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β subunits of GABA A receptors form proton-gated chloride channels: Insights into the molecular basis. Commun Biol 2022; 5:784. [PMID: 35922471 PMCID: PMC9349252 DOI: 10.1038/s42003-022-03720-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/20/2021] [Accepted: 07/14/2022] [Indexed: 11/17/2022] Open
Abstract
Gamma-aminobutyric acid type A receptors (GABAARs) are ligand gated channels mediating inhibition in the central nervous system. Here, we identify a so far undescribed function of β-subunit homomers as proton-gated anion channels. Mutation of a single H267A in β3 subunits completely abolishes channel activation by protons. In molecular dynamic simulations of the β3 crystal structure protonation of H267 increased the formation of hydrogen bonds between H267 and E270 of the adjacent subunit leading to a pore stabilising ring formation and accumulation of Cl- within the transmembrane pore. Conversion of these residues in proton insensitive ρ1 subunits transfers proton-dependent gating, thus highlighting the role of this interaction in proton sensitivity. Activation of chloride and bicarbonate currents at physiological pH changes (pH50 is in the range 6- 6.3) and kinetic studies suggest a physiological role in neuronal and non-neuronal tissues that express beta subunits, and thus as potential novel drug target. Beta subunits of GABAA receptors are unexpectedly shown to form homomeric proton gated ion channels attributable to a single histidine residue.
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Jatczak-Śliwa M, Kisiel M, Czyzewska MM, Brodzki M, Mozrzymas JW. GABA A Receptor β 2E155 Residue Located at the Agonist-Binding Site Is Involved in the Receptor Gating. Front Cell Neurosci 2020; 14:2. [PMID: 32116555 PMCID: PMC7026498 DOI: 10.3389/fncel.2020.00002] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/26/2019] [Accepted: 01/06/2020] [Indexed: 12/02/2022] Open
Abstract
GABAA receptors (GABAARs) play a crucial role in mediating inhibition in the adult brain. In spite of progress in describing (mainly) the static structures of this receptor, the molecular mechanisms underlying its activation remain unclear. It is known that in the α1β2γ2L receptors, the mutation of the β2E155 residue, at the orthosteric binding site, strongly impairs the receptor activation, but the molecular and kinetic mechanisms of this effect remain elusive. Herein, we investigated the impact of the β2E155C mutation on binding and gating of the α1β2γ2L receptor. To this end, we combined the macroscopic and single-channel analysis, the use of different agonists [GABA and muscimol (MSC)] and flurazepam (FLU) as a modulator. As expected, the β2E155C mutation caused a vast right shift of the dose–response (for GABA and MSC) and, additionally, dramatic changes in the time course of current responses, indicative of alterations in gating. Mutated receptors showed reduced maximum open probability and enhanced receptor spontaneous activity. Model simulations for macroscopic currents revealed that the primary effect of the mutation was the downregulation of the preactivation (flipping) rate. Experiments with MSC and FLU further confirmed a reduction in the preactivation rate. Our single-channel analysis revealed the mutation impact mainly on the second component in the shut times distributions. Based on model simulations, this finding further confirms that this mutation affects mostly the preactivation transition, supporting thus the macroscopic data. Altogether, we provide new evidence that the β2E155 residue is involved in both binding and gating (primarily preactivation).
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Affiliation(s)
- Magdalena Jatczak-Śliwa
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland.,Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
| | - Magdalena Kisiel
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland
| | | | - Marek Brodzki
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław, Poland.,Department of Molecular Physiology and Neurobiology, University of Wrocław, Wrocław, Poland
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O'Neill N, Sylantyev S. The Functional Role of Spontaneously Opening GABA A Receptors in Neural Transmission. Front Mol Neurosci 2019; 12:72. [PMID: 30983968 PMCID: PMC6447609 DOI: 10.3389/fnmol.2019.00072] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/22/2019] [Accepted: 03/08/2019] [Indexed: 12/22/2022] Open
Abstract
Ionotropic type of γ-aminobutyric acid receptors (GABAARs) produce two forms of inhibitory signaling: phasic inhibition generated by rapid efflux of neurotransmitter GABA into the synaptic cleft with subsequent binding to GABAARs, and tonic inhibition generated by persistent activation of extrasynaptic and/or perisynaptic GABAARs by GABA continuously present in the extracellular space. It is widely accepted that phasic and tonic GABAergic inhibition is mediated by receptor groups of distinct subunit composition and modulated by different cytoplasmic mechanisms. Recently, however, it has been demonstrated that spontaneously opening GABAARs (s-GABAARs), which do not need GABA binding to enter an active state, make a significant input into tonic inhibitory signaling. Due to GABA-independent action mode, s-GABAARs promise new safer options for therapy of neural disorders (such as epilepsy) devoid of side effects connected to abnormal fluctuations of GABA concentration in the brain. However, despite the potentially important role of s-GABAARs in neural signaling, they still remain out of focus of neuroscience studies, to a large extent due to technical difficulties in their experimental research. Here, we summarize present data on s-GABAARs functional properties and experimental approaches that allow isolation of s-GABAARs effects from those of conventional (GABA-dependent) GABAARs.
