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Actomyosin Rings: The Riddle of the Sphincter

2002; Elsevier BV; Volume: 12; Issue: 1 Linguagem: Inglês

10.1016/s0960-9822(01)00639-x

1879-0445

William M. Bement,

Tissue Engineering and Regenerative Medicine

Contractile rings consisting of actin filaments and myosin-2 have long been known to power cytokinesis, but recent work has shown that single-cell and multicellular actomyosin rings participate in processes ranging from wound healing to extrusion of apoptotic cells from epithelia. To the casual student of cell locomotion, epithelial cells would seem a rather peculiar choice for research material. When isolated, their rates of movement compare poorly to even the most indolent of fibroblasts and when in their natural state — an epithelium — they appear completely inert. This view is reinforced by the fact that epithelia are charged with maintaining a selective barrier between the outside world and the interior tissues of an organism, a role that presumably would not benefit from the kind of wanderlust displayed by other, more lively cell types. In fact, however, epithelia can be very dynamic and their ability to move is actually critical for barrier maintenance in the face of wounding, when translocation of epithelial cells repairs holes in the epithelial monolayer [1.Trinkaus J.P Cells into organs.The forces that shape the embryo. 2nd edn. Prentice-Hall, Inc, Englewood Cliffs, NJ1984Google Scholar]. But in contrast to most cells used as models for cell locomotion, which tend to move in isolation, epithelial cells have the ability to move en masse, while maintaining their cell–cell junctions [1.Trinkaus J.P Cells into organs.The forces that shape the embryo. 2nd edn. Prentice-Hall, Inc, Englewood Cliffs, NJ1984Google Scholar], a feature which reflects their role as living bulwarks against unwanted incursions by ions and pathogens. The danger to epithelial function from wounding is obvious, but there is another danger that is less obvious, but just as pervasive — programmed cell death, or apoptosis. As is common with highly differentiated cells, those that comprise the epithelium have a finite lifespan, and their demise would result in a breach in the epithelium unless somehow compensated. The cellular mechanism underlying this compensation has been revealed in a study published recently in Current Biology, in which Rosenblatt and colleagues examined the effects of ultraviolet radiation-triggered apoptosis in simple epithelia [2.Rosenblatt J Raff M.C Cramer L.P An epithelial cell destined for apoptosis signals its neighbors to extrude it by an actin- and myosin-dependent mechanism.Curr. Biol. 2001; 11: 1847-1857Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar]. Cells bordering apoptotic cells assemble a multicellular ring of actin filaments (F-actin) and myosin-2 around their doomed neighbor well before overt signs of cell death, and, like overly enthusiastic pallbearers, contract this structure to squeeze the apoptotic cell into the extracellular space (Fig. 1). Several aspects of this study are immediately fascinating. It not only extends older demonstrations of multicellular contractile rings assembled in epithelia during early development [3.Young P.E Pesacreta T.C Kiehart D.P Dynamic changes in the distribution of cytoplasmic myosin during Drosophila embryogenesis.Development. 1991; 111: 1-14Crossref PubMed Google Scholar, 4.Williams-Masson E.M Malik A.N Hardin J An actin-mediated two-step mechanism is required for ventral enclosure of the C. elegans hypodermis.Development. 1997; 124: 2889-2901PubMed Google Scholar], and wound healing [5.Martin P Lewis J Actin cables and epidermal movement in embryonic wound healing.Nature. 1992; 360: 179-183Crossref PubMed Scopus (311) Google Scholar, 6.Bement W.M Forscher P Mooseker M.S A novel cytoskeletal structure involved in purse string wound closure and cell polarity maintenance.J. Cell Biol. 1993; 121: 565-578Crossref PubMed Scopus (248) Google Scholar] to a novel situation, it also shows that such contractile rings serve a function that is not anticipated by previous results. Namely, the authors find that normal levels of transepithelial resistance (a measure of epithelial barrier integrity) are maintained during extrusion, even when a large proportion of the epithelial cells have been induced to undergo apoptosis. This is apparently a direct result of the extrusion mechanism itself: the closing contractile ring maintains tight contact with the apoptotic cell as it is extruded and this, in conjunction with an apparent upregulation of tight junction components (see also [7.Madara J.L Maintenance of the macromolecular barrier at cell extrusion sites in intestinal epithelium: physiological rearrangement of tight junctions.J. Memb. Biol. 1990; 116: 177-184Crossref PubMed Scopus (191) Google Scholar]), prevents any measurable influx of ions past the dying cell. In addition, by selective inhibition of contractility in either the apoptotic cell or those cells that surround it, the authors demonstrate that the force for the contraction comes from the surrounding cells. In a prescient study of epithelial cell extrusion in situ[7.Madara J.L Maintenance of the macromolecular barrier at cell extrusion sites in intestinal epithelium: physiological rearrangement of tight junctions.J. Memb. Biol. 1990; 116: 177-184Crossref PubMed Scopus (191) Google Scholar], Madara obtained ultrastructural evidence suggesting that dying cells might be squeezed out of the monolayer. However, at the time it was impossible to determine whether this resulted from the contraction of the neighboring cell, the apoptotic cell, or both. Here, however, inhibition of contraction in the apoptotic cell by injection of the Rho GTPase inhibitor, C3 exotransferase, did not prevent extrusion, while injection into the surrounding cells did. Rosenblatt et al.