Exon organization, coding sequence, physical mapping, and polymorphic intragenic markers for the human neuronal sodium channel gene SCN8A

Genetic Background of Epilepsy and Antiepileptic Treatments

Advanced identification of the gene mutations causing epilepsy syndromes is expected to translate into faster diagnosis and more effective treatment of these conditions. Over the last 5 years, approximately 40 clinical trials on the treatment of genetic epilepsies have been conducted. As a result, some medications that are not regular antiseizure drugs (e.g., soticlestat, fenfluramine, or ganaxolone) have been introduced to the treatment of drug-resistant seizures in Dravet, Lennox-Gastaut, maternally inherited chromosome 15q11.2-q13.1 duplication (Dup 15q) syndromes, and protocadherin 19 (PCDH 19)-clusterig epilepsy. And although the effects of soticlestat, fenfluramine, and ganaxolone are described as promising, they do not significantly affect the course of the mentioned epilepsy syndromes. Importantly, each of these syndromes is related to mutations in several genes. On the other hand, several mutations can occur within one gene, and different gene variants may be manifested in different disease phenotypes. This complex pattern of inheritance contributes to rather poor genotype-phenotype correlations. Hence, the detection of a specific mutation is not synonymous with a precise diagnosis of a specific syndrome. Bearing in mind that seizures develop as a consequence of the predominance of excitatory over inhibitory processes, it seems reasonable that mutations in genes encoding sodium and potassium channels, as well as glutamatergic and gamma-aminobutyric (GABA) receptors, play a role in the pathogenesis of epilepsy. In some cases, different pathogenic variants of the same gene can result in opposite functional effects, determining the effectiveness of therapy with certain medications. For instance, seizures related to gain-of-function (GoF) mutations in genes encoding sodium channels can be successfully treated with sodium channel blockers. On the contrary, the same drugs may aggravate seizures related to loss-of-function (LoF) variants of the same genes. Hence, knowledge of gene mutation-treatment response relationships facilitates more favorable selection of drugs for anticonvulsant therapy.

A mutation in the neonatal isoform of SCN2A causes neonatal-onset epilepsy

SCN2A (sodium channel 2A) encodes the Nav1.2 channel protein in excitatory neurons in the brain. Nav1.2 is a critical voltage-gated sodium channel of the central nervous system. Mutations in SCN2A are responsible for a broad phenotypic spectrum ranging from autism and developmental delay to severe encephalopathy with neonatal or early infantile onset. SCN2A can be spliced into two different isoforms, a neonatal (6N) and an adult (6A) form. After birth, there is an equal or higher amount of the 6N isoform, protecting the brain from the increased neuronal excitability of the infantile brain. During postnatal development, 6N is gradually replaced by 6A. In an infant carrying the novel SCN2A mutation c.643G > A (p.Ala215Thr) only in the neonatal transcript, seizures started immediately after birth. The clinical presentation evolved from a burst-suppression pattern with 30-50 tonic seizures per day to hypsarrhythmia. The first exome analysis, focusing only on common transcripts, missed the diagnosis and delayed early therapy. A reevaluation including all transcripts revealed the SCN2A variant.

Distinctive Properties and Powerful Neuromodulation of Nav1.6 Sodium Channels Regulates Neuronal Excitability

Voltage-gated sodium channels (Navs) are critical determinants of cellular excitability. These ion channels exist as large heteromultimeric structures and their activity is tightly controlled. In neurons, the isoform Na_v1.6 is highly enriched at the axon initial segment and nodes, making it critical for the initiation and propagation of neuronal impulses. Changes in Na_v1.6 expression and function profoundly impact the input-output properties of neurons in normal and pathological conditions. While mutations in Na_v1.6 may cause channel dysfunction, aberrant changes may also be the result of complex modes of regulation, including various protein-protein interactions and post-translational modifications, which can alter membrane excitability and neuronal firing properties. Despite decades of research, the complexities of Na_v1.6 modulation in health and disease are still being determined. While some modulatory mechanisms have similar effects on other Nav isoforms, others are isoform-specific. Additionally, considerable progress has been made toward understanding how individual protein interactions and/or modifications affect Na_v1.6 function. However, there is still more to be learned about how these different modes of modulation interact. Here, we examine the role of Na_v1.6 in neuronal function and provide a thorough review of this channel’s complex regulatory mechanisms and how they may contribute to neuromodulation.

Sodium channelopathies in neurodevelopmental disorders

The voltage-gated sodium channel α-subunit genes comprise a highly conserved gene family. Mutations of three of these genes, SCN1A, SCN2A and SCN8A, are responsible for a significant burden of neurological disease. Recent progress in identification and functional characterization of patient variants is generating new insights and novel approaches to therapy for these devastating disorders. Here we review the basic elements of sodium channel function that are used to characterize patient variants. We summarize a large body of work using global and conditional mouse mutants to characterize the in vivo roles of these channels. We provide an overview of the neurological disorders associated with mutations of the human genes and examples of the effects of patient mutations on channel function. Finally, we highlight therapeutic interventions that are emerging from new insights into mechanisms of sodium channelopathies.

SCN8A Mutation in Infantile Epileptic Encephalopathy: Report of Two Cases

Early infantile epileptic encephalopathy type 13 is a severe form of epilepsy caused by mutations in the sodium channel 8 alpha (SCN8A) gene. This gene encodes the neuronal voltage-gated sodium channel which plays vital role in neuronal excitability. Here we present two cases with SCN8A encephalopathy. Both cases had mutation in p.Arg1872Gin the SCN8A gene, which was detected by targeted next generation sequencing. Case 1 was a 14-month old boy, who had a normal birth history with normal development up to 6 months and then developed repeated generalized seizure, which was nonresponsive to multiple antiepileptic drugs. He also had neuroregression and dystonia. His electroencephalogram (EEG) showed progressive background abnormality with burst suppression pattern. His metabolic panel was normal and had partial response to carbamazepine. The second case was for an 11-month old boy with the onset of seizure at the age of 7 months. Seizure was generalized, resistant to multiple antiepileptic drugs. He had developmental delay from beginning, no movement disorder. EEG showed focal discharge from left temporal and occipital region. He showed partial response to oxcarbazepine. Our cases had similarities with the previously reported cases. The detailed discussion of our cases would contribute to early detection and targeted treatment of SCN8A encephalopathy. This also gives special emphasis on a genetic test in infants with intractable epilepsy, movement disorder and developmental delay.

Prominent role of forebrain excitatory neurons in SCN8A encephalopathy

De novo mutations of the sodium channel gene SCN8A result in an epileptic encephalopathy with refractory seizures, developmental delay, and elevated risk of sudden death. p.Arg1872Trp is a recurrent de novo SCN8A mutation reported in 14 unrelated individuals with epileptic encephalopathy that included seizure onset in the prenatal or infantile period and severe verbal and ambulatory comorbidities. The major biophysical effect of the mutation was previously shown to be impaired channel inactivation accompanied by increased current density. We have generated a conditional mouse mutation in which expression of this severe gain-of-function mutation is dependent upon Cre recombinase. Global activation of p.Arg1872Trp by EIIa-Cre resulted in convulsive seizures and lethality at 2 weeks of age. Neural activation of the p.Arg1872Trp mutation by Nestin-Cre also resulted in early onset seizures and death. Restriction of p.Arg1872Trp expression to excitatory neurons using Emx1-Cre recapitulated seizures and juvenile lethality between 1 and 2 months of age. In contrast, activation of p.Arg1872Trp in inhibitory neurons by Gad2-Cre or Dlx5/6-Cre did not induce seizures or overt neurological dysfunction. The sodium channel modulator GS967/Prax330 prolonged survival of mice with global expression of R1872W and also modulated the activity of the mutant channel in transfected cells. Activation of the p.Arg1872Trp mutation in adult mice was sufficient to generate seizures and death, indicating that successful therapy will require lifelong treatment. These findings provide insight into the pathogenic mechanism of this gain-of-function mutation of SCN8A and identify excitatory neurons as critical targets for therapeutic intervention.

