1
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Cottone G, Chiodo L, Maragliano L. Thermodynamics and Kinetics of Ion Permeation in Wild-Type and Mutated Open Active Conformation of the Human α7 Nicotinic Receptor. J Chem Inf Model 2020; 60:5045-5056. [PMID: 32803965 PMCID: PMC8011927 DOI: 10.1021/acs.jcim.0c00549] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
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Molecular
studies of human pentameric ligand-gated ion channels
(LGICs) expressed in neurons and at neuromuscular junctions are of
utmost importance in the development of therapeutic strategies for
neurological disorders. We focus here on the nicotinic acetylcholine
receptor nAChR-α7, a homopentameric channel widely expressed
in the human brain, with a proven role in a wide spectrum of disorders
including schizophrenia and Alzheimer’s disease. By exploiting
an all-atom structural model of the full (transmembrane and extracellular)
protein in the open, agonist-bound conformation we recently developed,
we evaluate the free energy and the mean first passage time of single-ion
permeation using molecular dynamics simulations and the milestoning
method with Voronoi tessellation. The results for the wild-type channel
provide the first available mapping of the potential of mean force
in the full-length α7 nAChR, reveal its expected cationic nature,
and are in good agreement with simulation data for other channels
of the LGIC family and with experimental data on nAChRs. We then investigate
the role of a specific mutation directly related to ion selectivity
in LGICs, the E-1′ → A-1′ substitution at the
cytoplasmatic selectivity filter. We find that the mutation strongly
affects sodium and chloride permeation in opposite directions, leading
to a complete inversion of selectivity, at variance with the limited
experimental results available that classify this mutant as cationic.
We thus provide structural determinants for the observed cationic-to-anionic
inversion, revealing a key role of the protonation state of residue
rings far from the mutation, in the proximity of the hydrophobic channel
gate.
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Affiliation(s)
- Grazia Cottone
- Department of Physics and Chemistry-Emilio Segrè, University of Palermo, Viale delle Scienze Ed. 17, 90128 Palermo, Italy
| | - Letizia Chiodo
- Department of Engineering, Campus Bio-Medico University of Rome, Via Á. del Portillo 21, 00128 Rome, Italy
| | - Luca Maragliano
- Center for Synaptic Neuroscience and Technology (NSYN@UniGe), Istituto Italiano di Tecnologia, Largo Rosanna Benzi, 10, 16132 Genova, Italy.,IRCCS Ospedale Policlinico San Martino, Largo Rosanna Benzi, 10, 16132 Genova, Italy
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2
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Shen XM, Brengman JM, Shen S, Durmus H, Preethish-Kumar V, Yuceyar N, Vengalil S, Nalini A, Deymeer F, Sine SM, Engel AG. Mutations causing congenital myasthenia reveal principal coupling pathway in the acetylcholine receptor ε-subunit. JCI Insight 2018; 3:97826. [PMID: 29367459 DOI: 10.1172/jci.insight.97826] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2017] [Accepted: 12/12/2017] [Indexed: 11/17/2022] Open
Abstract
We identify 2 homozygous mutations in the ε-subunit of the muscle acetylcholine receptor (AChR) in 3 patients with severe congenital myasthenia: εR218W in the pre-M1 region in 2 patients and εE184K in the β8-β9 linker in 1 patient. Arg218 is conserved in all eukaryotic members of the Cys-loop receptor superfamily, while Glu184 is conserved in the α-, δ-, and ε-subunits of AChRs from all species. εR218W reduces channel gating efficiency 338-fold and AChR expression on the cell surface 5-fold, whereas εE184K reduces channel gating efficiency 11-fold but does not alter AChR cell surface expression. Determinations of the effective channel gating rate constants, combined with mutant cycle analyses, demonstrate strong energetic coupling between εR218 and εE184, and between εR218 and εE45 from the β1-β2 linker, as also observed for equivalent residues in the principal coupling pathway of the α-subunit. Thus, efficient and rapid gating of the AChR channel is achieved not only by coupling between conserved residues within the principal coupling pathway of the α-subunit, but also between corresponding residues in the ε-subunit.
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Affiliation(s)
- Xin-Ming Shen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota, USA
| | - Joan M Brengman
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota, USA
| | - Shelley Shen
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota, USA
| | - Hacer Durmus
- Department of Neurology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey
| | - Veeramani Preethish-Kumar
- Department of Neurology and.,Clinical Neurosciences, National Institute of Mental Health and Neurosciences, Bangalore, India
| | - Nur Yuceyar
- Department of Neurology, Ege University, Izmir, Turkey
| | | | | | - Feza Deymeer
- Department of Neurology, Istanbul Faculty of Medicine, Istanbul University, Istanbul, Turkey
| | - Steven M Sine
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota, USA.,Departments of Physiology and Biomedical Engineering and of Pharmacology and Experimental Therapeutics, Mayo Clinic, Rochester, Minnesota, USA
| | - Andrew G Engel
- Department of Neurology and Neuromuscular Research Laboratory, Mayo Clinic, Rochester, Minnesota, USA
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3
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Abstract
Agonists turn on receptors because they have a higher affinity for active versus resting conformations of the protein. Activation can occur by either of two pathways that connect to form a cycle: Agonists bind to resting receptors that then become active, or resting receptors activate and then bind agonists. We used mutations to construct endplate acetylcholine receptors (AChRs) having only one functional neurotransmitter-binding site and single-channel electrophysiology to measure independently binding constants for four different agonists, to both resting and active conformations of each site. For all agonists and sites, the total free energy change in each pathway was the same, confirming the activation cycle without external energy. Other results show that (i) there is no cooperativity between sites; (ii) agonist association is slower than diffusion in resting receptors but nearly diffusional in active receptors; (iii) whereas resting affinity is determined mainly by agonist association, active affinity is determined mainly by agonist dissociation; and (iv) at each site and for all agonists, receptor activation approximately doubles the agonist-binding free energy. We discuss a two-step mechanism for binding that involves diffusion and a local conformational change ("catch") that is modulated by receptor activation. The results suggest that binding to a resting site and the switch to high affinity are both integral parts of a single allosteric transition. We hypothesize that catch ensures proper signal recognition in complex chemical environments and that binding site compaction is a determinant of both resting and active affinity.
