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

Loss-of-function variant in KCNH3 is associated with global developmental delay, autistic behavior, insomnia, and nocturnal seizures

INTRODUCTION

The KCNH gene family encodes voltage-gated potassium (Kv) channels of the EAG subtype covering three subfamilies (Kv10-12). EAG channels are involved in the control of cardiac and neuronal excitation, and pathogenic variants in KCNH genes encoding Kv10 (eag) and Kv11 (erg) subfamily members cause a broad clinical spectrum ranging from cardiac arrhythmia to neurodevelopmental syndromes. However, no pathogenic variants have been hitherto reported for KCNH genes encoding Kv12 (elk) subfamily members.

METHODS

Clinical, genomic, and functional studies were performed, including voltage-clamp experiments using heterologous channel expression in Xenopus oocytes.

RESULTS

We examined an eight-year-old girl presenting with global developmental delay, intellectual disability, autistic and aggressive behavior, hyperactivity, insomnia, and nocturnal seizures. Focal seizures were successfully treated with sulthiame, which reduced the occurrence of temporo-parietal spike-wave paroxysms. Trio exome sequencing revealed a heterozygous de novo missense variant, NM_012284.3:c.1112C>T; p.(Ala371Val), in KCNH3, which encodes the Kv channel α-subunit Kv12.2. The amino acid substitution associated with the KCNH3 variant identified in the patient is located at a site highly conserved in EAG channels. The analogous variant in KCNH2 causes long-QT-syndrome 2, and has also been associated with epilepsy. Electrophysiological characterization of the KCNH3 p.(Ala371Val) variant demonstrated loss-of-function of the mutant Kv12.2 channels and strongly reduced current amplitudes upon co-expression of wildtype and mutant channel subunits in a dominant-negative manner.

CONCLUSION

Our results propose KCNH3, which is primarily expressed in the nervous system, as a new disease gene associated with a neurodevelopmental phenotype including seizures.

A Novel De Novo Variant in KCNH5 in a Patient with Refractory Epileptic Encephalopathy

We herein report a novel de novo KCNH5 variant in a patient with refractory epileptic encephalopathy. The patient exhibited seizures at 1 year and 7 months old, which gradually worsened, leading to a bedridden status. Brain magnetic resonance imaging (MRI) showed cerebral atrophy and cerebellar hypoplasia. A trio whole-exome sequence analysis identified a de novo heterozygous c.640A>C, p.Lys214Gln variant in KCNH5 that was predicted to be deleterious. Recent studies have linked KCNH5 to various epileptic encephalopathies, with many patients showing normal MRI findings. The present case expands the clinical spectrum of the disease, as it is characterized by severe neurological prognosis, cerebral atrophy, and cerebellar hypoplasia.

Direct effects of antipsychotics on potassium channels

Schizophrenia (SCZ) and bipolar disorder (BD) and are severe psychiatric conditions that contribute to disability and increased healthcare costs globally. Although first-, second-, and third-generation antipsychotics are available for treating BD and SCZ, most have various side effects unrelated to their unique functions. Many antipsychotics affect K+ channels (Kv, KCa, Kir, K2P, and other channels), which change the functions of various organs. This review summarizes the biological actions of antipsychotics, including off-target side effects involving K+ channels.

Voltage-gated potassium channels and genetic epilepsy

Recent advances in exome and targeted sequencing have significantly improved the aetiological diagnosis of epilepsy, revealing an increasing number of epilepsy-related pathogenic genes. As a result, the diagnosis and treatment of epilepsy have become more accessible and more traceable. Voltage-gated potassium channels (Kv) regulate electrical excitability in neuron systems. Mutate Kv channels have been implicated in epilepsy as demonstrated in case reports and researches using gene-knockout mouse models. Both gain and loss-of-function of Kv channels lead to epilepsy with similar phenotypes through different mechanisms, bringing new challenges to the diagnosis and treatment of epilepsy. Research on genetic epilepsy is progressing rapidly, with several drug candidates targeting mutated genes or channels emerging. This article provides a brief overview of the symptoms and pathogenesis of epilepsy associated with voltage-gated potassium ion channels dysfunction and highlights recent progress in treatments. Here, we reviewed case reports of gene mutations related to epilepsy in recent years and summarized the proportion of Kv genes. Our focus is on the progress in precise treatments for specific voltage-gated potassium channel genes linked to epilepsy, including KCNA1, KCNA2, KCNB1, KCNC1, KCND2, KCNQ2, KCNQ3, KCNH1, and KCNH5.

Neurocardiac pathologies associated with potassium channelopathies

Voltage-gated potassium channels are expressed throughout the human body and are essential for physiological functions. These include delayed rectifiers, A-type channels, outward rectifiers, and inward rectifiers. They impact electrical function in the heart (repolarization) and brain (repolarization and stabilization of the resting membrane potential). KCNQx and KCNHx encode Kv7.x and Kv11.x proteins, which form delayed rectifier potassium channels. KCNQx and KCNHx channelopathies are associated with both cardiac and neuronal pathologies. These include electrocardiographic abnormalities, cardiac arrhythmias, sudden cardiac death (SCD), epileptiform discharges, seizures, bipolar disorder, and sudden unexpected death in epilepsy (SUDEP). Due to the ubiquitous expression of KCNQx and KCNHx channels, abnormalities in their function can be particularly harmful, increasing the risk of sudden death. For example, KCNH2 variants have a dual role in both cardiac and neuronal pathologies, whereas KCNQ2 and KCNQ3 variants are associated with severe and refractory epilepsy. Recurrent and uncontrolled seizures lead to secondary abnormalities, which include autonomics, cardiac electrical function, respiratory drive, and neuronal electrical activity. Even with a wide array of anti-seizure therapies available on the market, one-third of the more than 70 million people worldwide with epilepsy have uncontrolled seizures (i.e., intractable/drug-resistant epilepsy), which negatively impact neurodevelopment and quality of life. To capture the current state of the field, this review examines KCNQx and KCNHx expression patterns and electrical function in the brain and heart. In addition, it discusses several KCNQx and KCNHx variants that have been clinically and electrophysiologically characterized. Because these channel variants are associated with multi-system pathologies, such as epileptogenesis, Kv7 channel modulators provide a potential anti-seizure therapy, particularly for people with intractable epilepsy. Ultimately an increased understanding of the role of Kv channels throughout the body will fuel the development of innovative, safe, and effective therapies for people at a high risk of sudden death (SCD and SUDEP).

