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Regeneration Tensed Up: Polyploidy Takes the Lead
Developmental Cell
Previews Regeneration Tensed Up: Polyploidy Takes the Lead Zolta´n Spiro´1 and Carl-Philipp Heisenberg1,* 1Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2017.09.008
The cellular mechanisms allowing tissues to efficiently regenerate are not fully understood. In this issue of Developmental Cell, Cao et al. (2017) discover that during zebrafish heart regeneration, epicardial cells at the leading edge of regenerating tissue undergo endoreplication, possibly due to increased tissue tension, thereby boosting their regenerative capacity. Cell division represents a key process during regeneration and tissue repair. While cell division is typically associated with cytokinesis generating two diploid daughter cells, it can also occur in the absence of cytokinesis with cells replicating their DNA but not, or only incompletely, dividing. This type of cell division is known as endoreplication and gives rise to polyploid cells. Interestingly, cells have been observed to undergo endoreplication in tissue regeneration and repair. Yet the role endoreplication plays in these processes remained unclear. In this issue of Developmental Cell, Cao et al. (2017) shed light on the role of endoreplication for epicardial regeneration in zebrafish. By studying epicardial regeneration and migration in vivo and ex vivo, Cao et al. show that the regenerating epicardium consists of two distinct cell subpopulations: large, polyploid ‘‘leader’’ cells at the front of the regenerating tissue and small, diploid ‘‘follower’’ cells trailing behind the leader cells. Leader cells were able to migrate faster and had the capacity of covering the injured area by a lower number of cells as compared with follower cells. Polyploidy of leader cells was the consequence of cytokinetic failure stemming from increased mechanical tension in the front of the regenerating tissue. By modulating the proportion of leader to follower cells through chemical treatments, Cao et al. (2017) further show that both cell types were able to regenerate the whole epicardium but that leader cells had a greater capacity for surface coverage. There is increasing evidence for endoreplication being critical for regeneration and wound healing in various pathological processes. During wound repair in the
adult Drosophila epithelium, for instance, quiescent epithelial cells adjacent to the wound re-enter S phase without undergoing cytokinesis (Losick et al., 2013). Some of these cells fuse and thereby give rise to a large syncytium, while others undergo endocycling, leading to the formation of large polyploid cells. It has further been suggested that the polyploid cells might be required for replacing the synthetic capacity of the diploid cells lost upon wounding, while the large syncytium might serve as a mechanical stabilizer for wound repair via its robust cytoskeletal elements. Interestingly, these cell fusion and polyploidization processes in Drosophila were shown to be dependent on Yorkie, the Drosophila homolog of the mechanosensitive YAP/TAZ pathway. This points to the intriguing possibility that—similar to the findings of Cao et al. (2017) in zebrafish heart regeneration— tissue tension also controls cell fusion and polyploidization in Drosophila wound repair and that the modulation of YAP/ TAZ signaling by tension might be critical for triggering endoreplication in both Drosophila and zebrafish regeneration. But how does tension control endoreplication underlying leader cell formation during zebrafish heart regeneration? The observation by Cao et al. (2017) that endoreplication was the result of failed or incomplete cytokinesis of leader cells suggests that tissue tension functions in leader cell endoreplication by modulating cytokinesis. Interestingly, Cao et al. (2017) also show that in endoreplicating leader cells, formation and constriction of the cleavage furrow appeared normal, but these cells were unable to sever the intercellular bridge by which they remain connected (the abscission process). This
eventually led to resolution of the cleavage furrow and re-fusion of the transiently divided daughter cells. This suggests that tension affects leader cell abscission during cytokinesis and that delayed abscission leads to resolution of the cleavage furrow and fusion of the daughter cells. Consistent with this notion, previous studies have reported that cytokinetic abscission is dependent on tension release at the cytoplasmic bridge and that tension blocks cytokinetic abscission by inhibiting the assembly of the ESCRTIII complex at the bridge required for successful abscission (Lafaurie-Janvore et al., 2013). Thus, it is conceivable that the failure to release tension at the cytoplasmic bridge in dividing leader cells blocks cell abscission by inhibiting the recruitment of essential molecules mediating this process. Although this explains how tension might induce endoreplication, it remains less clear how the specific distribution and level of tension within the regenerating tissue functions in this process. Cao et al. (2017) provide evidence that tension within the plane of the tissue (planar tension) is higher in leader compared to follower cells when the two cell populations are already present. However, to determine how tension triggers the generation of these two cell types, one would ideally need to know about the distribution of tension right at the time when endoreplication occurs, thereby revealing whether and how planar tension distribution affects endoreplication in vivo. Given that tension might trigger endoreplication by directly acting at the abscission plane (see above), one would also need to know how global tissue tension translates to tension at the intercellular bridge. The
Developmental Cell 42, September 25, 2017 ª 2017 Published by Elsevier Inc. 559
Developmental Cell
Previews analysis of tissue tension will further be complicated by previous observations of tissue tension affecting cell division orientation, as well as the orientation of cell division and fusion feeding back on the distribution of tissue tension (Campinho et al., 2013). Based on those studies, it is conceivable that epicardial cell division axis is aligned to the main axis of tension within the regenerating tissue, and thus that failed cytokinesis and fusion of epicardial cells also predominantly occurs along this axis. Whether and how such hypothetical alignment feeds back on global tension distribution within the tissue and, consequently, the spreading capacity of the tissue during regeneration remains an interesting question for the future. Endoreplication of leader cells might also affect regeneration in ways other than regulating global tissue tension. Interestingly, Cao et al. (2017) found that polyploid leader cells display faster and more directed migration than mononucleated follower cells, suggesting that polyploidy enhances cell migration speed and directionality during collective migration. How polyploidy achieves these effects is yet unclear, but it is tempting to speculate that the associated increase in cell volume and size and resulting decrease in cell number promotes better cell movement coordination during collective migration. Beyond their function in cell migration, polyploid leader cells might also display an increased barrier function, similar to polyploid superneurial glia cells that form the blood-brain barrier in Drosophila. Furthermore, previous findings that large polyploid keratinocytes in mammals possess higher resistance to mechanical tension (Zanet et al., 2010) and that polyploid hepatocytes show
increased metabolic activity and are more resistant to metabolic stress and injury (Duncan et al., 2010, 2012; Miyaoka et al., 2012) point to the possibility that leader cells might possess similar functions during heart regeneration. Several questions remain as to the function and fate of cells undergoing endoreplication during regeneration. Studies in Drosophila have provided compelling evidence that the absence of polyploid cells interferes with wound closure (Losick et al., 2013). To address the capacity of leader versus follower cells for epicardial regeneration, Cao et al. (2017) successfully modulated the proportion of these cell types by using chemical inhibition of different signaling pathways. Surprisingly, both cell types alone were in principle able to regenerate the epicardium, although their capacities in doing so appeared different: whereas follower cells alone were able to cover and regenerate the heart faster than leader cells, leader cells were regenerating the heart with around 60% fewer cells than follower cells. Although these findings highlight interesting differences in the regenerative capacity between leader and follower cells, the specific function of these different cell types in the native regeneration process remains unclear. One possibility, which cannot be formally excluded based on the available data, is that the polyploid state of leader cells is merely the consequence of increased mechanical tension at the tissue front and that endoreplication as such has no definite function in heart regeneration. Besides the regenerative capacity of leader cells, it is also not yet entirely clear why and how those cells undergo apoptosis following regeneration. One
