The Role of the Persistent Sodium Current in Epilepsy

Persistent sodium currents in neurons: potential mechanisms and pharmacological blockers

Persistent sodium current (INaP) is an important activity-dependent regulator of neuronal excitability. It is involved in a variety of physiological and pathological processes, including pacemaking, prolongation of sensory potentials, neuronal injury, chronic pain and diseases such as epilepsy and amyotrophic lateral sclerosis. Despite its importance, neither the molecular basis nor the regulation of INaP are sufficiently understood. Of particular significance is a solid knowledge and widely accepted consensus about pharmacological tools for analysing the function of INaP and for developing new therapeutic strategies. However, the literature on INaP is heterogeneous, with varying definitions and methodologies used across studies. To address these issues, we provide a systematic review of the current state of knowledge on INaP, with focus on mechanisms and effects of this current in the central nervous system. We provide an overview of the specificity and efficacy of the most widely used INaP blockers: amiodarone, cannabidiol, carbamazepine, cenobamate, eslicarbazepine, ethosuximide, gabapentin, GS967, lacosamide, lamotrigine, lidocaine, NBI-921352, oxcarbazepine, phenytoine, PRAX-562, propofol, ranolazine, riluzole, rufinamide, topiramate, valproaic acid and zonisamide. We conclude that there is strong variance in the pharmacological effects of these drugs, and in the available information. At present, GS967 and riluzole can be regarded bona fide INaP blockers, while phenytoin and lacosamide are blockers that only act on the slowly inactivating component of sodium currents.

New epilepsy therapies in development

Epilepsy is a common brain disorder, characterized by spontaneous recurrent seizures, with associated neuropsychiatric and cognitive comorbidities and increased mortality. Although people at risk can often be identified, interventions to prevent the development of the disorder are not available. Moreover, in at least 30% of patients, epilepsy cannot be controlled by current antiseizure medications (ASMs). As a result of considerable progress in epilepsy genetics and the development of novel disease models, drug screening technologies and innovative therapeutic modalities over the past 10 years, more than 200 novel epilepsy therapies are currently in the preclinical or clinical pipeline, including many treatments that act by new mechanisms. Assisted by diagnostic and predictive biomarkers, the treatment of epilepsy is undergoing paradigm shifts from symptom-only ASMs to disease prevention, and from broad trial-and-error treatments for seizures in general to mechanism-based treatments for specific epilepsy syndromes. In this Review, we assess recent progress in ASM development and outline future directions for the development of new therapies for the treatment and prevention of epilepsy.

Intraparenchymal delivery of adeno-associated virus vectors for the gene therapy of neurological diseases

INTRODUCTION

In gene therapy with adeno-associated virus (AAV) vectors for diseases of the central nervous system, the vectors can be administered into blood vessels, cerebrospinal fluid space, or the brain parenchyma. When gene transfer to a large area of the brain is required, the first two methods are used, but for diseases in which local gene transfer is expected to be effective, vectors are administered directly into the brain parenchyma.

AREAS COVERED

Strategies for intraparenchymal vector delivery in gene therapy for Parkinson's disease, aromatic l-amino acid decarboxylase (AADC) deficiency, and epilepsy are reviewed.

EXPERT OPINION

Stereotactic intraparenchymal injection of AAV vectors allows precise gene delivery to the target site. Although more surgically invasive than intravascular or intrathecal administration, intraparenchymal vector delivery has the advantage of a lower vector dose, and preexisting neutralizing antibodies have little effect on the transduction efficacy. This approach improves motor function in AADC deficiency and led to regulatory approval of an AAV vector for the disease in the EU. Although further validation through clinical studies is needed, direct infusion of viral vectors into the brain parenchyma is expected to be a novel treatment for Parkinson's disease and drug-resistant epilepsy.

