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Cetin H, Beeson D, Vincent A, Webster R. The Structure, Function, and Physiology of the Fetal and Adult Acetylcholine Receptor in Muscle. Front Mol Neurosci 2020; 13:581097. [PMID: 33013323 PMCID: PMC7506097 DOI: 10.3389/fnmol.2020.581097] [Citation(s) in RCA: 38] [Impact Index Per Article: 9.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2020] [Accepted: 08/13/2020] [Indexed: 12/31/2022] Open
Abstract
The neuromuscular junction (NMJ) is a highly developed synapse linking motor neuron activity with muscle contraction. A complex of molecular cascades together with the specialized NMJ architecture ensures that each action potential arriving at the motor nerve terminal is translated into an action potential in the muscle fiber. The muscle-type nicotinic acetylcholine receptor (AChR) is a key molecular component located at the postsynaptic muscle membrane responsible for the generation of the endplate potential (EPP), which usually exceeds the threshold potential necessary to activate voltage-gated sodium channels and triggers a muscle action potential. Two AChR isoforms are found in mammalian muscle. The fetal isoform is present in prenatal stages and is involved in the development of the neuromuscular system whereas the adult isoform prevails thereafter, except after denervation when the fetal form is re-expressed throughout the muscle. This review will summarize the structural and functional differences between the two isoforms and outline congenital and autoimmune myasthenic syndromes that involve the isoform specific AChR subunits.
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Affiliation(s)
- Hakan Cetin
- Department of Neurology, Medical University of Vienna, Vienna, Austria.,Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
| | - David Beeson
- Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
| | - Angela Vincent
- Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
| | - Richard Webster
- Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom
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2
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Progress in nicotinic receptor structural biology. Neuropharmacology 2020; 171:108086. [PMID: 32272141 DOI: 10.1016/j.neuropharm.2020.108086] [Citation(s) in RCA: 50] [Impact Index Per Article: 12.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2020] [Accepted: 03/31/2020] [Indexed: 02/07/2023]
Abstract
Here we begin by briefly reviewing landmark structural studies on the nicotinic acetylcholine receptor. We highlight challenges that had to be overcome to push through resolution barriers, then focus on what has been gleaned in the past few years from crystallographic and single particle cryo-EM studies of different nicotinic receptor subunit assemblies and ligand complexes. We discuss insights into ligand recognition, ion permeation, and allosteric gating. We then highlight some foundational aspects of nicotinic receptor structural biology that remain unresolved and are areas ripe for future exploration. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.
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3
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Turner MW, Marquart LA, Phillips PD, McDougal OM. Mutagenesis of α-Conotoxins for Enhancing Activity and Selectivity for Nicotinic Acetylcholine Receptors. Toxins (Basel) 2019; 11:E113. [PMID: 30781866 PMCID: PMC6409848 DOI: 10.3390/toxins11020113] [Citation(s) in RCA: 17] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2019] [Revised: 02/10/2019] [Accepted: 02/11/2019] [Indexed: 02/04/2023] Open
Abstract
Nicotinic acetylcholine receptors (nAChRs) are found throughout the mammalian body and have been studied extensively because of their implication in a myriad of diseases. α-Conotoxins (α-CTxs) are peptide neurotoxins found in the venom of marine snails of genus Conus. α-CTxs are potent and selective antagonists for a variety of nAChR isoforms. Over the past 40 years, α-CTxs have proven to be valuable molecular probes capable of differentiating between closely related nAChR subtypes and have contributed greatly to understanding the physiological role of nAChRs in the mammalian nervous system. Here, we review the amino acid composition and structure of several α-CTxs that selectively target nAChR isoforms and explore strategies and outcomes for introducing mutations in native α-CTxs to direct selectivity and enhance binding affinity for specific nAChRs. This review will focus on structure-activity relationship studies involving native α-CTxs that have been rationally mutated and molecular interactions that underlie binding between ligand and nAChR isoform.
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Affiliation(s)
- Matthew W Turner
- Biomolecular Sciences Graduate Programs, Boise State University; Boise, ID 83725, USA.
| | - Leanna A Marquart
- Department of Chemistry and Biochemistry, Boise State University; Boise, ID 83725, USA.
| | - Paul D Phillips
- Department of Chemistry and Biochemistry, Boise State University; Boise, ID 83725, USA.
| | - Owen M McDougal
- Department of Chemistry and Biochemistry, Boise State University; Boise, ID 83725, USA.
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4
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Liu W, Li MD. Insights Into Nicotinic Receptor Signaling in Nicotine Addiction: Implications for Prevention and Treatment. Curr Neuropharmacol 2018; 16:350-370. [PMID: 28762314 PMCID: PMC6018190 DOI: 10.2174/1570159x15666170801103009] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/17/2017] [Revised: 06/18/2017] [Accepted: 07/28/2017] [Indexed: 11/22/2022] Open
Abstract
BACKGROUND Nicotinic acetylcholine receptors (nAChRs) belong to the Cys-loop ligandgated ion-channel (LGIC) superfamily, which also includes the GABA, glycine, and serotonin receptors. Many nAChR subunits have been identified and shown to be involved in signal transduction on binding to them of either the neurotransmitter acetylcholine or exogenous ligands such as nicotine. The nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and they are expressed at neuromuscular junctions throughout the nervous system. METHODS AND RESULTS Because different nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes, and different nAChRs are implicated in various physiological functions and pathophysiological conditions, nAChRs represent potential molecular targets for drug addiction and medical therapeutic research. This review intends to provide insights into recent advances in nAChR signaling, considering the subtypes and subunits of nAChRs and their roles in nicotinic cholinergic systems, including structure, diversity, functional allosteric modulation, targeted knockout mutations, and rare variations of specific subunits, and the potency and functional effects of mutations by focusing on their effects on nicotine addiction (NA) and smoking cessation (SC). Furthermore, we review the possible mechanisms of action of nAChRs in NA and SC based on our current knowledge. CONCLUSION Understanding these cellular and molecular mechanisms will lead to better translational and therapeutic operations and outcomes for the prevention and treatment of NA and other drug addictions, as well as chronic diseases, such as Alzheimer's and Parkinson's. Finally, we put forward some suggestions and recommendations for therapy and treatment of NA and other chronic diseases.
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Affiliation(s)
- Wuyi Liu
- State Key Laboratory for Diagnosis and Treatment of Infectious Diseases, The First Affiliated Hospital, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, Zhejiang University School of Medicine, Hangzhou, China.,School of Biological Sciences and Food Engineering, Fuyang Normal University, Fuyang, Anuhi 236041, China
| | - Ming D Li
- State Key Laboratory for Diagnosis and Treatment of Infectious Diseases, The First Affiliated Hospital, Collaborative Innovation Center for Diagnosis and Treatment of Infectious Diseases, Zhejiang University School of Medicine, Hangzhou, China.,Research Center for Air Pollution and Health, Zhejiang University, Hangzhou, China.,Institute of NeuroImmune Pharmacology, Seton Hall University, South Orange, NJ, United States
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5
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Basak S, Schmandt N, Gicheru Y, Chakrapani S. Crystal structure and dynamics of a lipid-induced potential desensitized-state of a pentameric ligand-gated channel. eLife 2017; 6:23886. [PMID: 28262093 PMCID: PMC5378477 DOI: 10.7554/elife.23886] [Citation(s) in RCA: 52] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2016] [Accepted: 03/04/2017] [Indexed: 12/14/2022] Open
Abstract
Desensitization in pentameric ligand-gated ion channels plays an important role in regulating neuronal excitability. Here, we show that docosahexaenoic acid (DHA), a key ω−3 polyunsaturated fatty acid in synaptic membranes, enhances the agonist-induced transition to the desensitized state in the prokaryotic channel GLIC. We determined a 3.25 Å crystal structure of the GLIC-DHA complex in a potentially desensitized conformation. The DHA molecule is bound at the channel-periphery near the M4 helix and exerts a long-range allosteric effect on the pore across domain-interfaces. In this previously unobserved conformation, the extracellular-half of the pore-lining M2 is splayed open, reminiscent of the open conformation, while the intracellular-half is constricted, leading to a loss of both water and permeant ions. These findings, in combination with spin-labeling/EPR spectroscopic measurements in reconstituted-membranes, provide novel mechanistic details of desensitization in pentameric channels. DOI:http://dx.doi.org/10.7554/eLife.23886.001 The nerve cells (or neurons) in the brain communicate with each other by releasing chemicals called neurotransmitters that bind to ion channels on neighboring neurons. This ultimately causes ions to flow in or out of the receiving neuron through these ion channels; this ion flow determines how the neuron responds. One family of ion channels that is found at the junction between neurons, and between neurons and muscle fibers, is known as the pentameric ligand-gated ion channels (or pLGICs). These channels act as ‘gates’ that open to allow ions through them when a neurotransmitter binds to the channel. In addition to the open ‘active’ state, the channels can take on two different ‘inactive’ states that do not allow ions to pass through the channel: a closed (resting) state and a desensitized state (that is still bound to the neurotransmitter). Understanding how channels switch between these states is important for designing drugs that correct problems that cause the channels to work incorrectly. Problems that affect the desensitized state have been linked to neurological disorders such as epilepsy. Medically important molecules such as anesthetics and alcohols are thought to affect desensitization, and drugs that target desensitized ion channels may present ways of treating neurological disorders with fewer side effects. Docosahexaenoic acid (DHA) is an abundant lipid molecule that is present in the membranes of neurons. It is one of the key ingredients in fish oil supplements and is thought to enhance learning and memory. DHA affects the desensitization of pLGICs but it is not clear exactly how it does so. Basak et al. now show that DHA affects a bacterial pLGIC in the same way as it affects human channels – by enhancing desensitization. Using a technique called X-ray crystallography to analyze the channel while bound to DHA revealed a previously unobserved channel structure. The DHA molecule binds to a site at the edge of the channel and causes a change in its structure that leaves the upper part of the channel open while the lower part is constricted. Basak et al. predict that molecules such as anesthetics target this desensitized state. The next step will be to obtain the structures of bacterial and human pLGIC channels in a natural membrane environment. This will allow us to better understand the changes in structure that the channels go through as they transmit signals between neurons, and so help in the development of new treatments for neurological disorders. DOI:http://dx.doi.org/10.7554/eLife.23886.002
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Affiliation(s)
- Sandip Basak
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Nicolaus Schmandt
- Department of Neuroscience, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Yvonne Gicheru
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
| | - Sudha Chakrapani
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, United States
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6
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Matthies D, Dalmas O, Borgnia MJ, Dominik PK, Merk A, Rao P, Reddy BG, Islam S, Bartesaghi A, Perozo E, Subramaniam S. Cryo-EM Structures of the Magnesium Channel CorA Reveal Symmetry Break upon Gating. Cell 2016; 164:747-56. [PMID: 26871634 DOI: 10.1016/j.cell.2015.12.055] [Citation(s) in RCA: 94] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2015] [Revised: 11/10/2015] [Accepted: 12/23/2015] [Indexed: 11/30/2022]
Abstract
CorA, the major Mg(2+) uptake system in prokaryotes, is gated by intracellular Mg(2+) (KD ∼ 1-2 mM). X-ray crystallographic studies of CorA show similar conformations under Mg(2+)-bound and Mg(2+)-free conditions, but EPR spectroscopic studies reveal large Mg(2+)-driven quaternary conformational changes. Here, we determined cryo-EM structures of CorA in the Mg(2+)-bound closed conformation and in two open Mg(2+)-free states at resolutions of 3.8, 7.1, and 7.1 Å, respectively. In the absence of bound Mg(2+), four of the five subunits are displaced to variable extents (∼ 10-25 Å) by hinge-like motions as large as ∼ 35° at the stalk helix. The transition between a single 5-fold symmetric closed state and an ensemble of low Mg(2+), open, asymmetric conformational states is, thus, the key structural signature of CorA gating. This mechanism is likely to apply to other structurally similar divalent ion channels.
