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

Photoinhibition of the hERG potassium channel PAS domain by ultraviolet light speeds channel closing

hERG potassium channels are critical for cardiac excitability. hERG channels have a Per-Arnt-Sim (PAS) domain at their N-terminus, and here, we examined the mechanism for PAS domain regulation of channel opening and closing (gating). We used TAG codon suppression to incorporate the noncanonical amino acid 4-benzoyl-L-phenylalanine (BZF), which is capable of forming covalent cross-links after photoactivation by ultraviolet (UV) light, at three locations (G47, F48, and E50) in the PAS domain. We found that hERG-G47BZF channels had faster closing (deactivation) when irradiated in the open state (at 0 mV) but showed no measurable changes when irradiated in the closed state (at -100 mV). hERG-F48BZF channels had slower activation, faster deactivation, and a marked rightward shift in the voltage dependence of activation when irradiated in the open (at 0 mV) or closed (at -100 mV) state. hERG-E50BZF channels had no measurable changes when irradiated in the open state (at 0 mV) but had slower activation, faster deactivation, and a rightward shift in the voltage dependence of activation when irradiated in the closed state (at -100mV), indicating that hERG-E50BZF had a state-dependent difference in UV photoactivation, which we interpret to mean that PAS underwent molecular motions between the open and closed states. Moreover, we propose that UV-dependent biophysical changes in hERG-G47BZF, F48BZF, and E50BZF were the direct result of photochemical cross-linking that reduced dynamic motions in the PAS domain and broadly stabilized the closed state relative to the open state of the channel.

Photo-crosslinking hERG channels causes a U.V.-driven, state-dependent disruption of kinetics and voltage dependence of activation

Human ether-à-go-go related gene (hERG) voltage-activated potassium channels are critical for cardiac excitability. Characteristic slow closing (deactivation) in hERG is regulated by direct interaction between the N-terminal Per-Arnt-Sim (PAS) domain and the C-terminal cyclic nucleotide binding homology domain (CNBHD). We aim to understand how the PAS domain that is distal to the pore rearranges during gating to allosterically regulate the channel pore (and ion flux). To achieve this, we utilized the non-canonical amino acid 4-Benzoyl-L-phenylalanine (BZF) which is a photo-activatable cross-linkable probe, that when irradiated with ultraviolet (U.V.) light forms a double radical capable of forming covalent cross-links with C-H bond-containing groups, enabling selective and potent U.V.-driven photoinactivation of ion channel dynamics. Here we incorporate BZF directly into the hERG potassium channel PAS domain at three locations (G47, F48, and E50) using TAG codon suppression technology. hERG channels with BZF incorporated into the PAS domain (hERG-BZF) showed a significant change in the biophysical properties of the channel. hERG-G47BZF activated slowly when irradiated in the closed state (-100mV) but deactivated quickly when irradiated in both the open (0mV) and closed state. hERG-F48BZF channels showed a state independent and U.V. dose-dependent change in channel activation (slowing down) and channel deactivation (speeding up), as well as a marked change (right-shift) in the voltage-dependence of conductance. When irradiated at -100 mV hERG-E50BZF showed a state dependent and U.V. dose-dependent change in a channel activation (slowing down) and deactivation (speeding up) of channel deactivation, as well as a marked change (right-shift) in the voltage-dependence of conductance that occurred only when the channel was irradiated in the closed state (-100mV). This approach demonstrated that direct photo-crosslinking of the PAS domain in hERG channels causes a measurable change in biophysical parameters and more broadly stabilized the closed state of the channel. We propose that altered channel gating is as a direct result of reduced dynamic motions in the PAS domain of hERG due to photo-chemical crosslinking.

Characterising ion channel structure and dynamics using fluorescence spectroscopy techniques

Ion channels undergo major conformational changes that lead to channel opening and ion conductance. Deciphering these structure-function relationships is paramount to understanding channel physiology and pathophysiology. Cryo-electron microscopy, crystallography and computer modelling provide atomic-scale snapshots of channel conformations in non-cellular environments but lack dynamic information that can be linked to functional results. Biophysical techniques such as electrophysiology, on the other hand, provide functional data with no structural information of the processes involved. Fluorescence spectroscopy techniques help bridge this gap in simultaneously obtaining structure-function correlates. These include voltage-clamp fluorometry, Förster resonance energy transfer, ligand binding assays, single molecule fluorescence and their variations. These techniques can be employed to unearth several features of ion channel behaviour. For instance, they provide real time information on local and global rearrangements that are inherent to channel properties. They also lend insights in trafficking, expression, and assembly of ion channels on the membrane surface. These methods have the advantage that they can be carried out in either native or heterologous systems. In this review, we briefly explain the principles of fluorescence and how these have been translated to study ion channel function. We also report several recent advances in fluorescence spectroscopy that has helped address and improve our understanding of the biophysical behaviours of different ion channel families.

Mutations within the cGMP-binding domain of CNGA1 causing autosomal recessive retinitis pigmentosa in human and animal model

Retinitis pigmentosa is a group of progressive inherited retinal dystrophies that may present clinically as part of a syndromic entity or as an isolated (nonsyndromic) manifestation. In an Indian family suffering from retinitis pigmentosa, we identified a missense variation in CNGA1 affecting the cyclic nucleotide binding domain (CNBD) and characterized a mouse model developed with mutated CNBD. A gene panel analysis comprising 105 known RP genes was used to analyze a family with autosomal-recessive retinitis pigmentosa (arRP) and revealed that CNGA1 was affected. From sperm samples of ENU mutagenesis derived F₁ mice, we re-derived a mutant with a Cnga1 mutation. Homozygous mutant mice, developing retinal degeneration, were examined for morphological and functional consequences of the mutation. In the family, we identified a rare CNGA1 variant (NM_001379270.1) c.1525 G > A; (p.Gly509Arg), which co-segregated among the affected family members. Homozygous Cnga1 mice harboring a (ENSMUST00000087213.12) c.1526 A > G (p.Tyr509Cys) mutation showed progressive degeneration in the retinal photoreceptors from 8 weeks on. This study supports a role for CNGA1 as a disease gene for arRP and provides new insights on the pathobiology of cGMP-binding domain mutations in CNGA1-RP.

