1
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Hu Z, Yang J. Structural basis of properties, mechanisms, and channelopathy of cyclic nucleotide-gated channels. Channels (Austin) 2023; 17:2273165. [PMID: 37905307 PMCID: PMC10761061 DOI: 10.1080/19336950.2023.2273165] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2023] [Accepted: 10/07/2023] [Indexed: 11/02/2023] Open
Abstract
Recent years have seen an outpouring of atomic or near atomic resolution structures of cyclic nucleotide-gated (CNG) channels, captured in closed, transition, pre-open, partially open, and fully open states. These structures provide unprecedented molecular insights into the activation, assembly, architecture, regulation, and channelopathy of CNG channels, as well as mechanistic explanations for CNG channel biophysical and pharmacological properties. This article summarizes recent advances in CNG channel structural biology, describes key structural features and elements, and illuminates a detailed conformational landscape of activation by cyclic nucleotides. The review also correlates structures with findings and properties delineated in functional studies, including nonselective monovalent cation selectivity, Ca2+ permeation and block, block by L-cis-diltiazem, location of the activation gate, lack of voltage-dependent gating, and modulation by lipids and calmodulin. A perspective on future research is also offered.
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Affiliation(s)
- Zhengshan Hu
- Department of Biological Sciences, Columbia University, New York, NY, USA
| | - Jian Yang
- Department of Biological Sciences, Columbia University, New York, NY, USA
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2
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Gibb AJ, Ogden KK, McDaniel MJ, Vance KM, Kell SA, Butch C, Burger P, Liotta DC, Traynelis SF. A structurally derived model of subunit-dependent NMDA receptor function. J Physiol 2018; 596:4057-4089. [PMID: 29917241 PMCID: PMC6117563 DOI: 10.1113/jp276093] [Citation(s) in RCA: 30] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/04/2018] [Accepted: 06/11/2018] [Indexed: 12/14/2022] Open
Abstract
Key points The kinetics of NMDA receptor (NMDAR) signalling are a critical aspect of the physiology of excitatory synaptic transmission in the brain. Here we develop a mechanistic description of NMDAR function based on the receptor tetrameric structure and the principle that each agonist‐bound subunit must undergo some rate‐limiting conformational change after agonist binding, prior to channel opening. By fitting this mechanism to single channel data using a new MATLAB‐based software implementation of maximum likelihood fitting with correction for limited time resolution, rate constants were derived for this mechanism that reflect distinct structural changes and predict the properties of macroscopic and synaptic NMDAR currents. The principles applied here to develop a mechanistic description of the heterotetrameric NMDAR, and the software used in this analysis, can be equally applied to other heterotetrameric glutamate receptors, providing a unifying mechanistic framework to understanding the physiology of glutamate receptor signalling in the brain.
Abstract NMDA receptors (NMDARs) are tetrameric complexes comprising two glycine‐binding GluN1 and two glutamate‐binding GluN2 subunits. Four GluN2 subunits encoded by different genes can produce up to 10 different di‐ and triheteromeric receptors. In addition, some neurological patients contain a de novo mutation or inherited rare variant in only one subunit. There is currently no mechanistic framework to describe tetrameric receptor function that can be extended to receptors with two different GluN1 or GluN2 subunits. Here we use the structural features of glutamate receptors to develop a mechanism describing both single channel and macroscopic NMDAR currents. We propose that each agonist‐bound subunit undergoes some rate‐limiting conformational change after agonist binding, prior to channel opening. We hypothesize that this conformational change occurs within a triad of interactions between a short helix preceding the M1 transmembrane helix, the highly conserved M3 motif encoded by the residues SYTANLAAF, and the linker preceding the M4 transmembrane helix of the adjacent subunit. Molecular dynamics simulations suggest that pre‐M1 helix motion is uncorrelated between subunits, which we interpret to suggest independent subunit‐specific conformational changes may influence these pre‐gating steps. According to this interpretation, these conformational changes are the main determinants of the key kinetic properties of NMDA receptor activation following agonist binding, and so these steps sculpt their physiological role. We show that this structurally derived tetrameric model describes both single channel and macroscopic data, giving a new approach to interpreting functional properties of synaptic NMDARs that provides a logical framework to understanding receptors with non‐identical subunits. The kinetics of NMDA receptor (NMDAR) signalling are a critical aspect of the physiology of excitatory synaptic transmission in the brain. Here we develop a mechanistic description of NMDAR function based on the receptor tetrameric structure and the principle that each agonist‐bound subunit must undergo some rate‐limiting conformational change after agonist binding, prior to channel opening. By fitting this mechanism to single channel data using a new MATLAB‐based software implementation of maximum likelihood fitting with correction for limited time resolution, rate constants were derived for this mechanism that reflect distinct structural changes and predict the properties of macroscopic and synaptic NMDAR currents. The principles applied here to develop a mechanistic description of the heterotetrameric NMDAR, and the software used in this analysis, can be equally applied to other heterotetrameric glutamate receptors, providing a unifying mechanistic framework to understanding the physiology of glutamate receptor signalling in the brain.
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Affiliation(s)
- Alasdair J Gibb
- Department of Neuroscience, Physiology and Pharmacology, University College London, Gower Street, London, WC1E 6BT, UK
| | - Kevin K Ogden
- Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road, Atlanta, GA, 30322, USA
| | - Miranda J McDaniel
- Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road, Atlanta, GA, 30322, USA
| | - Katie M Vance
- Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road, Atlanta, GA, 30322, USA
| | - Steven A Kell
- Department of Chemistry, Emory University School, 1515 Dickey Drive, Atlanta, GA, 30322, USA
| | - Chris Butch
- Department of Chemistry, Emory University School, 1515 Dickey Drive, Atlanta, GA, 30322, USA
| | - Pieter Burger
- Department of Chemistry, Emory University School, 1515 Dickey Drive, Atlanta, GA, 30322, USA
| | - Dennis C Liotta
- Department of Chemistry, Emory University School, 1515 Dickey Drive, Atlanta, GA, 30322, USA
| | - Stephen F Traynelis
- Department of Pharmacology, Emory University School of Medicine, Rollins Research Center, 1510 Clifton Road, Atlanta, GA, 30322, USA
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3
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Kowal J, Chami M, Baumgartner P, Arheit M, Chiu PL, Rangl M, Scheuring S, Schröder GF, Nimigean CM, Stahlberg H. Ligand-induced structural changes in the cyclic nucleotide-modulated potassium channel MloK1. Nat Commun 2015; 5:3106. [PMID: 24469021 PMCID: PMC4086158 DOI: 10.1038/ncomms4106] [Citation(s) in RCA: 53] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/10/2013] [Accepted: 12/13/2013] [Indexed: 12/25/2022] Open
Abstract
Cyclic nucleotide-modulated ion channels are important for signal transduction and pacemaking in eukaryotes. The molecular determinants of ligand gating in these channels are still unknown, mainly because of a lack of direct structural information. Here we report ligand-induced conformational changes in full-length MloK1, a cyclic nucleotide-modulated potassium channel from the bacterium Mesorhizobium loti, analysed by electron crystallography and atomic force microscopy. Upon cAMP binding, the cyclic nucleotide-binding domains move vertically towards the membrane, and directly contact the S1–S4 voltage sensor domains. This is accompanied by a significant shift and tilt of the voltage sensor domain helices. In both states, the inner pore-lining helices are in an ‘open’ conformation. We propose a mechanism in which ligand binding can favour pore opening via a direct interaction between the cyclic nucleotide-binding domains and voltage sensors. This offers a simple mechanistic hypothesis for the coupling between ligand gating and voltage sensing in eukaryotic HCN channels. The molecular determinants underlying ligand gating of cyclic nucleotide-modulated ion channels remain unclear. Kowal et al. determine the conformational changes underlying cAMP binding to the bacterial channel MloK1, and propose a mechanism for coupling of ligand gating and voltage sensing in eukaryotic HCN channels.