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Affiliation(s)
- Doreen Matthies
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
| | - Olivier Dalmas
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL 60637, USA
| | - Mario J Borgnia
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
| | - Pawel K Dominik
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL 60637, USA
| | - Alan Merk
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
| | - Prashant Rao
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
| | - Bharat G Reddy
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL 60637, USA
| | - Shahidul Islam
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL 60637, USA
| | - Alberto Bartesaghi
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA
| | - Eduardo Perozo
- Department of Biochemistry and Molecular Biology, Institute for Biophysical Dynamics, The University of Chicago, Chicago, IL 60637, USA.
| | - Sriram Subramaniam
- Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, NIH, Bethesda, MD 20892, USA.
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7
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Tavoulari S, Margheritis E, Nagarajan A, DeWitt DC, Zhang YW, Rosado E, Ravera S, Rhoades E, Forrest LR, Rudnick G. Two Na+ Sites Control Conformational Change in a Neurotransmitter Transporter Homolog. J Biol Chem 2015; 291:1456-71. [PMID: 26582198 DOI: 10.1074/jbc.m115.692012] [Citation(s) in RCA: 50] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/11/2015] [Indexed: 12/13/2022] Open
Abstract
In LeuT, a prokaryotic homolog of neurotransmitter transporters, Na(+) stabilizes outward-open conformational states. We examined how each of the two LeuT Na(+) binding sites contributes to Na(+)-dependent closure of the cytoplasmic pathway using biochemical and biophysical assays of conformation. Mutating either of two residues that contribute to the Na2 site completely prevented cytoplasmic closure in response to Na(+), suggesting that Na2 is essential for this conformational change, whereas Na1 mutants retained Na(+) responsiveness. However, mutation of Na1 residues also influenced the Na(+)-dependent conformational change in ways that varied depending on the position mutated. Computational analyses suggest those mutants influence the ability of Na1 binding to hydrate the substrate pathway and perturb an interaction network leading to the extracellular gate. Overall, the results demonstrate that occupation of Na2 stabilizes outward-facing conformations presumably through a direct interaction between Na(+) and transmembrane helices 1 and 8, whereas Na(+) binding at Na1 influences conformational change through a network of intermediary interactions. The results also provide evidence that N-terminal release and helix motions represent distinct steps in cytoplasmic pathway opening.
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Affiliation(s)
| | | | - Anu Nagarajan
- the Computational Structural Biology Section, NINDS, National Institutes of Health, Rockville, Maryland 20852
| | - David C DeWitt
- Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520 and
| | | | | | | | - Elizabeth Rhoades
- Molecular Biophysics and Biochemistry, Yale University, New Haven, Connecticut 06520 and
| | - Lucy R Forrest
- the Computational Structural Biology Section, NINDS, National Institutes of Health, Rockville, Maryland 20852
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8
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Kinde MN, Chen Q, Lawless MJ, Mowrey DD, Xu J, Saxena S, Xu Y, Tang P. Conformational Changes Underlying Desensitization of the Pentameric Ligand-Gated Ion Channel ELIC. Structure 2015; 23:995-1004. [PMID: 25960405 DOI: 10.1016/j.str.2015.03.017] [Citation(s) in RCA: 32] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2014] [Revised: 03/24/2015] [Accepted: 03/30/2015] [Indexed: 11/18/2022]
Abstract
Structural rearrangements underlying functional transitions of pentameric ligand-gated ion channels (pLGICs) are not fully understood. Using (19)F nuclear magnetic resonance and electron spin resonance spectroscopy, we found that ELIC, a pLGIC from Erwinia chrysanthemi, expanded the extracellular end and contracted the intracellular end of its pore during transition from the resting to an apparent desensitized state. Importantly, the contraction at the intracellular end of the pore likely forms a gate to restrict ion transport in the desensitized state. This gate differs from the hydrophobic gate present in the resting state. Conformational changes of the TM2-TM3 loop were limited to the N-terminal end. The TM4 helices and the TM3-TM4 loop appeared relatively insensitive to agonist-mediated structural rearrangement. These results indicate that conformational changes accompanying functional transitions are not uniform among different ELIC regions. This work also revealed the co-existence of multiple conformations for a given state and suggested asymmetric conformational arrangements in a homomeric pLGIC.
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Affiliation(s)
- Monica N Kinde
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Qiang Chen
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Matthew J Lawless
- Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - David D Mowrey
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Computational & Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Jiawei Xu
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Sunil Saxena
- Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - Yan Xu
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Pei Tang
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Computational & Systems Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA.
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9
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Forman SA, Chiara DC, Miller KW. Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:169-77. [PMID: 25316107 DOI: 10.1016/j.neuropharm.2014.10.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 09/22/2014] [Accepted: 10/02/2014] [Indexed: 11/25/2022]
Abstract
General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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Affiliation(s)
- Stuart A Forman
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - David C Chiara
- Dept. of Neurobiology, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - Keith W Miller
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
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10
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Divito CB, Underhill SM. Excitatory amino acid transporters: roles in glutamatergic neurotransmission. Neurochem Int 2014; 73:172-80. [PMID: 24418112 DOI: 10.1016/j.neuint.2013.12.008] [Citation(s) in RCA: 51] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/18/2013] [Revised: 12/20/2013] [Accepted: 12/23/2013] [Indexed: 01/04/2023]
Abstract
Excitatory amino acid transporters or EAATs are the major transport mechanism for extracellular glutamate in the nervous system. This family of five carriers not only displays an impressive ability to regulate ambient extracellular glu concentrations but also regulate the temporal and spatial profile of glu after vesicular release. This dynamic form of regulation mediates several characteristic of synaptic, perisynaptic, and spillover activation of ionotropic and metabotropic receptors. EAATs function through a secondary active, electrogenic process but also possess a thermodynamically uncoupled ligand gated anion channel activity, both of which have been demonstrated to play a role in regulation of cellular activity. This review will highlight the inception of EAATs as a focus of research, the transport and channel functionality of the carriers, and then describe how these properties are used to regulate glutamatergic neurotransmission.
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Affiliation(s)
- Christopher B Divito
- Center for Neuroscience, Department of Neurobiology, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15261, United States
| | - Suzanne M Underhill
- Laboratory of Cellular and Molecular Neuroscience, National Institute of Mental Health, National Institute of Health, Bethesda, MD 20892, United States.
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11
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Abstract
Ion channels, as membrane proteins, are the sensors of the cell. They act as the first line of communication with the world beyond the plasma membrane and transduce changes in the external and internal environments into unique electrical signals to shape the responses of excitable cells. Because of their importance in cellular communication, ion channels have been intensively studied at the structural and functional levels. Here, we summarize the diverse approaches, including molecular and cellular, chemical, optical, biophysical, and computational, used to probe the structural and functional rearrangements that occur during channel activation (or sensitization), inactivation (or desensitization), and various forms of modulation. The emerging insights into the structure and function of ion channels by multidisciplinary approaches allow the development of new pharmacotherapies as well as new tools useful in controlling cellular activity.
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Affiliation(s)
- Wei-Guang Li
- Neuroscience Division, Department of Biochemistry and Molecular Cell Biology, Institute of Medical Sciences, Shanghai Jiao Tong University School of Medicine, Shanghai 200025, China
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12
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Velisetty P, Chalamalasetti SV, Chakrapani S. Conformational transitions underlying pore opening and desensitization in membrane-embedded Gloeobacter violaceus ligand-gated ion channel (GLIC). J Biol Chem 2012; 287:36864-72. [PMID: 22977232 DOI: 10.1074/jbc.m112.401067] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022] Open
Abstract
Direct structural insight into the mechanisms underlying activation and desensitization remain unavailable for the pentameric ligand-gated channel family. Here, we report the structural rearrangements underlying gating transitions in membrane-embedded GLIC, a prokaryotic homologue, using site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy. We particularly probed the conformation of pore-lining second transmembrane segment (M2) under conditions that favor the closed and the ligand-bound desensitized states. The spin label mobility, intersubunit spin-spin proximity, and the solvent-accessibility parameters in the two states clearly delineate the underlying protein motions within M2. Our results show that during activation the extracellular hydrophobic region undergoes major changes involving an outward translational movement, away from the pore axis, leading to an increase in the pore diameter, whereas the lower end of M2 remains relatively immobile. Most notably, during desensitization, the intervening polar residues in the middle of M2 move closer to form a solvent-occluded barrier and thereby reveal the location of a distinct desensitization gate. In comparison with the crystal structure of GLIC, the structural dynamics of the channel in a membrane environment suggest a more loosely packed conformation with water-accessible intrasubunit vestibules penetrating from the extracellular end all the way to the middle of M2 in the closed state. These regions have been implicated to play a major role in alcohol and drug modulation. Overall, these findings represent a key step toward understanding the fundamentals of gating mechanisms in this class of channels.
