Dravet syndrome (DS) is an infantile epileptic encephalopathy that is caused by loss-of-function mutations in the SCN1A gene, which encodes the voltage-gated sodium channel, NaV1.1. Haploinsufficiency of NaV1.1 in DS patients leads to imbalanced excitability across brain circuits, resulting in a broad phenotypic profile including drug-resistant convulsive and non-convulsive (absence) seizures, cognitive impairment, ataxia, and sleep disruption. Dysfunction in the somatosensory corticothalamic (CT) circuit underlies several DS phenotypes including absence seizures and sleep disturbances. Yet, the precise mechanisms underlying somatosensory CT circuit dysfunction in DS remain unclear. Here, we sought to identify the cellular and synaptic mechanisms underlying somatosensory CT circuit dysfunction in a haploinsufficiency DS mouse model. This work reveals that NaV1.1 haploinsufficiency leads to cell-type-specific changes in the excitability of reticular thalamic (nRT), ventral posterolateral (VPL), and ventral posteromedial (VPM) neurons. Further, we identified alterations in both glutamatergic and GABAergic synaptic connectivity within the somatosensory CT circuit in DS mice. These findings introduce glutamatergic neuron dysfunction and synaptic alterations as novel disease mechanisms underlying thalamic circuit dysfunction in DS, providing new targets for therapeutic intervention. In addition, we reveal that VPL and VPM neurons exhibit distinct firing properties in a healthy CT circuit, suggesting they differentially contribute to circuit-wide function in health and dysfunction in disease. / Doctor of Philosophy / The brain is composed of biological circuits made up of excitatory and inhibitory neurons, which are connected through synapses. Proper balance between excitatory and inhibitory activity in these circuits is essential for maintaining healthy brain function. Dravet syndrome (DS) is an infantile-onset epilepsy caused by mutations in the SCN1A gene, which encodes the voltage-gated sodium channel, Nav1.1. Loss of this protein in the brain leads to an imbalance of excitation and inhibition across a variety of brain circuits, resulting in drug-resistant seizures and cognitive, motor, and learning deficits. Disrupted excitability in the somatosensory corticothalamic (CT) circuit specifically leads to non-convulsive seizures and sleep disruption in DS. However, the mechanisms underlying this circuit's dysfunction remain unclear. Revealing these mechanisms is critical for identifying therapeutic targets by which we can correct circuit function. In this work, we used a mouse model of DS to reveal changes in the excitability of three distinct cell populations of the somatosensory CT circuit. Importantly, changes were exhibited in both excitatory and inhibitory thalamic neuron populations. We further identified impairments in the synapses, both excitatory and inhibitory, connecting the somatosensory CT circuit. These cell-type-specific changes in excitability and synaptic connectivity provide novel targets for therapeutic intervention in DS.

Title: Hypersensitive and circadian effects of acebutolol administration in Scn1b-/- mice. Rationale: Dravet syndrome (DS) is a severe form of pediatric epilepsy with characterizations of pharmacoresistant seizures and developmental delay. A rarer variant of the DS model is caused by homozygous loss-of-function mutations in SCN1B, which is essential in regulating sodium channel gating, expression, localization, and the firing of action potentials. Mutations in SCN1B result in severe seizures as well as a higher risk of Sudden Unexpected Death in EPilepsy (SUDEP). Factors underlying SUDEP are poorly understood, although cardiac arrhythmias have been implicated. Acebutolol (ACE) is a common beta-blocker used in the treatment of arrhythmias and hypertension. We hypothesized that treating mice with ACE will decrease cardiac arrhythmias and the incidence of SUDEP, prolonging lifespan of Scn1b null mice. Methods: Wild-type (WT) and null (KO) mice were given daily injections of 10 mg/kg ACE or saline starting at postnatal day 15 (after typical seizure onset) either during the day (09:00) or at night (21:00). In the day group, ECG was recorded daily from P13 until animal death. Starting at P15 mice were recorded both pre- and post- injection to analyze the long-term and acute effects of treatment. Results: A modest, but significant, increase in survival curves in our KO animals was observed compared to saline treated mice for those given injections during the day (a 2 day increase in median survival). In addition, in this group, the onset of animal death was delayed. To investigate the timing of drug delivery, a subset of mice was given