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Shahoei R, Tajkhorshid E. Menthol Binding to the Human α4β2 Nicotinic Acetylcholine Receptor Facilitated by Its Strong Partitioning in the Membrane. J Phys Chem B 2020; 124:1866-1880. [PMID: 32048843 PMCID: PMC7094167 DOI: 10.1021/acs.jpcb.9b10092] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
We utilize various computational methodologies to study menthol's interaction with multiple organic phases, a lipid bilayer, and the human α4β2 nicotinic acetylcholine receptor (nAChR), the most abundant nAChR in the brain. First, force field parameters developed for menthol are validated in alchemical free energy perturbation simulations to calculate solvation free energies of menthol in water, dodecane, and octanol and compare the results against experimental data. Next, umbrella sampling is used to construct the free energy profile of menthol permeation across a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. The results from a flooding simulation designed to study the water-membrane partitioning of menthol in a POPC lipid bilayer are used to determine the penetration depth and the preferred orientation of menthol in the bilayer. Finally, employing both docking and flooding simulations, menthol is shown to bind to different sites on the human α4β2 nAChR. The most likely binding mode of menthol to a desensitized membrane-embedded α4β2 nAChR is identified to be via a membrane-mediated pathway in which menthol binds to the sites at the lipid-protein interface after partitioning in the membrane. A rare but distinct binding mode in which menthol binds to the extracellular opening of receptor's ion permeation pore is also reported.
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Affiliation(s)
- Rezvan Shahoei
- Department of Physics, NIH Center for Macromolecular Modeling and Bioinformatics, and Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Emad Tajkhorshid
- Department of Biochemistry, NIH Center for Macromolecular Modeling and Bioinformatics, Beckman Institute for Advanced Science and Technology, and Center for Biophysics and Quantitative Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
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2
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Wu B, Jayakar SS, Zhou X, Titterton K, Chiara DC, Szabo AL, Savechenkov PY, Kent DE, Cohen JB, Forman SA, Miller KW, Bruzik KS. Inhibitable photolabeling by neurosteroid diazirine analog in the β3-Subunit of human hetereopentameric type A GABA receptors. Eur J Med Chem 2018; 162:810-824. [PMID: 30544077 DOI: 10.1016/j.ejmech.2018.11.019] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2018] [Revised: 11/06/2018] [Accepted: 11/08/2018] [Indexed: 12/22/2022]
Abstract
Pregnanolone and allopregnanolone-type ligands exert general anesthetic, anticonvulsant and anxiolytic effects due to their positive modulatory interactions with the GABAA receptors in the brain. Binding sites for these neurosteroids have been recently identified at subunit interfaces in the transmembrane domain (TMD) of homomeric β3 GABAA receptors using photoaffinity labeling techniques, and in homomeric chimeric receptors containing GABAA receptor α subunit TMDs by crystallography. Steroid binding sites have yet to be determined in human, heteromeric, functionally reconstituted, full-length, glycosylated GABAA receptors. Here, we report on the synthesis and pharmacological characterization of several photoaffinity analogs of pregnanolone and allopregnanolone, of which 21-[4-(3-(trifluoromethyl)-3H-diazirin-3-yl)benzoxy]allopregnanolone (21-pTFDBzox-AP) was the most potent ligand. It is a partial positive modulator of the human α1β3 and α1β3γ2L GABAA receptors at sub-micromolar concentrations. [3H]21-pTFDBzox-AP photoincorporated in a pharmacologically specific manner into the α and β subunits of those receptors, with the β3 subunit photolabeled most efficiently. Importantly, photolabeling by [3H]21-pTFDBzox-AP was inhibited by the positive steroid modulators alphaxalone, pregnanolone and allopregnanolone, but not by inhibitory neurosteroid pregnenolone sulfate or by two potent general anesthetics and GABAAR positive allosteric modulators, etomidate and an anesthetic barbiturate. The latter two ligands bind to sites at subunit interfaces in the GABAAR that are different from those interacting with neurosteroids. 21-pTFDBzox-AP's potency and pharmacological specificity of photolabeling indicate its suitability for characterizing neurosteroid binding sites in native GABAA receptors.
