Parameters that specify the timing of cytokinesis

Lipid Polarization during Cytokinesis

The plasma membrane of eukaryotic cells is composed of a large number of lipid species that are laterally segregated into functional domains as well as asymmetrically distributed between the outer and inner leaflets. Additionally, the spatial distribution and organization of these lipids dramatically change in response to various cellular states, such as cell division, differentiation, and apoptosis. Division of one cell into two daughter cells is one of the most fundamental requirements for the sustenance of growth in all living organisms. The successful completion of cytokinesis, the final stage of cell division, is critically dependent on the spatial distribution and organization of specific lipids. In this review, we discuss the properties of various lipid species associated with cytokinesis and the mechanisms involved in their polarization, including forward trafficking, endocytic recycling, local synthesis, and cortical flow models. The differences in lipid species requirements and distribution in mitotic vs. male meiotic cells will be discussed. We will concentrate on sphingolipids and phosphatidylinositols because their transbilayer organization and movement may be linked via the cytoskeleton and thus critically regulate various steps of cytokinesis.

Echinoderm eggs as a model for discoveries in cell biology

I happen to have been trained in cell and developmental biology in the early 1970s, which was near the beginning of the explosive growth of the field of cell biology. The American Society for Cell Biology had been founded in 1960 and so the field was in its early days. Cell biology research was dominated by the use of the electron microscope and by protein biochemistry. Molecular biology and the use of genetics were in their infancy. When we track the path of discoveries in cell biology contributed by research using echinoderm eggs, we follow the development of new technologies in genetics, molecular biology, biochemistry and biophysics, bioengineering, and imaging. The changes in approaches and methods have led to many key discoveries in cell biology through the use of sea urchin, sand dollar and sea star eggs. These include the discovery of cyclin, cytoplasmic dynein, rho activation for cytokinesis, new membrane addition as a late event in cytokinesis, multiple kinesins playing multiple roles, how flagella beat, the dynamics of microtubules in the mitotic apparatus, control over centrosomes and cell cycle checkpoints, the process of nuclear envelope breakdown for cell division, the discovery of 1-methyl adenine (hormones) as the trigger for meiotic maturation, Ca⁺⁺ transients controlling cell activation and exocytosis among others. What I hope to provide in this perspective is to highlight some of those wonderful discoveries as my own career evolved to contribute to the field.

Control of Homeostasis and Dendritic Cell Survival by the GTPase RhoA

Tissues accommodate defined numbers of dendritic cells (DCs) in highly specific niches where different intrinsic and environmental stimuli control DC life span and numbers. DC homeostasis in tissues is important, because experimental changes in DC numbers influence immunity and tolerance toward various immune catastrophes and inflammation. However, the precise molecular mechanisms regulating DC life span and homeostasis are unclear. We report that the GTPase RhoA controls homeostatic proliferation, cytokinesis, survival, and turnover of cDCs. Deletion of RhoA strongly decreased the numbers of CD11b(-)CD8(+) and CD11b(+)Esam(hi) DC subsets, whereas CD11b(+)Esam(lo) DCs were not affected in conditional RhoA-deficient mice. Proteome analyses revealed a defective prosurvival pathway via PI3K/protein kinase B (Akt1)/Bcl-2-associated death promoter in the absence of RhoA. Taken together, our findings identify RhoA as a central regulator of DC homeostasis, and its deletion decreases DC numbers below critical thresholds for immune protection and homeostasis, causing aberrant compensatory DC proliferation.

The RhoGAP ARHGAP19 controls cytokinesis and chromosome segregation in T lymphocytes

Small GTP-binding proteins of the Rho family orchestrate the cytoskeleton remodelling events required for cell division. Guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) promote cycling of Rho GTPases between the active GTP-bound and the inactive GDP-bound conformations. We report that ARHGAP19, a previously uncharacterised protein, is predominantly expressed in hematopoietic cells and has an essential role in the division of T lymphocytes. Overexpression of ARHGAP19 in lymphocytes delays cell elongation and cytokinesis. Conversely, silencing of ARHGAP19 or expression of a GAP-deficient mutant induces precocious mitotic cell elongation and cleavage furrow ingression, as well as excessive blebbing. In relation to these phenotypes, we show that ARHGAP19 acts as a GAP for RhoA, and controls recruitment of citron and myosin II to the plasma membrane of mitotic lymphocytes as well as Rock2-mediated phosphorylation of vimentin, which is crucial to maintain the stiffness and shape of lymphocytes. In addition to its effects on cell shape, silencing of ARHGAP19 in lymphocytes also impairs chromosome segregation.

RhoA GTPase controls cytokinesis and programmed necrosis of hematopoietic progenitors

The GTPase RhoA is required for the appropriate division and survival of hematopoietic progenitor cells.

Hematopoietic progenitor cells (HPCs) are central to hematopoiesis as they provide large numbers of lineage-defined blood cells necessary to sustain blood homeostasis. They are one of the most actively cycling somatic cells, and their precise control is critical for hematopoietic homeostasis. The small GTPase RhoA is an intracellular molecular switch that integrates cytokine, chemokine, and adhesion signals to coordinate multiple context-dependent cellular processes. By using a RhoA conditional knockout mouse model, we show that RhoA deficiency causes a multilineage hematopoietic failure that is associated with defective multipotent HPCs. Interestingly, RhoA^−/− hematopoietic stem cells retained long-term engraftment potential but failed to produce multipotent HPCs and lineage-defined blood cells. This multilineage hematopoietic failure was rescued by reconstituting wild-type RhoA into the RhoA^−/− Lin⁻Sca-1⁺c-Kit⁺ compartment. Mechanistically, RhoA regulates actomyosin signaling, cytokinesis, and programmed necrosis of the HPCs, and loss of RhoA results in a cytokinesis failure of HPCs manifested by an accumulation of multinucleated cells caused by failed abscission of the cleavage furrow after telophase. Concomitantly, the HPCs show a drastically increased death associated with increased TNF–RIP-mediated necrosis. These results show that RhoA is a critical and specific regulator of multipotent HPCs during cytokinesis and thus essential for multilineage hematopoiesis.

