- Email: [email protected]
Chromosome Segregation: Spindle Mechanics Come To Life
Current Biology Vol 21 No 18 R688
13. Carvalho, A., and Clark, A. (2005). Y chromosome of D. pseudoobscura is not homologous to the ancestral Drosophila Y. Science 307, 108–110. 14. Bernasconi, G., Antonovics, J., Biere, A., Charlesworth, D., Delph, L.F., Filatov, D., Giraud, T., Hood, M.E., Marais, G.A., McCauley, D., et al. (2009). Silene as a model system in ecology and evolution. Heredity 103, 5–14. 15. Bergero, R., Forrest, A., Kamau, E., and Charlesworth, D. (2007). Evolutionary strata on the X chromosomes of the dioecious plant Silene latifolia: evidence from new sex-linked genes. Genetics 175, 1945–1954.
16. Marais, G.A., Nicolas, M., Bergero, R., Chambrier, P., Kejnovsky, E., Mone´ger, F., Hobza, R., Widmer, A., and Charlesworth, D. (2008). Evidence for degeneration of the Y chromosome in the dioecious plant Silene latifolia. Curr. Biol. 18, 545–549. 17. Heilbuth, J.C. (2000). Lower species richness in dioecious clades. Am. Nat. 156, 221–241. 18. Bachtrog, D. (2008). The temporal dynamics of processes underlying Y chromosome degeneration. Genetics 179, 1513–1525. 19. Gelbart, M.E., and Kuroda, M.I. (2009). Drosophila dosage compensation: a complex voyage to the X chromosome. Development 136, 1399–1410.
Chromosome Segregation: Spindle Mechanics Come To Life Chromosome segregation is a mechanical process, and the spindle generates, and is subject to, mechanical force. A recent study probes how the mechanical architecture of the spindle allows it to maintain mechanical integrity despite these forces. Sophie Dumont The spindle is a dynamic, self-organizing microtubule assembly whose function is essentially mechanical: to physically segregate chromosomes. The spindle generates forces using motor proteins and microtubule polymerization dynamics, and it also responds to forces, for example to position its poles and chromosomes relative to each other [1], and ultimately to segregate chromosomes [2]. The spindle self-assembles from a long parts list every time it is needed, forms a long-lived mm-scale structure using short-lived nm-sized parts, and is able to reorganize itself to self-repair and correct chromosome attachment errors. How does the spindle maintain its structural integrity and function while being subject to forces, and while being such a dynamic and adaptable machine? The spindle’s material properties are at the heart of this paradox — a paradox that has puzzled me since I first heard all that a spindle can do. We now have a near complete list of spindle molecules [3,4] and have made much progress in understanding their nm-scale dynamics and mechanics. However, our understanding of the emergent properties of the whole assembly, i.e. mm-scale integrated spindle architecture and mechanics, is still poor, making it difficult to bridge the nm- and mm-scale activities of chromosome segregation.
Spindle function relies on both active and passive mechanical forces, but only the former have been much studied. Molecular motors and microtubule dynamics consume fuel (ATP and GTP, respectively) to actively generate piconewton forces per molecule. These have been the subject of much investigation at the single molecule level [5,6]. How motors and microtubules act collectively, to generate nanonewton forces and mm-scale movements, is much less understood. How an object responds to force depends on its material properties, specifically its viscosity and elasticity. These properties manifest as opposing responses to deformation: they constitute passive molecular forces that are generated in response to, and tend to oppose, active force generators. Compared to homogeneous polymer solutions, biological assemblies (active matter) have a much richer set of nanoscale components and interactions — and their bulk material properties reflect this richness. The viscosity and elasticity of biological assemblies stem from specific dynamic molecular interactions, such as bonds between polymers and crosslinking molecules, and these interactions can be tuned by evolution, and can thus exhibit complex temporal and directional dependencies. Probing the material properties of the spindle is technically very challenging, and spindle viscosity and elasticity
20. Ross, J.A., Urton, J.R., Boland, J., Shapiro, M.D., and Peichel, C.L. (2009). Turnover of sex chromosomes in the stickleback fishes (gasterosteidae). PLoS Genet. 5, e1000391.
