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A Ring-like Template for Abscission
Developmental Cell 578
Sharad Ramanathan1,2 and Peter S. Swain3 1 Bauer Center for Genomics Research Harvard University Cambridge, Massachusetts 02138 2 Bell Laboratories 600 Mountain Avenue Murray Hill, New Jersey 07974 3 Centre for Non-Linear Dynamics Department of Physiology McGill University 3655 Promenade Sir William Osler Montreal, Quebec H3G 1Y6 Canada
Selected Reading Colman-Lerner, A., Gordon, A., Serra, E., Chin, T., Resnekov, O., Endy, D., Pesce, C.G., and Brent, R. (2005). Nature 437, 699–706. Elowitz, M.B., Levine, A.J., Siggia, E.D., and Swain, P.S. (2002). Science 297, 1183–1186. Heiman, M.G., and Walter, P. (2000). J. Cell Biol. 151, 719–730. Pedraza, J.M., and van Oudenaarden, A. (2005). Science 307, 1965–1969. Raser, J.M., and O’Shea, E.K. (2004). Science 304, 1811–1814. Rosenfeld, N., Young, J.W., Alon, U., Swain, P.S., and Elowitz, M.B. (2005). Science 307, 1962–1965. Swain, P.S., Elowitz, M.B., and Siggia, E.D. (2002). Proc. Natl. Acad. Sci. USA 99, 12795–12800. White, J.M., and Rose, M.D. (2001). Curr. Biol. 11, R16–R20.
Developmental Cell, Vol. 9, November, 2005, Copyright ©2005 by Elsevier Inc. DOI 10.1016/j.devcel.2005.10.003
A Ring-like Template for Abscission In the October 7th issue of Cell, Gromley et al. (2005) show that a ring-like structure containing the centrosomal protein centriolin acts as a local recruitment site for the membrane fusion machinery that controls abscission. The most obvious event in animal cell cytokinesis is ingression of the cleavage furrow, which is powered by the actomyosin contractile ring and which effects the majority of the physical task of cell division. While furrowing will likely never lose its cachet, it is increasingly clear that much of what happens during furrowing is directed toward the mechanistically distinct event abscission, in which daughter cells actually become physiologically distinct. Obviously, abscission must involve some topological transformation of the plasma membrane. Recent reports that the midbody contains various proteins involved in membrane trafficking (Low et al., 2003) and older work demonstrating membrane accumulation in the midbody region (Skop et al., 2001) suggest that the cell manages this rearrangement essentially by secreting itself apart. A new paper (Gromley et al., 2005) substantiates this hypothesis by demonstrating that the centrosomal protein centriolin accumulates at the midbody and recruits both vesicle targeting and vesicle fusion machinery. Centriolin was originally identified as a marker of centrosome maturation, such that newly made daughter centrioles possess none until cells enter mitosis (Gromley et al., 2003). But surprisingly, depletion of centriolin by siRNA had no detectable effect on microtubule organization but instead results in a consistent delay or suppression of abscission. The phenotype of centriolin depletion, however, is much different than that of many other late cytokinesis defects in which the furrow ingresses to various extents, and then rapidly regresses, such that the daughter cells rapidly become
a single, binucleate cell. Instead, cells depleted of centriolin remain connected by a long cytoplasmic bridge for hours, indicating that the problem isn’t that they can’t maintain the constriction, but rather cannot sever it. This study not only provides an explanation for this defect, it also reveals the existence of a structure the authors term the “midbody ring,” which serves as the site of centriolin recruitment as well as the point at which secretory vesicles eventually accumulate within the midbody. The midbody ring is equivalent to the socalled Flemming body, a phase-dense structure that bulges outward in the approximate middle of the midbody, like the neck of a chicken choking on a particularly stale donut (Figure 1). Gromley et al. (2005) note that centriolin begins accumulating in a ring-like structure not long after the onset of furrowing and show that the centriolin-containing midbody ring is clearly evident later in cleavage, five or more micrometers from the plasma membrane. Strikingly, recruitment of centriolin to the midbody ring is dependent on the components of the centralspindlin complex, MKLP1 and MgcRacGAP (Mishima et al., 2002), the same players that control assembly of the contractile ring (Somers and Saint, 2003). While most studies have focused on localization of centralspindlin to the spindle midzone and just beneath the contractile ring, in fact MKLP and MgcRacGAP also localized in a tight spot in the center of the midzone (see Figure 5 in Somers and Saint, 2003) before the onset of furrowing, suggesting that the midbody ring might be templated at the same time as the contractile ring. In contrast to the contractile ring, once formed, the midbody ring maintains a constant size. Further, it is remarkably stable, in that components of it persist as a coherent structure well into the next cell cycle. Thus, while both the beginning and end of cytokinesis depend on macromolecular rings, many of the features of the two rings are distinctly different. These differences reflect their different roles: the contractile ring dynamically narrows the cell to the point where abscission can take place, while the midbody ring acts as a landing site for machinery involved in
