Assembly of GABAAreceptors (Review)

Modular Structure and Polymerization Status of GABAA Receptors Illustrated with EM Analysis and AlphaFold2 Prediction

Type-A γ-aminobutyric acid (GABAA) receptors are channel proteins crucial to mediating neuronal balance in the central nervous system (CNS). The structure of GABAA receptors allows for multiple binding sites and is key to drug development. Yet the formation mechanism of the receptor's distinctive pentameric structure is still unknown. This study aims to investigate the role of three predominant subunits of the human GABAA receptor in the formation of protein pentamers. Through purifying and refolding the protein fragments of the GABAA receptor α1, β2, and γ2 subunits, the particle structures were visualised with negative staining electron microscopy (EM). To aid the analysis, AlphaFold2 was used to compare the structures. Results show that α1 and β2 subunit fragments successfully formed homo-oligomers, particularly homopentameric structures, while the predominant heteropentameric GABAA receptor was also replicated through the combination of the three subunits. However, homopentameric structures were not observed with the γ2 subunit proteins. A comparison of the AlphaFold2 predictions and the previously obtained cryo-EM structures presents new insights into the subunits' modular structure and polymerization status. By performing experimental and computational studies, a deeper understanding of the complex structure of GABAA receptors is provided. Hopefully, this study can pave the way to developing novel therapeutics for neuropsychiatric diseases.

Gender-related variation expressions of neuroplastin TRAF6, GluA1, GABA(A) receptor, and PMCA in cortex, hippocampus, and brainstem in an experimental epilepsy model

Epileptic seizures are seen as a result of changing excitability balance depending on the deterioration in synaptic plasticity in the brain. Neuroplastin, and its related molecules which are known to play a role in synaptic plasticity, neurotransmitter activities that provide balance of excitability and, different neurological diseases, have not been studied before in epilepsy. In this study, a total of 34 Sprague-Dawley male and female rats, 2 months old, weighing 250-300 g were used. The epilepsy model in rats was made via pentylenetetrazole (PTZ). After the completion of the experimental procedure, the brain tissue of the rats were taken and the histopathological changes in the hippocampus and cortex parts and the brain stem were investigated, as well as the immunoreactivity of the proteins related to the immunohistochemical methods. As a result of the histopathological evaluation, it was determined that neuron degeneration and the number of dilated blood vessels in the hippocampus, frontal cortex, and brain stem were higher in the PTZ status epilepticus (SE) groups than in the control groups. It was observed that neuroplastin and related proteins TNF receptor-associated factor 6 (TRAF6), Gamma amino butyric acid type A receptors [(GABA(A)], and plasma membrane Ca2+ ATPase (PMCA) protein immunoreactivity levels increased especially in the male hippocampus, and only AMPA receptor subunit type 1 (GluA1) immunoreactivity decreased, unlike other proteins. We believe this may be caused by a problem in the mechanisms regulating the interaction of neuroplastin and GluA1 and may cause problems in synaptic plasticity in the experimental epilepsy model. It may be useful to elucidate this mechanism and target GluA1 when determining treatment strategies.

Hormonal Changes in Women with Epilepsy

Epilepsy is a prevalent neurological disorder among women globally, often requiring long-term treatment. Hormonal fluctuations in women with epilepsy (WWE) can have reciprocal effects on epilepsy and antiseizure medications (ASMs), posing significant challenges for WWE. Notably, WWE commonly experience endocrine alterations such as thyroid dysfunctions, low bone metabolism, and reproductive hormone irregularities. On the one hand, the presence of hormones in women with epilepsy affects their susceptibility to epilepsy as well as the metabolism of antiseizure medications in various ways. On the other hand, epilepsy itself and the use of antiseizure medications impact the production, secretion, and metabolism of hormones, resulting in low fertility, increased risk of pregnancy complications, negative offspring outcomes, and so on. In order to develop more precise treatment strategies in the future, it is necessary to comprehend the explicit relationships between hormones, epilepsy, and antiseizure medications, as well as to elucidate the currently known mechanisms underlying these interactions.

