Dystrophin is not essential for the integrity of the cytoskeleton

Dystrophin is localized, in normal muscle fibers, on the cytoplasmic surface of the sarcolemma. The function of this protein is not known but, according to its structure and intracellular distribution, it seems likely that dystrophin interacts with other cytoskeletal proteins to form a complex linkage between myofibrils, sarcolemma and extracellular matrix. To evaluate the possibility that dystrophin deficiency induces, per se, a disarray in the cytoskeleton, we studied three components of this structure in muscle fibers of the dystrophic mdx mouse in a phase preceding the onset of necrosis. Vinculin, abundant in sarcolemmal structures called costameres, desmin, the principal component of intermediate filaments and nebulin, constituent of the so-called "third filament" within the sarcomere, were stained with the indirect immunofluorescence technique in cryostat sections. The same monoclonal antibodies were used in Western blots of proteins extracted from the same muscles. No difference was observed in the distribution or in the relative abundance of the three proteins, comparing muscles from 18 day-old mdx and control mice. Our results indicate that the lack of dystrophin does not induce, per se, alterations in the structures linking the sarcolemma to the contractile apparatus. It is likely that the structural damage in dystrophin-less muscle fibers is initially confined to limited portions of the plasma membrane. These focal lesions, impairing intracellular calcium homeostasis, can lead to muscle fiber necrosis.