A gene in human chromosome band Xq28 (GABRE) defines a putative new subunit class of the GABAA neurotransmitter receptor

Structure-function Studies of GABA (A) Receptors and Related computer-aided Studies

The γ-aminobutyric acid type A receptor (GABA (A) receptor) is a membrane protein activated by the neurotransmitter GABA. Structurally, this major inhibitory neurotransmitter receptor in the human central nervous system is a pentamer that can be built from a selection of 19 subunits consisting of α(1,2,3,4,5 or 6), β (1,2 or 3), γ (1,2 or 3), ρ (1,2 or 3), and δ, π, θ, and ε. This creates several possible pentameric arrangements, which also influence the pharmacological and physiological properties of the receptor. The complexity and heterogeneity of the receptors are further increased by the addition of short and long splice variants in several subunits and the existence of multiple allosteric binding sites and expansive ligands that can bind to the receptors. Therefore, a comprehensive understanding of the structure and function of the receptors is required to gain novel insights into the consequences of receptor dysfunction and subsequent drug development studies. Notably, advancements in computational-aided studies have facilitated the elucidation of residual interactions and exploring energy binding, which may otherwise be challenging to investigate. In this review, we aim to summarize the current understanding of the structure and function of GABA (A) receptors obtained from advancements in computational-aided applications.

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].

Roles of GABAA receptor α5 subunit on locomotion and working memory in transient forebrain ischemia in mice

The γ-aminobutyric acid A (GABA_A) receptor, which contains a chloride channel, is a typical inhibitory neurotransmitter receptor in the central nervous system. Although the GABAergic neurotransmitter system has been discovered to be involved in various psychological behaviors, such as anxiety, convulsions, and cognitive function, its functional changes under conditions of ischemic pathological situation are still uncovered. In the present study, we attempted to elucidate the functional changes in the GABAergic system after transient forebrain ischemia in mice. A bilateral common carotid artery occlusion for 20 min was used to establish a model of transient forebrain ischemia/reperfusion (tI/R). Delayed treatment with diazepam, a positive allosteric modulator of the GABA_A receptor, increased locomotor activity in the open field test and spontaneous alternations in the Y-maze test in tI/R mice, but not in shams. Delayed treatment with diazepam did not alter neuronal death or the number of GABAergic neurons in tI/R mice. However, tI/R induced changes in the protein levels of GABA_A receptor subunits in the hippocampus. In particular, the most marked increase in the tI/R group was found in the level of α5 subunit of the GABA_A receptor. Similar to delayed treatment with diazepam, delayed treatment with imidazenil, an α5-sensitive benzodiazepine, increased spontaneous alternations in the Y-maze in tI/R mice, whereas zolpidem, an α5-insensitive benzodiazepine, failed to show such effects. These results suggest that tI/R-induced changes in the level of the α5 subunit of the GABA_A receptor can alter the function of GABAergic drugs in a mouse model of forebrain ischemia.

MiR224-5p Inhibitor Restrains Neuronal Apoptosis by Targeting NR4A1 in the Oxygen-Glucose Deprivation (OGD) Model

This study was designed to investigate the molecular mechanism of stroke and to explore the effect of miR-224-5p in hypoxic cortical neurons. Firstly, we established a middle cerebral artery occlusion (MCAO) model with Sprague–Dawley rats. Triphenyltetrazolium chloride (TTC) staining showed the brain infarction of an MCAO rat. Longa scores of rats were significantly increased in 12th, 24th, and 48th hours after MCAO. Then, we found that miR-224-5p was increased after MCAO in rats by qRT-PCR. In order to investigate the effect of miR-224-5p in hypoxic neurons, we established an oxygen-glucose deprivation (OGD) model with cortical neurons. MiR-224-5p was also upregulated in neurons after OGD by qRT-PCR. After transfection of the miR-224-5p inhibitor, the number of neurons in the anti-miR-224-5p group significantly increased (P < 0.01) in comparison to the anti-NC group. Furthermore, Tuj1⁺ (neuronal marker) staining and TUNEL assay (to detect apoptotic cells) were performed in neurons. The survival of neurons in the anti-miR-224-5p group was significantly improved (P < 0.01), while the apoptosis of neurons in the anti-miR-224-5p group was significantly decreased (P < 0.01), when compared with that of the anti-NC group. In addition, we predicted that potential target genes of miR-224-5p were nuclear receptor subfamily 4 group A member 1 (NR4A1), interleukin 1 receptor antagonist (IL1RN), and ring finger protein 38 (RNF38) with bioinformatics databases, such as TargetScan, miRDB, miRmap, and miRanda. The result of qRT-PCR confirmed that NR4A1 was significantly decreased after hypoxic injury (P < 0.01). Meanwhile, luciferase reporter’s assay indicated that NR4A1 was the direct target of miR-224-5p. Compared with the anti-miR-224-5p + siNC group, the number of cortical neurons and the length of the neuron axon in the anti-miR-224-5p + si-NR4A1 group were significantly decreased (P < 0.01), and the number of neuronal apoptosis in the anti-miR-224-5p + si-NR4A1 group was increased (P < 0.01). In conclusion, miR-224-5p played a crucial role in hypoxic neuron injury through NR4A1, which might be an important regulatory mechanism in OGD injury of neurons.

Molecular Characterization of GABA-A Receptor Subunit Diversity within Major Peripheral Organs and Their Plasticity in Response to Early Life Psychosocial Stress

Gamma aminobutyric acid (GABA) subtype A receptors (GABA_ARs) are integral membrane ion channels composed of five individual proteins or subunits. Up to 19 different GABA_AR subunits (α1–6, β1–3, γ1–3, δ, ε, θ, π, and ρ1–3) have been identified, resulting in anatomically, physiologically, and pharmacologically distinct multiple receptor subtypes, and therefore GABA-mediated inhibition, across the central nervous system (CNS). Additionally, GABA_AR-modulating drugs are important tools in clinical medicine, although their use is limited by adverse effects. While significant advances have been made in terms of characterizing the GABA_AR system within the brain, relatively less is known about the molecular phenotypes within the peripheral nervous system of major organ systems. This represents a potentially missed therapeutic opportunity in terms of utilizing or repurposing clinically available GABA_AR drugs, as well as promising research compounds discarded due to their poor CNS penetrance, for the treatment of peripheral disorders. In addition, a broader understanding of the peripheral GABA_AR subtype repertoires will contribute to the design of therapies which minimize peripheral side-effects when treating CNS disorders. We have recently provided a high resolution molecular and function characterization of the GABA_ARs within the enteric nervous system of the mouse colon. In this study, the aim was to determine the constituent GABA_AR subunit expression profiles of the mouse bladder, heart, liver, kidney, lung, and stomach, using reverse transcription polymerase chain reaction and western blotting with brain as control. The data indicate that while some subunits are expressed widely across various organs (α3–5), others are restricted to individual organs (γ2, only stomach). Furthermore, we demonstrate complex organ-specific developmental expression plasticity of the transporters which determine the chloride gradient within cells, and therefore whether GABA_AR activation has a depolarizing or hyperpolarizing effect. Finally, we demonstrate that prior exposure to early life psychosocial stress induces significant changes in peripheral GABA_AR subunit expression and chloride transporters, in an organ- and subunit-specific manner. Collectively, the data demonstrate the molecular diversity of the peripheral GABA_AR system and how this changes dynamically in response to life experience. This provides a molecular platform for functional analyses of the GABA–GABA_AR system in health, and in diseases affecting various peripheral organs.

