Differential targeting of nNOS and AQP4 to dystrophin-deficient sarcolemma by membrane-directed -dystrobrevin

-Dystrobrevin associates with and is a homologue of dystrophin, the protein linked to Duchenne and Becker muscular dystrophies. We used a transgenic approach to restore -dystrobrevin to the sarcolemma in mice that lack dystrophin (mdx mice) to study two interrelated functions: (1) the ability of -dystrobrevin to rescue components of the dystrophin complex in the absence of dystrophin and (2) the ability of sarcolemmal -dystrobrevin to ameliorate the dystrophic phenotype. We generated transgenic mice expressing -dystrobrevin-2a linked to a palmitoylation signal sequence and bred them onto the -dystrobrevin-null and mdx backgrounds. Expression of palmitoylated -dystrobrevin prevented the muscular dystrophy observed in the -dystrobrevin-null mice, demonstrating that the altered form of -dystrobrevin was functional. On the mdx background, the palmitoylated form of -dystrobrevin was expressed on the sarcolemma but did not significantly ameliorate the muscular dystrophy phenotype. Palmitoylated dystrobrevin restored -syntrophin and aquaporin-4 (AQP4) to the mdx sarcolemma but was unable to recruit -dystroglycan or the sarcoglycans. Despite restoration of sarcolemmal -syntrophin, neuronal nitric oxide synthase (nNOS) was not localized to the sarcolemma, suggesting that nNOS requires both dystrophin and -syntrophin for correct localization. Thus, although nNOS and AQP4 both require interaction with the PDZ domain of -syntrophin for sarcolemmal association, their localization is regulated differentially.