MicroRNA function in muscle development

MicroRNAs (miRNAs) are a class of non-coding RNAs of [approximately]22 nucleotides in length that post-transcriptionally regulate gene expression. While there are more than 600 miRNAs identified in human genome, the biological functions of miRNAs are largely unknown. Here we show that microRNA-1 (miR-1) and microRNA-133 (miR-133) are specifically expressed in cardiac and skeletal muscle. Paradoxically, miR-1 and miR-133 exert opposing effects during skeletal muscle development in vitro and in vivo. miR-1 promotes myogenesis by targeting histone deacetylase 4 (HDAC4), a signal dependent chromatin regulator that represses MEF2 activity. MEF2, in turn, potently activates miR-1/-133 expression. In contrast, miR-133 enhances myoblast proliferation by repressing serum response factor (SRF), an essential regulator for muscle proliferation and differentiaiton. Together, these findings suggest that miR-1 and miR-133 are integrated into existing genetic circuits that control skeletal muscle development. We address the function of miRNAs during heart development using mouse genetic approaches. Cardiac-specific deletion of Dicer, a RNase III endonuclease responsible for miRNA maturation, leads to rapidly progressive dilated cardiomyopathy (DCM), heart failure, and postnatal lethality. Dicer mutant mice show misexpression of cardiac contractile proteins and profound sarcomere disarray. Functional analyses indicate significantly reduced heart rates and decreased fractional shortening of Dicer mutant hearts. Consistent with the role of Dicer in animal hearts, Dicer expression was decreased in end-stage human DCM and failing hearts and, most importantly, a significant increase of Dicer expression was observed in those hearts after left ventricle assist devices were inserted to improve cardiac function. Together, our studies demonstrate essential roles of Dicer in cardiac contraction and indicate that miRNAs play critical functions in normal cardiac development and function To examine the potential involvement of miRNAs in muscle stem cells, known as satellite cells, we examined miRNA expression profiles during satellite cell differentiation and skeletal muscle regeneration. miR-1 and its isoform, miR-206, are sharply up-regulated during satellite cell differentiation and down-regulated in muscle injury. miR-1 and -206 facilitate satellite cell differentiation by restricting satellite cell proliferative potential. We identify Pax7, an essential stem cell maintenance gene of satellite cells, as one of direct regulatory targets of miR-1 and -206. Knock down in vivo using antagomirs specifically against miR-1 and -206 in neonatal mouse skeletal muscle enhances satellite cell proliferation and Pax7 protein levels. Conversly, sustained Pax7 expression due to the loss of miR-1 and -206 repressive elements at Pax7 3' UTR inhibits myogenic progenitor differentiation. Our studies reveal a critical role of miR-1 and -206 in satellite cells and suggest that miRNAs participate in a regulatory circuit that allows rapid gene program transiton from cell proliferation to differentiation by repressing the expression of stem cell maintenance gene