Structural memory in the contractile ring makes the duration of cytokinesis independent of cell size

Mechanical and biochemical feedback combine to generate complex contractile oscillations in cytokinesis

The actomyosin cortex is an active material that generates force to drive shape changes via cytoskeletal remodeling. Cytokinesis is the essential cell division event during which a cortical actomyosin ring closes to separate two daughter cells. Our active gel theory predicted that actomyosin systems controlled by a biochemical oscillator and experiencing mechanical strain would exhibit complex spatiotemporal behavior. To test whether active materials in vivo exhibit spatiotemporally complex kinetics, we imaged the C. elegans embryo with unprecedented temporal resolution and discovered that sections of the cytokinetic cortex undergo periodic phases of acceleration and deceleration. Contractile oscillations exhibited a range of periodicities, including those much longer periods than the timescale of RhoA pulses, which was shorter in cytokinesis than in any other biological context. Modifying mechanical feedback in vivo or in silico revealed that the period of contractile oscillation is prolonged as a function of the intensity of mechanical feedback. Fast local ring ingression occurs where speed oscillations have long periods, likely due to increased local stresses and, therefore, mechanical feedback. Fast ingression also occurs where material turnover is high, in vivo and in silico. We propose that downstream of initiation by pulsed RhoA activity, mechanical feedback, including but not limited to material advection, extends the timescale of contractility beyond that of biochemical input and, therefore, makes it robust to fluctuations in activation. Circumferential propagation of contractility likely allows for sustained contractility despite cytoskeletal remodeling necessary to recover from compaction. Thus, like biochemical feedback, mechanical feedback affords active materials responsiveness and robustness.

Coherent light scattering from cellular dynamics in living tissues

This review examines the biological physics of intracellular transport probed by the coherent optics of dynamic light scattering from optically thick living tissues. Cells and their constituents are in constant motion, composed of a broad range of speeds spanning many orders of magnitude that reflect the wide array of functions and mechanisms that maintain cellular health. From the organelle scale of tens of nanometers and upward in size, the motion inside living tissue is actively driven rather than thermal, propelled by the hydrolysis of bioenergetic molecules and the forces of molecular motors. Active transport can mimic the random walks of thermal Brownian motion, but mean-squared displacements are far from thermal equilibrium and can display anomalous diffusion through Lévy or fractional Brownian walks. Despite the average isotropic three-dimensional environment of cells and tissues, active cellular or intracellular transport of single light-scattering objects is often pseudo-one-dimensional, for instance as organelle displacement persists along cytoskeletal tracks or as membranes displace along the normal to cell surfaces, albeit isotropically oriented in three dimensions. Coherent light scattering is a natural tool to characterize such tissue dynamics because persistent directed transport induces Doppler shifts in the scattered light. The many frequency-shifted partial waves from the complex and dynamic media interfere to produce dynamic speckle that reveals tissue-scale processes through speckle contrast imaging and fluctuation spectroscopy. Low-coherence interferometry, dynamic optical coherence tomography, diffusing-wave spectroscopy, diffuse-correlation spectroscopy, differential dynamic microscopy and digital holography offer coherent detection methods that shed light on intracellular processes. In health-care applications, altered states of cellular health and disease display altered cellular motions that imprint on the statistical fluctuations of the scattered light. For instance, the efficacy of medical therapeutics can be monitored by measuring the changes they induce in the Doppler spectra of livingex vivocancer biopsies.

Mechanical positive feedback and biochemical negative feedback combine to generate complex contractile oscillations in cytokinesis

Contractile force generation by the cortical actomyosin cytoskeleton is essential for a multitude of biological processes. The actomyosin cortex behaves as an active material that drives local and large-scale shape changes via cytoskeletal remodeling in response to biochemical cues and feedback loops. Cytokinesis is the essential cell division event during which a cortical actomyosin ring generates contractile force to change cell shape and separate two daughter cells. Our recent work with active gel theory predicts that actomyosin systems under the control of a biochemical oscillator and experiencing mechanical strain will exhibit complex spatiotemporal behavior, but cytokinetic contractility was thought to be kinetically simple. To test whether active materials in vivo exhibit spatiotemporally complex kinetics, we used 4-dimensional imaging with unprecedented temporal resolution and discovered sections of the cytokinetic cortex undergo periodic phases of acceleration and deceleration. Quantification of ingression speed oscillations revealed wide ranges of oscillation period and amplitude. In the cytokinetic ring, activity of the master regulator RhoA pulsed with a timescale of approximately 20 seconds, shorter than that reported for any other biological context. Contractility oscillated with 20-second periodicity and with much longer periods. A combination of in vivo and in silico approaches to modify mechanical feedback revealed that the period of contractile oscillation is prolonged as a function of the intensity of mechanical feedback. Effective local ring ingression is characterized by slower speed oscillations, likely due to increased local stresses and therefore mechanical feedback. Fast ingression also occurs where material turnover is high, in vivo and in silico . We propose that downstream of initiation by pulsed RhoA activity, mechanical positive feedback, including but not limited to material advection, extends the timescale of contractility beyond that of biochemical input and therefore makes it robust to fluctuations in activation. Circumferential propagation of contractility likely allows sustained contractility despite cytoskeletal remodeling necessary to recover from compaction. Our work demonstrates that while biochemical feedback loops afford systems responsiveness and robustness, mechanical feedback must also be considered to describe and understand the behaviors of active materials in vivo .

Cell shape instability during cytokinesis in tetraploid HCT116 cells

Tetraploidy is a hallmark of broad cancer types, but it remains largely unknown which aspects of cellular processes are influenced by tetraploidization in human cells. Here, we found that tetraploid HCT116 cells manifested severe cell shape instability during cytokinesis, unlike their diploid counterparts. The cell shape instability accompanied the formation of protrusive deformation at the cell poles, indicating ectopic contractile activity of the cell cortex. While cytokinesis regulators such as RhoA and anillin correctly accumulated at the equatorial cortex, myosin II was over-accumulated at the cell poles, specifically in tetraploid cells. Suppression of myosin II activity by Y27632 treatment restored smooth cell shape in tetraploids during cytokinesis, indicating dysregulation of myosin II as a primary cause of the cell shape instability in the tetraploid state. Our results demonstrate a new aspect of the dynamic cellular process profoundly affected by tetraploidization in human cells, which provides a clue to molecular mechanisms of tetraploidy-driven pathogenic processes.

Contractile ring composition dictates kinetics of in silico contractility

Constriction kinetics of the cytokinetic ring are expected to depend on dynamic adjustment of contractile ring composition, but the impact of ring component abundance dynamics on ring constriction is understudied. Computational models generally assume that contractile networks maintain constant total amounts of components, which is not always true. To test how compositional dynamics affect constriction kinetics, we first measured F-actin, non-muscle myosin II, septin, and anillin during Caenorhabditis elegans zygotic mitosis. A custom microfluidic device that positioned the cell with the division plane parallel to a light sheet allowed even illumination of the cytokinetic ring. Measured component abundances were implemented in a three-dimensional agent-based model of a membrane-associated contractile ring. With constant network component amounts, constriction completed with biologically unrealistic kinetics. However, imposing the measured changes in component quantities allowed this model to elicit realistic constriction kinetics. Simulated networks were more sensitive to changes in motor and filament amounts than those of crosslinkers and tethers. Our findings highlight the importance of network composition for actomyosin contraction kinetics.

