The enigmatic cytoplasmic regions of KCNH channels

Gene and stem cell therapy for inherited cardiac arrhythmias

Inherited cardiac arrhythmias are a group of genetic diseases predisposing to sudden cardiac arrest, mainly resulting from variants in genes encoding cardiac ion channels or proteins involved in their regulation. Currently available therapeutic options (pharmacotherapy, ablative therapy and device-based therapy) can not preclude the occurrence of arrhythmia events and/or provide complete protection. With growing understanding of the genetic background and molecular mechanisms of inherited cardiac arrhythmias, advancing insight of stem cell technology, and development of vectors and delivery strategies, gene therapy and stem cell therapy may be promising approaches for treatment of inherited cardiac arrhythmias. Recent years have witnessed impressive progress in the basic science aspects and there is a clear and urgent need to be translated into the clinical management of arrhythmic events. In this review, we present a succinct overview of gene and cell therapy strategies, and summarize the current status of gene and cell therapy. Finally, we discuss future directions for implementation of gene and cell therapy in the therapy of inherited cardiac arrhythmias.

Inhibitory Perturbations of Fluvastatin on Afterhyperpolarization Current, Erg-mediated K+ Current, and Hyperpolarization-activated Cation Current in Both Pituitary GH3 Cells and Primary Embryonic Mouse Cortical Neurons

Fluvastatin (FLV), the first synthetically derived 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, is a potent inhibitor of cholesterol biosynthesis. While its primary mechanism of action is to reduce cholesterol levels, there is some evidence suggesting that it may also have effects on K+ channels. However, the overall effects of fluvastatin on ionic currents are not yet well understood. The whole-cell clamp recordings were applied to evaluate the ionic currents and action potentials of cells. Here, we have demonstrated that FLV can effectively inhibit the amplitude of erg-mediated K+ current (IK(erg)) in pituitary tumor (GH3) cells, with an IC50 of approximately 3.2 µM. In the presence of FLV, the midpoint in the activation curve of IK(erg) was distinctly shifted to a less negative potential by 10 mV, with minimal modification of the gating charge. However, the magnitude of hyperpolarization-activated cation current (Ih) elicited by long-lasting membrane hyperpolarization was progressively decreased, with an IC50 value of 8.7 µM, upon exposure to FLV. More interestingly, we also found that FLV (5 µM) could regulate the action potential and afterhyperpolarization properties in primary embryonic mouse cortical neurons. Our study presents compelling evidence indicating that FLV has the potential to impact both the amplitude and gating of the ion channels IK(erg) and Ih. We also provide credible evidence suggesting that this drug has the potential to modify the properties of action potentials and the afterhyperpolarization current in electrically excitable cells. However, the assumption that these findings translate to similar in-vivo results remains unclear.

The N-linker region of hERG1a upregulates hERG1b potassium channels

A major physiological role of hERG1 (human Ether-á-go-go-Related Gene 1) potassium channels is to repolarize cardiac action potentials. Two isoforms, hERG1a and hERG1b, associate to form the potassium current IKr in cardiomyocytes. Inherited mutations in hERG1a or hERG1b cause prolonged cardiac repolarization, long QT syndrome, and sudden death arrhythmia. hERG1a subunits assemble with and enhance the number of hERG1b subunits at the plasma membrane, but the mechanism for the increase in hERG1b by hERG1a is not well understood. Here, we report that the hERG1a N-terminal region expressed in trans with hERG1b markedly increased hERG1b currents and increased biotin-labeled hERG1b protein at the membrane surface. hERG1b channels with a deletion of the N-terminal 1b domain did not have a measurable increase in current or biotinylated protein when coexpressed with hERG1a N-terminal regions, indicating that the 1b domain was required for the increase in hERG1b. Using a biochemical pull-down interaction assay and a FRET hybridization experiment, we detected a direct interaction between the hERG1a N-terminal region and the hERG1b N-terminal region. Using engineered deletions and alanine mutagenesis, we identified a short span of amino acids at positions 216 to 220 within the hERG1a "N-linker" region that were necessary for the upregulation of hERG1b. We propose that direct structural interactions between the hERG1a N-linker region and the hERG1b 1b domain increase hERG1b at the plasma membrane. Mechanisms regulating hERG1a and hERG1b are likely critical for cardiac function, may be disrupted by long QT syndrome mutants, and serve as potential targets for therapeutics.

Gating and regulation of KCNH (ERG, EAG, and ELK) channels by intracellular domains

The KCNH family comprises the ERG, EAG, and ELK voltage-activated, potassium-selective channels. Distinct from other K channels, KCNH channels contain unique structural domains, including a PAS (Per-Arnt-Sim) domain in the N-terminal region and a CNBHD (cyclic nucleotide-binding homology domain) in the C-terminal region. The intracellular PAS domains and CNBHDs interact directly and regulate some of the characteristic gating properties of each type of KCNH channel. The PAS-CNBHD interaction regulates slow closing (deactivation) of hERG channels, the kinetics of activation and pre-pulse dependent population of closed states (the Cole-Moore shift) in EAG channels and voltage-dependent potentiation in ELK channels. KCNH channels are all regulated by an intrinsic ligand motif in the C-terminal region which binds to the CNBHD. Here, we focus on some recent advances regarding the PAS-CNBHD interaction and the intrinsic ligand.