Previews 579
Figure 1. Schematic Model Showing the Potential Relationship between the Contractile Ring, the Midbody Ring, and Secretory Vesicles Microtubules are shown as black lines. Contractile ring, orange; midbody ring, green; secretory vesicles, pink. During cytokinesis (or cleavage), the contractile ring pinches the cell inward in concert with formation of the midbody ring. During abscission, the midbody ring serves as a recruitment site for secretory vesicles recruited from only one of the forming daughter cells. These vesicles undergo fusion with each other and the plasma membrane on one side of the midbody ring, accomplishing abscission.
membrane fusion. Accordingly, the exocyst complex and SNARE proteins localize to the midbody ring in a centriolin-dependent manner, and their knockdown by RNAi mimics the abscission phenotype resulting from centriolin knockdown. The deposition of this machinery is followed by a dramatic and rapid recruitment of secretory vesicles to the midbody ring (Figure 1). These vesicles apparently then undergo heterotypic fusion with the PM and (presumably) homotypic fusion with each other, thereby accomplishing abscission. Remarkably, the vesicles are recruited from only one of the daughter cells. The basis for this asymmetry is unknown, although it is reminiscent of previous work showing that the mother centriole of one of the daughter cells is transiently recruited to the midbody region shortly before abscission (Piel et al., 2001). However, Gromley et al. note that they do not consistently observe such centriole translocation, suggesting that whatever changes in the microtubule cytoskeleton direct asymmetric vesicle recruitment, they are not strictly dependent on en mass centriole movement per se. Conceivably, centrosome translocation might reflect a recruitment process in which centriolin is dragged off the centriole (by MKLP1) in large patches, sometimes giving a tug sufficient to haul the centriole with it, sometimes not. If indeed centriolin recruits in clots, and if its arrival at the midbody in sufficient quantity also signals the cessation of recruitment, then it might explain how vesicle targeting might be made asymmetric: presumably one cell or the other would be first to contribute the requisite centriolin clot. By identifying the midbody ring, characterizing its
role, and showing that centriolin is one of its key components, Gromley et al. (2005) have unified several disparate observations—the inhibition of cytokinesis by centrosome ablation (Khodjakov and Rieder, 2001), the localization of secretory proteins to the midbody (Low et al., 2003), and the accumulation of membrane in the midbody region (Skop et al., 2001). As described by the authors, this study also supports the notion that the terminal stages of animal cell division resemble in many ways cytokinesis in both plants, which is driven by vesicle fusion, and budding yeast, which is finished by local concentration of the exocyst complex at the bud site. Finally, these findings underscore the mechanistic difference between the beginning and end of cytokinesis in animal cells. In fact, it would probably make sense for cell biologists to stop referring to abscission as “late cytokinesis” or the “completion of cytokinesis.” We are talking about two mechanisms, as fundamentally different from one another as the process of spindle assembly is from furrow formation, not two different stages of one process. The progression of the cytokinetic furrow helps to create the preconditions for abscission, just as the geometry of the mitotic apparatus influences the development of the furrow. Of course, it may be that cells vary in the extent to which contractile furrowing is temporally segregated from abscission: in the HeLa cells studied by Gromley et al., abscission begins long after the furrow has reduced itself to a slender canal; in large embryonic cells with a fast cell cycle, vesicle delivery and fusion may overlap with some or all of furrowing (Danilchik et al., 2003). It will therefore be of considerable interest to follow the distribution of
Developmental Cell 580
centriolin and other midbody ring components in other model systems.
