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

ZFYVE19 deficiency: a ciliopathy involving failure of cell division, with cell death

BACKGROUND AND AIMS

Variants in ZFYVE19 underlie a disorder characterised by progressive portal fibrosis, portal hypertension and eventual liver decompensation. We aim to create an animal model to elucidate the pathogenic mechanism.

METHODS

Zfyve19 knockout (Zfyve19-/- ) mice were generated and exposed to different liver toxins. Their livers were characterised at the tissue, cellular and molecular levels. Findings were compared with those in wild-type mice and in ZFYVE19-deficient patients. ZFYVE19 knockout and knockdown retinal pigment epithelial-1 cells and mouse embryonic fibroblasts were generated to study cell division and cell death.

RESULTS

The Zfyve19-/- mice were normal overall, particularly with respect to hepatobiliary features. However, when challenged with α-naphthyl isothiocyanate, Zfyve19-/- mice developed changes resembling those in ZFYVE19-deficient patients, including elevated serum liver injury markers, increased numbers of bile duct profiles with abnormal cholangiocyte polarity and biliary fibrosis. Failure of cell division, centriole and cilia abnormalities, and increased cell death were observed in knockdown/knockout cells. Increased cell death and altered mRNA expression of cell death-related signalling pathways was demonstrated in livers from Zfyve19-/- mice and patients. Transforming growth factor-β (TGF-β) and Janus kinase-Signal Transducer and Activator of Transcription 3 (JAK-STAT3) signalling pathways were upregulated in vivo, as were chemokines such as C-X-C motif ligands 1, 10 and 12.

CONCLUSIONS

Our findings demonstrated that ZFYVE19 deficiency is a ciliopathy with novel histological features. Failure of cell division with ciliary abnormalities and cell death activates macrophages and may thus lead to biliary fibrosis via TGF-β pathway in the disease.

Methylation of ESCRT-III components regulates the timing of cytokinetic abscission

Abscission is the final stage of cytokinesis, which cleaves the intercellular bridge (ICB) connecting two daughter cells. Abscission requires tight control of the recruitment and polymerization of the Endosomal Protein Complex Required for Transport-III (ESCRT-III) components. We explore the role of post-translational modifications in regulating ESCRT dynamics. We discover that SMYD2 methylates the lysine 6 residue of human CHMP2B, a key ESCRT-III component, at the ICB, impacting the dynamic relocation of CHMP2B to sites of abscission. SMYD2 loss-of-function (genetically or pharmacologically) causes CHMP2B hypomethylation, delayed CHMP2B polymerization and delayed abscission. This is phenocopied by CHMP2B lysine 6 mutants that cannot be methylated. Conversely, SMYD2 gain-of-function causes CHMP2B hypermethylation and accelerated abscission, specifically in cells undergoing cytokinetic challenges, thereby bypassing the abscission checkpoint. Additional experiments highlight the importance of CHMP2B methylation beyond cytokinesis, namely during ESCRT-III-mediated HIV-1 budding. We propose that lysine methylation signaling fine-tunes the ESCRT-III machinery to regulate the timing of cytokinetic abscission and other ESCRT-III dependent functions.

Human ESCRT-I and ALIX function as scaffolding helical filaments in vivo

The Endosomal Sorting Complex Required for Transport (ESCRT) is an evolutionarily conserved machinery that performs reverse-topology membrane scission in cells universally required from cytokinesis to budding of enveloped viruses. Upstream acting ESCRT-I and ALIX control these events and link recruitment of viral and cellular partners to late-acting ESCRT-III CHMP4 through incompletely understood mechanisms. Using structure-function analyses combined with super-resolution imaging, we show that ESCRT-I and ALIX function as distinct helical filaments in vivo . Together, they are essential for optimal structural scaffolding of HIV-1 nascent virions, the retention of viral and human genomes through defined functional interfaces, and recruitment of CHMP4 that itself assembles into corkscrew-like filaments intertwined with ESCRT-I or ALIX helices. Disruption of filament assembly or their conformationally clustered RNA binding interfaces in human cells impaired membrane abscission, resulted in major structural instability and leaked nucleic acid from nascent virions and nuclear envelopes. Thus, ESCRT-I and ALIX function as helical filaments in vivo and serve as both nucleic acid-dependent structural scaffolds as well as ESCRT-III assembly templates.

Significance statement

When cellular membranes are dissolved or breached, ESCRT is rapidly deployed to repair membranes to restore the integrity of intracellular compartments. Membrane sealing is ensured by ESCRT-III filaments assembled on the inner face of membrane; a mechanism termed inverse topology membrane scission. This mechanism, initiated by ESCRT-I and ALIX, is universally necessary for cytokinesis, wound repair, budding of enveloped viruses, and more. We show ESCRT-I and ALIX individually oligomerize into helical filaments that cluster newly discovered nucleic acid-binding interfaces and scaffold-in genomes within nascent virions and nuclear envelopes. These oligomers additionally appear to serve as ideal templates for ESCRT-III polymerization, as helical filaments of CHMP4B were found intertwined ESCRT-I or ALIX filaments in vivo . Similarly, corkscrew-like filaments of ALIX are also interwoven with ESCRT-I, supporting a model of inverse topology membrane scission that is synergistically reinforced by inward double filament scaffolding.

Role of Rabenosyn-5 and Rab5b in host cell cytosol uptake reveals conservation of endosomal transport in malaria parasites

Vesicular trafficking, including secretion and endocytosis, plays fundamental roles in the unique biology of Plasmodium falciparum blood-stage parasites. Endocytosis of host cell cytosol (HCC) provides nutrients and room for parasite growth and is critical for the action of antimalarial drugs and parasite drug resistance. Previous work showed that PfVPS45 functions in endosomal transport of HCC to the parasite's food vacuole, raising the possibility that malaria parasites possess a canonical endolysosomal system. However, the seeming absence of VPS45-typical functional interactors such as rabenosyn 5 (Rbsn5) and the repurposing of Rab5 isoforms and other endolysosomal proteins for secretion in apicomplexans question this idea. Here, we identified a parasite Rbsn5-like protein and show that it functions with VPS45 in the endosomal transport of HCC. We also show that PfRab5b but not PfRab5a is involved in the same process. Inactivation of PfRbsn5L resulted in PI3P and PfRab5b decorated HCC-filled vesicles, typical for endosomal compartments. Overall, this indicates that despite the low sequence conservation of PfRbsn5L and the unusual N-terminal modification of PfRab5b, principles of endosomal transport in malaria parasite are similar to that of model organisms. Using a conditional double protein inactivation system, we further provide evidence that the PfKelch13 compartment, an unusual apicomplexa-specific endocytosis structure at the parasite plasma membrane, is connected upstream of the Rbsn5L/VPS45/Rab5b-dependent endosomal route. Altogether, this work indicates that HCC uptake consists of a highly parasite-specific part that feeds endocytosed material into an endosomal system containing more canonical elements, leading to the delivery of HCC to the food vacuole.

The human AAA-ATPase VPS4A isoform and its co-factor VTA1 have a unique function in regulating mammalian cytokinesis abscission

Mutations in the human AAA-ATPase VPS4 isoform, VPS4A, cause severe neurodevelopmental defects and congenital dyserythropoietic anemia (CDA). VPS4 is a crucial component of the endosomal sorting complex required for transport (ESCRT) system, which drives membrane remodeling in numerous cellular processes, including receptor degradation, cell division, and neural pruning. Notably, while most organisms encode for a single VPS4 gene, human cells have 2 VPS4 paralogs, namely VPS4A and VPS4B, but the functional differences between these paralogs is mostly unknown. Here, we set out to investigate the role of the human VPS4 paralogs in cytokinetic abscission using a series of knockout cell lines. We found that VPS4A and VPS4B hold both overlapping and distinct roles in abscission. VPS4A depletion resulted in a more severe abscission delay than VPS4B and was found to be involved in earlier stages of abscission. Moreover, VPS4A and a monomeric-locked VPS4A mutant bound the abscission checkpoint proteins CHMP4C and ANCHR, while VPS4B did not, indicating a regulatory role for the VPS4A isoform in abscission. Depletion of VTA1, a co-factor of VPS4, disrupted VPS4A-ANCHR interactions and accelerated abscission, suggesting that VTA1 is also involved in the abscission regulation. Our findings reveal a dual role for VPS4A in abscission, one that is canonical and can be compensated by VPS4B, and another that is regulatory and may be delivered by its monomeric form. These observations provide a potential mechanistic explanation for the neurodevelopmental defects and other related disorders reported in VPS4A-mutated patients with a fully functional VPS4B paralog.

