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Severi I, Perugini J, Ruocco C, Coppi L, Pedretti S, Di Mercurio E, Senzacqua M, Ragni M, Imperato G, Valerio A, Mitro N, Crestani M, Nisoli E, Giordano A. Activation of a non-neuronal cholinergic system in visceral white adipose tissue of obese mice and humans. Mol Metab 2024; 79:101862. [PMID: 38141849 PMCID: PMC10792749 DOI: 10.1016/j.molmet.2023.101862] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 09/07/2023] [Revised: 12/07/2023] [Accepted: 12/20/2023] [Indexed: 12/25/2023] Open
Abstract
BACKGROUND AND OBJECTIVES Since white adipose tissue (WAT) lacks parasympathetic cholinergic innervation, the source of the acetylcholine (ACh) acting on white adipocyte cholinergic receptors is unknown. This study was designed to identify ACh-producing cells in mouse and human visceral WAT and to determine whether a non-neuronal cholinergic system becomes activated in obese inflamed WAT. METHODS Mouse epididymal WAT (eWAT) and human omental fat were studied in normal and obese subjects. The expression of the key molecules involved in cholinergic signaling was evaluated by qRT-PCR and western blotting whereas their tissue distribution and cellular localization were investigated by immunohistochemistry, confocal microscopy and in situ hybridization. ACh levels were measured by liquid chromatography/tandem mass spectrometry. The cellular effects of ACh were assessed in cultured human multipotent adipose-derived stem cell (hMADS) adipocytes. RESULTS In mouse eWAT, diet-induced obesity modulated the expression of key cholinergic molecular components and, especially, raised the expression of choline acetyltransferase (ChAT), the ACh-synthesizing enzyme, which was chiefly detected in interstitial macrophages, in macrophages forming crown-like structures (CLSs), and in multinucleated giant cells (MGCs). The stromal vascular fraction of obese mouse eWAT contained significantly higher ACh and choline levels than that of control mice. ChAT was undetectable in omental fat from healthy subjects, whereas it was expressed in a number of interstitial macrophages, CLSs, and MGCs from some obese individuals. In hMADS adipocytes stressed with tumor necrosis factor α, ACh, alone or combined with rivastigmine, significantly blunted monocyte chemoattractant protein 1 and interleukin 6 expression, it partially but significantly, restored adiponectin and GLUT4 expression, and promoted glucose uptake. CONCLUSIONS In mouse and human visceral WAT, obesity induces activation of a macrophage-dependent non-neuronal cholinergic system that is capable of exerting anti-inflammatory and insulin-sensitizing effects on white adipocytes.
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Affiliation(s)
- Ilenia Severi
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Jessica Perugini
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Chiara Ruocco
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Lara Coppi
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Silvia Pedretti
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Eleonora Di Mercurio
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Martina Senzacqua
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy
| | - Maurizio Ragni
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Gabriele Imperato
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Alessandra Valerio
- Department of Molecular and Translational Medicine, University of Brescia, 25123 Brescia, Italy
| | - Nico Mitro
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy; Department of Experimental Oncology, IEO, European Institute of Oncology IRCCS, Milano, Italy
| | - Maurizio Crestani
- Department of Pharmacological and Biomolecular Sciences, University of Milan, 20122 Milano, Italy
| | - Enzo Nisoli
- Center for Study and Research on Obesity, Department of Medical Biotechnology and Translational Medicine, University of Milan, 20129 Milano, Italy
| | - Antonio Giordano
- Department of Experimental and Clinical Medicine, Marche Polytechnic University, 60126 Ancona, Italy; Center of Obesity, Marche Polytechnic University-United Hospitals, Ancona, Italy.
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Hardege I, Morud J, Courtney A, Schafer WR. A Novel and Functionally Diverse Class of Acetylcholine-Gated Ion Channels. J Neurosci 2023; 43:1111-1124. [PMID: 36604172 PMCID: PMC9962794 DOI: 10.1523/jneurosci.1516-22.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/08/2022] [Revised: 12/02/2022] [Accepted: 12/14/2022] [Indexed: 01/07/2023] Open
Abstract
Fast cholinergic neurotransmission is mediated by acetylcholine-gated ion channels; in particular, excitatory nicotinic acetylcholine receptors play well established roles in virtually all nervous systems. Acetylcholine-gated inhibitory channels have also been identified in some invertebrate phyla, yet their roles in the nervous system are less well understood. We report the existence of multiple new inhibitory ion channels with diverse ligand activation properties in Caenorhabditis elegans We identify three channels, LGC-40, LGC-57, and LGC-58, whose primary ligand is choline rather than acetylcholine, as well as the first evidence of a truly polymodal channel, LGC-39, which is activated by both cholinergic and aminergic ligands. Using our new ligand-receptor pairs we uncover the surprising extent to which single neurons in the hermaphrodite nervous system express both excitatory and inhibitory channels, not only for acetylcholine but also for the other major neurotransmitters. The results presented in this study offer new insight into the potential evolutionary benefit of a vast and diverse repertoire of ligand-gated ion channels to generate complexity in an anatomically compact nervous system.SIGNIFICANCE STATEMENT Here we describe the diversity of cholinergic signaling in the nematode Caenorhabditis elegans We identify and characterize a novel family of ligand-gated ion channels and show that they are preferentially gated by choline rather than acetylcholine and expressed broadly in the nervous system. Interestingly, we also identify one channel gated by chemically diverse ligands including acetylcholine and aminergic ligands. By using our new knowledge of these ligand-gated ion channels, we built a model to predict the synaptic polarity in the C. elegans connectome. This model can be used for generating hypotheses on neural circuit function.
