Abstract
Xenopus oocyte maturation is analogous to G2/M transition and characterized by germinal vesicle breakdown (GVBD), spindle formation, activation of MPF and Mos‐Xp42^Mpk1^ pathways. It is accompanied prior to GVBD by a transient increase in intracellular pH. We determined that a well known acidifying compound, NH~4~Cl, delayed progesterone‐induced GVBD in a dose‐dependent manner. GVBD~50~ was delayed up to 2.3‐fold by 10 mM NH~4~Cl. Cyclin B2 phosphorylation, Cdk1 Tyr15 dephosphorylation as well as p39^Mos^ accumulation, Xp42^Mpk1^ and p90^Rsk^ phosphorylation induced by progesterone were also delayed by incubation of oocyte in NH~4~Cl. The delay induced by NH~4~Cl was prevented by injection of MOPS buffer pH 7.7. In contrast to acidifying medium, alkalyzing treatment such as Tris buffer pH 9 injections, accelerated GVBD, MPF and Xp42^Mpk1^ activation, indicating that pHi changes control early steps of G2/M dynamics. When injected in an immature recipient oocyte, egg cytoplasm triggers GVBD through MPF auto‐amplification, independently of protein synthesis. In these conditions, GVBD and Xp42^Mpk1^ activation were delayed by high concentration of NH~4~Cl, which never prevented or delayed MPF activation. Strickingly, NH~4~Cl strongly inhibited thiophosphorylated active MAPK‐induced GVBD and MPF activation. Nevertheless, Tris pH 9 did not have any effects on egg cytoplasm‐ or active MAPK‐induced GVBD. Taken together, our results suggest that dynamic of early events driving Xp42^Mpk1^ and MPF activation induced by progesterone may be negatively or positively regulated by pH~i~ changes. However Xp42^Mpk1^ pathway was inhibited by acidification alone. Finally, MPF auto‐amplification loop was not sensitive to pH~i~ changes. J. Cell. Biochem. 98: 287–300, 2006. © 2006 Wiley‐Liss, Inc.