[2.Rosenblatt J Raff M.C Cramer L.P An epithelial cell destined for apoptosis signals its neighbors to extrude it by an actin- and myosin-dependent mechanism.Curr. Biol. 2001; 11: 1847-1857Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar] also provide evidence that induction of the multicellular contractile ring results from cell–cell communication, rather than simple physical effects. That is, previous studies of wound-induced contractile rings were unable to distinguish between assembly triggered by wound-induced tension changes, or assembly triggered by chemical signals induced by wounding. While the present study does not rule out contributions by tension redistribution, the finding that the signal for assembly of the contractile ring comes far in advance of the morphological signs of programmed cell death suggests that the assembly must be at least partly triggered by chemical signals traveling from the dying cell to its neighbors. Further, it was found that simply adding apoptotic cells to the surface of epithelial monolayers was sufficient to induce local accumulation of F-actin and myosin-2 in those cells contacting the added cells. It is difficult to envision how, under these circumstances, tension changes are responsible for triggering localized actomyosin assembly. Of course, it is possible that completely different processes trigger contractile ring formation around wounds and it seems likely that changes in tension will play at least some role in wound closure. On the other hand, it also seems likely that at least some epithelial damage that occurs in vivo will not be immediately lethal, in which case the same mechanisms used for apoptotic contractile rings might operate. Finally, the apoptotic model system developed by the authors has several advantages for the study of multicellular contractile rings relative to those induced by wounding. The most important of these is the ability to control the number of apoptotic cells and therefore the extent of contractile ring assembly, simply by varying the extent of ultraviolet radiation exposure. This flexibility should allow quantitative analyses of biochemical features of multicellular contractile ring assembly that would be difficult, if not impossible, using wound model systems. In addition to its immediate implications, this work is also of interest because it strengthens the emerging idea that contractile actomyosin rings represent a basic cellular motility mechanism that has been repeatedly adapted throughout evolution to drive a variety of diverse biological processes [6.Bement W.M Forscher P Mooseker M.S A novel cytoskeletal structure involved in purse string wound closure and cell polarity maintenance.J. Cell Biol. 1993; 121: 565-578Crossref PubMed Scopus (248) Google Scholar, 8.Bement W.M Mandato C Kirsch M.N Wound-induced assembly and closure of an actomyosin purse string in Xenopus oocytes.Curr Biol. 1999; 9: 579-587Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar, 9.Jacinto A Martinez-Arias A Martin P Mechanisms of epithelial fusion and repair.Nat. Cell Biol. 2001; 3: E117-E123Crossref PubMed Scopus (274) Google Scholar]. Put simply, the notion is that actomyosin contractile rings have a common origin and therefore may not only share functional properties, but may also be subject to common mechanisms of regulation. One simple evolutionary scenario is that actomyosin contractile rings evolved first to dispatch basic single-cell processes, such as cell division and/or plasma membrane wound repair, and were subsequently co-opted to drive activities specific to metazoans including erythrocyte enucleation (for example, see [10.Koury S.T Koury M.J Bondurant M.C Cytoskeletal distribution and function during the maturation and enucleation of mammalian erythroblasts.J. Cell Biol. 1989; 109: 3005-3013Crossref PubMed Scopus (107) Google Scholar]), epithelial closure during early development, and repair of simple epithelia following wounding and in the face of programmed cell death (Fig. 1). Several observations are consistent with this scenario. Firstly, single-cell contractile rings are clearly ancient, in that they accompany cytokinesis not only in animal cells, but also in fungi and slime molds [11.Field C Li R Oogema K Cytokinesis in eukaryotes: a mechanistic comparison.Curr. Opin. Cell Biol. 1999; 11: 68-80Crossref PubMed Scopus (195) Google Scholar]. Secondly, the stimuli required to trigger contractile ring assembly are remarkably diverse. Specifically, the contractile rings that drive cytokinesis, erythrocyte enucleation, epithelial closure, and apoptotic cell extrusion result from cell cycle or developmentally regulated triggers, while damage triggers formation of the single and multicellular contractile rings that mediate wound repair. Thirdly, single and multicellular contractile rings share features not anticipated by convergent evolution. In particular, recent work on epithelial closure in Caenorhabditis elegans[12.Raich W.B Agbunag C Hardin J Rapid epithelial-sheet sealing in the Caenorhabditis elegans embryo requires cadherin-dependent