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Affiliation(s)
- Julia Kowal
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Mohamed Chami
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Paul Baumgartner
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | - Marcel Arheit
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
| | | | - Martina Rangl
- U1006 INSERM, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, 13009 Marseille, France
| | - Simon Scheuring
- U1006 INSERM, Aix-Marseille Université, Parc Scientifique et Technologique de Luminy, 163 Avenue de Luminy, 13009 Marseille, France
| | - Gunnar F Schröder
- 1] Forschungszentrum Jülich, Institute of Complex Systems, ICS-6: Structural Biochemistry, 52425 Jülich, Germany [2] Department of Physics, Heinrich-Heine University Düsseldorf, 40225 Düsseldorf, Germany
| | - Crina M Nimigean
- Departments of Anesthesiology, Physiology and Biophysics, and Biochemistry, Weill Cornell Medical College, 1300 York Ave, New York, New York 10065, USA
| | - Henning Stahlberg
- Center for Cellular Imaging and NanoAnalytics, Biozentrum, University of Basel, Mattenstrasse 26, CH-4058 Basel, Switzerland
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4
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Dutta-Roy R, Rosenmund C, Edelstein SJ, Le Novère N. Ligand-dependent opening of the multiple AMPA receptor conductance states: a concerted model. PLoS One 2015; 10:e0116616. [PMID: 25629405 PMCID: PMC4309570 DOI: 10.1371/journal.pone.0116616] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2014] [Accepted: 11/21/2014] [Indexed: 01/27/2023] Open
Abstract
Modulation of the properties of AMPA receptors at the post-synaptic membrane is one of the main suggested mechanisms underlying fast synaptic transmission in the central nervous system of vertebrates. Electrophysiological recordings of single channels stimulated with agonists showed that both recombinant and native AMPA receptors visit multiple conductance states in an agonist concentration dependent manner. We propose an allosteric model of the multiple conductance states based on concerted conformational transitions of the four subunits, as an iris diaphragm. Our model predicts that the thermodynamic behaviour of the conductance states upon full and partial agonist stimulations can be described with increased affinity of receptors as they progress to higher conductance states. The model also predicts the existence of AMPA receptors in non-liganded conductive substates. However, the probability of spontaneous openings decreases with increasing conductances. Finally, we predict that the large conductance states are stabilized within the rise phase of a whole-cell EPSC in glutamatergic hippocampal neurons. Our model provides a mechanistic link between ligand concentration and conductance states that can explain thermodynamic and kinetic features of AMPA receptor gating.
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Affiliation(s)
- Ranjita Dutta-Roy
- Department of Medicine Solna, Karolinska Insitutet, 171 76 Stockholm, Sweden
- NWFZ, Charite Universitatsmedizin, 101 17 Berlin, Germany
- European Bioinformatics Institute (EMBL-EBI), Wellcome Trust Genome Campus, Hinxton, Cambridgeshire CB10 1SD, UK
| | | | | | - Nicolas Le Novère
- European Bioinformatics Institute (EMBL-EBI), Wellcome Trust Genome Campus, Hinxton, Cambridgeshire CB10 1SD, UK
- The Babraham Institute, Babraham, Cambridgeshire CB22 3AT, UK
- * E-mail:
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5
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Telezhkin V, Brown DA, Gibb AJ. Distinct subunit contributions to the activation of M-type potassium channels by PI(4,5)P2. ACTA ACUST UNITED AC 2012; 140:41-53. [PMID: 22689829 PMCID: PMC3382723 DOI: 10.1085/jgp.201210796] [Citation(s) in RCA: 29] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
Abstract
Low-threshold voltage-gated M-type potassium channels (M channels) are tetraheteromers, commonly of two Kv7.2 and two Kv7.3 subunits. Though gated by voltage, the channels have an absolute requirement for binding of the membrane phospholipid phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)) to open. We have investigated the quantitative relation between the concentration of a water-soluble PI(4,5)P(2) analog, dioctanoyl-PI(4,5)P(2) (DiC(8)-PI(4,5)P(2)), and channel open probability (P(open)) by fast application of increasing concentrations of DiC(8)-PI(4,5)P(2) to the inside face of membrane patches excised from Chinese hamster ovary cells expressing M channels as heteromeric Kv7.2/7.3 subunits. The rationale for the experiments is that this will mimic the effect of changes in membrane PI(4,5)P(2) concentration. Single-channel conductances from channel current-voltage relations in cell-attached mode were 9.2 ± 0.1 pS with a 2.5-mM pipette [K(+)]. Plots of P(open) against DiC(8)-PI(4,5)P(2) concentration were best fitted using a two-component concentration-P(open) relationship with high and low affinity, half-maximal effective concentration (EC(50)) values of 1.3 ± 0.14 and 75.5 ± 2.5 µM, respectively, and Hill slopes of 1.4 ± 0.06. In contrast, homomeric channels from cells expressing only Kv7.2 or Kv7.3 constructs yielded single-component curves with EC(50) values of 76.2 ± 19.9 or 3.6 ± 1.0 µM, respectively. When wild-type (WT) Kv7.2 was coexpressed with a mutated Kv7.3 subunit with >100-fold reduced sensitivity to PI(4,5)P(2), the high-affinity component of the activation curve was lost. Fitting the data for WT and mutant channels to an activation mechanism with independent PI(4,5)P(2) binding to two Kv7.2 and two Kv7.3 subunits suggests that the two components of the M-channel activation curve correspond to the interaction of PI(4,5)P(2) with the Kv7.3 and Kv7.2 subunits, respectively, that channels can open when only the two Kv7.3 subunits have bound DiC(8)-PI(4,5)P(2), and that maximum channel opening requires binding to all four subunits.
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Affiliation(s)
- Vsevolod Telezhkin
- Department of Neuroscience, Physiology, and Pharmacology, University College London, London, WC1E 6BT, England, UK
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6
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Cukkemane A, Seifert R, Kaupp UB. Cooperative and uncooperative cyclic-nucleotide-gated ion channels. Trends Biochem Sci 2010; 36:55-64. [PMID: 20729090 DOI: 10.1016/j.tibs.2010.07.004] [Citation(s) in RCA: 53] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/07/2010] [Revised: 07/06/2010] [Accepted: 07/14/2010] [Indexed: 12/31/2022]
Abstract
Ion channels gated by cyclic nucleotides serve multiple functions in sensory signaling in diverse cell types ranging from neurons to sperm. Newly discovered members from bacteria and marine invertebrates provide a wealth of structural and functional information on this channel family. A hallmark of classical tetrameric cyclic-nucleotide-gated channels is their cooperative activation by binding of several ligands. By contrast, the new members seem to be uncooperative, and binding of a single ligand molecule suffices to open these channels. These new findings provide a fresh look at the mechanism of allosteric activation of ion channels.
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Affiliation(s)
- Abhishek Cukkemane
- Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
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7
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Bönigk W, Loogen A, Seifert R, Kashikar N, Klemm C, Krause E, Hagen V, Kremmer E, Strünker T, Kaupp UB. An atypical CNG channel activated by a single cGMP molecule controls sperm chemotaxis. Sci Signal 2009; 2:ra68. [PMID: 19861689 DOI: 10.1126/scisignal.2000516] [Citation(s) in RCA: 64] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Sperm of the sea urchin Arbacia punctulata can respond to a single molecule of chemoattractant released by an egg. The mechanism underlying this extreme sensitivity is unknown. Crucial signaling events in the response of A. punctulata sperm to chemoattractant include the rapid synthesis of the intracellular messenger guanosine 3',5'-monophosphate (cGMP) and the ensuing membrane hyperpolarization that results from the opening of potassium-selective cyclic nucleotide-gated (CNGK) channels. Here, we use calibrated photolysis of caged cGMP to show that approximately 45 cGMP molecules are generated during the response to a single molecule of chemoattractant. The CNGK channel can respond to such small cGMP changes because it is exquisitely sensitive to cGMP and activated in a noncooperative fashion. Like voltage-activated Ca(v) and Na(v) channels, the CNGK polypeptide consists of four homologous repeat sequences. Disabling each of the four cyclic nucleotide-binding sites through mutagenesis revealed that binding of a single cGMP molecule to repeat 3 is necessary and sufficient to activate the CNGK channel. Thus, CNGK has developed a mechanism of activation that is different from the activation of other CNG channels, which requires the cooperative binding of several ligands and operates in the micromolar rather than the nanomolar range.