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Affiliation(s)
- Bo Wu
- Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 833 South Wood Street, Chicago, IL, 60612, USA
| | - Selwyn S Jayakar
- Department of Neurobiology, 220 Longwood Avenue, Harvard Medical School, Boston, MA, 02115, USA
| | - Xiaojuan Zhou
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, 32 Fruit Street, Boston, MA, 02114, USA
| | - Katherine Titterton
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, 32 Fruit Street, Boston, MA, 02114, USA
| | - David C Chiara
- Department of Neurobiology, 220 Longwood Avenue, Harvard Medical School, Boston, MA, 02115, USA
| | - Andrea L Szabo
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, 32 Fruit Street, Boston, MA, 02114, USA
| | - Pavel Y Savechenkov
- Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 833 South Wood Street, Chicago, IL, 60612, USA
| | - Daniel E Kent
- Department of Health Science, Northeastern University, 360 Huntington Ave, Boston, MA, 02115, USA
| | - Jonathan B Cohen
- Department of Neurobiology, 220 Longwood Avenue, Harvard Medical School, Boston, MA, 02115, USA
| | - Stuart A Forman
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, 32 Fruit Street, Boston, MA, 02114, USA
| | - Keith W Miller
- Department of Anesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, 32 Fruit Street, Boston, MA, 02114, USA; Department of Biological Chemistry and Molecular Pharmacology, 220 Longwood Avenue, Harvard Medical School, Boston, MA, 02115, USA
| | - Karol S Bruzik
- Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 833 South Wood Street, Chicago, IL, 60612, USA.
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Cobo R, Nikolaeva M, Alberola-Die A, Fernández-Ballester G, González-Ros JM, Ivorra I, Morales A. Mechanisms Underlying the Strong Inhibition of Muscle-Type Nicotinic Receptors by Tetracaine. Front Mol Neurosci 2018; 11:193. [PMID: 30135641 PMCID: PMC6092513 DOI: 10.3389/fnmol.2018.00193] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2018] [Accepted: 05/15/2018] [Indexed: 11/13/2022] Open
Abstract
Nicotinic acetylcholine (ACh) receptors (nAChRs) are included among the targets of a variety of local anesthetics, although the molecular mechanisms of blockade are still poorly understood. Some local anesthetics, such as lidocaine, act on nAChRs by different means through their ability to present as both charged and uncharged molecules. Thus, we explored the mechanisms of nAChR blockade by tetracaine, which at physiological pH is almost exclusively present as a positively charged local anesthetic. The nAChRs from Torpedo electroplaques were transplanted to Xenopus oocytes and the currents elicited by ACh (IACh s), either alone or co-applied with tetracaine, were recorded. Tetracaine reversibly blocked IACh , with an IC50 (i.e., the concentration required to inhibit half the maximum IACh ) in the submicromolar range. Notably, at very low concentrations (0.1 μM), tetracaine reduced IACh in a voltage-dependent manner, the more negative potentials produced greater inhibition, indicating open-channel blockade. When the tetracaine concentration was increased to 0.7 μM or above, voltage-independent inhibition was also observed, indicating closed-channel blockade. The IACh inhibition by pre-application of just 0.7 μM tetracaine before superfusion of ACh also corroborated the notion of tetracaine blockade of resting nAChRs. Furthermore, tetracaine markedly increased nAChR desensitization, mainly at concentrations equal or higher than 0.5 μM. Interestingly, tetracaine did not modify desensitization when its binding within the channel pore was prevented by holding the membrane at positive potentials. Tetracaine-nAChR interactions were assessed by virtual docking assays, using nAChR models in the closed and open states. These assays revealed that tetracaine binds at different sites of the nAChR located at the extracellular and transmembrane domains, in both open and closed conformations. Extracellular binding sites seem to be associated with closed-channel blockade; whereas two sites within the pore, with different affinities for tetracaine, contribute to open-channel blockade and the enhancement of desensitization, respectively. These results demonstrate a concentration-dependent heterogeneity of tetracaine actions on nAChRs, and contribute to a better understanding of the complex modulation of muscle-type nAChRs by local anesthetics. Furthermore, the combination of functional and virtual assays to decipher nAChR-tetracaine interactions has allowed us to tentatively assign the main nAChR residues involved in these modulating actions.
