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Speculation on How RIC-3 and Other Chaperones Facilitate α7 Nicotinic Receptor Folding and Assembly. MOLECULES (BASEL, SWITZERLAND) 2022; 27:molecules27144527. [PMID: 35889400 PMCID: PMC9318448 DOI: 10.3390/molecules27144527] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 06/22/2022] [Revised: 07/07/2022] [Accepted: 07/09/2022] [Indexed: 11/17/2022]
Abstract
The process of how multimeric transmembrane proteins fold and assemble in the endoplasmic reticulum is not well understood. The alpha7 nicotinic receptor (α7 nAChR) is a good model for multimeric protein assembly since it has at least two independent and specialized chaperones: Resistance to Inhibitors of Cholinesterase 3 (RIC-3) and Nicotinic Acetylcholine Receptor Regulator (NACHO). Recent cryo-EM and NMR data revealed structural features of α7 nAChRs. A ser-ala-pro (SAP) motif precedes a structurally important but unique "latch" helix in α7 nAChRs. A sampling of α7 sequences suggests the SAP motif is conserved from C. elegans to humans, but the latch sequence is only conserved in vertebrates. How RIC-3 and NACHO facilitate receptor subunits folding into their final pentameric configuration is not known. The artificial intelligence program AlphaFold2 recently predicted structures for NACHO and RIC-3. NACHO is highly conserved in sequence and structure across species, but RIC-3 is not. This review ponders how different intrinsically disordered RIC-3 isoforms from C. elegans to humans interact with α7 nAChR subunits despite having little sequence homology across RIC-3 species. Two models from the literature about how RIC-3 assists α7 nAChR assembly are evaluated considering recent structural information about the receptor and its chaperones.
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2
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Tillman TS, Choi Z, Xu Y, Tang P. Functional Tolerance to Cysteine Mutations in Human α7 Nicotinic Acetylcholine Receptors. ACS Chem Neurosci 2020; 11:242-247. [PMID: 31951367 DOI: 10.1021/acschemneuro.9b00647] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022] Open
Abstract
The α7 nicotinic acetylcholine receptor (α7 nAChR) is involved in various intracellular signaling pathways that mediate addiction, chronic pain, and other diseases, but its intracellular domain structures remain undetermined. The presence of 17 native cysteines in α7 nAChR provides opportunities for extracting structural information through site-directed labeling of chemical probes in strategic locations, but it also creates uncertainties in channel function when those native cysteines must be mutated. Using site-directed mutagenesis and two-electrode voltage clamp electrophysiology measurements, we found that α7 nAChR's function was well tolerated for mutations of all 13 cysteines as long as two pairs of disulfide-bond cysteines remained in the extracellular domain. Furthermore, surface plasmon resonance measurements showed that the cysteine mutations did not affect α7 nAChR binding to the intracellular protein PICK1. The study suggests that a high native cysteine content does not necessarily preclude the use of single cysteine labeling for acquiring structural information on functional proteins.
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Affiliation(s)
- Tommy S. Tillman
- Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
| | - Zachary Choi
- Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
| | - Yan Xu
- Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Department of Structural Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
| | - Pei Tang
- Department of Anesthesiology and Perioperative Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Department of Computational and Systems Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
- Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States
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α7 nicotinic acetylcholine receptor upregulation by anti-apoptotic Bcl-2 proteins. Nat Commun 2019; 10:2746. [PMID: 31227712 PMCID: PMC6588605 DOI: 10.1038/s41467-019-10723-x] [Citation(s) in RCA: 21] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/17/2018] [Accepted: 05/24/2019] [Indexed: 01/07/2023] Open
Abstract
Nicotinic acetylcholine receptors (nAChRs) mediate and modulate synaptic transmission throughout the brain, and contribute to learning, memory, and behavior. Dysregulation of α7-type nAChRs in neuropsychiatric as well as immunological and oncological diseases makes them attractive targets for pharmaceutical development. Recently, we identified NACHO as an essential chaperone for α7 nAChRs. Leveraging the robust recombinant expression of α7 nAChRs with NACHO, we utilized genome-wide cDNA library screening and discovered that several anti-apoptotic Bcl-2 family proteins further upregulate receptor assembly and cell surface expression. These effects are mediated by an intracellular motif on α7 that resembles the BH3 binding domain of pro-apoptotic Bcl-2 proteins, and can be blocked by BH3 mimetic Bcl-2 inhibitors. Overexpression of Bcl-2 member Mcl-1 in neurons enhanced surface expression of endogenous α7 nAChRs, while a combination of chemotherapeutic Bcl2-inhibitors suppressed neuronal α7 receptor assembly. These results demonstrate that Bcl-2 proteins link α7 nAChR assembly to cell survival pathways. The α7 nicotinic acetylcholine receptor (nAChR) plays a major role in shaping the activity of neuronal circuits and contributes to the pathophysiology of several neurological disorders. Following cDNA library screening, the authors identify anti-apoptotic, Bcl-2 family proteins as enhancers of α7 nAChR assembly, acting through an intracellular BH3-like domain during receptor biogenesis in the endoplasmic reticulum.
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Torrealba D, Balasch JC, Criado M, Tort L, Mackenzie S, Roher N. Functional evidence for the inflammatory reflex in teleosts: A novel α7 nicotinic acetylcholine receptor modulates the macrophage response to dsRNA. DEVELOPMENTAL AND COMPARATIVE IMMUNOLOGY 2018; 84:279-291. [PMID: 29501534 DOI: 10.1016/j.dci.2018.02.020] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/29/2017] [Revised: 02/28/2018] [Accepted: 02/28/2018] [Indexed: 06/08/2023]
Abstract
The inflammatory reflex modulates the innate immune system, keeping in check the detrimental consequences of overstimulation. A key player controlling the inflammatory reflex is the alpha 7 acetylcholine receptor (α7nAChR). This receptor is one of the signalling molecules regulating cytokine expression in macrophages. In this study, we characterize a novel teleost α7nAChR. Protein sequence analysis shows a high degree of conservation with mammalian orthologs and trout α7nAChR has all the features and essential amino acids to form a fully functional receptor. We demonstrate that trout macrophages can bind α-bungarotoxin (α-BTX), a competitive antagonist for α7nAChRs. Moreover, nicotine stimulation produces a decrease in pro-inflammatory cytokine expression after stimulation with poly(I:C). These results suggest the presence of a functional α7nAChR in the macrophage plasma membrane. Further, in vivo injection of poly(I:C) induced an increase in serum ACh levels in rainbow trout. Our results manifest for the first time the functional conservation of the inflammatory reflex in teleosts.
