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Munafó JP, Biscussi B, Obiol D, Costabel M, Bouzat C, Murray AP, Antollini S. New Multitarget Molecules Derived from Caffeine as Potentiators of the Cholinergic System. ACS Chem Neurosci 2024; 15:994-1009. [PMID: 38407056 DOI: 10.1021/acschemneuro.3c00710] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/27/2024] Open
Abstract
Cholinergic deficit is a characteristic factor of several pathologies, such as myasthenia gravis, some types of congenital myasthenic syndromes, and Alzheimer's Disease. Two molecular targets for its treatment are acetylcholinesterase (AChE) and nicotinic acetylcholine receptor (nAChR). In previous studies, we found that caffeine behaves as a partial nAChR agonist and confirmed that it inhibits AChE. Here, we present new bifunctional caffeine derivatives consisting of a theophylline ring connected to amino groups by different linkers. All of them were more potent AChE inhibitors than caffeine. Furthermore, although some of them also activated muscle nAChR as partial agonists, not all of them stabilized nAChR in its desensitized conformation. To understand the molecular mechanism underlying these results, we performed docking studies on AChE and nAChR. The nAChR agonist behavior of the compounds depends on their accessory group, whereas their ability to stabilize the receptor in a desensitized state depends on the interactions of the linker at the binding site. Our results show that the new compounds can inhibit AChE and activate nAChR with greater potency than caffeine and provide further information on the modulation mechanisms of pharmacological targets for the design of novel therapeutic interventions in cholinergic deficit.
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Affiliation(s)
- Juan Pablo Munafó
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Camino La Carrindanga km 7, Bahía Blanca 8000, Argentina
| | - Brunella Biscussi
- Instituto de Química del Sur, Departamento de Química, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Avda. Alem 1253, Bahía Blanca 8000, Argentina
| | - Diego Obiol
- Grupo de Biofísica, Instituto de Física del Sur, Departamento de Física, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Avda. Alem 1253, Bahía Blanca 8000, Argentina
| | - Marcelo Costabel
- Grupo de Biofísica, Instituto de Física del Sur, Departamento de Física, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Avda. Alem 1253, Bahía Blanca 8000, Argentina
| | - Cecilia Bouzat
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Camino La Carrindanga km 7, Bahía Blanca 8000, Argentina
| | - Ana Paula Murray
- Instituto de Química del Sur, Departamento de Química, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Avda. Alem 1253, Bahía Blanca 8000, Argentina
| | - Silvia Antollini
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, Departamento de Biología, Bioquímica y Farmacia, Universidad Nacional del Sur and Consejo Nacional de Investigaciones Científicas y Técnicas, Camino La Carrindanga km 7, Bahía Blanca 8000, Argentina
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Grillberger K, Cöllen E, Trivisani CI, Blum J, Leist M, Ecker GF. Structural Insights into Neonicotinoids and N-Unsubstituted Metabolites on Human nAChRs by Molecular Docking, Dynamics Simulations, and Calcium Imaging. Int J Mol Sci 2023; 24:13170. [PMID: 37685977 PMCID: PMC10487998 DOI: 10.3390/ijms241713170] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/28/2023] [Revised: 08/10/2023] [Accepted: 08/12/2023] [Indexed: 09/10/2023] Open
Abstract
Neonicotinoid pesticides were initially designed in order to achieve species selectivity on insect nicotinic acetylcholine receptors (nAChRs). However, concerns arose when agonistic effects were also detected in human cells expressing nAChRs. In the context of next-generation risk assessments (NGRAs), new approach methods (NAMs) should replace animal testing where appropriate. Herein, we present a combination of in silico and in vitro methodologies that are used to investigate the potentially toxic effects of neonicotinoids and nicotinoid metabolites on human neurons. First, an ensemble docking study was conducted on the nAChR isoforms α7 and α3β4 to assess potential crucial molecular initiating event (MIE) interactions. Representative docking poses were further refined using molecular dynamics (MD) simulations and binding energy calculations using implicit solvent models. Finally, calcium imaging on LUHMES neurons confirmed a key event (KE) downstream of the MIE. This method was also used to confirm the predicted agonistic effect of the metabolite descyano-thiacloprid (DCNT).
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Affiliation(s)
- Karin Grillberger
- Department of Pharmaceutical Sciences, University of Vienna, 1090 Vienna, Austria
| | - Eike Cöllen
- In Vitro Toxicology and Biomedicine, University of Konstanz, 78457 Konstanz, Germany
| | | | - Jonathan Blum
- In Vitro Toxicology and Biomedicine, University of Konstanz, 78457 Konstanz, Germany
| | - Marcel Leist
- In Vitro Toxicology and Biomedicine, University of Konstanz, 78457 Konstanz, Germany
| | - Gerhard F. Ecker
- Department of Pharmaceutical Sciences, University of Vienna, 1090 Vienna, Austria
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3
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Batista VS, Gonçalves AM, Nascimento-Júnior NM. Pharmacophore Mapping Combined with dbCICA Reveal New Structural Features for the Development of Novel Ligands Targeting α4β2 and α7 Nicotinic Acetylcholine Receptors. MOLECULES (BASEL, SWITZERLAND) 2022; 27:molecules27238236. [PMID: 36500328 PMCID: PMC9735964 DOI: 10.3390/molecules27238236] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 10/12/2022] [Revised: 11/04/2022] [Accepted: 11/10/2022] [Indexed: 11/29/2022]
Abstract
The neuronal nicotinic acetylcholine receptors (nAChRs) belong to the ligand-gated ion channel (GLIC) group, presenting a crucial role in several biological processes and neuronal disorders. The α4β2 and α7 nAChRs are the most abundant in the central nervous system (CNS), being involved in challenging diseases such as epilepsy, Alzheimer's disease, schizophrenia, and anxiety disorder, as well as alcohol and nicotine dependencies. In addition, in silico-based strategies may contribute to revealing new insights into drug design and virtual screening to find new drug candidates to treat CNS disorders. In this context, the pharmacophore maps were constructed and validated for the orthosteric sites of α4β2 and α7 nAChRs, through a docking-based Comparative Intermolecular Contacts Analysis (dbCICA). In this sense, bioactive ligands were retrieved from the literature for each receptor. A molecular docking protocol was developed for all ligands in both receptors by using GOLD software, considering GoldScore, ChemScore, ASP, and ChemPLP scoring functions. Output GOLD results were post-processed through dbCICA to identify critical contacts involved in protein-ligand interactions. Moreover, Crossminer software was used to construct a pharmacophoric map based on the most well-behaved ligands and negative contacts from the dbCICA model for each receptor. Both pharmacophore maps were validated by using a ROC curve. The results revealed important features for the ligands, such as the presence of hydrophobic regions, a planar ring, and hydrogen bond donor and acceptor atoms for α4β2. Parallelly, a non-planar ring region was identified for α7. These results can enable fragment-based drug design (FBDD) strategies, such as fragment growing, linking, and merging, allowing an increase in the activity of known fragments. Thus, our results can contribute to a further understanding of structural subunits presenting the potential for key ligand-receptor interactions, favoring the search in molecular databases and the design of novel ligands.
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Affiliation(s)
- Victor S. Batista
- Laboratory of Medicinal Chemistry, Organic Synthesis and Molecular Modeling (LaQMedSOMM), Department of Biochemistry and Organic Chemistry, Institute of Chemistry, São Paulo State University (Unesp), Rua Professor Francisco Degni, 55, Jardim Quitandinha, Araraquara 14800-060, SP, Brazil
| | - Adriano Marques Gonçalves
- Laboratory of Medicinal Chemistry, Organic Synthesis and Molecular Modeling (LaQMedSOMM), Department of Biochemistry and Organic Chemistry, Institute of Chemistry, São Paulo State University (Unesp), Rua Professor Francisco Degni, 55, Jardim Quitandinha, Araraquara 14800-060, SP, Brazil
- Department of Biological and Health Sciences, University of Araraquara (Uniara), Rua Carlos Gomes, 1217, Centro, Araraquara 14801-340, SP, Brazil
| | - Nailton M. Nascimento-Júnior
- Laboratory of Medicinal Chemistry, Organic Synthesis and Molecular Modeling (LaQMedSOMM), Department of Biochemistry and Organic Chemistry, Institute of Chemistry, São Paulo State University (Unesp), Rua Professor Francisco Degni, 55, Jardim Quitandinha, Araraquara 14800-060, SP, Brazil
- Correspondence:
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4
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Conformational transitions and ligand-binding to a muscle-type nicotinic acetylcholine receptor. Neuron 2022; 110:1358-1370.e5. [PMID: 35139364 DOI: 10.1016/j.neuron.2022.01.013] [Citation(s) in RCA: 40] [Impact Index Per Article: 20.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2021] [Revised: 11/02/2021] [Accepted: 01/10/2022] [Indexed: 12/15/2022]
Abstract
Fast synaptic communication requires receptors that respond to the presence of neurotransmitter by opening an ion channel across the post-synaptic membrane. The muscle-type nicotinic acetylcholine receptor from the electric fish, Torpedo, is the prototypic ligand-gated ion channel, yet the structural changes underlying channel activation remain undefined. Here we use cryo-EM to solve apo and agonist-bound structures of the Torpedo nicotinic receptor embedded in a lipid nanodisc. Using both a direct biochemical assay to define the conformational landscape and molecular dynamics simulations to assay flux through the pore, we correlate structures with functional states and elucidate the motions that lead to pore activation of a heteromeric nicotinic receptor. We highlight an underappreciated role for the complementary subunit in channel gating, establish the structural basis for the differential agonist affinities of α/δ versus α /γ sites, and explain why nicotine is less potent at muscle nicotinic receptors compared to neuronal ones.