Antiepileptic Drug Tiagabine Does Not Directly Target Key Cardiac Ion Channels Kv11.1, Nav1.5 and Cav1.2

Tiagabine is an antiepileptic drug used for the treatment of partial seizures in humans. Recently, this drug has been found useful in several non-epileptic conditions, including anxiety, chronic pain and sleep disorders. Since tachycardia-an impairment of cardiac rhythm due to cardiac ion channel dysfunction-is one of the most commonly reported non-neurological adverse effects of this drug, in the present paper we have undertaken pharmacological and numerical studies to assess a potential cardiovascular risk associated with the use of tiagabine. A chemical interaction of tiagabine with a model of human voltage-gated ion channels (VGICs) is described using the molecular docking method. The obtained in silico results imply that the adverse effects reported so far in the clinical cardiological of tiagabine could not be directly attributed to its interactions with VGICs. This is also confirmed by the results from the isolated organ studies (i.e., calcium entry blocking properties test) and in vivo (electrocardiogram study) assays of the present research. It was found that tachycardia and other tiagabine-induced cardiac complications are not due to a direct effect of this drug on ventricular depolarization and repolarization.

Structure of KCNH2 cyclic nucleotide-binding homology domain reveals a functionally vital salt-bridge

Human KCNH2 channels (hKCNH2, ether-à-go-go [EAG]–related gene, hERG) are best known for their contribution to cardiac action potential repolarization and have key roles in various pathologies. Like other KCNH family members, hKCNH2 channels contain a unique intracellular complex, consisting of an N-terminal eag domain and a C-terminal cyclic nucleotide-binding homology domain (CNBHD), which is crucial for channel function. Previous studies demonstrated that the CNBHD is occupied by an intrinsic ligand motif, in a self-liganded conformation, providing a structural mechanism for the lack of KCNH channel regulation by cyclic nucleotides. While there have been significant advancements in the structural and functional characterization of the CNBHD of KCNH channels, a high-resolution structure of the hKCNH2 intracellular complex has been missing. Here, we report the 1.5 Å resolution structure of the hKCNH2 channel CNBHD. The structure reveals the canonical fold shared by other KCNH family members, where the spatial organization of the intrinsic ligand is preserved within the β-roll region. Moreover, measurements of small-angle x-ray scattering profile in solution, as well as comparison with a recent NMR analysis of hKCNH2, revealed high agreement with the crystallographic structure, indicating an overall low flexibility in solution. Importantly, we identified a novel salt-bridge (E807-R863) which was not previously resolved in the NMR and cryo-EM structures. Electrophysiological analysis of charge-reversal mutations revealed the bridge’s crucial role in hKCNH2 function. Moreover, comparison with other KCNH members revealed the structural conservation of this salt-bridge, consistent with its functional significance. Together with the available structure of the mouse KCNH1 intracellular complex and previous electrophysiological and spectroscopic studies of KCNH family members, we propose that this salt-bridge serves as a strategically positioned linchpin to support both the spatial organization of the intrinsic ligand and the maintenance of the intracellular complex interface.

Molecular Insights Into the Gating Kinetics of the Cardiac hERG Channel, Illuminated by Structure and Molecular Dynamics

The rapidly activating delayed rectifier K⁺ current generated by the cardiac hERG potassium channel encoded by KCNH2 is the most important reserve current for cardiac repolarization. The unique inward rectification characteristics of the hERG channel depend on the gating regulation, which involves crucial structural domains and key single amino acid residues in the full-length hERG channel. Identifying critical molecules involved in the regulation of gating kinetics for the hERG channel requires high-resolution structures and molecular dynamics simulation models. Based on the latest progress in hERG structure and molecular dynamics simulation research, summarizing the molecules involved in the changes in the channel state helps to elucidate the unique gating characteristics of the channel and the reason for its high affinity to cardiotoxic drugs. In this review, we aim to summarize the significant advances in understanding the voltage gating regulation of the hERG channel based on its structure obtained from cryo-electron microscopy and computer simulations, which reveal the critical roles of several specific structural domains and amino acid residues.

Kv11 (ether-à-go-go-related gene) voltage-dependent K+ channels promote resonance and oscillation of subthreshold membrane potentials

Key points

Some ion channels are known to behave as inductors and make up the parallel resonant circuit in the plasma membrane of neurons, which enables neurons to respond to current inputs with a specific frequency (so‐called ‘resonant properties’).

Here, we report that heterologous expression of mouse Kv11 voltage‐dependent K⁺ channels generate resonance and oscillation at depolarized membrane potentials in HEK293 cells; expressions of individual Kv11 subtypes generate resonance and oscillation with different frequency properties.

Kv11.3‐expressing HEK293 cells exhibited transient conductance changes that opposed the current changes induced by voltage steps; this probably enables Kv11 channels to behave like an inductor.

The resonance and oscillation of inferior olivary neurons were impaired at the resting membrane potential in Kv11.3 knockout mice.

This study helps to elucidate basic ion channel properties that are crucial for the frequency responses of neurons.

