Maternal MRNAs With Distinct 3'UTRs Define The Temporal Pattern Of Ccnb1 Synthesis During Mouse Oocyte Meiotic Maturation

In fully grown mammalian oocytes,de novo mRNA synthesis is silenced,with gene expression driven by a program of selective unmasking,translation,and degradation of maternal mRNAs.Regulated translation of maternal mRNAs accumulated in the growth phase is essential for the progression of oocyte meiotic maturation.After reentry into meiosis,de novo cyclin B1 synthesis is thought to be indispensable for maintaining MPF activity,for chromosome condensation and spindle assembly,and for the aditional steps leading to development of the oocyte.In silico analysis of cyclin B1 3'UTR(3' untranslated region)suggests the presence of alternative polyadenylation signals.The presence of Ccnbl transcripts with distinct 3'UTR was confirmed by mapping RiboTag IP-RNAseq reads and EST(expressed sequence tag)available respectively to the full-length 3'UTR of cyclin B1.Anchored PCR has also shown that three isoforms of cyclin B1 mRNAs are transcribed in mouse oocytes,which differ in the length of their 3'UTR.Functional analysis of these different transcripts further confirms that they are differentially recruited to the polysomes and account for the time course of cyclin B1 protein accumulation.Since the Ccnbl translation is dependent on the RNA binding protein CPEB1 expressed in the oocyte,we analyzed the contribution of the different CPEs to the differential regulation of the three transcripts by mutagenesis.Here we demonstrate that the timing of Ccnbl mRNA translation in mouse oocytes is dependent on the presence of transcripts with different 3' UTRs.This 3'UTR heterogeneity directs distinct temporal patterns of translational activation or repression.Inclusion or exclusion of cis-acting elements is responsible for these divergent regulations.Our findings reveal an additional layer of translation control through alternative polyadenylation usage required to fine-tune the timing of meiosis progression.The presence of multiple 3'UTRs driving Ccnbl mRNA translation is also likely important during mitosis,as the preli'minary anchored PCR shows the presence of multiple 3'UTRs also in mouse embryonic stem cells.Analyasis of RiboTag IP-RNAseq data reveals the presence of 2276 transcripts in the oocyte containing multiple PAS.This global analysis predicts the presence and differential ribosome association of mRNAs with multiple PASs in the oocyte.Thus,an additional layer of complexity controls the translation of mRNAs in mammals,which is a critical property for future investigation.