal model individually. This can be achieved through techniques such as evaluating microdissected brain samples, conducting in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011), and MRS (Davidovic et al., 2011). Subsequently, based on such findings, it is also necessary to conduct functional analyses for each model animal by manipulating pH or lactate levels in specific brain regions/cell types and evaluating behavioral phenotypes relevant to neuropsychiatric disorders."

• Discuss in detail the studies that show the neuroprotective effects of lactate and reconcile these with the authors' conclusions.

As suggested by the reviewer, we have discussed in more detail the positive effect of lactate at molecular, cellular, and behavioral levels as below (page 39, line 11):

Original text

"Moreover, increased lactate may have a positive or beneficial effect on memory function to compensate for its impairment, as lactate administration with an associated increase in brain lactate levels attenuates cognitive deficits in human patients (Bisri et al., 2016) and rodent models (Rice et al., 2002) of traumatic brain injury. In addition, lactate administration exerts antidepressant effects in a mouse model of depression (Carrard et al., 2016)."

Revised text

"Moreover, increased lactate may have a positive or beneficial effect on memory function to compensate for its impairment, as lactate administration with an associated increase in brain lactate levels attenuates cognitive deficits in human patients (Bisri et al., 2016) and rodent models (Rice et al., 2002) of traumatic brain injury. In addition, lactate administration exerts antidepressant effects in a mouse model of depression (Carrard et al., 2021, 2016; Karnib et al., 2019; Shaif et al., 2018). Lactate has also shown to promote learning and memory (Descalzi G et al., 2019; Dong et al., 2017; Hayek et al. 2019; Lu et al., 2019; Roumes et al., 2021; Suzuki et al., 2011), synaptic plasticity (Herrera-López et al., 2020; Yang et al., 2014; Zhou et al., 2021), adult hippocampal neurogenesis (Lev-Vachnish et al., 2019), and mitochondrial biogenesis and antioxidant defense (Akter et al., 2023), while its effects on adult hippocampal neurogenesis and learning and memory are controversial (Ikeda et al., 2021; Lev-Vachnish et al., 2019; Wang et al., 2019)."

• Conduct experiments whereby you decrease/deplete/modulate lactate levels in animal models and show that there is amelioration of the symptoms.

Regarding this point, kindly refer to the responses we provided in the first comment from the reviewer. We have mentioned this limitation by revising the discussion section of this study as follows (page 43, line 5):

Original text

"Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). The brain region-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Further studies are needed to address this issue by measuring microdissected brain samples and performing in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011) and MRS (Davidovic et al., 2011)."

Revised text:

"The major limitations of this study include the absence of analyses specific to brain regions or cell types and the lack of functional investigations. Because we used whole brain samples to measure pH and lactate levels, we could not determine whether the observed changes in pH and/or lactate levels occurred ubiquitously throughout the brain or selectively in specific brain region(s) in each strain/condition of the models. It is known that certain molecular expression profiles and signaling pathways display brain region-specific alterations, and in some cases, even exhibit opposing changes in neuropsychiatric disease models (Hosp et al., 2017; Floriou-Servou et al. 2018; Reim et al., 2017). Indeed, brain region-specific increases in lactate levels were observed in human patients with ASD in an MRS study (Goh et al., 2014). Furthermore, while increased lactate levels were observed in whole-brain measurements in mice with chronic social defeat stress (Figure S7) (Hagihara et al., 2021a), decreased lactate levels were found in the dorsomedial prefrontal cortex (Yao et al., 2023). Additionally, it has been reported that the basal intracellular pH differs between neurons and astrocytes (lower in astrocytes than in neurons), and their responsiveness to conditions simulating neural hyperexcitation and the metabolic acidosis in terms of intracellular pH also varies (Raimondo et al., 2016; Salameh et al., 2017). It would also be possible that the brain region/cell type-specific changes may occur even in animal models in which undetectable changes were observed in the present study. This could be due to the masking of such changes in the analysis when using whole-brain samples. Given the assumption that the brain regions and cell types responsible for pH and lactate changes vary across different strains/conditions, comprehensive studies are needed to thoroughly examine this issue for each animal model individually. This can be achieved through techniques such as evaluating microdissected brain samples, conducting in vivo analyses using pH- or lactate-sensitive biosensor electrodes (Marunaka et al., 2014; Newman et al., 2011), and MRS (Davidovic et al., 2011). Subsequently, based on such findings, it is also necessary to conduct functional analyses for each model animal by manipulating pH or lactate levels in specific brain regions/cell types and evaluating behavioral phenotypes relevant to neuropsychiatric disorders."

Other corrections

Title page and Acknowledgements:

We have revised the affiliation information for the following co-authors: Drs. Anja Urbach8, Mohamed Darwish19, 20, Keizo Takao20, 22, Bong-Kiun Kaang53, 54, Michihiro Igarashi74, 75, Rie Ohashi87-89, and Nobuyuki Shiina87-89.

Page 56, line 12:

The term ‘The International Brain pH Consortium’ has been corrected to ‘The International Brain pH Project Consortium’.

Supplementary Table 1: Supplementary References:

1. Oota-Ishigaki A, Takao K, Yamada D, Sekiguchi M, Itoh M, Koshidata Y, et al. (2022): Prolonged contextual fear memory in AMPA receptor palmitoylation-deficient mice. Neuropsychopharmacology 47: 2150–2159.

We have updated the name of the mouse strain from “patDp” to “15q dup” throughout the manuscript.

We have made the following revisions to enhance readability.

Page 24, line 9: According to a simple correlation analysis, working memory measures (correct responses in the maze test) were significantly negatively correlated with brain lactate levels (r = -0.76, P = 1.93 × 10-5; Figure 1F).

Page 27, line 1:

Revised text

"We found that working memory measures (correct responses in the maze test) were the most frequently selected behavioral measures for constructing a successful prediction model (Figure 2E), which is consistent with the results of the exploratory study (Figure 1E)."

Figure 1 legend:

Revised text

"(F–H) Scatter plot showing correlations between actual brain lactate levels and measures of working memory (correct responses in the maze test) (F), the number of transitions in the light/dark transition test (G), and the percentage of immobility in the forced swim test (H)."

Figure 2 legend:

Revised text

"(F–H) Scatter plots showing correlations between actual brain lactate levels and working memory measures (correct responses in the maze test) (F), the acoustic startle response at 120 dB (G), and the time spent in dark room in the light/dark transition test (H)."

Page 30, line 2:

Original text

"The high to moderate-high pH/low to moderate-low lactate group included mouse models of ASD or developmental delay, such as Shank2 KO, Fmr1 KO, BTBR, Stxbp1 KO, Dyrk1 KO, Auts2 KO, and patDp mice (Table S1, Figure S7)."

Revised text

"The high pH/low lactate group and moderate-high pH/moderate-low lactate group included mouse models of ASD or developmental delay, such as Shank2 KO, Fmr1 KO, BTBR, Stxbp1 KO, Dyrk1 KO, Auts2 KO, and 15q dup mice (Table S1, Figure S7)."

Page 40, line 7:

Original text

"Moreover, increased lactate levels may also be involved in behavioral changes other than memory deficits such as anxiety."

Revised text

"Moreover, increased lactate levels may also be involved in behavioral changes other than memory deficits, such as anxiety."

eLife assessment

The manuscript offers useful descriptive insights into the potential influence of whole-brain lactate and pH levels on the manifestation of behavioral phenotypes seen in diverse animal models of neuropsychiatric disorders. However, reviewers have raised concerns about the potential loss of specificity in capturing regional and cell-type-specific effects when relying solely on whole-brain analysis methods. While the evidence supporting the conclusions is largely solid, the robustness of these conclusions could be enhanced by the inclusion of additional data and further analysis.

Reviewer #1 (Public Review):

Summary<br /> In this manuscript, Hagihara et al. characterized the relationship between the changes in lactate and pH and the behavioral phenotypes in different animal models of neuropsychiatric disorders at a large-scale level. The authors have previously reported that increased lactate levels and decreased pH are commonly observed in the brains of five genetic mouse models of schizophrenia (SZ), bipolar disorder (BD), and autism spectrum disorder (ASD). In this study, they expanded the detection range to 109 strains or conditions of animal models, covering neuropsychiatric disorders and neurodegenerative disorders. Through statistical analysis of the first 65 strains/conditions of animal models which were set as exploratory cohort, the authors found that most strains showed decreased pH and increased lactate levels in the brains. There was a significant negative correlation between pH and lactate levels both at the strain/condition level and the individual animal level. Besides, only working memory was negatively correlated with brain lactate levels. These results were successfully duplicated by studying the confirmative cohort, including 44 strains/conditions of animal models. In all strains/conditions, the lactate levels were not correlated with age, sex, or storage duration of brain samples.

Strengths<br /> 1. The manuscript is well-written and structured. In particular, the discussion is really nice, covering many potential mechanisms for the altered lactate levels in these disease models.<br /> 2. Tremendous efforts were made to recruit a huge number of various animal models, giving the conclusions sufficient power.

Weaknesses<br /> 1. The biggest concern of this study is the limited novelty. The point of "altered pH and/or lactate levels in the brains from human and rodent animals of neuropsychiatric disorders" has been reported by the same lab and other groups in many previous papers.<br /> 2. This study is mostly descriptive, lacking functional investigations. Although a larger cohort of animal models were studied which makes the conclusion more solid, limited conceptual advance is contributed to the relevant field, as we are still not clear about what the altered levels of pH and lactate mean for the pathogenesis of neuropsychiatric disorders.<br /> 3. The experiment procedure is also a concern. The brains from animal models were acutely collected without cardiac perfusion in this study, which suggests that resident blood may contaminate the brain samples. The lactate is enriched in the blood, making it a potential confounded factor to affect the lactate levels as well as pH in the brain samples.<br /> 4. The lactate and pH levels may also be affected by other confounded factors, such as circadian period, and locomotor activity before the mice were sacrificed. This should also be discussed in the paper.<br /> 5. Another concern is the animal models. Although previous studies have demonstrated that dysfunctions of these genes could cause related phenotypes for certain disorders, many of them are not acknowledged by the field as reliable disease models. Besides, gene deficiency could also cause many known or unknown unrelated phenotypes, which may contribute to the altered levels of lactate and pH, too. In this circumstance, the conclusion "pH and lactate levels are transdiagnostic endophenotype of neuropsychiatric disorders" is somewhat overstated.<br /> 6. The negative correlationship between pH and lactate is rather convincing. However, how much the contribution of lactate to pH is not tested. In addition, regarding pH and lactate, which factor contributes most to the pathogenesis of neuropsychiatric disorders is also unclear. These questions may need to be addressed in the future study.<br /> 7. The authorship is open to question. Most authors listed in this paper may only provide mice strains or brain samples. Maybe it is better just to acknowledge them in the acknowledgements section.<br /> 8. The last concern is about the significance of this study. Although the majority of strains showed increased lactate, some still showed decreased lactate levels in the brains. These results suggested that lactate or pH is an endophenotype for neuropsychiatric disorders, but it is hard to serve as a good diagnostic index as the change is not unidirectional in different disorders. In other words, the relationship between lactate level and neuropsychiatric disorders is not exclusive.

Reviewer #2 (Public Review):

Hagihara et al. conducted a study investigating the correlation between decreased brain pH, increased brain lactate, and poor working memory. They found altered brain pH and lactate levels in animal models of neuropsychiatric and neurodegenerative disorders. Their study suggests that poor working memory performance may predict higher brain lactate levels.

However, the study has some significant limitations. One major concern is that the authors examined whole-brain pH and lactate levels, which might not fully represent the complexity of disease states. Different brain regions and cell types may have distinct protein and metabolite profiles, leading to diverse disease outcomes. For instance, certain brain regions like the hippocampus and nucleus accumbens exhibit opposite protein/signaling pathways in neuropsychiatric disease models.

Moreover, the memory tests used in the study are specific to certain brain regions, but the authors did not measure lactate levels in those regions. Without making lactate measurements in brain-regions and cell types involved in these diseases, any conclusions regarding the role of lactate in CNS diseases is premature.

Additionally, evidence suggests that exogenous treatment with lactate has positive effects, such as antidepressant effects in multiple disease models (Carrard et al., 2018, Carrard et al., 2021, Karnib et al., 2019, Shaif et al., 2018). It also promotes learning, memory formation, neurogenesis, and synaptic plasticity (Suzuki et al., 2011, Yang et al., 2014, Weitian et al., 2015, Dong et al., 2017, El Hayek et al. 2019, Wang et al., 2019, Lu et al., 2019, Lev-Vachnish et a.l, 2019, Descalzi G et al., 2019, Herrera-López et al., 2020, Ikeda et al., 2021, Zhou et al., 2021,Roumes et al., 2021, Frame et al., 2023, Akter et al., 2023).

In conclusion, the relevance of total brain pH and lactate levels as indicators of the observed correlations is controversial, and evidence points towards lactate having more positive rather than negative effects. It is important that the authors perform studies looking at brain-region-specific concentrations of lactate and that they modulate lactate levels (decrease) in animal models of disease to validate their conclusions. It is also important to consider the above-mentioned studies before concluding that "altered brain pH and lactate levels are rather involved in the underlying pathophysiology of some patients with neuropsychiatric disorders" and that "lactate can serve as a potential therapeutic target for neuropsychiatric disorders".

eLife assessment

This valuable study highlights how the diversity of the malaria parasite population diminishes following the initiation of effective control interventions but quickly rebounds as control wanes. The data presented is solid and the work shows how genetic studies could be used to monitor changes in disease transmission.

Reviewer #1 (Public Review):

Tiedje et al. investigated the transient impact of indoor residual spraying (IRS) followed by seasonal malaria chemoprevention (SMC) on the plasmodium falciparum parasite population in a high transmission setting. The parasite population was characterized by sequencing the highly variable DBL$\alpha$ tag as a proxy for var genes, a method known as varcoding. Varcoding presents a unique opportunity due to the extraordinary diversity observed as well as the extremely low overlap of repertoires between parasite strains. The authors also present a new Bayesian approach to estimating individual multiplicity of infection (MOI) from the measured DBL$\alpha$ repertoire, addressing some of the potential shortcomings of the approach that have been previously discussed. The authors also present a new epidemiological endpoint, the so-called "census population size", to evaluate the impact of interventions.

This study provides a nice example of how varcoding technology can be leveraged, as well as the importance of using diverse genetic markers for characterizing populations, especially in the context of high transmission. The data are robust and clearly show the transient impact of IRS in a high transmission setting, however, some aspects of the analysis are confusing.

1) Approaching MOI estimation with a Bayesian framework is a well-received addition to the varcoding methodology that helps to address the uncertainty associated with not knowing the true repertoire size. It's unfortunate that while the authors clearly explored the ability to estimate the population MOI distribution, they opted to use only MAP estimates. Embracing the Bayesian methodology fully would have been interesting, as the posterior distribution of population MOI could have been better explored.

2) The "census population size" endpoint has unclear utility. It is defined as the sum of MOI across measured samples, making it sensitive to the total number of samples collected and genotyped. This means that the values are not comparable outside of this study, and are only roughly comparable between strata in the context of prevalence where we understand that approximately the same number of samples were collected. In contrast, mean MOI would be insensitive to differences in sample size, why was this not explored? It's also unclear in what way this is a "census". While the sample size is certainly large, it is nowhere near a complete enumeration of the parasite population in question, as evidenced by the extremely low level of pairwise type sharing in the observed data.

3) The extraordinary diversity of DBL$\alpha$ presents challenges to analyzing the data. The authors explore the variability in repertoire richness and frequency over the course of the study, noting that richness rapidly declined following IRS and later rebounded, while the frequency of rare types increased, and then later declined back to baseline levels. The authors attribute this to fundamental changes in population structure. While there may have been some changes to the population, the observed differences in richness as well as frequency before and after IRS may also be compatible with simply sampling fewer cases, and thus fewer DBL$\alpha$ sequences. The shift back to frequency and richness that is similar to pre-IRS also coincides with a similar total number of samples collected. The authors explore this to some degree with their survival analysis, demonstrating that a substantial number of rare sequences did not persist between timepoints and that rarer sequences had a higher probability of dropping out. This might also be explained by the extreme stochasticity of the highly diverse DBL$\alpha$, especially for rare sequences that are observed only once, rather than any fundamental shifts in the population structure.

Reviewer #2 (Public Review):

In this manuscript, Tiedje and colleagues longitudinally track changes in parasite numbers across four time points as a way of assessing the effect of malaria control interventions in Ghana. Some of the study results have been reported previously, and in this publication, the authors focus on age-stratification of the results. Malaria prevalence was lower in all age groups after IRS. Follow-up with SMC, however, maintained lower parasite prevalence in the targeted age group but not the population as a whole. Additionally, they observe that diversity measures rebounds more slowly than prevalence measures. Overall, I found these results clear, convincing, and well-presented. They add to a growing literature that demonstrates the relevance of asymptomatic reservoirs.

There is growing interest in developing an expanded toolkit for genomic epidemiology in malaria, and detecting changes in transmission intensity is one major application. As the authors summarize, there is no one-size-fits-all approach, and the Bayesian MOIvar estimate developed here has the potential to complement currently used methods. I find its extension to a calculation of absolute parasite numbers appealing as this could serve as both a conceptually straightforward and biologically meaningful metric. However, I am not fully convinced the current implementation will be applied meaningfully across additional studies.

1. I find the term "census population size" problematic as the groups being analyzed (hosts grouped by age at a single time point) do not delineate distinct parasite populations. Separate parasite lineages are not moving through time within these host bins. Rather, there is a single parasite population that is stochastically divided across hosts at each time point. I find this distinction important for interpreting the results and remaining mindful that the 2,000 samples at each time point comprise a subsample of the true population. Instead of "census population size", I suggest simplifying it to "census count" or "parasite lineage count".

It would be fascinating to use the obtained results to model absolute parasite numbers at the whole population level (taking into account, for instance, the age structure of the population), and I do hope this group takes that on at some point even if it remains outside the scope of this paper. Such work could enable calculations of absolute---rather than relative---fitness and help us further understand parasite distributions across hosts.

2. I'm uncertain how to contextualize the diversity results without taking into account the total number of samples analyzed in each group. Because of this, I would like a further explanation as to why the authors consider absolute parasite count more relevant than the combined MOI distribution itself (which would have sample count as a denominator). It seems to me that the "per host" component is needed to compare across age groups and time points---let alone different studies.

3. Thinking about the applicability of this approach to other studies, I would be interested in a larger treatment of how overlapping DBLa repertoires would impact MOIvar estimates. Is there a definable upper bound above which the method is unreliable? Alternatively, can repertoire overlap be incorporated into the MOI estimator?

Smaller comments:<br /> - Figure 1 provides confidence intervals for the prevalence estimates, but these aren't carried through on the other plots (and Figure 5 has lost CIs for both metrics). The relationship between prevalence and diversity is one of the interesting points in this paper, and it would be helpful to have CIs for both metrics when they are directly compared.

Reviewer #3 (Public Review):

Summary:<br /> The manuscript coins a term "the census population size" which they define from the diversity of malaria parasites observed in the human community. They use it to explore changes in parasite diversity in more than 2000 people in Ghana following different control interventions.

Strengths:<br /> This is a good demonstration of how genetic information can be used to augment routinely recorded epidemiological and entomological data to understand the dynamics of malaria and how it is controlled. The genetic information does add to our understanding, though by how much is currently unclear (in this setting it says the same thing as age-stratified parasite prevalence), and its relevance moving forward will depend on the practicalities and cost of the data collection and analysis. Nevertheless, this is a great dataset with good analysis and a good attempt to understand more about what is going on in the parasite population.

Weaknesses:<br /> Overall the manuscript is well-written and generally comprehensively explained. Some terms could be clarified to help the reader and I had some issues with a section of the methods and some of the more definitive statements given the evidence supporting them.

eLife assessment

The work by Han and collaborators describes valuable findings on the role of Akkermansia muciniphila during ETEC infection. If confirmed, these findings will add to a growing list of beneficial properties of this organism. However, as it stands, the strength of the evidence used to justify the conclusions in the manuscript is incomplete.

Reviewer #1 (Public Review):

Summary:<br /> In this paper, the authors investigate the impact of fecal microbiota transfer (FMT) on intestinal recovery from enterotoxigenic E. coli infection following antibiotic treatment. Using a piglet model of intestinal infection, the authors demonstrate that FMT reduces weight loss and diarrhea and enhances the expression of tight junction proteins. Sequencing analysis of the intestinal microbiota following FMT showed significant increases in Akkermansia muciniphila and Bacteroides fragilis. Using additional mouse and organoid models, the authors examine the impact of these microbes on intestinal recovery and modulation of the Wnt signaling pathway. Overall, the data support the notion that FMT following ETEC infection is beneficial, however, additional investigation is required to fully elucidate the mechanisms involved.

Strengths:<br /> Initial experiments used a piglet model of infection to test the value of FMT on recovery from E. coli. The FMT treatment was beneficial and the authors provide solid evidence that the treatment increased the diversity of the microbiota and enhanced the recovery of the intestinal epithelium. Sequencing data highlighted an increase in Akkermansia muciniphila and Bacteroides fragilis after FMT.

The mouse data are consistent with the observations in pigs, and reveal that daily gavage with A. muciniphila or B. fragilis enhances intestinal recovery based on histological analysis, expression of tight junction proteins, and analysis of intestinal barrier function.

The authors demonstrate the benefit of probiotic treatment following infection using a range of model systems.

Weaknesses:<br /> Without sequencing the pre-infection pig microbiota or the FMT input material itself, it's challenging to firmly say that the observed bloom in Akkermansia muciniphila and Bacteroides fragilis stemmed from the FMT.

The lack of details for the murine infection model, such as weight loss and quantification of bacterial loads over time, make it challenging for a reader to fully appreciate how treatment with Akkermansia muciniphila and Bacteroides fragilis is altering the course of infection. Bacterial loads of E. coli were only quantified at one time point, and the mice that received A. muciniphila and B. fragilis had very low levels of E. coli. Therefore, it is not clear if all mice were subjected to the same level of infection in the first place. The reduced translocation of E. coli to the organs and enhanced barrier function may just reflect the low level of infection in these mice. Further, the authors' conclusion that the effect is specific to A. muciniphila or B. fragilis would be more convincing if the experiments included an inert control bacterium, to demonstrate that gavage with any commensal microbe would not elicit a similar effect.

Many of the conclusions in the study are drawn from the microscopy results. However, the methods describing both light microscopy and electron microscopy lack sufficient detail. For example, it is not clear how many sections and fields of view were imaged or how the SEM samples were prepared and dehydrated. The mucus layer does not appear to be well preserved, which would make it challenging to accurately measure the thickness of the mucus layer.

Gene expression data appears to vary across the different models, for example, Wnt3 expression in mice versus organoids. Additional experiments may be required to clarify the mechanisms involved. Considering that both of the bacteria tested elicited similar changes in Wnt signaling, this pathway might be broadly modulated by the microbiota.

The unconventional choice to not include references in the results section makes it challenging for the reader to put the results in context with what is known in the field. Similarly, there is a lack of discussion acknowledging that B. fragilis is a potential pathogen, associated with intestinal inflammation and cancer (Haghi et al. BMC Cancer 19, 879 (2019) ), and how this would impact its utility as a potential probiotic.

Reviewer #2 (Public Review):

Ma X. et al proposed that A. muciniphila was a key strain that promotes the proliferation and differentiation of intestinal stem cells by acting on the Wnt/b-catenin signaling pathway. They used various models, such as the piglet model, mouse model, and intestinal organoids to address how A. muciniphila and B. fragilis offer protection against ETEC infection. They showed that FMT with fecal samples, A. muciniphila or B. fragilis protected piglets and/or mice from ETEC infection, and this protection is manifested as reduced intestinal inflammation/bacterial colonization, increased tight junction/Muc2 proteins, as well as proper Treg/Th17 cells. Additionally, they demonstrated that A. muciniphila protected basal-out and/or apical-out intestinal organoids against ETEC infection via Wnt signaling. While a large body of work has been performed in this study, there are quite a few questions to be addressed.

Major comments:

- The similar protective effect of FMT with fecal samples, A. muciniphila or B. fragilis is perhaps not that surprising, considering that FMT likely restores microbiota-mediated colonization resistance against ETEC infection. While FMT with fecal samples increases SCFAs, it is unclear whether/how FMT with A. muciniphila or B. fragilis alter the microbiota composition/abundance as well as metabolites in the current models in a way that offers protection.

- Does ETEC infection in piglets/mice cause histological damage in the intestines? These data should be shown.

- Line 447, "ETEC adheres to intestinal epithelial cells". However, there is no data showing the adherence (or invasion) of ETEC to intestinal epithelial cells, irrespective of piglets/mouse/organoids.

- In both basal-out and apical-out intestinal organoid models, A. muciniphila protects organoids against ETEC infection. Did ETEC enter into intestinal epithelial cells at all after only one hour of infection? Is the protection through certain A. muciniphila metabolites?

Reviewer #3 (Public Review):

Summary:<br /> The manuscript by Ma et al. describes a multi-model (pig, mouse, organoid) investigation into how fecal transplants protect against E. coli infection. The authors identify A. muciniphila and B. fragilis as two important strains and characterize how these organisms impact the epithelium by modulating host signaling pathways, namely the Wnt pathway in lgr5 intestinal stem cells.

Strengths:<br /> The strengths of this manuscript include the use of multiple model systems and follow-up mechanistic investigations to understand how A. muciniphila and B. fragilis interacted with the host to impact epithelial physiology.

Weaknesses:<br /> The major weakness is that, as presented, the manuscript is quite difficult to follow, even for someone familiar with the field. The lack of detail in figure legends, organization of the text, and frequent use of non-intuitive abbreviated group names without a clear key (ex. EP/EF, or C E A B) make comprehension challenging. The results section is perhaps too succinct and does not provide sufficient information to understand experimental design and interpretation without reading the methods section first or skipping to the discussion (as an example: WNT-c59 treatment). Extensive revisions could be encouraged to aid in communicating the potentially exciting findings.

The bioinformatics section of the methods requires revision and may indicate issues in the pipeline. Merging the forward and reverse reads may represent a problem for denoising. Also since these were sequenced on a NovaSeq, the error learning would have to be modified or the diversity estimates would be inappropriately multiplied. "Alpha diversity and beta diversity were calculated by normalized to the same sequence randomly." Not sure what this means, does this mean subsampled? "Blast was used for sequence alignment", does this mean the taxonomic alignment? This would need to be elaborated on and database versions should be included. The methods, including if any form of multiple testing was included, for LEFSE was also not included.

eLife assessment

This valuable study introduces a new simulation model to explain the wide-spread occurrence of genetic inversions in fruit-fly genomes, based on sexually antagonistic alleles and a trade-off between male reproduction and survival. The evidence supporting the conclusions is currently incomplete, but it might be possible to address this with additional simulations and experiments as well as more rigorous analysis of the model and the data. The work will be of interest to population geneticists beyond the fruit-fly system.

Reviewer #1 (Public Review):

The idea is that inversions capture genetic variants that have antagonistic effects on male sexual success (via some display traits) and survival of females (or both sexes) until reproduction. A series of simulations are presented and show that the scenario works at least under some conditions. While a polymorphism at a single locus with large antagonistic effects can be maintained for a certain range of parameters, a second such variant with somewhat smaller effects tends to be lost unless closely linked. It becomes much more likely for genomically distant variants that add to the antagonism to spread if they get trapped in an inversion; the model predicts this should drive the accumulation of sexually antagonistic variants on the inversion versus standard haplotype, leading to the evolution of haplotypes with very strong cumulative antagonistic pleiotropic effects. This idea has some analogies with one of the predominant hypotheses for the evolution of sex chromosomes, and the authors discuss these similarities. To provide empirical support for this idea, the authors study the dynamics of inversions in population cages over one generation, tracking their frequencies through amplicon sequencing, from the parental generation through embryos to aged adults of either sex. Out of four inversions included in the experiment, two show patterns consistent with antagonistic effects on male sexual success (competitive paternity) and the survival of offspring, especially females, until old age, which the authors interpret as consistent with their theory.

This is an interesting idea, and the authors should be praised for combining a model with experimental data. However, in addition to the potential for improvement of presentation (details below), the study has some substantial weaknesses that could be addressed with additional simulations and additional experiments.

(1) The authors claim that the negative frequency dependence that maintains polymorphism in their model results from a non-linear relationship between the display trait and sexual success. I am not convinced about that. It seems to me that the "best of n" female choice implemented in the model (l. 741ff and Figure 2) does not lead to negative frequency dependence. Let p be the frequency of the competitively inferior male genotype. Assuming no noise in the male display, a female will mate with an inferior male only if all males among the n males sampled by the female are of the inferior genotype, which will be the fraction p^n, the remaining 1-p^n matings will go to the superior males. Thus, per capita, the inferior males will achieve (p^n)/p or p^(n-1) matings while the per-capita matings per superior male will be (1-p^n)/(1-p). Thus, the ratio of the mating success of the inferior to the superior males will be (1-p) p^(n-1) / (1- p^n). For the range of p from 0 to 1, this is an increasing function of p. E.g., with n = 2, the sexual fitness of the inferior genotype relative to that of the superior phenotype is p/(1+p). Thus, at least in the absence of noise in the mate choice, this generates positive rather than negative frequency dependence. Maybe I missed something, but the authors do not provide support for their claim about the negative frequency-dependence of sexual selection in their simulations. To do so they could (1) extract the relationship between the relative mating success of the two male types from the simulations and (2) demonstrate that polymorphism is not maintained if the relationship between male display trait and mating success is linear.