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4
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Nemecz Á, Prevost MS, Menny A, Corringer PJ. Emerging Molecular Mechanisms of Signal Transduction in Pentameric Ligand-Gated Ion Channels. Neuron 2017; 90:452-70. [PMID: 27151638 DOI: 10.1016/j.neuron.2016.03.032] [Citation(s) in RCA: 159] [Impact Index Per Article: 22.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2015] [Revised: 01/07/2016] [Accepted: 03/24/2016] [Indexed: 10/21/2022]
Abstract
Nicotinic acetylcholine, serotonin type 3, γ-amminobutyric acid type A, and glycine receptors are major players of human neuronal communication. They belong to the family of pentameric ligand-gated ion channels, sharing a highly conserved modular 3D structure. Recently, high-resolution structures of both open- and closed-pore conformations have been solved for a bacterial, an invertebrate, and a vertebrate receptor in this family. These data suggest that a common gating mechanism occurs, coupling neurotransmitter binding to pore opening, but they also pinpoint significant differences among subtypes. In this Review, we summarize the structural and functional data in light of these gating models and speculate about their mechanistic consequences on ion permeation, pathological mutations, as well as functional regulation by orthosteric and allosteric effectors.
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Affiliation(s)
- Ákos Nemecz
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France
| | - Marie S Prevost
- Institute of Structural and Molecular Biology, University College London and Birkbeck, Malet Street, London WC1E 7HX, UK
| | - Anaïs Menny
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France; Université Pierre et Marie Curie (UPMC), Cellule Pasteur, 75005 Paris, France
| | - Pierre-Jean Corringer
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France.
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5
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Secondary Ammonium Agonists Make Dual Cation-π Interactions in α4β2 Nicotinic Receptors. eNeuro 2017; 4:eN-NWR-0032-17. [PMID: 28589175 PMCID: PMC5458768 DOI: 10.1523/eneuro.0032-17.2017] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2017] [Revised: 02/28/2017] [Accepted: 03/02/2017] [Indexed: 11/21/2022] Open
Abstract
A cation-π interaction between the ammonium group of an agonist and a conserved tryptophan termed TrpB is a near universal feature of agonist binding to nicotinic acetylcholine receptors (nAChRs). TrpB is one of five residues that form the aromatic box of the agonist binding site, and for the prototype agonists ACh and nicotine, only TrpB makes a functional cation-π interaction. We report that, in addition to TrpB, a significant cation-π interaction is made to a second aromatic, TyrC2, by the agonists metanicotine, TC299423, varenicline, and nornicotine. A common structural feature of these agonists, and a distinction from ACh and nicotine, is a protonated secondary amine that provides the cation for the cation-π interaction. These results indicate a distinction in binding modes between agonists with subtly different structures that may provide guidance for the development of subtype-selective agonists of nAChRs.
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6
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Grazioso G, Sgrignani J, Capelli R, Matera C, Dallanoce C, De Amici M, Cavalli A. Allosteric Modulation of Alpha7 Nicotinic Receptors: Mechanistic Insight through Metadynamics and Essential Dynamics. J Chem Inf Model 2015; 55:2528-39. [PMID: 26569022 DOI: 10.1021/acs.jcim.5b00459] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Increasing attention has recently been devoted to allosteric modulators, as they can provide inherent advantages over classic receptor agonists. In the field of nicotinic receptors (nAChRs), the main advantage is that allosteric modulators can trigger pharmacological responses, limiting receptor desensitization. Most of the known allosteric ligands are "positive allosteric modulators" (PAMs), which increase both sensitivity to receptor agonists and current amplitude. Intriguingly, some allosteric modulators are also able to activate the α7 receptor (α7-nAChR) even in the absence of orthosteric agonists. These compounds have been named "ago-allosteric modulators" and GAT107 has been studied in depth because of its unique mechanism of action. We here investigate by molecular dynamics simulations, metadynamics, and essential dynamics the activation mechanism of α7-nAChR, in the presence of different nicotinic modulators. We determine the free energy profiles associated with the closed-to-open motion of the loop C, and we highlight mechanistic differences observed in the presence of different modulators. In particular, we demonstrate that GAT107 triggers conformational motions and cross-talk similar to those observed when the α7-nACh receptor is in complex with both an agonist and an allosteric modulator.
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Affiliation(s)
- Giovanni Grazioso
- Dipartimento di Scienze Farmaceutiche, Sezione di Chimica Farmaceutica "Pietro Pratesi", Università degli Studi di Milano , Via L. Mangiagalli 25, 20133 Milan, Italy
| | - Jacopo Sgrignani
- Institute of Research in Biomedicine (IRB) , Via Vincenzo Vela 6, 6500 Bellinzona, Switzerland
| | - Romina Capelli
- Dipartimento di Scienze Farmaceutiche, Sezione di Chimica Farmaceutica "Pietro Pratesi", Università degli Studi di Milano , Via L. Mangiagalli 25, 20133 Milan, Italy
| | - Carlo Matera
- Dipartimento di Scienze Farmaceutiche, Sezione di Chimica Farmaceutica "Pietro Pratesi", Università degli Studi di Milano , Via L. Mangiagalli 25, 20133 Milan, Italy
| | - Clelia Dallanoce
- Dipartimento di Scienze Farmaceutiche, Sezione di Chimica Farmaceutica "Pietro Pratesi", Università degli Studi di Milano , Via L. Mangiagalli 25, 20133 Milan, Italy
| | - Marco De Amici
- Dipartimento di Scienze Farmaceutiche, Sezione di Chimica Farmaceutica "Pietro Pratesi", Università degli Studi di Milano , Via L. Mangiagalli 25, 20133 Milan, Italy
| | - Andrea Cavalli
- Drug Discovery and Development-Computation, Istituto Italiano di Tecnologia , Via Morego 30, 16163 Genoa, Italy.,Department of Pharmacy and Biotecnology, University of Bologna , Via Belmeloro 6, 40126 Bologna, Italy
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7
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Di Maio D, Chandramouli B, Brancato G. Pathways and Barriers for Ion Translocation through the 5-HT3A Receptor Channel. PLoS One 2015; 10:e0140258. [PMID: 26465896 PMCID: PMC4605793 DOI: 10.1371/journal.pone.0140258] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/06/2015] [Accepted: 09/12/2015] [Indexed: 11/29/2022] Open
Abstract
Pentameric ligand gated ion channels (pLGICs) are ionotropic receptors that mediate fast intercellular communications at synaptic level and include either cation selective (e.g., nAChR and 5-HT3) or anion selective (e.g., GlyR, GABAA and GluCl) membrane channels. Among others, 5-HT3 is one of the most studied members, since its first cloning back in 1991, and a large number of studies have successfully pinpointed protein residues critical for its activation and channel gating. In addition, 5-HT3 is also the target of a few pharmacological treatments due to the demonstrated benefits of its modulation in clinical trials. Nonetheless, a detailed molecular analysis of important protein features, such as the origin of its ion selectivity and the rather low conductance as compared to other channel homologues, has been unfeasible until the recent crystallization of the mouse 5-HT3A receptor. Here, we present extended molecular dynamics simulations and free energy calculations of the whole 5-HT3A protein with the aim of better understanding its ion transport properties, such as the pathways for ion permeation into the receptor body and the complex nature of the selectivity filter. Our investigation unravels previously unpredicted structural features of the 5-HT3A receptor, such as the existence of alternative intersubunit pathways for ion translocation at the interface between the extracellular and the transmembrane domains, in addition to the one along the channel main axis. Moreover, our study offers a molecular interpretation of the role played by an arginine triplet located in the intracellular domain on determining the characteristic low conductance of the 5-HT3A receptor, as evidenced in previous experiments. In view of these results, possible implications on other members of the superfamily are suggested.
