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 Xenopus, Mos is involved in the initiation of oocyte maturation as an initiator and in the arrest at the metaphase II stage (MII) as a component of the cytostatic factor (CSF). The function of Mos is mediated by MAP kinase (MAPK). We investigated the function of the Mos/MAPK pathway during goldfish oocyte maturation induced by 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-DP), a natural maturation-inducing hormone in fishes. Mos was absent in immature goldfish oocytes. It appeared before the onset of germinal vesicle breakdown (GVBD), increased to a maximum in mature oocytes arrested at MII and disappeared after fertilization. MAPK was activated after Mos synthesis but before maturation-promoting factor (MPF) activation, and its activity reached maximum at MII. Injection of either Xenopus or goldfish c-mos mRNA into one blastomere of 2-cell-stage Xenopus and goldfish embryos induced metaphase arrest, suggesting that goldfish Mos has a CSF activity. Injection of constitutively active Xenopus c-mos mRNA into immature goldfish oocytes 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 17α,20β-DP-treated oocytes, but these oocytes underwent GVBD. These results indicate that the Mos/MAPK pathway is not essential for initiating goldfish oocyte maturation despite its general function as a CSF. We discuss the general role of Mos/MAPK during oocyte maturation, with reference to the difference in contents of inactive MPF (pre-MPF) stored in immature oocytes. Received: 10 February 2000 / Accepted: 25 April 2000     

Synthesis and function of mos: The control switch of vertebrate oocyte meiosis

One distinguishing feature of vertebrate oocyte meiosis is its discontinuity; oocytes are released from their prophase I arrest, usually by hormonal stimulation, only to again halt at metaphase II, where they await fertilization. The product of the c-mos proto-oncogene, Mos, is a key regulator of this maturation process. Mos is a serine-threonine kinase that activates and/or stabilizes maturation-promoting factor (MPF), the master cell cycle switch, through a pathway that involves the mitogen-activated protein kinase (MAPK) cascade. Oocytes arrested at prophase I lack detectable levels of Mos, which must be synthesized from a pool of maternal mRNAs for proper maturation. While Mos is necessary throughout maturation in Xenopus, it seems to be required only for meiosis II in the mouse. The translational activation of c-mos mRNA at specific times during meiosis requires cytoplasmic polyadenylation. Cis- and trans-acting factors for polyadenylation are, therefore, essential elements of maturation.     

Localization of human c-mos to chromosome band 8q11 in leukemic cells with the t(8;21) (q22;q22)

Summary Chromosome in situ hybridization studies locate c-mos to chromosome band 8q11 in leukemic cells carrying the t(8;21) (q22;q22). This amends the previous assignment of c-mos to chromosome band 8q22 and conforms with its recent assignment to 8q11 in normal cells and in a cell line with a structurally abnormal chromosome 8. C-mos lies proximally to, and distant from, the breakpoint at 8q22 in the t(8;21) and is unlikely to have a role in the onset of acute myeloid leukemia characterized by this translocation.