Increased Neuronal Firing in Computer Simulations of Sodium Channel Mutations That Cause Generalized Epilepsy With Febrile Seizures Plus

Multi-scale modelling of the epileptic brain: advantages of computational therapy exploration

Objective: Epilepsy is a complex disease spanning across multiple scales, from ion channels in neurons to neuronal circuits across the entire brain. Over the past decades, computational models have been used to describe the pathophysiological activity of the epileptic brain from different aspects. Traditionally, each computational model can aid in optimizing therapeutic interventions, therefore, providing a particular view to design strategies for treating epilepsy. As a result, most studies are concerned with generating specific models of the epileptic brain that can help us understand the certain machinery of the pathological state. Those specific models vary in complexity and biological accuracy, with system-level models often lacking biological details.Approach: Here, we review various types of computational model of epilepsy and discuss their potential for different therapeutic approaches and scenarios, including drug discovery, surgical strategies, brain stimulation, and seizure prediction. We propose that we need to consider an integrated approach with a unified modelling framework across multiple scales to understand the epileptic brain. Our proposal is based on the recent increase in computational power, which has opened up the possibility of unifying those specific epileptic models into simulations with an unprecedented level of detail.Main results: A multi-scale epilepsy model can bridge the gap between biologically detailed models, used to address molecular and cellular questions, and brain-wide models based on abstract models which can account for complex neurological and behavioural observations.Significance: With these efforts, we move toward the next generation of epileptic brain models capable of connecting cellular features, such as ion channel properties, with standard clinical measures such as seizure severity.

Enhanced slow inactivation contributes to dysfunction of a recurrent SCN2A mutation associated with developmental and epileptic encephalopathy

KEY POINTS

The recurrent SCN2A mutation R853Q is associated with developmental and epileptic encephalopathy with typical onset after the first months of life. Heterologously expressed R853Q channels exhibit an overall loss-of-function as a result of multiple defects in time- and voltage-dependent channel properties. A previously unrecognized enhancement of slow inactivation is conferred by the R853Q mutation and is a major driver of loss-of-function. Enhanced slow inactivation is potentiated in the canonical splice isoform of the channel and this may explain the later onset of symptoms associated with R853Q.

ABSTRACT

Mutations in voltage gated sodium (NaV ) channel genes, including SCN2A (encoding NaV 1.2), are associated with diverse neurodevelopmental disorders with or without epilepsy that present clinically with varying severity, age-of-onset and pharmacoresponsiveness. We examined the functional properties of the most recurrent SCN2A mutation (R853Q) to determine whether developmentally-regulated alternative splicing impacts dysfunction severity and to investigate effects of the mutation on slow inactivation. We engineered the R853Q mutation into neonatal and adult NaV 1.2 splice isoforms. Channel constructs were heterologously co-expressed in HEK293T cells with human β1 and β2 subunits. Whole-cell patch clamp recording was used to compare time- and voltage-dependent properties of mutant and wild-type channels. The R853Q mutation exhibits an overall loss-of-function attributed to multiple functional defects including a previously undiscovered enhancement of slow inactivation. The mutation exhibited altered voltage dependence of activation and inactivation, slower recovery from inactivation and decreased channel availability during high-frequency depolarizations. More notable were effects on slow inactivation, including a 10-fold slower rate of recovery from slow inactivation exhibited by mutant channels. The impairments in fast inactivation properties were more severe in the neonatal splice isoform, whereas slow inactivation was more pronounced in the splice isoform of the channel expressed predominantly in later childhood. Enhanced later-onset slow inactivation may be a primary driver of the later onset of neurological features associated with this mutation.

Deciphering in silico the Role of Mutated Na V 1.1 Sodium Channels in Enhancing Trigeminal Nociception in Familial Hemiplegic Migraine Type 3

Familial hemiplegic migraine type 3 (FHM3) is caused by gain-of-function mutations in the SCN1A gene that encodes the α1 subunit of voltage-gated Na_V1.1 sodium channels. The high level of expression of Na_V1.1 channels in peripheral trigeminal neurons may lead to abnormal nociceptive signaling thus contributing to migraine pain. Na_V1.1 dysfunction is relevant also for other neurological disorders, foremost epilepsy and stroke that are comorbid with migraine. Here we used computer modeling to test the functional role of FHM3-mutated Na_V1.1 channels in mechanisms of trigeminal pain. The activation of Aδ-fibers was studied for two algogens, ATP and 5-HT, operating through P2X3 and 5-HT3 receptors, respectively, at trigeminal nerve terminals. In WT Aδ-fibers of meningeal afferents, Na_V1.1 channels efficiently participate in spike generation induced by ATP and 5-HT supported by Na_V1.6 channels. Of the various FHM3 mutations tested, the L263V missense mutation, with a longer activation state and lower activation voltage, resulted in the most pronounced spiking activity. In contrast, mutations that result in a loss of Na_V1.1 function largely reduced firing of trigeminal nerve fibers. The combined activation of P2X3 and 5-HT3 receptors and branching of nerve fibers resulted in very prolonged and high-frequency spiking activity in the mutants compared to WT. We identified, in silico, key determinants of long-lasting nociceptive activity in FHM3-mutated Aδ-fibers that naturally express P2X3 and 5-HT3 receptors and suggest mutant-specific correction options. Modeled trigeminal nerve firing was significantly higher for FHM3 mutations, compared to WT, suggesting that pronounced nociceptive signaling may contribute to migraine pain.

