Human ether-a-go-go related gene (hERG) K+ channels: function and dysfunction

Blockade of permeation by potassium but normal gating of the G628S nonconducting hERG channel mutant

G628S is a mutation in the signature sequence that forms the selectivity filter of the human ether-a-go-go-related gene (hERG) channel (GFG) and is associated with long-QT2 syndrome. G628S channels are known to have a dominant-negative effect on hERG currents, and the mutant is therefore thought to be nonfunctional. This study aims to assess the physiological mechanism that prevents the surface-expressing G628S channels from conducting ions. We used voltage-clamp fluorimetry along with two-microelectrode voltage clamping in Xenopus oocytes to confirm that the channels express well at the surface, and to show that they are actually functional, with activation kinetics comparable to that of wild-type, and that the mutation leads to a reduced selectivity to potassium. Although ionic currents are not detected in physiological solutions, removing extracellular K(+) results in the appearance of an inward Na(+)-dependent current. Using whole-cell patch clamp in mammalian transfected cells, we demonstrate that the G628S channels conduct Na(+), but that this can be blocked by both intracellular and higher-than-physiological extracellular K(+). Using solutions devoid of K(+) allows the appearance of nA-sized Na(+) currents with activation and inactivation gating analogous to wild-type channels. The G628S channels are functionally conducting but are normally blocked by intracellular K(+).

Partially dominant mutant channel defect corresponding with intermediate LQT2 phenotype

BACKGROUND

The hereditary Long QT Syndrome is a common cardiac disorder where ventricular repolarization is delayed, abnormally prolonging the QTc interval on electrocardiograms. LQTS is linked to various genetic loci, including the KCNH2 (HERG) gene that encodes the α-subunit of the cardiac potassium channel that carries I(Kr). Here, we report and characterize a novel pathologic missense mutation, G816V HERG, in a patient with sudden cardiac death.

METHODS

Autopsy-derived tissue sample was used for DNA extraction and sequencing from an unexpected sudden death victim. The G816V HERG mutation was studied using heterologous expression in mammalian cell culture, whole cell patch clamp, confocal immunofluorescence, and immunochemical analyses.

RESULTS

The mutant G816V HERG channel has reduced protein expression and shows a trafficking defective phenotype that is incapable of carrying current when expressed at physiological temperatures. The mutant channel showed reduced cell surface localization compared to wild-type HERG (WT HERG) but the mutant and wild-type subunits are capable of interacting. Expression studies at reduced temperatures enabled partial rescue of the trafficking defect with appearance of potassium currents, albeit with reduced current density and altered voltage-dependent activation. Lastly, we examined a potential role for hypokalemia as a contributory factor to the patient's lethal arrhythmia by possible low-potassium-induced degradation of WT HERG and haplo-insufficiency of G816V HERG.

CONCLUSION

The G816V mutation in HERG causes a trafficking defect that acts in a partially dominant negative manner. This intermediate severity defect agrees with the mild clinical presentation in other family members harboring the same mutation. Possible hypokalemia in the proband induced WT HERG degradation combined with haplo-insufficiency may have further compromised repolarization reserve and contributed to the lethal arrhythmia.

K(+) channels as therapeutic targets in oncology

Ion channels are involved in a variety of tumors. In particular, potassium channels are expressed abnormally in many cancer types, where their pharmacologic manipulation impairs tumor progression. Since this group of molecules has been successfully targeted for decades in other therapeutic areas, there is a significant body of knowledge on the pharmacology of potassium channels. Several groups of potassium channels with defined molecular identities have been proposed as candidates for therapeutic intervention. The strategies put forward range from classical small molecule blockade to gene therapy approaches, and include the use of potassium channels as targets for adjuvant therapy. We will discuss the reasons for these proposals and explore possible future developments.

