KV7 channelopathies

Dynamic protein-protein interactions of KCNQ1 and KCNE1 measured by EPR line shape analysis

KCNQ1, also known as Kv7.1, is a voltage gated potassium channel that associates with the KCNE protein family. Mutations in this protein has been found to cause a variety of diseases including Long QT syndrome, a type of cardiac arrhythmia where the QT interval observed on an electrocardiogram is longer than normal. This condition is often aggravated during strenuous exercise and can cause fainting spells or sudden death. KCNE1 is an ancillary protein that interacts with KCNQ1 in the membrane at varying molar ratios. This interaction allows for the flow of potassium ions to be modulated to facilitate repolarization of the heart. The interaction between these two proteins has been studied previously with cysteine crosslinking and electrophysiology. In this study, electron paramagnetic resonance (EPR) spectroscopy line shape analysis in tandem with site directed spin labeling (SDSL) was used to observe changes in side chain dynamics as KCNE1 interacts with KCNQ1. KCNE1 was labeled at different sites that were found to interact with KCNQ1 based on previous literature, along with sites outside of that range as a control. Once labeled KCNE1 was incorporated into vesicles, KCNQ1 (helices S1-S6) was titrated into the vesicles. The line shape differences observed upon addition of KCNQ1 are indicative of an interaction between the two proteins. This method provides a first look at the interactions between KCNE1 and KCNQ1 from a dynamics perspective using the full transmembrane portion of KCNQ1.

Advances in the Study of Etiology and Molecular Mechanisms of Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL), a multifactorial progressive disorder, results from a complex interplay of genetic and environmental factors, with its underlying mechanisms remaining unclear. Several pathological factors are believed to contribute to SNHL, including genetic factors, ion homeostasis, cell apoptosis, immune inflammatory responses, oxidative stress, hormones, metabolic syndrome, human cytomegalovirus infection, mitochondrial damage, and impaired autophagy. These factors collectively interact and play significant roles in the onset and progression of SNHL. The present review offers a comprehensive overview of the various factors that contribute to SNHL, emphasizes recent developments in understanding its etiology, and explores relevant preventive and intervention measures.

The Kv7 channel opener Retigabine reduces neuropathology and alleviates behavioral deficits in APP/PS1 transgenic mice

Hyperexcitability of neuronal networks is central to the pathogenesis of Alzheimer's disease (AD). Pharmacological activation of Kv7 channels is an effective way to reduce neuronal firing. Our results showed that that pharmacologically activating the Kv7 channel with Retigabine (RTG) can alleviate cognitive impairment in mice without affecting spontaneous activity. RTG could also ameliorate damage to the Nissl bodies in cortex and hippocampal CA and DG regions in 9-month-old APP/PS1 mice. Additionally, RTG could reduce the Aβ plaque number in the hippocampus and cortex of both 6-month-old and 9-month-old mice. By recordings of electroencephalogram, we showed that a decrease in the number of abnormal discharges in the brains of the AD model mice when the Kv7 channel was opened. Moreover, Western blot analysis revealed a reduction in the expression of the p-Tau protein in both the hippocampus and cortex upon Kv7 channel opening. These findings suggest that Kv7 channel opener RTG may ameliorate cognitive impairment in AD, most likely by reducing brain excitability.

Effect of a sensing charge mutation on the deactivation of KV7.2 channels

Potassium-selective, voltage-gated channels of the KV7 family are critical regulators of electrical excitability in many cell types. Removing the outermost putative sensing charge (R198) of the human KV7.2 shifts its activation voltage dependence toward more negative potentials. This suggests that removing a charge "at the top" of the fourth (S4) segment of the voltage-sensing domain facilitates activation. Here, we hypothesized that restoring that charge would bring back the activation to its normal voltage range. We introduced the mutation R198H in KV7.2 with the idea that titrating the introduced histidine with protons would reinstate the sensing charge. As predicted, the mutant's activation voltage dependence changed as a function of the external pH (pHEXT) while modest changes in the activation voltage dependence were observed with the wild-type (WT) channel. On the other hand, the deactivation kinetics of the R198H mutant was remarkably sensitive to pHEXT changes, readily deactivating at pHEXT 6, while becoming slower to deactivate at pHEXT 8. In contrast, the KV7.2 WT displayed modest changes in the deactivation kinetics as a function of pHEXT. This suggested that the charge of residue 198 was critical for deactivation. However, in a surprising turn, the mutant R198Q-a non-titratable mutation-also displayed a high pHEXT sensitivity activity. We thus concluded that rather than the charge at position 198, the protonation status of the channel's extracellular face modulates the open channel stabilization and that the charge of residue 198 is required for the voltage sensor to effectively deactivate the channel, overcoming the stabilizing effect of high pHEXT.

Recent advances in genetic etiology of non-syndromic deafness in children

Congenital auditory impairment is a prevalent anomaly observed in approximately 2-3 per 1,000 infants. The consequences associated with hearing loss among children encompass the decline of verbal communication, linguistic skills, educational progress, social integration, cognitive aptitude, and overall well-being. Approaches to reversing or preventing genetic hearing loss are limited. Patients with mild and moderate hearing loss can only use hearing aids, while those with severe hearing loss can only acquire speech and language through cochlear implants. Both environmental and genetic factors contribute to the occurrence of congenital hearing loss, and advancements in our understanding of the pathophysiology and molecular mechanisms underlying hearing loss, coupled with recent progress in genetic testing techniques, will facilitate the development of innovative approaches for treatment and screening. In this paper, the latest research progress in genetic etiology of non-syndromic deafness in children with the highest incidence is summarized in order to provide help for personalized diagnosis and treatment of deafness in children.

Sensing its own permeant ion: KCNQ1 channel inhibition by external K

External potassium inhibits KCNQ1 channel through a mechanism involving increased occupancy of the filter S0 site by K+o.

Potassium channelopathies associated with epilepsy-related syndromes and directions for therapeutic intervention

A number of mutations to members of several CNS potassium (K) channel families have been identified which result in rare forms of neonatal onset epilepsy, or syndromes of which one prominent characteristic is a form of epilepsy. Benign Familial Neonatal Convulsions or Seizures (BFNC or BFNS), also referred to as Self-Limited Familial Neonatal Epilepsy (SeLNE), results from mutations in 2 members of the K_V7 family (KCNQ) of K channels; while generally self-resolving by about 15 weeks of age, these mutations significantly increase the probability of generalized seizure disorders in the adult, in some cases they result in more severe developmental syndromes. Epilepsy of Infancy with Migrating Focal Seizures (EIMSF), or Migrating Partial Seizures of Infancy (MMPSI), is a rare severe form of epilepsy linked primarily to gain of function mutations in a member of the sodium-dependent K channel family, KCNT1 or SLACK. Finally, KCNMA1 channelopathies, including Liang-Wang syndrome (LIWAS), are rare combinations of neurological symptoms including seizure, movement abnormalities, delayed development and intellectual disabilities, with Liang-Wang syndrome an extremely serious polymalformative syndrome with a number of neurological sequelae including epilepsy. These are caused by mutations in the pore-forming subunit of the large-conductance calcium-activated K channel (BK channel) KCNMA1. The identification of these rare but significant channelopathies has resulted in a resurgence of interest in their treatment by direct pharmacological or genetic modulation. We will briefly review the genetics, biophysics and pharmacology of these K channels, their linkage with the 3 syndromes described above, and efforts to more effectively target these syndromes.