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Affiliation(s)
- Iris Hardege
- MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom
| | - Julia Morud
- MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom
| | - Amy Courtney
- MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom
| | - William R Schafer
- MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, United Kingdom
- Department of Biology, KU Leuven, 3000 Leuven, Belgium
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3
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Phillips MB, Nigam A, Johnson JW. Interplay between Gating and Block of Ligand-Gated Ion Channels. Brain Sci 2020; 10:brainsci10120928. [PMID: 33271923 PMCID: PMC7760600 DOI: 10.3390/brainsci10120928] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/27/2020] [Revised: 11/21/2020] [Accepted: 11/26/2020] [Indexed: 02/03/2023] Open
Abstract
Drugs that inhibit ion channel function by binding in the channel and preventing current flow, known as channel blockers, can be used as powerful tools for analysis of channel properties. Channel blockers are used to probe both the sophisticated structure and basic biophysical properties of ion channels. Gating, the mechanism that controls the opening and closing of ion channels, can be profoundly influenced by channel blocking drugs. Channel block and gating are reciprocally connected; gating controls access of channel blockers to their binding sites, and channel-blocking drugs can have profound and diverse effects on the rates of gating transitions and on the stability of channel open and closed states. This review synthesizes knowledge of the inherent intertwining of block and gating of excitatory ligand-gated ion channels, with a focus on the utility of channel blockers as analytic probes of ionotropic glutamate receptor channel function.
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Affiliation(s)
- Matthew B. Phillips
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA; (M.B.P.); (A.N.)
- Center for Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - Aparna Nigam
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA; (M.B.P.); (A.N.)
| | - Jon W. Johnson
- Department of Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA; (M.B.P.); (A.N.)
- Center for Neuroscience, University of Pittsburgh, Pittsburgh, PA 15260, USA
- Correspondence: ; Tel.: +1-(412)-624-4295
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4
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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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Thuma JB, Hooper SL. Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na + is alone not evidence of Na +-activated K + currents. J Neurophysiol 2018; 120:3217-3233. [PMID: 30354793 DOI: 10.1152/jn.00871.2017] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/19/2022] Open
Abstract
Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.
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Affiliation(s)
- Jeffrey B Thuma
- Department of Biological Sciences, Irvine Hall, Ohio University , Athens, Ohio
| | - Scott L Hooper
- Department of Biological Sciences, Irvine Hall, Ohio University , Athens, Ohio
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6
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Gielen M, Corringer P. The dual-gate model for pentameric ligand-gated ion channels activation and desensitization. J Physiol 2018; 596:1873-1902. [PMID: 29484660 PMCID: PMC5978336 DOI: 10.1113/jp275100] [Citation(s) in RCA: 73] [Impact Index Per Article: 12.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/24/2018] [Revised: 01/17/2018] [Accepted: 01/17/2018] [Indexed: 12/15/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) mediate fast neurotransmission in the nervous system. Their dysfunction is associated with psychiatric, neurological and neurodegenerative disorders such as schizophrenia, epilepsy and Alzheimer's disease. Understanding their biophysical and pharmacological properties, at both the functional and the structural level, thus holds many therapeutic promises. In addition to their agonist-elicited activation, most pLGICs display another key allosteric property, namely desensitization, in which they enter a shut state refractory to activation upon sustained agonist binding. While the activation mechanisms of several pLGICs have been revealed at near-atomic resolution, the structural foundation of desensitization has long remained elusive. Recent structural and functional data now suggest that the activation and desensitization gates are distinct, and are located at both sides of the ion channel. Such a 'dual gate mechanism' accounts for the marked allosteric effects of channel blockers, a feature illustrated herein by theoretical kinetics simulations. Comparison with other classes of ligand- and voltage-gated ion channels shows that this dual gate mechanism emerges as a common theme for the desensitization and inactivation properties of structurally unrelated ion channels.
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Affiliation(s)
- Marc Gielen
- Channel Receptors UnitInstitut PasteurCNRS UMR 3571ParisFrance
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7
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A Missense Mutation A384P Associated with Human Hyperekplexia Reveals a Desensitization Site of Glycine Receptors. J Neurosci 2018; 38:2818-2831. [PMID: 29440552 DOI: 10.1523/jneurosci.0674-16.2018] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/29/2016] [Revised: 01/22/2018] [Accepted: 02/06/2018] [Indexed: 11/21/2022] Open
Abstract
Hyperekplexia, an inherited neuronal disorder characterized by exaggerated startle responses with unexpected sensory stimuli, is caused by dysfunction of glycinergic inhibitory transmission. From analysis of newly identified human hyperekplexia mutations in the glycine receptor (GlyR) α1 subunit, we found that an alanine-to-proline missense mutation (A384P) resulted in substantially higher desensitization level and lower agonist sensitivity of homomeric α1 GlyRs when expressed in HEK cells. The incorporation of the β subunit fully reversed the reduction in agonist sensitivity and partially reversed the desensitization of α1A384P The heteromeric α1A384Pβ GlyRs showed enhanced desensitization but unchanged agonist-induced maximum responses, surface expression, main channel conductance, and voltage dependence compared with that of the wild-type α1β (α1WTβ) GlyRs. Coexpression of the R392H and A384P mutant α1 subunits, which mimic the expression of the compound heterozygous mutation in a hyperekplexia patient, resulted in channel properties similar to those with α1A384P subunit expression alone. In comparison, another human hyperekplexia mutation α1P250T, which was previously reported to enhance desensitization, caused a strong reduction in maximum currents in addition to the altered desensitization. These results were further confirmed by overexpression of α1P250T or α1A384P subunits in cultured neurons isolated from SD rats of either sex. Moreover, the IPSC-like responses of cells expressing α1A384Pβ induced by repeated glycine pulses showed a stronger frequency-dependent reduction than those expressing α1WTβ. Together, our findings demonstrate that A384 is associated with the desensitization site of the α1 subunit and its proline mutation produced enhanced desensitization of GlyRs, which contributes to the pathogenesis of human hyperekplexia.SIGNIFICANCE STATEMENT Human startle disease is caused by impaired synaptic inhibition in the brainstem and spinal cord, which is due to either direct loss of GlyR channel function or reduced number of synaptic GlyRs. Considering that fast decay kinetics of GlyR-mediated inhibitory synaptic responses, the question was raised whether altered desensitization of GlyRs will cause dysfunction of glycine transmission and disease phenotypes. Here, we found that the α1 subunit mutation A384P, identified from startle disease patients, results in enhanced desensitization and leads to rapidly decreasing responses in the mutant GlyRs when they are activated repeatedly by the synaptic-like simulation. These observations suggest that the enhanced desensitization of postsynaptic GlyRs could be the primary pathogenic mechanism of human startle disease.
