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Mapping General Anesthetic Sites in Heteromeric ?-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes.

ABSTRACT: IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing ?-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity also show remarkably selective interactions with distinct interfacial sites. These results suggest strategies for developing new drugs that selectively modulate distinct GABAA receptor subtypes.

SUBMITTER: Forman SA

PROVIDER: S-EPMC5073028 | biostudies-literature | 2016 Nov

REPOSITORIES: biostudies-literature

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Mapping General Anesthetic Sites in Heteromeric γ-Aminobutyric Acid Type A Receptors Reveals a Potential For Targeting Receptor Subtypes.

Forman Stuart A SA Miller Keith W KW

Anesthesia and analgesia 20161101 5

IV general anesthetics, including propofol, etomidate, alphaxalone, and barbiturates, produce important actions by enhancing γ-aminobutyric acid type A (GABAA) receptor activation. In this article, we review scientific studies that have located and mapped IV anesthetic sites using photoaffinity labeling and substituted cysteine modification protection. These anesthetics bind in transmembrane pockets between subunits of typical synaptic GABAA receptors, and drugs that display stereoselectivity al ...[more]

PMID: 27167687

Similar Datasets

General Anesthetic Binding Sites in Human α4β3δ γ-Aminobutyric Acid Type A Receptors (GABAARs).

Project description:Extrasynaptic γ-aminobutyric acid type A receptors (GABAARs),which contribute generalized inhibitory tone to the mammalian brain, are major targets for general anesthetics. To identify anesthetic binding sites in an extrasynaptic GABAAR, we photolabeled human α4β3δ GABAARs purified in detergent with [3H]azietomidate and a barbiturate, [3H]R-mTFD-MPAB, photoreactive anesthetics that bind with high selectivity to distinct but homologous intersubunit binding sites in the transmembrane domain of synaptic α1β3γ2 GABAARs. Based upon 3H incorporation into receptor subunits resolved by SDS-PAGE, there was etomidate-inhibitable labeling by [3H]azietomidate in the α4 and β3 subunits and barbiturate-inhibitable labeling by [3H]R-mTFD-MPAB in the β3 subunit. These sites did not bind the anesthetic steroid alphaxalone, which enhanced photolabeling, or DS-2, a δ subunit-selective positive allosteric modulator, which neither enhanced nor inhibited photolabeling. The amino acids labeled by [3H]azietomidate or [3H]R-mTFD-MPAB were identified by N-terminal sequencing of fragments isolated by HPLC fractionation of enzymatically digested subunits. No evidence was found for a δ subunit contribution to an anesthetic binding site. [3H]azietomidate photolabeling of β3Met-286 in βM3 and α4Met-269 in αM1 that was inhibited by etomidate but not by R-mTFD-MPAB established that etomidate binds to a site at the β3+-α4- interface equivalent to its site in α1β3γ2 GABAARs. [3H]Azietomidate and [3H]R-mTFD-MPAB photolabeling of β3Met-227 in βM1 established that these anesthetics also bind to a homologous site, most likely at the β3+-β3- interface, which suggests a subunit arrangement of β3α4β3δβ3.

| S-EPMC5159512 | biostudies-literature

Mapping general anesthetic binding site(s) in human ?1?3 ?-aminobutyric acid type A receptors with [³H]TDBzl-etomidate, a photoreactive etomidate analogue.

Project description:The ?-aminobutyric acid type A receptor (GABA(A)R) is a target for general anesthetics of diverse chemical structures, which act as positive allosteric modulators at clinical doses. Previously, in a heterogeneous mixture of GABA(A)Rs purified from bovine brain, [³H]azietomidate photolabeling of ?Met-236 and ?Met-286 in the ?M1 and ?M3 transmembrane helices identified an etomidate binding site in the GABA(A)R transmembrane domain at the interface between the ? and ? subunits [Li, G. D., et.al. (2006) J. Neurosci. 26, 11599-11605]. To further define GABA(A)R etomidate binding sites, we now use [³H]TDBzl-etomidate, an aryl diazirine with broader amino acid side chain reactivity than azietomidate, to photolabel purified human FLAG-?1?3 GABA(A)Rs and more extensively identify photolabeled GABA(A)R amino acids. [³H]TDBzl-etomidate photolabeled in an etomidate-inhibitable manner ?3Val-290, in the ?3M3 transmembrane helix, as well as ?1Met-236 in ?1M1, a residue photolabeled by [³H]azietomidate, while no photolabeling of amino acids in the ?M2 and ?M2 helices that also border the etomidate binding site was detected. The location of these photolabeled amino acids in GABA(A)R homology models derived from the recently determined structures of prokaryote (GLIC) or invertebrate (GluCl) homologues and the results of computational docking studies predict the orientation of [³H]TDBzl-etomidate bound in that site and the other amino acids contributing to this GABA(A)R intersubunit etomidate binding site. Etomidate-inhibitable photolabeling of ?3Met-227 in ?M1 by [³H]TDBzl-etomidate and [³H]azietomidate also provides evidence of a homologous etomidate binding site at the ?3-?3 subunit interface in the ?1?3 GABA(A)R.

| S-EPMC3274767 | biostudies-literature

Molecular mapping of general anesthetic sites in a voltage-gated ion channel.

