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Gibbs E, Chakrapani S. Structure, Function and Physiology of 5-Hydroxytryptamine Receptors Subtype 3. Subcell Biochem 2021; 96:373-408. [PMID: 33252737 DOI: 10.1007/978-3-030-58971-4_11] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
5-hydroxytryptamine receptor subtype 3 (5-HT3R) is a pentameric ligand-gated ion channel (pLGIC) involved in neuronal signaling. It is best known for its prominent role in gut-CNS signaling though there is growing interest in its other functions, particularly in modulating non-serotonergic synaptic activity. Recent advances in structural biology have provided mechanistic understanding of 5-HT3R function and present new opportunities for the field. This chapter gives a broad overview of 5-HT3R from a physiological and structural perspective and then discusses the specific details of ion permeation, ligand binding and allosteric coupling between these two events. Biochemical evidence is summarized and placed within a physiological context. This perspective underscores the progress that has been made as well as outstanding challenges and opportunities for future 5-HT3R research.
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Affiliation(s)
- Eric Gibbs
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH, 44106-4970, USA.
| | - Sudha Chakrapani
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH, 44106-4970, USA. .,Department of Neuroscience, School of Medicine, Case Western Reserve University, Cleveland, OH, 44106-4970, USA.
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Spirova EN, Ivanov IA, Kasheverov IE, Kudryavtsev DS, Shelukhina IV, Garifulina AI, Son LV, Lummis SCR, Malca-Garcia GR, Bussmann RW, Hennig L, Giannis A, Tsetlin VI. Curare alkaloids from Matis Dart Poison: Comparison with d-tubocurarine in interactions with nicotinic, 5-HT3 serotonin and GABAA receptors. PLoS One 2019; 14:e0210182. [PMID: 30608952 PMCID: PMC6319706 DOI: 10.1371/journal.pone.0210182] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2018] [Accepted: 12/18/2018] [Indexed: 12/16/2022] Open
Abstract
Several novel bisbenzylisoquinoline alkaloids (BBIQAs) have recently been isolated from a Matis tribe arrow poison and shown by two-electrode voltage-clamp to inhibit mouse muscle nicotinic acetylcholine receptors (nAChR). Here, using radioligand assay with Aplysia californica AChBP and radioiodinated α-bungarotoxin ([125I]-αBgt), we show that BBIQA1, BBIQA2, and d-tubocurarine (d-TC) have similar affinities to nAChR orthosteric site. However, a competition with [125I]-αBgt for binding to the Torpedo californica muscle-type nAChR revealed that BBIQAs1, 2, and 3 are less potent (IC50s = 26.3, 8.75, and 17.0 μM) than d-TC (IC50 = 0.39 μM), while with α7 nAChR in GH4C1 cells, BBIQA1 was less potent that d-TC (IC50s = 162 μM and 7.77 μM, respectively), but BBIQA2 was similar (IC50 = 5.52 μM). In inhibiting the Ca2+ responses induced by acetylcholine in Neuro2a cells expressing the mouse adult α1β1εδ nAChR or human α7 nAChR, BBIQAs1 and 2 had similar potencies to d-TC (IC50s in the range 0.75-3.08 μM). Our data suggest that BBIQA1 and BBIQA2 can inhibit adult muscle α1β1εδ nAChR by both competitive and noncompetitive mechanisms. Further experiments on neuronal α3β2, α4β2, and α9α10 nAChRs, expressed in Xenopus laevis oocytes, showed that similar potencies for BBIQAs1, 2, and d-TC. With α3β2γ2 GABAAR currents were almost completely inhibited by d-TC at a high (100 μM) concentration, but BBIQAs1 and 2 were less potent (only 40-50% inhibition), whereas in competition with Alexa Fluor 546-α-cobratoxin for binding to α1β3γ2 GABAAR in Neuro2a cells, d-TC and these analogs had comparable affinities. Especially interesting effects of BBIQAs1 and 2 in comparison with d-TC were observed for 5-HT3AR: BBIQA1 and BBIQA2 were 5- and 87-fold less potent than d-TC (IC50 = 22.63 nM). Thus, our results reveal that these BBIQAs differ from d-TC in their potencies towards certain Cys-loop receptors, and we suggest that understanding the reasons behind this might be useful for future drug design.