Modeling Neurotransmission: Computational Tools to Investigate Neurological Disorders

The investigation of synaptic functions remains one of the most fascinating challenges in the field of neuroscience and a large number of experimental methods have been tuned to dissect the mechanisms taking part in the neurotransmission process. Furthermore, the understanding of the insights of neurological disorders originating from alterations in neurotransmission often requires the development of (i) animal models of pathologies, (ii) invasive tools and (iii) targeted pharmacological approaches. In the last decades, additional tools to explore neurological diseases have been provided to the scientific community. A wide range of computational models in fact have been developed to explore the alterations of the mechanisms involved in neurotransmission following the emergence of neurological pathologies. Here, we review some of the advancements in the development of computational methods employed to investigate neuronal circuits with a particular focus on the application to the most diffuse neurological disorders.

Biophysical Psychiatry-How Computational Neuroscience Can Help to Understand the Complex Mechanisms of Mental Disorders

The brain is the most complex of human organs, and the pathophysiology underlying abnormal brain function in psychiatric disorders is largely unknown. Despite the rapid development of diagnostic tools and treatments in most areas of medicine, our understanding of mental disorders and their treatment has made limited progress during the last decades. While recent advances in genetics and neuroscience have a large potential, the complexity and multidimensionality of the brain processes hinder the discovery of disease mechanisms that would link genetic findings to clinical symptoms and behavior. This applies also to schizophrenia, for which genome-wide association studies have identified a large number of genetic risk loci, spanning hundreds of genes with diverse functionalities. Importantly, the multitude of the associated variants and their prevalence in the healthy population limit the potential of a reductionist functional genetics approach as a stand-alone solution to discover the disease pathology. In this review, we outline the key concepts of a "biophysical psychiatry," an approach that employs large-scale mechanistic, biophysics-founded computational modelling to increase transdisciplinary understanding of the pathophysiology and strive toward robust predictions. We discuss recent scientific advances that allow a synthesis of previously disparate fields of psychiatry, neurophysiology, functional genomics, and computational modelling to tackle open questions regarding the pathophysiology of heritable mental disorders. We argue that the complexity of the increasing amount of genetic data exceeds the capabilities of classical experimental assays and requires computational approaches. Biophysical psychiatry, based on modelling diseased brain networks using existing and future knowledge of basic genetic, biochemical, and functional properties on a single neuron to a microcircuit level, may allow a leap forward in deriving interpretable biomarkers and move the field toward novel treatment options.

Compartment models for the electrical stimulation of retinal bipolar cells

Bipolar cells of the retina are among the smallest neurons of the nervous system. For this reason, compared to other neurons, their delay in signaling is minimal. Additionally, the small bipolar cell surface combined with the low membrane conductance causes very little attenuation in the signal from synaptic input to the terminal. The existence of spiking bipolar cells was proven over the last two decades, but until now no complete model including all important ion channel types was published. The present study amends this and analyzes the impact of the number of model compartments on simulation accuracy. Characteristic features like membrane voltages and spike generation were tested and compared for one-, two-, four- and 117-compartment models of a macaque bipolar cell. Although results were independent of the compartment number for low membrane conductances (passive membranes), nonlinear regimes such as spiking required at least a separate axon compartment. At least a four compartment model containing the functionally different segments dendrite, soma, axon and terminal was needed for understanding signaling in spiking bipolar cells. Whereas for intracellular current application models with small numbers of compartments showed quantitatively correct results in many cases, the cell response to extracellular stimulation is sensitive to spatial variation of the electric field and accurate modeling therefore demands for a large number of short compartments even for passive membranes.

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.

Use of SU8 as a stable and biocompatible adhesion layer for gold bioelectrodes

Gold is the most widely used electrode material for bioelectronic applications due to its high electrical conductivity, good chemical stability and proven biocompatibility. However, it adheres only weakly to widely used substrate materials such as glass and silicon oxide, typically requiring the use of a thin layer of chromium between the substrate and the metal to achieve adequate adhesion. Unfortunately, this approach can reduce biocompatibility relative to pure gold films due to the risk of the underlying layer of chromium becoming exposed. Here we report on an alternative adhesion layer for gold and other metals formed from a thin layer of the negative-tone photoresist SU-8, which we find to be significantly less cytotoxic than chromium, being broadly comparable to bare glass in terms of its biocompatibility. Various treatment protocols for SU-8 were investigated, with a view to attaining high transparency and good mechanical and biochemical stability. Thermal annealing to induce partial cross-linking of the SU-8 film prior to gold deposition, with further annealing after deposition to complete cross-linking, was found to yield the best electrode properties. The optimized glass/SU8-Au electrodes were highly transparent, resilient to delamination, stable in biological culture medium, and exhibited similar biocompatibility to glass.

Ion Channels in Genetic Epilepsy: From Genes and Mechanisms to Disease-Targeted Therapies

Epilepsy is a common and serious neurologic disease with a strong genetic component. Genetic studies have identified an increasing collection of disease-causing genes. The impact of these genetic discoveries is wide reaching-from precise diagnosis and classification of syndromes to the discovery and validation of new drug targets and the development of disease-targeted therapeutic strategies. About 25% of genes identified in epilepsy encode ion channels. Much of our understanding of disease mechanisms comes from work focused on this class of protein. In this study, we review the genetic, molecular, and physiologic evidence supporting the pathogenic role of a number of different voltage- and ligand-activated ion channels in genetic epilepsy. We also review proposed disease mechanisms for each ion channel and highlight targeted therapeutic strategies.

