Multiple mechanisms of global microRNA suppression in vertebrate oocytes [RNA-seq]

Mouse oocyte maturation, fertilization, and reprogramming occur in the absence of transcription and thus must be regulated post-transcriptionally. Surprisingly, a major form of post-transcriptional regulation, microRNA-based transcript destabilization and translational inhibition, is lost during this developmental window. Here we evaluate the conservation, timing, and mechanism behind the loss of microRNA activity in oocytes. In both mouse and frogs, microRNA function was active in growing oocytes, but then lost during oocyte maturation. RNA-sequencing of the maturing oocytes uncovered expression of an alternative isoform of Ago2 lacking domains critical for its function. Introduction of full-length Ago2 together with an exogenous microRNA destabilized microRNA luciferase reporters. However, endogenous targets were still largely unaffected. These findings suggest that while it is possible to re-activate some aspects of microRNA activity by introducing full length Ago2, there are additional mechanisms to protect endogenous transcripts from microRNA activity in oocytes.