The S4-S5 linker couples voltage sensing and activation of pacemaker channels.

Voltage-gated channels are normally opened by depolarization and closed by repolarization of the membrane. Despite sharing significant sequence homology with voltage-gated K(+) channels, the gating of hyperpolarization-activated, cyclic-nucleotide-gated (HCN) pacemaker channels has the opposite dependence on membrane potential: hyperpolarization opens, whereas depolarization closes, these channels. The mechanism and structural basis of the process that couples voltage sensor movement to HCN channel opening and closing is not understood. On the basis of our previous studies of a mutant HERG (human ether-a-go-go-related gene) channel, we hypothesized that the intracellular linker that connects the fourth and fifth transmembrane domains (S4-S5 linker) of HCN channels might be important for channel gating. Here, we used alanine-scanning mutagenesis of the HCN2 S4-S5 linker to identify three residues, E324, Y331, and R339, that when mutated disrupted normal channel closing. Mutation of a basic residue in the S4 domain (R318Q) prevented channel opening, presumably by disrupting S4 movement. However, channels with R318Q and Y331S mutations were constitutively open, suggesting that these channels can open without a functioning S4 domain. We conclude that the S4-S5 linker mediates coupling between voltage sensing and HCN channel activation. Our findings also suggest that opening of HCN and related channels corresponds to activation of a gate located near the inner pore, rather than recovery of channels from a C-type inactivated state.

Long-QT syndrome-associated missense mutations in the pore helix of the HERG potassium channel.

BACKGROUND: Mutations in the human ether-à-go-go-related gene (HERG) cause chromosome 7-linked long-QT syndrome (LQTS), an inherited disorder of cardiac repolarization that predisposes affected individuals to arrhythmia and sudden death. METHODS AND RESULTS: Here, we characterize the physiological consequences of 3 LQTS-associated missense mutations (V612L, T613M, and L615V) located in the pore helix of the HERG channel subunit. Mutant HERG subunits were heterologously expressed in Xenopus oocytes alone or in combination with wild-type HERG subunits. Two-microelectrode voltage-clamp techniques were used to record currents, and a single oocyte chemiluminescence assay was used to assay surface expression of epitope-tagged subunits. When expressed alone, V612L and T613M HERG subunits did not induce detectable currents, and L615V induced very small currents. Coexpression of mutant and wild-type HERG subunits caused a dominant-negative effect that varied for each mutation. CONCLUSIONS: These findings define the physiological consequences of mutations in HERG that cause LQTS and indicate the importance of the pore helix of HERG for normal channel function.

Open channel block of HERG K(+) channels by vesnarinone.

Vesnarinone, a cardiotonic agent, blocks I(Kr) and, unlike other I(Kr) blockers, produces a frequency-dependent prolongation of action potential duration (APD). To elucidate the mechanisms, we studied the effects of vesnarinone on HERG, the cloned human I(Kr) channel, heterologously expressed in Xenopus laevis oocytes. Vesnarinone caused a concentration-dependent inhibition of HERG currents with an IC(50) value of 17.7 +/- 2.5 microM at 0 mV (n = 6). When HERG was coexpressed with the beta-subunit MiRP1, a similar potency for block was measured (IC(50): 15.0 +/- 3.0 microM at 0 mV, n = 5). Tonic block of the HERG channel current was minimal (<5% at 30 microM, n = 5). The rate of onset of block and the steady-state value for block of current were not significantly different for test potentials ranging from -40 to +40 mV [time constant (tau) = 372 +/- 76 ms at +40 mV, n = 4]. Recovery from block at -60, -90, and -120 mV was not significantly different (tau = 8.5 +/- 1.5 s at -90 mV, n = 4). Vesnarinone produced similar effects on inactivation-removed mutant (G628C/S631C) HERG channels. The IC(50) value was 10.7 +/- 3.7 microM at 0 mV (n = 5), and the onset and recovery from block of current findings were similar to those of wild-type HERG. Amino acids important for the binding of vesnarinone were identified using alanine-scanning mutagenesis of residues believed to line the inner cavity of the HERG channel. Six important residues were identified, including G648, F656, and V659 located in the S6 domain and T623, S624, and V625 located at the base of the pore helix. These residues are similar but not identical to those determined previously for MK-499, an antiarrhythmic drug. In conclusion, vesnarinone preferentially blocks open HERG channels, with little effect on channels in the rested or inactivated state. These actions may contribute to the favorable frequency-dependent prolongation in APD.

