Strain comparisons and developmental profile of the delta subunit of the murine GABAA receptor

Neuroinformatics: a new tool for studying the brain

BACKGROUND

Central nervous system diseases constitute a major target for drug development. Genes expressed by the nervous system may represent half or more of the mammalian genome, with literally tens of thousands of gene products.

METHODS

Better methods are therefore required to accelerate the pace of mapping gene expression patterns in the mouse brain and to evaluate the progressive phenotypic changes in genetic models of human brain diseases.

CONCLUSIONS

Recent studies of mouse models of Amyotrophic Lateral Sclerosis and Alzheimer's disease illustrate how such data could be used for drug development. Since these two diseases-- especially Alzheimer's Disease-- entail disordered behavior, cognition and emotions, the framework and the methodology described in this article might in the future find applications in research on affective disorders.

Role of the GABA(A)beta2, GABA(A)alpha6, GABA(A)alpha1 and GABA(A)gamma2 receptor subunit genes cluster in drug responses and the development of alcohol dependence

gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter of the central nervous system and it acts at the GABA(A) and GABA(B) receptors. A possible role for the GABA(A) receptors in alcohol action has been derived from in vitro cell models, animal studies and human research. GABA(A) subunit mRNA expression in cell models has suggested that the long form of the gamma2 subunit is essential for ethanol enhanced potentiation of GABA(A) receptors, by phosphorylation of a serine contained within the extra eight amino acids. Several animal studies have demonstrated that alterations in drug and alcohol responses may be caused by amino-acid differences at the GABA(A)alpha6 and GABA(A)gamma2 subunits. An Arg(100)/Glu(100) change at the GABA(A)alpha6 subunit conferring altered binding efficacy of the benzodiazepine inverse agonist Ro 15-4513, was found between the AT (alcohol tolerance) and ANT (alcohol non-tolerance) rats. Several loci related to alcohol withdrawal on mouse chromosome 11 which corresponds to the region containing four GABA(A) subunit (beta2, alpha6, alpha1 and gamma2) genes on human chromosome 5q33-34, were also identified. Gene knockout studies of the role of GABA(A)alpha6 and GABA(A)gamma2 subunit genes in mice have demonstrated an essential role in the modulation of other GABA(A) subunit expression and the efficacy of benzodiazepine binding. Absence of the GABA(A)gamma2 subunit gene has more severe effects with many of the mice dying shortly after birth. Disappointingly few studies have examined the effects of response to alcohol in these gene knockout mice. Human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have a role in the development of alcohol dependence, although their contributions may vary between ethnic group and phenotype. In summary, in vitro cell, animal and human genetic association studies have suggested that the GABA(A)beta2, alpha6, alpha1 and gamma2 subunit genes have an important role in alcohol related phenotypes (300 words).

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

GABA(A) receptor alpha4 subunit in DBA/2J and C57BL/6J mice

GABA(A) receptors are chloride channels in the brain activated by binding of gamma-aminobutyric acid (GABA). Several important classes of drugs, including alcohol and certain antiepileptic drugs, modulate the actions of GABA. We report the sequence and expression of alpha4 subunits of GABA(A) receptors in two inbred strains of mice, DBA/2J and C57BL/6J, which differ in susceptibility to seizures and to behavioral effects of alcohol. We find no differences between the two strains in cDNA sequence, or in levels of alpha4 mRNA in whole brains of the two strains at 21 days of age, when DBA/2J are most susceptible to audiogenic seizures. We also describe the pattern of developmental expression and brain regional distribution of this subunit in mice, finding the highest developmental expression at about 14 days of age in whole brains, and the highest regional levels in hippocampus and basal forebrain (including thalamus) in adults.

The alpha5 subunit of the murine type A GABA receptor

GABA[A] receptors in the brain convert binding of GABA (gamma-aminobutyric acid) to inhibition by chloride currents. Several important classes of drugs, including benzodiazepines and alcohol, modulate these receptors, which have also been implicated in epilepsy. We describe the alpha5 subunit of GABAA receptors in mice, comparing inbred DBA/2J mice, prone to juvenile audiogenic seizures, with seizure resistant C57BL/6J mice. We find no sequence differences between the strains, although there are several interesting amino acid differences from the rat. We also compare the expression of the alpha5 subunit in whole brains of DBA/2J mice to that in C57BL/6J mice at 21 days, the peak of the former's seizure susceptibility, again finding no significant difference. We further describe the pattern of expression of alpha5 mRNA during mouse brain development, with a peak at 3 days after birth, and among five brain regions in the adult mouse, with the highest levels in the hippocampus. Finally, we present preliminary evidence for rare alternative splicing of this subunit's message, in the N-terminal extracellular domain, to give a form not translatable into a functional protein.

