Developmental studies of dystrophin and other cytoskeletal proteins in cultured muscle cells.

We studied the developmental changes of localization of dystrophin and other cytoskeletal proteins, especially actin, spectrin and dystrophin related protein (DRP) using immunocytochemistry and quick-freezing and deep-etching (QF-DE) method. In developmental studies of mouse and human muscle cultures, some myoblasts had positive-reactions to spectrin, DRP, and F-actin, but not dystrophin. In aneurally cultured myotubes, dystrophin, DRP, and spectrin were localized diffusely in the cytoplasm and later in discontinuous patterns on the plasma membrane, when myotubes became mature. Spectrin and DRP had more positive reactions in immature myotubes, compared with those of dystrophin. In some areas of myotubes, dystrophin/spectrin and spectrin/actin were localized reciprocally. In innervated cultured human muscle cells, dystrophin and DRP were localized in neuro-muscular junctions, which were co-localized with clusters of acetylcholine receptors. By using the QF-DE method, dystrophin was localized just underneath the plasma membrane, and closely linked to actin-like filaments (8-10 nm in diameter), most of which were decorated with myosin subfragment 1. In actin-poor regions, spectrin was detected as well-organized filamentous structures in highly interconnected networks with various diameters. DRP was distributed irregularly with granular appearance inside the cytoplasm and also under the plasma membrane in immature mouse myotubes. Our present studies show that dystrophin, spectrin, and DRP are localized differently at the developmental stages of myotubes. These results suggest that dystrophin, spectrin, and DRP are organized independently in developing myotubes and these cytoskeletal proteins might play different functions in the preservation of plasma membrane stability in developing myotubes.