Effects of TRH on heteromeric rat erg1a/1b K+ channels are dominated by the rerg1b subunit

Ether-à-go-go K+ channels: effective modulators of neuronal excitability

Mammalian ether-à-go-go (EAG) channels are voltage-gated K⁺ channels. They are encoded by the KCNH gene family and divided into three subfamilies, eag (Kv10), erg (eag-related gene; Kv11) and elk (eag-like; Kv12). All EAG channel subtypes are expressed in the brain where they effectively modulate neuronal excitability. This Topical Review describes the biophysical properties of each of the EAG channel subtypes, their function in neurons and the neurological diseases induced by EAG channel mutations. In contrast to the function of erg currents in the heart, where they contribute to repolarization of the cardiac action potential, erg currents in neurons are involved in the maintenance of the resting potential, setting of action potential threshold and frequency accommodation. They can even support high frequency firing by preventing a depolarization-induced Na⁺ channel block. EAG channels are modulated differentially, e.g. eag channels by intracellular Ca²⁺ , erg channels by extracellular K⁺ and GPCRs, and elk channels by changes in pH. So far, only currents mediated by erg channels have been recorded in neurons with the help of selective blockers. Neuronal eag and elk currents have not been isolated due to the lack of suitable channel blockers. However, findings in KO mice indicate a physiological role of eag1 currents in synaptic transmission and an involvement of elk2 currents in cognitive performance. Human eag1 and eag2 gain-of-function mutations underlie syndromes associated with epileptic seizures.

Effects of Temperature on Heteromeric Kv11.1a/1b and Kv11.3 Channels

Kv11.1 channels are crucial in cardiac physiology, and there is increasing evidence of physiological roles of different Kv11 channels outside the heart. The HERG (human Kv11.1a) channel has previously been shown to carry substantially more current at elevated temperatures, and we have now comparably investigated the temperature dependence of neuronal Kv11.3 channels and the more ubiquitous heteromeric Kv11.1a/1b channels. Transiently expressed rat Kv11 channels were studied at 21°C, 30°C, and 35°C. At near-physiological temperature, the maximal sustained outward current density was almost three times the mean value obtained at room temperature for Kv11.1a/1b, and increased by ∼150% for Kv11.3. For both channels, reduced inactivation contributed to the current increase at higher temperature. Elevated temperature moved Kv11.1a/1b isochronal activation curves to more negative potentials, but shifted the potential of half-maximal Kv11.3 channel activation to more depolarized values and reduced its voltage sensitivity. Thus, increased temperature stabilized the open state over the closed state of Kv11.1a/1b channels and exerted the opposite effect on Kv11.3 channel activation. Both Kv11 channels exhibited an overall high temperature sensitivity of most gating parameters, with remarkably high Q10 factors of ∼5 for the rate of Kv11.1a/1b activation. The Q10 factors for Kv11.3 gating were more uniform, but still higher for activation than for inactivation kinetics. The results demonstrate that characteristic differences between Kv11.1a/1b and Kv11.3 determined at room temperature do not necessarily apply to physiological conditions. The data provided here can aid in the design of models that will enhance our understanding of the role of Kv11 currents in excitable cells.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Cytoplasmic domains and voltage-dependent potassium channel gating

The basic architecture of the voltage-dependent K⁺ channels (Kv channels) corresponds to a transmembrane protein core in which the permeation pore, the voltage-sensing components and the gating machinery (cytoplasmic facing gate and sensor–gate coupler) reside. Usually, large protein tails are attached to this core, hanging toward the inside of the cell. These cytoplasmic regions are essential for normal channel function and, due to their accessibility to the cytoplasmic environment, constitute obvious targets for cell-physiological control of channel behavior. Here we review the present knowledge about the molecular organization of these intracellular channel regions and their role in both setting and controlling Kv voltage-dependent gating properties. This includes the influence that they exert on Kv rapid/N-type inactivation and on activation/deactivation gating of Shaker-like and eag-type Kv channels. Some illustrative examples about the relevance of these cytoplasmic domains determining the possibilities for modulation of Kv channel gating by cellular components are also considered.

