Cytokinesis in eukaryotes.

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

Cytokinesis in Eukaryotes

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

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.     

Variations on a theme: the many modes of cytokinesis

Animal cell division is believed to be mediated primarily by the 'purse-string' mechanism, which entails furrowing of the equatorial region, driven by the interaction of actin and myosin II filaments within contractile rings. However, myosin II-null Dictyostelium cells on substrates divide efficiently in a cell cycle-coupled manner. This process, termed cytokinesis B, appears to be driven by polar traction forces. Data in the literature can be interpreted as suggesting that adherent higher animal cells also use a cytokinesis B-like mechanism for cytokinesis. An additional chemotaxis-based cytokinesis that involves a 'midwife' cell has also been reported. Collectively, these findings demonstrate an unexpected diversity of mechanisms by which animal cells carry out cytokinesis.     

Septins: traffic control at the cytokinesis intersection

The physical division of one cell into two requires the highly orchestrated separation of genetic and cytoplasmic contents during M phase of the cell cycle. Mitosis, the physical segregation of the genetic material of a cell into two daughter cells, has traditionally received more attention than cytokinesis, the partitioning of the cytoplasmic contents, yet clearly the two processes must be intimately co-ordinated and tightly regulated. While plant cells divide by the formation of a membranous cell barrier called the phragmoplast, animal cell division is largely driven by contraction of an actomyosin ring. However, recent evidence has suggested that membranes derived from one or more intracellular compartments are also required to break the cytoplasmic bridge connecting two dividing cells during late telophase. In this review, we focus on studies of animal cell cytokinesis that support a requirement for specific endomembrane fusion during fission, define molecular components of the membrane fusion apparatus that may be involved and point to possible roles for an emerging family of cytoskeletal proteins, the septins, in this process.     

Cytokinesis in Coleochaete orbicularis (Charophyceae): an ancestral mechanism inherited by plants

Recently, highly vacuolate cells of Arabidopsis were shown to exhibit "polarized" cytokinesis, in which the phragmoplast and cell plate contact the mother cell wall and then progress from one side of the cell to the other, rather than forming uniformly outward from the cell center (Cutler and Ehrhardt, 2002, Proceedings of the National Academy of Sciences, USA 99: 2812-2817). It was not known if such a mechanism was unique to flowering plants or whether it occurred more broadly in the plant clade. To determine if a polar mechanism of cell division might have been characteristic of the first plants, differential interference contrast optics were used to examine living cells of the charophycean green alga Coleochaete orbicularis, a close relative of plants, with cytokinesis involving a phragmoplast. By recording images in different focal planes over time, such "polarized" cytokinesis was found in cells dividing either parallel or perpendicular to the edge of this radially symmetrical organism. Previously reported differences between these two types of division in Coleochaete were clarified. Polarized cytokinesis appears to be an ancestral mechanism of plant cell division inherited from the highly vacuolate cells of the charophycean algal ancestors of plants.     

Demarcation of the cortical division zone in dividing plant cells

Somatic cytokinesis in higher plants involves, besides the actual construction of a new cell wall, also the determination of a division zone. Several proteins have been shown to play a part in the mechanism that somatic plant cells use to control the positioning of the new cell wall. Plant cells determine the division zone at an early stage of cell division and use a transient microtubular structure, the preprophase band (PPB), during this process. The PPB is formed at the division zone, leaving behind a mark that during cytokinesis is utilized by the phragmoplast to guide the expanding cell plate toward the correct cortical insertion site. This review discusses old and new observations with regard to mechanisms implicated in the orientation of cell division and determination of a cortical division zone.     

The molecular mechanism of targeted vesicle transport in cytokinesis

Recent studies have demonstrated that vesicle transport to cleavage furrow is indispensable for cytokinesis. Some animal and plant cells form distinct structures during cell division known as central spindle and phragmoplast, respectively. Several essential factors involved in the vesicle transport have been isolated so far. SNARE proteins and molecular motors play a central role in this process. For future research of cytokinesis, it is important to investigate these factors as well as cytoskeletal components of the contractile ring in detail. This review focuses on the molecular mechanism of targeted vesicle transport in cytokinesis.     