(2) The authors only explore versions of the model where the survival costs are paid by females or by both sexes. We do not know if polymorphism would be maintained or not if the survival cost only affected males, and thus if sexual antagonism is crucial.

(3) The authors assume no cost to aneuploidy, with no justification. Biologically, investment in aneuploid eggs would not be recoverable by Drosophila females and thus would potentially act against inversions when they are rare.

(4) The authors appear to define balanced polymorphism as a situation in which the average allele frequency from multiple simulation runs is intermediate between zero and one (e.g., Figure 3). However, a situation where 50% of simulation runs end up with the fixation of allele A and the rest with the fixation of allele B (average frequency of 0.5) is not a balanced polymorphism. The conditions for balanced polymorphism require that selection favors either variant when it is rare.

(5) Possibly the most striking result of the experiment is the fact that for 14 out of 16 combinations of inversion x maternal background, the changes in allele frequencies between embryo and adult appear greater in magnitude in females than in males irrespective of the direction of change, being the same in the remaining two combinations. The authors interpret this as consistent with sexually antagonistic pleiotropy in the case of In(3L)Ok and In(3R)K. The frequencies of adult inversion frequencies were, however, measured at the age of 2 months, at which point 80% of flies had died. For all we know, this may have been 90% of females and 70% of males that died at this point. If so, it might well be that the effects of inversion on longevity do not systematically differ between the ages and the difference in Figure 9B results from the fact that the sample includes 30% longest-lived males and 10% longest-lived females.

(6) Irrespective of the above problem, survival until the age of 2 months is arguably irrelevant from the viewpoint of fitness consequences and thus maintenance of inversion polymorphism in nature. It would seem that trade-offs in egg-to-adult survival (as assumed in the model), female fecundity, and possibly traits such as females resistance to male harm would be much more relevant to the maintenance of inversion polymorphisms.

(7) The experiment is rather minimalistic in size, with four cages in total; given that each cage contains a different female strain, it essentially means N=1. The lack of replication makes statements like " In(2L)t and In(2R)NS each showed elevated survival with all maternal strains except ZI418N" (l. 493) unsubstantiated because the claimed special effect of ZI418N is based on a single cage subject to genetic drift and sampling error. The same applies to statements on inversion x female background interaction (e.g., l. 550), as this is inseparable from residual variation. It is fortunate that the most interesting effects appear largely consistent across the cages/female backgrounds. Still, I am wondering why more replicates had not been included.

Reviewer #2 (Public Review):

Summary:

In their manuscript, the authors address the question of whether the inversion polymorphism in D. melanogaster can be explained by sexually antagonistic selection. They designed a new simulation tool to perform computer simulations, which confirmed their hypothesis. They also show a tradeoff between male reproduction and survival. Furthermore, some inversions display sex-specific survival.

Strengths:<br /> It is an interesting idea on how chromosomal inversions may be maintained

Weaknesses:<br /> General points:<br /> The manuscript lacks clarity of writing. It is impossible to fully grasp what the authors did in this study and how they reached their conclusions. Therefore, I will highlight some cases that I found problematic.<br /> Although this is an interesting idea, it clearly cannot explain the apparent influence of seasonal and clinal variation on inversion frequencies.

Specific points:<br /> The simulations are highly specific and make very strong assumptions, which are not well-justified.

Reviewer #3 (Public Review):

Summary:<br /> In this study, McAllester and Pool develop a new simulation model to explain the maintenance of balanced inversion polymorphism, based on (sexually) antagonistic alleles and a trade-off between male reproduction and survival (in females or both sexes). In support of the plausibility of this model, the authors use laboratory experiments on four naturally occurring inversion polymorphisms in Drosophila melanogaster, finding evidence for the existence of the above-mentioned trade-off in two out of the four cases.

Strengths:<br /> 1. The study develops and analyzes a new (Drosophila melanogaster-inspired) model for the maintenance of balanced inversion polymorphism, combining elements of (sexually) antagonistically (pleiotropic) alleles, negative frequency-dependent selection, and synergistic epistasis. To this end, the authors developed and used a new simulator (although it was not 100% clear as to why SLiM could not have been used as SLiM has been used to study inversions).

2. The above-mentioned model assumes, as a specific example, a trade-off between male reproductive display and survival; in the second part of their study, the authors perform laboratory experiments on four common D. melanogaster inversions to study whether these polymorphisms may be subject to such a trade-off. The authors find that two of the four inversions show suggestive evidence that is consistent with a trade-off between male reproduction and survival. The new amplicon sequencing approach to track inversion frequencies used by the authors seems promising in terms of studying fitness effects/trade-offs associated with polymorphic inversions and how such effects play out dynamically.

Weaknesses:<br /> 1. Mechanisms of balancing selection maintaining balanced inversion polymorphism. In Section 1.1 a better and more accurate overview of the different selective mechanisms that might contribute to the maintenance of balanced inversion polymorphisms should be given (for a recent review see Berdan et al. 2023). For example, negative frequency-dependent selection (NFDS), spatially and temporally varying selection are not mentioned here and are brought up only later, which is not really ideal. While I agree that in most cases our understanding of balanced inversion polymorphism is very limited, there are many empirical examples of these and other mechanisms of balancing selection being at play (e.g., NFDS: Wright & Dobzhansky 1946; Nassar et al. 1973; Álvarez-Castro and Álvarez 2005; Jay et al. 2021; and many examples of evidence for other mechanisms as well). Thus, while the prevalence of inversion polymorphisms is indeed in many cases "enigmatic", the reader should not be given the impression that we do not have yet any empirical evidence for specific mechanisms in particular cases. Similarly, the authors mention the classical (essentially Dobzhansky's) scenario of epistatically interacting loci only in passing, even though this coadaptation scenario may be simpler than the local adaptation mechanism of Kirkpatrick & Barton (2016): in its simplest form, epistatic coadaptation does not require any migration load or locally adaptive alleles à la Kirkpatrick & Barton, but just the capture of 2 overdominant loci, with inversion protecting this fittest double heterozygote from recombination load (Charlesworth 1974; also see Charlesworth & Charlesworth 1974; Charlesworth & Flatt 2021; also see discussion in Charlesworth and Barton 2018). On the other hand, the plausibility of the mutational load/associative overdominance (AOD) mechanism (Sturtevant & Mather 1938; Nei et al. 1967; Ohta 1971) seems to be given too much weight: new work by Charlesworth (2023; https://www.biorxiv.org/content/10.1101/2023.10.16.562579v1) suggests that load likely contributes only very modestly to heterokaryotypic advantage of inversions at intermediate frequencies, and that is very unlikely to provide a sufficient selective (heterotic) advantage to new autosomal inversions in order to explain their establishment (also see Nei et al. 1967; Connallon and Olito 2021; Jay et al. 2022).

2. The general reduction principle and inversion polymorphism. In Section 1.2., the authors state that "there has not been a proposed mechanism whereby alleles at multiple linked loci would directly benefit from linkage and thereby maintain an associated inversion polymorphism under indirect selection." Perhaps I am misunderstanding something, but in my reading, this statement is factually incorrect. In fact, the simplest version of Dobzhansky's epistatic coadaptation model (see Charlesworth 1974; also see Charlesworth and Charlesworth 1973 and discussion in Charlesworth & Flatt 2021; Berdan et al. 2023) seems to be an example of exactly what the authors seem to have in mind here: two loci experiencing overdominance, with the double heterozygote possessing the highest fitness (i.,e., 2 loci under epistatic selection, inducing some degree of LD between these loci), with subsequent capture by an inversion; in such a situation, a new inversion might capture a haplotype that is present in excess of random expectation (and which is thus fitter than average). The selective benefits of recombination suppression in the inversion heterokaryotype will then confer a heterozygote advantage to the inversion and prevent it from going to fixation (see Charlesworth 1974). This is probably the simplest (or one of the simplest) models of multilocus balancing selection that can act on inversions. Incidentally, this model represents a prime example of the "reduction principle", which the authors mention on two occasions in their paper: generally, any multi-locus polymorphism held at equilibrium by any type of balancing selection involving fitness epistasis will cause selection for reduced recombination (e.g., Feldman & Liberman, 1986; Zhivotovsky et al., 1994); notably, the example of inversion polymorphism is explicitly discussed in Altenberg's and Feldman's (1987) paper on the reduction principle. It is also noteworthy in this context that the 2-locus epistatic model of Charlesworth (1974) assumes constant fitness values/selection coefficients but actually leads to what one could call "apparent" frequency-dependent selection with different equilibria.

3. Trade-offs and antagonistic pleiotropy involved in maintaining inversions. Throughout the manuscript, previous work implicating trade-offs and/or antagonistic pleiotropy (AP) in the maintenance of inversion polymorphisms should be more adequately acknowledged and discussed - from the text as it currently stands one gains the impression that barely anything is known about the connection between inversions and pleiotropy/AP/trade-offs (e.g., Betrán et al. 1998; Mérot et al. 2020, which is cited but not really in the context of AP/trade-offs; Pei et al. 2023, etc.). The paper by Pei and colleagues is particularly relevant in the context of the present study: the authors find that the inverted allele has beneficial effects on male siring success and female fecundity but negative effects on survival. Generally, numerous studies have found that inversion polymorphisms have "pleiotropic" (albeit not always antagonistically pleiotropic) effects upon multiple fitness components (e.g., Etges 1989; Betrán et al. 1998; Küpper et al. 2016; Durmaz et al. 2018; Mérot et al. 2020). More broadly, the general role of AP in maintaining (life-history) polymorphisms should be mentioned by referring to previous theories (e.g., Rose 1982, 1985; Curtsinger et al. 1994; Charlesworth & Hughes 2000 chapter in Lewontin Festschrift; Conallon & Chenoweth 2019 - this latter paper is particularly relevant in terms of AP effects in the context of sexual antagonism).

4. Sexually antagonistic selection and inversion polymorphism. The authors' model of sexual antagonism being involved in maintaining an inversion polymorphism is novel and interesting, but again I felt that the authors' ideas could be better connected to what has been done before. First, several papers have made connections between sexual antagonism and inversion polymorphisms: for example, an important study that deserves discussion in this context is the paper by Natri and colleagues (2019) where the authors study sexual antagonism as a source of balancing selection that maintains an inversion polymorphism in the ruff. Similarly, another relevant study in this context is Hearn et al. 2022 on Littorina saxatilis snails. Also see Giraldo-Deck et al. (2022). A very interesting paper that may be worth discussing is Connallon & Chenoweth (2019) about dominance reversals of antagonistically selected alleles (even though C&C do not discuss inversions): AP alleles (with dominance reversals) affecting two or more life-history traits provide one example of such antagonistically selected alleles (also see Rose 1982, 1985; Curtsinger et al. 1994) and sexually antagonistically selected alleles provide another. The two are of course not necessarily mutually exclusive, thus making a conceptual connection to what the authors model here.

5. The model. In general, the description of the model and of the simulation results was somewhat hard to follow and vague. There are several aspects that could be improved: (1) it would help the reader if the terminology and distinction of inverted vs. standard arrangements and of the three karyotypes would be used throughout, wherever appropriate. (2) The mention of haploid populations/situations and haploid loci (e.g., legend to Figure 1) is somewhat confusing: the mechanism modelled here, of course, requires suppressed recombination in the inversion/standard heterokaryotype; and thus, while it may make sense to speak of haplotypes, we're dealing with an inherently diploid situation. (3) The authors have a situation in mind where the 2 karyotypes (INV vs. STD) in the heterokaryotype carry distinct sets of loci in LD with each other, with one karyotype/haplotype carrying antagonistic variants favoring high male display success and with the other karyotype/haplotype carrying non-antagonistic alternative alleles at these loci and which favor survival. Thus, at each of the linked loci, we have antagonistic alleles and non-antagonistic alleles - however, the authors don't mention or discuss the degree of dominance of these alleles. The degree of dominance of the alleles could be an important consideration, and I found it curious that this was not mentioned (or, for that matter, examined). (4) In many cases, the authors do not provide sufficient detail (in the main text and the main figures) about which parameter values they used for simulations; the same is true for the Materials & Methods section that describes the simulations. Conversely, when the text does mention specific values (e.g., 20N generations, 0.22-0.25M, etc.), little or no clear context or justification is being provided. (5) The authors sometimes refer to "inversion mutation(s)" - the meaning of this terminology is rather ambiguous.

6. Throughout the manuscript, especially in the description and the discussion of the model and simulations, a clearer conceptual distinction between initial "capture" and subsequent accumulation / "gain" of variants by an inversion should be made. This distinction is important in terms of understanding the initial establishment of an inversion polymorphism and its subsequent short- as well as long-term fate. For example, it is clear from the model/simulations that an inversion accumulates (sexually) antagonistic variants over time - but barely anything is said about the initial capture of such loci by a new inversion.

eLife assessment

The manuscript presents an analysis of different factors that are required for release of the lipid-linked morphogen Shh from cellular membranes., which will be useful in the field. The evidence is still incomplete as experiments rely on over-expression of Shh in a single cell line and are sometimes of a correlative nature. The study confirms and extends previous findings and will be of interest to developmental biologists who work on Hedgehog signaling.

Reviewer #1 (Public Review):

This manuscript presents a model in which combined action of the transporter-like protein DISP and the sheddases ADAM10/17 promote shedding of a mono-cholesteroylated Sonic Hedgehog (SHH) species following cleavage of palmitate from the dually lipidated precursor ligand. The authors propose that this leads to transfer of the cholesterol-modified SHH to HDL for solubilization. The minimal requirement for SHH release by this mechanism is proposed to be the covalently linked cholesterol modification because DISP could promote transfer of a cholesteroylated mCherry reporter protein to serum HDL. The authors used an in vitro system to demonstrate dependency on DISP/SCUBE2 for release of the cholesterol modified ligand. These results confirm previously published results from other groups (PMC3387659 and PMC3682496).

A strength of the work is the use of a bicistronic SHH-Hhat system to consistently generate dually-lipidated ligand to determine the quantity and lipidation status of SHH released into cell culture media.

Key shortcomings include the unusual normalization strategies used for many experiments and the lack of quantification/statistical analyses for several experiments. Due to these omissions, it is difficult to conclude that the data justify the conclusions. The significance of the data provided is overstated because many of the presented experiments confirm/support previously published work. The study provides a modest advance in understanding of the complex issue of SHH membrane extraction.

Reviewer #2 (Public Review):

Ehring et al. analyze contributions of Dispatched, Scube2, serum lipoproteins and Sonic Hedgehog lipid modifications to the generation of different Shh release forms. Hedgehog proteins are anchored in cellular membranes by N-terminal palmitate and C-terminal cholesterol modifications, yet spread through tissues and are released into the circulation. How Hedgehog proteins can be released, and in which form, remains controversial. The authors systematically dissect contributions of several previously identified factors, and present evidence that Disp, Scube2 and lipoproteins concertedly act to release a novel Shh variant that is cholesterol-modified but not palmitoylated. The results provide new insights into the function of Disp and Scube2 in Hedgehog release. The findings concerning the function of lipoproteins and cholesterol in Hedgehog release are largely confirmatory (PMID 23554573, 20685986). However, in light of the multitude of competing models for Hedgehog release, the present study is a valuable contribution that provides further insights into the relevance of lipoproteins in this process.

A novel and surprising finding of the present study is the differential removal of Shh N- or C-terminal lipid anchors depending on the presence of HDL and/or Disp. In particular, the identification of a non-palmitoylated but cholesterol-modified Shh variant that associates with lipoproteins is potentially important. The authors use RP-HPLC and defined controls to assess the properties of processed Shh forms, but their precise molecular identify remains to be defined. A caveat is the strong reliance on over-expression of Shh in a single cell line. The authors detect Shh variants that are released independently of Disp and Scube2 in secretion assays, which however are excluded from interpretation as experimental artifacts. Thus, it would be important to demonstrate key findings in cells that secrete Shh endogenously.

eLife assessment

This important study furthers our understanding of the antimicrobial properties of siderophores, and their potential use to battle opportunistic pathogens. The evidence supporting the conclusion is solid, based on rigorous biochemical, growth, and virulence assays. The work would benefit from a more in-depth discussion of the consequences and efficacy of 'siderophore therapy' in more complex communities/environments. The work will be of broad interest to colleagues in the fields of evolutionary ecology, microbiology, and medical sciences.

Reviewer #1 (Public Review):

Summary:<br /> In an era of increasing antibiotic resistance, there is a pressing need for the development of novel sustainable therapies to tackle problematic pathogens. In this study, the authors hypothesize that pyoverdines - metal-chelating compounds produced by fluorescent pseudomonads - can act as antibacterials by locking away iron, thereby arresting pathogen growth. Using biochemical, growth, and virulence assays on 12 opportunistic pathogens strains, the authors demonstrate that pyoverdines induce iron starvation, but this effect was highly context-dependent. This same effect has been demonstrated for plant pathogens, but not for human opportunistic pathogens exposed to natural siderophores. Only those pathogens lacking (1) a matching receptor to take up pyoverdine-bound iron and/or (2) the ability to produce strong iron chelators themselves experienced strong growth arrest. This would suggest that pyoverdines might not be effective against all pathogens, thereby potentially limiting the utility of pyoverdines as global antibacterials.

Strengths:<br /> The work addresses an important and timely question - can pyoverdines be used as an alternative strategy to deal with opportunistic pathogens? In general, the work is well conducted with rigorous biochemical, growth, and virulence assays. The work is clearly written and the findings are supported by high-quality figures.

Weaknesses:<br /> I do not think there are any 'weaknesses' as such. However, it is well known that siderophore production is highly plastic, typically being upregulated in response to metal limitation (as well as toxic metal stress). Did the authors quantify whether pyoverdine supplementation altered siderophore production in the focal pathogens (either through phenotypic assays / transcriptomics)? Could such a phenotypic plastic response result in an increased capacity to scavenge iron from the environment? Importantly, increased expression of siderophores has been shown to enhance pathogen virulence (e.g. Lear et al 2023: increased pyoverdine production is linked with increased virulence in Pseudomonas aeruginosa). I really appreciate the amount of work the authors have put into this study, but I would suggest expanding the discussion a bit to include a few sentences on (1) unintentional consequences of pyoverdine treatment (e.g. changes in gene expression and non-siderophore-related mutations (e.g. biofilm formation)) on disease dynamics/pathogen virulence , and (2) the efficacy of siderophore treatment under more natural conditions, i.e. when the pathogens have to compete with other species in the resident community (i.e. any other effects than resistance evolution through HGT of pyoverdine receptors as mentioned).

Reviewer #2 (Public Review):

In this work, Vollenweider et al. examine the effectiveness of using natural products, specifically molecules that chelate iron, to treat infectious agents. Through the purification of 320 environmental isolates, 25 potential candidates were identified from natural products based on inhibition assays and were further screened. The structural information and chemical composition were determined.

The paper is well-structured and thorough; targeting virulence factors in this manner is a great idea. My enthusiasm is dampened by the mediocre effects of the compounds. The lack of a dose-response curve in the survivability assays suggests a limited scope for these molecules. While it is encouraging that the best survivability occurred at the lowest toxicity level, it opens questions as to how effective such molecules can be. Either the reduction in mortality was offset by using higher concentrations, which was not observed in the compound-alone test, or there is no dose-response curve. The latter would suggest to me that the variation in survivability is not due to the addition of siderophores.

I would also like to see how these molecules compare to other iron-chelating molecules. Desferoxamine is a bacteria-derived siderophore that is FDA-approved. However, it is not used to treat infections. Would the author consider comparing their candidate molecules to well-studied molecules? This also raises questions about the novelty of this work; I think the authors could rephrase the discussion to better reflect that bioprospecting for iron-chelating molecules has previously occurred and been successful.

Finally, I am concerned about the few mutations reported in the resistance study. Looking at the SI, it appears that very few mutations were seen. It is unclear what filtering the authors used to arrive at such a low number of mutations. Even filtering against mutations that were selected by adaptation to the media, it seems low that only a handful of clones had distinct mutations.

This paper has a lot of strengths. The workflow is logical and well-executed; the only significant weakness is the effect of the molecules and the lack of an explanation for a dose-response curve in the survivability assay, especially when compared to the data reported in Figure 3. As the authors describe in lines 214-217.

eLife assessment

This important study uses an innovative set of reporter assays to probe the role of the TnpB protein in IS608 transposition. The work provides independent support for the recently reported homing activity of TnpB, where the transposon is restored following excision, and suggests an additional function for TnpB in enhancing the transposase activity of the TnpA transposase. The overall approach is solid, but the authors should consider how the activity of the TnpB protein used, or the levels of ωRNA, impact their model.

Reviewer #1 (Public Review):

Summary:<br /> The nuclease protein TnpB is ubiquitous across microorganisms. It is associated with the stabilization of transposable genetic elements and is a putative precursor of the RNA-guided endonuclease Cas9 from the CRISPR system. Despite its potential as a gene-editing tool, TnpB is not well understood. In this study, the authors use a fluorescence-based construct to individually quantify the transposable-element excision rate and the abundance of the two ancillary proteins TnpA and TnpB. They develop a mathematical model describing the role of TnpB in transposable-element stabilization.

Strengths:<br /> The methodology (with schematic shown in Figure 1A) is powerful and sophisticated. The authors are able to de-convolve excision events from the individual abundances of TnpA and TnpB.

Weaknesses:<br /> The claim that TnpB expression level correlates positively (and significantly) with the probability of a growth-disrupting integration (called 'b') is not well-supported by the data shown in Fig. 3D. The modelling of results shown in Figure 4 are not tied directly to the experimental data shown in Figures 1-3.

Reviewer #2 (Public Review):

Summary:<br /> Dr. Khulman and colleagues present a very interesting experimental and mathematical modeling work on IS608 transposition. The system has a number of unique advantages that create experimental possibilities utilized here to investigate transposition dynamics. This kind of approach is badly missing from the field and I believe the experiments and modeling work shown here and the type of results that can be derived are groundbreaking and certainly deserve high visibility.

Strengths:<br /> The attempt to measure and model transposition dynamics in cells.

Weaknesses:<br /> - Lack of controls using an active site mutant of TnpB.<br /> - Lack of control in RecA- cells as transposon restoration is proposed to be dependent on homologous recombination.<br /> - Lack of consideration of the levels of ωRNA present.

Reviewer #3 (Public Review):

The authors use a set of reporter assays to probe the impact of TnpB on IS605 transposition. This is an innovative approach to studying transposition that will be of interest to other groups. The authors conclude that TnpB likely has two activities: (ii) promoting "homing", where the transposon is restored following excision, and (ii) promoting the transposition activity of the transposase TnpA. The paper provides an independent validation of the recently reported homing activity of TnpB, where the transposon is restored following excision, and suggests an additional function for TnpB in enhancing the transposase activity of the TnpA transposase.

Strengths<br /> - The innovative use of reporter assays is an excellent way to infer the details of transposition, making a convincing case that TnpB plays two distinct roles.

Weaknesses<br /> - The authors need to discuss their conclusions in light of a very relevant recent paper from the Sternberg group demonstrating a role for TnpB in homing.<br /> - There doesn't appear to be an assessment of how well the model fits the experimental data, or any attempt to test how accurately the model can predict the effects of perturbing the system.<br /> - The figures could be presented more clearly.

eLife assessment

This is a compelling study on pro-inflammatory and anti-inflammatory lipids in relation to skeletal muscle injury. It convincingly identifies pro-inflammatory lipids during recovery predisposing to fibrosis, and maresin 1 as an anti-inflammatory lipid reducing fibrosis, improved muscle regeneration, partially restoring contractile function, of fundamental potential clinical application.

Reviewer #1 (Public Review):

Loss of skeletal muscle tissue from traumatic injury is debilitating. Restoring muscle mass and function remains a challenge. Using a mouse model, the authors performed punch biopsy injuries of the tibialis anterior in which the volume of muscle loss was varied to result in either successful muscle regeneration with a smaller injury or the unsuccessful outcome of fibrosis with a larger injury. For both conditions, a novel lipidomic profiling approach was used to evaluate pro-inflammatory and anti-inflammatory lipids at key time points post-injury with respect to collagen deposition, macrophage infiltration, muscle fiber regeneration, and force produced during isometric contractions. A key finding was that while all lipids increased at 3 days post-injury (dpi) and then declined through 14 dpi, pro-inflammatory lipids remained elevated during recovery from greater muscle loss which led to fibrosis. Maresin 1 was identified as an anti-inflammatory lipid that, when injected into injured muscle, reduced fibrosis, improved muscle regeneration, and partially restored the strength of contraction.

Strengths: The metabolipidomic profiling demonstrated here represents a novel approach to identifying pro-inflammatory and anti-inflammatory mediators of successful vs unsuccessful skeletal muscle regeneration. These findings may translate into a new therapeutic approach for promoting successful regeneration following volumetric muscle loss.

Weaknesses: Certain aspects of the data are overinterpreted; while some measures appear to have an adequate sample size to make sound conclusions, other measures are likely to lack sufficient statistical power given their variability. Presentation of the results would be strengthened by adhering to consistent terminology and labeling of figures throughout; specific examples are identified in recommendations to the authors. Several of the images used to illustrate differences between treatments are unconvincing because differences are not readily.

Reviewer #2 (Public Review):

The study is novel and valuable to the field and provides new and important insights into the role of lipid mediators in VML injuries. By expanding our understanding of the mechanisms that regulate muscle regeneration following VML injuries, the study has the potential to guide the development of novel therapeutic interventions that promote tissue repair and recovery. The data presented in the manuscript is of good quality. The findings and conclusions are supported by a variety of different analyses (e.g., gene expression, histology, flow cytometry).

Despite the strengths of the study, some limitations are identified. Specifically, the impact of maresin 1 on macrophage phenotypes (M1/M2) could have been explored in more detail using histological or protein expression analysis. Moreover, additional data are needed to substantiate the claims about increased muscle regeneration. Lastly, the study does not address myofiber innervation, myofiber-type transitions, or motor unit remodeling.

eLife assessment

George et al. present a convincing new Python toolbox ("RatInABox") that allows researchers to generate synthetic behavior and neural data specifically focusing on hippocampal functional cell types (place cells, grid cells, boundary vector cells, head direction cells).

This is valuable for theory-driven research where synthetic benchmarks should be used. Beyond just navigation, it can be highly useful for novel tool development that requires jointly modeling behavior and neural data. The authors provide convincing evidence of its utility with well documented and easy to use code and the corresponding manuscript.

Reviewer #1 (Public Review):

In this work George et al. describe RatInABox, a software system for generating surrogate locomotion trajectories and neural data to simulate the effects of a rodent moving about an arena. This work is aimed at researchers that study rodent navigation and its neural machinery.

Strengths:<br /> + The software contains several helpful features. It has the ability to import existing movement traces and interpolate data with lower sampling rates. It allows varying the degree to which rodents stay near the walls of the arena. It appears to be able to simulate place cells, grid cells, and some other features.<br /> + The architecture seems fine and the code is in a language that will be accessible to many labs.<br /> + There is convincing validation of velocity statistics. There are examples shown of position data, which seem to generally match between data and simulation.

Weaknesses:<br /> + There is little analysis of position statistics. I am not sure this is needed, but the software might end up more powerful and the paper higher impact if some position analysis was done. Based on the traces shown, it seems possible that some additional parameters might be needed to simulate position/occupancy traces whose statistics match the data.<br /> + The overall impact of this work is somewhat limited. It is not completely clear how many labs might use this, or have a need for it. The introduction could have provided more specificity about examples of past work that would have been better done with this tool.<br /> + Presentation: Some discussion of case studies in Introduction might address the above point on impact. It would be useful to have more discussion of how general the software is, and why the current feature set was chosen. For example, how well does RatInABox deal with environments of arbitrary shape? T-mazes? It might help illustrate the tool's generality to move some of the examples in supplementary figure to main text - or just summarize them in a main text figure/panel.

Reviewer #3 (Public Review):

George et al. present a convincing new Python toolbox that allows researchers to generate synthetic behavior and neural data specifically focusing on hippocampal functional cell types (place cells, grid cells, boundary vector cells, head direction cells). This is highly useful for theory-driven research where synthetic benchmarks should be used. Beyond just navigation, it can be highly useful for novel tool development that requires jointly modeling behavior and neural data. The code is well organized and written and it was easy for us to test.

We have a few constructive points that they might want to consider.

- Right now the code only supports X,Y movements, but Z is also critical and opens new questions in 3D coding of space (such as grid cells in bats, etc). Many animals effectively navigate in 2D, as a whole, but they certainly make a large number of 3D head movements, and modeling this will become increasingly important and the authors should consider how to support this.

- What about other environments that are not "Boxes" as in the name - can the environment only be a Box, what about a circular environment? Or Bat flight? This also has implications for the velocity of the agent, etc. What are the parameters for the motion model to simulate a bat, which likely has a higher velocity than a rat?

- Semi-related, the name suggests limitations: why Rat? Why Not Agent? (But its a personal choice)

- A future extension (or now) could be the ability to interface with common trajectory estimation tools; for example, taking in the (X, Y, (Z), time) outputs of animal pose estimation tools (like DeepLabCut or such) would also allow experimentalists to generate neural synthetic data from other sources of real-behavior.