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Affiliation(s)
- Ekaterina N. Spirova
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Igor A. Ivanov
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Igor E. Kasheverov
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
- Sechenov First Moscow State Medical University, Institute of Molecular Medicine, Moscow, Russia
| | - Denis S. Kudryavtsev
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Irina V. Shelukhina
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Alexandra I. Garifulina
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Lina V. Son
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - Sarah C. R. Lummis
- Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
| | - Gonzalo R. Malca-Garcia
- Department of Medicinal Chemistry and Pharmacognosy, College of Pharmacy, University of Illinois at Chicago, Chicago, IL, United States of America
| | | | - Lothar Hennig
- Institut für Organische Chemie, Fakultät für Chemie und Mineralogie, Universität Leipzig, Leipzig, Germany
| | - Athanassios Giannis
- Institut für Organische Chemie, Fakultät für Chemie und Mineralogie, Universität Leipzig, Leipzig, Germany
| | - Victor I. Tsetlin
- Department of Molecular Neuroimmune signaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
- PhysBio of MEPhI, Moscow, Russia
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Schaefer N, Roemer V, Janzen D, Villmann C. Impaired Glycine Receptor Trafficking in Neurological Diseases. Front Mol Neurosci 2018; 11:291. [PMID: 30186111 PMCID: PMC6110938 DOI: 10.3389/fnmol.2018.00291] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/30/2018] [Accepted: 08/02/2018] [Indexed: 12/21/2022] Open
Abstract
Ionotropic glycine receptors (GlyRs) enable fast synaptic neurotransmission in the adult spinal cord and brainstem. The inhibitory GlyR is a transmembrane glycine-gated chloride channel. The immature GlyR protein undergoes various processing steps, e.g., folding, assembly, and maturation while traveling from the endoplasmic reticulum to and through the Golgi apparatus, where post-translational modifications, e.g., glycosylation occur. The mature receptors are forward transported via microtubules to the cellular surface and inserted into neuronal membranes followed by synaptic clustering. The normal life cycle of a receptor protein includes further processes like internalization, recycling, and degradation. Defects in GlyR life cycle, e.g., impaired protein maturation and degradation have been demonstrated to underlie pathological mechanisms of various neurological diseases. The neurological disorder startle disease is caused by glycinergic dysfunction mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). In vitro studies have shown that most recessive forms of startle disease are associated with impaired receptor biogenesis. Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies. Binding of GlyR autoantibodies leads to enhanced receptor internalization. Here we focus on the normal life cycle of GlyRs concentrating on assembly and maturation, receptor trafficking, post-synaptic integration and clustering, and GlyR internalization/recycling/degradation. Furthermore, this review highlights findings on impairment of these processes under disease conditions such as disturbed neuronal ER-Golgi trafficking as the major pathomechanism for recessive forms of human startle disease. In SPS, enhanced receptor internalization upon autoantibody binding to the GlyR has been shown to underlie the human pathology. In addition, we discuss how the existing mouse models of startle disease increased our current knowledge of GlyR trafficking routes and function. This review further illuminates receptor trafficking of GlyR variants originally identified in startle disease patients and explains changes in the life cycle of GlyRs in patients with SPS with respect to structural and functional consequences at the receptor level.