Exosomal TNF-α mediates voltage-gated Na+ channel 1.6 overexpression and contributes to brain tumor-induced neuronal hyperexcitability

Patients affected by glioma frequently experience epileptic discharges; however, the causes of brain tumor-related epilepsy (BTRE) are still not completely understood. We investigated the mechanisms underlying BTRE by analyzing the effects of exosomes released by U87 glioma cells and by patient-derived glioma cells. Rat hippocampal neurons incubated for 24 hours with these exosomes exhibited increased spontaneous firing, while their resting membrane potential shifted positively by 10-15 mV. Voltage clamp recordings demonstrated that the activation of the Na+ current shifted toward more hyperpolarized voltages by 10-15 mV. To understand the factors inducing hyperexcitability, we focused on exosomal cytokines. Western blot and ELISAs showed that TNF-α was present inside glioma-derived exosomes. Remarkably, incubation with TNF-α fully mimicked the phenotype induced by exosomes, with neurons firing continuously, while their resting membrane potential shifted positively. Real-time PCR revealed that both exosomes and TNF-α induced overexpression of the voltage-gated Na+ channel Nav1.6, a low-threshold Na+ channel responsible for hyperexcitability. When neurons were preincubated with infliximab, a specific TNF-α inhibitor, the hyperexcitability induced by exosomes and TNF-α was drastically reduced. We propose that infliximab, an FDA-approved drug to treat rheumatoid arthritis, could ameliorate the conditions of glioma patients with BTRE.

Channelopathies in epilepsy: an overview of clinical presentations, pathogenic mechanisms, and therapeutic insights

Pathogenic variants in genes encoding ion channels are causal for various pediatric and adult neurological conditions. In particular, several epilepsy syndromes have been identified to be caused by specific channelopathies. These encompass a spectrum from self-limited epilepsies to developmental and epileptic encephalopathies spanning genetic and acquired causes. Several of these channelopathies have exquisite responses to specific antiseizure medications (ASMs), while others ASMs may prove ineffective or even worsen seizures. Some channelopathies demonstrate phenotypic pleiotropy and can cause other neurological conditions outside of epilepsy. This review aims to provide a comprehensive exploration of the pathophysiology of seizure generation, ion channels implicated in epilepsy, and several genetic epilepsies due to ion channel dysfunction. We outline the clinical presentation, pathogenesis, and the current state of basic science and clinical research for these channelopathies. In addition, we briefly look at potential precision therapy approaches emerging for these disorders.

Targeted blockade of aberrant sodium current in a stem cell-derived neuron model of SCN3A encephalopathy

Missense variants in SCN3A encoding the voltage-gated sodium (Na+) channel α subunit Nav1.3 are associated with SCN3A-related neurodevelopmental disorder (SCN3A-NDD), a spectrum of disease that includes epilepsy and malformation of cortical development. How genetic variation in SCN3A leads to pathology remains unclear, as prior electrophysiological work on disease-associated variants has been performed exclusively in heterologous cell systems. To further investigate the mechanisms of SCN3A-NDD pathogenesis, we used CRISPR/Cas9 gene editing to modify a control human induced pluripotent stem cell (iPSC) line to express the recurrent de novo missense variant SCN3A c.2624T>C (p.Ile875Thr). With the established Ngn2 rapid induction protocol, we generated glutamatergic forebrain-like neurons (iNeurons), which we showed to express SCN3A mRNA and Nav1.3-mediated Na+ currents. We performed detailed whole-cell patch clamp recordings to determine the effect of the SCN3A-p.Ile875Thr variant on endogenous Na+ currents in, and intrinsic excitability of, human neurons. Compared to control iNeurons, variant-expressing iNeurons exhibit markedly increased slowly-inactivating/persistent Na+ current, abnormal firing patterns with paroxysmal bursting and plateau-like potentials with action potential failure, and a hyperpolarized voltage threshold for action potential generation. We then validated these findings using a separate iPSC line generated from a patient harbouring the SCN3A-p.Ile875Thr variant compared to a corresponding CRISPR-corrected isogenic control line. Finally, we found that application of the Nav1.3-selective blocker ICA-121431 normalizes action potential threshold and aberrant firing patterns in SCN3A-p.Ile1875Thr iNeurons; in contrast, consistent with action as a Na+ channel blocker, ICA-121431 decreases excitability of control iNeurons. Our findings demonstrate that iNeurons can model the effects of genetic variation in SCN3A yet reveal a complex relationship between gain-of-function at the level of the ion channel versus impact on neuronal excitability. Given the transient expression of SCN3A in the developing human nervous system, selective blockade or suppression of Nav1.3-containing Na+ channels could represent a therapeutic approach towards SCN3A-NDD.

Bioelectricity in dental medicine: a narrative review

BACKGROUND

Bioelectric signals, whether exogenous or endogenous, play crucial roles in the life processes of organisms. Recently, the significance of bioelectricity in the field of dentistry is steadily gaining greater attention.