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Affiliation(s)
- Phanindra Velisetty
- Department of Physiology and Biophysics, School of Medicine, Case Western Reserve University, Cleveland, Ohio 44106, USA
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13
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Gündisch D, Eibl C. Nicotinic acetylcholine receptor ligands, a patent review (2006-2011). Expert Opin Ther Pat 2012; 21:1867-96. [PMID: 22098319 DOI: 10.1517/13543776.2011.637919] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
INTRODUCTION Nicotinic acetylcholine receptors (nAChRs), pentameric ligand-gated cation channels, are potential targets for the development of therapeutics for a variety of disease states. AREAS COVERED This article is reviewing recent advances in the development of small-molecule ligands for diverse nAChR subtypes and is a continuation of an earlier review in this journal. EXPERT OPINION The development of nAChR ligands with preference for α4β2 or α7 subtypes for the treatment of central nervous system disorders are in the most advanced developmental stage. In addition, there is a fast growing interest to generate so-called PAMs, positive allosteric modulators, to influence the channels' functionalities.
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Affiliation(s)
- Daniela Gündisch
- University of Hawai'i at Hilo, College of Pharmacy, Department of Pharmaceutical Sciences, 34 Rainbow Drive, Hilo, HI 96720, USA.
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14
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Ambrus JI, Halliday JI, Kanizaj N, Absalom N, Harpsøe K, Balle T, Chebib M, McLeod MD. Covalent attachment of antagonists to the α7 nicotinic acetylcholine receptor: synthesis and reactivity of substituted maleimides. Chem Commun (Camb) 2012; 48:6699-701. [DOI: 10.1039/c2cc32442c] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
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15
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Othman NA, Gallacher M, Deeb TZ, Baptista-Hon DT, Perry DC, Hales TG. Influences on blockade by t-butylbicyclo-phosphoro-thionate of GABA(A) receptor spontaneous gating, agonist activation and desensitization. J Physiol 2011; 590:163-78. [PMID: 22083597 DOI: 10.1113/jphysiol.2011.213249] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Picrotoxin and t-butylbicyclophosphorothionate (TBPS) are GABA(A) receptor (GABA(A)R) open channel blockers. However, picrotoxin displaceable [(35)S]TBPS binding to α1β2γ2 GABA(A)Rs occurs in the absence of GABA, suggesting that access to the binding site is independent of activation. Alternatively, spontaneous gating may provide access to the channel. In the absence of episodic GABA application, picrotoxin and TBPS blocked (by 91 ± 3% and 85 ± 5%, respectively) GABA-evoked currents mediated by α1β2γ2 receptors. We used two approaches to inhibit spontaneous GABA(A)R gating, bicuculline, which inhibits spontaneous current in the absence of exogenous agonist and the α1(K278M) mutant subunit. Whole-cell patch-clamp recordings demonstrated that α1(K278M)β2γ2 receptors have negligible spontaneous gating. Application of bicuculline to α1β2γ2 receptors in the absence of exogenous GABA caused a 35% reduction of current blockade by TBPS and reduced [(35)S]TBPS binding by 25%. Consistent with this, in the absence of exogenous GABA, α1(K278M)β2γ2 receptors exhibited reduced blockade by TBPS current compared to wild-type receptors. These data suggest that a decrease in spontaneous gating reduces accessibility of TBPS to its binding site. GABA application during picrotoxin or TBPS administration enhanced α1β2γ2 receptor blockade (to 98% in both cases). The GABA-dependent component of TBPS blockade accounts for the stimulation of [(35)S]TBPS binding to α1β2γ2 receptors seen with GABA (1 μm) application. Moreover, application of GABA at concentrations that cause significant steady-state desensitization reduced [(35)S]TBPS binding. The α1(K278M) subunit slowed desensitization kinetics and increased the rate of deactivation of GABA-evoked currents. Furthermore, there was a marked increase in the GABA EC(50) for desensitization of α1(K278M)β2γ2 receptors associated with a large increase in the GABA-dependent stimulation of [(35)S]TBPS binding. These data establish a relationship between GABA(A)R function and the three phases of [(35)S]TBPS binding seen in the absence and the presence of GABA.
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Affiliation(s)
- Nidaa A Othman
- The Institute of Academic Anaesthesia, Division of Neuroscience, University of Dundee, Dundee DD1 9SY, UK
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Nusair NA, Mayo DJ, Dorozenski TD, Cardon TB, Inbaraj JJ, Karp ES, Newstadt JP, Grosser SM, Lorigan GA. Time-resolved EPR immersion depth studies of a transmembrane peptide incorporated into bicelles. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2011; 1818:821-8. [PMID: 22100865 DOI: 10.1016/j.bbamem.2011.11.009] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/26/2011] [Revised: 10/31/2011] [Accepted: 11/02/2011] [Indexed: 11/25/2022]
Abstract
The reduction in EPR signal intensity of nitroxide spin-labels by ascorbic acid has been measured as a function of time to investigate the immersion depth of the spin-labeled M2δ AChR peptide incorporated into a bicelle system utilizing EPR spectroscopy. The corresponding decay curves of n-DSA (n=5, 7, 12, and 16) EPR signals have been used to (1) calibrate the depth of the bicelle membrane and (2) establish a calibration curve for measuring the depth of spin-labeled transmembrane peptides. The kinetic EPR data of CLS, n-DSA (n=5, 7, 12, and 16), and M2δ AChR peptide spin-labeled at Glu-1 and Ala-12 revealed excellent exponential and linear fits. For a model M2δ AChR peptide, the depth of immersion was calculated to be 5.8Å and 3Å for Glu-1, and 21.7Å and 19Å for Ala-12 in the gel-phase (298K) and L(α)-phases (318K), respectively. The immersion depth values are consistent with the pitch of an α-helix and the structural model of M2δ AChR incorporated into the bicelle system is in a good agreement with previous studies. Therefore, this EPR time-resolved kinetic technique provides a new reliable method to determine the immersion depth of membrane-bound peptides, as well as, explore the structural characteristics of the M2δ AChR peptide.
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17
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Wang Q, Lynch JW. Activation and desensitization induce distinct conformational changes at the extracellular-transmembrane domain interface of the glycine receptor. J Biol Chem 2011; 286:38814-38824. [PMID: 21917927 DOI: 10.1074/jbc.m111.273631] [Citation(s) in RCA: 26] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Most ligand-gated channels exhibit desensitization, which is the progressive fading of ionic current in the prolonged presence of agonist. This process involves conformational changes that close the channel despite continued agonist binding. Despite the physiological and pathological importance of desensitization, little is known about the conformational changes that underlie this process in any Cys-loop ion channel receptor. Here we employed voltage clamp fluorometry to identify conformational changes that occur with a similar time course as the current desensitization rate in both slow- and fast-desensitizing α1 glycine receptor chloride channels. Voltage clamp fluorometry provides a direct indication of conformational changes that occur in the immediate vicinity of residues labeled with environmentally sensitive fluorophores. We compared the rates of current desensitization and fluorescence changes at nine labeled extracellular sites in both wild type slow-desensitizing and mutated (A248L) fast-desensitizing glycine receptors. As labels attached to three sites at the interface between the ligand binding domain and transmembrane domain reported fluorescence responses that changed in parallel with the current desensitization rate, we concluded that they experienced local conformational changes associated with desensitization. These labeled sites included A52C in loop 2, Q219C in the pre-M1 domain, and M227C in the M1 domain. Activation and desensitization were accompanied by physically distinct conformational changes at each labeled site. Because activation is mediated by a specific reorganization of molecular interactions at the extracellular-transmembrane domain interface, we propose that desensitization is mediated by a distinct set of conformational changes that prevents this reorganization from occurring, thereby favoring channel closure.
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Affiliation(s)
- Qian Wang
- Queensland Brain Institute and School of Biomedical Sciences, University of Queensland, Brisbane QLD 4072, Australia
| | - Joseph W Lynch
- Queensland Brain Institute and School of Biomedical Sciences, University of Queensland, Brisbane QLD 4072, Australia.
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18
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Taly A, Colas C, Malliavin T, Blondel A, Nilges M, Corringer PJ, Joseph D. Discrimination of agonists versus antagonists of nicotinic ligands based on docking onto AChBP structures. J Mol Graph Model 2011; 30:100-9. [PMID: 21764343 DOI: 10.1016/j.jmgm.2011.06.008] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2011] [Revised: 05/27/2011] [Accepted: 06/22/2011] [Indexed: 11/26/2022]
Abstract
Numerous high-resolution crystallographic structures of the acetylcholine binding protein (AChBP), a molluscan cholinergic protein, homologous to the extracellular domain of nicotinic acetylcholine receptors, are available. This offers opportunities to model the interaction between various ligands and the acetylcholine binding site. Herein we present a study of the interplay between ligand binding and motions of the C-loop capping the binding site. Nicotinic agonists and antagonists were docked on AChBP X-ray structures. It is shown that the studied agonists and antagonists can be discriminated according to their higher affinities for structures respectively obtained in the presence of agonists or antagonists, highlighting the fact that AChBP structures retain a pharmacological footprint of the compound used in crystallography experiments. A detailed analysis of the binding site cavities suggests that this property is mainly related to the shape of the cavities.
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Affiliation(s)
- Antoine Taly
- Laboratoire de Conception et Application de Molécules Bioactives, UMR 7199 CNRS-Université de Strasbourg, 74 Route du Rhin-BP 60024, 67401 Illkirch Cedex, France.
