Localization of Rho GTPase in sea urchin eggs

Identification of a novel RhoA gene in the sea cucumber Apostichopus japonicus and its immune regulatory function via interacting with miR-2012-5p

RhoA (Ras homolog A) protein is a representative member of the Rho GTPase family and is involved in various cellular processes. The function of RhoA in sea cucumbers is unclear. In this study, we hypothesized that RhoA may regulate the innate immune response of Apostichopus japonicus. Our data showed that 1) the complete sequence of RhoA from A. japonicus (named AjRhoA) was 968 bp, with a high level sequence conservation across the echinoderms and other phyla; 2) tissue expression analysis showed that AjRhoA transcripts and protein exhibited higher abundance in coelomocytes, whereas the relative expression of miR-2012-5p was lower in coelomocytes; 3) interactive binding sites and a negative regulatory targeting relationship between AjRhoA and miR-2012-5p were confirmed through a dual-luciferase reporter assay and functional validation in vivo; 4) the relative expression levels of AjRhoA transcripts and protein were upregulated in coelomocytes 4- and 72-hour post infection (hpi) with Vibrio splendidus, whereas miR-2012-5p was expressed in the opposite pattern; 5) both AjRhoA silencing and miR-2012-5p overexpression suppressed the phagocytic capacity of A. japonicus compared with the control at 4 and 72 hpi. Our observations suggest that AjRhoA can regulate the pathogen-induced immune response of A. japonicus through the "AjRhoA-miR-2012-5p" module during the early infection, while miR-2012-5p plays a direct immunomodulatory role as the infection progresses.

Emerging Mechanisms and Roles for Asymmetric Cytokinesis

Cytokinesis completes cell division by physically separating the contents of the mother cell between the two daughter cells. This event requires the highly coordinated reorganization of the cytoskeleton within a precise window of time to ensure faithful genomic segregation. In addition, recent progress in the field highlighted the importance of cytokinesis in providing particularly important cues in the context of multicellular tissues. The organization of the cytokinetic machinery and the asymmetric localization or inheritance of the midbody remnants is critical to define the spatial distribution of mechanical and biochemical signals. After a brief overview of the conserved steps of animal cytokinesis, we review the mechanisms controlling polarized cytokinesis focusing on the challenges of epithelial cytokinesis. Finally, we discuss the significance of these asymmetries in defining embryonic body axes, determining cell fate, and ensuring the correct propagation of epithelial organization during proliferation.

Small GTPase RhoA regulates cytoskeleton dynamics during porcine oocyte maturation and early embryo development

Mammalian oocyte maturation is distinguished by asymmetric division that is regulated primarily by cytoskeleton, including microtubules and microfilaments. Small Rho GTPase RhoA is a key regulator of cytoskeletal organization which regulates cell polarity, migration, and division. In this study, we investigated the roles of RhoA in mammalian oocyte meiosis and early embryo cleavage. (1) Disrupting RhoA activity or knock down the expression of RhoA caused the failure of polar body emission. This may have been due to decreased actin assembly and subsequent spindle migration defects. The involvement of RhoA in this process may have been though its regulation of actin nucleators ROCK, p-Cofilin, and ARP2 expression. (2) In addition, spindle morphology was also disrupted and p-MAPK expression decreased in RhoA inhibited or RhoA KD oocytes, which indicated that RhoA also regulated MAPK phosphorylation for spindle formation. (3) Porcine embryo development was also suppressed by inhibiting RhoA activity. Two nuclei were observed in one blastomere, and actin expression was reduced, which indicated that RhoA regulated actin-based cytokinesis of porcine embryo. Thus, our results demonstrated indispensable roles for RhoA in regulating porcine oocyte meiosis and cleavage during early embryo development.

A complex of p190RhoGAP-A and anillin modulates RhoA-GTP and the cytokinetic furrow in human cells

The cytokinetic furrow is organized by the RhoA GTPase, which recruits actin and myosin II to the furrow and drives contractility. Here, we show that the RhoA GTPase-activting protein (GAP) p190RhoGAP-A (also known as ARHGAP35) has a role in cytokinesis and is involved in regulating levels of RhoA-GTP and contractility. Cells depleted of p190RhoGAP-A accumulate high levels of RhoA-GTP and markers of high RhoA activity in the furrow, resulting in failure of the cytokinetic furrow to progress to abscission. The loss of p190RhoGAP-A can be rescued by a low dose of the myosin II inhibitor blebbistatin, suggesting that cells fail cytokinesis because they have too much myosin activity. p190RhoGAP-A binds the cytokinetic organizer anillin, and mutants of p190RhoGAP-A that are unable to bind anillin or unable to inactivate RhoA fail to rescue cytokinesis defects in p190RhoGAP-A-depleted cells. Taken together, these data demonstrate that a complex of p190RhoGAP-A and anillin modulates RhoA-GTP levels in the cytokinetic furrow to ensure progression of cytokinesis.

