The Mos/MAPK pathway is involved in metaphase II arrest as a cytostatic factor but is neither necessary nor sufficient for initiating oocyte maturation in goldfish

Mos plays a crucial role in meiotic cell division in vertebrates. In Xenopu s, Mos is involved in the initiation of oocyte maturation as an initiator a nd in the arrest at the metaphase II stage (MII) as a component of the cyto static factor (CSF. The function of Mos is mediated by MAP kinase (MAPK). W e investigated the function of the Mos/MAPK pathway during goldfish oocyte maturation induced by 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha, 20 beta-DP), a natural maturation-inducing hormone in fishes. Mos was absen t in immature goldfish oocytes. It appeared before the onset of germinal ve sicle breakdown (GVBD), increased to a maximum in mature oocytes arrested a t MII and disappeared after fertilization. MAPK was activated after Mos syn thesis but before maturation-promoting factor (MPF) activation, and its act ivity reached maximum at MII. Injection of either Xenopus or goldfish c-mos mRNA into one blastomere of 2-cell-stage Xenopus and goldfish embryos indu ced metaphase arrest, suggesting that goldfish Mos has a CSF activity. Inje ction of constitutively active Xenopus c-mos mRNA into immature goldfish oo cytes induced MAPK activation, but neither MPF activation nor GVBD occurred . Conversely, the injection of goldfish c-mos antisense RNA inhibited both Mos synthesis and MAPK activation in the 17 alpha,20 beta-DP-treated oocyte s, but these oocytes underwent GVBD. These results indicate that the Mos/MA PK pathway is not essential For initiating goldfish oocyte maturation despi te its general function as a CSF. We discuss the general role of Mos/MAPK d uring oocyte maturation, with reference to the difference in contents of in active MPF (pre-MPF) stored in immature oocytes.