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Affiliation(s)
- Débora Torrealba
- Institut de Biotecnologia i de Biomedicina (IBB), Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain; Departament de Biologia Cel·lular, Fisiologia Animal i Immunologia, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain
| | - Joan Carles Balasch
- Departament de Biologia Cel·lular, Fisiologia Animal i Immunologia, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain
| | - Manuel Criado
- Instituto de Neurociencias, Universidad Miguel Hernández-CSIC, 03550 Sant Joan d'Alacant, Spain
| | - Lluís Tort
- Departament de Biologia Cel·lular, Fisiologia Animal i Immunologia, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain
| | - Simon Mackenzie
- Institute of Aquaculture, University of Stirling, Stirling, FK9 4LA, United Kingdom.
| | - Nerea Roher
- Institut de Biotecnologia i de Biomedicina (IBB), Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain; Departament de Biologia Cel·lular, Fisiologia Animal i Immunologia, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Spain.
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Kouvatsos N, Niarchos A, Zisimopoulou P, Eliopoulos E, Poulas K, Tzartos S. Purification and functional characterization of a truncated human α4β2 nicotinic acetylcholine receptor. Int J Biol Macromol 2014; 70:320-6. [PMID: 25014634 DOI: 10.1016/j.ijbiomac.2014.06.058] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2014] [Revised: 06/18/2014] [Accepted: 06/19/2014] [Indexed: 10/25/2022]
Abstract
Nicotinic acetylcholine receptors (nAChR) are abundant in the brain and are essential in cognitive function, learning and memory. Previous efforts on α4β2 nAChR had been focused on functional and pharmacological characterization, where high expression yield is not essential. For structural studies though, large amounts of pure protein is important but heterologous overexpression of membrane proteins can be a burdensome task, especially if high amounts are required. In the current study, a truncated mutant of the human α4β2 nAChR was designed in order to improve expression and solubility and to obtain material suitable for high resolution structural studies. We showed that the wild type α4β2 nAChR presented low expression and solubilization yield both of which were improved with the truncated construct. The truncated nAChR showed similar binding profile to the wild type, was purified by a two-step chromatography and isolated in high purity and adequate quantity.
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Affiliation(s)
- Nikolaos Kouvatsos
- Department of Neurobiology, Hellenic Pasteur Institute, 127 Vass. Sofias Avenue, GR11521 Athens, Greece
| | | | - Paraskevi Zisimopoulou
- Department of Neurobiology, Hellenic Pasteur Institute, 127 Vass. Sofias Avenue, GR11521 Athens, Greece
| | - Elias Eliopoulos
- Department of Agricultural Biotechnology, Agricultural University of Athens, Holy Street 75, GR11855 Athens, Greece
| | | | - Socrates Tzartos
- Department of Neurobiology, Hellenic Pasteur Institute, 127 Vass. Sofias Avenue, GR11521 Athens, Greece; Department of Pharmacy, University of Patras, GR26500 Patras, Greece.
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Bondarenko V, Mowrey DD, Tillman TS, Seyoum E, Xu Y, Tang P. NMR structures of the human α7 nAChR transmembrane domain and associated anesthetic binding sites. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2013; 1838:1389-95. [PMID: 24384062 DOI: 10.1016/j.bbamem.2013.12.018] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/29/2013] [Revised: 11/19/2013] [Accepted: 12/23/2013] [Indexed: 12/11/2022]
Abstract
The α7 nicotinic acetylcholine receptor (nAChR), assembled as homomeric pentameric ligand-gated ion channels, is one of the most abundant nAChR subtypes in the brain. Despite its importance in memory, learning and cognition, no structure has been determined for the α7 nAChR TM domain, a target for allosteric modulators. Using solution state NMR, we determined the structure of the human α7 nAChR TM domain (PDB ID: 2MAW) and demonstrated that the α7 TM domain formed functional channels in Xenopus oocytes. We identified the associated binding sites for the anesthetics halothane and ketamine; the former cannot sensitively inhibit α7 function, but the latter can. The α7 TM domain folds into the expected four-helical bundle motif, but the intra-subunit cavity at the extracellular end of the α7 TM domain is smaller than the equivalent cavity in the α4β2 nAChRs (PDB IDs: 2LLY; 2LM2). Neither drug binds to the extracellular end of the α7 TM domain, but two halothane molecules or one ketamine molecule binds to the intracellular end of the α7 TM domain. Halothane and ketamine binding sites are partially overlapped. Ketamine, but not halothane, perturbed the α7 channel-gate residue L9'. Furthermore, halothane did not induce profound dynamics changes in the α7 channel as observed in α4β2. The study offers a novel high-resolution structure for the human α7 nAChR TM domain that is invaluable for developing α7-specific therapeutics. It also provides evidence to support the hypothesis: only when anesthetic binding perturbs the channel pore or alters the channel motion, can binding generate functional consequences.
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Affiliation(s)
- Vasyl Bondarenko
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA
| | - David D Mowrey
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA; Department of Computational & Systems Biology, University of Pittsburgh School of Medicine, USA
| | - Tommy S Tillman
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA
| | - Edom Seyoum
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA
| | - Yan Xu
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA; Department of Structural Biology, University of Pittsburgh School of Medicine, USA; Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, USA
| | - Pei Tang
- Department of Anesthesiology, University of Pittsburgh School of Medicine, USA; Department of Computational & Systems Biology, University of Pittsburgh School of Medicine, USA; Department of Pharmacology & Chemical Biology, University of Pittsburgh School of Medicine, USA.