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Affiliation(s)
- Raúl Cobo
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, Alicante, Spain
| | - Magdalena Nikolaeva
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Alicante, Spain
| | - Armando Alberola-Die
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, Alicante, Spain
| | | | - José M González-Ros
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, Alicante, Spain
| | - Isabel Ivorra
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, Alicante, Spain
| | - Andrés Morales
- División de Fisiología, Departamento de Fisiología, Genética y Microbiología, Universidad de Alicante, Alicante, Spain
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Non-blocking modulation contributes to sodium channel inhibition by a covalently attached photoreactive riluzole analog. Sci Rep 2018; 8:8110. [PMID: 29802266 PMCID: PMC5970139 DOI: 10.1038/s41598-018-26444-y] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/14/2017] [Accepted: 05/04/2018] [Indexed: 11/08/2022] Open
Abstract
Sodium channel inhibitor drugs decrease pathological hyperactivity in various diseases including pain syndromes, myotonia, arrhythmias, nerve injuries and epilepsies. Inhibiting pathological but not physiological activity, however, is a major challenge in drug development. Sodium channel inhibitors exert their effects by a dual action: they obstruct ion flow ("block"), and they alter the energetics of channel opening and closing ("modulation"). Ideal drugs would be modulators without blocking effect, because modulation is inherently activity-dependent, therefore selective for pathological hyperactivity. Can block and modulation be separated? It has been difficult to tell, because the effect of modulation is obscured by conformation-dependent association/dissociation of the drug. To eliminate dynamic association/dissociation, we used a photoreactive riluzole analog which could be covalently bound to the channel; and found, unexpectedly, that drug-bound channels could still conduct ions, although with modulated gating. The finding that non-blocking modulation is possible, may open a novel avenue for drug development because non-blocking modulators could be more specific in treating hyperactivity-linked diseases.
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Chen Q, Wells MM, Tillman TS, Kinde MN, Cohen A, Xu Y, Tang P. Structural Basis of Alcohol Inhibition of the Pentameric Ligand-Gated Ion Channel ELIC. Structure 2016; 25:180-187. [PMID: 27916519 DOI: 10.1016/j.str.2016.11.007] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2016] [Revised: 09/30/2016] [Accepted: 11/07/2016] [Indexed: 11/18/2022]
Abstract
The structural basis for alcohol modulation of neuronal pentameric ligand-gated ion channels (pLGICs) remains elusive. We determined an inhibitory mechanism of alcohol on the pLGIC Erwinia chrysanthemi (ELIC) through direct binding to the pore. X-ray structures of ELIC co-crystallized with 2-bromoethanol, in both the absence and presence of agonist, reveal 2-bromoethanol binding in the pore near T237(6') and the extracellular domain (ECD) of each subunit at three different locations. Binding to the ECD does not appear to contribute to the inhibitory action of 2-bromoethanol and ethanol as indicated by the same functional responses of wild-type ELIC and mutants. In contrast, the ELIC-α1β3GABAAR chimera, replacing the ELIC transmembrane domain (TMD) with the TMD of α1β3GABAAR, is potentiated by 2-bromoethanol and ethanol. The results suggest a dominant role of the TMD in modulating alcohol effects. The X-ray structures and functional measurements support a pore-blocking mechanism for inhibitory action of short-chain alcohols.
