The degradation of two mitotic cyclins contributes to the timing of cytokinesis

The Expression of Cell Cycle Cyclins in a Human Megakaryoblast Cell Line Exposed to Simulated Microgravity

The study of the physiological and pathophysiological processes under extreme conditions facilitates a better understanding of the state of a healthy organism and can also shed light on the pathogenesis of diseases. In recent years, it has become evident that gravitational stress affects both the whole organism and individual cells. We have previously demonstrated that simulated microgravity inhibits proliferation, induces apoptosis, changes morphology, and alters the surface marker expression of megakaryoblast cell line MEG-01. In the present work, we investigate the expression of cell cycle cyclins in MEG-01 cells. We performed several experiments for 24 h, 72 h, 96 h and 168 h. Flow cytometry and Western blot analysis demonstrated that the main change in the levels of cyclins expression occurs under conditions of simulated microgravity after 96 h. Thus, the level of cyclin A expression showed an increase in the RPM group during the first 4 days, followed by a decrease, which, together with the peak of cyclin D, may indicate inhibition of the cell cycle in the G2 phase, before mitosis. In addition, based on the data obtained by PCR analysis, we were also able to see that both cyclin A and cyclin B expression showed a peak at 72 h, followed by a gradual decrease at 96 h. STED microscopy data also confirmed that the main change in cyclin expression of MEG-01 cells occurs at 96 h, under simulated microgravity conditions, compared to static control. These results suggested that the cell cycle disruption induced by RPM-simulated microgravity in MEG-01 cells may be associated with the altered expression of the main regulators of the cell cycle. Thus, these data implicate the development of cellular stress in MEG-01 cells, which may be important for proliferating human cells exposed to microgravity in real space.

Contribution of integrin adhesion to cytokinetic abscission and genomic integrity

Recent research shows that integrin-mediated adhesion contributes to the regulation of cell division at two key steps: the formation of the mitotic spindle at the mitotic entry and the final cytokinetic abscission at the mitotic exit. Failure in either of these processes will have a direct impact on the other in each round of the cell cycle and on the genomic integrity. This review aims to present how integrin signals are involved at these cell cycle stages under normal conditions and some safety mechanisms that may counteract the generation of aneuploid cells in cases of defective integrin signals.

Cyclin B3 activates the Anaphase-Promoting Complex/Cyclosome in meiosis and mitosis

In mitosis and meiosis, chromosome segregation is triggered by the Anaphase-Promoting Complex/Cyclosome (APC/C), a multi-subunit ubiquitin ligase that targets proteins for degradation, leading to the separation of chromatids. APC/C activation requires phosphorylation of its APC3 and APC1 subunits, which allows the APC/C to bind its co-activator Cdc20. The identity of the kinase(s) responsible for APC/C activation in vivo is unclear. Cyclin B3 (CycB3) is an activator of the Cyclin-Dependent Kinase 1 (Cdk1) that is required for meiotic anaphase in flies, worms and vertebrates. It has been hypothesized that CycB3-Cdk1 may be responsible for APC/C activation in meiosis but this remains to be determined. Using Drosophila, we found that mutations in CycB3 genetically enhance mutations in tws, which encodes the B55 regulatory subunit of Protein Phosphatase 2A (PP2A) known to promote mitotic exit. Females heterozygous for CycB3 and tws loss-of-function alleles lay embryos that arrest in mitotic metaphase in a maternal effect, indicating that CycB3 promotes anaphase in mitosis in addition to meiosis. This metaphase arrest is not due to the Spindle Assembly Checkpoint (SAC) because mutation of mad2 that inactivates the SAC does not rescue the development of embryos from CycB3-/+, tws-/+ females. Moreover, we found that CycB3 promotes APC/C activity and anaphase in cells in culture. We show that CycB3 physically associates with the APC/C, is required for phosphorylation of APC3, and promotes APC/C association with its Cdc20 co-activators Fizzy and Cortex. Our results strongly suggest that CycB3-Cdk1 directly activates the APC/C to promote anaphase in both meiosis and mitosis.

Jumu is required for circulating hemocyte differentiation and phagocytosis in Drosophila

Background

The regulatory mechanisms of hematopoiesis and cellular immunity show a high degree of similarity between insects and mammals, and Drosophila has become a good model for investigating cellular immune responses. Jumeau (Jumu) is a member of the winged-helix/forkhead (FKH) transcription factor family and is required for Drosophila development. Adult jumu mutant flies show defective hemocyte phagocytosis and a weaker defense capability against pathogen infection. Here, we further investigated the role of jumu in the regulation of larval hemocyte development and phagocytosis.

Methods

In vivo phagocytosis assays, immunohistochemistry, Real-time quantitative PCR and immunoblotting were performed to investigate the effect of Jumu on hemocyte phagocytosis. 5-Bromo-2-deoxyUridine (BrdU) labeling, phospho-histone H3 (PH3) and TdT-mediated dUTP Nick-End Labeling (TUNEL) staining were performed to analyze the proliferation and apoptosis of hemocyte; immunohistochemistry and Mosaic analysis with a repressible cell marker (MARCM) clone analysis were performed to investigate the role of Jumu in the activation of Toll pathway.

Results

Jumu indirectly controls hemocyte phagocytosis by regulating the expression of NimC1 and cytoskeleton reorganization. The loss of jumu also causes abnormal proliferation and differentiation in circulating hemocytes. Our results suggest that a severe deficiency of jumu leads to the generation of enlarged multinucleate hemocytes by affecting the normal cell mitosis process and induces numerous lamellocytes by activating the Toll pathway.