OBJECTIVE

This narrative review aims to comprehensively outline the theory, physiological effects, and practical applications of bioelectricity in dental medicine and to offer insights into its potential future direction. It attempts to provide dental clinicians and researchers with an electrophysiological perspective to enhance their clinical practice or fundamental research endeavors.

METHODS

An online computer search for relevant literature was performed in PubMed, Web of Science and Cochrane Library, with the keywords "bioelectricity, endogenous electric signal, electric stimulation, dental medicine."

RESULTS

Eventually, 288 documents were included for review. The variance in ion concentration between the interior and exterior of the cell membrane, referred to as transmembrane potential, forms the fundamental basis of bioelectricity. Transmembrane potential has been established as an essential regulator of intercellular communication, mechanotransduction, migration, proliferation, and immune responses. Thus, exogenous electric stimulation can significantly alter cellular action by affecting transmembrane potential. In the field of dental medicine, electric stimulation has proven useful for assessing pulp condition, locating root apices, improving the properties of dental biomaterials, expediting orthodontic tooth movement, facilitating implant osteointegration, addressing maxillofacial malignancies, and managing neuromuscular dysfunction. Furthermore, the reprogramming of bioelectric signals holds promise as a means to guide organism development and intervene in disease processes. Besides, the development of high-throughput electrophysiological tools will be imperative for identifying ion channel targets and precisely modulating bioelectricity in the future.

CONCLUSIONS

Bioelectricity has found application in various concepts of dental medicine but large-scale, standardized, randomized controlled clinical trials are still necessary in the future. In addition, the precise, repeatable and predictable measurement and modulation methods of bioelectric signal patterns are essential research direction.

Voltage- and Calcium-Gated Membrane Currents Tune the Plateau Potential Properties of Multiple Neuron Types

Many neurons exhibit regular firing that is limited to the duration and intensity of depolarizing stimuli. However, some neurons exhibit all-or-nothing plateau potentials that, once elicited, can lead to prolonged activity that is independent of stimulus intensity or duration. To better understand this diversity of information processing, we compared the voltage-gated and Ca2+-gated currents of three identified neurons from hermaphroditic Aplysia californica Two of these neurons, B51 and B64, generated plateau potentials and a third neuron, B8, exhibited regular firing and was incapable of generating a plateau potential. With the exception of the Ca2+-gated potassium current (I KCa), all three neuron types expressed a similar array of outward and inward currents, but with distinct voltage-dependent properties for each neuron type. Inhibiting voltage-gated Ca2+ channels with Ni+ prolonged the plateau potential, indicating I KCa is important for plateau potential termination. In contrast, inhibiting persistent Na+ (I NaP) blocked plateau potentials, empirically and in simulations. Surprisingly, the properties and level of expression of I NaP were similar in all three neurons, indicating that the presence of I NaP does not distinguish between regular-firing neurons and neurons capable of generating plateau potentials. Rather, the key distinguishing factor is the relationship between I NaP and outward currents such as the delayed outward current (I D), and I KCa We then demonstrated a technique for predicting complex physiological properties such as plateau duration, plateau amplitude, and action potential duration as a function of parameter values, by fitting a curve in parameter space and projecting the curve beyond the tested values.SIGNIFICANCE STATEMENT Plateau potentials are intrinsic properties of neurons that are important for information processing in a wide variety of nervous systems. We examined three identified neurons in Aplysia californica with different propensities to generate a plateau potential. No single conductance was found to distinguish plateau generating neurons. Instead, plateau generation depended on the ratio between persistent Na+ current (I NaP), which favored plateaus, and outward currents such as I KCa, which facilitated plateau termination. Computational models revealed a relationship between the individual currents that predicted the features of simulated plateau potentials. These results provide a more solid understanding of the conductances that mediate plateau generation.

Complexity in Genetic Epilepsies: A Comprehensive Review

Epilepsy is a highly prevalent neurological disorder, affecting between 5-8 per 1000 individuals and is associated with a lifetime risk of up to 3%. In addition to high incidence, epilepsy is a highly heterogeneous disorder, with variation including, but not limited to the following: severity, age of onset, type of seizure, developmental delay, drug responsiveness, and other comorbidities. Variable phenotypes are reflected in a range of etiologies including genetic, infectious, metabolic, immune, acquired/structural (resulting from, for example, a severe head injury or stroke), or idiopathic. This review will focus specifically on epilepsies with a genetic cause, genetic testing, and biomarkers in epilepsy.