Navigating the complex landscape of benzodiazepine- and Z-drug diversity: insights from comprehensive FDA adverse event reporting system analysis and beyond

Introduction

Medications which target benzodiazepine (BZD) binding sites of GABAA receptors (GABAARs) have been in widespread use since the nineteen-sixties. They carry labels as anxiolytics, hypnotics or antiepileptics. All benzodiazepines and several nonbenzodiazepine Z-drugs share high affinity binding sites on certain subtypes of GABAA receptors, from which they can be displaced by the clinically used antagonist flumazenil. Additional binding sites exist and overlap in part with sites used by some general anaesthetics and barbiturates. Despite substantial preclinical efforts, it remains unclear which receptor subtypes and ligand features mediate individual drug effects. There is a paucity of literature comparing clinically observed adverse effect liabilities across substances in methodologically coherent ways.

Methods

In order to examine heterogeneity in clinical outcome, we screened the publicly available U.S. FDA adverse event reporting system (FAERS) database for reports of individual compounds and analyzed them for each sex individually with the use of disproportionality analysis. The complementary use of physico-chemical descriptors provides a molecular basis for the analysis of clinical observations of wanted and unwanted drug effects.

Results and Discussion

We found a multifaceted FAERS picture, and suggest that more thorough clinical and pharmacoepidemiologic investigations of the heterogenous side effect profiles for benzodiazepines and Z-drugs are needed. This may lead to more differentiated safety profiles and prescription practice for particular compounds, which in turn could potentially ease side effect burden in everyday clinical practice considerably. From both preclinical literature and pharmacovigilance data, there is converging evidence that this very large class of psychoactive molecules displays a broad range of distinctive unwanted effect profiles - too broad to be explained by the four canonical, so-called "diazepam-sensitive high-affinity interaction sites". The substance-specific signatures of compound effects may partly be mediated by phenomena such as occupancy of additional binding sites, and/or synergistic interactions with endogenous substances like steroids and endocannabinoids. These in turn drive the wanted and unwanted effects and sex differences of individual compounds.

Fundamental Neurochemistry Review: GABAA receptor neurotransmission and epilepsy: Principles, disease mechanisms and pharmacotherapy

Epilepsy is a common neurological disorder associated with alterations of excitation-inhibition balance within brain neuronal networks. GABA_A receptor neurotransmission is the most prevalent form of inhibitory neurotransmission and is strongly implicated in both the pathophysiology and treatment of epilepsy, serving as a primary target for antiseizure medications for over a century. It is now established that GABA exerts a multifaceted influence through an array of GABA_A receptor subtypes that extends far beyond simply negating excitatory activity. As the role of GABA_A neurotransmission within inhibitory circuits is elaborated, this will enable the development of precision therapies that correct the network dysfunction underlying epileptic pathology.

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.

Molecular mechanisms of sex differences in epilepsy and seizure susceptibility in chemical, genetic and acquired epileptogenesis

This article provides a succinct overview of sex differences in epilepsy and putative molecular mechanisms underlying sex differences in seizure susceptibility in chemical, genetic, and acquired epileptogenesis. The susceptibility to excitability episodes and occurrence of epileptic seizures are generally higher in men than women. The precise molecular mechanisms remain unclear, but differences in regional morphology and neural circuits in men and women may explain differential vulnerability to seizures and epileptogenic cascades. Changes in seizure sensitivity can be attributed to steroid hormones, including fluctuations in neurosteroids as well as neuroplasticity in their receptor signaling systems. Other potential neurobiological bases for sex differences in epilepsies include differences in brain development, neurogenesis, neuronal chloride homeostasis, and neurotrophic and glial responses. In catamenial epilepsy, a gender-specific neuroendocrine condition, epileptic seizures are most often clustered around a specific menstrual period in adult women. A deeper understanding of the molecular and neural network basis of sex differences in seizures and response to antiepileptic drugs is highly warranted for designing effective, sex-specific therapies for epilepsy, epileptogenesis, and seizure disorders.

The Emerging Roles of π Subunit-Containing GABAA Receptors in Different Cancers

Cancer is one of the leading causes of death in both developed and developing countries. Due to its heterogenous nature, it occurs in various regions of the body and often goes undetected until later stages of disease progression. Feasible treatment options are limited because of the invasive nature of cancer and often result in detrimental side-effects and poor survival rates. Therefore, recent studies have attempted to identify aberrant expression levels of previously undiscovered proteins in cancer, with the hope of developing better diagnostic tools and pharmaceutical options. One class of such targets is the π-subunit-containing γ-aminobutyric acid type A receptors. Although these receptors were discovered more than 20 years ago, there is limited information available. They possess atypical functional properties and are expressed in several non-neuronal tissues. Prior studies have highlighted the role of these receptors in the female reproductive system. New research focusing on the higher expression levels of these receptors in ovarian, breast, gastric, cervical, and pancreatic cancers, their physiological function in healthy individuals, and their pro-tumorigenic effects in these cancer types is reviewed here.

