A healthy and independent life requires skeletal muscles to maintain optimal function throughout the lifespan, which is in turn dependent on efficient activation of processes that regulate muscle development, homeostasis, and metabolism. Thus, identifying mechanisms that modulate these processes is of crucial priority. Noncoding RNAs (ncRNAs), including microRNAs (miRNAs) and long noncoding RNAs (lncRNAs), have emerged as a class of previously unrecognized transcripts whose importance in a wide range of biological processes and human disease is only starting to be appreciated. In this review, we summarize the roles of recently identified miRNAs and lncRNAs during skeletal muscle development and pathophysiology. We also discuss several molecular mechanisms of these noncoding RNAs. Undoubtedly, further systematic understanding of these noncoding RNAs' functions and mechanisms will not only greatly expand our knowledge of basic skeletal muscle biology, but also significantly facilitate the development of therapies for various muscle diseases, such as muscular dystrophies, cachexia, and sarcopenia.

Cardiomyopathy is associated with altered expression of genes encoding contractile proteins. Here we show that Trbp (Tarbp2), an RNA-binding protein, is required for normal heart function. Cardiac-specific inactivation in mice of Trbp (Trbp(cKO)) caused progressive cardiomyopathy and lethal heart failure. Loss of Trbp function resulted in upregulation of Sox6, repression of genes encoding normal cardiac slow-twitch myofiber proteins and pathologically increased expression of genes encoding skeletal fast-twitch myofiber proteins. Remarkably, knockdown of Sox6 fully rescued the Trbp-mutant phenotype, whereas mice overexpressing Sox6phenocopied Trbp(cKO) mice. Trbp inactivation was mechanistically linked to Sox6 upregulation through altered processing of miR-208a, which is a direct inhibitor of Sox6. Transgenic overexpression of Mir208a sufficiently repressed Sox6, restored the balance in gene expression for fast- and slow-twitch myofiber proteins, and rescued cardiac function in Trbp(cKO) mice. Together, our studies identify a new Trbp-mediated microRNA-processing mechanism in the regulation of a linear genetic cascade essential for normal heartfunction.

GATA4 and GATA6 are central cardiac transcriptional regulators. The postnatal, stage-specific function of the cardiac transcription factors GATA4 and GATA6 have not been evaluated. In part, this is because current Cre-loxP approaches to cardiac gene inactivation require time consuming and costly breeding of Cre-expressing and "floxed" mouse lines, often with limited control of the extent or timing of gene inactivation. We investigated the stage-specific functions of GATA4 and GATA6 in the postnatal heart by using adeno-associated virus serotype 9 to control the timing and extent of gene inactivation by Cre. Systemic delivery of recombinant, adeno-associated virus 9 (AAV9) expressing Cre from the cardiac specific Tnnt2 promoter was well tolerated and selectively and efficiently recombined floxed target genes in cardiomyocytes. AAV9:Tnnt2-Cre efficiently inactivated Gata4 and Gata6. Neonatal Gata4/6inactivation caused severe, rapidly lethal systolic heart failure. In contrast, Gata4/6 inactivation in adult heart caused only mild systolic dysfunction but severe diastolic dysfunction. Reducing the dose of AAV9:Tnnt2-Cre generated mosaics in which scattered cardiomyocytes lacked Gata4/6. This mosaic knockout revealed that Gata4/6 are required cell autonomously for physiological cardiomyocyte growth. Our results define novel roles of GATA4 and GATA6 in the neonatal and adult heart. Furthermore, our data demonstrate that evaluation of gene function hinges on controlling the timing and extent of gene inactivation. AAV9:Tnnt2-Cre is a powerful tool for controlling these parameters.

The core functional machinery of the RNAi pathway is the RNA-induced silencing complex (RISC), wherein Argonaute2 (Ago2) is essential for siRNA-directed endonuclease activity and RNAi/microRNA-mediated gene silencing. Crystallin-αB (CryAB) is a small heat shock protein involved in preventing protein aggregation. We demonstrate that CryAB interacts with the N and C termini of Ago2, not the catalytic site defined by the convergence of the PAZ, MID, and PIWI domains. We further demonstrate significantly reduced Ago2 activity in the absence of CryAB, highlighting a novel role of CryAB in the mammalian RNAi/microRNA pathway. In skeletal muscle of CryAB null mice, we observe a shift in the hypertrophy-atrophy signaling axis toward atrophy under basal conditions. Moreover, loss of CryAB altered the capability of satellite cells to regenerate skeletal muscle. These studies establish that CryAB is necessary for normal Ago2/RISC activity and cellular homeostasis in skeletal muscle.