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Speculation on How RIC-3 and Other Chaperones Facilitate α7 Nicotinic Receptor Folding and Assembly. MOLECULES (BASEL, SWITZERLAND) 2022; 27:molecules27144527. [PMID: 35889400 PMCID: PMC9318448 DOI: 10.3390/molecules27144527] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Revised: 07/07/2022] [Accepted: 07/09/2022] [Indexed: 11/17/2022]
Abstract
The process of how multimeric transmembrane proteins fold and assemble in the endoplasmic reticulum is not well understood. The alpha7 nicotinic receptor (α7 nAChR) is a good model for multimeric protein assembly since it has at least two independent and specialized chaperones: Resistance to Inhibitors of Cholinesterase 3 (RIC-3) and Nicotinic Acetylcholine Receptor Regulator (NACHO). Recent cryo-EM and NMR data revealed structural features of α7 nAChRs. A ser-ala-pro (SAP) motif precedes a structurally important but unique "latch" helix in α7 nAChRs. A sampling of α7 sequences suggests the SAP motif is conserved from C. elegans to humans, but the latch sequence is only conserved in vertebrates. How RIC-3 and NACHO facilitate receptor subunits folding into their final pentameric configuration is not known. The artificial intelligence program AlphaFold2 recently predicted structures for NACHO and RIC-3. NACHO is highly conserved in sequence and structure across species, but RIC-3 is not. This review ponders how different intrinsically disordered RIC-3 isoforms from C. elegans to humans interact with α7 nAChR subunits despite having little sequence homology across RIC-3 species. Two models from the literature about how RIC-3 assists α7 nAChR assembly are evaluated considering recent structural information about the receptor and its chaperones.
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2
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Maldifassi MC, Martín-Sánchez C, Atienza G, Cedillo JL, Arnalich F, Bordas A, Zafra F, Giménez C, Extremera M, Renart J, Montiel C. Interaction of the α7-nicotinic subunit with its human-specific duplicated dupα7 isoform in mammalian cells: Relevance in human inflammatory responses. J Biol Chem 2018; 293:13874-13888. [PMID: 30006348 DOI: 10.1074/jbc.ra118.003443] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/12/2018] [Revised: 06/11/2018] [Indexed: 01/19/2023] Open
Abstract
The α7 nicotinic receptor subunit and its partially duplicated human-specific dupα7 isoform are coexpressed in neuronal and non-neuronal cells. In these cells, α7 subunits form homopentameric α7 nicotinic acetylcholine receptors (α7-nAChRs) implicated in numerous pathologies. In immune cells, α7-nAChRs are essential for vagal control of inflammatory response in sepsis. Recent studies show that the dupα7 subunit is a dominant-negative regulator of α7-nAChR activity in Xenopus oocytes. However, its biological significance in mammalian cells, particularly immune cells, remains unexplored, as the duplicated form is indistinguishable from the original subunit in standard tests. Here, using immunocytochemistry, confocal microscopy, coimmunoprecipitation, FRET, flow cytometry, and ELISA, we addressed this challenge in GH4C1 rat pituitary cells and RAW264.7 murine macrophages transfected with epitope- and fluorescent protein-tagged α7 or dupα7. We used quantitative RT-PCR of dupα7 gene expression levels in peripheral blood mononuclear cells (PBMCs) from patients with sepsis to analyze its relationship with PBMC α7 mRNA levels and with serum concentrations of inflammatory markers. We found that a physical interaction between dupα7 and α7 subunits in both cell lines generates heteromeric nAChRs that remain mainly trapped in the endoplasmic reticulum. The dupα7 sequestration of α7 subunits reduced membrane expression of functional α7-nAChRs, attenuating their anti-inflammatory capacity in lipopolysaccharide-stimulated macrophages. Moreover, the PBMC's dupα7 levels correlated inversely with their α7 levels and directly with the magnitude of the patients' inflammatory state. These results indicate that dupα7 probably reduces human vagal anti-inflammatory responses and suggest its involvement in other α7-nAChR-mediated pathophysiological processes.
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Affiliation(s)
- María C Maldifassi
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - Carolina Martín-Sánchez
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - Gema Atienza
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - José L Cedillo
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - Francisco Arnalich
- the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain.,the Servicio de Medicina Interna, Hospital Universitario La Paz de Madrid, 28046 Madrid
| | - Anna Bordas
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - Francisco Zafra
- the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain.,the Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, 28049 Madrid, and
| | - Cecilio Giménez
- the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain.,the Centro de Biología Molecular Severo Ochoa, Facultad de Ciencias, Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, 28049 Madrid, and
| | - María Extremera
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and.,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
| | - Jaime Renart
- the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain.,Instituto de Investigaciones Biomédicas "Alberto Sols," Consejo Superior de Investigaciones Científicas, Universidad Autónoma de Madrid, 28029 Madrid
| | - Carmen Montiel
- From the Departamento de Farmacología y Terapéutica, Facultad de Medicina and .,the Instituto de Investigación Sanitaria del Hospital Universitario La Paz, 28046 Madrid, Spain
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3
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Criado M, Mulet J, Sala F, Sala S, Colmena I, Gandía L, Bautista-Aguilera OM, Samadi A, Chioua M, Marco-Contelles J. N-Benzylpiperidine Derivatives as α7 Nicotinic Receptor Antagonists. ACS Chem Neurosci 2016; 7:1157-65. [PMID: 27254782 DOI: 10.1021/acschemneuro.6b00122] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022] Open
Abstract
A series of multitarget directed propargylamines, as well as other differently susbstituted piperidines have been screened as potential modulators of neuronal nicotinic acetylcholine receptors (nAChRs). Most of them showed antagonist actions on α7 nAChRs. Especially, compounds 13, 26, and 38 displayed submicromolar IC50 values on homomeric α7 nAChRs, whereas they were less effective on heteromeric α3β4 and α4β2 nAChRs (up to 20-fold higher IC50 values in the case of 13). Antagonism was concentration dependent and noncompetitive, suggesting that these compounds behave as negative allosteric modulators of nAChRs. Upon the study of a series of less complex derivatives, the N-benzylpiperidine motif, common to these compounds, was found to be the main pharmacophoric group. Thus, 2-(1-benzylpiperidin-4-yl)-ethylamine (48) showed an inhibitory potency comparable to the one of the previous compounds and also a clear preference for α7 nAChRs. In a neuroblastoma cell line, representative compounds 13 and 48 also inhibited, in a concentration-dependent manner, cytosolic Ca(2+) signals mediated by nAChRs. Finally, compounds 38 and 13 inhibited 5-HT3A serotonin receptors whereas they had no effect on α1 glycine receptors. Given the multifactorial nature of many pathologies in which nAChRs are involved, these piperidine antagonists could have a therapeutic potential in cases where cholinergic activity has to be negatively modulated.
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Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d’Alacant, 03550-Alicante, Spain
| | - José Mulet
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d’Alacant, 03550-Alicante, Spain
| | - Francisco Sala
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d’Alacant, 03550-Alicante, Spain
| | - Salvador Sala
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d’Alacant, 03550-Alicante, Spain
| | - Inés Colmena
- Departamento de Farmacología y Terapéutica, Universidad Autónoma de Madrid, C/Arzobispo Morcillo 4, 28029-Madrid, Spain
| | - Luis Gandía
- Departamento de Farmacología y Terapéutica, Universidad Autónoma de Madrid, C/Arzobispo Morcillo 4, 28029-Madrid, Spain
| | - Oscar M. Bautista-Aguilera
- Instituto de Quimica Orgánica General, Laboratory
of Medicinal Chemistry, Consejo Superior de Investigaciones Científicas, C/Juan de la Cierva 3, 28006-Madrid, Spain
| | - Abdelouahid Samadi
- Instituto de Quimica Orgánica General, Laboratory
of Medicinal Chemistry, Consejo Superior de Investigaciones Científicas, C/Juan de la Cierva 3, 28006-Madrid, Spain
| | - Mourad Chioua
- Instituto de Quimica Orgánica General, Laboratory
of Medicinal Chemistry, Consejo Superior de Investigaciones Científicas, C/Juan de la Cierva 3, 28006-Madrid, Spain
| | - José Marco-Contelles
- Instituto de Quimica Orgánica General, Laboratory
of Medicinal Chemistry, Consejo Superior de Investigaciones Científicas, C/Juan de la Cierva 3, 28006-Madrid, Spain
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Wong LW, Tae HS, Cromer BA. Role of the ρ1 GABA(C) receptor N-terminus in assembly, trafficking and function. ACS Chem Neurosci 2014; 5:1266-77. [PMID: 25347026 DOI: 10.1021/cn500220t] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
The GABAC receptor and closely related GABAA receptor are members of the pentameric ligand-gated ion channels (pLGICs) superfamily and mediate inhibitory fast synaptic transmission in the nervous system. Each pLGIC subunit comprises an N-terminal extracellular agonist-binding domain followed by a channel domain and a variable intracellular domain. Available structural information shows that the core of the agonist-binding domain is a β sandwich of ten β-strands, which form the agonist-binding pocket at the subunit interface. This β-sandwich is preceded by an N-terminal α-helix in eukaryotic structures but not in prokaryotic structures. The N-terminal α-helix has been shown to be functionally essential in α7 nicotinic acetylcholine receptors. Sequence analysis of GABAC and GABAA receptors predicts an α-helix in a similar position but preceded by 8 to 46 additional residues, of unknown function, which we term the N-terminal extension. To test the functional role of both the N-terminal extension and the putative N-terminal α-helix in the ρ1 GABAC receptor, we created a series of deletions from the N-terminus. The N-terminal extension was not functionally essential, but its removal did reduce both cell surface expression and cooperativity of agonist-gated channel function. Further deletion of the putative N-terminal α-helix abolished receptor function by preventing cell-surface expression. Our results further demonstrate the essential role of the N-terminal α-helix in the assembly and trafficking of eukaryotic pLGICs. They also provide evidence that the N-terminal extension, although not essential, contributes to receptor assembly, trafficking and conformational changes associated with ligand gating.
