Genetic and environmental interactions determine seizure susceptibility in epileptic EL mice

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Embryonic exposure to ethanol increases the susceptibility of larval zebrafish to chemically induced seizures

Prenatal ethanol exposure is known to cause neurodevelopmental disorders. While high prevalence of epilepsy is observed among the children whose mothers abused alcohol during pregnancy, the results from animal studies are conflicting. Here, we investigated whether embryonic exposure to ethanol can increase the susceptibility to pentylenetetrazole (PTZ)-induced seizures in larval zebrafish. Embryos at 3 hours post-fertilization (hpf) were exposed to ethanol at the concentrations ranging from 0.25% to 1% for 21 hours. Control and ethanol-exposed larvae were challenged with PTZ at 7 days post-fertilization (dpf) at the concentrations of 2.5, 5 or 15 mM. The seizure behavior of larvae was recorded and analyzed using EthoVision XT 11. We found that embryonic ethanol exposure increased the percentage of larvae exhibiting typical stage II and III seizure and resulted in a significant reduction in stage I, II and III seizure latency in an ethanol concentration-dependent manner. Embryonic exposure to ethanol also significantly increased the severity of PTZ-induced seizures in larvae, as demonstrated by increased total distance traveled and the duration of mobility. This is the first demonstration that ethanol exposure during early embryonic stage can reduce the threshold for chemically induced seizures and increase the severity of seizure behavior in larval fish.

Molecular regulation of dendritic spine dynamics and their potential impact on synaptic plasticity and neurological diseases

The structure and dynamics of dendritic spines reflect the strength of synapses, which are severely affected in different brain diseases. Therefore, understanding the ultra-structure, molecular signaling mechanism(s) regulating dendritic spine dynamics is crucial. Although, since last century, dynamics of spine have been explored by several investigators in different neurological diseases, but despite countless efforts, a comprehensive understanding of the fundamental etiology and molecular signaling pathways involved in spine pathology is lacking. The purpose of this review is to provide a contextual framework of our current understanding of the molecular mechanisms of dendritic spine signaling, as well as their potential impact on different neurodegenerative and psychiatric diseases, as a format for highlighting some commonalities in function, as well as providing a format for new insights and perspectives into this critical area of research. Additionally, the potential strategies to restore spine structure-function in different diseases are also pointed out. Overall, these informations should help researchers to design new drugs to restore the structure-function of dendritic spine, a "hot site" of synaptic plasticity.

Glucose reduces the anticonvulsant effects of the ketogenic diet in EL mice

The ketogenic diet (KD) is known to be anticonvulsant and anti-epileptogenic. While the mechanism behind this therapeutic benefit is unclear, a reduction of circulating glucose levels through calorie restriction (CR) has been implicated. Foods or drinks that elevate blood glucose are known to compromise the therapeutic benefit of the KD in some children with epilepsy. We therefore evaluated the effect of a calorie restricted KD (KD-R) with supplementation of glucose in the drinking water of EL mice, a natural model of idiopathic generalized epilepsy, prior to seizure testing to assess the effect of glucose on seizure generation. Mice were fed either a standard diet or the KD unrestricted (SD-UR and KD-UR, respectively), or the KD restricted (KD-R). d-Glucose (25 mM) was supplemented in the drinking water of KD-R fed mice for 0.5h or for 2.5h prior to seizure testing. Each restricted mouse served as its own body weight control to achieve a 15-18% body weight reduction. Seizure susceptibility, body weights, and plasma glucose and β-hydroxybutyrate levels were measured over a nine-week treatment period. Body weights and glucose levels remained high over the testing period in both the SD-UR and the KD-UR groups, but were significantly reduced in all R-fed groups. A significant increase in β-hydroxybutyrate levels was observed in all KD groups. Seizure susceptibility remained highest in the SD-UR group, was slightly reduced in the KD-UR group, and was significantly reduced after three weeks in all R-fed groups. Supplementation of glucose prior to seizure testing resulted in a decrease of seizure threshold for R-fed mice, but did not alter bodyweight or circulating glucose levels. The KD has both an anticonvulsant and antiepileptogenic effect in EL mice. Here we confirm that CR enhances the anticonvulsant action of the KD in EL mice. Additionally, we show for the first time that supplementation of glucose decreases the anticonvulsant action of the KD, which further supports the hypothesis that CR works through transitioning metabolism from glucose to ketone utilization for energy.

