New horizons for cytokinesis

Cell lineage-dependent chiral actomyosin flows drive cellular rearrangements in early Caenorhabditis elegans development

Proper positioning of cells is essential for many aspects of development. Daughter cell positions can be specified via orienting the cell division axis during cytokinesis. Rotatory actomyosin flows during division have been implied in specifying and reorienting the cell division axis, but how general such reorientation events are, and how they are controlled, remains unclear. We followed the first nine divisions of Caenorhabditis elegans embryo development and demonstrate that chiral counter-rotating flows arise systematically in early AB lineage, but not in early P/EMS lineage cell divisions. Combining our experiments with thin film active chiral fluid theory we identify a mechanism by which chiral counter-rotating actomyosin flows arise in the AB lineage only, and show that they drive lineage-specific spindle skew and cell reorientation events. In conclusion, our work sheds light on the physical processes that underlie chiral morphogenesis in early development.

Compressive Force Spectroscopy: From Living Cells to Single Proteins

One of the most successful applications of atomic force microscopy (AFM) in biology involves monitoring the effect of force on single biological molecules, often referred to as force spectroscopy. Such studies generally entail the application of pulling forces of different magnitudes and velocities upon individual molecules to resolve individualistic unfolding/separation pathways and the quantification of the force-dependent rate constants. However, a less recognized variation of this method, the application of compressive force, actually pre-dates many of these "tensile" force spectroscopic studies. Further, beyond being limited to the study of single molecules, these compressive force spectroscopic investigations have spanned samples as large as living cells to smaller, multi-molecular complexes such as viruses down to single protein molecules. Correspondingly, these studies have enabled the detailed characterization of individual cell states, subtle differences between seemingly identical viral structures, as well as the quantification of rate constants of functionally important, structural transitions in single proteins. Here, we briefly review some of the recent achievements that have been obtained with compressive force spectroscopy using AFM and highlight exciting areas of its future development.

Biomechanical profile of cancer stem-like cells derived from MHCC97H cell lines

Biomechanical properties and cytoskeletal organization of cancer cells are known to be closely related with their aggressive phenotype. In this study, based on atomic force microscopy (AFM), we aimed to evaluate the mechanical property of liver cancer stem-like cells (LCSCs) and compare it with human hepatoma cells (HHCs). LCSCs were enriched from human hepatoma cell line MHCC97H through a sphere culture system. AFM nanoindentation was investigated as a method for measuring the cell stiffness, and reflecting by Young׳s modulus. Microfilament bundles of F-actin were observed with immunofluorescence staining by confocal microscopy. We found that LCSCs show lower Young׳s modulus and higher migration ability compared to MHCC97H cells. Moreover, the decrease in Young׳s modulus is accompanied with a dramatic decline in F-actin content. These results demonstrated a close relationship between the cell Young׳s modulus and metastatic potential of HHCs, which suggest that Young׳s modulus detected by AFM can be used to evaluate metastatic potential of cancer cells.

Dephosphorylation of Iqg1 by Cdc14 regulates cytokinesis in budding yeast

Cytokinesis separates cells by contraction of a ring composed of filamentous actin (F-actin) and type II myosin. Iqg1, an IQGAP family member, is an essential protein in Saccharomyces cerevisiae required for assembly and contraction of the actomyosin ring. Localization of F-actin to the ring occurs only after anaphase and is mediated by the calponin homology domain (CHD) of Iqg1, but the regulatory mechanisms that temporally restrict actin ring assembly are not well defined. We tested the hypothesis that dephosphorylation of four perfect cyclin-dependent kinase (Cdk) sites flanking the CHD promotes actin ring formation, using site-specific alanine mutants. Cells expressing the nonphosphorylatable iqg1-4A allele formed actin rings before anaphase and exhibited defects in myosin contraction and cytokinesis. The Cdc14 phosphatase is required for normal cytokinesis and acts on specific Cdk phosphorylation sites. Overexpression of Cdc14 resulted in premature actin ring assembly, whereas inhibition of Cdc14 function prevented actin ring formation. Cdc14 associated with Iqg1, dependent on several CHD-flanking Cdk sites, and efficiently dephosphorylated these sites in vitro. Of importance, the iqg1-4A mutant rescued the inability of cdc14-1 cells to form actin rings. Our data support a model in which dephosphorylation of Cdk sites around the Iqg1 CHD by Cdc14 is both necessary and sufficient to promote actin ring formation. Temporal control of actin ring assembly by Cdk and Cdc14 may help to ensure that cytokinesis onset occurs after nuclear division is complete.

Overexpression of LIMK1 promotes tumor growth and metastasis in gastric cancer

Gastric cancer is the second-leading cause of cancer death in Asia. Despite improvement of therapies, the outcome in patients remains extremely poor because of metastasis. In the present study, we found that LIMK1 is overexpressed in gastric cancer, and its expression level correlate with tumor size, lymph node metastasis and TNM stage. Knockdown of LIMK1 expression could inhibit cell proliferation, migration and invasion in vitro, as well as suppress the activation of FAK/paxillin pathway. Moreover, knockdown of LIMK1 expression retarded tumor growth and peritoneal ametastasis in vivo. This highlights that LIMK1 might be used as a potential target in the treatment of gastric cancer.

Using cultured mammalian cells to study mitosis

Diverse cell types have been used to study various aspects of mitosis. Early investigators focused primarily on cells that were suited to morphological studies. More recently, experimental systems have been developed to study both morphology and the molecular basis of chromosome motion and cell-cycle regulation. This article briefly reviews cell types that have been used to study mitosis in live cells. It then discusses cell lines that have been used to examine mitosis in cultured mammalian cells and summarizes the methods that are used to culture and study these cells.

Mitotic spindle orients perpendicular to the forces imposed by dynamic shear

Orientation of the division axis can determine cell fate in the presence of morphogenetic gradients. Understanding how mitotic cells integrate directional cues is therefore an important question in embryogenesis. Here, we investigate the effect of dynamic shear forces on confined mitotic cells. We found that human epithelial cells (hTERT-RPE1) as well as MC3T3 osteoblasts align their mitotic spindle perpendicular to the external force. Spindle orientation appears to be a consequence of cell elongation along the zero-force direction in response to the dynamic shear. This process is a nonlinear response to the strain amplitude, requires actomyosin activity and correlates with redistribution of myosin II. Mechanosteered cells divide normally, suggesting that this mechanism is compatible with biological functions.

