Interneuron Dysfunction in a New Mouse Model of SCN1A GEFS

Are we there yet? A critical evaluation of sudden and unexpected death in epilepsy models

Although animal models have helped to elaborate meaningful hypotheses about the pathophysiology of sudden and unexpected death in epilepsy (SUDEP), specific prevention strategies are still lacking, potentially reflecting the limitations of these models and the intrinsic difficulties of investigating SUDEP. The interpretation of preclinical data and their translation to diagnostic and therapeutic developments in patients thus require a high level of confidence in their relevance to model the human situation. Preclinical models of SUDEP are heterogeneous and include rodent and nonrodent species. A critical aspect is whether the animals have isolated seizures exclusively induced by a specific trigger, such as models where seizures are elicited by electrical stimulation, pharmacological intervention, or DBA mouse strains, or whether they suffer from epilepsy with spontaneous seizures, with or without spontaneous SUDEP, either of nongenetic epilepsy etiology or from genetically based developmental and epileptic encephalopathies. All these models have advantages and potential disadvantages, but it is important to be aware of these limitations to interpret data appropriately in a translational perspective. The majority of models with spontaneous seizures are of a genetic basis, whereas SUDEP cases with a genetic basis represent only a small proportion of the total number. In almost all models, cardiorespiratory arrest occurs during the course of the seizure, contrary to that in patients observed at the time of death, potentially raising the issue of whether we are studying models of SUDEP or models of periseizure death. However, some of these limitations are impossible to avoid and can in part be dependent on specific features of SUDEP, which may be difficult to model. Several preclinical tools are available to address certain gaps in SUDEP pathophysiology, which can be used to further validate current preclinical models.

Anticonvulsant effect of equilibrative nucleoside transporters 1 inhibitor in a mouse model of Dravet syndrome

There are limited therapeutic options for patients with Dravet syndrome (DS). The equilibrative nucleoside transporters 1 (ENT1) mediate both the influx and efflux of adenosine across the cell membrane exerted beneficial effects in the treatment of epilepsy. This study aimed to evaluate the anticonvulsant effect of the ENT1 inhibitor in an animal model of DS (Scn1aE1099X/+ mice). J7 (5 mg/kg) treatment was efficacious in elevating seizure threshold in Scn1aE1099X/+ mice after hyperthermia exposure. Moreover, the J7 treatment significantly reduced the frequency of spontaneous excitatory post-synaptic currents (sEPSCs, ~35% reduction) without affecting the amplitude in dentate gyrus (DG) granule cells. Pretreatment with the adenosine A1 receptor (A1R) antagonist, DPCPX, abolished the J7 effects on sEPSCs. These observations suggest that the J7 shows an anticonvulsant effect in hyperthermia-induced seizures in Scn1aE1099X/+ mice. This effect possibly acts on presynaptic A1R-mediated signaling modulation in granule cells.

Inter and Intralaminar Excitation of Parvalbumin Interneurons in Mouse Barrel Cortex

Parvalbumin (PV) interneurons are inhibitory fast-spiking cells with essential roles in directing the flow of information through cortical circuits. These neurons set the balance between excitation and inhibition, control rhythmic activity, and have been linked to disorders including autism spectrum and schizophrenia. PV interneurons differ between cortical layers in their morphology, circuitry, and function, but how their electrophysiological properties vary has received little attention. Here we investigate responses of PV interneurons in different layers of primary somatosensory barrel cortex (BC) to different excitatory inputs. With the genetically-encoded hybrid voltage sensor, hVOS, we recorded voltage changes simultaneously in many L2/3 and L4 PV interneurons to stimulation in either L2/3 or L4. Decay-times were consistent across L2/3 and L4. Amplitude, half-width, and rise-time were greater for PV interneurons residing in L2/3 compared to L4. Stimulation in L2/3 elicited responses in both L2/3 and L4 with longer latency compared to stimulation in L4. These differences in latency between layers could influence their windows for temporal integration. Thus PV interneurons in different cortical layers of BC show differences in response properties with potential roles in cortical computations.