Impact of Electrode Position on the Elicitation of Sodium Spikes in Retinal Bipolar Cells

Bipolar cells of the magnocellular pathway in the primate retina can generate action potentials because they have an axonal segment with high sodium channel density, comparable to the sodium channel band in retinal ganglion cells or pyramidal cells. The similarity between the non-human primate and the human retina is of interest for the research on retinal implants for the blind, and especially, the conditions to elicit sodium spikes in bipolar cells using extracellular stimulation. A comparison of excitation characteristics of three model neurons, a bipolar cell, a retinal ganglion cell, and a cortical pyramidal cell, demonstrates the similarities and differences regarding stimulation with microelectrodes. Moving a microelectrode parallel to the axon of a neuron commonly allows to generate spikes for every position - and this rule holds both for cathodic and anodic pulses. However, for the simulated bipolar cell anodic pulses cannot generate sodium spikes directly. Further, there is only a small region for electrode placing where extracellular cathodic stimulation causes direct spike initiation in the sodium channel band. For all other positions, a sodium spike can only be generated by antidromic current flow originating from strongly depolarized terminals.

Temperature-dependent changes in neuronal dynamics in a patient with an SCN1A mutation and hyperthermia induced seizures

Dravet syndrome is the prototype of SCN1A-mutation associated epilepsies. It is characterised by prolonged seizures, typically provoked by fever. We describe the evaluation of an SCN1A mutation in a child with early-onset temperature-sensitive seizures. The patient carries a heterozygous missense variant (c3818C > T; pAla1273Val) in the Na_V1.1 brain sodium channel. We compared the functional effects of the variant vs. wild type Na_V1.1 using patch clamp recordings from channels expressed in Chinese Hamster Ovary Cells at different temperatures (32, 37, and 40 °C). The variant channels produced a temperature-dependent destabilization of activation and fast inactivation. Implementing these empirical abnormalities in a computational model predicts a higher threshold for depolarization block in the variant, particularly at 40 °C, suggesting a failure to autoregulate at high-input states. These results reveal direct effects of abnormalities in Na_V1.1 biophysical properties on neuronal dynamics. They illustrate the value of combining cellular measurements with computational models to integrate different observational scales (gene/channel to patient).

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.

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.

Altered PKA modulation in the Nav1.1 epilepsy variant I1656M

Genetic epilepsy with febrile seizures plus (GEFS+) is an inherited epilepsy that can result from mutations in at least four ion channel subunits. The majority of the known GEFS+ mutations have been identified in SCN1A, the gene encoding Nav1.1 α-subunit. Protein kinases as critical modulators of sodium channels have been closely related to the genesis of epilepsy. However, little is known about how protein kinases affect the GEFS+ mutant sodium channel. To gain insight into the protein kinases effect on channel properties and neuronal excitability of SCN1A mutant channels, we investigated the human SCN1A GEFS+ mutation I1656M by using whole cell patch-clamp technique and an established computational neuron model. The results showed that the PKA inhibition of sodium current amplitude significantly decreased in the I1656M mutant channels, but the PKC inhibition did not. The responses of the voltage-dependent activation and fast inactivation to PKA activator disappeared in the I1656M mutant channels, but the response of the voltage dependence of the slow inactivation did not. Computational model analysis suggested that changes of the I1656M mutant channel gating behaviors in response to PKA activation altered neuronal excitability. These results indicate that altered responses of the mutant channels to PKA signaling may impair the delicate balances between chemical and electrical harmony and lead to abnormal neuronal excitability.

Thrombin facilitates seizures through activation of persistent sodium current

OBJECTIVE

An epileptic seizure is frequently the presenting sign of intracerebral hemorrhage (ICH) caused by stroke, head trauma, hypertension, and a wide spectrum of disorders. However, the cellular mechanisms responsible for occurrence of seizures during ICH have not been established. During intracerebral bleeding, blood constituents enter the neuronal tissue and produce both an acute and a delayed effect on brain functioning. Among the blood components, only thrombin has been shown to evoke seizures immediately after entering brain tissue. In the present study, we tested the hypothesis that thrombin increases neuronal excitability in the immature brain through alteration of voltage-gated sodium channels.

METHODS

The thrombin effect on neuronal excitability and voltage-gated sodium channels was assessed using extracellular and intracellular recording techniques in the hippocampal slice preparation of immature rats.

RESULTS

We show that thrombin increased neuronal excitability in the immature hippocampus in an N-methyl-D-aspartate-independent manner. Application of thrombin did not alter transient voltage-gated sodium channels and action potential threshold. However, thrombin significantly depolarized the membrane potential and produced a hyperpolarizing shift of tetrodotoxin-sensitive persistent voltage-gated sodium channel activation. This effect of thrombin was attenuated by application of protease-activated receptor-1 and protein kinase C antagonists.

INTERPRETATION

Our data indicate that thrombin amplifies the persistent voltage-gated sodium current affecting resting membrane potential and seizure threshold at the network level. Our results provide a novel explanation as to how ICH in newborns results in seizures, which may provide avenues for therapeutic intervention in the prevention of post-ICH seizures.

