Functional analysis of a mouse brain Elk-type K+ channel

Interplay of Ca2+ and K+ signals in cell physiology and cancer

The cytoplasmic Ca2+ concentration and the activity of K+ channels on the plasma membrane regulate cellular processes ranging from mitosis to oriented migration. The interplay between Ca2+ and K+ signals is intricate, and different cell types rely on peculiar cellular mechanisms. Derangement of these mechanisms accompanies the neoplastic progression. The calcium signals modulated by voltage-gated (KV) and calcium-dependent (KCa) K+ channel activity regulate progression of the cell division cycle, the release of growth factors, apoptosis, cell motility and migration. Moreover, KV channels regulate the cell response to the local microenvironment by assembling with cell adhesion and growth factor receptors. This chapter summarizes the pathophysiological roles of Ca2+ and K+ fluxes in normal and cancer cells, by concentrating on several biological systems in which these functions have been studied in depth, such as early embryos, mammalian cell lines, T lymphocytes, gliomas and colorectal cancer cells. A full understanding of the underlying mechanisms will offer a comprehensive view of the ion channel implication in cancer biology and suggest potential pharmacological targets for novel therapeutic approaches in oncology.

Kv12-encoded K+ channels drive the day-night switch in the repetitive firing rates of SCN neurons

Considerable evidence suggests that day-night rhythms in the functional expression of subthreshold potassium (K+) channels regulate daily oscillations in the spontaneous firing rates of neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals. The K+ conductance(s) driving these daily rhythms in the repetitive firing rates of SCN neurons, however, have not been identified. To test the hypothesis that subthreshold Kv12.1/Kv12.2-encoded K+ channels play a role, we obtained current-clamp recordings from SCN neurons in slices prepared from adult mice harboring targeted disruptions in the Kcnh8 (Kv12.1-/-) or Kcnh3 (Kv12.2-/-) locus. We found that mean nighttime repetitive firing rates were higher in Kv12.1-/- and Kv12.2-/- than in wild type (WT), SCN neurons. In marked contrast, mean daytime repetitive firing rates were similar in Kv12.1-/-, Kv12.2-/-, and WT SCN neurons, and the day-night difference in mean repetitive firing rates, a hallmark feature of WT SCN neurons, was eliminated in Kv12.1-/- and Kv12.2-/- SCN neurons. Similar results were obtained with in vivo shRNA-mediated acute knockdown of Kv12.1 or Kv12.2 in adult SCN neurons. Voltage-clamp experiments revealed that Kv12-encoded current densities in WT SCN neurons are higher at night than during the day. In addition, the pharmacological block of Kv12-encoded currents increased the mean repetitive firing rate of nighttime, but not daytime, in WT SCN neurons. Dynamic clamp-mediated subtraction of modeled Kv12-encoded currents also selectively increased the mean repetitive firing rates of nighttime WT SCN neurons. Despite the elimination of the nighttime decrease in the mean repetitive firing rates of SCN neurons, however, locomotor (wheel-running) activity remained rhythmic in Kv12.1-/-, Kv12.2-/-, and Kv12.1-targeted shRNA-expressing, and Kv12.2-targeted shRNA-expressing animals.

Kv12-Encoded K + Channels Drive the Day-Night Switch in the Repetitive Firing Rates of SCN Neurons

Considerable evidence suggests that day-night rhythms in the functional expression of subthreshold potassium (K + ) channels regulate daily oscillations in the rates of spontaneous action potential firing of neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals. The K + conductance(s) driving these daily rhythms in repetitive firing rates, however, have not been identified. To test the hypothesis that subthreshold Kv12.1/Kv12.2-encoded K + channels play a role, we obtained current-clamp recordings from SCN neurons in slices prepared from adult mice harboring targeted disruptions in the Kcnh8 (Kv12.1 -/- ) or Kcnh3 (Kv12.2 -/- ) locus. We found that mean nighttime repetitive firing rates were higher in Kv12.1 -/- and Kv12.2 -/- , than in wild type (WT), SCN neurons. In marked contrast, mean daytime repetitive firing rates were similar in Kv12.1 -/- , Kv12.2 -/- and WT SCN neurons, and the day-night difference in mean repetitive firing rates, a hallmark feature of WT SCN neurons, was eliminated in Kv12.1 -/- and Kv12.2 -/- SCN neurons. Similar results were obtained with in vivo shRNA-mediated acute knockdown of Kv12.1 or Kv12.2 in adult SCN neurons. Voltage-clamp experiments revealed that Kv12-encoded current densities in WT SCN neurons are higher at night than during the day. In addition, pharmacological block of Kv12-encoded currents increased the mean repetitive firing rate of nighttime, but not daytime, in WT SCN neurons. Dynamic clamp-mediated subtraction of modeled Kv12-encoded currents also selectively increased the mean repetitive firing rates of nighttime WT SCN neurons. Despite the elimination of nighttime decrease in the mean repetitive firing rates of SCN neurons, however, locomotor (wheel-running) activity remained rhythmic in Kv12.1 -/- , Kv12.2 -/- , Kv12.1-targeted shRNA-expressing, and Kv12.2-targeted shRNA-expressing animals.

