1
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Pierce SR, Xu SQ, Germann AL, Steinbach JH, Akk G. Potentiation of the GABA AR reveals variable energetic contributions by etiocholanolone and propofol. Biophys J 2023:S0006-3495(23)00591-X. [PMID: 37752702 DOI: 10.1016/j.bpj.2023.09.014] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2023] [Revised: 09/14/2023] [Accepted: 09/21/2023] [Indexed: 09/28/2023] Open
Abstract
The properties of a potentiator are typically evaluated by measuring its ability to enhance the magnitude of the control response. Analysis of the ability of drugs to potentiate responses from receptor channels takes place in the context of particular models to extract parameters for functional effects. In the often-used coagonist model, the agonist generating control activity and the potentiator enhancing the control activity make additive energetic contributions to stabilize the active state of the receptor. The energetic contributions are fixed and, once known, enable calculation of predicted receptor behavior at any concentration combination of agonist and potentiator. Here, we have examined the applicability of the coagonist model by measuring the relationship between the magnitude of receptor potentiation and the level of background activity. Ternary αβγ GABAA receptors were activated by GABA or the allosteric agonist propofol, or by a gain-of-function mutation, and etiocholanolone- or propofol-mediated potentiation of peak responses was measured. We show that the free energy change contributed by the modulators etiocholanolone or propofol is reduced at higher levels of control activity, thereby being in disagreement with basic principles of the coagonist model. Possible mechanisms underlying this discrepancy are discussed.
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Affiliation(s)
- Spencer R Pierce
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri
| | - Sophia Q Xu
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri
| | - Allison L Germann
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri
| | - Joe Henry Steinbach
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri; The Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri; The Taylor Family Institute for Innovative Psychiatric Research, Washington University School of Medicine, St. Louis, Missouri.
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2
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Kłopotowski K, Michałowski MA, Gos M, Mosiądz D, Czyżewska MM, Mozrzymas JW. Mutation of valine 53 at the interface between extracellular and transmembrane domains of the β 2 principal subunit affects the GABA A receptor gating. Eur J Pharmacol 2023; 947:175664. [PMID: 36934960 DOI: 10.1016/j.ejphar.2023.175664] [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: 11/17/2022] [Revised: 02/09/2023] [Accepted: 03/16/2023] [Indexed: 03/21/2023]
Abstract
GABAA receptors (gamma-aminobutyric acid type A receptors) are pentameric ligand-gated ion channels mediating inhibition in adult mammalian brains. Their static structure has been intensely studied in the past years but the underlying molecular activatory mechanisms remain obscure. The interface between extracellular and transmembrane domains has been recognized as a key player in the receptor gating. However, the role of the valine 53 in the β1-β2 loop of the principal subunit (β2) remains controversial showing differences compared to homologous residues in some cys-loop counterparts such as nAChR. To address the role of the β2V53 residue in the α1β2γ2L receptor gating, we performed high resolution macroscopic and single-channel recordings. To explore underlying molecular mechanisms a variety of substituting amino acids were investigated: Glutamate and Lysine (different electric charge), Alanine (aliphatic, larger than Valine) and Histidine (same residue as in homologous α1H55). We report that mutation of the β2V53 residue results in alterations of nearly all gating transitions including opening/closing, preactivation and desensitization. A dramatic gating impairment was observed for glutamate substitution (β2V53E) but β2V53K mutation had a weak effect. The impact of histidine substitution was also small while β2V53A markedly affected the receptor but to a smaller extent than β2V53E. Considering available structures in desensitized and bicuculline blocked shut states we propose that strongly detrimental effect of β2V53E mutation on receptor activation results from electrostatic interaction between the glutamate and β2K274 on the loop M2-M3 which stabilizes the receptor in the shut state. We conclude that β2V53 is strongly involved in mechanisms underlying the receptor gating.
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Affiliation(s)
- Karol Kłopotowski
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland.
| | - Michał A Michałowski
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland
| | - Michalina Gos
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland; University of Wroclaw, Department of Molecular Physiology and Neurobiology, Sienkiewicza 21, Wrocław, Dolnośląskie, Pl 50-335, Poland
| | - Daniela Mosiądz
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland
| | - Marta M Czyżewska
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland
| | - Jerzy W Mozrzymas
- Wroclaw Medical University, Department of Biophysics and Neuroscience, Chałubińskiego 3a, Wrocław, Dolnośląskie, PL 50-368, Poland; University of Wroclaw, Department of Molecular Physiology and Neurobiology, Sienkiewicza 21, Wrocław, Dolnośląskie, Pl 50-335, Poland.
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3
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Cantor RS. Kinetic Model of Adsorption of Aqueous Solutes onto Lipid Bilayers: Modulation of the Activity of Membrane Proteins. J Phys Chem B 2023; 127:1598-1606. [PMID: 36763041 DOI: 10.1021/acs.jpcb.2c08714] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/11/2023]
Abstract
It has been argued that membrane proteins that are activated by agonist binding and whose activity depends on conformational transitions are sensitive to membrane adsorption of agonists as well as other solutes such as anesthetics. Ligand-gated ion channels such as GABAAR have been observed electrophysiologically to exhibit remarkable temporal complexity, with multiple time scales of desensitization and deactivation that depend on concentration over a very broad range. Earlier theoretical work was able to predict much of this complexity for GABAAR using a model that incorporates a simple Langmuir approximation of adsorption and desorption of neurotransmitters and solutes such as anesthetics, along with classical modeling of ligand binding and conformational transitions among the three canonical protein states. Here, a simple kinetic model is developed that improves on the Langmuir approach by incorporating the energetics of adsorbate-adsorbate (and adsorbate-bilayer) interactions. Predicted equilibrium isotherms are compared to experimental results, along with the time-course of adsorption/desorption, over a range of values of energetic parameters. Initial predictions of effects of adsorbate energetics on ion current traces involving long and short pulses of agonists show remarkably large effects on the time scale(s) of desensitization and deactivation.
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Affiliation(s)
- Robert S Cantor
- Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
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4
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Castellano D, Wu K, Keramidas A, Lu W. Shisa7-Dependent Regulation of GABA A Receptor Single-Channel Gating Kinetics. J Neurosci 2022; 42:8758-8766. [PMID: 36216503 PMCID: PMC9698691 DOI: 10.1523/jneurosci.0510-22.2022] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/14/2022] [Revised: 08/19/2022] [Accepted: 09/27/2022] [Indexed: 12/29/2022] Open
Abstract
GABAA receptors (GABAARs) mediate the majority of fast inhibitory transmission throughout the brain. Although it is widely known that pore-forming subunits critically determine receptor function, it is unclear whether their single-channel properties are modulated by GABAAR-associated transmembrane proteins. We previously identified Shisa7 as a GABAAR auxiliary subunit that modulates the trafficking, pharmacology, and deactivation properties of these receptors. However, whether Shisa7 also regulates GABAAR single-channel properties has yet to be determined. Here, we performed single-channel recordings of α2β3γ2L GABAARs cotransfected with Shisa7 in HEK293T cells and found that while Shisa7 does not change channel slope conductance, it reduced the frequency of receptor openings. Importantly, Shisa7 modulates GABAAR gating by decreasing the duration and open probability within bursts. Through kinetic analysis of individual dwell time components, activation modeling, and macroscopic simulations, we demonstrate that Shisa7 accelerates GABAAR deactivation by governing the time spent between close and open states during gating. Together, our data provide a mechanistic basis for how Shisa7 controls GABAAR gating and reveal for the first time that GABAAR single-channel properties can be modulated by an auxiliary subunit. These findings shed light on processes that shape the temporal dynamics of GABAergic transmission.SIGNIFICANCE STATEMENT Although GABAA receptor (GABAAR) single-channel properties are largely determined by pore-forming subunits, it remains unknown whether they are also controlled by GABAAR-associated transmembrane proteins. Here, we show that Shisa7, a recently identified GABAAR auxiliary subunit, modulates GABAAR activation by altering single-channel burst kinetics. These results reveal that Shisa7 primarily decreases the duration and open probability of receptor burst activity during gating, leading to accelerated GABAAR deactivation. These experiments are the first to assess the gating properties of GABAARs in the presence of an auxiliary subunit and provides a kinetic basis for how Shisa7 modifies temporal attributes of GABAergic transmission at the single-channel level.
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Affiliation(s)
- David Castellano
- Synapse and Neural Circuit Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892
| | - Kunwei Wu
- Synapse and Neural Circuit Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892
| | - Angelo Keramidas
- Institute for Molecular Bioscience, The University of Queensland, Brisbane, Queensland 4072, Australia
| | - Wei Lu
- Synapse and Neural Circuit Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892
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Masoli S, Rizza MF, Tognolina M, Prestori F, D’Angelo E. Computational models of neurotransmission at cerebellar synapses unveil the impact on network computation. Front Comput Neurosci 2022; 16:1006989. [PMID: 36387305 PMCID: PMC9649760 DOI: 10.3389/fncom.2022.1006989] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/29/2022] [Accepted: 10/10/2022] [Indexed: 11/25/2022] Open
Abstract
The neuroscientific field benefits from the conjoint evolution of experimental and computational techniques, allowing for the reconstruction and simulation of complex models of neurons and synapses. Chemical synapses are characterized by presynaptic vesicle cycling, neurotransmitter diffusion, and postsynaptic receptor activation, which eventually lead to postsynaptic currents and subsequent membrane potential changes. These mechanisms have been accurately modeled for different synapses and receptor types (AMPA, NMDA, and GABA) of the cerebellar cortical network, allowing simulation of their impact on computation. Of special relevance is short-term synaptic plasticity, which generates spatiotemporal filtering in local microcircuits and controls burst transmission and information flow through the network. Here, we present how data-driven computational models recapitulate the properties of neurotransmission at cerebellar synapses. The simulation of microcircuit models is starting to reveal how diverse synaptic mechanisms shape the spatiotemporal profiles of circuit activity and computation.