Sodium channels and the neurobiology of epilepsy

Voltage-gated sodium channels (VGSCs) are integral membrane proteins. They are essential for normal neurologic function and are, currently, the most common recognized cause of genetic epilepsy. This review summarizes the neurobiology of VGSCs, their association with different epilepsy syndromes, and the ways in which we can experimentally interrogate their function. The most important sodium channel subunit of relevance to epilepsy is SCN1A, in which over 650 genetic variants have been discovered. SCN1A mutations are associated with a variety of epilepsy syndromes; the more severe syndromes are associated with truncation or complete loss of function of the protein. SCN2A is another important subtype associated with epilepsy syndromes, across a range of severe and less severe epilepsies. This subtype is localized primarily to excitatory neurons, and mutations have a range of functional effects on the channel. SCN8A is the other main adult subtype found in the brain and has recently emerged as an epilepsy gene, with the first human mutation discovered in a severe epilepsy syndrome. Mutations in the accessory β subunits, thought to modulate trafficking and function of the α subunits, have also been associated with epilepsy. Genome sequencing is continuing to become more affordable, and as such, the amount of incoming genetic data is continuing to increase. Current experimental approaches have struggled to keep pace with functional analysis of these mutations, and it has proved difficult to build associations between disease severity and the precise effect on channel function. These mutations have been interrogated with a range of experimental approaches, from in vitro, in vivo, to in silico. In vitro techniques will prove useful to scan mutations on a larger scale, particularly with the advance of high-throughput automated patch-clamp techniques. In vivo models enable investigation of mutation in the context of whole brains with connected networks and more closely model the human condition. In silico models can help us incorporate the impact of multiple genetic factors and investigate epistatic interactions and beyond.

Computational modeling of epilepsy for an experimental neurologist

Computational modeling can be a powerful tool for an experimentalist, providing a rigorous mathematical model of the system you are studying. This can be valuable in testing your hypotheses and developing experimental protocols prior to experimenting. This paper reviews models of seizures and epilepsy at different scales, including cellular, network, cortical region, and brain scales by looking at how they have been used in conjunction with experimental data. At each scale, models with different levels of abstraction, the extraction of physiological detail, are presented. Varying levels of detail are necessary in different situations. Physiologically realistic models are valuable surrogates for experimental systems because, unlike in an experiment, every parameter can be changed and every variable can be observed. Abstract models are useful in determining essential parameters of a system, allowing the experimentalist to extract principles that explain the relationship between mechanisms and the behavior of the system. Modeling is becoming easier with the emergence of platforms dedicated to neuronal modeling and databases of models that can be downloaded. Modeling will never be a replacement for animal and clinical experiments, but it should be a starting point in designing experiments and understanding their results.

Channeling into the epilepsies

The Spectrum of SCN1A-Related Infantile Epileptic Encephalopathies. Harkin LA, McMahon JM, Iona X, Dibbens L, Pelekanos JT, Zuberi SM, Sadleir LG, Andermann E, Gill D, Farrell K, Connolly M, Stanley T, Harbord M, Andermann F, Wang J, Batish SD, Jones JG, Seltzer WK, Gardner A; Infantile Epileptic Encephalopathy Referral Consortium, Sutherland G, Berkovic SF, Mulley JC, Scheffer IE. Brain 2007;130(Pt 3):843–852. The relationship between severe myoclonic epilepsy of infancy (SMEI or Dravet syndrome) and the related syndrome SMEI-borderlands (SMEB) with mutations in the sodium channel alpha 1 subunit gene SCN1A is well established. To explore the phenotypic variability associated with SCN1A mutations, 188 patients with a range of epileptic encephalopathies were examined for SCN1A sequence variations by denaturing high performance liquid chromatography and sequencing. All patients had seizure onset within the first 2 years of life. A higher proportion of mutations were identified in patients with SMEI (52/66; 79%) compared to patients with SMEB (25/36; 69%). By studying a broader spectrum of infantile epileptic encephalopathies, we identified mutations in other syndromes including cryptogenic generalized epilepsy (24%) and cryptogenic focal epilepsy (22%). Within the latter group, a distinctive subgroup designated as severe infantile multifocal epilepsy had SCN1A mutations in three of five cases. This phenotype is characterized by early onset multifocal seizures and later cognitive decline. Knowledge of an expanded spectrum of epileptic encephalopathies associated with SCN1A mutations allows earlier diagnostic confirmation for children with these devastating disorders.

Altered function of the SCN1A voltage-gated sodium channel leads to gamma-aminobutyric acid-ergic (GABAergic) interneuron abnormalities

Voltage-gated sodium channels are required for the initiation and propagation of action potentials. Mutations in the neuronal voltage-gated sodium channel SCN1A are associated with a growing number of disorders including generalized epilepsy with febrile seizures plus (GEFS+),(7) severe myoclonic epilepsy of infancy, and familial hemiplegic migraine. To gain insight into the effect of SCN1A mutations on neuronal excitability, we introduced the human GEFS+ mutation SCN1A-R1648H into the orthologous mouse gene. Scn1a(RH/RH) mice homozygous for the R1648H mutation exhibit spontaneous generalized seizures and premature death between P16 and P26, whereas Scn1a(RH/+) heterozygous mice exhibit infrequent spontaneous generalized seizures, reduced threshold and accelerated propagation of febrile seizures, and decreased threshold to flurothyl-induced seizures. Inhibitory cortical interneurons from P5-P15 Scn1a(RH/+) and Scn1a(RH/RH) mice demonstrated slower recovery from inactivation, greater use-dependent inactivation, and reduced action potential firing compared with wild-type cells. Excitatory cortical pyramidal neurons were mostly unaffected. These results suggest that this SCN1A mutation predominantly impairs sodium channel activity in interneurons, leading to decreased inhibition. Decreased inhibition may be a common mechanism underlying clinically distinct SCN1A-derived disorders.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Molecular basis of severe myoclonic epilepsy in infancy