Gromley, A., Jurczyk, A., Sillibourne, J., Halilovic, E., Mogensen, M., Groisman, I., Blomberg, M., and Doxsey, S. (2003). J. Cell Biol. 161, 535–545.
George von Dassow1 and William M. Bement2 1 Center for Cell Dynamics Friday Harbor Labs Friday Harbor, Washington 98250 2 Department of Zoology University of Wisconsin-Madison Madison, Wisconsin 53706
Gromley, A., Yeaman, C., Rosa, J., Redick, S., Chen, C.T., Mirabelle, S., Guha, M., Sillibourne, J., and Doxsey, S.J. (2005). Cell 123, 75–87. Khodjakov, A., and Rieder, C.L. (2001). J. Cell Biol. 153, 237–242. Low, S.H., Li, X., Miura, M., Kudo, N., Quinones, B., and Weimbs, T. (2003). Dev. Cell 4, 753–759. Mishima, M., Kaitna, S., and Glotzer, M. (2002). Dev. Cell 2, 41–54. Piel, M., Nordberg, J., Euteneuer, U., and Bornens, M. (2001). Science 291, 1550–1553.
Selected Reading Danilchik, M.V., Bedrick, S.D., Brown, E.E., and Ray, K. (2003). J. Cell Sci. 116, 273–283.
Skop, A.R., Bergmann, D., Mohler, W.A., and White, J.G. (2001). Curr. Biol. 11, 735–746. Somers, W.G., and Saint, R.A. (2003). Dev. Cell 4, 29–39.
Sharad Ramanathan1,2 and Peter S. Swain3 1 Bauer Center for Genomics Research Harvard University Cambridge, Massachusetts 02138 2 Bell Laboratories 600 Mountain Avenue Murray Hill, New Jersey 07974 3 Centre for Non-Linear Dynamics Department of Physiology McGill University 3655 Promenade Sir William Osler Montreal, Quebec H3G 1Y6 Canada
Selected Reading Colman-Lerner, A., Gordon, A., Serra, E., Chin, T., Resnekov, O., Endy, D., Pesce, C.G., and Brent, R. (2005). Nature 437, 699–706. Elowitz, M.B., Levine, A.J., Siggia, E.D., and Swain, P.S. (2002). Science 297, 1183–1186. Heiman, M.G., and Walter, P. (2000). J. Cell Biol. 151, 719–730. Pedraza, J.M., and van Oudenaarden, A. (2005). Science 307, 1965–1969. Raser, J.M., and O’Shea, E.K. (2004). Science 304, 1811–1814. Rosenfeld, N., Young, J.W., Alon, U., Swain, P.S., and Elowitz, M.B. (2005). Science 307, 1962–1965. Swain, P.S., Elowitz, M.B., and Siggia, E.D. (2002). Proc. Natl. Acad. Sci. USA 99, 12795–12800. White, J.M., and Rose, M.D. (2001). Curr. Biol. 11, R16–R20.