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5
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Gharpure A, Teng J, Zhuang Y, Noviello CM, Walsh RM, Cabuco R, Howard RJ, Zaveri NT, Lindahl E, Hibbs RE. Agonist Selectivity and Ion Permeation in the α3β4 Ganglionic Nicotinic Receptor. Neuron 2019; 104:501-511.e6. [PMID: 31488329 DOI: 10.1016/j.neuron.2019.07.030] [Citation(s) in RCA: 122] [Impact Index Per Article: 24.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2019] [Revised: 07/01/2019] [Accepted: 07/25/2019] [Indexed: 12/16/2022]
Abstract
Nicotinic acetylcholine receptors are pentameric ion channels that mediate fast chemical neurotransmission. The α3β4 nicotinic receptor subtype forms the principal relay between the central and peripheral nervous systems in the autonomic ganglia. This receptor is also expressed focally in brain areas that affect reward circuits and addiction. Here, we present structures of the α3β4 nicotinic receptor in lipidic and detergent environments, using functional reconstitution to define lipids appropriate for structural analysis. The structures of the receptor in complex with nicotine, as well as the α3β4-selective ligand AT-1001, complemented by molecular dynamics, suggest principles of agonist selectivity. The structures further reveal much of the architecture of the intracellular domain, where mutagenesis experiments and simulations define residues governing ion conductance.
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Affiliation(s)
- Anant Gharpure
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Jinfeng Teng
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Yuxuan Zhuang
- Department of Biochemistry and Biophysics, Science for Life Laboratory, Stockholm University, Solna 17121, Sweden
| | - Colleen M Noviello
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Richard M Walsh
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Rico Cabuco
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA
| | - Rebecca J Howard
- Department of Biochemistry and Biophysics, Science for Life Laboratory, Stockholm University, Solna 17121, Sweden
| | | | - Erik Lindahl
- Department of Biochemistry and Biophysics, Science for Life Laboratory, Stockholm University, Solna 17121, Sweden; Department of Applied Physics, Swedish e-Science Research Center, KTH Royal Institute of Technology, Solna 17121, Sweden
| | - Ryan E Hibbs
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
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6
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Thompson AJ, Metzger S, Lochner M, Ruepp MD. The binding orientation of epibatidine at α7 nACh receptors. Neuropharmacology 2017; 116:421-428. [PMID: 28089847 PMCID: PMC5390772 DOI: 10.1016/j.neuropharm.2017.01.008] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2016] [Revised: 01/09/2017] [Accepted: 01/11/2017] [Indexed: 11/30/2022]
Abstract
Epibatidine is an alkaloid toxin that binds with high affinity to nicotinic and muscarinic acetylcholine receptors, and has been extensively used as a research tool. To examine binding interactions at the nicotinic receptor, it has been co-crystallised with the structural homologue acetylcholine binding protein (AChBP; PDB ID 2BYQ), and with an AChBP chimaera (3SQ6) that shares 64% sequence identity with the α7 nACh receptor. However, the binding orientations revealed by AChBP co-crystal structures may not precisely represent their receptor homologues and experimental evidence is needed to verify the ligand poses. Here we identify potential binding site interactions between epibatidine and AChBP residues, and substitute equivalent positions in the α7 nACh receptor. The effects of these are probed by [3H]epibatidine binding following the expression α7 nACh receptor cysteine mutants in HEK 293 cells. Of the sixteen mutants created, the affinity of epibatidine was unaffected by the substitutions Q55C, L106C, L116C, T146C, D160C and S162C, reduced by C186A and C187A, increased by Q114C and S144C, and abolished by W53C, Y91C, N104C, W145C, Y184C and Y191C. These results are consistent with the predicted orientations in AChBP and suggest that epibatidine is likely to occupy a similar location at α7 nACh receptors. We speculate that steric constraints placed upon the C-5 position of the pyridine ring in 3SQ6 may account for the relatively poor affinities of epibatidine derivatives that are substituted at this position.
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Affiliation(s)
| | - Simon Metzger
- Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland
| | - Martin Lochner
- Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland; Institute of Biochemistry and Molecular Medicine, University of Bern, Bern, Switzerland
| | - Marc-David Ruepp
- Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland.
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7
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Vij R, Purohit P, Auerbach A. Modal affinities of endplate acetylcholine receptors caused by loop C mutations. ACTA ACUST UNITED AC 2016; 146:375-86. [PMID: 26503719 PMCID: PMC4621750 DOI: 10.1085/jgp.201511503] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Modal activity at the nicotinic acetylcholine receptor, in which open channel probability switches reversibly between discrete values, arises from changes in the resting affinity at the agonist site. The time course of the endplate current is determined by the rate and equilibrium constants for acetylcholine receptor (AChR) activation. We measured these constants in single-channel currents from AChRs with mutations at the neurotransmitter-binding sites, in loop C. The main findings are: (a) Almost all perturbations of loop C generate heterogeneity in the channel open probability (“modes”). (b) Modes are generated by different affinities for ACh that can be either higher or lower than in the wild-type receptors. (c) The modes are stable, in so far as each receptor maintains its affinity for at least several minutes. (d) Different agonists show different degrees of modal activity. With the loop C mutation αP197A, there are four modes with ACh but only two with partial agonists. (e) The affinity variations arise exclusively from the αδ-binding site. (f) Substituting four γ-subunit residues into the δ subunit (three in loop E and one in the β5–β5′ linker) reduces modal activity. (g) At each neurotransmitter-binding site, affinity is determined by a core of five aromatic residues. Modes are eliminated by an alanine mutation at δW57 but not at the other aromatics. (h) Modes are eliminated by a phenylalanine substitution at all core aromatics except αY93. The results suggest that, at the αδ agonist site, loop C and the complementary subunit surface can each adopt alternative conformations and interact with each other to influence the position of δW57 with respect to the aromatic core and, hence, affinity.
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Affiliation(s)
- Ridhima Vij
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
| | - Prasad Purohit
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
| | - Anthony Auerbach
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
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8
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Crystal structure of a human neuronal nAChR extracellular domain in pentameric assembly: Ligand-bound α2 homopentamer. Proc Natl Acad Sci U S A 2016; 113:9635-40. [PMID: 27493220 DOI: 10.1073/pnas.1602619113] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
In this study we report the X-ray crystal structure of the extracellular domain (ECD) of the human neuronal α2 nicotinic acetylcholine receptor (nAChR) subunit in complex with the agonist epibatidine at 3.2 Å. Interestingly, α2 was crystallized as a pentamer, revealing the intersubunit interactions in a wild type neuronal nAChR ECD and the full ligand binding pocket conferred by two adjacent α subunits. The pentameric assembly presents the conserved structural scaffold observed in homologous proteins, as well as distinctive features, providing unique structural information of the binding site between principal and complementary faces. Structure-guided mutagenesis and electrophysiological data confirmed the presence of the α2(+)/α2(-) binding site on the heteromeric low sensitivity α2β2 nAChR and validated the functional importance of specific residues in α2 and β2 nAChR subunits. Given the pathological importance of the α2 nAChR subunit and the high sequence identity with α4 (78%) and other neuronal nAChR subunits, our findings offer valuable information for modeling several nAChRs and ultimately for structure-based design of subtype specific drugs against the nAChR associated diseases.
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9
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Dionisio L, Bergé I, Bravo M, Esandi MDC, Bouzat C. Neurotransmitter GABA Activates Muscle but Not α7 Nicotinic Receptors. Mol Pharmacol 2014; 87:391-400. [DOI: 10.1124/mol.114.095539] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
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10
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Forman SA, Chiara DC, Miller KW. Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:169-77. [PMID: 25316107 DOI: 10.1016/j.neuropharm.2014.10.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 09/22/2014] [Accepted: 10/02/2014] [Indexed: 11/25/2022]
Abstract
General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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Affiliation(s)
- Stuart A Forman
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - David C Chiara
- Dept. of Neurobiology, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - Keith W Miller
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
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11
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Lynagh T, Pless SA. Principles of agonist recognition in Cys-loop receptors. Front Physiol 2014; 5:160. [PMID: 24795655 PMCID: PMC4006026 DOI: 10.3389/fphys.2014.00160] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/11/2014] [Accepted: 04/04/2014] [Indexed: 12/22/2022] Open
Abstract
Cys-loop receptors are ligand-gated ion channels that are activated by a structurally diverse array of neurotransmitters, including acetylcholine, serotonin, glycine, and GABA. After the term "chemoreceptor" emerged over 100 years ago, there was some wait until affinity labeling, molecular cloning, functional studies, and X-ray crystallography experiments identified the extracellular interface of adjacent subunits as the principal site of agonist binding. The question of how subtle differences at and around agonist-binding sites of different Cys-loop receptors can accommodate transmitters as chemically diverse as glycine and serotonin has been subject to intense research over the last three decades. This review outlines the functional diversity and current structural understanding of agonist-binding sites, including those of invertebrate Cys-loop receptors. Together, this provides a framework to understand the atomic determinants involved in how these valuable therapeutic targets recognize and bind their ligands.
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Affiliation(s)
| | - Stephan A. Pless
- Department of Drug Design and Pharmacology, Center for Biopharmaceuticals, University of CopenhagenCopenhagen, Denmark
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12
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Van Arnam EB, Dougherty DA. Functional probes of drug-receptor interactions implicated by structural studies: Cys-loop receptors provide a fertile testing ground. J Med Chem 2014; 57:6289-300. [PMID: 24568098 PMCID: PMC4136689 DOI: 10.1021/jm500023m] [Citation(s) in RCA: 40] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
![]()
Structures
of integral membrane receptors provide valuable models
for drug–receptor interactions across many important classes
of drug targets and have become much more widely available in recent
years. However, it remains to be determined to what extent these images
are relevant to human receptors in their biological context and how
subtle issues such as subtype selectivity can be informed by them.
The high precision structural modifications enabled by unnatural amino
acid mutagenesis on mammalian receptors expressed in vertebrate cells
allow detailed tests of predictions from structural studies. Using
the Cys-loop superfamily of ligand-gated ion channels, we show that
functional studies lead to detailed binding models that, at times,
are significantly at odds with the structural studies on related invertebrate
proteins. Importantly, broad variations in binding interactions are
seen for very closely related receptor subtypes and for varying drugs
at a given binding site. These studies highlight the essential interplay
between structural studies and functional studies that can guide efforts
to develop new pharmaceuticals.