Conclusions

Jumu regulates circulating hemocyte differentiation and phagocytosis in Drosophila. Our findings provide new insight into the mechanistic roles of cytoskeleton regulatory proteins in phagocytosis and establish a basis for further analyses of the regulatory mechanism of the mammalian ortholog of Jumu in mammalian innate immunity.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0305-3) contains supplementary material, which is available to authorized users.

Cyclin B3 Deficiency Impairs Germline Stem Cell Maintenance and Its Overexpression Delays Cystoblast Differentiation in Drosophila Ovary

It is well known that cyclinB3 (cycB3) plays a key role in the control of cell cycle progression. However, whether cycB3 is involved in stem cell fate determination remains unknown. The Drosophila ovary provides an exclusive model for studying the intrinsic and extrinsic factors that modulate the fate of germline stem cells (GSCs). Here, using this model, we show that DrosophilacycB3 plays a new role in controlling the fate of germline stem cells (GSC). Results from cycB3 genetic analyses demonstrate that cycB3 is intrinsically required for GSC maintenance. Results from green fluorescent protein (GFP)-transgene reporter assays show that cycB3 is not involved in Dad-mediated regulation of Bmp signaling, or required for dpp-induced bam transcriptional silencing. Double mutants of bam and cycB3 phenocopied bam single mutants, suggesting that cycB3 functions in a bam-dependent manner in GSCs. Deficiency of cycB3 fails to cause apoptosis in GSCs or influence cystoblast (CB) differentiation into oocytes. Furthermore, overexpression of cycB3 dramatically increases the CB number in Drosophila ovaries, suggesting that an excess of cycB3 function delays CB differentiation. Given that the cycB3 gene is evolutionarily conserved, from insects to humans, cycB3 may also be involved in controlling the fate of GSCs in humans.

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

Rop, the Sec1/Munc18 homolog in Drosophila, is required for furrow ingression and stable cell shape during cytokinesis

Physically separating daughter cells during cytokinesis requires contraction of an actin-myosin ring and vesicle-mediated membrane addition at the cleavage furrow. To identify vesicle trafficking proteins that function in cytokinesis, we screened deficiencies and mutations of candidate genes by live imaging the mitotic domains of the Drosophila embryo. In embryos homozygous for some of these deficiencies, we observed several cytokinesis phenotypes, including slow furrow ingression and increased membrane blebbing. We also found that cytokinesis required the Sec1/Munc18 homolog Rop, which interacts with syntaxin and mediates exocytosis at the plasma membrane. In a temperature-sensitive Rop mutant (Rop(TS)), the contractile ring disassembled during furrow ingression, indicating that maintenance of the ring required vesicle addition. Furthermore, in some dividing Rop(TS) cells, the shape of the daughter cells became unstable, causing cytokinesis failure. These results further highlight the importance of vesicle trafficking in animal cytokinesis and show that vesicle fusion influences cell shape during cytokinesis.

Cyclin B3 is a mitotic cyclin that promotes the metaphase-anaphase transition

The timing mechanism for mitotic progression is still poorly understood. The spindle assembly checkpoint (SAC), whose reversal upon chromosome alignment is thought to time anaphase [1-3], is functional during the rapid mitotic cycles of the Drosophila embryo; but its genetic inactivation had no consequence on the timing of the early mitoses. Mitotic cyclins-Cyclin A, Cyclin B, and Cyclin B3-influence mitotic progression and are degraded in a stereotyped sequence [4-11]. RNAi knockdown of Cyclins A and B resulted in a Cyclin B3-only mitosis in which anaphase initiated prior to chromosome alignment. Furthermore, in such a Cyclin B3-only mitosis, colchicine-induced SAC activation failed to block Cyclin B3 destruction, chromosome decondensation, or nuclear membrane re-assembly. Injection of Cyclin B proteins restored the ability of SAC to prevent Cyclin B3 destruction. Thus, SAC function depends on particular cyclin types. Changing Cyclin B3 levels showed that it accelerated progress to anaphase, even in the absence of SAC function. The impact of Cyclin B3 on anaphase initiation appeared to decline with developmental progress. Our results show that different cyclin types affect anaphase timing differently in the early embryonic divisions. The early-destroyed cyclins-Cyclins A and B-restrain anaphase-promoting complex/cyclosome (APC/C) function, whereas the late-destroyed cyclin, Cyclin B3, stimulates function. We propose that the destruction schedule of cyclin types guides mitotic exit by affecting both Cdk1 and APC/C, whose activities change as each cyclin type is lost.

Spatial reorganization of the endoplasmic reticulum during mitosis relies on mitotic kinase cyclin A in the early Drosophila embryo

Mitotic cyclin-dependent kinase with their cyclin partners (cyclin:Cdks) are the master regulators of cell cycle progression responsible for regulating a host of activities during mitosis. Nuclear mitotic events, including chromosome condensation and segregation have been directly linked to Cdk activity. However, the regulation and timing of cytoplasmic mitotic events by cyclin:Cdks is poorly understood. In order to examine these mitotic cytoplasmic events, we looked at the dramatic changes in the endoplasmic reticulum (ER) during mitosis in the early Drosophila embryo. The dynamic changes of the ER can be arrested in an interphase state by inhibition of either DNA or protein synthesis. Here we show that this block can be alleviated by micro-injection of Cyclin A (CycA) in which defined mitotic ER clusters gathered at the spindle poles. Conversely, micro-injection of Cyclin B (CycB) did not affect spatial reorganization of the ER, suggesting CycA possesses the ability to initiate mitotic ER events in the cytoplasm. Additionally, RNAi-mediated simultaneous inhibition of all 3 mitotic cyclins (A, B and B3) blocked spatial reorganization of the ER. Our results suggest that mitotic ER reorganization events rely on CycA and that control and timing of nuclear and cytoplasmic events during mitosis may be defined by release of CycA from the nucleus as a consequence of breakdown of the nuclear envelope.