Analysis of the role of Ser1/Ser2/Thr9 phosphorylation on myosin II assembly and function in live cells

Background

Phosphorylation of non-muscle myosin II regulatory light chain (RLC) at Thr18/Ser19 is well established as a key regulatory event that controls myosin II assembly and activation, both in vitro and in living cells. RLC can also be phosphorylated at Ser1/Ser2/Thr9 by protein kinase C (PKC). Biophysical studies show that phosphorylation at these sites leads to an increase in the Km of myosin light chain kinase (MLCK) for RLC, thereby indirectly inhibiting myosin II activity. Despite unequivocal evidence that PKC phosphorylation at Ser1/Ser2/Thr9 can regulate myosin II function in vitro, there is little evidence that this mechanism regulates myosin II function in live cells.

Results

The purpose of these studies was to investigate the role of Ser1/Ser2/Thr9 phosphorylation in live cells. To do this we utilized phospho-specific antibodies and created GFP-tagged RLC reporters with phosphomimetic aspartic acid substitutions or unphosphorylatable alanine substitutions at the putative inhibitory sites or the previously characterized activation sites. Cell lines stably expressing the RLC-GFP constructs were assayed for myosin recruitment during cell division, the ability to complete cell division, and myosin assembly levels under resting or spreading conditions. Our data shows that manipulation of the activation sites (Thr18/Ser19) significantly alters myosin II function in a number of these assays while manipulation of the putative inhibitory sites (Ser1/Ser2/Thr9) does not.

Conclusions

These studies suggest that inhibitory phosphorylation of RLC is not a substantial regulatory mechanism, although we cannot rule out its role in other cellular processes or perhaps other types of cells or tissues in vivo.

Kinesins to the core: The role of microtubule-based motor proteins in building the mitotic spindle midzone

In mammalian cultured cells the initiation of cytokinesis is regulated - both temporally and spatially - by the overlapping, anti-parallel microtubules of the spindle midzone. This region recruits several key central spindle components: PRC-1, polo-like kinase 1 (Plk-1), the centralspindlin complex, and the chromosome passenger complex (CPC), which together serve to stabilize the microtubule overlap, and also to coordinate the assembly of the cortical actin/myosin cytoskeleton necessary to physically cleave the cell in two. The localization of these crucial elements to the spindle midzone requires members of the kinesin superfamily of microtubule-based motor proteins. Here we focus on reviewing the role played by a variety of kinesins in both building and operating the spindle midzone machinery during cytokinesis.

Cytokinesis: LET-ting the asters signal

Cytokinesis is regulated by both astral microtubules and the midzone microtubules of the mitotic apparatus. A new study in Caenorhabditis elegans has identified the polarity factor LET-99 and its heterotrimeric G-protein regulators as components of the signaling pathway downstream of astral microtubules.

A mechanosensory system controls cell shape changes during mitosis

Essential life processes are heavily controlled by a variety of positive and negative feedback systems. Cytokinesis failure, ultimately leading to aneuploidy, is appreciated as an early step in tumor formation in mammals and is deleterious for all cells. Further, the growing list of cancer predisposition mutations includes a number of genes whose proteins control mitosis and/or cytokinesis. Cytokinesis shape control is also an important part of pattern formation and cell-type specialization during multi-cellular development. Inherently mechanical, we hypothesized that mechanosensing and mechanical feedback are fundamental for cytokinesis shape regulation. Using mechanical perturbation, we identified a mechanosensory control system that monitors shape progression during cytokinesis. In this review, we summarize these findings and their implications for cytokinesis regulation and for understanding the cytoskeletal system architecture that governs shape control.

Mitosis-specific mechanosensing and contractile-protein redistribution control cell shape

Because cell-division failure is deleterious, promoting tumorigenesis in mammals, cells utilize numerous mechanisms to control their cell-cycle progression. Though cell division is considered a well-ordered sequence of biochemical events, cytokinesis, an inherently mechanical process, must also be mechanically controlled to ensure that two equivalent daughter cells are produced with high fidelity. Given that cells respond to their mechanical environment, we hypothesized that cells utilize mechanosensing and mechanical feedback to sense and correct shape asymmetries during cytokinesis. Because the mitotic spindle and myosin II are vital to cell division, we explored their roles in responding to shape perturbations during cell division. We demonstrate that the contractile proteins myosin II and cortexillin I redistribute in response to intrinsic and externally induced shape asymmetries. In early cytokinesis, mechanical load overrides spindle cues and slows cytokinesis progression while contractile proteins accumulate and correct shape asymmetries. In late cytokinesis, mechanical perturbation also directs contractile proteins but without apparently disrupting cytokinesis. Significantly, this response only occurs during anaphase through cytokinesis, does not require microtubules, and is independent of spindle orientation, but is dependent on myosin II. Our data provide evidence for a mechanosensory system that directs contractile proteins to regulate cell shape during mitosis.