KCNH5 deletion increases autism susceptibility by regulating neuronal growth through Akt/mTOR signaling pathway

Recent clinical studies have highlighted mutations in the voltage-gated potassium channel Kv10.2 encoded by the KCNH5 gene among individuals with autism spectrum disorder (ASD). Our preliminary study found that Kv10.2 was decreased in the hippocampus of valproic acid (VPA) - induced ASD rats. Nevertheless, it is currently unclear how KCNH5 regulates autism-like features, or becomes a new target for autism treatment. We employed KCNH5 knockout (KCNH5-/-) rats and VPA - induced ASD rats in this study. Then, we used behavioral assessments, combined with electrophysiological recordings and hippocampal brain slice, to elucidate the impact of KCNH5 deletion and environmental factors on neural development and function in rats. We found that KCNH5-/- rats showed early developmental delay, neuronal overdevelopment, and abnormal electroencephalogram (EEG) signals, but did not exhibit autism-like behavior. KCNH5-/- rats exposed to VPA (KCNH5-/--VPA) exhibit even more severe autism-like behaviors and abnormal neuronal development. The absence of KCNH5 excessively enhances the activity of the Protein Kinase B (Akt)/Mechanistic Target of Rapamycin (mTOR) signaling pathway in the hippocampus of rats after exposure to VPA. Overall, our findings underscore the deficiency of KCNH5 increases the susceptibility to autism under environmental exposures, suggesting its potential utility as a target for screening and diagnosis in ASD.

Clinical phenotypes of developmental and epileptic encephalopathy-related recurrent KCNH5 missense variant p.R327H in Chinese children

KCNH5 gene encodes for the voltage-gated potassium channel protein Kv10.2. Here, we investigated the clinical features of developmental and epileptic encephalopathy (DEE) in five Chinese pediatric patients with a missense mutation (p.R327H) in KCNH5 gene. These patients had undergone video EEG to evaluate background features and epileptiform activity, as well as 3.0 T MRI scans for structural analysis and intelligence assessments using the Gesell Developmental Observation or Wechsler Intelligence Scale for Children. Seizure onset occurs between 4 and 10 months of age, with focal and generalized tonic-clonic seizures being common. Initial EEG findings showed multiple multifocal sharp waves, sharp slow waves or spike slow waves, and spike waves. Brain MRI revealed widened extracerebral space in only one patient. Mechanistically, the KCNH5 mutation disrupts the two hydrogen bonds between Arg327 and Asp304 residues, potentially altering the protein's structural stability and function. Almost 80 % of patients receiving add-on valproic acid (VPA) therapy experienced a reduction in epileptic seizure frequency. Altogether, this study presents the first Chinese cohort of pediatric DEE patients with the KCNH5 p.R327H mutation, highlighting focal seizures as the predominant seizure type and incomplete mutation penetrance. Add-on VPA therapy was likely effective in the early stages of DEE pathogenesis.

Role of Potassium Ion Channels in Epilepsy: Focus on Current Therapeutic Strategies

BACKGROUND

Epilepsy is one of the prevalent neurological disorders characterized by disrupted synchronization between inhibitory and excitatory neurons. Disturbed membrane potential due to abnormal regulation of neurotransmitters and ion transport across the neural cell membrane significantly contributes to the pathophysiology of epilepsy. Potassium ion channels (KCN) regulate the resting membrane potential and are involved in neuronal excitability. Genetic alterations in the potassium ion channels (KCN) have been reported to result in the enhancement of the release of neurotransmitters, the excitability of neurons, and abnormal rapid firing rate, which lead to epileptic phenotypes, making these ion channels a potential therapeutic target for epilepsy. The aim of this study is to explore the variations reported in different classes of potassium ion channels (KCN) in epilepsy patients, their functional evaluation, and therapeutic strategies to treat epilepsy targeting KCN.

METHODOLOGY

A review of all the relevant literature was carried out to compile this article.

RESULTS

A large number of variations have been reported in different genes encoding various classes of KCN. These genetic alterations in KCN have been shown to be responsible for disrupted firing properties of neurons. Antiepileptic drugs (AEDs) are the main therapeutic strategy to treat epilepsy. Some patients do not respond favorably to the AEDs treatment, resulting in pharmacoresistant epilepsy.

CONCLUSION

Further to address the challenges faced in treating epilepsy, recent approaches like optogenetics, chemogenetics, and genome editing, such as clustered regularly interspaced short palindromic repeats (CRISPR), are emerging as target-specific therapeutic strategies.

Neurodevelopmental and Epilepsy Phenotypes in Individuals With Missense Variants in the Voltage-Sensing and Pore Domains of KCNH5

BACKGROUND AND OBJECTIVES

KCNH5 encodes the voltage-gated potassium channel EAG2/Kv10.2. We aimed to delineate the neurodevelopmental and epilepsy phenotypic spectrum associated with de novo KCNH5 variants.

METHODS

We screened 893 individuals with developmental and epileptic encephalopathies for KCNH5 variants using targeted or exome sequencing. Additional individuals with KCNH5 variants were identified through an international collaboration. Clinical history, EEG, and imaging data were analyzed; seizure types and epilepsy syndromes were classified. We included 3 previously published individuals including additional phenotypic details.