injections at night. In this group there was actually a decrease in lifespan, with an earlier onset of death compared to saline treated mice. On a daily basis from P13, the heart rate (HR) of KO mice was significantly lower than WT but remained steady until the day prior to animal death. HR the day prior to death consistently dropped ~50% (average 414 bpm to 193 bpm) in our saline group; this was prevented in KO animals treated with ACE (421 bpm). Analysis of acute recordings following ACE administration showed that KO mice had a significantly larger reduction in heart rate compared to WT (38% vs. 11%). Further analysis of heart rate variability in these recordings demonstrated that RMSSD (a measure of vagal control of the heart) was reduced in KO mice, with differences in both baseline and following ACE administration. Conclusions: Leading up to death, we believe it is possible ACE assisted in decreased cardiovascular deficits that could lead to SUDEP and contributed to the modestly increased lifespan. In addition, our results demonstrate the importance of timing in delivery of drugs targeted at SUDEP. Finally, these results suggest that there is a possible hypersensitivity to beta-adrenergic blockade in Scn1b-/- mice. Funding: This work was supported by a grant from the Research Development Committee at East Tennessee State University and NIH grant R21NS116647 to C.R.F.

Les mutations du gène SCN1A, sont impliquées dans des épilepsies du nourrisson : le Syndrome de Dravet (SD), une épilepsie rare et pharmaco-résistante ou l’Epilepsie généralisée avec crises fébriles plus (GEFS+), une épilepsie plus légère. GEFS+ et SD sont associés à des crises épileptiques fébriles dès l’âge de 6 mois. Dans le SD on voit apparaitre des retards mentaux mais également des déficits moteurs, visuels, langagiers et mnésiques au cours de l’évolution de la maladie. L’impact des crises épileptiques au cours l’enfance sur ces déficits cognitifs n’est pas connu. Le SD est considéré comme une encéphalopathie épileptique où les crises étaient les principales responsables du phénotype à l’âge adulte. Récemment, un rôle potentiel de la mutation dans les troubles cognitifs a été mis en évidence changeant la définition de SD d’encéphalopathie épileptique à une canalopathie. La question est quel est le rôle des crises épileptiques répétées sur les fonctions cognitives à l’âge adulte ? Nous avons utilisé un modèle murin de la maladie portant une mutation du gène Scn1a, et qui présente une pathologie très légère. Nous avons induit des crises épileptiques par hyperthermie à l’âge de 21 jours (10 jours) et testé les effets à long-terme à l’âge adulte. Nos résultats révèlent que l’induction de crises induit une hyperactivité, des altérations dans les interactions sociales et des déficits en mémoires hippocampo et cortex préfronto-dépendantes. Ainsi nous avons mis en évidence que les crises épileptiques répétées pendant le développement ont un fort impact sur la fonction cérébrale et qu’il est donc capital de les prévenir afin de diminuer, voir de prévenir, ces déficits. / The SCN1A gene codes for the voltage-gated sodium channel Nav1.1 α-subunit. SCN1A mutations cause genetic epilepsies, as Generalized Epilepsy with Febrile Seizures plus (GEFS+), a mild epilepsy, or Dravet Syndrome (DS), a rare, severe and drug-resistant epileptic encephalopathy (EE). DS patients show severe cognitive/behavioral impairments that, according to the definition of EE, should be caused by the recurrent epileptic activity. Yet, this causal relationship has never been proved and it is been challenged by studies in mouse models showing that the genetic mutation itself, which causes a decrease in GABAergic activity, can be responsible for DS cognitive outcome. We studied the implication of repeated seizures during childhood to the later long-term modifications on cognitive/behavioral and epileptic phenotypes by submitting the Scn1a mouse model carrying the R1648H missense mutation and presenting mild phenotype to a protocol of repeated seizures induction by hyperthermia (10 days/one seizure per day). We observed that early life seizures can worsen the epileptic phenotype and induce cognitive/behavioral defects notably by inducing hyperactivity, sociability deficits and hippocampus- and prefrontal cortex-dependent memory deficits. We found that early life seizures can worsen the epileptic phenotype and induce cognitive/behavioral defects. Although the effect of NaV1.1 dysfunction in altering brain synchrony and the effect of repeated seizure activity in the young brain are not mutually exclusive, we thus conclude that epileptic seizures are sufficient to convert a Scn1a mouse model carrying a mild phenotype into a severe phenotype.