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Affiliation(s)
- Stefano Masoli
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia, Italy
| | | | | | - Francesca Prestori
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia, Italy
- *Correspondence: Francesca Prestori,
| | - Egidio D’Angelo
- Department of Brain and Behavioral Sciences, University of Pavia, Pavia, Italy
- IRCCS Mondino Foundation, Brain Connectivity Center, Pavia, Italy
- Egidio D’Angelo,
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6
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Kaczor PT, Michałowski MA, Mozrzymas JW. α 1 Proline 277 Residues Regulate GABA AR Gating through M2-M3 Loop Interaction in the Interface Region. ACS Chem Neurosci 2022; 13:3044-3056. [PMID: 36219829 PMCID: PMC9634794 DOI: 10.1021/acschemneuro.2c00401] [Citation(s) in RCA: 6] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023] Open
Abstract
Cys-loop receptors are a superfamily of transmembrane, pentameric receptors that play a crucial role in mammalian CNS signaling. Physiological activation of these receptors is typically initiated by neurotransmitter binding to the orthosteric binding site, located at the extracellular domain (ECD), which leads to the opening of the channel pore (gate) at the transmembrane domain (TMD). Whereas considerable knowledge on molecular mechanisms of Cys-loop receptor activation was gathered for the acetylcholine receptor, little is known with this respect about the GABAA receptor (GABAAR), which mediates cellular inhibition. Importantly, several static structures of GABAAR were recently described, paving the way to more in-depth molecular functional studies. Moreover, it has been pointed out that the TMD-ECD interface region plays a crucial role in transduction of conformational changes from the ligand binding site to the channel gate. One of the interface structures implicated in this transduction process is the M2-M3 loop with a highly conserved proline (P277) residue. To address this issue specifically for α1β2γ2L GABAAR, we choose to substitute proline α1P277 with amino acids with different physicochemical features such as electrostatic charge or their ability to change the loop flexibility. To address the functional impact of these mutations, we performed macroscopic and single-channel patch-clamp analyses together with modeling. Our findings revealed that mutation of α1P277 weakly affected agonist binding but was critical for all transitions of GABAAR gating: opening/closing, preactivation, and desensitization. In conclusion, we provide evidence that conservative α1P277 at the interface is strongly involved in regulating the receptor gating.
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7
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Terejko K, Michałowski MA, Iżykowska I, Dominik A, Brzóstowicz A, Mozrzymas JW. Mutations at the M2 and M3 Transmembrane Helices of the GABA ARs α 1 and β 2 Subunits Affect Primarily Late Gating Transitions Including Opening/Closing and Desensitization. ACS Chem Neurosci 2021; 12:2421-2436. [PMID: 34101432 PMCID: PMC8291490 DOI: 10.1021/acschemneuro.1c00151] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023] Open
Abstract
![]()
GABA type A receptors
(GABAARs) belong to the pentameric
ligand-gated ion channel (pLGIC) family and play a crucial role in
mediating inhibition in the adult mammalian brain. Recently, a major
progress in determining the static structure of GABAARs
was achieved, although precise molecular scenarios underlying conformational
transitions remain unclear. The ligand binding sites (LBSs) are located
at the extracellular domain (ECD), very distant from the receptor
gate at the channel pore. GABAAR gating is complex, comprising
three major categories of transitions: openings/closings, preactivation,
and desensitization. Interestingly, mutations at, e.g., the ligand
binding site affect not only binding but often also more than one
gating category, suggesting that structural determinants for distinct
conformational transitions are shared. Gielen and co-workers (2015)
proposed that the GABAAR desensitization gate is located
at the second and third transmembrane segment. However, studies of
our and others’ groups indicated that other parts of the GABAAR macromolecule might be involved in this process. In the
present study, we asked how selected point mutations (β2G254V, α1G258V, α1L300V,
and β2L296V) at the M2 and M3 transmembrane segments
affect gating transitions of the α1β2γ2 GABAAR. Using high resolution macroscopic
and single-channel recordings and analysis, we report that these substitutions,
besides affecting desensitization, also profoundly altered openings/closings,
having some minor effect on preactivation and agonist binding. Thus,
the M2 and M3 segments primarily control late gating transitions of
the receptor (desensitization, opening/closing), providing a further
support for the concept of diffuse gating mechanisms for conformational
transitions of GABAAR.
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Affiliation(s)
- Katarzyna Terejko
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
| | - Michał A. Michałowski
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
- Department of Molecular Physiology and Neurobiology, University of Wrocław, ul. Sienkiewicza 21, 50-335 Wrocław, Poland
| | - Ilona Iżykowska
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
| | - Anna Dominik
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
| | - Aleksandra Brzóstowicz
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
| | - Jerzy W. Mozrzymas
- Department of Biophysics and Neuroscience, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368 Wrocław, Poland
- Department of Molecular Physiology and Neurobiology, University of Wrocław, ul. Sienkiewicza 21, 50-335 Wrocław, Poland
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Kaczor PT, Wolska AD, Mozrzymas JW. α 1 Subunit Histidine 55 at the Interface between Extracellular and Transmembrane Domains Affects Preactivation and Desensitization of the GABA A Receptor. ACS Chem Neurosci 2021; 12:562-572. [PMID: 33471498 PMCID: PMC7875458 DOI: 10.1021/acschemneuro.0c00781] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023] Open
Abstract
![]()
The
GABAA receptor is a member of the Cys-loop family
and plays a crucial role in the adult mammalian brain inhibition.
Although the static structure of this receptor is emerging, the molecular
mechanisms underlying its conformational transitions remain elusive.
It is known that in the Cys-loop receptors, the interface between
extracellular and transmembrane domains plays a key role in transmitting
the “activation wave” down to the channel gate in the
pore. It has been previously reported that histidine 55 (H55), located
centrally at the interfacial β1−β2 loop of the
α1 subunit, is important in the receptor activation,
but it is unknown which specific gating steps it is affecting. In
the present study, we addressed this issue by taking advantage of
the state-of-the-art macroscopic and single-channel recordings together
with extensive modeling. Considering that H55 is known to affect the
local electrostatic landscape and because it is neighbored by two
negatively charged aspartates, a well conserved feature in the α
subunits, we considered substitution with negative (E) and positive
(K) residues. We found that these mutations markedly affected the
receptor gating, altering primarily preactivation and desensitization
transitions. Importantly, opposite effects were observed for these
two mutations strongly suggesting involvement of electrostatic interactions.
Single-channel recordings suggested also a minor effect on opening/closing
transitions which did not depend on the electric charge of the substituting
amino acid. Altogether, we demonstrate that H55 mutations affect primarily
preactivation and desensitization most likely by influencing local
electrostatic interactions at the receptor interface.
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Affiliation(s)
- Przemyslaw T. Kaczor
- Department of Biophysics and Neuroscience, Wroclaw Medical University, Chalubinskiego 3a, Wroclaw, Dolnoślaskie 50-368, Poland
| | - Aleksandra D. Wolska
- Department of Biophysics and Neuroscience, Wroclaw Medical University, Chalubinskiego 3a, Wroclaw, Dolnoślaskie 50-368, Poland
| | - Jerzy W. Mozrzymas
- Department of Biophysics and Neuroscience, Wroclaw Medical University, Chalubinskiego 3a, Wroclaw, Dolnoślaskie 50-368, Poland
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Interaction between GABA A receptor α 1 and β 2 subunits at the N-terminal peripheral regions is crucial for receptor binding and gating. Biochem Pharmacol 2020; 183:114338. [PMID: 33189674 DOI: 10.1016/j.bcp.2020.114338] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2020] [Revised: 11/09/2020] [Accepted: 11/09/2020] [Indexed: 01/08/2023]
Abstract
Pentameric ligand gated ion channels (pLGICs) are crucial in electrochemical signaling but exact molecular mechanisms of their activation remain elusive. So far, major effort focused on the top-down molecular pathway between the ligand binding site and the channel gate. However, recent studies revealed that pLGIC activation is associated with coordinated subunit twisting in the membrane plane. This suggests a key role of intersubunit interactions but the underlying mechanisms remain largely unknown. Herein, we investigated a "peripheral" subunit interface region of GABAA receptor where structural modeling indicated interaction between N-terminal α1F14 and β2F31 residues. Our experiments underscored a crucial role of this interaction in ligand binding and gating, especially preactivation and opening, showing that the intersubunit cross-talk taking place outside (above) the top-down pathway can be strongly involved in receptor activation. Thus, described here intersubunit interaction appears to operate across a particularly long distance, affecting vast portions of the macromolecule.
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Absalom NL, Liao VW, Chebib M. Ligand-gated ion channels in genetic disorders and the question of efficacy. Int J Biochem Cell Biol 2020; 126:105806. [DOI: 10.1016/j.biocel.2020.105806] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2020] [Revised: 07/06/2020] [Accepted: 07/13/2020] [Indexed: 01/13/2023]
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Henry C, Cens T, Charnet P, Cohen-Solal C, Collet C, van-Dijk J, Guiramand J, de Jésus-Ferreira MC, Menard C, Mokrane N, Roussel J, Thibault JB, Vignes M, Rousset M. Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera. Br J Pharmacol 2020; 177:3924-3940. [PMID: 32436264 DOI: 10.1111/bph.15135] [Citation(s) in RCA: 13] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/31/2019] [Revised: 04/24/2020] [Accepted: 05/09/2020] [Indexed: 01/16/2023] Open
Abstract
BACKGROUND AND PURPOSE Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking. EXPERIMENTAL APPROACH We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques. KEY RESULTS The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels. CONCLUSION AND IMPLICATIONS For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.
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Affiliation(s)
| | - Thierry Cens
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | - Pierre Charnet
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | | | - Claude Collet
- UR 406 Abeilles et Environnement, INRAE, Avignon Cedex 9, France
| | | | | | | | - Claudine Menard
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | - Nawfel Mokrane
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | - Julien Roussel
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | | | - Michel Vignes
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
| | - Matthieu Rousset
- IBMM UMR5247, University of Montpellier, CNRS, Montpellier, France
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12
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Terejko K, Kaczor PT, Michałowski MA, Dąbrowska A, Mozrzymas JW. The C loop at the orthosteric binding site is critically involved in GABA A receptor gating. Neuropharmacology 2019; 166:107903. [PMID: 31972511 DOI: 10.1016/j.neuropharm.2019.107903] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/26/2019] [Revised: 11/15/2019] [Accepted: 12/02/2019] [Indexed: 02/02/2023]
Abstract
GABAA receptors (GABAARs) play a crucial role in mammalian adult brain inhibition. The dysfunction of GABAergic drive is related to such disorders as epilepsy, schizophrenia, and depression. Substantial progress has recently been made in describing the static structure of GABAARs, but the molecular mechanisms that underlie the activation process remain elusive. The C loop of the GABAAR structure shows the largest movement upon ligand binding to the orthosteric binding site, a phenomenon that is referred to as "capping." The C loop is known to be involved in agonist binding, but its role in the gating of Cys-loop receptors is still debated. Herein, we investigated this issue by analyzing the impact of a β2F200 residue mutation of the C loop on gating properties of α1β2γ2 GABAARs. Extensive analyses and the modeling of current responses to saturating agonist application demonstrated that this mutation strongly affected preactivation, opening, closing and desensitization, i.e. all considered gating steps. Single-channel analysis revealed that the β2F200 mutation slowed all shut time components, and open times were shortened. Model fitting of these single-channel data further confirmed that the β2F200 mutation strongly affected all of the gating characteristics. We also found that this mutation altered receptor sensitivity to the benzodiazepine flurazepam, which was attributable to a change in preactivation kinetics. In silico analysis indicated that the β2F200 mutation resulted in distortion of the C loop structure, causing the movement of its tip from the binding site. Altogether, we provide the first evidence that C loop critically controls GABAAR gating.
