Exercise, Skeletal Muscle and Circulating microRNAs.

Regular exercise stimulates numerous structural, metabolic, and morphological adaptations in skeletal muscle. These adaptations are vital to maintain human health over the life span. Exercise is therefore seen as a primary intervention to reduce the risk of chronic disease. Advances in molecular biology, biochemistry, and bioinformatics, combined with exercise physiology, have identified many key signaling pathways as well as transcriptional and translational processes responsible for exercise-induced adaptations. Noncoding RNAs, and specifically microRNAs (miRNAs), constitute a new regulatory component that may play a role in these adaptations. The short single-stranded miRNA sequences bind to the 3' untranslated region of specific messenger RNAs (mRNAs) on the basis of sequence homology. This results in the degradation of the target mRNA or the inhibition of protein translation causing repression of the corresponding protein. While tissue specificity or enrichment of certain miRNAs makes them ideal targets to manipulate and understand tissue development, function, health, and disease, other miRNAs are ubiquitously expressed; however, it is uncertain whether their mRNA/protein targets are conserved across different tissues. miRNAs are stable in plasma and serum and their altered circulating expression levels in disease conditions may provide important biomarker information. The emerging research into the role that miRNAs play in exercise-induced adaptations has predominantly focused on the miRNA species that are regulated in skeletal muscle or in circulation. This chapter provides an overview of these current research findings, highlights the strengths and weaknesses identified to date, and suggests where the exercise-miRNA field may move into the future.