1
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Indurthi DC. The effect of unliganded gating on agonist response in nicotinic receptors. Eur J Pharmacol 2024; 980:176830. [PMID: 39032761 DOI: 10.1016/j.ejphar.2024.176830] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/31/2024] [Revised: 07/02/2024] [Accepted: 07/18/2024] [Indexed: 07/23/2024]
Abstract
Understanding the agonist concentration-response curve (CRC) is the cornerstone in pharmacology. While CRC parameters, agonist potency (EC50) and efficacy (maximum response, Imax) are well-studied, the role of unliganded gating (minimum response, Imin) on CRC is often overlooked. This study explores the effect of unliganded gating on agonist response in muscle-type acetylcholine (ACh) receptors, focusing on the underexplored role of Imin in modulating EC50 and Imax. Three Gain-of-Function (GOF) mutations that increase, and two Loss-of-Function (LOF) mutations that decrease the unliganded gating equilibrium constant (L0) were studied using automated patch-clamp electrophysiology. GOF mutations enhanced agonist potency, whereas LOF mutations reduced it. The calculated CRC aligned well with empirical results, indicating that agonist CRC can be estimated from knowledge of L0. Reduction in agonist efficacy due to LOF mutations was calculated and subsequently validated using single-channel patch-clamp electrophysiology, a factor often obscured in normalized CRC. The study also evaluated the combined impact of mutations (L0) on CRC, confirming the predictive model. Further, no significant energetic coupling between distant residues (>15 Å) was found, indicating that the mutations' effects are localized and do not alter overall agonist affinity. These findings substantiate the role of unliganded gating in modulating agonist responses and establishes a predictive model for estimating CRC parameters from known changes in L0. The study highlights the importance of intrinsic activity in receptor theory.
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Affiliation(s)
- Dinesh C Indurthi
- Department of Physiology and Biophysics, University at Buffalo, State University of New York, Buffalo, United States.
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2
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Tessier CJG, Emlaw JR, Sturgeon RM, daCosta CJB. Derepression may masquerade as activation in ligand-gated ion channels. Nat Commun 2023; 14:1907. [PMID: 37019877 PMCID: PMC10076327 DOI: 10.1038/s41467-023-36770-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2022] [Accepted: 02/16/2023] [Indexed: 04/07/2023] Open
Abstract
Agonists are ligands that bind to receptors and activate them. In the case of ligand-gated ion channels, such as the muscle-type nicotinic acetylcholine receptor, mechanisms of agonist activation have been studied for decades. Taking advantage of a reconstructed ancestral muscle-type β-subunit that forms spontaneously activating homopentamers, here we show that incorporation of human muscle-type α-subunits appears to repress spontaneous activity, and furthermore that the presence of agonist relieves this apparent α-subunit-dependent repression. Our results demonstrate that rather than provoking channel activation/opening, agonists may instead 'inhibit the inhibition' of intrinsic spontaneous activity. Thus, agonist activation may be the apparent manifestation of agonist-induced derepression. These results provide insight into intermediate states that precede channel opening and have implications for the interpretation of agonism in ligand-gated ion channels.
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Affiliation(s)
- Christian J G Tessier
- Department of Chemistry and Biomolecular Sciences, Centre for Chemical and Synthetic Biology, University of Ottawa, Ottawa, ON, Canada
| | - Johnathon R Emlaw
- Department of Chemistry and Biomolecular Sciences, Centre for Chemical and Synthetic Biology, University of Ottawa, Ottawa, ON, Canada
| | - Raymond M Sturgeon
- Department of Chemistry and Biomolecular Sciences, Centre for Chemical and Synthetic Biology, University of Ottawa, Ottawa, ON, Canada
| | - Corrie J B daCosta
- Department of Chemistry and Biomolecular Sciences, Centre for Chemical and Synthetic Biology, University of Ottawa, Ottawa, ON, Canada.
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3
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Cowgill J, Chanda B. Charge-voltage curves of Shaker potassium channel are not hysteretic at steady state. J Gen Physiol 2023; 155:213823. [PMID: 36692860 PMCID: PMC9884579 DOI: 10.1085/jgp.202112883] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/26/2021] [Revised: 02/16/2022] [Accepted: 01/03/2023] [Indexed: 01/25/2023] Open
Abstract
Charge-voltage curves of many voltage-gated ion channels exhibit hysteresis but such curves are also a direct measure of free energy of channel gating and, hence, should be path-independent. Here, we identify conditions to measure steady-state charge-voltage curves and show that these are curves are not hysteretic. Charged residues in transmembrane segments of voltage-gated ion channels (VGICs) sense and respond to changes in the electric field. The movement of these gating charges underpins voltage-dependent activation and is also a direct metric of the net free-energy of channel activation. However, for most voltage-gated ion channels, the charge-voltage (Q-V) curves appear to be dependent on initial conditions. For instance, Q-V curves of Shaker potassium channel obtained by hyperpolarizing from 0 mV is left-shifted compared to those obtained by depolarizing from a holding potential of -80 mV. This hysteresis in Q-V curves is a common feature of channels in the VGIC superfamily and raises profound questions about channel energetics because the net free-energy of channel gating is a state function and should be path independent. Due to technical limitations, conventional gating current protocols are limited to test pulse durations of <500 ms, which raises the possibility that the dependence of Q-V on initial conditions reflects a lack of equilibration. Others have suggested that the hysteresis is fundamental thermodynamic property of voltage-gated ion channels and reflects energy dissipation due to measurements under non-equilibrium conditions inherent to rapid voltage jumps (Villalba-Galea. 2017. Channels. https://doi.org/10.1080/19336950.2016.1243190). Using an improved gating current and voltage-clamp fluorometry protocols, we show that the gating hysteresis arising from different initial conditions in Shaker potassium channel is eliminated with ultra-long (18-25 s) test pulses. Our study identifies a modified gating current recording protocol to obtain steady-state Q-V curves of a voltage-gated ion channel. Above all, these findings demonstrate that the gating hysteresis in Shaker channel is a kinetic phenomenon rather than a true thermodynamic property of the channel and the charge-voltage curve is a true measure of the net-free energy of channel gating.
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Affiliation(s)
- John Cowgill
- Departments of Anesthesiology, Neuroscience, Biochemistry and Molecular Biophysics, Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA,John Cowgill:
| | - Baron Chanda
- Departments of Anesthesiology, Neuroscience, Biochemistry and Molecular Biophysics, Center for Investigation of Membrane Excitability Diseases, Washington University School of Medicine, St. Louis, MO, USA,Correspondence to Baron Chanda:
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4
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Dual amplification strategy turns TRPM2 channels into supersensitive central heat detectors. Proc Natl Acad Sci U S A 2022; 119:e2212378119. [PMID: 36409885 PMCID: PMC9881722 DOI: 10.1073/pnas.2212378119] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
The Ca2+ and ADP ribose (ADPR)-activated cation channel TRPM2 is the closest homolog of the cold sensor TRPM8 but serves as a deep-brain warmth sensor. To unravel the molecular mechanism of heat sensing by the TRPM2 protein, we study here temperature dependence of TRPM2 currents in cell-free membrane patches across ranges of agonist concentrations. We find that channel gating remains strictly agonist-dependent even at 40°C: heating alone or in combination with just Ca2+, just ADPR, Ca2+ + cyclic ADPR, or H2O2 pretreatment only marginally activates TRPM2. For fully liganded TRPM2, pore opening is intrinsically endothermic, due to ~10-fold larger activation enthalpy for opening (~200 kJ/mol) than for closure (~20 kJ/mol). However, the temperature threshold is too high (>40°C) for unliganded but too low (<15°C) for fully liganded channels. Thus, warmth sensitivity around 37°C is restricted to narrow ranges of agonist concentrations. For ADPR, that range matches, but for Ca2+, it exceeds bulk cytosolic values. The supraphysiological [Ca2+] needed for TRPM2 warmth sensitivity is provided by Ca2+ entering through the channel's pore. That positive feedback provides further strong amplification to the TRPM2 temperature response (Q10 ~ 1,000), enabling the TRPM2 protein to autonomously respond to tiny temperature fluctuations around 37°C. These functional data together with published structures suggest a molecular mechanism for opposite temperature dependences of two closely related channel proteins.
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5
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Schirmeyer J, Eick T, Schulz E, Hummert S, Sattler C, Schmauder R, Benndorf K. Subunit promotion energies for channel opening in heterotetrameric olfactory CNG channels. PLoS Comput Biol 2022; 18:e1010376. [PMID: 35998156 PMCID: PMC9512249 DOI: 10.1371/journal.pcbi.1010376] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2022] [Revised: 09/26/2022] [Accepted: 07/11/2022] [Indexed: 11/18/2022] Open
Abstract
Cyclic nucleotide-gated (CNG) ion channels of olfactory sensory neurons contain three types of homologue subunits, two CNGA2 subunits, one CNGA4 subunit and one CNGB1b subunit. Each subunit carries an intracellular cyclic nucleotide binding domain (CNBD) whose occupation by up to four cyclic nucleotides evokes channel activation. Thereby, the subunits interact in a cooperative fashion. Here we studied 16 concatamers with systematically disabled, but still functional, binding sites and quantified channel activation by systems of intimately coupled state models transferred to 4D hypercubes, thereby exploiting a weak voltage dependence of the channels. We provide the complete landscape of free energies for the complex activation process of heterotetrameric channels, including 32 binding steps, in both the closed and open channel, as well as 16 closed-open isomerizations. The binding steps are specific for the subunits and show pronounced positive cooperativity for the binding of the second and the third ligand. The energetics of the closed-open isomerizations were disassembled to elementary subunit promotion energies for channel opening, ΔΔGfpn, adding to the free energy of the closed-open isomerization of the empty channel, E0. The ΔΔGfpn values are specific for the four subunits and presumably invariant for the specific patterns of liganding. In conclusion, subunit cooperativity is confined to the CNBD whereas the subunit promotion energies for channel opening are independent. Olfactory sensory neurons (OSNs) in the nose transmit the information of odor molecules to electrical signals that are conducted to central parts of the brain. Olfactory cyclic nucleotide-gated (CNG) ion channels, located in the cell membrane of the OSNs, are relevant proteins in this process. These olfactory CNG channels are formed by three types of homologue subunits and each of these subunits contains a cyclic nucleotide binding domain (CNBD). A channel is activated by the binding of up to four cyclic nucleotides. The process of channel activation is only poorly understood. Herein we analyzed this activation process in great detail by concatenating these four subunits, disabling the CNBDs by mutations and performing extended computational fit analyses providing all 32 constants for the different binding steps at different degrees of liganding and, in addition, elementary subunit promotion energies for channel opening for all subunits. Our data suggest that subunit cooperativity is confined to the action of the CNBD.
