Another mechanism for creating diversity in gamma-aminobutyrate type A receptors: RNA splicing directs expression of two forms of gamma 2 phosphorylation site

Cryo-EM structures reveal native GABAA receptor assemblies and pharmacology

Type A γ-aminobutyric acid receptors (GABAARs) are the principal inhibitory receptors in the brain and the target of a wide range of clinical agents, including anaesthetics, sedatives, hypnotics and antidepressants1-3. However, our understanding of GABAAR pharmacology has been hindered by the vast number of pentameric assemblies that can be derived from 19 different subunits4 and the lack of structural knowledge of clinically relevant receptors. Here, we isolate native murine GABAAR assemblies containing the widely expressed α1 subunit and elucidate their structures in complex with drugs used to treat insomnia (zolpidem (ZOL) and flurazepam) and postpartum depression (the neurosteroid allopregnanolone (APG)). Using cryo-electron microscopy (cryo-EM) analysis and single-molecule photobleaching experiments, we uncover three major structural populations in the brain: the canonical α1β2γ2 receptor containing two α1 subunits, and two assemblies containing one α1 and either an α2 or α3 subunit, in which the single α1-containing receptors feature a more compact arrangement between the transmembrane and extracellular domains. Interestingly, APG is bound at the transmembrane α/β subunit interface, even when not added to the sample, revealing an important role for endogenous neurosteroids in modulating native GABAARs. Together with structurally engaged lipids, neurosteroids produce global conformational changes throughout the receptor that modify the ion channel pore and the binding sites for GABA and insomnia medications. Our data reveal the major α1-containing GABAAR assemblies, bound with endogenous neurosteroid, thus defining a structural landscape from which subtype-specific drugs can be developed.

Altered Behavioral Responses Show GABA Sensitivity in Muscleblind-Like 2-Deficient Mice: Implications for CNS Symptoms in Myotonic Dystrophy

Considerable evidence from mouse models and human postmortem brain suggests loss of Muscleblind-like protein 2 (MBNL2) function in brain is a major driver of CNS symptoms in Myotonic dystrophy type 1 (DM1). Increased hypersomnia, fatigue, and surgical complications associated with general anesthesia suggest possible sensitivity to GABAergic inhibition in DM1. To test the hypothesis that MBNL2 depletion leads to behavioral sensitivity to GABA_A receptor (GABA_A-R) modulation, Mbnl2 knock-out (KO) and wild-type (WT) littermates were treated with the anesthetic sevoflurane, the benzodiazepine diazepam, the imidazopyridine zolpidem, and the benzodiazepine rescue agent, flumazenil (Ro 15-1788), and assessed for various behavioral metrics. Mbnl2 KO mice exhibited delayed recovery following sevoflurane, delayed emergence and recovery from zolpidem, and enhanced sleep time at baseline that was modulated by flumazenil. A significantly higher proportion of Mbnl2 KO mice also loss their righting reflex [loss of righting reflex (LORR)] from a standard diazepam dose. We further examined whether MBNL2 depletion affects total GABA_A-R mRNA subunit levels and validated RNA-sequencing data of mis-spliced Gabrg2, whose isoform ratios are known to regulate GABA sensitivity and associated behaviors. While no other GABA_A-R subunit mRNA levels tested were altered in Mbnl2 KO mouse prefrontal cortex, Gabrg2S/L mRNA ratio levels were significantly altered. Taken together, our findings indicate that loss of MBNL2 function affects GABAergic function in a mouse model of myotonic dystrophy (DM1).

Electroacupuncture at PC6 (Neiguan) Attenuates Angina Pectoris in Rats with Myocardial Ischemia-Reperfusion Injury Through Regulating the Alternative Splicing of the Major Inhibitory Neurotransmitter Receptor GABRG2

Angina pectoris is the most common manifestation of coronary heart disease, causing suffering in patients. Electroacupuncture at PC6 can effectively alleviate angina by regulating the expression of genes, whether the alternative splicing (AS) of genes is affected by acupuncture is still unknown. We established a rat model of myocardial ischemia-reperfusion by coronary artery ligation and confirmed electroacupuncture alleviated the abnormal discharge caused by angina pectoris measured in EMG electromyograms. Analysis of the GSE61840 dataset established that AS events were altered after I/R and regulated by electroacupuncture. I/R decreased the expression of splicing factor Nova1 while electroacupuncture rescued it. Further experiments in dorsal root ganglion cells showed Nova1 regulated the AS of the GABRG2, specifically on its exon 9 where an important phosphorylation site is present. In vivo, results also showed that electroacupuncture can restore AS of GABRG2. Our results proved that electroacupuncture alleviates angina results by regulating alternative splicing.