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Affiliation(s)
- Ethan B Van Arnam
- Division of Chemistry and Chemical Engineering, California Institute of Technology , Pasadena, California 91125, United States
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13
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Jadey S, Purohit P, Auerbach A. Action of nicotine and analogs on acetylcholine receptors having mutations of transmitter-binding site residue αG153. ACTA ACUST UNITED AC 2013; 141:95-104. [PMID: 23277476 PMCID: PMC3536520 DOI: 10.1085/jgp.201210896] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A primary target for nicotine is the acetylcholine receptor channel (AChR). Some of the ability of nicotine to activate differentially AChR subtypes has been traced to a transmitter-binding site amino acid that is glycine in lower affinity and lysine in higher affinity AChRs. We studied the effects of mutations of this residue (αG153) in neuromuscular AChRs activated by nicotine and eight other agonists including nornicotine and anabasine. All of the mutations increased the unliganded gating equilibrium constant. The affinity of the resting receptor (Kd) and the net binding energy from the agonist for gating (ΔGB) were estimated by cross-concentration fitting of single-channel currents. In all but one of the agonist/mutant combinations there was a moderate decrease in Kd and essentially no change in ΔGB. The exceptional case was nicotine plus lysine, which showed a large, >8,000-fold decrease in Kd but no change in ΔGB. The extraordinary specificity of this combination leads us to speculate that AChRs with a lysine at position αG153 may be exposed to a nicotine-like compound in vivo.
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Affiliation(s)
- Snehal Jadey
- Department of Physiology and Biophysics, State University of New York, Buffalo, NY 14214, USA
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14
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Abstract
In a multimeric receptor protein, the binding of a ligand can modulate the binding of a succeeding ligand. This phenomenon, called cooperativity, is caused by the interaction of the receptor subunits. By using a complex Markovian model and a set of parameters determined previously, we analyzed how the successive binding of four ligands leads to a complex cooperative interaction of the subunits in homotetrameric HCN2 pacemaker channels. The individual steps in the model were characterized by Gibbs free energies for the equilibria and activation energies, specifying the affinity of the binding sites and the transition rates, respectively. Moreover, cooperative free energies were calculated for each binding step in both the closed and the open channel. We show that the cooperativity sequence positive-negative-positive determined for the binding affinity is generated by the combined effect of very different cooperativity sequences determined for the binding and unbinding rates, which are negative-negative-positive and no-negative-no, respectively. It is concluded that in the ligand-induced activation of HCN2 channels, the sequence of cooperativity based on the binding affinity is caused by two even qualitatively different sequences of cooperativity that are based on the rates of ligand binding and unbinding.
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Affiliation(s)
- Klaus Benndorf
- Institut für Physiologie II, Universitätsklinikum Jena, Friedrich-Schiller-Universität Jena, Jena, Germany.
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15
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Auerbach A. The energy and work of a ligand-gated ion channel. J Mol Biol 2013; 425:1461-75. [PMID: 23357172 DOI: 10.1016/j.jmb.2013.01.027] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/22/2012] [Revised: 12/28/2012] [Accepted: 01/17/2013] [Indexed: 11/30/2022]
Abstract
Ligand-gated ion channels are allosteric membrane proteins that isomerize between C(losed) and O(pen) conformations. A difference in affinity for ligands in the two states influences the C↔O "gating" equilibrium constant. The energies associated with adult-type mouse neuromuscular nicotinic acetylcholine receptor (AChR) channel gating have been measured by using single-channel electrophysiology. Without ligands, the free energy, enthalpy and entropy of gating are ΔG0=+8.4, ΔH0=+10.9 and TΔS0=+2.5kcal/mol (-100mV, 23°C). Many mutations throughout the protein change ΔG0, including natural ones that cause disease. Agonists and most mutations change approximately independently the ground-state energy difference; thus, it is possible to forecast and engineer AChR responses simply by combining perturbations. The free energy of the low↔high affinity change for the neurotransmitter at each of two functionally equivalent binding sites is ΔGB(ACh)=-5.1kcal/mol. ΔGB(ACh) is set mainly by interactions of ACh with just three binding site aromatic groups. For a series of structurally related agonists, there is a correlation between the energies of low- and high-affinity binding, which implies that gating commences with the formation of the low-affinity complex. Brief, intermediate states in binding and gating have been detected. Several proposals for the nature of the gating transition-state energy landscape and the isomerization mechanism are discussed.
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Affiliation(s)
- Anthony Auerbach
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA.
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16
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Puskar NL, Lester HA, Dougherty DA. Probing the effects of residues located outside the agonist binding site on drug-receptor selectivity in the nicotinic receptor. ACS Chem Biol 2012; 7:841-6. [PMID: 22296725 DOI: 10.1021/cb200448j] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The nicotinic acetylcholine receptors (nAChRs) are a family of closely related but pharmacologically distinct neurotransmitter-gated ion channels. They are therapeutic targets for a wide range of neurological disorders, and a key issue in drug development is selective targeting among the more than 20 subtypes of nAChRs that are known. The present work evaluates a proposed hydrogen bonding interaction involving a residue known as the "loop B glycine" that distinguishes receptors that are highly responsive to ACh and nicotine from those that are much less so. We have performed structure-function studies on the loop B site, including unnatural amino acid mutagenesis, in three different nAChR subtypes and found that the correlation between agonist potency and this residue is strong. Low potency receptor subtypes have a glycine at this key site, and mutation to a residue with a side chain converts a low potency receptor to a high potency receptor. Innately high potency receptors have a lysine at the loop B site and show a decrease in potency for the reverse mutation (i.e., introducing a glycine). This residue lies outside of the agonist binding site, and studies of other residues at the agonist binding site show that the details of how changes at the loop B glycine site impact agonist potency vary for differing receptor subtypes. This suggests a model in which the loop B residue influences the global shape of the agonist binding site rather than modulating any specific interaction.
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Affiliation(s)
- Nyssa L. Puskar
- Division of Chemistry and Chemical
Engineering, California Institute of Technology, 164-30, Pasadena, California 91125, United States
| | - Henry A. Lester
- Division of Biology, California Institute of Technology, 156-29, Pasadena
, California 91125, United States
| | - Dennis A. Dougherty
- Division of Chemistry and Chemical
Engineering, California Institute of Technology, 164-30, Pasadena, California 91125, United States
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17
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Wang J, Papke RL, Stokes C, Horenstein NA. Potential state-selective hydrogen bond formation can modulate activation and desensitization of the α7 nicotinic acetylcholine receptor. J Biol Chem 2012; 287:21957-69. [PMID: 22556416 DOI: 10.1074/jbc.m112.339796] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
A series of arylidene anabaseines were synthesized to probe the functional impact of hydrogen bonding on human α7 nicotinic acetylcholine receptor (nAChR) activation and desensitization. The aryl groups were either hydrogen bond acceptors (furans), donors (pyrroles), or neither (thiophenes). These compounds were tested against a series of point mutants of the ligand-binding domain residue Gln-57, a residue hypothesized to be proximate to the aryl group of the bound agonist and a putative hydrogen bonding partner. Q57K, Q57D, Q57E, and Q57L were chosen to remove the dual hydrogen bonding donor/acceptor ability of Gln-57 and replace it with hydrogen bond donating, hydrogen bond accepting, or nonhydrogen bonding ability. Activation of the receptor was compromised with hydrogen bonding mismatches, for example, pairing a pyrrole with Q57K or Q57L, or a furan anabaseine with Q57D or Q57E. Ligand co-applications with the positive allosteric modulator PNU-120596 produced significantly enhanced currents whose degree of enhancement was greater for 2-furans or -pyrroles than for their 3-substituted isomers, whereas the nonhydrogen bonding thiophenes failed to show this correlation. Interestingly, the PNU-120596 agonist co-application data revealed that for wild-type α7 nAChR, the 3-furan desensitized state was relatively stabilized compared with that of 2-furan, a reversal of the relationship observed with respect to the barrier for entry into the desensitized state. These data highlight the importance of hydrogen bonding on the receptor-ligand state, and suggest that it may be possible to fine-tune features of agonists that mediate state selection in the nAChR.
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Affiliation(s)
- Jingyi Wang
- Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA
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18
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Rucktooa P, Haseler CA, van Elk R, Smit AB, Gallagher T, Sixma TK. Structural characterization of binding mode of smoking cessation drugs to nicotinic acetylcholine receptors through study of ligand complexes with acetylcholine-binding protein. J Biol Chem 2012; 287:23283-93. [PMID: 22553201 PMCID: PMC3390607 DOI: 10.1074/jbc.m112.360347] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Smoking cessation is an important aim in public health worldwide as tobacco smoking causes many preventable deaths. Addiction to tobacco smoking results from the binding of nicotine to nicotinic acetylcholine receptors (nAChRs) in the brain, in particular the α4β2 receptor. One way to aid smoking cessation is by the use of nicotine replacement therapies or partial nAChR agonists like cytisine or varenicline. Here we present the co-crystal structures of cytisine and varenicline in complex with Aplysia californica acetylcholine-binding protein and use these as models to investigate binding of these ligands binding to nAChRs. This analysis of the binding properties of these two partial agonists provides insight into differences with nicotine binding to nAChRs. A mutational analysis reveals that the residues conveying subtype selectivity in nAChRs reside on the binding site complementary face and include features extending beyond the first shell of contacting residues.
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Affiliation(s)
- Prakash Rucktooa
- Division of Biochemistry and Center for Biomedical Genetics, Netherlands Cancer Institute, 1066 CX Amsterdam, The Netherlands
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19
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Abstract
Since the discovery of the major excitatory and inhibitory neurotransmitters and their receptors in the brain, many have deliberated over their likely structures and how these may relate to function. This was initially satisfied by the determination of the first amino acid sequences of the Cys-loop receptors that recognized acetylcholine, serotonin, GABA, and glycine, followed later by similar determinations for the glutamate receptors, comprising non-NMDA and NMDA subtypes. The last decade has seen a rapid advance resulting in the first structures of Cys-loop receptors, related bacterial and molluscan homologs, and glutamate receptors, determined down to atomic resolution. This now provides a basis for determining not just the complete structures of these important receptor classes, but also for understanding how various domains and residues interact during agonist binding, receptor activation, and channel opening, including allosteric modulation. This article reviews our current understanding of these mechanisms for the Cys-loop and glutamate receptor families.
