Regulation of Xenopus oocyte meiosis arrest by G protein βγ subunits

AbstractBackground: Progesterone induces the resumption of meiosis (maturation) in Xenopus oocytes through a nongenomic mechanism involving inhibition of an oocyte adenylyl cyclase and reduction of intracellular cAMP. However, progesterone action in Xenopus oocytes is not blocked by pertussis toxin, and this finding indicates that the inhibition of the oocyte adenylyl cyclase is not mediated by the α subunits of classical Gi-type G proteins.Results: To investigate the possibility that G protein βγ subunits, rather than α subunits, play a key role in regulating oocyte maturation, we have employed two structurally distinct G protein βγ scavengers (Gtα and βARK-CCAAX) to sequester free Gβγ dimers. We demonstrated that the injection of mRNA encoding either of these Gβγ scavengers induced oocyte maturation. The Gβγ scavengers bound an endogenous, membrane-associated Gβ subunit, indistinguishable from Xenopus Gβ1 derived from mRNA injection. The injection of Xenopus Gβ1 mRNA, together with bovine Gγ2 mRNA, elevated oocyte cAMP levels and inhibited progesterone-induced oocyte maturation.Conclusion: An endogenous G protein βγ dimer, likely including Xenopus Gβ1, is responsible for maintaining oocyte meiosis arrest. Resumption of meiosis is induced by Gβγ scavengers in vitro or, naturally, by progesterone via a mechanism that suppresses the release of Gβγ

Inhibition of MEK or cdc2 Kinase Parthenogenetically Activates Mouse Eggs and Yields the Same Phenotypes as Mos−/− Parthenogenotes

AbstractMammalian eggs are arrested in metaphase II of meiosis until fertilization. Arrest is maintained by cytostatic factor (CSF) activity, which is dependent on the MOS–MEK–MAPK pathway. Inhibition of MEK1/2 with a specific inhibitor, U0126, parthenogenetically activated mouse eggs, producing phenotypes similar to Mos−/− parthenogenotes (premature, unequal cleavages and large polar bodies). U0126 inactivated MAPK in eggs within 1 h, in contrast to the 5 h required after fertilization, while the time course of MPF inactivation was similar in U0126-activated and fertilized eggs. We also found that inactivation of MPF by the cdc2 kinase inhibitor roscovitine induced parthenogenetic activation. Inactivation of MPF by roscovitine resulted in the subsequent inactivation of MAPK with a time course similar to that following fertilization. Notably, roscovitine also produced some Mos−/−-like phenotypes, indistinguishable from U0126 parthenogenotes. Simultaneous inhibition of both MPF and MAPK in eggs treated with roscovitine and U0126 produced a very high proportion of eggs with the more severe phenotype. These findings confirm that MEK is a required component of CSF in mammalian eggs and imply that the sequential inactivation of MPF followed by MAPK inactivation is required for normal spindle function and polar body emission