Short- and long-term effects of amiodarone on the two components of cardiac delayed rectifier K(+) current.

BACKGROUND: Amiodarone is the most promising drug for the treatment of life-threatening tachyarrhythmias in patients with structural heart disease. The pharmacological effects of amiodarone on cardiac ion channels are complex and may differ for short-term and long-term administration. METHODS AND RESULTS: The delayed rectifier K(+) current (I(K)) of ventricular myocytes isolated from rabbit hearts was recorded with the whole-cell voltage-clamp technique. I(K) was separated into 2 components by use of specific blockers for either I(Ks) (chromanol 293B, 30 micromol/L) or I(Kr) (E-4031, 10 micromol/L). Short-term application of amiodarone caused a concentration-dependent decrease in I(Kr) with an IC(50) of 2.8 micromol/L (n=8) but only a minimal reduction in I(Ks). The short-term effects of amiodarone were also determined in Xenopus oocytes expressing the cloned human channels that conduct I(Kr) and I(Ks) (HERG and KvLQT1/minK). HERG current in oocytes was reduced by amiodarone (IC(50)=38 micromol/L), whereas KvLQT1/minK current was unaffected by 300 micromol/L amiodarone. To study the effects of long-term drug administration, rabbits were treated for 4 weeks with oral amiodarone (100 mg. kg(-1). d(-1)) before cell isolation. Long-term administration of amiodarone decreased I(K) to 55% (n=10) in control rabbits and altered the relative density of I(Kr) and I(Ks). The majority (92%) of current was I(Kr). mRNA levels of rabbit ERG,KVLQT1, and minK in left ventricular myocardium did not differ between control and long-term amiodarone. CONCLUSIONS: Amiodarone has differential effects on the 2 components of I(K), depending on the application period; short-term treatment inhibits primarily I(Kr), whereas long-term treatment reduces I(Ks).

Molecular biology of K(+) channels and their role in cardiac arrhythmias.

The configuration of cardiac action potentials varies considerably from one region of the heart to another. These differences are caused by differential cellular expression of several types of K(+) channel genes. The channels encoded by these genes can be grouped into several classes depending on the stimulus that permits the channels to open and conduct potassium ions. K(+) channels are activated by changes in transmembrane voltage or binding of ligands. Voltage-gated channels are normally the most important players in determining the shape and duration of action potentials and include the delayed rectifiers and the transient outward potassium channels. Ligand-gated channels include those that probably have only minor roles in shaping repolarization under normal conditions but, when activated by extracellular acetylcholine or a decrease in the intracellular concentration of ATP, can substantially shorten action potential duration. Inward rectifier K(+) channels are unique in that they are basically stuck in the open state but can be blocked in a voltage-dependent manner by intracellular Mg(2+), Ca(2+), and polyamines. Other K(+) channels have been described that provide a small background leak conductance. Many of these cardiac K(+) channels have been cloned in the past decade, permitting detailed studies of the molecular basis of their function and facilitating the discovery of the molecular basis of several forms of congenital arrhythmias. Drugs that block cardiac K(+) channels and prolong action potential duration have been developed as antiarrhythmic agents. However, many of these same drugs, as well as other common medications that are structurally unrelated, can also cause long QT syndrome and induce ventricular arrhythmia.

A structural basis for drug-induced long QT syndrome.