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Affiliation(s)
- Katarzyna Terejko
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368, Wrocław, Poland.
| | - Przemysław T Kaczor
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368, Wrocław, Poland
| | - Michał A Michałowski
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368, Wrocław, Poland; Department of Molecular Physiology and Neurobiology, University of Wrocław, ul. Sienkiewicza 21, 50-335, Wrocław, Poland
| | - Agnieszka Dąbrowska
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368, Wrocław, Poland
| | - Jerzy W Mozrzymas
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, ul. Chałubińskiego 3A, 50-368, Wrocław, Poland; Department of Molecular Physiology and Neurobiology, University of Wrocław, ul. Sienkiewicza 21, 50-335, Wrocław, Poland.
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13
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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: 26] [Impact Index Per Article: 4.3] [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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14
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Kisiel M, Jatczak-Śliwa M, Mozrzymas JW. Protons modulate gating of recombinant α 1β 2γ 2 GABA A receptor by affecting desensitization and opening transitions. Neuropharmacology 2018; 146:300-315. [PMID: 30326242 DOI: 10.1016/j.neuropharm.2018.10.016] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2018] [Revised: 09/28/2018] [Accepted: 10/12/2018] [Indexed: 01/27/2023]
Abstract
Protons are potent modulators of GABAA receptors (GABAARs) and α1Phe64 residue was implicated in their pH sensitivity. Recently, we have demonstrated that this residue is involved in flipping transitions which precede channel opening. We thus re-addressed the mechanism of GABAAR modulation by protons by considering the gating scheme extended by flipping. The impact of pH changes was examined on currents mediated by wild-type α1β2γ2 receptors or by their α1Phe64Leu or α1Phe64Cys mutants and elicited by saturating concentrations of full (GABA) or partial (piperidine-4-sulfonic acid) agonists. To describe the impact of extracellular pH on receptor gating, we combined macroscopic analysis of currents elicited by rapid agonist applications with single-channel studies. Acidification (pH 6.0) increased current amplitudes (in the case of leucine mutants effect was stronger when P4S was used) and decreased the rate and the extent of desensitization whereas alkalization (pH 8.0) had the opposite but weaker effect. Deactivation kinetics for wild-type receptors was slowed down by acidification while in the case of mutants this effect was observed upon alkalization. Moreover, α1Phe64 mutations enhanced GABAAR sensitivity to alkaline pH. Single-channel analysis revealed that acidification prolonged burst durations and affected shut but not open time distributions. Model simulations for macroscopic and single-channel activity indicated a novel mechanism in which protons primarily affected opening and desensitization rates but not flipping/unflipping. This evidence for the impact of protons on the receptor gating together with previously demonstrated effect on the agonist binding, point to a complex effect of extracellular pH on GABAAR macromolecule.
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Affiliation(s)
- Magdalena Kisiel
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland.
| | - Magdalena Jatczak-Śliwa
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland; Department of Molecular Physiology and Neurobiology, Wrocław University, Wrocław 50-335, Poland
| | - Jerzy W Mozrzymas
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland.
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15
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Understanding Anesthetic Mechanisms: Analysis of the Complex Kinetics of Ligand-Gated Ion Channels. Methods Enzymol 2018. [PMID: 29588043 DOI: 10.1016/bs.mie.2018.02.002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register]
Abstract
Anesthetics modulate the response of ligand-gated ion channels to their neurotransmitter agonists, in a way that is consistent with clinical anesthesia: inhibition of synaptic transmission, by activation of inhibitory receptors and/or inhibition of excitatory receptors. Electrophysiological results for receptors such as GABAAR indicate that this modulation can be remarkably kinetically complex, characterized by concentration-dependent changes in the extent and (multiple) time scales of desensitization and deactivation. The full range of these features cannot be reproduced by a kinetic model in which anesthetic acts only by binding to putative protein sites, without having multiple sites with varying affinities, as well as many additional conformational states beyond the canonical set of three (resting, open, and desensitized). So, we discuss the implementation of a kinetic approach that incorporates only these three states, but accounts for effects of adsorption of anesthetic and agonist to the membrane in which the receptor is embedded, which modulates the conformational free energy landscape of the protein. As a result, the rate constants of conformational transitions become time dependent (non-Markovian), requiring nonstandard methods of kinetic analysis that can readily be implemented using available computational software.
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16
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Cantor RS. Path to the Desensitized State of Ligand-Gated Ion Channels: Why Are Inhibitory and Excitatory Receptors Different? J Phys Chem B 2018; 122:5368-5374. [DOI: 10.1021/acs.jpcb.7b10961] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Affiliation(s)
- Robert S. Cantor
- Dept. of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, United States
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17
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Kisiel M, Jatczak M, Brodzki M, Mozrzymas JW. Spontaneous activity, singly bound states and the impact of alpha 1Phe64 mutation on GABA AR gating in the novel kinetic model based on the single-channel recordings. Neuropharmacology 2017; 131:453-474. [PMID: 29162430 DOI: 10.1016/j.neuropharm.2017.11.030] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2017] [Revised: 10/25/2017] [Accepted: 11/17/2017] [Indexed: 12/31/2022]
Abstract
GABAA receptor is the primary mediator of inhibition in the adult mammalian brain. Our recent studies revealed that a classic gating scheme for GABAAR needed to be updated with an intermediate step (flipping) and that the α1Phe64 mutation at the GABA binding site affects this transition. However, description of flipping at the single-channel level remains incomplete. In particular, its role in singly-bound and spontaneous activity remains unknown. We have performed thus single-channel recordings over wide range of agonist concentration for wild-type α1β2γ2L receptors and α1Phe64 mutants. For WT receptors we observed relatively frequent brief spontaneous openings which were also present at low [GABA]. However, closed times distributions for spontaneous activity and at low [GABA] were clearly different indicating that a proportion of short-lived openings were due to liganded, most likely singly bound receptors. Increasing [GABA] resulted in prolongation of bursts and increased occurrence of bursts with long openings and short closures. Mutations of α1Phe64 residue dramatically affected the open and closed time distributions at high and saturating [GABA], especially in the case of cysteine mutants. However, this mutation weakly affected spontaneous or singly bound activity. Model fitting of our single-channel data led us to propose a novel and, to our knowledge, most complete GABAAR kinetic model in which flipping occurs in singly and doubly bound states. However, spontaneous activity did not reveal involvement of flipping. Moreover, we report that α1Phe64 mutation affects not only the flipping but also the opening/closing transitions indicating its generalized impact on the receptor gating.
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Affiliation(s)
- Magdalena Kisiel
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland.
| | - Magdalena Jatczak
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland; Department of Physiology and Molecular Neurobiology, Wrocław University, Wrocław 50-335, Poland
| | - Marek Brodzki
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland; Department of Physiology and Molecular Neurobiology, Wrocław University, Wrocław 50-335, Poland
| | - Jerzy W Mozrzymas
- Laboratory of Neuroscience, Department of Biophysics, Wrocław Medical University, Wrocław 50-368, Poland.
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18
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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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19
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Atif M, Estrada-Mondragon A, Nguyen B, Lynch JW, Keramidas A. Effects of glutamate and ivermectin on single glutamate-gated chloride channels of the parasitic nematode H. contortus. PLoS Pathog 2017; 13:e1006663. [PMID: 28968469 PMCID: PMC5638611 DOI: 10.1371/journal.ppat.1006663] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2017] [Revised: 10/12/2017] [Accepted: 09/22/2017] [Indexed: 12/12/2022] Open
Abstract
Ivermectin (IVM) is a widely-used anthelmintic that works by binding to and activating glutamate-gated chloride channel receptors (GluClRs) in nematodes. The resulting chloride flux inhibits the pharyngeal muscle cells and motor neurons of nematodes, causing death by paralysis or starvation. IVM resistance is an emerging problem in many pest species, necessitating the development of novel drugs. However, drug optimisation requires a quantitative understanding of GluClR activation and modulation mechanisms. Here we investigated the biophysical properties of homomeric α (avr-14b) GluClRs from the parasitic nematode, H. contortus, in the presence of glutamate and IVM. The receptor proved to be highly responsive to low nanomolar concentrations of both compounds. Analysis of single receptor activations demonstrated that the GluClR oscillates between multiple functional states upon the binding of either ligand. The G36’A mutation in the third transmembrane domain, which was previously thought to hinder access of IVM to its binding site, was found to decrease the duration of active periods and increase receptor desensitisation. On an ensemble macropatch level the mutation gave rise to enhanced current decay and desensitisation rates. Because these responses were common to both glutamate and IVM, and were observed under conditions where agonist binding sites were likely saturated, we infer that G36’A affects the intrinsic properties of the receptor with no specific effect on IVM binding mechanisms. These unexpected results provide new insights into the activation and modulatory mechanisms of the H. contortus GluClRs and provide a mechanistic framework upon which the actions of drugs can be reliably interpreted. IVM is a gold standard anti-parasitic drug that is used extensively to control invertebrate parasites pest species. The drug targets the glutamate-gated chloride channel receptor (GluClR) found on neurons and muscle cells of these organisms, causing paralysis and death. However, IVM resistance is becoming a serious problem in human and animal health, as well as human food production. We provide the first comprehensive investigation of the functional properties of the GluClR of H. contortus, which is a major parasite in grazing animals, such as sheep and goats. We compared glutamate and IVM induced activity of the wild-type and a mutant GluClR, G36’A, that markedly reduces IVM sensitivity in wild populations of pests. Our data demonstrate that the mutation reduces IVM sensitivity by altering the functional properties of the GluClR rather than specifically affecting the binding of IVM, even though the mutation occurs at the IVM binding site. This study provides a mechanistic framework upon which the actions of new candidate anthelmintic drugs can be interpreted.
