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J L Galzi

Publications and source records attributed to J L Galzi.

35 records · Page 2Linked to original sources
PubMed1992

Unconventional pharmacology of a neuronal nicotinic receptor mutated in the channel domain.

The putative channel-forming MII domains of the nicotinic, gamma-aminobutyric acid type A, and glycine receptors contain a highly conserved leucine residue. Mutation of this hydrophobic amino acid in the neuronal nicotinic receptor alpha 7 (Leu-247), reconstituted in Xenopus oocytes, modifies the ionic response to acetylcholine and alters desensitization. Furthermore, the Leu----Thr (L247T) mutant has two conducting states (46 pS and 80 pS), in contrast with the wild-type (WT) receptor, which has only one (45 pS). We now show that this mutant possesses a rather paradoxical pharmacology: antagonists of the WT receptor such as dihydro-beta-erythroidin, hexamethonium, or (+)-tubocurarine elicit ionic currents when applied to the L247T alpha 7 mutant and these responses are blocked by alpha-bungarotoxin. Furthermore, prolonged application of acetylcholine causes desensitization in the WT but leads to a potentiation of the responses to acetylcholine or dihydro-beta-erythroidin in the mutant. These data are consistent with a scheme in which mutation of Leu-247 renders a desensitized state in the WT channel a conducting state. They also strengthen the proposal that, in the WT, some competitive antagonists may stabilize desensitized states. Finally, these observations may shed light on properties of other ion channels, in particular the glutamate receptors, which display multiple conductance levels associated with various pharmacological agents.

Acetylcholine↗
PubMed1992

The acetylcholine receptor: a model of an allosteric membrane protein mediating intercellular communication.

Over the past 20 years the nicotinic acetylcholine receptor has become the prototype of a superfamily of ligand-gated ion channels. As a single macromolecular entity of M(r) about 300,000, the receptor protein mediates, altogether, the activation and the desensitization of the associated ion channel and the regulation of these processes by extracellular and intracellular signals. The notion is discussed that the acetylcholine receptor is a membrane-bound allosteric protein which possesses several categories of specific sites for neurotransmitters and for regulatory ligands, and undergoes conformational transitions which link these diverse sites together. At this elementary molecular level, interactions between signalling pathways may be mediated by membrane-bound allosteric receptors and/or by other categories of cytoplasmic allosteric proteins.

Allosteric Regulation↗
PubMed1992

Stratified organization of the nicotinic acetylcholine receptor channel.

Mutations of leucine 247 within the M2 channel domain of the alpha 7 neuronal nicotinic receptor, confer electrophysiological and pharmacological properties, which allow one of the desensitized states to become conductive. Here we show that, in Xenopus oocytes, the effects of the mutations were preserved when 1,2-bis(2-aminophenoxy)-ethane N,N,N',N'-tetraacetic acid (BAPTA) was injected in the cytoplasm to block Ca(2+)-dependent chloride currents, and that in agreement with the proposed interpretation, the ionic currents do not desensitize and rise slowly, in the time-scale of seconds, upon agonist application. Interestingly, similar effects were observed when the two rings (T244, V251) neighbouring L247 on the alpha-helix, but not the more distant ones (S240, L254/255), were mutated, thus supporting the proposal of a functional stratification of the channel domain.

Affinity Labels↗
PubMed1991

Functional significance of aromatic amino acids from three peptide loops of the alpha 7 neuronal nicotinic receptor site investigated by site-directed mutagenesis.

Three aromatic amino acids, Tyr92, Trp148 and Tyr187 belonging to three separate domains of the alpha 7-subunit of neuronal nicotinic acetylcholine receptor were mutated to phenylalanine, and the electrophysiological response of the resulting mutant receptors analyzed in the Xenopus oocyte expression system. All mutations significantly decreased the apparent affinities for acetylcholine and nicotine, and to a lesser extent, those for the competitive antagonists dihydro-beta-erythroidine and alpha-bungarotoxin. Other properties investigated, such as the voltage dependency of the ion response as well as its sensitivity to the open channel blocker QX222, were not significantly changed, indicating that the mutations affected selectively the recognition of cholinergic ligands by the receptor protein. The maximal rates for the rapid desensitization process were slightly modified, suggesting that the contribution of Tyr92, Trp148 and Tyr187 to the binding area might differ in the various conformations of the nicotinic receptor. Other mutations at nearby positions (S94N, W153F, G151D and G82E) did not affect the properties of the electrophysiological response. These data point to the functional significance of Tyr92, Trp148 and Tyr187 in the binding of cholinergic ligands and ion channel activation of the nicotinic receptor, thus supporting a multiple loop model [(1990) J. Biol. Chem. 265, 10430-10437] for the ligand binding area.

Acetylcholine↗
PubMed1991

Mutations in the channel domain alter desensitization of a neuronal nicotinic receptor.