Anterograde trafficking signals in GABAA subunits are required for functional expression

Pentameric GABA_A receptors are composed from 19 possible subunits. The GABA_A β subunit is unique because the β₁ and β₃ subunits can assemble and traffic to the cell surface as homomers, whereas most of the other subunits, including β₂, are heteromers. The intracellular domain (ICD) of the GABA_A subunits has been implicated in targeting and clustering GABA_A receptors at the plasma membrane. Here, we sought to test whether and how the ICD is involved in functional expression of the β₃ subunit. Since θ is the most homologous to β but does not form homomers, we created two reciprocal chimeric subunits, swapping the ICD between the β₃ and θ subunits, and expressed them in HEK293 cells. Surface expression was detected with immunofluorescence and functional expression was quantified using whole-cell patch-clamp recording with fast perfusion. Results indicate that, unlike β₃, neither the β₃/θ_IC nor the θ/β_3IC chimera can traffic to the plasma membrane when expressed alone; however, when expressed in combination with either wild-type α₃ or β₃, the β₃/θ_IC chimera was functionally expressed. This suggests that the ICD of α₃ and β₃ each contain essential anterograde trafficking signals that are required to overcome ER retention of assembled GABA_A homo- or heteropentamers.

Pathogenic Variants in STXBP1 and in Genes for GABAa Receptor Subunities Cause Atypical Rett/Rett-like Phenotypes

Rett syndrome (RTT) is a neurodevelopmental disorder, affecting 1 in 10,000 girls. Intellectual disability, loss of speech and hand skills with stereotypies, seizures and ataxia are recurrent features. Stringent diagnostic criteria distinguish classical Rett, caused by a MECP2 pathogenic variant in 95% of cases, from atypical girls, 40-73% carrying MECP2 variants, and rarely CDKL5 and FOXG1 alterations. A large fraction of atypical and RTT-like patients remain without genetic cause. Next Generation Sequencing (NGS) targeted to multigene panels/Whole Exome Sequencing (WES) in 137 girls suspected for RTT led to the identification of a de novo variant in STXBP1 gene in four atypical RTT and two RTT-like girls. De novo pathogenic variants-one in GABRB2 and, for first time, one in GABRG2-were disclosed in classic and atypical RTT patients. Interestingly, the GABRG2 variant occurred at low rate percentage in blood and buccal swabs, reinforcing the relevance of mosaicism in neurological disorders. We confirm the role of STXBP1 in atypical RTT/RTT-like patients if early psychomotor delay and epilepsy before 2 years of age are observed, indicating its inclusion in the RTT diagnostic panel. Lastly, we report pathogenic variants in Gamma-aminobutyric acid-A (GABAa) receptors as a cause of atypical/classic RTT phenotype, in accordance with the deregulation of GABAergic pathway observed in MECP2 defective in vitro and in vivo models.

Cryo-EM structure of the benzodiazepine-sensitive α1β1γ2S tri-heteromeric GABAA receptor in complex with GABA

Fast inhibitory neurotransmission in the mammalian nervous system is largely mediated by GABA_A receptors, chloride-selective members of the superfamily of pentameric Cys-loop receptors. Native GABA_A receptors are heteromeric assemblies sensitive to many important drugs, from sedatives to anesthetics and anticonvulsant agents, with mutant forms of GABA_A receptors implicated in multiple neurological diseases. Despite the profound importance of heteromeric GABA_A receptors in neuroscience and medicine, they have proven recalcitrant to structure determination. Here we present the structure of a tri-heteromeric α1β1γ2S_EM GABA_A receptor in complex with GABA, determined by single particle cryo-EM at 3.1–3.8 Å resolution, elucidating molecular principles of receptor assembly and agonist binding. Remarkable N-linked glycosylation on the α1 subunit occludes the extracellular vestibule of the ion channel and is poised to modulate receptor assembly and perhaps ion channel gating. Our work provides a pathway to structural studies of heteromeric GABA_A receptors and a framework for rational design of novel therapeutic agents.