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Affiliation(s)
- Mohammed Atif
- Queensland Brain Institute, The University of Queensland, Brisbane, Australia
| | | | - Bindi Nguyen
- Queensland Brain Institute, The University of Queensland, Brisbane, Australia
| | - Joseph W. Lynch
- Queensland Brain Institute, The University of Queensland, Brisbane, Australia
- School of Biomedical Sciences, The University of Queensland, Brisbane, Australia
- * E-mail: (AK); (JL)
| | - Angelo Keramidas
- Queensland Brain Institute, The University of Queensland, Brisbane, Australia
- * E-mail: (AK); (JL)
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20
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Shin DJ, Germann AL, Steinbach JH, Akk G. The Actions of Drug Combinations on the GABA A Receptor Manifest as Curvilinear Isoboles of Additivity. Mol Pharmacol 2017; 92:556-563. [PMID: 28790148 DOI: 10.1124/mol.117.109595] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/2017] [Accepted: 08/07/2017] [Indexed: 02/04/2023] Open
Abstract
Drug interactions are often analyzed in terms of isobolograms. In the isobologram, the line connecting the axial points corresponding to the concentrations of two different drugs that produce an effect of the same magnitude is termed an isobole of additivity. Although the isobole of additivity can be a straight line in some special cases, previous work has proposed that it is curvilinear when the two drugs differ in their maximal effects or Hill slopes. Modulators of transmitter-gated ion channels have a wide range of maximal effects as well as Hill slopes, suggesting that the isoboles for drug actions on ion channel function are not linear. In this study, we have conducted an analysis of direct activation and potentiation of the human α1β2γ2L GABAA receptor to demonstrate that: 1) curvilinear isoboles of additivity are predicted by a concerted transition model where the binding of each GABAergic drug additively and independently reduces the free energy of the open receptor compared with the closed receptor; and 2) experimental data for receptor activation using the agonist pair of GABA and propofol or potentiation of responses to a low concentration of GABA by the drug pair of alfaxalone and propofol agree very well with predictions. The approach assuming independent energetic contributions from GABAergic drugs enables, at least for the drug combinations tested, a straightforward method to accurately predict functional responses to any combination of concentrations.
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Affiliation(s)
- Daniel J Shin
- Department of Anesthesiology (D.J.S., A.L.G., J.H.S., G.A.), and the Taylor Family Institute for Innovative Psychiatric Research (J.H.S., G.A.), Washington University School of Medicine in St. Louis, St. Louis, Missouri
| | - Allison L Germann
- Department of Anesthesiology (D.J.S., A.L.G., J.H.S., G.A.), and the Taylor Family Institute for Innovative Psychiatric Research (J.H.S., G.A.), Washington University School of Medicine in St. Louis, St. Louis, Missouri
| | - Joe Henry Steinbach
- Department of Anesthesiology (D.J.S., A.L.G., J.H.S., G.A.), and the Taylor Family Institute for Innovative Psychiatric Research (J.H.S., G.A.), Washington University School of Medicine in St. Louis, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology (D.J.S., A.L.G., J.H.S., G.A.), and the Taylor Family Institute for Innovative Psychiatric Research (J.H.S., G.A.), Washington University School of Medicine in St. Louis, St. Louis, Missouri
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21
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Geng Y, Magleby KL. Modal gating of endplate acetylcholine receptors: A proposed mechanism. ACTA ACUST UNITED AC 2017; 146:435-9. [PMID: 26621772 PMCID: PMC4664830 DOI: 10.1085/jgp.201511534] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
Affiliation(s)
- Yanyan Geng
- Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136
| | - Karl L Magleby
- Department of Physiology and Biophysics, University of Miami Miller School of Medicine, Miami, FL 33136
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22
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Desai R, Savechenkov PY, Zolkowska D, Ge RL, Rogawski MA, Bruzik KS, Forman SA, Raines DE, Miller KW. Contrasting actions of a convulsant barbiturate and its anticonvulsant enantiomer on the α1 β3 γ2L GABAA receptor account for their in vivo effects. J Physiol 2016; 593:4943-61. [PMID: 26378885 DOI: 10.1113/jp270971] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2015] [Accepted: 09/11/2015] [Indexed: 11/08/2022] Open
Abstract
KEY POINTS Most barbiturates are anaesthetics but unexpectedly a few are convulsants whose mechanism of action is poorly understood. We synthesized and characterized a novel pair of chiral barbiturates that are capable of photolabelling their binding sites on GABAA receptors. In mice the S-enantiomer is a convulsant, but the R-enantiomer is an anticonvulsant. The convulsant S-enantiomer binds solely at an inhibitory site. It is both an open state inhibitor and a resting state inhibitor. Its action is pH independent, suggesting the pyrimidine ring plays little part in binding. The inhibitory site is not enantioselective because the R-enantiomer inhibits with equal affinity. In contrast, only the anticonvulsant R-enantiomer binds to the enhancing site on open channels, causing them to stay open longer. The enhancing site is enantioselective. The in vivo actions of the convulsant S-enantiomer are accounted for by its interactions with GABAA receptors. ABSTRACT Most barbiturates are anaesthetics but a few unexpectedly are convulsants. We recently located the anaesthetic sites on GABAA receptors (GABAA Rs) by photolabelling with an anaesthetic barbiturate. To apply the same strategy to locate the convulsant sites requires the creation and mechanistic characterization of a suitable agent. We synthesized enantiomers of a novel, photoactivable barbiturate, 1-methyl-5-propyly-5-(m-trifluoromethyldiazirinyl) phenyl barbituric acid (mTFD-MPPB). In mice, S-mTFD-MPPB acted as a convulsant, whereas R-mTFD-MPPB acted as an anticonvulsant. Using patch clamp electrophysiology and fast solution exchange on recombinant human α1 β3 γ2L GABAA Rs expressed in HEK cells, we found that S-mTFD-MPPB inhibited GABA-induced currents, whereas R-mTFD-MPPB enhanced them. S-mTFD-MPPB caused inhibition by binding to either of two inhibitory sites on open channels with bimolecular kinetics. It also inhibited closed, resting state receptors at similar concentrations, decreasing the channel opening rate and shifting the GABA concentration-response curve to the right. R-mTFD-MPPB, like most anaesthetics, enhanced receptor gating by rapidly binding to allosteric sites on open channels, initiating a rate-limiting conformation change to stabilized open channel states. These states had slower closing rates, thus shifting the GABA concentration-response curve to the left. Under conditions when most GABAA Rs were open, an inhibitory action of R-mTFD-MPPB was revealed that had a similar IC50 to that of S-mTFD-MPPB. Thus, the inhibitory sites are not enantioselective, and the convulsant action of S-mTFD-MPPB results from its negligible affinity for the enhancing, anaesthetic sites. Interactions with these two classes of barbiturate binding sites on GABAA Rs underlie the enantiomers' different pharmacological activities in mice.
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Affiliation(s)
- Rooma Desai
- Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA, 02114, USA
| | - Pavel Y Savechenkov
- Deparment of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, Chicago, IL, 60612, USA
| | - Dorota Zolkowska
- Department of Neurology, School of Medicine, University of California, Davis, Sacramento, CA, 95817, USA
| | - Ri Le Ge
- Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA, 02114, USA
| | - Michael A Rogawski
- Department of Neurology, School of Medicine, University of California, Davis, Sacramento, CA, 95817, USA
| | - Karol S Bruzik
- Deparment of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, Chicago, IL, 60612, USA
| | - Stuart A Forman
- Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA, 02114, USA
| | - Douglas E Raines
- Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA, 02114, USA
| | - Keith W Miller
- Department of Anaesthesia, Critical Care and Pain Medicine, Massachusetts General Hospital, Boston, MA, 02114, USA.,Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA, 02115, USA
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23
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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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24
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Chakrabarti S, Qian M, Krishnan K, Covey DF, Mennerick S, Akk G. Comparison of Steroid Modulation of Spontaneous Inhibitory Postsynaptic Currents in Cultured Hippocampal Neurons and Steady-State Single-Channel Currents from Heterologously Expressed α1β2γ2L GABA(A) Receptors. Mol Pharmacol 2016; 89:399-406. [PMID: 26769414 DOI: 10.1124/mol.115.102202] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/23/2015] [Accepted: 01/13/2016] [Indexed: 12/18/2022] Open
Abstract
Neuroactive steroids are efficacious modulators of γ-aminobutyric acid type A receptor (GABA(A)) receptor function. The effects of steroids on the GABA(A) receptor are typically determined by comparing steady-state single-channel open probability or macroscopic peak responses elicited by GABA in the absence and presence of a steroid. Due to differences in activation conditions (exposure duration, concentration of agonist), it is not obvious whether modulation measured using typical experimental protocols can be used to accurately predict the effect of a modulator on native receptors under physiologic conditions. In the present study, we examined the effects of 14 neuroactive steroids and analogs on the properties of spontaneous inhibitory postsynaptic currents (sIPSCs) in cultured rat hippocampal neurons. The goal was to determine whether the magnitude of modulation of the decay time course of sIPSCs correlates with the extent of modulation and kinetic properties of potentiation as determined in previous single-channel studies. The steroids were selected to cover a wide range of efficacy on heterologously expressed rat α1β2γ2L GABA(A) receptors, ranging from essentially inert to highly efficacious (strong potentiators of single-channel and macroscopic peak responses). The data indicate a strong correlation between prolongation of the decay time course of sIPSCs and potentiation of single-channel open probability. Furthermore, changes in intracluster closed time distributions were the single best predictor of prolongation of sIPSCs. We infer that the information obtained in steady-state single-channel recordings can be used to forecast modulation of synaptic currents.
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Affiliation(s)
- Sampurna Chakrabarti
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Mingxing Qian
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Kathiresan Krishnan
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Douglas F Covey
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Steven Mennerick
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
| | - Gustav Akk
- Department of Anesthesiology (S.C., G.A.), Department of Developmental Biology (M.Q., K.K., D.F.C.), and Department of Psychiatry (S.M.), and the Taylor Family Institute for Innovative Psychiatric Research (D.F.C., S.M., G.A.), Washington University School of Medicine, St. Louis, Missouri
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25
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Lee DK, Albershardt DJ, Cantor RS. Exploring the mechanism of general anesthesia: kinetic analysis of GABAA receptor electrophysiology. Biophys J 2016; 108:1081-93. [PMID: 25762320 DOI: 10.1016/j.bpj.2014.12.052] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2014] [Revised: 12/10/2014] [Accepted: 12/29/2014] [Indexed: 10/23/2022] Open
Abstract
A kinetic model of the effect of agonist and anesthetics on ligand-gated ion channels, developed in earlier work, is further refined and used to predict traces observed in fast-perfusion electrophysiological studies on recombinant GABAA receptors under a wide range of agonist and/or anesthetic concentrations. The model incorporates only three conformational states (resting, open, and desensitized) but allows for the modulation of the conformational free energy landscape connecting these states resulting from adsorption of agonist and/or anesthetic to the bilayer in which the protein is embedded. The model is shown to reproduce the diverse and complex features of experimental traces remarkably well, including both anesthetic-induced and agonist-induced traces, as well as the modulation of agonist-induced traces by anesthetic, either coapplied or continuously present. The solutions to the kinetic equations, which give the time-dependence of each of the nine protein states (three ligation states for each of the three conformations), describe the flow of probability among these states and thus reveal the kinetic underpinnings of the traces. Many of the parameters in the model, such as the desorption rate constants of anesthetic and agonist, are directly related to model-independent experimental measurements and thus can serve as a definitive test of its validity.
