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Gondarenko E, Mazur D, Masliakova M, Ryabukha Y, Kasheverov I, Utkin Y, Tsetlin V, Shahparonov M, Kudryavtsev D, Antipova N. Subtype-Selective Peptide and Protein Neurotoxic Inhibitors of Nicotinic Acetylcholine Receptors Enhance Proliferation of Patient-Derived Glioblastoma Cell Lines. Toxins (Basel) 2024; 16:80. [PMID: 38393158 PMCID: PMC10891657 DOI: 10.3390/toxins16020080] [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: 12/05/2023] [Revised: 01/24/2024] [Accepted: 01/30/2024] [Indexed: 02/25/2024] Open
Abstract
Glioblastoma multiforme (GBM) is the most aggressive type of brain cancer, with a poor prognosis. GBM cells, which develop in the environment of neural tissue, often exploit neurotransmitters and their receptors to promote their own growth and invasion. Nicotinic acetylcholine receptors (nAChRs), which play a crucial role in central nervous system signal transmission, are widely represented in the brain, and GBM cells express several subtypes of nAChRs that are suggested to transmit signals from neurons, promoting tumor invasion and growth. Analysis of published GBM transcriptomes revealed spatial heterogeneity in nAChR subtype expression, and functional nAChRs of α1*, α7, and α9 subtypes are demonstrated in our work on several patient-derived GBM microsphere cultures and on the U87MG GBM cell line using subtype-selective neurotoxins and fluorescent calcium mobilization assay. The U87MG cell line shows reactions to nicotinic agonists similar to those of GBM patient-derived culture. Selective α1*, α7, and α9 nAChR neurotoxins stimulated cell growth in the presence of nicotinic agonists. Several cultivating conditions with varying growth factor content have been proposed and tested. The use of selective neurotoxins confirmed that cell cultures obtained from patients are representative GBM models, but the use of media containing fetal bovine serum can lead to alterations in nAChR expression and functioning.
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Affiliation(s)
- Elena Gondarenko
- Department of Molecular Neuroimmune Signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (E.G.); (I.K.); (V.T.); (D.K.)
| | - Diana Mazur
- Department of Functioning of Living Systems, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (D.M.); (M.M.); (Y.R.); (M.S.); (N.A.)
| | - Marina Masliakova
- Department of Functioning of Living Systems, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (D.M.); (M.M.); (Y.R.); (M.S.); (N.A.)
| | - Yana Ryabukha
- Department of Functioning of Living Systems, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (D.M.); (M.M.); (Y.R.); (M.S.); (N.A.)
| | - Igor Kasheverov
- Department of Molecular Neuroimmune Signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (E.G.); (I.K.); (V.T.); (D.K.)
| | - Yuri Utkin
- Department of Molecular Neuroimmune Signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (E.G.); (I.K.); (V.T.); (D.K.)
| | - Victor Tsetlin
- Department of Molecular Neuroimmune Signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (E.G.); (I.K.); (V.T.); (D.K.)
| | - Mikhail Shahparonov
- Department of Functioning of Living Systems, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (D.M.); (M.M.); (Y.R.); (M.S.); (N.A.)
| | - Denis Kudryavtsev
- Department of Molecular Neuroimmune Signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (E.G.); (I.K.); (V.T.); (D.K.)
- Department of Biology and General Genetics, I.M. Sechenov First Moscow State Medical University, 119048 Moscow, Russia
| | - Nadine Antipova
- Department of Functioning of Living Systems, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry of Russian Academy of Sciences, 117997 Moscow, Russia; (D.M.); (M.M.); (Y.R.); (M.S.); (N.A.)
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Severi I, Perugini J, Ruocco C, Coppi L, Pedretti S, Di Mercurio E, Senzacqua M, Ragni M, Imperato G, Valerio A, Mitro N, Crestani M, Nisoli E, Giordano A. Activation of a non-neuronal cholinergic system in visceral white adipose tissue of obese mice and humans. Mol Metab 2024; 79:101862. [PMID: 38141849 PMCID: PMC10792749 DOI: 10.1016/j.molmet.2023.101862] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/07/2023] [Revised: 12/07/2023] [Accepted: 12/20/2023] [Indexed: 12/25/2023] Open
Abstract
BACKGROUND AND OBJECTIVES Since white adipose tissue (WAT) lacks parasympathetic cholinergic innervation, the source of the acetylcholine (ACh) acting on white adipocyte cholinergic receptors is unknown. This study was designed to identify ACh-producing cells in mouse and human visceral WAT and to determine whether a non-neuronal cholinergic system becomes activated in obese inflamed WAT. METHODS Mouse epididymal WAT (eWAT) and human omental fat were studied in normal and obese subjects. The expression of the key molecules involved in cholinergic signaling was evaluated by qRT-PCR and western blotting whereas their tissue distribution and cellular localization were investigated by immunohistochemistry, confocal microscopy and in situ hybridization. ACh levels were measured by liquid chromatography/tandem mass spectrometry. The cellular effects of ACh were assessed in cultured human multipotent adipose-derived stem cell (hMADS) adipocytes. RESULTS In mouse eWAT, diet-induced obesity modulated the expression of key cholinergic molecular components and, especially, raised the expression of choline acetyltransferase (ChAT), the ACh-synthesizing enzyme, which was chiefly detected in interstitial macrophages, in macrophages forming crown-like structures (CLSs), and in multinucleated giant cells (MGCs). The stromal vascular fraction of obese mouse eWAT contained significantly higher ACh and choline levels than that of control mice. ChAT was undetectable in omental fat from healthy subjects, whereas it was expressed in a number of interstitial macrophages, CLSs, and MGCs from some obese individuals. In hMADS adipocytes stressed with tumor necrosis factor α, ACh, alone or combined with rivastigmine, significantly blunted monocyte chemoattractant protein 1 and interleukin 6 expression, it partially but significantly, restored adiponectin and GLUT4 expression, and promoted glucose uptake. CONCLUSIONS In mouse and human visceral WAT, obesity induces activation of a macrophage-dependent non-neuronal cholinergic system that is capable of exerting anti-inflammatory and insulin-sensitizing effects on white adipocytes.
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Affiliation(s)
- Ilenia Severi
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Jessica Perugini
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Chiara Ruocco
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Lara Coppi
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Silvia Pedretti
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Eleonora Di Mercurio
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Martina Senzacqua
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Maurizio Ragni
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Gabriele Imperato
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Alessandra Valerio
- Department of Molecular and Translational Medicine, University of Brescia, 25123 Brescia, Italy
| | - Nico Mitro
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy; Department of Experimental Oncology, IEO, European Institute of Oncology IRCCS, Milano, Italy
| | - Maurizio Crestani
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Enzo Nisoli
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Antonio Giordano
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy; Center of Obesity, Marche Polytechnic University-United Hospitals, Ancona, Italy.
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Godellas NE, Cymes GD, Grosman C. An experimental test of the nicotinic hypothesis of COVID-19. Proc Natl Acad Sci U S A 2022; 119:e2204242119. [PMID: 36279466 PMCID: PMC9636949 DOI: 10.1073/pnas.2204242119] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/09/2022] [Accepted: 09/19/2022] [Indexed: 11/29/2022] Open
Abstract
The pathophysiological mechanisms underlying the constellation of symptoms that characterize COVID-19 are only incompletely understood. In an effort to fill these gaps, a "nicotinic hypothesis," which posits that nicotinic acetylcholine receptors (AChRs) act as additional severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptors, has recently been put forth. A key feature of the proposal (with potential clinical ramifications) is the suggested competition between the virus' spike protein and small-molecule cholinergic ligands for the receptor's orthosteric binding sites. This notion is reminiscent of the well-established role of the muscle AChR during rabies virus infection. To address this hypothesis directly, we performed equilibrium-type ligand-binding competition assays using the homomeric human α7-AChR (expressed on intact cells) as the receptor, and radio-labeled α-bungarotoxin (α-BgTx) as the orthosteric-site competing ligand. We tested different SARS-CoV-2 spike protein peptides, the S1 domain, and the entire S1-S2 ectodomain, and found that none of them appreciably outcompete [125I]-α-BgTx in a specific manner. Furthermore, patch-clamp recordings showed no clear effect of the S1 domain on α7-AChR-mediated currents. We conclude that the binding of the SARS-CoV-2 spike protein to the human α7-AChR's orthosteric sites-and thus, its competition with ACh, choline, or nicotine-is unlikely to be a relevant aspect of this complex disease.
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Affiliation(s)
- Nicole E. Godellas
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana–Champaign, Urbana, IL 61801
| | - Gisela D. Cymes
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana–Champaign, Urbana, IL 61801
| | - Claudio Grosman
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana–Champaign, Urbana, IL 61801
- Center for Biophysics and Quantitative Biology, University of Illinois at Urbana–Champaign, Urbana, IL 61801
- Neuroscience Program, University of Illinois at Urbana–Champaign, Urbana, IL 61801
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4
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Godellas NE, Grosman C. Probing function in ligand-gated ion channels without measuring ion transport. J Gen Physiol 2022; 154:213244. [PMID: 35612603 DOI: 10.1085/jgp.202213082] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2022] [Accepted: 04/28/2022] [Indexed: 12/11/2022] Open
Abstract
Although the functional properties of ion channels are most accurately assessed using electrophysiological approaches, a number of experimental situations call for alternative methods. Here, working on members of the pentameric ligand-gated ion channel (pLGIC) superfamily, we focused on the practical implementation of, and the interpretation of results from, equilibrium-type ligand-binding assays. Ligand-binding studies of pLGICs are by no means new, but the lack of uniformity in published protocols, large disparities between the results obtained for a given parameter by different groups, and a general disregard for constraints placed on the experimental observations by simple theoretical considerations suggested that a thorough analysis of this classic technique was in order. To this end, we present a detailed practical and theoretical study of this type of assay using radiolabeled α-bungarotoxin, unlabeled small-molecule cholinergic ligands, the human homomeric α7-AChR, and extensive calculations in the framework of a realistic five-binding-site reaction scheme. Furthermore, we show examples of the practical application of this method to tackle two longstanding questions in the field: our results suggest that ligand-binding affinities are insensitive to binding-site occupancy and that mutations to amino-acid residues in the transmembrane domain are unlikely to affect the channel's affinities for ligands that bind to the extracellular domain.