Mutations in the HERG K(+) channel gene cause inherited long QT syndrome (LQT), a disorder of cardiac repolarization that predisposes affected individuals to lethal arrhythmias [Curran, M. E. , Splawski, I., Timothy, K. W., Vincent, G. M., Green, E. D. & Keating, M. T. (1995) Cell 80, 795-804]. Acquired LQT is far more common and is most often caused by block of cardiac HERG K(+) channels by commonly used medications [Roden, D. M., Lazzara, R., Rosen, M., Schwartz, P. J., Towbin, J. & Vincent, G. M. (1996) Circulation 94, 1996-2012]. It is unclear why so many structurally diverse compounds block HERG channels, but this undesirable side effect now is recognized as a major hurdle in the development of new and safe drugs. Here we use alanine-scanning mutagenesis to determine the structural basis for high-affinity drug block of HERG channels by MK-499, a methanesulfonanilide antiarrhythmic drug. The binding site, corroborated with homology modeling, is comprised of amino acids located on the S6 transmembrane domain (G648, Y652, and F656) and pore helix (T623 and V625) of the HERG channel subunit that face the cavity of the channel. Other compounds that are structurally unrelated to MK-499, but cause LQT, also were studied. The antihistamine terfenadine and a gastrointestinal prokinetic drug, cisapride, interact with Y652 and F656, but not with V625. The aromatic residues of the S6 domain that interact with these drugs (Y652 and F656) are unique to eag/erg K(+) channels. Other voltage-gated K(+) (Kv) channels have Ile and Val (Ile) in the equivalent positions. These findings suggest a possible structural explanation for how so many commonly used medications block HERG but not other Kv channels and should facilitate the rational design of drugs devoid of HERG channel binding activity.

Functional roles of charged residues in the putative voltage sensor of the HCN2 pacemaker channel.

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels contribute to pacemaking activity in specialized neurons and cardiac myocytes. HCN channels have a structure similar to voltage-gated K(+) channels but have a much larger putative S4 transmembrane domain and open in response to membrane hyperpolarization instead of depolarization. As an initial attempt to define the structural basis of HCN channel gating, we have characterized the functional roles of the charged residues in the S2, S3, and S4 transmembrane domains. The nine basic residues and a single Ser in S4 were mutated individually to Gln, and the function of mutant channels was analyzed in Xenopus oocytes using two-microelectrode voltage clamp techniques. Surface membrane expression of hemagglutinin-epitope-tagged channel proteins was examined by chemiluminescence. Our results suggest that 1) Lys-291, Arg-294, Arg-297, and Arg-300 contribute to the voltage dependence of gating but not to channel folding or trafficking to the surface membrane; 2) Lys-303 and Ser-306 are essential for gating, but not for channel folding/trafficking; 3) Arg-312 is important for folding but not gating; and 4) Arg-309, Arg-315, and Arg-318 are crucial for normal protein folding/trafficking and may charge-pair with Asp residues located in the S2 and S3 domains.

Altered gating of HERG potassium channels by cobalt and lanthanum.

Activation of the rapid, delayed rectifier K current (IKr) is important for normal repolarization of cardiac action potentials, especially in mammalian ventricular muscle. The study of this current has been greatly aided by the discovery that the human ether-a-go-go-related gene (HERG) encodes the pore-forming alpha subunits of these channels. As for other voltage-activated K+ channels, divalent and trivalent cations affect the gating of HERG channels by screening negative membrane surface charges or by direct interaction with the channel gating mechanism. Previous studies have reported that IKr of myocytes, and HERG channels heterologously expressed in Xenopus oocytes, are reduced by external Co2+ and La3+. We have reinvestigated the "blocking" effect of Co2+ and La3+ on HERG channels expressed in Xenopus oocytes. At concentrations previously reported to block IKr or HERG current (IHERG), Co2+ (10 mM) and La3+ (10 microM) had only small effects on the magnitude of fully activated IHERG. The apparent block results from altered kinetics and voltage dependence of gating, similar to the effects of Ca2+ on HERG channels. Under control conditions, the half-points for voltage-dependent activation and inactivation of HERG were -35+/-2.1 and -76.3+/-1.7 mV, respectively. Co2+ and La3+ accelerated the rate of deactivation, decreased the rate of current activation, and shifted the half-point of the HERG channel activation curve by +53 and +65 mV, respectively. Co2+ shifted the voltage dependence of inactivation by + 14 mV, whereas La3+ had no effect. Co2+ also slowed the onset of IHERG inactivation and accelerated the rate of recovery from inactivation. These results indicate that reduction of IHERG by Co2+ (10 mM) and La3+ (10 microM) during depolarizing pulses is caused by a positive shift in the voltage dependence of activation, and does not result from pore block.

Long QT syndrome: ionic basis and arrhythmia mechanism in long QT syndrome type 1.