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Affiliation(s)
- Nathanael O'Neill
- Center for Clinical Brain Sciences, University of Edinburgh, Edinburgh, United Kingdom
| | - Sergiy Sylantyev
- Center for Clinical Brain Sciences, University of Edinburgh, Edinburgh, United Kingdom
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Wongsamitkul N, Maldifassi MC, Simeone X, Baur R, Ernst M, Sigel E. α subunits in GABA A receptors are dispensable for GABA and diazepam action. Sci Rep 2017; 7:15498. [PMID: 29138471 PMCID: PMC5686171 DOI: 10.1038/s41598-017-15628-7] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2017] [Accepted: 10/31/2017] [Indexed: 11/10/2022] Open
Abstract
The major isoform of the GABAA receptor is α1β2γ2. The binding sites for the agonist GABA are located at the β2+/α1− subunit interfaces and the modulatory site for benzodiazepines at α1+/γ2−. In the absence of α1 subunits, a receptor was formed that was gated by GABA and modulated by diazepam similarly. This indicates that alternative subunits can take over the role of the α1 subunits. Point mutations were introduced in β2 or γ2 subunits at positions homologous to α1− benzodiazepine binding and GABA binding positions, respectively. From this mutation work we conclude that the site for GABA is located at a β2+/β2− subunit interface and that the diazepam site is located at the β2+/γ2− subunit interface. Computational docking leads to a structural hypothesis attributing this non-canonical interaction to a binding mode nearly identical with the one at the α1+/γ2− interface. Thus, the β2 subunit can take over the role of the α1 subunit for the formation of both sites, its minus side for the GABA binding site and its plus side for the diazepam binding site.
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Affiliation(s)
- Nisa Wongsamitkul
- Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland
| | - Maria C Maldifassi
- Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland.,Centro Interdisciplinario de Neurociencia de Valparaíso. Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
| | - Xenia Simeone
- Department of Molecular Neurosciences, Center for Brain Research, Medical University of Vienna, Vienna, Austria
| | - Roland Baur
- Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland
| | - Margot Ernst
- Department of Molecular Neurosciences, Center for Brain Research, Medical University of Vienna, Vienna, Austria
| | - Erwin Sigel
- Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland.
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Inhibitory synapse deficits caused by familial α1 GABA A receptor mutations in epilepsy. Neurobiol Dis 2017; 108:213-224. [PMID: 28870844 DOI: 10.1016/j.nbd.2017.08.020] [Citation(s) in RCA: 14] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/04/2017] [Revised: 08/17/2017] [Accepted: 08/24/2017] [Indexed: 11/20/2022] Open
Abstract
Epilepsy is a spectrum of neurological disorders with many causal factors. The GABA type-A receptor (GABAAR) is a major genetic target for heritable human epilepsies. Here we examine the functional effects of three epilepsy-causing mutations to the α1 subunit (α1T10'I, α1D192N and α1A295D) on inhibitory postsynaptic currents (IPSCs) mediated by the major synaptic GABAAR isoform, α1β2γ2L. We employed a neuron - HEK293 cell heterosynapse preparation to record IPSCs mediated by mutant-containing GABAARs in isolation from other GABAAR isoforms. IPSCs were recorded in the presence of the anticonvulsant drugs, carbamazepine and midazolam, and at elevated temperatures (22, 37 and 40°C) to gain insight into mechanisms of febrile seizures. The mutant subunits were also transfected into cultured cortical neurons to investigate changes in synapse formation and neuronal morphology using fluorescence microscopy. We found that IPSCs mediated by α1T10'Iβ2γ2L, α1D192Nβ2γ2L GABAARs decayed faster than those mediated by α1β2γ2L receptors. IPSCs mediated by α1D192Nβ2γ2L and α1A295Dβ2γ2L receptors also exhibited a heightened temperature sensitivity. In addition, the α1T10'Iβ2γ2L GABAARs were refractory to modulation by carbamazepine or midazolam. In agreement with previous studies, we found that α1A295Dβ2γ2L GABAARs were retained intracellularly in HEK293 cells and neurons. However, pre-incubation with 100nM suberanilohydroxamic acid (SAHA) induced α1A295Dβ2γ2L GABAARs to mediate IPSCs that were indistinguishable in magnitude and waveform from those mediated by α1β2γ2L receptors. Finally, mutation-specific changes to synaptic bouton size, synapse number and neurite branching were also observed. These results provide new insights into the mechanisms of epileptogenesis of α1 epilepsy mutations and suggest possible leads for improving treatments for patients harbouring these mutations.