Autosomal dominant inheritance of brain cardiolipin fatty acid abnormality in VM/DK mice: association with hypoxic-induced cognitive insensitivity

Cardiolipin is a complex polyglycerol phospholipid found almost exclusively in the inner mitochondrial membrane and regulates numerous enzyme activities especially those related to oxidative phosphorylation and coupled respiration. Abnormalities in cardiolipin can impair mitochondrial function and bioenergetics. We recently demonstrated that the ratio of shorter chain saturated and monounsaturated fatty acids (C16:0; C18:0; C18:1) to longer chain polyunsaturated fatty acids (C18:2; C20:4; C22:6) was significantly greater in the brains of adult VM/DK (VM) inbred mice than in the brains of C57BL/6 J (B6) mice. The cardiolipin fatty acid abnormalities in VM mice are also associated with alterations in the activity of mitochondrial respiratory complexes. In this study we found that the abnormal brain fatty acid ratio in the VM strain was inherited as an autosomal dominant trait in reciprocal B6 × VM F1 hybrids. To evaluate the potential influence of brain cardiolipin fatty acid composition on cognitive sensitivity, we placed the parental B6 and VM mice and their reciprocal male and female B6VMF1 hybrid mice (3-month-old) in a hypoxic chamber (5 % O2). Cognitive awareness (conscientiousness) under hypoxia was significantly lower in the VM parental mice and F1 hybrid mice (11.4 ± 0.4  and 11.0 ± 0.4 min, respectively) than in the parental B6 mice (15.3 ± 1.4 min), indicating an autosomal dominant inheritance like that of the brain cardiolipin abnormalities. These findings suggest that impaired cognitive awareness under hypoxia is associated with abnormalities in neural lipid composition.

Neuronal mechanism of epileptogenesis in EL mouse

The convulsions of the EL mouse (EL) were described by Imaizumi et al. in 1954 and were established as epilepsy by Suzuki in 1976. The EL mouse has been kept as an inbred strain and is considered one of the best animal models originated in Japan. The mode of inheritance is autosomal dominant, and environmental risk factors for seizure occurrence are hypothesised to contribute to the polygenic background. Paroxysmal activities in the EL brain arise from the parietal cortex (PCX) and are augmented in the hippocampus, demonstrated by electrophysiology and autoradiography using 2-deoxy glucose when clinical symptoms of seizures appeared. The neurons in the EL PCX, where GABA activity is lower than that of DDY PCX demonstrate increased excitability to proprioceptive sensory input. After repetitive seizure-provoking stimuli, seizures are more easily induced, eventually occurring spontaneously. This phenomenon of "abnormal plasticity" is also observed in the EEG, decreasing GABA activity, expression of the immediately early gene, and various biochemical and molecular processes. This phenomenon is similar to the learning or progressive process of certain neurological diseases.

Methylmercury: a potential environmental risk factor contributing to epileptogenesis

Epilepsy or seizure disorder is one of the most common neurological diseases in humans. Although genetic mutations in ion channels and receptors and some other risk factors such as brain injury are linked to epileptogenesis, the underlying cause for the majority of epilepsy cases remains unknown. Gene-environment interactions are thought to play a critical role in the etiology of epilepsy. Exposure to environmental chemicals is an important risk factor. Methylmercury (MeHg) is a prominent environmental neurotoxicant, which targets primarily the central nervous system (CNS). Patients or animals with acute or chronic MeHg poisoning often display epileptic seizures or show increased susceptibility to seizures, suggesting that MeHg exposure may be associated with epileptogenesis. This mini-review highlights the effects of MeHg exposure, especially developmental exposure, on the susceptibility of humans and animals to seizures, and discusses the potential role of low level MeHg exposure in epileptogenesis. This review also proposes that a preferential effect of MeHg on the inhibitory GABAergic system, leading to disinhibition of excitatory glutamatergic function, may be one of the potential mechanisms underlying MeHg-induced changes in seizure susceptibility.