Cell cycle responses to Topoisomerase II inhibition: Molecular mechanisms and clinical implications

DNA Topoisomerase IIA (Topo IIA) is an enzyme that alters the topological state of DNA and is essential for the separation of replicated sister chromatids and the integrity of cell division. Topo IIA dysfunction activates cell cycle checkpoints, resulting in arrest in either the G2-phase or metaphase of mitosis, ultimately triggering the abscission checkpoint if non-disjunction persists. These events, which directly or indirectly monitor the activity of Topo IIA, have become of major interest as many cancers have deficiencies in Topoisomerase checkpoints, leading to genome instability. Recent studies into how cells sense Topo IIA dysfunction and respond by regulating cell cycle progression demonstrate that the Topo IIA G2 checkpoint is distinct from the G2-DNA damage checkpoint. Likewise, in mitosis, the metaphase Topo IIA checkpoint is separate from the spindle assembly checkpoint. Here, we integrate mechanistic knowledge of Topo IIA checkpoints with the current understanding of how cells regulate progression through the cell cycle to accomplish faithful genome transmission and discuss the opportunities this offers for therapy.

The abscission checkpoint senses chromatin bridges through Top2α recruitment to DNA knots

In response to chromatin bridges, the abscission checkpoint delays completion of cytokinesis to prevent chromosome breakage or tetraploidization. Here, we show that spontaneous or replication stress-induced chromatin bridges exhibit "knots" of catenated and overtwisted DNA next to the midbody. Topoisomerase IIα (Top2α) forms abortive Top2-DNA cleavage complexes (Top2ccs) on DNA knots; furthermore, impaired Top2α-DNA cleavage activity correlates with chromatin bridge breakage in cytokinesis. Proteasomal degradation of Top2ccs is required for Rad17 localization to Top2-generated double-strand DNA ends on DNA knots; in turn, Rad17 promotes local recruitment of the MRN complex and downstream ATM-Chk2-INCENP signaling to delay abscission and prevent chromatin breakage. In contrast, dicentric chromosomes that do not exhibit knotted DNA fail to activate the abscission checkpoint in human cells. These findings are the first to describe a mechanism by which the abscission checkpoint detects chromatin bridges, through generation of abortive Top2ccs on DNA knots, to preserve genome integrity.

Polymorphic variants involved in methylation regulation: a strategy to discover risk loci for pancreatic ductal adenocarcinoma

INTRODUCTION

Only a small number of risk factors for pancreatic ductal adenocarcinoma (PDAC) has been established. Several studies identified a role of epigenetics and of deregulation of DNA methylation. DNA methylation is variable across a lifetime and in different tissues; nevertheless, its levels can be regulated by genetic variants like methylation quantitative trait loci (mQTLs), which can be used as a surrogate.

MATERIALS AND METHODS

We scanned the whole genome for mQTLs and performed an association study in 14 705 PDAC cases and 246 921 controls. The methylation data were obtained from whole blood and pancreatic cancer tissue through online databases. We used the Pancreatic Cancer Cohort Consortium and the Pancreatic Cancer Case-Control Consortium genome-wide association study (GWAS) data as discovery phase and the Pancreatic Disease Research consortium, the FinnGen project and the Japan Pancreatic Cancer Research consortium GWAS as replication phase.

RESULTS

The C allele of 15q26.1-rs12905855 showed an association with a decreased risk of PDAC (OR=0.90, 95% CI 0.87 to 0.94, p=4.93×10-8 in the overall meta-analysis), reaching genome-level statistical significance. 15q26.1-rs12905855 decreases the methylation of a 'C-phosphate-G' (CpG) site located in the promoter region of the RCCD1 antisense (RCCD1-AS1) gene which, when expressed, decreases the expression of the RCC1 domain-containing (RCCD1) gene (part of a histone demethylase complex). Thus, it is possible that the rs12905855 C-allele has a protective role in PDAC development through an increase of RCCD1 gene expression, made possible by the inactivity of RCCD1-AS1.

CONCLUSION

We identified a novel PDAC risk locus which modulates cancer risk by controlling gene expression through DNA methylation.

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.

Genetic alterations and molecular mechanisms underlying hereditary intrahepatic cholestasis

Hereditary cholestatic liver disease caused by a class of autosomal gene mutations results in jaundice, which involves the abnormality of the synthesis, secretion, and other disorders of bile acids metabolism. Due to the existence of a variety of gene mutations, the clinical manifestations of children are also diverse. There is no unified standard for diagnosis and single detection method, which seriously hinders the development of clinical treatment. Therefore, the mutated genes of hereditary intrahepatic cholestasis were systematically described in this review.

Case Report: Add-on treatment with odevixibat in a new subtype of progressive familial intrahepatic cholestasis broadens the therapeutic horizon of genetic cholestasis

Odevixibat, an ileal bile acid transporter (IBAT) inhibitor, is effective for the treatment of pruritus in children diagnosed with progressive familial intrahepatic cholestasis (PFIC) type 1 and 2. There are no studies showing the efficacy of Odevixibat in children with different subtypes of PFIC. We describe the case of a 6-year-old girl with chronic cholestatic jaundice. In the last 12 months laboratory data showed high serum levels of bilirubin (total bilirubin x 2.5 ULN; direct bilirubin x 1.7 ULN) and bile acids (sBA x 70 ULN), elevated transaminases (x 3-4 ULN), and preserved synthetic liver function. Genetic testing showed homozygous mutation in ZFYVE19 gene, which is not included among the classic causative genes of PFIC and determined a new non-syndromic phenotype recently classified as PFIC9 (OMIM # 619849). Due to the persistent intensity of itching [score of 5 (very severe) at the Caregiver Global Impression of Severity (CaGIS)] and sleep disturbances not responsive to rifampicin and ursodeoxycholic acid (UDCA), Odevixibat treatment was started. After treatment with odevixibat we observed: (i) reduction in sBA from 458 to 71 μmol/L (absolute change from baseline: -387 μmol/L), (ii) reduction in CaGIS from 5 to 1, and (iii) resolution of sleep disturbances. The BMI z-score progressively increased from -0.98 to +0.56 after 3 months of treatment. No adverse drug events were recorded. Treatment with IBAT inhibitor was effective and safe in our patient suggesting that Odevixibat may be potentially considered for the treatment of cholestatic pruritus also in children with rare subtypes of PFIC. Further studies on a larger scale could lead to the increasing of patients eligible for this treatment.

Lipid Polarization during Cytokinesis

The plasma membrane of eukaryotic cells is composed of a large number of lipid species that are laterally segregated into functional domains as well as asymmetrically distributed between the outer and inner leaflets. Additionally, the spatial distribution and organization of these lipids dramatically change in response to various cellular states, such as cell division, differentiation, and apoptosis. Division of one cell into two daughter cells is one of the most fundamental requirements for the sustenance of growth in all living organisms. The successful completion of cytokinesis, the final stage of cell division, is critically dependent on the spatial distribution and organization of specific lipids. In this review, we discuss the properties of various lipid species associated with cytokinesis and the mechanisms involved in their polarization, including forward trafficking, endocytic recycling, local synthesis, and cortical flow models. The differences in lipid species requirements and distribution in mitotic vs. male meiotic cells will be discussed. We will concentrate on sphingolipids and phosphatidylinositols because their transbilayer organization and movement may be linked via the cytoskeleton and thus critically regulate various steps of cytokinesis.

