Muscle specific deletion of the vitamin-D receptor in mice is associated with diaphragm muscle weakness

Abstract

Diseases or conditions where diaphragm muscle (DIAm) function is impaired, including chronic obstructive pulmonary disease, cachexia, asthma and aging are associated withincreased risks of pulmonary symptoms, longer duration of hospitalizations and increasing requirements for mechanical ventilation. Vitamin-D deficiency is associated with proximal muscle weakness that resolves following therapy with vitamin D3. Skeletal muscle expresses the vitamin D receptor (VDR), which responds to the active form of vitamin D, 1,25-dihydroxyvitamin D3 by altering gene expression in target cells. In knock-out mice without skeletal muscle VDRs, there is marked atrophy of muscle fibers and a change in skeletal muscle biochemistry. We used a tamoxifen-inducible skeletal muscle Cre recombinase in Vdrfl/fl mice (Vdrfl/fl actin.iCre+) to assess the role of muscle-specific VDR signaling on DIAm specific force, fatigability and fiber type-dependent morphology. Vdrfl/fl actin.iCre+ mice treated with vehicle and Vdrfl/fl mice treated with tamoxifen served as controls. 7 days following the final treatment, mice were euthanized, the DIAm was removed and isometric force and fatigue were assessed in DIAm strips using direct muscle stimulation. The proportion and cross-sectional areas of DIAm fiber types were evaluated by immunolabeling with myosin heavy chain antibodies differentiating type I, IIa and IIx and/or IIb fibers. We show that in mice with skeletal muscle-specific VDR deletion, maximum specific force and residual force following fatigue are impaired, along with a selective atrophy of type IIx and/or IIb fibers. These results show that the VDR has a significant biological effect on DIAm function independent of systemic effects on mineral metabolism.