MITOGEN-ACTIVATED PROTEIN (MAP) KINASE DURING THE ACQUISITION OF MEIOTIC COMPETENCE BY GROWING OOCYTES OF THE MOUSE

During the growth phase of oogenesis, oocytes acquire the ability to undergo meiotic maturation, Although the molecular basis of this meiotic competence is unknown, specific differences in microtubular organization exist between incompetent and competent mammalian oocytes. Mitogen-activated protein (MAP) kinase has been implicated in microtubular regulation and is present in fully grown competent oocytes of mice, suggesting a possible role for this protein in the acquisition of meiotic competence. We report that the MAP kinase species, p42(ERK2) and p44(ERK1), were detectable by immunoblotting in incompetent oocytes at the early stages of oocyte growth and throughout subsequent growth and acquisition of competence. In partially competent oocytes, which can enter metaphase but cannot complete the first meiotic division, both p42(ERK2) and p44(ERK1) became phosphorylated, as judged by retarded electrophoretic mobility, and a morphologically normal spindle was assembled. In incompetent oocytes, which cannot enter metaphase, p42(ERK2) and p44(ERK1) remained nonphosphorylated. When these oocytes were treated with okadaic acid, an inhibitor of protein phosphatases 1 and 2A, a portion of them entered metaphase and the slow-migrating phosphorylated forms of p42(ERK2) and p44(ERK1) were observed. These phosphorylated forms appeared more rapidly, relative to the time of entry into metaphase, than during maturation of fully competent oocytes. The remaining incompetent oocytes, which did not enter metaphase during okadaic acid treatment, also did not generate slow-migrating p42(ERK2) and p44(ERK1). These results suggest that the acquisition of meiotic competence during oocyte growth is not linked to the de novo appearance of p42(ERK2) or p44(ERK1) that the failure of partially competent oocytes to complete meiosis I reflects a defect acting downstream or independently of MAP kinase phosphorylation, and that the ability of meiotically incompetent oocytes to generate phosphorylated forms of p42(ERK2) and p44(ERK1) in response to okadaic acid is linked to the ability to enter metaphase. (C) 1995 Wiley-Liss, Inc.