Introduction

MicroRNAs (miRNAs) represent an important class of short natural RNAs that act as post-transcriptional regulators of gene expression. Genetic studies in Caenorhabditis elegans and Drosophila revealed that miRNAs are involved in fine tuning the spatial and temporal regulation of developmental events, including precursor cell proliferation, differentiation and programmed death (Ambros, 2003; Brennecke et al., 2003; Sempere et al., 2003, Xu et al., 2003; Biemar et al., 2005). MiRNAs have been found essentially in every cell type analyzed to date. A recent systematic analysis of spatial expression of miRNA in developing zebrafish embryos showed that most tissues have a unique time-dependent pattern of miRNA expression (Wienholds et al., 2005). In silico methods predicted that the individual miRNAs have, on average, hundreds of target mRNAs, suggesting that miRNAs have enormous regulatory roles in different genetic programs (Lewis et al., 2003; Brennecke et al., 2005; Krek et al., 2005; Xie et al., 2005). However, the number of functional miRNA/target pairs experimentally characterized to date is minimal.

We have addressed the function of miRNAs in mammalian skeletal muscle. Muscle formation (Figure 30.1) involves the proliferation of myoblast precursor cells, which subsequently exit from the cell cycle and enter a terminal differentiation program that includes myoblast fusion into large multi-nucleated cells (myotubes) and expression of muscle specific markers such as myosin heavy chain (MHC) and muscle creatine kinase (MCK) (Figure 30.1). Differentiation can be recapitulated in ex vivo models, using either totipotent ES cells directed toward the muscle lineage (Dinsmore et al., 1998), or established myoblast cell lines that by default enter the skeletal muscle differentiation pathway when they are deprived of growth factors (Bains et al., 1984).