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Affiliation(s)
- Daniel K Lee
- Department of Chemistry, Dartmouth College, Hanover, New Hampshire
| | | | - Robert S Cantor
- Department of Chemistry, Dartmouth College, Hanover, New Hampshire; MEMPHYS Center for Biomembrane Physics, University of Southern Denmark, Odense, Denmark.
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26
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Dixon CL, Harrison NL, Lynch JW, Keramidas A. Zolpidem and eszopiclone prime α1β2γ2 GABAA receptors for longer duration of activity. Br J Pharmacol 2015; 172:3522-36. [PMID: 25817320 DOI: 10.1111/bph.13142] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2014] [Revised: 02/23/2015] [Accepted: 03/22/2015] [Indexed: 01/24/2023] Open
Abstract
BACKGROUND AND PURPOSE GABAA receptors mediate neuronal inhibition in the brain. They are the primary targets for benzodiazepines, which are widely used to treat neurological disorders including anxiety, epilepsy and insomnia. The mechanism by which benzodiazepines enhance GABAA receptor activity has been extensively studied, but there is little mechanistic information on how non-benzodiazepine drugs that bind to the same site exert their effects. Eszopiclone and zolpidem are two non-benzodiazepine drugs for which no mechanism of action has yet been proposed, despite their clinical importance as sleeping aids. Here we investigate how both drugs enhance the activity of α1β2γ2 GABAA receptors. EXPERIMENTAL APPROACH We used rapid ligand application onto macropatches and single-channel kinetic analysis to assess rates of current deactivation. We also studied synaptic currents in primary neuronal cultures and in heterosynapses, whereby native GABAergic nerve terminals form synapses with HEK293 cells expressing α1β2γ2 GABAA receptors. Drug binding and modulation was quantified with the aid of an activation mechanism. KEY RESULTS At the single-channel level, the drugs prolonged the duration of receptor activation, with similar KD values of ∼80 nM. Channel activation was prolonged primarily by increasing the equilibrium constant between two connected shut states that precede channel opening. CONCLUSIONS AND IMPLICATIONS As the derived mechanism successfully simulated the effects of eszopiclone and zolpidem on ensemble currents, we propose it as the definitive mechanism accounting for the effects of both drugs. Importantly, eszopiclone and zolpidem enhanced GABAA receptor currents via a mechanism that differs from that proposed for benzodiazepines.
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Affiliation(s)
- Christine L Dixon
- Queensland Brain Institute, University of Queensland, Brisbane, Qld, Australia
| | - Neil L Harrison
- Department of Anesthesiology and Department of Pharmacology, Columbia University, New York, NY, USA
| | - Joseph W Lynch
- Queensland Brain Institute, University of Queensland, Brisbane, Qld, Australia.,School of Biomedical Sciences, University of Queensland, Brisbane, Qld, Australia
| | - Angelo Keramidas
- Queensland Brain Institute, University of Queensland, Brisbane, Qld, Australia.,Department of Anesthesiology and Department of Pharmacology, Columbia University, New York, NY, USA
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27
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Stepanyuk A, Borisyuk A, Belan P. Maximum likelihood estimation of biophysical parameters of synaptic receptors from macroscopic currents. Front Cell Neurosci 2014; 8:303. [PMID: 25324721 PMCID: PMC4183100 DOI: 10.3389/fncel.2014.00303] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/30/2014] [Accepted: 09/09/2014] [Indexed: 11/13/2022] Open
Abstract
Dendritic integration and neuronal firing patterns strongly depend on biophysical properties of synaptic ligand-gated channels. However, precise estimation of biophysical parameters of these channels in their intrinsic environment is complicated and still unresolved problem. Here we describe a novel method based on a maximum likelihood approach that allows to estimate not only the unitary current of synaptic receptor channels but also their multiple conductance levels, kinetic constants, the number of receptors bound with a neurotransmitter, and the peak open probability from experimentally feasible number of postsynaptic currents. The new method also improves the accuracy of evaluation of unitary current as compared to the peak-scaled non-stationary fluctuation analysis, leading to a possibility to precisely estimate this important parameter from a few postsynaptic currents recorded in steady-state conditions. Estimation of unitary current with this method is robust even if postsynaptic currents are generated by receptors having different kinetic parameters, the case when peak-scaled non-stationary fluctuation analysis is not applicable. Thus, with the new method, routinely recorded postsynaptic currents could be used to study the properties of synaptic receptors in their native biochemical environment.
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Affiliation(s)
- Andrey Stepanyuk
- Laboratory of Molecular Biophysics, Bogomoletz Institute of Physiology Kiev, Ukraine ; State Key Laboratory of Molecular and Cellular Biology, Bogomoletz Institute of Physiology Kiev, Ukraine
| | - Anya Borisyuk
- Laboratory of Molecular Biophysics, Bogomoletz Institute of Physiology Kiev, Ukraine ; State Key Laboratory of Molecular and Cellular Biology, Bogomoletz Institute of Physiology Kiev, Ukraine
| | - Pavel Belan
- Laboratory of Molecular Biophysics, Bogomoletz Institute of Physiology Kiev, Ukraine ; State Key Laboratory of Molecular and Cellular Biology, Bogomoletz Institute of Physiology Kiev, Ukraine
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28
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α1F64 Residue at GABA(A) receptor binding site is involved in gating by influencing the receptor flipping transitions. J Neurosci 2014; 34:3193-209. [PMID: 24573278 DOI: 10.1523/jneurosci.2533-13.2014] [Citation(s) in RCA: 30] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
GABA receptors (GABAARs) mediate inhibition in the adult brain. These channels are heteropentamers and their ligand binding sites are localized at the β+ / α- interfaces. As expected, mutations of binding-site residues affect binding kinetics but accumulating evidence indicates that gating is also altered, although the underlying mechanisms are unclear. We investigated the impact of the hydrophobic box residue localized at α1(-), F64 (α1F64), on the binding and gating of rat recombinant α1β1γ2 receptors. The analysis of current responses to rapid agonist applications confirmed a marked effect of α1F64 mutations on agonist binding and revealed surprisingly strong effects on gating, including the disappearance of rapid desensitization, the slowing of current onset, and accelerated deactivation. Moreover, nonstationary variance analysis revealed that the α1F64C mutation dramatically reduced the maximum open probability without altering channel conductance. Interestingly, for wild-type receptors, responses to saturating concentration of a partial agonist, P4S, showed no rapid desensitization, similar to GABA-evoked responses mediated by α1F64C mutants. For the α1F64L mutation, the application of the high-affinity agonist muscimol partially rescued rapid desensitization compared with responses evoked by GABA. These findings suggest that α1F64 mutations do not disrupt desensitization mechanisms but rather affect other gating features that obscure it. Model simulations indicated that all of our observations related to α1F64 mutations could be properly reproduced by altering the flipped state transitions that occurred after agonist binding but preceded opening. In conclusion, we propose that the α1F64 residue may participate in linking binding and gating by influencing flipping kinetics.
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29
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Dixon C, Sah P, Lynch JW, Keramidas A. GABAA receptor α and γ subunits shape synaptic currents via different mechanisms. J Biol Chem 2014; 289:5399-411. [PMID: 24425869 DOI: 10.1074/jbc.m113.514695] [Citation(s) in RCA: 64] [Impact Index Per Article: 6.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Synaptic GABAA receptors (GABAARs) mediate most of the inhibitory neurotransmission in the brain. The majority of these receptors are comprised of α1, β2, and γ2 subunits. The amygdala, a structure involved in processing emotional stimuli, expresses α2 and γ1 subunits at high levels. The effect of these subunits on GABAAR-mediated synaptic transmission is not known. Understanding the influence of these subunits on GABAAR-mediated synaptic currents may help in identifying the roles and locations of amygdala synapses that contain these subunits. Here, we describe the biophysical and synaptic properties of pure populations of α1β2γ2, α2β2γ2, α1β2γ1 and α2β2γ1 GABAARs. Their synaptic properties were examined in engineered synapses, whereas their kinetic properties were studied using rapid agonist application, and single channel recordings. All macropatch currents activated rapidly (<1 ms) and deactivated as a function of the α-subunit, with α2-containing GABAARs consistently deactivating ∼10-fold more slowly. Single channel analysis revealed that the slower current decay of α2-containing GABAARs was due to longer burst durations at low GABA concentrations, corresponding to a ∼4-fold higher affinity for GABA. Synaptic currents revealed a different pattern of activation and deactivation to that of macropatch data. The inclusion of α2 and γ1 subunits slowed both the activation and deactivation rates, suggesting that receptors containing these subunits cluster more diffusely at synapses. Switching the intracellular domains of the γ2 and γ1 subunits substantiated this inference. Because this region determines post-synaptic localization, we hypothesize that GABAARs containing γ1 and γ2 use different mechanisms for synaptic clustering.
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Affiliation(s)
- Christine Dixon
- From the Queensland Brain Institute, University of Queensland, Brisbane, Queensland, Australia 4072
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30
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Co-expression of γ2 subunits hinders processing of N-linked glycans attached to the N104 glycosylation sites of GABAA receptor β2 subunits. Neurochem Res 2013; 39:1088-103. [PMID: 24213971 DOI: 10.1007/s11064-013-1187-9] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2013] [Revised: 10/18/2013] [Accepted: 10/22/2013] [Indexed: 10/26/2022]
Abstract
GABAA receptors, the major mediators of fast inhibitory neuronal transmission, are heteropentameric glycoproteins assembled from a panel of subunits, usually including α and β subunits with or without a γ2 subunit. The α1β2γ2 receptor is the most abundant GABAA receptor in brain. Co-expression of γ2 with α1 and β2 subunits causes conformational changes, increases GABAA receptor channel conductance, and prolongs channel open times. We reported previously that glycosylation of the three β2 subunit glycosylation sites, N32, N104 and N173, was important for α1β2 receptor channel gating. Here, we examined the hypothesis that steric effects or conformational changes caused by γ2 subunit co-expression alter the glycosylation of partnering β2 subunits. We found that co-expression of γ2 subunits hindered processing of β2 subunit N104 N-glycans in HEK293T cells. This γ2 subunit-dependent effect was strong enough that a decrease of γ2 subunit expression in heterozygous GABRG2 knockout (γ2(+/-)) mice led to appreciable changes in the endoglycosidase H digestion pattern of neuronal β2 subunits. Interestingly, as measured by flow cytometry, γ2 subunit surface levels were decreased by mutating each of the β2 subunit glycosylation sites. The β2 subunit mutation N104Q also decreased GABA potency to evoke macroscopic currents and reduced conductance, mean open time and open probability of single channel currents. Collectively, our data suggested that γ2 subunits interacted with β2 subunit N-glycans and/or subdomains containing the glycosylation sites, and that γ2 subunit co-expression-dependent alterations in the processing of the β2 subunit N104 N-glycans were involved in altering the function of surface GABAA receptors.