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Affiliation(s)
- Lik-Wei Wong
- Health
Innovation Research Institute, School of Medical Sciences, RMIT University, Bundoora, Victoria 3083, Australia
- Department
of Pharmacology and Therapeutics, University of Melbourne, Parkville, Victoria 3010, Australia
| | - Han-Shen Tae
- Health
Innovation Research Institute, School of Medical Sciences, RMIT University, Bundoora, Victoria 3083, Australia
| | - Brett A. Cromer
- Health
Innovation Research Institute, School of Medical Sciences, RMIT University, Bundoora, Victoria 3083, Australia
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Abstract
We have characterized the effect of triazine derivatives on neuronal nicotinic receptors expressed in Xenopus oocytes. All triazines investigated inhibit the current of α7 and α3β4 neuronal nicotinic receptors elicited by acetylcholine. The effect is concentration dependent, reversible, and noncompetitive. In contrast, some derivatives have a dual effect on α4β2 receptors, by potentiating the currents at intermediate concentration and causing inhibition at higher concentrations. Triazine derivatives also affect the macroscopic kinetics of the heteromeric receptors α3β4 and α4β2 accelerating the rise and decay time course of the currents, but have no significant effect on the kinetics of homomeric α7 receptors. Two simple kinetic models are presented. The first reproduces the effects of different concentrations of triazines both on the peak currents and on the macroscopic kinetics of α7 with a simple inhibitory result. The second model describes the behavior of α4β2 receptors involving a more complex dual action.
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6
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Wang Y, Xiao C, Indersmitten T, Freedman R, Leonard S, Lester HA. The duplicated α7 subunits assemble and form functional nicotinic receptors with the full-length α7. J Biol Chem 2014; 289:26451-26463. [PMID: 25056953 DOI: 10.1074/jbc.m114.582858] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The α7 nicotinic acetylcholine receptor gene (CHRNA7) is linked to schizophrenia. A partial duplication of CHRNA7 (CHRFAM7A) is found in humans on 15q13-14. Exon 6 of CHRFAM7A harbors a 2-bp deletion polymorphism, CHRFAM7AΔ2bp, which is also associated with schizophrenia. To understand the effects of the duplicated subunits on α7 receptors, we fused α7, dupα7, and dupΔα7 subunits with various fluorescent proteins. The duplicated subunits co-localized with full-length α7 subunits in mouse neuroblastoma cells (Neuro2a) as well as rat hippocampal neurons. We investigated the interaction between the duplicated subunits and full-length α7 by measuring Förster resonance energy transfer using donor recovery after photobleaching and fluorescence lifetime imaging microscopy. The results revealed that the duplicated proteins co-assemble with α7. In electrophysiological studies, Leu at the 9'-position in the M2 membrane-spanning segment was replaced with Cys in dupα7 or dupΔα7, and constructs were co-transfected with full-length α7 in Neuro2a cells. Exposure to ethylammonium methanethiosulfonate inhibited acetylcholine-induced currents, showing that the assembled functional nicotinic acetylcholine receptors (nAChRs) included the duplicated subunit. Incorporation of dupα7 and dupΔα7 subunits modestly changes the sensitivity of receptors to choline and varenicline. Thus, the duplicated proteins are assembled and transported to the cell membrane together with full-length α7 subunits and alter the function of the nAChRs. The characterization of dupα7 and dupΔα7 as well as their influence on α7 nAChRs may help explain the pathophysiology of schizophrenia and may suggest therapeutic strategies.
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Affiliation(s)
- Ying Wang
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125 and
| | - Cheng Xiao
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125 and
| | - Tim Indersmitten
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125 and
| | - Robert Freedman
- Department of Psychiatry, University of Colorado at Denver, Denver, Colorado 80045
| | - Sherry Leonard
- Department of Psychiatry, University of Colorado at Denver, Denver, Colorado 80045
| | - Henry A Lester
- Division of Biology and Biological Engineering, California Institute of Technology, Pasadena, California 91125 and.
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7
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MacDonald K, Buxton S, Kimber MJ, Day TA, Robertson AP, Ribeiro P. Functional characterization of a novel family of acetylcholine-gated chloride channels in Schistosoma mansoni. PLoS Pathog 2014; 10:e1004181. [PMID: 24945827 PMCID: PMC4055736 DOI: 10.1371/journal.ppat.1004181] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/12/2013] [Accepted: 04/28/2014] [Indexed: 12/25/2022] Open
Abstract
Acetylcholine is the canonical excitatory neurotransmitter of the mammalian neuromuscular system. However, in the trematode parasite Schistosoma mansoni, cholinergic stimulation leads to muscle relaxation and a flaccid paralysis, suggesting an inhibitory mode of action. Information about the pharmacological mechanism of this inhibition is lacking. Here, we used a combination of techniques to assess the role of cholinergic receptors in schistosome motor function. The neuromuscular effects of acetylcholine are typically mediated by gated cation channels of the nicotinic receptor (nAChR) family. Bioinformatics analyses identified numerous nAChR subunits in the S. mansoni genome but, interestingly, nearly half of these subunits carried a motif normally associated with chloride-selectivity. These putative schistosome acetylcholine-gated chloride channels (SmACCs) are evolutionarily divergent from those of nematodes and form a unique clade within the larger family of nAChRs. Pharmacological and RNA interference (RNAi) behavioral screens were used to assess the role of the SmACCs in larval motor function. Treatment with antagonists produced the same effect as RNAi suppression of SmACCs; both led to a hypermotile phenotype consistent with abrogation of an inhibitory neuromuscular mediator. Antibodies were then generated against two of the SmACCs for use in immunolocalization studies. SmACC-1 and SmACC-2 localize to regions of the peripheral nervous system that innervate the body wall muscles, yet neither appears to be expressed directly on the musculature. One gene, SmACC-1, was expressed in HEK-293 cells and characterized using an iodide flux assay. The results indicate that SmACC-1 formed a functional homomeric chloride channel and was activated selectively by a panel of cholinergic agonists. The results described in this study identify a novel clade of nicotinic chloride channels that act as inhibitory modulators of schistosome neuromuscular function. Additionally, the iodide flux assay used to characterize SmACC-1 represents a new high-throughput tool for drug screening against these unique parasite ion channels.
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Affiliation(s)
- Kevin MacDonald
- Institute of Parasitology, McGill University, Macdonald Campus, Ste. Anne de Bellevue, Quebec, Canada
| | - Samuel Buxton
- Department of Biomedical Sciences, Iowa State University, Ames, Iowa, United States of America
| | - Michael J. Kimber
- Department of Biomedical Sciences, Iowa State University, Ames, Iowa, United States of America
| | - Tim A. Day
- Department of Biomedical Sciences, Iowa State University, Ames, Iowa, United States of America
| | - Alan P. Robertson
- Department of Biomedical Sciences, Iowa State University, Ames, Iowa, United States of America
| | - Paula Ribeiro
- Institute of Parasitology, McGill University, Macdonald Campus, Ste. Anne de Bellevue, Quebec, Canada
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8
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Criado M, Valor LM, Mulet J, Gerber S, Sala S, Sala F. Expression and functional properties of α7 acetylcholine nicotinic receptors are modified in the presence of other receptor subunits. J Neurochem 2012; 123:504-14. [DOI: 10.1111/j.1471-4159.2012.07931.x] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2012] [Revised: 07/30/2012] [Accepted: 08/20/2012] [Indexed: 01/03/2023]
Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
| | - Luis M. Valor
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
| | - José Mulet
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
| | - Susana Gerber
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
| | - Salvador Sala
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
| | - Francisco Sala
- Instituto de Neurociencias; Universidad Miguel Hernández-CSIC; Alicante Spain
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9
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Criado M, Mulet J, Gerber S, Sala S, Sala F. Mutants of β-strand β3 and the loop B in the interface between α7 subunits of a homomeric acetylcholine receptor show functional and pharmacological alterations. J Neurochem 2011; 118:968-78. [DOI: 10.1111/j.1471-4159.2011.07381.x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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10
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Criado M, Svobodová L, Mulet J, Sala F, Sala S. Substitutions of amino acids in the pore domain of homomeric α7 nicotinic receptors for analogous residues present in heteromeric receptors modify gating, rectification and binding properties. J Neurochem 2011; 119:40-9. [PMID: 21790604 DOI: 10.1111/j.1471-4159.2011.07398.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We have studied the role of different amino acids in the M2 transmembrane domain of the α7 neuronal nicotinic receptor by mutating residues that differ from the ones located at the same positions in other α (α2-α10) or β (β2-β4) subunits. Our aim was to investigate the contribution of these amino acids to the peculiar kinetic and inward rectification properties that differentiate the homomeric α7 receptor from other nicotinic receptors. Mutations of several residues strongly modified receptor function. We found that Thr245 had the most profound effect when mutated to serine, an amino acid present in all heteromeric receptors composed of α and β subunits, by dramatically increasing the maximal current, decreasing the decaying rate of the currents and decreasing receptor rectification. Some mutants also showed altered agonist-binding properties as revealed by shifts in the dose-response curves for acetylcholine. We conclude that residues in the M2 segment and flanking regions contribute to the unusual properties of the α7 receptor, especially to its characteristic fast kinetic behavior and strong inward rectification and furthermore to the potency of agonists.