The Role of Voltage-Gated Sodium Channels in Pain Signaling

Acute pain signaling has a key protective role and is highly evolutionarily conserved. Chronic pain, however, is maladaptive, occurring as a consequence of injury and disease, and is associated with sensitization of the somatosensory nervous system. Primary sensory neurons are involved in both of these processes, and the recent advances in understanding sensory transduction and human genetics are the focus of this review. Voltage-gated sodium channels (VGSCs) are important determinants of sensory neuron excitability: they are essential for the initial transduction of sensory stimuli, the electrogenesis of the action potential, and neurotransmitter release from sensory neuron terminals. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are all expressed by adult sensory neurons. The biophysical characteristics of these channels, as well as their unique expression patterns within subtypes of sensory neurons, define their functional role in pain signaling. Changes in the expression of VGSCs, as well as posttranslational modifications, contribute to the sensitization of sensory neurons in chronic pain states. Furthermore, gene variants in Nav1.7, Nav1.8, and Nav1.9 have now been linked to human Mendelian pain disorders and more recently to common pain disorders such as small-fiber neuropathy. Chronic pain affects one in five of the general population. Given the poor efficacy of current analgesics, the selective expression of particular VGSCs in sensory neurons makes these attractive targets for drug discovery. The increasing availability of gene sequencing, combined with structural modeling and electrophysiological analysis of gene variants, also provides the opportunity to better target existing therapies in a personalized manner.

12q13.12q13.13 microdeletion encompassing ACVRL1 and SCN8A genes: Clinical report of a new contiguous gene syndrome

Hereditary hemorrhagic telangiectasia is usually linked to the presence of a pathogenic mutation ACVRL1 or ENG. Thus, apparently there is no benefit to perform an array CGH in case of HHT. However, ENG has been involved in a contiguous gene syndrome due to a de novo 9q33.3q34.11 microdeletion. We describe here a new contiguous gene syndrome involving ACVRL1 gene. A 50-year-old female patient had a typical clinical presentation of hereditary hemorrhagic telangiectasia (HHT) with epistaxis, cutaneous-mucous telangiectases, arteriovenous malformation. She also presented a cognitive disability. Cognitive assessment showed a heterogeneous cognitive disorder predominating in the executive sphere without intellectual deficiency. She had no peculiar morphological feature. Neurological examination disclosed the presence of contralateral mirror movements during voluntary movement of each hand. A heterozygous deletion of the whole ACVRL1 gene (exons 1 to 10) was found to be responsible for the HHT features. To investigate further the dysexecutive syndrome and the mirror movements, we performed oligonucleotide array comparative genomic hybridization (array CGH) study (180K, Agilent, Santa-Clara, CA, USA). This study revealed a de novo 1.58 Mb deletion on chromosome 12q13.12q13.13 encompassing the ACVRL1 and SCN8A genes. To our knowledge, this deletion has not been previously reported and defines a new contiguous gene syndrome. The loss of one ACVRL1 allele is likely to be responsible for the HHT phenotype, while the deletion of the SCN8A gene is likely to be the cause of the mild cognitive disorder. SCN8A haploinsufficiency might also be involved in the occurrence of mirror movements. This report highlights the benefit of searching for large rearrangements in cases including unusual symptoms in association with HHT. On the other hand, an early diagnosis of 12q13.12q13.13 microdeletion based on the presence of a dysexecutive syndrome and/or mirror movement may allow to prevent HHT complications.

De novo variants in the alternative exon 5 of SCN8A cause epileptic encephalopathy

Purpose

As part of the Epilepsy Genetics Initiative, we re-evaluated clinically generated exome sequence data from 54 epilepsy patients and their unaffected parents to identify molecular diagnoses not provided in the initial diagnostic interpretation.

Methods

We compiled and analyzed exome sequence data from 54 genetically undiagnosed trios using a validated analysis pipeline. We evaluated the significance of the genetic findings by reanalyzing sequence data generated at Ambry Genetics, and from a number of additional case and control cohorts.

Results

In 54 previously undiagnosed trios, we identified two de novo missense variants in SCN8A in the highly expressed alternative exon 5 A—an exon only recently added to the Consensus Coding Sequence database. One additional undiagnosed epilepsy patient harboring a de novo variant in exon 5 A was found in the Ambry Genetics cohort. Missense variants in SCN8A exon 5 A are extremely rare in the population, further supporting the pathogenicity of the de novo alterations identified.

Conclusion

These results expand the range of SCN8A variants in epileptic encephalopathy patients and illustrate the necessity of ongoing reanalysis of negative exome sequences, as advances in the knowledge of disease genes and their annotations will permit new diagnoses to be made.

Is there a role for voltage-gated Na+ channels in the aggressiveness of breast cancer?

Breast cancer is the most common cancer among women and its metastatic potential is responsible for numerous deaths. Thus, the need to find new targets for improving treatment, and even finding the cure, becomes increasingly greater. Ion channels are known to participate in several physiological functions, such as muscle contraction, cell volume regulation, immune response and cell proliferation. In breast cancer, different types of ion channels have been associated with tumorigenesis. Recently, voltage-gated Na⁺ channels (VGSC) have been implicated in the processes that lead to increased tumor aggressiveness. To explain this relationship, different theories, associated with pH changes, gene expression and intracellular Ca²⁺, have been proposed in an attempt to better understand the role of these ion channels in breast cancer. However, these theories are having difficulty being accepted because most of the findings are contrary to the present scientific knowledge. Several studies have shown that VGSC are related to different types of cancer, making them a promising pharmacological target against this debilitating disease. Molecular biology and cell electrophysiology have been used to look for new forms of treatment aiming to reduce aggressiveness and the disease progress.

The association of bacterial C9-based TTX-like compounds with Prorocentrum minimum opens new uncertainties about shellfish seafood safety

In 2012, Tetrodotoxin (TTX) was identified in mussels and linked to the presence of Prorocentrum minimum (P. minimum) in Greece. The connexion between TTX and P. minimum was further studied in this paper. First, the presence of TTX-producer bacteria, Vibrio and Pseudomonas spp, was confirmed in Greek mussels. In addition these samples showed high activity as inhibitors of sodium currents (I_Na). P. minimum was before associated with neurotoxic symptoms, however, the nature and structure of toxins produced by this dinoflagellate remains unknown. Three P. minimum strains, ccmp1529, ccmp2811 and ccmp2956, growing in different conditions of temperature, salinity and light were used to study the production of toxic compounds. Electrophysiological assays showed no effect of ccmp2811 strain on I_Na, while ccmp1529 and ccmp2956 strains were able to significantly reduce I_Na in the same way as TTX. In these samples two new compounds, m/z 265 and m/z 308, were identified and characterized by liquid chromatography tandem high-resolution mass spectrometry. Besides, two TTX-related bacteria, Roseobacter and Vibrio sp, were observed. These results show for the first time that P. minimum produce TTX-like compounds with a similar ion pattern and C9-base to TTX analogues and with the same effect on I_Na.