Lineage-based scaling of germline intercellular bridges during oogenesis

The size of subcellular structures must be tightly controlled to maintain normal cell function; this is especially important when cells are part of developing tissues or organs. Despite its importance, few studies have determined how the size of organelles or other structures is maintained during tissue growth, when cells are growing, dividing, and rearranging. The developing egg chamber is a powerful model in which to study the relative growth rates of subcellular structures. The egg chamber contains a cluster of sixteen germ cells, which are connected through intercellular bridges called ring canals. Ring canals are formed following incomplete cytokinesis after each of four germ cell divisions. As the egg chamber grows, the nurse cells and the ring canals that connect them increase in size. Here, we demonstrate that ring canal size scaling is related to their lineage; the largest, "first born" ring canals grow at a relatively slower rate than ring canals derived from subsequent mitotic divisions. This lineage-based scaling relationship is maintained even if directed transport is reduced, ring canal size is altered, or if the germ cells go through an additional mitotic division. Further, we propose that changes in ring canal scaling could provide a mechanism to alter egg size.

Stability of asymmetric cell division: A deformable cell model of cytokinesis applied to C. elegans

Cell division during early embryogenesis is linked to key morphogenic events such as embryo symmetry breaking and tissue patterning. It is thought that the physical surrounding of cells together with cell intrinsic cues act as a mechanical "mold," guiding cell division to ensure these events are robust. To quantify how cell division is affected by the mechanical and geometrical environment, we present a novel computational mechanical model of cytokinesis, the final phase of cell division. Simulations with the model reproduced experimentally observed furrow dynamics and describe the volume ratio of daughter cells in asymmetric cell divisions, based on the position and orientation of the mitotic spindle. For dividing cells in geometrically confined environments, we show how the orientation of confinement relative to the division axis modulates the volume ratio in asymmetric cell division. Further, we quantified how cortex viscosity and surface tension determine the shape of a dividing cell and govern bubble-instabilities in asymmetric cell division. Finally, we simulated the formation of the three body axes via sequential (a)symmetric divisions up until the six-cell stage of early C. elegans development, which proceeds within the confines of an eggshell. We demonstrate how model input parameters spindle position and orientation provide sufficient information to reliably predict the volume ratio of daughter cells during the cleavage phase of development. However, for egg geometries perturbed by compression, the model predicts that a change in confinement alone is insufficient to explain experimentally observed differences in cell volume. This points to an effect of the compression on the spindle positioning mechanism. Additionally, the model predicts that confinement stabilizes asymmetric cell divisions against bubble-instabilities.

Diversity is the spice of life: An overview of how cytokinesis regulation varies with cell type

Cytokinesis is required to physically cleave a cell into two daughters at the end of mitosis. Decades of research have led to a comprehensive understanding of the core cytokinesis machinery and how it is regulated in animal cells, however this knowledge was generated using single cells cultured in vitro, or in early embryos before tissues develop. This raises the question of how cytokinesis is regulated in diverse animal cell types and developmental contexts. Recent studies of distinct cell types in the same organism or in similar cell types from different organisms have revealed striking differences in how cytokinesis is regulated, which includes different threshold requirements for the structural components and the mechanisms that regulate them. In this review, we highlight these differences with an emphasis on pathways that are independent of the mitotic spindle, and operate through signals associated with the cortex, kinetochores, or chromatin.

Cytokinetic diversity in mammalian cells is revealed by the characterization of endogenous anillin, Ect2 and RhoA

Cytokinesis is required to physically separate the daughter cells at the end of mitosis. This crucial process requires the assembly and ingression of an actomyosin ring, which must occur with high fidelity to avoid aneuploidy and cell fate changes. Most of our knowledge of mammalian cytokinesis was generated using over-expressed transgenes in HeLa cells. Over-expression can introduce artefacts, while HeLa are cancerous human cells that have lost their epithelial identity, and the mechanisms controlling cytokinesis in these cells could be vastly different from other cell types. Here, we tagged endogenous anillin, Ect2 and RhoA with mNeonGreen and characterized their localization during cytokinesis for the first time in live human cells. Comparing anillin localization in multiple cell types revealed cytokinetic diversity with differences in the duration and symmetry of ring closure, and the timing of cortical recruitment. Our findings show that the breadth of anillin correlates with the rate of ring closure, and support models where cell size or ploidy affects the cortical organization, and intrinsic mechanisms control the symmetry of ring closure. This work highlights the need to study cytokinesis in more diverse cell types, which will be facilitated by the reagents generated for this study.

Powering morphogenesis: multiscale challenges at the interface of cell adhesion and the cytoskeleton

Among the defining features of the animal kingdom is the ability of cells to change shape and move. This underlies embryonic and postembryonic development, tissue homeostasis, regeneration, and wound healing. Cell shape change and motility require linkage of the cell's force-generating machinery to the plasma membrane at cell-cell and cell-extracellular matrix junctions. Connections of the actomyosin cytoskeleton to cell-cell adherens junctions need to be both resilient and dynamic, preventing tissue disruption during the dramatic events of embryonic morphogenesis. In the past decade, new insights radically altered the earlier simple paradigm that suggested simple linear linkage via the cadherin-catenin complex as the molecular mechanism of junction-cytoskeleton interaction. In this Perspective we provide a brief overview of our current state of knowledge and then focus on selected examples highlighting what we view as the major unanswered questions in our field and the approaches that offer exciting new insights at multiple scales from atomic structure to tissue mechanics.

Cell size and polarization determine cytokinesis furrow ingression dynamics in mouse embryos

The final step of cell division, termed cytokinesis, comprises the constriction of a furrow that divides the cytoplasm to form two daughter cells. Although cytokinesis is well studied in traditional cell systems, how cytokinesis is regulated in complex multicellular settings and during cell-fate decisions is less well understood. Here, using live imaging and physical and molecular interventions, we find that the emergence of cell polarity during mouse embryo morphogenesis dramatically impacts cytokinesis mechanisms. Specifically, the assembly of the apical domain in outer cells locally inhibits the cytokinetic machinery, leading to an unexpected laterally biased cytokinesis.

Cytokinesis is the final step of cell division during which a contractile ring forms a furrow that partitions the cytoplasm in two. How furrow ingression is spatiotemporally regulated and how it is adapted to complex cellular environments and developmental transitions remain poorly understood. Here, we examine furrow ingression dynamics in the context of the early mouse embryo and find that cell size is a powerful determinant of furrow ingression speed during reductive cell divisions. In addition, the emergence of cell polarity and the assembly of the apical domain in outer cells locally inhibits the recruitment of cytokinesis components and thereby negatively regulates furrow ingression specifically on one side of the furrow. We show that this biasing of cytokinesis is not dependent upon cell–cell adhesion or shape but rather is cell intrinsic and is caused by a paucity of cytokinetic machinery in the apical domain. The results thus reveal that in the mouse embryo cell polarity directly regulates the recruitment of cytokinetic machinery in a cell-autonomous manner and that subcellular organization can instigate differential force generation and constriction speed in different zones of the cytokinetic furrow.