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19
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Zimmermann I, Dutzler R. Ligand activation of the prokaryotic pentameric ligand-gated ion channel ELIC. PLoS Biol 2011; 9:e1001101. [PMID: 21713033 PMCID: PMC3119659 DOI: 10.1371/journal.pbio.1001101] [Citation(s) in RCA: 89] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/17/2011] [Accepted: 05/31/2011] [Indexed: 11/18/2022] Open
Abstract
While the pentameric ligand-gated ion channel ELIC has recently provided first insight into the architecture of the family at high resolution, its detailed investigation was so far prevented by the fact that activating ligands were unknown. Here we describe a study on the functional characterization of ELIC by electrophysiology and X-ray crystallography. ELIC is activated by a class of primary amines that include the neurotransmitter GABA at high micro- to millimolar concentrations. The ligands bind to a conserved site and evoke currents that slowly desensitize over time. The protein forms cation selective channels with properties that resemble the nicotinic acetylcholine receptor. The high single channel conductance and the comparably simple functional behavior make ELIC an attractive model system to study general mechanisms of ion conduction and gating in this important family of neurotransmitter receptors.
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Affiliation(s)
- Iwan Zimmermann
- Department of Biochemistry, University of Zurich, Zurich, Switzerland
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20
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Zhang J, Xue F, Whiteaker P, Li C, Wu W, Shen B, Huang Y, Lukas RJ, Chang Y. Desensitization of alpha7 nicotinic receptor is governed by coupling strength relative to gate tightness. J Biol Chem 2011; 286:25331-40. [PMID: 21610071 DOI: 10.1074/jbc.m111.221754] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
Binding of a neurotransmitter to its membrane receptor opens an integral ion conducting pore. However, prolonged exposure to the neurotransmitter drives the receptor to a refractory state termed desensitization, which plays an important role in shaping synaptic transmission. Despite intensive research in the past, the structural mechanism of desensitization is still elusive. Using mutagenesis and voltage clamp in an oocyte expression system, we provide several lines of evidence supporting a novel hypothesis that uncoupling between binding and gating machinery is the underlying mechanism for α7 nicotinic receptor (nAChR) desensitization. First, the decrease in gate tightness was highly correlated to the reduced desensitization. Second, nonfunctional mutants in three important coupling loops (loop 2, loop 7, and the M2-M3 linker) could be rescued by a gating mutant. Furthermore, the decrease in coupling strength in these rescued coupling loop mutants reversed the gating effect on desensitization. Finally, coupling between M1 and hinge region of the M2-M3 linker also influenced the receptor desensitization. Thus, the uncoupling between N-terminal domain and transmembrane domain, governed by the balance of coupling strength and gate tightness, underlies the mechanism of desensitization for the α7 nAChR.
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Affiliation(s)
- Jianliang Zhang
- Division of Neurobiology, Barrow Neurological Institute and St. Joseph's Hospital and Medical Center, Phoenix, Arizona 85013, USA
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21
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Tolino LA, Okumura S, Kashlan OB, Carattino MD. Insights into the mechanism of pore opening of acid-sensing ion channel 1a. J Biol Chem 2011; 286:16297-307. [PMID: 21388961 DOI: 10.1074/jbc.m110.202366] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
Acid-sensing ion channels (ASICs) are trimeric cation channels that undergo activation and desensitization in response to extracellular acidification. The underlying mechanism coupling proton binding in the extracellular region to pore gating is unknown. Here we probed the reactivity toward methanethiosulfonate (MTS) reagents of channels with cysteine-substituted residues in the outer vestibule of the pore of ASIC1a. We found that positively-charged MTS reagents trigger pore opening of G428C. Scanning mutagenesis of residues in the region preceding the second transmembrane spanning domain indicated that the MTSET-modified side chain of Cys at position 428 interacts with Tyr-424. This interaction was confirmed by double-mutant cycle analysis. Strikingly, Y424C-G428C monomers were associated by intersubunit disulfide bonds and were insensitive to MTSET. Despite the spatial constraints introduced by these intersubunit disulfide bonds in the outer vestibule of the pore, Y424C-G428C transitions between the resting, open, and desensitized states in response to extracellular acidification. This finding suggests that the opening of the ion conductive pathway involves coordinated rotation of the second transmembrane-spanning domains.
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Affiliation(s)
- Lindsey A Tolino
- Renal-Electrolyte Division, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
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22
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Pittel I, Witt-Kehati D, Degani-Katzav N, Paas Y. Probing pore constriction in a ligand-gated ion channel by trapping a metal ion in the pore upon agonist dissociation. J Biol Chem 2010; 285:26519-31. [PMID: 20466725 PMCID: PMC2924088 DOI: 10.1074/jbc.m110.102327] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2010] [Revised: 04/24/2010] [Indexed: 12/16/2022] Open
Abstract
Eukaryotic pentameric ligand-gated ion channels (pLGICs) are receptors activated by neurotransmitters to rapidly transport ions across cell membranes, down their electrochemical gradients. Recent crystal structures of two prokaryotic pLGICs were interpreted to imply that the extracellular side of the transmembrane pore constricts to close the channel (Hilf, R. J., and Dutzler, R. (2009) Nature 457, 115-118; Bocquet, N., Nury, H., Baaden, M., Le Poupon, C., Changeux, J. P., Delarue, M., and Corringer, P. J. (2009) Nature 457, 111-114). Here, we utilized a eukaryotic acetylcholine (ACh)-serotonin chimeric pLGIC that was engineered with histidines to coordinate a metal ion within the channel pore, at its cytoplasmic side. In a previous study, the access of Zn(2+) ions to the engineered histidines had been explored when the channel was either at rest (closed) or active (open) (Paas, Y., Gibor, G., Grailhe, R., Savatier-Duclert, N., Dufresne, V., Sunesen, M., de Carvalho, L. P., Changeux, J. P., and Attali, B. (2005) Proc. Natl. Acad. Sci. U.S.A. 102, 15877-15882). In this study, the interactions of Zn(2+) with the pore were probed upon agonist (ACh) dissociation that triggers the transition of the receptor from the active conformation to the resting conformation (i.e. during deactivation). Application of Zn(2+) onto ACh-bound open receptors obstructed their pore and prevented ionic flow. Removing ACh from its extracellular binding sites to trigger deactivation while Zn(2+) is still bound led to tight trapping of Zn(2+) within the pore. Together with single-channel recordings, made to explore single pore-blocking events, we show that dissociation of ACh causes the gate to shut on a Zn(2+) ion that effectively acts as a "foot in the door." We infer that, upon deactivation, the cytoplasmic side of the pore of the ACh-serotonin receptor chimera constricts to close the channel.
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Affiliation(s)
- Ilya Pittel
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Dvora Witt-Kehati
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Nurit Degani-Katzav
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
| | - Yoav Paas
- From the Laboratory of Ion Channels, The Mina and Everard Goodman Faculty of Life Sciences, Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat Gan 52900, Israel
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Heiny JA, Kravtsova VV, Mandel F, Radzyukevich TL, Benziane B, Prokofiev AV, Pedersen SE, Chibalin AV, Krivoi II. The nicotinic acetylcholine receptor and the Na,K-ATPase alpha2 isoform interact to regulate membrane electrogenesis in skeletal muscle. J Biol Chem 2010; 285:28614-26. [PMID: 20595385 DOI: 10.1074/jbc.m110.150961] [Citation(s) in RCA: 47] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The nicotinic acetylcholine receptor (nAChR) and the Na,K-ATPase functionally interact in skeletal muscle (Krivoi, I. I., Drabkina, T. M., Kravtsova, V. V., Vasiliev, A. N., Eaton, M. J., Skatchkov, S. N., and Mandel, F. (2006) Pflugers Arch. 452, 756-765; Krivoi, I., Vasiliev, A., Kravtsova, V., Dobretsov, M., and Mandel, F. (2003) Ann. N.Y. Acad. Sci. 986, 639-641). In this interaction, the specific binding of nanomolar concentrations of nicotinic agonists to the nAChR stimulates electrogenic transport by the Na,K-ATPase alpha2 isozyme, causing membrane hyperpolarization. This study examines the molecular nature and membrane localization of this interaction. Stimulation of Na,K-ATPase activity by the nAChR does not require ion flow through open nAChRs. It can be induced by nAChR desensitization alone, in the absence of nicotinic agonist, and saturates when the nAChR is fully desensitized. It is enhanced by noncompetitive blockers of the nAChR (proadifen, QX-222), which promote non-conducting or desensitized states; and retarded by tetracaine, which stabilizes the resting nAChR conformation. The interaction operates at the neuromuscular junction as well as on extrajunctional sarcolemma. The Na,K-ATPase alpha2 isozyme is enriched at the postsynaptic neuromuscular junction and co-localizes with nAChRs. The nAChR and Na,K-ATPase alpha subunits specifically coimmunoprecipitate with each other, phospholemman, and caveolin-3. In a purified membrane preparation from Torpedo californica enriched in nAChRs and the Na,K-ATPase, a ouabain-induced conformational change of the Na,K-ATPase enhances a conformational transition of the nAChR to a desensitized state. These results suggest a mechanism by which the nAChR in a desensitized state with high apparent affinity for agonist interacts with the Na,K-ATPase to stimulate active transport. The interaction utilizes a membrane-delimited complex involving protein-protein interactions, either directly or through additional protein partners. This interaction is expected to enhance neuromuscular transmission and muscle excitation.
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Affiliation(s)
- Judith A Heiny
- Department of General Physiology, St. Petersburg State University, St. Petersburg 199034, Russia
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24
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Abstract
The New Anticonvulsant Retigabine Favors Voltage-dependent Opening of the Kv7.2 (KCNQ2) Channel by Binding to Its Activation Gate Wuttke TV, Seebohm G, Bail S, Maljevic S, Lerche H Mol Pharmacol 2005;67:1009–1017 Retigabine (RTG) is an anticonvulsant drug with a novel mechanism of action. It activates neuronal KCNQ-type K+ channels by inducing a large hyperpolarizing shift of steady-state activation. To identify the structural determinants of KCNQ channel activation by RTG, we constructed a set of chimeras by using the neuronal KV7.2 ( KCNQ2) channel, which is activated by RTG, and the cardiac KV7.1 ( KCNQ1) channel, which is not affected by this drug. Substitution of either the S5 or the S6 segment in KV7.2 by the respective parts of KV7.1 led to a complete loss of activation by RTG. Trp236 in the cytoplasmic part of S5 and the conserved Gly301 in S6 (KV7.2), considered as the gating hinge (Ala336 in KV7.1), were found to be crucial for the RTG effect: mutation of these residues could either knock out the effect in KV7.2 or restore it partially in KV7.1/KV7.2 chimeras. We propose that RTG binds to a hydrophobic pocket formed upon channel opening between the cytoplasmic parts of S5 and S6 involving Trp236 and the channel's gate, which could well explain the strong shift in voltage-dependent activation.