Dysfunctional sodium channel kinetics as a novel epilepsy mechanism in chromosome 15q11-q13 duplication syndrome

OBJECTIVE

Duplication of the maternal chromosome 15q11.2-q13.1 region causes Dup15q syndrome, a highly penetrant neurodevelopmental disorder characterized by severe autism and refractory seizures. Although UBE3A, the gene encoding the ubiquitin ligase E3A, is thought to be the main driver of disease phenotypes, the cellular and molecular mechanisms that contribute to the development of the syndrome are yet to be determined. We previously established the necessity of UBE3A overexpression for the development of cellular phenotypes in human Dup15q neurons, including increased action potential firing and increased inward current density, which prompted us to further investigate sodium channel kinetics.

METHODS

We used a Dup15q patient-derived induced pluripotent stem cell line that was CRISPR-edited to remove the supernumerary chromosome and create an isogenic control line. We performed whole cell patch clamp electrophysiology on Dup15q and corrected control neurons at two time points of in vitro development.

RESULTS

Compared to corrected neurons, Dup15q neurons showed increased sodium current density and a depolarizing shift in steady-state inactivation. Moreover, onset of slow inactivation was delayed, and a faster recovery from both fast and slow inactivation processes was observed in Dup15q neurons. A fraction of sodium current in Dup15q neurons (~15%) appeared to be resistant to slow inactivation. Not unexpectedly, a higher fraction of persistent sodium current was also observed in Dup15q neurons. These phenotypes were modulated by the anticonvulsant drug rufinamide.

SIGNIFICANCE

Sodium channels play a crucial role in the generation of action potentials, and sodium channelopathies have been uncovered in multiple forms of epilepsy. For the first time, our work identifies in Dup15q neurons dysfunctional inactivation kinetics, which have been previously linked to multiple forms of epilepsy. Our work can also guide therapeutic approaches to epileptic seizures in Dup15q patients and emphasize the role of drugs that modulate inactivation kinetics, such as rufinamide.

Transient Receptor Potential (TRP) Channels in Pain, Neuropsychiatric Disorders, and Epilepsy

Pharmacomodulation of membrane channels is an essential topic in the study of physiological conditions and disease status. Transient receptor potential (TRP) channels are one such family of nonselective cation channels that have an important influence. In mammals, TRP channels consist of seven subfamilies with a total of twenty-eight members. Evidence shows that TRP channels mediate cation transduction in neuronal signaling, but the full implication and potential therapeutic applications of this are not entirely clear. In this review, we aim to highlight several TRP channels which have been shown to mediate pain sensation, neuropsychiatric disorders, and epilepsy. Recent findings suggest that TRPM (melastatin), TRPV (vanilloid), and TRPC (canonical) are of particular relevance to these phenomena. The research reviewed in this paper validates these TRP channels as potential targets of future clinical treatment and offers patients hope for more effective care.

An inhibitory mechanism for suppressing high salt intake in Drosophila

High concentrations of dietary salt are harmful to health. Like most animals, Drosophila melanogaster are attracted to foods that have low concentrations of salt, but show strong taste avoidance of high salt foods. Salt in known on multiple classes of taste neurons, activating Gr64f sweet-sensing neurons that drive food acceptance and 2 others (Gr66a bitter and Ppk23 high salt) that drive food rejection. Here we find that NaCl elicits a bimodal dose-dependent response in Gr64f taste neurons, which show high activity with low salt and depressed activity with high salt. High salt also inhibits the sugar response of Gr64f neurons, and this action is independent of the neuron's taste response to salt. Consistent with the electrophysiological analysis, feeding suppression in the presence of salt correlates with inhibition of Gr64f neuron activity, and remains if high salt taste neurons are genetically silenced. Other salts such as Na2SO4, KCl, MgSO4, CaCl2, and FeCl3 act on sugar response and feeding behavior in the same way. A comparison of the effects of various salts suggests that inhibition is dictated by the cationic moiety rather than the anionic component of the salt. Notably, high salt-dependent inhibition is not observed in Gr66a neurons-response to a canonical bitter tastant, denatonium, is not altered by high salt. Overall, this study characterizes a mechanism in appetitive Gr64f neurons that can deter ingestion of potentially harmful salts.

