Centriolin anchoring of exocyst and SNARE complexes at the midbody is required for secretory-vesicle-mediated abscission

Cell and molecular biology of septins

Septins are a family of GTP-binding proteins that assemble into cytoskeletal filaments. Unlike other cytoskeletal components, septins form ordered arrays of defined stoichiometry that can polymerize into long filaments and bundle laterally. Septins associate directly with membranes and have been implicated in providing membrane stability and serving as diffusion barriers for membrane proteins. In addition, septins bind other proteins and have been shown to function as multimolecular scaffolds by recruiting components of signaling pathways. Remarkably, septins participate in a spectrum of cellular processes including cytokinesis, ciliogenesis, cell migration, polarity, and cell-pathogen interactions. Given their breadth of functions, it is not surprising that septin abnormalities have also been linked to human diseases. In this review, we discuss the current knowledge of septin structure, assembly and function, and discuss these in the context of human disease.

ESCRT function in cytokinesis: location, dynamics and regulation by mitotic kinases

Mammalian cytokinesis proceeds by constriction of an actomyosin ring and furrow ingression, resulting in the formation of the midbody bridge connecting two daughter cells. At the centre of the midbody resides the Flemming body, a dense proteinaceous ring surrounding the interlocking ends of anti-parallel microtubule arrays. Abscission, the terminal step of cytokinesis, occurs near the Flemming body. A series of broad processes govern abscission: the initiation and stabilisation of the abscission zone, followed by microtubule severing and membrane scission-The latter mediated by the endosomal sorting complex required for transport (ESCRT) proteins. A key goal of cell and developmental biologists is to develop a clear understanding of the mechanisms that underpin abscission, and how the spatiotemporal coordination of these events with previous stages in cell division is accomplished. This article will focus on the function and dynamics of the ESCRT proteins in abscission and will review recent work, which has begun to explore how these complex protein assemblies are regulated by the cell cycle machinery.

Novel functions for the endocytic regulatory proteins MICAL-L1 and EHD1 in mitosis

During interphase, recycling endosomes mediate the transport of internalized cargo back to the plasma membrane. However, in mitotic cells, recycling endosomes are essential for the completion of cytokinesis, the last phase of mitosis that promotes the physical separation the two daughter cells. Despite recent advances, our understanding of the molecular determinants that regulate recycling endosome dynamics during cytokinesis remains incomplete. We have previously demonstrated that Molecule Interacting with CasL Like-1 (MICAL-L1) and C-terminal Eps15 Homology Domain protein 1 (EHD1) coordinately regulate receptor transport from tubular recycling endosomes during interphase. However, their potential roles in controlling cytokinesis had not been addressed. In this study, we show that MICAL-L1 and EHD1 regulate mitosis. Depletion of either protein resulted in increased numbers of bi-nucleated cells. We provide evidence that bi-nucleation in MICAL-L1- and EHD1-depleted cells is a consequence of impaired recycling endosome transport during late cytokinesis. However, depletion of MICAL-L1, but not EHD1, resulted in aberrant chromosome alignment and lagging chromosomes, suggesting an EHD1-independent function for MICAL-L1 earlier in mitosis. Moreover, we provide evidence that MICAL-L1 and EHD1 differentially influence microtubule dynamics during early and late mitosis. Collectively, our new data suggest several unanticipated roles for MICAL-L1 and EHD1 during the cell cycle.

Phosphoinositides: Lipids with informative heads and mastermind functions in cell division

Phosphoinositides are low abundant but essential phospholipids in eukaryotic cells and refer to phosphatidylinositol and its seven polyphospho-derivatives. In this review, we summarize our current knowledge on phosphoinositides in multiple aspects of cell division in animal cells, including mitotic cell rounding, longitudinal cell elongation, cytokinesis furrow ingression, intercellular bridge abscission and post-cytokinesis events. PtdIns(4,5)P₂production plays critical roles in spindle orientation, mitotic cell shape and bridge stability after furrow ingression by recruiting force generator complexes and numerous cytoskeleton binding proteins. Later, PtdIns(4,5)P₂hydrolysis and PtdIns3P production are essential for normal cytokinesis abscission. Finally, emerging functions of PtdIns3P and likely PtdIns(4,5)P₂have recently been reported for midbody remnant clearance after abscission. We describe how the multiple functions of phosphoinositides in cell division reflect their distinct roles in local recruitment of protein complexes, membrane traffic and cytoskeleton remodeling. This article is part of a Special Issue entitled Phosphoinositides.

An integrated overview of spatiotemporal organization and regulation in mitosis in terms of the proteins in the functional supercomplexes

Eukaryotic cells may divide via the critical cellular process of cell division/mitosis, resulting in two daughter cells with the same genetic information. A large number of dedicated proteins are involved in this process and spatiotemporally assembled into three distinct super-complex structures/organelles, including the centrosome/spindle pole body, kinetochore/centromere and cleavage furrow/midbody/bud neck, so as to precisely modulate the cell division/mitosis events of chromosome alignment, chromosome segregation and cytokinesis in an orderly fashion. In recent years, many efforts have been made to identify the protein components and architecture of these subcellular organelles, aiming to uncover the organelle assembly pathways, determine the molecular mechanisms underlying the organelle functions, and thereby provide new therapeutic strategies for a variety of diseases. However, the organelles are highly dynamic structures, making it difficult to identify the entire components. Here, we review the current knowledge of the identified protein components governing the organization and functioning of organelles, especially in human and yeast cells, and discuss the multi-localized protein components mediating the communication between organelles during cell division.

Engulfment of the midbody remnant after cytokinesis in mammalian cells

The midbody remnant (MBR) that is generated after cytokinetic abscission has recently attracted a lot of attention, because it might have crucial consequences for cell differentiation and tumorigenesis in mammalian cells. In these cells, it has been reported that the MBR is either released into the extracellular medium or retracted into one of the two daughter cells where it can be degraded by autophagy. Here, we describe a major alternative pathway in a variety of human and mouse immortalized cells, cancer cells and primary stem cells. Using correlative light and scanning electron microscopy and quantitative assays, we found that sequential abscissions on both sides of the midbody generate free MBRs, which are tightly associated with the cell surface through a Ca(2+)/Mg(2+)-dependent receptor. Surprisingly, MBRs move over the cell surface for several hours, before being eventually engulfed by an actin-dependent phagocytosis-like mechanism. Mathematical modeling combined with experimentation further demonstrates that lysosomal activities fully account for the clearance of MBRs after engulfment. This study changes our understanding of how MBRs are inherited and degraded in mammalian cells and suggests a mechanism by which MBRs might signal over long distances between cells.

