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Díaz-Rodríguez SM, Ivorra I, Espinosa J, Vegar C, Herrero-Turrión MJ, López DE, Gómez-Nieto R, Alberola-Die A. Enhanced Membrane Incorporation of H289Y Mutant GluK1 Receptors from the Audiogenic Seizure-Prone GASH/Sal Model: Functional and Morphological Impacts on Xenopus Oocytes. Int J Mol Sci 2023; 24:16852. [PMID: 38069190 PMCID: PMC10706347 DOI: 10.3390/ijms242316852] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2023] [Revised: 11/21/2023] [Accepted: 11/23/2023] [Indexed: 12/18/2023] Open
Abstract
Epilepsy is a neurological disorder characterized by abnormal neuronal excitability, with glutamate playing a key role as the predominant excitatory neurotransmitter involved in seizures. Animal models of epilepsy are crucial in advancing epilepsy research by faithfully replicating the diverse symptoms of this disorder. In particular, the GASH/Sal (genetically audiogenic seizure-prone hamster from Salamanca) model exhibits seizures resembling human generalized tonic-clonic convulsions. A single nucleotide polymorphism (SNP; C9586732T, p.His289Tyr) in the Grik1 gene (which encodes the kainate receptor GluK1) has been previously identified in this strain. The H289Y mutation affects the amino-terminal domain of GluK1, which is related to the subunit assembly and trafficking. We used confocal microscopy in Xenopus oocytes to investigate how the H289Y mutation, compared to the wild type (WT), affects the expression and cell-surface trafficking of GluK1 receptors. Additionally, we employed the two-electrode voltage-clamp technique to examine the functional effects of the H289Y mutation. Our results indicate that this mutation increases the expression and incorporation of GluK1 receptors into an oocyte's membrane, enhancing kainate-evoked currents, without affecting their functional properties. Although further research is needed to fully understand the molecular mechanisms responsible for this epilepsy, the H289Y mutation in GluK1 may be part of the molecular basis underlying the seizure-prone circuitry in the GASH/Sal model.
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Affiliation(s)
- Sandra M. Díaz-Rodríguez
- Neuroscience Institute of Castilla y León (INCyL), University of Salamanca, E-37007 Salamanca, Spain; (S.M.D.-R.); (M.J.H.-T.); (R.G.-N.)
- Institute of Biomedical Research of Salamanca (IBSAL), E-37007 Salamanca, Spain
| | - Isabel Ivorra
- Department of Physiology, Genetics and Microbiology, University of Alicante, E-03690 Alicante, Spain; (I.I.); (J.E.); (C.V.); (A.A.-D.)
| | - Javier Espinosa
- Department of Physiology, Genetics and Microbiology, University of Alicante, E-03690 Alicante, Spain; (I.I.); (J.E.); (C.V.); (A.A.-D.)
| | - Celia Vegar
- Department of Physiology, Genetics and Microbiology, University of Alicante, E-03690 Alicante, Spain; (I.I.); (J.E.); (C.V.); (A.A.-D.)
| | - M. Javier Herrero-Turrión
- Neuroscience Institute of Castilla y León (INCyL), University of Salamanca, E-37007 Salamanca, Spain; (S.M.D.-R.); (M.J.H.-T.); (R.G.-N.)
- Institute of Biomedical Research of Salamanca (IBSAL), E-37007 Salamanca, Spain
- Neurological Tissue Bank INCYL (BTN-INCYL), University of Salamanca, E-37007 Salamanca, Spain
| | - Dolores E. López
- Neuroscience Institute of Castilla y León (INCyL), University of Salamanca, E-37007 Salamanca, Spain; (S.M.D.-R.); (M.J.H.-T.); (R.G.-N.)
- Institute of Biomedical Research of Salamanca (IBSAL), E-37007 Salamanca, Spain
| | - Ricardo Gómez-Nieto
- Neuroscience Institute of Castilla y León (INCyL), University of Salamanca, E-37007 Salamanca, Spain; (S.M.D.-R.); (M.J.H.-T.); (R.G.-N.)