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Feng HJ, Macdonald RL. Barbiturates require the N terminus and first transmembrane domain of the delta subunit for enhancement of alpha1beta3delta GABAA receptor currents. J Biol Chem 2010; 285:23614-21. [PMID: 20525684 DOI: 10.1074/jbc.m110.122564] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
GABA(A) receptors are composed predominantly of alphabetagamma receptors, which mediate primarily synaptic inhibition, and alphabetadelta receptors, which mediate primarily extrasynaptic inhibition. At saturating GABA concentrations, the barbiturate pentobarbital substantially increased the amplitude and desensitization of the alpha1beta3delta receptor but not the alpha1beta3gamma2L receptor currents. To explore the structural domains of the delta subunit that are involved in pentobarbital potentiation and increased desensitization of alpha1beta3delta currents, chimeric cDNAs were constructed by progressive replacement of gamma2L subunit sequence with a delta subunit sequence or a delta subunit sequence with a gamma2L subunit sequence, and HEK293T cells were co-transfected with alpha1 and beta3 subunits or alpha1 and beta3 subunits and a gamma2L, delta, or chimeric subunit. Currents evoked by a saturating concentration of GABA or by co-application of GABA and pentobarbital were recorded using the patch clamp technique. By comparing the extent of enhancement and changes in kinetic properties produced by pentobarbital among chimeric and wild type receptors, we concluded that although potentiation of alpha1beta3delta currents by pentobarbital required the delta subunit sequence from the N terminus to proline 241 in the first transmembrane domain (M1), increasing desensitization of alpha1beta3delta currents required a delta subunit sequence from the N terminus to isoleucine 235 in M1. These findings suggest that the delta subunit N terminus and N-terminal portion of the M1 domain are, at least in part, involved in transduction of the allosteric effect of pentobarbital to enhance alpha1beta3delta currents and that this effect involves a distinct but overlapping structural domain from that involved in altering desensitization.
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Affiliation(s)
- Hua-Jun Feng
- Department of Neurology, Vanderbilt University Medical Center, Nashville, TN 37232, USA.
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McCormack T, Petrovich RM, Mercier KA, DeRose EF, Cuneo MJ, Williams J, Johnson KL, Lamb PW, London RE, Yakel JL. Identification and functional characterization of a novel acetylcholine-binding protein from the marine annelid Capitella teleta. Biochemistry 2010; 49:2279-87. [PMID: 20136097 DOI: 10.1021/bi902023y] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
We identified a homologue of the molluscan acetylcholine-binding protein (AChBP) in the marine polychaete Capitella teleta, from the annelid phylum. The amino acid sequence of C. teleta AChBP (ct-AChBP) is 21-30% identical with those of known molluscan AChBPs. Sequence alignments indicate that ct-AChBP has a shortened Cys loop compared to other Cys loop receptors, and a variation on a conserved Cys loop triad, which is associated with ligand binding in other AChBPs and nicotinic ACh receptor (nAChR) alpha subunits. Within the D loop of ct-AChBP, a conserved aromatic residue (Tyr or Trp) in nAChRs and molluscan AChBPs, which has been implicated directly in ligand binding, is substituted with an isoleucine. Mass spectrometry results indicate that Asn122 and Asn216 of ct-AChBP are glycosylated when expressed using HEK293 cells. Small-angle X-ray scattering data suggest that the overall shape of ct-AChBP in the apo or unliganded state is similar to that of homologues with known pentameric crystal structures. NMR experiments show that acetylcholine, nicotine, and alpha-bungarotoxin bind to ct-AChBP with high affinity, with K(D) values of 28.7 microM, 209 nM, and 110 nM, respectively. Choline bound with a lower affinity (K(D) = 163 microM). Our finding of a functional AChBP in a marine annelid demonstrates that AChBPs may exhibit variations in hallmark motifs such as ligand-binding residues and Cys loop length and shows conclusively that this neurotransmitter binding protein is not limited to the phylum Mollusca.
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Affiliation(s)
- Thomas McCormack
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, P.O. Box 12233, Research Triangle Park, North Carolina 27709, USA
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Liu LT, Haddadian EJ, Willenbring D, Xu Y, Tang P. Higher susceptibility to halothane modulation in open- than in closed-channel alpha4beta2 nAChR revealed by molecular dynamics simulations. J Phys Chem B 2010; 114:626-32. [PMID: 20014754 DOI: 10.1021/jp908944e] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
The neuronal alpha4beta2 nicotinic acetylcholine receptor (nAChR) is a potential molecular target for general anesthetics. It is unclear, however, whether anesthetic action produces the same effect on the open and closed channels. Computations parallel to our previous open channel study (J. Phys. Chem. B 2009, 113, 12581) were performed on the closed-channel alpha4beta2 nAChR to investigate the conformation-dependent anesthetic effects on channel structures and dynamics. Flexible ligand docking and over 20 ns molecular dynamics simulations revealed similar halothane-binding sites in the closed and open channels. The sites with relatively high binding affinities (approximately -6.0 kcal/mol) were identified at the interface of extracellular (EC) and transmembrane (TM) domains or at the interface between alpha4 and beta2 subunits. Despite similar sites for halothane binding, the closed-channel conformation showed much less sensitivity than the open channel to the structural and dynamical perturbations from halothane. Compared to the systems without anesthetics, the amount of water inside the pore decreased by 22% in the presence of halothane in the open channel but only by 6% in the closed channel. Comparison of the nonbonded interactions at the EC/TM interfaces suggested that the beta2 subunits were more prone than the alpha4 subunits to halothane binding. In addition, our data support the notion that halothane exerts its effect by disturbing the quaternary structure and dynamics of the channel. The study concludes that sensitivity and global dynamics responsiveness of alpha4beta2 nAChR to halothane are conformation dependent. The effect of halothane on the global dynamics of the open-channel conformation might also account for the action of other inhaled general anesthetics.
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Affiliation(s)
- Lu Tian Liu
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261, USA
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Pantoja R, Rodriguez EA, Dibas MI, Dougherty DA, Lester HA. Single-molecule imaging of a fluorescent unnatural amino acid incorporated into nicotinic receptors. Biophys J 2010; 96:226-37. [PMID: 19134478 DOI: 10.1016/j.bpj.2008.09.034] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2008] [Accepted: 09/29/2008] [Indexed: 10/21/2022] Open
Abstract
We report on the first, to our knowledge, successful detection of a fluorescent unnatural amino acid (fUAA), Lys(BODIPYFL), incorporated into a membrane protein (the muscle nicotinic acetylcholine receptor, nAChR) in a living cell. Xenopus oocytes were injected with a frameshift-suppressor tRNA, amino-acylated with Lys(BODIPYFL) and nAChR (alpha/beta19'GGGU/gamma/delta) mRNAs. We measured fluorescence from oocytes expressing nAChR beta19'Lys(BODIPYFL), using time-resolved total internal reflection fluorescence microscopy. Under conditions of relatively low receptor density (<0.1 receptors/microm(2)), we observed puncta with diffraction-limited profiles that were consistent with the point-spread function of our microscope. Furthermore, diffraction-limited puncta displayed step decreases in fluorescence intensity, consistent with single-molecule photobleaching. The puncta densities agreed with macroscopic ACh-induced current densities, showing that the fUAA was incorporated, and that receptors were functional. Dose-response relations for the nAChR beta19'Lys(BODIPYFL) receptors were similar to those for wild-type receptors. We also studied nAChR beta19'Lys(BODIPYFL) receptors labeled with alpha-bungarotoxin monoconjugated with Alexa488 (alphaBtxAlexa488). The nAChR has two alphaBtx binding sites, and puncta containing the Lys(BODIPYFL) labeled with alphaBtxAlexa488 yielded the expected three discrete photobleaching steps. We also performed positive control experiments with a nAChR containing enhanced green fluorescent protein in the gamma-subunit M3-M4 loop, which confirmed our nAChR beta19'Lys(BODIPYFL) measurements. Thus, we report on the cell-based single-molecule detection of nAChR beta19'Lys(BODIPYFL).
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Affiliation(s)
- Rigo Pantoja
- Division of Biology, California Institute of Technology, Pasadena, California 91125, USA
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29
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Corringer PJ, Baaden M, Bocquet N, Delarue M, Dufresne V, Nury H, Prevost M, Van Renterghem C. Atomic structure and dynamics of pentameric ligand-gated ion channels: new insight from bacterial homologues. J Physiol 2010; 588:565-72. [PMID: 19995852 PMCID: PMC2828131 DOI: 10.1113/jphysiol.2009.183160] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2009] [Accepted: 12/03/2009] [Indexed: 12/11/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) are widely expressed in the animal kingdom and are key players of neurotransmission by acetylcholine (ACh), gamma-amminobutyric acid (GABA), glycine and serotonin. It is now established that this family has a prokaryotic origin, since more than 20 homologues have been discovered in bacteria. In particular, the GLIC homologue displays a ligand-gated ion channel function and is activated by protons. The prokaryotic origin of these membrane proteins facilitated the X-ray structural resolution of the first members of this family. ELIC was solved at 3.3 A in a closed-pore conformation, and GLIC at up to 2.9 A in an apparently open-pore conformation. These data reveal many structural features, notably the architecture of the pore, including its gate and its selectivity filter, and the interactions between the protein and lipids. In addition, comparison of the structures of GLIC and ELIC hints at a mechanism of channel opening, which consists of both a quaternary twist and a tertiary deformation. This mechanism couples opening-closing motions of the channel with a global reorganization of the protein, including the subunit interface that holds the neurotransmitter binding sites in eukaryotic pLGICs.
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Affiliation(s)
- Pierre-Jean Corringer
- Pasteur Institute, G5 Group of Channel-Receptor, CNRS URA 2182, 75015 Paris, France.