The GABA(A) receptor gamma1-subunit in seizure prone (DBA/2) and resistant (C57BL/6) mice

Gamma-aminobutyric acid (GABA)A receptors are the sites of action for many antiepileptic drugs such as benzodiazepines and barbiturates. We report the results of molecular cloning of the gamma1-subunit from seizure prone DBA/2J and resistant C57BL/6J inbred mice, and analyses of nucleotide sequences and expression of the gamma1-subunit messenger RNA (mRNA) in DBA/2 and C57BL/6 inbred mice. The mouse gamma1-subunit complementary DNA (cDNA) shares 98% similarity with that of the rat at the level of amino acid sequence. Northern blot hybridization indicates that the gamma1-subunit mRNA is expressed predominantly in areas other than the cerebral cortex and cerebellum and shows little change with postnatal development. No differences have been found for the subunit between DBA/2 and C57BL/6 mice either for nucleotide sequence or for level of expression of the subunit's mRNA in whole brain by Northern blots at 3 weeks of age.

Structural and pharmacological aspects of the GABAA receptor: involvement in behavioral pathogenesis

The gamma-aminobutyric acidA (GABAA) receptor is a complex hetero-oligomeric protein. It is composed of several subunits which assemble to form a functional chloride channel. The precise molecular organization of the receptor is as yet unknown. In the first part, we review recent literature dealing with the molecular and pharmacological aspects of the GABAA receptor, the second part will review some of the pathologies probably associated with gene defects and/or quantitative differential expression of transcripts encoding GABAA receptor subunits.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

GABAA receptor beta 1, beta 2, and beta 3 subunits: comparisons in DBA/2J and C57BL/6J mice

GABAA receptors link binding of GABA (gamma-aminobutyric acid) to inhibitory chloride flux in the brain. They are the site of action of several important classes of drugs, and have been implicated in animal models of epilepsy and in the actions of alcohol. We compare the sequence and expression of the beta 1, beta 2 and beta 3 subunits of GABAA receptors in two inbred strains of mice, DBA/2J and C57BL/6J, which differ markedly in seizure susceptibility and in a variety of behaviors related to alcohol. Only the beta 3 subunit had strain differences in cDNA nucleotide sequence, which did not affect amino acid sequence but which did create restriction fragment length polymorphisms (RFLPs) potentially useful in gene mapping. We have also tested mouse beta 1 and beta 2 subunits for internal alternative splicing, detecting none.

GABAA, GABAC, and NMDA receptor subunit expression in the suprachiasmatic nucleus and other brain regions

Identification of the neurotransmitter receptor subtypes within the suprachiasmatic nuclei (SCN) will further understanding of the mechanism of the biological clock and may provide targets to manipulate circadian rhythms pharmacologically. We have focused on the ionotropic GABA and glutamate receptors because these appear to account for the majority of synaptic communication in the SCN. Of the 15 genes known to code for GABA receptor subunits in mammals we have examined the expression of 12 in the SCN, neglecting only the alpha 6, gamma 3, and rho 2 subunits. Among glutamate receptors, we have focused on the five known genes coding for the NMDA receptor subunits, and two subunits which help comprise the kainate-selective receptors. Expression was characterized by Northern analysis with RNA purified from a large number of mouse SCN and compared to expression in the remaining hypothalamus, cortex and cerebellum. This approach provided a uniform source of RNA to generate many replicate blots, each of which was probed repeatedly. The most abundant GABA receptor subunit mRNAs in the SCN were alpha 2, alpha 5, beta 1, beta 3, gamma 1 and gamma 2. The rho 1 (rho 1) subunit, which produces GABAC pharmacology, was expressed primarily in the retina in three different species and was not detectable in the mouse SCN despite a common embryological origin with the retina. For several GABA subunits we detected additional mRNA species not previously described. High expression of both genes coding for glutamic acid decarboxylase (GAD65 and GAD67) was also found in the SCN. Among the NMDA receptor subunits, NR1 was most highly expressed in the SCN followed in order of abundance by NR2B, NR2A, NR2C and NR2D. In addition, both GluR5 and GluR6 show clear expression in the SCN, with GluR5 being the most SCN specific. This approach provides a simple measure of receptor subtype expression, complements in situ hybridization studies, and may suggest novel isoforms of known subunits.