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Affiliation(s)
- Wolfgang Bönigk
- Center of Advanced European Studies and Research, Abteilung Molekulare Neurosensorik, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany
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8
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Wang Z, Jiang Y, Lu L, Huang R, Hou Q, Shi F. Molecular mechanisms of cyclic nucleotide-gated ion channel gating. J Genet Genomics 2009; 34:477-85. [PMID: 17601606 DOI: 10.1016/s1673-8527(07)60052-6] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2006] [Accepted: 01/08/2007] [Indexed: 01/09/2023]
Abstract
Cyclic nucleotide-gated ion channels (CNGs) are distributed most widely in the neuronal cell. Great progress has been made in molecular mechanisms of CNG channel gating in the recent years. Results of many experiments have indicated that the stoichiometry and assembly of CNG channels affect their property and gating. Experiments of CNG mutants and analyses of cysteine accessibilities show that cyclic nucleotide-binding domains (CNBD) bind cyclic nucleotides and subsequently conformational changes occurred followed by the concerted or cooperative conformational change of all four subunits during CNG gating. In order to provide theoretical assistances for further investigation on CNG channels, especially regarding the disease pathogenesis of ion channels, this paper reviews the latest progress on mechanisms of CNG channels, functions of subunits, processes of subunit assembly, and conformational changes of subunit regions during gating.
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Affiliation(s)
- Zhengchao Wang
- College of Animal Science and Technology, Nanjing Agricultural University, Nanjing 210095, China
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9
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Four-mode gating model of fast inactivation of sodium channel Nav1.2a. Pflugers Arch 2008; 457:103-19. [DOI: 10.1007/s00424-008-0500-y] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/10/2008] [Revised: 03/18/2008] [Accepted: 03/18/2008] [Indexed: 12/19/2022]
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10
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Chiu PL, Pagel MD, Evans J, Chou HT, Zeng X, Gipson B, Stahlberg H, Nimigean CM. The structure of the prokaryotic cyclic nucleotide-modulated potassium channel MloK1 at 16 A resolution. Structure 2007; 15:1053-64. [PMID: 17850745 PMCID: PMC2000844 DOI: 10.1016/j.str.2007.06.020] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2007] [Revised: 06/11/2007] [Accepted: 06/14/2007] [Indexed: 11/24/2022]
Abstract
The gating ring of cyclic nucleotide-modulated channels is proposed to be either a two-fold symmetric dimer of dimers or a four-fold symmetric tetramer based on high-resolution structure data of soluble cyclic nucleotide-binding domains and functional data on intact channels. We addressed this controversy by obtaining structural data on an intact, full-length, cyclic nucleotide-modulated potassium channel, MloK1, from Mesorhizobium loti, which also features a putative voltage-sensor. We present here the 3D single-particle structure by transmission electron microscopy and the projection map of membrane-reconstituted 2D crystals of MloK1 in the presence of cAMP. Our data show a four-fold symmetric arrangement of the CNBDs, separated by discrete gaps. A homology model for full-length MloK1 suggests a vertical orientation for the CNBDs. The 2D crystal packing in the membrane-embedded state is compatible with the S1-S4 domains in the vertical "up" state.
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Affiliation(s)
- Po-Lin Chiu
- Molecular and Cellular Biology, College of Biological Sciences, University of California-Davis, 1 Shields Avenue, Davis, CA 95616, USA
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11
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Gopalakrishnan M, Borowski P, Jülicher F, Zapotocky M. Response and fluctuations of a two-state signaling module with feedback. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2007; 76:021904. [PMID: 17930062 DOI: 10.1103/physreve.76.021904] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/04/2006] [Revised: 04/16/2007] [Indexed: 05/25/2023]
Abstract
We study the stochastic kinetics of a signaling module consisting of a two-state stochastic point process with negative feedback. In the active state, a product is synthesized which increases the active-to-inactive transition rate of the process. We analyze this simple autoregulatory module using a path-integral technique based on the temporal statistics of state flips of the process. We develop a systematic framework to calculate averages, autocorrelations, and response functions by treating the feedback as a weak perturbation. Explicit analytical results are obtained to first order in the feedback strength. Monte Carlo simulations are performed to test the analytical results in the weak feedback limit and to investigate the strong feedback regime. We conclude by relating some of our results to experimental observations in the olfactory and visual sensory systems.
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Affiliation(s)
- Manoj Gopalakrishnan
- Max Planck Institute for the Physics of Complex Systems, 01187 Dresden, Germany.
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12
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Lisman JE, Raghavachari S, Tsien RW. The sequence of events that underlie quantal transmission at central glutamatergic synapses. Nat Rev Neurosci 2007; 8:597-609. [PMID: 17637801 DOI: 10.1038/nrn2191] [Citation(s) in RCA: 171] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The properties of synaptic transmission were first elucidated at the neuromuscular junction. More recent work has examined transmission at synapses within the brain. Here we review the remarkable progress in understanding the biophysical and molecular basis of the sequential steps in this process. These steps include the elevation of Ca2+ in microdomains of the presynaptic terminal, the diffusion of transmitter through the fusion pore into the synaptic cleft and the activation of postsynaptic receptors. The results give insight into the factors that control the precision of quantal transmission and provide a framework for understanding synaptic plasticity.
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Affiliation(s)
- John E Lisman
- Brandeis University, Department of Biology, MS 008, 415 South Street, Waltham, Massachusetts 02454-9110, USA.
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13
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Biskup C, Kusch J, Schulz E, Nache V, Schwede F, Lehmann F, Hagen V, Benndorf K. Relating ligand binding to activation gating in CNGA2 channels. Nature 2007; 446:440-3. [PMID: 17322905 DOI: 10.1038/nature05596] [Citation(s) in RCA: 107] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/24/2006] [Accepted: 01/12/2007] [Indexed: 11/09/2022]
Abstract
Cyclic nucleotide-gated (CNG) ion channels mediate sensory signal transduction in photoreceptors and olfactory cells. Structurally, CNG channels are heterotetramers composed of either two or three homologue subunits. Although it is well established that activation is a cooperative process of these subunits, it remains unknown whether the cooperativity is generated by the ligand binding, the gating, or both, and how the subunits interact. In this study, the action of homotetrameric olfactory-type CNGA2 channels was studied in inside-out membrane patches by simultaneously determining channel activation and ligand binding, using the fluorescent cGMP analogue 8-DY547-cGMP as the ligand. At concentrations of 8-DY547-cGMP < 1 microM, steady-state binding was larger than steady-state activation, whereas at higher concentrations it was smaller, generating a crossover of the steady-state relationships. Global analysis of these relationships together with multiple activation time courses following cGMP jumps showed that four ligands bind to the channels and that there is significant interaction between the binding sites. Among the binding steps, the second is most critical for channel opening: its association constant is three orders of magnitude smaller than the others and it triggers a switch from a mostly closed to a maximally open state. These results contribute to unravelling the role of the subunits in the cooperative mechanism of CNGA2 channel activation and could be of general relevance for the action of other ion channels and receptors.
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Affiliation(s)
- Christoph Biskup
- Institut für Physiologie II, Friedrich-Schiller-Universität Jena, D 07740 Jena, Germany
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14
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Contreras JE, Holmgren M. Access of quaternary ammonium blockers to the internal pore of cyclic nucleotide-gated channels: implications for the location of the gate. ACTA ACUST UNITED AC 2006; 127:481-94. [PMID: 16606688 PMCID: PMC2151523 DOI: 10.1085/jgp.200509440] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Cyclic nucleotide-gated (CNG) channels play important roles in the transduction of visual and olfactory information by sensing changes in the intracellular concentration of cyclic nucleotides. We have investigated the interactions between intracellularly applied quaternary ammonium (QA) ions and the alpha subunit of rod cyclic nucleotide-gated channels. We have used a family of alkyl-triethylammonium derivatives in which the length of one chain is altered. These QA derivatives blocked the permeation pathway of CNG channels in a concentration- and voltage-dependent manner. For QA compounds with tails longer than six methylene groups, increasing the length of the chain resulted in higher apparent affinities of approximately 1.2 RT per methylene group added, which is consistent with the presence of a hydrophobic pocket within the intracellular mouth of the channel that serves as part of the receptor binding site. At the single channel level, decyltriethyl ammonium (C10-TEA) ions did not change the unitary conductance but they did reduce the apparent mean open time, suggesting that the blocker binds to open channels. We provide four lines of evidence suggesting that QA ions can also bind to closed channels: (1) the extent of C10-TEA blockade at subsaturating [cGMP] was larger than at saturating agonist concentration, (2) under saturating concentrations of cGMP, cIMP, or cAMP, blockade levels were inversely correlated with the maximal probability of opening achieved by each agonist, (3) in the closed state, MTS reagents of comparable sizes to QA ions were able to modify V391C in the inner vestibule of the channel, and (4) in the closed state, C10-TEA was able to slow the Cd2+ inhibition observed in V391C channels. These results are in stark contrast to the well-established QA blockade mechanism in Kv channels, where these compounds can only access the inner vestibule in the open state because the gate that opens and closes the channel is located cytoplasmically with respect to the binding site of QA ions. Therefore, in the context of Kv channels, our observations suggest that the regions involved in opening and closing the permeation pathways in these two types of channels are different.