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Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, 03550-Alicante, Spain
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11
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Wu J, Lukas RJ. Naturally-expressed nicotinic acetylcholine receptor subtypes. Biochem Pharmacol 2011; 82:800-7. [PMID: 21787755 DOI: 10.1016/j.bcp.2011.07.067] [Citation(s) in RCA: 93] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/25/2011] [Revised: 07/01/2011] [Accepted: 07/05/2011] [Indexed: 12/31/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) warrant attention, as they play many critical roles in brain and body function and have been implicated in a number of neurological and psychiatric disorders, including nicotine dependence. nAChRs are composed as diverse subtypes containing specific combinations of genetically-distinct subunits and that have different functional properties, distributions, and pharmacological profiles. There had been confidence that the rules that define ranges of assembly partners for specific subunits were well-established, especially for the more prominent nAChR subtypes. However, we review here some newer findings indicating that nAChRs having largely the same, major subunits exist as isoforms with unexpectedly different properties. Moreover, we also summarize our own studies indicating that novel nAChR subtypes exist and/or have distributions not heretofore described. Importantly, the nAChRs that exist as new isoforms or subtypes or have interesting distributions require alteration in thinking about their roles in health and disease.
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Affiliation(s)
- Jie Wu
- Division of Neurology, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, AZ 85013, United States.
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12
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de Lucas-Cerrillo AM, Maldifassi MC, Arnalich F, Renart J, Atienza G, Serantes R, Cruces J, Sánchez-Pacheco A, Andrés-Mateos E, Montiel C. Function of partially duplicated human α77 nicotinic receptor subunit CHRFAM7A gene: potential implications for the cholinergic anti-inflammatory response. J Biol Chem 2010; 286:594-606. [PMID: 21047781 DOI: 10.1074/jbc.m110.180067] [Citation(s) in RCA: 101] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
The neuronal α7 nicotinic receptor subunit gene (CHRNA7) is partially duplicated in the human genome forming a hybrid gene (CHRFAM7A) with the novel FAM7A gene. The hybrid gene transcript, dupα7, has been identified in brain, immune cells, and the HL-60 cell line, although its translation and function are still unknown. In this study, dupα7 cDNA has been cloned and expressed in GH4C1 cells and Xenopus oocytes to study the pattern and functional role of the expressed protein. Our results reveal that dupα7 transcript was natively translated in HL-60 cells and heterologously expressed in GH4C1 cells and oocytes. Injection of dupα7 mRNA into oocytes failed to generate functional receptors, but when co-injected with α7 mRNA at α7/dupα7 ratios of 5:1, 2:1, 1:1, 1:5, and 1:10, it reduced the nicotine-elicited α7 current generated in control oocytes (α7 alone) by 26, 53, 75, 93, and 94%, respectively. This effect is mainly due to a reduction in the number of functional α7 receptors reaching the oocyte membrane, as deduced from α-bungarotoxin binding and fluorescent confocal assays. Two additional findings open the possibility that the dominant negative effect of dupα7 on α7 receptor activity observed in vitro could be extrapolated to in vivo situations. (i) Compared with α7 mRNA, basal dupα7 mRNA levels are substantial in human cerebral cortex and higher in macrophages. (ii) dupα7 mRNA levels in macrophages are down-regulated by IL-1β, LPS, and nicotine. Thus, dupα7 could modulate α7 receptor-mediated synaptic transmission and cholinergic anti-inflammatory response.
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13
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Aldea M, Castillo M, Mulet J, Sala S, Criado M, Sala F. Role of the extracellular transmembrane domain interface in gating and pharmacology of a heteromeric neuronal nicotinic receptor. J Neurochem 2010; 113:1036-45. [DOI: 10.1111/j.1471-4159.2010.06665.x] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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14
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Criado M, Castillo M, Mulet J, Sala F, Sala S. Role of loop 9 on the function of neuronal nicotinic receptors. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2010; 1798:654-9. [PMID: 20043866 DOI: 10.1016/j.bbamem.2009.12.013] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/28/2009] [Revised: 12/16/2009] [Accepted: 12/18/2009] [Indexed: 10/20/2022]
Abstract
We have studied the role of loop 9 in the function of neuronal nicotinic receptors. By systematically mutating the residues in the loop we have determined that the most important amino acids determining the coupling of binding to gating are the ones closer to the transmembrane region. Single mutations at location E173 in homomeric alpha7 receptors destroyed their function by completely abolishing the current while preserving the expression at the membrane. In contrast, heteromeric receptor alpha3beta4 with the same mutations retained some function. We conclude that loop 9 has a different role in the function of homomeric and heteromeric receptors.
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Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, 03550-Alicante, Spain
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15
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Criado M, Mulet J, Castillo M, Gerber S, Sala S, Sala F. The loop between β-strands β2 and β3 and its interaction with the N-terminal α-helix is essential for biogenesis of α7 nicotinic receptors. J Neurochem 2010; 112:103-11. [DOI: 10.1111/j.1471-4159.2009.06439.x] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Millar NS. A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors. Biochem Pharmacol 2009; 78:766-76. [PMID: 19540210 DOI: 10.1016/j.bcp.2009.06.015] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2009] [Revised: 06/08/2009] [Accepted: 06/10/2009] [Indexed: 11/18/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.
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Affiliation(s)
- Neil S Millar
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, UK.
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Castillo M, Mulet J, Aldea M, Gerber S, Sala S, Sala F, Criado M. Role of the N-terminal α-helix in biogenesis of α7 nicotinic receptors. J Neurochem 2009; 108:1399-409. [DOI: 10.1111/j.1471-4159.2009.05924.x] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
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Bernal JA, Mulet J, Castillo M, Criado M, Sala F, Sala S. Single-channel study of the binding-gating coupling in the slowly desensitizing chimeric α7-5HT3A receptor. FEBS Lett 2009; 583:1045-51. [DOI: 10.1016/j.febslet.2009.02.026] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2008] [Revised: 02/13/2009] [Accepted: 02/16/2009] [Indexed: 11/26/2022]
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19
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Lee HK, Gwalani L, Mishra V, Anandjiwala P, Sala F, Sala S, Ballesta JJ, O'Malley D, Criado M, Loring RH. Investigating the role of protein folding and assembly in cell-type dependent expression of alpha7 nicotinic receptors using a green fluorescent protein chimera. Brain Res 2009; 1259:7-16. [PMID: 19368825 DOI: 10.1016/j.brainres.2009.01.046] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2008] [Revised: 01/12/2009] [Accepted: 01/17/2009] [Indexed: 11/25/2022]
Abstract
To test the hypothesis that cell-dependent expression of alpha7 receptors is due to differences in protein folding or assembly, we constructed a chimeric rat alpha7 subunit with green fluorescent protein (GFP) at the receptor C-terminal. Expression of alpha7-GFP in Xenopus oocytes resulted in currents that were indistinguishable from wild type receptors but were only 33% of control. (125)I-alpha-bungarotoxin (alphaBGT) binding at the oocyte surface was reduced to 23% of wild type. Transfection of alpha7-GFP into GH4C1 cells produced fluorescence that was less intense than GFP alone, but showed significant alpha-BGT binding compared to transfection with GFP. In contrast, alpha7-GFP transfection in SH-EP1, HEK293 and CHO-CAR cells produced fluorescence without alphaBGT binding. Flow cytometry of cells transfected with alpha7-GFP indicated fluorescence in both SH-EP1 and GH4C1 cells, but surface toxin binding sites and sites immunoprecipitated using anti-GFP antibodies were undetectable in SH-EP1 cells, suggesting a problem in folding/assembly rather than trafficking. Surprisingly, integrated fluorescence intensities in GH4C1 cells transfected with alpha7-GFP did not correlate with amounts of cell surface or immunoprecipitable alphaBGT binding. Therefore, GFP folding at the C-terminal of the alpha7-GFP chimera is cell-line independent, but toxin binding is highly cell-line dependent, suggesting that if altered protein folding is involved in the cell-type dependence of alpha7 receptor expression, the phenomenon is restricted to specific protein domains. Further, C-terminal GFP-labeled alpha7 receptors decreased the efficiency of folding/assembly not only of chimeric subunits, but also wild-type subunits, suggesting that the C-terminal is an important domain for alpha7 receptor assembly.
