Fast inactivation causes rectification of the IKr channel

The S4-S5 linker acts as a signal integrator for HERG K+ channel activation and deactivation gating

Human ether-à-go-go-related gene (hERG) K(+) channels have unusual gating kinetics. Characterised by slow activation/deactivation but rapid inactivation/recovery from inactivation, the unique gating kinetics underlie the central role hERG channels play in cardiac repolarisation. The slow activation and deactivation kinetics are regulated in part by the S4-S5 linker, which couples movement of the voltage sensor domain to opening of the activation gate at the distal end of the inner helix of the pore domain. It has also been suggested that cytosolic domains may interact with the S4-S5 linker to regulate activation and deactivation kinetics. Here, we show that the solution structure of a peptide corresponding to the S4-S5 linker of hERG contains an amphipathic helix. The effects of mutations at the majority of residues in the S4-S5 linker of hERG were consistent with the previously identified role in coupling voltage sensor movement to the activation gate. However, mutations to Ser543, Tyr545, Gly546 and Ala548 had more complex phenotypes indicating that these residues are involved in additional interactions. We propose a model in which the S4-S5 linker, in addition to coupling VSD movement to the activation gate, also contributes to interactions that stabilise the closed state and a separate set of interactions that stabilise the open state. The S4-S5 linker therefore acts as a signal integrator and plays a crucial role in the slow deactivation kinetics of the channel.

Molecular coupling in the human ether-a-go-go-related gene-1 (hERG1) K+ channel inactivation pathway

Emerging evidence suggests that K(+) channel inactivation involves coupling between residues in adjacent regions of the channel. Human ether-a-go-go-related gene-1 (hERG1) K(+) channels undergo a fast inactivation gating process that is crucial for maintaining electrical stability in the heart. The molecular mechanisms that drive inactivation in hERG1 channels are unknown. Using alanine scanning mutagenesis, we show that a pore helix residue (Thr-618) that points toward the S5 segment is critical for normal inactivation gating. Amino acid substitutions at position 618 modulate the free energy of inactivation gating, causing enhanced or reduced inactivation. Mutation of an S5 residue that is predicted to be adjacent to Thr-618 (W568L) abolishes inactivation and alters ion selectivity. The introduction of the Thr-618-equivalent residue in Kv1.5 enhances inactivation. Molecular dynamic simulations of the Kv1.2 tetramer reveal van der Waals coupling between hERG1 618- and 568-equivalent residues and a significant increase in interaction energies when threonine is introduced at the 618-equivalent position. We propose that coupling between the S5 segment and pore helix may participate in the inactivation process in hERG1 channels.

Molecular determinants of interactions between the N-terminal domain and the transmembrane core that modulate hERG K+ channel gating

A conserved eag domain in the cytoplasmic amino terminus of the human ether-a-go-go-related gene (hERG) potassium channel is critical for its slow deactivation gating. Introduction of gene fragments encoding the eag domain are able to restore normal deactivation properties of channels from which most of the amino terminus has been deleted, and also those lacking exclusively the eag domain or carrying a single point mutation in the initial residues of the N-terminus. Deactivation slowing in the presence of the recombinant domain is not observed with channels carrying a specific Y542C point mutation in the S4–S5 linker. On the other hand, mutations in some initial positions of the recombinant fragment also impair its ability to restore normal deactivation. Fluorescence resonance energy transfer (FRET) analysis of fluorophore-tagged proteins under total internal reflection fluorescence (TIRF) conditions revealed a substantial level of FRET between the introduced N-terminal eag fragments and the eag domain-deleted channels expressed at the membrane, but not between the recombinant eag domain and full-length channels with an intact amino terminus. The FRET signals were also minimized when the recombinant eag fragments carried single point mutations in the initial portion of their amino end, and when Y542C mutated channels were used. These data suggest that the restoration of normal deactivation gating by the N-terminal recombinant eag fragment is an intrinsic effect of this domain directed by the interaction of its N-terminal segment with the gating machinery, likely at the level of the S4–S5 linker.

Cardiac ventricular repolarization reserve: a principle for understanding drug-related proarrhythmic risk

Cardiac repolarization abnormalities can be caused by a wide range of cardiac and non-cardiac compounds and may lead to the development of life-threatening Torsades de Pointes (TdP) ventricular arrhythmias. Drug-induced torsades de pointes is associated with unexpected and unexplained sudden cardiac deaths resulting in the withdrawal of several compounds in the past. To better understand the mechanism of such unexpected sudden cardiac deaths, the concept of repolarization reserve has recently emerged. According to this concept, pharmacological, congenital or acquired impairment of one type of transmembrane ion channel does not necessarily result in excessive repolarization changes because other repolarizing currents can take over and compensate. In this review, the major factors contributing to repolarization reserve are discussed in the context of their clinical significance in physiological and pathophysiological conditions including drug administration, genetic defects, heart failure, diabetes mellitus, gender, renal failure, hypokalaemia, hypothyroidism and athletes' sudden deaths. In addition, pharmacological support of repolarization reserve as a possible therapeutic option is discussed. Some methods for the quantitative estimation of repolarization reserve are also recommended. It is concluded that repolarization reserve should be considered by safety pharmacologists to better understand, predict and prevent previously unexplained drug-induced sudden cardiac deaths.

hERG potassium channel gating is mediated by N- and C-terminal region interactions

Human ether-á-go-go–related gene (hERG) potassium channels have voltage-dependent closing (deactivation) kinetics that are unusually slow. A Per-Arnt-Sim (PAS) domain in the cytoplasmic N-terminal region of hERG regulates slow deactivation by making a direct interaction with another part of the hERG channel. The mechanism for slow deactivation is unclear, however, because the other regions of the channel that participate in regulation of deactivation are not known. To identify other functional determinants of slow deactivation, we generated hERG channels with deletions of the cytoplasmic C-terminal regions. We report that hERG channels with deletions of the cyclic nucleotide–binding domain (CNBD) had accelerated deactivation kinetics that were similar to those seen in hERG channels lacking the PAS domain. Channels with dual deletions of the PAS domain and the CNBD did not show further acceleration in deactivation, indicating that the PAS domain and the CNBD regulate deactivation by a convergent mechanism. A recombinant PAS domain that we previously showed could directly regulate PAS domain–deleted channels did not regulate channels with dual deletions of the PAS domain and CNBD, suggesting that the PAS domain did not interact with CNBD-deleted channels. Biochemical protein interaction assays showed that glutathione S-transferase (GST)–PAS (but not GST) bound to a CNBD-containing fusion protein. Coexpression of PAS domain–deleted subunits (with intact C-terminal regions) and CNBD-deleted subunits (with intact N-terminal regions) resulted in channels with partially restored slow deactivation kinetics, suggesting regulatory intersubunit interactions between PAS domains and CNBDs. Together, these data suggest that the mechanism for regulation of slow deactivation in hERG channels is an interaction between the N-terminal PAS domain and the C-terminal CNBD.