Piperine Increases Pentagamavunon-1 Anti-cancer Activity on 4T1 Breast Cancer Through Mitotic Catastrophe Mechanism and Senescence with Sharing Targeting on Mitotic Regulatory Proteins

Pentagamavunon-1 performs more potent anti-cancer effects than curcumin against various cancer cells, but it remains to be optimized. Piperine shows the activity as an enhancer of a therapeutic agent. This study expects to achieve higher effectiveness of PGV-1 on 4T1 breast cancer cells through co-treatment with piperine with exploring the effect of cytotoxicity, mitotic catastrophe, cellular senescence, and target proteins of PGV-1 and piperine on the regulation of mitosis in TNBC cells (4T1). The assays emphasize MTT assay, May Grünwald-Giemsa staining, SA-β-galactosidase assay, and bioinformatics analysis, respectively, to elicit the respected activities. The results revealed that PGV-1 performed a cytotoxic effect with an IC50 value of 9 µM while piperine showed a lower cytotoxic effect with an IC50 value of 800 µM on 4T1 cells 24 h treatment. However, the combination treatment of both showed a synergistic cytotoxic enhancement effect with an average CI value < 1. Furthermore, the combination of PGV-1 and piperine induced mitotic catastrophe and senescence better than the single treatment. Treatment of 1 µM of PGV-1 and 400 µM of piperine increased the percentage of senescent cells by 33%. Bioinformatics analysis revealed that PGV-1 and piperine target proteins play a role in mitotic regulation, namely CDK1, KIF11, AURKA, AURKB, and PLK1, to contribute to mitotic catastrophe. Therefore, piperine increases the effectiveness of PGV-1 to suppress 4T1 cells growth synergistically that may occur through mitotic catastrophe and senescence targeting on mitotic regulatory proteins.

Mechanics and regulation of cytokinetic abscission

Cytokinetic abscission leads to the physical cut of the intercellular bridge (ICB) connecting the daughter cells and concludes cell division. In different animal cells, it is well established that the ESCRT-III machinery is responsible for the constriction and scission of the ICB. Here, we review the mechanical context of abscission. We first summarize the evidence that the ICB is initially under high tension and explain why, paradoxically, this can inhibit abscission in epithelial cells by impacting on ESCRT-III assembly. We next detail the different mechanisms that have been recently identified to release ICB tension and trigger abscission. Finally, we discuss whether traction-induced mechanical cell rupture could represent an ancient alternative mechanism of abscission and suggest future research avenues to further understand the role of mechanics in regulating abscission.

Minor Kinases with Major Roles in Cytokinesis Regulation

Cytokinesis, the conclusive act of cell division, allows cytoplasmic organelles and chromosomes to be faithfully partitioned between two daughter cells. In animal organisms, its accurate regulation is a fundamental task for normal development and for preventing aneuploidy. Cytokinesis failures produce genetically unstable tetraploid cells and ultimately result in chromosome instability, a hallmark of cancer cells. In animal cells, the assembly and constriction of an actomyosin ring drive cleavage furrow ingression, resulting in the formation of a cytoplasmic intercellular bridge, which is severed during abscission, the final event of cytokinesis. Kinase-mediated phosphorylation is a crucial process to orchestrate the spatio-temporal regulation of the different stages of cytokinesis. Several kinases have been described in the literature, such as cyclin-dependent kinase, polo-like kinase 1, and Aurora B, regulating both furrow ingression and/or abscission. However, others exist, with well-established roles in cell-cycle progression but whose specific role in cytokinesis has been poorly investigated, leading to considering these kinases as "minor" actors in this process. Yet, they deserve additional attention, as they might disclose unexpected routes of cell division regulation. Here, we summarize the role of multifunctional kinases in cytokinesis with a special focus on those with a still scarcely defined function during cell cleavage. Moreover, we discuss their implication in cancer.

Identification of pyroptosis-related genes and long non-coding RNAs signatures in osteosarcoma

Osteosarcoma is a highly malignant tumor, with very high disability and fatality rates. However, the overall prognosis is not optimistic. Pyroptosis is a newly discovered cell death modality accompanied by inflammation, which is closely related to varieties of cancers. In this study, the RNA-seq data were downloaded from public databases, the differences in the expression of the pyroptosis-related genes (PRGs) were identified, and the six PRGs signature was established through the univariate and LASSO Cox analysis. The patients were grouped according to the PRGs signature, and the prognosis between the two groups was further compared. In addition, a ten pyroptosis-related lncRNAs (PRLs) prognostic signature was also constructed. Through functional analysis of the differentially expressed genes (DEGs), the immune-related pathways were found to be enriched. The Pearson correlation analysis showed a strong correlation between the pyroptosis-related biomarkers. Finally, we identified a promising biomarker, CHMP4C, which is highly expressed in osteosarcoma. Overexpression of CHMP4C promoted the proliferation, migration and invasion of the osteosarcoma cell. Our results thus provide new evidence for exploring prognostic biomarkers and therapeutic targets of osteosarcoma.

Comprehensive analysis of the human ESCRT-III-MIT domain interactome reveals new cofactors for cytokinetic abscission

The 12 related human ESCRT-III proteins form filaments that constrict membranes and mediate fission, including during cytokinetic abscission. The C-terminal tails of polymerized ESCRT-III subunits also bind proteins that contain Microtubule-Interacting and Trafficking (MIT) domains. MIT domains can interact with ESCRT-III tails in many different ways to create a complex binding code that is used to recruit essential cofactors to sites of ESCRT activity. Here, we have comprehensively and quantitatively mapped the interactions between all known ESCRT-III tails and 19 recombinant human MIT domains. We measured 228 pairwise interactions, quantified 60 positive interactions, and discovered 18 previously unreported interactions. We also report the crystal structure of the SPASTIN MIT domain in complex with the IST1 C-terminal tail. Three MIT enzymes were studied in detail and shown to: (1) localize to cytokinetic midbody membrane bridges through interactions with their specific ESCRT-III binding partners (SPASTIN-IST1, KATNA1-CHMP3, and CAPN7-IST1), (2) function in abscission (SPASTIN, KATNA1, and CAPN7), and (3) function in the 'NoCut' abscission checkpoint (SPASTIN and CAPN7). Our studies define the human MIT-ESCRT-III interactome, identify new factors and activities required for cytokinetic abscission and its regulation, and provide a platform for analyzing ESCRT-III and MIT cofactor interactions in all ESCRT-mediated processes.

A function for ataxia telangiectasia and Rad3-related (ATR) kinase in cytokinetic abscission

Abscission, the final stage of cytokinesis, occurs when the cytoplasmic canal connecting two emerging daughter cells is severed either side of a large proteinaceous structure, the midbody. Here, we expand the functions of ATR to include a cell-cycle-specific role in abscission, which is required for genome stability. All previously characterized roles for ATR depend upon its recruitment to replication protein A (RPA)-coated single-stranded DNA (ssDNA). However, we establish that in each cell cycle ATR, as well as ATRIP, localize to the midbody specifically during late cytokinesis and independently of RPA or detectable ssDNA. Rather, midbody localization and ATR-dependent regulation of abscission requires the known abscission regulator-charged multivesicular body protein 4C (CHMP4C). Intriguingly, this regulation is also dependent upon the CDC7 kinase and the known ATR activator ETAA1. We propose that in addition to its known RPA-ssDNA-dependent functions, ATR has further functions in preventing premature abscission.

