Sodium channelopathies of skeletal muscle and brain.

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease.

The transient receptor potential ankyrin (TRPA) channels are Ca2+-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH2 terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca2+, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Channelopathies in fragile X syndrome.

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and the leading monogenic cause of autism. The condition stems from loss of fragile X mental retardation protein (FMRP), which regulates a wide range of ion channels via translational control, protein-protein interactions and second messenger pathways. Rapidly increasing evidence demonstrates that loss of FMRP leads to numerous ion channel dysfunctions (that is, channelopathies), which in turn contribute significantly to FXS pathophysiology. Consistent with this, pharmacological or genetic interventions that target dysregulated ion channels effectively restore neuronal excitability, synaptic function and behavioural phenotypes in FXS animal models. Recent studies further support a role for direct and rapid FMRP-channel interactions in regulating ion channel function. This Review lays out the current state of knowledge in the field regarding channelopathies and the pathogenesis of FXS, including promising therapeutic implications.

Structure and Pharmacology of Voltage-Gated Sodium and Calcium Channels.

Voltage-gated sodium and calcium channels are evolutionarily related transmembrane signaling proteins that initiate action potentials, neurotransmission, excitation-contraction coupling, and other physiological processes. Genetic or acquired dysfunction of these proteins causes numerous diseases, termed channelopathies, and sodium and calcium channels are the molecular targets for several major classes of drugs. Recent advances in the structural biology of these proteins using X-ray crystallography and cryo-electron microscopy have given new insights into the molecular basis for their function and pharmacology. Here we review this recent literature and integrate findings on sodium and calcium channels to reveal the structural basis for their voltage-dependent activation, fast and slow inactivation, ion conductance and selectivity, and complex pharmacology at the atomic level. We conclude with the theme that new understanding of the diseases and therapeutics of these channels will be derived from application of the emerging structural principles from these recent structural analyses.

Central Role of Subthreshold Currents in Myotonia.

It is generally thought that muscle excitability is almost exclusively controlled by currents responsible for generation of action potentials. We propose that smaller ion channel currents that contribute to setting the resting potential and to subthreshold fluctuations in membrane potential can also modulate excitability in important ways. These channels open at voltages more negative than the action potential threshold and are thus termed subthreshold currents. As subthreshold currents are orders of magnitude smaller than the currents responsible for the action potential, they are hard to identify and easily overlooked. Discovery of their importance in regulation of excitability opens new avenues for improved therapy for muscle channelopathies and diseases of the neuromuscular junction. ANN NEUROL 2020;87:175-183.

Pharmacological Approaches for the Modulation of the Potassium Channel KV4.x and KChIPs.

Ion channels are macromolecular complexes present in the plasma membrane and intracellular organelles of cells. Dysfunction of ion channels results in a group of disorders named channelopathies, which represent an extraordinary challenge for study and treatment. In this review, we will focus on voltage-gated potassium channels (KV), specifically on the KV4-family. The activation of these channels generates outward currents operating at subthreshold membrane potentials as recorded from myocardial cells (ITO, transient outward current) and from the somata of hippocampal neurons (ISA). In the heart, KV4 dysfunctions are related to Brugada syndrome, atrial fibrillation, hypertrophy, and heart failure. In hippocampus, KV4.x channelopathies are linked to schizophrenia, epilepsy, and Alzheimer's disease. KV4.x channels need to assemble with other accessory subunits (ß) to fully reproduce the ITO and ISA currents. ß Subunits affect channel gating and/or the traffic to the plasma membrane, and their dysfunctions may influence channel pharmacology. Among KV4 regulatory subunits, this review aims to analyze the KV4/KChIPs interaction and the effect of small molecule KChIP ligands in the A-type currents generated by the modulation of the KV4/KChIP channel complex. Knowledge gained from structural and functional studies using activators or inhibitors of the potassium current mediated by KV4/KChIPs will better help understand the underlying mechanism involving KV4-mediated-channelopathies, establishing the foundations for drug discovery, and hence their treatments.

Altered neuronal excitability in a Hodgkin-Huxley model incorporating channelopathies of the delayed rectifier potassium channel.