Cell type influences the molecular mechanisms involved in hormonal regulation of ERG K+ channels

While the thyrotropin-releasing hormone (TRH) effect of raising intracellular Ca(2+) levels has been shown to rely on G(q/11) and PLC activation, the molecular mechanisms involved in the regulation of ERG K(+) channels by TRH are still partially unknown. We have analysed the effects of βγ scavengers, Akt/PKB inactivation, and TRH receptor (TRH-R) overexpression on such regulation in native and heterologous expression cell systems. In native rat pituitary GH(3) cells β-ARK/CT, Gα(t), and phosducin significantly reduced TRH inhibition of rERG currents, whereas in HEK-H36/T1 cells permanently expressing TRH-R and hERG, neither of the βγ scavengers affected the TRH-induced shift in V (1/2). Use of specific siRNAs to knock Akt/PKB expression down abolished the TRH effect on HEK-H36/T1 cell hERG, but not on rERG from GH(3) cells. Indeed, wortmannin or long insulin pretreatment also blocked TRH regulation of ERG currents in HEK-H36/T1 but not in GH(3) cells. To determine whether these differences could be related to the amount of TRH-Rs in the cell, we studied the TRH concentration dependence of the Ca(2+) and ERG responses in GH(3) cells overexpressing the receptors. The data indicated that independent of the receptor number additional cellular factor(s) contribute differently to couple the TRH-R to hERG channel modulation in HEK-H36/T1 cells. We conclude that regulation of ERG currents by TRH and its receptor is transduced in GH(3) and HEK-H36/T1 cell systems through common and different elements, and hence that the cell type influences the signalling pathways involved in the TRH-evoked responses.

Gonadotropin-releasing hormone inhibits ether-à-go-go-related gene K+ currents in mouse gonadotropes

Secretion of LH from gonadotropes is initiated by a GnRH-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) is the result of Ca(2+) release from intracellular stores and Ca(2+) influx through voltage-dependent Ca(2+) channels. Here we describe an ether-à-go-go-related gene (erg) K(+) current in primary mouse gonadotropes and its possible function in the control of Ca(2+) influx. To detect gonadotropes, we used a knock-in mouse strain, in which GnRH receptor-expressing cells are fluorescently labeled. Erg K(+) currents were recorded in 80-90% of gonadotropes. Blockage of erg currents by E-4031 depolarized the resting potential by 5-8 mV and led to an increase in [Ca(2+)](i), which was abolished by nifedipine. GnRH inhibited erg currents by a reduction of the maximal erg current and in some cells additionally by a shift of the activation curve to more positive potentials. In conclusion, the erg current contributes to the maintenance of the resting potential in gonadotropes, thereby securing a low [Ca(2+)](i) by restricting Ca(2+) influx. In addition, the erg channels are modulated by GnRH by an as-yet unknown signal cascade.

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.

Differential expression of hERG1 channel isoforms reproduces properties of native I(Kr) and modulates cardiac action potential characteristics

BACKGROUND

The repolarizing cardiac rapid delayed rectifier current, I(Kr), is composed of ERG1 channels. It has been suggested that two isoforms of the ERG1 protein, ERG1a and ERG1b, both contribute to I(Kr). Marked heterogeneity in the kinetic properties of native I(Kr) has been described. We hypothesized that the heterogeneity of native I(Kr) can be reproduced by differential expression of ERG1a and ERG1b isoforms. Furthermore, the functional consequences of differential expression of ERG1 isoforms were explored as a potential mechanism underlying native heterogeneity of action potential duration (APD) and restitution.

METHODOLOGY/PRINCIPAL FINDINGS

The results show that the heterogeneity of native I(Kr) can be reproduced in heterologous expression systems by differential expression of ERG1a and ERG1b isoforms. Characterization of the macroscopic kinetics of ERG1 currents demonstrated that these were dependent on the relative abundance of ERG1a and ERG1b. Furthermore, we used a computational model of the ventricular cardiomyocyte to show that both APD and the slope of the restitution curve may be modulated by varying the relative abundance of ERG1a and ERG1b. As the relative abundance of ERG1b was increased, APD was gradually shortened and the slope of the restitution curve was decreased.

CONCLUSIONS/SIGNIFICANCE

Our results show that differential expression of ERG1 isoforms may explain regional heterogeneity of I(Kr) kinetics. The data demonstrate that subunit dependent changes in channel kinetics are important for the functional properties of ERG1 currents and hence I(Kr). Importantly, our results suggest that regional differences in the relative abundance of ERG1 isoforms may represent a potential mechanism underlying the heterogeneity of both APD and APD restitution observed in mammalian hearts.

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.