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Chua HC, Absalom NL, Hanrahan JR, Viswas R, Chebib M. The Direct Actions of GABA, 2'-Methoxy-6-Methylflavone and General Anaesthetics at β3γ2L GABAA Receptors: Evidence for Receptors with Different Subunit Stoichiometries. PLoS One 2015; 10:e0141359. [PMID: 26496640 PMCID: PMC4619705 DOI: 10.1371/journal.pone.0141359] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2015] [Accepted: 10/07/2015] [Indexed: 12/13/2022] Open
Abstract
2'-Methoxy-6-methylflavone (2'MeO6MF) is an anxiolytic flavonoid which has been shown to display GABAA receptor (GABAAR) β2/3-subunit selectivity, a pharmacological profile similar to that of the general anaesthetic etomidate. Electrophysiological studies suggest that the full agonist action of 2'MeO6MF at α2β3γ2L GABAARs may mediate the flavonoid's in vivo effects. However, we found variations in the relative efficacy of 2'MeO6MF (2'MeO6MF-elicited current responses normalised to the maximal GABA response) at α2β3γ2L GABAARs due to the presence of mixed receptor populations. To understand which receptor subpopulation(s) underlie the variations observed, we conducted a systematic investigation of 2'MeO6MF activity at all receptor combinations that could theoretically form (α2, β3, γ2L, α2β3, α2γ2L, β3γ2L and α2β3γ2L) in Xenopus oocytes using the two-electrode voltage clamp technique. We found that 2'MeO6MF activated non-α-containing β3γ2L receptors. In an attempt to establish the optimal conditions to express a uniform population of these receptors, we found that varying the relative amounts of β3:γ2L subunit mRNAs resulted in differences in the level of constitutive activity, the GABA concentration-response relationships, and the relative efficacy of 2'MeO6MF activation. Like 2'MeO6MF, general anaesthetics such as etomidate and propofol also showed distinct levels of relative efficacy across different injection ratios. Based on these results, we infer that β3γ2L receptors may form with different subunit stoichiometries, resulting in the complex pharmacology observed across different injection ratios. Moreover, the discovery that GABA and etomidate have direct actions at the α-lacking β3γ2L receptors raises questions about the structural requirements for their respective binding sites at GABAARs.
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Affiliation(s)
- Han Chow Chua
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
| | - Nathan L Absalom
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
| | - Jane R Hanrahan
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
| | - Raja Viswas
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
| | - Mary Chebib
- Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia
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Absalom N, Yamamoto I, O'Hagan D, Hunter L, Chebib M. Probing the Mode of Neurotransmitter Binding to GABA Receptors Using Selectively Fluorinated GABA Analogues. Aust J Chem 2015. [DOI: 10.1071/ch14456] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Stereoselective fluorination is a useful technique for controlling the conformations of organic molecules. This concept has been exploited to create conformationally biased analogues of the neurotransmitter gamma-aminobutyric acid (GABA). Mono- and di-fluorinated GABA analogues are found to adopt different conformations, due to subtle stereoelectronic effects associated with the C–F bond. These conformationally biased GABA analogues exhibit different shape-dependent selectivity patterns towards GABAA, GABAB, and GABAC receptors, providing valuable information on the binding modes of the natural ligand at these medicinally important targets.
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Dash B, Lukas RJ. Modulation of gain-of-function α6*-nicotinic acetylcholine receptor by β3 subunits. J Biol Chem 2012; 287:14259-69. [PMID: 22315221 DOI: 10.1074/jbc.m111.322610] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
We previously have shown that β3 subunits either eliminate (e.g. for all-human (h) or all-mouse (m) α6β4β3-nAChR) or potentiate (e.g. for hybrid mα6hβ4hβ3- or mα6mβ4hβ3-nAChR containing subunits from different species) function of α6*-nAChR expressed in Xenopus oocytes, and that nAChR hα6 subunit residues Asn-143 and Met-145 in N-terminal domain loop E are important for dominant-negative effects of nAChR hβ3 subunits on hα6*-nAChR function. Here, we tested the hypothesis that these effects of β3 subunits would be preserved even if nAChR α6 subunits harbored gain-of-function, leucine- or valine-to-serine mutations at 9' or 13' positions (L9'S or V13'S) in their second transmembrane domains, yielding receptors with heightened functional activity and more amenable to assessment of effects of β3 subunit incorporation. However, coexpression with β3 subunits potentiates rather than suppresses function of all-human, all-mouse, or hybrid α6((L9'S or V13'S))β4*- or α6(N143D+M145V)(L9'S)β2*-nAChR. This contrasts with the lack of consistent function when α6((L9'S or V13'S)) and β2 subunits are expressed alone or in the presence of wild-type β3 subunits. These results provide evidence that gain-of-function hα6hβ2*-nAChR (i.e. hα6(N143D+M145V)(L9'S)hβ2hβ3 nAChR) could be produced in vitro. These studies also indicate that nAChR β3 subunits can be assembly partners in functional α6*-nAChR and that 9' or 13' mutations in the nAChR α6 subunit second transmembrane domain can act as gain-of-function and/or reporter mutations. Moreover, our findings suggest that β3 subunit coexpression promotes function of α6*-nAChR.