Molecular Characterization of Voltage-Gated Sodium Channels and Their Relations with Paralytic Shellfish Toxin Bioaccumulation in the Pacific Oyster Crassostrea gigas

Paralytic shellfish toxins (PST) bind to voltage-gated sodium channels (Nav) and block conduction of action potential in excitable cells. This study aimed to (i) characterize Nav sequences in Crassostrea gigas and (ii) investigate a putative relation between Nav and PST-bioaccumulation in oysters. The phylogenetic analysis highlighted two types of Nav in C. gigas: a Nav1 (CgNav1) and a Nav2 (CgNav2) with sequence properties of sodium-selective and sodium/calcium-selective channels, respectively. Three alternative splice transcripts of CgNav1 named A, B and C, were characterized. The expression of CgNav1, analyzed by in situ hybridization, is specific to nervous cells and to structures corresponding to neuromuscular junctions. Real-time PCR analyses showed a strong expression of CgNav1A in the striated muscle while CgNav1B is mainly expressed in visceral ganglia. CgNav1C expression is ubiquitous. The PST binding site (domain II) of CgNav1 variants possess an amino acid Q that could potentially confer a partial saxitoxin (STX)-resistance to the channel. The CgNav1 genotype or alternative splicing would not be the key point determining PST bioaccumulation level in oysters.

Cardiac arrhythmia in a mouse model of sodium channel SCN8A epileptic encephalopathy

Patients with early infantile epileptic encephalopathy (EIEE) are at increased risk for sudden unexpected death in epilepsy (SUDEP). De novo mutations of the sodium channel gene SCN8A, encoding the sodium channel Na_v1.6, result in EIEE13 (OMIM 614558), which has a 10% risk of SUDEP. Here, we investigated the cardiac phenotype of a mouse model expressing the gain of function EIEE13 patient mutation p.Asn1768Asp in Scn8a (Na_v1.6-N1768D). We tested Scn8a^N1768D/+ mice for alterations in cardiac excitability. We observed prolongation of the early stages of action potential (AP) repolarization in mutant myocytes vs. controls. Scn8a^N1768D/+ myocytes were hyperexcitable, with a lowered threshold for AP firing, increased incidence of delayed afterdepolarizations, increased calcium transient duration, increased incidence of diastolic calcium release, and ectopic contractility. Calcium transient duration and diastolic calcium release in the mutant myocytes were tetrodotoxin-sensitive. A selective inhibitor of reverse mode Na/Ca exchange blocked the increased incidence of diastolic calcium release in mutant cells. Scn8a^N1768D/+ mice exhibited bradycardia compared with controls. This difference in heart rate dissipated after administration of norepinephrine, and there were no differences in heart rate in denervated ex vivo hearts, implicating parasympathetic hyperexcitability in the Scn8a^N1768D/+ animals. When challenged with norepinephrine and caffeine to simulate a catecholaminergic surge, Scn8a^N1768D/+ mice showed ventricular arrhythmias. Two of three mutant mice under continuous ECG telemetry recording experienced death, with severe bradycardia preceding asystole. Thus, in addition to central neuron hyperexcitability, Scn8a^N1768D/+ mice have cardiac myoycte and parasympathetic neuron hyperexcitability. Simultaneous dysfunction in these systems may contribute to SUDEP associated with mutations of Scn8a.

Lack of variation in voltage-gated sodium channels of common bottlenose dolphins (Tursiops truncatus) exposed to neurotoxic algal blooms

In coastal marine ecosystems, neurotoxins produced by harmful algal blooms (HABs) often result in large-scale mortality events of many marine species. Historical and frequent exposure to HABs therefore may provide a strong selective pressure for adaptations that result in toxin resistance. Neurotoxin resistance has independently evolved in a variety of terrestrial and marine species via mutations in genes encoding the toxin binding sites within the voltage-gated sodium channel gene complex. Accordingly, we tested the hypothesis that genetic variation in the putative binding site of brevetoxins in common bottlenose dolphins (Tursiops truncatus) explains differences among individuals or populations in resistance to harmful Karenia brevis blooms in the Gulf of Mexico. We found very little variation in the sodium channel exons encoding the putative brevetoxin binding site among bottlenose dolphins from central-west Florida and the Florida Panhandle. Our study included samples from several bottlenose dolphin mortality events associated with HABs, but we found no association between genetic variation and survival. We observed a significant effect of geographic region on genetic variation for some sodium channel isoforms, but this can be primarily explained by rare private alleles and is more likely a reflection of regional genetic differentiation than the cause of different levels of HAB resistance between regions. In contrast to many other previously studied neurotoxin-resistant species, we conclude that bottlenose dolphins have not evolved resistance to HABs via mutations in genes encoding the brevetoxin binding site on the voltage-gated sodium channels.

Characterization of a de novo SCN8A mutation in a patient with epileptic encephalopathy

OBJECTIVE

Recently, de novo SCN8A missense mutations have been identified as a rare dominant cause of epileptic encephalopathies (EIEE13). Functional studies on the first described case demonstrated gain-of-function effects of the mutation. We describe a novel de novo mutation of SCN8A in a patient with epileptic encephalopathy, and functional characterization of the mutant protein.

DESIGN

Whole exome sequencing was used to discover the variant. We generated a mutant cDNA, transfected HEK293 cells, and performed Western blotting to assess protein stability. To study channel functional properties, patch-clamp experiments were carried out in transfected neuronal ND7/23 cells.

RESULTS

The proband exhibited seizure onset at 6 months of age, diffuse brain atrophy, and more profound developmental impairment than the original case. The mutation p.Arg233Gly in the voltage sensing transmembrane segment D1S4 was present in the proband and absent in both parents. This mutation results in a temperature-sensitive reduction in protein expression as well as reduced sodium current amplitude and density and a relative increased response to a slow ramp stimulus, though this did not result in an absolute increased current at physiological temperatures.

CONCLUSION

The new de novo SCN8A mutation is clearly deleterious, resulting in an unstable protein with reduced channel activity. This differs from the gain-of-function attributes of the first SCN8A mutation in epileptic encephalopathy, pointing to heterogeneity of mechanisms. Since Nav1.6 is expressed in both excitatory and inhibitory neurons, a differential effect of a loss-of-function of Nav1.6 Arg223Gly on inhibitory interneurons may underlie the epilepsy phenotype in this patient.

Regulation of membrane excitability: a convergence on voltage-gated sodium conductance

The voltage-gated sodium channel (Nav) plays a key role in regulation of neuronal excitability. Aberrant regulation of Nav expression and/or function can result in an imbalance in neuronal activity which can progress to epilepsy. Regulation of Nav activity is achieved by coordination of a multitude of mechanisms including RNA alternative splicing and translational repression. Understanding of these regulatory mechanisms is complicated by extensive genetic redundancy: the mammalian genome encodes ten Navs. By contrast, the genome of the fruitfly, Drosophila melanogaster, contains just one Nav homologue, encoded by paralytic (DmNa v ). Analysis of splicing in DmNa v shows variants exhibit distinct gating properties including varying magnitudes of persistent sodium current (INaP). Splicing by Pasilla, an identified RNA splicing factor, alters INaP magnitude as part of an activity-dependent mechanism. Enhanced INaP promotes membrane hyperexcitability that is associated with seizure-like behaviour in Drosophila. Nova-2, a mammalian Pasilla homologue, has also been linked to splicing of Navs and, moreover, mouse gene knockouts display seizure-like behaviour.Expression level of Navs is also regulated through a mechanism of translational repression in both flies and mammals. The translational repressor Pumilio (Pum) can bind to Na v transcripts and repress the normal process of translation, thus regulating sodium current (INa) density in neurons. Pum2-deficient mice exhibit spontaneous EEG abnormalities. Taken together, aberrant regulation of Nav function and/or expression is often epileptogenic. As such, a better understanding of regulation of membrane excitability through RNA alternative splicing and translational repression of Navs should provide new leads to treat epilepsy.