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.

Histidine at position 462 determines the low quinine sensitivity of ether-à-go-go channel superfamily member Kv 12.1

BACKGROUND AND PURPOSE

The ether-à-go-go (Eag) Kv superfamily comprises closely related Kv 10, Kv 11, and Kv 12 subunits. Kv 11.1 (termed hERG in humans) gained much attention, as drug-induced inhibition of these channels is a frequent cause of sudden death in humans. The exclusive drug sensitivity of Kv 11.1 can be explained by central drug-binding pockets that are absent in most other channels. Currently, it is unknown whether Kv 12 channels are equipped with an analogous drug-binding pocket and whether drug-binding properties are conserved in all Eag superfamily members.

EXPERIMENTAL APPROACH

We analysed sensitivity of recombinant Kv 12.1 channels to quinine, a substituted quinoline that blocks Kv 10.1 and Kv 11.1 at low micromolar concentrations.

KEY RESULTS

Quinine inhibited Kv 12.1, but its affinity was 10-fold lower than for Kv 11.1. Contrary to Kv 11.1, quinine inhibited Kv 12.1 in a largely voltage-independent manner and induced channel opening at more depolarised potentials. Low sensitivity of Kv 12.1 and characteristics of quinine-dependent inhibition were determined by histidine 462, as site-directed mutagenesis of this residue into the homologous tyrosine of Kv 11.1 conferred Kv 11.1-like quinine block to Kv 12.1(H462Y). Molecular modelling demonstrated that the low affinity of Kv 12.1 was determined by only weak interactions of residues in the central cavity with quinine. In contrast, more favourable interactions can explain the higher quinine sensitivity of Kv 12.1(H462Y) and Kv 11.1 channels.

CONCLUSIONS AND IMPLICATIONS

The quinoline-binding "motif" is not conserved within the Eag superfamily, although the overall architecture of these channels is apparently similar. Our findings highlight functional and pharmacological diversity in this group of evolutionary-conserved channels.

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.

Kv12.1 channels are not sensitive to GqPCR-triggered activation of phospholipase Cβ

K_v12.1 K⁺ channels are expressed in several brain areas, but no physiological function could be attributed to these subunits so far. As genetically-modified animal models are not available, identification of native K_v12.1 currents must rely on characterization of distinct channel properties. Recently, it was shown in Xenopus laevis oocytes that K_v12.1 channels were modulated by membrane PI(4,5)P₂. However, it is not known whether these channels are also sensitive to physiologically-relevant PI(4,5)P₂ dynamics. We thus studied whether K_v12.1 channels were modulated by activation of phospholipase C β (PLCβ) and found that they were insensitive to receptor-triggered depletion of PI(4,5)P₂. Thus, K_v12.1 channels add to the growing list of K⁺ channels that are insensitive to PLCβ signaling, although modulated by PI(4,5)P₂ in Xenopus laevis oocytes.

Coupling Drosophila melanogaster Cryptochrome Light Activation and Oxidation of the Kvβ Subunit Hyperkinetic NADPH Cofactor

Motivated by the observations on the involvement of light-induced processes in the Drosophila melanogaster cryptochrome (DmCry) in regulation of the neuronal firing rate, which is achieved by a redox-state change of its voltage-dependent K⁺ channel Kvβ subunit hyperkinetic (Hk) reduced nicotinamide adenine dinucleotide phosphate (NADPH) cofactor, we propose in this work two hypothetical pathways that may potentially enable such coupling. In the first pathway, triggered by blue-light-induced formation of a radical pair [FAD^•-TRP^•+] in DmCry, the hole (TRP^•+) may hop to Hk, for example, through a tryptophan chain and oxidize NADPH, possibly leading to inhibition of the N-terminus inactivation in the K⁺ channel. In a second possible pathway, DmCry's FAD^•- is reoxidized by molecular oxygen, producing H₂O₂, which then diffuses to Hk and oxidizes NADPH. In this work, by applying a combination of quantum and empirical-based methods for free-energy calculations, we find that the oxidation of NADPH by TRP^•+ or H₂O₂ and the reoxidation of FAD^•- by O₂ are thermodynamically feasible. Our results may have an implication in identifying a magnetic sensing signal transduction pathway, specifically upon Drosophila's Hk NADPH cofactor oxidation, with a subsequent inhibition of the K⁺ channel N-terminus inactivation gate, permitting K⁺ flux.