RESULTS

We report a cohort of 17 patients, including 9 with a recurrent de novo missense variant p.Arg327His, 4 with a recurrent missense variant p.Arg333His, and 4 additional novel missense variants. All variants were located in or near the functionally critical voltage-sensing or pore domains, absent in the general population, and classified as pathogenic or likely pathogenic using the American College of Medical Genetics and Genomics criteria. All individuals presented with epilepsy with a median seizure onset at 6 months. They had a wide range of seizure types, including focal and generalized seizures. Cognitive outcomes ranged from normal intellect to profound impairment. Individuals with the recurrent p.Arg333His variant had a self-limited drug-responsive focal or generalized epilepsy and normal intellect, whereas the recurrent p.Arg327His variant was associated with infantile-onset DEE. Two individuals with variants in the pore domain were more severely affected, with a neonatal-onset movement disorder, early-infantile DEE, profound disability, and childhood death.

DISCUSSION

We describe a cohort of 17 individuals with pathogenic or likely pathogenic missense variants in the voltage-sensing and pore domains of Kv10.2, including 14 previously unreported individuals. We present evidence for a putative emerging genotype-phenotype correlation with a spectrum of epilepsy and cognitive outcomes. Overall, we expand the role of EAG proteins in human disease and establish KCNH5 as implicated in a spectrum of neurodevelopmental disorders and epilepsy.

A novel loss-of-function mutation of the voltage-gated potassium channel Kv10.2 involved in epilepsy and autism

Background

Novel developmental mutations associated with disease are a continuous challenge in medicine. Clinical consequences caused by these mutations include neuron and cognitive alterations that can lead to epilepsy or autism spectrum disorders. Often, it is difficult to identify the physiological defects and the appropriate treatments.

Results

We have isolated and cultured primary cells from the skin of a patient with combined epilepsy and autism syndrome. A mutation in the potassium channel protein Kv10.2 was identified. We have characterised the alteration of the mutant channel and found that it causes loss of function (LOF). Primary cells from the skin displayed a very striking growth defect and increased differentiation. In vitro treatment with various carbonic anhydrase inhibitors with various degrees of specificity for potassium channels, (Brinzolamide, Acetazolamide, Retigabine) restored the activation capacity of the mutated channel. Interestingly, the drugs also recovered in vitro the expansion capacity of the mutated skin cells. Furthermore, treatment with Acetazolamide clearly improved the patient regarding epilepsy and cognitive skills. When the treatment was temporarily halted the syndrome worsened again.

Conclusions

By in vitro studying primary cells from the patient and the activation capacity of the mutated protein, we could first, find a readout for the cellular defects and second, test pharmaceutical treatments that proved to be beneficial. The results show the involvement of a novel LOF mutation of a Potassium channel in autism syndrome with epilepsy and the great potential of in vitro cultures of primary cells in personalised medicine of rare diseases.

Clinical Feature, Treatment, and KCNH5 Mutations in Epilepsy

The voltage-gated Kv10.2 potassium channel, encoded by KCNH5, is broadly expressed in mammalian tissues, including the brain. Its potential mechanism remains unclear. According to previous studies, dysfunction of Kv10.2 may be associated with epileptic encephalopathies and autism spectrum disorder (ASD). To date, only one disease-causing mutation of KCNH5 has been reported, and it involves a case that presented with seizures and autism symptoms. In this study, we discovered and characterized three de novo mutations in KCNH5 that potentially caused severe conditions observed in three Chinese children. All of them experienced seizures, two of them presented with epileptic encephalopathy, one of them presented with ASD, and one did not relapse after drug withdrawal. Notably, treatment with antiepileptic drugs (AEDs) was effective in all patients whose epileptic seizures were controlled. The structures of the proteins resulting from the mutations were predicted in two of the three cases. This provides powerful insight into clinical heterogeneity and genotype-phenotype correlation in KCNH5-related diseases.

NRXN1α+/- is associated with increased excitability in ASD iPSC-derived neurons

Background

NRXN1 deletions are identified as one of major rare risk factors for autism spectrum disorder (ASD) and other neurodevelopmental disorders. ASD has 30% co-morbidity with epilepsy, and the latter is associated with excessive neuronal firing. NRXN1 encodes hundreds of presynaptic neuro-adhesion proteins categorized as NRXN1α/β/γ. Previous studies on cultured cells show that the short NRXN1β primarily exerts excitation effect, whereas the long NRXN1α which is more commonly deleted in patients involves in both excitation and inhibition. However, patient-derived models are essential for understanding functional consequences of NRXN1α deletions in human neurons. We recently derived induced pluripotent stem cells (iPSCs) from five controls and three ASD patients carrying NRXN1α^+/- and showed increased calcium transients in patient neurons.

Methods

In this study we investigated the electrophysiological properties of iPSC-derived cortical neurons in control and ASD patients carrying NRXN1α^+/- using patch clamping. Whole genome RNA sequencing was carried out to further understand the potential underlying molecular mechanism.

Results

NRXN1α^{+/-} cortical neurons were shown to display larger sodium currents, higher AP amplitude and accelerated depolarization time. RNASeq analyses revealed transcriptomic changes with significant upregulation glutamatergic synapse and ion channels/transporter activity including voltage-gated potassium channels (GRIN1, GRIN3B, SLC17A6, CACNG3, CACNA1A, SHANK1), which are likely to couple with the increased excitability in NRXN1α^{+/-} cortical neurons.

Conclusions

Together with recent evidence of increased calcium transients, our results showed that human NRXN1α^{+/-} isoform deletions altered neuronal excitability and non-synaptic function, and NRXN1α^{+/-} patient iPSCs may be used as an ASD model for therapeutic development with calcium transients and excitability as readouts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12868-021-00661-0.