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7
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Carland JE, Cooper MA, Livesey MR, Hales TG, Peters JA, Lambert JJ. Mutagenic analysis of the intracellular portals of the human 5-HT3A receptor. J Biol Chem 2013; 288:31592-601. [PMID: 24030822 PMCID: PMC3814755 DOI: 10.1074/jbc.m113.503300] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/18/2013] [Revised: 09/11/2013] [Indexed: 11/15/2022] Open
Abstract
Structural models of Cys-loop receptors based on homology with the Torpedo marmorata nicotinic acetylcholine receptor infer the existence of cytoplasmic portals within the conduction pathway framed by helical amphipathic regions (termed membrane-associated (MA) helices) of adjacent intracellular M3-M4 loops. Consistent with these models, two arginine residues (Arg(436) and Arg(440)) within the MA helix of 5-hydroxytryptamine type 3A (5-HT3A) receptors act singularly as rate-limiting determinants of single-channel conductance (γ). However, there is little conservation in primary amino acid sequences across the cytoplasmic loops of Cys-loop receptors, limiting confidence in the fidelity of this particular aspect of the 5-HT3A receptor model. We probed the majority of residues within the MA helix of the human 5-HT3A subunit using alanine- and arginine-scanning mutagenesis and the substituted cysteine accessibility method to determine their relative influences upon γ. Numerous residues, prominently those at the 435, 436, 439, and 440 positions, were found to markedly influence γ. This approach yielded a functional map of the 5-HT3A receptor portals, which agrees well with the homology model.
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MESH Headings
- Animals
- Cell Line
- Humans
- Models, Molecular
- Mutagenesis, Site-Directed
- Protein Structure, Secondary
- Protein Structure, Tertiary
- Receptors, Serotonin, 5-HT3/chemistry
- Receptors, Serotonin, 5-HT3/genetics
- Receptors, Serotonin, 5-HT3/metabolism
- Structural Homology, Protein
- Torpedo
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Affiliation(s)
- Jane E. Carland
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Michelle A. Cooper
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Matthew R. Livesey
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Tim G. Hales
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - John A. Peters
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
| | - Jeremy J. Lambert
- From the Division of Neuroscience, Medical Research and Medical Education Institutes, Ninewells Hospital and Medical School, University of Dundee, Dundee DD1 9SY, Scotland, United Kingdom
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Goyal R, Salahudeen AA, Jansen M. Engineering a prokaryotic Cys-loop receptor with a third functional domain. J Biol Chem 2011; 286:34635-42. [PMID: 21844195 DOI: 10.1074/jbc.m111.269647] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022] Open
Abstract
Prokaryotic members of the Cys-loop receptor ligand-gated ion channel superfamily were recently identified. Previously, Cys-loop receptors were only known from multicellular organisms (metazoans). Contrary to the metazoan Cys-loop receptors, the prokaryotic ones consist of an extracellular (ECD) and a transmembrane domain (TMD), lacking the large intracellular domain (ICD) present in metazoa (between transmembrane segments M3 and M4). Using a chimera approach, we added the 115-amino acid ICD from mammalian serotonin type 3A receptors (5-HT(3A)) to the prokaryotic proton-activated Gloeobacter violaceus ligand-gated ion channel (GLIC). We created 12 GLIC-5-HT(3A)-ICD chimeras by replacing a variable number of amino acids in the short GLIC M3M4 linker with the entire 5-HT(3A)-ICD. Two-electrode voltage clamp recordings after expression in Xenopus laevis oocytes showed that only two chimeras were functional and produced currents upon acidification. The pH(50) was comparable with wild-type GLIC. 5-HT(3A) receptor expression can be inhibited by the chaperone protein RIC-3. We have shown previously that the 5-HT(3A)-ICD is required for the attenuation of 5-HT-induced currents when RIC-3 is co-expressed with 5-HT(3A) receptors in X. laevis oocytes. Expression of both functional 5-HT(3A) chimeras was inhibited by RIC-3 co-expression, indicating appropriate folding of the 5-HT(3A)-ICD in the chimeras. Our results indicate that the ICD can be considered a separate domain that can be removed from or added to the ECD and TMD while maintaining the overall structure and function of the ECD and TMD.
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Affiliation(s)
- Raman Goyal
- Department of Cell Physiology and Molecular Biophysics, School of Medicine, Texas Tech University Health Sciences Center, Lubbock, Texas 79430, USA
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Criado M, Mulet J, Gerber S, Sala S, Sala F. A small cytoplasmic region adjacent to the fourth transmembrane segment of the α7 nicotinic receptor is essential for its biogenesis. FEBS Lett 2011; 585:2477-80. [DOI: 10.1016/j.febslet.2011.06.028] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2011] [Revised: 06/21/2011] [Accepted: 06/22/2011] [Indexed: 10/18/2022]
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Mukherjee J, Kuryatov A, Moss SJ, Lindstrom JM, Anand R. Mutations of cytosolic loop residues impair assembly and maturation of alpha7 nicotinic acetylcholine receptors. J Neurochem 2009; 110:1885-94. [PMID: 19627445 DOI: 10.1111/j.1471-4159.2009.06285.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Mechanisms that regulate early events in the biogenesis of the alpha7 nicotinic acetylcholine receptor (alpha7 AChR) are not well understood. Data presented here show that single amino acid mutations in the cytoplasmic loop of the alpha7 AChR, between position 335 and 343, abolish or attenuate expression of mature pentameric alpha7 AChRs in both human embryonic kidney tsA201 (HEK) and neuronal SH-SY5Y cells. Although the number of mature alpha7 AChRs is increased significantly in the presence of the chaperone protein resistant to inhibitors of cholineesterase-3 in HEK cells, sucrose gradient sedimentation reveals that the vast majority of alpha7 subunits are aggregated or improperly assembled. Transfection of alpha7 AChRs in SH-SY5Y cells, which endogenously express the alpha7 AChR, results in a much larger fraction of subunits assembled into mature AChRs. Thus, efficient assembly of alpha7 AChRs is influenced by several regions of the large cytoplasmic domain, as well perhaps by other parts of its structure, and requires as yet unknown factors not required by other AChR subtypes.
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Affiliation(s)
- Jayanta Mukherjee
- Neuroscience Center of Excellence, Louisiana State University Health Sciences Center, New Orleans, LA, USA
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12
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Zouridakis M, Zisimopoulou P, Poulas K, Tzartos SJ. Recent advances in understanding the structure of nicotinic acetylcholine receptors. IUBMB Life 2009; 61:407-23. [PMID: 19319967 DOI: 10.1002/iub.170] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Nicotinic acetylcholine receptors (nAChRs), members of the Cys-loop ligand-gated ion channels (LGICs) superfamily, are involved in signal transduction upon binding of the neurotransmitter acetylcholine or exogenous ligands, such as nicotine. nAChRs are pentameric assemblies of homologous subunits surrounding a central pore that gates cation flux, and are expressed at the neuromuscular junction and in the nervous system and several nonneuronal cell types. The 17 known nAChR subunits assemble into a variety of pharmacologically distinct receptor subtypes. nAChRs are implicated in a range of physiological functions and pathophysiological conditions related to muscle contraction, learning and memory, reward, motor control, arousal, and analgesia, and therefore present an important target for drug research. Such studies would be greatly facilitated by knowledge of the high-resolution structure of the nAChR. Although this information is far from complete, important progress has been made mainly based on electron microscopy studies of Torpedo nAChR and the high-resolution X-ray crystal structures of the homologous molluscan acetylcholine-binding proteins, the extracellular domain of the mouse nAChR alpha1 subunit, and two prokaryotic pentameric LGICs. Here, we review some of the latest advances in our understanding of nAChR structure and gating.