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Affiliation(s)
- H K Lee
- Department of Pharmaceutical Science, Northeastern Univ, Boston, MA 02115, USA
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20
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Bernal JA, Mulet J, Castillo M, Criado M, Sala S, Sala F. Binding-gating coupling in a nondesensitizing alpha7 nicotinic receptor A single channel pharmacological study. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2008; 1788:410-6. [PMID: 19063861 DOI: 10.1016/j.bbamem.2008.11.004] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/20/2008] [Revised: 10/31/2008] [Accepted: 11/04/2008] [Indexed: 11/29/2022]
Abstract
The highly conserved alphaLys145 has been suggested to play an important role in the early steps of activation of the nicotinic acetylcholine receptor (nAChR) by acetylcholine. Both macroscopic and single channel currents were recorded in the slowly desensitizing mutants L248T- and K145A-L248T-alpha7 receptors expressed in Xenopus oocytes. On ACh-evoked currents, substitution of Lys145 by alanine showed the same effects that in wild type receptors: moderately decreased gating function and a more-than-expected loss of ACh potency, thus validating the experimental model. Single channel analysis quantitatively agreed with macroscopic data and revealed that impaired gating function in the double mutant alpha7K145A/L248T is the consequence of a slower opening rate, beta. Several nicotinic agonists were also studied, showing important features. Particularly, dimethylphenylpiperazinium (DMPP), acting as an antagonist in alpha7K145A, became a full agonist in alpha7K145A/L248T. Single channel analysis of DMPP-evoked currents showed effects of Lys145 removal similar to those observed with ACh. Data suggest that alpha7Lys145 facilitates the early steps of channel activation. Moreover, the slowly desensitizing mutant alpha7L248T could be an interesting tool for the study of channel activation in alpha7 receptors. Nevertheless, its extensively altered pharmacology precludes the simple extrapolation of pharmacological data obtained in singly mutated alpha7 receptors.
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Affiliation(s)
- José Antonio Bernal
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, 03550-Alicante, Spain
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21
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Millar NS, Harkness PC. Assembly and trafficking of nicotinic acetylcholine receptors (Review). Mol Membr Biol 2008; 25:279-92. [PMID: 18446614 DOI: 10.1080/09687680802035675] [Citation(s) in RCA: 96] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/22/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) are members of an extensive super-family of neurotransmitter-gated ion channels. In humans, nAChRs are expressed within the nervous system and at the neuromuscular junction and are important targets for pharmaceutical drug discovery. They are also the site of action for neuroactive pesticides in insects and other invertebrates. Nicotinic receptors are complex pentameric transmembrane proteins which are assembled from a large family of subunits; seventeen nAChR subunits (alpha1-alpha10, beta1-beta4, gamma, delta and epsilon) have been identified in vertebrate species. This review will discuss nAChR subunit diversity and factors influencing receptor assembly and trafficking.
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Affiliation(s)
- Neil S Millar
- Department of Pharmacology, University College London, London, UK.
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22
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Castelán F, Castillo M, Mulet J, Sala S, Sala F, Domínguez del Toro E, Criado M. Molecular characterization and localization of the RIC-3 protein, an effector of nicotinic acetylcholine receptor expression. J Neurochem 2008; 105:617-27. [DOI: 10.1111/j.1471-4159.2007.05169.x] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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23
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Influence of the M3-M4 intracellular domain upon nicotinic acetylcholine receptor assembly, targeting and function. Br J Pharmacol 2008; 153:1474-84. [PMID: 18204482 DOI: 10.1038/sj.bjp.0707676] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
BACKGROUND AND PURPOSE The aim of this study was to investigate the influence of the intracellular domain of nicotinic acetylcholine receptor (nAChR) subunits upon receptor assembly, targeting and functional properties. EXPERIMENTAL APPROACH Because most nAChR subunits form functional receptors only as heteromeric complexes, it can be difficult to examine the influence of individual subunits or subunit domains in isolation. A series of subunit chimaeras was constructed which contain the intracellular loop region (located between the M3 and M4 transmembrane domains) from nAChR subunits alpha1-alpha10 or beta1-beta4. All of these chimaeras contain common extracellular and transmembrane domains (from the nAChR alpha7 subunit and the 5-hydroxytryptamine receptor 5-HT(3A) subunit, respectively), thereby facilitating both homomeric receptor assembly and detection with radiolabelled or fluorescent alpha-bungarotoxin. KEY RESULTS The nAChR M3-M4 intracellular loop domain had no significant effect upon levels of total subunit protein detected in transfected cells but had a significant influence upon levels of both cell surface and intracellular assembled receptors. Comparisons of functional properties revealed a significant influence of the intracellular loop domain upon both single-channel conductance and receptor desensitization. In addition, studies conducted in polarized epithelial cells demonstrate that the nAChR loop can influence receptor targeting, resulting in either polarized (apical) or non-polarized distribution. CONCLUSIONS AND IMPLICATIONS Evidence has been obtained which demonstrates that the large intracellular loop domain of nAChR subunits can exert a profound influence upon receptor assembly, targeting and ion channel properties.
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Criado M, Mulet J, Castillo M, Aldea M, Sala S, Sala F. Interactions between loop 5 and beta-strand beta6' are involved in alpha7 nicotinic acetylcholine receptors channel gating. J Neurochem 2007; 104:719-30. [PMID: 17961148 DOI: 10.1111/j.1471-4159.2007.05010.x] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Binding of agonists to nicotinic acetylcholine receptors (nAChR) is coupled to channel opening through local rearrangements of different domains of the protein. Recent structural data suggest that two of these regions could be the loop 5 (L5) and the beta-strand beta6', both forming the inner part of the N-terminal domain. Amino acids in these domains were mutated in alpha7 nAChRs, and expression levels and functional responses of mutant receptors were measured. Mutations located at the putative apex of L5, Asp97 and Glu98, and also at Phe100, gave receptors with smaller currents, showing qualitative differences with respect to muscle nAChRs. In contrast, mutations in the beta-strand beta6' (at Phe124 and Lys125) showed increased functional responses. Mutations affected equally the responses to acetylcholine and dimethylphenylpiperazinium, except in Phe100 where the latter was sevenfold less effective than in wild-type. Currents in mutants decayed with almost the same kinetics, ruling out large effects on desensitization. Analysis of double mutants demonstrated a functional coupling among the three electrically charged amino acids Asp97, Glu98, and Lys125, and also between Phe100 and Phe124. The results are compatible with the involvement of functional interactions between L5 and beta-strand beta6' during nAChR activation.
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Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, Alicante, Spain
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25
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Aldea M, Mulet J, Sala S, Sala F, Criado M. Non-charged amino acids from three different domains contribute to link agonist binding to channel gating in alpha7 nicotinic acetylcholine receptors. J Neurochem 2007; 103:725-35. [PMID: 17635664 DOI: 10.1111/j.1471-4159.2007.04771.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Binding of agonists to nicotinic acetylcholine receptors results in channel opening. Previously, we have shown that several charged residues at three different domains of the alpha7 nicotinic receptor are involved in coupling binding and gating, probably through a network of electrostatic interactions. This network, however, could also be integrated by other residues. To test this hypothesis, non-charged amino acids were mutated and expression levels and electrophysiological responses of mutant receptors were determined. Mutants at positions Asn47 and Gln48 (loop 2), Ile130, Trp134, and Gln140 (loop 7), and Thr264 (M2-M3 linker) showed poor or null functional responses, despite significant membrane expression. By contrast, mutants F137A and S265A exhibited a gain of function effect. In all cases, changes in dose-response relationships were small, EC(50) values being between threefold smaller and fivefold larger, arguing against large modifications of agonist binding. Peak currents decayed at the same rate in all receptors except two, excluding large effects on desensitization. Thus, the observed changes could be mostly caused by alterations of the gating characteristics. Moreover, analysis of double mutants showed an interconnection between some residues in these domains, especially Gln48 with Ile130, suggesting a potential coupling between agonist binding and channel gating through these amino acids.
