The Mechanism Of Translational Regulation Of Maternal MRNA In Mouse Oocvte Meiosis

During the process of mammalian oogenesis,active transcription and translation events accumulate a large amount of maternal mRNAs and proteins for oocytes.The cytoplasmic polyadenylation and translational activation of these maternal mRNAs drive the process of meiosis.Cytoplasmic polyadenylation is regulated by multiple proteins,including poly(A)polymerase(PAP),splicing and polyadenylation specific factors(CPSF).During the development of mouse oocytes,different maternal mRNAs are not simultaneously polyadenylated and translated into proteins.The spatiotemporal specificity of cytoplasmic polyadenylation is mediated by the cis-elements on the3?-UTR of maternal mRNAs,mainly including polyadenylation signal(PAS)and cytoplasmic polyadenylation element(CPE),which bind to CPSF complex and CPE binding protein(CPEB)respectively.For a long time,people have not been very clear about the spatiotemporal specific regulation of mRNA cytoplasmic polyadenylation during oocyte meiosis and the poly(A)polymerase that regulates this process.In this study,I used the 3?-UTR of mouse Cpeb1,Btg4 and Cnot6 l mRNA to decipher the combinatorial code that regulates mRNA translation at specific developmental stages during meiotic maturation:(i)Translation of maternal transcripts in the GV(Germinal vesical)period requires more than one PASs far away from the CPEs;(ii)The distal and proximal PASs to the 3?-end of the transcript,they can effectively regulate the translation in the GV period,as long as they are not close to CPEs;(iii)Translational inhibition in the GV stage and translational activation after GVBD(Germinal vesical breakdown)require at least one PASs adjacent to the CPEs;(iv)The number and location of the CPEs adjacent to the PASs on the 3?-UTR of a specific transcript determines its repression efficiency in GV stage oocytes.These studies revealed a previously undiscovered mechanism,namely how the proximal PAS regulates polyadenylation and translation at the 3?-end of the maternal transcript.In addition,I also revealed that PAP? is the elusive poly(A)polymerase that regulates mRNA cytoplasmic polyadenylation during mouse oocyte maturation.My study found that PAP? is mainly located in the nucleus at the GV stage,but dispersed in the cytoplasm after GVBD.Inhibition of PAP? activity will impair the polyadenylation and translation of cytoplasmic mRNA,thereby impeding the progression of the meiotic cell cycle.After the oocyte resumes meiosis,the activated CDK1 and ERK1/2 jointly regulate the phosphorylation of the three serine residues(S537,S545 and S558)of PAP?,which leads to an increase in PAP? activity.This mechanism enables translational activation of transcripts without CPE on 3?-UTR.After GVBD,activated PAP? can also stimulate the polyadenylation and translation of Papola mRNA through a positive feedback pathway.In addition,ERK1/2 can also promote Papola mRNA translation in a CPE-dependent manner.Through these mechanisms,the activity and protein levels of PAP? are significantly enhanced,thereby increasing the global level of mRNA polyadenylation and translation,which is conducive to the normal progression of the meiotic cell cycle.