Modulation of sodium current in mammalian cells by an epilepsy-correlated beta 1-subunit mutation

Voltage Gated Sodium Channel Genes in Epilepsy: Mutations, Functional Studies, and Treatment Dimensions

Genetic epilepsy occurs as a result of mutations in either a single gene or an interplay of different genes. These mutations have been detected in ion channel and non-ion channel genes. A noteworthy class of ion channel genes are the voltage gated sodium channels (VGSCs) that play key roles in the depolarization phase of action potentials in neurons. Of huge significance are SCN1A, SCN1B, SCN2A, SCN3A, and SCN8A genes that are highly expressed in the brain. Genomic studies have revealed inherited and de novo mutations in sodium channels that are linked to different forms of epilepsies. Due to the high frequency of sodium channel mutations in epilepsy, this review discusses the pathogenic mutations in the sodium channel genes that lead to epilepsy. In addition, it explores the functional studies on some known mutations and the clinical significance of VGSC mutations in the medical management of epilepsy. The understanding of these channel mutations may serve as a strong guide in making effective treatment decisions in patient management.

A mutation of SCN1B associated with GEFS+ causes functional and maturation defects of the voltage-dependent sodium channel

Voltage-dependent sodium channels are responsible of the rising phase of the action potential in excitable cells. These integral membrane proteins are composed of a pore-forming α-subunit, and one or more auxiliary β subunits. Mutation p.Asp25Asn (D25N; c.73G > A) of the β1 subunit, coded by the gene SCN1B, has been reported in a patient with generalized epilepsy with febrile seizure plus type 1 (GEFS+). In human embryonic kidney 293 (HEK) cells, the heterologous coexpression of D25N-β1 subunit with Nav1.2, Nav1.4, and Nav1.5 α subunits, representative of brain, skeletal muscle, and heart voltage gated sodium channels, determines a reduced sodium channel functional expression and a negative shift of the activation and inactivation steady state curves. The D25N mutation of the β1 subunit causes a maturation (glycosylation) defect of the protein, leading to a reduced targeting to the plasma membrane. Also the β1-dependent gating properties of the sodium channels are abolished by the mutation, suggesting that D25N is no more able to interact with the α subunit. Our work underscores the role played by the β1 subunit, highlighting how a defective interaction between the sodium channel constituents could lead to a disabling pathological condition, and opens the possibility to design a mutation-specific GEFS+ treatment based on protein maturation.

Voltage-gated sodium channel β subunits: The power outside the pore in brain development and disease

Voltage gated sodium channels (VGSCs) were first identified in terms of their role in the upstroke of the action potential. The underlying proteins were later identified as saxitoxin and scorpion toxin receptors consisting of α and β subunits. We now know that VGSCs are heterotrimeric complexes consisting of a single pore forming α subunit joined by two β subunits; a noncovalently linked β1 or β3 and a covalently linked β2 or β4 subunit. VGSC α subunits contain all the machinery necessary for channel cell surface expression, ion conduction, voltage sensing, gating, and inactivation, in one central, polytopic, transmembrane protein. VGSC β subunits are more than simple accessories to α subunits. In the more than two decades since the original cloning of β1, our knowledge of their roles in physiology and pathophysiology has expanded immensely. VGSC β subunits are multifunctional. They confer unique gating mechanisms, regulate cellular excitability, affect brain development, confer distinct channel pharmacology, and have functions that are independent of the α subunits. The vast array of functions of these proteins stems from their special station in the channelome: being the only known constituents that are cell adhesion and intra/extracellular signaling molecules in addition to being part of channel complexes. This functional trifecta and how it goes awry demonstrates the power outside the pore in ion channel signaling complexes, broadening the term channelopathy beyond defects in ion conduction. This article is part of the Special Issue entitled 'Channelopathies.'

Mutation E87Q of the β1-subunit impairs the maturation of the cardiac voltage-dependent sodium channel

Voltage-dependent sodium channels are responsible of the rising phase of the action potential in excitable cells. These membrane integral proteins are composed by a pore-forming α-subunit, and one or more auxiliary β subunits. Mutation E87Q of the β1 subunit is correlated with Brugada syndrome, a genetic disease characterised by ventricular fibrillation, right precordial ST segment elevation on ECG and sudden cardiac death. Heterologous expression of E87Q-β1 subunit in CHO cells determines a reduced sodium channel functional expression. The effect the E87Q mutation of the β1 subunit on sodium currents and α protein expression is correlated with a reduced availability of the mature form of the α subunit in the plasma membrane. This finding offers a new target for the treatment of the Brugada syndrome, based on protein maturation management. This work highlights the role played by the β1 subunit in the maturation and expression of the entire sodium channel complex and underlines how the defective interaction between the sodium channel constituents could lead to a disabling pathological condition.

β1-C121W Is Down But Not Out: Epilepsy-Associated Scn1b-C121W Results in a Deleterious Gain-of-Function