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Affiliation(s)
- Natascha Schaefer
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Vera Roemer
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Dieter Janzen
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
| | - Carmen Villmann
- Institute for Clinical Neurobiology, Julius-Maximilians-University Würzburg, Würzburg, Germany
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Mihalik B, Pálvölgyi A, Bogár F, Megyeri K, Ling I, Barkóczy J, Bartha F, Martinek TA, Gacsályi I, Antoni FA. Loop-F of the α-subunit determines the pharmacologic profile of novel competitive inhibitors of GABA A receptors. Eur J Pharmacol 2017; 798:129-136. [DOI: 10.1016/j.ejphar.2017.01.033] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2016] [Revised: 01/25/2017] [Accepted: 01/25/2017] [Indexed: 12/11/2022]
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Thompson AJ, Lummis SCR. Discriminating between 5-HT₃A and 5-HT₃AB receptors. Br J Pharmacol 2014; 169:736-47. [PMID: 23489111 PMCID: PMC3687655 DOI: 10.1111/bph.12166] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2012] [Revised: 01/07/2013] [Accepted: 02/04/2013] [Indexed: 12/19/2022] Open
Abstract
The 5-HT3B subunit was first cloned in 1999, and co-expression with the 5-HT3A subunit results in heteromeric 5-HT₃AB receptors that are functionally distinct from homomeric 5-HT₃A receptors. The affinities of competitive ligands at the two receptor subtypes are usually similar, but those of non-competitive antagonists that bind in the pore often differ. A competitive ligand and allosteric modulator that distinguishes 5-HT₃A from 5-HT₃AB receptors has recently been described, and the number of non-competitive antagonists identified with this ability has increased in recent years. In this review, we discuss the differences between 5-HT₃A and 5-HT₃AB receptors and describe the possible sites of action of compounds that can distinguish between them.
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Affiliation(s)
- A J Thompson
- Department of Biochemistry, University of Cambridge, Cambridge, UK.
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6
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De Rienzo F, Moura Barbosa AJ, Perez MA, Fernandes PA, Ramos MJ, Menziani MC. The extracellular subunit interface of the 5-HT3receptors: a computational alanine scanning mutagenesis study. J Biomol Struct Dyn 2012; 30:280-98. [DOI: 10.1080/07391102.2012.680029] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/28/2022]
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Carpenter TS, Lau EY, Lightstone FC. A role for loop F in modulating GABA binding affinity in the GABA(A) receptor. J Mol Biol 2012; 422:310-23. [PMID: 22659322 DOI: 10.1016/j.jmb.2012.05.025] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/05/2011] [Revised: 05/08/2012] [Accepted: 05/17/2012] [Indexed: 10/28/2022]
Abstract
The brain's major inhibitory neuroreceptor is the ligand-gated ion channel γ-aminobutyric acid (GABA) type A receptor (GABAR). GABARs exist in a variety of different subunit combinations that act to modulate the physiological behavior of GABAR by altering its pharmacological profile, as well as its affinity for GABA. While the α(1)β(2)γ(2) subtype is one of the most prevalent GABARs, the less populous α(6)β(3)δ subtype has much higher GABA sensitivity. Previous studies identified residues crucial for GABA binding; however, the specific molecular differences responsible for this diverse sensitivity are not known. Furthermore, the role of loop F is a divisive subject, with conflicting evidence for ligand binding function. Using homology modeling, ligand docking, and molecular dynamics simulations, we investigated the GABA binding sites of the two receptor subtypes. Simulations identified seven residues that consistently interacted with GABA in both subtypes: αF65, αR132, βL99, βE155, βR/K196, βY205, and βR207. Residue substitution at position β196 (arginine in α(6)β(3)δ, lysine in α(1)β(2)γ(2)) resulted in a shift in GABA binding. However, the major difference between the two binding sites was the magnitude of loop F involvement, with a greater contribution in the α(6)β(3)δ receptor. Free energy calculations confirm that the α(6)β(3)δ binding pocket has an increased affinity for GABA. Thus, the possible role for loop F across the GABAR family is to modulate GABA affinity.
