miR-206 and-486 Induce Myoblast Differentiation by

The Pax7 transcription factor is required for muscle satellite cell biogenesis and specification of the myogenic precursor lineage. Pax7 is expressed in proliferating myoblasts but is rapidly downregulated during differentiation. Here we report that miR-206 and-486 are induced during myoblast differentiation and downregulate Pax7 by directly targeting its 3 untranslated region (UTR). Expression of either of these microRNAs in myoblasts accelerates differentiation, whereas inhibition of these microRNAs causes persistence of Pax7 protein and delays differentiation. A microRNA-resistant form of Pax7 is sufficient to inhibit differ-entiation. Since both these microRNAs are induced by MyoD and since Pax7 promotes the expression of Id2, an inhibitor of MyoD, our results revealed a bistable switch that exists either in a Pax7-driven myoblast state or a MyoD-driven myotube state. Skeletal muscle is largely composed of multinucleated mus-cle fibers. Postnatal growth or the regeneration capacity of adult skeletal muscle is dependent on satellite cells (15, 36). Satellite cells give rise to myoblast cells that undergo multiple rounds of division before terminal differentiation. Skeletal muscle development during embryogenesis and during regen