Cell surface expression of homomeric GABAA receptors depends on single residues in subunit transmembrane domains

Cell surface expression of type A GABA receptors (GABA_ARs) is a critical determinant of the efficacy of inhibitory neurotransmission. Pentameric GABA_ARs are assembled from a large pool of subunits according to precise co-assembly rules that limit the extent of receptor structural diversity. These rules ensure that particular subunits, such as ρ1 and β3, form functional cell surface ion channels when expressed alone in heterologous systems, whereas other brain-abundant subunits, such as α and γ, are retained within intracellular compartments. Why some of the most abundant GABA_AR subunits fail to form homomeric ion channels is unknown. Normally, surface expression of α and γ subunits requires co-assembly with β subunits via interactions between their N-terminal sequences in the endoplasmic reticulum. Here, using molecular biology, imaging, and electrophysiology with GABA_AR chimeras, we have identified two critical residues in the transmembrane domains of α and γ subunits, which, when substituted for their ρ1 counterparts, permit cell surface expression as homomers. Consistent with this, substitution of the ρ1 transmembrane residues for the α subunit equivalents reduced surface expression and altered channel gating, highlighting their importance for GABA_AR trafficking and signaling. Although not ligand-gated, the formation of α and γ homomeric ion channels at the cell surface was revealed by incorporating a mutation that imparts the functional signature of spontaneous channel activity. Our study identifies two single transmembrane residues that enable homomeric GABA_AR subunit cell surface trafficking and demonstrates that α and γ subunits can form functional ion channels.

Expanding the phenotypic spectrum of GABRG2 variants: a recurrent GABRG2 missense variant associated with a severe phenotype

Pathogenic missense and truncating variants in the GABRG2 gene cause a spectrum of epilepsies, from Dravet syndrome to milder simple febrile seizures. In most cases, pathogenic missense variants in the GABRG2 gene segregate with a febrile seizure phenotype. In this case series, we report a recurrent, de novo missense variant (c0.316 G > A; p.A106T) in the GABRG2 gene that was identified in five unrelated individuals. These patients were described to have a more severe phenotype than previously reported for GABRG2 missense variants. Common features include variable early-onset seizures, significant motor and speech delays, intellectual disability, hypotonia, movement disorder, dysmorphic features and vision/ocular issues. Our report further explores a recurrent pathogenic missense variant within the GABRG2 variant family and broadens the spectrum of associated phenotypes for GABRG2-associated disorders.

Isolation, purification, and partial characterization of a membrane-bound Cl-/HCO3--activated ATPase complex from rat brain with sensitivity to GABAAergic ligands

This study describes the isolation and purification of a protein complex with [Formula: see text]-ATPase activity and sensitivity to GABA_Aergic ligands from rat brain plasma membranes. The ATPase complex was enriched using size-exclusion, affinity, and ion-exchange chromatography. The fractions obtained at each purification step were subjected to SDS-polyacrylamide gel electrophoresis (SDS-PAGE), which revealed four subunits with molecular mass ∼48, 52, 56, and 59 kDa; these were retained at all stages of the purification process. Autoradiography revealed that the ∼52 and 56 kDa subunits could bind [³H]muscimol. The [Formula: see text]-ATPase activity of this enriched protein complex was regulated by GABA_Aergic ligands but was not sensitive to blockers of the NKCC or KCC cotransporters.

Ionotropic GABA receptor antagonism-induced adverse outcome pathways for potential neurotoxicity biomarkers

Antagonism of ionotropic GABA receptors (iGABARs) can occur at three distinct types of receptor binding sites causing chemically induced epileptic seizures. Here we review three adverse outcome pathways, each characterized by a specific molecular initiating event where an antagonist competitively binds to active sites, negatively modulates allosteric sites or noncompetitively blocks ion channel on the iGABAR. This leads to decreased chloride conductance, followed by depolarization of affected neurons, epilepsy-related death and ultimately decreased population. Supporting evidence for causal linkages from the molecular to population levels is presented and differential sensitivity to iGABAR antagonists in different GABA receptors and organisms discussed. Adverse outcome pathways are poised to become important tools for linking mechanism-based biomarkers to regulated outcomes in next-generation risk assessment.

