Phosphorylation of p90rsk during meiotic maturation and parthenogenetic activation of rat oocytes:: correlation with MAP kinases

This paper reports on the activation of p90(rsk) during meiotic maturation and the inactivation of p90(rsk) after electrical parthenogenetic activation of rat oocytes. In addition, the correlation between p90(rsk) and MAP kinases after different treatments was studied. We assessed p90(rsk) activity by examining its electrophoretic mobility shift on SDS-PAGE and evaluated ERK1+2 activity by both mobility shift and a specific antibody against phospho-MAP kinase. The phosphorylation of p90(rsk) during rat oocyte maturation was a sequential process that may be divided into two stages: the first stage was partial phosphorylation, which was irrelevant with MAP kinases because p90(rsk) phosphorylation took place prior to activation of MAP kinases. The second stage inferred full activation occurred at the time when MAP kinases began to be activated (3 h after germinal visicle breakdown). Evidence for the involvement of MAP kinases in the p90(rsk) phosphorylation was further obtained by the following approaches: (1) okadaic acid (OA) accelerated the phosphorylation of both MAP kinases and p90(rsk); (2) OA induced phosphorylation of both MAP kinases and p90(rsk) in the presence of IBMX; (3) when activation of MAP kinases was inhibited by cycloheximide, p90(rsk) phosphorylation was also abolished; (4) dephosphorylation of p90(rsk) began to take place at 3 h post-activation, temporally correlated with the completion of MAP kinase inactivation; (5) phosphorylation of both kinases was maintained in oocytes that failed to form pronuclei after stimulation; (6) OA abolished the dephosphorylation of both kinases after parthenogenetic activation. Our data suggest that MAP kinases are not required for early partial activation of p90(rsk) but are required for full activation of p90(rsk) during rat oocyte maturation, and that p90(rsk) dephosphorylation occurs following MAP kinase inactivation after parthenogenetic activation of rat oocytes.