Demonstration of physical proximity between the N terminus and the S4-S5 linker of the human ether-a-go-go-related gene (hERG) potassium channel

Potassium channels encoded by the human ether-à-go-go-related gene (hERG) contribute to cardiac repolarization as a result of their characteristic gating properties. The hERG channel N terminus acts as a crucial determinant in gating. It is also known that the S4-S5 linker couples the voltage-sensing machinery to the channel gate. Moreover, this linker has been repeatedly proposed as an interaction site for the distal portion of the N terminus controlling channel gating, but direct evidence for such an interaction is still lacking. In this study, we used disulfide bond formation between pairs of engineered cysteines to demonstrate the close proximity between the beginning of the N terminus and the S4-S5 linker. Currents from channels with introduced cysteines were rapidly and strongly attenuated by an oxidizing agent, this effect being maximal for cysteine pairs located around amino acids 3 and 542 of the hERG sequence. The state-dependent modification of the double-mutant channels, but not the single-cysteine mutants, and the ability to readily reverse modification with the reducing agent dithiothreitol indicate that a disulfide bond is formed under oxidizing conditions, locking the channels in a non-conducting state. We conclude that physical interactions between the N-terminal-most segment of the N terminus and the S4-S5 linker constitute an essential component of the hERG gating machinery, thus providing a molecular basis for previous data and indicating an important contribution of these cytoplasmic domains in controlling its unusual gating and hence determining its physiological role in setting the electrical behavior of cardiac and other cell types.

Resonance assignment and secondary structure prediction of the N-terminal domain of hERG (Kv11.1)

The hERG (human ether-à-go-go related gene) channel is a member of the eag voltage-gated K(+) channel family. In common with other members of this family, it has a subunit topology of six trans-membrane helices that tetramerise to form a functional ion-channel. In addition, hERG has an N-terminal PAS (Per, Arnt and Sim) domain and a C-terminal cyclic nucleotide binding domain (cNBD). Both these cytosolic domains are involved in regulation of the gating of the ion channel as demonstrated by inheritable mutations in these domains that result in either a loss, or a gain, in function. Here we report near complete backbone and side chain (15)N, (13)C and (1)H assignments for the N-terminal domain (residues 1-135) including the functionally critical first 26 residues. Comparison with the secondary structure of the crystal structure (residues 26-135) suggests that the solution and crystal structures are very similar except that the solution structure contains an additional helix between residues 12-23; a region of the protein important for channel gating.

Mutations within the S4-S5 linker alter voltage sensor constraints in hERG K+ channels

Human ether-a-go-go related gene (hERG) channel gating is associated with slow activation, yet the mechanistic basis for this is unclear. Here, we examine the effects of mutation of a unique glycine residue (G546) in the S4-S5 linker on voltage sensor movement and its coupling to pore gating. Substitution of G546 with residues possessing different physicochemical properties shifted activation gating by ∼-50 mV (with the exception of G546C). With the activation shift taken into account, the time constant of activation was also accelerated, suggesting a stabilization of the closed state by ∼1.6-4.3 kcal/mol (the energy equivalent of one to two hydrogen bonds). Predictions of the α-helical content of the S4-S5 linker suggest that the presence of G546 in wild-type hERG provides flexibility to the helix. Deactivation gating was affected differentially by the G546 substitutions. G546V induced a pronounced slow component of closing that was voltage-independent. Fluorescence measurements of voltage sensor movement in G546V revealed a slow component of voltage sensor return that was uncoupled from charge movement, suggesting a direct effect of the mutation on voltage sensor movement. These data suggest that G546 plays a critical role in channel gating and that hERG channel closing involves at least two independently modifiable reconfigurations of the voltage sensor.

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.

Mechanistic insight into human ether-à-go-go-related gene (hERG) K+ channel deactivation gating from the solution structure of the EAG domain

Human ether-à-go-go-related gene (hERG) K⁺ channels have a critical role in cardiac repolarization. hERG channels close (deactivate) very slowly, and this is vital for regulating the time course and amplitude of repolarizing current during the cardiac action potential. Accelerated deactivation is one mechanism by which inherited mutations cause long QT syndrome and potentially lethal arrhythmias. hERG deactivation is highly dependent upon an intact EAG domain (the first 135 amino acids of the N terminus). Importantly, deletion of residues 2–26 accelerates deactivation to a similar extent as removing the entire EAG domain. These and other experiments suggest the first 26 residues (NT1–26) contain structural elements required to slow deactivation by stabilizing the open conformation of the pore. Residues 26–135 form a Per-Arnt-Sim domain, but a structure for NT1–26 has not been forthcoming, and little is known about its site of interaction on the channel. In this study, we present an NMR structure for the entire EAG domain, which reveals that NT1–26 is structurally independent from the Per-Arnt-Sim domain and contains a stable amphipathic helix with one face being positively charged. Mutagenesis and electrophysiological studies indicate that neutralizing basic residues and breaking the amphipathic helix dramatically accelerate deactivation. Furthermore, scanning mutagenesis and molecular modeling studies of the cyclic nucleotide binding domain suggest that negatively charged patches on its cytoplasmic surface form an interface with the NT1–26 domain. We propose a model in which NT1–26 obstructs gating motions of the cyclic nucleotide binding domain to allosterically stabilize the open conformation of the pore.

The calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide directly blocks human ether à-go-go-related gene potassium channels stably expressed in human embryonic kidney 293 cells

BACKGROUND AND PURPOSE

N-(6-aminohexyl)-5-chloro-1-naphthalene sulphonamide (W-7) is a well-known calmodulin inhibitor used to study calmodulin regulation of intracellular Ca(2+) signalling-related process. Here, we have determined whether W-7 would inhibit human ether gene (hERG or K(v) 11.1) potassium channels, hK(v) 1.5 channels or hK(IR) 2.1 channels expressed in human embryonic kidney (HEK) 293 cells.