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Affiliation(s)
- Jorge E Contreras
- Porter Neuroscience Research Center, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 02892, USA
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15
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Nache V, Kusch J, Hagen V, Benndorf K. Gating of cyclic nucleotide-gated (CNGA1) channels by cGMP jumps and depolarizing voltage steps. Biophys J 2006; 90:3146-54. [PMID: 16473910 PMCID: PMC1432111 DOI: 10.1529/biophysj.105.078667] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We expressed rod-type homotetrameric cyclic nucleotide-gated (CNGA1) channels in Xenopus oocytes and studied activation by photolysis-induced jumps of the 3',5'-cyclic guanosine monophosphate (cGMP) concentration and by voltage steps. cGMP jumps to increasing concentrations up to the EC50 value of 46.5 microM decelerate the activation gating, indicative that even at concentrations of cGMP << EC50 binding is not rate limiting. Above the EC50 value, activation by cGMP jumps is again accelerated to the higher concentrations. At the same cGMP concentration, the speed of the activation gating by depolarizing voltage steps is roughly similar to that by cGMP jumps. Permeating ions passing the pore more slowly (Rb+ > K+ > Na+) slow down the activation time course. At the single-channel level, cGMP jumps to high concentrations cause openings directly to the main open level without passing sublevels. From these results it is concluded that at both low and high cGMP the gating of homotetrameric CNGA1 channels is not rate-limited by the cGMP binding but by conformational changes of the channel which are voltage dependent and include movements in the pore region.
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Affiliation(s)
- Vasilica Nache
- Institut für Physiologie II, Friedrich-Schiller-Universität Jena, Jena, Germany
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16
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Brown RL, Strassmaier T, Brady JD, Karpen JW. The pharmacology of cyclic nucleotide-gated channels: emerging from the darkness. Curr Pharm Des 2006; 12:3597-613. [PMID: 17073662 PMCID: PMC2467446 DOI: 10.2174/138161206778522100] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.
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Affiliation(s)
- R. Lane Brown
- Neurological Sciences Institute, Oregon Health & Science University, Portland, OR 97239, USA
| | - Timothy Strassmaier
- Department of Physiology and Pharmacology, Oregon Health & Science University, Portland, OR 97239, USA
| | - James D. Brady
- Neurological Sciences Institute, Oregon Health & Science University, Portland, OR 97239, USA
| | - Jeffrey W. Karpen
- Department of Physiology and Pharmacology, Oregon Health & Science University, Portland, OR 97239, USA
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17
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Hallermann S, Heckmann S, Dudel J, Heckmann M. Short openings in high resolution single channel recordings of mouse nicotinic receptors. J Physiol 2005; 563:645-62. [PMID: 15677689 PMCID: PMC1665609 DOI: 10.1113/jphysiol.2004.080606] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/04/2004] [Accepted: 01/21/2005] [Indexed: 11/08/2022] Open
Abstract
The temporal fine structure of single channel currents was studied to obtain information on how agonists open nicotinic acetylcholine receptor channels. Currents were recorded from mouse myoballs with quartz pipettes in the on-cell mode of the patch-clamp technique. With 62 kHz filter cut-off and root mean square (r.m.s.) noise levels as low as 1.45 pA at 200 mV hyperpolarization, events down to 6 micros duration could be resolved with negligible error rate. Three types of openings with mean durations of 750 micros, 89 micros and 4 micros were identified with 0.1-10 microM suberyldicholine (SubCh). The relative frequencies of the three types of openings were 84% for long, 5% for medium and 11% for short openings with 1 microM SubCh. Stability plots and single channel current amplitude comparisons suggest that the three types of openings arise from a homogenous channel population. Above 10 microM SubCh, the three types of openings could not be discerned because channel openings occurred too closely spaced and open channels were increasingly blocked. Three types of openings can be generated with a mechanistic receptor model with two unequal binding sites, short and medium openings arising from one or the other monoliganded state, and long openings from the fully liganded state of the receptor. Maximum likelihood fitting of the rate constants of this model directly to the sequence of observed open and shut times accurately predicted the main physiological properties of the receptors with 0.1 microM SubCh. However, fitting recordings with 0.1-10 microM SubCh simultaneously revealed that this model cannot reproduce the weak influence of SubCh concentration on the proportions of the three types of openings. Therefore we conclude that short and medium openings are unlikely to arise preferentially from one or the other monoliganded state of nicotinic acetylcholine receptor channels.
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Affiliation(s)
- Stefan Hallermann
- Physiologisches Institut, Hermann-Herder-Str. 7, D-79104 Freiburg i. Br., Germany.
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18
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Abstract
We have used Monte Carlo simulations to understand the generation of quantal responses at the single active zones of CA1 synapses. We constructed a model of AMPA channel activation that accounts for the responses to controlled glutamate application and a model of glutamate diffusion in the synaptic cleft. With no further adjustments to these models, we simulated the response to the release of glutamate from a single vesicle. The predicted response closely matches the rise time of observed responses, which recent measurements show is much faster (<100 μs) than previously thought. The simulations show that initial channel opening is driven by a brief (<100 μs) glutamate spike near the site of vesicle fusion, producing a hotspot of channel activation (diameter: ∼250 nm) smaller than many synapses. Quantal size therefore depends more strongly on the density of channels than their number, a finding that has important implications for measuring synaptic strength. Recent measurements allow estimation of AMPA receptor density at CA1 synapses. Using this value, our simulations correctly predicts a quantal amplitude of ∼10 pA. We have also analyzed the properties of excitatory postsynaptic currents (EPSCs) generated by the multivesicular release that can occur during evoked responses. We find that summation is nearly linear and that the existence of multiple narrow peaks in amplitude histograms can be accounted for. It has been unclear how to reconcile the existence of these narrow peaks, which indicate that the variation of quantal amplitude is small (CV < 0.2) with the highly variable amplitude of miniature EPSCs (mEPSCs; CV ∼ 0.6). According to one theory, mEPSC variability arises from variation in vesicle glutamate content. However, both our modeling results and recent experimental results indicate that this view cannot account for the observed rise time/amplitude correlation of mEPSCs. In contrast, this correlation and the high mEPSC variability can be accounted for if some mEPSCs are generated by two or more vesicles released with small temporal jitter. We conclude that a broad range of results can be accounted for by simple principles: quantal amplitude (∼10 pA) is stereotyped, some mEPSCs are multivesicular at moderate and large synapses, and evoked responses are generated by quasi-linear summation of multiple quanta.