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Affiliation(s)
- Danilo Di Maio
- Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126, Pisa, Italy
| | | | - Giuseppe Brancato
- Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126, Pisa, Italy
- * E-mail:
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8
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Lykhmus O, Gergalova G, Koval L, Zhmak M, Komisarenko S, Skok M. Mitochondria express several nicotinic acetylcholine receptor subtypes to control various pathways of apoptosis induction. Int J Biochem Cell Biol 2014; 53:246-52. [PMID: 24880090 DOI: 10.1016/j.biocel.2014.05.030] [Citation(s) in RCA: 65] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2014] [Revised: 05/07/2014] [Accepted: 05/19/2014] [Indexed: 11/18/2022]
Abstract
Nicotinic acetylcholine receptors control survival, proliferation and cytokine release in non-excitable cells. Previously we reported that α7 nicotinic receptors were present in the outer membranes of mouse liver mitochondria to regulate mitochondrial pore formation and cytochrome c release. Here we used a wide spectrum of nicotinic receptor subunit-specific antibodies to show that mitochondria express several nicotinic receptor subtypes in a tissue-specific manner: brain and liver mitochondria contain α7β2, α4β2 and less α3β2 nicotinic receptors, while mitochondria from the lung express preferentially α3β4 receptor subtype; all of them are non-covalently connected to voltage-dependent anion channels and control cytochrome c release. By using selective ligands of different nicotinic receptor subtypes (acetylcholine (1 μM) or dihydro-β-erythroidine (1 μM) for α4β2), conotoxin MII (1 nM) for α3β2, MLA (50 nM) for α7β2 and acetylcholine (10 μM) for all subtypes) and apoptogenic agents triggering different mitochondrial signaling pathways (1 μM wortmannin, 90 μM Ca(2+) or 0.5 mM H₂O₂) it was found that α7β2 receptors affect mainly PI₃K/Akt pathway, while α3β2 and α4β2 nAChRs also significantly influence CaKMII- and Src-dependent pathways. It is concluded that cholinergic regulation in mitochondria is realized through multiple nicotinic receptor subtypes, which control various pathways inducing mitochondrial type of apoptosis.
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Affiliation(s)
- Olena Lykhmus
- Palladin Institute of Biochemistry, 9, Leontovicha str., 01601 Kyiv, Ukraine
| | - Galyna Gergalova
- Palladin Institute of Biochemistry, 9, Leontovicha str., 01601 Kyiv, Ukraine
| | - Lyudmyla Koval
- Palladin Institute of Biochemistry, 9, Leontovicha str., 01601 Kyiv, Ukraine
| | - Maxim Zhmak
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, 16/10, Miklukho-Maklaya str., 117997 Moscow, Russian Federation
| | - Sergiy Komisarenko
- Palladin Institute of Biochemistry, 9, Leontovicha str., 01601 Kyiv, Ukraine
| | - Maryna Skok
- Palladin Institute of Biochemistry, 9, Leontovicha str., 01601 Kyiv, Ukraine.
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9
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Salari R, Murlidaran S, Brannigan G. Pentameric Ligand-gated Ion Channels : Insights from Computation. MOLECULAR SIMULATION 2014; 40:821-829. [PMID: 25931676 PMCID: PMC4412168 DOI: 10.1080/08927022.2014.896462] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
Abstract
Pentameric ligand-gated ion channels (pLGICs) conduct upon the binding of an agonist and are fundamental to neurotransmission. New insights into the complex mechanisms underlying pLGIC gating, ion selectivity, and modulation have recently been gained via a series of crystal structures in prokaryotes and C .elegans, as well as computational studies relying on these structures. Here we review contributions from a variety of computational approaches, including normal mode analysis, automated docking, and fully atomistic molecular dynamics simulation. Examples from our own research, particularly concerning interactions with general anesthetics and lipids, are used to illustrate predictive results complementary to crystallographic studies.