Protection of neurons from high glucose-induced injury by deletion of MAD2B

Diabetic encephalopathy may lead to cognitive deficits in diabetic patients and diminish quality of life. It has been shown that protracted hyperglycaemia is directly associated with neuronal apoptosis, which is involved in diabetic encephalopathy. The anaphase-promoting complex (APC) is essential for the survival of post-mitotic neurons. In our previous study, we found that the mitotic arrest deficient protein MAD2B, one of APC inhibitors, was expressed in neurons in central nervous system. However, whether MAD2B is involved in hyperglycaemia-induced apoptosis and thus takes part in diabetic encephalopathy is still unknown. To address this issue, we first explored the expression of MAD2B and cyclin B1 detected by immunofluorescence and Western blot. It was found that hyperglycaemia remarkably increased the expression of MAD2B and accumulation of cyclin B1 in cortices of diabetes mellitus rat model and in cultured primary neurons. To further explore the role of MAD2B in hyperglycaemia-induced neuronal injury, we depleted MAD2B expression by a specifically targeted shRNA against MAD2B. We observed that MAD2B deficiency alleviated cyclin B1 expression and apoptotic neuronal death. These results demonstrate that MAD2B expression is the main culprit for accumulation of cyclin B1 and apoptosis in neurons under high glucose. Moreover, inhibition of the expression of MAD2B prevented neurons from entering an aberrant S phase that led differentiated neurons into apoptotic cell death. These results suggest that hyperglycaemia induced neuronal apoptosis through inducing expression of MAD2B, which represents a novel mechanism of diabetic encephalopathy.

Cell cycle alterations induced by urban PM2.5 in bronchial epithelial cells: characterization of the process and possible mechanisms involved

BACKGROUND

This study explores and characterizes cell cycle alterations induced by urban PM2.5 in the human epithelial cell line BEAS-2B, and elucidates possible mechanisms involved.

METHODS

The cells were exposed to a low dose (7.5 μg/cm(2)) of Milan winter PM2.5 for different time points, and the cell cycle progression was analyzed by fluorescent microscopy and flow cytometry. Activation of proteins involved in cell cycle control was investigated by Western blotting and DNA damage by (32)P-postlabelling, immunostaining and comet assay. The formation of reactive oxygen species (ROS) was quantified by flow cytometry. The role of PM organic fraction versus washed PM on the cell cycle alterations was also examined. Finally, the molecular pathways activated were further examined using specific inhibitors.

RESULTS

Winter PM2.5 induced marked cell cycle alteration already after 3 h of exposure, represented by an increased number of cells (transient arrest) in G2. This effect was associated with an increased phosphorylation of Chk2, while no changes in p53 phosphorylation were observed at this time point. The increase in G2 was followed by a transient arrest in the metaphase/anaphase transition point (10 h), which was associated with the presence of severe mitotic spindle aberrations. The metaphase/anaphase delay was apparently followed by mitotic slippage at 24 h, resulting in an increased number of tetraploid G1 cells and cells with micronuclei (MN), and by apoptosis at 40 h. Winter PM2.5 increased the level of ROS at 2 h and DNA damage (8-oxodG, single- and double stand breaks) was detected after 3 h of exposure. The PM organic fraction caused a similar G2/M arrest and augmented ROS formation, while washed PM had no such effects. DNA adducts were detected after 24 h. Both PM-induced DNA damage and G2 arrest were inhibited by the addition of antioxidants and α-naphthoflavone, suggesting the involvement of ROS and reactive electrophilic metabolites formed via a P450-dependent reaction.

CONCLUSIONS

Milan winter PM2.5 rapidly induces severe cell cycle alterations, resulting in increased frequency of cells with double nuclei and MN. This effect is related to the metabolic activation of PM2.5 organic chemicals, which cause damages to DNA and spindle apparatus.

Aurora B and cyclin B have opposite effects on the timing of cytokinesis abscission in Drosophila germ cells and in vertebrate somatic cells

Abscission is the last step of cytokinesis that physically separates the cytoplasm of sister cells. As the final stage of cell division, abscission is poorly characterized during animal development. Here, we show that Aurora B and Survivin regulate the number of germ cells in each Drosophila egg chamber by inhibiting abscission during differentiation. This inhibition is mediated by an Aurora B-dependent phosphorylation of Cyclin B, as a phosphomimic form of Cyclin B rescues premature abscission caused by a loss of function of Aurora B. We show that Cyclin B localizes at the cytokinesis bridge, where it promotes abscission. We propose that mutual inhibitions between Aurora B and Cyclin B regulate the duration of abscission and thereby the number of sister cells in each cyst. Finally, we show that inhibitions of Aurora B and Cyclin-dependent kinase 1 activity in vertebrate cells also have opposite effects on the timing of abscission, suggesting a possible conservation of these mechanisms.

Gravitational force modulates G2/M phase exit in mechanically unloaded myoblasts

Prolonged spaceflight gives rise to muscle loss and reduced strength, a condition commonly referred to as space atrophy. During exposure to microgravity, skeletal muscle myoblasts are mechanically unloaded and respond with attenuated cell proliferation, slowed cell cycle progression, and modified protein expression. To elucidate the underlying mechanisms by which muscle mass declines in response to prolonged microgravity exposure, we grew C2C12 mouse muscle cells under conditions of simulated microgravity (SM) and analyzed their proliferative capacity, cell cycle progression, and cyclin B and D expression. We demonstrated that the retarded cell growth observed in SM was correlated with an approximate 16 h delay in G₂/M phase progression, where cells accumulated specifically between the G₂ checkpoint and the onset of anaphase, concomitantly with a positive expression for cyclin B. The effect was specific for gravitational mechanical unloading as cells grown under conditions of hypergravity (HG, 4 g) for similar durations of time exhibited normal proliferation and normal cell cycle progression. Our results show that SM and HG exert phenomenological distinct responses over cell cycle progression. The deficits of SM can be restored by terrestrial gravitational force, whereas the effects of HG are indistinguishable from the 1 g control. This suggests that the mechanotransduction apparatus of cells responds differently to mechanical unloading and loading.