A comprehensive atlas of fetal splicing patterns in the brain of adult myotonic dystrophy type 1 patients

In patients with myotonic dystrophy type 1 (DM1), dysregulation of RNA-binding proteins like MBNL and CELF1 leads to alternative splicing of exons and is thought to induce a return to fetal splicing patterns in adult tissues, including the central nervous system (CNS). To comprehensively evaluate this, we created an atlas of developmentally regulated splicing patterns in the frontal cortex of healthy individuals and DM1 patients, by combining RNA-seq data from BrainSpan, GTEx and DM1 patients. Thirty-four splice events displayed an inclusion pattern in DM1 patients that is typical for the fetal situation in healthy individuals. The regulation of DM1-relevant splicing patterns could partly be explained by changes in mRNA expression of the splice regulators MBNL1, MBNL2 and CELF1. On the contrary, interindividual differences in splicing patterns between healthy adults could not be explained by differential expression of these splice regulators. Our findings lend transcriptome-wide evidence to the previously noted shift to fetal splicing patterns in the adult DM1 brain as a consequence of an imbalance in antagonistic MBNL and CELF1 activities. Our atlas serves as a solid foundation for further study and understanding of the cognitive phenotype in patients.

The Hair Cell α9α10 Nicotinic Acetylcholine Receptor: Odd Cousin in an Old Family

Nicotinic acetylcholine receptors (nAChRs) are a subfamily of pentameric ligand-gated ion channels with members identified in most eumetazoan clades. In vertebrates, they are divided into three subgroups, according to their main tissue of expression: neuronal, muscle and hair cell nAChRs. Each receptor subtype is composed of different subunits, encoded by paralogous genes. The latest to be identified are the α9 and α10 subunits, expressed in the mechanosensory hair cells of the inner ear and the lateral line, where they mediate efferent modulation. α9α10 nAChRs are the most divergent amongst all nicotinic receptors, showing marked differences in their degree of sequence conservation, their expression pattern, their subunit co-assembly rules and, most importantly, their functional properties. Here, we review recent advances in the understanding of the structure and evolution of nAChRs. We discuss the functional consequences of sequence divergence and conservation, with special emphasis on the hair cell α9α10 receptor, a seemingly distant cousin of neuronal and muscle nicotinic receptors. Finally, we highlight potential links between the evolution of the octavolateral system and the extreme divergence of vertebrate α9α10 receptors.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Electrophysiology of ionotropic GABA receptors

GABA_A receptors are ligand-gated chloride channels and ionotropic receptors of GABA, the main inhibitory neurotransmitter in vertebrates. In this review, we discuss the major and diverse roles GABA_A receptors play in the regulation of neuronal communication and the functioning of the brain. GABA_A receptors have complex electrophysiological properties that enable them to mediate different types of currents such as phasic and tonic inhibitory currents. Their activity is finely regulated by membrane voltage, phosphorylation and several ions. GABA_A receptors are pentameric and are assembled from a diverse set of subunits. They are subdivided into numerous subtypes, which differ widely in expression patterns, distribution and electrical activity. Substantial variations in macroscopic neural behavior can emerge from minor differences in structure and molecular activity between subtypes. Therefore, the diversity of GABA_A receptors widens the neuronal repertoire of responses to external signals and contributes to shaping the electrical activity of neurons and other cell types.

Looking for Novelty in an "Old" Receptor: Recent Advances Toward Our Understanding of GABAARs and Their Implications in Receptor Pharmacology

Diverse populations of GABA_A receptors (GABA_ARs) throughout the brain mediate fast inhibitory transmission and are modulated by various endogenous ligands and therapeutic drugs. Deficits in GABA_AR signaling underlie the pathophysiology behind neurological and neuropsychiatric disorders such as epilepsy, anxiety, and depression. Pharmacological intervention for these disorders relies on several drug classes that target GABA_ARs, such as benzodiazepines and more recently neurosteroids. It has been widely demonstrated that subunit composition and receptor stoichiometry impact the biophysical and pharmacological properties of GABA_ARs. However, current GABA_AR-targeting drugs have limited subunit selectivity and produce their therapeutic effects concomitantly with undesired side effects. Therefore, there is still a need to develop more selective GABA_AR pharmaceuticals, as well as evaluate the potential for developing next-generation drugs that can target accessory proteins associated with native GABA_ARs. In this review, we briefly discuss the effects of benzodiazepines and neurosteroids on GABA_ARs, their use as therapeutics, and some of the pitfalls associated with their adverse side effects. We also discuss recent advances toward understanding the structure, function, and pharmacology of GABA_ARs with a focus on benzodiazepines and neurosteroids, as well as newly identified transmembrane proteins that modulate GABA_ARs.