Characterization of cell elasticity correlated with cell morphology by atomic force microscope

Biomechanical properties of cells have been identified as an important factor in a broad range of biological processes. Based on measurements of mechanical properties by atomic force microscopy (AFM) particularly cell elasticity has been linked with human diseases, such as cancer. AFM has been widely used as a nanomechanical tool to probe the elasticity of living cells, however, standard methods for characterizing cell elasticity are still lacking. The local elasticity of a cell is conventionally used to represent the mechanical property of the cell. However, since cells have highly heterogeneous regions, elasticity mapping over the entire cell, rather than at a few points of measurement, is required. Using human aortic endothelial cells (HAECs) as a model, we have developed in this study a new method to evaluate cell elasticity more quantitatively. Based on the height information of the cell, a new characterization method was proposed to evaluate the elasticity of a cell. Using this method, elasticities of cells on different substrates were compared. Results showed that the elasticity of HAECs on softer substrate also has higher value compared to those on harder substrate given a certain height where the statistical distribution analysis confirmed that higher actin filaments density was located. Thus, the elasticity of small portions of a cell could not represent the entire cell property and may lead to invalid characterization. In order to gain a more comprehensive and detailed understanding of biomechanical properties for future clinical use, elasticity and cell morphology should therefore be correlated with discussion.

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.

Immune pathology associated with altered actin cytoskeleton regulation

The actin cytoskeleton plays a crucial role in a variety of important cellular processes required for normal immune function, including locomotion, intercellular interactions, endocytosis, cytokinesis, signal transduction, and maintenance of cell morphology. Recent studies have uncovered not only many of the components and mechanisms that regulate the cortical actin cytoskeleton but have also revealed significant immunopathological consequences associated with genetic alteration of actin cytoskeletal regulatory genes. These advances have provided new insights into the role of cortical actin cytoskeletal regulation in a number of immune cell functions and have identified cytoskeletal regulatory proteins critical for normal immune system activity and susceptibility to autoimmunity.

Confocal and force probe imaging system for simultaneous three-dimensional optical and mechanical spectroscopic evaluation of biological samples

We present the design and operation of a novel instrument for the simultaneous three-dimensional measurements of localized properties using optical and mechanical probes. In this instrument the mechanical and optical probes are stationary relative to the instrument frame while the specimen can be navigated in three-dimensional space in the probing field, translating over a range of 64.5 microm by 49.7 microm by 31.5 microm in each axis, respectively, at closed loop speeds of 10 Hz. A large aperture is provided in the center of the moving platform so that an optical lens can image the specimen from below. An additional z-direction translator has been integrated with this instrument to independently move a force probe that contacts the specimen from above with a translation range of 16 microm. Furthermore, there is an additional seven degrees of freedom providing adjustments to independently position and/or align the scanner and force probe relative to the optical imaging lens. Initial results of both optical and mechanical scans demonstrate 6 nm localization from single molecule fluorescence measurements, as well as single pair fluorescence energy transfer measurements indicating molecular separations of about 2 nm.

Chemotaxis-mediated scission contributes to efficient cytokinesis in Dictyostelium

Interphase amoeba of Entamoeba invadens are attracted to the furrowing region of a neighboring dividing cell to assist with the division. A seemingly similar behavior has been observed in Dictyostelium discoideum, but in this case, it has not been shown whether the movements were truly directed toward the furrowing region or whether they have any relevance. We thus used myosin II-null cells, which spend more time than wild type cells in cytokinesis, and successfully demonstrated that nearly half of the division events involve the attraction of a neighbor cell to the furrowing region. Cells lacking the beta subunit of the trimeric G protein (Gbeta), which are incapable of chemotaxis, did not show such midwifery. Culturing wild type cells flattened under agarose sheets also slowed the cytokinesis process, and this allowed us to demonstrate that phosphatidylinositol trisphosphate was enriched in the anterior region of midwifing cells, consistent with the view that midwifery in D. discoideum is also chemotaxis. On substrates, while only 3.6% of wild type cells were multinucleate, 8.1% of Gbeta-null cells were multinucleate, and this was reduced to 3.4% when they were surrounded by wild type cells. Conversely, multinucleated wild type cells increased to 6.8% when they were surrounded by Gbeta-null cells. Thus, Gbeta-null cells frequently fail to divide because they cannot assist each other's division and midwifery ensures successful cytokinesis in Dictyostelium discoideum.

Calcium signalling during the cleavage period of zebrafish development

Imaging studies, using both luminescent and fluorescent Ca(2+)-sensitive reporters, have revealed that during the first few meroblastic cleavages of the large embryos of teleosts, localized elevations of intracellular Ca(2+) accompany positioning, propagation, deepening and apposition of the cleavage furrows. Here, we will review the Ca(2+) transients reported during the cleavage period in these embryos, with reference mainly to that of the zebrafish (Danio rerio). We will also present the latest findings that support the proposal that Ca(2+) transients are an essential feature of embryonic cytokinesis. In addition, the potential upstream triggers and downstream targets of the different cytokinetic Ca(2+) transients will be discussed. Finally, we will present a hypothetical model that summarizes what has been suggested to be the various roles of Ca(2+) signalling during cytokinesis in teleost embryos.

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

Equatorial organization of myosin II and actin has been recognized as a universal event in cytokinesis of animal cells. Current models for the formation of equatorial cortex favor either directional cortical transport toward the equator or localized de novo assembly. However, this process has never been analyzed directly in dividing mammalian cells at a high resolution. Here we applied total internal reflection fluorescence microscope (TIRF-M), coupled with spatial temporal image correlation spectroscopy (STICS) and a new analytical approach termed temporal differential microscopy (TDM), to image the dynamics of myosin II and actin during the assembly of equatorial cortex. Our results indicated distinct and at least partially independent mechanisms for the early equatorial recruitment of myosin and actin filaments. Cortical myosin showed no detectable directional flow during early cytokinesis. In addition to equatorial assembly, we showed that localized inhibition of disassembly contributed to the formation of the equatorial myosin band. In contrast to myosin, actin filaments underwent a striking flux toward the equator. Myosin motor activity was required for the actin flux, but not for actin concentration in the furrow, suggesting that there was a flux-independent, de novo mechanism for actin recruitment along the equator. Our results indicate that cytokinesis involves signals that regulate both assembly and disassembly activities and argue against mechanisms that are coupled to global cortical movements.

Gelsolin and diseases

Gelsolin is a calcium-activated actin filament severing and capping protein found in many cell types and as a secreted form in the plasma of vertebrates. Mutant mice for gelsolin as well as clinical studies have shown that gelsolin is linked to a number of pathological conditions such as inflammation, cancer and amyloidosis. The tight regulation of gelsolin by calcium is crucial for its physiological role and constitutive activation leads to apoptosis. In the following we will give an overview on how gelsolin is regulated by calcium, and which clinical conditions have been linked to lack or misregulation of gelsolin.