Department of Integrative Biology, University of California Berkeley, Berkeley, CA 9470, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2011.08.027
have been largely ignored: we know neither their magnitudes, nor the molecules responsible for them. In a recent study, Shimamoto et al. [7] developed an elegant approach to measure the spindle’s material properties and identify their dominant architectural determinants. The authors measured the deformation of the spindle to different externally applied forces, and from this extracted the spindle’s viscosity and elasticity [8,9]. These measurements were performed on spindles assembled in Xenopus egg extracts, a model cytoplasm that maintains the physiological milieu characteristic of egg cytoplasm, i.e. a living cell-free system. This system allows physical access to the spindle without having to penetrate the plasma membrane, as well as easy biochemical and chemical perturbations. By repeating their physical perturbation– response studies in different molecular backgrounds, the authors could thus begin to relate viscosity and elasticity to known spindle molecules. To mechanically perturb the spindle, the authors skewered two microneedles into it (Figure 1A): a stiff needle was programmed to move the spindle in a given direction at a given velocity, while a flexible needle reported forces exerted by the spindle in response to this perturbation. The stiffness of the flexible needle was calibrated such that its bending, measured via imaging, could be mapped to force. Forces up to two nanonewtons were applied. Nanonewtons is an appropriate force scale for a large assembly like the spindle, which consists of thousands of molecules that can each produce piconewton-scale forces. Indeed, this is the force scale required to stall chromosome movement [10]. Roughly speaking, the steady-state bending amplitude of the flexible needle
Dispatch R689
A
B
Force
Dynamic crosslinks Kinetochore-microtubules Non-kinetochore-microtubules
Short times
Intermediate times
Long times Current Biology
Figure 1. Spindle mechanical elements and their response dynamics. (A) Sketch of a spindle and its key mechanical elements. Shimamoto et al. [7] probed the spindle’s material properties by using one needle (top) to controllably move it, and another (bottom) to measure its mechanical response. (B) The spindle behaves as a viscoelastic material along its short axis. It behaves as a solid (elastic spring) at short and long timescales, and as a liquid (viscous dashpot) at intermediate ones. Dominant mechanical elements are shown in a grey ellipse and thickness of spring scales with stiffness. Colored legend applies to A and B.
post-perturbation informs on spindle elasticity, a solid-like property, while the time-dependence of its bending relaxation informs on spindle viscosity, a liquid-like property. The spindle is an intrinsically anisotropic and dynamic structure. Spindle microtubules are predominantly oriented along the pole-to-pole axis (Figure 1A), and most of these microtubules have an average lifetime of tens of seconds [11] due to rapid length fluctuations [12] — although a subset connected to chromosomes at kinetochores may live longer [13] (kinetochore-microtubules, or kMTs). Microtubules are inter-connected and spatially organized by cross-linking molecules that include plus- and minus-end directed motor proteins. Material properties of the spindle must depend on the orientation of the mechanical perturbation [14,15]. They must also depend on the timescale of the perturbation because the spindle can relax and reorganize in different ways over different timescales. For example, motors associate and dissociate their heads over milliseconds to seconds (depending on their processivity), while microtubules shrink and are replaced over tens of seconds. As we might except from the spindle’s anisotropic structure and function, Shimamoto et al. [7] found strong anisotropy in physical responses. When the spindle was displaced along its long axis, the
force-measuring needle bent, and relaxed back until it reached its original unbent position without affecting spindle shape. The authors detected only viscosity, and no elasticity, along the long axis. The measured viscosity was at least 100 times greater than that measured outside the spindle. Thus simple fluid drag forces are negligible in the spindle, and the high viscosity must emerge from structural spindle elements that oppose forced movements, for example protein crosslinks between microtubules. Biologically, it may make sense that no elastic spring works to permanently oppose motion parallel to the axis of chromosome segregation, and that the spindle is liquid-like along this axis. If such an elastic material existed, it might tend to block chromosome segregation. When the spindle was displaced along its short axis, the force-measuring needle bent, relaxed at similar rates as above, but never returned to its unbent original position and the spindle never recovered its original shape. Along its short axis, the spindle is a viscoelastic material: it is not only viscous (liquid-like), but also elastic (solid-like). This too may make biological sense. Structural integrity normal to the spindle axis is required to preserve the spindle’s shape, given that no microtubules grow in this direction. It is particularly notable that the spindle never relaxed, given that most of its molecules turn over on tens of seconds timescales.
