Dysregulation of Cdc25B in PDE3 mice oocytes

The molecular machinery controlling the meiotic, as well as mitotic, cell cycle is centered around the regulation of the activity of MPF, a complex composed of catalytic Cdc2 and the cyclin B regulatory subunit which triggers germinal vesicle breakdown and reentry into the meiotic cell cycle. MPF activity depends on the balance between Wee Kinase activity and cdc25B activity. Wee kinase phosphorylates and inactivates Cdc2. Conversely, Cdc25B dephosphorylates and activates cdc2. Although high levels of cAMP and protein kinase A (PKA) play a critical role in maintaining inactive Cdc2, the steps downstream of PKA in the oocyte remain unknown. In cultured PDE3A knock out (KO) oocytes (arrested at G2 stage), activities of MPF, Wee kinase and Cdc25B, and amounts of cyclin B1, Cdc2 and phospho‐Cdc2(Thr14/Tyr15) were assessed. Consistent with our earlier data, during incubation of oocytes MPF activity increased in wild type (WT), but not in KO, oocytes. During a 4 hr incubation, Wee kinase activity decreased both in WT and KO oocytes, and Cdc25B activity increased in WT, but not in KO, oocytes. Whereas, during this time period, immunoreactive cyclin B1 increased in WT oocytes, in KO oocytes, it increased for 2 hours, and then significantly decreased. Phospho‐Cdc2(Thr14/Tyr15) was decreased in WT, but not in KO oocytes. Our data indicate that Cdc25B, not Wee kinase, maybe a critical PKA substrate that directly or indirectly blocks MPF activity, and thereby induces arrest of oocytes at G2 stage. Since MPF, which could affect Cyclin B degradation via APC activation, is not activated in KO oocytes, other mechanisms for regulation of cyclin B degradation will be assessed.