Differential Expression of hERG1 Channel Isoforms Reproduces Properties of Native I Kr and Modulates Cardiac Action Potential Characteristics Anders Peter Larsen*, Søren-Peter Olesen The Danish National Research Foundation Centre for Cardiac Arrhythmia, Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark Abstract Background: The repolarizing cardiac rapid delayed rectifier current, IKr, is composed of ERG1 channels. It has been suggested that two isoforms of the ERG1 protein, ERG1a and ERG1b, both contribute to IKr. Marked heterogeneity in the kinetic properties of native IKr has been described. We hypothesized that the heterogeneity of native IKr can be reproduced by differential expression of ERG1a and ERG1b isoforms. Furthermore, the functional consequences of differential expression of ERG1 isoforms were explored as a potential mechanism underlying native heterogeneity of action potential duration (APD) and restitution. Methodology/Principal Findings: The results show that the heterogeneity of native IKr can be reproduced in heterologous expression systems by differential expression of ERG1a and ERG1b isoforms. Characterization of the macroscopic kinetics of ERG1 currents demonstrated that these were dependent on the relative abundance of ERG1a and ERG1b. Furthermore, we used a computational model of the ventricular cardiomyocyte to show that both APD and the slope of the restitution curve may be modulated by varying the relative abundance of ERG1a and ERG1b. As the relative abundance of ERG1b was increased, APD was gradually shortened and the slope of the restitution curve was decreased. Conclusions/Significance: Our results show that differential expression of ERG1 isoforms may explain regional heterogeneity of IKr kinetics. The data demonstrate that subunit dependent changes in channel kinetics are important for the functional properties of ERG1 currents and hence IKr. Importantly, our results suggest that regional differences in the relative abundance of ERG1 isoforms may represent a potential mechanism underlying the heterogeneity of both APD and APD restitution observed in mammalian hearts. Citation: Larsen AP, Olesen S-P (2010) Differential Expression of hERG1 Channel Isoforms Reproduces Properties of Native IKr and Modulates Cardiac Action Potential Characteristics. PLoS ONE 5(2): e9021. doi:10.1371/journal.pone.0009021 Editor: Arnold Schwartz, University of Cincinnati, United States of America Received October 13, 2009; Accepted January 14, 2010; Published February 2, 2010 Copyright: 2010 Larsen, Olesen. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by the Danish National Research Foundation (http://www.dg.dk/). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: spo@mfi.ku.dk Introduction The molecular correlate of the cardiac rapid delayed rectifier current, IKr, is the ERG1 (Kv11.1) ion channel [1,2]. Mutations in the human variant of ERG1 (hERG1) has been linked to the long QT [3] and short QT syndromes [4]. Two isoforms of ERG1, ERG1a and ERG1b, have been detected on the protein level in both rat, canine and human cardiac tissue [5]. The isoforms differ only in the N-terminal region. Compared to ERG1a, the N- terminus of ERG1b is 340 amino acids shorter. However, the initial 36 residues of ERG1b are unique to this isoform [6,7]. Quantification of mRNA from healthy human hearts have shown that hERG1b accounts for 10–20% of the total hERG1 transcripts [8]. Very recently, a mutation in the unique N-terminus of hERG1b was discovered in a patient with long QT syndrome [9] highlighting the importance of this isoform in cardiac repolariza- tion. However, little is known about regional differences in the relative abundance of hERG1a and hERG1b or the potential functional consequences thereof. Gintant [10] used voltage ramps to characterize IKr transients in isolated canine cardiomyocytes. The IKr transient showed only little variation in the peak current density but a large variation (20–30 mV) in the membrane potential at which the current peaked. This variation in ‘peak potential’ was evident in both atrial, ventricular and purkinje fiber cells. These observations suggest that the kinetic properties of IKr are different between individual cardiac cells. Theoretically, in terms of ERG1 channel gating, differences in kinetics of recovery from inactivation and kinetics of deactivation could explain such a variation in ‘peak potential’. Interestingly, it has been shown that the deactivation rate of IKr is faster in epicardial than in midmyocardial cells obtained from canine left ventricular muscle [11] supporting that regional differences in the kinetics of IKr exist in mammalian hearts. In heterologous expression systems, heteromeric ERG1a/b, homomeric ERG1a and homomeric ERG1b channels differ in their kinetic properties [6,8,9,12]. Potentially, by varying the relative abundance of ERG1 isoforms, the macroscopic kinetics of PLoS ONE | www.plosone.org 1 February 2010 | Volume 5 | Issue 2 | e9021