Assembly, trafficking and function of α1β2γ2 GABAA receptors are regulated by N-terminal regions, in a subunit-specific manner

GABAA receptors are pentameric ligand-gated ion channels that mediate inhibitory fast synaptic transmission in the central nervous system. Consistent with recent pentameric ligand-gated ion channels structures, sequence analysis predicts an α-helix near the N-terminus of each GABAA receptor subunit. Preceding each α-helix are 8-36 additional residues, which we term the N-terminal extension. In homomeric GABAC receptors and nicotinic acetylcholine receptors, the N-terminal α-helix is functionally essential. Here, we determined the role of the N-terminal extension and putative α-helix in heteromeric α1β2γ2 GABAA receptors. This role was most prominent in the α1 subunit, with deletion of the N-terminal extension or further deletion of the putative α-helix both dramatically reduced the number of functional receptors at the cell surface. Conversely, deletion of the β2 or γ2 N-terminal extension had little effect on the number of functional cell surface receptors. Additional deletion of the putative α-helix in the β2 or γ2 subunits did, however, decrease both functional cell surface receptors and incorporation of the γ2 subunit into mature receptors. In the β2 subunit only, α-helix deletions affected GABA sensitivity and desensitization. Our findings demonstrate that N-terminal extensions and α-helices make key subunit-specific contributions to assembly, consistent with both regions being involved in inter-subunit interactions. N-terminal α-helices and preceding sequences of eukaryotic pentameric ligand-gated ion channels are absent in prokaryotic homologues, suggesting they may not be functionally essential. Here, we show that in heteropentameric α1β2γ2 GABAA receptors, the role of these segments is highly subunit dependent. The extension preceding the α-helix in the α subunit is crucial for assembly and trafficking, but is of little importance in β and γ subunits. Indeed, robust receptor levels remain when the extension and α-helix are removed in β or γ subunits.

Midazolam as an anticonvulsant antidote for organophosphate intoxication--A pharmacotherapeutic appraisal

OBJECTIVE

This review summarizes the therapeutic potential of midazolam as an anticonvulsant antidote for organophosphate (OP) intoxication.

METHODS

Benzodiazepines are widely used to treat acute seizures and status epilepticus (SE), a neurologic emergency of persistent seizures that can lead to severe neuronal damage or death. Midazolam is a benzodiazepine hypnotic with a rapid onset and short duration of action.

RESULTS

Midazolam is considered the new drug of choice for persistent acute seizures and SE, including those caused by neurotoxic OPs and nerve agents. Midazolam is a positive allosteric modulator of synaptic γ-aminobutyric acid (GABA)A receptors in the brain. It potentiates GABAergic inhibition and thereby controls hyperexcitability and seizures. Midazolam is administered intravenously or intramuscularly to control acute seizures and SE. Due to its favorable pharmacokinetic features, midazolam is being considered as a replacement anticonvulsant for diazepam in the antidote kit for nerve agents. Clinical studies such as the recent Rapid Anticonvulsant Medication Prior to Arrival Trial (RAMPART) trial have confirmed the anticonvulsant efficacy of midazolam in SE in prehospital settings.

SIGNIFICANCE

In experimental models, midazolam is effective when given at the onset of seizures caused by nerve agents. However, benzodiazepines are less effective at terminating seizures when given 30 min or later after OP exposure or seizure onset, likely because of internalization or downregulation of synaptic, but not extrasynaptic, GABAA receptors, which can lead to diminished potency and seizure recurrence.

Antiseizure Activity of Midazolam in Mice Lacking δ-Subunit Extrasynaptic GABA(A) Receptors

