Requirement of mitogen-activated protein kinase activation for the meiotic resumption of porcine oocytes

The requirement of mitogen-activated protein kinase (MAPK) for the initiation of meiotic maturation in porcine oocytes was examined using U0126, an inhibitor of MAPK activator. Immunoblot analysis and kinase assay showed that U0126 inhibited the phosphorylation and activation of MAPK incompletely in porcine oocytes. In this condition, maturation/M-phase promoting factor (MPF) activity was also inhibited and the rate of germinal vesicle breakdown (GVBD, 95%) was reduced to 42%. The classification of oocytes remaining at germinal vesicle stage (GV) and reached to second metaphase stage (M2) after 48h culture with U0126 revealed that GV oocytes had low MAPK and MPF activities while M2 oocytes had high activities in both kinases, suggesting the indispensability of MAPK activation for meiotic resumption of porcine oocytes. In mouse oocytes, however, the inhibition of MAPK activation by U0126 had no effect on GVBD nor MPF activation, consistent with previous reports using mos deficient mice. Additionally, we found that the inhibitory effects of U0126 needed the presence of cumulus cells because of the ineffectiveness of U0126 on denuded porcine oocytes. These results suggest that MAPK act as indispensable mediator of MPF activation and GVBD in porcine oocytes, like in Xenopus oocytes but not in mouse oocytes.