Channelopathies involving acquired or genetic modifications of the delayed rectifier K+ channel Kv1.1 include phenotypes characterized by enhanced neuronal excitability. Affected Kv1.1 channels exhibit combinations of altered expression, voltage sensitivity, and rates of activation and deactivation. Computational modeling and analysis can reveal the potential of particular channelopathies to alter neuronal excitability. A dynamical systems approach was taken to study the excitability and underlying dynamical structure of the Hodgkin-Huxley (HH) model of neural excitation as properties of the delayed rectifier K+ channel were altered. Bifurcation patterns of the HH model were determined as the amplitude of steady injection current was varied simultaneously with single parameters describing the delayed rectifier rates of activation and deactivation, maximal conductance, and voltage sensitivity. Relatively modest changes in the properties of the delayed rectifier K+ channel analogous to what is described for its channelopathies alter the bifurcation structure of the HH model and profoundly modify excitability of the HH model. Channelopathies associated with Kv1.1 can reduce the threshold for onset of neural activity. These studies also demonstrate how pathological delayed rectifier K+ channels could lead to the observation of the generalized Hopf bifurcation and, perhaps, other variants of the Hopf bifurcation. The observed bifurcation patterns collectively demonstrate that properties of the nominal delayed rectifier in the HH model appear optimized to permit activation of the HH model over the broadest possible range of input currents.

Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes as Models for Cardiac Channelopathies: A Primer for Non-Electrophysiologists.

Life threatening ventricular arrhythmias leading to sudden cardiac death are a major cause of morbidity and mortality. In the absence of structural heart disease, these arrhythmias, especially in the younger population, are often an outcome of genetic defects in specialized membrane proteins called ion channels. In the heart, exceptionally well-orchestrated activity of a diversity of ion channels mediates the cardiac action potential. Alterations in either the function or expression of these channels can disrupt the configuration of the action potential, leading to abnormal electrical activity of the heart that can sometimes initiate an arrhythmia. Understanding the pathophysiology of inherited arrhythmias can be challenging because of the complexity of the disorder and lack of appropriate cellular and in vivo models. Recent advances in human induced pluripotent stem cell technology have provided remarkable progress in comprehending the underlying mechanisms of ion channel disorders or channelopathies by modeling these complex arrhythmia syndromes in vitro in a dish. To fully realize the potential of induced pluripotent stem cells in elucidating the mechanistic basis and complex pathophysiology of channelopathies, it is crucial to have a basic knowledge of cardiac myocyte electrophysiology. In this review, we will discuss the role of the various ion channels in cardiac electrophysiology and the molecular and cellular mechanisms of arrhythmias, highlighting the promise of human induced pluripotent stem cell-cardiomyocytes as a model for investigating inherited arrhythmia syndromes and testing antiarrhythmic strategies. Overall, this review aims to provide a basic understanding of the electrical activity of the heart and related channelopathies, especially to clinicians or research scientists in the cardiovascular field with limited electrophysiology background.

Epilepsy and brain channelopathies from infancy to adulthood.

Genetic brain channelopathies result from inherited or de novo mutations of genes encoding ion channel subunits within the central nervous system. Most neurological channelopathies arise in childhood with paroxysmal or episodic symptoms, likely because of a transient impairment of homeostatic mechanisms regulating membrane excitability, and the prototypical expression of this impairment is epilepsy. Migraine, episodic ataxia and alternating hemiplegia can also occur, as well as chronic phenotypes, such as spinocerebellar ataxias, intellectual disability and autism spectrum disorder. Voltage-gated and ligand-gated channels may be involved. In most cases, a single gene may be associated with a phenotypical spectrum that shows variable expressivity. Different clinical features may arise at different ages and the adult phenotype may be remarkably modified from the syndrome onset in childhood or adolescence. Recognizing the prominent phenotypical traits of brain channelopathies is essential to perform appropriate diagnostic investigations and to provide the better care not only in the paediatric setting but also for adult patients and their caregivers. Herein, we provide an overview of genetic brain channelopathies associated with epilepsy, highlight the different molecular mechanisms and describe the different clinical characteristics which may prompt the clinician to suspect specific syndromes and to possibly establish tailored treatments.

Changes of Resurgent Na+ Currents in the Nav1.4 Channel Resulting from an SCN4A Mutation Contributing to Sodium Channel Myotonia.