The Difficult Path to the Discovery of Novel Treatments in Psychiatric Disorders

CNS diseases, including psychiatric disorders, represent a significant opportunity for the discovery and development of new drugs and therapeutic treatments with the potential to have a significant impact on human health. CNS diseases, however, present particular challenges to therapeutic discovery efforts, and psychiatric diseases/disorders may be among the most difficult. With specific exceptions such as psychostimulants for ADHD, a large number of psychiatric patients are resistant to existing treatments. In addition, clinicians have no way of knowing which psychiatric patients will respond to which drugs. By definition, psychiatric diagnoses are syndromal in nature; determinations of efficacy are often self-reported, and drug discovery is largely model-based. While such models of psychiatric disease are amenable to screening for new drugs, whether cellular or whole-animal based, they have only modest face validity and, more importantly, predictive validity. Multiple academic, pharmaceutical industry, and government agencies are dedicated to the translation of new findings about the neurobiology of major psychiatric disorders into the discovery and advancement of novel therapies. The collaboration of these agencies provide a pathway for developing new therapeutics. These efforts will be greatly helped by recent advances in understanding the genetic bases of psychiatric disorders, the ongoing search for diagnostic and therapy-responsive biomarkers, and the validation of new animal models.

Pacemaker activity and ion channels in the sinoatrial node cells: MicroRNAs and arrhythmia

The primary pacemaking activity of the heart is determined by a spontaneous action potential (AP) within sinoatrial node (SAN) cells. This unique AP generation relies on two mechanisms: membrane clocks and calcium clocks. Nonhomologous arrhythmias are caused by several functional and structural changes in the myocardium. MicroRNAs (miRNAs) are essential regulators of gene expression in cardiomyocytes. These miRNAs play a vital role in regulating the stability of cardiac conduction and in the remodeling process that leads to arrhythmias. Although it remains unclear how miRNAs regulate the expression and function of ion channels in the heart, these regulatory mechanisms may support the development of emerging therapies. This study discusses the spread and generation of AP in the SAN as well as the regulation of miRNAs and individual ion channels. Arrhythmogenicity studies on ion channels will provide a research basis for miRNA modulation as a new therapeutic target.

Purification and membrane interactions of human KCNQ1100-370 potassium ion channel

KCNQ1 (Kv7.1 or KvLQT1) is a voltage-gated potassium ion channel that is involved in the ventricular repolarization following an action potential in the heart. It forms a complex with KCNE1 in the heart and is the pore forming subunit of slow delayed rectifier potassium current (Iks). Mutations in KCNQ1, leading to a dysfunctional channel or loss of activity have been implicated in a cardiac disorder, long QT syndrome. In this study, we report the overexpression, purification, biochemical characterization of human KCNQ1100-370, and lipid bilayer dynamics upon interaction with KCNQ1100-370. The recombinant human KCNQ1 was expressed in Escherichia coli and purified into n-dodecylphosphocholine (DPC) micelles. The purified KCNQ1100-370 was biochemically characterized by SDS-PAGE electrophoresis, western blot and nano-LC-MS/MS to confirm the identity of the protein. Circular dichroism (CD) spectroscopy was utilized to confirm the secondary structure of purified protein in vesicles. Furthermore, 31P and 2H solid-state NMR spectroscopy in DPPC/POPC/POPG vesicles (MLVs) indicated a direct interaction between KCNQ100-370 and the phospholipid head groups. Finally, a visual inspection of KCNQ1100-370 incorporated into MLVs was confirmed by transmission electron microscopy (TEM). The findings of this study provide avenues for future structural studies of the human KCNQ1 ion channel to have an in depth understanding of its structure-function relationship.

The Pathological Mechanisms of Hearing Loss Caused by KCNQ1 and KCNQ4 Variants

Deafness-associated genes KCNQ1 (also associated with heart diseases) and KCNQ4 (only associated with hearing loss) encode the homotetrameric voltage-gated potassium ion channels Kv7.1 and Kv7.4, respectively. To date, over 700 KCNQ1 and over 70 KCNQ4 variants have been identified in patients. The vast majority of these variants are inherited dominantly, and their pathogenicity is often explained by dominant-negative inhibition or haploinsufficiency. Our recent study unexpectedly identified cell-death-inducing cytotoxicity in several Kv7.1 and Kv7.4 variants. Elucidation of this cytotoxicity mechanism and identification of its modifiers (drugs) have great potential for aiding the development of a novel pharmacological strategy against many pathogenic KCNQ variants. The purpose of this review is to disseminate this emerging pathological role of Kv7 variants and to underscore the importance of experimentally characterizing disease-associated variants.

Two small-molecule activators share similar effector sites in the KCNQ1 channel pore but have distinct effects on voltage sensor movements

ML277 and R-L3 are two small-molecule activators of KCNQ1, the pore-forming subunit of the slowly activating potassium channel I_Ks. KCNQ1 loss-of-function mutations prolong cardiac action potential duration and are associated with long QT syndrome, which predispose patients to lethal ventricular arrhythmia. ML277 and R-L3 enhance KCNQ1 current amplitude and slow deactivation. However, the presence of KCNE1, an auxiliary subunit of I_Ks channels, renders the channel insensitive to both activators. We found that ML277 effects are dependent on several residues in the KCNQ1 pore domain. Some of these residues are also necessary for R-L3 effects. These residues form a putative hydrophobic pocket located between two adjacent KCNQ1 subunits, where KCNE1 subunits are thought to dwell, thus providing an explanation for how KCNE1 renders the I_Ks channel insensitive to these activators. Our experiments showed that the effect of R-L3 on voltage sensor movement during channel deactivation was much more prominent than that of ML277. Simulations using a KCNQ1 kinetic model showed that the effects of ML277 and R-L3 could be reproduced through two different effects on channel gating: ML277 enhances KCNQ1 channel function through a pore-dependent and voltage sensor-independent mechanism, while R-L3 affects both channel pore and voltage sensor.

