Distinct pathways for the early recruitment of myosin II and actin to the cytokinetic furrow

Cytokinetic contractile ring structural progression in an early embryo: positioning of scaffolding proteins, recruitment of α-actinin, and effects of myosin II inhibition

Our knowledge of the assembly and dynamics of the cytokinetic contractile ring (CR) in animal cells remains incomplete. We have previously used super-resolution light microscopy and platinum replica electron microscopy to elucidate the ultrastructural organization of the CR in first division sea urchin embryos. To date, our studies indicate that the CR initiates as an equatorial band of clusters containing myosin II, actin, septin and anillin, which then congress over time into patches which coalesce into a linear array characteristic of mature CRs. In the present study, we applied super-resolution interferometric photoactivated localization microscopy to confirm the existence of septin filament-like structures in the developing CR, demonstrate the close associations between septin2, anillin, and myosin II in the CR, as well as to show that septin2 appears consistently submembranous, whereas anillin is more widely distributed in the early CR. We also provide evidence that the major actin cross-linking protein α-actinin only associates with the linearized, late-stage CR and not with the early CR clusters, providing further support to the idea that α-actinin associates with actomyosin structures under tension and can serve as a counterbalance. In addition, we show that inhibition of actomyosin contraction does not stop the assembly of the early CR clusters but does arrest the progression of these structures to the aligned arrays required for functional cytokinesis. Taken together our results reinforce and extend our model for a cluster to patch to linear structural progression of the CR in sea urchin embryos and highlight the evolutionary relationships with cytokinesis in fission yeast.

Dynamics of actomyosin filaments in the contractile ring revealed by ultrastructural analysis

Cytokinesis, the final process of cell division, involves the accumulation of actin and myosin II filaments at the cell's equator, forming a contractile ring that facilitates the division into two daughter cells. While light microscopy has provided valuable insights into the molecular mechanism of this process, it has limitations in examining individual filaments in vivo. In this study, we utilized transmission electron microscopy to observe actin and myosin II filaments in the contractile rings of dividing Dictyostelium cells. To synchronize cytokinesis, we developed a novel method that allowed us to visualize dividing cells undergoing cytokinesis with a frequency as high as 18%. This improvement enabled us to examine the lengths and alignments of individual filaments within the contractile rings. As the furrow constricted, the length of actin filaments gradually decreased. Moreover, both actin and myosin II filaments reoriented perpendicularly to the long axis during furrow constriction. Through experiments involving myosin II null cells, we discovered that myosin II plays a role in regulating both the lengths and alignments of actin filaments. Additionally, dynamin-like protein A was found to contribute to regulating the length of actin filaments, while cortexillins were involved in regulating their alignment.

Periodicity, mixed-mode oscillations, and multiple timescales in a phosphoinositide-Rho GTPase network

While rhythmic contractile behavior is commonly observed at the cellular cortex, the primary focus has been on excitable or periodic events described by simple activator-delayed inhibitor mechanisms. We show that Rho GTPase activation in nocodazole-treated mitotic cells exhibits both simple oscillations and complex mixed-mode oscillations. Rho oscillations with a 20- to 30-s period are regulated by phosphatidylinositol (3,4,5)-trisphosphate (PIP3) via an activator-delayed inhibitor mechanism, while a slow reaction with period of minutes is regulated by phosphatidylinositol 4-kinase via an activator-substrate depletion mechanism. Conversion from simple to complex oscillations can be induced by modulating PIP3 metabolism or altering membrane contact site protein E-Syt1. PTEN depletion results in a period-doubling intermediate, which, like mixed-mode oscillations, is an intermediate state toward chaos. In sum, this system operates at the edge of chaos. Small changes in phosphoinositide metabolism can confer cells with the flexibility to rapidly enter ordered states with different periodicities.

The Role of non-muscle actin paralogs in cell cycle progression and proliferation

Uncontrolled cell proliferation leads to several pathologies, including cancer. Thus, this process must be tightly regulated. The cell cycle accounts for cell proliferation, and its progression is coordinated with changes in cell shape, for which cytoskeleton reorganization is responsible. Rearrangement of the cytoskeleton allows for its participation in the precise division of genetic material and cytokinesis. One of the main cytoskeletal components is filamentous actin-based structures. Mammalian cells have at least six actin paralogs, four of which are muscle-specific, while two, named β- and γ-actin, are abundantly present in all types of cells. This review summarizes the findings that establish the role of non-muscle actin paralogs in regulating cell cycle progression and proliferation. We discuss studies showing that the level of a given non-muscle actin paralog in a cell influences the cell's ability to progress through the cell cycle and, thus, proliferation. Moreover, we elaborate on the non-muscle actins' role in regulating gene transcription, interactions of actin paralogs with proteins involved in controlling cell proliferation, and the contribution of non-muscle actins to different structures in a dividing cell. The data cited in this review show that non-muscle actins regulate the cell cycle and proliferation through varying mechanisms. We point to the need for further studies addressing these mechanisms.

Rapid assembly of a polar network architecture drives efficient actomyosin contractility

Actin network architecture and dynamics play a central role in cell contractility and tissue morphogenesis. RhoA-driven pulsed contractions are a generic mode of actomyosin contractility, but the mechanisms underlying how their specific architecture emerges and how this architecture supports the contractile function of the network remain unclear. Here we show that, during pulsed contractions, the actin network is assembled by two subpopulations of formins: a functionally inactive population (recruited) and formins actively participating in actin filament elongation (elongating). We then show that elongating formins assemble a polar actin network, with barbed ends pointing out of the pulse. Numerical simulations demonstrate that this geometry favors rapid network contraction. Our results show that formins convert a local RhoA activity gradient into a polar network architecture, causing efficient network contractility, underlying the key function of kinetic controls in the assembly and mechanics of cortical network architectures.