Rho GTPases as regulators of mitosis and cytokinesis in mammalian cells

Rho GTPases regulate a diverse range of cellular functions primarily through their ability to modulate microtubule dynamics and the actin-myosin cytoskeleton. Both of these cytoskeletal structures are crucial for a mitotic cell division. Specifically, their assembly and disassembly is tightly regulated in a temporal manner to ensure that each mitotic stage occurs in the correct sequential order and not prematurely until the previous stage is completed. Thus, it is not surprising that the Rho GTPases, RhoA, and Cdc42, have reported roles in several stages of mitosis: cell cortex stiffening during cell rounding, mitotic spindle formation, and bi-orient attachment of the spindle microtubules to the kinetochore and during cytokinesis play multiple roles in establishing the division plane, assembly, and activation of the contractile ring, membrane ingression, and abscission. Here, I review the molecular mechanisms regulating the spatial and temporal activation of RhoA and Cdc42 during mitosis, and how this is critical for mitotic progression and completion.

Asymmetric cell division in polyploid giant cancer cells and low eukaryotic cells

Asymmetric cell division is critical for generating cell diversity in low eukaryotic organisms. We previously have reported that polyploid giant cancer cells (PGCCs) induced by cobalt chloride demonstrate the ability to use an evolutionarily conserved process for renewal and fast reproduction, which is normally confined to simpler organisms. The budding yeast, Saccharomyces cerevisiae, which reproduces by asymmetric cell division, has long been a model for asymmetric cell division studies. PGCCs produce daughter cells asymmetrically in a manner similar to yeast, in that both use budding for cell polarization and cytokinesis. Here, we review the results of recent studies and discuss the similarities in the budding process between yeast and PGCCs.

Plant cytokinesis is orchestrated by the sequential action of the TRAPPII and exocyst tethering complexes

Plant cytokinesis is initiated in a transient membrane compartment, the cell plate, and completed by a process of maturation during which the cell plate becomes a cross wall. How the transition from juvenile to adult stages occurs is poorly understood. In this study, we monitor the Arabidopsis transport protein particle II (TRAPPII) and exocyst tethering complexes throughout cytokinesis. We show that their appearance is predominantly sequential, with brief overlap at the onset and end of cytokinesis. The TRAPPII complex is required for cell plate biogenesis, and the exocyst is required for cell plate maturation. The TRAPPII complex sorts plasma membrane proteins, including exocyst subunits, at the cell plate throughout cytokinesis. We show that the two tethering complexes physically interact and propose that their coordinated action may orchestrate not only plant but also animal cytokinesis.

Cytokinesis defines a spatial landmark for hepatocyte polarization and apical lumen formation

By definition, all epithelial cells have apical-basal polarity, but it is unclear how epithelial polarity is acquired and how polarized cells engage in tube formation. Here, we show that hepatocyte polarization is linked to cytokinesis using the rat hepatocyte cell line Can 10. Before abscission, polarity markers are delivered to the site of cell division in a strict spatiotemporal order. Immediately after abscission, daughter cells remain attached through a unique disc-shaped structure, which becomes the site for targeted exocytosis, resulting in the formation of a primitive bile canaliculus. Subsequently, oriented cell division and asymmetric cytokinesis occur at the bile canaliculus midpoint, resulting in its equal partitioning into daughter cells. Finally, successive cycles of oriented cell division and asymmetric cytokinesis lead to the formation of a tubular bile canaliculus, which is shared by two rows of hepatocytes. These findings define a novel mechanism for cytokinesis-linked polarization and tube formation, which appears to be broadly conserved in diverse cell types.

BRCA2 phosphorylated by PLK1 moves to the midbody to regulate cytokinesis mediated by nonmuscle myosin IIC

Cytokinesis is the critical final step in cell division. BRCA2 disruption during cytokinesis is associated with chromosome instability, but mechanistic information is lacking that could be used to prevent cancer cell division. In this study, we report that BRCA2 phosphorylation by the mitotic polo-like kinase (PLK1) governs the localization of BRCA2 to the Flemming body at the central midbody, permitting an interaction with nonmuscle myosin IIC (NM-IIC). Formation of an NM-IIC ring-like structure at the Flemming body shows that the IIC-ring relies on its ATPase activity stimulated by interaction with BRCA2 and associated proteins. Notably, inhibiting this binding inactivated the ATPase activity, causing disassembly of the IIC-ring, defective formation of the midbody, and interruption of cytokinesis. An analysis of cancer-associated mutations in BRCA2 at the PLK1-binding site suggests that they may contribute to cytokinetic defects by altering BRCA2 localization. Our findings suggest that BRCA2-dependent IIC-ring formation is a critical step in proper formation of the midbody, offering an explanation for how chromosome instability may arise in breast cancer.

Incisive imaging and computation for cellular mysteries: lessons from abscission

The final cleavage event that terminates cell division, abscission of the small, dense intercellular bridge, has been particularly challenging to resolve. Here, we describe imaging innovations that helped answer long-standing questions about the mechanism of abscission. We further explain how computational modeling of high-resolution data was employed to test hypotheses and generate additional insights. We present the model that emerges from application of these complimentary approaches. Similar experimental strategies will undoubtedly reveal exciting details about other underresolved cellular structures.