Autism Spectrum Disorder and a De Novo Kcnq2 Gene Mutation: A Case Report

The KCNQ2 gene, encoding for the Kv7.2 subunits of the Kv7 voltage potassium channel, is involved in the modulation of neuronal excitability and plays a crucial role in brain morphogenesis and maturation during embryonic life. De novo heterozygous mutations in KCNQ2 genes are associated with early-onset epileptic encephalopathy and neurodevelopmental disorders including developmental delay and intellectual disability. However, little is known about the socio-communicative phenotype of children affected by the KCNQ2 mutation, and a detailed behavioral characterization focused on autistic symptoms has not yet been conducted. This case report describes the clinical behavioral phenotype of a 6-year-old boy carrying a de novo heterozygous KCNQ2 mutation, affected by early-onset seizures and autism spectrum disorder (ASD). We performed a neuropsychiatric assessment of cognitive, adaptive, socio-communicative and autistic symptoms through the administration of standardized tools. The main contribution of this case report is to provide a detailed developmental and behavioral characterization focused on ASD symptoms in a child with [c.812 G > A; p. (Gly271Asp)]mutation in the KCNQ2 gene.

Cell death-inducing cytotoxicity in truncated KCNQ4 variants associated with DFNA2 hearing loss

KCNQ4 encodes the homotetrameric voltage-dependent potassium ion channel Kv7.4, and is the causative gene for autosomal dominant nonsyndromic sensorineural hearing loss, DFNA2. Dominant-negative inhibition accounts for the observed dominant inheritance of many DFNA2-associated KCNQ4 variants. In addition, haploinsufficiency has been presumed as the pathological mechanism for truncated Kv7.4 variants lacking the C-terminal tetramerization region, as they are unlikely to exert a dominant-negative inhibitory effect. Such truncated Kv7.4 variants should result in relatively mild hearing loss when heterozygous; however, this is not always the case. In this study, we characterized Kv7.4^Q71fs (c.211delC), Kv7.4^W242X (c.725G>A) and Kv7.4^A349fs (c.1044_1051del8) in heterologous expression systems and found that expression of these truncated Kv7.4 variants induced cell death. We also found similar cell death-inducing cytotoxic effects in truncated Kv7.1 (KCNQ1) variants, suggesting that the generality of our findings could account for the dominant inheritance of many, if not most, truncated Kv7 variants. Moreover, we found that the application of autophagy inducers can ameliorate the cytotoxicity, providing a novel insight for the development of alternative therapeutic strategies for Kv7.4 variants.

Summary: Expression of truncated KCNQ4 variants lacking the C-terminal tetramerization domain results in cell-death inducing cytotoxicity, providing novel insight into the development of alternative therapeutic strategies for DFNA2 hearing loss.

PIP2-dependent coupling of voltage sensor and pore domains in Kv7.2 channel

Phosphatidylinositol-4,5-bisphosphate (PIP₂) is a signaling lipid which regulates voltage-gated K_v7/KCNQ potassium channels. Altered PIP₂ sensitivity of neuronal K_v7.2 channel is involved in KCNQ2 epileptic encephalopathy. However, the molecular action of PIP₂ on K_v7.2 gating remains largely elusive. Here, we use molecular dynamics simulations and electrophysiology to characterize PIP₂ binding sites in a human K_v7.2 channel. In the closed state, PIP₂ localizes to the periphery of the voltage-sensing domain (VSD). In the open state, PIP₂ binds to 4 distinct interfaces formed by the cytoplasmic ends of the VSD, the gate, intracellular helices A and B and their linkers. PIP₂ binding induces bilayer-interacting conformation of helices A and B and the correlated motion of the VSD and the pore domain, whereas charge-neutralizing mutations block this coupling and reduce PIP₂ sensitivity of K_v7.2 channels by disrupting PIP₂ binding. These findings reveal the allosteric role of PIP₂ in K_v7.2 channel activation.

Pant et al. describe the mechanism by which PIP2 might regulate homomeric Kv7.2 channels. They identify sites important in the binding of the PIP2 lipid to Kv7.2 channels and propose that the PIP2 binding to a specific site results in the coupling between the voltage sensor domain (VSD) and pore domain (PD), which stabilizes the open state of the channel.

Chemical modulation of Kv7 potassium channels

The rising interest in Kv7 modulators originates from their ability to evoke fundamental electrophysiological perturbations in a tissue-specific manner. A large number of therapeutic applications are, in part, based on the clinical experience with two broad-spectrum Kv7 agonists, flupirtine and retigabine. Since precise molecular structures of human Kv7 channel subtypes in closed and open states have only very recently started to emerge, computational studies have traditionally been used to analyze binding modes and direct the development of more potent and selective Kv7 modulators with improved safety profiles. Herein, the synthetic and medicinal chemistry of small molecule modulators and the representative biological properties are summarized. Furthermore, new therapeutic applications supported by in vitro and in vivo assay data are suggested.

Developmental and epileptic encephalopathies: what we do and do not know

Developmental encephalopathies, including intellectual disability and autistic spectrum disorder, are frequently associated with infant epilepsy. Epileptic encephalopathy is used to describe an assumed causal relationship between epilepsy and developmental delay. Developmental encephalopathies pathogenesis more independent from epilepsy is supported by the identification of several gene variants associated with both developmental encephalopathies and epilepsy, the possibility for gene-associated developmental encephalopathies without epilepsy, and the continued development of developmental encephalopathies even when seizures are controlled. Hence, 'developmental and epileptic encephalopathy' may be a more appropriate term than epileptic encephalopathy. This update considers the best studied 'developmental and epileptic encephalopathy' gene variants for illustrative support for 'developmental and epileptic encephalopathy' over epileptic encephalopathy. Moreover, the interaction between epilepsy and developmental encephalopathies is considered with respect to influence on treatment decisions. Continued research in genetic testing will increase access to clinical tests, earlier diagnosis, better application of current treatments, and potentially provide new molecular-investigated treatments.

Activation of KCNQ4 as a Therapeutic Strategy to Treat Hearing Loss

Potassium voltage-gated channel subfamily q member 4 (KCNQ4) is a voltage-gated potassium channel that plays essential roles in maintaining ion homeostasis and regulating hair cell membrane potential. Reduction of the activity of the KCNQ4 channel owing to genetic mutations is responsible for nonsyndromic hearing loss, a typically late-onset, initially high-frequency loss progressing over time. In addition, variants of KCNQ4 have also been associated with noise-induced hearing loss and age-related hearing loss. Therefore, the discovery of small compounds activating or potentiating KCNQ4 is an important strategy for the curative treatment of hearing loss. In this review, we updated the current concept of the physiological role of KCNQ4 in the inner ear and the pathologic mechanism underlying the role of KCNQ4 variants with regard to hearing loss. Finally, we focused on currently developed KCNQ4 activators and their pros and cons, paving the way for the future development of specific KCNQ4 activators as a remedy for hearing loss.

