The role of GABAA receptor biogenesis, structure and function in epilepsy

Association between the SLC6A11 rs2304725 and GABRG2 rs211037 polymorphisms and drug-resistant epilepsy: a meta-analysis

Background: Previous studies have shown that SLC6A11 and GABRG2 are linked to drug-resistant epilepsy (DRE), although there have been conflicting results in the literature. In this study, we systematically assessed the relationship between DRE and these two genes. Methods: We systematically searched the PubMed, Embase, Cochrane Library, Web of Science, Google Scholar, Wanfang Data, CNKI, and VIP databases. To clarify whether heterogeneity existed between studies, tools such as the Q-test and I ² statistic were selected. According to study heterogeneity, we chose fixed- or random-effects models for analysis. We then used the chi-squared ratio to evaluate any bias of the experimental data. Results: In total, 11 trials and 3,813 patients were selected. To investigate the relationship with DRE, we performed model tests on the two genes separately. The results showed that SLC6A11 rs2304725 had no significant correlation with DRE risk in the allele, dominant, recessive, and additive models in a pooled population. However, for the over-dominant model, DRE was correlated with rs2304725 (OR = 1.08, 95% CI: 0.92-1.27, p = 0.33) in a pooled population. Similarly, rs211037 was weakly significantly correlated with DRE for the dominant, recessive, over-dominant, and additive models in a pooled population. The subgroup analysis results showed that rs211037 expressed a genetic risk of DRE in allele (OR = 1.01, 95% CI: 0.76-1.35, p = 0.94), dominant (OR = 1.08, 95% CI: 0.77-1.50, p = 0.65), and additive models (OR = 1.14, 95% CI: 0.62-2.09, p = 0.67) in an Asian population. Conclusion: In this meta-analysis, our results showed that SLC6A11 rs2304725 and GABRG2 rs211037 are not significantly correlated with DRE. However, in the over-dominant model, rs2304725 was significantly correlated with DRE. Likewise, rs211037 conveyed a genetic risk for DRE in an Asian population in the allele, dominant, and additive models.

Seizure susceptibility to various convulsant stimuli in the BTBR mouse model of autism spectrum disorders

Background: Autism spectrum disorders (ASDs) are one of the most severe chronic childhood disorders in terms of prevalence, morbidity, and impact on society. Interestingly, several systematic reviews and meta-analyses documented a bidirectional link between epilepsy and ASD, supporting the hypothesis that both disorders may have common neurobiological pathways. According to this hypothesis, an imbalance of the excitatory/inhibitory (E/I) ratio in several brain regions may represent a causal mechanism underpinning the co-occurrence of these neurological diseases. Methods: To investigate this bidirectional link, we first tested the seizure susceptibility to chemoconvulsants acting on GABAergic and glutamatergic systems in the BTBR mice, in which an imbalance between E/I has been previously demonstrated. Subsequently, we performed the PTZ kindling protocol to study the impact of seizures on autistic-like behavior and other neurological deficits in BTBR mice. Results: We found that BTBR mice have an increased susceptibility to seizures induced by chemoconvulsants impairing GABA_A neurotransmission in comparison to C57BL/6J control mice, whereas no significant difference in seizure susceptibility was observed after administration of AMPA, NMDA, and Kainate. This data suggests that deficits in GABAergic neurotransmission can increase seizure susceptibility in this strain of mice. Interestingly, BTBR mice showed a longer latency in the development of kindling compared to control mice. Furthermore, PTZ-kindling did not influence autistic-like behavior in BTBR mice, whereas it was able to significantly increase anxiety and worsen cognitive performance in this strain of mice. Interestingly, C57BL/6J displayed reduced sociability after PTZ injections, supporting the hypothesis that a tight connection exists between ASD and epilepsy. Conclusion: BTBR mice can be considered a good model to study epilepsy and ASD contemporarily. However, future studies should shed light on the mechanisms underpinning the co-occurrence of these neurological disorders in the BTBR model.