RhoA-driven actomyosin contractility plays a key role in driving cell and tissue contractility during morphogenesis. Tracking individual formins, Costache et al. show that the network assembled downstream of RhoA displays a polar architecture, barbed ends pointing outward, a feature that supports efficient contractility and force transmission during pulsed contractions.

Building the cytokinetic contractile ring in an early embryo: Initiation as clusters of myosin II, anillin and septin, and visualization of a septin filament network

The cytokinetic contractile ring (CR) was first described some 50 years ago, however our understanding of the assembly and structure of the animal cell CR remains incomplete. We recently reported that mature CRs in sea urchin embryos contain myosin II mini-filaments organized into aligned concatenated arrays, and that in early CRs myosin II formed discrete clusters that transformed into the linearized structure over time. The present study extends our previous work by addressing the hypothesis that these myosin II clusters also contain the crucial scaffolding proteins anillin and septin, known to help link actin, myosin II, RhoA, and the membrane during cytokinesis. Super-resolution imaging of cortices from dividing embryos indicates that within each cluster, anillin and septin2 occupy a centralized position relative to the myosin II mini-filaments. As CR formation progresses, the myosin II, septin and anillin containing clusters enlarge and coalesce into patchy and faintly linear patterns. Our super-resolution images provide the initial visualization of anillin and septin nanostructure within an animal cell CR, including evidence of a septin filament-like network. Furthermore, Latrunculin-treated embryos indicated that the localization of septin or anillin to the myosin II clusters in the early CR was not dependent on actin filaments. These results highlight the structural progression of the CR in sea urchin embryos from an array of clusters to a linearized purse string, the association of anillin and septin with this process, and provide the visualization of an apparent septin filament network with the CR structure of an animal cell.

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.

Ventral stress fibers induce plasma membrane deformation in human fibroblasts

Interactions between the actin cytoskeleton and the plasma membrane are important in many eukaryotic cellular processes. During these processes, actin structures deform the cell membrane outward by applying forces parallel to the fiber's major axis (as in migration) or they deform the membrane inward by applying forces perpendicular to the fiber's major axis (as in the contractile ring during cytokinesis). Here we describe a novel actin-membrane interaction in human dermal myofibroblasts. When labeled with a cytosolic fluorophore, the myofibroblasts displayed prominent fluorescent structures on the ventral side of the cell. These structures are present in the cell membrane and colocalize with ventral actin stress fibers, suggesting that the stress fibers bend the membrane to form a "cytosolic pocket" that the fluorophores diffuse into, creating the observed structures. The existence of this pocket was confirmed by transmission electron microscopy. While dissolving the stress fibers, inhibiting fiber protein binding, or inhibiting myosin II binding of actin removed the observed pockets, modulating cellular contractility did not remove them. Taken together, our results illustrate a novel actin-membrane bending topology where the membrane is deformed outward rather than being pinched inward, resembling the topological inverse of the contractile ring found in cytokinesis.

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.

Connecting cell polarity signals to the cytokinetic machinery in yeast and metazoan cells

Polarized growth and cytokinesis are two fundamental cellular processes that exist in virtually all cell types. Mechanisms for asymmetric distribution of materials allow for cells to grow in a polarized manner. This gives rise to a variety of cell shapes seen throughout all cell types. Following polarized growth during interphase, dividing cells assemble a cytokinetic ring containing the protein machinery to constrict and separate daughter cells. Here, we discuss how cell polarity signaling pathways act on cytokinesis, with a focus on direct regulation of the contractile actomyosin ring (CAR). Recent studies have exploited phosphoproteomics to identify new connections between cell polarity kinases and CAR proteins. Existing evidence suggests that some polarity kinases guide the local organization of CAR proteins and structures while also contributing to global organization of the division plane within a cell. We provide several examples of this regulation from budding yeast, fission yeast, and metazoan cells. In some cases, kinase-substrate connections point to conserved processes in these different organisms. We point to several examples where future work can indicate the degree of conservation and divergence in the cell division process of these different organisms.

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.

Chromokinesin KIF4A teams up with stathmin 1 to regulate abscission in a SUMO-dependent manner

Cell division ends when two daughter cells physically separate via abscission, the cleavage of the intercellular bridge. It is not clear how the anti-parallel microtubule bundles bridging daughter cells are severed. Here, we present a novel abscission mechanism. We identified chromokinesin KIF4A, which is adjacent to the midbody during cytokinesis, as being required for efficient abscission. KIF4A is regulated by post-translational modifications. We evaluated modification of KIF4A by the ubiquitin-like protein SUMO. We mapped lysine 460 in KIF4A as the SUMO acceptor site and employed CRISPR-Cas9-mediated genome editing to block SUMO conjugation of endogenous KIF4A. Failure to SUMOylate this site in KIF4A delayed cytokinesis. SUMOylation of KIF4A enhanced the affinity for the microtubule destabilizer stathmin 1 (STMN1). We here present a new level of abscission regulation through the dynamic interactions between KIF4A and STMN1 as controlled by SUMO modification of KIF4A.