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.

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.

Inverse Modulation of Neuronal Kv12.1 and Kv11.1 Channels by 4-Aminopyridine and NS1643

The three members of the ether-à-go-go-gene-like (Elk; K_v12.1-K_v12.3) family of voltage-gated K⁺ channels are predominantly expressed in neurons, but only little information is available on their physiological relevance. It was shown that K_v12.2 channels modulate excitability of hippocampal neurons, but no native current could be attributed to K_v12.1 and K_v12.3 subunits yet. This may appear somewhat surprising, given high expression of their mRNA transcripts in several brain areas. Native K_v12 currents may have been overlooked so far due to limited knowledge on their biophysical properties and lack of specific pharmacology. Except for K_v12.2, appropriate genetically modified mouse models have not been described; therefore, identification of K_v12-mediated currents in native cell types must rely on characterization of unique properties of the channels. We focused on recombinant human K_v12.1 to identify distinct properties of these channels. We found that K_v12.1 channels exhibited significant mode shift of activation, i.e., stabilization of the voltage sensor domain in a “relaxed” open state after prolonged channel activation. This mode shift manifested by a slowing of deactivation and, most prominently, a significant shift of voltage dependence to hyperpolarized potentials. In contrast to related K_v11.1, mode shift was not sensitive to extracellular Na⁺, which allowed for discrimination between these isoforms. Sensitivity of K_v12.1 and K_v11.1 to the broad-spectrum K⁺ antagonist 4-aminopyridine was similar. However, 4-AP strongly activated K_v12.1 channels, but it was an inhibitor of K_v11 channels. Interestingly, the agonist of K_v11 channels NS1643 also differentially modulated the activity of these channels, i.e., NS1643 activated K_v11.1, but strongly inhibited K_v12.1 channels. Thus, these closely related channels are distinguished by inverse pharmacological profiles. In summary, we identified unique biophysical and pharmacological properties of K_v12.1 channels and established straightforward experimental protocols to characterize K_v12.1-mediated currents. Identification of currents in native cell types with mode shift that are activated through 4-AP and inhibited by NS1643 can provide strong evidence for contribution of K_v12.1 to whole cell currents.

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.

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.

Concerted all-or-none subunit interactions mediate slow deactivation of human ether-à-go-go-related gene K+ channels

During the repolarization phase of a cardiac action potential, hERG1 K(+) channels rapidly recover from an inactivated state then slowly deactivate to a closed state. The resulting resurgence of outward current terminates the plateau phase and is thus a key regulator of action potential duration of cardiomyocytes. The intracellular N-terminal domain of the hERG1 subunit is required for slow deactivation of the channel as its removal accelerates deactivation 10-fold. Here we investigate the stoichiometry of hERG1 channel deactivation by characterizing the kinetic properties of concatenated tetramers containing a variable number of wild-type and mutant subunits. Three mutations known to accelerate deactivation were investigated, including R56Q and R4A/R5A in the N terminus and F656I in the S6 transmembrane segment. In all cases, a single mutant subunit induced the same rapid deactivation of a concatenated channel as that observed for homotetrameric mutant channels. We conclude that slow deactivation gating of hERG1 channels involves a concerted, fully cooperative interaction between all four wild-type channel subunits.

Functional evolution of Erg potassium channel gating reveals an ancient origin for IKr