Global gene expression analysis of the effects of vinblastine on endothelial cells, when eluted from a thermo-responsive polymer

In-stent restenosis remains a significant problem associated with bare metal stents. This drawback has prompted research into improving stent design and the development of novel coatings, including drug-eluting stents. A number of drug-eluting stents are currently on the market; however, the success rate of these stents in complex situations has been found to be quite low. Thus, there remains potential for the development of more suitable drug-eluting stents. The aims of this study were to use a thermoresponsive polymer to develop a system to locally deliver vinblastine, an antimitotic agent currently used as an anticancer drug, and in addition, assess the effects of this drug at the gene expression level in vitro. An N-isopropylacrylamide/N-tert-butylacrylamide (NiPAAm/NtBAAm) copolymer solution in the ratio 65:35 was prepared and appropriate volumes of vinblastine were added to generate two final drug concentrations of 22 nanomoles/film or 0.022 nanomoles/film. Stainless steel discs (316) were coated with the copolymer solution or this solution containing drug. Human endothelial cells were cultured on collagen type 1 gels and then incubated with the coated discs for 24 h. Gene expression studies using oligonucleotide microarray analysis and quantitative RT-PCR were then performed. Microarray analysis revealed that vinblastine causes the differential expression of a range of genes involved in a variety of different functions, including cell cycle and apoptosis. The changes in expression of some of these genes culminate in cell cycle arrest and apoptotic pathways.

The yeast actin cytoskeleton: from cellular function to biochemical mechanism

All cells undergo rapid remodeling of their actin networks to regulate such critical processes as endocytosis, cytokinesis, cell polarity, and cell morphogenesis. These events are driven by the coordinated activities of a set of 20 to 30 highly conserved actin-associated proteins, in addition to many cell-specific actin-associated proteins and numerous upstream signaling molecules. The combined activities of these factors control with exquisite precision the spatial and temporal assembly of actin structures and ensure dynamic turnover of actin structures such that cells can rapidly alter their cytoskeletons in response to internal and external cues. One of the most exciting principles to emerge from the last decade of research on actin is that the assembly of architecturally diverse actin structures is governed by highly conserved machinery and mechanisms. With this realization, it has become apparent that pioneering efforts in budding yeast have contributed substantially to defining the universal mechanisms regulating actin dynamics in eukaryotes. In this review, we first describe the filamentous actin structures found in Saccharomyces cerevisiae (patches, cables, and rings) and their physiological functions, and then we discuss in detail the specific roles of actin-associated proteins and their biochemical mechanisms of action.

Cytokinesis in plant and animal cells: endosomes 'shut the door'

For many years, cytokinesis in eukaryotic cells was considered to be a process that took a variety of forms. This is rather surprising in the face of an apparently conservative mitosis. Animal cytokinesis was described as a process based on an actomyosin-based contractile ring, assembling, and acting at the cell periphery. In contrast, cytokinesis of plant cells was viewed as the centrifugal generation of a new cell wall by fusion of Golgi apparatus-derived vesicles. However, recent advances in animal and plant cell biology have revealed that many features formerly considered as plant-specific are, in fact, valid also for cytokinetic animal cells. For example, vesicular trafficking has turned out to be important not only for plant but also for animal cytokinesis. Moreover, the terminal phase of animal cytokinesis based on midbody microtubule activity resembles plant cytokinesis in that interdigitating microtubules play a decisive role in the recruitment of cytokinetic vesicles and directing them towards the cytokinetic spaces which need to be plugged by fusing endosomes. Presently, we are approaching another turning point which brings cytokinesis in plant and animal cells even closer. As an unexpected twist, new studies reveal that both plant and animal cytokinesis is driven not so much by Golgi-derived vesicles but rather by homotypically and heterotypically fusing endosomes. These are generated from cytokinetic cortical sites defined by preprophase microtubules and contractile actomyosin ring, which induce local endocytosis of both the plasma membrane and cell wall material. Finally, plant and animal cytokinesis meet together at the physical separation of daughter cells despite obvious differences in their preparatory events.

Wound-induced contractile ring: a model for cytokinesis

The actomyosin-based contractile ring is required for several biological processes, such as wound healing and cytokinesis of animal cells. Despite progress in defining the roles of this structure in both wound closure and cell division, we still do not fully understand how an actomyosin ring is spatially and temporally assembled, nor do we understand the molecular mechanism of its contraction. Recent results have demonstrated that microtubule-dependent local assembly of F-actin and myosin-II is present in wound closure and is similar to that in cytokinesis in animal cells. Furthermore, signalling factors such as small Rho GTPases have been shown to be involved in the regulation of actin dynamics during both processes. In this review we address recent findings in an attempt to better understand the dynamics of actomyosin contractile rings during wound healing as compared with the final step of animal cell division.

Behavior and function of paternally inherited centrioles in brown algal zygotes

In brown algal cells, the centrosome, consisting of a pair of centrioles and the pericentriolar material, is primarily involved in the organization of microtubules (MTs) throughout the cell cycle. In motile cells, the centrioles participate in the formation of flagellar axoneme as flagellar basal bodies, and in somatic cells they play a crucial role in many cellular activities as a part of the centrosome. With respect to the role of the centrosome as a microtubule organizing center (MTOC), brown algal cells resemble animal cells. In most animal fertilization processes, the sperm cell introduces centrioles, the core of the centrosome, into the egg cytoplasm. In this study, the behavior of centrioles from gametogenesis and fertilization to the first cell division of the zygote was examined in the three sexual reproduction patterns occurring in brown algae, i.e., oogamy, anisogamy and isogamy, by electron- and immunofluorescence-microscopy. The pair of centrioles contained in somatic cells was shown to be derived from the male gamete, irrespective of the sexual reproductive pattern. The paternally derived centrioles were duplicated before mitosis and were involved in spindle pole formation. Moreover, MTs from the centrosome play a crucial role in the process of cytokinesis, as the position of centrosomes accompanying daughter nuclei seems to determine the cytokinetic plane. A new approach to clarifying the mode of cytokinesis in brown algae is presented in this study.