Dynamics and Stability of the Contractile Actomyosin Ring in the Cell

Contraction of the cytokinetic ring during cell division leads to physical partitioning of a eukaryotic cell into two daughter cells. This involves flows of actin filaments and myosin motors in the growing membrane interface at the midplane of the dividing cell. Assuming boundary driven alignment of the actomyosin filaments at the inner edge of the interface, we explore how the resulting active stresses influence the flow. Using the continuum gel theory framework, we obtain exact axisymmetric solutions of the dynamical equations. These solutions are consistent with experimental observations on closure rate. Using these solutions, we perform linear stability analysis for the contracting ring under nonaxisymmetric deformations. Our analysis shows that few low wave number modes, which are unstable during onset of the constriction, later on become stable when the ring shrinks to smaller radii, which is a generic feature of actomyosin ring closure. Our theory also captures how the effective tension in the ring decreases with its radius, causing significant slowdown in the contraction process at later times.

Spatial and Temporal Scaling of Microtubules and Mitotic Spindles

During cell division, the mitotic spindle, a macromolecular structure primarily comprised of microtubules, drives chromosome alignment and partitioning between daughter cells. Mitotic spindles can sense cellular dimensions in order to adapt their length and mass to cell size. This scaling capacity is particularly remarkable during early embryo cleavage when cells divide rapidly in the absence of cell growth, thus leading to a reduction of cell volume at each division. Although mitotic spindle size scaling can occur over an order of magnitude in early embryos, in many species the duration of mitosis is relatively short, constant throughout early development and independent of cell size. Therefore, a key challenge for cells during embryo cleavage is not only to assemble a spindle of proper size, but also to do it in an appropriate time window which is compatible with embryo development. How spatial and temporal scaling of the mitotic spindle is achieved and coordinated with the duration of mitosis remains elusive. In this review, we will focus on the mechanisms that support mitotic spindle spatial and temporal scaling over a wide range of cell sizes and cellular contexts. We will present current models and propose alternative mechanisms allowing cells to spatially and temporally coordinate microtubule and mitotic spindle assembly.

Filament-guided filament assembly provides structural memory of filament alignment during cytokinesis

During cytokinesis, animal cells rapidly remodel the equatorial cortex to build an aligned array of actin filaments called the contractile ring. Local reorientation of filaments by active equatorial compression is thought to underlie the emergence of filament alignment during ring assembly. Here, combining single molecule analysis and modeling in one-cell C. elegans embryos, we show that filaments turnover is far too fast for reorientation of individual filaments by equatorial compression to explain the observed alignment, even if favorably oriented filaments are selectively stabilized. By tracking single formin/CYK-1::GFP particles to monitor local filament assembly, we identify a mechanism that we call filament-guided filament assembly (FGFA), in which existing filaments serve as templates to orient the growth of new filaments. FGFA sharply increases the effective lifetime of filament orientation, providing structural memory that allows cells to build highly aligned filament arrays in response to equatorial compression, despite rapid turnover of individual filaments.

A 'dynamic adder model' for cell size homeostasis in Dictyostelium cells

After a cell divides into two daughter cells, the total cell surface area of the daughter cells should increase to the original size to maintain cell size homeostasis in a single cell cycle. Previously, three models have been proposed to explain the regulation of cell size homeostasis: sizer, timer, and adder models. Here, we precisely measured the total cell surface area of Dictyostelium cells in a whole cell cycle by using the agar-overlay method, which eliminated the influence of surface membrane reservoirs, such as microvilli and membrane wrinkles. The total cell surface area exponentially increased during interphase, slightly decreased at metaphase, and then increased by approximately 20% during cytokinesis. From the analysis of the added surface area, we concluded that the cell size was regulated by the adder or near-adder model in interphase. This adder model is not caused by a simple cell membrane addition, but is more dynamic due to the rapid cell membrane turnover. We propose a 'dynamic adder model' to explain cell size homeostasis in interphase.

The Ringleaders: Understanding the Apicomplexan Basal Complex Through Comparison to Established Contractile Ring Systems

The actomyosin contractile ring is a key feature of eukaryotic cytokinesis, conserved across many eukaryotic kingdoms. Recent research into the cell biology of the divergent eukaryotic clade Apicomplexa has revealed a contractile ring structure required for asexual division in the medically relevant genera Toxoplasma and Plasmodium; however, the structure of the contractile ring, known as the basal complex in these parasites, remains poorly characterized and in the absence of a myosin II homolog, it is unclear how the force required of a cytokinetic contractile ring is generated. Here, we review the literature on the basal complex in Apicomplexans, summarizing what is known about its formation and function, and attempt to provide possible answers to this question and suggest new avenues of study by comparing the Apicomplexan basal complex to well-studied, established cytokinetic contractile rings and their mechanisms in organisms such as S. cerevisiae and D. melanogaster. We also compare the basal complex to structures formed during mitochondrial and plastid division and cytokinetic mechanisms of organisms beyond the Opisthokonts, considering Apicomplexan diversity and divergence.

Polar relaxation by dynein-mediated removal of cortical myosin II

Nearly six decades ago, Lewis Wolpert proposed the relaxation of the polar cell cortex by the radial arrays of astral microtubules as a mechanism for cleavage furrow induction. While this mechanism has remained controversial, recent work has provided evidence for polar relaxation by astral microtubules, although its molecular mechanisms remain elusive. Here, using C. elegans embryos, we show that polar relaxation is achieved through dynein-mediated removal of myosin II from the polar cortexes. Mutants that position centrosomes closer to the polar cortex accelerated furrow induction, whereas suppression of dynein activity delayed furrowing. We show that dynein-mediated removal of myosin II from the polar cortexes triggers a bidirectional cortical flow toward the cell equator, which induces the assembly of the actomyosin contractile ring. These results provide a molecular mechanism for the aster-dependent polar relaxation, which works in parallel with equatorial stimulation to promote robust cytokinesis.

Spindle scaling mechanisms

The mitotic spindle robustly scales with cell size in a plethora of different organisms. During development and throughout evolution, the spindle adjusts to cell size in metazoans and yeast in order to ensure faithful chromosome separation. Spindle adjustment to cell size occurs by the scaling of spindle length, spindle shape and the velocity of spindle assembly and elongation. Different mechanisms, depending on spindle structure and organism, account for these scaling relationships. The limited availability of critical spindle components, protein gradients, sequestration of spindle components, or post-translational modification and differential expression levels have been implicated in the regulation of spindle length and the spindle assembly/elongation velocity in a cell size-dependent manner. In this review, we will discuss the phenomenon and mechanisms of spindle length, spindle shape and spindle elongation velocity scaling with cell size.