The ERG1 K+ Channel and Its Role in Neuronal Health and Disease

The ERG1 potassium channel, encoded by KCNH2, has long been associated with cardiac electrical excitability. Yet, a growing body of work suggests that ERG1 mediates physiology throughout the human body, including the brain. ERG1 is a regulator of neuronal excitability, ERG1 variants are associated with neuronal diseases (e.g., epilepsy and schizophrenia), and ERG1 serves as a potential therapeutic target for neuronal pathophysiology. This review summarizes the current state-of-the-field regarding the ERG1 channel structure and function, ERG1’s relationship to the mammalian brain and highlights key questions that have yet to be answered.

Investigation of PAS and CNBH domain interactions in hERG channels and effects of long-QT syndrome-causing mutations with surface plasmon resonance

Human ether-á-go-go-related gene (hERG) channels are key regulators of cardiac repolarization, neuronal excitability, and tumorigenesis. hERG channels contain N-terminal Per-Arnt-Sim (PAS) and C-terminal cyclic nucleotide-binding homology (CNBH) domains with many long-QT syndrome (LQTS)-causing mutations located at the interface between these domains. Despite the importance of PAS/CNBH domain interactions, little is known about their affinity. Here, we used the surface plasmon resonance (SPR) technique to investigate interactions between isolated PAS and CNBH domains and the effects of LQTS-causing mutations R20G, N33T, and E58D, located at the PAS/CNBH domain interface, on these interactions. We determined that the affinity of the PAS/CNBH domain interactions was ∼1.4 μM. R20G and E58D mutations had little effect on the domain interaction affinity, while N33T abolished the domain interactions. Interestingly, mutations in the intrinsic ligand, a conserved stretch of amino acids occupying the beta-roll cavity in the CNBH domain, had little effect on the affinity of PAS/CNBH domain interactions. Additionally, we determined that the isolated PAS domains formed oligomers with an interaction affinity of ∼1.6 μM. Coexpression of the isolated PAS domains with the full-length hERG channels or addition of the purified PAS protein inhibited hERG currents. These PAS/PAS interactions can have important implications for hERG function in normal and pathological conditions associated with increased surface density of channels or interaction with other PAS-domain-containing proteins. Taken together, our study provides the first account of the binding affinities for wild-type and mutant hERG PAS and CNBH domains and highlights the potential functional significance of PAS/PAS domain interactions.

Conformation-sensitive antibody reveals an altered cytosolic PAS/CNBh assembly during hERG channel gating

The human ERG (hERG) K⁺ channel has a crucial function in cardiac repolarization, and mutations or channel block can give rise to long QT syndrome and catastrophic ventricular arrhythmias. The cytosolic assembly formed by the Per-Arnt-Sim (PAS) and cyclic nucleotide binding homology (CNBh) domains is the defining structural feature of hERG and related KCNH channels. However, the molecular role of these two domains in channel gating remains unclear. We have previously shown that single-chain variable fragment (scFv) antibodies can modulate hERG function by binding to the PAS domain. Here, we mapped the scFv2.12 epitope to a site overlapping with the PAS/CNBh domain interface using NMR spectroscopy and mutagenesis and show that scFv binding in vitro and in the cell is incompatible with the PAS interaction with CNBh. By generating a fluorescently labeled scFv2.12, we demonstrate that association with the full-length hERG channel is state dependent. We detect Förster resonance energy transfer (FRET) with scFv2.12 when the channel gate is open but not when it is closed. In addition, state dependence of scFv2.12 FRET signal disappears when the R56Q mutation, known to destabilize the PAS-CNBh interaction, is introduced in the channel. Altogether, these data are consistent with an extensive structural alteration of the PAS/CNBh assembly when the cytosolic gate opens, likely favoring PAS domain dissociation from the CNBh domain.

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.

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.

Insights into the Molecular Inhibition of the Oncogenic Channel KV10.1 by Globular Toxins

Inhibition of the expression of the human ether-à-go-go (hEAG1 or hK_V10.1) channel is associated with a dramatic reduction in the growth of several cancerous tumors. The modulation of this channel's activity is a promising target for the development of new anticancer drugs. Although some small molecules have shown inhibitory activity against K_V10.1, their lack of specificity has prevented their use in humans. In vitro studies have recently identified a limited number of peptide toxins with proven specificity in their hK_V10.1 channel inhibitory effect. These peptide toxins have become desirable candidates to use as lead compounds to design more potent and specific hK_V10.1 inhibitors. However, the currently available studies lack the atomic resolution needed to characterize the molecular features that favor their binding to hK_V10.1. In this work, we present the first attempt to locate the possible hK_V10.1 binding sites of the animal peptide toxins APETx4, Aa1a, Ap1a, and k-hefutoxin 1, all of which described as hK_V10.1 inhibitors. Our studies incorporated homology modeling to construct a robust three-dimensional (3D) model of hK_V10.1, applied protein docking, and multiscale molecular dynamics techniques to reveal in atomic resolution the toxin-channel interactions. Our approach suggests that some peptide toxins bind in the outer vestibule surrounding the pore of hK_V10.1; it also identified the channel residues Met397 and Asp398 as possible anchors that stabilize the binding of the evaluated toxins. Finally, a description of the possible mechanism for inhibition and gating is presented.

External Cd2+ and protons activate the hyperpolarization-gated K+ channel KAT1 at the voltage sensor

The functionally diverse cyclic nucleotide binding domain (CNBD) superfamily of cation channels contains both depolarization-gated (e.g., metazoan EAG family K+ channels) and hyperpolarization-gated channels (e.g., metazoan HCN pacemaker cation channels and the plant K+ channel KAT1). In both types of CNBD channels, the S4 transmembrane helix of the voltage sensor domain (VSD) moves outward in response to depolarization. This movement opens depolarization-gated channels and closes hyperpolarization-gated channels. External divalent cations and protons prevent or slow movement of S4 by binding to a cluster of acidic charges on the S2 and S3 transmembrane domains of the VSD and therefore inhibit activation of EAG family channels. However, a similar divalent ion/proton binding pocket has not been described for hyperpolarization-gated CNBD family channels. We examined the effects of external Cd2+ and protons on Arabidopsisthaliana KAT1 expressed in Xenopus oocytes and found that these ions strongly potentiate voltage activation. Cd2+ at 300 µM depolarizes the V50 of KAT1 by 150 mV, while acidification from pH 7.0 to 4.0 depolarizes the V50 by 49 mV. Regulation of KAT1 by Cd2+ is state dependent and consistent with Cd2+ binding to an S4-down state of the VSD. Neutralization of a conserved acidic charge in the S2 helix in KAT1 (D95N) eliminates Cd2+ and pH sensitivity. Conversely, introduction of acidic residues into KAT1 at additional S2 and S3 cluster positions that are charged in EAG family channels (N99D and Q149E in KAT1) decreases Cd2+ sensitivity and increases proton potentiation. These results suggest that KAT1, and presumably other hyperpolarization-gated plant CNBD channels, can open from an S4-down VSD conformation homologous to the divalent/proton-inhibited conformation of EAG family K+ channels.