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ATR localises non-canonically to the midbody during late cytokinesis

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Absence of ATR function results in faster abscission and increased binucleates

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CDC7 kinase and the ESCRT protein, CHMP4C are required for ATR midbody localisation

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ATR functions upstream of known abscission regulators, CHMP4B and ANCHR

Caveolae promote successful abscission by controlling intercellular bridge tension during cytokinesis

During cytokinesis, the intercellular bridge (ICB) connecting the daughter cells experiences pulling forces, which delay abscission by preventing the assembly of the ESCRT scission machinery. Abscission is thus triggered by tension release, but how ICB tension is controlled is unknown. Here, we report that caveolae, which are known to regulate membrane tension upon mechanical stress in interphase cells, are located at the midbody, at the abscission site, and at the ICB/cell interface in dividing cells. Functionally, the loss of caveolae delays ESCRT-III recruitment during cytokinesis and impairs abscission. This is the consequence of a twofold increase of ICB tension measured by laser ablation, associated with a local increase in myosin II activity at the ICB/cell interface. We thus propose that caveolae buffer membrane tension and limit contractibility at the ICB to promote ESCRT-III assembly and cytokinetic abscission. Together, this work reveals an unexpected connection between caveolae and the ESCRT machinery and the first role of caveolae in cell division.

Selective Recognition of Phosphatidylinositol Phosphate Receptors by C-Terminal Tail of Mitotic Kinesin-like Protein 2 (MKlp2)

The mitotic kinesin-like protein 2 (MKlp2) plays a key role in the proper completion of cytokinetic abscission. Specifically, the C-terminal tail of MKlp2 (CTM peptides) offers a stable tethering on the plasma membrane and microtubule cytoskeleton in the midbody during abscission. However, little is known about the underlying mechanism of how the CTM peptides bind to the plasma membrane of the intercellular bridge. Herein, we identify the specific molecular interaction between the CTM peptides and phosphatidylinositol phosphate (PIP) receptors using quartz crystal microbalance-dissipation and atomic force microscopy force spectroscopic measurements. To systematically examine the effects of amino acids, we designed a series of synthetic 33-mer peptides derived from the wild-type (CTM1). First, we evaluated the peptide binding amount caused by electrostatic interactions based on 100% zwitterionic and 30% negatively charged model membranes, whereby the nonspecific attractions were nearly proportional to the net charge of peptides. Upon incubating with PIP-containing model membranes, the wild-type CTM1 and its truncated mutation showed significant PI(3)P-specific binding, which was evidenced by a 15-fold higher binding mass and 6-fold stronger adhesion force compared to other negatively charged membranes. The extent of the specific binding was predominantly dependent on the existence of S21, whereby substitution or deletion of S21 significantly hindered the binding affinity. Taken together, our findings based on a correlative measurement platform enabled the quantification of the nonelectrostatic, selective binding interactions of the C-terminal of MKlp2 to certain PIP receptors and contributed to understanding the molecular mechanisms on complete cytokinetic abscission in cells.

PI(3,4)P2-mediated cytokinetic abscission prevents early senescence and cataract formation

Cytokinetic membrane abscission is a spatially and temporally regulated process that requires ESCRT (endosomal sorting complexes required for transport)–dependent control of membrane remodeling at the midbody, a subcellular organelle that defines the cleavage site. Alteration of ESCRT function can lead to cataract, but the underlying mechanism and its relation to cytokinesis are unclear. We found a lens-specific cytokinetic process that required PI3K-C2α (phosphatidylinositol-4-phosphate 3-kinase catalytic subunit type 2α), its lipid product PI(3,4)P2 (phosphatidylinositol 3,4-bisphosphate), and the PI(3,4)P2–binding ESCRT-II subunit VPS36 (vacuolar protein-sorting-associated protein 36). Loss of each of these components led to impaired cytokinesis, triggering premature senescence in the lens of fish, mice, and humans. Thus, an evolutionarily conserved pathway underlies the cell type–specific control of cytokinesis that helps to prevent early onset cataract by protecting from senescence.

The Abscission Checkpoint: A Guardian of Chromosomal Stability

The abscission checkpoint contributes to the fidelity of chromosome segregation by delaying completion of cytokinesis (abscission) when there is chromatin lagging in the intercellular bridge between dividing cells. Although additional triggers of an abscission checkpoint-delay have been described, including nuclear pore defects, replication stress or high intercellular bridge tension, this review will focus only on chromatin bridges. In the presence of such abnormal chromosomal tethers in mammalian cells, the abscission checkpoint requires proper localization and optimal kinase activity of the Chromosomal Passenger Complex (CPC)-catalytic subunit Aurora B at the midbody and culminates in the inhibition of Endosomal Sorting Complex Required for Transport-III (ESCRT-III) components at the abscission site to delay the final cut. Furthermore, cells with an active checkpoint stabilize the narrow cytoplasmic canal that connects the two daughter cells until the chromatin bridges are resolved. Unsuccessful resolution of chromatin bridges in checkpoint-deficient cells or in cells with unstable intercellular canals can lead to chromatin bridge breakage or tetraploidization by regression of the cleavage furrow. In turn, these outcomes can lead to accumulation of DNA damage, chromothripsis, generation of hypermutation clusters and chromosomal instability, which are associated with cancer formation or progression. Recently, many important questions regarding the mechanisms of the abscission checkpoint have been investigated, such as how the presence of chromatin bridges is signaled to the CPC, how Aurora B localization and kinase activity is regulated in late midbodies, the signaling pathways by which Aurora B implements the abscission delay, and how the actin cytoskeleton is remodeled to stabilize intercellular canals with DNA bridges. Here, we review recent progress toward understanding the mechanisms of the abscission checkpoint and its role in guarding genome integrity at the chromosome level, and consider its potential implications for cancer therapy.

An ATM-Chk2-INCENP pathway activates the abscission checkpoint

During cell division, in response to chromatin bridges, the chromosomal passenger complex (CPC) delays abscission to prevent chromosome breakage or tetraploidization. Here, we show that inhibition of ATM or Chk2 kinases impairs CPC localization to the midbody center, accelerates midbody resolution in normally segregating cells, and correlates with premature abscission and chromatin breakage in cytokinesis with trapped chromatin. In cultured human cells, ATM activates Chk2 at late midbodies. In turn, Chk2 phosphorylates human INCENP-Ser91 to promote INCENP binding to Mklp2 kinesin and CPC localization to the midbody center through Mklp2 association with Cep55. Expression of truncated Mklp2 that does not bind to Cep55 or nonphosphorylatable INCENP-Ser91A impairs CPC midbody localization and accelerates abscission. In contrast, expression of phosphomimetic INCENP-Ser91D or a chimeric INCENP protein that is targeted to the midbody center rescues the abscission delay in Chk2-deficient or ATM-deficient cells. Furthermore, the Mre11–Rad50–Nbs1 complex is required for ATM activation at the midbody in cytokinesis with chromatin bridges. These results identify an ATM–Chk2–INCENP pathway that imposes the abscission checkpoint by regulating CPC midbody localization.

Identification of abscission checkpoint bodies as structures that regulate ESCRT factors to control abscission timing

The abscission checkpoint regulates the ESCRT membrane fission machinery and thereby delays cytokinetic abscission to protect genomic integrity in response to residual mitotic errors. The checkpoint is maintained by Aurora B kinase, which phosphorylates multiple targets, including CHMP4C, a regulatory ESCRT-III subunit necessary for this checkpoint. We now report the discovery that cytoplasmic abscission checkpoint bodies (ACBs) containing phospho-Aurora B and tri-phospho-CHMP4C develop during an active checkpoint. ACBs are derived from mitotic interchromatin granules, transient mitotic structures whose components are housed in splicing-related nuclear speckles during interphase. ACB formation requires CHMP4C, and the ESCRT factor ALIX also contributes. ACB formation is conserved across cell types and under multiple circumstances that activate the checkpoint. Finally, ACBs retain a population of ALIX, and their presence correlates with delayed abscission and delayed recruitment of ALIX to the midbody where it would normally promote abscission. Thus, a cytoplasmic mechanism helps regulate midbody machinery to delay abscission.

