An accumulation of p34(cdc2) at the end of mouse oocyte growth correlates with the acquisition of meiotic competence

Growing incompetent mouse oocytes released from follicular cells are unable to spontaneously resume meiosis in vitro. To identify the reasons for meiotic incompetence in these cells, the levels of p34(cdc)2/cyclin B kinase and p42(MAPK) between incompetent and competent oocytes were compared. p34(cdc2) was present at very low levels in incompetent oocytes and accumulated abruptly at the time of meiotic competence acquisition. By contrast, cyclin B and p42(MAPK) were present at similar concentrations in both types of oocytes. Okadaic acid induced centrosome phosphorylation and meiotic reinitiation in incompetent oocytes, without inducing an increase in p34(cdc2) concentration. However, the p34(cdc2) present in incompetent oocytes was activated and all events following germinal vesicle breakdown were induced up to the formation of a metaphase I spindle including p42(MAPK) activation, sustained increase in p34(cdc2) kinase activity, and translational activation of a dormant mRNA. We suggest that a threshold level of p34(cdc2) has to be reached for meiotic reinitiation to be spontaneously triggered: competence is restricted at a point preceding MPF activation. Whatever the mechanism involved in this restriction point, i.e., subthreshold concentration of p34(cdc2) and/or lack of an activator or presence of an inhibitor, it is bypassed by okadaic acid. Downstream of this point meiosis progresses up to metaphase I, even though p34(cdc2) concentration remains low. (C) 1996 Academic Press, Inc.