Septation and cytokinesis in fungi

Cytokinesis is the ultimate step of a cell cycle resulting in the generation of two progeny. Failure of correct cell division may be lethal for both, mother and daughter cells, and thus such a process must be tightly regulated with other events of the cell cycle. Differing solutions to the same problem have been developed in bacteria and plants while cytokinesis in animal and fungal cells is highly similar and requires a contractile ring containing actomyosin. Cytokinesis in fungi can be viewed as a three-stage process: (i) selection of a division site, (ii) orderly assembly of protein complexes, and finally (iii) dynamic events that lead to a constriction of the contractile ring and septum construction. Elaborate mechanisms known as the Mitotic Exit Network (MEN) and the Septation Initiation Network (SIN) have evolved to link these events, particularly the final steps of cytokinesis, with nuclear division. The purpose of this review was to discuss the latest developments in the fungal field and to describe the central known players required for key steps on the road to cell division. Differences in the cytokinesis of yeast-like fungi that result in complete cell separation in contrast to septation which leads to the compartmentalization of fungal hyphae are highlighted.     

Genetic dissection of cytokinesis

Higher plants have evolved specific mechanisms for partitioning the cytoplasm of dividing cells. In the predominant mode of phragmoplast-assisted cytokinesis, a cell wall and flanking plasma membranes are made de novo from a transient membrane compartment, the cell plate, which in turn forms by vesicle fusion from the centre to the periphery of the dividing cell. Other modes of cytokinesis appear to occur in meiotic cells and developing gametophytes. Here we review recent progress in the analysis of plant cytokinesis, focusing on genetic studies in Arabidopsis which are beginning to identify structural and regulatory components of phragmoplast-assisted cytokinesis. Two classes of mutations have been described. In one class, the defects appear to be confined to cell plate formation, suggesting that the execution of cytokinesis is specifically affected. Mutations in the other class display more general defects in cell division. We also discuss possible roles of proteins that have been localised in cytokinetic cells but not characterised genetically. Finally, mutations affecting meiotic or gametophytic cell divisions suggest that mechanistically different modes of cytokinesis occur in higher plants.     

Membrane trafficking in cytokinesis

Until recently, two distinct types of cytokinesis were thought to be responsible for the division of plant and animal cells. Plant cells divide through the formation of a membrane plate between the daughter cells, while animal cells divide by the constriction of a cortical actin-based ring around the cell. However, accumulating evidence suggests that the two mechanisms may have more in common than previously thought. In this review we will focus on recent developments that raise the possibility of unexpected similarities between the final steps in cytokinesis in animal and plant cells.     

The mammalian septin MSF localizes with microtubules and is required for completion of cytokinesis

Cytokinesis in animal cells involves the contraction of an actomyosin ring formed at the cleavage furrow. Nuclear division, or karyokinesis, must be precisely timed to occur before cytokinesis in order to prevent genetic anomalies that would result in either cell death or uncontrolled cell division. The septin family of GTPase proteins has been shown to be important for cytokinesis although little is known about their role during this process. Here we investigate the distribution and function of the mammalian septin MSF. We show that during interphase, MSF colocalizes with actin, microtubules, and another mammalian septin, Nedd5, and coprecipitates with six septin proteins. In addition, transfections of various MSF isoforms reveal that MSF-A specifically localizes with microtubules and that this localization is disrupted by nocodazole treatment. Furthermore, MSF isoforms localize primarily with tubulin at the central spindle during mitosis, whereas Nedd5 is mainly associated with actin. Microinjection of affinity-purified anti-MSF antibodies into synchronized cells, or depletion of MSF by small interfering RNAs, results in the accumulation of binucleated cells and in cells that have arrested during cytokinesis. These results reveal that MSF is required for the completion of cytokinesis and suggest a role that is distinct from that of Nedd5.     