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Affiliation(s)
- Reza Salari
- Center for Computational and Integrative Biology, Rutgers University, Camden, NJ
- Department of Physics, Rutgers University, Camden, NJ
| | - Sruthi Murlidaran
- Center for Computational and Integrative Biology, Rutgers University, Camden, NJ
| | - Grace Brannigan
- Center for Computational and Integrative Biology, Rutgers University, Camden, NJ
- Department of Physics, Rutgers University, Camden, NJ
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10
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daCosta CJB, Baenziger JE. Gating of pentameric ligand-gated ion channels: structural insights and ambiguities. Structure 2014; 21:1271-83. [PMID: 23931140 DOI: 10.1016/j.str.2013.06.019] [Citation(s) in RCA: 79] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2013] [Revised: 05/31/2013] [Accepted: 06/26/2013] [Indexed: 01/09/2023]
Abstract
Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic communication by converting chemical signals into an electrical response. Recently solved agonist-bound and agonist-free structures greatly extend our understanding of these complex molecular machines. A key challenge to a full description of function, however, is the ability to unequivocally relate determined structures to the canonical resting, open, and desensitized states. Here, we review current understanding of pLGIC structure, with a focus on the conformational changes underlying channel gating. We compare available structural information and review the evidence supporting the assignment of each structure to a particular conformational state. We discuss multiple factors that may complicate the interpretation of crystal structures, highlighting the potential influence of lipids and detergents. We contend that further advances in the structural biology of pLGICs will require deeper insight into the nature of pLGIC-lipid interactions.
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Affiliation(s)
- Corrie J B daCosta
- Receptor Biology Laboratory, Departments of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, MN 55905, USA
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11
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Mohammad Hosseini Naveh Z, Malliavin TE, Maragliano L, Cottone G, Ciccotti G. Conformational changes in acetylcholine binding protein investigated by temperature accelerated molecular dynamics. PLoS One 2014; 9:e88555. [PMID: 24551117 PMCID: PMC3923797 DOI: 10.1371/journal.pone.0088555] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2013] [Accepted: 01/07/2014] [Indexed: 11/19/2022] Open
Abstract
Despite the large number of studies available on nicotinic acetylcholine receptors, a complete account of the mechanistic aspects of their gating transition in response to ligand binding still remains elusive. As a first step toward dissecting the transition mechanism by accelerated sampling techniques, we study the ligand-induced conformational changes of the acetylcholine binding protein (AChBP), a widely accepted model for the full receptor extracellular domain. Using unbiased Molecular Dynamics (MD) and Temperature Accelerated Molecular Dynamics (TAMD) simulations we investigate the AChBP transition between the apo and the agonist-bound state. In long standard MD simulations, both conformations of the native protein are stable, while the agonist-bound structure evolves toward the apo one if the orientation of few key sidechains in the orthosteric cavity is modified. Conversely, TAMD simulations initiated from the native conformations are able to produce the spontaneous transition. With respect to the modified conformations, TAMD accelerates the transition by at least a factor 10. The analysis of some specific residue-residue interactions points out that the transition mechanism is based on the disruption/formation of few key hydrogen bonds. Finally, while early events of ligand dissociation are observed already in standard MD, TAMD accelerates the ligand detachment and, at the highest TAMD effective temperature, it is able to produce a complete dissociation path in one AChBP subunit.
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Affiliation(s)
| | - Therese E. Malliavin
- Institut Pasteur and CNRS UMR 3528, Unité de Bioinformatique Structurale, Paris, France
| | - Luca Maragliano
- Department of Neuroscience and Brain Technologies, Istituto Italiano di Tecnologia, Genoa, Italy
| | - Grazia Cottone
- School of Physics, University College Dublin, Dublin, Ireland
- Department of Physics and Chemistry, University of Palermo, Palermo, Italy
- * E-mail:
| | - Giovanni Ciccotti
- School of Physics, University College Dublin, Dublin, Ireland
- Department of Physics, University of Roma “La Sapienza”, Rome, Italy
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12
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Intra-subunit flexibility underlies activation and allosteric modulation of neuronal nicotinic acetylcholine receptors. Neuropharmacology 2013; 79:420-31. [PMID: 24373904 DOI: 10.1016/j.neuropharm.2013.12.017] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2013] [Revised: 12/12/2013] [Accepted: 12/14/2013] [Indexed: 01/20/2023]
Abstract
Allosteric modulation is a general feature of nicotinic acetylcholine receptors, yet the structural components and movements important for conversions among functional states are not well understood. In this study, we examine the communication between the binding sites for agonist and the modulator morantel (Mor) of neuronal α3β2 receptors, measuring evoked currents of receptors expressed in Xenopus oocytes with the two-electrode voltage-clamp method. We hypothesized that movement along an interface of β sheets connecting the agonist and modulator sites is necessary for allosteric modulation. To address this, we created pairs of substituted cysteines that span the cleft formed where the outer β sheet meets the β sheet constituting the (-)-face of the α3 subunit; the three pairs were L158C-A179C, L158C-G181C and L158C-K183C. Employing a disulfide trapping approach in which bonds are formed between neighboring cysteines under oxidation conditions, we found that oxidation treatments decreased the amplitude of currents evoked by either the agonist (ACh) or co-applied agonist and modulator (ACh + Mor), by as much as 51%, consistent with the introduced bond decreasing channel efficacy. Reduction treatment increased evoked currents up to 89%. The magnitude of the oxidation effects depended on whether agonists were present during oxidation and on the cysteine pair. Additionally, the cysteine mutations themselves decreased Mor potentiation, implicating these residues in modulation. Our findings suggest that these β sheets in the α3 subunit move with respect to each other during activation and modulation, and the residues studied highlight the contribution of this intramolecular allosteric pathway to receptor function.
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13
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Nicotinic acetylcholine receptor and the structural basis of neuromuscular transmission: insights from Torpedo postsynaptic membranes. Q Rev Biophys 2013; 46:283-322. [PMID: 24050525 PMCID: PMC3820380 DOI: 10.1017/s0033583513000061] [Citation(s) in RCA: 97] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
The nicotinic acetylcholine (ACh) receptor, at the neuromuscular junction, is a neurotransmitter-gated ion channel that has been fine-tuned through evolution to transduce a chemical signal into an electrical signal with maximum efficiency and speed. It is composed from three similar and two identical polypeptide chains, arranged in a ring around a narrow membrane pore. Central to the design of this assembly is a hydrophobic gate in the pore, more than 50 Å away from sites in the extracellular domain where ACh binds. Although the molecular properties of the receptor have been explored intensively over the last few decades, only recently have structures emerged revealing its complex architecture and illuminating how ACh entering the binding sites opens the distant gate. Postsynaptic membranes isolated from the (muscle-derived) electric organ of the Torpedo ray have underpinned most of the structural studies: the membranes form tubular vesicles having receptors arranged on a regular surface lattice, which can be imaged directly in frozen physiological solutions. Advances in electron crystallographic techniques have also been important, enabling analysis of the closed- and open-channel forms of the receptor in unreacted tubes or tubes reacted briefly with ACh. The structural differences between these two forms show that all five subunits participate in a concerted conformational change communicating the effect of ACh binding to the gate, but that three of them (αγ, β and δ) play a dominant role. Flexing of oppositely facing pore-lining α-helices is the principal motion determining the closed/open state of the gate. These results together with the findings of biochemical, biophysical and other structural studies allow an integrated description of the receptor and of its mode of action at the synapse.