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Affiliation(s)
- Qiang Chen
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Marta M Wells
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Computational and System Biology, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - Tommy S Tillman
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Monica N Kinde
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA
| | - Aina Cohen
- Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
| | - Yan Xu
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA 15260, USA; Department of Structural Biology, University of Pittsburgh, Pittsburgh, PA 15260, USA
| | - Pei Tang
- Department of Anesthesiology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260, USA; Department of Pharmacology and Chemical Biology, University of Pittsburgh, Pittsburgh, PA 15260, USA; Department of Computational and System Biology, University of Pittsburgh, Pittsburgh, PA 15260, USA.
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Forman SA, Chiara DC, Miller KW. Anesthetics target interfacial transmembrane sites in nicotinic acetylcholine receptors. Neuropharmacology 2014; 96:169-77. [PMID: 25316107 DOI: 10.1016/j.neuropharm.2014.10.002] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/22/2014] [Revised: 09/22/2014] [Accepted: 10/02/2014] [Indexed: 11/25/2022]
Abstract
General anesthetics are a heterogeneous group of small amphiphilic ligands that interact weakly at multiple allosteric sites on many pentameric ligand gated ion channels (pLGICs), resulting in either inhibition, potentiation of channel activity, or both. Allosteric principles imply that modulator sites must change configuration and ligand affinity during receptor state transitions. Thus, general anesthetics and related compounds are useful both as state-dependent probes of receptor structure and as potentially selective modulators of pLGIC functions. This review focuses on general anesthetic sites in nicotinic acetylcholine receptors, which were among the first anesthetic-sensitive pLGIC experimental models studied, with particular focus on sites formed by transmembrane domain elements. Structural models place many of these sites at interfaces between two or more pLGIC transmembrane helices both within subunits and between adjacent subunits, and between transmembrane helices and either lipids (the lipid-protein interface) or water (i.e. the ion channel). A single general anesthetic may bind at multiple allosteric sites in pLGICs, producing a net effect of either inhibition (e.g. blocking the ion channel) or enhanced channel gating (e.g. inter-subunit sites). Other general anesthetic sites identified by photolabeling or crystallography are tentatively linked to functional effects, including intra-subunit helix bundle sites and the lipid-protein interface. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.
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Affiliation(s)
- Stuart A Forman
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - David C Chiara
- Dept. of Neurobiology, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
| | - Keith W Miller
- Dept. of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, 55 Fruit Street, MA 02114, USA; Dept. of Anaesthesia, Harvard Medical School, 25 Shattuck Street, Boston, MA 02115, USA.
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Weiser BP, Woll KA, Dailey WP, Eckenhoff RG. Mechanisms revealed through general anesthetic photolabeling. CURRENT ANESTHESIOLOGY REPORTS 2013; 4:57-66. [PMID: 24563623 DOI: 10.1007/s40140-013-0040-7] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
General anesthetic photolabels are used to reveal molecular targets and molecular binding sites of anesthetic ligands. After identification, the relevance of anesthetic substrates or binding sites can be tested in biological systems. Halothane and photoactive analogs of isoflurane, propofol, etomidate, neurosteroids, anthracene, and long chain alcohols have been used in anesthetic photolabeling experiments. Interrogated protein targets include the nicotinic acetylcholine receptor, GABAA receptor, tubulin, leukocyte function-associated antigen-1, and protein kinase C. In this review, we summarize insights revealed by photolabeling these targets, as well as general features of anesthetics, such as their propensity to partition to mitochondria and bind voltage-dependent anion channels. The theory of anesthetic photolabel design and the experimental application of photoactive ligands are also discussed.