Cytokinesis in eukaryotes: a mechanistic comparison

Cytokinesis is a crucial but poorly understood process of cell proliferation. Recently, molecular genetic analyses of fungal cytokinesis have led to an appreciation of contractile mechanisms in simple eukaryotes, and studies in animal and plant cells have led to new insights into the role of microtubules in the cleavage process. These findings suggest that fundamental mechanisms of cytokinesis may be highly conserved among eukaryotic organisms.     

The cytoplast concept in dividing plant cells: cytoplasmic domains and the evolution of spatially organized cell

The unique cytokinetic apparatus of higher plant cells comprises two cytoskeletal systems: a predictive preprophase band of microtubules (MTs), which defines the future division site, and the phragmoplast, which mediates crosswall formation after mitosis. We review features of plant cell division in an evolutionary context and from the viewpoint that the cell is a domain of cytoplasm (cytoplast) organized around the nucleus by a cytoskeleton consisting of a single "tensegral" unit. The term "tensegrity" is a contraction of "tensional integrity" and the concept proposes that the whole cell is organized by an integrated cytoskeleton of tension elements (e.g., actin fibers) extended over compression-resistant elements (e.g., MTs).During cell division, a primary role of the spindle is seen as generating two cytoplasts from one with separation of chromosomes a later, derived function. The telophase spindle separates the newly forming cytoplasts and the overlap between half spindles (the shared edge of two new domains) dictates the position at which cytokinesis occurs. Wall MTs of higher plant cells, like the MT cytoskeleton in animal and protistan cells, spatially define the interphase cytoplast. Redeployment of actin and MTs into the preprophase band (PPB) is the overt signal that the boundary between two nascent cytoplasts has been delineated. The "actin-depleted zone" that marks the site of the PPB throughout mitosis may be a more persistent manifestation of this delineation of two domains of cortical actin. The growth of the phragmoplast is controlled by these domains, not just by the spindle. These domains play a major role in controlling the path of phragmoplast expansion. Primitive land plants show different morphological changes that reveal that the plane of division, with or without the PPB, has been determined well in advance of mitosis.The green alga Spirogyra suggests how the phragmoplast system might have evolved: cytokinesis starts with cleavage and then actin-related determinants stimulate and positionally control cell-plate formation in a phragmoplast arising from interzonal MTs from the spindle. Actin in the PPB of higher plants may be assembling into a potential furrow, imprinting a cleavage site whose persistent determinants (perhaps actin) align the outgrowing edge of the phragmoplast, as in Spirogyra. Cytochalasin spatially disrupts polarized mitosis and positioning of the phragmoplast. Thus, the tensegral interaction of actin with MTs (at the spindle pole and in the phragmoplast) is critical to morphogenesis, just as they seem to be during division of animal cells. In advanced green plants, intercalary expansion driven by turgor is controlled by MTs, which in conjunction with actin, may act as stress detectors, thereby affecting the plane of division (a response clearly evident after wounding of tissue). The PPB might be one manifestation of this strain detection apparatus.     

Microtubules are required for completion of cytokinesis in sea urchin eggs.

Completion of cytokinesis, abscission, has been studied little despite the intensive studies of the onset and contractile mechanism of the earlier phases of division. It has been well documented that microtubule (MT) disruption before furrow stimulation prevents furrowing, while MT disruption after furrow stimulation allows division to proceed. We have confirmed those findings using the MT inhibitors, nocodazole and demecolcine. In addition, we have found that MT disruption after furrow stimulation but before completion of division prevents abscission as evidenced by the observation that prospective daughter cells in MT-disrupted eggs maintain electrical continuity. Continued observation of eggs revealed that the furrow in MT-disrupted eggs did not result in abscission, but rather held steady until the time when controls underwent second cleavage, at which point the furrows regressed. These findings extend the recent reports that MTs are required for completion of division in mammalian tissue culture cells and frog eggs, to invertebrates, suggesting a common mechanism of abscission for animal cells.     