Adhesion-dependent and contractile ring-independent equatorial furrowing during cytokinesis in mammalian cells

Myosin II-dependent contraction of the contractile ring drives equatorial furrowing during cytokinesis in animal cells. Nonetheless, myosin II-null cells of the cellular slime mold Dictyostelium divide efficiently when adhering to substrates by making use of polar traction forces. Here, we show that in the presence of 30 microM blebbistatin, a potent myosin II inhibitor, normal rat kidney (NRK) cells adhering to fibronectin-coated surfaces formed equatorial furrows and divided in a manner strikingly similar to myosin II-null Dictyostelium cells. Such blebbistatin-resistant cytokinesis was absent in partially detached NRK cells and was disrupted in adherent cells if the advance of their polar lamellipodia was disturbed by neighboring cells. Y-27632 (40 microM), which inhibits Rho-kinase, was similar to 30 microM blebbistatin in that it inhibited cytokinesis of partially detached NRK cells but only prolonged furrow ingression in attached cells. In the presence of 100 microM blebbistatin, most NRK cells that initiated anaphase formed tight furrows, although scission never occurred. Adherent HT1080 fibrosarcoma cells also formed equatorial furrows efficiently in the presence of 100 microM blebbistatin. These results provide direct evidence for adhesion-dependent, contractile ring-independent equatorial furrowing in mammalian cells and demonstrate the importance of substrate adhesion for cytokinesis.

The association of calmodulin with central spindle regulates the initiation of cytokinesis in HeLa cells

Calmodulin is a major cytoplasmic calcium receptor that performs multiple functions in the cell including cytokinesis. Central spindle appears between separating chromatin masses after metaphase-anaphase transition. The interaction of microtubules from central spindle with cell cortex regulates the cleavage furrow formation. In this paper, we use green fluorescence protein (GFP)-tagged calmodulin as a living cell probe to examine the detailed dynamic redistribution and co-localization of calmodulin with central spindle during cytokinesis and the function of this distribution pattern in a tripolar HeLa cell model. We found that calmodulin is associated with spindle microtubules during mitosis and begins to aggregate with central spindle after anaphase initiation. The absence of either central spindle or central spindle-distributed calmodulin is correlated with the defect in the formation of cleavage furrow, where contractile ring-distributed CaM is also extinct. Further analysis found that both the assembly of central spindle and the formation of cleavage furrow are affected by the W7 treatment. The microtubule density of central spindle was decreased after the treatment. Only less than 10% of the synchronized cells enter cytokinesis when treated with 25 microM W7, and the completion time of furrow regression is also delayed from 10 min to at least 40 min. It is suggested that calmodulin plays a significant role in cytokinesis including furrow formation and regression, The understanding of the interaction between calmodulin and microtubules may give us insight into the mechanism through which calmodulin regulates cytokinesis.

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.

Dynamics of Yeast Myosin I

Cortical actin patches are dynamic structures required for endocytosis in yeast. Recent studies have shown that components of cortical patches localize to the plasma membrane in a precisely orchestrated manner, and their movements at and away from the plasma membrane may define the endocytic membrane invagination and vesicle scission events, respectively. Here, through live-cell imaging, we analyze the dynamics of the highly conserved class I unconventional myosin, Myo5, which also localizes to cortical patches and is known to be involved in endocytosis and actin nucleation. Myo5 exhibits a pattern of dynamic localization different from all cortical patch components analyzed to date. Myo5 associates with cortical patches only transiently and remains stationary during its brief cortical lifespan. The peak of Myo5 association with cortical patches immediately precedes the fast movement of Arp2/3 complex-associated structures away from the plasma membrane, thus correlating precisely with the proposed vesicle scission event. To further test the role of Myo5, we generated a temperature-sensitive mutant myo5 allele. In the myo5 mutant cells, Myo5 exhibits a significantly extended cortical lifespan as a result of a general impairment of Myo5 function, and Arp2 patches exhibit an extended slow-movement phase prior to the fast movement toward the cell interior. The myo5 mutant cells are defective in fluid-phase endocytosis and exhibit an increased number of invaginations on the membrane. Based on these results, we hypothesize that the myosin I motor protein facilitates the membrane fusion/vesicle scission event of endocytosis.

Multiple Parallelisms in Animal Cytokinesis

The process of cytokinesis in animal cells is usually presented as a relatively simple picture: A cleavage plane is first positioned in the equatorial region by the astral microtubules of the anaphase mitotic apparatus, and a contractile ring made up of parallel filaments of actin and myosin II is formed and encircles the cortex at the division site. Active sliding between the two filament systems constricts the perimeter of the cortex, leading to separation of two daughter cells. However, recent studies in both animal cells and lower eukaryotic model organisms have demonstrated that cytokinesis is actually far more complex. It is now obvious that the three key processes of cytokinesis, cleavage plane determination, equatorial furrowing, and scission, are driven by different mechanisms in different types of cells. In some cases, moreover, multiple pathways appear to have redundant functions in a single cell type. In this review, we present a novel hypothesis that incorporates recent observations on the activities of mitotic microtubules and the biochemistry of Rho-type GTPase proteins and postulates that two different sets of microtubules are responsible for the two known mechanisms of cleavage plane determination and also for two distinct mechanisms of equatorial furrowing.

Actin cytoskeleton remodeling during early Drosophila furrow formation requires recycling endosomal components Nuclear-fallout and Rab11

Cytokinesis requires a dramatic remodeling of the cortical cytoskeleton as well as membrane addition. The Drosophila pericentrosomal protein, Nuclear-fallout (Nuf), provides a link between these two processes. In nuf-derived embryos, actin remodeling and membrane recruitment during the initial stages of metaphase and cellular furrow formation are disrupted. Nuf is a homologue of arfophilin-2, an ADP ribosylation factor effector that binds Rab11 and influences recycling endosome (RE) organization. Here, we show that Nuf is an important component of the RE, and that these phenotypes are a consequence of Nuf activities at the RE. Nuf exhibits extensive colocalization with Rab11, a key RE component. GST pull-downs and the presence of a conserved Rab11-binding domain in Nuf demonstrate that Nuf and Rab11 physically associate. In addition, Nuf and Rab11 are mutually required for their localization to the RE. Embryos with reduced levels of Rab11 produce membrane recruitment and actin remodeling defects strikingly similar to nuf-derived embryos. These analyses support a common role for Nuf and Rab11 at the RE in membrane trafficking and actin remodeling during the initial stages of furrow formation.

Actomyosin transports microtubules and microtubules control actomyosin recruitment during Xenopus oocyte wound healing

BACKGROUND

Interactions between microtubules and actin filaments (F-actin) are critical for cellular motility processes ranging from directed cell locomotion to cytokinesis. However, the cellular bases of these interactions remain poorly understood. We have analyzed the role of microtubules in generation of a contractile array comprised of F-actin and myosin-2 that forms around wounds made in Xenopus oocytes.