filopodial priming.Curr. Biol. 1999; 9: 1139-1146Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar] and Drosophila[13.Jacinto A Wood W Balayo T Turmaine M Martinez-Arias A Martin P Dynamic actin-based epithelial adhesion and cell matching during Drosophila dorsal closure.Curr. Biol. 2000; 10: 1420-1426Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar] embryos has revealed that the leading edge of the epithelia that contains the contractile ring extends filopodia that contact each other across the opposing side of the closing ring, and pull the epithelial edges toward each other. While this might appear to be a phenomenon likely to be specific to multicellular contractile rings, in fact filopodia on opposing sides of single-cell wounds have recently been shown to contact each other and help pull wound edges together [14.Mandato C.A Bement W.M Contraction and polymerization cooperate to assemble and close actomyosin rings around Xenopus oocyte wounds.J. Cell Biol. 2001; 154: 785-797Crossref PubMed Scopus (117) Google Scholar]. Further, published electron micrographs of cytokinesis in ascidian embryos reveal that filopodia on opposite sides of the cleavage furrow make contact and link the forming blastomeres together [15.Satoh N Deno T Periodic appearance and disappearance of microvilli associated with cleavage cycles in the egg of the ascidian, Halocynthia roretzi.Dev. Biol. 1984; 102: 488-492Crossref PubMed Scopus (13) Google Scholar]. Finally, transmission electron microscopy analysis of apoptotic cell extrusion in situ has shown that elongate, F-actin-filled protrusions extend from the neighboring (non-extruding) cells and contact both the apoptotic cell and the extrusions extending from other neighbor cells [7.Madara J.L Maintenance of the macromolecular barrier at cell extrusion sites in intestinal epithelium: physiological rearrangement of tight junctions.J. Memb. Biol. 1990; 116: 177-184Crossref PubMed Scopus (191) Google Scholar]. Regardless of whether or not one accepts a common origin for single and multicellular contractile rings, the challenges facing those interested in these structures are similar. In particular, two central riddles concerning contractile rings have yet to be answered. First, what exactly is the nature of the processes that lead to their establishment? For example, in cytokinesis (for example [16.Kishi K Sasaki T Kuroda S Itoh T Takai Y Regulation of cytoplasmic division of Xenopus embryo by rho p21 and its inhibitor GDP/GTP exchange protein (rho GDI).J. Cell Biol. 1993; 120: 1187-1195Crossref PubMed Scopus (305) Google Scholar]), single-cell wound healing [8.Bement W.M Mandato C Kirsch M.N Wound-induced assembly and closure of an actomyosin purse string in Xenopus oocytes.Curr Biol. 1999; 9: 579-587Abstract Full Text Full Text PDF PubMed Scopus (151) Google Scholar], multicellular wound healing [17.Brock J Midwinter K Lewis J Martin P Healing of incisional wounds in the embryonic chick wing bud: characterization of the actin purse-string and demonstration of a requirement for Rho activation.J. Cell Biol. 1996; 135: 1097-1107Crossref PubMed Scopus (181) Google Scholar], and epithelial closure during development (for example [18.Harden N Ricos M Ong Y.M Chia W Lim L Participation of small GTPases in dorsal closure of the Drosophila embryo: Distinct roles for Rho subfamily proteins in epithelial morphogenesis.J. Cell Sci. 1999; 112: 273-284PubMed Google Scholar]), it is clear that the Rho GTPases are involved, but the upstream signals that lead to their activation in these systems are somewhat obscure. Thus, the demonstration that the signal is given within a few seconds of the induction of the apoptotic pathway has the potential to be extremely informative, assuming that our understanding of the processes unleashed by apoptosis induction continues to increase at their current rate. The second riddle concerns the exact mechanism(s) by which contractile rings work. Nature has an irritating tendency to clothe complex processes in apparently simple garb, and contractile rings are no exception. While it is clear that circumferential contraction is an important function of actomyosin rings, it is also becoming increasingly apparent that circumferential contraction cannot completely explain their ability to drive cytokinesis (for example [19.O'Connell C.B Warner A.K Wang Y Distinct roles of the equatorial and polar cortices in the cleavage of adherent cells.Curr. Biol. 2001; 11: 702-707Abstract Full Text Full Text PDF PubMed Scopus (72) Google Scholar]), single-cell wound healing [14.Mandato C.A Bement W.M Contraction and polymerization cooperate to assemble and close actomyosin rings around Xenopus oocyte wounds.J. Cell Biol. 2001; 154: 785-797Crossref PubMed Scopus (117) Google Scholar], and epithelial closure during development [20.Kiehart D.P Galbraith C.G Edwards K.A Rickoll W.L Montague R.A Multiple forces contribute to cell sheet morphogenesis for dorsal closure in Drosophila.J. Cell Biol. 2000; 149: 471-490Crossref PubMed Scopus (494) Google Scholar]. Hopefully, the development of new systems such as that described by Rosenblatt et al.[2.Rosenblatt J Raff M.C Cramer L.P An epithelial cell destined for apoptosis signals its neighbors to extrude it by an actin- and myosin-dependent mechanism.Curr. Biol. 2001; 11: 1847-1857Abstract Full Text Full Text PDF PubMed Scopus (362) Google Scholar], as well as increased scrutiny of other model systems, will provide answers to these riddles.