Sarcoglycan Complex In Muscle Research Articles

The 16 kDa subunit of vacuolar H +-ATPase is a novel sarcoglycan-interacting protein

The sarcoglycan complex in muscle consists of α-, β-, γ- and δ-sarcoglycan and is part of the larger dystrophin–glycoprotein complex (DGC), which is essential for maintaining muscle membrane integrity. Mutations in any of the four sarcoglycans cause limb-girdle muscular dystrophies (LGMD). In this report, we have identified a novel interaction between δ-sarcoglycan and the 16 kDa subunit c (16K) of vacuolar H +-ATPase. Co-expression studies in heterologous cell system revealed that 16K interacts specifically with δ-sarcoglycan and the highly related γ-sarcoglycan through the transmembrane domains. In cultured C2C12 myotubes, 16K forms a complex with sarcoglycans at the plasma membrane. Loss of sarcoglycans in the sarcoglycan-deficient BIO14.6 hamster destabilizes the DGC and alters the localization of 16K at the sarcolemma. In addition, the steady state level of β 1-integrin is increased. Recent studies have shown that 16K also interacts directly with β 1-integrin and our data demonstrated that sarcoglycans, 16K and β 1-integrin were immunoprecipitated together in C2C12 myotubes. Since sarcoglycans have been proposed to participate in bi-directional signaling with integrins, our findings suggest that 16K might mediate the communication between sarcoglycans and integrins and play an important role in the pathogenesis of muscular dystrophy.

Secondary Coronary Artery Vasospasm Promotes Cardiomyopathy Progression

Genetic defects in the plasma membrane-associated sarcoglycan complex produce cardiomyopathy characterized by focal degeneration. The infarct-like pattern of cardiac degeneration has led to the hypothesis that coronary artery vasospasm underlies cardiomyopathy in this disorder. We evaluated the coronary vasculature of gamma-sarcoglycan mutant mice and found microvascular filling defects consistent with arterial vasospasm. However, the vascular smooth muscle sarcoglycan complex was intact in the coronary arteries of gamma-sarcoglycan hearts with perturbation of the sarcoglycan complex only within the adjacent myocytes. Thus, in this model, coronary artery vasospasm derives from a vascular smooth muscle-cell extrinsic process. To reduce this secondary vasospasm, we treated gamma-sarcoglycan-deficient mice with the calcium channel antagonist verapamil. Verapamil treatment eliminated evidence of vasospasm and ameliorated histological and functional evidence of cardiomyopathic progression. Echocardiography of verapamil-treated, gamma-sarcoglycan-null mice showed an improvement in left ventricular fractional shortening (44.3 +/- 13.3% treated versus 37.4 +/- 15.3% untreated), maximal velocity at the aortic outflow tract (114.9 +/- 27.9 cm/second versus 92.8 +/- 22.7 cm/second), and cardiac index (1.06 +/- 0.30 ml/minute/g versus 0.67 +/- 0.16 ml/minute/g, P < 0.05). These data indicate that secondary vasospasm contributes to the development of cardiomyopathy and is an important therapeutic target to limit cardiomyopathy progression.

Gene transfer establishes primacy of striated vs. smooth muscle sarcoglycan complex in limb-girdle muscular dystrophy.

Limb-girdle muscular dystrophy types 2E and F are characterized by skeletal muscle weakness and often cardiomyopathy and are due to mutations in the genes encoding beta- and delta-sarcoglycan. We previously demonstrated that loss of sarcoglycans in smooth muscle leads to constrictions of the microvasculature that contributes to the cardiac phenotype. It is unclear how vasculature abnormalities affect skeletal muscle. We injected recombinant beta- or delta-sarcoglycan adenoviruses into skeletal muscles of corresponding null mice. We hypothesized that the adenoviruses would not transduce vascular smooth muscle, and we would only target skeletal muscle. Indeed, sustained expression of intact sarcoglycan-sarcospan complex was noted at the sarcolemma, neuromuscular junction, myotendinous junction, and in peripheral nerve, but not in vascular smooth muscle. Gene transfer of the corresponding deleted sarcoglycan gene preserved sarcolemmal integrity, prevented pathological dystrophy and hypertrophy, and protected against exercised-induced damage. We conclude that vascular dysfunction is not a primary cause of beta- and delta-sarcoglycan-deficient muscular dystrophy. In addition, we show successful functional rescue of entire muscles after adenovirus-mediated gene delivery. Thus, virus-mediated gene transfer of sarcoglycans to skeletal muscle in combination with pharmacological prevention of cardiomyopathy constitute promising therapeutic strategies for limb-girdle muscular dystrophies.