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Affiliation(s)
- Trevor G Smart
- Department of Neuroscience, Physiology & Pharmacology, University College London, London, United Kingdom.
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20
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Bar-Lev DD, Degani-Katzav N, Perelman A, Paas Y. Molecular dissection of Cl--selective Cys-loop receptor points to components that are dispensable or essential for channel activity. J Biol Chem 2011; 286:43830-43841. [PMID: 21987577 DOI: 10.1074/jbc.m111.282715] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022] Open
Abstract
Cys-loop receptors are pentameric ligand-gated ion channels (pLGICs) that bind neurotransmitters to open an intrinsic transmembrane ion channel pore. The recent crystal structure of a prokaryotic pLGIC from the cyanobacterium Gloeobacter violaceus (GLIC) revealed that it naturally lacks an N-terminal extracellular α helix and an intracellular domain that are typical of eukaryotic pLGICs. GLIC does not respond to neurotransmitters acting at eukaryotic pLGICs but is activated by protons. To determine whether the structural differences account for functional differences, we used a eukaryotic chimeric acetylcholine-glutamate pLGIC that was modified to carry deletions corresponding to the sequences missing in the prokaryotic homolog GLIC. Deletions made in the N-terminal extracellular α helix did not prevent the expression of receptor subunits and the appearance of receptor assemblies on the cell surface but abolished the capability of the receptor to bind α-bungarotoxin (a competitive antagonist) and to respond to the neurotransmitter. Other truncated chimeric receptors that lacked the intracellular domain did bind ligands; displayed robust acetylcholine-elicited responses; and shared with the full-length chimeric receptor similar anionic selectivity, effective open pore diameter, and unitary conductance. We suggest that the integrity of the N-terminal α helix is crucial for ligand accommodation because it stabilizes the intersubunit interfaces adjacent to the neurotransmitter-binding pocket(s). We also conclude that the intracellular domain of the chimeric acetylcholine-glutamate receptor does not modulate the ion channel conductance and is not involved in positioning of the pore-lining helices in the conformation necessary for coordinating a Cl- ion within the intracellular vestibule of the ion channel pore.
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Affiliation(s)
- Dekel D Bar-Lev
- Laboratory of Ion Channels, Bar-Ilan University, Ramat Gan 52900, Israel; Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Nurit Degani-Katzav
- Laboratory of Ion Channels, Bar-Ilan University, Ramat Gan 52900, Israel; Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Alexander Perelman
- Scientific Equipment Unit, The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Yoav Paas
- Laboratory of Ion Channels, Bar-Ilan University, Ramat Gan 52900, Israel; Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel.
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21
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Bouzat C. New insights into the structural bases of activation of Cys-loop receptors. ACTA ACUST UNITED AC 2011; 106:23-33. [PMID: 21995938 DOI: 10.1016/j.jphysparis.2011.09.012] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2011] [Revised: 09/07/2011] [Accepted: 09/26/2011] [Indexed: 11/27/2022]
Abstract
Neurotransmitter receptors of the Cys-loop superfamily mediate rapid synaptic transmission throughout the nervous system, and include receptors activated by ACh, GABA, glycine and serotonin. They are involved in physiological processes, including learning and memory, and in neurological disorders, and they are targets for clinically relevant drugs. Cys-loop receptors assemble either from five copies of one type of subunit, giving rise to homomeric receptors, or from several types of subunits, giving rise to heteromeric receptors. Homomeric receptors are invaluable models for probing fundamental relationships between structure and function. Receptors contain a large extracellular domain that carries the binding sites and a transmembrane region that forms the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50Å to the ion channel gate is central to understanding receptor function. Depending on the receptor subtype, occupancy of either two, as in the prototype muscle nicotinic receptor, or three binding sites, as in homomeric receptors, is required for full activation. The conformational changes initiated at the binding sites are propagated to the gate through the interface between the extracellular and transmembrane domains. This region forms a network that relays structural changes from the binding site towards the pore, and also contributes to open channel lifetime and rate of desensitization. Thus, this coupling region controls the beginning and duration of a synaptic response. Here we review recent advances in the molecular mechanism by which Cys-loop receptors are activated with particular emphasis on homomeric receptors.
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Affiliation(s)
- Cecilia Bouzat
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur and CONICET, 8000 Bahía Blanca, Argentina.
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22
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Puskar NL, Xiu X, Lester HA, Dougherty DA. Two neuronal nicotinic acetylcholine receptors, alpha4beta4 and alpha7, show differential agonist binding modes. J Biol Chem 2011; 286:14618-27. [PMID: 21343288 DOI: 10.1074/jbc.m110.206565] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nicotinic acetylcholine receptors (nAChRs) are pentameric, neurotransmitter-gated ion channels responsible for rapid excitatory neurotransmission in the central and peripheral nervous systems, resulting in skeletal muscle tone and various cognitive effects in the brain. These complex proteins are activated by the endogenous neurotransmitter ACh as well as by nicotine and structurally related agonists. Activation and modulation of nAChRs has been implicated in the pathology of multiple neurological disorders, and as such, these proteins are established therapeutic targets. Here we use unnatural amino acid mutagenesis to examine the ligand binding mechanisms of two homologous neuronal nAChRs: the α4β4 and α7 receptors. Despite sequence identity among the residues that form the core of the agonist-binding site, we find that the α4β4 and α7 nAChRs employ different agonist-receptor binding interactions in this region. The α4β4 receptor utilizes a strong cation-π interaction to a conserved tryptophan (TrpB) of the receptor for both ACh and nicotine, and nicotine participates in a strong hydrogen bond with a backbone carbonyl contributed by TrpB. Interestingly, we find that the α7 receptor also employs a cation-π interaction for ligand recognition, but the site has moved to a different aromatic amino acid of the agonist-binding site depending on the agonist. ACh participates in a cation-π interaction with TyrA, whereas epibatidine participates in a cation-π interaction with TyrC2.
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Affiliation(s)
- Nyssa L Puskar
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
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23
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Pittel I, Witt-Kehati D, Degani-Katzav N, Paas Y. Probing pore constriction in a ligand-gated ion channel by trapping a metal ion in the pore upon agonist dissociation. J Biol Chem 2010; 285:26519-31. [PMID: 20466725 PMCID: PMC2924088 DOI: 10.1074/jbc.m110.102327] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2010] [Revised: 04/24/2010] [Indexed: 12/16/2022] Open
Abstract
Eukaryotic pentameric ligand-gated ion channels (pLGICs) are receptors activated by neurotransmitters to rapidly transport ions across cell membranes, down their electrochemical gradients. Recent crystal structures of two prokaryotic pLGICs were interpreted to imply that the extracellular side of the transmembrane pore constricts to close the channel (Hilf, R. J., and Dutzler, R. (2009) Nature 457, 115-118; Bocquet, N., Nury, H., Baaden, M., Le Poupon, C., Changeux, J. P., Delarue, M., and Corringer, P. J. (2009) Nature 457, 111-114). Here, we utilized a eukaryotic acetylcholine (ACh)-serotonin chimeric pLGIC that was engineered with histidines to coordinate a metal ion within the channel pore, at its cytoplasmic side. In a previous study, the access of Zn(2+) ions to the engineered histidines had been explored when the channel was either at rest (closed) or active (open) (Paas, Y., Gibor, G., Grailhe, R., Savatier-Duclert, N., Dufresne, V., Sunesen, M., de Carvalho, L. P., Changeux, J. P., and Attali, B. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 15877-15882). In this study, the interactions of Zn(2+) with the pore were probed upon agonist (ACh) dissociation that triggers the transition of the receptor from the active conformation to the resting conformation (i.e. during deactivation). Application of Zn(2+) onto ACh-bound open receptors obstructed their pore and prevented ionic flow. Removing ACh from its extracellular binding sites to trigger deactivation while Zn(2+) is still bound led to tight trapping of Zn(2+) within the pore. Together with single-channel recordings, made to explore single pore-blocking events, we show that dissociation of ACh causes the gate to shut on a Zn(2+) ion that effectively acts as a "foot in the door." We infer that, upon deactivation, the cytoplasmic side of the pore of the ACh-serotonin receptor chimera constricts to close the channel.
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Affiliation(s)
- Ilya Pittel
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Dvora Witt-Kehati
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Nurit Degani-Katzav
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Yoav Paas
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
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24
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Cysteine accessibility analysis of the human alpha7 nicotinic acetylcholine receptor ligand-binding domain identifies L119 as a gatekeeper. Neuropharmacology 2010; 60:159-71. [PMID: 20650284 DOI: 10.1016/j.neuropharm.2010.07.014] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2010] [Revised: 07/06/2010] [Accepted: 07/13/2010] [Indexed: 01/09/2023]
Abstract
A large number of structurally diverse ligands have been produced to selectively target α7 nicotinic acetylcholine receptors (nAChRs). We applied the method of scanning cysteine accessibility mutations (SCAM) to the ligand-binding domain of the α7 nAChR to identify subdomains of particular importance to the binding and subsequent activation by select agonists. We evaluated the activity of four structurally distinct α7 agonists on wild-type human α7 and 44 targeted mutants expressed in Xenopus oocytes. Responses were measured prior and subsequent to the application of the sulfhydryl reagent methanethiosulfonate ethylammonium (MTSEA). One mutant (C116S) served as a Cys-null control, and the additional mutants were made in the C116S background. In many cases, the insertion of free cysteines into the agonist-binding site had a negative effect on function, with 12 of 44 mutants showing no detectable responses to ACh, and with only 19 of the 44 mutants showing sufficiently large responses to permit further study. Several of the cysteine mutations, including W55C, showed selectively reduced responses to the largest agonist tested, 2-methoxy,4-hydroxy-benzylidene anabaseine. Interestingly, although homology models suggest that most of the introduced cysteine mutations should have had good solvent accessibility, application of MTSEA had no effect or produced only modest changes in the agonist response profile of most mutants. Consistent with previous studies implicating W55 to play important roles in agonist activation, MTSEA treatment further decreased the functional responses of W55C to all the test agonists. While the cysteine mutation at L119 itself had relatively little effect on receptor function, treatment of L119C receptors with MTSEA or alternative cationic sulfhydryl reagents profoundly decreased activation by all agonists tested, suggesting a general block of gating. The homologous mutation in heteromeric nAChRs produced similar results, provided that the mutation was placed in the beta subunit complementary surface of the ligand-binding domain. Structural models locate the L119 residue directly across the subunit interface from the C-loop of the primary face of the binding domain. Our data suggest that a covalent modification of L119C by MTSEA or other cationic reagents might block the binding of even small agonists such as TMA through electrostatic interactions. Reaction of L119C with small non-polar reagents increases activation by small agonists but can block the access of large ligands such as benzylidene anabaseines to the ligand-binding domain.
