Cytokinetic abscission: cellular dynamics at the midbody

The mammalian midbody and midbody remnant are assembly sites for RNA and localized translation

Long ignored as a vestigial remnant of cytokinesis, the mammalian midbody (MB) is released post-abscission inside large extracellular vesicles called MB remnants (MBRs). Recent evidence suggests that MBRs can modulate cell proliferation and cell fate decisions. Here, we demonstrate that the MB matrix is the site of ribonucleoprotein assembly and is enriched in mRNAs that encode proteins involved in cell fate, oncogenesis, and pluripotency, which we are calling the MB granule. Both MBs and post-abscission MBRs are sites of spatiotemporally regulated translation, which is initiated when nascent daughter cells re-enter G1 and continues after extracellular release. MKLP1 and ARC are necessary for the localization and translation of RNA in the MB dark zone, whereas ESCRT-III is necessary to maintain translation levels in the MB. Our work reveals a unique translation event that occurs during abscission and within a large extracellular vesicle.

Insights into the function of ESCRT and its role in enveloped virus infection

The endosomal sorting complex required for transport (ESCRT) is an essential molecular machinery in eukaryotic cells that facilitates the invagination of endosomal membranes, leading to the formation of multivesicular bodies (MVBs). It participates in various cellular processes, including lipid bilayer remodeling, cytoplasmic separation, autophagy, membrane fission and re-modeling, plasma membrane repair, as well as the invasion, budding, and release of certain enveloped viruses. The ESCRT complex consists of five complexes, ESCRT-0 to ESCRT-III and VPS4, along with several accessory proteins. ESCRT-0 to ESCRT-II form soluble complexes that shuttle between the cytoplasm and membranes, mainly responsible for recruiting and transporting membrane proteins and viral particles, as well as recruiting ESCRT-III for membrane neck scission. ESCRT-III, a soluble monomer, directly participates in vesicle scission and release, while VPS4 hydrolyzes ATP to provide energy for ESCRT-III complex disassembly, enabling recycling. Studies have confirmed the hijacking of ESCRT complexes by enveloped viruses to facilitate their entry, replication, and budding. Recent research has focused on the interaction between various components of the ESCRT complex and different viruses. In this review, we discuss how different viruses hijack specific ESCRT regulatory proteins to impact the viral life cycle, aiming to explore commonalities in the interaction between viruses and the ESCRT system.

Inhomogeneous Canham-Helfrich Abscission in Catenoid Necks under Critical Membrane Mosaicity

The mechanical effects of membrane compositional inhomogeneities are analyzed in a process analogous to neck formation in cellular membranes. We cast on the Canham-Helfrich model of fluid membranes with both the spontaneous curvature and the surface tension being non-homogeneous functions along the cell membrane. The inhomogeneous distribution of necking forces is determined by the equilibrium mechanical equations and the boundary conditions as considered in the axisymmetric setting compatible with the necking process. To establish the role played by mechanical inhomogeneity, we focus on the catenoid, a surface of zero mean curvature. Analytic solutions are shown to exist for the spontaneous curvature and the constrictive forces in terms of the border radii. Our theoretical analysis shows that the inhomogeneous distribution of spontaneous curvature in a mosaic-like neck constrictional forces potentially contributes to the membrane scission under minimized work in living cells.

DENND2B activates Rab35 at the intercellular bridge, regulating cytokinetic abscission and tetraploidy

Cytokinesis relies on membrane trafficking pathways regulated by Rabs and guanine nucleotide exchange factors (GEFs). During cytokinesis, the intercellular cytokinetic bridge (ICB) connecting daughter cells undergoes abscission, which requires actin depolymerization. Rab35 recruits MICAL1 to oxidize and depolymerize actin filaments. We show that DENND2B, a protein linked to cancer and congenital disorders, functions as a Rab35 GEF, recruiting and activating Rab35 at the ICB. DENND2B's N-terminal region also interacts with an active form of Rab35, suggesting that DENND2B is both a Rab35 GEF and effector. Knockdown of DENND2B delays abscission, leading to multinucleated cells and filamentous actin (F-actin) accumulation at the ICB, impairing recruitment of ESCRT-III at the abscission site. Additionally, F-actin accumulation triggers the formation of a chromatin bridge, activating the NoCut/abscission checkpoint, and DENND2B knockdown activates Aurora B kinase, a hallmark of checkpoint activation. Thus, our study identifies DENND2B as a crucial player in cytokinetic abscission.

The role of Vps4 in cancer development

VPS4 series proteins play a crucial role in the endosomal sorting complexes required for the transport (ESCRT) pathway, which is responsible for sorting and trafficking cellular proteins and is involved in various cellular processes, including cytokinesis, membrane repair, and viral budding. VPS4 proteins are ATPases that mediate the final steps of membrane fission and protein sorting as part of the ESCRT machinery. They disassemble ESCRT-III filaments, which are vital for forming multivesicular bodies (MVBs) and the release of intraluminal vesicles (ILVs), ultimately leading to the sorting and degradation of various cellular proteins, including those involved in cancer development and progression. Recent studies have shown a potential relationship between VPS4 series proteins and cancer. Evidence suggests that these proteins may have crucial roles in cancer development and progression. Several experiments have explored the association between VPS4 and different types of cancer, including gastrointestinal and reproductive system tumors, providing insight into the underlying mechanisms. Understanding the structure and function of VPS4 series proteins is critical in assessing their potential role in cancer. The evidence supporting the involvement of VPS4 series proteins in cancer provides a promising avenue for future research and therapeutic development. However, further researches are necessary to fully understand the mechanisms underlying the relationship between VPS4 series proteins and cancer and to develop effective strategies for targeting these proteins in cancer therapy. This article aims to review the structures and functions of VPS4 series proteins and the previous experiments to analyze the relationship between VPS4 series proteins and cancer.

Advances in Rice Seed Shattering

Seed shattering is an important trait that wild rice uses to adapt to the natural environment and maintain population reproduction, and weedy rice also uses it to compete with the rice crop. The loss of shattering is a key event in rice domestication. The degree of shattering is not only one of the main reasons for rice yield reduction but also affects its adaptability to modern mechanical harvesting methods. Therefore, it is important to cultivate rice varieties with a moderate shattering degree. In this paper, the research progress on rice seed shattering in recent years is reviewed, including the physiological basis, morphological and anatomical characteristics of rice seed shattering, inheritance and QTL/gene mapping of rice seed shattering, the molecular mechanism regulating rice seed shattering, the application of seed-shattering genes, and the relationship between seed-shattering genes and domestication.

An anillin-CIN85-SEPT9 complex promotes intercellular bridge maturation required for successful cytokinesis

Cleavage of one cell into two is the most dramatic event in the life of a cell. Plasma membrane fission occurs within a narrow intercellular bridge (ICB) between the daughter cells, but the mechanisms underlying ICB formation and maturation are poorly understood. Here we identify CIN85 as an ICB assembly factor and demonstrate its requirement for robust and timely cytokinesis. CIN85 interacts directly with the N-terminal region of anillin and SEPT9 and thereby facilitates SEPT9-containing filament localization to the plasma membrane of the ICB. In contrast, the C-terminal pleckstrin homology (PH) domain of anillin binds to septin units lacking SEPT9 but enriched in SEPT11. Anillin's interactions with distinct septin units are required to promote ICB elongation and maturation that, we propose, generate the physical space into which the abscission machinery is recruited to drive the final membrane scission event releasing two independent daughter cells.