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Affiliation(s)
- Timothy S Carpenter
- Biosciences and Biotechnology Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
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Baptista-Hon DT, Deeb TZ, Othman NA, Sharp D, Hales TG. The 5-HT3B subunit affects high-potency inhibition of 5-HT3 receptors by morphine. Br J Pharmacol 2012; 165:693-704. [PMID: 21740409 DOI: 10.1111/j.1476-5381.2011.01582.x] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
Abstract
BACKGROUND AND PURPOSE Morphine is an antagonist at 5-HT(3) A receptors. 5-HT(3) and opioid receptors are expressed in many of the same neuronal pathways where they modulate gut motility, pain and reinforcement. There is increasing interest in the 5-HT3B subunit, which confers altered pharmacology to 5-HT(3) receptors. We investigated the mechanisms of inhibition by morphine of 5-HT(3) receptors and the influence of the 5-HT3B subunit. EXPERIMENTAL APPROACH 5-HT-evoked currents were recorded from voltage-clamped HEK293 cells expressing human 5-HT3A subunits alone or in combination with 5-HT3B subunits. The affinity of morphine for the orthosteric site of 5-HT(3) A or 5-HT(3) AB receptors was assessed using radioligand binding with the antagonist [(3) H]GR65630. KEY RESULTS When pre-applied, morphine potently inhibited 5-HT-evoked currents mediated by 5-HT(3) A receptors. The 5-HT3B subunit reduced the potency of morphine fourfold and increased the rates of inhibition and recovery. Inhibition by pre-applied morphine was insurmountable by 5-HT, was voltage-independent and occurred through a site outside the second membrane-spanning domain. When applied simultaneously with 5-HT, morphine caused a lower potency, surmountable inhibition of 5-HT(3) A and 5-HT(3) AB receptors. Morphine also fully displaced [(3) H]GR65630 from 5-HT(3) A and 5-HT(3) AB receptors with similar potency. CONCLUSIONS AND IMPLICATIONS These findings suggest that morphine has two sites of action, a low-affinity, competitive site and a high-affinity, non-competitive site that is not available when the channel is activated. The affinity of morphine for the latter is reduced by the 5-HT3B subunit. Our results reveal that morphine causes a high-affinity, insurmountable and subunit-dependent inhibition of human 5-HT(3) receptors.
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Affiliation(s)
- Daniel T Baptista-Hon
- The Institute of Academic Anaesthesia, Centre for Neuroscience, University of Dundee, Dundee, UK
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Machu TK. Therapeutics of 5-HT3 receptor antagonists: current uses and future directions. Pharmacol Ther 2011; 130:338-47. [PMID: 21356241 PMCID: PMC3103470 DOI: 10.1016/j.pharmthera.2011.02.003] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2011] [Accepted: 02/09/2011] [Indexed: 12/14/2022]
Abstract
The 5-Hydroxytryptamine3 (5-HT3) receptor is a member of the cys-loop family of ligand gated ion channels, of which the nicotinic acetylcholine receptor is the prototype. All other 5-HT receptors identified to date are metabotropic receptors. The 5-HT3 receptor is present in the central and peripheral nervous systems, as well as a number of non-nervous tissues. As an ion channel that is permeable to the cations, Na(+), K(+), and Ca(2+), the 5-HT3 receptor mediates fast depolarizing responses in pre- and post-synaptic neurons. As such, 5-HT3 receptor antagonists that are used clinically block afferent and efferent synaptic transmission. The most well established physiological roles of the 5-HT3 receptor are to coordinate emesis and regulate gastrointestinal motility. Currently marketed 5-HT3 receptor antagonists are indicated for the treatment of chemotherapy, radiation, and anesthesia-induced nausea and vomiting, as well as irritable bowel syndrome. Other therapeutic uses that have been explored include pain and drug addiction. The 5-HT3 receptor is one of a number of receptors that play a role in mediating nausea and vomiting, and as such, 5-HT3 receptor antagonists demonstrate the greatest anti-emetic efficacy when administered in combination with other drug classes.
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Affiliation(s)
- Tina K Machu
- Dept. of Medical Education and Dept. of Pharmacology and Neuroscience, University of North Texas Health Science Center, 3500 Camp Bowie Blvd. Fort Worth, TX 76107-2699, USA.
