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> Stages de M2 > Liste des stages proposés pour l année 2018-2019 > The Transition from Meiosis I to Meiosis II in Mammalian Oocytes

The Transition from Meiosis I to Meiosis II in Mammalian Oocytes

proposé par Katja WASSMANN , Biologie du Développement, UMR7622 CNRS

Projet We study cell cycle regulation and spindle assembly checkpoint (SAC) control between meiosis I and II. Meiosis is a special form of cell division, because it consists of two divisions without intermediate DNA-replication (S-phase). In the somatic cell cycle, interphase (G1, S-phase, G2) ensures the inactivation of enzymes such as Separase, that bring about the separation of chromosomes, whereas between meiosis I and II it is not entirely clear how these enzymes are inactivated. Apart from correctly executing the two meiotic divisions, a further challenge in oocytes is the fact that they have to mount a cell cycle arrest named CSF arrest in meiosis II, but not meiosis I, to await fertilization. This M2 project will focus on the transition from meiosis I to meiosis II. The candidate will address how the cell cycle is controlled during this transition. We want to elucidate how Separase, the protease required for cleavage of Cohesin, is maintained inactive as oocytes enter meiosis II, and again activated at anaphase onset in meiosis II. We have a combination of conditional knock-out mouse models and specific inhibitors at our disposition to dissect the molecular mechanisms of cell cycle progression and Separase inhibition. We are interested in the role of B-type cyclins at this stage, and how Cohesin cleavage is brought about by Separase in meiosis I and meiosis II. We have recently developed a biosensor to follow Separase activity by confocal live imaging during progression through the meiotic divisions, and this sensor may now be used to determine how cyclin-dependent kinases control Separase in mouse oocytes. In addition, accessibility of Cohesin for Separase cleavage in meiosis I and II (phosphorylation of Cohesin, protection of Cohesin from cleavage by Separase) will be determined, and the student will take part in independent, key aspects of this study, under close supervision by senior researchers in the group. In the long term, we hope that our study will give important insights into our understanding of oocyte maturation and to identify key players for correct chromosome segregation to generate healthy eggs for fertilization.

Techniques mises en œuvre par le stagiaire : To understand why oocytes missegregate chromosomes at such high rates we use state-of-art imaging approaches, chemical genetics, and conditional knock-out mouse models, and some of these appoaches will be used by the student, using mouse oocytes. Proteins required for Cohesin removal will be analysed by immunofluorescence staining on chromosome spreads. Progression through meiosis I and II will be studied by live imaging of oocytes that have been microinjected with mRNAs coding for proteins of our choice. In parallel we use frog oocytes for biochemistry, and depending on the interest of the candidate he/she will be able to use frog oocytes for biochemical approaches, in addition to sophisticated imaging techniques in mouse oocytes that have been set up in the lab and that are available to adress questions related to female fertility

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