Sodium channel SCN8A (Nav1.6): properties and de novo mutations in epileptic encephalopathy and intellectual disability

The sodium channel Nav1.6, encoded by the gene SCN8A, is one of the major voltage-gated channels in human brain. The sequences of sodium channels have been highly conserved during evolution, and minor changes in biophysical properties can have a major impact in vivo. Insight into the role of Nav1.6 has come from analysis of spontaneous and induced mutations of mouse Scn8a during the past 18 years. Only within the past year has the role of SCN8A in human disease become apparent from whole exome and genome sequences of patients with sporadic disease. Unique features of Nav1.6 include its contribution to persistent current, resurgent current, repetitive neuronal firing, and subcellular localization at the axon initial segment (AIS) and nodes of Ranvier. Loss of Nav1.6 activity results in reduced neuronal excitability, while gain-of-function mutations can increase neuronal excitability. Mouse Scn8a (med) mutants exhibit movement disorders including ataxia, tremor and dystonia. Thus far, more than ten human de novo mutations have been identified in patients with two types of disorders, epileptic encephalopathy and intellectual disability. We review these human mutations as well as the unique features of Nav1.6 that contribute to its role in determining neuronal excitability in vivo. A supplemental figure illustrating the positions of amino acid residues within the four domains and 24 transmembrane segments of Nav1.6 is provided to facilitate the location of novel mutations within the channel protein.

Activity-dependent alternative splicing increases persistent sodium current and promotes seizure

Activity of voltage-gated Na channels (Na(v)) is modified by alternative splicing. However, whether altered splicing of human Na(v)s contributes to epilepsy remains to be conclusively shown. We show here that altered splicing of the Drosophila Na(v) (paralytic, DmNa(v)) contributes to seizure-like behavior in identified seizure mutants. We focus attention on a pair of mutually exclusive alternate exons (termed K and L), which form part of the voltage sensor (S4) in domain III of the expressed channel. The presence of exon L results in a large, non-inactivating, persistent I(Nap). Many forms of human epilepsy are associated with an increase in this current. In wild-type (WT) Drosophila larvae, ∼70-80% of DmNa(v) transcripts contain exon L, and the remainder contain exon K. Splicing of DmNa(v) to include exon L is increased to ∼100% in both the slamdance and easily-shocked seizure mutants. This change to splicing is prevented by reducing synaptic activity levels through exposure to the antiepileptic phenytoin or the inhibitory transmitter GABA. Conversely, enhancing synaptic activity in WT, by feeding of picrotoxin is sufficient to increase I(Nap) and promote seizure through increased inclusion of exon L to 100%. We also show that the underlying activity-dependent mechanism requires the presence of Pasilla, an RNA-binding protein. Finally, we use computational modeling to show that increasing I(Nap) is sufficient to potentiate membrane excitability consistent with a seizure phenotype. Thus, increased synaptic excitation favors inclusion of exon L, which, in turn, further increases neuronal excitability. Thus, at least in Drosophila, this self-reinforcing cycle may promote the incidence of seizure.

Increased expression of the beta3 subunit of voltage-gated Na+ channels in the spinal cord of the SOD1G93A mouse

Amyotrophic lateral sclerosis (ALS) is an adult-onset disease characterized by the progressive degeneration of motoneurons (MNs). Altered electrical properties have been described in familial and sporadic ALS patients. Cortical and spinal neurons cultured from the mutant Cu,Zn superoxide dismutase 1 (SOD1G93A) mouse, a murine model of ALS, exhibit a marked increase in the persistent Na+ currents. Here, we investigated the effects of the SOD1G93A mutation on the expression of the voltage-gated Na+ channel alpha subunit SCN8A (Nav1.6) and the beta subunits SCN1B (beta1), SCN2B (beta2), and SCN3B (beta3) in MNs of the spinal cord in presymptomatic (P75) and symptomatic (P120) mice. We observed a significant increase, within lamina IX, of the beta3 transcript and protein expression. On the other hand, the beta1 transcript was significantly decreased, in the same area, at the symptomatic stage, while the beta2 transcript levels were unaltered. The SCN8A transcript was significantly decreased at P120 in the whole spinal cord. These data suggest that the SOD1G93A mutation alters voltage-gated Na+ channel subunit expression. Moreover, the increased expression of the beta3 subunit support the hypothesis that altered persistent Na+ currents contribute to the hyperexcitability observed in the ALS-affected MNs.

Voltage-gated potassium channel KCNV2 (Kv8.2) contributes to epilepsy susceptibility

Mutations in voltage-gated ion channels are responsible for several types of epilepsy. Genetic epilepsies often exhibit variable severity in individuals with the same mutation, which may be due to variation in genetic modifiers. The Scn2a(Q54) transgenic mouse model has a sodium channel mutation and exhibits epilepsy with strain-dependent severity. We previously mapped modifier loci that influence Scn2a(Q54) phenotype severity and identified Kcnv2, encoding the voltage-gated potassium channel subunit Kv8.2, as a candidate modifier. In this study, we demonstrate a threefold increase in hippocampal Kcnv2 expression associated with more severe epilepsy. In vivo exacerbation of the phenotype by Kcnv2 transgenes supports its identification as an epilepsy modifier. The contribution of KCNV2 to human epilepsy susceptibility is supported by identification of two nonsynonymous variants in epilepsy patients that alter function of Kv2.1/Kv8.2 heterotetrameric potassium channels. Our results demonstrate that altered potassium subunit function influences epilepsy susceptibility and implicate Kcnv2 as an epilepsy gene.

Regulation of sodium channel activity by phosphorylation

Voltage-gated sodium channels carry the major inward current responsible for action potential depolarization in excitable cells as well as providing additional inward current that modulates overall excitability. Both their expression and function is under tight control of protein phosphorylation by specific kinases and phosphatases and this control is particular to each type of sodium channel. This article examines the impact and mechanism of phosphorylation for isoforms where it has been studied in detail in an attempt to delineate common features as well as differences.

Cell-specific patterns of alternative splicing of voltage-gated ion channels in single identified neurons

CbNa(v) and CbIH encode channels that carry voltage-gated sodium and hyperpolarization activated cation currents respectively in the crab, Cancer borealis. We cloned and sequenced full length cDNAs for both CbNa(v) and CbIH and found nine different regions of alternative splicing for the CbNa(v) gene and four regions of alternative splicing for CbIH. We used RT-PCR to determine tissue-specific differences in splicing of both channel genes among cardiac muscle, skeletal muscle, brain, and stomatogastric ganglion (STG) tissue. We then examined the splice variant isoforms present in single, unambiguously identified neurons of the STG. We found cell-type specific patterns of alternative splicing for CbNa(v), indicating unique cell-specific pattern of post-transcriptional modification. Furthermore, we detected possible differences in cellular localization of alternatively spliced CbNa(v) transcripts; distinct mRNA isoforms are present between the cell somata and the axons of the neurons. In contrast, we found no qualitative differences among different cell types for CbIH variants present, although this analysis did not represent the full spectrum of all possible CbIH variants. CbIH mRNA was not detected in axon samples. Finally, although cell-type specific patterns of splicing were detected for CbNa(v), the same cell type within and between animals also displayed variability in which splice forms were detected. These results indicate that channel splicing is differentially regulated at the level of single neurons of the same neural network, providing yet another mechanism by which cell-specific neuronal output can be achieved.