Mammalian Ether-a-go-go related gene (Erg) family voltage-gated K(+) channels possess an unusual gating phenotype that specializes them for a role in delayed repolarization. Mammalian Erg currents rectify during depolarization due to rapid, voltage-dependent inactivation, but rebound during repolarization due to a combination of rapid recovery from inactivation and slow deactivation. This is exemplified by the mammalian Erg1 channel, which is responsible for IKr, a current that repolarizes cardiac action potential plateaus. The Drosophila Erg channel does not inactivate and closes rapidly upon repolarization. The dramatically different properties observed in mammalian and Drosophila Erg homologs bring into question the evolutionary origins of distinct Erg K(+) channel functions. Erg channels are highly conserved in eumetazoans and first evolved in a common ancestor of the placozoans, cnidarians, and bilaterians. To address the ancestral function of Erg channels, we identified and characterized Erg channel paralogs in the sea anemone Nematostella vectensis. N. vectensis Erg1 (NvErg1) is highly conserved with respect to bilaterian homologs and shares the IKr-like gating phenotype with mammalian Erg channels. Thus, the IKr phenotype predates the divergence of cnidarians and bilaterians. NvErg4 and Caenorhabditis elegans Erg (unc-103) share the divergent Drosophila Erg gating phenotype. Phylogenetic and sequence analysis surprisingly indicates that this alternate gating phenotype arose independently in protosomes and cnidarians. Conversion from an ancestral IKr-like gating phenotype to a Drosophila Erg-like phenotype correlates with loss of the cytoplasmic Ether-a-go-go domain. This domain is required for slow deactivation in mammalian Erg1 channels, and thus its loss may partially explain the change in gating phenotype.

A functional Kv1.2-hERG chimaeric channel expressed in Pichia pastoris

Members of the six-transmembrane segment family of ion channels share a common structural design. However, there are sequence differences between the members that confer distinct biophysical properties on individual channels. Currently, we do not have 3D structures for all members of the family to help explain the molecular basis for the differences in their biophysical properties and pharmacology. This is due to low-level expression of many members in native or heterologous systems. One exception is rat Kv1.2 which has been overexpressed in Pichia pastoris and crystallised. Here, we tested chimaeras of rat Kv1.2 with the hERG channel for function in Xenopus oocytes and for overexpression in Pichia. Chimaera containing the S1-S6 transmembrane region of HERG showed functional and pharmacological properties similar to hERG and could be overexpressed and purified from Pichia. Our results demonstrate that rat Kv1.2 could serve as a surrogate to express difficult-to-overexpress members of the six-transmembrane segment channel family.

Modulation of voltage-gated ion channels by sialylation

Control and modulation of electrical signaling is vital to normal physiology, particularly in neurons, cardiac myocytes, and skeletal muscle. The orchestrated activities of variable sets of ion channels and transporters, including voltage-gated ion channels (VGICs), are responsible for initiation, conduction, and termination of the action potential (AP) in excitable cells. Slight changes in VGIC activity can lead to severe pathologies including arrhythmias, epilepsies, and paralyses, while normal excitability depends on the precise tuning of the AP waveform. VGICs are heavily posttranslationally modified, with upward of 30% of the mature channel mass consisting of N- and O-glycans. These glycans are terminated typically by negatively charged sialic acid residues that modulate voltage-dependent channel gating directly. The data indicate that sialic acids alter VGIC activity in isoform-specific manners, dependent in part, on the number/location of channel sialic acids attached to the pore-forming alpha and/or auxiliary subunits that often act through saturating electrostatic mechanisms. Additionally, cell-specific regulation of sialylation can affect VGIC gating distinctly. Thus, channel sialylation is likely regulated through two mechanisms that together contribute to a dynamic spectrum of possible gating motifs: a subunit-specific mechanism and regulated (aberrant) changes in the ability of the cell to glycosylate. Recent studies showed that neuronal and cardiac excitability is modulated through regulated changes in voltage-gated Na(+) channel sialylation, suggesting that both mechanisms of differential VGIC sialylation contribute to electrical signaling in the brain and heart. Together, the data provide insight into an important and novel paradigm involved in the control and modulation of electrical signaling.

C-terminal β9-strand of the cyclic nucleotide-binding homology domain stabilizes activated states of Kv11.1 channels

Kv11.1 potassium channels are important for regulation of the normal rhythm of the heartbeat. Reduced activity of Kv11.1 channels causes long QT syndrome type 2, a disorder that increases the risk of cardiac arrhythmias and sudden cardiac arrest. Kv11.1 channels are members of the KCNH subfamily of voltage-gated K⁺ channels. However, they also share many similarities with the cyclic nucleotide gated ion channel family, including having a cyclic nucleotide-binding homology (cNBH) domain. Kv11.1 channels, however, are not directly regulated by cyclic nucleotides. Recently, crystal structures of the cNBH domain from mEAG and zELK channels, both members of the KCNH family of voltage-gated potassium channels, revealed that a C-terminal β9-strand in the cNBH domain occupied the putative cyclic nucleotide-binding site thereby precluding binding of cyclic nucleotides. Here we show that mutations to residues in the β9-strand affect the stability of the open state relative to the closed state of Kv11.1 channels. We also show that disrupting the structure of the β9-strand reduces the stability of the inactivated state relative to the open state. Clinical mutations located in this β9-strand result in reduced trafficking efficiency, which suggests that binding of the C-terminal β9-strand to the putative cyclic nucleotide-binding pocket is also important for assembly and trafficking of Kv11.1 channels.