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Affiliation(s)
- Sridhar Raghavachari
- Dept. of Biology and Volen Center for Complex Systems, Brandeis University MS 008, 415 South S., Waltham, MA 02454, USA
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19
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Kusch J, Nache V, Benndorf K. Effects of permeating ions and cGMP on gating and conductance of rod-type cyclic nucleotide-gated (CNGA1) channels. J Physiol 2004; 560:605-16. [PMID: 15308684 PMCID: PMC1665289 DOI: 10.1113/jphysiol.2004.070193] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Cyclic nucleotide-gated (CNG) channels are tetrameric non-specific cation channels. They mediate the receptor potentials in photoreceptors and cells of the olfactory epithelium and they are activated by the binding of cyclic nucleotides such as cGMP and cAMP. Previous studies in homotetrameric CNGA1 channels, activated with covalently bound cGMP, presented evidence that partially liganded channels cause partial channel opening (Ruiz & Karpen, 1997, 1999). Here, homotetrameric CNGA1 channels were expressed in Xenopus oocytes. Conductance and gating of these channels were studied as a function of the concentration of freely diffusible cGMP and with different permeating ions. At saturating cGMP the current levels distributed around a single mean in a Gaussian fashion and the open times were long. At low cGMP, however, the current levels were heterogeneous: they were smaller than those at saturating cGMP, equal, or larger. The open times were short. Ions generating the larger single-channel currents (Na(+) > K(+) > Rb(+)) concomitantly increased the heterogeneity of current levels and decreased the open probability and open times. The results suggest that the activation of CNGA1 channels by cGMP and ions staying longer in the pore is associated with less extensive and less frequent conformational fluctuations of the channel pore.
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Affiliation(s)
- Jana Kusch
- Institut für Physiologie II, Friedrich-Schiller-Universität Jena, 07740 Jena, Germany
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20
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Ulens C, Siegelbaum SA. Regulation of hyperpolarization-activated HCN channels by cAMP through a gating switch in binding domain symmetry. Neuron 2004; 40:959-70. [PMID: 14659094 DOI: 10.1016/s0896-6273(03)00753-0] [Citation(s) in RCA: 82] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Recent X-ray structures show that the binding domains of tetrameric ligand-gated channels form either a 4-fold symmetric gating ring or a 2-fold symmetric dimer of dimers. To determine how such structures function to coordinate the binding of multiple ligands during channel activation, we examined the action of cAMP to enhance the opening of the hyperpolarization-activated HCN2 channels, whose cytoplasmic C terminus forms a gating ring in the presence of cAMP. Using tandem dimers and tetramers in which cAMP binding to selected HCN2 subunits was prevented by a point mutation or deletion, we provide the first direct determination of the energetic effects on gating of each of four ligand binding events and demonstrate the importance of the gating ring for cAMP regulation. We suggest that cAMP binding enhances channel opening by promoting assembly of the gating ring from an unliganded state in which the four subunits interact as a 2-fold symmetric dimer of dimers.
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Affiliation(s)
- Chris Ulens
- Center for Neurobiology and Behavior, Department of Pharmacology, Howard Hughes Medical Institute, Columbia University, 722 West 168 Street, New York, NY 10032, USA
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21
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Nai Q, McIntosh JM, Margiotta JF. Relating neuronal nicotinic acetylcholine receptor subtypes defined by subunit composition and channel function. Mol Pharmacol 2003; 63:311-24. [PMID: 12527802 DOI: 10.1124/mol.63.2.311] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Neuronal nicotinic acetylcholine receptors (nAChRs) are widespread, diverse ion channels involved in synaptic signaling, addiction, and disease. Despite their importance, the relationship between native nAChR subunit composition and function remains poorly defined. Chick ciliary ganglion neurons express two major nAChR types: those recognized by alpha-bungarotoxin (alphaBgt), nearly all of which contain only alpha7 subunits (alpha7-nAChRs) and those insensitive to alphaBgt, which contain alpha3, alpha5, beta4, and, in some cases, beta2 subunits (alpha3*-nAChRs). We explored the relationship between nAChR composition and channel function using toxins recognizing alpha7 subunits (alphaBgt), and alpha3/beta4 (alpha-conotoxin-AuIB), or alpha3/beta2 (alpha-conotoxin-MII) subunit interfaces to perturb responses induced by nicotine, alpha7-, or alpha3-selective agonists (GTS-21 or epibatidine, respectively). Using these reagents, fast-decaying whole-cell current components were attributed solely to alpha7-nAChRs, and slow-decaying components mostly to alpha3*-nAChRs. In outside-out patches, nicotine activated brief 60- and 80-pS single nAChR channel events, and mixed-duration 25- and 40-pS nAChR events. Subsequently, 60- and 80-pS nAChR events and most brief 25- and 40-pS events were attributed to alpha7-nAChRs, and long 25- and 40-pS events to alpha3*-nAChRs. alpha3*-nAChRs lacking beta2 subunits seemed responsible for long 25 pS nAChR events, whereas those containing beta2 subunits mediated the long 40 pS nAChR events that dominate single-channel records. These results reveal greater functional heterogeneity for alpha7-nAChRs than previously expected and indicate that beta2 subunits contribute importantly to alpha3*-nAChR function. By linking structural to functional nAChR subtypes, the findings also illustrate a useful pharmacological strategy for selectively targeting nAChRs.
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Affiliation(s)
- Qiang Nai
- Department of Anatomy and Neurobiology, Medical College of Ohio, Toledo, Ohio 43614, USA
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22
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Abstract
Cyclic nucleotide-gated (CNG) channels have been shown to be blocked by diltiazem, tetracaine, polyamines, toxins, divalent cations, and other compounds. Dequalinium is an organic divalent cation which suppresses the rat small conductance Ca(2+)-activated K(+) channel 2 (rSK2) and the activity of protein kinase C. In this study, we have tested the ability of dequalinium to block CNGA1 channels and heteromeric CNGA1+CNGB1 channels. When applied to the intracellular side of inside-out excised patches from Xenopus oocytes, dequalinium blocks CNGA1 channels with a K(1/2) approximately 190 nM and CNGA1+CNGB1 channels with a K(1/2) approximately 385 nM, at 0 mV. This block occurs in a state-independent fashion, and is voltage dependent with a zdelta approximately 1. Our data also demonstrate that dequalinium interacts with the permeant ion probably because it occupies a binding site in the ion conducting pathway. Dequalinium applied to the extracellular surface also produced block, but with a voltage dependence that suggests it crosses the membrane to block from the inside. We also show that at the single-channel level, dequalinium is a slow blocker that does not change the unitary conductance of CNGA1 channels. Thus, dequalinium should be a useful tool for studying permeation and gating properties of CNG channels.
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Affiliation(s)
- Tamara Rosenbaum
- Department of Ophthalmology, University of Washington, Seattle, WA 98195, USA
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23
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Abstract
The advent of the patch-clamp technique 25 years ago revolutionized the study of ion channels. This method also made it possible to measure the kinetic behavior of single protein molecules. The low-noise recordings of ionic currents through single channels, coupled with other cutting-edge technologies, have revealed a rich complexity of functional states that are not readily explained by simple allosteric protein models such as the popular concerted model and the sequential model. Although these models can each account for elements of ion channel function, we propose that variations or extensions of the lesser-known general allosteric model provide a more promising framework for explaining the intricate behaviors of ion channels.
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Affiliation(s)
- Jeffrey W Karpen
- Department of Physiology and Biophysics, University of Colorado Health Sciences Center, 4200 East Ninth Avenue, C240, Denver, CO 80262, USA.
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24
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Abstract
Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.
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Affiliation(s)
- U Benjamin Kaupp
- Institut für Biologische Informationsverarbeitung, Forschungszentrum Jülich, Jülich, Germany.
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25
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Abstract
Potassium (K+) channels and ionotropic glutamate receptors (iGluRs) fulfill divergent roles in vertebrate nervous systems. Despite this, however, recent work suggests that these ion channels are structurally homologous, sharing an ancestral protein, architectural design, and tetrameric subunit stoichiometry. Their gating mechanisms also are speculated to have overlapping features. Here we show that the mechanism of iGluR desensitization is unique. Unlike K+ channels, AMPA- and kainate-type iGluR subunits desensitize in several ordered conformational steps. AMPA receptors operate as dimers, whereas the functional stoichiometry of kainate receptor desensitization is dependent on external ions. Contrary to conventional understanding, kinetic models suggest that partially desensitized AMPA and kainate receptors conduct ions and are likely participants in synaptic signaling. Although sharing many structural correlates with K+ channels, iGluRs have evolved unique subunit-subunit interactions, tailoring their gating behavior to fulfill distinct roles in neuronal signaling.