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Affiliation(s)
- Jana Schirmeyer
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
| | - Thomas Eick
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
| | - Eckhard Schulz
- Schmalkalden University of Applied Sciences, Faculty of Electrical Engineering, Blechhammer, Schmalkalden, Germany
| | - Sabine Hummert
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
- Schmalkalden University of Applied Sciences, Faculty of Electrical Engineering, Blechhammer, Schmalkalden, Germany
| | - Christian Sattler
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
| | - Ralf Schmauder
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
| | - Klaus Benndorf
- Institute of Physiology II, Jena University Hospital, Friedrich Schiller University Jena, Jena, Germany
- * E-mail:
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6
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Khatoon N, Adusumilli S, Dey P, Sharma R, Kampani P, Shandilya J, Nayak TK. Protein engineering and design in ion channels and receptors. Methods Cell Biol 2022; 169:143-168. [DOI: 10.1016/bs.mcb.2021.12.031] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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7
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Cecchini M, Changeux JP. Nicotinic receptors: From protein allostery to computational neuropharmacology. Mol Aspects Med 2021; 84:101044. [PMID: 34656371 DOI: 10.1016/j.mam.2021.101044] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/27/2021] [Revised: 09/28/2021] [Accepted: 09/30/2021] [Indexed: 11/15/2022]
Abstract
We propose an extension and further development of the Monod-Wyman-Changeux model for allosteric transitions of regulatory proteins to brain communications and specifically to neurotransmitters receptors, with the nicotinic acetylcholine receptor (nAChR) as a model of ligand-gated ion channels. The present development offers an expression of the change of the gating isomerization constant caused by pharmacological ligand binding in terms of its value in the absence of ligands and several "modulation factors", which vary with orthosteric ligand binding (agonists/antagonists), allosteric ligand binding (positive allosteric modulators/negative allosteric modulators) and receptor desensitization. The new - explicit - formulation of such "modulation factors", provides expressions for the pharmacological attributes of potency, efficacy, and selectivity for the modulatory ligands (including endogenous neurotransmitters) in terms of their binding affinity for the active, resting, and desensitized states of the receptor. The current formulation provides ways to design neuroactive compounds with a controlled pharmacological profile, opening the field of computational neuro-pharmacology.
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Affiliation(s)
- Marco Cecchini
- Institut de Chimie de Strasbourg, UMR7177, CNRS, Université de Strasbourg, F-67083, Strasbourg Cedex, France.
| | - Jean-Pierre Changeux
- Kavli Institute for Brain & Mind University of California, San Diego La Jolla, CA, 92093, USA; Institut Pasteur, URA 2182, CNRS, F-75015, France; Collège de France, F-75005 Paris, France.
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8
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Thermodynamic profile of mutual subunit control in a heteromeric receptor. Proc Natl Acad Sci U S A 2021; 118:2100469118. [PMID: 34301910 PMCID: PMC8325370 DOI: 10.1073/pnas.2100469118] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Cyclic nucleotide-gated (CNG) ion channels of olfactory neurons are tetrameric membrane receptors that are composed of two A2 subunits, one A4 subunit, and one B1b subunit. Each subunit carries a cyclic nucleotide-binding domain in the carboxyl terminus, and the channels are activated by the binding of cyclic nucleotides. The mechanism of cooperative channel activation is still elusive. Using a complete set of engineered concatenated olfactory CNG channels, with all combinations of disabled binding sites and fit analyses with systems of allosteric models, the thermodynamics of microscopic cooperativity for ligand binding was subunit- and state-specifically quantified. We show, for the closed channel, that preoccupation of each of the single subunits increases the affinity of each other subunit with a Gibbs free energy (ΔΔG) of ∼-3.5 to ∼-5.5 kJ ⋅ mol-1, depending on the subunit type, with the only exception that a preoccupied opposite A2 subunit has no effect on the other A2 subunit. Preoccupation of two neighbor subunits of a given subunit causes the maximum affinity increase with ΔΔG of ∼-9.6 to ∼-9.9 kJ ⋅ mol-1 Surprisingly, triple preoccupation leads to fewer negative ΔΔG values for a given subunit as compared to double preoccupation. Channel opening increases the affinity of all subunits. The equilibrium constants of closed-open isomerizations systematically increase with progressive liganding. This work demonstrates, on the example of the heterotetrameric olfactory CNG channel, a strategy to derive detailed insights into the specific mutual control of the individual subunits in a multisubunit membrane receptor.
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9
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Lam AKM, Dutzler R. Mechanism of pore opening in the calcium-activated chloride channel TMEM16A. Nat Commun 2021; 12:786. [PMID: 33542228 PMCID: PMC7862263 DOI: 10.1038/s41467-020-20788-8] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/05/2020] [Accepted: 12/16/2020] [Indexed: 01/14/2023] Open
Abstract
The anion channel TMEM16A is activated by intracellular Ca2+ in a highly cooperative process. By combining electrophysiology and autocorrelation analysis, we investigated the mechanism of channel activation and the concurrent rearrangement of the gate in the narrow part of the pore. Features in the fluctuation characteristics of steady-state current indicate the sampling of intermediate conformations that are successively occupied during gating. The initial step is related to conformational changes induced by Ca2+ binding, which is ensued by rearrangements that open the pore. Mutations in the gate shift the equilibrium of transitions in a manner consistent with a progressive destabilization of this region during pore opening. We come up with a mechanism of channel activation where the binding of Ca2+ induces conformational changes in the protein that, in a sequential manner, propagate from the binding site and couple to the gate in the narrow pore to allow ion permeation.
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Affiliation(s)
- Andy K M Lam
- Department of Biochemistry, University of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland.
| | - Raimund Dutzler
- Department of Biochemistry, University of Zurich, Winterthurerstrasse 190, CH-8057, Zurich, Switzerland.
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10
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Han L, Shan Q. Pair of Residue Substitutions at the Outer Mouth of the Channel Pore Act as Inputs for a Boolean Logic "OR" Gate Based on the Glycine Receptor. ACS Chem Neurosci 2020; 11:3409-3417. [PMID: 32970400 DOI: 10.1021/acschemneuro.0c00522] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
The glycine receptor (GlyR) is a ligand-activated chloride channel, whose mutations are the major cause of hereditary hyperekplexia. The hyperekplexia-causing R271Q mutation, which is located at the extracellular outer mouth of the channel pore, dramatically impairs the GlyR function manifesting a reduced sensitivity toward glycine. This study reports that a second mutation, S273D, rescues the function of the R271Q GlyR to that of the wild-type (WT) GlyR. Surprisingly, the S273D mutation, when introduced to the WT GlyR, does not further increase the receptor function. In other words, the compromised function of the 271Q 273S GlyR (i.e., the R271Q GlyR) can be rescued to WT levels by the introduction of either, or both, of the Q271R and S273D substitutions. From the perspective of Boolean logic gates, the Q271R and S273D substitutions act as inputs for an OR gate based on the GlyR. Further experiments revealed that the negative-charge carried by the 273 residue is essential for the expression of the OR gate and that the expression of the OR gate is residue-position-specific. In addition, mechanistic investigation implied that the 273 residue influences the 271 residue, which might underpin the unique nonadditive OR gate relationship between these two residues. Such an ion-channel-based OR gate, expressing output in the form of electrical current, could potentially be developed to digitally manipulate neuronal activity.
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Affiliation(s)
- Lu Han
- Zhejiang Key Laboratory of Organ Development and Regeneration, College of Life and Environmental Sciences, Hangzhou Normal University, Hangzhou, Zhejiang 311121, China
| | - Qiang Shan
- Laboratory for Synaptic Plasticity, Shantou University Medical College, Shantou, Guangdong 515041, China
- School of Basic Medical Sciences, Wenzhou Medical University, Wenzhou, Zhejiang 325035, China
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11
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Pierce SR, Germann AL, Evers AS, Steinbach JH, Akk G. Reduced Activation of the Synaptic-Type GABA A Receptor Following Prolonged Exposure to Low Concentrations of Agonists: Relationship between Tonic Activity and Desensitization. Mol Pharmacol 2020; 98:762-769. [PMID: 32978327 DOI: 10.1124/molpharm.120.000088] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/11/2020] [Accepted: 09/09/2020] [Indexed: 01/16/2023] Open
Abstract
Synaptic GABAA receptors are alternately exposed to short pulses of a high, millimolar concentration of GABA and prolonged periods of low, micromolar concentration of the transmitter. Prior work has indicated that exposure to micromolar concentrations of GABA can both activate the postsynaptic receptors generating sustained low-amplitude current and desensitize the receptors, thereby reducing the peak amplitude of subsequent synaptic response. However, the precise relationship between tonic activation and reduction of peak response is not known. Here, we have measured the effect of prolonged exposure to GABA or the combination of GABA and the neurosteroid allopregnanolone, which was intended to desensitize a fraction of receptors, on a subsequent response to a high concentration of agonist in human α1β3γ2L receptors expressed in Xenopus oocytes. We show that the reduction in the peak amplitude of the post-exposure test response correlates with the open probability of the preceding desensitizing response. Curve fitting of the inhibitory relationship yielded an IC50 of 12.5 µM and a Hill coefficient of -1.61. The activation and desensitization data were mechanistically analyzed in the framework of a three-state Resting-Active-Desensitized model. Using the estimated affinity, efficacy, and desensitization parameters, we calculated the amount of desensitization that would accumulate during a long (2-minute) application of GABA or GABA plus allopregnanolone. The results indicate that accumulation of desensitization depends on the level of activity rather than agonist or potentiator concentration per se. We estimate that in the presence of 1 µM GABA, approximately 5% of α1β3γ2L receptors are functionally eliminated because of desensitization. SIGNIFICANCE STATEMENT: We present an analytical approach to quantify and predict the loss of activatable GABAA receptors due to desensitization in the presence of transmitter and the steroid allopregnanolone. The findings indicate that the peak amplitude of the synaptic response is influenced by ambient GABA and that changes in ambient concentrations of the transmitter and other GABAergic agents can modify tonically and phasically activated synaptic receptors in opposite directions.
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Affiliation(s)
- Spencer R Pierce
- Department of Anesthesiology (S.R.P., A.L.G., A.S.E., J.H.S., G.A.) and the Taylor Family Institute for Innovative Psychiatric Research (A.S.E., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Allison L Germann
- Department of Anesthesiology (S.R.P., A.L.G., A.S.E., J.H.S., G.A.) and the Taylor Family Institute for Innovative Psychiatric Research (A.S.E., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Alex S Evers
- Department of Anesthesiology (S.R.P., A.L.G., A.S.E., J.H.S., G.A.) and the Taylor Family Institute for Innovative Psychiatric Research (A.S.E., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Joe Henry Steinbach
- Department of Anesthesiology (S.R.P., A.L.G., A.S.E., J.H.S., G.A.) and the Taylor Family Institute for Innovative Psychiatric Research (A.S.E., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology (S.R.P., A.L.G., A.S.E., J.H.S., G.A.) and the Taylor Family Institute for Innovative Psychiatric Research (A.S.E., J.H.S., G.A.), Washington University School of Medicine, St. Louis, Missouri
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12
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Çetiner U, Raz O, Sukharev S, Jarzynski C. Recovery of Equilibrium Free Energy from Nonequilibrium Thermodynamics with Mechanosensitive Ion Channels in E. coli. PHYSICAL REVIEW LETTERS 2020; 124:228101. [PMID: 32567892 DOI: 10.1103/physrevlett.124.228101] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/12/2019] [Accepted: 04/21/2020] [Indexed: 06/11/2023]
Abstract
In situ measurements of the free energy difference between the open and closed states of ion channels are challenging due to hysteresis effects and inactivation. Exploiting recent developments in statistical physics, we present a general formalism to extract the free energy difference ΔF between the closed and open states of mechanosensitive ion channels from nonequilibrium work distributions associated with the opening and closing of the channels (gating) in response to ramp stimulation protocols recorded in native patches. We show that the work distributions obtained from the gating of MscS channels in E. coli membrane satisfy the strong symmetry relation predicted by the Crooks fluctuation theorem. Our approach enables the determination of ΔF using patch-clamp experiments, which are often inherently restricted to the nonequilibrium regime.