Sorcin links calcium signaling to vesicle trafficking, regulates Polo-like kinase 1 and is necessary for mitosis

Sorcin, a protein overexpressed in many multi-drug resistant cancers, dynamically localizes to distinct subcellular sites in 3T3-L1 fibroblasts during cell-cycle progression. During interphase sorcin is in the nucleus, in the plasma membrane, in endoplasmic reticulum (ER) cisternae, and in ER-derived vesicles localized along the microtubules. These vesicles are positive to RyR, SERCA, calreticulin and Rab10. At the beginning of mitosis, sorcin-containing vesicles associate with the mitotic spindle, and during telophase are concentrated in the cleavage furrow and, subsequently, in the midbody. Sorcin regulates dimensions and calcium load of the ER vesicles by inhibiting RYR and activating SERCA. Analysis of sorcin interactome reveals calcium-dependent interactions with many proteins, including Polo-like kinase 1 (PLK1), Aurora A and Aurora B kinases. Sorcin interacts physically with PLK1, is phosphorylated by PLK1 and induces PLK1 autophosphorylation, thereby regulating kinase activity. Knockdown of sorcin results in major defects in mitosis and cytokinesis, increase in the number of rounded polynucleated cells, blockage of cell progression in G2/M, apoptosis and cell death. Sorcin regulates calcium homeostasis and is necessary for the activation of mitosis and cytokinesis.

Exocyst proteins in cytokinesis: Regulation by Rab11

The Exocyst is an octameric protein complex comprised of Sec3, Sec5, Sec6, Sec8, Sec10, Sec15, Exo70, and Exo84 subunits.(1, 2) This complex was first identified in budding yeast where it acts to target vesicles to the bud tip and the cleavage furrow.(3) Here, we show that all Exocyst subunits are required for cytokinesis in mammalian cells. We further show that a subset of Exocyst proteins are differentially regulated by Rab11, consistent with recent studies implicating Rab11 vesicles in Exocyst protein trafficking.

ARF-Like (ARL) Proteins

The ARF-like (ARL) proteins, within the ARF family, are a collection of functionally diverse GTPases that share extensive (>40 %) identity with the ARFs and each other and are assumed to share basic mechanisms of regulation and a very incompletely documented degree of overlapping regulators. At least four ARLs were already present in the last eukaryotic common ancestor, along with one ARF, and these have been expanded to >20 members in mammals. We know little about the majority of these proteins so our review will focus on those about which the most is known, including ARL1, ARL2, ARL3, ARL4s, ARL6, ARL13s, and ARFRP1. From this fragmentary information we extract some generalizations and conclusions regarding the sources and extent of specificity and functions of the ARLs.

How genome-wide SNP-SNP interactions relate to nasopharyngeal carcinoma susceptibility

This study is the first to use genome-wide association study (GWAS) data to evaluate the multidimensional genetic architecture underlying nasopharyngeal cancer. Since analysis of data from GWAS confirms a close and consistent association between elevated risk for nasopharyngeal carcinoma (NPC) and major histocompatibility complex class 1 genes, our goal here was to explore lesser effects of gene-gene interactions. We conducted an exhaustive genome-wide analysis of GWAS data of NPC, revealing two-locus interactions occurring between single nucleotide polymorphisms (SNPs), and identified a number of suggestive interaction loci which were missed by traditional GWAS analyses. Although none of the interaction pairs we identified passed the genome-wide Bonferroni-adjusted threshold for significance, using independent GWAS data from the same population (Stage 2), we selected 66 SNP pairs in 39 clusters with P<0.01. We identified that in several chromosome regions, multiple suggestive interactions group to form a block-like signal, effectively reducing the rate of false discovery. The strongest cluster of interactions involved the CREB5 gene and a SNP rs1607979 on chromosome 17q22 (P = 9.86×10(-11)) which also show trans-expression quantitative loci (eQTL) association in Chinese population. We then detected a complicated cis-interaction pattern around the NPC-associated HLA-B locus, which is immediately adjacent to copy-number variations implicated in male susceptibility for NPC. While it remains to be seen exactly how and to what degree SNP-SNP interactions such as these affect susceptibility for nasopharyngeal cancer, future research on these questions holds great promise for increasing our understanding of this disease's genetic etiology, and possibly also that of other gene-related cancers.

KIFC3 promotes mitotic progression and integrity of the central spindle in cytokinesis

Kinesin-14 motor proteins play a variety of roles during metaphase and anaphase. However, it is not known whether members of this family of motors also participate in the dramatic changes in mitotic spindle organization during the transition from telophase to cytokinesis. We have identified the minus-end-directed motor, KIFC3, as an important contributor to central bridge morphology at this stage. KIFC3's unique motor-dependent localization at the central bridge allows it to congress microtubules, promoting efficient progress through cytokinesis. Conversely, when KIFC3 function is perturbed, abscission is delayed, and the central bridge is both widened and extended. Examination of KIFC3 on growing microtubules in interphase indicates that it caps microtubules released from the centrosome, both in the region of the centrosome and in the cell periphery. In line with other kinesin-14 family members, KIFC3 may guide free microtubules to their destination at the bridge and/or may slide and crosslink central bridge microtubules in order to stage the cells for abscission.

Centrosome-dependent asymmetric inheritance of the midbody ring in Drosophila germline stem cell division

The midbody ring (MR) is asymmetrically segregated during asymmetric divisions of germline stem cells (GSCs) in Drosophila. Male GSCs, which inherit the mother centrosome, exclude the MR, whereas female GSCs, which inherit the daughter centrosome, inherit the MR. Moreover, stem cell identity correlates with the mode of MR inheritance.

Many stem cells, including Drosophila germline stem cells (GSCs), divide asymmetrically, producing one stem cell and one differentiating daughter. Cytokinesis is often asymmetric, in that only one daughter cell inherits the midbody ring (MR) upon completion of abscission even in apparently symmetrically dividing cells. However, whether the asymmetry in cytokinesis correlates with cell fate or has functional relevance has been poorly explored. Here we show that the MR is asymmetrically segregated during GSC divisions in a centrosome age–dependent manner: male GSCs, which inherit the mother centrosome, exclude the MR, whereas female GSCs, which we here show inherit the daughter centrosome, inherit the MR. We further show that stem cell identity correlates with the mode of MR inheritance. Together our data suggest that the MR does not inherently dictate stem cell identity, although its stereotypical inheritance is under the control of stemness and potentially provides a platform for asymmetric segregation of certain factors.