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Affiliation(s)
- Brian P Weiser
- Department of Anesthesiology & Critical Care, University of Pennsylvania Perelman School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104 ; Department of Pharmacology, University of Pennsylvania Perelman School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104
| | - Kellie A Woll
- Department of Anesthesiology & Critical Care, University of Pennsylvania Perelman School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104 ; Department of Pharmacology, University of Pennsylvania Perelman School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104
| | - William P Dailey
- Department of Chemistry, University of Pennsylvania School of Arts and Sciences, 231 S. 34th Street, Philadelphia, PA 19104
| | - Roderic G Eckenhoff
- Department of Anesthesiology & Critical Care, University of Pennsylvania Perelman School of Medicine, 3620 Hamilton Walk, Philadelphia, PA 19104
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Bhargava Y, Rettinger J, Mourot A. Allosteric nature of P2X receptor activation probed by photoaffinity labelling. Br J Pharmacol 2013; 167:1301-10. [PMID: 22725669 DOI: 10.1111/j.1476-5381.2012.02083.x] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
BACKGROUND AND PURPOSE In P2X receptors, agonist binding at the interface between neighbouring subunits is efficiently transduced to ion channel gating. However, the relationship between binding and gating is difficult to study because agonists continuously bind and unbind. Here, we covalently incorporated agonists in the binding pocket of P2X receptors and examined how binding site occupancy affects the ability of the channel to gate. EXPERIMENTAL APPROACH We used a strategy for tethering agonists to their ATP-binding pocket, while simultaneously probing ion channel gating using electrophysiology. The agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP), a photoaffinity analogue of ATP, enabled us to trap rat homomeric P2X2 receptor and a P2X2/1 receptor chimera in different agonist-bound states. UV light was used to control the degree of covalent occupancy of the receptors. KEY RESULTS Irradiation of the P2X2/1 receptor chimera - BzATP complex resulted in a persistent current that lasted even after extensive washout, consistent with photochemical tethering of the agonist BzATP and trapping of the receptors in an open state. Partial labelling with BzATP primed subsequent agonist binding and modulated gating efficiency for both full and partial agonists. CONCLUSIONS AND IMPLICATIONS Our photolabelling strategy provides new molecular insights into the activation mechanism of the P2X receptor. We show here that priming with full agonist molecules leads to an increase in gating efficiency after subsequent agonist binding.
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Affiliation(s)
- Y Bhargava
- Department of Biophysical Chemistry, Max-Planck-Institute of Biophysics, Frankfurt am Main, Germany.
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Chen ZW, Chen LH, Akentieva N, Lichti CF, Darbandi R, Hastings R, Covey DF, Reichert DE, Townsend RR, Evers AS. A neurosteroid analogue photolabeling reagent labels the colchicine-binding site on tubulin: a mass spectrometric analysis. Electrophoresis 2012; 33:666-74. [PMID: 22451060 PMCID: PMC3690291 DOI: 10.1002/elps.201100434] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Previous studies have shown that the neurosteroid analogue, 6-Azi-pregnanolone (6-AziP), photolabels voltage-dependent anion channels and proteins of approximately 55 kDa in rat brain membranes. The present study used two-dimensional electrophoresis and nanoelectrospray ionization ion-trap mass spectrometry (nano-ESI-MS) to identify the 55 kDa proteins (isoelectric point 4.8) as isoforms of β-tubulin. This identification was confirmed by immunoblot and immunoprecipitation of photolabeled protein with anti-β-tubulin antibody and by the demonstration that 6-AziP photolabels purified bovine brain tubulin in a concentration-dependent pattern. To identify the photolabeling sites, purified bovine brain tubulin was photolabeled with 6-AziP, digested with trypsin, and analyzed by matrix-assisted laser desorption/ionization MS (MALDI). A 6-AziP adduct of TAVCDIPPR(m/z = 1287.77), a β-tubulin specific peptide, was detected by MALDI. High-resolution liquid chromatography-MS/MS analysis identified that 6-AziP was covalently bound to cysteine 354 (Cys-354), previously identified as a colchicine-binding site. 6-AziP photolabeling was inhibited by 2-methoxyestradiol, an endogenous derivative of estradiol thought to bind to the colchicine site. Structural modeling predicted that neurosteroids could dock in this colchicine site at the interface between α- and β-tubulin with the photolabeling group of 6-AziP positioned proximate to Cys-354.