La résistance aux médicaments antiépileptiques (MAEs) est un des problèmes majeurs des épilepsies infantiles, comme par exemple le syndrome de Dravet. La pharmacoresistance de l’épilepsie pourrait s’expliquer par une diminution du passage des MAEs dans le cerveau, à travers la Barrière Hémato-Encéphalique (BHE). La BHE comporte des transporteurs des familles « ATP-binding cassette » (ABC) et « SoLute Carrier » (SLC) localisés au niveau de la membrane des cellules endothéliales qui contrôlent leur passage entre le sang et le cerveau. La pharmacoresistance des épilepsies a été associée à ces transporteurs car des MAEs ont été identifiés comme substrats de transporteurs comme la glycoprotéine-P (P-gP) et la « Breast Cancer Resistance Protein » (BCRP). L’hypothèse de cette relation est confortée par l’observation de l’augmentation de l’expression de ces transporteurs d’efflux dans le foyer épileptogène et par l’identification des polymorphismes dans les gènes des transporteurs chez des patients pharmacorésistants. L’interaction au cours du développement cérébral entre les cellules endothéliales et les neurones et astrocytes pourrait modifier le profil des transporteurs de la BHE. Les MAEs sont aussi connus pour être soit des inducteurs, soit des inhibiteurs des enzymes du métabolisme des médicaments et des transporteurs membranaires. Ces données nous permettent de faire les hypothèses suivantes: 1) La BHE en développement présente un profil de transporteurs différent de la BHE mature qui pourrait modifier le passage des MAEs vers le cerveau ; et 2) le traitement chronique administré au cours du syndrome de Dravet pourrait changer le phénotype des transporteurs de la BHE en développement. Nous résultats ont montré que la P-gP et la BCRP augment leur expression au cours du développement. La maturation de la BHE a aussi un impact sur le passage des MAEs étudiés. Nous avons constaté une augmentation de l’expression des différents transporteurs ABC et SLC étudiés pendant le développement de la BHE, suite au traitement chronique avec la thérapie du Syndrome de Dravet. L’acide valproïque, un des MAEs utilisé dans ce traitement, diminue l’activité d’efflux de la P-gP chez les rats en développement et adultes, ce qui a été confirmé dans un modèle in-vitro de BHE immature. Ces résultats mettent en évidence l’interaction entre la BHE en développement et le traitement chronique par les MAEs peut modifier leur distribution au niveau du cerveau et la réponse aux MAEs. / Resistance to Antiepileptic Drugs (AEDs) has been a major concern in infantile epilepsies such as for example the Dravet Syndrome. One hypothesis concerning the pharmacoresistance in epilepsy is that a decreased delivery of these drugs to the brain may occur in relation to changes in the Blood-Brain Barrier (BBB) function. BBB exhibits ATP-binding cassette (ABC) and SoLute Carrier (SLC) transporters at the surface of endothelial cells that control the blood-brain transport. Pharmacoresistance in epilepsy may be linked to changes in the functions of these transporters since some AEDs are substrates of the P-glycoprotein (P-gP) and Breast Cancer Resistance Protein (BCRP) transporters. The increased expression of efflux transporters in epileptogenic tissue and the identification of polymorphisms in the efflux transporters genes of resistant patients further support this potential relationship. The interaction of endothelial cells with astrocytes and neurons during brain development could change the pattern of transporters in the BBB. AEDs are also known as either inducers or inhibitors of drug metabolic enzymes and membrane transporters. Taken together, these facts led us to test the following hypothesis: 1) the developing BBB in immature animals presents a different pattern of transporters that could change AEDs disposition in the brain of immature subjects; and 2) the chronic pharmacotherapy used in infantile epilepsies such as the Dravet Syndrome may change the transporters phenotype of the BBB. Our work showed that the expression of P-gP and BCRP increases during development as a function of age. We also showed the maturation of the BBB has an impact on brain disposition of the studied AEDs. We finally observed an increase in the expression of various ABC and SLC transporters induced by the pharmacotherapy of the Dravet Syndrome in immature animals. One of the drugs used, valproic acid, appeared nonetheless to reduce the efflux activity of P-gP in developing and adult animals, which was confirmed in an in-vitro model of the immature BBB. Taken together, these results demonstrated that the interaction between the developing BBB and the AEDs chronic treatment may lead to differences in brain disposition of the AEDs that may impact on the response to AEDs.

