Extensive but Coordinated Reorganization of the Membrane Skeleton in Myofibers of Dystrophic (mdx) Mice

Abstract

We used immunofluorescence techniques and confocal imaging to study the organization of the membrane skeleton of skeletal muscle fibers of mdx mice, which lack dystrophin. β-Spectrin is normally found at the sarcolemma in costameres, a rectilinear array of longitudinal strands and elements overlying Z and M lines. However, in the skeletal muscle of mdx mice, β-spectrin tends to be absent from the sarcolemma over M lines and the longitudinal strands may be disrupted or missing. Other proteins of the membrane and associated cytoskeleton, including syntrophin, β-dystroglycan, vinculin, and Na,K-ATPase are also concentrated in costameres, in control myofibers, and mdx muscle. They also distribute into the same altered sarcolemmal arrays that contain β-spectrin. Utrophin, which is expressed in mdx muscle, also codistributes with β-spectrin at the mutant sarcolemma. By contrast, the distribution of structural and intracellular membrane proteins, including α-actinin, the Ca-ATPase and dihydropyridine receptors, is not affected, even at sites close to the sarcolemma. Our results suggest that in myofibers of the mdx mouse, the membrane- associated cytoskeleton, but not the nearby myoplasm, undergoes widespread coordinated changes in organization. These changes may contribute to the fragility of the sarcolemma of dystrophic muscle.

Specific Interaction of the Actin-binding Domain of Dystrophin with Intermediate Filaments Containing Keratin 19

“…Whatever the mechanism, our results suggest that K19 can compete with the normal binding sites for dystrophin, which would otherwise concentrate at the sarcolemma. Because previous studies have shown that the costameric distribution of dystrophin-associated proteins, including ␤-dystroglycan, is compromised when dystrophin is absent from the sarcolemma in dystrophic muscle (Williams and Bloch, 1999b), it is not surprising that the ability of ␤-dystroglycan to concentrate at costameres is altered in myofibers that overexpress K19. Dystroglycan also redistributes in the plane of the sarcolemma in other muscular dystrophies and myopathies (O'Neill et al, 2002;Reed et al, 2004).…”

“…It suggests that these vesicles, present in a subset of fibers that overexpress K19, represent a distinct membrane population that associates with membrane-cytoskeletal proteins, such as ␤-spectrin and dystrophin, but that does not contain all of the integral membrane proteins that anchor these proteins at the sarcolemma. The presence of ␤-spectrin at these structures is unlikely to be due to interactions of that molecule or its ABD with K19 directly (Figure 2, G-I), but is instead more likely linked to the ability of ␤-spectrin to associate with muscle membranes in a manner that is independent of dystrophin (Porter et al, 1992;Ehmer et al, 1997;Williams and Bloch, 1999b). Further research will be needed to establish the nature of the aggregates and vesicles induced by the overexpression of K19, the mechanism by which they form, and their relationship to structures seen in myopathies (De Bleecker et al, 1993).…”

“…When it is present, the dystrophin complex helps to link the contractile apparatus to the sarcolemma and through the membrane to the extracellular matrix (Ibraghimov-Beskrovnaya et al, 1992;Ervasti and Campbell, 1993;Rybakova et al, 2000;Rybakova et al, 2002; Michele et al, 2003). When dystrophin is absent, links between the contractile apparatus and the sarcolemma can weaken and eventually break (Porter et al, 1992;Ehmer et al, 1997;Williams and Bloch, 1999b;Rybakova et al, 2000), damaging the sarcolemma (Mokri and Engel, 1975), and, ultimately, killing the myofiber (reviewed in Emery, 1993).…”

“…When it is present, the dystrophin complex helps to link the contractile apparatus to the sarcolemma and through the membrane to the extracellular matrix (Ibraghimov-Beskrovnaya et al, 1992;Ervasti and Campbell, 1993;Rybakova et al, 2000;Rybakova et al, 2002; Michele et al, 2003). When dystrophin is absent, links between the contractile apparatus and the sarcolemma can weaken and eventually break (Porter et al, 1992;Ehmer et al, 1997;Williams and Bloch, 1999b;Rybakova et al, 2000), damaging the sarcolemma (Mokri and Engel, 1975), and, ultimately, killing the myofiber (reviewed in Emery, 1993).Several filamentous proteins, including actin, link the contractile apparatus to the dystrophin complex at costameres. Dystrophin binds actin (Senter et al, 1993;Renley et al, 1998;Sutherland-Smith et al, 2003), and this activity is thought to be physiologically important for the health of the muscle fiber (Corrado et al, 1996;Eraslan et al, 1999;Warner et al, 2002).…”

Utrophin suppresses low frequency oscillations and coupled gating of mechanosensitive ion channels in dystrophic skeletal muscle

“…[1][2][3][4][5] Contractioninduced stresses are generated both longitudinally as well as laterally. Lateral stresses disrupt alignment of sarcomeres and orientation of the t-tubular system, which reduce muscle force generation.…”