Mitotic exit and separation of mother and daughter cells

Productive cell proliferation involves efficient and accurate splitting of the dividing cell into two separate entities. This orderly process reflects coordination of diverse cytological events by regulatory systems that drive the cell from mitosis into G1. In the budding yeast Saccharomyces cerevisiae, separation of mother and daughter cells involves coordinated actomyosin ring contraction and septum synthesis, followed by septum destruction. These events occur in precise and rapid sequence once chromosomes are segregated and are linked with spindle organization and mitotic progress by intricate cell cycle control machinery. Additionally, critical paarts of the mother/daughter separation process are asymmetric, reflecting a form of fate specification that occurs in every cell division. This chapter describes central events of budding yeast cell separation, as well as the control pathways that integrate them and link them with the cell cycle.

Rho GTPases in animal cell cytokinesis: an occupation by the one percent

Rho GTPases are molecular switches that elicit distinct effects on the actomyosin cytoskeleton to accurately promote cytokinesis. Although they represent less than 1% of the human genome, Rho GTPases exert disproportionate control over cell division. Crucial to this master regulatory role is their localized occupation of specific domains of the cell to ensure the assembly of a contractile ring at the proper time and place. RhoA occupies the division plane and is the central positive Rho family regulator of cytokinesis. Rac1 is a negative regulator of cytokinesis and is inactivated within the division plane while active Rac1 occupies the cell poles. Cdc42 regulation during cytokinesis is less studied, but thus far a clear role has only been shown during polar body emission. Here we review what is known about the function of Rho family GTPases during cell division, as well as their upstream regulators and known downstream cytokinetic effectors.

Rho-kinase in sea urchin eggs and embryos

The activation of sea urchin eggs at fertilization provides an ideal system for studying the molecular events involved in cellular activation. Rho GTPases, which are key signaling enzymes in eukaryotes, are involved in sustaining the activation of sea urchin eggs; however, their downstream effectors have not yet been characterized. In somatic cells, RhoA regulates a serine/threonine kinase known as Rho-kinase (ROCK). The activity of ROCK in early sea urchin development has been inferred, but not tested directly. A ROCK gene was identified in the sea urchin (Strongylocentrotus purpuratus) genome and the sequence of its cDNA determined. The sea urchin ROCK (SpROCK) sequence predicts a protein of 158 kDa with >72% and 45% identities with different protein orthologues of the kinase catalytic domain and the complete protein sequence, respectively. SpROCK mRNA levels are high in unfertilized eggs and decrease to 35% after 15 min postfertilization and remain low up to the 4 cell stage. Antibodies to the human ROCK-I kinase domain revealed SpROCK to be concentrated in the cortex of eggs and early embryos. Co-immunoprecipitation assays indicate that RhoA and SpROCK are physically associated. This association is destroyed by treatment with the C3 exoenzyme and with the ROCK antagonist H-1152. H-1152 also inhibited DNA synthesis in embryos. We conclude that the Rho-dependent signaling pathway, via SpROCK, is essential for early embryonic development.

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

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

Mitotic down-regulation of p190RhoGAP is required for the successful completion of cytokinesis

p190RhoGAP-A (p190) is a GTPase-activating protein known to regulate actin cytoskeleton dynamics by decreasing RhoGTP levels through activation of Rho intrinsic GTPase activity. We have previously shown that p190 protein levels are cell cycle-regulated, decreasing in mitosis, and that this decrease is mediated by the ubiquitin-proteasome pathway. In addition, overexpression of p190 results in decreased RhoGTP levels at the cleavage furrow during cytokinesis, p190 and the RhoGEF Ect2 play opposing roles in cytokinesis, and sustained levels of p190 in mitosis are associated with cytokinesis failure, all findings that suggest but do not directly demonstrate that completion of cytokinesis is dependent on reduced levels of p190. Here we report, using an RNAi reconstitution approach with a degradation-resistant mutant, that decreased p190 levels are required for successful cytokinesis. We also show that the multinucleation phenotype is dependent on p190 RhoGAP activity, determine that the N-terminal GBDS1 region is necessary and sufficient for p190 mitotic ubiquitination and degradation, and identify four N-terminal residues as necessary for the degradation of p190 in mitosis. Our data indicate that in addition to activation of RhoGEF(s), reduction of RhoGAP (p190) is a critical mechanism by which increased RhoGTP levels are achieved in late mitosis, thereby ensuring proper cell division.

