Cleavage furrow: timing of emergence of contractile ring actin filaments and establishment of the contractile ring by filament bundling in sea urchin eggs

ROCK and the actomyosin network control biomineral growth and morphology during sea urchin skeletogenesis

Biomineralization had apparently evolved independently in different phyla, using distinct minerals, organic scaffolds, and gene regulatory networks (GRNs). However, diverse eukaryotes from unicellular organisms, through echinoderms to vertebrates, use the actomyosin network during biomineralization. Specifically, the actomyosin remodeling protein, Rho-associated coiled-coil kinase (ROCK) regulates cell differentiation and gene expression in vertebrates' biomineralizing cells, yet, little is known on ROCK's role in invertebrates' biomineralization. Here, we reveal that ROCK controls the formation, growth, and morphology of the calcite spicules in the sea urchin larva. ROCK expression is elevated in the sea urchin skeletogenic cells downstream of the Vascular Endothelial Growth Factor (VEGF) signaling. ROCK inhibition leads to skeletal loss and disrupts skeletogenic gene expression. ROCK inhibition after spicule formation reduces the spicule elongation rate and induces ectopic spicule branching. Similar skeletogenic phenotypes are observed when ROCK is inhibited in a skeletogenic cell culture, indicating that these phenotypes are due to ROCK activity specifically in the skeletogenic cells. Reduced skeletal growth and enhanced branching are also observed under direct perturbations of the actomyosin network. We propose that ROCK and the actomyosin machinery were employed independently, downstream of distinct GRNs, to regulate biomineral growth and morphology in Eukaryotes.

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.

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.

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.

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.

High resolution imaging of the cortex isolated from sea urchin eggs and embryos

The cellular cortex-consisting of the plasma membrane and the adjacent outer few microns of the cytoplasm-is a critically important, dynamic and complex region in the sea urchin egg and embryo. Some 40 years ago it was discovered that isolated cortices could be obtained from eggs adhered to glass coverslips and since that time this preparation has been used in a wide range of studies, including seminal research on fertilization, exocytosis, the cytoskeleton, and cytokinesis. In this chapter, we discuss methods for isolating cortices from eggs and embryos, including those undergoing cell division. We also provide protocols for analyzing cortical architecture and dynamics using specific localization methods combined with super-resolution Structured Illumination and Stimulated Emission Depletion light microscopy and platinum replica transmission electron microscopy.

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.

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.

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.

Antagonistic Behaviors of NMY-1 and NMY-2 Maintain Ring Channels in the C. elegans Gonad

Contractile rings play critical roles in a number of biological processes, including oogenesis, wound healing, and cytokinesis. In many cases, the activity of motor proteins such as nonmuscle myosins is required for appropriate constriction of these contractile rings. In the gonad of the nematode worm Caenorhabditis elegans, ring channels are a specialized form of contractile ring that are maintained at a constant diameter before oogenesis. We propose a model of ring channel maintenance that explicitly incorporates force generation by motor proteins that can act normally or tangentially to the ring channel opening. We find that both modes of force generation are needed to maintain the ring channels. We demonstrate experimentally that the type II myosins NMY-1 and NMY-2 antagonize each other in the ring channels by producing force in perpendicular directions: the experimental depletion of NMY-1/theoretical decrease in orthogonal force allows premature ring constriction and cellularization, whereas the experimental depletion of NMY-2/theoretical decrease in tangential force opens the ring channels and prevents cellularization. Together, our experimental and theoretical results show that both forces, mediated by NMY-1 and NMY-2, are crucial for maintaining the appropriate ring channel diameter and dynamics throughout the gonad.

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.

Constriction model of actomyosin ring for cytokinesis by fission yeast using a two-state sliding filament mechanism

We developed a model describing the structure and contractile mechanism of the actomyosin ring in fission yeast, Schizosaccharomyces pombe. The proposed ring includes actin, myosin, and α-actinin, and is organized into a structure similar to that of muscle sarcomeres. This structure justifies the use of the sliding-filament mechanism developed by Huxley and Hill, but it is probably less organized relative to that of muscle sarcomeres. Ring contraction tension was generated via the same fundamental mechanism used to generate muscle tension, but some physicochemical parameters were adjusted to be consistent with the proposed ring structure. Simulations allowed an estimate of ring constriction tension that reproduced the observed ring constriction velocity using a physiologically possible, self-consistent set of parameters. Proposed molecular-level properties responsible for the thousand-fold slower constriction velocity of the ring relative to that of muscle sarcomeres include fewer myosin molecules involved, a less organized contractile configuration, a low α-actinin concentration, and a high resistance membrane tension. Ring constriction velocity is demonstrated as an exponential function of time despite a near linear appearance. We proposed a hypothesis to explain why excess myosin heads inhibit constriction velocity rather than enhance it. The model revealed how myosin concentration and elastic resistance tension are balanced during cytokinesis in S. pombe.

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.