Genetic Variants Identified from Epilepsy of Unknown Etiology in Chinese Children by Targeted Exome Sequencing

Genetic factors play a major role in the etiology of epilepsy disorders. Recent genomics studies using next generation sequencing (NGS) technique have identified a large number of genetic variants including copy number (CNV) and single nucleotide variant (SNV) in a small set of genes from individuals with epilepsy. These discoveries have contributed significantly to evaluate the etiology of epilepsy in clinic and lay the foundation to develop molecular specific treatment. However, the molecular basis for a majority of epilepsy patients remains elusive, and furthermore, most of these studies have been conducted in Caucasian children. Here we conducted a targeted exome-sequencing of 63 trios of Chinese epilepsy families using a custom-designed NGS panel that covers 412 known and candidate genes for epilepsy. We identified pathogenic and likely pathogenic variants in 15 of 63 (23.8%) families in known epilepsy genes including SCN1A, CDKL5, STXBP1, CHD2, SCN3A, SCN9A, TSC2, MBD5, POLG and EFHC1. More importantly, we identified likely pathologic variants in several novel candidate genes such as GABRE, MYH1, and CLCN6. Our results provide the evidence supporting the application of custom-designed NGS panel in clinic and indicate a conserved genetic susceptibility for epilepsy between Chinese and Caucasian children.

The MicroRNA-224 Inhibitor Prevents Neuronal Apoptosis via Targeting Spastic Paraplegia 7 After Cerebral Ischemia

Recently, the study of microRNA expression profile has shown that miR-224 was implicated in neuron injury, but the mechanism of miR-224 on regulating neuronal apoptosis is completely unclear until now. Therefore, the current study aims to illuminate the miR-224 and its target gene on the modulation of neuronal cell apoptosis induced by ischemic injury. In this study, we used oxygen/glucose deprivation (OGD)-induced human-derived HCN-2 cells to establish the model of cerebral ischemia injury. We found that miR-224 was upregulated in injured cells (human brain cortical neuron). Using bioinformatics analyses, we found that miR-224 targeted the 3'UTR of spastic paraplegia 7 (SPG7) and the miR-224 inhibitor promoted expression of SPG7 and promoter activity of SPG7 3'UTR. In addition, we further found that miR-224 inhibitor enhanced interaction SPG7 with mitochondrial voltage-dependent anion channel (VDAC1) detected by co-immunoprecipitation in injured cells. The knockdown of SPG7 reduced mitochondrial membrane potential and caused higher mitochondrial calcium retention in injured cells. Knockdown of SPG7 inhibits expression of nicotinic acetylcholine receptor. Besides, the miR-224 inhibitor reduced neuronal cell apoptosis was increased by knockdown of either SPG7 or VDAC1. Overall, miR-224 inhibitor may prevent neuronal cell apoptosis by targeting SPG7 3'UTR and promote interaction SPG7 with VDAC1 after cerebral ischemia. Downregulation of SPG7 induces VDAC1 to form mitochondria permeability transition pore probably by inhibiting expression of nicotinic acetylcholine receptor, resulting in mitochondrial membrane depolarization and higher mitochondrial calcium retention.

Perils of paediatric anaesthesia and novel molecular approaches: An evidence-based review

Evolution of anaesthesia has been largely helped by progress of evidence-based medicine. In spite of many advancements in anaesthesia techniques and availability of newer and safer drugs, much more needs to be explored scientifically for the development of anaesthesia. Over the last few years, the notion that the actions of the anaesthesiologist have only immediate or short-term consequences has largely been challenged. Evidences accumulated in the recent years have shown that anaesthesia exposure may have long-term consequences particularly in the extremes of ages. However, most of the studies conducted so far are in vitro or animal studies, the results of which have been extrapolated to humans. There have been confounding evidences linking anaesthesia exposure in the developing brain with poor neurocognitive outcome. The results of animal studies and human retrospective studies have raised concern over the potential detrimental effects of general anaesthetics on the developing brain. The purpose of this review is to highlight the long-term perils of anaesthesia in the very young and the potential of improving anaesthesia delivery with the novel molecular approaches.

Hypermethylation of the GABRE~miR-452~miR-224 promoter in prostate cancer predicts biochemical recurrence after radical prostatectomy

PURPOSE

Available tools for prostate cancer diagnosis and prognosis are suboptimal and novel biomarkers are urgently needed. Here, we investigated the regulation and biomarker potential of the GABRE∼miR-452∼miR-224 genomic locus.

EXPERIMENTAL DESIGN

GABRE/miR-452/miR-224 transcriptional expression was quantified in 80 nonmalignant and 281 prostate cancer tissue samples. GABRE∼miR-452∼miR-224 promoter methylation was determined by methylation-specific qPCR (MethyLight) in 35 nonmalignant, 293 prostate cancer [radical prostatectomy (RP) cohort 1] and 198 prostate cancer tissue samples (RP cohort 2). Diagnostic/prognostic biomarker potential of GABRE∼miR-452∼miR-224 methylation was evaluated by ROC, Kaplan-Meier, uni- and multivariate Cox regression analyses. Functional roles of miR-224 and miR-452 were investigated in PC3 and DU145 cells by viability, migration, and invasion assays and gene-set enrichment analysis (GSEA) of posttransfection transcriptional profiling data.

RESULTS

GABRE∼miR-452∼miR-224 was significantly downregulated in prostate cancer compared with nonmalignant prostate tissue and had highly cancer-specific aberrant promoter hypermethylation (AUC = 0.98). Functional studies and GSEA suggested that miR-224 and miR-452 inhibit proliferation, migration, and invasion of PC3 and DU145 cells by direct/indirect regulation of pathways related to the cell cycle and cellular adhesion and motility. Finally, in uni- and multivariate analyses, high GABRE∼miR-452∼miR-224 promoter methylation was significantly associated with biochemical recurrence in RP cohort 1, which was successfully validated in RP cohort 2.

CONCLUSION

The GABRE∼miR-452∼miR-224 locus is downregulated and hypermethylated in prostate cancer and is a new promising epigenetic candidate biomarker for prostate cancer diagnosis and prognosis. Tumor-suppressive functions of the intronic miR-224 and miR-452 were demonstrated in two prostate cancer cell lines, suggesting that epigenetic silencing of GABRE∼miR-452∼miR-224 may be selected for in prostate cancer.