Diversity of voltage-gated potassium channels and cyclic nucleotide-binding domain-containing channels in eukaryotes

Voltage-gated potassium channels (K_v) and cyclic nucleotide-binding domain-containing cation channels HCN, CNG, and KCNH are the evolutionarily related families of ion channels in animals. Their homologues were found in several lineages of eukaryotes and prokaryotes; however, the actual phylogenetic and structural diversity of these ion channels remains unclear. In this work, we present a taxonomically broad investigation of evolutionary relationships and structural diversity of K_v, HCN, CNG, and KCNH and their homologues in eukaryotes focusing on channels from different protistan groups. We demonstrate that both groups of channels consist of a more significant number of lineages than it was shown before, and these lineages can be grouped in two clusters termed K_v-like channels and CNBD-channels. Moreover, we, for the first time, report the unusual two-repeat tandem K_v-like channels and CNBD-channels in several eukaryotic groups, i.e. dinoflagellates, oomycetes, and chlorarachniophytes. Our findings reveal still underappreciated phylogenetic and structural diversity of eukaryotic ion channel lineages.

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.

hERG Function in Light of Structure

The human ether-a-go-go-related gene1 (hERG) ion channel has been the subject of fascination since it was identified as a target of long QT syndrome more than 20 years ago. In this Biophysical Perspective, we look at what makes hERG intriguing and vexingly unique. By probing recent high-resolution structures in the context of functional and biochemical data, we attempt to summarize new insights into hERG-specific function and articulate important unanswered questions. X-ray crystallography and cryo-electron microscopy have revealed features not previously on the radar-the "nonswapped" transmembrane architecture, an "intrinsic ligand," and hydrophobic pockets off a pore cavity that is surprisingly small. Advances in our understanding of drug block and inactivation mechanisms are noted, but a full picture will require more investigation.

High potassium seawater inhibits ascidian sperm chemotaxis, but does not affect the male gamete chemotaxis of a brown alga

Male gamete chemotaxis towards the female gamete is a general strategy to facilitate the sexual reproduction in many marine eukaryotes. Biochemical studies of chemoattractants for male gametes of brown algae have advanced in the 1970s and 1980s, but the molecular mechanism of male gamete responses to the attractants remains elusive. In sea urchin, a K+ channel called the tetraKCNG channel plays a fundamental role in sperm chemotaxis and inhibition of K+ efflux through this channel by high K+ seawater blocks almost all cell responses to the chemoattractant. This signalling mechanism could be conserved in marine invertebrates as tetraKCNG channels are conserved in the marine invertebrates that exhibit sperm chemotaxis. We confirmed that high K+ seawater also inhibited sperm chemotaxis in ascidian, Ciona intestinalis (robusta), in this study. Conversely, the male gamete chemotaxis towards the female gamete of a brown alga, Mutimo cylindricus, was preserved even in high K+ seawater. This result indicates that none of the K+ channels is essential for male gamete chemotaxis in the brown alga, suggesting that the signalling mechanism for chemotaxis in this brown alga is quite different from that of marine invertebrates. Correlated to this result, we revealed that the channels previously proposed as homologues of tetraKCNG in brown algae have a distinct domain composition from that of the tetraKCNG. Namely, one of them possesses two repeats of the six transmembrane segments (diKCNG) instead of four. The structural analysis suggests that diKCNG is a cyclic nucleotide-modulated and/or voltage-gated K+ channel.

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.

N- and C-terminal interactions in KCNH channels: The spotlight on the intrinsic ligand

Brelidze examines recent data revealing the new role of the intrinsic ligand in hERG potassium channel gating.

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.

Two mutations at different positions in the CNBH domain of the hERG channel accelerate deactivation and impair the interaction with the EAG domain

KEY POINTS

In the human ether-a-go-go related gene (hERG) channel, both the ether-a-go-go (EAG) domain in the N-terminal and the cyclic nucleotide (CN) binding homology (CNBH) domain in the C-terminal cytoplasmic region are known to contribute to the characteristic slow deactivation. Mutations of Phe860 in the CNBH domain, reported to fill the CN binding pocket, accelerate the deactivation and decrease the fluorescence resonance energy transfer (FRET) efficiencies between the EAG and CNBH domains. An electrostatic interaction between Arg696 and Asp727 in the C-linker domain, critical for HCN and CNG channels, is not formed in the hERG channel. Mutations of newly identified electrostatically interacting pair, Asp727 in the C-linker and Arg752 in the CNBH domains, accelerate the deactivation and decrease FRET efficiency. Voltage-dependent changes in FRET efficiency were not detected. These results suggest that the acceleration of the deactivation by mutations of C-terminal domains is a result of the lack of interaction between the EAG and CNBH domains.

ABSTRACT

The human ether-a-go-go related gene (hERG) channel shows characteristic slow deactivation, and the contribution of both of the N-terminal cytoplasmic ether-a-go-go (EAG) domain and the C-terminal cytoplasmic cyclic nucleotide (CN) binding homology (CNBH) domain is well known. The interaction between these domains is known to be critical for slow deactivation. We analysed the effects of mutations in the CNBH domain and its upstream C-linker domain on slow deactivation and the interaction between the EAG and CNBH domains by electrophysiological and fluorescence resonance energy transfer (FRET) analyses using Xenopus oocyte and HEK293T cell expression systems. We first observed that mutations of Phe860 in the CNBH domain, which is reported to fill the CN binding pocket as an intrinsic ligand, accelerate deactivation and eliminate the inter-domain interaction. Next, we observed that the salt bridge between Arg696 and Asp727 in the C-linker domain, which is reported to be critical for the function of CN-regulated channels, is not formed. We newly identified an electrostatically interacting pair critical for slow deactivation: Asp727 and Arg752 in the CNBH domain. Their mutations also impaired the inter-domain interaction. Taking these results together, both mutations of the intrinsic ligand (Phe860) and a newly identified salt bridge pair (Asp727 and Arg752) in the hERG channel accelerated deactivation and also decreased the interaction between the EAG and CNBH domains. Voltage-dependent changes in FRET efficiency between the two domains were not detected. The results suggest that the CNBH domain contributes to slow deactivation of the hERG channel by a mechanism involving the EAG domain.