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31
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Lewis R, Feetham CH, Gentles L, Penny J, Tregilgas L, Tohami W, Mobasheri A, Barrett-Jolley R. Benzamil sensitive ion channels contribute to volume regulation in canine chondrocytes. Br J Pharmacol 2013; 168:1584-96. [PMID: 22928819 PMCID: PMC3605868 DOI: 10.1111/j.1476-5381.2012.02185.x] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/07/2011] [Revised: 07/20/2012] [Accepted: 07/29/2012] [Indexed: 12/02/2022] Open
Abstract
BACKGROUND AND PURPOSE Chondrocytes exist within cartilage and serve to maintain the extracellular matrix. It has been postulated that osteoarthritic (OA) chondrocytes lose the ability to regulate their volume, affecting extracellular matrix production. In previous studies, we identified expression of epithelial sodium channels (ENaC) in human chondrocytes, but their function remained unknown. Although ENaC typically has Na(+) transport roles, it is also involved in the cell volume regulation of rat hepatocytes. ENaC is a member of the degenerin (Deg) family, and ENaC/Deg-like channels have a low conductance and high sensitivity to benzamil. In this study, we investigated whether canine chondrocytes express functional ENaC/Deg-like ion channels and, if so, what their function may be. EXPERIMENTAL APPROACH Canine chondrocytes were harvested from dogs killed for unassociated welfare reasons. We used immunohistochemistry and patch-clamp electrophysiology to investigate ENaC expression and video microscopy to analyse the effects of pharmacological inhibition of ENaC/Deg on cell volume regulation. KEY RESULTS Immunofluorescence showed that canine chondrocytes expressed ENaC protein. Single-channel recordings demonstrated expression of a benzamil-sensitive Na(+) conductance (9 pS), and whole-cell experiments show this to be approximately 1.5 nS per cell with high selectivity for Na(+) . Benzamil hyperpolarized chondrocytes by approximately 8 mV with a pD2 8.4. Chondrocyte regulatory volume decrease (RVI) was inhibited by benzamil (pD2 7.5) but persisted when extracellular Na(+) ions were replaced by Li(+) . CONCLUSION AND IMPLICATIONS Our data suggest that benzamil inhibits RVI by reducing the influx of Na(+) ions through ENaC/Deg-like ion channels and present ENaC/Deg as a possible target for pharmacological modulation of chondrocyte volume.
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Affiliation(s)
- R Lewis
- Musculoskeletal Biology, CIMA, Faculty of Health & Life Sciences, University of Liverpool, Liverpool, UK
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32
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Keramidas A, Lynch JW. An outline of desensitization in pentameric ligand-gated ion channel receptors. Cell Mol Life Sci 2013; 70:1241-53. [PMID: 22936353 PMCID: PMC11113241 DOI: 10.1007/s00018-012-1133-z] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/27/2012] [Revised: 07/28/2012] [Accepted: 08/13/2012] [Indexed: 10/27/2022]
Abstract
Pentameric ligand-gated ion channel (pLGIC) receptors exhibit desensitization, the progressive reduction in ionic flux in the prolonged presence of agonist. Despite its pathophysiological importance and the fact that it was first described over half a century ago, surprisingly little is known about the structural basis of desensitization in this receptor family. Here, we explain how desensitization is defined using functional criteria. We then review recent progress into reconciling the structural and functional basis of this phenomenon. The extracellular-transmembrane domain interface is a key locus. Activation is well known to involve conformational changes at this interface, and several lines of evidence suggest that desensitization involves a distinct conformational change here that is incompatible with activation. However, major questions remain unresolved, including the structural basis of the desensitization-induced agonist affinity increase and the mechanism of pore closure during desensitization.
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Affiliation(s)
- Angelo Keramidas
- Queensland Brain Institute, The University of Queensland, Brisbane, QLD 4072 Australia
| | - Joseph W. Lynch
- Queensland Brain Institute, The University of Queensland, Brisbane, QLD 4072 Australia
- School of Biomedical Sciences, The University of Queensland, Brisbane, QLD 4072 Australia
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33
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Abstract
Volatile anesthetics serve as useful probes of a conserved biological process that is essential to the proper functioning of the central nervous system. A kinetic and thermodynamic analysis of their unusual pharmacological and physiological characteristics has led to a general, predictive theory in which small molecules that adsorb to membranes modulate ion channel function by altering physical properties of membrane bilayers. A kinetic model that is both parsimonious and falsifiable has been developed to test this mechanism. This theory leads to predictions about the structure, function, origin, and evolution of synapses, the etiology of several diseases and disease symptoms affecting the brain, and the mechanism of action of several drugs that are used therapeutically. Neuronal membranes may offer an appealing drug target, given the large number of compounds that adsorb to interfaces and hence membranes.
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Affiliation(s)
- James M Sonner
- Department of Anesthesia and Perioperative Care, University of California, San Francisco, California 94143-0464, USA.
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34
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Benzodiazepines modulate GABAA receptors by regulating the preactivation step after GABA binding. J Neurosci 2012; 32:5707-15. [PMID: 22539833 DOI: 10.1523/jneurosci.5663-11.2012] [Citation(s) in RCA: 83] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
GABA(A) receptors (GABA(A)Rs) composed of αβγ subunits are allosterically modulated by the benzodiazepines (BDZs). Agonists at the BDZ binding site potentiate submaximal GABA responses by increasing the apparent affinity of GABA(A)Rs for GABA. Although BDZs were initially thought to affect the binding of GABA agonists, recent studies suggest an effect on receptor gating; however, the involvement of preactivation steps in the modulation by BDZs has not been considered. Consequently, we examined whether BDZ agonists could exert their modulatory effect by displacing the equilibrium between resting and preactivated states of recombinant α1β2γ2 GABA(A)Rs expressed in Xenopus oocytes. For GABA and the partial agonists 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol and piperidine-4-sulfonic acid, we examined BDZ modulation using a simple three-step model incorporating agonist binding, receptor preactivation, and channel opening. The model accounted for diazepam modulation simply by increasing the preactivation constant by approximately fourfold. To assess whether BDZs preferentially affected a specific GABA binding site, pentameric concatamers were used. This demonstrated that single GABA-binding site mutant receptors were equally sensitive to modulation by BDZs compared with wild-type counterparts. Overall, our results suggest that BDZs affect the preactivation step to cause a global conformational rearrangement of GABA(A)Rs, thereby modulating receptor function.
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35
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Stepanyuk AR, Borisyuk AL, Belan PV. Efficient maximum likelihood estimation of kinetic rate constants from macroscopic currents. PLoS One 2012; 6:e29731. [PMID: 22242142 PMCID: PMC3248447 DOI: 10.1371/journal.pone.0029731] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2011] [Accepted: 12/03/2011] [Indexed: 01/24/2023] Open
Abstract
A new method is described that accurately estimates kinetic constants, conductance and number of ion channels from macroscopic currents. The method uses both the time course and the strength of correlations between different time points of macroscopic currents and utilizes the property of semiseparability of covariance matrix for computationally efficient estimation of current likelihood and its gradient. The number of calculation steps scales linearly with the number of channel states as opposed to the cubic dependence in a previously described method. Together with the likelihood gradient evaluation, which is almost independent of the number of model parameters, the new approach allows evaluation of kinetic models with very complex topologies. We demonstrate applicability of the method to analysis of synaptic currents by estimating accurately rate constants of a 7-state model used to simulate GABAergic macroscopic currents.
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Affiliation(s)
- Andrey R. Stepanyuk
- Bogomoletz Institute of Physiology, Kiev, Ukraine
- State Key Laboratory of Molecular and Cellular Biology, Kiev, Ukraine
| | - Anya L. Borisyuk
- Bogomoletz Institute of Physiology, Kiev, Ukraine
- State Key Laboratory of Molecular and Cellular Biology, Kiev, Ukraine
| | - Pavel V. Belan
- Bogomoletz Institute of Physiology, Kiev, Ukraine
- State Key Laboratory of Molecular and Cellular Biology, Kiev, Ukraine
- * E-mail:
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36
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Lachance-Touchette P, Brown P, Meloche C, Kinirons P, Lapointe L, Lacasse H, Lortie A, Carmant L, Bedford F, Bowie D, Cossette P. Novel α1 and γ2 GABAA receptor subunit mutations in families with idiopathic generalized epilepsy. Eur J Neurosci 2011; 34:237-49. [PMID: 21714819 DOI: 10.1111/j.1460-9568.2011.07767.x] [Citation(s) in RCA: 90] [Impact Index Per Article: 6.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
Abstract
Epilepsy is a heterogeneous neurological disease affecting approximately 50 million people worldwide. Genetic factors play an important role in both the onset and severity of the condition, with mutations in several ion-channel genes being implicated, including those encoding the GABA(A) receptor. Here, we evaluated the frequency of additional mutations in the GABA(A) receptor by direct sequencing of the complete open reading frame of the GABRA1 and GABRG2 genes from a cohort of French Canadian families with idiopathic generalized epilepsy (IGE). Using this approach, we have identified three novel mutations that were absent in over 400 control chromosomes. In GABRA1, two mutations were found, with the first being a 25-bp insertion that was associated with intron retention (i.e. K353delins18X) and the second corresponding to a single point mutation that replaced the aspartate 219 residue with an asparagine (i.e. D219N). Electrophysiological analysis revealed that K353delins18X and D219N altered GABA(A) receptor function by reducing the total surface expression of mature protein and/or by curtailing neurotransmitter effectiveness. Both defects would be expected to have a detrimental effect on inhibitory control of neuronal circuits. In contrast, the single point mutation identified in the GABRG2 gene, namely P83S, was indistinguishable from the wildtype subunit in terms of surface expression and functionality. This finding was all the more intriguing as the mutation exhibited a high degree of penetrance in three generations of one French Canadian family. Further experimentation will be required to understand how this mutation contributes to the occurrence of IGE in these individuals.
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Affiliation(s)
- Pamela Lachance-Touchette
- Centre for Excellence in Neuromics of University of Montreal, CHUM Research Center, 1560 Sherbrooke est, Montreal, QC, Canada H2L 4M1
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37
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Abstract
To reach the open state, the GABA(A) receptor (GABA(A)R) is assumed to bind two agonist molecules. Although it is currently believed that GABA(A)R could also operate in the monoliganded state, the gating properties of singly bound GABA(A)R are poorly understood and their physiological role is still obscure. In the present study, we characterize for the first time the gating properties of singly bound GABA(A)Rs by using a mutagenesis approach and we propose that monoliganded GABA(A)R contribute in shaping synaptic responses. At saturating GABA concentrations, currents mediated by recombinant GABA(A)Rs with a single functional binding site display slow onset, fast deactivation kinetics, and slow rate of desensitization-resensitization. GABA(A)Rs with two binding sites activated by brief pulses of subsaturating GABA concentrations (in the range of the GABA concentration profile in the synaptic cleft) could also mediate fast deactivating currents, displaying deactivation kinetics similar to those mediated by GABA(A)Rs with a single functional binding site. Model simulations of receptors activated by realistic synaptic GABA waves revealed that a considerable proportion of GABA(A) receptors open in the monoliganded state during synaptic transmission, therefore contributing in shaping IPSCs.