Downregulation of GABAA receptor protein subunits α6, β2, δ, ε, γ2, θ, and ρ2 in superior frontal cortex of subjects with autism

We measured protein and mRNA levels for nine gamma-aminobutyric acid A (GABAA) receptor subunits in three brain regions (cerebellum, superior frontal cortex, and parietal cortex) in subjects with autism versus matched controls. We observed changes in mRNA for a number of GABAA and GABAB subunits and overall reduced protein expression for GABAA receptor alpha 6 (GABRα6), GABAA receptor beta 2 (GABRβ2), GABAA receptor delta (GABRδ), GABAA receptor epsilon (GABRε), GABAA receptor gamma 2 (GABRγ2), GABAA receptor theta (GABRθ), and GABAA receptor rho 2 (GABRρ2) in superior frontal cortex from subjects with autism. Our data demonstrate systematic changes in GABAA&B subunit expression in brains of subjects with autism, which may help explain the presence of cognitive abnormalities in subjects with autism.

Highlights from the first symposium on upper tract urothelial carcinoma

OBJECTIVES

Upper tract urothelial carcinoma (UTUC) is a rare disease in Western countries and garners little focused attention in urologic and oncologic circles. We report highlights from the first symposium on UTUC.

METHODS

All participants were asked to provide a summary of their presentation to be included as part of these proceedings. Submitted summaries were synthesized into this document. All contributors reviewed and provided input on the final draft.

RESULTS

Five highlights are included in this report, including landmark research that not only reveals the likely cause of Balkan endemic nephropathy and associated UTUC but also links it directly to UTUC in Taiwan. Because of the ubiquitous use of Aristolochia plants in these herbal remedies, a public health problem of considerable magnitude is anticipated in Asian countries. Gene expression signatures reveal some differential expression in bladder carcinoma, such as CLCA2 and GABRE. Few urinary markers have proven utility for the diagnosis and follow-up of UTUC, and no tissue or blood-based markers are currently undergoing clinical application. Novel endoscopic therapies provide some hope of improving tissue sampling, diagnosis, and kidney-sparing therapeutics, but the greatest potential lies in improving clinical (preoperative) risk stratification, which is critically limited in this disease. Biomarkers, currently untested, hold promise in identifying patients most likely to benefit from perioperative chemotherapy and at high risk from cisplatin-induced nephrotoxicity.

CONCLUSIONS

Despite its rarity in the West, UTUC is reaching potentially epidemic proportions in the East because of exposure to carcinogenic herbal remedies. Critical trials are needed to improve our understanding and treatment of UTUC. Because of the broad range of comorbid conditions in patients suffering from this disease, it is the consensus of the participants that future clinical trials should be practical in design and applicable to a broad range of patients, diverging from the current dogma of narrow patient selection criteria in clinical trials. Practical designs would maximize accrual for a still uncommon disease, and their findings would be applicable to a larger proportion of patients than current narrowly selected designs.

Evolution of neurotransmitter gamma-aminobutyric acid, glutamate and their receptors

Gamma-aminobutyric acid (GABA) and glutamate are two important amino acid neurotransmitters widely present in the nervous systems of mammals, insects, round worm, and platyhelminths, while their receptors are quite diversified across different animal phyla. However, the evolutionary mechanisms between the two conserved neurotransmitters and their diversified receptors remain elusive, and antagonistic interactions between GABA and glutamate signal transduction systems, in particular, have begun to attract significant attention. In this review, we summarize the extant results on the origin and evolution of GABA and glutamate, as well as their receptors, and analyze possible evolutionary processes and phylogenetic relationships of various GABAs and glutamate receptors. We further discuss the evolutionary history of Excitatory/Neutral Amino Acid Transporter (EAAT), a transport protein, which plays an important role in the GABA-glutamate "yin and yang" balanced regulation. Finally, based on current advances, we propose several potential directions of future research.

A factor analysis of global GABAergic gene expression in human brain identifies specificity in response to chronic alcohol and cocaine exposure

Although expression patterns of GABAergic genes in rodent brain have largely been elucidated, no comprehensive studies have been performed in human brain. The purpose of this study was to identify global patterns of GABAergic gene expression in healthy adults, including trans and cis effects in the GABA_A gene clusters, before determining the effects of chronic alcohol and cocaine exposure on gene expression in the hippocampus. RNA-Seq data from ‘BrainSpan’ was obtained across 16 brain regions from postmortem samples from nine adults. A factor analysis was performed on global expression of 21 GABAergic pathway genes. Factor specificity for response to chronic alcohol/cocaine exposure was subsequently determined from the analysis of RNA-Seq data from postmortem hippocampus of eight alcoholics, eight cocaine addicts and eight controls. Six gene expression factors were identified. Most genes loaded (≥0.5) onto one factor; six genes loaded onto two. The largest factor (0.30 variance) included the chromosome 5 gene cluster that encodes the most common GABA_A receptor, α₁β₂γ₂, and genes encoding the α₃β₃γ₂ receptor. Genes within this factor were largely unresponsive to chronic alcohol/cocaine exposure. In contrast, the chromosome 4 gene cluster factor (0.14 variance) encoding the α₂β₁γ₁ receptor was influenced by chronic alcohol/cocaine exposure. Two other factors (0.17 and 0.06 variance) showed expression changes in alcoholics/cocaine addicts; these factors included genes involved in GABA synthesis and synaptic transport. Finally there were two factors that included genes with exceptionally low (0.10 variance) and high (0.09 variance) expression in the cerebellum; the former factor was unaffected by alcohol/cocaine exposure. This study has shown that there appears to be specificity of GABAergic gene groups, defined by covariation in expression, for response to chronic alcohol/cocaine exposure. These findings might have implications for combating stress-related craving and relapse.

Significant differences in gene expression of GABA receptors in peripheral blood leukocytes of migraineurs

Migraine is a debilitating neurovascular disorder, with a substantial genetic component. The exact cause of a migraine attack is unknown; however cortical hyperexcitability is thought to play a role. As Gamma-aminobutyric Acid (GABA) is the major inhibitory neurotransmitter in the brain, malfunctioning of this system may be a cause of the hyperexcitability. To date, there has been limited research examining the gene expression or genetics of GABA receptors in relation to migraine. The aim of our study was to determine if GABA receptors play a role in migraine by investigating their gene expression using profile in migraine affected individuals and non-affected controls by Q-PCR. Gene expression of GABA(A) receptor subunit isoforms (GABRA3, GABRB3, GABRQ) and GABA(B) receptor 2 (GABBR2) was quantified in mRNA obtained from peripheral blood leukocytes from 28 migraine subjects and 22 healthy control subjects. Analysis of results showed that two of the tested genes, GABRA3 and GABBR2, were significantly down regulated in migraineurs (P=0.018; P=0.017), compared to controls. Results from the other tested genes did not show significant gene expression variation. The results indicate that there may be specific GABA receptor gene expression variation in migraine, particularly involving the GABRA3 and GABBR2 genes. This study also identifies GABRA3 and GABBR2 as potential biomarkers to select migraineurs that may be more responsive to GABA agonists with future investigations in this area warranted.

