Regulation of Oocyte Meiotic Maturation: Unraveling the Interplay between PKA Inhibition and Cdk1 Activation

Oocyte meiosis is arrested at the first prophase stage, until hormonal stimulation triggers progression into the meiotic divisions. This process, called meiotic maturation, depends on extensive post-transcriptional events. In all vertebrates, two bottleneck events orchestrate meiosis resumption: first, the inhibition of PKA and, second, the activation of Cdk1, the master regulator of eukaryotic cell division. However, the molecular events occurring between these two steps are almost unknown. To address this issue, we took advantage of a Cdk1 inhibitor to identify the early events that depend on PKA downregulation and occur independently of Cdk1 activity. Unexpectedly, we show that accumulation of Cyclin B1 and Mos, the kinase responsible for MAPK activation in oocytes, are regulated in an opposing manner by a two-step mechanism. PKA downregulation induces first the accumulation of Cyclin B1 without any increase of its translation, independently of Cdk1 activation. Subsequently, the rate of Cyclin B1 translation increases in response to Cdk1 activation. In contrast, Mos translation begins downstream PKA inhibition, but the protein does not accumulate until Cdk1 is activated. These intertwined regulations create the positive feedback loops required for the full activation of Cdk1. Additionally, we show that two consecutive waves of translation occur during the G2-M transition, the first induced by PKA inhibition and the second by Cdk1 activation. Finally, we demonstrate that Arpp19, the only known early substrate of PKA in Xenopus oocytes, is not involved in the control of these early events. This study reveals that PKA downregulation promotes multiple molecular pathways that converge on the activation of Cdk1 to induce the G2/M transition in vertebrate oocytes.