The value of mechanistic biophysical information for systems-level understanding of complex biological processes such as cytokinesis

This review illustrates the value of quantitative information including concentrations, kinetic constants and equilibrium constants in modeling and simulating complex biological processes. Although much has been learned about some biological systems without these parameter values, they greatly strengthen mechanistic accounts of dynamical systems. The analysis of muscle contraction is a classic example of the value of combining an inventory of the molecules, atomic structures of the molecules, kinetic constants for the reactions, reconstitutions with purified proteins and theoretical modeling to account for the contraction of whole muscles. A similar strategy is now being used to understand the mechanism of cytokinesis using fission yeast as a favorable model system.

Cdk1-dependent phosphorylation of Iqg1 governs actomyosin ring assembly prior to cytokinesis

Contraction of the actomyosin ring (AMR) provides the centripetal force that drives cytokinesis. In budding yeast (Saccharomyces cerevisiae), assembly and contraction of the AMR is coordinated with membrane deposition and septum formation at the bud neck. A central player in this process is Iqg1, which promotes recruitment of actin to the myosin ring and links AMR assembly with that of septum-forming components. We observed early actin recruitment in response to inhibition of cyclin-dependent kinase 1 (Cdk1) activity, and we find that the Cdk1-dependent phosphorylation state of Iqg1 is a determining factor in the timing of bud neck localization of both Iqg1 and actin, with both proteins accumulating prematurely in cells expressing nonphosphorylatable Iqg1 mutants. We also identified the primary septum regulator Hof1 as a binding partner of Iqg1, providing a regulatory link between the septation and contractile pathways that cooperate to complete cytokinesis.

Biphasic assembly of the contractile apparatus during the first two cell division cycles in zebrafish embryos

The large and optically clear embryos of the zebrafish provide an excellent model system in which to study the dynamic assembly of the essential contractile band components, actin and myosin, via double fluorescent labelling in combination with confocal microscopy. We report the rapid appearance (i.e. within <2 min) of a restricted arc of F-actin patches along the prospective furrow plane in a central, apical region of the blastodisc cortex. These patches then fused with each other end-to-end forming multiple actin cables, which were subsequently bundled together forming an F-actin band. During this initial assembly phase, the F-actin-based structure did not elongate laterally, but was still restricted to an arc extending ~15° either side of the blastodisc apex. This initial assembly phase was then followed by an extension phase, where additional F-actin patches were added to each end of the original arc, thus extending it out to the edges of the blastodisc. The dynamics of phosphorylated myosin light chain 2 (MLC2) recruitment to this F-actin scaffold also reflect the two-phase nature of the contractile apparatus assembly. MLC2 was not associated with the initial F-actin arc, but MLC2 clusters were recruited and assembled into the extending ends of the band. We propose that the MLC2-free central region of the contractile apparatus acts to position and then extend the cleavage furrow in the correct plane, while the actomyosin ends alone generate the force required for furrow ingression. This biphasic assembly strategy may be required to successfully divide the early cells of large embryos.

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.

Cytoskeleton organization during the cell cycle in two stramenopile microalgae, Ochromonas danica (Chrysophyceae) and Heterosigma akashiwo (Raphidophyceae), with special reference to F-actin organization and its role in cytokinesis

F-actin organization during the cell cycle was investigated in two stramenopile microalgae, Ochromonas danica (Chrysophyceae; UTEX LB1298) and Heterosigma akashiwo (Raphidophyceae; NIES-6) using FITC-phalloidin. In the interphase cell of O. danica, F-actin bundles were localized forming a network structure in the cortical region, which converged from the anterior region to the posterior, whereas in the interphase cell of H. akashiwo, F-actin bundles were observed forming a network structure in the cortical region without any polarity. In both O. danica and H. akashiwo, at the initial stage of mitosis the cortical F-actin disappeared, and then during cytokinesis assembly of an actin-based ring-like structure occurred in the cell cortex in the plane of cytokinesis. The ring-like structure initiated from aster-like structures was composed of F-actin in both O. danica and H. akashiwo. Different from animal cells, later stages of cytokinesis of O. danica seemed to be promoted by microtubules, although the early stages of cytokinesis progressed with a constriction of the ring-like structure, whereas cytokinesis of H. akashiwo was apparently completed by constriction of the cell mediated by the F-actin ring, as in animal cells.

