Transient reactivation of CSF in parthenogenetic one-cell mouse embryos

During meiosis, the cytostatic factor (CSF) activity stabilizes the activit y of the M-phase promoting factor (MPF) in metaphase II arrested vertebrate oocytes. Upon oocyte activation, the inactivation of both MPF and CSF enab les the entry into the first embryonic mitotic cell cycle. Using a biologic al assay based on cell-fusion (hybrid between a parthenogenetically activat ed egg entering the first mitotic division and an activated oocyte), we obs erved that in activated mouse oocytes a first drop in CSF activity is detec table as early as 20 min post-activation. This suggests that CSF is inactiv ated upon MPF inactivation. However, CSF activity increases again to reach a maximum 60 min postactivation and gradually disappears during the followi ng 40 min. Thus, in activated mouse oocytes (undergoing the transition to i nterphase) CSF activity fluctuates before definitive inactivation. We found that hybrids arrested in M-phase, thus containing CSF activity after oocyt e activation, have activated forms of MAP kinases while hybrids in interpha se have inactive forms of these enzymes. We postulate that CSF inactivation in mouse oocytes proceeds in two steps. The initial inactivation of CSF, r equired for MPF inactivation, is transient and does not require MAP kinase inactivation. The final inactivation of CSF, required for normal embryonic cell cycle progression, is dependent upon the inactivation of MAP kinases. (C) 1999 Editions scientifiques et medicales Elsevier SAS.