The solute carrier SLC9C1 is a Na+/H+-exchanger gated by an S4-type voltage-sensor and cyclic-nucleotide binding

Voltage-sensing (VSD) and cyclic nucleotide-binding domains (CNBD) gate ion channels for rapid electrical signaling. By contrast, solute carriers (SLCs) that passively redistribute substrates are gated by their substrates themselves. Here, we study the orphan sperm-specific solute carriers SLC9C1 that feature a unique tripartite structure: an exchanger domain, a VSD, and a CNBD. Voltage-clamp fluorimetry shows that SLC9C1 is a genuine Na⁺/H⁺ exchanger gated by voltage. The cellular messenger cAMP shifts the voltage range of activation. Mutations in the transport domain, the VSD, or the CNBD strongly affect Na⁺/H⁺ exchange, voltage gating, or cAMP sensitivity, respectively. Our results establish SLC9C1 as a phylogenetic chimaera that combines the ion-exchange mechanism of solute carriers with the gating mechanism of ion channels. Classic SLCs slowly readjust changes in the intra- and extracellular milieu, whereas voltage gating endows the Na⁺/H⁺ exchanger with the ability to produce a rapid pH response that enables downstream signaling events.

The sperm-specific solute carrier SLC9C1 is a phylogenetic chimaera that carries a voltage-sensing (VSD) and a cyclic nucleotide-binding domain (CNBD). Here authors show by electrophysiology and fluorimetry that SLC9C1 is a genuine Na⁺/H⁺ exchanger gated by voltage and cAMP.

Ligand binding and activation properties of the purified bacterial cyclic nucleotide-gated channel SthK

SthK is a bacterial cyclic nucleotide–gated ion channel from Spirochaeta thermophila. By optimizing the expression and purification of SthK, Schmidpeter et al. show that cAMP and cGMP bind to the channel with similar affinity but activate it with different efficacy.

Cyclic nucleotide–modulated ion channels play several essential physiological roles. They are involved in signal transduction in photoreceptors and olfactory sensory neurons as well as pacemaking activity in the heart and brain. Investigations of the molecular mechanism of their actions, including structural and electrophysiological characterization, are restricted by the availability of stable, purified protein obtained from accessible systems. Here, we establish that SthK, a cyclic nucleotide–gated (CNG) channel from Spirochaeta thermophila, is an excellent model for investigating the gating of eukaryotic CNG channels at the molecular level. The channel has high sequence similarity with its eukaryotic counterparts and was previously reported to be activated by cyclic nucleotides in patch-clamp experiments with Xenopus laevis oocytes. We optimized protein expression and purification to obtain large quantities of pure, homogeneous, and active recombinant SthK protein from Escherichia coli. A negative-stain electron microscopy (EM) single-particle analysis indicated that this channel is a promising candidate for structural studies with cryo-EM. Using radioactivity and fluorescence flux assays, as well as single-channel recordings in lipid bilayers, we show that the protein is partially activated by micromolar concentrations of cyclic adenosine monophosphate (cAMP) and that channel activity is increased by depolarization. Unlike previous studies, we find that cyclic guanosine monophosphate (cGMP) is also able to activate SthK, but with much lower efficiency than cAMP. The distinct sensitivities to different ligands resemble eukaryotic CNG and hyperpolarization-activated and cyclic nucleotide–modulated channels. Using a fluorescence binding assay, we show that cGMP and cAMP bind to SthK with similar apparent affinities, suggesting that the large difference in channel activation by cAMP or cGMP is caused by the efficacy with which each ligand promotes the conformational changes toward the open state. We conclude that the functional characteristics of SthK reported here will permit future studies to analyze ligand gating and discrimination in CNG channels.

Dynamic rearrangement of the intrinsic ligand regulates KCNH potassium channels

KCNH potassium channels possess an intrinsic ligand in their cyclic nucleotide-binding homology domain, located at the N- and C-terminal domain interface. Dai et al. show that this intrinsic ligand regulates voltage-dependent potentiation via a rearrangement between the ligand and its binding site.

KCNH voltage-gated potassium channels (EAG, ERG, and ELK) play significant roles in neuronal and cardiac excitability. They contain cyclic nucleotide-binding homology domains (CNBHDs) but are not directly regulated by cyclic nucleotides. Instead, the CNBHD ligand-binding cavity is occupied by an intrinsic ligand, which resides at the intersubunit interface between the N-terminal eag domain and the C-terminal CNBHD. We show that, in Danio rerio ELK channels, this intrinsic ligand is critical for voltage-dependent potentiation (VDP), a process in which channel opening is stabilized by prior depolarization. We demonstrate that an exogenous peptide corresponding to the intrinsic ligand can bind to and regulate zebrafish ELK channels. This exogenous intrinsic ligand inhibits the channels before VDP and potentiates the channels after VDP. Furthermore, using transition metal ion fluorescence resonance energy transfer and a fluorescent noncanonical amino acid L-Anap, we show that there is a rearrangement of the intrinsic ligand relative to the CNBHD during VDP. We propose that the intrinsic ligand switches from antagonist to agonist as a result of a rearrangement of the eag domain–CNBHD interaction during VDP.

How an intrinsic ligand tunes the activity of a potassium channel

Khoo and Pless examine new work that provides mechanistic insight into the role of the intrinsic ligand in KCNH ion channels.​

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.

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.

Kv10.1 potassium channel: from the brain to the tumors

The KCNH1 gene encodes the Kv10.1 (Eag1) ion channel, a member of the EAG (ether-à-go-go) family of voltage-gated potassium channels. Recent studies have demonstrated that KCHN1 mutations are implicated in Temple–Baraitser and Zimmermann–Laband syndromes and other forms of developmental deficits that all present with mental retardation and epilepsy, suggesting that Kv10.1 might be important for cognitive development in humans. Although the Kv10.1 channel is mainly expressed in the mammalian brain, its ectopic expression occurs in 70% of human cancers. Cancer cells and tumors expressing Kv10.1 acquire selective advantages that favor cancer progression through molecular mechanisms that involve several cellular pathways, indicating that protein–protein interactions may be important for Kv10.1 influence in cell proliferation and tumorigenesis. Several studies on transcriptional and post-transcriptional regulation of Kv10.1 expression have shown interesting mechanistic insights about Kv10.1 role in oncogenesis, increasing the importance of identifying the cellular factors that regulate Kv10.1 expression in tumors.