Elongation factors are involved in cytokinesis of sea urchin eggs

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

p190RhoGAP negatively regulates Rho activity at the cleavage furrow of mitotic cells

Previous studies demonstrated that p190RhoGAP (p190) negatively affects cytokinesis in a RhoGAP-dependent manner, suggesting that regulation of Rho may be a critical mechanism of p190 action during cytokinesis. P190 localizes to the cleavage furrow (CF) of dividing cells, and its levels decrease during late mitosis by an ubiquitin-mediated mechanism, consistent with the hypothesis that high RhoGTP levels are required for completion of cytokinesis. To determine whether RhoGTP levels in the CF are affected by p190 and to define the phase(s) of cytokinesis in which p190 is involved, we used FRET analysis alone or in combination with time-lapse microscopy. In normal cell division activated Rho accumulated at the cell equator in early anaphase and in the contractile ring, where it co-localized with p190. Real-time movies revealed that cells expressing elevated levels of p190 exhibited multiple cycles of abnormal CF site selection and ingression/regression, which resulted in failed or prolonged cytokinesis. This was accompanied by mislocalization of active Rho at the aberrant CF sites. Quantified data revealed that in contrast to ECT2 and dominate negative p190 (Y1283Ap190), which resulted in hyper-activated Rho, Rho activity in the CF was reduced by wild type p190 in a dose-dependent manner. These results suggest that p190 regulates cytokinesis through modulation of RhoGTP levels, thereby affecting CF specification site selection and subsequent ring contraction.

Functional implications of caspase-mediated RhoGDI2 processing during apoptosis of HL60 and K562 leukemia cells

RhoGDI2, a cytosolic regulator of Rho GTPase, is cleaved during apoptosis in a caspase-3 dependent fashion. By using 2D-gel electrophoresis, mass spectrometry and Western blotting we investigate in this paper the functional consequences of RhoGDI2 processing. We can show that loss of the N-terminal 19 amino acids results in a shift of the isoelectric point of the truncated RhoGDI2 (NDelta19) to a more basic value due to the removal of 9 acidic amino acids from the N-terminus, which may be responsible for enhanced retention of the N-terminally truncated protein within the nuclear compartment. Fusion of the p53 nuclear export signaling sequence MFRELNEALELK to NDelta19 (NDelta19NES) abolished its apoptosis promoting properties, while overexpression of NDelta19 significantly increased the susceptibility to apoptosis induction by the proteasome inhibitor PSI and by staurosporine. These results suggest that cleavage of RhoGDI2 by caspase-3 is not a functionally irrelevant bystander effect of caspase activation during apoptosis, but rather expedites progression of the apoptotic process.

Rho, Rho-kinase, and the actin cytoskeleton regulate the Na+–H+ exchanger in sea urchin eggs

At fertilization, the sea urchin egg undergoes an internal pH (pHi) increase mediated by a Na+ -H+ exchanger. We used antibodies against the mammalian antiporters NHE1 and NHE3 to characterize this exchanger. In unfertilized eggs, only anti-NHE3 cross-reacted specifically with a protein of 81-kDa, which localized to the plasma membrane and cortical granules. Cytochalasin D, C3 exotoxin (blocker of RhoGTPase function), and Y-27632 (inhibitor of Rho-kinase) prevented the pHi change in fertilized eggs. These inhibitors blocked the first cleavage division of the embryo, but not the cortical granule exocytosis. Thus, the sea urchin egg has an epithelial NHE3-like Na+ -H+ exchanger which can be responsible for the pHi change at fertilization. Determinants of this pHi change can be: (i) the increase of exchangers in the plasma membrane (via cortical granule exocytosis) and (ii) Rho, Rho-kinase, and optimal organization of the actin cytoskeleton as regulators, among others, of the intrinsic activity of the exchanger.