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Affiliation(s)
- Nicole E Godellas
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL
| | - Claudio Grosman
- Department of Molecular and Integrative Physiology, University of Illinois at Urbana-Champaign, Urbana, IL.,Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, IL.,Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL
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5
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Rahman MM, Basta T, Teng J, Lee M, Worrell BT, Stowell MHB, Hibbs RE. Structural mechanism of muscle nicotinic receptor desensitization and block by curare. Nat Struct Mol Biol 2022; 29:386-394. [PMID: 35301478 DOI: 10.1038/s41594-022-00737-3] [Citation(s) in RCA: 29] [Impact Index Per Article: 14.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2021] [Accepted: 02/04/2022] [Indexed: 11/09/2022]
Abstract
Binding of the neurotransmitter acetylcholine to its receptors on muscle fibers depolarizes the membrane and thereby triggers muscle contraction. We sought to understand at the level of three-dimensional structure how agonists and antagonists alter nicotinic acetylcholine receptor conformation. We used the muscle-type receptor from the Torpedo ray to first define the structure of the receptor in a resting, activatable state. We then determined the receptor structure bound to the agonist carbachol, which stabilizes an asymmetric, closed channel desensitized state. We find conformational changes in a peripheral membrane helix are tied to recovery from desensitization. To probe mechanisms of antagonism, we obtained receptor structures with the active component of curare, a poison arrow toxin and precursor to modern muscle relaxants. d-Tubocurarine stabilizes the receptor in a desensitized-like state in the presence and absence of agonist. These findings define the transitions between resting and desensitized states and reveal divergent means by which antagonists block channel activity of the muscle-type nicotinic receptor.
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Affiliation(s)
- Md Mahfuzur Rahman
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Tamara Basta
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO, USA
| | - Jinfeng Teng
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA
| | - Myeongseon Lee
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO, USA
| | - Brady T Worrell
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO, USA
| | - Michael H B Stowell
- Department of Molecular, Cellular and Developmental Biology, University of Colorado, Boulder, CO, USA.
| | - Ryan E Hibbs
- Department of Neuroscience, University of Texas Southwestern Medical Center, Dallas, TX, USA.
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Signal transduction through Cys-loop receptors is mediated by the nonspecific bumping of closely apposed domains. Proc Natl Acad Sci U S A 2021; 118:2021016118. [PMID: 33785596 DOI: 10.1073/pnas.2021016118] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
One of the most fundamental questions in the field of Cys-loop receptors (pentameric ligand-gated ion channels, pLGICs) is how the affinity for neurotransmitters and the conductive/nonconductive state of the transmembrane pore are correlated despite the ∼60-Å distance between the corresponding domains. Proposed mechanisms differ, but they all converge into the idea that interactions between wild-type side chains across the extracellular-transmembrane-domain (ECD-TMD) interface are crucial for this phenomenon. Indeed, the successful design of fully functional chimeras that combine intact ECD and TMD modules from different wild-type pLGICs has commonly been ascribed to the residual conservation of sequence that exists at the level of the interfacial loops even between evolutionarily distant parent channels. Here, using mutagenesis, patch-clamp electrophysiology, and radiolabeled-ligand binding experiments, we studied the effect of eliminating this residual conservation of sequence on ion-channel function and cell-surface expression. From our results, we conclude that proper state interconversion ("gating") does not require conservation of sequence-or even physicochemical properties-across the ECD-TMD interface. Wild-type ECD and TMD side chains undoubtedly interact with their surroundings, but the interactions between them-straddling the interface-do not seem to be more important for gating than those occurring elsewhere in the protein. We propose that gating of pLGICs requires, instead, that the overall structure of the interfacial loops be conserved, and that their relative orientation and distance be the appropriate ones for changes in one side to result in changes in the other, in a phenomenon akin to the nonspecific "bumping" of closely apposed domains.
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Shen XM, Milone M, Wang HL, Banwell B, Selcen D, Sine SM, Engel AG. Slow-channel myasthenia due to novel mutation in M2 domain of AChR delta subunit. Ann Clin Transl Neurol 2019; 6:2066-2078. [PMID: 31560172 PMCID: PMC6801167 DOI: 10.1002/acn3.50902] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2019] [Accepted: 09/01/2019] [Indexed: 11/09/2022] Open
Abstract
OBJECTIVE To characterize the molecular and phenotypic basis of a severe slow-channel congenital myasthenic syndrome (SCCMS). METHODS Intracellular and single-channel recordings from patient endplates; alpha-bungarotoxin binding studies; direct sequencing of AChR genes; microsatellite analysis; kinetic analysis of AChR activation; homology modeling of adult human AChR structure. RESULTS Among 24 variants reported to cause SCCMS only two appear in the AChR δ-subunit. We here report a 16-year-old patient harboring a novel δL273F mutation (δL294F in HGVS nomenclature) in the second transmembrane domain (M2) of the AChR δ subunit. Kinetic analyses with ACh and the weak agonist choline indicate that δL273F prolongs the channel opening bursts 9.4-fold due to a 75-fold increase in channel gating efficiency, whereas a previously identified εL269F mutation (εL289F in HGVS nomenclature) at an equivalent location in the AChR ε-subunit prolongs channel opening bursts 4.4-fold due to a 30-fold increase in gating efficiency. Structural modeling of AChR predicts that inter-helical hydrophobic interactions between the mutant residue in the δ and ε subunit and nearby M2 domain residues in neighboring α subunits contribute to structural stability of the open relative to the closed channel states. INTERPRETATION The greater increase in gating efficiency by δL273F than by εL269F explains why δL273F has more severe clinical effects. Both δL273F and εL269F impair channel gating by disrupting hydrophobic interactions with neighboring α-subunits. Differences in the extent of impairment of channel gating in δ and ε mutant receptors suggest unequal contributions of ε/α and δ/α subunit pairs to gating efficiency.
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Affiliation(s)
- Xin-Ming Shen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota
| | - Margherita Milone
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota
| | - Hang-Long Wang
- Department of Neurology and Vesicular Biology Laboratory, Mayo Clinic, Rochester, Minnesota
| | - Brenda Banwell
- Division of Neurology, Department of Pediatrics, Children's Hospital of Philadelphia, Philadelphia, Pennsylvania
| | - Duygu Selcen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota
| | - Steven M Sine
- Department of Physiology and Biomedical Engineering and Receptor Biology Laboratory, Mayo Clinic, Rochester, Minnesota.,Department of Pharmacology and Experimental Therapeutics, Mayo Clinic, Rochester, Minnesota.,Department of Neurology, Mayo Clinic, Rochester, Minnesota
| | - Andrew G Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota
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Extreme-QTL mapping of monepantel resistance in Haemonchus contortus. Parasit Vectors 2019; 12:403. [PMID: 31412938 PMCID: PMC6693152 DOI: 10.1186/s13071-019-3663-9] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2019] [Accepted: 08/09/2019] [Indexed: 11/25/2022] Open
Abstract
Background Haemonchus contortus, a gastrointestinal nematode parasite of sheep, is mainly controlled by anthelmintics; the occurrence of anthelmintic resistance leads to treatment failures and increases economic burden. Because molecular mechanisms involved in drug resistance can be elucidated by genomic studies, an extreme quantitative trait locus (X-QTL) mapping approach was used to identify co-segregation of the resistance phenotype with genetic markers to detect the genome-wide variants associated with monepantel resistance in H. contortus. Methods A cross between H. contortus isolates using parental susceptible (Par-S) males and monepantel resistant (Par-R) females resulted in SR progeny, while reciprocal cross resulted in RS progeny. Pools (n = 30,000) of infective larvae (L3) recovered from Par-R, and from SR and RS populations in the F3 generation, collected both before (unselected group) and 7 days after (selected group) selection with monepantel treatment in sheep hosts, were subjected to genome sequencing (Pool-Seq). Pairwise comparisons of allele frequencies between unselected and selected groups were performed for each population by Fisher’s exact test (FET) and for both populations combined by a Cochran-Mantel-Haenszel (CMH) test. Results Mapping rates varied from 80.29 to 81.77% at a 90.4X mean coverage of aligned reads. After correction for multiple testing, significant (P < 0.05) changes in allele frequencies were detected by FET for 6 and 57 single nucleotide polymorphisms (SNPs) in the SR and RS populations, respectively, and by the CMH test for 124 SNPs in both populations. The significant variants located on chromosome 2 generated a selection signal in a genomic region harboring the mptl-1, deg-3 and des-2 genes, previously reported as candidates for monepantel resistance. In addition, three new variants were identified in the mptl-1 gene. Conclusions This study expands knowledge on genome-wide molecular events underlying H. contortus resistance to monepantel. The identification of a genome region harboring major genes previously associated with monepantel resistance supports the results of the employed X-QTL approach. In addition, a deletion in exon 11 of the mptl-1 gene should be further investigated as the putative causal mutation leading to monepantel resistance. Electronic supplementary material The online version of this article (10.1186/s13071-019-3663-9) contains supplementary material, which is available to authorized users.
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Ion BF, Wells MM, Chen Q, Xu Y, Tang P. Ketamine Inhibition of the Pentameric Ligand-Gated Ion Channel GLIC. Biophys J 2017; 113:605-612. [PMID: 28793215 DOI: 10.1016/j.bpj.2017.06.041] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2017] [Revised: 06/14/2017] [Accepted: 06/22/2017] [Indexed: 12/11/2022] Open
Abstract
Ketamine inhibits pentameric ligand-gated ion channels (pLGICs), including the bacterial pLGIC from Gloeobacter violaceus (GLIC). The crystal structure of GLIC shows R-ketamine bound to an extracellular intersubunit cavity. Here, we performed molecular dynamics simulations of GLIC in the absence and presence of R- or S-ketamine. No stable binding of S-ketamine in the original cavity was observed in the simulations, largely due to its unfavorable access to residue D154, which provides important electrostatic interactions to stabilize R-ketamine binding. Contrary to the symmetric binding shown in the crystal structure, R-ketamine moved away from some of the binding sites and was bound to GLIC asymmetrically at the end of simulations. The asymmetric binding is consistent with the experimentally measured negative cooperativity of ketamine binding to GLIC. In the presence of R-ketamine, all subunits showed changes in structure and dynamics, irrespective of binding stability; the extracellular intersubunit cavity expanded and intersubunit electrostatic interactions involved in channel activation were altered. R-ketamine binding promoted a conformational shift toward closed GLIC. Conformational changes near the ketamine-binding site were propagated to the interface between the extracellular and transmembrane domains, and further to the pore-lining TM2 through two pathways: pre-TM1 and the β1-β2 loop. Both signaling pathways have been predicted previously using the perturbation-based Markovian transmission model. The study provides a structural and dynamics basis for the inhibitory modulation of ketamine on pLGICs.