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8
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Gonzalez-Gutierrez G, Grosman C. The atypical cation-conduction and gating properties of ELIC underscore the marked functional versatility of the pentameric ligand-gated ion-channel fold. J Gen Physiol 2015; 146:15-36. [PMID: 26078054 PMCID: PMC4485021 DOI: 10.1085/jgp.201411333] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/25/2014] [Accepted: 05/14/2015] [Indexed: 01/07/2023] Open
Abstract
The superfamily of pentameric ligand-gated ion channels (pLGICs) is unique among ionotropic receptors in that the same overall structure has evolved to generate multiple members with different combinations of agonist specificities and permeant-ion charge selectivities. However, aside from these differences, pLGICs have been typically regarded as having several invariant functional properties. These include pore blockade by extracellular quaternary-ammonium cations in the micromolar-to-millimolar concentration range (in the case of the cation-selective members), and a gain-of-function phenotype, which manifests as a slower deactivation time course, as a result of mutations that reduce the hydrophobicity of the transmembrane pore lining. Here, we tested this notion on three distantly related cation-selective members of the pLGIC superfamily: the mouse muscle nicotinic acetylcholine receptor (nAChR), and the bacterial GLIC and ELIC channels. Remarkably, we found that, whereas low millimolar concentrations of TMA(+) and TEA(+) block the nAChR and GLIC, neither of these two quaternary-ammonium cations blocks ELIC at such concentrations; instead, both carry measurable inward currents when present as the only cations on the extracellular side. Also, we found that, whereas lidocaine binding speeds up the current-decay time courses of the nAChR and GLIC in the presence of saturating concentrations of agonists, the binding of lidocaine to ELIC slows this time course down. Furthermore, whereas mutations that reduce the hydrophobicity of the side chains at position 9' of the M2 α-helices greatly slowed the deactivation time course of the nAChR and GLIC, these mutations had little effect--or even sped up deactivation--when engineered in ELIC. Our data indicate that caution should be exercised when generalizing results obtained with ELIC to the rest of the pLGICs, but more intriguingly, they hint at the possibility that ELIC is a representative of a novel branch of the superfamily with markedly divergent pore properties despite a well-conserved three-dimensional architecture.
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Affiliation(s)
- Giovanni Gonzalez-Gutierrez
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
| | - Claudio Grosman
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, IL 61801
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9
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The desensitization gate of inhibitory Cys-loop receptors. Nat Commun 2015; 6:6829. [PMID: 25891813 PMCID: PMC4410641 DOI: 10.1038/ncomms7829] [Citation(s) in RCA: 98] [Impact Index Per Article: 10.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/04/2014] [Accepted: 03/03/2015] [Indexed: 11/09/2022] Open
Abstract
Cys-loop neurotransmitter-gated ion channels are vital for communication throughout the nervous system. Following activation, these receptors enter into a desensitized state in which the ion channel shuts even though the neurotransmitter molecules remain bound. To date, the molecular determinants underlying this most fundamental property of Cys-loop receptors have remained elusive. Here we present a generic mechanism for the desensitization of Cys-loop GABAA (GABAARs) and glycine receptors (GlyRs), which both mediate fast inhibitory synaptic transmission. Desensitization is regulated by interactions between the second and third transmembrane segments, which affect the ion channel lumen near its intracellular end. The GABAAR and GlyR pore blocker picrotoxin prevented desensitization, consistent with its deep channel-binding site overlapping a physical desensitization gate.
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10
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Forman SA, Chiara DC, Miller KW. Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:169-77. [PMID: 25316107 DOI: 10.1016/j.neuropharm.2014.10.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 09/22/2014] [Accepted: 10/02/2014] [Indexed: 11/25/2022]
Abstract
General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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Affiliation(s)
- Stuart A Forman
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - David C Chiara
- Dept. of Neurobiology, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - Keith W Miller
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
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11
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Bruhova I, Gregg T, Auerbach A. Energy for wild-type acetylcholine receptor channel gating from different choline derivatives. Biophys J 2013; 104:565-74. [PMID: 23442907 DOI: 10.1016/j.bpj.2012.11.3833] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2012] [Revised: 11/25/2012] [Accepted: 11/27/2012] [Indexed: 02/01/2023] Open
Abstract
Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.
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Affiliation(s)
- Iva Bruhova
- Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, New York, USA
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Keramidas A, Lynch JW. An outline of desensitization in pentameric ligand-gated ion channel receptors. Cell Mol Life Sci 2013; 70:1241-53. [PMID: 22936353 PMCID: PMC11113241 DOI: 10.1007/s00018-012-1133-z] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2012] [Revised: 07/28/2012] [Accepted: 08/13/2012] [Indexed: 10/27/2022]
Abstract
Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.