Potassium channels and autism spectrum disorder: An overview

Autism spectrum disorder (ASD) comprises a group of neurodevelopmental disorders characterized by impaired social interaction and communication, and restricted, repetitive patterns of behaviors, interests, or activities. It had been demonstrated that potassium channels played a key role in regulating neuronal excitability, which was closely associated with neurological diseases including epilepsy, ataxia, myoclonus, and psychiatric disorders. In recent years, a growing body of evidence from whole-genome sequencing and whole-exome sequencing had identified several ASD susceptibility genes of potassium channels in ASD subjects. Genetically dysfunction of potassium channels may be involved in altered neuronal excitability and abnormal brain function in the pathogenesis of ASD. This review summarizes current findings on the features of ASD-risk genes (KCND2, KCNQ2, KCNQ3, KCNH5, KCNJ2, KCNJ10, and KCNMA1) and further expatiate their potential role in the pathogenicity of ASD.

Impaired neuronal activity and differential gene expression in STXBP1 encephalopathy patient iPSC-derived GABAergic neurons

Syntaxin-binding protein 1 (STXBP1; also called MUNC18-1), encoded by STXBP1, is an essential component of the molecular machinery that controls synaptic vesicle docking and fusion. De novo pathogenic variants of STXBP1 cause a complex set of neurological disturbances, namely STXBP1 encephalopathy (STXBP1-E) that includes epilepsy, neurodevelopmental disorders and neurodegeneration. Several animal studies have suggested the contribution of GABAergic dysfunction in STXBP1-E pathogenesis. However, the pathophysiological changes in GABAergic neurons of these patients are still poorly understood. Here, we exclusively generated GABAergic neurons from STXBP1-E patient-derived induced pluripotent stem cells (iPSCs) by transient expression of the transcription factors ASCL1 and DLX2. We also generated CRISPR/Cas9-edited isogenic iPSC-derived GABAergic (iPSC GABA) neurons as controls. We demonstrated that the reduction in STXBP1 protein levels in patient-derived iPSC GABA neurons was slight (approximately 20%) compared to the control neurons, despite a 50% reduction in STXBP1 mRNA levels. Using a microelectrode array-based assay, we found that patient-derived iPSC GABA neurons exhibited dysfunctional maturation with reduced numbers of spontaneous spikes and bursts. These findings reinforce the idea that GABAergic dysfunction is a crucial contributor to STXBP1-E pathogenesis. Moreover, gene expression analysis revealed specific dysregulation of genes previously implicated in epilepsy, neurodevelopment and neurodegeneration in patient-derived iPSC GABA neurons, namely KCNH1, KCNH5, CNN3, RASGRF1, SEMA3A, SIAH3 and INPP5F. Thus, our study provides new insights for understanding the biological processes underlying the widespread neuropathological features of STXBP1-E.

Reduced GABAergic Neuron Excitability, Altered Synaptic Connectivity, and Seizures in a KCNT1 Gain-of-Function Mouse Model of Childhood Epilepsy

Gain-of-function (GOF) variants in K+ channels cause severe childhood epilepsies, but there are no mechanisms to explain how increased K+ currents lead to network hyperexcitability. Here, we introduce a human Na+-activated K+ (KNa) channel variant (KCNT1-Y796H) into mice and, using a multiplatform approach, find motor cortex hyperexcitability and early-onset seizures, phenotypes strikingly similar to those of human patients. Although the variant increases KNa currents in cortical excitatory and inhibitory neurons, there is an increase in the KNa current across subthreshold voltages only in inhibitory neurons, particularly in those with non-fast-spiking properties, resulting in inhibitory-neuron-specific impairments in excitability and action potential (AP) generation. We further observe evidence of synaptic rewiring, including increases in homotypic synaptic connectivity, accompanied by network hyperexcitability and hypersynchronicity. These findings support inhibitory-neuron-specific mechanisms in mediating the epileptogenic effects of KCNT1 channel GOF, offering cell-type-specific currents and effects as promising targets for therapeutic intervention.

Deletion of Kv10.2 Causes Abnormal Dendritic Arborization and Epilepsy Susceptibility

The abnormal function of the voltage-gated potassium channel Kv10.2 can induce epilepsy. However, the physiological function of Kv10.2 in the central nervous system remains unclear. In this study, we found that Kv10.2 knockout (KO) increased the complexity of neurons in the CA3 subarea of hippocampus. Kv10.2 KO led to enlarged somata, elongated dendritic length, and increased the number of dendritic tips in cultured rat hippocampus neurons. Kv10.2 KO also increased Synapsin I and PSD95 protein density in cultured rat hippocampal neurons. Whole cell patch-clamp recordings of brain slices in the CA3 subarea of hippocampus revealed that Kv10.2 KO increased the amplitude of spontaneous excitatory postsynaptic currents (sEPSC) and miniature excitatory postsynaptic currents (mEPSC), depolarized the resting membrane potential and increased the action potential firing, reduced the rheobase and increased the input resistance, which results in enhanced neuronal excitability. Furthermore, we made electroencephalogram (EEG) recordings of brain activity in freely moving rats before and after inducing seizures by pentylenetetrazole (PTZ) injection. Kv10.2 KO rats dramatically increased the EEG amplitude during epilepsy. Behavioral observation after seizure induction revealed that Kv10.2 KO rats demonstrated shortened onset latency, prolonged duration, and increased seizure severity when compared with wild type rats. Therefore, this study provides a new link between Kv10.2 and neuronal morphology and higher intrinsic excitability.

Sox11 is an Activity-Regulated Gene with Dentate-Gyrus-Specific Expression Upon General Neural Activation

Neuronal activity initiates transcriptional programs that shape long-term changes in plasticity. Although neuron subtypes differ in their plasticity response, most activity-dependent transcription factors (TFs) are broadly expressed across neuron subtypes and brain regions. Thus, how region- and neuronal subtype-specific plasticity are established on the transcriptional level remains poorly understood. We report that in young adult (i.e., 6-8 weeks old) mice, the developmental TF SOX11 is induced in neurons within 6 h either by electroconvulsive stimulation or by exploration of a novel environment. Strikingly, SOX11 induction was restricted to the dentate gyrus (DG) of the hippocampus. In the novel environment paradigm, SOX11 was observed in a subset of c-FOS expressing neurons (ca. 15%); whereas around 75% of SOX11+ DG granule neurons were c-FOS+, indicating that SOX11 was induced in an activity-dependent fashion in a subset of neurons. Environmental enrichment or virus-mediated overexpression of SOX11 enhanced the excitability of DG granule cells and downregulated the expression of different potassium channel subunits, whereas conditional Sox11/4 knock-out mice presented the opposite phenotype. We propose that Sox11 is regulated in an activity-dependent fashion, which is specific to the DG, and speculate that activity-dependent Sox11 expression may participate in the modulation of DG neuron plasticity.

