The role of anillin in meiotic cytokinesis of Drosophila males

Germ fate determinants protect germ precursor cell division by reducing septin and anillin levels at the cell division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formincyk-1(ts) mutant Caenorhabditis elegans 4-cell embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide with greatly reduced F-actin levels at the cell division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septinUNC-59 and anillinANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into the regulation of cytokinesis in other cell types, especially in stem cells with high potency.

Germ fate determinants protect germ precursor cell division by restricting septin and anillin levels at the division plane

Animal cell cytokinesis, or the physical division of one cell into two, is thought to be driven by constriction of an actomyosin contractile ring at the division plane. The mechanisms underlying cell type-specific differences in cytokinesis remain unknown. Germ cells are totipotent cells that pass genetic information to the next generation. Previously, using formin cyk-1 (ts) mutant C. elegans embryos, we found that the P2 germ precursor cell is protected from cytokinesis failure and can divide without detectable F-actin at the division plane. Here, we identified two canonical germ fate determinants required for P2-specific cytokinetic protection: PIE-1 and POS-1. Neither has been implicated previously in cytokinesis. These germ fate determinants protect P2 cytokinesis by reducing the accumulation of septin UNC-59 and anillin ANI-1 at the division plane, which here act as negative regulators of cytokinesis. These findings may provide insight into cytokinetic regulation in other cell types, especially in stem cells with high potency.

Fusome topology and inheritance during insect gametogenesis

From insects to mammals, oocytes and sperm develop within germline cysts comprising cells connected by intercellular bridges (ICBs). In numerous insects, formation of the cyst is accompanied by growth of the fusome-a membranous organelle that permeates the cyst. Fusome composition and function are best understood in Drosophila melanogaster: during oogenesis, the fusome dictates cyst topology and size and facilitates oocyte selection, while during spermatogenesis, the fusome synchronizes the cyst's response to DNA damage. Despite its distinct and sex-specific roles during insect gametogenesis, elucidating fusome growth and inheritance in females and its structure and connectivity in males has remained challenging. Here, we take advantage of advances in three-dimensional (3D) confocal microscopy and computational image processing tools to reconstruct the topology, growth, and distribution of the fusome in both sexes. In females, our experimental findings inform a theoretical model for fusome assembly and inheritance and suggest that oocyte selection proceeds through an 'equivalency with a bias' mechanism. In males, we find that cell divisions can deviate from the maximally branched pattern observed in females, leading to greater topological variability. Our work consolidates existing disjointed experimental observations and contributes a readily generalizable computational approach for quantitative studies of gametogenesis within and across species.

Evolutionary conserved relocation of chromatin remodeling complexes to the mitotic apparatus

Background

ATP-dependent chromatin remodeling complexes are multi-protein machines highly conserved across eukaryotic genomes. They control sliding and displacing of the nucleosomes, modulating histone-DNA interactions and making nucleosomal DNA more accessible to specific binding proteins during replication, transcription, and DNA repair, which are processes involved in cell division. The SRCAP and p400/Tip60 chromatin remodeling complexes in humans and the related Drosophila Tip60 complex belong to the evolutionary conserved INO80 family, whose main function is promoting the exchange of canonical histone H2A with the histone variant H2A in different eukaryotic species. Some subunits of these complexes were additionally shown to relocate to the mitotic apparatus and proposed to play direct roles in cell division in human cells. However, whether this phenomenon reflects a more general function of remodeling complex components and its evolutionary conservation remains unexplored.

Results

We have combined cell biology, reverse genetics, and biochemical approaches to study the subcellular distribution of a number of subunits belonging to the SRCAP and p400/Tip60 complexes and assess their involvement during cell division progression in HeLa cells. Interestingly, beyond their canonical chromatin localization, the subunits under investigation accumulate at different sites of the mitotic apparatus (centrosomes, spindle, and midbody), with their depletion yielding an array of aberrant outcomes of mitosis and cytokinesis, thus causing genomic instability. Importantly, this behavior was conserved by the Drosophila melanogaster orthologs tested, despite the evolutionary divergence between fly and humans has been estimated at approximately 780 million years ago.

Conclusions

Overall, our results support the existence of evolutionarily conserved diverse roles of chromatin remodeling complexes, whereby subunits of the SRCAP and p400/Tip60 complexes relocate from the interphase chromatin to the mitotic apparatus, playing moonlighting functions required for proper execution of cell division.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01365-5.

A meiotic switch in lysosome activity supports spermatocyte development in young flies but collapses with age

Gamete development ultimately influences animal fertility. Identifying mechanisms that direct gametogenesis, and how they deteriorate with age, may inform ways to combat infertility. Recently, we found that lysosomes acidify during oocyte maturation in Caenorhabditis elegans, suggesting that a meiotic switch in lysosome activity promotes female germ-cell health. Using Drosophila melanogaster, we report that lysosomes likewise acidify in male germ cells during meiosis. Inhibiting lysosomes in young-male testes causes E-cadherin accumulation and loss of germ-cell partitioning membranes. Notably, analogous changes occur naturally during aging; in older testes, a reduction in lysosome acidity precedes E-cadherin accumulation and membrane dissolution, suggesting one potential cause of age-related spermatocyte abnormalities. Consistent with lysosomes governing the production of mature sperm, germ cells with homozygous-null mutations in lysosome-acidifying machinery fail to survive through meiosis. Thus, lysosome activation is entrained to meiotic progression in developing sperm, as in oocytes, and lysosomal dysfunction may instigate male reproductive aging.

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Lysosomes acidify at the mitotic-meiotic transition in the testis

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Acidic lysosomes support germ-cell membrane stability

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Lysosome acidity naturally declines in the aging male germline

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Lysosome acidification is required for mature sperm production

Microtubule and Actin Cytoskeletal Dynamics in Male Meiotic Cells of Drosophila melanogaster

Drosophila dividing spermatocytes offer a highly suitable cell system in which to investigate the coordinated reorganization of microtubule and actin cytoskeleton systems during cell division of animal cells. Like male germ cells of mammals, Drosophila spermatogonia and spermatocytes undergo cleavage furrow ingression during cytokinesis, but abscission does not take place. Thus, clusters of primary and secondary spermatocytes undergo meiotic divisions in synchrony, resulting in cysts of 32 secondary spermatocytes and then 64 spermatids connected by specialized structures called ring canals. The meiotic spindles in Drosophila males are substantially larger than the spindles of mammalian somatic cells and exhibit prominent central spindles and contractile rings during cytokinesis. These characteristics make male meiotic cells particularly amenable to immunofluorescence and live imaging analysis of the spindle microtubules and the actomyosin apparatus during meiotic divisions. Moreover, because the spindle assembly checkpoint is not robust in spermatocytes, Drosophila male meiosis allows investigating of whether gene products required for chromosome segregation play additional roles during cytokinesis. Here, we will review how the research studies on Drosophila male meiotic cells have contributed to our knowledge of the conserved molecular pathways that regulate spindle microtubules and cytokinesis with important implications for the comprehension of cancer and other diseases.

