Protein phosphatases control MAP kinase activation and microtubule organization during rat oocyte maturation.

Mitogen-activated protein kinase (MAP) is involved in many signal transduction pathways and is activated during meiotic maturation in various species. In this study, we used the rat oocyte to identify some of the control mechanisms involved in MAP kinase activation which is triggered 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 of 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 activation 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 activation of MAP kinase was accelerated and began 1 h after the resumption of meiosis, i.e. 2 h earlier than in control oocytes. We also showed that protein synthesis was required to activate MAP kinase, but not cdc2 kinase. However, once MAP kinase was activated, ongoing protein synthesis was not necessary to maintain its activity. These results suggest that a negative regulation of MAP kinase slows down its activation at the resumption of meiosis, mediated through the level of phosphatase activity. Moreover, MAP kinase activation requires protein synthesis, even upon phosphatase inactivation by OA, suggesting also the existence of a positive control pathway. We observed that during the first meiotic M-phase, the spindle did not form immediately after cdc2 kinase activation, but that its formation coincided with the appearance of MAP kinase activity. However, earlier activation of MAP kinase by treatment with OA did not lead to premature spindle formation, but instead a large aster formed consisting of long microtubules radiating from the condensed chromatin. In OA-treated oocytes, spindles did not form and an interphase network of microtubule developed with time. Thus, MAP kinase is unable to substitute for MPF under these conditions, its activity alone being insufficient to maintain the progression through meiotic maturation.