Voltage-gated sodium channel (VGSC) β subunits signal through multiple pathways on multiple time scales. In addition to modulating sodium and potassium currents, β subunits play nonconducting roles as cell adhesion molecules, which allow them to function in cell-cell communication, neuronal migration, neurite outgrowth, neuronal pathfinding, and axonal fasciculation. Mutations in SCN1B, encoding VGSC β1 and β1B, are associated with epilepsy. Autosomal-dominant SCN1B-C121W, the first epilepsy-associated VGSC mutation identified, results in genetic epilepsy with febrile seizures plus (GEFS+). This mutation has been shown to disrupt both the sodium-current-modulatory and cell-adhesive functions of β1 subunits expressed in heterologous systems. The goal of this study was to compare mice heterozygous for Scn1b-C121W (Scn1b(+/W)) with mice heterozygous for the Scn1b-null allele (Scn1b(+/-)) to determine whether the C121W mutation results in loss-of-function in vivo We found that Scn1b(+/W) mice were more susceptible than Scn1b(+/-) and Scn1b(+/+) mice to hyperthermia-induced convulsions, a model of pediatric febrile seizures. β1-C121W subunits are expressed at the neuronal cell surface in vivo However, despite this, β1-C121W polypeptides are incompletely glycosylated and do not associate with VGSC α subunits in the brain. β1-C121W subcellular localization is restricted to neuronal cell bodies and is not detected at axon initial segments in the cortex or cerebellum or at optic nerve nodes of Ranvier of Scn1b(W/W) mice. These data, together with our previous results showing that β1-C121W cannot participate in trans-homophilic cell adhesion, lead to the hypothesis that SCN1B-C121W confers a deleterious gain-of-function in human GEFS+ patients.

SIGNIFICANCE STATEMENT

The mechanisms underlying genetic epilepsy syndromes are poorly understood. Closing this gap in knowledge is essential to the development of new medicines to treat epilepsy. We have used mouse models to understand the mechanism of a mutation in the sodium channel gene SCN1B linked to genetic epilepsy with febrile seizures plus. We report that sodium channel β1 subunit proteins encoded by this mutant gene are expressed at the surface of neuronal cell bodies; however, they do not associate with the ion channel complex nor are they transported to areas of the axon that are critical for proper neuronal firing. We conclude that this disease-causing mutation is not simply a loss-of-function, but instead results in a deleterious gain-of-function in the brain.

Role of Sodium Channels in Epilepsy

Voltage-gated sodium channels (VGSCs) are fundamentally important for the generation and coordinated transmission of action potentials throughout the nervous system. It is, therefore, unsurprising that they have been shown to play a central role in the genesis and alleviation of epilepsy. Genetic studies on patients with epilepsy have identified more than 700 mutations among the genes that encode for VGSCs attesting to their role in pathogenesis. Further, many common antiepileptic drugs act on VGSCs to suppress seizure activity. Here, we present an account of the role of VGSCs in epilepsy, both through their pathogenic dysfunction and as targets for pharmacotherapy.

Differential gene expression profiles of two excitable rat cell lines after over-expression of WT- and C121W-β1 sodium channel subunits

Voltage-dependent sodium channels are membrane proteins essential for cell excitability. They are composed by a pore-forming α-subunit, encoded in mammals by up to nine different genes, and four different ancillary β-subunits. The expression pattern of the α subunit isoforms confers the distinctive functional and pharmacological properties to different excitable tissues. β-Subunits are important modulators of channel function and expression. Mutation C121W of the β1-subunit causes an autosomal dominant epileptic syndrome without cardiac symptoms. In neuroectoderm GH3 and cardiac H9C2 cells, the over-expression of β1 subunit augments α subunit mRNA and protein levels as well as sodium current density. Interestingly, the introduction of the epileptogenic C121W-β1 subunit produces additional changes in the α-subunit expression pattern of H9C2 cells, leaving unaltered the sodium channel isoform composition of GH3 cells. The challenge of the present work was to identify those genes that were differentially expressed in response to WT- or C121W-β1 subunit over-expression in the two rat cell lines under analysis. Hence, we analyzed the total mRNA extracted from control-untransfected and from WT- and C121W-β1-transfected GH3 and H9C2 cells by DNA-microarray. We found that, in agreement with their different embryonal origin, the over-expression of WT- and C121W-β1 subunits modifies the expression of different gene sets in GH3 and H9C2 cells. Focusing on the effects of the C121W mutation, we found that it causes the modification of 214 genes, most of them were down-regulated (202) in GH3 cells; on the contrary, it determined the up-regulation of only five genes in H9C2 cells. Interestingly, most genes modified by the C121W β1 subunit are involved in pivotal processes of the cell such as cellular communication and protein expression. Our results confirm the important role of the sodium channel β1 subunit in the control of NaCh gene expression, and highlight once more the tissue-specific effect of the C121W mutation.

On the multiple roles of the voltage gated sodium channel β1 subunit in genetic diseases

Voltage-gated sodium channels are intrinsic plasma membrane proteins that initiate the action potential in electrically excitable cells. They are composed of a pore-forming α-subunit and associated β-subunits. The β1-subunit was the first accessory subunit to be cloned. It can be important for controlling cell excitability and modulating multiple aspects of sodium channel physiology. Mutations of β1 are implicated in a wide variety of inherited pathologies, including epilepsy and cardiac conduction diseases. This review summarizes β1-subunit related channelopathies pointing out the current knowledge concerning their genetic background and their underlying molecular mechanisms.

Antisense-mediated post-transcriptional silencing of SCN1B gene modulates sodium channel functional expression

BACKGROUND INFORMATION

Voltage-dependent sodium channels are membrane proteins essential for cell excitability. They are composed by a pore-forming α-subunit and one or more β subunits. Nine α subunit and five β subunit isoforms have been identified in mammals: β1, its splice variant β1B, β2, β3 and β4. Although they do not form the ion channel pore, β subunits modulate both function as well as expression of sodium channels on cell membrane.

RESULTS

To investigate the role of β1 subunit on the modulation of sodium channel expression, we silenced this auxiliary subunit with specific antisense oligonucleotides (ASONs) in two rat cell lines, the GH3 and the H9C2, from neuro-ectoderm and cardiac myocyte origin, respectively. Treatment of cells with ASONs determined a reduction of about 50% of β1 subunit mRNA and protein expression in both cell lines. We found that this level of β1 subunit silencing resulted in an overall decrease of α subunit mRNA, protein expression and a decrease of sodium current density, without altering significantly the voltage-dependent and kinetic properties of the currents. In GH3 cells, the β1 subunit silencing reduced the expression of Nav1.1, Nav1.3 and Nav1.6 isoforms, whereas the Nav 1.2 isoform expression remained unaltered. The expression of the only α subunit present in H9C2 cells, the Nav1.5, was also reduced by β1 subunit silencing.