Severe myoclonic epilepsy (SMEI) or Dravet syndrome is caused by mutations of the SCN1A gene that encodes voltage-gated sodium channel alpha-1 subunit. Recently, we generated and characterized a knock-in (KI) mice with an SCN1A nonsense mutation that appeared in three independent SMEI patients. The SCN1A-KI mice well reproduced the SMEI disease phenotypes. Both homozygous and heterozygous knock-in mice developed epileptic seizures within the first postnatal month. In heterozygous knock-in mice, trains of evoked action potentials in inhibitory neurons exhibited pronounced spike amplitude decrement late in the burst but not in pyramidal neurons. We further showed that in wild-type mice the Nav1.1 protein is expressed dominantly in axons and moderately in somata of parbalbumin (PV) - positive inhibitory interneurons. Our immunohistochemical observations of the Nav1.1 are clearly distinct to the previous studies, and our findings has corrected the view of the Nav1.1 protein distribution. The data indicate that Nav1.1 plays critical roles in the spike output from PV interneurons and further, that the specifically altered function of these inhibitory circuits may contribute to epileptic seizures in the mice. These information should contribute to the understanding of molecular pathomechanism of SMEI and to develop its effective therapies.

Computer modelling of epilepsy

Epilepsy is a complex set of disorders that can involve many areas of the cortex, as well as underlying deep-brain systems. The myriad manifestations of seizures, which can be as varied as déjà vu and olfactory hallucination, can therefore give researchers insights into regional functions and relations. Epilepsy is also complex genetically and pathophysiologically: it involves microscopic (on the scale of ion channels and synaptic proteins), macroscopic (on the scale of brain trauma and rewiring) and intermediate changes in a complex interplay of causality. It has long been recognized that computer modelling will be required to disentangle causality, to better understand seizure spread and to understand and eventually predict treatment efficacy. Over the past few years, substantial progress has been made in modelling epilepsy at levels ranging from the molecular to the socioeconomic. We review these efforts and connect them to the medical goals of understanding and treating the disorder.

Ionic mechanisms of endogenous bursting in CA3 hippocampal pyramidal neurons: a model study

A critical property of some neurons is burst firing, which in the hippocampus plays a primary role in reliable transmission of electrical signals. However, bursting may also contribute to synchronization of electrical activity in networks of neurons, a hallmark of epilepsy. Understanding the ionic mechanisms of bursting in a single neuron, and how mutations associated with epilepsy modify these mechanisms, is an important building block for understanding the emergent network behaviors. We present a single-compartment model of a CA3 hippocampal pyramidal neuron based on recent experimental data. We then use the model to determine the roles of primary depolarizing currents in burst generation. The single compartment model incorporates accurate representations of sodium (Na⁺) channels (Na_V1.1) and T-type calcium (Ca²⁺) channel subtypes (Ca_V3.1, Ca_V3.2, and Ca_V3.3). Our simulations predict the importance of Na⁺ and T-type Ca²⁺ channels in hippocampal pyramidal cell bursting and reveal the distinct contribution of each subtype to burst morphology. We also performed fast-slow analysis in a reduced comparable model, which shows that our model burst is generated as a result of the interaction of two slow variables, the T-type Ca²⁺ channel activation gate and the Ca²⁺-dependent potassium (K⁺) channel activation gate. The model reproduces a range of experimentally observed phenomena including afterdepolarizing potentials, spike widening at the end of the burst, and rebound. Finally, we use the model to simulate the effects of two epilepsy-linked mutations: R1648H in Na_V1.1 and C456S in Ca_V3.2, both of which result in increased cellular excitability.

Neutralization of a negative charge in the S1-S2 region of the KV7.2 (KCNQ2) channel affects voltage-dependent activation in neonatal epilepsy

The voltage-gated potassium channels KV7.2 and KV7.3 (genes KCNQ2 and KCNQ3) constitute a major component of the M-current controlling the firing rate in many neurons. Mutations within these two channel subunits cause benign familial neonatal convulsions (BFNC). Here we identified a novel BFNC-causing mutation (E119G) in the S1-S2 region of KV7.2. Electrophysiological investigations in Xenopus oocytes using two-microelectrode voltage clamping revealed that the steady-state activation curves for E119G alone and its coexpressions with KV7.2 and/or KV7.3 wild-type (WT) channels were significantly shifted in the depolarizing direction compared to KV7.2 or KV7.2/KV7.3. These shifts reduced the relative current amplitudes for mutant channels particularly in the subthreshold range of an action potential (about 45% reduction at --50 mV for E119G compared to KV7.2, and 33% for E119G/KV7.3 compared to KV7.2/KV7.3 channels). Activation kinetics were significantly slowed for mutant channels. Our results indicate that small changes in channel gating at subthreshold voltages are sufficient to cause neonatal seizures and demonstrate the importance of the M-current for this voltage range. This was confirmed by a computer model predicting an increased burst duration for the mutation. On a molecular level, these results reveal a critical role in voltage sensing of the negatively charged E119 in S1-S2 of KV7.2, a region that-- according to molecular modelling - might interact with a positive charge in the S4 segment.