Enhancement of GABAergic activity: neuropharmacological effects of benzodiazepines and therapeutic use in anesthesiology

GABA is the major inhibitory neurotransmitter in the central nervous system (CNS). The type A GABA receptor (GABA(A)R) system is the primary pharmacological target for many drugs used in clinical anesthesia. The α1, β2, and γ2 subunit-containing GABA(A)Rs located in the various parts of CNS are thought to be involved in versatile effects caused by inhaled anesthetics and classic benzodiazepines (BZD), both of which are widely used in clinical anesthesiology. During the past decade, the emergence of tonic inhibitory conductance in extrasynaptic GABA(A)Rs has coincided with evidence showing that these receptors are highly sensitive to the sedatives and hypnotics used in anesthesia. Anesthetic enhancement of tonic GABAergic inhibition seems to be preferentially increased in regions shown to be important in controlling memory, awareness, and sleep. This review focuses on the physiology of the GABA(A)Rs and the pharmacological properties of clinically used BZDs. Although classic BZDs are widely used in anesthesiological practice, there is a constant need for new drugs with more favorable pharmacokinetic and pharmacodynamic effects and fewer side effects. New hypnotics are currently developed, and promising results for one of these, the GABA(A)R agonist remimazolam, have recently been published.

Ubc9 promotes breast cell invasion and metastasis in a sumoylation-independent manner

Ubc9 is an E2-conjugating enzyme that transfers the activated small ubiquitin-like modifier (SUMO) to protein substrates, and thus it has an important function in sumoylation-mediated cellular pathways. We have earlier reported that Ubc9 promotes tumor growth in the xenograft mouse model using breast cancer cell line MCF-7 in part through regulation of Bcl-2 expression. In this study, we show that ectopic expression of wild-type Ubc9 (Ubc9-WT) promotes cell invasion and metastasis. Surprisingly, the dominant negative mutant Ubc9 (Ubc9-DN) also causes the same phenotype, indicating that the ability of Ubc9 to promote invasion and metastasis is distinct from its ability to conjugate SUMO to protein substrates. Of considerable interest, several microRNAs such as miR-224 are regulated by Ubc9. Although ectopic expression of Ubc9 causes downregulation of miR-224, suppression of Ubc9 by Ubc9-siRNAs leads to its upregulation. We further show that miR-224 can inhibit cell invasion and directly targets CDC42 and CXCR4, and that suppression of CDC42 and CXCR4 by RNAi causes inhibition of Ubc9-mediated invasion. Together, these results show a molecular link between Ubc9 and the metastasis genes such as CDC42 and CXCR4, and thus provide new insight into the mechanism by which Ubc9 promotes tumor invasion and metastasis.

Investigation of gamma-aminobutyric acid (GABA) A receptors genes and migraine susceptibility

BACKGROUND

Migraine is a neurological disorder characterized by recurrent attacks of severe headache, affecting around 12% of Caucasian populations. It is well known that migraine has a strong genetic component, although the number and type of genes involved is still unclear. Prior linkage studies have reported mapping of a migraine gene to chromosome Xq 24-28, a region containing a cluster of genes for GABA A receptors (GABRE, GABRA3, GABRQ), which are potential candidate genes for migraine. The GABA neurotransmitter has been implicated in migraine pathophysiology previously; however its exact role has not yet been established, although GABA receptors agonists have been the target of therapeutic developments. The aim of the present research is to investigate the role of the potential candidate genes reported on chromosome Xq 24-28 region in migraine susceptibility. In this study, we have focused on the subunit GABA A receptors type epsilon (GABRE) and type theta (GABRQ) genes and their involvement in migraine.

METHODS

We have performed an association analysis in a large population of case-controls (275 unrelated Caucasian migraineurs versus 275 controls) examining a set of 3 single nucleotide polymorphisms (SNPs) in the coding region (exons 3, 5 and 9) of the GABRE gene and also the I478F coding variant of the GABRQ gene.

RESULTS

Our study did not show any association between the examined SNPs in our test population (P>0.05).

CONCLUSION

Although these particular GABA receptor genes did not show positive association, further studies are necessary to consider the role of other GABA receptor genes in migraine susceptibility.

An investigation of potential genetic determinants of propofol requirements and recovery from anaesthesia

BACKGROUND AND OBJECTIVE

The objectives of this study were, firstly, to characterize the inter-patient variability in the dose of propofol required to achieve a bispectral index <70 and 'time to eye opening' following propofol infusion and, secondly, to determine if the pharmacodynamic parameter 'time to achieve bispectral index <70' was influenced by genotype of the sex-linked drug receptor gene GABRE or if pharmacokinetic parameters such as clearance and 'time to eye opening' were influenced by the genotype of the metabolizing enzyme CYP2B6.

METHODS

One hundred and fifty patients received a standardized anaesthetic. Apparent systemic clearance values were estimated. Correlation was sought between carriers of different CYP2B6 and GABRE genotypes and apparent systemic clearance, 'time to achieve bispectral index <70' and 'time to eye opening'.

RESULTS

Propofol induction/emergence characteristics varied, with slow recovery times in a subset of males. Time to loss of verbal contact and time to bispectral index <70 varied 6.6- and 4.3-fold, respectively. At emergence, there was a 15.5- to 111-fold variability in the measured time intervals. Clearance varied from 9.1 to 55.8 mL min-1 kg-1. The CYP2B6 C1459T (R487C) genotype frequencies were TT 1%, TC 22% and CC 67%. The three major haplotypes of CYP2B6 (R487C, K262R and Q172H variants) were not significantly associated with time to eye opening or clearance. Clearance was similar in 487C carriers and 487RR genotypes. There was no statistically significant correlation between the four major haplotypes of GABRE variants investigated ([mRNA358]G/T, 20118C/T, 20326C/T and 20502 A/T) and the observed anaesthesia induction time.

CONCLUSIONS

Great inter-patient variability exists in the dose of propofol required to achieve bispectral index <70, apparent systemic propofol clearance and time to eye opening. Common haplotypic differences at the CYP2B6 and GABRE genes do not appear to account for the majority of the observed inter-patient variability.