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Affiliation(s)
- Zi-Wei Chen
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
| | - Li-Hai Chen
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
- Department of Anesthesiology, Ruijin Hospital, Shanghai Jiaotong University School of Medicine
| | - Natalia Akentieva
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
| | - Cheryl F. Lichti
- Department of Internal Medicine Washington University School of Medicine, St. Louis, Missouri 63110
| | - Ramin Darbandi
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
| | - Randy Hastings
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
| | - Douglas F. Covey
- Department of Developmental Biology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - David E. Reichert
- Department of Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - R. Reid Townsend
- Department of Internal Medicine Washington University School of Medicine, St. Louis, Missouri 63110
- Department of Cell Biology and Physiology, Washington University School of Medicine, St. Louis, Missouri 63110
| | - Alex S. Evers
- Department of Anesthesiology Washington University School of Medicine, St. Louis, Missouri 63110
- Department of Internal Medicine Washington University School of Medicine, St. Louis, Missouri 63110
- Department of Developmental Biology, Washington University School of Medicine, St. Louis, Missouri 63110
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10
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Das J. Aliphatic diazirines as photoaffinity probes for proteins: recent developments. Chem Rev 2011; 111:4405-17. [PMID: 21466226 DOI: 10.1021/cr1002722] [Citation(s) in RCA: 200] [Impact Index Per Article: 15.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Affiliation(s)
- Joydip Das
- Department of Pharmacological and Pharmaceutical Sciences, College of Pharmacy, University of Houston, Houston, Texas 77204, USA.
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12
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Spitzmaul G, Gumilar F, Dilger JP, Bouzat C. The local anaesthetics proadifen and adiphenine inhibit nicotinic receptors by different molecular mechanisms. Br J Pharmacol 2009; 157:804-17. [PMID: 19422391 DOI: 10.1111/j.1476-5381.2009.00214.x] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
Abstract
BACKGROUND AND PURPOSE Many local anaesthetics are non-competitive inhibitors of nicotinic receptors (acetylcholine receptor, AChR). Proadifen induces a high-affinity state of the receptor, but its mechanism of action and that of an analogue, adiphenine, is unknown. EXPERIMENTAL APPROACH We measured the effects of proadifen and adiphenine on single-channel and macroscopic currents of adult mouse muscle AChR (wild-type and mutant). We assessed the results in terms of mechanisms and sites of action. KEY RESULTS Both proadifen and adiphenine decreased the frequency of ACh-induced single-channel currents. Proadifen did not change cluster properties, but adiphenine decreased cluster duration (36-fold at 100 micromolxL(-1)). Preincubation with proadifen decreased the amplitude (IC(50)= 19 micromolxL(-1)) without changing the decay rate of macroscopic currents. In contrast, adiphenine did not change amplitude but increased the decay rate (IC(50)= 15 micromolxL(-1)). Kinetic measurements demonstrate that proadifen acts on the resting state to induce a desensitized state whose kinetics of recovery resemble those of ACh-induced desensitization. Adiphenine accelerates desensitization from the open state, but previous application of the drug to resting receptors is required. Both drugs stabilize desensitized states, as evidenced by the decrease in the number of clusters elicited by high ACh concentrations. The inhibition by adiphenine is not affected by proadifen, and the mutation alphaE262K decreases the sensitivity of the AChR only for adiphenine, indicating that these drugs act at different sites. CONCLUSIONS AND IMPLICATIONS Two analogous local anaesthetics bind to different sites and inhibit AChR activity via different mechanisms and conformational states. These results provide new information on drug modulation of AChR.