A variety of ligand-gated ion channels undergo a fast activation process after the rapid application of agonist and also a slower transition towards desensitized or inactivated closed channel states when exposure to agonist is prolonged. Desensitization involves at least two distinct closed states in the acetylcholine receptor, each with an affinity for agonists higher than those of the resting or active conformations. Here we investigate how structural elements could be involved in the desensitization of the acetylcholine-gated ion channel from the chick brain alpha-bungarotoxin sensitive homo-oligomeric alpha 7 receptor, using site-directed mutagenesis and expression in Xenopus oocytes. Mutations of the highly conserved leucine 247 residue from the uncharged MII segment of alpha 7 suppress inhibition by the open-channel blocker QX-222, indicating that this residue, like others from MII, faces the lumen of the channel. But, unexpectedly, the same mutations decrease the rate of desensitization of the response, increase the apparent affinity for acetylcholine and abolish current rectification. Moreover, unlike wild-type alpha 7, which has channels with a single conductance level, the leucine-to-threonine mutant has an additional conducting state active at low acetylcholine concentrations. It is possible that mutation of Leu 247 renders conductive one of the high-affinity desensitized states of the receptor.

Acetylcholine↗
PubMed1991

Allosteric transitions of the acetylcholine receptor probed at the amino acid level with a photolabile cholinergic ligand.

Structural changes occurring upon desensitization of the Torpedo marmorata acetylcholine receptor were monitored with tritiated p-(N,N-dimethyl)aminobenzenediazonium fluoroborate, a reversible competitive antagonist in the dark, which may serve as a photoaffinity probe of the area of the receptor molecule with which cholinergic ligands interact. Addition of meproadifen, an allosteric effector that stabilizes the high-affinity desensitized state of the receptor upon binding to a site topographically distinct from the cholinergic ligand-binding domains, caused a major increase in labeling of the alpha subunit, a smaller increase in the delta subunit, and decreased labeling in the gamma subunit, thus revealing changes in the alpha and non-alpha subunits' contribution to cholinergic ligand binding. Also, in agreement with the tighter binding of cholinergic ligands to the desensitized receptor, differential labeling of three peptide loops of the alpha subunit was detected: while Tyr-190, Cys-192, and Cys-193 were labeled in a roughly identical manner in both resting and desensitized conformations, the labeling of Tyr-93 and Trp-149 increased up to 6-fold in the desensitized state. Tyr-93 and Trp-149 belong to separate regions containing strictly conserved "canonical" amino acids, common to all nicotinic, gamma-aminobutyrate, and glycine receptor subunits. These regions are thus likely to play a critical role in the regulation of ligand-gated ion channels.

Affinity Labels↗
PubMed1990

Identification of a novel amino acid alpha-tyrosine 93 within the cholinergic ligands-binding sites of the acetylcholine receptor by photoaffinity labeling. Additional evidence for a three-loop model of the cholinergic ligands-binding sites.

The native, membrane-bound, acetylcholine receptor from Torpedo marmorata was photolabeled by the competitive antagonist p-[3H]dimethylaminobenzene-diazonium fluoroborate (DDF) in the presence of the noncompetitive blocker phencyclidine and under energy transfer conditions. The isolated alpha-subunits were treated with cyanogen bromide and fractionation of the resulting fragments yielded three radiolabeled peptides, at the level of which, incorporation of [3H]DDF (i) was equally inhibited by the agonist carbamoylcholine and the competitive antagonist alpha-bungarotoxin and (ii) was insensitive to "scavenging" reagents. Subfragmentation of cyanogen bromide peptide III with omicron-iodosobenzoic acid or trypsin and sequence analysis of the fragments led to the identification of a novel amino acid alpha-Tyr-93 (and possibly Trp-86) as labeled by [3H]DDF in a carbamoylcholine-sensitive manner. alpha-Tyr-93 is conserved in the muscle and neuronal alpha-subunits but not in the other subunits of muscle receptor. This result provides evidence for a site involving at least a third loop of the alpha-subunit amino-terminal hydrophilic domain, in addition to the ones previously identified (Dennis, M., Giraudat, J., Kotzyba-Hibert, F., Goeldner, M., Hirth, C., Chang, J. Y., Lazure, C., Chretien, M., and Changeux, J. P. (1988) Biochemistry 27, 2346-2357). Possible contribution of tyrosine side-chains to the complexation of the quaternary ammonium group of cholinergic ligands is discussed.

Affinity Labels↗
PubMed1990

Photoaffinity labelling of the mu-opioid receptor is dependent on the nature of the photosensitive group of carfentanil derivatives.

Arylazido and aryldiazonium derivatives of carfentanil, bearing their photoactivatable function at the same position on the molecule, were synthesized. In the dark both of them exhibited similar binding affinity profiles and behaved as mu-selective and reversible ligands of the opioid receptor sites. Following irradiation, only the azido analog displayed the properties of an efficient, irreversible and selective label of the mu-opioid receptor, allowing physicochemical requirements for alkylation of this receptor subtype to be examined. Evidence is presented to consider this azido compound a promising tool for characterizing the mu-opioid receptor protein.