CONCLUSIONS

These results indicate that the β1 subunit may exert an isoform-specific fine-tuned modulation of sodium channel expression.

A thermosensitive mutation alters the effects of lacosamide on slow inactivation in neuronal voltage-gated sodium channels, NaV1.2

Epilepsy is a disorder characterized by seizures and convulsions. The basis of epilepsy is an increase in neuronal excitability that, in some cases, may be caused by functional defects in neuronal voltage gated sodium channels (Na_Vs). The C121W mutation of the β1 subunit, in particular, gives rise to the thermosensitive generalized epilepsy with febrile seizures plus (GEFS+) phenotype. Lacosamide is used to treat epileptic seizures and is distinct from other anti-seizure drugs by targeting Na_V slow-inactivation. We studied the effects of a physiologically relevant concentration of lacosamide on the biophysical properties of Na_V1.2 channels associated with either WT-β1 or the mutant C121W-β1 subunit. Biophysical parameters were measured at both normal (22°C) and elevated (34°C) temperatures to elicit the differential temperature-sensitivity of C121W. Lacosamide was more effective in Na_V1.2 associated with the WT-β1 than with C121W-β1 at either temperature. There is also a more potent effect by lacosamide on slow inactivation at elevated temperatures. Our data suggest a modulatory role is imparted by the β1 subunit in the interaction between the drug and the channel.

Voltage-gated sodium channels: pharmaceutical targets via anticonvulsants to treat epileptic syndromes

Epilepsy is a brain disorder characterized by seizures and convulsions. The basis of epilepsy is an increase in neuronal excitability that, in some cases, may be caused by functional defects in neuronal voltage gated sodium channels, Nav1.1 and Nav1.2. The effects of antiepileptic drugs (AEDs) as effective therapies for epilepsy have been characterized by extensive research. Most of the classic AEDs targeting Nav share a common mechanism of action by stabilizing the channel's fast-inactivated state. In contrast, novel AEDs, such as lacosamide, stabilize the slow-inactivated state in neuronal Nav1.1 and Nav1.7 isoforms. This paper reviews the different mechanisms by which this stabilization occurs to determine new methods for treatment.

Identification of an intra-molecular disulfide bond in the sodium channel β1-subunit

The sodium channel β1 subunit is non-covalently associated with the pore-forming α-subunits, and has been proposed to act as a modulator of channel activity, regulator of channel cell surface expression and cell adhesion molecule. Its importance is evident since mutations of the β1 subunit cause neurologic and cardiovascular disorders. The first described β1 subunit mutation is the C121W, that is related to generalized epilepsy with febrile seizures plus (GEFS+), a childhood genetic epilepsy syndrome. This mutation changed a conserved cysteine residue in position 121 into a tryptophan, putatively disrupting a disulfide bridge that should normally maintain the β1 extracellular immunoglobulin-like fold. Using the 2-D-diagonal-SDS-PAGE technique, we demonstrated the existence of this putative disulfide bridge in the Ig-like extracellular domain of the β1 subunit and its disruption in the epileptogenic C121W mutant.

A thermoprotective role of the sodium channel β1 subunit is lost with the β1 (C121W) mutation

PURPOSE

A mutation in the β(1) subunit of the voltage-gated sodium (Na(V)) channel, β(1) (C121W), causes genetic epilepsy with febrile seizures plus (GEFS+), a pediatric syndrome in which febrile seizures are the predominant phenotype. Previous studies of molecular mechanisms underlying neuronal hyperexcitability caused by this mutation were conducted at room temperature. The prevalence of seizures during febrile states in patients with GEFS+, however, suggests that the phenotypic consequence of β(1) (C121W) may be exacerbated by elevated temperature. We investigated the putative mechanism underlying seizure generation by the β(1) (C121W) mutation with elevated temperature.

METHODS

Whole-cell voltage clamp experiments were performed at 22 and 34°C using Chinese Hamster Ovary (CHO) cells expressing the α subunit of neuronal Na(V) channel isoform, Na(V) 1.2. Voltage-dependent properties were recorded from CHO cells expressing either Na(V) 1.2 alone, Na(V) 1.2 plus wild-type (WT) β(1) subunit, or Na(V) 1.2 plus β(1) (C121W).

KEY FINDINGS

Our results suggest WT β(1) is protective against increased channel excitability induced by elevated temperature; protection is lost in the absence of WT β(1) or with expression of β(1) (C121W). At 34°C, Na(V) 1.2 + β(1) (C121W) channel excitability increased compared to NaV1.2 + WT β(1) by the following mechanisms: decreased use-dependent inactivation, increased persistent current and window current, and delayed onset of, and accelerated recovery from, fast inactivation.

SIGNIFICANCE

Temperature-dependent changes found in our study are consistent with increased neuronal excitability of GEFS+ patients harboring C121W. These results suggest a novel seizure-causing mechanism for β(1) (C121W): increased channel excitability at elevated temperature.

