The GABAA receptor alpha 6 subunit gene (Gabra6) is tightly linked to the alpha 1-gamma 2 subunit cluster on mouse chromosome 11

GABAA Receptor Subunit α3 in Network Dynamics in the Medial Entorhinal Cortex

Layer II of the medial entorhinal cortex (MEC LII) contains the largest number of spatially modulated grid cells and is one of the first regions in the brain to express Alzheimer's disease (AD)-related pathology. The most common principal cell type in MEC LII, reelin-expressing stellate cells, are grid cell candidates. Recently we found evidence that γ-aminobutyric acid (GABA)_A receptor subunits show a specific distribution in MEC LII, in which GABA_A α3 is selectively associated with reelin-positive neurons, with limited association with the other principal cell type, calbindin (CB)-positive pyramidal neurons. Furthermore, the expression of α3 subunit decreases in mice between P15 and P25, which coincides with the emergence of stable grid cell activity. It has been shown that the α3 subunit undergoes specific developmental changes and that it may exert pro-inflammatory actions if improperly regulated. In this review article, we evaluate the changing kinetics of α3-GABA_A receptors (GABA_ARs). during development in relation to α3-subunit expression pattern in MEC LII and conclude that α3 could be closely related to the stabilization of grid cell activity and theta oscillations. We further conclude that dysregulated α3 may be a driving factor in early AD pathology.

A systematic review: Candidate gene and environment interaction on alcohol use and misuse among adolescents and young adults

BACKGROUND AND OBJECTIVES

Youth drinking is a pervasive public health concern with serious negative developmental implications. Candidate gene and environment interaction studies (cGxE) show that environmental effects on drinking behaviors may differ by individuals' genotypes. Yet little is known about whether genetic and environmental effects on drinking behaviors are developmentally specific.

METHODS

This systematic review evaluated 42 cGxE studies of drinking in adolescence and young adulthood.

RESULTS

Although there are mixed findings, studies of cGxE effects involving DRD4, 5-HTTLPR, DRD2, and OPRM1 genotypes showed relatively consistent patterns. The effects of under-controlled environments (eg, low levels of parental monitoring) on early and middle adolescent drinking appeared to differ across DRD2 or OPRM1 genotypes. Effects of alcohol-facilitating environments (eg, heavy drinking peers) on late adolescent and young adult drinking appeared to differ across DRD4 or OPRM1 genotypes. Interactions between 5-HTTLPR genotype with stressful environments (eg, negative life events) were found throughout adolescence and young adulthood, although there were some inconsistencies regarding the risk-conferring allele. There was limited evidence for other cGxE effects due to the small number of studies.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

This review suggests that GxE findings may advance our knowledge regarding which developmentally specific conditions result in the expression of candidate genes that influence youth alcohol use and misuse. However, since a significant number of studies had small sample sizes and most studies had small effect sizes, findings need replication across independent studies with large samples. (Am J Addict 2018;XX:1-19).

The cerebellar GABAAR α6-R100Q polymorphism alters ligand binding in outbred Sprague-Dawley rats in a similar manner as in selectively bred AT and ANT rats

The alcohol-tolerant AT and alcohol-nontolerant ANT rat lines have been selectively bred for innate sensitivity to ethanol-induced motor impairment. The cerebellar GABAA receptor (GABAAR) α6 subunit alleles α6-100R and α6-100Q are segregated in the AT and ANT rats, respectively. This α6 polymorphism might explain various differences in pharmacological properties and density of GABAARs between the rat lines. In the present study, we have used nonselected outbred Sprague-Dawley rats homozygous for the α6-100RR (RR) and α6-100QQ (QQ) genotypes to show that these RR and QQ rats display similar differences between genotypes as AT and ANT rat lines. The genotypes differed in their affinity for [3H]Ro 15-4513 and classic benzodiazepines (BZs) to cerebellar "diazepam-insensitive" (DZ-IS) binding sites, in density of cerebellar [3H]muscimol binding and in the antagonizing effect of furosemide on GABA-induced inhibition of [3H]EBOB binding. The results suggest the involvement of α6-R100Q polymorphism in these line differences and in the differences previously found between AT and ANT rats. In addition, the α6-R100Q polymorphism induces striking differences in [3H]Ro 15-4513 binding kinetics to recombinant α6β3γ2s receptors and cerebellar DZ-IS sites. Association of [3H]Ro 15-4513 binding was ∼10-fold faster and dissociation was ∼3-4-fold faster in DZ-IS α6βγ2 receptors containing the α6-100Q allele, with a resulting change of ∼2.5-fold in equilibrium dissociation constant (KD). The results indicate that in addition to the central role of the homologous α6-100R/Q (α1-101H) residue in BZ binding and efficacy, this critical BZ binding site residue has a major impact on BZ binding kinetics.