Systematic characterisation of GABRP expression in sporadic breast cancer and normal breast tissue

The GABRP gene has been previously identified by in silico analysis of four million ESTs as a candidate gene differentially expressed in breast cancer. GABRP is located on chromosome 5q34 and it encodes the pi-subunit of the gamma-aminobutyric acid (GABA) receptor, a transmembrane protein expressed in the brain and several nonneuronal tissues. Using cDNA dot blot hybridisation (cancer profiling array), quantitative RT-PCR and non-radioisotopic in situ hybridisation (ISH), we have analysed GABRP expression in breast cancer and normal breast tissues as well as in nontumorigenic and tumorigenic breast cell lines. Analysis of the cancer profiling array revealed a more than 2-fold downregulation of GABRP (p < 0.001) in 76% of primary breast carcinomas (n = 50) compared to corresponding normal tissues. Quantitative RT-PCR in a panel of 23 normal human tissues showed that the GABRP expression level was most abundant in the normal breast tissues compared to other human tissues. GABRP downregulation in breast cancer was confirmed by quantitative RT-PCR in cryopreserved breast tumour and normal breast tissue specimens (n = 22), in archival formalin-fixed, paraffin-embedded tissue specimens (n = 32), as well as in breast cancer cell lines (n = 8). Furthermore, a significant downregulation of GABRP was noted in large (pT3-pT4) (p = 0.044) primary breast tumours. Non-radioisotopic ISH showed strong GABRP expression in normal epithelial and benign papilloma breast cells, but no signal could be detected in invasive ductal carcinoma. Altogether, these data suggest that GABRP is progressively down-regulated with tumour-progression, and that it may be useful as a prognostic marker in breast cancer.

Consequences of the Evolution of the GABAA Receptor Gene Family

1. This paper reviews the evolution of the family of genes present in mammals and other vertebrates that encode gamma-aminobutyric acid (GABA) type A (GABA(A)) receptors, which are the major inhibitory neurotransmitter receptors in the central nervous system (CNS). In mammals, 16 different polypeptides (alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and theta) have been identified, using recombinant DNA techniques, each of which is encoded by a distinct gene. The products of these genes assemble in diverse combinations to form a variety of receptor subtypes that have different sensitivities to a number of clinically relevant compounds, such as the benzodiazepines (BZs). 2. Based on a number of chromosomal mapping techniques, the majority of the GABA(A) receptor genes have been localized, in man, in four clusters on chromosomes 4, 5, 15, and the X. Furthermore, the genes that are present within these clusters have a conserved transcriptional orientation. It has, therefore, been proposed that the clusters arose largely as a consequence of two whole-genome doublings that occurred during chordate evolution, and that the ancestral cluster contained an "alpha-like," a "beta-like," and a "gamma-like" subunit gene. 3. Our laboratory has identified two additional GABA(A) receptor polypeptides (the beta4 and gamma4 subunits) in a number of vertebrate species; these do not appear to be present in mammals. We discuss here the relationship of the corresponding genes to other GABA(A) receptor genes, and conclude that their products are orthologous to the mammalian theta and epsilon subunits, respectively. 4. The GABA(A) receptor has a number of binding sites for compounds such as BZs, barbiturates, neurosteroids, and certain volatile anaesthetics. However, the only site at which endogenous compounds are thought to be active is the steroid site; this binds steroids such as certain metabolites of progesterone and deoxycorticosterone, which are synthesized in the periphery and CNS. Since the in vivo functional relevance, if any, of binding sites for other classes of compounds (such as the BZs) is unknown, the significance of differences in primary sequence, between different receptor subunits, is uncertain. We suggest that a possibly more important consequence of gene duplication is that it permitted greater flexibility in the level, pattern and regulation of expression of GABA(A) receptor genes.

Analysis of the Set of GABAA Receptor Genes in the Human Genome

The genes of the ionotropic gamma-aminobutyric acid receptor (GABR) subunits have shown an unusual chromosomal clustering, but only now can this be fully specified by analyses of the human genome. We have characterized the genes encoding the 18 known human GABR subunits, plus one now located here, for their precise locations, sizes, and exon/intron structures. Clusters of 17 of the 19, distributed between five chromosomes, are specified in detail, and their possible significance is considered. By applying search algorithms designed to recognize sequences of all known GABR-type subunits in species from man down to nematodes, we found no new GABR subunit is detectable in the human genome. However, the sequence of the human orthologue of the rat GABR rho3 receptor subunit was uncovered by these algorithms, and its gene could be analyzed. Consistent with those search results, orthologues of the beta4 and gamma4 subunits from the chicken, not cloned from mammals, were not detectable in the human genome by specific searches for them. The relationships are consistent with the mammalian subunit being derived from the beta line and epsilon from the gamma line, with mammalian loss of beta4 and gamma4. In their structures the human GABR genes show a basic pattern of nine coding exons, with six different genomic mechanisms for the alternative splicing found in various subunits. Additional noncoding exons occur for certain subunits, which can be regulatory. A dicysteine loop and its exon show remarkable constancy between all GABR subunits and species, of deduced functional significance.

Characterization of the interaction of indiplon, a novel pyrazolopyrimidine sedative-hypnotic, with the GABAA receptor

Clinically used benzodiazepine and nonbenzodiazepine sedative-hypnotic agents for the treatment of insomnia produce their therapeutic effects through allosteric enhancement of the effects of the inhibitory neurotransmitter GABA at the GABA(A) receptor. Indiplon is a novel pyrazolopyrimidine sedative-hypnotic agent, currently in development for insomnia. Using radioligand binding studies, indiplon inhibited the binding of [(3)H]Ro 15-1788 (flumazenil) to rat cerebellar and cerebral cortex membranes with high affinity (K(i) values of 0.55 and 0.45 nM, respectively). [(3)H]Indiplon binding to rat cerebellar and cerebral cortex membranes was reversible and of high affinity, with K(D) values of 1.01 and 0.45 nM, respectively, with a pharmacological specificity consistent with preferential labeling of GABA(A) receptors containing alpha1 subunits. In "GABA shift" experiments and in measurements of GABA-induced chloride conductance in rat cortical neurons in culture, indiplon behaved as an efficacious potentiator of GABA(A) receptor function. In both the radioligand binding and electrophysiological experiments, indiplon had a higher affinity than zolpidem or zaleplon. These in vitro properties are consistent with the in vivo properties of indiplon as an effective sedative-hypnotic acting through allosteric potentiation of the GABA(A) receptor.

GABAA receptors: building the bridge between subunit mRNAs, their promoters, and cognate transcription factors

The type A gamma-aminobutyric acid (GABA(A)) receptors mediate the majority of fast inhibitory neurotransmission in the CNS, and alterations in GABA(A) receptor function is believed to be involved in the pathology of several neurological and psychiatric illnesses, such as epilepsy, anxiety, Alzheimer's disease, and schizophrenia. GABA(A) receptors can be assembled from eight distinct subunit families defined by sequence similarity: alpha(1-6), beta(1-3), gamma(1-3), delta, pi, theta, and rho(1-3). The regulation of GABA(A) receptor function in the brain is a highly compensating system, influencing both the number and the composition of receptors at the cell surface. While transcriptional and translational points of control operate in parallel, it is becoming increasingly evident that many functional changes in GABA(A) receptors reflect the differential gene regulation of its subunits. The fact that certain GABA(A) receptor subunit genes are transcribed in distinct cell types during specific periods of development strongly suggests that genetic control plays a major role in the choice of subunit variants available for receptor assembly. This review focuses on the physiological conditions that alter subunit mRNA levels, the promoters that may control such levels, and the use of a conceptual framework created by bioinformatics to study coordinate and independent GABA(A) receptor subunit gene regulation. As this exciting field moves closer to identifying the language hidden inside the chromatin of GABA(A) receptor subunit gene clusters, future experiments will be aimed at testing models generated by computational analysis with biologically relevant in vivo and in vitro assays. It is hoped that through this functional genomic approach there will be the identification of new targets for therapeutic intervention.