CORTICAL F-ACTIN REORGANIZATION AND A CONTRACTILE RING-LIKE STRUCTURE FOUND DURING THE CELL CYCLE IN THE RED CRYPTOMONAD, PYRENOMONAS HELGOLANDII(1)

Cortical F-actin reorganization during the cell cycle was observed in Pyrenomonas helgolandii U. J. Santore (SAG 28.87) for the first time in Cryptophyta using fluorescein-isothiocyanate (FITC)-phalloidin staining. In interphase, a number of F-actin bundles were observed as straight lines running parallel to the long axis of the cell on the cell cortical region. They extended from an F-actin bundle that runs along the margin of the vestibulum. Although the F-actin bundles running parallel to the long axis of the cell disappeared during anaphase, they gradually reappeared in telophase. By contrast, the F-actin bundle along the vestibulum margin remained visible during cytokinesis and dynamically changed following the enlargement of the vestibulum, suggesting that F-actin was involved in the mechanism of vestibulum enlargement. F-actins were not found in the cytoplasmic and nucleoplasmic regions throughout the cell cycle. In addition, a contractile ring-like structure appeared at the cleavage furrow during cytokinesis. Treatment with cytochalasin B and latrunculin B significantly inhibited the formation of cleavage furrow, resulting in forming an abnormal cell with two nuclei, suggesting that cytokinesis in P. helgolandii is controlled by the contractile ring-like structure constituted of F-actin.

A functional analysis of MELK in cell division reveals a transition in the mode of cytokinesis during Xenopus development

MELK is a serine/threonine kinase involved in several cell processes, including the cell cycle, proliferation, apoptosis and mRNA processing. However, its function remains elusive. Here, we explored its role in the Xenopus early embryo and show by knockdown that xMELK (Xenopus MELK) is necessary for completion of cell division. Consistent with a role in cell division, endogenous xMELK accumulates at the equatorial cortex of anaphase blastomeres. Its relocalization is highly dynamic and correlates with a conformational rearrangement in xMELK. Overexpression of xMELK leads to failure of cytokinesis and impairs accumulation at the division furrow of activated RhoA - a pivotal regulator of cytokinesis. Furthermore, endogenous xMELK associates and colocalizes with the cytokinesis organizer anillin. Unexpectedly, our study reveals a transition in the mode of cytokinesis correlated to cell size and that implicates xMELK. Collectively, our findings disclose the importance of xMELK in cytokinesis during early development and show that the mechanism of cytokinesis changes during Xenopus early development.

Concordance and interaction of guanine nucleotide dissociation inhibitor (RhoGDI) with RhoA in oogenesis and early development of the sea urchin

Rho GTPases are Ras-related GTPases that regulate a variety of cellular processes. In the sea urchin Strongylocentrotus purpuratus, RhoA in the oocyte associates with the membrane of the cortical granules and directs their movement from the cytoplasm to the cell cortex during maturation to an egg. RhoA also plays an important role regulating the Na(+) -H(+) exchanger activity, which determines the internal pH of the cell during the first minutes of embryogenesis. We investigated how this activity may be regulated by a guanine-nucleotide dissociation inhibitor (RhoGDI). The sequence of this RhoA regulatory protein was identified in the genome on the basis of its similarity to other RhoGDI species, especially for key segments in the formation of the isoprenyl-binding pocket and in interactions with the Rho GTPase. We examined the expression and the subcellular localization of RhoGDI during oogenesis and in different developmental stages. We found that RhoGDI mRNA levels were high in eggs and during cleavage divisions until blastula, when it disappeared, only to reappear in gastrula stage. RhoGDI localization overlaps the presence of RhoA during oogenesis and in embryonic development, reinforcing the regulatory premise of the interaction. By use of recombinant protein interactions in vitro, we also find that these two proteins selectively interact. These results support the hypothesis of a functional relationship in vivo and now enable mechanistic insight for the cellular and organelle rearrangements that occur during oogenesis and embryonic development.

Impact of marine drugs on cytoskeleton-mediated reproductive events

Marine organisms represent an important source of novel bioactive compounds, often showing unique modes of action. Such drugs may be useful tools to study complex processes such as reproduction; which is characterized by many crucial steps that start at gamete maturation and activation and virtually end at the first developmental stages. During these processes cytoskeletal elements such as microfilaments and microtubules play a key-role. In this review we describe: (i) the involvement of such structures in both cellular and in vitro processes; (ii) the toxins that target the cytoskeletal elements and dynamics; (iii) the main steps of reproduction and the marine drugs that interfere with these cytoskeleton-mediated processes. We show that marine drugs, acting on microfilaments and microtubules, exert a wide range of impacts on reproductive events including sperm maturation and motility, oocyte maturation, fertilization, and early embryo development.

Elongation factors are involved in cytokinesis of sea urchin eggs

Cleavage furrows (CFs) have been isolated from dividing sea urchin eggs and the protein constituents have been analyzed by two-dimensional gel electrophoresis (Fujimoto & Mabuchi, J. Biochem. 122, 518-524, 1997). Two proteins of 51 and 32 kDa, respectively, have been found to be enriched in the CF preparation. Here, we show that these proteins are identical to the protein elongation factor 1alpha (EF-1alpha) and 1beta (EF-1beta), respectively. Furthermore, the CF 51-kDa protein is identical to the 51-kDa protein which had been isolated as a component of the microtubule organizing granules of mitotic sea urchin eggs. The 51-kDa protein bundles F-actin in vitro. This activity is suppressed by Ca(2+)/calmodulin or GTPgammaS. The 32-kDa protein binds EF-1alpha both in vitro and in cell extract, and is shown to suppress the F-actin-bundling activity of the 51-kDa protein. Microinjection of a monoclonal antibody against the 51-kDa protein or that of His-tagged 32-kDa protein into dividing sea urchin eggs at the onset of cleavage leads to failure of cytokinesis. These results strongly suggest that EF-1alpha is involved in maintenance of the structure of the contractile ring and EF-1beta regulates the F-actin-bundling activity of EF-1alpha.