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Affiliation(s)
- Marios Zouridakis
- Department of Biochemistry, Hellenic Pasteur Institute, Athens, Greece
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Millar NS. A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors. Biochem Pharmacol 2009; 78:766-76. [PMID: 19540210 DOI: 10.1016/j.bcp.2009.06.015] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/20/2009] [Revised: 06/08/2009] [Accepted: 06/10/2009] [Indexed: 11/18/2022]
Abstract
Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.
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Affiliation(s)
- Neil S Millar
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, UK.
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Nicotinic acetylcholine receptor alpha7 subunits with a C2 cytoplasmic loop yellow fluorescent protein insertion form functional receptors. Acta Pharmacol Sin 2009; 30:828-41. [PMID: 19498423 DOI: 10.1038/aps.2009.78] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023] Open
Abstract
AIM Several nicotinic acetylcholine receptor (nAChR) subunits have been engineered as fluorescent protein (FP) fusions and exploited to illuminate features of nAChRs. The aim of this work was to create a FP fusion in the nAChR alpha7 subunit without compromising formation of functional receptors. METHODS A gene construct was generated to introduce yellow fluorescent protein (YFP), in frame, into the otherwise unaltered, large, second cytoplasmic loop between the third and fourth transmembrane domains of the mouse nAChR alpha7 subunit (alpha7Y). SH-EP1 cells were transfected with mouse nAChR wild type alpha7 subunits (alpha7) or with alpha7Y subunits, alone or with the chaperone protein, hRIC-3. Receptor function was assessed using whole-cell current recording. Receptor expression was measured with (125)I-labeled alpha-bungarotoxin (I-Bgt) binding, laser scanning confocal microscopy, and total internal reflectance fluorescence (TIRF) microscopy. RESULTS Whole-cell currents revealed that alpha7Y nAChRs and alpha7 nAChRs were functional with comparable EC(50) values for the alpha7 nAChR-selective agonist, choline, and IC(50) values for the alpha7 nAChR-selective antagonist, methyllycaconitine. I-Bgt binding was detected only after co-expression with hRIC-3. Confocal microscopy revealed that alpha7Y had primarily intracellular rather than surface expression. TIRF microscopy confirmed that little alpha7Y localized to the plasma membrane, typical of alpha7 nAChRs. CONCLUSION nAChRs composed as homooligomers of alpha7Y subunits containing cytoplasmic loop YFP have functional, ligand binding, and trafficking characteristics similar to those of alpha7 nAChRs. alpha7Y nAChRs may be used to elucidate properties of alpha7 nAChRs and to identify and develop novel probes for these receptors, perhaps in high-throughput fashion.
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Lee HK, Gwalani L, Mishra V, Anandjiwala P, Sala F, Sala S, Ballesta JJ, O'Malley D, Criado M, Loring RH. Investigating the role of protein folding and assembly in cell-type dependent expression of alpha7 nicotinic receptors using a green fluorescent protein chimera. Brain Res 2009; 1259:7-16. [PMID: 19368825 DOI: 10.1016/j.brainres.2009.01.046] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/25/2008] [Revised: 01/12/2009] [Accepted: 01/17/2009] [Indexed: 11/25/2022]
Abstract
To test the hypothesis that cell-dependent expression of alpha7 receptors is due to differences in protein folding or assembly, we constructed a chimeric rat alpha7 subunit with green fluorescent protein (GFP) at the receptor C-terminal. Expression of alpha7-GFP in Xenopus oocytes resulted in currents that were indistinguishable from wild type receptors but were only 33% of control. (125)I-alpha-bungarotoxin (alphaBGT) binding at the oocyte surface was reduced to 23% of wild type. Transfection of alpha7-GFP into GH4C1 cells produced fluorescence that was less intense than GFP alone, but showed significant alpha-BGT binding compared to transfection with GFP. In contrast, alpha7-GFP transfection in SH-EP1, HEK293 and CHO-CAR cells produced fluorescence without alphaBGT binding. Flow cytometry of cells transfected with alpha7-GFP indicated fluorescence in both SH-EP1 and GH4C1 cells, but surface toxin binding sites and sites immunoprecipitated using anti-GFP antibodies were undetectable in SH-EP1 cells, suggesting a problem in folding/assembly rather than trafficking. Surprisingly, integrated fluorescence intensities in GH4C1 cells transfected with alpha7-GFP did not correlate with amounts of cell surface or immunoprecipitable alphaBGT binding. Therefore, GFP folding at the C-terminal of the alpha7-GFP chimera is cell-line independent, but toxin binding is highly cell-line dependent, suggesting that if altered protein folding is involved in the cell-type dependence of alpha7 receptor expression, the phenomenon is restricted to specific protein domains. Further, C-terminal GFP-labeled alpha7 receptors decreased the efficiency of folding/assembly not only of chimeric subunits, but also wild-type subunits, suggesting that the C-terminal is an important domain for alpha7 receptor assembly.
