MSTN and chronic kidney disease: Repletion of 25(OH)D3 significantly decreased mRNA expression of negative regulators of skeletal muscle mass (IL-1β, IL-6 and TNF-α as well as Atrogin-1, Murf-1 and Myostatin), while increasing the expression of positive regulators of skeletal muscle mass (Myod, Myogenin and Pax7) relative to repletion of 1,25(OH)2D3 in CKD mice (Figure 5).