Numerous microRNPs in neuronal cells containing novel microRNAs

Spinal muscular atrophy (SMA) is a common neurodegenerative disease that is caused by deletions or loss-of-function mutations in the Survival of Motor Neuron (SMN) protein. SMN is part of a large complex that functions in the assembly/restructuring of ribonucleoprotein (RNP) complexes. We recently showed in HeLa cells that two components of the SMN complex, Gemin3 and Gemin4, together with the argonaute protein eIF2C2, also associate with microRNAs (miRNAs) as part of a novel class of RNPs termed miRNPs. Here we report on miRNPs isolated from neuronal cell lines of mouse and human, and describe 53 novel miRNAs. Several of these miRNAs are conserved in divergent organisms, including rat, zebrafish, pufferfish, and the nematode Caenorhabditis elegans. The chromosomal locations of most of the novel miRNAs were identified and indicate some phylogenetic conservation of the likely precursor structures. Interestingly the gene locus of one miRNA, miR-175, is a candidate region for two neurologic diseases: early-onset parkinsonism (Waisman syndrome) and X-linked mental retardation (MRX3). Also, several miRNAs identified as part of miRNPs in these cells appear to constitute two distinct subfamilies. These subfamilies comprise multiple copies of miRNAs on different chromosomes, suggesting an important function in the regulation of gene expression.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Complete genomic sequence of 195 Kb of human DNA containing the gene GABRG2

GABA (gamma-aminobutyric acid), as the main inhibitory neurotransmitter in the brain, plays an essential role for the overall balance between neuronal excitation and inhibition by acting on GABAA receptors, which are ligand-gated chloride channels. Impaired GABAergic function contributes to certain forms of epilepsy, schizophrenia, Alzheimer's Disease, and other neurological disorders. In order to identify possible genetic features and to further study biological regulation of GABAA receptor genes whose promoter elements and sequence anomalies may contribute to epileptic disorders, as an initial step, we shot-gun sequenced a BAC clone, dj082c10 (195,909-bp in size), encompassing human gamma(2) subunit of GABAA receptor (GABRG2). It is, we believe, the first genomic sequence of the GABA receptor gamma subunit family. Four contigs were assembled from 2950 reads prior to gap in an average redundancy of eight folds over the entire region. The precision of the consensus sequence was predicted to be 99.999% after closing gaps and finishing weak regions. The nine exons of GABRG2 spans an 85-kb region that had 81 SINEs comprising 22.32%, and nine L1 elements comprising 3.40%, respectively. However, the density of L1 in the regions flanking GABRG2 gene (29.45% by 45 elements) is significantly higher than that within the gene. The length of GABRG2 introns varies in the range of 1.5 kb to 38.1 kb.

Identification of gamma-aminobutyric acid receptor subunit types in human and rat liver

GABA is a potent inhibitory neurotransmitter that binds to heterooligomeric receptors in the mammalian brain. In a previous study, we documented specific GABA binding to isolated rat hepatocytes that resulted in inhibition of hepatocyte proliferation. The purpose of the present study was to define the nature of hepatic GABA(A) receptors and to document their expression during rapid liver growth (after partial hepatectomy). PCRs with gene-specific primers derived from published sequences were performed with Marathon-ready human and rat liver cDNA. Two GABA(A) receptor subunit types (beta3 and epsilon) were expressed in human liver and one subunit type (beta3) in rat liver. PCR amplification of the human GABA(A) receptorbeta3-subunit produced a single product (molecular mass 53-59 kDa). In the case of the epsilon-subunit, two PCR products were identified. After partial hepatectomy, GABA(A) receptorbeta3-subunit expression inversely correlated with regenerative activity (r = -0.527, P = 0.006). In conclusion, these results indicate that in the human liver GABA(A) receptors consist of the beta3- and epsilon-subunit types, whereas in the rat liver only the beta3-subunit type is expressed. The results also support the hypothesis that GABAergic activity serves to maintain hepatocytes in a quiescent state.

GABA(A) receptor epsilon and theta subunits display unusual structural variation between species and are enriched in the rat locus ceruleus

Previously, GABA(A) receptor epsilon and theta subunits have been identified only in human. Here, we describe properties of the epsilon and theta subunit genes from mouse and rat that reveal an unusually high level of divergence from their human homologs. In addition to a low level of amino acid sequence conservation ( approximately 70%), the rodent epsilon subunit cDNAs encode a unique Pro/Glx motif of approximately 400 residues within the N-terminal extracellular domain of the subunits. Transcripts of the rat epsilon subunit were detected in brain and heart, whereas the mouse theta subunit mRNA was detectable in brain, lung, and spleen by Northern blot analysis. In situ hybridization revealed a particularly strong signal for both subunit mRNAs in rat locus ceruleus in which expression was detectable from the first postnatal day. Lower levels of coexpression were also detected in other brainstem nuclei and in the hypothalamus. However, the expression pattern of theta subunit mRNA was more widespread than that of epsilon subunit, being found also in the cerebral cortex of rat pups. In contrast to primate brain, neither subunit was expressed in the hippocampus or substantia nigra. The results indicate that GABA(A) receptor epsilon and theta subunits are evolving at a much faster rate than other known GABA(A) receptor subunits and that their expression patterns and functional properties may differ significantly between species.

FXY2/MID2, a gene related to the X-linked Opitz syndrome gene FXY/MID1, maps to Xq22 and encodes a FNIII domain-containing protein that associates with microtubules

Opitz G/BBB syndrome (OS) is a genetically heterogeneous disorder with an X-linked locus and an autosomal locus linked to 22q11.2. OS affects multiple organ systems with often variable severity even between siblings. The clinical features, which include hypertelorism, cleft lip and palate, defects of cardiac septation, hypospadias, and anorectal anomalies, indicate an underlying disturbance of the developing ventral midline of the embryo. The gene responsible for X-linked OS, FXY/MID1, is located on the short arm of the human X chromosome within Xp22.3 and encodes a protein with both an RBCC (RING finger, B-box, coiled coil) and a B30.2 domain. The Fxy gene in mice is also located on the X chromosome but spans the pseudoautosomal boundary in this species. Here we describe a gene closely related to FXY/MID1, called FXY2, which also maps to the X chromosome within Xq22. The mouse Fxy2 gene is located on the distal part of the mouse X chromosome within a region syntenic to Xq22. Analysis of genes flanking both FXY/MID1 and FXY2 (as well as their counterparts in mouse) suggests that these regions may have arisen as a result of an intrachromosomal duplication on an ancestral X chromosome. We have also identified in both FXY2 and FXY/MID1 proteins a conserved fibronectin type III domain located between the RBCC and B30.2 domains that has implications for understanding protein function. The FXY/MID1 protein has previously been shown to colocalize with microtubules, and here we show that the FXY2 protein similarly associates with microtubules in a manner that is dependent on the carboxy-terminal B30.2 domain.