A localized calcium transient and polar body abscission

Polar body emission is a special form of cytokinesis in oocyte meiosis that ensures the correct number of chromosomes in reproduction-competent eggs. The molecular mechanism of the last step, polar body abscission, is poorly understood. While it has been proposed that Ca²⁺ signaling plays important roles in embryonic cytokinesis, to date transient increases in intracellular free Ca²⁺ have been difficult to document in oocyte meiosis except for the global Ca²⁺ wave induced by sperm at fertilization. Here, we find that microinjection of the calcium chelator dibromo-BAPTA inhibits polar body abscission in Xenopus laevis oocytes. Using a novel, microtubule-targeted ratio-metric calcium sensor, we detected a calcium transient that is focused at the contractile ring-associated plasma membrane and which occurred after anaphase and constriction of the contractile ring but prior to abscission. This calcium transient was confirmed by mobile calcium probes. Further, the Ca²⁺-sensitive protein kinase Cβ C2 domain transiently translocated to the contractile ring-associated membrane simultaneously with the calcium transient. Collectively, these results demonstrate that a calcium transient, apparently originating at the contractile ring-associated plasma membrane, promotes polar body abscission.

Fine-tuning cell organelle dynamics during mitosis by small GTPases

During mitosis, the allocation of genetic material concurs with organelle transformation and distribution. The coordination of genetic material inheritance with organelle dynamics directs accurate mitotic progression, cell fate determination, and organismal homeostasis. Small GTPases belonging to the Ras superfamily regulate various cell organelles during division. Being the key regulators of membrane dynamics, the dysregulation of small GTPases is widely associated with cell organelle disruption in neoplastic and non-neoplastic diseases, such as cancer and Alzheimer's disease. Recent discoveries shed light on the molecular properties of small GTPases as sophisticated modulators of a remarkably complex and perfect adaptors for rapid structure reformation. This review collects current knowledge on small GTPases in the regulation of cell organelles during mitosis and highlights the mediator role of small GTPase in transducing cell cycle signaling to organelle dynamics during mitosis.

Functional midbody assembly in the absence of a central spindle

Contractile ring constriction during cytokinesis is thought to compact central spindle microtubules to form the midbody, an antiparallel microtubule bundle at the intercellular bridge. In Caenorhabditis elegans, central spindle microtubule assembly requires targeting of the CLASP family protein CLS-2 to the kinetochores in metaphase and spindle midzone in anaphase. CLS-2 targeting is mediated by the CENP-F–like HCP-1/2, but their roles in cytokinesis and midbody assembly are not known. We found that although HCP-1 and HCP-2 mostly function cooperatively, HCP-1 plays a more primary role in promoting CLS-2–dependent central spindle microtubule assembly. HCP-1/2 codisrupted embryos did not form central spindles but completed cytokinesis and formed functional midbodies capable of supporting abscission. These central spindle–independent midbodies appeared to form via contractile ring constriction–driven bundling of astral microtubules at the furrow tip. This work suggests that, in the absence of a central spindle, astral microtubules can support midbody assembly and that midbody assembly is more predictive of successful cytokinesis than central spindle assembly.

The Cockayne syndrome group A and B proteins are part of a ubiquitin-proteasome degradation complex regulating cell division

Cytokinesis is monitored by a molecular machinery that promotes the degradation of the intercellular bridge, a transient protein structure connecting the two daughter cells. Here, we found that CSA and CSB, primarily defined as DNA repair factors, are located at the midbody, a transient structure in the middle of the intercellular bridge, where they recruit CUL4 and MDM2 ubiquitin ligases and the proteasome. As a part of this molecular machinery, CSA and CSB contribute to the ubiquitination and the degradation of proteins such as PRC1, the Protein Regulator of Cytokinesis, to ensure the correct separation of the two daughter cells. Defects in CSA or CSB result in perturbation of the abscission leading to the formation of long intercellular bridges and multinucleated cells, which might explain part of the Cockayne syndrome phenotypes. Our results enlighten the role played by CSA and CSB as part of a ubiquitin/proteasome degradation process involved in transcription, DNA repair, and cell division.

F-actin disassembly factor MICAL1 binding to Myosin Va mediates cargo unloading during cytokinesis

Motor-mediated intracellular trafficking requires motors to position cargoes at proper locations. Myosin Va (MyoVa), an actin-based motor, is a classic model for studying cargo transport. However, the molecular basis underlying cargo unloading in MyoVa-mediated transport has remained enigmatic. We have identified MICAL1, an F-actin disassembly regulator, as a binding partner of MyoVa and shown that MICAL1-MyoVa interaction is critical for localization of MyoVa at the midbody. By binding to MICAL1, MyoVa-mediated transport is terminated, resulting in vesicle unloading at the midbody for efficient cytokinesis. The MyoVa/MICAL1 complex structure reveals that MICAL1 and F-actin assembly factors, Spires, share an overlapped binding surface on MyoVa, suggesting a regulatory role of F-actin dynamics in cargo unloading. Down-regulating F-actin disassembly by a MICAL1 mutant significantly reduces MyoVa and vesicles accumulating at the midbody. Collectively, our findings demonstrate that MyoVa binds to MICAL1 at the midbody destination and triggers F-actin disassembly to unload the vesicle cargo.

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

Cytokinesis is the last step of cell division that partitions the cellular organelles and cytoplasm of one cell into two. In animal cells, cytokinesis requires Rho-GTPase-dependent assembly of F-actin and myosin II (actomyosin) to form an equatorial contractile ring (CR) that bisects the cell. Despite 50 years of research, the precise mechanisms of CR assembly, tension generation and closure remain elusive. This hypothesis article considers a holistic view of the CR that, in addition to actomyosin, includes another Rho-dependent cytoskeletal sub-network containing the scaffold protein, Anillin, and septin filaments (collectively termed anillo-septin). We synthesize evidence from our prior work in Drosophila S2 cells that actomyosin and anillo-septin form separable networks that are independently anchored to the plasma membrane. This latter realization leads to a simple conceptual model in which CR assembly and closure depend upon the micro-management of the membrane microdomains to which actomyosin and anillo-septin sub-networks are attached. During CR assembly, actomyosin contractility gathers and compresses its underlying membrane microdomain attachment sites. These microdomains resist this compression, which builds tension. During CR closure, membrane microdomains are transferred from the actomyosin sub-network to the anillo-septin sub-network, with which they flow out of the CR as it advances. This relative outflow of membrane microdomains regulates tension, reduces the circumference of the CR and promotes actomyosin disassembly all at the same time. According to this hypothesis, the metazoan CR can be viewed as a membrane microdomain gathering, compressing and sorting machine that intrinsically buffers its own tension through coordination of actomyosin contractility and anillo-septin-membrane relative outflow, all controlled by Rho. Central to this model is the abandonment of the dogmatic view that the plasma membrane is always readily deformable by the underlying cytoskeleton. Rather, the membrane resists compression to build tension. The notion that the CR might generate tension through resistance to compression of its own membrane microdomain attachment sites, can account for numerous otherwise puzzling observations and warrants further investigation using multiple systems and methods.