Seizure-like afterdischarges simulated in a model neuron

To explore non-synaptic mechanisms in paroxysmal discharges, we used a computer model of a simplified hippocampal pyramidal cell, surrounded by interstitial space and a "glial-endothelial" buffer system. Ion channels for Na+, K+, Ca2+ and Cl- ion antiport 3Na/Ca, and "active" ion pumps were represented in the neuron membrane. The glia had "leak" conductances and an ion pump. Fluxes, concentration changes and cell swelling were computed. The neuron was stimulated by injecting current. Afterdischarge (AD) followed stimulation if depolarization due to rising interstitial K+ concentration ([K+]o) activated persistent Na+ current (INa.P). AD was either simple or self-regenerating; either regular (tonic) or burst-type (clonic); and always self-limiting. Self-regenerating AD required sufficient INa.P to ensure re-excitation. Burst firing depended on activation of dendritic Ca2+ currents and Ca-dependent K+ current. Varying glial buffer function influenced [K+]o accumulation and afterdischarge duration. Variations in Na+ and K+ currents influenced the threshold and the duration of AD. The data show that high [K+]o and intrinsic membrane currents can produce the feedback of self-regenerating afterdischarges without synaptic input. The simulated discharge resembles neuron behavior during paroxysmal firing in living brain tissue.

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.

Computational analysis of the R85C and R85H epilepsy mutations in Na+ channel β1 subunits

Mutations in Na+ channels cause a variety of epilepsy syndromes. Analysis of these mutations shows a range of simultaneous functional consequences, each of which may increase or decrease membrane excitability, making it difficult to predict the combined effect on neuron firing. This may be addressed by building mathematical models of Na+ channel gating and using them in neuron models to predict responses to natural stimuli. The R85C and R85H mutations of the beta1 subunit cause generalized epilepsy syndromes in humans, and an experimental study showed that these mutations shift steady-state activation in the negative direction, which predicts increased excitability, and shift fast inactivation in the negative direction, which predicts decreased excitability. In addition, the R85C also shifts slow inactivation in the negative direction. To predict changes in neuron excitability resulting from these contradictory effects we built Na+ channel models based on our earlier data and on new measurements of the rate of slow inactivation over a range of potentials. Use of these Na+ channel models in simple neuron models revealed that both mutations cause an increase in excitability but the R85H mutation was more excitable. This is due to differences in steady-state slow inactivation and to subtle differences in fast kinetics captured by the model fitting process. To understand the effect of changes in different gating processes and to provide a simple guide for interpreting changes caused by mutations, we performed a sensitivity analysis. Using the wild-type model we shifted each activation curve by +/-5 mV or altered gating rates up or down by 20%. Excitability was most sensitive to changes in voltage dependence of activation, followed by voltage dependence of inactivation and then slow inactivation. By contrast, excitability was relatively insensitive to gating rates.

A childhood epilepsy mutation reveals a role for developmentally regulated splicing of a sodium channel

Seizure susceptibility is high in human infants compared to adults, presumably because of developmentally regulated changes in neural excitability. Benign familial neonatal-infantile seizures (BFNIS), characterized by both early onset and remission, are caused by mutations in the gene encoding a human sodium channel (NaV1.2). We analyzed neonatal and adult splice forms of NaV1.2 with a BFNIS mutation (L1563V) in human embryonic kidney cells. Computer modeling revealed that neonatal channels are less excitable than adult channels. Introduction of the mutation increased excitability in the neonatal channels to a level similar to adult channels. By contrast, the mutation did not affect the adult channel variant. This "adult-like" increased excitability is likely to be the mechanism underlying BFNIS in infants with this mutation. More generally, developmentally regulated NaV1.2 splicing may be one mechanism that counters the normally high excitability of neonatal neurons and helps to reduce seizure susceptibility in normal human infants.

Opposing Modifications in Intrinsic Currents and Synaptic Inputs in Post-Traumatic Mossy Cells: Evidence for Single-Cell Homeostasis in a Hyperexcitable Network

Recent experimental and modeling results demonstrated that surviving mossy cells in the dentate gyrus play key roles in the generation of network hyperexcitability. Here we examined if mossy cells exhibit long-term plasticity in the posttraumatic, hyperexcitable dentate gyrus. Mossy cells 1 wk after fluid percussion head injury did not show alterations in their current-firing frequency ( I-F) and current-membrane voltage ( I-V) relationships. In spite of the unchanged I-F and I-V curves, mossy cells showed extensive modifications in Na+, K+ and h-currents, indicating the coordinated nature of these opposing modifications. Computational experiments in a realistic large-scale model of the dentate gyrus demonstrated that individually, these perturbations could significantly affect network activity. Synaptic inputs also displayed systematic, opposing modifications. Miniature excitatory postsynaptic current (EPSC) amplitudes were decreased, whereas miniature inhibitory postsynaptic current (IPSC) amplitudes were increased as expected from a homeostatic response to network hyperexcitability. In addition, opposing alterations in miniature and spontaneous synaptic event frequencies and amplitudes were observed for both EPSCs and IPSCs. Despite extensive changes in synaptic inputs, cannabinoid-mediated depolarization-induced suppression of inhibition was not altered in posttraumatic mossy cells. These data demonstrate that many intrinsic and synaptic properties of mossy cells undergo highly specific, long-term alterations after traumatic brain injury. The systematic nature of such extensive and opposing alterations suggests that single-cell properties are significantly influenced by homeostatic mechanisms in hyperexcitable circuits.