Investigating cyclic nucleotide and cyclic dinucleotide binding to HCN channels by surface plasmon resonance

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels control cardiac and neuronal rhythmicity. HCN channels contain cyclic nucleotide-binding domain (CNBD) in their C-terminal region linked to the pore-forming transmembrane segment with a C-linker. The C-linker couples the conformational changes caused by the direct binding of cyclic nucleotides to the HCN pore opening. Recently, cyclic dinucleotides were shown to antagonize the effect of cyclic nucleotides in HCN4 but not in HCN2 channels. Based on the structural analysis and mutational studies it has been proposed that cyclic dinucleotides affect HCN4 channels by binding to the C-linker pocket (CLP). Here, we first show that surface plasmon resonance (SPR) can be used to accurately measure cyclic nucleotide binding affinity to the C-linker/CNBD of HCN2 and HCN4 channels. We then used SPR to investigate cyclic dinucleotide binding in HCN channels. To our surprise, we detected no binding of cyclic dinucleotides to the isolated monomeric C-linker/CNBDs of HCN4 channels with SPR. The binding of cyclic dinucleotides was further examined with isothermal calorimetry (ITC), which indicated no binding of cyclic dinucleotides to both monomeric and tetrameric C-linker/CNBDs of HCN4 channels. Taken together, our results suggest that interaction of the C-linker/CNBD with other parts of the channel is necessary for cyclic-dinucleotide binding in HCN4 channels.

FRET-based binding assay between a fluorescent cAMP analogue and a cyclic nucleotide-binding domain tagged with a CFP

The cyclic nucleotide-binding domain (CNBD) functions as a regulatory domain of many proteins involved in cyclic nucleotide signalling. We developed a straightforward and reliable binding assay based on intermolecular fluorescence resonance energy transfer (FRET) between an adenosine-3', 5'-cyclic monophosphate analogue labelled with fluorescein and a recombinant CNBD of human EPAC1 tagged with a cyan fluorescence protein (CFP). The high FRET efficiency of this method (~ 80%) allowed us to perform several types of binding experiments with nanomolar range of sample using conventional equipment. In addition, the CFP tag on the CNBD enabled us to perform a specific binding experiment using an unpurified protein. Considering these advantages, this technique is useful to study poorly characterized CNBDs.

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

CryoEM structure of a prokaryotic cyclic nucleotide-gated ion channel

Cyclic nucleotide-gated (CNG) and hyperpolarization-activated cyclic nucleotide-regulated (HCN) ion channels play crucial physiological roles in phototransduction, olfaction, and cardiac pace making. These channels are characterized by the presence of a carboxyl-terminal cyclic nucleotide-binding domain (CNBD) that connects to the channel pore via a C-linker domain. Although cyclic nucleotide binding has been shown to promote CNG and HCN channel opening, the precise mechanism underlying gating remains poorly understood. Here we used cryoEM to determine the structure of the intact LliK CNG channel isolated from Leptospira licerasiae-which shares sequence similarity to eukaryotic CNG and HCN channels-in the presence of a saturating concentration of cAMP. A short S4-S5 linker connects nearby voltage-sensing and pore domains to produce a non-domain-swapped transmembrane architecture, which appears to be a hallmark of this channel family. We also observe major conformational changes of the LliK C-linkers and CNBDs relative to the crystal structures of isolated C-linker/CNBD fragments and the cryoEM structures of related CNG, HCN, and KCNH channels. The conformation of our LliK structure may represent a functional state of this channel family not captured in previous studies.

Eag1 K+ Channel: Endogenous Regulation and Functions in Nervous System

Ether-à-go-go1 (Eag1, Kv10.1, KCNH1) K⁺ channel is a member of the voltage-gated K⁺ channel family mainly distributed in the central nervous system and cancer cells. Like other types of voltage-gated K⁺ channels, the EAG1 channels are regulated by a variety of endogenous signals including reactive oxygen species, rendering the EAG1 to be in the redox-regulated ion channel family. The role of EAG1 channels in tumor development and its therapeutic significance have been well established. Meanwhile, the importance of hEAG1 channels in the nervous system is now increasingly appreciated. The present review will focus on the recent progress on the channel regulation by endogenous signals and the potential functions of EAG1 channels in normal neuronal signaling as well as neurological diseases.

The intrinsically liganded cyclic nucleotide-binding homology domain promotes KCNH channel activation

hEAG1 is a member of the KCNH family of ion channels, which are characterized by C-terminal regions with homology to cyclic nucleotide–binding domains (CNBhDs). Zhao et al. show that an “intrinsic ligand” occupying the CNBhD binding pocket promotes the activated and open state of the channel.

Channels in the ether-à-go-go or KCNH family of potassium channels are characterized by a conserved, C-terminal domain with homology to cyclic nucleotide–binding homology domains (CNBhDs). Instead of cyclic nucleotides, two amino acid residues, Y699 and L701, occupy the binding pocket, forming an “intrinsic ligand.” The role of the CNBhD in KCNH channel gating is still unclear, however, and a detailed characterization of the intrinsic ligand is lacking. In this study, we show that mutating both Y699 and L701 to alanine, serine, aspartate, or glycine impairs human EAG1 channel function. These mutants slow channel activation and shift the conductance–voltage (G–V) relation to more depolarized potentials. The mutations affect activation and the G-V relation progressively, indicating that the gating machinery is sensitive to multiple conformations of the CNBhD. Substitution with glycine at both sites (GG), which eliminates the side chains that interact with the binding pocket, also reduces the ability of voltage prepulses to populate more preactivated states along the activation pathway (i.e., the Cole–Moore effect), as if stabilizing the voltage sensor in deep resting states. Notably, deletion of the entire CNBhD (577–708, ΔCNBhD) phenocopies the GG mutant, suggesting that GG is a loss-of-function mutation and the CNBhD requires an intrinsic ligand to exert its functional effects. We developed a kinetic model for both wild-type and ΔCNBhD mutant channels that describes all our observations on activation kinetics, the Cole–Moore shift, and G-V relations. These findings support a model in which the CNBhD both promotes voltage sensor activation and stabilizes the open pore. The intrinsic ligand is critical for these functional effects.