Mechanochemical Crosstalk Produces Cell-Intrinsic Patterning of the Cortex to Orient the Mitotic Spindle

Proliferating animal cells are able to orient their mitotic spindles along their interphase cell axis, setting up the axis of cell division, despite rounding up as they enter mitosis. This has previously been attributed to molecular memory and, more specifically, to the maintenance of adhesions and retraction fibers in mitosis [1-6], which are thought to act as local cues that pattern cortical Gαi, LGN, and nuclear mitotic apparatus protein (NuMA) [3, 7-18]. This cortical machinery then recruits and activates Dynein motors, which pull on astral microtubules to position the mitotic spindle. Here, we reveal a dynamic two-way crosstalk between the spindle and cortical motor complexes that depends on a Ran-guanosine triphosphate (GTP) signal [12], which is sufficient to drive continuous monopolar spindle motion independently of adhesive cues in flattened human cells in culture. Building on previous work [1, 12, 19-23], we implemented a physical model of the system that recapitulates the observed spindle-cortex interactions. Strikingly, when this model was used to study spindle dynamics in cells entering mitosis, the chromatin-based signal was found to preferentially clear force generators from the short cell axis, so that cortical motors pulling on astral microtubules align bipolar spindles with the interphase long cell axis, without requiring a fixed cue or a physical memory of interphase shape. Thus, our analysis shows that the ability of chromatin to pattern the cortex during the process of mitotic rounding is sufficient to translate interphase shape into a cortical pattern that can be read by the spindle, which then guides the axis of cell division.

Intercellular Bridge Mediates Ca2+ Signals between Micropatterned Cells via IP3 and Ca2+ Diffusion

Intercellular bridges are plasma continuities formed at the end of the cytokinesis process that facilitate intercellular mass transport between the two daughter cells. However, it remains largely unknown how the intercellular bridge mediates Ca²⁺ communication between postmitotic cells. In this work, we utilize BV-2 microglial cells planted on dumbbell-shaped micropatterned assemblies to resolve spatiotemporal characteristics of Ca²⁺ signal transfer over the intercellular bridges. With the use of such micropatterns, considerably longer and more regular intercellular bridges can be obtained than in conventional cell cultures. The initial Ca²⁺ signal is evoked by mechanical stimulation of one of the daughter cells. A considerable time delay is observed between the arrivals of passive Ca²⁺ diffusion and endogenous Ca²⁺ response in the intercellular-bridge-connected cell, indicating two different pathways of the Ca²⁺ communication. Extracellular Ca²⁺ and the paracrine pathway have practically no effect on the endogenous Ca²⁺ response, demonstrated by application of Ca²⁺-free medium, exogenous ATP, and P2Y₁₃ receptor antagonist. In contrast, the endoplasmic reticulum Ca²⁺-ATPase inhibitor thapsigargin and inositol trisphosphate (IP₃) receptor blocker 2-aminoethyl diphenylborate significantly inhibit the endogenous Ca²⁺ increase, which signifies involvement of IP₃-sensitive calcium store release. Notably, passive Ca²⁺ diffusion into the connected cell can clearly be detected when IP₃-sensitive calcium store release is abolished by 2-aminoethyl diphenylborate. Those observations prove that both passive Ca²⁺ diffusion and IP₃-mediated endogenous Ca²⁺ response contribute to the Ca²⁺ increase in intercellular-bridge-connected cells. Moreover, a simulation model agreed well with the experimental observations.

MED12 exerts an emerging role in actin-mediated cytokinesis via LIMK2/cofilin pathway in NSCLC

Background

Mediator complex subunit 12 (MED12) is an essential hub for transcriptional regulation, in which mutations and overexpression were reported to be associated with several kinds of malignancies. Nevertheless, the role of MED12 in non-small cell lung cancer (NSCLC) remains to be elucidated.

Methods

MED12 mutation was detected by Next-generation sequencing. The expression of MED12 in 179 human NSCLC tissue samples and 73 corresponding adjacent normal lung tissue samples was measured by immunohistochemistry (IHC). CRISPR-Cas9 was used to knock out MED12 in PC9 and SPC-A1 cells. MED12 rescued stable cell lines were generated by lentivirus infection. We traced cell division process by live cell imaging. The molecular mechanism of aborted cytokinesis resulted by MED12 knockout was investigated by RNA-seq. Effects of MED12 deletion on the proliferation of NSCLC cells were determined by MTT assay and Colony-formation assay in vitro and xenograft tumor model in nude mouse. Cell senescence was measured by SA-β-gal staining.

Results

In our study, no MED12 exon mutation was detected in NSCLC samples, whereas we found that MED12 was overexpressed in human NSCLC tissues, which positively correlated with the tumor volume and adversely affected patient survival. Furthermore, knockout MED12 in NSCLC cell lines resulted in cytokinesis failure, displayed a multinuclear phenotype, and disposed to senescence, and become non-viable. Lack of MED12 decreased the proliferative potential of NSCLC cells and limited the tumor growth in vivo. Mechanism investigations revealed that MED12 knockout activated LIMK2, caused aberrant actin cytoskeleton remodeling, and disrupted the abscission of intercellular bridge, which led to the cytokinesis failure. Reconstitution of exogenous MED12 restored actin dynamics, normal cytokinesis and cell proliferation capacity in MED12 knockout cells.

Conclusions

These results revealed a novel role of MED12 as an important regulator for maintaining accurate cytokinesis and survival in NSCLC cells, which may offer a therapeutic strategy to control tumor growth for NSCLC patients especially those highly expressed MED12.

Electronic supplementary material

The online version of this article (10.1186/s12943-019-1020-4) contains supplementary material, which is available to authorized users.

Anionic Lipids: A Pipeline Connecting Key Players of Plant Cell Division

How cells position their division plane is a critical component of cell division. Indeed, it defines whether the two daughter cells divide symmetrically (with equal volumes) or not, and as such is critical for cell differentiation and lineage specification across eukaryotes. However, oriented cell divisions are of special significance for organisms with cell walls, such as plants, because their cells are embedded and cannot relocate. Correctly positioning the division plane is therefore of prevailing importance in plants, as it controls not only the occurrence of asymmetric cell division, but also tissue morphogenesis and organ integrity. While cytokinesis is executed in radically different manners in animals and plants, they both rely on the dynamic interplay between the cytoskeleton and membrane trafficking to precisely deliver molecular components to the future site of cell division. Recent research has shown that strict regulation of the levels and distribution of anionic lipids, which are minor components of the cell membrane's lipids, is required for successful cytokinesis in non-plant organisms. This review focused on the recent evidence pointing to whether such signaling lipids have roles in plant cell division.