When a cell divides, it must first carefully duplicate its genetic information and package these copies into compartments housed in the two new cells. Errors in this process lead to genetic mistakes that trigger cancer or other harmful biological events.

Quality control checks exist to catch errors before it is too late. This includes a final ‘abscission’ checkpoint right before the end of division, when the two new cells are still connected by a thin membrane bridge. If cells fail to pass this ‘no cut’ checkpoint, they delay severing their connection until the mistake is fixed.

A group of proteins called ESCRTs is responsible for splitting the two cells apart if nothing is amiss. The abscission checkpoint blocks this process by altering certain proteins in the ESCRT complex, but exactly how this works is not yet clear.

To find out more, Strohacker et al. imaged ESCRT factors in a new experimental system in which the abscission checkpoint is active in many cells. This showed that, in this context, certain ESCRT components were rerouted from the thread of membrane between the daughter cells to previously unknown structures, which Strohacker et al. named abscission checkpoint bodies. These entities also sequestered other factors that participate in the abscission checkpoint and factors that contribute to gene expression.

These results are key to better understand how cells regulate their division; in particular, they provide a new framework to explore when this process goes wrong and contributes to cancer.

Co-ordinated control of the Aurora B abscission checkpoint by PKCε complex assembly, midbody recruitment and retention

A requirement for PKCε in exiting from the Aurora B dependent abscission checkpoint is associated with events at the midbody, however, the recruitment, retention and action of PKCε in this compartment are poorly understood. Here, the prerequisite for 14-3-3 complex assembly in this pathway is directly linked to the phosphorylation of Aurora B S227 at the midbody. However, while essential for PKCε control of Aurora B, 14-3-3 association is shown to be unnecessary for the activity-dependent enrichment of PKCε at the midbody. This localisation is demonstrated to be an autonomous property of the inactive PKCε D532N mutant, consistent with activity-dependent dissociation. The C1A and C1B domains are necessary for this localisation, while the C2 domain and inter-C1 domain (IC1D) are necessary for retention at the midbody. Furthermore, it is shown that while the IC1D mutant retains 14-3-3 complex proficiency, it does not support Aurora B phosphorylation, nor rescues division failure observed with knockdown of endogenous PKCε. It is concluded that the concerted action of multiple independent events facilitates PKCε phosphorylation of Aurora B at the midbody to control exit from the abscission checkpoint.

The Last Chance Saloon

Accurate chromosome segregation requires the removal of all chromatin bridges, which link chromosomes before cell division. When chromatin bridges fail to be removed, cell cycle progression may halt, or cytokinesis failure and ensuing polyploidization may occur. Conversely, the inappropriate severing of chromatin bridges leads to chromosome fragmentation, excessive genome instability at breakpoints, micronucleus formation, and chromothripsis. In this mini-review, we first describe the origins of chromatin bridges, the toxic processing of chromatin bridges by mechanical force, and the TREX1 exonuclease. We then focus on the abscission checkpoint (NoCut) which can confer a transient delay in cytokinesis progression to facilitate bridge resolution. Finally, we describe a recently identified mechanism uncovered in C. elegans where the conserved midbody associated endonuclease LEM-3/ANKLE1 is able to resolve chromatin bridges generated by various perturbations of DNA metabolism at the final stage of cell division. We also discuss how LEM-3 dependent chromatin bridge resolution may be coordinated with abscission checkpoint (NoCut) to achieve an error-free cleavage, therefore acting as a "last chance saloon" to facilitate genome integrity and organismal survival.

Mitotic syndicates Aurora Kinase B (AURKB) and mitotic arrest deficient 2 like 2 (MAD2L2) in cohorts of DNA damage response (DDR) and tumorigenesis

Aurora Kinase B (AURKB) and Mitotic Arrest Deficient 2 Like 2 (MAD2L2) are emerging anticancer therapeutic targets. AURKB and MAD2L2 are the least well studied members of their protein families, compared to AURKA and MAD2L1. Both AURKB and MAD2L2 play a critical role in mitosis, cell cycle checkpoint, DNA damage response (DDR) and normal physiological processes. However, the oncogenic roles of AURKB and MAD2L2 in tumorigenesis and genomic instability have also been reported. DDR acts as an arbitrator for cell fate by either repairing the damage or directing the cell to self-destruction. While there is strong evidence of interphase DDR, evidence of mitotic DDR is just emerging and remains largely unelucidated. To date, inhibitors of the DDR components show effective anti-cancer roles. Contrarily, long-term resistance towards drugs that target only one DDR target is becoming a challenge. Targeting interactions between protein-protein or protein-DNA holds prominent therapeutic potential. Both AURKB and MAD2L2 play critical roles in the success of mitosis and their emerging roles in mitotic DDR cannot be ignored. Small molecule inhibitors for AURKB are in clinical trials. A few lead compounds towards MAD2L2 inhibition have been discovered. Targeting mitotic DDR components and their interaction is emerging as a potent next generation anti-cancer therapeutic target. This can be done by developing small molecule inhibitors for AURKB and MAD2L2, thereby targeting DDR components as anti-cancer therapeutic targets and/or targeting mitotic DDR. This review focuses on AURKB and MAD2L2 prospective synergy to deregulate the p53 DDR pathway and promote favourable conditions for uncontrolled cell proliferation.

Why Cells and Viruses Cannot Survive without an ESCRT

Intracellular organelles enwrapped in membranes along with a complex network of vesicles trafficking in, out and inside the cellular environment are one of the main features of eukaryotic cells. Given their central role in cell life, compartmentalization and mechanisms allowing their maintenance despite continuous crosstalk among different organelles have been deeply investigated over the past years. Here, we review the multiple functions exerted by the endosomal sorting complex required for transport (ESCRT) machinery in driving membrane remodeling and fission, as well as in repairing physiological and pathological membrane damages. In this way, ESCRT machinery enables different fundamental cellular processes, such as cell cytokinesis, biogenesis of organelles and vesicles, maintenance of nuclear–cytoplasmic compartmentalization, endolysosomal activity. Furthermore, we discuss some examples of how viruses, as obligate intracellular parasites, have evolved to hijack the ESCRT machinery or part of it to execute/optimize their replication cycle/infection. A special emphasis is given to the herpes simplex virus type 1 (HSV-1) interaction with the ESCRT proteins, considering the peculiarities of this interplay and the need for HSV-1 to cross both the nuclear-cytoplasmic and the cytoplasmic-extracellular environment compartmentalization to egress from infected cells.

Synthetic Lethal Interaction between the ESCRT Paralog Enzymes VPS4A and VPS4B in Cancers Harboring Loss of Chromosome 18q or 16q

Few therapies target the loss of tumor suppressor genes in cancer. We examine CRISPR-SpCas9 and RNA-interference loss-of-function screens to identify new therapeutic targets associated with genomic loss of tumor suppressor genes. The endosomal sorting complexes required for transport (ESCRT) ATPases VPS4A and VPS4B score as strong synthetic lethal dependencies. VPS4A is essential in cancers harboring loss of VPS4B adjacent to SMAD4 on chromosome 18q and VPS4B is required in tumors with co-deletion of VPS4A and CDH1 (E-cadherin) on chromosome 16q. We demonstrate that more than 30% of cancers selectively require VPS4A or VPS4B. VPS4A suppression in VPS4B-deficient cells selectively leads to ESCRT-III filament accumulation, cytokinesis defects, nuclear deformation, G2/M arrest, apoptosis, and potent tumor regression. CRISPR-SpCas9 screening and integrative genomic analysis reveal other ESCRT members, regulators of abscission, and interferon signaling as modifiers of VPS4A dependency. We describe a compendium of synthetic lethal vulnerabilities and nominate VPS4A and VPS4B as high-priority therapeutic targets for cancers with 18q or 16q loss.