Midazolam is a benzodiazepine anticonvulsant with rapid onset and short duration of action. Midazolam is the current drug of choice for acute seizures and status epilepticus, including those caused by organophosphate nerve agents. The antiseizure activity of midazolam is thought to result from its allosteric potentiation of synaptic GABA(A) receptors in the brain. However, there are indications that benzodiazepines promote neurosteroid synthesis via the 18-kDa cholesterol transporter protein (TSPO). Therefore, we investigated the role of neurosteroids and their extrasynaptic GABA(A) receptor targets in the antiseizure activity of midazolam. Here, we used δ-subunit knockout (DKO) mice bearing a targeted deletion of the extrasynaptic receptors to investigate the contribution of the extrasynaptic receptors to the antiseizure activity of midazolam using the 6-Hz and hippocampus kindling seizure models. In both models, midazolam produced rapid and dose-dependent protection against seizures (ED50, 0.4 mg/kg). Moreover, the antiseizure potency of midazolam was undiminished in DKO mice compared with control mice. Pretreatment with PK11195 [1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide], a TSPO blocker, or finasteride, a 5α-reductase neurosteroid inhibitor, did not affect the antiseizure effect of midazolam. The antiseizure activity of midazolam was significantly reversed by pretreatment with flumazenil, a benzodiazepine antagonist. Plasma and brain levels of the neurosteroid allopregnanolone were not significantly greater in midazolam-treated animals. These studies therefore provide strong evidence that neurosteroids and extrasynaptic GABA(A) receptors are not involved in the antiseizure activity of midazolam, which mainly occurs through synaptic GABA(A) receptors via direct binding to benzodiazepine sites. This study reaffirms midazolam's use for controlling acute seizures and status epilepticus.

Extended access oxycodone self-administration and neurotransmitter receptor gene expression in the dorsal striatum of adult C57BL/6 J mice

RATIONALE

Although non-medical use of oxycodone continues to be a growing problem in the United States, there are no animal studies examining the effects of long-term oxycodone self-administration (SA).

OBJECTIVES

The current study was designed to examine chronic oxycodone SA by mice (14 days), in novel extended (4 h) SA sessions and its effect on selective striatal neurotransmitter receptor mRNA expression.

METHODS

Adult male C57/BL6J mice were either allowed to self-administer oxycodone (0.25 mg/kg/infusion, FR1) or served as yoked-saline controls in an extended access paradigm. Mice self-administered oxycodone for 4 h/day for 14 consecutive days. Comparison groups with 14-days exposure to 1-h SA sessions were also studied. Within 1 h of the last extended SA session, mice were sacrificed, dorsal striatum was isolated and selective neurotransmitter receptor mRNA levels were examined.

RESULTS

The oxycodone groups poked the active hole significantly more times than the yoked controls. The number of nose pokes at the active hole rose over the 14 days in the oxycodone group with extended access. The expression of 13 neurotransmitter receptor mRNAs was significantly altered in the dorsal striatum, including the gamma-aminobutyric acid (GABA) A receptor beta 2 subunit (Gabrb2) showing experiment-wise significant decrease, as a result of extended oxycodone SA.

CONCLUSION

C57BL/6 J mice escalated the amount of oxycodone self-administered across 14 consecutive daily extended sessions, but not 1-h sessions. Decreases in Gabrb2 mRNA levels may underlie escalation of oxycodone intake in the extended access SA sessions.

Molecular dynamics investigation of Cl(-) transport through the closed and open states of the 2α12β2γ2 GABA(A) receptor

The α1β2γ2 gamma-aminobutyric type A receptor (GABA(A)R) is one of the most widely expressed GABA(A)R subtypes in the mammalian brain. GABA(A)Rsbelonging to the Cys-loop superfamily of ligand-gated ion channels have been identified as key targets for many clinical drugs, and the motions that govern the gating mechanism are still not well understood. In this study, an open-state GABA(A)R was constructed using the structure of the glutamate-gated chloride channel (GluCl), which has a high sequence identity to GABA(A)R. A closed-state model was constructed using the structure of the nicotinic acetylcholine receptor (nAChR). Molecular dynamics simulations of the open-state and closed-state GABA(A)R were performed. We calculated the electrostatic potential of the two conformations, the pore radius of the two ion channels and the root-mean-square fluctuation. We observed the presence of two positively charged girdles around the ion channel and found flexible regions in the GABA(A)R. Then, the free-energy of chloride ion permeations through the closed-state and open-state G GABA(A)R has been estimated using adaptive biasing force (ABF) simulation. For the closed-state G GABA(A)R, we observed two major energy barriers for chloride ion translocation in the transmembrane domain (TMD). For the open-state GABA(A)R, there was only one energy barrier formed by two Thr261 (α1), two Thr255 (β2) and one Thr271 (γ2). By using ABF simulation, the overall free-energy profile is obtained for Cl(-) transporting through GABA(A)R, which gives a complete map of the ion channel of Cl(-) permeation.