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26
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Abstract
In cyclic nucleotide-gated (CNG) ion channels, binding of cGMP or cAMP drives a conformational change that leads to opening of an ion-conducting pore. One region implicated in the coupling of ligand binding to opening of the pore is the C linker region. Here, we used crosslinking of endogenous cysteines to study interregion proximity. We demonstrate that an individual amino acid--C481--in the C linker region of each of two neighboring subunits can form a disulfide bond. Further, using tandem dimers, we show that a disulfide bond between C35 in the N-terminal region and C481 in the C linker region can form either within a subunit or between subunits. From our data on proximity between individual amino acids and previous studies, a picture emerges of the C linker as a potential dimerization interface.
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Affiliation(s)
- Tamara Rosenbaum
- Department of Ophthalmology, University of Washington, Seattle, WA 98195, USA
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27
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Laver DR, Lenz GK, Lamb GD. Regulation of the calcium release channel from rabbit skeletal muscle by the nucleotides ATP, AMP, IMP and adenosine. J Physiol 2001; 537:763-78. [PMID: 11744753 PMCID: PMC2279010 DOI: 10.1111/j.1469-7793.2001.00763.x] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/25/2001] [Accepted: 08/06/2001] [Indexed: 11/29/2022] Open
Abstract
1. Nucleotide activation of skeletal muscle ryanodine receptors (RyRs) was studied in planar lipid bilayers in order to understand RyR regulation in vivo under normal and fatigued conditions. With 'resting' calcium (100 nM cytoplasmic and 1 mM luminal), RyRs had an open probability (P(o)) of approximately 0.01 in the absence of nucleotides and magnesium. ATP reversibly activated RyRs with P(o) at saturation (P(max)) approximately 0.33 and K(a) (concentration for half-maximal activation) approximately 0.36 mM and with a Hill coefficient (n(H)) of approximately 1.8 in RyRs when P(max) < 0.5 and approximately 4 when P(max) > 0.5. 2. AMP was a much weaker agonist (P(max) approximately 0.09) and adenosine was weaker still (P(max) approximately 0.01-0.02), whereas inosine monophosphate (IMP), the normal metabolic end product of ATP hydrolysis, produced no activation at all. 3. Adenosine acted as a competitive antagonist that reversibly inhibited ATP- and AMP-activated RyRs with n(H) approximately 1 and K(i) approximately 0.06 mM at [ATP] < 0.5 mM, increasing 4-fold for each 2-fold increase in [ATP] above 0.5 mM. This is explained by the binding of a single adenosine preventing the cooperative binding of two ATP or AMP molecules, with dissociation constants of 0.4, 0.45 and 0.06 mM for ATP, AMP and adenosine, respectively. Importantly, IMP (< or = 8 mM) had no inhibitory effect whatsoever on ATP-activated RyRs. 4. Mean open (tau(o)) and closed (tau(c)) dwell-times were more closely related to P(o) than to the nucleotide species or individual RyRs. At P(o) < 0.2, RyR regulation occurred via changes in tau(c), whereas at higher P(o) this also occurred via changes in tau(o). The detailed properties of activation and competitive inhibition indicated complex channel behaviour that could be explained in terms of a model involving interactions between different subunits of the RyR homotetramer. 5. The results also show how deleterious adenosine accumulation is to the function of RyRs in skeletal muscle and, by comparison with voltage sensor-controlled Ca(2+) release, indicate that voltage sensor activation requires ATP binding to the RyR to be effective.
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Affiliation(s)
- D R Laver
- School of Biochemistry and Molecular Biology, Faculty of Science, Australian National University, Canberra ACT 0200, Australia.
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28
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Abstract
Visual transduction captures widespread interest because its G-protein signaling motif recurs throughout nature yet is uniquely accessible for study in the photoreceptor cells. The light-activated currents generated at the photoreceptor outer segment provide an easily observed real-time measure of the output of the signaling cascade, and the ease of obtaining pure samples of outer segments in reasonable quantity facilitates biochemical experiments. A quiet revolution in the study of the mechanism has occurred during the past decade with the advent of gene-targeting techniques. These have made it possible to observe how transduction is perturbed by the deletion, overexpression, or mutation of specific components of the transduction apparatus.
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Affiliation(s)
- M E Burns
- Department of Neurobiology, Stanford University Medical Center, Stanford, California 94305, USA.
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29
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Abstract
Most AMPA-type glutamate receptors (GluRs) exhibit rapid and virtually complete desensitization when activated by glutamate, and at some central synapses it is largely desensitization that determines the decay of EPSCs. However, the mechanisms underlying the conformation change that results in desensitization are not fully understood. AMPA receptor subunits that contain a single amino acid substitution have been shown to form homomeric channels that show markedly reduced desensitization. We show here that the coexpression of wild-type GluR1 with one such mutant, GluR1(L497Y), results in heteromeric channels that show desensitization behavior that is intermediate between wild-type and mutant homomers. The relative amplitudes of the multiple exponential components present in the decay of glutamate-evoked currents depended on the relative abundance of wild-type and mutant subunits and were described by the combinatorial distribution of the two types of subunits into tetrameric, but not pentameric, assemblies. Our results are consistent with recent structural data suggesting that AMPA receptors are tetrameric assemblies composed of two dimers.
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30
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Robert A, Irizarry SN, Hughes TE, Howe JR. Subunit interactions and AMPA receptor desensitization. J Neurosci 2001; 21:5574-86. [PMID: 11466429 PMCID: PMC6762644] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/20/2023] Open
Abstract
Most AMPA-type glutamate receptors (GluRs) exhibit rapid and virtually complete desensitization when activated by glutamate, and at some central synapses it is largely desensitization that determines the decay of EPSCs. However, the mechanisms underlying the conformation change that results in desensitization are not fully understood. AMPA receptor subunits that contain a single amino acid substitution have been shown to form homomeric channels that show markedly reduced desensitization. We show here that the coexpression of wild-type GluR1 with one such mutant, GluR1(L497Y), results in heteromeric channels that show desensitization behavior that is intermediate between wild-type and mutant homomers. The relative amplitudes of the multiple exponential components present in the decay of glutamate-evoked currents depended on the relative abundance of wild-type and mutant subunits and were described by the combinatorial distribution of the two types of subunits into tetrameric, but not pentameric, assemblies. Our results are consistent with recent structural data suggesting that AMPA receptors are tetrameric assemblies composed of two dimers.
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Affiliation(s)
- A Robert
- Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06520-8066, USA
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31
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Molokanova E, Kramer RH. Mechanism of inhibition of cyclic nucleotide-gated channel by protein tyrosine kinase probed with genistein. J Gen Physiol 2001; 117:219-34. [PMID: 11222626 PMCID: PMC2225614 DOI: 10.1085/jgp.117.3.219] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/03/2022] Open
Abstract
Rod cyclic nucleotide-gated (CNG) channels are modulated by changes in tyrosine phosphorylation catalyzed by protein tyrosine kinases (PTKs) and phosphatases (PTPs). We used genistein, a PTK inhibitor, to probe the interaction between the channel and PTKs. Previously, we found that in addition to inhibiting tyrosine phosphorylation of the rod CNG channel alpha-subunit (RETalpha), genistein triggers a noncatalytic inhibitory interaction between the PTK and the channel. These studies suggest that PTKs affects RETalpha channels in two ways: (1) by catalyzing phosphorylation of the channel protein, and (2) by allosterically regulating channel activation. Here, we study the mechanism of noncatalytic inhibition. We find that noncatalytic inhibition follows the same activity dependence pattern as catalytic modulation (phosphorylation): the efficacy and apparent affinity of genistein inhibition are much higher for closed than for fully activated channels. Association rates with the genistein-PTK complex were similar for closed and fully activated channels and independent of genistein concentration. Dissociation rates were 100 times slower for closed channels, which is consistent with a much higher affinity for genistein-PTK. Genistein-PTK affects channel gating, but not single channel conductance or the number of active channels. By analyzing single channel gating during genistein-PTK dissociation, we determined the maximal open probability for normal and genistein-PTK-bound channels. genistein-PTK decreases open probability by increasing the free energy required for opening, making opening dramatically less favorable. Ni(2+), which potentiates RETalpha channel gating, partially relieves genistein inhibition, possibly by disrupting the association between the genistein-PTK and the channel. Studies on chimeric channels containing portions of RETalpha, which exhibits genistein inhibition, and the rat olfactory CNG channel alpha-subunit, which does not, reveals that a domain containing S6 and flanking regions is the crucial for genistein inhibition and may constitute the genistein-PTK binding site. Thus, genistein-PTK stabilizes the closed state of the channel by interacting with portions of the channel that participate in gating.
