The hERG1 PAS Domain: An Anti-arrhythmic Drug Target and Master Regulator of IKr

Abstract

The hERG1 potassium channel conducts the cardiac repolarizing current IKr. It is one of the first currents to appear in the heart and its disruption is associated with cardiac disorders. hERG1 channels comprise at least two subunits, hERG1a and hERG1b. hERG1a channels contain an N-terminal Per-Arnt-Sim (PAS) domain that interacts with the C-terminal cyclic nucleotide binding homology domain (CNBHD) to regulate channel gating. hERG1b is identical to hERG1a except for its unique N-terminus which is much shorter and has no PAS domain. Shifts in the relative abundance of hERG1a and hERG1b alter IKr gating and repolarization which modulates cardiac function. The goal of this dissertation was to identify a novel hERG1 antiarrhythmic drug target and elucidate the role of hERG1 subunit abundance in cardiac development and pathophysiology. In Chapter 1 we tested the antiarrhythmic capacity of a single chain variable fragment antibody, scFv2.10, in a model of long QT syndrome (LQTS). scFv2.10 selectively binds the hERG1a PAS domain and disrupts its interaction with the CNBHD. This causes a two-fold increase in the time course of hERG1 deactivation, inactivation recovery, and increases hERG1 current in HEK293 cells. In human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), scFv2.10 increases IKr and shortens action potential (AP) duration. We hypothesized that the hERG1 PAS domain could represent a novel therapeutic target for diseases of impaired cardiac repolarization. To test this, we recorded cardiac currents and APs from an hiPSC-CM line derived from a patient with Jervell and Lange Nielsen syndrome (JLN), a severe form of LQTS. Compared to hiPSC-CMs derived from a healthy patient, JLN hiPSC-CMs display hallmarks of proarrhythmia including prolonged AP duration (APD), increased AP variability, and early after depolarizations (EADs). scFv2.10 expression shortened APD and reduced AP variability and the incidence of EADs in JLN hiPSC-CMs, compared to GFP controls. Thus, disabling the PAS domain may be a viable approach for treating disrupted cardiac excitability. In Chapter 2 we investigated the role of hERG1 subunit abundance in early cardiac physiology. hERG1 variants are linked with intrauterine fetal death and sudden infant death syndrome (SIDS), yet little is known about hERG1’s role in developing cardiomyocytes. We used substrate-mediated hiPSC-CM maturation model immature and matured cardiomyocytes to determine how maturation impacts hERG1 function and subunit abundance. Immature hiPSC-CMs had reduced hERG1a mRNA and protein levels and elevated hERG1b mRNA and protein levels compared to matured hiPSC-CMs. This apparent shift in subunit abundance coincided with reduced IKr density in immature hiPSC-CMs. Extracellular acidosis, which is proposed to promote SIDS, inhibits hERG1 channels and has a greater inhibitory effect on hERG1b. We hypothesized that acidosis preferentially inhibits IKr of the immature myocardium where hERG1b is upregulated, which could promote arrhythmia and SIDS. We screened the impact of extracellular acidosis on native IKr at pH 6.3 and pH 7.4. Acidic extracellular pH significantly reduced IKr magnitude in immature and matured hiPSC-CMs and the magnitude of IKr inhibition was significantly greater in immature hiPSC-CMs compared to matured hiPSC-CMs. PAS expression, which effectively converts hERG1b to hERG1a channels, reduced the magnitude of IKr inhibition. These data demonstrate that hERG1 subunit abundance modulates IKr sensitivity to acidosis and may be a contributing factor to SIDS. Together, these findings highlight the crucial role of the PAS domain as a potential antiarrhythmic target and structural determinant for the onset of SIDS.