The relevance of individual genetic background and its role in animal models of epilepsy

Growing evidence has indicated that genetic factors contribute to the etiology of seizure disorders. Most epilepsies are multifactorial, involving a combination of additive and epistatic genetic variables. However, the genetic factors underlying epilepsy have remained unclear, partially due to epilepsy being a clinically and genetically heterogeneous syndrome. Similar to the human situation, genetic background also plays an important role in modulating both seizure susceptibility and its neuropathological consequences in animal models of epilepsy, which has too often been ignored or not been paid enough attention to in published studies. Genetic homogeneity within inbred strains and their general amenability to genetic manipulation have made them an ideal resource for dissecting the physiological function(s) of individual genes. However, the inbreeding that makes inbred mice so useful also results in genetic divergence between them. This genetic divergence is often unaccounted for but may be a confounding factor when comparing studies that have utilized distinct inbred strains. The purpose of this review is to discuss the effects of genetic background strain on epilepsy phenotypes of mice, to remind researchers that the background genetics of a knockout strain can have a profound influence on any observed phenotype, and outline the means by which to overcome potential genetic background effects in experimental models of epilepsy.

A new locus for autosomal dominant generalized epilepsy associated with mild mental retardation on chromosome 3p

PURPOSE

Generalized epilepsies are clinically and genetically heterogeneous syndromes. Idiopathic generalized epilepsy (IGE), which has a strong genetic background, is not associated with any additional clinical features, such as mental retardation (MR). Herein we report results of linkage analysis in a large family with autosomal dominant (AD) generalized epilepsy associated with MR.

METHODS

We identified a four-generation kindred with several affected members with generalized epilepsy without any evidence for secondary causes. Electroencephalography (EEG) studies and magnetic resonance imaging (MRI) results were reviewed when available. We performed a genome-wide linkage analysis.

KEY FINDINGS

Fourteen individuals were classified as affected and an additional three were considered as nonpenetrant obligatory carriers. Thirteen affected individual had a history of generalized tonic-clonic seizures, and absence seizures were reported in nine affected individuals. There was no history of preceding febrile seizures. MR was present in nine affected individuals with epilepsy but the other affected individuals had normal intelligence. Neuroimaging did not reveal any structural abnormalities and EEG studies were consistent with IGE rather than symptomatic generalized epilepsy. Genetic analysis detected a group of markers with logarithmic (base 10) of odds (LOD) score >3 on chromosome 3p spanning a 5.5 Mbp region. Sequencing of several candidate genes, including dynein light chain-A, golgin subfamily a4, leucine rich repeat (in FLII) interacting gene, serine/threonine-protein kinase DCAMKL3 (doublecortin- like and CAM kinase-like 3), laforin (EPM2A) interacting protein 1 (EPM2AIP1, programmed cell death 6 interacting protein, and CLIP-associating protein 2 (cytoplasmic linker-associated protein 2) (hOrbit2) genes did not identify the disease-causing mutations.

SIGNIFICANCE

We report the identification of a genetic locus for generalized epilepsy associated with MR on chromosome 3p. Affected individuals have a form of genetic epilepsy with generalized seizures variably associated with MR. Despite the presence of MR in several affected patients, epilepsy phenotype was not fully consistent with symptomatic epilepsy and suggests a biologic continuum between symptomatic epilepsies and IGE.

The EL mouse: a natural model of autism and epilepsy

PURPOSE

Autism is a multifactorial disorder that involves impairments in social interactions and communication, as well as restricted and repetitive behaviors. About 30% of individuals with autism develop epilepsy by adulthood. The EL mouse has long been studied as a natural model of multifactorial idiopathic generalized epilepsy with complex partial seizures. Because epilepsy is a comorbid trait of autism, we evaluated the EL mouse for behaviors associated with autism.

METHODS

We compared the behavior of EL mice to age-matched control DDY mice, a genetically related nonepileptic strain. The mice were compared in the open field and in the light-dark compartment tests to measure activity, exploratory behavior, and restricted and repetitive behaviors. The social transmission of food preference test was employed to evaluate social communication. Home-cage behavior was also evaluated in EL and DDY mice as a measure of repetitive activity.

KEY FINDINGS

We found that EL mice displayed several behavioral abnormalities characteristic of autism. Impairments in social interaction and restricted patterns of interest were evident in EL mice. Activity, exploratory behavior, and restricted behavior were significantly greater in EL mice than in DDY mice. EL mice exhibited impairment in the social transmission of food preference assay. In addition, a stereotypic myoclonic jumping behavior was observed in EL mice, but was not seen in DDY mice. It is of interest to note that seizure activity within 24 h of testing exacerbated the autistic behavioral abnormalities found in EL mice.