Genetic Landscape of Common Epilepsies: Advancing towards Precision in Treatment

Epilepsy, a neurological disease characterized by recurrent seizures, is highly heterogeneous in nature. Based on the prevalence, epilepsy is classified into two types: common and rare epilepsies. Common epilepsies affecting nearly 95% people with epilepsy, comprise generalized epilepsy which encompass idiopathic generalized epilepsy like childhood absence epilepsy, juvenile myoclonic epilepsy, juvenile absence epilepsy and epilepsy with generalized tonic-clonic seizure on awakening and focal epilepsy like temporal lobe epilepsy and cryptogenic focal epilepsy. In 70% of the epilepsy cases, genetic factors are responsible either as single genetic variant in rare epilepsies or multiple genetic variants acting along with different environmental factors as in common epilepsies. Genetic testing and precision treatment have been developed for a few rare epilepsies and is lacking for common epilepsies due to their complex nature of inheritance. Precision medicine for common epilepsies require a panoramic approach that incorporates polygenic background and other non-genetic factors like microbiome, diet, age at disease onset, optimal time for treatment and other lifestyle factors which influence seizure threshold. This review aims to comprehensively present a state-of-art review of all the genes and their genetic variants that are associated with all common epilepsy subtypes. It also encompasses the basis of these genes in the epileptogenesis. Here, we discussed the current status of the common epilepsy genetics and address the clinical application so far on evidence-based markers in prognosis, diagnosis, and treatment management. In addition, we assessed the diagnostic predictability of a few genetic markers used for disease risk prediction in individuals. A combination of deeper endo-phenotyping including pharmaco-response data, electro-clinical imaging, and other clinical measurements along with genetics may be used to diagnose common epilepsies and this marks a step ahead in precision medicine in common epilepsies management.

Kv7 Channels in Lung Diseases

Lung diseases constitute a global health concern causing disability. According to WHO in 2016, respiratory diseases accounted for 24% of world population mortality, the second cause of death after cardiovascular diseases. The Kv7 channels family is a group of voltage-dependent K⁺ channels (Kv) encoded by KCNQ genes that are involved in various physiological functions in numerous cell types, especially, cardiac myocytes, smooth muscle cells, neurons, and epithelial cells. Kv7 channel α-subunits are regulated by KCNE1–5 ancillary β-subunits, which modulate several characteristics of Kv7 channels such as biophysical properties, cell-location, channel trafficking, and pharmacological sensitivity. Kv7 channels are mainly expressed in two large groups of lung tissues: pulmonary arteries (PAs) and bronchial tubes. In PA, Kv7 channels are expressed in pulmonary artery smooth muscle cells (PASMCs); while in the airway (trachea, bronchus, and bronchioles), Kv7 channels are expressed in airway smooth muscle cells (ASMCs), airway epithelial cells (AEPs), and vagal airway C-fibers (VACFs). The functional role of Kv7 channels may vary depending on the cell type. Several studies have demonstrated that the impairment of Kv7 channel has a strong impact on pulmonary physiology contributing to the pathophysiology of different respiratory diseases such as cystic fibrosis, asthma, chronic obstructive pulmonary disease, chronic coughing, lung cancer, and pulmonary hypertension. Kv7 channels are now recognized as playing relevant physiological roles in many tissues, which have encouraged the search for Kv7 channel modulators with potential therapeutic use in many diseases including those affecting the lung. Modulation of Kv7 channels has been proposed to provide beneficial effects in a number of lung conditions. Therefore, Kv7 channel openers/enhancers or drugs acting partly through these channels have been proposed as bronchodilators, expectorants, antitussives, chemotherapeutics and pulmonary vasodilators.

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.

β-Secretase BACE1 Is Required for Normal Cochlear Function

Cleavage of amyloid precursor protein (APP) by β-secretase BACE1 initiates the production and accumulation of neurotoxic amyloid-β peptides, which is widely considered an essential pathogenic mechanism in Alzheimer's disease (AD). Here, we report that BACE1 is essential for normal auditory function. Compared with wild-type littermates, BACE1^-/- mice of either sex exhibit significant hearing deficits, as indicated by increased thresholds and reduced amplitudes in auditory brainstem responses (ABRs) and decreased distortion product otoacoustic emissions (DPOAEs). Immunohistochemistry revealed aberrant synaptic organization in the cochlea and hypomyelination of auditory nerve fibers as predominant neuropathological substrates of hearing loss in BACE1^-/- mice. In particular, we found that fibers of spiral ganglion neurons (SGN) close to the organ of Corti are disorganized and abnormally swollen. BACE1 deficiency also engenders organization defects in the postsynaptic compartment of SGN fibers with ectopic overexpression of PSD95 far outside the synaptic region. During postnatal development, auditory fiber myelination in BACE1^-/- mice lags behind dramatically and remains incomplete into adulthood. We relate the marked hypomyelination to the impaired processing of Neuregulin-1 when BACE1 is absent. To determine whether the cochlea of adult wild-type mice is susceptible to AD treatment-like suppression of BACE1, we administered the established BACE1 inhibitor NB-360 for 6 weeks. The drug suppressed BACE1 activity in the brain, but did not impair hearing performance and, upon neuropathological examination, did not produce the characteristic cochlear abnormalities of BACE1^-/- mice. Together, these data strongly suggest that the hearing loss of BACE1 knock-out mice represents a developmental phenotype.SIGNIFICANCE STATEMENT Given its crucial role in the pathogenesis of Alzheimer's disease (AD), BACE1 is a prime pharmacological target for AD prevention and therapy. However, the safe and long-term administration of BACE1-inhibitors as envisioned in AD requires a comprehensive understanding of the various physiological functions of BACE1. Here, we report that BACE1 is essential for the processing of auditory signals in the inner ear, as BACE1-deficient mice exhibit significant hearing loss. We relate this deficit to impaired myelination and aberrant synapse formation in the cochlea, which manifest during postnatal development. By contrast, prolonged pharmacological suppression of BACE1 activity in adult wild-type mice did not reproduce the hearing deficit or the cochlear abnormalities of BACE1 null mice.

Measurement of axonal excitability: Consensus guidelines

Measurement of axonal excitability provides an in vivo indication of the properties of the nerve membrane and of the ion channels expressed on these axons. Axonal excitability techniques have been utilised to investigate the pathophysiological mechanisms underlying neurological diseases. This document presents guidelines derived for such studies, based on a consensus of international experts, and highlights the potential difficulties when interpreting abnormalities in diseased axons. The present manuscript provides a state-of-the-art review of the findings of axonal excitability studies and their interpretation, in addition to suggesting guidelines for the optimal performance of excitability studies.