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38
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Poon K, Nowak LM, Oswald RE. Characterizing single-channel behavior of GluA3 receptors. Biophys J 2010; 99:1437-46. [PMID: 20816055 DOI: 10.1016/j.bpj.2010.06.058] [Citation(s) in RCA: 32] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/12/2010] [Revised: 06/14/2010] [Accepted: 06/29/2010] [Indexed: 10/19/2022] Open
Abstract
AMPA receptors play a major role in excitatory neurotransmission in the CNS and are involved in numerous neurological disorders. Agonists bind to each of four bilobed LBDs of this tetrameric receptor, and upon binding, the lobes close to envelope the agonist, leading to channel activation. However, AMPA receptors exhibit complex activation kinetics, the mechanism of which has not yet been determined. We report here single-channel studies of a homomeric AMPA receptor (GluA3) activated by the full agonist, glutamate, and a partial agonist, fluorowillardiine. Both agonists activate the channel to the same three open conductance levels but with different open probabilities in each level. The closed probability (P(c)) varied within records, particularly at low agonist concentrations. By sorting discrete segments of the record according to P(c) using the X-means algorithm, we defined five modes of activity. The kinetic behavior could then be analyzed for both agonists over a range of agonist concentrations with a relatively simple model (three closed states and two open states for each open conductance level). The structural mechanism underlying the modal behavior is not clear; however, it occurs on a timescale consistent with hydrogen bonding across the lobe interface in the LBD.
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Affiliation(s)
- Kinning Poon
- Department of Molecular Medicine, Cornell University, Ithaca, New York, USA
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39
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Amico-Ruvio SA, Popescu GK. Stationary gating of GluN1/GluN2B receptors in intact membrane patches. Biophys J 2010; 98:1160-9. [PMID: 20371315 DOI: 10.1016/j.bpj.2009.12.4276] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2009] [Revised: 11/19/2009] [Accepted: 12/01/2009] [Indexed: 11/30/2022] Open
Abstract
NMDA receptors are heteromeric glutamate-gated channels composed of GluN1 and GluN2 subunits. Receptor isoforms that differ in their GluN2-subunit type (A-D) are expressed differentially throughout the central nervous system and have distinct kinetic properties in recombinant systems. How specific receptor isoforms contribute to the functions generally attributed to NMDA receptors remains unknown, due in part to the incomplete functional characterization of individual receptor types and unclear molecular composition of native receptors. We examined the stationary gating kinetics of individual rat recombinant GluN1/GluN2B receptors in cell-attached patches of transiently transfected HEK293 cells and used kinetic analyses and modeling to describe the full range of this receptor's gating behaviors. We found that, like GluN1/GluN2A receptors, GluN1/GluN2B receptors have three gating modes that are distinguishable by their mean open durations. However, for GluN1/GluN2B receptors, the modes also differed markedly in their mean closed durations and thus generated a broader range of open probabilities. We also found that regardless of gating mode, glutamate dissociation occurred approximately 4-fold more slowly (k(-) = 15 s(-1)) compared to that observed in GluN1/GluN2A receptors. On the basis of these results, we suggest that slow glutamate dissociation and modal gating underlie the long heterogeneous activations of GluN1/GluN2B receptors.
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Affiliation(s)
- Stacy A Amico-Ruvio
- Department of Biochemistry, State University of New York at Buffalo, Buffalo, New York, USA
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40
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Lo WY, Lagrange AH, Hernandez CC, Harrison R, Dell A, Haslam SM, Sheehan JH, Macdonald RL. Glycosylation of {beta}2 subunits regulates GABAA receptor biogenesis and channel gating. J Biol Chem 2010; 285:31348-61. [PMID: 20639197 DOI: 10.1074/jbc.m110.151449] [Citation(s) in RCA: 35] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
γ-aminobutyric acid type A (GABA(A)) receptors are heteropentameric glycoproteins. Based on consensus sequences, the GABA(A) receptor β2 subunit contains three potential N-linked glycosylation sites, Asn-32, Asn-104, and Asn-173. Homology modeling indicates that Asn-32 and Asn-104 are located before the α1 helix and in loop L3, respectively, near the top of the subunit-subunit interface on the minus side, and that Asn-173 is located in the Cys-loop near the bottom of the subunit N-terminal domain. Using site-directed mutagenesis, we demonstrated that all predicted β2 subunit glycosylation sites were glycosylated in transfected HEK293T cells. Glycosylation of each site, however, produced specific changes in α1β2 receptor surface expression and function. Although glycosylation of Asn-173 in the Cys-loop was important for stability of β2 subunits when expressed alone, results obtained with flow cytometry, brefeldin A treatment, and endo-β-N-acetylglucosaminidase H digestion suggested that glycosylation of Asn-104 was required for efficient α1β2 receptor assembly and/or stability in the endoplasmic reticulum. Patch clamp recording revealed that mutation of each site to prevent glycosylation decreased peak α1β2 receptor current amplitudes and altered the gating properties of α1β2 receptor channels by reducing mean open time due to a reduction in the proportion of long open states. In addition to functional heterogeneity, endo-β-N-acetylglucosaminidase H digestion and glycomic profiling revealed that surface β2 subunit N-glycans at Asn-173 were high mannose forms that were different from those of Asn-32 and N104. Using a homology model of the pentameric extracellular domain of α1β2 channel, we propose mechanisms for regulation of GABA(A) receptors by glycosylation.
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Affiliation(s)
- Wen-Yi Lo
- Department of Neurology, Vanderbilt University, Nashville, Tennessee 37232, USA
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Keramidas A, Harrison NL. The activation mechanism of alpha1beta2gamma2S and alpha3beta3gamma2S GABAA receptors. ACTA ACUST UNITED AC 2010; 135:59-75. [PMID: 20038526 PMCID: PMC2806416 DOI: 10.1085/jgp.200910317] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
The α1β2γ2 and α3β3γ2 are two isoforms of γ-aminobutyric acid type A (GABAA) receptor that are widely distributed in the brain. Both are found at synapses, for example in the thalamus, where they mediate distinctly different inhibitory postsynaptic current profiles, particularly with respect to decay time. The two isoforms were expressed in HEK293 cells, and single-channel activity was recorded from outside-out patches. The kinetic characteristics of both isoforms were investigated by analyzing single-channel currents over a wide range of GABA concentrations. α1β2γ2 channels exhibited briefer active periods than α3β3γ2 channels over the entire range of agonist concentrations and had lower intraburst open probabilities at subsaturating concentrations. Activation mechanisms were constructed by fitting postulated schemes to data recorded at saturating and subsaturating GABA concentrations simultaneously. Reaction mechanisms were ranked according to log-likelihood values and how accurately they simulated ensemble currents. The highest ranked mechanism for both channels consisted of two sequential binding steps, followed by three conducting and three nonconducting configurations. The equilibrium dissociation constant for GABA at α3β3γ2 channels was ∼2.6 µM compared with ∼19 µM for α1β2γ2 channels, suggesting that GABA binds to the α3β3γ2 channels with higher affinity. A notable feature of the mechanism was that two consecutive doubly liganded shut states preceded all three open configurations. The lifetime of the third shut state was briefer for the α3β3γ2 channels. The longer active periods, higher affinity, and preference for conducting states are consistent with the slower decay of inhibitory currents at synapses that contain α3β3γ2 channels. The reaction mechanism we describe here may also be appropriate for the analysis of other types of GABAA receptors and provides a framework for rational investigation of the kinetic effects of a variety of therapeutic agents that activate or modulate GABAA receptors and hence influence synaptic and extrasynaptic inhibition in the central nervous system.
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Affiliation(s)
- Angelo Keramidas
- Department of Anesthesiology, Columbia University, New York, NY 10032, USA.
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O'Shea SM, Williams CA, Jenkins A. Inverse effects on gating and modulation caused by a mutation in the M2-M3 Linker of the GABA(A) receptor gamma subunit. Mol Pharmacol 2009; 76:641-51. [PMID: 19553237 DOI: 10.1124/mol.109.055111] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
M2-M3 linkers are receptor subunit domains known to be critical for the normal function of cysteine-loop ligand-gated ion channels. Previous studies of alpha and beta subunits of type "A" GABA receptors suggest that these linkers couple extracellular elements involved in GABA binding to the transmembrane segments that control the opening of the ion channel. To study the importance of the gamma subunit M2-M3 linker, we examined the macroscopic and single-channel effects of an engineered gamma2(L287A) mutation on GABA activation and propofol modulation. In the macroscopic analysis, we found that the gamma2(L287A) mutation decreased GABA potency but increased the ability of propofol to enhance both GABA potency and efficacy compared with wild-type receptors. Indeed, although propofol had significant effects on GABA potency in wild-type receptors, we found that propofol produced no corresponding increase in GABA efficacy. At the single-channel level, mutant receptors showed a loss in the longest of three open-time components compared with wild-type receptors under GABA activation. Furthermore, propofol reduced the duration of one closed-time component, increased the duration of two open-time components, and generated a third open component with a longer lifetime in mutant compared with wild-type receptors. Taken together, we conclude that although the gamma subunit is not required for the binding of GABA or propofol, the M2-M3 linker of this subunit plays a critical role in channel gating by GABA and allosteric modulation by propofol. Our results also suggest that in wild-type receptors, propofol exerts its enhancing effects by mechanisms extrinsic to channel gating.
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Affiliation(s)
- Sean M O'Shea
- Department of Anesthesiology, Emory University University School of Medicine, Atlanta, Georgia, USA.