The timescale of mechanical perturbation will determine whether the spindle responds more like a liquid or a solid along its short axis. Step-like perturbations can be used to measure long timescale responses, and oscillatory perturbations of different frequencies to measure shorter timescsale responses. The author’s results are striking: the spindle’s short axis has a solid-like elastic response to both slow (>100 s) and fast (<1–10 s) perturbations, and a liquid-like viscous response to perturbations of intermediate (10–100s) timescale. Both spindle viscosity and elasticity are timescale-dependent. Spindle viscosity is maximum (as high as that of peanut butter!) for intermediate timescale perturbations. Meanwhile, the spindle is stiffest to fast perturbations, and most compliant to slow perturbations (while always more compliant than even Jell-O!). In order to understand how this works, the authors consider the simplest model for a viscoelastic material (Figure 1B): an elastic spring and viscous dashpot in series, connected to a second spring in parallel. Fitting the data to this model suggested that the former spring is much stiffer than the latter, and a battery of biochemical and chemical perturbations suggested which spindle molecular and structural components are responsible for each mechanical element (Figure 1A). Spindle viscosity likely stems from dynamic
Current Biology Vol 21 No 18 R690
protein crosslinks (Figure 1B, red dashpot) between microtubules. Upon mechanical perturbation, spindle microtubules could relax and reorganize as microtubule crosslinking proteins detach and reattach, or as microtubules simply shrink and regrow, for example. The latter is especially appealing since both the microtubule lifetime and measured mechanical relaxation times are of tens of seconds. As for spindle elasticity, it depends on spindle pole integrity and likely stems from the rigidity of microtubules (Figure 1B, springs): the authors link the rigidity of non-kMTs (green and stiffer spring in series) to short-term elasticity, and that of kMTs (purple and more compliant spring in parallel) to long-term elasticity. Repeating this experiment in spindles assembled without kinetochores and kMTs [16] would allow us to determine whether both microtubule populations do indeed behave as distinct mechanical entities. The data provided by Shimamoto et al. [7] suggest that the spindle can be a mechanically versatile machine by exploiting different functional timescales and axes. Along its long axis the spindle is liquid-like, while along its short axis it can be more liquid- or solid-like at different timescales. Over short timescales (Figure 1B, left), the dynamic microtubule crosslinks do not have time to relieve strain and the stiffest spring, non-KMT rigidity, dominates: non-kMTs, with their short lifetimes, help the spindle robustly keep its integrity in the face of rapid yanks. Over intermediate timescales (Figure 1B,
center), these dynamic crosslinks reorganize themselves locally and dominate the response until the system is equilibrated: if the spindle is deformed at such velocities, for example when a chromosome squeezes through, it can accommodate big deformations locally while minimizing an elastic response and maintaining global integrity. Over long timescales (Figure 1B, right), the same dynamic crosslinks reach a new equilibrium, and the most compliant spring, kMT rigidity, dominates the response: kMTs, with their longer lifetimes, give the spindle a long-term mechanical memory of its architecture. The force the spindle exerts back on its components can thus be very different depending on how fast these move, in which direction they go and where they are in the spindle [17]. Thus not only do nm-scale activities lead to mm-scale material properties, but these material properties may inform — and help coordinate – nm-scale dynamics. 1. Dumont, S., and Mitchison, T. (2009). Force and length in the mitotic spindle. Curr. Biol. 19, R749–R761. 2. Li, X., and Nicklas, R.B. (1995). Mitotic forces control a cell-cycle checkpoint. Nature 373, 630–632. 3. Hutchins, J.R., Toyoda, Y., Hegemann, B., Poser, I., He´riche´, J.K., Sykora, M.M., Augsburg, M., Hudecz, O., Buschhorn, B.A., Bulkescher, J., et al. (2010). Systematic analysis of human protein complexes identifies chromosome segregation proteins. Science 328, 593–599. 4. Neumann, B., Walter, T., He´riche´, J.K., Bulkescher, J., Erfle, H., Conrad, C., Rogers, P., Poser, I., Held, M., Liebel, U., et al. (2010). Phenotypic profiling of the human genome by time-lapse microscopy reveals cell division genes. Nature 464, 721–727. 5. Grishchuk, E.L., Molodtsov, M.I., Ataullakhanov, F.I., and McIntosh, J.R. (2005).