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Affiliation(s)
- Angelo Keramidas
- Queensland Brain Institute, The University of Queensland, Brisbane, QLD 4072 Australia
| | - Joseph W. Lynch
- Queensland Brain Institute, The University of Queensland, Brisbane, QLD 4072 Australia
- School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072 Australia
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13
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Cervantes B, Vega R, Limón A, Soto E. Identity, expression and functional role of the sodium-activated potassium current in vestibular ganglion afferent neurons. Neuroscience 2013; 240:163-75. [PMID: 23466807 DOI: 10.1016/j.neuroscience.2013.02.052] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2012] [Revised: 02/19/2013] [Accepted: 02/20/2013] [Indexed: 10/27/2022]
Abstract
Vestibular afferent neurons (VANs) transmit information from the vestibular end organs to the central nuclei. This information is encoded within the firing pattern of these cells and is heavily influenced by the K⁺ conductances expressed by vestibular neurons. In the present study, we describe the presence of a previously unidentified Na⁺-activated K⁺ conductance (KNa) in these cells. We observed that the blocking of Na⁺ channels by tetrodotoxin (TTX) or the substitution of choline for Na⁺ in the extracellular solution during voltage clamp pulses resulted in the reduction of a sustained outward current that was dependent on the Na⁺ current. Furthermore, increases in the intracellular concentration of Na⁺ that were made by blocking the Na⁺/K⁺ ATPase with ouabain increased the amplitude of the outward current, and reduction of the intracellular Cl⁻ concentration reduced the TTX-sensitive outward current. The substitution of Li⁺ for Na⁺ in the extracellular solution significantly reduced the amplitude of the outward current in voltage clamp pulses and decreased the afterhyperpolarization (AHP) of the action potentials in current clamp experiments. These electrophysiological results are consistent with the presence of mRNA transcripts for the KNa subunits Slick and Slack in the vestibular ganglia and in the sensory epithelium, which were detected using reverse-transcription polymerase chain reaction (RT-PCR). These results are also consistent with the immunolabeling of Slick and Slack protein in isolated vestibular neurons, in the vestibular ganglion and in the vestibular sensory epithelium. These results indicate that KNa channels are expressed in VANs and in their terminals. Furthermore, these data indicate that these channels may contribute to the firing pattern of vestibular neurons.
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Affiliation(s)
- B Cervantes
- Instituto de Fisiología, Universidad Autónoma de Puebla, 14 Sur 6301, Puebla C.P. 72570, Pue., Mexico
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14
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Zhang J, Xue F, Liu Y, Yang H, Wang X. The structural mechanism of the Cys-loop receptor desensitization. Mol Neurobiol 2013; 48:97-108. [PMID: 23397136 DOI: 10.1007/s12035-013-8420-z] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/26/2012] [Accepted: 01/30/2013] [Indexed: 11/25/2022]
Abstract
The cys-loop receptors are neurotransmitter-operated ion channels, which mediate fast synaptic transmission for communication between neurons. However, prolonged exposure to the neurotransmitter drives the receptor to a desensitization state, which plays an important role in shaping synaptic transmission. Much progress has been made through more than half a century's research since Katz and Thesleff first descried desensitization for muscle nicotinic acetylcholine receptor. In this review, we summarized recent research developments of receptor desensitization. Now, it has been identified that many parts of the receptor, such as the pore domain (including the hinge in the M2-M3 linker), the binding domain, the coupling region, and the intracellular domain, are all involved in the cys-loop receptor desensitization and that uncoupling between the amino-terminal domain and channel lining domain seems to play a central role in desensitization. This uncoupling is mainly governed by the balance between coupling strength and relative tightness of gating machinery and influenced by other parts of the receptor. Agonist binding induces conformational change to overcome the gating barrier to open the channel through the stressed coupling region, which is subsequently broken, causing receptor desensitization. With rapid advancement in structural biology of membrane receptors, final validation of this mechanism is expected to occur in the near future when the high-resolution structure of the desensitized state is available.
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Affiliation(s)
- Jianliang Zhang
- Beijing Institute of Brain Disorders, Capital Medical University, Key Laboratory for Neurodegenerative Disease of the Ministry of Education, Beijing Center of Neural Regeneration and Repair, Beijing Key Laboratory of Brain Major Disorders-State Key Lab Incubation Base, Beijing Neuroscience Disciplines, #10 Xitoutiao, Youanmenwai, Beijing, 100069, China.
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15
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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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Jha A, Gupta S, Zucker SN, Auerbach A. The energetic consequences of loop 9 gating motions in acetylcholine receptor-channels. J Physiol 2011; 590:119-29. [PMID: 22025664 DOI: 10.1113/jphysiol.2011.213892] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Acetylcholine receptor-channels (AChRs) mediate fast synaptic transmission between nerve and muscle. In order to better-understand the mechanism by which this protein assembles and isomerizes between closed- and open-channel conformations we measured changes in the diliganded gating equilibrium constant (E(2)) consequent to mutations of residues at the C-terminus of loop 9 (L9) in the α and ε subunits of mouse neuromuscular AChRs. These amino acids are close to two interesting interfaces, between the extracellular and transmembrane domain within a subunit (E–T interface) and between primary and complementary subunits (P–C interface). Most α subunit mutations modestly decreased E(2) (mainly by slowing the channel-opening rate constant) and sometimes produced AChRs that had heterogeneous gating kinetic properties. Mutations in the ε subunit had a larger effect and could either increase or decrease E(2), but did not induce kinetic heterogeneity. There are broad-but-weak energetic interactions between αL9 residues and others at the αE–T interface, as well as between the εL9 residue and others at the P–C interface (in particular, the M2–M3 linker). These interactions serve, in part, to maintain the structural integrity of the AChR assembly at the E–T interface. Overall, the energy changes of L9 residues are significant but smaller than in other regions of the protein.