560 Developmental Cell 42, September 25, 2017
way to test the role of leader cell elimination would be to follow the fate of the regenerated epicardium upon inhibition of apoptosis. It is possible that maintaining a polyploid cell population long-term might have detrimental effects, because polyploid cells re-entering the cell cycle can lead to aneuploidy and genome instability, a hallmark of tumor formation and other genetic malformations (reviewed in Tang and Amon, 2013). Answering these open questions would further advance our understanding about the mechanisms linking endoreplication to regeneration. REFERENCES Campinho, P., Behrndt, M., Ranft, J., Risler, T., Minc, N., and Heisenberg, C.-P. (2013). Nat. Cell Biol. 15, 1405–1414. Cao, J., Wang, J., Jackman, C.P., Cox, A.H., Trembley, M.A., Balowski, J.J., Cox, B.D., De Simone, A., Dickson, A.L., Di Talia, S., et al. (2017). Dev. Cell 42, this issue, 600–615. Duncan, A.W., Taylor, M.H., Hickey, R.D., Hanlon Newell, A.E., Lenzi, M.L., Olson, S.B., Finegold, M.J., and Grompe, M. (2010). Nature 467, 707–710. Duncan, A.W., Hanlon Newell, A.E., Bi, W., Finegold, M.J., Olson, S.B., Beaudet, A.L., and Grompe, M. (2012). J. Clin. Invest. 122, 3307–3315. Lafaurie-Janvore, J., Maiuri, P., Wang, I., Pinot, M., Manneville, J.-B., Betz, T., Balland, M., and Piel, M. (2013). Science 339, 1625–1629. Losick, V.P., Fox, D.T., and Spradling, A.C. (2013). Curr. Biol. 23, 2224–2232. Miyaoka, Y., Ebato, K., Kato, H., Arakawa, S., Shimizu, S., and Miyajima, A. (2012). Curr. Biol. 22, 1166–1175. Tang, Y.C., and Amon, A. (2013). Cell 152, 394–405. Zanet, J., Freije, A., Ruiz, M., Coulon, V., Sanz, J.R., Chiesa, J., and Gandarillas, A. (2010). PLoS One 5, e15701.
Previews Regeneration Tensed Up: Polyploidy Takes the Lead Zolta´n Spiro´1 and Carl-Philipp Heisenberg1,* 1Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria *Correspondence: [email protected] http://dx.doi.org/10.1016/j.devcel.2017.09.008
The cellular mechanisms allowing tissues to efficiently regenerate are not fully understood. In this issue of Developmental Cell, Cao et al. (2017) discover that during zebrafish heart regeneration, epicardial cells at the leading edge of regenerating tissue undergo endoreplication, possibly due to increased tissue tension, thereby boosting their regenerative capacity. Cell division represents a key process during regeneration and tissue repair. While cell division is typically associated with cytokinesis generating two diploid daughter cells, it can also occur in the absence of cytokinesis with cells replicating their DNA but not, or only incompletely, dividing. This type of cell division is known as endoreplication and gives rise to polyploid cells. Interestingly, cells have been observed to undergo endoreplication in tissue regeneration and repair. Yet the role endoreplication plays in these processes remained unclear. In this issue of Developmental Cell, Cao et al. (2017) shed light on the role of endoreplication for epicardial regeneration in zebrafish. By studying epicardial regeneration and migration in vivo and ex vivo, Cao et al. show that the regenerating epicardium consists of two distinct cell subpopulations: large, polyploid ‘‘leader’’ cells at the front of the regenerating tissue and small, diploid ‘‘follower’’ cells trailing behind the leader cells. Leader cells were able to migrate faster and had the capacity of covering the injured area by a lower number of cells as compared with follower cells. Polyploidy of leader cells was the consequence of cytokinetic failure stemming from increased mechanical tension in the front of the regenerating tissue. By modulating the proportion of leader to follower cells through chemical treatments, Cao et al. (2017) further show that both cell types were able to regenerate the whole epicardium but that leader cells had a greater capacity for surface coverage. There is increasing evidence for endoreplication being critical for regeneration and wound healing in various pathological processes. During wound repair in the
