MicroRNA Directly Enhances Mitochondrial Translation during Muscle Differentiation

MicroRNAs are well known to mediate translational repression and mRNA degradation in the cytoplasm. Various microRNAs have also been detected in membrane-compartmentalized organelles, but the functional significance has remained elusive. Here, we report that miR-1, a microRNA specifically induced during myogenesis, efficiently enters the mitochondria where it unexpectedly stimulates, rather than represses, the translation of specific mitochondrial genome-encoded transcripts. We show that this positive effect requires specific miR:mRNA base-pairing and Ago2, but not its functional partner GW182, which is excluded from the mitochondria. We provide evidence for the direct action of Ago2 in mitochondrial translation by crosslinking immuno-precipitation coupled with deep sequencing (CLIP-seq), functional rescue with mitochondria-targeted Ago2, and selective inhibition of the microRNA machinery in the cytoplasm. These findings unveil a positive function of microRNA in mitochondrial translation and suggest a highly coordinated myogenic program via miR-1-mediated translational stimulation in the mitochondria and repression in the cytoplasm.