A novel rat model of Dravet syndrome recapitulates clinical hallmarks

Dravet syndrome (DS) is a debilitating infantile epileptic encephalopathy characterized by seizures induced by high body temperature (hyperthermia), sudden unexpected death in epilepsy (SUDEP), cognitive impairment, and behavioral disturbances. The most common cause of DS is haploinsufficiency of the SCN1A gene, which encodes the voltage-gated sodium channel Nav_1.1. In current mouse models of DS, the epileptic phenotype is strictly dependent on the genetic background and most mouse models exhibit drastically higher SUDEP rates than patients. Therefore, we sought to develop an alternative animal model for DS. Here, we report the generation and characterization of a Scn1a halploinsufficiency rat model of DS by disrupting the Scn1a allele. Scn1a^+/- rats show reduced Scn1a expression in the cerebral cortex, hippocampus and thalamus. Homozygous null rats die prematurely. Heterozygous animals are highly susceptible to heat-induced seizures, the clinical hallmark of DS, but are otherwise normal in survival, growth, and behavior without seizure induction. Hyperthermia-induced seizures activate distinct sets of neurons in the hippocampus and hypothalamus in Scn1a^+/- rats. Electroencephalogram (EEG) recordings in Scn1a^+/- rats reveal characteristic ictal EEG with high amplitude bursts with significantly increased delta and theta power. After the initial hyperthermia-induced seizures, non-convulsive, and convulsive seizures occur spontaneously in Scn1a^+/- rats. In conclusion, we generate a Scn1a haploinsufficiency rat model with phenotypes closely resembling DS, providing a unique platform for establishing therapies for DS.

The long-term neurodevelopmental outcomes of febrile seizures and underlying mechanisms

Febrile seizures (FSs) are convulsions caused by a sudden increase in body temperature during a fever. FSs are one of the commonest presentations in young children, occurring in up to 4% of children between the ages of about 6 months and 5 years old. FSs not only endanger children's health, cause panic and anxiety to families, but also have many adverse consequences. Both clinical and animal studies show that FSs have detrimental effects on neurodevelopment, that cause attention deficit hyperactivity disorder (ADHD), increased susceptibility to epilepsy, hippocampal sclerosis and cognitive decline during adulthood. However, the mechanisms of FSs in developmental abnormalities and disease occurrence during adulthood have not been determined. This article provides an overview of the association of FSs with neurodevelopmental outcomes, outlining both the underlying mechanisms and the possible appropriate clinical biomarkers, from histological changes to cellular molecular mechanisms. The hippocampus is the brain region most significantly altered after FSs, but the motor cortex and subcortical white matter may also be involved in the development disorders induced by FSs. The occurrence of multiple diseases after FSs may share common mechanisms, and the long-term role of inflammation and γ-aminobutyric acid (GABA) system are currently well studied.

Hidden behavioral fingerprints in epilepsy

Epilepsy is a major disorder affecting millions of people. Although modern electrophysiological and imaging approaches provide high-resolution access to the multi-scale brain circuit malfunctions in epilepsy, our understanding of how behavior changes with epilepsy has remained rudimentary. As a result, screening for new therapies for children and adults with devastating epilepsies still relies on the inherently subjective, semi-quantitative assessment of a handful of pre-selected behavioral signs of epilepsy in animal models. Here, we use machine learning-assisted 3D video analysis to reveal hidden behavioral phenotypes in mice with acquired and genetic epilepsies and track their alterations during post-insult epileptogenesis and in response to anti-epileptic drugs. These results show the persistent reconfiguration of behavioral fingerprints in epilepsy and indicate that they can be employed for rapid, automated anti-epileptic drug testing at scale.

