Posttranscriptional regulation of maternal Pou5f1/Oct4 during mouse oogenesis and early embryogenesis

Protein syntheses at appropriate timings are important for promoting diverse biological processes and are controlled at the levels of transcription and translation. Pou5f1/Oct4 is a transcription factor that is essential for vertebrate embryonic development. However, the precise timings when the mRNA and protein of Pou5f1/Oct4 are expressed during oogenesis and early stages of embryogenesis remain unclear. We analyzed the expression patterns of mRNA and protein of Pou5f1/Oct4 in mouse oocytes and embryos by using a highly sensitive in situ hybridization method and a monoclonal antibody specific to Pou5f1/Oct4, respectively. Pou5f1/Oct4 mRNA was detected in growing oocytes from the primary follicle stage to the fully grown GV stage during oogenesis. In contrast, Pou5f1/Oct4 protein was undetectable during oogenesis, oocyte maturation and the first cleavage stage but subsequently became detectable in the nuclei of early 2-cell-stage embryos. Pou5f1/Oct4 protein at this stage was synthesized from maternal mRNAs stored in oocytes. The amount of Pou5f1/Oct4 mRNA in the polysomal fraction was small in GV-stage oocytes but was significantly increased in fertilized eggs. Taken together, our results indicate that the synthesis of Pou5f1/Oct4 protein during oogenesis and early stages of embryogenesis is controlled at the level of translation and suggest that precise control of the amount of this protein by translational regulation is important for oocyte development and early embryonic development.