Myotonia congenita (MC) is a rare disorder characterized by stiffness and weakness of the limb and trunk muscles. Mutations in the SCN4A gene encoding the alpha-subunit of the voltage-gated sodium channel Nav1.4 have been reported to be responsible for sodium channel myotonia (SCM). The Nav1.4 channel is expressed in skeletal muscles, and its related channelopathies affect skeletal muscle excitability, which can manifest as SCM, paramyotonia and periodic paralysis. In this study, the missense mutation p.V445M was identified in two individual families with MC. To determine the functional consequences of having a mutated Nav1.4 channel, whole-cell patch-clamp recording of transfected Chinese hamster ovary cells was performed. Evaluation of the transient Na+ current found that a hyperpolarizing shift occurs at both the activation and inactivation curves with an increase of the window currents in the mutant channels. The Nav1.4 channel's co-expression with the Navß4 peptide can generate resurgent Na+ currents at repolarization following a depolarization. The magnitude of the resurgent currents is higher in the mutant than in the wild-type (WT) channel. Although the decay kinetics are comparable between the mutant and WT channels, the time to the peak of resurgent Na+ currents in the mutant channel is significantly protracted compared with that in the WT channel. These findings suggest that the p.V445M mutation in the Nav1.4 channel results in an increase of both sustained and resurgent Na+ currents, which may contribute to hyperexcitability with repetitive firing and is likely to facilitate recurrent myotonia in SCM patients.

Acquired cardiac channelopathies in epilepsy: Evidence, mechanisms, and clinical significance.

There is growing evidence that cardiac dysfunction in patients with chronic epilepsy could play a pathogenic role in sudden unexpected death in epilepsy (SUDEP). Recent animal studies have revealed that epilepsy secondarily alters the expression of cardiac ion channels alongside abnormal cardiac electrophysiology and remodeling. These molecular findings represent novel evidence for an acquired cardiac channelopathy in epilepsy, distinct from inherited ion channels mutations associated with cardiocerebral phenotypes. Specifically, seizure activity has been shown to alter the messenger RNA (mRNA) and protein expression of voltage-gated sodium channels (Nav 1.1, Nav 1.5), voltage-gated potassium channels (Kv 4.2, Kv 4.3), sodium-calcium exchangers (NCX1), and nonspecific cation-conducting channels (HCN2, HCN4). The pathophysiology may involve autonomic dysfunction and structural cardiac disease, as both are independently associated with epilepsy and ion channel dysregulation. Indeed, in vivo and in vitro studies of cardiac pathology reveal a complex network of signaling pathways and transcription factors regulating ion channel expression in the setting of sympathetic overactivity, cardiac failure, and hypertrophy. Other mechanisms such as circulating inflammatory mediators or exogenous effects of antiepileptic medications lack evidence. Moreover, an acquired cardiac channelopathy may underlie the electrophysiologic cardiac abnormalities seen in chronic epilepsy, potentially contributing to the increased risk of malignant arrhythmias and sudden death. Therefore, further investigation is necessary to establish whether cardiac ion channel dysregulation similarly occurs in patients with epilepsy, and to characterize any pathogenic relationship with SUDEP.

SCN2A channelopathies: Mechanisms and models.

Variants in the SCN2A gene, encoding the voltage-gated sodium channel NaV 1.2, cause a variety of neuropsychiatric syndromes with different severity ranging from self-limiting epilepsies with early onset to developmental and epileptic encephalopathy with early or late onset and intellectual disability (ID), as well as ID or autism without seizures. Functional analysis of channel defects demonstrated a genotype-phenotype correlation and suggested effective treatment options for one group of affected patients carrying gain-of-function variants. Here, we sum up the functional mechanisms underlying different phenotypes of patients with SCN2A channelopathies and present currently available models that can help in understanding SCN2A-related disorders.

A common co-morbidity modulates disease expression and treatment efficacy in inherited cardiac sodium channelopathy.

Aims: Management of patients with inherited cardiac ion channelopathy is hindered by variability in disease severity and sudden cardiac death (SCD) risk. Here, we investigated the modulatory role of hypertrophy on arrhythmia and SCD risk in sodium channelopathy. Methods and results: Follow-up data was collected from 164 individuals positive for the SCN5A-1795insD founder mutation and 247 mutation-negative relatives. A total of 38 (obligate) mutation-positive patients died suddenly or suffered life-threatening ventricular arrhythmia. Of these, 18 were aged >40 years, a high proportion of which had a clinical diagnosis of hypertension and/or cardiac hypertrophy. While pacemaker implantation was highly protective in preventing bradycardia-related SCD in young mutation-positive patients, seven of them aged >40 experienced life-threatening arrhythmic events despite pacemaker treatment. Of these, six had a diagnosis of hypertension/hypertrophy, pointing to a modulatory role of this co-morbidity. Induction of hypertrophy in adult mice carrying the homologous mutation (Scn5a1798insD/+) caused SCD and excessive conduction disturbances, confirming a modulatory effect of hypertrophy in the setting of the mutation. The deleterious effects of the interaction between hypertrophy and the mutation were prevented by genetically impairing the pro-hypertrophic response and by pharmacological inhibition of the enhanced late sodium current associated with the mutation. Conclusion: This study provides the first evidence for a modulatory effect of co-existing cardiac hypertrophy on arrhythmia risk and treatment efficacy in inherited sodium channelopathy. Our findings emphasize the need for continued assessment and rigorous treatment of this co-morbidity in SCN5A mutation-positive individuals.

