The neural stem cell renewal factor, Musashi, plays a critical role in cell cycle progression

Meiotic progression of oocytes in Xenopus laevis requires a strictly regulated temporal order of maternal mRNA translation. This timed translation of the mRNAs encoding cell cycle control proteins is mediated by elements in the 3'UTR (untranslated region). A recent study (Pique et al. (Cell 132:434; 2008)) has proposed that this timing is dictated strictly through various combinations of cytoplasmic polyadenylation elements (CPEs) within the 3'UTR and can be classified as cdc2-independent "early" (eg. Mos) or cdc2-dependent "late" (eg. cyclin B1). However, we have previously demonstrated that the early translational activation of the Mos mRNA is independent of CPE sequences and rather requires a 3' UTR Musashi-binding element (MBE). In this study, we reiterate and extend the requirements for MBE sequences and the Musashi protein in early maternal mRNA translational activation. We report that antisense oligonucleotide mediated knockdown of endogenous Musashi protein synthesis blocks oocyte cell cycle progression. Notably, knockdown of Musashi also prevents CPE-dependent mRNA translation, although Musashi does not bind the CPE itself. In addition we have employed mass spectrometry to identify a progesterone-dependent phosphorylation site of Musashi. Musashi phosphorylation is an early event and occurs coincident with the early activation of Mos mRNA. These analyses demonstrate that a CPE-dependent model is incomplete in describing control of early class mRNA translation and we propose a more comprehensive model wherein a hierarchy of distinct regulatory pathways enforces the temporal order of mRNA translational activation during Xenopus oocyte maturation. In this model, Musashi mediates the progesterone "trigger" pathway and activation of early class mRNAs, including the Mos mRNA, resulting in the subsequent activation of mRNAs governed by a later-acting CPE combinatorial code.