Abstract

The plasma membranes of some neurons preferentially respond to current inputs with a specific frequency, and output as large voltage changes. This property is called resonance, and is thought to be mediated by ion channels that show inductor‐like behaviour. However, details of the candidate ion channels remain unclear. In this study, we mainly focused on the functional roles of Kv11 potassium (K⁺) channels, encoded by ether‐á‐go‐go‐related genes, in resonance in mouse inferior olivary (IO) neurons. We transfected HEK293 cells with long or short splice variants of Kv11.1 (Merg1a and Merg1b) or Kv11.3, and examined membrane properties using whole‐cell recording. Transfection with Kv11 channels reproduced resonance at membrane potentials depolarized from the resting state. Frequency ranges of Kv11.3‐, Kv11.1(Merg1b)‐ and Kv11.1(Merg1a)‐expressing cells were 2–6 Hz, 2–4 Hz, and 0.6–0.8 Hz, respectively. Responses of Kv11.3 currents to step voltage changes were essentially similar to those of inductor currents in the resistor–inductor–capacitor circuit. Furthermore, Kv11 transfections generated membrane potential oscillations. We also confirmed the contribution of HCN1 channels as a major mediator of resonance at more hyperpolarized potentials by transfection into HEK293 cells. The Kv11 current kinetics and properties of Kv11‐dependent resonance suggested that Kv11.3 mediated resonance in IO neurons. This finding was confirmed by the impairment of resonance and oscillation at –30 to –60 mV in Kcnh7 (Kv11.3) knockout mice. These results suggest that Kv11 channels have important roles in inducing frequency‐dependent responses in a subtype‐dependent manner from resting to depolarized membrane potentials.

Some ion channels are known to behave as inductors and make up the parallel resonant circuit in the plasma membrane of neurons, which enables neurons to respond to current inputs with a specific frequency (so‐called ‘resonant properties’).

Here, we report that heterologous expression of mouse Kv11 voltage‐dependent K⁺ channels generate resonance and oscillation at depolarized membrane potentials in HEK293 cells; expressions of individual Kv11 subtypes generate resonance and oscillation with different frequency properties.

Kv11.3‐expressing HEK293 cells exhibited transient conductance changes that opposed the current changes induced by voltage steps; this probably enables Kv11 channels to behave like an inductor.

The resonance and oscillation of inferior olivary neurons were impaired at the resting membrane potential in Kv11.3 knockout mice.

This study helps to elucidate basic ion channel properties that are crucial for the frequency responses of neurons.

The EAG Voltage-Dependent K+ Channel Subfamily: Similarities and Differences in Structural Organization and Gating

EAG (ether-à-go-go or KCNH) are a subfamily of the voltage-gated potassium (Kv) channels. Like for all potassium channels, opening of EAG channels drives the membrane potential toward its equilibrium value for potassium, thus setting the resting potential and repolarizing action potentials. As voltage-dependent channels, they switch between open and closed conformations (gating) when changes in membrane potential are sensed by a voltage sensing domain (VSD) which is functionally coupled to a pore domain (PD) containing the permeation pathway, the potassium selectivity filter, and the channel gate. All Kv channels are tetrameric, with four VSDs formed by the S1-S4 transmembrane segments of each subunit, surrounding a central PD with the four S5-S6 sections arranged in a square-shaped structure. Structural information, mutagenesis, and functional experiments, indicated that in "classical/Shaker-type" Kv channels voltage-triggered VSD reorganizations are transmitted to PD gating via the α-helical S4-S5 sequence that links both modules. Importantly, these Shaker-type channels share a domain-swapped VSD/PD organization, with each VSD contacting the PD of the adjacent subunit. In this case, the S4-S5 linker, acting as a rigid mechanical lever (electromechanical lever coupling), would lead to channel gate opening at the cytoplasmic S6 helices bundle. However, new functional data with EAG channels split between the VSD and PD modules indicate that, in some Kv channels, alternative VSD/PD coupling mechanisms do exist. Noticeably, recent elucidation of the architecture of some EAG channels, and other relatives, showed that their VSDs are non-domain swapped. Despite similarities in primary sequence and predicted structural organization for all EAG channels, they show marked kinetic differences whose molecular basis is not completely understood. Thus, while a common general architecture may establish the gating system used by the EAG channels and the physicochemical coupling of voltage sensing to gating, subtle changes in that common structure, and/or allosteric influences of protein domains relatively distant from the central gating machinery, can crucially influence the gating process. We consider here the latest advances on these issues provided by the elucidation of eag1 and erg1 three-dimensional structures, and by both classical and more recent functional studies with different members of the EAG subfamily.

Chlorpromazine binding to the PAS domains uncovers the effect of ligand modulation on EAG channel activity

Ether-a-go-go (EAG) potassium selective channels are major regulators of neuronal excitability and cancer progression. EAG channels contain a Per-Arnt-Sim (PAS) domain in their intracellular N-terminal region. The PAS domain is structurally similar to the PAS domains in non-ion channel proteins, where these domains frequently function as ligand-binding domains. Despite the structural similarity, it is not known whether the PAS domain can regulate EAG channel function via ligand binding. Here, using surface plasmon resonance, tryptophan fluorescence, and analysis of EAG currents recorded in Xenopus laevis oocytes, we show that a small molecule chlorpromazine (CH), widely used as an antipsychotic medication, binds to the isolated PAS domain of EAG channels and inhibits currents from these channels. Mutant EAG channels that lack the PAS domain show significantly lower inhibition by CH, suggesting that CH affects currents from EAG channels directly through the binding to the PAS domain. Our study lends support to the hypothesis that there are previously unaccounted steps in EAG channel gating that could be activated by ligand binding to the PAS domain. This has broad implications for understanding gating mechanisms of EAG and related ERG and ELK K+ channels and places the PAS domain as a new target for drug discovery in EAG and related channels. Up-regulation of EAG channel activity is linked to cancer and neurological disorders. Our study raises the possibility of repurposing the antipsychotic drug chlorpromazine for treatment of neurological disorders and cancer.