Kainate down-regulates a subset of GABAA receptor subunits expressed in cultured mouse cerebellar granule cells

The effect of kainate, an agonist selective for ionotropic AMPA/kainate type of glutamate receptors, on GABAA receptor subunit expression in cultured mouse cerebellar granule cells was studied using quantitative RT-PCR, ligand binding and electrophysiology. Chronic kainate treatment, without producing excitotoxicity, resulted in preferential, dose- and time-dependent down-regulation of alpha1, alpha6 and beta2 subunit mRNA expression, the expression of beta3, gamma2 and delta subunit mRNAs being less affected. The down-regulation was reversed by DNQX, an AMPA/kainate-selective glutamate receptor antagonist. A 14-day kainate treatment resulted in 46% decrease of total [3H]Ro 15-4513 binding to the benzodiazepine sites. Diazepam-insensitive [3H]Ro 15-4513 binding was decreased by 89% in accordance with very low amount of alpha6 subunit mRNA present. Diazepam-sensitive [3H]Ro 154513 binding was decreased only by 40%, contrasting >90% decrease in alpha1 subunit mRNA expression. However, this was consistent with lower potentiation of GABA-evoked currents in kainate-treated than control cells by the alpha1-selective benzodiazepine site ligand zolpidem, suggesting compensatory expression of alpha5 (and/or alpha2 or alpha3) subunits producing diazepam-sensitive but zolpidem-insensitive receptor subtypes. In conclusion, chronic kainate treatment of cerebellar granule cells selectively down-regulates oil, alpha6 and beta2 subunits resulting in altered GABAA receptor pharmacology.

Function of gamma-aminobutyric acid receptor/channel rho 1 subunits in spinal cord

gamma-Aminobutyric acid (GABA) receptor/channel rho 1 subunits are important components in inhibitory pathways in the central nervous system. However, the precise locations and roles of these receptors in the central nervous system are unknown. We studied the expression localization of GABA receptor/channel rho 1 subunit in mouse spinal cord and dorsal root ganglia (DRG). The immunohistochemistry results indicated that GABA receptor/channel rho 1 subunits were expressed in mouse spinal cord superficial dorsal horn (lamina I and lamina II) and in DRG. To understand the functions of the GABA receptor/channel rho 1 subunit in these crucial sites of sensory transmission in vivo, we generated GABA receptor/channel rho 1 subunit mutant mice (rho 1-/-). GABA receptor/channel rho 1 subunit expression in the rho 1-/- mice was eliminated completely, whereas the gross neuroanatomical structures of the rho 1-/- mice spinal cord and DRG were unchanged. Electrophysiological recording showed that GABA-mediated spinal cord response was altered in the rho 1-/- mice. A decreased threshold for mechanical pain in the rho 1-/- mice compared with control mice was observed with the von Frey filament test. These findings indicate that the GABA receptor/channel rho 1 subunit plays an important role in modulating spinal cord pain transmission functions in vivo.

Molecular characterization of new polymorphisms at the beta2, alpha1, gamma2 GABA(A) receptor subunit genes associated to a rat nonpreferring ethanol phenotype

Recent preclinical and clinical studies have indicated a possible involvement of the genes encoding for the GABA(A) receptor subunits alpha6, beta2, alpha1 and gamma2 in the genetic susceptibility to alcohol abuse. We have recently found an (R) to (Q) mutation in codon 100 of the alpha6 GABA(A) subunit, that segregated in a rat line selectively bred for its voluntary ethanol aversion, Sardinian alcohol nonpreferring (sNP), but not in their Sardinian alcohol preferring (sP) counterpart, selected for its ethanol preference. In the present study the molecular composition of other GABA(A) subunits (beta2, alpha1 and gamma2) were analyzed in order to further investigate the involvement of the GABA(A) receptors in the genetic predisposition to voluntary alcohol intake. Automated sequencing analysis indicated the presence of six new silent substitutions (289 T-->C in the beta2 gene; 115 G-->A in the alpha1 gene; 157 G-->A, 174 C-->T, 347 A-->G and 385 A-->T in the gamma2 gene), in sNP but not in sP rats. These polymorphisms were linked to the alpha6 R100Q mutation previously described in sNP rats. The strict association between the alpha6 point mutation and the new polymorphisms found in the beta2, alpha1 and gamma2 genes, demonstrate that such genes belong to the same cluster and are inherited together in the rat. These results sustain the synteny for these clusters between the rodent and human genomes, and suggest that mutated GABA(A) beta2, alpha6, alpha1 and gamma2 subunit genes might contribute to the expression of an ethanol nonpreferring phenotype in a rat line that voluntarily avoids alcoholic solutions.

Elimination of the rho1 subunit abolishes GABA(C) receptor expression and alters visual processing in the mouse retina

Inhibition is crucial for normal function in the nervous system. In the CNS, inhibition is mediated primarily by the amino acid GABA via activation of two ionotropic GABA receptors, GABA(A) and GABA(C). GABA(A) receptor composition and function have been well characterized, whereas much less is known about native GABA(C) receptors. Differences in molecular composition, anatomical distributions, and physiological properties strongly suggest that GABA(A) receptors and GABA(C) receptors have distinct functional roles in the CNS. To determine the functional role of GABA(C) receptors, we eliminated their expression in mice using a knock-out strategy. Although native rodent GABA(C) receptors are composed of rho1 and rho2 subunits, we show that after rho1 subunit expression was selectively eliminated there was no GABA(C) receptor expression. We assessed GABA(C) receptor function in the retina because GABA(C) receptors are highly expressed on the axon terminals of rod bipolar cells and because this site modulates the visual signal to amacrine and ganglion cells. In GABA(C)rho1 null mice, GABA-evoked responses, normally mediated by GABA(C) receptors, were eliminated, and signaling from rod bipolar cells to third order cells was altered. These data demonstrate that elimination of the GABA(C)rho1 subunit, via gene targeting, results in the absence of GABA(C) receptors in the retina and selective alterations in normal visual processing.