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Affiliation(s)
- Bogdan F Ion
- Departments of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Marta M Wells
- Departments of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Computational and Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Qiang Chen
- Departments of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Yan Xu
- Departments of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania
| | - Pei Tang
- Departments of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Computational and Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; Pharmacology and Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania.
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Abstract
The 97 amino acid bitopic membrane protein M2 of influenza A forms a tetrameric bundle in which two of the monomers are covalently linked via a cysteine bridge. In its tetrameric assembly the protein conducts protons across the viral envelope and within intracellular compartments during the infectivity cycle of the virus. A key residue in the translocation of the protons is His-37 which forms a planar tetrad in the configuration of the bundle accepting and translocating the incoming protons from the N terminal side, exterior of the virus, to the C terminal side, inside the virus. With experimentally available data from NMR spectroscopy of the transmembrane domains of the tetrameric M2 bundle classical MD simulations are conducted with the protein bundle in different protonation stages in respect to His-37. A full correlation analysis (FCA) of the data sets with the His-37 tetrad either in a fully four times unprotonated or protonated state, assumed to mimic high and low pH in vivo, respectively, in both cases reveal asymmetric backbone dynamics. His-37 side chain rotation dynamics is increased at full protonation of the tetrad compared to the dynamics in the fully unprotonated state. The data suggest that proton translocation can be achieved by decoupled side chain or backbone dynamics. Graphical abstract Visualization of the tetrameric bundle of the transmembrane domains of M2 of influenza A after 200 ns of MD simulations (upper left). The four histidine residues 37 are either not protonated as in M20 or fully protonated is in M24+. The asymmetric dynamics of the backbones are shown after a full correlation analysis (FCA) in blue (lower left). The rotational dynamics of the χ2 dihedral angles of the histidines in M20 (upper right) are less than those in M24+ (lower right).
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Affiliation(s)
- Monoj Mon Kalita
- Institute of Biophotonics and Biophotonics & Molecular Imaging Research Center (BMIRC), School of Biomedical Science and Engineering, National Yang-Ming University, 155, Li-Nong St., Sec. 2, Taipei, 112, Taiwan
| | - Wolfgang B Fischer
- Institute of Biophotonics and Biophotonics & Molecular Imaging Research Center (BMIRC), School of Biomedical Science and Engineering, National Yang-Ming University, 155, Li-Nong St., Sec. 2, Taipei, 112, Taiwan.
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11
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Alberola-Die A, Fernández-Ballester G, González-Ros JM, Ivorra I, Morales A. Muscle-Type Nicotinic Receptor Modulation by 2,6-Dimethylaniline, a Molecule Resembling the Hydrophobic Moiety of Lidocaine. Front Mol Neurosci 2016; 9:127. [PMID: 27932949 PMCID: PMC5121239 DOI: 10.3389/fnmol.2016.00127] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2016] [Accepted: 11/07/2016] [Indexed: 11/25/2022] Open
Abstract
To identify the molecular determinants responsible for lidocaine blockade of muscle-type nAChRs, we have studied the effects on this receptor of 2,6-dimethylaniline (DMA), which resembles lidocaine’s hydrophobic moiety. Torpedo marmorata nAChRs were microtransplanted to Xenopus oocytes and currents elicited by ACh (IACh), either alone or co-applied with DMA, were recorded. DMA reversibly blocked IACh and, similarly to lidocaine, exerted a closed-channel blockade, as evidenced by the enhancement of IACh blockade when DMA was pre-applied before its co-application with ACh, and hastened IACh decay. However, there were marked differences among its mechanisms of nAChR inhibition and those mediated by either the entire lidocaine molecule or diethylamine (DEA), a small amine resembling lidocaine’s hydrophilic moiety. Thereby, the IC50 for DMA, estimated from the dose-inhibition curve, was in the millimolar range, which is one order of magnitude higher than that for either DEA or lidocaine. Besides, nAChR blockade by DMA was voltage-independent in contrast to the increase of IACh inhibition at negative potentials caused by the more polar lidocaine or DEA molecules. Accordingly, virtual docking assays of DMA on nAChRs showed that this molecule binds predominantly at intersubunit crevices of the transmembrane-spanning domain, but also at the extracellular domain. Furthermore, DMA interacted with residues inside the channel pore, although only in the open-channel conformation. Interestingly, co-application of ACh with DEA and DMA, at their IC50s, had additive inhibitory effects on IACh and the extent of blockade was similar to that predicted by the allotopic model of interaction, suggesting that DEA and DMA bind to nAChRs at different loci. These results indicate that DMA mainly mimics the low potency and non-competitive actions of lidocaine on nAChRs, as opposed to the high potency and voltage-dependent block by lidocaine, which is emulated by the hydrophilic DEA. Furthermore, it is pointed out that the hydrophobic (DMA) and hydrophilic (DEA) moieties of the lidocaine molecule act differently on nAChRs and that their separate actions taken together account for most of the inhibitory effects of the whole lidocaine molecule on nAChRs.
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Affiliation(s)
- Armando Alberola-Die
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante Alicante, Spain
| | | | - José M González-Ros
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández Alicante, Spain
| | - Isabel Ivorra
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante Alicante, Spain
| | - Andrés Morales
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante Alicante, Spain
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12
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Shen XM, Okuno T, Milone M, Otsuka K, Takahashi K, Komaki H, Giles E, Ohno K, Engel AG. Mutations Causing Slow-Channel Myasthenia Reveal That a Valine Ring in the Channel Pore of Muscle AChR is Optimized for Stabilizing Channel Gating. Hum Mutat 2016; 37:1051-9. [PMID: 27375219 DOI: 10.1002/humu.23043] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2016] [Revised: 06/18/2016] [Accepted: 06/23/2016] [Indexed: 12/22/2022]
Abstract
We identify two novel mutations in acetylcholine receptor (AChR) causing a slow-channel congenital myasthenia syndrome (CMS) in three unrelated patients (Pts). Pt 1 harbors a heterozygous βV266A mutation (p.Val289Ala) in the second transmembrane domain (M2) of the AChR β subunit (CHRNB1). Pts 2 and 3 carry the same mutation at an equivalent site in the ε subunit (CHRNE), εV265A (p.Val285Ala). The mutant residues are conserved across all AChR subunits of all species and are components of a valine ring in the channel pore, which is positioned four residues above the leucine ring. Both βV266A and εV265A reduce the amino acid size and lengthen the channel opening bursts by fourfold by enhancing gating efficiency by approximately 30-fold. Substitution of alanine for valine at the corresponding position in the δ and α subunit prolongs the burst duration four- and eightfold, respectively. Replacing valine at ε codon 265 either by a still smaller glycine or by a larger leucine also lengthens the burst duration. Our analysis reveals that each valine in the valine ring contributes to channel kinetics equally, and the valine ring has been optimized in the course of evolution to govern channel gating.
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Affiliation(s)
- Xin-Ming Shen
- Department of Neurology, Mayo Clinic, Rochester, Minnesota. ,
| | - Tatsuya Okuno
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | | | - Kenji Otsuka
- Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Koji Takahashi
- Department of Child Neurology, National Center Hospital of Neurology and Psychiatry, Tokyo, Japan
| | - Hirofumi Komaki
- Department of Child Neurology, National Center Hospital of Neurology and Psychiatry, Tokyo, Japan
| | | | - Kinji Ohno
- Department of Neurology, Mayo Clinic, Rochester, Minnesota.,Division of Neurogenetics, Center for Neurological Diseases and Cancer, Nagoya University Graduate School of Medicine, Nagoya, Japan
| | - Andrew G Engel
- Department of Neurology, Mayo Clinic, Rochester, Minnesota
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13
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Sivilotti L, Colquhoun D. In praise of single channel kinetics. J Gen Physiol 2016; 148:79-88. [PMID: 27432998 PMCID: PMC4969800 DOI: 10.1085/jgp.201611649] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2016] [Accepted: 06/30/2016] [Indexed: 11/20/2022] Open
Affiliation(s)
- Lucia Sivilotti
- Department of Neuroscience, Physiology, and Pharmacology, University College London, WC1E 6BT, London, England, UK
| | - David Colquhoun
- Department of Neuroscience, Physiology, and Pharmacology, University College London, WC1E 6BT, London, England, UK
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14
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Kalita MM, Fischer WB. Asymmetric dynamics of ion channel forming proteins - Hepatitis C virus (HCV) p7 bundles. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2016; 1858:1462-70. [PMID: 27079148 DOI: 10.1016/j.bbamem.2016.04.004] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/17/2015] [Revised: 04/07/2016] [Accepted: 04/08/2016] [Indexed: 01/01/2023]
Abstract
Protein p7 of hepatitis C virus (HCV) is a short 63 amino acid membrane protein which homo-oligomerises in the lipid membrane to form ion and proton conducting bundles. Two different genotypes (GTs) of p7, 1a and 5a, are used to simulate hexameric bundles of the protein embedded in a fully hydrated lipid bilayer during 400 ns molecular dynamics (MD) simulations. Whilst the bundle of GT 1a is based on a fully computational derived structure, the bundle of GT 5a is based on NMR spectroscopic data. Results of a full correlation analysis (FCA) reveal that albeit structural differences both bundles screen local minima during the simulation. The collective motion of the protein domains is asymmetric. No 'breathing-mode'-like dynamics is observed. The presence of divalent ions, such as Ca-ions affects the dynamics of especially solvent exposed parts of the protein, but leaves the asymmetric domain motion unaffected.
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Affiliation(s)
- Monoj Mon Kalita
- Institute of Biophotonics and Biophotonics and Molecular Imaging Research Center (BMIRC), School of Biomedical Science and Engineering, National Yang-Ming University, Taipei, Taiwan, ROC
| | - Wolfgang B Fischer
- Institute of Biophotonics and Biophotonics and Molecular Imaging Research Center (BMIRC), School of Biomedical Science and Engineering, National Yang-Ming University, Taipei, Taiwan, ROC.