The ATPase SRCAP is associated with the mitotic apparatus, uncovering novel molecular aspects of Floating-Harbor syndrome

Background

A variety of human genetic diseases is known to be caused by mutations in genes encoding chromatin factors and epigenetic regulators, such as DNA or histone modifying enzymes and members of ATP-dependent chromatin remodeling complexes. Floating-Harbor syndrome is a rare genetic disease affecting human development caused by dominant truncating mutations in the SRCAP gene, which encodes the ATPase SRCAP, the core catalytic subunit of the homonymous chromatin-remodeling complex. The main function of the SRCAP complex is to promote the exchange of histone H2A with the H2A.Z variant. According to the canonical role played by the SRCAP protein in epigenetic regulation, the Floating-Harbor syndrome is thought to be a consequence of chromatin perturbations. However, additional potential physiological functions of SRCAP have not been sufficiently explored.

Results

We combined cell biology, reverse genetics, and biochemical approaches to study the subcellular localization of the SRCAP protein and assess its involvement in cell cycle progression in HeLa cells. Surprisingly, we found that SRCAP associates with components of the mitotic apparatus (centrosomes, spindle, midbody), interacts with a plethora of cytokinesis regulators, and positively regulates their recruitment to the midbody. Remarkably, SRCAP depletion perturbs both mitosis and cytokinesis. Similarly, DOM-A, the functional SRCAP orthologue in Drosophila melanogaster, is found at centrosomes and the midbody in Drosophila cells, and its depletion similarly affects both mitosis and cytokinesis.

Conclusions

Our findings provide first evidence suggesting that SRCAP plays previously undetected and evolutionarily conserved roles in cell division, independent of its functions in chromatin regulation. SRCAP may participate in two different steps of cell division: by ensuring proper chromosome segregation during mitosis and midbody function during cytokinesis. Moreover, our findings emphasize a surprising scenario whereby alterations in cell division produced by SRCAP mutations may contribute to the onset of Floating-Harbor syndrome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01109-x.

Knockdown of Anillin Actin Binding Protein Blocks Cytokinesis in Hepatocytes and Reduces Liver Tumor Development in Mice Without Affecting Regeneration

BACKGROUND & AIMS

Cytokinesis can fail during normal postnatal liver development, leading to polyploid hepatocytes. We investigated whether inhibiting cytokinesis in the liver slows tumor growth without compromising the health of normal hepatocytes. We inhibited cytokinesis in cancer cells by knocking down ANLN, a cytoskeletal scaffolding protein that regulates cytokinesis and might promote tumorigenesis, in mice with liver disease.

METHODS

We analyzed clinical and gene expression data from The Cancer Genome Atlas, Oncomine, PrognoScan, and a hepatocellular carcinoma (HCC) tissue microarray. We knocked down ANLN with small interfering RNAs (siRNAs) in H2.35 liver cells and performed image analyses of cells undergoing cytokinesis. siRNAs were delivered to LAP-MYC mice, which develop hepatoblastoma, using lipid nanoparticles. H2.35 cells with knockdown of ANLN or control cells were injected into FRG mice, which develop chronic liver damage, and tumor growth was monitored. We also developed mice with inducible expression of transgenes encoding small hairpin RNAs (shRNAs) against Anln messenger RNA and studied liver tumorigenesis after administration of diethylnitrosamine and carbon tetrachloride. siRNAs against Anln messenger RNA were conjugated to N-acetylgalactosamine to reduce toxicity and increase hepatocyte tropism; their effects were studied in mouse models of liver cancer and regeneration.

RESULTS

Levels of ANLN messenger RNA were increased in human HCC tissues compared to non-tumor liver tissues. siRNA knockdown of ANLN blocked cytokinesis in H2.35 liver cells. Administration of siRNA against ANLN increased survival times of LAP-MYC mice, compared to mice given a control siRNA. H2.35 liver cells with shRNA knockdown of ANLN formed tumors more slowly in FRG mice than control H2.35 cells. Mice with inducible expression of shRNAs against Anln mRNA developed fewer liver tumors after administration of diethylnitrosamine and carbon tetrachloride than control mice. Knockdown of ANLN did not affect liver regeneration after acute and chronic liver injuries.

CONCLUSIONS

Knockdown of ANLN in liver cells blocks cytokinesis and inhibits development of liver tumors in mice. Agents that inhibit ANLN in the liver might be effective for prevention or treatment of HCC.

Dual strands of the miR-223 duplex (miR-223-5p and miR-223-3p) inhibit cancer cell aggressiveness: targeted genes are involved in bladder cancer pathogenesis

Analyses of microRNA (miRNA) expression signatures obtained by RNA sequencing revealed that some passenger miRNAs (miR-144-5p, miR-145-3p, miR-149-3p, miR-150-3p, and miR-199a-3p) acted as anti-tumor miRNAs in several types of cancer cells. The involvement of passenger strands in the pathogenesis of human cancer is a novel concept. Based on the miRNA signature of bladder cancer (BC) obtained by RNA sequencing, we focused on both strands of the miR-223-duplex (miR-223-5p and miR-223-3p) and investigated their functional significance in BC cells. Ectopic expression of these miRNAs showed that both miR-223-3p (the guide strand) and miR-223-5p (the passenger strand) inhibited cancer cell migration and invasion of BC cells. The role of miR-223-5p (the passenger strand) has not been well studied. Combining gene expression studies and in silico database analyses, we demonstrated the presence of 20 putative target genes that could be regulated by miR-223-5p in BC cells. Among these targets, high expression of five genes (ANLN, INHBA, OIP5, CCNB1, and CDCA2) was significantly associated with poor prognosis of BC patients based on The Cancer Genome Atlas (TCGA) database. Moreover, we showed that a gene (ANLN) encoding a multifunctional actin-binding protein was directly regulated by miR-223-5p in BC cells. Overexpression of ANLN was observed in BC clinical specimens and high expression of ANLN was significantly associated with poor prognosis of BC patients. We suggest that studies of regulatory cancer networks, including the passenger strands of miRNAs, may provide new insights into the pathogenic mechanisms of BC.

Phosphoinositide signaling in sperm development

Phosphatidylinositol phosphates (PIPs)¹ are membrane lipids with crucial roles during cell morphogenesis, including the establishment of cytoskeletal organization, membrane trafficking, cell polarity, cell-cycle control and signaling. Recent studies in mice (Mus musculus), fruit flies (Drosophila melanogaster) and other organisms have defined germ cell intrinsic requirements for these lipids and their regulatory enzymes in multiple aspects of sperm development. In particular, PIP levels are crucial in germline stem cell maintenance, spermatogonial proliferation and survival, spermatocyte cytokinesis, spermatid polarization, sperm tail formation, nuclear shaping, and production of mature, motile sperm. Here, we briefly review the stages of spermatogenesis and discuss the roles of PIPs and their regulatory enzymes in male germ cell development.