Structure of the voltage-gated K⁺ channel Eag1 reveals an alternative voltage sensing mechanism

Voltage-gated potassium (K(v)) channels are gated by the movement of the transmembrane voltage sensor, which is coupled, through the helical S4-S5 linker, to the potassium pore. We determined the single-particle cryo-electron microscopy structure of mammalian K(v)10.1, or Eag1, bound to the channel inhibitor calmodulin, at 3.78 angstrom resolution. Unlike previous K(v) structures, the S4-S5 linker of Eag1 is a five-residue loop and the transmembrane segments are not domain swapped, which suggest an alternative mechanism of voltage-dependent gating. Additionally, the structure and position of the S4-S5 linker allow calmodulin to bind to the intracellular domains and to close the potassium pore, independent of voltage-sensor position. The structure reveals an alternative gating mechanism for K(v) channels and provides a template to further understand the gating properties of Eag1 and related channels.

Bimodal regulation of an Elk subfamily K+ channel by phosphatidylinositol 4,5-bisphosphate

PIP₂ mediates the bimodal regulation of the EAG family K⁺ channel ELK1 to produce an overall inhibitory effect.

Phosphatidylinositol 4,5-bisphosphate (PIP₂) regulates Shaker K⁺ channels and voltage-gated Ca²⁺ channels in a bimodal fashion by inhibiting voltage activation while stabilizing open channels. Bimodal regulation is conserved in hyperpolarization-activated cyclic nucleotide–gated (HCN) channels, but voltage activation is enhanced while the open channel state is destabilized. The proposed sites of PIP₂ regulation in these channels include the voltage-sensor domain (VSD) and conserved regions of the proximal cytoplasmic C terminus. Relatively little is known about PIP₂ regulation of Ether-á-go-go (EAG) channels, a metazoan-specific family of K⁺ channels that includes three gene subfamilies, Eag (Kv10), Erg (Kv11), and Elk (Kv12). We examined PIP₂ regulation of the Elk subfamily potassium channel human Elk1 to determine whether bimodal regulation is conserved within the EAG K⁺ channel family. Open-state stabilization by PIP₂ has been observed in human Erg1, but the proposed site of regulation in the distal C terminus is not conserved among EAG family channels. We show that PIP₂ strongly inhibits voltage activation of Elk1 but also stabilizes the open state. This stabilization produces slow deactivation and a mode shift in voltage gating after activation. However, removal of PIP₂ has the net effect of enhancing Elk1 activation. R347 in the linker between the VSD and pore (S4–S5 linker) and R479 near the S6 activation gate are required for PIP₂ to inhibit voltage activation. The ability of PIP₂ to stabilize the open state also requires these residues, suggesting an overlap in sites central to the opposing effects of PIP₂ on channel gating. Open-state stabilization in Elk1 requires the N-terminal eag domain (PAS domain + Cap), and PIP₂-dependent stabilization is enhanced by a conserved basic residue (K5) in the Cap. Our data shows that PIP₂ can bimodally regulate voltage gating in EAG family channels, as has been proposed for Shaker and HCN channels. PIP₂ regulation appears fundamentally different for Elk and KCNQ channels, suggesting that, although both channel types can regulate action potential threshold in neurons, they are not functionally redundant.

Crystal structure of the PAS domain of the hEAG potassium channel

KCNH voltage-gated potassium channels play critical roles in regulating cellular functions. The channel is composed of four subunits, each of which contains six transmembrane helices forming the central pore. The cytoplasmic parts of the subunits present a Per-Arnt-Sim (PAS) domain at the N-terminus and a cyclic nucleotide-binding homology domain at the C-terminus. PAS domains are conserved from prokaryotes to eukaryotes and are involved in sensing signals and cellular responses. To better understand the functional roles of PAS domains in KCNH channels, the structure of this domain from the human ether-à-go-go channel (hEAG channel) was determined. By comparing it with the structures of the Homo sapiens EAG-related gene (hERG) channel and the Drosophila EAG-like K(+) (dELK) channel and analyzing the structural features of the hEAG channel, it was identified that a hydrophobic patch on the β-sheet may mediate interaction between the PAS domain and other regions of the channel to regulate its functions.

Deciphering the function of the CNGB1b subunit in olfactory CNG channels

Olfactory cyclic nucleotide-gated (CNG) ion channels are key players in the signal transduction cascade of olfactory sensory neurons. The second messengers cAMP and cGMP directly activate these channels, generating a depolarizing receptor potential. Olfactory CNG channels are composed of two CNGA2 subunits and two modulatory subunits, CNGA4, and CNGB1b. So far the exact role of the modulatory subunits for channel activation is not fully understood. By measuring ligand binding and channel activation simultaneously, we show that in functional heterotetrameric channels not only the CNGA2 subunits and the CNGA4 subunit but also the CNGB1b subunit binds cyclic nucleotides and, moreover, also alone translates this signal to open the pore. In addition, we show that the CNGB1b subunit is the most sensitive subunit in a heterotetrameric channel to cyclic nucleotides and that it accelerates deactivation to a similar extent as does the CNGA4 subunit. In conclusion, the CNGB1b subunit participates in ligand-gated activation of olfactory CNG channels and, particularly, contributes to rapid termination of odorant signal in an olfactory sensory neuron.

Calmodulin Regulates Human Ether à Go-Go 1 (hEAG1) Potassium Channels through Interactions of the Eag Domain with the Cyclic Nucleotide Binding Homology Domain

The ether à go-go family of voltage-gated potassium channels is structurally distinct. The N terminus contains an eag domain (eagD) that contains a Per-Arnt-Sim (PAS) domain that is preceded by a conserved sequence of 25–27 amino acids known as the PAS-cap. The C terminus contains a region with homology to cyclic nucleotide binding domains (cNBHD), which is directly linked to the channel pore. The human EAG1 (hEAG1) channel is remarkably sensitive to inhibition by intracellular calcium (Ca²⁺_i) through binding of Ca²⁺-calmodulin to three sites adjacent to the eagD and cNBHD. Here, we show that the eagD and cNBHD interact to modulate Ca²⁺-calmodulin as well as voltage-dependent gating. Sustained elevation of Ca²⁺_i resulted in an initial profound inhibition of hEAG1 currents, which was followed by a phase when current amplitudes partially recovered, but activation gating was slowed and shifted to depolarized potentials. Deletion of either the eagD or cNBHD abolished the inhibition by Ca²⁺_i. However, deletion of just the PAS-cap resulted in a >15-fold potentiation in response to elevated Ca²⁺_i. Mutations of residues at the interface between the eagD and cNBHD have been linked to human cancer. Glu-600 on the cNBHD, when substituted with residues with a larger volume, resulted in hEAG1 currents that were profoundly potentiated by Ca²⁺_i in a manner similar to the ΔPAS-cap mutant. These findings provide the first evidence that eagD and cNBHD interactions are regulating Ca²⁺-dependent gating and indicate that the binding of the PAS-cap with the cNBHD is required for the closure of the channels upon CaM binding.