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25
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Iturriaga-Vásquez P, Carbone A, García-Beltrán O, Livingstone PD, Biggin PC, Cassels BK, Wonnacott S, Zapata-Torres G, Bermudez I. Molecular determinants for competitive inhibition of alpha4beta2 nicotinic acetylcholine receptors. Mol Pharmacol 2010; 78:366-75. [PMID: 20547737 DOI: 10.1124/mol.110.065490] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The Erythrina alkaloids erysodine and dihydro-beta-erythroidine (DHbetaE) are potent and selective competitive inhibitors of alpha4beta2 nicotinic acetylcholine receptors (nAChRs), but little is known about the molecular determinants of the sensitivity of this receptor subtype to inhibition by this class of antagonists. We addressed this issue by examining the effects of DHbetaE and a range of aromatic Erythrina alkaloids on [(3)H]cytisine binding and receptor function in conjunction with homology models of the alpha4beta2 nAChR, mutagenesis, and functional assays. The lactone group of DHbetaE and a hydroxyl group at position C-16 in aromatic Erythrina alkaloids were identified as major determinants of potency, which was decreased when the conserved residue Tyr126 in loop A of the alpha4 subunit was substituted by alanine. Sensitivity to inhibition was also decreased by substituting the conserved aromatic residues alpha4Trp182 (loop B), alpha4Tyr230 (loop C), and beta2Trp82 (loop D) and the nonconserved beta2Thr84; however, only alpha4Trp182 was predicted to contact bound antagonist, suggesting alpha4Tyr230, beta2Trp82, and beta2Thr84 contribute allosterically to the closed state elicited by bound antagonist. In addition, homology modeling predicted strong ionic interactions between the ammonium center of the Erythrina alkaloids and beta2Asp196, leading to the uncapping of loop C. Consistent with this, beta2D196A abolished sensitivity to inhibition by DHbetaE or erysodine but not by epierythratidine, which is not predicted to form ionic bonds with beta2Asp196. This residue is not conserved in subunits that comprise nAChRs with low sensitivity to inhibition by DHbetaE or erysodine, which highlights beta2Asp196 as a major determinant of the receptor selectivity of Erythrina alkaloids.
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26
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Miller PS, Smart TG. Binding, activation and modulation of Cys-loop receptors. Trends Pharmacol Sci 2010; 31:161-74. [PMID: 20096941 DOI: 10.1016/j.tips.2009.12.005] [Citation(s) in RCA: 233] [Impact Index Per Article: 16.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2009] [Revised: 12/17/2009] [Accepted: 12/18/2009] [Indexed: 11/19/2022]
Abstract
It is over forty years since the major neurotransmitters and their protein receptors were identified, and over twenty years since determination of the first amino-acid sequences of the Cys-loop receptors that recognize acetylcholine, serotonin, GABA and glycine. The last decade has seen the first structures of these proteins (and related bacterial and molluscan homologues) determined to atomic resolution. Hopefully over the next decade, more detailed molecular structures of entire Cys-loop receptors in drug-bound and drug-free conformations will become available. These, together with functional studies, will provide a clear picture of how these receptors participate in neurotransmission and how structural variations between receptor subtypes impart their unique characteristics. This insight should facilitate the design of novel and improved therapeutics to treat neurological disorders. This review considers our current understanding about the processes of agonist binding, receptor activation and channel opening, as well as allosteric modulation of the Cys-loop receptor family.
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Affiliation(s)
- Paul S Miller
- Department of Neuroscience, Physiology & Pharmacology, University College London, Gower Street, London WC1E 6BT, UK
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27
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Arias HR. Positive and negative modulation of nicotinic receptors. ADVANCES IN PROTEIN CHEMISTRY AND STRUCTURAL BIOLOGY 2010; 80:153-203. [PMID: 21109220 DOI: 10.1016/b978-0-12-381264-3.00005-9] [Citation(s) in RCA: 58] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
Nicotinic acetylcholine receptors (AChRs) are one of the best characterized ion channels from the Cys-loop receptor superfamily. The study of acetylcholine binding proteins and prokaryotic ion channels from different species has been paramount for the understanding of the structure-function relationship of the Cys-loop receptor superfamily. AChR function can be modulated by different ligand types. The neurotransmitter ACh and other agonists trigger conformational changes in the receptor, finally opening the intrinsic cation channel. The so-called gating process couples ligand binding, located at the extracellular portion, to the opening of the ion channel, located at the transmembrane region. After agonist activation, in the prolonged presence of agonists, the AChR becomes desensitized. Competitive antagonists overlap the agonist-binding sites inhibiting the pharmacological action of agonists. Positive allosteric modulators (PAMs) do not bind to the orthostetic binding sites but allosterically enhance the activity elicited by agonists by increasing the gating process (type I) and/or by decreasing desensitization (type II). Instead, negative allosteric modulators (NAMs) produce the opposite effects. Interestingly, this negative effect is similar to that found for another class of allosteric drugs, that is, noncompetitive antagonists (NCAs). However, the main difference between both categories of drugs is based on their distinct binding site locations. Although both NAMs and NCAs do not bind to the agonist sites, NACs bind to sites located in the ion channel, whereas NAMs bind to nonluminal sites. However, this classification is less clear for NAMs interacting at the extracellular-transmembrane interface where the ion channel mouth might be involved. Interestingly, PAMs and NAMs might be developed as potential medications for the treatment of several diseases involving AChRs, including dementia-, skin-, and immunological-related diseases, drug addiction, and cancer. More exciting is the potential combination of specific agonists with specific PAMs. However, we are still in the beginning of understanding how these compounds act and how these drugs can be used therapeutically.
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Affiliation(s)
- Hugo R Arias
- Department of Pharmaceutical Sciences, Midwestern University, Glendale, AZ, USA
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28
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Bartos M, Corradi J, Bouzat C. Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region. Mol Neurobiol 2009; 40:236-52. [PMID: 19859835 DOI: 10.1007/s12035-009-8084-x] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2009] [Accepted: 09/22/2009] [Indexed: 10/25/2022]
Abstract
Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.
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Affiliation(s)
- Mariana Bartos
- Instituto de Investigaciones Bioquímicas, UNS-CONICET, Bahía Blanca, Argentina
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30
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Abstract
Binding of a neurotransmitter to its ionotropic receptor opens a distantly located ion channel, a process termed allosteric activation. Here we review recent advances in the molecular mechanism by which the cys-loop receptors are activated with emphasis on the best studied nicotinic acetylcholine receptors (nAChRs). With a combination of affinity labeling, mutagenesis, electrophysiology, kinetic modeling, electron microscopy (EM), and crystal structure analysis, the allosteric activation mechanism is emerging. Specifically, the binding domain and gating domain are interconnected by an allosteric activation network. Agonist binding induces conformational changes, resulting in the rotation of a beta sheet of amino-terminal domain and outward movement of loop 2, loop F, and cys-loop, which are coupled to the M2-M3 linker to pull the channel to open. However, there are still some controversies about the movement of the channel-lining domain M2. Nine angstrom resolution EM structure of a nAChR imaged in the open state suggests that channel opening is the result of rotation of the M2 domain. In contrast, recent crystal structures of bacterial homologues of the cys-loop receptor family in apparently open state have implied an M2 tilting model with pore dilation and quaternary twist of the whole pentameric receptor. An elegant study of the nAChR using protonation scanning of M2 domain supports a similar pore dilation activation mechanism with minimal rotation of M2. This remains to be validated with other approaches including high resolution structure determination of the mammalian cys-loop receptors in the open state.
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31
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Xiu X, Puskar NL, Shanata JAP, Lester HA, Dougherty DA. Nicotine binding to brain receptors requires a strong cation-pi interaction. Nature 2009; 458:534-7. [PMID: 19252481 PMCID: PMC2755585 DOI: 10.1038/nature07768] [Citation(s) in RCA: 282] [Impact Index Per Article: 18.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2008] [Accepted: 01/09/2009] [Indexed: 11/29/2022]
Abstract
Nicotine addiction begins with high-affinity binding of nicotine to acetylcholine (ACh) receptors in the brain. The end result is over 4,000,000 smoking-related deaths annually worldwide and the largest source of preventable mortality in developed countries. Stress reduction, pleasure, improved cognition, and other CNS effects are strongly associated with smoking. But, if nicotine activated ACh receptors found in muscle as potently as it does brain receptors, smoking would cause intolerable and perhaps fatal muscle contractions. Despite extensive pharmacological, functional, and structural studies of ACh receptors, the basis for the differential action of nicotine on brain vs. muscle ACh receptors has not been determined. Here we show that at the α4β2 brain receptors thought to underlie nicotine addiction, the high affinity of nicotine is the result of a strong cation-п interaction to a specific aromatic amino acid of the receptor, TrpB. In contrast, the low affinity of nicotine at the muscle-type receptor is largely due to the fact that this key interaction is absent, even though the immediate binding site residues, including the key TrpB, are identical in the brain and muscle receptors. At the same time a hydrogen bond from nicotine to the backbone carbonyl of TrpB is enhanced in the neuronal receptor relative to the muscle-type. A point mutation near TrpB that differentiates α4β2 and muscle-type receptors appears to influence the shape of the binding site, allowing nicotine to interact more strongly with TrpB in the neuronal receptor. ACh receptors are established therapeutic targets for Alzheimer’s disease, schizophrenia, Parkinson’s disease, smoking cessation, pain, attention deficit-hyperactivity disorder, epilepsy, autism, and depression1. Along with solving a chemical mystery in nicotine addiction, our results provide guidance for efforts to develop drugs that target specific types of nicotinic receptors.