- Institute of Biomedical Research of Salamanca (IBSAL), E-37007 Salamanca, Spain
| | - Armando Alberola-Die
- Department of Physiology, Genetics and Microbiology, University of Alicante, E-03690 Alicante, Spain; (I.I.); (J.E.); (C.V.); (A.A.-D.)
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Miller B, Powell A, Gutierrez BA, Limon A. Microtransplantation of Synaptic Membranes to Reactivate Human Synaptic Receptors for Functional Studies. J Vis Exp 2022:10.3791/64024. [PMID: 35938847 PMCID: PMC10729793 DOI: 10.3791/64024] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023] Open
Abstract
Excitatory and inhibitory ionotropic receptors are the major gates of ion fluxes that determine the activity of synapses during physiological neuronal communication. Therefore, alterations in their abundance, function, and relationships with other synaptic elements have been observed as a major correlate of alterations in brain function and cognitive impairment in neurodegenerative diseases and mental disorders. Understanding how the function of excitatory and inhibitory synaptic receptors is altered by disease is of critical importance for the development of effective therapies. To gain disease-relevant information, it is important to record the electrical activity of neurotransmitter receptors that remain functional in the diseased human brain. So far this is the closest approach to assess pathological alterations in receptors' function. In this work, a methodology is presented to perform microtransplantation of synaptic membranes, which consists of reactivating synaptic membranes from snap frozen human brain tissue containing human receptors, by its injection and posterior fusion into the membrane of Xenopus laevis oocytes. The protocol also provides the methodological strategy to obtain consistent and reliable responses of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and γ-aminobutyric acid (GABA) receptors, as well as novel detailed methods that are used for normalization and rigorous data analysis.
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Affiliation(s)
- Brice Miller
- The Mitchell Center for Neurodegenerative Diseases, Department of Neurology, University of Texas Medical Branch
| | - Ashli Powell
- The Mitchell Center for Neurodegenerative Diseases, Department of Neurology, University of Texas Medical Branch
| | - Berenice A Gutierrez
- The Mitchell Center for Neurodegenerative Diseases, Department of Neurology, University of Texas Medical Branch
| | - Agenor Limon
- The Mitchell Center for Neurodegenerative Diseases, Department of Neurology, University of Texas Medical Branch;
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Antagonistic effect of dopamine structural analogues on human GABAρ1 receptor. Sci Rep 2017; 7:17385. [PMID: 29234054 PMCID: PMC5727059 DOI: 10.1038/s41598-017-17530-8] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2017] [Accepted: 11/24/2017] [Indexed: 12/12/2022] Open
Abstract
GABAergic and dopaminergic pathways are co-localized in several areas of the central nervous system and recently several reports have shown co-release of both neurotransmitters. The GABA-A receptor (β and ρ1 subunits) is modulated by dopamine (DA) and, interestingly, GABAρ1 can be modulated by several biogenic amines. Here we explored the effects of the metabolites of the dopaminergic pathway and other structural analogues of DA on GABAρ1 and the DA gated ion channel (LGC-53) from Caenorhabditis elegans expressed in Xenopus laevis oocytes. Our findings show an antagonistic effect of the metabolite 3-Methoxytyramine (3-MT, IC50 = 285 ± 30 µM) with similar potency compared to DA on induced GABA currents; however, it was inactive on LGC-53. The structural DA analogues and metabolites, 3, 4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 2-phenylethylamine (β-PEA) and 4-amino-1-butanol (4-AM-1-OH), antagonized GABAρ1 currents, whereas β-PEA acted as partial agonists on LGC-53, indicating that the putative binding sites of both receptors may share structural characteristics. These results suggest that the DA metabolites 3-MT, DOPAC and HVA modulate GABAρ1 and possibly affect the activity of the receptors that include this subunit in vivo.