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Affiliation(s)
- Uğur Çetiner
- Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
- Maryland Biophysics Program, University of Maryland, College Park, Maryland 20742, USA
- Department of Biology, University of Maryland, College Park, Maryland 20742, USA
| | - Oren Raz
- Department of Physics of Complex Systems, Faculty of Physics, Weizmann Institute of Science, Rehovot 7610001, Israel
| | - Sergei Sukharev
- Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
- Maryland Biophysics Program, University of Maryland, College Park, Maryland 20742, USA
- Department of Biology, University of Maryland, College Park, Maryland 20742, USA
| | - Christopher Jarzynski
- Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
- Maryland Biophysics Program, University of Maryland, College Park, Maryland 20742, USA
- Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA
- Department of Physics, University of Maryland, College Park, Maryland 20742, USA
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13
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Germann AL, Steinbach JH, Akk G. Application of the Co-Agonist Concerted Transition Model to Analysis of GABAA Receptor Properties. Curr Neuropharmacol 2020; 17:843-851. [PMID: 30520374 PMCID: PMC7052843 DOI: 10.2174/1570159x17666181206092418] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/02/2018] [Revised: 11/16/2018] [Accepted: 11/28/2018] [Indexed: 11/22/2022] Open
Abstract
The co-agonist concerted transition model is a simple and practical solution to analyze various aspects of GABAA receptor function. Several model-based predictions have been verified experimentally in previous reports. We review here the practical implications of the model and demonstrate how it enables simplification of the experimental procedure and data analysis to characterize the effects of mutations or properties of novel ligands. Specifically, we show that the value of EC50 and the magnitude of current response are directly affected by basal activity, and that coapplication of a background agonist acting at a distinct site or use of a gain-of-function mutation can be employed to enable studies of weak activators or mutated receptors with impaired gating. We also show that the ability of one GABAergic agent to potentiate the activity elicited by another is a computable value that depends on the level of constitutive activity of the ion channel and the ability of each agonist to directly activate the receptor. Significantly, the model accurately accounts for situations where the paired agonists interact with the same site compared to distinct sites on the receptor.
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Affiliation(s)
- Allison L Germann
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO, United States
| | - Joe Henry Steinbach
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO, United States.,Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, MO, United States
| | - Gustav Akk
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, MO, United States.,Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, MO, United States
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14
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Changeux JP. The nicotinic acetylcholine receptor: a typical 'allosteric machine'. Philos Trans R Soc Lond B Biol Sci 2019; 373:rstb.2017.0174. [PMID: 29735728 DOI: 10.1098/rstb.2017.0174] [Citation(s) in RCA: 47] [Impact Index Per Article: 9.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Accepted: 12/06/2017] [Indexed: 12/26/2022] Open
Abstract
The concept of allosteric interaction was initially proposed to account for the inhibitory feedback mechanism mediated by bacterial regulatory enzymes. In contrast with the classical mechanism of competitive, steric, interaction between ligands for a common site, allosteric interactions take place between topographically distinct sites and are mediated by a discrete and reversible conformational change of the protein. The concept was soon extended to membrane receptors for neurotransmitters and shown to apply to the signal transduction process which, in the case of the acetylcholine nicotinic receptor (nAChR), links the ACh binding site to the ion channel. Pharmacological effectors, referred to as allosteric modulators, such as Ca2+ ions and ivermectin, were discovered that enhance the transduction process when they bind to sites distinct from the orthosteric ACh site and the ion channel. The recent X-ray and electron microscopy structures, at atomic resolution, of the resting and active conformations of several homologues of the nAChR, in combination with atomistic molecular dynamics simulations reveal a stepwise quaternary transition in the transduction process with tertiary changes modifying the boundaries between subunits. These interfaces host orthosteric and allosteric modulatory sites which structural organization changes in the course of the transition. The nAChR appears as a typical allosteric machine. The model emerging from these studies has led to the conception and development of several new pharmacological agents.This article is part of a discussion meeting issue 'Allostery and molecular machines'.
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Affiliation(s)
- Jean-Pierre Changeux
- CNRS UMR 3571, Institut Pasteur, Paris 75724, France .,Communications Cellulaires, Collège de France, Paris 75005, France
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15
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Galstyan V, Funk L, Einav T, Phillips R. Combinatorial Control through Allostery. J Phys Chem B 2019; 123:2792-2800. [PMID: 30768906 DOI: 10.1021/acs.jpcb.8b12517] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/29/2022]
Abstract
Many instances of cellular signaling and transcriptional regulation involve switch-like molecular responses to the presence or absence of input ligands. To understand how these responses come about and how they can be harnessed, we develop a statistical mechanical model to characterize the types of Boolean logic that can arise from allosteric molecules following the Monod-Wyman-Changeux (MWC) model. Building upon previous work, we show how an allosteric molecule regulated by two inputs can elicit AND, OR, NAND, and NOR responses but is unable to realize XOR or XNOR gates. Next, we demonstrate the ability of an MWC molecule to perform ratiometric sensing-a response behavior where activity depends monotonically on the ratio of ligand concentrations. We then extend our analysis to more general schemes of combinatorial control involving either additional binding sites for the two ligands or an additional third ligand and show how these additions can cause a switch in the logic behavior of the molecule. Overall, our results demonstrate the wide variety of control schemes that biological systems can implement using simple mechanisms.
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Affiliation(s)
| | - Luke Funk
- Harvard-MIT Division of Health Sciences and Technology and the Broad Institute of MIT and Harvard , Massachusetts Institute of Technology , Cambridge , Massachusetts 02139 , United States
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16
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Steinbach JH, Akk G. Applying the Monod-Wyman-Changeux Allosteric Activation Model to Pseudo-Steady-State Responses from GABA A Receptors. Mol Pharmacol 2018; 95:106-119. [PMID: 30333132 DOI: 10.1124/mol.118.113787] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/25/2018] [Accepted: 10/12/2018] [Indexed: 12/16/2022] Open
Abstract
The Monod-Wyman-Changeux (MWC) cyclic model was described as a kinetic scheme to explain enzyme function and modulation more than 50 years ago and was proposed as a model for understanding the activation of transmitter-gated channels soon afterward. More recently, the MWC model has been used to describe the activation of the GABAA receptor by the transmitter, GABA, and drugs that bind to separate sites on the receptor. It is most interesting that the MWC formalism can also describe the interactions among drugs that activate the receptor. In this review, we describe properties of the MWC model that have been explored experimentally using the GABAA receptor, summarize analytical expressions for activation and interaction for drugs, and briefly review experimental results.
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Affiliation(s)
- Joe Henry Steinbach
- Department of Anesthesiology, and Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology, and Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, Missouri
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17
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Kudryavtsev DS, Spirova EN, Shelukhina IV, Son LV, Makarova YV, Utkina NK, Kasheverov IE, Tsetlin VI. Makaluvamine G from the Marine Sponge Zyzzia fuliginosa Inhibits Muscle nAChR by Binding at the Orthosteric and Allosteric Sites. Mar Drugs 2018; 16:md16040109. [PMID: 29597332 PMCID: PMC5923396 DOI: 10.3390/md16040109] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2018] [Revised: 03/16/2018] [Accepted: 03/23/2018] [Indexed: 12/19/2022] Open
Abstract
Diverse ligands of the muscle nicotinic acetylcholine receptor (nAChR) are used as muscle relaxants during surgery. Although a plethora of such molecules exists in the market, there is still a need for new drugs with rapid on/off-set, increased selectivity, and so forth. We found that pyrroloiminoquinone alkaloid Makaluvamine G (MG) inhibits several subtypes of nicotinic receptors and ionotropic γ-aminobutiric acid receptors, showing a higher affinity and moderate selectivity toward muscle nAChR. The action of MG on the latter was studied by a combination of electrophysiology, radioligand assay, fluorescent microscopy, and computer modeling. MG reveals a combination of competitive and un-competitive inhibition and caused an increase in the apparent desensitization rate of the murine muscle nAChR. Modeling ion channel kinetics provided evidence for MG binding in both orthosteric and allosteric sites. We also demonstrated that theα1 (G153S) mutant of the receptor, associated with the myasthenic syndrome, is more prone to inhibition by MG. Thus, MG appears to be a perspective hit molecule for the design of allosteric drugs targeting muscle nAChR, especially for treating slow-channel congenital myasthenic syndromes.
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Affiliation(s)
- Denis S Kudryavtsev
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
| | - Ekaterina N Spirova
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
| | - Irina V Shelukhina
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
| | - Lina V Son
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
- Moscow Institute of Physics and Technology, Institutsky Per. 9, Dolgoprudny, 141700 Moscow Region, Russia.
| | - Yana V Makarova
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
| | - Natalia K Utkina
- G.B. Elyakov Pacific Institute of Bioorganic Chemistry (PIBOC), Russian Academy of Sciences, Prospect 100 let Vladivostoku, 159, 690022 Vladivostok, Russia.
| | - Igor E Kasheverov
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
| | - Victor I Tsetlin
- Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Russian Academy of Sciences, Miklukho-Maklaya Street, 16/10, 117997 Moscow, Russia.
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18
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Tuning Transcriptional Regulation through Signaling: A Predictive Theory of Allosteric Induction. Cell Syst 2018; 6:456-469.e10. [PMID: 29574055 PMCID: PMC5991102 DOI: 10.1016/j.cels.2018.02.004] [Citation(s) in RCA: 39] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2017] [Revised: 02/02/2018] [Accepted: 02/09/2018] [Indexed: 02/02/2023]
Abstract
Allosteric regulation is found across all domains of life, yet we still lack simple, predictive theories that directly link the experimentally tunable parameters of a system to its input-output response. To that end, we present a general theory of allosteric transcriptional regulation using the Monod-Wyman-Changeux model. We rigorously test this model using the ubiquitous simple repression motif in bacteria by first predicting the behavior of strains that span a large range of repressor copy numbers and DNA binding strengths and then constructing and measuring their response. Our model not only accurately captures the induction profiles of these strains, but also enables us to derive analytic expressions for key properties such as the dynamic range and [EC50]. Finally, we derive an expression for the free energy of allosteric repressors that enables us to collapse our experimental data onto a single master curve that captures the diverse phenomenology of the induction profiles.