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15
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Mowrey D, Cheng MH, Liu LT, Willenbring D, Lu X, Wymore T, Xu Y, Tang P. Asymmetric ligand binding facilitates conformational transitions in pentameric ligand-gated ion channels. J Am Chem Soc 2013; 135:2172-80. [PMID: 23339564 DOI: 10.1021/ja307275v] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/28/2022]
Abstract
The anesthetic propofol inhibits the currents of the homopentameric ligand-gated ion channel GLIC, yet the crystal structure of GLIC with five propofol molecules bound symmetrically shows an open-channel conformation. To address this dilemma and determine if the symmetry of propofol binding sites affects the channel conformational transition, we performed a total of 1.5 μs of molecular dynamics simulations for different GLIC systems with propofol occupancies of 0, 1, 2, 3, and 5. GLIC without propofol binding or with five propofol molecules bound symmetrically, showed similar channel conformation and hydration status over multiple replicates of 100-ns simulations. In contrast, asymmetric binding to one, two or three equivalent sites in different subunits accelerated the channel dehydration, increased the conformational heterogeneity of the pore-lining TM2 helices, and shifted the lateral and radial tilting angles of TM2 toward a closed-channel conformation. The results differentiate two groups of systems based on the propofol binding symmetry. The difference between symmetric and asymmetric groups is correlated with the variance in the propofol-binding cavity adjacent to the hydrophobic gate and the force imposed by the bound propofol. Asymmetrically bound propofol produced greater variance in the cavity size that could further elevate the conformation heterogeneity. The force trajectory generated by propofol in each subunit over the course of a simulation exhibits an ellipsoidal shape, which has the larger component tangential to the pore. Asymmetric propofol binding creates an unbalanced force that expedites the channel conformation transitions. The findings from this study not only suggest that asymmetric binding underlies the propofol functional inhibition of GLIC, but also advocate for the role of symmetry breaking in facilitating channel conformational transitions.
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Affiliation(s)
- David Mowrey
- Department of Anesthesiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
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16
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Nayak TK, Purohit PG, Auerbach A. The intrinsic energy of the gating isomerization of a neuromuscular acetylcholine receptor channel. ACTA ACUST UNITED AC 2012; 139:349-58. [PMID: 22547665 PMCID: PMC3343375 DOI: 10.1085/jgp.201110752] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Nicotinic acetylcholine receptor (AChR) channels at neuromuscular synapses rarely open in the absence of agonists, but many different mutations increase the unliganded gating equilibrium constant (E0) to generate AChRs that are active constitutively. We measured E0 for two different sets of mutant combinations and by extrapolation estimated E0 for wild-type AChRs. The estimates were 7.6 and 7.8 × 10−7 in adult-type mouse AChRs (−100 mV at 23°C). The values are in excellent agreement with one obtained previously by using a completely different method (6.5 × 10−7, from monoliganded gating). E0 decreases with depolarization to the same extent as does the diliganded gating equilibrium constant, e-fold with ∼60 mV. We estimate that at −100 mV the intrinsic energy of the unliganded gating isomerization is +8.4 kcal/mol (35 kJ/mol), and that in the absence of a membrane potential, the intrinsic chemical energy of this global conformational change is +9.4 kcal/mol (39 kJ/mol). Na+ and K+ in the extracellular solution have no measureable effect on E0, which suggests that unliganded gating occurs with only water occupying the transmitter binding sites. The results are discussed with regard to the energy changes in receptor activation and the competitive antagonism of ions in agonist binding.
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Affiliation(s)
- Tapan K Nayak
- Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, NY 14214, USA
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17
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Ligand-binding domain of an α7-nicotinic receptor chimera and its complex with agonist. Nat Neurosci 2011; 14:1253-9. [PMID: 21909087 DOI: 10.1038/nn.2908] [Citation(s) in RCA: 160] [Impact Index Per Article: 12.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2011] [Accepted: 07/20/2011] [Indexed: 02/02/2023]
Abstract
The α(7) acetylcholine receptor (AChR) mediates pre- and postsynaptic neurotransmission in the central nervous system and is a potential therapeutic target in neurodegenerative, neuropsychiatric and inflammatory disorders. We determined the crystal structure of the extracellular domain of a receptor chimera constructed from the human α(7) AChR and Lymnaea stagnalis acetylcholine binding protein (AChBP), which shares 64% sequence identity and 71% similarity with native α(7). We also determined the structure with bound epibatidine, a potent AChR agonist. Comparison of the structures revealed molecular rearrangements and interactions that mediate agonist recognition and early steps in signal transduction in α(7) AChRs. The structures further revealed a ring of negative charge within the central vestibule, poised to contribute to cation selectivity. Structure-guided mutational studies disclosed distinctive contributions to agonist recognition and signal transduction in α(7) AChRs. The structures provide a realistic template for structure-aided drug design and for defining structure-function relationships of α(7) AChRs.
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18
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Cymes GD, Grosman C. Tunable pKa values and the basis of opposite charge selectivities in nicotinic-type receptors. Nature 2011; 474:526-30. [PMID: 21602825 PMCID: PMC3121909 DOI: 10.1038/nature10015] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/05/2010] [Accepted: 03/21/2011] [Indexed: 11/09/2022]
Abstract
Among ion channels, only the nicotinic-receptor superfamily has evolved to generate both cation- and anion-selective members. Although other, structurally unrelated, neurotransmitter-gated cation channels exist, no other type of neurotransmitter-gated anion channel, and thus no other source of fast synaptic inhibitory signals, has been described so far. In addition to the seemingly straightforward electrostatic effect of the presence (in the cation-selective members) or absence (in the anion-selective ones) of a ring of pore-facing carboxylates, mutational studies have identified other features of the amino-acid sequence near the intracellular end of the pore-lining transmembrane segments (M2) that are also required to achieve the high charge selectivity shown by native channels. However, the mechanism underlying this more subtle effect has remained elusive and a subject of speculation. Here we show, using single-channel electrophysiological recordings to estimate the protonation state of native ionizable side chains, that anion-selective-type sequences favour whereas cation-selective-type sequences prevent the protonation of the conserved, buried basic residues at the intracellular entrance of the pore (the M2 0' position). We conclude that the previously unrecognized tunable charge state of the 0' ring of buried basic side chains is an essential feature of these channels' versatile charge-selectivity filter.
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Affiliation(s)
- Gisela D Cymes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
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19
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Design and control of acetylcholine receptor conformational change. Proc Natl Acad Sci U S A 2011; 108:4328-33. [PMID: 21368211 DOI: 10.1073/pnas.1016617108] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Allosteric proteins use energy derived from ligand binding to promote a global change in conformation. The "gating" equilibrium constant of acetylcholine receptor-channels (AChRs) is influenced by ligands, mutations, and membrane voltage. We engineered AChRs to have specific values of this constant by combining these perturbations, and then calculated the corresponding values for a reference condition. AChRs were designed to have specific rate and equilibrium constants simply by adding multiple, energetically independent mutations with known effects on gating. Mutations and depolarization (to remove channel block) changed the diliganded gating equilibrium constant only by changing the unliganded gating equilibrium constant (E(0)) and did not alter the energy from ligand binding. All of the tested perturbations were approximately energetically independent. We conclude that naturally occurring mutations mainly adjust E(0) and cause human disease because they generate AChRs that have physiologically inappropriate values of this constant. The results suggest that the energy associated with a structural change of a side chain in the gating isomerization is dissipated locally and is mainly independent of rigid body or normal mode motions of the protein. Gating rate and equilibrium constants are estimated for seven different AChR agonists using a stepwise engineering approach.
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20
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Purohit P, Auerbach A. Energetics of gating at the apo-acetylcholine receptor transmitter binding site. ACTA ACUST UNITED AC 2010; 135:321-31. [PMID: 20351060 PMCID: PMC2847916 DOI: 10.1085/jgp.200910384] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Acetylcholine receptor channels switch between conformations that have a low versus high affinity for the transmitter and conductance for ions (R↔R*; gating). The forward isomerization, which begins at the transmitter binding sites and propagates ∼50 Å to the narrow region of the pore, occurs by approximately the same sequence of molecular events with or without agonists present at the binding sites. To pinpoint the forces that govern the R versus R* agonist affinity ratio, we measured single-channel activation parameters for apo-receptors having combinations of mutations of 10 transmitter binding site residues in the α (Y93, G147, W149, G153, Y190, C192, and Y198), ε (W55 and P121), or δ (W57) subunit. Gating energy changes were largest for the tryptophan residues. The αW149 energy changes were coupled with those of the other aromatic amino acids. Mutating the aromatic residues to Phe reduces the R/R* equilibrium dissociation constant ratio, with αY190 and αW149 being the most sensitive positions. Most of the mutations eliminated long-lived spontaneous openings. The results provide a foundation for understanding how ligands trigger protein conformational change.
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Affiliation(s)
- Prasad Purohit
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA
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21
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Moriconi C, Di Castro MA, Fucile S, Eusebi F, Grassi F. Mechanism of verapamil action on wild-type and slow-channel mutant human muscle acetylcholine receptor. J Neurochem 2010; 114:1231-40. [PMID: 20533996 DOI: 10.1111/j.1471-4159.2010.06842.x] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Verapamil, a Ca(2+) channel blocker widely used in clinical practice, also affects the properties of frog and mouse muscle acetylcholine receptor (AChR). Here, we examine the mechanism of action of verapamil on human wild-type and slow-channel mutant muscle AChRs harboring in any subunit a valine-to-alanine mutation of 13' residue of the pore-lining M2 transmembrane segment. Verapamil, after a pre-treatment of 0.5-10 s, accelerated the decay of whole-cell or macroscopic outside-out currents within milliseconds of ACh application even at clinically attainable doses. Recordings of unitary events in the cell-attached and outside-out configurations showed that verapamil does not alter single-channel conductance, but reduces channel open probability, by prolonging the dwell time into the closed state for wild-type and all mutant AChR. The duration of channel openings decreased only for the epsilonV265A-AChR, by shortening the longest exponential component of the open-time distribution. These results provide a rationale for the therapeutic use of verapamil in the slow-channel syndrome and emphasize the major role played by epsilon subunit in controlling the functional properties of human muscle AChR, as revealed by the peculiar alterations imparted by mutations in this subunit.
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Affiliation(s)
- Claudia Moriconi
- Dipartimento di Fisiologia e Farmacologia, Università La Sapienza, Roma, Italy
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22
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Lape R, Krashia P, Colquhoun D, Sivilotti LG. Agonist and blocking actions of choline and tetramethylammonium on human muscle acetylcholine receptors. J Physiol 2009; 587:5045-72. [PMID: 19752108 PMCID: PMC2790248 DOI: 10.1113/jphysiol.2009.176305] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2009] [Accepted: 09/08/2009] [Indexed: 11/08/2022] Open
Abstract
Choline has been used widely as an agonist for the investigation of gain-of-function mutants of the nicotinic acetylcholine receptor. It is useful because it behaves like a partial agonist. The efficacy of choline is difficult to measure because choline blocks the channel at concentrations about four times lower than those that activate it. We have fitted activation mechanisms to single-channel activity elicited from HEK-expressed human recombinant muscle nicotinic receptors by choline and by tetramethylammonium (TMA). Channel block by the agonist was incorporated into the mechanisms that were fitted, and block was found not to be selective for the open state. The results also suggest that channel block is very fast and that the channel can shut almost as fast as normal when the blocker was bound. Single-channel data are compatible with a mechanism in which choline is actually a full agonist, its maximum response being limited only by channel block. However, they are also compatible with a mechanism incorporating a pre-opening conformation change ('flip') in which choline is a genuine partial agonist. The latter explanation is favoured by concentration jump experiments, and by the fact that only this mechanism fits the TMA data. We propose that choline, like TMA, is a partial agonist because it is very ineffective (approximately 600-fold less than acetylcholine) at eliciting the initial, pre-opening conformation change. Once flipping has occurred, all agonists, even choline, open the channel with similar efficiency.