Alternative splicing of Na(V)1.7 exon 5 increases the impact of the painful PEPD mutant channel I1461T

Alternative splicing is known to alter pharmacological sensitivities, kinetics, channel distribution under pathological conditions, and developmental regulation of VGSCs. Mutations that alter channel properties in Na(V)1.7 have been genetically implicated in patients with bouts of extreme pain classified as inherited erythromelalgia (IEM) or paroxysmal extreme pain disorder (PEPD). Furthermore, patients with IEM or PEPD report differential age onsets. A recent study reported that alternative splicing of Na(V)1.7 exon 5 affects ramp current properties. Since IEM and PEPD mutations also alter Na(V)1.7 ramp current properties we speculated that alternative splicing might impact the functional consequences of IEM or PEPD mutations. We compared the effects alternative splicing has on the biophysical properties of Na(V)1.7 wild-type, IEM (I136V) and PEPD (I1461T) channels. Our major findings demonstrate that although the 5A splice variant of the IEM channel had no functional impact, the 5A splice variant of the PEPD channel significantly hyperpolarized the activation curve, slowed deactivation and closed-state inactivation, shifted the ramp current activation to more hyperpolarized potentials, and increased ramp current amplitude. We hypothesize a D1/S3-S4 charged residue difference between the 5N (Asn) and the 5A (Asp) variants within the coding region of exon 5 may contribute to shifts in channel activation and deactivation. Taken together, the additive effects observed on ramp currents from exon 5 splicing and the PEPD mutation (I1461T) are likely to impact the disease phenotype and may offer insight into how alternative splicing may affect specific intramolecular interactions critical for voltage-dependent gating.

Alternative splicing in the voltage-gated sodium channel DmNav regulates activation, inactivation, and persistent current

Diversity in neuronal signaling is a product not only of differential gene expression, but also of alternative splicing. However, although recognized, the precise contribution of alternative splicing in ion channel transcripts to channel kinetics remains poorly understood. Invertebrates, with their smaller genomes, offer attractive models to examine the contribution of splicing to neuronal function. In this study we report the sequencing and biophysical characterization of alternative splice variants of the sole voltage-gated Na+ gene (DmNav, paralytic), in late-stage embryos of Drosophila melanogaster. We identify 27 unique splice variants, based on the presence of 15 alternative exons. Heterologous expression, in Xenopus oocytes, shows that alternative exons j, e, and f primarily influence activation kinetics: when present, exon f confers a hyperpolarizing shift in half-activation voltage (V1/2), whereas j and e result in a depolarizing shift. The presence of exon h is sufficient to produce a depolarizing shift in the V1/2 of steady-state inactivation. The magnitude of the persistent Na+ current, but not the fast-inactivating current, in both oocytes and Drosophila motoneurons in vivo is directly influenced by the presence of either one of a pair of mutually exclusive, membrane-spanning exons, termed k and L. Transcripts containing k have significantly smaller persistent currents compared with those containing L. Finally, we show that transcripts lacking all cytoplasmic alternatively spliced exons still produce functional channels, indicating that splicing may influence channel kinetics not only through change to protein structure, but also by allowing differential modification (i.e., phosphorylation, binding of cofactors, etc.). Our results provide a functional basis for understanding how alternative splicing of a voltage-gated Na+ channel results in diversity in neuronal signaling.

Fine mapping of an epilepsy modifier gene on mouse Chromosome 19

Mutations in voltage-gated sodium channels are associated with several types of human epilepsy. Variable expressivity and penetrance are common features of inherited epilepsy caused by sodium channel mutations, suggesting that genetic modifiers may influence clinical severity. The mouse model Scn2a(Q54) has an epilepsy phenotype due to a mutation in Scn2a that results in elevated persistent sodium current. Phenotype severity in Scn2a(Q54) mice is dependent on the genetic background. Congenic C57BL/6J.Q54 mice have delayed onset and low seizure frequency compared to (C57BL/6J x SJL/J)F1.Q54 mice. Previously, we identified two modifier loci that influence the Scn2a(Q54) epilepsy phenotype: Moe1 (modifier of epilepsy 1) on Chromosome 11 and Moe2 on Chromosome 19. We have constructed interval-specific congenic strains to further refine the position of Moe2 on Chromosome 19 to a 5-Mb region. Sequencing and expression analyses of genes in the critical interval suggested two potential modifier candidates: (1) voltage-gated potassium channel subunit subfamily V, member 2 (Kcnv2), and (2) SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily a, member 2 (Smarca2). Based on its biological role in regulating membrane excitability and the association between ion channel variants and seizures, Kcnv2 is a strong functional candidate for Moe2. Modifier genes affecting the epilepsy phenotype of Scn2a(Q54) mice may contribute to variable expressivity and penetrance in human epilepsy patients with sodium channel mutations.

Cerebellum-related characteristics of Scn8a-mutant mice

Among ten sodium channel alpha-subunit genes mapped in human and mouse genomes, the SCN8A gene is primarily expressed in neurons and glia. Mice with two types of Scn8a null mutations--Scn8a ( med ) and Scn8a ( medTg )--live for only 21-24 days, but those with incomplete mutations-Scn8a ( medJ ) and Scn8a ( medJo )--and those with knockout of Scn8a only in cerebellar Purkinje cells live to adult age. We review here previous work on cerebellum and related regions of Scn8a mutant mice and include some newer immunohistochemical and microchemical results. The resurgent sodium current that underlies the repeated firing of Purkinje cells is reduced in Scn8a mutant and knockout mice. Purkinje cells of mutant mice have greatly reduced spontaneous activity, as do the analogous cartwheel cells of the dorsal cochlear nucleus. Up-regulation of GABA(A) receptors in regions to which Purkinje cells project may partially compensate for their decreased activity in the mutant mice. The somata of cerebellar Purkinje cells of Scn8a ( medJ ) and Scn8a ( medJo ) mice, as revealed by PEP-19 immunoreaction, are slightly smaller than normal, and their axons, especially in Scn8a ( medJo ) mice, sometimes show enlargements similar to those in other types of mutant mice. Density of GABA-like immunoreactivity is decreased in Purkinje somata and regions of termination in deep cerebellar and vestibular nuclei of Scn8a ( medJ ) mice, but measured GABA concentration is not significantly reduced in microdissected samples of these regions. The concentrations of taurine and glutamine are significantly increased in cerebellar-related regions of Scn8a ( medJ ) mice, possibly suggesting up-regulation of glial amino acid metabolism.

An emerging role for voltage-gated Na+ channels in cellular migration: regulation of central nervous system development and potentiation of invasive cancers

Voltage-gated Na(+) channels (VGSCs) exist as macromolecular complexes containing a pore-forming alpha subunit and one or more beta subunits. The VGSC alpha subunit gene family consists of 10 members, which have distinct tissue-specific and developmental expression profiles. So far, four beta subunits (beta1-beta4) and one splice variant of beta1 (beta1A, also called beta1B) have been identified. VGSC beta subunits are multifunctional, serving as modulators of channel activity, regulators of channel cell surface expression, and as members of the immunoglobulin superfamily, cell adhesion molecules (CAMs). beta subunits are substrates of beta-amyloid precursor protein-cleaving enzyme (BACE1) and gamma-secretase, yielding intracellular domains (ICDs) that may further modulate cellular activity via transcription. Recent evidence shows that beta1 regulates migration and pathfinding in the developing postnatal CNS in vivo. The alpha and beta subunits, together with other components of the VGSC signaling complex, may have dynamic interactive roles depending on cell/tissue type, developmental stage, and pathophysiology. In addition to excitable cells like nerve and muscle, VGSC alpha and beta subunits are functionally expressed in cells that are traditionally considered nonexcitable, including glia, vascular endothelial cells, and cancer cells. In particular, the alpha subunits are up-regulated in line with metastatic potential and are proposed to enhance cellular migration and invasion. In contrast to the alpha subunits, beta1 is more highly expressed in weakly metastatic cancer cells, and evidence suggests that its expression enhances cellular adhesion. Thus, novel roles are emerging for VGSC alpha and beta subunits in regulating migration during normal postnatal development of the CNS as well as during cancer metastasis.