The eag domain regulates hERG channel inactivation gating via a direct interaction

Human ether-á-go-go (eag)-related gene (hERG) potassium channel kinetics are characterized by rapid inactivation upon depolarization, along with rapid recovery from inactivation and very slow closing (deactivation) upon repolarization. These factors combine to create a resurgent hERG current, where the current amplitude is paradoxically larger with repolarization than with depolarization. Previous data showed that the hERG N-terminal eag domain regulated deactivation kinetics by making a direct interaction with the C-terminal region of the channel. A primary mechanism for fast inactivation depends on residues in the channel pore; however, inactivation was also shown to be slower after deletion of a large N-terminal region. The mechanism for N-terminal region regulation of inactivation is unclear. Here, we investigated the contributions of the large N-terminal domains (amino acids 1-354), including the eag domain (amino acids 1-135), to hERG channel inactivation kinetics and steady-state inactivation properties. We found that N-deleted channels lacking just the eag domain (Δ2-135) or both the eag domain and the adjacent proximal domain (Δ2-354) had less rectifying current-voltage (I-V) relationships, slower inactivation, faster recovery from inactivation, and lessened steady-state inactivation. We coexpressed genetically encoded N-terminal fragments for the eag domain (N1-135) or the eag domain plus the proximal domain (N1-354) with N-deleted hERG Δ2-135 or hERG Δ2-354 channels and found that the resulting channels had more rectifying I-V relationships, faster inactivation, slower recovery from inactivation, and increased steady-state inactivation, similar to those properties measured for wild-type (WT) hERG. We also found that the eag domain-containing fragments regulated the time to peak and the voltage at the peak of a resurgent current elicited with a ramp voltage protocol. The eag domain-containing fragments effectively converted N-deleted channels into WT-like channels. Neither the addition of the proximal domain to the eag domain in N1-354 fragments nor the presence of the proximal domain in hERG Δ2-135 channels measurably affected inactivation properties; in contrast, the proximal region regulated steady-state activation in hERG Δ2-135 channels. The results show that N-terminal region-dependent regulation of channel inactivation and resurgent current properties are caused by a direct interaction of the eag domain with the rest of the hERG channel.

Structure of the carboxy-terminal region of a KCNH channel

The KCNH family of ion channels, comprising ether-à-go-go (EAG), EAG-related gene (ERG), and EAG-like (ELK) K⁺ channel subfamilies, is crucial for repolarization of the cardiac action potential¹, regulation of neuronal excitability², and proliferation of tumor cells³. The C-terminal region of KCNH channels contains a cyclic nucleotide-binding homology domain (CNBHD) and C-linker that couples the CNBHD to the pore⁴. The C-linker/CNBHD is essential for proper function and trafficking of ion channels in the KCNH family^5–9. However, despite the importance of the C-linker/CNBHD for the function of KCNH channels, the structural basis of ion channel regulation by the C-linker/CNBHD is unknown. Here we report the crystal structure of the C-linker/CNBHD of zebrafish ELK channels at 2.2 Å resolution. While the overall structure of the C-linker/CNBHD of zELK channels is similar to the cyclic nucleotide-binding domain (CNBD) structure of the related HCN channels¹⁰, there are dramatic differences. Unlike the CNBD of HCN, the CNBHD of zELK displays a negatively charged electrostatic profile that explains the lack of binding and regulation of KCNH channels by cyclic nucleotides^4,11. Instead of cyclic nucleotide, the binding pocket is occupied by a short β-strand. Mutations of the β-strand shift the voltage dependence of activation to more depolarized voltages, implicating the β-strand as an intrinsic ligand for the CNBHD of zELK channels. In both zELK and HCN channels the C-linker is the site of virtually all of the intersubunit interactions in the C-terminal region. However, in the zELK structure there is a reorientation in the C-linker so the subunits form dimers instead of tetramers as observed in HCN channels. These results provide a structural framework for understanding the regulation of ion channels in the KCNH family by the C-linker/CNBHD and may guide the design of specific drugs.

