PP2A-B55SUR-6 promotes nuclear envelope breakdown in C. elegans embryos.

Nuclear envelope (NE) disassembly during mitosis is critical to ensure faithful segregation of the genetic material. NE disassembly is a phosphorylation-dependent process wherein mitotic kinases hyper-phosphorylate lamina and nucleoporins to initiate nuclear envelope breakdown (NEBD). In this study, we uncover an unexpected role of the PP2A phosphatase B55SUR-6 in NEBD during the first embryonic division of Caenorhabditis elegans embryo. B55SUR-6 depletion delays NE permeabilization and stabilizes lamina and nucleoporins. As a result, the merging of parental genomes and chromosome segregation is impaired. NEBD defect upon B55SUR-6 depletion is not due to delayed mitotic onset or mislocalization of mitotic kinases. Importantly, we demonstrate that microtubule-dependent mechanical forces synergize with B55SUR-6 for efficient NEBD. Finally, our data suggest that the lamin LMN-1 is likely a bona fide target of PP2A-B55SUR-6. These findings establish a model highlighting biochemical crosstalk between kinases, PP2A-B55SUR-6 phosphatase, and microtubule-generated mechanical forces in timely NE dissolution.

NuMA interaction with chromatin is vital for proper chromosome decondensation at the mitotic exit.

NuMA is an abundant long coiled-coil protein that plays a prominent role in spindle organization during mitosis. In interphase, NuMA is localized to the nucleus and hypothesized to control gene expression and chromatin organization. However, because of the prominent mitotic phenotype upon NuMA loss, its precise function in the interphase nucleus remains elusive. Here, we report that NuMA is associated with chromatin in interphase and prophase but released upon nuclear envelope breakdown (NEBD) by the action of Cdk1. We uncover that NuMA directly interacts with DNA via evolutionarily conserved sequences in its C-terminus. Notably, the expression of the DNA-binding-deficient mutant of NuMA affects chromatin decondensation at the mitotic exit, and nuclear shape in interphase. We show that the nuclear shape defects observed upon mutant NuMA expression are due to its potential to polymerize into higher-order fibrillar structures. Overall, this work establishes the spindle-independent function of NuMA in choreographing proper chromatin decompaction and nuclear shape by directly associating with the DNA.

Centrosome Aurora A gradient ensures single polarity axis in C. elegans embryos.

Cellular asymmetries are vital for generating cell fate diversity during development and in stem cells. In the newly fertilized Caenorhabditis elegans embryo, centrosomes are responsible for polarity establishment, i.e. anterior-posterior body axis formation. The signal for polarity originates from the centrosomes and is transmitted to the cell cortex, where it disassembles the actomyosin network. This event leads to symmetry breaking and the establishment of distinct domains of evolutionarily conserved PAR proteins. However, the identity of an essential component that localizes to the centrosomes and promotes symmetry breaking was unknown. Recent work has uncovered that the loss of Aurora A kinase (AIR-1 in C. elegans and hereafter referred to as Aurora A) in the one-cell embryo disrupts stereotypical actomyosin-based cortical flows that occur at the time of polarity establishment. This misregulation of actomyosin flow dynamics results in the occurrence of two polarity axes. Notably, the role of Aurora A in ensuring a single polarity axis is independent of its well-established function in centrosome maturation. The mechanism by which Aurora A directs symmetry breaking is likely through direct regulation of Rho-dependent contractility. In this mini-review, we will discuss the unconventional role of Aurora A kinase in polarity establishment in C. elegans embryos and propose a refined model of centrosome-dependent symmetry breaking.

Centrosome Aurora A regulates RhoGEF ECT-2 localisation and ensures a single PAR-2 polarity axis in C. elegans embryos.

Proper establishment of cell polarity is essential for development. In the one-cell C. elegans embryo, a centrosome-localised signal provides spatial information for polarity establishment. It is hypothesised that this signal causes local inhibition of the cortical actomyosin network, and breaks symmetry to direct partitioning of the PAR proteins. However, the molecular nature of the centrosomal signal that triggers cortical anisotropy in the actomyosin network to promote polarity establishment remains elusive. Here, we discover that depletion of Aurora A kinase (AIR-1 in C. elegans) causes pronounced cortical contractions on the embryo surface, and this creates more than one PAR-2 polarity axis. This function of AIR-1 appears to be independent of its role in microtubule nucleation. Importantly, upon AIR-1 depletion, centrosome positioning becomes dispensable in dictating the PAR-2 axis. Moreover, we uncovered that a Rho GEF, ECT-2, acts downstream of AIR-1 in regulating contractility and PAR-2 localisation, and, notably, AIR-1 depletion influences ECT-2 cortical localisation. Overall, this study provides a novel insight into how an evolutionarily conserved centrosome Aurora A kinase inhibits promiscuous PAR-2 domain formation to ensure singularity in the polarity establishment axis.

Plk1 regulates spindle orientation by phosphorylating NuMA in human cells.

Proper orientation of the mitotic spindle defines the correct division plane and is essential for accurate cell division and development. In metazoans, an evolutionarily conserved complex comprising of NuMA/LGN/Gαi regulates proper orientation of the mitotic spindle by orchestrating cortical dynein levels during metaphase. However, the molecular mechanisms that modulate the spatiotemporal dynamics of this complex during mitosis remain elusive. Here, we report that acute inactivation of Polo-like kinase 1 (Plk1) during metaphase enriches cortical levels of dynein/NuMA/LGN and thus influences spindle orientation. We establish that this impact of Plk1 on cortical levels of dynein/NuMA/LGN is through NuMA, but not via dynein/LGN. Moreover, we reveal that Plk1 inhibition alters the dynamic behavior of NuMA at the cell cortex. We further show that Plk1 directly interacts and phosphorylates NuMA. Notably, NuMA-phosphorylation by Plk1 impacts its cortical localization, and this is needed for precise spindle orientation during metaphase. Overall, our finding connects spindle-pole pool of Plk1 with cortical NuMA and answers a long-standing puzzle about how spindle-pole Plk1 gradient dictates proper spindle orientation for error-free mitosis.

Ovarian Sertoli-Leydig Cell Tumour with Heterologus Elements Masquerading as Mucinous Tumour on Frozen Section: A Case Report.

Sertoli-Leydig cell tumour (SLCT) is an extremely rare ovarian neoplasm. This tumour is characterized by excessive proliferation of normal testicular structures sertoli and leydig cells. These cells are seen in varying proportions and exhibit varying degrees of differentiation. We report a case of primary ovarian SLCT with heterologus elements in a 17-year-old girl which was misdiagnosed on frozen section as mucinous cystic neoplasm. We discuss the clinicopathologic features of SLCT along with the unusual features seen in this case.

A Review on Mechanisms of Anti Tumor Activity of Chalcones.

Chalcones comprise a characteristic framework of 1, 3-diaryl-2-propen-1-one. They have been heralded as promising anti cancer drugs and have received much attention in the field of cancer research and drug development. The cytotoxicity of these potent pharmacophores is attributable to a wide spectrum of biological activities like anti inflammatory, anti proliferative, anti fungal, etc. These anti tumor activities are effectuated through apoptosis, cell cycle arrest, anti tubulin and so forth. This review summarizes the recent developments on anti tumor activity of synthetic and natural chalcones and their detailed underlying mechanisms as reported in the past.