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Neuhäuser B, Ludewig U. Uncoupling of ionic currents from substrate transport in the plant ammonium transporter AtAMT1;2. J Biol Chem 2014; 289:11650-11655. [PMID: 24634212 PMCID: PMC4002075 DOI: 10.1074/jbc.c114.552802] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/23/2014] [Revised: 03/13/2014] [Indexed: 12/20/2022] Open
Abstract
The ammonium flux across prokaryotic, plant, and animal membranes is regulated by structurally related ammonium transporters (AMT) and/or related Rhesus (Rh) glycoproteins. Several plant AMT homologs, such as AtAMT1;2 from Arabidopsis, elicit ionic, ammonium-dependent currents when expressed in oocytes. By contrast, functional evidence for the transport of NH3 and the lack of coupled ionic currents has been provided for many Rh proteins. Furthermore, despite high resolution structures the transported substrate in many bacterial homologs, such as AmtB from Escherichia coli, is still unclear. In a heterologous genetic screen in yeast, AtAMT1;2 mutants with reduced transport activity were identified based on the resistance of yeast to the toxic transport analog methylamine. When expressed in oocytes, the reduced transport capacity was confirmed for either of the mutants Q67K, M72I,and W145S. Structural alignments suggest that these mutations were dispersed at subunit contact sites of trimeric AMTs, without direct contact to the pore lumen. Surprisingly, and in contrast to the wild type AtAMT1;2 transporter, ionic currents were not associated with the substrate transport in these mutants. Whether these data suggest that the wild type AtAMT1;2 functions as H(+)/NH3 co-transporter, as well as how the strict substrate coupling with protons is lost by the mutations, is discussed.
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Affiliation(s)
- Benjamin Neuhäuser
- Institute of Crop Science, Nutritional Crop Physiology, University of Hohenheim, Fruwirthstrasse 20, D-70593 Stuttgart, Germany.
| | - Uwe Ludewig
- Institute of Crop Science, Nutritional Crop Physiology, University of Hohenheim, Fruwirthstrasse 20, D-70593 Stuttgart, Germany
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Dossou KSS, Devkota KP, Kavanagh PV, Beutler JA, Egan JM, Moaddel R. Development and preliminary validation of a plate-based CB1/CB2 receptor functional assay. Anal Biochem 2013; 437:138-43. [PMID: 23481912 DOI: 10.1016/j.ab.2013.02.025] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2012] [Revised: 02/23/2013] [Accepted: 02/27/2013] [Indexed: 11/19/2022]
Abstract
Cannabinoid (CB) receptors are being targeted therapeutically for the treatment of anxiety, obesity, movement disorders, glaucoma, and pain. More recently, cannabinoid agonists have displayed antiproliferative activity against breast cancer and prostate cancer in animal models. To study cannabinoid receptor ligands, we have developed a novel plate-based assay that measures internalization of CB1/CB2 receptors by determining the change in the intracellular levels of the radiolabeled agonists: [(3)H]Win55-212-2 for CB1 and [(3)H]CP55-940 for CB2. The developed plate-based assay was validated by determining IC50 values for known antagonists: AM251, AM281, AM630, and AM6545. The data obtained were consistent with previously reported values, thereby confirming that the assay can be used to determine the functional binding activities (IC50) of antagonists for the CB1 and CB2 receptors. In addition, we demonstrated that the plate-based assay may be used for screening against complex matrices. Specifically, we demonstrated that the plate-based assay was able to identify which extracts of several species of the genus Zanthoxylum had activity at the CB1/CB2 receptors.
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Affiliation(s)
- K S S Dossou
- Biomedical Research Center, National Institute on Aging, National Institutes of Health, Baltimore, MD 21224, USA
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Martínez-Delgado G, Estrada-Mondragón A, Miledi R, Martínez-Torres A. An Update on GABAρ Receptors. Curr Neuropharmacol 2011; 8:422-33. [PMID: 21629448 PMCID: PMC3080597 DOI: 10.2174/157015910793358141] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2009] [Revised: 04/08/2010] [Accepted: 06/21/2010] [Indexed: 01/29/2023] Open
Abstract
The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.