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19
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Activation mechanism of the calcium-activated chloride channel TMEM16A revealed by cryo-EM. Nature 2017; 552:421-425. [PMID: 29236691 DOI: 10.1038/nature24652] [Citation(s) in RCA: 197] [Impact Index Per Article: 28.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2017] [Accepted: 10/18/2017] [Indexed: 12/21/2022]
Abstract
The calcium-activated chloride channel TMEM16A is a ligand-gated anion channel that opens in response to an increase in intracellular Ca2+ concentration. The protein is broadly expressed and contributes to diverse physiological processes, including transepithelial chloride transport and the control of electrical signalling in smooth muscles and certain neurons. As a member of the TMEM16 (or anoctamin) family of membrane proteins, TMEM16A is closely related to paralogues that function as scramblases, which facilitate the bidirectional movement of lipids across membranes. The unusual functional diversity of the TMEM16 family and the relationship between two seemingly incompatible transport mechanisms has been the focus of recent investigations. Previous breakthroughs were obtained from the X-ray structure of the lipid scramblase of the fungus Nectria haematococca (nhTMEM16), and from the cryo-electron microscopy structure of mouse TMEM16A at 6.6 Å (ref. 14). Although the latter structure disclosed the architectural differences that distinguish ion channels from lipid scramblases, its low resolution did not permit a detailed molecular description of the protein or provide any insight into its activation by Ca2+. Here we describe the structures of mouse TMEM16A at high resolution in the presence and absence of Ca2+. These structures reveal the differences between ligand-bound and ligand-free states of a calcium-activated chloride channel, and when combined with functional experiments suggest a mechanism for gating. During activation, the binding of Ca2+ to a site located within the transmembrane domain, in the vicinity of the pore, alters the electrostatic properties of the ion conduction path and triggers a conformational rearrangement of an α-helix that comes into physical contact with the bound ligand, and thereby directly couples ligand binding and pore opening. Our study describes a process that is unique among channel proteins, but one that is presumably general for both functional branches of the TMEM16 family.
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20
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Akk G, Shin DJ, Germann AL, Steinbach JH. GABA Type A Receptor Activation in the Allosteric Coagonist Model Framework: Relationship between EC 50 and Basal Activity. Mol Pharmacol 2017; 93:90-100. [PMID: 29150461 DOI: 10.1124/mol.117.110569] [Citation(s) in RCA: 26] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2017] [Accepted: 11/15/2017] [Indexed: 01/08/2023] Open
Abstract
The concerted transition model for multimeric proteins is a simple formulation for analyzing the behavior of transmitter-gated ion channels. We used the model to examine the relationship between the EC50 for activation of the GABA type A (GABAA) receptor by the transmitter GABA and basal activity employing concatemeric ternary GABAA receptors expressed in Xenopus oocytes. Basal activity, reflecting the receptor function in the absence of the transmitter, can be changed either by mutation to increase constitutive activity or by the addition of a second agonist (acting at a different site) to increase background activity. The model predicts that either mechanism for producing a change in basal activity will result in identical effects on the EC50 We examined receptor activation by GABA while changing the level of basal activity with the allosterically acting anesthetics propofol, pentobarbital, or alfaxalone. We found that the relationship between EC50 and basal activity was well described by the concerted transition model. Changes in the basal activity by gain-of-function mutations also resulted in predictable changes in the EC50 Finally, we altered the number of GABA-binding sites by a mutation and again found that the relationship could be well described by the model. Overall, the results support the idea that interactions between the transmitter GABA and the allosteric agonists propofol, pentobarbital, or alfaxalone can be understood as reflecting additive and independent free energy changes, without assuming any specific interactions.
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Affiliation(s)
- Gustav Akk
- Department of Anesthesiology (G.A., D.J.S., A.L.G., J.H.S.) and Taylor Family Institute for Innovative Psychiatric Research (G.A., J.H.S.), Washington University School of Medicine, St. Louis, Missouri
| | - Daniel J Shin
- Department of Anesthesiology (G.A., D.J.S., A.L.G., J.H.S.) and Taylor Family Institute for Innovative Psychiatric Research (G.A., J.H.S.), Washington University School of Medicine, St. Louis, Missouri
| | - Allison L Germann
- Department of Anesthesiology (G.A., D.J.S., A.L.G., J.H.S.) and Taylor Family Institute for Innovative Psychiatric Research (G.A., J.H.S.), Washington University School of Medicine, St. Louis, Missouri
| | - Joe Henry Steinbach
- Department of Anesthesiology (G.A., D.J.S., A.L.G., J.H.S.) and Taylor Family Institute for Innovative Psychiatric Research (G.A., J.H.S.), Washington University School of Medicine, St. Louis, Missouri
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21
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Abstract
Agonists turn on receptors because they have a higher affinity for active versus resting conformations of the protein. Activation can occur by either of two pathways that connect to form a cycle: Agonists bind to resting receptors that then become active, or resting receptors activate and then bind agonists. We used mutations to construct endplate acetylcholine receptors (AChRs) having only one functional neurotransmitter-binding site and single-channel electrophysiology to measure independently binding constants for four different agonists, to both resting and active conformations of each site. For all agonists and sites, the total free energy change in each pathway was the same, confirming the activation cycle without external energy. Other results show that (i) there is no cooperativity between sites; (ii) agonist association is slower than diffusion in resting receptors but nearly diffusional in active receptors; (iii) whereas resting affinity is determined mainly by agonist association, active affinity is determined mainly by agonist dissociation; and (iv) at each site and for all agonists, receptor activation approximately doubles the agonist-binding free energy. We discuss a two-step mechanism for binding that involves diffusion and a local conformational change ("catch") that is modulated by receptor activation. The results suggest that binding to a resting site and the switch to high affinity are both integral parts of a single allosteric transition. We hypothesize that catch ensures proper signal recognition in complex chemical environments and that binding site compaction is a determinant of both resting and active affinity.
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22
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Nemecz Á, Prevost MS, Menny A, Corringer PJ. Emerging Molecular Mechanisms of Signal Transduction in Pentameric Ligand-Gated Ion Channels. Neuron 2017; 90:452-70. [PMID: 27151638 DOI: 10.1016/j.neuron.2016.03.032] [Citation(s) in RCA: 154] [Impact Index Per Article: 22.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2015] [Revised: 01/07/2016] [Accepted: 03/24/2016] [Indexed: 10/21/2022]
Abstract
Nicotinic acetylcholine, serotonin type 3, γ-amminobutyric acid type A, and glycine receptors are major players of human neuronal communication. They belong to the family of pentameric ligand-gated ion channels, sharing a highly conserved modular 3D structure. Recently, high-resolution structures of both open- and closed-pore conformations have been solved for a bacterial, an invertebrate, and a vertebrate receptor in this family. These data suggest that a common gating mechanism occurs, coupling neurotransmitter binding to pore opening, but they also pinpoint significant differences among subtypes. In this Review, we summarize the structural and functional data in light of these gating models and speculate about their mechanistic consequences on ion permeation, pathological mutations, as well as functional regulation by orthosteric and allosteric effectors.
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Affiliation(s)
- Ákos Nemecz
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France
| | - Marie S Prevost
- Institute of Structural and Molecular Biology, University College London and Birkbeck, Malet Street, London WC1E 7HX, UK
| | - Anaïs Menny
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France; Université Pierre et Marie Curie (UPMC), Cellule Pasteur, 75005 Paris, France
| | - Pierre-Jean Corringer
- Channel-Receptors Unit, Institut Pasteur, 75015 Paris, France; CNRS UMR 3571, 75015 Paris, France.
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23
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Soh MS, Estrada-Mondragon A, Durisic N, Keramidas A, Lynch JW. Probing the Structural Mechanism of Partial Agonism in Glycine Receptors Using the Fluorescent Artificial Amino Acid, ANAP. ACS Chem Biol 2017; 12:805-813. [PMID: 28121133 DOI: 10.1021/acschembio.6b00926] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The efficacy of an agonist at a pentameric ligand-gated ion channel is determined by the rate at which it induces a conformational change from the resting closed state to a preopen ("flip") state. If the ability of an agonist to promote this isomerization is sufficiently low, then it becomes a partial agonist. As partial agonists at pentameric ligand-gated ion channels show considerable promise as therapeutics, understanding the structural basis of the resting-flip-state isomerization may provide insight into therapeutic design. Accordingly, we sought to identify structural correlates of the resting-flip conformational change in the glycine receptor chloride channel. We used nonsense suppression to introduce the small, fluorescent amino acid, 3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (ANAP), into specific sites in the extracellular and transmembrane domains. Then, under voltage-clamp conditions in Xenopus oocytes, we simultaneously quantified current and fluorescence responses induced by structurally similar agonists with high, medium, and low efficacies (glycine, β-alanine, and taurine, respectively). Analyzing results from nine ANAP-incorporated sites, we show that glycine receptor activation by agonists with graded efficacies manifests structurally as correspondingly graded movements of the β1-β2 loop, the β8-β9 loop, and the Cys-loop from the extracellular domain and the TM2-TM3 linker in the transmembrane domain. We infer that the resting-flip transition involves an efficacy-dependent molecular reorganization at the extracellular-transmembrane domain interface that primes receptors for efficacious opening.
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Affiliation(s)
- Ming S. Soh
- Queensland Brain
Institute, The University of Queensland, Brisbane, Queensland 4072, Australia
| | - Argel Estrada-Mondragon
- Queensland Brain
Institute, The University of Queensland, Brisbane, Queensland 4072, Australia
| | - Nela Durisic
- Queensland Brain
Institute, The University of Queensland, Brisbane, Queensland 4072, Australia
| | - Angelo Keramidas
- Queensland Brain
Institute, The University of Queensland, Brisbane, Queensland 4072, Australia
| | - Joseph W. Lynch
- Queensland Brain
Institute, The University of Queensland, Brisbane, Queensland 4072, Australia
- School
of Biomedical Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia
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24
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Einav T, Phillips R. Monod-Wyman-Changeux Analysis of Ligand-Gated Ion Channel Mutants. J Phys Chem B 2017; 121:3813-3824. [PMID: 28134524 DOI: 10.1021/acs.jpcb.6b12672] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022]
Abstract
We present a framework for computing the gating properties of ligand-gated ion channel mutants using the Monod-Wyman-Changeux (MWC) model of allostery. We derive simple analytic formulas for key functional properties such as the leakiness, dynamic range, half-maximal effective concentration ([EC50]), and effective Hill coefficient, and explore the full spectrum of phenotypes that are accessible through mutations. Specifically, we consider mutations in the channel pore of nicotinic acetylcholine receptor (nAChR) and the ligand binding domain of a cyclic nucleotide-gated (CNG) ion channel, demonstrating how each mutation can be characterized as only affecting a subset of the biophysical parameters. In addition, we show how the unifying perspective offered by the MWC model allows us, perhaps surprisingly, to collapse the plethora of dose-response data from different classes of ion channels into a universal family of curves.