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Affiliation(s)
- Remigijus Lape
- Department of Neuroscience, Physiology and Pharmacology, Medical Sciences Building, University College London, Gower St, London WC1E 6BT, UK
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23
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Spitzmaul G, Corradi J, Bouzat C. Mechanistic contributions of residues in the M1 transmembrane domain of the nicotinic receptor to channel gating. Mol Membr Biol 2009; 21:39-50. [PMID: 14668137 DOI: 10.1080/09687680310001607341] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
Abstract
The nicotinic receptor (AChR) is a pentamer of homologous subunits with an alpha(2)betaepsilondelta composition in adult muscle. Each subunit contains four transmembrane domains (M1-M4). Position 15' of the M1 domain is phenylalanine in alpha subunits while it is isoleucine in non-alpha subunits. Given this peculiar conservation pattern, we studied its contribution to muscle AChR activation by combining mutagenesis with single-channel kinetic analysis. AChRs containing the mutant alpha subunit (alphaF15'I) as well as those containing the reverse mutations in the non-alpha subunits (betaI15'F, deltaI15'F, and epsilonI15'F) show prolonged lifetimes of the diliganded open channel resulting from a slower closing rate with respect to wild-type AChRs. The kinetic changes are not equivalent among subunits, the beta subunit, being the one that produces the most significant stabilization of the open state. Kinetic analysis of betaI15'F of AChR channels activated by the low-efficacious agonist choline revealed a 10-fold decrease in the closing rate, a 2.5-fold increase in the opening rate, a 28-fold increase in the gating equilibrium constant in the diliganded receptor, and a significant increase opening in the absence of agonist. Mutations at betaI15' showed that the structural bases of its contribution to gating is complex. Rate-equilibrium linear free-energy relationships suggest an approximately 70% closed-state-like environment for the beta15' position at the transition state of gating. The overall results identify position 15' as a subunit-selective determinant of channel gating and add new experimental evidence that gives support to the involvement of the M1 domain in the operation of the channel gating apparatus.
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Affiliation(s)
- Guillermo Spitzmaul
- Instituto de Investigaciones Bioquímicas, UNS-CONICET, Bahía Blanca, Argentina
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24
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Tantama M, Licht S. Functional equivalence of the nicotinic acetylcholine receptor transmitter binding sites in the open state. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2009; 1788:936-44. [PMID: 19366595 DOI: 10.1016/j.bbamem.2009.01.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/30/2008] [Revised: 12/14/2008] [Accepted: 01/21/2009] [Indexed: 10/21/2022]
Abstract
The subunits of the muscle-type nicotinic acetylcholine receptor (AChR) are not uniformly oriented in the resting closed conformation: the two alpha subunits are rotated relative to its non-alpha subunits. In contrast, all the subunits overlay well with one another when agonist is bound to the AChR, suggesting that they are uniformly oriented in the open receptor. This gating-dependent increase in orientational uniformity due to rotation of the alpha subunits might affect the relative affinities of the two transmitter binding sites, making the two affinities dissimilar (functionally non-equivalent) in the initial ligand-bound closed state but similar (functionally equivalent) in the open state. To test this hypothesis, we measured single-channel activity of the alphaG153S gain-of-function mutant receptor evoked by choline, and estimated the resting closed-state and open-state affinities of the two transmitter binding sites. Both model-independent analyses and maximum-likelihood estimation of microscopic rate constants indicate that channel opening makes the binding sites' affinities more similar to each other. These results support the hypothesis that open-state affinities to the transmitter binding sites are primarily determined by the alpha subunits.
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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, USA
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25
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Elenes S, Decker M, Cymes GD, Grosman C. Decremental response to high-frequency trains of acetylcholine pulses but unaltered fractional Ca2+ currents in a panel of "slow-channel syndrome" nicotinic receptor mutants. ACTA ACUST UNITED AC 2009; 133:151-69. [PMID: 19171769 PMCID: PMC2638206 DOI: 10.1085/jgp.200810089] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
The slow-channel congenital myasthenic syndrome (SCCMS) is a disorder of the neuromuscular junction caused by gain-of-function mutations to the muscle nicotinic acetylcholine (ACh) receptor (AChR). Although it is clear that the slower deactivation time course of the ACh-elicited currents plays a central role in the etiology of this disease, it has been suggested that other abnormal properties of these mutant receptors may also be critical in this respect. We characterized the kinetics of a panel of five SCCMS AChRs (αS269I, βV266M, εL221F, εT264P, and εL269F) at the ensemble level in rapidly perfused outside-out patches. We found that, for all of these mutants, the peak-current amplitude decreases along trains of nearly saturating ACh pulses delivered at physiologically relevant frequencies in a manner that is consistent with enhanced entry into desensitization during the prolonged deactivation phase. This suggests that the increasingly reduced availability of activatable AChRs upon repetitive stimulation may well contribute to the fatigability and weakness of skeletal muscle that characterize this disease. Also, these results emphasize the importance of explicitly accounting for entry into desensitization as one of the pathways for burst termination, if meaningful mechanistic insight is to be inferred from the study of the effect of these naturally occurring mutations on channel function. Applying a novel single-channel–based approach to estimate the contribution of Ca2+ to the total cation currents, we also found that none of these mutants affects the Ca2+-conduction properties of the AChR to an extent that seems to be of physiological importance. Our estimate of the Ca2+-carried component of the total (inward) conductance of wild-type and SCCMS AChRs in the presence of 150 mM Na+, 1.8 mM Ca2+, and 1.7 mM Mg2+ on the extracellular side of cell-attached patches turned out be in the 5.0–9.4 pS range, representing a fractional Ca2+ current of ∼14%, on average. Remarkably, these values are nearly identical to those we estimated for the NR1-NR2A N-methyl-d-aspartate receptor (NMDAR), which has generally been considered to be the main neurotransmitter-gated pathway of Ca2+ entry into the cell. Our estimate of the rat NMDAR Ca2+ conductance (using the same single-channel approach as for the AChR but in the nominal absence of extracellular Mg2+) was 7.9 pS, corresponding to a fractional Ca2+ current of 13%.
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Affiliation(s)
- Sergio Elenes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, 61801, USA
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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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27
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Haddadian EJ, Cheng MH, Coalson RD, Xu Y, Tang P. In silico models for the human alpha4beta2 nicotinic acetylcholine receptor. J Phys Chem B 2008; 112:13981-90. [PMID: 18847252 DOI: 10.1021/jp804868s] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The neuronal alpha4beta2 nicotinic acetylcholine receptor (nAChR) is one of the most widely expressed nAChR subtypes in the brain. Its subunits have high sequence identity (54 and 46% for alpha4 and beta2, respectively) with alpha and beta subunits in Torpedo nAChR. Using the known structure of the Torpedo nAChR as a template, the closed-channel structure of the alpha4beta2 nAChR was constructed through homology modeling. Normal-mode analysis was performed on this closed structure and the resulting lowest frequency mode was applied to it for a "twist-to-open" motion, which increased the minimum pore radius from 2.7 to 3.4 A and generated an open-channel model. Nicotine could bind to the predicted agonist binding sites in the open-channel model but not in the closed one. Both models were subsequently equilibrated in a ternary lipid mixture via extensive molecular dynamics (MD) simulations. Over the course of 11 ns MD simulations, the open channel remained open with filled water, but the closed channel showed a much lower water density at its hydrophobic gate comprised of residues alpha4-V259 and alpha4-L263 and their homologous residues in the beta2 subunits. Brownian dynamics simulations of Na+ permeation through the open channel demonstrated a current-voltage relationship that was consistent with experimental data on the conducting state of alpha4beta2 nAChR. Besides establishment of the well-equilibrated closed- and open-channel alpha4beta2 structural models, the MD simulations on these models provided valuable insights into critical factors that potentially modulate channel gating. Rotation and tilting of TM2 helices led to changes in orientations of pore-lining residue side chains. Without concerted movement, the reorientation of one or two hydrophobic side chains could be enough for channel opening. The closed- and open-channel structures exhibited distinct patterns of electrostatic interactions at the interface of extracellular and transmembrane domains that might regulate the signal propagation of agonist binding to channel opening. A potential prominent role of the beta2 subunit in channel gating was also elucidated in the study.
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Affiliation(s)
- Esmael J Haddadian
- Department of Anesthesiology, University of Pittsburgh School of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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28
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Influence of membrane potentials upon reversible protonation of acidic residues from the OmpF eyelet. Biophys Chem 2008; 135:32-40. [DOI: 10.1016/j.bpc.2008.02.018] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2008] [Revised: 02/29/2008] [Accepted: 02/29/2008] [Indexed: 11/23/2022]
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29
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Taly A, Changeux JP. Functional Organization and Conformational Dynamics of the Nicotinic Receptor. Ann N Y Acad Sci 2008; 1132:42-52. [DOI: 10.1196/annals.1405.008] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
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30
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Cymes GD, Grosman C. Pore-opening mechanism of the nicotinic acetylcholine receptor evinced by proton transfer. Nat Struct Mol Biol 2008; 15:389-96. [PMID: 18376414 DOI: 10.1038/nsmb.1407] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/19/2007] [Accepted: 02/25/2008] [Indexed: 01/06/2023]
Abstract
The conformational changes underlying cysteine-loop receptor channel gating remain elusive and controversial. We previously developed a single-channel electrophysiological method that allows structural inferences about the transient open-channel conformation to be made from the effect and properties of introduced charges on systematically engineered ionizable amino acids. Here we have applied this methodology to the entire M1 and M3 segments of the muscle nicotinic acetylcholine receptor, two transmembrane alpha-helices that pack against the pore-lining M2 alpha-helix. Together with our previous results on M2, these data suggest that the pore dilation that underlies channel opening involves only a subtle rearrangement of these three transmembrane helices. Such a limited conformational change seems optimal to allow rapid closed-open interconversion rates, and hence a fast postsynaptic response upon neurotransmitter binding. Thus, this receptor-channel seems to have evolved to take full advantage of the steep dependence of ion- and water-conduction rates on pore diameter that is characteristic of model hydrophobic nanopores.