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Affiliation(s)
- Archana Jha
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA
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17
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Akk G, Li P, Bracamontes J, Wang M, Steinbach JH. Pharmacology of structural changes at the GABA(A) receptor transmitter binding site. Br J Pharmacol 2011; 162:840-50. [PMID: 20958292 DOI: 10.1111/j.1476-5381.2010.01083.x] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
BACKGROUND AND PURPOSE The binding of transmitter to specialized binding pockets leads to rearrangements in the structure of the receptor eventually resulting in channel opening. We used voltage-clamp fluorometry to investigate the pharmacological basis and biophysical processes that underlie structural changes at the transmitter binding site of the rat α1β2γ2L GABA(A) receptor. EXPERIMENTAL APPROACH Simultaneous electrophysiological and site-specific fluorescence measurements were conducted on receptors expressed in Xenopus oocytes and labelled with an environmentally-sensitive fluorophore, Alexa 546 maleimide, at the α1L127C site. KEY RESULTS Receptors activated by GABA demonstrate a concentration-dependent increase in fluorescence intensity, indicating that the environment surrounding the fluorophore becomes less polar upon activation. Qualitatively similar responses were observed with other GABA site ligands such as piperidine-4-sulphonic acid, muscimol, β-alanine and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol. Fluorescence changes were not affected by the direction of current flow. During long applications of GABA significant desensitization developed, which was not accompanied by additional changes in fluorescence. Pentobarbital was an efficacious agonist of the labelled mutant receptor but did not cause changes in fluorescence. Direct activation by etomidate or the steroid allopregnanolone also did not result in fluorescence changes. Functional potentiation of GABA-activated receptors by allopregnanolone or etomidate enhanced both the GABA-elicited functional response and the fluorescence change. In contrast, potentiation by pentobarbital was not accompanied by an enhanced fluorescence response. CONCLUSIONS AND IMPLICATIONS The data indicate that there is no direct correlation between current flow or position of the activation gate and the structural changes as detected by Alexa 546-labelled α1L127Cβ2γ2L GABA(A) receptors. Channel potentiation by pentobarbital qualitatively differs from potentiation by etomidate or allopregnanolone.
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Affiliation(s)
- Gustav Akk
- Department of Anesthesiology, Washington University School of Medicine, St Louis, MO, USA.
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18
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Temperature dependence of acetylcholine receptor channels activated by different agonists. Biophys J 2011; 100:895-903. [PMID: 21320433 DOI: 10.1016/j.bpj.2010.12.3727] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2010] [Revised: 12/14/2010] [Accepted: 12/16/2010] [Indexed: 11/20/2022] Open
Abstract
The temperature dependence of agonist binding and channel gating were measured for wild-type adult neuromuscular acetylcholine receptors activated by acetylcholine, carbamylcholine, or choline. With acetylcholine, temperature changed the gating rate constants (Q(10) ≈ 3.2) but had almost no effect on the equilibrium constant. The enthalpy change associated with gating was agonist-dependent, but for all three ligands it was approximately equal to the corresponding free-energy change. The equilibrium dissociation constant of the resting conformation (K(d)), the slope of the rate-equilibrium free-energy relationship (Φ), and the acetylcholine association and dissociation rate constants were approximately temperature-independent. In the mutant αG153S, the choline association and dissociation rate constants were temperature-dependent (Q(10) ≈ 7.4) but K(d) was not. By combining two independent mutations, we were able to compensate for the catalytic effect of temperature on the decay time constant of a synaptic current. At mouse body temperature, the channel-opening and -closing rate constants are ∼400 and 16 ms(-1). We hypothesize that the agonist dependence of the gating enthalpy change is associated with differences in ligand binding, specifically to the open-channel conformation of the protein.
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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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20
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Jha A, Auerbach A. Acetylcholine receptor channels activated by a single agonist molecule. Biophys J 2010; 98:1840-6. [PMID: 20441747 DOI: 10.1016/j.bpj.2010.01.025] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2009] [Revised: 01/12/2010] [Accepted: 01/14/2010] [Indexed: 11/30/2022] Open
Abstract
The neuromuscular acetylcholine receptor (AChR) is an allosteric protein that alternatively adopts inactive versus active conformations (R<-->R). The R shape has a higher agonist affinity and ionic conductance than R. To understand how agonists trigger this gating isomerization, we examined single-channel currents from adult mouse muscle AChRs that isomerize normally without agonists but have only a single site able to use agonist binding energy to motivate gating. We estimated the monoliganded gating equilibrium constant E(1) and the energy change associated with the R versus R change in affinity for agonists. AChRs with only one operational binding site gave rise to a single population of currents, indicating that the two transmitter binding sites have approximately the same affinity for the transmitter ACh. The results indicated that E(1) approximately 4.3 x 10(-3) with ACh, and approximately 1.7 x 10(-4) with the partial-agonist choline. From these values and the diliganded gating equilibrium constants, we estimate that the unliganded AChR gating constant is E(0) approximately 6.5 x 10(-7). Gating changes the stability of the ligand-protein complex by approximately 5.2 kcal/mol for ACh and approximately 3.3 kcal/mol for choline.