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Affiliation(s)
- E Molokanova
- Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA
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32
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He Y, Karpen JW. Probing the interactions between cAMP and cGMP in cyclic nucleotide-gated channels using covalently tethered ligands. Biochemistry 2001; 40:286-95. [PMID: 11141082 DOI: 10.1021/bi002014n] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Cyclic nucleotide-gated channels contain four ligand-binding subunits, and they are directly activated by the binding of cGMP or cAMP. Channels with different combinations of subunits are known to have different sensitivities to the two nucleotides. However, the consequences of mixed occupancy by cGMP and cAMP are not well understood, and may have important implications for understanding the functions of these channels in different cell types. We studied the activation of homomeric and heteromeric retinal rod cyclic nucleotide-gated channels with the four ligand-binding sites occupied by different combinations of cGMP (a strong agonist) and cAMP (a weak agonist). Control of occupancy was obtained by covalently tethering different numbers of cGMP moieties using the photoaffinity analogue 8-p-azidophenacylthio-cGMP; the remaining sites were then saturated with cAMP, or cGMP, for comparison. The fractional current activated by cAMP increased dramatically as the number of tethered cGMP moieties increased. In homomeric channels comprised of the alpha subunit, cAMP became an effective agonist only after three of the four sites were occupied by tethered cGMP moieties. In contrast, in heteromeric channels comprised of two alpha and two beta subunits, cAMP caused significant activation after two sites were occupied by tethered cGMP moieties. In agreement with earlier work, a single residue on the beta subunit, N1201, accounted for much of the increased efficacy of cAMP on heteromeric channels. The results are consistent with significant interactions between subunits, including the two types of subunits in heteromeric channels.
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Affiliation(s)
- Y He
- Department of Physiology & Biophysics, University of Colorado School of Medicine, Denver, Colorado 80262, USA
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33
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Abstract
The permeability ratio between K(+) and Na(+) ions in cyclic nucleotide-gated channels is close to 1, and the single channel conductance has almost the same value in the presence of K(+) or Na(+). Therefore, K(+) and Na(+) ions are thought to permeate with identical properties. In the alpha-subunit from bovine rods there is a loop of three prolines at positions 365 to 367. When proline 365 is mutated to a threonine, a cysteine, or an alanine, mutant channels exhibit a complex interaction between K(+) and Na(+) ions. Indeed K(+), Rb(+) and Cs(+) ions do not carry any significant macroscopic current through mutant channels P365T, P365C and P365A and block the current carried by Na(+) ions. Moreover in mutant P365T the presence of K(+) in the intracellular (or extracellular) medium caused the appearance of a large transient inward (or outward) current carried by Na(+) when the voltage command was quickly stepped to large negative (or positive) membrane voltages. This transient current is caused by a transient potentiation, i.e., an increase of the open probability. The permeation of organic cations through these mutant channels is almost identical to that through the wild type (w.t.) channel. Also in the w.t. channel a similar but smaller transient current is observed, associated to a slowing down of the channel gating evident when intracellular Na(+) is replaced with K(+). As a consequence, a rather simple mechanism can explain the complex behavior here described: when a K(+) ion is occupying the pore there is a profound blockage of the channel and a potentiation of gating immediately after the K(+) ion is driven out. Potentiation occurs because K(+) ions slow down the rate constant K(off) controlling channel closure. These results indicate that K(+) and Na(+) ions do not permeate through CNG channels in the same way and that K(+) ions influence the channel gating.
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Affiliation(s)
- K Gamel
- Scuola Internazionale Superiore di Studi Avanzati and Instituto Nationale di Fiscia del la Materia-Unita' di Trieste, 34014 Trieste, Italy
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34
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Boehning D, Joseph SK. Direct association of ligand-binding and pore domains in homo- and heterotetrameric inositol 1,4,5-trisphosphate receptors. EMBO J 2000; 19:5450-9. [PMID: 11032812 PMCID: PMC313997 DOI: 10.1093/emboj/19.20.5450] [Citation(s) in RCA: 94] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of intracellular Ca(2+) channels that exist as homo- or heterotetramers. In order to determine whether the N-terminal ligand-binding domain is in close physical proximity to the C-terminal pore domain, we prepared microsomal membranes from COS-7 cells expressing recombinant type I and type III IP(3)R isoforms. Trypsin digestion followed by cross-linking and co-immunoprecipitation of peptide fragments suggested an inter-subunit N- and C-terminal interaction in both homo- and heterotetramers. This observation was further supported by the ability of in vitro translated C-terminal peptides to interact specifically with an N-terminal fusion protein. Using a (45)Ca(2+) flux assay, we provide functional evidence that the ligand-binding domain of one subunit can gate the pore domain of an adjacent subunit. We conclude that common structural motifs are shared between the type I and type III IP(3)Rs and propose that the gating mechanism of IP(3)R Ca(2+) channels involves the association of the N-terminus of one subunit with the C-terminus of an adjacent subunit in both homo- and heterotetrameric complexes.
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MESH Headings
- Animals
- COS Cells
- Calcium/metabolism
- Calcium Channels/chemistry
- Calcium Channels/classification
- Calcium Channels/genetics
- Calcium Channels/metabolism
- Calcium Signaling
- Cross-Linking Reagents/metabolism
- Inositol 1,4,5-Trisphosphate/pharmacology
- Inositol 1,4,5-Trisphosphate Receptors
- Ion Channel Gating/drug effects
- Ligands
- Membrane Proteins/chemistry
- Membrane Proteins/metabolism
- Microsomes/metabolism
- Peptide Fragments/chemistry
- Peptide Fragments/immunology
- Peptide Fragments/metabolism
- Precipitin Tests
- Protein Binding
- Protein Isoforms
- Protein Structure, Quaternary/drug effects
- Protein Structure, Tertiary/drug effects
- Quaternary Ammonium Compounds/pharmacology
- Receptors, Cytoplasmic and Nuclear/chemistry
- Receptors, Cytoplasmic and Nuclear/classification
- Receptors, Cytoplasmic and Nuclear/genetics
- Receptors, Cytoplasmic and Nuclear/metabolism
- Recombinant Fusion Proteins/chemistry
- Recombinant Fusion Proteins/metabolism
- Succinimides/metabolism
- Trypsin/metabolism
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Affiliation(s)
- D Boehning
- Department of Pathology and Cell Biology, Thomas Jefferson University School of Medicine, Philadelphia, PA 19107, USA
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35
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Middendorf TR, Aldrich RW. Effects of ultraviolet modification on the gating energetics of cyclic nucleotide-gated channels. J Gen Physiol 2000; 116:253-82. [PMID: 10919870 PMCID: PMC2229492 DOI: 10.1085/jgp.116.2.253] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Middendorf et al. (Middendorf, T.R., R.W. Aldrich, and D.A. Baylor. 2000. J. Gen. Physiol. 116:227-252) showed that ultraviolet light decreases the current through cloned cyclic nucleotide-gated channels from bovine retina activated by high concentrations of cGMP. Here we probe the mechanism of the current reduction. The channels' open probability before irradiation, P(o)(0), determined the sign of the change in current amplitude that occurred upon irradiation. UV always decreased the current through channels with high initial open probabilities [P(o)(0) > 0.3]. Manipulations that promoted channel opening antagonized the current reduction by UV. In contrast, UV always increased the current through channels with low initial open probabilities [P(o)(0) < or = 0.02], and the magnitude of the current increase varied inversely with P(o)(0). The dual effects of UV on channel currents and the correlation of both effects with P(o)(0) suggest that the channels contain two distinct classes of UV target residues whose photochemical modification exerts opposing effects on channel gating. We present a simple model based on this idea that accounts quantitatively for the UV effects on the currents and provides estimates for the photochemical quantum yields and free energy costs of modifying the UV targets. Simulations indicate that UV modification may be used to produce and quantify large changes in channel gating energetics in regimes where the associated changes in open probability are not measurable by existing techniques.