Microexons: at the nexus of nervous system development, behaviour and autism spectrum disorder

The discovery and characterization of a network of highly conserved neuronal microexons has provided fundamental new insight into mechanisms underlying nervous system development and function, as well as an important basis for pathway convergence in autism spectrum disorder. In the past few years, considerable progress has been made in comprehensively determining the repertoires of factors that control neuronal microexons. These results have illuminated molecular mechanisms that activate the splicing of microexons, including those that control gene expression programs critical for neurogenesis, as well as synaptic protein translation and neuronal activity. Remarkably, individual disruption of specific microexons in these pathways results in autism-like phenotypes and cognitive impairment in mice. This review discusses these findings and their implications for delivering new therapeutic strategies for neurological disorders.

Genome-Wide Analysis of Differential Gene Expression and Splicing in Excitatory Neurons and Interneuron Subtypes

Cortical circuit activity is shaped by the parvalbumin (PV) and somatostatin (SST) interneurons that inhibit principal excitatory (EXC) neurons and the vasoactive intestinal peptide (VIP) interneurons that suppress activation of other interneurons. To understand the molecular-genetic basis of functional specialization and identify potential drug targets specific to each neuron subtype, we performed a genome wide assessment of both gene expression and splicing across EXC, PV, SST and VIP neurons from male and female mouse brains. These results reveal numerous examples where neuron subtype-specific gene expression, as well as splice-isoform usage, can explain functional differences between neuron subtypes, including in presynaptic plasticity, postsynaptic receptor function, and synaptic connectivity specification. We provide a searchable web resource for exploring differential mRNA expression and splice form usage between excitatory, PV, SST, and VIP neurons (http://research-pub.gene.com/NeuronSubtypeTranscriptomes). This resource, combining a unique new dataset and novel application of analysis methods to multiple relevant datasets, identifies numerous potential drug targets for manipulating circuit function, reveals neuron subtype-specific roles for disease-linked genes, and is useful for understanding gene expression changes observed in human patient brains.SIGNIFICANCE STATEMENT Understanding the basis of functional specialization of neuron subtypes and identifying drug targets for manipulating circuit function requires comprehensive information on cell-type-specific transcriptional profiles. We sorted excitatory neurons and key inhibitory neuron subtypes from mouse brains and assessed differential mRNA expression. We used a genome-wide analysis which not only examined differential gene expression levels but could also detect differences in splice isoform usage. This analysis reveals numerous examples of neuron subtype-specific isoform usage with functional importance, identifies potential drug targets, and provides insight into the neuron subtypes involved in psychiatric disease. We also apply our analysis to two other relevant datasets for comparison, and provide a searchable website for convenient access to the resource.

γ2 GABAAR Trafficking and the Consequences of Human Genetic Variation

GABA type A receptors (GABA_ARs) mediate the majority of fast inhibitory neurotransmission in the central nervous system (CNS). Most prevalent as heteropentamers composed of two α, two β, and a γ2 subunit, these ligand-gated ionotropic chloride channels are capable of extensive genetic diversity (α1-6, β1-3, γ1-3, δ, 𝜀, 𝜃, π, ρ1-3). Part of this selective GABA_AR assembly arises from the critical role for γ2 in maintaining synaptic receptor localization and function. Accordingly, mutations in this subunit account for over half of the known epilepsy-associated genetic anomalies identified in GABA_ARs. Fundamental structure-function studies and cellular pathology investigations have revealed dynamic GABA_AR trafficking and synaptic scaffolding as critical regulators of GABAergic inhibition. Here, we introduce in vitro and in vivo findings regarding the specific role of the γ2 subunit in receptor trafficking. We then examine γ2 subunit human genetic variation and assess disease related phenotypes and the potential role of altered GABA_AR trafficking. Finally, we discuss new-age imaging techniques and their potential to provide novel insight into critical regulatory mechanisms of GABA_AR function.

Towards a Better Understanding of GABAergic Remodeling in Alzheimer's Disease

γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the vertebrate brain. In the past, there has been a major research drive focused on the dysfunction of the glutamatergic and cholinergic neurotransmitter systems in Alzheimer’s disease (AD). However, there is now growing evidence in support of a GABAergic contribution to the pathogenesis of this neurodegenerative disease. Previous studies paint a complex, convoluted and often inconsistent picture of AD-associated GABAergic remodeling. Given the importance of the GABAergic system in neuronal function and homeostasis, in the maintenance of the excitatory/inhibitory balance, and in the processes of learning and memory, such changes in GABAergic function could be an important factor in both early and later stages of AD pathogenesis. Given the limited scope of currently available therapies in modifying the course of the disease, a better understanding of GABAergic remodeling in AD could open up innovative and novel therapeutic opportunities.