Alternative paradigms for ion channelopathies: disorders of ion channel membrane trafficking and posttranslational modification.

Channelopathies are a diverse set of disorders associated with defects in ion channel (and transporter) function. Although the vast majority of channelopathies are linked with inherited mutations that alter ion channel biophysical properties, another group of similar disorders has emerged that alter ion channel synthesis, membrane trafficking, and/or posttranslational modifications. In fact, some electrical and episodic disorders have now been identified that are not defects in the ion channel but instead reflect dysfunction in an ion channel (or transporter) regulatory protein. This review focuses on alternative paradigms for physiological disorders associated with protein biosynthesis, folding, trafficking, and membrane retention. Furthermore, the review highlights the role of aberrant posttranslational modifications in acquired channelopathies.

Sodium channel ß subunits: emerging targets in channelopathies.

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in excitable cells. VGSCs in mammalian brain are heterotrimeric complexes of α and ß subunits. Although ß subunits were originally termed auxiliary, we now know that they are multifunctional signaling molecules that play roles in both excitable and nonexcitable cell types and with or without the pore-forming α subunit present. ß subunits function in VGSC and potassium channel modulation, cell adhesion, and gene regulation, with particularly important roles in brain development. Mutations in the genes encoding ß subunits are linked to a number of diseases, including epilepsy, sudden death syndromes like SUDEP and SIDS, and cardiac arrhythmia. Although VGSC ß subunit-specific drugs have not yet been developed, this protein family is an emerging therapeutic target.

[Neuropathic pain syndromes and channelopathies]. / Neuropathische Schmerzsyndrome bei Ionenkanalerkrankungen.

BACKGROUND: The causes for neuropathic pain are manifold and remain unexplained in the majority of cases. In recent years a growing number of pain syndromes have been attributed to mutations in genes encoding voltage-gated sodium channels. Hence, this group of rare diseases should be considered in the differential diagnostics of neuropathic pain. MATERIAL AND METHODS: Evaluation of topic-related literature and discussion of own experiences as well as consideration of current guidelines. RESULTS: Alterations in the electrical excitability of nociceptive neurons by pathogenic mutations in sodium channels lead to disease patterns, such as small fiber neuropathy and various pain syndromes. This article summarizes the knowledge on these genetic diseases and discusses the differential diagnosis of neuropathic pain. Current treatment concepts are presented and the predominantly experimental approaches to targeted modulation of sodium channels are discussed. CONCLUSION: The treatment of patients with chronic neuropathic pain requires interdisciplinary cooperation and is often difficult due to an unsatisfactory treatment response. Increasing knowledge on rare genetically determined channelopathies can contribute to the development of novel pharmaceuticals since ion channels are central players in the processing of pain.

Irritable bowel syndrome patients have SCN5A channelopathies that lead to decreased NaV1.5 current and mechanosensitivity.

