Cloning and Functional Characterization of the Smooth Muscle Ether-a-go-go-related Gene K+ Channel

Abstract

The human ether-a-go-go-related gene (HERG) product forms the pore-forming subunit of the delayed rectifier K ؉ channel in the heart. Unlike the cardiac isoform, the erg K ؉ channels in native smooth muscle demonstrate gating properties consistent with a role in maintaining resting potential. We have cloned the smooth muscle isoform of HERG, denoted as erg1-sm, from human and rabbit colon. erg1-sm is truncated by 101 amino acids in the C terminus due to a single nucleotide deletion in the 14th exon. Sequence alignment against HERG showed a substitution of alanine for valine in the S4 domain. When expressed in Xenopus oocytes, erg1-sm currents had much faster activation and deactivation kinetics compared with HERG.Step depolarization positive to ؊20 mV consistently produced a transient outward component. The threshold for activation of erg1-sm was ؊60 mV and steady-state conductance was ϳ10-fold greater than HERG near the resting potential of smooth muscle. Site-directed mutagenesis of alanine to valine in the S4 region of erg1-sm converted many of the properties to that of the cardiac HERG, including shifts in the voltage dependence of activation and slowing of deactivation. These studies define the functional role of a novel isoform of the ethera-go-go-related gene K ؉ channel in smooth muscle.The human ether-a-go-go related gene (HERG) 1 encodes for a K ϩ channel that is essential for normal repolarization of the cardiac action potential. HERG was originally cloned from the human hippocampal cDNA library by homology to the Drosophila K ϩ channel gene, eag (1), and according to the latest International Union of Pharmacology (IUPHAR) nomenclature has been termed as Kv 11.1. It is strongly expressed in the mammalian heart, and inherited mutations in this gene cause one form of long Q-T syndrome, LQT2 (2-4). HERG forms the pore-forming subunit of the rapidly activating delayed rectifier K ϩ current, IK r , in native cardiac myocytes (5, 6), and heterologous expression of the cardiac HERG channel in Xenopus oocytes and mammalian cells (7,8) has demonstrated the inwardly rectifying properties of this current. HERG channels slowly activate on depolarization and demonstrate fast inactivation at positive potentials, resulting in small outward currents at potentials positive to 0 mV; hence, inward rectification (9, 10). On repolarization close to resting potentials, large outward tail currents occur that slowly deactivate. These features of HERG channel dictate its role in repolarization of the cardiac action potential and frequency-dependent modulation of the action potential duration (11). Recent studies also suggest that an inwardly rectifying K ϩ conductance that is active near the resting membrane potential of gastrointestinal smooth muscle cells (12, 13), pituitary cells (14 -16), carotid body (15-17), and microglia (18) has properties similar to that of HERG channels. In these cells HERG conductance has been directly demonstrated in single cells, and in esophageal and stomach smooth muscle the presence of a "wind...

Expression and Functional Phenotype of MouseERGK+Channels in the Inner Ear: Potential Role in K+Regulation in the Inner Ear

“…First cloned from the cDNA library of the hippocampus by homology to ether-a-go-go, a Drosophila K ϩ channel gene (Warmke and Ganetzky, 1994), the channel has been identified in several tissues, including neurons (Emmi et al, 2000;Gullo et al, 2003) and muscle (Shoeb et al, 2003); however, only the high expression of the HERG protein in cardiac myocytes and the significant role of the channel current in the plateau phase of the cardiac action potential have been studied exhaustively . The importance of HERG protein in cardiac function is underpinned by the fact that mutations in the channel, as well as drugs blocking the current, result in long QT syndrome Vandenberg et al, 2001).…”

In vivo identification of genes that modify ether-a-go-go-related gene activity in Caenorhabditis elegans may also affect human cardiac arrhythmia

“…This possibility appears to be the case with mammalian ERG as well. In the mammalian heart, ERG is involved in repolarization of the cardiac action potential, whereas in other tissues, ERG appears to have alternate functions (most notably, setting resting-membrane potentials) (19)(20)(21)(22). We propose that the C. elegans unc-103 (n500) mutant strain provides a powerful in vivo system for the identification of ERG-modifying proteins that could prove to be crucial modulators of HERG and proarrhythmic risk in aLQTS.…”

Diminished hERG K+ channel activity facilitates strong human labour contractions but is dysregulated in obese women

“…The voltage dependence of the rate of deactivation of hERG current in smooth muscle cells is much weaker than that of hERG in cardiomyocytes 13,15,26 . Furthermore, in mouse myometrium the voltage dependence appears weaker in late pregnancy compared with non-pregnant tissue 8 .…”

“…The level of b-subunit protein increases in late pregnancy in mouse uterus 8 but the situation in labour has not been addressed. hERG has been identified in a range of smooth muscle tissues and pharmacological manipulation of its activity has an impact on contractile function [8][9][10][11][12][13][14][15] . Contractile function of uterine smooth muscle is impaired in obese women, who are more likely to experience failure to go into spontaneous labour and failure to progress through to vaginal delivery (18%) compared with normal weight controls (5%) [16][17][18] .…”

“…To test whether hERG regulated AP duration we first recorded membrane potential and contraction simultaneously in strips of myometrium from lean women (body mass index (BMI) o30) at term before labour onset. We used dofetilide (1 mM) and E-4031 (1 mM) as they are selective for hERG blockade 19 and have been commonly used in smooth [8][9][10][11][12][13][14][15] and cardiac 5 muscles. Both blockers caused a striking prolongation of the AP plateau and contraction (Fig.…”