PLK1 plays dual roles in centralspindlin regulation during cytokinesis

Cytokinesis begins upon anaphase onset. An early step involves local activation of the small GTPase RhoA, which triggers assembly of an actomyosin-based contractile ring at the equatorial cortex. Here, we delineated the contributions of PLK1 and Aurora B to RhoA activation and cytokinesis initiation in human cells. Knock-down of PRC1, which disrupts the spindle midzone, revealed the existence of two pathways that can initiate cleavage furrow ingression. One pathway depends on a well-organized spindle midzone and PLK1, while the other depends on Aurora B activity and centralspindlin at the equatorial cortex and can operate independently of PLK1. We further show that PLK1 inhibition sequesters centralspindlin onto the spindle midzone, making it unavailable for Aurora B at the equatorial cortex. We propose that PLK1 activity promotes the release of centralspindlin from the spindle midzone through inhibition of PRC1, allowing centralspindlin to function as a regulator of spindle midzone formation and as an activator of RhoA at the equatorial cortex.

PRR14L mutations are associated with chromosome 22 acquired uniparental disomy, age-related clonal hematopoiesis and myeloid neoplasia

Acquired uniparental disomy (aUPD, also known as copy-neutral loss of heterozygosity) is a common feature of cancer cells and characterized by extended tracts of somatically-acquired homozygosity without any concurrent loss or gain of genetic material. The presumed genetic targets of many regions of aUPD remain unknown. Here we describe the association of chromosome 22 aUPD with mutations that delete the C-terminus of PRR14L in patients with chronic myelomonocytic leukemia (CMML), related myeloid neoplasms and age-related clonal hematopoiesis (ARCH). Myeloid panel analysis identified a median of 3 additional mutated genes (range 1-6) in cases with a myeloid neoplasm (n=8), but no additional mutations in cases with ARCH (n=2) suggesting that mutated PRR14L alone may be sufficient to drive clonality. PRR14L has very limited homology to other proteins and its function is unknown. ShRNA knockdown of PRR14L in human CD34+ cells followed by in vitro growth and differentiation assays showed an increase in monocytes and decrease in neutrophils consistent, with a CMML-like phenotype. RNA-Seq and cellular localization studies suggest a role for PRR14L in cell division. PRR14L is thus a novel, biallelically mutated gene and potential founding abnormality in myeloid neoplasms.

Dynamic localization of α-tubulin acetyltransferase ATAT1 through the cell cycle in human fibroblastic KD cells

Acetylation of α-tubulin is a well-studied posttranscriptional modification, which is mostly catalyzed by α-tubulin N-acetyltransferase (ATAT1). ATAT1 possibly affects various cellular functions related with microtubules, such as intracellular transport, cell motility, cilia formation, and neuronal signaling. Here, we analyzed the subcellular localization of immunolabeled ATAT1 in human fibroblast KD cells through the cell cycle using confocal laser scanning microscopy. ATAT1 dramatically changed its localization through the cell cycle, depending on the mitotic phase. In interphase, immunolabeled ATAT1 was observed in centrioles, nuclei, and basal bodies if the cells projected primary cilia. ATAT1 was intensely detected as clusters in the nuclei in the G1-G2 phase. In telophase, ATAT1 colocalized with chromatids and spindle poles, and ultimately migrated to the daughter nucleus, newly synthesized centrioles, and midbody. The nucleolus is a core region of ribosomal RNA transcription, and the midbody is associated with severing and depolymerizing of microtubules in the stembody. The specific distributions of ATAT1 through the cell cycle suggest multiple functions of ATAT1, which could include acetylation of microtubules, RNA transcription activity, severing microtubules, and completion of cytokinesis.

Slow calcium waves mediate furrow microtubule reorganization and germ plasm compaction in the early zebrafish embryo

Zebrafish germ plasm ribonucleoparticles (RNPs) become recruited to furrows of early zebrafish embryos through their association with astral microtubules ends. During the initiation of cytokinesis, microtubules are remodeled into a furrow microtubule array (FMA), which is thought to be analogous to the mammalian midbody involved in membrane abscission. During furrow maturation, RNPs and FMA tubules transition from their original distribution along the furrow to enrichments at the furrow distal ends, which facilitates germ plasm mass compaction. We show that nebel mutants exhibit reduced furrow-associated slow calcium waves (SCWs), caused at least in part by defective enrichment of calcium stores. RNP and FMA distal enrichment mirrors the medial-to-distal polarity of SCWs, and inhibition of calcium release or downstream mediators such as Calmodulin affects RNP and FMA distal enrichment. Blastomeres with reduced or lacking SCWs, such as early blastomeres in nebel mutants and wild-type blastomeres at later stages, exhibit medially bundling microtubules similar to midbodies in other cell types. Our data indicate that SCWs provide medial-to-distal directionality along the furrow to facilitate germ plasm RNP enrichment at the furrow ends.

Aurora kinase B dependent phosphorylation of 53BP1 is required for resolving merotelic kinetochore-microtubule attachment errors during mitosis

Defects in resolving kinetochore-microtubule attachment mistakes during mitosis is linked to chromosome instability associated with carcinogenesis as well as resistance to cancer therapy. Here we report for the first time that tumor suppressor p53-binding protein 1 (53BP1) is phosphorylated at serine 1342 (S1342) by Aurora kinase B both in vitro and in human cells, which is required for optimal recruitment of 53BP1 at kinetochores. Furthermore, 53BP1 staining normally localized on the outer kinetochore, extended to the whole kinetochore when it is merotelically-attached, in concert with mitotic centromere-associated kinesin. Kinetochore-binding of pS1342-53BP1 is essential for efficient resolving of merotelic attachment, a spontaneous kinetochore-microtubule connection error that usually causes aneuploidy. Consistently, loss of 53BP1 results in significant increase in lagging chromosome events, micronuclei formation and aneuploidy, due to the unresolved merotely in both cancer and primary cells, which is prevented by ectopic wild type 53BP1 but not by the nonphophorylable S1342A mutant. We thus document a novel DNA damage-independent function of 53BP1 in maintaining faithful chromosome segregation during mitosis.

Motor activity of centromere-associated protein-E contributes to its localization at the center of the midbody to regulate cytokinetic abscission

Accurate control of cytokinesis is critical for genomic stability to complete high-fidelity transmission of genetic material to the next generation. A number of proteins accumulate in the intercellular bridge (midbody) during cytokinesis, and the dynamics of these proteins are temporally and spatially orchestrated to complete the process. In this study, we demonstrated that localization of centromere-associated protein-E (CENP-E) at the midbody is involved in cytokinetic abscission. The motor activity of CENP-E and the C-terminal midbody localization domain, which includes amino acids 2659-2666 (RYFDNSSL), are involved in the anchoring of CENP-E to the center of the midbody. Furthermore, CENP-E motor activity contributes to the accumulation of protein regulator of cytokinesis 1 (PRC1) in the midbody during cytokinesis. Midbody localization of PRC1 is critical to the antiparallel microtubule structure and recruitment of other midbody-associated proteins. Therefore, CENP-E motor activity appears to play important roles in the organization of these proteins to complete cytokinetic abscission. Our findings will be helpful for understanding how each step of cytokinesis is regulated to complete cytokinetic abscission.