Long QT syndrome type 1 (LQT1) causes torsades de pointes arrhythmia, ventricular fibrillation, and sudden death. It usually is inherited as an autosomal dominant trait (Romano-Ward syndrome). The primary defect in LQT1 is a mutation in KVLQT1, a gene that encodes the pore-forming alpha-subunit of a K+ channel. KvLQT1 alpha-subunits coassemble with minK beta-subunits to form channels that conduct the slow delayed rectifier K+ current (I(Ks)) in the heart. Recessive mutations in KVLQT1 cause Jervell and Lange-Nielsen syndrome, which is characterized by more severe arrhythmias and congenital neural deafness. Heterologous expression studies demonstrated that mutations in KVLQT1 reduce I(Ks) by causing loss of channel function, altered channel gating, and/or a dominant-negative effect. It remains to be proven that an understanding of the molecular basis of LQT1 will lead to more effective therapy.

Functional consequences of chloride channel gene (CLCN1) mutations causing myotonia congenita.

OBJECTIVE: To determine the functional consequences of missense mutations within the skeletal muscle chloride channel gene CLCN1 that cause myotonia congenita. BACKGROUND: Myotonia congenita is a genetic muscle disease associated with abnormalities in the skeletal muscle voltage-gated chloride (ClC-1) channel. In order to understand the molecular basis of this inherited disease, it is important to determine the physiologic consequences of mutations found in patients affected by it. METHODS: The authors used a mammalian cell (human embryonic kidney 293) expression system and the whole-cell voltage-clamp technique to functionally express and physiologically characterize five CLCN1 mutations. RESULTS: The I329T mutation shifted the voltage dependence of open probability of ClC-1 channels to the right by 192 mV, and the R338Q mutation shifted it to the right by 38 mV. In addition, the I329T ClC-1 channels deactivated to a lesser extent than normal at negative potentials. The V165G, F167L, and F413C ClC-1 channels also shifted the voltage dependence of open probability, but only by +14 to +20 mV. CONCLUSIONS: The functional consequences of these mutations form the physiologic argument that these are disease-causing mutations and could lead to myotonia congenita by impairing the ability of the skeletal muscle voltage-gated chloride channels to maintain normal muscle excitability. Understanding of genetic and physiologic defects may ultimately lead to better diagnosis and treatment of patients with myotonia congenita.

Trapping of a methanesulfonanilide by closure of the HERG potassium channel activation gate.

Deactivation of voltage-gated potassium (K(+)) channels can slow or prevent the recovery from block by charged organic compounds, a phenomenon attributed to trapping of the compound within the inner vestibule by closure of the activation gate. Unbinding and exit from the channel vestibule of a positively charged organic compound should be favored by membrane hyperpolarization if not impeded by the closed gate. MK-499, a methanesulfonanilide compound, is a potent blocker (IC(50) = 32 nM) of HERG K(+) channels. This bulky compound (7 x 20 A) is positively charged at physiological pH. Recovery from block of HERG channels by MK-499 and other methanesulfonanilides is extremely slow (Carmeliet 1992; Ficker et al. 1998), suggesting a trapping mechanism. We used a mutant HERG (D540K) channel expressed in Xenopus oocytes to test the trapping hypothesis. D540K HERG has the unusual property of opening in response to hyperpolarization, in addition to relatively normal gating and channel opening in response to depolarization (Sanguinetti and Xu 1999). The hyperpolarization-activated state of HERG was characterized by long bursts of single channel reopening. Channel reopening allowed recovery from block by 2 microM MK-499 to occur with time constants of 10.5 and 52.7 s at -160 mV. In contrast, wild-type HERG channels opened only briefly after membrane hyperpolarization, and thus did not permit recovery from block by MK-499. These findings provide direct evidence that the mechanism of slow recovery from HERG channel block by methanesulfonanilides is due to trapping of the compound in the inner vestibule by closure of the activation gate. The ability of HERG channels to trap MK-499, despite its large size, suggests that the vestibule of this channel is larger than the well studied Shaker K(+) channel.

Mechanism of inverted activation of ClC-1 channels caused by a novel myotonia congenita mutation.