Lineage-specific expansions provide genomic complexity among sea urchin GTPases

In every organism, GTP-binding proteins control many aspects of cell signaling. Here, we examine in silico several GTPase families from the Strongylocentrotus purpuratus genome: the monomeric Ras superfamily, the heterotrimeric G proteins, the dynamin superfamily, the SRP/SR family, and the "protein biosynthesis" translational GTPases. Identified were 174 GTPases, of which over 90% are expressed in the embryo as shown by tiling array and expressed sequence tag data. Phylogenomic comparisons restricted to Drosophila, Ciona, and humans (protostomes, urochordates, and vertebrates, respectively) revealed both common and unique elements in the expected composition of these families. Galpha and dynamin families contain vertebrate expansions, consistent with whole genome duplications, whereas SRP/SR and translational GTPases are highly conserved. Unexpectedly, Ras superfamily analyses revealed several large (5+) lineage-specific expansions in the sea urchin. For Rho, Rab, Arf, and Ras subfamilies, comparing total human gene numbers to the number of sea urchin genes with vertebrate orthologs suggests reduced genomic complexity in the sea urchin. However, gene duplications in the sea urchin increase overall numbers such that total sea urchin gene numbers approximate vertebrate gene numbers for each monomeric GTPase family. These findings suggest that lineage-specific expansions may be an important component of genomic evolution in signal transduction.

RhoA regulates initiation of invagination, but not convergent extension, during sea urchin gastrulation

During gastrulation, the archenteron is formed using cell shape changes, cell rearrangements, filopodial extensions, and convergent extension movements to elongate and shape the nascent gut tube. How these events are coordinated remains unknown, although much has been learned from careful morphological examinations and molecular perturbations. This study reports that RhoA is necessary to trigger archenteron invagination in the sea urchin embryo. Inhibition of RhoA results in a failure to initiate invagination movements, while constitutively active RhoA induces precocious invagination of the archenteron, complete with the actin rearrangements and extracellular matrix secretions that normally accompany the onset of invagination. Although RhoA activity has been reported to control convergent extension movements in vertebrate embryos, experiments herein show that RhoA activity does not regulate convergent extension movements during sea urchin gastrulation. Instead, the results support the hypothesis that RhoA serves as a trigger to initiate invagination, and once initiation occurs, RhoA activity is no longer involved in subsequent gastrulation movements.

Centralspindlin regulates ECT2 and RhoA accumulation at the equatorial cortex during cytokinesis

During determination of the cell division plane, an actomyosin contractile ring is induced at the equatorial cell cortex by signals from the mitotic apparatus and contracts to cause cleavage furrow progression. Although the small GTPase RhoA is known to regulate the progression, probably by controlling actin filament assembly and enhancing actomyosin interaction, any involvement of RhoA in division plane determination is unknown. In this study, using a trichloroacetic acid (TCA) fixation protocol we recently developed, we show that RhoA accumulates at the equatorial cortex before furrow initiation and continues to concentrate at the cleavage furrow during cytokinesis. We also demonstrate that both Rho activity and microtubule organization are required for RhoA localization and proper furrowing. Selective disruption of microtubule organization revealed that both astral and central spindle microtubules can recruit RhoA at the equatorial cortex. We find that centralspindlin and ECT2 are required for RhoA localization and furrowing. Centralspindlin is localized both to central spindle microtubules and at the tips of astral microtubules near the equatorial cortex and recruits ECT2. Positional information for division plane determination from microtubules is transmitted to the cell cortex to organize actin cytoskeleton through a mechanism involving these proteins.

Cytokinesis: welcome to the Rho zone

Cytokinesis follows nuclear division and generates two distinct daughter cells, each replete with a full complement of the genome and cytoplasmic organelles. Members of the Rho family of GTPases are crucial regulators of this process in a wide variety of species. In many cell types, cytokinesis is mediated by a discretely localized contractile ring that is rich in actin and myosin. In this article (which is part of the Cytokinesis series), we review recent studies in animal cells that have shown that local assembly of the contractile ring is mediated by a discrete pool of GTP-bound, active RhoA. Advances in detecting the active pool of RhoA have allowed insights into the mechanisms and the molecules that promote the accumulation of active RhoA at the correct time and place in the cell.

Small GTPase RhoA is required for ooplasmic segregation and spindle rotation, but not for spindle organization and chromosome separation during mouse oocyte maturation, fertilization, and early cleavage

RhoA, a small GTPase, plays versatile roles in many aspects of cell function such as stress fiber formation, cytokinesis, and cell polarization. In this study, we investigated the subcellular localization of RhoA and its possible roles during oocyte maturation and fertilization. RhoA was localized in the cytoplasm of eggs from the germinal vesicle (GV) stage to 2-cell stage, especially concentrating in the midbody of telophase spindle when oocyte extruded PB1 and PB2. The RhoA kinases (ROCKs) specific inhibitor Y-27632 blocked GV breakdown (GVBD) and first polar body extrusion, but did not affect apparatus formation and anaphase/telophase I entry. Anti-RhoA antibody microinjection into the oocytes showed similar results. RhoA inhibitor caused abnormal organization of microfilaments, failure of spindle rotation, PB2 extrusion as well as cleavage furrow formation, while sister chromatid separation was not affected. Microinjection of RhoA antibody also blocked PB2 emission. Our findings indicate that RhoA, by regulating microfilament organization, regulates several important events including GVBD, polar body emission, spindle rotation, and cleavage.