Project description:Several voltage-gated ion channels are modulated by clinically relevant doses of general anesthetics. However, the structural basis of this modulation is not well understood. Previous work suggested that n-alcohols and inhaled anesthetics stabilize the closed state of the Shaw2 voltage-gated (Kv) channel (K-Shaw2) by directly interacting with a discrete channel site. We hypothesize that the inhibition of K-Shaw2 channels by general anesthetics is governed by interactions between binding and effector sites involving components of the channel's activation gate. To investigate this hypothesis, we applied Ala/Val scanning mutagenesis to the S4-S5 linker and the post-PVP S6 segment, and conducted electrophysiological analysis to evaluate the energetic impact of the mutations on the inhibition of the K-Shaw2 channel by 1-butanol and halothane. These analyses identified residues that determine an apparent binding cooperativity and residue pairs that act in concert to modulate gating upon anesthetic binding. In some instances, due to their critical location, key residues also influence channel gating. Complementing these results, molecular dynamics simulations and in silico docking experiments helped us visualize possible anesthetic sites and interactions. We conclude that the inhibition of K-Shaw2 by general anesthetics results from allosteric interactions between distinct but contiguous binding and effector sites involving inter- and intrasubunit interfaces.

| S-EPMC3183804 | biostudies-other

Specificity of intersubunit general anesthetic-binding sites in the transmembrane domain of the human ?1?3?2 ?-aminobutyric acid type A (GABAA) receptor.

Project description:GABA type A receptors (GABAAR), the brain's major inhibitory neurotransmitter receptors, are the targets for many general anesthetics, including volatile anesthetics, etomidate, propofol, and barbiturates. How such structurally diverse agents can act similarly as positive allosteric modulators of GABAARs remains unclear. Previously, photoreactive etomidate analogs identified two equivalent anesthetic-binding sites in the transmembrane domain at the ?(+)-?(-) subunit interfaces, which also contain the GABA-binding sites in the extracellular domain. Here, we used R-[(3)H]5-allyl-1-methyl-5-(m-trifluoromethyl-diazirynylphenyl) barbituric acid (R-mTFD-MPAB), a potent stereospecific barbiturate anesthetic, to photolabel expressed human ?1?3?2 GABAARs. Protein microsequencing revealed that R-[(3)H]mTFD-MPAB did not photolabel the etomidate sites at the ?(+)-?(-) subunit interfaces. Instead, it photolabeled sites at the ?(+)-?(-) and ?(+)-?(-) subunit interfaces in the transmembrane domain. On the (+)-side, ?1M3 was labeled at Ala-291 and Tyr-294 and ?2M3 at Ser-301, and on the (-)-side, ?3M1 was labeled at Met-227. These residues, like those in the etomidate site, are located at subunit interfaces near the synaptic side of the transmembrane domain. The selectivity of R-etomidate for the ?(+)-?(-) interface relative to the ?(+)-?(-)/?(+)-?(-) interfaces was >100-fold, whereas that of R-mTFD-MPAB for its sites was >50-fold. Each ligand could enhance photoincorporation of the other, demonstrating allosteric interactions between the sites. The structural heterogeneity of barbiturate, etomidate, and propofol derivatives is accommodated by varying selectivities for these two classes of sites. We hypothesize that binding at any of these homologous intersubunit sites is sufficient for anesthetic action and that this explains to some degree the puzzling structural heterogeneity of anesthetics.

| S-EPMC3707639 | biostudies-literature

Neurosteroids allosterically modulate binding of the anesthetic etomidate to gamma-aminobutyric acid type A receptors.