- What if a place cell is not encoding place but is influenced by reward or encodes a more abstract concept? Should a PlaceCell class inherit from an AbstractPlaceCell class, which could be used for encoding more conceptual spaces? How could their tool support this?

- This a bit odd in the Discussion: "If there is a small contribution you would like to make, please open a pull request. If there is a larger contribution you are considering, please contact the corresponding author3" This should be left to the repo contribution guide, which ideally shows people how to contribute and your expectations (code formatting guide, how to use git, etc). Also this can be very off-putting to new contributors: what is small? What is big? we suggest use more inclusive language.

- Could you expand on the run time for BoundaryVectorCells, namely, for how long of an exploration period? We found it was on the order of 1 min to simulate 30 min of exploration (which is of course fast, but mentioning relative times would be useful).

- Regarding the Geometry and Boundary conditions, would supporting hyperbolic distance might be useful, given the interest in alternative geometry of representations (ie, https://www.nature.com/articles/s41593-022-01212-4)?

- In general, the set of default parameters might want to be included in the main text (vs in the supplement).

- It still says you can only simulate 4 velocity or head directions, which might be limiting.

- The code license should be mentioned in the Methods.

eLife assessment

The authors introduce a potentially valuable novel method that provides trial-by-trial probabilistic estimates of learning and decision-making strategies inferred from choice behavior across species. This approach could prove more useful over traditional techniques for arbitrating between strategies and detecting when learning happens, and because it is computationally lightweight. Reviewers identified several concerns that limit the strength of the evidence provided, rendering the findings incomplete.

Reviewer #1 (Public Review):

This article proposes a new statistical approach to identify which of several experimenter-defined strategies best describes a biological agent's decisions when such strategies are not fully observable by choices made in a given trial. The statistical approach is described as Bayesian but can be understood instead as computing a smoothed running average (with decay) of the strategies' success at matching choices, with a winner-take-all inference across the rules. The article tests the validity of this statistical approach by applying it to both simulated agents and real data sets in mice and humans. It focuses on dynamically changing environments, where the strategy best describing a biological agent may change rapidly.

The paper asks an important question, and the analysis is well conducted; the paper is well-written and easy to follow. However, there are several concerns that limit the strength of the contribution. Major concerns include the framing of the method, considerations around the strategy space, limitations in how useful the technique may be, and missing details in analyses.

Reviewer #2 (Public Review):

In this study, the goal is to leverage the power of Bayesian inference to estimate online the probability that any given arbitrarily chosen strategy is being used by the decision-maker. By computing the trial-by-trial MAP and variance of the posterior distribution for each candidate strategy, the authors can not only see which strategy is primarily being used at every given time during the task and when strategy changes occur but also detect when the target rule of a learning task becomes the front-running strategy, i.e., when successful learning occurs.

Strengths:<br /> 1. The proposed approach adds to recent methods for capturing the dynamics of decision-making at finer temporal resolution (trials) (Roy et al., 2021; Ashwood et al., 2022) but it is novel and differs from these in that it is suited especially well for analyzing when learning occurs, or when a rule switches and learning must recommence, and it does not necessitate large numbers of trials.

2. The manuscript starts with a validation of the approach using synthetic data and then is applied to datasets of trial-based two-alternative forced choice tasks ranging from rodent to non-human primate to human, providing solid evidence of its utility.

3. Compared to classic procedures for identifying when an animal has learned a contingency which typically needs to be conservative in favor of better accuracy, this method retrieves signs of learning happening earlier (~30 trials earlier on average). This is achieved by identifying the moment (trial) when the posterior probability of the correct "target" rule surpasses the probability of all other strategies. Having greater temporal precision in detecting when learning happens may have a very significant impact on studies of the neural mechanisms of learning.

4. This approach seems amenable to testing many different strategies depending on the purpose of the analysis. In the manuscript, the authors test target versus non-target strategies (correct versus incorrect) and also in another version of the analysis, they test what they call "exploratory" strategies.

5. One of the main appeals of this method is its apparent computational simplicity. It necessitates only updating on every trial the parameters of a beta distribution (prior distribution for a given strategy) with the evidence that the behavior on trial was either consistent or inconsistent with the strategy. Two scalars, the mode of the posterior (MAP) and the inverse of the variance, are all that are required for identifying the decision criterion (highest MAP and if tied lowest variance) and the learning criterion (first trial where MAP for target strategy is higher than chance).

Weaknesses:<br /> 1. It seems like a limitation of this approach is that the candidate strategies to arbitrate between must be known ex-ante. It is not clear how this approach could be applied to uncover latent strategies that are not mixtures of the strategies selected.

2. Different strategies may be indistinguishable from each other and thus it may not be possible to distinguish between them. Similarly, the fact that two strategies seem to be competing for the highest MAP doesn't necessarily mean that those are correct strategies and perhaps interchangeable as the manuscript seems to suggest.

3. The decay parameter is a necessary component to make the strategy selection non-stationary and accommodate data sets where the rules are changing throughout the task. However, the choice of the decay parameter value bounds does not seem very principled. Having this parameter as a free-parameter adds a flexibility that seems to have significant effects on when the strategy switch is detected and how stable the detected switch is.

4. This method is a useful approach for arbitrating between strategies and describing the behavior with a temporal precision that may prove important for studies attempting to tie these precise events to changes in neural activity. However, it seems limited in its explanatory power. In its current form, this method does not provide a prediction of the probability to transition from one strategy to another. And, because the MAP of different strategies may be close at any given moment, it is hard to imagine using this approach to tease out the different "mental states" that represent each strategy being at play.

Author Response

The following is the authors’ response to the original reviews.

Reviewer #1 (Public Review):

The experimental design presented cannot clearly show that the effect of passive exposure was due to the specific exposure to task-relevant stimuli since there is no control group exposed to irrelevant stimuli.

We acknowledge the possibility that exposure to task-irrelevant stimuli could result in improvements in learning. Testing this possibility would be a worthwhile goal of future experiments, but it is outside the scope of our current study. We have been careful in our paper to only draw conclusions about the effects of exposure to task-relevant stimuli compared to no exposure. We have added a discussion of this point and relevant references to the literature in the Discussion section of our manuscript.

The conclusion that "passive exposure influences responses to sounds not used during training" (line 147) does not seem fully supported by the authors' analysis. The authors show that there is an increase in accuracy for intermediate sweep speeds despite the fact that this is the first time the animals encounter them in the active session. However, it seems impossible to exclude that this effect is not simply due to the increased accuracy of the extreme sounds that the animals had been trained on.

We have modified this sentence to emphasize that it refers to “intermediate” sounds. Regarding the reviewer’s concern, the conclusion is drawn from Figure 3, in which we show that mice exhibit an improvement on non-extreme stimuli after training on extreme stimuli. Panel 3D illustrates that the observed improvements are not just changes in psychometric performance driven by the extreme sounds. In the context of this result, the conclusion relates to generalization in performance on task-relevant stimuli that are closely related to the training stimuli. In our view, it was not entirely obvious a priori that this result would have to occur, since it is possible that performance could improve at the extremes without improving at the intermediate stimuli.

In the modelling section, the authors adjusted the hyper-parameters to maximize the difference between pure active and passive/active learning. This makes a comparison of learning rates between models somewhat confusing.

We apologize for the confusion. None of our conclusions are based on comparisons of learning speed between models, but perhaps this was not pointed out sufficiently clearly. The relevant comparisons between conditions for each specific model are made using the same hyperparameters. We have clarified this point in the modeling section of our manuscript.

The description of the sound does not state whether when reducing the slope of the sweeps the center or the onset frequency of the sounds is preserved.

Frequency modulated sounds of different FM slopes were generated such that the center frequency was always the same. This is now clarified in the updated version of the manuscript.

Reviewer #1 (Recommendations for the authors):

As mentioned, the specificity of the stimuli presented during the passive period is not explicitly addressed in either modelling or behaviour. For modelling, this could be quite straightforward to assess by manipulating the input stimuli during passive episodes. For the behaviour, this would require repeating the experiment with passive sessions during which unrelated sounds are presented (for example varying in frequency or intensity instead of frequency slope). I mainly include this suggestion to clarify my previous comment because this would require a huge amount of work.

We agree that varying the extent to which the presented passive stimuli are task-related to the task is an interesting point to study for future experiments. However, doing so for the experiments is outside the scope of the current study, and we believe exploring this only in the modeling part would add little value to the current study, because the outcome will highly depend on the details of the implementation.

Reviewer #2 (Public Review):

One limitation here is that the presented analysis is somewhat simplistic, does not include any detailed psychometric analysis (bias, lapse rates etc), and primarily focuses on learning speed.

In our preliminary analyses of trials that included extreme and intermediate stimuli after animals had learned the task (Figure 3), we investigated some metrics of the type that the reviewer suggests here. However, since such additional psychometric analyses were somewhat tangential to our main results (which are about learning speed and responses to sounds not included during training), we did not include these in our manuscript. In agreement with the reviewer’s concern, a main limitation of our study is that the available data does not allow for an analysis of psychometrics during the initial learning stages, since only the extreme stimuli were presented during the task.

Reviewer #2 (Recommendations for the authors):

The International Brain Lab has shown quite nicely that psychometric curves continue to improve (increased slope, decreased bias) across learning. This was not really discussed or presented in your data - is this observed during the S4 training portion?

We indeed saw improvements in the psychometric performance during stage S4, in particular for the active-only learners, as can be seen in Figure 3. We quantified these changes (now presented in the Results section), and added a discussion to the main text.

Why use a linear fit to extract the various quantities of interest? All of these quantities could be extracted from the raw behavioral data itself.

Because of the large variations in performance from day-to-day, a linear fit allowed us to extract a more reliable estimate of quantities like “Time to achieve 70%” and “Performance at 21 days” for each animal.

The analysis presented was focussed primarily on the fast learners. What about the slow learners? Are the ANN models able to recapitulate different aspects of their behavior?

We agree with the reviewer that the observation that the learners clustered into two groups calls for further investigation. In this study, we focused on the mice that learned more efficiently, because those allowed us to address our main research question about the influence of passive exposure. We believe, the slow learners could be modeled with ANNs that start with a less-easily discriminable input representation, which limits the performance that the trained network is ultimately able to achieve. This additional analysis is outside the scope of the current manuscript, but we hope to address these questions in the future.

Although I appreciate the thoroughness of the modeling, I was not entirely convinced by the narrative underlying models 1-5, since none of these models were able to successfully recapitulate your core findings. Would it not make more sense to focus primarily on the final model?

By starting with the simplest possible model that incorporates supervised and unsupervised learning, we were able to determine which ingredients were necessary to capture the behavioral data. We believe this could not have been clearly established by considering the final model alone.

Reviewer #3 (Public Review):

The first [major weakness] is that even Model 5 differs from their data. For example, the A+P (passive interleaved condition) learning curve in Figure 7 seems to be non-monotonic, and has some sort of complex eigenvalue in its decay to the steady state performance as trials increase. This wasn't present in their experimental data (Figure 2D), and implies a subtle but important difference. There also appear to be differences in how quickly the initial learning (during early trials) occurs for the A+P and A:P conditions. While both A+P and A:P conditions learn faster than A only in M5, A+P and A:P seem to learn in different ways, which isn't supported in their data.

The reviewer is correct that there are subtle differences between the two learning curves produced by Model 5. Due to expected variability in the experimental data, however, it is difficult to conclude whether such subtle distinctions also appear in the learning curves of the mice. Further, the slight overshoot of the learning curve that the reviewer mentions is not constrained by the experimental data due to different mice reaching asymptotic performance at different times, and many of them not having even reached asymptotic performance by the end of the training period.

However, even if there are minor discrepancies between the learning curves produced by the final version of the model and by the mice, we do not see this as being especially surprising or problematic. As in any model, there are a large number of potentially important features that are not included in any of our models–for example, realistic spectrotemporal neural responses, nonlinearity in neural activations, heterogeneity across mice, and many others. The aim of our modeling was to choose a space of possible models (which is inevitably restricted) and show which model version within that space best captures our experimental observations. Expanding the space of possible models that we considered to capture further nuances in the data will be a task for future work.

The second major weakness is that the authors also don't generate any predictions with M5. Can they test this model of learning somehow in follow-up behavioural experiments in mice? ... Without follow-up experiments to test their mechanism of why passive exposure helps in a schedule-independent way, the impact of this paper will be limited.

Although testing predictions from our models was beyond the scope of the current study, we do generate specific predictions with model M5 (in particular, about neural representations). Our model produces predictions about neural representations and the ways in which they evolve through learning, and we hope to test these predictions in future work.

I believe the authors need to place this work in the context of a large amount of existing literature on passive (unsupervised) and active (supervised) learning interactions. This field is broad both experimentally and computationally. For example, there is an entire sub-field of machine learning, called semi-supervised learning that is not mentioned at all in this work.

We thank the reviewer for pointing this out. The Discussion section of the updated manuscript now includes a discussion on how our results fit in with this literature.

Reviewer #3 (Recommendations for the authors):

All points made by the reviewer in their Recommendations For The Authors are associated with those presented in the Public Review and they are addressed in our response above.

eLife assessment

This study reports valuable behavioral and computational observations regarding how passive exposure to auditory stimuli can facilitate auditory categorization. The combination of behavioral results in mice with a study of artificial neural network models provides solid evidence for the authors' conclusions. This paper will likely be of broad interest to the general neuroscience community.

Reviewer #1 (Public Review):

Schmid et al. investigate the question of how sensory learning in animals and artificial networks is driven both by passive exposure to the environment (unsupervised) and from reinforcing feedback (supervised) and how these two systems interact. They first demonstrate in mice that passive exposure to the same auditory stimuli used in a discrimination task modify learning and performance in the task. Based on this data, they then tested how the interaction of supervised and unsupervised learning in an artificial network could account for the behavioural results.

The clear behavioural impact of the passive exposure to sounds on accelerating learning is a major strength of the paper. Moreover, the observation that passive exposure had a positive impact on learning whether it was prior to the task or interleaved with learning sessions provides interesting constraints for modelling the interaction between supervised and unsupervised learning. A practical fallout for labs performing long training procedures is that the periods of active learning that require water-restriction could be reduced by using passive sessions. This could increase both experimental efficiency and animal well-being.

The modelling section clearly exhibits the differences between models and the step-by-step presentation building to the final model provides the reader with a lot of intuition about how supervised and unsupervised learning interact. In particular the authors highlight situations in which the task-relevant discrimination does not align with the directions of highest variance, thus reinforcing the relevance of their conclusions for the complex structure of sensory stimuli. A great strength of these models is that they generate clear predictions about how neural activity should evolve during the different training regimes that would be exciting to test.

As the authors acknowledge, the experimental design presented cannot clearly show that the effect of passive exposure was due to the specific exposure to task-relevant stimuli since there is no control group exposed to irrelevant stimuli. Studies have shown that exposure to a richer sensory environment, even in the adult, swiftly (ie within days) enhances responses even in the adult and even when the stimuli are different from those present in the task (1-3). Clearly distinguishing between these two options would require further experiments and could be a possible direction for future research.

1. Mandairon, N., Stack, C. & Linster, C. Olfactory enrichment improves the recognition of individual components in mixtures. Physiol. Behav. 89, 379-384 (2006).<br /> 2. Alwis, D. S. & Rajan, R. Environmental enrichment and the sensory brain: The role of enrichment in remediating brain injury. Front. Syst. Neurosci. 8, 1-20 (2014).<br /> 3. Polley, D. B., Kvašňák, E. & Frostig, R. D. Naturalistic experience transforms sensory maps in the adult cortex of caged animals. Nature 429, 67-71 (2004).

Reviewer #2 (Public Review):

Schmid et al present a lovely study looking at the effect of passive auditory exposure on learning a categorization task.<br /> The authors utilize a two-alternative choice task where mice have to discriminate between upward and downward moving frequency sweeps. Once mice learn to discriminate easy stimuli, the task is made psychometric and additional intermediate stimuli are introduced (as is standard in the literature). The authors introduce an additional two groups of animals, one that was passively exposed to the task stimuli before any behavioral shaping, and one that had passive exposure interleaved with learning. The major behavioral finding is that passive exposure to sounds improves learning speed. The authors show this in a number of ways through linear fits to the learning curves. Additionally, by breaking down performance based on the "extreme" vs "psychometric" stimuli, the authors show that passive exposure can influence responses to sounds that were not present during the initial training period. One limitation here is that the presented analysis is somewhat simplistic, does not include any detailed psychometric analysis (bias, lapse rates etc), and primarily focuses on learning speed. Ultimately though, the behavioral results are interesting and seem supported by the data.

To investigate the neural mechanisms that may underlie their behavioral findings, the authors turn to a family of artificial neural network models and evaluate the consequences of different learning algorithms and schedules, network architectures, and stimulus distributions, on the learning outcomes. The authors work through five different architectures that fail to recapitulate the primary behavior findings before settling on a final model, utilizing a combination of supervised and unsupervised learning, that was capable of reproducing the key aspects of the experiments. Ultimately, the behavioral results presented are consistent with network models that build latent representations of task-relevant features that are determined by statistical properties of the input distribution.

Reviewer #3 (Public Review):

Summary of Author's Results/Intended Achievements<br /> The authors were trying to ascertain the underlying learning mechanisms and network structure that could explain their primary experimental finding: passive exposure to a stimulus (independent of when the exposure occurs) can lead to improvements in active (supervised) learning. They modeled their task with 5 progressively more complex shallow neural networks classifying vectors drawn from multi-variate Gaussian distributions.

Account of Major Strengths:<br /> Overall, the experimental findings were interesting. The modelling was also appropriate, with a solid attempt at matching the experimental condition to simplified network models.

Author Response

The following is the authors’ response to the previous reviews.

eLife assessment

This is a valuable study of Eph-Ephrin signaling mechanisms generating pathological changes in amyotropic lateral sclerosis. There are exciting findings bearing on the role of glial cells in this pathology. The study emerges with solid evidence for a novel astrocyte-mediated mechanism for disease propagation. It may help identify potential therapeutic targets.

Response to Editor’s decision letter: Drs. Huang and Zaidi: Thank you for considering this re-revision of our manuscript for potential publication in eLife. We have addressed the remaining comments of reviewer #2. We have included detailed response-to-reviewer comments below to address each of these remaining specific points from reviewer #2, and we have highlighted all the changes in the manuscript text (using a red font color) made in response to these comments. Based on the reviewers’ critiques, we feel our re-working of the manuscript has made for a greatly improved study.

Reviewer #1 (Recommendations For The Authors):

Reviewer comment: All questions/concerns have been addressed.

Response: We thank Reviewer #1 for the previous helpful comments that we used to improve our manuscript. As Reviewer #1 has no new comments, we have provided no additional responses to address this reviewer’s input. Instead, we only focus (in this new “Response to Reviewer Comments” document) on the remaining points from Reviewer #2 below.

Reviewer #2 (Recommendations For The Authors):

Overall, the authors have addressed most concerns raised in the prior review. A couple of very minor points remain, which would improve the clarity of the report.

Reviewer comment 1: The abstract has not been edited and still emphasizes that astrocyte-mediated upregulation in ephrinB2 signaling underlies pathogenicity in mutant SOD1-associated ALS. There is certainly sufficient evidence to suggest a large role for astrocytes, however, without a thorough investigation of other key cell types in the spinal cord, this cannot be concluded specifically. Especially given that a non-specific promoter (U6) was employed in the viral constructs.

Response: We apoplogize for this mistake. In response to the reviewer’s previous comment in the first round of review, we made changes throughout the manuscript to address this issue; however, we failed to do this in the Abstract. In this re-revised manucript, we now also make the necessary changes to the Abstract.

Reviewer comment 2: It is interesting to note that a non-specific promoter, U6, exhibited such large specificity to astrocytes in the cord as compared to neurons (Fig 2M). This is worth discussing briefly in the discussion and how this result compares to those in the literature.

Response: We have now added a brief discussion of this issue to the Discussion section, including describing our previous studies that used the Gfa2 promotor to achieve astrocyte-specific transduction when employing viral vectors in the rodent spinal cord.

Reviewer comment 3: I appreciate the authors including a supplemental figure on the expression of ephrinA4 receptors in the cervical ventral horn. Unfortunately, the quality of this image is very poor in conveying the receptor expression. The detailed discussion point on the expression of EphB receptors in the cervical ventral horn should be sufficient for readers to take into consideration.

Response: We have now removed this supplemental figure and keep only the text from the rerevised manuscript.

Reviewer comment 4: A few instances of motor neuron diameter being attributed to a 200μm2 size remain (e.g. pg 14).

Response: We have corrected this issue throughout the re-revised manuscript. The correct information is: somal diameter greater than 20 μm.

Reviewer comment 5: It is still a little unclear in the result text as to when assessment of lentiviral transduction was conducted following intraspinal injections.

Response: We have now added this detail about the time point of assessing transduction to both the Results section and the Materials/Methods section.

Reviewer comment 6: Some figures are missing markers of significance (e.g. Fig 2M).

Response: Below are our comments about significance markers for each graph in all figures.

Figure 1:

Panel E: We have now added asterisks for any statistically-significant comparisons. In addition, we provide the details of this statistical analysis in the text of the re-revised manuscript.

Figure 2:

Panel M: We have now added asterisks for statistical comparisons, as well as details in the text.

Panel N: The asterisk was already shown in the previous version of the figure.

Figure 3:

Panels B and G: The asterisks were already shown in the previous version of the figure.

Figure 4:

All panels: There are no significant differences; therefore, no asterisks are needed.

Figure 5:

Panel F and G: The asterisks were already shown in the previous version of the figure.

Panel H: The difference is not statistically-signficant.

Figure 6: No graphs are shown in this figure.

Reviewer comment 7: Since a wild type mouse control has not been included in the quantification of diaphragm NMJ innervation with and without ephrin knock-down, it would be useful to include a description or discussion on the phenotype of NMJ denervation exhibited in the SOD1G93A mouse model of ALS.

Response: We have now added description of diaphragm NMJ denervation that occurs in SOD1G93A mice, in particular at the age/time point of our NMJ analysis.

eLife assessment

This is a valuable study of Eph-Ephrin signaling mechanisms generating pathological changes in amyotropic lateral sclerosis. There are exciting findings bearing on the role of glial cells in this pathology. The study emerges with solid evidence for a novel astrocyte-mediated mechanism for disease propagation. It may help identify potential therapeutic targets.

Reviewer #1 (Public Review):

In the manuscript by Urban et al., the authors attempt to further delineate the role with which non-neuronal CNS cells play in the development of ALS. Towards this goal, the transmembrane signaling molecule ephrinB2 was studied. It was found that there is an increased expression of ephrinB2 in astrocytes within the cervical ventral horn of the spinal cord in a rodent model of ALS. Moreover, reduction of ephrinB2 reduced motoneuron loss and prevented respiratory dysfunction at the NMJ. Further driving the importance of ephrinB2 is an increased expression in the spinal cords of human ALS individuals. Collectively, these findings present compelling evidence implicating ephrinB2 as a contributing factor towards the development of ALS.

Reviewer #2 (Public Review):

The contribution of glial cells to the pathogenesis of amyotrophic lateral sclerosis (ALS) is of substantial interest and the investigators have contributed significantly to this emerging field via prior publications. In the present study, authors use a SOD1G93A mouse model to elucidate the role of astrocyte ephrinB2 signaling in ALS disease progression. Erythropoietin-producing human hepatocellular receptors (Ephs) and the Eph receptor-interacting proteins (ephrins) signaling is an important mediators of signaling between neurons and non-neuronal cells in the nervous system. Recent evidence suggests that dysregulated Eph-ephrin signaling in the mature CNS is a feature of neurodegenerative diseases. In the ALS model, upregulated Eph4A expression in motor neurons has been linked to disease pathogenesis. In the present study, authors extend previous findings to a new class of ephrinB2 ligands. Urban et al. hypothesize that upregulated ephrinB2 signaling contributes to disease pathogenesis in ALS mice. The authors successfully test this hypothesis and their results generally support their conclusion.

Major strengths of this work include a robust study design, a well-defined translational model, and complementary biochemical and experimental methods such that correlated findings are followed up by interventional studies. Authors show that ephrinB2 ligand expression is progressively upregulated in the ventral horn of the cervical and lumbar spinal cord through pre-symptomatic to end stages of the disease. This novel association was also observed in lumbar spinal cord samples from post-mortem samples of human ALS donors with a SOD1 mutation. Further, they use a lentiviral approach to drive knock-down of ephrinB2 in the central cervical region of SOD1G93A mice at the pre-symptomatic stage. Interestingly, in spite of using a non-specific promoter, authors note that the lentiviral expression was preferentially driven in astrocytes.

Since respiratory compromise is a leading cause of morbidity in the ALS population, the authors proceed to characterize the impact of ephrinB2 knockdown on diaphragm muscle output. In mice approaching the end stage of the disease, electrophysiological recordings from the diaphragm muscle show that animals in the knock-down group exhibited a ~60% larger amplitude. This functional preservation of diaphragm function was also accompanied with the preservation of diaphragm neuromuscular innervation. However, it must be noted that this cervical ephrinB2 knockdown approach had no impact on disease onset, disease duration, or animal survival. Furthermore, there was no impact of ephrinB2 knockdown on forelimb or hindlimb function. This is an expected result, given the fairly focal approach of ephrinB2 knockdown in C3-C5 spinal segments.

The major limitation of the study is the conclusion that the preservation of diaphragm output following ephrinB2 knockdown in SOD1 mice is mediated primarily (if not entirely) by astrocytes. The authors present convincing evidence that a reduction in ephrinB2 is observed in local astrocytes (~56% transduction) following the intraspinal injection of the lentivirus. However, the proportion of cell types assessed for transduction with the lentivirus in the spinal cord was limited to neurons, astrocytes, and oligodendrocyte lineage cells. Microglia comprise a large proportion of the glial population in the spinal grey matter and have been shown to associate closely with respiratory motor pools. This cell type, amongst the many other that comprise the ventral gray matter, have not been investigated in this study. Nonetheless, there is convincing evidence to suggest astrocytes play a significant role, as compared to oligodendrocytes in promoting ALS pathogenesis.

In summary, this study by Urban et al. provides a valuable framework for Eph-Ephrin signaling mechanisms imposing pathological changes in an ALS mouse model. The role of glial cells in ALS pathology is a very exciting and upcoming field of investigation. The current study proposes a novel astrocyte-mediated mechanism for the propagation of disease that may eventually help to identify potential therapeutic targets.

Author Response

The following is the authors’ response to the original reviews.

eLife Assessment

This valuable manuscript investigates the roles of DKK3 in AD synapse integrity. Although previous work has identified the involvement of Wnt and DKK1 in synaptic physiology, this study provides compelling evidence that suppression of DKK3 rescues the changes in excitatory synapse numbers, as well as memory deficits in an established AD model mice. The authors provide both gain and loss of function data that support the main conclusion and advance our understanding of the mechanisms by which Wnt pathway mediates early synaptic dysfunction in AD models.

Public Reviews:

Reviewer #1 (Public Review):

In this study, Nuria Martin-Flores, Marina Podpolny and colleagues investigate the role of Dickkopf-3 (DKK3), a Wnt antagonist in synaptic dysfunction in Alzheimer's disease. Loss of synapses is a feature of Alzheimer's and other forms of dementia such as frontotemporal dementia and linked amyotrophic lateral sclerosis (FTD). The authors utilise a broad range of experimental approaches. They show that DKK3 levels are increased in Alzheimer's disease and that this occurs early in disease. This is an important finding since early disease changes are believed to be the most important. They also show increases in DKK3 in transgenic mouse models of Alzheimer's disease and that DKK3 knockdown restores synapse number and memory in one such model. Finally, they link these DKK3 increases to loss of excitatory synapses via the blockade of the Wnt pathway and subsequent activation of GSK3B; GSK3B is strongly linked to both Alzheimer's disease and FTD. The quality of the data is good and the conclusions well supported by these data. There are no major weaknesses. The findings support studies that target the Wnt pathway as a potential therapeutic for Alzheimer's disease.

Reviewer #2 (Public Review):

This manuscript by Martin-Flores et al., has examined the role of DKK3 in Alzheimer's disease, focusing on the regulation of synaptic numbers. By using human AD brain databases and tissue samples, the authors showed that DKK3 protein and mRNA levels are increased in the brains of AD patients. DKK3 is expressed in the excitatory neurons in WT mouse brains and accumulates at atrophic neurites around amyloid plaques in AD mouse brains. Interestingly, secretion of DKK3 appears to be regulated by NMDAR antagonist as well as chemical LTD. Through gain and loss of function studies, the authors showed that DKK3 regulates the number of excitatory as well as inhibitory synapses with distinct downstream pathways. Finally, the authors investigated the contribution of DKK3 to synaptic changes in AD and found that DKK3 loss of function rescues both the excitatory and inhibitory synaptic defects, resulting in the improvement of memory function in J20 mice.

Overall, the data is clearly presented and deals with novel roles of DKK3 in controlling excitatory and inhibitory synapses. The finding that shRNA expression of DKK3 in AD model mice rescues synaptic phenotypes and memory impairment is potentially interesting and may provide a new strategy for AD treatment.