Cytokinesis and cancer

Cytokinesis is the final stage of cell division during which the two daughter cells separate completely. Although less well understood than some of the earlier phases of the cell cycle, recent discoveries have shed light on the mechanisms that orchestrate this process, including cleavage furrow formation, midbody maturation and abscission. One of the reasons why research on cytokinesis has been attracting increasing attention is the concept that failure of this process in mammals is associated with carcinogenesis. In this minireview, we will discuss the possible links between cytokinesis and cancer, and highlight key mechanisms that connect these processes.     

Multinucleation in mouse fibroblasts cultured in methocel medium

3T3-4E cells formed multinucleate cells with high frequency when incubated in methocel medium. The experiment with hydroxyurea and the cytological observation of mitoses showed that multinucleate cells were produced by nuclear division in the absence of cytokinesis. When transferred onto a solid substratum, most of the multinucleate cells divided within seven hours into mononucleate cells through the process of cytoplasmic division, indicating that cell spreading induced cytokinesis. Other mouse fibroblast lines examined so far showed only the low frequency of multinucleation. These findings indicate that in 3T3-4E cells cultivated in methocel, nuclear division occurred independently of cytokinesis, and that cytokinesis was also anchorage-dependent. This system will be available for studying cytoplasmic division of mammalian cells.     

Human ECT2 is an exchange factor for Rho GTPases, phosphorylated in G2/M phases, and involved in cytokinesis

Animal cells divide into two daughter cells by the formation of an actomyosin-based contractile ring through a process called cytokinesis. Although many of the structural elements of cytokinesis have been identified, little is known about the signaling pathways and molecular mechanisms underlying this process. Here we show that the human ECT2 is involved in the regulation of cytokinesis. ECT2 catalyzes guanine nucleotide exchange on the small GTPases, RhoA, Rac1, and Cdc42. ECT2 is phosphorylated during G2 and M phases, and phosphorylation is required for its exchange activity. Unlike other known guanine nucleotide exchange factors for Rho GTPases, ECT2 exhibits nuclear localization in interphase, spreads throughout the cytoplasm in prometaphase, and is condensed in the midbody during cytokinesis. Expression of an ECT2 derivative, containing the NH(2)-terminal domain required for the midbody localization but lacking the COOH-terminal catalytic domain, strongly inhibits cytokinesis. Moreover, microinjection of affinity-purified anti-ECT2 antibody into interphase cells also inhibits cytokinesis. These results suggest that ECT2 is an important link between the cell cycle machinery and Rho signaling pathways involved in the regulation of cell division.     

Membrane traffic: a driving force in cytokinesis

Dividing animal and plant cells maintain a constant chromosome content through temporally separated rounds of replication and segregation. Until recently, the mechanisms by which animal and plant cells maintain a constant surface area have been considered to be distinct. The prevailing view was that surface area was maintained in dividing animal cells through temporally separated rounds of membrane expansion and membrane invagination. The latter event, known as cytokinesis, produces two physically distinct daughter cells and has been thought to be primarily driven by actomyosin-based constriction. By contrast, membrane addition seems to be the primary mechanism that drives cytokinesis in plants and, thus, the two events are linked mechanistically and temporally. In this article (which is part of the Cytokinesis series), we discuss recent studies of a variety of organisms that have made a convincing case for membrane trafficking at the cleavage furrow being a key component of both animal and plant cytokinesis.     

Cell division: plant-like properties of animal cell cytokinesis.

Recent evidence that a syntaxin is required for cytokinesis in Caenorhabditis elegans embryos suggests that the mechanism of cell division in plant and animal cells may be more similar than previously imagined.