Project description:Photoaffinity labeling of gamma-aminobutyric acid type A (GABA(A))-receptors (GABA(A)R) with an etomidate analog and mutational analyses of direct activation of GABA(A)R by neurosteroids have each led to the proposal that these structurally distinct general anesthetics bind to sites in GABA(A)Rs in the transmembrane domain at the interface between the beta and alpha subunits. We tested whether the two ligand binding sites might overlap by examining whether neuroactive steroids inhibited etomidate analog photolabeling. We previously identified (Li, G. D., Chiara, D. C., Sawyer, G. W., Husain, S. S., Olsen, R. W., and Cohen, J. B. (2006) J. Neurosci. 26, 11599-11605) azietomidate photolabeling of GABA(A)R alpha1Met-236 and betaMet-286 (in alphaM1 and betaM3). Positioning these two photolabeled amino acids in a single type of binding site at the interface of beta and alpha subunits (two copies per pentamer) is consistent with a GABA(A)R homology model based upon the structure of the nicotinic acetylcholine receptor and with recent alphaM1 to betaM3 cross-linking data. Biologically active neurosteroids enhance rather than inhibit azietomidate photolabeling, as assayed at the level of GABA(A)R subunits on analytical SDS-PAGE, and protein microsequencing establishes that the GABA(A)R-modulating neurosteroids do not inhibit photolabeling of GABA(A)R alpha1Met-236 or betaMet-286 but enhance labeling of alpha1Met-236. Thus modulatory steroids do not bind at the same site as etomidate, and neither of the amino acids identified as neurosteroid activation determinants (Hosie, A. M., Wilkins, M. E., da Silva, H. M., and Smart, T. G. (2006) Nature 444, 486-489) are located at the subunit interface defined by our etomidate site model.

| S-EPMC2673245 | biostudies-literature

Differential Inhibition of Neuronal Sodium Channel Subtypes by the General Anesthetic Isoflurane.

Project description:Volatile anesthetics depress neurotransmitter release in a brain region- and neurotransmitter-selective manner by unclear mechanisms. Voltage-gated sodium channels (Navs), which are coupled to synaptic vesicle exocytosis, are inhibited by volatile anesthetics through reduction of peak current and modulation of gating. Subtype-selective effects of anesthetics on Nav might contribute to observed neurotransmitter-selective anesthetic effects on release. We analyzed anesthetic effects on Na+ currents mediated by the principal neuronal Nav subtypes Nav1.1, Nav1.2, and Nav1.6 heterologously expressed in ND7/23 neuroblastoma cells using whole-cell patch-clamp electrophysiology. Isoflurane at clinically relevant concentrations induced a hyperpolarizing shift in the voltage dependence of steady-state inactivation and slowed recovery from fast inactivation in all three Nav subtypes, with the voltage of half-maximal steady-state inactivation significantly more positive for Nav1.1 (-49.7 ± 3.9 mV) than for Nav1.2 (-57.5 ± 1.2 mV) or Nav1.6 (-58.0 ± 3.8 mV). Isoflurane significantly inhibited peak Na+ current (I Na) in a voltage-dependent manner: at a physiologically relevant holding potential of -70 mV, isoflurane inhibited peak I Na of Nav1.2 (16.5% ± 5.5%) and Nav1.6 (18.0% ± 7.8%), but not of Nav1.1 (1.2% ± 0.8%). Since Nav subtypes are differentially expressed both between neuronal types and within neurons, greater inhibition of Nav1.2 and Nav1.6 compared with Nav1.1 could contribute to neurotransmitter-selective effects of isoflurane on synaptic transmission.

| S-EPMC6439525 | biostudies-literature

Assembly and Trafficking of Homomeric and Heteromeric Kainate Receptors with Impaired Ligand Binding Sites.

Project description:Kainate receptors (KARs) are a subfamily of ionotropic glutamate receptors (iGluRs) mediating excitatory synaptic transmission. Cell surface expressed KARs modulate the excitability of neuronal networks. The transfer of iGluRs from the endoplasmic reticulum (ER) to the cell surface requires occupation of the agonist binding sites. Here we used molecular modelling to produce a range of ligand binding domain (LBD) point mutants of GluK1-3 KAR subunits with and without altered agonist efficacy to further investigate the role of glutamate binding in surface trafficking and activation of homomeric and heteromeric KARs using endoglycosidase digestion, cell surface biotinylation and imaging of changes in intracellular Ca2+ concentration [Ca2+]i. Mutations of conserved amino acid residues in the LBD that disrupt agonist binding to GluK1-3 (GluK1-T675V, GluK2-A487L, GluK2-T659V and GluK3-T661V) reduced both the total expression levels and cell surface delivery of all of these mutant subunits compared to the corresponding wild type in transiently transfected human embryonic kidney 293 (HEK293) cells. In contrast, the exchange of non-conserved residues in the LBD that convert antagonist selectivity of GluK1-3 (GluK1-T503A, GluK2-A487T, GluK3-T489A, GluK1-N705S/S706N, GluK2-S689N/N690S, GluK3-N691S) did not alter the biosynthesis and trafficking of subunit proteins. Co-assembly of mutant GluK2 with an impaired LBD and wild type GluK5 subunits enables the cell surface expression of both subunits. However, [Ca2+]i imaging indicates that the occupancy of both GluK2 and GluK5 LBDs is required for the full activation of GluK2/GluK5 heteromeric KAR channels.

| S-EPMC6420462 | biostudies-literature

Identification of binding sites contributing to volatile anesthetic effects on GABA type A receptors.