The sea urchin kinome: a first look

This paper reports a preliminary in silico analysis of the sea urchin kinome. The predicted protein kinases in the sea urchin genome were identified, annotated and classified, according to both function and kinase domain taxonomy. The results show that the sea urchin kinome, consisting of 353 protein kinases, is closer to the Drosophila kinome (239) than the human kinome (518) with respect to total kinase number. However, the diversity of sea urchin kinases is surprisingly similar to humans, since the urchin kinome is missing only 4 of 186 human subfamilies, while Drosophila lacks 24. Thus, the sea urchin kinome combines the simplicity of a non-duplicated genome with the diversity of function and signaling previously considered to be vertebrate-specific. More than half of the sea urchin kinases are involved with signal transduction, and approximately 88% of the signaling kinases are expressed in the developing embryo. These results support the strength of this nonchordate deuterostome as a pivotal developmental and evolutionary model organism.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

Temporal change in local forces and total force all over the surface of the sea urchin egg during cytokinesis

We determined the tension over the entire surface of the sea urchin eggs during cytokinesis, on the basis of the intracellular pressure and cell shape. This allowed us to determine the temporal changes in both the distribution of local forces and the total force produced in the whole cell cortex. A spike-like peak at anaphase and a broader peak at the onset of furrowing were observed in the time-course of the total force. Treatment of the eggs with cytochalasin D, blebbistatin, ML-9, or ML-7 significantly lowered the total force when they inhibited cytokinesis, suggesting that the tension results mainly from the interaction between intact actin filaments and activated myosin II. Myosin II would function as a motor, not only in the furrow region, but over a wide area of the cell surface, because the sum of the tensions outside the furrow region was larger than that inside the furrow region throughout cytokinesis. The distribution of the local force revealed that a global increase in the cortical force started well before the onset of furrowing, and that the force inside the furrow region continued to increase despite the decrease in the force outside the furrow region after the onset of furrowing. The spatial and temporal patterns of the force over the entire surface support the hypothesis that there are two separate but coordinated actomyosin activation mechanisms, one of which induces global activation of the cortex and the other of which then maintains the contractility only inside the furrow region.

Animal cytokinesis: from parts list to mechanisms

The mechanism underlying cytokinesis, the final step in cell division, remains one of the major unsolved questions in basic cell biology. Thanks to advances in functional genomics and proteomics, we are now able to assemble a "parts list" of proteins involved in cytokinesis. In this review, we discuss how to relate this parts list to biological mechanism. For easier analysis, we split cytokinesis into discrete steps: cleavage plane specification, rearrangement of microtubule structures, contractile ring assembly, ring ingression, and completion. We report on the advances that have been made to understand these steps and how they can be integrated into a global understanding of cytokinesis. We also discuss the extent to which classic questions have been answered and identify major outstanding questions.

Calcium-responsive contractility during fertilization in sea urchin eggs

Fertilization triggers a reorganization of oocyte cytoskeleton, and in sea urchins, there is a dramatic increase in cortical F-actin. However, the role that myosin II plays during fertilization remains largely unexplored. Myosin II is localized to the cortical cytoskeleton both before and after fertilization and to examine myosin II contractility in living cells, Lytechinus pictus eggs were observed by time-lapse microscopy. Upon sperm binding, a cell surface deflection traversed the egg that was followed by and dependent on the calcium wave. The calcium-dependence of surface contractility could be reproduced in unfertilized eggs, where mobilization of intracellular calcium in unfertilized eggs under compression resulted in a marked contractile response. Lastly, inhibition of myosin II delayed absorption of the fertilization cone, suggesting that myosin II not only responds to the same signals that activate eggs but also participates in the remodeling of the cortical actomyosin cytoskeleton during the first zygotic cell cycle.

Movement of membrane domains and requirement of membrane signaling molecules for cytokinesis

Plasma membrane subdomains enriched in sphingolipids, cholesterol, and signaling proteins are critical for organization of actin, membrane trafficking, and cell polarity, but the role of such domains in cytokinesis in animal cells is unknown. Here, we show that eggs form a plasma membrane domain enriched in ganglioside G(M1) and cholesterol where tyrosine phosphorylated proteins occur at late anaphase at the contractile ring. The equatorial membrane domain forms by movement-specific lipids and proteins and is dependent on anaphase onset, myosin light chain phosphorylation, actin, and microtubules. Isolated detergent-resistant membranes contain Src and PLCgamma, which become tyrosine phosphorylated at cytokinesis, and whose activation is required for furrow progression. These studies suggest that membrane domains at the cleavage furrow possess a signaling pathway that contributes to cytokinesis.

Induction of cytokinesis is independent of precisely regulated microtubule dynamics

Astral microtubules (MTs) emanating from the mitotic apparatus (MA) during anaphase are required for stimulation of cytokinesis in eggs. We have used green fluorescent protein-labeled EB1 to observe MT dynamics during mitosis and cytokinesis in normal sea urchin eggs. Analysis of astral MT growth rates during anaphase shows that MTs contact the polar cortex earlier than the equatorial cortex after anaphase onset but that a normal cleavage furrow is not induced until contact with MTs has been achieved throughout the cortex. To assess the role of MT dynamics in initiation of cytokinesis, we used a collection of small molecule drugs to affect dynamics. Hexylene glycol resulted in rapid astral elongation due to decreased MT catastrophe and precocious furrowing. Taxol suppressed MT dynamics but did not inhibit furrow induction when the MA was manipulated toward the cortex. Urethane resulted in short, highly dynamic astral MTs with increased catastrophe that also stimulated furrowing upon being brought into proximity to the cortex. Our findings indicate that astral MT contact with the cortex is necessary for furrow initiation but that the dynamic state of astral MTs does not affect their competency to stimulate furrowing.

Comparative Analysis of Cytokinesis in Budding Yeast, Fission Yeast and Animal Cells

Cytokinesis is a temporally and spatially regulated process through which the cellular constituents of the mother cell are partitioned into two daughter cells, permitting an increase in cell number. When cytokinesis occurs in a polarized cell it can create daughters with distinct fates. In eukaryotes, cytokinesis is carried out by the coordinated action of a cortical actomyosin contractile ring and targeted membrane deposition. Recent use of model organisms with facile genetics and improved light-microscopy methods has led to the identification and functional characterization of many proteins involved in cytokinesis. To date, this analysis indicates that some of the basic components involved in cytokinesis are conserved from yeast to humans, although their organization into functional machinery that drives cytokinesis and the associated regulatory mechanisms bear species-specific features. Here, we briefly review the current status of knowledge of cytokinesis in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and animal cells, in an attempt to highlight both the common and the unique features. Although these organisms diverged from a common ancestor about a billion years ago, there are eukaryotes that are far more divergent. To evaluate the overall evolutionary conservation of cytokinesis, it will be necessary to include representatives of these divergent branches. Nevertheless, the three species discussed here provide substantial mechanistic diversity.