Imaging cellular structures in super-resolution with SIM, STED and Localisation Microscopy: A practical comparison

Many biological questions require fluorescence microscopy with a resolution beyond the diffraction limit of light. Super-resolution methods such as Structured Illumination Microscopy (SIM), STimulated Emission Depletion (STED) microscopy and Single Molecule Localisation Microscopy (SMLM) enable an increase in image resolution beyond the classical diffraction-limit. Here, we compare the individual strengths and weaknesses of each technique by imaging a variety of different subcellular structures in fixed cells. We chose examples ranging from well separated vesicles to densely packed three dimensional filaments. We used quantitative and correlative analyses to assess the performance of SIM, STED and SMLM with the aim of establishing a rough guideline regarding the suitability for typical applications and to highlight pitfalls associated with the different techniques.

An Overview of Alternating Electric Fields Therapy (NovoTTF Therapy) for the Treatment of Malignant Glioma

As with many cancer treatments, tumor treating fields (TTFields) target rapidly dividing tumor cells. During mitosis, TTFields-exposed cells exhibit uncontrolled membrane blebbing at the onset of anaphase, resulting in aberrant mitotic exit. Based on these criteria, at least two protein complexes have been proposed as TTFields’ molecular targets, including α/β-tubulin and the septin 2, 6, 7 heterotrimer. After aberrant mitotic exit, cells exhibited abnormal nuclei and signs of cellular stress, including decreased cellular proliferation and p53 dependence, and exhibit the hallmarks of immunogenic cell death, suggesting that TTFields treatment may induce an antitumor immune response. Clinical trials lead to Food and Drug Administration approval for their treatment of recurrent glioblastoma. Detailed modeling of TTFields within the brain suggests that the location of the tumor may affect treatment efficacy. These observations have a profound impact on the use of TTFields in the clinic, including what co-therapies may be best applied to boost its efficacy.

Tumor treating fields perturb the localization of septins and cause aberrant mitotic exit

The anti-tumor effects of chemotherapy and radiation are thought to be mediated by triggering G₁/S or G₂/M cell cycle checkpoints, while spindle poisons, such as paclitaxel, block metaphase exit by initiating the spindle assembly checkpoint. In contrast, we have found that 150 kilohertz (kHz) alternating electric fields, also known as Tumor Treating Fields (TTFields), perturbed cells at the transition from metaphase to anaphase. Cells exposed to the TTFields during mitosis showed normal progression to this point, but exhibited uncontrolled membrane blebbing that coincided with metaphase exit. The ability of such alternating electric fields to affect cellular physiology is likely to be dependent on their interactions with proteins possessing high dipole moments. The mitotic Septin complex consisting of Septin 2, 6 and 7, possesses a high calculated dipole moment of 2711 Debyes (D) and plays a central role in positioning the cytokinetic cleavage furrow, and governing its contraction during ingression. We showed that during anaphase, TTFields inhibited Septin localization to the anaphase spindle midline and cytokinetic furrow, as well as its association with microtubules during cell attachment and spreading on fibronectin. After aberrant metaphase exit as a consequence of TTFields exposure, cells exhibited aberrant nuclear architecture and signs of cellular stress including an overall decrease in cellular proliferation, followed by apoptosis that was strongly influenced by the p53 mutational status. Thus, TTFields are able to diminish cell proliferation by specifically perturbing key proteins involved in cell division, leading to mitotic catastrophe and subsequent cell death.

Anillin localization suggests distinct mechanisms of division plane specification in mouse oogenic meiosis I and II

Anillin is a conserved cytokinetic ring protein implicated in actomyosin cytoskeletal organization and cytoskeletal-membrane linkage. Here we explored anillin localization in the highly asymmetric divisions of the mouse oocyte that lead to the extrusion of two polar bodies. The purposes of polar body extrusion are to reduce the chromosome complement within the egg to haploid, and to retain the majority of the egg cytoplasm for embryonic development. Anillin's proposed roles in cytokinetic ring organization suggest that it plays important roles in achieving this asymmetric division. We report that during meiotic maturation, anillin mRNA is expressed and protein levels steadily rise. In meiosis I, anillin localizes to a cortical cap overlying metaphase I spindles, and a broad ring over anaphase spindles that are perpendicular to the cortex. Anillin is excluded from the cortex of the prospective first polar body, and highly enriched in the cytokinetic ring that severs the polar body from the oocyte. In meiosis II, anillin is enriched in a cortical stripe precisely coincident with and overlying the meiotic spindle midzone. These results suggest a model in which this cortical structure contributes to spindle re-alignment in meiosis II. Thus, localization of anillin as a conserved cytokinetic ring marker illustrates that the geometry of the cytokinetic ring is distinct between the two oogenic meiotic cytokineses in mammals.

Cytokinesis in Drosophila male meiosis

Cytokinesis separates the cytoplasm and the duplicated genome into two daughter cells at the end of cell division. This process must be finely regulated to maintain ploidy and prevent tumor formation. Drosophila male meiosis provides an excellent cell system for investigating cytokinesis. Mutants affecting this process can be easily identified and spermatocytes are large cells particularly suitable for cytological analysis of cytokinetic structures. Over the past decade, the powerful tools of Drosophila genetics and the unique characteristics of this cell system have led researchers to identify molecular players of the cell cleavage machinery and to address important open questions. Although spermatocyte cytokinesis is incomplete, resulting in formation of stable intercellular bridges, the molecular mechanisms are largely conserved in somatic cells. Thus, studies of Drosophila male meiosis will shed new light on the complex cell circuits regulating furrow ingression and substantially further our knowledge of cancer and other human diseases.

Orbit/CLASP is required for myosin accumulation at the cleavage furrow in Drosophila male meiosis

Peripheral microtubules (MTs) near the cell cortex are essential for the positioning and continuous constriction of the contractile ring (CR) in cytokinesis. Time-lapse observations of Drosophila male meiosis showed that myosin II was first recruited along the cell cortex independent of MTs. Then, shortly after peripheral MTs made contact with the equatorial cortex, myosin II was concentrated there in a narrow band. After MT contact, anillin and F-actin abruptly appeared on the equatorial cortex, simultaneously with myosin accumulation. We found that the accumulation of myosin did not require centralspindlin, but was instead dependent on Orbit, a Drosophila ortholog of the MT plus-end tracking protein CLASP. This protein is required for stabilization of central spindle MTs, which are essential for cytokinesis. Orbit was also localized in a mid-zone of peripheral MTs, and was concentrated in a ring at the equatorial cortex during late anaphase. Fluorescence resonance energy transfer experiments indicated that Orbit is closely associated with F-actin in the CR. We also showed that the myosin heavy chain was in close proximity with Orbit in the cleavage furrow region. Centralspindlin was dispensable in Orbit ring formation. Instead, the Polo-KLP3A/Feo complex was required for the Orbit accumulation independently of the Orbit MT-binding domain. However, orbit mutations of consensus sites for the phosphorylation of Cdk1 or Polo did not influence the Orbit accumulation, suggesting an indirect regulatory role of these protein kinases in Orbit localization. Orbit was also necessary for the maintenance of the CR. Our data suggest that Orbit plays an essential role as a connector between MTs and the CR in Drosophila male meiosis.

