Condensation properties of stress granules and processing bodies are compromised in myotonic dystrophy type 1

  1. Selma Gulyurtlu
  2. Monika S. Magon
  3. Patrick Guest
  4. Panagiotis P. Papavasiliou
  5. Kim D. Morrison
  6. Alan R. Prescott
  7. Judith E. Sleeman

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Abstract

RNA regulation in mammalian cells requires complex physical compartmentalisation, using structures thought to be formed by liquid-liquid phase separation. Disruption of these structures is implicated in numerous degenerative diseases. Myotonic dystrophy type 1 (DM1) is a multi-systemic trinucleotide repeat disorder resulting from an expansion of nucleotides CTG (CTGexp) in the DNA encoding DM1 protein kinase (DMPK). The cellular hallmark of DM1 is the formation of nuclear foci that contain expanded DMPK RNA (CUGexp) (with thymine instead of uracil). We report here the deregulation of stress granules (SGs) and processing bodies (P-bodies), two cytoplasmic structures key for mRNA regulation, in cell culture models of DM1. Alterations to the rates of formation and dispersal of SGs suggest an altered ability of cells to respond to stress associated with DM1, while changes to the structure and dynamics of SGs and P-bodies suggest that a widespread alteration to the biophysical properties of cellular structures is a consequence of the presence of CUGexp RNA.

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  1. Version published to 10.1242/dmm.049294
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    Reply to the reviewers

    Reviewer #1

    This manuscript by Gulyrutlu and co-workers addresses the role of CUG expanded repeat RNA associated with DM1 in regulating the formation of higher order RNP assemblies such as stress granules and P-bodies in the cell. The authors used lens epithelial cells (hLECs) derived from a DM1 patient

    We used cell lines from several patients and age-matched controls to avoid effects of individual cell-line variation. We will make sure that this is clear in the text.

    or a HeLa cell inducible model of DM1 to investigate whether expression of the CUG repeat-associated protein MBNL1 and CUGBP1 affected the formation and dispersal of stress granules and P-bodies. The authors show that MBNL1 and CUGBP1 are components of SGs and PBs in hLECs and HeLa cells. In cells expressing the CUG repeat, there are minor alterations in the dispersal of stress granules as well as in the formation of P-bodies.

    The alterations in the formation and dispersal of stress granules are not minor. For example, in the HeLa cell model, stress granules take more than twice as long to form in cell expressing the CUGexp repeats associated with DM1 and disperse in half the time. __These data are already in the results section, but we will highlight them in a revision and have included an additional representation of the data to the figure, using graphs of ‘proportion of cells with stress granules’ against time. __ The changes we see are as large, or larger, then results published elsewhere (see appendix below)

    MBNL1 could affect the formation and dispersal of SGs independent of the CUG repeat.

    In fact, we present data in HeLa cells with MBNL1 almost completely removed by shRNA revealing that this has a much smaller effect on stress granules than does the expression of CUGexp RNA. This is an important point, as it is widely assumed that most of the cellular defects in DM1 are caused by the ‘sequestration’ of MBNL1 in the CUGexp foci. Since only . This is not what our data show. In the hexanediol experiments, both cell lines over-express MBNL1 in similar amounts. The difference between them is that one cell line expresses a DMPK1 mini-gene with a CUG expansion and the other expresses a mini-gene without the expansion. Again, our results show that the alteration to P-body responses to 1,6-hexanediol can be attributed to the presence of the CUGexp RNA, rather than altered levels of MBNL1. We will revise the results and discussion to further emphasise this point.

    Finally, in HeLa cells, overexpression of MBNL1 can reduce the dispersal of P-bodies upon 1,6-hexanediol treatment.

    This is not what our data show. In the hexanediol experiments, both cell lines over-express MBNL1 in similar amounts. The difference between them is that one cell line expresses a DMPK1 mini-gene with a CUG expansion and the other expresses a mini-gene without the expansion. Again, our results show that the alteration to P-body responses to 1,6-hexanediol can be attributed to the presence of the CUGexp RNA, rather than altered levels of MBNL1. We will revise the results and discussion to further emphasise this point.

    Major comments:

    One limitation of the work is that the perturbations seen with stress granules or P-bodies are all relatively small, and no evidence for a functional consequence on gene expression is demonstrated. Specifically, the authors observe only minor alterations in the formation or disaggregation of PBs and SGs in these DM1 models. Further, some of the effects observed are independent of the CUG repeat expression, suggesting that MBNL1 and CUGBP1 might have independent roles in modulating some properties of SG and PB formation or dispersal.

    As above, the changes we see in SG formation and dispersal are not small. There are already numerous studies of the effects of DM1 on gene expression and mRNA splicing. This is not what we set out to study: we are interested in perturbations to the organisation of cellular structures associated with the expression of the CUGexp repeat RNA characteristic of DM1. We do show some data relating specifically to the proteins MBNL1 and CUGBP1 in the paper. shRNA resulting in almost complete loss of these proteins has much smaller effects that the expression of CUGexp RNA, suggesting that the major part of the effects caused by expression of the CUGexp RNA is not mediated through changes in MBNL1 or CUGBP1 levels. MBNL1 and CUGBP1 levels may well contribute to alterations in SG dynamics, but our data suggest that they are minor contributors. This is an advance in our current knowledge

    1. The authors could investigate whether the CUG repeat RNA itself is localized to SGs or PBs in their models, and whether the presence of the repeat RNA is absolutely necessary for regulating the dynamics of SG or PB formation.