Identification of human Ether-à-go-go related gene modulators by three screening platforms in an academic drug-discovery setting

The human Ether-à-go-go related gene (hERG) potassium channel is responsible for the rapid delayed rectifier potassium current that plays a critical role in the repolarization of cardiomyocytes during the cardiac action potential. In humans, inhibition of hERG by drugs can prolong the electrocardiographic QT interval, which, in rare instance, leads to ventricular arrhythmia and sudden cardiac death. As such, several medications that block hERG channels in vitro have been withdrawn from the market due to QT prolongation and arrhythmias. The current FDA guidelines recommend that drug candidates destined for human use be evaluated for potential hERG activity ( www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/ucm074963.pdf ). Here, we employed automated planar patch clamp (APPC), high-throughput fluorescent Tl(+) flux, and moderate-throughput [³H]dofetilide competition binding assays to characterize a panel of 49 drugs for their activities at the hERG channel. Notably, we used the same HEK293-hERG cell line for all assays, facilitating comparisons of hERG potencies across screening platforms. In general, hERG inhibitors were most potent in APPC assays, intermediate potent in [³H]dofetilide binding assays, and least potent in Tl(+) flux assays. Binding affinity constants (pK(i) values) and Tl(+) flux potencies (pEC₅₀ values) correlated well with APPC pEC₅₀ values. Further, the inhibitory potencies of many known hERG inhibitors in APPC matched literature values from manual and/or automated patch clamp systems. We also developed a novel fluorescent Tl(+) flux assays to measure the effects of drugs that modulate hERG trafficking and surface expression.

Activation of human ether-a-go-go related gene (hERG) potassium channels by small molecules

Human ether-a-go-go related gene (hERG) potassium (K(+)) channels play a critical role in cardiac action potential repolarization. Mutations that reduce hERG conductance or surface expression may cause congenital long QT syndrome (LQTS). However, the channels can be inhibited by structurally diverse small molecules, resulting in an acquired form of LQTS. Consequently, small molecules that increase the hERG current may be of value for treatment for LQTS. So far, nine hERG activators have been reported. The aim of this review is to discuss recent advances concerning the identification and action mechanism of hERG activators.

The N-terminal tail of hERG contains an amphipathic α-helix that regulates channel deactivation

The cytoplasmic N–terminal domain of the human ether–a–go–go related gene (hERG) K⁺ channel is critical for the slow deactivation kinetics of the channel. However, the mechanism(s) by which the N–terminal domain regulates deactivation remains to be determined. Here we show that the solution NMR structure of the N–terminal 135 residues of hERG contains a previously described Per–Arnt–Sim (PAS) domain (residues 26–135) as well as an amphipathic α–helix (residues 13–23) and an initial unstructured segment (residues 2–9). Deletion of residues 2–25, only the unstructured segment (residues 2–9) or replacement of the α–helix with a flexible linker all result in enhanced rates of deactivation. Thus, both the initial flexible segment and the α–helix are required but neither is sufficient to confer slow deactivation kinetics. Alanine scanning mutagenesis identified R5 and G6 in the initial flexible segment as critical for slow deactivation. Alanine mutants in the helical region had less dramatic phenotypes. We propose that the PAS domain is bound close to the central core of the channel and that the N–terminal α–helix ensures that the flexible tail is correctly orientated for interaction with the activation gating machinery to stabilize the open state of the channel.

Eag and HERG potassium channels as novel therapeutic targets in cancer

Voltage gated potassium channels have been extensively studied in relation to cancer. In this review, we will focus on the role of two potassium channels, Ether à-go-go (Eag), Human ether à-go-go related gene (HERG), in cancer and their potential therapeutic utility in the treatment of cancer. Eag and HERG are expressed in cancers of various organs and have been implicated in cell cycle progression and proliferation of cancer cells. Inhibition of these channels has been shown to reduce proliferation both in vitro and vivo studies identifying potassium channel modulators as putative inhibitors of tumour progression. Eag channels in view of their restricted expression in normal tissue may emerge as novel tumour biomarkers.

Rescue of mutated cardiac ion channels in inherited arrhythmia syndromes

Inherited arrhythmia syndromes comprise an increasingly complex group of diseases involving mutations in multiple genes encoding ion channels, ion channel accessory subunits and channel interacting proteins, and various regulatory elements. These mutations serve to disrupt normal electrophysiology in the heart, leading to increased arrhythmogenic risk and death. These diseases have added impact as they often affect young people, sometimes without warning. Although originally thought to alter ion channel function, it is now increasingly recognized that mutations may alter ion channel protein and messenger RNA processing, to reduce the number of channels reaching the surface membrane. For many of these mutations, it is also known that several interventions may restore protein processing of mutant channels to increase their surface membrane expression toward normal. In this article, we reviewed inherited arrhythmia syndromes, focusing on long QT syndrome type 2, and discuss the complex biology of ion channel trafficking and pharmacological rescue of disease-causing mutant channels. Pharmacological rescue of misprocessed mutant channel proteins, or their transcripts providing appropriate small molecule drugs can be developed, has the potential for novel clinical therapies in some patients with inherited arrhythmia syndromes.