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14
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Mukhtasimova N, Sine SM. Nicotinic receptor transduction zone: invariant arginine couples to multiple electron-rich residues. Biophys J 2013; 104:355-67. [PMID: 23442857 DOI: 10.1016/j.bpj.2012.12.013] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/02/2012] [Revised: 12/04/2012] [Accepted: 12/06/2012] [Indexed: 11/26/2022] Open
Abstract
Gating of the muscle-type acetylcholine receptor (AChR) channel depends on communication between the ACh-binding site and the remote ion channel. A key region for this communication is located within the structural transition zone between the ligand-binding and pore domains. Here, stemming from β-strand 10 of the binding domain, the invariant αArg209 lodges within the hydrophobic interior of the subunit and is essential for rapid and efficient channel gating. Previous charge-reversal experiments showed that the contribution of αArg209 to channel gating depends strongly on αGlu45, also within this region. Here we determine whether the contribution of αArg209 to channel gating depends on additional anionic or electron-rich residues in this region. Also, to reconcile diverging findings in the literature, we compare the dependence of αArg209 on αGlu45 in AChRs from different species, and compare the full agonist ACh with the weak agonist choline. Our findings reveal that the contribution of αArg209 to channel gating depends on additional nearby electron-rich residues, consistent with both electrostatic and steric contributions. Furthermore, αArg209 and αGlu45 show a strong interdependence in both human and mouse AChRs, whereas the functional consequences of the mutation αE45R depend on the agonist. The emerging picture shows a multifaceted network of interdependent residues that are required for communication between the ligand-binding and pore domains.
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Affiliation(s)
- Nuriya Mukhtasimova
- Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota, USA
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15
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Abstract
During the past five years an increasing number of patients have been diagnosed with congenital myasthenic syndromes (CMS) and a number of novel syndromes have been recognized and investigated. This presentation focuses on the CMS caused by defects in choline acetyltransferase, novel fast-channel syndromes that hinder isomerization of the acetylcholine receptor from the closed to the open state, the consequences of deleterious mutations in the intermediate filament linker plectin, altered neuromuscular transmission in a centronuclear myopathy, and two recently identified CMS caused by congenital defects in glycosylation.
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Affiliation(s)
- Andrew G Engel
- Neuromuscular Research Laboratory, Department of Neurology, Mayo Clinic, Rochester, Minnesota, USA.
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16
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Unwin N, Fujiyoshi Y. Gating movement of acetylcholine receptor caught by plunge-freezing. J Mol Biol 2012; 422:617-634. [PMID: 22841691 PMCID: PMC3443390 DOI: 10.1016/j.jmb.2012.07.010] [Citation(s) in RCA: 144] [Impact Index Per Article: 12.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/16/2012] [Revised: 06/06/2012] [Accepted: 07/10/2012] [Indexed: 11/19/2022]
Abstract
The nicotinic acetylcholine (ACh) receptor converts transiently to an open-channel form when activated by ACh released into the synaptic cleft. We describe here the conformational change underlying this event, determined by electron microscopy of ACh-sprayed and freeze-trapped postsynaptic membranes. ACh binding to the α subunits triggers a concerted rearrangement in the ligand-binding domain, involving an ~1-Å outward displacement of the extracellular portion of the β subunit where it interacts with the juxtaposed ends of α-helices shaping the narrow membrane-spanning pore. The β-subunit helices tilt outward to accommodate this displacement, destabilising the arrangement of pore-lining helices, which in the closed channel bend inward symmetrically to form a central hydrophobic gate. Straightening and tangential motion of the pore-lining helices effect channel opening by widening the pore asymmetrically and increasing its polarity in the region of the gate. The pore-lining helices of the α(γ) and δ subunits, by flexing between alternative bent and straight conformations, undergo the greatest movements. This coupled allosteric transition shifts the structure from a tense (closed) state toward a more relaxed (open) state.
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Affiliation(s)
- Nigel Unwin
- MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 0QH, UK.
| | - Yoshinori Fujiyoshi
- Department of Biophysics, Faculty of Science, Kyoto University, Oiwake, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan
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17
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Abstract
The synapse is a localized neurohumoral contact between a neuron and an effector cell and may be considered the quantum of fast intercellular communication. Analogously, the postsynaptic neurotransmitter receptor may be considered the quantum of fast chemical to electrical transduction. Our understanding of postsynaptic receptors began to develop about a hundred years ago with the demonstration that electrical stimulation of the vagus nerve released acetylcholine and slowed the heart beat. During the past 50 years, advances in understanding postsynaptic receptors increased at a rapid pace, owing largely to studies of the acetylcholine receptor (AChR) at the motor endplate. The endplate AChR belongs to a large superfamily of neurotransmitter receptors, called Cys-loop receptors, and has served as an exemplar receptor for probing fundamental structures and mechanisms that underlie fast synaptic transmission in the central and peripheral nervous systems. Recent studies provide an increasingly detailed picture of the structure of the AChR and the symphony of molecular motions that underpin its remarkably fast and efficient chemoelectrical transduction.
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Affiliation(s)
- Steven M Sine
- Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA.