    We have now done this. The CUG repeat RNA is not localised in stress granules or PBs to a detectable extent. This suggests that the effect we see on these structures by expression of the expanded RNA occurs despite the absence of the RNA from the structures. This is similar to the effects of the ALS-associated paraspeckle protein FUS, which can affect the integrity of nuclear LLPS structures (gems) despite not co-localising with them https://doi.org/10.1016/j.celrep.2012.08.025 We have added these data to the manuscript, as part of draft figure 8, and will add text emphasising this as an additional example of disease-causing macromolecules affecting the structure of LLPS domains in which they are not found.

    1. The authors use 1,6-hexanediol to suggests that PBs and SGs in HeLa cells show behavior analogous to LLPS. However, the use of 1,6,-hexanediol to establish an assembly as a LLPS is a relatively limited analysis (despite its widespread use in the field), since this compound can affect the formation of multiple cellular substructures that are not always LLPS (for example, see Wheeler et al, 2016, eLife).

    We are aware of and have cited this publication. Our comments about LLPS structures are measured, as there is still controversy about how to definitively identify them in cells. SGs and PBs have, however, previously been widely published to be formed by LLPS. The rapid exchange of SG and PB components during FRAP and the ability of SGs to both fuse and bud (seen in our supplementary movies) are also supportive of these structures behaving as LLPS structures in our models. Wheeler et showed that, in yeast, the nuclear pore complex and some cytoskeletal structures were affected by 1,6-hexanediol but membrane-bound structures such as the ER and mitochondria were not. The disruption of the nuclear pore complex is not unexpected, since phase separation is involved in cargo shuttling through the NPC (reviewed in https://doi.org/10.1016/j.devcel.2020.06.033). We will revise our discussion to make it more clear that we are not relying only on the use of 1,6-hexanediol to define SGs and PBs as LLPS structures but also on other aspects of their dynamic behaviour and on extensive prior literature.

    Significance

    This study would be of interest to the field if the impact of the DM! repeat RNAs on PB and SG were more substantial...

    As above, the effects we see on SG formation and loss are substantial. Tissue types affected in DM1 are prone to stress, particularly the lens of the eye, so alterations to cellular response to stress associated with the presence of CUG repeats are of key importance to understanding the cellular pathology of DM1.

    ...and if some functional consequences were demonstrated.

    In terms of function, we show altered responses to stress caused by the expression of CUGexp RNA and probably mediated through alterations in the propensity of LLPS cytoplasmic structures (SGs and PBs) to form and be resolved. Additionally, we can now show that SGs in HeLa cells expressing CUGexp RNA contain less total polyA RNA than is seen in controls, and that ‘docking’ events between SGs and PBs are compromised in cells with CUGexp RNA. These docking events are proposed to mediate transfer of RNA from SGs to PBs (reviewed in https://doi.org/10.1007/978-1-4614-5107-5_12). These new data demonstrate functional impairment of SGs and PBs associated with DM1. We have included this as an additional draft figure 8.

    The lack of a strong effect on SG or PB formation in the DM1 models, along with the CUG repeat-independent effect of MBNL1 on the formation and dispersal of these complexes, argues that MBNL1/CUGBP1 may not significantly affect the formation or dispersal of SGs and PBs.

    We are actually not arguing that MBNL1 and CUGBP1 are the main effectors in the changes we see to SGs and PBs, but that the CUGexp RNA is the key player, so are a little confused by this comment.

    __Reviewer #2: __

    In the current study, the authors compared the dynamics of P-bodies (PBs) and stress granules (SGs) between control and several DM1 cell lines. They found that MBNL1 and CUGBP1, two CUG repeat RNA-binding proteins that are primarily nuclear, could also co-localize with PBs in the cytoplasm and re-localize to SGs under stress. Small differences were observed in SG assembly and disassembly dynamics between control and DM1 HLECs, between HeLa cells expressing either CTG12 or CTG960, and between HeLa cells with and without shRNAs targeting CUGBP1 or MBNL1.

    As detailed above, the alterations in SG assembly and disassembly in cells expressing CUGexp RNA are not small, in contrast to those in cells will lowered expression of MBNL1 and CUGBP1, which are much smaller suggesting that the changes caused by CUGexp RNA largely do not result from loss of MBNL1 (or CUGBP1). We have inserted additional graphs of ‘proportion of cells with stress granules’ against time' and will modify the text to emphasise both of these points.

    Overall, the experiments were clearly described and the results properly presented. However, critical controls, as detailed below, are missing in multiple analyses. The mechanisms underlying these apparent differences are also unknown.

    We do not consider that any ‘critical controls’ are missing, but can supply all of the additional analysis of our data that the reviewer requests below. We can also now provide additional mechanistic insight and will add an additional figure showing lowered amount of polyA RNA in stress granules in cells expressing CUGexp RNA and compromised docking events between stress granules and P-bodies, suggesting impaired communication between them.

    Major concerns:

    1. Throughout the study, the authors compared MBNL1 and CUGBP1 association with PBs and SGs without considering the potential differences in their cytoplasmic abundance between control and DM1 cell lines, which seems to be case for MBNL1 abundance in CTG960-expressing HeLa cells (Fig. 3). Provided that PBs and SGs exchange components with the cytosol at an equilibrium, if the cytoplasmic abundance of, for example, MBNL1 is decreased in DM1, one would expect the equilibrium being shifted resulting in less MBNL1 associated with PB/SG. Therefore, before measuring the association or the assembly/disassembly kinetics of PB and SG, the authors should first test whether MBNL1 and CUGBP1 abundance may be different between control and DM cell lines.