RESULTS

After wounding, microtubules are transported to the wound edge in association with F-actin that is itself recruited to wound borders via actomyosin-powered cortical flow. This transport generates sufficient force to buckle and break microtubules at the wound edge. Transport is complemented by local microtubule assembly around wound borders. The region of microtubule breakage and assembly coincides with a zone of actin assembly, and perturbation of the microtubule cytoskeleton disrupts this zone as well as local recruitment of the Arp2/3 complex and myosin-2.

CONCLUSIONS

The results reveal transport of microtubules in association with F-actin that is pulled to wound borders via actomyosin-based contraction. Microtubules, in turn, focus zones of actin assembly and myosin-2 recruitment at the wound border. Thus, wounding triggers the formation of a spatially coordinated feedback loop in which transport and assembly of microtubules maintains actin and myosin-2 in close proximity to the closing contractile array. These results are surprisingly reminiscent of recent findings in locomoting cells, suggesting that similar feedback interactions may be generally employed in a variety of fundamental cell motility processes.

Identification of a novel microtubule-associated protein that regulates microtubule organization and cytokinesis by using a GFP-screening strategy

Microtubules play critical roles in a variety of cell processes, including mitosis, organelle transport, adhesion and migration, and the maintenance of cell polarity. Microtubule-associated proteins (MAPs) regulate the dynamic organization and stability of microtubules, often through either cell-specific or cell division stage-specific interactions. To identify novel cytoskeletal-associated proteins and peptides that regulate microtubules and other cytoskeletal and adhesive structures, we have developed a GFP cDNA screening strategy based on identifying gene products that localize to these structures. Using this approach, we have identified a novel MAP, GLFND, that shows homology to the Opitz syndrome gene product [6], localizes to a subpopulation of microtubules that are acetylated, and protects microtubules from depolymerization with nocodazole. Expression of an N-terminal deletion binds microtubules but alters their organization. During the cell cycle, GLFND dissociates from microtubules at the beginning of mitosis and then reassociates at cytokinesis. Furthermore, ectopic expression of GLFND inhibits cell division and cytokinesis in CHO cells. These observations make GLFND unique among MAPs characterized thus far.

An IQGAP-related protein, encoded by AgCYK1, is required for septation in the filamentous fungus Ashbya gossypii

In filamentous ascomycetes hyphae are compartmentalized by septation in which the cytoplasm of the compartments are interconnected via septal pores. Thus, septation in filamentous fungi is different from cytokinesis in yeast like fungi. We have identified an Ashbya gossypii orthologue of the Saccharomyces cerevisiae CYK1 gene which belongs to the IQGAP-protein family. In contrast to S. cerevisiae disruption of AgCYK1 yields viable mutant strains that exhibit wildtype-like polarized hyphal growth rates. In the Agcyk1 mutant cortical actin patches localize to growing hyphal tips like wildtype, however, mutant hyphae are totally devoid of actin rings at presumptive septal sites. Septation in wildtype results in the formation of chitin rings. Agcyk1 mutant hyphae are aseptate and do not accumulate chitin in their cell walls. Agcyk1 mutant strains are completely asporogenous indicating that septation is essential for the formation of sporangia in A. gossypii. AgCyk1p-GFP localizes to sites of future septation as a ring prior to chitin depositioning. Furthermore, decrease in Cyk1p-ring diameter was found to be a prerequisite for the accumulation of chitin and septum formation.

Nir2, a human homolog of Drosophila melanogaster retinal degeneration B protein, is essential for cytokinesis

Cytokinesis, the final stage of eukaryotic cell division, ensures the production of two daughter cells. It requires fine coordination between the plasma membrane and cytoskeletal networks, and it is known to be regulated by several intracellular proteins, including the small GTPase Rho and its effectors. In this study we provide evidence that the protein Nir2 is essential for cytokinesis. Microinjection of anti-Nir2 antibodies into interphase cells blocks cytokinesis, as it results in the production of multinucleate cells. Immunolocalization studies revealed that Nir2 is mainly localized in the Golgi apparatus in interphase cells, but it is recruited to the cleavage furrow and the midbody during cytokinesis. Nir2 colocalizes with the small GTPase RhoA in the cleavage furrow and the midbody, and it associates with RhoA in mitotic cells. Its N-terminal region, which contains a phosphatidylinositol transfer domain and a novel Rho-inhibitory domain (Rid), is required for normal cytokinesis, as overexpression of an N-terminal-truncated mutant blocks cytokinesis completion. Time-lapse videomicroscopy revealed that this mutant normally initiates cytokinesis but fails to complete it, due to cleavage furrow regression, while Rid markedly affects cytokinesis due to abnormal contractility. Rid-expressing cells exhibit aberrant ingression and ectopic cleavage sites; the cells fail to segregate into daughter cells and they form a long unseparated bridge-like cytoplasmic structure. These results provide new insight into the cellular functions of Nir2 and introduce it as a novel regulator of cytokinesis.

Influence of the centrosome in cytokinesis of brown algae: polyspermic zygotes of Scytosiphon lomentaria (Scytosiphonales, Phaeophyceae)

We examined the relationship between the spindle orientation and the determination site of cytokinesis in brown algal cells using polyspermic zygotes of Scytosiphon lomentaria. When two male gametes fuse with one female gamete, the zygote has two pairs of centrioles derived from male gametes and three chloroplasts from two male and one female gametes. Just before mitosis, two pairs of centrioles duplicate and migrate towards the future mitotic poles. Spindle MTs develop and three or four spindle poles are formed. In a tri-polar spindle, one pair of centrioles shifts away from the spindle, otherwise, two pairs of centrioles exist adjoining at one spindle pole. Chromosomes arrange at several equators of the spindle. As a result of these multipolar mitoses, three or four daughter nuclei developed. Subsequently, these daughter nuclei form a line along the long axis of the cell. Cell partition always takes place between daughter nuclei, perpendicular to the long axis of the cell. Three or four daughter cells are produced by cytokinesis. Some of the daughter cells after cytokinesis do not have a nucleus, but all of them always contain the centrosome and chloroplast. Therefore, the number of daughter cells always coincides with the number of centrosomes or microtubule organizing centers (MTOCs). These results show that the cytokinetic plane in the brown algae is determined by the position of centrosomes after mitosis and is not dependent on the spindle position.

