Susceptibility of the hERG Potassium Channel to Proteases and its Clinical Implications
Cardiovascular diseases are the leading cause of death worldwide. Cardiac ischemia accounts for approximately 50% of all cardiovascular-related deaths. Cardiac ischemia is associated with high incidence of QT prolongation and cardiac arrhythmias. Hypoxia is a common pathophysiological factor experienced during cardiac ischemia. Inflammation and leukocyte infiltration is also enhanced during ischemic insults. Additionally, the activation, upregulation and secretion of several different proteases occur during cardiac ischemia. The mechanisms surrounding ischemia-induced QT prolongation and cardiac arrhythmias are not fully understood. The aim of this thesis was to gain a better understanding of the underlying mechanisms of ischemia-induced cardiac arrhythmias and sudden death. Dysfunction of cardiac ion channels is directly responsible for the generation of cardiac arrhythmias. In particular, loss of function of the potassium channel, hERG, leads to QT interval prolongation and cardiac arrhythmias. The hERG potassium channel has also been suggested to be uniquely susceptible to extracellular proteases. Thus, we hypothesized that hERG dysfunction due to protease cleavage of the channel during cardiac ischemia plays a role in ischemia-induced QT prolongation and cardiac arrhythmias. We observed that proteases selectively cleaved hERG/IKr extracellularly in the S5-pore linker of the channel. We demonstrated that glycosylation which occurs in the S5-pore linker of hERG, as well as the BeKm-1 scorpion toxin, which binds to the S5-pore linker of hERG, protects the channel from protease cleavage. To determine whether proteases upregulated, activated and secreted during cardiac ischemia affect hERG, coronary ligation of the circumflex artery was performed on a rabbit model. Among the proteases shown to be upregulated during ischemia, calpain-1, which was secreted to the extracellular milieu, cleaved hERG in the S5-pore linker. Consistent with a decrease in hERG function in ischemic conditions, a calpain-mediated reduction of hERG was also observed in hypoxic conditions. The effect was abrogated with the treatment of BeKm-1 and calpain inhibitors. In summary, proteases upregulated and secreted, cleave hERG during ischemic and hypoxic insults, which can be prevented with protease inhibitors and hERG targeting peptides. The present work extends our understanding of the mechanisms that underlie cardiovascular disease-related mortality while revealing potential preventative treatment options to combat ischemia-induced arrhythmogenesis.