Comparative Analysis of the Roles of Non-muscle Myosin-IIs in Cytokinesis in Budding Yeast, Fission Yeast, and Mammalian Cells

The contractile ring, which plays critical roles in cytokinesis in fungal and animal cells, has fascinated biologists for decades. However, the basic question of how the non-muscle myosin-II and actin filaments are assembled into a ring structure to drive cytokinesis remains poorly understood. It is even more mysterious why and how the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe, and humans construct the ring structure with one, two, and three myosin-II isoforms, respectively. Here, we provide a comparative analysis of the roles of the non-muscle myosin-IIs in cytokinesis in these three model systems, with the goal of defining the common and unique features and highlighting the major questions regarding this family of proteins.

Integrating cellular dimensions with cell differentiation during early development

Early embryo development is characterized by alteration of cellular dimensions and fating of blastomeres. An emerging concept is that cell size and shape drive cellular differentiation during early embryogenesis in a variety of model organisms. In this review, we summarize recent advances that elucidate the contribution of the physical dimensions of a cell to major embryonic transitions and cell fate specification in vivo. We also highlight techniques and newly evolving methods for manipulating the sizes and shapes of cells and whole embryos in situ and ex vivo. Finally, we provide an outlook for addressing fundamental questions in the field and more broadly uncovering how changes to cell size control decision making in a variety of biological contexts.

Going with the flow: insights from Caenorhabditis elegans zygote polarization

Cell polarity is the asymmetric distribution of cellular components along a defined axis. Polarity relies on complex signalling networks between conserved patterning proteins, including the PAR (partitioning defective) proteins, which become segregated in response to upstream symmetry breaking cues. Although the mechanisms that drive the asymmetric localization of these proteins are dependent upon cell type and context, in many cases the regulation of actomyosin cytoskeleton dynamics is central to the transport, recruitment and/or stabilization of these polarity effectors into defined subcellular domains. The transport or advection of PAR proteins by an actomyosin flow was first observed in the Caenorhabditis elegans zygote more than a decade ago. Since then a multifaceted approach, using molecular methods, high-throughput screens, and biophysical and computational models, has revealed further aspects of this flow and how polarity regulators respond to and modulate it. Here, we review recent findings on the interplay between actomyosin flow and the PAR patterning networks in the polarization of the C. elegans zygote. We also discuss how these discoveries and developed methods are shaping our understanding of other flow-dependent polarizing systems. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

Cytokinesis is the last step of cell division that partitions the cellular organelles and cytoplasm of one cell into two. In animal cells, cytokinesis requires Rho-GTPase-dependent assembly of F-actin and myosin II (actomyosin) to form an equatorial contractile ring (CR) that bisects the cell. Despite 50 years of research, the precise mechanisms of CR assembly, tension generation and closure remain elusive. This hypothesis article considers a holistic view of the CR that, in addition to actomyosin, includes another Rho-dependent cytoskeletal sub-network containing the scaffold protein, Anillin, and septin filaments (collectively termed anillo-septin). We synthesize evidence from our prior work in Drosophila S2 cells that actomyosin and anillo-septin form separable networks that are independently anchored to the plasma membrane. This latter realization leads to a simple conceptual model in which CR assembly and closure depend upon the micro-management of the membrane microdomains to which actomyosin and anillo-septin sub-networks are attached. During CR assembly, actomyosin contractility gathers and compresses its underlying membrane microdomain attachment sites. These microdomains resist this compression, which builds tension. During CR closure, membrane microdomains are transferred from the actomyosin sub-network to the anillo-septin sub-network, with which they flow out of the CR as it advances. This relative outflow of membrane microdomains regulates tension, reduces the circumference of the CR and promotes actomyosin disassembly all at the same time. According to this hypothesis, the metazoan CR can be viewed as a membrane microdomain gathering, compressing and sorting machine that intrinsically buffers its own tension through coordination of actomyosin contractility and anillo-septin-membrane relative outflow, all controlled by Rho. Central to this model is the abandonment of the dogmatic view that the plasma membrane is always readily deformable by the underlying cytoskeleton. Rather, the membrane resists compression to build tension. The notion that the CR might generate tension through resistance to compression of its own membrane microdomain attachment sites, can account for numerous otherwise puzzling observations and warrants further investigation using multiple systems and methods.

Equatorial Non-muscle Myosin II and Plastin Cooperate to Align and Compact F-actin Bundles in the Cytokinetic Ring

Cytokinesis is the last step of cell division that physically partitions the mother cell into two daughter cells. Cytokinesis requires the assembly and constriction of a contractile ring, a circumferential array of filamentous actin (F-actin), non-muscle myosin II motors (myosin), and actin-binding proteins that forms at the cell equator. Cytokinesis is accompanied by long-range cortical flows from regions of relaxation toward regions of compression. In the C. elegans one-cell embryo, it has been suggested that anterior-directed cortical flows are the main driver of contractile ring assembly. Here, we use embryos co-expressing motor-dead and wild-type myosin to show that cortical flows can be severely reduced without major effects on contractile ring assembly and timely completion of cytokinesis. Fluorescence recovery after photobleaching in the ingressing furrow reveals that myosin recruitment kinetics are also unaffected by the absence of cortical flows. We find that myosin cooperates with the F-actin crosslinker plastin to align and compact F-actin bundles at the cell equator, and that this cross-talk is essential for cytokinesis. Our results thus argue against the idea that cortical flows are a major determinant of contractile ring assembly. Instead, we propose that contractile ring assembly requires localized concerted action of motor-competent myosin and plastin at the cell equator.

Stoichiometric interactions explain spindle dynamics and scaling across 100 million years of nematode evolution

The spindle shows remarkable diversity, and changes in an integrated fashion, as cells vary over evolution. Here, we provide a mechanistic explanation for variations in the first mitotic spindle in nematodes. We used a combination of quantitative genetics and biophysics to rule out broad classes of models of the regulation of spindle length and dynamics, and to establish the importance of a balance of cortical pulling forces acting in different directions. These experiments led us to construct a model of cortical pulling forces in which the stoichiometric interactions of microtubules and force generators (each force generator can bind only one microtubule), is key to explaining the dynamics of spindle positioning and elongation, and spindle final length and scaling with cell size. This model accounts for variations in all the spindle traits we studied here, both within species and across nematode species spanning over 100 million years of evolution.