Neggers, Paolella, and colleagues identify the ATPases VPS4A and VPS4B as selective vulnerabilities and potential therapeutic targets in cancers harboring loss of chromosome 18q or 16q. In VPS4B-deficient cancers, VPS4A suppression leads to ESCRT-III dysfunction, nuclear deformation, and abscission defects. Moreover, ESCRT proteins and interferons can modulate dependency on VPS4A.

A Structural View on ESCRT-Mediated Abscission

The endosomal sorting complex required for transport (ESCRT) mediates cellular processes that are related to membrane remodeling, such as multivesicular body (MVB) formation, viral budding and cytokinesis. Abscission is the final stage of cytokinesis that results in the physical separation of the newly formed two daughter cells. Although abscission has been investigated for decades, there are still fundamental open questions related to the spatio-temporal organization of the molecular machinery involved in this process. Reviewing knowledge obtained from in vitro as well as in vivo experiments, we give a brief overview on the role of ESCRT components in abscission mainly focussing on mammalian cells.

A cancer-associated, genome protective programme engaging PKCε

Associated with their roles as targets for tumour promoters, there has been a long-standing interest in how members of the protein kinase C (PKC) family act to modulate cell growth and division. This has generated a great deal of observational data, but has for the most part not afforded clear mechanistic insights into the control mechanisms at play. Here, we review the roles of PKCε in protecting transformed cells from non-disjunction. In this particular cell cycle context, there is a growing understanding of the pathways involved, affording biomarker and interventional insights and opportunities.

Biallelic loss-of-function ZFYVE19 mutations are associated with congenital hepatic fibrosis, sclerosing cholangiopathy and high-GGT cholestasis

BACKGROUND

For many children with intrahepatic cholestasis and high-serum gamma-glutamyl transferase (GGT) activity, a genetic aetiology of hepatobiliary disease remains undefined. We sought to identify novel genes mutated in children with idiopathic high-GGT intrahepatic cholestasis, with clinical, histopathological and functional correlations.

METHODS

We assembled a cohort of 25 children with undiagnosed high-GGT cholestasis and without clinical features of biliary-tract infection or radiological features of choledochal malformation, sclerosing cholangitis or cholelithiasis. Mutations were identified through whole-exome sequencing and targeted Sanger sequencing. We reviewed histopathological findings and assessed phenotypical effects of ZFYVE19 deficiency in cultured cells by immunofluorescence microscopy.

RESULTS

Nine Han Chinese children harboured biallelic, predictedly complete loss-of-function pathogenic mutations in ZFYVE19 (c.314C>G, p.S105X; c.379C>T, p.Q127X; c.514C>T, p.R172X; c.547C>T, p.R183X; c.226A>G, p.M76V). All had portal hypertension and, at liver biopsy, histopathological features of the ductal plate malformation (DPM)/congenital hepatic fibrosis (CHF). Four children required liver transplantation for recurrent gastrointestinal haemorrhage. DPM/CHF was confirmed at hepatectomy, with sclerosing small-duct cholangitis. Immunostaining for two primary-cilium axonemal proteins found expression that was deficient intraluminally and ectopic within cholangiocyte cytoplasm. ZFYVE19 depletion in cultured cells yielded abnormalities of centriole and axoneme.

CONCLUSION

Biallelic ZFYVE19 mutations can lead to high-GGT cholestasis and DPM/CHF in vivo. In vitro, they can lead to centriolar and axonemal abnormalities. These observations indicate that mutation in ZFYVE19 results, through as yet undefined mechanisms, in a ciliopathy.

The Correct Localization of Borealin in Midbody during Cytokinesis Depends on IQGAP1

Borealin is a key component of chromosomal passenger complex, which is vital in cytokinesis. IQ domain-containing GTPase-activating protein 1 (IQGAP1) also participates in cytokinesis. The correlation between Borealin and IQGAP1 during cytokinesis is not yet clear. Here, we used mass spectrometry and endogenous coimmunoprecipitation experiments to investigate the interaction between IQGAP1 and Borealin. Results of the current study showed that Borealin interacted directly with IQGAP1 both in vitro and in vivo. Knockdown of IQGAP1 resulted in an abnormal location of Borealin in the midbody. Knocking down Borealin alone, IQGAP1 alone, or Borealin and IQGAP1 at the same time inhibited the completion of cytokinesis and formed multinucleated cells. Our results indicated that IQGAP1 interacts with Borealin during cytokinesis, and the correct localization of Borealin in the midbody during cytokinesis is determined by IQGAP1, and IQGAP1 may play an important role in regulating Borealin function in cytokinesis.

Understanding the birth of rupture-prone and irreparable micronuclei

Micronuclei are extra-nuclear bodies mainly derived from ana-telophase lagging chromosomes/chromatins (LCs) that are not incorporated into primary nuclei at mitotic exit. Unlike primary nuclei, most micronuclei are enclosed by nuclear envelope (NE) that is highly susceptible to spontaneous and irreparable rupture. Ruptured micronuclei act as triggers of chromothripsis-like chaotic chromosomal rearrangements and cGAS-mediated innate immunity and inflammation, raising the view that micronuclei play active roles in human aging and tumorigenesis. Thus, understanding the ways in which micronuclear envelope (mNE) goes awry acquires increased importance. Here, we review the data to present a general framework for this question. We firstly describe NE reassembly after mitosis and NE repair during interphase. Simultaneously, we briefly discuss how mNE is organized and how mNE rupture controls the fate of micronuclei and micronucleated cells. As a focus of this review, we highlight current knowledge about why mNE is rupture-prone and irreparable. For this, we survey observations from a series of elegant studies to provide a systematic overview. We conclude that the birth of rupture-prone and irreparable micronuclei may be the cumulative effects of their intracellular geographic origins, biophysical properties, and specific mNE features. We propose that DNA damage and immunogenicity in micronuclei increase stepwise from altered mNE components, mNE rupture, and refractory to repair. Throughout our discussion, we note interesting issues in mNE fragility that have yet to be resolved.

The EGFR-ZNF263 signaling axis silences SIX3 in glioblastoma epigenetically

The homeotic protein SIX3 is a transcription factor vital for neurogenesis and has a bivalent promoter. We previously showed that SIX3 can be transcriptionally silenced by DNA hypermethylation, functions as a tumor suppressor gene, and inhibits human glioblastoma transcriptionally. Here, we show that the activation of epidermal growth factor (EGFR) induces DNA methylation of SIX3 promoter through the MAPK pathway. ERK, when activated, binds with ZNF263, consequently abrogating the ubiquitination of ZNF263 and leading to its stabilization. ZNF263 binds to the core promoter region of SIX3 and recruits the KAP1/HATS/DNMT corepressor complex to induce transcriptional silencing of SIX3 through H3K27me3 and methylation of SIX3 promoter. Activation of the EGFR-ZNF263 signaling axis in phenotypically normal astrocytes or glioblastoma cells triggers or enhances tumorigenic activities, while elevated expression of the EGFR-ZNF263 signaling components in glioblastoma tissues is associated with poor prognosis of the patients. Together, our findings demonstrate that epigenetic silencing of SIX3 is controlled by a sophisticated and highly ordered oncogenic signaling pathway and therefore provide new insights into initiation and progression of glioblastoma.