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

Cytokinesis is accomplished by constriction of a cortical contractile ring. We show that during the early embryonic divisions in C. elegans, ring constriction occurs in two phases--an initial phase at a constant rate followed by a second phase during which the constriction rate decreases in proportion to ring perimeter. Cytokinesis completes in the same amount of time, despite the reduction in cell size during successive divisions, due to a strict proportionality between initial ring size and the constant constriction rate. During closure, the myosin motor in the ring decreases in proportion to perimeter without turning over. We propose a "contractile unit" model to explain how the ring retains a structural memory of its initial size as it disassembles. The scalability of constriction may facilitate coordination of mitotic events and cytokinesis when cell size, and hence the distance traversed by the ring, varies during embryogenesis and in other contexts.

Quantitative Analysis of Cortical Actin Filaments during Polar Body Formation in Starfish Oocytes

Polar body formation is an extremely unequal cell division. In order to understand the mechanism of polar body formation, morphological changes at the animal pole were investigated in living oocytes of the starfish, Asterina pectinifera, and the amounts of cortical actin filaments were quantitatively estimated after staining the maturing oocytes with fluorescently-labeled phallotoxins using a computer and image-processing software. Formation of a bulge, which is presumed to become a polar body, and the anaphase separation of chromosomes occurred simultaneously. When the bulge became large, one group of chromatids moved into the bulge. The dividing furrow then formed and finally a polar body formed. Just at the time of bulge formation, the intensity of the fluorescence produced by the actin filaments at the top of the animal pole began to decrease, and subsequently the intensity at the top fell to half of the original value. On the other hand, the fluorescence intensity at the base of the bulge increased gradually. This actin accumulation at the base created a dividing furrow around the top of the animal pole as the bulge grew. Even when the polar body formation was inhibited mechanically, a similar pattern of actin deficiency and accumulation in the cortex near the animal pole was observed. This indicates that such regulation of filamentous actin can take place without bulging. Therefore, polar body formation is initiated by the bulging of the cortex weakened by actin deficiency and followed by contraction of the base of the bulge reinforced by actin accumulation.

Myosin II activity is not essential for recruitment of myosin II to the furrow in dividing HeLa cells

To elucidate whether phosphorylation of myosin II regulatory light chain (MRLC) is essential for myosin II recruitment to the furrow during cytokinesis, HeLa cells transfected with three types of GFP-tagged recombinant MRLCs, wild-type MRLC, non-phosphorylated form of MRLC, and phosphorylated form of MRLC, were examined. Living cell-imaging showed that both phosphorylated and non-phosphorylated form of MRLCs were recruited to the equator at the same time after anaphase onset, suggesting that phosphorylation of MRLC is not responsible for recruitment of myosin II to the equator. Moreover, the treatment with an inhibitor of myosin II activity, blebbistatin, induced no effect on recruitment of those three recombinant MRLCs. During cytokinesis, phosphorylated but not non-phosphorylated form of MRLC was retained in the equator. These results suggest that phosphorylation of MRLC is essential for retainment of myosin II in the furrow but not for initial recruitment of myosin II to the furrow in dividing HeLa cells.

Regulation of dynamic events by microfilaments during oocyte maturation and fertilization

Actin filaments (microfilaments) regulate various dynamic events during oocyte meiotic maturation and fertilization. In most species, microfilaments are not required for germinal vesicle breakdown and meiotic spindle formation, but they mediate peripheral nucleus (chromosome) migration, cortical spindle anchorage, homologous chromosome separation, cortex development/maintenance, polarity establishment, and first polar body emission during oocyte maturation. Peripheral cortical granule migration is controlled by microfilaments, while mitochondria movement is mediated by microtubules. During fertilization, microfilaments are involved in sperm incorporation, spindle rotation (mouse), cortical granule exocytosis, second polar body emission and cleavage ring formation, but are not required for pronuclear apposition (except for the mouse). Many of the events are driven by the dynamic interactions between myosin and actin filaments whose polymerization is regulated by RhoA, Cdc42, Arp2/3 and other signaling molecules. Studies have also shown that oocyte cortex organization and polarity formation mediated by actin filaments are regulated by mitogen-activated protein kinase, myosin light-chain kinase, protein kinase C and its substrate p-MARKS as well as PAR proteins. The completion of several dynamic events, including homologous chromosome separation, spindle anchorage, spindle rotation, vesicle organelle transport and pronuclear apposition (mouse), requires interactions between microfilaments and microtubules, but determination of how the two systems of the cytoskeleton precisely cross-link, and which proteins link microfilaments to microtubules to perform functions in eggs, requires further studies. Finally, the meaning of microfilament-mediated oocyte polarity versus embryo polarity and embryo development in different species (Drosophila, Xenopus and mouse) is discussed.