Cytokinesis: progress on all fronts

Cell multiplication requires sequestration of the duplicated and segregated genome into two daughter cells. The mitotic spindle is critical for orchestrating sister chromatid separation and division plane positioning. During anaphase, spindle microtubules become bundled to form the central spindle, which is essential for completion of cytokinesis. Central spindle assembly is mediated by a microtubule-associated protein and a kinesin-RhoGAP complex, both of which are regulated by phosphorylation/dephosphorylation. The central spindle also plays a role in cleavage furrow positioning, which appears to involve activation of RhoA. New results have provided some initial clues as to how furrow positioning is achieved. Particularly notable is the discovery that a protein activated by RhoA, formin, has actin nucleation activity.

Actin-based centripetal flow: phosphatase inhibition by calyculin-A alters flow pattern, actin organization, and actomyosin distribution

Previous studies have suggested that the actin-based centripetal flow process in sea urchin coelomocytes is the result of a two-part mechanism, actin polymerization at the cell edge coupled with actomyosin contraction at the cell center. In the present study, we have extended the testing of this two-part model by attempting to stimulate actomyosin contraction via treatment of coelomocytes with the phosphatase inhibitor Calyculin A (CalyA). The effects of this drug were studied using digitally-enhanced video microscopy of living cells combined with immunofluorescent localization and scanning electron microscopy. Under the influence of CalyA, the coelomocyte actin cytoskeleton undergoes a radical reorganization from a dense network to one displaying an array of tangential arcs and radial rivulets in which actin and the Arp2/3 complex concentrate. In addition, the structure and dynamics of the cell center are transformed due to the accumulation of actin and membrane in this region and the constriction of the central actomyosin ring. Physiological evidence of an increase in actomyosin-based contractility following CalyA treatment was demonstrated in experiments in which cells generated tears in their cell centers in response to the drug. Western blotting and immunofluorescent localization with antibodies against the phosphorylated form of the myosin regulatory light chain (MRLC) suggested that the demonstrated constriction of actomyosin distribution was the result of CalyA-induced phosphorylation of MRLC. Overall, the results suggest that there is significant cross talk between the two underlying mechanisms of actin polymerization and actomyosin contraction, and indicate that changes in actomyosin tension may be translated into alterations in the structural organization of the actin cytoskeleton.

Dispatch. Cell division: the place and time of cytokinesis

Recent studies have provided evidence that, during cytokinesis, activation of the Pbl-Rho1 pathway by a protein complex located at the spindle midzone, and inhibition of this pathway by two mitotic cyclins, may be major contributing factors controlling the place and timing of the cleavage furrow.

The degradation of two mitotic cyclins contributes to the timing of cytokinesis

BACKGROUND

Cytokinesis occurs just as chromosomes complete segregation and reform nuclei. It has been proposed that cyclin/Cdk kinase inhibits cytokinesis until exit from mitosis; however, the timer of cytokinesis has not been experimentally defined. Whereas expression of a stable version of Drosophila cyclin B blocks cytokinesis along with numerous events of mitotic exit, stable cyclin B3 allows cytokinesis even though it blocks late events of mitotic exit. We examined the interface between mitotic cyclin destruction and the timing of cytokinesis.

RESULTS

In embryonic mitosis 14, the cytokinesis furrow appeared 60 s after the metaphase/anaphase transition and closed 90 s later during telophase. In cyclin B or cyclin B3 mutant cells, the cytokinesis furrow appeared at an earlier stage of mitosis. Expression of stable cyclin B3 delayed and prolonged furrow invagination; nonetheless, cytokinesis completed during the extended mitosis. Reduced function of Pebble, a Rho GEF required for cytokinesis, also delayed and slowed furrow invagination, but incomplete furrows were aborted at the time of mitotic exit. In functional and genetic tests, cyclin B and cyclin B3 inhibited Pebble contributions to cytokinesis.

CONCLUSIONS

Temporal coordination of mitotic events involves inhibition of cytokinesis by cyclin B and cyclin B3 and punctual relief of the inhibition by destruction of these cyclins. Both cyclins inhibit Pebble-dependent activation of cytokinesis, whereas cyclin B can inhibit cytokinesis by additional modes. Stable cyclin B3 also blocks the later return to interphase that otherwise appears to impose a deadline for the completion of cytokinesis.

RhoA is required for cortical retraction and rigidity during mitotic cell rounding

Mitotic cell rounding is the process of cell shape change in which a flat interphase cell becomes spherical at the onset of mitosis. Rearrangement of the actin cytoskeleton, de-adhesion, and an increase in cortical rigidity accompany mitotic cell rounding. The molecular mechanisms that contribute to this process have not been defined. We show that RhoA is required for cortical retraction but not de-adhesion during mitotic cell rounding. The mitotic increase in cortical rigidity also requires RhoA, suggesting that increases in cortical rigidity and cortical retraction are linked processes. Rho-kinase is also required for mitotic cortical retraction and rigidity, indicating that the effects of RhoA on cell rounding are mediated through this effector. Consistent with a role for RhoA during mitotic entry, RhoA activity is elevated in rounded, preanaphase mitotic cells. The activity of the RhoA inhibitor p190RhoGAP is decreased due to its serine/threonine phosphorylation at this time. Cumulatively, these results suggest that the mitotic increase in RhoA activity leads to rearrangements of the cortical actin cytoskeleton that promote cortical rigidity, resulting in mitotic cell rounding.