Identification of undecylenic acid as EAG channel inhibitor using surface plasmon resonance-based screen of KCNH channels

BACKGROUND

KCNH family of potassium channels is responsible for diverse physiological functions ranging from the regulation of neuronal excitability and cardiac contraction to the regulation of cancer progression. KCNH channels contain a Per-Arn-Sim (PAS) domain in their N-terminal and cyclic nucleotide-binding homology (CNBH) domain in their C-terminal regions. These intracellular domains shape the function of KCNH channels and are important targets for drug development.

METHODS

Here we describe a surface plasmon resonance (SPR)-based screening method aimed in identifying small molecule binders of PAS and CNBH domains for three KCNH channel subfamilies: ether-à-go-go (EAG), EAG-related gene (ERG), and EAG-like K+ (ELK). The method involves purification of the PAS and CNBH domains, immobilization of the purified domains on the SPR senor chip and screening small molecules in a chemical library for binding to the immobilized domains using changes in the SPR response as a reporter of the binding. The advantages of this method include low quantity of purified PAS and CNBH domains necessary for the implementation of the screen, direct assessment of the small molecule binding to the PAS and CNBH domains and easiness of assessing KCNH subfamily specificity of the small molecule binders.

RESULTS

Using the SPR-based method we screened the Spectrum Collection Library of 2560 compounds against the PAS and CNBH domains of the three KCNH channel subfamilies and identified a pool of small molecules that bind to the PAS or CNBH domains. To further evaluate the effectiveness of the screen we tested the functional effect of one of the identified mEAG PAS domain specific small molecule binders on currents recorded from EAG channels. Undecylenic acid inhibited currents recorded from EAG channels in a concentration-dependent manner with IC50 of ~ 1 μM.

CONCLUSION

Our results show that the SPR-based method is well suited for identifying small molecule binders of KCNH channels and can facilitate drug discovery for other ion channels as well.

New Structures and Gating of Voltage-Dependent Potassium (Kv) Channels and Their Relatives: A Multi-Domain and Dynamic Question

Voltage-dependent potassium channels (Kv channels) are crucial regulators of cell excitability that participate in a range of physiological and pathophysiological processes. These channels are molecular machines that display a mechanism (known as gating) for opening and closing a gate located in a pore domain (PD). In Kv channels, this mechanism is triggered and controlled by changes in the magnitude of the transmembrane voltage sensed by a voltage-sensing domain (VSD). In this review, we consider several aspects of the VSD–PD coupling in Kv channels, and in some relatives, that share a common general structure characterized by a single square-shaped ion conduction pore in the center, surrounded by four VSDs located at the periphery. We compile some recent advances in the knowledge of their architecture, based in cryo-electron microscopy (cryo-EM) data for high-resolution determination of their structure, plus some new functional data obtained with channel variants in which the covalent continuity between the VSD and PD modules has been interrupted. These advances and new data bring about some reconsiderations about the use of exclusively a classical electromechanical lever model of VSD–PD coupling by some Kv channels, and open a view of the Kv-type channels as allosteric machines in which gating may be dynamically influenced by some long-range interactional/allosteric mechanisms.

The hERG potassium channel intrinsic ligand regulates N- and C-terminal interactions and channel closure

An intersubunit interaction between the N-terminal PAS domain and C-terminal cyclic nucleotide binding homology domain (CNBHD) regulates slow deactivation in hERG potassium channels. By mutating the intrinsic ligand, Codding and Trudeau disrupt slow deactivation and prevent the PAS-CNBHD interaction.

Human ether-à-go-go–related gene (hERG, KCNH2) voltage-activated potassium channels are critical for cardiac excitability. hERG channels have characteristic slow closing (deactivation), which is auto-regulated by a direct interaction between the N-terminal Per-Arnt-Sim (PAS) domain and the C-terminal cyclic nucleotide binding homology domain (CNBHD). hERG channels are not activated by the binding of extrinsic cyclic nucleotide ligands, but rather bind an “intrinsic ligand” that is composed of residues 860–862 within the CNBHD and mimics a cyclic nucleotide. The intrinsic ligand is located at the PAS–CNBHD interface, but its mechanism of action in hERG is not well understood. Here we use whole-cell patch-clamp electrophysiology and FRET spectroscopy to examine how the intrinsic ligand regulates gating. To carry out this work, we coexpress PAS (a PAS domain fused to cyan fluorescent protein) in trans with hERG “core” channels (channels with a deletion of the PAS domain fused to citrine fluorescent protein). The PAS domain in trans with hERG core channels has slow (regulated) deactivation, like that of WT hERG channels, as well as robust FRET, which indicates there is a direct functional and structural interaction of the PAS domain with the channel core. In contrast, PAS in trans with hERG F860A core channels has intermediate deactivation and intermediate FRET, indicating perturbation of the PAS domain interaction with the CNBHD. Furthermore, PAS in trans with hERG L862A core channels, or PAS in trans with hERG F860G,L862G core channels, has fast (nonregulated) deactivation and no measurable FRET, indicating abolition of the PAS and CNBHD interaction. These results indicate that the intrinsic ligand is necessary for the functional and structural interaction between the PAS domain and the CNBHD, which regulates the characteristic slow deactivation gating in hERG channels.

