On 2019-05-20 18:59:04, user Aaron Gitler wrote:
Comments on Bolognesi et al.
By Aaron Gitler and Steven Boeynaems (Stanford University)
Many human neurodegenerative diseases are associated with protein aggregation. In Alzheimer disease, Amyloid-beta (A) and tau form aggregates, in Parkinson disease, synuclein forms aggregates, and in Huntington disease, a form of huntingtin with an expanded polyglutamine tract forms aggregates. Whether or not these aggregates cause disease is an area of intense study, with important therapeutic implications. For example, if the aggregates are what is causing disease, then strategies to dissolve or degrade the aggregates would be warranted. But if the aggregates were potentially protective, then therapeutic strategies to promote aggregate formation of toxic entities would be warranted. For the neurodegenerative diseases ALS and FTD, the RNA-binding protein TDP-43 is mislocalized to the cytoplasm and forms aggregates in the degenerating neurons of patients with these diseases.
TDP-43 harbors a prion-like domain, which is able to drive liquid-liquid phase separation. The liquid phases formed by TDP-43 are thought to lead to the formation of solid insoluble aggregates. Several mutations in the gene encoding TDP-43 have been identified in familial and sporadic ALS patients. Almost all of these mutations are located in the prion-like domain. Some reports have indicated that these pathogenic mutations can increase the aggregation-propensity of TDP-43. Whether the loss of TDP-43 from the nucleus (where it regulates a diverse array of mRNA targets) or rather its accumulation in the cytoplasm in an aggregated form (perhaps causing dysregulation of other RNAs and RNA-binding proteins) causes ALS (i.e., loss of function vs. gain of function) is still unresolved and an area of intense interest.
In this manuscript by Bolognesi and colleagues, the authors performed a systematic analysis of the prion-like domain of TDP-43 – engineering over 50,000 mutations – assessing effects on aggregation and toxicity in a yeast model. They made several key findings:
1) Mutations that change hydrophobicity and aggregation are predictive of changes in toxicity<br />
2) Surprisingly, the mutations they introduced that increased hydrophobicity and aggregation actually decreased toxicity! <br />
3) The mutations that increase the liquid-like properties of TDP-43 increased toxicity<br />
4) Using a clever analysis of double mutant combinations they were able to infer the existence of specific structures within TDP-43’s prion-like domain.
Overall, this is a very exciting new story that provides a comprehensive and deep analysis of TDP-43 prion-like domain with surprising findings that mutations that increase aggregation actually decrease toxicity and the ones that increase the liquid phases of TDP-43 are the more toxic ones. This study has important implications for thinking about how TDP-43 contributes to ALS and FTD and also provides a framework and new method to study other neurodegenerative disease-associated aggregating proteins.
We have several comments and suggestions for the authors to consider.
1) RNA-binding seems to be required for TDP-43 toxicity in yeast (PMID: 18434538 and PMID: 20740007). The authors focused their mutational scan on the prion-like domain of TDP-43, which does not include the two upstream RNA-recognition motifs (RRMs). The specific mutations they introduced to the prion-like domain could potentially influence the ability of TDP-43 to bind RNA. It would be interesting for the authors to test several of their mutations for impact on RNA-binding. The binding site for TDP-43 (UGUGUGUG) has been characterized and several of its targets are known. It is possible that the mutations that decrease toxicity do so by decreasing TDP-43’s ability to bind RNA. This result would still be interesting and important and would provide insight into potential communication between the prion-like domain and the RRMs. The authors could also introduce additional mutations in the RRMs (e.g., FW) in the context of their most toxic mutants in order to define if the enhanced toxicity of these new mutations depends on RNA-binding or not.
2) Kaganovich, Kopito, and Frydman have described distinct subcellular quality control compartments, to which distinct misfolded proteins are routed (PMID: 18756251). These include a juxtanuclear compartment (JUNQ) and a peripheral perivacuolar compartment named insoluble protein deposit (IPOD). Proteins in the JUNQ are more mobile than those in the IPOD. Toxicity of a model misfolded protein can be reduced by directing it away the JUNQ and towards the IPOD (PMID: 22967507). The authors should consider analyzing whether their toxic and non-toxic mutants accumulate in yeast JUNQs or IPODs, using the established markers for these compartments.
3) Expression level of TDP-43 strongly correlates with toxicity in yeast (and other systems). Surprisingly, the authors see (in Extended Fig. 5b), that the toxic mutants are expressed at much lower levels than the non-toxic ones. This result is unexpected and it would be helpful to extend the analysis to additional mutants, and potentially longer time of protein induction (e.g., >8 hours).
4) While recurrent ALS mutations in TDP-43 were in general moderately more toxic when expressed in yeast, they do not necessarily associate with increased charge or decreased hydrophobicity. Rather, they seem enriched for G/A/N to S/T mutations (5/12). While such mutations could increase hydrophilicity substantially via phosphorylation, these human disease mutations do not replicate the pattern observed for the very toxic yeast mutations. An explanation why “severe” mutants are not observed in humans is that such mutations are potentially not tolerated during development, leaving only the possibility for moderate mutations to cause age-related disease. Hence, while the signature of increased hydrophilicity/decreased hydrophobicity is clearly associated with toxicity in yeast, this finding does not necessarily illuminate the biophysics behind TDP-43 toxicity in humans.
5) Some of the well-established TDP-32 ALS mutants have been previously studied in vitro, where authors have shown that these mutants perturb liquid phase separation of TDP-43 (PMID: 27545621). It would be great if the authors could specifically image these mutants in their yeast system. Additionally, evaluation of key mutants, both ALS-relevant ones and the ones with strong yeast phenotypes, in a human cellular context would be crucial for interpreting these results. It is known that condensates may present with different material properties between yeast and human cells (PMID: 26238190), and yeast cells potentially lack the (non-)canonical chaperone systems that may alter TDP-43 phase behavior (e.g. PMID: 29677512, PMID: 30100264).
6) Currently, the round, dynamic, perinuclear TDP-43 condensates are described as being liquid(-like). However, this claim is only weakly supported. Micronscale dynamic liquid behavior should be observed before such claims can be made (i.e., plastic deformation, wetting, fusion/fission). As mentioned above, investigating the relationship of these condensates to IPOD/JUNQ will be important.
7) This paper once more provides evidence for the fact that aggregates may be beneficial by sequestering toxic protein species (e.g. oligomers, liquid droplets). Interestingly, a recent study identified stress-induced liquid-like wildtype TDP-43 condensates that were toxic to human cells (PMID: 30853299). However, it remains unclear what the effect of ALS-associated mutations is on the latter ones, and how they relate to the TDP-43 pathology in human patients. While these two studies suggest that indeed “liquid droplet toxicity” can indeed happen in yeast and human cell culture, it remains to be tested whether this relates to the disease. Especially, in the context of yeast: Overexpression of TDP-43 out of its endogenous context induces spontaneous cytoplasmic condensation and toxicity, which is generally not observed in human systems. This is likely caused by TDP-43 perturbing endogenous yeast RNA metabolism and proteostasis pathways. Any molecular perturbation (i.e. mutations) that make TDP-43 inert for yeast clients (i.e. aggregation in this study, RNA-binding mutants in PMID: 18434538 and PMID: 20740007), are expected to reduce toxicity. While loss of RNA binding is indeed beneficial in yeast, in the human system RNA binding protects against toxicity (PMID: 30826182). The latter observation suggests that toxicity in yeast –and the associated biophysical features of TDP-43– may not always be correlated with toxicity in human system