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Affiliation(s)
- Guillermo Spitzmaul
- Instituto de Investigaciones Bioquímicas, Universidad Nacional del Sur-CONICET, Bahía Blanca, Argentina
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13
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Dopico AM, Lovinger DM. Acute alcohol action and desensitization of ligand-gated ion channels. Pharmacol Rev 2009; 61:98-114. [PMID: 19270242 DOI: 10.1124/pr.108.000430] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Ethanol exerts its biological actions through multiple receptors, including ion channels. Ion channels that are sensitive to pharmacologically relevant ethanol concentrations constitute a heterogeneous set, including structurally unrelated proteins solely sharing the property that their gating is regulated by a ligand(s). Receptor desensitization is almost universal among these channels, and its modulation by ethanol may be a crucial aspect of alcohol pharmacology and effects in the body. We review the evidence documenting interactions between ethanol and ionotropic receptor desensitization, and the contribution of this interaction to overall ethanol action on channel function. In some cases, such as type 3 serotonin, nicotinic acetylcholine, GABA-A, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, ethanol actions on apparent desensitization play a significant role in acute drug action on receptor function. In a few cases, mutagenesis helped to identify different areas within a receptor protein that differentially sense n-alcohols, resulting in differential modulation of receptor desensitization. However, desensitization of a receptor is linked to a variety of biochemical processes that may alter protein conformation, such as the lipid microenvironment, post-translational channel modification, and channel subunit composition, the relative contribution of these processes to ethanol interactions with channel desensitization remains unclear. Understanding interactions between ethanol and ionotropic receptor desensitization may help to explain different ethanol actions 1) when ethanol is evaluated in vitro on cloned channel proteins, 2) under physiological or pathological conditions or in distinct cell domains with modified ligand concentration and/or receptor conformation. Finally, receptor desensitization is likely to participate in molecular and, possibly, behavioral tolerance to ethanol, which is thought to contribute to the risk of alcoholism.
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Affiliation(s)
- Alex M Dopico
- Department of Pharmacology, University of Tennessee Health Science Center, 874 Union Avenue, Memphis, TN 38163-0001, USA.
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Chiara DC, Hong FH, Arevalo E, Husain SS, Miller KW, Forman SA, Cohen JB. Time-resolved photolabeling of the nicotinic acetylcholine receptor by [3H]azietomidate, an open-state inhibitor. Mol Pharmacol 2009; 75:1084-95. [PMID: 19218367 DOI: 10.1124/mol.108.054353] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Azietomidate is a photoreactive analog of the general anesthetic etomidate that acts as a nicotinic acetylcholine receptor (nAChR) noncompetitive antagonist. We used rapid perfusion electrophysiological techniques to characterize the state dependence and kinetics of azietomidate inhibition of Torpedo californica nAChRs and time-resolved photolabeling to identify the nAChR binding sites occupied after exposure to [(3)H]azietomidate and agonist for 50 ms (open state) or at equilibrium (desensitized state). Azietomidate acted primarily as an open channel inhibitor characterized by a bimolecular association rate constant of k(+) = 5 x 10(5) M(-1) s(-1) and a dissociation rate constant of <3s(-1). Azietomidate at 10 microM, when perfused with acetylcholine (ACh), inhibited the ACh response by approximately 50% after 50 ms; when preincubated for 10 s, it decreased the peak initial response by approximately 15%. Comparison of the kinetics of recovery of ACh responses after exposure to ACh and azietomidate or to ACh alone indicated that at subsecond times, azietomidate inhibited nAChRs without enhancing the kinetics of agonist-induced desensitization. In nAChRs frozen after 50-ms exposure to agonist and [(3)H]azietomidate, amino acids were photolabeled in the ion channel [position M2-20 (alphaGlu-262, betaAsp-268, deltaGln-276)], in deltaM1 (deltaCys-236), and in alphaMA/alphaM4 (alphaGlu-390, alphaCys-412) that were also photolabeled in nAChRs in the equilibrium desensitized state at approximately half the efficiency. These results identify azietomidate binding sites at the extracellular end of the ion channel, in the delta subunit helix bundle, and in the nAChR cytoplasmic domain that seem similar in structure and accessibility in the open and desensitized states of the nAChR.