Investigating spermatogenesis in Drosophila melanogaster

The process of spermatogenesis in Drosophila melanogaster provides a powerful model system to probe a variety of developmental and cell biological questions, such as the characterization of mechanisms that regulate stem cell behavior, cytokinesis, meiosis, and mitochondrial dynamics. Classical genetic approaches, together with binary expression systems, FRT-mediated recombination, and novel imaging systems to capture single cell behavior, are rapidly expanding our knowledge of the molecular mechanisms regulating all aspects of spermatogenesis. This methods chapter provides a detailed description of the system, a review of key questions that have been addressed or remain unanswered thus far, and an introduction to tools and techniques available to probe each stage of spermatogenesis.

Formin' cellular structures: Physiological roles of Diaphanous (Dia) in actin dynamics

Members of the Diaphanous (Dia) protein family are key regulators of fundamental actin driven cellular processes, which are conserved from yeast to humans. Researchers have uncovered diverse physiological roles in cell morphology, cell motility, cell polarity, and cell division, which are involved in shaping cells into tissues and organs. The identification of numerous binding partners led to substantial progress in our understanding of the differential functions of Dia proteins. Genetic approaches and new microscopy techniques allow important new insights into their localization, activity, and molecular principles of regulation.

Drosophila rae1 is required for male meiosis and spermatogenesis

Previous studies of RAE1, a conserved WD40 protein, in Schizosaccharomyces pombe and mouse revealed a role in mRNA export and cell cycle progression in mitotic cells. Studies of RAE1 in Drosophila showed that the protein localizes to the nuclear envelope and is required for progression through the G1 phase of the cell cycle but not RNA export in tissue culture cells. Drosophila RAE1 also plays an essential developmental role, as it is required for viability and synaptic growth regulation as a component of an E3 ubiquitin ligase complex. Here we describe characterization of a new Drosophila rae1 mutant that is viable but results in male sterility. The mutant showed striking defects in primary spermatocyte nuclear integrity, meiotic chromosome condensation, segregation, and spindle morphology. These defects led to a failure to complete meiosis but allowed several aspects of spermatid differentiation to proceed, including axoneme formation and elongation. A GFP-RAE1 fusion protein that rescued most of the cytological defects showed a dynamic localization to the nuclear envelope, chromatin and other structures depending on the stage of spermatogenesis. A role for RAE1 in male meiosis, as well as mitotic cells, was also indicated by the defects induced by expression of rae1-RNAi. These studies in Drosophila provide the first evidence for an essential meiotic role of RAE1, and thus define RAE1 as a protein required for both meiotic and mitotic cell cycles.

Spatiotemporal control of Cindr at ring canals during incomplete cytokinesis in the Drosophila male germline

During male and female gametogenesis in species ranging from insects to mammals, germ cell cyst formation by incomplete cytokinesis involves the stabilization of cleavage furrows and the formation of stable intercellular bridges called ring canals. Accurate regulation of incomplete cytokinesis is required for both female and male fertility in Drosophila melanogaster. Nevertheless, the molecular mechanisms controlling complete versus incomplete cytokinesis are largely unknown. Here, we show that the scaffold protein Cindr is a novel component of both mitotic and meiotic ring canals during Drosophila spermatogenesis. Strikingly, unlike other male germline ring canal components, including Anillin and Pavarotti, Cindr and contractile ring F-actin dissociate from mitotic ring canals and translocate to the fusome upon completion of the mitotic germ cell divisions. We provide evidence that the loss of Cindr from mitotic ring canals is coordinated by signals that mediate the transition from germ cell mitosis to differentiation. Interestingly, Cindr loss from ring canals coincides with completion of the mitotic germ cell divisions in both Drosophila females and males, thus marking a common step of gametogenesis. We also show that Cindr co-localizes with Anillin at mitotic and meiotic ring canals and is recruited to the contractile ring by Anillin during male germ cell meiotic cytokinesis. Taken together, our analyses reveal a key step of incomplete cytokinesis at the endpoint of the mitotic germ cell divisions in D. melanogaster.

What Drosophila spermatocytes tell us about the mechanisms underlying cytokinesis

Cytokinesis separates the genomic material and organelles of a dividing cell equitably into two physically distinct daughter cells at the end of cell division. This highly choreographed process involves coordinated reorganization and regulated action of the actin and microtubule cytoskeletal systems, an assortment of motor proteins, and membrane trafficking components. Due to their large size, the ease with which exquisite cytological analysis may be performed on them, and the availability of numerous mutants and other genetic tools, Drosophila spermatocytes have provided an excellent system for exploring the mechanistic basis for the temporally programmed and precise spatially localized events of cytokinesis. Mutants defective in male meiotic cytokinesis can be easily identified in forward genetic screens by the production of multinucleate spermatids. In addition, the weak spindle assembly checkpoint in spermatocytes, which causes only a small delay of anaphase onset in the presence of unattached chromosomes, allows investigation of whether gene products required for spindle assembly and chromosome segregation are also involved in cytokinesis. Perhaps due to the large size of spermatocytes and the requirement for two rapid-fire rounds of division without intervening S or growth phases during meiosis, male meiotic mutants have also revealed much about molecular mechanisms underlying new membrane addition during cytokinesis. Finally, cell type-specific differences in the events that set up and complete cytokinesis are emerging from comparison of spermatocytes with cells undergoing mitosis either elsewhere in the organism or in tissue culture.

Mutations in Cog7 affect Golgi structure, meiotic cytokinesis and sperm development during Drosophila spermatogenesis

The conserved oligomeric Golgi (COG) complex plays essential roles in Golgi function, vesicle trafficking and glycosylation. Deletions in the human COG7 gene are associated with a rare multisystemic congenital disorder of glycosylation that causes mortality within the first year of life. In this paper, we characterise the Drosophila orthologue of COG7 (Cog7). Loss-of-function Cog7 mutants are viable but male sterile. The Cog7 gene product is enriched in the Golgi stacks and in Golgi-derived structures throughout spermatogenesis. Mutations in the Cog7 gene disrupt Golgi architecture and reduce the number of Golgi stacks in primary spermatocytes. During spermiogenesis, loss of the Cog7 protein impairs the assembly of the Golgi-derived acroblast in spermatids and affects axoneme architecture. Similar to the Cog5 homologue, four way stop (Fws), Cog7 enables furrow ingression during cytokinesis. We show that the recruitment of the small GTPase Rab11 and the phosphatidylinositol transfer protein Giotto (Gio) to the cleavage site requires a functioning wild-type Cog7 gene. In addition, Gio coimmunoprecipitates with Cog7 and with Rab11 in the testes. Our results altogether implicate Cog7 as an upstream component in a gio-Rab11 pathway controlling membrane addition during cytokinesis.