SCN8A epileptic encephalopathy mutations display a gain-of-function phenotype and divergent sensitivity to antiepileptic drugs

De novo missense mutations in SCN8A gene encoding voltage-gated sodium channel NaV1.6 are linked to a severe form of early infantile epileptic encephalopathy named early infantile epileptic encephalopathy type13 (EIEE13). The majority of the patients with EIEE13 does not respond favorably to the antiepileptic drugs (AEDs) in clinic and has a significantly increased risk of death. Although more than 60 EIEE13-associated mutations have been discovered, only few mutations have been functionally analyzed. In this study we investigated the functional influences of mutations N1466T and N1466K, two EIEE13-associated mutations located in the inactivation gate, on sodium channel properties. Sodium currents were recorded from CHO cells expressing the mutant and wide-type (WT) channels using the whole-cell patch-clamp technique. We found that, in comparison with WT channels, both the mutant channels exhibited increased window currents, persistent currents (INaP) and ramp currents, suggesting that N1466T and N1466K were gain-of-function (GoF) mutations. Sodium channel inhibition is one common mechanism of currently available AEDs, in which topiramate (TPM) was effective in controlling seizures of patients carrying either of the two mutations. We found that TPM (100 µM) preferentially inhibited INaP and ramp currents but did not affect transient currents (INaT) mediated by N1466T or N1466K. Among the other 6 sodium channel-inhibiting AEDs tested, phenytoin and carbamazepine displayed greater efficacy than TPM in suppressing both INaP and ramp currents. Functional characterization of mutants N1466T and N1466K is beneficial for understanding the pathogenesis of EIEE13. The divergent effects of sodium channel-inhibiting AEDs on INaP and ramp currents provide insight into the development of therapeutic strategies for the N1466T and N1466K-associated EIEE13.

Effective Perturbations by Small-Molecule Modulators on Voltage-Dependent Hysteresis of Transmembrane Ionic Currents

The non-linear voltage-dependent hysteresis (Hys_(V)) of voltage-gated ionic currents can be robustly activated by the isosceles-triangular ramp voltage (V_ramp) through digital-to-analog conversion. Perturbations on this Hys_(V) behavior play a role in regulating membrane excitability in different excitable cells. A variety of small molecules may influence the strength of Hys_(V) in different types of ionic currents elicited by long-lasting triangular V_ramp. Pirfenidone, an anti-fibrotic drug, decreased the magnitude of I_h's Hys_(V) activated by triangular V_ramp, while dexmedetomidine, an agonist of α₂-adrenoceptors, effectively suppressed I_h as well as diminished the Hys_(V) strength of I_h. Oxaliplatin, a platinum-based anti-neoplastic drug, was noted to enhance the I_h's Hys_(V) strength, which is thought to be linked to the occurrence of neuropathic pain, while honokiol, a hydroxylated biphenyl compound, decreased I_h's Hys_(V). Cell exposure to lutein, a xanthophyll carotenoid, resulted in a reduction of I_h's Hys_(V) magnitude. Moreover, with cell exposure to UCL-2077, SM-102, isoplumbagin, or plumbagin, the Hys_(V) strength of erg-mediated K⁺ current activated by triangular V_ramp was effectively diminished, whereas the presence of either remdesivir or QO-58 respectively decreased or increased Hys_(V) magnitude of M-type K⁺ current. Zingerone, a methoxyphenol, was found to attenuate Hys_(V) (with low- and high-threshold loops) of L-type Ca²⁺ current induced by long-lasting triangular V_ramp. The Hys_(V) properties of persistent Na⁺ current (I_Na(P)) evoked by triangular V_ramp were characterized by a figure-of-eight (i.e., ∞) configuration with two distinct loops (i.e., low- and high-threshold loops). The presence of either tefluthrin, a pyrethroid insecticide, or t-butyl hydroperoxide, an oxidant, enhanced the Hys_(V) strength of I_Na(P). However, further addition of dapagliflozin can reverse their augmenting effects in the Hys_(V) magnitude of the current. Furthermore, the addition of esaxerenone, mirogabalin, or dapagliflozin was effective in inhibiting the strength of I_Na(P). Taken together, the observed perturbations by these small-molecule modulators on Hys_(V) strength in different types of ionic currents evoked during triangular V_ramp are expected to influence the functional activities (e.g., electrical behaviors) of different excitable cells in vitro or in vivo.