Heterozygous loss of epilepsy gene KCNQ2 alters social, repetitive and exploratory behaviors

KCNQ/K_v7 channels conduct voltage‐dependent outward potassium currents that potently decrease neuronal excitability. Heterozygous inherited mutations in their principle subunits K_v7.2/KCNQ2 and K_v7.3/KCNQ3 cause benign familial neonatal epilepsy whereas patients with de novo heterozygous K_v7.2 mutations are associated with early‐onset epileptic encephalopathy and neurodevelopmental disorders characterized by intellectual disability, developmental delay and autism. However, the role of K_v7.2‐containing K_v7 channels in behaviors especially autism‐associated behaviors has not been described. Because pathogenic K_v7.2 mutations in patients are typically heterozygous loss‐of‐function mutations, we investigated the contributions of K_v7.2 to exploratory, social, repetitive and compulsive‐like behaviors by behavioral phenotyping of both male and female KCNQ2^+/− mice that were heterozygous null for the KCNQ2 gene. Compared with their wild‐type littermates, male and female KCNQ2^+/− mice displayed increased locomotor activity in their home cage during the light phase but not the dark phase and showed no difference in motor coordination, suggesting hyperactivity during the inactive light phase. In the dark phase, KCNQ2^+/− group showed enhanced exploratory behaviors, and repetitive grooming but decreased sociability with sex differences in the degree of these behaviors. While male KCNQ2^+/− mice displayed enhanced compulsive‐like behavior and social dominance, female KCNQ2^+/− mice did not. In addition to elevated seizure susceptibility, our findings together indicate that heterozygous loss of K_v7.2 induces behavioral abnormalities including autism‐associated behaviors such as reduced sociability and enhanced repetitive behaviors. Therefore, our study is the first to provide a tangible link between loss‐of‐function K_v7.2 mutations and the behavioral comorbidities of K_v7.2‐associated epilepsy.

KCNQ/M-channels regulate mouse vagal bronchopulmonary C-fiber excitability and cough sensitivity

Increased airway vagal sensory C-fiber activity contributes to the symptoms of inflammatory airway diseases. The KCNQ/Kv7/M-channel is a well-known determinant of neuronal excitability, yet whether it regulates the activity of vagal bronchopulmonary C-fibers and airway reflex sensitivity remains unknown. Here we addressed this issue using single-cell RT-PCR, patch clamp technique, extracellular recording of single vagal nerve fibers innervating the mouse lungs, and telemetric recording of cough in free-moving mice. Single-cell mRNA analysis and biophysical properties of M-current (IM) suggest that KCNQ3/Kv7.3 is the major M-channel subunit in mouse nodose neurons. The M-channel opener retigabine negatively shifted the voltage-dependent activation of IM, leading to membrane hyperpolarization, increased rheobase, and suppression of both evoked and spontaneous action potential (AP) firing in nodose neurons in an M-channel inhibitor XE991-sensitive manner. Retigabine also markedly suppressed the α,β-methylene ATP-induced AP firing in nodose C-fiber terminals innervating the mouse lungs, and coughing evoked by irritant gases in awake mice. In conclusion, KCNQ/M-channels play a role in regulating the excitability of vagal airway C-fibers at both the cell soma and nerve terminals. Drugs that open M-channels in airway sensory afferents may relieve the sufferings associated with pulmonary inflammatory diseases such as chronic coughing.

How to replace the lost keys? Strategies toward safer KV7 channel openers

The highly structurally similar drugs flupirtine and retigabine have been regarded as safe and effective for many years but lately they turned out to exert intolerable side effects. While the twin molecules share the mode of action, both stabilize the open state of voltage-gated potassium channels, the form and severity of adverse effects is different. The analgesic flupirtine caused drug-induced liver injury in rare but fatal cases, whereas prolonged use of the antiepileptic retigabine led to blue tissue discoloration. Because the adverse effects seem unrelated to the mode of action, it is likely, that both drugs that occupied important therapeutic niches, could be replaced. Reasons for the clinically relevant toxicity will be clarified and future substitutes for these drugs presented in this review.

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

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

Association between BACE1 gene polymorphisms and focal seizures in a Chinese Han population

BACKGROUND

Beta-secretase 1 (BACE1) is attracting increasing attention for its vital role in pathogenesis of many neuropsychiatric disorders and many studies also have indicated BACE1 as a possible risk factor for seizures, but not any studies have reported association between BACE1 gene polymorphisms and seizures. Therefore, we investigated the possible association between focal seizures and BACE1 gene polymorphisms in the present study.

METHODS

A total of 162 patients and 211 health controls were enrolled in this study and polymorphisms of BACE1 gene were detected using polymerase chain reaction (PCR)-ligase detection reaction method.

RESULTS

The frequency of genotype AT for BACE1 rs535860 (A>T) was significantly higher (24.1%) in patients compared to controls (14.7%) (OR = 1.836, 95% CI = 1.086-3.102, P = .023). Intriguingly, we only found the significant difference of BACE1 SNP genotype and allele frequency among males but not females. However, no statistically significant results were presented for the genotype distributions of rs525493 (G>T) and rs638405(C>G) polymorphisms between patients and controls.

CONCLUSION

Our study demonstrated there may exist an association between BACE1 rs535860 (A>T) polymorphism and focal seizures in Chinese Han males.

Regulation of Kv7.2/Kv7.3 channels by cholesterol: Relevance of an optimum plasma membrane cholesterol content

Kv7.2/Kv7.3 channels are the molecular correlate of the M-current, which stabilizes the membrane potential and controls neuronal excitability. Previous studies have shown the relevance of plasma membrane lipids on both M-currents and Kv7.2/Kv7.3 channels. Here, we report the sensitive modulation of Kv7.2/Kv7.3 channels by membrane cholesterol level. Kv7.2/Kv7.3 channels transiently expressed in HEK-293 cells were significantly inhibited by decreasing the cholesterol level in the plasma membrane by three different pharmacological strategies: methyl-β-cyclodextrin (MβCD), Filipin III, and cholesterol oxidase treatment. Surprisingly, Kv7.2/Kv7.3 channels were also inhibited by membrane cholesterol loading with the MβCD/cholesterol complex. Depletion or enrichment of plasma membrane cholesterol differentially affected the biophysical parameters of the macroscopic Kv7.2/Kv7.3 currents. These results indicate a complex mechanism of Kv7.2/Kv7.3 channels modulation by membrane cholesterol. We propose that inhibition of Kv7.2/Kv7.3 channels by membrane cholesterol depletion involves a loss of a direct cholesterol-channel interaction. However, the inhibition of Kv7.2/Kv7.3 channels by membrane cholesterol enrichment could include an additional direct cholesterol-channel interaction, or changes in the physical properties of the plasma membrane. In summary, our results indicate that an optimum cholesterol level in the plasma membrane is required for the proper functioning of Kv7.2/Kv7.3 channels.