We would like to thank the Editors and the Reviewers for their very insightful suggestions. We are delighted to receive very positive reviews of our manuscript. In response to the comments made by the reviewers, we have carried out an extensive revision of our manuscript. In the revised manuscript, we have addressed all the comments made by the reviewers.

Recommendations for the authors:

Reviewer #1:

My only comment regards the role of GSK3B activation in synaptic dysfunction and its targets. GSK3B is a Tau kinase but is also involved in IP3 receptor delivery of Ca2+ to mitochondria. This delivery is major regulator of mitochondrial ATP production and synaptic function is heavily dependent on ATP. Both Alzheimer's disease and FTD insults have been linked to GSK3B activation -see for e.g. Szabo EMBO R 2023, Gomez-Suaga Aging Cell 2022. It might be valuable to readers for the authors to speculate briefly on potential GSK3B synaptic targets in the Discussion.

We appreciate the reviewer for this suggestion. In the Discussion, we now included how GSK3β may contribute to synaptic dysfunction and loss in the context of increased DKK3 levels and in Alzheimer’s disease.

Reviewer #2:

1. In Fig 1B, the authors showed that soluble DKK3 levels were increased in Braak 1-3 patients, while no changes were observed in Braak 4-5. If the secretion of DKK3 is dependent on NMDAR activity, does this data imply that Braak 4-5 patients have reduced NMDAR activity in general, resulting in the reduced DKK3 release even with the increased mRNA levels? It would be interesting to test this hypothesis in a mouse AD model.

In Figure 1B, we analyzed the levels of soluble and insoluble DKK3 in the hippocampus of AD patients at different disease stages based on their Braak stages. As the reviewer indicated, soluble levels of DKK3 were increased in patients with Braak I-III but not at later stages. Importantly, DKK3 levels were also elevated in Braak IV-VI patients, but only in the insoluble fraction (Figure 1C), suggesting that DKK3 could accumulate within Aβ aggregates. Based on these findings, we cannot conclude that DKK3 release is reduced at later stages of the disease in patients.

To explore the underlying mechanisms regulating DKK3 levels, we used cultured hippocampal neurons and AD mouse brain slices. In mouse models, we have demonstrated that extracellular DKK3 levels (secreted DKK3 fraction) depends on NMDAR activation early in the disease progression (Figure 2E, F). Moreover, we also provide new data showing that antagonizing NMDAR partially blocks the increase of DKK3 extracellular levels induced by oligomeric Aβ (see response to question 4 of this reviewer and Figure S2G, H). It is well established that oligomeric Aβ promotes hyperexcitability through, in part, the aberrant activation of NMDAR (Li S et al., 2011, PMID: 21543591; Mucke L and Selkoe DJ et al., 2012, PMID: 22762015). In line with this, NMDAR blockers prevent Aβ-induced synapse loss and improve cognition in AD models (Hu NW et al., 2009, PMID: 19918059; Ye C et al., 2004, PMID: 15288443). In addition, an NMDAR antagonist is currently approved as a drug treatment for AD patients (Cumming J 2021, PMID: 33441154). Together, our findings in dissociated neurons, AD mouse brain and human samples indicate that soluble Aβ oligomers promote the release of DKK3 through NMDAR activation and suggest that this mechanism might also be occurring in the brain of AD patients.

1. Recent work (Yuan et al., 2022, Nature) has shown that dystrophic neurites/axonal spheroids found around Aβ deposits are filled with neuronal endolysosomes. Are DKK3 in ThioS positive amyloid plaques located in endolysosomes of these axonal spheroids? If so, does this data mean that DKK3 in Fig 2B-D represents the entrapped DKK3 protein population that fails to be secreted from dystrophic neurites?

The reviewer points an interesting question. Our results show that secretion of DKK3 is increased in two AD models before substantial plaque load. Later in the disease, DKK3 accumulates in dystrophic neurites (visualized as axonal spheroids) surrounding amyloid plaques. To address if DKK3 protein is located in vesicles of the endolysosomal pathway within axonal spheroids, we performed co-localization analyses of DKK3 and the endolysosomal marker LAMP1. We found that DKK3 colocalized with LAMP1 (Figure 2D) indicating the presence of DKK3 in axonal spheroids. These results indeed suggest that DKK3 is present in abnormally enlarged vesicles in dystrophic neurites around Aβ plaques. This could affect the axonal transport of DKK3. Given that proteins present in dystrophic neurites have been correlated with defects in bidirectional transport in the axon (Stokin GB et al., 2005, PMID: 15731448; Sadleir KR et al., 2016, PMID: 26993139), both DKK3 turnover and secretion could be affected.

1. Why does only LTD induce DKK3 release? Why not general activation of neuronal activity? It would be important to test the relationship between DKK3 secretion and neuronal activity with optogenetics and chemogenetics.

We tested whether neuronal activity triggered increased extracellular DKK3 levels by subjecting neurons to chemical long-term potentiation (cLTP) or long-term depression (cLTD). However, only cLTD increased extracellular DKK3, which we then confirmed in brain slices (Figure S3). This finding is not unexpected as it is well described that different patterns of activity can lead to different molecular outcomes. For example, high-frequency stimulation (HFS; an activity pattern that resembles LTP) and low-frequency stimulation (LFS; a different activity pattern resembling LTD) leads to opposing effects on surface levels of the Wnt receptor Frizzled-5 (Fz5) (Sahores M et al., 2010, PMID: 20530549). Furthermore, cLTP increases Fz5 s-acylation, an important post-translational modification that regulates the surface levels of Fz5, whereas cLTD decreases it (Teo S et al., 2023, PMID: 37557176). Another example is the BDNF receptor TrkB. Surface TrkB is increased by tetanic stimulation, which also induces LTP as HFS or cLTP, but not by LFS (Du J et al., 2000, PMID: 10995446). Our findings suggest that DKK3 might contribute to synaptic changes underlying cLTD. Future experiments using chemogenetics or optogenetics might elucidate the role of DKK3 in activity-induced synaptic changes.

1. Are Abeta oligomer treatment-dependent increases in DKK3 protein levels in the cellular lysate and the extracellular fraction also suppressed by APV?

Our results in AD mice indicate that increased DKK3 release is dependent on NMDAR activation. To investigate if amyloid-β oligomers (Aβo) increase DKK3 levels in the cell lysate and extracellular fractions through NMDAR, we blocked these receptors in hippocampal neurons using AP-V (Figure S2G, H). In these experiments, we use a lower concentration of Aβo (200nM of Aβ1-42) to avoid any potential cytotoxic effect. In line with our previous results using a higher concentration of Aβo, we observed that Aβo markedly increased DKK3 levels both in the cell lysate and in the extracellular fraction compared to the reverse Aβ42-1 control peptide. Kruskal-Wallis with Dunn’s test showed a trend to a reduced levels of DKK3 in the extracellular fraction when we compared neurons treated with Aβo and APV with those neurons treated with Aβ and vehicle (p = 0.0726). However, this reduced levels of DKK3 in the extracellular fraction reached statistical significance using a t-test (p = 0.0384). No differences were observed between the reverse control peptide and Aβo and APV conditions. These results suggest that blockade of the NMDAR partially occludes the ability of Aβo to increase DKK3 levels in the extracellular fraction.

1. Why does DKK3 shRNA only downregulate inhibitory synapses but not excitatory synapses in the WT brain slice? Does this mean that in the WT brain, other DKK proteins (without changes in their expression as shown in Fig S6) are sufficiently expressed and compensate for the roles of DKK3 in excitatory synapse integrity?

The reviewer points out an interesting result. In J20 mice, DKK3 knockdown affects both excitatory and inhibitory synapse density (Figure 6B, C). In Figure 3B, D, we show that in vivo downregulation of DKK3 leads to an increased number of inhibitory synapses without affecting excitatory ones in the brain of WT animals. These results indicate that in a healthy brain (WT), DKK3 is required for the maintenance of inhibitory synapses but not for excitatory synapses under our experimental conditions. Furthermore, DKK3 partially shares the mechanism of action with DKK1 as both DKK proteins promote excitatory synapse loss through the Wnt/GSK3β pathway (Figure 4A-C) (Marzo A et al., 2016, PMID: 27593374). Therefore, it is possible that endogenous DKK1 levels in the hippocampus could compensate for the reduced expression of DKK3 resulting in the lack of changes in excitatory synapse number when DKK3 is knockdown in WT animals.

1. Manipulating DKK3 in WT brains only affects Gephyrin but not VGAT, but in J20, both Gephyrin and VGAT seem to be affected by DKK3 shRNA (Fig 6). The authors need to provide the pre vs post synapse number in Fig 6 and discuss the potential differences.

We have now included the quantification of excitatory and inhibitory pre- and postsynaptic puncta for 4-months old (Figure S6B, C) and 9-months old (Figure S6D, E) WT and J20 mice. At 4-months old, the density of Homer1 puncta for excitatory synapses and both vGAT and Gephyrin for inhibitory synapses was increased and decreased respectively by knocking down DKK3 in the J20 mice. At 9-months, strong trends were observed in all the synaptic markers when downregulating DKK3, but significance was only reached for Homer1 puncta.

1. Where are the Wnt receptors expressed? Are they exclusively expressed in neurons? Can the authors exclude the potential involvement of glial cells in this process?

In neurons, Wnt receptors can be expressed in the synaptic terminals. For example, Wnt receptor Frizzled-5 is located at the presynaptic terminal and the dendritic shaft but not at spines (Sahores M et al., 2010, PMID: 20530549; McLeod F et al., 2018, PMID: 29694885), whereas Frizzled-7 is located at the dendritic shaft and spines (McLeod F et al., 2018, PMID: 29694885). In addition, the Wnt co-receptor LRP6 is present at both pre- and postsynaptic sites in excitatory synapses (Jones ME et al., 2023, PMID: 36638182). Kremen1, another receptor for Dkk proteins, is also highly expressed in the brain and our unpublished superresolution results show that this receptor is present in both pre- and postsynaptic sites of 53% of excitatory and 30% of inhibitory synapses. However, these receptors are not exclusively expressed in neurons and many of them are also highly expressed in astrocytes (Zhang Y et al., 2016, PMID: 25186741). Based on the literature and our findings, we cannot rule out the possibility that DKK3 may signal to other cell types such as astrocytes, which could also contribute to changes in synapse density. However, recombinant DKK3 induces structural and functional changes in excitatory and inhibitory synapses within 3-4h (Figure 3), suggesting that DKK3 acts on neurons leading to synaptic changes.

1. Does the shRNA treatment of DKK3 affect the size and number of amyloid plaques in the AD mice?

We thank the reviewer for raising this very important question. We have now evaluated the impact of DKK3 knockdown in Aβ pathology in the J20 mice. We did not observe differences in the Aβ coverage nor the averaged number and size of Aβ plaques when DKK3 was silenced in the CA3 (Figure S6F). Therefore, the changes we observe in excitatory and inhibitory synapse density around plaques after knocking down DKK3 are unlikely to be due to changes in Aβ plaques.

eLife assessment

This important manuscript investigates the roles of DKK3 in AD synapse integrity. Although previous work has identified the involvement of Wnt and DKK1 in synaptic physiology, this study provides compelling evidence that suppression of DKK3 rescues the changes in excitatory synapse numbers, as well as memory deficits in an established AD model mice. The authors provide both gain and loss of function data that support the main conclusion and advance our understanding of the mechanisms by which Wnt pathway mediates early synaptic dysfunction in AD models.

Reviewer #1 (Public Review):

In this study, Nuria Martin-Flores, Marina Podpolny and colleagues investigate the role of Dickkopf-3 (DKK3), a Wnt antagonist in synaptic dysfunction in Alzheimer's disease. Loss of synapses is a feature of Alzheimer's and other forms of dementia such as frontotemporal dementia and linked amyotrophic lateral sclerosis (FTD). The authors utilise a broad range of experimental approaches. They show that DKK3 levels are increased in Alzheimer's disease and that this occurs early in disease. This is an important finding since early disease changes are believed to be the most important. They also show increases in DKK3 in transgenic mouse models of Alzheimer's disease and that DKK3 knockdown restores synapse number and memory in one such model. Finally, they link these DKK3 increases to loss of excitatory synapses via the blockade of the Wnt pathway and subsequent activation of GSK3B; GSK3B is strongly linked to both Alzheimer's disease and FTD. The quality of the data is good and the conclusions well supported by these data. There are no major weaknesses. The findings support studies that target the Wnt pathway as a potential therapeutic for Alzheimer's disease.

Reviewer #2 (Public Review):

Summary:<br /> This manuscript by Martin-Flores et al. examines the role of DKK3 in Alzheimer's disease, focusing on the regulation of synaptic number. Using human AD brain databases and tissue samples, the authors demonstrate increased levels of DKK3 protein and mRNA in the brains of AD patients. DKK3 is expressed in excitatory neurons in WT mouse brains and accumulates at atrophic neurites around amyloid plaques in AD mouse brains. Interestingly, the secretion of DKK3 appears to be regulated by NMDAR antagonists, as well as chemical LTD. Through gain and loss of function studies, the authors reveal that DKK3 regulates the number of both excitatory and inhibitory synapses with distinct downstream pathways. Finally, the authors investigate the contribution of DKK3 to synaptic changes in AD and find that DKK3 loss of function rescues both excitatory and inhibitory synaptic defects, resulting in improved memory function in J20 mice.

Strengths:<br /> Overall, the data is clearly presented and deals with the novel roles of DKK3 in controlling excitatory and inhibitory synapses. The finding that shRNA expression of DKK3 in AD model mice rescues synaptic phenotypes and memory impairment is potentially interesting and may provide a new strategy for AD treatment.

Weaknesses:<br /> There are no major weaknesses.

Author Response

eLife assessment

This study presents a valuable finding on the distinct subpopulation of adipocytes during brown-to-white conversion in perirenal adipose tissue (PRAT) at different ages. The evidence supporting the claims of the authors is convincing, although specific lineage tracing of this subpopulation of cells and mechanistic studies would expand the work. The work will be of interest to scientists working on adipose and kidney biology.

Public Reviews:

Reviewer #1 (Public Review):

Summary:

In this manuscript, the authors performed single nucleus RNA-seq for perirenal adipose tissue (PRAT) at different ages. They concluded a distinct subpopulation of adipocytes arises through brown-to-white conversion and can convert to a thermogenic phenotype upon cold exposure.

Strengths:

PRAT adipose tissue has been reported as an adipose tissue that undergoes browning. This study confirms that brown-to-white and white-to-beige conversions also exist in PRAT, as previously reported in the subcutaneous adipose tissue.

We did not observe any white-to-beige conversion in PRAT under regular condition. The adipocyte population that arises from brown-to-white conversion (mPRAT-ad2) can respond to cold and restore their UCP1 expression. However, brown adipocytes that arise from the mPRAT-ad2 subpopulation after cold exposure have a distinct transcriptome to that of cold-induced beige adipocyte in iWAT (Figure S6K) and are more related to iBAT brown adipocytes (Figure 6E).

Weaknesses:

1. There is overall a disconnection between single nucleus RNA-seq data and the lineage chasing data. No specific markers of this population have been validated by staining.

We are not sure what “this population” refers to. We suspect it is the Ucp1-&Cidea+ mPRAT-ad2 adipocyte subpopulation. If so, we did not identify specific markers for these adipocytes as shown in Figure 1H and statement in the Discussion. mPRAT-ad2 is negative for Ucp1 and Cyp2e1, which are markers for mPRAT-ad1 and mPRAT-ad3&4, respectively. Therefore, we plan to stain the mPRAT with Ucp1, Cyp2e1 and Perilipin (a pan adipocyte marker) antibodies. Cells that are Perilipin+&Ucp1-&Cyp2e1- will represent the mPRAT-ad2 subpopulation.

1. It would be nice to provide more evidence to support the conclusion shown in lines 243 to 245 "These results indicated that new BAs induced by cold exposure were mainly derived from UCP1- adipocytes rather than de novo ASPC differentiation in puPRAT". Pdgfra-negative progenitor cells may also contribute to these new beige adipocytes.

Our sequencing data and many previous studies (Angueira et al., 2021; Burl et al., 2022; Dong et al., 2022) have shown that Pdgfra is a marker for all ASPCs. We will also check adipocyte labelling pattern of mPRAT in the PdgfraCre;Ai14 mice. If all adipocytes are Tomato+, it suggests that adipocytes in mPRAT are all derived from Pdgfra-expressing cells. Also, the cold-induced adipocytes in mPRAT resemble more to the brown adipocytes of iBAT than the beige adipocytes of iWAT (Figure 6E and S6K).

Angueira, A.R., Sakers, A.P., Holman, C.D., Cheng, L., Arbocco, M.N., Shamsi, F., Lynes, M.D., Shrestha, R., Okada, C., Batmanov, K., et al. (2021). Defining the lineage of thermogenic perivascular adipose tissue. Nat Metab 3, 469-484. 10.1038/s42255-021-00380-0.

Burl, R.B., Rondini, E.A., Wei, H., Pique-Regi, R., and Granneman, J.G. (2022). Deconstructing cold-induced brown adipocyte neogenesis in mice. Elife 11. 10.7554/eLife.80167.

Dong, H., Sun, W., Shen, Y., Balaz, M., Balazova, L., Ding, L., Loffler, M., Hamilton, B., Kloting, N., Bluher, M., et al. (2022). Identification of a regulatory pathway inhibiting adipogenesis via RSPO2. Nat Metab 4, 90-105. 10.1038/s42255-021-00509-1.

1. The UCP1Cre-ERT2; Ai14 system should be validated by showing Tomato and UCP1 co-staining right after the Tamoxifen treatment.

We will inject Ucp1CreERT2;Ai14 mice at 1- and 6-month-old of age with tamoxifen and collect one day after the last injection to check the overlap between the Tomato signal and UCP1 immunofluorescent staining.

Reviewer #2 (Public Review):

Summary:

In the present manuscript, Zhang et al utilize single-nuclei RNA-Seq to investigate the heterogeneity of perirenal adipose tissue. The perirenal depot is interesting because it contains both brown and white adipocytes, a subset of which undergo functional "whitening" during early development. While adipocyte thermogenic transdifferentiation has been previously reported, there remain many unanswered questions regarding this phenomenon and the mechanisms by which it is regulated.

Strengths:

The combination of UCP1-lineage tracing with the single nuclei analysis allowed the authors to identify four populations of adipocytes with differing thermogenic potential, including a "whitened" adipocyte (mPRAT-ad2) that retains the capacity to rapidly revert to a brown phenotype upon cold exposure. They also identify two populations of white adipocytes that do not undergo browning with acute cold exposure.

Anatomically distinct adipose depots display interesting functional differences, and this work contributes to our understanding of one of the few brown depots present in humans.

Weaknesses:

The most interesting aspect of this work is the identification of a highly plastic mature adipocyte population with the capacity to switch between a white and brown phenotype. The authors attempt to identify the transcriptional signature of this ad2 subpopulation, however, the limited sequencing depth of single nuclei somewhat lessens the impact of these findings. Furthermore, the lack of any form of mechanistic investigation into the regulation of mPRAT whitening limits the utility of this manuscript. However, the combination of well-executed lineage tracing with comprehensive cross-depot single-nuclei presented in this manuscript could still serve as a useful reference for the field.

The sequencing depth of our data is comparable, if not better than previously published snRNA-seq studies on adipose tissue (Burl et al., 2022; Sarvari et al., 2021; Sun et al., 2020). Therefore, the depth of our data has reached the limit of the 3’ sequencing methods. Unfortunately, due to size limitation of the adipocytes, it is also not feasible to sort them for Smart-seq.

Burl, R.B., Rondini, E.A., Wei, H., Pique-Regi, R., and Granneman, J.G. (2022). Deconstructing cold-induced brown adipocyte neogenesis in mice. Elife 11. 10.7554/eLife.80167.

Sarvari, A.K., Van Hauwaert, E.L., Markussen, L.K., Gammelmark, E., Marcher, A.B., Ebbesen, M.F., Nielsen, R., Brewer, J.R., Madsen, J.G.S., and Mandrup, S. (2021). Plasticity of Epididymal Adipose Tissue in Response to Diet-Induced Obesity at Single-Nucleus Resolution. Cell Metab 33, 437-453 e435. 10.1016/j.cmet.2020.12.004.

Sun, W., Dong, H., Balaz, M., Slyper, M., Drokhlyansky, E., Colleluori, G., Giordano, A., Kovanicova, Z., Stefanicka, P., Balazova, L., et al. (2020). snRNA-seq reveals a subpopulation of adipocytes that regulates thermogenesis. Nature 587, 98-102. 10.1038/s41586-020-2856-x.

eLife assessment

This study presents a valuable finding on the distinct subpopulation of adipocytes during brown-to-white conversion in perirenal adipose tissue (PRAT) at different ages. The evidence supporting the claims of the authors is convincing, although specific lineage tracing of this subpopulation of cells and mechanistic studies would expand the work. The work will be of interest to scientists working on adipose and kidney biology.

Reviewer #1 (Public Review):

Summary:

In this manuscript, the authors performed single nucleus RNA-seq for perirenal adipose tissue (PRAT) at different ages. They concluded a distinct subpopulation of adipocytes arises through brown-to-white conversion and can convert to a thermogenic phenotype upon cold exposure.

Strengths:

PRAT adipose tissue has been reported as an adipose tissue that undergoes browning. This study confirms that brown-to-white and white-to-beige conversions also exist in PRAT, as previously reported in the subcutaneous adipose tissue.

Weaknesses:

1. There is overall a disconnection between single nucleus RNA-seq data and the lineage chasing data. No specific markers of this population have been validated by staining.<br /> 2. It would be nice to provide more evidence to support the conclusion shown in lines 243 to 245 "These results indicated that new BAs induced by cold exposure were mainly derived from UCP1- adipocytes rather than de novo ASPC differentiation in puPRAT". Pdgfra-negative progenitor cells may also contribute to these new beige adipocytes.<br /> 3. The UCP1Cre-ERT2; Ai14 system should be validated by showing Tomato and UCP1 co-staining right after the Tamoxifen treatment.

Reviewer #2 (Public Review):

Summary:

In the present manuscript, Zhang et al utilize single-nuclei RNA-Seq to investigate the heterogeneity of perirenal adipose tissue. The perirenal depot is interesting because it contains both brown and white adipocytes, a subset of which undergo functional "whitening" during early development. While adipocyte thermogenic transdifferentiation has been previously reported, there remain many unanswered questions regarding this phenomenon and the mechanisms by which it is regulated.

Strengths:

The combination of UCP1-lineage tracing with the single nuclei analysis allowed the authors to identify four populations of adipocytes with differing thermogenic potential, including a "whitened" adipocyte (mPRAT-ad2) that retains the capacity to rapidly revert to a brown phenotype upon cold exposure. They also identify two populations of white adipocytes that do not undergo browning with acute cold exposure.

Anatomically distinct adipose depots display interesting functional differences, and this work contributes to our understanding of one of the few brown depots present in humans.

Weaknesses:

The most interesting aspect of this work is the identification of a highly plastic mature adipocyte population with the capacity to switch between a white and brown phenotype. The authors attempt to identify the transcriptional signature of this ad2 subpopulation, however, the limited sequencing depth of single nuclei somewhat lessens the impact of these findings. Furthermore, the lack of any form of mechanistic investigation into the regulation of mPRAT whitening limits the utility of this manuscript. However, the combination of well-executed lineage tracing with comprehensive cross-depot single-nuclei presented in this manuscript could still serve as a useful reference for the field.

Author Response

The following is the authors’ response to the original reviews.

Public Reviews:

The study could also valuably explore what kinds of genes experienced what forms of expression evolution. A brief description of GO terms frequently represented in genes which showed strong patterns of expression evolution might be suggestive of which selective pressures led to the changes in expression in the C. bursa-pastoris lineage, and to what extent they related to adaptation to polyploidization (e.g. cell-cycle regulators), compensating for the initial pollen and seed inviability or adapting to selfing (endosperm- or pollen-specific genes), or adaptation to abiotic conditions. ”

We did not include a gene ontology (GO) analysis in the first place as we did not have a clear expectation on the GO terms that would be enriched in the genes that are differentially expressed between resynthesized and natural allotetraploids. Even if we only consider adaptive changes, the modifications could occur in various aspects, such as stabilizing meiosis, adapting to the new cell size, reducing hybrid incompatibility and adapting to self-fertilization. And each of these modifications involves numerous biological processes and molecular functions. As we could make post-hoc stories for too many GO terms, extrapolating at this stage have limited implications and could be misleading.

Nonetheless, we are not the only study that compared newly resynthesized and established allopolyploids. GO terms that were repeatedly revealed by this type of exploratory analysis may give a hint for future studies. For this reason, now we have reported the results of a simple GO analysis.

Recommendations for the authors: please note that you control which, if any, revisions, to undertake

The majority of concerns from reviewers and the reviewing editor are in regards to the presentation of the manuscript; that the framing of the manuscript does not help the general reader understand how this work advances our knowledge of allopolyploid evolution in the broad sense. The manuscript may be challenging to read for those who aren't familiar with the study system or the genetic basis of polyploidy/gene expression regulation. Further, it is difficult to understand from the introduction how this work is novel compared to the recently published work from Duan et al and compared to other systems. Because eLife is a journal that caters to a broad readership, re-writing the introduction to bring home the novelty for the reader will be key.

Additionally, the writing is quite technical and contains many short-hands and acronyms that can be difficult to keep straight. Revising the full text for clarity (and additionally not using acronyms) would help highlight the findings for a larger audience.

Reviewer #1 (Recommendations For The Authors):

Most of my suggestions on this interesting and well-written study are minor changes to clarify the writing and the statistical approaches.

The use of abbreviations throughout for both transcriptional phenomena and lines is logical because of word limits, but for me as a reader, it really added to the cognitive burden. Even though writing out "homoeolog expression bias" or "hybridization-first" every time would add length, I would find it easier to follow and suspect others would too.

Thank you for this suggestion. Indeed, using less uncommon acronyms or short-hands should increase the readability of the text for broader audience. Now in most places, we refer to “Sd/Sh” and “Cbp” as “resynthesized allotetraploids” and “natural allotetraploids”, respectively. We have also replaced the most occurrences of the acronyms for transcriptional phenomena (ELD, HEB and TRE) with full phrases, unless there are extra attributes before them (such as “Cg-/Co-ELD” and “relic/Cbp-specific ELD”).

It would be helpful to include complete sample sizes to either a slightly modified Figure 1 or the beginning of the methods, just to reduce mental arithmetic ("Each of the five groups was represented by six "lines", and each line had six individuals" so there were 180 total plants, of which 167 were phenotyped - presumably the other 13 died? - and 30 were sequenced).

The number 167 only applied to floral morphorlogical traits (“Floral morphological traits were measured for all five groups on 167 plants…”), but the exact total sample size for other traits differed. Now the total sample sizes of other traits have also been added to beginning of the second paragraph of the methods.

For this study 180 seedings have been transplanted from Petri dishes to soil, but 8 seedlings died right after transplanting, seemingly caused by mechanical damage and insufficient moistening. Later phenotyping (2020.02-2020.05) was also disrupted by the COVID-19 pandemic, and some individuals were not measured as we missed the right life stages. Specifically, 5 individuals were missing for floral morphological traits (sepal width, sepal length, petal width, petal length, pistil width, pistil length, and stamen length), 30 for pollen traits, 1 for stem length, and 2 for flowering time. As for seed traits, we only measured individuals with more than ten fruits, so apart from the reasons mentioned above, individuals that were self-incompatible and had insufficient hand-pollination were also excluded. We spotted another mistake during the revision: two individuals with floral morphological measurements had no positional information (tray ID). These measurements were likely mis-sampled or mislabeled, and were therefore excluded from analysis. We assumed most of these missing values resulted from random technical mistakes and were not directly related to the measured traits.

In general, the methods did a thorough job of describing the genomics approaches but could have used more detail for the plant growth (were plants randomized in the growth chamber, can you rule out block/position effects) and basic statistics (what statistical software was used to perform which tests comparing groups in each section, after the categories were identified).

When describing the methods, mention whether the plants; this should be straightforward as a linear model with position as a covariate.

Data used in the present study and a previously published work (Duan et al., 2023) were different subsets of a single experiment. For this reason, we spent fewer words in describing shared methods in this manuscript but tried to summarize some methods that were essential for understanding the current paper. But as you have pointed out, we did miss many important details that should have been kept. Now we have added some description and a table (Supplementary file 1) in the “Plant material” section for explaining randomization, and added more information of the software used for performing statistic tests in the “Phenotyping” section.

Although we did not mention in the present manuscript, we used a randomized block design for the experiment (Author response image 1).

Author response image 1.

Plant positions inside the growth chamber.

Plants used in the present study and Duan et al. (2023) were different subsets of a single experiment. The entire experiment had eight plant groups, including the five plant groups used in the present study (diploid C. orientalis (Co2), diploid C. grandiflora (Cg2), “whole-genome-duplication-first” (Sd) and “hybridization-first”(Sh) resynthesized allotetraploids, and natural allotetraploids, C. bursa pastoris (Cbp), as well as three plant groups that were only used in Duan et al. (2023; tetraploid C. orientalis (Co4), tetraploid C. grandiflora (Cg4) and diploid hybrids (F)). Each of the eight plant groups had six lines and each line represented by six plants, resulting in 288 plants (8 groups x 6 lines x 6 individuals = 288 plants). The 288 plants were grown in 36 trays placed on six shelves inside the same growth chamber. Each tray had exactly one plant from each of the eight groups, and the position of the eight plants within each tray (A-H) were randomized with random.shuffle() method in Python (Supplementary file 1). The position of the 36 trays inside the growth room (1-36) was also random and the positions of all trays were shuffled once again 28 days after germination (randomized with RAND() and sorting in Microsoft Excel Spreadsheet). (a) Plant distribution; (b) An example of one tray; (c) A view inside the growth chamber, showing the six benches.