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

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

Regulation of myosin II during cytokinesis in higher eukaryotes

Cellular myosin II is the principal motor responsible for cytokinesis. In higher eukaryotes, phosphorylation of the regulatory light chain (MLC) of myosin II is a primary means of activating myosin II and is known to be crucial for the execution of cell division. Because signals transmitted by the mitotic spindle coordinate key spatial and temporal aspects of cytokinesis, such signals should ultimately function to activate myosin II. Thus, it follows that identification of regulatory factors involved in MLC phosphorylation should elucidate the nature of spindle-derived regulatory signals and lead to a model for how they control cytokinesis. However, the identity of these upstream molecules remains elusive. This review (which is part of the Cytokinesis series) summarizes current views of the regulatory pathway controlling MLC phosphorylation and features four candidate molecules that are likely immediate upstream myosin regulators. I discuss proposed functions for MLCK, ROCK, citron kinase and myosin phosphatase during cytokinesis and consider the possibility of a link between these molecules and the signals transmitted by the mitotic spindle.

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

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

Rho localization in cells and tissues

Rho family small GTPases regulate cytoskeletal organization. Although their spatiotemporal activities appear to be important for cellular morphogenesis, there has been little characterization of the localization of Rho family GTPases in cells and tissues. Here we show precise localization of Rho subfamily proteins in mammalian cultured cells and tissues through evaluation of anti-Rho antibodies and fixation protocols. Although Rho is not a structural protein but functions as a switching molecule, it often localizes at several distinct domains or structures of cells. In cultured epithelial cells, Rho was highly accumulated at lateral membranes. However, in fibroblastic cells, Rho appeared to be distributed evenly in the cytoplasm. Rho concentration at the cleavage furrow at cytokinesis was generally observed. In A431 cells, Rho translocation from the cytoplasm to elongating microvilli at the apical membrane within 30 s after EGF stimulation was clearly demonstrated. Also, Myc- or GFP-tagged RhoA did not always reflect the localization of endogenous Rho, indicating a drawback of protein-tagging methods for localization research. In mouse tissues, Rho localization differed depending on cell type, probably reflecting the functional differences of each cell type.

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

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

A Rho GTPase controls the rate of protein synthesis in the sea urchin egg

Fertilization of the sea urchin egg triggers a Ca(2+)-dependent cortical granule exocytosis and cytoskeletal reorganization, both of which are accompanied by an accelerated protein synthesis. The signaling mechanisms leading to these events are not completely understood. The possible role of Rho GTPases in sea urchin egg activation was studied using the Clostridium botulinum C3 exotoxin, which specifically ADP-ribosylates Rho proteins and inactivates them. We observed that incubation of eggs with C3 resulted in in situ ADP-ribosylation of Rho. Following fertilization, C3-treated eggs were capable of performing cortical granule exocytosis but not the first cytokinesis. C3 caused in both unfertilized eggs and early embryos alterations in the state of actin polymerization and inhibition of the spindle formation. Moreover, C3 diminished markedly the rate of protein synthesis. These findings suggested that Rho is involved in regulating the acceleration of protein synthesis that accompanies the egg activation by sperm.

Activity of Rho-family GTPases during cell division as visualized with FRET-based probes

Rho-family GTPases regulate many cellular functions. To visualize the activity of Rho-family GTPases in living cells, we developed fluorescence resonance energy transfer (FRET)-based probes for Rac1 and Cdc42 previously (Itoh, R.E., K. Kurokawa, Y. Ohba, H. Yoshizaki, N. Mochizuki, and M. Matsuda. 2002. Mol. Cell. Biol. 22:6582-6591). Here, we added two types of probes for RhoA. One is to monitor the activity balance between guanine nucleotide exchange factors and GTPase-activating proteins, and another is to monitor the level of GTP-RhoA. Using these FRET probes, we imaged the activities of Rho-family GTPases during the cell division of HeLa cells. The activities of RhoA, Rac1, and Cdc42 were high at the plasma membrane in interphase, and decreased rapidly on entry into M phase. From after anaphase, the RhoA activity increased at the plasma membrane including cleavage furrow. Rac1 activity was suppressed at the spindle midzone and increased at the plasma membrane of polar sides after telophase. Cdc42 activity was suppressed at the plasma membrane and was high at the intracellular membrane compartments during cytokinesis. In conclusion, we could use the FRET-based probes to visualize the complex spatio-temporal regulation of Rho-family GTPases during cell division.

