Activation of both Mos and Cdc25 is required for G2-M transition in perch oocyte

Resumption of meiosis from diplotene arrest during the first meiotic prophase in vertebrate oocytes is universally controlled by MPF, a heterodimer of Cdk1 and cyclin B. Activation of MPF depends on the withdrawal of Cdk1 inhibition by Wee1/Myt1 kinase on the one hand and the activation of Cdk1 by Cdc25 phosphatase on the other. It is relevant to know whether both these pathways are necessary to rescue diplotene arrest or if either one of them is sufficient. In MIH (17alpha, 20beta dihydroxy-4-pregnen-3-one) incubated perch (Anabas testudineus) oocytes we have examined these possibilities. Perch oocyte extract following MIH incubation showed a significant increase in Myt1 phosphorylation from 12 to 16 hr indicating its progressive deactivation. MIH induced Mos expression markedly increased at 16 hr effecting 95% GVBD. Cycloheximide inhibited MIH induced Mos expression and its phosphorylation, which in turn reduced Myt1 phosphorylation and GVBD. Myt1 phosphorylation was blocked in Mos immunodepleted oocytes. All these suggest the involvement of Mos in Myt1 phosphorylation. Oocytes incubated in MIH for 16 hr activated Cdc25, but such activation could not rescue the inhibition of GVBD due to Myt1 in Mos immunodepleted oocytes. Blocking Cdc25 with an antisense oligo significantly inhibited GVBD even though Myt1 remained deactivated during this period. Taken together, our findings indicate that MIH requires both pathways for perch oocyte maturation: the expression and activation of Mos, which is linked to Myt1 deactivation on the one hand, and the activation of Cdc25 on the other, as blocking either pathway compromised G2-M transition in perch oocytes.