Constitutive recycling of the store-operated Ca2+ channel Orai1 and its internalization during meiosis

The egg's competency to activate at fertilization and transition to embryogenesis is dependent on its ability to generate a fertilization-specific Ca(2+) transient. To endow the egg with this capacity, Ca(2+) signals remodel during oocyte maturation, including inactivation of the primary Ca(2+) influx pathway store-operated Ca(2+) entry (SOCE). SOCE inactivation is coupled to internalization of the SOCE channel, Orai1. In this study, we show that Orai1 internalizes during meiosis through a caveolin (Cav)- and dynamin-dependent endocytic pathway. Cav binds to Orai1, and we map a Cav consensus-binding site in the Orai1 N terminus, which is required for Orai1 internalization. Furthermore, at rest, Orai1 actively recycles between an endosomal compartment and the cell membrane through a Rho-dependent endocytic pathway. A significant percentage of total Orai1 is intracellular at steady state. Store depletion completely shifts endosomal Orai1 to the cell membrane. These results define vesicular trafficking mechanisms in the oocyte that control Orai1 subcellular localization at steady state, during meiosis, and after store depletion.

Intracellular regions of the Eag potassium channel play a critical role in generation of voltage-dependent currents

Folding, assembly, and trafficking of ion channels are tightly controlled processes and are important for biological functions relevant to health and disease. Here, we report that functional expression of the Eag channel is temperature-sensitive by a mechanism that is independent of trafficking or surface targeting of the channel protein. Eag channels in cells grown at 37 °C exhibit voltage-evoked gating charge movements but fail to conduct K(+) ions. By mutagenesis and chimeric channel studies, we show that the N- and C-terminal regions are involved in controlling a step after movement of the voltage sensor, as well as in regulating biophysical properties of the Eag channel. Synthesis and assembly of Eag at high temperature disrupt the ability of these domains to carry out their function. These results suggest an important role of the intracellular regions in the generation of Eag currents.

A major role for HERG in determining frequency of reentry in neonatal rat ventricular myocyte monolayer

RATIONALE

the rapid delayed rectifier potassium current, I(Kr), which flows through the human ether-a-go-go-related (hERG) channel, is a major determinant of the shape and duration of the human cardiac action potential (APD). However, it is unknown whether the time dependency of I(Kr) enables it to control APD, conduction velocity (CV), and wavelength (WL) at the exceedingly high activation frequencies that are relevant to cardiac reentry and fibrillation.

OBJECTIVE

to test the hypothesis that upregulation of hERG increases functional reentry frequency and contributes to its stability.

METHODS AND RESULTS

using optical mapping, we investigated the effects of I(Kr) upregulation on reentry frequency, APD, CV, and WL in neonatal rat ventricular myocyte (NRVM) monolayers infected with GFP (control), hERG (I(Kr)), or dominant negative mutant hERG G628S. Reentry frequency was higher in the I(Kr)-infected monolayers (21.12 ± 0.8 Hz; n=43 versus 9.21 ± 0.58 Hz; n=16; P<0.001) but slightly reduced in G628S-infected monolayers. APD(80) in the I(Kr)-infected monolayers was shorter (>50%) than control during pacing at 1 to 5 Hz. CV was similar in both groups at low frequency pacing. In contrast, during high-frequency reentry, the CV measured at varying distances from the center of rotation was significantly faster in I(Kr)-infected monolayers than controls. Simulations using a modified NRVM model predicted that rotor acceleration was attributable, in part, to a transient hyperpolarization immediately following the AP. The transient hyperpolarization was confirmed experimentally.

CONCLUSIONS

hERG overexpression dramatically accelerates reentry frequency in NRVM monolayers. Both APD and WL shortening, together with transient hyperpolarization, underlies the increased rotor frequency and stability.