Recent findings report the selective internalization of core planar cell polarity components during mitosis followed by cell-non-autonomous polarized recycling. This novel mechanistic model explains how tissue polarity is inherited in daughter cells of proliferative tissue.
Planar cell polarity (PCP) is an evolutionarily conserved mechanism
7.
8.
9.
10.
11.
12.
13.
14.
15.
References
Tissue Polarity: PCP Inheritance Ensured by Selective Mitotic Endocytosis
Nabila Founounou and Roland Le Borgne
6.
enabling epithelial cells to individually polarize perpendicular to their apicobasal axis. Establishment and maintenance of PCP have been extensively studied in the developing
16.
17.
Force production by disassembling microtubules. Nature 438, 384–388. Svoboda, K., and Block, S.M. (1994). Force and velocity measured for single kinesin molecules. Cell 77, 773–784. Shimamoto, Y., Maeda, Y., Ishiwata, S., Libchaber, A., and Kapoor, T.M. (2011). Insights into the micromechanical properties of the metaphase spindle. Cell 145, 1062–1074. Fletcher, D.A., and Mullins, R.D. (2010). Cell mechanics and the cytoskeleton. Nature 463, 485–492. Howard, J. (2001). Mechanics of Motor Proteins and the Cytoskeleton (Sunderland: Sinauer Associates Incorporated). Nicklas, R.B. (1983). Measurements of the force produced by the mitotic spindle in anaphase. J. Cell Biol. 97, 532–548. Sawin, K.E., and Mitchison, T.J. (1991). Poleward microtubule flux in mitotic spindles assembled in vitro. J. Cell Biol. 112, 941–954. Mitchison, T., and Kirschner, M. (1984). Dynamic instability of microtubule growth. Nature 312, 237–242. Zhai, Y., Kronebusch, P.J., and Borisy, G.G. (1995). Kinetochore microtubule dynamics and the metaphase-anaphase transition. J. Cell Biol. 131, 731–734. Dumont, S., and Mitchison, T.J. (2009). Compression regulates spindle length by a mechanochemical switch at the poles. Curr. Biol. 19, 1086–1095. Itabashi, T., Takagi, J., Shimamoto, Y., Onoe, H., Kuwana, K., Shimoyama, I., Gaetz, J., Kapoor, T.M., and Ishiwata, S. (2009). Probing the mechanical architecture of the vertebrate meiotic spindle. Nat. Methods 6, 167–172. Heald, R., Tournebize, R., Blank, T., Sandaltzopoulos, R., Becker, P., Hyman, A., and Karsenti, E. (1996). Self-organization of microtubules into bipolar spindles around artificial chromosomes in Xenopus egg extracts. Nature 382, 420–425. Gatlin, J.C., Matov, A., Danuser, G., Mitchison, T.J., and Salmon, E.D. (2010). Directly probing the mechanical properties of the spindle and its matrix. J. Cell Biol. 188, 481–489.
Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA. E-mail: [email protected]
DOI: 10.1016/j.cub.2011.08.008
Drosophila epidermis [1–5]. Highly regenerative tissues, such as mammalian skin, also exhibit the features of PCP [6,7]. A challenging task for such proliferative tissue is to maintain and accurately propagate PCP information while cells keep dividing at high frequency. A recent study [8], published in Nature Cell Biology by the team of Elaine Fuchs, addresses this issue in mouse basal epithelial cells — progenitors that generate hair follicles and outer stratified skin layers. In this study, which combines cutting-edge mouse genetics and state-of-the-art cell biology approaches, Devenport and
13. Carvalho, A., and Clark, A. (2005). Y chromosome of D. pseudoobscura is not homologous to the ancestral Drosophila Y. Science 307, 108–110. 14. Bernasconi, G., Antonovics, J., Biere, A., Charlesworth, D., Delph, L.F., Filatov, D., Giraud, T., Hood, M.E., Marais, G.A., McCauley, D., et al. (2009). Silene as a model system in ecology and evolution. Heredity 103, 5–14. 15. Bergero, R., Forrest, A., Kamau, E., and Charlesworth, D. (2007). Evolutionary strata on the X chromosomes of the dioecious plant Silene latifolia: evidence from new sex-linked genes. Genetics 175, 1945–1954.
16. Marais, G.A., Nicolas, M., Bergero, R., Chambrier, P., Kejnovsky, E., Mone´ger, F., Hobza, R., Widmer, A., and Charlesworth, D. (2008). Evidence for degeneration of the Y chromosome in the dioecious plant Silene latifolia. Curr. Biol. 18, 545–549. 17. Heilbuth, J.C. (2000). Lower species richness in dioecious clades. Am. Nat. 156, 221–241. 18. Bachtrog, D. (2008). The temporal dynamics of processes underlying Y chromosome degeneration. Genetics 179, 1513–1525. 19. Gelbart, M.E., and Kuroda, M.I. (2009). Drosophila dosage compensation: a complex voyage to the X chromosome. Development 136, 1399–1410.
Chromosome Segregation: Spindle Mechanics Come To Life Chromosome segregation is a mechanical process, and the spindle generates, and is subject to, mechanical force. A recent study probes how the mechanical architecture of the spindle allows it to maintain mechanical integrity despite these forces. Sophie Dumont The spindle is a dynamic, self-organizing microtubule assembly whose function is essentially mechanical: to physically segregate chromosomes. The spindle generates forces using motor proteins and microtubule polymerization dynamics, and it also responds to forces, for example to position its poles and chromosomes relative to each other [1], and ultimately to segregate chromosomes [2]. The spindle self-assembles from a long parts list every time it is needed, forms a long-lived mm-scale structure using short-lived nm-sized parts, and is able to reorganize itself to self-repair and correct chromosome attachment errors. How does the spindle maintain its structural integrity and function while being subject to forces, and while being such a dynamic and adaptable machine? The spindle’s material properties are at the heart of this paradox — a paradox that has puzzled me since I first heard all that a spindle can do. We now have a near complete list of spindle molecules [3,4] and have made much progress in understanding their nm-scale dynamics and mechanics. However, our understanding of the emergent properties of the whole assembly, i.e. mm-scale integrated spindle architecture and mechanics, is still poor, making it difficult to bridge the nm- and mm-scale activities of chromosome segregation.
Spindle function relies on both active and passive mechanical forces, but only the former have been much studied. Molecular motors and microtubule dynamics consume fuel (ATP and GTP, respectively) to actively generate piconewton forces per molecule. These have been the subject of much investigation at the single molecule level [5,6]. How motors and microtubules act collectively, to generate nanonewton forces and mm-scale movements, is much less understood. How an object responds to force depends on its material properties, specifically its viscosity and elasticity. These properties manifest as opposing responses to deformation: they constitute passive molecular forces that are generated in response to, and tend to oppose, active force generators. Compared to homogeneous polymer solutions, biological assemblies (active matter) have a much richer set of nanoscale components and interactions — and their bulk material properties reflect this richness. The viscosity and elasticity of biological assemblies stem from specific dynamic molecular interactions, such as bonds between polymers and crosslinking molecules, and these interactions can be tuned by evolution, and can thus exhibit complex temporal and directional dependencies. Probing the material properties of the spindle is technically very challenging, and spindle viscosity and elasticity
20. Ross, J.A., Urton, J.R., Boland, J., Shapiro, M.D., and Peichel, C.L. (2009). Turnover of sex chromosomes in the stickleback fishes (gasterosteidae). PLoS Genet. 5, e1000391.