Fibroblast growth factor homologous factors serve as a molecular rheostat in tuning arrhythmogenic cardiac late sodium current

Voltage-gated sodium (Nav1.5) channels support the genesis and brisk spatial propagation of action potentials in the heart. Disruption of NaV1.5 inactivation results in a small persistent Na influx known as late Na current (I Na,L), which has emerged as a common pathogenic mechanism in both congenital and acquired cardiac arrhythmogenic syndromes. Here, using low-noise multi-channel recordings in heterologous systems, LQTS3 patient-derived iPSCs cardiomyocytes, and mouse ventricular myocytes, we demonstrate that the intracellular fibroblast growth factor homologous factors (FHF1-4) tune pathogenic I Na,L in an isoform-specific manner. This scheme suggests a complex orchestration of I Na,L in cardiomyocytes that may contribute to variable disease expressivity of NaV1.5 channelopathies. We further leverage these observations to engineer a peptide-inhibitor of I Na,L with a higher efficacy as compared to a well-established small-molecule inhibitor. Overall, these findings lend insights into molecular mechanisms underlying FHF regulation of I Na,L in pathophysiology and outline potential therapeutic avenues.

NBI-921352, a first-in-class, NaV1.6 selective, sodium channel inhibitor that prevents seizures in Scn8a gain-of-function mice, and wild-type mice and rats

NBI-921352 (formerly XEN901) is a novel sodium channel inhibitor designed to specifically target Na_V1.6 channels. Such a molecule provides a precision-medicine approach to target SCN8A-related epilepsy syndromes (SCN8A-RES), where gain-of-function (GoF) mutations lead to excess Na_V1.6 sodium current, or other indications where Na_V1.6 mediated hyper-excitability contributes to disease (Gardella and Møller, 2019; Johannesen et al., 2019; Veeramah et al., 2012). NBI-921352 is a potent inhibitor of Na_V1.6 (IC₅₀0.051 µM), with exquisite selectivity over other sodium channel isoforms (selectivity ratios of 756 X for Na_V1.1, 134 X for Na_V1.2, 276 X for Na_V1.7, and >583 Xfor Na_V1.3, Na_V1.4, and Na_V1.5). NBI-921352is a state-dependent inhibitor, preferentially inhibiting inactivatedchannels. The state dependence leads to potent stabilization of inactivation, inhibiting Na_V1.6 currents, including resurgent and persistent Na_V1.6 currents, while sparing the closed/rested channels. The isoform-selective profile of NBI-921352 led to a robust inhibition of action-potential firing in glutamatergic excitatory pyramidal neurons, while sparing fast-spiking inhibitory interneurons, where Na_V1.1 predominates. Oral administration of NBI-921352 prevented electrically induced seizures in a Scn8a GoF mouse,as well as in wild-type mouse and ratseizure models. NBI-921352 was effective in preventing seizures at lower brain and plasma concentrations than commonly prescribed sodium channel inhibitor anti-seizure medicines (ASMs) carbamazepine, phenytoin, and lacosamide. NBI-921352 waswell tolerated at higher multiples of the effective plasma and brain concentrations than those ASMs. NBI-921352 is entering phase II proof-of-concept trials for the treatment of SCN8A-developmental epileptic encephalopathy (SCN8A-DEE) and adult focal-onset seizures.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

The novel persistent sodium current inhibitor PRAX-562 has potent anticonvulsant activity with improved protective index relative to standard of care sodium channel blockers

OBJECTIVE

This study investigates the effects of PRAX-562 on sodium current (I_Na ), intrinsic neuronal excitability, and protection from evoked seizures to determine whether a preferential persistent I_Na inhibitor would exhibit improved preclinical efficacy and tolerability compared to two standard voltage-gated sodium channel (Na_V ) blockers.

METHODS

Inhibition of I_Na  was characterized using patch clamp analysis. The effect on intrinsic excitability was measured using evoked action potentials recorded from hippocampal CA1 pyramidal neurons in mouse brain slices. Anticonvulsant activity was evaluated using the maximal electroshock seizure (MES) model, and tolerability was assessed by measuring spontaneous locomotor activity (sLMA).