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Affiliation(s)
- Bhagirathi Dash
- Division of Neurobiology, Barrow Neurological Institute, Phoenix, Arizona 85013, USA
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Dash B, Chang Y, Lukas RJ. Reporter mutation studies show that nicotinic acetylcholine receptor (nAChR) α5 Subunits and/or variants modulate function of α6*-nAChR. J Biol Chem 2011; 286:37905-37918. [PMID: 21873428 DOI: 10.1074/jbc.m111.264044] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
To further the understanding of functional α6α5*-nicotinic acetylcholine receptors (nAChR; the asterisk (*) indicates known or possible presence of other subunits), we have heterologously expressed in oocytes different, mouse or human, nAChR subunit combinations. Coexpression with wild-type α5 subunits or chimeric α5/β3 subunits (in which the human α5 subunit N-terminal, extracellular domain is linked to the remaining domains of the human β3 subunit) almost completely abolishes the very small amount of function seen for α6β4*-nAChR and does not induce function of α6β2*-nAChR. Coexpression with human α5(V9)'(S) subunits bearing a valine 290 to serine mutation in the 9' position of the second transmembrane domain does not rescue the function of α6β4*-nAChR or induce function of α6β2*-nAChR. However, coexpression with mutant chimeric α5/β3(V9)'(S) subunits has a gain-of-function effect (higher functional expression and agonist sensitivity and spontaneous opening inhibited by mecamylamine) on α6β4*-nAChR. Moreover, N143D + M145V mutations in the α6 subunit N-terminal domain enable α5/β3(V9)'(S) subunits to have a gain-of-function effect on α6β2*-nAChR. nAChR containing chimeric α6/α3 subunits plus either β2 or β4 subunits have some function that is modulated in the presence of α5 or α5/β3 subunits. Coexpression with α5/β3(V9)'(S) subunits has a gain-of-function effect more pronounced than that in the presence of α5(V9)'(S) subunits. Gain-of-function effects are dependent, sometimes subtly, on the nature and apparently the extracellular, cytoplasmic, and/or transmembrane domain topology of partner subunits. These studies yield insight into assembly of functional α6α5*-nAChR and provide tools for development of α6*-nAChR-selective ligands that could be important in the treatment of nicotine dependence, and perhaps other neurological diseases.
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Affiliation(s)
- Bhagirathi Dash
- Division of Neurobiology, Barrow Neurological Institute, Phoenix, Arizona 85013
| | - Yongchang Chang
- Division of Neurobiology, Barrow Neurological Institute, Phoenix, Arizona 85013
| | - Ronald J Lukas
- Division of Neurobiology, Barrow Neurological Institute, Phoenix, Arizona 85013.
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Chen L, Xue L, Giacomini KM, Casida JE. GABAA receptor open-state conformation determines non-competitive antagonist binding. Toxicol Appl Pharmacol 2010; 250:221-8. [PMID: 21111751 DOI: 10.1016/j.taap.2010.11.010] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2010] [Revised: 11/10/2010] [Accepted: 11/15/2010] [Indexed: 10/18/2022]
Abstract
The γ-aminobutyric acid (GABA) type A receptor (GABA(A)R) is one of the most important targets for insecticide action. The human recombinant β3 homomer is the best available model for this binding site and 4-n-[(3)H]propyl-4'-ethynylbicycloorthobenzoate ([(3)H]EBOB) is the preferred non-competitive antagonist (NCA) radioligand. The uniquely high sensitivity of the β3 homomer relative to the much-less-active but structurally very-similar β1 homomer provides an ideal comparison to elucidate structural and functional features important for NCA binding. The β1 and β3 subunits were compared using chimeragenesis and mutagenesis and various combinations with the α1 subunit and modulators. Chimera β3/β1 with the β3 subunit extracellular domain and the β1 subunit transmembrane helices retained the high [(3)H]EBOB binding level of the β3 homomer while chimera β1/β3 with the β1 subunit extracellular domain and the β3 subunit transmembrane helices had low binding activity similar to the β1 homomer. GABA at 3μM stimulated heteromers α1β1 and α1β3 binding levels more than 2-fold by increasing the open probability of the channel. Addition of the α1 subunit rescued the inactive β1/β3 chimera close to wildtype α1β1 activity. EBOB binding was significantly altered by mutations β1S15'N and β3N15'S compared with wildtype β1 and β3, respectively. However, the binding activity of α1β1S15'N was insensitive to GABA and α1β3N15'S was stimulated much less than wildtype α1β3 by GABA. The inhibitory effect of etomidate on NCA binding was reduced more than 5-fold by the mutation β3N15'S. Therefore, the NCA binding site is tightly regulated by the open-state conformation that largely determines GABA(A) receptor sensitivity.