Na+ channelopathies and epilepsy: recent advances and new perspectives

Mutations of ion channel genes have a major role in the pathogenesis of several epilepsies, confirming that some epilepsies are disorders due to the impairment of ion channel function (channelopathies). Voltage-gated Na(+) channels (VGSCs) play an essential role in neuronal excitability; it is, therefore, not surprising that most mutations associated with epilepsy have been identified in genes coding for VGSCs subunits. Epilepsies linked to VGSCs mutations range in severity from mild disorders, such as benign neonatal-infantile familial seizures and febrile seizures, to severe and drug-resistant epileptic encephalopathies. SCN1A is the most clinically relevant of all of the known epilepsy genes, several hundred mutations have been identified in this gene. This review will summarize recent advances and new perspectives on Na(+) channels and epilepsy. A better understanding of the genetic basis and of how gene defects cause seizures is mandatory to direct future research for newer selective and more efficacious treatments.

Epileptogenic ion channel mutations: from bedside to bench and, hopefully, back again

Mutations of genes coding for ion channels cause several genetically determined human epileptic syndromes. The identification of a gene variant linked to a particular disease gives important information, but it is usually necessary to perform functional studies in order to completely disclose the pathogenic mechanisms. The functional consequences of epileptogenic mutations have been studied both in vitro and in vivo with several experimental systems, studies that have provided significant knowledge on the pathogenic mechanisms that leads to inherited human epilepsies, and possibly also on the pathogenic mechanisms of non-genetic human epilepsies due to "acquired channelopathies". However, several open issues remain and difficulties in the interpretation of the experimental data have arisen that limit translational applications. We will highlight the value and the limits of different approaches to the study of epileptogenic channelopathies, focussing on the importance of the experimental systems in the assessment of the functional effects of the mutations and on the possible applications of the obtained results to the clinical practice.

Electrophysiology and beyond: multiple roles of Na+ channel β subunits in development and disease

Voltage-gated Na+ channel (VGSC) β Subunits are not "auxiliary." These multi-functional molecules not only modulate Na+ current (I(Na)), but also function as cell adhesion molecules (CAMs)-playing roles in aggregation, migration, invasion, neurite outgrowth, and axonal fasciculation. β subunits are integral members of VGSC signaling complexes at nodes of Ranvier, axon initial segments, and cardiac intercalated disks, regulating action potential propagation through critical intermolecular and cell-cell communication events. At least in vitro, many β subunit cell adhesive functions occur both in the presence and absence of pore-forming VGSC α subunits, and in vivo β subunits are expressed in excitable as well as non-excitable cells, thus β subunits may play important functional roles on their own, in the absence of α subunits. VGSC β1 subunits are essential for life and appear to be especially important during brain development. Mutations in β subunit genes result in a variety of human neurological and cardiovascular diseases. Moreover, some cancer cells exhibit alterations in β subunit expression during metastasis. In short, these proteins, originally thought of as merely accessory to α subunits, are critical players in their own right in human health and disease. Here we discuss the role of VGSC β subunits in the nervous system.

A functional null mutation of SCN1B in a patient with Dravet syndrome

Dravet syndrome (also called severe myoclonic epilepsy of infancy) is one of the most severe forms of childhood epilepsy. Most patients have heterozygous mutations in SCN1A, encoding voltage-gated sodium channel Na(v)1.1 alpha subunits. Sodium channels are modulated by beta1 subunits, encoded by SCN1B, a gene also linked to epilepsy. Here we report the first patient with Dravet syndrome associated with a recessive mutation in SCN1B (p.R125C). Biochemical characterization of p.R125C in a heterologous system demonstrated little to no cell surface expression despite normal total cellular expression. This occurred regardless of coexpression of Na(v)1.1 alpha subunits. Because the patient was homozygous for the mutation, these data suggest a functional SCN1B null phenotype. To understand the consequences of the lack of beta1 cell surface expression in vivo, hippocampal slice recordings were performed in Scn1b(-/-) versus Scn1b(+/+) mice. Scn1b(-/-) CA3 neurons fired evoked action potentials with a significantly higher peak voltage and significantly greater amplitude compared with wild type. However, in contrast to the Scn1a(+/-) model of Dravet syndrome, we found no measurable differences in sodium current density in acutely dissociated CA3 hippocampal neurons. Whereas Scn1b(-/-) mice seize spontaneously, the seizure susceptibility of Scn1b(+/-) mice was similar to wild type, suggesting that, like the parents of this patient, one functional SCN1B allele is sufficient for normal control of electrical excitability. We conclude that SCN1B p.R125C is an autosomal recessive cause of Dravet syndrome through functional gene inactivation.

Sodium channel beta1 regulatory subunit deficiency reduces pancreatic islet glucose-stimulated insulin and glucagon secretion

Glucose-stimulated insulin and glucagon release regulates glucose homeostasis by an excitation-secretion coupling pathway beginning with ATP-sensitive K(+) channel closure, membrane depolarization, and entry of calcium ions to stimulate exocytosis. The contribution of voltage-gated sodium channels to this release pathway is still being elucidated. We demonstrate that loss of Scn1b, a major regulatory subunit expressed with Na(v)1.7 protein in mouse pancreatic islets, reduces glucose-stimulated insulin and glucagon secretion in vitro and in vivo, resulting in severe fed and fasting hypoglycemia. This genetic mouse model is the first to demonstrate that sodium channelopathy impairs the physiological excitation-release coupling pathway for pancreatic insulin and glucagon release.

