Enforcing temporal control of maternal mRNA translation during oocyte cell-cycle progression

Abstract

Meiotic cell-cycle progression in progesterone-stimulated Xenopus oocytes requires that the translation of pre-existing maternal mRNAs occur in a strict temporal order. Timing of translation is regulated through elements within the mRNA 3 0 untranslated region (3 0 UTR), which respond to cell cycle-dependant signalling. One element that has been previously implicated in the temporal control of mRNA translation is the cytoplasmic polyadenylation element (CPE). In this study, we show that the CPE does not direct early mRNA translation. Rather, early translation is directed through specific early factors, including the Musashi-binding element (MBE) and the MBE-binding protein, Musashi. Our findings indicate that although the cyclin B5 3 0 UTR contains both CPEs and an MBE, the MBE is the critical regulator of early translation. The cyclin B2 3 0 UTR contains CPEs, but lacks an MBE and is translationally activated late in maturation. Finally, utilizing antisense oligonucleotides to attenuate endogenous Musashi synthesis, we show that Musashi is critical for the initiation of early class mRNA translation and for the subsequent activation of CPE-dependant mRNA translation.

Context-dependent regulation of Musashi-mediated mRNA translation and cell cycle regulation.

“…Thus, during early phases of neuronal differentiation, Musashi exerts stimulusdependent activation of target mRNAs, similar to the translational activation of Musashi target mRNAs during Xenopus oocyte maturation. 20,21 The Fluc reporter mRNAs were expressed at equivalent levels (Fig. 4B).…”

“…Recent findings have demonstrated that Musashi can unexpectedly act as an mRNA translational activator, rather than a translational repressor, in promotion of cell cycle progression in Xenopus oocytes. 20,21 This study attempts to reconcile these apparently opposing roles for Musashi function in mRNA translational control during cell cycle progression and previews areas of promising research into the mechanism and regulation of Musashi function. 29 As a consequence, Musashi is thought to disrupt recruitment of the large ribosomal subunit and prevent target mRNA translation.…”

“…We have previously demonstrated that this knockdown strategy caused failure to polyadenylate and subsequently translate Musashi target mRNAs and that both target mRNA polyadenylation (a requirement for translational activation) and cell cycle progression can be rescued by ectopic expression of wild-type Xenopus Musashi1 protein. 20 Expression of mouse Musashi1 rescued cell cycle progression ( Fig. 2A) and polyadenylation of the Musashi-regulated cyclin B5 mRNA (Fig.…”

“…20,21 Inhibition of Musashi function in oocytes demonstrates that Musashi is a master regulator of oocyte maternal mRNA translational recruitment and functions upstream of subsequent mRNA translation controlled by cytoplasmic polyadenylation elements (CPEs) and the CPE binding protein, CPEB. 28 Thus, in stem cells and oocytes Musashi acts by exerting opposite control on target mRNA translation to promote cell cycle progression.…”

Translational regulation of the cell cycle: when, where, how and why?

“…Pum2 exerts its effects at least in part by affecting the interaction between the RNA-binding protein DAZL (deleted in azoospermia-like) and the embryonic PABP, but it also may affect polyadenylation [31]. Another translational regulator, Musashi, is implicated in promoting the translation of Mos and Cyclin B5 by binding to the MBE (Musashi-binding element) and acting together with CPE to influence polyadenylation [94,99,100].…”