Modulation of Kv3.4 channel N-type inactivation by protein kinase C shapes the action potential in dorsal root ganglion neurons

Fast inactivation of heterologously expressed Kv3.4 channels is dramatically slowed upon phosphorylation of the channel's N-terminal (N-type) inactivation gate by protein kinase C (PKC). However, the presence and physiological importance of this exquisite modulation in excitable tissues were unknown. Here, we employed minimally invasive cell-attached patch-clamping, single-cell qPCR and specific siRNAs to unambiguously demonstrate that fast-inactivating Kv3.4 channels underlie a robust high voltage-activated A-type K(+) current (I(AHV)) in nociceptive dorsal root ganglion neurons from 7-day-old rats. We also show that PKC activation with phorbol 12,13-dibutyrate (PDBu) causes a 4-fold slowing of Kv3.4 channel inactivation and, consequently, accelerates the repolarization of the action potential (AP) by 22%, which shortens the AP duration by 14%. G-protein coupled receptor (GPCR) agonists eliminate I(AHV) fast inactivation in a membrane-delimited manner, suggesting a Kv3.4 channel signalling complex. Preincubation of the neurons with the PKC inhibitor bisindolylmaleimide II inhibits the effect of GPCR agonists and PDBu. Furthermore, activation of PKC via GPCR agonists recapitulates the effects of PDBu on the AP. Finally, transfection of the neurons with Kv3.4 siRNA prolongs the AP by 25% and abolishes the GPCR agonist-induced acceleration of the AP repolarization. These results show that Kv3.4 channels help shape the repolarization of the nociceptor AP, and that modulation of Kv3.4 channel N-type inactivation by PKC regulates AP repolarization and duration. We propose that the dramatic modulation of I(AHV) fast inactivation by PKC represents a novel mechanism of neural plasticity with potentially significant implications in the transition from acute to chronic pain.

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

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

Deletion of the potassium channel Kv12.2 causes hippocampal hyperexcitability and epilepsy

We found the voltage-gated K+ channel Kv12.2 to be a potent regulator of excitability in hippocampal pyramidal neurons. Genetic deletion and pharmacologic block of Kv12.2 substantially reduced the firing threshold of these neurons. Kv12.2-/- (also known as Kcnh3-/-) mice showed signs of persistent neuronal hyperexcitability including frequent interictal spiking, spontaneous seizures and increased sensitivity to the chemoconvulsant pentylenetetrazol.

Divalent cations slow activation of EAG family K+ channels through direct binding to S4

Voltage-gated K+ channels share a common voltage sensor domain (VSD) consisting of four transmembrane helices, including a highly mobile S4 helix that contains the major gating charges. Activation of ether-a-go-go (EAG) family K+ channels is sensitive to external divalent cations. We show here that divalent cations slow the activation rate of two EAG family channels (Kv12.1 and Kv10.2) by forming a bridge between a residue in the S4 helix and acidic residues in S2. Histidine 328 in the S4 of Kv12.1 favors binding of Zn2+ and Cd2+, whereas the homologous residue Serine 321 in Kv10.2 contributes to effects of Mg2+ and Ni2+. This novel finding provides structural constraints for the position of transmembrane VSD helices in closed, ion-bound EAG family channels. Homology models of Kv12.1 and Kv10.2 VSD structures based on a closed-state model of the Shaker family K+ channel Kv1.2 match these constraints. Our results suggest close conformational conservation between closed EAG and Shaker family channels, despite large differences in voltage sensitivity, activation rates, and activation thresholds.

Triple N-glycosylation in the long S5-P loop regulates the activation and trafficking of the Kv12.2 potassium channel

Mammalian voltage-dependent potassium (Kv) channels regulate the excitability of nerve and muscle cells. Kv12.2 features the longest S5-P loop among all known mammalian Kv channels with the most N-linked glycosylation sites (three sites). Despite its unique structural features, Kv12.2 is not well characterized. Because glycosylation plays important roles in the folding, trafficking, and function of various Kv channels, we focused on the N-glycosylation of Kv12.2. We show that Kv12.2 is N-glycosylated in Chinese hamster ovary (CHO) cells and in cultured neurons as well as in the mouse brain. As an effect of N-glycosylation on the function of Kv12.2, we demonstrate that removal of sugar chains causes a depolarizing shift in the steady-state activation without a significant reduction in current amplitude. Unlike the previously reported shift for Shaker-type Kv channels, this shift does not appear to be due to negatively charged sialic acid residues in the sugar chains. We next examined the trafficking in CHO cells to address whether the unglycosylated Kv12.2 channels are utilized in vivo. Although double mutants, retaining only one glycosylation site, are trafficked to the surface of CHO cells irrespective of the position of the glycosylated site, unglycosylated channels are not trafficked to the cell surface. Furthermore, we could not detect unglycosylated channels in the mouse brain. Our data suggest that only glycosylated Kv12.2 channels show proper voltage dependence and are utilized in vivo.