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Affiliation(s)
- Tal Einav
- Department of Physics, California Institute of Technology , Pasadena, California 91125, United States
| | - Rob Phillips
- Department of Applied Physics and Division of Biology and Biological Engineering, California Institute of Technology , Pasadena, California 91125, United States
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25
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Bruhova I, Auerbach A. Molecular recognition at cholinergic synapses: acetylcholine versus choline. J Physiol 2016; 595:1253-1261. [PMID: 27779761 DOI: 10.1113/jp273291] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/13/2016] [Accepted: 10/12/2016] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS Neuromuscular acetylcholine (ACh) receptors have a high affinity for the neurotransmitter ACh and a low affinity for its metabolic product choline. At each transmitter binding site three aromatic groups determine affinity, and together provide ∼50% more binding energy for ACh than for choline. Deprotonation of αY190 by a nearby lysine strengthens the interaction between this aromatic ring and both ACh and choline. H-bonds position ACh and choline differently in the aromatic cage to generate the different affinities. ABSTRACT Acetylcholine (ACh) released at the vertebrate nerve-muscle synapse is hydrolysed rapidly to choline (Cho), so endplate receptors (AChRs) are exposed to high concentrations of both of these structurally related ligands. To understand how these receptors distinguish ACh and Cho, we used single-channel electrophysiology to measure resting affinities (binding free energies) of these and other agonists in adult-type mouse AChRs having a mutation(s) at the transmitter-binding sites. The aromatic rings of αY190, αW149 and αY198 each provide ∼50% less binding energy for Cho compared to ACh. At αY198 a phenylalanine substitution had no effect, but at αY190 this substitution caused a large, agonist-independent loss in binding energy that depended on the presence of αK145. The results suggest that (1) αY190 is deprotonated by αK145 to strengthen the interaction between this benzene ring and the agonist's quaternary ammonium (QA) and (2) AChRs respond strongly to ACh because an H-bond positions the QA to interact optimally with the rings, and weakly to Cho because a different H-bond tethers the ligand to misalign the QA and form weaker interactions with the aromatic groups. The results suggest that the difference in ACh versus Cho binding energies is determined by different ligand positions within a fixed protein structure.
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Affiliation(s)
- Iva Bruhova
- Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, NY, 14214, USA
| | - Anthony Auerbach
- Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, NY, 14214, USA
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26
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Gupta S, Chakraborty S, Vij R, Auerbach A. A mechanism for acetylcholine receptor gating based on structure, coupling, phi, and flip. J Gen Physiol 2016; 149:85-103. [PMID: 27932572 PMCID: PMC5217088 DOI: 10.1085/jgp.201611673] [Citation(s) in RCA: 36] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/21/2016] [Revised: 09/20/2016] [Accepted: 11/10/2016] [Indexed: 01/29/2023] Open
Abstract
Gupta et al. use single-channel electrophysiology to investigate the gating mechanism of acetylcholine receptor ion channels. They propose that channel opening starts at the M2–M3 linker and ligand-binding sites and proceeds through four brief intermediate conformations before ending with the collapse of a gate bubble. Nicotinic acetylcholine receptors are allosteric proteins that generate membrane currents by isomerizing (“gating”) between resting and active conformations under the influence of neurotransmitters. Here, to explore the mechanisms that link the transmitter-binding sites (TBSs) with the distant gate, we use mutant cycle analyses to measure coupling between residue pairs, phi value analyses to sequence domain rearrangements, and current simulations to reproduce a microsecond shut component (“flip”) apparent in single-channel recordings. Significant interactions between amino acids separated by >15 Å are rare; an exception is between the αM2–M3 linkers and the TBSs that are ∼30 Å apart. Linker residues also make significant, local interactions within and between subunits. Phi value analyses indicate that without agonists, the linker is the first region in the protein to reach the gating transition state. Together, the phi pattern and flip component suggest that a complete, resting↔active allosteric transition involves passage through four brief intermediate states, with brief shut events arising from sojourns in all or a subset. We derive energy landscapes for gating with and without agonists, and propose a structure-based model in which resting→active starts with spontaneous rearrangements of the M2–M3 linkers and TBSs. These conformational changes stabilize a twisted extracellular domain to promote transmembrane helix tilting, gate dilation, and the formation of a “bubble” that collapses to initiate ion conduction. The energy landscapes suggest that twisting is the most energetically unfavorable step in the resting→active conformational change and that the rate-limiting step in the reverse process is bubble formation.
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Affiliation(s)
- Shaweta Gupta
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214
| | - Srirupa Chakraborty
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214
| | - Ridhima Vij
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214
| | - Anthony Auerbach
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, NY 14214
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27
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Vij R, Purohit P, Auerbach A. Modal affinities of endplate acetylcholine receptors caused by loop C mutations. ACTA ACUST UNITED AC 2016; 146:375-86. [PMID: 26503719 PMCID: PMC4621750 DOI: 10.1085/jgp.201511503] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Modal activity at the nicotinic acetylcholine receptor, in which open channel probability switches reversibly between discrete values, arises from changes in the resting affinity at the agonist site. The time course of the endplate current is determined by the rate and equilibrium constants for acetylcholine receptor (AChR) activation. We measured these constants in single-channel currents from AChRs with mutations at the neurotransmitter-binding sites, in loop C. The main findings are: (a) Almost all perturbations of loop C generate heterogeneity in the channel open probability (“modes”). (b) Modes are generated by different affinities for ACh that can be either higher or lower than in the wild-type receptors. (c) The modes are stable, in so far as each receptor maintains its affinity for at least several minutes. (d) Different agonists show different degrees of modal activity. With the loop C mutation αP197A, there are four modes with ACh but only two with partial agonists. (e) The affinity variations arise exclusively from the αδ-binding site. (f) Substituting four γ-subunit residues into the δ subunit (three in loop E and one in the β5–β5′ linker) reduces modal activity. (g) At each neurotransmitter-binding site, affinity is determined by a core of five aromatic residues. Modes are eliminated by an alanine mutation at δW57 but not at the other aromatics. (h) Modes are eliminated by a phenylalanine substitution at all core aromatics except αY93. The results suggest that, at the αδ agonist site, loop C and the complementary subunit surface can each adopt alternative conformations and interact with each other to influence the position of δW57 with respect to the aromatic core and, hence, affinity.
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Affiliation(s)
- Ridhima Vij
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
| | - Prasad Purohit
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
| | - Anthony Auerbach
- Neuroscience Program and Department of Physiology, State University of New York at Buffalo, Buffalo, NY 14214
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Díaz-Franulic I, Caceres-Molina J, Sepulveda RV, Gonzalez-Nilo F, Latorre R. Structure-Driven Pharmacology of Transient Receptor Potential Channel Vanilloid 1. Mol Pharmacol 2016; 90:300-8. [PMID: 27335334 DOI: 10.1124/mol.116.104430] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/26/2016] [Accepted: 06/16/2016] [Indexed: 01/08/2023] Open
Abstract
The transient receptor potential vanilloid 1 (TRPV1) ion channel is a polymodal receptor that mediates the flux of cations across the membrane in response to several stimuli, including heat, voltage, and ligands. The best known agonist of TRPV1 channels is capsaicin, the pungent component of "hot" chili peppers. In addition, peptides found in the venom of poisonous animals, along with the lipids phosphatidylinositol 4,5-biphosphate, lysophosphatidic acid, and cholesterol, bind to TRPV1 with high affinity to modulate channel gating. Here, we discuss the functional evidence regarding ligand-dependent activation of TRPV1 channels in light of structural data recently obtained by cryoelectron microscopy. This review focuses on the mechanistic insights into ligand binding and allosteric gating of TRPV1 channels and the relevance of accurate polymodal receptor biophysical characterization for drug design in novel pain therapies.
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Affiliation(s)
- Ignacio Díaz-Franulic
- Centro Interdisciplinario de Neurociencias de Valparaíso, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile (I.D.-F., R.L., F.G.-N.); Centro de Bioinformática y Biología Integrativa, Universidad Andrés Bello, Santiago, Chile (I.D.-F., J.C.-M., R.V.S., F.G.-N.); and Fraunhofer Chile Research, Santiago, Chile (I.D.-F.)
| | - Javier Caceres-Molina
- Centro Interdisciplinario de Neurociencias de Valparaíso, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile (I.D.-F., R.L., F.G.-N.); Centro de Bioinformática y Biología Integrativa, Universidad Andrés Bello, Santiago, Chile (I.D.-F., J.C.-M., R.V.S., F.G.-N.); and Fraunhofer Chile Research, Santiago, Chile (I.D.-F.)
| | - Romina V Sepulveda
- Centro Interdisciplinario de Neurociencias de Valparaíso, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile (I.D.-F., R.L., F.G.-N.); Centro de Bioinformática y Biología Integrativa, Universidad Andrés Bello, Santiago, Chile (I.D.-F., J.C.-M., R.V.S., F.G.-N.); and Fraunhofer Chile Research, Santiago, Chile (I.D.-F.)
| | - Fernando Gonzalez-Nilo
- Centro Interdisciplinario de Neurociencias de Valparaíso, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile (I.D.-F., R.L., F.G.-N.); Centro de Bioinformática y Biología Integrativa, Universidad Andrés Bello, Santiago, Chile (I.D.-F., J.C.-M., R.V.S., F.G.-N.); and Fraunhofer Chile Research, Santiago, Chile (I.D.-F.)
| | - Ramon Latorre
- Centro Interdisciplinario de Neurociencias de Valparaíso, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile (I.D.-F., R.L., F.G.-N.); Centro de Bioinformática y Biología Integrativa, Universidad Andrés Bello, Santiago, Chile (I.D.-F., J.C.-M., R.V.S., F.G.-N.); and Fraunhofer Chile Research, Santiago, Chile (I.D.-F.)
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29
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Wang W, Hong JS, Rab A, Sorscher EJ, Kirk KL. Robust Stimulation of W1282X-CFTR Channel Activity by a Combination of Allosteric Modulators. PLoS One 2016; 11:e0152232. [PMID: 27007499 PMCID: PMC4805204 DOI: 10.1371/journal.pone.0152232] [Citation(s) in RCA: 28] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/10/2015] [Accepted: 03/10/2016] [Indexed: 11/18/2022] Open
Abstract
W1282X is a common nonsense mutation among cystic fibrosis patients that results in the production of a truncated Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) channel. Here we show that the channel activity of the W1282X-CFTR polypeptide is exceptionally low in excised membrane patches at normally saturating doses of ATP and PKA (single channel open probability (PO) < 0.01). However, W1282X-CFTR channels were stimulated by two CFTR modulators, the FDA-approved VX-770 and the dietary compound curcumin. Each of these compounds is an allosteric modulator of CFTR gating that promotes channel activity in the absence of the native ligand, ATP. Although W1282X-CFTR channels were stimulated by VX-770 in the absence of ATP their activities remained dependent on PKA phosphorylation. Thus, activated W1282X-CFTR channels should remain under physiologic control by cyclic nucleotide signaling pathways in vivo. VX-770 and curcumin exerted additive effects on W1282X-CFTR channel gating (opening/closing) in excised patches such that the Po of the truncated channel approached unity (> 0.9) when treated with both modulators. VX-770 and curcumin also additively stimulated W1282X-CFTR mediated currents in polarized FRT epithelial monolayers. In this setting, however, the stimulated W1282X-CFTR currents were smaller than those mediated by wild type CFTR (3-5%) due presumably to lower expression levels or cell surface targeting of the truncated protein. Combining allosteric modulators of different mechanistic classes is worth considering as a treatment option for W1282X CF patients perhaps when coupled with maneuvers to increase expression of the truncated protein.