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Affiliation(s)
- H K Lee
- Department of Pharmaceutical Science, Northeastern Univ, Boston, MA 02115, USA
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Influence of the M3-M4 intracellular domain upon nicotinic acetylcholine receptor assembly, targeting and function. Br J Pharmacol 2008; 153:1474-84. [PMID: 18204482 DOI: 10.1038/sj.bjp.0707676] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022] Open
Abstract
BACKGROUND AND PURPOSE The aim of this study was to investigate the influence of the intracellular domain of nicotinic acetylcholine receptor (nAChR) subunits upon receptor assembly, targeting and functional properties. EXPERIMENTAL APPROACH Because most nAChR subunits form functional receptors only as heteromeric complexes, it can be difficult to examine the influence of individual subunits or subunit domains in isolation. A series of subunit chimaeras was constructed which contain the intracellular loop region (located between the M3 and M4 transmembrane domains) from nAChR subunits alpha1-alpha10 or beta1-beta4. All of these chimaeras contain common extracellular and transmembrane domains (from the nAChR alpha7 subunit and the 5-hydroxytryptamine receptor 5-HT(3A) subunit, respectively), thereby facilitating both homomeric receptor assembly and detection with radiolabelled or fluorescent alpha-bungarotoxin. KEY RESULTS The nAChR M3-M4 intracellular loop domain had no significant effect upon levels of total subunit protein detected in transfected cells but had a significant influence upon levels of both cell surface and intracellular assembled receptors. Comparisons of functional properties revealed a significant influence of the intracellular loop domain upon both single-channel conductance and receptor desensitization. In addition, studies conducted in polarized epithelial cells demonstrate that the nAChR loop can influence receptor targeting, resulting in either polarized (apical) or non-polarized distribution. CONCLUSIONS AND IMPLICATIONS Evidence has been obtained which demonstrates that the large intracellular loop domain of nAChR subunits can exert a profound influence upon receptor assembly, targeting and ion channel properties.
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17
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Gee VJ, Kracun S, Cooper ST, Gibb AJ, Millar NS. Identification of domains influencing assembly and ion channel properties in alpha 7 nicotinic receptor and 5-HT3 receptor subunit chimaeras. Br J Pharmacol 2007; 152:501-12. [PMID: 17721553 PMCID: PMC2050819 DOI: 10.1038/sj.bjp.0707429] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/09/2007] [Revised: 06/26/2007] [Accepted: 07/02/2007] [Indexed: 02/06/2023] Open
Abstract
BACKGROUND AND PURPOSE Nicotinic acetylcholine receptors (nAChRs) and 5-hydroxytryptamine type 3 receptors (5-HT(3)Rs) are members of the superfamily of neurotransmitter-gated ion channels. Both contain five subunits which assemble to form either homomeric or heteromeric subunit complexes. With the aim of identifying the influence of subunit domains upon receptor assembly and function, a series of chimaeras have been constructed containing regions of the neuronal nAChR alpha 7 subunit and the 5-HT(3) receptor (3A) subunit. EXPERIMENTAL APPROACH A series of subunit chimaeras containing alpha 7 and 5-HT(3A) subunit domains have been constructed and expressed in cultured mammalian cells. Properties of the expressed receptors have been examined by means of radioligand binding, agonist-induced changes in intracellular calcium and patch-clamp electrophysiology. KEY RESULTS Subunit domains which influence properties such as rectification, desensitization and conductance have been identified. In addition, the influence of subunit domains upon subunit folding, receptor assembly and cell-surface expression has been identified. Co-expression studies with the nAChR-associated protein RIC-3 revealed that, in contrast to the potentiating effect of RIC-3 on alpha 7 nAChRs, RIC-3 caused reduced levels of cell-surface expression of some alpha 7/5-HT(3A) chimaeras. CONCLUSIONS AND IMPLICATIONS Evidence has been obtained which demonstrates that subunit transmembrane domains are critical for efficient subunit folding and assembly. In addition, functional characterization of subunit chimaeras revealed that both extracellular and cytoplasmic domains exert a dramatic and significant influence upon single-channel conductance. These data support a role for regions other than hydrophobic transmembrane domains in determining ion channel properties.
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MESH Headings
- Animals
- Binding Sites
- Binding, Competitive
- Bungarotoxins/metabolism
- Calcium/metabolism
- Cell Line
- Cell Membrane/metabolism
- Humans
- Intracellular Fluid/chemistry
- Intracellular Fluid/metabolism
- Iodine Radioisotopes
- Ion Channels/genetics
- Ion Channels/metabolism
- Ion Channels/physiology
- Membrane Potentials/physiology
- Patch-Clamp Techniques
- Protein Structure, Tertiary
- Protein Subunits/genetics
- Protein Subunits/metabolism
- Radioligand Assay
- Rats
- Receptors, Nicotinic/genetics
- Receptors, Nicotinic/metabolism
- Receptors, Nicotinic/physiology
- Receptors, Serotonin, 5-HT3/genetics
- Receptors, Serotonin, 5-HT3/metabolism
- Receptors, Serotonin, 5-HT3/physiology
- Recombinant Fusion Proteins/chemistry
- Recombinant Fusion Proteins/metabolism
- Recombinant Fusion Proteins/physiology
- Transfection
- alpha7 Nicotinic Acetylcholine Receptor
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Affiliation(s)
- V J Gee
- Department of Pharmacology, University College London London, UK
| | - S Kracun
- Department of Pharmacology, University College London London, UK
| | - S T Cooper
- Department of Pharmacology, University College London London, UK
| | - A J Gibb
- Department of Pharmacology, University College London London, UK
| | - N S Millar
- Department of Pharmacology, University College London London, UK
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18
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Castillo M, Mulet J, Gutiérrez LM, Ortiz JA, Castelán F, Gerber S, Sala S, Sala F, Criado M. Role of the RIC-3 protein in trafficking of serotonin and nicotinic acetylcholine receptors. J Mol Neurosci 2007; 30:153-6. [PMID: 17192664 DOI: 10.1385/jmn:30:1:153] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/30/1999] [Revised: 11/30/1999] [Accepted: 11/30/1999] [Indexed: 11/11/2022]
Abstract
Neurotransmitter-gated receptors are assembled in the endoplasmic reticulum and transported to the cell surface through a process that might be of central importance to regulate the efficacy of synaptic transmission (Kneussel and Betz, 2000; Kittler and Moss, 2003). This process is relatively inefficient- what may be the consequence of tight quality controls that guarantee the functional competence of the final product. For this purpose, specific proteins involved in assembly and trafficking of receptors might be required (Keller and Taylor, 1999; Millar, 2003; Wanamaker et al., 2003). The RIC-3 protein could be one of them, as mutations in the ric-3 gene affect maturation of nicotinic acetylcholine receptors (nAChRs) in Caenorhabditis elegans (Halevi et al., 2002). Moreover, the human homolog hRIC-3 showed differential effects when coexpressed with several ligand-gated receptors (Halevi et al., 2003). Thus, it enhanced alpha7 nAChR expression while inhibiting expression of other nAChR subtypes (alpha4beta2 and alpha3beta4) and 5-HT3 serotonin receptors (5-HT3Rs). These opposite effects suggested that the RIC-3 protein might play a key role in the biogenesis of some ligand-gated receptors and prompted us to investigate how it performs its action. Here, we show that the RIC-3 protein acts as a barrier for some receptors like alpha4beta2 nAChRs and 5-HT3Rs, stopping the traffic of mature receptors to the membrane. In contrast, the inefficient transport of alpha7 nAChRs is enhanced by RIC-3 in a process in which certain amino acids at the amphipathic helix located at the C-terminal region of the large cytoplasmic domain are involved.