LIS1 determines cleavage plane positioning by regulating actomyosin-mediated cell membrane contractility

Heterozygous loss of human PAFAH1B1 (coding for LIS1) results in the disruption of neurogenesis and neuronal migration via dysregulation of microtubule (MT) stability and dynein motor function/localization that alters mitotic spindle orientation, chromosomal segregation, and nuclear migration. Recently, human- induced pluripotent stem cell (iPSC) models revealed an important role for LIS1 in controlling the length of terminal cell divisions of outer radial glial (oRG) progenitors, suggesting cellular functions of LIS1 in regulating neural progenitor cell (NPC) daughter cell separation. Here, we examined the late mitotic stages NPCs in vivo and mouse embryonic fibroblasts (MEFs) in vitro from Pafah1b1-deficient mutants. Pafah1b1-deficient neocortical NPCs and MEFs similarly exhibited cleavage plane displacement with mislocalization of furrow-associated markers, associated with actomyosin dysfunction and cell membrane hyper-contractility. Thus, it suggests LIS1 acts as a key molecular link connecting MTs/dynein and actomyosin, ensuring that cell membrane contractility is tightly controlled to execute proper daughter cell separation.

A direct interaction between survivin and myosin II is required for cytokinesis

An acto-myosin contractile ring, which forms after anaphase onset and is highly regulated in time and space, mediates cytokinesis, the final step of mitosis. The chromosomal passenger complex (CPC), composed of Aurora-B kinase, INCENP, borealin and survivin (also known as BIRC5), regulates various processes during mitosis, including cytokinesis. It is not understood, however, how CPC regulates cytokinesis. We show that survivin binds to non-muscle myosin II (NMII), regulating its filament assembly. Survivin and NMII interact mainly in telophase, and Cdk1 regulates their interaction in a mitotic-phase-specific manner, revealing the mechanism for the specific timing of survivin-NMII interaction during mitosis. The survivin-NMII interaction is indispensable for cytokinesis, and its disruption leads to multiple mitotic defects. We further show that only the survivin homodimer binds to NMII, attesting to the biological importance for survivin homodimerization. We suggest a novel function for survivin in regulating the spatio-temporal formation of the acto-NMII contractile ring during cytokinesis and we elucidate the role of Cdk1 in regulating this process.This article has an associated First Person interview with the first author of the paper.

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.

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.

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.

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.

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.

Reconsidering an active role for G-actin in cytoskeletal regulation

Globular (G)-actin, the actin monomer, assembles into polarized filaments that form networks that can provide structural support, generate force and organize the cell. Many of these structures are highly dynamic and to maintain them, the cell relies on a large reserve of monomers. Classically, the G-actin pool has been thought of as homogenous. However, recent work has shown that actin monomers can exist in distinct groups that can be targeted to specific networks, where they drive and modify filament assembly in ways that can have profound effects on cellular behavior. This Review focuses on the potential factors that could create functionally distinct pools of actin monomers in the cell, including differences between the actin isoforms and the regulation of G-actin by monomer binding proteins, such as profilin and thymosin β4. Owing to difficulties in studying and visualizing G-actin, our knowledge over the precise role that specific actin monomer pools play in regulating cellular actin dynamics remains incomplete. Here, we discuss some of these unanswered questions and also provide a summary of the methodologies currently available for the imaging of G-actin.

Cytokinesis requires localized β-actin filament production by an actin isoform specific nucleator

Cytokinesis is initiated by the localized assembly of the contractile ring, a dynamic actomyosin structure that generates a membrane furrow between the segregating chromosomal masses to divide a cell into two. Here we show that the stabilization and organization of the cytokinetic furrow is specifically dependent on localized β-actin filament assembly at the site of cytokinesis. β-actin filaments are assembled directly at the furrow by an anillin-dependent pathway that enhances RhoA-dependent activation of the formin DIAPH3, an actin nucleator. DIAPH3 specifically generates homopolymeric filaments of β-actin in vitro. By employing enhancers and activators, cells can achieve acute spatio-temporal control over isoform-specific actin arrays that are required for distinct cellular functions.

Cytokinesis is initiated by the localized assembly of the contractile ring. Here the authors show that the stabilization and organization of the cytokinetic furrow requires localized β-actin filament assembly at the site of cytokinesis by an actin isoform specific nucleator.

Analysis of interphase node proteins in fission yeast by quantitative and superresolution fluorescence microscopy

We used quantitative confocal microscopy and FPALM superresolution microscopy of live fission yeast to investigate the structures and assembly of two types of interphase nodes-multiprotein complexes associated with the plasma membrane that merge together and mature into the precursors of the cytokinetic contractile ring. During the long G2 phase of the cell cycle, seven different interphase node proteins maintain constant concentrations as they accumulate in proportion to cell volume. During mitosis, the total numbers of type 1 node proteins (cell cycle kinases Cdr1p, Cdr2p, Wee1p, and anillin Mid1p) are constant even when the nodes disassemble. Quantitative measurements provide strong evidence that both types of nodes have defined sizes and numbers of constituent proteins, as observed for cytokinesis nodes. Type 1 nodes assemble in two phases-a burst at the end of mitosis, followed by steady increase during interphase to double the initial number. Type 2 nodes containing Blt1p, Rho-GEF Gef2p, and kinesin Klp8p remain intact throughout the cell cycle and are constituents of the contractile ring. They are released from the contractile ring as it disassembles and then associate with type 1 nodes around the equator of the cell during interphase.

Cytokinesis in vertebrate cells initiates by contraction of an equatorial actomyosin network composed of randomly oriented filaments

The actomyosin ring generates force to ingress the cytokinetic cleavage furrow in animal cells, yet its filament organization and the mechanism of contractility is not well understood. We quantified actin filament order in human cells using fluorescence polarization microscopy and found that cleavage furrow ingression initiates by contraction of an equatorial actin network with randomly oriented filaments. The network subsequently gradually reoriented actin filaments along the cell equator. This strictly depended on myosin II activity, suggesting local network reorganization by mechanical forces. Cortical laser microsurgery revealed that during cytokinesis progression, mechanical tension increased substantially along the direction of the cell equator, while the network contracted laterally along the pole-to-pole axis without a detectable increase in tension. Our data suggest that an asymmetric increase in cortical tension promotes filament reorientation along the cytokinetic cleavage furrow, which might have implications for diverse other biological processes involving actomyosin rings.