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.

A K(+)-selective CNG channel orchestrates Ca(2+) signalling in zebrafish sperm

Calcium in the flagellum controls sperm navigation. In sperm of marine invertebrates and mammals, Ca(2+) signalling has been intensely studied, whereas for fish little is known. In sea urchin sperm, a cyclic nucleotide-gated K(+) channel (CNGK) mediates a cGMP-induced hyperpolarization that evokes Ca(2+) influx. Here, we identify in sperm of the freshwater fish Danio rerio a novel CNGK family member featuring non-canonical properties. It is located in the sperm head rather than the flagellum and is controlled by intracellular pH, but not cyclic nucleotides. Alkalization hyperpolarizes sperm and produces Ca(2+) entry. Ca(2+) induces spinning-like swimming, different from swimming of sperm from other species. The "spinning" mode probably guides sperm into the micropyle, a narrow entrance on the surface of fish eggs. A picture is emerging of sperm channel orthologues that employ different activation mechanisms and serve different functions. The channel inventories probably reflect adaptations to species-specific challenges during fertilization.

Ether-à-go-go family voltage-gated K+ channels evolved in an ancestral metazoan and functionally diversified in a cnidarian-bilaterian ancestor

We examined the evolutionary origins of the ether-à-go-go (EAG) family of voltage-gated K(+) channels, which have a strong influence on the excitability of neurons. The bilaterian EAG family comprises three gene subfamilies (Eag, Erg and Elk) distinguished by sequence conservation and functional properties. Searches of genome sequence indicate that EAG channels are metazoan specific, appearing first in ctenophores. However, phylogenetic analysis including two EAG family channels from the ctenophore Mnemiopsis leidyi indicates that the diversification of the Eag, Erg and Elk gene subfamilies occurred in a cnidarian/bilaterian ancestor after divergence from ctenophores. Erg channel function is highly conserved between cnidarians and mammals. Here we show that Eag and Elk channels from the sea anemone Nematostella vectensis (NvEag and NvElk) also share high functional conservation with mammalian channels. NvEag, like bilaterian Eag channels, has rapid kinetics, whereas NvElk activates at extremely hyperpolarized voltages, which is characteristic of Elk channels. Potent inhibition of voltage activation by extracellular protons is conserved between mammalian and Nematostella EAG channels. However, characteristic inhibition of voltage activation by Mg(2+) in Eag channels and Ca(2+) in Erg channels is reduced in Nematostella because of mutation of a highly conserved aspartate residue in the voltage sensor. This mutation may preserve sub-threshold activation of Nematostella Eag and Erg channels in a high divalent cation environment. mRNA in situ hybridization of EAG channels in Nematostella suggests that they are differentially expressed in distinct cell types. Most notable is the expression of NvEag in cnidocytes, a cnidarian-specific stinging cell thought to be a neuronal subtype.

Functional Characterization of Cnidarian HCN Channels Points to an Early Evolution of Ih

HCN channels play a unique role in bilaterian physiology as the only hyperpolarization-gated cation channels. Their voltage-gating is regulated by cyclic nucleotides and phosphatidylinositol 4,5-bisphosphate (PIP₂). Activation of HCN channels provides the depolarizing current in response to hyperpolarization that is critical for intrinsic rhythmicity in neurons and the sinoatrial node. Additionally, HCN channels regulate dendritic excitability in a wide variety of neurons. Little is known about the early functional evolution of HCN channels, but the presence of HCN sequences in basal metazoan phyla and choanoflagellates, a protozoan sister group to the metazoans, indicate that the gene family predates metazoan emergence. We functionally characterized two HCN channel orthologs from Nematostella vectensis (Cnidaria, Anthozoa) to determine which properties of HCN channels were established prior to the emergence of bilaterians. We find Nematostella HCN channels share all the major functional features of bilaterian HCNs, including reversed voltage-dependence, activation by cAMP and PIP₂, and block by extracellular Cs⁺. Thus bilaterian-like HCN channels were already present in the common parahoxozoan ancestor of bilaterians and cnidarians, at a time when the functional diversity of voltage-gated K⁺ channels was rapidly expanding. NvHCN1 and NvHCN2 are expressed broadly in planulae and in both the endoderm and ectoderm of juvenile polyps.

Alternatively Spliced Isoforms of KV10.1 Potassium Channels Modulate Channel Properties and Can Activate Cyclin-dependent Kinase in Xenopus Oocytes

K_V10.1 is a voltage-gated potassium channel expressed selectively in the mammalian brain but also aberrantly in cancer cells. In this study we identified short splice variants of K_V10.1 resulting from exon-skipping events (E65 and E70) in human brain and cancer cell lines. The presence of the variants was confirmed by Northern blot and RNase protection assays. Both variants completely lacked the transmembrane domains of the channel and produced cytoplasmic proteins without channel function. In a reconstituted system, both variants co-precipitated with the full-length channel and induced a robust down-regulation of K_V10.1 current when co-expressed with the full-length form, but their effect was mechanistically different. E65 required a tetramerization domain and induced a reduction in the overall expression of full-length K_V10.1, whereas E70 mainly affected its glycosylation pattern. E65 triggered the activation of cyclin-dependent kinases in Xenopus laevis oocytes, suggesting a role in cell cycle control. Our observations highlight the relevance of noncanonical functions for the oncogenicity of K_V10.1, which need to be considered when ion channels are targeted for cancer therapy.

Getting to the heart of hERG K(+) channel gating

Potassium ion channels encoded by the human ether-a-go-go related gene (hERG) form the ion-conducting subunit of the rapid delayed rectifier potassium current (IKr ). Although hERG channels exhibit a widespread tissue distribution they play a particularly important role in the heart. There has been considerable interest in hERG K(+) channels for three main reasons. First, they have very unusual gating kinetics, most notably rapid and voltage-dependent inactivation coupled to slow deactivation, which has led to the suggestion that they may play a specific role in the suppression of arrhythmias. Second, mutations in hERG are the cause of 30-40% of cases of congenital long QT syndrome (LQTS), the commonest inherited primary arrhythmia syndrome. Third, hERG is the molecular target for the vast majority of drugs that cause drug-induced LQTS, the commonest cause of drug-induced arrhythmias and cardiac death. Drug-induced LQTS has now been reported for a large range of both cardiac and non-cardiac drugs, in which this side effect is entirely undesired. In recent years there have been comprehensive reviews published on hERG K(+) channels (Vandenberg et al. 2012) and we will not re-cover this ground. Rather, we focus on more recent work on the structural basis and dynamics of hERG gating with an emphasis on how the latest developments may facilitate translational research in the area of stratifying risk of arrhythmias.