Developmental Cell, Vol. 9, November, 2005, Copyright ©2005 by Elsevier Inc. DOI 10.1016/j.devcel.2005.10.003
A Ring-like Template for Abscission In the October 7th issue of Cell, Gromley et al. (2005) show that a ring-like structure containing the centrosomal protein centriolin acts as a local recruitment site for the membrane fusion machinery that controls abscission. The most obvious event in animal cell cytokinesis is ingression of the cleavage furrow, which is powered by the actomyosin contractile ring and which effects the majority of the physical task of cell division. While furrowing will likely never lose its cachet, it is increasingly clear that much of what happens during furrowing is directed toward the mechanistically distinct event abscission, in which daughter cells actually become physiologically distinct. Obviously, abscission must involve some topological transformation of the plasma membrane. Recent reports that the midbody contains various proteins involved in membrane trafficking (Low et al., 2003) and older work demonstrating membrane accumulation in the midbody region (Skop et al., 2001) suggest that the cell manages this rearrangement essentially by secreting itself apart. A new paper (Gromley et al., 2005) substantiates this hypothesis by demonstrating that the centrosomal protein centriolin accumulates at the midbody and recruits both vesicle targeting and vesicle fusion machinery. Centriolin was originally identified as a marker of centrosome maturation, such that newly made daughter centrioles possess none until cells enter mitosis (Gromley et al., 2003). But surprisingly, depletion of centriolin by siRNA had no detectable effect on microtubule organization but instead results in a consistent delay or suppression of abscission. The phenotype of centriolin depletion, however, is much different than that of many other late cytokinesis defects in which the furrow ingresses to various extents, and then rapidly regresses, such that the daughter cells rapidly become
a single, binucleate cell. Instead, cells depleted of centriolin remain connected by a long cytoplasmic bridge for hours, indicating that the problem isn’t that they can’t maintain the constriction, but rather cannot sever it. This study not only provides an explanation for this defect, it also reveals the existence of a structure the authors term the “midbody ring,” which serves as the site of centriolin recruitment as well as the point at which secretory vesicles eventually accumulate within the midbody. The midbody ring is equivalent to the socalled Flemming body, a phase-dense structure that bulges outward in the approximate middle of the midbody, like the neck of a chicken choking on a particularly stale donut (Figure 1). Gromley et al. (2005) note that centriolin begins accumulating in a ring-like structure not long after the onset of furrowing and show that the centriolin-containing midbody ring is clearly evident later in cleavage, five or more micrometers from the plasma membrane. Strikingly, recruitment of centriolin to the midbody ring is dependent on the components of the centralspindlin complex, MKLP1 and MgcRacGAP (Mishima et al., 2002), the same players that control assembly of the contractile ring (Somers and Saint, 2003). While most studies have focused on localization of centralspindlin to the spindle midzone and just beneath the contractile ring, in fact MKLP and MgcRacGAP also localized in a tight spot in the center of the midzone (see Figure 5 in Somers and Saint, 2003) before the onset of furrowing, suggesting that the midbody ring might be templated at the same time as the contractile ring. In contrast to the contractile ring, once formed, the midbody ring maintains a constant size. Further, it is remarkably stable, in that components of it persist as a coherent structure well into the next cell cycle. Thus, while both the beginning and end of cytokinesis depend on macromolecular rings, many of the features of the two rings are distinctly different. These differences reflect their different roles: the contractile ring dynamically narrows the cell to the point where abscission can take place, while the midbody ring acts as a landing site for machinery involved in
Previews 579
Figure 1. Schematic Model Showing the Potential Relationship between the Contractile Ring, the Midbody Ring, and Secretory Vesicles Microtubules are shown as black lines. Contractile ring, orange; midbody ring, green; secretory vesicles, pink. During cytokinesis (or cleavage), the contractile ring pinches the cell inward in concert with formation of the midbody ring. During abscission, the midbody ring serves as a recruitment site for secretory vesicles recruited from only one of the forming daughter cells. These vesicles undergo fusion with each other and the plasma membrane on one side of the midbody ring, accomplishing abscission.