LET-99 functions in the astral furrowing pathway, where it is required for myosin enrichment in the contractile ring

LET-99 is required for furrowing during cytokinesis in both symmetrically and asymmetrically dividing cells. This function is distinct from the role of LET-99 in spindle positioning with Gα signaling. LET-99 is localized to the furrow, where it acts to promote myosin enrichment.

The anaphase spindle determines the position of the cytokinesis furrow, such that the contractile ring assembles in an equatorial zone between the two spindle poles. Contractile ring formation is mediated by RhoA activation at the equator by the centralspindlin complex and midzone microtubules. Astral microtubules also inhibit RhoA accumulation at the poles. In the Caenorhabditis elegans one-cell embryo, the astral microtubule–dependent pathway requires anillin, NOP-1, and LET-99. LET-99 is well characterized for generating the asymmetric cortical localization of the Gα-dependent force-generating complex that positions the spindle during asymmetric division. However, whether the role of LET-99 in cytokinesis is specific to asymmetric division and whether it acts through Gα to promote furrowing are unclear. Here we show that LET-99 contributes to furrowing in both asymmetrically and symmetrically dividing cells, independent of its function in spindle positioning and Gα regulation. LET-99 acts in a pathway parallel to anillin and is required for myosin enrichment into the contractile ring. These and other results suggest a positive feedback model in which LET-99 localizes to the presumptive cleavage furrow in response to the spindle and myosin. Once positioned there, LET-99 enhances myosin accumulation to promote furrowing in both symmetrically and asymmetrically dividing cells.

Myosin IIA-related Actomyosin Contractility Mediates Oxidative Stress-induced Neuronal Apoptosis

Oxidative stress-induced neuronal apoptosis plays an important role in the progression of central nervous system (CNS) diseases. In our study, when neuronal cells were exposed to hydrogen peroxide (H₂O₂), an exogenous oxidant, cell apoptosis was observed with typical morphological changes including membrane blebbing, neurite retraction and cell contraction. The actomyosin system is considered to be responsible for the morphological changes, but how exactly it regulates oxidative stress-induced neuronal apoptosis and the distinctive functions of different myosin II isoforms remain unclear. We demonstrate that myosin IIA was required for neuronal contraction, while myosin IIB was required for neuronal outgrowth in normal conditions. During H₂O₂-induced neuronal apoptosis, myosin IIA, rather than IIB, interacted with actin filaments to generate contractile forces that lead to morphological changes. Moreover, myosin IIA knockout using clustered regularly interspaced short palindromic repeats/CRISPR-associated protein-9 nuclease (CRISPR/Cas9) reduced H₂O₂-induced neuronal apoptosis and the associated morphological changes. We further demonstrate that caspase-3/Rho-associated kinase 1 (ROCK1) dependent phosphorylation of myosin light chain (MLC) was required for the formation of the myosin IIA-actin complex. Meanwhile, either inhibition of myosin II ATPase with blebbistatin or knockdown of myosin IIA with siRNA reversely attenuated caspase-3 activation, suggesting a positive feedback loop during oxidative stress-induced apoptosis. Based on our observation, myosin IIA-actin complex contributes to actomyosin contractility and is associated with the positive feedback loop of caspase-3/ROCK1/MLC pathway. This study unravels the biochemical and mechanistic mechanisms during oxidative stress-induced neuronal apoptosis and may be applicable for the development of therapies for CNS diseases.

The ultrastructural organization of actin and myosin II filaments in the contractile ring: new support for an old model of cytokinesis

Despite recent advances in our understanding of the components and spatial regulation of the contractile ring (CR), the precise ultrastructure of actin and myosin II within the animal cell CR remains an unanswered question. We used superresolution light microscopy and platinum replica transmission electron microscopy (TEM) to determine the structural organization of actin and myosin II in isolated cortical cytoskeletons prepared from dividing sea urchin embryos. Three-dimensional structured illumination microscopy indicated that within the CR, actin and myosin II filaments were organized into tightly packed linear arrays oriented along the axis of constriction and restricted to a narrow zone within the furrow. In contrast, myosin II filaments in earlier stages of cytokinesis were organized into small, discrete, and regularly spaced clusters. TEM showed that actin within the CR formed a dense and anisotropic array of elongate, antiparallel filaments, whereas myosin II was organized into laterally associated, head-to-head filament chains highly reminiscent of mammalian cell stress fibers. Together these results not only support the canonical "purse-string" model for contractile ring constriction, but also suggest that the CR may be derived from foci of myosin II filaments in a manner similar to what has been demonstrated in fission yeast.

Local pulsatile contractions are an intrinsic property of the myosin 2A motor in the cortical cytoskeleton of adherent cells

The role of nonmuscle myosin 2 (NM2) pulsatile dynamics in generating contractile forces required for developmental morphogenesis has been characterized, but whether these pulsatile contractions are an intrinsic property of all actomyosin networks is not known. Here we used live-cell fluorescence imaging to show that transient, local assembly of NM2A "pulses" occurs in the cortical cytoskeleton of single adherent cells of mesenchymal, epithelial, and sarcoma origin, independent of developmental signaling cues and cell-cell or cell-ECM interactions. We show that pulses in the cortical cytoskeleton require Rho-associated kinase- or myosin light chain kinase (MLCK) activity, increases in cytosolic calcium, and NM2 ATPase activity. Surprisingly, we find that cortical cytoskeleton pulses specifically require the head domain of NM2A, as they do not occur with either NM2B or a 2B-head-2A-tail chimera. Our results thus suggest that pulsatile contractions in the cortical cytoskeleton are an intrinsic property of the NM2A motor that may mediate its role in homeostatic maintenance of tension in the cortical cytoskeleton of adherent cells.

