Expression and localization of protein inhibitor of neuronal nitric oxide synthase in Duchenne muscular dystrophy

In skeletal muscle fibers, nitric oxide is synthesized by neuronal nitric oxide synthase (nNOS), which normally associates with the dystrophin complex in close proximity to the sarcolemma. Many reports have documented that very low levels of nNOS protein exist in muscle fibers of Duchenne muscular dystrophy (DMD) patients. In this study we investigated the functional significance of PIN (protein inhibitor of nNOS) in targeting of nNOS to the sarcolemma and the association between nNOS and the dystrophin complex in normal and dystrophic muscle fibers. Northern blotting for PIN mRNA in normal mouse muscles and muscles of mdx mice (an animal model of DMD) revealed a significant rise in PIN mRNA in dystrophic muscles compared with normal muscles. Immunohistochemical analysis showed that, in normal mouse muscle fibers, PIN expression was localized at the sarcolemma, peripheral nuclei, and the sarcoplasm. By comparison, PIN protein in muscles from mdx mice was more concentrated around the sarcolemma and central nuclei. The presence of PIN protein expression in muscles from mdx mice was evident despite the significant reduction in nNOS and dystrophin protein expressions in these fibers. In muscle sections of DMD patients, the absence of nNOS protein expression was accompanied by maintained PIN expression. Prominent PIN expression was also detectable in macrophages infiltrating dystrophic muscle fibers both in mdx mice and DMD patients. These results suggest that PIN expression in muscles from mdx mice and DMD patients is controlled by factors different from those involved in the regulation of nNOS and dystrophin. Moreover, our results indicate that PIN is not an integral component of the dystrophin complex inside skeletal muscle fibers.