Impairment of ubiquitin-proteasome system by E334K cMyBPC modifies channel proteins, leading to electrophysiological dysfunction.

Cardiac arrhythmogenesis is regulated by channel proteins whose protein levels are in turn regulated by the ubiquitin-proteasome system (UPS). We have previously reported on UPS impairment induced by E334K cardiac myosin-binding protein C (cMyBPC), which causes hypertrophic cardiomyopathy (HCM) accompanied by arrhythmia. We hypothesized that UPS impairment induced by E334K cMyBPC causes accumulation of cardiac channel proteins, leading to electrophysiological dysfunction. Wild-type or E334K cMyBPC was overexpressed in HL-1 cells and primary cultured neonatal rat cardiac myocytes. The expression of E334K cMyBPC suppressed cellular proteasome activities. The protein levels of K(v)1.5, Na(v)1.5, Hcn4, Ca(v)3.2, Ca(v)1.2, Serca, RyR2, and Ncx1 were significantly higher in cells expressing E334K cMyBPC than in wild type. They further increased in cells pretreated with MG132 and had longer protein decays. The channel proteins retained the correct localization. Cells expressing E334K cMyBPC exhibited higher Ca(2+) transients and longer action potential durations (APDs), accompanied by afterdepolarizations and higher apoptosis. Those augments of APD and Ca(2+) transients were recapitulated by a simulation model. Although a Ca(2+) antagonist, azelnidipine, neither protected E334K cMyBPC from degradation nor affected E334K cMyBPC incorporation into the sarcomere, it normalized the APD and Ca(2+) transients and partially reversed the levels of those proteins regulating apoptosis, thereby attenuating apoptosis. In conclusion, UPS impairment caused by E334K cMyBPC may modify the levels of channel proteins, leading to electrophysiological dysfunction. Therefore, UPS impairment due to a mutant cMyBPC may partly contribute to the observed clinical arrhythmias in HCM patients.