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Affiliation(s)
- Mar Castillo
- Instituto de Neurociencias de Alicante, UMH-CSIC, 03550 Sant Joan d'Alacant, Spain
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19
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Peters JA, Carland JE, Cooper MA, Livesey MR, Deeb TZ, Hales TG, Lambert JJ. Novel structural determinants of single-channel conductance in nicotinic acetylcholine and 5-hydroxytryptamine type-3 receptors. Biochem Soc Trans 2007; 34:882-6. [PMID: 17052220 DOI: 10.1042/bst0340882] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
Nicotinic ACh (acetylcholine) and 5-HT3 (5-hydroxytryptamine type-3) receptors are cation-selective ion channels of the Cys-loop transmitter-gated ion channel superfamily. Numerous lines of evidence indicate that the channel lining domain of such receptors is formed by the alpha-helical M2 domain (second transmembrane domain) contributed by each of five subunits present within the receptor complex. Specific amino acid residues within the M2 domain have accordingly been demonstrated to influence both single-channel conductance (gamma) and ion selectivity. However, it is now clear from work performed on the homomeric 5-HT3A receptor, heteromeric 5-HT3A/5-HT3B receptor and 5-HT3A/5-HT3B receptor subunit chimaeric constructs that an additional major determinant of gamma resides within a cytoplasmic domain of the receptor termed the MA-stretch (membrane-associated stretch). The MA-stretch, within the M3-M4 loop, is not traditionally thought to be implicated in ion permeation and selection. Here, we describe how such observations extend to a representative neuronal nicotinic ACh receptor composed of alpha4 and beta2 subunits and, by inference, probably other members of the Cys-loop family. In addition, we will attempt to interpret our results within the context of a recently developed atomic scale model of the nicotinic ACh receptor of Torpedo marmorata (marbled electric ray).
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Affiliation(s)
- J 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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20
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Castelán F, Mulet J, Aldea M, Sala S, Sala F, Criado M. Cytoplasmic regions adjacent to the M3 and M4 transmembrane segments influence expression and function of ?7 nicotinic acetylcholine receptors. A study with single amino acid mutants. J Neurochem 2007; 100:406-15. [PMID: 17076762 DOI: 10.1111/j.1471-4159.2006.04202.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
We studied the role of the cytoplasmic regions adjacent to the M3 and M4 transmembrane segments of alpha7 nicotinic receptors in the expression of functional channels. For this purpose, a total of 50 amino acids were mutated throughout the mentioned regions. Mutants close to M3, from Arg294 to Leu321, showed slight modifications in the levels of alpha-bungarotoxin binding sites and acetylcholine-evoked currents. Exceptions were mutants located at two clusters (His296 to Pro300 and Ile312 to Trp316), which exhibited low expression levels. In addition, some mutants showed altered functional responses. Many mutants close to M4 showed increased receptor expression, especially the ones located at the hydrophobic face of a putative amphipathic helix. This effect seems to be the consequence of a combination of increased receptor biosynthesis, higher transport efficiency and delayed degradation, such that we postulate that elements in the amphipathic domain strongly influence receptor stability. Finally, some mutants in this region showed altered functional responses: elimination of positively charged residues (Arg424 and Arg426) increased currents, whereas the opposite was observed upon suppression of negatively charged ones (Glu430 and Glu432). These results suggest that the cytoplasmic regions close to M3 and M4 play important structural and functional roles.
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Affiliation(s)
- Francisco Castelán
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-CSIC, Alicante, Spain
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21
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Morley BJ. Nicotinic cholinergic intercellular communication: implications for the developing auditory system. Hear Res 2005; 206:74-88. [PMID: 16081000 DOI: 10.1016/j.heares.2005.02.012] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/03/2004] [Accepted: 02/24/2005] [Indexed: 02/02/2023]
Abstract
In this paper, research on the temporal and spatial distribution of cholinergic-related molecules in the lower auditory brainstem, with an emphasis on nicotinic acetylcholine receptors (nAChRs), is reviewed. The possible functions of acetylcholine (ACh) in driving selective auditory neurons before the onset of hearing, inducing glutamate receptor gene expression, synaptogenesis, differentiation, and cell survival are discussed. Experiments conducted in other neuronal and non-neuronal systems are drawn on extensively to discuss putative functions of ACh and nAChRs. Data from other systems may provide insight into the functions of ACh and nAChRs in auditory processing. The mismatch of presynaptic and postsynaptic markers and novel endogenous agonists of nAChRs are discussed in the context of non-classical interneuronal communication. The molecular mechanism that may underlie the many functions of ACh and its agonists is the regulation of intracellular calcium through nAChRs. The possible reorganization that may take place in the auditory system by the exposure to nicotine during critical developmental periods is also briefly considered.
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Affiliation(s)
- Barbara J Morley
- Boys Town National Research Hospital, Neurochemistry Laboratory, 555 North 30th Street, Omaha, NE 68131, USA.
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22
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Peters JA, Hales TG, Lambert JJ. Molecular determinants of single-channel conductance and ion selectivity in the Cys-loop family: insights from the 5-HT3 receptor. Trends Pharmacol Sci 2005; 26:587-94. [PMID: 16194573 DOI: 10.1016/j.tips.2005.09.011] [Citation(s) in RCA: 53] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/29/2005] [Revised: 08/19/2005] [Accepted: 09/13/2005] [Indexed: 10/25/2022]
Abstract
The molecular determinants of the ionic selectivity and single-channel conductance of the Cys-loop family of transmitter-gated ion channels are beginning to be understood with increasing precision, in part, as a result of the recent availability of refined ultrastructural information for the archetype of the family, the nicotinic acetylcholine receptor (nAChR). Studies of another member of this family, the 5-HT(3) receptor, have now provided insight into the structure of its channel pore, the location of its gate and mechanisms of ion selectivity and translocation. The anomaly of the extremely low single-channel conductance of the homo-oligomeric 5-HT(3A) receptor has recently been solved, revealing that an intracellular domain of the protein is an important determinant of single-channel conductance. Such data are interpreted, in this article, in light of the most recent developments in structural characterization of the nAChR.