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Affiliation(s)
- Gisela D Cymes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology and Neuroscience Program, University of Illinois at Urbana-Champaign, 407 South Goodwin Avenue, 524 Burrill Hall, Urbana, Illinois 61801, USA
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31
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The L293 residue in transmembrane domain 2 of the 5-HT3A receptor is a molecular determinant of allosteric modulation by 5-hydroxyindole. Neuropharmacology 2008; 54:1153-65. [PMID: 18436267 DOI: 10.1016/j.neuropharm.2008.03.009] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2007] [Revised: 03/10/2008] [Accepted: 03/14/2008] [Indexed: 11/22/2022]
Abstract
Allosteric modulation of ligand-gated ion channels can play important roles in shaping synaptic transmission. The function of the 5-hydroxytryptamine (serotonin) type 3 (5-HT(3)) receptor, a member of the Cys-loop ligand-gated ion channel superfamily, is modulated by a variety of compounds such as alcohols, anesthetics and 5-hydroxyindole (5-HI). In this study, the molecular determinants of allosteric modulation by 5-HI were explored in N1E-115 neuroblastoma cells expressing the native 5-HT(3) receptor and HEK 293 cells transfected with the recombinant 5-HT(3A) receptor using molecular biology and whole-cell patch-clamp techniques. 5-HI potentiated 5-HT-activated currents in both N1E-115 cells and HEK 293 cells, and significantly decreased current desensitization and deactivation. Substitution of Leu293 (L293, L15') in the second transmembrane domain (TM2) with cysteine (L293C) or serine (L293S) abolished 5-HI modulation. Other mutations in the TM2 domain, such as D298A and T284F, failed to alter 5-HI modulation. The L293S mutation enhanced dopamine efficacy and converted 5-HI into a partial agonist at the mutant receptor. These data suggest that 5-HI stabilizes the 5-HT(3A) receptor in the open state by decreasing both desensitization and 5-HT unbinding/channel closing; and L293 is a common site for both channel gating and allosteric modulation by 5-HI. Our observations also indicate existence of a second 5-HI recognition site on the 5-HT(3A) receptor, which may overlap with the 5-HT binding site and is not involved in the positive modulation by 5-HI. These findings support the idea that there are two discrete sites for 5-HI allosteric modulation and direct activation in the 5-HT(3A) receptor.
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32
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Jha A, Cadugan DJ, Purohit P, Auerbach A. Acetylcholine receptor gating at extracellular transmembrane domain interface: the cys-loop and M2-M3 linker. ACTA ACUST UNITED AC 2008; 130:547-58. [PMID: 18040057 PMCID: PMC2151658 DOI: 10.1085/jgp.200709856] [Citation(s) in RCA: 77] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Acetylcholine receptor channel gating is a propagated conformational cascade that links changes in structure and function at the transmitter binding sites in the extracellular domain (ECD) with those at a “gate” in the transmembrane domain (TMD). We used Φ-value analysis to probe the relative timing of the gating motions of α-subunit residues located near the ECD–TMD interface. Mutation of four of the seven amino acids in the M2–M3 linker (which connects the pore-lining M2 helix with the M3 helix), including three of the four residues in the core of the linker, changed the diliganded gating equilibrium constant (Keq) by up to 10,000-fold (P272 > I274 > A270 > G275). The average Φ-value for the whole linker was ∼0.64. One interpretation of this result is that the gating motions of the M2–M3 linker are approximately synchronous with those of much of M2 (∼0.64), but occur after those of the transmitter binding site region (∼0.93) and loops 2 and 7 (∼0.77). We also examined mutants of six cys-loop residues (V132, T133, H134, F135, P136, and F137). Mutation of V132, H134, and F135 changed Keq by 2800-, 10-, and 18-fold, respectively, and with an average Φ-value of 0.74, similar to those of other cys-loop residues. Even though V132 and I274 are close, the energetic coupling between I and V mutants of these positions was small (≤0.51 kcal mol−1). The M2–M3 linker appears to be the key moving part that couples gating motions at the base of the ECD with those in TMD. These interactions are distributed along an ∼16-Å border and involve about a dozen residues.
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Affiliation(s)
- Archana Jha
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA
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33
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De Rosa MJ, Corradi J, Bouzat C. Subunit-selective role of the M3 transmembrane domain of the nicotinic acetylcholine receptor in channel gating. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2008; 1778:521-9. [DOI: 10.1016/j.bbamem.2007.10.026] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2007] [Revised: 09/25/2007] [Accepted: 10/29/2007] [Indexed: 10/22/2022]
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34
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Szarecka A, Xu Y, Tang P. Dynamics of heteropentameric nicotinic acetylcholine receptor: implications of the gating mechanism. Proteins 2007; 68:948-60. [PMID: 17546671 DOI: 10.1002/prot.21462] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
The dynamics characteristics of the currently available structure of Torpedo nicotinic acetylcholine receptor (nAChR), including the extracellular, transmembrane, and intracellular domains (ICDs), were analyzed using the Gaussian Network Model (GNM) and Anisotropic Network Model (ANM). We found that a symmetric quaternary twist motion, reported previously in the literature in a homopentameric receptor (Cheng et al. J Mol Biol 2006;355:310-324; Taly et al. Biophys J 2005;88:3954-3965), occurred also in the heteropentameric Torpedo nAChR. We believe, however, that the symmetric twist alone is not sufficient to explain a large body of experimental data indicating asymmetry and subunit nonequivalence during gating. Here we report our results supporting the hypothesis that a combination of symmetric and asymmetric motions opens the gate. We show that the asymmetric motion involves tilting of the TM2 helices. Furthermore, our study reveals three additional aspects of channel dynamics: (1) loop A serves as an allosteric mediator between the ligand binding loops and those at the domain interface, particularly the linker between TM2 and TM3; (2) the ICD can modulate the pore dynamics and thus should not be neglected in gating studies; and (3) the F loops, which are peculiarly longer and poorly-conserved in non-alpha-subunits, have important dynamical implications.
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Affiliation(s)
- Agnieszka Szarecka
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA
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35
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Cadugan DJ, Auerbach A. Conformational dynamics of the alphaM3 transmembrane helix during acetylcholine receptor channel gating. Biophys J 2007; 93:859-65. [PMID: 17513382 PMCID: PMC1913136 DOI: 10.1529/biophysj.107.105171] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Muscle acetylcholine receptors are synaptic ion channels that "gate" between closed- and open-channel conformations. We used Phi-value analysis to probe the transition state of the diliganded gating reaction with regard to residues in the M3, membrane-spanning helix of the muscle acetylcholine receptor alpha-subunit. Phi (a fraction between 1 and 0) parameterizes the extent to which a mutation changes the opening versus the closing rate constant and, for a linear reaction mechanism, the higher the Phi-value, the "earlier" the gating motion. In the upper half of alphaM3 the gating motions of all five tested residues were temporally correlated (Phi approximately 0.30) and serve to link structural changes occurring at the middle of the M2, pore-lining helix with those occurring at the interface of the extracellular and transmembrane domains. alphaM3 belongs to a complex and diverse set of synchronously moving parts that change structure relatively late in the channel-opening process. The propagation of the gating Brownian conformational cascade has a complex spatial distribution in the transmembrane domain.
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Affiliation(s)
- David J Cadugan
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, New York, USA
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36
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Danelon C, Grandl J, Hovius R, Vogel H. Modulation of proton-induced current fluctuations in the human nicotinic acetylcholine receptor channel. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2007; 1768:76-89. [PMID: 16956576 DOI: 10.1016/j.bbamem.2006.07.015] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/20/2006] [Revised: 06/12/2006] [Accepted: 07/13/2006] [Indexed: 02/03/2023]
Abstract
The nicotinic acetylcholine receptor (nAChR) is a ligand-gated ion channel that switches upon activation from a closed state to a full conducting state. We found that the mutation delta S268K, located at 12' position of the second transmembrane domain of the delta subunit of the human nAChR generates a long-lived intermediate conducting state, from which openings to a wild-type like conductance level occur on a submillisecond time scale. Aiming to understand the interplay between structural changes near the 12' position and channel gating, we investigated the influence of various parameters: different ligands (acetylcholine, choline and epibatidine), ligand concentrations, transmembrane voltages and both fetal and adult nAChRs. Since sojourns in the high conductance state are not fully resolved in time, spectral noise analysis was used as a complement to dwell time analysis to determine the gating rate constants. Open channel current fluctuations are described by a two-state Markov model. The characteristic time of the process is markedly influenced by the ligand and the receptor type, whereas the frequency of openings to the high conductance state increases with membrane hyperpolarization. Conductance changes are discussed with regard to reversible transfer reaction of single protons at the lysine 12' side chain.
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Affiliation(s)
- Christophe Danelon
- Laboratory of Physical Chemistry of Polymers and Membranes, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
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37
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Elenes S, Ni Y, Cymes GD, Grosman C. Desensitization contributes to the synaptic response of gain-of-function mutants of the muscle nicotinic receptor. ACTA ACUST UNITED AC 2006; 128:615-27. [PMID: 17074980 PMCID: PMC2151585 DOI: 10.1085/jgp.200609570] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Although the muscle nicotinic receptor (AChR) desensitizes almost completely in the steady presence of high concentrations of acetylcholine (ACh), it is well established that AChRs do not accumulate in desensitized states under normal physiological conditions of neurotransmitter release and clearance. Quantitative considerations in the framework of plausible kinetic schemes, however, lead us to predict that mutations that speed up channel opening, slow down channel closure, and/or slow down the dissociation of neurotransmitter (i.e., gain-of-function mutations) increase the extent to which AChRs desensitize upon ACh removal. In this paper, we confirm this prediction by applying high-frequency trains of brief (∼1 ms) ACh pulses to outside-out membrane patches expressing either lab-engineered or naturally occurring (disease-causing) gain-of-function mutants. Entry into desensitization was evident in our experiments as a frequency-dependent depression in the peak value of succesive macroscopic current responses, in a manner that is remarkably consistent with the theoretical expectation. We conclude that the comparatively small depression of the macroscopic currents observed upon repetitive stimulation of the wild-type AChR is due, not to desensitization being exceedingly slow but, rather, to the particular balance between gating, entry into desensitization, and ACh dissociation rate constants. Disruption of this fine balance by, for example, mutations can lead to enhanced desensitization even if the kinetics of entry into, and recovery from, desensitization themselves are not affected. It follows that accounting for the (usually overlooked) desensitization phenomenon is essential for the correct interpretation of mutagenesis-driven structure–function relationships and for the understanding of pathological synaptic transmission at the vertebrate neuromuscular junction.