The voltage-gated chloride channel ClC-1 is the major contributor of membrane conductance in skeletal muscle and has been associated with the inherited muscular disorder myotonia congenita. Here, we report a novel mutation identified in a recessive myotonia congenita family. This mutation, Gly-499 to Arg (G499R) is located in the putative transmembrane domain 10 of the ClC-1 protein. In contrast to normal ClC-1 channels that deactivate upon hyperpolarization, functional expression of G499R ClC-1 yielded a hyperpolarization-activated chloride current when measured in the presence of a high (134 mM) intracellular chloride concentration. Current was abolished when measured with a physiological chloride transmembrane gradient. Electrophysiological analysis of other Gly-499 mutants (G499K, G499Q, and G499E) suggests that the positive charge introduced by the G499R mutation may be responsible for this unique gating behavior. To further explore the function of domain 10, we mutated two charged residues near Gly-499 of ClC-1. Functional analyses of R496Q, R496Q/G499R, R496K, and E500Q mutant channels suggest that the charged residues in domain 10 are important for normal channel function. Study of these mutants may shed further light on the structure and voltage-gating of this channel.

Biophysical properties and molecular basis of cardiac rapid and slow delayed rectifier potassium channels.

Normal cardiac action potential repolarization is dependent on activation of several K(+) currents, including I(Kr) and I(Ks). I(Kr) activates rapidly at positive potentials, exhibits inward rectification caused by C-type inactivation, and is potently blocked by methanesulfon-anilide antiarrhythmic drugs and several other common medications. I(Ks) activates very slowly, does not inactivate and is blocked by some benzodiazepines and a chromanol. HERG encodes subunits that form channels that mediate I(Kr). KVLQT1 and minK encode subunits that coassemble to form channels that mediate I(Ks). Mutations in any of these genes cause long QT syndrome, a disorder of cardiac repolarization that predisposes individuals to lethal arrhythmias. In this review, we summarize recent studies of the biophysical and pharmacological properties of HERG and KvLQT1/minK K(+) channels.

Long QT syndrome-associated mutations in the S4-S5 linker of KvLQT1 potassium channels modify gating and interaction with minK subunits.

Long QT syndrome is an inherited disorder of cardiac repolarization caused by mutations in cardiac ion channel genes, including KVLQT1. In this study, the functional consequences of three long QT-associated missense mutations in KvLQT1 (R243C, W248R, E261K) were characterized using the Xenopus oocyte heterologous expression system and two-microelectrode voltage clamp techniques. These mutations are located in or near the intracellular linker between the S4 and S5 transmembrane domains, a region implicated in activation gating of potassium channels. The E261K mutation caused loss of function and did not interact with wild-type KvLQT1 subunits. R243C or W248R KvLQT1 subunits formed functional channels, but compared with wild-type KvLQT1 current, the rate of activation was slower, and the voltage dependence of activation and inactivation was shifted to more positive potentials. Co expression of minK and KvLQT1 channel subunits induces a slow delayed rectifier K(+) current, I(Ks), characterized by slow activation and a markedly increased magnitude compared with current induced by KvLQT1 subunits alone. Coexpression of minK with R243C or W248R KvLQT1 subunits suppressed current, suggesting that coassembly of mutant subunits with minK prevented normal channel gating. The decrease in I(Ks) caused by loss of function or altered gating properties explains the prolonged QT interval and increased risk of arrhythmia and sudden death associated with these mutations in KVLQT1.

Dysfunction of delayed rectifier potassium channels in an inherited cardiac arrhythmia.

The rapid (IKr) and slow (IKs) delayed rectifier K+ currents are key regulators of cardiac repolarization. HERG encodes the Kr channel, and KVLQT1 and hminK encode subunits that coassemble to form Ks channels. Mutations in any one of these genes cause Romano-Ward syndrome, an autosomal dominant form of long QT syndrome (LQT). Mutations in KVLQT1 and HERG are the most common cause of LQT. Not all missense mutations of HERG or KVLQT1 have the same effect on K+ channel function. Most mutations result in a dominant-negative effect, but the severity of the resulting phenotype varies widely, as judged by reduction of current induced by coexpression of wild-type and mutant subunits in heterologous expression systems. Mutations in hminK (S74L, D76N) reduce IKs by shifting the voltage dependence of activation and accelerating channel deactivation. A recessive form of LQT is caused by mutations in either KVLQT1 or hminK. The functional consequences of mutations in delayed rectifier K+ channel subunits are delayed cardiac repolarization, lengthened QT interval, and an increased risk of torsade de pointes and sudden death.