Cytokinetic actomyosin ring formation and septation in fission yeast are dependent on the full recruitment of the polo-like kinase Plo1 to the spindle pole body and a functional spindle assembly checkpoint

In dividing cells, the assembly and contraction of the cytokinetic actomyosin ring (CAR) is precisely coordinated with spindle formation and chromosome segregation. Despite having a cell wall, the fission yeast Schizosaccharomyces pombe forms a CAR reminiscent of the structure responsible for the cleavage of cells with flexible boundaries. We used the myo2-gc fission yeast strain in which the chromosomal copy of the type II myosin gene, myo2(+), is fused to the gene encoding green fluorescent protein (GFP) to investigate the dynamics of Myo2 recruitment to the cytokinetic actomyosin ring in living cells. Analysis of CAR formation in relation to spindle pole body (SPB) and centromere separation enabled us to pinpoint the timing of Myo2 recruitment into a stable CAR structure to the onset of anaphase A. Depolymerisation of actin with latrunculin B did not affect the timing of Myo2 accumulation at the cell equator (although Myo2 no longer formed a ring), whereas depolymerisation of microtubules with either thiabendazole (TBZ) or methyl 2-benzimidazolecarbamate (MBC) resulted in a delay of up to 90 minutes in CAR formation. Microtubule depolymerisation also delayed the localisation of other CAR components such as actin and Mid1/Dmf1. The delay of cytokinesis in response to loss of microtubule integrity was abolished in cells lacking the spindle assembly checkpoint protein Mad2 or containing non-functional Cdc16, a component of the fission yeast septation initiation network (SIN). The delay was also abolished in cells lacking Zfs1, a component of the previously described S. pombe cytokinesis checkpoint. Recruitment of the polo-related kinase, Plo1, a key regulator of CAR formation, to the SPBs was substantially reduced in TBZ in a Mad2-dependent manner. Loading of Cdc7, a component of the SIN and downstream of Plo1 in the cytokinesis pathway, onto the the SPBs was also delayed in TBZ to the same extent as CAR formation. We conclude that CAR formation is subject to regulation by the spindle assembly checkpoint via the loading of Plo1 onto the SPBs and the consequent activation of the SIN.

Plane of cell cleavage and numb distribution during cell division relative to cell differentiation in the developing retina

Progenitor cells in the early developing nervous system can divide symmetrically, giving rise to two daughter cells that divide again, or asymmetrically, giving rise to one cell that differentiates and one that divides again. It has been suggested that the orientation of the cell cleavage plane during mitosis determines the type of division. A marker of early cell differentiation, the RA4 antigen, was used to identify regions of the developing chick retina with and without differentiating cells, and the orientation of the cleavage plane was characterized for mitotic figures in each region. No difference was found in the frequency of any orientation between the regions with or without differentiating cells. Furthermore, in the region of the retina with differentiating cells, the RA4 antigen was present in mitotic figures with every possible orientation. Thus, the orientation of the cleavage plane appears to be unrelated to whether or not a division produces a cell that differentiates. It has also been suggested that the intracellular protein Numb mediates neurogenesis via asymmetric localization during cell division. Numb localization was compared with expression of markers of early cell differentiation, the RA4 antigen and Delta. Differentiating and nondifferentiating cells were found both with and without Numb expression. Cells with a cleavage plane parallel to the retinal surface were polarized, such that Numb and/or the RA4 antigen, when present, were only in the daughter cell farthest from the ventricle. These findings indicate a need to reconsider current hypotheses regarding the key features underlying symmetric and asymmetric divisions in the developing nervous system.

Plane of cell cleavage and numb distribution during cell division relative to cell differentiation in the developing retina

Progenitor cells in the early developing nervous system can divide symmetrically, giving rise to two daughter cells that divide again, or asymmetrically, giving rise to one cell that differentiates and one that divides again. It has been suggested that the orientation of the cell cleavage plane during mitosis determines the type of division. A marker of early cell differentiation, the RA4 antigen, was used to identify regions of the developing chick retina with and without differentiating cells, and the orientation of the cleavage plane was characterized for mitotic figures in each region. No difference was found in the frequency of any orientation between the regions with or without differentiating cells. Furthermore, in the region of the retina with differentiating cells, the RA4 antigen was present in mitotic figures with every possible orientation. Thus, the orientation of the cleavage plane appears to be unrelated to whether or not a division produces a cell that differentiates. It has also been suggested that the intracellular protein Numb mediates neurogenesis via asymmetric localization during cell division. Numb localization was compared with expression of markers of early cell differentiation, the RA4 antigen and Delta. Differentiating and nondifferentiating cells were found both with and without Numb expression. Cells with a cleavage plane parallel to the retinal surface were polarized, such that Numb and/or the RA4 antigen, when present, were only in the daughter cell farthest from the ventricle. These findings indicate a need to reconsider current hypotheses regarding the key features underlying symmetric and asymmetric divisions in the developing nervous system.

Cortical recruitment of nonmuscle myosin II in early syncytial Drosophila embryos: its role in nuclear axial expansion and its regulation by Cdc2 activity

The nuclei of early syncytial Drosophila embryos migrate dramatically toward the poles. The cellular mechanisms driving this process, called axial expansion, are unclear, but myosin II activity is required. By following regulatory myosin light chain (RLC)-green fluorescent protein dynamics in living embryos, we observed cycles of myosin recruitment to the cortex synchronized with mitotic cycles. Cortical myosin is first seen in a patch at the anterocentral part of the embryo at cycle 4. With each succeeding cycle, the patch expands poleward, dispersing at the beginning of each mitosis and reassembling at the end of telophase. Each cycle of actin and myosin recruitment is accompanied by a cortical contraction. The cortical myosin cycle does not require microtubules but correlates inversely with Cdc2/cyclinB (mitosis-promoting factor) activity. A mutant RLC lacking inhibitory phosphorylation sites was fully functional with no effect on the cortical myosin cycle, indicating that Cdc2 must be modulating myosin activity by some other mechanism. An inhibitor of Rho kinase blocks the cortical myosin recruitment cycles and provokes a concomitant failure of axial expansion. These studies suggest a model in which cycles of myosin-mediated contraction and relaxation, tightly linked to Cdc2 and Rho kinase activity, are directly responsible for the axial expansion of the syncytial nuclei.