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Affiliation(s)
- Marcos Aldea
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, Alicante, Spain
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26
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Castillo M, Mulet J, Gutiérrez LM, Ortiz JA, Castelán F, Gerber S, Sala S, Sala F, Criado M. Role of the RIC-3 protein in trafficking of serotonin and nicotinic acetylcholine receptors. J Mol Neurosci 2007; 30:153-6. [PMID: 17192664 DOI: 10.1385/jmn:30:1:153] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/1999] [Revised: 11/30/1999] [Accepted: 11/30/1999] [Indexed: 11/11/2022]
Abstract
Neurotransmitter-gated receptors are assembled in the endoplasmic reticulum and transported to the cell surface through a process that might be of central importance to regulate the efficacy of synaptic transmission (Kneussel and Betz, 2000; Kittler and Moss, 2003). This process is relatively inefficient- what may be the consequence of tight quality controls that guarantee the functional competence of the final product. For this purpose, specific proteins involved in assembly and trafficking of receptors might be required (Keller and Taylor, 1999; Millar, 2003; Wanamaker et al., 2003). The RIC-3 protein could be one of them, as mutations in the ric-3 gene affect maturation of nicotinic acetylcholine receptors (nAChRs) in Caenorhabditis elegans (Halevi et al., 2002). Moreover, the human homolog hRIC-3 showed differential effects when coexpressed with several ligand-gated receptors (Halevi et al., 2003). Thus, it enhanced alpha7 nAChR expression while inhibiting expression of other nAChR subtypes (alpha4beta2 and alpha3beta4) and 5-HT3 serotonin receptors (5-HT3Rs). These opposite effects suggested that the RIC-3 protein might play a key role in the biogenesis of some ligand-gated receptors and prompted us to investigate how it performs its action. Here, we show that the RIC-3 protein acts as a barrier for some receptors like alpha4beta2 nAChRs and 5-HT3Rs, stopping the traffic of mature receptors to the membrane. In contrast, the inefficient transport of alpha7 nAChRs is enhanced by RIC-3 in a process in which certain amino acids at the amphipathic helix located at the C-terminal region of the large cytoplasmic domain are involved.
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Affiliation(s)
- Mar Castillo
- Instituto de Neurociencias de Alicante, UMH-CSIC, 03550 Sant Joan d'Alacant, Spain
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27
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Castelán F, Mulet J, Aldea M, Sala S, Sala F, Criado M. Cytoplasmic regions adjacent to the M3 and M4 transmembrane segments influence expression and function of ?7 nicotinic acetylcholine receptors. A study with single amino acid mutants. J Neurochem 2007; 100:406-15. [PMID: 17076762 DOI: 10.1111/j.1471-4159.2006.04202.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We studied the role of the cytoplasmic regions adjacent to the M3 and M4 transmembrane segments of alpha7 nicotinic receptors in the expression of functional channels. For this purpose, a total of 50 amino acids were mutated throughout the mentioned regions. Mutants close to M3, from Arg294 to Leu321, showed slight modifications in the levels of alpha-bungarotoxin binding sites and acetylcholine-evoked currents. Exceptions were mutants located at two clusters (His296 to Pro300 and Ile312 to Trp316), which exhibited low expression levels. In addition, some mutants showed altered functional responses. Many mutants close to M4 showed increased receptor expression, especially the ones located at the hydrophobic face of a putative amphipathic helix. This effect seems to be the consequence of a combination of increased receptor biosynthesis, higher transport efficiency and delayed degradation, such that we postulate that elements in the amphipathic domain strongly influence receptor stability. Finally, some mutants in this region showed altered functional responses: elimination of positively charged residues (Arg424 and Arg426) increased currents, whereas the opposite was observed upon suppression of negatively charged ones (Glu430 and Glu432). These results suggest that the cytoplasmic regions close to M3 and M4 play important structural and functional roles.
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Affiliation(s)
- Francisco Castelán
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Alicante, Spain
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28
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Villarroya M, López MG, de Pascual R, García AG. Preclinical profile of PF9404C, a nitric oxide donor with beta receptor blocking properties. ACTA ACUST UNITED AC 2006; 23:149-60. [PMID: 16007231 DOI: 10.1111/j.1527-3466.2005.tb00162.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
PF9404C ((2'S),(2S)-3-isopropylamine, 1-[4-(2,3-dinitroxy)propoxymethyl]-phenoxy-2'-propranol) is the S-S diesteroisomer of a novel blocker of beta-adrenergic receptors with vasorelaxing properties. It causes a concentration-dependent relaxation of rat aorta helical strips precontracted with 10(-6) M norepinephrine (NE; IC50 33 nM). It is equipotent to nitroglycerin (NTG; IC50 49 nM), but much more potent than isosorbide dinitrate (ISD; IC50 15,000 nM). In rat aorta smooth muscle cells, at 10 microM, PF9404C increased the formation of cGMP from 3 pmol/mg protein in basal conditions to 53 pmol/mg protein, suggesting that the mechanism of its vasorelaxing effects involves the slow generation of NO. This is supported by the facts that (i) ODQ (a blocker of guanylate cyclase) inhibited the vasodilatory effects of PF9404C; and (ii) PF9404C generates NO, as indirectly measured by the Griess reaction. In the electrically driven guinea pig left atrium, PF9404C blocks the inotropic effects of isoproterenol in a concentration-dependent manner. Its IC50 (30 nM) was similar to that of S-propranolol (22.4 nM) and lower than that of metoprolol (120 nM) or atenolol (192 nM). The beta adrenergic ligand (-)-[3H]-CGP12177 (4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one hydrochloride) (0.2 nM) is displaced from its binding sites in rat brain membranes with a K(i) of 7, 17, 170, and 1200 nM for PF9404C, S-(-)propranolol, metoprolol, and atenolol, respectively. PF9404C blocks 45Ca2+ entry into bovine adrenal chromaffin cells induced by direct depolarization with 70 mM K+ or by the nicotinic agonist dimethylphenylpiperazinium (DMPP). PF9404C exhibits about 3-fold higher potency than NTG to relax the majority of the vessels studied, especially when they were contracted with K+, and shows a certain selectivity of action for the renal artery. It produces auto-tolerance that is ca. 20-fold less pronounced than that observed with NTG. Cross-tolerance in preparations pre-exposed to PF9404C and later relaxed with NTG, was much greater than auto-tolerance. This makes PF9404C a useful pharmacological tool for the development of novel NO-donor compounds with a lesser degree of vascular tolerance than those currently available.
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Affiliation(s)
- Mercedes Villarroya
- Instituto de Farmacologia Teófilo Hernando, Facultad de Medicina, Universidad Autonoma de Madrid, Arzobispo Morcillo 4, 28029 Madrid, Spain.
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Castillo M, Mulet J, Bernal JA, Criado M, Sala F, Sala S. Improved gating of a chimeric α7-5HT3Areceptor upon mutations at the M2-M3 extracellular loop. FEBS Lett 2005; 580:256-60. [PMID: 16364316 DOI: 10.1016/j.febslet.2005.12.010] [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] [Received: 10/14/2005] [Revised: 12/01/2005] [Accepted: 12/06/2005] [Indexed: 11/27/2022]
Abstract
Acetylcholine-evoked currents of the receptor chimera alpha7-5HT3A V201 expressed in Xenopus oocytes are strikingly small when compared to the amount of alpha-bungarotoxin binding sites detected at the oocyte membrane. Since the chimeric receptor is made of the extracellular N-terminal region of the rat alpha7 nicotinic acetylcholine receptor and the C-terminal region of the mouse 5-HT3A receptor, which includes the ion channel, we hypothesized that communication between these two regions was not optimal. Here, we show that mutating to aspartate several adjacent positions in the M2-M3 extracellular linker increases current amplitudes to different extents, thus confirming the important role of this region on receptor gating.
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Affiliation(s)
- Mar Castillo
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-C.S.I.C., 03550-Sant Joan d'Alacant, Alicante, Spain
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30
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Ortiz JA, Castillo M, del Toro ED, Mulet J, Gerber S, Valor LM, Sala S, Sala F, Gutiérrez LM, Criado M. The cysteine-rich with EGF-like domains 2 (CRELD2) protein interacts with the large cytoplasmic domain of human neuronal nicotinic acetylcholine receptor alpha4 and beta2 subunits. J Neurochem 2005; 95:1585-96. [PMID: 16238698 DOI: 10.1111/j.1471-4159.2005.03473.x] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Using a yeast two-hybrid screening we report the isolation of a novel human protein, hCRELD2beta, that interacts specifically with the large cytoplasmic regions of human nicotinic acetylcholine receptor (nAChR) alpha4 and beta2 subunits, both in yeast cells and in vitro. This interaction is not detected with nAChR alpha7 and alpha3 subunits. The hCRELD2 gene encodes for multiple transcripts, likely to produce multiple protein isoforms. A previously reported one has been renamed as CRELD2alpha. Isoforms alpha and beta are expressed in all tissues examined and have the same N-terminal and central regions but alternative C-terminal regions. Both isoforms interact with the alpha4 subunit. Within this subunit the interaction was localized to the N-terminal region of the large cytoplasmic loop. The CRELD2beta protein is present at the endoplasmic reticulum where colocalized with alpha4beta2 nAChRs upon cell transfection. Immunohistochemistry experiments demonstrated the presence of CRELD2 in the rat brain at sites where alpha4beta2 receptors have been previously detected. Labeling was restricted to neuronal perikarya. Finally, CRELD2 decreases the functional expression and impairs membrane transport of alpha4beta2 nAChRs in Xenopus leavis oocytes, without affecting alpha3beta4 and alpha7 nAChR expression. These results suggest that CRELD2 can act as a specific regulator of alpha4beta2 nAChR expression.