Inducing "cytokinesis" without mitosis in unfertilized Drosophila eggs

Selection of the cleavage plane during cytokinesis in dividing cells is linked to the position of the mitotic spindle. A major player in cleavage plane positioning is believed to be the anaphase central spindle and its associated signaling complex called centralspindlin, composed of MgcRacGap and MKLP1. Centralspindlin has the capacity to induce furrowing of the cell cortex by promoting the localized activation of RhoA, which in turn promotes assembly of the contractile ring. We have found a way to induce a cytokinesis-like process in unfertilized Drosophila eggs and very early embryos, when spindle structures are few and located far from invaginating egg cortex. The simple injection of a small molecule inhibitor of Cdk1/Cyclin B (either Roscovitin or RO3306) is sufficient to promote membrane invagination near the site of injection. The furrow generated is in many respects similar to a classical cleavage furrow. Actin, myosin, anillin and MKLP1 are all associated with the forming furrow, which in some cases can entirely circumscribe the unfertilized egg. A similar furrow can also be generated by the localized injection of constitutively active RhoA protein, suggesting that Cdk1 is normally an upstream inhibitor of RhoA activation. We show further that this process apparently is not associated with microtubules. Since simple localized inhibition of Cdk1 is sufficient to induce a furrow, we suggest that in real cytokinesis in normal cells, the localized downregulation of Cdk1 activity at the metaphase-anaphase transition may contribute, along with the spindle, to the positioning of the cleavage furrow.

Cytokinesis in animal cells

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

Cytokinetic abscission: cellular dynamics at the midbody

The intercellular canal containing the midbody is one of the most prominent structures in dividing animal cells, yet its function in the completion of cytokinesis by abscission remains largely unknown. This is because of its small size, which makes it difficult to investigate the cytoskeletal and membrane dynamics underlying abscission by standard light microscopy. The advent of new fluorescent probes and imaging technologies, along with sophisticated perturbation tools, provides new possibilities to elucidate the molecular control of this essential cell biological process. Here we discuss the control of midbody assembly and current models for the mechanism of abscission in animal cells. We highlight new methodologies that will facilitate testing and refining of these models.

Influence of cyclin type and dose on mitotic entry and progression in the early Drosophila embryo

Cyclins are key cell cycle regulators, yet few analyses test their role in timing the events that they regulate. We used RNA interference and real-time visualization in embryos to define the events regulated by each of the three mitotic cyclins of Drosophila melanogaster, CycA, CycB, and CycB3. Each individual and pairwise knockdown results in distinct mitotic phenotypes. For example, mitosis without metaphase occurs upon knockdown of CycA and CycB. To separate the role of cyclin levels from the influences of cyclin type, we knocked down two cyclins and reduced the gene dose of the one remaining cyclin. This reduction did not prolong interphase but instead interrupted mitotic progression. Mitotic prophase chromosomes formed, centrosomes divided, and nuclei exited mitosis without executing later events. This prompt but curtailed mitosis shows that accumulation of cyclin function does not directly time mitotic entry in these early embryonic cycles and that cyclin function can be sufficient for some mitotic events although inadequate for others.

Vesicles and actin are targeted to the cleavage furrow via furrow microtubules and the central spindle

During cytokinesis, cleavage furrow invagination requires an actomyosin-based contractile ring and addition of new membrane. Little is known about how this actin and membrane traffic to the cleavage furrow. We address this through live analysis of fluorescently tagged vesicles in postcellularized Drosophila melanogaster embryos. We find that during cytokinesis, F-actin and membrane are targeted as a unit to invaginating furrows through formation of F-actin–associated vesicles. F-actin puncta strongly colocalize with endosomal, but not Golgi-derived, vesicles. These vesicles are recruited to the cleavage furrow along the central spindle and a distinct population of microtubules (MTs) in contact with the leading furrow edge (furrow MTs). We find that Rho-specific guanine nucleotide exchange factor mutants, pebble (pbl), severely disrupt this F-actin–associated vesicle transport. These transport defects are a consequence of the pbl mutants' inability to properly form furrow MTs and the central spindle. Transport of F-actin–associated vesicles on furrow MTs and the central spindle is thus an important mechanism by which actin and membrane are delivered to the cleavage furrow.

Moesin and its activating kinase Slik are required for cortical stability and microtubule organization in mitotic cells

Cell division requires cell shape changes involving the localized reorganization of cortical actin, which must be tightly linked with chromosome segregation operated by the mitotic spindle. How this multistep process is coordinated remains poorly understood. In this study, we show that the actin/membrane linker moesin, the single ERM (ezrin, radixin, and moesin) protein in Drosophila melanogaster, is required to maintain cortical stability during mitosis. Mitosis onset is characterized by a burst of moesin activation mediated by a Slik kinase–dependent phosphorylation. Activated moesin homogenously localizes at the cortex in prometaphase and is progressively restricted at the equator in later stages. Lack of moesin or inhibition of its activation destabilized the cortex throughout mitosis, resulting in severe cortical deformations and abnormal distribution of actomyosin regulators. Inhibiting moesin activation also impaired microtubule organization and precluded stable positioning of the mitotic spindle. We propose that the spatiotemporal control of moesin activation at the mitotic cortex provides localized cues to coordinate cortical contractility and microtubule interactions during cell division.