The small GTPase Rho4 is involved in controlling cell morphology and septation in fission yeast

BACKGROUND

Rho family small GTPases have been shown to be involved in various cellular activities, including the organization of actin cytoskeleton in eukaryotic cells. There are six rho genes in the fission yeast Schizosaccharomyces pombe. Cdc42 is known to control the polarity of the cell. Rho1, Rho2 and Rho3 play important roles in controlling cell shape and septation. On the other hand, Rho4 and Rho5 have not yet been characterized. Here we report the function of rho4+ in fission yeast.

RESULTS

Gene disruption revealed that rho4+ is not essential for cell growth. However, rho4-null cells were abnormally elongated and had multiple septa of irregular shape at 37 degrees C. In these cells, F-actin patches were randomly localized all over the cell periphery, and cytoplasmic microtubules (MTs) were misoriented. On the other hand, the exogenous expression of a constitutively active Rho4-G23V or Rho4-Q74L in wild-type cells induced depolarization of F-actin patches and cytoplasmic MTs. Rho4 was localized to the cell periphery during interphase and septum during mitosis. Both the binding of GTP and isoprenylation of its C-terminus were necessary for the localization. Furthermore, the localization of Rho4 was likely to be controlled by Rho GAP and Rho GDI.

CONCLUSION

Rho4 may control cell morphogenesis and septation by regulating both the actin cytoskeleton and cytoplasmic MTs.

Microtuble organization in Xenopus eggs during the first cleavage and its role in cytokinesis

It has been suggested that the organization of microtubules during mitosis plays an important role in cytokinesis in animal cells. We studied the organization of microtubules during the first cleavage and its role in cytokinesis of Xenopus eggs. First, we examined the immunofluorescent localization of microtubules in Xenopus eggs at various stages during the first cleavage. The astral microtubules that extend from each of the two centrosomes towards the division plane meet and connect with each other at the division plane as cytokinesis proceeds. The microtubular connection thus advances from the animal pole to the vegetal pole, and its leading edge is located approximately beneath the leading edge of the cleavage furrow. Furthermore, an experiment using nocodazole suggests that microtubules have an essential role in advancement of the cleavage furrow, but neither in contraction nor maintenance of the already formed contractile ring which underlies the cleavage furrow membrane. These results suggest that the astral microtubules play an important role in controlling the formation of the contractile ring in Xenopus eggs.

Molecular dissection of cytokinesis by RNA interference in Drosophila cultured cells

We have used double-stranded RNA-mediated interference (RNAi) to study Drosophila cytokinesis. We show that double-stranded RNAs for anillin, acGAP, pavarotti, rho1, pebble, spaghetti squash, syntaxin1A, and twinstar all disrupt cytokinesis in S2 tissue culture cells, causing gene-specific phenotypes. Our phenotypic analyses identify genes required for different aspects of cytokinesis, such as central spindle formation, actin accumulation at the cell equator, contractile ring assembly or disassembly, and membrane behavior. Moreover, the cytological phenotypes elicited by RNAi reveal simultaneous disruption of multiple aspects of cytokinesis. These phenotypes suggest interactions between central spindle microtubules, the actin-based contractile ring, and the plasma membrane, and lead us to propose that the central spindle and the contractile ring are interdependent structures. Finally, our results indicate that RNAi in S2 cells is a highly efficient method to detect cytokinetic genes, and predict that genome-wide studies using this method will permit identification of the majority of genes involved in Drosophila mitotic cytokinesis.

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.

Sites of Ca2+ wave initiation move with caveolae to the trailing edge of migrating cells

The caveola is a membrane domain that compartmentalizes signal transduction at the cell surface. Normally in endothelial cells, groups of caveolae are found clustered along stress fibers or at the lateral margins in all regions of the cell. Subsets of these clusters appear to contain the signaling machinery for initiating Ca2+ wave formation. Here we report that induction of cell migration, either by wounding a cell monolayer or by exposing cells to laminar shear stress, causes caveolae to move to the trailing edge of the cell. Concomitant with the relocation of the caveolae,sites of Ca2+ wave initiation move to the same location. In as much as the relocated caveolae contain elements of the signaling machinery required for ATP-stimulated release of Ca2+ from the ER, these results suggest that caveolae function as containers that carry this machinery to different cellular locations.