Department of Integrative Biology, University of California Berkeley, Berkeley, CA 9470, USA. E-mail: [email protected] DOI: 10.1016/j.cub.2011.08.027
have been largely ignored: we know neither their magnitudes, nor the molecules responsible for them. In a recent study, Shimamoto et al. [7] developed an elegant approach to measure the spindle’s material properties and identify their dominant architectural determinants. The authors measured the deformation of the spindle to different externally applied forces, and from this extracted the spindle’s viscosity and elasticity [8,9]. These measurements were performed on spindles assembled in Xenopus egg extracts, a model cytoplasm that maintains the physiological milieu characteristic of egg cytoplasm, i.e. a living cell-free system. This system allows physical access to the spindle without having to penetrate the plasma membrane, as well as easy biochemical and chemical perturbations. By repeating their physical perturbation– response studies in different molecular backgrounds, the authors could thus begin to relate viscosity and elasticity to known spindle molecules. To mechanically perturb the spindle, the authors skewered two microneedles into it (Figure 1A): a stiff needle was programmed to move the spindle in a given direction at a given velocity, while a flexible needle reported forces exerted by the spindle in response to this perturbation. The stiffness of the flexible needle was calibrated such that its bending, measured via imaging, could be mapped to force. Forces up to two nanonewtons were applied. Nanonewtons is an appropriate force scale for a large assembly like the spindle, which consists of thousands of molecules that can each produce piconewton-scale forces. Indeed, this is the force scale required to stall chromosome movement [10]. Roughly speaking, the steady-state bending amplitude of the flexible needle
Dispatch R689
A
B
Force
Dynamic crosslinks Kinetochore-microtubules Non-kinetochore-microtubules
Short times
Intermediate times
Long times Current Biology
Figure 1. Spindle mechanical elements and their response dynamics. (A) Sketch of a spindle and its key mechanical elements. Shimamoto et al. [7] probed the spindle’s material properties by using one needle (top) to controllably move it, and another (bottom) to measure its mechanical response. (B) The spindle behaves as a viscoelastic material along its short axis. It behaves as a solid (elastic spring) at short and long timescales, and as a liquid (viscous dashpot) at intermediate ones. Dominant mechanical elements are shown in a grey ellipse and thickness of spring scales with stiffness. Colored legend applies to A and B.
post-perturbation informs on spindle elasticity, a solid-like property, while the time-dependence of its bending relaxation informs on spindle viscosity, a liquid-like property. The spindle is an intrinsically anisotropic and dynamic structure. Spindle microtubules are predominantly oriented along the pole-to-pole axis (Figure 1A), and most of these microtubules have an average lifetime of tens of seconds [11] due to rapid length fluctuations [12] — although a subset connected to chromosomes at kinetochores may live longer [13] (kinetochore-microtubules, or kMTs). Microtubules are inter-connected and spatially organized by cross-linking molecules that include plus- and minus-end directed motor proteins. Material properties of the spindle must depend on the orientation of the mechanical perturbation [14,15]. They must also depend on the timescale of the perturbation because the spindle can relax and reorganize in different ways over different timescales. For example, motors associate and dissociate their heads over milliseconds to seconds (depending on their processivity), while microtubules shrink and are replaced over tens of seconds. As we might except from the spindle’s anisotropic structure and function, Shimamoto et al. [7] found strong anisotropy in physical responses. When the spindle was displaced along its long axis, the
force-measuring needle bent, and relaxed back until it reached its original unbent position without affecting spindle shape. The authors detected only viscosity, and no elasticity, along the long axis. The measured viscosity was at least 100 times greater than that measured outside the spindle. Thus simple fluid drag forces are negligible in the spindle, and the high viscosity must emerge from structural spindle elements that oppose forced movements, for example protein crosslinks between microtubules. Biologically, it may make sense that no elastic spring works to permanently oppose motion parallel to the axis of chromosome segregation, and that the spindle is liquid-like along this axis. If such an elastic material existed, it might tend to block chromosome segregation. When the spindle was displaced along its short axis, the force-measuring needle bent, relaxed at similar rates as above, but never returned to its unbent original position and the spindle never recovered its original shape. Along its short axis, the spindle is a viscoelastic material: it is not only viscous (liquid-like), but also elastic (solid-like). This too may make biological sense. Structural integrity normal to the spindle axis is required to preserve the spindle’s shape, given that no microtubules grow in this direction. It is particularly notable that the spindle never relaxed, given that most of its molecules turn over on tens of seconds timescales.