Mitosis-specific activation of LIM motif-containing protein kinase and roles of cofilin phosphorylation and dephosphorylation in mitosis

Actin filament dynamics play a critical role in mitosis and cytokinesis. LIM motif-containing protein kinase 1 (LIMK1) regulates actin reorganization by phosphorylating and inactivating cofilin, an actin-depolymerizing and -severing protein. To examine the role of LIMK1 and cofilin during the cell cycle, we measured cell cycle-associated changes in the kinase activity of LIMK1 and in the level of cofilin phosphorylation. Using synchronized HeLa cells, we found that LIMK1 became hyperphosphorylated and activated in prometaphase and metaphase, then gradually returned to the basal level as cells entered into telophase and cytokinesis. Although Rho-associated kinase and p21-activated protein kinase phosphorylate and activate LIMK1, they are not likely to be involved in mitosis-specific activation and phosphorylation of LIMK1. Immunoblot and immunofluorescence analyses using an anti-phosphocofilin-specific antibody revealed that the level of cofilin phosphorylation, similar to levels of LIMK1 activity, increased during prometaphase and metaphase then gradually declined in telophase and cytokinesis. Ectopic expression of LIMK1 increased the level of cofilin phosphorylation throughout the cell cycle and induced the formation of multinucleate cells. These results suggest that LIMK1 is involved principally in control of mitosis-specific cofilin phosphorylation and that dephosphorylation and reactivation of cofilin at later stages of mitosis play a critical role in cytokinesis of mammalian cells.

Mapping the G-actin binding surface of cofilin using synchrotron protein footprinting

Cofilin is an actin regulatory protein that binds to both monomeric and filamentous actin, and has filament severing activity. Although crystal structures for the monomeric forms of both G-actin and cofilin have been described, the structure of the binary cofilin-G-actin complex is not available. Synchrotron protein footprinting is used to identify specific side chain residues on the cofilin surface that are buried in the formation of the cofilin-G-actin binary complex. Exposure to synchrotron X-rays results in stable oxidative modifications of aromatic, aliphatic, and sulfur-containing side chains, with the rate of modification for a particular residue being dependent on its intrinsic reactivity and solvent accessibility. The rates of modification were monitored for a number of peptides generated by digestion of oxidized cofilin, both in isolation and in its binary complex with G-actin. After binding to G-actin takes place, a significant decrease in modification rates, indicating protection of side chain groups, is seen for cofilin peptides corresponding to residues 4-20, 10-17, 83-96, 91-105, and 106-117. A number of other peptides show no change in reactivity, and are presumed to represent regions distal to the binding site. Tandem mass spectrometry demonstrates that residues Leu 13, Pro 94, Met 99, and Leu 108 and 112 directly participate in the binding interface. These results are generally consistent with, and complementary to, the results of previous site-directed mutagenesis studies and extend our understanding of the G-actin binding surface of cofilin.

Inhibition of anchorage-independent growth of transformed NIH3T3 cells by epithelial protein lost in neoplasm (EPLIN) requires localization of EPLIN to actin cytoskeleton

Epithelial protein lost in neoplasm (EPLIN) is a cytoskeleton-associated protein characterized by the presence of a single centrally located lin-11, isl-1, and mec-3 (LIM) domain. We have reported previously that EPLIN is down-regulated in transformed cells. In this study, we have investigated whether ectopic expression of EPLIN affects transformation. In untransformed NIH3T3 cells, retroviral-mediated transduction of EPLIN did not alter the cell morphology or growth. NIH3T3 cells expressing EPLIN, however, failed to form colonies when transformed by the activated Cdc42 or the chimeric nuclear oncogene EWS/Fli-1. This suppression of anchorage-independent growth was not universal because EPLIN failed to inhibit the colony formation of Ras-transformed cells. Interestingly, the localization of EPLIN to the actin cytoskeleton was maintained in the EWS/Fli-1- or Cdc42-transformed cells, but not in Ras-transformed cells where it was distributed heterogeneously in the cytoplasm. Using truncated EPLIN constructs, we demonstrated that the NH(2)-terminal region of EPLIN is necessary for both the localization of EPLIN to the actin cytoskeleton and suppression of anchorage-independent growth of EWS/Fli-1-transformed cells. The LIM domain or the COOH-terminal region of EPLIN could be deleted without affecting its cytoskeletal localization or ability to suppress anchorage-dependent growth. Our study indicates EPLIN may function in growth control by associating with and regulating the actin cytoskeleton.

Cytokinesis in budding yeast: the relationship between actomyosin ring function and septum formation

Cytokinesis in budding yeast is accomplished by the concerted action of actomyosin ring function and septum formation. The actomyosin ring is not essential for cell viability, but it is required for efficient cell division. Deletion of the actomyosin ring results in abnormal septum formation, and a delay in cytokinesis and cell separation. In contrast, septum formation is essential for cell viability. Block of septum formation prevents the contraction, but not the formation of the actomyosin ring. Here we review and provide additional evidence that defines the functional and molecular relationship between actomyosin ring function and septum formation.

On the mechanism of cleavage furrow ingression in Dictyostelium

The ability of Dictyostelium cells to divide without myosin II in a cell cycle-coupled manner has opened two questions about the mechanism of cleavage furrow ingression. First, are there other possible functions for myosin II in this process except for generating contraction of the furrow by a sliding filament mechanism? Second, what could be an alternative mechanical basis for the furrowing? Using aberrant changes of the cell shape and anomalous localization of the actin-binding protein cortexillin I during asymmetric cytokinesis in myosin II-deficient cells as clues, it is proposed that myosin II filaments act as a mechanical lens in cytokinesis. The mechanical lens serves to focus the forces that induce the furrowing to the center of the midzone, a cortical region where cortexillins are enriched in dividing cells. Additionally, continual disassembly of a filamentous actin meshwork at the midzone is a prerequisite for normal ingression of the cleavage furrow and a successful cytokinesis. If this process is interrupted, as it occurs in cells that lack cortexillins, an overassembly of filamentous actin at the midzone obstructs the normal cleavage. Disassembly of the crosslinked actin network can generate entropic contractile forces in the cortex, and may be considered as an alternative mechanism for driving ingression of the cleavage furrow. Instead of invoking different types of cytokinesis that operate under attached and unattached conditions in Dictyostelium, it is anticipated that these cells use a universal multifaceted mechanism to divide, which is only moderately sensitive to elimination of its constituent mechanical processes.