Two appendages homologous between basal bodies and centrioles are formed using distinct Odf2 domains

Analysis of Odf2 deletion mutants reveals regions important for the formation of basal body transition fibers and centriole distal appendages and distinct regions required for basal feet and subdistal appendages.

Ciliogenesis is regulated by context-dependent cellular cues, including some transduced through appendage-like structures on ciliary basal bodies called transition fibers and basal feet. However, the molecular basis for this regulation is not fully understood. The Odf2 gene product, ODF2/cenexin, is essential for both ciliogenesis and the formation of the distal and subdistal appendages on centrioles, which become basal bodies. We examined the effects of Odf2 deletion constructs on ciliogenesis in Odf2-knockout F9 cells. Electron microscopy revealed that ciliogenesis and transition fiber formation required the ODF2/cenexin fragment containing amino acids (aa) 188–806, whereas basal foot formation required aa 1–59 and 188–806. These sequences also formed distal and subdistal appendages, respectively, indicating that the centriole appendages are molecularly analogous to those on basal bodies. We used the differential formation of appendages by Odf2 deletion constructs to study the incorporation and function of molecules associated with each appendage type. We found that transition fibers and distal appendages were required for ciliogenesis and subdistal appendages stabilized the centrosomal microtubules.

Regulation of cytokinesis by exocyst subunit SEC6 and KEULE in Arabidopsis thaliana

Proper vesicle tethering and membrane fusion at the cell plate are essential for cytokinesis. Both the vesicle tethering complex exocyst and membrane fusion regulator KEULE were shown to function in cell plate formation, but the exact mechanisms still remain to be explored. In this study, using yeast two-hybrid (Y-2-H) assay, we found that SEC6 interacted with KEULE, and that a small portion of C-terminal region of KEULE was required for the interaction. The direct SEC6-KEULE interaction was supported by further studies using in vitro pull-down assay, immunoprecipitation, and in vivo bimolecular fluorescence complementation (BIFC) microscopy. sec6 mutants were male gametophytic lethal as reported; however, pollen-rescued sec6 mutants (PRsec6) displayed cytokinesis defects in the embryonic cells and later in the leaf pavement cells and the guard cells. SEC6 and KEULE proteins were co-localized to the cell plate during cytokinesis in transgenic Arabidopsis. Furthermore, only SEC6 but not other exocyst subunits located in the cell plate interacted with KEULE in vitro. These results demonstrated that, like KEULE, SEC6 plays a physiological role in cytokinesis, and the SEC6-KEULE interaction may serve as a novel molecular linkage between arriving vesicles and membrane fusion machinery or directly regulate membrane fusion during cell plate formation in plants.

The vertebrate-specific Kinesin-6, Kif20b, is required for normal cytokinesis of polarized cortical stem cells and cerebral cortex size

Mammalian neuroepithelial stem cells divide using a polarized form of cytokinesis, which is not well understood. The cytokinetic furrow cleaves the cell by ingressing from basal to apical, forming the midbody at the apical membrane. The midbody mediates abscission by recruiting many factors, including the Kinesin-6 family member Kif20b. In developing embryos, Kif20b mRNA is most highly expressed in neural stem/progenitor cells. A loss-of-function mutant in Kif20b, magoo, was found in a forward genetic screen. magoo has a small cerebral cortex, with reduced production of progenitors and neurons, but preserved layering. In contrast to other microcephalic mouse mutants, mitosis and cleavage furrows of cortical stem cells appear normal in magoo. However, apical midbodies show changes in number, shape and positioning relative to the apical membrane. Interestingly, the disruption of abscission does not appear to result in binucleate cells, but in apoptosis. Thus, Kif20b is required for proper midbody organization and abscission in polarized cortical stem cells and has a crucial role in the regulation of cerebral cortex growth.

[Temporal regulation of abscission, the last step of cell division]

Cell division is one of the most tightly controlled steps of the cell cycle. Indeed, the many steps of cell division have to be perfectly coordinated both in time and space in order to ensure an error-free division and an accurate transmission of the genome from the mother cell to the two daughter cells. Abscission, the last step of cytokinesis, consists in the severing of the intercellular bridge that connects the two daughter cells after the contraction of the acto-myosin ring. As is the case for any other step of cell division, abscission has to be precisely regulated in order to take place at the right time and the proper place. Whereas the spatial regulation of abscission is quite well understood, the study of temporal regulation is in its infancy. This review begins by describing the formation of the intercellular bridge, its organization, and its composition. Next the different models of abscission are discussed. Finally, the current understanding of the temporal regulation of abscission is detailed. In particular, I present my recent results on the role of forces exerted by the daughter cells on the intercellular bridge.

Syntaxin 16 is a master recruitment factor for cytokinesis

Syntaxin 16 is a key regulator of cytokinesis, as it is required for the recruitment of both recycling endosome–associated Exocyst and ESCRT machinery during late telophase. Therefore these two distinct facets of cytokinesis are inextricably linked.

Recently it was shown that both recycling endosome and endosomal sorting complex required for transport (ESCRT) components are required for cytokinesis, in which they are believed to act in a sequential manner to bring about secondary ingression and abscission, respectively. However, it is not clear how either of these complexes is targeted to the midbody and whether their delivery is coordinated. The trafficking of membrane vesicles between different intracellular organelles involves the formation of soluble N-ethylmalei­mide–sensitive factor attachment protein receptor (SNARE) complexes. Although membrane traffic is known to play an important role in cytokinesis, the contribution and identity of intracellular SNAREs to cytokinesis remain unclear. Here we demonstrate that syntaxin 16 is a key regulator of cytokinesis, as it is required for recruitment of both recycling endosome–associated Exocyst and ESCRT machinery during late telophase, and therefore that these two distinct facets of cytokinesis are inextricably linked.

Cytokinesis: centralspindlin moonlights as a membrane anchor

The hardest working complex in animal cell division has a new gig. This extraordinary machine, the centralspindlin complex, works overtime, contributing to nearly every step in cytokinesis. It has now been shown to stabilize an association between the plasma membrane and the midbody microtubules prior to abscission.

