BRCA2 localization to the midbody by filamin A regulates cep55 signaling and completion of cytokinesis

A cancer-associated mutation inactivates a region of the high-mobility group protein HMG20b essential for cytokinesis

Defects in the completion of cell division by cytokinesis have long been proposed to foster carcinogenesis by engendering chromosome instability, but few tumor suppressor mechanisms controlling this process have so far been identified. Here, we identify a carboxyl (C)-terminal region of the high-mobility group protein HMG20b that is essential for cytokinesis, and report that it is inactivated by a cancer-associated mutation. We find that a C-terminal region of HMG20b spanning residues 173-317 is necessary and sufficient not only for its localization to cytokinetic structures, but also for its interaction with the tumor suppressor BRCA2, implicated in the abscission step of cytokinesis. Indeed, expression of this C-terminal HMG20b region suffices to restore cytokinesis in HMG20b-depleted cells. The non-conservative substitution of HMG20b residue Ala247 with Pro, reported in human lung cancer, disrupts these activities of HMG20b, impairing cytokinesis in a trans-dominant manner. Our findings provide fresh insight into the mechanism by which the HMG20b-BRCA2 complex controls mitotic cell division, and implicate heterozygous HMG20b mutations affecting cytokinesis regulation in the genesis of human cancers.

Beyond cytokinesis: the emerging roles of CEP55 in tumorigenesis

CEP55 was initially identified as a pivotal component of abscission, the final stage of cytokinesis, serving to regulate the physical separation of two daughter cells. Over the past 10 years, several studies have illuminated additional roles for CEP55 including regulating the PI3K/AKT pathway and midbody fate. Concurrently, CEP55 has been studied in the context of cancers including those of the breast, lung, colon and liver. CEP55 overexpression has been found to significantly correlate with tumor stage, aggressiveness, metastasis and poor prognosis across multiple tumor types and therefore has been included as part of several prognostic 'gene signatures' for cancer. Here by discussing in depth the functions of CEP55 across different effector pathways, and also its roles as a biomarker and driver of tumorigenesis, we assemble an exhaustive review, thus commemorating a decade of research on CEP55.

ALIX and ESCRT-III coordinately control cytokinetic abscission during germline stem cell division in vivo

Abscission is the final step of cytokinesis that involves the cleavage of the intercellular bridge connecting the two daughter cells. Recent studies have given novel insight into the spatiotemporal regulation and molecular mechanisms controlling abscission in cultured yeast and human cells. The mechanisms of abscission in living metazoan tissues are however not well understood. Here we show that ALIX and the ESCRT-III component Shrub are required for completion of abscission during Drosophila female germline stem cell (fGSC) division. Loss of ALIX or Shrub function in fGSCs leads to delayed abscission and the consequent formation of stem cysts in which chains of daughter cells remain interconnected to the fGSC via midbody rings and fusome. We demonstrate that ALIX and Shrub interact and that they co-localize at midbody rings and midbodies during cytokinetic abscission in fGSCs. Mechanistically, we show that the direct interaction between ALIX and Shrub is required to ensure cytokinesis completion with normal kinetics in fGSCs. We conclude that ALIX and ESCRT-III coordinately control abscission in Drosophila fGSCs and that their complex formation is required for accurate abscission timing in GSCs in vivo.

Cytokinesis, the final step of cell division, concludes with a process termed abscission, during which the two daughter cells physically separate. In spite of their importance, the molecular machineries controlling abscission are poorly characterized especially in the context of living metazoan tissues. Here we provide molecular insight into the mechanism of abscission using the fruit fly Drosophila melanogaster as a model organism. We show that the scaffold protein ALIX and the ESCRT-III component Shrub are required for completion of abscission in Drosophila female germline stem cells (fGSCs). ESCRT-III has been implicated in topologically similar membrane scission events as abscission, namely intraluminal vesicle formation at endosomes and virus budding. Here we demonstrate that ALIX and Shrub co-localize and interact to promote abscission with correct timing in Drosophila fGSCs. We thus show that ALIX and ESCRT-III coordinately control abscission in Drosophila fGSCs cells and report an evolutionarily conserved function of the ALIX/ESCRT-III pathway during cytokinesis in a multi-cellular organism.