Assembly of the cytokinetic contractile ring from a broad band of nodes in fission yeast

We observed live fission yeast expressing pairs of functional fluorescent fusion proteins to test the popular model that the cytokinetic contractile ring assembles from a single myosin II progenitor or a Cdc12p-Cdc15p spot. Under our conditions, the anillin-like protein Mid1p establishes a broad band of small dots or nodes in the cortex near the nucleus. These nodes mature by the addition of conventional myosin II (Myo2p, Cdc4p, and Rlc1p), IQGAP (Rng2p), pombe Cdc15 homology protein (Cdc15p), and formin (Cdc12p). The nodes coalesce laterally into a compact ring when Cdc12p and profilin Cdc3p stimulate actin polymerization. We did not observe assembly of contractile rings by extension of a leading cable from a single spot or progenitor. Arp2/3 complex and its activators accumulate in patches near the contractile ring early in anaphase B, but are not concentrated in the contractile ring and are not required for assembly of the contractile ring. Their absence delays late steps in cytokinesis, including septum formation and cell separation.

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

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

The mechanism of cortical ingression during early cytokinesis: thinking beyond the contractile ring hypothesis

Owing to the rapid advances in genomic, proteomic and imaging technologies, the field of cytokinesis has seen rapid advances during the past decade. However, the basic model for the early stage of ingression, known as the contractile ring hypothesis, remains largely unchanged. From recent observations, it is becoming clear that early cytokinesis of animal cells involves a more extensive set of events, both temporally and spatially, than what is encompassed by the original contractile ring hypothesis. Activities relevant to cytokinesis, such as cortical contraction, can initiate well before onset of anaphase. Furthermore, equatorial ingression can involve multiple events in different regions of the cortex, including the establishment of anterior-posterior polarity, the modulation of cortical deformability, the expansion and compression of the cell cortex, and forces directed towards the interior of the cell or away from the equator. In this article (which is part of the Cytokinesis series), I evaluate critically key observations on when, where and how early ingression of animal cells takes place.

Molecular model of the contractile ring

We present a model for the actin contractile ring of adherent animal cells. The model suggests that the actin concentration within the ring and consequently the power that the ring exerts both increase during contraction. We demonstrate the crucial role of actin polymerization and depolymerization throughout cytokinesis, and the dominance of viscous dissipation in the dynamics. The physical origin of two phases in cytokinesis dynamics ("biphasic cytokinesis") follows from a limitation on the actin density. The model is consistent with a wide range of measurements of the midzone of dividing animal cells.

Polar body formation in Spisula oocytes: function of the peripheral aster

Activated Spisula oocytes proceed through meiotic stages rapidly and in near synchrony, providing an excellent system for analyzing polar body formation. Our previous studies suggested that cortical spreading of the metaphase peripheral aster determines spatial features of the cortical F-actin ring that is generated prior to extrusion of the polar body. We tested this hypothesis by experimentally altering the number and cortical contact patterns of peripheral asters. Such alteration was achieved by (a) lovastatin-induced arrest at metaphase I, with and without hexylene glycol modification, followed by washout; and (b) cytochalasin-D inhibition of extrusion of the first polar body, with washout before extrusion of the second polar body. Both methods induced simultaneous formation of two or more cortically spreading asters, correlated with subsequent formation of double, or even triple, overlapping F-actin rings during anaphase. Regardless of pattern, ring F-actin was deposited near regions of greatest astral microtubule density, indicating that microtubules provided a positive stimulus to which the cortex responded indiscriminately. These results strongly support the proposed causal relationship between peripheral aster spreading and biogenesis of the F-actin ring involved in polar body formation.

A MAPK pathway is involved in the control of mitosis after fertilization of the sea urchin egg

Activation and role of mitogen-activated protein (MAP) kinase (MAPK) during mitosis are still matters of controversy in early embryos. We report here that an ERK-like protein is present and highly phosphorylated in unfertilized sea urchin eggs. This MAPK becomes dephosphorylated after fertilization and a small pool of it is transiently reactivated during mitosis. The phosphorylated ERK-like protein is localized to the nuclear region and then to the mitotic poles and the mitotic spindle. Treatment of eggs after fertilization with two different MEK inhibitors, PD 98059 and U0126, at low concentrations capable to selectively induce dephosphorylation of this ERK-like protein, or expression of a dominant-negative MEK1/2, perturbed mitotic progression. Our results suggest that an ERK-like cascade is part of a control mechanism that regulates mitotic spindle formation and the attachment of chromosomes to the spindle during the first mitosis of the sea urchin embryo.

