Dgcr8 knockout embryonic stem (ES) cells lack microprocessor activity and hence all canonical microRNAs (miRNAs). These cells proliferate slowly and accumulate in G1 phase of the cell cycle1. Here, by screening a comprehensive library of individual miRNAs in the background of the Dgcr8 knockout ES cells, we report that multiple ES cell-specific miRNAs, members of the miR-290 family, rescue the ES cell proliferation defect. Furthermore, rescued cells no longer accumulate in the G1 phase of the cell cycle. These miRNAs function by suppressing several key regulators of the G1/S transition. These results show that post-transcriptional regulation by miRNAs promotes the G1/S transition of the ES cell cycle enabling their rapid proliferation. Furthermore, our screening strategy provides an alternative and powerful approach for uncovering the role of individual miRNAs in biological processes as it overcomes the common problem of redundancy and saturation in the miRNA system.

Summary Dicer, which is required for the processing of both micro-RNAs (miRNAs) and small interfering RNAs (siRNAs), is essential for oocyte maturation [1, 2]. Oocytes express both miRNAs and endogenous siRNAs (endo-siRNAs) [3, 4]. To determine whether the abnormalities in Dicer knockout oocytes during meiotic maturation are secondary to the loss of endo-siRNAs and/or miRNAs, we deleted Dgcr8, which encodes an RNA-binding protein specifically required for miRNA processing. In striking contrast to Dicer, Dgcr8-deficient oocytes matured normally and, when fertilized with wild-type sperm, produced healthy-appearing offspring, even though miRNA levels were reduced to similar levels as Dicer-deficient oocytes. Furthermore, the deletion of both maternal and zygotic Dgcr8 alleles did not impair preimplantation development, including the determination of the inner cell mass and trophectoderm. Most surprisingly, the mRNA profiles of wild-type and Dgcr8 null oocytes were essentially identical, whereas Dicer null oocytes showed hundreds of misregulated transcripts. These findings show that miRNA function is globally suppressed during oocyte maturation and preimplantation development and that endo-siRNAs, rather than miRNAs, underlie the Dicer knockout phenotype in oocytes.

microRNAs (miRNAs) are important modulators of development. Owing to their ability to simultaneously silence hundreds of target genes, they have key roles in large-scale transcriptomic changes that occur during cell fate transitions. In somatic stem and progenitor cells — such as those involved in myogenesis, haematopoiesis, skin and neural development — miRNA function is carefully regulated to promote and stabilize cell fate choice. miRNAs are integrated within networks that form both positive and negative feedback loops. Their function is regulated at multiple levels, including transcription, biogenesis, stability, availability and/or number of target sites, as well as their cooperation with other miRNAs and RNA-binding proteins. Together, these regulatory mechanisms result in a refined molecular response that enables proper cellular differentiation and function.