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Affiliation(s)
- Wei Wang
- Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
- Department of Cell, Developmental, and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
- * E-mail: (WW); (KLK)
| | - Jeong S. Hong
- Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
- Department of Cell, Developmental, and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
| | - Andras Rab
- Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
| | - Eric J. Sorscher
- Department of Pediatrics, Emory University, Atlanta, GA, 30322, United States of America
| | - Kevin L. Kirk
- Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
- Department of Cell, Developmental, and Integrative Biology, University of Alabama at Birmingham, Birmingham, AL, 35294, United States of America
- * E-mail: (WW); (KLK)
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30
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Bertozzi C, Zimmermann I, Engeler S, Hilf RJC, Dutzler R. Signal Transduction at the Domain Interface of Prokaryotic Pentameric Ligand-Gated Ion Channels. PLoS Biol 2016; 14:e1002393. [PMID: 26943937 PMCID: PMC4778918 DOI: 10.1371/journal.pbio.1002393] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2015] [Accepted: 01/27/2016] [Indexed: 11/18/2022] Open
Abstract
Pentameric ligand-gated ion channels are activated by the binding of agonists to a site distant from the ion conduction path. These membrane proteins consist of distinct ligand-binding and pore domains that interact via an extended interface. Here, we have investigated the role of residues at this interface for channel activation to define critical interactions that couple conformational changes between the two structural units. By characterizing point mutants of the prokaryotic channels ELIC and GLIC by electrophysiology, X-ray crystallography and isothermal titration calorimetry, we have identified conserved residues that, upon mutation, apparently prevent activation but not ligand binding. The positions of nonactivating mutants cluster at a loop within the extracellular domain connecting β-strands 6 and 7 and at a loop joining the pore-forming helix M2 with M3 where they contribute to a densely packed core of the protein. An ionic interaction in the extracellular domain between the turn connecting β-strands 1 and 2 and a residue at the end of β-strand 10 stabilizes a state of the receptor with high affinity for agonists, whereas contacts of this turn to a conserved proline residue in the M2-M3 loop appear to be less important than previously anticipated. When mapping residues with strong functional phenotype on different channel structures, mutual distances are closer in conducting than in nonconducting conformations, consistent with a potential role of contacts in the stabilization of the open state. Our study has revealed a pattern of interactions that are crucial for the relay of conformational changes from the extracellular domain to the pore region of prokaryotic pentameric ligand-gated ion channels. Due to the strong conservation of the interface, these results are relevant for the entire family.
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Affiliation(s)
- Carlo Bertozzi
- Department of Biochemistry, University of Zürich, Zürich, Switzerland
| | - Iwan Zimmermann
- Department of Biochemistry, University of Zürich, Zürich, Switzerland
| | - Sibylle Engeler
- Department of Biochemistry, University of Zürich, Zürich, Switzerland
| | | | - Raimund Dutzler
- Department of Biochemistry, University of Zürich, Zürich, Switzerland
- * E-mail:
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31
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Subunit stoichiometry and arrangement in a heteromeric glutamate-gated chloride channel. Proc Natl Acad Sci U S A 2016; 113:E644-53. [PMID: 26792524 DOI: 10.1073/pnas.1423753113] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The invertebrate glutamate-gated chloride-selective receptors (GluClRs) are ion channels serving as targets for ivermectin (IVM), a broad-spectrum anthelmintic drug used to treat human parasitic diseases like river blindness and lymphatic filariasis. The native GluClR is a heteropentamer consisting of α and β subunit types, with yet unknown subunit stoichiometry and arrangement. Based on the recent crystal structure of a homomeric GluClαR, we introduced mutations at the intersubunit interfaces where Glu (the neurotransmitter) binds. By electrophysiological characterization of these mutants, we found heteromeric assemblies with two equivalent Glu-binding sites at β/α intersubunit interfaces, where the GluClβ and GluClα subunits, respectively, contribute the "principal" and "complementary" components of the putative Glu-binding pockets. We identified a mutation in the IVM-binding site (far away from the Glu-binding sites), which significantly increased the sensitivity of the heteromeric mutant receptor to both Glu and IVM, and improved the receptor subunits' cooperativity. We further characterized this heteromeric GluClR mutant as a receptor having a third Glu-binding site at an α/α intersubunit interface. Altogether, our data unveil heteromeric GluClR assemblies having three α and two β subunits arranged in a counterclockwise β-α-β-α-α fashion, as viewed from the extracellular side, with either two or three Glu-binding site interfaces.
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32
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Auerbach A. Dose-Response Analysis When There Is a Correlation between Affinity and Efficacy. Mol Pharmacol 2015; 89:297-302. [PMID: 26655305 DOI: 10.1124/mol.115.102509] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2015] [Accepted: 12/10/2015] [Indexed: 11/22/2022] Open
Abstract
The shape of a concentration-response curve (CRC) is determined by underlying equilibrium constants for agonist binding and receptor conformational change. Typically, agonists are characterized by the empirical CRC parameters efficacy (the maximum response), EC(50) (the concentration that produces a half-maximum response), and the Hill coefficient (the maximum slope of the response). Ligands activate receptors because they bind with higher affinity to the active versus resting conformation, and in skeletal muscle nicotinic acetylcholine receptors there is an exponential relationship between these two equilibrium dissociation constants. Consequently, knowledge of two receptor-specific, agonist-independent constants--the activation equilibrium constant without agonists (E(0)) and the affinity-correlation exponent (M)--allows an entire CRC to be calculated from a measurement of either efficacy or affinity. I describe methods for estimating the CRCs of partial agonists in receptors that have a correlation between affinity and efficacy.
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Affiliation(s)
- Anthony Auerbach
- Department of Physiology and Biophysics, State University of New York at Buffalo, Buffalo, New York
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33
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Wei S, Roessler BC, Icyuz M, Chauvet S, Tao B, Hartman JL, Kirk KL. Long-range coupling between the extracellular gates and the intracellular ATP binding domains of multidrug resistance protein pumps and cystic fibrosis transmembrane conductance regulator channels. FASEB J 2015; 30:1247-62. [PMID: 26606940 DOI: 10.1096/fj.15-278382] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2015] [Accepted: 11/16/2015] [Indexed: 12/22/2022]
Abstract
The ABCC transporter subfamily includes pumps, the long and short multidrug resistance proteins (MRPs), and an ATP-gated anion channel, the cystic fibrosis transmembrane conductance regulator (CFTR). We show that despite their thermodynamic differences, these ABCC transporter subtypes use broadly similar mechanisms to couple their extracellular gates to the ATP occupancies of their cytosolic nucleotide binding domains. A conserved extracellular phenylalanine at this gate was a prime location for producing gain of function (GOF) mutants of a long MRP in yeast (Ycf1p cadmium transporter), a short yeast MRP (Yor1p oligomycin exporter), and human CFTR channels. Extracellular gate mutations rescued ATP binding mutants of the yeast MRPs and CFTR by increasing ATP sensitivity. Control ATPase-defective MRP mutants could not be rescued by this mechanism. A CFTR double mutant with an extracellular gate mutation plus a cytosolic GOF mutation was highly active (single-channel open probability >0.3) in the absence of ATP and protein kinase A, each normally required for CFTR activity. We conclude that all 3 ABCC transporter subtypes use similar mechanisms to couple their extracellular gates to ATP occupancy, and highly active CFTR channels that bypass defects in ATP binding or phosphorylation can be produced.
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Affiliation(s)
- Shipeng Wei
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - Bryan C Roessler
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - Mert Icyuz
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - Sylvain Chauvet
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - Binli Tao
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - John L Hartman
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
| | - Kevin L Kirk
- *Department of Cell, Developmental, and Integrative Biology, Department of Genetics, and Department of Neurobiology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama, USA
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34
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Marabelli A, Lape R, Sivilotti L. Mechanism of activation of the prokaryotic channel ELIC by propylamine: a single-channel study. ACTA ACUST UNITED AC 2015; 145:23-45. [PMID: 25548135 PMCID: PMC4278187 DOI: 10.1085/jgp.201411234] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Prokaryotic channels, such as Erwinia chrysanthemi ligand-gated ion channel (ELIC) and Gloeobacter violaceus ligand-gated ion channel, give key structural information for the pentameric ligand-gated ion channel family, which includes nicotinic acetylcholine receptors. ELIC, a cationic channel from E. chrysanthemi, is particularly suitable for single-channel recording because of its high conductance. Here, we report on the kinetic properties of ELIC channels expressed in human embryonic kidney 293 cells. Single-channel currents elicited by the full agonist propylamine (0.5-50 mM) in outside-out patches at -60 mV were analyzed by direct maximum likelihood fitting of kinetic schemes to the idealized data. Several mechanisms were tested, and their adequacy was judged by comparing the predictions of the best fit obtained with the observable features of the experimental data. These included open-/shut-time distributions and the time course of macroscopic propylamine-activated currents elicited by fast theta-tube applications (50-600 ms, 1-50 mM, -100 mV). Related eukaryotic channels, such as glycine and nicotinic receptors, when fully liganded open with high efficacy to a single open state, reached via a preopening intermediate. The simplest adequate description of their activation, the "Flip" model, assumes a concerted transition to a single intermediate state at high agonist concentration. In contrast, ELIC open-time distributions at saturating propylamine showed multiple components. Thus, more than one open state must be accessible to the fully liganded channel. The "Primed" model allows opening from multiple fully liganded intermediates. The best fits of this type of model showed that ELIC maximum open probability (99%) is reached when at least two and probably three molecules of agonist have bound to the channel. The overall efficacy with which the fully liganded channel opens was ∼ 102 (∼ 20 for α1β glycine channels). The microscopic affinity for the agonist increased as the channel activated, from 7 mM for the resting state to 0.15 mM for the partially activated intermediate state.
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Affiliation(s)
- Alessandro Marabelli
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, England, UK
| | - Remigijus Lape
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, England, UK
| | - Lucia Sivilotti
- Department of Neuroscience, Physiology and Pharmacology, University College London, London WC1E 6BT, England, UK
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35
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Changeux JP. Protein dynamics and the allosteric transitions of pentameric receptor channels. Biophys Rev 2014; 6:311-321. [PMID: 25505495 PMCID: PMC4256460 DOI: 10.1007/s12551-014-0149-z] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2014] [Accepted: 11/13/2014] [Indexed: 01/30/2023] Open
Abstract
The recent application of molecular dynamics (MD) methodology to investigate the allosteric transitions of the acetylcholine receptor and its prokaryotic and eukaryotic pentameric homologs has yielded new insights into the mechanisms of signal transduction by these receptors. Combined with available data on X-ray structures, MD techniques enable description of the dynamics of the conformational change at the atomic level, intra-molecular propagation of this signal transduction mechanism as a concerted stepwise process at physiological timescales and the control of this process by allosteric modulators, thereby offering new perspectives for drug design.
