HERG1 Expression Increases Calpain Activity in C2C12 Myotubes

Skeletal muscle atrophy is the loss of muscle size and strength caused by an imbalance in protein degradation and protein synthesis. It occurs with normal aging, neural and skeletal muscle injuries, and with diseases such as diabetes, cancer cachexia, AIDS, Muscular Dystrophy, etc. If unabated, skeletal muscle atrophy can be very debilitating and even cause death. Three proteolytic systems are known to be upregulated in atrophic muscle: 1) ubiquitin proteasome proteolysis (UPP); 2) calcium activated calpains; and 3) lysosomal cathepsins. HERG1 is a K+ ion channel shown to be up‐regulated in atrophying skeletal muscle. We have shown that it transiently increases intracellular calcium concentration in cultured C2C12 myotubes. Thus our objective was to test our hypothesis that myotubes with increased levels of the HERG1 protein would have increased calpain activity. We transfected C2C12 myotubes with either an adenovirus encoding HERG1 or an appropriate control virus and, in both experimental groups, measured: 1) myotube size using ImageJ; 2) MuRF1 protein abundance using immunoblot; and 3) calpain activity in depolarized and non‐depolarized myotubes using a Calpain‐Glo Assay Kit (ProMega; Madison, WI). Indeed, our data reveal that, relative to controls, myotubes expressing HERG1 undergo atrophy, experiencing: 1) a 55% decrease in size; 2) an increased abundance of the UPP E3 ligase MuRF1; and 3) an 18% increase in calpain activity. We conclude that HERG1 increases proteolytic calpain activity in cultured myotubes.Support or Funding InformationThis research was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health under Award Number NIH NIAMS 1R03AR053706‐01A2 to ALP. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.