[ES] El síndrome de Dravet es una epilepsia severa rara causada por mutaciones en el gen SCN1A, el cual codifica para la proteína Nav1.1, subunidad α de los canales de sodio regulados por voltaje. En esta tesis se ha generado mediante recombinación homóloga, una nueva mutación en el gen para que hemos denominado paraKO, el cual cumple la misma función en Drosophila melanogaster. Estas moscas han mostrado un fenotipo epiléptico inducido por altas temperaturas, y muerte súbita en el caso de las crisis de larga duración. También se han observado alteraciones musculares en ensayos de geotaxis negativa, vuelo y locomoción. Del mismo modo, han presentado problemas cognitivos como la ansiedad y dificultades en el aprendizaje. El uso de imanes como terapia contra el fenotipo epiléptico ha tenido buenos resultados retrasando la aparición de las crisis y disminuyendo su duración y la cantidad de moscas que las padecen. El perfil metabolómico de las cabezas de estas moscas mostró un incremento en la concentración de aminoácidos, succinato y lactato, alteraciones que se pueden relacionar con la epilepsia y la disfunción mitocondrial. El neurotransmisor GABA, principal implicado en el síndrome de Dravet, mostró niveles superiores en el modelo generado. El análisis electrofisiológico de las corrientes de sodio de las motoneuronas aCC en estadío de larva señaló aumentos en las corrientes persistentes de sodio y su ratio con las transitorias, lo cual podría justificar las crisis epilépticas. Además, la excitabilidad y el tamaño de estas células fueron menores. Todos estos cambios presentes en los mutantes KO generados hacen de estas moscas un buen modelo para estudio de la epilepsia en general, y del síndrome de Dravet en particular. Este modelo ofrece nuevas herramientas para entender la patofisiología de la enfermedad y la búsqueda de biomarcadores y tratamientos. Finalmente la búsqueda de modificadores genéticos a través de ensayos de supervivencia, tiempo de recuperación a crisis y vuelo empleando el modelo parabss1 obtuvo buenos resultados con los genes nAchRα4 y KCNQ. El gen toy por el contrario resultó ser intensificador. La variabilidad en los resultados obtenidos en este apartado cuestiona la manera de llevar a cabo este tipo de estudios en modelos animales y pacientes del síndrome de Dravet. / [CA] La síndrome de Dravet és una epilèpsia severa rara causada por mutacions en el gen SCN1A, el qual codifica para la proteïna Nav1.1, subunitat α dels canals de sodi regulats por voltatge. En aquesta tesis s'ha generat, mitjançant recombinació homòloga, una nova mutació en el gen para, anomenada paraKO, el qual té la mateixa funció en Drosophila melanogaster. Aquestes mosques han mostrat un fenotip epilèptic induït por altes temperatures, y mort súbdita en el cas de les crisis de llarga duració. També s'han observat alteracions musculars en assajos de geotaxis negativa, vol y locomoció. De la mateixa manera, han presentat problemes cognitius como l'ansietat i dificultats en l'aprenentatge. L'ús d'imants com teràpia contra el fenotip epilèptic ha tingut bons resultats endarrerint l'aparició de les crisis i disminuint la seua durada i la quantitat de mosques que les pateixen. El perfil metabolòmic dels caps d'aquestes mosques mostrà increments en la concentració d'aminoàcids, succinat i lactat, alteracions les quals es poden relacionar amb l'epilèpsia y la disfunció mitocondrial. El neurotransmissor GABA, principal implicat en la síndrome de Dravet, mostrà nivells superiores en el model generat. L'anàlisi electrofisiològic de las corrents de sodi de les motoneurones aCC en estadi de larva assenyalà augments en les corrents persistents de sodi y el seu ràtio amb las transitòries, lo qual podria justificar las crisis epilèptiques. A més a més, l'excitabilitat y el tamany d'aquestes cèl·lules va ser menor. Todos aquests canvis presents en els mutants KO generats fan d'aquestes mosques un model per a l'estudi de l'epilèpsia en general, i de la síndrome de Dravet en particular. Aquest model ofereix noves ferramentes per a entendre la patofisiología de la malaltia i la recerca de biomarcadors y tractaments. Finalment la recerca de modificadors genètics a través d'assajos de supervivència, temps de recuperació a crisis y vol mitjançant el model parabss1 va obtindre bons resultats amb els gens nAchRα4 y KCNQ. El gen toy pel contrari resultà ser intensificador. La variabilitat en els resultats obtinguts en aquest apartat qüestiona la manera de fer aquest tipus d'estudis en models animals i pacients de la síndrome de Dravet. / [EN] 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. In this thesis we have made a new mutation in a gene called paraKO through homologous recombination, the single Drosophila melanogaster gene encoding this type of protein. 