Gephyrin, the enigmatic organizer at GABAergic synapses

GABA_A receptors are clustered at synaptic sites to achieve a high density of postsynaptic receptors opposite the input axonal terminals. This allows for an efficient propagation of GABA mediated signals, which mostly result in neuronal inhibition. A key organizer for inhibitory synaptic receptors is the 93 kDa protein gephyrin that forms oligomeric superstructures beneath the synaptic area. Gephyrin has long been known to be directly associated with glycine receptor β subunits that mediate synaptic inhibition in the spinal cord. Recently, synaptic GABA_A receptors have also been shown to directly interact with gephyrin and interaction sites have been identified and mapped within the intracellular loops of the GABA_A receptor α1, α2, and α3 subunits. Gephyrin-binding to GABA_A receptors seems to be at least one order of magnitude weaker than to glycine receptors (GlyRs) and most probably is regulated by phosphorylation. Gephyrin not only has a structural function at synaptic sites, but also plays a crucial role in synaptic dynamics and is a platform for multiple protein-protein interactions, bringing receptors, cytoskeletal proteins and downstream signaling proteins into close spatial proximity.

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.

New insights into the GABA(A) receptor structure and orthosteric ligand binding: receptor modeling guided by experimental data

GABA(A) receptors (GABA(A)Rs) are ligand gated chloride ion channels that mediate overall inhibitory signaling in the CNS. A detailed understanding of their structure is important to gain insights in, e.g., ligand binding and functional properties of this pharmaceutically important target. Homology modeling is a necessary tool in this regard because experimentally determined structures are lacking. Here we present an exhaustive approach for creating a high quality model of the α(1)β(2)γ(2) subtype of the GABA(A)R ligand binding domain, and we demonstrate its usefulness in understanding details of orthosteric ligand binding. The model was constructed by using multiple templates and by incorporation of knowledge from biochemical/pharmacological experiments. It was validated on the basis of objective energy functions, its ability to account for available residue specific information, and its stability in molecular dynamics (MD) compared with that of the two homologous crystal structures. We then combined the model with extensive structure-activity relationships available from two homologous series of orthosteric GABA(A)R antagonists to create a detailed hypothesis for their binding modes. Excellent agreement with key experimental data was found, including the ability of the model to accommodate and explain a previously developed pharmacophore model. A coupling to agonist binding was thereby established and discussed in relation to activation mechanisms. Our results highlight the importance of critical evaluation and optimization of each step in the homology modeling process. The approach taken here can greatly aid in increasing the understanding of GABA(A)Rs and related receptors where structural insight is limited and reliable models are difficult to obtain.

GABAA receptor open-state conformation determines non-competitive antagonist binding

The γ-aminobutyric acid (GABA) type A receptor (GABA(A)R) is one of the most important targets for insecticide action. The human recombinant β3 homomer is the best available model for this binding site and 4-n-[(3)H]propyl-4'-ethynylbicycloorthobenzoate ([(3)H]EBOB) is the preferred non-competitive antagonist (NCA) radioligand. The uniquely high sensitivity of the β3 homomer relative to the much-less-active but structurally very-similar β1 homomer provides an ideal comparison to elucidate structural and functional features important for NCA binding. The β1 and β3 subunits were compared using chimeragenesis and mutagenesis and various combinations with the α1 subunit and modulators. Chimera β3/β1 with the β3 subunit extracellular domain and the β1 subunit transmembrane helices retained the high [(3)H]EBOB binding level of the β3 homomer while chimera β1/β3 with the β1 subunit extracellular domain and the β3 subunit transmembrane helices had low binding activity similar to the β1 homomer. GABA at 3μM stimulated heteromers α1β1 and α1β3 binding levels more than 2-fold by increasing the open probability of the channel. Addition of the α1 subunit rescued the inactive β1/β3 chimera close to wildtype α1β1 activity. EBOB binding was significantly altered by mutations β1S15'N and β3N15'S compared with wildtype β1 and β3, respectively. However, the binding activity of α1β1S15'N was insensitive to GABA and α1β3N15'S was stimulated much less than wildtype α1β3 by GABA. The inhibitory effect of etomidate on NCA binding was reduced more than 5-fold by the mutation β3N15'S. Therefore, the NCA binding site is tightly regulated by the open-state conformation that largely determines GABA(A) receptor sensitivity.