Cortical flow aligns actin filaments to form a furrow

Cytokinesis in eukaryotic cells is often accompanied by actomyosin cortical flow. Over 30 years ago, Borisy and White proposed that cortical flow converging upon the cell equator compresses the actomyosin network to mechanically align actin filaments. However, actin filaments also align via search-and-capture, and to what extent compression by flow or active alignment drive furrow formation remains unclear. Here, we quantify the dynamical organization of actin filaments at the onset of ring assembly in the C. elegans zygote, and provide a framework for determining emergent actomyosin material parameters by the use of active nematic gel theory. We characterize flow-alignment coupling, and verify at a quantitative level that compression by flow drives ring formation. Finally, we find that active alignment enhances but is not required for ring formation. Our work characterizes the physical mechanisms of actomyosin ring formation and highlights the role of flow as a central organizer of actomyosin network architecture.

Molecular organization of cytokinesis nodes and contractile rings by super-resolution fluorescence microscopy of live fission yeast

Cytokinesis in animals, fungi, and amoebas depends on the constriction of a contractile ring built from a common set of conserved proteins. Many fundamental questions remain about how these proteins organize to generate the necessary tension for cytokinesis. Using quantitative high-speed fluorescence photoactivation localization microscopy (FPALM), we probed this question in live fission yeast cells at unprecedented resolution. We show that nodes, protein assembly precursors to the contractile ring, are discrete structural units with stoichiometric ratios and distinct distributions of constituent proteins. Anillin Mid1p, Fes/CIP4 homology-Bin/amphiphysin/Rvs (F-BAR) Cdc15p, IQ motif containing GTPase-activating protein (IQGAP) Rng2p, and formin Cdc12p form the base of the node that anchors the ends of myosin II tails to the plasma membrane, with myosin II heads extending into the cytoplasm. This general node organization persists in the contractile ring where nodes move bidirectionally during constriction. We observed the dynamics of the actin network during cytokinesis, starting with the extension of short actin strands from nodes, which sometimes connected neighboring nodes. Later in cytokinesis, a broad network of thick bundles coalesced into a tight ring around the equator of the cell. The actin ring was ∼125 nm wide and ∼125 nm thick. These observations establish the organization of the proteins in the functional units of a cytokinetic contractile ring.

Flow-dependent myosin recruitment during Drosophila cellularization requires zygotic dunk activity

Actomyosin contractility underlies force generation in morphogenesis ranging from cytokinesis to epithelial extension or invagination. In Drosophila, the cleavage of the syncytial blastoderm is initiated by an actomyosin network at the base of membrane furrows that invaginate from the surface of the embryo. It remains unclear how this network forms and how it affects tissue mechanics. Here, we show that during Drosophila cleavage, myosin recruitment to the cleavage furrows proceeds in temporally distinct phases of tension-driven cortical flow and direct recruitment, regulated by different zygotic genes. We identify the gene dunk, which we show is transiently transcribed when cellularization starts and functions to maintain cortical myosin during the flow phase. The subsequent direct myosin recruitment, however, is Dunk-independent but requires Slam. The Slam-dependent direct recruitment of myosin is sufficient to drive cleavage in the dunk mutant, and the subsequent development of the mutant is normal. In the dunk mutant, cortical myosin loss triggers misdirected flow and disrupts the hexagonal packing of the ingressing furrows. Computer simulation coupled with laser ablation suggests that Dunk-dependent maintenance of cortical myosin enables mechanical tension build-up, thereby providing a mechanism to guide myosin flow and define the hexagonal symmetry of the furrows.

Summary: During Drosophila cellularisation, myosin recruitment to the cleavage furrows proceeds in temporally and mechanistically distinct phases separately regulated by dunk and slam.

Avoiding artefacts when counting polymerized actin in live cells with LifeAct fused to fluorescent proteins

When tagged with a fluorescent protein, actin is not fully functional, so the LifeAct peptide fused to a fluorescent protein is widely used to localize actin filaments in live cells. However, we find that these fusion proteins have many concentration-dependent effects on actin assembly in vitro and in fission yeast cells. mEGFP-LifeAct inhibits actin assembly during endocytosis as well as assembly and constriction of the cytokinetic contractile ring. Purified mEGFP-LifeAct and LifeAct-mCherry bind actin filaments with Kd values of ∼10 μM. LifeAct-mCherry can promote actin filament nucleation and either promote or inhibit filament elongation. Both separately and together, profilin and formins suppress these effects. LifeAct-mCherry can also promote or inhibit actin filament severing by cofilin. These concentration-dependent effects mean that caution is necessary when overexpressing LifeAct fusion proteins to label actin filaments in cells. Therefore, we used low micromolar concentrations of tagged LifeAct to follow assembly and disassembly of actin filaments in cells. Careful titrations also gave an estimate of a peak of ∼190,000 actin molecules (∼500 μm) in the fission yeast contractile ring. These filaments shorten from ∼500 to ∼100 subunits as the ring constricts.