The role of vertebrate nonmuscle Myosin II in development and human disease

Three different genes each located on a different chromosome encode the heavy chains of nonmuscle myosin II in humans and mice. This review explores the functional consequences of the presence of three isoforms during embryonic development and beyond. The roles of the various isoforms in cell division, cell-cell adhesion, blood vessel formation and neuronal cell migration are addressed in animal models and at the cellular level. Particular emphasis is placed on the role of nonmuscle myosin II during cardiac and brain development, and during closure of the neural tube and body wall. Questions addressed include the consequences on organ development, of lowering or ablating a particular isoform as well as the effect of substituting one isoform for another, all in vivo. Finally the roles of the three isoforms in human diseases such as cancer as well as in syndromes affecting a variety of organs in humans are reviewed.

The role of the endosomal sorting complexes required for transport (ESCRT) in tumorigenesis

The endosomal sorting complexes required for transport (ESCRT) are needed for three distinct cellular functions in higher eukaryotes: (i) Multivesicular body formation for the degradation of transmembrane proteins in lysosomes, (ii) midbody abscission during cytokinesis and (iii) retroviral budding. Not surprisingly, loss of ESCRT function has severe consequences, which include the failure to down-regulate growth factor receptors leading to deregulated mitogenic signaling. While it is clear that the function of the ESCRT machinery is important for embryonic development, its role in cancer is more controversial. Various experimental approaches in different model organisms arrive at partially divergent conclusions regarding the contribution of ESCRTs to tumorigenesis. Therefore the aim of this review is to provide an overview on different model systems used to study the role of the ESCRT machinery in cancer development, to highlight common grounds and present certain controversies in the field.

The negative interplay between Aurora A/B and BRCA1/2 controls cancer cell growth and tumorigenesis via distinct regulation of cell cycle progression, cytokinesis, and tetraploidy

It is well known that the activation of Aurora A/B (Aur A/B) or inactivation of BRCA1/2 induces tumor formation. Others and we have reported that the mutual suppression between Aur A/B and BRCA1/2 may manipulate cancer cell growth and tumorigenesis, however, the interactive regulation and mechanism between these molecules are still elusive. In this study, by consecutive silencing of Aur A/B or/and BRCA1/2 with specific shRNAs, we showed that, in BRCA2-deficient pancreatic cancer cell line Capan-1 and in ovarian cancer cell line OVCA433, Aur A/B and BRCA1/2 inversely regulated the expression of each other likely through proteasome-mediated proteolysis but not through gene transcription. Aur A/B and BRCA1/2 conversely regulated cell cycle progression mainly through control of p53 and cyclin A. Moreover, the disruption of Aur A/B blocked abnormal cytokinesis and decreased cell multinuclearity and chromosome tetraploidy, whereas the deprivation of BRCA1/2 promoted the abnormal cytokinesis and enhanced the cell multinuclearity and tetraploidy. Furthermore, we showed by animal assays that the depletion of Aur A/B inhibited tumor growth of both cell lines, while the knockdown of BRCA1/2 promoted the tumor growth. However, the concurrent silencing of Aur A/B and BRCA1/2 diminished the effects of these molecules on the regulation of cell cycle, cytokinesis, and tetraploidy, leading to the burdened tumor sizes similar to those induced by scrambled shRNA-treated control cells. In summary, our study revealed that the negative interplay between Aur A/B and BRCA1/2 inversely controls the cell proliferation, cell cycle progression, cell multinuclearity, and tetraploidization to modulate tumorigenesis.

Cancer suppression by the chromosome custodians, BRCA1 and BRCA2

Germline mutations in BRCA1 and BRCA2 predispose to common human malignancies, most notably tumors of the breast and ovaries. The proteins encoded by these genes have been implicated in a plethora of biochemical interactions and biological functions, confounding attempts to coherently explain how their inactivation promotes carcinogenesis. Here, I argue that tumor suppression by BRCA1 and BRCA2 originates from their fundamental role in controlling the assembly and activity of macromolecular complexes that monitor chromosome duplication, maintenance, and segregation across the cell cycle. A tumor-suppressive role for the BRCA proteins as "chromosome custodians" helps to explain the clinical features of cancer susceptibility after their inactivation, provides foundations for the rational therapy of BRCA-deficient cancers, and offers general insights into the mechanisms opposing early steps in human carcinogenesis.

