MICROTUBULE AND CHROMATIN BEHAVIOR FOLLOW MAP KINASE-ACTIVITY BUT NOTMPF ACTIVITY DURING MEIOSIS IN MOUSE OOCYTES

Oocyte meiotic maturation is triggered by different stimuli (hormones, unknown signals through cell interactions) in different species. Thes e stimuli indirectly lead to the activation of a major cell cycle regu lating activity, the maturation promoting factor (MPF). Other factors, such as the product of the proto-oncogene c-mos or enzymes of the MAP kinase family, are also involved in the process of maturation. MAP ki nase activation occurs during meiotic maturation in oocytes from diffe rent species with different kinetics. The relationshipsbetween MPF act ivation and MAP kinase activation have been well studied in species su ch as clam and Xenopus. In this paper, we study the precise timing of MAP kinase activation (as measured by phosphorylation of exogenous mye lin basic protein and shifts in mobility of ERK 1 and ERK 2) versus MP F activation (asmeasured by phosphorylation of exogenous histone H1) d uring mouse oocyte maturation and, in parallel, morphological events s uch as changes in microtubule organization and chromatin condensation. We observed that MAP kinase activationwas delayed after MPF activatio n and that this activity persisted throughout maturation whereas MPF a ctivity dropped between the two meiotic metaphases. After parthenogene tic activation of ovulated eggs, MAP kinase inactivation was very slow compared to MPF inactivation. During the first mitotic cell cycle, a rise in myelin basic protein kinase activity at M-phase was observed b ut it was not related to MAP kinase activation. Furthermore, microtubu les and chromatin remained in a metaphase-like state during the comple te period of maturation (including the period between the two meiotic metaphases) and a few hours after activation. Thus, during meiosis but not during mitosis, the changes in microtubule organization and chrom atin condensation correlate with MAP kinase activity rather than with MPF activity. We discuss the possible role of MAP kinasein the mainten ance of a metaphasic state during meiosis when MPF is inactive.