A novel actin isoform is expressed in the ovotestis of Aplysia californica

The actin family encodes a large number of protein isoforms with quasi-identical primary structure but distinct function and localization. In oocytes, actin is known to play important roles in different processes such as those leading to fertilization or to mRNA localization during oogenesis. In this paper, we report the characterization of a novel actin isoform (apACTov) in Aplysia californica that is specifically expressed in ovotestis. The apACTov cDNA codes for a putative protein of 376 amino acids that shows 96% and 94% sequence identity with two other actin isoforms previously characterized in Aplysia. In situ hybridization experiments showed that the apACTov transcript is not uniformly distributed but is found in crescent or filipodia-like structures at the surface of the oocyte. Our results suggest that apACTov may contribute to the differential distribution of critical material during egg division and/or cell differentiation.

Dissection of septin actin interactions using actin overexpression in Saccharomyces cerevisiae

Although many proteins can be overexpressed several fold without much effect on cell viability and morphology, some become toxic upon a slight increase in their intracellular level. This is particularly true for cytoskeletal proteins and has proven useful in the past for studying the cytoskeleton. In yeast, actin and tubulin are examples of proteins that cannot be overexpressed without affecting cell viability. Here, we have analysed the effect of actin overexpression in Saccharomyces cerevisiae. We show that actin overexpression interferes differently with distinct aspects of actin function. For example, two- to fourfold overexpression of actin did not affect the establishment of actin polarity, whereas it abrogated its maintenance. Also, actin structures that are barely visible in wild-type cells could be observed upon actin overexpression. This allowed us to identify a new ring-like actin structure genetically distinguishable from the actomyosin contractile ring. Formation of this actin structure upon actin overexpression was dependent on the septin cytoskeleton, the poorly understood cytokinetic protein Hof1 and the Arp2/3 complex. In contrast to the actomyosin ring, the ring formed upon actin overexpression required neither Myo1 nor formins for assembly. Therefore, we propose that Hof1 acts as a linker between actin and septins. Furthermore, we found that, in the absence of actin overexpression, a novel, Hof1-dependent actin belt is formed at the bud neck of anaphase cells. The physiological role of this belt might be related to that of the similar structure observed in dividing fission yeast.

Zebrafish maternal-effect mutations causing cytokinesis defect without affecting mitosis or equatorial vasa deposition

Maternal-effect genes play essential roles in early embryogenesis particularly before activation of the zygotic genes. A genetic screen for mutations affecting such maternal-effect genes was carried out employing an F3 screen strategy, identifying six recessive mutations out of 60 mutagenized genomes. Three of the mutations (acytokinesis mutations: ackkt5, ackkt62 and ackkt119) caused absence of cell cleavage in the embryos derived from homozygous females regardless of the paternal genotype, without affecting nuclear divisions. These embryos are defective in generating contractile rings, ackkt62 mutation abolishing reactions to organize cortical F-actin, while other mutations causing abortive contractile ring-like structures at ectopic sites. Defect of contractile ring formation in the affected embryos leads to the absence of microtubule arrays at the prospective cleavage plane. Thus, these mutations reveal the sequence of events associated with cytokinesis, in particular, the cortical actin dynamics. It is remarkable that in all acytokinetic embryos, daughter nuclei after mitosis are arranged in spatially normal positions, and maternal vasa mRNAs accumulate in the prospective planes of the first and second cell cleavages in the total absence of cytokinesis. This indicates that the basic cell architectures of early embryos are largely established by the autonomous activities of the mitotic apparatus, without much dependence on the cell cleavage machinery.

An IQGAP-like protein is involved in actin assembly together with Cdc42 in the sea urchin egg

We isolated a gene homologous to human cdc42 (ucdc42) from a sea urchin cDNA library. The GTPgammaS-bound UCdc42 induced actin assembly in sea urchin egg extract. Proteins that are involved in this actin assembly system were searched using UCdc42-bound agarose beads. A 180-kDa protein (p180), which showed a homology to human IQGAPs, bound to the GTPgammaS-UCdc42 beads. Immunodepletion of p180 from the sea urchin egg extract abolished this actin assembly on the UCdc42 beads. Immunofluorescent localization of p180 was similar to that of the actin cytoskeleton in the egg cortex and it was concentrated in the cleavage furrow during cytokinesis. A possible role of p180 in actin assembly is discussed.

