nNos localization, muscle function and atrophy in skeletal muscle disorders

Abstract

In skeletal muscle, loss of neuronal nitric oxide synthase (nNOS) from the sarcolemma has been observed in a few muscular dystrophies and myopathies. However, the extent of this phenomenon, its mechanism, and its physiological impact are not well understood. Using immunofluorescent staining for nNOS, a survey of 161 patient biopsies found absent or reduced sarcolemmal nNOS in 43% of patients. Patient mobility and muscle functional status correlated with nNOS mislocalization from the sarcolemma. Mouse models of inherited and acquired myopathies showed similar loss of sarcolemmal nNOS and impaired mobility and muscle function. A proteomic approach, using mass spectrometry and differentially labeled control and steroid-induced myopathy (SIM) mouse samples, found novel nNOS binding proteins including alpha-actinin-3 (ACTN3), which exhibited decreased interaction with nNOS after steroid treatment. It revealed a potential explanation for impaired muscle function in SIM as nNOS interactions were lost at the sarcomere and gained at the sarcoplasmic reticulum impairing contractility. Treating nNOS-deficient mice with steroids demonstrated that loss of sarcolemmal nNOS reduces muscle contractility and strength in SIM through increased nitric oxide (NO) signaling. In SIM mice treated with a nitric oxide donor and steroids, nitric oxide partially protects the muscle from atrophy and improves muscle fatigability and recovery suggesting nNOS mislocalization also decreases NO availability. These findings show that loss of sarcolemmal nNOS is a common phenomenon that negatively impacts muscle function. Therapeutic strategies targeting nNOS or NO signaling need to allow for the complexity of local nitric oxide content and cellular context.