Molecular targets for antiepileptic drug development

This review considers how recent advances in the physiology of ion channels and other potential molecular targets, in conjunction with new information on the genetics of idiopathic epilepsies, can be applied to the search for improved antiepileptic drugs (AEDs). Marketed AEDs predominantly target voltage-gated cation channels (the alpha subunits of voltage-gated Na+ channels and also T-type voltage-gated Ca2+ channels) or influence GABA-mediated inhibition. Recently, alpha2-delta voltage-gated Ca2+ channel subunits and the SV2A synaptic vesicle protein have been recognized as likely targets. Genetic studies of familial idiopathic epilepsies have identified numerous genes associated with diverse epilepsy syndromes, including genes encoding Na+ channels and GABA(A) receptors, which are known AED targets. A strategy based on genes associated with epilepsy in animal models and humans suggests other potential AED targets, including various voltage-gated Ca2+ channel subunits and auxiliary proteins, A- or M-type voltage-gated K+ channels, and ionotropic glutamate receptors. Recent progress in ion channel research brought about by molecular cloning of the channel subunit proteins and studies in epilepsy models suggest additional targets, including G-protein-coupled receptors, such as GABA(B) and metabotropic glutamate receptors; hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel subunits, responsible for hyperpolarization-activated current Ih; connexins, which make up gap junctions; and neurotransmitter transporters, particularly plasma membrane and vesicular transporters for GABA and glutamate. New information from the structural characterization of ion channels, along with better understanding of ion channel function, may allow for more selective targeting. For example, Na+ channels underlying persistent Na+ currents or GABA(A) receptor isoforms responsible for tonic (extrasynaptic) currents represent attractive targets. The growing understanding of the pathophysiology of epilepsy and the structural and functional characterization of the molecular targets provide many opportunities to create improved epilepsy therapies.

SCN1A mutations and epilepsy

SCN1A is part of the SCN1A-SCN2A-SCN3A gene cluster on chromosome 2q24 that encodes for alpha pore forming subunits of sodium channels. The 26 exons of SCN1A are spread over 100 kb of genomic DNA. Genetic defects in the coding sequence lead to generalized epilepsy with febrile seizures plus (GEFS+) and a range of childhood epileptic encephalopathies of varied severity (e.g., SMEI). All published mutations are collated. More than 100 novel mutations are spread throughout the gene with the more debilitating usually de novo. Some clustering of mutations is observed in the C-terminus and the loops between segments 5 and 6 of the first three domains of the protein. Functional studies so far show no consistent relationship between changes to channel properties and clinical phenotype. Of all the known epilepsy genes SCN1A is currently the most clinically relevant, with the largest number of epilepsy related mutations so far characterized.

Impaired inactivation gate stabilization predicts increased persistent current for an epilepsy-associated SCN1A mutation

Mutations in SCN1A (encoding the neuronal voltage-gated sodium channel alpha1 subunit, Na(V)1.1, or SCN1A) are associated with genetic epilepsy syndromes including generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy of infancy. Here, we present the formulation and use of a computational model for SCN1A to elucidate molecular mechanisms underlying the increased persistent sodium current exhibited by the GEFS+ mutant R1648H. Our model accurately reproduces all experimentally measured SCN1A whole-cell biophysical properties including biphasic whole-cell current decay, channel activation, and entry into and recovery from fast and slow inactivation. The model predicts that SCN1A open-state inactivation results from a two-step process that can be conceptualized as initial gate closure, followed by recruitment of a mechanism ("latch") to stabilize the inactivated state. Selective impairment of the second latching step results in an increase in whole-cell persistent current similar to that observed for the GEFS+ mutant R1648H. These results provide a deeper level of understanding of mutant SCN1A dysfunction in an inherited epilepsy syndrome, which will enable more precise computational studies of abnormal neuronal activity in epilepsy and may help guide new targeted therapeutic strategies.

A conserved ring of charge in mammalian Na+ channels: a molecular regulator of the outer pore conformation during slow inactivation

The molecular mechanisms underlying slow inactivation in sodium channels are elusive. Our results suggest that EEDD, a highly conserved ring of charge in the external vestibule of mammalian voltage-gated sodium channels, undermines slow inactivation. By employing site-directed mutagenesis, we found that charge alterations in this asymmetric yet strong local electrostatic field of the EEDD ring significantly altered the kinetics of slow inactivation gating. Using a non-linear Poisson-Boltzmann equation, quantitative computations of the electrostatic field in a sodium channel structural model suggested a significant electrostatic repulsion between residues E403 and E758 at close proximity. Interestingly, when this electrostatic interaction was eliminated by the double mutation E403C + E758C, the kinetics of recovery from slow inactivation of the double-mutant channel was retarded by 2500% compared to control. These data suggest that the EEDD ring, located within the asymmetric electric field, is a molecular motif that critically modulates slow inactivation in sodium channels.

De-novo mutations of the sodium channel gene SCN1A in alleged vaccine encephalopathy: a retrospective study

BACKGROUND

Vaccination, particularly for pertussis, has been implicated as a direct cause of an encephalopathy with refractory seizures and intellectual impairment. We postulated that cases of so-called vaccine encephalopathy could have mutations in the neuronal sodium channel alpha1 subunit gene (SCN1A) because of a clinical resemblance to severe myoclonic epilepsy of infancy (SMEI) for which such mutations have been identified.