Alcohol actions on GABA(A) receptors: from protein structure to mouse behavior

This article represents the proceedings of a symposium at the 2000 ISBRA Meeting in Yokohama, Japan. The chairs were R. Adron Harris and Susumu Ueno. The presentations were (1) Protein kinase Cepsilon-regulated sensitivity of gamma-aminobutyric acid type A (GABAA) receptors to allosteric agonists, by Robert O. Messing, A. M. Sanchez-Perez, C. W. Hodge, T. McMahon, D. Wang, K. K. Mehmert, S. P. Kelley, A. Haywood, and M. F. Olive; (2) Genetic and functional analysis of a GABAA receptor gamma2 subunit variant: A candidate for quantitative trait loci involved in alcohol sensitivity and withdrawal, by Kari J. Buck and Heather M. Hood; (3) Tryptophan-scanning mutagenesis in GABAA receptor subunits: Channel gating and alcohol actions, by Susumu Ueno; and (4) Can a single binding site account for actions of alcohols on GABAA and glycine receptors? by R. Adron Harris, Yuri Blednov, Geoffrey Findlay, and Maria Paola Mascia.

Long-range interactions in neuronal gene expression: evidence from gene targeting in the GABA(A) receptor beta2-alpha6-alpha1-gamma2 subunit gene cluster

Clustering of GABA(A) receptor alpha1, alpha6, beta2, and gamma2 subunit genes on mouse chromosome 11/human chromosome 5 may have functional significance for coordinating expression patterns, but until now there has been no evidence for cross-talk between the genes. However, altering the structure of the alpha6 gene, specifically expressed in the cerebellum, with neomycin gene insertions in two different experiments unexpectedly reduced the expression of the widespread alpha1 and beta2 genes in the forebrain. There were corresponding reductions in the levels of alpha1 and beta2 subunit proteins and in autoradiographic ligand binding densities to GABA(A) receptors in the forebrain of alpha6-/- mice. The gamma2 mRNA level was not changed, nor were beta3 and delta mRNAs. The data suggest that elements in the neo gene may have an influence over long distances in the GABA(A) subunit gene complex on as yet undefined structures coordinating the expression of the alpha1 and beta2 genes.

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.

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.

Directing gene expression to cerebellar granule cells using gamma-aminobutyric acid type A receptor alpha6 subunit transgenes

Expression of the gamma-aminobutyric acid type A receptor alpha6 subunit gene is restricted to differentiated granule cells of the cerebellum and cochlear nucleus. The mechanisms underlying this limited expression are unknown. Here we have characterized the expression of a series of alpha6-based transgenes in adult mouse brain. A DNA fragment containing a 1-kb portion upstream of the start site(s), together with exons 1-8, can direct high-level cerebellar granule cell-specific reporter gene expression. Thus powerful granule cell-specific determinants reside within the 5' half of the alpha6 subunit gene body. This intron-containing transgene appears to lack the cochlear nucleus regulatory elements. It therefore provides a cassette to deliver gene products solely to adult cerebellar granule cells.

Quantitative trait loci involved in genetic predisposition to acute alcohol withdrawal in mice

Alcohol dependence (alcoholism) is accompanied by evidence of tolerance, withdrawal (physiological dependence), or compulsive behavior related to alcohol use. Studies of strain and individual differences using animal models for acute physiological dependence liability are useful means to identify potential genetic determinants of liability in humans. Behavioral and quantitative trait analyses were conducted using animal models for high risk versus resistance to acute physiological dependence. Using a two-step genetic mapping strategy, loci on mouse chromosomes 1, 4, and 11 were mapped that contain genes that influence alcohol withdrawal severity. In the aggregate, these three risk markers accounted for 68% of the genetic variability in alcohol withdrawal. Candidate genes in proximity to the chromosome 11 locus include genes encoding the alpha1, alpha6, and gamma2 subunits of type-A receptors for the inhibitory neurotransmitter, GABA. In addition, suggestive linkage is indicated for two loci on mouse chromosome 2, one near Gad1 encoding glutamic acid decarboxylase, and the other near the El2 locus which influences the seizure phenotype in the neurological mutant strain El. The present analyses detect and map some of the loci that increase risk to develop physiological dependence and may facilitate identification of genes related to the development of alcoholism. Syntenic conservation between human and mouse chromosomes suggests that human homologs of genes that increase risk for physiological dependence may localize to 1q21-q32, 2q24-q37/11p13, 9p21-p23/1p32-p22.1, and 5q32-q35.