Cell development relies on elaborate changes in gene expression in order to transition 38 through different phenotypic and functional stages that ultimately lead to terminal differentiation. 39Changes in gene expression are achieved through transcriptional and post-transcriptional 40 regulations. Although transcriptional regulation is understood in considerable detail 1, 2 , much less 41 is known about the molecular machinery involved in translation regulation. 42Large oligomeric complexes involving proteins and non-coding RNAs are assembled on 43 the mRNA 3 to regulate its interaction with ribosomes, its translation rate, and its stability 4, 5 . In 44 somatic cells, numerous observations indicate that translation is intimately coupled with 45 degradation of mRNAs 5, 6 . Proteins recruited on the mRNA interact with elements located 46 throughout the length of the transcript 3, 7 . However, complexes nucleated around the 5' and 3' 47 polyadenylation element-binding protein (CPEB) is considered a master regulator of 63 polyadenylation and translation 18,19 . Much less is known about the role of CPEB in mammalian 64 oocytes. Here, we have used a genome-wide approach to investigate the role of this RNA-binding 65 protein (RBP) during the transition from quiescence to re-entry into the meiosis. Through a 66 detailed time course, we have investigated the temporal association between maternal mRNA 67 translation and the different steps involved in oocyte re-entry into and progression through the 68 meiosis. Using a RiboTag/RNA-Seq strategy, we describe a genome-wide switch in the 69 translation program of maternal mRNAs, and define new, critical functions of CPEB in the control 70 of this switch. 71 4
Results
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Re-entry into meiosis coincides with rapid translational changes of stable mRNAs 73We have used a RiboTag/RNA-Seq strategy to characterize mRNA translation in oocytes 74 arrested at prophase I (GV) and in those undergoing meiotic maturation. (Fig. 1a). This strategy 75 has been previously validated 20, 21 and additional quality controls are reported (Supplementary 76 Fig. 1a-d). Upon meiotic resumption, there were both progressive increases and decreases in 77 ribosome loading of maternal mRNAs (Fig. 1b). By late metaphase I (MI), mRNAs were either 78 constitutively translated (n = 5580, CONSTITUTIVE), translationally repressed (n = 1092, 79 DOWN), or translationally activated (n = 871, UP) (FDR ≤ 0.05 and -1 ≥ log2(FC) (LFC) ≥ 1, 80 Supplementary Fig. 1e). Total mRNA levels remained stable up to MI and significant 81 destabilization was only detectable for 3% of maternal mRNAs at late MI ( Fig. 1b and 82 Supplementary Fig. 1f). Comparison of changes in total mRNA levels (transcriptome) to changes 83 in ribosome-associated mRNA levels (translatome) confirms this late MI destabilization (Fig. 1c), 84 which became prominent later during MII arrest, with a subset of mRNAs remaining stable even 85 if their translation was repressed. Changes in translation that initiate during MI were extended into 86 and amplified ...