Phragmoplast microtubule dynamics - a game of zones

Plant morphogenesis relies on the accurate positioning of the partition (cell plate) between dividing cells during cytokinesis. The cell plate is synthetized by a specialized structure called the phragmoplast, which consists of microtubules, actin filaments, membrane compartments and associated proteins. The phragmoplast forms between daughter nuclei during the transition from anaphase to telophase. As cells are commonly larger than the originally formed phragmoplast, the construction of the cell plate requires phragmoplast expansion. This expansion depends on microtubule polymerization at the phragmoplast forefront (leading zone) and loss at the back (lagging zone). Leading and lagging zones sandwich the 'transition' zone. A population of stable microtubules in the transition zone facilitates transport of building materials to the midzone where the cell plate assembly takes place. Whereas microtubules undergo dynamic instability in all zones, the overall balance appears to be shifted towards depolymerization in the lagging zone. Polymerization of microtubules behind the lagging zone has not been reported to date, suggesting that microtubule loss there is irreversible. In this Review, we discuss: (1) the regulation of microtubule dynamics in the phragmoplast zones during expansion; (2) mechanisms of the midzone establishment and initiation of cell plate biogenesis; and (3) signaling in the phragmoplast.

Tension-induced cytokinetic abscission in human fibroblasts

Previous studies have shown that cytokinetic abscission at the end of mitosis is executed by the ESCRT machinery in mammalian cells, and that the process is dependent on adhesion-induced integrin signalling via a FAK-PLK1-CEP55-TSG101/Alix-CHMP4B pathway. The present study identified an alternative abscission mechanism driven by mechanical force. In the absence of integrin signals (non-adherent conditions), cytokinesis in non-transformed human fibroblasts proceeds to CEP55 accumulation at the midbody, but after prolonged time (>3 hours) the major midbody components Aurora B, MKLP1 and CEP55 were no longer detected in the area. Upon adhesion to fibronectin, such cells were able to complete abscission without re-appearance of midbody proteins. Live-cell imaging revealed that re-plating on stiff fibronectin matrix (64 KPa) allowed >95% of the cells to complete abscission within 9 hours while the corresponding number was 40% on soft fibronectin matrix (0.5 KPa). The cells re-plated on poly-L-lysine were not able to generate tension and did not divide. Thus, mechanical tension can cause cytokinetic abscission by stretching of the intercellular bridge between the two daughter cells until it eventually ruptures without the involvement of ESCRT complexes. Importantly, regression of the cleavage furrow and formation of bi-nucleated cells did not occur in most of the suspension-treated mitotic cells after re-plating on fibronectin. Septin, which stabilizes the membrane associated with the midbody, was found to remain along the ingressed membrane, suggesting that this filament system maintains the membrane bridge although the midbody had dissolved, thereby preventing regression and allowing tension to act on the narrow intercellular bridge.

PRC1: Linking Cytokinesis, Chromosomal Instability, and Cancer Evolution

Cytokinesis is the final event of the cell cycle dividing one cell into two daughter cells. The protein regulator of cytokinesis (PRC)1 is essential for cytokinesis and normal cell cleavage. Deregulation of PRC1 causes cytokinesis defects that promote chromosomal instability (CIN) and thus tumor heterogeneity and cancer evolution. Consistently, abnormal PRC1 expression correlates with poor patient outcome in various malignancies, which may be caused by PRC1-mediated CIN and aneuploidy. Here, we review the physiological functions of PRC1 in cell cycle regulation and its contribution to tumorigenesis and intratumoral heterogeneity. We discuss targeting PRC1 within the complementary approaches of either normalizing CIN in aneuploid cancers or creating chromosomal chaos in genomically stable cancers to induce apoptosis.

Midbody localization of vinexin recruits rhotekin to facilitate cytokinetic abscission

Vinexin is a SH3 domain-containing adaptor protein that has diverse roles in cell adhesion, signal transduction, gene regulation and stress granule assembly. In this study, we found that vinexin localizes at the midbody during cell division and facilitates cytokinesis. Knockdown of vinexin in HeLa cells delayed the mitotic cell cycle progression and increased the time of cell abscission and the failure to resolve the cytoplasmic bridge. Midbody-localized vinexin is essential for recruiting rhotekin to this structure for cytokinesis because overexpression of a vinexin mutant without a rhotekin-binding motif or knockdown of rhotekin also impaired cytokinetic abscission and increased the number of cells arrested at the midbody stage. Aberrant expression of vinexin and rhotekin in various cancers has been implicated to promote metastasis because of their functions in cell adhesion and signaling. Our findings reveal a novel role of vinexin and rhotekin in cytokinetic abscission and provide another perspective of how both molecules may affect oncogenic transformation via this fundamental cell cycle process.

Keep it on the edge: The post-mitotic midbody as a polarity signal unit

The maintenance of the epithelial architecture during tissue proliferation is achieved by apical positioning of the midbody after cell division. Consequently, midbody mislocalization contributes to epithelial architecture disruption, a fundamental event during epithelial tumorigenesis. Studies in 3D polarized epithelial MDCK or Caco2 cell models, where midbody misplacement leads to multiple ectopic but fully polarized lumen-containing cysts, revealed that this phenotype can be caused by 2 different scenarios: the loss of mitotic spindle orientation or the loss of asymmetric abscission. In addition, we have recently proposed a third cellular mechanism where the midbody mislocalization is achieved through cytokinesis acceleration driven by the cancer-promoting phosphatase of regenerating liver (PRL)-3. Here we critically review these findings, and we furthermore present new data indicating that midbodies themselves might act as signal unit for polarization since they can infer apical characteristics to a basal membrane.

Functional assignment of multiple ESCRT-III homologs in cell division and budding in Sulfolobus islandicus

The archaea Sulfolobus utilizes the ESCRT-III-based machinery for cell division. This machinery comprises three proteins: CdvA, Eukaryotic-like ESCRT-III and Vps4. In addition to ESCRT-III, Sulfolobus cells also encode three other ESCRT-III homologs termed ESCRT-III-1, -2 and -3. Herein, we show that ESCRT-III-1 and -2 in S. islandicus REY15A are localized at midcell between segregating chromosomes, indicating that both are involved in cell division. Genetic analysis reveals that escrt-III-2 is indispensable for cell viability and cells with reduced overall level of ESCRT-III-1 exhibit growth retardation and cytokinesis defect with chain-like cell morphology. In contrast, escrt-III-3 is dispensable for cell division. We show that S. islandicus REY15A cells generate buds when infected with S. tengchongensis spindle shaped-virus 2 (STSV2) or when ESCRT-III-3 is over-expressed. Interestingly, Δescrt-III-3 cells infected with STSV2 do not produce buds. These results suggest that ESCRT-III-3 plays an important role in budding. In addition, cells over-expressing the C-terminal truncated mutants of ESCRT-III, ESCRT-III-1 and ESCRT-III-2 are maintained predominantly at the early, late, and membrane abscission stages of cell division respectively, suggesting a crucial role of the ESCRTs at different stages of membrane ingression. Intriguingly, intercellular bridge and midbody-like structures are observed in cells over-expressing MIM2-truncated mutant of ESCRT-III-2.

