Functional distinction of human EAG1 and EAG2 potassium channels

The hEag1 K+ Channel Inhibitor Astemizole Stimulates Ca2+ Deposition in SaOS-2 and MG-63 Osteosarcoma Cultures

The hEag1 (Kv10.1) K⁺ channel is normally found in the brain, but it is ectopically expressed in tumor cells, including osteosarcoma. Based on the pivotal role of ion channels in osteogenesis, we tested whether pharmacological modulation of hEag1 may affect osteogenic differentiation of osteosarcoma cell lines. Using molecular biology (RT-PCR), electrophysiology (patch-clamp) and pharmacology (astemizole sensitivity, IC₅₀ = 0.135 μM) we demonstrated that SaOS-2 osteosarcoma cells also express hEag1 channels. SaOS-2 cells also express to KCa1.1 K⁺ channels as shown by mRNA expression and paxilline sensitivity of the current. The inhibition of hEag1 (2 μM astemizole) or KCa1.1 (1 mM TEA) alone did not induce Ca²⁺ deposition in SaOS-2 cultures, however, these inhibitors, at identical concentrations, increased Ca²⁺ deposition evoked by the classical or pathological (inorganic phosphate, Pi) induction pathway without causing cytotoxicity, as reported by three completer assays (LDH release, MTT assay and SRB protein assay). We observed a similar effect of astemizole on Ca²⁺ deposition in MG-63 osteosarcoma cultures as well. We propose that the increase in the osteogenic stimuli-induced mineral matrix formation of osteosarcoma cell lines by inhibiting hEag1 may be a useful tool to drive terminal differentiation of osteosarcoma.

Nitidine chloride inhibits fibroblast like synoviocytes-mediated rheumatoid synovial inflammation and joint destruction by targeting KCNH1

OBJECTIVE

Nitidine chloride (NC), a natural small molecular compound from traditional Chinese herbal medicine zanthoxylum nitidum, has been shown to exhibit anti-tumor effect. However, its role in autoimmune diseases such as rheumatoid arthritis (RA) is unknown. Here, we investigate the effect of NC in controlling fibroblast-like synoviocytes (FLS)-mediated synovial inflammation and joint destruction in RA and further explore its underlying mechanism(s).

METHODS

FLSs were separated from synovial tissues obtained from patients with RA. Protein expression was analyzed by Western blot or immunohistochemistry. Gene expression was measured using quantitative RT-PCR. ELISA was used to measure the levels of cytokines and MMPs. Cell proliferation was detected using EdU incorporation. Migration and invasion were evaluated by Boyden chamber assay. RNA sequencing analysis was used to identify the target of NC. Collagen-induced arthritis (CIA) model was used to evaluate the in vivo effect of NC.

RESULTS

NC treatment reduced the proliferation, migration, invasion, and lamellipodia formation but not apoptosis of RA FLSs. We also demonstrated the inhibitory effect of NC on TNF-α-induced expression and secretion of IL-6, IL-8, CCL-2, MMP-1 and MMP-13. Furthermore, we identified KCNH1, a gene that encodes ether-à-go-go-1 channel, as a novel targeting gene of NC in RA FLSs. KCNH1 expression was increased in FLSs and synovial tissues from patients with RA compared to healthy controls. KCNH1 knockdown or NC treatment decreased the TNF-α-induced phosphorylation of AKT. Interestingly, NC treatment ameliorated the severity of arthritis and reduced synovial KCNH1 expression in mice with CIA.

CONCLUSIONS

Our data demonstrate that NC treatment inhibits aggressive and inflammatory actions of RA FLSs by targeting KCNH1 and sequential inhibition of AKT phosphorylation. Our findings suggest that NC might control FLS-mediated rheumatoid synovial inflammation and joint destruction, and be a novel therapeutic agent for RA.

Molecular Dynamics-Derived Pharmacophore Model Explaining the Nonselective Aspect of KV10.1 Pore Blockers

The K_V10.1 voltage-gated potassium channel is highly expressed in 70% of tumors, and thus represents a promising target for anticancer drug discovery. However, only a few ligands are known to inhibit K_V10.1, and almost all also inhibit the very similar cardiac hERG channel, which can lead to undesirable side-effects. In the absence of the structure of the K_V10.1–inhibitor complex, there remains the need for new strategies to identify selective K_V10.1 inhibitors and to understand the binding modes of the known K_V10.1 inhibitors. To investigate these binding modes in the central cavity of K_V10.1, a unique approach was used that allows derivation and analysis of ligand–protein interactions from molecular dynamics trajectories through pharmacophore modeling. The final molecular dynamics-derived structure-based pharmacophore model for the simulated K_V10.1–ligand complexes describes the necessary pharmacophore features for K_V10.1 inhibition and is highly similar to the previously reported ligand-based hERG pharmacophore model used to explain the nonselectivity of K_V10.1 pore blockers. Moreover, analysis of the molecular dynamics trajectories revealed disruption of the π–π network of aromatic residues F359, Y464, and F468 of K_V10.1, which has been reported to be important for binding of various ligands for both K_V10.1 and hERG channels. These data indicate that targeting the K_V10.1 channel pore is also likely to result in undesired hERG inhibition, and other potential binding sites should be explored to develop true K_V10.1-selective inhibitors as new anticancer agents.

Overcoming challenges of HERG potassium channel liability through rational design: Eag1 inhibitors for cancer treatment

Two decades of research have proven the relevance of ion channel expression for tumor progression in virtually every indication, and it has become clear that inhibition of specific ion channels will eventually become part of the oncology therapeutic arsenal. However, ion channels play relevant roles in all aspects of physiology, and specificity for the tumor tissue remains a challenge to avoid undesired effects. Eag1 (K_V 10.1) is a voltage-gated potassium channel whose expression is very restricted in healthy tissues outside of the brain, while it is overexpressed in 70% of human tumors. Inhibition of Eag1 reduces tumor growth, but the search for potent inhibitors for tumor therapy suffers from the structural similarities with the cardiac HERG channel, a major off-target. Existing inhibitors show low specificity between the two channels, and screenings for Eag1 binders are prone to enrichment in compounds that also bind HERG. Rational drug design requires knowledge of the structure of the target and the understanding of structure-function relationships. Recent studies have shown subtle structural differences between Eag1 and HERG channels with profound functional impact. Thus, although both targets' structure is likely too similar to identify leads that exclusively bind to one of the channels, the structural information combined with the new knowledge of the functional relevance of particular residues or areas suggests the possibility of selective targeting of Eag1 in cancer therapies. Further development of selective Eag1 inhibitors can lead to first-in-class compounds for the treatment of different cancers.