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Affiliation(s)
- José A Ortiz
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Sant Joan d'Alacant, Alicante, Spain
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31
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Criado M, Mulet J, Bernal JA, Gerber S, Sala S, Sala F. Mutations of a Conserved Lysine Residue in the N-Terminal Domain of α7 Nicotinic Receptors Affect Gating and Binding of Nicotinic Agonists. Mol Pharmacol 2005; 68:1669-77. [PMID: 16129734 DOI: 10.1124/mol.105.015446] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Activation of nicotinic acetylcholine receptors is initiated by binding of agonists, and as a consequence, specific domains transmit the chemical signal to the channel gate through a sequence of conformational changes. Recent high-resolution structural data from a snail acetylcholine binding protein have shown that the side chain of a lysine residue, located in the beta-strand beta7 and strictly conserved in alpha subunits of nicotinic receptors, systematically moves upon agonist binding, suggesting that it might be involved in both binding and gating. To test this hypothesis in neuronal nicotinic receptors, Lys145 was substituted by other amino acids in the alpha7 nicotinic receptor, and expression levels and electrophysiological responses for several nicotinic agonists and antagonists were determined. Substitutions of Lys145 showed a variety of functional effects: 1) strong reductions in the functional responses to acetylcholine, nicotine, and dimethylphenylpiperazinium, the latter becoming an antagonist; 2) increases in the agonist EC50 values (up to 80-fold with acetylcholine); 3) heterogeneous behavior of the different agonists, with epibatidine and cytisine being less affected by the substitutions; 4) decreases of agonist affinities for the desensitized receptors; and 5) small changes in the affinity of nicotinic antagonists. It is concluded that the presence of a polar or positively charged side chain at this position improves the gating function with acetylcholine and nicotine, although the lysine side chain seems to be necessary for retaining the binding properties of acetylcholine. The results are compatible with the involvement of Lys145 in the early steps of channel activation by acetylcholine.
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Affiliation(s)
- Manuel Criado
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-Consejo Superior de Investigaciones Cientificas, Apartado 18, 03550-Sant Joan d'Alacant, Alicante, Spain
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32
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Castillo M, Mulet J, Gutiérrez LM, Ortiz JA, Castelán F, Gerber S, Sala S, Sala F, Criado M. Dual Role of the RIC-3 Protein in Trafficking of Serotonin and Nicotinic Acetylcholine Receptors. J Biol Chem 2005; 280:27062-8. [PMID: 15927954 DOI: 10.1074/jbc.m503746200] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of alpha7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of alpha4beta2 nicotinic and 5-HT3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of alpha7 and 5-HT3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of alpha7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the alpha7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.
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Affiliation(s)
- Mar Castillo
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-Consejo Superior de Investigaciones Científicas, Apartado 18, 03550 Sant Joan d'Alacant, Alicante, Spain
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33
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Sala F, Mulet J, Sala S, Gerber S, Criado M. Charged amino acids of the N-terminal domain are involved in coupling binding and gating in alpha7 nicotinic receptors. J Biol Chem 2004; 280:6642-7. [PMID: 15611071 DOI: 10.1074/jbc.m411048200] [Citation(s) in RCA: 38] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
Abstract
Binding of agonists to nicotinic acetylcholine receptors generates a sequence of conformational changes resulting in channel opening. Previously, we have shown that the aspartate residue Asp-266 at the M2-M3 linker of the alpha7 nicotinic receptor is involved in connecting binding and gating. High resolution structural data suggest that this region could interact with the so-called loops 2 and 7 of the extracellular N-terminal region. In this case, certain charged amino acids present in these loops could integrate together with Asp-266 and other amino acids, a mechanism involved in channel activation. To test this hypothesis, all charged residues in these loops, Asp-42, Asp-44, Glu-45, Lys-46, Asp-128, Arg-130, and Asp-135, were substituted with other amino acids, and expression levels and electrophysiological responses of mutant receptors were determined. Mutants at positions Glu-45, Lys-46, and Asp-135 exhibited poor or null functional responses to different nicotinic agonists regardless of significant membrane expression, whereas D128A showed a gain of function effect. Because the double reverse charge mutant K46D/D266K did not restore receptor function, a gating mechanism controlled by the pairwise electrostatic interaction between these residues is not likely. Rather, a network of interactions formed by residues Lys-46, Asp-128, Asp-135, Asp-266, and possibly others appears to link agonist binding to channel gating.
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Affiliation(s)
- Francisco Sala
- Instituto de Neurociencias, Universidad Miguel Hernández-Consejo Superior de Investigaciones Cientificas, Apartado 18, 03550-Sant Joan d'Alacant, Alicante, Spain
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34
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Functional mapping and Ca2+ regulation of nicotinic acetylcholine receptor channels in rat hippocampal CA1 neurons. J Neurosci 2003. [PMID: 14534236 DOI: 10.1523/jneurosci.23-27-09024.2003] [Citation(s) in RCA: 95] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Diverse subtypes of nicotinic acetylcholine receptors (nAChRs), including fast-desensitizing alpha7-containing receptors thought to be Ca2+-permeable, are expressed in the CNS, where they appear to regulate cognitive processing and synaptic plasticity. To understand the physiological role of nAChRs in regulating neuronal excitability, it is important to know the distribution of functional receptors along the surface of neurons, whether they can increase [Ca2+]i, and/or are regulated by Ca2+. We mapped the distribution of receptors on the membrane of rat hippocampal CA1 stratum radiatum interneurons and pyramidal cells in acute slices by recording nAChR-mediated currents elicited by local UV laser-based photolysis of caged carbachol in patch-clamped neurons. The local application (approximately 7 microm patches) allowed mapping of functional nAChRs along the soma and dendritic tree, whereas the fast uncaging minimized the effects of desensitization of alpha7-containing nAChRs and allowed us to measure the kinetics of responses. The alpha7-containing nAChRs were the predominant subtype on interneurons, and were located primarily at perisomatic sites (<70 microm from the soma; in contrast to the more uniform distribution of glutamate receptors); no currents were detectable on pyramidal neurons. The activation of nAChRs increased [Ca2+]i, indicating that these native receptors in acute slices are significantly Ca2+-permeable, consistent with previous observations made with recombinant receptors. In addition, they exhibited strong desensitization, the rate of recovery from which was controlled by [Ca2+]i. Our results demonstrate the strategic location and Ca2+ regulation of alpha7-containing nAChRs, which may contribute to understanding their involvement in hippocampal plasticity.
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35
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Halevi S, Yassin L, Eshel M, Sala F, Sala S, Criado M, Treinin M. Conservation within the RIC-3 gene family. Effectors of mammalian nicotinic acetylcholine receptor expression. J Biol Chem 2003; 278:34411-7. [PMID: 12821669 DOI: 10.1074/jbc.m300170200] [Citation(s) in RCA: 149] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In Caenorhabditis elegans, the ric-3 gene is required for the maturation of multiple nicotinic acetylcholine receptors (nAChRs), whereas other neurotransmittergated channels expressed within the same cells are unaffected by the presence of RIC-3. Here we show that RIC-3 is a member of a conserved gene family with representatives in both vertebrates and invertebrates. All members of this family have two transmembrane domains followed by a coiled-coil domain. Expression of the human ric-3 homolog, hric3, like the C. elegans ric-3, enhances C. elegans DEG-3/DES-2, rat alpha 7, and human alpha 7 nAChR-dependent whole-cell current amplitudes in Xenopus leavis oocytes, thus demonstrating functional conservation. However, hric3 also reduces human alpha 4 beta 2 and alpha 3 beta 4 nAChR-dependent whole-cell current amplitudes. Thus, hric3 shows differential effects on human nAChRs unlike the observed uniform effect of ric-3 on C. elegans nAChRs. Moreover, hric3 totally abolished currents evoked by 5-HT3 serotonin receptors, whereas it barely modified alpha1 glycine receptor currents. With this caveat, RIC-3 belongs to a conserved family of genes likely to regulate nAChR-mediated transmission throughout evolution. Analysis of transcripts encoded by the hric3 locus shows that it encodes for multiple transcripts, likely to produce multiple hric3 isoforms, and that hric3 is expressed in neurons and muscles, thus enabling its interactions with nAChRs in vivo.