Microexons: discovery, regulation, and function

The importance of RNA splicing in numerous cellular processes is well established. However, an underappreciated aspect is the ability of the spliceosome to recognize a set of very small (3-30 nucleotide, 1-10 amino acid) exons named microexons. Despite their small size, microexons and their regulation through alternative splicing have now been shown to play critical roles in protein and system function. Here we review the discovery of microexons over time and the mechanisms by which their splicing is regulated, including recent progress made through deep RNA sequencing. We also discuss the functional role of microexons in biology and disease. WIREs RNA 2017, 8:e1418. doi: 10.1002/wrna.1418 For further resources related to this article, please visit the WIREs website.

Characterization of the γ-aminobutyric acid signaling system in the zebrafish (Danio rerio Hamilton) central nervous system by reverse transcription-quantitative polymerase chain reaction

In the vertebrate brain, inhibition is largely mediated by γ-aminobutyric acid (GABA). This neurotransmitter comprises a signaling machinery of GABA_A, GABA_B receptors, transporters, glutamate decarboxylases (gads) and 4-aminobutyrate aminotransferase (abat), and associated proteins. Chloride is intimately related to GABA_A receptor conductance, GABA uptake, and GADs activity. The response of target neurons to GABA stimuli is shaped by chloride-cation co-transporters (CCCs), which strictly control Cl^- gradient across plasma membranes. This research profiled the expression of forty genes involved in GABA signaling in the zebrafish (Danio rerio) brain, grouped brain regions and retinas. Primer pairs were developed for reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The mRNA levels of the zebrafish GABA system share similarities with that of mammals, and confirm previous studies in non-mammalian species. Proposed GABA_A receptors are α₁β₂γ₂, α₁β₂δ, α_2bβ₃γ₂, α_2bβ₃δ, α₄β₂γ₂, α₄β₂δ, α_6bβ₂γ₂ and α_6bβ₂δ. Regional brain differences were documented. Retinal hetero- or homomeric ρ-composed GABA_A receptors could exist, accompanying α₁β_yγ₂, α₁β_yδ, α_6aβ_yγ₂, α_6aβ_yδ. Expression patterns of α_6a and α_6b were opposite, with the former being more abundant in retinas, the latter in brains. Given the stoichiometry α_6wβ_yγ_z, α_6a- or α_6b-containing receptors likely have different regulatory mechanisms. Different gene isoforms could originate after the rounds of genome duplication during teleost evolution. This research depicts that one isoform is generally more abundantly expressed than the other. Such observations also apply to GABA_B receptors, GABA transporters, GABA-related enzymes, CCCs and GABA_A receptor-associated proteins, whose presence further strengthens the proof of a GABA system in zebrafish.

Alternative RNA splicing: contribution to pain and potential therapeutic strategy

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Alternative pre-mRNA splicing generates multiple proteins from a single gene.

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Control of alternative splicing is a likely therapy in cancer and other disorders.

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Key molecules in pain pathways – GPCRs and channels – are alternatively spliced.

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It is proposed that alternative splicing may be a therapeutic target in pain.

Since the sequencing of metazoan genomes began, it has become clear that the number of expressed proteins far exceeds the number of genes. It is now estimated that more than 98% of human genes give rise to multiple proteins through alternative pre-mRNA splicing. In this review, we highlight the known alternative splice variants of many channels, receptors, and growth factors that are important in nociception and pain. Recently, pharmacological control of alternative splicing has been proposed as potential therapy in cancer, wet age-related macular degeneration, retroviral infections, and pain. Thus, we also consider the effects that known splice variants of molecules key to nociception/pain have on nociceptive processing and/or analgesic action, and the potential for control of alternative pre-mRNA splicing as a novel analgesic strategy.

Abnormal subcellular localization of GABAA receptor subunits in schizophrenia brain

Inhibitory neurotransmission is primarily mediated by γ-aminobutyric acid (GABA) activating synaptic GABA type A receptors (GABA(A)R). In schizophrenia, presynaptic GABAergic signaling deficits are among the most replicated findings; however, postsynaptic GABAergic deficits are less well characterized. Our lab has previously demonstrated that although there is no difference in total protein expression of the α1-6, β1-3 or γ2 GABA(A)R subunits in the superior temporal gyrus (STG) in schizophrenia, the α1, β1 and β2 GABA(A)R subunits are abnormally N-glycosylated. N-glycosylation is a posttranslational modification that has important functional roles in protein folding, multimer assembly and forward trafficking. To investigate the impact that altered N-glycosylation has on the assembly and trafficking of GABA(A)Rs in schizophrenia, this study used western blot analysis to measure the expression of α1, α2, β1, β2 and γ2 GABA(A)R subunits in subcellular fractions enriched for endoplasmic reticulum (ER) and synapses (SYN) from STG of schizophrenia (N = 16) and comparison (N = 14) subjects and found evidence of abnormal localization of the β1 and β2 GABA(A)R subunits and subunit isoforms in schizophrenia. The β2 subunit is expressed as three isoforms at 52 kDa (β2(52 kDa)), 50 kDa (β2(50 kDa)) and 48 kDa (β2(48 kDa)). In the ER, we found increased total β2 GABA(A)R subunit (β2(ALL)) expression driven by increased β2(50 kDa), a decreased ratio of β(248 kDa):β2(ALL) and an increased ratio of β2(50 kDa):β2(48 kDa). Decreased ratios of β1:β2(ALL) and β1:β2(50 kDa) in both the ER and SYN fractions and an increased ratio of β2(52 kDa):β(248 kDa) at the synapse were also identified in schizophrenia. Taken together, these findings provide evidence that alterations of N-glycosylation may contribute to GABAergic signaling deficits in schizophrenia by disrupting the assembly and trafficking of GABA(A)Rs.

