Toward modeling of a general mechanism of MPF formation during oocyte maturation in vertebrates

Abstract

Vertebrate oocytes arrested at meiotic prophase I are still immature even when they reach their fully grown stage. For the acquisition of fertilizability, the fully grown oocytes must undergo oocyte maturation, during which the meiosis is released from prophase I arrest and stops again at metaphase II until inseminated. The resumption of meiosis from prophase I to metaphase II is induced by the action of maturation-promoting factor (MPF). The molecular structure of MPF is common to all eukaryotes, but the mechanisms of its formation and activation vary in cell types and in species. In this review, I summarize the mechanisms of MPF formation during oocyte maturation in two amphibian species, Xenopus laevis and Rana japonica. In Xenopus, immature oocytes are equipped with inactive MPF (pre-MPF) sufficient for completing oocyte maturation, and therefore only its activation is required after hormonal stimulation. In contrast, immature Rana oocytes contain no pre-MPF. Therefore, MPF must be newly formed during oocyte maturation, as is the case in fishes and other amphibians (toads and newts). The mechanism of MPF formation in Xenopus therefore seems to be different to that in other lower vertebrates. However, I wish to propose a new mechanism of MPF formation that might be appropriate for all species of lower vertebrates, inclusive of Xenopus, based on the novel concept that pre-MPF is an artifact produced under unnatural conditions and is not an essential molecule for initiating oocyte maturation, in contradiction to the generally believed notion that pre-MPF is actively stocked for the initiation of oocyte maturation. The standpoint that pre-MPF is not indispensable for initiating oocyte maturation might provide a new insight into the mechanism of MPF formation during oocyte maturation, allowing us to model a comprehensive mechanism applicable to all vertebrates, including mammals.