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30
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Yamodo IH, Chiara DC, Cohen JB, Miller KW. Conformational changes in the nicotinic acetylcholine receptor during gating and desensitization. Biochemistry 2010; 49:156-65. [PMID: 19961216 DOI: 10.1021/bi901550p] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The nicotinic acetylcholine receptor (nAChR) is a member of the important Cys loop ligand-gated ion channel superfamily that modulates neuronal excitability. After they respond to their agonists, their actions are terminated either by removal of ligand or by fast and slow desensitization, processes that play an important role in modulating the duration of conducting states and hence of integrated neuronal behavior. We monitored structural changes occurring during fast and slow desensitization in the transmembrane domain of the Torpedo nAChR using time-resolved photolabeling with the hydrophobic probe 3-(trifluoromethyl)-3-(m-iodophenyl)diazirine (TID). After channel opening, TID photolabels a residue on the delta-subunit's M2-M3 loop and a cluster of four residues on deltaM1 and deltaM2, defining an open state pocket [Arevalo, E., et al. (2005) J. Biol. Chem. 280, 13631-13640]. We now find that photolabeling of this pocket persists during the transition to the fast desensitized state, the extent of photoincorporation decreasing only with the transition to the slow desensitized state. In contrast, the extent of photoincorporation in the channel lumen at the conserved 9'-leucines on the second transmembrane helix (M2-9') decreased successively during the resting to open and open to fast desensitized state transitions, implying that the local conformation is different in each state, a conclusion consistent with the hypothesis that there are separate gates for channel opening and desensitization. Thus, although during fast desensitization there is a conformation change in the channel lumen at the level of M2-9', there is none in the regions of the delta-subunit's M2-M3 loop and the interior of its M1-M4 helix bundle until slow desensitization occurs.
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Affiliation(s)
- Innocent H Yamodo
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, Massachusetts 02114, USA
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31
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Nicotinic receptors: allosteric transitions and therapeutic targets in the nervous system. Nat Rev Drug Discov 2009; 8:733-50. [PMID: 19721446 DOI: 10.1038/nrd2927] [Citation(s) in RCA: 521] [Impact Index Per Article: 34.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
Nicotinic receptors - a family of ligand-gated ion channels that mediate the effects of the neurotransmitter acetylcholine - are among the most well understood allosteric membrane proteins from a structural and functional perspective. There is also considerable interest in modulating nicotinic receptors to treat nervous-system disorders such as Alzheimer's disease, schizophrenia, depression, attention deficit hyperactivity disorder and tobacco addiction. This article describes both recent advances in our understanding of the assembly, activity and conformational transitions of nicotinic receptors, as well as developments in the therapeutic application of nicotinic receptor ligands, with the aim of aiding novel drug discovery by bridging the gap between these two rapidly developing fields.
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32
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Lee TE, Chuang AR, Marek MS, Doniach S, Fairclough RH. Redistribution of terbium ions across acetylcholine receptor-enriched membranes induced by agonist desensitization. Biophys J 2009; 96:2637-47. [PMID: 19348747 PMCID: PMC2711283 DOI: 10.1016/j.bpj.2008.08.006] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2007] [Revised: 08/15/2008] [Accepted: 08/20/2008] [Indexed: 11/21/2022] Open
Abstract
Using small-angle x-ray diffraction from centrifugally oriented acetylcholine receptor (AChR) enriched membranes coupled with anomalous scattering from terbium ions (Tb3+) titrated into presumed Ca2+ binding sites, we have mapped the distribution of Tb3+ perpendicular to the membrane plane using a heavy atom refinement algorithm. We have compared the distribution of Tb3+ in the closed resting state with that in the carbamylcholine-desensitized state. In the closed resting state we find 45 Tb3+ ions distributed in 10 narrow peaks perpendicular to the membrane plane. Applying the same refinement procedure to the data from carbamylcholine desensitized AChR we find 18 fewer Tb3+ ions in eight peaks, and slight rearrangements of Tb3+ density in the peaks near the ends of the AChR ion channel pore. These agonist dependent changes in the Tb3+ stoichiometry and distribution suggest a likely role for multivalent cations in stabilizing the different functional states of the AChR, and the changes in the Tb3+ distribution at the two ends of the pore suggest a potential role for multivalent cations in the gating of the ion channel.
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Affiliation(s)
- Thomas E. Lee
- Department of Information Technology, Gilead Sciences, Foster City, California
| | | | - Matthew S. Marek
- Cellular and Molecular Biology Program, University of Michigan Medical School, Ann Arbor, Michigan
| | - Sebastian Doniach
- Departments of Physics and Applied Physics, Stanford University, Palo Alto, California
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33
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Barnes NM, Hales TG, Lummis SC, Peters JA. The 5-HT3 receptor--the relationship between structure and function. Neuropharmacology 2009; 56:273-84. [PMID: 18761359 PMCID: PMC6485434 DOI: 10.1016/j.neuropharm.2008.08.003] [Citation(s) in RCA: 174] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/03/2008] [Revised: 07/31/2008] [Accepted: 08/01/2008] [Indexed: 12/15/2022]
Abstract
The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.
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Affiliation(s)
- Nicholas M. Barnes
- Cellular and Molecular Neuropharmacology Research Group, Department of Pharmacology, Division of Neuroscience, The Medical School, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
| | - Tim G. Hales
- Department of Pharmacology and Physiology, The George Washington University, Washington, DC 20037, USA
| | - Sarah C.R. Lummis
- Department of Biochemistry, University of Cambridge, Tennis Court Road, Cambridge CB2 1QW, UK
| | - John A. Peters
- Neurosciences Institute, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, UK
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34
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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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35
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The insecticide fipronil and its metabolite fipronil sulphone inhibit the rat alpha1beta2gamma2L GABA(A) receptor. Br J Pharmacol 2008; 155:783-94. [PMID: 18660823 DOI: 10.1038/bjp.2008.309] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
BACKGROUND AND PURPOSE Fipronil is the active ingredient in a number of widely used insecticides. Human exposure to fipronil leads to symptoms (headache, nausea and seizures) typically associated with the antagonism of GABA(A) receptors in the brain. In this study, we have examined the modulation of the common brain GABA(A) receptor subtype by fipronil and its major metabolite, fipronil sulphone. EXPERIMENTAL APPROACH Whole-cell and single-channel recordings were made from HEK 293 cells transiently expressing rat alpha1beta2gamma2L GABA(A) receptors. KEY RESULTS The major effect of fipronil was to increase the rate of current decay in macroscopic recordings. In single-channel recordings, the presence of fipronil resulted in shorter cluster durations without affecting the intracluster open and closed time distributions or the single-channel conductance. The alpha1V256S mutation, previously shown alleviate channel inhibition by inhibitory steroids and several insecticides, had a relatively small effect on channel block by fipronil. The mode of action of fipronil sulphone was similar to that of its parent compound but the metabolite was less potent at inhibiting the alpha1beta2gamma2L receptor. CONCLUSIONS AND IMPLICATIONS We conclude that exposure to fipronil induces accumulation of receptors in a novel, long-lived blocked state. This process proceeds in parallel with and independently of, channel desensitization. The lower potency of fipronil sulphone indicates that the conversion serves as a detoxifying process in mammalian brain.
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36
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Changeux JP, Taly A. Nicotinic receptors, allosteric proteins and medicine. Trends Mol Med 2008; 14:93-102. [PMID: 18262468 DOI: 10.1016/j.molmed.2008.01.001] [Citation(s) in RCA: 78] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/11/2007] [Revised: 01/16/2008] [Accepted: 01/16/2008] [Indexed: 01/20/2023]
Abstract
The nicotinic acetylcholine receptor (nAChR) was the first ion channel and membrane receptor of a neurotransmitter to be isolated and chemically identified and is one of the best known membrane proteins involved in signal transduction. Subsequently, nAChRs have been a target for drug discovery because of their potential to impact numerous brain diseases and disorders. Here, we consider recent developments in our understanding of nAChR structure and of the conformational transitions that link the acetylcholine (ACh)-binding site and the ion channel to mediate fast neurotransmission. The knowledge of such allosteric mechanisms is essential to understand pathologies such as congenital myasthenia, autosomal dominant nocturnal frontal lobe epilepsies, sudden infant death syndrome, attention deficit hyperactivity disorder and nicotine addiction and to design novel therapies.
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Affiliation(s)
- Jean-Pierre Changeux
- Research unit, National Center of Scientific Research 2182, Department of Neuroscience, Pasteur Institute, Paris, France.
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37
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Tavoulari S, Frillingos S. Substrate Selectivity of the Melibiose Permease (MelY) from Enterobacter cloacae. J Mol Biol 2008; 376:681-93. [DOI: 10.1016/j.jmb.2007.12.015] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/15/2007] [Revised: 12/01/2007] [Accepted: 12/07/2007] [Indexed: 10/22/2022]
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38
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Jepson JEC, Reenan RA. RNA editing in regulating gene expression in the brain. BIOCHIMICA ET BIOPHYSICA ACTA-GENE REGULATORY MECHANISMS 2007; 1779:459-70. [PMID: 18086576 DOI: 10.1016/j.bbagrm.2007.11.009] [Citation(s) in RCA: 92] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/15/2007] [Accepted: 11/13/2007] [Indexed: 01/26/2023]
Abstract
Adenosine to inosine RNA editing, catalyzed by Adenosine Deaminases Acting on RNA (ADARs), represents an evolutionary conserved post-transcriptional mechanism which harnesses RNA structures to produce proteins that are not literally encoded in the genome. The species-specific alteration of functionally important residues in a multitude of neuronal ion channels and pre-synaptic proteins through RNA editing has been shown to have profound importance for normal nervous system function in a wide range of invertebrate and vertebrate model organisms. ADARs have also been shown to regulate neuronal gene expression through a remarkable variety of disparate processes, including modulation of the RNAi pathway, the creation of alternative splice sites, and the abolition of stop codons. In addition, ADARs have recently been revealed to have a novel role in the primate lineage: the widespread editing of Alu elements, which comprise approximately 10% of the human genome. Thus, as well as enabling the cell-specific regulation of RNAi and selfish genetic elements, the unshackling of the proteome from the constraints of the genome through RNA editing may have been fundamental to the evolution of complex behavior.
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Affiliation(s)
- James E C Jepson
- Department of Molecular Biology, Cell Biology and Biochemistry, SFH Life Sciences Building, Brown University, 185 Meeting Street, Providence, RI 02912, USA.