Project description:Most general anesthetics enhance GABA type A (GABAA) receptor activity at clinically relevant concentrations. Sites of action of volatile anesthetics on the GABAA receptor remain unknown, whereas sites of action of many intravenous anesthetics have been identified in GABAA receptors by using photolabeling. Here, we used photoactivatable analogs of isoflurane (AziISO) and sevoflurane (AziSEVO) to locate their sites on ?1?3?2L and ?1?3 GABAA receptors. As with isoflurane and sevoflurane, AziISO and AziSEVO enhanced the currents elicited by GABA. AziISO and AziSEVO each labeled 10 residues in ?1?3 receptors and 9 and 8 residues, respectively, in ?1?3?2L receptors. Photolabeled residues were concentrated in transmembrane domains and located in either subunit interfaces or in the interface between the extracellular domain and the transmembrane domain. The majority of these transmembrane residues were protected from photolabeling with the addition of excess parent anesthetic, which indicated specificity. Binding sites were primarily located within ?+/?- and ?+/?- subunit interfaces, but residues in the ?+/?- interface were also identified, which provided a basis for differential receptor subtype sensitivity. Isoflurane and sevoflurane did not always share binding sites, which suggests an unexpected degree of selectivity.-Woll, K. A., Zhou, X., Bhanu, N. V., Garcia, B. A., Covarrubias, M., Miller, K. W., Eckenhoff, R. G. Identification of binding sites contributing to volatile anesthetic effects on GABA type A receptors.

| S-EPMC6044061 | biostudies-literature

The general anesthetic propofol excites nociceptors by activating TRPV1 and TRPA1 rather than GABAA receptors.

Project description:Anesthetic agents can induce a paradox activation and sensitization of nociceptive sensory neurons and, thus, potentially facilitate pain processing. Here we identify distinct molecular mechanisms that mediate an activation of sensory neurons by 2,6-diisopropylphenol (propofol), a commonly used intravenous anesthetic known to elicit intense pain upon injection. Clinically relevant concentrations of propofol activated the recombinant transient receptor potential (TRP) receptors TRPA1 and TRPV1 heterologously expressed in HEK293t cells. In dorsal root ganglion (DRG) neurons, propofol-induced activation correlated better to expression of TRPA1 than of TRPV1. However, pretreatment with the protein kinase C activator 4β-phorbol 12-myristate 13-acetate (PMA) resulted in a significantly sensitized propofol-induced activation of TRPV1 in DRG neurons as well as in HEK293t cells. Pharmacological and genetic silencing of both TRPA1 and TRPV1 only partially abrogated propofol-induced responses in DRG neurons. The remaining propofol-induced activation was abolished by the selective γ-aminobutyric acid, type A (GABA(A)) receptor antagonist picrotoxin. Propofol but not GABA evokes a release of calcitonin gene-related peptide, a key component of neurogenic inflammation, from isolated peripheral nerves of wild-type but not TRPV1 and TRPA1-deficient mice. Moreover, propofol but not GABA induced an intense pain upon intracutaneous injection. As both the release of calcitonin gene-related peptide and injection pain by propofol seem to be independent of GABA(A) receptors, our data identify TRPV1 and TRPA1 as key molecules for propofol-induced excitation of sensory neurons. This study warrants further investigations into the role of anesthetics to induce nociceptor sensitization and to foster postoperative pain.

| S-EPMC2966094 | biostudies-literature

Multiple binding sites for the general anesthetic isoflurane identified in the nicotinic acetylcholine receptor transmembrane domain.

Project description:An extensive search for isoflurane binding sites in the nicotinic acetylcholine receptor (nAChR) and the proton gated ion channel from Gloebacter violaceus (GLIC) has been carried out based on molecular dynamics (MD) simulations in fully hydrated lipid membrane environments. Isoflurane introduced into the aqueous phase readily partitions into the lipid membrane and the membrane-bound protein. Specifically, isoflurane binds persistently to three classes of sites in the nAChR transmembrane domain: (i) An isoflurane dimer occludes the pore, contacting residues identified by previous mutagenesis studies; analogous behavior is observed in GLIC. (ii) Several nAChR subunit interfaces are also occupied, in a site suggested by photoaffinity labeling and thought to positively modulate the receptor; these sites are not occupied in GLIC. (iii) Isoflurane binds to the subunit centers of both nAChR alpha chains and one of the GLIC chains, in a site that has had little experimental targeting. Interpreted in the context of existing structural and physiological data, the present MD results support a multisite model for the mechanism of receptor-channel modulation by anesthetics.

| S-EPMC2922517 | biostudies-literature

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