A theoretical model of cytokinesis implicates feedback between membrane curvature and cytoskeletal organization in asymmetric cytokinetic furrowing

Furrow ingression is asymmetric in cytokinesis in the Caenorhabditis elegans zygote. A combination of quantitative high-resolution live-cell microscopy and theoretical modeling revealed a mechanistic basis for asymmetry: feedback among membrane curvature, cytoskeletal alignment, and contractility. The model also suggests that asymmetry promotes energy efficiency.

During cytokinesis, the cell undergoes a dramatic shape change as it divides into two daughter cells. Cell shape changes in cytokinesis are driven by a cortical ring rich in actin filaments and nonmuscle myosin II. The ring closes via actomyosin contraction coupled with actin depolymerization. Of interest, ring closure and hence the furrow ingression are nonconcentric (asymmetric) within the division plane across Metazoa. This nonconcentricity can occur and persist even without preexisting asymmetric cues, such as spindle placement or cellular adhesions. Cell-autonomous asymmetry is not explained by current models. We combined quantitative high-resolution live-cell microscopy with theoretical modeling to explore the mechanistic basis for asymmetric cytokinesis in the Caenorhabditis elegans zygote, with the goal of uncovering basic principles of ring closure. Our theoretical model suggests that feedback among membrane curvature, cytoskeletal alignment, and contractility is responsible for asymmetric cytokinetic furrowing. It also accurately predicts experimental perturbations of conserved ring proteins. The model further suggests that curvature-mediated filament alignment speeds up furrow closure while promoting energy efficiency. Collectively our work underscores the importance of membrane–cytoskeletal anchoring and suggests conserved molecular mechanisms for this activity.

F-actin localization dynamics during appressorium formation in Colletotrichum graminicola

Appressoria are essential penetration structures for many phytopathogenic fungi. Here F-actin localization dynamics were documented during appressorium formation in vitro and in planta in Colletotrichum graminicola Four discernible stages of dynamic F-actin distribution occurring in a programmed order were documented from differentiation of appressoria to formation of penetration pores: (stage A) from germ tube enlargement to complete expansion of the appressorium; (stage S) septation occurs; (stage L) a long period of low F-actin activity; (stage P) the penetration pore forms. The F-actin subcellular localization corresponded to each stage. A distinct redistribution of actin cables occurred at the transition from stage A to stage S. The in planta assays revealed that F-actin also assembled in invasive hyphae and that actin cables might play an essential role for penetration-peg development. The F-actin localization distribution may be used as a subcellular marker to define the developmental stages during appressorium formation.

Cytokinetic abscission: molecular mechanisms and temporal control

Cytokinesis mediates the physical separation of dividing cells after chromosome segregation. In animal cell cytokinesis, a contractile ring, mainly composed of actin and myosin filaments, ingresses a cleavage furrow midway between the two spindle poles. A distinct machinery, involving the endosomal sorting complex required for transport III (ESCRT-III), subsequently splits the plasma membrane of nascent daughter cells in a process termed abscission. Here, we provide a brief overview of early cytokinesis events in animal cells and then cover in depth recently emerging models for the assembly and function of the abscission machinery and its temporal coordination with chromosome segregation.

New single-molecule speckle microscopy reveals modification of the retrograde actin flow by focal adhesions at nanometer scales

This paper introduces a new, easy-to-use method of fluorescence single-molecule speckle microscopy for actin with nanometer-scale accuracy. This new method reveals that actin flows in front of mature focal adhesions (FAs) are fast and biased toward FAs, suggesting that mature FAs are actively engaged in pulling and remodeling the local actin network.

Speckle microscopy directly visualizes the retrograde actin flow, which is believed to promote cell-edge protrusion when linked to focal adhesions (FAs). However, it has been argued that, due to rapid actin turnover, the use of green fluorescent protein–actin, the lack of appropriate analysis algorithms, and technical difficulties, speckle microscopy does not necessarily report the flow velocities of entire actin populations. In this study, we developed a new, user-friendly single-molecule speckle (SiMS) microscopy using DyLight dye-labeled actin. Our new SiMS method enables in vivo nanometer-scale displacement analysis with a low localization error of ±8–8.5 nm, allowing accurate flow-velocity measurement for actin speckles with lifetime <5 s. In lamellipodia, both short- and long-lived F-actin molecules flow with the same speed, indicating they are part of a single actin network. These results do not support coexistence of F-actin populations with different flow speeds, which is referred to as the lamella hypothesis. Mature FAs, but not nascent adhesions, locally obstruct the retrograde flow. Interestingly, the actin flow in front of mature FAs is fast and biased toward FAs, suggesting that mature FAs attract the flow in front and actively remodel the local actin network.

An equatorial contractile mechanism drives cell elongation but not cell division

A cytokinesis-like contractile mechanism is co-opted in a different developmental scenario to achieve cell elongation instead of cell division in Ciona intestinalis.

Cell shape changes and proliferation are two fundamental strategies for morphogenesis in animal development. During embryogenesis of the simple chordate Ciona intestinalis, elongation of individual notochord cells constitutes a crucial stage of notochord growth, which contributes to the establishment of the larval body plan. The mechanism of cell elongation is elusive. Here we show that although notochord cells do not divide, they use a cytokinesis-like actomyosin mechanism to drive cell elongation. The actomyosin network forming at the equator of each notochord cell includes phosphorylated myosin regulatory light chain, α-actinin, cofilin, tropomyosin, and talin. We demonstrate that cofilin and α-actinin are two crucial components for cell elongation. Cortical flow contributes to the assembly of the actomyosin ring. Similar to cytokinetic cells, membrane blebs that cause local contractions form at the basal cortex next to the equator and participate in force generation. We present a model in which the cooperation of equatorial actomyosin ring-based constriction and bleb-associated contractions at the basal cortex promotes cell elongation. Our results demonstrate that a cytokinesis-like contractile mechanism is co-opted in a completely different developmental scenario to achieve cell shape change instead of cell division. We discuss the occurrences of actomyosin rings aside from cell division, suggesting that circumferential contraction is an evolutionally conserved mechanism to drive cell or tissue elongation.