SIGNIFICANCE

These findings suggest that the EL mouse expresses behavioral abnormalities similar to those seen in persons with autism. We propose that the EL mouse can be utilized as a natural model of autism and epilepsy.

Quantitative trait loci for electrical seizure threshold mapped in C57BLKS/J and C57BL/10SnJ mice

We mapped the quantitative trait loci (QTL) that contribute to the robust difference in maximal electroshock seizure threshold (MEST) between C57BLKS/J (BKS) and C57BL10S/J (B10S) mice. BKS, B10S, BKS × B10S F1 and BKS × B10S F2 intercross mice were tested for MEST at 8-9 weeks of age. Results of F2 testing showed that, in this cross, MEST is a continuously distributed trait determined by polygenic inheritance. Mice from the extremes of the trait distribution were genotyped using microarray technology. MEST correlated significantly with body weight and sex; however, because of the high correlation between these factors, the QTL mapping was conditioned on sex alone. A sequential series of statistical analyses was used to map QTLs including single-point, multipoint and multilocus methods. Two QTLs reached genome-wide levels of significance based upon an empirically determined permutation threshold: chromosome 6 (LOD = 6.0 at ∼69 cM) and chromosome 8 (LOD = 5.7 at ∼27 cM). Two additional QTLs were retained in a multilocus regression model: chromosome 3 (LOD = 2.1 at ∼68 cM) and chromosome 5 (LOD = 2.7 at ∼73 cM). Together the four QTLs explain one third of the total phenotypic variance in the mapping population. Lack of overlap between the major MEST QTLs mapped here in BKS and B10S mice and those mapped previously in C57BL/6J and DBA/2J mice (strains that are closely related to BKS and B10S) suggest that BKS and B10S represent a new polygenic mouse model for investigating susceptibility to seizures.

Evaluation of animal models of neurobehavioral disorders

Animal models play a central role in all areas of biomedical research. The process of animal model building, development and evaluation has rarely been addressed systematically, despite the long history of using animal models in the investigation of neuropsychiatric disorders and behavioral dysfunctions. An iterative, multi-stage trajectory for developing animal models and assessing their quality is proposed. The process starts with defining the purpose(s) of the model, preferentially based on hypotheses about brain-behavior relationships. Then, the model is developed and tested. The evaluation of the model takes scientific and ethical criteria into consideration.Model development requires a multidisciplinary approach. Preclinical and clinical experts should establish a set of scientific criteria, which a model must meet. The scientific evaluation consists of assessing the replicability/reliability, predictive, construct and external validity/generalizability, and relevance of the model. We emphasize the role of (systematic and extended) replications in the course of the validation process. One may apply a multiple-tiered 'replication battery' to estimate the reliability/replicability, validity, and generalizability of result.Compromised welfare is inherent in many deficiency models in animals. Unfortunately, 'animal welfare' is a vaguely defined concept, making it difficult to establish exact evaluation criteria. Weighing the animal's welfare and considerations as to whether action is indicated to reduce the discomfort must accompany the scientific evaluation at any stage of the model building and evaluation process. Animal model building should be discontinued if the model does not meet the preset scientific criteria, or when animal welfare is severely compromised. The application of the evaluation procedure is exemplified using the rat with neonatal hippocampal lesion as a proposed model of schizophrenia.In a manner congruent to that for improving animal models, guided by the procedure expounded upon in this paper, the developmental and evaluation procedure itself may be improved by careful definition of the purpose(s) of a model and by defining better evaluation criteria, based on the proposed use of the model.

Quality of rearing guides expression of behavioral and neural seizure phenotypes in EL mice

The present studies employed behavioral and neural markers of seizure-related plasticity to examine the relative contributions of genetic predisposition versus rearing environment in generating adult phenotypes in EL mice, a stress-induced animal model of epilepsy. Early environment was manipulated by cross-fostering pups of the EL strain to a seizure-resistant CD-1 control strain of mouse. The impact of changes in rearing quality on growth,exploratory and stress-reactivity phenotypes were examined, with a focus on the role of maternal care in shaping seizure susceptibility and neural cF os activation. Improvement in maternal care imposed by replacing biological EL dams with foster CD-1 mothers was sufficient to decrease pup mortality, to increase body weight gain (+0.1 g/day) and to delay the onset of seizure susceptibility in EL offspring beyond post-natal day 80–90. Moreover,hypoactivity in hippocampus and cortex among EL offspring cross-fostered to EL, but not CD-1 control, dams suggests that changes in rearing environment were accompanied by enduring changes in brain plasticity. Thus, neural and behavioral phenotypes of EL mice are dependent upon post-partum maternal care which if systematically enhanced can postpone seizure expression.