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Affiliation(s)
- Sergio Elenes
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
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38
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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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39
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Cheng X, Wang H, Grant B, Sine SM, McCammon JA. Targeted molecular dynamics study of C-loop closure and channel gating in nicotinic receptors. PLoS Comput Biol 2006; 2:e134. [PMID: 17009865 PMCID: PMC1584325 DOI: 10.1371/journal.pcbi.0020134] [Citation(s) in RCA: 105] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2006] [Accepted: 08/23/2006] [Indexed: 11/24/2022] Open
Abstract
The initial coupling between ligand binding and channel gating in the human α7 nicotinic acetylcholine receptor (nAChR) has been investigated with targeted molecular dynamics (TMD) simulation. During the simulation, eight residues at the tip of the C-loop in two alternating subunits were forced to move toward a ligand-bound conformation as captured in the crystallographic structure of acetylcholine binding protein (AChBP) in complex with carbamoylcholine. Comparison of apo- and ligand-bound AChBP structures shows only minor rearrangements distal from the ligand-binding site. In contrast, comparison of apo and TMD simulation structures of the nAChR reveals significant changes toward the bottom of the ligand-binding domain. These structural rearrangements are subsequently translated to the pore domain, leading to a partly open channel within 4 ns of TMD simulation. Furthermore, we confirmed that two highly conserved residue pairs, one located near the ligand-binding pocket (Lys145 and Tyr188), and the other located toward the bottom of the ligand-binding domain (Arg206 and Glu45), are likely to play important roles in coupling agonist binding to channel gating. Overall, our simulations suggest that gating movements of the α7 receptor may involve relatively small structural changes within the ligand-binding domain, implying that the gating transition is energy-efficient and can be easily modulated by agonist binding/unbinding. Nicotinic acetylcholine receptors are ligand-gated ion channels responsible for neurotransmitter-mediated signal transduction at synapses throughout the central and peripheral nervous systems. Binding of neurotransmitter molecules to subunit interfaces in the N-terminal extracellular domain induces structural rearrangements of the membrane-spanning domain permitting the influx of cations. A full understanding of how the conformational changes propagate from the ligand-binding site to the pore domain is of great interest to biologists, yet remains to be established. Using a special simulation technique known as targeted molecular dynamics, Cheng and colleagues probed the early stages of ligand-induced conformational rearrangements that may lead to channel opening. During the simulation, Cheng et al. observed a sequence of conformational changes that stem from the ligand-binding site to the transmembrane domain resulting in a wider channel. From these results, they suggest that gating movements may entail only small structural changes in the ligand-binding domain, implying that channel gating is energy-efficient and can readily be modulated by the binding/unbinding of agonist molecules.
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Affiliation(s)
- Xiaolin Cheng
- Howard Hughes Medical Institute, University of California San Diego, La Jolla, California, United States of America.
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40
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Purohit Y, Grosman C. Estimating binding affinities of the nicotinic receptor for low-efficacy ligands using mixtures of agonists and two-dimensional concentration-response relationships. ACTA ACUST UNITED AC 2006; 127:719-35. [PMID: 16735756 PMCID: PMC2151536 DOI: 10.1085/jgp.200509438] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
Abstract
The phenomenon of ligand-induced ion channel gating hinges upon the ability of a receptor channel to bind ligand molecules with conformation-specific affinities. However, our understanding of this fundamental phenomenon is notably limited, not only because the changes in binding site structure and ligand conformation that occur upon gating are largely unknown but, also, because the strength of these ligand–receptor interactions are experimentally elusive. Both high- and low-efficacy ligands pose a number of analytical and experimental challenges that can render the estimation of their conformation-specific binding affinities impossible. In this paper, we present a novel assay that overcomes some of the hurdles presented by weak agonists of the muscle nicotinic receptor and allows the estimation of their closed-state affinities. The method, which we have termed the “activation-competition” assay, consists of a single-channel concentration–response assay performed in the presence of a binary mixture of ligands of widely different efficacies. By plotting the channel response (i.e., the open probability) as a function of the concentration of each agonist in the mixture, interpreting the observed response in the framework of a plausible kinetic scheme, and fitting the open probability surface with the corresponding function, the affinities of the closed receptor for the two agonists can be simultaneously extracted as free parameters. Here, we applied this methodology to estimate the closed-state affinity of the muscle nicotinic receptor for choline (a very weak agonist) using acetylcholine (ACh) as the partner in the mixture. We estimated the dissociation equilibrium constant of choline (KD) from the wild type's closed state to be 4.1 ± 0.5 mM (and that of ACh to be 106 ± 6 μM). We also discuss the use of accurate estimates of affinities for low-efficacy agonists as a tool to discriminate between binding and gating effects of mutations, and in the context of the rational design of therapeutic drugs.
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Affiliation(s)
- Yamini Purohit
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, 61801, USA
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41
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Purohit Y, Grosman C. Block of muscle nicotinic receptors by choline suggests that the activation and desensitization gates act as distinct molecular entities. ACTA ACUST UNITED AC 2006; 127:703-17. [PMID: 16735755 PMCID: PMC2151541 DOI: 10.1085/jgp.200509437] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Ion channel block in muscle acetylcholine nicotinic receptors (AChRs) is an extensively reported phenomenon. Yet, the mechanisms underlying the interruption of ion flow or the interaction of the blocker with the channel's gates remain incompletely characterized. In this paper, we studied fast channel block by choline, a quaternary-ammonium cation that is also an endogenous weak agonist of this receptor, and a valuable tool in structure-function studies. Analysis of the single-channel current amplitude as a function of both choline concentration and voltage revealed that extracellular choline binds to the open-channel pore with millimolar apparent affinity (K(B) congruent with 12 mM in the presence of approximately 155 mM monovalent and 3.5 mM divalent, inorganic cations), and that it permeates the channel faster than acetylcholine. This, together with its relatively small size ( approximately 5.5 A along its longest axis), suggests that the pore-blocking choline binding site is the selectivity filter itself, and that current blockages simply reflect the longer-lived sojourns of choline at this site. Kinetic analysis of single-channel traces indicated that increasing occupancy of the pore-blocking site by choline (as judged from the reduction of the single-channel current amplitude) is accompanied by the lengthening of (apparent) open interval durations. Consideration of a number of possible mechanisms firmly suggests that this prolongation results from the local effect of choline interfering with the operation of the activation gate (closure of blocked receptors is slower than that of unblocked receptors by a factor of approximately 13), whereas closure of the desensitization gate remains unaffected. Thus, we suggest that these two gates act as distinct molecular entities. Also, the detailed understanding gained here on how choline distorts the observed open-time durations can be used to compensate for this artifact during activation assays. This correction is necessary if we are to understand how choline binds to and gates the AChR.
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Affiliation(s)
- Yamini Purohit
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, 61801, USA
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42
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Grandl J, Danelon C, Hovius R, Vogel H. Functional asymmetry of transmembrane segments in nicotinic acetylcholine receptors. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2006; 35:685-93. [PMID: 16838147 DOI: 10.1007/s00249-006-0078-2] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/13/2006] [Revised: 05/22/2006] [Accepted: 06/08/2006] [Indexed: 10/24/2022]
Abstract
Nicotinic acetylcholine receptors are heteropentameric ion channels that open upon activation to a single conducting state. The second transmembrane segments of each subunit were identified as channel-forming elements, but their respective contribution in the gating process remains unclear. Moreover, the detailed impact of variations of the membrane potential, such as occurring during an action potential, on the transmembrane domains, is unknown. Residues at the 12' position, close to the center of each second transmembrane segment, play a key role in channel gating. We examined their functional symmetry by substituting a lysine to that position of each subunit and measuring the electrical activity of single channels. For 12' lysines in the alpha, gamma and delta subunits rapid transitions between an intermediate and large conductance appeared, which are interpreted as single lysine protonation events. From the kinetics of these transitions we calculated the pK (a) values of respective lysines and showed that they vary differently with membrane hyperpolarization. Respective mutations in beta or epsilon subunits gave receptors with openings of either intermediate or large conductance, suggesting extreme pK (a) values in two open state conformations. The results demonstrate that these parts of the highly homologous transmembrane domains, as probed by the 12' lysines, sense unequal microenvironments and are differently affected by physiologically relevant voltage changes. Moreover, observation of various gating events for mutants of alpha subunits suggests that the open channel pore exists in multiple conformations, which in turn supports the notion of functional asymmetry of the channel.
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Affiliation(s)
- Jörg Grandl
- Laboratory of Physical Chemistry of Polymers and Membranes, Ecole Polytechnique Fédérale de Lausanne, Faculté des Sciences de Base, Station 6, Lausanne, Switzerland
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Shen XM, Deymeer F, Sine SM, Engel AG. Slow-channel mutation in acetylcholine receptor alphaM4 domain and its efficient knockdown. Ann Neurol 2006; 60:128-36. [PMID: 16685696 DOI: 10.1002/ana.20861] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
OBJECTIVE To identify the genetic basis of a slow-channel myasthenic syndrome, characterize functional properties of the mutant receptor, and selectively silence the mutant allele. METHODS We performed nutation analysis, cloning, and patch-clamp analysis of the functional properties of the mutant receptor; screening for a small interfering RNA with check plasmid; and assessed of the efficacy of small interfering RNA at the messenger RNA, protein, and functional levels. RESULTS We traced the cause of a slow-channel myasthenic syndrome to a C418W mutation in the M4 domain of the acetylcholine receptor alpha subunit. The mutation is the first one to occur spontaneously in an M4 domain of the receptor, and it is positioned within a stripe of hydrophobic residues facing the lipid bilayer. Kinetic analysis shows that alphaC418W enhances the channel opening equilibrium constant 26-fold without altering agonist affinity. Using a check plasmid as a screening tool, we identified a small interfering RNA that markedly suppresses the mutant but not the wild-type allele at the messenger RNA, protein, and functional levels. INTERPRETATION alphaC418W occurring in humans causes a slow-channel syndrome by enhancing the relative stability of the channel open state. Efficient and selective knockdown of the mutant allele holds promise of therapeutic gene silencing.