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39
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Gay EA, Yakel JL. Gating of nicotinic ACh receptors; new insights into structural transitions triggered by agonist binding that induce channel opening. J Physiol 2007; 584:727-33. [PMID: 17823204 PMCID: PMC2276999 DOI: 10.1113/jphysiol.2007.142554] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Nicotinic acetylcholine receptors (nAChRs) are in the superfamily of Cys-loop ligand-gated ion channels, and are pentameric assemblies of five subunits, with each subunit arranged around the central ion-conducting pore. The binding of ACh to the extracellular interface between two subunits induces channel opening. With the recent 4 A resolution of the Torpedo nAChR, and the crystal structure of the related molluscan ACh binding protein, much has been learned about the structure of the ligand binding domain and the channel pore, as well as major structural rearrangements that may confer channel opening. For example, the putative pathway coupling agonist binding to channel gating may include a major rearrangement of the C-loop within the ligand binding pocket, and the disruption of a salt bridge between an arginine residue at the end of the beta10 strand and a glutamate residue in the beta1-beta2 linker. Here we will review and discuss the latest structural findings aiming to further refine the transduction pathway linking binding to gating for the nAChR channels, and discuss similarities and differences among the different members of this Cys-loop superfamily of receptors.
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Affiliation(s)
- Elaine A Gay
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, NC 27709, USA
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40
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Taly A. Opened by a twist: a gating mechanism for the nicotinic acetylcholine receptor. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2007; 36:911-8. [PMID: 17609938 DOI: 10.1007/s00249-007-0189-4] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 02/12/2007] [Revised: 04/27/2007] [Accepted: 05/07/2007] [Indexed: 12/25/2022]
Affiliation(s)
- Antoine Taly
- Unité de Recherche Associée Centre National de la Recherche Scientifique 2182 "Récepteurs et Cognition", Département de Neuroscience, Institut Pasteur, 25 Rue du Dr Roux, 75724, Paris Cedex 15, France.
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41
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Ursell T, Huang KC, Peterson E, Phillips R. Cooperative gating and spatial organization of membrane proteins through elastic interactions. PLoS Comput Biol 2007; 3:e81. [PMID: 17480116 PMCID: PMC1864995 DOI: 10.1371/journal.pcbi.0030081] [Citation(s) in RCA: 87] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2007] [Accepted: 03/21/2007] [Indexed: 11/24/2022] Open
Abstract
Biological membranes are elastic media in which the presence of a transmembrane protein leads to local bilayer deformation. The energetics of deformation allow two membrane proteins in close proximity to influence each other's equilibrium conformation via their local deformations, and spatially organize the proteins based on their geometry. We use the mechanosensitive channel of large conductance (MscL) as a case study to examine the implications of bilayer-mediated elastic interactions on protein conformational statistics and clustering. The deformations around MscL cost energy on the order of 10 kBT and extend ∼3 nm from the protein edge, as such elastic forces induce cooperative gating, and we propose experiments to measure these effects. Additionally, since elastic interactions are coupled to protein conformation, we find that conformational changes can severely alter the average separation between two proteins. This has important implications for how conformational changes organize membrane proteins into functional groups within membranes. Membranes form flexible boundaries between the interior of a cell and its surrounding environment. Proteins that reside in the membrane are responsible for transporting materials and transmitting signals across these membranes to regulate processes crucial for cellular survival. These proteins respond to stimuli by altering their shape to perform specific tasks, such as channel proteins, which allow the flow of ions in only one conformation. However, the membrane is not just a substrate for these proteins, rather it is an elastic medium that bends and changes thickness to accommodate the proteins embedded in it. Thus, the membrane plays a role in the function of many proteins by affecting which conformation is energetically favorable. Using a physical model that combines membrane elastic properties with the structure of a typical membrane protein, we show that the membrane can communicate structural and hence conformational information between membrane proteins in close proximity. Hence, proteins can “talk” and “respond” to each other using the membrane as a generic “voice.” We show that these membrane-mediated elastic forces can ultimately drive proteins of the same shape to cluster together, leading to spatial organization of proteins within the membrane.
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Affiliation(s)
- Tristan Ursell
- Department of Applied Physics, California Institute of Technology, Pasadena, California, United States of America
| | - Kerwyn Casey Huang
- Department of Molecular Biology, Princeton University, Princeton, New Jersey, United States of America
| | - Eric Peterson
- Department of Physics, California Institute of Technology, Pasadena, California, United States of America
| | - Rob Phillips
- Department of Applied Physics, California Institute of Technology, Pasadena, California, United States of America
- Kavli Nanoscience Institute, Pasadena, California, United States of America
- * To whom correspondence should be addressed. E-mail:
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42
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Cheng X, Ivanov I, Wang H, Sine SM, McCammon JA. Nanosecond-timescale conformational dynamics of the human alpha7 nicotinic acetylcholine receptor. Biophys J 2007; 93:2622-34. [PMID: 17573436 PMCID: PMC1989720 DOI: 10.1529/biophysj.107.109843] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We explore the conformational dynamics of a homology model of the human alpha7 nicotinic acetylcholine receptor using molecular dynamics simulation and analyses of root mean-square fluctuations, block partitioning of segmental motion, and principal component analysis. The results reveal flexible regions and concerted global motions of the subunits encompassing extracellular and transmembrane domains of the subunits. The most relevant motions comprise a bending, hinged at the beta10-M1 region, accompanied by concerted tilting of the M2 helices that widens the intracellular end of the channel. Despite the nanosecond timescale, the observations suggest that tilting of the M2 helices may initiate opening of the pore. The results also reveal direct coupling between a twisting motion of the extracellular domain and dynamic changes of M2. Covariance analysis of interresidue motions shows that this coupling arises through a network of residues within the Cys and M2-M3 loops where Phe135 is stabilized within a hydrophobic pocket formed by Leu270 and Ile271. The resulting concerted motion causes a downward shift of the M2 helices that disrupts a hydrophobic girdle formed by 9' and 13' residues.
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Affiliation(s)
- Xiaolin Cheng
- Howard Hughes Medical Institute, National Science Foundation Center for Theoretical Biophysics, Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, USA.
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43
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Abstract
Considerable controversy surrounds the location of the closed channel gate in members of the Cys-loop receptor family of neurotransmitter-gated ion channels that includes the GABAA, glycine, acetylcholine, and 5-HT3 receptors. Cysteine-accessibility studies concluded that the gate is near the cytoplasmic end of the channel in acetylcholine and GABAA receptors but in the middle of the 5-HT3A receptor channel. Zn2+ accessibility studies in a chimeric 5-HT3-ACh receptor suggested the gate is near the channel's cytoplasmic end. In the 4-Å resolution structure of the acetylcholine receptor closed state determined by cryoelectron microscopy, the narrowest region, inferred to be the gate, is in the channel's midsection from 9' to 14' but the M1–M2 loop residues at the channel's cytoplasmic end were not resolved in that structure. We used blocker trapping experiments with picrotoxin, a GABAA receptor open channel blocker, to determine whether a gate exists at a position more extracellular than the picrotoxin binding site, which is in the vicinity of α1Val257 (2') near the channel's cytoplasmic end. We show that picrotoxin can be trapped in the channel after removal of GABA. By using the state-dependent accessibility of engineered cysteines as reporters for the channel's structural state we infer that after GABA washout, with picrotoxin trapped in the channel, the channel appears to be in the closed state. We infer that a gate exists between the picrotoxin binding site and the channel's extracellular end, consistent with a closed channel gate in the middle of the channel. Given the homology with acetylcholine and 5-HT3 receptors there is probably a similar gate in those channels as well. This does not preclude the existence of an additional gate at a more cytoplasmic location.
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Affiliation(s)
- Moez Bali
- Department of Physiology, Albert Einstein College of Medicine of Yeshiva University, Bronx, NY 10461, USA
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44
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Taly A, Corringer PJ, Grutter T, de Carvalho LP, Karplus M, Changeux JP. Implications of the quaternary twist allosteric model for the physiology and pathology of nicotinic acetylcholine receptors. Proc Natl Acad Sci U S A 2006; 103:16965-70. [PMID: 17077146 PMCID: PMC1629088 DOI: 10.1073/pnas.0607477103] [Citation(s) in RCA: 83] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Nicotinic acetylcholine receptors (nAChR) are pentameric ligand-gated ion channels composed of subunits that consist of an extracellular domain that carries the ligand-binding site and a distinct ion-pore domain. Signal transduction results from the allosteric coupling between the two domains: the distance from the binding site to the gate of the pore domain is 50 A. Normal mode analysis with a C(alpha) Gaussian network of a new structural model of the neuronal alpha7 nAChR showed that the lowest mode involves a global quaternary twist motion that opens the ion pore. A molecular probe analysis, in which the network is modified at each individual amino acid residue, demonstrated that the major effect is to change the frequency, but not the form, of the twist mode. The largest effects were observed for the ligand-binding site and the Cys-loop. Most (24/27) of spontaneous mutations known to cause congenital myasthenia and autosomal dominant nocturnal frontal lobe epilepsy are located either at the interface between subunits or, within a given subunit, at the interface between rigid blocks. These interfaces are modified significantly by the twist mode. The present analysis, thus, supports the quaternary twist model of the nAChR allosteric transition and provides a qualitative interpretation of the effect of the mutations responsible for several receptor pathologies.