Novel mRNA isoforms of the sodium channels Na(v)1.2, Na(v)1.3 and Na(v)1.7 encode predicted two-domain, truncated proteins

The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 mRNAs. We isolated novel mRNA isoforms of Na(v)1.2 and Na(v)1.3 from adult mouse and rat dorsal root ganglia (DRG), Na(v)1.3 and Na(v)1.7 from adult mouse brain, and Na(v)1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Na(v)1.2 exon 17A and topologically equivalent Na(v)1.7 exon 16A) or exon pair (Na(v)1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94-100% identities), as are the paralogous Na(v)1.2 and Na(v)1.3 exons (93% identity in mouse) to which the Na(v)1.7 exon has only 60% identity. Previously, Na(v)1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Na(v)1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Na(v)1.3 mRNA expression is upregulated by 33% (P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77-89% and 72-76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.

Biophysical properties of human Na v1.7 splice variants and their regulation by protein kinase A

The sodium channel Na(v)1.7 is preferentially expressed in nociceptive neurons and is believed to play a crucial role in pain sensation. Four alternative splice variants are expressed in human dorsal root ganglion neurons, two of which differ in exon 5 by two amino acids in the S3 segment of domain I (exons 5A and 5N). Two others differ in exon 11 by the presence (11L) or absence (11S) of an 11 amino acid sequence in the loop between domains I and II, an important region for PKA regulation. In the present study, we used the whole cell configuration of the patch-clamp technique to investigate the biophysical properties and 8-bromo-cyclic adenosine monophosphate (8Br-cAMP) modulation of these splice variants expressed in tsA201 cells in the presence of the beta(1)-subunit. The alternative splicing of Na(v)1.7 had no effect on most of the biophysical properties of this channel, including activation, inactivation, and recovery from inactivation. However, development of inactivation experiments revealed that the isoform containing exon 5A had slower kinetics of inactivation for negative potentials than that of the variant containing exon 5N. This difference was associated with higher ramp current amplitudes for isoforms containing exon 5A. Moreover, 8Br-cAMP-mediated phosphorylation induced a negative shift of the activation curve of variants containing exon 11S, whereas inactivation properties were unchanged. Isoforms with exon 11L were not modulated by 8Br-cAMP-induced phosphorylation. We conclude that alternative splicing of human Na(v)1.7 can specifically modulate the biophysical properties and cAMP-mediated regulation of this channel. Changing the proportions of these variants may thus influence neuronal excitability and pain sensation.

Nav1.7 and Nav1.3 are the only tetrodotoxin-sensitive sodium channels expressed by the adult guinea pig enteric nervous system

The types of sodium channels that are expressed by neurons shape the rising phase of action potentials and influence patterns of action potential discharge. With regard to the enteric nervous system (ENS), there is uncertainty about which channels are expressed, and in particular it is unknown whether Na(v)1.7 is present. We designed specific probes for the guinea pig Na(v)1.7 alpha subunit as well as for the other tetrodotoxin (TTX)-sensitive alpha subunits (Na(v)1.1, Na(v)1.2, Na(v)1.3, and Na(v)1.6) in order to perform in situ hybridization (ISH) histochemistry on guinea pig myenteric ganglia. We established that only Na(v)1.7 mRNA and Na(v)1.3 mRNA are expressed in these ganglia. The ISH signal for Na(v)1.7 transcripts was found in seemingly all the myenteric neurons. The expression of the Na(v)1.3 alpha subunit was confirmed by immunohistochemistry in a large proportion (62%) of the myenteric neuron population. This population included enteric sensory neurons. Na(v)1.6 immunoreactivity, absent from myenteric neurons, was detected in glial cells only when a high anti-Na(v)1.6 antibody concentration was used. This suggests that the Na(v)1.6 alpha subunit and mRNA are present only at low levels, which is consistent with the fact that no Na(v)1.6 mRNA could be detected in the ENS by ISH. The fact that adult myenteric neurons are endowed with only two TTX-sensitive alpha subunits, namely, Na(v)1.3 and Na(v)1.7, emphasizes the singularity of the ENS. Both these subunits, known to have slow-inactivation kinetics, are well adapted for generating action potentials from slow excitatory postsynaptic potentials, a mode of synaptic transmission that applies to all ENS neuron types.

Characterization of 5' untranslated regions of the voltage-gated sodium channels SCN1A, SCN2A, and SCN3A and identification of cis-conserved noncoding sequences

The human voltage-gated sodium channel gene cluster on chromosome 2q24 contains three paralogs, SCN1A, SCN2A, and SCN3A, which are expressed in the central nervous system. Mutations in SCN1A and SCN2A cause several subtypes of idiopathic epilepsy. Furthermore, many SCN1A mutations are predicted to reduce protein levels, emphasizing the importance of precise sodium channel gene regulation. To investigate the genetic factors that regulate the expression of SCN1A, SCN2A, and SCN3A, we characterized the 5' untranslated region of each gene. We identified multiple noncoding exons and observed brain region differences in the expression levels of noncoding exons. Comparative sequence analysis revealed 33 conserved noncoding sequences (CNSs) between the orthologous mammalian genes and 6 CNSs between the three human paralogs. Seven CNSs corresponded to noncoding exons. Twelve CNSs were evaluated for their ability to alter the transcription of a luciferase reporter gene, and 3 resulted in a modest, but statistically significant change.

Subunit dependence of Na channel slow inactivation and open channel block in cerebellar neurons

Purkinje and cerebellar nuclear neurons both have Na currents with resurgent kinetics. Previous observations, however, suggest that their Na channels differ in their susceptibility to entering long-lived inactivated states. To compare fast inactivation, slow inactivation, and open-channel block, we recorded voltage-clamped, tetrodotoxin-sensitive Na currents in Purkinje and nuclear neurons acutely isolated from mouse cerebellum. In nuclear neurons, recovery from all inactivated states was slower, and open-channel unblock was less voltage-dependent than in Purkinje cells. To test whether specific subunits contributed to this differential stability of inactivation, experiments were repeated in Na(V)1.6-null (med) mice. In med Purkinje cells, recovery times were prolonged and the voltage dependence of open-channel block was reduced relative to control cells, suggesting that availability of Na(V)1.6 is quickly restored at negative potentials. In med nuclear cells, however, currents were unchanged, suggesting that Na(V)1.6 contributes little to wild-type nuclear cells. Extracellular Na(+) prevented slow inactivation more effectively in Purkinje than in nuclear neurons, consistent with a resilience of Na(V)1.6 to slow inactivation. The tendency of nuclear Na channels to inactivate produced a low availability during trains of spike-like depolarization. Hyperpolarizations that approximated synaptic inhibition effectively recovered channels, suggesting that increases in Na channel availability promote rebound firing after inhibition.

Radiation hybrid mapping of 18 positional and physiological candidate genes for arthrogryposis multiplex congenita on porcine chromosome 5

We report the chromosomal assignment of 18 porcine genes to human homologues using the INRA-Minnesota swine radiation hybrid panel (IMpRH). These genes (CACNA1C, COL2A1, CPNE8, C3F, C12ORF4, DDX11, GDF11, HOXC8, KCNA1, MDS028, TMEM106C, NR4A1, PHB2, PRICKLE1, Q6ZUQ4, SCN8A, TUBA8 and USP18) are located on porcine chromosome 5 (SSC5) and represent positional and functional candidates for arthrogryposis multiplex congenita (AMC), which maps to SSC5. CPNE8, PRICKLE1, Q6ZUQ4 and TUBA8 were mapped to the interval for pig AMC between microsatellites SW152 and SW904. Three SNPs in TUBA8 co-segregated with the AMC phenotype in 230 pigs of our research population without recombination and could be used as a genetic marker test for AMC. In addition, we provide evidence that a small chromosomal region of HSA22q11.2 evolutionarily corresponds to SSC5q12-q22 (and contains the human homologues of porcine SW152, Q6ZUQ4, TUBA8 and USP18), while the regions flanking HSA22q11.2 on SSC5 correspond to HSA12p13 and HSA12q12. We identified seven distinct chromosomal blocks, further supporting extensive rearrangements between genes on HSA12 and HSA22 in the AMC region on SSC5.