Leukemia-associated RhoGEF (LARG) is a novel RhoGEF in cytokinesis and required for the proper completion of abscission

This study demonstrates a novel and unexpected role in cytokinesis for leukemia-associated RhoGEF (LARG). Depletion of LARG results in delayed abscission, and thus LARG is the first RhoGEF to be implicated in late cytokinesis.

Proper completion of mitosis requires the concerted effort of multiple RhoGEFs. Here we show that leukemia-associated RhoGEF (LARG), a RhoA-specific RGS-RhoGEF, is required for abscission, the final stage of cytokinesis, in which the intercellular membrane is cleaved between daughter cells. LARG colocalizes with α-tubulin at the spindle poles before localizing to the central spindle. During cytokinesis, LARG is condensed in the midbody, where it colocalizes with RhoA. HeLa cells depleted of LARG display apoptosis during cytokinesis with unresolved intercellular bridges, and rescue experiments show that expression of small interfering RNA–resistant LARG prevents this apoptosis. Moreover, live cell imaging of LARG-depleted cells reveals greatly delayed fission kinetics in abscission in which a population of cells with persistent bridges undergoes apoptosis; however, the delayed fission kinetics is rescued by Aurora-B inhibition. The formation of a Flemming body and thinning of microtubules in the intercellular bridge of cells depleted of LARG is consistent with a defect in late cytokinesis, just before the abscission event. In contrast to studies of other RhoGEFs, particularly Ect2 and GEF-H1, LARG depletion does not result in cytokinetic furrow regression nor does it affect internal mitotic timing. These results show that LARG is a novel and temporally distinct RhoGEF required for completion of abscission.

Coiled-coil domain containing protein 124 is a novel centrosome and midbody protein that interacts with the Ras-guanine nucleotide exchange factor 1B and is involved in cytokinesis

Cytokinetic abscission is the cellular process leading to physical separation of two postmitotic sister cells by severing the intercellular bridge. The most noticeable structural component of the intercellular bridge is a transient organelle termed as midbody, localized at a central region marking the site of abscission. Despite its major role in completion of cytokinesis, our understanding of spatiotemporal regulation of midbody assembly is limited. Here, we report the first characterization of coiled-coil domain-containing protein-124 (Ccdc124), a eukaryotic protein conserved from fungi-to-man, which we identified as a novel centrosomal and midbody protein. Knockdown of Ccdc124 in human HeLa cells leads to accumulation of enlarged and multinucleated cells; however, centrosome maturation was not affected. We found that Ccdc124 interacts with the Ras-guanine nucleotide exchange factor 1B (RasGEF1B), establishing a functional link between cytokinesis and activation of localized Rap2 signaling at the midbody. Our data indicate that Ccdc124 is a novel factor operating both for proper progression of late cytokinetic stages in eukaryotes, and for establishment of Rap2 signaling dependent cellular functions proximal to the abscission site.

Mitotic exit and separation of mother and daughter cells

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

Knowing when to cut and run: mechanisms that control cytokinetic abscission

Abscission, the final step of cytokinesis, mediates the severing of the membrane tether, or midbody, that connects two daughter cells. It is now recognized that abscission is a complex process requiring tight spatiotemporal regulation of its machinery to ensure equal chromosome segregation and cytoplasm content distribution between daughter cells. Failure to coordinate these events results in genetic damage. Here, we review recent evidence suggesting that proper abscission timing is coordinated by cytoskeletal rearrangements and recruitment of regulators of the Endosomal Sorting Complex Required for Transport (ESCRT) machinery such as CEP55 and MIT-domain-containing protein 1 (MITD1) to the abscission site. Additionally, we discuss the surveillance mechanism known as the Aurora B-mediated abscission checkpoint (NoCut), which prevents genetic damage by ensuring proper abscission delay when chromatin is trapped at the midbody.

FAM190A deficiency creates a cell division defect

Like the p16, SMAD4, and RB1 genes, FAM190A (alias CCSER1) lies at a consensus site of homogeneous genomic deletions in human cancer. FAM190A transcripts in 40% of cancers also contain in-frame deletions of evolutionarily conserved exons. Its gene function was unknown. We found an internal deletion of the FAM190A gene in a pancreatic cancer having prominent focal multinuclearity. The experimental knockdown of FAM190A expression by shRNA caused focal cytokinesis defects, multipolar mitosis, and multinuclearity as observed in time-lapse microscopy. FAM190A was localized to the γ-tubulin ring complex of early mitosis and to the midbody in late cytokinesis by immunofluorescence assay and was present in the nuclear fraction of unsynchronized cells by immunoblot. FAM190A interacted with EXOC1 and Ndel1, which function in cytoskeletal organization and the cell division cycle. Levels of FAM190A protein peaked 12 hours after release from thymidine block, corresponding to M-phase. Slower-migrating phosphorylated forms accumulated toward M-phase and disappeared after release from a mitotic block and before cytokinesis. Studies of FAM190A alterations may provide mechanistic insights into mitotic dysregulation and multinuclearity in cancer. We propose that FAM190A is a regulator or structural component required for normal mitosis and that both the rare truncating mutations and common in-frame deletion alteration of FAM190A may contribute to the chromosomal instability of cancer.

A molecular rheostat at the interface of cancer and diabetes

Epidemiology studies revealed the connection between several types of cancer and type 2 diabetes (T2D) and suggested that T2D is both a symptom and a risk factor of pancreatic cancer. High level of circulating insulin (hyperinsulinemia) in obesity has been implicated in promoting aggressive types of cancers. Insulin resistance, a symptom of T2D, pressures pancreatic β-cells to increase insulin secretion, leading to hyperinsulinemia, which in turn leads to a gradual loss of functional β-cell mass, thus indicating a fine balance and interplay between β-cell function and mass. While the mechanisms of these connections are unclear, the mTORC1-Akt signaling pathway has been implicated in controlling β-cell function and mass, and in mediating the link of cancer and T2D. However, incomplete understating of how the pathway is regulated and how it integrates body metabolism has hindered its efficacy as a clinical target. The IQ motif containing GTPase activating protein 1 (IQGAP1)-Exocyst axis is a growth factor- and nutrient-sensor that couples cell growth and division. Here we discuss how IQGAP1-Exocyst, through differential interactions with Rho-type of small guanosine triphosphatases (GTPases), acts as a rheostat that modulates the mTORC1-Akt and MAPK signals, and integrates β-cell function and mass with insulin signaling, thus providing a molecular mechanism for cancer initiation in diabetes. Delineating this regulatory pathway may have the potential of contributing to optimizing the efficacy and selectivity of future therapies for cancer and diabetes.