The timescale of mechanical perturbation will determine whether the spindle responds more like a liquid or a solid along its short axis. Step-like perturbations can be used to measure long timescale responses, and oscillatory perturbations of different frequencies to measure shorter timescsale responses. The author’s results are striking: the spindle’s short axis has a solid-like elastic response to both slow (>100 s) and fast (<1–10 s) perturbations, and a liquid-like viscous response to perturbations of intermediate (10–100s) timescale. Both spindle viscosity and elasticity are timescale-dependent. Spindle viscosity is maximum (as high as that of peanut butter!) for intermediate timescale perturbations. Meanwhile, the spindle is stiffest to fast perturbations, and most compliant to slow perturbations (while always more compliant than even Jell-O!). In order to understand how this works, the authors consider the simplest model for a viscoelastic material (Figure 1B): an elastic spring and viscous dashpot in series, connected to a second spring in parallel. Fitting the data to this model suggested that the former spring is much stiffer than the latter, and a battery of biochemical and chemical perturbations suggested which spindle molecular and structural components are responsible for each mechanical element (Figure 1A). Spindle viscosity likely stems from dynamic
Current Biology Vol 21 No 18 R690
protein crosslinks (Figure 1B, red dashpot) between microtubules. Upon mechanical perturbation, spindle microtubules could relax and reorganize as microtubule crosslinking proteins detach and reattach, or as microtubules simply shrink and regrow, for example. The latter is especially appealing since both the microtubule lifetime and measured mechanical relaxation times are of tens of seconds. As for spindle elasticity, it depends on spindle pole integrity and likely stems from the rigidity of microtubules (Figure 1B, springs): the authors link the rigidity of non-kMTs (green and stiffer spring in series) to short-term elasticity, and that of kMTs (purple and more compliant spring in parallel) to long-term elasticity. Repeating this experiment in spindles assembled without kinetochores and kMTs [16] would allow us to determine whether both microtubule populations do indeed behave as distinct mechanical entities. The data provided by Shimamoto et al. [7] suggest that the spindle can be a mechanically versatile machine by exploiting different functional timescales and axes. Along its long axis the spindle is liquid-like, while along its short axis it can be more liquid- or solid-like at different timescales. Over short timescales (Figure 1B, left), the dynamic microtubule crosslinks do not have time to relieve strain and the stiffest spring, non-KMT rigidity, dominates: non-kMTs, with their short lifetimes, help the spindle robustly keep its integrity in the face of rapid yanks. Over intermediate timescales (Figure 1B,
center), these dynamic crosslinks reorganize themselves locally and dominate the response until the system is equilibrated: if the spindle is deformed at such velocities, for example when a chromosome squeezes through, it can accommodate big deformations locally while minimizing an elastic response and maintaining global integrity. Over long timescales (Figure 1B, right), the same dynamic crosslinks reach a new equilibrium, and the most compliant spring, kMT rigidity, dominates the response: kMTs, with their longer lifetimes, give the spindle a long-term mechanical memory of its architecture. The force the spindle exerts back on its components can thus be very different depending on how fast these move, in which direction they go and where they are in the spindle [17]. Thus not only do nm-scale activities lead to mm-scale material properties, but these material properties may inform — and help coordinate – nm-scale dynamics. 1. Dumont, S., and Mitchison, T. (2009). Force and length in the mitotic spindle. Curr. Biol. 19, R749–R761. 2. Li, X., and Nicklas, R.B. (1995). Mitotic forces control a cell-cycle checkpoint. Nature 373, 630–632. 3. Hutchins, J.R., Toyoda, Y., Hegemann, B., Poser, I., He´riche´, J.K., Sykora, M.M., Augsburg, M., Hudecz, O., Buschhorn, B.A., Bulkescher, J., et al. (2010). Systematic analysis of human protein complexes identifies chromosome segregation proteins. Science 328, 593–599. 4. Neumann, B., Walter, T., He´riche´, J.K., Bulkescher, J., Erfle, H., Conrad, C., Rogers, P., Poser, I., Held, M., Liebel, U., et al. (2010). Phenotypic profiling of the human genome by time-lapse microscopy reveals cell division genes. Nature 464, 721–727. 5. Grishchuk, E.L., Molodtsov, M.I., Ataullakhanov, F.I., and McIntosh, J.R. (2005).