Role of MgcRacGAP/Cyk4 as a regulator of the small GTPase Rho family in cytokinesis and cell differentiation

To identify the key molecules that regulate differentiation of hematopoietic cells, we carried out retrovirus-mediated functional screening for cDNAs whose expression suppresses IL-6-induced differentiation of mouse myeloid leukemic M1 cells. From this screening, we obtained a full length cDNA encoding a mouse homologue of human MgcRacGAP. Overexpression of the anti-sense MgcRacGAP profoundly inhibited IL-6-induced macrophage-differentiation of M1 cells. On the other hand, overexpression of the full-length form of MgcRacGAP alone enhanced macrophage differentiation of M1 cells in response to IL-6, and induced macrophage differentiation of HL-60 leukemic cells. To determine how this protein regulates differentiation and proliferation, an antibody against MgcRacGAP was prepared. Immunohistochemical studies revealed that MgcRacGAP mainly localizes in the nucleus in interphase, accumulates on the mitotic spindle in metaphase, and is condensed in the midbody during cytokinesis. Overexpression of an N-terminal domain deletion mutant, which lacks the ability to localize to the midbody through association with tubulins, or a GAP-inactive mutant resulted in the formation of multinucleated cells in HeLa cells as well as in hemopoietic cells. Interestingly, MgcRacGAP in the midbody was phosphorylated probably on serine and threonine residues. These results indicate that MgcRacGAP regulates cytokinesis and cellular differentiation as a regulator of Rho family of GTPase and suggest that this process is controlled by some serine/threonine kinases.

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.

MgcRacGAP is involved in cytokinesis through associating with mitotic spindle and midbody

We have recently cloned a cDNA for a full-length form of MgcRacGAP. Here we show using anti-MgcRacGAP antibodies that, unlike other known GAPs for Rho family, MgcRacGAP localized to the nucleus in interphase, accumulated to the mitotic spindle in metaphase, and was condensed in the midbody during cytokinesis. Overexpression of an N-terminal deletion mutant resulted in the production of multinucleated cells in HeLa cells. This mutant lost the ability to localize in the mitotic spindle and midbody. MgcRacGAP was also found to bind alpha-, beta-, and gamma-tubulins through its N-terminal myosin-like domain. These results indicate that MgcRacGAP dynamically moves during cell cycle progression probably through binding to tubulins and plays critical roles in cytokinesis. Furthermore, using a GAP-inactive mutant, we have shown that the GAP activity of MgcRacGAP is required for cytokinesis, suggesting that inactivation of the Rho family of GTPases may be required for normal progression of cytokinesis.

Localization of a mammalian homolog of diaphanous, mDia1, to the mitotic spindle in HeLa cells

mDia1 is a mammalian homolog of Drosophila diaphanous and works as an effector of the small GTPase Rho. It is a member of the formin homology (FH) proteins and contains the Rho-binding domain and an FH3 region in its N terminus, an FH1 region containing polyproline stretches in the middle and an FH2 region in the C terminus. Several lines of evidence indicate that mDia1 and diaphanous are essential in cytokinesis. mDia1 is present in a large amount in the cytoplasm of both interphase and mitotic cells. Using the instantaneous fixation method that preferentially extracts soluble components, we have analyzed localization of mDia1 in mitotic HeLa cells. Immunocytochemistry using polyclonal anti-mDia1 antibody revealed specific immunofluorescence localized to the mitotic spindle. This localization was seen from prophase to telophase. Western blot analysis also detected anti-mDia1 immunoreactivity in the mitotic spindle fraction isolated from mitotic HeLa cells. Consistently, expression of full-length mDia1 as a fusion protein with green fluorescence protein (GFP) revealed the GFP fluorescence again in the mitotic spindle in HeLa cells. Expression of GFP fusions of various truncated mutants of mDia1 identified that this localization is determined by a 173 amino acid-long sequence between the Rho-binding domain and the FH1 region, which contains the C-terminal part of the FH3 region. Point mutation analysis revealed that Leu(434) and Leu(455) in the FH3 region are essential in localization to the mitotic spindle. Neither electroporation of botulinum C3 exoenzyme nor microinjection of Val14RhoA into mitotic cells affected the localization of endogenous mDia1 to the mitotic spindle, suggesting that mDia1 localizes to the mitotic spindle independent of Rho activity. The present study has thus established the mDia1 localization in the mitotic spindle. This localization suggests a role of mDia1 in the spindle-cleavage furrow interaction during cell division.

