Repression of AGO1 by AGO2 via let-7 microRNAs facilitates embryonic stem cell differentiation.

Argonaute (AGO) proteins are critical regulators of gene expression. Of the four AGOs in mammals, AGO1 and AGO2 are expressed in mouse embryonic stem cells (mESCs). These two proteins have opposing functions in controlling mESCs' fate decisions between pluripotency and differentiation. AGO2 promotes differentiation predominantly via the let-7 microRNAs, whereas AGO1 maintains pluripotency via modulating protein folding independent of small RNAs. These recent findings raise the question of whether and how these two AGOs are mutually regulated in mESCs. Here, using loss-of-function and gain-of-function approaches, we show that AGO2 represses the expression of mRNA via a conserved let-7-microRNA-binding site in its 3' UTR. Mutating this binding site at the endogenous locus abolishes the AGO2-mediated repression of mRNA and compromises the exit pluripotency of mESCs. These results indicate that the posttranscriptional regulation of AGO1 by AGO2 and let-7 microRNAs is important for stem cell differentiation, but also reveal a regulatory mechanism between the two AGO paralogs with opposing functions in controlling stem cell fate decisions.