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 locomotion tests. Moreover, they also manifested some cognitive alterations such as anxiety and difficulties in learning. Using magnets as a therapy for epileptic phenotype, seizures start was delayed, and its duration and the quantity of flies affected was lower. Metabolomic profile of these flies' brains showed an increase in the amount of aminoacids, succinate and lactate, alterations that could be related with epilepsy and mitochondrial dysfunction. GABA, the main neurotransmitter involved in Dravet syndrome, was higher in the paraKO model. Electrophysiological sodium current analysis from aCC motoneurons in larvae stage revealed an increase in persistent currents and their ratio with transients, which is a symptom for epileptic seizures. Cell size and excitability were lower in these cells too. 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. Finally, a search for genetic modifiers through survival, recuperation time and flight using parabss1 flies obtained good results with nAchR¿4 y KCNQ. Otherwise, toy gene was an enhancer. However, variability observed in these type of assays dispute how modifiers search is made with model animals and Dravet syndrome patients. / Tapia González, A. (2022). Generación de nuevos modelos y búsqueda de modificadores para el Síndrome de Dravet en Drosophila Melanogaster [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/186189 / TESIS

Sudden unexpected death in epilepsy (SUDEP) is the most common cause of death in chronic refractory epilepsy patients. Dravet Syndrome (DS) is an infantile-onset epilepsy with severe seizures commonly due to mutations of the sodium channel gene SCN1A. DS patients have a high risk of SUDEP, but the mechanisms of death are not well defined. The principal risk factor for SUDEP is a high frequency of seizures. The recent MORTality in Epilepsy Monitoring Unit Study (MORTEMUS) reported the largest series of SUDEP cases while in Epilepsy Monitoring Units (EMUs). Most cases occurred after a generalized seizure, and were associated with both cardiac and respiratory dysfunction. None of the SUDEP cases in the MORTEMUS study had direct measurements of breathing, but visual analysis of video recording suggested that apnea occurred early in the sequence of events preceding death. Since SUDEP is always identified after death and is impossible to predict its incidence, it is not possible to run clinical trials in patients. Therefore, developing animal models of SUDEP is beneficial. In a DS mouse model with an Scn1aR1407X/+ mutation, death occurs after spontaneous and induced seizures. This postictal death is likely to be relevant to the mechanisms of SUDEP in DS patients. In a previous study, electrocardiogram (EKG) recordings from DS mice have shown that postictal death after heat-induced seizures is due to progressive bradycardia and asystole. However, postictal breathing has not been measured during experiments studying postictal death in these mice, so it is unknown if respiratory dysfunction, such as central apnea, contributes to postictal death. The first goal of this dissertation is to design and develop a mouse EMU that monitored electroencephalogram (EEG), nuchal electromyography (EMG), EKG, video, whole body plethysmography (breathing), body temperature, room temperature, and humidity from mice until the occurrence of postictal death. Using a mouse EMU we sought to evaluate the primary cause of death in multiple mouse strains with seizure-induced death, and to determine whether they