Cleavage-furrow formation without F-actin in Chlamydomonas

It is widely believed that cleavage-furrow formation during cytokinesis is driven by the contraction of a ring containing F-actin and type-II myosin. However, even in cells that have such rings, they are not always essential for furrow formation. Moreover, many taxonomically diverse eukaryotic cells divide by furrowing but have no type-II myosin, making it unlikely that an actomyosin ring drives furrowing. To explore this issue further, we have used one such organism, the green alga Chlamydomonas reinhardtii We found that although F-actin is associated with the furrow region, none of the three myosins (of types VIII and XI) is localized there. Moreover, when F-actin was eliminated through a combination of a mutation and a drug, furrows still formed and the cells divided, although somewhat less efficiently than normal. Unexpectedly, division of the large Chlamydomonas chloroplast was delayed in the cells lacking F-actin; as this organelle lies directly in the path of the cleavage furrow, this delay may explain, at least in part, the delay in cytokinesis itself. Earlier studies had shown an association of microtubules with the cleavage furrow, and we used a fluorescently tagged EB1 protein to show that microtubules are still associated with the furrows in the absence of F-actin, consistent with the possibility that the microtubules are important for furrow formation. We suggest that the actomyosin ring evolved as one way to improve the efficiency of a core process for furrow formation that was already present in ancestral eukaryotes.

Cell size sets the diameter of the budding yeast contractile ring

The formation and maintenance of subcellular structures and organelles with a well-defined size is a key requirement for cell function, yet our understanding of the underlying size control mechanisms is limited. While budding yeast cell polarization and subsequent assembly of a septin ring at the site of bud formation has been successfully used as a model for biological self-assembly processes, the mechanisms that set the size of the septin ring at the bud neck are unknown. Here, we use live-cell imaging and genetic manipulation of cell volume to show that the septin ring diameter increases with cell volume. This cell-volume-dependence largely accounts for modulations of ring size due to changes in ploidy and genetic manipulation of cell polarization. Our findings suggest that the ring diameter is set through the dynamic interplay of septin recruitment and Cdc42 polarization, establishing it as a model for size homeostasis of self-assembling organelles.

Budding yeast cell polarization is known to self-assemble, but it is still not clear what controls the size of the resulting septin ring. Here the authors show that the septin ring diameter is set by cell volume, ensuring that larger cells have larger rings.

ESCRT Machinery Mediates Cytokinetic Abscission in the Unicellular Red Alga Cyanidioschyzon merolae

In many eukaryotes, cytokinesis proceeds in two successive steps: first, ingression of the cleavage furrow and second, abscission of the intercellular bridge. In animal cells, the actomyosin contractile ring is involved in the first step, while the endosomal sorting complex required for transport (ESCRT), which participates in various membrane fusion/fission events, mediates the second step. Intriguingly, in archaea, ESCRT is involved in cytokinesis, raising the hypothesis that the function of ESCRT in eukaryotic cytokinesis descended from the archaeal ancestor. In eukaryotes other than in animals, the roles of ESCRT in cytokinesis are poorly understood. To explore the primordial core mechanisms for eukaryotic cytokinesis, we investigated ESCRT functions in the unicellular red alga Cyanidioschyzon merolae that diverged early in eukaryotic evolution. C. merolae provides an excellent experimental system. The cell has a simple organelle composition. The genome (16.5 Mb, 5335 genes) has been completely sequenced, transformation methods are established, and the cell cycle is synchronized by a light and dark cycle. Similar to animal and fungal cells, C. merolae cells divide by furrowing at the division site followed by abscission of the intercellular bridge. However, they lack an actomyosin contractile ring. The proteins that comprise ESCRT-I-IV, the four subcomplexes of ESCRT, are partially conserved in C. merolae. Immunofluorescence of native or tagged proteins localized the homologs of the five ESCRT-III components [charged multivesicular body protein (CHMP) 1, 2, and 4-6], apoptosis-linked gene-2-interacting protein X (ALIX), the ESCRT-III adapter, and the main ESCRT-IV player vacuolar protein sorting (VPS) 4, to the intercellular bridge. In addition, ALIX was enriched around the cleavage furrow early in cytokinesis. When the ESCRT function was perturbed by expressing dominant-negative VPS4, cells with an elongated intercellular bridge accumulated-a phenotype resulting from abscission failure. Our results show that ESCRT mediates cytokinetic abscission in C. merolae. The fact that ESCRT plays a role in cytokinesis in archaea, animals, and early diverged alga C. merolae supports the hypothesis that the function of ESCRT in cytokinesis descended from archaea to a common ancestor of eukaryotes.

Genetically encoded orientation probes for F-actin for fluorescence polarization microscopy

Fluorescence polarization microscopy, which can visualize both position and orientation of fluorescent molecules, is useful for analyzing architectural dynamics of proteins in vivo, especially that of cytoskeletal proteins such as actin. Fluorescent phalloidin conjugates and SiR-actin can be used as F-actin orientation probes for fluorescence polarization microscopy, but a lack of appropriate methods for their introduction to living specimens especially to tissues, embryos, and whole animals hampers their applications to image the orientation of F-actin. To solve this problem, we have developed genetically encoded F-actin orientation probes for fluorescence polarization microscopy. We rigidly connected circular permutated green fluorescent protein (GFP) to the N-terminal α-helix of actin-binding protein Lifeact or utrophin calponin homology domain (UtrCH), and normal mEGFP to the C-terminal α-helix of UtrCH. After evaluation of ensemble and single particle fluorescence polarization with the instantaneous FluoPolScope, one of the constructs turned out to be suitable for practical usage in live cell imaging. Our new, genetically encoded F-actin orientation probe, which has a similar property of an F-actin probe to conventional GFP-UtrCH, is expected to report the 3D architecture of the actin cytoskeleton with fluorescence polarization microscopy, paving the way for both the single molecular orientation imaging in cultured cells and the sub-optical resolution architectural analysis of F-actin networks analysis of F-actin in various living systems.

Using the Four-Cell C. elegans Embryo to Study Contractile Ring Dynamics During Cytokinesis

Cytokinesis is the process that completes cell division by partitioning the contents of the mother cell between the two daughter cells. It involves the highly regulated assembly and constriction of an actomyosin contractile ring, whose function is to pinch the mother cell in two. Research on the contractile ring has particularly focused on the signaling mechanisms that dictate when and where the ring is formed. In vivo studies of ring constriction are however scarce and its mechanistic understanding is therefore limited. Here we present several experimental approaches for monitoring ring constriction in vivo, using the four-cell C. elegans embryo as model. These approaches allow for the ring to be perturbed only after it forms and include the combination of live imaging with acute drug treatments, temperature-sensitive mutants and rapid temperature shifts, as well as laser microsurgery. In addition, we explain how to combine these with RNAi-mediated depletion of specific components of the cytokinetic machinery.

Crosslinking activity of non-muscle myosin II is not sufficient for embryonic cytokinesis in C. elegans

Cytokinesis in animal cells requires the assembly and constriction of a contractile actomyosin ring. Non-muscle myosin II is essential for cytokinesis, but the role of its motor activity remains unclear. Here, we examine cytokinesis in C. elegans embryos expressing non-muscle myosin motor mutants generated by genome editing. Two non-muscle motor-dead myosins capable of binding F-actin do not support cytokinesis in the one-cell embryo, and two partially motor-impaired myosins delay cytokinesis and render rings more sensitive to reduced myosin levels. Further analysis of myosin mutants suggests that it is myosin motor activity, and not the ability of myosin to crosslink F-actin, that drives the alignment and compaction of F-actin bundles during contractile ring assembly, and that myosin motor activity sets the pace of contractile ring constriction. We conclude that myosin motor activity is required at all stages of cytokinesis. Finally, characterization of the corresponding motor mutations in C. elegans major muscle myosin shows that motor activity is required for muscle contraction but is dispensable for F-actin organization in adult muscles.

