Expression of Alternative Ago2 Isoform Associated with Loss of microRNA-Driven Translational Repression in Mouse Oocytes

Mouse oocyte maturation, fertilization, and reprogramming occur in the absence of transcription, and thus, changes in mRNA levels and translation rate are regulated through post-transcriptional mechanisms [1]. Surprisingly, microRNA function, which is a major form of post-transcriptional regulation, is absent during this critical period of mammalian development [2, 3]. Here, we investigated the mechanisms underlying the global suppression of microRNA activity. In both mouse and frogs, microRNA function was active in growing oocytes but then absent during oocyte maturation. RNA sequencing (RNA-seq) of mouseoocytes uncovered that the microRNA effector protein AGO2 is predominantly expressed as an alternative isoform that encodes a truncated protein lacking all of the known essential domains. Full-length Ago2 as well as the related Argonautes (Ago1, Ago3, and Ago4) were lowly expressed in maturing mouse oocytes. Reintroduction of full-length AGO2 together with an exogenous microRNA in either mouse or frog oocytes restored translational repression of a target reporter. However, levels of endogenous transcripts remained unchanged. Consistent with a lack of microRNA activity, analysis of transcripts with alternative polyadenylation sites showed increased stability of transcripts with a longer 30 UTR during oocyte maturation. Redundant mechanisms protecting endogenous transcripts and the conserved loss of microRNA activity suggest a strong selection for suppressing microRNA function in vertebrate oocytes.