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18
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Shen XM, Brengman JM, Sine SM, Engel AG. Myasthenic syndrome AChRα C-loop mutant disrupts initiation of channel gating. J Clin Invest 2012; 122:2613-21. [PMID: 22728938 DOI: 10.1172/jci63415] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/16/2012] [Accepted: 04/18/2012] [Indexed: 01/19/2023] Open
Abstract
Congenital myasthenic syndromes (CMSs) are neuromuscular disorders that can be caused by defects in ace-tylcholine receptor (AChR) function. Disease-associated point mutants can reveal the unsuspected functional significance of mutated residues. We identified two pathogenic mutations in the extracellular domain of the AChR α subunit (AChRα) in a patient with myasthenic symptoms since birth: a V188M mutation in the C-loop and a heteroallelic G74C mutation in the main immunogenic region. The G74C mutation markedly reduced surface AChR expression in cultured cells, whereas the V188M mutant was expressed robustly but had severely impaired kinetics. Single-channel patch-clamp analysis indicated that V188M markedly decreased the apparent AChR channel opening rate and gating efficiency. Mutant cycle analysis of energetic coupling among conserved residues within or dispersed around the AChRα C-loop revealed that V188 is functionally linked to Y190 in the C-loop and to D200 in β-strand 10, which connects to the M1 transmembrane domain. Furthermore, V188M weakens inter-residue coupling of K145 in β-strand 7 with Y190 and with D200. Cumulatively, these results indicate that V188 of AChRα is part of an interdependent tetrad that contributes to rearrangement of the C-loop during the initial coupling of agonist binding to channel gating.
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Affiliation(s)
- Xin-Ming Shen
- Neuromuscular Research Laboratory, Department of Neurology, Mayo Clinic, Rochester, MN, USA.
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19
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Cheng Y, Kekenes-Huskey P, Hake J, Holst M, McCammon J, Michailova A. Multi-Scale Continuum Modeling of Biological Processes: From Molecular Electro-Diffusion to Sub-Cellular Signaling Transduction. ACTA ACUST UNITED AC 2012; 5. [PMID: 23505398 DOI: 10.1088/1749-4699/5/1/015002] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
This article provides a brief review of multi-scale modeling at the molecular to cellular scale, with new results for heart muscle cells. A finite element-based simulation package (SMOL) was used to investigate the signaling transduction at molecular and sub-cellular scales (http://mccammon.ucsd.edu/smol/, http://FETK.org) by numerical solution of time-dependent Smoluchowski equations and a reaction-diffusion system. At the molecular scale, SMOL has yielded experimentally-validated estimates of the diffusion-limited association rates for the binding of acetylcholine to mouse acetylcholinesterase using crystallographic structural data. The predicted rate constants exhibit increasingly delayed steady-state times with increasing ionic strength and demonstrate the role of an enzyme's electrostatic potential in influencing ligand binding. At the sub-cellular scale, an extension of SMOL solves a non-linear, reaction-diffusion system describing Ca2+ ligand buffering and diffusion in experimentally-derived rodent ventricular myocyte geometries. Results reveal the important role for mobile and stationary Ca2+ buffers, including Ca2+ indicator dye. We found that the alterations in Ca2+-binding and dissociation rates of troponin C (TnC) and total TnC concentration modulate subcellular Ca2+ signals. Model predicts that reduced off-rate in whole troponin complex (TnC, TnI, TnT) versus reconstructed thin filaments (Tn, Tm, actin) alters cytosolic Ca2+ dynamics under control conditions or in disease-linked TnC mutations. The ultimate goal of these studies is to develop scalable methods and theories for integration of molecular-scale information into simulations of cellular-scale systems.
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Affiliation(s)
- Y Cheng
- Department of Bioengineering, University of California, San Diego, La Jolla, CA 92093, USA
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20
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Wang HL, Cheng X, Sine SM. Intramembrane proton binding site linked to activation of bacterial pentameric ion channel. J Biol Chem 2011; 287:6482-9. [PMID: 22084238 DOI: 10.1074/jbc.m111.305839] [Citation(s) in RCA: 38] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Prokaryotic orthologs of eukaryotic Cys-loop receptor channels recently emerged as structural and mechanistic surrogates to investigate this superfamily of intercellular signaling proteins. Here, we examine proton activation of the prokaryotic ortholog GLIC using patch clamp electrophysiology, mutagenesis, and molecular dynamics (MD) simulations. Whole-cell current recordings from human embryonic kidney (HEK) 293 cells expressing GLIC show half-maximal activation at pH 6, close to the pK(a) of histidine, implicating the three native His residues in proton sensing linked to activation. The mutation H235F abolishes proton activation, H277Y is without effect, and all nine mutations of His-127 prevent expression on the cell surface. In the GLIC crystal structure, His-235 on transmembrane (TM) α-helix 2, hydrogen bonds to the main chain carbonyl oxygen of Ile-259 on TM α-helix 3. MD simulations show that when His-235 is protonated, the hydrogen bond persists, and the channel remains in the open conformation, whereas when His-235 is deprotonated, the hydrogen bond dissociates, and the channel closes. Mutations of the proximal Tyr-263, which also links TM α-helices 2 and 3 via a hydrogen bond, alter proton sensitivity over a 1.5 pH unit range. MD simulations show that mutations of Tyr-263 alter the hydrogen bonding capacity of His-235. The overall findings show that His-235 in the TM region of GLIC is a novel proton binding site linked to channel activation.
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Affiliation(s)
- Hai-Long Wang
- Department of Physiology and Biomedical Engineering, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA
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21
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Nemecz Á, Taylor P. Creating an α7 nicotinic acetylcholine recognition domain from the acetylcholine-binding protein: crystallographic and ligand selectivity analyses. J Biol Chem 2011; 286:42555-42565. [PMID: 22009746 DOI: 10.1074/jbc.m111.286583] [Citation(s) in RCA: 57] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Determining the structure of the ligand-binding domain of the nicotinic acetylcholine receptor (nAChR) has been a long standing goal in the design of selective drugs useful in implicated diseases for this prevalent receptor family. Acetylcholine-binding proteins have proven to be valuable surrogates with structural similarity and sequence identity to the extracellular domain of the nicotinic receptor, yet these soluble proteins have their unique features and do not serve as exact replicates of the nAChRs of interest. Here we systematically modify the sequence of these proteins toward the homomeric human α7 nAChR. These chimeric proteins exhibit a shift in affinities to reflect α7 binding characteristics yet maintain expression levels and stability conducive for crystallization. We also present a pentameric humanoid nAChR extracellular domain with the structural determination of the α7 nAChR glycosylation site.
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Affiliation(s)
- Ákos Nemecz
- Departments of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093-0650; Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, California 92093-0650
| | - Palmer Taylor
- Department of Pharmacology, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California, San Diego, La Jolla, California 92093-0650.