Integrins on eggs: the betaC subunit is essential for formation of the cortical actin cytoskeleton in sea urchin eggs

Eggs of several metazoans have been demonstrated to express integrins; however, their function is unclear. Previous studies have shown that the betaC integrin subunit is expressed on unfertilized sea urchin eggs and proteolytically removed at fertilization. Here we report that the betaC subunit is reexpressed on the egg surface immediately after fertilization. Using morpholino antisense oligonucleotides to block translation, we show that without betaC expression, eggs undergo cleavage resulting in loosely adherent cells that fail to develop beyond a blastula. Without betaC containing integrins, the cortical actin network of the egg does not form, yet contractile rings appear. Coinjection of RNA encoding the betaC or chicken beta1 subunit, but lacking the morpholino target sequence, rescues the cortical actin network and normal embryos result. Coinjection of RNA encoding the betaC subunit lacking the cytoplasmic domain fails to rescue. These studies demonstrate that the cortical actin cytoskeleton is anchored by betaC integrins and contractile ring actin is not. We suggest that one important function of egg integrins is to organize the actin cortex.

Effects of the phosphatase inhibitors, okadaic acid, ATPgammaS, and calyculin A on the dividing sand dollar egg

The effects of the phosphatase inhibitors, okadaic acid (OA), adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS), and calyculin A (CL-A) on anaphase chromosome movement, cytokinesis, and cytoskeletal structures at cell division were examined by being microinjected into mitotic sand dollar eggs. When OA was injected, chromosome movement was inhibited and, moreover, chromosomes were ejected from the polar regions of the mitotic apparatus. By immunofluorescence, microtubules were observed to be severed in the OA-injected eggs, causing the smooth cell surface to be changed to an irregular surface. When ATPgammaS and CL-A were injected, the effect on cell shape was remarkable: In dividing eggs, furrowing stopped within several seconds after injection, small blebs appeared on the cell surface and became large, spherical or dumbbell cell shapes then changed to irregular forms, and subsequently cytoplasmic flow occurred. Microfilament detection revealed that actin accumulation in the cortex, which was not limited to the furrow cortex, occurred shortly after injection. Cortical accumulation of actin is thought to induce force generation and random cortical contraction, and accordingly to result in bleb extrusion from the cortex. Consequently, the phosphatase inhibitors inhibited the transition from mitosis to interphase by mediating cortical accumulation of actin filaments and/or fragmentation of microtubules.

F-actin ring formation and the role of F-actin cables in the fission yeastSchizosaccharomyces pombe

Cells of the fission yeast Schizosaccharomyces pombe divide by the contraction of the F-actin ring formed at the medial region of the cell. We investigated the process of F-actin ring formation in detail using optical sectioning and three-dimensional reconstruction fluorescence microscopy. In wild-type cells, formation of an aster-like structure composed of F-actin cables and accumulation of F-actin cables were recognized at the medial cortex of the cell during prophase to metaphase. The formation of the aster-like structure seemed to initiate from branching of the longitudinal F-actin cables at a site near the spindle pole bodies, which had been duplicated but not yet separated. A single cable extended from the aster and encircled the cell at the equator to form a primary F-actin ring during metaphase. During anaphase,the accumulated F-actin cables were linked to the primary F-actin ring, and then all of these structures seemed to be packed to form the F-actin ring. These observations suggest that formation of the aster-like structure and the accumulation of the F-actin cables at the medial region of the cell during metaphase may be required to initiate the F-actin ring formation. In the nda3 mutant, which has a mutation in ß-tubulin and has been thought to be arrested at prophase, an F-actin ring with accumulated F-actin cables similar to that of anaphase wild-type cells was formed at a restrictive temperature. Immediately after shifting to a permissive temperature, this structure changed into a tightly packed ring. This suggests that the F-actin ring formation progresses beyond prophase in the nda3 cells once the cells enter prophase. We further examined F-actin structures in both cdc12 and cdc15 early cytokinesis mutants. As a result,Cdc12 seemed to be required for the primary F-actin ring formation during prophase, whereas Cdc15 may be involved in both packing the F-actin cables to form the F-actin ring and rearrangement of the F-actin after anaphase. In spg1, cdc7 and sid2 septum initiation mutants, the F-actin ring seemed to be formed in order.

Response of the cortex to the mitotic apparatus during polar body formation in the starfish oocyte of Asterina pectinifera

In order to understand the mechanism of unequal division, polar body formation was investigated using the oocytes of the starfish, Asterina pectinifera. Cortical actin filaments were quantitatively measured after staining the maturing oocytes with fluorescently labeled phalloidin using a computer and image-processing software. Before polar body formation, at first the actin filaments at the animal pole decreased and subsequently the animal pole bulged. On the other hand, actin filaments surrounding the animal pole increased gradually and made a cleavage furrow around the animal pole as the bulge grew. Then the furrow ingressed and finally a polar body formed. When the surface force was calculated according to the cell shape, the surface force decreased at the animal pole but the force at the contractile ring increased. When by micromanipulation the mitotic apparatus was detached and translocated to the cortex other than the animal pole, polar body formation occurred all over the cortex of the oocyte, which indicates that the response of the whole cortex to the mitotic apparatus is equal. These results indicate that the decrease in the actin filaments and surface force near the centrosome of the mitotic apparatus as well as the increase in the actin filaments and surface force at some distance of the centrosome is important for cytokinesis.