Chlorpromazine binding to the PAS domains uncovers the effect of ligand modulation on EAG channel activity

Ether-a-go-go (EAG) potassium selective channels are major regulators of neuronal excitability and cancer progression. EAG channels contain a Per-Arnt-Sim (PAS) domain in their intracellular N-terminal region. The PAS domain is structurally similar to the PAS domains in non-ion channel proteins, where these domains frequently function as ligand-binding domains. Despite the structural similarity, it is not known whether the PAS domain can regulate EAG channel function via ligand binding. Here, using surface plasmon resonance, tryptophan fluorescence, and analysis of EAG currents recorded in Xenopus laevis oocytes, we show that a small molecule chlorpromazine (CH), widely used as an antipsychotic medication, binds to the isolated PAS domain of EAG channels and inhibits currents from these channels. Mutant EAG channels that lack the PAS domain show significantly lower inhibition by CH, suggesting that CH affects currents from EAG channels directly through the binding to the PAS domain. Our study lends support to the hypothesis that there are previously unaccounted steps in EAG channel gating that could be activated by ligand binding to the PAS domain. This has broad implications for understanding gating mechanisms of EAG and related ERG and ELK K+ channels and places the PAS domain as a new target for drug discovery in EAG and related channels. Up-regulation of EAG channel activity is linked to cancer and neurological disorders. Our study raises the possibility of repurposing the antipsychotic drug chlorpromazine for treatment of neurological disorders and cancer.

Genetic potassium channel-associated epilepsies: Clinical review of the Kv family

Next-generation sequencing has enhanced discovery of many disease-associated genes in previously unexplained epilepsies, mainly in developmental and epileptic encephalopathies and familial epilepsies. We now classify these disorders according to the underlying molecular pathways, which encompass a diverse array of cellular and sub-cellular compartments/signalling processes including voltage-gated ion-channel defects. With the aim to develop and increase the use of precision medicine therapies, understanding the pathogenic mechanisms and consequences of disease-causing variants has gained major relevance in clinical care. The super-family of voltage-gated potassium channels is the largest and most diverse family among the ion channels, encompassing approximately 80 genes. Key potassium channelopathies include those affecting the K_V, K_Ca and K_ir families, a significant proportion of which have been implicated in neurological disease. As for other ion channel disorders, different pathogenic variants within any individual voltage-gated potassium channel gene tend to affect channel protein function differently, causing heterogeneous clinical phenotypes. The focus of this review is to summarise recent clinical developments regarding the key voltage-gated potassium (K_V) family-related epilepsies, which now encompasses approximately 12 established disease-associated genes, from the KCNA-, KCNB-, KCNC-, KCND-, KCNV-, KCNQ- and KCNH-subfamilies.

Myotonia in a patient with a mutation in an S4 arginine residue associated with hypokalaemic periodic paralysis and a concomitant synonymous CLCN1 mutation

The sarcolemmal voltage gated sodium channel Na_V1.4 conducts the key depolarizing current that drives the upstroke of the skeletal muscle action potential. It contains four voltage-sensing domains (VSDs) that regulate the opening of the pore domain and ensuing permeation of sodium ions. Mutations that lead to increased Na_V1.4 currents are found in patients with myotonia or hyperkalaemic periodic paralysis (HyperPP). Myotonia is also caused by mutations in the CLCN1gene that result in loss-of-function of the skeletal muscle chloride channel ClC-1. Mutations affecting arginine residues in the fourth transmembrane helix (S4) of the Na_V1.4 VSDs can result in a leak current through the VSD and hypokalemic periodic paralysis (HypoPP), but these have hitherto not been associated with myotonia. We report a patient with an Nav1.4 S4 arginine mutation, R222Q, presenting with severe myotonia without fulminant paralytic episodes. Other mutations affecting the same residue, R222W and R222G, have been found in patients with HypoPP. We show that R222Q channels have enhanced activation, consistent with myotonia, but also conduct a leak current. The patient carries a concomitant synonymous CLCN1 variant that likely worsens the myotonia and potentially contributes to the amelioration of muscle paralysis. Our data show phenotypic variability for different mutations affecting the same S4 arginine that have implications for clinical therapy.

Central changes in the Kv10.2 potassium channel in stress-induced hypertension rats

Ion channels play as a pivotal role in hypertension in the processes of maintenance of vascular tone and sympathetic excitement of hypertension. The Kv10.2 channel (encoded by the Kcnh5 gene) belongs to the EAG voltage-gated superfamily. It is distributed widely in the brain, such as the hippocampus, the cortex, and the olfactory bulb. To date, the expression of Kv10.2 in central nervous system nuclei that regulates cardiovascular function and its inter-relationship with hypertension are still unclear. Here, electric foot-shock stressors with noise were used to establish the stress-induced hypertensive (SIH) rat model. The expression of Kv10.2 in the rostral ventrolateral medulla, the nucleus tractus solitarius, and the paraventricular nucleus (PVN) was examined by immunohistochemical staining and western blots. The following results were obtained: (a) the expression level of Kv10.2 was increased obviously in the paraventricular nucleus of SIH rats, whereas no significant difference was found in the rostral ventrolateral medulla and the nucleus tractus solitarius. (b) Kv10.2 was located in neurons. (c) Vesicular glutamate transporter 1 as a protein mark of glutamate neurons was increased in the paraventricular nucleus of the SIH group. (d) The expression of vesicular glutamate transporter 1 protein in neurons was significantly decreased when the Kcnh5 gene was knocked down by small interfering RNA in vitro. These findings indicate that the changes in Kv10.2 may be related to SIH, which may provide a potential avenue for further investigation of SIH.