Haploinsufficiency of glutamine synthetase increases susceptibility to experimental febrile seizures

Glutamine synthetase (GS) is a pivotal glial enzyme in the glutamate-glutamine cycle. GS is important in maintaining low extracellular glutamate concentrations and is downregulated in the hippocampus of temporal lobe epilepsy patients with mesial-temporal sclerosis, an epilepsy syndrome that is frequently associated with early life febrile seizures (FS). Human congenital loss of GS activity has been shown to result in brain malformations, seizures and death within days after birth. Recently, we showed that GS knockout mice die during embryonic development and that haploinsufficient GS mice have no obvious abnormalities or behavioral seizures. In the present study, we investigated whether reduced expression/activity of GS in haploinsufficient GS mice increased the susceptibility to experimentally induced FS. FS were elicited by warm-air-induced hyperthermia in 14-day-old mice and resulted in seizures in most animals. FS susceptibility was measured as latencies to four behavioral FS characteristics. Our phenotypic data show that haploinsufficient mice are more susceptible to experimentally induced FS (P < 0.005) than littermate controls. Haploinsufficient animals did not differ from controls in hippocampal amino acid content, structure (Nissl and calbindin), glial properties (glial fibrillary acidic protein and vimentin) or expression of other components of the glutamate-glutamine cycle (excitatory amino acid transporter-2 and vesicular glutamate transporter-1). Thus, we identified GS as a FS susceptibility gene. GS activity-disrupting mutations have been described in the human population, but heterozygote mutations were not clearly associated with seizures or epilepsy. Our results indicate that individuals with reduced GS activity may have reduced FS seizure thresholds. Genetic association studies will be required to test this hypothesis.

Paternal care paradoxically increases offspring seizure susceptibility in the El mouse model of epilepsy

The El mouse is a model of idiopathic epilepsy in which seizures emerge on Postnatal Days (PNDs) 80-90, although time to first seizure can be modified by experiential factors including handling during development and history of past seizures. This study tested the hypothesis that a significant increase in the amount of parental investment would impact seizure susceptibility in adult El offspring. The study used a single dam control, in which the litter was reared by a female biological parent, and a biparental experimental group, in which both biological parents reared the litter. Components of parental care and pup body weights were quantified on PNDs 2-21, and adult offspring were examined using a handling-induced seizure susceptibility (HISS) test on PNDs 80-90 to assess the long-term impact of alterations in the perinatal environment. As expected, presence of both parents did increase parental/pup contact time by 350% relative to single-mother parenting and also reduced body weight, an index of perinatal stressor exposure, in already underweight El offspring. Accordingly, HISS testing of adult El offspring revealed a deleterious effect of biparental rearing, which increased seizure incidence to 30% relative to 0% for the single dam condition. These results suggest that the presence of a second care provider in addition to the dam constitutes a form of stressor exposure in El pups and, as a consequence, reduces the time to first seizure in genetically susceptible offspring.

Chemoconvulsant-induced seizure susceptibility: toward a common genetic basis?

Despite the efforts employed, understanding the genetic architecture underlying epilepsy remains difficult. To reach this aim, convulsive epilepsies are classically modeled in mice, where genetic studies are less constricting than in humans. Pharmacogenetic approaches are one major source of investigation where kainic acid, pentylenetetrazol, and the ss-carboline family represent compounds that are used extensively. Several quantitative trait loci (QTLs) influencing the convulsant effects of these drugs have been mapped using either recombinant inbred strains (RIS) or segregating F2 populations (or both). In our laboratory, we have recently mapped two QTLs for methyl 6, 7-dimethoxy-4-ethyl-ss-carboline-3-carboxylate (DMCM), and seizure response using an F2 method. One is located on the distal part of Chromosome 1, a region implicated in a number of other studies. Here, we address the general importance of this chromosomal fragment for influencing seizure susceptibility.