Cytokinesis in eukaryotes

Cytokinesis is the final event of the cell division cycle, and its completion results in irreversible partition of a mother cell into two daughter cells. Cytokinesis was one of the first cell cycle events observed by simple cell biological techniques; however, molecular characterization of cytokinesis has been slowed by its particular resistance to in vitro biochemical approaches. In recent years, the use of genetic model organisms has greatly advanced our molecular understanding of cytokinesis. While the outcome of cytokinesis is conserved in all dividing organisms, the mechanism of division varies across the major eukaryotic kingdoms. Yeasts and animals, for instance, use a contractile ring that ingresses to the cell middle in order to divide, while plant cells build new cell wall outward to the cortex. As would be expected, there is considerable conservation of molecules involved in cytokinesis between yeast and animal cells, while at first glance, plant cells seem quite different. However, in recent years, it has become clear that some aspects of division are conserved between plant, yeast, and animal cells. In this review we discuss the major recent advances in defining cytokinesis, focusing on deciding where to divide, building the division apparatus, and dividing. In addition, we discuss the complex problem of coordinating the division cycle with the nuclear cycle, which has recently become an area of intense research. In conclusion, we discuss how certain cells have utilized cytokinesis to direct development.

Protein kinase C mediates phosphorylation of the regulatory light chain of myosin-II during mitosis

Phosphorylation of the regulatory light chain (RLC) of myosin-II is cell cycle dependent. Early in mitosis the RLC is phosphorylated predominantly on Ser-1/2, while during cytokinesis the primary site of phosphorylation is Ser-19 (Yamakita et al., 1994). To identify candidate kinases likely to mediate the mitotic phosphorylation on Ser-1/2, we assayed RLC kinase activity in mitotic cell extracts and measured apparent steady-state kinetic constants using purified enzymes. The mitotic RLC kinase is distinct from cdc2 kinase, protein kinase A and protein kinase G, as activators or inhibitors specific for these kinases do not affect the mitotic kinase activity. The activity of the mitotic RLC kinase is enhanced by the addition of Ca2+ and DAG and/or phorbol esters, characteristics of a conventional protein kinase C (PKC). Moreover, the PKC inhibitors, Gö6983 and Gö6976, significantly attenuate the phosphorylation of the RLC in mitotic extracts. Apparent steady-state kinetic studies indicate that several PKC isoforms display high specificity for myosin-II. These results suggest that current models describing Ser-1/2 phosphorylation during mitosis need to be re-evaluated.

Transitions regulating the timing of cytokinesis in embryonic cells

Anaphase, mitotic exit, and cytokinesis proceed in rapid succession, and while mitotic exit is a requirement for cytokinesis in yeast, it may not be a direct requirement for furrow initiation in animal cells. In this report, we physically manipulated the proximity of the mitotic apparatus (MA) to the cell cortex in combination with microinjection of effectors of the spindle checkpoint and CDK1 activity to determine how the initiation of cytokinesis is coupled to the onset of anaphase and mitotic exit. Whereas precocious contact between the MA and the cell surface advanced the onset of cytokinesis into early anaphase A, furrowing could not be advanced prior to the metaphase-anaphase transition. Additionally, while cells arrested in anaphase could be induced to initiate cleavage furrows, cells arrested in metaphase could not. Finally, activation of the mitotic checkpoint in one spindle of a binucleate cell failed to arrest cytokinesis induced by the control spindle but did inhibit the formation of furrows between the arrested MA and the control, nonarrested MA. Our experiments suggest that the competence of the mitotic apparatus to initiate cytokinesis is not dependent on cyclin degradation but does require anaphase-promoting complex (APC) activity and, thus, inactivation of the mitotic checkpoint.

Cytokinesis: regulated by destruction

Important transitions in the cell cycle are regulated by ubiquitin-dependent degradation of specific proteins. Recent data indicate that cytokinesis is also regulated by proteolysis.

Targeted new membrane addition in the cleavage furrow is a late, separate event in cytokinesis

Cytokinesis in animal cells is accomplished in part by an actomyosin contractile ring. Recent work on amphibian, Drosophila, and Caenorhabditis elegans embryos implicates membrane trafficking and delivery as essential for cytokinesis. However, the relative contributions of contractile ring constriction versus membrane insertion to cytokinesis and the temporal relationship between these processes are largely unexplored. Here we monitor secretion of the extracellular matrix protein, hyalin, as a marker for new plasma membrane addition in dividing sea urchin zygotes. We find that new membrane addition occurs specifically in the cleavage furrow late in telophase independent of contractile ring constriction. The directed equatorial deposition of new furrow membrane requires astral microtubules and release of internal stores of Ca(2+), but not the presence of a central spindle. Further, cells arrested in M phase do not secrete hyalin, suggesting that mitotic exit is required for new membrane addition. These results demonstrate that astral overlap in equilaterally dividing cells not only serves to specify positioning and contraction of the contractile ring, but also to direct the delivery of new membrane to the furrow as a late, independent event during cytokinesis.

Animal cell cytokinesis

Cytokinesis creates two daughter cells endowed with a complete set of chromosomes and cytoplasmic organelles. This conceptually simple event is mediated by a complex and dynamic interplay between the microtubules of the mitotic spindle, the actomyosin cytoskeleton, and membrane fusion events. For many decades the study of cytokinesis was driven by morphological studies on specimens amenable to physical manipulation. The studies led to great insights into the cellular structures that orchestrate cell division, but the underlying molecular machinery was largely unknown. Molecular and genetic approaches have now allowed the initial steps in the development of a molecular understanding of this fundamental event in the life of a cell. This review provides an overview of the literature on cytokinesis with a particular emphasis on the molecular pathways involved in the division of animal cells.