    There is, in fact, no difference in the relative cytoplasmic abundance of GFP-MBNL1 between CTG12 and CTG960- expressing HeLa cells. Each has approximately a 50/50 split between nucleus and cytoplasm, with <3% of nuclear GFP-MBNL1 found in nuclear CUGexp foci when they are present. We have added a graph demonstrating this to the supplementary data. The abundance of total endogenous MBNL1 is also not altered in DM1 patient-derived lens cell lines compared to controls, as shown by semi-quantitative western blot analysis, which we have also added to the supplementary data. However, if the expression of CUGexp RNA did cause a major loss of cytoplasmic MBNL1, this change would be reflective of the situation seen in DM1 and would not invalidate our results or conclusions.

    The same caveat applies to MBNL1/CUGBP1 knockdown experiments, where knocking down one may change the abundance of the other.

    To carry out FRAP experiments or live cell analysis of SG formation and loss, it is necessary to over-express a tagged version of the protein being studied. For the knockdown experiments shown in figure 6, therefore, when we knocked down MBNL1, CUGBP1 was present in excess as a GFP-tagged protein and when we knocked down CUGBP1, MBNL1 was present in excess as a GFP-tagged protein. Thus, any effects of the knockdowns on expression of the endogenous proteins being analysed would be highly unlikely to influence the results.

    1. Similarly, the authors did not consider the possibility that changes in SG/PB dynamics may be due to changes in the abundance/availability of essential SG/PB components such as GE1 and G3BP1.

    From our immunofluorescence experiments, there was certainly no obvious reduction in GE1 or TIA1 abundance (we did not assess G3BP1). We have quantitative proteomic analysis (unpublished) from a similar pair of cell lines, expressing CUGexp RNA alongside GFP rather than GFP-MBNL1. This shows no change in GE1 or G3BP1, so we would not expect to see any here either. We can easily carry out a quantitative western blot analysis to confirm and will add this to the supplementary data

    1. Most of the observed differences between control and DM cell lines were modest, leaving one wonder whether it could be simply due to cell line-to-cell line variability. Whenever possible, the authors should present results for each individual lines. For example, in Fig.2, 3 DM1 lines and 2 control lines were used. Was the difference in SG disassembly (Fig. 2B) observed in each of the 3 lines?

    Some of the alterations were modest and there is cell line-to-cell line variability in the lens cell lines. This is why we pooled the data: on average, DM1 cells disperse their SGs more quickly than control cell lines do on average. This is not an unusual way to present data from patient cell lines of diverse genetic background. We have added data for stress granule loss in the individual cell lines to the supplementary data. These data show a consistent trend towards quicker dispersal of stress granules in patient cell lines. The variability between the patient lens cell lines was also the primary reason for us to develop the inducible system in HeLa cells, on a fixed genetic background, as explained in the manuscript.

    Minor points:

    1. Western blot in Fig. 3 shows two protein products from both endogenous and overexpressed MBNL1. Please explain.

    Many of the commercially available anti-MBNL1 antibodies show this double-band in some cell lines as evidenced in numerous publications and on manufacturers’ websites (for example https://abclonal.com/catalog-antibodies/MBNL1RabbitmAb/A5149, https://www.ptglab.com/products/MBNL1-Antibody-66837-1-Ig.htm). We haven’t analysed the two bands in detail, but assume this to be a result of a post translational modification of some sort. Since GFP-MBNL1 and endogenous MBNL1 show the same thing, we do not consider it to be a major concern. We do mention the double-band as ‘characteristic’ in the figure legend for figure 3 so are not seeking to conceal anything here.

    1. No data were shown to substantiate the statement that "MBNL1 localises to CUGexp foci and CUGBP1 does not" (page 6).

    This has been published many times and is shown in figure 1A. However, we will add in a citation for this and have added an additional supplementary figure showing the lack of co-localisation in the foci from figure 1A more clearly together with separate data confirming that MBNL1 and CUGexp RNA do not co-localise with CUGBP1 in the nuclei of line HeLa_CTG960_GFPMBNL1.

    1. The y-axis of Fig. 4D should not go beyond 1.

    We will trim the axis. There are no data points above 1.0, just the indicator of statistical significance

    Significance:

    The nature of the current study is highly descriptive with little mechanistic insights.

    Our work is not descriptive, as we observed a change in stress granules in patient cells, which we could then replicate (and enhance) in a novel inducible model of DM1 designed to abrogate the unavoidable variation in patient-derived cell lines. We also now have additional mechanistic insights (see above) and have added an additional figure (draft figure 8) detailing these.

    For the subtle differences observed between control and DM1 cell lines, it remains unclear whether it may be due to cell line-to-cell line variation (see above).

    We cannot completely rule out an influence of cell line-to-cell line variation in the patient-derived lens cell lines (see above), though we think this unlikely as we saw the same effect repeated and amplified in the inducible HeLa-derived cell model, which was designed to minimise this concern. Furthermore, for stress granule loss, we see a larger effect in the HeLa cell model after 72hrs of induction than after 24hrs (figure 5C). This argues strongly that the effects seen are due to the expression of CUGexp RNA and we will emphasise this point more strongly.

    Some difference appear to be specific to one model but not the others (e.g., SG formation is slower in HeLa-CTG960 cells but not in DM1 HLECs).

    Even for the observations that seem consistent between models, the current results yielded little novel biological insights into whether and how these subtle differences in PB/SG dynamics may relate to DM1 pathogenesis. Collectively, these weaknesses render the current study incremental at best.