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Affiliation(s)
- Gustavo Martínez-Delgado
- Instituto de Neurbiología, Departamento de Neurobiología Celular y Molecular, Laboratorio D15, Campus UNAM Juriquilla. Querétaro 76230, México
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7
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Use of multicomponent reactions in developing small-molecule tools to study GABAA receptor mechanism and function. Future Med Chem 2011; 3:243-50. [PMID: 21428818 DOI: 10.4155/fmc.10.302] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022] Open
Abstract
We discuss the potential use of multicomponent reactions in developing small-molecule probes of GABA(A) receptor function. Two examples that illustrate this approach are presented: the synthesis of a class of compounds that specifically modulate the function of GABA(A) receptors containing the δ-subunit, and also 'caged' GABA derivatives. A caged GABA is a photolabile precursor of GABA that releases GABA upon photolysis.
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8
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Functional characterization of N-terminally GFP-tagged GLP-1 receptor. J Biomed Biotechnol 2009; 2009:498149. [PMID: 19859570 PMCID: PMC2765688 DOI: 10.1155/2009/498149] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2009] [Revised: 06/19/2009] [Accepted: 08/10/2009] [Indexed: 11/24/2022] Open
Abstract
The glucagon-like peptide-1 receptor (GLP-1 receptor) mediates important effects on peripheral tissues and the central nervous system. It seems one of the most promising therapeutic targets for treatment of diabetes mellitus type 2. Surprisingly, very little is known about the cellular mechanisms that regulate its function in vivo. One of the approaches to study receptor dynamics, expression, or signaling is using GFP-tagged fluorescent proteins. In this study, we synthesized and characterized N-terminally GFP-tagged GLP-1 (GFP-GLP-1) receptor in CHO cells. We demonstrated that GFP-GLP-1 receptor is weakly expressed in the plasma membranes and is functionally coupled to adenylyl cyclase via heterotrimeric G-proteins, similarly as its wild type.
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Microtransplantation of ligand-gated receptor-channels from fresh or frozen nervous tissue into Xenopus oocytes: A potent tool for expanding functional information. Prog Neurobiol 2009; 88:32-40. [DOI: 10.1016/j.pneurobio.2009.01.008] [Citation(s) in RCA: 46] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/27/2008] [Revised: 12/21/2008] [Accepted: 01/29/2009] [Indexed: 02/05/2023]
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10
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Limon A, Reyes-Ruiz JM, Eusebi F, Miledi R. Properties of GluR3 receptors tagged with GFP at the amino or carboxyl terminus. Proc Natl Acad Sci U S A 2007; 104:15526-30. [PMID: 17881566 PMCID: PMC2000508 DOI: 10.1073/pnas.0706773104] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Anatomical visualization of neurotransmitter receptor localization is facilitated by tagging receptors, but this process can alter their functional properties. We have evaluated the distribution and properties of WT glutamate receptor 3 (GluR3) alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (WT GluR3) and two receptors in which GFP was tagged to the amino terminus (GFP-GluR3) or to the carboxyl terminus (GluR3-GFP). Although the fluorescence in Xenopus oocytes was stronger in the vegetal hemisphere because of localization of internal structures (probable sites of production, storage or recycling of receptors), the insertion of receptors into the plasma membrane was polarized to the animal hemisphere. The fluorescence intensity of oocytes injected with GluR3-GFP RNA was approximately double that of oocytes injected with GFP-GluR3 RNA. Accordingly, GluR3-GFP oocytes generated larger kainate-induced currents than GFP-GluR3 oocytes, with similar EC(50) values. Currents elicited by glutamate, or AMPA coapplied with cyclothiazide, were also larger in GluR3-GFP oocytes. The glutamate- to kainate-current amplitude ratios differed, with GluR3-GFP being activated more efficiently by glutamate than the WT or GFP-GluR3 receptors. This pattern correlates with the slower decay of glutamate-induced currents generated by GluR3-GFP receptors. These changes were not observed when GFP was tagged to the amino terminus, and these receptors behaved like the WT. The antagonistic effects of 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were not altered in any of the tagged receptors. We conclude that GFP is a useful and convenient tag for visualizing these proteins. However, the effects of different sites of tag insertion on receptor characteristics must be taken into account in assessing the roles played by these receptor proteins.