Eugenol inhibits the GABAA current in trigeminal ganglion neurons

Eugenol has sedative, antioxidant, anti-inflammatory, and analgesic effects, but also serves as an irritant through the regulation of a different set of ion channels. Activation of gamma aminobutyric acid (GABA) receptors on sensory neurons leads to the stabilization of neuronal excitability but contributes to formalin-induced inflammatory pain. In this study, we examined the effect of eugenol on the GABA-induced current in rat trigeminal ganglia (TG) neurons and in human embryonic kidney (HEK) 293 cells expressing the GABAA receptor α1β2γ2 subtype using the whole-cell patch clamp technique. RT-PCR and Western blot analysis were used to confirm the expression of GABAA receptor γ2 subunit mRNA and protein in the TG and hippocampus. Eugenol decreased the amplitude ratio of the GABA-induced current to 27.5 ± 3.2% (p < 0.05) in TG neurons, which recovered after a 3-min washout. In HEK 293 cells expressing the α1β2γ2 subtype, eugenol inhibited GABA-induced currents in a dose-dependent manner. Application of eugenol also decreased the GABA response in the presence of a G-protein blocker. Eugenol pretreatment with different concentrations of GABA resulted in similar inhibition of the GABA-induced current in a non-competitive manner. In conclusion, eugenol inhibits the GABA-induced current in TG neurons and HEK 293 cells expressing the GABAA receptor in a reversible, dose-dependent, and non-competitive manner, but not via the G-protein pathway. We suggest that the GABAA receptor could be a molecular target for eugenol in the modulation of nociceptive information.

Regulation of GABAARs by phosphorylation

γ-Aminobutyric acid type A receptors (GABAARs) are the principal mediators of fast synaptic inhibition in the brain as well as the low persistent extrasynaptic inhibition, both of which are fundamental to proper brain function. Thus unsurprisingly, deficits in GABAARs are implicated in a number of neurological disorders and diseases. The complexity of GABAAR regulation is determined not only by the heterogeneity of these receptors but also by its posttranslational modifications, the foremost, and best characterized of which is phosphorylation. This review will explore the details of this dynamic process, our understanding of which has barely scratched the surface. GABAARs are regulated by a number of kinases and phosphatases, and its phosphorylation plays an important role in governing its trafficking, expression, and interaction partners. Here, we summarize the progress in understanding the role phosphorylation plays in the regulation of GABAARs. This includes how phosphorylation can affect the allosteric modulation of GABAARs, as well as signaling pathways that affect GABAAR phosphorylation. Finally, we discuss the dysregulation of GABAAR phosphorylation and its implication in disease processes.

Neurochemical and behavioral features in genetic absence epilepsy and in acutely induced absence seizures

The absence epilepsy typical electroencephalographic pattern of sharp spikes and slow waves (SWDs) is considered to be due to an interaction of an initiation site in the cortex and a resonant circuit in the thalamus. The hyperpolarization-activated cyclic nucleotide-gated cationic I h pacemaker channels (HCN) play an important role in the enhanced cortical excitability. The role of thalamic HCN in SWD occurrence is less clear. Absence epilepsy in the WAG/Rij strain is accompanied by deficiency of the activity of dopaminergic system, which weakens the formation of an emotional positive state, causes depression-like symptoms, and counteracts learning and memory processes. It also enhances GABAA receptor activity in the striatum, globus pallidus, and reticular thalamic nucleus, causing a rise of SWD activity in the cortico-thalamo-cortical networks. One of the reasons for the occurrence of absences is that several genes coding of GABAA receptors are mutated. The question arises: what the role of DA receptors is. Two mechanisms that cause an infringement of the function of DA receptors in this genetic absence epilepsy model are proposed.