Contractile ring formation in Xenopus egg and fission yeast

How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.

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

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

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

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

Reorganization of actin cytoskeleton at the growing end of the cleavage furrow of Xenopus egg during cytokinesis

We studied reorganization of actin-myosin cytoskeleton at the growing ends of the cleavage furrow of Xenopus eggs in order to understand how the contractile ring is formed during cytokinesis. Reorganization of F-actin structures during the furrow formation was demonstrated by rhodamine-phalloidin staining of the cleavage furrow and by time-lapse scanning with laser scanning microscopy of F-actin structures in the cleavage furrow of live eggs to which rhodamine-G-actin had been injected. Actin filaments assemble to form small clusters that we call ‘F-actin patches’ at the growing end of the furrow. In live recordings, we observed emergence and rapid growth of F-actin patches in the furrow region. These patches then align in tandem, elongate and fuse with each other to form short F-actin bundles. The short bundles then form long F-actin bundles that compose the contractile ring. During the furrow formation, a cortical movement towards the division plane occurs at the growing ends of the furrow, as shown by monitoring wheatgerm agglutinin-conjugated fluorescent beads attached to the egg surface. As a result, wheatgerm agglutinin-binding sites accumulate and form ‘bleb-like’ structures on the surface of the furrow region. The F-actin patch forms and grows underneath this structure. The slope of F-actin accumulation in the interior region of the furrow exceeds that of accumulation of the cortex transported by the cortical movement. In addition, rhodamine-G-actin microinjected at the growing end is immediately incorporated into the F-actin patches. These data, together with the rapid growth of F-actin patches in the live image, suggest that actin polymerization occurs in the contractile ring formation. Distribution of myosin II in the cleavage furrow was also examined by immunofluorescence microscopy. Myosin II assembles as spots at the growing end underneath the bleb-like structure. It was suggested that myosin is transported and accumulates as spots by way of the cortical movement. F-actin accumulates at the position of the myosin spot a little later as the F-actin patches. The myosin spots and the F-actin patches are then simultaneously reorganized to form the contractile ring bundles

Contractile apparatus of the normal and abortive cytokinetic cells during mouse male meiosis

Mouse male meiotic cytokinesis was studied using immunofluorescent probes against various elements of cytokinetic apparatus and electron microscopy. In normal mice, some spermatocytes fail to undergo cytokinesis after meiotic I or II nuclear divisions, forming syncytial secondary spermatocytes and spermatids. Abnormal cytokinetic cells develop sparse and dispersed midzone spindles during the early stage. However, during late stages, single and compact midzone spindles are formed as in normal cells, but localize asymmetrically and attach to the cortex. Myosin and f-actin were observed in the midzone spindle and midbody regions of normally cleaving cells as well as in those cells that failed to develop a cytokinetic furrow, implying that cytokinetic failure is unlikely to be due to defect in myosin or actin assembly. Depolymerization of microtubules by nocodazole resulted in the loss of the midbody-associated f-actin and myosin. These observations suggest that actin-myosin localization in the midbody could be a microtubule-dependent process that may not play a direct role in cytokinetic furrowing. Anti-centrin antibody labels the putative centrioles while anti-(gamma)-tubulin antibody labels the minus-ends of the midzone spindles of late-stage normal and abnormal cytokinetic cells, suggesting that the centrosome and midzone spindle nucleation in abnormal cytokinetic cells is not different from those of normally cleaving cells. Possible use of mouse male meiotic cells as a model system to study cytokinesis has been discussed.

Characterization of sea urchin unconventional myosins and analysis of their patterns of expression during early embryogenesis

Early sea urchin development requires a dynamic reorganization of both the actin cytoskeleton and cytoskeletal interactions with cellular membranes. These events may involve the activities of multiple members of the superfamily of myosin motor proteins. Using RT-PCR with degenerate myosin primers, we identified 11 myosin mRNAs expressed in unfertilized eggs and coelomocytes of the sea urchin Strongylocentrotus purpuratus. Seven of these sea urchin myosins belonged to myosin classes Igamma, II, V, VI, VII, IX, and amoeboid-type I, and the remaining four may be from novel classes. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I were either completely or partially cloned and their molecular structures characterized. Sea urchin myosins-V, -VI, -VII, and amoeboid-type-I shared a high degree of sequence identity with their respective family members from vertebrates and they retained their class-specific structure and domain organization. Analysis of expression of myosin-V, -VI, -VII, and amoeboid-type-I mRNAs during development revealed that each myosin mRNA displayed a distinct temporal pattern of expression, suggesting that myosins might be involved in specific events of early embryogenesis. Interestingly, the onset of gastrulation appeared to be a pivotal point in modulation of myosin mRNA expression. The presence of multiple myosin mRNAs in eggs and embryos provides insight into the potential involvement of multiple specific motor proteins in the actin-dependent events of embryo development.