This review reflects comments and contributions by Vaishnavi Ananthanarayanan, Xianrui Cheng, Joachim Goedhart, Arthur Molines, Eric Peterman, Sanjana Pillay, Pablo Ranea-Robles, Mugdha Sathe and Zara Weinberg
The study by Kidwell et al. reports that lateral transfer of mitochondria from macrophages to breast cancer cells activates signaling pathways that promote cell proliferation. The study uses single-cell imaging strategies to show transfer of fluorescently labeled mitochondria; the transfer is rare, but it is convincingly demonstrated. The study reports that transferred mitochondria were depolarized and accumulate ROS, which signaled through ERK and promoted proliferation of the recipient cells as well as their daughter cells who also inherited the transferred mitochondria.
The report sheds light on the mechanism by which macrophages stimulate cancer cell proliferation in the tumor microenvironment, which has strong implications in the cancer field. One of the novelties of this study is the fact that the transferred mitochondria are rather dysfunctional (depolarized), in contrast with most of the previous reports on mitochondrial transfer, which was thought to be a way to "rejuvenate" the mitochondrial pool of a cell.
The authors make some interesting observations and the methods are described in detail. With the current data, there are questions as to whether all effects are due to biological changes, and there may be a need to re-evaluate some of the data and nuance the interpretation of the results.
General comments are outlined below followed by annotations/specific comments on manuscript content (in order of appearance).
- One of the most important claims is that mitochondria are the organelles responsible for the activation of the signals of cell proliferation. However, a previous report by the last author reported that macrophages transfer cytoplasm to recipient cells. It cannot be excluded that other organelles or cellular fragments are transferred as well and contribute to the observed effects (ERK activity). Perhaps a good way to solve this would be the use of macrophages that are devoid of mitochondria. At least, this aspect should be discussed in the manuscript.
- Most of the positive examples of transferred mitochondria discussed appeared in a small clump. However, there also appears to be another population that was more diffuse and co-localizes with host mitochondria (e.g., Fig2B, bottom right panels). It would be helpful to show results of these sibling mitochondria for assays performed on their clumpy siblings. If they behave differently, it would be helpful to provide some explanation.
- The effects that are attributed to the transferred mitochondria are highly variable (figures 1F, 3A,E) and often due to a subpopulation of samples that show a few extreme values (e.g. figures 2D, 3E, S4B, S4D). This might be expected from effects that are caused by a single mitochondria (which has a small volume) that is transferred to a complete cell. This complicates the study of the transfer process and effects and should be discussed. Also, do the authors have ideas how to improve the system, to make it more robust and easier to study the effects?
- The authors conclude that the transfer of dysfunctional mitochondria generated a signal mediated by ROS that activates cell proliferation signals. The statement that "transferred mitochondria act as a signaling source that promotes cancer cell proliferation" is too strong. There is increased ROS production from mitochondria, yes, but an experiment in which ROS are decreased would be needed to properly sustain that conclusion. The title and abstract could be changed to better reflect the data.
- The study may benefit from more direct evidence to support its conclusion of increased proliferation after mitochondrial transfer. While the RNA-seq, flow cytometry, counting of completion of cytokinesis and dry mass measurements provided in the present study do lend some support to the proliferation hypothesis, they all seem indirect. With the biomarkers labeling the mitochondria of donor and potential recipient cells, high content imaging and tracking of cells could be used to monitor cell division. A comparison of cell division rates of transfer-positive cells and transfer-negative cells will provide a more pertinent test of whether mitochondrial transfer promotes recipient cell proliferation.
The authors have used such a tracking-based approach on a very small scale (n=5) to measure daughter cell growth rate. However, the data do not show a statistically significant difference between the growth rates of daughters that inherited transferred mitochondria and those who did not (Fig S3). Increasing the case number via high content imaging would help obtain sufficient data points for a reliable statistical test. In addition, as suggested above, an accounting of the daughter cells' division rate for transfer positive and negative cells would provide another line of evidence to either prove or disprove the increased proliferation rate hypothesis. The same suggestion goes to the optically induced ERK activation experiments shown in Fig3F. It is also helpful to include references that studied how ERK signaling promotes proliferation and compare the evidence here with evidence or assays used in those studies as a benchmark.
Specific comments
Figure S1A - The authors could perhaps use a more aggressive gating strategy here, clipping closer to the 231 population described in Fig S1A - picking only the center of the cluster in the upper left of the RFP vs CD11b plot would likely not affect results but make them more convincing by unequivocally excluding macrophages.
Figure S1B - Could perhaps be an interesting follow-up question for future works re: differences between cell lines and propensities to transfer mitochondria. Did the authors attempt to use other cell lines (ie, MDCK, HeLa, iPSCs, etc)?
Figure S1B - Did the authors see an increase in growth rate in MCF10A line despite the lower growth rate?