Affinity Labels↗
PubMed1990

Photoactivatable opiate derivatives as irreversible probes of the mu-opioid receptor.

The synthesis of aryldiazonium and arylazido derivatives of carfentanil, etonitazene, and naltrexone and of a triazaspirodecane derivative is described. The chemical stability and the spectral characteristics of these compounds were verified, and their binding affinity constants for the different opioid receptor classes were determined, in the absence of light, from competition experiments. With the exception of the naltrexyl derivatives, which remained nonselective, all compounds tested displayed a pronounced mu-binding selectivity with mu/delta and mu/kappa ratios ranging from 12 to 1000. After irradiation, only the arylazido probes led to an irreversible mu-binding-site inactivation. This inactivation fulfilled the criteria for photoaffinity labeling such as protection against inactivation by other opiate ligands and absence of an effect of scavengers on the extent of the inactivation. Most of the photoactivatable probes formed long-lasting reversible complexes with the opioid binding sites: an efficient dissociation procedure was thus required to discriminate between pseudoirreversible and covalent complexes. The marked differences in labeling efficacy between aryldiazonium salts and their corresponding arylazido derivatives are discussed.

Affinity Labels↗
PubMed1990

The noncompetitive blocker [3H]chlorpromazine labels three amino acids of the acetylcholine receptor gamma subunit: implications for the alpha-helical organization of regions MII and for the structure of the ion channel.

Labeling studies of Torpedo marmorata nicotinic acetylcholine receptor with the noncompetitive channel blocker [3H]chlorpromazine have led to the initial identification of amino acids plausibly participating to the walls of the ion channel on the alpha, beta, and delta subunits. We report here results obtained with the gamma subunit, which bring additional information on the structure of the channel. After photolabeling of the membrane-bound receptor under equilibrium conditions in the presence of agonist and with or without phencyclidine (a specific ligand for the high-affinity site for noncompetitive blockers), the purified labeled gamma subunit was digested with trypsin, and the resulting fragments were fractionated by HPLC. Sequence analysis of peptide mixtures containing various amounts of highly hydrophobic fragments showed that three amino acids are labeled by [3H]chlorpromazine in a phencyclidine-sensitive manner: Thr-253, Ser-257, and Leu-260. These residues all belong to the hydrophobic and putative transmembrane region MII of the gamma subunit. Their distribution along the sequence is consistent with an alpha-helical organization of this segment. The [3H]chlorpromazine-labeled amino acids are conserved at homologous positions in the known sequences of other ligand-gated ion channels and may, thus, play a critical role in ion-transport mechanisms.

Allosteric Site↗
PubMed1988

A mu-opioid receptor-filter assay. Rapid estimation of binding affinity of ligands and reversibility of long-lasting ligand-receptor complexes.

A filter-associated binding technique, originally described by Leysen and Gommeren [J. Receptor Res. 4, 817 (1984); Drug Dev. Res. 8, 119 (1986)], was focused on the study of mu-opioid receptor sites. The interesting binding features of 3H-Sufentanil, a mu-selective radioligand, permitted such a filter procedure to be performed. Its application was 3-fold. A 5-min binding assay allowed us to verify that specific 3H-Sufentanil binding to filter-absorbed rat brain membranes was endowed with the known kinetic and equilibrium binding properties of membrane-bound mu-opioid receptor sites. Moreover, the filtration technique allowed us to get rapid information about the dissociation rates of unlabelled compounds from the receptor sites. In practice, membranes, preincubated with high concentrations of cold drugs, were absorbed to filters. Dissociation was achieved by repeatedly applying buffer samples on the filter, and monitored by the recovery in free specific 3H-Sufentanil binding sites. Finally, such a dissociation procedure, improved by a washing buffer at high ionic strength, was found to be much more efficient and attractive than the classical dilution-centrifugation procedure, especially for slowly-dissociating compounds. Special attention was paid to the discrimination between pseudo-irreversible binding of drugs and stable covalent labelling of mu-opioid receptor sites (either by affinity or photoaffinity probes), particularly when unlabelled ligands were the only tools available.

Animals↗
PubMedDate not supplied

Fluorescent muscarinic EGFP-hM1 chimeric receptors: design, ligand binding and functional properties.

We describe the construction, expression and characterization of recombinant proteins comprising the enhanced green fluorescent protein (EGFP) fused to the amino-terminal part of the muscarinic hM1 receptor together or not with an additional hexahistidine tag placed at the C-terminal end of the receptor. Expression of the fluorescent proteins reaches levels identical to those of the wt hM1 receptor, provided that fusion takes place at the very N-terminal end of the receptor. Also correct protein folding and targeting to plasma membrane is obtained upon addition of a signal peptide promoting amino-terminal domain translocation through the membrane. Ligand binding properties of--and activation of the calcium release response by--the fusion proteins are almost identical to those of the wild-type muscarinic receptor, indicating that such fluorescently-labelled receptors are valuable model systems for further functional, biochemical and structural studies.

Base Sequence↗