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22
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Bouzat C. New insights into the structural bases of activation of Cys-loop receptors. ACTA ACUST UNITED AC 2011; 106:23-33. [PMID: 21995938 DOI: 10.1016/j.jphysparis.2011.09.012] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/06/2011] [Revised: 09/07/2011] [Accepted: 09/26/2011] [Indexed: 11/27/2022]
Abstract
Neurotransmitter receptors of the Cys-loop superfamily mediate rapid synaptic transmission throughout the nervous system, and include receptors activated by ACh, GABA, glycine and serotonin. They are involved in physiological processes, including learning and memory, and in neurological disorders, and they are targets for clinically relevant drugs. Cys-loop receptors assemble either from five copies of one type of subunit, giving rise to homomeric receptors, or from several types of subunits, giving rise to heteromeric receptors. Homomeric receptors are invaluable models for probing fundamental relationships between structure and function. Receptors contain a large extracellular domain that carries the binding sites and a transmembrane region that forms the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50Å to the ion channel gate is central to understanding receptor function. Depending on the receptor subtype, occupancy of either two, as in the prototype muscle nicotinic receptor, or three binding sites, as in homomeric receptors, is required for full activation. The conformational changes initiated at the binding sites are propagated to the gate through the interface between the extracellular and transmembrane domains. This region forms a network that relays structural changes from the binding site towards the pore, and also contributes to open channel lifetime and rate of desensitization. Thus, this coupling region controls the beginning and duration of a synaptic response. Here we review recent advances in the molecular mechanism by which Cys-loop receptors are activated with particular emphasis on homomeric receptors.
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Affiliation(s)
- Cecilia Bouzat
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur and CONICET, 8000 Bahía Blanca, Argentina.
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23
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Mazzaferro S, Benallegue N, Carbone A, Gasparri F, Vijayan R, Biggin PC, Moroni M, Bermudez I. Additional acetylcholine (ACh) binding site at alpha4/alpha4 interface of (alpha4beta2)2alpha4 nicotinic receptor influences agonist sensitivity. J Biol Chem 2011; 286:31043-31054. [PMID: 21757735 DOI: 10.1074/jbc.m111.262014] [Citation(s) in RCA: 105] [Impact Index Per Article: 8.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Nicotinic acetylcholine receptor (nAChR) α4 and β2 subunits assemble in two alternate stoichiometries to produce (α4β2)(2)α4 and (α4β2)(2)β2, which display different agonist sensitivities. Functionally relevant agonist binding sites are thought to be located at α4(+)/β2(-) subunit interfaces, but because these interfaces are present in both receptor isoforms, it is unlikely that they account for differences in agonist sensitivities. In contrast, incorporation of either α4 or β2 as auxiliary subunits produces isoform-specific α4(+)/α4(-) or β2(+)/β2(-) interfaces. Using fully concatenated (α4β2)(2)α4 nAChRs in conjunction with structural modeling, chimeric receptors, and functional mutagenesis, we have identified an additional site at the α4(+)/α4(-) interface that accounts for isoform-specific agonist sensitivity of the (α4β2)(2)α4 nAChR. The additional site resides in a region that also contains a potentiating Zn(2+) site but is engaged by agonists to contribute to receptor activation. By engineering α4 subunits to provide a free cysteine in loop C at the α4(+)α4(-) interface, we demonstrated that the acetylcholine responses of the mutated receptors are attenuated or enhanced, respectively, following treatment with the sulfhydryl reagent [2-(trimethylammonium)ethyl]methanethiosulfonate or aminoethyl methanethiosulfonate. The findings suggest that agonist occupation of the site at the α4(+)/(α4(-) interface leads to channel gating through a coupling mechanism involving loop C. Overall, we propose that the additional agonist site at the α4(+)/α4(-) interface, when occupied by agonist, contributes to receptor activation and that this additional contribution underlies the agonist sensitivity signature of (α4β2)(2)α4 nAChRs.
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Affiliation(s)
- Simone Mazzaferro
- School of Life Sciences, Oxford Brookes University, Oxford OX3 0BP, United Kingdom
| | - Naïl Benallegue
- School of Life Sciences, Oxford Brookes University, Oxford OX3 0BP, United Kingdom
| | - Anna Carbone
- Leibniz-Institut für Molekulare Pharmakologie and Neurocure Initiative Charité Universitäts Medizin, 13125 Berlin, Germany
| | - Federica Gasparri
- School of Life Sciences, Oxford Brookes University, Oxford OX3 0BP, United Kingdom
| | - Ranjit Vijayan
- Structural Bioinformatics and Computational Biochemistry, Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom
| | - Philip C Biggin
- Structural Bioinformatics and Computational Biochemistry, Department of Biochemistry, University of Oxford, Oxford OX1 3QU, United Kingdom
| | - Mirko Moroni
- Department of Pharmacology, University College London, London WC1E 6BT, United Kingdom
| | - Isabel Bermudez
- School of Life Sciences, Oxford Brookes University, Oxford OX3 0BP, United Kingdom.
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24
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Cheng MH, Coalson RD, Tang P. Molecular dynamics and brownian dynamics investigation of ion permeation and anesthetic halothane effects on a proton-gated ion channel. J Am Chem Soc 2010; 132:16442-9. [PMID: 20979415 PMCID: PMC3071019 DOI: 10.1021/ja105001a] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
Bacterial Gloeobacter violaceus pentameric ligand-gated ion channel (GLIC) is activated to cation permeation upon lowering the solution pH. Its function can be modulated by anesthetic halothane. In the present work, we integrate molecular dynamics (MD) and Brownian dynamics (BD) simulations to elucidate the ion conduction, charge selectivity, and halothane modulation mechanisms in GLIC, based on recently resolved X-ray crystal structures of the open-channel GLIC. MD calculations of the potential of mean force (PMF) for a Na(+) revealed two energy barriers in the extracellular domain (R109 and K38) and at the hydrophobic gate of transmembrane domain (I233), respectively. An energy well for Na(+) was near the intracellular entrance: the depth of this energy well was modulated strongly by the protonation state of E222. The energy barrier for Cl(-) was found to be 3-4 times higher than that for Na(+). Ion permeation characteristics were determined through BD simulations using a hybrid MD/continuum electrostatics approach to evaluate the energy profiles governing the ion movement. The resultant channel conductance and a near-zero permeability ratio (P(Cl)/P(Na)) were comparable to experimental data. On the basis of these calculations, we suggest that a ring of five E222 residues may act as an electrostatic gate. In addition, the hydrophobic gate region may play a role in charge selectivity due to a higher dehydration energy barrier for Cl(-) ions. The effect of halothane on the Na(+) PMF was also evaluated. Halothane was found to perturb salt bridges in GLIC that may be crucial for channel gating and open-channel stability, but had no significant impact on the single ion PMF profiles.
