Physiological Properties of hERG 1a/1b Heteromeric Currents and a hERG 1b-Specific Mutation Associated With Long-QT Syndrome

Department of Physiology, University of Wisconsin, Madison, WI, USA.
Circulation Research (Impact Factor: 11.02). 10/2008; 103(7):e81-95. DOI: 10.1161/CIRCRESAHA.108.185249
Source: PubMed

ABSTRACT Cardiac I Kr is a critical repolarizing current in the heart and a target for inherited and acquired long-QT syndrome (LQTS). Biochemical and functional studies have demonstrated that I Kr channels are heteromers composed of both hERG 1a and 1b subunits, yet our current understanding of I Kr functional properties derives primarily from studies of homooligomers of the original hERG 1a isolate. Here, we examine currents produced by hERG 1a and 1a/1b channels expressed in HEK-293 cells at near-physiological temperatures. We find that heteromeric hERG 1a/1b currents are much larger than hERG 1a currents and conduct 80% more charge during an action potential. This surprising difference corresponds to a 2-fold increase in the apparent rates of activation and recovery from inactivation, thus reducing rectification and facilitating current rebound during repolarization. Kinetic modeling shows these gating differences account quantitatively for the differences in current amplitude between the 2 channel types. Drug sensitivity was also different. Compared to homomeric 1a channels, heteromeric 1a/1b channels were inhibited by E-4031 with a slower time course and a corresponding 4-fold shift in the IC50. The importance of hERG 1b in vivo is supported by the identification of a 1b-specific A8V missense mutation in 1/269 unrelated genotype-negative LQTS patients that was absent in 400 control alleles. Mutant 1bA8V expressed alone or with hERG 1a in HEK-293 cells dramatically reduced 1b protein levels. Thus, mutations specifically disrupting hERG 1b function are expected to reduce cardiac I Kr and enhance drug sensitivity, and represent a potential mechanism underlying inherited or acquired LQTS.

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Available from: Gail Robertson, Jul 08, 2014
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    • "The model revealed that IKr enhancement could be explained by dose-dependent loss of deactivation; point mutation analysis provided the structural mechanism behind this model prediction. Going a step further, Sale et al. [85] simulated the effects of the drug E4031, known to block IKr in its open state in a use dependent manner [86] on the human ventricular action potential. Drug E4031 is known to block IKr in its open state, in a use-dependent manner [86]. "
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    • "Smaller outward hERG currents are proarrhythmic in computational (Clancy and Rudy, 2001), animal (Arnaout et al., 2007; Brunner et al., 2008), and human stem cell models (Itzhaki et al., 2011) of type 2 LQTS. Furthermore, hERG1a currents are proarrhythmic in computational models, whereas hERG1a/hERG1b currents are not (Sale et al., 2008). Our results are most consistent with a mechanism where hERG1a NTRs dysregulate hERG1a/hERG1b channels, convert them into channels with hERG1a-like functional properties, and decrease outward repolarizing current. "
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    • "ards MTSES . None of the mutants showed diminished MTSES sensitivity ( not shown ) . Since a mutant without any cysteine in the N terminus was not electrophysiologically functional , we next utilized a naturally occurring splice variant that differs in the N terminus . The splice variant hERG1b is expressed in cardiac tissue ( Jones et al . 2004 ; Sale et al . 2008 ) and , compared with the more common variant hERG1a , has a truncated N terminus with a unique sequence of only 29 residues ( Fig . 3A ) . We found that homomeric hERG1b channels expressed in HEK293 cells , showing much faster deactivation kinetics , were dramatically less sensitive to MTSES ; ≥85% of the tail current remained in the p"
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