Genetic epilepsy is a common disorder with phenotypic variation, but the basis for the variation is unknown. Comparing the molecular pathophysiology of mutations in the same epilepsy gene may provide mechanistic insights into the phenotypic heterogeneity. GABRG2 is an established epilepsy gene, and mutations in it produce epilepsy syndromes with varying severities. The disease phenotype in some cases may be caused by simple loss of subunit function (functional haploinsufficiency), while others may be caused by loss-of-function plus dominant negative suppression and other cellular toxicity. Detailed molecular defects and the corresponding seizures and related comorbidities resulting from haploinsufficiency and dominant negative mutations, however, have not been compared. Here we compared two mouse models of GABRG2 loss-of-function mutations associated with epilepsy with different severities, Gabrg2 knockin (KI) and Gabrg2 knockout (KO) mice. Heterozygous Gabrg2 KI mice are associated with a severe epileptic encephalopathy due to a dominant negative effect of the mutation, while heterozygous Gabrg2 KO mice are associated with mild absence epilepsy due to simple haploinsufficiency. Unchanged at the transcriptional level, KI mice with severe epilepsy had neuronal accumulation of mutant γ2 subunits, reduced remaining functional wild-type subunits in dendrites and synapses, while KO mice with mild epilepsy had no intracellular accumulation of the mutant subunits and unaffected biogenesis of the remaining wild-type subunits. Consequently, KI mice with dominant negative mutations had much less wild-type receptor expression, more severe seizures and behavioural comorbidities than KO mice. This work provides insights into the pathophysiology of epilepsy syndrome heterogeneity and designing mechanism-based therapies.

Seizures are a known co-occurring symptom of Alzheimer's disease, and they can accelerate cognitive and neuropathological dysfunction. Sub-optimal vitamin C (ascorbic acid) deficiency, that is low levels that do not lead the sufferer to present with clinical signs of scurvy (e.g. lethargy, hemorrhage, hyperkeratosis), are easily obtainable with insufficient dietary intake, and may contribute to the oxidative stress environment of both Alzheimer's disease and epilepsy. The purpose of this study was to test whether mice that have diminished brain ascorbic acid in addition to carrying human Alzheimer's disease mutations in the amyloid precursor protein (APP) and presenilin 1 (PSEN1) genes, had altered electrical activity in the brain (electroencephalography; EEG), and were more susceptible to pharmacologically-induced seizures. Brain ascorbic acid was decreased in APP/PSEN1 mice by crossing them with sodium vitamin C transporter 2 (SVCT2) heterozygous knockout mice. These mice have an approximately 30% decrease in brain ascorbic acid due to lower levels of SVCT2 that supplies the brain with ASC. SVCT2+/−APP/PSEN1 mice had decreased ascorbic acid and increased oxidative stress in brain, increased mortality, faster seizure onset latency following treatment with kainic acid (10 mg/kg i.p.), and more ictal events following pentylenetetrazol (50 mg/kg i.p.) treatment. Furthermore, we report the entirely novel phenomenon that ascorbic acid deficiency alone increased the severity of kainic acid-and pentylenetetrazol-induced seizures. These data suggest that avoiding ascorbic acid deficiency may be particularly important in populations at increased risk for epilepsy and seizures, such as Alzheimer's disease.

Ascorbate (vitamin C) is critical as a first line of defense antioxidant within the brain, and specifically within the synapse. Ascorbate is released by astrocytes during glutamate clearance and disruption of this exchange mechanism may be critical in mediating glutamate toxicity within the synapse. This is likely even more critical in neurodegenerative disorders with associated excitotoxicity and seizures, in particular Alzheimer's disease, in which ascorbate levels are often low. Using Gulo mice that are dependent on dietary ascorbate, we established that low brain ascorbate increased sensitivity to kainic acid as measured via behavioral observations, electroencephalography (EEG) measurements, and altered regulation of several glutamatergic system genes. Kainic acid-induced immobility was improved in wild-type mice following treatment with ceftriaxone, which upregulates glutamate transporter GLT-1. The same effect was not observed in ascorbate-deficient mice in which sufficient ascorbate is not available for release. A single, mild seizure event was sufficient to disrupt performance in the water maze in low-ascorbate mice and in APP/PSEN1 mice. Together, the data support the critical role of brain ascorbate in maintaining protection during glutamatergic hyperexcitation events, including seizures. The study further supports a role for mild, subclinical seizures in cognitive decline in Alzheimer's disease.

These results suggest that the ISS-induced immobility is mediated through the NE system and may represent a model for atypical depression.

It has been established that febrile seizures and its extended syndromes like generalized epilepsy with febrile seizures (FS) plus (GEFS+) and Dravet syndrome have been associated with mutations especially in SCN1A and GABRG2 genes. In patients, the onset of FS is likely due to the combined effect of temperature and inflammation in genetically vulnerable individuals because fever is often associated with infection. Much effort has been spent to understand the mechanisms underlying fever induction of seizures. In addition to the role of cytokines in FS, previous studies in Scn1a+/− knockout mice, a model of Dravet syndrome, indicated that temperature elevation alone could result in seizure generation, and the effect of elevated temperature inducing seizures was age-dependent. Here, we report the thermal effect in a different mouse model of Dravet syndrome, the Gabrg2+/Q390X knockin mouse. We demonstrated age-dependent dysregulated temperature control and that temperature elevation produced myoclonic jerks, generalized tonic clonic seizures (GTCSs) and heightened anxietylike symptoms in Gabrg2+/Q390X mice. The study indicated that regardless of other inflammatory factors, brief heat alone increased brain excitability and induced multiple types of seizures in Gabrg2+/Q390X mice, suggesting that mutations like GABRG2(Q390X) may alter brain thermal regulation and precipitate seizures during temperature elevations.