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Affiliation(s)
- Agenor Limon
- *Department of Neurobiology and Behavior, University of California, Irvine, CA 92697-4550
| | | | - Fabrizio Eusebi
- Dipartimento di Fisiologia Umana & Farmacologia, Universita' di Roma “Sapienza,” and Neuromed Via Atinese 18, I86077 Isernia, Italy
| | - Ricardo Miledi
- *Department of Neurobiology and Behavior, University of California, Irvine, CA 92697-4550
- Instituto de Neurobiología, Laboratorio de Neurobiología Molecular y Celular, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Juriquilla, Querétaro, 76230, México
- To whom correspondence should be addressed. E-mail:
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Ilegems E, Pick H, Deluz C, Kellenberger S, Vogel H. Ligand Binding Transmits Conformational Changes across the Membrane-Spanning Region to the Intracellular Side of the 5-HT3 Serotonin Receptor. Chembiochem 2005; 6:2180-5. [PMID: 16254942 DOI: 10.1002/cbic.200500191] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Affiliation(s)
- Erwin Ilegems
- Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, 1015 Lausanne, Switzerland
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López-Chávez A, Miledi R, Martínez-Torres A. Cloning and functional expression of the bovine GABA(C) rho2 subunit. Molecular evidence of a widespread distribution in the CNS. Neurosci Res 2005; 53:421-7. [PMID: 16213047 DOI: 10.1016/j.neures.2005.08.014] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2005] [Revised: 08/30/2005] [Accepted: 08/31/2005] [Indexed: 11/17/2022]
Abstract
GABA(C) receptors were first described as a non-desensitizing, bicuculline- and baclofen-insensitive component in Xenopus oocytes expressing bovine retina mRNA. However, the expression, tissue distribution and functional properties of GABA(C) receptors from other areas of the CNS still remain controversial. In previous experiments, the injection of rat cerebellum mRNA into Xenopus oocytes induced the expression of receptors that generated currents with both GABA(A) and GABA(C) characteristics; the latter component apparently being given by the rho2 subunit, suggesting the expression of GABA(C) receptors in the CNS and the formation of homooligomeric receptors. In this study, using RT-PCR, we found that the rho1 and rho2 subunits are widely expressed in the CNS including areas where they have not been previously described such as the bulb, pons and the caudate nucleus. To determine if the GABA(C) component of the GABA-currents elicited by oocytes expressing cerebellum mRNA was caused by activation of homomeric GABA rho2 receptors, we cloned the corresponding cDNA and expressed it in Xenopus oocytes. It was found that oocytes injected with rho2 cDNA, efficiently formed GABA-gated homooligomeric receptors. The GABA-dose-current response gave an EC50=1.19muM and the currents were resistant to bicuculline and reversibly antagonized by the specific GABA(C) receptor antagonist TPMPA. Altogether, our results indicate a widespread distribution of both rho1 and rho2 subunits in the bovine CNS and show further that the rho2 subunit cDNA isolated from cerebellum, forms fully functional receptors when expressed in Xenopus oocytes.
