The mechanisms of skeletal muscle atrophy in response to transient knockdown of the vitamin D receptor in vivo

Objective Vitamin‐D deficiency is estimated to affect ∼40% of the world's population and has been associated with impaired muscle maintenance. Vitamin‐D exerts its actions through the Vitamin‐D‐receptor (VDR), the expression of which was recently confirmed in skeletal muscle, and its down‐regulation is linked to reduced muscle mass and functional decline. To identify potential mechanisms underlying muscle atrophy, we studied the impact of VDR knockdown (KD) on mature skeletal muscle in vivo, and myogenic regulation in vitro in C2C12 cells. Methods Male Wistar rats underwent in vivo electrotransfer (IVE) to knock down the VDR in hind‐limb tibialis anterior (TA) muscle for 10 days. Comprehensive metabolic and physiological analysis was undertaken to define the influence loss of the VDR on muscle fibre composition, protein synthesis, anabolic and catabolic signalling, mitochondrial phenotype, and gene expression. Finally, in vitro lentiviral transfection was used to induce sustained VDR‐KD in C2C12 cells to analyse myogenic regulation. Results Muscle VDR‐KD elicited atrophy through a reduction in total protein content, resulting in lower myofibre area. Activation of autophagic processes was observed, with no effect upon muscle protein synthesis or anabolic signalling. Furthermore, RNA‐Seq analysis identified systematic down‐regulation of multiple mitochondrial respiration related protein and genesets. Finally, in vitro VDR‐knockdown impaired myogenesis (cell cycling, differentiation and myotube formation). Conclusion Taken together, these data indicate a fundamental regulatory role of the VDR in the regulation of myogenesis and muscle mass; whereby it acts to maintain muscle mitochondrial function and limit autophagy. Joseph Bass completed his PhD in Medicine and Health in 2017 at The University of Nottingham, where he is currently a Research Fellow. Joe is interested in examining the mechanistic regulation of musculoskeletal health, particularly factors impacting muscle atrophy susceptibility.