Myogenic factors that regulate expression of muscle-specific microRNAs.

Since their discovery as key regulators of early animal development, microRNAs now are recognized as widespread regulators of gene expression. Despite their abundance, little is known regarding the regulation of microRNA biogenesis. We show that three highly conserved muscle-specific microRNAs, miR-1, miR-133 and miR-206, are robustly induced during the myoblast-myotube transition, both in primary human myoblasts and in the mouse mesenchymal C2C12 stem cell line. These microRNAs were not induced during osteogenic conversion of C2C12 cells. Moreover, both loci encoding miR-1, miR-1-1, and miR-1-2, and two of the three encoding miR-133, miR-133a-1 and miR-133a-2, are strongly induced during myogenesis. Some of the induced microRNAs are in intergenic regions, whereas two are transcribed in the opposite direction to the nonmuscle-specific gene in which they are embedded. By using CHIP analysis, we demonstrate that the myogenic factors Myogenin and MyoD bind to regions upstream of these microRNAs and, therefore, are likely to regulate their expression. Because miR-1 and miR-206 are predicted to repress similar mRNA targets, our work suggests that induction of these microRNAs is important in regulating the expression of muscle-specific proteins.