Reduced protein diffusion rate by cytoskeleton in vegetative and polarized dictyostelium cells

Fluorescence recovery after photobleaching measurements with high spatial resolution are performed to elucidate the impact of the actin cytoskeleton on translational mobility of green fluorescent protein (GFP) in aqueous domains of Dictyostelium discoideum amoebae. In vegetative Dictyostelium cells, GFP molecules experience a 3.6-fold reduction of their translational mobility relative to dilute aqueous solutions. In disrupting the actin filamentous network using latrunculin-A, the intact actin cytoskeletal network is shown to contribute an effective viscosity of 1.36 cP, which accounts for 53% of the restrained molecular diffusion of GFP. The remaining 47% of hindered protein motions is ascribed to other mechanical barriers and the viscosity of the cell liquid. A direct correlation between the density of the actin network and its limiting action on protein diffusion is furthermore established from measurements under different osmotic conditions. In highly locomotive polarized cells, the obstructing effect of the actin filamentous network is seen to decline to 0.46 cP in the non-cortical regions of the cell. Our results indicate that the meshwork of actin filaments constitutes the primary mechanical barrier for protein diffusion and that any noticeable reorganization of the network is accompanied by altered intracellular protein mobility.

Rho-dependent transfer of Citron-kinase to the cleavage furrow of dividing cells

Citron-kinase (Citron-K) is a Rho effector working in cytokinesis. It is enriched in cleavage furrow, but how Rho mobilizes Citron-K remains unknown. Using anti-Citron antibody and a Citron-K Green Fluorescence Protein (GFP)-fusion, we monitored its localization in cell cycle. We have found: (1) Citron-K is present as aggregates in interphase cells, disperses throughout the cytoplasm in prometaphase, translocates to cell cortex in anaphase and accumulates in cleavage furrow in telophase; (2) Rho colocalizes with Citron-K in the cortex of ana- to telophase cells and the two proteins are concentrated in the cleavage furrow and to the midbody; (3) inactivation of Rho by C3 exoenzyme does not affect the dispersion of Citron-K in prometaphase, but prevented its transfer to the cell cortex, and Citron-K stays in association with the midzone spindles of C3 exoenzyme-treated cells. To clarify further the mechanism of the Rho-mediated transfer and concentration of Citron-K in cleavage furrow, we expressed active Val14RhoA in interphase cells expressing GFP-Citron-K. Val14RhoA expression transferred Citron-K to the ventral cortex of interphase cells, where it formed band-like structures in a complex with Rho. This structure was localized at the same plane as actin stress fibers, and they exclude each other. Disruption of F-actin abolished the band and dispersed the Citron-K-Rho-containing patches throughout the cell cortex. Similarly, in dividing cells, a structure composed of Rho and Citron-K in cleavage furrow excludes cortical actin cytoskeleton, and disruption of F-actin disperses Citron-K throughout the cell cortex. These results suggest that Citron-K is a novel type of a passenger protein, which is dispersed to the cytoplasm in prometaphase and associated with midzone spindles by a Rho-independent signal. Rho is then activated, binds to Citron-K and translocates it to cell cortex, where the complex is then concentrated in the cleavage furrow by the action of actin cytoskeleton beneath the equator of dividing cells.

Direct, high-resolution measurement of furrow stiffening during division of adherent cells

It is unclear whether cell division is driven by cortical relaxation outside the equatorial region or cortical contractility within the developing furrow alone. To approach this question, a technique is required that can monitor spatially-resolved changes in cortical stiffness with good time resolution. We employed atomic force microscopy (AFM), in force-mapping mode, to track dynamic changes in the stiffness of the cortex of adherent cultured cells along a single scan-line during M phase, from metaphase to cytokinesis. Video microscopy, which we used to correlate the AFM data with mitotic events identified by light microscopy, indicated that the AFM force-mapping technique does not perturb dividing cells. Here we show that cortical stiffening occurs over the equatorial region about 160 seconds before any furrow appears, and that this stiffening markedly increases as the furrow starts. By contrast, polar relaxation of cells does not seem to be an obligatory event for cell division to occur.

CYTOKINESIS AND BUILDING OF THE CELL PLATE IN PLANTS

Cytokinesis in plant cells is more complex than in animals, as it involves building a cell plate as the final step in generating two cells. The cell plate is built in the center of phragmoplast by fusion of Golgi-derived vesicles. This step imposes an architectural problem where ballooning of the fused structures has to be avoided to create a plate instead. This is apparently achieved by squeezing the vesicles into dumbbell-shaped vesicle-tubule-vesicle (VTV) structures with the help of phragmoplastin, a homolog of dynamin. These structures are fused at their ends in a star-shaped body creating a tubulovesicular "honeycomb-like" structure sandwiched between the positive ends of the phragmoplast microtubules. This review summarizes our current understanding of various mechanisms involved in budding-off of Golgi vesicles, delivery and fusion of vesicles to initiate cell plate, and the synthesis of polysaccharides at the forming cell plate. Little is known about the molecular mechanisms involved in determining the site, direction, and the point of attachment of the growing cell plate with the parental cell wall. These gaps may be filled soon, as many genes that have been identified by mutations are analyzed and functions of their products are deciphered.

Cytokinesis in prokaryotes and eukaryotes: common principles and different solutions

Cytokinesis requires duplication of cellular structures followed by bipolarization of the predivisional cell. As a common principle, this applies to prokaryotes as well as eukaryotes. With respect to eukaryotes, the discussion has focused mainly on Saccharomyces cerevisiae and on Schizosaccharomyces pombe. Escherichia coli and to a lesser extent Bacillus subtilis have been used as prokaryotic examples. To establish a bipolar cell, duplication of a eukaryotic origin of DNA replication as well as its genome is not sufficient. Duplication of the microtubule-organizing center is required as a prelude to mitosis, and it is here that the dynamic cytoskeleton with all its associated proteins comes to the fore. In prokaryotes, a cytoskeleton that pervades the cytoplasm appears to be absent. DNA replication and the concomitant DNA segregation seem to occur without help from extensive cytosolic supramacromolecular assemblies but with help from the elongating cellular envelope. Prokaryotic cytokinesis proceeds through a contracting ring, which has a roughly 100-fold-smaller circumference than its eukaryotic counterpart. Although the ring contains proteins that can be considered as predecessors of actin, tubulin, and microtubule-associated proteins, its macromolecular composition is essentially different.

The Tem1 small GTPase controls actomyosin and septin dynamics during cytokinesis

Cytokinesis in budding yeast involves an actomyosin-based ring which assembles in a multistepped fashion during the cell cycle and constricts during cytokinesis. In this report, we have investigated the structural and regulatory events that occur at the onset of cytokinesis. The septins, which form an hour-glass like structure during early stages of the cell cycle, undergo dynamic rearrangements prior to cell division: the hourglass structure splits into two separate rings. The contractile ring, localized between the septin double rings, immediately undergoes contraction. Septin ring splitting is independent of actomyosin ring contraction as it still occurs in mutants where contraction fails. We hypothesize that septin ring splitting may remove a structural barrier for actomyosin ring to contract. Because the Tem1 small GTPase (Tem1p) is required for the completion of mitosis, we investigated its role in regulating septin and actomyosin ring dynamics in the background of the net1-1 mutation, which bypasses the anaphase cell cycle arrest in Tem1-deficient cells. We show that Tem1p plays a specific role in cytokinesis in addition to its function in cell cycle progression. Tem1p is not required for the assembly of the actomyosin ring but controls actomyosin and septin dynamics during cytokinesis.