A Calmodulin C-Lobe Ca2+-Dependent Switch Governs Kv7 Channel Function

Kv7 (KCNQ) voltage-gated potassium channels control excitability in the brain, heart, and ear. Calmodulin (CaM) is crucial for Kv7 function, but how this calcium sensor affects activity has remained unclear. Here, we present X-ray crystallographic analysis of CaM:Kv7.4 and CaM:Kv7.5 AB domain complexes that reveal an Apo/CaM clamp conformation and calcium binding preferences. These structures, combined with small-angle X-ray scattering, biochemical, and functional studies, establish a regulatory mechanism for Kv7 CaM modulation based on a common architecture in which a CaM C-lobe calcium-dependent switch releases a shared Apo/CaM clamp conformation. This C-lobe switch inhibits voltage-dependent activation of Kv7.4 and Kv7.5 but facilitates Kv7.1, demonstrating that mechanism is shared by Kv7 isoforms despite the different directions of CaM modulation. Our findings provide a unified framework for understanding how CaM controls different Kv7 isoforms and highlight the role of membrane proximal domains for controlling voltage-gated channel function. VIDEO ABSTRACT.

Age-dependent electrical and morphological remodeling of the Drosophila heart caused by hERG/seizure mutations

Understanding the cellular-molecular substrates of heart disease is key to the development of cardiac specific therapies and to the prevention of off-target effects by non-cardiac targeted drugs. One of the primary targets for therapeutic intervention has been the human ether a go-go (hERG) K⁺ channel that, together with the KCNQ channel, controls the rate and efficiency of repolarization in human myocardial cells. Neither of these channels plays a major role in adult mouse heart function; however, we show here that the hERG homolog seizure (sei), along with KCNQ, both contribute significantly to adult heart function as they do in humans. In Drosophila, mutations in or cardiac knockdown of sei channels cause arrhythmias that become progressively more severe with age. Intracellular recordings of semi-intact heart preparations revealed that these perturbations also cause electrical remodeling that is reminiscent of the early afterdepolarizations seen in human myocardial cells defective in these channels. In contrast to KCNQ, however, mutations in sei also cause extensive structural remodeling of the myofibrillar organization, which suggests that hERG channel function has a novel link to sarcomeric and myofibrillar integrity. We conclude that deficiency of ion channels with similar electrical functions in cardiomyocytes can lead to different types or extents of electrical and/or structural remodeling impacting cardiac output.

We have used the fruit fly cardiac model to show that seizure, the fly homolog of the human ether a go-go K⁺ channel hERG, is functional in the fly heart. This channel plays a major role in cardiac repolarization in humans but not in adult rodent hearts. Loss of channel function in the fly causes bradycardia, electrical arrhythmia and altered myofibrillar structure. Gene expression analysis indicates that Wnt signaling is affected and we show a genetic interaction between sei and pygopus, a Wnt pathway component, on heart function.

Modification of multiple ion channel functions in vivo by pharmacological inhibition: observation by threshold tracking and modeling

Maintenance of axonal excitability relies on complex balance by multiple ion currents, but its evaluation is limited by in vitro single channel neurophysiological study on overall behavior. We sought to evaluate behaviors of multiple ion currents by pharmacological blockade. The threshold tracking technique was used to measure multiple excitability indices on tail sensory nerve of normal male mice before and after administration of either BaCl₂ or ivabradine. Mathematical modeling was used to identify the interval changes of the channel parameters. After administration of BaCl₂ and ivabradine, the following changes were present: greater threshold changes of both depolarizing and hyperpolarizing threshold electrotonus by both; additionally, reduced S2 accommodation, reduced late subexcitability and increased superexcitability by BaCl₂, increased S3 accommodation by ivabradine. Mathematical modelling implied reduction of slow K⁺ conductance, along with reduction of H conductance (Ih) by BaCl₂; and reduction of Ih while augmentation of K⁺ conductances by ivabradine. Pharmacological blockade of a selective ion channel may be compensated by other ion channels. Unintended effects by ion channel modification could be caused by secondary current alteration by multiple ion channels. J. Med. Invest. 64: 30-38, February, 2017.

Genetic basis of pediatric epilepsy syndromes

Childhood epilepsy affects ~0.5-1% in the general population worldwide. Early-onset epileptic encephalopathies are considered to be severe neurological disorders, which lead to impaired motor, cognitive, and sensory development due to recurrence of seizures. Many of the observed epilepsy phenotypes are associated with specific chromosomal imbalances and thus display gene dosage effects, and also specific mutations of a variety of genes ranging from ion channels to transcription factors. High throughput sequencing technologies and whole exome sequencing have led to the recognition of several new candidate genes with a possible role in the pathogenesis of epileptic encephalopathies. The mutations causing channelopathies can be either a gain or a loss of ion channel function and contribute to the pathogenesis of epilepsy syndrome. Nearly 300 mutations of SCN1A gene coding for the Nav1.1 channel protein have been identified that contribute to the pathology of epilepsy. Besides Na, potassium and calcium channels are also implicated in epileptic encephalopathies. Therapeutic management of epileptic encephalopathies has been challenging as the majority of the medications are not efficient and often have many undesirable side effects. A better understanding of the molecular nature of epilepsy in an individual is important to design a personalized medication, considering the number of possible genetic mutations that can contribute to epileptic encephalopathies.

KCNE1 induces fenestration in the Kv7.1/KCNE1 channel complex that allows for highly specific pharmacological targeting

Most small-molecule inhibitors of voltage-gated ion channels display poor subtype specificity because they bind to highly conserved residues located in the channel's central cavity. Using a combined approach of scanning mutagenesis, electrophysiology, chemical ligand modification, chemical cross-linking, MS/MS-analyses and molecular modelling, we provide evidence for the binding site for adamantane derivatives and their putative access pathway in Kv7.1/KCNE1 channels. The adamantane compounds, exemplified by JNJ303, are highly potent gating modifiers that bind to fenestrations that become available when KCNE1 accessory subunits are bound to Kv7.1 channels. This mode of regulation by auxiliary subunits may facilitate the future development of potent and highly subtype-specific Kv channel inhibitors.

Specificity of inhibitors of voltage-gated ion channels is crucial for their use as therapeutics. Here, the authors show that adamantane derivatives interact with a specific binding site on fenestrations that only become available when accessory subunits are bound to the channel.

Protein arginine methylation facilitates KCNQ channel-PIP2 interaction leading to seizure suppression

KCNQ channels are critical determinants of neuronal excitability, thus emerging as a novel target of anti-epileptic drugs. To date, the mechanisms of KCNQ channel modulation have been mostly characterized to be inhibitory via Gq-coupled receptors, Ca²⁺/CaM, and protein kinase C. Here we demonstrate that methylation of KCNQ by protein arginine methyltransferase 1 (Prmt1) positively regulates KCNQ channel activity, thereby preventing neuronal hyperexcitability. Prmt1+/- mice exhibit epileptic seizures. Methylation of KCNQ2 channels at 4 arginine residues by Prmt1 enhances PIP₂ binding, and Prmt1 depletion lowers PIP₂ affinity of KCNQ2 channels and thereby the channel activities. Consistently, exogenous PIP₂ addition to Prmt1+/- neurons restores KCNQ currents and neuronal excitability to the WT level. Collectively, we propose that Prmt1-dependent facilitation of KCNQ-PIP₂ interaction underlies the positive regulation of KCNQ activity by arginine methylation, which may serve as a key target for prevention of neuronal hyperexcitability and seizures.