HAT- and HDAC-Targeted Protein Acetylation in the Occurrence and Treatment of Epilepsy

Epilepsy is a common and severe chronic neurological disorder. Recently, post-translational modification (PTM) mechanisms, especially protein acetylation modifications, have been widely studied in various epilepsy models or patients. Acetylation is regulated by two classes of enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs). HATs catalyze the transfer of the acetyl group to a lysine residue, while HDACs catalyze acetyl group removal. The expression of many genes related to epilepsy is regulated by histone acetylation and deacetylation. Moreover, the acetylation modification of some non-histone substrates is also associated with epilepsy. Various molecules have been developed as HDAC inhibitors (HDACi), which have become potential antiepileptic drugs for epilepsy treatment. In this review, we summarize the changes in acetylation modification in epileptogenesis and the applications of HDACi in the treatment of epilepsy as well as the mechanisms involved. As most of the published research has focused on the differential expression of proteins that are known to be acetylated and the knowledge of whole acetylome changes in epilepsy is still minimal, a further understanding of acetylation regulation will help us explore the pathological mechanism of epilepsy and provide novel ideas for treating epilepsy.

GABRG2 C588T Polymorphism Is Associated with Idiopathic Generalized Epilepsy but Not with Antiepileptic Drug Resistance in Pakistani Cohort

Idiopathic generalized epilepsy (IGE) is the most prevalent type of epilepsy with genetic origin. Mutations in ion channel genes have been identified as a common cause of IGE. Several studies have reported various epilepsy risk variants of GABRG2 (gamma-aminobutyric acid type A receptor subunit gamma2 subunit) gene in different ethnic groups, but the results are inconsistent. The purpose of this case-control research is to determine if GABRG2 polymorphisms contribute to IGE susceptibility and antiepileptic drug resistance in Pakistani population. For this purpose, we genotyped exon2, exon5 (C540T and C588T), exon7 (T813C), exon8 (K289M), and exon9 of GABRG2 gene by restriction fragment length polymorphism and Sanger's sequencing in 87 drug-responsive idiopathic generalized epilepsy patients, 55 drug-resistant epilepsy patients, and 83 healthy controls. Restriction fragment length polymorphism (RFLP) and sequencing results indicated only C588T polymorphism in the studied subjects. The comparison of genotypic and allelic frequencies showed significant differences between IGE patients and control groups (P = 0.008 and odds ratio = 4.2) and nonsignificant association of C588T polymorphism in antiseizure medication-resistant patients (P = 0.9). Our findings showed that C588T polymorphism of GABRG2 is a risk variant for IGE in Pakistani population. Further studies are required to validate the results.

Gamma-Aminobutyric Acid Type A Receptor Genes and Their Related Epilepsies

Gamma-aminobutyric acid type A (GABA-A) receptor subunit gene mutations, which include GABRA1, GABRB3, GABRD, and GABRG2, are often involved in several genetic epilepsy syndromes and other neuropsychiatric diseases like autism spectrum disorder, schizophrenia, and anxiety. GABA-A are ligand-gated ionic channels, and are involved firstly in the fast inhibitory synaptic transmission of the central nervous system. The GABA receptors include the ionotropic GABA-A and GABA-C receptors and the metabotropic GABA-B receptors. According to the site in which mutations occur, they cause disorders in channel opening, “lock-and-pull” receptor system functioning, and capable of causing a specific epilepsy phenotype. The aim of this article is to summarize the most recent literature findings, considering genetic mutations, clinical features, genotype/phenotype correlation, and therapy about neurodevelopment diseases correlated to GABA receptors dysfunction, in particular epilepsy. According to our findings, we conclude that further mutation analysis could permit genotype–phenotype correlation and give more information about the best efficient treatment, even if—at present—more clinical and genetic studies are necessary.

Modeling poststroke epilepsy and preclinical development of drugs for poststroke epilepsy

Stroke is a severe clinical issue for global public health, representing the third leading cause of death and a major cause of disability in developed countries. Progresses in the pharmacological treatment of the acute stroke have given rise to a significant decrease in its mortality rate. However, as a result, there has been an increasing number of stroke survivors living with disability worldwide. Poststroke epilepsy (PSE) is a common clinical complication following stroke. Seizures can arise in close temporal association with stroke damage and/or after a variably longer interval. Overall, PSE have a good prognosis; in fact, its responding rate to antiepileptic drugs (AEDs) is higher than other types of epilepsy. However, regarding pharmacological treatment, some issues are still unresolved. To this aim, a deeper understanding of mechanisms underlying the transformation of infarcted tissue into an epileptic focus or better from a nonepileptic brain to an epileptic brain is also mandatory for PSE. However, studying epileptogenesis in patients with PSE clearly has several limitations and difficulties; therefore, modeling PSE is crucial. Until now, different experimental models have been used to study the etiopathology of cerebrovascular stroke with or without infarction, but few studies focused on poststroke epileptogenesis and PSE. In this review, we show a brief overview on the features emerging from preclinical research into experimental PSE, which could affect the discovery of biomarkers and therapy strategies for poststroke epileptogenesis. This article is part of the Special Issue "Seizures & Stroke".