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43
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Number and locations of agonist binding sites required to activate homomeric Cys-loop receptors. J Neurosci 2009; 29:6022-32. [PMID: 19420269 DOI: 10.1523/jneurosci.0627-09.2009] [Citation(s) in RCA: 95] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022] Open
Abstract
Homo-pentameric Cys-loop receptors contain five identical agonist binding sites, each formed at a subunit interface. To determine the number and locations of binding sites required to generate a stable active state, we constructed a receptor subunit with a mutation that disables the agonist binding site and a reporter mutation that alters unitary conductance and coexpressed mutant and nonmutant subunits. Although receptors with a range of different subunit compositions are produced, patch-clamp recordings reveal that the amplitude of each single-channel opening event reports the number and, for certain subunit combinations, the locations of subunits with intact binding sites. We find that receptors with three binding sites at nonconsecutive subunit interfaces exhibit maximal mean channel open time, receptors with binding sites at three consecutive or two nonconsecutive interfaces exhibit intermediate open time, and receptors with binding sites at two consecutive or one interface exhibit brief open time. Macroscopic recordings after rapid application of agonist reveal that channel activation slows and the extent of desensitization decreases as the number of binding sites per receptor decreases. The overall results provide a framework for defining mechanisms of activation and drug modulation for homo-pentameric Cys-loop receptors.
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Feng HJ, Botzolakis EJ, Macdonald RL. Context-dependent modulation of alphabetagamma and alphabetadelta GABA A receptors by penicillin: implications for phasic and tonic inhibition. Neuropharmacology 2009; 56:161-73. [PMID: 18775733 PMCID: PMC2661208 DOI: 10.1016/j.neuropharm.2008.08.010] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2007] [Revised: 08/11/2008] [Accepted: 08/12/2008] [Indexed: 01/25/2023]
Abstract
Penicillin, an open-channel blocker of GABA(A) receptors, was recently reported to inhibit phasic, but not tonic, currents in hippocampal neurons. To distinguish between isoform-specific and context-dependent modulation as possible explanations for this selectivity, the effects of penicillin were evaluated on recombinant GABA(A) receptors expressed in HEK293T cells. When co-applied with saturating GABA, penicillin decreased peak amplitude, induced rebound, and prolonged deactivation of currents evoked from both synaptic and extrasynaptic receptor isoforms. However, penicillin had isoform-specific effects on the extent of desensitization, reflecting its ability to differentially modulate peak (non-equilibrium) and residual (near-equilibrium) currents. This suggested that the context of activation could determine the apparent sensitivity of a given receptor isoform to penicillin. To test this hypothesis, we explored the ability of penicillin to modulate synaptic and extrasynaptic isoform currents that were activated under more physiologically relevant conditions. Interestingly, while currents evoked from synaptic isoforms under phasic conditions (transient activation by a saturating concentration of GABA) were substantially inhibited by penicillin, currents evoked from extrasynaptic isoforms under tonic conditions (prolonged application by a sub-saturating concentration of GABA) were minimally affected. We therefore concluded that the reported inability of penicillin to modulate tonic currents could not simply be attributed to insensitivity of extrasynaptic receptors, but rather, reflected an inability to modulate these receptors in their native context of activation.
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Affiliation(s)
- Hua-Jun Feng
- Department of Neurology, Vanderbilt University School of Medicine, Nashville, TN 37232
| | | | - Robert L. Macdonald
- Department of Neurology, Vanderbilt University School of Medicine, Nashville, TN 37232
- Department of Molecular Physiology and Biophysics, Vanderbilt University School of Medicine, Nashville, TN 37232
- Department of Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37232
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Mercado J, Czajkowski C. Gamma-aminobutyric acid (GABA) and pentobarbital induce different conformational rearrangements in the GABA A receptor alpha1 and beta2 pre-M1 regions. J Biol Chem 2008; 283:15250-7. [PMID: 18387955 DOI: 10.1074/jbc.m708638200] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
Gamma-aminobutyric acid (GABA) binding to GABA(A) receptors (GABA(A)Rs) triggers conformational movements in the alpha(1) and beta(2) pre-M1 regions that are associated with channel gating. At high concentrations, the barbiturate pentobarbital opens GABA(A)R channels with similar conductances as GABA, suggesting that their open state structures are alike. Little, however, is known about the structural rearrangements induced by barbiturates. Here, we examined whether pentobarbital activation triggers movements in the GABA(A)R pre-M1 regions. Alpha(1)beta(2) GABA(A)Rs containing cysteine substitutions in the pre-M1 alpha(1) (K219C, K221C) and beta(2) (K213C, K215C) subunits were expressed in Xenopus oocytes and analyzed using two-electrode voltage clamp. The cysteine substitutions had little to no effect on GABA and pentobarbital EC(50) values. Tethering chemically diverse thiol-reactive methanethiosulfonate reagents onto alpha(1)K219C and alpha(1)K221C affected GABA- and pentobarbital-activated currents differently, suggesting that the pre-M1 structural elements important for GABA and pentobarbital current activation are distinct. Moreover, pentobarbital altered the rates of cysteine modification by methanethiosulfonate reagents differently than GABA. For alpha(1)K221Cbeta(2) receptors, pentobarbital decreased the rate of cysteine modification whereas GABA had no effect. For alpha(1)beta(2)K215C receptors, pentobarbital had no effect whereas GABA increased the modification rate. The competitive GABA antagonist SR-95531 and a low, non-activating concentration of pentobarbital did not alter their modification rates, suggesting that the GABA- and pentobarbital-mediated changes in rates reflect gating movements. Overall, the data indicate that the pre-M1 region is involved in both GABA- and pentobarbital-mediated gating transitions. Pentobarbital, however, triggers different movements in this region than GABA, suggesting their activation mechanisms differ.
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Affiliation(s)
- Jose Mercado
- Department of Physiology, University of Wisconsin-Madison, 601 Science Drive, Madison, WI 53706, USA
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Keramidas A, Harrison NL. Agonist-dependent single channel current and gating in alpha4beta2delta and alpha1beta2gamma2S GABAA receptors. ACTA ACUST UNITED AC 2008; 131:163-81. [PMID: 18227274 PMCID: PMC2213567 DOI: 10.1085/jgp.200709871] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The family of γ-aminobutyric acid type A receptors (GABAARs) mediates two types of inhibition in the mammalian brain. Phasic inhibition is mediated by synaptic GABAARs that are mainly comprised of α1, β2, and γ2 subunits, whereas tonic inhibition is mediated by extrasynaptic GABAARs comprised of α4/6, β2, and δ subunits. We investigated the activation properties of recombinant α4β2δ and α1β2γ2S GABAARs in response to GABA and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3(2H)-one (THIP) using electrophysiological recordings from outside-out membrane patches. Rapid agonist application experiments indicated that THIP produced faster opening rates at α4β2δ GABAARs (β ∼1600 s−1) than at α1β2γ2S GABAARs (β ∼ 460 s−1), whereas GABA activated α1β2γ2S GABAARs more rapidly (β ∼1800 s−1) than α4β2δ GABAARs (β < 440 s−1). Single channel recordings of α1β2γ2S and α4β2δ GABAARs showed that both channels open to a main conductance state of ∼25 pS at −70 mV when activated by GABA and low concentrations of THIP, whereas saturating concentrations of THIP elicited ∼36 pS openings at both channels. Saturating concentrations of GABA elicited brief (<10 ms) openings with low intraburst open probability (PO ∼ 0.3) at α4β2δ GABAARs and at least two “modes” of single channel bursting activity, lasting ∼100 ms at α1β2γ2S GABAARs. The most prevalent bursting mode had a PO of ∼0.7 and was described by a reaction scheme with three open and three shut states, whereas the “high” PO mode (∼0.9) was characterized by two shut and three open states. Single channel activity elicited by THIP in α4β2δ and α1β2γ2S GABAARs occurred as a single population of bursts (PO ∼0.4–0.5) of moderate duration (∼33 ms) that could be described by schemes containing two shut and two open states for both GABAARs. Our data identify kinetic properties that are receptor-subtype specific and others that are agonist specific, including unitary conductance.
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Affiliation(s)
- Angelo Keramidas
- CV Starr Laboratory for Molecular Neuropharmacology, Department of Anesthesiology, Weill Medical College of Cornell University, NY, NY 10021, USA.
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Steinbach JH. A slip ‘twixt the cup and the lip: a new way to impair function of transmitter-gated channels. J Physiol 2007; 581:3. [PMID: 17379629 PMCID: PMC2075217 DOI: 10.1113/jphysiol.2007.131367] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Affiliation(s)
- J H Steinbach
- Department of Anaesthesiology, Washington University, Campus Box 8054, 660 South Euclid Ave., St Louis, MO 63110, USA.
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Mokrab Y, Bavro VN, Mizuguchi K, Todorov NP, Martin IL, Dunn SMJ, Chan SL, Chau PL. Exploring ligand recognition and ion flow in comparative models of the human GABA type A receptor. J Mol Graph Model 2007; 26:760-74. [PMID: 17544304 DOI: 10.1016/j.jmgm.2007.04.012] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/24/2007] [Accepted: 04/29/2007] [Indexed: 11/25/2022]
Abstract
We present two comparative models of the GABA(A) receptor. Model 1 is based on the 4-A resolution structure of the nicotinic acetylcholine receptor from Torpedo marmorata and represents the unliganded receptor. Two agonists, GABA and muscimol, two benzodiazepines, flunitrazepam and alprazolam, together with the general anaesthetic halothane, have been docked to this model. The ion flow is also explored in model 1 by evaluating the interaction energy of a chloride ion as it traverses the extracellular, transmembrane and intracellular domains of the protein. Model 2 differs from model 1 only in the extracellular domain and represents the liganded receptor. Comparison between the two models not only allows us to explore commonalities and differences with comparative models of the nicotinic acetylcholine receptor, but also suggests possible protein sub-domain interactions with the GABA(A) receptor not previously addressed.
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Affiliation(s)
- Younes Mokrab
- Department of Biochemistry, University of Cambridge, Cambridge CB2 1GA, United Kingdom
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Farrant M, Kaila K. The cellular, molecular and ionic basis of GABA(A) receptor signalling. PROGRESS IN BRAIN RESEARCH 2007; 160:59-87. [PMID: 17499109 DOI: 10.1016/s0079-6123(06)60005-8] [Citation(s) in RCA: 266] [Impact Index Per Article: 15.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
Abstract
GABA(A) receptors mediate fast synaptic inhibition in the CNS. Whilst this is undoubtedly true, it is a gross oversimplification of their actions. The receptors themselves are diverse, being formed from a variety of subunits, each with a different temporal and spatial pattern of expression. This diversity is reflected in differences in subcellular targetting and in the subtleties of their response to GABA. While activation of the receptors leads to an inevitable increase in membrane conductance, the voltage response is dictated by the distribution of the permeant Cl(-) and HCO(3)(-) ions, which is established by anion transporters. Similar to GABA(A) receptors, the expression of these transporters is not only developmentally regulated but shows cell-specific and subcellular variation. Untangling all these complexities allows us to appreciate the variety of GABA-mediated signalling, a diverse set of phenomena encompassing both synaptic and non-synaptic functions that can be overtly excitatory as well as inhibitory.
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Affiliation(s)
- Mark Farrant
- Department of Pharmacology, UCL (University College London), Gower Street, London WC1E 6BT, UK.
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