An ECT2-centralspindlin complex regulates the localization and function of RhoA

In anaphase, the spindle dictates the site of contractile ring assembly. Assembly and ingression of the contractile ring involves activation of myosin-II and actin polymerization, which are triggered by the GTPase RhoA. In many cells, the central spindle affects division plane positioning via unknown molecular mechanisms. Here, we dissect furrow formation in human cells and show that the RhoGEF ECT2 is required for cortical localization of RhoA and contractile ring assembly. ECT2 concentrates on the central spindle by binding to centralspindlin. Depletion of the centralspindlin component MKLP1 prevents central spindle localization of ECT2; however, RhoA, F-actin, and myosin still accumulate on the equatorial cell cortex. Depletion of the other centralspindlin component, CYK-4/MgcRacGAP, prevents cortical accumulation of RhoA, F-actin, and myosin. CYK-4 and ECT2 interact, and this interaction is cell cycle regulated via ECT2 phosphorylation. Thus, central spindle localization of ECT2 assists division plane positioning and the CYK-4 subunit of centralspindlin acts upstream of RhoA to promote furrow assembly.

A global, myosin light chain kinase-dependent increase in myosin II contractility accompanies the metaphase-anaphase transition in sea urchin eggs

Myosin II is the force-generating motor for cytokinesis, and although it is accepted that myosin contractility is greatest at the cell equator, the temporal and spatial cues that direct equatorial contractility are not known. Dividing sea urchin eggs were placed under compression to study myosin II-based contractile dynamics, and cells manipulated in this manner underwent an abrupt, global increase in cortical contractility concomitant with the metaphase-anaphase transition, followed by a brief relaxation and the onset of furrowing. Prefurrow cortical contractility both preceded and was independent of astral microtubule elongation, suggesting that the initial activation of myosin II preceded cleavage plane specification. The initial rise in contractility required myosin light chain kinase but not Rho-kinase, but both signaling pathways were required for successful cytokinesis. Last, mobilization of intracellular calcium during metaphase induced a contractile response, suggesting that calcium transients may be partially responsible for the timing of this initial contractile event. Together, these findings suggest that myosin II-based contractility is initiated at the metaphase-anaphase transition by Ca2+-dependent myosin light chain kinase (MLCK) activity and is maintained through cytokinesis by both MLCK- and Rho-dependent signaling. Moreover, the signals that initiate myosin II contractility respond to specific cell cycle transitions independently of the microtubule-dependent cleavage stimulus.

Animal cytokinesis: from parts list to mechanisms

The mechanism underlying cytokinesis, the final step in cell division, remains one of the major unsolved questions in basic cell biology. Thanks to advances in functional genomics and proteomics, we are now able to assemble a "parts list" of proteins involved in cytokinesis. In this review, we discuss how to relate this parts list to biological mechanism. For easier analysis, we split cytokinesis into discrete steps: cleavage plane specification, rearrangement of microtubule structures, contractile ring assembly, ring ingression, and completion. We report on the advances that have been made to understand these steps and how they can be integrated into a global understanding of cytokinesis. We also discuss the extent to which classic questions have been answered and identify major outstanding questions.

Rho-kinase controls cell shape changes during cytokinesis

BACKGROUND

Animal cell cytokinesis is characterized by a sequence of dramatic cortical rearrangements. How these are coordinated and coupled with mitosis is largely unknown. To explore the initiation of cytokinesis, we focused on the earliest cell shape change, cell elongation, which occurs during anaphase B and prior to cytokinetic furrowing.

RESULTS

Using RNAi and live video microscopy in Drosophila S2 cells, we implicate Rho-kinase (Rok) and myosin II in anaphase cell elongation. rok RNAi decreased equatorial myosin II recruitment, prevented cell elongation, and caused a remarkable spindle defect where the spindle poles collided with an unyielding cell cortex and the interpolar microtubules buckled outward as they continued to extend. Disruption of the actin cytoskeleton with Latrunculin A, which abolishes cortical rigidity, suppressed the spindle defect. rok RNAi also affected furrowing, which was delayed and slowed, sometimes distorted, and in severe cases blocked altogether. Codepletion of the myosin binding subunit (Mbs) of myosin phosphatase, an antagonist of myosin II activation, only partially suppressed the cell-elongation defect and the furrowing delay, but prevented cytokinesis failures induced by prolonged rok RNAi. The marked sensitivity of cell elongation to Rok depletion was highlighted by RNAi to other genes in the Rho pathway, such as pebble, racGAP50C, and diaphanous, which had profound effects on furrowing but lesser effects on elongation.

CONCLUSIONS

We show that cortical changes underlying cell elongation are more sensitive to depletion of Rok and myosin II, in comparison to other regulators of cytokinesis, and suggest that a distinct regulatory pathway promotes cell elongation.

The Cdc14p phosphatase affects late cell-cycle events and morphogenesis in Candida albicans

We have characterized the CDC14 gene, which encodes a dual-specificity protein phosphatase in Candida albicans, and demonstrated that its deletion results in defects in cell separation, mitotic exit and morphogenesis. The C. albicans cdc14delta mutants formed large aggregates of cells that resembled those found in ace2-null strains. In cdc14delta cells, expression of Ace2p target genes was reduced and Ace2p did not accumulate specifically in daughter nuclei. Taken together, these results imply that Cdc14p is required for the activation and daughter-specific nuclear accumulation of Ace2p. Consistent with a role in cell separation, Cdc14p was targeted to the septum region during the M-G1 transition in yeast-form cells. Interestingly, hypha-inducing signals abolished the translocation of Cdc14p to the division plate, and this regulation depended on the cyclin Hgc1p, since hgc1delta mutants were able to accumulate Cdc14p in the septum region of the germ tubes. In addition to its role in cytokinesis, Cdc14p regulated mitotic exit, since synchronous cultures of cdc14delta cells exhibited a severe delay in the destruction of the mitotic cyclin Clb2p. Finally, deletion of CDC14 resulted in decreased invasion of solid agar medium and impaired true hyphal growth.

