Contribution of JNK, Mek, Mos and PI-3K signaling to GVBD in Xenopus oocytes

In Xenopus oocytes, induction of the G2/M transition by progesterone is a complex process that is promoted by a network of signaling molecules whose cumulative effect results in the activation of maturation promoting factor (MPF) and germinal vesicle breakdown (GVBD). We examined the role of Mos, Mek, PI-3 kinase and c-Jun N-terminal kinase (JNK) in progesterone stimulation of GVBD. Expression of an activated form of JNK neither induced nor enhanced progesterone-mediated GVBD in oocytes, suggesting a limited role in cell-cycle progression. We blocked Mek, Mos and PI-3 kinase activities by a variety of means that included expression of dominant-negative kinase suppressor of Ras (DnKSR), expression of a dominant-negative PI-3 kinase (DnPI3K), treatment of oocytes with a Mek inhibitor (U1026) or PI-3 kinase (LY294002) inhibitor, and introduction of Mos antisense morpholinos. Inhibition of any one pathway alone failed to block GVBD induced by either high or low concentrations of progesterone. In contrast, inhibiting Mos or Mek function in addition to abrogating PI-3 kinase activity effectively blocked oocyte maturation. Furthermore, by expressing suboptimal amounts of Mos in conjunction with an activated form of Mek and an activated form of the p110 catalytic subunit of PI-3 kinase, we show cooperation among these signaling molecules toward the induction of GVBD. Moreover, expression of optimal amounts of these three proteins in conjunction with inhibitors of Mos, Mek or PI-3 kinase demonstrated that activated Mek-induced GVBD is independent of Mos or PI-3 kinase activity. In addition, Mos-induced GVBD is dependent upon Mek activity, but does not require PI-3 kinase activity. Finally, Mos appears to be a major contributor to GVBD induced by activated PI-3 kinase, while Mek is a minor contributor to this process.