Recent findings report the selective internalization of core planar cell polarity components during mitosis followed by cell-non-autonomous polarized recycling. This novel mechanistic model explains how tissue polarity is inherited in daughter cells of proliferative tissue.
Planar cell polarity (PCP) is an evolutionarily conserved mechanism
7.
8.
9.
10.
11.
12.
13.
14.
15.
References
Tissue Polarity: PCP Inheritance Ensured by Selective Mitotic Endocytosis
Nabila Founounou and Roland Le Borgne
6.
enabling epithelial cells to individually polarize perpendicular to their apicobasal axis. Establishment and maintenance of PCP have been extensively studied in the developing
16.
17.
Force production by disassembling microtubules. Nature 438, 384–388. Svoboda, K., and Block, S.M. (1994). Force and velocity measured for single kinesin molecules. Cell 77, 773–784. Shimamoto, Y., Maeda, Y., Ishiwata, S., Libchaber, A., and Kapoor, T.M. (2011). Insights into the micromechanical properties of the metaphase spindle. Cell 145, 1062–1074. Fletcher, D.A., and Mullins, R.D. (2010). Cell mechanics and the cytoskeleton. Nature 463, 485–492. Howard, J. (2001). Mechanics of Motor Proteins and the Cytoskeleton (Sunderland: Sinauer Associates Incorporated). Nicklas, R.B. (1983). Measurements of the force produced by the mitotic spindle in anaphase. J. Cell Biol. 97, 532–548. Sawin, K.E., and Mitchison, T.J. (1991). Poleward microtubule flux in mitotic spindles assembled in vitro. J. Cell Biol. 112, 941–954. Mitchison, T., and Kirschner, M. (1984). Dynamic instability of microtubule growth. Nature 312, 237–242. Zhai, Y., Kronebusch, P.J., and Borisy, G.G. (1995). Kinetochore microtubule dynamics and the metaphase-anaphase transition. J. Cell Biol. 131, 731–734. Dumont, S., and Mitchison, T.J. (2009). Compression regulates spindle length by a mechanochemical switch at the poles. Curr. Biol. 19, 1086–1095. Itabashi, T., Takagi, J., Shimamoto, Y., Onoe, H., Kuwana, K., Shimoyama, I., Gaetz, J., Kapoor, T.M., and Ishiwata, S. (2009). Probing the mechanical architecture of the vertebrate meiotic spindle. Nat. Methods 6, 167–172. Heald, R., Tournebize, R., Blank, T., Sandaltzopoulos, R., Becker, P., Hyman, A., and Karsenti, E. (1996). Self-organization of microtubules into bipolar spindles around artificial chromosomes in Xenopus egg extracts. Nature 382, 420–425. Gatlin, J.C., Matov, A., Danuser, G., Mitchison, T.J., and Salmon, E.D. (2010). Directly probing the mechanical properties of the spindle and its matrix. J. Cell Biol. 188, 481–489.
Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA. E-mail: [email protected]
DOI: 10.1016/j.cub.2011.08.008
Drosophila epidermis [1–5]. Highly regenerative tissues, such as mammalian skin, also exhibit the features of PCP [6,7]. A challenging task for such proliferative tissue is to maintain and accurately propagate PCP information while cells keep dividing at high frequency. A recent study [8], published in Nature Cell Biology by the team of Elaine Fuchs, addresses this issue in mouse basal epithelial cells — progenitors that generate hair follicles and outer stratified skin layers. In this study, which combines cutting-edge mouse genetics and state-of-the-art cell biology approaches, Devenport and