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MESH Headings
- Amino Acid Sequence
- Animals
- Cysteine/chemistry
- Humans
- Ion Channel Gating
- Ion Channels/chemistry
- Ion Channels/genetics
- Ion Channels/metabolism
- Ions
- Models, Molecular
- Molecular Sequence Data
- Mutation
- Protein Structure, Quaternary
- Protein Structure, Tertiary
- Receptors, Nicotinic/chemistry
- Receptors, Nicotinic/genetics
- Receptors, Nicotinic/metabolism
- Receptors, Serotonin, 5-HT3/chemistry
- Receptors, Serotonin, 5-HT3/genetics
- Receptors, Serotonin, 5-HT3/metabolism
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Affiliation(s)
- 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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23
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Castillo M, Mulet J, Gutiérrez LM, Ortiz JA, Castelán F, Gerber S, Sala S, Sala F, Criado M. Dual Role of the RIC-3 Protein in Trafficking of Serotonin and Nicotinic Acetylcholine Receptors. J Biol Chem 2005; 280:27062-8. [PMID: 15927954 DOI: 10.1074/jbc.m503746200] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The ric-3 gene is required for maturation of nicotinic acetylcholine receptors in Caenorhabditis elegans. The human homolog of RIC-3, hRIC-3, enhances expression of alpha7 nicotinic receptors in Xenopus laevis oocytes, whereas it totally abolishes expression of alpha4beta2 nicotinic and 5-HT3 serotonergic receptors. Both the N-terminal region of hRIC-3, which contains two transmembrane segments, and the C-terminal region are needed for these differential effects. hRIC-3 inhibits receptor expression by hindering export of mature receptors to the cell membrane. By using chimeric proteins made of alpha7 and 5-HT3 receptors, we have shown that the presence of an extracellular isoleucine close to the first transmembrane receptor fragment is responsible for the transport arrest induced by hRIC-3. Enhancement of alpha7 receptor expression occurs, at least, at two levels: by increasing the number of mature receptors and facilitating its transport to the membrane. Certain amino acids of a putative amphipathic helix present at the large cytoplasmic region of the alpha7 subunit are required for these actions. Therefore, hRIC-3 can act as a specific regulator of receptor expression at different levels.
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Affiliation(s)
- Mar Castillo
- Instituto de Neurociencias de Alicante, Universidad Miguel Hernández-Consejo Superior de Investigaciones Científicas, Apartado 18, 03550 Sant Joan d'Alacant, Alicante, Spain
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24
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Kuo YP, Xu L, Eaton JB, Zhao L, Wu J, Lukas RJ. Roles for Nicotinic Acetylcholine Receptor Subunit Large Cytoplasmic Loop Sequences in Receptor Expression and Function. J Pharmacol Exp Ther 2005; 314:455-66. [PMID: 15833891 DOI: 10.1124/jpet.105.084954] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
To evaluate possible physiological roles of the large cytoplasmic loops (C2) and neighboring transmembrane domains of nicotinic acetylcholine receptor (nAChR) subunits, we generated novel fusion constructs in which human nAChR alpha4, beta2, or beta4 subunit C2 or C2 and neighboring sequences were replaced by corresponding sequences from the mouse serotonin type 3A (5-HT3A) receptor subunit. Following stable expression in human SH-EP1 cells, we found that extensive sequence substitutions involving third and fourth transmembrane domains and neighboring "proximal" C2 sequences (e.g., beta2 H322-V335 and V449-R460) did not allow functional expression of nAChR containing chimeric subunits. However, expression of functional nAChR was achieved containing wild-type alpha4 subunits and chimeric beta2 (beta2chi) subunits whose "nested" C2 domain sequences K336-S448 were replaced with the corresponding 5-HT3A subunit sequences. Whereas these findings suggested indispensable roles for M3/M4 transmembrane and/or proximal C2 sequences in alpha4beta2-nAChR function, nested C2 sequences in the beta2 subunit are not essential for functional receptor expression. Ligand-binding analyses also revealed only subtle differences in pharmacological profiles of alpha4beta2-nAChR compared with alpha4beta2chi-nAChR. Nevertheless, there was heightened emergence of agonist-mediated self-inhibition of alpha4beta2chi function, greater sensitivity to functional blockade by a number of antagonists, and faster and more complete acute desensitization of alpha4beta2chi-nAChR than for alpha4beta2-nAChR. These studies are consistent with unexpected roles of nested C2 sequences in nAChR function.
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Affiliation(s)
- Yen-Ping Kuo
- Department of Neurobiology, Barrow Neurological Institute, 350 West Thomas Road, Phoenix, AZ 85013, USA.
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25
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Arun KHS, Kaul CL, Ramarao P. Green fluorescent proteins in receptor research: An emerging tool for drug discovery. J Pharmacol Toxicol Methods 2005; 51:1-23. [PMID: 15596111 DOI: 10.1016/j.vascn.2004.07.006] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2003] [Accepted: 07/27/2004] [Indexed: 01/20/2023]
Abstract
In the last five years, green fluorescent protein (GFP) has emerged from being a mere curiosity to become a reliable tool for molecular pharmacological research. GFP produces an intense and stable green fluorescence noncatalytically by absorbing blue light maximally at 395 nm and emitting green light with a peak at 509 nm. It consists of 238 amino acids and its molecular mass is 27-30 kDa. GFP fluorescence occurs without cofactors and this property allows GFP fluorescence to be utilised in nonnative organisms, wherein it can be used as a reporter. This use of GFP permits real-time analysis of receptor dynamics. The emitted fluorescence can be used as a nontoxic marker and detected using fluorescence-activated cell sorting (FACS), thus avoiding any staining procedure, expensive mRNA analysis or hazardous radiolabeled binding assays. The potential value of GFP has also been recognized in orphan receptor research, where various GFP-tagged therapeutic proteins have been constructed in an attempt to identify the endogenous ligand(s). These chimeric proteins have been used to determine the site and time course of receptor expression and to relate receptor dynamics with therapeutic outcome. The preparation of new GFP constructs for identifying germ layer cells (endodermal, ectodermal, and mesodermal), as well as neuronal, haematopoietic, endothelial, and cartilage cells, has provided a useful battery of tissue/receptor-specific screening assays for new chemical entities. Genetically engineered cells with GFP expression have provided a valuable tool for automated analysis, and can be adapted for high-throughput systems. GFP is being increasingly utilised for the study of receptor dynamics, where, having already proved beneficial, it will likely continue to contribute towards the search for new classes of drugs, as well as to "de-orphaning" orphan receptors.