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Affiliation(s)
- Sarah Halevi
- Department of Physiology, Hebrew University-Hadassah Medical School, Jerusalem 91120, Israel
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36
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Zhao L, Kuo YP, George AA, Peng JH, Purandare MS, Schroeder KM, Lukas RJ, Wu J. Functional properties of homomeric, human alpha 7-nicotinic acetylcholine receptors heterologously expressed in the SH-EP1 human epithelial cell line. J Pharmacol Exp Ther 2003; 305:1132-41. [PMID: 12626641 DOI: 10.1124/jpet.103.048777] [Citation(s) in RCA: 68] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
alpha 7-Nicotinic acetylcholine receptors (alpha 7-nAChRs) are broadly distributed in the central nervous system, where they play important roles in chemical and electrical signaling, and perhaps in neurite outgrowth, synaptic plasticity, and neuronal death/survival. To help elucidate their normal and pathophysiological roles, we have heterologously expressed human alpha 7-nAChR in transfected SH-EP1 human epithelial cells. Reverse transcription-polymerase chain reaction and mRNA fluorescence in situ hybridization analyses demonstrate expression of human alpha 7 subunits as messenger RNA. Patch-clamp recordings exploiting a novel strategy to prevent functional rundown of whole-cell peak current responses to repeated acute challenges with nicotinic agonists show successful expression of functional alpha 7-nAChR that mediate inward currents characterized by rapid phases of activation and inactivation. Concentration-response curves show that nicotine, acetylcholine, and choline are efficacious agonists at human alpha 7-nAChRs. Current-voltage relationships show inward rectification for agonist-induced currents. Human alpha 7-nAChRs exhibit some sensitivity to alpha 7-nAChR antagonists alpha-bungarotoxin (Bgt) or methyllycaconitine (MLA) when applied coincidentally with agonist, but much higher affinity block occurs when cells and alpha 7-nAChRs are pre-exposed to antagonists for 2 min before challenge with agonist. Both Bgt and MLA are competitive inhibitors of alpha 7-nAChR function. Whole-cell current peak amplitudes and half-times for inactivation of alpha 7-nAChR functional responses to nicotine are dramatically reduced in the absence of extracellular Ca2+, suggestive of high Ca2+ permeability of the alpha 7-nAChR channel. Thus, heterologously expressed human alpha 7-nAChR in mammalian cells have properties of native alpha 7-nAChR or of alpha 7-nAChR heterologously expressed in other systems and serve as excellent models for studies of molecular bases of alpha 7-nAChR function.
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Affiliation(s)
- Lingke Zhao
- Division of Neurology, Barrow Neurological Institute, 350 West Thomas Rd., Phoenix, AZ 85013-4496, USA
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37
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Downie DL, Vicente-Agullo F, Campos-Caro A, Bushell TJ, Lieb WR, Franks NP. Determinants of the anesthetic sensitivity of neuronal nicotinic acetylcholine receptors. J Biol Chem 2002; 277:10367-73. [PMID: 11741933 DOI: 10.1074/jbc.m107847200] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Some neurotransmitter-gated ion channels are very much more sensitive to general anesthetics than others, even when they are genetically and structurally related. The most striking example of this is the extreme sensitivity of heteromeric neuronal nicotinic acetylcholine receptors to inhalational general anesthetics compared with the marked insensitivity of the closely related homomeric neuronal nicotinic receptors. Here we investigate the role of the alpha subunit in determining the anesthetic sensitivity of these receptors by using alpha(3)/alpha(7) chimeric subunits that are able to form functional homomeric receptors. By comparing the sensitivities of a number of chimeras to the inhalational agent halothane we show that the short (13 amino acids) putative extracellular loop connecting the second and third transmembrane segments is a critical determinant of anesthetic sensitivity. In addition, using site-directed mutagenesis, we show that two particular amino acids in this loop play a dominant role. When mutations are made in this loop, there is a good correlation between increasing anesthetic sensitivity and decreasing acetylcholine sensitivity. We conclude that this extracellular loop probably does not participate directly in anesthetic binding, but rather determines receptor sensitivity indirectly by playing a critical role in transducing anesthetic binding into an effect on channel gating.
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Affiliation(s)
- David Lindsay Downie
- Biophysics Group, The Blackett Laboratory, Imperial College of Science, Technology & Medicine, London SW7 2BW, United Kingdom
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38
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Abstract
Neuronal nicotinic acetylcholine receptors are a prototype of ligand-gated channels that mediate transmission in the central and peripheral nervous system. Structure-function studies performed at the amino acid level are now unraveling the determinant residues either for the properties of the ligand-binding domain or the ionic pore. In this work we review, in the light of the latest finding, the structure-function relationship of these receptors and their implication in neurological diseases.
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Affiliation(s)
- V Itier
- Department of Physiology, CMU, 1 rue Michel Servet, CH-1211, Geneva 4, Switzerland
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39
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Quiram PA, Ohno K, Milone M, Patterson MC, Pruitt NJ, Brengman JM, Sine SM, Engel AG. Mutation causing congenital myasthenia reveals acetylcholine receptor beta/delta subunit interaction essential for assembly. J Clin Invest 1999; 104:1403-10. [PMID: 10562302 PMCID: PMC409847 DOI: 10.1172/jci8179] [Citation(s) in RCA: 57] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
We describe a severe postsynaptic congenital myasthenic syndrome with marked endplate acetylcholine receptor (AChR) deficiency caused by 2 heteroallelic mutations in the beta subunit gene. One mutation causes skipping of exon 8, truncating the beta subunit before its M1 transmembrane domain, and abolishing surface expression of pentameric AChR. The other mutation, a 3-codon deletion (beta426delEQE) in the long cytoplasmic loop between the M3 and M4 domains, curtails but does not abolish expression. By coexpressing beta426delEQE with combinations of wild-type subunits in 293 HEK cells, we demonstrate that beta426delEQE impairs AChR assembly by disrupting a specific interaction between beta and delta subunits. Studies with related deletion and missense mutants indicate that secondary structure in this region of the beta subunit is crucial for interaction with the delta subunit. The findings imply that the mutated residues are positioned at the interface between beta and delta subunits and demonstrate contribution of this local region of the long cytoplasmic loop to AChR assembly.
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MESH Headings
- Acetylcholinesterase/metabolism
- Alleles
- Amino Acid Sequence
- Animals
- Child
- Codon
- Exons
- Female
- Humans
- Macromolecular Substances
- Male
- Molecular Sequence Data
- Motor Endplate/pathology
- Motor Endplate/physiology
- Muscle, Skeletal/metabolism
- Muscle, Skeletal/pathology
- Muscle, Skeletal/physiopathology
- Myasthenia Gravis, Neonatal/genetics
- Myasthenia Gravis, Neonatal/pathology
- Myasthenia Gravis, Neonatal/physiopathology
- Nuclear Family
- Pedigree
- Protein Structure, Secondary
- Receptors, Cholinergic/chemistry
- Receptors, Cholinergic/genetics
- Receptors, Cholinergic/metabolism
- Reference Values
- Sequence Alignment
- Sequence Deletion
- Sequence Homology, Amino Acid
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Affiliation(s)
- P A Quiram
- Receptor Biology Laboratory, Department of Physiology and Biophysics, Mayo Clinic, Rochester, Minnesota 55905, USA
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40
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Rovira JC, Ballesta JJ, Vicente-Agulló F, Campos-Caro A, Criado M, Sala F, Sala S. A residue in the middle of the M2-M3 loop of the beta4 subunit specifically affects gating of neuronal nicotinic receptors. FEBS Lett 1998; 433:89-92. [PMID: 9738939 DOI: 10.1016/s0014-5793(98)00889-8] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
An aspartate residue in the M2-M3 loop of neuronal nicotinic receptor alpha7 subunits is a major determinant of the channel functional response. This residue is conserved in most beta4 subunits, e.g. human and rat, but not in others, e.g. bovine. We have used these differences to examine the mechanism by which this residue alters the functional properties of alpha3beta4 receptors. Having ruled out an effect on the macroscopic binding ability of the agonist, the level of receptor expression, or the single channel conductance, the results suggest that receptors lacking that residue have a deficient coupling between binding and gating.
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Affiliation(s)
- J C Rovira
- Departamento de Farmacología, Instituto de Neurociencias, Universidad Miguel Hernández, Campus de San Juan, Alicante, Spain
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41
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Recovery from desensitization of neuronal nicotinic acetylcholine receptors of rat chromaffin cells is modulated by intracellular calcium through distinct second messengers. J Neurosci 1998. [PMID: 9502806 DOI: 10.1523/jneurosci.18-07-02458.1998] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
The mechanisms through which changes in intracellular Ca2+ concentration ([Ca2+]i) might influence desensitization of neuronal nicotinic receptors (nAChRs) of rat chromaffin cells were investigated by simultaneous patch-clamp recording of membrane currents and confocal microscopy imaging of [Ca2+]i induced by nicotine. Increases in [Ca2+]i that were induced by membrane depolarization or occurred spontaneously did not influence inward currents elicited by focally applied test pulses (10 msec) of nicotine, indicating that raised [Ca2+]i per se did not trigger desensitization of nAChRs. Desensitization of nAChRs, evoked by 2 sec focal application of nicotine, which largely raised [Ca2+]i, was not affected by intracellular application of agents that activate or depress protein kinase C (PKC) or A (PKA) or inhibit phosphatase 1, 2 A and B. Conversely, recovery from desensitization was facilitated by the phorbol ester phorbol 12-myristate 13-acetate (PMA) or the phosphatase 2 B inhibiting complex of cyclosporin A-cyclophilin A, whereas it was impaired by the broad spectrum kinase inhibitor staurosporine. The effects of PMA or staurosporine were prevented by the intracellularly applied Ca2+ chelator BAPTA. The adenylate cyclase activator forskolin accelerated recovery, whereas the selective PKA antagonist Rp-cAMPS had an opposite effect. The action of staurosporine and Rp-cAMPS on recovery from desensitization was additive. It is proposed that when nAChRs are desensitized, they become susceptible to modulation by [Ca2+]i via intracellular second messengers such as serine/threonine kinases and calcineurin. Thus, the phosphorylation state of neuronal nAChRs appears to regulate their rate of recovery from desensitization.