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Highlighted Article: The motor activity of non-muscle myosin II is essential for cytokinesis and contributes to all stages of the process in C. elegans embryos.

Anchoring of actin to the plasma membrane enables tension production in the fission yeast cytokinetic ring

The cytokinetic ring generates tensile force that drives cell division, but how tension emerges from the relatively disordered ring organization remains unclear. Long ago, a musclelike sliding filament mechanism was proposed, but evidence for sarcomeric order is lacking. Here we present quantitative evidence that in fission yeast, ring tension originates from barbed-end anchoring of actin filaments to the plasma membrane, providing resistance to myosin forces that enables filaments to develop tension. The role of anchoring was highlighted by experiments on isolated fission yeast rings, where sections of ring became unanchored from the membrane and shortened ∼30-fold faster than normal. The dramatically elevated constriction rates are unexplained. Here we present a molecularly explicit simulation of constricting partially anchored rings as studied in these experiments. Simulations accurately reproduced the experimental constriction rates and showed that following anchor release, a segment becomes tensionless and shortens via a novel noncontractile reeling-in mechanism at about the velocity of load-free myosin II. The ends are reeled in by barbed end-anchored actin filaments in adjacent segments. Other actin anchoring schemes failed to constrict rings. Our results quantitatively support a specific organization and anchoring scheme that generate tension in the cytokinetic ring.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Molecular Mechanism of Cytokinesis

Division of amoebas, fungi, and animal cells into two daughter cells at the end of the cell cycle depends on a common set of ancient proteins, principally actin filaments and myosin-II motors. Anillin, formins, IQGAPs, and many other proteins regulate the assembly of the actin filaments into a contractile ring positioned between the daughter nuclei by different mechanisms in fungi and animal cells. Interactions of myosin-II with actin filaments produce force to assemble and then constrict the contractile ring to form a cleavage furrow. Contractile rings disassemble as they constrict. In some cases, knowledge about the numbers of participating proteins and their biochemical mechanisms has made it possible to formulate molecularly explicit mathematical models that reproduce the observed physical events during cytokinesis by computer simulations.

Molecular form and function of the cytokinetic ring

Animal cells, amoebas and yeast divide using a force-generating, actin- and myosin-based contractile ring or 'cytokinetic ring' (CR). Despite intensive research, questions remain about the spatial organization of CR components, the mechanism by which the CR generates force, and how other cellular processes are coordinated with the CR for successful membrane ingression and ultimate cell separation. This Review highlights new findings about the spatial relationship of the CR to the plasma membrane and the arrangement of molecules within the CR from studies using advanced microscopy techniques, as well as mechanistic information obtained from in vitro approaches. We also consider advances in understanding coordinated cellular processes that impact the architecture and function of the CR.

Rho-dependent control of the Citron kinase, Sticky, drives midbody ring maturation

Rho-dependent proteins control assembly of the cytokinetic contractile ring, yet it remains unclear how those proteins guide ring closure and how they promote subsequent formation of a stable midbody ring. Citron kinase is one important component required for midbody ring formation but its mechanisms of action and relationship with Rho are controversial. Here, we conduct a structure-function analysis of the Drosophila Citron kinase, Sticky, in Schneider's S2 cells. We define two separable and redundant RhoGEF/Pebble-dependent inputs into Sticky recruitment to the nascent midbody ring and show that each input is subsequently required for retention at, and for the integrity of, the mature midbody ring. The first input is via an actomyosin-independent interaction between Sticky and Anillin, a key scaffold also required for midbody ring formation. The second input requires the Rho-binding domain of Sticky, whose boundaries we have defined. Collectively, these results show how midbody ring biogenesis depends on the coordinated actions of Sticky, Anillin, and Rho.

Network Contractility During Cytokinesis-from Molecular to Global Views

Cytokinesis is the last stage of cell division, which partitions the mother cell into two daughter cells. It requires the assembly and constriction of a contractile ring that consists of a filamentous contractile network of actin and myosin. Network contractility depends on network architecture, level of connectivity and myosin motor activity, but how exactly is the contractile ring network organized or interconnected and how much it depends on motor activity remains unclear. Moreover, the contractile ring is not an isolated entity; rather, it is integrated into the surrounding cortex. Therefore, the mechanical properties of the cell cortex and cortical behaviors are expected to impact contractile ring functioning. Due to the complexity of the process, experimental approaches have been coupled to theoretical modeling in order to advance its global understanding. While earlier coarse-grained descriptions attempted to provide an integrated view of the process, recent models have mostly focused on understanding the behavior of an isolated contractile ring. Here we provide an overview of the organization and dynamics of the actomyosin network during cytokinesis and discuss existing theoretical models in light of cortical behaviors and experimental evidence from several systems. Our view on what is missing in current models and should be tested in the future is provided.

Scaling behaviour in steady-state contracting actomyosin networks

Contractile actomyosin network flows are crucial for many cellular processes including cell division and motility, morphogenesis and transport. How local remodeling of actin architecture tunes stress production and dissipation and regulates large-scale network flows remains poorly understood. Here, we generate contracting actomyosin networks with rapid turnover in vitro, by encapsulating cytoplasmic Xenopus egg extracts into cell-sized 'water-in-oil' droplets. Within minutes, the networks reach a dynamic steady-state with continuous inward flow. The networks exhibit homogeneous, density-independent contraction for a wide range of physiological conditions, implying that the myosin-generated stress driving contraction and the effective network viscosity have similar density dependence. We further find that the contraction rate is roughly proportional to the network turnover rate, but this relation breaks down in the presence of excessive crosslinking or branching. Our findings suggest that cells use diverse biochemical mechanisms to generate robust, yet tunable, actin flows by regulating two parameters: turnover rate and network geometry.

Identification and Characterization of the Anillin Gene in Silkworm

Anillin is an actin binding protein and plays crucial roles during mitotic cell cycle progression in metazoan. However, the sequence and functions of the Anillin gene have not been yet characterized in the silkworm, Bombyx mori. In this study, we cloned the full-length cDNA sequence of the silkworm Anillin (BmAnillin) gene. The deduced amino acid sequence for BmAnillin protein comprises an Anillin homology region (AHR) covering an Anillin homology domain and a pleckstrin homology domain. Phylogenetic analysis and multiple alignments of the Anillin genes from silkworm and other organisms indicated evolutionary conservation in the AHR containing conserved phosphorylation sites. Reverse transcription-PCR experiments confirmed that the BmAnillin gene was highly expressed during larval development of gonads in which cells undergo mitotic cycles and exhibited an unexpected high expression in silk gland with endocycle during larval molting. RNA interference-mediated knockdown of the BmAnillin gene in silkworm BmN4-SID1 cells derived from ovary disrupted chromosome separation and resulted in a loss of the F-actin filament at cleavage furrow during anaphase, suggesting that the BmAnillin gene is essential for cytokinesis in silkworm.