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Affiliation(s)
- Archana Jha
- Department of Physiology and Biophysics, State University of New York, Buffalo, New York, USA
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21
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Chin C, Kim IK, Lim DY, Kim KS, Lee HA, Kim EJ. Gold nanoparticle-choline complexes can block nicotinic acetylcholine receptors. Int J Nanomedicine 2010; 5:315-21. [PMID: 20517475 PMCID: PMC2875724 DOI: 10.2147/ijn.s10466] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022] Open
Abstract
We identified a novel class of direct ion-channel blockers of ligand-gated ion channels called the gold nanoparticle–choline complex. Negatively charged gold nanoparticles (1.4 nm) block ion pores by binding to the sulfur group of the cysteine loop of nicotinic acetylcholine receptors (nAChRs), and currents evoked by acetylcholine (Ach) can break these bonds. The current evoked by ACh in nAChRs was blocked directly in ion pores by the gold nanoparticle– choline complex. In adrenal-gland perfusion studies, the complex also blocked nAChRs by diminishing catecholamine release by about 75%. An in vivo study showed muscle relaxation in rats after injection of the complex. These results will foster the application of gold nanoparticles as a direct ion-channel blocker.
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Affiliation(s)
- Chur Chin
- Department of Pediatrics, Fatima Hospital, Daegu, Korea
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22
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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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23
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Lape R, Krashia P, Colquhoun D, Sivilotti LG. Agonist and blocking actions of choline and tetramethylammonium on human muscle acetylcholine receptors. J Physiol 2009; 587:5045-72. [PMID: 19752108 PMCID: PMC2790248 DOI: 10.1113/jphysiol.2009.176305] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2009] [Accepted: 09/08/2009] [Indexed: 11/08/2022] Open
Abstract
Choline has been used widely as an agonist for the investigation of gain-of-function mutants of the nicotinic acetylcholine receptor. It is useful because it behaves like a partial agonist. The efficacy of choline is difficult to measure because choline blocks the channel at concentrations about four times lower than those that activate it. We have fitted activation mechanisms to single-channel activity elicited from HEK-expressed human recombinant muscle nicotinic receptors by choline and by tetramethylammonium (TMA). Channel block by the agonist was incorporated into the mechanisms that were fitted, and block was found not to be selective for the open state. The results also suggest that channel block is very fast and that the channel can shut almost as fast as normal when the blocker was bound. Single-channel data are compatible with a mechanism in which choline is actually a full agonist, its maximum response being limited only by channel block. However, they are also compatible with a mechanism incorporating a pre-opening conformation change ('flip') in which choline is a genuine partial agonist. The latter explanation is favoured by concentration jump experiments, and by the fact that only this mechanism fits the TMA data. We propose that choline, like TMA, is a partial agonist because it is very ineffective (approximately 600-fold less than acetylcholine) at eliciting the initial, pre-opening conformation change. Once flipping has occurred, all agonists, even choline, open the channel with similar efficiency.
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Affiliation(s)
- Remigijus Lape
- Department of Neuroscience, Physiology and Pharmacology, Medical Sciences Building, University College London, Gower St, London WC1E 6BT, UK
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24
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Jha A, Purohit P, Auerbach A. Energy and structure of the M2 helix in acetylcholine receptor-channel gating. Biophys J 2009; 96:4075-84. [PMID: 19450479 DOI: 10.1016/j.bpj.2009.02.030] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2008] [Revised: 02/20/2009] [Accepted: 02/23/2009] [Indexed: 11/18/2022] Open
Abstract
We studied single-channel currents from neuromuscular acetylcholine receptor-channels with mutations in the pore-lining, M2 helix of the epsilon-subunit. Three parameters were quantified: 1), the diliganded gating equilibrium constant (E(2)), which reflects the energy difference between C(losed) and O(pen) conformations; 2), the correlation between the opening rate constant and E(2) on a log-log scale (Phi), which illuminates the energy character of the residue (C- versus O-like) within the C<-->O isomerization process; and 3), the open-channel current amplitude (i(0)), which reports whether a mutation alters the energetics of ion permeation. The largest E(2) changes were observed in the cytoplasmic half of epsilonM2 (5', 9', 12', 13', and 16'), with smaller changes apparent for residues > or =17'. Phi was approximately 0.54 for most epsilonM2 residues, but was approximately 0.32 at the positions that had largest E(2) changes. An arginine substitution reduced i(0) significantly at six positions, with the magnitude of the reduction increasing, 16'-->2'. The measurements suggest that the 9', 12', and 13' residues experience large and late free-energy changes in the channel-opening process. We speculate that in the gating isomerization the pore-facing residues >6' and <16' experience multiple energy perturbations associated with changes in protein structure and, perhaps, hydration.
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Affiliation(s)
- Archana Jha
- Department of Physiology and Biophysics, State University of New York, Buffalo, New York, USA
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25
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Tantama M, Licht S. Functional equivalence of the nicotinic acetylcholine receptor transmitter binding sites in the open state. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2009; 1788:936-44. [PMID: 19366595 DOI: 10.1016/j.bbamem.2009.01.009] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/30/2008] [Revised: 12/14/2008] [Accepted: 01/21/2009] [Indexed: 10/21/2022]
Abstract
The subunits of the muscle-type nicotinic acetylcholine receptor (AChR) are not uniformly oriented in the resting closed conformation: the two alpha subunits are rotated relative to its non-alpha subunits. In contrast, all the subunits overlay well with one another when agonist is bound to the AChR, suggesting that they are uniformly oriented in the open receptor. This gating-dependent increase in orientational uniformity due to rotation of the alpha subunits might affect the relative affinities of the two transmitter binding sites, making the two affinities dissimilar (functionally non-equivalent) in the initial ligand-bound closed state but similar (functionally equivalent) in the open state. To test this hypothesis, we measured single-channel activity of the alphaG153S gain-of-function mutant receptor evoked by choline, and estimated the resting closed-state and open-state affinities of the two transmitter binding sites. Both model-independent analyses and maximum-likelihood estimation of microscopic rate constants indicate that channel opening makes the binding sites' affinities more similar to each other. These results support the hypothesis that open-state affinities to the transmitter binding sites are primarily determined by the alpha subunits.