RESULTS

PRAX-562 potently and preferentially inhibited persistent I_Na induced by ATX-II or the SCN8A mutation N1768D (half-maximal inhibitory concentration [IC₅₀ ] = 141 and 75 nmol·L^-1 , respectively) relative to peak I_Na tonic/resting block (60× preference). PRAX-562 also exhibited potent use-dependent block (31× preference to tonic block). This profile is considerably different from standard Na_V blockers, including carbamazepine (CBZ; persistent I_Na IC₅₀ = 77 500 nmol·L^-1 , preference ratios of 30× [tonic block], less use-dependent block observed at various frequencies). In contrast to CBZ, PRAX-562 reduced neuronal intrinsic excitability with only a minor reduction in action potential amplitude. PRAX-562 (10 mg/kg po) completely prevented evoked seizures without affecting sLMA (MES unbound brain half-maximal efficacious concentration = 4.3 nmol·L^-1 , sLMA half-maximal tolerated concentration = 69.7 nmol·L^-1 , protective index [PI] = 16×). In contrast, CBZ and lamotrigine (LTG) had PIs of approximately 5.5×, with significant overlap between doses that were anticonvulsant and that reduced locomotor activity.

SIGNIFICANCE

PRAX-562 demonstrated robust preclinical anticonvulsant activity similar to CBZ but improved compared to LTG. PRAX-562 exhibited significantly improved preclinical tolerability compared with standard Na_V blockers (CBZ and LTG), potentially due to the preference for persistent I_Na . Preferential targeting of persistent I_Na may represent a differentiated therapeutic option for diseases of hyperexcitability, where standard Na_V blockers have demonstrated efficacy but poor tolerability.

The newest TRP channelopathy: Gain of function TRPM3 mutations cause epilepsy and intellectual disability

Transient Receptor Potential Melastatin 3 (TRPM3) is a Ca2+ permeable nonselective cation channel, activated by heat and chemical agonists, such as the endogenous neuro-steroid Pregnenolone Sulfate (PregS) and the chemical compound CIM0216. TRPM3 is expressed in peripheral sensory neurons of the dorsal root ganglia (DRG), and its role in noxious heat sensation in mice is well established. TRPM3 is also expressed in a number of other tissues, including the brain, but its role there has been largely unexplored. Recent reports showed that two mutations in TRPM3 are associated with a developmental and epileptic encephalopathy, pointing to an important role of TRPM3 in the human brain. Subsequent reports found that the two disease-associated mutations increased basal channel activity, and sensitivity of the channel to activation by heat and chemical agonists. This review will discuss these mutations in the context of human diseases caused by mutations in other TRP channels, and in the context of the biophysical properties and physiological functions of TRPM3.

Generation and Characterization of the Drosophila melanogaster paralytic Gene Knock-Out as a Model for Dravet Syndrome

Dravet syndrome is a severe rare epileptic disease caused by mutations in the SCN1A gene coding for the Nav1.1 protein, a voltage-gated sodium channel alpha subunit. We have made a knock-out of the paralytic gene, the single Drosophila melanogaster gene encoding this type of protein, by homologous recombination. These flies showed a heat-induced seizing phenotype, and sudden death in long term seizures. In addition to seizures, neuromuscular alterations were observed in climbing, flight, and walking tests. Moreover, they also manifested some cognitive alterations, such as anxiety and problems in learning. Electrophysiological analyses from larval motor neurons showed a decrease in cell capacitance and membrane excitability, while persistent sodium current increased. To detect alterations in metabolism, we performed an NMR metabolomic profiling of heads, which revealed higher levels in some amino acids, succinate, and lactate; and also an increase in the abundance of GABA, which is the main neurotransmitter implicated in Dravet syndrome. All these changes in the paralytic knock-out flies indicate that this is a good model for epilepsy and specifically for Dravet syndrome. This model could be a new tool to understand the pathophysiology of the disease and to find biomarkers, genetic modifiers and new treatments.