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

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

Distribution and functional properties of human KCNH8 (Elk1) potassium channels

The Elk subfamily of the Eag K+ channel gene family is represented in mammals by three genes that are highly conserved between humans and rodents. Here we report the distribution and functional properties of a member of the human Elk K+ channel gene family, KCNH8. Quantitative RT-PCR analysis of mRNA expression patterns showed that KCNH8, along with the other Elk family genes, KCNH3 and KCNH4, are primarily expressed in the human nervous system. KCNH8 was expressed at high levels, and the distribution showed substantial overlap with KCNH3. In Xenopus oocytes, KCNH8 gives rise to slowly activating, voltage-dependent K+ currents that open at hyperpolarized potentials (half-maximal activation at -62 mV). Coexpression of KCNH8 with dominant-negative KCNH8, KCNH3, and KCNH4 subunits led to suppression of the KCNH8 currents, suggesting that Elk channels can form heteromultimers. Similar experiments imply that KCNH8 subunits are not able to form heteromultimers with Eag, Erg, or Kv family K+ channels.

Kinetic analyses of three distinct potassium conductances in ventral cochlear nucleus neurons

Neurons in the ventral cochlear nucleus (VCN) express three distinct K+ currents that differ in their voltage and time dependence, and in their inactivation behavior. In the present study, we quantitatively analyze the voltage-dependent kinetics of these three currents to gain further insight into how they regulate the discharge patterns of VCN neurons and to provide supporting data for the identification of their channel components. We find the transient A-type K+ current (IA) exhibits fourth-order activation kinetics (a4), and inactivates with one or two time constants. A second inactivation rate (leading to an a4bc kinetic description) is required to explain its recovery from inactivation. The dendrotoxin-sensitive low-threshold K+ current (ILT) also activates with fourth-order kinetics (w4) but shows slower, incomplete inactivation. The high-threshold K+ current (IHT) appears to consist of two kinetically distinct components (n2 + p). The first component activates approximately 10 mV positive to the second and has second-order kinetics. The second component activates with first-order kinetics. These two components also contribute to two kinetically distinct currents upon deactivation. The kinetic behavior of IHT was indistinguishable amongst cell types, suggesting the current is mediated by the same K+ channels amongst VCN neurons. Together these results provide a basis for more realistic modeling of VCN neurons, and provide clues regarding the molecular basis of the three K+ currents.

The functional properties of the human ether-à-go-go-like (HELK2) K+ channel

The voltage-dependent K+ channels belonging to the ether-à-go-go family (eag, erg, elk) are widely expressed in the mammalian CNS. Their neuronal function, however, is poorly understood. Among the elk clones, elk2 is the most abundantly expressed in the brain. We have characterized the human ELK2 channel (HELK2) expressed in mammalian cell lines. Moreover, we have detected helk2 mRNA and ELK2-like currents in freshly dissociated human astrocytoma cells. HELK2 was inhibited by Cs+ in a voltage-dependent way (Kd was 0.7 mm, at -120 mV). It was not affected by Way 123398 (5 micro m), dofetilide (10 micro m), quinidine (10 micro m), verapamil (20 micro m), haloperidol (2 micro m), astemizole (1 micro m), terfenadine (1 micro m) and hydroxyzine (30 micro m), compounds known to inhibit the biophysically related HERG channel. The crossover of the activation and inactivation curves produced a steady state 'window' current with a peak around -20 mV and considerably broader than it usually is in voltage-dependent channels, including HERG. Similar features were observed in the ELK2 clone from rat, in the same experimental conditions. Thus, ELK2 channels are active within a wide range of membrane potentials, both sub- and suprathreshold. Moreover, the kinetics of channel deactivation and removal of inactivation was about one order of magnitude quicker in HELK2, compared to HERG. Overall, these properties suggest that ELK2 channels are very effective at dampening the neuronal excitability, but less so at producing adaptation of action potential firing frequency. In addition, we suggest experimental ways to recognize HELK2 currents in vivo and raise the issue of the possible function of these channels in astrocytoma.