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Affiliation(s)
- Thomas R. Middendorf
- Neurobiology Department, Howard Hughes Medical Institute, School of Medicine, Stanford University, Stanford, California 94305
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, School of Medicine, Stanford University, Stanford, California 94305
| | - Richard W. Aldrich
- Department of Molecular and Cellular Physiology, Howard Hughes Medical Institute, School of Medicine, Stanford University, Stanford, California 94305
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36
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Karpen JW, Ruiz M, Brown RL. Covalent tethering of ligands to retinal rod cyclic nucleotide-gated channels: binding site structure and allosteric mechanism. Methods Enzymol 2000; 315:755-72. [PMID: 10736739 DOI: 10.1016/s0076-6879(00)15880-x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/15/2023]
Affiliation(s)
- J W Karpen
- Department of Physiology and Biophysics, University of Colorado School of Medicine, Denver 80262, USA
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37
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Pagès F, Ildefonse M, Ragno M, Crouzy S, Bennett N. Coexpression of alpha and beta subunits of the rod cyclic GMP-gated channel restores native sensitivity to cyclic AMP: role of D604/N1201. Biophys J 2000; 78:1227-39. [PMID: 10692312 PMCID: PMC1300725 DOI: 10.1016/s0006-3495(00)76680-x] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Coexpression of the betawt and alphawt subunits of the bovine rod channel restores two characteristics of the native channels: higher sensitivity to cAMP and potentiation of cGMP-induced currents by low cAMP concentrations. To test whether the increased sensitivity to cAMP is due to the uncharged nature of the asparagine residue (N1201) situated in place of aspartate D604 in the beta subunit as previously suggested (, Neuron. 15:619-625), we compared currents from wild-type (alphawt and alphawt/betawt) and from mutated channels (alphaD604N, alphaD604N/betawt, and alphawt/betaN1201D). The results show that the sensitivity to cAMP and cAMP potentiation is partly but not entirely determined by the charge of residue 1201 in the beta subunit. The D604N mutation in the alpha subunit and, to a lesser extent, coexpression of the betawt subunit with the alphawt subunit reduce the open probability for cGMP compared to that of the alphawt channel. Interpretation of the data with the MWC allosteric model (model of Monod, Wyman, Changeux;, J. Mol. Biol. 12:88-118) suggests that the D604N mutation in the alpha subunits and coassembly of alpha and beta subunits alter the free energy of gating by cAMP more than that of cAMP binding.
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Affiliation(s)
- F Pagès
- Laboratoire de Biophysique Moléculaire et Cellulaire (URA CNRS 520), DBMS, C.E.A.-Grenoble, Grenoble, France
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38
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Anthony TL, Brooks HL, Boassa D, Leonov S, Yanochko GM, Regan JW, Yool AJ. Cloned human aquaporin-1 is a cyclic GMP-gated ion channel. Mol Pharmacol 2000; 57:576-88. [PMID: 10692499 DOI: 10.1124/mol.57.3.576] [Citation(s) in RCA: 135] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
Aquaporin-1 (AQP1) is a member of the membrane intrinsic protein (MIP) gene family and is known to provide pathways for water flux across cell membranes. We show here that cloned human AQP1 not only mediates water flux but also serves as a cGMP-gated ion channel. Two-electrode voltage-clamp analyses showed consistent activation of an ionic conductance in wild-type AQP1-expressing oocytes after the direct injection of cGMP (50 nl of 100 mM). Current activation was not observed in control (water-injected) oocytes or in AQP5-expressing oocytes with osmotic water permeabilities equivalent to those seen with AQP1. Patch-clamp recordings revealed large conductance channels (150 pS in K(+) saline) in excised patches from AQP1-expressing oocytes after the application of cGMP to the internal side. Amino acid sequence alignments between AQP1 and sensory cyclic-nucleotide-gated channels showed similarities between the cyclic-nucleotide-gated binding domain and the AQP1 carboxyl terminus that were not present in AQP5. Competitive radioligand-binding assays with [(3)H]cGMP demonstrated specific binding (K(D) = 0.2 microM) in AQP1-expressing Sf9 cells but not in controls. These results indicate that AQP1 channels have the capacity to participate in ionic signaling after the activation of cGMP second-messenger pathways.
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Affiliation(s)
- T L Anthony
- Department of Pharmacology, University of Arizona College of Pharmacy, Tucson, Arizona, USA
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39
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Brown RL, Haley TL, Snow SD. Irreversible activation of cyclic nucleotide-gated ion channels by sulfhydryl-reactive derivatives of cyclic GMP. Biochemistry 2000; 39:432-41. [PMID: 10631005 DOI: 10.1021/bi9920735] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
First discovered in the sensory epithelium of the visual and olfactory systems, cyclic nucleotide-gated (CNG) ion channels have now been found in tissues throughout the body. Native rod CNG channels are tetramers composed of homologous, but distinct, alpha- and beta-subunits. The goal of this study was to develop a novel method for targeting covalent attachment of cGMP to individual subunit types. Toward this goal, we have found that treatment of membrane patches expressing rod alpha-subunit channels with sulfhydryl-reactive derivatives of cGMP resulted in irreversible activation. The persistent currents were sensitive to block by both Mg(2+) and tetracaine. Pretreatment of the patch with the sulfhydryl-blocking reagents N-ethylmaleimide (NEM) and bis-dithionitrobenzoic acid (DTNB) prevented covalent activation; the effect of DTNB was reversed by reduction with DTT. Furthermore, the process of covalent activation was dramatically slowed by the presence of an excess of 8-Br-cGMP. These results suggested that covalent activation resulted from the tethering of cGMP near the channel's ligand-binding sites by reaction with an endogenous cysteine. The alpha-subunit of the rod channel contains seven cysteine residues, and we set out to determine the site of attachment by site-directed mutagenesis. Surprisingly, irreversible activation was not abolished by elimination of all seven cysteine residues. This result suggests that the site of attachment is on a tightly associated protein, rather than on the channel protein itself. To further investigate these results, we treated patches containing irreversibly activated channels with 100 microg/mL trypsin and discovered two modes of covalent activation. One type developed rapidly and was removed by trypsin treatment, and the second developed slowly and was resistant to trypsin treatment. Both types of covalent activation were present in all mutants tested and were also present when CNG channels were expressed in HEK-293 cells. These results suggest that CNG channel subunits may associate with endogenous proteins when they are expressed in heterologous systems.
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Affiliation(s)
- R L Brown
- Neurological Sciences Institute, Oregon Health Sciences University, 1120 NW 20th Avenue, Portland, Oregon 97209, USA.
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40
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Ruiz M, Brown RL, He Y, Haley TL, Karpen JW. The single-channel dose-response relation is consistently steep for rod cyclic nucleotide-gated channels: implications for the interpretation of macroscopic dose-response relations. Biochemistry 1999; 38:10642-8. [PMID: 10451358 DOI: 10.1021/bi990532w] [Citation(s) in RCA: 30] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Cyclic nucleotide-gated channels contain four subunits, each with a C-terminal binding site for cGMP or cAMP. The dose-response relation for activation is usually fit with the Hill equation, I/I(max) = [cGMP]n/([cGMP]n + K(1/2)n, where I/I(max) is the fraction of maximal current, K(1/2) is the concentration of cGMP that gives a half-maximal current, and n is the Hill coefficient, taken as the minimum number of ligands required for significant activation. The dose-response relations in multichannel patches are often fit with Hill coefficients of </=2.0, even though other lines of evidence indicate that these channels contain four binding sites and that the binding of three or four ligands is required for significant opening. We have measured dose-response relations for a large number of single cyclic nucleotide-gated channels composed of the bovine rod alpha subunit. We find that the single-channel Hill coefficient is consistently higher than 2.5, with an average of 3.0 +/- 0.37 over 16 patches. In both multichannel and single-channel patches, large variations in K(1/2) have been observed, and are thought to arise from modifications such as phosphorylation. Here we show that mixtures of single channels with high Hill coefficients and variable K(1/2) values will give rise to shallow macroscopic dose-response relations with anomalously low Hill coefficients. This is because activation occurs over a broad range of cGMP concentrations. Thus, dose-response relations from multichannel patches should be interpreted with caution, particularly when detailed mechanistic issues such as cooperativity are being investigated.
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Affiliation(s)
- M Ruiz
- Department of Physiology and Biophysics, University of Colorado School of Medicine, Denver 80262, USA
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