Gene duplications and evolution of vertebrate voltage-gated sodium channels

Voltage-gated sodium channels underlie action potential generation in excitable tissue. To establish the evolutionary mechanisms that shaped the vertebrate sodium channel alpha-subunit (SCNA) gene family and their encoded Nav1 proteins, we identified all SCNA genes in several teleost species. Molecular cloning revealed that teleosts have eight SCNA genes, compared to ten in another vertebrate lineage, mammals. Prior phylogenetic analyses have indicated that the genomes of both teleosts and tetrapods contain four monophyletic groups of SCNA genes, and that tandem duplications expanded the number of genes in two of the four mammalian groups. However, the number of genes in each group varies between teleosts and tetrapods, suggesting different evolutionary histories in the two vertebrate lineages. Our findings from phylogenetic analysis and chromosomal mapping of Danio rerio genes indicate that tandem duplications are an unlikely mechanism for generation of the extant teleost SCNA genes. Instead, analyses of other closely mapped genes in D. rerio as well as of SCNA genes from several teleost species all support the hypothesis that a whole-genome duplication was involved in expansion of the SCNA gene family in teleosts. Interestingly, despite their different evolutionary histories, mRNA analyses demonstrated a conservation of expression patterns for SCNA orthologues in teleosts and tetrapods, suggesting functional conservation.

Severe epilepsy resulting from genetic interaction between Scn2a and Kcnq2

A mutation in the voltage-gated sodium-channel Scn2a results in moderate epilepsy in transgenic Scn2a(Q54) mice maintained on a C57BL/6J strain background. The onset of progressive epilepsy begins in adults with short-duration partial seizures that originate in the hippocampus. The underlying abnormality is an increase in persistent sodium current in hippocampal neurons. The voltage-gated potassium channel Kcnq2 is responsible for generating M current (I(KM)) that is thought to control excitability and limit repetitive firing of hippocampal neurons. To determine whether impaired M current would exacerbate the seizure phenotype of Scn2a(Q54) mice, we carried out genetic crosses with two mutant alleles of Kcnq2. Szt1 mice carry a spontaneous deletion that removes the C-terminal domain of Kcnq2. A novel Kcnq2 missense mutation V182M was identified by screening the offspring of ENU-treated males for reduced threshold to electrically evoked minimal clonic seizures. Double mutant mice carrying the Scn2a(Q54) transgene together with either of the Kcnq2 mutations exhibited severe epilepsy with early onset, generalized tonic-clonic seizures and juvenile lethality by 3 weeks of age. This dramatic exacerbation of the sodium-channel mutant phenotype indicates that M current plays a critical role in preventing seizure initiation and spreading in this animal model. The genetic interaction between Scn2a and Kcnq2 demonstrates that combinations of mild alleles of monogenic epilepsy genes can result in severe disease and provides a model for complex inheritance of human epilepsy. The data suggest that interaction between these genes might contribute to the variable expressivity observed in human families with sodium-channel mutations. In a screen of 23 SMEI patients with missense mutations of SCN1A, no second-site mutations in KCNQ2 were identified.

Voltage-gated sodium channels: action players with many faces

Voltage-gated sodium channels are responsible for the upstroke of the action potential and thereby play an important role in propagation of the electrical impulse in excitable tissues like muscle, nerve and the heart. Duplication of the sodium channels encoding genes during evolution generated the sodium channel gene family with the different isoforms differing in biophysical properties and tissue distribution. In this review article, mutations in these genes leading to various inherited disorders are discussed.

A novel polyclonal antibody specific for the Na(v)1.5 voltage-gated Na(+) channel 'neonatal' splice form

Voltage-gated Na(+) channel (VGSC) diversity is achieved through a number of mechanisms: multiple subunits, multiple genes encoding the pore-forming VGSC alpha-subunit and multiple gene isoforms generated by alternative splicing. A major type of VGSCalpha alternative splicing is in D1:S3, which has been proposed to be developmentally regulated. We recently reported a D1:S3 spliced form of Na(v)1.5 in human metastatic breast cancer cells. This novel 'neonatal' isoform differs from the counterpart 'adult' form at seven amino acids (in the extracellular loop between S3-S4 of D1). Here, we generated an anti-peptide polyclonal antibody, named NESOpAb, which specifically recognised 'neonatal' but not 'adult' Na(v)1.5 when tested on cells specifically over-expressing one or other of these Na(v)1.5 spliced forms. The antibody was used to investigate developmental expression of 'neonatal' Na(v)1.5 (nNa(v)1.5) in a range of mouse tissues by immunohistochemistry. Overall, the results were consistent with nNa(v)1.5 protein being more abundantly expressed in selected tissues (particularly heart and brain) from neonate as compared to adult animals. Importantly, NESOpAb blocked functional nNa(v)1.5 ion conductance when applied extracellularly at concentrations as low as 0.05 ng/ml. Possible biological and clinical applications of NESOpAb are discussed.

Heterozygosity for a protein truncation mutation of sodium channel SCN8A in a patient with cerebellar atrophy, ataxia, and mental retardation

BACKGROUND

The SCN8A gene on chromosome 12q13 encodes the voltage gated sodium channel Na(v)1.6, which is widely expressed in neurons of the CNS and PNS. Mutations in the mouse ortholog of SCN8A result in ataxia and other movement disorders.

METHODS

We screened the 26 coding exons of SCN8A in 151 patients with inherited or sporadic ataxia.

RESULTS

A 2 bp deletion in exon 24 was identified in a 9 year old boy with mental retardation, pancerebellar atrophy, and ataxia. This mutation, Pro1719ArgfsX6, introduces a translation termination codon into the pore loop of domain 4, resulting in removal of the C-terminal cytoplasmic domain and predicted loss of channel function. Three additional heterozygotes in the family exhibit milder cognitive and behavioural deficits including attention deficit hyperactivity disorder (ADHD). No additional occurrences of this mutation were observed in 625 unrelated DNA samples (1250 chromosomes).

CONCLUSIONS

The phenotypes of the heterozygous individuals suggest that mutations in SCN8A may result in motor and cognitive deficits of variable expressivity, but the study was limited by lack of segregation in the small pedigree and incomplete information about family members. Identification of additional families will be required to confirm the contribution of the SCN8A mutation to the clinical features in ataxia, cognition and behaviour disorders.

Genetic modifiers affecting severity of epilepsy caused by mutation of sodium channelScn2a

Mutations in the voltage-gated sodium channels SCN 1 A and SCN 2 A are responsible for several types of human epilepsy. Variable expressivity among family members is a common feature of these inherited epilepsies, suggesting that genetic modifiers may influence the clinical manifestation of epilepsy. The transgenic mouse model Scn 2 a(Q 54) has an epilepsy phenotype as a result of a mutation in Scn 2 a that slows channel inactivation. The mice display progressive epilepsy that begins with short-duration partial seizures that appear to originate in the hippocampus. The partial seizures become more frequent and of longer duration with age and often induce secondary generalized seizures. Clinical severity of the Scn 2 a(Q 54) phenotype is influenced by genetic background. Congenic C57BL/6J.Q 54 mice exhibit decreased incidence of spontaneous seizures, delayed seizure onset, and longer survival in comparison with [C57BL/6J x SJL/J]F(1).Q 54 mice. This observation indicates that strain SJL/J carries dominant modifier alleles at one or more loci that determine the severity of the epilepsy phenotype. Genome-wide interval mapping in an N(2) backcross revealed two modifier loci on Chromosomes 11 and 19 that influence the clinical severity of of this sodium channel-induced epilepsy. Modifier genes affecting clinical severity in the Scn 2 a(Q 54) mouse model may contribute to the variable expressivity seen in epilepsy patients with sodium channel mutations.