    The key biological insight the results provide is that the presence of the CUGexp repeat RNA results in defects in LLPS structures that are largely separable from any sequestration of MBNL1 in nuclear foci. With many researchers attributing the cellular defects in DM1 simply to the loss of MBNL1 by sequestration into nuclear foci, both this separation of altered stress response from MBNL1 levels and the involvement of altered LLPS formation (evidenced by the changes in PB behaviour on 1,6-hexanediol treatment) are novel biological insights into the cellular pathology of DM1. Additionally, our results shift the emphasis from nuclear effects to those seen in the cytoplasm.

    In terms of specific DM1 pathogenesis, the eye lens is subject to constant repeated stress and is subject to continued growth throughout the life span, relying on lens epithelial cells as a stem cell pool. Epithelial cells are also vital to the homeostatic regulation of ions, growth factor and nutrient flow from the aqueous humor to the underlying fibre cells. Any alterations in the response of lens epithelial cells, in particular, to stress is highly relevant to the pathology of cataract seen in DM1. We will revise our discussion to emphasise these key points more strongly.

    Reviewer #3

    The manuscript entiled "Phase-separated stress granules and processing bodies are compromised in Myotonic Dystrophy Type 1" by Gulyurtlu et al., characterizes the composition and ydnamics of stress granules and P-bodies in two Myotonic Dystrophy Type 1 (DM1) cell models, human lens epithelial cells from DM1 patients and age-matched controls and HeLa_CTG12_GFPMBNL1 and HeLa_CTG960_GFPMBNL1 cell lines. The manuscript is somewhat descriptive with lack of functional data and some discrepancies. For example, in the discussion section, the authors conclude that "MBNL1 appears to be absent from P-bodies in cells with CUGexp foci in their nuclei. This observation suggests that the role of MBNL1 in P-bodies may be disrupted by the presence of CUGexp RNA." Figure 4A shows that "P-bodies in the DM1 model line, HeLa_CTG960_GFPMBNL1 do not contain detectable amounts of GFPMBNL1". However, Figure 4E shows similar levels of total cellular MBNL1 per PB between the control CTG12 and mutated CTG960 lines.

    There is no discrepancy here. The reviewer has misinterpreted our data. PBs in the HeLa CTG960 cell line do not contain detectable amounts of GFP-MBNL1 under normal growth conditions, as shown in figure 4A. The data shown in figure 4E concern arsenite-treated cells, where some PBs in the CTG960 line do contain detectable levels of GFP-MBNL1, but significantly less than in the control CTG12 cells. We will reword these sections to make sure this is clear.

    Most importantly, in Figure S3 the authors show that CUGexp foci are present in 1-2 % of the cells. The claim appears to be too strong for the data presented in the manuscript.

    This is not what is shown in figure S3. The reviewer has misinterpreted our data. Figure S3 shows that in cells from line CTG960, only 1-2% of the total nuclear GFP-MBNL1 signal is found in the CUGexp foci, despite the intensity of the signal within them. Virtually all of the cells from the CTG960 cell line contain CUGexp foci (>95%). We will add a statement to this effect into the results section. We would not have continued working with a cell line in which only 1-2% of cells showed the DM1 phenotype of nuclear CUGexp RNA foci.

    Although the findings are interesting and of potential impact for a better understanding of the implications of RNA-protein condensate dynamics in the pathogenesis of DM1, the work presented here is still descriptive and preliminary in my opinion. In summary, the conclusions are not so convincing and additional experiments are essential to support the authors claims.

    This reviewer seems to have misinterpreted several of our data sets, including the specific points above, leading to the assertion that our conclusions are not convincing.

    Several months of works will be required to consolidate data and reorganize and ameliorate the manuscript, including the way data are presented and quantified.

    We already have data with which to address the majority of the queries posed, so should be able to make the adjustments relatively quickly.

    Specific comments:

    "On removal of stress, clearance of stress granules is mediated largely by a form of autophagy." This statement is not correct since the majority of stress granules disassemble and are not targeted to autophagy; in healthy cells only 5 % (or less) of the total SGs tend to persist in presence of autophagy or lysosome inhibitors, while the vast majority disassembles. Please rephrase carefully.

    The degree and manner of dispersal of stress granules in healthy cells on removal of stress is not well understood, but is known to differ depending on the type and duration of the stress (DOI: 10.1126/science.abj2400). We do not yet know how this may be altered in DM1, however, compromised autophagy is implicated in cataract formation, which is of relevance to our study. We will re-phrase this section of the discussion carefully to reflect the complex situation.

    Figure 1: RNA-protein complexes have heterogeneous composition. In HLECs, do all PBs colocalize with MBNL1 and CUGBP1 or only a fraction of them?

    We do not routinely see PBs without MBNL1 or CUGBP1 in the HLECs, in contrast to the situation in the HeLa CTG960 line. We have data available in order to quantify this and will add the numbers to the text of the results.

    Figure 2: Stress granules and P-Bodies are known to touch each-other, a process referred to as a "kissing event". The authors have studied the mobility of GFP-MBNL1 inside these two types of assemblies. It would be important also to quantify the "kissing" events. Is this altered in DM1 cells?

    We couldn’t find reference in the literature to ‘kissing events’ between SGs and PBs, but found several references to ‘docking’ events. We have noticed such interactions between PBs and SGs in our models. We are currently quantifying this and our first experiments (one in the HeLa cell model and one comparing one of the patient-derived lens cell lines to a control) suggest that there is a change in the frequency and/or size of such interactions in the HeLa CTG960 cell line compared to the CTG12 control and in the DM1 lens cell line derived to control. If this holds true in our repeat experiments (currently in progress), this would also provide the mechanistic insight requested by reviewer 2. We have included this, together with our data showing a decrease in total polyA RNA in stress granules in HeLa cells expressing CUGexp RNA, as an additional draft figure 8.