Regulation of persistent Na current by interactions between beta subunits of voltage-gated Na channels

The beta subunits of voltage-gated Na channels (Scnxb) regulate the gating of pore-forming alpha subunits, as well as their trafficking and localization. In heterologous expression systems, beta1, beta2, and beta3 subunits influence inactivation and persistent current in different ways. To test how the beta4 protein regulates Na channel gating, we transfected beta4 into HEK (human embryonic kidney) cells stably expressing Na(V)1.1. Unlike a free peptide with a sequence from the beta4 cytoplasmic domain, the full-length beta4 protein did not block open channels. Instead, beta4 expression favored open states by shifting activation curves negative, decreasing the slope of the inactivation curve, and increasing the percentage of noninactivating current. Consequently, persistent current tripled in amplitude. Expression of beta1 or chimeric subunits including the beta1 extracellular domain, however, favored inactivation. Coexpressing Na(V)1.1 and beta4 with beta1 produced tiny persistent currents, indicating that beta1 overcomes the effects of beta4 in heterotrimeric channels. In contrast, beta1(C121W), which contains an extracellular epilepsy-associated mutation, did not counteract the destabilization of inactivation by beta4 and also required unusually large depolarizations for channel opening. In cultured hippocampal neurons transfected with beta4, persistent current was slightly but significantly increased. Moreover, in beta4-expressing neurons from Scn1b and Scn1b/Scn2b null mice, entry into inactivated states was slowed. These data suggest that beta1 and beta4 have antagonistic roles, the former favoring inactivation, and the latter favoring activation. Because increased Na channel availability may facilitate action potential firing, these results suggest a mechanism for seizure susceptibility of both mice and humans with disrupted beta1 subunits.

Periodicity in Na+ channel properties alters excitability of a model neuron

The voltage gated Na channels play vital role in action potential waveform shaping and propagation. We have shown earlier that the duration and amplitude of a prolonged depolarization alter all the steady state and kinetic parameters of rNa(v)1.2a voltage gated Na channel in a pseudo-oscillatory fashion. In the present study, we show that the Hodgkin-Huxley voltage and time dependent rate constants of activation (alpha(m) and beta(m)) and fast inactivation (alpha(h) and beta(h)), obtained from the analyses of Na currents and steady state activation and inactivation plots, following application of prepulses in both slow (1-100s) and fast (100-1000ms) ranges, vary with the duration of a prepulse in a pseudo-oscillatory manner. Using these Hodgkin-Huxley kinetic parameters in simulation, the excitability and firing pattern of a model neuron are shown to vary in a history dependent periodic fashion.

Generalized epilepsy with febrile seizures plus-associated sodium channel beta1 subunit mutations severely reduce beta subunit-mediated modulation of sodium channel function

Two novel mutations (R85C and R85H) on the extracellular immunoglobulin-like domain of the sodium channel beta1 subunit have been identified in individuals from two families with generalized epilepsy with febrile seizures plus (GEFS+). The functional consequences of these two mutations were determined by co-expression of the human brain NaV1.2 alpha subunit with wild type or mutant beta1 subunits in human embryonic kidney (HEK)-293T cells. Patch clamp studies confirmed the regulatory role of beta1 in that relative to NaV1.2 alone the NaV1.2+beta1 currents had right-shifted voltage dependence of activation, fast and slow inactivation and reduced use dependence. In addition, the NaV1.2+beta1 current entered fast inactivation slightly faster than NaV1.2 channels alone. The beta1(R85C) subunit appears to be a complete loss of function in that none of the modulating effects of the wild type beta1 were observed when it was co-expressed with NaV1.2. Interestingly, the beta1(R85H) subunit also failed to modulate fast kinetics, however, it shifted the voltage dependence of steady state slow inactivation in the same way as the wild type beta1 subunit. Immunohistochemical studies revealed cell surface expression of the wild type beta1 subunit and undetectable levels of cell surface expression for both mutants. The functional studies suggest association of the beta1(R85H) subunit with the alpha subunit where its influence is limited to modulating steady state slow inactivation. In summary, the mutant beta1 subunits essentially fail to modulate alpha subunits which could increase neuronal excitability and underlie GEFS+ pathogenesis.

Neurotoxins from invertebrates as anticonvulsants: From basic research to therapeutic application

Invertebrate venoms have attracted considerable interest as a potential source of bioactive substances, especially neurotoxins. These molecules have proved to be extremely useful tools for the understanding of synaptic transmission events, and they have contributed to the design of novel drugs for the treatment of neurological disorders and pain. In this context, as epilepsy involves neuronal substrates, which are sites of action of many neurotoxins; venoms may be particularly useful for antiepileptic drug (AED) research. Epilepsy is a chronic disease whose treatment consists of controlling seizures with antiepileptics that very often induce strong undesirable side effects that may limit treatment. Here, we review the vast, but yet unexplored, world of neurotoxins from invertebrates used as probes in pharmacological screening for novel and less toxic antiepileptics. We briefly review (1) the molecular basis of epilepsy, as well as the sites of action of commonly used anticonvulsants (we bring a comprehensive review of the elements from invertebrate venoms which are mostly studied in neuroscience research and may be useful for drug development); (2) peptides from conus snails; (3) peptides and polyamine toxins from spiders and wasps; and (4) peptides from scorpions.

Epileptogenic Channelopathies: Experimental Models of Human Pathologies

The discovery of genetically determined epileptic syndromes associated with specific mutations of genes codifying for subunits of voltage or ligand-activated ion channels highlights the role of ion channels in epileptogenesis. In vitro and in vivo models of channel pathology have been used to define the functional consequence of the mutations identified in human epilepsies. The evaluation of gene-channel mutations based on molecular and physiological techniques have provided significant knowledge on the cellular mechanisms leading to inherited human epilepsies, and possibly to nongenetic human epilepsies due to "acquired" channel pathologies. We review some of the studies that have explored human epileptic disorders through experimental manipulations of these channels, highlighting some of the difficulties that have arisen using "in vitro" preparations or rodent models. These findings underscore the need for further studies to address the mechanisms involved in mutated-channel dysfunctions.

