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Comparative study of the molecular mechanisms of oocyte maturation in amphibians
N Yoshida1, K Mita, M Yamashita
1Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan.
Abstract:
Maturation-promoting factor (MPF), a complex of Cdc2 and cyclin B, is the final inducer of oocyte maturation. Its activity is controlled by inhibitory phosphorylation of Cdc2 on Tyr15/Thr14 and activating phosphorylation on Thr161. Full-grown immature oocytes of the African clawed frog Xenopus laevis contain inactive MPF (pre-MPF) that comprises cyclin B-bound Cdc2 phosphorylated on Tyr15/Thr14 and Thr161. The synthesis of Mos, but not cyclin B, after stimulation by the maturation-inducing steroid progesterone, is believed to be necessary for initiating Xenopus oocyte maturation through Tyr15/Thr14 dephosphorylation of pre-MPF. In contrast, amphibians other than Xenopus (and also fishes) employ a different mechanism. Full-grown immature oocytes of these species contain monomeric Cdc2 but not cyclin B. MPF is formed after hormonal stimulation by binding of the newly produced cyclin B to the pre-existing Cdc2 and is immediately activated through Thr161 phosphorylation. Mos/MAP kinase is neither necessary nor sufficient for initiating maturation in fishes and amphibians except for Xenopus. We propose a new model of MPF formation and activation during oocyte maturation that is applicable to all amphibians (as well as fishes), based on a novel concept that pre-MPF is an artificial molecule that is not essential for inducing oocyte maturation.
Insights
Maturation-promoting factor (MPF) formation differs between Xenopus and other amphibians/fishes. A new model suggests pre-MPF is not essential for oocyte maturation, proposing a unified mechanism for amphibians and fishes.
Area of Science:
- Cell Biology
- Developmental Biology
- Reproductive Biology
Background:
- Maturation-promoting factor (MPF), a Cdc2/cyclin B complex, induces oocyte maturation.
- MPF activity is regulated by phosphorylation of Cdc2 at Tyr15/Thr14 and Thr161.
- Xenopus laevis oocytes contain inactive pre-MPF, requiring Mos synthesis for maturation initiation via Tyr15/Thr14 dephosphorylation.
Purpose of the Study:
- To investigate the mechanism of MPF formation and activation in oocyte maturation across different amphibian and fish species.
- To propose a new model for MPF regulation applicable to all amphibians and fishes, challenging the necessity of pre-MPF.
Main Methods:
- Comparative analysis of MPF formation and activation pathways in Xenopus versus other amphibians and fishes.
- Review of existing literature on oocyte maturation signaling pathways, including Mos/MAP kinase and cyclin B synthesis.
Main Results:
- Amphibians other than Xenopus and fishes form MPF after hormonal stimulation by cyclin B binding to monomeric Cdc2, followed by Thr161 phosphorylation.
- Mos/MAP kinase is not essential for initiating oocyte maturation in these species.
- Pre-MPF, as defined in Xenopus, is not present or required in immature oocytes of other amphibians and fishes.
Conclusions:
- Oocyte maturation mechanisms vary significantly, with Xenopus employing a unique pre-MPF pathway.
- A unified model of MPF formation and activation, independent of pre-MPF, is proposed for all amphibians and fishes.
- This revised model highlights the conserved role of cyclin B-Cdc2 complex formation and activation in oocyte maturation across diverse vertebrate species.