    Figure 3: In HeLa cells overexpressing CTG960_GFPMBNL1, beside the accumulation of one bright CUGexp puncta, several intranuclear GFPMBNL1 protein foci are visible. This subcellular distribution is different from the one observed in the control HeLa_CTG12_ GFPMBNL1. Can the author describe what these intranuclear GFPMBNL1 protein foci are?

    In most cells expressing CUGexp RNA, several nuclear foci form (usually one large and several smaller) and all of them contain MBNL1 (or GFP-MBNL1 in the HeLa_CTG960_GFPMBNL1 cell line). Figure S3 shows object identification using MBNL1 in this cell line, with two clear foci detected as the reviewer points out. We have added an additional panel to supplementary figure 1 to confirm that the additional foci are also CUGexp RNA foci and will clarify in the text of the results that there is not a single focus of CUGexp RNA in each nucleus.

    Is GFPMBNL1 accumulating at the level of splicing speckles? Or paraspeckles? Or other types on intranuclear condensates such as e.g. PML nuclear bodies? The different intranuclear distribution of GFPMBNL1 should be better characterized.

    The sub-nuclear distribution of MBNL1 is, indeed, very complex. MBNL1 also sometimes co-localises to splicing speckles/interchromatin granule clusters as we have previously reported in lens epithelial cell lines (DOI: 10.1042/BJ20130870 ) . The details of differences in the nuclear distribution of MBNL1, beyond its accumulation in CUGexp RNA foci, in DM1 cells compared to controls is the subject of another manuscript we have in preparation but are beyond the scope of the current study.

    Moreover, the % of cells expressing CTG960_GFPMBNL1 and forming intranuclear CUGexp foci is only mentioned in the discussion (Figure S3); for clarity it should be reported in the main text when describing Figure 3.

    The number of cells forming nuclear CUGexp foci on expression of CTG960_GFP-MBNL1 is >95% and we will add this to the text of the results section.

    "Figure S2: Quantitation of GFPMBNL1 in P-bodies in HeLa cell model of DM1." The authors report in the legend "Some, but not all, of these P-bodies contain detectable amounts of GFPMBNL1". However, the figure only shows a representative image of cells without quantification. Quantitation should be provided.

    We have data available to provide this simple quantitation. Approximately 38% of PBs in arsenite-treated cells from line HeLa_CTG960_GFPMBNL1 contain detectable levels of GFPMBNL1 using a manually-assigned cut-off intensity. We will add this to the relevant figure legend (now figure S5). However, this method of analysis requires an intensity to be manually set above which GFP-MBNL1 signal is considered ‘detectable’. This is hugely subjective, and in our opinion, the automatically generated quantitative comparison of “% total cellular MBNL1 per P-body” as shown in figure 4E is a more experimentally robust way to demonstrate a small loss of MBNL1 from P-bodies in cells from line HeLa_CTG960_GFPMBNL1 treated with arsenite compared to the relevant control.

    The authors report "a subtle change in stress granule architecture associated with the presence of CUGexp RNA". This statement is not supported by experimental data and should be omitted.

    We will qualify this statement to make it clear that we are referring to a subtle alteration in the co-localisation between CUGBP1 and MBNL1 specifically in the SGs, as our experimental data shown in figure 4D clearly support that, showing a statistically significant increase in the Pearson’s co-efficient of cololcalisation between MBNL1 and CUGBP1 in cell containing CUGexp RNA compared to the relevant control (0.90+/-0.05 for CTG960; 0.87+/-0.07 for CTG12).

    Figure 4. MBNL1 and CUGBP1 co-localise in P-bodies. What is the % of colocalization?

    We’re not sure exactly what is being requested here or what biological question the reviewer is asking us to address. MBNL1 and CUGBP1 co-localise in virtually all PBs (except in the HeLa CTG960 line where MBNL1 is undetectable in PBs under normal growth conditions). Figure 4E shows that, in cells with PBs upregulated by sodium arsenite, the mean amount of total cellular MBNL1 per PB is 0.1%, so it will be similar in cells grown under normal conditions as the PB sizes are similar and they appear to be of similar brightness by immunofluorescence. Again, this would be straightforward to quantify with our existing data if this is, indeed what the reviewer is requesting, but we question the biological significance. We would be reluctant to derive a Pearson’s co-efficient for the degree of co-localisation between CUGBP1 and MBNL1 in P-bodies as the structures are too small in size for this to be meaningful within the limits of imaging capabilities. We could, however, provide this if this is a specific request.

    Figure 5: "Treatment with sodium arsenite was then carried out under time-lapse microscopy, with Z-stacks of images taken every 4 minutes until stress granule formation was clearly seen (Fig.5A). This revealed a pronounced delay in formation of stress granules in cells containing CUGexp foci (HeLa CTG960 GFPMBNL1, 36 min +/- 12) compared to those without (HeLa CTG12 GFPMBNL1, 15 min +/- 2) (Fig.5B)." Data representation in Figure 5 is unclear and the pronounced delay in stress granule formation is not appreciated. Since the authors performed a live imaging taking pictures every 4 minutes, it would me more informative to plot the data and show the assembly and disassembly kinetics over time for both control and CTG960_ GFPMBNL1 cell lines (similar to what shown in e.g. Gwon et al., Science 2021, Ubiquitination of G3BP1 mediates stress granule disassembly in a context-specific manner, Figure 2G).