EXPERIMENTAL APPROACH

The hERG channel current, hK(v) 1.5 channel current or hK(IR) 2.1 channel current was recorded with a whole-cell patch clamp technique.

KEY RESULTS

It was found that the calmodulin inhibitor W-7 blocked hERG, hK(v) 1.5 and hK(IR) 2.1 channels. W-7 decreased the hERG current (I(hERG) ) in a concentration-dependent manner (IC(50) : 3.5 µM), and the inhibition was more significant at depolarization potentials between +10 and +60 mV. The hERG mutations in the S6 region Y652A and F656V, and in the pore helix S631A, had the IC(50) s of 5.5, 9.8 and 25.4 µM respectively. In addition, the compound inhibited hK(v) 1.5 and hK(IR) 2.1 channels with IC(50) s of 6.5 and 13.4 µM respectively.

CONCLUSION AND IMPLICATIONS

These results indicate that the calmodulin inhibitor W-7 exerts a direct channel-blocking effect on hERG, hK(v) 1.5 and hK(IR) 2.1 channels stably expressed in HEK 293 cells. Caution should be taken in the interpretation of calmodulin regulation of ion channels with W-7.

Overlapping LQT1 and LQT2 phenotype in a patient with long QT syndrome associated with loss-of-function variations in KCNQ1 and KCNH2

Long QT syndrome (LQTS) is an inherited disorder characterized by prolonged QT intervals and potentially life-threatening arrhythmias. Mutations in 12 different genes have been associated with LQTS. Here we describe a patient with LQTS who has a mutation in KCNQ1 as well as a polymorphism in KCNH2. The proband (MMRL0362), a 32-year-old female, exhibited multiple ventricular extrasystoles and one syncope. Her ECG (QT interval corrected for heart rate (QTc) = 518ms) showed an LQT2 morphology in leads V4-V6 and LQT1 morphology in leads V1-V2. Genomic DNA was isolated from lymphocytes. All exons and intron borders of 7 LQTS susceptibility genes were amplified and sequenced. Variations were detected predicting a novel missense mutation (V110I) in KCNQ1, as well as a common polymorphism in KCNH2 (K897T). We expressed wild-type (WT) or V110I Kv7.1 channels in CHO-K1 cells cotransfected with KCNE1 and performed patch-clamp analysis. In addition, WT or K897T Kv11.1 were also studied by patch clamp. Current-voltage (I-V) relations for V110I showed a significant reduction in both developing and tail current densities compared with WT at potentials >+20 mV (p < 0.05; n = 8 cells, each group), suggesting a reduction in IKs currents. K897T- Kv11.1 channels displayed a significantly reduced tail current density compared with WT-Kv11.1 at potentials >+10 mV. Interestingly, channel availability assessed using a triple-pulse protocol was slightly greater for K897T compared with WT (V0.5 = -53.1 ± 1.13 mV and -60.7 ± 1.15 mV for K897T and WT, respectively; p < 0.05). Comparison of the fully activated I-V revealed no difference in the rectification properties between WT and K897T channels. We report a patient with a loss-of-function mutation in KCNQ1 and a loss-of-function polymorphism in KCNH2. Our results suggest that a reduction of both IKr and IKs underlies the combined LQT1 and LQT2 phenotype observed in this patient.

Functional effects of a missense mutation in HERG associated with type 2 long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is characterized by a prolonged QT interval that can lead to severe ventricular arrhythmias (torsades de pointes) and sudden death. Congenital LQTS type 2 (LQT2) is due to loss-of-function mutations in the KCNH2 gene encoding Kv11.1 channels responsible for the rapid component of the delayed rectifier current.

OBJECTIVE

The purpose of this study was to determine the functional properties of the LQT2-associated mutation p.E637G found in a Spanish family.

METHODS

Wild-type (WT) and p.E637G Kv11.1 channels were transiently transfected in Chinese hamster ovary cells, and currents were recorded using the patch-clamp technique.

RESULTS

The p.E637G channels lost inward rectification and K(+) selectivity, generating small but measurable slowly activating, noninactivating currents. These important alterations were corrected neither by cotransfection with WT channels nor by incubation at low temperatures or with pharmacological chaperones. As a consequence of its effects on channel gating, the mutation significantly reduced the outward repolarizing current during the action potential (AP), resulting in a marked lengthening of the duration of a simulated human ventricular AP.

CONCLUSION

We have identified and characterized an LQT2-associated mutation that through removal of C-type inactivation and reduction of K(+) selectivity causes the QT prolongation observed in the patients carrying the mutation. Moreover, the results obtained demonstrate the importance of the glutamic acid at position 637 for the inactivation process and K(+) selectivity of Kv11.1 channels.

Revealing the structural basis of action of hERG potassium channel activators and blockers

Human ether-á-go-go related gene (hERG) potassium (K(+)) channels play a critical role in cardiac action potential repolarization. This is due, in large part, to the unique gating properties of these channels, which are characterized by relatively slow activation and an unusually fast and voltage-dependent inactivation. A large number of structurally diverse compounds bind to hERG and carry an unacceptably high risk of causing arrhythmias. On the other hand, drugs that increase hERG current may, at least in principle, prove useful for treatment of long QT syndrome. A few blockers have been shown to increase hERG current at potentials close to the threshold for channel activation--a process referred to as facilitation. More recently, a novel group of hERG channel activators have been identified that slow deactivation and/or attenuate inactivation. Structural determinants for the action of two different types of activators have been identified. These compounds bind at sites that are distinct from each other and also separate from the binding site of high affinity blockers. They reveal not only novel ways of chemically manipulating hERG channel function, but also interactions between structural domains that are critical to normal activation and inactivation gating.

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.

Trafficking-competent KCNQ1 variably influences the function of HERG long QT alleles

BACKGROUND

Mutations in the KCNQ1 and human ether-a-go-go-related gene (HERG) genes cause the long QT syndromes, LQTS1 and LQTS2, due to reductions in the cardiac repolarizing I(Ks) and I(Kr) currents, respectively. It was previously reported that KCNQ1 coexpression modulates HERG function by enhancing membrane expression of HERG, and that the 2 proteins coimmunoprecipitate, and colocalize in myocytes. In vivo studies in genetically modified rabbits also support a HERG-KCNQ1 interaction.

