PROTEIN PHOSPHATASES CONTROL MAP KINASE ACTIVATION AND MICROTUBULE ORGANIZATION DURING RAT OOCYTE MATURATION

Mitogen-activated protein kinase (MAP) is involved in many signal tran sduction pathways and is activated during meiotic maturation in variou s species. In this study, we used the rat oocyte to identify some of t he control mechanisms involved in MAP kinase activation which is trigg ered at resumption of meiosis. We examined the respective contribution of this kinase and maturation promoting factor (MPF), or cdc2 kinase, in the regulation of microtubule behavior and in the reorganization o f chromatin during meiotic maturation. We found that the resumption of meiotic division in rat oocytes coincided with the activation of MPF and was followed 3 h later by the activation of MAP kinase. The activa tion of the two kinases also occurred in oocytes undergoing maturation in the presence of the protein phosphatase inhibitor okadaic acid (OA ). However, the activation of cdc2 kinase was only partial, whereas ac tivation of MAP kinase was accelerated and began Ih after the resumpti on of meiosis, i.e. 2 h earlier than in control oocytes. We also showe d that protein synthesis was required to activate MAP kinase, but not cdc2 kinase. However, once MAP kinase was activated, ongoing protein s ynthesis was not necessary to maintain its activity. These results sug gest that a negative regulation of MAP kinase slows down its activatio n at the resumption of meiosis, mediated through the level of phosphat ase activity. Moreover, MAP kinase activation requires protein synthes is, even upon phosphatase inactivation by OA, suggesting also the exis tence of a positive control pathway. We observed that during the first meiotic M-phase, the spindle did not form immediately after cdc2 kina se activation, but that its formation coincided with the appearance of MAP kinase activity However, earlier activation of MAP kinase by trea tment with OA did not lead to premature spindle formation, but instead a large aster formed consisting of long microtubules radiating from t ile condensed chromatin. In OA-treated oocytes, spindles did not form and an interphase network of microtubule developed with time. Thus, MA P kinase is unable to substitute for MPF under these conditions, its a ctivity alone being insufficient to maintain the progression through m eiotic maturation.