β1-subunit modulates the Nav1.4 sodium channel by changing the surface charge

Voltage-gated sodium channels, comprised of a pore-forming alpha-subunit and additional regulatory (beta) subunits, play a critical role in regulation of neuronal excitability. Mechanisms of regulation of beta-subunits remain elusive. We have tested the functional effects of beta1 sodium channel subunit on surface charges as a mechanism for channel modulation. HEK-293 cell lines permanently transfected with the sole rat skeletal muscle sodium channel alpha-subunit (Nav1.4), or co-expressing the sodium channel alpha-subunit and beta1-subunit were studied with the whole-cell mode of the patch-clamp technique. At physiological extracellular Ca2+ concentration (2 mM), expression of beta1-subunit did not produce any significant effect on the voltage-dependent properties of sodium currents. However, a shift of half-activation potentials of sodium channel by changing the extracellular Ca2+ was potentiated when beta1 was co-expressed with alpha-subunit. In contrast, the expression of beta1-subunit did not affect the Ca2+ binding to the open or to the closed sodium channel pore, difference of the effect provoked by extracellular Ca2+ could therefore be attributed to an increased in negative surface charge determined by the presence of beta1-subunit. These data are in agreement with the hypothesis of a modulation of the sodium current by the expression of the highly sialylated beta1-subunit, which would alter the channel gating by increasing the density of surface negative charges in the vicinity of the sodium channel voltage sensing machinery.

Fast Pseudo-Periodic Oscillation in the Rat Brain Voltage-gated Sodium Channel α Subunit

In the work reported here, we have investigated the changes in the activation and fast inactivation properties of the rat brain voltage-gated sodium channel (rNa(v) 1.2a) alpha subunit, expressed heterologously in the Chinese Hamster Ovary (CHO) cells, by short depolarizing prepulses (10-1000 ms). The time constant of recovery from fast inactivation (tau(fast)) and steady-state parameters for activation and inactivation varied in a pseudo-oscillatory fashion with the duration and amplitude of a sustained prepulse. A consistent oscillation was observed in most of the steady-state and non-inactivating current parameters with a time period close to 225 ms, although a faster oscillation of time period 125 ms was observed in the tau(fast). The studies on the non-inactivating current and steady-state activation indicate that the phase of oscillation varies from cell to cell. Co-expression of the beta1 subunit with the alpha subunit channel suppressed the oscillation in the charge movement per single channel and free energy of steady-state inactivation, although the oscillation in the half steady-state inactivation potential remained unaltered. Incidentally, the frequencies of oscillation in the sodium channel parameters (4-8 Hz) correspond to the theta component of network oscillation. This fast pseudo-oscillatory mechanism, together with the slow pseudo-oscillatory mechanism found in these channels earlier, may contribute to the oscillations in the firing properties observed in various neuronal subtypes and many pathological conditions.

The epileptic seizures induced by BmK I, a modulator of sodium channels

In the present study, the susceptibility to rat epileptic seizures induced by the intrahippocampal administration of BmK I, a modulator of sodium channels purified from the venom of Chinese scorpion, has been investigated. The results showed that the strong epileptic behaviors and discharges in the hippocampus were evoked by BmK I dose-dependently. The hippocampal c-Fos expression displayed two peak waves in a specific spatio-temporal pattern elicited by BmK I. The whole cell patch clamp recordings showed that the inactivation of sodium currents in rat cultured hippocampal neurons was prolonged significantly by BmK I, and restored partially after washing. These results indicated that the rat hippocampus is a susceptible target for the proconvulsant effects of BmK I, and the induction of epileptic seizures may be ascribed to the modulation of BmK I on the inactivation of voltage-gated sodium channels distributing in the rat hippocampal neurons.

Mutation in the Na+ channel subunit SCN1B produces paradoxical changes in peripheral nerve excitability

To determine the effect of an established mutation of the beta1 subunit of Na(+) channels on nerve excitability, studies were undertaken in patients diagnosed with generalized epilepsy with febrile seizures plus (GEFS+). Multiple nerve excitability measurements were used to investigate the membrane properties of sensory and motor axons in five patients (aged 18-55 years) who were currently experiencing no seizures and were not on anticonvulsants. There was no history of paraesthesiae, fasciculation or cramps to suggest hyperexcitability of peripheral nerve axons. The median nerve was stimulated at the wrist, and compound muscle action potentials (CMAPs) were recorded from abductor pollicis brevis and the antidromic compound sensory nerve action potential (CSAPs) from digit 2. Stimulus-response behaviour, strength-duration time constant, threshold electrotonus, current-threshold relationship and the recovery of excitability following a supramaximal conditioning stimulus were recorded using threshold tracking. Compared with normal controls (n = 29), the axons of patients were of higher threshold. CMAPs and CSAPs were relatively small, although individual values remained within the normal ranges. Refractoriness and relative refractory period (markers of transient Na(+) channel function) were significantly reduced in GEFS+ patients with established mutations in SCN1B (P < 0.05), and strength-duration time constants (dependent on persistent Na(+) conductances) were reduced. It is suggested that, in peripheral nerve axons, the mutation underlying GEFS+ reduces the number of functioning Na(+) channels at the node of Ranvier and that this rather than any change in gating of individual channels dominates axonal excitability in these patients.

Primary structure, behavioral and electroencephalographic effects of an epileptogenic peptide from the sea anemone Bunodosoma cangicum

The primary structure of cangitoxin (CGX), a 4958 Da peptide from the sea anemone Bunodosoma cangicum, was determined: GVACRCDSDGPTVRGNSLSGTLWLTGGCPSGWHNCRGSGPFIGYCCKK. CGX contains all the 11 residues that are conserved and the 5 that are conservatively substituted within or between the type 1 and type 2 sequences of sea anemone peptides with specific action on voltage-sensitive sodium channels. Furthermore, it also has 6 identities (Asp9, Arg14, Asn16, Leu18, Trp33 and Lys48) and 1 homology (Arg36) in the 8 residues of the pharmacophore of the sea anemone ApB which are essential for interaction with mammalian sodium channels. The intrahippocampal injection of CGX induces several sequential behavioral alterations--episodes of akinesia alternating with facial automatisms and head tremor, salivation, rearing, jumping, barrel-rolling, wet dog shakes and forelimb clonic movements--and the electroencephalography analysis shows that they were followed by important seizure periods that gradually evolved to status epilepticus that lasted 8-12 h, similar to that observed in the acute phase of the pilocarpine model of epilepsy. These results suggest that CGX may be an important tool to develop a new experimental model of status epilepticus which may contribute to understanding the etiology of epilepsy and to test the effects of new antiepileptic drugs.

