POTENTIAL ROLE OF MITOGEN-ACTIVATED PROTEIN-KINASE DURING MEIOSIS RESUMPTION IN BOVINE OOCYTES

During meiotic maturation, numerous cytoplasmic and nuclear events tak e place that prepare the oocytes for fertilization. These changes are initiated by an increase in the activity of several kinases, most nota bly maturation-promoting factor, also called histone H1 kinase. Anothe r kinase, mitogen-activated protein (MAP) kinase, is also stimulated d uring this period. In this study, we investigated the role of MAP kina se in bovine oocyte maturation. First, the kinetics of activation of h istone H1 and MAP kinases during maturation were assessed simultaneous ly by evaluating their catalytic activities in vitro. We found that th ey are activated at approximately the same time, around germinal vesic le breakdown (GVBD). Then, at approximately 15 h of maturation, the ac tivity of H1 kinase temporarily decreases, whereas MAP kinase remains high through the metaphase II stage. Second, the activation and cataly tic activity of MAP kinase was directly evaluated by Western blotting and by an in-gel kinase assay. We determined that MAP kinase becomes a ctivated and exhibits a decreased mobility through SDS-polyacrylamide gels, and that its catalytic activity increases as maturation progress es. In our system, most of the MAP kinase activity can be attributed t o p42(MAPK2). Third, the activation pathway of MAP kinase was explored . In Xenopus oocytes, MAP kinase is activated by a kinase cascade that includes several upstream activators; one of them is the product of t he proto-oncogene mos. In bovine oocytes, injection of Mos RNA elicite d a rapid and maximal activation of MAP kinase that resulted in accele rated resumption of meiosis and GVBD. These results were thought to be mediated by an overexpression of a kinase-active Mos product because injection of a kinase-inactive Mos RNA failed to activate MAP kinase. Together, these results suggest a role for MAP kinase during the initi ation and progression of meiosis in bovine oocytes. The data also sugg est the presence of an MAP kinase-activating cascade that can be initi ated by the Mos protein.