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Affiliation(s)
- Jean-Pierre Changeux
- UMR 3571 CNRS, Institut Pasteur, 25 rue du Docteur Roux, 75015 Paris, France
- Collège de France, 11 Place Marcelin Berthelot, 75005 Paris, France
- Kavli Brain-Mind Institute University of California, San Diego, CA USA
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36
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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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37
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What ligand-gated ion channels can tell us about the allosteric regulation of G protein-coupled receptors. PROGRESS IN MOLECULAR BIOLOGY AND TRANSLATIONAL SCIENCE 2014; 115:291-347. [PMID: 23415097 DOI: 10.1016/b978-0-12-394587-7.00007-5] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/26/2023]
Abstract
The GABA(A) receptor is the target for a number of important allosteric drugs used in medicine, including benzodiazepines and anesthetics. These modulators have variable effects on the potency and maximal response of macroscopic currents elicited by different GABA(A) receptor agonists, yet this modulation is consistent with a two-state model in which the allosteric ligand has invariant affinity constants for the active and inactive states. Analysis of the effects of an allosteric agonist, like etomidate, on the population current provides a means of estimating the gating constant of the unliganded GABA(A) receptor (∼10(-4)). In contrast, allosteric interactions at the M(2) muscarinic receptor are often inconsistent with a two-state model. Analyzing allosterism within the constraints of a two-state model, nonetheless, provides an unbiased measure of probe dependence as well as clues to the mechanism of allosteric modulation. The rather simple allosteric effect of affinity-only modulation is difficult to explain and suggests modulation of a peripheral orthosteric ligand-docking site on the M(2) muscarinic receptor.
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38
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Wei S, Roessler BC, Chauvet S, Guo J, Hartman JL, Kirk KL. Conserved allosteric hot spots in the transmembrane domains of cystic fibrosis transmembrane conductance regulator (CFTR) channels and multidrug resistance protein (MRP) pumps. J Biol Chem 2014; 289:19942-57. [PMID: 24876383 DOI: 10.1074/jbc.m114.562116] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
ATP-binding cassette (ABC) transporters are an ancient family of transmembrane proteins that utilize ATPase activity to move substrates across cell membranes. The ABCC subfamily of the ABC transporters includes active drug exporters (the multidrug resistance proteins (MRPs)) and a unique ATP-gated ion channel (cystic fibrosis transmembrane conductance regulator (CFTR)). The CFTR channel shares gating principles with conventional ligand-gated ion channels, but the allosteric network that couples ATP binding at its nucleotide binding domains (NBDs) with conformational changes in its transmembrane helices (TMs) is poorly defined. It is also unclear whether the mechanisms that govern CFTR gating are conserved with the thermodynamically distinct MRPs. Here we report a new class of gain of function (GOF) mutation of a conserved proline at the base of the pore-lining TM6. Multiple substitutions of this proline promoted ATP-free CFTR activity and activation by the weak agonist, 5'-adenylyl-β,γ-imidodiphosphate (AMP-PNP). TM6 proline mutations exhibited additive GOF effects when combined with a previously reported GOF mutation located in an outer collar of TMs that surrounds the pore-lining TMs. Each TM substitution allosterically rescued the ATP sensitivity of CFTR gating when introduced into an NBD mutant with defective ATP binding. Both classes of GOF mutations also rescued defective drug export by a yeast MRP (Yor1p) with ATP binding defects in its NBDs. We conclude that the conserved TM6 proline helps set the energy barrier to both CFTR channel opening and MRP-mediated drug efflux and that CFTR channels and MRP pumps utilize similar allosteric mechanisms for coupling conformational changes in their translocation pathways to ATP binding at their NBDs.
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Affiliation(s)
- Shipeng Wei
- From the Departments of Cell, Developmental, and Integrative Biology
| | - Bryan C Roessler
- From the Departments of Cell, Developmental, and Integrative Biology
| | - Sylvain Chauvet
- From the Departments of Cell, Developmental, and Integrative Biology
| | | | | | - Kevin L Kirk
- From the Departments of Cell, Developmental, and Integrative Biology, Neurobiology and the Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham Alabama 35294-0005
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39
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Taly A, Hénin J, Changeux JP, Cecchini M. Allosteric regulation of pentameric ligand-gated ion channels: an emerging mechanistic perspective. Channels (Austin) 2014; 8:350-60. [PMID: 25478624 PMCID: PMC4203737 DOI: 10.4161/chan.29444] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/08/2014] [Revised: 06/03/2014] [Accepted: 06/03/2014] [Indexed: 12/22/2022] Open
Abstract
Pentameric ligand-gated ion channels (pLGICs) play a central role in intercellular communications in the nervous system by converting the binding of a chemical messenger—a neurotransmitter—into an ion flux through the postsynaptic membrane. They are oligomeric assemblies that provide prototypical examples of allosterically regulated integral membrane proteins. Here, we present an overview of the most recent advances on the signal transduction mechanism based on the X-ray structures of both prokaryotic and invertebrate eukaryotic pLGICs and atomistic Molecular Dynamics simulations. The present results suggest that ion gating involves a large structural reorganization of the molecule mediated by two distinct quaternary transitions, a global twisting and the blooming of the extracellular domain, which can be modulated by ligand binding at the topographically distinct orthosteric and allosteric sites. The emerging model of gating is consistent with a wealth of functional studies and will boost the development of novel pharmacological strategies.
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Affiliation(s)
- Antoine Taly
- Laboratoire de Biochimie Théorique; IBPC; CNRS and Université Paris Diderot; Paris, France
| | - Jérôme Hénin
- Laboratoire de Biochimie Théorique; IBPC; CNRS and Université Paris Diderot; Paris, France
| | - Jean-Pierre Changeux
- CNRS; URA 2182; F-75015 & Collège de France; Paris, France
- Kavli Institute for Brain & Mind University of California; San Diego La Jolla, CA USA
| | - Marco Cecchini
- ISIS; UMR 7006 CNRS; Université de Strasbourg; F-67083 Strasbourg Cedex, France
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40
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Crystal structures of a pentameric ligand-gated ion channel provide a mechanism for activation. Proc Natl Acad Sci U S A 2013; 111:966-71. [PMID: 24367074 DOI: 10.1073/pnas.1314997111] [Citation(s) in RCA: 149] [Impact Index Per Article: 13.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023] Open
Abstract
Pentameric ligand-gated ion channels mediate fast chemical transmission of nerve signals. The structure of a bacterial proton-gated homolog has been established in its open and locally closed conformations at acidic pH. Here we report its crystal structure at neutral pH, thereby providing the X-ray structures of the two end-points of the gating mechanism in the same pentameric ligand-gated ion channel. The large structural variability in the neutral pH structure observed in the four copies of the pentamer present in the asymmetric unit has been used to analyze the intrinsic fluctuations in this state, which are found to prefigure the transition to the open state. In the extracellular domain (ECD), a marked quaternary change is observed, involving both a twist and a blooming motion, and the pore in the transmembrane domain (TMD) is closed by an upper bend of helix M2 (as in locally closed form) and a kink of helix M1, both helices no longer interacting across adjacent subunits. On the tertiary level, detachment of inner and outer β sheets in the ECD reshapes two essential cavities at the ECD-ECD and ECD-TMD interfaces. The first one is the ligand-binding cavity; the other is close to a known divalent cation binding site in other pentameric ligand-gated ion channels. In addition, a different crystal form reveals that the locally closed and open conformations coexist as discrete ones at acidic pH. These structural results, together with site-directed mutagenesis, physiological recordings, and coarse-grained modeling, have been integrated to propose a model of the gating transition pathway.
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Colquhoun D, Lape R. Perspectives on: conformational coupling in ion channels: allosteric coupling in ligand-gated ion channels. ACTA ACUST UNITED AC 2013. [PMID: 23183696 PMCID: PMC3514732 DOI: 10.1085/jgp.201210844] [Citation(s) in RCA: 38] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Affiliation(s)
- David Colquhoun
- Department of Neuroscience, Physiology, and Pharmacology, University College London, London WC1E 6BT, England, UK.
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Nicotinic acetylcholine receptor and the structural basis of neuromuscular transmission: insights from Torpedo postsynaptic membranes. Q Rev Biophys 2013; 46:283-322. [PMID: 24050525 PMCID: PMC3820380 DOI: 10.1017/s0033583513000061] [Citation(s) in RCA: 97] [Impact Index Per Article: 8.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
The nicotinic acetylcholine (ACh) receptor, at the neuromuscular junction, is a neurotransmitter-gated ion channel that has been fine-tuned through evolution to transduce a chemical signal into an electrical signal with maximum efficiency and speed. It is composed from three similar and two identical polypeptide chains, arranged in a ring around a narrow membrane pore. Central to the design of this assembly is a hydrophobic gate in the pore, more than 50 Å away from sites in the extracellular domain where ACh binds. Although the molecular properties of the receptor have been explored intensively over the last few decades, only recently have structures emerged revealing its complex architecture and illuminating how ACh entering the binding sites opens the distant gate. Postsynaptic membranes isolated from the (muscle-derived) electric organ of the Torpedo ray have underpinned most of the structural studies: the membranes form tubular vesicles having receptors arranged on a regular surface lattice, which can be imaged directly in frozen physiological solutions. Advances in electron crystallographic techniques have also been important, enabling analysis of the closed- and open-channel forms of the receptor in unreacted tubes or tubes reacted briefly with ACh. The structural differences between these two forms show that all five subunits participate in a concerted conformational change communicating the effect of ACh binding to the gate, but that three of them (αγ, β and δ) play a dominant role. Flexing of oppositely facing pore-lining α-helices is the principal motion determining the closed/open state of the gate. These results together with the findings of biochemical, biophysical and other structural studies allow an integrated description of the receptor and of its mode of action at the synapse.
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Cornish-Bowden A. Understanding allosteric and cooperative interactions in enzymes. FEBS J 2013; 281:621-32. [DOI: 10.1111/febs.12469] [Citation(s) in RCA: 47] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/31/2013] [Revised: 07/24/2013] [Accepted: 07/30/2013] [Indexed: 01/25/2023]
Affiliation(s)
- Athel Cornish-Bowden
- Unité de Bioénergétique et Ingénierie des Protéines; Institut de Microbiologie de la Méditerranée; Centre National de la Recherche Scientifique and Aix-Marseille Université; France
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Bruhova I, Gregg T, Auerbach A. Energy for wild-type acetylcholine receptor channel gating from different choline derivatives. Biophys J 2013; 104:565-74. [PMID: 23442907 DOI: 10.1016/j.bpj.2012.11.3833] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/20/2012] [Revised: 11/25/2012] [Accepted: 11/27/2012] [Indexed: 02/01/2023] Open
Abstract
Agonists, including the neurotransmitter acetylcholine (ACh), bind at two sites in the neuromuscular ACh receptor channel (AChR) to promote a reversible, global change in protein conformation that regulates the flow of ions across the muscle cell membrane. In the synaptic cleft, ACh is hydrolyzed to acetate and choline. Replacement of the transmitter's ester acetyl group with a hydroxyl (ACh→choline) results in a + 1.8 kcal/mol reduction in the energy for gating generated by each agonist molecule from a low- to high-affinity change of the transmitter binding site (ΔG(B)). To understand the distinct actions of structurally related agonist molecules, we measured ΔG(B) for 10 related choline derivatives. Replacing the hydroxyl group of choline with different substituents, such as hydrogen, chloride, methyl, or amine, increased the energy for gating (i.e., it made ΔG(B) more negative relative to choline). Extending the ethyl hydroxide tail of choline to propyl and butyl hydroxide also increased this energy. Our findings reveal the amount of energy that is available for the AChR conformational change provided by different, structurally related agonists. We speculate that a hydrogen bond between the choline hydroxyl and the backbone carbonyl of αW149 positions this agonist's quaternary ammonium group so as to reduce the cation-π interaction between this moiety and the aromatic groups at the binding site.