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Affiliation(s)
- Ariel López-Chávez
- Instituto de Neurobiología, Universidad Nacional Autónoma de México-Campus Juriquilla, AP1-1141 Querétaro, Mexico
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Martínez-Martínez A, Reyes-Ruiz JM, Martínez-Torres A, Miledi R. Functional expression in frog oocytes of human rho 1 receptors produced in Saccharomyces cerevisiae. Proc Natl Acad Sci U S A 2004; 101:682-6. [PMID: 14704273 PMCID: PMC327208 DOI: 10.1073/pnas.0307564100] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The yeast Saccharomyces cerevisiae was engineered to express the rho 1 subunit of the human gamma-aminobutyric acid rho 1 (GABA rho 1) receptor. RNA that was isolated from several transformed yeast strains produced fully functional GABA receptors in Xenopus oocytes. The GABA currents elicited in the oocytes were fast, nondesensitizing chloride currents; and the order of agonist potency was GABA > beta-alanine > glycine. Moreover, the receptors were resistant to bicuculline, strongly antagonized by (1,2,5,6 tetrahydropyridine-4-yl)methylphosphinic acid, and modulated by zinc and lanthanum. Thus, the GABA receptors expressed by the yeast mRNA retained all of the principal characteristics of receptors expressed by cRNA or native retina mRNAs. Western blot assays showed immunoreactivity in yeast plasma membrane preparations, and a rho 1-GFP fusion gene showed mostly intracellular distribution with a faint fluorescence toward the plasma membrane. In situ immunodetection of rho 1 in yeast demonstrated that some receptors reach the plasma membrane. Furthermore, microtransplantation of yeast plasma membranes to frog oocytes resulted in the incorporation of a small number of functional yeast rho 1 receptors into the oocyte plasma membrane. These results show that yeast may be useful to produce complete functional ionotropic receptors suitable for structural analysis.
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Affiliation(s)
- Alejandro Martínez-Martínez
- Laboratory of Cellular and Molecular Neurobiology, Department of Neurobiology and Behavior, University of California, McGaugh Hall 1115, Irvine, CA 92697-4550, USA
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Fucile S, Palma E, Martinez-Torres A, Miledi R, Eusebi F. The single-channel properties of human acetylcholine alpha 7 receptors are altered by fusing alpha 7 to the green fluorescent protein. Proc Natl Acad Sci U S A 2002; 99:3956-61. [PMID: 11891309 PMCID: PMC122630 DOI: 10.1073/pnas.052699599] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/26/2001] [Indexed: 11/18/2022] Open
Abstract
Neuronal nicotinic acetylcholine (AcCho) receptors composed of alpha7-subunits (alpha7-AcChoRs) are involved in many physiological activities. Nevertheless, very little is known about their single-channel characteristics. By using outside-out patch-clamp recordings from Xenopus oocytes expressing wild-type (wt) alpha7-AcChoRs, we identified two classes of channel conductance: a low conductance (gamma(L)) of 72 pS and a high one (gamma(H)) of 87 pS, with mean open-times (tau(op)) of 0.6 ms. The same classes of conductances, but longer tau(op) (3 ms), were seen in experiments with chimeric alpha7 receptors in which the wtalpha7 extracellular C terminus was fused to the green fluorescent protein (wtalpha7-GFP AcChoRs). In contrast, channels with three different conductances were gated by AcCho in oocytes expressing alpha7 receptors carrying a Leu-to-Thr 248 mutation (mutalpha7) or oocytes expressing chimeric mutalpha7-GFP receptors. These conductance levels were significantly smaller, and their mean open-times were larger, than those of wtalpha7-AcChoRs. Interestingly, in the absence of AcCho, these oocytes showed single-channel openings of the same conductances, but shorter tau(op), than those activated by AcCho. Accordingly, human homomeric wtalpha7 receptors open channels of high conductance and brief lifetime, and fusion to GFP lengthens their lifetime. In contrast, mutalpha7 receptors open channels of lower conductance and longer lifetime than those gated by wtalpha7-AcChoRs, and these parameters are not greatly altered by fusing the mutalpha7 to GFP. All this evidence shows that GFP-tagging can alter importantly receptor kinetics, a fact that has to be taken into account whenever tagged proteins are used to study their function.