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Affiliation(s)
- Ligong Chen
- Environmental Chemistry and Toxicology Laboratory, Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720, USA
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Miller PS, Smart TG. Binding, activation and modulation of Cys-loop receptors. Trends Pharmacol Sci 2010; 31:161-74. [PMID: 20096941 DOI: 10.1016/j.tips.2009.12.005] [Citation(s) in RCA: 245] [Impact Index Per Article: 17.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2009] [Revised: 12/17/2009] [Accepted: 12/18/2009] [Indexed: 11/19/2022]
Abstract
It is over forty years since the major neurotransmitters and their protein receptors were identified, and over twenty years since determination of the first amino-acid sequences of the Cys-loop receptors that recognize acetylcholine, serotonin, GABA and glycine. The last decade has seen the first structures of these proteins (and related bacterial and molluscan homologues) determined to atomic resolution. Hopefully over the next decade, more detailed molecular structures of entire Cys-loop receptors in drug-bound and drug-free conformations will become available. These, together with functional studies, will provide a clear picture of how these receptors participate in neurotransmission and how structural variations between receptor subtypes impart their unique characteristics. This insight should facilitate the design of novel and improved therapeutics to treat neurological disorders. This review considers our current understanding about the processes of agonist binding, receptor activation and channel opening, as well as allosteric modulation of the Cys-loop receptor family.
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Affiliation(s)
- Paul S Miller
- Department of Neuroscience, Physiology & Pharmacology, University College London, Gower Street, London WC1E 6BT, UK
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Gamma-amino butyric acid type A receptor mutations at beta2N265 alter etomidate efficacy while preserving basal and agonist-dependent activity. Anesthesiology 2010; 111:774-84. [PMID: 19741491 DOI: 10.1097/aln.0b013e3181b55fae] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022]
Abstract
BACKGROUND Etomidate acts at gamma-Aminobutyric acid type A (GABAA) receptors containing beta2 or beta3, but not beta1 subunits. Mutations at beta residue 265 (Ser in beta1; Asn in beta2 or beta3) profoundly affect etomidate sensitivity. Whether these mutations alter etomidate binding remains uncertain. METHODS Heterologously expressed alpha1beta2gamma2L GABAA receptors and receptors with beta2(N265S) or beta2(N265M) mutations were studied electrophysiologically in both Xenopus oocytes and HEK293 cells. Experiments quantified the impact of beta2N265 mutations or substituting beta1 for beta2 on basal channel activation, GABA EC50, maximal GABA efficacy, etomidate-induced leftward shift in GABA responses, etomidate direct activation, and rapid macrocurrent kinetics. Results were analyzed in the context of an established allosteric co-agonist mechanism. RESULTS Mutations produced only small changes in basal channel activity, GABA EC50, maximal GABA efficacy, and macrocurrent kinetics. Relative to wild-type, beta2(N265S) reduced etomidate enhancement of apparent GABA affinity six-fold, and it reduced etomidate direct activation efficacy 14-fold. beta2(N265M) totally eliminated both etomidate modulation of GABA responses and direct channel activation. Mechanism-based analysis showed that the function of both mutants remains consistent with the allosteric co-agonist model and that beta2(N265S) reduced etomidate allosteric efficacy five-fold, whereas etomidate-binding affinity dropped threefold. Experiments swapping beta2 subunits for beta1 indicated that etomidate efficacy is reduced 34-fold, whereas binding affinity drops less than two-fold. CONCLUSIONS Mutations at beta2N265 profoundly alter etomidate sensitivity with only small changes in basal and GABA-dependent channel activity. Mutations at the beta2N265 residue or replacement of beta2 with beta1 influence etomidate efficacy much more than binding to inactive receptors.