Identification of MKRN1 as a second E3 ligase for Eag1 potassium channels reveals regulation via differential degradation

Mutations in the human gene encoding the neuron-specific Eag1 voltage-gated K⁺ channel are associated with neurodevelopmental diseases, indicating an important role of Eag1 during brain development. A disease-causing Eag1 mutation is linked to decreased protein stability that involves enhanced protein degradation by the E3 ubiquitin ligase cullin 7 (CUL7). The general mechanisms governing protein homeostasis of plasma membrane- and endoplasmic reticulum (ER)-localized Eag1 K⁺ channels, however, remain unclear. By using yeast two-hybrid screening, we identified another E3 ubiquitin ligase, makorin ring finger protein 1 (MKRN1), as a novel binding partner primarily interacting with the carboxyl-terminal region of Eag1. MKRN1 mainly interacts with ER-localized immature core-glycosylated, as well as nascent nonglycosylated, Eag1 proteins. MKRN1 promotes polyubiquitination and ER-associated proteasomal degradation of immature Eag1 proteins. Although both CUL7 and MKRN1 contribute to ER quality control of immature core-glycosylated Eag1 proteins, MKRN1, but not CUL7, associates with and promotes degradation of nascent, nonglycosylated Eag1 proteins at the ER. In direct contrast to the role of CUL7 in regulating both ER and peripheral quality controls of Eag1, MKRN1 is exclusively responsible for the early stage of Eag1 maturation at the ER. We further demonstrated that both CUL7 and MKRN1 contribute to protein quality control of additional disease-causing Eag1 mutants associated with defective protein homeostasis. Our data suggest that the presence of this dual ubiquitination system differentially maintains Eag1 protein homeostasis and may ensure efficient removal of disease-associated misfolded Eag1 mutant channels.

The functional analysis of Cullin 7 E3 ubiquitin ligases in cancer

Cullin (CUL) proteins have critical roles in development and cancer, however few studies on CUL7 have been reported due to its characteristic molecular structure. CUL7 forms a complex with the ROC1 ring finger protein, and only two F-box proteins Fbxw8 and Fbxw11 have been shown to bind to CUL7. Interestingly, CUL7 can interact with its substrates by forming a novel complex that is independent of these two F-box proteins. The biological implications of CUL-ring ligase 7 (CRL7) suggest that the CRL7 may not only perform a proteolytic function but may also play a non-proteolytic role. Among the existing studied CRL7-based E3 ligases, CUL7 exerts both tumor promotion and suppression in a context-dependent manner. Currently, the mechanism of CUL7 in cancer remains unclear, and no studies have addressed potential therapies targeting CUL7. Consistent with the roles of the various CRL7 adaptors exhibit, targeting CRL7 might be an effective strategy for cancer prevention and treatment. We systematically describe the recent major advances in understanding the role of the CUL7 E3 ligase in cancer and further summarize its potential use in clinical therapy.

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.

Monoclonal Antibodies Targeting Ion Channels and Their Therapeutic Potential

Monoclonal antibodies (mAbs) represent a rapidly growing pharmaceutical class of protein drugs that becomes an important part of the precision therapy. mAbs are characterized by their high specificity and affinity for the target antigen, which is mostly present on the cell surface. Ion channels are a large family of transmembrane proteins that control ion transport across the cell membrane. They are involved in almost all biological processes in both health and disease and are widely considered as prospective targets. However, no antibody-based drug targeting ion channel has been developed so far that has progressed to clinical use. Thus, we provide a comprehensive review of the elaborated mAbs against ion channels, describe their mechanisms of action, and discuss their therapeutic potential.

Cardiac Targeting Peptide, a Novel Cardiac Vector: Studies in Bio-Distribution, Imaging Application, and Mechanism of Transduction

Our previous work identified a 12-amino acid peptide that targets the heart, termed cardiac targeting peptide (CTP). We now quantitatively assess the bio-distribution of CTP, show a clinical application with the imaging of the murine heart, and study its mechanisms of transduction. Bio-distribution studies of cyanine5.5-N-Hydroxysuccinimide (Cy5.5) labeled CTP were undertaken in wild-type mice. Cardiac targeting peptide was labeled with Technetium 99m (^99mTc) using the chelator hydrazino-nicotinamide (HYNIC), and imaging performed using micro-single photon emission computerized tomography/computerized tomography (SPECT/CT). Human-induced pluripotent stem cell (iPSC)-derived cardiomyocytes (CMCs) were incubated with dual-labeled CTP, and imaged using confocal microscopy. TriCEPs technology was utilized to study the mechanism of transduction. Bio-distribution studies showed peak uptake of CTP at 15 min. ^99mTc-HYNIC-CTP showed heart-specific uptake. Robust transduction of beating human iPSC-derived CMCs was seen. TriCEPs experiments revealed five candidate binding partners for CTP, with Kcnh5 being felt to be the most likely candidate as it showed a trend towards being competed out by siRNA knockdown. Transduction efficiency was enhanced by increasing extracellular potassium concentration, and with Quinidine, a Kcnh5 inhibitor, that blocks the channel in an open position. We demonstrate that CTP transduces the normal heart as early as 15 min. ^99mTc-HYNIC-CTP targets the normal murine heart with substantially improved targeting compared with ^99mTc Sestamibi. Cardiac targeting peptide's transduction ability is not species limited and has human applicability. Cardiac targeting peptide appears to utilize Kcnh5 to gain cell entry, a phenomenon that is affected by pre-treatment with Quinidine and changes in potassium levels.

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.

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.

Cullin 7 mediates proteasomal and lysosomal degradations of rat Eag1 potassium channels

Mammalian Eag1 (Kv10.1) potassium (K⁺) channels are widely expressed in the brain. Several mutations in the gene encoding human Eag1 K⁺ channel have been associated with congenital neurodevelopmental anomalies. Currently very little is known about the molecules mediating protein synthesis and degradation of Eag1 channels. Herein we aim to ascertain the protein degradation mechanism of rat Eag1 (rEag1). We identified cullin 7 (Cul7), a member of the cullin-based E3 ubiquitin ligase family, as a novel rEag1 binding partner. Immunoprecipitation analyses confirmed the interaction between Cul7 and rEag1 in heterologous cells and neuronal tissues. Cul7 and rEag1 also exhibited significant co-localization at synaptic regions in neurons. Over-expression of Cul7 led to reduced protein level, enhanced ubiquitination, accelerated protein turn-over, and decreased current density of rEag1 channels. We provided further biochemical and morphological evidence suggesting that Cul7 targeted endoplasmic reticulum (ER)- and plasma membrane-localized rEag1 to the proteasome and the lysosome, respectively, for protein degradation. Cul7 also contributed to protein degradation of a disease-associated rEag1 mutant. Together, these results indicate that Cul7 mediates both proteasomal and lysosomal degradations of rEag1. Our findings provide a novel insight to the mechanisms underlying ER and peripheral protein quality controls of Eag1 channels.