Pharmacological modulation of voltage-gated potassium channels as a therapeutic strategy

IMPORTANCE OF THE FIELD

The human genome encodes at least 40 distinct voltage-gated potassium channel subtypes, which vary in regional expression, pharmacological and biophysical properties. Voltage-dependent potassium (Kv) channels help orchestrate many of the physiological and pathophysiological processes that promote and sometimes hinder the healthy functioning of our bodies.

AREAS COVERED IN THIS REVIEW

This review summarizes patent and scientific literature reports from the past decade highlighting the opportunities that Kv channels offer for the development of new therapeutic interventions for a wide variety of disorders.

WHAT THE READER WILL GAIN

The reader will gain an insight from an analysis of the associations of different Kv family members with disease processes, summary and evaluation of the development of therapeutically relevant pharmacological modulators of these channels, particularly focusing on proprietary agents being developed.

TAKE HOME MESSAGE

Development of new drugs that target Kv channels continue to be of great interest but is proving to be challenging. Nevertheless, opportunities for Kv channel modulators to have an impact on a wide range of disorders in the future remain an exciting prospect.

Intracellular linkers are involved in Mg2+-dependent modulation of the Eag potassium channel

Modulation of activation kinetics by divalent ions is one of the characteristic features of Eag channels. Here, we report that Mg(2+)-dependent deceleration of Eag channel activation is significantly attenuated by a G297E mutation, which exhibits a gain-of-function phenotype in Drosophila by suppressing the effect of shaker mutation on behavior and neuronal excitability. The G297 residue is located in the intracellular linker of transmembrane segments S2 and S3, and is thus not involved in direct binding of Mg(2+) ions. Moreover, mutation of the only positively charged residue in the other intracellular linker between S4 and S5 also results in a dramatic reduction of Mg(2+)-dependent modulation of Eag activation kinetics. Collectively, the two mutations in eag eliminate or even paradoxically reverse the effect of Mg(2+) on channel activation and inactivation kinetics. Together, these results suggest an important role of the intracellular linker regions in gating processes of Eag channels.

Two four-marker haplotypes on 7q36.1 region indicate that the potassium channel gene HERG1 (KCNH2, Kv11.1) is related to schizophrenia: a case control study

Background

The pathobiology of schizophrenia is still unclear. Its current treatment mainly depends on antipsychotic drugs. A leading adverse effect of these medications is the acquired long QT syndrome, which results from the blockade of cardiac HERG1 channels (human ether-a-go-go-related gene potassium channels 1) by antipsychotic agents. The HERG1 channel is encoded by HERG1 (KCNH2, Kv11.1) gene and is most highly expressed in heart and brain. Genetic variations in HERG1 predispose to acquired long QT syndrome. We hypothesized that the blockade of HERG1 channels by antipsychotics might also be significant for their therapeutic mode of action, indicating a novel mechanism in the pathogenesis of schizophrenia.

Methods

We genotyped four single nucleotide polymorphisms (SNPs) in 7q36.1 region (two SNPs, rs1805123 and rs3800779, located on HERG1, and two SNPs, rs885684 and rs956642, at the 3'-downstream intergenic region) and then performed single SNP and haplotype association analyses in 84 patients with schizophrenia and 74 healthy controls after the exclusion of individuals having prolonged or shortened QT interval on electrocardiogram.

Results

Our analyses revealed that both genotype and allele frequencies of rs3800779 (c.307+585G>T) were significantly different between populations (P = 0.023 and P = 0.018, respectively). We also identified that two previously undescribed four-marker haplotypes which are nearly allelic opposite of each other and located in chr7:150225599-150302147bp position encompassing HERG1 were either overrepresented (A-A-A-T, the at-risk haplotype, P = 0.0007) or underrepresented (C-A-C-G, the protective haplotype, P = 0.005) in patients compared to controls.

Conclusions

Our results indicate that the potassium channel gene HERG1 is related to schizophrenia. Our findings may also implicate the whole family of HERG channels (HERG1, HERG2 and HERG3) in the pathogenesis of psychosis and its treatment.