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MESH Headings
- Adult
- Bungarotoxins/metabolism
- Bungarotoxins/pharmacology
- Cells, Cultured
- DNA Mutational Analysis
- Down-Regulation/genetics
- Gene Expression
- Humans
- Iodine Radioisotopes
- Kidney/cytology
- Male
- Mutagenesis, Site-Directed
- Myasthenic Syndromes, Congenital/genetics
- Myasthenic Syndromes, Congenital/physiopathology
- Patch-Clamp Techniques
- Plasmids
- Protein Structure, Tertiary
- Protein Subunits/chemistry
- Protein Subunits/genetics
- Protein Subunits/metabolism
- RNA, Messenger/genetics
- RNA, Small Interfering
- Radioligand Assay
- Receptors, Nicotinic/chemistry
- Receptors, Nicotinic/genetics
- Receptors, Nicotinic/metabolism
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Affiliation(s)
- Xin-Ming Shen
- Neuromuscular Research Laboratory and Department of Neurology, Mayo Clinic, Rochester, MN 55905, USA
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Xiu X, Hanek AP, Wang J, Lester HA, Dougherty DA. A Unified View of the Role of Electrostatic Interactions in Modulating the Gating of Cys Loop Receptors. J Biol Chem 2005; 280:41655-66. [PMID: 16216879 DOI: 10.1074/jbc.m508635200] [Citation(s) in RCA: 117] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
In the Cys loop superfamily of ligand-gated ion channels, a global conformational change, initiated by agonist binding, results in channel opening and the passage of ions across the cell membrane. The detailed mechanism of channel gating is a subject that has lent itself to both structural and electrophysiological studies. Here we defined a gating interface that incorporates elements from the ligand binding domain and transmembrane domain previously reported as integral to proper channel gating. An overall analysis of charged residues within the gating interface across the entire superfamily showed a conserved charging pattern, although no specific interacting ion pairs were conserved. We utilized a combination of conventional mutagenesis and the high precision methodology of unnatural amino acid incorporation to study extensively the gating interface of the mouse muscle nicotinic acetylcholine receptor. We found that charge reversal, charge neutralization, and charge introduction at the gating interface are often well tolerated. Furthermore, based on our data and a reexamination of previously reported data on gamma-aminobutyric acid, type A, and glycine receptors, we concluded that the overall charging pattern of the gating interface, and not any specific pairwise electrostatic interactions, controls the gating process in the Cys loop superfamily.
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MESH Headings
- Acetylcholine/chemistry
- Amino Acid Sequence
- Amino Acids/chemistry
- Animals
- Biochemistry/methods
- Blotting, Western
- Bungarotoxins/chemistry
- Cations
- Cell Membrane/metabolism
- Cysteine/chemistry
- Cystine/chemistry
- Databases, Protein
- Dose-Response Relationship, Drug
- Electrodes
- Electrophysiology
- Glycine/chemistry
- Ions
- Kinetics
- Ligands
- Mice
- Models, Chemical
- Models, Molecular
- Molecular Sequence Data
- Mutagenesis
- Mutation
- Oocytes/metabolism
- Protein Binding
- Protein Conformation
- Protein Structure, Secondary
- Protein Structure, Tertiary
- RNA, Messenger/metabolism
- Receptors, Nicotinic/chemistry
- Sequence Homology, Amino Acid
- Static Electricity
- Torpedo
- Xenopus laevis
- gamma-Aminobutyric Acid/chemistry
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Affiliation(s)
- Xinan Xiu
- Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, USA
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Zhou Y, Pearson JE, Auerbach A. Phi-value analysis of a linear, sequential reaction mechanism: theory and application to ion channel gating. Biophys J 2005; 89:3680-5. [PMID: 16183877 PMCID: PMC1366938 DOI: 10.1529/biophysj.105.067215] [Citation(s) in RCA: 64] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/23/2005] [Accepted: 08/01/2005] [Indexed: 11/18/2022] Open
Abstract
We derive the analytical form of a rate-equilibrium free-energy relationship (with slope Phi) for a bounded, linear chain of coupled reactions having arbitrary connecting rate constants. The results confirm previous simulation studies showing that Phi-values reflect the position of the perturbed reaction within the chain, with reactions occurring earlier in the sequence producing higher Phi-values than those occurring later in the sequence. The derivation includes an expression for the transmission coefficients of the overall reaction based on the rate constants of an arbitrary, discrete, finite Markov chain. The results indicate that experimental Phi-values can be used to calculate the relative heights of the energy barriers between intermediate states of the chain but provide no information about the energies of the wells along the reaction path. Application of the equations to the case of diliganded acetylcholine receptor channel gating suggests that the transition-state ensemble for this reaction is nearly flat. Although this mechanism accounts for many of the basic features of diliganded and unliganded acetylcholine receptor channel gating, the experimental rate-equilibrium free-energy relationships appear to be more linear than those predicted by the theory.
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Affiliation(s)
- Yu Zhou
- Center for Single Molecule Biophysics and Department of Physiology & Biophysics, State University of New York at Buffalo, Buffalo, New York 14214, USA
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Groot-Kormelink PJ, Broadbent S, Beato M, Sivilotti LG. Constraining the expression of nicotinic acetylcholine receptors by using pentameric constructs. Mol Pharmacol 2005; 69:558-63. [PMID: 16269534 DOI: 10.1124/mol.105.019356] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Much of our understanding of ligand-gated ion channels comes from heterologous expression studies. However, this technique cannot produce receptors with a predetermined subunit composition for channels formed by several different subunits and cannot insert a single mutation copy if the subunit of interest is present in several copies in the channel. Here, we describe a novel approach that overcomes these problems by expressing pentameric constructs, in which the code of the five subunits is linked (i.e., beta4_beta4_alpha3_beta4_alpha3). This is the first time that a concatemer of the complete pentameric receptor has been expressed for channels in the cysteine-loop superfamily. The presence of the linker did not change the agonist or antagonist sensitivity of alpha3beta4 nicotinic receptors. We show evidence that the expressed receptors were made up of alpha3 and beta4 subunits in one pentameric fusion protein as designed in the construct. This approach can be applied to any nicotinic superfamily receptor to produce channels with a defined subunit arrangement and to introduce specific mutations at any desired location of the pentameric fusion protein.
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Mitra A, Cymes GD, Auerbach A. Dynamics of the acetylcholine receptor pore at the gating transition state. Proc Natl Acad Sci U S A 2005; 102:15069-74. [PMID: 16217024 PMCID: PMC1257706 DOI: 10.1073/pnas.0505090102] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/17/2005] [Indexed: 11/18/2022] Open
Abstract
Neuromuscular acetylcholine receptors (AChRs) are ion channels that alternatively adopt stable conformations that either allow (open) or prohibit (closed) ionic conduction. We probed the dynamics of pore (M2) residues at the diliganded gating transition state by using single-channel kinetic and rate-equilibrium free energy relationship (phi-value) analyses of mutant AChRs. The mutations were at the equatorial (9') position of the alpha, beta, and epsilon subunits (n = 15) or at sites between the equator and the extracellular domain in the alpha-subunit (n = 8). We also studied AChRs having only one of the two alpha-subunits mutated. The results indicate that the alpha-subunit, like the delta-subunit, has a region of flexure near the middle of M2, that the two alpha-subunits experience distinct energy barriers to gating at the equator (but not elsewhere), and that the collective subunit motions at the equator are asymmetric during the AChR gating isomerization.
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Affiliation(s)
- Ananya Mitra
- Center for Single Molecule Biophysics and Department of Physiology and Biophysics, University at Buffalo, State University of New York, Buffalo, NY 14214, USA
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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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Mitra A, Tascione R, Auerbach A, Licht S. Plasticity of acetylcholine receptor gating motions via rate-energy relationships. Biophys J 2005; 89:3071-8. [PMID: 16113115 PMCID: PMC1366804 DOI: 10.1529/biophysj.105.068783] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Like other protein conformational changes, ion channel gating requires the protein to achieve a high-energy transition-state structure. It is not known whether ion channel gating takes place on a broad energy landscape on which many alternative transition state structures are accessible, or on a narrow energy landscape where only a few transition-state structures are possible. To address this question, we measured how rate-equilibrium free energy relationships (REFERs) for di-liganded and unliganded acetylcholine receptor gating vary as a function of the gating equilibrium constant. A large slope for the REFER plot indicates an openlike transition state, whereas a small slope indicates a closedlike transition state. Due to this relationship between REFERs and transition-state structure, the sensitivity of the REFER slope to mutation-induced energetic perturbations allows estimation of the breadth of the energy landscape underlying gating. The relatively large sensitivity of di-liganded REFER slopes to energetic perturbations suggests that the motions underlying di-liganded gating take place on a broad, shallow energy landscape where many alternative transition-state structures are accessible.
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Affiliation(s)
- Ananya Mitra
- Center for Single Molecule Biophysics and the Department of Physiology and Biophysics, The State University of New York, Buffalo, NY, USA
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Law RJ, Henchman RH, McCammon JA. A gating mechanism proposed from a simulation of a human alpha7 nicotinic acetylcholine receptor. Proc Natl Acad Sci U S A 2005; 102:6813-8. [PMID: 15857954 PMCID: PMC1100735 DOI: 10.1073/pnas.0407739102] [Citation(s) in RCA: 119] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/18/2004] [Indexed: 12/24/2022] Open
Abstract
The nicotinic acetylcholine receptor is a well characterized ligand-gated ion channel, yet a proper description of the mechanisms involved in gating, opening, closing, ligand binding, and desensitization does not exist. Until recently, atomic-resolution structural information on the protein was limited, but with the production of the x-ray crystal structure of the Lymnea stagnalis acetylcholine binding protein and the EM image of the transmembrane domain of the torpedo electric ray nicotinic channel, we were provided with a window to examine the mechanism by which this channel operates. A 15-ns all-atom simulation of a homology model of the homomeric human alpha7 form of the receptor was conducted in a solvated palmitoyl-2-oleoyl-sn-glycerol-phosphatidylcholine bilayer and examined in detail. The receptor was unliganded. The structure undergoes a twist-to-close motion that correlates movements of the C loop in the ligand binding domain, via the beta10-strand that connects the two, with the 10 degrees rotation and inward movement of two nonadjacent subunits. The Cys loop appears to act as a stator around which the alpha-helical transmembrane domain can pivot and rotate relative to the rigid beta-sheet binding domain. The M2-M3 loop may have a role in controlling the extent or kinetics of these relative movements. All of this motion, along with essential dynamics analysis, is suggestive of the direction of larger motions involved in gating of the channel.
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Affiliation(s)
- Richard J Law
- Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093, USA.
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