The organization of beta-spectrin and dystrophin at the sarcolemma of skeletal muscle

Abstract

Dystrophin, the protein missing in Duchennes muscular dystrophy, is structurally similar to, beta-spectrin. Immunofluorescence studies have shown that these two proteins have similar distributions within skeletal muscle fibers. I use immunofluorescence and immunoprecipitation to examine beta-spectrin and dystrophin in innervated and denervated fast and slow twitch muscle, and to determine how beta-spectrin is anchored to the membrane. I also examine the organization of the sarcolemma in mdx mice, which lack dystrophin, and in ja/ja mice which lack beta-spectrin. My results indicate that beta-spectrin and dystrophin colocalize at most areas of the sarcolemma in skeletal muscle, but that in fast twitch fibers, "intracostameric regions" and "juxtanuclear domains" contain dystrophin but little or no beta-spectrin. This indicates that the distribution of dystrophin at the sarcolemma is not dependent on beta-spectrin, and suggests that broad regions of the sarcolemma are likely to be poorly supported in the absence of dystrophin. I demonstrate that the distributions of both beta-spectrin and dystrophin are different in fast and slow twitch fibers, and that this distribution is affected by denervation. Denervating both fast and slow twitch fibers causes both proteins to redistribute in a pattern resembling that seen in healthy slow twitch fibers. This indicates that the distribution of these proteins is influenced by innervation. I determine that beta-spectrin is anchored to the membrane through its association with the alpha-subunit of the Na+/K+ ATPase. In ja/ja mice, the Na+/K+ ATPase distributes more uniformly in the sarcolemma. Dystrophin is believed to connect the membrane to the contractile apparatus through its interaction with actin and its membrane receptor, the dystrophin-associated glycoprotein complex. Through its ability to interact with the Na+/K+ ATPase, beta-spectrin, may serve a similar role. Finally I show that in mdx mice, beta-spectrin distributes abnormally. Other membrane-associated proteins like beta-dystroglycan, syntrophin, and the Na+/K+ ATPase distribute abnormally together with beta-spectrin. This suggests that the normal distribution of beta-spectrin and other proteins of the cytoskeleton may depend on dystrophin. It further suggests that the spectrin-based membrane skeleton and the dystrophin-based membrane skeleton in skeletal muscle are linked by a protein other than dystrophin itself.