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γ-Aminobutyric Acid Type A (GABAA) Receptor Subtype Inverse Agonists as Therapeutic Agents in Cognition. Methods Enzymol 2010; 485:197-211. [DOI: 10.1016/b978-0-12-381296-4.00011-7] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/05/2022]
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Saras A, Gisselmann G, Vogt-Eisele AK, Erlkamp KS, Kletke O, Pusch H, Hatt H. Histamine Action on Vertebrate GABAA Receptors. J Biol Chem 2008; 283:10470-5. [DOI: 10.1074/jbc.m709993200] [Citation(s) in RCA: 58] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
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Hisano K, Ozoe F, Huang J, Kong X, Ozoe Y. The channel-lining 6' amino acid in the second membrane-spanning region of ionotropic GABA receptors has more profound effects on 4'-ethynyl-4-n-propylbicycloorthobenzoate binding than the 2' amino acid. INVERTEBRATE NEUROSCIENCE 2007; 7:39-46. [PMID: 17205299 DOI: 10.1007/s10158-006-0035-x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/11/2006] [Accepted: 12/12/2006] [Indexed: 10/23/2022]
Abstract
The noncompetitive antagonist of ionotropic gamma-aminobutyric acid (GABA) receptors 4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) is a useful tool to probe the antagonist-binding site. In the present study, four mutants of the human GABA(A) receptor beta3 subunit were stably expressed in S2 cells and examined for their abilities to bind [(3)H]EBOB to identify the binding site of EBOB. The homo-oligomeric beta3 GABA receptor was used as a housefly GABA receptor model, as the beta3 subunit has a high sequence similarity with the housefly Rdl subunit in the second membrane-spanning (M2) region. The A274S mutation at the -1' position in the M2 region had no effect on [(3)H]EBOB binding. The A277S mutation at the 2' position led to a decrease in the affinity of EBOB for the GABA receptor. The T281V mutant at the 6' position and the A277S/T281V double mutant completely abolished the binding ability. A beta3 GABA receptor homology model predicts these interactions between the receptor and EBOB. These results suggest that EBOB interacts with threonine 281 and alanine 277, and that threonine 281 plays a more critical role in interacting with EBOB than alanine 277.
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Affiliation(s)
- Kazutoshi Hisano
- Department of Life Science and Biotechnology, Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane, 690-8504, Japan
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McCartney MR, Deeb TZ, Henderson TN, Hales TG. Tonically Active GABAA Receptors in Hippocampal Pyramidal Neurons Exhibit Constitutive GABA-Independent Gating. Mol Pharmacol 2006; 71:539-48. [PMID: 17090706 DOI: 10.1124/mol.106.028597] [Citation(s) in RCA: 72] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Phasic and tonic inhibitory currents of hippocampal pyramidal neurons exhibit distinct pharmacological properties. Picrotoxin and bicuculline methiodide inhibited both components, consistent with a role for GABAA receptors; however, gabazine, at a concentration that abolished miniature GABAergic inhibitory postsynaptic currents and responses to exogenous GABA, had no effect on tonic currents. Because all GABA-activated GABAA receptors in pyramidal neurons are gabazine-sensitive, it follows that tonic currents are not GABA-activated. Furthermore, picrotoxin-sensitive spontaneous single-channel events recorded from outside-out patches had the same chord conductance as GABA-activated channels and were gabazine-resistant. Therefore, we hypothesize that GABAA receptors, constitutively active in the absence of GABA, mediate tonic current; the failure of gabazine to block tonic current reflects a lack of negative intrinsic efficacy of the antagonist. We compared the negative efficacies of bicuculline and gabazine using the general anesthetic propofol to directly activate GABAA receptors native to pyramidal neurons or alpha1beta3gamma2 receptors recombinantly expressed in human embryonic kidney 293 cells. Propofol activated gabazine-resistant, bicuculline-sensitive currents when applied to either preparation. Although gabazine had negligible efficacy as an inhibitor of propofol-activated currents, it prevented inhibition by bicuculline, which acts as an inverse agonist inhibiting GABA-independent gating. Recombinant alpha1beta1/3gamma2 receptors also mediated agonist-independent tonic currents that were resistant to gabazine and inhibited by bicuculline. Thus, gabazine is a competitive antagonist with negligible negative efficacy and is therefore unable to inhibit GABAA receptors that are active in the absence of GABA because of either anesthetic or spontaneous gating. Moreover, spontaneously active GABAA receptors mediate gabazine-resistant tonic currents in pyramidal neurons.
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Affiliation(s)
- Melissa R McCartney
- Department of Pharmacology, The George Washington University, 2300 Eye Street NW, Washington, DC 20037, USA.
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17
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Chen Y, Reilly K, Chang Y. Evolutionarily conserved allosteric network in the Cys loop family of ligand-gated ion channels revealed by statistical covariance analyses. J Biol Chem 2006; 281:18184-92. [PMID: 16595655 DOI: 10.1074/jbc.m600349200] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022] Open
Abstract
The Cys loop family of ligand-gated ion channels mediate fast synaptic transmission for communication between neurons. They are allosteric proteins, in which binding of a neurotransmitter to its binding site in the extracellular amino-terminal domain triggers structural changes in distant transmembrane domains to open a channel for ion flow. Although the locations of binding site and channel gating machinery are well defined, the structural basis of the activation pathway coupling binding and channel opening remains to be determined. In this paper, by analyzing amino acid covariance in a multiple sequence alignment, we have identified an energetically interconnected network in the Cys loop family of ligand-gated ion channels. Statistical coupling and correlated mutational analyses along with clustering revealed a highly coupled cluster. Mapping the positions in the cluster onto a three-dimensional structural model demonstrated that these highly coupled positions form an interconnected network linking experimentally identified binding domains through the coupling region to the gating machinery. In addition, these highly coupled positions are also condensed in the transmembrane domains, which are a recent focus for the sites of action of many allosteric modulators. Thus, our results revealed a genetically interconnected network that potentially plays an important role in the allosteric activation and modulation of the Cys loop family of ligand-gated ion channels.