Cytoskeletal reorganization during the formation of oligodendrocyte processes and branches

During oligodendrocyte development, signals relevant to process formation must be transduced into appropriate changes in cytoskeletal organization. We have explored how microtubules and microfilaments interact during the outgrowth and branching of oligodendrocyte processes in culture. We observed that microfilaments are enriched in the peripheral region beneath the plasma membrane and constitute the major cytoskeletal element at the leading edge of the process, which is also enriched in the B-isoform of the non-muscle myosin II heavy chain. Microtubules form a dense bundle within the process and splay before extending into the leading edge and branches, following tracks laid by microfilaments. Pharmacologic disruption of microfilaments and microtubules compromised normal process outgrowth and branching. However, microtubules rapidly reinvaded most processes after removal of both antimicrotubule and antimicrofilament drugs, but the reinvasion was severely compromised if the antimicrofilament drug was retained. These results are consistent with the hypothesis that microfilaments guide the local reorganization of microtubules for the elongation of oligodendrocyte processes and the formation of new branches.

Occludin and claudin-1 concentrate in the midbody of immortalized mouse hepatocytes during cell division

It has been believed that epithelial cells maintain tight junctions at all times, including during cell division, to provide a continuous epithelial seal. However, changes in localization of integral tight junction proteins during cell division have not been examined. In this study, using SV40-immortalized mouse hepatocytes transfected with human Cx32 cDNA, in which tight junction strands and the endogenous tight junction proteins occludin, claudin-1, ZO-1, and ZO-2 were induced, we examined changes in localization of the tight junction proteins at all stages of cell division. All tight junction proteins were present between mitotic cells and neighboring cells throughout cell division. In late telophase, the integral tight junction proteins occludin and claudin-1, but not the cytoplasmic proteins ZO-1 and ZO-2, were concentrated in the midbody between the daughter cells and were observed at cell borders between the daugher and neighboring cells. These results indicate that the integral tight junction proteins are regulated in a different manner from the cytoplasmic proteins ZO-1 and ZO-2 during cytokinesis.

A novel function of Saccharomyces cerevisiae CDC5 in cytokinesis

Coordination of mitotic exit with timely initiation of cytokinesis is critical to ensure completion of mitotic events before cell division. The Saccharomyces cerevisiae polo kinase Cdc5 functions in a pathway leading to the degradation of mitotic cyclin Clb2, thereby permitting mitotic exit. Here we provide evidence that Cdc5 also plays a role in regulating cytokinesis and that an intact polo-box, a conserved motif in the noncatalytic COOH-terminal domain of Cdc5, is required for this event. Depletion of Cdc5 function leads to an arrest in cytokinesis. Overexpression of the COOH-terminal domain of Cdc5 (cdc5DeltaN), but not the corresponding polo-box mutant, resulted in connected cells. These cells shared cytoplasms with incomplete septa, and possessed aberrant septin ring structures. Provision of additional copies of endogenous CDC5 remedied this phenotype, suggesting a dominant-negative inhibition of cytokinesis. The polo-box-dependent interactions between Cdc5 and septins (Cdc11 and Cdc12) and genetic interactions between the dominant-negative cdc5DeltaN and Cyk2/Hof1 or Myo1 suggest that direct interactions between cdc5DeltaN and septins resulted in inhibition of Cyk2/Hof1- and Myo1-mediated cytokinetic pathways. Thus, we propose that Cdc5 may coordinate mitotic exit with cytokinesis by participating in both anaphase promoting complex activation and a polo-box-dependent cytokinetic pathway.

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.

Conditional expression of a truncated fragment of nonmuscle myosin II-A alters cell shape but not cytokinesis in HeLa cells

A truncated fragment of the nonmuscle myosin II-A heavy chain (NMHC II-A) lacking amino acids 1-591, delta N592, was used to examine the cellular functions of this protein. Green fluorescent protein (GFP) was fused to the amino terminus of full-length human NMHC II-A, NMHC II-B, and delta N592 and the fusion proteins were stably expressed in HeLa cells by using a conditional expression system requiring absence of doxycycline. The HeLa cell line studied normally expressed only NMHC II-A and not NMHC II-B protein. Confocal microscopy indicated that the GFP fusion proteins of full-length NMHC II-A, II-B, and delta N592 were localized to stress fibers. However, in vitro assays showed that baculovirus-expressed delta N592 did not bind to actin, suggesting that delta N592 was localized to actin stress fibers through incorporation into endogenous myosin filaments. There was no evidence for the formation of heterodimers between the full-length endogenous nonmuscle myosin and truncated nonmuscle MHCs. Expression of delta N592, but not full-length NMHC II-A or NMHC II-B, induced cell rounding with rearrangement of actin filaments and disappearance of focal adhesions. These cells returned to their normal morphology when expression of delta N592 was repressed by addition of doxycycline. We also show that GFP-tagged full-length NMHC II-A or II-B, but not delta N592, were localized to the cytokinetic ring during mitosis, indicating that, in vertebrates, the amino-terminus part of mammalian nonmuscle myosin II may be necessary for localization to the cytokinetic ring.

A phospholipid kinase regulates actin organization and intercellular bridge formation during germline cytokinesis

The endgame of cytokinesis can follow one of two pathways depending on developmental context: resolution into separate cells or formation of a stable intercellular bridge. Here we show that the four wheel drive (fwd) gene of Drosophila melanogaster is required for intercellular bridge formation during cytokinesis in male meiosis. In fwd mutant males, contractile rings form and constrict in dividing spermatocytes, but cleavage furrows are unstable and daughter cells fuse together, producing multinucleate spermatids. fwd is shown to encode a phosphatidylinositol 4-kinase (PI 4-kinase), a member of a family of proteins that perform the first step in the synthesis of the key regulatory membrane phospholipid PIP2. Wild-type activity of the fwd PI 4-kinase is required for tyrosine phosphorylation in the cleavage furrow and for normal organization of actin filaments in the constricting contractile ring. Our results suggest a critical role for PI 4-kinases and phosphatidylinositol derivatives during the final stages of cytokinesis.