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MESH Headings
- Allosteric Regulation
- Animals
- Epilepsy, Frontal Lobe/genetics
- Epilepsy, Frontal Lobe/physiopathology
- Genes, Dominant
- Humans
- Models, Molecular
- Models, Neurological
- Mutation
- Myasthenic Syndromes, Congenital/genetics
- Myasthenic Syndromes, Congenital/physiopathology
- Protein Structure, Quaternary
- Protein Subunits
- Receptors, Nicotinic/chemistry
- Receptors, Nicotinic/physiology
- Torpedo
- alpha7 Nicotinic Acetylcholine Receptor
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Affiliation(s)
- Antoine Taly
- *Recepteurs et Cognition, Unité de Recherche Associée Centre National de la Recherche Scientifique 2182, Institut Pasteur, 25 Rue du Dr Roux, 75724 Paris Cedex 15, France
- To whom correspondence may be sent at the present address:
Laboratoire de Chimie Biophysique, Université Louis Pasteur, 8, Allée Gaspard Monge, B.P. 70028, F-67083 Strasbourg Cedex, France. E-mail:
| | - Pierre-Jean Corringer
- *Recepteurs et Cognition, Unité de Recherche Associée Centre National de la Recherche Scientifique 2182, Institut Pasteur, 25 Rue du Dr Roux, 75724 Paris Cedex 15, France
| | - Thomas Grutter
- *Recepteurs et Cognition, Unité de Recherche Associée Centre National de la Recherche Scientifique 2182, Institut Pasteur, 25 Rue du Dr Roux, 75724 Paris Cedex 15, France
| | - Lia Prado de Carvalho
- *Recepteurs et Cognition, Unité de Recherche Associée Centre National de la Recherche Scientifique 2182, Institut Pasteur, 25 Rue du Dr Roux, 75724 Paris Cedex 15, France
| | - Martin Karplus
- Laboratoire de Chimie Biophysique, Institut de Science et d'Ingénierie Supramoléculaire, Université Louis Pasteur, 8, Allée Gaspard Monge, B.P. 70028, F-67083 Strasbourg Cedex, France; and
- Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138
| | - Jean-Pierre Changeux
- *Recepteurs et Cognition, Unité de Recherche Associée Centre National de la Recherche Scientifique 2182, Institut Pasteur, 25 Rue du Dr Roux, 75724 Paris Cedex 15, France
- To whom correspondence may be addressed. E-mail:
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45
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Aridon P, Marini C, Di Resta C, Brilli E, De Fusco M, Politi F, Parrini E, Manfredi I, Pisano T, Pruna D, Curia G, Cianchetti C, Pasqualetti M, Becchetti A, Guerrini R, Casari G. Increased sensitivity of the neuronal nicotinic receptor alpha 2 subunit causes familial epilepsy with nocturnal wandering and ictal fear. Am J Hum Genet 2006; 79:342-50. [PMID: 16826524 PMCID: PMC1559502 DOI: 10.1086/506459] [Citation(s) in RCA: 163] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2006] [Accepted: 06/01/2006] [Indexed: 11/03/2022] Open
Abstract
Sleep has traditionally been recognized as a precipitating factor for some forms of epilepsy, although differential diagnosis between some seizure types and parasomnias may be difficult. Autosomal dominant frontal lobe epilepsy is characterized by nocturnal seizures with hyperkinetic automatisms and poorly organized stereotyped movements and has been associated with mutations of the alpha 4 and beta 2 subunits of the neuronal nicotinic acetylcholine receptor. We performed a clinical and molecular genetic study of a large pedigree segregating sleep-related epilepsy in which seizures are associated with fear sensation, tongue movements, and nocturnal wandering, closely resembling nightmares and sleep walking. We identified a new genetic locus for familial sleep-related focal epilepsy on chromosome 8p12.3-8q12.3. By sequencing the positional candidate neuronal cholinergic receptor alpha 2 subunit gene (CHRNA2), we detected a heterozygous missense mutation, I279N, in the first transmembrane domain that is crucial for receptor function. Whole-cell recordings of transiently transfected HEK293 cells expressing either the mutant or the wild-type receptor showed that the new CHRNA2 mutation markedly increases the receptor sensitivity to acetylcholine, therefore indicating that the nicotinic alpha 2 subunit alteration is the underlying cause. CHRNA2 is the third neuronal cholinergic receptor gene to be associated with familial sleep-related epilepsies. Compared with the CHRNA4 and CHRNB2 mutations reported elsewhere, CHRNA2 mutations cause a more complex and finalized ictal behavior.
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Affiliation(s)
- Paolo Aridon
- Human Molecular Genetics Unit, Dibit San Raffaele Scientific Institute, Milan, Italy
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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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Mourot A, Grutter T, Goeldner M, Kotzyba-Hibert F. Dynamic Structural Investigations on the Torpedo Nicotinic Acetylcholine Receptor by Time-Resolved Photoaffinity Labeling. Chembiochem 2006; 7:570-83. [PMID: 16538695 DOI: 10.1002/cbic.200500526] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
An increasing number of high-resolution structures of membrane-embedded ion channels (or soluble homologues) have emerged during the last couple of years. The most pressing need now is to understand the complex mechanism underlying ion-channel function. Time-resolved photoaffinity labeling is a suitable tool for investigating the molecular function of membrane proteins, especially when high-resolution structures of related proteins are available. However until now this methodology has only been used on the Torpedo nicotinic acetylcholine receptor (nAChR). nAChRs are allosteric cation-selective receptor channels that are activated by the neurotransmitter acetylcholine (ACh) and implicated in numerous physiological and pathological processes. Time-resolved photoaffinity labeling has already enabled local motions of nAChR subdomains (i.e. agonist binding sites, ion channel, subunit interface) to be understood at the molecular level, and has helped to explain how small molecules can exert their physiological effect, an important step toward the development of drug design. Recent analytical and technical improvements should allow the application of this powerful methodology to other membrane proteins in the near future.
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Affiliation(s)
- Alexandre Mourot
- Biophysical Chemistry Department, Max Planck Institut für Biophysik, Max-von-Laue Strasse 3, 60438 Frankfurt am Main, Germany.
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Mourot A, Rodrigo J, Kotzyba-Hibert F, Bertrand S, Bertrand D, Goeldner M. Probing the reorganization of the nicotinic acetylcholine receptor during desensitization by time-resolved covalent labeling using [3H]AC5, a photoactivatable agonist. Mol Pharmacol 2005; 69:452-61. [PMID: 16269537 DOI: 10.1124/mol.105.017566] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The structural reorganizations occurring on the nicotinic acetylcholine receptor (nAChR) during activation and subsequent desensitization have been investigated through time-resolved photoaffinity labeling using a photoactivatable nicotinic agonist. [(3)H]AC5 is a photosensitive nicotinic probe with high affinity for the desensitized state of the Torpedo marmorata receptor (K(D) = 5 nM) that displays full agonist activity on the Torpedo californica receptor expressed in oocytes (EC(50) = 1.2 microM). Photoaffinity labeling of this receptor in the desensitized state showed a predominant specific labeling of gamma and delta subunits, whereas the alpha subunit was barely labeled. Using a stopped-flow device combined with a flash photolysis quenching system, we investigated the covalent mapping of the subunits as a function of incubation time of the receptor with [(3)H]AC5 (17 ms-1.25 h). During agonist-induced desensitization, specific labeling increased substantially, with similar time constants for gamma and delta subunits (0.016 s(-1)), whereas labeling of the alpha subunit remained relatively low. Therefore, the repartition of radioactivity shifted during desensitization from a weak but predominant labeling of the alpha and gamma subunits toward a substantial labeling of gamma and delta subunits. The observed time-dependent labeling pattern together with AC5 docking into a homology model of the T. californica nAChR suggest a subunit reorganization during agonist-induced desensitization, leading to a tightly packed arrangement that corresponds to a stable high affinity state for agonists.
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Affiliation(s)
- Alexandre Mourot
- Laboratoire de Chimie Bioorganique, Unité Mixte de Recherche (UMR) 7514 Centre National de la Recherche Scientifique (CNRS), Faculté de Pharmacie, Université Louis Pasteur Strasbourg, Illkirch, France.
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Paas Y, Gibor G, Grailhe R, Savatier-Duclert N, Dufresne V, Sunesen M, de Carvalho LP, Changeux JP, Attali B. Pore conformations and gating mechanism of a Cys-loop receptor. Proc Natl Acad Sci U S A 2005; 102:15877-82. [PMID: 16247006 PMCID: PMC1276086 DOI: 10.1073/pnas.0507599102] [Citation(s) in RCA: 60] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Neurons regulate the propagation of chemoelectric signals throughout the nervous system by opening and closing ion channels, a process known as gating. Here, histidine-based metal-binding sites were engineered along the intrinsic pore of a chimeric Cys-loop receptor to probe state-dependent Zn(2+)-channel interactions. Patterns of Zn(2+) ion binding within the pore reveal that, in the closed state, the five pore-lining segments adopt an oblique orientation relative to the axis of ion conduction and constrict into a physical gate at their intracellular end. The interactions of Zn(2+) with the open state indicate that the five pore-lining segments should rigidly tilt to enable the movement of their intracellular ends away from the axis of ion conduction, so as to open the constriction (i.e., the gate). Alignment of the functional results with the 3D structure of an acetylcholine receptor allowed us to generate structural models accounting for the closed and open pore conformations and for a gating mechanism of a Cys-loop receptor.
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Affiliation(s)
- Yoav Paas
- Récepteurs et Cognition, Unité de Recherche Associée 2182 Centre National de la Recherche Scientifique, Institut Pasteur, 25 Rue du Docteur Roux, 75724 Paris Cedex 15, France.
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50
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Cheng MH, Cascio M, Coalson RD. Theoretical studies of the M2 transmembrane segment of the glycine receptor: models of the open pore structure and current-voltage characteristics. Biophys J 2005; 89:1669-80. [PMID: 15951389 PMCID: PMC1366671 DOI: 10.1529/biophysj.105.060368] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The pentameric glycine receptor (GlyR), a member of the nicotinicoid superfamily of ligand-gated ion channels, is an inhibitory Cl(-) channel that is gated by glycine. Using recently published NMR data of the second transmembrane segment (M2) of the human alpha1 GlyR, structural models of pentameric assemblies embedded in a lipid bilayer were constructed using a combination of experimentally determined constraints coupled with all-atom energy minimization. Based on this structure of the pentameric M2 "pore", Brownian dynamics simulations of ion permeation through this putative conducting open state of the channel were carried out. Simulated I-V curves were in good agreement with published experimental current-voltage curves and the anion/cation permeability ratio, suggesting that our open-state model may be representative of the conducting channel of the full-length receptor. These studies also predicted regions of chloride occupancy and suggested residues critical to anion permeation. Calculations of the conductance of the cation-selective mutant A251E channel are also consistent with experimental data. In addition, both rotation and untilting of the pore helices of our model were found to be broadly consistent with closing of the channel, albeit at distinct regions that may reflect alternate gates of the receptor.
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Affiliation(s)
- Mary Hongying Cheng
- Department of Chemistry,University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
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