OBJECTIVE

We sought to determine whether KCNQ1 influences the current characteristics of HERG genetic variants.

METHODS

This study used expression of HERG and KCNQ1 wild-type (WT) and mutant channels in heterologous systems, combined with whole-cell patch clamp analysis and biochemistry.

RESULTS

Supporting the notion that KCNQ1 needs to be trafficking competent to influence HERG function, we found that although the tail current density of HERG expressed in Chinese Hamster Ovary (CHO) cells was approximately doubled by WT KCNQ1 coexpression, it was not altered in the presence of the trafficking-defective KCNQ1(T587M) variant. Activation and deactivation kinetics of HERG variants were not altered. The HERG(M124T) variant, previously shown to be mildly impaired functionally, was restored to WT levels by KCNQ1-WT but not KCNQ1(T587M) coexpression. The tail current densities of the severely trafficking-impaired HERG(G601S) and HERG(F805C) variants were only slightly improved by KCNQ1 coexpression. The trafficking competent but incompletely processed HERG(N598Q), and a mutation in the selectivity filter, HERG(G628S), were not improved by KCNQ1 coexpression.

CONCLUSION

These findings suggest a functional codependence of HERG on KCNQ1 during channel biogenesis. Moreover, KCNQ1 variably modulates LQTS2 mutations with distinct underlying pathologies.

Regulation of the voltage-insensitive step of HERG activation by extracellular pH

Human ether-à-go-go-related gene (HERG, Kv11.1, KCNH2) voltage-gated K(+) channels dominate cardiac action potential repolarization. In addition, HERG channels play a role in neuronal and smooth cell excitability as well as cancer pathology. Extracellular pH (pH(o)) is modified during myocardial ischemia, inflammation, and respiratory alkalosis, so understanding the response of HERG channels to changes in pH is of clinical significance. The relationship between pH(o) and HERG channel gating appears complex. Acidification has previously been reported to speed, slow, or have no effect on activation. We therefore undertook comprehensive analysis of the effect of pH(o) on HERG channel activation. HERG channels have unique and complex activation gating characteristics with both voltage-sensitive and voltage-insensitive steps in the activation pathway. Acidosis decreased the activation rate, suppressed peak current, and altered the sigmoidicity of gating near threshold potentials. At positive voltages, where the voltage-insensitive transition is rate limiting, pH(o) modified the voltage-insensitive step with a pK(a) similar to that of histidine. Hill coefficient analysis was incompatible with a coefficient of 1 but was well described by a Hill coefficient of 4. We derived a pH(o)-sensitive term for a five-state Markov model of HERG channel gating. This model demonstrates the mechanism of pH(o) sensitivity in HERG channel activation. Our experimental data and mathematical model demonstrate that the pH(o) sensitivity of HERG channel activation is dominated by the pH(o) sensitivity of the voltage-insensitive step, in a fashion that is compatible with the presence of at least one proton-binding site on each subunit of the channel tetramer.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Pharmacological removal of human ether-à-go-go-related gene potassium channel inactivation by 3-nitro-N-(4-phenoxyphenyl) benzamide (ICA-105574)

Human ether-à-go-go-related gene (hERG) potassium channel activity helps shape the cardiac action potential and influences its duration. In this study, we report the discovery of 3-nitro-N-(4-phenoxyphenyl) benzamide (ICA-105574), a potent and efficacious hERG channel activator with a unique mechanism of action. In whole-cell patch-clamp studies of recombinant hERG channels, ICA-105574 steeply potentiated current amplitudes more than 10-fold with an EC(50) value of 0.5 +/- 0.1 microM and a Hill slope (n(H)) of 3.3 +/- 0.2. The effect on hERG channels was confirmed because the known hERG channel blockers, N-[4-[[1-[2-(6-methyl-2-pyridinyl)ethyl]-4-piperidinyl]carbonyl]phenyl]methanesulfonamide, 2HCl (E-4031) and BeKm-1, potently blocked the stimulatory effects of ICA-105574. The primary mechanism by which ICA-105574 potentiates hERG channel activity is by removing hERG channel inactivation, because ICA-105574 (2 microM) shifts the midpoint of the voltage-dependence of inactivation by >180 mV from -86 to +96 mV. In addition to the effects on inactivation, greater concentrations of ICA-105574 (3 microM) produced comparatively small hyperpolarizing shifts (up to 11 mV) in the voltage-dependence of channel activation and a 2-fold slowing of channel deactivation. In isolated guinea pig ventricular cardiac myocytes, ICA-105574 induced a concentration-dependent shortening of action potential duration (>70%, 3 microM) that could be prevented by preincubation with E-4031. In conclusion, we identified a novel agent that can prevent the inactivation of hERG potassium channels. This compound may provide a useful tool to further understand the mechanism by which hERG channels inactivate and affect cardiac function in addition to the role of hERG channels in other cell systems.

Transfer of rolf S3-S4 linker to HERG eliminates activation gating but spares inactivation

Studies in Shaker, a voltage-dependent potassium channel, suggest a coupling between activation and inactivation. This coupling is controversial in hERG, a fast-inactivating voltage-dependent potassium channel. To address this question, we transferred to hERG the S3-S4 linker of the voltage-independent channel, rolf, to selectively disrupt the activation process. This chimera shows an intact voltage-dependent inactivation process consistent with a weak coupling, if any, between both processes. Kinetic models suggest that the chimera presents only an open and an inactivated states, with identical transition rates as in hERG. The lower sensitivity of the chimera to BeKm-1, a hERG preferential closed-state inhibitor, also suggests that the chimera presents mainly open and inactivated conformations. This chimera allows determining the mechanism of action of hERG blockers, as exemplified by the test on ketoconazole.