Impaired surface αβγ GABA(A) receptor expression in familial epilepsy due to a GABRG2 frameshift mutation

The purpose of the study was to explore the pathogenic mechanisms underlying generalized epilepsy and febrile seizures plus (GEFS+) in a family with a novel γ2 subunit gene (GABRG2) frameshift mutation. Four affected and one unaffected individuals carried a c.1329delC GABRG2 mutation resulting in a subunit [γ2S(S443delC)] with a modified and elongated carboxy-terminus that is different from that of the wildtype γ2S subunit. We expressed the wildtype γ2S subunit and the predicted mutant γ2S(S443delC) subunit cDNAs in HEK293T cells and performed immunoblotting, flow cytometry and electrophysiology studies. The mutant subunit was translated as a stable protein that was larger than the wildtype γ2S subunit and was retained in the ER and not expressed on the cell surface membrane, suggesting GABRG2 haploinsufficiency. Peak GABA-evoked currents recorded from cells cotransfected with wildtype α1 and β2 subunits and mutant γ2S subunits were significantly decreased and were comparable to α1β2 receptor currents. S443delC is the first GABR epilepsy mutation predicted to abolish the natural stop codon and produce a stop codon in the 3' UTR that leads to translation of an extended peptide. The GEFS+ phenotype observed in this family is likely caused by γ2S subunit loss-of-function and possibly to dominant-negative suppression of α1β2γ2 receptors. Many GABRG2 truncation mutations result in GEFS+, but the spectrum of phenotypic severity is wider, ranging from asymptomatic individuals to the Dravet syndrome. Mechanisms influencing the severity of the phenotype are therefore complex and difficult to correlate with its demonstrable functional effects.

Complex control of GABA(A) receptor subunit mRNA expression: variation, covariation, and genetic regulation

GABA type-A receptors are essential for fast inhibitory neurotransmission and are critical in brain function. Surprisingly, expression of receptor subunits is highly variable among individuals, but the cause and impact of this fluctuation remains unknown. We have studied sources of variation for all 19 receptor subunits using massive expression data sets collected across multiple brain regions and platforms in mice and humans. Expression of Gabra1, Gabra2, Gabrb2, Gabrb3, and Gabrg2 is highly variable and heritable among the large cohort of BXD strains derived from crosses of fully sequenced parents—C57BL/6J and DBA/2J. Genetic control of these subunits is complex and highly dependent on tissue and mRNA region. Remarkably, this high variation is generally not linked to phenotypic differences. The single exception is Gabrb3, a locus that is linked to anxiety. We identified upstream genetic loci that influence subunit expression, including three unlinked regions of chromosome 5 that modulate the expression of nine subunits in hippocampus, and that are also associated with multiple phenotypes. Candidate genes within these loci include, Naaa, Nos1, and Zkscan1. We confirmed a high level of coexpression for subunits comprising the major channel—Gabra1, Gabrb2, and Gabrg2—and identified conserved members of this expression network in mice and humans. Gucy1a3, Gucy1b3, and Lis1 are novel and conserved associates of multiple subunits that are involved in inhibitory signaling. Finally, proximal and distal regions of the 3′ UTRs of single subunits have remarkably independent expression patterns in both species. However, corresponding regions of different subunits often show congruent genetic control and coexpression (proximal-to-proximal or distal-to-distal), even in the absence of sequence homology. Our findings identify novel sources of variation that modulate subunit expression and highlight the extraordinary capacity of biological networks to buffer 4–100 fold differences in mRNA levels.

Flavonoids as GABAA receptor ligands: the whole story?

Benzodiazepines are the most widely prescribed class of psychoactive drugs in current therapeutic use, despite the important unwanted side effects that they produce, such as sedation, myorelaxation, ataxia, amnesia, and ethanol and barbiturate potentiation and tolerance. They exert their therapeutic effects via binding to the benzodiazepine binding site of gamma-aminobutyric acid (GABA) type A receptors, and allosterically modulating the chloride flux through the ion channel complex. First isolated from plants used as tranquilizers in folkloric medicine, some natural flavonoids have been shown to possess selective affinity for the benzodiazepine binding site with a broad spectrum of central nervous system effects. Since the initial search for alternative benzodiazepine ligands amongst the flavonoids, a list of successful synthetic derivatives has been generated with enhanced activities. This review provides an update on research developments that have established the activity of natural and synthetic flavonoids on GABA type A receptors. Flavonoids are prominent drugs in the treatment of mental disorders, and can also be used as tools to study modulatory sites at GABA type A receptors and to develop GABA type A selective agents further.