Epilepsy Characteristics in Neurodevelopmental Disorders: Research from Patient Cohorts and Animal Models Focusing on Autism Spectrum Disorder

Epilepsy, a heterogeneous group of brain-related diseases, has continued to significantly burden society and families. Epilepsy comorbid with neurodevelopmental disorders (NDDs) is believed to occur due to multifaceted pathophysiological mechanisms involving disruptions in the excitation and inhibition (E/I) balance impeding widespread functional neuronal circuitry. Although the field has received much attention from the scientific community recently, the research has not yet translated into actionable therapeutics to completely cure epilepsy, particularly those comorbid with NDDs. In this review, we sought to elucidate the basic causes underlying epilepsy as well as those contributing to the association of epilepsy with NDDs. Comprehensive emphasis is put on some key neurodevelopmental genes implicated in epilepsy, such as MeCP2, SYNGAP1, FMR1, SHANK1-3 and TSC1, along with a few others, and the main electrophysiological and behavioral deficits are highlighted. For these genes, the progress made in developing appropriate and valid rodent models to accelerate basic research is also detailed. Further, we discuss the recent development in the therapeutic management of epilepsy and provide a briefing on the challenges and caveats in identifying and testing species-specific epilepsy models.

Human iPSC Modeling of Genetic Febrile Seizure Reveals Aberrant Molecular and Physiological Features Underlying an Impaired Neuronal Activity

Mutations in SCN1A gene, encoding the voltage-gated sodium channel (VGSC) NaV1.1, are widely recognized as a leading cause of genetic febrile seizures (FS), due to the decrease in the Na+ current density, mainly affecting the inhibitory neuronal transmission. Here, we generated induced pluripotent stem cells (iPSCs)-derived neurons (idNs) from a patient belonging to a genetically well-characterized Italian family, carrying the c.434T > C mutation in SCN1A gene (hereafter SCN1AM145T). A side-by-side comparison of diseased and healthy idNs revealed an overall maturation delay of SCN1AM145T cells. Membranes isolated from both diseased and control idNs were injected into Xenopus oocytes and both GABA and AMPA currents were successfully recorded. Patch-clamp measurements on idNs revealed depolarized action potential for SCN1AM145T, suggesting a reduced excitability. Expression analyses of VGSCs and chloride co-transporters NKCC1 and KCC2 showed a cellular “dysmaturity” of mutated idNs, strengthened by the high expression of SCN3A, a more fetal-like VGSC isoform, and a high NKCC1/KCC2 ratio, in mutated cells. Overall, we provide strong evidence for an intrinsic cellular immaturity, underscoring the role of mutant NaV1.1 in the development of FS. Furthermore, our data are strengthening previous findings obtained using transfected cells and recordings on human slices, demonstrating that diseased idNs represent a powerful tool for personalized therapy and ex vivo drug screening for human epileptic disorders.

Dravet Variant SCN1A A1783V Impairs Interneuron Firing Predominantly by Altered Channel Activation

Dravet syndrome (DS) is a developmental epileptic encephalopathy mainly caused by functional Na_V1.1 haploinsufficiency in inhibitory interneurons. Recently, a new conditional mouse model expressing the recurrent human p.(Ala1783Val) missense variant has become available. In this study, we provided an electrophysiological characterization of this variant in tsA201 cells, revealing both altered voltage-dependence of activation and slow inactivation without reduced sodium peak current density. Based on these data, simulated interneuron (IN) firing properties in a conductance-based single-compartment model suggested surprisingly similar firing deficits for Na_V1.1^A1783V and full haploinsufficiency as caused by heterozygous truncation variants. Impaired Na_V1.1^A1783V channel activation was predicted to have a significantly larger impact on channel function than altered slow inactivation and is therefore proposed as the main mechanism underlying IN dysfunction. The computational model was validated in cortical organotypic slice cultures derived from conditional Scn1a^A1783V mice. Pan-neuronal activation of the p.Ala1783V in vitro confirmed a predicted IN firing deficit and revealed an accompanying reduction of interneuronal input resistance while demonstrating normal excitability of pyramidal neurons. Altered input resistance was fed back into the model for further refinement. Taken together these data demonstrate that primary loss of function (LOF) gating properties accompanied by altered membrane characteristics may match effects of full haploinsufficiency on the neuronal level despite maintaining physiological peak current density, thereby causing DS.