Inhibition of the human ether-a-go-go-related gene (HERG) K+ channels by Lindera erythrocarpa

Lindera erythrocarpa Makino (Lauraceae) is used as a traditional medicine for analgesic, antidote, and antibacterial purposes and shows anti-tumor activity. We studied the effects of Lindera erythrocarpa on the human ether-a-go-go-related gene (HERG) channel, which appears of importance in favoring cancer progression in vivo and determining cardiac action potential duration. Application of MeOH extract of Lindera erythrocarpa showed a dose-dependent decrease in the amplitudes of the outward currents measured at the end of the pulse (I(HERG)) and the tail currents of HERG (I(tail)). When the BuOH fraction and H(2)O fraction of Lindera erythrocarpa were added to the perfusate, both I(HERG) and I(tail) were suppressed, while the hexane fraction, CHCl(3) fraction, and EtOAc fraction did not inhibit either I(HERG) or I(tail). The potential required for half-maximal activation caused by EtOAc fraction, BuOH fraction, and H(2)O fraction shifted significantly. The BuOH fraction and H(2)O fraction (100 microg/mL) decreased g(max) by 59.6% and 52.9%, respectively. The H(2)O fraction- and BuOH fraction-induced blockades of I(tail) progressively decreased with increasing depolarization, showing the voltage-dependent block. Our findings suggest that Lindera erythrocarpa, a traditional medicine, blocks HERG channel, which could contribute to its anticancer and cardiac arrhythmogenic effect.

Absence of direct cyclic nucleotide modulation of mEAG1 and hERG1 channels revealed with fluorescence and electrophysiological methods

Similar to CNG and HCN channels, EAG and ERG channels contain a cyclic nucleotide binding domain (CNBD) in their C terminus. While cyclic nucleotides have been shown to facilitate opening of CNG and HCN channels, their effect on EAG and ERG channels is less clear. Here we explored cyclic nucleotide binding and modulation of mEAG1 and hERG1 channels with fluorescence and electrophysiology. Binding of cyclic nucleotides to the isolated CNBD of mEAG1 and hERG1 channels was examined with two independent fluorescence-based methods: changes in tryptophan fluorescence and fluorescence of an analog of cAMP, 8-NBD-cAMP. As a positive control for cyclic nucleotide binding we used changes in the fluorescence of the isolated CNBD of mHCN2 channels. Our results indicated that cyclic nucleotides do not bind to the isolated CNBD domain of mEAG1 channels and bind with low affinity (K(d) > or = 51 microm) to the isolated CNBD of hERG1 channels. Consistent with the results on the isolated CNBD, application of cyclic nucleotides to inside-out patches did not affect currents recorded from mEAG1 channels. Surprisingly, despite its low affinity binding to the isolated CNBD, cAMP also had no effect on currents from hERG1 channels even at high concentrations. Our results indicate that cyclic nucleotides do not directly modulate mEAG1 and hERG1 channels. Further studies are necessary to determine if the CNBD in the EAG family of K(+) channels might harbor a binding site for a ligand yet to be uncovered.

Functional characterization of cardiomyocytes derived from murine induced pluripotent stem cells in vitro

Several types of terminally differentiated somatic cells can be reprogrammed into a pluripotent state by ectopic expression of Klf4, Oct3/4, Sox2, and c-Myc. Such induced pluripotent stem (iPS) cells have great potential to serve as an autologous source of cells for tissue repair. In the process of developing iPS-cell-based therapies, the major goal is to determine whether differentiated cells derived from iPS cells, such as cardiomyocytes (CMs), have the same functional properties as their physiological in vivo counterparts. Therefore, we differentiated murine iPS cells to CMs in vitro and characterized them by RT-PCR, immunocytochemistry, and electrophysiology. As key markers of cardiac lineages, transcripts for Nkx2.5, alphaMHC, Mlc2v, and cTnT could be identified. Immunocytochemical stainings revealed the presence of organized sarcomeric actinin but the absence of mature atrial natriuretic factor. We examined characteristics and developmental changes of action potentials, as well as functional hormonal regulation and sensitivity to channel blockers. In addition, we determined expression patterns and functionality of cardiac-specific voltage-gated Na+, Ca2+, and K+ channels at early and late differentiation stages and compared them with CMs derived from murine embryonic stem cells (ESCs) as well as with fetal CMs. We conclude that iPS cells give rise to functional CMs in vitro, with established hormonal regulation pathways and functionally expressed cardiac ion channels; CMs generated from iPS cells have a ventricular phenotype; and cardiac development of iPS cells is delayed compared with maturation of native fetal CMs and of ESC-derived CMs. This difference may reflect the incomplete reprogramming of iPS cells and should be critically considered in further studies to clarify the suitability of the iPS model for regenerative medicine of heart disorders.

Delayed rectifier K(+) currents and cardiac repolarization

The two components of the cardiac delayed rectifier current have been the subject of numerous studies since firstly described. This current controls the action potential duration and is highly regulated. After identification of the channel subunits underlying IKs, KCNQ1 associated with KCNE1, and IKr, HERG, their involvement in human cardiac channelopathies have provided various models allowing the description of the molecular mechanisms of the KCNQ1 and HERG channels trafficking, activity and regulation. More recently, studies have been focusing on the unveiling of different partners of the pore-forming proteins that contribute to their maturation, trafficking, activity and/or degradation, on one side, and on their respective expression in the heterogeneous cardiac tissue, on the other side. The aim of this review is to report and discuss the major works on IKs and IKr and the most recent ones that help to understand the precise function of these currents in the heart.

A recombinant N-terminal domain fully restores deactivation gating in N-truncated and long QT syndrome mutant hERG potassium channels

Human ether á go-go related gene (hERG) potassium channels play a central role in cardiac repolarization where channel closing (deactivation) regulates current density during action potentials. Consequently, mutations in hERG that perturb deactivation are linked to long QT syndrome (LQTS), a catastrophic cardiac arrhythmia. Interactions between an N-terminal domain and the pore-forming "core" of the channel were proposed to regulate deactivation, however, despite its central importance the mechanistic basis for deactivation is unclear. Here, to more directly examine the mechanism for regulation of deactivation, we genetically fused N-terminal domains to fluorescent proteins and tested channel function with electrophysiology and protein interactions with Förster resonance energy transfer (FRET) spectroscopy. Truncation of hERG N-terminal regions markedly sped deactivation, and here we report that reapplication of gene fragments encoding N-terminal residues 1-135 (the "eag domain") was sufficient to restore regulation of deactivation. We show that fluorophore-tagged eag domains and N-truncated channels were in close proximity at the plasma membrane as determined with FRET. The eag domains with Y43A or R56Q (a LQTS locus) mutations showed less regulation of deactivation and less FRET, whereas eag domains restored regulation of deactivation gating to full-length Y43A or R56Q channels and showed FRET. This study demonstrates that direct, noncovalent interactions between the eag domain and the channel core were sufficient to regulate deactivation gating, that an LQTS mutation perturbed physical interactions between the eag domain and the channel, and that small molecules such as the eag domain represent a novel method for restoring function to channels with disease-causing mutations.