    The bar graph in figure 5B shows that cells from the CTG960 line take more than twice as long to form SGs compared to controls and are lost in half the time, with the precise numbers given in the text. A simple bar graph seemed the clearest way to present this. However, we have plotted our existing data in a similar manner similar to that in the cited reference and added this to figure 5. These graphs clearly show that the differences we see are at least as great as in other published literature, including the reference given by the reviewer (see below).

    Concerning Figure 1, the authors report no difference in the kinetic of stress granule formation in HLECs. However, they only report data after 45 and 60 min of arsenite treatment; at these time-points the assembly step is maximal. Thus, for consistency, the authors should include earlier time-points to the analysis of stress granule assembly also in HLECs, similar to what done in HeLa cells in Figure 5.

    The assembly step is not ‘maximal’ in these cell lines after 45 minutes. Figure 2A clearly shows that only ~30% of cells have SGs after 45 minutes of treatment, compared with 100% of cells after 90 minutes shown in figure 2B. We have additional data at 10, 20 and 30 minutes all showing no significant differences. We had omitted them to keep the graph simple, but have now included them as a graph of ‘% of cells with stress granules against time’ in figure 2.

    "Having established that MBNL1 and CUGBP1 co-localise closely in stress granules": the authors investigated the colocalization of each of these two proteins with stress granule markers but they did not verify whether MBNL1 and CUGBP1 co-localise.

    In figure 1B we show that endogenous CUGBP1 and endogenous MBNL1 both co-localise with the stress granule marker TIA1 in stress granules in lens epithelial cells. It would, therefore, be highly unlikely that CUGBP1 and MBNL1 would not co-localise with each other in stress granules. We have also previously verified that GFPMBNL1 behaves identically to its endogenous counterpart (Coleman et al, 2014). Furthermore, in figure 4C and 4D, we show close co-localisation between endogenous CUGBP1 and GFPMBNL1 in stress granules in our HeLa cell model, using high-resolution AiryScan microscopy for which we provide detailed quantitation.

    This aspect should be addressed experimentally since the authors also conclude that "a complex relationship between MBNL1 and CUGBP1 in stress granules" exists. Thus, the authors need to assess the colocalization of GFPMBNL1 with endogenous CUGBP1 in stress granules and the one of GFPCUGBP1 with endogenous MBNL1.

    The complex relationship we propose is based on the effects of CUGBP1 or MBNL1 knockdown on the dynamic behaviours of each other by FRAP assay and not solely on their co-localisation, although we have already analysed their co-localisation in detail as above.

    Figure 6: Please add antibody labeling to microscopy panels A and B.

    Certainly, this was an accidental omission and has been added

    Moreover, specify is the numbers refer to minutes in panel F. The data representation is also unclear - see comment above, Figure 5.

    As stated in the figure legend and on the graph axes, these numbers have been normalised to the mean time taken for SG formation/loss in the control CTG12 cell line (set at 100%). The precise numbers in minutes for mean and SD are given in the text. We have added additional graphs of ‘% of cells with stress granules against time’ to this figure, with the values in minutes given to clarify the exact time-scale.

    Figure 7: was 1,6-hexanediol added in presence of arsenite? Or was arsenite removed?

    Arsenite was not removed (neither was Doxycycline) as we wanted to examine the effect of 1,6-hexanediol on SGs and PBs without the added complication of the effects of stress removal. We will clarify this point in the methods/results.

    Aberrant persistent stress granules have been implicated in age-related (Mateju et al., 2017) and neurodegenerative diseases (Protter and Parker, 2016), such as ALS and FTD (Jain et al., 2016; Markmiller et al., 2018; Zhang et al., 2018). These are proposed to result from increased liquid-to-solid phase transitions within the stress granules (Mateju et al., 2017)." The authors should better define what are aberrant stress granules (e.g. see Ganassi et al., 2016; Turakhiya et al., 2018, PMID: 29804830).*

    We will expand on this subject in the discussion

    "Persistent stress granules have long been associated with degenerative conditions, notably ALS (Li et al., 2013)". I suggest updating the reference adding a more recent one.

    We selected this 2013 review to emphasise that there is a long history of association of persistent stress granules with degenerative conditions. We will add in an additional, more recent review.

    Significance

    The work is descriptive; thus, in this form I do not consider that it is strongly advancing the field.

    Having noted alterations to stress granule disassembly in lens epithelial cells from DM1 patients, we went on to develop a novel inducible model in which we replicated and enhanced these effects by expressing the large CUGexp RNA that causes DM1 as part of a DMPK mini-gene mimicking the genetic mutation seen in DM1 patients. This is not purely descriptive. Furthermore, we are now in a position to add an additional figure showing two pieces of evidence for functional defects in stress granules associated with CUGexp RNA expression 1) reduced accumulation of total PolyA RNA in stress granules indicating compromised function and 2) compromised ‘docking’ events between stress granules and P-bodies, a process proposed to be integral to the function of both structures.

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    Referee #3

    Evidence, reproducibility and clarity

    The manuscript entiled " Phase-separated stress granules and processing bodies are compromised in Myotonic Dystrophy Type 1" by Gulyurtlu et al., characterizes the composition and ydnamics of stress granules and P-bodies in two Myotonic Dystrophy Type 1 (DM1) cell models, human lens epithelial cells from DM1 patients and age-matched controls and HeLa_CTG12_GFPMBNL1 and HeLa_CTG960_GFPMBNL1 cell lines. The manuscript is somewhat descriptive with lack of functional data and some discrepancies. For example, in the discussion section, the authors conclude that "MBNL1 appears to be absent from P-bodies in cells with CUGexp foci in their nuclei. This observation suggests that the role of MBNL1 in P-bodies may be disrupted by the presence of CUGexp RNA." Figure 4A shows that "P-bodies in the DM1 model line, HeLa_CTG960_GFPMBNL1 do not contain detectable amounts of GFPMBNL1". However, Figure 4E shows similar levels of total cellular MBNL1 per PB between the control CTG12 and mutated CTG960 lines. Most importantly, in Figure S3 the authors show that CUGexp foci are present in 1-2 % of the cells. The claim appears to be too strong for the data presented in the manuscript.