Cep57 protein is required for cytokinesis by facilitating central spindle microtubule organization

Cytokinesis is the final stage of cell division in which the cytoplasm of a cell is divided into two daughter cells after the segregation of genetic material, and the central spindle and midbody are considered to be the essential structures required for the initiation and completion of cytokinesis. Here, we determined that the centrosome protein Cep57, which is localized to the central spindle and midbody, acts as a spindle organizer and is required for cytokinesis. Depletion of Cep57 disrupted microtubule assembly of the central spindle and further led to abnormal midbody localization of MKLP1, Plk1, and Aurora B, which resulted in cytokinesis failure and the formation of binuclear cells. Furthermore, we found that Cep57 directly recruited Tektin 1 to the midbody matrix to regulate microtubule organization. Thus, our data reveal that Cep57 is essential for cytokinesis via regulation of central spindle assembly and formation of the midbody.

Mutations in LRRC50 predispose zebrafish and humans to seminomas

Seminoma is a subclass of human testicular germ cell tumors (TGCT), the most frequently observed cancer in young men with a rising incidence. Here we describe the identification of a novel gene predisposing specifically to seminoma formation in a vertebrate model organism. Zebrafish carrying a heterozygous nonsense mutation in Leucine-Rich Repeat Containing protein 50 (lrrc50 also called dnaaf1), associated previously with ciliary function, are found to be highly susceptible to the formation of seminomas. Genotyping of these zebrafish tumors shows loss of heterozygosity (LOH) of the wild-type lrrc50 allele in 44.4% of tumor samples, correlating with tumor progression. In humans we identified heterozygous germline LRRC50 mutations in two different pedigrees with a family history of seminomas, resulting in a nonsense Arg488* change and a missense Thr590Met change, which show reduced expression of the wild-type allele in seminomas. Zebrafish in vivo complementation studies indicate the Thr590Met to be a loss-of-function mutation. Moreover, we show that a pathogenic Gln307Glu change is significantly enriched in individuals with seminoma tumors (13% of our cohort). Together, our study introduces an animal model for seminoma and suggests LRRC50 to be a novel tumor suppressor implicated in human seminoma pathogenesis.

Asymmetric stem cell division: precision for robustness

Asymmetric cell division (ACD) produces two daughter cells with distinct fates or characteristics. Many adult stem cells use ACD as a means of maintaining stem cell number and thus tissue homeostasis. Here, we review recent progress on ACD, discussing conservation between stem and non-stem cell systems, molecular mechanisms, and the biological meaning of ACD.

A simple model for the fate of the cytokinesis midbody remnant: implications for remnant degradation by autophagy

When a cell divides, it produces two daughter cells initially connected by a cytokinesis bridge, which is eventually cut through abscission. One of the two daughter cells inherits a bridge "remnant", which has been proposed to be degraded by autophagy. The fate and function of remnants is attracting increasing attention, as their accumulation appears to influence proliferation versus differentiation of the daughter cells. Here, we present a simple model for bridge and remnant turnover in a dynamic cell population. We demonstrate that remnant proportions depend on the ratio of remnant and bridge lifetimes to the cell population doubling time. Our results yield new alternative interpretations for published experimental data, leading us to believe that autophagy-independent pathways for remnant degradation may exist. In addition, using the model, we determined experimentally inaccessible parameters such as remnant lifetime. Our model proves to be a useful tool for studying bridge and remnant populations.

Phagocytosis and cytokinesis: do cells use common tools to cut and to eat? Highlights on common themes and differences

Eukaryotic cells with specialized functions often use and adapt common molecular machineries. Recent findings have highlighted that actin polymerization, contractile activity and membrane remodelling with exocytosis of internal compartments are required both for successful phagocytosis, the internalization of particulate material and for cytokinesis, the last step of cell division. Phagocytosis is induced by the triggering of specific cell surface receptors, which leads to membrane deformation, pseudopod extension and contraction to engulf particles. Cytokinesis relies on intense contractile activity and eventually leads to the physical scission of sister cells. In this review, shared features of signalling, cytoskeletal reorganization and vesicular trafficking used in both phagocytosis and cytokinesis will be described, but non-common mechanisms and questions that remain open in these dynamic areas of research are also highlighted.

Roles for focal adhesion kinase (FAK) in blastomere abscission and vesicle trafficking during cleavage in the sea urchin embryo

Is focal adhesion kinase (FAK) needed for embryonic cleavage? We find that FAK is expressed during early cleavage divisions of sea urchin embryos as determined by polyclonal antibodies to the Lytechinus variegatus protein. FAK is absent in eggs and zygotes and then cycles in abundance during the first cleavages after fertilization. It is maximal at anaphase, similar to the destruction and synthesis of cyclin proteins. To investigate whether FAK is needed during early cleavage, we interfered with its function by microinjecting eggs with anti-FAK antibodies or with FAK antisense morpholino oligonucleotides. Both treatments led to regression of the cleavage furrow. FAK knockdown with antibodies or morpholino oligonucleotides also resulted in an over-accumulation of endocytic vesicles. Thus, FAK could be restricting endocytosis or increasing exocytosis in localized areas important for abscission. FAK appears to be necessary for successful cleavage. These results are the first to document a functional role for FAK during embryonic cleavage.

Visualization of the exocyst complex dynamics at the plasma membrane of Arabidopsis thaliana

The exocyst complex localizes to distinct foci at the plasma membrane of Arabidopsis thaliana cells. Their localization at the plasma membrane is insensitive to BFA treatment but is decreased in an exocyst-subunit mutant. In turn, exocyst-subunit mutants show decreased exocytosis.