The Interaction between the Drosophila EAG Potassium Channel and the Protein Kinase CaMKII Involves an Extensive Interface at the Active Site of the Kinase

The Drosophila EAG (dEAG) potassium channel is the founding member of the superfamily of KNCH channels, which are involved in cardiac repolarization, neuronal excitability and cellular proliferation. In flies, dEAG is involved in regulation of neuron firing and assembles with CaMKII to form a complex implicated in memory formation. We have characterized the interaction between the kinase domain of CaMKII and a 53-residue fragment of the dEAG channel that includes a canonical CaMKII recognition sequence. Crystal structures together with biochemical/biophysical analysis show a substrate-kinase complex with an unusually tight and extensive interface that appears to be strengthened by phosphorylation of the channel fragment. Electrophysiological recordings show that catalytically active CaMKII is required to observe active dEAG channels. A previously identified phosphorylation site in the recognition sequence is not the substrate for this crucial kinase activity, but rather contributes importantly to the tight interaction of the kinase with the channel. The available data suggest that the dEAG channel is a docking platform for the kinase and that phosphorylation of the channel's kinase recognition sequence modulates the strength of the interaction between the channel and the kinase.

Determinants of Isoform-Specific Gating Kinetics of hERG1 Channel: Combined Experimental and Simulation Study

I_Kr is the rapidly activating component of the delayed rectifier potassium current, the ion current largely responsible for the repolarization of the cardiac action potential. Inherited forms of long QT syndrome (LQTS) (Lees-Miller et al., 1997) in humans are linked to functional modifications in the Kv11.1 (hERG) ion channel and potentially life threatening arrhythmias. There is little doubt now that hERG-related component of I_Kr in the heart depends on the tetrameric (homo- or hetero-) channels formed by two alternatively processed isoforms of hERG, termed hERG1a and hERG1b. Isoform composition (hERG1a- vs. the b-isoform) has recently been reported to alter pharmacologic responses to some hERG blockers and was proposed to be an essential factor pre-disposing patients for drug-induced QT prolongation. Very little is known about the gating and pharmacological properties of two isoforms in heart membranes. For example, how gating mechanisms of the hERG1a channels differ from that of hERG1b is still unknown. The mechanisms by which hERG 1a/1b hetero-tetramers contribute to function in the heart, or what role hERG1b might play in disease are all questions to be answered. Structurally, the two isoforms differ only in the N-terminal region located in the cytoplasm: hERG1b is 340 residues shorter than hERG1a and the initial 36 residues of hERG1b are unique to this isoform. In this study, we combined electrophysiological measurements for HEK cells, kinetics and structural modeling to tease out the individual contributions of each isoform to Action Potential formation and then make predictions about the effects of having various mixture ratios of the two isoforms. By coupling electrophysiological data with computational kinetic modeling, two proposed mechanisms of hERG gating in two homo-tetramers were examined. Sets of data from various experimental stimulation protocols (HEK cells) were analyzed simultaneously and fitted to Markov-chain models (M-models). The minimization procedure presented here, allowed assessment of suitability of different Markov model topologies and the corresponding parameters that describe the channel kinetics. The kinetics modeling pointed to key differences in the gating kinetics that were linked to the full channel structure. Interactions between soluble domains and the transmembrane part of the channel appeared to be critical determinants of the gating kinetics. The structures of the full channel in the open and closed states were compared for the first time using the recent Cryo-EM resolved structure for full open hERG channel and an homology model for the closed state, based on the highly homolog EAG1 channel. Key potential interactions which emphasize the importance of electrostatic interactions between N-PAS cap, S4-S5, and C-linker are suggested based on the structural analysis. The derived kinetic parameters were later used in higher order models of cells and tissue to track down the effect of varying the ratios of hERG1a and hERG1b on cardiac action potentials and computed electrocardiograms. Simulations suggest that the recovery from inactivation of hERG1b may contribute to its physiologic role of this isoform in the action potential. Finally, the results presented here contribute to the growing body of evidence that hERG1b significantly affects the generation of the cardiac I_kr and plays an important role in cardiac electrophysiology. We highlight the importance of carefully revisiting the Markov models previously proposed in order to properly account for the relative abundance of the hERG1 a- and b- isoforms.

Gene and Blood Analysis Reveal That Transfer from Brackish Water to Freshwater Is More Stressful to the Silverside Odontesthes humensis

Silversides are fish that inhabit marine coastal waters, coastal lagoons, and estuarine regions in southern South America. The freshwater (FW) silversides have the ability to tolerate salinity variations. Odontesthes humensis have similar habitats and biological characteristics of congeneric O. bonariensis, the most studied silverside species and with great economic importance. Studies revealed that O. bonariensis is not fully adapted to FW, despite inhabiting hyposmotic environments in nature. However, there is little information about stressful environments for cultivation of silverside O. humensis. Thus, the aim of this study was to evaluate the stress and osmoregulation responses triggered by the osmotic transfers on silverside O. humensis. Silversides were acclimated to FW (0 ppt) and to brackish water (BW, 10 ppt) and then they were exposed to opposite salinity treatment. Silverside gills and blood were sampled on pre-transfer (D0) and 1, 7, and 15 days (D1, D7, and D15) after changes in environmental salinity, the expression levels of genes atp1a3a, slc12a2b, kcnh1, and hspa1a were determined by quantitative reverse transcription-PCR for evaluation of osmoregulatory and stress responses. Furthermore, glycemia, hematocrit, and osmolality were also evaluated. The expression of atp1a3a was up- and down-regulated at D1 after the FW-BW and BW-FW transfers, respectively. Slc12a2b was up-regulated after FW-BW transfer. Similarly, kcnh1 and hspa1a were up-regulated at D1 after the BW-FW transfer. O. humensis blood osmolality decreased after the exposure to FW. It remained stable after exposure to BW, indicating an efficient hyposmoregulation. The glycemia had a peak at D1 after BW-FW transfer. No changes were observed in hematocrit. The return to the pre-transfer levels at D7 after the significant increases in responses of almost all evaluated molecular and blood parameters indicated that this period is enough for acclimation to the experimental conditions. In conclusion, our results suggest that BW-FW transfer is more stressful to O. humensis than FW-BW transfer and the physiology of O. humensis is only partially adapted to FW.