‘physically separated from macrophages by a 0.4μM trans-well insert’ - should this read 0.4 micrometer?
Figure S1F - The authors wrote that they used a two-way ANOVA analysis, could you report the factors used for that analysis in the Figure legend.
Figure 1B - It is difficult to see the arrowheads in 1B, suggest moving them so they are not covering the magenta fluorescence, have them point from a different angle, and make them more brightly colored. Insets here would help the reader. A negative control image from a monoculture would also be helpful, to ensure the GFP signal is not an artifact of culture conditions.
Figure 1D - Not sure about the 0.2% baseline assigned for the monoculture of cancer cells (that does not have the macrophages with the Emerald mitochondria). It is determined with cytometry - I am no expert on that topic, so maybe I missed something - but it looks weird to see some cells with transfer when there is a monoculture.
Figure 1F - For graphs that do not show zero (as in 1F), the bar should be omitted. In these cases the length of the bar does not reflect the average of the data (as it does in 1D).
Figure 1 - Given that these data are fractions of a population (i.e. can be described via a contingency table), isn't something like a Fisher's exact test a better measure of significance here?
Single cell RNA- sequencing - In the methods section the authors mention doing a differential analysis between the cells that received the mitochondria and the cells that didn’t. It might be worth introducing a figure (a heatmap or a U-MAP) relating to this analysis. Single cell sequencing would not only affirm the heterogeneity between these two populations but also help in highlighting the novel cell surface markers associated with the two populations.
Figure S3 - There is no statistical test to check for ‘increase in their rate of change of dry mass over time versus sister cells that did not inherit macrophage mitochondria’. What are the colours indicative of in S3B? Can this be reported in the figure legend.
‘mito-mEm+ mitochondria remained distinct from the recipient host mitochondrial network, with no detectable loss of the fluorescent signal for over 15 hours’ - It is surprising that the transferred mitochondria do (or cannot) fuse with the host 231 mitochondria. It is unclear in these images, but the 231 mitochondria appear fragmented too. Is it possible that the mitochondrial fusion machinery (Opa1 or Mfn1/2) are inactive?
Figure 2B - What does the MTDR staining of the macrophage mitochondria prior to transfer look like? Important to check this to confirm that only the transferred mitochondria had lower membrane potential.
Figure 2A, 2BB and S1D - How were the colocalizations assessed? Was it just a visual assessment? Given the importance of these experiments for the whole story, having a quantification of the level of colocalization with each dye would be important.
Figure S1D - The paper makes an argument about mitochondria transferred from Macrophages (marked green) having positive DNA stain (gray), but appearing depolarized (negative TMRM stain). The image in FigS1D is peculiar, as the majority of the 231 cells' mitochondria appear to not have any DNA stain but maintain membrane potential (positive in TMRM), while some (just above the green macrophage mitochondria) do have both DNA stain and membrane potential. The authors might want to clarify whether this is a typical scenario, and if so perhaps offer an explanation as to why the 231 mitochondria exhibit such heterogeneity.
‘we confirmed that 91% of transferred mitochondria were not encapsulated by a membranous structure, thus excluding sequestration as a mechanism for explaining the lack of degradation or interaction with the endogenous mitochondrial network’ - This is based on co-staining with MemBrite 640/660, which is a dye that "covalently labels the surface of live cells", thus there is a concern as to whether this approach allows to study whether the mitochondrium is encapsulated by an endomembrane.
Figure 2 ‘Majority (57%) of donated mitochondria do not colocalize with LysoTracker signal (N=24 cells, 4 donors) - Here the paper implies that some transferred mitochondria do co-localize with lysoTracker signal. More importantly, they co-localize with host mitochondria. It raises the question of whether they signal through ROS and ERK like their clumpy siblings who are in the limelight of most figures.
‘macrophage mitochondria are depolarized but remain in the recipient cancer cell’ - Did the authors examine the extent of cancer cell death in their co-culture system (due to the activation of apoptosis by the depolarized mitochondria)?
‘significantly higher ratios of oxidized:reduced protein were associated with the transferred mitochondria versus the host network’ - Here too, it would be important to check the mito-Grx1-roGFP2 readout of macrophage mitochondria prior to transfer.
Figure 2C–D - Like in Fig 2B, in the bottom left of panel of Fig 2C there are a lot of donor mitochondria not in highly oxidized state and the growth/proliferation phenotypes apply mostly to donor mitochondria that appear 'clumpy'. Perhaps it is worth commenting on whether there is a link between donor mitochondrial morphology and the suspected proliferation-enhancing phenotype.