DOI:http://dx.doi.org/10.7554/eLife.17159.001

In the brain, cells called neurons transmit information along their length in the form of electrical signals. To generate electrical signals, ions move into and out of neurons through ion channel proteins – such as the KCNQ channel – in the surface of these cells, which open and close to control the electrical response of the neuron.

Abnormally intense bursts of electrical activity from many neurons at once can cause seizures such as those experienced by people with epilepsy. A significant proportion of patients do not respond to current anti-seizure medications. Openers of KCNQ channels have emerged as a potential new class of anti-epileptic drugs. A better understanding of how KCNQ channels work, and how their opening by PIP₂lipid signals is regulated, could help to develop more effective therapies for epilepsy.

A process called methylation controls many biological tasks by changing the structure of key proteins inside cells. Although methylation occurs throughout the brain, its role in controlling how easily neurons are activated (a property known as “excitability”) remains unclear.

Kim, Jeong, Kim, Jung et al. now show that a protein called Prmt1 methylates the KCNQ channels in mice, and that this methylation is essential for suppressing seizures. Mice born without the Prmt1 protein developed epileptic seizures and the KCNQ channels in their neurons featured a reduced level of methylation. However, increasing the amount of PIP₂ in these neurons restored their excitability back to normal levels. The methylation of KCNQ channel proteins increases their affinity for PIP2, which is critical to open KCNQ channels. Kim et al. propose that these “opening” controllers balance the action of known “closers” of KCNQ channels to maintain neurons in a healthy condition.

In future, Kim et al. plan to investigate whether methylation affects the activity of other ion channels controlled by PIP₂. Such experiments will complement a more widespread investigation into other ways in which the Prtmt1 protein may control the activity of neurons.

DOI:http://dx.doi.org/10.7554/eLife.17159.002

Ion channel regulation by β-secretase BACE1 - enzymatic and non-enzymatic effects beyond Alzheimer's disease

β-site APP-cleaving enzyme 1 (BACE1) has become infamous for its pivotal role in the pathogenesis of Alzheimer's disease (AD). Consequently, BACE1 represents a prime target in drug development. Despite its detrimental involvement in AD, it should be quite obvious that BACE1 is not primarily present in the brain to drive mental decline. In fact, additional functions have been identified. In this review, we focus on the regulation of ion channels, specifically voltage-gated sodium and KCNQ potassium channels, by BACE1. These studies provide evidence for a highly unexpected feature in the functional repertoire of BACE1. Although capable of cleaving accessory channel subunits, BACE1 exerts many of its physiologically significant effects through direct, non-enzymatic interactions with main channel subunits. We discuss how the underlying mechanisms can be conceived and develop scenarios how the regulation of ion conductances by BACE1 might shape electric activity in the intact and diseased brain and heart.

Retigabine holds KV7 channels open and stabilizes the resting potential

The anticonvulsant Retigabine is a KV7 channel agonist used to treat hyperexcitability disorders in humans. Retigabine shifts the voltage dependence for activation of the heteromeric KV7.2/KV7.3 channel to more negative potentials, thus facilitating activation. Although the molecular mechanism underlying Retigabine's action remains unknown, previous studies have identified the pore region of KV7 channels as the drug's target. This suggested that the Retigabine-induced shift in voltage dependence likely derives from the stabilization of the pore domain in an open (conducting) conformation. Testing this idea, we show that the heteromeric KV7.2/KV7.3 channel has at least two open states, which we named O1 and O2, with O2 being more stable. The O1 state was reached after short membrane depolarizations, whereas O2 was reached after prolonged depolarization or during steady state at the typical neuronal resting potentials. We also found that activation and deactivation seem to follow distinct pathways, suggesting that the KV7.2/KV7.3 channel activity displays hysteresis. As for the action of Retigabine, we discovered that this agonist discriminates between open states, preferentially acting on the O2 state and further stabilizing it. Based on these findings, we proposed a novel mechanism for the therapeutic effect of Retigabine whereby this drug reduces excitability by enhancing the resting potential open state stability of KV7.2/KV7.3 channels. To address this hypothesis, we used a model for action potential (AP) in Xenopus laevis oocytes and found that the resting membrane potential became more negative as a function of Retigabine concentration, whereas the threshold potential for AP firing remained unaltered.

The Sensorless Pore Module of Voltage-gated K+ Channel Family 7 Embodies the Target Site for the Anticonvulsant Retigabine

KCNQ (voltage-gated K(+) channel family 7 (Kv7)) channels control cellular excitability and underlie the K(+) current sensitive to muscarinic receptor signaling (the M current) in sympathetic neurons. Here we show that the novel anti-epileptic drug retigabine (RTG) modulates channel function of pore-only modules (PMs) of the human Kv7.2 and Kv7.3 homomeric channels and of Kv7.2/3 heteromeric channels by prolonging the residence time in the open state. In addition, the Kv7 channel PMs are shown to recapitulate the single-channel permeation and pharmacological specificity characteristics of the corresponding full-length proteins in their native cellular context. A mutation (W265L) in the reconstituted Kv7.3 PM renders the channel insensitive to RTG and favors the conductive conformation of the PM, in agreement to what is observed when the Kv7.3 mutant is heterologously expressed. On the basis of the new findings and homology models of the closed and open conformations of the Kv7.3 PM, we propose a structural mechanism for the gating of the Kv7.3 PM and for the site of action of RTG as a Kv7.2/Kv7.3 K(+) current activator. The results validate the modular design of human Kv channels and highlight the PM as a high-fidelity target for drug screening of Kv channels.

Structural basis of a Kv7.1 potassium channel gating module: studies of the intracellular c-terminal domain in complex with calmodulin

Kv7 channels tune neuronal and cardiomyocyte excitability. In addition to the channel membrane domain, they also have a unique intracellular C-terminal (CT) domain, bound constitutively to calmodulin (CaM). This CT domain regulates gating and tetramerization. We investigated the structure of the membrane proximal CT module in complex with CaM by X-ray crystallography. The results show how the CaM intimately hugs a two-helical bundle, explaining many channelopathic mutations. Structure-based mutagenesis of this module in the context of concatemeric tetramer channels and functional analysis along with in vitro data lead us to propose that one CaM binds to one individual protomer, without crosslinking subunits and that this configuration is required for proper channel expression and function. Molecular modeling of the CT/CaM complex in conjunction with small-angle X-ray scattering suggests that the membrane proximal region, having a rigid lever arm, is a critical gating regulator.