adult Drosophila epithelium, for instance, quiescent epithelial cells adjacent to the wound re-enter S phase without undergoing cytokinesis (Losick et al., 2013). Some of these cells fuse and thereby give rise to a large syncytium, while others undergo endocycling, leading to the formation of large polyploid cells. It has further been suggested that the polyploid cells might be required for replacing the synthetic capacity of the diploid cells lost upon wounding, while the large syncytium might serve as a mechanical stabilizer for wound repair via its robust cytoskeletal elements. Interestingly, these cell fusion and polyploidization processes in Drosophila were shown to be dependent on Yorkie, the Drosophila homolog of the mechanosensitive YAP/TAZ pathway. This points to the intriguing possibility that—similar to the findings of Cao et al. (2017) in zebrafish heart regeneration— tissue tension also controls cell fusion and polyploidization in Drosophila wound repair and that the modulation of YAP/ TAZ signaling by tension might be critical for triggering endoreplication in both Drosophila and zebrafish regeneration. But how does tension control endoreplication underlying leader cell formation during zebrafish heart regeneration? The observation by Cao et al. (2017) that endoreplication was the result of failed or incomplete cytokinesis of leader cells suggests that tissue tension functions in leader cell endoreplication by modulating cytokinesis. Interestingly, Cao et al. (2017) also show that in endoreplicating leader cells, formation and constriction of the cleavage furrow appeared normal, but these cells were unable to sever the intercellular bridge by which they remain connected (the abscission process). This
eventually led to resolution of the cleavage furrow and re-fusion of the transiently divided daughter cells. This suggests that tension affects leader cell abscission during cytokinesis and that delayed abscission leads to resolution of the cleavage furrow and fusion of the daughter cells. Consistent with this notion, previous studies have reported that cytokinetic abscission is dependent on tension release at the cytoplasmic bridge and that tension blocks cytokinetic abscission by inhibiting the assembly of the ESCRTIII complex at the bridge required for successful abscission (Lafaurie-Janvore et al., 2013). Thus, it is conceivable that the failure to release tension at the cytoplasmic bridge in dividing leader cells blocks cell abscission by inhibiting the recruitment of essential molecules mediating this process. Although this explains how tension might induce endoreplication, it remains less clear how the specific distribution and level of tension within the regenerating tissue functions in this process. Cao et al. (2017) provide evidence that tension within the plane of the tissue (planar tension) is higher in leader compared to follower cells when the two cell populations are already present. However, to determine how tension triggers the generation of these two cell types, one would ideally need to know about the distribution of tension right at the time when endoreplication occurs, thereby revealing whether and how planar tension distribution affects endoreplication in vivo. Given that tension might trigger endoreplication by directly acting at the abscission plane (see above), one would also need to know how global tissue tension translates to tension at the intercellular bridge. The
Developmental Cell 42, September 25, 2017 ª 2017 Published by Elsevier Inc. 559
Developmental Cell
Previews analysis of tissue tension will further be complicated by previous observations of tissue tension affecting cell division orientation, as well as the orientation of cell division and fusion feeding back on the distribution of tissue tension (Campinho et al., 2013). Based on those studies, it is conceivable that epicardial cell division axis is aligned to the main axis of tension within the regenerating tissue, and thus that failed cytokinesis and fusion of epicardial cells also predominantly occurs along this axis. Whether and how such hypothetical alignment feeds back on global tension distribution within the tissue and, consequently, the spreading capacity of the tissue during regeneration remains an interesting question