Micromanipulation of daughter cells for the study of cytokinetic abscission

The last step of cytokinesis, abscission, consists in the severing of the intercellular bridge connecting the two daughter cells. Because daughter cells move randomly on regular cell culture substrates, the use of adhesive micropatterns facilitates the observation of the intercellular bridge and its severing. Here we propose general rules to design micropatterns optimized to study this process. In particular, these micropatterns allow a good stabilization of the daughter cells and a predictable positioning of the intercellular bridge. We suggest a series of micropatterns controlling various cellular parameters such as distance between daughter cells or daughter cells polarization. We give recommendations for videomicroscopy acquisition during cell division and propose automated image analysis methods using kymograph analysis or bridge detection. Finally, we detail methods to artificially cut the intercellular bridge using UV-based laser ablation or using two-photons laser ablation.

PRL-3 disrupts epithelial architecture by altering the post-mitotic midbody position

Disruption of epithelial architecture is a fundamental event during epithelial tumorigenesis. We show that the expression of the cancer-promoting phosphatase PRL-3 (PTP4A3), which is overexpressed in several epithelial cancers, in polarized epithelial MDCK and Caco2 cells leads to invasion and the formation of multiple ectopic, fully polarized lumens in cysts. Both processes disrupt epithelial architecture and are hallmarks of cancer. The pathological relevance of these findings is supported by the knockdown of endogenous PRL-3 in MCF-7 breast cancer cells grown in three-dimensional branched structures, showing the rescue from multiple-lumen- to single-lumen-containing branch ends. Mechanistically, it has been previously shown that ectopic lumens can arise from midbodies that have been mislocalized through the loss of mitotic spindle orientation or through the loss of asymmetric abscission. Here, we show that PRL-3 triggers ectopic lumen formation through midbody mispositioning without altering the spindle orientation or asymmetric abscission, instead, PRL-3 accelerates cytokinesis, suggesting that this process is an alternative new mechanism for ectopic lumen formation in MDCK cysts. The disruption of epithelial architecture by PRL-3 revealed here is a newly recognized mechanism for PRL-3-promoted cancer progression.

Specific membrane dynamics during neural stem cell division

Neural stem and progenitor cells in the developing cerebral cortex, but also when grown in culture, display a range of distinct phenomena during cytokinesis. Cleavage furrow ingression in neural progenitor cells can bisect their basal processes and, later on, result in midbody formation at the apical surface. After abscission, these midbodies are released as membrane-bound particles into the extracellular space, in contrast to uptake and degradation of postabscission midbodies in other cell types. Whether these cellular dynamics are unique to neural stem cells, or more ubiquitously found, and what biological significance these processes have for cell differentiation or cell-cell communication, are open questions that require a combination of approaches. Here, we discuss techniques to study the specific membrane dynamics underlying the basal process splitting and postabscission midbody release in neural stem cells. We provide some basic concepts and protocols to isolate, enrich and stain released midbodies, and follow midbody dynamics over time. Moreover, we discuss techniques to prepare cortical sections for high-voltage electron microscopy to visualize the fine basal processes of progenitor cells.

Integrin signaling via FAK-Src controls cytokinetic abscission by decelerating PLK1 degradation and subsequent recruitment of CEP55 at the midbody

Adhesion to extracellular matrix is required for cell cycle progression through the G1 phase and for the completion of cytokinesis in normal adherent cells. Cancer cells acquire the ability to proliferate anchorage-independently, a characteristic feature of malignantly transformed cells. However, the molecular mechanisms underlying this escape of the normal control mechanisms remain unclear. The current study aimed to identify adhesion-induced reactions regulating the cytokinesis of non-transformed human fibroblasts.The adhesion-dependent control of cytokinesis was found to occur at a late stage close to the abscission, during which the endosomal sorting complex required for transport (ESCRT) severs the thin intercellular bridge connecting two nascent daughter cells. CEP55, a key protein involved in the abscission process, was localized at the midbody in both adherent and non-adherent fibroblasts, but it was unable to efficiently recruit ALIX, TSG101, and consequently the ESCRT-III subunit CHMP4B was missing in the non-adherent cells. PLK1, a kinase that prevents premature recruitment of CEP55 to the midbody, disappeared from this site more rapidly in the non-adherent cells. A FAK-Src signaling pathway downstream of integrin-mediated cell adhesion was found to decelerate both PLK1 degradation and CEP55 accumulation at the midbody. These data identify the regulation of PLK1 and CEP55 as steps where integrins exert control over the cytokinetic abscission.

Psychosine-triggered endomitosis is modulated by membrane sphingolipids through regulation of phosphoinositide 4,5-bisphosphate production at the cleavage furrow

Endomitosis is a special type of mitosis in which only cytokinesis-the final step of the cell division cycle-is defective, resulting in polyploid cells. Although endomitosis is biologically important, its regulatory aspects remain elusive. Psychosine, a lysogalactosylceramide, prevents proper cytokinesis when supplemented to proliferating cells. Cytokinetic inhibition by psychosine does not inhibit genome duplication. Consequently cells undergo multiple rounds of endomitotic cell cycles, resulting in the formation of giant multiploid cells. Here we successfully quantified psychosine-triggered multiploid cell formation, showing that membrane sphingolipids ratios modulate psychosine-triggered polyploidy in Namalwa cells. Among enzymes that experimentally remodel cellular sphingolipids, overexpression of glucosylceramide synthase to biosynthesize glycosylsphingolipids (GSLs) and neutral sphingomyelinase 2 to hydrolyze sphingomyelin (SM) additively enhanced psychosine-triggered multiploidy; almost all of the cells became polyploid. In the presence of psychosine, Namalwa cells showed attenuated cell surface SM clustering and suppression of phosphatidylinositol 4,5-bisphosphate production at the cleavage furrow, both important processes for cytokinesis. Depending on the sphingolipid balance between GSLs and SM, Namalwa cells could be effectively converted to viable multiploid cells with psychosine.

Beclin-1 knockdown shows abscission failure but not autophagy defect during oocyte meiotic maturation

Cytokinesis is the final step in cell division that results in the separation of a parent cell into daughter cells. Unlike somatic cells that undergo symmetric division, meiotic division is highly asymmetric, allowing the preservation of maternal resources for embryo development. Beclin-1/BECN1, the mammalian homolog of yeast Atg6, is a key molecule of autophagy. As part of a class III phosphatidylinositol 3-kinase (PI3K-III) complex, BECN1 initiates autophagosome formation by coordinating membrane trafficking. However, emerging evidence suggests that BECN1 regulates chromosome segregation and cytokinesis during mitosis. Thus, we investigated the function of BECN1 during oocyte meiotic maturation. BECN1 was widely distributed during meiotic maturation forming small vesicles. Interestingly, BECN1 is also detected at the midbody ring during cytokinesis. Depletion of BECN1 impaired the cytokinetic abscission, perturbing the recruitment of ZFYVE26 at the midbody. Similar phenotypes were observed when PI3K-III activity was inhibited. However, inhibition of autophagy by depleting Atg14L did not disturb meiotic maturation. Therefore, our results not only demonstrate that BECN1 as a PI3K-III component is essential for cytokinesis, but also suggest that BECN1 is not associated with autophagy pathway in mouse oocytes.