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Affiliation(s)
- David C Chiara
- Department of Neurobiology, Harvard Medical School, Boston, MA 02115, USA
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Bafna PA, Purohit PG, Auerbach A. Gating at the mouth of the acetylcholine receptor channel: energetic consequences of mutations in the alphaM2-cap. PLoS One 2008; 3:e2515. [PMID: 18575616 PMCID: PMC2429975 DOI: 10.1371/journal.pone.0002515] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/22/2008] [Accepted: 05/16/2008] [Indexed: 11/18/2022] Open
Abstract
Gating of nicotinic acetylcholine receptors from a C(losed) to an O(pen) conformation is the initial event in the postsynaptic signaling cascade at the vertebrate nerve-muscle junction. Studies of receptor structure and function show that many residues in this large, five-subunit membrane protein contribute to the energy difference between C and O. Of special interest are amino acids located at the two transmitter binding sites and in the narrow region of the channel, where C↔O gating motions generate a low↔high change in the affinity for agonists and in the ionic conductance, respectively. We have measured the energy changes and relative timing of gating movements for residues that lie between these two locations, in the C-terminus of the pore-lining M2 helix of the α subunit (‘αM2-cap’). This region contains a binding site for non-competitive inhibitors and a charged ring that influences the conductance of the open pore. αM2-cap mutations have large effects on gating but much smaller effects on agonist binding, channel conductance, channel block and desensitization. Three αM2-cap residues (αI260, αP265 and αS268) appear to move at the outset of channel-opening, about at the same time as those at the transmitter binding site. The results suggest that the αM2-cap changes its secondary structure to link gating motions in the extracellular domain with those in the channel that regulate ionic conductance.
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Affiliation(s)
- Pallavi A. Bafna
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
| | - Prasad G. Purohit
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
| | - Anthony Auerbach
- Department of Biophysics and Physiology, State University of New York at Buffalo, Buffalo, New York, United States of America
- * E-mail:
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Photo-activated azi-etomidate, a general anesthetic photolabel, irreversibly enhances gating and desensitization of gamma-aminobutyric acid type A receptors. Anesthesiology 2008; 108:103-12. [PMID: 18156888 DOI: 10.1097/01.anes.0000296074.33999.52] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
BACKGROUND The general anesthetic etomidate acts via gamma-aminobutyric acid type A (GABA(A)) receptors, enhancing activation at low GABA and prolonging deactivation. Azi-etomidate is a photo-reactive etomidate derivative with similar pharmacological actions, which has been used to identify putative binding sites. The authors examine the irreversible effects of azi-etomidate photo-modification on functional GABA(A) receptors in cell membranes. METHODS GABA(A) receptors (alpha1beta2gamma2L) were expressed in both Xenopus oocytes and human embryonic kidney cells exposed to 365 nm light-activated azi-etomidate with or without GABA, then extensively washed. Receptor-mediated chloride currents were measured using voltage clamp electrophysiology to assess the ratio of peak responses at 10 microm and 1 mm GABA (I10/I1000) and deactivation time course. RESULTS After azi-etomidate photo-modification, I10/I1000 ratios were persistently enhanced and deactivation was prolonged, mimicking reversible azi-etomidate actions. Azi-etomidate and ultraviolet light were required to produce irreversible receptor modulation. Adding GABA during photo-modification greatly enhanced irreversible modulation. Azi-etomidate modification also dose-dependently reduced maximal GABA-activated currents, consistent with accumulation of permanently desensitized receptors. Excess etomidate during azi-etomidate photo-modification competitively reduced permanent desensitization. Persistent channel modulation was blocked by 320-fold excess etomidate but enhanced when 32-fold excess etomidate was present. CONCLUSIONS Azi-etomidate efficiently photo-modifies etomidate sites on GABA(A) receptors in intact cells, producing persistent functional changes that mimic its reversible effects. The results demonstrate sequential modification at more than one etomidate site per receptor. The sites display reciprocal positive cooperativity. In combination with focal photo-activation, azi-etomidate may prove useful for studies of anesthetic actions in neural circuits.
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