METHODS

We retrospectively studied 14 patients with alleged vaccine encephalopathy in whom the first seizure occurred within 72 h of vaccination. We reviewed the relation to vaccination from source records and assessed the specific epilepsy phenotype. Mutations in SCN1A were identified by PCR amplification and denaturing high performance liquid chromatography analysis, with subsequent sequencing. Parental DNA was examined to ascertain the origin of the mutation.

FINDINGS

SCN1A mutations were identified in 11 of 14 patients with alleged vaccine encephalopathy; a diagnosis of a specific epilepsy syndrome was made in all 14 cases. Five mutations predicted truncation of the protein and six were missense in conserved regions of the molecule. In all nine cases where parental DNA was available the mutations arose de novo. Clinical-molecular correlation showed mutations in eight of eight cases with phenotypes of SMEI, in three of four cases with borderline SMEI, but not in two cases with Lennox-Gastaut syndrome.

INTERPRETATION

Cases of alleged vaccine encephalopathy could in fact be a genetically determined epileptic encephalopathy that arose de novo. These findings have important clinical implications for diagnosis and management of encephalopathy and, if confirmed in other cohorts, major societal implications for the general acceptance of vaccination.

Na channel gene mutations in epilepsy--the functional consequences

Mutations of voltage-gated sodium channel genes SCN1A, SCN2A, and SCN1B have been identified in several types of epilepsies including generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy in infancy (SMEI). In both SCN1A and SCN2A, missense mutations tend to result in benign idiopathic epilepsy, whereas truncation mutations lead to severe and intractable epilepsy. However, the results obtained by the biophysical analyses using cultured cell systems still remain elusive. Now studies in animal models harboring sodium channel gene mutations should be eagerly pursued.

An epilepsy mutation in the sodium channel SCN1A that decreases channel excitability

Mutations in three voltage-gated sodium channel genes, SCN1A, SCN2A, and SCN1B, and two GABAA receptor subunit genes, GABRG2 and GABRD, have been identified in families with generalized epilepsy with febrile seizures plus (GEFS+). A novel mutation, R859C, in the Nav1.1 sodium channel was identified in a four-generation, 33-member Caucasian family with a clinical presentation consistent with GEFS+. The mutation neutralizes a positively charged arginine in the domain 2 S4 voltage sensor of the Nav1.1 channel alpha subunit. This residue is conserved in mammalian sodium channels as well as in sodium channels from lower organisms. When the mutation was placed in the rat Nav1.1 channel and expressed in Xenopus oocytes, the mutant channel displayed a positive shift in the voltage dependence of sodium channel activation, slower recovery from slow inactivation, and lower levels of current compared with the wild-type channel. Computational analysis suggests that neurons expressing the mutant channel have higher thresholds for firing a single action potential and for firing multiple action potentials, along with decreased repetitive firing. Therefore, this mutation should lead to decreased neuronal excitability, in contrast to most previous GEFS+ sodium channel mutations, which have changes predicted to increase neuronal firing.

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.

Sodium channel mutations in epilepsy and other neurological disorders

Since the first mutations of the neuronal sodium channel SCN1A were identified 5 years ago, more than 150 mutations have been described in patients with epilepsy. Many are sporadic mutations and cause loss of function, which demonstrates haploinsufficiency of SCN1A. Mutations resulting in persistent sodium current are also common. Coding variants of SCN2A, SCN8A, and SCN9A have also been identified in patients with seizures, ataxia, and sensitivity to pain, respectively. The rapid pace of discoveries suggests that sodium channel mutations are significant factors in the etiology of neurological disease and may contribute to psychiatric disorders as well.

A novel epilepsy mutation in the sodium channel SCN1A identifies a cytoplasmic domain for beta subunit interaction

A mutation in the sodium channel SCN1A was identified in a small Italian family with dominantly inherited generalized epilepsy with febrile seizures plus (GEFS+). The mutation, D1866Y, alters an evolutionarily conserved aspartate residue in the C-terminal cytoplasmic domain of the sodium channel alpha subunit. The mutation decreased modulation of the alpha subunit by beta1, which normally causes a negative shift in the voltage dependence of inactivation in oocytes. There was less of a shift with the mutant channel, resulting in a 10 mV difference between the wild-type and mutant channels in the presence of beta1. This shift increased the magnitude of the window current, which resulted in more persistent current during a voltage ramp. Computational analysis suggests that neurons expressing the mutant channels will fire an action potential with a shorter onset delay in response to a threshold current injection, and that they will fire multiple action potentials with a shorter interspike interval at a higher input stimulus. These results suggest a causal relationship between a positive shift in the voltage dependence of sodium channel inactivation and spontaneous seizure activity. Direct interaction between the cytoplasmic C-terminal domain of the wild-type alpha subunit with the beta1 or beta3 subunit was first demonstrated by yeast two-hybrid analysis. The SCN1A peptide K1846-R1886 is sufficient for beta subunit interaction. Coimmunoprecipitation from transfected mammalian cells confirmed the interaction between the C-terminal domains of the alpha and beta1 subunits. The D1866Y mutation weakens this interaction, demonstrating a novel molecular mechanism leading to seizure susceptibility.

Epilepsy and sodium channel gene mutations: gain or loss of function?

Mutations in voltage-gated sodium channel genes (SCN1A, SCN2A, SCN1B) have been reported to be responsible for some epilepsies. Although studying such mutations to elucidate the disease mechanisms would be indispensable for the development of effective therapies, the functional consequences of these mutations remain controversial. Here, I propose a novel hypothesis for an epileptic disease mechanism which could drive the design of further studies to understand the molecular pathology of these diseases.