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.

Fluid flow modulates electrical activity in cardiac hERG potassium channels

Fluid movement within the heart generates substantial shear forces, but the effect of this mechanical stress on the electrical activity of the human heart has not been examined. The fast component of the delayed rectifier potassium currents responsible for repolarization of the cardiac action potential, Ikr, is encoded by the human ether-a-go-go related gene (hERG) channel. Here, we exposed hERG1a channel-expressing HEK293T cells to laminar shear stress (LSS) and observed that this mechanical stress increased the whole-cell current by 30-40%. LSS shifted the voltage dependence of steady-state activation of the hERG channel to the hyperpolarizing direction, accelerated the time course of activation and recovery from inactivation, slowed down deactivation, and shifted the steady-state inactivation to the positive direction, all of which favored the hERG open state. In contrast, the time course of inactivation was faster, favoring the closed state. Using specific inhibitors of focal adhesion kinase, a regulator of mechano-transduction via the integrin pathway, we also found that the LSS-induced modulation of the whole-cell current depended on the integrin pathway. The hERG1b channel variant, which lacks the Per-Arnt-Sim (PAS) domain, and long QT syndrome-associated variants having point mutations in the PAS domain were unaffected by LSS, suggesting that the PAS domain in hERG1a channel may be involved in sensing mechanical shear stress. We conclude that a mechano-electric feedback pathway modulates hERG channel activity through the integrin pathway, indicating that mechanical forces in the heart influence its electrical activity.

Structural insights into the mechanisms of CNBD channel function

James and Zagotta discuss how recent cryoEM structures inform our understanding of cyclic nucleotide–binding domain channels.

Cyclic nucleotide-binding domain (CNBD) channels are a family of ion channels in the voltage-gated K⁺ channel superfamily that play crucial roles in many physiological processes. CNBD channels are structurally similar but functionally very diverse. This family includes three subfamilies: (1) the cyclic nucleotide-gated (CNG) channels, which are cation-nonselective, voltage-independent, and cyclic nucleotide-gated; (2) the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are weakly K⁺ selective, hyperpolarization-activated, and cyclic nucleotide-gated; and (3) the ether-à-go-go-type (KCNH) channels, which are strongly K⁺ selective, depolarization-activated, and cyclic nucleotide-independent. Recently, several high-resolution structures have been reported for intact CNBD channels, providing a structural framework to better understand their diverse function. In this review, we compare and contrast the recent structures and discuss how they inform our understanding of ion selectivity, voltage-dependent gating, and cyclic nucleotide–dependent gating within this channel family.

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.

Molecular mechanism of voltage-dependent potentiation of KCNH potassium channels

EAG-like (ELK) voltage-gated potassium channels are abundantly expressed in the brain. These channels exhibit a behavior called voltage-dependent potentiation (VDP), which appears to be a specialization to dampen the hyperexitability of neurons. VDP manifests as a potentiation of current amplitude, hyperpolarizing shift in voltage sensitivity, and slowing of deactivation in response to a depolarizing prepulse. Here we show that VDP of D. rerio ELK channels involves the structural interaction between the intracellular N-terminal eag domain and C-terminal CNBHD. Combining transition metal ion FRET, patch-clamp fluorometry, and incorporation of a fluorescent noncanonical amino acid, we show that there is a rearrangement in the eag domain-CNBHD interaction with the kinetics, voltage-dependence, and ATP-dependence of VDP. We propose that the activation of ELK channels involves a slow open-state dependent rearrangement of the direct interaction between the eag domain and CNBHD, which stabilizes the opening of the channel.

DOI:http://dx.doi.org/10.7554/eLife.26355.001

In humans and other animals, electrical signals trigger the heart to beat and carry information around the brain and nervous system. Particular cells can generate these signals by regulating the flow of ions into and out of the cell via proteins called ion channels. These proteins sit in the membrane that surrounds the cell and will open or close in response to specific signals. For example, an ion channel in humans called hERG allows positively-charged potassium ions to flow out of a heart cell to help the cell return to its “resting” state after producing an electrical signal. Defects in hERG can alter the rhythm at which the heart beats, leading to a serious condition called Long QT syndrome.

The human hERG channel is part of a family of related channels known as the KCNH channels. These channels are made of four protein subunits that assemble to form a pore that spans the cell membrane. When a cell is resting before producing an electrical signal, KCNH channels are generally closed. However, once an electrical signal starts, the flow of ions through other ion channels in the cell membrane changes an electrical property across the membrane known as the “voltage”. This change in voltage causes KCNH channels to open.