Cytokinesis in Drosophila melanogaster

Cytokinesis is the process that physically separates two sibling cells, ensuring the proper partitioning of the nuclear and cytoplasmic contents at the end of cell division. Cytokinesis requires a fine-tuned molecular machinery that has to be assembled with high spatiotemporal precision. Drosophila melanogaster is an ideal model system to investigate this cellular process. Genetic screens performed in spermatocytes, neuroblasts, and Schneider cells revealed numerous evolutionary conserved components. These genetically amenable systems have proven to be very useful to further elucidate the cellular and molecular mechanism of cytokinesis, significantly contributing to our current understanding of this important cellular process. As in other organisms, cytokinesis is largely dependent on the mitotic spindle, providing positional cues for cleavage furrow placement and progression. However, spindle-independent mechanisms could also be important during special cases of cytokinesis, such as asymmetric cell division. Thus, powerful fly genetics combined with single-cell analysis, live imaging, and biochemical assays will continue to provide important insights into the mechanism of cytokinesis.

The Drosophila RZZ complex - roles in membrane trafficking and cytokinesis

The Zw10 protein, in the context of the conserved Rod-Zwilch-Zw10 (RZZ) complex, is a kinetochore component required for proper activity of the spindle assembly checkpoint in both Drosophila and mammals. In mammalian and yeast cells, the Zw10 homologues, together with the conserved RINT1/Tip20p and NAG/Sec39p proteins, form a second complex involved in vesicle transport between Golgi and ER. However, it is currently unknown whether Zw10 and the NAG family member Rod are also involved in Drosophila membrane trafficking. Here we show that Zw10 is enriched at both the Golgi stacks and the ER of Drosophila spermatocytes. Rod is concentrated at the Golgi but not at the ER, whereas Zwilch does not accumulate in any membrane compartment. Mutations in zw10 and RNAi against the Drosophila homologue of RINT1 (rint1) cause strong defects in Golgi morphology and reduce the number of Golgi stacks. Mutations in rod also affect Golgi morphology, whereas zwilch mutants do not exhibit gross Golgi defects. Loss of either Zw10 or Rint1 results in frequent failures of spermatocyte cytokinesis, whereas Rod or Zwilch are not required for this process. Spermatocytes lacking zw10 or rint1 function assemble regular central spindles and acto-myosin rings, but furrow ingression halts prematurely due to defective plasma membrane addition. Collectively, our results suggest that Zw10 and Rint1 cooperate in the ER-Golgi trafficking and in plasma membrane formation during spermatocyte cytokinesis. Our findings further suggest that Rod plays a Golgi-related function that is not required for spermatocyte cytokinesis.

Anillin acts as a bifunctional linker coordinating midbody ring biogenesis during cytokinesis

Animal cell cytokinesis proceeds via constriction of an actomyosin-based contractile ring (CR) [1, 2]. Upon reaching a diameter of ~1 μm [3], a midbody ring (MR) forms to stabilize the intercellular bridge until abscission [4-6]. How MR formation is coupled to CR closure and how plasma membrane anchoring is maintained at this key transition is unknown. Time-lapse microscopy of Drosophila S2 cells depleted of the scaffold protein Anillin [7-9] revealed that Anillin is required for complete closure of the CR and formation of the MR. Truncation analysis revealed that Anillin N termini connected with the actomyosin CR and supported formation of stable MR-like structures, but these could not maintain anchoring of the plasma membrane. Conversely, Anillin C termini failed to connect with the CR or MR but recruited the septin Peanut to ectopic structures at the equatorial cortex. Peanut depletion mimicked truncation of the Anillin C terminus, resulting in MR-like structures that failed to anchor the membrane. These data demonstrate that Anillin coordinates the transition from CR to MR and that it does so by linking two distinct cortical cytoskeletal elements. One apparently acts as the core structural template for MR assembly, while the other ensures stable anchoring of the plasma membrane beyond the CR stage.

Actin filament severing by cofilin is more important for assembly than constriction of the cytokinetic contractile ring

When fission yeast express mutant cofilin that is inefficient at actin filament severing, cytokinetic contractile ring formation is severely impaired, but ring contraction proceeds efficiently.

We created two new mutants of fission yeast cofilin to investigate why cytokinesis in many organisms depends on this small actin-binding protein. These mutant cofilins bound actin monomers normally, but bound and severed ADP-actin filaments much slower than wild-type cofilin. Cells depending on mutant cofilins condensed nodes, precursors of the contractile ring, into clumps rather than rings. Starting from clumped nodes, mutant cells slowly assembled rings from diverse intermediate structures including spiral strands containing actin filaments and other contractile ring proteins. This process in mutant cells depended on α-actinin. These slowly assembled contractile rings constricted at a normal rate but with more variability, indicating ring constriction is not very sensitive to defects in severing by cofilin. Computer simulations of the search-capture-pull and release model of contractile ring formation predicted that nodes clump when the release step is slow, so cofilin severing of actin filament connections between nodes likely contributes to the release step.

Role of Survivin in cytokinesis revealed by a separation-of-function allele

A mutation in Drosophila Survivin that enables metaphase functions but impairs cytokinesis demonstrates a role for Survivin and CPC in Rho activation and contractile ring assembly and highlights striking differences in regulation of cytokinesis in different cell systems.

The chromosomal passenger complex (CPC), containing Aurora B kinase, Inner Centromere Protein, Survivin, and Borealin, regulates chromosome condensation and interaction between kinetochores and microtubules at metaphase, then relocalizes to midzone microtubules at anaphase and regulates central spindle organization and cytokinesis. However, the precise role(s) played by the CPC in anaphase have been obscured by its prior functions in metaphase. Here we identify a missense allele of Drosophila Survivin that allows CPC localization and function during metaphase but not cytokinesis. Analysis of mutant cells showed that Survivin is essential to target the CPC and the mitotic kinesin-like protein 1 orthologue Pavarotti (Pav) to the central spindle and equatorial cell cortex during anaphase in both larval neuroblasts and spermatocytes. Survivin also enabled localization of Polo kinase and Rho at the equatorial cortex in spermatocytes, critical for contractile ring assembly. In neuroblasts, in contrast, Survivin function was not required for localization of Rho, Polo, or Myosin II to a broad equatorial cortical band but was required for Myosin II to transition to a compact, fully constricted ring. Analysis of this “separation-of-function” allele demonstrates the direct role of Survivin and the CPC in cytokinesis and highlights striking differences in regulation of cytokinesis in different cell systems.

The myriad roles of Anillin during cytokinesis

Anillin is a highly conserved multidomain protein that interacts with cytoskeletal components as well as their regulators. Throughout phylogeny, Anillins contribute to cytokinesis, the cell shape change that occurs at the end of meiosis and mitosis to separate a cell into daughter cells. Failed cytokinesis results in binucleation, which can lead to genomic instability. Study of Anillin in several model organisms has provided us with insight into how the cytoskeleton is coordinated to ensure that cytokinesis occurs with high fidelity. Here we review Anillin's interacting partners and the relevance of these interactions in vivo. We also discuss questions of how these interactions are coordinated, and finally provide some perspective regarding Anillin's role in cancer.