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Affiliation(s)
- Mathew Tantama
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 16, Room 573B, Cambridge, Massachusetts 02139, USA
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26
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Bernal JA, Mulet J, Castillo M, Criado M, Sala F, Sala S. Single-channel study of the binding-gating coupling in the slowly desensitizing chimeric α7-5HT3A receptor. FEBS Lett 2009; 583:1045-51. [DOI: 10.1016/j.febslet.2009.02.026] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2008] [Revised: 02/13/2009] [Accepted: 02/16/2009] [Indexed: 11/26/2022]
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27
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Tantama M, Licht S. Use of calculated cation-pi binding energies to predict relative strengths of nicotinic acetylcholine receptor agonists. ACS Chem Biol 2008; 3:693-702. [PMID: 19032090 DOI: 10.1021/cb800189y] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Agonists and antagonists of the nicotinic acetylcholine receptor (nAChR) are used to treat nicotine addiction, neuromuscular disorders, and neurological diseases. In designing small molecule therapeutics with the nAChR as a target, it is useful to identify chemical parameters that correlate with ability to activate the receptor. Previous studies have shown that cation-pi interactions at the transmitter binding sites of the nAChR are important for receptor activation by strong agonists such as acetylcholine. We hypothesized that a calculated estimate of cation-pi binding ability could be used to predict the efficiency for channel opening (i.e., the gating efficiency) associated with activation of the acetylcholine receptor by a series of structurally related organic cations. We demonstrate that the calculated cation-pi energy is strongly correlated with gating efficiency but only weakly correlated with closed-state binding affinity. Our results suggest that cation-pi interactions contribute significantly to the open-state affinity of these cations and that the calculated cation-pi energy will be a useful parameter for designing nAChR agonists and antagonists.
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Affiliation(s)
- Mathew Tantama
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 16, Room 573B, Cambridge, Massachusetts 02139
| | - Stuart Licht
- Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 16, Room 573B, Cambridge, Massachusetts 02139
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28
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Bafna PA, Purohit PG, Auerbach A. Gating at the mouth of the acetylcholine receptor channel: energetic consequences of mutations in the alphaM2-cap. PLoS One 2008; 3:e2515. [PMID: 18575616 PMCID: PMC2429975 DOI: 10.1371/journal.pone.0002515] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2008] [Accepted: 05/16/2008] [Indexed: 11/18/2022] Open
Abstract
Gating of nicotinic acetylcholine receptors from a C(losed) to an O(pen) conformation is the initial event in the postsynaptic signaling cascade at the vertebrate nerve-muscle junction. Studies of receptor structure and function show that many residues in this large, five-subunit membrane protein contribute to the energy difference between C and O. Of special interest are amino acids located at the two transmitter binding sites and in the narrow region of the channel, where C↔O gating motions generate a low↔high change in the affinity for agonists and in the ionic conductance, respectively. We have measured the energy changes and relative timing of gating movements for residues that lie between these two locations, in the C-terminus of the pore-lining M2 helix of the α subunit (‘αM2-cap’). This region contains a binding site for non-competitive inhibitors and a charged ring that influences the conductance of the open pore. αM2-cap mutations have large effects on gating but much smaller effects on agonist binding, channel conductance, channel block and desensitization. Three αM2-cap residues (αI260, αP265 and αS268) appear to move at the outset of channel-opening, about at the same time as those at the transmitter binding site. The results suggest that the αM2-cap changes its secondary structure to link gating motions in the extracellular domain with those in the channel that regulate ionic conductance.
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Affiliation(s)
- Pallavi A. Bafna
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
| | - Prasad G. Purohit
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
| | - Anthony Auerbach
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
- * E-mail:
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29
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Wells GB. Structural answers and persistent questions about how nicotinic receptors work. FRONTIERS IN BIOSCIENCE : A JOURNAL AND VIRTUAL LIBRARY 2008; 13:5479-510. [PMID: 18508600 PMCID: PMC2430769 DOI: 10.2741/3094] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The electron diffraction structure of nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata and the X-ray crystallographic structure of acetylcholine binding protein (AChBP) are providing new answers to persistent questions about how nAChRs function as biophysical machines and as participants in cellular and systems physiology. New high-resolution information about nAChR structures might come from advances in crystallography and NMR, from extracellular domain nAChRs as high fidelity models, and from prokaryotic nicotinoid proteins. At the level of biophysics, structures of different nAChRs with different pharmacological profiles and kinetics will help describe how agonists and antagonists bind to orthosteric binding sites, how allosteric modulators affect function by binding outside these sites, how nAChRs control ion flow, and how large cytoplasmic domains affect function. At the level of cellular and systems physiology, structures of nAChRs will help characterize interactions with other cellular components, including lipids and trafficking and signaling proteins, and contribute to understanding the roles of nAChRs in addiction, neurodegeneration, and mental illness. Understanding nAChRs at an atomic level will be important for designing interventions for these pathologies.
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Affiliation(s)
- Gregg B Wells
- Department of Molecular and Cellular Medicine, College of Medicine, Texas A&M Health Science Center, College Station, TX 77843-1114, USA.