Microtubules, membranes and cytokinesis

Proper division of the cell requires coordination between chromosome segregation by the mitotic spindle and cleavage of the cell by the cytokinetic apparatus. Interactions between the mitotic spindle, the contractile ring and the plasma membrane ensure that the cleavage furrow is properly placed between the segregating chromosomes and that new membrane compartments are formed to produce two daughter cells. The microtubule midzone is able to stimulate the cortex of the cell to ensure proper ingression and completion of the cleavage furrow. Specialized microtubule structures are responsible for directing membrane vesicles to the site of cell cleavage, and vesicle fusion is required for the proper completion of cytokinesis.

Accumulation of GTP-bound RhoA during cytokinesis and a critical role of ECT2 in this accumulation

We developed a new pull-down assay for GTP-Rho and examined its level during cell cycle. HeLa cells were arrested in the S phase by thymidine and were enriched in the prometaphase, metaphase, telophase, and G(1) phase by collecting at 0, 45, 90, and 180 min after the release from the nocodazole arrest, respectively. The level of GTP-Rho did not change significantly from the S phase to the prometaphase, but increased thereafter, peaking in the telophase, and returned to the original level in the G(1) phase. The GDP-GTP exchange activity for Rho measured in cell lysates in parallel increased also during the mitosis with a peak in the metaphase. Using this system, we examined a role of ECT2, an exchanger for Rho GTPases, suggested to be involved in cytokinesis (Tatsumoto, T., Xie, X., Blumenthal, R., Okamoto, I., and Miki., T. (1999) J. Cell. Biol. , 147, 921-928). Expression of the dominant negative form of ECT2 completely suppressed both the rise of GTP-Rho in the telophase and the increased GDP-GTP exchange activity in the mitotic cell extracts. These results suggest a critical role of ECT2 in Rho activation during cytokinesis.

The GTP-binding protein RhoA localizes to the cortical granules of Strongylocentrotus purpuratas sea urchin egg and is secreted during fertilization

The sea urchin egg has thousands of secretory vesicles known as cortical granules. Upon fertilization, these vesicles undergo a Ca2+-dependent exocytosis. G-protein-linked mechanisms may take place during the egg activation. In somatic cells from mammals, GTP-binding proteins of the Rho family regulate a number of cellular processes, including organization of the actin cytoskeleton. We report here that a crude membrane fraction from homogenates of Strongylocentrotus purpuratus sea urchin eggs, incubated with C3 (which ADP-ribosylates specifically Rho proteins) and [32P]NAD, displayed an [32P]ADP-ribosylated protein of 25 kDa that had the following characteristics: i) identical electrophoretic mobility in SDS-PAGE gels as the [32P]ADP-ribosylated Rho from sea urchin sperm; ii) identical mobility in isoelectro focusing gels as human RhoA; iii) positive cross-reactivity by immunoblotting with an antibody against mammalian RhoA. Thus, unfertilized S. purpuratus eggs contain a mammalian RhoA-like protein. Immunocytochemical analyses indicated that RhoA was localized preferentially to the cortical granules; this was confirmed by experiments of [32P]ADP-ribosylation with C3 in isolated cortical granules. Rho was secreted and retained in the fertilization membrane after insemination or activation with A23187. It was observed that the Rho protein present in the sea urchin sperm acrosome was also secreted during the exocytotic acrosome reaction. Thus, Rho could participate in those processes related to the cortical granules, i.e., in the Ca2+-regulated exocytosis or actin reorganization that accompany the egg activation.

A putative exchange factor for Rho1 GTPase is required for initiation of cytokinesis in Drosophila

Cytokinesis ensures the successful completion of the cell cycle and distribution of chromosomes, organelles, and cytoplasm between daughter cells. It is accomplished by formation and constriction of an actomyosin contractile ring that drives the progression of a cleavage furrow. Microinjection experiments and in vitro transfection assays have suggested a requirement for small GTPases of the Rho family in cytokinesis. Yet, the identity of proteins regulating Rho signaling pathways during cytokinesis remains unknown. Here we show that in Drosophila, Pebble (Pbl), a putative exchange factor for Rho GTPases (RhoGEF), is required for the formation of the contractile ring and initiation of cytokinesis. The dynamics of Pbl expression and its distribution during mitosis, as well as structure-function analysis, indicate that it is a key regulatory component of the pathway. pbl interacts genetically with Rho1, but not with Rac1 or Cdc42, and Pbl and Rho1 proteins interact in vivo in yeast. Similar to mutations in pbl, loss of Rho1 or expression of a dominant-negative Rho1 blocks cytokinesis. Our results identify Pbl as a RhoGEF specifically required for cytokinesis and linked through Rho1 activity to the reorganization of the actin cytoskeleton at the cleavage furrow.