The structural basis of function in Cys-loop receptors

Cys-loop receptors are membrane-spanning neurotransmitter-gated ion channels that are responsible for fast excitatory and inhibitory transmission in the peripheral and central nervous systems. The best studied members of the Cys-loop family are nACh, 5-HT3, GABAA and glycine receptors. All these receptors share a common structure of five subunits, pseudo-symmetrically arranged to form a rosette with a central ion-conducting pore. Some are cation selective (e.g. nACh and 5-HT3) and some are anion selective (e.g. GABAA and glycine). Each receptor has an extracellular domain (ECD) that contains the ligand-binding sites, a transmembrane domain (TMD) that allows ions to pass across the membrane, and an intracellular domain (ICD) that plays a role in channel conductance and receptor modulation. Cys-loop receptors are the targets for many currently used clinically relevant drugs (e.g. benzodiazepines and anaesthetics). Understanding the molecular mechanisms of these receptors could therefore provide the catalyst for further development in this field, as well as promoting the development of experimental techniques for other areas of neuroscience.In this review, we present our current understanding of Cys-loop receptor structure and function. The ECD has been extensively studied. Research in this area has been stimulated in recent years by the publication of high-resolution structures of nACh receptors and related proteins, which have permitted the creation of many Cys loop receptor homology models of this region. Here, using the 5-HT3 receptor as a typical member of the family, we describe how homology modelling and ligand docking can provide useful but not definitive information about ligand interactions. We briefly consider some of the many Cys-loop receptors modulators. We discuss the current understanding of the structure of the TMD, and how this links to the ECD to allow channel gating, and consider the roles of the ICD, whose structure is poorly understood. We also describe some of the current methods that are beginning to reveal the differences between different receptor states, and may ultimately show structural details of transitions between them.

Early history of glycine receptor biology in Mammalian spinal cord circuits

In this review we provide an overview of key in vivo experiments undertaken in the cat spinal cord in the 1950s and 1960s, and point out their contributions to our present understanding of glycine receptor (GlyR) function. Importantly, some of these discoveries were made well before an inhibitory receptor, or its agonist, was identified. These contributions include the universal acceptance of a chemical mode of synaptic transmission; that GlyRs are chloride channels; are involved in reciprocal and recurrent spinal inhibition; are selectively blocked by strychnine; and can be distinguished from the GABA_A receptor by their insensitivity to bicuculline. The early in vivo work on inhibitory mechanisms in spinal neurons also contributed to several enduring principles on synaptic function, such as the time associated with synaptic delay, the extension of Dale's hypothesis (regarding the chemical unity of nerve cells and their terminals) to neurons within the central nervous system, and the importance of inhibition for synaptic integration in motor and sensory circuits. We hope the work presented here will encourage those interested in GlyR biology and inhibitory mechanisms to seek out and read some of the “classic” articles that document the above discoveries.

The alpha5(H105R) mutation impairs alpha5 selective binding properties by altered positioning of the alpha5 subunit in GABAA receptors containing two distinct types of alpha subunits

GABA(A) receptors are pentameric ligand-gated ion channels that are major mediators of fast inhibitory neurotransmission. Clinically relevant GABA(A) receptor subtypes are assembled from alpha5(1-3, 5), beta1-3 and the gamma2 subunit. They exhibit a stoichiometry of two alpha, two beta and one gamma subunit, with two GABA binding sites located at the alpha/beta and one benzodiazepine binding site located at the alpha/gamma subunit interface. Introduction of the H105R point mutation into the alpha5 subunit, to render alpha5 subunit-containing receptors insensitive to the clinically important benzodiazepine site agonist diazepam, unexpectedly resulted in a reduced level of alpha5 subunit protein in alpha5(H105R) mice. In this study, we show that the alpha5(H105R) mutation did not affect cell surface expression and targeting of the receptors or their assembly into macromolecular receptor complexes but resulted in a severe reduction of alpha5-selective ligand binding. Immunoprecipitation studies suggest that the diminished alpha5-selective binding is presumably due to a repositioning of the alpha5(H105R) subunit in GABA(A) receptor complexes containing two different alpha subunits. These findings imply an important role of histidine 105 in determining the position of the alpha5 subunit within the receptor complex by determining the affinity for assembly with the gamma2 subunit.