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

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

Chromosome instability and carcinogenesis: insights from murine models of human pancreatic cancer associated with BRCA2 inactivation

Chromosomal instability is a hallmark of human cancer cells, but its role in carcinogenesis remains poorly resolved. Insights into this role have emerged from studies on the tumour suppressor BRCA2, whose inactivation in human cancers causes chromosomal instability through the loss of essential functions of the BRCA2 protein in the normal mechanisms responsible for the replication, repair and segregation of DNA during cell division. Humans who carry heterozygous germline mutations in the BRCA2 gene are highly predisposed to cancers of the breast, ovary, pancreas, prostate and other tissues. Here, we review recent studies that describe genetically engineered mouse models (GEMMs) for pancreatic cancer associated with BRCA2 mutations. These studies not only surprisingly show that BRCA2 does not follow the classical Knudson "two hit" paradigm for tumour suppression, but also highlight features of the interplay between TP53 inactivation and carcinogenesis in the context of BRCA2 deficiency. Thus, the models reveal novel aspects of cancer evolution in carriers of germline BRCA2 mutations, provide new insights into the tumour suppressive role of BRCA2, and establish valuable new preclinical settings for testing approaches to pancreatic cancer therapy; together, these features emphasize the value of GEMMs in cancer research.

Tumour suppressor mechanisms in the control of chromosome stability: insights from BRCA2

Cancer is unique amongst human diseases in that its cellular manifestations arise and evolve through the acquisition of somatic alterations in the genome. In particular, instability in the number and structure of chromosomes is a near-universal feature of the genomic alterations associated with epithelial cancers, and is triggered by the inactivation of tumour suppressor mechanisms that preserve chromosome integrity in normal cells. The nature of these mechanisms, and how their inactivation promotes carcinogenesis, remains enigmatic. I will review recent work from our laboratory on the tumour suppressor BRCA2 that addresses these issues, focusing on new insights into cancer pathogenesis and therapy that are emerging from improved understanding of the molecular basis of chromosomal instability in BRCA2-deficient cancer cells.

Functional assays for analysis of variants of uncertain significance in BRCA2

Missense variants in the BRCA2 gene are routinely detected during clinical screening for pathogenic mutations in patients with a family history of breast and ovarian cancer. These subtle changes frequently remain of unknown clinical significance because of the lack of genetic information that may help establish a direct correlation with cancer predisposition. Therefore, alternative ways of predicting the pathogenicity of these variants are urgently needed. Since BRCA2 is a protein involved in important cellular mechanisms such as DNA repair, replication, and cell cycle control, functional assays have been developed that exploit these cellular activities to explore the impact of the variants on protein function. In this review, we summarize assays developed and currently utilized for studying missense variants in BRCA2. We specifically depict details of each assay, including variants of uncertain significance analyzed, and describe a validation set of (genetically) proven pathogenic and neutral missense variants to serve as a golden standard for the validation of each assay. Guidelines are proposed to enable implementation of laboratory-based methods to assess the impact of the variant on cancer risk.

Integrins promote cytokinesis through the RSK signaling axis

Cytokinesis is the final stage in cell division. Although integrins can regulate cytokinesis, the mechanisms involved are not fully understood. In this study, we demonstrate that integrin-regulated ERK (extracellular signal-related kinase) and RSK (p90 ribosomal S6 kinase) signaling promotes successful cytokinesis. Inhibiting the activation of ERK and RSK in CHO cells by a mutation in the integrin β1 cytoplasmic tail or with pharmacological inhibitors results in the accumulation of cells with midbodies and the formation of binucleated cells. Activation of ERK and RSK signaling by the expression of constitutively active RAF1 suppresses the mutant phenotype in a RSK-dependent manner. Constitutively active RSK2 also restores cytokinesis inhibited by the mutant integrin. Importantly, the regulatory role of the RSK pathway is not specific to CHO cells. MCF-10A human mammary epithelial cells and HPNE human pancreatic ductal epithelial cells exhibit a similar dependence on RSK for successful cytokinesis. In addition, depriving mitotic MCF10A cells of integrin-mediated adhesion by incubating them in suspension suppressed ERK and RSK activation and resulted in a failure of cytokinesis. Furthermore, inhibition of RSK or integrins within the 3D context of a developing salivary gland organ explant also leads to an accumulation of epithelial cells with midbodies, suggesting a similar defect in cytokinesis. Interestingly, neither ERK nor RSK regulates cytokinesis in human fibroblasts, suggesting cell-type specificity. Taken together, our results identify the integrin-RSK signaling axis as an important regulator of cytokinesis in epithelial cells. We propose that the proper interaction of cells with their microenvironment through integrins contributes to the maintenance of genomic stability by promoting the successful completion of cytokinesis.