Cytokinesis regulator ECT2 changes its conformation through phosphorylation at Thr-341 in G2/M phase

The Rho activator ECT2 functions as a key regulator in cytokinesis. ECT2 is phosphorylated during G2/M phase, but the physiological significance of this event is not well known. In this study, we show that phosphorylation of ECT2 at threonine-341 (T341) affects the autoregulatory mechanism of ECT2. In G2/M phase, ECT2 was phosphorylated at T341 most likely by Cyclin B/Cyclin-dependent kinase 1 (Cdk1), and then dephosphorylated before cytokinesis. Depletion of ECT2 by RNA interference (RNAi) efficiently induced multinucleate cells. Expression of the phospho-deficient mutant of ECT2 at T341 suppressed the multinucleation induced by RNAi to ECT2, indicating that ECT2 is biologically active even when it is not phosphorylated at T341. However, the phospho-mimic mutation at T341 weakly stimulates the catalytic activity of ECT2 as detected by serum response element reporter gene assays. As T341 is located at the hinge region of the N-terminal regulatory domain and C-terminal catalytic domain, phosphorylation of T341 may help accessing downstream signaling molecules to further activate ECT2. We found that the phospho-mimic mutation T341D increases binding with itself or the N-terminal half of ECT2. These results suggest a conformational change of ECT2 upon phosphorylation at T341. Therefore, ECT2 activity might be regulated by the phosphorylation status of T341. We propose that T341 phosphorylation by Cyclin B/Cdk1 could be a trigger for further activation of ECT2.

Tum/RacGAP50C provides a critical link between anaphase microtubules and the assembly of the contractile ring in Drosophila melanogaster

A central question in understanding cytokinesis is how the cleavage plane is positioned. Although the positioning signal is likely to be transmitted via the anaphase microtubule array to the cell cortex, exactly how the microtubule array determines the site of contractile ring formation remains unresolved. By analysing tum/RacGAP50C mutant Drosophila embryos we show that cells lacking Tum do not form furrows and fail to localise the key cytokinetic components Pebble (a RhoGEF), Aurora B kinase, Diaphanous, Pav-KLP and Anillin. The GAP activity of Tum is required for cytokinesis: in its absence cytokinesis fails early even though Tum is present on microtubules at the cell equator where the furrow should form. Disruption of the Pebble-interacting domain leaves Tum localised to the cell equator on cortically associated microtubules, again with no evidence of furrowing. These data support a model in which Tum/RacGAP, via its interaction with Pbl, provides a critical link between the anaphase microtubule spindle and cytokinetic furrow formation in Drosophila cells.

Dissecting the role of Rho-mediated signaling in contractile ring formation

In anaphase, microtubules provide a specification signal for positioning of the contractile ring. However, the nature of the signal remains unknown. The small GTPase Rho is a potent regulator of cytokinesis, but the involvement of Rho in contractile ring formation is disputed. Here, we show that Rho serves as a microtubule-dependent signal that specifies the position of the contractile ring. We found that Rho translocates to the equatorial region before furrow ingression. The Rho-specific inhibitor C3 exoenzyme and small interfering RNA to the Rho GDP/GTP exchange factor ECT2 prevent this translocation and disrupt contractile ring formation, indicating that active Rho is required for contractile ring formation. ECT2 forms a complex with the GTPase-activating protein MgcRacGAP and the kinesinlike protein MKLP1 at the central spindle, and the localization of ECT2 at the central spindle depends on MgcRacGAP and MKLP1. In addition, we show that the bundled microtubules direct Rho-mediated signaling molecules to the furrowing site and regulate furrow formation. Our study provides strong evidence for the requirement of Rho-mediated signaling in contractile ring formation.

Inhibition of cyclin-dependent kinase 1 induces cytokinesis without chromosome segregation in an ECT2 and MgcRacGAP-dependent manner

Cleavage furrow formation marks the onset of cell division during early anaphase. The small GTPase RhoA and its regulators ECT2 and MgcRacGAP have been implicated in furrow ingression in mammalian cells, but the signaling upstream of these molecules remains unclear. We now show that the inhibition of cyclin-dependent kinase (Cdk)1 is sufficient to initiate cytokinesis. When mitotically synchronized cells were treated with the Cdk-specific inhibitor BMI-1026, the initiation of cytokinesis was induced precociously before chromosomal separation. Cytokinesis was also induced by the Cdk1-specific inhibitor purvalanol A but not by Cdk2/Cdk5- or Cdk4-specific inhibitors. Consistent with initiation of precocious cytokinesis by Cdk1 inhibition, introduction of anti-Cdk1 monoclonal antibody resulted in cells with aberrant nuclei. Depolymerization of mitotic spindles by nocodazole inhibited BMI-1026-induced precocious cytokinesis. However, in the presence of a low concentration of nocodazole, BMI-1026 induced excessive membrane blebbing, which appeared to be caused by formation of ectopic cleavage furrows. Depletion of ECT2 or MgcRacGAP by RNA interference abolished both of the phenotypes (precocious furrowing after nocodazole release and excessive blebbing in the presence of nocodazole). RNA interference of RhoA or expression of dominant-negative RhoA efficiently reduced both phenotypes. RhoA was localized at the cleavage furrow or at the necks of blebs. We propose that Cdk1 inactivation is sufficient to activate a signaling pathway leading to cytokinesis, which emanates from mitotic spindles and is regulated by ECT2, MgcRacGAP, and RhoA. Chemical induction of cytokinesis will be a valuable tool to study the initiation mechanism of cytokinesis.