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Affiliation(s)
- Sergio Fucile
- Istituto Pasteur-Fondazione Cenci Bolognetti and Dipartimento di Fisiologia Umana e Farmacologia, Universita' di Roma La Sapienza, P.le A Moro 5, I00185 Rome, Italy
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15
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Palma E, Mileo AM, Martinez-Torres A, Eusebi F, Miledi R. Some properties of human neuronal alpha 7 nicotinic acetylcholine receptors fused to the green fluorescent protein. Proc Natl Acad Sci U S A 2002; 99:3950-5. [PMID: 11891308 PMCID: PMC122629 DOI: 10.1073/pnas.052699299] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/26/2001] [Indexed: 11/18/2022] Open
Abstract
The functional properties and cellular localization of the human neuronal alpha7 nicotinic acetylcholine (AcCho) receptor (alpha7 AcChoR) and its L248T mutated (mut) form were investigated by expressing them alone or as gene fusions with the enhanced version of the green fluorescent protein (GFP). Xenopus oocytes injected with wild-type (wt), mutalpha7, or the chimeric subunit cDNAs expressed receptors that gated membrane currents when exposed to AcCho. As already known, AcCho currents generated by wtalpha7 receptors decay much faster than those elicited by the mutalpha7 receptors. Unexpectedly, the fusion of GFP to the wt and mutated alpha7 receptors led to opposite results: the AcCho-current decay of the wt receptors became slower, whereas that of the mutated receptors was accelerated. Furthermore, repetitive applications of AcCho led to a considerable "run-down" of the AcCho currents generated by mutalpha7-GFP receptors, whereas those of the wtalpha7-GFP receptors remained stable or increased in amplitude. The AcCho-current run-down of mutalpha7-GFP oocytes was accompanied by a marked decrease of alpha-bungarotoxin binding activity. Fluorescence, caused by the chimeric receptors expressed, was seen over the whole oocyte surface but was more intense and abundant in the animal hemisphere, whereas it was much weaker in the vegetal hemisphere. We conclude that fusion of GFP to wtalpha7 and mutalpha7 receptors provides powerful tools to study the distribution and function of alpha7 receptors. We also conclude that fused genes do not necessarily recapitulate all of the properties of the original receptors. This fact must be borne close in mind whenever reporter genes are attached to proteins.
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Affiliation(s)
- Eleonora Palma
- Istituto Pasteur-Fondazione Cenci Bolognetti and Dipartimento di Fisiologia Umana e Farmacologia, Universita' di Roma La Sapienza, P. le A. Moro 5, I00185 Roma, Italy
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Abstract
In the central nervous system inhibitory neurotransmission is primarily achieved through activation of receptors for gamma-aminobutyric acid (GABA). Three types of GABA receptors have been identified on the basis of their pharmacological and electrophysiological properties. The predominant type, termed GABA(A), and a recently identified GABA(C) type, form ligand-gated chloride channels, whereas GABA(B) receptors activate separate cation channels via G proteins. Based on their homology to nicotinic acetylcholine receptors, GABA(C) receptors are believed to be oligomeric protein complexes composed of five subunits in a pentameric arrangement. To date up to five different GABA(C) receptors subunits have been identified in various species. Recent studies have shed new light on the biological characteristics of GABA(C) receptors, including the chromosomal localization of its subunit genes and resulting links to deseases, the cloning of new splice variants, the identification of GABA(C) receptor-associated proteins, the identification of domains involved in subunit assembly, and finally structure/function studies examining functional consequences of introduced mutations. This review summarizes recent data in view of the molecular structure of GABA(C) receptors and presents new insights into the biological function of this protein in the retina.
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Affiliation(s)
- R Enz
- Emil-Fischer Zentrum, Institut für Biochemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
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