Antero-posterior gradients of cell plasticity and proliferation modulate posterior regeneration in the annelid Platynereis

  1. Loïc Bideau
  2. Loeiza Baduel
  3. Marianne Basso
  4. Pascale Gilardi-Hebenstreit
  5. Vanessa Ribes
  6. Michel Vervoort
  7. Eve Gazave

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Abstract

Regenerative abilities are extremely variable among animals and may be substantial in some phyla, such as the annelids. So far, the cellular mechanisms underlying regeneration in annelids remain elusive. To precisely determine the origin(s), plasticity and fate of the cells participating in the blastema formation during posterior regeneration in the annelid Platynereis dumerilii , we developed specific tools to track proliferative cells as well as gut epithelial cells. We showed that two populations of progenitors are at play during regeneration and that, among them, gut progenitors from differentiated tissues are lineage-restricted. Strikingly, gut progenitors from less differentiated and more proliferative tissues are much more plastic and can produce ectodermal and mesodermal derivatives, in addition to gut cells. However, their plasticity is de facto limited as exemplified by their inability to regenerate populations of stem cells responsible for the constant growth of the worms. We evidenced that those stem cells are from local origin ( i.e. from the segment abutting the amputation plan) as most of the blastema cells. Our results are in favour of a hybrid and flexible cellular model for posterior regeneration in Platynereis relying on a gradient of cell plasticity along the antero-posterior axis of the animal.

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    The authors do not wish to provide a response at this time.

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    Referee #3

    Evidence, reproducibility and clarity

    In this interesting paper Bideau et al. report an antero-posterior gradient in the plasticity of tail tissues during tail regeneration in the annelid worm Platynereis dumerilii. The experiments are well designed and thoroughly quantified, the figures are of high quality.

    Major comments:

    The microscopic images lack scale bars. These should be added to all figures.

    The authors should provide the source data for all quantifications as txt files. They should also provide as supplement representative confocal stacks for the various stainings.

    The authors use LatrunculinB treatment to investigate the role of cell migration in regeneration. However, since LatB inhibits f-actin, it could also interfere with cell proliferation and other processes. The authors should check if this is the case and provide control data.

    Minor comments:

    Sometimes the language is a bit quite cryptic. For example, the title of Figure 4 is "Cell proliferation and migration, as well as tissue maturity modulate the plasticity of posteriorized gut progenitors through regeneration"
    in short: 'cell migration modulates the plasticity of progenitors' - this is just to say that inhibiting cell migration reduces regeneration

    The authors should attempt to simplify the language.

    Language:

    "is an tremendous and essential process in animals" - not clear what 'tremendous' means here - please revise

    "Those regeneration processes have been studied from a long" - for a long time

    "more EdU+ cells in S1 than in S6 or S7 regardless the EdU incubation time" - regardless of the

    "It showed that the gut is composed of" - The stainings showed that

    "Indeed, cell labelled with a rather short EdU" - cells labelled

    "tissues plays a major role on the reformation" - in the formation

    The paper will be of interest to animal developmental biologists and scientists working on the plasticity of tissues during regeneration.

    Referees cross-commenting

    I agree with the comments made by the other reviewers. The authors need to be more clear and careful in interpreting their data. I don't think that new data are needed (unless they would like to demonstrate a 'gradient' with more positions) and the comments could be addressed by substantially rewriting the text and revising the claims.

    Significance

    The paper will be of interest to animal developmental biologists and scientists working on the plasticity of tissues during regeneration.

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    Referee #2

    Evidence, reproducibility and clarity

    Bideau et al. studied the origin, plasticity and fate of the cells participating in blastema formation during posterior regeneration in the annelid Platynereis dumerilii. To label and track the fate of proliferative cells, the authors applied EdU/BrDU incorporation coupled with mRNA in situ hybridization and fluorescent beads labelling, on a wide array of regeneration assays. They also performed drug treatments to assess the role of proliferation and cell migration during posterior regeneration. Interestingly, the Authors showed that some proliferative gut cells can participate in the formation of ectodermal and mesodermal tissues during regeneration, in case of two successive posterior regeneration events, suggesting that gut cycling cells residing in a regenerating segment are more plastic than those located in a non-regenerating segment. They also suggested the existence of two different cell populations - "slowly" and "quickly" cycling cells - acting during regeneration. Overall, the experiments are well presented, and methods clearly described. The Authors concluded that posterior regeneration in Platynereis relies on a gradient of cell plasticity and cell proliferation, along the antero-posterior axis of the animal.

    Major comments

    Two of the main conclusions of this study are, in my opinion, not supported by the data:

    As indicated in the title of the manuscript, the Authors put forward a cellular model for posterior regeneration relying on gradients of cell proliferation, cell differentiation and cell plasticity along the the antero-posterior axis of the animal. I am not convinced that the Authors have provided strong enough evidence to prove any of these gradients. They showed that there are differences between the region directly adjacent the most posterior segment and a region located more anteriorly (6 or 7 segments from the posterior end). However, by comparing only two positions, they cannot distinguish between graded or clearly regionalized contexts. To prove the existence of a gradient along the animal's antero-posterior axis, the authors would need to compare cell proliferation, cell dynamics and cell differentiation between multiple regions at increasing anterior positions, and show that their responses are indeed graded. This would represent a quite substantial amount of work. Instead, I would suggest removing the reference to a gradient in the paper entirely.