Zeta-sarcoglycan, a novel component of the sarcoglycan complex, is reduced in muscular dystrophy.

The dystrophin glycoprotein complex (DGC) is found at the plasma membrane of muscle cells, where it provides a link between the cytoskeleton and the extracellular matrix. A subcomplex within the DGC, the sarcoglycan complex, associates with dystrophin and mediates muscle membrane stability. Mutations in sarcoglycan genes lead to muscular dystrophy and cardiomyopathy in both humans and mice. In invertebrates, there are three sarcoglycan genes, while in mammals there are additional sarcoglycan genes that probably arose from gene duplication events. We identified a novel mammalian sarcoglycan, zeta-sarcoglycan, that is highly related to gamma-sarcoglycan and delta-sarcoglycan. We generated a zeta-sarcoglycan-specific antibody and found that zeta-sarcoglycan associated with other members of the sarcoglycan complex at the plasma membrane. Additionally, zeta-sarcoglycan was reduced at the membrane in muscular dystrophy, consistent with a role in mediating membrane stability. zeta-Sarcoglycan was also found as a component of the vascular smooth muscle sarcoglycan complex. Together, these data demonstrate that zeta-sarcoglycan is an integral component of the sarcoglycan complex and, as such, is important in the pathogenesis of muscular dystrophy.

ε-Sarcoglycan Replaces α-Sarcoglycan in Smooth Muscle to Form a Unique Dystrophin-Glycoprotein Complex

The sarcoglycan complex has been well characterized in striated muscle, and defects in its components are associated with muscular dystrophy and cardiomyopathy. Here, we have characterized the smooth muscle sarcoglycan complex. By examination of embryonic muscle lineages and biochemical fractionation studies, we demonstrated that epsilon-sarcoglycan is an integral component of the smooth muscle sarcoglycan complex along with beta- and delta-sarcoglycan. Analysis of genetically defined animal models for muscular dystrophy supported this conclusion. The delta-sarcoglycan-deficient cardiomyopathic hamster and mice deficient in both dystrophin and utrophin showed loss of the smooth muscle sarcoglycan complex, whereas the complex was unaffected in alpha-sarcoglycan null mice in agreement with the finding that alpha-sarcoglycan is not expressed in smooth muscle cells. In the cardiomyopathic hamster, the smooth muscle sarcoglycan complex, containing epsilon-sarcoglycan, was fully restored following intramuscular injection of recombinant delta-sarcoglycan adenovirus. Together, these results demonstrate a tissue-dependent variation in the sarcoglycan complex and show that epsilon-sarcoglycan replaces alpha-sarcoglycan as an integral component of the smooth muscle dystrophin-glycoprotein complex. Our results also suggest a molecular basis for possible differential smooth muscle dysfunction in sarcoglycan-deficient patients.

Stable restoration of the sarcoglycan complex in dystrophic muscle perfused with histamine and a recombinant adeno-associated viral vector.

Limb-girdle muscular dystrophies 2C-F represent a family of autosomal recessive diseases caused by defects in sarcoglycan genes. The cardiomyopathic hamster is a naturally occurring model for limb-girdle muscular dystrophy caused by a primary deficiency in delta-sarcoglycan. We show here that acute sarcolemmal disruption occurs in this animal model during forceful muscle contraction. A recombinant adeno-associated virus vector encoding human delta-sarcoglycan conferred efficient and stable genetic reconstitution in the adult cardiomyopathic hamster when injected directly into muscle. A quantitative assay demonstrated that vector-transduced muscle fibers are stably protected from sarcolemmal disruption; there was no associated inflammation or immunologic response to the vector-encoded protein. Efficient gene transduction with rescue of the sarcoglycan complex in muscle fibers of the distal hindlimb was also obtained after infusion of recombinant adeno-associated virus into the femoral artery in conjunction with histamine-induced endothelial permeabilization. This study provides a strong rationale for the development of gene therapy for limb-girdle muscular dystrophy.