Anillin is a substrate of anaphase-promoting complex/cyclosome (APC/C) that controls spatial contractility of myosin during late cytokinesis

Anillin, an actin-binding protein localized at the cleavage furrow, is required for cytokinesis. Through an in vitro expression screen, we identified anillin as a substrate of the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that controls mitotic progression. We found that the levels of anillin fluctuate in the cell cycle, peaking in mitosis and dropping drastically during mitotic exit. Ubiquitination of anillin required a destruction-box and was mediated by Cdh1, an activator of APC/C. Overexpression of Cdh1 reduced the levels of anillin, whereas inactivation of APC/C(Cdh1) increased the half-life of anillin. Functionally, anillin was required for the completion of cytokinesis. In anillin knockdown cells, the cleavage furrow ingressed but failed to complete the ingression. At late cytokinesis, the cytosol and DNA in knockdown cells underwent rapid myosin-based oscillatory movement across the furrow. During this movement, RhoA and active myosin were absent from the cleavage furrow, and myosin was redistributed to cortical patches, which powers the random oscillatory movement. We concluded that anillin functions to maintain the localization of active myosin, thereby ensuring the spatial control of concerted contraction during cytokinesis.

YB-1 Provokes Breast Cancer through the Induction of Chromosomal Instability That Emerges from Mitotic Failure and Centrosome Amplification

YB-1 protein levels are elevated in most human breast cancers, and high YB-1 levels have been correlated with drug resistance and poor clinical outcome. YB-1 is a stress-responsive, cell cycle-regulated transcription factor with additional functions in RNA metabolism and translation. In this study, we show in a novel transgenic mouse model that human hemagglutinin-tagged YB-1 provokes remarkably diverse breast carcinomas through the induction of genetic instability that emerges from mitotic failure and centrosome amplification. The increase of centrosome numbers proceeds during breast cancer development and explanted tumor cell cultures show the phenotype of ongoing numerical chromosomal instability. These data illustrate a mechanism that might contribute to human breast cancer development.

twin, a CCR4 homolog, regulates cyclin poly(A) tail length to permit Drosophila oogenesis

Cyclins regulate progression through the cell cycle. Control of cyclin levels is essential in Drosophila oogenesis for the four synchronous divisions that generate the 16 cell germ line cyst and for ensuring that one cell in each cyst, the oocyte, is arrested in meiosis, while the remaining fifteen cells become polyploid nurse cells. Changes in cyclin levels could be achieved by regulating transcription, translation or protein stability. The proteasome limits cyclin protein levels in the Drosophila ovary, but the mechanisms regulating RNA turnover or translation remain largely unclear. Here, we report the identification of twin, a homolog of the yeast CCR4 deadenylase. We show that twin is important for the number and synchrony of cyst divisions and oocyte fate. Consistent with the deadenylase activity of CCR4 in yeast, our data suggest that Twin controls germ line cyst development by regulating poly(A) tail lengths of several targets including Cyclin A (CycA) RNA. twin mutants exhibit very low expression of Bag-of-marbles (Bam), a regulator of cyst division, indicating that Twin/Ccr4 activity is necessary for wild-type Bam expression. Lowering the levels of CycA or increasing the levels of Bam suppresses the defects we observe in twin ovaries, implicating CycA and Bam as downstream effectors of Twin. We propose that Twin/Ccr4 functions during early oogenesis to coordinate cyst division, oocyte fate specification and egg chamber maturation.

The E2-C vihar is required for the correct spatiotemporal proteolysis of cyclin B and itself undergoes cyclical degradation

BACKGROUND

Proteolytic degradation of mitotic regulatory proteins first requires these targets to be ubiquitinated. This is regulated at the level of conjugation of ubiquitin to substrates by the anaphase-promoting complex/cyclosome (APC/C) ubiquitin-protein ligase. Substrate specificity and temporal activity of the APC/C has been thought to lie primarily with its two activators, Cdc20/Fizzy and Cdh1/Fizzy-related.

RESULTS

Here, we show that reduction in the E2 ubiquitin-conjugating enzyme (UBC) of the E2-C family that is encoded by the Drosophila gene vihar (vih), by either mutation or RNAi, leads to an accumulation of cells in a metaphase-like state. Cyclin B accumulates to high levels in all mitotic vih cells, particularly at the spindle poles. Vihar E2-C is present in the cytoplasm of mitotic cells but also associates with centrosomes, and its own degradation is initiated at the metaphase-anaphase transition. Expression of destruction D box mutants of vihar in the syncytial embryo results in mitotic arrest at late anaphase. In contrast to hypomorphic mutants, Cyclin B is degraded at the spindle poles and accumulates in the equatorial region of the spindle.

CONCLUSIONS

In Drosophila, the Vihar E2 UBC contributes to the spatiotemporal control of Cyclin B degradation that first occurs at the spindle poles. APC/C-mediated proteolysis of Vihar E2-C autoinactivates the APC/C at the centrosome before a second wave of proteolysis to degrade Cyclin B on the rest of the spindle and elsewhere in the cell.

Mechanics and regulation of cytokinesis

Recent advances are revealing quantitative aspects of cytokinesis. Further, genetic analyses and cell imaging are providing insights into the molecular dynamics of cleavage furrow ingression as well as further refining our knowledge of the zones of the mitotic spindle that regulate the contractile properties of the overlying cortex. Ultimately, however, cortical mechanics are the result of signals that emanate from the mitotic spindle. A genuine quantitative understanding of cytokinesis must include a thorough analysis of the mechanical properties of the cortex and how signals modify these properties to dictate a well-controlled, error-free cytokinesis.