Mutations in GABRG2 receptor gene are not a major factor in the pathogenesis of mesial temporal lobe epilepsy in Indian population

AIM

This study is focused on GABRG2 gene sequence variations in patients with mesial temporal lobe epilepsy (mTLE). The GABAAreceptor is a heteropentameric receptor and alpha-1 beta-2 gamma-2 subunits combination is most abundant and present in almost all regions of the brain. The gamma-2 subunit (GABRG2) gene mutations have been reported in different epilepsy pathologies. In the present study we have looked for GABRG2 gene sequence variations in patients with mTLE.

MATERIALS AND METHODS

Twenty patients (12 females and eight males, age 4.6-38 years) with MTLE were recruited for this investigation. Patients were recommended for epilepsy surgery after all clinical investigations as per the epilepsy protocol. Ethnically matched glioma or meningioma patients were considered as nonepileptic controls. During temporal lobectomy of amygdalohippocampectomy, hippocampal brain tissue samples were resected guided by intraoperative electrocorticography (ECoG) activity. All 11 exons of GABRG2 gene with their flanking intronic regions were amplified by polymerase chain reaction (PCR) and screened by DNA sequencing analysis for sequence variations.

STATISTICAL ANALYSIS USED

Comparison of allele frequencies between patient and control groups was determined using a c(2) test.

RESULTS AND CONCLUSIONS

Total five DNA sequence variations were identified, three in exonic regions (c.643A > G, rs211035), (c.T > A, rs424740), and (c.C > T, rs418210) and two in intronic regions (c.751 + 41A > G, rs211034) and (c.751 + 52G > A, rs 34281163). Allele frequencies of variants identified in this study did not differ between patients and normal controls. Thus, we conclude that GABRG2 gene may not be playing significant role in the development of epilepsy or as a susceptibility gene in patients with MTLE in Indian population.

Epigenetic Suppression of GADs Expression is Involved in Temporal Lobe Epilepsy and Pilocarpine-Induced Mice Epilepsy

Recent studies have shown that histone acetylation is involved with the regulation of enzyme glutamate decarboxylases (GADs), including GAD67 and GAD65. Here, we investigated the histone acetylation modifications of GADs in the pathogenesis of epilepsy and explored the therapeutic effect of a novel second-generation histone deacetylase inhibitor (HDACi) JNJ-26481585 in epilepsy animals. We revealed the suppression of GADs protein and mRNA level, and histone hypoacetylation in patients with temporal lobe epilepsy and pilocarpine-induced epilepsy mice model. Double-immunofluorescence also indicated that the hypoacetyl-H3 was located in hippocampal GAD67/GAD65 positive neurons in epilepsy mice. JNJ-26481585 significantly reversed the decrease of the GAD67/GAD65 both protein and mRNA levels, and the histone hypoacetylation of GABAergic neurons in epilepsy mice. Meanwhile, single-cell real-time PCR performed in GFP-GAD67/GAD65 transgenic mice demonstrated that JNJ-26481585 induced increase of GAD67/GAD65 mRNA level in GABAergic neurons. Furthermore, JNJ-26481585 significantly alleviated the epileptic seizures in mice model. Together, our findings demonstrate inhibition of GADs gene via histone acetylation plays an important role in the pathgenesis of epilepsy, and suggest JNJ-26481585 as a promising therapeutic strategy for epilepsy.