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Affiliation(s)
- Yonghui Chen
- Department of Computer and Information Sciences, University of Alabama at Birmingham, Birmingham, AL 35294, USA
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18
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McCann CM, Bracamontes J, Steinbach JH, Sanes JR. The cholinergic antagonist alpha-bungarotoxin also binds and blocks a subset of GABA receptors. Proc Natl Acad Sci U S A 2006; 103:5149-54. [PMID: 16549768 PMCID: PMC1458809 DOI: 10.1073/pnas.0600847103] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The polypeptide snake toxin alpha-bungarotoxin (BTX) has been used in hundreds of studies on the structure, function, and development of the neuromuscular junction because it binds tightly and specifically to the nicotinic acetylcholine receptors (nAChRs) at this synapse. We show here that BTX also binds to and blocks a subset of GABA(A) receptors (GABA(A)Rs) that contain the GABA(A)R beta3 subunit. These results introduce a previously unrecognized tool for analysis of GABA(A)Rs but may complicate interpretation of some studies on neuronal nAChRs.
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Affiliation(s)
- Corey M. McCann
- *Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138; and
| | - John Bracamontes
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO 63110
| | - Joe Henry Steinbach
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO 63110
| | - Joshua R. Sanes
- *Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA 02138; and
- To whom correspondence should be addressed at:
Department of Molecular and Cellular Biology, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138. E-mail:
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19
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Jones BL, Whiting PJ, Henderson LP. Mechanisms of anabolic androgenic steroid inhibition of mammalian epsilon-subunit-containing GABAA receptors. J Physiol 2006; 573:571-93. [PMID: 16543268 PMCID: PMC1779744 DOI: 10.1113/jphysiol.2006.106534] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022] Open
Abstract
GABAergic transmission regulates the activity of gonadotrophin-releasing hormone (GnRH) neurons in the preoptic area/hypothalamus that control the onset of puberty and the expression of reproductive behaviours. One of the hallmarks of illicit use of anabolic androgenic steroids (AAS) is disruption of behaviours under neuroendocrine control. GnRH neurons are among a limited population of cells that express high levels of the epsilon-subunit of the GABAA receptor. To better understand the actions of AAS on neuroendocrine mechanisms, we have characterized modulation of GABAA receptor-mediated currents in mouse native GnRH neurons and in heterologous cells expressing recombinant alpha2beta3epsilon-receptors. GnRH neurons exhibited robust currents in response to millimolar concentrations of GABA and a picrotoxin (PTX)-sensitive, bicuculline-insensitive current that probably arises from spontaneous openings of GABAA receptors. The AAS 17alpha-methyltestosterone (17alpha-MeT) inhibited spontaneous and GABA-evoked currents in GnRH neurons. For recombinant alpha2beta3epsilon-receptors, 17alpha-MeT inhibited phasic and tonic GABA-elicited responses, accelerated desensitization and slowed paired pulse response recovery. Single channel analysis indicated that GABA-evoked events could be described by three open dwell components and that 17alpha-MeT enhanced residence in the intermediate dwell state. This AAS also inhibited a PTX-sensitive, spontaneous current (open probability, approximately 0.15-0.2) in a concentration-dependent fashion (IC50 approximately 9 microm). Kinetic modelling indicated that the inhibition induced by 17alpha-MeT occurs by an allosteric block in which the AAS interacts preferentially with a closed state and promotes accumulation in that state. Finally, studies with a G302S mutant epsilon-subunit suggest that this residue within the transmembrane domain TM2 plays a role in mediating AAS binding and modulation. In sum, our results indicate that inclusion of the epsilon-subunit significantly alters the profile of AAS modulation and that this allosteric inhibition of native GnRH neurons should be considered with regard to AAS disruption of neuroendocrine control.