Actomyosin contraction during cellularization is regulated in part by Src64 control of Actin 5C protein levels

Src64 is required for actomyosin contraction during cellularization of the Drosophila embryonic blastoderm. The mechanism of actomyosin ring constriction is poorly understood even though a number of cytoskeletal regulators have been implicated in the assembly, organization, and contraction of these microfilament rings. How these cytoskeletal processes are regulated during development is even less well understood. To investigate the role of Src64 as an upstream regulator of actomyosin contraction, we conducted a proteomics screen to identify proteins whose expression levels are controlled by src64. Global levels of actin are reduced in src64 mutant embryos. Furthermore, we show that reduction of the actin isoform Actin 5C causes defects in actomyosin contraction during cellularization similar to those caused by src64 mutation, indicating that a relatively high level of Actin 5C is required for normal actomyosin contraction and furrow canal structure. However, reduction of Actin 5C levels only slows down actomyosin ring constriction rather than preventing it, suggesting that src64 acts not only to modulate actin levels, but also to regulate the actomyosin cytoskeleton by other means.

Mitotic Cell Division in Caenorhabditis elegans

Mitotic cell divisions increase cell number while faithfully distributing the replicated genome at each division. The Caenorhabditis elegans embryo is a powerful model for eukaryotic cell division. Nearly all of the genes that regulate cell division in C. elegans are conserved across metazoan species, including humans. The C. elegans pathways tend to be streamlined, facilitating dissection of the more redundant human pathways. Here, we summarize the virtues of C. elegans as a model system and review our current understanding of centriole duplication, the acquisition of pericentriolar material by centrioles to form centrosomes, the assembly of kinetochores and the mitotic spindle, chromosome segregation, and cytokinesis.

Mechanisms of contractile ring tension production and constriction

The contractile ring is a remarkable tension-generating cellular machine that constricts and divides cells into two during cytokinesis, the final stage of the cell cycle. Since the ring's discovery, the parallels with muscle have been emphasized. Both are contractile actomyosin machineries, and long ago, a muscle-like sliding filament mechanism was proposed for the ring. This review focuses on the mechanisms that generate ring tension and constrict contractile rings. The emphasis is on fission yeast, whose contractile ring is sufficiently well characterized that realistic mathematical models are feasible, and possible lessons from fission yeast that may apply to animal cells are discussed. Recent discoveries relevant to the organization in fission yeast rings suggest a stochastic steady-state version of the classic sliding filament mechanism for tension. The importance of different modes of anchoring for tension production and for organizational stability of constricting rings is discussed. Possible mechanisms are discussed that set the constriction rate and enable the contractile ring to meet the technical challenge of maintaining structural integrity and tension-generating capacity while continuously disassembling throughout constriction.

Molecular mechanisms of contractile-ring constriction and membrane trafficking in cytokinesis

In this review, we discuss the molecular mechanisms of cytokinesis from plants to humans, with a focus on contribution of membrane trafficking to cytokinesis. Selection of the division site in fungi, metazoans, and plants is reviewed, as well as the assembly and constriction of a contractile ring in fungi and metazoans. We also provide an introduction to exocytosis and endocytosis, and discuss how they contribute to successful cytokinesis in eukaryotic cells. The conservation in the coordination of membrane deposition and cytoskeleton during cytokinesis in fungi, metazoans, and plants is highlighted.

Regulation and Assembly of Actomyosin Contractile Rings in Cytokinesis and Cell Repair

Cytokinesis and single-cell wound repair both involve contractile assemblies of filamentous actin (F-actin) and myosin II organized into characteristic ring-like arrays. The assembly of these actomyosin contractile rings (CRs) is specified spatially and temporally by small Rho GTPases, which trigger local actin polymerization and myosin II contractility via a variety of downstream effectors. We now have a much clearer view of the Rho GTPase signaling cascade that leads to the formation of CRs, but some factors involved in CR positioning, assembly, and function remain poorly understood. Recent studies show that this regulation is multifactorial and goes beyond the long-established Ca²⁺ -dependent processes. There is substantial evidence that the Ca²⁺ -independent changes in cell shape, tension, and plasma membrane composition that characterize cytokinesis and single-cell wound repair also regulate CR formation. Elucidating the regulation and mechanistic properties of CRs is important to our understanding of basic cell biology and holds potential for therapeutic applications in human disease. In this review, we present a primer on the factors influencing and regulating CR positioning, assembly, and contraction as they occur in a variety of cytokinetic and single-cell wound repair models. Anat Rec, 301:2051-2066, 2018. © 2018 Wiley Periodicals, Inc.

Syncytial germline architecture is actively maintained by contraction of an internal actomyosin corset

Syncytial architecture is an evolutionarily-conserved feature of the germline of many species and plays a crucial role in their fertility. However, the mechanism supporting syncytial organization is largely unknown. Here, we identify a corset-like actomyosin structure within the syncytial germline of Caenorhabditis elegans, surrounding the common rachis. Using laser microsurgery, we demonstrate that actomyosin contractility within this structure generates tension both in the plane of the rachis surface and perpendicular to it, opposing membrane tension. Genetic and pharmacological perturbations, as well as mathematical modeling, reveal a balance of forces within the gonad and show how changing the tension within the actomyosin corset impinges on syncytial germline structure, leading, in extreme cases, to sterility. Thus, our work highlights a unique tissue-level cytoskeletal structure, and explains the critical role of actomyosin contractility in the preservation of a functional germline.

Germline cells in many species are fused to form a syncytium but the mechanics behind the maintenance of these structures are poorly defined. Here, the authors propose an inner contractile actomyosin corset provides a supportive framework to maintain germline architecture in C. elegans.

The ARP2/3 complex prevents excessive formin activity during cytokinesis

Cytokinesis completes cell division by constriction of an actomyosin contractile ring that separates the two daughter cells. Here we use the early Caenorhabditis elegans embryo to explore how the actin filament network in the ring and the surrounding cortex is regulated by the single cytokinesis formin CYK-1 and the ARP2/3 complex, which nucleate nonbranched and branched filaments, respectively. We show that CYK-1 and the ARP2/3 complex are the predominant F-actin nucleators responsible for generating distinct cortical F-actin architectures and that depletion of either nucleator affects the kinetics of cytokinesis. CYK-1 is critical for normal F-actin levels in the contractile ring, and acute inhibition of CYK-1 after furrow ingression slows ring constriction rate, suggesting that CYK-1 activity is required throughout ring constriction. Surprisingly, although the ARP2/3 complex does not localize in the contractile ring, depletion of the ARP2 subunit or treatment with ARP2/3 complex inhibitor delays contractile ring formation and constriction. We present evidence that the delays are due to an excess in formin-nucleated cortical F-actin, suggesting that the ARP2/3 complex negatively regulates CYK-1 activity. We conclude that the kinetics of cytokinesis are modulated by interplay between the two major actin filament nucleators.