The exocyst complex, an effector of Rho and Rab GTPases, is believed to function as an exocytotic vesicle tether at the plasma membrane before soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE) complex formation. Exocyst subunits localize to secretory-active regions of the plasma membrane, exemplified by the outer domain of Arabidopsis root epidermal cells. Using variable-angle epifluorescence microscopy, we visualized the dynamics of exocyst subunits at this domain. The subunits colocalized in defined foci at the plasma membrane, distinct from endocytic sites. Exocyst foci were independent of cytoskeleton, although prolonged actin disruption led to changes in exocyst localization. Exocyst foci partially overlapped with vesicles visualized by VAMP721 v-SNARE, but the majority of the foci represent sites without vesicles, as indicated by electron microscopy and drug treatments, supporting the concept of the exocyst functioning as a dynamic particle. We observed a decrease of SEC6–green fluorescent protein foci in an exo70A1 exocyst mutant. Finally, we documented decreased VAMP721 trafficking to the plasma membrane in exo70A1 and exo84b mutants. Our data support the concept that the exocyst-complex subunits dynamically dock and undock at the plasma membrane to create sites primed for vesicle tethering.

MTR120/KIAA1383, a novel microtubule-associated protein, promotes microtubule stability and ensures cytokinesis

Microtubules (MTs) are the major constituent of the mitotic apparatus. Deregulation of MT dynamics leads to chromosome missegregation, cytokinesis failure and improper inheritance of genetic materials. Here, we describe the identification and characterization of KIAA1383/MTR120 (microtubule regulator 120 kDa) as a novel MT-associated protein. We found that MTR120 localizes to stabilized MTs during interphase and to the mitotic apparatus during mitosis. MTR120 overexpression results in MT bundling and acetylation. In vitro, purified MTR120 protein binds to and bundles preassembled MTs. Moreover, depletion of MTR120 by RNA interference leads to cytokinesis failure and polyploidy. These phenotypes can be rescued by wild-type MTR120 but not by the MT non-binding mutant of MTR120. Together, these data suggest that MTR120 is a novel MT-associated protein that directly stabilizes MTs and hence ensures the fidelity of cell division.

Cep164 mediates vesicular docking to the mother centriole during early steps of ciliogenesis

Cep164 provides a molecular link between the mother centriole and the ciliary membrane biogenesis machinery by interacting with the GEF Rabin8 and the GTPase Rab8.

Cilia formation is a multi-step process that starts with the docking of a vesicle at the distal part of the mother centriole. This step marks the conversion of the mother centriole into the basal body, from which axonemal microtubules extend to form the ciliary compartment. How vesicles are stably attached to the mother centriole to initiate ciliary membrane biogenesis is unknown. Here, we investigate the molecular role of the mother centriolar component Cep164 in ciliogenesis. We show that Cep164 was indispensable for the docking of vesicles at the mother centriole. Using biochemical and functional assays, we identified the components of the vesicular transport machinery, the GEF Rabin8 and the GTPase Rab8, as interacting partners of Cep164. We propose that Cep164 is targeted to the apical domain of the mother centriole to provide the molecular link between the mother centriole and the membrane biogenesis machinery that initiates cilia formation.

Resurrecting remnants: the lives of post-mitotic midbodies

Around a century ago, the midbody (MB) was described as a structural assembly within the intercellular bridge during cytokinesis that served to connect the two future daughter cells. The MB has become the focus of intense investigation through the identification of a growing number of diverse cellular and molecular pathways that localize to the MB and contribute to its cytokinetic functions, ranging from selective vesicle trafficking and regulated microtubule (MT), actin, and endosomal sorting complex required for transport (ESCRT) filament assembly and disassembly to post-translational modification, such as ubiquitination. More recent studies have revealed new and unexpected functions of MBs in post-mitotic cells. In this review, we provide a historical perspective, discuss exciting new roles for MBs beyond their cytokinetic function, and speculate on their potential contributions to pluripotency.

Exocyst complex component Sec8: a presumed component in the progression of human oral squamous-cell carcinoma by secretion of matrix metalloproteinases

PURPOSE

Sec8, a component of the exocyst complex, has been implicated in tethering of secretory vesicles to specific regions on the plasma membrane. To investigate the involvement of Sec8 in oral squamous-cell carcinoma (OSCC), we evaluated the expression status and effect of Sec8 in OSCC cell lines.

METHODS

Sec8 mRNA and protein expressions in human OSCC cell lines were assessed by quantitative reverse transcriptase-polymerase chain reaction and immunoblotting. Functional analyses, proliferation assay, invasiveness assay, and gelatin zymography in Sec8 knockdown cells were performed. Also the correlation between Sec8 expression and the clinicopathological features in 98 primary OSCCs samples was evaluated by immunohistochemistry.

RESULTS

Sec8 mRNA and protein expression were significantly up-regulated in all cell lines (p < 0.05). Sec8 knockdown cells were characterized by reduced cellular proliferation, invasiveness, and secretion of matrix metalloproteinases (MMPs) (MMP-2, proMMP-2, and proMMP-9). Sec8 protein expression in primary OSCCs also was significantly (p < 0.05) greater than in normal counterparts, and higher Sec8 expression was correlated with tumor size (p = 0.03).

CONCLUSIONS

Our results suggested for the first time that Sec8 might play a specific role in OSCC progression by mediating MMP secretion.

Membrane dynamics during cytokinesis

Endocytic membrane transport has recently emerged as a key process required for the successful completion of cytokinesis. Specific endocytic membranes act in concert with the cytoskeleton and ESCRT proteins to regulate the various stages of cytokinesis. In this review, we focus on the different endocytic Arf and Rab GTPases and their interaction proteins that regulate organelle transport to the intracellular bridge during cytokinesis. The identity and function of these endocytic organelles during the late stages of cell division will also be discussed.

Plasma membrane growth during the cell cycle: unsolved mysteries and recent progress

Growth of the plasma membrane is as fundamental to cell reproduction as DNA replication, chromosome segregation and ribosome biogenesis, yet little is known about the underlying mechanisms. Membrane growth during the cell cycle requires mechanisms that control the initiation, location, and extent of membrane growth, as well as mechanisms that coordinate membrane growth with cell cycle progression. Recent experiments have established links between membrane growth and core cell cycle regulators. Further analysis of these links will yield insights into conserved and fundamental mechanisms of cell growth. A better understanding of the post-Golgi pathways by which membrane growth occurs will be essential for future progress.