Induction of pseudo-periodic oscillation in voltage-gated sodium channel properties is dependent on the duration of prolonged depolarization

The neuronal voltage-gated sodium channels play a vital role in the action potential waveform shaping and propagation. Here, we report the effects of prolonged depolarization (1-160 s) on the detailed kinetics of activation, fast inactivation and recovery from slow inactivation in the rNa(v)1.2a voltage-gated sodium channel alpha-subunit expressed in Chinese hamster ovary (CHO) cells. Wavelet analysis revealed that the duration and amplitude of a prolonged sustained depolarization altered all the steady state and kinetic parameters of the channel in a pseudo-oscillatory fashion with time-variable period and amplitude, often superimposed on a linear trend. The half steady state activation potential showed a reversible depolarizing shift of 5-10 mV with duration of prolonged depolarization, while half steady state inactivation potential showed a hyperpolarizing shift of 43-55 mV. The time periods for most of the parameters relating to activation and fast and slow inactivation, lie close to 28-30 s, suggesting coupling of these kinetic processes through an oscillatory mechanism. Co-expression of the beta1-subunit affected the time periods of oscillation (close to 22 s for alpha + beta1) in steady state activation parameters. Application of a pulse protocol that mimicked paroxysmal depolarizing shift (PDS), a kind of depolarization seen in epileptic discharges, instead of a sustained depolarization, also caused oscillatory behaviour in the rNav1.2a alpha-subunit. This inherent pseudo-oscillatory mechanism may regulate excitability of the neurons, account for the epileptic discharges and subthreshold membrane potential oscillation and offer a molecular memory mechanism intrinsic to the neurons, independent of synaptic plasticity.

Données récentes sur l’implications des canaux ioniques dans les formes familiales d’épilepsies généralisées idiopathiques associées ou non à des convulsions fébriles

Major advances have recently been made in the understanding of the genetic bases of monogenic inherited epilepsies. For several idiopathic epilepsies, mutations in genes encoding subunits of ion channels or ligand receptors have been demonstrated. This is the case for some generalized idiopathic epilepsies and generalized epilepsies associated with febrile seizures. In this Article, we review the recent clinical and genetic data of these forms of epilepsy.

Monogenic idiopathic epilepsies

Major advances have recently been made in our understanding of the genetic bases of monogenic inherited epilepsies. Direct molecular diagnosis is now possible in numerous inherited symptomatic epilepsies. Progress has also been spectacular with respect to several idiopathic epilepsies that are caused by mutations in genes encoding subunits of ion channels or neurotransmitter receptors. Although these findings concern only a few families and sporadic cases, their potential importance is great, because these genes are implicated in a wide range of more common epileptic disorders and seizure types as well as some rare syndromes. Functional studies of these mutations, while leading to further progress in the neurobiology of the epilepsies, will help to refine genotype-phenotype relations and increase our understanding of responses to antiepileptic drugs. In this article, we review the clinical and genetic data on most of the idiopathic human epilepsies and epileptic contexts in which the association of epilepsy and febrile convulsions is genetically determined.

TSII toxin isolated from Tityus serrulatus scorpion venom: behavioral, electroencephalographic, and histopathologic studies

We have reported earlier that intrahippocampal administration of the C-pool from Tityus serrulatus scorpion venom induces convulsions in rats. Here we report the effects of seven toxins isolated from the C-pool. The strongest effects were seen after toxin 5C, which was sequenced and identified as TSII, a beta-type toxin that affects Na+ channel activation. Unilateral injection of TSII in the rat hippocampus (1.7 microg/microl) induced clusters of spikes and epileptic discharges of mainly moderate intensity, convulsion-related behavioral changes (wet dog shakes, staring, masticatory jaw movements, facial automatisms, orofacial movements, intense sniffing, blinking, and forelimb clonus with rearing and falling) and a massive neuronal loss of pyramidal cells in the ipsilateral CA1, CA3, and CA4 subfields and of granulate cells of the ipsilateral dentate gyrus. Toxins C3, C4, and C6 induced weaker changes in the EEG and behavioral changes and failed to induce cell death, and toxins C1, C2, and C7 had no effects. The similarities in the effects of TsTx, a alpha-type toxin that affects Na+ channel, suggest that the loss of modulation of activation of the sodium channel caused by TSII increases glutamate release, leading to long-lasting increases in intracellular Ca2+ and cell death.

TsTx toxin isolated from Tityus serrulatus scorpion venom induces spontaneous recurrent seizures and mossy fiber sprouting

PURPOSE

To characterize the long-term behavioral, electroencephalographic (EEG) and histopathologic features after a single TsTx microinjection into the hippocampus of rats.

METHODS

TsTx, 2 microg, or 1 microl of 0.1 M phosphate buffer was injected into the right dorsal hippocampus of the rat. EEG records and behavioral observations were made over a period of 10 h after injection. For a period of 4 months, the animals were observed for the occurrence of convulsive seizures. At the end of the experiment, the brains were processed by the neo-Timm and Nissl methods.