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Affiliation(s)
- Iva Bruhova
- Department of Physiology and Biophysics, SUNY at Buffalo, Buffalo, New York, USA
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Changeux JP. The concept of allosteric interaction and its consequences for the chemistry of the brain. J Biol Chem 2013; 288:26969-26986. [PMID: 23878193 DOI: 10.1074/jbc.x113.503375] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Throughout this Reflections article, I have tried to follow up on the genesis in the 1960s and subsequent evolution of the concept of allosteric interaction and to examine its consequences within the past decades, essentially in the field of the neuroscience. The main conclusion is that allosteric mechanisms built on similar structural principles operate in bacterial regulatory enzymes, gene repressors (and the related nuclear receptors), rhodopsin, G-protein-coupled receptors, neurotransmitter receptors, ion channels, and so on from prokaryotes up to the human brain yet with important features of their own. Thus, future research on these basic cybernetic sensors is expected to develop in two major directions: at the elementary level, toward the atomic structure and molecular dynamics of the conformational changes involved in signal recognition and transduction, but also at a higher level of organization, the contribution of allosteric mechanisms to the modulation of brain functions.
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Affiliation(s)
- Jean-Pierre Changeux
- Collège de France, 75005 Paris and the Institut Pasteur, 75724 Paris Cedex 15, France.
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Jadey S, Purohit P, Auerbach A. Action of nicotine and analogs on acetylcholine receptors having mutations of transmitter-binding site residue αG153. ACTA ACUST UNITED AC 2013; 141:95-104. [PMID: 23277476 PMCID: PMC3536520 DOI: 10.1085/jgp.201210896] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
A primary target for nicotine is the acetylcholine receptor channel (AChR). Some of the ability of nicotine to activate differentially AChR subtypes has been traced to a transmitter-binding site amino acid that is glycine in lower affinity and lysine in higher affinity AChRs. We studied the effects of mutations of this residue (αG153) in neuromuscular AChRs activated by nicotine and eight other agonists including nornicotine and anabasine. All of the mutations increased the unliganded gating equilibrium constant. The affinity of the resting receptor (Kd) and the net binding energy from the agonist for gating (ΔGB) were estimated by cross-concentration fitting of single-channel currents. In all but one of the agonist/mutant combinations there was a moderate decrease in Kd and essentially no change in ΔGB. The exceptional case was nicotine plus lysine, which showed a large, >8,000-fold decrease in Kd but no change in ΔGB. The extraordinary specificity of this combination leads us to speculate that AChRs with a lysine at position αG153 may be exposed to a nicotine-like compound in vivo.
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Affiliation(s)
- Snehal Jadey
- Department of Physiology and Biophysics, State University of New York, Buffalo, NY 14214, USA
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Okeyo G, Wang W, Wei S, Kirk KL. Converting nonhydrolyzable nucleotides to strong cystic fibrosis transmembrane conductance regulator (CFTR) agonists by gain of function (GOF) mutations. J Biol Chem 2013; 288:17122-33. [PMID: 23620589 DOI: 10.1074/jbc.m112.442582] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Cystic fibrosis transmembrane conductance regulator (CFTR) is the only ligand-gated ion channel that hydrolyzes its agonist, ATP. CFTR gating has been argued to be tightly coupled to its enzymatic activity, but channels do open occasionally in the absence of ATP and are reversibly activated (albeit weakly) by nonhydrolyzable nucleotides. Why the latter only weakly activates CFTR is not understood. Here we show that CFTR activation by adenosine 5'-O-(thiotriphosphate) (ATPγS), adenosine 5'-(β,γ-imino)triphosphate (AMP-PNP), and guanosine 5'-3-O-(thio)triphosphate (GTPγS) is enhanced substantially by gain of function (GOF) mutations in the cytosolic loops that increase unliganded activity. This enhancement correlated with the base-line nucleotide-independent activity for several GOF mutations. AMP-PNP or ATPγS activation required both nucleotide binding domains (NBDs) and was disrupted by a cystic fibrosis mutation in NBD1 (G551D). GOF mutant channels deactivated very slowly upon AMP-PNP or ATPγS removal (τdeac ∼ 100 s) implying tight binding between the two NBDs. Despite this apparently tight binding, neither AMP-PNP nor ATPγS activated even the strongest GOF mutant as strongly as ATP. ATPγS-activated wild type channels deactivated more rapidly, indicating that GOF mutations in the cytosolic loops reciprocally/allosterically affect nucleotide occupancy of the NBDs. A GOF mutation substantially rescued defective ATP-dependent gating of G1349D-CFTR, a cystic fibrosis NBD2 signature sequence mutant. Interestingly, the G1349D mutation strongly disrupted activation by AMP-PNP but not by ATPγS, indicating that these analogs interact differently with the NBDs. We conclude that poorly hydrolyzable nucleotides are less effective than ATP at opening CFTR channels even when they bind tightly to the NBDs but are converted to stronger agonists by GOF mutations.
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Affiliation(s)
- George Okeyo
- Department of Cell, Developmental, and Integrative Biology, Gregory Fleming James Cystic Fibrosis Research Center, University of Alabama at Birmingham, Birmingham, Alabama 35294-0005, USA
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Stewart DS, Hotta M, Desai R, Forman SA. State-dependent etomidate occupancy of its allosteric agonist sites measured in a cysteine-substituted GABAA receptor. Mol Pharmacol 2013; 83:1200-8. [PMID: 23525330 DOI: 10.1124/mol.112.084558] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
A central axiom of ligand-receptor theory is that agonists bind more tightly to active than to inactive receptors. However, measuring agonist affinity in inactive receptors is confounded by concomitant activation. We identified a cysteine substituted mutant γ-aminobutyric acid type A (GABAA) receptor with unique characteristics allowing the determination of allosteric agonist site occupancy in both inactive and active receptors. Etomidate, the allosteric agonist, is an anesthetic that activates or modulates α1β2γ2L GABAA receptors via transmembrane sites near β2M286 residues in M3 domains. Voltage-clamp electrophysiology studies of α1β2M286Cγ2L receptors show that GABA is an efficacious agonist and that etomidate modulates GABA-activated activity, but direct etomidate agonism is absent. Quantitative analysis of mutant activity using an established Monod-Wyman-Changeux (MWC) allosteric model indicates that the intrinsic efficacy of etomidate, defined as its relative affinity for active versus inactive receptors, is lower than in wild-type receptors. Para-chloromercuribenzene sulfonate covalently modifies β2M286C side-chain sulfhydryls, irreversibly altering GABA-induced currents. Etomidate concentration dependently reduces the apparent rate of β2M286C-pCMBS bond formation, tracked electrophysiologically. High etomidate concentrations completely protect the β2M286C suflhydryl from covalent modification, suggesting close steric interactions. The 50% protective etomidate concentration (PC50) is 14 μM in inactive receptors and 1.1 to 2.2 μM during GABA-activation, experimentally demonstrating that activated receptors bind etomidate more avidly than do inactive receptors. The experimental PC50 values are remarkably close to, and therefore validate, MWC model predictions for etomidate dissociation constants in both inactive and active receptors. Our results support MWC models as valid frameworks for understanding the agonism, coagonism, and modulation of ligand-gated ion channels.
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Affiliation(s)
- Deirdre S Stewart
- Department of Anesthesia Critical Care & Pain Medicine, Beecher-Mallinckrodt Research Laboratories, and Department of Anesthesia Critical Care & Pain Medicine, Massachusetts General Hospital, Boston, MA, USA
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Corringer PJ, Poitevin F, Prevost MS, Sauguet L, Delarue M, Changeux JP. Structure and pharmacology of pentameric receptor channels: from bacteria to brain. Structure 2012; 20:941-56. [PMID: 22681900 DOI: 10.1016/j.str.2012.05.003] [Citation(s) in RCA: 173] [Impact Index Per Article: 14.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2012] [Revised: 05/16/2012] [Accepted: 05/17/2012] [Indexed: 01/21/2023]
Abstract
Orthologs of the pentameric receptor channels that mediate fast synaptic transmission in the central and peripheral nervous systems have been found in several bacterial species and in a single archaea genus. Recent X-ray structures of bacterial and invertebrate pentameric receptors point to a striking conservation of the structural features within the whole family, even between distant prokaryotic and eukaryotic members. These structural data reveal general principles of molecular organization that allow allosteric membrane proteins to mediate chemoelectric transduction. Notably, several conformations have been solved, including open and closed channels with distinct global tertiary and quaternary structure. The data reveal features of the ion channel architecture and of diverse categories of binding sites, such as those that bind orthosteric ligands, including neurotransmitters, and those that bind allosteric modulators, such as general anesthetics, ivermectin, or lipids. In this review, we summarize the most recent data, discuss insights into the mechanism of action in these systems, and elaborate on newly opened avenues for drug design.
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Jadey S, Auerbach A. An integrated catch-and-hold mechanism activates nicotinic acetylcholine receptors. ACTA ACUST UNITED AC 2012; 140:17-28. [PMID: 22732309 PMCID: PMC3382718 DOI: 10.1085/jgp.201210801] [Citation(s) in RCA: 50] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In neuromuscular acetylcholine (ACh) receptor channels (AChRs), agonist molecules bind with a low affinity (LA) to two sites that can switch to high affinity (HA) and increase the probability of channel opening. We measured (by using single-channel kinetic analysis) the rate and equilibrium constants for LA binding and channel gating for several different agonists of adult-type mouse AChRs. Almost all of the variation in the equilibrium constants for LA binding was from differences in the association rate constants. These were consistently below the limit set by diffusion and were substantially different even though the agonists had similar sizes and the same charge. This suggests that binding to resting receptors is not by diffusion alone and, hence, that each binding site can undergo two conformational changes ("catch" and "hold") that connect three different structures (apo-, LA-bound, and HA-bound). Analyses of ACh-binding protein structures suggest that this binding site, too, may adopt three discrete structures having different degrees of loop C displacement ("capping"). For the agonists we tested, the logarithms of the equilibrium constants for LA binding and LA↔HA gating were correlated. Although agonist binding and channel gating have long been considered to be separate processes in the activation of ligand-gated ion channels, this correlation implies that the catch-and-hold conformational changes are energetically linked and together comprise an integrated process having a common structural basis. We propose that loop C capping mainly reflects agonist binding, with its two stages corresponding to the formation of the LA and HA complexes. The catch-and-hold reaction coordinate is discussed in terms of preopening states and thermodynamic cycles of activation.
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Affiliation(s)
- Snehal Jadey
- Department of Physiology and Biophysics, State University of New York, Buffalo, NY 14214, USA
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