    Using "short" (5h) and "long" (48h) EdU pulses, the Authors claim they have established the existence of two cell populations, namely "slowly-cycling cells" and "quickly-cycling cells" (first paragraph of the result section - pages 5/6 "We exposed uninjured worms to EdU, either for 5 or 48h to discriminate quickly-cycling cells from cells harboring a slower replication rate"). I am not convinced that the Authors provide strong enough evidence to demonstrate the existence of two such cell populations. Given that about 20% of cells incorporated EdU after 5h of exposure, that almost all of them have done so after 48h, and that only a fraction of proliferative cells are in S-phase at any given time, it is well possible that a majority of cells stained after 5h and 48h are from the same cell cycling population. To show the existence of different populations of cells, cycling at different rates, the Authors would need to compare staining after an equal EdU exposure time, following a period of chase of different duration. Without this set of experiments, I would refrain from distinguishing between several slower and faster cell cycling populations.

    Minor comments

    Page 1. Please correct "is an tremendous" into "is a tremendous".

    Page 7. "The huge majority of the EdU+ cells colocalize with FoxA". Please provide quantification.

    Page 11 "Quickly-cycling gut progenitors.... cannot give rise to neural progenitors and probably not to stem cells from the ectodermal growth zone"; Page 12 "cannot regenerate neural tissues"; Page 20 "posterior gut progenitors cannot produce nervous system or putative posterior stem cells". What the authors show in their experiments, is that labeled gut cycling cells likely do not generate neural cells or stem cells, in the assessed context. However, the Authors do not show that those cells 'cannot' do so. Please rephrase.

    Page 11. "migration, through actin polymerization (LatrunculinB or LatB) widely used inhibitors". Please add a reference to justify the use of LatB as a cell migration inhibitor.

    Significance

    This is a thorough, well executed and interesting study on a tractable annelid regeneration model. The experiments are neatly performed and the manuscript reads well. As stated in my major comments, two of the main conclusions of the study (gradient of cell proliferation/plasticity/differentiation; identification of two types of progenitors differing in their cell cycle rates) have not been demonstrated properly and would need to be either strengthened or deleted from the manuscript. Several other findings, notably the increased plasticity of cells that recently participated in posterior regeneration (notably gut cells) are well demonstrated and of interest. Overall, this manuscript significantly advances our understanding of the cellular mechanisms that occur during posterior regeneration in Platynereis. It will be of interest to anyone working on comparative regeneration, but may be of lesser interest to researchers working outside this field.

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    Referee #1

    Evidence, reproducibility and clarity

    Bideau et al. describe posterior regeneration in the annelid Platynereis. The authors aimed to identify patterns of proliferative cells. Pulse-chase and double labeling by EdU/BrdU was used to track cells. Finally, they attempted to reveal the identity of cycling cells and their contribution to regeneration. Platynereis is a relatively new regeneration model. Understanding the cellular source of regeneration in this annelid would be of considerable interest.

    The authors performed EdU labeling of intact worms to find a posterior-anterior decreasing gradient of S-phase cells. Histological sections showed that proliferative cells are located in all tissues in the posterior-most segment, but mostly restricted to the gut epithelium in more anterior segments. Using fluorescent beads that are taken up specifically by gut epithelial cells, they show that gut epithelial cells of the intact animal contribute only to gut regeneration, i.e., they are lineage restricted. The authors also performed immunostaining and mRNA in situ hybridization experiments to better understand the tissue identity of proliferative cells.

    The following are my specific comments:

    1. I am not sure I understand how the authors identify slow- or fast-cycling cells. EdU gets incorporated in S-phase; the longer the incubation time the more cells will be labeled until saturation is reached. The length of the cell cycle and the number of different populations cannot be directly derived from this experiment. I think it would be fair to conclude that there are more cycling cells in posterior segments and in the gut of anterior segments but the conclusion of two distinct populations is unsupported in my opinion.
    2. The Results and Discussion sections will have to be revised to address the above issue. The two supported conclusions are (1) the gradient of proliferative cells (but w/o reference to the number of distinct populations); (2) the fate-restricted nature of gut epithelial cells. The plasticity gradient is unsupported because worms can regenerate if amputated at segment #5. This suggests the presence of either resident stem cells (with broad potential or lineage-specific), cells that can dedifferentiate, or a combination of both. The authors' experiments cannot discriminate between the alternatives.
    3. The text would benefit from copy editing to improve the language and making it more accessible. In its current form, it is rather difficult to read, with descriptions of experiments that are not easy to follow.
    4. The figures and their legends can be improved. Re the legends, one has to read the full text to understand what each panel shows. The figures are very complex. It would already be easier if usage of A', A', A' was avoided. The figures could also be improved by direct annotation. Finally, consider simplifying the main figures and moving some material to the supplement.
    5. How do the authors know that proliferative cells in the gut are "gut progenitors"? They might simply be proliferative gut epithelial cells.
    6. The conclusions drawn from the drug experiments are overstated.

    Referees cross-commenting

    The comments made by the two other reviewers are complementary to mine. Either the authors extensively revise the text to remove unsupported conclusions or they perform additional experiments.

    Significance

    Little is known about the cellular basis of Platynereis posterior regeneration.

  9. Version published to 10.1101/2023.06.12.544593v1 on bioRxiv