Voltage-dependent activation in EAG channels follows a ligand-receptor rather than a mechanical-lever mechanism

Ether-a-go-go family (EAG) channels play a major role in many physiological processes in humans, including cardiac repolarization and cell proliferation. Cryo-EM structures of two of them, K_V10.1 and human ether-a-go-go-related gene (hERG or K_V11.1), have revealed an original nondomain-swapped structure, suggesting that the mechanism of voltage-dependent gating of these two channels is quite different from the classical mechanical-lever model. Molecular aspects of hERG voltage-gating have been extensively studied, indicating that the S4-S5 linker (S4-S5_L) acts as a ligand binding to the S6 gate (S6 C-terminal part, S6_T) and stabilizes it in a closed state. Moreover, the N-terminal extremity of the channel, called N-Cap, has been suggested to interact with S4-S5_L to modulate channel voltage-dependent gating, as N-Cap deletion drastically accelerates hERG channel deactivation. In this study, using COS-7 cells, site-directed mutagenesis, electrophysiological measurements, and immunofluorescence confocal microscopy, we addressed whether these two major mechanisms of voltage-dependent gating are conserved in K_V10.2 channels. Using cysteine bridges and S4-S5_L-mimicking peptides, we show that the ligand/receptor model is conserved in K_V10.2, suggesting that this model is a hallmark of EAG channels. Truncation of the N-Cap domain, Per-Arnt-Sim (PAS) domain, or both in K_V10.2 abolished the current and altered channel trafficking to the membrane, unlike for the hERG channel in which N-Cap and PAS domain truncations mainly affected channel deactivation. Our results suggest that EAG channels function via a conserved ligand/receptor model of voltage gating, but that the N-Cap and PAS domains have different roles in these channels.

Potassium Channel Gain of Function in Epilepsy: An Unresolved Paradox

Exome and targeted sequencing have revolutionized clinical diagnosis. This has been particularly striking in epilepsy and neurodevelopmental disorders, for which new genes or new variants of preexisting candidate genes are being continuously identified at increasing rates every year. A surprising finding of these efforts is the recognition that gain of function potassium channel variants are actually associated with certain types of epilepsy, such as malignant migrating partial seizures of infancy or early-onset epileptic encephalopathy. This development has been difficult to understand as traditionally potassium channel loss-of-function, not gain-of-function, has been associated with hyperexcitability disorders. In this article, we describe the current state of the field regarding the gain-of-function potassium channel variants associated with epilepsy (KCNA2, KCNB1, KCND2, KCNH1, KCNH5, KCNJ10, KCNMA1, KCNQ2, KCNQ3, and KCNT1) and speculate on the possible cellular mechanisms behind the development of seizures and epilepsy in these patients. Understanding how potassium channel gain-of-function leads to epilepsy will provide new insights into the inner working of neural circuits and aid in developing new therapies.

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.

Studying Kv Channels Function using Computational Methods

In recent years, molecular modeling techniques, combined with MD simulations, provided significant insights on voltage-gated (Kv) potassium channels intrinsic properties. Among the success stories are the highlight of molecular level details of the effects of mutations, the unraveling of several metastable intermediate states, and the influence of a particular lipid, PIP₂, in the stability and the modulation of Kv channel function. These computational studies offered a detailed view that could not have been reached through experimental studies alone. With the increase of cross disciplinary studies, numerous experiments provided validation of these computational results, which endows an increase in the reliability of molecular modeling for the study of Kv channels. This chapter offers a description of the main techniques used to model Kv channels at the atomistic level.

Arthropod toxins acting on neuronal potassium channels

Arthropod venoms are a rich mixture of biologically active compounds exerting different physiological actions across diverse phyla and affecting multiple organ systems including the central nervous system. Venom compounds can inhibit or activate ion channels, receptors and transporters with high specificity and affinity providing essential insights into ion channel function. In this review, we focus on arthropod toxins (scorpions, spiders, bees and centipedes) acting on neuronal potassium channels. A brief description of the K⁺ channels classification and structure is included and a compendium of neuronal K⁺ channels and the arthropod toxins that modify them have been listed. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Nav1.7-A1632G Mutation from a Family with Inherited Erythromelalgia: Enhanced Firing of Dorsal Root Ganglia Neurons Evoked by Thermal Stimuli

Voltage-gated sodium channel Nav1.7 is a central player in human pain. Mutations in Nav1.7 produce several pain syndromes, including inherited erythromelalgia (IEM), a disorder in which gain-of-function mutations render dorsal root ganglia (DRG) neurons hyperexcitable. Although patients with IEM suffer from episodes of intense burning pain triggered by warmth, the effects of increased temperature on DRG neurons expressing mutant Nav1.7 channels have not been well documented. Here, using structural modeling, voltage-clamp, current-clamp, and multielectrode array recordings, we have studied a newly identified Nav1.7 mutation, Ala1632Gly, from a multigeneration family with IEM. Structural modeling suggests that Ala1632 is a molecular hinge and that the Ala1632Gly mutation may affect channel gating. Voltage-clamp recordings revealed that the Nav1.7-A1632G mutation hyperpolarizes activation and depolarizes fast-inactivation, both gain-of-function attributes at the channel level. Whole-cell current-clamp recordings demonstrated increased spontaneous firing, lower current threshold, and enhanced evoked firing in rat DRG neurons expressing Nav1.7-A1632G mutant channels. Multielectrode array recordings further revealed that intact rat DRG neurons expressing Nav1.7-A1632G mutant channels are more active than those expressing Nav1.7 WT channels. We also showed that physiologically relevant thermal stimuli markedly increase the mean firing frequencies and the number of active rat DRG neurons expressing Nav1.7-A1632G mutant channels, whereas the same thermal stimuli only increase these parameters slightly in rat DRG neurons expressing Nav1.7 WT channels. The response of DRG neurons expressing Nav1.7-A1632G mutant channels upon increase in temperature suggests a cellular basis for warmth-triggered pain in IEM.

