Meiosis resumption, calcium-sensitive period, and PLC- beta 1 relocation into the nucleus in the mouse oocyte

We aimed to determine whether the breakdown of the germinal vesicle of the mouse oocyte and the nuclear import of phospholipase C- beta 1 were calcium- dependent. We chelated Ca super(2+) ions with BAPTA-dextran at different times after the release of the oocyte from the ovarian follicle, i.e. after meiosis resumption has started, and we studied the effects on the kinetics of germinal vesicle breakdown, and on the migration of phospholipase C- beta 1. We discriminate between two key-periods of calcium-sensitivity during the process of meiosis resumption. During the first hour, changes in the cytosolic Ca super(2+) especially promoted the migration of phospholipase C- beta 1 into the nucleus, whereas changes in the nuclear concentration of Ca super(2+) were not implicated. Moreover, at this time, the cytosolic calcium pathway is PLC- beta 1-dependent. By contrast, during the second hour following the onset of meiosis resumption, and thus just previous GVBD, the PLC- beta 1-dependent Ca super(2+) signals in both cellular compartments were equally necessary for the resumption of meiosis. This particular period of the meiotic process corresponds to the moment when the phospholipase C- beta 1 has strongly migrated into the nucleus. Our results highlight also the role played by the nucleus during the second key-period in the control of the GVBD via a Ca super(2+)-dependent pathway.