Differential Effects of Toluene and Ethanol on Dopaminergic Neurons of the Ventral Tegmental Area

Drugs of abuse increase the activity of dopaminergic neurons of the ventral tegmental area (VTA), and output from the VTA is critical for both natural and drug-induced reward and reinforcement. Ethanol and the abused inhalant toluene both enhance VTA neuronal firing, but the mechanisms of this effect is not fully known. In this study, we used extracellular recordings to compare the actions of toluene and ethanol on DA VTA neurons. Both ethanol and toluene increased the firing rate of DA neurons, although toluene was ~100 times more potent than ethanol. The mixed ion channel blocker quinine (100 μM) blocked the increases in firing produced by ethanol and toluene, indicating some similarity in mechanisms of excitation. A mixture of antagonists of GABA and cholinergic receptors did not prevent toluene-induced or ethanol-induced excitation, and toluene-induced excitation was not altered by co-administration of ethanol, suggesting independent mechanisms of excitation for ethanol and toluene. Concurrent blockade of NMDA, AMPA, and metabotropic glutamate receptors enhanced the excitatory effect of toluene while having no significant effect on ethanol excitation. Nicotine increased firing of DA VTA neurons, and this was blocked by the nicotinic antagonist mecamylamine (1 μM). Mecamylamine did not alter ethanol or toluene excitation of firing but the muscarinic antagonist atropine (5 μM) or a combination of GABA antagonists (bicuculline and CGP35348, 10 μM each) reduced toluene-induced excitation without affecting ethanol excitation. The Ih current blocker ZD7288 abolished the excitatory effect of toluene but unlike the block of ethanol excitation, the effect of ZD7288 was not reversed by the GIRK channel blocker barium, but was reversed by GABA antagonists. These results demonstrate that the excitatory effects of ethanol and toluene have some similarity, such as block by quinine and ZD7288, but also indicate that there are important differences between these two drugs in their modulation by glutamatergic, cholinergic, and GABAergic receptors. These findings provide important information regarding the actions of abused inhalants on central reward pathways, and suggest that regulation of the activation of central dopamine pathways by ethanol and toluene partially overlap.

An antiarrhythmic agent as a promising lead compound for targeting the hEAG1 ion channel in cancer therapy: insights from molecular dynamics simulations

Experimental evidence suggests that hERG and hEAG potassium channels may serve as important cancer therapy targets because either of the channel blockade or inactivation by different methods leads to inhibition of cancer cells growth and proliferation. However, there is no known hEAG specific blocker, and hERG blockade leads to adverse cardiac side effects, although it is currently used in treating certain types of arrhythmias. There have been some attempts to explain the channels blockade by clofilium, an antiarrhythmic agent, and the results lead to different possible binding modes. This study investigates for the first time the potential of using clofilium as a lead compound for finding a novel cancer therapy agent which may target ion channels. The implied findings from a comparative assessment of literature studies were verified using molecular dynamics simulations. The results indicate a particular structural difference between the two channels that could provide a novel and realistic way of using clofilium analogs which may target the hEAG1 ion channel in cancer therapy.

Cyclic expression of the voltage-gated potassium channel KV10.1 promotes disassembly of the primary cilium

The primary cilium, critical for morphogenic and growth factor signaling, is assembled upon cell cycle exit, but the links between ciliogenesis and cell cycle progression are unclear. KV10.1 is a voltage-gated potassium channel frequently overexpressed in tumors. We have previously reported that expression of KV10.1 is temporally restricted to a time period immediately prior to mitosis in healthy cells. Here, we provide microscopical and biochemical evidence that KV10.1 localizes to the centrosome and the primary cilium and promotes ciliary disassembly. Interference with KV10.1 ciliary localization abolishes not only the effects on ciliary disassembly, but also KV10.1-induced tumor progression in vivo Conversely, upon knockdown of KV10.1, ciliary disassembly is impaired, proliferation is delayed, and proliferating cells show prominent primary cilia. Thus, modulation of ciliogenesis by KV10.1 can explain the influence of KV10.1 expression on the proliferation of normal cells and is likely to be a major mechanism underlying its tumorigenic effects.

Involvement of potassium channels in the progression of cancer to a more malignant phenotype

Potassium channels are a diverse group of pore-forming transmembrane proteins that selectively facilitate potassium flow through an electrochemical gradient. They participate in the control of the membrane potential and cell excitability in addition to different cell functions such as cell volume regulation, proliferation, cell migration, angiogenesis as well as apoptosis. Because these physiological processes are essential for the correct cell function, K+ channels have been associated with a growing number of diseases including cancer. In fact, different K+ channel families such as the voltage-gated K+ channels, the ether à-go-go K+ channels, the two pore domain K+ channels and the Ca2+-activated K+ channels have been associated to tumor biology. Potassium channels have a role in neoplastic cell-cycle progression and their expression has been found abnormal in many types of tumors and cancer cells. In addition, the expression and activity of specific K+ channels have shown a significant correlation with the tumor malignancy grade. The aim of this overview is to summarize published data on K+ channels that exhibit oncogenic properties and have been linked to a more malignant cancer phenotype. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

The Eag domain regulates the voltage-dependent inactivation of rat Eag1 K+ channels

Eag (Kv10) and Erg (Kv11) belong to two distinct subfamilies of the ether-à-go-go K+ channel family (KCNH). While Erg channels are characterized by an inward-rectifying current-voltage relationship that results from a C-type inactivation, mammalian Eag channels display little or no voltage-dependent inactivation. Although the amino (N)-terminal region such as the eag domain is not required for the C-type inactivation of Erg channels, an N-terminal deletion in mouse Eag1 has been shown to produce a voltage-dependent inactivation. To further discern the role of the eag domain in the inactivation of Eag1 channels, we generated N-terminal chimeras between rat Eag (rEag1) and human Erg (hERG1) channels that involved swapping the eag domain alone or the complete cytoplasmic N-terminal region. Functional analyses indicated that introduction of the homologous hERG1 eag domain led to both a fast phase and a slow phase of channel inactivation in the rEag1 chimeras. By contrast, the inactivation features were retained in the reverse hERG1 chimeras. Furthermore, an eag domain-lacking rEag1 deletion mutant also showed the fast phase of inactivation that was notably attenuated upon co-expression with the rEag1 eag domain fragment, but not with the hERG1 eag domain fragment. Additionally, we have identified a point mutation in the S4-S5 linker region of rEag1 that resulted in a similar inactivation phenotype. Biophysical analyses of these mutant constructs suggested that the inactivation gating of rEag1 was distinctly different from that of hERG1. Overall, our findings are consistent with the notion that the eag domain plays a critical role in regulating the inactivation gating of rEag1. We propose that the eag domain may destabilize or mask an inherent voltage-dependent inactivation of rEag1 K+ channels.

The subfamily-specific assembly of Eag and Erg K+ channels is determined by both the amino and the carboxyl recognition domains

A functional voltage-gated K(+) (Kv) channel comprises four pore-forming α-subunits, and only members of the same Kv channel subfamily may co-assemble to form heterotetramers. The ether-à-go-go family of Kv channels (KCNH) encompasses three distinct subfamilies: Eag (Kv10), Erg (Kv11), and Elk (Kv12). Members of different ether-à-go-go subfamilies, such as Eag and Erg, fail to form heterotetramers. Although a short stretch of amino acid sequences in the distal C-terminal section has been implicated in subfamily-specific subunit assembly, it remains unclear whether this region serves as the sole and/or principal subfamily recognition domain for Eag and Erg. Here we aim to ascertain the structural basis underlying the subfamily specificity of ether-à-go-go channels by generating various chimeric constructs between rat Eag1 and human Erg subunits. Biochemical and electrophysiological characterizations of the subunit interaction properties of a series of different chimeric and truncation constructs over the C terminus suggested that the putative C-terminal recognition domain is dispensable for subfamily-specific assembly. Further chimeric analyses over the N terminus revealed that the N-terminal region may also harbor a subfamily recognition domain. Importantly, exchanging either the N-terminal or the C-terminal domain alone led to a virtual loss of the intersubfamily assembly boundary. By contrast, simultaneously swapping both recognition domains resulted in a reversal of subfamily specificity. Our observations are consistent with the notion that both the N-terminal and the C-terminal recognition domains are required to sustain the subfamily-specific assembly of rat Eag1 and human Erg.

The punctate localization of rat Eag1 K+ channels is conferred by the proximal post-CNBHD region

Background

In mammals, Eag K⁺ channels (K_V10) are exclusively expressed in the brain and comprise two isoforms: Eag1 (K_V10.1) and Eag2 (K_V10.2). Despite their wide presence in various regions of the brain, the functional role of Eag K⁺ channels remains obscure. Here we address this question by characterizing the subcellular localization of rat Eag1 (rEag1) and rat Eag2 (rEag2) in hippocampal neurons, as well as determining the structural basis underlying their different localization patterns.

Results

Immunofluorescence analysis of young and mature hippocampal neurons in culture revealed that endogenous rEag1 and rEag2 K⁺ channels were present in both the dendrosomatic and the axonal compartments. Only rEag1 channels displayed a punctate immunostaining pattern and showed significant co-localization with PSD-95. Subcellular fractionation analysis further demonstrated a distinct enrichment of rEag1 in the synaptosomal fraction. Over-expression of recombinant GFP-tagged Eag constructs in hippocampal neurons also showed a significant punctate localization of rEag1 channels. To identify the protein region dictating the Eag channel subcellular localization pattern, we generated a variety of different chimeric constructs between rEag1 and rEag2. Quantitative studies of neurons over-expressing these GFP-tagged chimeras indicated that punctate localization was conferred by a segment (A723-R807) within the proximal post-cyclic nucleotide-binding homology domain (post-CNBHD) region in the rEag1 carboxyl terminus.

Conclusions

Our findings suggest that Eag1 and Eag2 K⁺ channels may modulate membrane excitability in both the dendrosomatic and the axonal compartments and that Eag1 may additionally regulate neurotransmitter release and postsynaptic signaling. Furthermore, we present the first evidence showing that the proximal post-CNBHD region seems to govern the Eag K⁺ channel subcellular localization pattern.

External pH modulates EAG superfamily K+ channels through EAG-specific acidic residues in the voltage sensor

The Ether-a-go-go (EAG) superfamily of voltage-gated K⁺ channels consists of three functionally distinct gene families (Eag, Elk, and Erg) encoding a diverse set of low-threshold K⁺ currents that regulate excitability in neurons and muscle. Previous studies indicate that external acidification inhibits activation of three EAG superfamily K⁺ channels, Kv10.1 (Eag1), Kv11.1 (Erg1), and Kv12.1 (Elk1). We show here that Kv10.2, Kv12.2, and Kv12.3 are similarly inhibited by external protons, suggesting that high sensitivity to physiological pH changes is a general property of EAG superfamily channels. External acidification depolarizes the conductance–voltage (GV) curves of these channels, reducing low threshold activation. We explored the mechanism of this high pH sensitivity in Kv12.1, Kv10.2, and Kv11.1. We first examined the role of acidic voltage sensor residues that mediate divalent cation block of voltage activation in EAG superfamily channels because protons reduce the sensitivity of Kv12.1 to Zn²⁺. Low pH similarly reduces Mg²⁺ sensitivity of Kv10.1, and we found that the pH sensitivity of Kv11.1 was greatly attenuated at 1 mM Ca²⁺. Individual neutralizations of a pair of EAG-specific acidic residues that have previously been implicated in divalent block of diverse EAG superfamily channels greatly reduced the pH response in Kv12.1, Kv10.2, and Kv11.1. Our results therefore suggest a common mechanism for pH-sensitive voltage activation in EAG superfamily channels. The EAG-specific acidic residues may form the proton-binding site or alternatively are required to hold the voltage sensor in a pH-sensitive conformation. The high pH sensitivity of EAG superfamily channels suggests that they could contribute to pH-sensitive K⁺ currents observed in vivo.

Multistate structural modeling and voltage-clamp analysis of epilepsy/autism mutation Kv10.2-R327H demonstrate the role of this residue in stabilizing the channel closed state

Voltage-gated potassium channel Kv10.2 (KCNH5) is expressed in the nervous system, but its functions and involvement in human disease are poorly understood. We studied a human Kv10.2 channel mutation (R327H) recently identified in a child with epileptic encephalopathy and autistic features. Using multistate structural modeling, we demonstrate that the Arg327 residue in the S4 helix of voltage-sensing domain has strong ionic interactions with negatively charged residues within the S1-S3 helices in the resting (closed) and early-activation state but not in the late-activation and fully-activated (open) state. The R327H mutation weakens ionic interactions between residue 327 and these negatively charged residues, thus favoring channel opening. Voltage-clamp analysis showed a strong hyperpolarizing (∼70 mV) shift of voltage dependence of activation and an acceleration of activation. Our results demonstrate the critical role of the Arg327 residue in stabilizing the channel closed state and explicate for the first time the structural and functional change of a Kv10.2 channel mutation associated with neurological disease.

Potassium ion channels in retinal ganglion cells (review)

Retinal ganglion cells (RGCs) consolidate visual processing and constitute the last step prior to the transmission of signals to higher brain centers. RGC death is a major cause of visual impairment in optic neuropathies, including glaucoma, age‑related macular degeneration, diabetic retinopathy, uveoretinitis and vitreoretinopathy. Discharge patterns of RGCs are primarily determined by the presence of ion channels. As the most diverse group of ion channels, potassium (K+) channels play key roles in modulating the electrical properties of RGCs. Biochemical, molecular and pharmacological studies have identified a number of K+ channels in RGCs, including inwardly rectifying K+ (Kir), ATP‑sensitive K+ (KATP), tandem‑pore domain K+ (TASK), voltage‑gated K+ (Kv), ether‑à‑go‑go (Eag) and Ca2+‑activated K+ (KCa) channels. Kir channels are important in the maintenance of the resting membrane potential and controlling RGC excitability. KATP channels are involved in RGC survival and neuroprotection. TASK channels are hypothesized to contribute to the regulation of resting membrane potentials and firing patterns of RGCs. Kv channels are important regulators of cellular excitability, functioning to modulate the amplitude, duration and frequency of action potentials and subthreshold depolarizations, and are also important in RGC development and protection. Eag channels may contribute to dendritic repolarization during excitatory postsynaptic potentials and to the attenuation of the back propagation of action potentials. KCa channels have been observed to contribute to repetitive firing in RGCs. Considering these important roles of K+ channels in RGCs, the study of K+ channels may be beneficial in elucidating the pathophysiology of RGCs and exploring novel RGC protection strategies.

Behavioural and functional characterization of Kv10.1 (Eag1) knockout mice

K_v10.1 (Eag1), member of the K_v10 family of voltage-gated potassium channels, is preferentially expressed in adult brain. The aim of the present study was to unravel the functional role of K_v10.1 in the brain by generating knockout mice, where the voltage sensor and pore region of K_v10.1 were removed to render non-functional proteins through deletion of exon 7 of the KCNH1 gene using the ‘3 Lox P strategy’. K_v10.1-deficient mice show no obvious alterations during embryogenesis and develop normally to adulthood; cortex, hippocampus and cerebellum appear anatomically normal. Other tests, including general health screen, sensorimotor functioning and gating, anxiety, social behaviour, learning and memory did not show any functional aberrations in K_v10.1 null mice. K_v10.1 null mice display mild hyperactivity and longer-lasting haloperidol-induced catalepsy, but there was no difference between genotypes in amphetamine sensitization and withdrawal, reactivity to apomorphine and haloperidol in the prepulse inhibition tests or to antidepressants in the haloperidol-induced catalepsy. Furthermore, electrical properties of K_v10.1 in cerebellar Purkinje cells did not show any difference between genotypes. Bearing in mind that K_v10.1 is overexpressed in over 70% of all human tumours and that its inhibition leads to a reduced tumour cell proliferation, the fact that deletion of K_v10.1 does not show a marked phenotype is a prerequisite for utilizing K_v10.1 blocking and/or reduction techniques, such as siRNA, to treat cancer.

Cortactin controls surface expression of the voltage-gated potassium channel K(V)10.1

K_V10.1 is a voltage-gated potassium channel aberrantly expressed in many cases of cancer, and participates in cancer initiation and tumor progression. Its action as an oncoprotein can be inhibited by a functional monoclonal antibody, indicating a role for channels located at the plasma membrane, accessible to the antibody. Cortactin is an actin-interacting protein implicated in cytoskeletal architecture and often amplified in several types of cancer. In this study, we describe a physical and functional interaction between cortactin and K_V10.1. Binding of these two proteins occurs between the C terminus of K_V10.1 and the proline-rich domain of cortactin, regions targeted by many post-translational modifications. This interaction is specific for K_V10.1 and does not occur with K_V10.2. Cortactin controls the abundance of K_V10.1 at the plasma membrane and is required for functional expression of K_V10.1 channels.

Electrophysiological properties and synaptic function of mesenchymal stem cells during neurogenic differentiation - a mini-review

INTRODUCTION

Mesenchymal stem cells (MSCs) have gained considerable interest due to their potential use in cell therapies and tissue engineering. They have been reported to differentiate into various anchorage-dependent cell types, including bone, cartilage, and tendon. Our focus is on the differentiation of MSCs into neuron-like cells through the use of soluble chemical stimuli or specific growth factor supplements. The resulting cells appear to adopt neural phenotypes and express some typical neuronal markers, however, their electrophysiological properties and synaptic function remains unclear.

RESULTS

This mini-review illustrates how particular characteristics, electrophysiological properties, and synaptic functions of MSCs change during their neuronal differentiation. In particular we focus on changes in ion currents, ion channels, synaptic communication, and neurotransmitter release. We also highlight conflicting results, caused by inconsistencies in the experimental conditions used and in the methodologies adopted.

CONCLUSIONS

We conclude that there is insufficient data and that further, carefully controlled investigations are required in order to ascertain whether MSC-derived neuron-like cells can exhibit the necessary neuronal functions to become clinically relevant for use in neural repairs.

Kcnh1 voltage-gated potassium channels are essential for early zebrafish development

The Kcnh1 gene encodes a voltage-gated potassium channel highly expressed in neurons and involved in tumor cell proliferation, yet its physiological roles remain unclear. We have used the zebrafish as a model to analyze Kcnh1 function in vitro and in vivo. We found that the kcnh1 gene is duplicated in teleost fish (i.e. kcnh1a and kcnh1b) and that both genes are maternally expressed during early development. In adult zebrafish, kcnh1a and kcnh1b have distinct expression patterns but share expression in brain and testis. Heterologous expression of both genes in Xenopus oocytes revealed a strong conservation of characteristic functional properties between human and fish channels, including a unique sensitivity to intracellular Ca²⁺/calmodulin and modulation of voltage-dependent gating by extracellular Mg²⁺. Using a morpholino antisense approach, we demonstrate a strong kcnh1 loss-of-function phenotype in developing zebrafish, characterized by growth retardation, delayed hindbrain formation, and embryonic lethality. This late phenotype was preceded by transcriptional up-regulation of known cell-cycle inhibitors (p21, p27, cdh2) and down-regulation of pro-proliferative factors, including cyclin D1, at 70% epiboly. These results reveal an unanticipated basic activity of kcnh1 that is crucial for early embryonic development and patterning.

Statistical epistasis and functional brain imaging support a role of voltage-gated potassium channels in human memory

Despite the current progress in high-throughput, dense genome scans, a major portion of complex traits' heritability still remains unexplained, a phenomenon commonly termed “missing heritability.” The negligence of analytical approaches accounting for gene-gene interaction effects, such as statistical epistasis, is probably central to this phenomenon. Here we performed a comprehensive two-way SNP interaction analysis of human episodic memory, which is a heritable complex trait, and focused on 120 genes known to show differential, memory-related expression patterns in rat hippocampus. Functional magnetic resonance imaging was also used to capture genotype-dependent differences in memory-related brain activity. A significant, episodic memory-related interaction between two markers located in potassium channel genes (KCNB2 and KCNH5) was observed (P_{nominal combined} = 0.000001). The epistatic interaction was robust, as it was significant in a screening (P_nominal = 0.0000012) and in a replication sample (P_nominal = 0.01). Finally, we found genotype-dependent activity differences in the parahippocampal gyrus (P_nominal = 0.001) supporting the behavioral genetics finding. Our results demonstrate the importance of analytical approaches that go beyond single marker statistics of complex traits.

Calmodulin interaction with hEAG1 visualized by FRET microscopy

Background

Ca²⁺-mediated regulation of ion channels provides a link between intracellular signaling pathways and membrane electrical activity. Intracellular Ca²⁺ inhibits the voltage-gated potassium channel EAG1 through the direct binding of calmodulin (CaM). Three CaM binding sites (BD-C1: 674-683, BD-C2: 711-721, BD-N: 151-165) have been identified in a peptide screen and were proposed to mediate binding. The participation of the three sites in CaM binding to the native channel, however, remains unclear.

Methodology/Principal Findings

Here we studied the binding of Ca²⁺/CaM to the EAG channel by visualizing the interaction between YFP-labeled CaM and Cerulean-labeled hEAG1 in mammalian cells by FRET. The results of our cellular approach substantiate that two CaM binding sites are predominantly involved; the high-affinity 1-8-14 based CaM binding domain in the N-terminus and the second C-terminal binding domain BD-C2. Mutations at these sites completely abolished CaM binding to hEAG1.

Conclusions/Significance

We demonstrated that the BD-N and BD-C2 binding domains are sufficient for CaM binding to the native channel, and, therefore, that BD-C1 is unable to bind CaM independently.

Eag1 expression interferes with hypoxia homeostasis and induces angiogenesis in tumors

Ether-á-go-go-1 (Eag1) is a CNS-localized voltage-gated potassium channel that is found ectopically expressed in a majority of extracranial solid tumors. While circumstantial evidence linking Eag1 to tumor biology has been well established, the mechanisms by which the channel contributes to tumor progression remain elusive. In this study, we have used in vivo and in vitro techniques to identify a candidate mechanism. A mutation that eliminates ion permeation fails to completely abolish xenograft tumor formation by transfected cells, indicating that Eag1 contributes to tumor progression independently of its primary function as an ion channel. Our data suggest that Eag1 interferes with the cellular mechanism for maintaining oxygen homeostasis, increasing HIF-1 activity, and thereby VEGF secretion and tumor vascularization.

Intracellular regions of potassium channels: Kv2.1 and heag

Intracellular regions of voltage-gated potassium channels often comprise the largest part of the channel protein, and yet the functional role of these regions is not fully understood. For the Kv2.1 channel, although there are differences in activation kinetics between rat and human channels, there are, for instance, no differences in movement of the S4 region between the two channels, and indeed our mutagenesis studies have identified interacting residues in both the N- and C -terminal intracellular regions that are responsible for these functional effects. Furthermore, using FRET with fluorescent-tagged Kv2.1 channels, we have shown movement of the C-termini relative to the N-termini during activation. Such interactions and movements of the intracellular regions of the channel appear to form part of the channel gating machinery. Heag1 and heag2 channels also display differing activation properties, despite their considerable homology. By a chimeric approach, we have shown that these differences in activation kinetics are determined by multiple interacting regions in the N-terminus and membrane-spanning regions. Furthermore, alanine mutations of many residues in the C-terminal cyclic nucleotide binding domain affect activation kinetics. The data again suggest interacting regions between N- and C- termini that participate in the conformational changes during channel activation. Using a mass-spectrometry approach, we have identified alpha-tubulin and a heat shock protein as binding to the C-terminus of the heag2 channel, and alpha-tubulin itself has functional effects on channel activation kinetics. Clearly, the intracellular regions of these ion channels (and most likely many other ion channels too) are important regions in determining channel function.

Differential localization of rat Eag1 and Eag2 potassium channels in the retina

Despite of their wide expression in the brain, the precise neurophysiological role of rat Eag1 (rEag1) and Eag2 (rEag2) K(+) channels remains elusive. Our previous studies in hippocampal pyramidal neurons demonstrate a somatodendritic localization of rEag1 and rEag2 channels, suggesting that the two channel isoforms may contribute to setting the membrane excitability of somas and dendrites. Here, we aim to further characterize the cellular and subcellular localization patterns of rEag1 and rEag2 proteins by studying their laminar distribution in the retina. Confocal microscopic analyses of immunofluorescence data revealed that rEag1 and rEag2 K(+) channels exhibit distinct cellular expression pattern in the retina. rEag1 immunoreactivity was most prominent in the outer half of the inner plexiform layer, whereas strong rEag2 immunostain was found in the outer and inner segments of photoreceptor cells, the outer plexiform layer, and the inner nuclear layer. These results suggest that rEag1 and rEag2 K(+) channels may play a significant role in the transmission of electrical signals along the retinal neuronal circuits. We also performed double-labeling experiments to demonstrate that rEag1 and rEag2 are predominantly expressed in the somatodendritic compartment of retinal neurons. In addition, we presented evidence suggesting that rEag1 channels may be expressed in the GABAergic amacrine cell. Finally, based on their different immunostaining patterns over the inner region of the retina, we propose that compared to rEag2, rEag1 expression encompasses a significantly broader range of the somatodendritic compartment of the retinal ganglion cell.

Ether à go-go potassium channel and malignant tumors

Recently, increasing evidence from cell biology and pharmacology demonstrates that cancer cells exhibit ion channel expression patterns, ion conductances and electric properties that are very different from those of resting cells. These peculiar properties are functionally involved in cancer pathogenesis. In particular, because of its oncogenic properties, distribution, modulation and pharmacology, human ether à go-go potassium channel (Eag1, K_v10.1, KCNH1) is considered a critical ion channel-encoding gene involved in the establishment and maintenance of neoplastic growth. This review summarizes most of the findings regarding Eag1 channels and malignant tumors, focusing on cellular mechanisms, mRNA and protein expression in tissues, oncogenic properties, modulation and pharmacology.

Monoclonal antibody blockade of the human Eag1 potassium channel function exerts antitumor activity

The potassium channel ether à go-go has been directly linked to cellular proliferation and transformation, although its physiologic role(s) are as of yet unknown. The specific blockade of human Eag1 (hEag1) may not only allow the dissection of the role of the channel in distinct physiologic processes, but because of the implication of hEag1 in tumor biology, it may also offer an opportunity for the treatment of cancer. However, members of the potassium channel superfamily are structurally very similar to one another, and it has been notoriously difficult to obtain specific blockers for any given channel. Here, we describe and validate the first rational design of a monoclonal antibody that selectively inhibits a potassium current in intact cells. Specifically blocking hEag1 function using this antibody inhibits tumor cell growth both in vitro and in vivo. Our data provide a proof of concept that enables the generation of functional antagonistic monoclonal antibodies against ion channels with therapeutic potential. The particular antibody described here, as well as the technique developed to make additional functional antibodies to Eag1, makes it possible to evaluate the potential of the channel as a target for cancer therapy.

Activation of hEAG1 potassium channels by arachidonic acid

The depolarisation activated human ether à go-go (hEAG) potassium channels are primarily expressed in neuronal tissue but their appearance in various tumour entities is also indicative of an oncogenic role. Because upregulation of hEAG channels may yield to an enhanced cell proliferation, interventions increasing hEAG1 currents may serve similar purposes. We therefore investigated the effects of polyunsaturated fatty acids on hEAG1 channels. Arachidonic acid (AA) lowered their activation threshold, accelerated the activation kinetics and increased the open probability with a half-maximal concentration of about 4 microM. This effect correlated with the number of double bonds (db) in the fatty acids, increasing from oleic acid (1 db), linolenic acid (3 db), AA (4 db) to eicosapentaenoic acid (5 db). Unlike other voltage-gated K(+) channels, hEAG1 channels are not blocked by arachidonic acid. Therefore, in particular at typical resting potentials of tumour cells (-30 mV), AA potently activated hEAG1 channels in a reversible manner. Proliferation and metabolic activity of hEAG1-expressing human melanoma cells increased when cells were exposed to AA concentrations of 5 microM and this effect was suppressed in the presence of the hEAG1 blocker LY97241 suggesting that the proliferative effect of AA is in part mediated by activation of hEAG channels.

Neural differentiation of human mesenchymal stem cells: evidence for expression of neural markers and eag K+ channel types

OBJECTIVE

Mesenchymal stem cells (MSCs) are multipotent cells that can self-renew, proliferate, and exhibit elevated cellular plasticity. To investigate their possible neural fate, we studied human mesenchymal stem cells (hMSCs) in different cell culture conditions from morphological, immunochemical, gene expression, and physiological points of view.

MATERIALS AND METHODS

We tested hMSCs in three previously reported experimental conditions made of alpha-modified minimum essential medium (alpha-MEM)/1 mM beta-mercaptoethanol (betaME), 10 microM alpha-MEM/retinoic acid (RA) or alpha-MEM/2% dimethylsulfoxide (DMSO) + 200 microM beta-hydroxyanisole (BHA), respectively, and in a new experimental condition with neural progenitor maintenance medium (NPMM).

RESULTS

hMSCs were isolated from bone marrow and expanded for several passages. In betaME, cells became immunoreactive for neuronal nuclear antigen (NeuN), neuron-specific enolase (NSE), Nestin, and glial fibrillary acidic protein (GFAP). In experimental conditions with RA and DMSO/BHA, hMSCs were NeuN and NSE-positive while in NPMM they were positive for GFAP and NSE. Untreated hMSCs showed a weak mRNA expression for microtubule-associated protein, NSE, and neurofilament protein-medium and GFAP, which strongly increased in NPMM-treated hMSCs. In the electrophysiological study, NPMM-differentiated hMSCs expressed two delayed rectifier K+ currents related to two ether-à-go-go K+ channels (eag1, eag2), which are fundamental for setting the negative resting potentials required for neuronal survival and basal cell activity. The two K+ channels were absent in undifferentiated hMSCs. These data were confirmed by real-time polymerase chain reaction.

CONCLUSION

In our new culture condition, hMSCs acquired new morphological characteristics, neural markers, and electrophysiological properties, which are suggestive of neural differentiation. This might lead to clinical use of hMSCs in neural degenerative diseases.

Ethanol inhibition of m-current and ethanol-induced direct excitation of ventral tegmental area dopamine neurons

Ethanol-induced excitation of ventral tegmental area dopamine (DA VTA) neurons is thought to be critical for the reinforcing effects of ethanol. Although ligand-gated ion channels are known to be the targets of ethanol, ethanol modulation of voltage-dependent ion channels of central neurons has not been well studied. We have demonstrated that ethanol excites DA VTA neurons by the reduction of sustained K(+) currents and recently reported that M-current (I(M)) regulates action potential generation through fast and slow afterhyperpolarization phases. In the present study we thus examined whether ethanol inhibition of I(M) contributes to the excitation of DA VTA neurons using nystatin-perforated patch current- and voltage-clamp recordings. Ethanol (20-120 mM) reduced I(M) in a concentration-dependent manner and increased the spontaneous firing frequency of DA VTA neurons. Ethanol-induced increase in spontaneous firing frequency correlated positively with ethanol inhibition of I(M) with a slope value of 1.3. Specific I(M) inhibition by XE991 (0.3-10 microM) increased spontaneous firing frequency which correlated positively with I(M) inhibition with a slope value of 0.5. In the presence of 10 muM XE991, a concentration that produced maximal inhibition of I(M), ethanol still increased the spontaneous firing frequency of DA VTA neurons in a concentration-dependent manner. Thus we conclude that, although ethanol causes inhibition of I(M) and this results in some increase in the firing frequency of DA VTA neurons, another effect of ethanol is primarily responsible for the ethanol-induced increase in firing rate in these neurons.

Molecular regions responsible for differences in activation between heag channels

The ether-a-go-go potassium channels heag1 and heag2 are highly homologous; however, the activation properties between the two channels are different. We have studied the molecular regions that determine differences in activation properties by making chimeras between the two channels, expressing them in oocytes, and recording currents with two-electrode voltage-clamp. The activation time course has an initial sigmoidal component dependent on the Cole-Moore shift, followed by a faster component. We show that not only is the extreme N terminus involved in differences between heag1 and heag2 channels, but also the PAS domain itself. Also multiple regions of the membrane-spanning part of the channel appear to be involved, with different regions involved for the early and late time courses, reflecting their different mechanisms. The later time course involved S1 and P-S6 regions. Taken together, our data show that activation involves multiple regions of the N terminal region and membrane-spanning regions of the channel.

Role of voltage-gated potassium channels in cancer

Ion channels are being associated with a growing number of diseases including cancer. This overview summarizes data on voltage-gated potassium channels (VGKCs) that exhibit oncogenic properties: ether-à-go-go type 1 (Eag1). Normally, Eag1 is expressed almost exclusively in tissue of neural origin, but its ectopic expression leads to uncontrolled proliferation, while inhibition of Eag1 expression produces a concomitant reduction in proliferation. Specific monoclonal antibodies against Eag1 recognize an epitope in over 80% of human tumors of diverse origins, endowing it with diagnostic and therapeutic potential. Eag1 also possesses unique electrophysiological properties that simplify its identification. This is particularly important, as specific blockers of Eag1 currents are not available. Molecular imaging of Eag1 in live tumor models has been accomplished with dye-tagged antibodies using 3-D imaging techniques in the near-infrared spectral range.

Voltage-gated potassium channels in cell proliferation

It is commonly accepted that cells require K(+) channels to proliferate. The role(s) of K(+) channels in the process is, however, poorly understood. Cloning of K(+) channel genes opened the possibility to approach this problem in a way more independent from pharmacological tools. Recent work shows that several identified K(+) channels are important in both physiological and pathological cell proliferation and open a promising pathway for novel targeted therapies.

Molecular determinants for high-affinity block of human EAG potassium channels by antiarrhythmic agents

Undesired block of human ERG1 potassium channels is the basis for cardiac side effects of many different types of drugs. Therefore, it is important to know exactly why some drugs particularly bind to these channels with high affinity. Upon expression in mammalian cells and Xenopus laevis oocytes, we investigated the inhibition of the closely related hEAG1 and hEAG2 channels by agents that have previously been reported to block hERG1 channels. Clofilium inhibited hEAG1 and hERG1 with the same potency, whereas hEAG2 was about 150-fold less sensitive to this antiarrhythmic agent. The molecular determinants for this difference are residues Ser436 and Val437 in the inner cavity of the pore and Ala453, which is located in S6 (i.e., remote from the inner cavity). A modeling approach that allowed for partial conformational relaxation of hEAG model structures upon ligand docking suggests that high-affinity block of ether à go-go channels is mediated by an anchoring of the clofilium alkane tail between S6 and the pore helices. In qualitative agreement with experiments, the mutations of hEAG1 residues Ser436 and Val437 to the corresponding larger hEAG2 residues (Thr432, Ile433) resulted in reduced sterical fit between the ligand and the binding cavity. The model is further supported by functional assays involving (+)-N-[1'-(6-cyano-1,2,3,4-tetrahydro-2(R)-naphthalenyl)-3,4-dihydro-4(R)-hydroxyspiro(2H-1-benzopyran-2,4'-piperidin)-6-yl]methanesulfonamide monohydrochloride (MK-499), terfenadine, quinidine, and tetrabutylammonium that are differentially affected by mutations in the pore pocket.

Ca(2+)-inactivated K+ current is modulated by endothelin-1 in B-16 murine melanoma cells

It is well established that endothelin-1 (ET-1) plays a role in differentiation and proliferation in a variety of cells such as fibroblasts and human melanoma cells via a receptor-mediated mechanism. However, whether ET-1 modulates ion channel activity in these cell types is still unknown. In this report, we recorded the voltage-dependent outward K+ current in cultured B16 melanoma cells using the patch-clamp technique. Biophysical and pharmacological properties of the K+ current, and the effect of ET-1 on the K+ current were investigated. When cells were loaded with a Ca(2+)-chelating agent (EGTA or BAPTA), the K+ current amplitude gradually increased with time after establishment of the whole cell configuration. Replacement of Ca2+ with Co2+ in the extracellular medium caused no significant modulation of the K+ current amplitude. Addition of BaCl2 or quinidine to the extracellular solution reduced the K+ current amplitude, whereas the K+ current was insensitive to tetraethylammonium. ET-1 (10 nM) reversibly decreased the K+ current amplitude and accelerated the decay of the K+ current. The ET-1-induced inhibitory effect displayed no desensitization following repeated ET-1 application. Pretreatment with pertussis toxin (PTX) or perfusion of cells with the protein kinase C (PKC) inhibitor H-7 abolished the inhibitory effect of ET-1 on the K+ current. We conclude that the outward K+ current recorded in murine B-16 melanoma cells represents a Ca(2+)-inactivated K+ current, and that the inhibitory effect of ET-1 on the K+ current may reveal a novel mechanism to control the differentiation and proliferation of melanoma cells.

Ethanol excitation of dopaminergic ventral tegmental area neurons is blocked by quinidine

The dopaminergic (DA) neurons in the ventral tegmental area (VTA) are important for the reinforcing effects of ethanol. We have shown that ethanol directly excites DA VTA neurons and reduces the afterhyperpolarization (AHP) that follows spontaneous action potentials in these neurons. These data suggested that ethanol may be increasing the firing rate of DA VTA neurons by modulating currents that contribute to the AHP, either by reducing a K+ current or by increasing the inward current Ih. In the present study, different blockers of K+ channels and Ih were tested to determine whether any could prevent the ethanol excitation of DA VTA neurons. Extracellular single-unit recordings and whole-cell patch-clamp recordings were made from DA VTA neurons in brain slices from Fischer-344 rats and ethanol (40-120 mM) and channel blockers were applied in the bath. Ethanol excitation was not reduced by blockade of Ih with cesium (5 mM) or ZD7288 (30 microM), or by block of G-protein-coupled inwardly rectifying K+ channels with barium (500 microM). Tetraethylammonium (TEA) ion (2-10 mM), which blocks the large conductance calcium-dependent potassium K+ current and some types of delayed rectifier currents, had no effect on the ethanol-induced excitation. Interestingly, ethanol excitation of DA VTA neurons was blocked by quinidine (20-80 microM), a drug that blocks many types of delayed rectifier K+ channels, including some insensitive to TEA. This effect of quinidine was concentration-dependent and reversible. These results suggest that ethanol excites DA VTA neurons by reducing a quinidine-sensitive K+ current.

Inhibition of hEAG1 and hERG1 potassium channels by clofilium and its tertiary analogue LY97241

1 We investigated the inhibition of hEAG1 potassium channels, expressed in mammalian cells and Xenopus oocytes, by several blockers that have previously been reported to be blockers of hERG1 channels. 2 In the whole-cell mode of mammalian cells, LY97241 was shown to be a potent inhibitor of both hEAG1 and hERG1 channels (IC(50) of 4.9 and 2.2 nM, respectively). Clofilium, E4031, and haloperidol apparently inhibited hEAG1 channels with lower potency than hERG1 channels, but they cannot be considered hERG1-specific. 3 The block of hEAG1 channels by LY97241 and clofilium was time-, use-, and voltage-dependent, best explained by an open-channel block mechanism. 4 Both drugs apparently bind from the intracellular side of the membrane at (a) specific site(s) within the central cavity of the channel pore. They can be trapped by closure of the activation gate. 5 In inside-out patches from Xenopus oocytes, hEAG1 block by clofilium was stronger than by LY97241 (IC(50) of 0.8 and 1.9 nM, respectively). In addition, hEAG1 block by clofilium was much faster than by LY97241 although there was no difference in the voltage dependence of the on-rate of block. 6 Physico-chemical differences of clofilium and the weak base LY97241 determine the access of the drugs to the binding site and thereby the influence of the recording mode on the apparent block potencies. This phenomenon must be considered when assessing the inhibitory action of drugs on ion channels.