Characterization of protein phosphatases in mouse oocytes

Okadaic acid (OA) enhances the resumption of meiosis in mouse oocytes, indi cating that serine/threonine protein phosphatase-1 (PP1) and/or PP2A is inv olved. However, specific identification of PP1 and/or PP2A in mouse oocytes has not been reported. Here we demonstrate that fully grown germinal vesic le-intact (GVI) mouse oocytes contain mRNA corresponding to two isotypes of PP1, PP1(alpha) and PP1(gamma). In addition, the transcript for PP2A was a lso present. At the protein level only PP1, and PP2A were recognized in ful ly grown GVI oocytes by Western blot analysis. Neither of the PP1(gamma) sp liced variant proteins, PP1(gamma 1) and PP1(gamma 2), was detectable. Immu nohistochemical analysis of ovarian tissue from gonadotropin-stimulated adu lt mice resulted in subcellular localization of both PP1(alpha) and PP2A, b ut not PP1(gamma), in oocytes from all stages of folliculogenesis. In primo rdial oocytes, PP1(alpha) and PP2A were present in the cytoplasm. In more a dvanced stages of oogenesis, PP1(alpha), although still present in the cyto plasm, was highly concentrated in the nucleus, whereas PP2A was predominant ly cytoplasmic with a distinct reduction in the nuclear area. Both PP1(alph a) and PP2A were immunodetectable in oocytes during the prepubertal period. Eleven-day-old mouse oocytes, considered OA-insensitive and germinal vesic le breakdown (GVB)-incompetent, displayed both PP1(alpha) and PP2A predomin antly in the cytoplasm. By 15 days of age mouse oocytes, which are beginnin gs to acquire OA sensitivity and GVB competence, showed a relocation of PP1 (alpha) into the nucleoplasm while PP2A remained predominantly cytoplasmic. This is the first specific identification of PP1(alpha) and PP2A in mouse oocytes. The differential localization of PP1(alpha) and PP2A, in addition to the relocation of PP1(alpha) during the acquisition of meiotic competenc e, suggests that these PPs have distinct regulatory roles during the resump tion of meiosis. (C) 1998 Academic Press.