membrane fusion. Accordingly, the exocyst complex and SNARE proteins localize to the midbody ring in a centriolin-dependent manner, and their knockdown by RNAi mimics the abscission phenotype resulting from centriolin knockdown. The deposition of this machinery is followed by a dramatic and rapid recruitment of secretory vesicles to the midbody ring (Figure 1). These vesicles apparently then undergo heterotypic fusion with the PM and (presumably) homotypic fusion with each other, thereby accomplishing abscission. Remarkably, the vesicles are recruited from only one of the daughter cells. The basis for this asymmetry is unknown, although it is reminiscent of previous work showing that the mother centriole of one of the daughter cells is transiently recruited to the midbody region shortly before abscission (Piel et al., 2001). However, Gromley et al. note that they do not consistently observe such centriole translocation, suggesting that whatever changes in the microtubule cytoskeleton direct asymmetric vesicle recruitment, they are not strictly dependent on en mass centriole movement per se. Conceivably, centrosome translocation might reflect a recruitment process in which centriolin is dragged off the centriole (by MKLP1) in large patches, sometimes giving a tug sufficient to haul the centriole with it, sometimes not. If indeed centriolin recruits in clots, and if its arrival at the midbody in sufficient quantity also signals the cessation of recruitment, then it might explain how vesicle targeting might be made asymmetric: presumably one cell or the other would be first to contribute the requisite centriolin clot. By identifying the midbody ring, characterizing its
role, and showing that centriolin is one of its key components, Gromley et al. (2005) have unified several disparate observations—the inhibition of cytokinesis by centrosome ablation (Khodjakov and Rieder, 2001), the localization of secretory proteins to the midbody (Low et al., 2003), and the accumulation of membrane in the midbody region (Skop et al., 2001). As described by the authors, this study also supports the notion that the terminal stages of animal cell division resemble in many ways cytokinesis in both plants, which is driven by vesicle fusion, and budding yeast, which is finished by local concentration of the exocyst complex at the bud site. Finally, these findings underscore the mechanistic difference between the beginning and end of cytokinesis in animal cells. In fact, it would probably make sense for cell biologists to stop referring to abscission as “late cytokinesis” or the “completion of cytokinesis.” We are talking about two mechanisms, as fundamentally different from one another as the process of spindle assembly is from furrow formation, not two different stages of one process. The progression of the cytokinetic furrow helps to create the preconditions for abscission, just as the geometry of the mitotic apparatus influences the development of the furrow. Of course, it may be that cells vary in the extent to which contractile furrowing is temporally segregated from abscission: in the HeLa cells studied by Gromley et al., abscission begins long after the furrow has reduced itself to a slender canal; in large embryonic cells with a fast cell cycle, vesicle delivery and fusion may overlap with some or all of furrowing (Danilchik et al., 2003). It will therefore be of considerable interest to follow the distribution of
Developmental Cell 580
centriolin and other midbody ring components in other model systems.
Gromley, A., Jurczyk, A., Sillibourne, J., Halilovic, E., Mogensen, M., Groisman, I., Blomberg, M., and Doxsey, S. (2003). J. Cell Biol. 161, 535–545.
George von Dassow1 and William M. Bement2 1 Center for Cell Dynamics Friday Harbor Labs Friday Harbor, Washington 98250 2 Department of Zoology University of Wisconsin-Madison Madison, Wisconsin 53706
Gromley, A., Yeaman, C., Rosa, J., Redick, S., Chen, C.T., Mirabelle, S., Guha, M., Sillibourne, J., and Doxsey, S.J. (2005). Cell 123, 75–87. Khodjakov, A., and Rieder, C.L. (2001). J. Cell Biol. 153, 237–242. Low, S.H., Li, X., Miura, M., Kudo, N., Quinones, B., and Weimbs, T. (2003). Dev. Cell 4, 753–759. Mishima, M., Kaitna, S., and Glotzer, M. (2002). Dev. Cell 2, 41–54. Piel, M., Nordberg, J., Euteneuer, U., and Bornens, M. (2001). Science 291, 1550–1553.
Selected Reading Danilchik, M.V., Bedrick, S.D., Brown, E.E., and Ray, K. (2003). J. Cell Sci. 116, 273–283.
Skop, A.R., Bergmann, D., Mohler, W.A., and White, J.G. (2001). Curr. Biol. 11, 735–746. Somers, W.G., and Saint, R.A. (2003). Dev. Cell 4, 29–39.