With the randomized block design and one round of shuffling, positional effect is very unlikely to bias the comparison among the five plant groups. The main risk of not adding positions to the statistical model is increasing error variance and decreasing the statistical power for detecting group effect. As we had already observed significant among-group variation in all phenotypic traits (p-value <2.2e-16 for group effect in most tests), further increasing statistical power is not our primary concern. In addition, during the experiment we did not notice obvious difference in plant growth related to positions. Although we could have added more variables to account for potential positional effects (tray ID, shelf ID, positions in a tray etc.), adding variables with little effect may reduce statistical power due to the loss of degree of freedom.

Due to one round of random shuffling, positions cannot be easily added as a single continuous variable. Now we have redone all the statistical tests on phenotypic traits and included tray ID as a categorical factor (Figure 2-Source Data 1). In general, the results were similar to the models without tray ID. The F-values of group effect was only slightly changed, and p-values were almost unchanged in most cases (still < 2.2e-16). The tray effect (df=35) was not significant in most tests and was only significant in petal length (p-value=0.0111), sepal length (p-value=0.0242) and the number of seeds in ten fruits (p-value=0.0367). As expected, positions (tray ID) had limited effect on phenotypic traits.

Figure 2 - I assume the numbers at the top indicate sample sizes but perhaps add this to the figure caption.

Statistical power depends on both the total sample size and the sample size of each group, especially the group with the fewest observations. We lost different number of measurements in each phenotypic trait, and for pollen traits we did have a notable loss, so we chose to show sample sizes above each group to increase transparency. Since we had five different sets of sample sizes (for floral morphological traits, stem length, days to flowering, pollen traits and seed traits, respectively), it would be cumbersome to introduce all 25 numbers in figure caption and could be hard for readers to match the sample sizes with results. For this reason, we would like to keep the sample sizes in the figure, and now we have modified the legend to clarify that the numbers above groups are sample sizes.

’The trend has been observed in a wide range of organisms, including ...’ - perhaps group Brassica and Raphanobrassica into one clause in the sentence, since separating them out undermines the diversity somewhat.

Indeed, it is very strange to put “cotton” between two representatives from Brassicaceae. Now the sentence is changed to “… including Brassica (Wu et al., 2018; Li et al., 2020; Wei et al., 2021) and Raphanobrassica (Ye et al., 2016), cotton (Yoo et al., 2013)…”

The diagrams under the graph in Figure 4B are particularly helpful for understanding the expression patterns under consideration! I appreciated them a lot!

Thank you for the comment. We also feel the direction of expression level dominance is convoluted and hard to remember, so we adopted the convention of showing the directions with diagrams.

Reviewer #2 (Recommendations For The Authors):

The science is very interesting and thorough, so my comments are mostly meant to improve the clarity of the manuscript text:

• I found it challenging to remember the acronyms for the different gene expression phenomena and had to consistently cross-reference different parts of the manuscript to remind myself. I think using the full phrase once or twice at the start of a paragraph to remind readers what the acronym stands for could improve readability.

Thank you for this reasonable suggestion. Now we have replaced the most occurrence of acronyms with the full phrases.

• There are some technical terms, such as "homoeologous synapsis" and "disomic inheritance", which I think are under-defined in the current text.

Indeed these terms were not well-defined before using in the manuscript. Now we have added a brief explanation for each term.

• Under the joint action of these forces, allopolyploid subgenomes are further coordinated and degenerated, and subgenomes are often biasedly fractionated" This sentence has some unclear terminology. Does "coordinated" mean co-adapted, co-inherited, or something else? Is "biasedly fractionated" referring to biased inheritance or evolution of one of the parental subgenomes?

We apologize for not using accurate terms. With “coordinated” we emphasized the evolution of both homoeologs depends on the selection on total expression of both homoeologs, and on both relative and absolute dosages, which may have shifted away from optima after allopolyploidization. “Co-evolved” or “co-adapted” might be a better word.

But the term "biasedly fractionation" has been commonly used for referring to the phenomenon that genes from one subgenome of polyploids are preferentially retained during diploidization (Woodhouse et al., 2014; Wendel, 2015). Instead of inventing a new term, we prefer to keep the same term for consistency, so readers could link our findings with numerous studies in this field. Now the sentence is changed to “Under the joint action of these forces, allopolyploid subgenomes are further co-adapted and degenerated, and subgenomes are often biasedly retained, termed biased fractionation”.

• There are a series of paragraphs in the results, starting with "Resynthesized allotetraploids and the natural Cbp had distinct floral morphologies", which consistently reference Figure 1 where they should be referencing Figure 2.

Thank you for spotting this mistake! Now the numbers have been corrected.

• ‘The number of pollen grains per flower decreased in natural Cbp’ this wording implies it's the effect of some experimental treatment on Cbp, rather than just measured natural variation.

Yes, it is not scientifically precise to say this in the Results section, especially when describing details of results. We meant that assuming resynthesized allopolyploids are good approximation of the initial state of natural allotetraploid C. bursa-pastoris, our results indicate that the number of pollen grains had decreased in natural C. bursa-pastoris. But this is an implication, rather than an observation, so the sentence is better rewritten as “Natural allotetraploids had less pollen grains per flower.”

• ‘The percentage of genes showing complete ELD was altogether limited but doubled between resynthesized allotetraploid groups and natural allotetraploids’ for clarity, I would suggest revising this to something like "doubled in natural allotetraploids relative to resynthesized allotetraploids

Thank you for the suggestion. The sentence has been revised as suggested.

• I'm not sure I understand what the difference is between expression-level dominance and homeolog expression bias. It seems to me like the former falls under the umbrella of the latter.

Expression-level dominance and homeolog expression bias are easily confused, but they are conceptually independent. One gene could have expression-level dominance without any homeolog expression bias, or strong homeolog expression bias without any expression-level dominance. The concepts were well explained in Grover et al., (2012) with nice figures.

Expression level dominance compares the total expression level of both homoeologs in allopolyploids with the expression of the same gene in parental species, and judges whether the total expression level in allopolyploids is only similar to one of the parental species. The contributions from different homoeologs are not distinguished.

While homoeolog expression bias compares the relative expression level of each homoeologs in allopolyploids, with no implication on the total expression of both homoeologs.

Let the expression level of one gene in parental species X and Y be e(X) and e(Y), respectively. And let the expression level of x homoeolog (from species X) and y homoeolog (from species Y) in allopolyploids be e(x) and e(y), respectively.

Then a (complete) expression level dominance toward species X means: e(x)+e(y)=e(X) and e(x)+e(y)≠e(Y);

While a homoeolog expression bias toward species X means: e(x) > e(y), or e(x)/e(y) > e(X)/e(Y), depending on the definition of studies.

Both expression-level dominance and homeolog expression bias have been widely studied in allopolyploids (Combes et al., 2013; Li et al., 2014; Yoo et al., 2014; Hu & Wendel, 2019). As the two phenomena could be in opposite directions, and may be caused by different mechanisms, we think adopting the definitions in Grover et al., (2012) and distinguishing the two concepts would facilitate communication.

• Is it possible to split up the results in Figure 7 to show which of the two homeologs was lost (i.e. orientalis vs. grandiflora)? Or at least clarify in the legend that these scenarios are pooled together in the figure?

Maybe using acronyms without explanation made the figure titles hard to understand, but in the original Figure 7 the loss of two homoeologs were shown separately. Figure 7a,c showed the loss of C. orientalis-homoeolog (“co-expession loss”), and Figure 7b,d showed the loss of C. grandiflora-homoeolog (“cg-expession loss”). Now the legends have been modified to explain the Figure.

• The paragraph starting with "The extant diploid species" is too long, should probably be split into two paragraphs and edited for clarity.

The whole paragraph was used to explain why the resynthesized allotetraploids could be a realistic approximation of the early stage of C. bursa-pastoris with two arguments:

1) The further divergence between C. grandiflora and C. orientalis after the formation of C. bursa-pastoris should be small compared to the total divergence between the two parental species; 2) The mating systems of real parental populations were most likely the same as today. Now the two arguments were separated as two paragraphs, and the second paragraph has been shortened.

• On the other hand, the number of seeds per fruit" implies this is evidence for an alternative hypothesis, when I think it's really just more support for the same idea.

“On the other hand” was used to contrast the reduced number of pollen grains and the increased number of seeds in natural allotetraploids. As both changes are typical selfing syndrome, indeed the two support the same idea. We replaced the “On the other hand” with “Moreover”.

• ‘has become self-compatible before the formation" "has become" should be "became".

The tense of the word has been changed.

• If natural C. bursa-pastoris indeed originated from the hybridization between C. grandiflora-like outcrossing plants and C. orientalis-like self-fertilizing plants, the selfing syndrome in C. bursa-pastoris does not reflect the instant dominance effect of the C. orientalis alleles, but evolved afterward.’ This sentence should be closer to the end of the paragraph, after the main morphological results are summarized.

Thank you for the suggestion. The paragraph is indeed more coherent after moving the conclusion sentence.

References

Combes, M.C., Dereeper, A., Severac, D., Bertrand, B. & Lashermes, P. (2013) Contribution of subgenomes to the transcriptome and their intertwined regulation in the allopolyploid Coffea arabica grown at contrasted temperatures. New Phytologist, 200, 251–260.

Grover, C.E., Gallagher, J.P., Szadkowski, E.P., Yoo, M.J., Flagel, L.E. & Wendel, J.F. (2012) Homoeolog expression bias and expression level dominance in allopolyploids. New Phytologist, 196, 966–971.

Hu, G. & Wendel, J.F. (2019) Cis – trans controls and regulatory novelty accompanying allopolyploidization. New Phytologist, 221, 1691–1700.

Li, A., Liu, D., Wu, J., Zhao, X., Hao, M., Geng, S., et al. (2014) mRNA and Small RNA Transcriptomes Reveal Insights into Dynamic Homoeolog Regulation of Allopolyploid Heterosis in

Nascent Hexaploid Wheat. The Plant Cell, 26, 1878–1900. Wendel, J.F. (2015) The wondrous cycles of polyploidy in plants. American Journal of Botany, 102, 1753–1756.

Woodhouse, M.R., Cheng, F., Pires, J.C., Lisch, D., Freeling, M. & Wang, X. (2014) Origin, inheritance, and gene regulatory consequences of genome dominance in polyploids. Proceedings of the National Academy of Sciences of the United States of America, 111, 5283–5288.

Yoo, M.J., Liu, X., Pires, J.C., Soltis, P.S. & Soltis, D.E. (2014) Nonadditive Gene Expression in Polyploids. https://doi.org/10.1146/annurev-genet-120213-092159, 48, 485–517.

Reviewer #3 (Public Review):

Summary:<br /> The authors aim to demonstrate the effectiveness of their developed methodology, which utilizes super-resolution microscopy and single-molecule tracking in live cells on a high-throughput scale. Their study focuses on measuring the diffusion state of a molecule target, the estrogen receptor, in both ligand-bound and unbound forms in live cells. By showcasing the ability to screen 5067 compounds and measure the diffusive state of the estrogen receptor for each compound in live cells, they illustrate the capability and power of their methodology.

Strengths:<br /> Readers are well introduced to the principles in the initial stages of the manuscript with highly convincing video examples. The methods and metrics used (fbound) are robust. The authors demonstrate high reproducibility of their screening method (R2=0.92). They also showcase the great sensitivity of their method in predicting the proliferation/viability state of cells (R2=0.84). The outcome of the screen is sound, with multiple compounds clustering identified in line with known estrogen receptor biology.

Weaknesses:<br /> - Potential overstatement on the relationship of low diffusion state of ER bound to compound and chromatin state without any work on chromatin level.<br /> - Could the authors clarify if the identified lead compound effects are novel at any level?<br /> - More video example cases on the final lead compounds identified would be a good addition to the current data package.

eLife assessment

This work presents an important technological advance, in the form of a high throughput platform for Single Particle Tracking allowing us to measure millions of cells and thousands of compounds per day. Analysis of the diffusional behaviour of fluorescently-tagged targets permits the identification of, and differentiation between, small molecules that bind directly or affect the target indirectly. The evidence provided is compelling, although some methodological information is undisclosed.

Reviewer #1 (Public Review):

Summary:<br /> The authors set up a pipeline for automated high-throughput single-molecule fluorescence imaging (htSMT) in living cells and analysis of molecular dynamics.

Strengths:<br /> htSMT reveals information on the diffusion and bound fraction of molecules, dose-response curves, relative estimates of binding rates, and temporal changes of parameters. It enables the screening of thousands of compounds in a reasonable time and proves to be more sensitive and faster than classical cell-growth assays. If the function of a compound is coupled to the mobility of the protein of interest, or affects an interaction partner, which modulates the mobility of the protein of interest, htSMT allows identifying the modulator and getting the first indication of the mechanism of action or interaction networks, which can be a starting point for more in-depth analysis.

Weaknesses:<br /> While elegantly showcasing the power of high-throughput measurements, the authors disclose little information on their microscope setup and analysis procedures. Thus, reproduction by other scientists is limited. Moreover, a critical discussion about the limits of the approach in determining dynamic parameters, the mechanism of action of compounds, and network reconstruction for the protein of interest is missing. In addition, automated imaging and analysis procedures require implementing sensitive measures to assure data and analysis quality, but a description of such measures is missing.

Reviewer #2 (Public Review):

Summary:<br /> McSwiggen et al present a high throughput platform for SPT that allows them to identify pharmaceutics interactions with the diffusional behavior of receptors and in turn to identify potent new ligands and cellular mechanisms. The manuscript is well written, it provides a solid new mentor and a proper experimental foundation

Strengths:<br /> The method capitalizes and extends to existing high throughput toolboxes and is directly applied to multiple receptors and ligands. The outcomes are important and relevant for society. 10^6 cells and >400 ligands per is a significant achievement.

The method can detect functionally relevant changes in transcription factor dynamics and accurately differentiate the ligand/target specificity directly within the cellular environment. This will be instrumental in screening libraries of compounds to identify starting points for the development of new therapeutics. Identifying hitherto unknown networks of biochemical signaling pathways will propel the field of single-particle live cell and quantitative microscopy in the area of diagnostics. The manuscript is well-written and clearly conveys its message.

Weaknesses:<br /> There are a few elements, that if rectified would improve the claims of the manuscript.

The authors claim that they measure receptor dynamics. In essence, their readout is a variation in diffusional behavior that correlates to ligand binding. While ligand binding can result in altered dynamics or /and shift in conformational equilibrium, SPT is not recording directly protein structural dynamics, but their effect on diffusion. They should correct and elaborate on this.

L 148 What do the authors mean 'No correlation between diffusion and monomeric protein size was observed, highlighting the differences between cellular protein dynamics versus purified systems'. This is not justified by data here or literature reference. How do the authors know these are individual molecules? Intensity distributions or single bleaching steps should be presented.

Along the same lines, the data in Figs 2 and 4 show that not only the immobile fraction is increased but also that the diffusion coefficient of the fast-moving (attributed to free) is reduced. The authors mention this and show an extended Fig 5 but do not provide an explanation. How do potential transient ligand binding and the time-dependent heterogeneity in motion (see comment above) contribute to this? Also, in line 216 the authors write "with no evidence" of transient diffusive states. How do they define transient diffusive states? While there are toolboxes to directly extract the existence and abundance of these either by HMM analysis or temporal segmentation, the authors do not discuss or use them.

The authors discuss the methods for extracting kinetic information of ligand binding by diffusion. They should consider the temporal segmentation of heterogenous diffusion. There are numerous methods published in journals or BioRxiv based on analytical or deep learning tools to perform temporal segmentation. This could elevate their analysis of Kon and Koff.

Reviewer #1 (Public Review):

Farhat-Younis and colleagues demonstrate tumor-specific IgM's capacity to induce tumor cell death in monocyte-derived dendritic cell cultures. They subsequently designed a chimeric receptor based on high-affinity FcRI. However, the authors found that the transfection process was more efficient when either the variable light or heavy chain was transfected individually rather than the entire scFv. This scFv construct led to an endoplasmic reticulum (ER) stress response and scFv degradation. A considerable portion of the manuscript is dedicated to the negative scFv expression results. The authors pivoted to a modified FcgRI capable of transmitting IgM signals. This represents a tremendous amount of work in the development of this chimeric receptor, the critical experiment showing efficacy in vivo was not presented, and instead various in vitro assays are shown. Thus, this manuscript will markedly benefit from showing improved responses to tumors in vivo when macrophages express FcgRI-IgM.

1. In a mouse tumor model, the authors demonstrated that monocyte-derived dendritic cells (MoDCs) treated with IgG immune complexes (ICs) were more effective at preventing tumor growth compared to those treated with IgM ICs (as shown in Figure 1B). In Figure 1C, their in vitro experiments revealed that IgM resulted in tumor cell death, as well as increased production of nitric oxide (NO) and granzyme B.<br /> How do the authors reconcile IgG IC-treated MoDCs performing better in preventing tumors in vivo than IgM IC-treated MoDCs, despite the in vitro results with IgM-ICs. The authors speculate that IgG IC-treated MoDCs might trigger T cell immunity but do not show T cell involvement.

2. The authors report distinct functional consequences of MoDCs incubated with tumor-IgG complexes and tumor IgM complexes. Tumor growth was inhibited and T cell immunity induced with the former. The latter, however, elicited robust anti-tumor killing. What happens if MoDCs are incubated with both IgG and IgM complexes? If this combined treatment induces effective killing and T cell memory, would this impact the design of the chimeric receptor to include IgG responsiveness as well?

3. In Figure 5H, the authors demonstrate the ability of the chimeric receptor construct to deplete tumor cells in vitro. The ms would improve if the authors could show the chimeric receptor construct results in tumor cell death and/or prevention in an in vivo model. Similarly, if combined stimulation with IgG and IgM complexes enhances tumor response, this should be incorporated into the therapeutic strategy.

eLife assessment

The significance of this work is important in that the authors propose a novel method to therapeutically harness myeloid cells which can be otherwise immunosuppressive and hamper T cell and immunotherapy responses. The strength of evidence is convincing but requires critical pieces of in vivo work to validate the therapeutic efficacy of this approach.

Reviewer #2 (Public Review):

Summary:

While a significant portion of immunotherapy research has focused on the pivotal role of T cells in tumor immunity, their effectiveness may be limited by the suppressive nature of the tumor environment. On the other hand, myeloid cells are commonly found within tumors and can withstand these adverse conditions. However, these cells often adopt an immunosuppressive phenotype when infiltrating tumors. Therefore, manipulating myeloid cells could potentially enhance the anti-tumor potential of immunotherapy.

In this manuscript, Farhat-Younes and colleagues have demonstrated that activating the IgM receptor signaling in myeloid cells induces an oxygen burst, the secretion of Granzyme B, and the lysis of adjacent tumor cells. Furthermore, they have outlined a strategy to utilize these features to generate CAR macrophages. However, they have identified a limitation: the expression of scFv in myeloid cells induces ER stress and the degradation of misfolded proteins. To address this issue, chimeric receptors were designed based on the high-affinity FcγRI for IgG. When macrophages transfected with these receptors were exposed to tumor-binding IgG, extensive tumor cell killing, and the release of reactive oxygen species and Granzyme B were observed.

Strengths:<br /> In general, I consider this work to be significant, and the results are compelling. It emphasizes the specific considerations and requirements for successful manipulation in myeloid cells, which could further advance the field of cellular engineering for the benefit of immunotherapy

Weaknesses:

Nevertheless, there are several minor issues that should be addressed:

1- TCR fragments are commonly used to induce ER stress in non-immune cells. Therefore, it would be interesting to investigate whether TCR fragments can be expressed in myeloid cells and if they induce ER stress. Addressing this issue would support the notion that these cells lack the ER chaperones required for folding immunoglobulin variable chains.<br /> 2- It would be valuable to determine whether, after the degradation of scFv fragments by myeloid cells, they are presented on MHC-I and MHC-II.<br /> 3- Some methodological details, such as the vaccination protocol and high-resolution microscopy procedures, are missing from the text.

eLife assessment

This important study combines disparate results from both psychophysics and neural silencing experiments to suggest a new interpretation of how animals and humans represent and interpret recent events in our memory. A key aspect of the model put forward here is the presence of discrete jumps in neural activity within the posterior parietal region of the cortex. The model is distinct from other models, and the authors provide convincing evidence to support it both from existing results as well as from novel experiments.

Reviewer #1 (Public Review):

This paper aims to explain recent experimental results that showed deactivating the PPC in rats reduced both the contraction bias and the recent history bias during working memory tasks. The authors propose a two-component attractor model, with a slow PPC area and a faster WM area (perhaps mPFC, but unspecified). Crucially, the PPC memory has slow adaptation that causes it to eventually decay and then suddenly jump to the value of the last stimulus. These discrete jumps lead to an effective sampling of the distribution of stimuli, as opposed to a gradual drift towards the mean that was proposed by other models. Because these jumps are single-trial events, and behavior on single events is binary, various statistical measures are proposed to support this model. To facilitate this comparison, the authors derive a simple probabilistic model that is consistent with both the mechanistic model and behavioral data from humans and rats. The authors show data consistent with model predictions: longer interstimulus intervals (ISIs) increase biases due to a longer effect over the WM, while longer intertrial intervals (ITIs) reduce biases. Finally, they perform new experiments using skewed or bimodal stimulus distributions, in which the new model better fits the data compared to Bayesian models.

The mechanistic proposed model is simple and elegant, and it captures both biases that were previously observed in behavior, and how these are affected by the ISI and ITI (as explained above). Their findings help rethink whether our understanding of contraction bias is correct.

On the other hand, the main proposal - discrete jumps in PPC - is only indirectly verified. The majority of the behavioral predictions stem from the probabilistic model, which is consistent with the mechanistic one, but does not necessitate it.<br /> The revised submission uses the self-paced nature of the experiments to confirm the systematic change in bias with inter-trial-interval, as predicted by the model. This analysis strengthens the hypothesis.

Reviewer #2 (Public Review):

Working memory is not error free. Behavioral reports of items held in working memory display several types of bias, including contraction bias and serial dependence. Recent work from Akrami and colleagues demonstrates that inactivating rodent PPC reduces both forms of bias, raising the possibility of a common cause.

In the present study, Boboeva, Pezotta, Clopath, and Akrami introduce circuit and descriptive variants of a model in which the contents of working memory can be replaced samples from recent sensory history. This volatility manifests as contraction bias and serial dependence in simulated behavior, parsimoniously explaining both sources of bias. The authors validate their model by showing that it can recapitulate previously published and novel behavioral results in rodents and neurotypical and atypical humans.

Both the modeling and the experimental work is rigorous, providing convincing evidence that a model of working memory in which reports sometimes sample past experience can produce both contraction bias and serial dependence, and that this model is consistent with behavioral observations across rodents and humans in the parametric working memory (PWM) task.

These efforts constitute an admirable initial validation of the proposed model, and the authors present several novel predictions that will allow for further tests in future experiments. First, the authors note that their circuit model predicts a bimodal error distribution in delayed estimation paradigms (due to noisy sampling of sensory history on a subset of trials) that merges into a unimodal distribution when recent sensory history and the current to-be-reported stimulus have very similar values (Fig. 5c). Analysis of extent delayed estimation datasets (e.g., https://osf.io/jmkc9/) or new experiments will provide the opportunity for a straightforward test of this hypothesis.

Second, the bulk of the modeling efforts presented here are devoted to a circuit-level description of how putative posterior parietal cortex (PPC) and working-memory (WM) related networks may interact to produce such volatility and biases in memory. This effort is extremely useful because it allows the model to be constrained by neural observations and manipulations in addition to behavior, and the authors begin this line of inquiry here (by showing that the circuit model can account for effects of optogenetic inactivation of rodent PPC). As the authors note, population electrophysiology in PPC and WM-related areas and single-trial analyses will play an important role in the ultimate validation of this model.

Finally, it is noteworthy that, in the spirit of moving away from an overreliance on p-values (e.g., Amrhein et al., PeerJ 2017), the authors eschew conventional hypothesis testing when reporting their experimental results. The p-values aren't missed, in large part thanks to excellent visualizations and apparently large effect sizes. It's unclear how well this approach would generalize to other questions and datasets; nevertheless, this study provides an interesting data point in the ongoing conversation around reproducibility and rigor.

Reviewer #1 (Public Review):

In this manuscript, the authors describe an improved miniscope they name "E-scope" combining in vivo calcium imaging with electrophysiological recording and use it to examine neural correlates of social interactions with respect to cerebellar and cortical circuits. Through correlations between electrophysiological single units of Purkinje cells and dentate nucleus neurons as well as with calcium signals imaging of neurons from the anterior cingulate cortex, the authors provide correlative data supporting the view that intracerebellar circuits and cerebello-cortical communications take part in the modulation of social behavior. In particular, the electrophysiological dataset reflects the PC-DN connection and strongly suggests its involvement in social interactions. Cross-correlations analyses between PC / DN single units and ACC calcium signals suggest that the recorded cerebellar and cortical structures both take part in the brain networks at play in social behavior.

Comments on revised submission:

While the authors have, to some extent, replied to most of my comments, they seem to have chosen not to respond to the part concerning the different types of social interactions that are not addressed in the manuscript, as also pointed out by reviewer 3. However, I feel that given the scope of the paper, which aims at demonstrating the value of the E-scope new device, this should not preclude the current study from being published.

Reviewer #2 (Public Review):

This report by Hur et al. examines simultaneous activity in the cerebellum and anterior cingulate cortex (ACC) to determine how activity in these regions is coordinated during social behavior. To accomplish this, the authors developed a recording device named the E-scope, which combines a head-mounted mini-scope for in vivo Ca2+ imaging with an extracellular recording probe (in the manuscript they use a 32-channel silicon probe). Using the E-scope, the authors find subpopulations of cerebellar neurons with social-interaction-related activity changes. The activity pattern is predominantly decreased firing in PCs and increases in DNs, which is the expected reciprocal relationship between these populations. They also find social-interaction-related activity in the ACC. The authors nicely show the absence of locomotion onset and offset activity in PCs and DNs ruling out that is movement driven. Analysis showed high correlations between cerebellar and ACC populations (namely, Soc+ACC and Soc+DN cells). The finding of correlated activity is interesting because non-motor functions of the cerebellum are relatively little explored. However, the causal relationship is far from established with the methods used, leaving it unclear if these two brain regions are similarly engaged by the behavior or if they form a pathway/loop. Overall, the data are presented clearly, and the manuscript is well written, however the biological insight gained is rather limited.

Reviewer #3 (Public Review):

Complex behavior requires complex neural control involving multiple brain regions. The currently available tools to measure neural activity in multiple brain regions in small animals are limited and often involve obligatory head-fixation. The latter, obviously, impacts the behaviors under study. Hur and colleagues present a novel recording device, the E-Scope, that combines optical imaging of fluorescent calcium imaging in one brain region with high-density electrodes in another. Importantly, the E-Scope can be implanted and is, therefore, compatible with usage in freely moving mice. The authors used their new E-Scope to study neural activity during social interactions in mice. They demonstrate the presence of neural correlates of social interaction that happen simultaneously in the cerebellum and the anterior cingulate cortex.

The major accomplishment of this study is the development and introduction of the E-Scope. The evaluation of this part can be short: it works, so the authors succeeded.

The authors managed to reduce the weight of the implant to 4.5 g, which is - given all functionality - quite an accomplishment in my view. However, a mouse weighs between 20 and 40 g, so that an implant of 4.5 g is still quite considerable. It can be expected that this has an impact on the behavior and, possibly, the well-being of the animals. Whether this is the case or not, is not really addressed in this study. The authors suffice with the statement that "Recorded animals made more contact with the other mouse than with the object (Figure 2A), suggesting a normal preference for social contact with the E-Scope attached." A direct comparison between mice before and after implant, or between mice with and without an implant would provide more insight into the putative impact of the E-Scope on (social) behavior.

In Figure 1 D-G, the authors present raw data from the neurophysiological recordings. In panel D, we see events with vastly different amplitudes. It would be very insightful if the authors would describe which events they considered to be action potentials, and which not. Similarly, indicating the detected complex spikes in the raw traces of Figure 1E would provide more insight into the interpretation of the data. Although the authors mention to consider the occurrence of complex spikes and simple spikes, a clear definition of what is considered a single unit recording is lacking. As there is quite a wide range in reported firing rates in Figure 2 - figure supplement 3, more clarity on this aspect would be insightful. Furthermore, in their text, the authors state that the pause in simple spike firing following a complex spike normally lasts until around 40 ms, and for this statement they refer to Figure 1G that shows a pause of less than 10 ms.

The number of Purkinje cells recorded during social interactions is quite low: only 11 cells showed a modulation in their spiking activity (unclear whether in complex spikes, simple spikes or both. During object interaction, only 4 cells showed a significant modulation. Unclear is whether the latter 4 are a subset of the former 11, or whether "social cells" and "object cells" are different categories. Having so few cells, and with these having different types of modulation, the group of cells for each type of modulation is really small, going down to 2 cells/group. The small group sizes complicate the interpretation of the data - in particular also on the analysis of movement-related activity that is now very noisy (Figure 2 - figure supplement 4).