Dynamic Phosphorylation of NudC by Aurora B in Cytokinesis

Nuclear distribution protein C (NudC) is a mitotic regulator that plays a role in cytokinesis. However, how NudC is regulated during cytokinesis remains unclear. Here, we show that NudC is phosphorylated by Aurora B, a kinase critical for cell abscission. NudC is co-localized with Aurora B at the midbody and co-immunoprecipitated with Aurora B in mitosis. Inhibition of Aurora B by ZM447439 reduced NudC phosphorylation, suggesting that NudC is an Aurora B substrate in vivo. We identified T40 on NudC as an Aurora B phosphorylation site. NudC depletion resulted in cytokinesis failure with a dramatic elongation of the intercellular bridge between daughter cells, sustained Aurora B activity at the midbody, and reduced cell abscission. These cytokinetic defects can be rescued by the ectopic expression of wild-type NudC. Reconstitution with T40A phospho-defective NudC was found to rescue the cytokinesis defect. In contrast, reconstitution with the T40D phospho-mimetic NudC was inefficient in supporting the completion of cytokinesis. These results suggest that that dynamic phosphorylation of NudC by Aurora B regulates cytokinesis.

SATB2 is localized to the centrosome and spindle maintenance and its knockdown leads to downregulation of CDK2

Though special AT-rich sequence-binding protein 2 (SATB2) is reported as a transcriptional regulator of skeletal development and osteogenic differentiation, the underlying mechanism of SATB2 is not fully understood. Herein, we report that SATB2 is localized to the mitotic microtubules, the centrosome, and midbodies in mitotic cells with alpha-tubulin. Moreover, siRNA-mediated disruption of SATB2 in H460 cells caused the defect of nuclear morphology and multinucleate cells. SATB2 siRNA knockdown reduced the viability and downregulated the CDK2 expression in SKOV3 cells. Consistently, cell cycle analysis demonstrated that the silencing of SATB2 induced cell-cycle G1 arrest. Furthermore, proteosomal inhibitor MG132 treatment rescued the downregulation of CDK2 in SATB2-silenced SKOV3 cells. Taken together, our findings suggest that SATB2 regulates the mitosis of cell cycle and affects G1 cell cycle via interaction with CDK2.

Two-pore channel 1 interacts with citron kinase, regulating completion of cytokinesis

Two-pore channels (TPC1, 2, and 3) are recently identified endolysosmal ion channels, but remain poorly characterized. In this study, we show for the first time a role for TPC1 in cytokinesis, the final step in cell division. HEK 293 T-REx cells inducibly overexpressing TPC1 demonstrated a lack of proliferation accompanied by multinucleation and an increase in G2/M cycling cells. Increased TPC1 was associated with a concomitant accumulation of active RhoGTP and a decrease in phosphorylated myosin light chain (MLC). Finally, we demonstrated a novel interaction between TPC1 and citron kinase (CIT). These results identify TPC1 as a central component of cytokinetic control, specifically during abscission, and introduce a means by which the endolysosomal system may play an active role in this process.

SHCBP1 is required for midbody organization and cytokinesis completion

The centralspindlin complex, which is composed of MKLP1 and MgcRacGAP, is one of the crucial factors involved in cytokinesis initiation. Centralspindlin is localized at the middle of the central spindle during anaphase and then concentrates at the midbody to control abscission. A number of proteins that associate with centralspindlin have been identified. These associating factors regulate furrowing and abscission in coordination with centralspindlin. A recent study identified a novel centralspindlin partner, called Nessun Dorma, which is essential for germ cell cytokinesis in Drosophila melanogaster. SHCBP1 is a human ortholog of Nessun Dorma that associates with human centralspindlin. In this report, we analyzed the interaction of SHCBP1 with centralspindlin in detail and determined the regions that are required for the interaction. In addition, we demonstrate that the central region is necessary for the SHCBP1 dimerization. Both MgcRacGAP and MKLP1 are degraded once cells exit mitosis. Similarly, endogenous and exogenous SHCBP1 were degraded with mitosis progression. Interestingly, SHCBP1 expression was significantly reduced in the absence of centralspindlin, whereas centralspindlin expression was not affected by SHCBP1 knockdown. Finally, we demonstrate that SHCBP1 depletion promotes midbody structure disruption and inhibits abscission, a final stage of cytokinesis. Our study gives novel insight into the role of SHCBP in cytokinesis completion.

The novel function of JADE1S in cytokinesis of epithelial cells

JADE1 belongs to a small family of PHD zinc finger proteins that interacts with histone acetyl transferase (HAT) HBO1 and is associated with chromatin. We recently reported JADE1 chromatin shuttling and phosphorylation during G2/M to G1 transition, which was sensitive to Aurora A inhibition. In the current study we examined mechanisms of the cell cycle regulation by the small isoform of JADE1 protein, JADE1S, and report data showing that JADE1S has a novel function in the regulation of cytokinesis. Using FACS assays, we show that, JADE1S depletion facilitated rates of G1-cells accumulation in synchronously dividing HeLa cell cultures. Depletion of JADE1S protein in asynchronously dividing cells decreased the proportion of cytokinetic cells, and increased the proportion of multi-nuclear cells, indicative of premature and failed cytokinesis. In contrast, moderate overexpression of JADE1S increased the number of cytokinetic cells in time- and dose- dependent manner, indicating cytokinetic delay. Pharmacological inhibition of Aurora B kinase resulted in the release of JADE1S-mediated cytokinetic delay and allowed progression of abscission in cells over-expressing JADE1S. Finally, we show that JADE1S protein localized to centrosomes in interphase and mitotic cells, while during cytokinesis JADE1S localized to the midbody. Neither JADE1L nor partner of JADE1, HAT HBO1 was localized to the centrosomes or midbodies. Our study identifies the novel role for JADE1S in regulation of cytokinesis and suggests function in Aurora B kinase-mediated cytokinesis checkpoint.