Somatostatin-Positive Interneurons Contribute to Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy is a devastating epilepsy syndrome caused by mutant SCN8A, which encodes the voltage-gated sodium channel Na_V1.6. To date, it is unclear if and how inhibitory interneurons, which express Na_V1.6, influence disease pathology. Using both sexes of a transgenic mouse model of SCN8A epileptic encephalopathy, we found that selective expression of the R1872W SCN8A mutation in somatostatin (SST) interneurons was sufficient to convey susceptibility to audiogenic seizures. Patch-clamp electrophysiology experiments revealed that SST interneurons from mutant mice were hyperexcitable but hypersensitive to action potential failure via depolarization block under normal and seizure-like conditions. Remarkably, GqDREADD-mediated activation of WT SST interneurons resulted in prolonged electrographic seizures and was accompanied by SST hyperexcitability and depolarization block. Aberrantly large persistent sodium currents, a hallmark of SCN8A mutations, were observed and were found to contribute directly to aberrant SST physiology in computational modeling and pharmacological experiments. These novel findings demonstrate a critical and previously unidentified contribution of SST interneurons to seizure generation not only in SCN8A epileptic encephalopathy, but epilepsy in general.SIGNIFICANCE STATEMENT SCN8A epileptic encephalopathy is a devastating neurological disorder that results from de novo mutations in the sodium channel isoform Na_v1.6. Inhibitory neurons express Na_V1.6, yet their contribution to seizure generation in SCN8A epileptic encephalopathy has not been determined. We show that mice expressing a human-derived SCN8A variant (R1872W) selectively in somatostatin (SST) interneurons have audiogenic seizures. Physiological recordings from SST interneurons show that SCN8A mutations lead to an elevated persistent sodium current which drives initial hyperexcitability, followed by premature action potential failure because of depolarization block. Furthermore, chemogenetic activation of WT SST interneurons leads to audiogenic seizure activity. These findings provide new insight into the importance of SST inhibitory interneurons in seizure initiation, not only in SCN8A epileptic encephalopathy, but for epilepsy broadly.

The role of sodium channels in direct current stimulation-axonal perspective

Transcranial neurostimulation methods are utilized as therapies for various neuropsychiatric disorders. Primarily, they entail the delivery of weak subthreshold currents across the brain, which modulate neuronal excitability. However, it is still a puzzle how such weak electrical fields actuate their effects. Previous studies showed that axons are the most sensitive subcellular compartment for direct current stimulation, and maximal polarization is achieved at their terminals. Nonetheless, polarization of axon terminals according to models was predicted to be weak, and the mechanism for substantial axon terminals polarization was obscure. Here, we show that a weak subthreshold electrical field modifies the conductance of voltage-dependent sodium channels in axon terminals, subsequently amplifying their membrane polarization. Moreover, we show that this amplification has substantial effects on synaptic functioning. Finally, we employ analytical modeling to explain how sodium currents modifications enhance axon terminal polarization. These findings relate to the mechanistic aspects of any neurostimulation technique.

The NaVy paradox: reducing sodium currents increases excitability

Mutations that increase sodium currents in excitatory neurons typically produce hyperexcitability and epileptic seizures. Paradoxically, mutations that reduce Na_V1.2 sodium currents also have a similar effect. Two research groups (Spratt et al. and Zhang et al.) have now found that in some excitatory neurons, loss of Na_V1.2 increases intrinsic excitability by altering activation and/or expression of potassium channels.

Subcellular Distribution of Persistent Sodium Conductance in Cortical Pyramidal Neurons

Cortical pyramidal neurons possess a persistent Na+ current (I NaP), which, in contrast to the larger transient current, does not undergo rapid inactivation. Although relatively quite small, I NaP is active at subthreshold voltages and therefore plays an important role in neuronal input-output processing. The subcellular distribution of channels responsible for I NaP and the mechanisms that render them persistent are not known. Using high-speed fluorescence Na+ imaging and whole-cell recordings in brain slices obtained from mice of either sex, we reconstructed the I NaP elicited by slow voltage ramps in soma and processes of cortical pyramidal neurons. We found that in all neuronal compartments, the relationship between persistent Na+ conductance and membrane voltage has the shape of a Boltzmann function. Although the density of channels underlying I NaP was about twofold lower in the axon initial segment (AIS) than in the soma, the axonal channels were activated by ∼10 mV less depolarization than were somatic channels. This difference in voltage dependence explains why, at functionally critical subthreshold voltages, most I NaP originates in the AIS. Finally, we show that endogenous polyamines constrain I NaP availability in both somatodendritic and axonal compartments of nondialyzed cortical neurons.SIGNIFICANCE STATEMENT The most salient characteristic of neuronal sodium channels is fast inactivation. However, a fraction of the sodium current does not inactivate. In cortical neurons, persistent current (I NaP) plays a prominent role in many important functions. Its subcellular distribution and generation mechanisms are, however, elusive. Using high-speed fluorescence Na+ imaging and electrical recordings, we reconstructed the I NaP in soma and processes of cortical pyramidal neurons. We found that at near-threshold voltages I NaP originates predominately from the axon, because of the distinctive voltage dependence of the underlying channels and not because of their high density. Finally, we show that the presence of endogenous polyamines significantly constrains I NaP availability in all compartments of nondialyzed cortical neurons.