    Although the findings are interesting and of potential impact for a better understanding of the implications of RNA-protein condensate dynamics in the pathogenesis of DM1, the work presented here is still descriptive and preliminary in my opinion. In summary, the conclusions are not so convincing and additional experiments are essential to support the authors claims. Several months of works will be required to consolidate data and reorganize and ameliorate the manuscript, including the way data are presented and quantified.

    Specific comments:

    "On removal of stress, clearance of stress granules is mediated largely by a form of autophagy." This statement is not correct since the majority of stress granules disassemble and are not targeted to autophagy; in healthy cells only 5 % (or less) of the total SGs tend to persist in presence of autophagy or lysosome inhibitors, while the vast majority disassembles. Please rephrase carefully.

    Figure 1: RNA-protein complexes have heterogeneous composition. In HLECs, do all PBs colocalize with MBNL1 and CUGBP1 or only a fraction of them?

    Figure 2: Stress granules and P-Bodies are known to touch each-other, a process referred to as a "kissing event". The authors have studied the mobility of GFP-MBNL1 inside these two types of assemblies. It would be important also to quantify the "kissing" events. Is this altered in DM1 cells?

    Figure 3: In HeLa cells overexpressing CTG960_GFPMBNL1, beside the accumulation of one bright CUGexp puncta, several intranuclear GFPMBNL1 protein foci are visible. This subcellular distribution is different from the one observed in the control HeLa_CTG12_ GFPMBNL1. Can the author describe what these intranuclear GFPMBNL1 protein foci are? Is GFPMBNL1 accumulating at the level of splicing speckles? Or paraspeckles? Or other types on intranuclear condensates such as e.g. PML nuclear bodies? The different intranuclear distribution of GFPMBNL1 should be better characterized. Moreover, the % of cells expressing CTG960_GFPMBNL1 and forming intranuclear CUGexp foci is only mentioned in the discussion (Figure S3); for clarity it should be reported in the main text when describing Figure 3.

    "Figure S2: Quantitation of GFPMBNL1 in P-bodies in HeLa cell model of DM1." The authors report in the legend "Some, but not all, of these P-bodies contain detectable amounts of GFPMBNL1". However, the figure only shows a representative image of cells without quantification. Quantitation should be provided.

    The authors report "a subtle change in stress granule architecture associated with the presence of CUGexp RNA". This statement is not supported by experimental data and should be omitted.

    Figure 4. MBNL1 and CUGBP1 co-localise in P-bodies. What is the % of colocalization?

    Figure 5: "Treatment with sodium arsenite was then carried out under time-lapse microscopy, with Z-stacks of images taken every 4 minutes until stress granule formation was clearly seen (Fig.5A). This revealed a pronounced delay in formation of stress granules in cells containing CUGexp foci (HeLa_CTG960_ GFPMBNL1, 36 min +/- 12) compared to those without (HeLa_CTG12_ GFPMBNL1, 15 min +/- 2) (Fig.5B)." Data representation in Figure 5 is unclear and the pronounced delay in stress granule formation is not appreciated. Since the authors performed a live imaging taking pictures every 4 minutes, it would me more informative to plot the data and show the assembly and disassembly kinetics over time for both control and CTG960_ GFPMBNL1 cell lines (similar to what shown in e.g. Gwon et al., Science 2021, Ubiquitination of G3BP1 mediates stress granule disassembly in a context-specific manner, Figure 2G). Concerning Figure 1, the authors report no difference in the kinetic of stress granule formation in HLECs. However, they only report data after 45 and 60 min of arsenite treatment; at these time-points the assembly step is maximal. Thus, for consistency, the authors should include earlier time-points to the analysis of stress granule assembly also in HLECs, similar to what done in HeLa cells in Figure 5.

    "Having established that MBNL1 and CUGBP1 co-localise closely in stress granules": the authors investigated the colocalization of each of these two proteins with stress granule markers but they did not verify whether MBNL1 and CUGBP1 co-localise. This aspect should be addressed experimentally since the authors also conclude that "a complex relationship between MBNL1 and CUGBP1 in stress granules" exists. Thus, the authors need to assess the colocalization of GFPMBNL1 with endogenous CUGBP1 in stress granules and the one of GFPCUGBP1 with endogenous MBNL1.

    Figure 6: Please add antibody labeling to microscopy panels A and B. Moreover, specify is the numbers refer to minutes in panel F. The data representation is also unclear - see comment above, Figure 5.

    Figure 7: was 1,6-hexanediol added in presence of arsenite? Or was arsenite removed?

    Minor comments:

    Aberrant persistent stress granules have been implicated in age-related (Mateju et al., 2017) and neurodegenerative diseases (Protter and Parker, 2016), such as ALS and FTD (Jain et al., 2016; Markmiller et al., 2018; Zhang et al., 2018). These are proposed to result from increased liquid-to-solid phase transitions within the stress granules (Mateju et al., 2017)." The authors should better define what are aberrant stress granules (e.g. see Ganassi et al., 2016; Turakhiya et al., 2018, PMID: 29804830).

    "Persistent stress granules have long been associated with degenerative conditions, notably ALS (Li et al., 2013)". I suggest updating the reference adding a more recent one.