Functional effects of mutations in KvLQT1 that cause long QT syndrome.

INTRODUCTION: The long QT syndrome (LQT) is caused by mutations in genes encoding ion channels that modulate the duration of ventricular action potentials. One of these genes, KVLQT1, encodes an alpha subunit that coassembles with another subunit, hminK, to form the cardiac slow delayed rectifier (I(Ks)) K+ channel. METHODS AND RESULTS: The functional effects of seven mutations in KVLQT1 were assessed using two-microelectrode voltage clamp and the Xenopus oocyte expression system. Most mutations in KVLQT1 caused loss of function when expressed alone. Oocytes were also injected with equal amounts of wild-type (WT) KVLQT1 and mutant KVLQT1 cRNA (with or without coinjection of hminK) and the resulting currents compared to currents induced by WT KvLQT1 alone. A341V, R190Q, or G189R KVLQT1 subunits did not affect expression of WT KvLQT1. The other mutations in KVLQT1 caused a variable degree of dominant-negative suppression of I(Ks). The order of potency for this effect was G345E > G306R = V254M > A341E. CONCLUSIONS: LQT1-associated mutations in KVLQT1 caused a spectrum of dysfunction in I(Ks) and KvLQT1 channels. The degree of I(Ks) dysfunction did not correlate with the QTc interval or the presence of symptoms in the respective gene carriers. In contrast to previous reports, we found that loss of function mutations are not exclusive to recessively inherited LQT.

Long QT syndrome-associated mutations in the Per-Arnt-Sim (PAS) domain of HERG potassium channels accelerate channel deactivation.

Mutations in the human ether-a-go-go-related gene (HERG) cause long QT syndrome, an inherited disorder of cardiac repolarization that predisposes affected individuals to life-threatening arrhythmias. HERG encodes the cardiac rapid delayed rectifier potassium channel that mediates repolarization of ventricular action potentials. In this study, we used the oocyte expression system and voltage clamp techniques to determine the functional consequences of eight long QT syndrome-associated mutations located in the amino-terminal region of HERG (F29L, N33T, G53R, R56Q, C66G, H70R, A78P, and L86R). Mutant subunits formed functional channels with altered gating properties when expressed alone in oocytes. Deactivation was accelerated by all mutations. Some mutants shifted the voltage dependence of channel availability to more positive potentials. Voltage ramps indicated that fast deactivation of mutant channels would reduce outward current during the repolarization phase of the cardiac action potential and cause prolongation of the corrected QT interval, QTc. The amino-terminal region of HERG was recently crystallized and shown to possess a Per-Arnt-Sim (PAS) domain. The location of these mutations suggests they may disrupt the PAS domain and interfere with its interaction with the S4-S5 linker of the HERG channel.

Mutations of the S4-S5 linker alter activation properties of HERG potassium channels expressed in Xenopus oocytes.

1. The structural basis for the activation gate of voltage-dependent K+ channels is not known, but indirect evidence has implicated the S4-S5 linker, the cytoplasmic region between the fourth and fifth transmembrane domains of the channel subunit. We have studied the effects of mutations in the S4-S5 linker of HERG (human ether-á-go-go-related gene), a human delayed rectifier K+ channel, in Xenopus oocytes. 2. Mutation of acidic residues (D540, E544) in the S4-S5 linker of HERG channels to neutral (Ala) or basic (Lys) residues accelerated the rate of channel deactivation. Most mutations greatly accelerated the rate of activation. However, E544K HERG channels activated more slowly than wild-type HERG channels. 3. Mutation of residues in the S4-S5 linker had little or no effect on fast inactivation, consistent with independence of HERG channel activation and inactivation 4. In response to large hyperpolarizations, D540K HERG channels can reopen into a state that is distinct from the normal depolarization-induced open state. It is proposed that substitution of a negatively charged Asp with the positively charged Lys disrupts a subunit interaction that normally stabilizes the channel in a closed state at negative transmembrane potentials. 5. The results indicate that the S4-S5 linker is a crucial component of the activation gate of HERG channels.