for the future. Endoreplication of leader cells might also affect regeneration in ways other than regulating global tissue tension. Interestingly, Cao et al. (2017) found that polyploid leader cells display faster and more directed migration than mononucleated follower cells, suggesting that polyploidy enhances cell migration speed and directionality during collective migration. How polyploidy achieves these effects is yet unclear, but it is tempting to speculate that the associated increase in cell volume and size and resulting decrease in cell number promotes better cell movement coordination during collective migration. Beyond their function in cell migration, polyploid leader cells might also display an increased barrier function, similar to polyploid superneurial glia cells that form the blood-brain barrier in Drosophila. Furthermore, previous findings that large polyploid keratinocytes in mammals possess higher resistance to mechanical tension (Zanet et al., 2010) and that polyploid hepatocytes show
increased metabolic activity and are more resistant to metabolic stress and injury (Duncan et al., 2010, 2012; Miyaoka et al., 2012) point to the possibility that leader cells might possess similar functions during heart regeneration. Several questions remain as to the function and fate of cells undergoing endoreplication during regeneration. Studies in Drosophila have provided compelling evidence that the absence of polyploid cells interferes with wound closure (Losick et al., 2013). To address the capacity of leader versus follower cells for epicardial regeneration, Cao et al. (2017) successfully modulated the proportion of these cell types by using chemical inhibition of different signaling pathways. Surprisingly, both cell types alone were in principle able to regenerate the epicardium, although their capacities in doing so appeared different: whereas follower cells alone were able to cover and regenerate the heart faster than leader cells, leader cells were regenerating the heart with around 60% fewer cells than follower cells. Although these findings highlight interesting differences in the regenerative capacity between leader and follower cells, the specific function of these different cell types in the native regeneration process remains unclear. One possibility, which cannot be formally excluded based on the available data, is that the polyploid state of leader cells is merely the consequence of increased mechanical tension at the tissue front and that endoreplication as such has no definite function in heart regeneration. Besides the regenerative capacity of leader cells, it is also not yet entirely clear why and how those cells undergo apoptosis following regeneration. One
560 Developmental Cell 42, September 25, 2017
way to test the role of leader cell elimination would be to follow the fate of the regenerated epicardium upon inhibition of apoptosis. It is possible that maintaining a polyploid cell population long-term might have detrimental effects, because polyploid cells re-entering the cell cycle can lead to aneuploidy and genome instability, a hallmark of tumor formation and other genetic malformations (reviewed in Tang and Amon, 2013). Answering these open questions would further advance our understanding about the mechanisms linking endoreplication to regeneration. REFERENCES Campinho, P., Behrndt, M., Ranft, J., Risler, T., Minc, N., and Heisenberg, C.-P. (2013). Nat. Cell Biol. 15, 1405–1414. Cao, J., Wang, J., Jackman, C.P., Cox, A.H., Trembley, M.A., Balowski, J.J., Cox, B.D., De Simone, A., Dickson, A.L., Di Talia, S., et al. (2017). Dev. Cell 42, this issue, 600–615. Duncan, A.W., Taylor, M.H., Hickey, R.D., Hanlon Newell, A.E., Lenzi, M.L., Olson, S.B., Finegold, M.J., and Grompe, M. (2010). Nature 467, 707–710. Duncan, A.W., Hanlon Newell, A.E., Bi, W., Finegold, M.J., Olson, S.B., Beaudet, A.L., and Grompe, M. (2012). J. Clin. Invest. 122, 3307–3315. Lafaurie-Janvore, J., Maiuri, P., Wang, I., Pinot, M., Manneville, J.-B., Betz, T., Balland, M., and Piel, M. (2013). Science 339, 1625–1629. Losick, V.P., Fox, D.T., and Spradling, A.C. (2013). Curr. Biol. 23, 2224–2232. Miyaoka, Y., Ebato, K., Kato, H., Arakawa, S., Shimizu, S., and Miyajima, A. (2012). Curr. Biol. 22, 1166–1175. Tang, Y.C., and Amon, A. (2013). Cell 152, 394–405. Zanet, J., Freije, A., Ruiz, M., Coulon, V., Sanz, J.R., Chiesa, J., and Gandarillas, A. (2010). PLoS One 5, e15701.