The actomyosin cytoskeleton is the primary force that drives cell shape changes. These fibers are organized in elaborate structures that form sarcomeres in the muscle and the contractile ring during cytokinesis. In cytokinesis, the establishment of an equatorial actomyosin ring is preceded and regulated by many cell cycle events, and the ring itself is a complex and dynamic structure. Here we report the presence of an equatorial circumferential actomyosin structure with remarkable similarities to the cytokinetic ring formed in postmitotic notochord cells of sea squirt Ciona intestinalis. The notochord is a transient rod-like structure found in all embryos that belong to the phylum Chordata, and in Ciona, a simple chordate, it consists of only 40 cylindrical cells arranged in a single file, which elongate individually during development. Our study shows that the activity of the equatorial actomyosin ring is required for the elongation of the notochord cells. We also find that cortical flow contributes significantly to the formation of the ring at the equator. Similar to cytokinetic cells, we observe the formation of membrane blebs outside the equatorial region. Our analyses suggest that cooperation of actomyosin ring-based circumferential constriction and bleb-associated contractions drive cell elongation in Ciona. We conclude that cells can utilize a cytokinesis-like force generation mechanism to promote cell shape change instead of cell division.

The formins Cdc12 and For3 cooperate during contractile ring assembly in cytokinesis

Both de novo-assembled actin filaments at the division site and existing filaments recruited by directional cortical transport contribute to contractile ring formation during cytokinesis. However, it is unknown which source is more important. Here, we show that fission yeast formin For3 is responsible for node condensation into clumps in the absence of formin Cdc12. For3 localization at the division site depended on the F-BAR protein Cdc15, and for3 deletion was synthetic lethal with mutations that cause defects in contractile ring formation. For3 became essential in cells expressing N-terminal truncations of Cdc12, which were more active in actin assembly but depended on actin filaments for localization to the division site. In tetrad fluorescence microscopy, double mutants of for3 deletion and cdc12 truncations were severely defective in contractile ring assembly and constriction, although cortical transport of actin filaments was normal. Together, these data indicate that different formins cooperate in cytokinesis and that de novo actin assembly at the division site is predominant for contractile ring formation.

A conserved RhoGAP limits M phase contractility and coordinates with microtubule asters to confine RhoA during cytokinesis

During animal cell cytokinesis, the spindle directs contractile ring assembly by activating RhoA in a narrow equatorial zone. Rapid GTPase activating protein (GAP)-mediated inactivation (RhoA flux) is proposed to limit RhoA zone dimensions. Testing the significance of RhoA flux has been hampered by the fact that the GAP targeting RhoA is not known. Here, we identify M phase GAP (MP-GAP) as the primary GAP targeting RhoA during mitosis and cytokinesis. MP-GAP inhibition caused excessive RhoA activation in M phase, leading to the uncontrolled formation of large cortical protrusions and late cytokinesis failure. RhoA zone width was broadened by attenuation of the centrosomal asters but was not affected by MP-GAP inhibition alone. Simultaneous aster attenuation and MP-GAP inhibition led to RhoA accumulation around the entire cell periphery. These results identify the major GAP restraining RhoA during cell division and delineate the relative contributions of RhoA flux and centrosomal asters in controlling RhoA zone dimensions.

A nu-space for ICS: characterization and application to measure protein transport in live cells

We introduce a new generalized theoretical framework for image correlation spectroscopy (ICS). Using this framework, we extend the ICS method in time-frequency (ν, nu) space to map molecular flow of fluorescently tagged proteins in individual living cells. Even in the presence of a dominant immobile population of fluorescent molecules, nu-space ICS (nICS) provides an unbiased velocity measurement, as well as the diffusion coefficient of the flow, without requiring filtering. We also develop and characterize a tunable frequency-filter for STICS that allows quantification of the density, the diffusion coefficient and the velocity of biased diffusion. We show that the techniques are accurate over a wide range of parameter space in computer simulation. We then characterize the retrograde flow of adhesion proteins (α6- and αLβ2-GFP integrins and mCherry-paxillin) in CHO.B2 cells plated on laminin and ICAM ligands respectively. STICS with a tunable frequency filter, in conjunction with nICS, measures two new transport parameters, the density and transport bias coefficient (a measure of the diffusive character of a flow/biased diffusion), showing that molecular flow in this cell system has a significant diffusive component. Our results suggest that the integrinligand interaction, along with the internal myosin-motor generated force, varies for different integrin-ligand pairs, consistent with previous results.

Nonmedially assembled F-actin cables incorporate into the actomyosin ring in fission yeast

In many eukaryotes, cytokinesis requires the assembly and constriction of an actomyosin-based contractile ring. Despite the central role of this ring in cytokinesis, the mechanism of F-actin assembly and accumulation in the ring is not fully understood. In this paper, we investigate the mechanism of F-actin assembly during cytokinesis in Schizosaccharomyces pombe using lifeact as a probe to monitor actin dynamics. Previous work has shown that F-actin in the actomyosin ring is assembled de novo at the division site. Surprisingly, we find that a significant fraction of F-actin in the ring was recruited from formin-Cdc12p nucleated long actin cables that were generated at multiple nonmedial locations and incorporated into the ring by a combination of myosin II and myosin V activities. Our results, together with findings in animal cells, suggest that de novo F-actin assembly at the division site and directed transport of F-actin cables assembled elsewhere can contribute to ring assembly.

