Kinterova, V. | bioRxiv Science

bioRxiv2021

Physiologically relevant miRNAs in mammalian oocytes are rare and highly abundant.

miRNAs, ~22nt small RNAs associated with Argonaute (AGO) proteins, are important negative regulators of gene expression in mammalian cells. However, mammalian maternal miRNAs show negligible repressive activity and the miRNA pathway is dispensable for oocytes and maternal-to-zygotic transition. The stoichiometric hypothesis proposed that this is caused by dilution of maternal miRNAs during oocyte growth. As the dilution affects miRNAs but not mRNAs, it creates unfavorable miRNA:mRNA stoichiometry for efficient repression of cognate mRNAs. Here we report that porcine ssc-miR-205 and bovine bta-miR-10b are exceptional miRNAs, which resist the diluting effect of oocyte growth and can efficiently suppress gene expression. Additional analysis of ssc-miR-205 shows that it has higher stability, reduces expression of endogenous targets, and contributes to porcine oocyte-to-embryo transition. Consistent with the stoichiometric hypothesis, our results show that the endogenous miRNA pathway in mammalian oocytes is intact and that maternal miRNAs can efficiently suppress gene expression when a favorable miRNA:mRNA stoichiometry is established.

molecular biology↗

bioRxiv2019

Low miRNA abundance disables microRNA pathway in mammalian oocytes

MicroRNAs (miRNAs) are ubiquitous small RNAs, which guide post-transcriptional gene repression in countless biological processes. However, the miRNA pathway in mouse oocytes appears inactive and is dispensable for development. We report that miRNAs do not accumulate like maternal mRNAs during oocyte growth. The most abundant miRNAs total tens of thousands of molecules in fully-grown oocytes, a number similar to that observed in much smaller fibroblasts. The lack of miRNA accumulation acts like miRNA knock-down, where miRNAs can engage their targets but are not abundant enough to produce significant silencing effect. Injection of 100,000 miRNAs was sufficient to restore reporter repression in oocytes, confirming that miRNA inactivity primarily stems from low miRNA abundance and not from an active repression of the miRNA pathway per se. Similar situation was observed in rat, hamster, porcine, and bovine oocytes arguing that miRNA inactivity is not a mouse-specific adaptation but a common mammalian oocyte feature.

molecular biology↗