Nitric Oxide (NO) is formed endogenously by a family of enzymes known as NO synthases (NOS). Three distinct isoforms of NOS have been identified: nNOS being the isoform first found in neuronal tissue, iNOS which is inducible in a wide range of cells and tissues, and eNOS, which was first characterized in vascular endothelial cells. nNOS plays several important roles in the brain including regulation of synaptic signaling and plasticity. Additionally, high levels of nNOS protein are present in skeletal muscle where NO controls muscle contractility and local blood flow .NO is generated via a five-electron oxidation of a terminal guanidine nitrogen on L-Arginine by nNOS. The reaction yields L-Citrulline in addition to NO in a 1:1 stoichiometry. The most important regulator of nNOS activity is free cytosolic Ca2+, which stimulates nNOS through interaction with CALM. Action potentials activate calcium channels (CaCn) in the neurolemma and stimulates the release of Ca2+ from intracellular stores. This elevates cytosolic Ca2+ concentrations required for CALM binding to nNOS, thereby activating the enzyme. When the concentration of Ca2+ falls, it dissociates from CALM, which in turn dissociates from nNOS, thus acting as a switch that turns the enzyme on and off. Skeletal muscle nNOSμ is bound to the dystrophin-associated protein complex through its PDZ domain. nNOS does not bind to dystrophin directly, but rather is targeted to the sarcolemma via syntrophins (SNTα1, SNTβ1, SNTβ2) which are predominantly expressed in skeletal and cardiac muscle. This dystrophin-nNOS-syntrophin complex is essential for the sarcolemmal association of nNOS. The loss of dystrophin in muscular dystrophy prevents assembly of the dystroglycan complex. As a result, NO signaling in response to muscle contraction is disrupted, and the blood vessel dilation of contracting skeletal muscle that is normally mediated by NO is abolished. Similar to nNOSα in brain, nNOSμ protein turnover in skeletal muscle is also regulated by Ca2+-dependent calpain degradation.
Disruption of NO signaling plays a major role in muscular dystrophy pathophysiology. Several muscular diseases have been linked to a dystrophin deficiency which in turn affects NO signaling. A mutation in the rod-like domain of dystrophin causes Becker's dystrophy and results in a loss of sarcolemmal nNOS, while other components of the dystrophin complex are preserved. Sarcolemmal instability in Duchenne dystrophy leads to a repeated cycle of myofiber degeneration and subsequent regeneration. Redistribution of nNOS from sarcolemma to cytosol is involved in myofiber necrosis. NO derived from nNOSμ in skeletal muscle fibers plays a major role in dilating blood vessels adjacent to contracting skeletal muscle. This physiological response, titled functional hyperemia, plays an important role in increasing blood flow to contracting skeletal muscles to support their enhanced metabolic needs.