Loss of functional K+ channels encoded by ether-à-go-go-related genes in mouse myometrium prior to labour onset

There is a growing appreciation that ion channels encoded by the ether-à-go-go-related gene family have a functional impact in smooth muscle in addition to their accepted role in cardiac myocytes and neurones. This study aimed to assess the expression of ERG1-3 (KCNH1-3) genes in the murine myometrium (smooth muscle layer of the uterus) and determine the functional impact of the ion channels encoded by these genes in pregnant and non-pregnant animals. Quantitative RT-PCR did not detect message for ERG2 and 3 in whole myometrial tissue extracts. In contrast, message for two isoforms of mERG1 were readily detected with mERG1a more abundant than mERG1b. In isometric tension studies of non-pregnant myometrium, the ERG channel blockers dofetilide (1 microM), E4031 (1 microM) and Be-KM1 (100 nM) increased spontaneous contractility and ERG activators (PD118057 and NS1643) inhibited spontaneous contractility. In contrast, neither ERG blockade nor activation had any effect on the inherent contractility in myometrium from late pregnant (19 days gestation) animals. Moreover, dofetilide-sensitive K(+) currents with distinctive 'hooked' kinetics were considerably smaller in uterine myocytes from late pregnant compared to non-pregnant animals. Expression of mERG1 isoforms did not alter throughout gestation or upon delivery, but the expression of genes encoding auxillary subunits (KCNE) were up-regulated considerably. This study provides the first evidence for a regulation of ERG-encoded K(+) channels as a precursor to late pregnancy physiological activity.

Ether-à-gogo-related gene (erg1) potassium channels shape the dark response of horizontal cells in the mammalian retina

Postsynaptic to photoreceptors, horizontal cells face prolonged exposure to glutamate in the dark. Therefore, efficient hyperpolarizing mechanisms are crucial to keep horizontal cells within an operating range and to reduce glutamate-induced excitotoxicity. Combining electrophysiology, single-cell reverse transcriptase polymerase chain reaction, and immunocytochemistry, we found that horizontal cell bodies but not their axon terminals express the ether-à-gogo-related gene isoform 1 (erg1) K(+) channel. Erg1-mediated outward currents displayed voltage-dependent activation and C-type inactivation. Recovery from inactivation involved a transient open state. Gating of erg1 channels kept the voltage response to glutamate brief and at physiological amplitudes. With erg1 channels blocked, the response of horizontal cells to the onset of darkness was significantly enhanced. These results indicate a functional dichotomy between horizontal cell bodies and axon terminals in the processing of photoreceptor signals. The dark response thus reflects a finely tuned balance determined by the successive gating of ionotropic glutamate receptors and erg1 channels.

The pore domain outer helix contributes to both activation and inactivation of the HERG K+ channel

Ion flow in many voltage-gated K(+) channels (VGK), including the (human ether-a-go-go-related gene) hERG channel, is regulated by reversible collapse of the selectivity filter. hERG channels, however, exhibit low sequence homology to other VGKs, particularly in the outer pore helix (S5) domain, and we hypothesize that this contributes to the unique activation and inactivation kinetics in hERG K(+) channels that are so important for cardiac electrical activity. The S5 domain in hERG identified by NMR spectroscopy closely corresponded to the segment predicted by bioinformatics analysis of 676 members of the VGK superfamily. Mutations to approximately every third residue, from Phe(551) to Trp(563), affected steady state activation, whereas mutations to approximately every third residue on an adjacent face and spanning the entire S5 segment perturbed inactivation, suggesting that the whole span of S5 experiences a rearrangement associated with inactivation. We refined a homology model of the hERG pore domain using constraints from the mutagenesis data with residues affecting inactivation pointing in toward S6. In this model the three residues with maximum impact on activation (W563A, F559A, and F551A) face out toward the voltage sensor. In addition, the residues that when mutated to alanine, or from alanine to valine, that did not express (Ala(561), His(562), Ala(565), Trp(568), and Ile(571)), all point toward the pore helix and contribute to close hydrophobic packing in this region of the channel.

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

The human Ether-a-go-go Related Gene (hERG) potassium channel plays a central role in regulating cardiac excitability and maintenance of normal cardiac rhythm. Mutations in hERG cause a third of all cases of congenital long QT syndrome, a disorder of cardiac repolarisation characterised by prolongation of the QT interval on the surface electrocardiogram, abnormal T waves, and a risk of sudden cardiac death due to ventricular arrhythmias. Additionally, the hERG channel protein is the molecular target for almost all drugs that cause the acquired form of long QT syndrome. Advances in understanding the structural basis of hERG gating, its traffic to the cell surface, and the molecular architecture involved in drug-block of hERG, are providing the foundation for rational treatment and prevention of hERG associated long QT syndrome. This review summarises the current knowledge of hERG function and dysfunction, and the areas of ongoing research.

A new C-terminal hERG mutation A915fs+47X associated with symptomatic LQT2 and auditory-trigger syncope

BACKGROUND

A novel mutation of hERG (A915fs+47X) was discovered in a 32-year-old woman with torsades de pointes, long QTc interval (515 ms), and syncope upon auditory trigger.

OBJECTIVE

We explored whether the properties of this mutation could explain the pathology.

METHODS

Whole-cell A915fs+47X (del) and wild-type (WT) currents were recorded in transiently transfected COS7 cells or Xenopus oocytes. Western blots and sedimentation analysis of del/WT hERG were used to analyze protein expression, assembly, and trafficking.

RESULTS

The tail current density at -40 mV after a 2-s depolarization to +40 mV in COS7 cells expressing del was 36% of that for WT. Inactivation was 1.9-fold to 2.8-fold faster in del versus WT between -60 and +60 mV. In the range -60 to -10 mV, we found that a nondeactivating fraction of current was increased in del at the expense of a rapidly deactivating fraction, with a slowly deactivating fraction being unchanged. In Xenopus oocytes, expression of del alone produced 38% of WT currents, whereas coexpression of 1/2 WT + 1/2 del produced 49.8%. Furthermore, the expression of del protein at the cell surface was reduced by about 50%. This suggests that a partial trafficking defect of del contributes to the reduction in del current densities and to the dominant negative effect when coexpressed with WT. In model simulations, the mutation causes a 10% prolongation of action potential duration.