Syntaxin 16 is a master recruitment factor for cytokinesis

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

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

Rac1 is deactivated at integrin activation sites through an IQGAP1-filamin-A-RacGAP1 pathway

Cell migration makes a fundamental contribution to both normal physiology and disease pathogenesis. Integrin engagement with extracellular ligands spatially controls, via the cyclical activation and deactivation of the small GTPase Rac1, the dynamic membrane protrusion and cytoskeletal reorganization events that are required for directional migration. Although the pathways that control integrin-mediated Rac1 activation are reasonably well defined, the mechanisms that are responsible for switching off activity are poorly understood. Here, proteomic analysis of activated integrin-associated complexes suggests filamin-A and IQ-motif-containing GTPase-activating protein 1 (IQGAP1) as candidates that link β1 integrin to Rac1. siRNA-mediated knockdown of either filamin-A or IQGAP1 induced high, dysregulated Rac1 activity during cell spreading on fibronectin. Using immunoprecipitation and immunocytochemistry, filamin-A and IQGAP1 were shown to be part of a complex that is recruited to active β1 integrin. Mass spectrometric analysis of individual filamin-A, IQGAP1 and Rac1 pull-downs and biochemical analysis, identified RacGAP1 as a novel IQGAP1 binding partner. Further immunoprecipitation and immunocytochemistry analyses demonstrated that RacGAP1 is recruited to IQGAP1 and active β1 integrin, and that suppression of RacGAP1 expression triggered elevated Rac1 activity during spreading on fibronectin. Consistent with these findings, reduced expression of filamin-A, IQGAP1 or RacGAP1 triggered unconstrained membrane protrusion and disrupted directional cell migration on fibrillar extracellular matrices. These findings suggest a model whereby integrin engagement, followed by filamin-A, IQGAP1 and RacGAP1 recruitment, deactivates Rac1 to constrain its activity spatially and thereby coordinate directional cell migration.

Characterization of a novel cell line from the caudal fin of koi carp Cyprinus carpio

A continuous cell line (KF-101) derived from the caudal fin of the koi carp Cyprinus carpio was established and characterized. The KF-101 cell line multiplied abundantly in Leibovitz's L-15 medium containing 10% foetal bovine serum at 25° C, and was subcultured for >90 passages over a period of 3 years. Immunocytochemistry revealed that the KF-101 cells contain keratin, junction proteins connexin-43 and occludin, and ectodermal stem-cell marker Pax-6, but not vimentin. Furthermore, the KF-101 cells reacted with anti-human DARPP-32 and anti-human GATA-4 antibodies, and the labelling was regulated according to the cell cycle. The labels of the DARPP-32 and GATA-4 antibodies in the KF-101 cells were the suggested phosphatase-1 inhibitor-1 and GATA-3, respectively. In addition, the KF-101 cells were susceptible to koi herpesvirus but were resistant to eel herpesvirus, iridovirus, grouper nodavirus and chum salmon (Oncorhynchus keta) virus. The results indicate that the KF-101 cells are suitable materials for investigating biological and virological development.

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

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

Male breast cancer, age and sex chromosome aneuploidy

Background:

In cultured, dividing transformed T lymphocytes and in dividing bone marrow cells from normal men and those with a haematological malignancy, sex chromosome aneuploidy has been found to increase in prevalence and degree with age. This has rarely been investigated in non-dividing uncultured blood samples. The loss and gain of the X chromosome in dividing transformed lymphocytes in women with age is much more frequent than that of the Y chromosome in males. However, paradoxically X chromosome aneuploidy is rarely seen in the dividing cells of bone marrow of females.

Methods:

In blood samples from 565 men with breast cancer and 54 control men from the England and Wales general population, 80 cell nuclei per sample were scored for presence of X and Y chromosomes using fluorescent centromeric probes.

Results:

Sex chromosome aneuploidy, largely Y chromosome loss, was present in 63% of cases and 57% of controls, with the prevalence and degree of aneuploidy increasingly sharply and highly significantly with age. At ages 65–80 years, 71% of cases and 85% of controls showed aneuploidy and 15% and 25%, respectively, had ⩾10% of cells aneuploid. Allowing for age, aneuploidy was less prevalent (P=0.03) in cases than controls.

Conclusion:

Sex chromosome aneuploidy in non-dividing nuclei of peripheral blood cells is frequent in adult men, the prevalence and degree increasing sharply with age. The possible relation of sex chromosome aneuploidy to breast cancer risk in men, and to cancer risk generally, needs further investigation, ideally in cohort studies.