SIGNIFICANCE STATEMENT

Inherited erythromelalgia (IEM), a severe pain syndrome characterized by episodes of intense burning pain triggered by warmth, is caused by mutations in sodium channel Nav1.7, which are preferentially expressed in sensory and sympathetic neurons. More than 20 gain-of-function Nav1.7 mutations have been identified from IEM patients, but the question of how warmth triggers episodes of pain in IEM has not been well addressed. Combining multielectrode array, voltage-clamp, and current-clamp recordings, we assessed a newly identified IEM mutation (Nav1.7-A1632G) from a multigeneration family. Our data demonstrate gain-of-function attributes at the channel level and differential effects of physiologically relevant thermal stimuli on the excitability of DRG neurons expressing mutant and WT Nav1.7 channels, suggesting a cellular mechanism for warmth-triggered pain episodes in IEM patients.

Insights into channel dysfunction from modelling and molecular dynamics simulations

Developments in structural biology mean that the number of different ion channel structures has increased significantly in recent years. Structures of ion channels enable us to rationalize how mutations may lead to channelopathies. However, determining the structures of ion channels is still not trivial, especially as they necessarily exist in many distinct functional states. Therefore, the use of computational modelling can provide complementary information that can refine working hypotheses of both wild type and mutant ion channels. The simplest but still powerful tool is homology modelling. Many structures are available now that can provide suitable templates for many different types of ion channels, allowing a full three-dimensional interpretation of mutational effects. These structural models, and indeed the structures themselves obtained by X-ray crystallography, and more recently cryo-electron microscopy, can be subjected to molecular dynamics simulations, either as a tool to help explore the conformational dynamics in detail or simply as a means to refine the models further. Here we review how these approaches have been used to improve our understanding of how diseases might be linked to specific mutations in ion channel proteins. This article is part of the Special Issue entitled 'Channelopathies.'

Ion Channel Genes and Epilepsy: Functional Alteration, Pathogenic Potential, and Mechanism of Epilepsy

Ion channels are crucial in the generation and modulation of excitability in the nervous system and have been implicated in human epilepsy. Forty-one epilepsy-associated ion channel genes and their mutations are systematically reviewed. In this paper, we analyzed the genotypes, functional alterations (funotypes), and phenotypes of these mutations. Eleven genes featured loss-of-function mutations and six had gain-of-function mutations. Nine genes displayed diversified funotypes, among which a distinct funotype-phenotype correlation was found in SCN1A. These data suggest that the funotype is an essential consideration in evaluating the pathogenicity of mutations and a distinct funotype or funotype-phenotype correlation helps to define the pathogenic potential of a gene.

Familial gain-of-function Nav1.9 mutation in a painful channelopathy

OBJECTIVE

Gain-of-function mutations in Na_v1.9 have been identified in three families with rare heritable pain disorders, and in patients with painful small-fibre neuropathy. Identification and functional assessment of new Na_v1.9 mutations will help to elucidate the phenotypic spectrum of Na_v1.9 channelopathies.

METHODS

Patients from a large family with early-onset pain symptoms were evaluated by clinical examination and genomic screening for mutations in SCN9A and SCN11A. Electrophysiological recordings and multistate modelling analysis were implemented for functional analyses.

RESULTS

A novel Na_v1.9 mutation, p.Arg222His, was identified in patients with early-onset pain in distal extremities including joints and gastrointestinal disturbances, but was absent from an asymptomatic blood relative. This mutation alters channel structure by substituting the highly conserved first arginine residue in transmembrane segment 4 (domain 1), the voltage sensor, with histidine. Voltage-clamp recordings demonstrate a hyperpolarising shift and acceleration of activation of the p.Arg222His mutant channel, which make it easier to open the channel. When expressed in dorsal root ganglion neurons, mutant p.Arg222His channels increase excitability via a depolarisation of resting potential and increased evoked firing.

CONCLUSIONS

This study expands the spectrum of heritable pain disorders linked to gain-of-function mutations in Na_v1.9, strengthening human validation of this channel as a potential therapeutic target for pain.

Epilepsy-associated genes

Development in genetic technology has led to the identification of an increasing number of genes associated with epilepsy. These discoveries will both provide the basis for including genetic tests in clinical practice and improve diagnosis and treatment of epilepsy. By searching through several databases (OMIM, HGMD, and EpilepsyGene) and recent publications on PubMed, we found 977 genes that are associated with epilepsy. We classified these genes into 4 categories according to the manifestation of epilepsy in phenotypes. We found 84 genes that are considered as epilepsy genes: genes that cause epilepsies or syndromes with epilepsy as the core symptom. 73 genes were listed as neurodevelopment-associated genes: genes associated with both brain-development malformations and epilepsy. Several genes (536) were epilepsy-related: genes associated with both physical or other systemic abnormalities and epilepsy or seizures. We found 284 additional genes putatively associated with epilepsy; this requires further verification. These integrated data will provide new insights useful for both including genetic tests in the clinical practice and evaluating the results of genetic tests. We also summarized the epilepsy-associated genes according to their function, with the goal to better characterize the association between genes and epilepsies and to further understand the mechanisms underlying epilepsy.

