The purpose was to investigate the contribution of mechanical damage to sarcomeres to the greater susceptibility of dystrophic muscle fibers to contraction-induced injury. Single stretches provide an effective method for studying mechanical factors that contribute to the initiation of contraction-induced injury. We hypothesized that, after single stretches, the deficits in isometric force would be greater for muscles of mdx than C57BL/10 mice, whereas membrane damage would be minimal for all muscles. Extensor digitorum longus (EDL) and soleus muscles of mice were removed under anesthesia with Avertin (tribromoethanol). During the plateau of a maximum isometric contraction in vitro, muscles were stretched through single strains of 20-60% fiber length. Isometric force was remeasured 1 min later, and muscles were then incubated in procion orange dye to identify fibers with membrane damage. Force deficits at 1 min were two- to threefold greater for EDL muscles of mdx compared with C57BL/10 mice, whereas no significant differences were observed between soleus muscles of mdx and C57BL/10 mice. For all muscles, membrane damage was minimal and not significantly increased by single stretches for either strain of mice. These data support a critical role of dystrophin maintaining sarcomere stability in EDL muscles, whereas soleus muscles retain abilities, in the absence of dystrophin, not different from control muscles to resist sarcomere damage.

Sarcospan is an integral membrane component of the dystrophin-glycoprotein complex (DGC) found at the sarcolemma of striated and smooth muscle. The DGC plays important roles in muscle function and viability as evidenced by defects in components of the DGC, which cause muscular dystrophy. Sarcospan is unique among the components of the complex in that it contains four transmembrane domains with intracellular Nand C-terminal domains and is a member of the tetraspan superfamily of proteins. Sarcospan is tightly linked to the sarcoglycans, and together these proteins form a subcomplex within the DGC. Stable expression of sarcospan at the sarcolemma is dependent upon expression of the sarcoglycans. Here we describe the generation and analysis of mice carrying a null mutation in the Sspn gene. Surprisingly, the Sspn-deficient muscle maintains expression of other components of the DGC at the sarcolemma, and no gross histological abnormalities of muscle from the mice are observed. The Sspn-deficient muscle maintains sarcolemmal integrity as determined by serum creatine kinase and Evans blue uptake assays, and the Sspn-deficient muscle maintains normal force and power generation capabilities. These data suggest either that sarcospan is not required for normal DGC function or that the Sspn-deficient muscle is compensating for the absence of sarcospan, perhaps by utilizing another protein to carry out its function.The dystrophin-glycoprotein complex (DGC) is a multisubunit protein complex composed of integral membrane, peripheral membrane, and cytoplasmic proteins expressed at the sarcolemma of striated muscle fibers (see references 7, 41, 54, and 62 for reviews). Isolation and cloning of proteins within this complex have provided key insights into the function of the DGC and its role in normal muscle physiology. The skeletal muscle DGC is composed of dystrophin (9, 31); the syntrophins (22); ␣-and ␤-dystroglycan (33); ␣-, ␤-, ␥-, and ␦-sarcoglycan (3, 36, 42, 48, 50, 51, 55, 56); and sarcospan (14). Recently, the DGC within smooth muscle fibers has been characterized (64). This complex differs from the striated muscle DGC in at least two respects. First, ε-sarcoglycan replaces ␣-sarcoglycan within the sarcoglycan subcomplex, and second, differences occur in the glycosylation pattern of ␣-dystroglycan in the smooth muscle DGC.One likely function of the DGC is to provide a structural link between the extracellular matrix and the actin cytoskeleton, thereby maintaining the stability of the sarcolemma under contractile forces (10, 21). This link occurs through dystrophin, which binds to filamentous actin (31, 58, 59), and dystroglycan-with its ␤-subunit binding to dystrophin (37) and its ␣-subunit interacting with the extracellular matrix component laminin-2 (24). The DGC likely plays other roles in normal muscle physiology by interacting with cell signaling molecules or other proteins at the sarcolemma. The skeletal muscle DGC has been the most fully characterized in this respect, and several laboratories have demonstrated i...

alpha-Sarcoglycan-deficient (Sgca-null) mice provide potential for elucidating the pathogenesis of limb girdle muscular dystrophy type 2D (LGMD 2D) as well as for studying the effectiveness of therapeutic strategies. Skeletal muscles of Sgca-null mice demonstrate an early onset of extensive fiber necrosis, degeneration, and regeneration, but the progression of the pathology and the effects on muscle structure and function throughout the life span are not known. Thus the phenotypic accuracy of the Sgca-null mouse as a model of LGMD 2D has not been fully established. To investigate skeletal muscle structure and function in the absence of alpha-sarcoglycan throughout the life span, we analyzed extensor digitorum longus and soleus muscles of male and female Sgca-null and wild-type mice at 3, 6, 12, and 18 mo of age. Maximum isometric forces and powers were measured in vitro at 25 degrees C. Also determined were individual myofiber cross-sectional areas and numbers, water content, and the proportion of the cross section occupied by connective tissue. Muscle masses were 40-100% larger for Sgca-null compared with age- and gender-matched wild-type mice, with the majority of the increased muscle mass for Sgca-null mice attributable to greater connective tissue and water contents. Although the greater mass of muscles in Sgca-null mice was primarily noncontractile material, absolute forces and powers were maintained near control levels at all ages, indicating a successful adaptation to the deficiency in alpha-sarcoglycan not observed at any age in LGMD 2D patients.