MicroRNAs (miRNAs) are ubiquitous small RNAs guiding post-transcriptional gene repression in countless biological processes. However, the miRNA pathway in mouse oocytes appears inactive and dispensable for development. We propose that marginalization of the miRNA pathway activity stems from the constraints and adaptations of RNA metabolism elicited by the diluting effects of oocyte growth. We report that miRNAs do not accumulate like mRNAs during the oocyte growth because miRNA turnover has not adapted to it. The most abundant miRNAs total tens of thousands of molecules in growing (∅ 40 μm) and fully grown (∅ 80 μm) oocytes, a number similar to that observed in much smaller fibroblasts. The lack of miRNA accumulation results in a 100-fold lower miRNA concentration in fully grown oocytes than in somatic cells. This brings a knock-down-like effect, where diluted miRNAs engage targets but are not abundant enough for significant repression. Low-miRNA concentrations were observed in rat, hamster, porcine and bovine oocytes, arguing that miRNA inactivity is not mouse-specific but a common mammalian oocyte feature. Injection of 250,000 miRNA molecules was sufficient to restore reporter repression in mouse and porcine oocytes, suggesting that miRNA inactivity comes from low-miRNA abundance and not from some suppressor of the pathway.

• MeSH
• 3T3 Cells MeSH
• Species Specificity MeSH
• Cricetinae MeSH
• Rats MeSH
• Cells, Cultured MeSH
• RNA, Messenger genetics   metabolism   MeSH
• MicroRNAs genetics   metabolism   MeSH
• Mice, Inbred C57BL MeSH
• Mice MeSH
• Oocytes cytology   metabolism   MeSH
• Oogenesis * MeSH
• Swine MeSH
• Cattle MeSH
• Models, Theoretical MeSH
• Gene Expression Regulation, Developmental MeSH
• Animals MeSH
• Check Tag
• Cricetinae MeSH
• Rats MeSH
• Mice MeSH
• Cattle MeSH
• Female MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• RNA, Messenger MeSH
• MicroRNAs MeSH