Cytokinetic abscission in animal cells

Cytokinesis leads to the separation of dividing cells, which in animal cells involves the contraction of an actin-myosin ring and subsequent fission during abscission. Abscission requires a series of dynamic events, including midbody-targeted vesicle secretion, specialization of plasma membrane domains, disassembly of midbody-associated microtubule bundles and plasma membrane fission. A large number of molecular factors required for abscission have been identified through localization, loss-of-function and proteomics studies, but their coordinate function in abscission is still poorly understood. Here, we review the structural elements and molecular factors known to contribute to abscission, and discuss their potential role in the context of proposed models for the abscission mechanism.

Stabilization of the actomyosin ring enables spermatocyte cytokinesis in Drosophila

The scaffolding protein anillin recruits septins to the cleavage furrow and constrains actomyosin contractility. Expression of E-cadherin suppresses the cytokinesis defects caused by anillin knockdown and stabilizes F-actin in the furrow, thereby providing an alternate means of coupling the actomyosin ring to the plasma membrane during cytokinesis.

The scaffolding protein anillin is required for completion of cytokinesis. Anillin binds filamentous (F) actin, nonmuscle myosin II, and septins and in cell culture models has been shown to restrict actomyosin contractility to the cleavage furrow. Whether anillin also serves this function during the incomplete cytokinesis that occurs in developing germ cells has remained unclear. Here, we show that anillin is required for cytokinesis in dividing Drosophila melanogaster spermatocytes and that anillin, septins, and myosin II stably associate with the cleavage furrow in wild-type cells. Anillin is necessary for recruitment of septins to the cleavage furrow and for maintenance of F-actin and myosin II at the equator in late stages of cytokinesis. Remarkably, expression of DE-cadherin suppresses the cytokinesis defect of anillin-depleted spermatocytes. DE-cadherin recruits β-catenin (armadillo) and α-catenin to the cleavage furrow and stabilizes F-actin at the equator. Similarly, E-cadherin expression suppresses the cytokinesis defect caused by anillin knockdown in mouse L-fibroblast cells. Our results show that the anillin-septin and cadherin–catenin complexes can serve as alternative cassettes to promote tight physical coupling of F-actin and myosin II to the cleavage furrow and successful completion of cytokinesis.

How to scaffold the contractile ring for a safe cytokinesis - lessons from Anillin-related proteins

The ingression of a cleavage furrow separates the two daughter cells at the end of cell division. In many organisms this furrow ingression is driven by the assembly and contraction of actomyosin filaments, forming a contractile ring. To achieve a successful cytokinesis, these actomyosin filaments need to be assembled in an organized manner. For this purpose, a network of cytoskeletal proteins is built at the cleavage site to act as a scaffold for actomyosin filaments and to connect them to the plasma membrane. The Drosophila melanogaster protein Anillin, and its related proteins in other organisms, has a pivotal role in the organization of this scaffold in many species, ranging from yeast to humans. Recent studies indicate that Anillin-related proteins interact not only with the structural components of the contractile ring, but also with the signalling factors that control their dynamics. In addition, Drosophila Anillin connects the actomyosin ring to the spindle microtubules through its interaction with the RacGAP component of the centralspindlin complex. Here I review the structures and functions of Anillin and Anillin-related proteins in various model systems, and aim to highlight both the common and distinctive features of these essential organizers of the molecular machinery that drives furrow ingression.

Anillin: a pivotal organizer of the cytokinetic machinery

Cytokinesis is a dynamic and plastic process involving the co-ordinated regulation of many components. Accordingly, many proteins, including the putative scaffold protein anillin, localize to the cleavage furrow and are required for cytokinesis, but how they function together is poorly understood. Anillin can bind to numerous other furrow components, including F-actin, septins and myosin II, but its molecular functions are unclear. Recent data suggest that anillin participates in a previously unrecognized Rho-dependent pathway that can promote the association of anillin with the plasma membrane, septins, myosin II and microtubules. Studies using the inhibitor of F-actin assembly, Lat A (Latrunculin A), have revealed that these associations occur independently of F-actin; indeed they appear to be stabilized by the loss of F-actin. This pathway may explain previously described requirements for anillin in maintaining stable furrow positioning and for forming a stable midbody, and supports the notion that anillin is a central organizer at the hub of the cytokinetic machinery.

Australin: a chromosomal passenger protein required specifically for Drosophila melanogaster male meiosis

The chromosomal passenger complex (CPC), which is composed of conserved proteins aurora B, inner centromere protein (INCENP), survivin, and Borealin/DASRA, localizes to chromatin, kinetochores, microtubules, and the cell cortex in a cell cycle–dependent manner. The CPC is required for multiple aspects of cell division. Here we find that Drosophila melanogaster encodes two Borealin paralogues, Borealin-related (Borr) and Australin (Aust). Although Borr is a passenger in all mitotic tissues studied, it is specifically replaced by Aust for the two male meiotic divisions. We analyzed aust mutant spermatocytes to assess the effects of fully inactivating the Aust-dependent functions of the CPC. Our results indicate that Aust is required for sister chromatid cohesion, recruitment of the CPC to kinetochores, and chromosome alignment and segregation but not for meiotic histone phosphorylation or spindle formation. Furthermore, we show that the CPC is required earlier in cytokinesis than previously thought; cells lacking Aust do not initiate central spindle formation, accumulate anillin or actin at the cell equator, or undergo equatorial constriction.

Anillin is a scaffold protein that links RhoA, actin, and myosin during cytokinesis

Cell division after mitosis is mediated by ingression of an actomyosin-based contractile ring. The active, GTP-bound form of the small GTPase RhoA is a key regulator of contractile-ring formation. RhoA concentrates at the equatorial cell cortex at the site of the nascent cleavage furrow. During cytokinesis, RhoA is activated by its RhoGEF, ECT2. Once activated, RhoA promotes nucleation, elongation, and sliding of actin filaments through the coordinated activation of both formin proteins and myosin II motors (reviewed in [1, 2]). Anillin is a 124 kDa protein that is highly concentrated in the cleavage furrow in numerous animal cells in a pattern that resembles that of RhoA [3-7]. Although anillin contains conserved N-terminal actin and myosin binding domains and a PH domain at the C terminus, its mechanism of action during cytokinesis remains unclear. Here, we show that human anillin contains a conserved C-terminal domain that is essential for its function and localization. This domain shares homology with the RhoA binding protein Rhotekin and directly interacts with RhoA. Further, anillin is required to maintain active myosin in the equatorial plane during cytokinesis, suggesting it functions as a scaffold protein to link RhoA with the ring components actin and myosin. Although furrows can form and initiate ingression in the absence of anillin, furrows cannot form in anillin-depleted cells in which the central spindle is also disrupted, revealing that anillin can also act at an early stage of cytokinesis.