    Significance

    The work is descriptive; thus, in this form I do not consider that it is strongly advancing the field.

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    Referee #2

    Evidence, reproducibility and clarity

    In the current study, the authors compared the dynamics of P-bodies (PBs) and stress granules (SGs) between control and several DM1 cell lines. They found that MBNL1 and CUGBP1, two CUG repeat RNA-binding proteins that are primarily nuclear, could also co-localize with PBs in the cytoplasm and re-localize to SGs under stress. Small differences were observed in SG assembly and disassembly dynamics between control and DM1 HLECs, between HeLa cells expressing either CTG12 or CTG960, and between HeLa cells with and without shRNAs targeting CUGBP1 or MBNL1. Overall, the experiments were clearly described and the results properly presented. However, critical controls, as detailed below, are missing in multiple analyses. The mechanisms underlying these apparent differences are also unknown.

    Major concerns:

    1. Throughout the study, the authors compared MBNL1 and CUGBP1 association with PBs and SGs without considering the potential differences in their cytoplasmic abundance between control and DM1 cell lines, which seems to be case for MBNL1 abundance in CTG960-expressing HeLa cells (Fig. 3). Provided that PBs and SGs exchange components with the cytosol at an equilibrium, if the cytoplasmic abundance of, for example, MBNL1 is decreased in DM1, one would expect the equilibrium being shifted resulting in less MBNL1 associated with PB/SG. Therefore, before measuring the association or the assembly/disassembly kinetics of PB and SG, the authors should first test whether MBNL1 and CUGBP1 abundance may be different between control and DM cell lines. The same caveat applies to MBNL1/CUGBP1 knockdown experiments, where knocking down one may change the abundance of the other.
    2. Similarly, the authors did not consider the possibility that changes in SG/PB dynamics may be due to changes in the abundance/availability of essential SG/PB components such as GE1 and G3BP1.
    3. Most of the observed differences between control and DM cell lines were modest, leaving one wonder whether it could be simply due to cell line-to-cell line variability. Whenever possible, the authors should present results for each individual lines. For example, in Fig.2, 3 DM1 lines and 2 control lines were used. Was the difference in SG disassembly (Fig. 2B) observed in each of the 3 lines?

    Minor points:

    1. Western blot in Fig. 3 shows two protein products from both endogenous and overexpressed MBNL1. Please explain.
    2. No data were shown to substantiate the statement that "MBNL1 localises to CUGexp foci and CUGBP1 does not" (page 6).
    3. The y-axis of Fig. 4D should not go beyond 1.

    Significance

    The nature of the current study is highly descriptive with little mechanistic insights. For the subtle differences observed between control and DM1 cell lines, it remains unclear whether it may be due to cell line-to-cell line variation (see above). Some difference appear to be specific to one model but not the others (e.g., SG formation is slower in HeLa-CTG960 cells but not in DM1 HLECs). Even for the observations that seem consistent between models, the current results yielded little novel biological insights into whether and how these subtle differences in PB/SG dynamics may relate to DM1 pathogenesis. Collectively, these weaknesses render the current study incremental at best.

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    Referee #1

    Evidence, reproducibility and clarity

    This manuscript by Gulyrutlu and co-workers addresses the role of CUG expanded repeat RNA associated with DM1 in regulating the formation of higher order RNP assemblies such as stress granules and P-bodies in the cell. The authors used lens epithelial cells (hLECs) derived from a DM1 patient or a HeLa cell inducible model of DM1 to investigate whether expression of the CUG repeat-associated protein MBNL1 and CUGBP1 affected the formation and dispersal of stress granules and P-bodies. The authors show that MBNL1 and CUGBP1 are components of SGs and PBs in hLECs and HeLa cells. In cells expressing the CUG repeat, there are minor alterations in the dispersal of stress granules as well as in the formation of P-bodies. MBNL1 could affect the formation and dispersal of SGs independent of the CUG repeat. Finally, in HeLa cells, overexpression of MBNL1 can reduce the dispersal of P-bodies upon 1,6-hexanediol treatment.

    Major comments:

    One limitation of the work is that the perturbations seen with stress granules or P-bodies are all relatively small, and no evidence for a functional consequence on gene expression is demonstrated. Specifically, the authors observe only minor alterations in the formation or disaggregation of PBs and SGs in these DM1 models. Further, some of the effects observed are independent of the CUG repeat expression, suggesting that MBNL1 and CUGBP1 might have independent roles in modulating some properties of SG and PB formation or dispersal.

    1. The authors could investigate whether the CUG repeat RNA itself is localized to SGs or PBs in their models, and whether the presence of the repeat RNA is absolutely necessary for regulating the dynamics of SG or PB formation.
    2. The authors use 1,6-hexanediol to suggests that PBs and SGs in HeLa cells show behavior analogous to LLPS. However, the use of 1,6,-hexanediol to establish an assembly as a LLPS is a relatively limited analysis (despite its widespread use in the field), since this compound can affect the formation of multiple cellular substructures that are not always LLPS (for example, see Wheeler et al, 2016, eLife).

    Significance

    This study would be of interest to the field if the impact of the DM! repeat RNAs on PB and SG were more substantial, and if some functional consequences were demonstrated. The lack of a strong effect on SG or PB formation in the DM1 models, along with the CUG repeat-independent effect of MBNL1 on the formation and dispersal of these complexes, argues that MBNL1/CUGBP1 may not significantly affect the formation or dispersal of SGs and PBs.

  6. Version published to 10.1101/2021.06.14.448303v1 on bioRxiv