‘At 24 hours, we observed a similar trend, but no statistically significant difference (Fig. S4D). These results indicate ROS accumulates at the site of transferred mitochondria in recipient cancer cells’ - if a specific sensor fails to show a significant oxidation at 24 hours compared mito-Grx1-roGFP2 which reports on mitochondrial glutathione redox state, does that mean there are ROS independent ways to oxidize Glutathione? The authors did see cell growth phenotype both in 24 and 48 hours which suggests that something is happening in 24 hours despite no significant difference in ROS H2O2 sensor.
The differences in ratio for the two sensors used are not very convincing. In Fig 2D and Fig S4B and D the “host” and “transfer” populations are very similar. The difference seems only due to the presence of a few outliers in the “transfer” populations. More importantly, sometimes it seems that these outliers come mostly from one donor rather than being present in all 3 donors. It could be good to show histograms of the two populations for each replicate/donor and maybe redo the stats excluding these outliers.
Figure S5C - it seems like the percentage of cells that divided is the same for unstimulated cells and cells with stimulated mito-KillerRed. Isn't this contrary to the expectation? The figure shows that photobleaching cytoplasm decreased % cell division, which is puzzling.
‘ROS induces several downstream signaling pathways’ - We would not expect the authors to investigate every signaling pathway, but wonder if the PI3K pathway was explored? It seems to be the other major cancer/proliferative pathway induced by ROS.
‘Recipient 231 cells had significantly higher cytoplasmic to nuclear (C/N) ERK-KTR ratios compared to cells that did not receive transfer’ - Since two different quantification styles with opposite fraction values were used, is it possible to please specify which one was used here.
Figure 3A - In the 'cyto' condition 6 out of 13 fields have no cells that divide. Is that expected? What is the percentage of dividing cells for cells that were not illuminated at all (a control that is lacking)? There is large variation, ranging from 0% to 22%. The evidence that illumination of KillerRed leads to increased proliferation is rather weak. Also, since Cyto and Mito are different cells, is a "paired" statistical test the right kind of test to use here?
Figure 3B - Please show the outlines of the nuclei and that of the cell.
Figure 3C - Please omit bar, see comment on panel 1F.
Figure 3D - it is peculiar that ERK-KTR in Fig 3D is so strongly cytosolic while in Fig 3B it is almost exclusively nuclear. If this sensor behaves differently in different situations, the authors may want to comment on how that would affect their conclusions.
Figure 3E - The effect of 'opto-induced' ERK activity is weak. The initial ERK-KTR is 1 at time point zero (as the data is normalized to this timepoint) and around 1 for both the cyto and mito condition. A statistical difference is observed, but the effect is minor and it is unclear whether it is biologically meaningful. The 'cyto' condition shows an average below 1 and the mito condition remains 1, suggesting that ERK activity remains constant when ROS are produced in the mitochondria.
Also from S8C and 3E it appears cyto actually shows a decrease rather than mito showing an increase, could the authors comment on this?
‘Furthermore, treatment with an ERK inhibitor (ERKi) was sufficient to inhibit ERK activity ‘- curious as to whether antioxidant treatment would reverse any proliferative phenotypes?
‘patient-derived xenografts (PDxOs)’ - As a control it would be relevant to include a normal mammary organoid model perhaps from the same patient to demonstrate that the transfer of mitochondria specifically to the cancer cells is more beneficial.
‘macrophages to both HCI-037 and HCI-038 PDxO cells (Fig. 4G)’ - Why is M0 able to transfer efficiently to HCL-037 tumour when its mitochondrial network is less fragmented as M2? Are mito transfer from M0 depolarised and accumulate ROS or show increased ERK activity or increased cell proliferation?
‘M2-like macrophages preferentially transferred mitochondria to the bone metastasis PDxO cells (HCI-038) when compared to primary breast tumor PDxO cells (HCI-037)’ -The authors may want to check this statement here as it is in consistent with their data plot. In Fig. 4G, M2/PDxO transfer percentages for HCI-037 and HCI-038 are about the same, unless the authors provide statistical tests to prove otherwise. Instead, M0 appears to transfer mitochondria to HCI-037 much more efficiently than it does HCI-038.
‘M2-like macrophages exhibit mitochondrial fragmentation’ - Is there a correlation between the status of the mitochondrial network in the donor and the % of transfer to the recipient? If so, this would be a correlation that would support the conclusions.
‘accumulate ROS, leading to increased ERK activity’ - Did the authors obtain similar results with the PDXOs? It would be an interesting observation if the primary samples also exhibit a mechanism similar to established cell lines wherein there are more accumulated genetic changes. It would also be interesting to examine whether there is any difference in the ROS-ERK mechanism for primary and metastatic tumour.
‘in cancer cells that receive exogenous mitochondria’ - Since these macrophages also transfer mitochondria to non-malignant cells, such as MCF10A cells shown in Fig S1B, perhaps the authors could comment on whether this is part of a physiological process that would also promote normal cell growth?