Phosphorylation of MAP kinase and p90rsk and its regulation during in vitro maturation of cumulus-enclosed rabbit oocytes

Numerous studies have demonstrated that activation of the mitogen-activated protein (MAP) kinase is involved in the maturation of oocytes. In this study, the expression and phosphorylation of MAP kinase and p90(rsk), one of the substrates of MAP kinase, during rabbit oocyte maturation were studied. The results showed that MAP kinase phosphorylation began to occur after germinal vesicle breakdown (GVBD) and the active form was maintained until metaphase II. p90(rsk) was also activated after GVBD following MAP kinase activation. Immunofluorescent analysis showed that p90(rsk) was enriched in the nuclear area after GVBD and was gradually localised to the spindle. When GVBD was inhibited by increased cAMP or decreased protein kinase C activity, the phosphorylation of both MAP kinase and p90(rsk) was blocked. Our data suggest that (1) MAP kinase/p90(rsk) activation is not necessary for GVBD, but plays an important role in the post-GVBD events including spindle assembly in rabbit oocytes; and (2) MAP kinase/p90(rsk) activation is down-regulated by cAMP and up-regulated by protein kinase C in cumulus-enclosed rabbit oocytes.