Effects of anti-myosin drugs on anaphase chromosome movement and cytokinesis in crane-fly primary spermatocytes

To investigate whether myosin is involved in crane-fly primary spermatocyte division, we studied the effects of myosin inhibitors on chromosome movement and on cytokinesis. With respect to chromosome movement, the myosin ATPase inhibitor 2,3-butanedione 2-monoxime (BDM) added during autosomal anaphase reversibly perturbed the movements of all autosomes: autosomes stopped, slowed, or moved backwards during treatment. BDM added before anaphase onset altered chromosome movement less than when BDM was added during anaphase: chromosome movements only rarely were stopped. They often were normal initially and then, if altered at all, were slowed. To confirm that the effects of BDM were due to myosin inhibition, we treated cells with ML-7, a drug that inhibits myosin light chain kinase (MLCK), an enzyme necessary to activate myosin. ML-7 affected anaphase movement only when added in early prometaphase: this treatment prevented chromosome attachment to the spindle. We treated cells with H-7 as a control for possible non-myosin effects of ML-7. H-7, which has a lower affinity than ML-7 for MLCK but a higher affinity than ML-7 for other potential targets, had no effect. These data confirm that the BDM effect is on myosin and indicate that the myosin used for chromosome movement is activated near the start of prometaphase. With respect to cytokinesis, BDM did not block furrow initiation but did block subsequent contraction of the contractile ring. When BDM was added after initiation of the furrow, the contractile ring either stalled or relaxed. ML-7 blocked contractile ring contraction when added at all stages after autosomal anaphase onset, including when added during cytokinesis. H-7 had no effect. These results confirm that the effects of BDM are on myosin and indicate that the myosin used for cytokinesis is activated starting from autosomal anaphase and continuing throughout cytokinesis.

Role of myosin light chain phosphorylation in the regulation of cytokinesis

Phosphorylation of regulatory light chain (RMLC) of myosin II at Ser19/Thr18 is likely to play important roles in controlling the morphological changes seen during cell division of cultured mammalian cells. Phosphorylation of RMLC regulates the activity of myosin II, an essntial motor for cytokinesis, and phosphorylation of RMLC shows dramatic changes during mitosis. Two exzymes, myosin phosphatase and kinase, control phosphorvlation of RMLC. Myosin phosphatase is activated during mitosis, apparently as a result of mitosis-specific phosphorylation of the myosin phosphatase targeting subunit (MYPT). This activation of myosin phosphatase is likely to result in RMLC dephosphorylation, causing the disassemly of stress fibers and focal adhesions during prophase. The phosphorylation of MYPT is lost in cyotokinesis, which would decrease myosin phosphatase activity. At the same time, ROCK (Rho-kinase) probably phosphorylates MYPT at its inhibitory sites, further decreasing the activity of myosin phosphatase. These changes in MYPT phosphorylation would raise RMLC phosphorylation, leading to the activation of myosin II for cyotokinesis. RMLC phosphorylation is also regulated by several RMLC kinases including ROCK (Rho-kinase), MLCK and citron kinase, all of which are localized at cleavage furrows. Future studies should examine whether these multiple kinases are redundant or whether they control distinct aspects of cell division.

Calyculin-A, an inhibitor for protein phosphatases, induces cortical contraction in unfertilized sea urchin eggs

When an unfertilized sea urchin egg was exposed to calyculin-A (CL-A), an inhibitor of protein phosphatases, for a short period and then lysed, the cortex contracted to exclude cytoplasm and became a cup-shaped mass. We call the contracted cortex "actin cup" since actin filaments were major structural components. Electron microscopic observation revealed that the cup consisted of inner electron-dense layer, middle microfilamentous layer, and outermost granular region. Microfilaments were heavily accumulated in the inner electron-dense layer. The middle layer also contained numerous microfilaments, which were determined to be actin filaments by myosin S1 decoration, and they were aligned so that their barbed ends directed toward the outermost region. Myosin II, Arp2, Arp3, and spectrin were concentrated in the actin cup. Immuno-electron microscopy revealed that myosin II was localized to the electron-dense layer. We further found that the cortical tension of the egg increased just after application of CL-A and reached maximum within 10 min. Cytochalasin B or butanedione monoxime blocked the contraction, which suggested that both actin filaments and myosin ATPase activity were required for the contraction. Myosin regulatory light chain (MRLC) in the actin cup was shown to be phosphorylated at the activation sites Ser-19 and Thr-18, by immunoblotting with anti-phosphoepitope antibodies. The phosphorylation of MRLC was also confirmed by a (32)P in vivo labeling experiment. The CL-A-induced cortical contraction may be a good model system for studying the mechanism of cytokinesis.

Simulation of the mechanism of determining the position of the cleavage furrow in cytokinesis of sea urchin eggs

In cytokinesis of sea urchin eggs, the numerical density of astral microtubules extending close to the cell surface has been thought to determine the position of the cleavage furrow. In the present study, a new model was constructed to simulate the relationship between the microtubule density and the furrow formation. In the model, gradients of the microtubule density drive fluid membrane proteins whose accumulation triggers the formation of contractile-ring microfilaments. The model could explain the behavior of the cleavage furrow under various experimental conditions. These simulations revealed two aspects of furrow formation. One is that in some cases, the cleavage furrow appears in a surface region where the microtubule density has neither a minimum nor a maximum. In all furrow regions, however, the second derivative of the microtubule-density function has large positive values. Membrane proteins greatly slow down to accumulate in such a region. The other is that the cleavage furrow is mobile, not fixed in one position, because of the fluidity of membrane proteins. These results strongly suggested that the mitotic apparatus determines the position of the cleavage furrow by redistributing membrane proteins through gradients of the microtubule density at the cell surface.