CONCLUSION

Decreased current levels caused by a trafficking defect may explain the long QT syndrome observed in our patient.

Rapid kinetic interactions of ranolazine with HERG K+ current

Ranolazine, an anti-ischemic agent, inhibits I Kr [encoded by the human ether-a-go-go-related gene (HERG)] and causes a small QT interval prolongation without any proarrhythmic events. The objective of this study was to elucidate the biophysical characteristics of inhibition of HERG K+ current (IHERG) by ranolazine. We investigated the effects of ranolazine using voltage-clamp and Western blot analyses of HERG channels stably expressed in HEK293 cells. Ranolazine reduced IHERG with the half-maximal inhibitory concentration of 12.0 microM. Block of IHERG by ranolazine was reversible and voltage-dependent but frequency-independent. At 0 mV, the time constants for development of block were 76.6 +/- 1.6, 35.8 +/- 2.4, and 19.4 +/- 1.7 msec with 10, 30, and 100 microM ranolazine (n = 4), respectively. The apparent dissociation constant estimated from the time course of ranolazine-induced IHERG decay was 22.5 microM. After repolarization at -80 and -100 mV, IHERG recovery from ranolazine block followed a monophasic time course with tau values of 204.3 +/- 51.5 and 155.0 +/- 31.9 msec (n = 5), respectively. Intracellular but not extracellular application of a membrane-impermeable (permanently charged) ranolazine analogue caused rapid block of IHERG. Ranolazine did not alter HERG protein trafficking to the plasma membrane. In conclusion, ranolazine caused a time- and voltage-dependent, but frequency-independent, block of IHERG. The kinetics of IHERG inhibition (at positive potentials) and unblock (upon hyperpolarization) by ranolazine were rapid. These distinct and rapid kinetic interactions of ranolazine with IHERG may partially contribute to the observations that the drug is not proarrhythmic despite causing a small prolongation of action potentials and QT intervals.

Physiological properties of hERG 1a/1b heteromeric currents and a hERG 1b-specific mutation associated with Long-QT syndrome

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.

Electrophysiological properties and expression of the delayed rectifier potassium (ERG) channels in the heart of thermally acclimated rainbow trout

In ectotherms, compensatory changes in ion channel number and activity are needed to maintain proper cardiac function at variable temperatures. The rapid component of the delayed rectifier K+current ( IKr) is important for repolarization of cardiac action potential and, therefore, crucial for regulation of cellular excitability and heart rate. To examine temperature plasticity of cardiac IKr, we cloned the ether-à- go- go-related gene (ERG) channel and measured its electrophysiological properties in thermally acclimated rainbow trout ( Oncorhynchus mykiss; omERG). The present findings demonstrate a complete thermal compensation in the whole cell conductance of the atrial IKrin rainbow trout acclimated to 4°C (cold acclimation) and 18°C (warm acclimation). In situ hybridization indicates that transcripts of the omERG channel are present throughout the muscular tissue of the heart, and quantitative PCR shows increased expression of the omERG in cold-acclimated trout compared with warm-acclimated trout. In both acclimation groups, omERG expression is higher in atrium than ventricle. In addition, the omERG has some functional features that support IKractivity at low temperatures. Voltage dependence of steady-state activation is completely resistant to temperature changes, and steady-state inactivation and activation kinetics are little affected by temperatures below 11°C. Collectively, these findings suggest that high density of cardiac IKris achieved by cold-induced increase in the number of functional omERG channels and inherent insensitivity of the omERG to temperature below 11°C. These adaptations are probably important in maintaining high heart rates and proper excitability and contractility of trout cardiac myocytes in the cold.

The 5-HT2 antagonist ketanserin is an open channel blocker of human cardiac ether-à-go-go-related gene (hERG) potassium channels

BACKGROUND AND PURPOSE

Ketanserin, a selective 5-HT receptor antagonist, prolongs the QT interval of ECG in patients. The purpose of the present study was to determine whether ketanserin would block human cardiac ether-à-go-go-related gene (hERG) potassium channels.

EXPERIMENTAL APPROACH

Whole-cell patch voltage-clamp technique was used to record membrane currents in HEK 293 cells expressing wild type or mutant hERG channel genes.

KEY RESULTS

Ketanserin blocked hERG current (I(hERG)) in a concentration-dependent manner (IC50=0.11 microM). The drug showed an open channel blocking property, the block increasing significantly at depolarizing voltages between +10 to +60 mV. Voltage-dependence for inactivation of hERG channels was negatively shifted by 0.3 microM ketanserin. A 2.8 fold attenuation of inhibition by elevation of external K+ concentration (from 5.0 to 20 mM) was observed, whereas the inactivation-deficient mutants S620T and S631A had the IC50s of 0.84 +/- 0.2 and 1.7 +/-0.4 microM (7.6 and 15.4 fold attenuation of block). In addition, the hERG mutants in pore helix and S6 also significantly reduced the channel block (2-59 fold) by ketanserin.

CONCLUSIONS AND IMPLICATIONS

These results suggest that ketanserin binds to and blocks the open hERG channels in the pore helix and the S6 domain; channel inactivation is also involved in the blockade of hERG channels. Blockade of hERG channels most likely contributes to the prolongation of QT intervals in ECG observed clinically at therapeutic concentrations of ketanserin.

FRET with multiply labeled HERG K(+) channels as a reporter of the in vivo coarse architecture of the cytoplasmic domains

The intracellular N-terminus of human ether-a-go-go-related gene (HERG) potassium channels constitutes a key determinant of activation and deactivation characteristics and is necessary for hormone-induced modifications of gating properties. However, the general organization of the long amino and carboxy HERG terminals remains unknown. In this study we performed fluorescence resonance energy transfer (FRET) microscopy with a library of fluorescent HERG fusion proteins obtained combining site-directed and transposon-based random insertion of GFP variants into multiple sites of HERG. Determinations of FRET efficiencies with functional HERG channels labeled in different combinations localize the fluorophores, introduced in the amino and carboxy ends, in two quadratic planes of 7.8 and 8.6 nm lateral size, showing a vertical separation of nearly 8 nm without major angular torsion between the planes. Similar analysis using labels at positions 345 and 905 of the amino and carboxy terminals, located them slightly above the planes delimited by the amino and carboxy end labels, respectively. Our data also indicate an almost vertical arrangement of the fluorophores introduced in the NH(2) and COOH ends and at position 905, but a near 45 degrees angular rotation between the planes delimited by these labels and the 345-located fluorophores. Systematic triangulation using interfluorophore distances coming from multiply labeled channels provides an initial constraint on the overall in vivo arrangement of the HERG cytoplasmic domains, suggesting that the C-linker/CNBD region of HERG hangs centrally below the transmembrane core, with the initial portion of the amino terminus around its top and side surfaces directed towards the gating machinery.