have a common final pathway of death. We induced seizures acutely in multiple non-epileptic mouse strains that are prone to sudden death in response to seizures: 1) maximal electroshock (MES)-induced seizures in Lmx1bf/f/p mice, 2) MES-induced seizures in C57Bl6 mice, and 3) audiogenic seizures in DBA/1 mice. These seizures caused immediate and permanent respiratory arrest (terminal apnea) in all 3 strains of mice. In each strain, EKG activity continued for 3 to 5 minutes after terminal apnea. We interpret these data as indicating that the primary cause of postictal death was central apnea, and the resulting hypoxia then caused bradycardia and asystole. The second goal of this dissertation is to understand the mechanism(s) of SUDEP in DS. Here we found that DS patients have frequent postictal respiratory dysfunction, while cardiac activity was normal. One of these patients who had severe postictal hypoventilation later died of SUDEP. Also, we studied mice with an Scn1aR1407X/+ mutation to determine the role of respiratory dysfunction in postictal death after spontaneous and acutely induced seizures. In DS mice, death occurred after spontaneous, heat-induced, and MES seizures while monitoring in a mouse EMU. Death always occurred after a severe seizure with tonic extension. We found that both non-epileptic and epileptic mice have consistently died by the same primary mechanisms of central apnea. Death could be prevented after heat-induced and MES seizures by mechanical ventilation. We conclude that the primary cause of postictal death was central apnea that began during the seizures and induced secondary bradycardia due to the hypoxia, ultimately leading to terminal asystole. The final goal of this dissertation is to propose a new alternative dietary therapy for DS patients. Some epilepsy children with refractory seizures, especially DS patients, have been able to reduce their seizures by following a strict high-fat and low-carbohydrate ketogenic diet (KD). Although the exact anti-epileptic mechanisms of KD diet are unknown, producing ketone bodies and creating ketosis have been widely believed to contribute to the anti-epileptic effects. Our collaborator (Toshi Kitamoto, PhD, Univ. of Iowa) has found that a diet containing milk whey was able to prevent seizures in Drosophila with an orthologous sodium channel mutation. Here we tested the effect of both KD and milk whey supplementation on DS mice. Two types of KD (with and without milk additives), KD with glucose water to eliminate ketone formation, milk whey supplementation, and standard diet were administered to growing DS mice age from P16 to P40. Compared to a standard diet, all KDs, KD with glucose water, and milk whey supplementation had beneficial effects on seizure control and prevention of postictal death. Compared to a standard diet, all KDs greatly elevated ketone body levels (β-hydroxybutyrate) and mice consistently weighed less, whereas the milk whey diet had no effect on ketosis or weight. KD with glucose water did not produce high ketone levels, but mice weighed less. These results demonstrate that ketone bodies are not the main reason for the anti-epileptic property of ketogenic diets. Taken together, these data indicate it is important to obtain data on both cardiac and respiratory function to make conclusions about the mechanisms of SUDEP in both humans and animals. Data in this dissertation show that severe postictal respiratory dysfunction in DS patients may play a major role in causing SUDEP, and may be a biomarker for those at highest risk. Death in DS mice with spontaneous seizures may be directly related to the mechanism of SUDEP in humans. Defining the specific mechanisms of postictal apnea may help to identify methods for prevention of SUDEP. We propose a SUDEP prevention strategy in DS through a new alternative diet supplemented with a milk whey compound. Milk whey supplementation of diet has a great potential to prevent postictal death in an economical and non-invasive manner without detrimental metabolic outcomes, such as ketosis and weight reduction. Milk whey supplementation of diet may be a new treatment to prevent SUDEP in DS patients.