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Jha A, Cadugan DJ, Purohit P, Auerbach A. Acetylcholine receptor gating at extracellular transmembrane domain interface: the cys-loop and M2-M3 linker. ACTA ACUST UNITED AC 2008; 130:547-58. [PMID: 18040057 PMCID: PMC2151658 DOI: 10.1085/jgp.200709856] [Citation(s) in RCA: 77] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Acetylcholine receptor channel gating is a propagated conformational cascade that links changes in structure and function at the transmitter binding sites in the extracellular domain (ECD) with those at a “gate” in the transmembrane domain (TMD). We used Φ-value analysis to probe the relative timing of the gating motions of α-subunit residues located near the ECD–TMD interface. Mutation of four of the seven amino acids in the M2–M3 linker (which connects the pore-lining M2 helix with the M3 helix), including three of the four residues in the core of the linker, changed the diliganded gating equilibrium constant (Keq) by up to 10,000-fold (P272 > I274 > A270 > G275). The average Φ-value for the whole linker was ∼0.64. One interpretation of this result is that the gating motions of the M2–M3 linker are approximately synchronous with those of much of M2 (∼0.64), but occur after those of the transmitter binding site region (∼0.93) and loops 2 and 7 (∼0.77). We also examined mutants of six cys-loop residues (V132, T133, H134, F135, P136, and F137). Mutation of V132, H134, and F135 changed Keq by 2800-, 10-, and 18-fold, respectively, and with an average Φ-value of 0.74, similar to those of other cys-loop residues. Even though V132 and I274 are close, the energetic coupling between I and V mutants of these positions was small (≤0.51 kcal mol−1). The M2–M3 linker appears to be the key moving part that couples gating motions at the base of the ECD with those in TMD. These interactions are distributed along an ∼16-Å border and involve about a dozen residues.
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Affiliation(s)
- Archana Jha
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214, USA
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Baran I, Ganea C, Baran V. A two-gate model for the ryanodine receptor with allosteric modulation by caffeine and quercetin. EUROPEAN BIOPHYSICS JOURNAL: EBJ 2008; 37:793-806. [DOI: 10.1007/s00249-008-0271-6] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/27/2007] [Revised: 12/22/2007] [Accepted: 01/17/2008] [Indexed: 12/01/2022]
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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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Corradi J, Spitzmaul G, De Rosa MJ, Costabel M, Bouzat C. Role of pairwise interactions between M1 and M2 domains of the nicotinic receptor in channel gating. Biophys J 2006; 92:76-86. [PMID: 17028140 PMCID: PMC1697868 DOI: 10.1529/biophysj.106.088757] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The adult form of the nicotinic acetylcholine receptor (AChR) consists of five subunits (alpha(2)betaepsilondelta), each having four transmembrane domains (M1-M4). The atomic model of the nicotinic acetylcholine receptor shows that the pore-lining M2 domains make no extensive contacts with the rest of the transmembrane domains. However, there are several sites where close appositions between segments occur. It has been suggested that the pair alphaM1-F15' and alphaM2-L11' is one of the potential interactions between segments. To determine experimentally if these residues are interacting and to explore if this interhelical interaction is essential for channel gating, we combined mutagenesis with single-channel kinetic analysis. Mutations in alphaM1-F15' lead to profound changes in the opening rate and slighter changes in the closing rate. Channel gating is impaired as the volume of the residue increases. Rate-equilibrium linear free-energy relationship analysis reveals an approximately 70% open-state-like environment for alphaM1-F15' at the transition state of the gating reaction, suggesting that it moves early during the gating process. Replacing the residue at alphaM1-15' by that at alphaM2-11' and vice versa profoundly alters gating, but the combination of the two mutations restores gating to near normal, indicating that alphaM1-F15' and alphaM2-L11' are interchangeable. Double-mutant cycle analysis shows that these residues are energetically coupled. Thus, the interaction between M1 and M2 plays a key role in channel gating.
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Affiliation(s)
- Jeremías Corradi
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-CONICET, Bahía Blanca, Argentina
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Purohit Y, Grosman C. Estimating binding affinities of the nicotinic receptor for low-efficacy ligands using mixtures of agonists and two-dimensional concentration-response relationships. ACTA ACUST UNITED AC 2006; 127:719-35. [PMID: 16735756 PMCID: PMC2151536 DOI: 10.1085/jgp.200509438] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022]
Abstract
The phenomenon of ligand-induced ion channel gating hinges upon the ability of a receptor channel to bind ligand molecules with conformation-specific affinities. However, our understanding of this fundamental phenomenon is notably limited, not only because the changes in binding site structure and ligand conformation that occur upon gating are largely unknown but, also, because the strength of these ligand–receptor interactions are experimentally elusive. Both high- and low-efficacy ligands pose a number of analytical and experimental challenges that can render the estimation of their conformation-specific binding affinities impossible. In this paper, we present a novel assay that overcomes some of the hurdles presented by weak agonists of the muscle nicotinic receptor and allows the estimation of their closed-state affinities. The method, which we have termed the “activation-competition” assay, consists of a single-channel concentration–response assay performed in the presence of a binary mixture of ligands of widely different efficacies. By plotting the channel response (i.e., the open probability) as a function of the concentration of each agonist in the mixture, interpreting the observed response in the framework of a plausible kinetic scheme, and fitting the open probability surface with the corresponding function, the affinities of the closed receptor for the two agonists can be simultaneously extracted as free parameters. Here, we applied this methodology to estimate the closed-state affinity of the muscle nicotinic receptor for choline (a very weak agonist) using acetylcholine (ACh) as the partner in the mixture. We estimated the dissociation equilibrium constant of choline (KD) from the wild type's closed state to be 4.1 ± 0.5 mM (and that of ACh to be 106 ± 6 μM). We also discuss the use of accurate estimates of affinities for low-efficacy agonists as a tool to discriminate between binding and gating effects of mutations, and in the context of the rational design of therapeutic drugs.
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Affiliation(s)
- Yamini Purohit
- Department of Molecular and Integrative Physiology, Center for Biophysics and Computational Biology, and Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, 61801, USA
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