Identification of a posttranslational mechanism for the regulation of hERG1 K+ channel expression and hERG1 current density in tumor cells

A common feature of tumor cells is the aberrant expression of ion channels on their plasma membrane. The molecular mechanisms regulating ion channel expression in cancer cells are still poorly known. K(+) channels that belong to the human ether-a-go-go-related gene 1 (herg1) family are frequently misexpressed in cancer cells compared to their healthy counterparts. We describe here a posttranslational mechanism for the regulation of hERG1 channel surface expression in cancer cells. This mechanism is based on the activity of hERG1 isoforms containing the USO exon. These isoforms (i) are frequently overexpressed in human cancers, (ii) are retained in the endoplasmic reticulum, and (iii) form heterotetramers with different proteins of the hERG family. (iv) The USO-containing heterotetramers are retained intracellularly and undergo ubiquitin-dependent degradation. This process results in decreased hERG1 current (I(hERG1)) density. We detailed such a mechanism in heterologous systems and confirmed its functioning in tumor cells that endogenously express hERG1 proteins. The silencing of USO-containing hERG1 isoforms induces a higher I(hERG1) density in tumors, an effect that apparently regulates neurite outgrowth in neuroblastoma cells and apoptosis in leukemia cells.

Erg K+ channels modulate contractile activity in the bovine epididymal duct

The expression and functional role of ether-à-go-go-related gene (erg) K+ channels were examined in the bovine epididymal duct. Sperm transit through the epididymal duct relies on spontaneous phasic contractions (SC) of the peritubular smooth muscle wall. Isometric tension studies revealed SC-enhancing effects of the erg channel blockers E-4031, dofetilide, cisapride, and haloperidol and SC-suppressing effects of the activator NS-1643. In the corpus epididymidis, EC50 values of 32 nM and 8.3 microM were determined for E-4031 and NS-1643, respectively. E-4031 was also able to elicit contraction in epithelium-denuded corpus segments, which lacked SC. In the cauda region, E-4031 and NS-1643 exerted effects on agonist-induced contraction similar to those observed in the proximal duct. Experiments with nifedipine and thapsigargin suggested that the excitatory effects of E-4031 depended mainly on external calcium influx and not on intracellular calcium release. Western blot and RT-PCR assays revealed the expression of both, erg1a and erg1b, in all duct regions. Because erg1b appears to predominate in the epididymal duct, patch-clamp experiments were performed on heterologously expressed erg1b channels to investigate the sensitivity of this splice variant to NS-1643. In contrast to its effects on erg1a, NS-1643 induced a concentration-dependent current increase mainly due to a marked leftward shift in erg1b channel activation by approximately 30 mV at 10 microM, explaining the inhibitory effect of the drug on epididymal SC. In summary, these data provide strong evidence for a physiological role of erg1 channels in regulating epididymal motility patterns.

HERG1 currents in native K562 leukemic cells

The human ether-a-go-go related gene (HERG1) K+ channel is expressed in neoplastic cells, in which it was proposed to play a role in proliferation, differentiation and/or apoptosis. K562 cells (a chronic myeloid leukemic human cell line) express both the full-length (herg1a) and the N-terminally truncated (herg1b) isoforms of the gene, and this was confirmed with Western blots and coimmunoprecipitation experiments. Whole-cell currents were studied with a tail protocol. Seventy-eight percent of cells showed a HERG1-like current: repolarization to voltages negative to -40 mV produced a transient peak inward tail current, characteristic of HERG1 channels. Cells were exposed to a HERG-specific channel blocker, E4031. Half-maximal inhibitory concentration (IC50) of the blocker was 4.69 nM: The kinetics of the HERG1 current in K562 cells resembled the rapid component of the native cardiac delayed rectifier current, known to be conducted by heterotetrameric HERG1 channels. Fast and slow deactivation time constants at -120 mV were 27.5 and 239.5 ms, respectively. Our results in K562 cells suggest the assembling of heterotetrameric channels, with some parameters being dominated by one of the isoforms and other parameters being intermediate. Hydrogen peroxide was shown to increase HERG1a K+ current in heterologous expression systems, which constitutes an apoptotic signal. However, we found that K562 HERG1 whole-cell currents were not activated by H2O2.

Toxins interacting with ether-à-go-go-related gene voltage-dependent potassium channels

The critical role that ether-à-go-go-related gene (erg) K(+) channels play in mating in Caenorhabditis elegans, neuronal seizures in Drosophila and cardiac action potential repolarization in humans has been well documented. Three erg genes (erg1, erg2 and erg3) have been identified and characterized. A structurally diverse number of compounds block these channels, but do not display specificity among the different channel isoforms. In this review we describe the blocking properties of several peptides, purified from scorpion, sea anemone and spider venoms, which are selective for certain members of the ERG family of channels. These peptides do not behave as classical pore blockers and appear to modify the gating properties of the channel. Genomic studies predict the existence of many other novel peptides with the potential of being more selective for ERG channels than those discussed here.