The dynamic modulation of GABA(A) receptor trafficking and its role in regulating the plasticity of inhibitory synapses

Inhibition in the adult mammalian central nervous system (CNS) is mediated by γ-aminobutyric acid (GABA). The fast inhibitory actions of GABA are mediated by GABA type A receptors (GABA(A)Rs); they mediate both phasic and tonic inhibition in the brain and are the principle sites of action for anticonvulsant, anxiolytic, and sedative-hypnotic agents that include benzodiazepines, barbiturates, neurosteroids, and some general anesthetics. GABA(A)Rs are heteropentameric ligand-gated ion channels that are found concentrated at inhibitory postsynaptic sites where they mediate phasic inhibition and at extrasynaptic sites where they mediate tonic inhibition. The efficacy of inhibition and thus neuronal excitability is critically dependent on the accumulation of specific GABA(A)R subtypes at inhibitory synapses. Here we evaluate how neurons control the number of GABA(A)Rs on the neuronal plasma membrane together with their selective stabilization at synaptic sites. We then go on to examine the impact that these processes have on the strength of synaptic inhibition and behavior.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

The effects of midazolam and sevoflurane on the GABA(A) receptors with alternatively spliced variants of the γ2 subunit

Background

Emergence agitation after sevoflurane anesthesia in children can be prevented by midazolam. Alternative splicing of the GABA_A receptor changes with age. Therefore, we hypothesized that alternative splicing of the γ2 subunit affects the GABA current when applying sevoflurane and midazolam.

Methods

We performed the whole-cell patch clamp technique on human embryonic kidney 293 cells that were transfected with α1β2γ2L or α1β2γ2S. The concentration-response relations were recorded for midazolam and sevoflurane, and the co-application responses were measured at concentrations of 1.5 nM, 15 nM and 300 nM of midazolam and 0.5%, 2.0% and 4.0% of sevoflurane. Each GABA current was compared with that produced by 5 µM of GABA.

Results

The concentration-response relationships for midazolam and sevoflurane were dose-dependent without any differences between the α1β2γ2L and α1β2γ2S subtypes. 1.5 nM and 15 nM of midazolam did not significantly enhance the current after treatment with 0.5% sevoflurane for both subtypes. The current after treatment with 2.0% sevoflurane was enhanced by 1.5 nM midazolam for the α1β2γ2S subtype, but not for the α1β2γ2L subtype. In the case of 2.0% sevoflurane with 15 nM of midazolam, and 4.0% sevoflurane with 300 nM of midazolam, the GABA currents were significantly enhanced for both subtypes.

Conclusions

These results show that the difference in the γ2 subunit cannot explain the emergence agitation after sevoflurane anesthesia in children in vitro. This suggests that co-application of sevoflurane and midazolam enhances the GABA current according to the alternative splicing of the γ2 subunit and the concentration of both drugs.

The short splice variant of the gamma 2 subunit acts as an external modulator of GABA(A) receptor function

GABA(A) receptors (GABA(A)Rs) regulate the majority of fast inhibition in the mammalian brain and are the target for multiple drug types, including sleep aids, anti-anxiety medication, anesthetics, alcohol, and neurosteroids. A variety of subunits, including the highly distributed gamma2, allow for pharmacologic and kinetic differences in particular brain regions. The two common splice variants gamma2S (short) and gamma2L (long) show different patterns of regional distribution both in adult brain and during the course of development, but show few notable differences when incorporated into pentameric receptors. However, results presented here show that the gamma2S variant can strongly affect both GABA(A)R pharmacology and kinetics by acting as an external modulator of fully formed receptors. Mutation of one serine residue can confer gamma2S-like properties to gamma2L subunits, and addition of a modified gamma2 N-terminal polypeptide to the cell surface recapitulates the pharmacological effect. Thus, rather than incorporation of a separate accessory protein as with voltage-gated channels, this is an example of an ion channel using a common subunit for dual purposes. The modified receptor properties conferred by accessory gamma2S have implications for understanding GABA(A)R pharmacology, receptor kinetics, stoichiometry, GABAergic signaling in the brain during development, and altered function in disease states such as epilepsy.

Genetic disruption of the autism spectrum disorder risk gene PLAUR induces GABAA receptor subunit changes

Disruption of the GABAergic system has been implicated in multiple developmental disorders, including epilepsy, autism spectrum disorder and schizophrenia. The human gene encoding uPAR (PLAUR) has been shown recently to be associated with the risk of autism. The uPAR(-/-) mouse exhibits a regionally-selective reduction in GABAergic interneurons in frontal and parietal regions of the cerebral cortex as well as in the CA1 and dentate gyrus subfields of the hippocampus. Behaviorally, these mice exhibit increased sensitivity to pharmacologically-induced seizures, heightened anxiety, and atypical social behavior. Here, we explore potential alterations in GABAergic circuitry that may occur in the context of altered interneuron development. Analysis of gene expression for 13 GABA(A) receptor subunits using quantitative real-time polymerase chain reaction (PCR) indicates seven subunit mRNAs (alpha(1), alpha(2), alpha(3), beta(2), beta(3), gamma(2S) and gamma(2L)) of interest. Semi-quantitative in situ hybridization analysis focusing on these subunit mRNAs reveals a complex pattern of potential gene regulatory adaptations. The levels of alpha(2) subunit mRNAs increase in frontal cortex, CA1 and CA3, while those of alpha3 decrease in frontal cortex and CA1. In contrast, alpha(1) subunit mRNAs are unaltered in any region examined. beta(2) subunit mRNAs are increased in frontal cortex whereas beta(3) subunit mRNAs are decreased in parietal cortex. Finally, gamma(2S) subunit mRNAs are increased in parietal cortex while gamma(2L) subunit mRNAs are increased in the dentate gyrus, potentially altering the gamma(2S):gamma(2L) ratio in these two regions. For all subunits, no changes were observed in forebrain regions where GABAergic interneuron numbers are normal. We propose that disrupted differentiation of GABAergic neurons specifically in frontal and parietal cortices leads to regionally-selective alterations in local circuitry and subsequent adaptive changes in receptor subunit composition. Future electrophysiological studies will be useful in determining how alterations in network activity in the cortex and hippocampus relate to the observed behavioral phenotype.

