Ringo/Cyclin-dependent Kinase and Mitogen-activated Protein Kinase Signaling Pathways Regulate the Activity of the Cell Fate Determinant Musashi to Promote Cell Cycle Re-entry in Xenopus Oocytes

Karthik Arumugam,Melanie C Macnicol,Yiying Wang,Chad E Cragle,Alan J Tackett,Linda L Hardy,Angus M Macnicol

doi:10.1074/jbc.m111.300681

Abstract

Cell cycle re-entry during vertebrate oocyte maturation is mediated through translational activation of select target mRNAs, culminating in the activation of mitogen-activated protein kinase and cyclin B/cyclin-dependent kinase (CDK) signaling. The temporal order of targeted mRNA translation is crucial for cell cycle progression and is determined by the timing of activation of distinct mRNA-binding proteins. We have previously shown in oocytes from Xenopus laevis that the mRNA-binding protein Musashi targets translational activation of early class mRNAs including the mRNA encoding the Mos proto-oncogene. However, the molecular mechanism by which Musashi function is activated is unknown. We report here that activation of Musashi1 is mediated by Ringo/CDK signaling, revealing a novel role for early Ringo/CDK function. Interestingly, Musashi1 activation is subsequently sustained through mitogen-activated protein kinase signaling, the downstream effector of Mos mRNA translation, thus establishing a positive feedback loop to amplify Musashi function. The identified regulatory sites are present in mammalian Musashi proteins, and our data suggest that phosphorylation may represent an evolutionarily conserved mechanism to control Musashi-dependent target mRNA translation.

Highlights

The mechanisms that regulate the activity of the mRNA translational regulator, Musashi, are unknown
In this study we present evidence that Musashi functions downstream of progesterone-stimulated Ringo/cyclin-dependent kinase (CDK) activation and upstream of MAP kinase and MPF signaling in the promotion of oocyte cell cycle reentry (Fig. 4C)
Our findings further suggest the action of a signal amplification step, where progesterone-stimulated translation of the Musashi-dependent Mos mRNA and subsequent activation of MAP kinase generates a positive feedback loop in which MAP kinase serves to induce additional Musashi activation

Summary

Background

The mechanisms that regulate the activity of the mRNA translational regulator, Musashi, are unknown. Musashi-dependent translation of the early class Mos mRNA results in MAP kinase activation and subsequent CPEB-mediated, late class mRNA translation The sequential function of Musashi- and CPEB-dependent translational control promotes and maintains M-phase promoting factor activity (MPF, cyclin B/CDK) and cell cycle progression up to metaphase of meiosis II [9]. An immediate early response to progesterone stimulation is the inactivation of Pumilio as a repressor protein and translation of the Pumilio target mRNA encoding Ringo/Speedy, an atypical activator of cyclin-dependent kinases 1 and 2 (Cdk and Cdk2) (8, 14 –16). We demonstrate that Musashi phosphorylation and activation is initially mediated through Ringo/CDK signaling and is subsequently amplified through MAP kinase signaling as the oocytes progress through the meiotic cell cycle. We further show that mammalian Musashi undergoes regulated phosphorylation and that a phospho-mimetic mutant of mammalian Musashi promotes translation of a Musashi target mRNA in NIH3T3 cells

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION