Inhibition of human ether à go-go potassium channels by Ca(2+)/calmodulin

The Role of the Swollen State in Cell Proliferation

Cell swelling is known to be involved in various stages of the growth of plant cells and microorganisms but in mammalian cells how crucial a swollen state is for determining the fate of the cellular proliferation remains unclear. Recent evidence has increased our understanding of how the loss of the cell surface interactions with the extracellular matrix at early mitosis decreases the membrane tension triggering curvature changes in the plasma membrane and the activation of the sodium/hydrogen (Na +/H +) exchanger (NHE1) that drives osmotic swelling. Such a swollen state is temporary, but it is critical to alter essential membrane biophysical parameters that are required to activate Ca2 + channels and modulate the opening of K + channels involved in setting the membrane potential. A decreased membrane potential across the mitotic cell membrane enhances the clustering of Ras proteins involved in the Ca2 + and cytoskeleton-driven events that lead to cell rounding. Changes in the external mechanical and osmotic forces also have an impact on the lipid composition of the plasma membrane during mitosis.

Fentanyl blockade of K+ channels contribute to Wooden Chest Syndrome

Fentanyl is a potent synthetic opioid widely used perioperatively and illicitly as a drug of abuse 1,2. It is well established that fentanyl acts as a μ-opioid receptor agonist, signaling through Gαi/o intracellular pathways to inhibit electrical excitability, resulting in analgesia and respiratory depression 3,4. However, fentanyl uniquely also triggers muscle rigidity, including respiratory muscles, hindering the ability to execute central respiratory commands or to receive external resuscitation. This potentially lethal condition is termed Wooden Chest Syndrome (WCS), the mechanisms of which are poorly understood 5-7. Here we show that fentanyl directly blocks a subset of EAG-class potassium channels 8. Our results also demonstrate that these channels are widely expressed in cervical spinal motoneurons, including those innervating the diaphragm. A significant fraction of these motoneurons is excited by fentanyl, concomitant with blockade of voltage-dependent non-inactivating K+ currents. In vivo electromyography revealed a persistent tonic component of diaphragmatic muscle activity elicited by fentanyl, but not morphine. Taken together our results identify a novel off-target mechanism for fentanyl action, independent of μ-opioid receptor activation, with a paradoxical excitatory effect that may underlie WCS. We anticipate these findings may inform the design of safer analgesics and generalize to other neuronal circuits implicated in fentanyl-related maladaptive behaviors.

Calmodulin binding is required for calcium mediated TRPA1 desensitization

Calcium (Ca2+) ions affect nearly all aspects of biology. Excessive Ca2+ entry is cytotoxic and Ca2+-mobilizing receptors have evolved diverse mechanisms for tight regulation that often include Calmodulin (CaM). TRPA1, an essential Ca2+-permeable ion channel involved in pain signaling and inflammation, exhibits complex Ca2+ regulation with initial channel potentiation followed by rapid desensitization. The molecular mechanisms of TRPA1 Ca2+ regulation and whether CaM plays a role remain elusive. We find that TRPA1 binds CaM best at basal Ca2+ concentration, that they co-localize in resting cells, and that CaM suppresses TRPA1 activity. Combining biochemical, biophysical, modeling, NMR spectroscopy, and functional approaches, we identify an evolutionarily conserved, high-affinity CaM binding element in the distal TRPA1 C-terminus (DCTCaMBE). Genetic or biochemical perturbation of Ca2+/CaM binding to the TRPA1 DCTCaMBE yields hyperactive channels that exhibit drastic slowing of desensitization with no effect on potentiation. Ca2+/CaM TRPA1 regulation does not require the N-lobe, raising the possibility that CaM is not the Ca2+ sensor, per se. Higher extracellular Ca2+ can partially rescue slowed desensitization suggesting Ca2+/CaM binding to the TRPA1 DCTCaMBE primes an intrinsic TRPA1 Ca2+ binding site that, upon binding Ca2+, triggers rapid desensitization. Collectively, our results identify a critical regulatory element in an unstructured TRPA1 region highlighting the importance of these domains, they reveal Ca2+/CaM is an essential TRPA1 auxiliary subunit required for rapid desensitization that establishes proper channel function with implications for all future TRPA1 work, and they uncover a mechanism for receptor regulation by Ca2+/CaM that expands the scope of CaM biology.

Revealing a hidden conducting state by manipulating the intracellular domains in KV10.1 exposes the coupling between two gating mechanisms

The KCNH family of potassium channels serves relevant physiological functions in both excitable and non-excitable cells, reflected in the massive consequences of mutations or pharmacological manipulation of their function. This group of channels shares structural homology with other voltage-gated K+ channels, but the mechanisms of gating in this family show significant differences with respect to the canonical electromechanical coupling in these molecules. In particular, the large intracellular domains of KCNH channels play a crucial role in gating that is still only partly understood. Using KCNH1(KV10.1) as a model, we have characterized the behavior of a series of modified channels that could not be explained by the current models. With electrophysiological and biochemical methods combined with mathematical modeling, we show that the uncovering of an open state can explain the behavior of the mutants. This open state, which is not detectable in wild-type channels, appears to lack the rapid flicker block of the conventional open state. Because it is accessed from deep closed states, it elucidates intermediate gating events well ahead of channel opening in the wild type. This allowed us to study gating steps prior to opening, which, for example, explain the mechanism of gating inhibition by Ca2+-Calmodulin and generate a model that describes the characteristic features of KCNH channels gating.

Voltage-gated ion channels are expressed in the Malpighian tubules and anal papillae of the yellow fever mosquito (Aedes aegypti), and may regulate ion transport during salt and water imbalance

Vectors of infectious disease include several species of Aedes mosquitoes. The life cycle of Aedes aegypti, the yellow fever mosquito, consists of a terrestrial adult and an aquatic larval life stage. Developing in coastal waters can expose larvae to fluctuating salinity, causing salt and water imbalance, which is addressed by two prime osmoregulatory organs - the Malpighian tubules (MTs) and anal papillae (AP). Voltage-gated ion channels (VGICs) have recently been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects. In the current study, we: (i) generated MT transcriptomes of freshwater-acclimated and brackish water-exposed larvae of Ae. aegypti, (ii) detected expression of several voltage-gated Ca2+, K+, Na+ and non-ion-selective ion channels in the MTs and AP using transcriptomics, PCR and gel electrophoresis, (iii) demonstrated that mRNA abundance of many altered significantly following brackish water exposure, and (iv) immunolocalized CaV1, NALCN, TRP/Painless and KCNH8 in the MTs and AP of larvae using custom-made antibodies. We found CaV1 to be expressed in the apical membrane of MTs of both larvae and adults, and its inhibition to alter membrane potentials of this osmoregulatory epithelium. Our data demonstrate that multiple VGICs are expressed in osmoregulatory epithelia of Ae. aegypti and may play an important role in the autonomous regulation of ion transport.

Voltage-sensor movements in the Eag Kv channel under an applied electric field

Voltage-dependent ion channels regulate the opening of their pores by sensing the membrane voltage. This process underlies the propagation of action potentials and other forms of electrical activity in cells. The voltage dependence of these channels is governed by the transmembrane displacement of the positive charged S4 helix within their voltage-sensor domains. We use cryo-electron microscopy to visualize this movement in the mammalian Eag voltage-dependent potassium channel in lipid membrane vesicles with a voltage difference across the membrane. Multiple structural configurations show that the applied electric field displaces S4 toward the cytoplasm by two helical turns, resulting in an extended interfacial helix near the inner membrane leaflet. The position of S4 in this down conformation is sterically incompatible with an open pore, thus explaining how movement of the voltage sensor at hyperpolarizing membrane voltages locks the pore shut in this kind of voltage-dependent K⁺ (K_v) channel. The structures solved in lipid bilayer vesicles detail the intricate interplay between K_v channels and membranes, from showing how arginines are stabilized deep within the membrane and near phospholipid headgroups, to demonstrating how the channel reshapes the inner leaflet of the membrane itself.

Intracellular hemin is a potent inhibitor of the voltage-gated potassium channel Kv10.1

Heme, an iron-protoporphyrin IX complex, is a cofactor bound to various hemoproteins and supports a broad range of functions, such as electron transfer, oxygen transport, signal transduction, and drug metabolism. In recent years, there has been a growing recognition of heme as a non-genomic modulator of ion channel functions. Here, we show that intracellular free heme and hemin modulate human ether à go-go (hEAG1, Kv10.1) voltage-gated potassium channels. Application of hemin to the intracellular side potently inhibits Kv10.1 channels with an IC₅₀ of about 4 nM under ambient and 63 nM under reducing conditions in a weakly voltage-dependent manner, favoring inhibition at resting potential. Functional studies on channel mutants and biochemical analysis of synthetic and recombinant channel fragments identified a heme-binding motif CxHx₈H in the C-linker region of the Kv10.1 C terminus, with cysteine 541 and histidines 543 and 552 being important for hemin binding. Binding of hemin to the C linker may induce a conformational constraint that interferes with channel gating. Our results demonstrate that heme and hemin are endogenous modulators of Kv10.1 channels and could be exploited to modulate Kv10.1-mediated cellular functions.

Blending physiology and RNAseq to provide new insights into regulation of epithelial transport: switching between ion secretion and reabsorption

This Review addresses the means by which epithelia change the direction of vectorial ion transport. Recent studies have revealed that insect Malpighian (renal) tubules can switch from secreting to reabsorbing K+. When the gut of larval lepidopterans is empty (during the moult cycle) or when the larvae are reared on K+-deficient diet, the distal ileac plexus segment of the tubule secretes K+ from the haemolymph into the tubule lumen. By contrast, in larvae reared on K+-rich diet, ions and fluid are reabsorbed from the rectal lumen into the perinephric space surrounding the cryptonephridial tubules of the rectal complex. Ions and fluid are then transported from the perinephric space into the lumen of the cryptonephridial tubules, thus supplying the free segments of the tubule downstream. Under these conditions, some of the K+ and water in the tubule lumen is reabsorbed across the cells of the distal ileac plexus, allowing for expansion of haemolymph volume in the rapidly growing larvae, as well as recycling of K+ and base equivalents. RNA sequencing data reveal large-scale changes in gene transcription that are associated with the switch between ion secretion and ion reabsorption by the distal ileac plexus. An unexpected finding is the presence of voltage-gated, ligand-gated and mechanosensitive ion channels, normally seen in excitable cells, in Malpighian tubules. Transcriptomic surveys indicate that these types of channels are also present in multiple other types of vertebrate and invertebrate epithelia, suggesting that they may play novel roles in epithelial cell signalling and regulation of epithelial ion transport.

Monitoring of compound resting membrane potentials of cell cultures with ratiometric genetically encoded voltage indicators

The cellular resting membrane potential (V_m) not only determines electrical responsiveness of excitable cells but also plays pivotal roles in non-excitable cells, mediating membrane transport, cell-cycle progression, and tumorigenesis. Studying these processes requires estimation of V_m, ideally over long periods of time. Here, we introduce two ratiometric genetically encoded V_m indicators, rArc and rASAP, and imaging and analysis procedures for measuring differences in average resting V_m between cell groups. We investigated the influence of ectopic expression of K⁺ channels and their disease-causing mutations involved in Andersen-Tawil (Kir2.1) and Temple-Baraitser (K_V10.1) syndrome on median resting V_m of HEK293T cells. Real-time long-term monitoring of V_m changes allowed to estimate a 40–50 min latency from induction of transcription to functional Kir2.1 channels in HEK293T cells. The presented methodology is readily implemented with standard fluorescence microscopes and offers deeper insights into the role of the resting V_m in health and disease.

Rühl et al. report the generation of ratiometric genetically encoded voltage indicators (GEVIs) and establish that they can be used in high-throughput automated imaging to measure compound membrane potential (Vm) in mammalian cells. This method is implementable with standard fluorescence microscopes and has the potential to offer insights into the role of the resting Vm in health and disease.

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.

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.

Voltage-gated calcium channels regulate K+ transport in the Malpighian tubules of the larval cabbage looper, Trichoplusia ni

Transporting epithelia are tissues that specialize in the directional movements of ions and water and are typically either secretory or reabsorptive. Recent work on the Malpighian tubule of larval lepidopterans (caterpillars) demonstrated that the distal ileac plexus segment of this epithelium is capable of rapidly switching between ion secretion and reabsorption. Subsequent transcriptomic studies suggested expression of voltage-gated ion channels in the lepidopteran MTs (which are not contractile and not innervated). The present study shows that isolated MTs of larval Trichoplusia ni express α1, β2, and α2δ4 subunits of voltage-gated Ca²⁺ channel Ca_V1 and that pan-Ca_Vα immunoreactivity is present in the apical and basolateral membranes of the principal cells. Basolateral membrane potential (V_bl) in isolated MTs of larval Trichoplusia ni was influenced by Ca_V1 functioning; pharmacological inhibition of Ca_V1 reversed V_bl from inside-negative to inside-positive, and also reduced transepithelial potential (V_te), lowered [Ca²⁺]_i and reversed the direction of K⁺ transport from secretion to reabsorption. Thus, our findings indicate that a functional Ca_V1 channel is necessary for constitutive K⁺ secretion observed in isolated preparations of lepidopteran MTs. Lastly, V_te and V_bl of isolated MTs were influenced by changes in bathing saline [K⁺]. Our findings suggest that epithelia may rely on Ca_V channels to enable robust ion secretion and downregulation of Ca_V channels, together with other transcriptional changes, enables ion reabsorption.

Dynamic Na+/H+ exchanger 1 (NHE1) - calmodulin complexes of varying stoichiometry and structure regulate Ca2+-dependent NHE1 activation

Calmodulin (CaM) engages in Ca²⁺-dependent interactions with numerous proteins, including a still incompletely understood physical and functional interaction with the human Na⁺/H⁺-exchanger NHE1. Using nuclear magnetic resonance (NMR) spectroscopy, isothermal titration calorimetry, and fibroblasts stably expressing wildtype and mutant NHE1, we discovered multiple accessible states of this functionally important complex existing in different NHE1:CaM stoichiometries and structures. We determined the NMR solution structure of a ternary complex in which CaM links two NHE1 cytosolic tails. In vitro, stoichiometries and affinities could be tuned by variations in NHE1:CaM ratio and calcium ([Ca²⁺]) and by phosphorylation of S648 in the first CaM-binding α-helix. In cells, Ca²⁺-CaM-induced NHE1 activity was reduced by mimicking S648 phosphorylation and by mutation of the first CaM-binding α-helix, whereas it was unaffected by inhibition of Akt, one of several kinases phosphorylating S648. Our results demonstrate a diversity of NHE1:CaM interaction modes and suggest that CaM may contribute to NHE1 dimerization and thereby augment NHE1 regulation. We propose that a similar structural diversity is of relevance to many other CaM complexes.

Novel KCNH1 Mutations Associated with Epilepsy: Broadening the Phenotypic Spectrum of KCNH1-Associated Diseases

Here, we describe four patients suffering from a rather broad spectrum of epilepsy-related disorders, ranging from developmental and epileptic encephalopathy with intellectual disability (DEE) to genetic generalized epilepsy (GGE), which all harbor novel KCNH1 mutations. In one family, we found a weak association of a novel nonsense mutation with epilepsy, suggesting reduced penetrance, and which shows, in agreement with previous findings, that gain-of-function effects rather than haploinsufficiency are important for the pathogenicity of mutations. De novo missense variants in the pore region of the channel result in severe phenotypes presenting usually with DEE with various malformations. The potential pathogenicity of a novel KCNH1 germline mutation located outside of the critical pore domain observed in a GGE patient with a milder phenotype is supported by the fact that the very same amino acid exchange was detected as a somatic mutation in the resected brain tissue of a patient suffering from a focal cortical dysplasia type IIb. Thus, our case series broadens the phenotypic spectrum of KCNH1-associated diseases.

Kv10.1 Regulates Microtubule Dynamics during Mitosis

Kv10.1 (potassium voltage-gated channel subfamily H member 1, known as EAG1 or Ether-à-go-go 1), is a voltage-gated potassium channel, prevailingly expressed in the central nervous system. The aberrant expression of Kv10.1 is detected in over 70% of all human tumor tissues and correlates with poorer prognosis. In peripheral tissues, Kv10.1 is expressed almost exclusively during the G2/M phase of the cell cycle and regulates its progression-downregulation of Kv10.1 extends the duration of the G2/M phase both in cancer and healthy cells. Here, using biochemical and imaging techniques, such as live-cell measurements of microtubule growth and of cytosolic calcium, we elucidate the mechanisms of Kv10.1-mediated regulation at the G2/M phase. We show that Kv10.1 has a dual effect on mitotic microtubule dynamics. Through the functional interaction with ORAI1 (calcium release-activated calcium channel protein 1), it modulates cytosolic calcium oscillations, thereby changing microtubule behavior. The inhibition of either Kv10.1 or ORAI1 stabilizes the microtubules. In contrast, the knockdown of Kv10.1 increases the dynamicity of mitotic microtubules, resulting in a stronger spindle assembly checkpoint, greater mitotic spindle angle, and a decrease in lagging chromosomes. Understanding of Kv10.1-mediated modulation of the microtubule architecture will help to comprehend how cancer tissue benefits from the presence of Kv10.1, and thereby increase the efficacy and safety of Kv10.1-directed therapeutic strategies.

Potassium and Calcium Channel Complexes as Novel Targets for Cancer Research

The intracellular Ca²⁺ concentration is mainly controlled by Ca²⁺ channels. These channels form complexes with K⁺ channels, which function to amplify Ca²⁺ flux. In cancer cells, voltage-gated/voltage-dependent Ca²⁺ channels and non-voltage-gated/voltage-independent Ca²⁺ channels have been reported to interact with K⁺ channels such as Ca²⁺-activated K⁺ channels and voltage-gated K⁺ channels. These channels are activated by an increase in cytosolic Ca²⁺ concentration or by membrane depolarization, which induces membrane hyperpolarization, increasing the driving force for Ca²⁺ flux. These complexes, composed of K⁺ and Ca²⁺ channels, are regulated by several molecules including lipids (ether lipids and cholesterol), proteins (e.g. STIM), receptors (e.g. S1R/SIGMAR1), and peptides (e.g. LL-37) and can be targeted by monoclonal antibodies, making them novel targets for cancer research.

Membrane Transporters and Channels in Melanoma

Recent research has revealed that ion channels and transporters can be important players in tumor development, progression, and therapy resistance in melanoma. For example, members of the ABC family were shown to support cancer stemness-like features in melanoma cells, while several members of the TRP channel family were reported to act as tumor suppressors.Also, many transporter proteins support tumor cell viability and thus suppress apoptosis induction by anticancer therapy. Due to the high number of ion channels and transporters and the resulting high complexity of the field, progress in understanding is often focused on single molecules and is in total rather slow. In this review, we aim at giving an overview about a broad subset of ion transporters, also illustrating some aspects of the field, which have not been addressed in detail in melanoma. In context with the other chapters in this special issue on "Transportome Malfunctions in the Cancer Spectrum," a comparison between melanoma and these tumors will be possible.

Procyanidin B1, a novel and specific inhibitor of Kv10.1 channel, suppresses the evolution of hepatoma

Recently, we and other groups revealed that aberrant expression of Kv10.1 channel, a voltage-gated potassium ion channel, contributes to a variety of tumorigenesis process.Potent and selective inhibitor of Kv10.1 is urgently needed, both as pharmacological tools for studying the physiological functions of this enigmatic channel and as potential leads for development of anti-tumor drugs. In this study, Procyanidin B1, a natural compound extracted from the grape seed, was identified as a potent, specific inhibitor, which can inhibit the Kv10.1 channel in a concentration-dependent manner (IC₅₀ = 10.38 ± 0.87 μM), but has negligible effects on other potassium channels, including Kir2.1, HERG or KCNQ1. It was demonstrated that Procyanidin B1 directly binds to Kv10.1 channel and inhibits its currents, without increasing intracellular Ca²⁺. Further, three amino acids, I550, T552, and Q557 in the C-linker domain of Kv10.1 were found critical for forming the binding pocket of Procyanidin B1 with Kv10.1 channel.In addition, Procyanidin B1 inhibits migration and proliferation of liver cancer cells (HuH-7 cells, HepG2 cells) through inhibiting Kv10.1, but not in Kv10.1 negatively expressed cell lines. Next, we assayed the tumor suppressing effect of Procyanidin B1 on cell line-derived xenograft mouse model. Our data showed that 15 mg/kg Procyanidin B1 can significantly suppress the growth of the tumor (HepG2) with an inhibition rate of about 60.25%. Compared with cisplatin, Procyanidin B1 has no side effect on the normal metabolismof the mice. The present work indicated that Procyanidin B1 is a proming liver cancer anti-tumor drug, and also confirmed that Kv10.1 can serve as a potential, tumor-specific drug target.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

hERG Function in Light of Structure

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

A novel anti-cancer mechanism of Nutlin-3 through downregulation of Eag1 channel and PI3K/AKT pathway

Nutlin-3 shows a potent antitumor efficacy through downregulation of the cancerogenic ether à go-go 1 (Eag1) channel. However, the molecular mechanisms responsible for the regulation of Eag1 by Nutlin-3 in cancer cells remain unclear. In this study, we propose a novel anticancer mechanism of Nutlin-3, in which Nutlin-3 acts through the p53-Eag1-PI3K/AKT pathway. We first confirmed that Eag1 was downregulated through the activation of p53 by Nutlin-3. We then revealed that the inhibition of Eag1 electrophysiological function resulted in the decrease of viability, migration and invasion of HeLa cells. It is worth noting that the antitumor effect of Nutlin-3 was abolished in the Eag1 knockdown HeLa cell lines by siRNA. And Nutlin-3 can decrease the cell viability of H8 cells which were stably transfected with Eag1, but has no obvious inhibitory effect on blank H8 cells. Finally, we demonstrated that the decrease in Eag1 channel activity induced by Nutlin-3 treatment exerts anticancer activity by inhibiting the PI3K/AKT pathway. Our study therefore fills the gap between p53 pathway and its cellular function mediated by Eag1, shedding light on the new anti-cancer mechanism of Nutlin-3.

Regulation of Eag1 gating by its intracellular domains

Voltage-gated potassium channels (K_vs) are gated by transmembrane voltage sensors (VS) that move in response to changes in membrane voltage. K_v10.1 or Eag1 also has three intracellular domains: PAS, C-linker, and CNBHD. We demonstrate that the Eag1 intracellular domains are not required for voltage-dependent gating but likely interact with the VS to modulate gating. We identified specific interactions between the PAS, CNBHD, and VS that modulate voltage-dependent gating and provide evidence that VS movement destabilizes these interactions to promote channel opening. Additionally, mutation of these interactions renders Eag1 insensitive to calmodulin inhibition. The structure of the calmodulin insensitive mutant in a pre-open conformation suggests that channel opening may occur through a rotation of the intracellular domains and calmodulin may prevent this rotation by stabilizing interactions between the VS and intracellular domains. Intracellular domains likely play a similar modulatory role in voltage-dependent gating of the related K_v11-12 channels.

Tetrandrine, a novel inhibitor of ether-à-go-go-1 (Eag1), targeted to cervical cancer development

Mortality-to-incidence ratios in patients with cancer are extremely high, positioning cancer as a major cause of death worldwide. Ether-à-go-go-1 (Eag1) is an ion channel that plays important roles in tumour proliferation, malignant transformation, invasion, metastasis, recurrence, and prognosis. Therefore, identifying potent and specific Eag1 channel inhibitors is crucial. In this study, we identified the first natural inhibitor of Eag1, the traditional Chinese medicine agent tetrandrine, and explored the underlying mechanism. Tetrandrine directly interacted with Eag1 and inhibited the currents in a concentration-dependent manner (IC50 of 69.97 ± 5.2 μM), and the amino acids Ile 550 , Thr 552 , and Gln 557 in the Eag1 C-linker domain were critical for tetrandrine's inhibitory effect. Moreover, tetrandrine reduced the proliferation of HeLa cells and Chinese hamster ovary (CHO) cells stably expressing Eag1 in a concentration-dependent manner. Finally, tetrandrine (30 mg/kg/day) inhibited tumor growth in mice, demonstrating a 64.21% inhibitory rate of HeLa cell-transplanted tumors. These results suggest that tetrandrine is a potent and selective Eag1 channel inhibitor, and could act as a leading compound in the development of therapies for Eag1 ion channel dysfunction-induced diseases.

Malpighian tubules of caterpillars: blending RNAseq and physiology to reveal regional functional diversity and novel epithelial ion transport control mechanisms

Malpighian tubules (MTs) and hindgut constitute the functional kidney of insects. MTs are outpouches of the gut and in most insects demonstrate proximodistal heterogeneity in function. In most insects, such heterogeneity is confined to ion/fluid secretion in the distal portion and ion/fluid reabsorption in the proximal portion. In contrast, MTs of larval Lepidoptera (caterpillars of butterflies and moths), are comprised of five regions that differ in their association with the gut, their structure, and ion/fluid transport function. Recent studies have shown that several regions can rapidly and reversibly switch between ion secretion and reabsorption. The current study employed RNAseq, pharmacology and electrophysiology to characterize four distinct regions of the MT in larval Trichoplusia ni. Luminal microelectrode measurements indicate changes in [K+], [Na+] and pH as fluid passes through different regions of the tubule. In addition, the regions examined differ in gene ontology enrichment, and demonstrate robust gradients in expression of ion transporters and endocrine ligand receptors. Lastly, the study provides evidence for direct involvement of voltage-gated and ligand-gated ion channels in epithelial ion transport of insect MTs.

Regulation of Eag by Ca2+/calmodulin controls presynaptic excitability in Drosophila

Drosophila ether-à-go-go ( eag) is the founding member of a large family of voltage-gated K+ channels, the KCNH family, which includes Kv10, 11, and 12. Concurrent binding of calcium/calmodulin (Ca2+/CaM) to NH2- and COOH-terminal sites inhibits mammalian EAG1 channels at submicromolar Ca2+ concentrations, likely by causing pore constriction. Although the Drosophila EAG channel was believed to be Ca2+-insensitive (Schönherr R, Löber K, Heinemann SH. EMBO J 19: 3263-3271, 2000.), both the NH2- and COOH-terminal sites are conserved. In this study we show that Drosophila EAG is inhibited by high Ca2+ concentrations that are only present at plasma membrane Ca2+ channel microdomains. To test the role of this regulation in vivo, we engineered mutations that block CaM-binding to the major COOH-terminal site of the endogenous eag locus, disrupting Ca2+-dependent inhibition. eag CaMBD mutants have reduced evoked release from larval motor neuron presynaptic terminals and show decreased Ca2+ influx in stimulated adult projection neuron presynaptic terminals, consistent with an increase in K+ conductance. These results are predicted by a conductance-based multicompartment model of the presynaptic terminal in which some fraction of EAG is localized to the Ca2+ channel microdomains that control neurotransmitter release. The reduction of release in the larval neuromuscular junction drives a compensatory increase in motor neuron somatic excitability. This misregulation of synaptic and somatic excitability has consequences for systems-level processes and leads to defects in associative memory formation in adults. NEW & NOTEWORTHY Regulation of excitability is critical to tuning the nervous system for complex behaviors. We demonstrate in this article that the EAG family of voltage-gated K+ channels exhibit conserved gating by Ca2+/CaM. Disruption of this inhibition in Drosophila results in decreased evoked neurotransmitter release due to truncated Ca2+ influx in presynaptic terminals. In adults, disrupted Ca2+ dynamics cripples memory formation. These data demonstrate that the biophysical details of channels have important implications for cell function and behavior.

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

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

Structural insights into the mechanisms of CNBD channel function

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

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

Direct visualization of interaction between calmodulin and connexin45

Calmodulin (CaM) is an intracellular Ca²⁺ transducer involved in numerous activities in a broad Ca²⁺ signaling network. Previous studies have suggested that the Ca²⁺/CaM complex may participate in gap junction regulation via interaction with putative CaM-binding motifs in connexins; however, evidence of direct interactions between CaM and connexins has remained elusive to date due to challenges related to the study of membrane proteins. Here, we report the first direct interaction of CaM with Cx45 (connexin45) of γ-family in living cells under physiological conditions by monitoring bioluminescence resonance energy transfer. The interaction between CaM and Cx45 in cells is strongly dependent on intracellular Ca²⁺ concentration and can be blocked by the CaM inhibitor, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide hydrochloride (W7). We further reveal a CaM-binding site at the cytosolic loop (residues 164-186) of Cx45 using a peptide model. The strong binding (K_d ∼ 5 nM) observed between CaM and Cx45 peptide, monitored by fluorescence-labeled CaM, is found to be Ca²⁺-dependent. Furthermore, high-resolution nuclear magnetic resonance spectroscopy reveals that CaM and Cx45 peptide binding leads to global chemical shift changes of ¹⁵N-labeled CaM, but does not alter the size of the structure. Observations involving both N- and C-domains of CaM to interact with the Cx45 peptide differ from the embraced interaction with Cx50 from another connexin family. Such interaction further increases Ca²⁺ sensitivity of CaM, especially at the N-terminal domain. Results of the present study suggest that both helicity and the interaction mode of the cytosolic loop are likely to contribute to CaM's modulation of connexins.

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

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.

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

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

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

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

Crystal structure of the PAS domain of the hEAG potassium channel

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

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

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

Human EAG channels are directly modulated by PIP2 as revealed by electrophysiological and optical interference investigations

Voltage-gated ether à go-go (EAG) K⁺ channels are expressed in various types of cancer cells and also in the central nervous system. Aberrant overactivation of human EAG1 (hEAG1) channels is associated with cancer and neuronal disorders such as Zimmermann-Laband and Temple-Baraitser syndromes. Although hEAG1 channels are recognized as potential therapeutic targets, regulation of their functional properties is only poorly understood. Here, we show that the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂) is a potent inhibitory gating modifier of hEAG1 channels. PIP₂ inhibits the channel activity by directly binding to a short N-terminal segment of the channel important for Ca²⁺/calmodulin (CaM) binding as evidenced by bio-layer interferometry measurements. Conversely, depletion of endogenous PIP₂ either by serotonin-induced phospholipase C (PLC) activation or by a rapamycin-induced translocation system enhances the channel activity at physiological membrane potentials, suggesting that PIP₂ exerts a tonic inhibitory influence. Our study, combining electrophysiological and direct binding assays, demonstrates that hEAG1 channels are subject to potent inhibitory modulation by multiple phospholipids and suggests that manipulations of the PIP₂ signaling pathway may represent a strategy to treat hEAG1 channel-associated diseases.

Ca(2+)-binding protein centrin 4 is a novel binding partner of rat Eag1 K(+) channels

Eag1 is neuron-specific K(+) channel abundantly expressed in the brain and retina. Subcellular localization and physiological analyses in neurons reveal that Eag1 may participate in Ca(2+)-signaling processes in the synapse. Here, we searched for rat Eag1 (rEag1)-binding proteins that may contribute to Ca(2+) regulation of the K(+) channel. Yeast two-hybrid screening identified centrin 4, a member of the centrin family of Ca(2+)-binding proteins. GST pull-down and immunoprecipitation assays in brain and retina lysates confirm the interaction of centrin with rEag1 in neurons. Centrin 4 binds to rEag1 in the absence of Ca(2+). Raising Ca(2+) concentration enhances the association efficiency of centrin 4 and rEag1, and is required for the suppression of rEag1 currents by centrin 4. Altogether, our data suggest that centrin 4 is a novel binding partner that may contribute to Ca(2+) regulation of rEag1 in neurons.

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

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

Ca(2+)/calmodulin regulates Kvβ1.1-mediated inactivation of voltage-gated K(+) channels

A-type K⁺ channels open on membrane depolarization and undergo subsequent rapid inactivation such that they are ideally suited for fine-tuning the electrical signaling in neurons and muscle cells. Channel inactivation mostly follows the so-called ball-and-chain mechanism, in which the N-terminal structures of either the K⁺ channel’s α or β subunits occlude the channel pore entry facing the cytosol. Inactivation of Kv1.1 and Kv1.4 channels induced by Kvβ1.1 subunits is profoundly decelerated in response to a rise in the intracellular Ca²⁺ concentration, thus making the affected channel complexes negative feedback regulators to limit neuronal overexcitation. With electrophysiological and biochemical experiments we show that the Ca²⁺ dependence is gained by binding of calmodulin to the “chain” segment of Kvβ1.1 thereby compromising the mobility of the inactivation particle. Furthermore, inactivation regulation via Ca²⁺/calmodulin does not interfere with the β subunit’s enzymatic activity as an NADPH-dependent oxidoreductase, thus rendering the Kvβ1.1 subunit a multifunctional receptor that integrates cytosolic signals to be transduced to altered electrical cellular activity.

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

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

The enigmatic cytoplasmic regions of KCNH channels

KCNH channels are expressed across a vast phylogenetic and evolutionary spectrum. In humans, they function in a wide range of tissues and serve as biomarkers and targets for diseases such as cancer and cardiac arrhythmias. These channels share a general architecture with other voltage-gated ion channels but are distinguished by the presence of an N-terminal PAS (Per-Arnt-Sim) domain and a C-terminal domain with homology to cyclic nucleotide binding domains (referred to as the CNBh domain). Cytosolic regions outside these domains show little conservation between KCNH families but are strongly conserved across species within a family, likely reflecting variability that confers specificity to individual channel types. PAS and CNBh domains participate in channel gating, but at least twice in evolutionary history, the PAS domain has been lost and it is omitted by alternate transcription to create a distinct channel subunit in one family. In this focused review, we present current knowledge of the structure and function of these cytosolic regions, discuss their evolution as modular domains and provide our perspective on the important questions moving forward.