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Affiliation(s)
- Xinan Xiu
- Division of Chemistry and Chemical Engineering, California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA
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32
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Tantama M, Licht S. Use of calculated cation-pi binding energies to predict relative strengths of nicotinic acetylcholine receptor agonists. ACS Chem Biol 2008; 3:693-702. [PMID: 19032090 DOI: 10.1021/cb800189y] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Agonists and antagonists of the nicotinic acetylcholine receptor (nAChR) are used to treat nicotine addiction, neuromuscular disorders, and neurological diseases. In designing small molecule therapeutics with the nAChR as a target, it is useful to identify chemical parameters that correlate with ability to activate the receptor. Previous studies have shown that cation-pi interactions at the transmitter binding sites of the nAChR are important for receptor activation by strong agonists such as acetylcholine. We hypothesized that a calculated estimate of cation-pi binding ability could be used to predict the efficiency for channel opening (i.e., the gating efficiency) associated with activation of the acetylcholine receptor by a series of structurally related organic cations. We demonstrate that the calculated cation-pi energy is strongly correlated with gating efficiency but only weakly correlated with closed-state binding affinity. Our results suggest that cation-pi interactions contribute significantly to the open-state affinity of these cations and that the calculated cation-pi energy will be a useful parameter for designing nAChR agonists and antagonists.
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Affiliation(s)
- Mathew Tantama
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 16, Room 573B, Cambridge, Massachusetts 02139
| | - Stuart Licht
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 16, Room 573B, Cambridge, Massachusetts 02139
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33
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A conserved salt bridge critical for GABA(A) receptor function and loop C dynamics. Proc Natl Acad Sci U S A 2008; 105:13604-9. [PMID: 18757734 DOI: 10.1073/pnas.0801854105] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Chemical signaling in the brain involves rapid opening and closing of ligand gated ion channels (LGICs). LGICs are allosteric membrane proteins that transition between multiple conformational states (closed, open, and desensitized) in response to ligand binding. While structural models of cys-loop LGICs have been recently developed, our understanding of the protein movements underlying these conformational transitions is limited. Neurotransmitter binding is believed to initiate an inward capping movement of the loop C region of the ligand-binding site, which ultimately triggers channel gating. Here, we identify a critical intrasubunit salt bridge between conserved charged residues (betaE153, betaK196) in the GABA(A) receptor (GABA(A)R) that is involved in regulating loop C position. Charge reversals (E153K, K196E) increased the EC(50) for GABA and for the allosteric activators pentobarbital (PB) and propofol indicating that these residues are critical for channel activation, and charge swap (E153K-K196E) significantly rescued receptor function suggesting a functional electrostatic interaction. Mutant cycle analysis of alanine substitutions indicated that E153 and K196 are energetically coupled. By monitoring disulfide bond formation between cysteines substituted at these positions (E153C-K196C), we probed the mobility of loop C in resting and ligand-bound states. Disulfide bond formation was significantly reduced in the presence of GABA or PB suggesting that agonist activation of the GABA(A)R proceeds via restricting loop C mobility.
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Corradi J, Spitzmaul G, De Rosa MJ, Costabel M, Bouzat C. Role of pairwise interactions between M1 and M2 domains of the nicotinic receptor in channel gating. Biophys J 2006; 92:76-86. [PMID: 17028140 PMCID: PMC1697868 DOI: 10.1529/biophysj.106.088757] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The adult form of the nicotinic acetylcholine receptor (AChR) consists of five subunits (alpha(2)betaepsilondelta), each having four transmembrane domains (M1-M4). The atomic model of the nicotinic acetylcholine receptor shows that the pore-lining M2 domains make no extensive contacts with the rest of the transmembrane domains. However, there are several sites where close appositions between segments occur. It has been suggested that the pair alphaM1-F15' and alphaM2-L11' is one of the potential interactions between segments. To determine experimentally if these residues are interacting and to explore if this interhelical interaction is essential for channel gating, we combined mutagenesis with single-channel kinetic analysis. Mutations in alphaM1-F15' lead to profound changes in the opening rate and slighter changes in the closing rate. Channel gating is impaired as the volume of the residue increases. Rate-equilibrium linear free-energy relationship analysis reveals an approximately 70% open-state-like environment for alphaM1-F15' at the transition state of the gating reaction, suggesting that it moves early during the gating process. Replacing the residue at alphaM1-15' by that at alphaM2-11' and vice versa profoundly alters gating, but the combination of the two mutations restores gating to near normal, indicating that alphaM1-F15' and alphaM2-L11' are interchangeable. Double-mutant cycle analysis shows that these residues are energetically coupled. Thus, the interaction between M1 and M2 plays a key role in channel gating.
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Affiliation(s)
- Jeremías Corradi
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-CONICET, Bahía Blanca, Argentina
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35
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Bartos M, Rayes D, Bouzat C. Molecular determinants of pyrantel selectivity in nicotinic receptors. Mol Pharmacol 2006; 70:1307-18. [PMID: 16825485 DOI: 10.1124/mol.106.026336] [Citation(s) in RCA: 19] [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
Nicotinic receptors (acetylcholine receptors, AChRs) play key roles in synaptic transmission throughout the nervous system. AChRs mediate neuromuscular transmission in nematodes, and they are targets for antiparasitic drugs. The anthelmintic agents levamisole and pyrantel, which are potent agonists of nematode muscle AChRs, are partial agonists of mammalian muscle AChRs. To further explore the structural basis of the differential activation of AChR subtypes by anthelmintics, we studied the activation of alpha7 AChRs using the high-conductance form of the alpha7-5-hydroxytryptamine-3A receptor, which is a good model for pharmacological studies involving the extracellular region of alpha7. Macroscopic and single-channel current recordings show that levamisole is a weak agonist of alpha7. It is interesting that pyrantel is a more potent agonist of alpha7 than acetylcholine (ACh). To identify determinants of this differential activation, we replaced residues of the complementary face of the binding site by the homologous residues in the muscle epsilon subunit and evaluated changes in activation. The mutation Q57G does not affect the activation by either ACh or levamisole. However, it increases EC50 values and decreases the maximal response to pyrantel. Kinetic analysis shows that gating of the mutant channel activated by pyrantel is profoundly impaired. The decreased sensitivity of alpha7-Q57G to pyrantel agrees with its weak action at muscle AChRs, indicating that when glycine occupies position 57, as in the mammalian muscle AChR, pyrantel behaves as a partial agonist. Thus, position 57 located at the complementary face of the binding site plays a key role in the selective activation of AChRs by pyrantel.
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Affiliation(s)
- Mariana Bartos
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-Consejo de Investigaciones Científicas y Técnicas, Camino La Carrindanga Km 7, B8000FWB, Bahía Blanca, Argentina
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36
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Grutter T, Prado de Carvalho L, Virginie D, Taly A, Fischer M, Changeux JP. A chimera encoding the fusion of an acetylcholine-binding protein to an ion channel is stabilized in a state close to the desensitized form of ligand-gated ion channels. C R Biol 2006; 328:223-34. [PMID: 15810546 DOI: 10.1016/j.crvi.2004.11.004] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
To understand the mechanism of allosteric coupling between the ligand-binding domain and the ion channel of the Cys-loop ligand-gated ion channels (LGICs), we fused the soluble acetylcholine-binding protein (AChBP), which lacks an ion channel, to either the cationic serotonin type-3A ion channel (5HT(3A)) or the anionic glycine ion channel. Both linear chimeras expressed in HEK-293 cells display high affinity for the nicotinic agonist epibatidine (K(D) = 0.2-0.5 nM), but are not targeted to the cell surface. Only after substituting a ring of three loops located at the putative membrane side of the AChBP three-dimensional structure by the homologous residues of 5HT(3A), the resulting chimera AChBP(ring)/5HT(3A) (i) still displayed on intact cells an apparent high affinity for epibatidine, yet with a fourfold decrease (K(D) = 2.1 nM), (ii) displayed a high proportion of low affinity sites (11 +/- 7 microM) for the resting state stabilizing competitive antagonist alpha-bungarotoxin and (iii) was successfully targeted to the cell surface, as seen by immunofluorescence labelling. The AChBP(ring)/5HT(3A) chimera forms a pentameric structure, as revealed by sucrose gradient sedimentation. However, no whole-cell patch-clamp currents were detectable. Interestingly, binding assays with membrane fragments prepared from cells expressing AChBP(ring)/5HT(3A) showed a decrease in the apparent affinity for the agonists nicotine and epibatidine (5-fold), concomitant with an increase in the proportion of high-affinity sites (48 +/- 1 nM) for alpha-bungarotoxin. These results indicate that fusion of AChBP to an ion channel forms a pentameric receptor exposed to the cell surface and able to convert between discrete allosteric states, but stabilized in a high affinity state for epibatidine that likely corresponds to a desensitized form of LGICs. These artificial chimeras might offer a useful system to investigate signal transduction in LGICs.
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Affiliation(s)
- Thomas Grutter
- Récepteurs et Cognition, URA CNRS 2182, Institut Pasteur, 25, rue du Docteur-Roux, 75724 Paris, France.