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Brams M, Pandya A, Kuzmin D, van Elk R, Krijnen L, Yakel JL, Tsetlin V, Smit AB, Ulens C. A structural and mutagenic blueprint for molecular recognition of strychnine and d-tubocurarine by different cys-loop receptors. PLoS Biol 2011; 9:e1001034. [PMID: 21468359 PMCID: PMC3066128 DOI: 10.1371/journal.pbio.1001034] [Citation(s) in RCA: 84] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2010] [Accepted: 02/09/2011] [Indexed: 11/18/2022] Open
Abstract
Cys-loop receptors (CLR) are pentameric ligand-gated ion channels that mediate fast excitatory or inhibitory transmission in the nervous system. Strychnine and d-tubocurarine (d-TC) are neurotoxins that have been highly instrumental in decades of research on glycine receptors (GlyR) and nicotinic acetylcholine receptors (nAChR), respectively. In this study we addressed the question how the molecular recognition of strychnine and d-TC occurs with high affinity and yet low specificity towards diverse CLR family members. X-ray crystal structures of the complexes with AChBP, a well-described structural homolog of the extracellular domain of the nAChRs, revealed that strychnine and d-TC adopt multiple occupancies and different ligand orientations, stabilizing the homopentameric protein in an asymmetric state. This introduces a new level of structural diversity in CLRs. Unlike protein and peptide neurotoxins, strychnine and d-TC form a limited number of contacts in the binding pocket of AChBP, offering an explanation for their low selectivity. Based on the ligand interactions observed in strychnine- and d-TC-AChBP complexes we performed alanine-scanning mutagenesis in the binding pocket of the human α1 GlyR and α7 nAChR and showed the functional relevance of these residues in conferring high potency of strychnine and d-TC, respectively. Our results demonstrate that a limited number of ligand interactions in the binding pocket together with an energetic stabilization of the extracellular domain are key to the poor selective recognition of strychnine and d-TC by CLRs as diverse as the GlyR, nAChR, and 5-HT3R. Ligand-gated ion channels play an important role in fast electrochemical signaling in the brain. Cys-loop receptors are a class of pentameric ligand-gated ion channels that are activated by specific neurotransmitters, including acetylcholine (ACh), serotonin (5-HT), glycine (Gly), and γ-aminobutyric acid (GABA). Each type of cys-loop receptor contains an extracellular domain that specifically recognizes only one of these four neurotransmitters and opens an ion-conducting channel pore upon ligand binding. In this study, we investigated the poor specificity with which two potent neurotoxic inhibitors, namely strychnine and d-tubocurarine, are recognized by different cys-loop receptors. Using X-ray crystallography we solved 3-dimensional structures of strychnine or d-tubocurarine in complex with ACh binding protein (AChBP), a well-recognized structural homolog of the nicotinic ACh receptor. Based on ligand-receptor interactions observed in AChBP structures we designed mutant GlyR and α7 nAChR to identify hot spots in the binding pocket of these receptors that define potent inhibition by strychnine and d-tubocurarine, respectively. Combined, our results offer detailed understanding of the molecular recognition of antagonists that have high affinity but poor specificity for different cys-loop receptors.