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Affiliation(s)
- K H S Arun
- Cardiovascular and Receptorology Laboratory, Department of Pharmacology and Toxicology, National Institute of Pharmaceutical Education and Research, Phase-X, Sector 67, S.A.S. Nagar (Mohali)-160 062, Punjab, India
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26
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Kottwitz D, Kukhtina V, Dergousova N, Alexeev T, Utkin Y, Tsetlin V, Hucho F. Intracellular domains of the δ-subunits of Torpedo and rat acetylcholine receptors—expression, purification, and characterization. Protein Expr Purif 2004; 38:237-47. [PMID: 15555939 DOI: 10.1016/j.pep.2004.07.017] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/25/2004] [Indexed: 11/19/2022]
Abstract
There are quite detailed structural data on the extracellular ligand-binding domain and the intramembrane channel-forming domain of the nicotinic acetylcholine receptors (nAChR). However, the structure of the intracellular domain, which has variable amino acid sequences in different nAChR subunits, remains unknown. We expressed in Escherichia coli the intracellular loops (between transmembrane fragments TM3 and TM4) of the delta-subunits from the Torpedo californica and Rattus norvegicus muscle nAChRs. To facilitate purification, (His)6-tags were attached with or without linkers, and the effects of protein truncations at C- or N-termini were examined. The proteins were purified from inclusion bodies under denaturing conditions by Ni-NTA-chromatography. Molecular weight and peptide mass fingerprint was determined by MALDI mass spectrometry. Size-exclusion chromatography revealed that the Torpedo intracellular delta-loop refolded in an aqueous buffer was present in solution as a dimer. Phosphorylation of this protein with protein kinase A and tyrosine kinase (Abl) occurred at the same serine and tyrosine residues as in the native receptor. According to CD spectra, the secondary structure was not sensitive to phosphorylation. The rat intracellular loops could be solubilized only in the presence of non-ionic detergents or lipids. CD spectra indicate that the Torpedo and rat proteins have differences in their secondary structure. In the presence of dodecylphosphocholine, high concentrations (up to 6 mg/ml) of the Torpedo and rat intracellular loops were achieved. The results suggest that the spatial structure of the intracellular loops is dependent on environment and species, but is not changed significantly upon enzymatic phosphorylation.
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Affiliation(s)
- Denise Kottwitz
- Institute of Biochemistry, Free University of Berlin, Thielallee 63, 14195 Berlin, Germany
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27
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Gault J, Hopkins J, Berger R, Drebing C, Logel J, Walton C, Short M, Vianzon R, Olincy A, Ross RG, Adler LE, Freedman R, Leonard S. Comparison of polymorphisms in the alpha7 nicotinic receptor gene and its partial duplication in schizophrenic and control subjects. Am J Med Genet B Neuropsychiatr Genet 2003; 123B:39-49. [PMID: 14582144 DOI: 10.1002/ajmg.b.20061] [Citation(s) in RCA: 70] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/16/2023]
Abstract
The hypothesis that the 15q13-15 region of chromosome 15 contains a gene that contributes to the etiology of schizophrenia is supported by multiple genetic linkage studies. The alpha7 neuronal nicotinic acetylcholine receptor (CHRNA7) gene was selected as the best candidate gene in this region for molecular investigation, based on these linkage findings and biological evidence in both human and rodent models. CHRNA7 receptors are decreased in expression in postmortem brain of schizophrenic subjects. A dinucleotide marker, D15S1360, in intron two of the CHRNA7 gene is genetically linked to an auditory gating deficit found in schizophrenics and half of the first-degree relatives of patients. Single strand conformation polymorphism (SSCP) and sequence analyses of DNA from schizophrenic and control individuals identified 33 variants in the coding region and intron/exon borders of the CHRNA7 gene and its partial duplication, dupCHRNA7; common polymorphisms were mapped. Twenty-one variants were found in the exons, but non-synonymous changes were rare. Although the expression of CHRNA7 is decreased in schizophrenia, the general structure of the remaining receptors is likely to be normal.
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Affiliation(s)
- Judith Gault
- Department of Psychiatry, University of Colorado Health Sciences Center, Denver, Colorado 80262, USA
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28
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Leite JF, Blanton MP, Shahgholi M, Dougherty DA, Lester HA. Conformation-dependent hydrophobic photolabeling of the nicotinic receptor: electrophysiology-coordinated photochemistry and mass spectrometry. Proc Natl Acad Sci U S A 2003; 100:13054-9. [PMID: 14569028 PMCID: PMC240743 DOI: 10.1073/pnas.2133028100] [Citation(s) in RCA: 39] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2003] [Accepted: 08/22/2003] [Indexed: 11/18/2022] Open
Abstract
We characterized the differential accessibility of the nicotinic acetylcholine receptor alpha1 subunit in the open, closed, and desensitized states by using electrophysiology-coordinated photolabeling by several lipophilic probes followed by mass spectrometric analysis. Voltage-clamped oocytes expressing receptors were preincubated with one of the lipophilic probes and were continually exposed to acetylcholine; UV irradiation was applied during 500-ms pulses to + 40 or to -140 mV (which produced closed or approximately 50% open receptors, respectively). In the open state, there was specific probe incorporation within the N-terminal domain at residues that align with the beta8-beta9 loop of the acetylcholine-binding protein. In the closed state, probe incorporation was identified at several sites of the N-terminal domain within the conserved cysteine loop (residues 128-142), the cytoplasmic loop (M3-M4), and M4. The labeling pattern in the M4 region is consistent with previous results, further defining the lipid-exposed face of this transmembrane alpha-helix. These results show regions within the N-terminal domain that are involved in gating-dependent conformational shifts, confirm that the cysteine loop resides at or near the protein-membrane interface, and show that segments of the M3-M4 loop are near to the lipid bilayer.
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Affiliation(s)
- John F Leite
- Divisions of Biology and Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
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