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Grutter T, de Carvalho LP, Dufresne V, Taly A, Edelstein SJ, Changeux JP. Molecular tuning of fast gating in pentameric ligand-gated ion channels. Proc Natl Acad Sci U S A 2005; 102:18207-12. [PMID: 16319224 PMCID: PMC1312410 DOI: 10.1073/pnas.0509024102] [Citation(s) in RCA: 91] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Neurotransmitters such as acetylcholine (ACh) and glycine mediate fast synaptic neurotransmission by activating pentameric ligand-gated ion channels (LGICs). These receptors are allosteric transmembrane proteins that rapidly convert chemical messages into electrical signals. Neurotransmitters activate LGICs by interacting with an extracellular agonist-binding domain (ECD), triggering a tertiary/quaternary conformational change in the protein that results in the fast opening of an ion pore domain (IPD). However, the molecular mechanism that determines the fast opening of LGICs remains elusive. Here, we show by combining whole-cell and single-channel recordings of recombinant chimeras between the ECD of alpha7 nicotinic receptor (nAChR) and the IPD of the glycine receptor (GlyR) that only two GlyR amino acid residues of loop 7 (Cys-loop) from the ECD and at most five alpha7 nAChR amino acid residues of the M2-M3 loop (2-3L) from the IPD control the fast activation rates of the alpha7/Gly chimera and WT GlyR. Mutual interactions of these residues at a critical pivot point between the agonist-binding site and the ion channel fine-tune the intrinsic opening and closing rates of the receptor through stabilization of the transition state of activation. These data provide a structural basis for the fast opening of pentameric LGICs.
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Affiliation(s)
- Thomas Grutter
- Laboratoire Récepteurs et Cognition, Institut Pasteur, Paris, France.
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38
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Mourot A, Rodrigo J, Kotzyba-Hibert F, Bertrand S, Bertrand D, Goeldner M. Probing the reorganization of the nicotinic acetylcholine receptor during desensitization by time-resolved covalent labeling using [3H]AC5, a photoactivatable agonist. Mol Pharmacol 2005; 69:452-61. [PMID: 16269537 DOI: 10.1124/mol.105.017566] [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
The structural reorganizations occurring on the nicotinic acetylcholine receptor (nAChR) during activation and subsequent desensitization have been investigated through time-resolved photoaffinity labeling using a photoactivatable nicotinic agonist. [(3)H]AC5 is a photosensitive nicotinic probe with high affinity for the desensitized state of the Torpedo marmorata receptor (K(D) = 5 nM) that displays full agonist activity on the Torpedo californica receptor expressed in oocytes (EC(50) = 1.2 microM). Photoaffinity labeling of this receptor in the desensitized state showed a predominant specific labeling of gamma and delta subunits, whereas the alpha subunit was barely labeled. Using a stopped-flow device combined with a flash photolysis quenching system, we investigated the covalent mapping of the subunits as a function of incubation time of the receptor with [(3)H]AC5 (17 ms-1.25 h). During agonist-induced desensitization, specific labeling increased substantially, with similar time constants for gamma and delta subunits (0.016 s(-1)), whereas labeling of the alpha subunit remained relatively low. Therefore, the repartition of radioactivity shifted during desensitization from a weak but predominant labeling of the alpha and gamma subunits toward a substantial labeling of gamma and delta subunits. The observed time-dependent labeling pattern together with AC5 docking into a homology model of the T. californica nAChR suggest a subunit reorganization during agonist-induced desensitization, leading to a tightly packed arrangement that corresponds to a stable high affinity state for agonists.
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Affiliation(s)
- Alexandre Mourot
- Laboratoire de Chimie Bioorganique, Unité Mixte de Recherche (UMR) 7514 Centre National de la Recherche Scientifique (CNRS), Faculté de Pharmacie, Université Louis Pasteur Strasbourg, Illkirch, France.
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39
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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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40
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Rayes D, Spitzmaul G, Sine SM, Bouzat C. Single-channel kinetic analysis of chimeric alpha7-5HT3A receptors. Mol Pharmacol 2005; 68:1475-83. [PMID: 16118362 DOI: 10.1124/mol.105.015438] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The receptor chimera alpha7-5HT3A has served as a prototype for understanding the pharmacology of alpha7 neuronal nicotinic receptors, yet its low single channel conductance has prevented studies of the activation kinetics of single receptor channels. In this study, we show that introducing mutations in the M3-M4 cytoplasmic linker of the chimera alters neither the apparent affinity for the agonist nor the EC50 but increases the amplitude of agonist-evoked single channel currents to enable kinetic analysis. Channel events appear as single brief openings flanked by long closings or as bursts of several openings in quick succession. Both the open and closed time distributions are described as the sum of multiple exponential components, but these do not change over a wide range of acetylcholine (ACh), nicotine, or choline concentrations. Bursts elicited by a saturating concentration of ACh contain brief and long openings and closings, and a cyclic scheme containing two open and two closed states is found to adequately describe the data. The analysis indicates that once fully occupied, the receptor opens rapidly and efficiently, and closes and reopens several times before it desensitizes. Channel closing and desensitization occur at similar rates and account for the invariant open and closed time distributions.
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Affiliation(s)
- Diego Rayes
- Instituto de Investigaciones Bioquímicas, Camino La Carrindanga Km 7, 8000 Bahía Blanca, Argentina.
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41
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Newell JG, McDevitt RA, Czajkowski C. Mutation of glutamate 155 of the GABAA receptor beta2 subunit produces a spontaneously open channel: a trigger for channel activation. J Neurosci 2005; 24:11226-35. [PMID: 15601928 PMCID: PMC6730373 DOI: 10.1523/jneurosci.3746-04.2004] [Citation(s) in RCA: 66] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Protein movements underlying ligand-gated ion channel activation are poorly understood. The binding of agonist initiates a series of conformational movements that ultimately lead to the opening of the ion channel pore. Although little is known about local movements within the GABA-binding site, a recent structural model of the GABA(A) receptor (GABA(A)R) ligand-binding domain predicts that beta2Glu155 is a key residue for direct interactions with the neurotransmitter (Cromer et al., 2002). To elucidate the role of the beta2Ile154-Asp163 region in GABA(A)R activation, each residue was individually mutated to cysteine and coexpressed with wild-type alpha1 subunits in Xenopus laevis oocytes. Seven mutations increased the GABA EC50 value (8- to 3400-fold), whereas three mutations (E155C, S156C, and G158C) also significantly increased the 2-(3-carboxypropyl)-3-amino-6-(4-methoxyphenyl) pyridazinium (SR-95531) K(I) value. GABA, SR-95531, and pentobarbital slowed N-biotinylaminoethyl methanethiosulfonate modification of T160C and D163C, indicating that beta2Thr160 and beta2Asp163 are located in or near the GABA-binding site and that this region undergoes structural rearrangements during channel gating. Cysteine substitution of beta2Glu155 resulted in spontaneously open GABA(A)Rs and differentially decreased the GABA, piperidine-4-sulfonic acid (partial agonist), and SR-95531 sensitivities, indicating that the mutation perturbs ligand binding as well as channel gating. Tethering thiol-reactive groups onto beta2E155C closed the spontaneously open channels, suggesting that beta2Glu155 is a control element involved in coupling ligand binding to channel gating. Structural modeling suggests that the beta2 Ile154-Asp163 region is a protein hinge that forms a network of interconnections that couples binding site movements to the cascade of events leading to channel opening.
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Affiliation(s)
- J Glen Newell
- Department of Physiology, University of Wisconsin, Madison, Wisconsin 53706, USA
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42
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Sallette J, Pons S, Devillers-Thiery A, Soudant M, Prado de Carvalho L, Changeux JP, Corringer PJ. Nicotine Upregulates Its Own Receptors through Enhanced Intracellular Maturation. Neuron 2005; 46:595-607. [PMID: 15944128 DOI: 10.1016/j.neuron.2005.03.029] [Citation(s) in RCA: 195] [Impact Index Per Article: 10.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2004] [Revised: 11/19/2004] [Accepted: 03/24/2005] [Indexed: 10/25/2022]
Abstract
Chronic exposure to nicotine elicits upregulation of high-affinity nicotinic receptors in the smoker's brain. To address the molecular mechanism of upregulation, we transfected HEK293 cells with human alpha4beta2 receptors and traced the subunits throughout their intracellular biosynthesis, using metabolic labeling and immunoprecipitation techniques. We show that high-mannose glycosylated subunits mature and assemble into pentamers in the endoplasmic reticulum and that only pentameric receptors reach the cell surface following carbohydrate processing. Nicotine is shown to act inside the cell and to increase the amount of beta subunits immunoprecipitated by the conformation-dependent mAb290, indicating that nicotine enhances a critical step in the intracellular maturation of these receptors. This effect, which also takes place at concentrations of nicotine found in the blood of smokers upon expression of alpha4beta2 in SH-SY5Y neuroblastoma cells, may play a crucial role in nicotine addiction and possibly implement a model of neural plasticity.
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Affiliation(s)
- Jérôme Sallette
- CNRS URA D2182 Récepteurs et Cognition, Institut Pasteur, Paris, France
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43
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Abstract
Ligand-gated ion channels (LGICs) are fast-responding channels in which the receptor, which binds the activating molecule (the ligand), and the ion channel are part of the same nanomolecular protein complex. This paper will describe the properties and functions of the nicotinic acetylcholine LGIC superfamily, which plays a critical role in the fast chemical transmission of electrical signals between nerve cells and between nerve and muscle cells. The superfamily will mainly be exemplified by the excitatory nicotinic acetylcholine receptor (nAChR) and the inhibitory glycine receptor (GlyR) channels.
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Affiliation(s)
- Peter H Barry
- School of Medical Sciences at the University of New South Wales, Sydney, NSW 2052, Australia.