Modulation of cardiac ERG1 K(+) channels by cGMP signaling

Different K(+) currents have been implicated in the myocardial action potential repolarization including the I(Kr). ERG1 alpha subunits, identified as the molecular correlate of I(Kr), have been shown to form heteromultimeric channels in the heart and their activity is modulated by a complex interplay of signal transduction events. Using electrophysiological techniques, we examined the effects of the cGMP-analogue 8-Br-cGMP on rat and guinea-pig papillary action potential duration (APD), on the biophysical properties of heterologously expressed homo- and heteromeric ERG1 channels, and on cardiac I(Kr). 8-Br-cGMP prolonged APD by about 25% after pharmacological inhibition of L-type Ca(2+) currents and I(Ks). The prolongation was completely abolished by prior application of the hERG channel blocker E-4031 or the protein kinase G (PKG) inhibitor Rp-8-Br-cGMPS. Expression analysis revealed the presence of both ERG1a and -1b subunits in rat papillary muscle. Both 8-Br-cGMP and ANP inhibited heterologously expressed ERG1b and even stronger ERG1a/1b channels, whereas ERG1a channels remained unaffected. The inhibitory 8-Br-cGMP effects were PKG-dependent and involved a profound ERG current reduction, which was also observed with cardiac AP clamp recordings. Measurements of I(Kr) from isolated mouse cardiomyocytes using Cs(+) as charge carrier exhibited faster deactivation kinetics in atrial than in ventricular myocytes consistent with a higher relative expression of ERG1b transcripts in atria than in ventricles. 8-Br-cGMP significantly reduced I(Kr) in atrial, but not in ventricular myocytes. These findings provide first evidence that through heteromeric assembly ERG1 channels become a critical target of cGMP-PKG signaling linking cGMP accumulation to cardiac I(Kr) modulation.

Bioelectric controls of cell proliferation: ion channels, membrane voltage and the cell cycle

All cells possess long-term, steady-state voltage gradients across the plasma membrane. These transmembrane potentials arise from the combined activity of numerous ion channels, pumps and gap junction complexes. Increasing data from molecular physiology now reveal that the role of changes in membrane voltage controls, and is in turn controlled by, progression through the cell cycle. We review recent functional data on the regulation of mitosis by bioelectric signals, and the function of membrane voltage and specific potassium, sodium and chloride ion channels in the proliferation of embryonic, somatic and neoplastic cells. Its unique properties place this powerful, well-conserved, but still poorly-understood signaling system at the center of the coordinated cellular interactions required for complex pattern formation. Moreover, disregulation of ion channel expression and function is increasingly observed to be not only a useful marker but likely a functional element in oncogenesis. New advances in genomics and the development of in vivo biophysical techniques suggest exciting opportunities for molecular medicine, bioengineering and regenerative approaches to human health.

HERG1 channelopathies

Human ether a go-go-related gene type 1 (hERG1) K+ channels conduct the rapid delayed rectifier K+ current and mediate action potential repolarization in the heart. Mutations in KCNH2 (the gene that encodes hERG1) causes LQT2, one of the most common forms of long QT syndrome, a disorder of cardiac repolarization that predisposes affected subjects to ventricular arrhythmia and increases the risk of sudden cardiac death. Hundreds of LQT2-associated mutations have been described, and most cause a loss of function by disrupting subunit folding, assembly, or trafficking of the channel to the cell surface. Loss-of-function mutations in hERG1 channels have also recently been implicated in epilepsy. A single gain-of-function mutation has been described that causes short QT syndrome and cardiac arrhythmia. In addition, up-regulation of hERG1 channel expression has been demonstrated in specific tumors and has been associated with skeletal muscle atrophy in mice.

Ion-selective optodes measure extracellular potassium flux in excitable cells

Optodes have been used for detection of ionic concentrations and fluxes for several years. However, their uses in biomedical applications have not yet been fully explored. This study investigates optodes as a potential sensor platform for monitoring cellular ion flux with attendant implications in the field of drug screening and toxicology. A prototype system was developed to quantitatively measure extracellular potassium flux from a monolayer of cardiomyocytes. Optodes were created and immobilized on a glass coverslip for fluorescent imaging. The system detected potassium (K(+) ) ion flux during the repolarization phase of the cardiac action potential and further detected a decrease in the magnitude of the flux in the presence of a known K(+) channel inhibitor by optically monitoring local K(+) ion concentrations during field stimulation of the cardiomyocyte monolayer.