TheDrosophilaphosphatidylinositol transfer protein encoded byvibratoris essential to maintain cleavage-furrow ingression in cytokinesis

Cytokinesis requires the coordination of cytoskeletal and plasma membrane dynamics. A role for phosphatidylinositol lipids has been proposed for the successful completion of cytokinesis but this is still poorly characterised. Here, we show mutants of the gene vibrator, previously found to encode the Drosophila phosphatidylinositol transfer protein, produce multinucleate cells indicative of cytokinesis failure in male meiosis. Examination of fixed preparations of mutant spermatocytes showed contractile rings of anillin and actin that were of normal appearance at early stages but were larger and less well organised at later stages of cytokinesis than in wild-type cells. Time-lapse imaging revealed sequential defects in cytokinesis of vibrator spermatocytes. In cells that fail cytokinesis, central spindle formation occurred correctly, but furrow ingression was delayed and the central spindle did not become compressed to the extent seen in wild-type cells. Cells then stalled at this point before the apparent connection between the constricted cytoskeleton and the plasma membrane was lost; the furrow then underwent elastic regression. We discuss these defects in relation to multiple functions of phosphoinositol lipids in regulating actin dynamics and membrane synthesis.

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.

The septin-binding protein anillin is overexpressed in diverse human tumors

Anillin is an actin-binding protein that can bind septins and is a component of the cytokinetic ring. We assessed the anillin expression in 7,579 human tissue samples and cell lines by DNA microarray analysis. Anillin is expressed ubiquitously but with variable levels of expression, being highest in the central nervous system. The median level of anillin mRNA expression was higher in tumors than normal tissues (median fold increase 2.58; 95% confidence intervals, 2.19-5.68, P < 0.0001) except in the central nervous system where anillin mRNA levels were lower in tumors. We developed a sensitive reverse transcription-PCR strategy to show that anillin mRNA is expressed in cell lines and in cDNA panels derived from fetal and adult tissues, thus validating the microarray data. We compared anillin with Ki67 mRNA expression and found a significant linear relationship between anillin and Ki67 mRNA expression (Spearmann r approximately 0.6, P < 0.0001). Anillin mRNA expression was analyzed during tumor progression in breast, ovarian, kidney, colorectal, hepatic, lung, endometrial, and pancreatic tumors and in all tissues there was progressive increase in anillin mRNA expression from normal to benign to malignant to metastatic disease. Finally, we used anti-anillin sera and found nuclear anillin immunoreactivity to be widespread in normal tissues, often not correlating with proliferative compartments. These data provide insight into the existence of nonproliferation-associated activities of anillin and roles in interphase nuclei. Thus, anillin is overexpressed in diverse common human tumors, but not simply as a consequence of being a proliferation marker. Anillin may have potential as a novel biomarker.

Cleavage furrow formation and ingression during animal cytokinesis: a microtubule legacy

Cytokinesis ensures the proper partitioning of the nuclear and cytoplasmic contents into independent daughter cells at the end of cell division. Although the metazoan mitotic spindle has been implicated in the placement and advancement of the cleavage furrow, the molecules responsible for these processes have remained elusive. Recent studies have provided insights into the role of different microtubule structures and associated proteins in cleavage furrow positioning and ingression together with the signalling events that regulate the dynamics of the equatorial cell cortex during cytokinesis. We try to unify these findings into a general model of cytokinesis in which both astral and central spindle microtubules have the ability to induce furrowing. We further propose that the evolutionarily conserved centralspindlin complex serves as a master controller of cell cleavage in Drosophila by promoting both furrow formation and ingression. The same mechanism might be conserved in other organisms.

Molecular basis for dissimilar nuclear trafficking of the actin-bundling protein isoforms T- and L-plastin

T- and L-plastin are highly similar actin-bundling proteins implicated in the regulation of cell morphology, lamellipodium protrusion, bacterial invasion and tumor progression. We show that T-plastin localizes predominantly to the cytoplasm, whereas L-plastin distributes between nucleus and cytoplasm in HeLa or Cos cells. T-plastin shows nuclear accumulation upon incubation of cells with the CRM1 antagonist leptomycin B (LMB). We identified a Rev-like nuclear export sequence (NES) in T-plastin that is able to export an otherwise nuclear protein in an LMB-dependent manner. Deletion of the NES promotes nuclear accumulation of T-plastin. Mutation of residues L17, F21 or L26 in the T-plastin NES inhibits nuclear efflux. L-plastin harbors a less conserved NES and lacks the F21 T-plastin residue. Insertion of a Phe residue in the L-plastin NES specifically enhances its export activity. These findings explain why both isoforms exhibit specific distribution patterns in eukaryotic cells.

Klp67A destabilises pre-anaphase microtubules but subsequently is required to stabilise the central spindle

Klp67A is a member of the Kip3 subfamily of microtubule destabilising kinesins, the loss of which results in abnormally long and stable pre-anaphase microtubules. Here we examine its role during cytokinesis in Drosophila primary spermatocytes that require the coordinated interaction of an interior and peripheral set of central spindle microtubules. In mutants anaphase B spindles elongated with normal kinetics but bent towards the cortex. Both peripheral and interior spindle microtubules then formed diminished bundles of abnormally positioned central spindle microtubules associated with the pavarotti-KLP and KLP3A motor proteins. The minus ends of these were poorly aligned as revealed by Asp protein localisation. Furrows always initiated at the sites of central spindle bundles but could be unilateral or nonequatorially positioned. Ectopic furrows were stimulated by the interior central spindle and formed only after this structure buckled and contacted the cortex. Furrows often halted and regressed as they could not be sustained by the central spindles that became increasing unstable over time and often completely degraded. Consistent with this, actin and anillin failed to form homogenous bands. Thus, the Klp67A microtubule catastrophe factor is required for cytokinesis by regulating both the formation and stability of the central spindle.

Terminal cytokinesis events uncovered after an RNAi screen

Much of our understanding of animal cell cytokinesis centers on the regulation of the equatorial acto-myosin contractile ring that drives the rapid ingression of a deep cleavage furrow. However, the central part of the mitotic spindle collapses to a dense structure that impedes the furrow and keeps the daughter cells connected via an intercellular bridge. Factors involved in the formation, maintenance, and resolution of this bridge are largely unknown. Using a library of 7,216 double-stranded RNAs (dsRNAs) representing the conserved genes of Drosophila, we performed an RNA interference (RNAi) screen for cytokinesis genes in Schneider's S2 cells. We identified both familiar and novel genes whose inactivation induced a multi-nucleate phenotype. Using live video microscopy, we show that three genes: anillin, citron-kinase (CG10522), and soluble N-ethylmaleimide sensitive factor (NSF) attachment protein (alpha-SNAP), are essential for the terminal (post-furrowing) events of cytokinesis. anillin RNAi caused gradual disruption of the intercellular bridge after furrowing; citron-kinase RNAi destabilized the bridge at a later stage; alpha-SNAP RNAi caused sister cells to fuse many hours later and by a different mechanism. We have shown that the stability of the intercellular bridge is essential for successful cytokinesis and have defined genes contributing to this stability.