In conclusion, the authors present a novel method to record neural activity with single cell-resolution in two brain regions in freely moving mice. Given the challenges associated with understanding of complex behaviors, this approach can be useful for many neuroscientists. The authors demonstrate the potential of their approach by studying social interactions in mice. Clearly, there are correlations in activity of neurons in the anterior cingulate cortex and the cerebellum related to social interactions. To bring our understanding of these patterns to a higher level, more detailed analyses (and probably also larger group sizes of cerebellar neurons) are required, though.

eLife assessment

This paper reports valuable results regarding the potential role and time course of the prefrontal cortex in conscious perception. Although the sample size is small, the results are convincing, and strengths include the use of several complementary analysis methods. The behavioral test includes subject report such that the study does not allow for distinguishing between (phenomenal) awareness and conscious access; nevertheless, results do advance our understanding of the contribution of prefrontal cortex to conscious perception.

Reviewer #1 (Public Review):

This is a clear and rigorous study of intracranial EEG signals in the prefrontal cortex during a visual awareness task. The results are convincing and worthwhile, and strengths include the use of several complementary analysis methods and clear results. The only methodological weakness is relatively small sample size of only 6 participants compared to other studies in the field. Interpretation weaknesses are claims that their task removes the confound of report (it does not), and claims of primacy in showing early prefrontal cortical involvement in visual perception using intracranial EEG (several studies already have shown this). Also the shorter reaction times for perceived vs not perceived stimuli (confident vs not confident responses) has been described many times previously and is not a new result.

Reviewer #2 (Public Review):

The authors attempt to address a long-standing controversy in the study of the neural correlates of visual awareness, namely whether neurons in prefrontal cortex are necessarily involved in conscious perception. Several leading theories of consciousness propose a necessary role for (at least some sub-regions of) PFC in basic perceptual awareness (e.g., global neuronal workspace theory, higher order theories), while several other leading theories posit that much of the previously reported PFC contributions to perceptual awareness may have been confounded by task-based cognition that co-varied between the aware and unaware reports (e.g., recurrent processing theory, integrated information theory). By employing intracranial EEG in human patients and a threshold detection task on low-contrast visual stimuli, the authors assessed the timing and location of neural populations in PFC that are differentially activated by stimuli that are consciously perceived vs. not perceived. Overall, the reported results support the view that certain regions of PFC do contribute to visual awareness, but at time-points earlier than traditionally predicted by GNWT and HOTs.

Major strengths of this paper include the straightforward visual threshold detection task including the careful calibration of the stimuli and the separate set of healthy control subjects used for validation of the behavioral and eye tracking results, the high quality of the neural data in six epilepsy patients, the clear patterns of differential high gamma activity and temporal generalization of decoding for seen versus unseen stimuli, and the authors' interpretation of these results within the larger research literature on this topic. This study appears to have been carefully conducted, the data were analyzed appropriately, and the overall conclusions seem warranted given the main patterns of results.

Weaknesses include the saccadic reaction time results and the potential flaws in the design of the reporting task. As the authors acknowledge, this is not a "no report" paradigm, rather, it's a paradigm aimed at balancing the post-perceptual cognitive and motor requirements between the seen and unseen trials. On each trial, subjects/patients either perceived the stimulus or not, and had to briefly maintain this "yes/no" judgment until a fixation cross changed color, and the color change indicated how to respond (saccade to the left or right). Differences in saccadic RTs (measured from the time of the fixation color change to moving the eyes to the left or right response square) were evident between the seen and unseen trials (faster for seen). In the discussion, the authors summarize several alternative explanations of the saccade results and limitations of their report paradigm that will help guide future research.

The current results help advance our understanding of the contribution of PFC to visual awareness. These results, when situated within the larger context of the rapidly developing literature on this topic provide converging evidence that some sub-regions of PFC contribute to visual awareness, but at latencies earlier than originally predicted by proponents of, especially, global neuronal workspace theory. Three recent studies that used "no report paradigms", but with clearly visible stimuli, reported very similar results in PFC (Vishne et al., 2023; Broday-Dvir et al., 2023; Cogitate et al., 2023). The current study uses a report paradigm, but with consciously seen vs. unseen conditions, to fill the gap left by these previous studies, i.e., it remained unclear whether the PFC results from the previous studies were related to conscious or unconscious processing. Taken as a whole, evidence appears to be converging for a limited and early-in-time (200-300ms) contribution of PFC to visual awareness, after task and motor confounds are minimized.

Reviewer #3 (Public Review):

The authors report a study in which they use intracranial recordings to dissociate subjectively aware and subjectively unaware stimuli, focusing mainly on prefrontal cortex.

The authors have dealt successfully with some of my previous concerns, especially the more direct link to the Gaillard et al., (2009) paper, and the associated analyses, has improved the manuscript. Some of my other concerns regarding the theoretical embedding of the findings have only been partially mitigated and some interesting results derived from suggestions for additional analyses will be used for future papers.

eLife assessment

This study provides an important starting point for unraveling the molecular basis of the pathological phenotypes of the repeat expansion in the gene associated with open reading frame 72 in human chromosome 9. The coarse-grained simulation method used by the authors goes beyond the state of the art, investigating a compelling number of binding partners. The evidence supporting the claims of the authors is solid, although experimental validation of the results would strengthen the major conclusions of the work. The work will be of broad interest to biophysicists and biochemists.

Reviewer #1 (Public Review):

Jafarinia et al. have made an interesting contribution to unravel the molecular mechanisms underlying pathological phenotypes of repeat expansion of the C9orf72 gene.

The repeat expression leads to expression of polyPR proteins. Using coarse-grained molecular dynamics simulations, the authors identify putative binding partners involved in nucleocytoplasmic transport (NCT), and conjecture that polyPR affects essential processes by binding to NCT-related proteins.

The results are well-reported, but only putative, and need experimental support to be more conclusive. Also, comparison with results from all-atom MD simulations in explicit water could help verify the results. But even without these, the work is very useful as a first step to unravel the role of polyPR and related peptides.

Reviewer #3 (Public Review):

Summary:<br /> Onck and co-workers present in this work the identification of binding partners and sites of polyPR on various nuclear transport components and elucidate how polyPR might potentially influence the transport process. It's interesting to note that some interaction sites on transport components also serve as their inherent/functional binding sites (Figure 3). The difference in the effects between short polyPR (PR7) and long polyPR (PR50) is also evident, although the authors might need to clarify the mechanisms better. Overall, I find this manuscript well organized and concisely written, and it would greatly enhance our understanding of the toxicity induced by polyPR.

Strengths:<br /> The 1-bead per atom force field model used in the study is well-tuned for studying the interactions between polyPR and proteins, as the essential cation-pi interactions (between Arg and Phe/Tyr/Trp) was included using a 8-6 LJ model.

Weaknesses:<br /> To cite the author's response: "At the moment, accurately capturing the binding of NCT components to their native binding targets and the competition with polyPR are best resolved by all-atom molecular dynamics simulations, which come with significant computational demands. This level of detail and computation-intensive analyses is beyond the scope of the current study."

eLife assessment

In this important study, Guma and colleagues describe the use of structural neuroimaging to assess the cross-species convergence of sex differences in global and regional brain volumes in humans and mice. The goal of the work is to inform to what extent mouse studies of these aforementioned sex differences have relevance to humans. The authors suggest which aspects of brain anatomy (as measured by volume) are conserved or not, across species, which has theoretical and practical implications beyond a single sub-field. The evidence to support the findings is solid, it uses methods and data analysis that are appropriate and validated.

Joint Public Review:

Summary:<br /> Guma and colleagues set out to compare to what extent differences in total and regional brain volumes, as measured by structural magnetic resonance imaging (MRI) are conserved or not, between humans and mice. The rationale for this work is to inform the best use of the mouse as a model system to carry out mechanistic studies of how sex differences arise in brain volumes, based on convergence to humans. This has practical implications for multiple fields in neuroscience. The authors find a modest convergence on these measures highlighting important areas for further mechanistic study.

Strengths:<br /> The main strengths of the study lie in the use of a cross-species technology, i.e. structural MRI, using tools and methods that have been extensively validated.

Weaknesses:<br /> Limitations of the study include, as acknowledged by the authors, the focus on a specific age range in mice and humans (which may not be congruent) and the lack of information regarding sex differences earlier or later in life. This has relevance with regard to the ages of onset for psychiatric and neurological disorders for example, which show apparent sex differences in prevalence. The paper also does provide data for an intermediate phylogenic level of analysis, such as data from primates. Lastly, these data do not provide any evidence as to the mechanisms underlying sex differences, when they arise, and to what extent they impact behavior.

eLife assessment

This important work advances our understanding of how brains flexibly gate actions in different contexts, based on dynamically reconfiguring neural dynamics in motor circuits. The findings, using analyses of many neurons recorded simultaneously during mouse behavior, as well as causal perturbations, are clear and compelling. This work will be of interest to systems neuroscientists and to researchers studying context-dependent computation generally.

Reviewer #1 (Public Review):

Summary: Using a cross-modal sensory selection task in head-fixed mice, the authors attempted to characterize how different rules reconfigured representations of sensory stimuli and behavioral reports in sensory (S1, S2) and premotor cortical areas (medial motor cortex or MM, and ALM). They used silicon probe recordings during behavior, a combination of single-cell and population-level analyses of neural data, and optogenetic inhibition during the task.

Strengths: A major strength of the manuscript was the clarity of the writing and motivation for experiments and analyses. The behavioral paradigm is somewhat simple but well-designed and well-controlled. The neural analyses were sophisticated, clearly presented, and generally supported the authors' interpretations. The statistics are clearly reported and easy to interpret. In general, my view is that the authors achieved their aims. They found that different rules affected preparatory activity in premotor areas, but not sensory areas, consistent with dynamical systems perspectives in the field that hold that initial conditions are important for determining trial-based dynamics.

Weaknesses: The manuscript was generally strong. The main weakness in my view was in interpreting the optogenetic results. While the simplicity of the task was helpful for analyzing the neural data, I think it limited the informativeness of the perturbation experiments. The behavioral read-out was low dimensional -a change in hit rate or false alarm rate- but it was unclear what perceptual or cognitive process was disrupted that led to changes in these read-outs. This is a challenge for the field, and not just this paper, but was the main weakness in my view. I have some minor technical comments in the recommendations for authors that might address other minor weaknesses.

I think this is a well-performed, well-written, and interesting study that shows differences in rule representations in sensory and premotor areas and finds that rules reconfigure preparatory activity in the motor cortex to support flexible behavior.

Reviewer #2 (Public Review):

Summary:<br /> Chang et al. investigate neuronal activity firing patterns across various cortical regions in an interesting context-dependent tactile vs visual detection task, developed previously by the authors (Chevee et al., 2021; doi: 10.1016/j.neuron.2021.11.013). The authors report the important involvement of a medial frontal cortical region (MM, probably a similar location to wM2 as described in Esmaeili et al., 2021 & 2022; doi: 10.1016/j.neuron.2021.05.005; doi: 10.1371/journal.pbio.3001667) in mice for determining task rules.

Strengths:<br /> The experiments appear to have been well carried out and the data well analysed. The manuscript clearly describes the motivation for the analyses and reaches clear and well-justified conclusions. I find the manuscript interesting and exciting!

Weaknesses:<br /> I did not find any major weaknesses.

Reviewer #3 (Public Review):

This study examines context-dependent stimulus selection by recording neural activity from several sensory and motor cortical areas along a sensorimotor pathway, including S1, S2, MM, and ALM. Mice are trained to either withhold licking or perform directional licking in response to visual or tactile stimulus. Depending on the task rule, the mice have to respond to one stimulus modality while ignoring the other. Neural activity to the same tactile stimulus is modulated by task in all the areas recorded, with significant activity changes in a subset of neurons and population activity occupying distinct activity subspaces. Recordings further reveal a contextual signal in the pre-stimulus baseline activity that differentiates task context. This signal is correlated with subsequent task modulation of stimulus activity. Comparison across brain areas shows that this contextual signal is stronger in frontal cortical regions than in sensory regions. Analyses link this signal to behavior by showing that it tracks the behavioral performance switch during task rule transitions. Silencing activity in frontal cortical regions during the baseline period impairs behavioral performance.

Overall, this is a superb study with solid results and thorough controls. The results are relevant for context-specific neural computation and provide a neural substrate that will surely inspire follow-up mechanistic investigations. We only have a couple of suggestions to help the authors further improve the paper.

1. We have a comment regarding the calculation of the choice CD in Fig S3. The text on page 7 concludes that "Choice coding dimensions change with task rule". However, the motor choice response is different across blocks, i.e. lick right vs. no lick for one task and lick left vs. no lick for the other task. Therefore, the differences in the choice CD may be simply due to the motor response being different across the tasks and not due to the task rule per se. The authors may consider adding this caveat in their interpretation. This should not affect their main conclusion.

2. We have a couple of questions about the effect size on single neurons vs. population dynamics. From Fig 1, about 20% of neurons in frontal cortical regions show task rule modulation in their stimulus activity. This seems like a small effect in terms of population dynamics. There is somewhat of a disconnect from Figs 4 and S3 (for stimulus CD), which show remarkably low subspace overlap in population activity across tasks. Can the authors help bridge this disconnect? Is this because the neurons showing a difference in Fig 1 are disproportionally stimulus selective neurons?

eLife assessment

This manuscript reveals important insights into the role of ipsilateral descending pathways in locomotion, especially following unilateral spinal cord injury. The study provides solid evidence that this method improves the injured side's ability to support weight, and as such the findings may lead to new treatments for stroke, spinal cord injuries, or unilateral cerebral injuries. However, the methods and results need to be better detailed, and some of the statistical analysis enhanced.

Reviewer #1 (Public Review):

Summary:<br /> This manuscript provides potentially important new information about ipsilateral cortical impact on locomotion. A number of issues need to be addressed.

Strengths:<br /> The primary appeal and contribution of this manuscript are that it provides a range of different measures of ipsilateral cortical impact on locomotion in the setting of impaired contralateral control. While the pathways and mechanisms underlying these various measures are not fully defined and their functional impacts remain uncertain, they comprise a rich body of results that can inform and guide future efforts to understand cortical control of locomotion and to develop more effective rehabilitation protocols.

Weaknesses:

1. The authors state that they used a cortical stimulation location that produced the largest ankle flexion response (lines 102-104). Did other stimulation locations always produce similar, but smaller responses (aside from the two rats that showed ipsilateral neuromodulation)? Was there any site-specific difference in response to stimulation location?

2. Figure 2: There does not appear to be a strong relationship between the percentage of spared tissue and the ladder score. For example, the animal with the mild injury (based on its ladder score) in the lower left corner of Figure 2A has less than 50% spared tissue, which is less spared tissue than in any animal other than the two severe injuries with the most tissue loss. Is it possible that the ladder test does not capture the deficits produced by this spinal cord injury? Have the authors looked for a region of the spinal cord that correlates better with the deficits that the ladder test produces? The extent of damage to the region at the base of the dorsal column containing the corticospinal tract would be an appropriate target area to quantify and compare with functional measures.

3. Lines 219-221: The authors state that "phase-coherent stimulation reinstated the function of this muscle, leading to increased burst duration (90{plus minus}18% of the deficit, p=0.004, t-test, Fig. 4B) and total activation (56{plus minus}13% of the deficit, p=0.014, t-test, Fig. 3B). This way of expressing the data is unclear. For example, the previous sentence states that after SCI, burst duration decreased by 72%. Does this mean that the burst duration after stimulation was 90% higher than the -72% level seen with SCI alone, i.e., 90% + -72% = +18%? Or does it mean that the stimulation recovered 90% of the portion of the burst duration that had been lost after SCI, i.e., -72% * (100%-90%)= -7%? The data in Figure 4 suggests the latter. It would be clearer to express both these SCI alone and SCI plus stimulation results in the text as a percent of the pre-SCI results, as done in Figure 4.

4. Lines 227-229: The authors claim that the phase-dependent stimulation effects in SCI rats are immediate, but they don't say how long it takes for these effects to be expressed. Are these effects evident in the response to the first stimulus train, or does it take seconds or minutes for the effects to be expressed? After the initial expression of these effects, are there any gradual changes in the responses over time, e.g., habituation or potentiation?

5. Awake motor maps (lines 250-277): The analysis of the motor maps appears to be based on measurements of the percentage of channels in which a response can be detected. This analytic approach seems incomplete in that it only assesses the spatial aspect of the cortical drive to the musculature. One channel could have a just-above-threshold response, while another could have a large response; in either case, the two channels would be treated as the same positive result. An additional analysis that takes response intensity into account would add further insight into the data, and might even correlate with the measures of functional recovery. Also, a single stimulation intensity was used; the results may have been different at different stimulus intensities.

6. Lines 858-860: The authors state that "All tests were one-sided because all hypotheses were strictly defined in the direction of motor improvement." By using the one-sided test, the authors are using a lower standard for assessing statistical significance that the overwhelming majority of studies in this field use. More importantly, ipsilateral stimulation of particular kinds or particular sites might conceivably impair function, and that is ignored if the analysis is confined to detecting improvement. Thus, a two-sided analysis or comparable method should be used. This appropriate change would not greatly modify the authors' current conclusions about improvements.

Reviewer #2 (Public Review):

Summary:<br /> The authors' long-term goals are to understand the utility of precisely phased cortex stimulation regimes on recovery of function after spinal cord injury (SCI). In prior work, the authors explored the effects of contralesion cortex stimulation. Here, they explore ipsilesion cortex stimulation in which the corticospinal fibers that cross at the pyramidal decussation are spared. The authors explore the effects of such stimulation in intact rats and rats with a hemisection lesion at the thoracic level ipsilateral to the stimulated cortex. The appropriately phased microstimulation enhances contralateral flexion and ipsilateral extension, presumably through lumbar spinal cord crossed-extension interneuron systems. This microstimulation improves weight bearing in the ipsilesion hindlimb soon after injury, before any normal recovery of function would be seen. The contralateral homologous cortex can be lesioned in intact rats without impacting the microstimulation effect on flexion and extension during gait. In two rats ipsilateral flexion responses are noted, but these are not clearly demonstrated to be independent of the contralateral homologous cortex remaining intact.

Strengths:<br /> This paper adds to prior data on cortical microstimulation by the laboratory in interesting ways. First, the strong effects of the spared crossed fibers from the ipsi-lesional cortex in parts of the ipsi-lesion leg's step cycle and weight support function are solidly demonstrated. This raises the interesting possibility that stimulating the contra-lesion cortex as reported previously may execute some of its effects through callosal coordination with the ipsi-lesion cortex tested here. This is not fully discussed by the authors but may represent a significant aspect of these data. The authors demonstrate solidly that ablation of the contra-lesional cortex does not impede the effects reported here. I believe this has not been shown for the contra-lesional cortex microstimulation effects reported earlier, but I may be wrong.

Effects and neuroprosthetic control of these effects are explored well in the ipsi-lesion cortex tests here.

Weaknesses:<br /> Some data is based on very few rats. For example (N=2) for ipsilateral flexion effects of microstimulation. N=3 for homologous cortex ablation, and only ipsi extension is tested it seems. There is no explicit demonstration that the ipsilateral flexion effects in only 2 rats reported can survive the contra-lateral cortex ablation.

Some improvements in clarity and precision of descriptions are needed, as well as fuller definitions of terms and algorithms.

Likely Impacts:<br /> This data adds in significant ways to prior work by the authors, and an understanding of how phased stimulation in cortical neuroprosthetics may aid in recovery of function after SCI, especially if a few ambiguities in writing and interpretation are fully resolved.

Reviewer #3 (Public Review):

Summary:<br /> This article aims to investigate the impact of neuroprosthesis (intracortical microstimulation) implanted unilaterally on the lesion side in the context of locomotor recovery following unilateral thoracic spinal cord injury.

Strength:<br /> The study reveals that stimulating the left motor cortex, on the same side as the lesion, not only activates the expected right (contralateral) muscle activity but also influences unexpected muscle activity on the left (ipsilateral) side. These muscle activities resulted in a substantial enhancement in lift during the swing phase of the contralateral limb and improved trunk-limb support for the ipsilateral limb. They used different experimental and stimulation conditions to show the ipsilateral limb control evoked by the stimulation. This outcome holds significance, shedding light on the engagement of the "contralateral projecting" corticospinal tract in activating not only the contralateral but also the ipsilateral spinal network.

The experimental design and findings align with the investigation of the stimulation effect of contralateral projecting corticospinal tracts. They carefully examined the recovery of ipsilateral limb control with motor maps. They also tested the effective sites of cortical stimulation. The study successfully demonstrates the impact of electrical stimulation on the contralateral projecting neurons on ipsilateral limb control during locomotion, as well as identifying important stimulation spots for such an effect. These results contribute to our understanding of how these neurons influence bilateral spinal circuitry. The study's findings contribute valuable insights to the broader neuroscience and rehabilitation communities.

Weakness:<br /> The term "ipsilateral" lacks a clear definition in the title, abstract, introduction, and discussion, potentially causing confusion for the reader.

The unexpected ipsilateral (left) muscle activity is most likely due to the left corticospinal neurons recruiting not only the right spinal network but also the left spinal network. This is probably due to the joint efforts of the neuroprosthesis and activation of spinal motor networks which work bilaterally at the spinal level.

However, in my opinion, readers can easily link the ipsilateral cortical network to the ipsilateral-projecting corticospinal tract, which is less likely to play a role in ipsilateral limb control in this study since this tract is disrupted by the thoracic spinal injury.

eLife assessment

This is an important study that investigates BMP signaling mechanisms in the developing chick cerebellum to better understand germinal layer formation, cellular amplification and neuronal differentiation. The data from human tissue is compelling and lends support to the possible links of these processes to medulloblastoma, although these specific statements could be toned down and presented only as part of the discussion. Overall, this is a solid piece of work with beautifully presented findings.

Reviewer #1 (Public Review):

Summary:<br /> Rook et al examined the role of BMP signaling in cerebellum development, using chick as a model alongside human tissue samples. They first examined p-SMADs and found differences between the species, with human samples retaining high p-SMAD after foliation, while in chick, BMP signaling appears to decrease following foliation. To understand the role of BMP during early development, they then used early chick embryos to modulate BMP, using either a constitutively active BMP regulator to increase BMP signaling or overexpressing the negative intracellular BMP regulator to decrease BMP signaling. After validating the constructs in ovo, the authors then examined GNP morphology and migration. They then determined whether the effects were cell autonomous.

Strengths:<br /> The experiments were well-designed and well-controlled. The figures were extremely clear and convincing, and the accompanying drawings help orient the reader to easily understand the experimental set up. These studies also help clarify the role of BMP at different stages of cerebellum development, suggesting early BMP signaling is required for dorsalization, not rhombic lip induction, and that later BMP signaling is needed to regulate the timing of migration and maturation of granule neurons.

Weaknesses:<br /> Given the species-specific differences in pSmad localization and abundance in human and chick cerebellum, caution is warranted when making the link to the treatment of human medulloblastoma through modulation BMP signaling. While these studies certainly hint that BMP modulation may affect tumor growth, this was not explicitly tested here. Future studies are required to generalize the functional role of BMP signaling in normal cerebellum development to malignant growth.

Reviewer #2 (Public Review):

Summary:<br /> This is a fundamental and elegant study showing the role of BMP signaling in cerebellar development. This is an important question because there are multiple diseases, including aggressive childhood cancers, which involve granule cell precursors. Thus understanding of the factors that govern the formation of the granule cell layer is important both from a basic science and a disease perspective.

Overall, the manuscript is clear and well-written. The figures are extremely clear, wonderfully informative, and overall quite beautiful.

Figures 1-3 show the experimental design and report how BMP activity is altered over development in both the chick and the human developing cerebellum. Both data are very impressive and convincing.

They then go on to modulate BMP activity in the developing chick, using a complex electroporation paradigm that allows them to label cells with GFP as well as with cell-specific reporters of BMP activity levels. They bidirectionally modulate BMP levels and then can look at both cell-specific and non-specific alterations in the formation of the external and internal granule cell layer, across different developmental timepoints. These are really elegant and rigorous experiments, as they look at both sagittal and transverse sections to collect this data. This makes the data extremely compelling. With these rigorous techniques, they show that BMP signaling serves more than one function across development: it is involved in the initial tangential migration from the rhombic lip, but at a later time, both up- and down-regulation of BMP activity reduces the density of amplifying cells in the external granule cell layer.

Strengths:<br /> Overall, I think the paper is interesting and important and the data is strong. The use of both chick and human tissue strengthens the findings. They are extremely rigorous, analyzing data from multiple planes at multiple ages, which also really strengthens their findings. The dual electroporation approach is extremely elegant, providing beautiful visual representations of their findings.

Weaknesses:<br /> I find no significant weaknesses.

eLife assessment

The manuscript aims to better understand the mechanisms underlying the behavioral responses of C. elegans to hydrogen sulfide, a toxin known to exert remarkable effects on animal physiology in a range of contexts. To this end, the authors provide a series of useful findings regarding the mechanisms by which hydrogen sulfide may be sensed, their relationships to other gas-sensing pathways, and the role of a variety of physiological pathways in responding to hydrogen sulfide exposure. While some of the findings are solid, other aspects of the paper are incomplete, such that some claims are incompletely supported, and an integrated understanding of the authors' observations does not clearly emerge.

Reviewer #1 (Public Review):

Summary:<br /> This paper sets out to achieve a deeper understanding of the effects of hydrogen sulfide on C. elegans behavior and physiology, with a focus on behavior, detection mechanism(s), physiological responses, and detoxification mechanisms.

Strengths:<br /> The paper takes full advantage of the experimental tractability of C. elegans, with thorough, well-designed genetic analyses.<br /> Some evidence suggests that H2S may be directly detected by the ASJ sensory neurons.<br /> The paper provides interesting and convincing evidence for complex interactions between responses to different gaseous stimuli, particularly an antagonistic role between H2S and O2 detection/response.<br /> Intriguing roles for mitochondria and iron homeostasis are identified, opening the door to future studies to better understand the roles of these components and processes.

Weaknesses:<br /> The claim that worms' behavioral responses to H2S are mediated by direct detection is incompletely supported. While a role for the chemosensory neuron ASJ is implicated, it remains unclear whether this reflects direct detection. Other possibilities, including indirect effects of ASJ and the guanylyl cyclase daf-11 on O2 responses, are also consistent with the authors' data.

The role of H2S-mediated damage in behavioral responses, particularly when detoxification pathways are disrupted, remains unclear.

The findings of the paper are somewhat disjointed, such that a clear picture of the relationships between H2S detection, detoxification mechanisms, mitochondria, and iron does not emerge from these studies. Most importantly, the relative roles of H2S detection and integration, vs. general and acute mitochondrial crisis, in generating behavioral responses are not convincingly resolved.

Reviewer #2 (Public Review):

Summary:

H2S is a gas that is toxic to many animals and causes avoidance in animals such as C. elegans. The authors show that H2S increases the frequency of turning and the speed of locomotion. The response was shown to be modulated by a number of neurons and signaling pathways as well as by ambient oxygen concentrations. The long-term adaptation involved gene expression changes that may be related to iron homeostasis as well as the homeostasis of mitochondria.

Strengths:

Overall, the authors provide many pieces that will be important for solving how H2S signals through neuronal circuits to change gene expression and physiological programs. The experiments rely mostly on a behavioral assay that measures the increase of locomotion speed upon exposure to H2S. This assay is then combined with manipulations of environmental factors, different wild-type strains, and mutants. The mutants analyzed were obtained as candidates from the literature and from transcriptional profiling that the authors carried out in worms that were exposed to H2S. These studies imply several genetic signaling pathways, some neurons, and metabolism-related factors in the response to H2S. Hence the data provided should be useful for the field.

Weaknesses:

On the other hand, many important aspects of the underlying mechanisms remain unsolved and the reader is left with many loose ends. For example, it is not clear how H2S is actually sensed, how sensory neurons are activated and signal to downstream circuits, and what the role of ciliated and RMG neurons is in this circuit. It remains unclear how signals lead to gene expression and physiological changes such as metabolic rewiring. Solving all this would clearly be beyond the scope of a single manuscript. Yet, the manuscript also does not focus on understanding one of these central aspects and rather is all over the place, which makes it harder to understand for readouts that are not in this core field. Multiple additional methods and approaches exist to dig deeper into these mechanisms in the future, such as neuronal calcium imaging, optogenetics, and metabolic analysis. To generate a story that will be interesting to a broad readership substantial additional experimentation would be required. Further, in the current manuscript, it is often difficult to understand the rationales of the experiments, why they were carried out, and how to place them into a context. This could be improved in terms of documentation, narration/explanation, and visualization.

Reviewer #3 (Public Review):

Summary:<br /> The manuscript explores the behavioral responses of C. elegans to hydrogen sulfide, which is known to exert remarkable effects on animal physiology in a range of contexts. The possibility of genetic and precise neuronal dissection of responses to H2S motivates the study of responses in C. elegans. The manuscript is well-written in communicating the complex physiology around C. elegans behavioral responses to H2S and in appropriately citing prior and related relevant work.

There are three parts to the manuscript.