β-Secretase BACE1 regulates hippocampal and reconstituted M-currents in a β-subunit-like fashion

The β-secretase BACE1 is widely known for its pivotal role in the amyloidogenic pathway leading to Alzheimer's disease, but how its action on transmembrane proteins other than the amyloid precursor protein affects the nervous system is only beginning to be understood. We report here that BACE1 regulates neuronal excitability through an unorthodox, nonenzymatic interaction with members of the KCNQ (Kv7) family that give rise to the M-current, a noninactivating potassium current with slow kinetics. In hippocampal neurons from BACE1(-/-) mice, loss of M-current enhanced neuronal excitability. We relate the diminished M-current to the previously reported epileptic phenotype of BACE1-deficient mice. In HEK293T cells, BACE1 amplified reconstituted M-currents, altered their voltage dependence, accelerated activation, and slowed deactivation. Biochemical evidence strongly suggested that BACE1 physically associates with channel proteins in a β-subunit-like fashion. Our results establish BACE1 as a physiologically essential constituent of regular M-current function and elucidate a striking new feature of how BACE1 impacts on neuronal activity in the intact and diseased brain.

A novel frameshift mutation in KCNQ4 in a family with autosomal recessive non-syndromic hearing loss

Mutation of KCNQ4 has been reported to cause autosomal dominant non-syndromic hearing loss (DFNA2A) that usually presents as progressive hearing loss starting from mild to moderate hearing loss during childhood. Here, we identified a novel KCNQ4 mutation, c.1044_1051del8, in a family with autosomal recessive non-syndromic hearing loss. The proband was homozygous for the mutation and was born to consanguineous parents; she showed severe hearing loss that was either congenital or of early childhood onset. The proband had a sister who was heterozygous for the mutation but showed normal hearing. The mutation caused a frameshift that eliminated most of the cytoplasmic C-terminus, including the A-domain, which has an important role for protein tetramerization, and the B-segment, which is a binding site for calmodulin (CaM) that regulates channel function via Ca ions. The fact that the heterozygote had normal hearing indicates that sufficient tetramerization and CaM binding sites were present to preserve a normal phenotype even when only half the proteins contained an A-domain and B-segment. On the other hand, the severe hearing loss in the homozygote suggests that complete loss of the A-domain and B-segment in the protein caused loss of function due to the failure of tetramer formation and CaM binding. This family suggests that some KCNQ4 mutations can cause autosomal recessive hearing loss with more severe phenotype in addition to autosomal dominant hearing loss with milder phenotype. This genotype-phenotype correlation is analogous to that in KCNQ1 which causes autosomal dominant hereditary long QT syndrome 1 with milder phenotype and the autosomal recessive Jervell and Lange-Nielsen syndrome 1 with more severe phenotype due to deletion of the cytoplasmic C-terminus of the potassium channel.

Functional analysis of potassium channels in Kv7.2 G271V mutant causing early onset familial epilepsy

Kv7 (KCNQ) channels underlying a class of voltage-gated K+ current are best known for regulating neuronal excitability. The first glycine (G) residue in the pore helix of Kv7.2 (KCNQ2) subunit is highly conserved among different classes of Kv7 channel family. A missense mutation causing the replacement of the corresponding G residues with a valine (p.G271V) in Kv7.2 was found in a large, four-generation pedigree. Here, we set out to examine the molecular pathomechanism of G271V mutants using patch clamp technology combined with biochemical and immunocytochemical techniques in transiently transfected human embryonic kidney (HEK) 293 cells. The expression of Kv7.2 protein had the same intensity for both wild type (WT) and G271V. In transfected HEK cells, G271V mutants induced large depolarizing shifts of the conductance-voltage relationships and marked slowing of current activation kinetics compared to WT. In addition, G271V mutants abolished currents in homomeric channels, and resulted in about 50% reduction of current in Kv7.2/G271V/Kv7.3 heteromultimeric condition, indicating a more severe functional defect. To test for G271V mutant channel expression in surface membrane, we performed fluorescence confocal microscopy imaging, which revealed no differences between the mutant and WT, suggesting that G271V channels fail to open in response to depolarization even though they are present in the membrane. Furthermore, pharmacologic intervention experiments revealed that upon specific incubation of transfected HEK 293 cells expressing G271V heteromultimeric channels in presence of Kv7 channel enhancer retigabine (ezogabine), the potassium currents increased significantly, suggesting the potential of retigabine as gene-specific therapy.

New antiepileptic medication linked to blue discoloration of the skin and eyes

Ezogabine is an antiepileptic medication approved in June 2011 by the US Food and Drug Administration (FDA) as an adjunctive treatment for partial seizures. Minimal drug interactions and a novel mechanism of action made ezogabine an appealing new treatment option. However, adverse effects reported during clinical trials and following drug approval have been alarming. A Risk Evaluation Mitigation Strategy (REMS) program has been established for urinary retention. A safety alert was published in April 2013 warning ezogabine may cause retinal pigment abnormalities and/or blue-gray discoloration, most notably on or near the lips, nail beds, sclera and conjunctiva with long-term use. In October 2013, the FDA announced a formal label change to ezogabine to include a black boxed warning emphasizing the previously reported warnings of eye and skin discoloration and permanent vision changes. Given the unknown nature of the pathophysiology, consequences and potential for reversibility of these effects, GlaxoSmithKline and the FDA have published recommendations for patients currently receiving ezogabine. Further data from published case reports and long-term safety trials in the future may lend additional insight into these concerning effects.

The human ether-a-go-go-related gene activator NS1643 enhances epilepsy-associated KCNQ channels

Human ether-a-go-go-related gene (hERG) and KCNQ channels are two classes of voltage-gated potassium channels. Specific mutations have been identified that are causal for type II long QT (LQT2) syndrome, neonatal epilepsy, and benign familial neonatal convulsions. Increasing evidence from clinical studies suggests that LQT2 and epilepsy coexist in some patients. Therefore, an integral approach to investigating and treating the two diseases is likely more effective. In the current study, we found that NS1643 [1,3-bis-(2-hydroxy-5-trifluoromethyl-phenyl)-urea], a previously reported hERG activator, is also an activator of KCNQ channels. It potentiates the neuronal KCNQ2, KCNQ4, and KCNQ2/Q3 channels, but not the cardiac KCNQ1. The effects of NS1643 on the KCNQ2 channel include left shifting of voltage for reaching 50% of the maximum conductance and slowing of deactivation. Analysis of the dose-response curve of NS1643 revealed an EC50 value of 2.44 ± 0.25 μM. A hydrophobic phenylalanine (F137) located at the middle region of the voltage-sensing domain was identified as critical for NS1643 activity on KCNQ2. When testing NS1643 effects in rescuing LQT2 hERG mutants and the KCNQ2 BFNC mutants, we found it is particularly efficacious in some cases. Considering the substantial relationship between LQT2 and epilepsy, these findings reveal that NS1643 is a useful compound to elucidate the causal connection of LQT2 and epilepsy. More generally, this may provide a strategy in the development of therapeutics for LQT2 and epilepsy.