Associated proteins: The universal toolbox controlling ligand gated ion channel function

Ligand gated ion channels are integral multimeric membrane proteins that can detect with high sensitivity the presence of a specific transmitter in the extracellular space and transduce this signal into an ion flux. While these receptors are widely expressed in the nervous system, their expression is not limited to neurons or their postsynaptic targets but extends to non-neuronal cells where they participate in many physiological responses. Cells have developed complex regulatory mechanisms allowing for the precise control and modulation of ligand gated ion channels. In this overview the roles of accessory subunits and associated proteins in these regulatory mechanisms are reviewed and their relevance illustrated by examples at different ligand gated ion channel types, with emphasis on nicotinic acetylcholine receptors. Dysfunction of ligand gated ion channels can result in neuromuscular, neurological or psychiatric disorders. A better understanding of the precise function of associated proteins and how they impact on ligand gated ion channels will provide new therapeutic opportunities for clinical intervention.

CaMKII phosphorylation of the GABA(A) receptor: receptor subtype- and synapse-specific modulation

As a major inhibitory neurotransmitter, GABA plays a vital role in the brain by controlling the extent of neuronal excitation. This widespread role is reflected by the ubiquitous distribution of GABA(A) receptors throughout the central nervous system. To regulate the level of neuronal inhibition requires some endogenous control over the release of GABA and/or its postsynaptic response. In this context, Ca(2+) ions are often used as primary or secondary messengers frequently resulting in the activation of protein kinases and phosphatases. One such kinase, Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), can target the GABA(A) receptor to cause its phosphorylation. Evidence is now emerging, which is reviewed here, that GABA(A) receptors are indeed substrates for CaMKII and that this covalent modification alters the expression of cell surface receptors and their function. This type of regulation can also feature at inhibitory synapses leading to long-term inhibitory synaptic plasticity. Most recently, CaMKII has now been proposed to differentially phosphorylate particular isoforms of GABA(A) receptors in a synapse-specific context.

Polymorphism in cytochrome P450 2E1 and interaction with other genetic risk factors and susceptibility to alcoholic liver cirrhosis

The association of polymorphism in cytochrome P450 2E1 (CYP2E1), the major microsomal ethanol metabolizing enzyme and its interaction with genes, involved in detoxification of reactive oxygen species, such as glutathione-S-transferases M1 (GSTM1) and alcohol intake, gamma-aminobutyric acid receptor gamma2 (GABRG2) was studied with the risk to alcoholic cirrhosis in a case-control study. A total of 160 alcoholic cirrhotic and 125 non-alcoholic cirrhotic cases, visiting the OPD facility of Gastroenterology Department of Sanjay Gandhi Post Graduate Institute of Medical Sciences (SGPGI), Lucknow, India and 250 non-alcoholic and 100 alcoholic controls having no evidence of liver disease were included in the study. PCR-based RFLP methodology was followed for genotyping studies. Our data revealed that the variant genotypes of CYP2E1 5B exhibited significant association with the alcoholic liver cirrhosis when compared to non-alcoholic controls (OR: 4.3; 95%CI: 1.5-12.4; p: 0.003) or non-alcoholic cirrhosis patients (OR: 5.4; 95%CI: 1.2-24.5; p: 0.01) or alcoholic controls (OR: 4.3; 95%CI: 0.95-19.62; p: 0.04). Haplotype approach revealed that haplotype T-A-T was found to be associated with more than 5-fold increase in risk for alcoholic cirrhosis. Likewise, combination of variant genotype of CYP2E1 5B with null genotype of GSTM1, a phase II detoxification enzyme, resulted in several fold increase in risk in alcoholic cirrhotic patients when compared with non-alcoholic controls or non-alcoholic cirrhotic patients. Further, the combination of variant genotype of CYP2E1 5B with GABRG2, significantly increased the risk upto 6.5-fold in alcoholic cirrhotic patients when compared with non-alcoholic controls thereby suggesting the role of gene-gene interaction in alcoholic cirrhosis.