Drosophila citron kinase is required for the final steps of cytokinesis

The mechanisms underlying completion of cytokinesis are still poorly understood. Here, we show that the Drosophila orthologue of mammalian Citron kinases is essential for the final events of the cytokinetic process. Flies bearing mutations in the Drosophila citron kinase (dck) gene were defective in both neuroblast and spermatocyte cytokinesis. In both cell types, early cytokinetic events such as central spindle assembly and contractile ring formation were completely normal. Moreover, cytokinetic rings constricted normally, leading to complete furrow ingression. However late telophases of both cell types displayed persistent midbodies associated with disorganized F actin and anillin structures. Similar defects were observed in dck RNA interference (RNAi) telophases, which, in addition to abnormal F actin and anillin rings, also displayed aberrant membrane protrusions at the cleavage site. Together, these results indicate that mutations in the dck gene result in morphologically abnormal intercellular bridges and in delayed resolution of these structures, suggesting that the wild-type function of dck is required for abscission at the end of cytokinesis. The phenotype of Dck-depleted cells is different from those observed in most Drosophila cytokinesis mutants but extraordinarily similar to that caused by anillin RNAi, suggesting that Dck and anillin are in the same pathway for completion of cytokinesis.

Mutations in sticky lead to defective organization of the contractile ring during cytokinesis and are enhanced by Rho and suppressed by Rac

The contractile ring is a highly dynamic structure, but how this dynamism is accomplished remains unclear. Here, we report the identification and analysis of a novel Drosophila gene, sticky (sti), essential for cytokinesis in all fly proliferating tissues. sti encodes the Drosophila orthologue of the mammalian Citron kinase. RNA interference–mediated silencing of sti in cultured cells causes them to become multinucleate. Components of the contractile ring and central spindle are recruited normally in such STICKY-depleted cells that nevertheless display asymmetric furrowing and aberrant blebbing. Together with an unusual distribution of F-actin and Anillin, these phenotypes are consistent with defective organization of the contractile ring. sti shows opposite genetic interactions with Rho and Rac genes suggesting that these GTPases antagonistically regulate STICKY functions. Similar genetic evidence indicates that RacGAP50C inhibits Rac during cytokinesis. We discuss that antagonism between Rho and Rac pathways may control contractile ring dynamics during cytokinesis.

Mutations in orbit/mast reveal that the central spindle is comprised of two microtubule populations, those that initiate cleavage and those that propagate furrow ingression

We address the relative roles of astral and central spindle microtubules (MTs) in cytokinesis of Drosophila melanogaster primary spermatocytes. Time-lapse imaging studies reveal that the central spindle is comprised of two MT populations, “interior” central spindle MTs found within the spindle envelope and “peripheral” astral MTs that probe the cytoplasm and initiate cleavage furrows where they contact the cortex and form overlapping bundles. The MT-associated protein Orbit/Mast/CLASP concentrates on interior rather than peripheral central spindle MTs. Interior MTs are preferentially affected in hypomorphic orbit mutants, and consequently the interior central spindle fails to form or is unstable. In contrast, peripheral MTs still probe the cortex and form regions of overlap that recruit the Pav-KLP motor and Aurora B kinase. orbit mutants have disorganized or incomplete anillin and actin rings, and although cleavage furrows initiate, they ultimately regress. Our work identifies a new function for Orbit/Mast/CLASP and identifies a novel MT population involved in cleavage furrow initiation.

Genetic dissection of meiotic cytokinesis in Drosophila males

We have used Drosophila male meiosis as a model system for genetic dissection of the cytokinesis mechanism. Drosophila mutants defective in meiotic cytokinesis can be easily identified by their multinucleate spermatids. Moreover, the large size of meiotic spindles allows characterization of mutant phenotypes with exquisite cytological resolution. We have screened a collection of 1955 homozygous mutant male sterile lines for those with multinucleate spermatids, and thereby identified mutations in 19 genes required for cytokinesis. These include 16 novel loci and three genes, diaphanous, four wheel drive, and pebble, already known to be involved in Drosophila cytokinesis. To define the primary defects leading to failure of cytokinesis, we analyzed meiotic divisions in male mutants for each of these 19 genes. Examination of preparations stained for tubulin, anillin, KLP3A, and F-actin revealed discrete defects in the components of the cytokinetic apparatus, suggesting that these genes act at four major points in a stepwise pathway for cytokinesis. Our results also indicated that the central spindle and the contractile ring are interdependent structures that interact throughout cytokinesis. Moreover, our genetic and cytological analyses provide further evidence for a cell type-specific control of Drosophila cytokinesis, suggesting that several genes required for meiotic cytokinesis in males are not required for mitotic cytokinesis.

The Drosophila EAST protein associates with a nuclear remnant during mitosis and constrains chromosome mobility

Drosophila EAST protein associates with an interior nonchromosomal compartment of the interphase nucleus. Because overexpression of east can dramatically alter nuclear architecture we investigated a potential role for EAST in changing chromosome organization during the cell cycle. Following nuclear envelope breakdown, EAST remains enriched in the mitotic remnant of the interphase nucleus until the onset of anaphase. Loss of east expression affects the accuracy of division cycles. In female meiosis, east mutations lead to abnormal segregation of nonexchange chromosomes. During the syncytial nuclear cycles, increased frequency of mitotic errors leads to the depletion of surface nuclei. In the post-syncytial cell cycles, abnormal congression of chromosomes in prometaphase delays the onset of anaphase. Loss of east expression also results in abnormal chromosome morphology in male meiosis. We propose that EAST constitutes a component of a nucleoskeleton that helps to constrain the mobility of chromosomes in interphase, mitosis and meiosis.

Spindle assembly and cytokinesis in the absence of chromosomes during Drosophila male meiosis

A large body of work indicates that chromosomes play a key role in the assembly of both a centrosomal and centrosome-containing spindles. In animal systems, the absence of chromosomes either prevents spindle formation or allows the assembly of a metaphase-like spindle that fails to evolve into an ana-telophase spindle. Here, we show that Drosophila secondary spermatocytes can assemble morphologically normal spindles in the absence of chromosomes. The Drosophila mutants fusolo and solofuso are severely defective in chromosome segregation and produce secondary spermatocytes that are devoid of chromosomes. The centrosomes of these anucleated cells form robust asters that give rise to bipolar spindles that undergo the same ana-telophase morphological transformations that characterize normal spindles. The cells containing chromosome-free spindles are also able to assemble regular cytokinetic structures and cleave normally. In addition, chromosome-free spindles normally accumulate the Aurora B kinase at their midzones. This suggests that the association of Aurora B with chromosomes is not a prerequisite for its accumulation at the central spindle, or for its function during cytokinesis.