Novel α1 and γ2 GABAA receptor subunit mutations in families with idiopathic generalized epilepsy

Epilepsy is a heterogeneous neurological disease affecting approximately 50 million people worldwide. Genetic factors play an important role in both the onset and severity of the condition, with mutations in several ion-channel genes being implicated, including those encoding the GABA(A) receptor. Here, we evaluated the frequency of additional mutations in the GABA(A) receptor by direct sequencing of the complete open reading frame of the GABRA1 and GABRG2 genes from a cohort of French Canadian families with idiopathic generalized epilepsy (IGE). Using this approach, we have identified three novel mutations that were absent in over 400 control chromosomes. In GABRA1, two mutations were found, with the first being a 25-bp insertion that was associated with intron retention (i.e. K353delins18X) and the second corresponding to a single point mutation that replaced the aspartate 219 residue with an asparagine (i.e. D219N). Electrophysiological analysis revealed that K353delins18X and D219N altered GABA(A) receptor function by reducing the total surface expression of mature protein and/or by curtailing neurotransmitter effectiveness. Both defects would be expected to have a detrimental effect on inhibitory control of neuronal circuits. In contrast, the single point mutation identified in the GABRG2 gene, namely P83S, was indistinguishable from the wildtype subunit in terms of surface expression and functionality. This finding was all the more intriguing as the mutation exhibited a high degree of penetrance in three generations of one French Canadian family. Further experimentation will be required to understand how this mutation contributes to the occurrence of IGE in these individuals.

Downregulation of gephyrin in temporal lobe epilepsy neurons in humans and a rat model

Gephyrin, which is a postsynaptic scaffolding protein participated in clustering GABA(A) receptors at inhibitory synapses, has been reported to be involved in temporal lobe epilepsy (TLE) recently. Here, we investigate gephyrin protein expression in the temporal lobe epileptic foci in epileptic patients and experimental animals in order to explore the probable relationship between gephyrin expression and TLE. Using immunohistochemistry, immunofluorescence, and western blot analysis, gephyrin expression was examined in 30 human temporal neocortex samples from patients who underwent surgery to treat drug-refractory TLE and 10 histological normal temporal neocortex from the controls. Meanwhile, we investigated the gephyrin expression in the hippocampus and adjacent neocortex from experimental rats on 24 h, 48 h, 1 week, 2 weeks, 1 month, and 2 months postseizure and from control rats. Gephyrin protein was mainly expressed in the membrane and cytoplasm of neurons in temporal lobe epileptic foci in humans and experimental rats. Gephyrin expression was significantly lower in the temporal neocortex of TLE patients compared to the controls. In experimental rats, the expression of gephyrin in temporal lobe was downregulated in epileptic groups compared to the control group. Gephyrin expression gradually decreased during the acute period and the latent period, but then began to increase below the levels seen in controls during the chronic phase. Our findings suggest that gephyrin may be involved in the development of TLE.

Functional excitatory GABAA receptors precede ionotropic glutamate receptors in radial glia-like neural stem cells

The involvement of neurotransmission in neuronal development is a generally accepted concept. Nevertheless, the precise regulation of neurotransmitter receptor expression is still unclear. To investigate the expression profiles of the most important ionotropic neurotransmitter receptors, namely GABA(A) receptors (GABA(A)Rs), NMDA receptors (NMDARs), and AMPA receptors (AMPARs), quantitative RT-PCR, immunoblot analysis and patch clamp studies were performed in in vitro-generated neural stem cells (NSCs). This clearly defined cell line is closely related to radial glia cells, the stem cells in the neonate brain. We found functional GABA(A)Rs of the subunit composition alpha2, beta3, and gamma1 to be expressed. Unexpectedly, functional ionotropic glutamate receptors were absent. However, NSCs expressed the NMDAR subunits NR2A and NR3A, and the AMPAR subunit GluR4 at the protein level, and GluR3 at the mRNA level. The overexpression of functional NMDARs in NSCs led to an increased mRNA level of AMPAR subunits, indicating a role in synaptogenesis. Early neuronal markers remained unchanged. These data extend our knowledge about ionotropic neurotransmitter receptor expression during neuronal development and will aid further investigations on activity-dependent neurogenesis.