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Affiliation(s)
- Brian L Jones
- Department of Physiology, Dartmouth Medical School, Hanover, New Hampshire 03755, USA
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20
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Sarto-Jackson I, Ramerstorfer J, Ernst M, Sieghart W. Identification of amino acid residues important for assembly of GABA receptor alpha1 and gamma2 subunits. J Neurochem 2006; 96:983-95. [PMID: 16412095 DOI: 10.1111/j.1471-4159.2005.03626.x] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Comparative models of GABA(A) receptors composed of alpha1 beta3 gamma2 subunits were generated using the acetylcholine-binding protein (AChBP) as a template and were used for predicting putative engineered cross-link sites between the alpha1 and the gamma2 subunit. The respective amino acid residues were substituted by cysteines and disulfide bond formation between subunits was investigated on co-transfection into human embryonic kidney (HEK) cells. Although disulfide bond formation between subunits could not be observed, results indicated that mutations studied influenced assembly of GABA(A) receptors. Whereas residue alpha1A108 was important for the formation of assembly intermediates with beta3 and gamma2 subunits consistent with its proposed location at the alpha1(+) side of GABA(A) receptors, residues gamma2T125 and gamma2P127 were important for assembly with beta3 subunits. Mutation of each of these residues also caused an impaired expression of receptors at the cell surface. In contrast, mutated residues alpha1F99C, alpha1S106C or gamma2T126C only impaired the formation of receptors at the cell surface when co-expressed with subunits in which their predicted interaction partner was also mutated. These data are consistent with the prediction that the mutated residue pairs are located close to each other.
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Affiliation(s)
- Isabella Sarto-Jackson
- Division of Biochemistry and Molecular Biology, Centre for Brain Research, Medical University of Vienna, Austria
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21
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Wagner DA, Goldschen-Ohm MP, Hales TG, Jones MV. Kinetics and spontaneous open probability conferred by the epsilon subunit of the GABAA receptor. J Neurosci 2005; 25:10462-8. [PMID: 16280584 PMCID: PMC6725813 DOI: 10.1523/jneurosci.1658-05.2005] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2005] [Revised: 09/28/2005] [Accepted: 10/02/2005] [Indexed: 12/26/2022] Open
Abstract
GABAA receptors mediate synaptic and extrasynaptic inhibition. Native receptors consist of alpha and beta subunits, which are required for function, and another "modulatory" subunit, for example, gamma, delta, or epsilon. Of these, the epsilon subunit has the most restricted distribution, confers resistance to neurosteroid and anesthetic modulation, and causes spontaneous channel opening. Little is known, however, about how epsilon affects receptor kinetics, which in turn shape responses to both ambient and synaptic GABA exposure. Here, we expressed human alpha2beta1, alpha2beta1gamma2, or alpha2beta1epsilon subunit combinations in human embryonic kidney 293 cells and used rapid solution exchange to study receptor kinetics in outside-out patches. The epsilon subunit greatly slowed deactivation and recovery after brief GABA pulses. During long, saturating GABA pulses, the rate of desensitization was slower for alpha2beta1epsilon and alpha2beta1gamma2 than for alpha2beta1. However, in alpha2beta1epsilon, the final extent of desensitization was large compared with that of alpha2beta1gamma2. Responses in alpha2beta1epsilon, but not the others, were often followed by an "overshoot" above the baseline, suggesting that a fraction of channels are spontaneously open and are transiently silenced by receptor activation and subsequent desensitization. The baseline current and associated noise were reduced by picrotoxin, revealing that epsilon-containing channels are open approximately 4% of the time in the absence of GABA. These results suggest that, if epsilon-containing receptors are expressed at synapses, the synaptic currents would be long-lasting but may rundown quickly under high-frequency activation. In addition, silencing of spontaneous openings by desensitization raises the possibility that tonic inhibition mediated by epsilon-containing receptors may be regulated by phasic inhibition.
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Affiliation(s)
- David A Wagner
- Department of Biological Sciences, Marquette University, Milwaukee, Wisconsin 53201, USA.
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22
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Schofield CM, Harrison NL. Transmembrane residues define the action of isoflurane at the GABAA receptor alpha-3 subunit. Brain Res 2005; 1032:30-5. [PMID: 15680938 DOI: 10.1016/j.brainres.2004.11.002] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 11/06/2004] [Indexed: 11/22/2022]
Abstract
The gamma-aminobutyric acid type A (GABA(A)) receptor is the target of a structurally diverse group of sedative, hypnotic, and anesthetic drugs, including the volatile anesthetic isoflurane. Previous studies on the GABA(A) receptor have suggested the existence of a cavity located between transmembrane (TM) segments 2 and 3 in both alpha-1 and alpha-2 subunits, within which volatile anesthetics might bind. In this study, we have used site-directed mutagenesis to investigate the involvement of homologous residues of the GABA(A) alpha-3 subunit in allosteric modulation by isoflurane. Mutation of serine residue 294 within the TM2 to histidine or tyrosine increased the potency of GABA and decreased positive modulation by isoflurane. Mutation of alanine residue 315 within the TM3 to tryptophan increased the potency of GABA and abolished isoflurane modulation. The activity of the intravenous anesthetic propofol was unaltered from wild-type at these mutant receptors. These findings are consistent with the action of isoflurane on a critical site within the transmembrane domains of the receptor and suggest a degree of functional homology between the GABA(A) alpha-1, -2, and -3 subunits.
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Affiliation(s)
- Claude M Schofield
- Graduate Program in Neuroscience, Weill Medical College of Cornell University, New York, NY 10021, USA.
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