Defective assembly of sarcoglycan complex in patients with β-sarcoglycan gene mutations. Study of aneural and innervated cultured myotubes

Mutations in the sarcoglycan (SG) genes cause autosomal recessive muscular dystrophies. The absence of each SG complex component in muscle impairs the proper assembly of the entire SG complex, resulting in sarcolemmal damage. We investigated the consequences of β-SG gene mutations in cultured muscle from two β-SG mutated patients, and analysed each individual SG protein expression by cross-sectional immunocytochemistry and Western blot in aneural and innervated myotubes. Patients' muscle biopsy showed total loss of SG complex; however, a limited amount of β-SG was detected in aneural and innervated myotubes, where the protein was localized to the plasma membrane. This paradoxical β-SG expression can be attributable to antibody cross-reaction or to the expression of an unknown SG isoform specific of immature muscle. In our cultured myotubes, the other components of the SG complex were absent, suggesting that β-SG gene mutations result in a defective assembly of the entire SG complex in early stages of muscle development, and that the role of β-SG is crucial for the normal structure and/or function of the SG complex in the sarcolemma.