Stretch-induced cell damage in sarcoglycan-deficient myotubes

Sarcoglycans (SGs) are components of the dystrophin-glycoprotein complex, genetic defects in which cause skeletal muscle dystrophy and cardiomyopathy in humans and animals. To obtain insight into the roles of SGs, we characterized properties of myotubes prepared from cells of the rat L6 line or primary myoblast cultures of rat gastrocnemius muscle that were made SG-deficient by treatment with antisense oligodeoxynucleotides (AS-ODNs). Immunoblot and immunoprecipitation analyses revealed that dystrophin and its remaining associated proteins were tightly associated in these cells despite SG deficiency. Ca-45(2+) influx into SG AS-ODN-treated L6 myotubes under resting conditions was significantly higher (1.7-fold at 6 min) than in controls, suggesting that Ca2+ influx is activated in these SG-deficient myotubes. When these cells were subjected to cyclic elongation of up to 20% for 1 h, a marked increase in creatine phosphokinase (CK) release into the medium was observed. Nifedipine, tranilast, FK506 and E64 or intracellular loading with 1,2-bis(2-aminophenoxy)ethane-N,N,N ' ,N ' -tetraacetic acid, tetrakis(acetoxymethyl)ester (BAPTA/AM) reduced the stretch-induced CK release; a raised extracellular [Ca2+] increased CK release. The stretch-induced damage to SG-deficient myotubes thus appears to be caused by alterations in cell Ca2+ homeostasis. A similar abnormality in Ca2+ handling has been reported for myocytes from mdx mice or dystrophin-deficient patients, in whom SGs are also greatly reduced or absent. Thus it is possible that SG deficiency may play a critical role in the pathology of dystrophin-deficient muscle.