Dai and Zagotta studied how a KCNH channel known as ELK from zebrafish responds to changes in membrane voltage. The experiments show that the manner in which ELK channels respond to the voltage is due to changes in how the subunits interact in the part of the channel that lies inside the cell. Further experiments using several new techniques reveal in much more detail how the shape of the channel alters as the voltage changes. These new techniques could also be used to observe how other KCNH channels in the heart and brain change shape in response to changes in voltage. This could lead to the design of new drugs to treat heart and neurological diseases.

DOI:http://dx.doi.org/10.7554/eLife.26355.002

Molecular Insights into the Mechanism of Calmodulin Inhibition of the EAG1 Potassium Channel

The human EAG1 potassium channel belongs to the superfamily of KCNH voltage-gated potassium channels that have roles in cardiac repolarization and neuronal excitability. EAG1 is strongly inhibited by Ca²⁺/calmodulin (CaM) through a mechanism that is not understood. We determined the binding properties of CaM with each one of three previously identified binding sites (BDN, BDC1, and BDC2), analyzed binding to protein stretches that include more than one site, and determined the effect of neighboring globular domains on the binding properties. The determination of the crystal structure of CaM bound to BDC2 shows the channel fragment interacting with only the C lobe of calmodulin and adopting an unusual bent conformation. Based on this structure and on a functional and biochemical analysis of mutants, we propose a model for the mechanism of inhibition whereby the local conformational change induced by CaM binding at BDC2 lies at the basis of channel modulation.

The human olfactory transcriptome

BACKGROUND

Olfaction is a versatile sensory mechanism for detecting thousands of volatile odorants. Although molecular basis of odorant signaling is relatively well understood considerable gaps remain in the complete charting of all relevant gene products. To address this challenge, we applied RNAseq to four well-characterized human olfactory epithelial samples and compared the results to novel and published mouse olfactory epithelium as well as 16 human control tissues.

RESULTS

We identified 194 non-olfactory receptor (OR) genes that are overexpressed in human olfactory tissues vs.

CONTROLS

The highest overexpression is seen for lipocalins and bactericidal/permeability-increasing (BPI)-fold proteins, which in other species include secreted odorant carriers. Mouse-human discordance in orthologous lipocalin expression suggests different mammalian evolutionary paths in this family. Of the overexpressed genes 36 have documented olfactory function while for 158 there is little or no previous such functional evidence. The latter group includes GPCRs, neuropeptides, solute carriers, transcription factors and biotransformation enzymes. Many of them may be indirectly implicated in sensory function, and ~70 % are over expressed also in mouse olfactory epithelium, corroborating their olfactory role. Nearly 90 % of the intact OR repertoire, and ~60 % of the OR pseudogenes are expressed in the olfactory epithelium, with the latter showing a 3-fold lower expression. ORs transcription levels show a 1000-fold inter-paralog variation, as well as significant inter-individual differences. We assembled 160 transcripts representing 100 intact OR genes. These include 1-4 short 5' non-coding exons with considerable alternative splicing and long last exons that contain the coding region and 3' untranslated region of highly variable length. Notably, we identified 10 ORs with an intact open reading frame but with seemingly non-functional transcripts, suggesting a yet unreported OR pseudogenization mechanism. Analysis of the OR upstream regions indicated an enrichment of the homeobox family transcription factor binding sites and a consensus localization of a specific transcription factor binding site subfamily (Olf/EBF).

CONCLUSIONS

We provide an overview of expression levels of ORs and auxiliary genes in human olfactory epithelium. This forms a transcriptomic view of the entire OR repertoire, and reveals a large number of over-expressed uncharacterized human non-receptor genes, providing a platform for future discovery.

Enhancement of hERG channel activity by scFv antibody fragments targeted to the PAS domain

The human human ether-à-go-go-related gene (hERG) potassium channel plays a critical role in the repolarization of the cardiac action potential. Changes in hERG channel function underlie long QT syndrome (LQTS) and are associated with cardiac arrhythmias and sudden death. A striking feature of this channel and KCNH channels in general is the presence of an N-terminal Per-Arnt-Sim (PAS) domain. In other proteins, PAS domains bind ligands and modulate effector domains. However, the PAS domains of KCNH channels are orphan receptors. We have uncovered a family of positive modulators of hERG that specifically bind to the PAS domain. We generated two single-chain variable fragments (scFvs) that recognize different epitopes on the PAS domain. Both antibodies increase the rate of deactivation but have different effects on channel activation and inactivation. Importantly, we show that both antibodies, on binding to the PAS domain, increase the total amount of current that permeates the channel during a ventricular action potential and significantly reduce the action potential duration recorded in human cardiomyocytes. Overall, these molecules constitute a previously unidentified class of positive modulators and establish that allosteric modulation of hERG channel function through ligand binding to the PAS domain can be attained.

Unfolding of a Temperature-Sensitive Domain Controls Voltage-Gated Channel Activation

Voltage-gated ion channels (VGICs) are outfitted with diverse cytoplasmic domains that impact function. To examine how such elements may affect VGIC behavior, we addressed how the bacterial voltage-gated sodium channel (BacNa(V)) C-terminal cytoplasmic domain (CTD) affects function. Our studies show that the BacNa(V) CTD exerts a profound influence on gating through a temperature-dependent unfolding transition in a discrete cytoplasmic domain, the neck domain, proximal to the pore. Structural and functional studies establish that the BacNa(V) CTD comprises a bi-partite four-helix bundle that bears an unusual hydrophilic core whose integrity is central to the unfolding mechanism and that couples directly to the channel activation gate. Together, our findings define a general principle for how the widespread four-helix bundle cytoplasmic domain architecture can control VGIC responses, uncover a mechanism underlying the diverse BacNa(V) voltage dependencies, and demonstrate that a discrete domain can encode the temperature-dependent response of a channel.