Analysis of turnover dynamics of the submembranous actin cortex

Two filament subpopulations with very different turnover rates make up the actin cortex in living cells: one with fast turnover dynamics and polymerization resulting from addition of monomers to free barbed ends, and one with slow turnover dynamics with polymerization resulting from formin-mediated filament growth.

The cell cortex is a thin network of actin, myosin motors, and associated proteins that underlies the plasma membrane in most eukaryotic cells. It enables cells to resist extracellular stresses, perform mechanical work, and change shape. Cortical structural and mechanical properties depend strongly on the relative turnover rates of its constituents, but quantitative data on these rates remain elusive. Using photobleaching experiments, we analyzed the dynamics of three classes of proteins within the cortex of living cells: a scaffold protein (actin), a cross-linker (α-actinin), and a motor (myosin). We found that two filament subpopulations with very different turnover rates composed the actin cortex: one with fast turnover dynamics and polymerization resulting from addition of monomers to free barbed ends, and one with slow turnover dynamics with polymerization resulting from formin-mediated filament growth. Our data suggest that filaments in the second subpopulation are on average longer than those in the first and that cofilin-mediated severing of formin-capped filaments contributes to replenishing the filament subpopulation with free barbed ends. Furthermore, α-actinin and myosin minifilaments turned over significantly faster than F-actin. Surprisingly, only one-fourth of α-actinin dimers were bound to two actin filaments. Taken together, our results provide a quantitative characterization of essential mechanisms under­lying actin cortex homeostasis.

Comparing contractile apparatus-driven cytokinesis mechanisms across kingdoms

Cytokinesis is the final stage of the cell cycle during which a cell physically divides into two daughters through the assembly of new membranes (and cell wall in some cases) between the forming daughters. New membrane assembly can either proceed centripetally behind a contractile apparatus, as in the case of prokaryotes, archaea, fungi, and animals or expand centrifugally, as in the case of higher plants. In this article, we compare the mechanisms of cytokinesis in diverse organisms dividing through the use of a contractile apparatus. While an actomyosin ring participates in cytokinesis in almost all centripetally dividing eukaryotes, the majority of bacteria and archaea (except Crenarchaea) divide using a ring composed of the tubulin-related protein FtsZ. Curiously, despite molecular conservation of the division machinery components, division site placement and its cell cycle regulation occur by a variety of unrelated mechanisms even among organisms from the same kingdom. While molecular motors and cytoskeletal polymer dynamics contribute to force generation during eukaryotic cytokinesis, cytoskeletal polymer dynamics alone appears to be sufficient for force generation during prokaryotic cytokinesis. Intriguingly, there are life forms on this planet that appear to lack molecules currently known to participate in cytokinesis and how these cells perform cytokinesis remains a mystery waiting to be unravelled.

Actin cortex mechanics and cellular morphogenesis

The cortex is a thin, crosslinked actin network lying immediately beneath the plasma membrane of animal cells. Myosin motors exert contractile forces in the meshwork. Because the cortex is attached to the cell membrane, it plays a central role in cell shape control. The proteic constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. The cortex has recently attracted increasing attention and its functions in cellular processes such as cytokinesis, cell migration, and embryogenesis are progressively being dissected. In this review, we summarize current knowledge on the structural organization, composition, and mechanics of the actin cortex, focusing on the link between molecular processes and macroscopic physical properties. We also highlight consequences of cortex dysfunction in disease.

Cytokinesis in animal cells

Cytokinesis, the final step in cell division, partitions the contents of a single cell into two. In animal cells, cytokinesis occurs through cortical remodeling orchestrated by the anaphase spindle. Cytokinesis relies on a tight interplay between signaling and cellular mechanics and has attracted the attention of both biologists and physicists for more than a century. In this review, we provide an overview of four topics in animal cell cytokinesis: (a) signaling between the anaphase spindle and cortex, (b) the mechanics of cortical remodeling, (c) abscission, and (d) regulation of cytokinesis by the cell cycle machinery. We report on recent progress in these areas and highlight some of the outstanding questions that these findings bring into focus.

α-Actinin and fimbrin cooperate with myosin II to organize actomyosin bundles during contractile-ring assembly

During cytokinesis in Schizosaccharomyces pombe, the transient connections between nodes allow them to condense into the contractile ring. We find that α-actinin and fimbrin, two actin cross-linking proteins, are critical for node condensation as they stabilize transient linear actomyosin structures and thus modulate the morphology of the actomyosin network.

The actomyosin contractile ring assembles through the condensation of a broad band of nodes that forms at the cell equator in fission yeast cytokinesis. The condensation process depends on actin filaments that interconnect nodes. By mutating or titrating actin cross-linkers α-actinin Ain1 and fimbrin Fim1 in live cells, we reveal that both proteins are involved in node condensation. Ain1 and Fim1 stabilize the actin cytoskeleton and modulate node movement, which prevents nodes and linear structures from aggregating into clumps and allows normal ring formation. Our computer simulations modeling actin filaments as semiflexible polymers reproduce the experimental observations and provide a model of how actin cross-linkers work with other proteins to regulate actin-filament orientations inside actin bundles and organize the actin network. As predicted by the simulations, doubling myosin II Myo2 level rescues the node condensation defects caused by Ain1 overexpression. Taken together, our work supports a cooperative process of ring self-organization driven by the interaction between actin filaments and myosin II, which is progressively stabilized by the cross-linking proteins.