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Affiliation(s)
- Marijke Brams
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
| | - Anshul Pandya
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, United States of America
| | - Dmitry Kuzmin
- Department of Molecular Basis of Neurosignaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - René van Elk
- Department of Molecular & Cellular Neurobiology, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University, Amsterdam, The Netherlands
| | - Liz Krijnen
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
| | - Jerrel L. Yakel
- Laboratory of Neurobiology, National Institute of Environmental Health Sciences, National Institutes of Health, Department of Health and Human Services, Research Triangle Park, North Carolina, United States of America
| | - Victor Tsetlin
- Department of Molecular Basis of Neurosignaling, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Moscow, Russia
| | - August B. Smit
- Department of Molecular & Cellular Neurobiology, Center for Neurogenomics and Cognitive Research, Neuroscience Campus Amsterdam, VU University, Amsterdam, The Netherlands
| | - Chris Ulens
- Laboratory of Structural Neurobiology, KULeuven, Leuven, Belgium
- * E-mail:
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Nyce HL, Stober ST, Abrams CF, White MM. Mapping spatial relationships between residues in the ligand-binding domain of the 5-HT3 receptor using a molecular ruler. Biophys J 2010; 98:1847-55. [PMID: 20441748 DOI: 10.1016/j.bpj.2010.01.034] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/11/2009] [Revised: 01/11/2010] [Accepted: 01/14/2010] [Indexed: 12/24/2022] Open
Abstract
The serotonin 5-HT(3) receptor (5-HT(3)R) is a member of the Cys-loop ligand-gated ion channel family. We used a combination of site-directed mutagenesis, homology modeling, and ligand-docking simulations to analyze antagonist-receptor interactions. Mutation of E236, which is near loop C of the binding site, to aspartate prevents expression of the receptor on the cell surface, and no specific ligand binding can be detected. On the other hand, mutation to glutamine, asparagine, or alanine produces receptors that are expressed on the cell surface, but decreases receptor affinity for the competitive antagonist d-tubocurarine (dTC) 5-35-fold. The results of a double-mutant cycle analysis employing a panel of dTC analogs to identify specific points of interactions between the dTC analogs and E236 are consistent with E236 making a direct physical interaction with the 12 -OH of dTC. dTC is a rigid molecule of known three-dimensional structure. Together with previous studies linking other regions of dTC to specific residues in the binding site, these data allow us to define the relative spatial arrangement of three different residues in the ligand-binding site: R92 (loop D), N128 (loop A), and E236 (near loop C). Molecular modeling employing these distance constraints followed by molecular-dynamics simulations produced a dTC/receptor complex consistent with the experimental data. The use of the rigid ligands as molecular rulers in conjunction with double-mutant cycle analysis provides a means of mapping the relative positions of various residues in the ligand-binding site of any ligand-receptor complex, and thus is a useful tool for delineating the architecture of the binding site.
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Affiliation(s)
- Heather L Nyce
- Department of Biochemistry, Drexel University College of Medicine, Philadelphia, Pennsylvania, USA
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12
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Eshaq RS, Stahl LD, Stone R, Smith SS, Robinson LC, Leidenheimer NJ. GABA acts as a ligand chaperone in the early secretory pathway to promote cell surface expression of GABAA receptors. Brain Res 2010; 1346:1-13. [PMID: 20580636 DOI: 10.1016/j.brainres.2010.05.030] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2009] [Revised: 05/06/2010] [Accepted: 05/11/2010] [Indexed: 10/19/2022]
Abstract
GABA (gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in brain. The fast inhibitory effect of GABA is mediated through the GABA(A) receptor, a postsynaptic ligand-gated chloride channel. We propose that GABA can act as a ligand chaperone in the early secretory pathway to facilitate GABA(A) receptor cell surface expression. Forty-two hours of GABA treatment increased the surface expression of recombinant receptors expressed in HEK 293 cells, an effect accompanied by an increase in GABA-gated chloride currents. In time-course experiments, a 1h GABA exposure, followed by a 5h incubation in GABA-free medium, was sufficient to increase receptor surface expression. A shorter GABA exposure could be used in HEK 293 cells stably transfected with the GABA transporter GAT-1. In rGAT-1HEK 293 cells, the GABA effect was blocked by the GAT-1 inhibitor NO-711, indicating that GABA was acting intracellularly. The effect of GABA was prevented by brefeldin A (BFA), an inhibitor of early secretory pathway trafficking. Coexpression of GABA(A) receptors with the GABA synthetic enzyme glutamic acid decarboxylase 67 (GAD67) also resulted in an increase in receptor surface levels. GABA treatment failed to promote the surface expression of GABA binding site mutant receptors, which themselves were poorly expressed at the surface. Consistent with an intracellular action of GABA, we show that GABA does not act by stabilizing surface receptors. Furthermore, GABA treatment rescued the surface expression of a receptor construct that was retained within the secretory pathway. Lastly, the lipophilic competitive antagonist (+)bicuculline promoted receptor surface expression, including the rescue of a secretory pathway-retained receptor. Our results indicate that a neurotransmitter can act as a ligand chaperone in the early secretory pathway to regulate the surface expression of its receptor. This effect appears to rely on binding site occupancy, rather than agonist-induced structural changes, since chaperoning is observed with both an agonist and a competitive antagonist.