Characterization of two de novoKCNT1 mutations in children with malignant migrating partial seizures in infancy

The KCNT1 gene encodes for subunits contributing to the Na(+)-activated K(+) current (KNa), expressed in many cell types. Mutations in KCNT1 have been found in patients affected with a wide spectrum of early-onset epilepsies, including Malignant Migrating Partial Seizures in Infancy (MMPSI), a severe early-onset epileptic encephalopathy characterized by pharmacoresistant focal seizures migrating from one brain region or hemisphere to another and neurodevelopment arrest or regression, resulting in profound disability. In the present study we report identification by whole exome sequencing (WES) of two de novo, heterozygous KCNT1 mutations (G288S and, not previously reported, M516V) in two unrelated MMPSI probands. Functional studies in a heterologous expression system revealed that channels formed by mutant KCNT1 subunits carried larger currents when compared to wild-type KCNT1 channels, both as homo- and heteromers with these last. Both mutations induced a marked leftward shift in homomeric channel activation gating. Interestingly, the KCNT1 blockers quinidine (3-1000μM) and bepridil (0.03-10μM) inhibited both wild-type and mutant KCNT1 currents in a concentration-dependent manner, with mutant channels showing higher sensitivity to blockade. This latter result suggests two genotype-tailored pharmacological strategies to specifically counteract the dysfunction of KCNT1 activating mutations in MMPSI patients.

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

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

Molecular pathophysiology and pharmacology of the voltage-sensing module of neuronal ion channels

Voltage-gated ion channels (VGICs) are membrane proteins that switch from a closed to open state in response to changes in membrane potential, thus enabling ion fluxes across the cell membranes. The mechanism that regulate the structural rearrangements occurring in VGICs in response to changes in membrane potential still remains one of the most challenging topic of modern biophysics. Na⁺, Ca²⁺ and K⁺ voltage-gated channels are structurally formed by the assembly of four similar domains, each comprising six transmembrane segments. Each domain can be divided into two main regions: the Pore Module (PM) and the Voltage-Sensing Module (VSM). The PM (helices S₅ and S₆ and intervening linker) is responsible for gate opening and ion selectivity; by contrast, the VSM, comprising the first four transmembrane helices (S₁–S₄), undergoes the first conformational changes in response to membrane voltage variations. In particular, the S₄ segment of each domain, which contains several positively charged residues interspersed with hydrophobic amino acids, is located within the membrane electric field and plays an essential role in voltage sensing. In neurons, specific gating properties of each channel subtype underlie a variety of biological events, ranging from the generation and propagation of electrical impulses, to the secretion of neurotransmitters and to the regulation of gene expression. Given the important functional role played by the VSM in neuronal VGICs, it is not surprising that various VSM mutations affecting the gating process of these channels are responsible for human diseases, and that compounds acting on the VSM have emerged as important investigational tools with great therapeutic potential. In the present review we will briefly describe the most recent discoveries concerning how the VSM exerts its function, how genetically inherited diseases caused by mutations occurring in the VSM affects gating in VGICs, and how several classes of drugs and toxins selectively target the VSM.

Early-onset epileptic encephalopathy caused by gain-of-function mutations in the voltage sensor of Kv7.2 and Kv7.3 potassium channel subunits

Mutations in Kv7.2 (KCNQ2) and Kv7.3 (KCNQ3) genes, encoding for voltage-gated K(+) channel subunits underlying the neuronal M-current, have been associated with a wide spectrum of early-onset epileptic disorders ranging from benign familial neonatal seizures to severe epileptic encephalopathies. The aim of the present work has been to investigate the molecular mechanisms of channel dysfunction caused by voltage-sensing domain mutations in Kv7.2 (R144Q, R201C, and R201H) or Kv7.3 (R230C) recently found in patients with epileptic encephalopathies and/or intellectual disability. Electrophysiological studies in mammalian cells transfected with human Kv7.2 and/or Kv7.3 cDNAs revealed that each of these four mutations stabilized the activated state of the channel, thereby producing gain-of-function effects, which are opposite to the loss-of-function effects produced by previously found mutations. Multistate structural modeling revealed that the R201 residue in Kv7.2, corresponding to R230 in Kv7.3, stabilized the resting and nearby voltage-sensing domain states by forming an intricate network of electrostatic interactions with neighboring negatively charged residues, a result also confirmed by disulfide trapping experiments. Using a realistic model of a feedforward inhibitory microcircuit in the hippocampal CA1 region, an increased excitability of pyramidal neurons was found upon incorporation of the experimentally defined parameters for mutant M-current, suggesting that changes in network interactions rather than in intrinsic cell properties may be responsible for the neuronal hyperexcitability by these gain-of-function mutations. Together, the present results suggest that gain-of-function mutations in Kv7.2/3 currents may cause human epilepsy with a severe clinical course, thus revealing a previously unexplored level of complexity in disease pathogenetic mechanisms.

Depolarized inactivation overcomes impaired activation to produce DRG neuron hyperexcitability in a Nav1.7 mutation in a patient with distal limb pain

Sodium channel Nav1.7, encoded by SCN9A, is expressed in DRG neurons and regulates their excitability. Genetic and functional studies have established a critical contribution of Nav1.7 to human pain disorders. We have now characterized a novel Nav1.7 mutation (R1279P) from a female human subject with distal limb pain, in which depolarized fast inactivation overrides impaired activation to produce hyperexcitability and spontaneous firing in DRG neurons. Whole-cell voltage-clamp recordings in human embryonic kidney (HEK) 293 cells demonstrated that R1279P significantly depolarizes steady-state fast-, slow-, and closed-state inactivation. It accelerates deactivation, decelerates inactivation, and facilitates repriming. The mutation increases ramp currents in response to slow depolarizations. Our voltage-clamp analysis showed that R1279P depolarizes channel activation, a change that was supported by our multistate structural modeling. Because this mutation confers both gain-of-function and loss-of-function attributes on the Nav1.7 channel, we tested the impact of R1279P expression on DRG neuron excitability. Current-clamp studies reveal that R1279P depolarizes resting membrane potential, decreases current threshold, and increases firing frequency of evoked action potentials within small DRG neurons. The populations of spontaneously firing and repetitively firing neurons were increased by expressing R1279P. These observations indicate that the dominant proexcitatory gating changes associated with this mutation, including depolarized steady-state fast-, slow-, and closed-state inactivation, faster repriming, and larger ramp currents, override the depolarizing shift of activation, to produce hyperexcitability and spontaneous firing of nociceptive neurons that underlie pain.