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44
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Henchman RH, Wang HL, Sine SM, Taylor P, McCammon JA. Ligand-induced conformational change in the alpha7 nicotinic receptor ligand binding domain. Biophys J 2005; 88:2564-76. [PMID: 15665135 PMCID: PMC1305353 DOI: 10.1529/biophysj.104.053934] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023] Open
Abstract
Molecular dynamics simulations of a homology model of the ligand binding domain of the alpha7 nicotinic receptor are conducted with a range of bound ligands to induce different conformational states. Four simulations of 15 ns each are run with no ligand, antagonist d-tubocurarine (dTC), agonist acetylcholine (ACh), and agonist ACh with potentiator Ca(2+), to give insight into the conformations of the active and inactive states of the receptor and suggest the mechanism for conformational change. The main structural factor distinguishing the active and inactive states is that a more open, symmetric arrangement of the five subunits arises for the two agonist simulations, whereas a more closed and asymmetric arrangement results for the apo and dTC cases. Most of the difference arises in the lower portion of the ligand binding domain near its connection to the adjacent transmembrane domain. The transfer of the more open state to the transmembrane domain could then promote ion flow through the channel. Variation in how subunits pack together with no ligand bound appears to give rise to asymmetry in the apo case. The presence of dTC expands the receptor but induces rotations in alternate directions in adjacent subunits that lead to an asymmetric arrangement as in the apo case. Ca(2+) appears to promote a slightly greater expansion in the subunits than ACh alone by stabilizing the C-loop and ACh positions. Although the simulations are unlikely to be long enough to view the full conformational changes between open and closed states, a collection of different motions at a range of length scales are observed that are likely to participate in the conformational change.
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Affiliation(s)
- Richard H Henchman
- Howard Hughes Medical Institute, NSF Center for Theoretical Biophysics, Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA.
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45
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Beene DL, Price KL, Lester HA, Dougherty DA, Lummis SCR. Tyrosine residues that control binding and gating in the 5-hydroxytryptamine3 receptor revealed by unnatural amino acid mutagenesis. J Neurosci 2005; 24:9097-104. [PMID: 15483128 PMCID: PMC6730062 DOI: 10.1523/jneurosci.2429-04.2004] [Citation(s) in RCA: 75] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/14/2023] Open
Abstract
The mechanism by which agonist binding triggers pore opening in ligand-gated ion channels is poorly understood. Here, we used unnatural amino acid mutagenesis to introduce subtle changes to the side chains of tyrosine residues (Tyr141, Tyr143, Tyr153, and Tyr234), which dominate the 5-HT3 receptor binding site. Heterologous expression in oocytes, combined with radioligand binding data and a model of 5-HT (serotonin) computationally docked into the binding site, has allowed us to determine which of these residues are responsible for binding and/or gating. We have shown that Tyr 143 forms a hydrogen bond that is essential for receptor gating but does not affect binding, whereas a hydrogen bond formed by Tyr153 is involved in both binding and gating of the receptor. The aromatic group of Tyr234 is essential for binding and gating, whereas its hydroxyl does not affect binding but plays a steric role in receptor gating. Tyr141 is not involved in agonist binding or receptor gating but is important for antagonist interactions. These data, combined with a new model of the nonliganded 5-HT3 receptor, lead to a mechanistic explanation of the interactions that initiate the conformational change leading to channel opening. Thus, we suggest that agonist entry into the binding pocket may displace Tyr143 and Tyr153 and results in their forming new hydrogen bonds. These bonds may form part of the network of bond rearrangements that trigger the conformational change leading to channel opening. Similar rearrangements may initiate gating in all Cys-loop receptors.
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Affiliation(s)
- Darren L Beene
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
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Gyermek L. Development of ultra short-acting muscle relaxant agents: History, research strategies, and challenges. Med Res Rev 2005; 25:610-54. [PMID: 16086361 DOI: 10.1002/med.20036] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Author has reviewed the literature and his own work related to the chemistry, pharmacology, and clinical aspects of new muscle relaxants. Emphasis has been placed on the basic science concepts and technologies (e.g. structure-activity relationships, nicotinic receptor pharmacology, and investigation of side effects) behind the development of rapidly and short acting nondepolarizing muscle relaxants.
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Affiliation(s)
- Laszlo Gyermek
- Department of Anesthesiology, Harbor-UCLA Medical Center, Box 10, 1000 W. Carson Street, Torrance, California 90509, USA.
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Pons S, Sallette J, Bourgeois JP, Taly A, Changeux JP, Devillers-Thiéry A. Critical role of the C-terminal segment in the maturation and export to the cell surface of the homopentameric alpha7-5HT3A receptor. Eur J Neurosci 2004; 20:2022-30. [PMID: 15450081 DOI: 10.1111/j.1460-9568.2004.03673.x] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Many neurological pathologies are related to misfolded proteins. During folding and assembly in the endoplasmic reticulum, the nicotinic acetylcholine receptor (nAChR) subunits undergo several conformational changes to acquire the ability to bind ligands. After folding and maturation, by mechanisms largely unknown, receptors are exported to the cell surface. We investigated the maturational role of the extracellular C-terminal segment located at the boundary between the extracellular and the transmembrane domains. In the functional chimeric alpha7-5HT3A receptor used as a model system, amino acids from the C-terminal segment were successively deleted or mutated. Upon progressive shortening of the peptide we observed less and less alpha-bungarotoxin binding sites until no sites could be detected when the entire peptide had been deleted (chimera Del 5). Protein synthesis and pentameric assembly were not altered. In Del 5 transfected cells, pentameric receptors present in the endoplasmic reticulum were not detected on the cell surface where Del 5 proteins appeared as patches. With the Del 5 chimera, export of proteins to the cell surface diminished to about half that of wild-type. We propose that the C-terminal segment plays a double role: (i) through an interaction between the penultimate tyrosine residue of the C-terminal segment and the Cys loop of the N-terminal domain, it locks the receptor in a mature alpha-bungarotoxin binding conformation; (ii) this mature conformation, in turn, masks a retention signal present in the first transmembrane segment allowing properly assembled and matured receptors to escape to the cell surface.
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Affiliation(s)
- S Pons
- Laboratoire Récepteurs et Cognition, Département de Neuroscience, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris 15, France
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Ibañez-Tallon I, Wen H, Miwa JM, Xing J, Tekinay AB, Ono F, Brehm P, Heintz N. Tethering Naturally Occurring Peptide Toxins for Cell-Autonomous Modulation of Ion Channels and Receptors In Vivo. Neuron 2004; 43:305-11. [PMID: 15294139 DOI: 10.1016/j.neuron.2004.07.015] [Citation(s) in RCA: 69] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/28/2004] [Revised: 07/06/2004] [Accepted: 07/07/2004] [Indexed: 11/18/2022]
Abstract
The physiologies of cells depend on electrochemical signals carried by ion channels and receptors. Venomous animals produce an enormous variety of peptide toxins with high affinity for specific ion channels and receptors. The mammalian prototoxin lynx1 shares with alpha-bungarotoxin the ability to bind and modulate nicotinic receptors (nAChRs); however, lynx1 is tethered to the membrane via a GPI anchor. We show here that several classes of neurotoxins, including bungarotoxins and cobratoxins, retain their selective antagonistic properties when tethered to the membrane. Targeted elimination of nAChR function in zebrafish can be achieved with tethered alpha-bungarotoxin, silencing synaptic transmission without perturbing synapse formation. These studies harness the pharmacological properties of peptide toxins for use in genetic experiments. When combined with specific methods of cell and temporal expression, the extension of this approach to hundreds of naturally occurring peptide toxins opens a new landscape for cell-autonomous regulation of cellular physiology in vivo.
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Affiliation(s)
- Inés Ibañez-Tallon
- Howard Hughes Medical Institute, Laboratory of Molecular Biology, The Rockefeller University, New York, NY 10021, USA
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Rayes D, De Rosa MJ, Bartos M, Bouzat C. Molecular Basis of the Differential Sensitivity of Nematode and Mammalian Muscle to the Anthelmintic Agent Levamisole. J Biol Chem 2004; 279:36372-81. [PMID: 15201284 DOI: 10.1074/jbc.m403096200] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Levamisole is an anthelmintic agent that exerts its therapeutic effect by acting as a full agonist of the nicotinic receptor (AChR) of nematode muscle. Its action at the mammalian muscle AChR has not been elucidated to date despite its wide use as an anthelmintic in humans and cattle. By single channel and macroscopic current recordings, we investigated the interaction of levamisole with the mammalian muscle AChR. Levamisole activates mammalian AChRs. However, single channel openings are briefer than those activated by acetylcholine (ACh) and do not appear in clusters at high concentrations. The peak current induced by levamisole is about 3% that activated by ACh. Thus, the anthelmintic acts as a weak agonist of the mammalian AChR. Levamisole also produces open channel blockade of the AChR. The apparent affinity for block (190 microm at -70 mV) is similar to that of the nematode AChR, suggesting that differences in channel activation kinetics govern the different sensitivity of nematode and mammalian muscle to anthelmintics. To identify the structural basis of this different sensitivity, we performed mutagenesis targeting residues in the alpha subunit that differ between vertebrates and nematodes. The replacement of the conserved alphaGly-153 with the homologous glutamic acid of nematode AChR significantly increases the efficacy of levamisole to activate channels. Channel activity takes place in clusters having two different kinetic modes. The kinetics of the high open probability mode are almost identical when the agonist is ACh or levamisole. It is concluded that alphaGly-153 is involved in the low efficacy of levamisole to activate mammalian muscle AChRs.
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Affiliation(s)
- Diego Rayes
- Instituto de Investigaciones Bioquímicas de Bahía Blanca, UNS-CONICET, Camino La Carrindanga, Km 7B-8000FWB Bahía Blanca, Argentina
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Tsetlin VI, Hucho F. Snake and snail toxins acting on nicotinic acetylcholine receptors: fundamental aspects and medical applications. FEBS Lett 2003; 557:9-13. [PMID: 14741333 DOI: 10.1016/s0014-5793(03)01454-6] [Citation(s) in RCA: 81] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
This review covers recent data on interactions of nicotinic acetylcholine receptors (AChR) with snake venom proteins (alpha- and kappa-neurotoxins, 'weak' toxins recently shown to act on AChRs), as well as with peptide alpha-conotoxins from Conus snails. Mutations of AChRs and toxins, X-ray/nuclear magnetic resonance structures of alpha-neurotoxin bound to AChR fragments, and the X-ray structure of the acetylcholine-binding protein were used by several groups to build models for the alpha-neurotoxin-AChR complexes. Application of snake toxins and alpha-conotoxins for pharmacological distinction of muscle, neuronal and neuronal-like AChR subtypes and for other medical purposes is briefly discussed.
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Affiliation(s)
- V I Tsetlin
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia.
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