Differential expression of genes encoding subthreshold-operating voltage-gated K+ channels in brain

The members of the three subfamilies (eag, erg, and elk) of the ether-a-go-go (EAG) family of potassium channel pore-forming subunits express currents that, like the M-current (I(M)), could have considerable influence on the subthreshold properties of neuronal membranes, and hence the control of excitability. A nonradioactive in situ hybridization (NR-ISH) study of the distribution of the transcripts encoding the eight known EAG family subunits in rat brain was performed to identify neuronal populations in which the physiological roles of EAG channels could be studied. These distributions were compared with those of the mRNAs encoding the components of the classical M-current (Kcnq2 and Kcnq3). NR-ISH was combined with immunohistochemistry to specific neuronal markers to help identify expressing neurons. The results show that each EAG subunit has a specific pattern of expression in rat brain. EAG and Kcnq transcripts are prominent in several types of excitatory neurons in the cortex and hippocampus; however, only one of these channel components (erg1) was consistently expressed in inhibitory interneurons in these areas. Some neuronal populations express more than one product of the same subfamily, suggesting that the subunits may form heteromeric channels in these neurons. Many neurons expressed multiple EAG family and Kcnq transcripts, such as CA1 pyramidal neurons, which contained Kcnq2, Kcnq3, eag1, erg1, erg3, elk2, and elk3. This indicates that the subthreshold current in many neurons may be complex, containing different components mediated by a number of channels with distinct properties and neuromodulatory responses.

Mg(2+) modulates voltage-dependent activation in ether-à-go-go potassium channels by binding between transmembrane segments S2 and S3

Extracellular Mg(2+) directly modulates voltage-dependent activation in ether-à-go-go (eag) potassium channels, slowing the kinetics of ionic and gating currents (Tang, C.-Y., F. Bezanilla, and D.M. Papazian. 2000. J. Gen. Physiol. 115:319-337). To exert its effect, Mg(2+) presumably binds to a site in or near the eag voltage sensor. We have tested the hypothesis that acidic residues unique to eag family members, located in transmembrane segments S2 and S3, contribute to the Mg(2+)-binding site. Two eag-specific acidic residues and three acidic residues found in the S2 and S3 segments of all voltage-dependent K(+) channels were individually mutated in Drosophila eag, mutant channels were expressed in Xenopus oocytes, and the effect of Mg(2+) on ionic current kinetics was measured using a two electrode voltage clamp. Neutralization of eag-specific residues D278 in S2 and D327 in S3 eliminated Mg(2+)-sensitivity and mimicked the slowing of activation kinetics caused by Mg(2+) binding to the wild-type channel. These results suggest that Mg(2+) modulates activation kinetics in wild-type eag by screening the negatively charged side chains of D278 and D327. Therefore, these residues are likely to coordinate the bound ion. In contrast, neutralization of the widely conserved residues D284 in S2 and D319 in S3 preserved the fast kinetics seen in wild-type eag in the absence of Mg(2+), indicating that D284 and D319 do not mediate the slowing of activation caused by Mg(2+) binding. Mutations at D284 affected the eag gating pathway, shifting the voltage dependence of Mg(2+)-sensitive, rate limiting transitions in the hyperpolarized direction. Another widely conserved residue, D274 in S2, is not required for Mg(2+) sensitivity but is in the vicinity of the binding site. We conclude that Mg(2+) binds in a water-filled pocket between S2 and S3 and thereby modulates voltage-dependent gating. The identification of this site constrains the packing of transmembrane segments in the voltage sensor of K(+) channels, and suggests a molecular mechanism by which extracellular cations modulate eag activation kinetics.

Cloning of components of a novel subthreshold-activating K(+) channel with a unique pattern of expression in the cerebral cortex

Potassium channels that are open at very negative membrane potentials govern the subthreshold behavior of neurons. These channels contribute to the resting potential and help regulate the degree of excitability of a neuron by affecting the impact of synaptic inputs and the threshold for action potential generation. They can have large influences on cell behavior even when present at low concentrations because few conductances are active at these voltages. We report the identification of a new K(+) channel pore-forming subunit of the ether-à-go-go (Eag) family, named Eag2, that expresses voltage-gated K(+) channels that have significant activation at voltages around -100 mV. Eag2 expresses outward-rectifying, non-inactivating voltage-dependent K(+) currents resembling those of Eag1, including a strong dependence of activation kinetics on prepulse potential. However, Eag2 currents start activating at subthreshold potentials that are 40-50 mV more negative than those reported for Eag1. Because they activate at such negative voltages and do not inactivate, Eag2 channels will contribute sustained outward currents down to the most negative membrane potentials known in neurons. Although Eag2 mRNA levels in whole brain appear to be low, they are highly concentrated in a few neuronal populations, most prominently in layer IV of the cerebral cortex. This highly restricted pattern of cortical expression is unlike that of any other potassium channel cloned to date and may indicate specific roles for this channel in cortical processing. Layer IV neurons are the main recipient of the thalamocortical input. Given their functional properties and specific distribution, Eag2 channels may play roles in the regulation of the behavioral state-dependent entry of sensory information to the cerebral cortex.