Eag Domains Regulate LQT Mutant hERG Channels in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Human Ether á go-go Related Gene potassium channels form the rapid component of the delayed-rectifier (IKr) current in the heart. The N-terminal 'eag' domain, which is composed of a Per-Arnt-Sim (PAS) domain and a short PAS-cap region, is a critical regulator of hERG channel function. In previous studies, we showed that isolated eag (i-eag) domains rescued the dysfunction of long QT type-2 associated mutant hERG R56Q channels, by substituting for defective eag domains, when the channels were expressed in Xenopus oocytes or HEK 293 cells.Here, our goal was to determine whether the rescue of hERG R56Q channels by i-eag domains could be translated into the environment of cardiac myocytes. We expressed hERG R56Q channels in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and measured electrical properties of the cells with whole-cell patch-clamp recordings. We found that, like in non-myocyte cells, hERG R56Q had defective, fast closing (deactivation) kinetics when expressed in hiPSC-CMs. We report here that i-eag domains slowed the deactivation kinetics of hERG R56Q channels in hiPSC-CMs. hERG R56Q channels prolonged the AP of hiPSCs, and the AP was shortened by co-expression of i-eag domains and hERG R56Q channels. We measured robust Förster Resonance Energy Transfer (FRET) between i-eag domains tagged with Cyan fluorescent protein (CFP) and hERG R56Q channels tagged with Citrine fluorescent proteins (Citrine), indicating their close proximity at the cell membrane in live iPSC-CMs. Together, functional regulation and FRET spectroscopy measurements indicated that i-eag domains interacted directly with hERG R56Q channels in hiPSC-CMs. These results mean that the regulatory role of i-eag domains is conserved in the cellular environment of human cardiomyocytes, indicating that i-eag domains may be useful as a biological therapeutic.

A sustained increase in the intracellular Ca²⁺ concentration induces proteolytic cleavage of EAG2 channel

Voltage-gated EAG2 channel is abundant in the brain and enhances cancer cell growth by controlling cell volume. The channel contains a cyclic nucleotide-binding homology (CNBH) domain and multiple calmodulin-binding motifs. Here we show that a raised intracellular Ca(2+) concentration causes proteolytic digestion of heterologously expressed and native EAG2 channels. A treatment of EAG2-expressing cells with the Ca(2+) ionophore A23187 for 1h reduces the full-length protein by ∼80% with a concomitant appearance of 30-35-kDa peptides. Similarly, a treatment with the Ca(2+)-ATPase inhibitor thapsigargin for 3h removes 30-35-kDa peptides from ∼1/3 of the channel protein. Moreover, an incubation of the isolated rat brain membrane with CaCl2 leads to the generation of fragments with similar sizes. This Ca(2+)-induced digestion is not seen with EAG1. Mutations in a C-terminal calmodulin-binding motif alter the degrees and positions of the cleavage. Truncated channels that mimic the digested proteins exhibit a reduced current density and altered channel gating. In particular, these shorter channels lack a rapid activation typical in EAG channels with more than 20-mV positive shifts in voltage dependence of activation. The truncation also eliminates the ability of EAG2 channel to reduce cell volume. These results suggest that a sustained increase in the intracellular Ca(2+) concentration leads to proteolytic cleavage at the C-terminal cytosolic region following the CNBH domain by altering its interaction with calmodulin. The observed Ca(2+)-induced proteolytic cleavage of EAG2 channel may act as an adaptive response under physiological and/or pathological conditions.

Large-scale mutational analysis of Kv11.1 reveals molecular insights into type 2 long QT syndrome

It has been suggested that deficient protein trafficking to the cell membrane is the dominant mechanism associated with type 2 Long QT syndrome (LQT2) caused by Kv11.1 potassium channel missense mutations, and that for many mutations the trafficking defect can be corrected pharmacologically. However, this inference was based on expression of a small number of Kv11.1 mutations. We performed a comprehensive analysis of 167 LQT2-linked missense mutations in four Kv11.1 structural domains and found that deficient protein trafficking is the dominant mechanism for all domains except for the distal carboxy-terminus. Also, most pore mutations--in contrast to intracellular domain mutations--were found to have severe dominant-negative effects when co-expressed with wild-type subunits. Finally, pharmacological correction of the trafficking defect in homomeric mutant channels was possible for mutations within all structural domains. However, pharmacological correction is dramatically improved for pore mutants when co-expressed with wild-type subunits to form heteromeric channels.

The Therapeutic Potential of hERG1 K+ Channels for Treating Cancer and Cardiac Arrhythmias

hERG potassium channels present pharmacologists and medicinal chemists with a dilemma. On the one hand hERG is a major reason for drugs being withdrawn from the market because of drug induced long QT syndrome and the associated risk of inducing sudden cardiac death, and yet hERG blockers are still widely used in the clinic to treat cardiac arrhythmias. Moreover, in the last decade overwhelming evidence has been provided that hERG channels are aberrantly expressed in cancer cells and that they contribute to tumour cell proliferation, resistance to apoptosis, and neoangiogenesis. Here we provide an overview of the properties of hERG channels and their role in excitable cells of the heart and nervous system as well as in cancer. We consider the therapeutic potential of hERG, not only with regard to the negative impact due to drug induced long QT syndrome, but also its future potential as a treatment in the fight against cancer.

Interactions between the N-terminal tail and the gating machinery of hERG K⁺ channels both in closed and open/inactive states

The N-terminal-most N-tail of the human ether-à-go-go-related gene (hERG) potassium channel is a crucial modulator of deactivation through its interactions with the S4-S5 loop and/or the C-linker/cNBD, leading to a stabilization of the channel's open state. Not only the N-terminal, but also the initial C-terminal region of the channel can modulate the transitions between the open and closed states either by direct or by indirect/allosteric interactions with the gating machinery. However, while a physical proximity of the N-tail to the gating machinery has been demonstrated in the closed state, data about their possible interaction in other channel conformations have been lacking. Using a site-directed cysteine mutagenesis and disulfide chemistry approach, we present here evidence that a physical proximity between the N-tail and the gating-related structures can also exist in channels held between pulses in the open/inactive state, highlighting the physiological and functional relevance of the direct interactions between the N-terminal tail and the S4-S5 loop and/or C-linker structures for modulation of channel.