The SCN5A-encoded voltage-gated mechanosensitive Na+ channel NaV1.5 is expressed in human gastrointestinal smooth muscle cells and interstitial cells of Cajal. NaV1.5 contributes to smooth muscle electrical slow waves and mechanical sensitivity. In predominantly Caucasian irritable bowel syndrome (IBS) patient cohorts, 2-3% of patients have SCN5A missense mutations that alter NaV1.5 function and may contribute to IBS pathophysiology. In this study we examined a racially and ethnically diverse cohort of IBS patients for SCN5A missense mutations, compared them with IBS-negative controls, and determined the resulting NaV1.5 voltage-dependent and mechanosensitive properties. All SCN5A exons were sequenced from somatic DNA of 252 Rome III IBS patients with diverse ethnic and racial backgrounds. Missense mutations were introduced into wild-type SCN5A by site-directed mutagenesis and cotransfected with green fluorescent protein into HEK-293 cells. NaV1.5 voltage-dependent and mechanosensitive functions were studied by whole cell electrophysiology with and without shear force. Five of 252 (2.0%) IBS patients had six rare SCN5A mutations that were absent in 377 IBS-negative controls. Six of six (100%) IBS-associated NaV1.5 mutations had voltage-dependent gating abnormalities [current density reduction (R225W, R433C, R986Q, and F1293S) and altered voltage dependence (R225W, R433C, R986Q, G1037V, and F1293S)], and at least one kinetic parameter was altered in all mutations. Four of six (67%) IBS-associated SCN5A mutations (R225W, R433C, R986Q, and F1293S) resulted in altered NaV1.5 mechanosensitivity. In this racially and ethnically diverse cohort of IBS patients, we show that 2% of IBS patients harbor SCN5A mutations that are absent in IBS-negative controls and result in NaV1.5 channels with abnormal voltage-dependent and mechanosensitive function. NEW & NOTEWORTHY The voltage-gated Na+ channel NaV1.5 contributes to smooth muscle physiology and electrical slow waves. In a racially and ethnically mixed irritable bowel syndrome cohort, 2% had mutations in the NaV1.5 gene SCN5A. These mutations were absent in irritable bowel syndrome-negative controls. Most mutant NaV1.5 channels were loss of function in voltage dependence or mechanosensitivity.

Neurological channelopathies.

Inherited ion channel mutations can affect the entire nervous system. Many cause paroxysmal disturbances of brain, spinal cord, peripheral nerve or skeletal muscle function, with normal neurological development and function in between attacks. To fully understand how mutations of ion channel genes cause disease, we need to know the normal location and function of the channel subunit, consequences of the mutation for biogenesis and biophysical properties, and possible compensatory changes in other channels that contribute to cell or circuit excitability. Animal models of monogenic channelopathies increasingly help our understanding. An important challenge for the future is to determine how more subtle derangements of ion channel function, which arise from the interaction of genetic and environmental influences, contribute to common paroxysmal disorders, including idiopathic epilepsy and migraine, that share features with rare monogenic channelopathies.

Signature and Pathophysiology of Non-canonical Pores in Voltage-Dependent Cation Channels.

Opening and closing of voltage-gated cation channels allows the regulated flow of cations such as Na(+), K(+), and Ca(2+) across cell membranes, which steers essential physiological processes including shaping of action potentials and triggering Ca(2+)-dependent processes. Classical textbooks describe the voltage-gated cation channels as membrane proteins with a single, central aqueous pore. In recent years, however, evidence has accumulated for the existence of additional ion permeation pathways in this group of cation channels, distinct from the central pore, which here we collectively name non-canonical pores. Whereas the first non-canonical pores were unveiled only after making specific point mutations in the voltage-sensor region of voltage-gated Na(+) and K(+) channels, recent evidence indicates that they may also be functional in non-mutated channels. Moreover, several channelopathies have been linked to mutations that cause the appearance of a non-canonical ion permeation pathway as a new pathological mechanism. This review provides an integrated overview of the biophysical properties of non-canonical pores described in voltage-dependent cation channels (KV, NaV, Cav, Hv1, and TRPM3) and of the (patho)physiological impact of opening of such pores.

Store depletion-induced h-channel plasticity rescues a channelopathy linked to Alzheimer's disease.

Voltage-gated ion channels are critical for neuronal integration. Some of these channels, however, are misregulated in several neurological disorders, causing both gain- and loss-of-function channelopathies in neurons. Using several transgenic mouse models of Alzheimer's disease (AD), we find that sub-threshold voltage signals strongly influenced by hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels progressively deteriorate over chronological aging in hippocampal CA1 pyramidal neurons. The degraded signaling via HCN channels in the transgenic mice is accompanied by an age-related global loss of their non-uniform dendritic expression. Both the aberrant signaling via HCN channels and their mislocalization could be restored using a variety of pharmacological agents that target the endoplasmic reticulum (ER). Our rescue of the HCN channelopathy helps provide molecular details into the favorable outcomes of ER-targeting drugs on the pathogenesis and synaptic/cognitive deficits in AD mouse models, and implies that they might have beneficial effects on neurological disorders linked to HCN channelopathies.