DNA damage response proteins regulating mitotic cell division: double agents preserving genome stability

The DNA damage response recognizes DNA lesions and coordinates a cell cycle arrest with the repair of the damaged DNA, or removal of the affected cells to prevent the passage of genetic alterations to the next generation. The mitotic cell division, on the other hand, is a series of processes that aims to accurately segregate the genomic material from the maternal to the two daughter cells. Despite their great importance in safeguarding genomic integrity, the DNA damage response and the mitotic cell division were long viewed as unrelated processes, mainly because animal cells that are irradiated during mitosis continue cell division without repairing the broken chromosomes. However, recent studies have demonstrated that DNA damage proteins play an important role in mitotic cell division. This is performed through regulation of the onset of mitosis, mitotic spindle formation, correction of misattached kinetochore-microtubules, spindle checkpoint signaling, or completion of cytokinesis (abscission), in the absence of DNA damage. In this review, we summarize the roles of DNA damage proteins in unperturbed mitosis, analyze the molecular mechanisms involved, and discuss the potential implications of these findings in cancer therapy.

Actin reduction by MsrB2 is a key component of the cytokinetic abscission checkpoint and prevents tetraploidy

Abscission is the terminal step of cytokinesis leading to the physical separation of the daughter cells. In response to the abnormal presence of lagging chromatin between dividing cells, an evolutionarily conserved abscission/NoCut checkpoint delays abscission and prevents formation of binucleated cells by stabilizing the cytokinetic intercellular bridge (ICB). How this bridge is stably maintained for hours while the checkpoint is activated is poorly understood and has been proposed to rely on F-actin in the bridge region. Here, we show that actin polymerization is indeed essential for stabilizing the ICB when lagging chromatin is present, but not in normal dividing cells. Mechanistically, we found that a cytosolic pool of human methionine sulfoxide reductase B2 (MsrB2) is strongly recruited at the midbody in response to the presence of lagging chromatin and functions within the ICB to promote actin polymerization there. Consistently, in MsrB2-depleted cells, F-actin levels are decreased in ICBs, and dividing cells with lagging chromatin become binucleated as a consequence of unstable bridges. We further demonstrate that MsrB2 selectively reduces oxidized actin monomers and thereby counteracts MICAL1, an enzyme known to depolymerize actin filaments by direct oxidation. Finally, MsrB2 colocalizes and genetically interacts with the checkpoint components Aurora B and ANCHR, and the abscission delay upon checkpoint activation by nuclear pore defects also depends on MsrB2. Altogether, this work reveals that actin reduction by MsrB2 is a key component of the abscission checkpoint that favors F-actin polymerization and limits tetraploidy, a starting point for tumorigenesis.

The many functions of ESCRTs

Cellular membranes can form two principally different involutions, which either exclude or contain cytosol. The 'classical' budding reactions, such as those occurring during endocytosis or formation of exocytic vesicles, involve proteins that assemble on the cytosol-excluding face of the bud neck. Inverse membrane involution occurs in a wide range of cellular processes, supporting cytokinesis, endosome maturation, autophagy, membrane repair and many other processes. Such inverse membrane remodelling is mediated by a heteromultimeric protein machinery known as endosomal sorting complex required for transport (ESCRT). ESCRT proteins assemble on the cytosolic (or nucleoplasmic) face of the neck of the forming involution and cooperate with the ATPase VPS4 to drive membrane scission or sealing. Here, we review similarities and differences of various ESCRT-dependent processes, with special emphasis on mechanisms of ESCRT recruitment.

Building bridges between chromosomes: novel insights into the abscission checkpoint

In the presence of chromatin bridges, mammalian cells delay completion of cytokinesis (abscission) to prevent chromatin breakage or tetraploidization by regression of the cleavage furrow. This abscission delay is called "the abscission checkpoint" and is dependent on Aurora B kinase. Furthermore, cells stabilize the narrow cytoplasmic canal between the two daughter cells until the DNA bridges are resolved. Impaired abscission checkpoint signaling or unstable intercellular canals can lead to accumulation of DNA damage, aneuploidy, or generation of polyploid cells which are associated with tumourigenesis. However, the molecular mechanisms involved have only recently started to emerge. In this review, we focus on the molecular pathways of the abscission checkpoint and describe newly identified triggers, Aurora B-regulators and effector proteins in abscission checkpoint signaling. We also describe mechanisms that control intercellular bridge stabilization, DNA bridge resolution, or abscission checkpoint silencing upon satisfaction, and discuss how abscission checkpoint proteins can be targeted to potentially improve cancer therapy.

CDK11p58-cyclin L1β regulates abscission site assembly

Rigorous spatiotemporal regulation of cell division is required to maintain genome stability. The final stage in cell division, when the cells physically separate (abscission), is tightly regulated to ensure that it occurs after cytokinetic events such as chromosome segregation. A key regulator of abscission timing is Aurora B kinase activity, which inhibits abscission and forms the major activity of the abscission checkpoint. This checkpoint prevents abscission until chromosomes have been cleared from the cytokinetic machinery. Here we demonstrate that the mitosis-specific CDK11^p58 kinase specifically forms a complex with cyclin L1β that, in late cytokinesis, localizes to the stem body, a structure in the middle of the intercellular bridge that forms between two dividing cells. Depletion of CDK11 inhibits abscission, and rescue of this phenotype requires CDK11^p58 kinase activity or inhibition of Aurora B kinase activity. Furthermore, CDK11^p58 kinase activity is required for formation of endosomal sorting complex required for transport III filaments at the site of abscission. Combined, these data suggest that CDK11^p58 kinase activity opposes Aurora B activity to enable abscission to proceed and result in successful completion of cytokinesis.

Getting out of mitosis: spatial and temporal control of mitotic exit and cytokinesis by PP1 and PP2A

Here, we will review the evidence showing that mitotic exit is initiated by regulated proteolysis and then driven by the PPP family of phosphoserine/threonine phosphatases. Rapid APC/CCDC20 and ubiquitin-dependent proteolysis of cyclin B and securin initiates sister chromatid separation, the first step of mitotic exit. Because proteolysis of Aurora and Polo family kinases dependent on APC/CCDH1 is relatively slow, this creates a new regulatory state, anaphase, different to G2 and M-phase. We will discuss how the CDK1-counteracting phosphatases PP1 and PP2A-B55, together with Aurora and Polo kinases, contribute to the temporal regulation and order of events in the different stages of mitotic exit from anaphase to cytokinesis. For PP2A-B55, these timing properties are created by the ENSA-dependent inhibitory pathway and differential recognition of phosphoserine and phosphothreonine. Finally, we will discuss how Aurora B and PP2A-B56 are needed for the spatial regulation of anaphase spindle formation and how APC/C-dependent destruction of PLK1 acts as a timer for abscission, the final event of cytokinesis.

A Septin Double Ring Controls the Spatiotemporal Organization of the ESCRT Machinery in Cytokinetic Abscission

Abscission is the terminal step of mitosis that physically separates two daughter cells [1, 2]. Abscission requires the endocytic sorting complex required for transport (ESCRT), a molecular machinery of multiple subcomplexes (ESCRT-I/II/III) that promotes membrane remodeling and scission [3-5]. Recruitment of ESCRT-I/II complexes to the midbody of telophase cells initiates ESCRT-III assembly into two rings, which subsequently expand into helices and spirals that narrow down to the incipient site of abscission [6-8]. ESCRT-III assembly is highly dynamic and spatiotemporally ordered, but the underlying mechanisms are poorly understood. Here, we report that, after cleavage furrow closure, septins form a membrane-bound double ring that controls the organization and function of ESCRT-III. The septin double ring demarcates the sites of ESCRT-III assembly into rings and disassembles before ESCRT-III rings expand into helices and spirals. We show that septin 9 (SEPT9) depletion, which abrogates abscission, impairs recruitment of VPS25 (ESCRT-II) and CHMP6 (ESCRT-III). Strikingly, ESCRT-III subunits (CHMP4B and CHMP2A/B) accumulate to the midbody, but they are highly disorganized, failing to form symmetric rings and to expand laterally into the cone-shaped helices and spirals of abscission. We found that SEPT9 interacts directly with the ubiquitin E2 variant (UEV) domain of ESCRT-I protein TSG101 through two N-terminal PTAP motifs, which are required for the recruitment of VPS25 and CHMP6, and the spatial organization of ESCRT-III (CHMP4B and CHMP2B) into functional rings. These results reveal that septins function in the ESCRT-I-ESCRT-II-CHMP6 pathway of ESCRT-III assembly and provide a framework for the spatiotemporal control of the ESCRT machinery of cytokinetic abscission.