In the first, an immediate behavioral response-increased locomotory rate-upon exposure to H2S is characterized. The experimental conditions are critical, and data are obtained from exposure of animals to 150ppm H2S at 7% O2. The authors provide evidence that this is a chemosensory response to H2S, showing a requirement for genes encoding components of the cilia apparatus and implicating a role for tax-4 and daf-11. Neuron-specific rescue in the ASJ neurons suggests the ASJ neurons contribute to the response to H2S. One caveat is that previous work has shown that the dauer-constitutive phenotype of daf-11 mutants can be suppressed by ASJ ablation, suggesting that there may be pervasive changes in animal nervous system signaling that are ASJ-dependent in daf-11 mutants, which may indirectly alter chemosensory responses to H2S. More direct methods to assess whether ASJ senses H2S, e.g. using calcium imaging, would better assess a direct role for the ASJ neurons in a behavioral response to H2S. The authors also point out interesting parallels between the response to H2S and CO2 though provide some genetic data separating the two responses. Importantly, the authors note that when aerotaxis (O2-sensing and movement) in the presence of bacterial food is intact, as in npr-1 215F animals, the response to H2S is abrogated. Mutation in gcy-35 in the npr-1 215F background restores the H2S chemosensory response.

There is a second part of the paper that conducts transcriptional profiling of the response to H2S that corroborates and extends prior work in this area.

The final part of the paper is the most intriguing, but for me, also the most problematic. The authors examine how H2S-evoked locomotory behavioral responses are affected in mutants defective in the stress and detoxification response to H2S, most notably hif-1. Prior genetic studies have established the pathways leading to HIF-1 activation/stabilization, as well as potential downstream mechanisms. The authors conduct logical genetic analysis to complement studies of the hif-1 mutant and in part motivated by their transcriptional profiling studies, examine the role of iron sequestration/free iron in the locomotory response to H2S, and further speculate on how the behavior of mutants defective in mitochondrial function might be affected by exposure to H2S.

In some regard, this part of the manuscript is interesting because the analysis begins to connect how the behavior of an animal to a toxic compound is affected by mutations that affect sensitivity to the toxic compound. However, what is unclear is what is being studied at this point. In the context, of noting that H2S at 150ppm is known to be lethal, its addition to mutants clearly sensitized to its effects would be anticipated to have pervasive effects on animal physiology and nervous system function. The authors note that the continued increased locomotion of wild-type animals upon H2S exposure might be due to the byproducts of detoxification or the detrimental effects of H2S. The latter explanation seems much more likely, in which case what one may be observing is the effects of general animal sickness, or even a bit more specifically, neuronal dysfunction in the presence of a toxic compound, on locomotion. As such, what is unclear is what conclusions can be taken away from this part of the work.

Strengths:<br /> 1. Characterization of a motor behavior response to H2S<br /> 2. Transcriptional profiling of the response to H2S corroborating prior work.

Weaknesses:<br /> Unclear significance and experimental challenges regarding the study of locomotory responses to animals sensitized to the toxic effects of H2S under exposure to H2S.

eLife assessment

This potentially valuable study provides some evidence that upregulation of sodium-activated potassium channels contributes to neuronal hyperexcitability and seizures following traumatic brain injury. However, the evidence supporting a direct link is incomplete. This work will be of interest to epilepsy and ion channel researchers.

Reviewer #1 (Public Review):

Summary<br /> General Comments<br /> The authors present an interesting study that aims to resolve the contribution of the sodium-activated potassium channel (KNa1.1) to acquired, trauma-induced epilepsy. To this end, the authors first aim to develop a mouse model that consistently generates seizures. Using controlled cortical impact (CCI) methods to induce traumatic brain injuries (TBIs) that range from mild to severe, the authors demonstrate that behavioral deficits correlate with the extent of brain damage. Interestingly, despite the differences in behavioral scores, the spontaneous seizure phenotype was similar across mice with a range of TBI-associated tissue loss. However, when challenged with the chemoconvulsant pentylenetetrazol (PTZ), mice with more severe TBI exhibited more severe seizures.

After establishing a model of moderate TBI, the authors then show that moderate brain injury transiently upregulates the perilesional expression of KNa1.1. Moreover, the authors provide some evidence that the expression of inhibitory neuron markers is downregulated in the perilesional region following moderate TBI, whereas the expression of excitatory neuron markers is unchanged. Consistent with this finding is the functional observation that neurons receive less inhibitory signaling following TBI, whereas excitatory signaling is unchanged. Inhibitory neurons also fire less robustly following moderate TBI.

The authors then show that deletion of KNa1.1 in mice provides a moderate level of protection against pharmacologically induced seizures following TBI. In aggregate, the authors propose a model wherein inhibitory neuron-specific upregulation of KNa1.1 following TBI selectively reduces the excitability of inhibitory neurons. In turn, this reduced excitability of inhibitory neurons promotes perilesional tissue hyperexcitability and, ultimately, seizures. Although this model is compelling, readers should be aware that the authors only utilized PTZ-induced seizures following TBI to resolve differences between WT and KO animals. It remains unclear whether WT mice have a robust spontaneous seizure phenotype 14 days after moderate TBI, and whether deleting KNa1.1 reduces this spontaneous seizure phenotype. In general, the combined use of TBI and a chemoconvulsant to evaluate epileptic phenotypes diminishes this reviewer's enthusiasm for the clinical impact of the author's conclusions; although, I appreciate that "capturing [spontaneous] seizures is challenging in terms of animal numbers and long-term recording, which hinders high-throughput studies" (line 302).

Finally, the authors seemed to have missed an opportunity to determine if the electrophysiological changes observed in WT mice following TBI (i.e., Figure 5) are eliminated in the KNa1.1 KO mouse. That is, the authors show that KNa1.1 contributes to the intrinsic firing properties of uninjured tissue (Figure 6). But does deleting KNa1.1 also restore inhibitory neuron excitability associated with TBI? The inclusion of such data would strengthen the conclusion that the reduced inhibitory neuron excitability following TBI is indeed the result of changes in KNa1.1 expression.

Strengths:<br /> (1) The development of a TBI model with a range of behavioral phenotypes.

(2) The inclusion of knockout mouse.

(3) The inclusion of functional data regarding the intrinsic and synaptic properties of neurons following TBI.

Weaknesses:<br /> (1) A missed opportunity to better utilize the knockout animal to test the hypothesis that KNa1.1 drives changes in intrinsic excitability following TBI.

(2) The combined use of TBI and chemoconvulsants to suss out differences in seizure phenotypes.

Reviewer #2 (Public Review):

Summary:<br /> Authors hypothesized that modulation of KNa1.1 channel specifically in inhibitory interneurons contributes to the hyperactivity of neurons in the peripheral cortex at the lesion site, enhances seizure susceptibility to PTZ-induced seizures, and promotes the occurrence of PTE. They test this hypothesis in a mouse model of TBI induced by controlled cortical impact in wild-type and kcnt1 knock-out (KO) mice. The authors performed a series of experiments including behavioral assessment, electrographic recordings in vivo and in vitro, western blotting, and immunofluorescence imaging with the goal of investigating the contributory role of KNa1.1 channel to post-traumatic epileptogenesis.

Strengths:<br /> The hypothesis is innovative, focusing on the specific role of the KNa1.1 channel in the development of PTE post-TBI. The use of a comprehensive set of techniques, including EEG, whole cell patch clamp, western blot, immunofluorescence imaging, and behavioral assessments, provides a diverse data set. The study makes initial steps in correlating specific molecular changes with functional outcomes in TBI models, offering potential pathways for therapeutic intervention.

Weaknesses:<br /> 1) The study presents interesting findings on early changes in protein expression and electrophysiological properties following TBI. However, I would like to draw attention to the timeline of EEG and cellular assessments that require further clarification or consideration. The patch clamp recordings and other assays were conducted within 14 days post-TBI, while EEG recordings with or without PTZ testing were performed at 3 months post-injury. This temporal gap leaves a period where changes in electrophysiological properties and PTE status are not accounted for. Since epileptogenesis post-TBI involves a dynamic process spanning from hyperacute and acute phases to chronic development, capturing these changes continuously or at least at more frequent intervals (on and off bi-weekly) could provide a more comprehensive understanding of this progression. In the current study design, the one-week duration of EEG recordings at the 3-month timepoint raises the possibility that some seizures might have occurred undetected between the early post-injury phase and the EEG recording period. This gap could potentially affect the interpretation of results, especially when correlating early post-injury cellular changes with later seizure activity and thresholds and hence is a significant limitation to data interpretation. Experiments using western blots, immunofluorescence, and patch clamp were done at an early timepoint hence the relevance of these datasets to PTE status outcome is not established.

2) While referencing Nichols et al., 2015, to justify the 14-day timeline for characterizing seizures is understandable, it is important to consider differences in animal models (juvenile rats in Nichols vs. adult mice in the present study) which might influence the generalizability of the findings.

3) Behavior: Authors performed behavioral assays using the rotarod technique evaluating the hanging time of mice with different severity of TBI (mild, moderate, severe). The purpose of the testing is explained as 'to sort out an appropriate TBI model'. The authors also measured mortality rates 'to attain a stable model'. It is not clear what is assumed by the terms 'appropriate' or 'stable' model. Furthermore, the relevance of this to post-traumatic epileptogenesis is unclear. Additionally, the mortality in the Sham group within 2 weeks of craniectomy is not explained.

4) Seizure assessment: authors report seizure severity in PTZ-induced seizures but no mention about the severity of spontaneous seizures between different TBI severity modalities. When characterizing the PTZ-induced seizures, the mild TBI group does not have generalized seizures. Does this mean that al all 6 tested animals in the mild TBI group had exclusively focal seizures? What about the spontaneous seizure occurrence: were all seizures generalized or were any focal too? Did that differ between mild, moderate, and severe TBI?

5) Experiments with KCNT1 KO mice: in all experiments with a mutant mouse line, authors only used them in TBI group. Without the Sham group, it is difficult to discern whether any observed changes in seizure susceptibility in the KCNT1 KO TBI group are due to gene deletion, the TBI, or a combination of both. This group would provide a crucial comparison point to isolate the effects of the KCNT1 knockout from those of TBI. This limits the ability to make comprehensive conclusions about the role of the KCNT1 gene in seizure susceptibility following TBI.

6) While the current study showed interesting data about the KNa1.1 changes early after TBI, the study design and disconnect between early and late electrophysiology experiments timeline, does not establish a correlative or causative link between KNa1.1 and post-traumatic epileptogenesis since it remained unresolved whether KCNT1 KO mice developed no PTE (or less severe/ less frequent seizures at 3 months) compared with WT mice and what are the seizure properties of KCNT1 KO Sham mice compared to WT TBI and Sham groups. The hypothesis was that modulation of KNa1.1 channel specifically in inhibitory interneurons contributes to the hyperactivity of neurons in the peripheral cortex at the lesion site, enhances seizure susceptibility to PTZ-induced seizures, and promotes the occurrence of PTE. The part about 'promotes the occurrence of PTE' was not established.

7) NeuN is not the best marker of neurons in the context of TBI since TBI affects its expression patterns which will influence the interpretation of co-localization results. Unlike NeuN, Nissl staining is less likely to be affected by factors that alter protein expression, such as TBI. Therefore, it can be a more stable marker for identifying neurons in injured brain tissue.

8) Statistics: Authors report only SEM, which shows the precision of the mean and it will decrease as the sample size increases and does not reflect the data variability.

eLife assessment

This important work provides insight into the activity and spatial organization of synapses during early postnatal development in the mouse visual cortex, using state-of-the-art tools to show that synapses are distributed in co-active clusters well before eye opening. The evidence supporting the claims is convincing, although additional methodological details are needed to fully assess the rigor of the analysis. This work is of particular interest to the field of developmental neuroscience and can also be used by computational neuroscientists studying dendritic integration.

Reviewer #1 (Public Review):

Summary:<br /> Using concurrent in vivo whole-cell patch clamp and dendritic calcium imaging, the authors characterized how functional synaptic inputs across dendritic arborizations of mouse primary visual cortex layer 2/3 neurons emerge during the second postnatal week. They were able to identify spatially and functionally separated domains of clustered synapses in these neurons even before eye-opening and characterize how the clustering changes from P8 to P13.

Strengths:<br /> The work is technically challenging and the findings are novel. The results support previous EM and immunostaining studies but provide in vivo evidence on the time course and the trajectory of how functional synaptic input develops.

Weaknesses:<br /> There are some missing details about how the experiments were performed, and I also have some questions about the analyses.

Reviewer #2 (Public Review):

In this study, Leighton et al performed remarkable experiments by combining in-vivo patch-clamp recording with two-photon dendritic Ca2+ imaging. The voltage-clamp mode is a major improvement over the pioneer versions of this combinatorial experiment that has led to major breakthroughs in the neuroscience field for visualizing and understanding synaptic input activities in single cells in-vivo (sharp electrodes: Svoboda et al, Nature 1997, Helmchen et al, Nature Neurosci 1999; whole-cell current-clamp: Jia et al, Nature 2010, Chen et al, Nature 2011. I suggest that these papers would be cited). This is because in voltage-clamp mode, despite the full control of membrane voltage in-vivo not being realistic, is nevertheless most effective in preventing back-propagation action potentials, which would severely confound the measurement of individual synaptically-induced Ca2+ influx events. Furthermore, clamping the cell body at a strongly depolarized potential (here the authors did -30mV) also facilitates the detection of synaptically-induced Ca2+ influx. As a result, the authors successfully recorded high-quality Ca2+ imaging data that can be used for precise analysis. To date, even in view of the rapid progress of voltage-sensitive indicators and relevant imaging technologies in recent years, this very old 'art' of combining single-cell electrophysiology and two-photon imaging (ordinary, raster-scanned, video-rate imaging) of Ca2+ signals still enables measurements of the best-level precision.

On the other hand, the interpretation of data in this study is a bit narrow-minded and lacks a comprehensive picture. Some suggestions to improve the manuscript are as follows:

1. The authors made a segregation of 'spine synapse' and 'shaft synapse' based solely on the two-photon images in-vivo. However, caution shall be taken here, because the optical resolution under in-vivo imaging conditions like this cannot reliably tell apart whether a bright spot within or partially overlapping a segment of the dendrite is a spine on top of (or below) it. Therefore, what the authors consider as a 'shaft synapse' (by detecting Ca2+ hotspots) has an unknown probability of being just a spine on top or below the dendrite. If there is other imaging data of higher axial resolution to validate or calibrate, the authors shall take some further considerations or analysis to check the consistency of their data, as the authors do need such a segregation between spine and shaft synapses to show how they evolve over the brain development stages.

2. The use of terminology 'bursts of spontaneous inputs' for describing voltage-clamp data seems improper. Conventionally, 'burst' refers to suprathreshold spike firing events, but here, the authors use 'burst' to refer to inward synaptic currents collected at the cell body. Not every excitatory synaptic input (or ensemble of inputs) activation will lead to spike firing under naturalistic conditions, therefore, these two concepts are not equivalent. It is recommended to use 'barrage of inputs' instead of 'burst of inputs'. Imagine a full picture of the entire dendritic tree, the fact that the authors could always capture spontaneous Ca2+ events here and there within a few pieces of dendrites within an arbitrary field-of-view suggests that, the whole dendritic tree must have many more such events going on as a barrage while the author's patch electrode picks up the summed current flow from the whole dendritic tree.

3. Following the above issue, an analysis of the temporal correlation between synaptic (not segregating 'spine' or 'shaft') Ca2+ events and EPSCs is absent. Again, the authors drew arbitrary time windows to clump the events for statistical analysis. However, the demonstrated example data already shows that the onset times of individual synaptic Ca2+ events do not necessarily align with the beginning of a 'barrage' inward current event.

4. The authors claim that "these observations indicate that the activity patterns investigated here are not or only slightly affected by low-level anesthesia". It would be nice to show some of the recordings in this work without any anesthesia to support this claim.

Reviewer #3 (Public Review):

Summary:<br /> There is a growing body of literature on the clustering of co-active synapses in adult mice, which has important implications for understanding dendritic integration and sensory processing more broadly. However, it has been unclear when this spatial organization of co-active synapses arises during development. In this manuscript, Leighton et al. investigate the emergence of spatially organized, co-active synapses on pyramidal dendrites in the mouse visual cortex before eye-opening. They find that some dendrite segments contain highly active synapses that are co-active with their neighbors as early as postnatal day (P) 8-10, and that these domains of co-active synapses increase their coverage of the dendritic arbor by P12-13. Interestingly, Leighton et al. demonstrate that synapses co-active with their neighbors are more likely to increase their activity across a single recording session, compared to synapses that are not co-active with their neighbors, suggesting local plasticity driven by coincident activity before eye-opening.

The current manuscript includes some replication of earlier results from the same research group (Winnubst et al., 2015), including the presence of clustered, co-active synapses in the visual cortex of mouse pups, and the finding that synapses co-active with their neighbors show an increase in transmission frequency during a recording session. The main novelty in the current study compared to Winnubst et al. (2015) is the inclusion of younger animals (P8-13 in the current study compared to P10-15 in Winnubst et al., 2015). The current manuscript is the first demonstration that active synapses are clustered on specific dendrite segments as early as P8-10 in the mouse visual cortex, and the first to show the progression in active synapse distribution along the dendrite during the 2nd postnatal week. These results from the visual cortex may help inform our understanding of sensory development more broadly.

Strengths:<br /> The authors ask a novel question about the emergence of synaptic spatial organization, and they use well-chosen techniques that directly address their questions despite the challenging nature of these techniques. To capture both structural and functional information from dendrites simultaneously, the authors performed a whole-cell voltage clamp to record synaptic currents arriving at the soma while imaging calcium influx at individual synaptic sites on dendrites. The simultaneous voltage clamp and calcium imaging allowed the authors to isolate individual synaptic inputs without their occlusion by widespread calcium influx from back-propagating action potentials. Achieving in vivo dendrite imaging in live mice that are as young as P8 is challenging, and the resulting data provides a unique view of synaptic activity along individual dendrites in the visual cortex at an early stage in development that is otherwise difficult to assess.

The authors provide convincing evidence that synapses are more likely to be co-active with their neighbors compared to synapses located farther away (Fig. 6F-H), and that synapses co-active with their neighbors increase their transmission frequency during a recording session (Figure 7C). These findings are particularly interesting given that the recordings occur before eye-opening, suggesting a relationship between co-activity and local synaptic plasticity even before the onset of detailed visual input. These results replicate previously published findings from P10-15 pups (Winnubst et al., 2015), increasing confidence in the reproducibility of the data.

The authors also provide novel data documenting for the first time spatially organized, co-active synapses in pups as young as P8. Comparing the younger (P8-10) and older (P12-13) pups, provides insight into how clusters of co-active synapses might emerge during development.

Weaknesses:<br /> This manuscript provides insufficient detail for assessing the rigor and reproducibility of the methods, particularly for age comparisons. The P8-10 vs P12-13 age comparisons are the primary novel finding in this manuscript, and it is, therefore, critical to avoid systematic age differences in the methods and analysis whenever possible. Specific concerns related to the age comparisons are listed below:

• Given that the same research group previously published P12-13 data (Winnubst et al., 2015), it is unclear whether both age groups in the current study were imaged/analyzed in parallel by the same researcher(s), or whether previous data was used for the P12-13 group.

• The authors mention that they used 2 different microscopes, and used a fairly wide range of imaging frame rates (5-15 Hz). It is unclear from the current manuscript whether the same imaging parameters were used across the two age groups. If data for the two experimental groups was collected separately, perhaps at different times, by a different person, or on a different microscope, there is a concern that some differences between the groups may not necessarily be due to age.

• It is unclear whether the image analysis was performed blind to age. Blinding to age during analysis is particularly important for this study, in which it was not possible to blind to age during imaging due to visible differences in size and developmental stage between younger and older pups.

• The relatively low N (where N is the number of dendrites or the number of mice) in this study is acceptable due to the challenging nature of the techniques used, but unintentional sampling bias is a concern. For example, if higher-order dendrites from the apical tuft were imaged at P12-13, while more segments of the apical trunk were imaged at P8-10, this could inadvertently create apparent age differences that were in fact due to dendrite location on the arbor or dendrite depth.

Additional general methodological concerns, which are not specifically related to the age comparisons, are listed below:

• The authors assert that clustered, co-active synapses emerge in the visual cortex before eye-opening, which is an important finding in that it suggests this phenomenon is driven by spontaneous activity rather than visual input. However, this finding hinges on the imaged cells being reliably located in the visual cortex, which is difficult to identify with certainty in animals that have not yet opened their eyes and therefore cannot undergo intrinsic signal imaging to demarcate the boundaries of the visual cortex. If the imaged cells were in, for example, nearby somatosensory cortex, then the observed spatial organization could be due to sensory input rather than spontaneous activity.

• It is unclear how the authors defined a synaptic transmission event in the GCaMP signal (e.g. whether there was a quantitative deltaF/F threshold).

• The authors' division of synapses into spine vs shaft is unconvincing due to the difficulty of identifying Z-projecting spines in images from 2-photon microscopy, where the Z resolution is insufficient to definitively identify Z-projecting spines, and the fact that spines in young animals may be thin and dim. The authors' examples of spine synapses (e.g. in Fig. 2A) are convincing, but some of the putative shaft synapses may in fact be on spines.

eLife assessment

This fundamental study advances substantially our understanding of sound encoding at synapses between single inner hair cells of the mouse cochlea and spiral ganglion neurons. Dual patch-clamp recordings-a technical tour-de force-and careful data analysis provide compelling evidence that the functional heterogeneity of these synapses contributes to the diversity of spontaneous and sound-evoked firing by the neurons. The work will be of broad interest to scientists in the field of auditory neuroscience.

Reviewer #1 (Public Review):

Summary:<br /> Tobón and Moser reveal a remarkable amount of presynaptic diversity in the fundamental Ca dependent exocytosis of synaptic vesicles at the afferent fiber bouton synapse onto the pilar or mediolar sides of single inner hair cells of mice. These are landmark findings with profound implications for understanding acoustic signal encoding and presynaptic mechanisms of synaptic diversity at inner hair cell ribbon synapses. The paper will have an immediate and long-lasting impact in the field of auditory neuroscience.

Main findings: 1) Synaptic delays and jitter of masker responses are significantly shorter (synaptic delay: 1.19 ms) at high SR fibers (pilar) than at low SR fibers (mediolar; 2.57 ms). 2) Masked evoked EPSC are significantly larger in high SR than in low SR. 3) Quantal content and RRP size are 14 vesicles in both high and low SR fibers. 4) Depression is faster in high SR synapses suggesting they have a higher release probability and tighter Ca nanodomain coupling to docked vesicles. 5) Recovery of master-EPSCs from depletion is similar for high and low SR synapses, although there is a slightly faster rate for low SR synapses that have bigger synaptic ribbons, which is very interesting. 6) High SR synapses had larger and more compact (monophasic) sEPSCs, well suited to trigger rapidly and faithfully spikes. 7) High SR synapses exhibit lower voltage (~sound pressure in vivo) dependent thresholds of exocytosis.

Strengths:<br /> Great care was taken to use physiological external pH buffers and physiological external Ca concentrations. Paired recordings were also performed at higher temperatures with IHCs at physiological resting membrane potentials and in more mature animals than previously done for paired recordings. This is extremely challenging because it becomes increasingly difficult to visualize bouton terminals when myelination becomes more prominent in the cochlear afferents. In addition, perforated patch recordings were used in the IHC to preserve its intracellular milieu intact and thus extend the viability of the IHCs. The experiments are tour-de-force and reveal several novel aspects of IHC ribbon synapses. The data set is rich and extensive. The analysis is detailed and compelling.

Weaknesses:<br /> 1) Materials and Methods: Please provide whole-cell Rs (series resistance ) and Cm (membrane capacitance) average +/- S.E.M. (or SD) values for IHC and afferent fiber bouton recordings. The Cm values for afferents have been estimated to be about 0.1 pF (Glowatzki and Fuchs, 2002) and it would be interesting to know if there are differences in these numbers for high and low SR afferents. Is it possible to estimate Cm from the capacitative transient time constant? Minimal electronic filtering would be required for that to work, so I realize the authors may not have this data and I also realize that the long cable of the afferents do not allow accurate Cm measurements, but some first order estimate would be very interesting to report, if possible.

2) Page 20, 26 and Figure 4: With regard to synaptic delays at auditory hair cell synapses: please see extensive studies done in Figure 11 of Chen and von Gersdorff (JNeurosci., 2019); this showed that synaptic delays are 1.26 ms in adult bullfrog auditory hair cells at 31oC, which is very similar to the High SR fibers (1.19 ms; Fig.4B and page 20). During ongoing depolarizations (e.g. during a sustained sine wave) the synaptic delay can be reduced to just 0.72 ms for probe EPSCs, which is a more usual number for mature fast synapses. This paper should, thus, be cited and briefly discussed in the Discussion. So a significant shortening of delay occurs for the probe response and this is also observed in young rat IHC synapses (see Goutman and Glowatzki, 2011).

3) Gaussian-like (and/or multi-peak) EPSC amplitude distributions were obtained in more mature rat IHCs by Grant et al. (see their Figure 4G; JNeurosci. 2010; postnatal day 19-21). The putative single quanta peak was at 50 pA and the main peak was at 375 pA. The large mean suggests a low CV (probably < 0.4). However, Fig. 2F shows a mean of about 100 pA and CV = 0.7 for spontaneous EPSCs. This major difference deserves some more discussion. I suppose that one possible explanation may be that the current paper holds the IHC membrane potential fixed at -58 mV, whereas Grant et al. (2010) did not control the IHC membrane potential and spontaneous fluctuations in the Vm may have depolarized the IHC, thus producing larger evoked EPSCs that are triggered by Ca channel openings. Some discussion that compares these differences and possible explanations would be quite useful for the readers.

Reviewer #2 (Public Review):

Summary: The study by Jaime-Tobon & Moser is a truly major effort to bridge the gap between classical observations on how auditory neurons respond to sounds and the synaptic basis of these phenomena. The so-called spiral ganglion neurons (SGNs) are the primary auditory neurons connecting the brain with hair cells in the cochlea. They all respond to sounds increasing their firing rates, but also present multiple heterogeneities. For instance, some present a low threshold to sound intensity, whereas others have high threshold. This property inversely correlates with the spontaneous rate, i.e., the rate at which each neuron fires in the absence of any acoustic input. These characteristics, along with others, have been studied by many reports over the years. However, the mechanisms that allow the hair cells-SGN synapses to drive these behaviors are not fully understood.

Strengths:<br /> The level of experimental complexity described in this manuscript is unparalleled, producing data that is hardly found elsewhere. The authors provide strong proof for heterogeneity in transmitter release thresholds at individual synapses and they do so in extremely complex experimental settings. In addition, the authors found other specific differences such as in synaptic latency and max EPSCs. A reasonable effort is put into bridging these observations with those extensively reported in in vivo SGNs recordings. Similarities are many and differences are not particularly worrying as experimental conditions cannot be perfectly matched, despite the authors' efforts in minimizing them.

Weaknesses:<br /> Some concern surges in relation to mismatches with previous reports of IHC-SGN synapses function. EPSCs at these synapses present a peculiar distribution of amplitudes, shapes, and rates. These characteristics are well-established and some do not seem to be paralleled in this study. Here, amplitude distributions are drastically shifted to smaller values, and rates of events are very low, all compared with previous evidence. The reasons for these discrepancies are unclear. The rate at which spontaneous EPSCs appear is an especially sensitive matter. A great part of the conclusions relies on the definition of which of the SGNs (or should say synapses) belong to the low end and which to the high end in the spectrum of spontaneous rates. The data presented by the authors seem a bit off and the criteria used to classify recordings are not well justified. The authors should clarify the origin of these differences since they do not seem to come from obvious reasons such as animal ages, recording techniques, mouse strain, or even species.

Reviewer #3 (Public Review):

Summary:

"Bridging the gap between presynaptic hair cell function and neural sound encoding" by Jaime Tobon and Moser uses patch-clamp electrophysiology in cochlear preparations to probe the pre- and post-synaptic specializations that give rise to the diverse activity of spiral ganglion afferent neurons (SGN). The experiments are quite an achievement! They use paired recordings from pre-synaptic cochlear inner hair cells (IHC) that allow precise control of voltage and therefore calcium influx, with post-synaptic recordings from type I SGN boutons directly opposed to the IHC for both presynaptic control of membrane voltage and post-synaptic measurement of synaptic function with great temporal resolution.

Strengths<br /> Any of these techniques by themselves are challenging, but the authors do them in pairs, at physiological temperatures, and in hearing animals, all of which combined make these experiments a real tour de force. The data is carefully analyzed and presented, and the results are convincing. In particular, the authors demonstrate that post-synaptic features that contribute to the spontaneous rate (SR) of predominantly monophasic post-synaptic currents (PSCs), shorter EPSC latency, and higher PSC rates are directly paired with pre-synaptic features such as a lower IHC voltage activation and tighter calcium channel coupling for release to give a higher probability of release and subsequent increase in synaptic depression. Importantly, IHCs paired with Low and High SR afferent fibers had the same total calcium currents, indicating that the same IHC can connect to both low and high SR fibers. These fibers also followed expected organizational patterns, with high SR fibers primarily contacting the pillar IHC face and low SR fibers primarily contacting the modiolar face. The authors also use in vivo-like stimulation paradigms to show different RRP and release dynamics that are similar to results from SGN in vivo recordings. Overall, this work systematically examines many features giving rise to specializations and diversity of SGN neurons.