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42
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Wells GB, Anand R, Wang F, Lindstrom J. Water-soluble nicotinic acetylcholine receptor formed by alpha7 subunit extracellular domains. J Biol Chem 1998; 273:964-73. [PMID: 9422757 DOI: 10.1074/jbc.273.2.964] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Water-soluble models of ligand-gated ion channels would be advantageous for structural studies. We investigated the suitability of three versions of the N-terminal extracellular domain (ECD) of the alpha7 subunit of the nicotinic acetylcholine receptor (AChR) family for this purpose by examining their ligand-binding and assembly properties. Two versions included the first transmembrane domain and were solubilized with detergent after expression in Xenopus oocytes. The third was truncated before the first transmembrane domain and was soluble without detergent. For all three, their equilibrium binding affinities for alpha-bungarotoxin, nicotine, and acetylcholine, combined with their velocity sedimentation profiles, were consistent with the formation of native-like AChRs. These characteristics imply that the alpha7 ECD can form a water-soluble AChR that is a model of the ECD of the full-length alpha7 AChR.
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Affiliation(s)
- G B Wells
- Department of Pathology and Laboratory Medicine, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6082, USA.
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43
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Gehle VM, Walcott EC, Nishizaki T, Sumikawa K. N-glycosylation at the conserved sites ensures the expression of properly folded functional ACh receptors. BRAIN RESEARCH. MOLECULAR BRAIN RESEARCH 1997; 45:219-29. [PMID: 9149096 DOI: 10.1016/s0169-328x(96)00256-2] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
The role of the conserved carbohydrate moiety in the expression of complete acetylcholine receptor (AChR), alpha2 beta gamma delta was re-investigated by expressing additional site-directed mutant subunits, lacking an N-glycosylation site, in Xenopus oocytes. All mutant subunits were stably expressed and appeared to associate with other normal subunits; however, removal of carbohydrate on the alpha subunit inhibited the formation of 125I-alpha-bungarotoxin (alpha-BuTX) binding sites and functional ACh-gated ion channels. 125I-alpha-BuTX binding to AChRs was also significantly reduced by removal of the conserved carbohydrate on the gamma or delta subunits. Immunoprecipitation with monoclonal antibodies that recognize the two distinct alpha-BuTX sites on the AChR indicated that the mutant gamma subunit did not interfere with efficient formation of the alpha-BuTX binding site at the alpha/delta interface, but loss of the carbohydrate did interfere with formation of the alpha-BuTX binding site at the alpha/mutant gamma interface. A similar result was obtained with the mutant delta subunit. Furthermore, the mutant gamma and mutant delta subunits were not incorporated efficiently into the mature (correct tertiary conformation capable of alpha-BuTX binding) alpha beta delta or alpha beta gamma complexes, respectively. Since both mutant gamma and mutant delta subunits were capable of assembling with the alpha subunits (immature assembly), these results suggest that the formation of the two alpha-BuTX binding sites requires correct folding of the alpha gamma and alpha delta complexes, which is aided by the conserved carbohydrate on the gamma and delta subunits. Electrophysiological experiments demonstrated that functional receptors containing mutant subunits were produced, but the functional properties of the mutant receptors were differentially altered, depending on the subunit mutated. Together, our results suggest that N-glycosylation of AChR subunits ensures the correct folding of important functional domains and expression of proper functional receptors in the plasma membrane.
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Affiliation(s)
- V M Gehle
- Department of Psychobiology, University of California at Irvine, 92697-4550, USA
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44
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Gready JE, Ranganathan S, Schofield PR, Matsuo Y, Nishikawa K. Predicted structure of the extracellular region of ligand-gated ion-channel receptors shows SH2-like and SH3-like domains forming the ligand-binding site. Protein Sci 1997; 6:983-98. [PMID: 9144769 PMCID: PMC2143702 DOI: 10.1002/pro.5560060504] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Fast synaptic neurotransmission is mediated by ligand-gated ion-channel (LGIC) receptors, which include receptors for acetylcholine, serotonin, GABA, glycine, and glutamate. LGICs are pentamers with extracellular ligand-binding domains and form integral membrane ion channels that are selective for cations (acetylcholine and serotonin 5HT3 receptors) or anions (GABAA and glycine receptors and the invertebrate glutamate-binding chloride channel). They form a protein superfamily with no sequence similarity to any protein of known structure. Using a 1D-3D structure mapping approach, we have modeled the extracellular ligand-binding domain based on a significant match with the SH2 and SH3 domains of the biotin repressor structure. Refinement of the model based on knowledge of the large family of SH2 and SH3 structures, sequence alignments, and use of structure templates for loop building, allows the prediction of both monomer and pentamer models. These are consistent with medium-resolution electron microscopy structures and with experimental structure/function data from ligand-binding, antibody-binding, mutagenesis, protein-labeling and subunit-linking studies, and glycosylation sites. Also, the predicted polarity of the channel pore calculated from electrostatic potential maps of pentamer models of superfamily members is consistent with known ion selectivities. Using the glycine receptor alpha 1 subunit, which forms homopentamers, the monomeric and pentameric models define the agonist and antagonist (strychnine) binding sites to a deep crevice formed by an extended loop, which includes the invariant disulfide bridge, between the SH2 and SH3 domains. A detailed binding site for strychnine is reported that is in strong agreement with known structure/function data. A site for interaction of the extracellular ligand-binding domain with the activation of the M2 transmembrane helix is also suggested.
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Affiliation(s)
- J E Gready
- Computational Molecular Biology and Drug Design Group, John Curtin School of Medical Research, Australian National University, Canberra, Australia.
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45
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Vicente-Agulló F, Rovira JC, Campos-Caro A, Rodríguez-Ferrer C, Ballesta JJ, Sala S, Sala F, Criado M. Acetylcholine receptor subunit homomer formation requires compatibility between amino acid residues of the M1 and M2 transmembrane segments. FEBS Lett 1996; 399:83-6. [PMID: 8980125 DOI: 10.1016/s0014-5793(96)01291-4] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The neuronal nicotinic acetylcholine receptor (nAChR) subunits alpha3 and alpha7 have different assembly behavior when expressed in heterologous expression systems: alpha3 subunits require other subunits to assemble functional nAChRs, whereas alpha7 subunits can produce homomeric nAChRs. A previous analysis of alpha7/alpha3 chimeric constructs identified a domain comprising the first putative membrane-spanning segment, M1, as essential to homomeric assembly. The present study dissected further this domain, identifying three amino acid residues, which are located at the most intracellular third of the M1 transmembrane segment, as important in the assembly of homomers. Moreover, formation of homooligomeric complexes seems to require a compatible accommodation between this region and certain residues of the second transmembrane segment, M2. Thus, compatibility between defined domains of the M1 and M2 transmembrane segments appears as a determinant factor governing homomer association of nAChR subunits.
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46
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Campos-Caro A, Sala S, Ballesta JJ, Vicente-Agulló F, Criado M, Sala F. A single residue in the M2-M3 loop is a major determinant of coupling between binding and gating in neuronal nicotinic receptors. Proc Natl Acad Sci U S A 1996; 93:6118-23. [PMID: 8650229 PMCID: PMC39199 DOI: 10.1073/pnas.93.12.6118] [Citation(s) in RCA: 76] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Abstract
Binding of agonists to nicotinic acetylcholine receptors generates a sequence of changes that activate a cation-selective conductance. By measuring electrophysiological responses in chimeric alpha7/alpha3 receptors expressed in Xenopus oocytes, we have showed the involvement of the M2-M3 loop in coupling agonist binding to the channel gate. An aspartate residue therein, Asp-266 in the alpha7 subunit, was identified by site-directed mutagenesis as crucial, since mutants at this position exhibited very poor functional responses to three different nicotinic agonists. We have extended this investigation to another neuronal nicotinic receptor (alpha3/beta4), and found that a homologous residue in the beta4 subunit, Asp-268, played a similar role in coupling. These findings are consistent with a hypothesis that the aspartate residue in the M2-M3 loop, which is conserved in all homomer-forming alpha-type subunits and all neuronal beta-type subunits that combine to form functional receptors, is a major determinant of information transmission from binding site to channel gate in all neuronal nicotinic receptors.
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Affiliation(s)
- A Campos-Caro
- Departamento de Neuroquímica, Universidad de Alicante, Spain
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