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Affiliation(s)
| | - Rob D. Coalson
- Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260
| | - Pei Tang
- Department of Anesthesiology, University of Pittsburgh, Pittsburgh, PA 15260
- Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA 15260
- Department of Computational Biology, University of Pittsburgh, Pittsburgh, PA 15260
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25
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Sancar F, Czajkowski C. Allosteric modulators induce distinct movements at the GABA-binding site interface of the GABA-A receptor. Neuropharmacology 2010; 60:520-8. [PMID: 21093460 DOI: 10.1016/j.neuropharm.2010.11.009] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2010] [Revised: 10/14/2010] [Accepted: 11/10/2010] [Indexed: 10/18/2022]
Abstract
Benzodiazepines (BZDs) and barbiturates exert their CNS actions by binding to GABA-A receptors (GABARs). The structural mechanisms by which these drugs allosterically modulate GABAR function, to either enhance or inhibit GABA-gated current, are poorly understood. Here, we used the substituted cysteine accessibility method to examine and compare structural movements in the GABA-binding site interface triggered by a BZD positive (flurazepam), zero (flumazenil) and negative (3-carbomethoxy-4-ethyl-6,7-dimethoxy-β-carboline, DMCM) modulator as well as the barbiturate pentobarbital. Ten residues located throughout the GABA-binding site interface were individually mutated to cysteine. Wild-type and mutant α(1)β(2)γ(2) GABARs were expressed in Xenopus laevis oocytes and functionally characterized using two-electrode voltage clamp. We measured and compared the rates of modification of the introduced cysteines by sulfhydryl-reactive methanethiosulfonate (MTS) reagents in the absence and presence of BZD-site ligands and pentobarbital. Flurazepam and DMCM each accelerated the rate of reaction at α(1)R131C and slowed the rate of reaction at α(1)E122C, whereas flumazenil had no effect indicating that simple occupation of the BZD binding site is not sufficient to cause movements near these positions. Therefore, BZD-induced movements at these residues are likely associated with the ability of the BZD to modulate GABAR function (BZD efficacy). Low, modulating concentrations of pentobarbital accelerated the rate of reaction at α(1)S68C and β(2)P206C, slowed the rate of reaction at α(1)E122C and had no effect at α(1)R131C. These findings indicate that pentobarbital and BZDs induce different movements in the receptor, providing evidence that the structural mechanisms underlying their allosteric modulation of GABAR function are distinct.
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Affiliation(s)
- Feyza Sancar
- Department of Physiology, University of Wisconsin-Madison, 601 Science Drive, Madison, WI 53711, USA
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26
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Cadugan DJ, Auerbach A. Linking the acetylcholine receptor-channel agonist-binding sites with the gate. Biophys J 2010; 99:798-807. [PMID: 20682257 DOI: 10.1016/j.bpj.2010.05.008] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2010] [Revised: 05/05/2010] [Accepted: 05/06/2010] [Indexed: 11/26/2022] Open
Abstract
The gating isomerization of neuromuscular acetylcholine receptors links the rearrangements of atoms at two transmitter-binding sites with those at a distant gate region in the pore. To explore the mechanism of this reversible process, we estimated the gating rate and equilibrium constants for receptors with point mutations of alpha-subunit residues located between the binding sites and the membrane domain (N95, A96, Y127, and I49). The maximum energy change caused by a side-chain substitution at alphaA96 was huge (approximately 8.6 kcal/mol, the largest value measured so far for any alpha-subunit amino acid). A Phi-value analysis suggests that alphaA96 experiences its change in energy (structure) approximately synchronously with residues alphaY127 and alphaI49, but after the agonist molecule and other residues in loop A. Double mutant-cycle experiments show that the energy changes at alphaA96 are strongly coupled with those of alphaY127 and alphaI49. We identify a column of mutation-sensitive residues in the alpha-subunit that may be a pathway for energy transfer through the extracellular domain in the gating isomerization.
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Affiliation(s)
- David J Cadugan
- Department of Physiology and Biophysics, State University of New York, Buffalo, New York, USA
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27
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Bartos M, Corradi J, Bouzat C. Structural basis of activation of cys-loop receptors: the extracellular-transmembrane interface as a coupling region. Mol Neurobiol 2009; 40:236-52. [PMID: 19859835 DOI: 10.1007/s12035-009-8084-x] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2009] [Accepted: 09/22/2009] [Indexed: 10/25/2022]
Abstract
Cys-loop receptors mediate rapid transmission throughout the nervous system by converting a chemical signal into an electric one. They are pentameric proteins with an extracellular domain that carries the transmitter binding sites and a transmembrane region that forms the ion pore. Their essential function is to couple the binding of the agonist at the extracellular domain to the opening of the ion pore. How the structural changes elicited by agonist binding are propagated through a distance of 50 A to the gate is therefore central for the understanding of the receptor function. A step forward toward the identification of the structures involved in gating has been given by the recently elucidated high-resolution structures of Cys-loop receptors and related proteins. The extracellular-transmembrane interface has attracted attention because it is a structural transition zone where beta-sheets from the extracellular domain merge with alpha-helices from the transmembrane domain. Within this zone, several regions form a network that relays structural changes from the binding site toward the pore, and therefore, this interface controls the beginning and duration of a synaptic response. In this review, the most recent findings on residues and pairwise interactions underlying channel gating are discussed, the main focus being on the extracellular-transmembrane interface.
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Affiliation(s)
- Mariana Bartos
- Instituto de Investigaciones Bioquímicas, UNS-CONICET, Bahía Blanca, Argentina
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