A new cap for kinetochore fibre minus ends

Mcrs1 regulates G2/M transition and spindle assembly during mouse oocyte meiosis

Microspherule protein 1 (Mcrs1) is a component of the nonspecific lethal (NSL) complex and the chromatin remodeling INO80 complex, which participates in transcriptional regulation during mitosis. Here, we investigate the roles of Mcrs1 during female meiosis in mice. We demonstrate that Mcrs1 is a novel regulator of the meiotic G2/M transition and spindle assembly in mouse oocytes. Mcrs1 is present in the nucleus and associates with spindle poles and chromosomes of oocytes during meiosis I. Depletion of Mcrs1 alters HDAC2-mediated H4K16ac, H3K4me2, and H3K9me2 levels in nonsurrounded nucleolus (NSN)-type oocytes, and reduces CDK1 activity and cyclin B1 accumulation, leading to G2/M transition delay. Furthermore, Mcrs1 depletion results in abnormal spindle assembly due to reduced Aurora kinase (Aurka and Aurkc) and Kif2A activities, suggesting that Mcrs1 also plays a transcription-independent role in regulation of metaphase I oocytes. Taken together, our results demonstrate that the transcription factor Mcrs1 has important roles in cell cycle regulation and spindle assembly in mouse oocyte meiosis.

The molecular characteristics and functional roles of microspherule protein 1 (MCRS1) in gene expression, cell proliferation, and organismic development

Accurate spatial and temporal regulation of cell cycle progression is essential for cell proliferation and organismic development. This review demonstrates the role of microspherule protein 58kD, commonly known as MCRS1, as a key cell cycle regulator of higher eukaryotic organisms. We discuss the isoforms and functional domains of MCRS1 as well as their subcellular localization at specific stages of the cell cycle. These molecular characteristics reveal MCRS1's dynamic regulatory role in gene expression, genome stability, cell proliferation, and organismic development. Furthermore, we discuss the molecular details of its seemingly opposite, tumor-suppressive or tumor-promoting, role in different types of cancer.

BAP1 loss induces mitotic defects in mesothelioma cells through BRCA1-dependent and independent mechanisms

The tumour suppressor BRCA1-associated protein 1 (BAP1) is the most frequently mutated cancer gene in mesothelioma. Here we report novel functions for BAP1 in mitotic progression highlighting the relationship between BAP1 and control of genome stability in mesothelioma cells with therapeutic implications. Depletion of BAP1 protein induced proteasome-mediated degradation of BRCA1 in mesothelioma cells while loss of BAP1 correlated with BRCA1 loss in mesothelioma patient tumour samples. BAP1 loss also led to mitotic defects that phenocopied the loss of BRCA1 including spindle assembly checkpoint failure, centrosome amplification and chromosome segregation errors. However, loss of BAP1 also led to additional mitotic changes that were not observed upon BRCA1 loss, including an increase in spindle length and enhanced growth of astral microtubules. Intriguingly, these consequences could be explained by loss of expression of the KIF18A and KIF18B kinesin motors that occurred upon depletion of BAP1 but not BRCA1, as spindle and astral microtubule defects were rescued by re-expression of KIF18A and KIF18B, respectively. We therefore propose that BAP1 inactivation causes mitotic defects through BRCA1-dependent and independent mechanisms revealing novel routes by which mesothelioma cells lacking BAP1 may acquire genome instability and exhibit altered responses to microtubule-targeted agents.

Reprint of "Nuclear transport factors: global regulation of mitosis"

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator – the γ-TuRC complex – and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.

Nuclear transport factors: global regulation of mitosis

The unexpected repurposing of nuclear transport proteins from their function in interphase to an equally vital and very different set of functions in mitosis was very surprising. The multi-talented cast when first revealed included the import receptors, importin alpha and beta, the small regulatory GTPase RanGTP, and a subset of nuclear pore proteins. In this review, we report that recent years have revealed new discoveries in each area of this expanding story in vertebrates: (a) The cast of nuclear import receptors playing a role in mitotic spindle regulation has expanded: both transportin, a nuclear import receptor, and Crm1/Xpo1, an export receptor, are involved in different aspects of spindle assembly. Importin beta and transportin also regulate nuclear envelope and pore assembly. (b) The role of nucleoporins has grown to include recruiting the key microtubule nucleator - the γ-TuRC complex - and the exportin Crm1 to the mitotic kinetochores of humans. Together they nucleate microtubule formation from the kinetochores toward the centrosomes. (c) New research finds that the original importin beta/RanGTP team have been further co-opted by evolution to help regulate other cellular and organismal activities, ranging from the actual positioning of the spindle within the cell perimeter, to regulation of a newly discovered spindle microtubule branching activity, to regulation of the interaction of microtubule structures with specific actin structures. (d) Lastly, because of the multitudinous roles of karyopherins throughout the cell cycle, a recent large push toward testing their potential as chemotherapeutic targets has begun to yield burgeoning progress in the clinic.