Vps4A mediates the localization and exosome release of β-catenin to inhibit epithelial-mesenchymal transition in hepatocellular carcinoma

We previously reported that Vps4A acted as a tumor suppressor by influencing the microRNA profiles of exosomes and their parental cells in hepatocellular carcinoma (HCC). However, the underlying mechanism and if Vps4A contributes to sorting proteins into exosomes are not well known. Here, we performed mass spectrometry analysis of the immunoprecipitated Vps4A complex and confirmed that Vps4A was associated with β-catenin and CHMP4B. Through this interaction, Vps4A promoted the plasma membrane (PM) localization and exosome release of β-catenin. Silencing Vps4A or CHMP4B decreased the PM localization and exosome sorting of β-catenin. Vps4A overexpression decreased β-catenin signaling pathway and inhibited epithelial-mesenchymal transition (EMT) and motility of HCC cells. And, silencing Vps4A or CHMP4B promoted EMT in HCC. Furthermore, the expression of Vps4A was significantly related to that of several EMT markers in HCC tissues and the level of exosomal β-catenin in patients with metastatic HCC was significantly lower compared to that of control patients. In conclusion, through the interaction with CHMP4B and β-catenin, Vps4A regulates the PM localization and exosome sorting of β-catenin, consequently decreases β-catenin signaling, and thereby inhibits EMT and metastasis in HCC.

ESCRT-III is necessary for the integrity of the nuclear envelope in micronuclei but is aberrant at ruptured micronuclear envelopes generating damage

Micronuclei represent the cellular attempt to compartmentalize DNA to maintain genomic integrity threatened by mitotic errors and genotoxic events. Some micronuclei show aberrant nuclear envelopes (NEs) that collapse, generating damaged DNA that can promote complex genome alterations. However, ruptured micronuclei also provide a pool of cytosolic DNA that can stimulate antitumor immunity, revealing the complexity of micronuclear impact on tumor progression. The ESCRT-III (Endosomal Sorting Complex Required for Transport-III) complex ensures NE reseals during late mitosis and is repaired in interphase. Therefore, ESCRT-III activity maybe crucial for maintaining the integrity of other genomic structures enclosed by a NE. ESCRT-III activity at the NE is coordinated by the subunit CHMP7. We show that CHMP7 and ESCRT-III protect against the genomic instability associated with micronuclei formation. Loss of ESCRT-III activity increases the population of micronuclei with ruptured NEs, revealing that its NE repair activity is also necessary to maintain micronuclei integrity. Surprisingly, aberrant accumulation of ESCRT-III are found at the envelope of most acentric collapsed micronuclei, suggesting that ESCRT-III is not recycled efficiently from these structures. Moreover, CHMP7 depletion relieves micronuclei from the aberrant accumulations of ESCRT-III. CHMP7-depleted cells display a reduction in micronuclei containing the DNA damage marker RPA and a sensor of cytosolic DNA. Thus, ESCRT-III activity appears to protect from the consequence of genomic instability in a dichotomous fashion: ESCRT-III membrane repair activity prevents the occurrence of micronuclei with weak envelopes, but the aberrant accumulation of ESCRT-III on a subset of micronuclei appears to exacerbate DNA damage and sustain proinflammatory pathways.

Toxoplasma gondii chromosomal passenger complex is essential for the organization of a functional mitotic spindle: a prerequisite for productive endodyogeny

The phylum Apicomplexa encompasses deadly pathogens such as malaria and Cryptosporidium. Apicomplexa cell division is mechanistically divergent from that of their mammalian host, potentially representing an attractive source of drug targets. Depending on the species, apicomplexan parasites can modulate the output of cell division, producing two to thousands of daughter cells at once. The inherent flexibility of their cell division mechanisms allows these parasites to adapt to different niches, facilitating their dissemination. Toxoplasma gondii tachyzoites divide using a unique form of cell division called endodyogeny. This process involves a single round of DNA replication, closed nuclear mitosis, and assembly of two daughter cells within a mother. In higher Eukaryotes, the four-subunit chromosomal passenger complex (CPC) (Aurora kinase B (ARKB)/INCENP/Borealin/Survivin) promotes chromosome bi-orientation by detaching incorrect kinetochore-microtubule attachments, playing an essential role in controlling cell division fidelity. Herein, we report the characterization of the Toxoplasma CPC (Aurora kinase 1 (Ark1)/INCENP1/INCENP2). We show that the CPC exhibits dynamic localization in a cell cycle-dependent manner. TgArk1 interacts with both TgINCENPs, with TgINCENP2 being essential for its translocation to the nucleus. While TgINCENP1 appears to be dispensable, interfering with TgArk1 or TgINCENP2 results in pronounced division and growth defects. Significant anti-cancer drug development efforts have focused on targeting human ARKB. Parasite treatment with low doses of hesperadin, a known inhibitor of human ARKB at higher concentrations, phenocopies the TgArk1 and TgINCENP2 mutants. Overall, our study provides new insights into the mechanisms underpinning cell cycle control in Apicomplexa, and highlights TgArk1 as potential drug target.

A cancer-associated polymorphism in ESCRT-III disrupts the abscission checkpoint and promotes genome instability

Cytokinetic abscission facilitates the irreversible separation of daughter cells. This process requires the endosomal-sorting complexes required for transport (ESCRT) machinery and is tightly regulated by charged multivesicular body protein 4C (CHMP4C), an ESCRT-III subunit that engages the abscission checkpoint (NoCut) in response to mitotic problems such as persisting chromatin bridges within the midbody. Importantly, a human polymorphism in CHMP4C (rs35094336, CHMP4CT232) increases cancer susceptibility. Here, we explain the structural and functional basis for this cancer association: The CHMP4CT232 allele unwinds the C-terminal helix of CHMP4C, impairs binding to the early-acting ESCRT factor ALIX, and disrupts the abscission checkpoint. Cells expressing CHMP4CT232 exhibit increased levels of DNA damage and are sensitized to several conditions that increase chromosome missegregation, including DNA replication stress, inhibition of the mitotic checkpoint, and loss of p53. Our data demonstrate the biological importance of the abscission checkpoint and suggest that dysregulation of abscission by CHMP4CT232 may synergize with oncogene-induced mitotic stress to promote genomic instability and tumorigenesis.

Butein suppresses hepatocellular carcinoma growth via modulating Aurora B kinase activity

Aurora B is aberrantly expressed in various tumors and shown to be a promising target for cancer therapy. Butein, a chalcone isolated from Rhus cerniciflua, has demonstrated antitumor activities in different cancers. In this study, we aimed to validate whether Aurora B kinase was the direct target of butein to exhibit its potency in hepatocellular carcinoma (HCC). Comparing with the normal cell line and tissue, Aurora B was overexpressed in all tested HCC cells and the majority of tumor tissue. Knocking down of Aurora B with shRNA substantially inhibited HCC cell proliferation, colony formation and delayed tumor growth in nude mice. Except computer docking, a series of kinase assays revealed butein directly interacted with Aurora B and inhibited its kinase activity. Along with the decrease of Aurora B and histone H3 phosphorylation, HCC cells were induced G2/M cell cycle arrest and subjected to cell apoptosis. Butein-mediated antitumor activities were substantially impaired in Aurora B knockdown cells, suggesting Aurora B was an important target of butein in HCC. Oral administration of butein substantially restrained HCC xenograft growth and the expressions of Ki67 and phosphor-histone H3 were significantly decreased in butein-treated tissue. To the best of our knowledge, our studies revealed that Aurora B was the direct target of butein in HCC.