The mouse GABA(A) receptor alpha3 subunit gene and promoter

Gamma-aminobutyric acid (GABA) type A receptors are multisubunit ligand-gated ion channels which mediate inhibition in the brain. The GABA(A) receptor alpha3 subunit gene exhibits extensive variation in its developmental and regional expression, but the detailed mechanisms governing the expression patterns of this gene remain unknown. We have cloned and begun to characterize the murine alpha3 subunit gene Gabra3. All but one of the 10 exons and the intron-exon boundaries have been sequenced; the first intron is in the 5' untranslated region (5'UTR) of the alpha3 mRNA. Rapid amplification of the cDNA 5'-end (5'-RACE) and RNase protection indicated many transcription start sites, with the major site (=+1) corresponding to a 5'UTR of 178 bases. Most sites were in or just downstream of a region of 55 (mouse) and 25 (human) GA repeats in the proximal promoter, as revealed by genome walking of Gabra3 and the human gene GABRA3. No canonical TATA or CAAT boxes or initiator (Inr) sites were found in either promoter, but both contained conserved consensus sites for several transcription factors. Progressive deletion of the mouse promoter produced positive or negative effects on expression of reporter (luciferase) constructs, with the highest observed activity in several types of transiently transfected cells for a construct containing bases -320 to +35. The GA repeats and a much shorter nearby series of four GC repeats, the first three of which are part of a consensus E2F site, appear to contribute significantly to mouse promoter activity. Upstream GA repeats enhanced activity of the SV40 promoter, and the GA repeat sequence bound nuclear proteins from several tissues.

Widespread expression of GABA(A) receptor subunits in peripheral tissues

The receptor subtypes involved in the physiological and pharmacological actions of gamma-amino butyric acid (GABA) in peripheral and endocrine tissues are not clear. Information about the molecular characteristics of GABA(A) receptors in peripheral endocrine tissues is only available for the pancreas and the adrenal medulla. Using reverse transcription (RT) polymerase chain reaction (PCR), the widespread expression of GABA(A) receptors subunits in rat peripheral tissues, including adrenal, ovary, testis, placenta, uterus, and small intestine is shown. It is shown that GABA(A) receptor subunits are expressed in multiple endocrine tissues in a tissue specific manner. These results give an insight into the likely pharmacological properties of these GABA(A) receptors in these tissues. The gonadal endocrine tissues such as the placenta, ovary and the testis express greater range of GABA(A) receptor subunits relative to the adrenal gland. The tissues with greater smooth muscle content, the small intestine and the uterus also express a smaller range of subunits subtypes.

GABA(A) receptors expressed in undifferentiated human teratocarcinoma NT2 cells differ from those expressed by differentiated NT2-N cells

During CNS development, changes occur in expression of GABA(A) receptor subunit subtypes and GABA(A) receptor pharmacological and biophysical properties. We used reverse transcription PCR and whole-cell-recording techniques to determine whether GABA(A) receptor expression and function also changed during retinoic acid-induced differentiation of human Ntera 2 (NT2) teratocarcinoma cells into neuron-like cells (NT2-N cells). In undifferentiated NT2 cells only alpha5, beta3, gamma3, and pi subtype mRNAs were detected. NT2 GABA(A) receptor currents had a maximal amplitude of 52 pA and an EC(50) of 4.0 microM, were relatively insensitive to enhancement by zolpidem and diazepam, and were enhanced by loreclezole and inhibited by lanthanum, zinc, and furosemide. In contrast, in NT2-N cells after 13 weeks of retinoic acid treatment, all GABA(A) receptor subtype mRNAs were detected. Maximal peak whole-cell currents were approximately 50-fold larger than NT2 cell currents, and the GABA EC(50) was higher (39.7 microM). In 13 week NT2-N cells, diazepam, zolpidem, loreclezole, and lanthanum had only small effects on GABA(A) receptor currents, and the zinc IC(50) for current inhibition was significantly higher than that for NT2 cells. In a previous study, we showed that NT2-N cells after 5 weeks of retinoic acid treatment had moderate peak currents, GABA EC(50,) and zinc IC(50) but that currents were robustly enhanced by diazepam, zolpidem, and loreclezole. During differentiation of NT2 cells to NT2-N cells, GABA(A) receptors underwent changes in subunit expression and pharmacology that were similar to many of the developmental changes in GABA(A) receptors that occur in CNS neurons.

theta, a novel gamma-aminobutyric acid type A receptor subunit

gamma-Aminobutyric acid type A (GABA-A) receptors are a major mediator of inhibitory neurotransmission in the mammalian central nervous system, and the site of action of a number of clinically important drugs. These receptors exist as a family of subtypes with distinct temporal and spatial patterns of expression and distinct properties that presumably underlie a precise role for each subtype. The newest member of this gene family is the theta subunit. The deduced polypeptide sequence is 627 amino acids long and has highest sequence identity (50.5%) with the beta1 subunit. Within the rat striatum, this subunit coassembles with alpha2, beta1, and gamma1, suggesting that gamma-aminobutyric acid type A receptors consisting of arrangements other than alpha beta + gamma, delta, or epsilon do exist. Expression of alpha2beta1gamma1theta in transfected mammalian cells leads to the formation of receptors with a 4-fold decrease in the affinity for gamma-aminobutyric acid compared with alpha2beta1gamma1. This subunit has a unique distribution, with studies so far suggesting significant expression within monoaminergic neurons of both human and monkey brain.

A Sequence-Ready BAC Contig of the GABAA Receptor Gene Cluster Gabrg1–Gabra2–Gabrb1 on Mouse Chromosome 5

The type-A receptors for the neurotransmitter GABA (γ-aminobutyric acid) are ligand-gated chloride channels that mediate postsynaptic inhibition. The functional diversity of these receptors comes from the use of a large repertoire of subunits encoded by separate genes, as well as from differences in subunit composition of individual receptors. In mammals, a majority of GABAAreceptor subunit genes are located in gene clusters that may be important for their regulated expression and function. We have established a high-resolution physical map of the cluster of genes encoding GABAA receptor subunits α2 (Gabra2), β1 (Gabrb1), and γ1 (Gabrg1) on mouse chromosome 5. Rat cDNA probes and specific sequence probes for all three GABAA receptor subunit genes have been used to initiate the construction of a sequence-ready contig of bacterial artificial chromosomes (BACs) encompassing this cluster. In the process of contig construction clones from 129/Sv and C57BL/6J BAC libraries were isolated. The assembled 1.3-Mb contig, consisting of 45 BACs, gives five- to sixfold coverage over the gene cluster and provides an average resolution of one marker every 32 kb. A number of BAC insert ends were sequenced, generating 30 new sequence tag sites (STS) in addition to 6 Gabr gene-based and 3 expressed sequence tag (EST)-based markers. STSs from, and surrounding, theGabrg1–Gabra2–Gabrb1 gene cluster were mapped in the T31 mouse radiation hybrid panel. The integration of the BAC contig with a map of loci ordered by radiation hybrid mapping suggested the most likely genomic orientation of this cluster on mouse chromosome 5: cen–D5Mit151–Gabrg1–Gabra2–Gabrb1–D5Mit58–tel. This established contig will serve as a template for genomic sequencing and for functional analysis of the GABAA gene cluster on mouse chromosome 5 and the corresponding region on human chromosome 4.