Expression of GABA A receptor alpha1 subunit mRNA and protein in rat neocortex following photothrombotic infarction

Photothrombotic infarcts of the neocortex result in structural and functional alterations of cortical networks, including decreased GABAergic inhibition, and can generate epileptic seizures within 1 month of lesioning. In our study, we assessed the involvement and potential changes of cortical GABA A receptor (GABA AR) alpha1 subunits at 1, 3, 7, and 30 days after photothrombosis. Quantitative competitive reverse transcription-polymerase chain reaction (cRT-PCR) and semi-quantitative Western blot analysis were used to investigate GABA AR alpha1 subunit mRNA and protein levels in proximal and distal regions of perilesional cortex and in homotopic areas of young adult Sprague-Dawley rats. GABA AR alpha1 subunit mRNA levels were decreased ipsilateral and contralateral to the infarct at 7 days, but were increased bilaterally at 30 days. GABA AR alpha1 subunit protein levels revealed no significant change in neocortical areas of both hemispheres of lesioned animals compared with protein levels of sham-operated controls at 1, 3, 7, and 30 days. At 30 days, GABA AR alpha1 subunit protein expression was significantly increased in lesioned animals within proximal and distal regions of perilesional cortex compared with distal neocortical areas contralaterally (Student's t-test, p<0.05). Short- and long-term alterations of mRNA and protein levels of the GABA AR alpha1 subunit ipsilateral and contralateral to the lesion may influence alterations in cell surface receptor subtype expression and GABA AR function following ischemic infarction and may be associated with formative mechanisms of poststroke epileptogenesis.

Ion channels in epilepsy

Neuronal excitability is determined by the flux of ions through ion channels. Many types of ion channels are expressed in the central nervous system, each responsible for its own aspect of neuronal excitability, from postsynaptic depolarization to action potential generation to neurotransmitter release. These mechanisms are tightly regulated to create a balance between excitation and inhibition. Disruption of this balance is thought to be key in many neurological disorders, including epilepsy syndromes. More and more ion channel mutations are being identified through genetic studies; however, their incidence is still small, suggesting the presence of undiscovered mutations or other causative mechanisms. Understanding wild-type channel function during epileptic activity may also provide vital insights into the remaining idiopathic epilepsies and provide targets for future antiepileptic drugs.

Gene characterization index: assessing the depth of gene annotation

Background

We introduce the Gene Characterization Index, a bioinformatics method for scoring the extent to which a protein-encoding gene is functionally described. Inherently a reflection of human perception, the Gene Characterization Index is applied for assessing the characterization status of individual genes, thus serving the advancement of both genome annotation and applied genomics research by rapid and unbiased identification of groups of uncharacterized genes for diverse applications such as directed functional studies and delineation of novel drug targets.

Methodology/Principal Findings

The scoring procedure is based on a global survey of researchers, who assigned characterization scores from 1 (poor) to 10 (extensive) for a sample of genes based on major online resources. By evaluating the survey as training data, we developed a bioinformatics procedure to assign gene characterization scores to all genes in the human genome. We analyzed snapshots of functional genome annotation over a period of 6 years to assess temporal changes reflected by the increase of the average Gene Characterization Index. Applying the Gene Characterization Index to genes within pharmaceutically relevant classes, we confirmed known drug targets as high-scoring genes and revealed potentially interesting novel targets with low characterization indexes. Removing known drug targets and genes linked to sequence-related patent filings from the entirety of indexed genes, we identified sets of low-scoring genes particularly suited for further experimental investigation.

Conclusions/Significance

The Gene Characterization Index is intended to serve as a tool to the scientific community and granting agencies for focusing resources and efforts on unexplored areas of the genome. The Gene Characterization Index is available from http://cisreg.ca/gci/.

Structure and trafficking of NMDA and GABAA receptors

The fidelity of synaptic function is dependent on the expression of the appropriate neurotransmitter receptor subtype, the targeting and trafficking of receptors to synapses as well as the regulation of the actual number of receptors at synapses. GABAA (γ-aminobutyric acid type A) receptors and NMDA (N-methyl-D-aspartate) receptors are both examples of ligand-gated, heteromeric neurotransmitter receptors whose cell-surface expression is dynamic and tightly regulated. NMDA receptors are localized at excitatory synapses. These synapses are highly structured but dynamic, with the interplay between NMDA receptors and NMDA receptor-associated scaffolding proteins regulating the expression of functional cell-surface synaptic and extrasynaptic receptors. Based on current information, inhibitory synapses seem to be less ordered, and a GABAA receptor equivalent of PSD-95 (postsynaptic density-95), the scaffolding molecule pivotal to the organization of NMDA receptor complexes at synapses, is yet to be validated. In the present paper, processes regulating the trafficking, assembly and molecular organization of both NMDA receptors and GABAA receptors will be discussed.