Oxidative modulation of voltage-gated potassium channels

SIGNIFICANCE

Voltage-gated K+ channels are a large family of K+-selective ion channel protein complexes that open on membrane depolarization. These K+ channels are expressed in diverse tissues and their function is vital for numerous physiological processes, in particular of neurons and muscle cells. Potentially reversible oxidative regulation of voltage-gated K+ channels by reactive species such as reactive oxygen species (ROS) represents a contributing mechanism of normal cellular plasticity and may play important roles in diverse pathologies including neurodegenerative diseases.

RECENT ADVANCES

Studies using various protocols of oxidative modification, site-directed mutagenesis, and structural and kinetic modeling provide a broader phenomenology and emerging mechanistic insights.

CRITICAL ISSUES

Physicochemical mechanisms of the functional consequences of oxidative modifications of voltage-gated K+ channels are only beginning to be revealed. In vivo documentation of oxidative modifications of specific amino-acid residues of various voltage-gated K+ channel proteins, including the target specificity issue, is largely absent.

FUTURE DIRECTIONS

High-resolution chemical and proteomic analysis of ion channel proteins with respect to oxidative modification combined with ongoing studies on channel structure and function will provide a better understanding of how the function of voltage-gated K+ channels is tuned by ROS and the corresponding reducing enzymes to meet cellular needs.

Gap junction regulation by calmodulin

Intracellular Ca(2+) activated calmodulin (CaM) inhibits gap junction channels in the low nanomolar to high micromolar range of [Ca(2+)]i. This regulation plays an essential role in numerous cellular processes that include hearing, lens transparency, and synchronized contractions of the heart. Previous studies have indicated that gap junction mediated cell-to-cell communication was inhibited by CaM antagonists. More recent evidence indicates a direct role of CaM in regulating several members of the connexin family. Since the intracellular loop and carboxyl termini of connexins are largely "invisible" in electron microscopy and X-ray crystallographic structures due to disorder in these domains, peptide models encompassing the putative CaM binding sites of several intracellular domains of connexins have been used to identify the Ca(2+)-dependent CaM binding sites of these proteins. This approach has been used to determine the CaM binding affinities of peptides derived from a number of different connexin-subfamilies.

Fixed charges in the gel matrix of sensor chips and dissociation in diffusion gradients influence the detection of fast protein-protein interactions

In molecular interaction studies based on surface plasmon resonance (SPR) measurements, the ligand is often immobilized in a thin carboxydextran gel matrix. Here we investigated the influence of the charged gel on the results of such SPR measurements. At physiological ionic strength, analytes with a net charge of more than about 5 are considerably enriched or depleted due to the Donnan potential under commonly applied experimental conditions. Below physiological ionic strength, enrichment was found to be even stronger than predicted by Donnan theory. The influence of the gel matrix on the apparent binding is prevented in competition experiments, in which SPR measurements are only used to discriminate between free and complexed analyte while the interaction between analyte and ligand is studied in solution. However, if the analyte-ligand interaction is very fast, thermodynamic equilibrium is disturbed near the interface where free analyte binds to the immobilized ligand due to mass transport limitation. Consequently, the soluble analyte-ligand complex dissociates, which results in an overestimation of free analyte. In experiments of calmodulin binding to fragments of the KCNH1 ion channel protein this mass-transport-induced dissociation led to a systematic underestimation of the affinity. We conclude that the insufficient discrimination between the true analyte-ligand binding and the complex interactions of the analyte with the gel phase may result in systematic errors. The theoretical framework for recognizing and avoiding such errors is provided.

KCNH1 potassium channels are expressed in cervical cytologies from pregnant patients and are regulated by progesterone

Potassium voltage-gated channel, subfamily H (eag-related), member 1 (KCNH1) potassium channels are potential tumour markers and cancer therapeutic targets and are up-regulated by oestrogens and human papilloma virus (HPV) oncogenes. However, the role of KCNH1 in normal tissues is poorly understood, and its expression in pregnancy is unknown. We wondered whether KCNH1 channels are expressed in cervical cells from pregnant patients and whether progesterone (P4) regulates KCNH1. The association with HPV was also investigated. KCNH1 protein expression was studied by immunocytochemistry in liquid-based cervical cytologies; 93 samples were obtained from pregnant patients at different trimesters, and 15 samples were obtained from non-pregnant women (controls). The presence ofHPVwas studied by PCR with direct sequencing and nested multiplex PCR. HeLa cervical cancer cells were transfected with human progesterone receptor-B (PR-B) and treated with P4.KCNH1mRNA expression in these cultures was studied by real-time PCR. KCNH1 protein was detected in 100% of the pregnancy samples and in 26% of the controls. We found 18 pregnant patients infected with HPV and detected 14 types ofHPV. There was no association between the percentage of cells expressing KCNH1 and either the presence or type of HPV. P4induced KCNH1 mRNA and protein expression in cells transfected with human PR-B. No regulation of KCNH1 by P4was observed in non-transfected cells. We show for the first time the expression of an ion channel during human pregnancy at different trimesters and KCNH1 regulation by P4in human cells. These data raise a new research field for KCNH1 channels in human tissues.