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Affiliation(s)
- Randa S Eshaq
- Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center-Shreveport, 1501 Kings Highway, Shreveport, LA 71130-3932, USA
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Abstract
Functional studies of the ligand gated ion channel family (nicotinic acetylcholine, serotonin Type 3, glycine and GABA receptors) along with the crystal structure of the acetylcholine binding protein (AChBP) and molecular dynamics simulations of the nAChR structure have resulted in a structural model in which the agonist-binding pocket comprises six loops (A-F) contributed by adjacent subunits. It is presumed that the binding of agonist results in a local structural rearrangement that is then transduced to the gate, causing the pore to open. Efforts are underway to better define the specific roles of the six binding loops. Several studies have suggested Loop F may play a direct role in linking the structural rearrangement within the binding pocket to the gate, although other investigations have indicated Loop F may be crucial for locking the agonist molecule into the binding site. This review will focus on the controversy surrounding the role of Loop F during GABA receptor activation.
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Affiliation(s)
- Alpa Khatri
- Department of Physiology, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA
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14
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de Oliveira-Pierce AN, Zhang R, Machu TK. Colchicine: a novel positive allosteric modulator of the human 5-hydroxytryptamine3A receptor. J Pharmacol Exp Ther 2009; 329:838-47. [PMID: 19188483 DOI: 10.1124/jpet.108.146522] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The actions of colchicine were examined with the two-electrode voltage-clamp technique and radioligand binding assays in mouse and human 5-hydroxytryptamine(3A) receptors (5-HT(3A)Rs) expressed in Xenopus laevis oocytes. Colchicine inhibited 5-hydroxytryptamine (5-HT)-evoked currents in oocytes expressing mouse 5-HT(3A)Rs, with an IC(50) of 59.5 +/- 3 microM. In contrast to the mouse receptor, coapplication of colchicine with 5-HT (<1 microM) strongly enhanced 5-HT-evoked currents in oocytes expressing human 5-HT(3A)Rs. Colchicine applied alone did not induce a detectable current. In the presence of 0.5 microM 5-HT, the potentiation was concentration-dependent and reached the maximum (approximately 100%) when 750 microM colchicine was applied. However, colchicine-dependent inhibition can be observed at 5-HT concentrations > 1 microM. In oocyte membranes expressing mouse or human receptors, binding studies with colchicine (25 nM-1 mM) revealed no displacement of 1-methyl-N-((1R,3r,5S)-9-methyl-9 azabicyclo [3.3.1]nonan-3yl)-1H-indazole-3 carboxamide ([(3)H]BRL-43694), suggesting that actions of colchicine do not occur at the ligand binding domain. Functional effects of colchicine on both receptors occurred in the absence of preincubation and after cold temperature incubation, suggesting that the microtubule-depolymerizing effects of colchicine play no role in modulation of receptor function. Studies with interspecies chimeric receptors demonstrated that the distal one third of the N terminus is responsible for the bidirectional modulation by colchicine. Collectively, these results suggest that colchicine modulates receptor function through loops C and/or F through a gating mechanism.
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Affiliation(s)
- A N de Oliveira-Pierce
- Department of Pharmacology and Neuroscience, University of North Texas Health Sciences Center, Fort Worth, TX 76107, USA.
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