The sequence data described in this paper have been submitted to the GenBank/GSS data libraries under accession nos.AF156490 and AQ589406–AQ589436.

Fluorescence imaging of GABAA receptor-mediated intracellular [Cl-] in P19-N cells reveals unique pharmacological properties

This study describes the pharmacological properties of GABAA receptors expressed in P19-N cells using fluorescence imaging of intracellular chloride with 6-methoxy-N-ethylquinolinium iodide (MEQ). We show that application of the GABA agonist, muscimol (10-200 microM), produces time- and concentration-dependent increases in intracellular [Cl-] that are blocked by bicuculline. Diazepam (10 microM) and pentobarbital (1 mM) potentiate muscimol-stimulation. These receptors exhibit novel pharmacological properties. The neurosteroid, 3alpha-hydroxy-5alpha-pregnane-20-one (1-10 microM) exhibited weak potency in enhancement of muscimol-stimulation. Ethanol (50 and 100 mM) exhibited high efficacy on muscimol responses, a 4- to 5-fold potentiation, respectively, of muscimol (10 microM) alone. GABA and muscimol allosterically modulated specific binding of [3H]flunitrazepam to differentiated P19 cells. Modulation of GABAA receptor mediated increases in intracellular [Cl-] demonstrated stability in response magnitude from 7 to 15 days following removal of retinoic acid. In concert, GABAA receptor subunit mRNA and protein expression patterns in these neuron-like cells were stable over the same period. Using RT-PCR we determined that differentiated P19 cells lack gamma1, gamma2L, alpha6 and delta subunit mRNAs while expressing alpha1, alpha2, alpha3, alpha4, alpha5, beta1, beta2, beta3, gamma2S and gamma3. Furthermore, subunit specific antibody immunocytochemical labeling of cells with a neuronal morphology indicated the presence of alpha1, alpha2, alpha4, and gamma2 subunits (the only subunits tested). Therefore, P19-N cells should prove useful to researchers in need of a model cell culture system in which to study function and regulation of neuronal GABAA receptors.

The GABA-A receptor gene family: new targets for therapeutic intervention

Until 1987, when the first GABA-A receptor subunit cDNAs were cloned and sequenced, it was thought that there were perhaps two subtypes of receptor in the brain. These were defined by the fact that benzodiazepines, which act through the GABA-A receptor, had two binding sites with different affinities. By 1991 it was known that the GABA-A receptor family existed as a family of subunits which coassembled to form a family of receptor subtypes in the brain. More recently, two additional GABA-A receptor subunits have been identified, epsilon and theta. The identification of these new members of the gene family, and the characterisation of the receptor subtypes into which they are incorporated, is reviewed.

Molecular and functional diversity of the expanding GABA-A receptor gene family

Fast inhibitory neurotransmission in the mammalian CNS is mediated primarily by the neurotransmitter gamma-aminobutyric acid (GABA), which, upon binding to its receptor, leads to opening of the intrinsic ion channel, allowing chloride to enter the cell. Over the past 10 years it has become clear that a family of GABA-A receptor subtypes exists, generated through the coassembly of polypeptides selected from alpha 1-alpha 6, beta 1-beta 3, gamma 1-gamma 3, delta, epsilon, and pie to form what is most likely a pentomeric macromolecule. The gene transcripts, and indeed the polypeptides, show distinct patterns of temporal and spatial expression, such that the GABA-A receptor subtypes have a defined localization that presumably reflects their physiological role. A picture is beginning to emerge of the properties conferred to receptor subtypes by the different subunits; these include different functional properties, differential modulation by protein kinases, and the targeting to different membrane compartments. These properties presumably underlie the different physiological roles of the various receptor subtypes. Recently we have identified a further member of the GABA-A receptor gene family, which we have termed theta, which appears to be most closely related to the beta subunits. The structure, function, and distribution of theta-containing receptors, and receptors containing the recently reported epsilon subunit, are described.

Evolution of GABA(A) receptor diversity in the human genome

Nowhere is the record of receptor evolution more accessible than in the organization of the 19 vertebrate genes coding for subunits of the major inhibitory neurotransmitter receptor in the central nervous system, the gamma-aminobutyric acid receptor (GABAAR). Co-expression of alpha, beta, and gamma subunit genes is necessary for the formation of a GABAAR that is potentiated by widely used anxiolytics, anticonvulsants, and hypnotics. The identification of alpha, beta, and gamma genes on chromosomes 4, 5, and 15 suggests that co-localization of a gamma gene with an alpha and beta may be important for brain function. We have now directly examined the organization of GABAAR subunit genes on human chromosomes. Estimates of physical distance using in situ hybridization to cells in interphase, and gene localization using hybridization to cells in metaphase demonstrate the existence of beta-alpha-alpha-gamma gene clusters in cytogenetic bands on chromosomes 4(p12) and 5(q34). Sequencing of PAC clones establishes intercluster conservation of a unique head-to-head configuration for alpha and beta genes on chromosomes 4, 5, and 15. Remarkably, phylogenetic tree analysis predicts the existence of a beta-alpha-gamma ancestral gene cluster in which internal duplication of an ancestral alpha was followed by cluster duplication, resulting in the relative chromosomal positions of modern GABAAR subunit genes in the human genome.

Isolation of human and mouse HMG2a cDNAs: evidence for an HMG2a-specific 3' untranslated region

We have isolated cDNAs of the human gene for high mobility group protein HMG2a, using the method of direct cDNA selection. The gene maps to chromosome band Xq28, and is located within 40 kb from marker DXS1684, at a distance of 5.4 Mb from the telomere. The deduced human HMG2a protein sequence has a length of 199 amino acids and is 97% identical to the sequence of chicken HMG2a. The 3' untranslated regions of the HMG2a gene in both species are highly homologous (87% identical nucleotides), and are even more conserved than the coding sequences (84% identical nucleotides). In addition, a partial cDNA sequence of the putative HMG2a gene from mouse was identified. The 3' untranslated regions from human and mouse are 90% identical. We conclude that the 3' untranslated sequences have been under strong selective pressure during evolution. Whereas expression of the chicken HMG2a gene has previously been demonstrated in liver of newly hatched chicken, the human HMG2a gene is transcribed mainly in placenta.