Nuclear receptor/microRNA circuitry links muscle fiber type to energy metabolism

The mechanisms involved in the coordinate regulation of the metabolic and structural programs controlling muscle fitness and endurance are unknown. Recently, the nuclear receptor PPAR beta/delta was shown to activate muscle endurance programs in transgenic mice. In contrast, muscle-specific transgenic overexpression of the related nuclear receptor, PPAR alpha, results in reduced capacity for endurance exercise. We took advantage of the divergent actions of PPAR beta/delta and PPAR alpha to explore the downstream regulatory circuitry that orchestrates the programs linking muscle fiber type with energy metabolism. Our results indicate that, in addition to the well-established role in transcriptional control of muscle metabolic genes, PPAR beta/delta and PPAR alpha participate in programs that exert opposing actions upon the type I fiber program through a distinct muscle microRNA (miRNA) network, dependent on the actions of another nuclear receptor, estrogen-related receptor gamma (ERR gamma). Gain-of-function and loss-of-function strategies in mice, together with assessment of muscle biopsies from humans, demonstrated that type I muscle fiber proportion is increased via the stimulatory actions of ERR gamma on the expression of miR-499 and miR-208b. This nuclear receptor/miRNA regulatory circuit shows promise for the identification of therapeutic targets aimed at maintaining muscle fitness in a variety of chronic disease states, such as obesity, skeletal myopathies, and heart failure.