Weaknesses / Comments / edits:<br /> 1) The careful analysis of calcium coupling and EPSC metrics is especially nice. Can the authors speculate as to why different synapses (likely in the same IHC) would have different calcium cooperativity?

2) On the bottom of page 6 it would be helpful to mention earlier how many pillar vs modiolar fibers were recorded from, otherwise the skewness of SRs (figure 2H could be thought to be due to predominantly recordings from modiolar fibers. As is, it reads a bit like a cliff-hanger.

3) The contrasts for some of the data could be used to point out that while significant differences occur between low and high SR fibers, some of these differences are no longer apparent when comparing modiolar vs pillar fibers (eg by contrasting Figure 2C and 2K). This can indicate that indeed there are differences between the fiber activity, but that the activity likely exists in a gradient across the hair cell faces. Pointing this out at the top of page 10 (end of the first paragraph) would be helpful, it would make the seemingly contradictory voltage-dependence data easier to understand on first read (voltage-dependence of release is significantly different between different SR fibers (figure 3) but is not significantly different between fibers on different HC faces (figure S3).

4) It should be acknowledged that although the use of post-hearing animals here (P14-23) ensures that SGN have begun to develop more mature activity patterns (Grant et al 2010), the features of the synapses and SGN activity may not be completely mature (Wu et al 2016 PMID: 27733610). Could this explain some of the 'challenges' (authors' section title) detailed on page 28, first full paragraph?

5) In the discussion on page 24, the authors compare their recorded SR of EPSCs to measure values in vivo which are higher. Could this indicate that in vivo, the resting membrane potential of IHCs is more depolarized than is currently used for in vitro cochlear experiments?

6) The results showing lower calcium cooperativity of high SR fibers are powerful, but do the authors have an explanation for why the calcium cooperativity of < 2 is different from that (m = 3-4) observed in other manuscripts?

eLife assessment

This valuable study on the molecular and cellular mechanisms of ingestion avoidance of high salt in insects is focused in scope, but the authors present convincing evidence that a specific subset of gustatory receptors in a pair of pharyngeal taste neurons are necessary and sufficient for avoiding ingestion of high salt during feeding. This work will be of interest to Drosophila neuroscientists interested in taste coding and feeding behavior.

Reviewer #1 (Public Review):

Summary:<br /> In this manuscript, Sang et al. proposed a pair of IR60b-expressing pharyngeal neurons in Drosophila use IR25a, IR76b, and IR60b channels to detect high Na+ and limit its consumption. Some of the key findings that support this thesis are: 1) animals that lacked any one of these channels - or with their IR60b-expressing neurons selectively silenced - showed much reduced rejection of high Na+, but restored rejection when these channels were reintroduced back in the IR60b neurons; 2) animals with TRPV artificially expressed in their IR60b neurons rejected capsaicin-laced food whereas WT did not; 3) IR60b-expressing neurons exhibited increased Ca2+ influx in response to high Na+ and such response went away when animals lacked any of the three channels.

Strengths:<br /> The experiments were thorough and well designed. The results are compelling and support the main claim. The development and the use of the DrosoX two-choice assay put forward for a more quantitative and automatic/unbiased assessment for ingestion volume and preference.

Weaknesses:<br /> There are a few inconsistencies with respect the the exact role by which IR60b neurons limit high salt consumption and the contribution of external (labellar) high-salt sensors in regulating high salt consumption. These weaknesses do not significantly impact the main conclusion, however.

Reviewer #2 (Public Review):

Summary:

In this paper, Sang et al. set out to identify gustatory receptors involved in salt taste sensation in Drosophila melanogaster. In a two-choice assay screen of 30 Ir mutants, they identified that Ir60b is required for avoidance of high salt. In addition, they demonstrate that activation of Ir60b neurons is sufficient for gustatory avoidance using either optogenetics or TRPV1 to specifically activate Ir60b neurons. Then, using tip recordings of labellar gustatory sensory neurons and proboscis extension response behavioral assays in Ir60b mutants, the authors demonstrate that Ir60b is dispensable for labellar taste neuron responses to high salt and the suppression of proboscis extension by high salt. Since external gustatory receptor neurons (GRNs) are not implicated, they look at Poxn mutants, which lack external chemosensory sensilla but have intact pharyngeal GRNs. High salt avoidance was reduced in Poxn mutants but was still greater than Ir60b mutants, suggesting that pharyngeal gustatory sensory neurons alone are sufficient for high salt avoidance. The authors use a new behavioral assay to demonstrate that Ir60b mutants ingest a higher volume of sucrose mixed with high salt than control flies do, suggesting that the action of Ir60b is to limit high salt ingestion. Finally, they identify that Ir60b functions within a single pair of gustatory sensory neurons in the pharynx, and that these neurons respond to high salt but not bitter tastants.

Strengths:

A great strength of this paper is that it rigorously corroborates previously published studies that have implicated specific Irs in salt taste sensation. It further introduces a new role for Ir60b in limiting high salt ingestion, demonstrating that Ir60b is necessary and sufficient for high salt avoidance and convincingly tracing the action of Ir60b to a particular subset of gustatory receptor neurons. Overall the authors have achieved their aim by identifying a new gustatory receptor involved in limiting high salt ingestion. They use rigorous genetic, imaging, and behavioral studies to achieve this aim, often confirming a given conclusion with multiple experimental approaches. They have further done a great service to the field by replicating published studies and corroborating the roles of a number of other Irs in salt taste sensation. An aspect of this study that merits further investigation is how the same gustatory receptor neurons and Ir in the pharynx can be responsible for regulating the ingestion of both appetitive (sugar) and aversive tastants (high salt).

Weaknesses:

There are several weaknesses that, if addressed, could greatly improve this work.<br /> 1) The authors combine the results and discussion but provide a very limited interpretation of their results. More discussion of the results would help to highlight what this paper contributes, how the authors interpret their results, and areas for future study.<br /> 2) The authors rename previously studied populations of labellar GRNs to arbitrary letters, which makes it difficult to understand the experiments and results in some places. These GRN populations would be better referred to according to the gustatory receptors they are known to express.<br /> 3) The conclusion that GRNs responsible for high salt aversion may be inhibited by those that function in low salt attraction is not well substantiated. This conclusion seems to come from the fact that overexpression of Ir60b in salt attraction and salt aversion sensory neurons still leads to salt aversion, but there need not be any interaction between these two types of sensory neurons if they act oppositely on downstream circuits.<br /> 4) The authors rely heavily on a new Droso-X behavioral apparatus that is not sufficiently described here or in the previous paper the authors cite. This greatly limits the reader's ability to interpret the results.

Reviewer #3 (Public Review):

Summary:<br /> Sang et al. successfully demonstrate that a set of single sensory neurons in the pharynx of _Drosophila_ promotes avoidance of food with high salt concentrations, complementing previous findings on Ir7c neurons with an additional internal sensing mechanism. The experiments are well-conducted and presented, convincingly supporting their important findings and extending the understanding of internal sensing mechanisms. However, a few suggestions could enhance the clarity of the work.

Strengths:<br /> The authors convincingly demonstrate the avoidance phenotype using different behavioral assays, thus comprehensively analyzing different aspects of the behavior. The experiments are straightforward and well-contextualized within existing literature.

Weaknesses:<br /> Discussion<br /> While the authors effectively relate their findings to existing literature, expanding the discussion on the surprising role of Ir60b neurons in both sucrose and salt rejection would add depth. Additionally, considering Yang et al. 2021's (https://doi.org/10.1016/j.celrep.2021.109983) result that Ir60b neurons activate feeding-promoting IN1 neurons, the authors should discuss how this aligns with their own findings.

Lines 187ff: The discussion primarily focuses on taste sensillae outside the labellum, neglecting peg-type sensillae on the inner surface. Clarification on whether these pegs contribute to the described behaviors and if the Poxn mutants described also affect the pegs would strengthen the discussion.

In line 261 the authors state: "We attempted to induce salt activation in the I-type sensilla by ectopically expressing Ir60b, similar to what was observed with Ir56b 8; however, this did not generate a salt receptor (Figures S6A)"<br /> An obvious explanation would be that these neurons are missing the identified necessary co-receptors Ir76b and Ir25a. The authors should discuss here if the Gr33a neurons they target also express these co-receptors, if yes this would strengthen their conclusion that an additional receptor might be missing.

Methods<br /> The description of the Droso-X assay seems to be missing some details. Currently, it is not obvious how the two-choice is established. Only one capillary is mentioned, I assume there were two used? Also, the meaning of the variables used in the equation (DrosoX and DrosoXD) are not explained.

The description of the ex-vivo calcium imaging prep. is unclear in several points:<br /> 1. It is lacking information on how the stimulus was applied (was it manually washed in? If so how was it removed?).<br /> 2. The authors write: "A mild swallow deep well was prepared for sample fixation." I assume they might have wanted to describe a "shallow well"?<br /> 3. "...followed by excising a small portion of the labellum in the extended proboscis region to facilitate tastant access to pharyngeal organs." It is not clear to me how one would excise a small portion of the labellum, the labellum depicts the most distal part of the proboscis that carries the sensillae and pegs. Did the authors mean to say that they cut a part of the proboscis?

eLife assessment

The authors have identified a media condition that maintains iPSCs in suspension cultures by inhibiting the PKCβ and Wnt signaling pathways. The manuscript is valuable for the pluripotent stem cell field. This is a solid study with substantial data to support the conclusions in the manuscript.

Reviewer #1 (Public Review):

Summary:<br /> The authors have presented data showing that there is a greater amount of spontaneous differentiation in human pluripotent cells cultured in suspension vs static and have used PKCβ and Wnt signaling pathway inhibitors to decrease the amount of differentiation in suspension culture.

Strengths:<br /> This is a very comprehensive study that uses a number of different rector designs and scales in addition to a number of unbiased outcomes to determine how suspension impacts the behaviour of the cells and in turn how the addition of inhibitors counteracts this effect. Furthermore, the authors were also able to derive new hiPSC lines in suspension with this adapted protocol.

Weaknesses:<br /> The main weakness of this study is the lack of optimization with each bioreactor change. It has been shown multiple times in the literature that the expansion and behaviour of pluripotent cells can be dramatically impacted by impeller shape, RPM, reactor design, and multiple other factors. It remains unclear to me how much of the results the authors observed (e.g. increased spontaneous differentiation) was due to not having an optimized bioreactor protocol in place (per bioreactor vessel type). For instance - was the starting seeding density, RPM, impeller shape, feeding schedule, and/or any other aspect optimized for any of the reactors used in the study, and if not, how were the values used in the study determined?

Reviewer #2 (Public Review):

This study by Matsuo-Takasaki et al. reported the development of a novel suspension culture system for hiPSC maintenance using Wnt/PKC inhibitors. The authors showed elegantly that inhibition of the Wnt and PKC signaling pathways would repress spontaneous differentiation into neuroectoderm and mesendoderm in hiPSCs, thereby maintaining cell pluripotency in suspension culture. This is a solid study with substantial data to demonstrate the quality of the hiPSC maintained in the suspension culture system, including long-term maintenance in >10 passages, robust effect in multiple hiPSC lines, and a panel of conventional hiPSC QC assays. Notably, large-scale expansion of a clinical grade hiPSC using a bioreactor was also demonstrated, which highlighted the translational value of the findings here. In addition, the author demonstrated a wide range of applications for the IWR1+LY suspension culture system, including support for freezing/thawing and PBMC-iPSC generation in suspension culture format. The novel suspension culture system reported here is exciting, with significant implications in simplifying the current culture method of iPSC and upscaling iPSC manufacturing.

Another potential advantage that perhaps wasn't well discussed in the manuscript is the reported suspension culture system does not require additional ECM to provide biophysical support for iPSC, which differentiates from previous studies using hydrogel and this should further simplify the hiPSC culture protocol.

Interestingly, although several hiPSC suspension media are currently available commercially, the content of these suspension media remained proprietary, as such the signaling that represses differentiation/maintains pluripotency in hiPSC suspension culture remained unclear. This study provided clear evidence that inhibition of the Wnt/PKC pathways is critical to repress spontaneous differentiation in hiPSC suspension culture.

I have several concerns that the authors should address, in particular, it is important to benchmark the reported suspension system with the current conventional culture system (eg adherent feeder-free culture), which will be important to evaluate the usefulness of the reported suspension system. Also, the manuscript lacks a clear description of a consistent robust effect in hiPSC maintenance across multiple cell lines. There are also several minor comments that should be addressed to improve readability, including some modifications to the wording to better reflect the results and conclusions.

Reviewer #3 (Public Review):

In the current manuscript, Matsuo-Takasaki et al. have demonstrated that the addition of PKCβ and WNT signaling pathway inhibitors to the suspension cultures of iPSCs suppresses spontaneous differentiation. These conditions are suitable for large-scale expansion of iPSCs. The authors have shown that they can perform single-cell cloning, direct cryopreservation, and iPSC derivation from PBMCs in these conditions. Moreover, the authors have performed a thorough characterization of iPSCs cultured in these conditions, including an assessment of undifferentiated stem cell markers and genetic stability. The authors have elegantly shown that iPSCs cultured in these conditions can be differentiated into derivatives of three germ layers. By differentiating iPSCs into dopaminergic neural progenitors, cardiomyocytes, and hepatocytes they have shown that differentiation is comparable to adherent cultures. This new method of expanding iPSCs will benefit the clinical applications of iPSCs.

Recently, multiple protocols have been optimized for culturing human pluripotent stem cells in suspension conditions and their expansion. Additionally, a variety of commercially available media for suspension cultures are also accessible. However, the authors have not adequately justified why their conditions are superior to previously published protocols (indicated in Table 1) and commercially available media. They have not conducted direct comparisons. Additionally, the authors have not adequately addressed the observed variability among iPSC lines. While they claim in the Materials and Methods section to have tested multiple pluripotent stem cell lines, they do not clarify in the Results section which line they used for specific experiments and the rationale behind their choices. There is a lack of comparison among the different cell lines. It would also be beneficial to include testing with human embryonic stem cell lines. Additionally, there is a lack of information regarding the specific role of the two small molecules in these conditions. The authors have not attempted to elucidate the underlying mechanism other than RNA expression analysis.

For these reasons some aspects of the manuscript need to be extended:

1. It is crucial for authors to specify the culture media used for suspension cultures. In the Materials and Methods section, the authors mentioned that cells in suspension were cultured in either StemFit AK02N medium, 415 StemFit AK03N (Cat# AK03N, Ajinomoto, Co., Ltd., Tokyo, Japan), or StemScale PSC416 suspension medium (A4965001, Thermo Fisher Scientific, MA, USA). The authors should clarify in the text which medium was used for suspension cultures and whether they observed any differences among these media.

2. In the Materials and Methods section, the authors mentioned that they used multiple cell lines for this study. However, it is not clear in the text which cell lines were used for various experiments. Since there is considerable variation among iPSC lines, I suggest that the authors simultaneously compare 2 to 3 pluripotent stem cell lines for expansion, differentiation, etc.

3. Single-cell sorting can be confusing. Can iPSCs grown in suspensions be single-cell sorted? Additionally, what was the cloning efficiency? The cloning efficiency should be compared with adherent cultures.

4. The authors have not addressed the naïve pluripotent state in their suspension cultures, even though PKC inhibition has been shown to drive cells toward this state. I suggest the authors measure the expression of a few naïve pluripotent state markers and compare them with adherent cultures

eLife assessment

This is an important study that develops a method to fluorescently label peptide MHC complexes on live dendritic cells to enable detection of antigen specific T cells in polyclonal populations. Solid evidence that this can be used to effectively identify antigen specific T cells in vitro and in vivo is provided for one model antigen systems (Ova-OTII). The approach has exciting potential as prior single step methods with directly conjugated single peptides have generally failed due to high background. Thus, this approach potentially moves the state of the art forward, but further work is needed to realise and determine the limits and ultimate utility of the approach.

Reviewer #1 (Public Review):

Summary:<br /> The authors develop a method to fluorescently tagged peptides loaded onto dendritic cells using a three step method. These include a pre blocking step to block endogenous cysteine motifs on the DC surface, loading a tetracystein motif modified peptide on surface MHC and a labelling step done on the surface of live DC using a dye with high affinity for the added motif. The results are convincing in demonstrating in vitro and in vivo T cell activation and efficient label transfer to specific T cells in vivo. The label transfer technique will be useful to identify T cell that have recognised a DC presenting a specific peptide antigen to allow the isolation of the T cell and cloning of its TCR subunits, for example. It may also be useful as a general assay for in vitro or in vivo T-DC communication that can allow detection of genetic or chemical modulators.

Strengths:<br /> The study include both in vitro and in vivo analysis including flow cytometry and two photon laser scanning microscopy. The results are convincing and the level of T cell labelling with the fluorescent pMHC is surprisingly robust and suggests that the approach is potentially revealing something about fundamental mechanisms beyond the state of the art. They also provide practical information about the challenges of the method and discuss limitations.

Weaknesses:<br /> The method is demonstrated only at high pMHC density and it would need to be re-optimised to determine if it can be used at lower densities that may often be encountered physiologically.

Reviewer #2 (Public Review):

Summary:

The authors have developed novel Ovalbumin model (OTII) peptide that can be labeled with a site-specific FlAsH dye to track agonist peptides both in vitro and in vivo. The utility of this tool could allow better tracking of activated polyclonal T cells particularly in novel systems. The authors have provided solid evidence that peptides are functional, capable of activating OTII T cells, and these peptides can undergo trogocytosis by cognate T cells only.

Strengths:<br /> -An extensive array of in vitro and in vivo studies are used to assess peptide functionality.<br /> -Nice use of cutting edge intravital imaging,<br /> -internal controls such as multiple non-cogate T cells were used to improve robustness of the results<br /> -One of the strengths is the direct labeling of the peptide, and the potential utility in other systems.

Weaknesses:<br /> -Peptide labeling specificity and efficiency is not clear. High levels of background labeling. While it was sufficient for demonstrating the system works, it may pose problems depending on the peptide sequence, and/or use at lower dose.<br /> -Only one peptide system was tested, namely OVA323-339 region.<br /> -Limited novel biological findings. This study mostly describes a new tool that may have exciting potential.

eLife assessment

The study presents a valuable finding on sexually-dimorphic patterns of osteocyte transcriptomics and low calcium diet-induce bone loss. The evidence supporting the claims of the authors regarding the protective role of Irisin/FNDC5-deficiency in lactation and low-calcium diet in female mice is incomplete, due to the relatively modest phenotypes observed and the insufficient link between Irisin/FNDC5 and calcium homeostasis or milk production.

Reviewer #1 (Public Review):

In this manuscript, Shimonty and colleagues study the effects of FNDC5/irisin deletion on osteocytes in a sex-specific manner using models of lactation-induced bone loss and bone loss due to low calcium diet (LCD). Consistent with the previous findings of Kim et al. (2018), the authors report 'protective' effects of irisin deficiency in lactating female FNDC5-null mice due to reduced osteocytic osteolysis. Interestingly, FNDC5 null mice show distinct changes when placed on LCD, with mutant females showing some protection from hyperparathyroidism-induced bone loss, while mutant males (which have more cortical bone at baseline) show increased LCD-induced bone loss. Furthermore, new insights into irisin's role in osteocytes regarding cellular energetic metabolism were provided by sex and gene-dependent transcriptomic datasets. Strengths of the well-written manuscript include a clear description of sex-dependent effects, strong transcriptomic datasets, and a focus on cortical bone changes using microCT, histomorphometry, BSEM, and serum analysis. Despite these strengths, important weaknesses are noted (below) which could be addressed to improve the impact of the work for a broad audience.

Major comments:

1. Overall, the magnitude of the effect size due to FNDC5 deficiency in both male and female mice is rather modest. Looking at the data from a qualitative perspective, it is clear that knockout females still lose bone during lactation and on the low calcium diet (LCD). It is difficult to assess the physiologic consequence of the modest quantitative 'protection' seen in FNDC5 mutants since the mutants still show clear and robust effects of lactation and LCD on all parameters measured. Similarly, the magnitude of the 'increased' cortical bone loss in FNDC5 mutant males is also modest and perhaps could be related to the fact that these mice are starting with slightly more cortical bone. Since the authors do not provide a convincing molecular explanation for why FNDC5 deficiency causes these somewhat subtle changes, I would like to offer a suggestion for the authors to consider (below, point #2) which might de-emphasize the focus of the manuscript on FNDC5. If the authors chose not to follow this suggestion, the manuscript could be strengthened by addressing the consequences of the modest changes observed in WT versus FNDC5 KO mice.

2. The bone RNA-seq findings reported in Figures 4-6 are quite interesting. Although Youlten et al previously reported that the osteocyte transcriptome is sex-dependent, the work here certainly advances that notion to a considerable degree and likely will be of high interest to investigators studying skeletal biology and sexual dimorphism in general. To this end, one direction for the authors to consider might be to refocus their manuscript toward sexually-dimorphic gene expression patterns in osteocytes and the different effects of LCD on male versus female mice. This would allow the authors to better emphasize these major findings, and to then use FNDC5 deficiency as an illustrative example of how sexually-dimorphic osteocytic gene expression patterns might be affected by deletion of an osteocyte-acting endocrine factor. Ideally, the authors would confirm RNA-seq data comparing male versus female mice in osteocytes using in situ hybridization or immunostaining.

3. Along the lines of point #2 (above), the presentation of the RNA-seq studies in Figures 4-6 is somewhat confusing in that the volcano plot titles seem to be reversed. For example, Figure 4A is titled "WT M: WT F", but the genes in the upper right quadrant appear to be up-regulated in female cortical bone RNA samples. Should this plot instead be titled "WT F: WT M"? If so, then all other volcano plots should be re-titled as well.

4. Have the authors compared male versus female transcriptomes of LCD mice?

5. It would be appreciated if the authors could provide additional serum parameters (if possible) to clarify incomplete data in both lactation and low-calcium diet models: RANKL/OPG ratio, Ctx, PTHrP, and 1,25-dihydroxyvitamin D levels.

6. Lastly, the data that overexpressing irisin improved bone properties in Fig 2G was somewhat confusing. Based on Kim et al.'s (2018) work, irisin injection increased sclerostin gene expression and serum levels, thus reducing bone formation. Were sclerostin levels affected by irisin overexpression in this study? Was irisin's role in modulating sclerostin levels attenuated with additional calcium deficiency?

Reviewer #2 (Public Review):

Summary:

The goal of this study was to examine the role of FNDC5 in the response of the murine skeleton to either lactation or a calcium-deficient diet. The authors find that female FNDC5 KO mice are somewhat protected from bone loss and osteocyte lacunar enlargement caused by either lactation or a calcium-deficient diet. In contrast, male FNDC5 KO mice lose more bone and have a greater enlargement of osteocyte lacunae than their wild-type controls. Based on these results, the authors conclude that in males irisin protects bone from calcium deficiency but that in females it promotes calcium removal from bone for lactation.

While some of the conclusions of this study are supported by the results, it is not clear that the modest effects of FNDC5 deletion have an impact on calcium homeostasis or milk production.

Specific comments:

1. The authors sometimes refer to FNDC5 and other times to irisin when describing causes for a particular outcome. Because irisin was not measured in any of the experiments, the authors should not conclude that lack of irisin is responsible. Along these lines, is there any evidence that either lactation or a calcium-deficient diet increases the production of irisin in mice?

2. The results of the irisin-rescue experiment shown in figure 2G cannot be appropriately interpreted without normal diet controls. In addition, some evidence that the AAV8-irisin virus actually increased irisin levels in the mice would strengthen the conclusion.

3. There is insufficient evidence to support the idea that the effect of FNDC5 on bone resorption and osteocytic osteolysis is important for the transfer of calcium from bone to milk. Previous studies by others have shown that bone resorption is not required to maintain milk or serum calcium when dietary calcium is sufficient but is critical if dietary calcium is low (Endo. 156:2762-73, 2015). To support the conclusions of the current study, it would be necessary to determine whether FNDC5 is required to maintain calcium levels when lactating mice lack sufficient dietary calcium.

4. The amount of cortical bone loss due to lactation is very similar in both WT and FNDC5 KO mice. The results of the statistical analysis of the data presented in figure 1B are surprising given the very similar effect size of lactation. The key result from the 2-way ANOVA is whether there is an effect of genotype on the effect size of lactation (genotype-lactation interaction). The interaction terms were not provided. Similar concerns are noted for the results shown in figure 1G and H.

5. It is not clear what justifies the term 'primed' or 'activated' for resorption. Is there evidence that a certain level of TRAP expression lowers the threshold for osteocytic osteolysis in response to a stimulus?

Reviewer #3 (Public Review):

Summary: Irisin has previously been demonstrated to be a muscle-secreted factor that affects skeletal homeostasis. Through the use of different experimental approaches, such as genetic knockout models, recombinant Irisin treatment, or different cell lines, the role of Irisin on skeletal homeostasis has been revealed to be more complex than previously thought and this warrants further examination of its role. Therefore, the current study sought to rigorously examine the effects of global Irisin knockout (KO) in male and female mouse bone. Authors demonstrated that in calcium-demanding settings, such as lactation or low-calcium diet, female Irisin KO mice lose less bone compared to wild-type (WT) female mice. Interestingly male Irisin KO mice exhibited worse skeletal deterioration compared to WT male mice when fed a low-calcium diet. When examined for transcriptomic profiles of osteocyte-enriched cortical bone, authors found that Irisin KO altered the expression of osteocytic osteolysis genes as well as steroid and fatty acid metabolism genes in males but not in females. These data support the authors' conclusion that Irisin regulates skeletal homeostasis in sex-dependent manner.

Strengths: The major strength of the study is the rigorous examination of the effects of Irisin deletion in the settings of skeletal maturity and increased calcium demands in female and male mice. Since many of the common musculoskeletal disorders are dependent on sex, examining both sexes in the preclinical setting is crucial. Had the investigators only examined females or males in this study, the conclusions from each sex would have contradicted each other regarding the role of Irisin on bone. Also, the approaches are thorough and comprehensive that assess the functional (mechanical testing), morphological (microCT, BSEM, and histology), and cellular (RNA-seq) properties of bone.

Weaknesses: One of the weaknesses of this study is a lack of detailed mechanistic analysis of why Irisin has a sex-dependent role on skeletal homeostasis. This absence is particularly notable in the osteocyte transcriptomic results where such data could have been used to further probe potential candidate pathways between LC females vs. LC males.

Another weakness is authors did not present data that convincingly demonstrate that Irisin secretion is altered in the skeletal muscle between female vs. male WT mice in response to calcium restriction. The supplement skeletal muscle data only present functional and electrophysiolgical outcomes. Since Itgav or Itgb5 were not different in any of the experimental groups, it is assumed that the changes in the level of Irisin is responsible for the phenotypes observed in WT mice. Assessing Irisin expression will further strengthen the conclusion based on observing skeletal changes that occur in Irisin KO male and female mice.

eLife assessment

This important study provides novel strategies to overcome certain limitations when investigating the metabolism of hematopoietic stem cells, mainly due to their low abundance. The study provides compelling evidence suggesting that proliferative hematopoietic stem cells mainly use glycolysis (rather than mitochondrial OXPHOS or TCA cycle) as their primary energy source during emergency hematopoiesis. The article provides direct links between metabolic features and cell proliferation and explores alternative energy sources, and is of great interest to stem cell biologists.

Review #1 (Public Review)

Watanuki et al used metabolomic tracing strategies of U-13C6-labeled glucose and 13C-MFA to quantitatively identify the metabolic programs of HSCs during steady-state, cell-cycling, and OXPHOS inhibition. They found that 5-FU administration in mice increased anaerobic glycolytic flux and decreased ATP concentration in HSCs, suggesting that HSC differentiation and cell cycle progression are closely related to intracellular metabolism and can be monitored by measuring ATP concentration. Using the GO-ATeam2 system to analyze ATP levels in single hematopoietic cells, they found that PFKFB3 can accelerate glycolytic ATP production during HSC cell cycling by activating the rate-limiting enzyme PFK of glycolysis. Additionally, by using Pfkfb3 knockout or overexpressing strategies and conducting experiments with cytokine stimulation or transplantation stress, they found that PFKFB3 governs cell cycle progression and promotes the production of differentiated cells from HSCs in proliferative environments by activating glycolysis. Overall, in their study, Watanuki et al combined metabolomic tracing to quantitatively identify metabolic programs of HSCs and found that PFKFB3 confers glycolytic dependence onto HSCs to help coordinate their response to stress.

Review #2 (Public Review)

In the manuscript Watanuki et al. define the metabolic profile of HSCs in stress/proliferative (myelosuppression with 5-FU), and mitochondrial inhibition and homeostatic conditions. Their conclusions are that during proliferation HSCs rely more on glycolysis (as other cell types) while HSCs in homeostatic conditions are mostly dependent on mitochondrial metabolism. Mitochondrial inhibition is used to demonstrate that blocking mitochondrial metabolism results in similar features of proliferative conditions.

The authors used state-of-the-art technologies that allow metabolic readout in a limited number of cells like rare HSCs. These applications could be of help in the field since one of the major issues in studying HSCs metabolism is the limited sensitivity of the "standard" assays, which make them not suitable for HSC studies.