Structure of the Cyclic Nucleotide-Binding Homology Domain of the hERG Channel and Its Insight into Type 2 Long QT Syndrome

The human ether-à-go-go related gene (hERG) channel is crucial for the cardiac action potential by contributing to the fast delayed-rectifier potassium current. Mutations in the hERG channel result in type 2 long QT syndrome (LQT2). The hERG channel contains a cyclic nucleotide-binding homology domain (CNBHD) and this domain is required for the channel gating though molecular interactions with the eag domain. Here we present solution structure of the CNBHD of the hERG channel. The structural study reveals that the CNBHD adopts a similar fold to other KCNH channels. It is self-liganded and it contains a short β-strand that blocks the nucleotide-binding pocket in the β-roll. Folding of LQT2-related mutations in this domain was shown to be affected by point mutation. Mutations in this domain can cause protein aggregation in E. coli cells or induce conformational changes. One mutant-R752W showed obvious chemical shift perturbation compared with the wild-type, but it still binds to the eag domain. The helix region from the N-terminal cap domain of the hERG channel showed unspecific interactions with the CNBHD.

Screening for Non-Pore-Binding Modulators of EAG K+ Channels

Members of the ether-à-go-go (EAG) family of voltage-gated K(+) channels are involved in several pathophysiological diseases, and there has been a great interest in screening for drugs that modulate the activity of these channels. Many drugs have been shown to bind in the pore of these channels, blocking ion flux and causing disease pathology. In this report, we present two independent screening campaigns in which we wanted to identify small molecules that bind to either the intracellular cytoplasmic amino terminal Per-Arnt-Sim (PAS) domain from the human EAG-related gene (ERG) channel or the amino or carboxy terminal globular domains from the mouse EAG1 channel, affecting their interaction. We report that in both cases, compounds were identified that showed weak, nonspecific binding. We suggest alternative routes should be pursued in future efforts to identify specific, high-affinity binders to these cytoplasmic domains.

'Splitting versus lumping': Temple-Baraitser and Zimmermann-Laband Syndromes

KCNH1 mutations have recently been described in six individuals with Temple-Baraitser syndrome (TMBTS) and six individuals with Zimmermann-Laband syndrome (ZLS). TMBTS is characterized by intellectual disability (ID), epilepsy, dysmorphic facial features, broad thumbs and great toes with absent/hypoplastic nails. ZLS is characterized by facial dysmorphism including coarsening of the face and a large nose, gingival enlargement, ID, hypoplasia of terminal phalanges and nails and hypertrichosis. In this study, we present four additional unrelated individuals with de novo KCNH1 mutations from ID cohorts. We report on a novel recurrent pathogenic KCNH1 variant in three individuals and add a fourth individual with a previously TMBTS-associated KCNH1 variant. Neither TMBTS nor ZLS was suspected clinically. KCNH1 encodes a voltage-gated potassium channel, which is not only highly expressed in the central nervous system, but also seems to play an important role during development. Clinical evaluation of our mutation-positive individuals revealed that one of the main characteristics of TMBTS/ZLS, namely the pronounced nail hypoplasia of the great toes and thumbs, can be mild and develop over time. Clinical comparison of all published KCNH1 mutation-positive individuals revealed a similar facial but variable limb phenotype. KCNH1 mutation-positive individuals present with severe ID, neonatal hypotonia, hypertelorism, broad nasal tip, wide mouth, nail a/hypoplasia, a proximal implanted and long thumb and long great toes. In summary, we show that the phenotypic variability of individuals with KCNH1 mutations is more pronounced than previously expected, and we discuss whether KCNH1 mutations allow for "lumping" or for "splitting" of TMBTS and ZLS.

hERG 1b is critical for human cardiac repolarization

The human ether-à-go-go-related gene (hERG; or KCNH2) encodes the voltage-gated potassium channel underlying IKr, a repolarizing current in the heart. Mutations in KCNH2 or pharmacological agents that reduce IKr slow action potential (AP) repolarization and can trigger cardiac arrhythmias associated with long QT syndrome. Two channel-forming subunits encoded by KCNH2 (hERG 1a and 1b) are expressed in cardiac tissue. In heterologous expression systems, these subunits avidly coassemble and exhibit biophysical and pharmacological properties distinct from those of homomeric hERG 1a channels. Despite these findings, adoption of hERG 1a/1b heteromeric channels as a model for cardiac IKr has been hampered by the lack of evidence for a direct functional role for the 1b subunit in native tissue. In this study, we measured IKr and APs at physiological temperature in cardiomyocytes derived from human induced pluripotent stem cells (iPSC-CMs). We found that specific knockdown of the 1b subunit using shRNA caused reductions in 1b mRNA, 1b protein levels, and IKr magnitude by roughly one-half. AP duration was increased and AP variability was enhanced relative to controls. Early afterdepolarizations, considered cellular substrates for arrhythmia, were also observed in cells with reduced 1b expression. Similar behavior was elicited when channels were effectively converted from heteromers to 1a homomers by expressing a fragment corresponding to the 1a-specific N-terminal Per-Arnt-Sim domain, which is omitted from hERG 1b by alternate transcription. These findings establish that hERG 1b is critical for normal repolarization and that loss of 1b is proarrhythmic in human cardiac cells.