Comparative Analysis of Cytokinesis in Budding Yeast, Fission Yeast and Animal Cells

Cytokinesis is a temporally and spatially regulated process through which the cellular constituents of the mother cell are partitioned into two daughter cells, permitting an increase in cell number. When cytokinesis occurs in a polarized cell it can create daughters with distinct fates. In eukaryotes, cytokinesis is carried out by the coordinated action of a cortical actomyosin contractile ring and targeted membrane deposition. Recent use of model organisms with facile genetics and improved light-microscopy methods has led to the identification and functional characterization of many proteins involved in cytokinesis. To date, this analysis indicates that some of the basic components involved in cytokinesis are conserved from yeast to humans, although their organization into functional machinery that drives cytokinesis and the associated regulatory mechanisms bear species-specific features. Here, we briefly review the current status of knowledge of cytokinesis in the budding yeast Saccharomyces cerevisiae, the fission yeast Schizosaccharomyces pombe and animal cells, in an attempt to highlight both the common and the unique features. Although these organisms diverged from a common ancestor about a billion years ago, there are eukaryotes that are far more divergent. To evaluate the overall evolutionary conservation of cytokinesis, it will be necessary to include representatives of these divergent branches. Nevertheless, the three species discussed here provide substantial mechanistic diversity.

Cytokinesis: progress on all fronts

Cell multiplication requires sequestration of the duplicated and segregated genome into two daughter cells. The mitotic spindle is critical for orchestrating sister chromatid separation and division plane positioning. During anaphase, spindle microtubules become bundled to form the central spindle, which is essential for completion of cytokinesis. Central spindle assembly is mediated by a microtubule-associated protein and a kinesin-RhoGAP complex, both of which are regulated by phosphorylation/dephosphorylation. The central spindle also plays a role in cleavage furrow positioning, which appears to involve activation of RhoA. New results have provided some initial clues as to how furrow positioning is achieved. Particularly notable is the discovery that a protein activated by RhoA, formin, has actin nucleation activity.

Animal cell division: a fellowship of the double ring?

Despite a century of research into the nature of animal cell division, a molecular explanation for the positioning of the actomyosin contractile ring has remained elusive. The discovery of a novel interaction between regulators of Rho family small GTPases has revealed a link between the mitotic microtubules and the contractile ring during the later stages of mitosis. The properties of the interacting Rho regulators suggest a molecular model for the positioning and initiation of contractile ring furrowing in animal cells. In this 'double ring' model, centralspindlin complexes, localized by the action of their kinesin-like protein component, position and activate a cortical equatorial ring of Rho GTPase exchange factors. The resulting ring of activated Rho would then trigger a cascade of events leading to formation and constriction of the contractile ring.

Inhibition of JNK2 disrupts anaphase and produces aneuploidy in mammalian cells

The JNK family members JNK1 and JNK2 regulate tumor growth and are essential for transformation by oncogenes such as constitutively activated Ras. The mechanisms downstream of JNK that regulate cell cycle progression and transformation are unclear. Here we show that inhibition of JNK2, but not JNK1, with either a dominant-negative mutant, a pharmacological inhibitor, or RNA interference caused an accumulation of mammalian cells with 4N DNA content. When observed by immunofluorescence, these cells progressed to metaphase without apparent defects in spindle formation or chromosome alignment to the metaphase plate, suggesting that the 4N accumulation is a result of postmetaphase defects. Consistent with this prediction, when JNK activity was suppressed, we observed defects in central spindle formation and chromosome segregation during anaphase. In contrast, cyclin-dependent kinase 1 activity, cyclin B1 protein, and Polo-like kinase 1 protein turnover remained intact when JNK was inhibited. In addition, continued inhibition of JNK activity did not block reentry into subsequent cell cycles but instead resulted in polyploidy. This evidence suggests that JNK2 functions in maintaining the genomic stability of mammalian cells by signaling that is independent of cyclin-dependent kinase 1/cyclin B1 down-regulation.

Linked for life: temporal and spatial coordination of late mitotic events

Establishing the temporal order of mitotic events is critical to ensure that each daughter cell receives a complete DNA complement. The spatial co-ordination of the cytokinetic ring site with the axis of chromosome segregation is likewise crucial. Recent studies in fungi indicate that regulators of chromosome segregation also participate in promoting mitotic exit and that the proteins that initiate mitotic exit, in turn, additionally regulate cytokinesis. These findings suggest that late mitotic events are coupled by employing one pathway to control multiple events. The regulatory mechanisms that ensure the spatial co-ordination of the mitotic spindle apparatus with the division site have also been elucidated recently in the asymmetrically dividing budding yeast. Interestingly, the spatial co-ordination of late mitotic events seems also to be important in higher eukaryotes.

Regulation of Cell Cycles in Drosophila Development: Intrinsic and Extrinsic Cues

An intriguing aspect of cell cycle regulation is how cell growth and division are coordinated with developmental signals to produce properly patterned organisms of the appropriate size. Using the foundation laid by a detailed understanding of the regulators that intrinsically control progression through the cell cycle, links between developmental signals and the cell cycle are being elucidated. Considerable progress has been made using Drosophila melanogaster, both in identifying new cell cycle regulators that respond to developmental cues and in defining the impact of extrinsic signals on homologs of mammalian oncogenes and tumor suppressors. In this review, we discuss each cell cycle phase, highlighting differences between archetypal and variant cell cycles employed for specific developmental strategies. We emphasize the interplay between developmental signals and cell cycle transitions. Developmental control of checkpoints, cell cycle exit, and cell growth are also addressed.

Dispatch. Cell division: the place and time of cytokinesis

Recent studies have provided evidence that, during cytokinesis, activation of the Pbl-Rho1 pathway by a protein complex located at the spindle midzone, and inhibition of this pathway by two mitotic cyclins, may be major contributing factors controlling the place and timing of the cleavage furrow.