The microtubule-associated Rho activating factor GEF-H1 interacts with exocyst complex to regulate vesicle traffic

The exocyst complex plays a critical role in targeting and tethering vesicles to specific sites of the plasma membrane. These events are crucial for polarized delivery of membrane components to the cell surface, which is critical for cell motility and division. Though Rho GTPases are involved in regulating actin dynamics and membrane trafficking, their role in exocyst-mediated vesicle targeting is not very clear. Herein, we present evidence that depletion of GEF-H1, a guanine nucleotide exchange factor for Rho proteins, affects vesicle trafficking. Interestingly, we found that GEF-H1 directly binds to exocyst component Sec5 in a Ral GTPase-dependent manner. This interaction promotes RhoA activation, which then regulates exocyst assembly/localization and exocytosis. Taken together, our work defines a mechanism for RhoA activation in response to RalA-Sec5 signaling and involvement of GEF-H1/RhoA pathway in the regulation of vesicle trafficking.

Polar expansion during cytokinesis

Vesicle trafficking and new membrane addition at the cleavage furrow have been extensively documented. However, less clear is the old idea that expansion at the cell poles occurs during cytokinesis. We find that new membrane is added to the cell poles during anaphase, causing the plasma membrane to expand coincident with the constriction of the contractile ring and may provide a pushing force for membrane ingression at the furrow. This membrane addition occurs earlier during mitosis than membrane addition at the furrow and is dependent on actin and astral microtubules. The membrane that is added at the polar regions is compositionally distinct from the original cell membrane in that it is devoid of GM(1) , a component of lipid rafts. These findings suggest that the growth of the plasma membrane at the cell poles during cell division is not due to stretching as previously thought, but due to the addition of compositionally unique new membrane.

SNAREs in HIV-1 assembly

Viruses have a limited number of genes but a complex life cycle and have evolved to utilize numerous host factors to complete their replication. The assembly and budding process of enveloped viruses utilizes numerous cellular factors to facilitate transport from one membrane bound compartment to the other. The host SNARE proteins are widely involved in late stages of vesicular mediated transport by catalyzing the docking and fusion of apposing membranes in the vesicle and target compartment. By generalized disruption of the SNARE sorting machinery, we recently demonstrated a role for these proteins in HIV-1 assembly by affecting Gag localization to the plasma membrane. Whether the observed phenomenon is specifically due to SNARE disruption or generalized disturbance of the cell sorting machinery and the involvement of specific “v” vs. “t” SNAREs in this phenomenon remains to be elucidated.

A novel big protein TPRBK possessing 25 units of TPR motif is essential for the progress of mitosis and cytokinesis

Through the comprehensive analysis of the genomic DNA sequence of human chromosome 22, we identified a novel gene of 702 kb encoding a big protein of 2481 amino acid residues, and named it as TPRBK (TPR containing big gene cloned at Keio). A novel protein TPRBK possesses 25 units of the TPR motif, which has been known to associate with a diverse range of biological functions. Orthologous genes of human TPRBK were found widely in animal species, from insecta to mammal, but not found in plants, fungi and nematoda. Northern blotting and RT-PCR analyses revealed that TPRBK gene is expressed ubiquitously in the human and mouse fetal tissues and various cell lines of human, monkey and mouse. Immunofluorescent staining of the synchronized monkey COS-7 cells with several relevant antibodies indicated that TPRBK changes its subcellular localization during the cell cycle: at interphase TPRBK locates on the centrosomes, during mitosis it translocates from spindle poles to mitotic spindles then to spindle midzone, and through a period of cytokinesis it stays on the midbody. Co-immunoprecipitation assay and immunofluorescent staining with adequate antibodies revealed that TPRBK binds to Aurora B, and those proteins together translocate throughout mitosis and cytokinesis. Treatments of cells with two drugs (Blebbistatin and Y-27632), that are known to inhibit the contractility of actin-myosin, disturbed the proper intracellular localization of TPRBK. Moreover, the knockdown of TPRBK expression by small interfering RNA (siRNA) suppressed the bundling of spindle midzone microtubules and disrupted the midbody formation, arresting the cells at G(2)+M phase. These observations indicated that a novel big protein TPRBK is essential for the formation and integrity of the midbody, hence we postulated that TPRBK plays a critical role in the progress of mitosis and cytokinesis during mammalian cell cycle.

Phosphoinositides and cytokinesis: the "PIP" of the iceberg

Phosphoinositides [Phosphatidylinositol (PtdIns), phosphatidylinositol 3-monophosphate (PtdIns3P), phosphatidylinositol 4-monophosphate (PtdIns4P), phosphatidylinositol 5-monophosphate (PtdIns5P), phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P(2) ), phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P(2) ), phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2) ), and phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P(3) )] are lowly abundant acidic lipids found at the cytosolic leaflet of the plasma membrane and intracellular membranes. Initially discovered as precursors of second messengers in signal transduction, phosphoinositides are now known to directly or indirectly control key cellular functions, such as cell polarity, cell migration, cell survival, cytoskeletal dynamics, and vesicular traffic. Phosphoinositides actually play a central role at the interface between membranes and cytoskeletons and contribute to the identity of the cellular compartments by recruiting specific proteins. Increasing evidence indicates that several phosphoinositides, particularly PtdIns(4,5)P(2) , are essential for cytokinesis, notably after furrow ingression. The present knowledge about the specific phosphoinositides and phosphoinositide modifying-enzymes involved in cytokinesis will be first presented. The review of the current data will then show that furrow stability and cytokinesis abscission require that both phosphoinositide production and hydrolysis are regulated in space and time. Finally, I will further discuss recent mechanistic insights on how phosphoinositides regulate membrane trafficking and cytoskeletal remodeling for successful furrow ingression and intercellular bridge abscission. This will highlight unanticipated connections between cytokinesis and enzymes implicated in human diseases, such as the Lowe syndrome.