The hERG potassium channel and hERG screening for drug-induced torsades de pointes

Drug-induced torsades de pointes (TdP) arrhythmia is a major safety concern in the process of drug design and development. The incidence of TdP tends to be low, so early pre-clinical screens rely on surrogate markers of TdP to highlight potential problems with new drugs. hERG (human ether-à-go-go-related gene, alternative nomenclature KCNH2) is responsible for channels mediating the 'rapid' delayed rectifier K+ current (IKr) which plays an important role in ventricular repolarization. Pharmacological inhibition of native IKr and of recombinant hERG channels is a shared feature of diverse drugs associated with TdP. In vitro hERG assays therefore form a key element of an integrated assessment of TdP liability, with patch-clamp electrophysiology offering a 'gold standard'. However, whilst clearly necessary, hERG assays cannot be assumed automatically to provide sufficient information, when considered in isolation, to differentiate 'safe' from 'dangerous' drugs. Other relevant factors include therapeutic plasma concentration, drug metabolism and active metabolites, severity of target condition and drug effects on other cardiac ion channels that may mitigate or exacerbate effects of hERG blockade. Increased understanding of the nature of drug-hERG channel interactions may ultimately help eliminate potential hERG blockade early in the design and development process. Currently, for promising drug candidates integration of data from hERG assays with information from other pre-clinical safety screens remains essential.

hERG potassium channels and the structural basis of drug-induced arrhythmias

hERG potassium channels have a critical role in the normal electrical activity of the heart. The block of hERG channels can cause the drug-induced form of long QT syndrome, a cardiac disorder that carries an increased risk of cardiac arrhythmias and sudden death. hERG channels are extraordinarily sensitive to block by large numbers of structurally diverse drugs. In previous years, the risk of compounds causing this cardiotoxic side effect has been a common reason for the failure of compounds in preclinical safety trials. Pharmaceutical companies have successfully utilized and developed higher throughput techniques for the early detection of compounds that block hERG, and this has helped reduce the number of compounds that fail in the late stages of development. Nevertheless, this screening-based approach is expensive, consumes chemistry resources, and bypasses the problem rather than shedding light on it. Crystal structures of potassium channels have facilitated studies into the structural basis for the gating and block of hERG channels. Most drugs bind within the inner cavity, and the individual amino acids that form the drug binding site have been identified by site-directed mutagenesis approaches. Gating processes have an important influence on the drug-binding site. Recent advances in our understanding of channel activation and inactivation are providing insight into why hERG channels are more susceptible to block than other K (+) channels. Knowledge of the structure of the drug-binding site and precise nature of interactions with drug molecules should assist efforts to develop drugs without the propensity to cause cardiac arrhythmias.

Structural basis for ether-a-go-go-related gene K+ channel subtype-dependent activation by niflumic acid

Niflumic acid [2-((3-(trifluoromethyl)phenyl)amino)-3-pyridinecarboxylic acid, NFA] is a nonsteroidal anti-inflammatory drug that also blocks or modulates the gating of a wide spectrum of ion channels. Here we investigated the mechanism of channel activation by NFA on ether-a-go-go-related gene (ERG) K(+) channel subtypes expressed in Xenopus laevis oocytes using two-electrode voltage-clamp techniques. NFA acted from the extracellular side of the membrane to differentially enhance ERG channel currents independent of channel state. At 1 mM, NFA shifted the half-point for activation by -6, -18, and -11 mV for ERG1, ERG2, and ERG3 channels, respectively. The half-point for channel inactivation was shifted by +5 to +9 mV by NFA. The structural basis for the ERG subtype-specific response to NFA was explored with chimeric channels and site-directed mutagenesis. The molecular determinants of enhanced sensitivity of ERG2 channels to NFA were isolated to an Arg and a Thr triplet in the extracellular S3-S4 linker.

2-[2-(3,4-dichloro-phenyl)-2,3-dihydro-1H-isoindol-5-ylamino]-nicotinic acid (PD-307243) causes instantaneous current through human ether-a-go-go-related gene potassium channels

Long and short QT syndromes associated with loss and gain of human ether-a-go-go-related gene (hERG) channel activity, respectively, can cause life-threatening arrhythmias. As such, modulation of hERG channel activity is an important consideration in the development of all new therapeutic agents. In the present study, we investigated the mechanisms of action of 2-[2-(3,4-dichloro-phenyl)-2,3-dihydro-1H-isoindol-5-ylamino]-nicotinic acid (PD-307243), a known hERG channel activator, on hERG channels stably expressed in Chinese hamster ovary (CHO) cells using the patch-clamp technique. In the whole-cell recordings, the extracellular application of PD-307243 concentration-dependently increased the hERG current and markedly slowed hERG channel deactivation and inactivation. PD-307243 had no effect on the selectivity filter of hERG channels. The activity of PD-307243 was use-dependent. PD-307243 (3 and 10 muM) induced instantaneous hERG current with little decay at membrane potentials from -120 to -40 mV. At more positive voltages, PD-307243 induced an I(to)-like upstroke of hERG current. The actions of PD-307243 on the rapid component of delayed rectifier K(+) current (I(Kr)) in rabbit ventricular myocytes were similar to those observed in hERG channel-transfected CHO cells. Inside-out patch experiments revealed that PD-307243 increased hERG tail currents by 2.1 +/- 0.6 (n = 7) and 3.4 +/- 0.3-fold (n = 4) at 3 and 10 muM, respectively, by slowing the channel deactivation but had no effect on channel activation. During a voltage-clamp protocol using a prerecorded cardiac action potential, 3 muM PD-307243 increased the total potassium ions passed through hERG channels by 8.8 +/- 1.0-fold (n = 5). Docking studies suggest that PD-307243 interacts with residues in the S5-P region of the channel.