RESULTS

After intrahippocampal TsTx injection, three distinct phases were observed: (a) an immediate period that lasted 1 day, during which the motor and electrographic seizures characteristic of status epilepticus (SE) were seen; (b) a silent period (31-49 days), characterized by normal EEG and behavior; and (c) a period of spontaneous recurrent seizures (SRSs). The seizure frequency was one to two per week. Four months after TsTx injection, hippocampal neuronal loss and mossy fiber sprouting in the supragranular layer of the dentate gyrus were observed.

CONCLUSIONS

The SRSs observed in this study may be associated with the TsTx-induced SE and brain damage. All animals injected with the toxin showed massive pyramidal neuronal loss in the dorsal hippocampus as well as intense gliosis and atrophy. Mossy fiber sprouting in the supragranular layer of the dentate gyrus was observed in those animals that had SRSs. The effects observed may be due, at least in part, to TsTx-enhanced release of glutamate in hippocampal pathways.

Mechanisms of sodium channel inactivation

Rapid inactivation of sodium channels is crucial for the normal electrical activity of excitable cells. There are many different types of inactivation, including fast, slow and ultra-slow, and each of these can be modulated by cellular factors or accessory subunits. Fast inactivation occurs by a 'hinged lid' mechanism in which an inactivating particle occludes the pore, whereas slow inactivation is most likely to involve a rearrangement of the channel pore. Subtle defects in either inactivation process can lead to debilitating human diseases, including periodic paralyses in muscle, ventricular fibrillation and long QT syndrome (delayed cardiac repolarization) in the heart, and epilepsy in the CNS.

Tonic and phasic guanidinium toxin-block of skeletal muscle Na channels expressed in Mammalian cells

The blockage of skeletal muscle sodium channels by tetrodotoxin (TTX) and saxitoxin (STX) have been studied in CHO cells permanently expressing rat Nav1.4 channels. Tonic and use-dependent blockage were analyzed in the framework of the ion-trapped model. The tonic affinity (26.6 nM) and the maximum affinity (7.7 nM) of TTX, as well as the "on" and "off" rate constants measured in this preparation, are in remarkably good agreement with those measured for Nav1.2 expressed in frog oocytes, indicating that the structure of the toxin receptor of Nav1.4 and Nav1.2 channels are very similar and that the expression method does not have any influence on the pore properties of the sodium channel. The higher affinity of STX for the sodium channels (tonic and maximum affinity of 1.8 nM and 0.74 nM respectively) is explained as an increase on the "on" rate constant (approximately 0.03 s(-1) nM(-1)), compared to that of TTX (approximately 0.003 s(-1) nM(-1)), while the "off" rate constant is the same for both toxins (approximately 0.02 s(-1)). Estimations of the free-energy differences of the toxin-channel interaction indicate that STX is bound in a more external position than TTX. Similarly, the comparison of the toxins free energy of binding to a ion-free, Na(+)- and Ca(2+)-occupied channel, is consistent with a binding site in the selectivity filter for Ca(2+) more external than for Na(+). This data may be useful in further attempts at sodium-channel pore modeling.

Sodium channel heterologous expression in mammalian cells and the role of the endogenous beta1-subunits

Sodium currents in cell lines transfected with the sole alpha-subunit, or constitutively expressing sodium channels, have an inactivation that is always prevalently mono-exponential. Differently, expression of alpha-subunit in Xenopus oocytes exerts slow inactivating currents with biphasic decay, while simultaneous co-transfection of alpha and beta1 restores a mono-exponential (normal) inactivation. A hypothesis for such differences is that an endogenous presence of beta1 or beta1-alternative splicing, beta1A, in cells could account for the normal inactivation. To test this hypothesis and to evaluate the role for the beta1A, we inhibited the expression of beta1/beta1A by antisense oligonucleotides on Nav1.4-transfected human embryonic cell line 293 (HEK) cells. Reduction of beta1/beta1A produces no significant functional effects in Nav1.4-HEK. This result invalidates the hypothesis that the lack of slow-mode in cell lines is simply due to a constitutive expression of beta1/beta1A.

Functional and biochemical analysis of a sodium channel beta1 subunit mutation responsible for generalized epilepsy with febrile seizures plus type 1

Generalized epilepsy with febrile seizures plus type 1 is an inherited human epileptic syndrome, associated with a cysteine-to-tryptophan (C121W) mutation in the extracellular immunoglobin domain of the auxiliary beta1 subunit of the voltage-gated sodium channel. The mutation disrupts beta1 function, but how this leads to epilepsy is not understood. In this study, we make several observations that may be relevant for understanding why this beta1 mutation results in seizures. First, using electrophysiological recordings from mammalian cell lines, coexpressing sodium channel alpha subunits and either wild-type beta1 or C121Wbeta1, we show that loss of beta1 functional modulation, caused by the C121W mutation, leads to increased sodium channel availability at hyperpolarized membrane potentials and reduced sodium channel rundown during high-frequency channel activity, compared with channels coexpressed with wild-type beta1. In contrast, neither wild-type beta1 nor C121Wbeta1 significantly affected sodium current time course or the voltage dependence of channel activation. We also show, using a Drosophila S2 cell adhesion assay, that the C121W mutation disrupts beta1-beta1 homophilic cell adhesion, suggesting that the mutation may alter the ability of beta1 to mediate protein-protein interactions critical for sodium channel localization. Finally, we demonstrate that neither functional modulation nor cell adhesion mediated by wild-type beta1 is occluded by coexpression of C121Wbeta1, arguing against the idea that the mutant beta1 acts as a dominant-negative subunit. Together, these data suggest that C121Wbeta1 causes subtle effects on channel function and subcellular distribution that bias neurons toward hyperexcitabity and epileptogenesis.