Hypoxia Reduces Mature HERG Channels through Calpain Up‐regulation and Activation

Human ether‐à‐go‐go related gene (hERG) encodes the pore‐forming subunit of the I Kr channel, which is important for cardiac repolarization. Loss of function of the hERG channel causes long QT syndrome, a cardiac disorder with high risk of cardiac arrhythmias and sudden death. Patients with medical conditions, such as cardiac ischemia, are associated with prolonged QT intervals and arrhythmias. Since proteases, such as calpain, are upregulated and activated during cardiac ischemia and hypoxia, we investigated the role of calpain in hypoxia‐mediated hERG reduction. We demonstrate that hypoxic (0.5% O 2 ) culture of hERG‐expressing HEK cells and neonatal rat cardiomyocytes led to a significant reduction of mature hERG expression and a concomitant increase in calpain expression. Transfection of hERG‐HEK cells with calpain‐1 reduced mature hERG expression and was accompanied by a significant calpain expression in the cell culture media. Treatment of cells with media from calpain‐1 transfected cells reduced mature hERG channel expression and function. The calpain‐1‐mediated hERG reduction was completely prevented by the membrane‐impermeable calpain inhibitor peptide B27. Replacing the S5‐S6 pore linker of hERG with that of EAG completely eliminated the calpain‐1 and hypoxic culture‐induced hERG reduction. Application of a peptide (BeKm‐1), which binds specifically to hERG's extracellular S5‐pore linker, prevented calpain‐1 and hypoxic culture‐induced hERG reduction. We conclude that hypoxia decreases mature hERG expression by activating calpain, which cleaves the channel protein at the extracellular S5‐S6 pore linker leading to subsequent channel degradation. Supported by the Canadian Institutes of Health Research & the Heart and Stroke Foundation of Ontario