Into the breach: how cells cope with wounds

Repair of wounds to individual cells is crucial for organisms to survive daily physiological or environmental stresses, as well as pathogen assaults, which disrupt the plasma membrane. Sensing wounds, resealing membranes, closing wounds and remodelling plasma membrane/cortical cytoskeleton are four major steps that are essential to return cells to their pre-wounded states. This process relies on dynamic changes of the membrane/cytoskeleton that are indispensable for carrying out the repairs within tens of minutes. Studies from different cell wound repair models over the last two decades have revealed that the molecular mechanisms of single cell wound repair are very diverse and dependent on wound type, size, and/or species. Interestingly, different repair models have been shown to use similar proteins to achieve the same end result, albeit sometimes by distinctive mechanisms. Recent studies using cutting edge microscopy and molecular techniques are shedding new light on the molecular mechanisms during cellular wound repair. Here, we describe what is currently known about the mechanisms underlying this repair process. In addition, we discuss how the study of cellular wound repair—a powerful and inducible model—can contribute to our understanding of other fundamental biological processes such as cytokinesis, cell migration, cancer metastasis and human diseases.

Microtubule Dynamics Scale with Cell Size to Set Spindle Length and Assembly Timing

Successive cell divisions during embryonic cleavage create increasingly smaller cells, so intracellular structures must adapt accordingly. Mitotic spindle size correlates with cell size, but the mechanisms for this scaling remain unclear. Using live cell imaging, we analyzed spindle scaling during embryo cleavage in the nematode Caenorhabditis elegans and sea urchin Paracentrotus lividus. We reveal a common scaling mechanism, where the growth rate of spindle microtubules scales with cell volume, which explains spindle shortening. Spindle assembly timing is, however, constant throughout successive divisions. Analyses in silico suggest that controlling the microtubule growth rate is sufficient to scale spindle length and maintain a constant assembly timing. We tested our in silico predictions to demonstrate that modulating cell volume or microtubule growth rate in vivo induces a proportional spindle size change. Our results suggest that scalability of the microtubule growth rate when cell size varies adapts spindle length to cell volume.

A positive-feedback-based mechanism for constriction rate acceleration during cytokinesis in Caenorhabditis elegans

To ensure timely cytokinesis, the equatorial actomyosin contractile ring constricts at a relatively constant rate despite its progressively decreasing size. Thus, the per-unit-length constriction rate increases as ring perimeter decreases. To understand this acceleration, we monitored cortical surface and ring component dynamics during the first cytokinesis of the Caenorhabditis elegans embryo. We found that, per unit length, the amount of ring components (myosin, anillin) and the constriction rate increase with parallel exponential kinetics. Quantitative analysis of cortical flow indicated that the cortex within the ring is compressed along the axis perpendicular to the ring, and the per-unit-length rate of cortical compression increases during constriction in proportion to ring myosin. We propose that positive feedback between ring myosin and compression-driven flow of cortex into the ring drives an exponential increase in the per-unit-length amount of ring myosin to maintain a high ring constriction rate and support this proposal with an analytical mathematical model.

Specific detection of fission yeast primary septum reveals septum and cleavage furrow ingression during early anaphase independent of mitosis completion

It is widely accepted in eukaryotes that the cleavage furrow only initiates after mitosis completion. In fission yeast, cytokinesis requires the synthesis of a septum tightly coupled to cleavage furrow ingression. The current cytokinesis model establishes that simultaneous septation and furrow ingression only initiate after spindle breakage and mitosis exit. Thus, this model considers that although Cdk1 is inactivated at early-anaphase, septation onset requires the long elapsed time until mitosis completion and full activation of the Hippo-like SIN pathway. Here, we studied the precise timing of septation onset regarding mitosis by exploiting both the septum-specific detection with the fluorochrome calcofluor and the high-resolution electron microscopy during anaphase and telophase. Contrarily to the existing model, we found that both septum and cleavage furrow start to ingress at early anaphase B, long before spindle breakage, with a slow ingression rate during anaphase B, and greatly increasing after telophase onset. This shows that mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. We found that the timing of septation during early anaphase correlates with the cell size and is regulated by the corresponding levels of SIN Etd1 and Rho1. Cdk1 inactivation was directly required for timely septation in early anaphase. Strikingly the reduced SIN activity present after Cdk1 loss was enough to trigger septation by immediately inducing the medial recruitment of the SIN kinase complex Sid2-Mob1. On the other hand, septation onset did not depend on the SIN asymmetry establishment, which is considered a hallmark for SIN activation. These results recalibrate the timing of key cytokinetic events in fission yeast; and unveil a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression to safeguard the proper chromosome segregation during cell division.

Fission yeast cytokinesis requires the invagination of the equatorial plasma membrane (cleavage furrow ingression) coupled to the synthesis of a special wall structure named septum (septation). Despite Cdk1 kinase is inactivated in early anaphase, it is believed that cleavage furrow ingression and septation onset require anaphase progression and mitosis completion, only initiating after the complete activation of the Hippo-like septation initiation network (SIN) after telophase onset. Here, we studied the precise timing of septation start with respect to mitosis through specific septum-staining and electron microscopy. We found that septum and cleavage furrow ingression initiate in early anaphase, showing first a slow ingression rate during anaphase B, and increasing to a fast ingression rate after telophase onset. Thus, mitosis and cleavage furrow ingression are not concatenated but simultaneous events in fission yeast. The timing of septation correlated with cell size and depended on the level of cytoplasmic activators like SIN Etd1 and Rho1. We further analyzed the mitotic mechanisms that control the septation onset during early anaphase. Cdk1 directly regulated the timing of septation onset during early anaphase, and the low SIN activity present after Cdk1 inactivation was enough to trigger septation. Globally, these results recalibrate the timing of the main cytokinetic events of fission yeast and reveal a size-dependent control mechanism that synchronizes simultaneous nuclei separation with septum and cleavage furrow ingression.

Employing the one-cell C. elegans embryo to study cell division processes

The one-cell Caenorhabditis elegans embryo offers many advantages for mechanistic analysis of cell division processes. Conservation of key genes and pathways involved in cell division makes findings in C. elegans broadly relevant. A key technical advantage of this system is the ability to penetrantly deplete essential gene products by RNA interference (RNAi) and replace them with wild-type or mutant versions expressed at endogenous levels from single copy RNAi-resistant transgene insertions. This ability to precisely perturb essential genes is complemented by the inherently highly reproducible nature of the zygotic division that facilitates development of quantitative imaging assays. Here, we detail approaches to generate targeted single copy transgene insertions that are RNAi-resistant, to engineer variants of individual genes employing transgene insertions as well as at the endogenous locus, and to in situ tag genes with fluorophores/purification tags. We also describe imaging assays and common image analysis tools employed to quantitatively monitor phenotypic effects of specific perturbations on meiotic and mitotic chromosome segregation, centrosome assembly/function, and cortical dynamics/cytokinesis.