live-cell-imaging-chromosome-segregation-during-mouse-oocyte

Live-Cell Imaging of Chromosome Segregation During Mouse Oocyte Meiosis

Accurate chromosome segregation during oocyte meiosis is essential for ensuring proper embryonic development and preventing aneuploidy-related disorders. ...

The University of Queensland

Targeted sequencing using capture probes has become increasingly popular in clinical applications due to its scalability and cost-effectiveness. The approach also allows for higher sequencing coverage of the targeted regions resulting in better analysis statistical power. However, because of the dynamics of the hybridization process, it is difficult to evaluate the efficiency of the probe design prior to the experiments which are time consuming and costly.

Simulating the dynamics of targeted capture sequencing with CapSim.

Female meiotic divisions are extremely asymmetric, producing large oocytes and small polar bodies (PBs). In mouse oocytes, the spindle relocates to the cortex before anaphase of meiosis I (MI). It is presumed that by displacing the future midzone, pre-anaphase spindle repositioning alone ensures asymmetry. But how subsequent anaphase events might contribute to asymmetric PB extrusion (PBE) is unknown. Here, we find that inactivation of cyclin-dependent kinase 1 (Cdk1) induces anaphase and simultaneously triggers cytoplasmic formin-mediated F-actin polymerisation that propels the spindle into the cortex causing it to protrude while anaphase progresses. Significantly, if post-anaphase-onset spindle migration fails, protrusion and asymmetry are severely threatened even with intact pre-anaphase migration. Conversely, post-anaphase migration can completely compensate for failed pre-anaphase migration. These data identify a cell-cycle-triggered phase of spindle displacement occurring after anaphase-onset, which, by inducing protrusion, is necessary for extreme asymmetry in mouse oocytes and uncover a pathway for maximising unequal division.

Cdk1 inactivation induces post-anaphase-onset spindle migration and membrane protrusion required for extreme asymmetry in mouse oocytes.

During mitosis, anaphase is triggered by anaphase-promoting complex (APC)-mediated destruction of securin and cyclin B1, which leads to inactivation of cyclin-dependent kinase 1 (Cdk1). By regulating APC activity, the mitotic spindle assembly checkpoint (SAC) therefore has robust control over anaphase timing to prevent chromosome mis-segregation. Mammalian oocytes are prone to aneuploidy, the reasons for which remain obscure. In mitosis, Cep55 is required post-anaphase for the final steps of cytokinesis. We found that Cep55-depleted mouse oocytes progress normally through early meiosis I, but that anaphase I fails as a result of persistent Cdk1 activity. Unexpectedly, Cdk1 inactivation was compromised following Cep55 depletion, despite on-time SAC silencing and intact APC-mediated proteolysis. We found that impaired Cdk1 inactivation was caused by inadequate inhibitory Cdk1 phosphorylation consequent upon failure to suppress Cdc25 phosphatase, identifying a proteolysis-independent step necessary for anaphase I. Thus, the SAC in oocytes does not exert exclusive control over anaphase I initiation, providing new insight into vulnerability to error.

First meiotic anaphase requires Cep55-dependent inhibitory cyclin-dependent kinase 1 phosphorylation.

Inactivation of cyclin-dependent kinase 1 (Cdk1), controlled by cyclin B1 proteolysis, orders events during mitotic exit. Here, we used a FRET biosensor to study Cdk1 activity while simultaneously monitoring anaphase II and pronuclear (PN) formation in live mouse eggs throughout fertilization. We find that Cdk1 inactivation occurs over two phases separated by a 3-h pause, the first induces anaphase II and the second induces PN formation. Although both phases require the inhibitory Cdk1 kinase Wee1B, only the first involves cyclin B1 proteolysis. Enforcing the 3-h pause is critical for providing the delay required for male PN formation and is mediated by spindle midzone-dependent sequestration of Wee1B between the first and second phases. Thus, unlike continuous Cdk1 inactivation driven by cyclin B1 proteolysis during mitotic exit, MII oocytes engineer a physiologically important pause during fertilization involving two different pathways to inactivate Cdk1, only the first of which requires proteolysis.

Article	Year
Simulating the dynamics of targeted capture sequencing with CapSim. Bioinformatics (Oxford, England)\| PubMed ID: 29092025	2018
Cdk1 inactivation induces post-anaphase-onset spindle migration and membrane protrusion required for extreme asymmetry in mouse oocytes. Nature communications\| PubMed ID: 30279413	2018
First meiotic anaphase requires Cep55-dependent inhibitory cyclin-dependent kinase 1 phosphorylation. Journal of cell science\| PubMed ID: 31427428	2019
The oocyte spindle midzone pauses Cdk1 inactivation during fertilization to enable male pronuclear formation and embryo development. Cell reports\| PubMed ID: 35508138	2022

Chenxi Zhou

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