Effects of the regulatory light chain phosphorylation of myosin II on mitosis and cytokinesis of mammalian cells

Myosin plays an important role in mitosis, especially during cytokinesis. Although it has been assumed that phosphorylation of regulatory light chain of myosin (RLC) controls motility of mammalian non-muscle cells, the functional significance of RLC phosphorylation remains uninvestigated. To address this problem, we have produced unphosphorylatable RLC (T18A/S19A RLC) and overexpressed it in COS-7 cells and normal rat kidney cells. Overexpression of T18A/S19A RLC but not wild type RLC almost completely abolished concanavalin A-induced receptor cap formation. The results indicate that myosin phosphorylation is critical for concanavalin A-induced gathering of surface receptors. T18A/S19A RLC overexpression resulted in the production of multinucleated cells, suggesting the failure of proper cell division in these cells. Video microscopic observation revealed that cells expressing T18A/S19A RLC showed abnormalities during mitosis in two respects. One is that the cells produced abnormal cleavage furrows, resulting in incomplete cytokinesis, which suggests that myosin phosphorylation is important for the normal recruitment of myosin molecules into the contractile ring structure. The other is that separation of chromosomes from the metaphase plate is disrupted in T18A/S19A RLC expressing cells, thus preventing proper transition from metaphase to anaphase. These results suggest that, in addition to cytokinesis, myosin and myosin phosphorylation play a role in the karyokinetic process.

Mitosis: shutting the door behind when you leave

Yeast cells must segregate sister chromosomes to the opposite sides of the bud neck during mitosis. A pathway has been identified, involving a small GTPase, which prevents the onset of cytokinesis until one of the spindle poles has migrated into the bud.

Multiple cyclin-dependent kinase complexes and phosphatases control G2/M progression in alfalfa cells

Reversible phosphorylation of proteins by kinases and phosphatases plays a key regulatory role in several eukaryotic cellular functions including the control of the division cycle. Increasing numbers of sequence and biochemical data show the involvement of cyclin-dependent kinases (CDKs) and cyclins in regulation of the cell cycle progression in higher plants. The complexity represented by different types of CDKs and cyclins in a single species such as alfalfa, indicates that multicomponent regulatory pathways control G2/M transition. A set of cdc2-related genes (cdc2Ms A, B, D and F) was expressed in G2 and M cells. Phosphorylation assays also revealed that at least three kinase complexes (Cdc2Ms A/B, D and F) were successively active in G2/M cells after synchronization. Interaction between alfalfa mitotic cyclin (Medsa;CycB2;1) and a kinase partner has been reported previously. The present yeast two-hybrid analyses showed differential interaction between defined D-type cyclins and Cdc2Ms kinases functioning in G2/M phases. Localization of Cdc2Ms F kinase to the preprophase band (PPB), the perinuclear ring in early prophase, the mitotic spindle and the phragmoplast indicated a pivotal role for this kinase in mitotic plant cells. So far limited research efforts have been devoted to the functions of phosphatases in the control of plant cell division. A homologue of dual phosphatase, cdc25, has not been cloned yet from alfalfa; however tyrosine phosphorylation was indicated in the case of Cdc2Ms A kinase and the p(13suc1)-bound kinase activity was increased by treatment of this complex with recombinant Drosophila Cdc25. The potential role of serine/threonine phosphatases can be concluded from inhibitor studies based on okadaic acid or endothall. Endothall elevated the kinase activity of p(13suc1)-bound fractions in G2-phase alfalfa cells. These biochemical data are in accordance with observed cytological abnormalities. The present overview with selected original data outlines a conclusion that emphasizes the complexity of G2/M regulatory events in flowering plants.

Towards a molecular understanding of cytokinesis

In this review, we focus on recent discoveries regarding the molecular basis of cleavage furrow positioning and contractile ring assembly and contraction during cytokinesis. However, some of these mechanisms might have different degrees of importance in different organisms. This synthesis attempts to uncover common themes and to reveal potential relationships that might contribute to the biochemical and mechanical aspects of cytokinesis. Because the information about cytokinesis is still fairly rudimentary, our goal is not to present a definitive model but to present testable hypotheses that might lead to a better mechanistic understanding of the process.

Assembly of cytoskeletal proteins into cleavage furrows of tissue culture cells

We review results obtained after fluorescent actin and myosin II probes were microinjected into interphase and prophase PtK2 and LLC-PK tissue culture cells to follow the changing distribution of these cytoskeletal proteins in the live cells during division. The fluorescent probes first begin to assemble into the future furrow region during mid-anaphase before any sign of initial contractions. The total concentrations of F-actin and myosin in the cleavage furrow begin to decrease a few minutes after the onset of furrow contraction. The cell's shape and the position of its mitotic spindle affect the deposition of cytoskeletal proteins in the forming cleavage furrow. In cells with two spindles, contractile proteins were recruited not only to the cortex bordering the former metaphase plates but also to the cortex midway between each pair of adjacent non-daughter poles or centrosomes. The furrowing between adjacent poles seen in these cultured cells are similar to the furrows observed by Rappaport [(1961) J Exp Zool 148:81-89] when echinoderm eggs were manipulated into a torus shape so that the poles of two mitotic spindles were adjacent to one another. These observations on injected tissue culture cells suggest that vertebrate cells share common mechanisms for the establishment of the cleavage furrow with echinoderm cells.

Cytokinesis: an emerging unified theory for eukaryotes?

In animal and fungal cells, cytokinesis involves an actomyosin ring that forms and contracts at the division plane. Important new details have emerged concerning the composition, assembly, and dynamics of these contractile rings. In addition, recent advances suggest that targeted membrane addition is a central feature of cytokinesis in animal cells - as it is in fungi and plants - and the coordination of actomyosin ring function with targeted exocytosis at the cleavage plane is being explored. Important new information has also emerged about the spatial and temporal regulation of cytokinesis, especially in relation to the function of the spindle midzone in animal cells and the control of cytokinesis by GTPase systems.