Myotubularin-related proteins 3 and 4 interact with polo-like kinase 1 and centrosomal protein of 55 kDa to ensure proper abscission

The myotubularins are a family of phosphatases that dephosphorylate the phosphatidylinositols phosphatidylinositol-3-phosphate and phosphatidylinositol-3,5-phosphate. Several family members are mutated in disease, yet the biological functions of the majority of myotubularins remain unknown. To gain insight into the roles of the individual enzymes, we have used affinity purification coupled to mass spectrometry to identify protein-protein interactions for the myotubularins. The myotubularin interactome comprises 66 high confidence (false discovery rate ≤1%) interactions, including 18 pairwise interactions between individual myotubularins. The results reveal a number of potential signaling contexts for this family of enzymes, including an intriguing, novel role for myotubularin-related protein 3 and myotubularin-related protein 4 in the regulation of abscission, the final step of mitosis in which the membrane bridge remaining between two daughter cells is cleaved. Both depletion and overexpression of either myotubularin-related protein 3 or myotubularin-related protein 4 result in abnormal midbody morphology and cytokinesis failure. Interestingly, myotubularin-related protein 3 and myotubularin-related protein 4 do not exert their effects through lipid regulation at the midbody, but regulate abscission during early mitosis, by interacting with the mitotic kinase polo-like kinase 1, and with centrosomal protein of 55 kDa (CEP55), an important regulator of abscission. Structure-function analysis reveals that, consistent with known intramyotubularin interactions, myotubularin-related protein 3 and myotubularin-related protein 4 interact through their respective coiled coil domains. The interaction between myotubularin-related protein 3 and polo-like kinase 1 relies on the divergent, nonlipid binding Fab1, YOTB, Vac1, and EEA1 domain of myotubularin-related protein 3, and myotubularin-related protein 4 interacts with CEP55 through a short GPPXXXY motif, analogous to endosomal sorting complex required for transport-I components. Disruption of any of these interactions results in abscission failure, by disrupting the proper recruitment of CEP55, and subsequently, of endosomal sorting complex required for transport-I, to the midbody. Our data suggest that myotubularin-related protein 3 and myotubularin-related protein 4 may act as a bridge between CEP55 and polo-like kinase 1, ensuring proper CEP55 phosphorylation and regulating CEP55 recruitment to the midbody. This work provides a novel role for myotubularin-related protein 3/4 heterodimers, and highlights the temporal and spatial complexity of the regulation of cytokinesis.

FLJ25439, a novel cytokinesis-associated protein, induces tetraploidization and maintains chromosomal stability via enhancing expression of endoplasmic reticulum stress chaperones

Investigation of the mechanisms leading to aneuploidy and polyploidy is critical to cancer research. Previous studies have provided strong evidence of the importance of tetraploidization as an early step in tumorigenesis. In cancer cells, tetraploid cells may contribute to abnormal mitotic progression, which may be associated with cytokinesis failure. Tetraploidy leads to genomic instability due to centrosome and chromosome over-replication. Until now, the mechanism by which cells maintain tetraploid status has been unknown. Here, we identified a novel D box-containing protein, FLJ25439, which displays a dynamic expression profile during mitosis/cytokinesis with the midbody as the most prominent associated structure. To understand the function of FLJ25439, we established stable cell lines overexpressing FLJ25439. FLJ25439-overexpression cells grew slower and displayed a tetraploid DNA content in comparison with diploid parental cells. They also showed aberrant mitosis and dysregulated expression of p53, pRb and p21, suggesting a defect in cell cycle progression. To explore the molecular mechanisms responsible for FLJ25439-induced tetraploidization, we conducted a comparative analysis of the global protein expression patterns of wild type and overexpressors using proteomics and bioinformatics approaches. Protein category profiling indicated that FLJ25439 is involved in pathways related to anti-apoptosis, protein folding, the cell cycle, and cytoskeleton regulation. Specifically, genotoxic-stress- and ER stress-related chaperone proteins greatly contributed to the FLJ25439 overexpression phenotypes. The results of this study pave the way to our further understanding of the role of this novel cytokinesis-related protein in protecting cells from environmental stress and tetraploid formation.

WD repeat-containing protein 5 (WDR5) localizes to the midbody and regulates abscission

Cytokinesis partitions the cytoplasm of a parent cell into two daughter cells and is essential for the completion of cell division. The final step of cytokinesis in animal cells is abscission, which is a process leading to the physical separation of two daughter cells. Abscission requires membrane traffic and microtubule disassembly at a specific midbody region called the secondary ingression. Here, we report that WD repeat-containing protein 5 (WDR5), a core subunit of COMPASS/MLL family histone H3 lysine 4 methyltransferase (H3K4MT) complexes, resides at the midbody and associates with a subset of midbody regulatory proteins, including PRC1 and CYK4/MKLP1. Knockdown of WDR5 impairs abscission and increases the incidence of multinucleated cells. Further investigation revealed that the abscission delay is primarily due to slower formation of secondary ingressions in WDR5 knockdown cells. Consistent with these defects, midbody microtubules in WDR5 knockdown cells also display enhanced resistance to depolymerization by nocodazole. Recruitment of WDR5 to the midbody dark zone appears to require integrity of the WDR5 central arginine-binding cavity, as mutations that disrupt histone H3 and MLL1 binding to this pocket also abolish the midbody localization of WDR5. Taken together, these data suggest that WDR5 is specifically targeted to the midbody in the absence of chromatin and that it promotes abscission, perhaps by facilitating midbody microtubule disassembly.

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.

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.

ANCHR mediates Aurora-B-dependent abscission checkpoint control through retention of VPS4

During the final stage of cell division, cytokinesis, the Aurora-B-dependent abscission checkpoint (NoCut) delays membrane abscission to avoid DNA damage and aneuploidy in cells with chromosome segregation defects. This arrest depends on Aurora-B-mediated phosphorylation of CHMP4C, a component of the endosomal sorting complex required for transport (ESCRT) machinery that mediates abscission, but the mechanism remains unknown. Here we describe ANCHR (Abscission/NoCut Checkpoint Regulator; ZFYVE19) as a key regulator of the abscission checkpoint, functioning through the most downstream component of the ESCRT machinery, the ATPase VPS4. In concert with CHMP4C, ANCHR associates with VPS4 at the midbody ring following DNA segregation defects to control abscission timing and prevent multinucleation in an Aurora-B-dependent manner. This association prevents VPS4 relocalization to the abscission zone and is relieved following inactivation of Aurora B to allow abscission. We propose that the abscission checkpoint is mediated by ANCHR and CHMP4C through retention of VPS4 at the midbody ring.

The 3M complex maintains microtubule and genome integrity

CUL7, OBSL1, and CCDC8 genes are mutated in a mutually exclusive manner in 3M and other growth retardation syndromes. The mechanism underlying the function of the three 3M genes in development is not known. We found that OBSL1 and CCDC8 form a complex with CUL7 and regulate the level and centrosomal localization of CUL7, respectively. CUL7 depletion results in altered microtubule dynamics, prometaphase arrest, tetraploidy, and mitotic cell death. These defects are recaptured in CUL7 mutated 3M cells and can be rescued by wild-type, but not by 3M patient-derived CUL7 mutants. Depletion of either OBSL1 or CCDC8 results in defects and sensitizes cells to microtubule damage similarly to loss of CUL7 function. Microtubule damage reduces the level of CCDC8 that is required for the centrosomal localization of CUL7. We propose that CUL7, OBSL1, and CCDC8 proteins form a 3M complex that functions in maintaining microtubule and genome integrity and normal development.