Distribution and functional characterization of human Nav1.3 splice variants

The focus of the present study is the molecular and functional characterization of four splice variants of the human Nav1.3 alpha subunit. These subtypes arise due to the use of alternative splice donor sites of exon 12, which encodes a region of the alpha subunit that resides in the intracellular loop between domains I and II. This region contains several important phosphorylation sites that modulate Na+ channel kinetics in related sodium channels, i.e. Nav1.2. While three of the four Nav1.3 isoforms, 12v1, 12v3 and 12v4 have been previously identified in human, 12v2 has only been reported in rat. Herein, we evaluate the distribution of these splice variants in human tissues and the functional characterization of each of these subtypes. We demonstrate by reverse transcriptase-polymerase chain reaction (RT-PCR) that each subtype is expressed in the spinal cord, thalamus, amygdala, cerebellum, adult and fetal whole brain and heart. To investigate the functional properties of these different splice variants, each alpha subunit isoform was cloned by RT-PCR from human fetal brain and expressed in Xenopus oocytes. Each isoform exhibited functional voltage-dependent Na+ channels with similar sensitivities to tetrodotoxin (TTX) and comparable current amplitudes. Subtle shifts in the V 1/2 of activation and inactivation (2-3 mV) were observed among the four isoforms, although the functional significance of these differences remains unclear. This study has demonstrated that all four human splice variants of the Nav1.3 channel alpha subunit are widely expressed and generate functional TTX-sensitive Na+ channels that likely modulate cellular excitability.

Allelic mutations of the sodium channel SCN8A reveal multiple cellular and physiological functions

Allelic mutations of Scn8a in the mouse have revealed the range of neurological disorders that can result from alternations of one neuronal sodium channel. Null mutations produce the most severe phenotype, with motor neuron failure leading to paralysis and juvenile lethality. Two less severe mutations cause ataxia, tremor, muscle weakness, and dystonia. The electrophysiological effects have been studied at the cellular level by recording from neurons from the mutant mice. The data demonstrate that Scn8a is required for the complex spiking of cerebellar Purkinje cells and for persistent sodium current in several classes of neurons, including some with pacemaker roles. The mouse mutations of Scn8a have also provided insight into the mode of inheritance of channelopathies, and led to the identification of a modifier gene that affects transcript splicing. These mutations demonstrate the value of mouse models to elucidate the pathophysiology of human disease.

Multiple transcripts of sodium channel SCN8A (NaV1.6) with alternative 5′- and 3′-untranslated regions and initial characterization of the SCN8A promoter

To identify the transcriptional start sites of the neuronal channel SCN8A, we carried out 5'-RACE (rapid amplification of cDNA ends) with RNA from human and mouse brain. We recovered four mutually exclusive 5'-untranslated exons (exon 1a to exon 1d) that map to a 1.8-kb region of genomic DNA located approximately 70 kb upstream of the first coding exon. The same 5'-untranslated exons are expressed in central, peripheral and sympathetic nervous system and in embryonic and adult brain. A 4.8-kb genomic fragment containing these 5' exons demonstrated promoter activity in transfected MN-1 cells. In transgenic mice, transcription of the 4.8-kb promoter was restricted to brain and spinal cord. The 4.8-kb promoter contains eight consensus Sp1-binding sites and two Inr sites. A potential NRSE/RE-1 site is located nearby. Two active polyadenylation sites identified by 3'-RACE are conserved in human, mouse, and chicken SCN8A. Sequence comparison of human and mouse SCN8A identified 12 conserved noncoding elements whose effect on transcription was tested in transfected cells.

Molecular diversity of voltage-gated sodium channel alpha subunits expressed in neuronal and non-neuronal excitable cells

In order to investigate the role of molecular diversity of voltage-activated sodium channel alpha-subunits in excitability of neuronal and non-neuronal cells, we carried out patch-clamp recordings and single-cell RT-PCR on two different types of mammalian excitable cells i.e. hippocampal neurons and non-neuronal utricular epithelial hair cells. In each cell type, multiple different combinations of sodium channel alpha-subunits exist from cell to cell despite similar sodium current properties. The mRNA isoforms, Nav1.2 and Nav1.6, are the most frequently detected by single cell analysis in the two cell types while Nav1.3 and Nav1.7 are also moderately expressed in embryonic hippocampal neurons and in neonatal utricular hair cells respectively. By investigating the particular alternate splice isoforms of Nav1.6 occurring at the exon 18 of the mouse orthologue SCN8A, we revealed that this subunit co-exist in the two cell types under different alternative spliced isoforms. The expression of non-functional isoforms of Nav1.6 in utricular epithelial hair cells excludes the involvement of this subunit in supporting their excitability. Thus, from a functional point of view, the present results suggest that, at the single cell level, both neuronal and non-neuronal excitable cells expressed different and complex patterns of sodium channel gene transcripts but this diversity alone cannot explain the sodium current properties of these cell types.

Use-dependent potentiation of the Nav1.6 sodium channel

Nav1.2 and Nav1.6 are two voltage-gated sodium channel isoforms that are abundant in the adult central nervous system. These channels are expressed in different cells and localized in different neuronal regions, which may reflect functional specialization. To examine this possibility, we compared the properties of Nav1.2 and Nav1.6 in response to a rapid series of repetitive depolarizations. Currents through Nav1.6 coexpressed with beta1 demonstrated use-dependent potentiation during a rapid train of depolarizations. This potentiation was in contrast to the use-dependent decrease in current for Nav1.2 with beta1. The voltage dependence of potentiation correlated with the voltage dependence of activation, and it still occurred when fast inactivation was removed by mutation. Rapid stimulation accelerated a slow phase of activation in the Nav1.6 channel that had fast inactivation removed, resulting in faster channel activation. Although the Nav1.2 channel with fast inactivation removed also demonstrated slightly faster activation, that channel showed very pronounced slow inactivation compared to Nav1.6. These results indicate that potentiation of Nav1.6 sodium currents results from faster channel activation, and that this effect is masked by slow inactivation in Nav1.2. The data suggest that Nav1.6 might be more resistant to inactivation, which might be helpful for high-frequency firing at nodes of Ranvier compared to Nav1.2.

Expression of alternatively spliced sodium channel alpha-subunit genes. Unique splicing patterns are observed in dorsal root ganglia

Molecular medicine requires the precise definition of drug targets, and tools are now in place to provide genome-wide information on the expression and alternative splicing patterns of any known gene. DNA microarrays were used to monitor transcript levels of the nine well-characterized alpha-subunit sodium channel genes across a broad range of tissues from cynomolgus monkey, a non-human primate model. Alternative splicing of human transcripts for a subset of the genes that are expressed in dorsal root ganglia, SCN8A (Na(v)1.6), SCN9A (Na(v)1.7), and SCN11A (Na(v)1.9) was characterized in detail. Genomic sequence analysis among gene family paralogs and between cross-species orthologs suggested specific alternative splicing events within transcripts of these genes, all of which were experimentally confirmed in human tissues. Quantitative PCR revealed that certain alternative splice events are uniquely expressed in dorsal root ganglia. In addition to characterization of human transcripts, alternatively spliced sodium channel transcripts were monitored in a rat model for neuropathic pain. Consistent down-regulation of all transcripts was observed, as well as significant changes in the splicing patterns of SCN8A and SCN9A.