Modulation of acetylcholine receptor states by thiol modification

Optochemical control of genetically engineered neuronal nicotinic acetylcholine receptors

Advances in synthetic chemistry, structural biology, molecular modelling and molecular cloning have enabled the systematic functional manipulation of transmembrane proteins. By combining genetically manipulated proteins with light-sensitive ligands, innately 'blind' neurobiological receptors can be converted into photoreceptors, which allows them to be photoregulated with high spatiotemporal precision. Here, we present the optochemical control of neuronal nicotinic acetylcholine receptors (nAChRs) with photoswitchable tethered agonists and antagonists. Using structure-based design, we produced heteromeric α3β4 and α4β2 nAChRs that can be activated or inhibited with deep-violet light, but respond normally to acetylcholine in the dark. The generation of these engineered receptors should facilitate investigation of the physiological and pathological functions of neuronal nAChRs and open a general pathway to photosensitizing pentameric ligand-gated ion channels.

Relative spatial position of a snake neurotoxin and the reduced disulfide bond alpha (Cys192-Cys193) at the alpha gamma interface of the nicotinic acetylcholine receptor

We determined the distances separating five functionally important residues (Gln(10), Lys(27), Trp(29), Arg(33), and Lys(47)) of a three-fingered snake neurotoxin from the reduced disulfide bond alpha(Cys(192)-Cys(193)) located at the alphagamma interface of the Torpedo nicotinic acetylcholine receptor. Each toxin position was substituted individually for a cysteine, which was then linked to a maleimido moiety through three different spacers, varying in length from 10 to 22 A. We estimated the coupling efficiency between the 15 toxin derivatives and the reduced cystine alpha(192-193) by gel densitometry of Coomassie Blue-stained gels. A nearly quantitative coupling was observed between alphaCys(192) and/or alphaCys(193) and all probes introduced at the tip of the first (position 10) and second (position 33) loops of Naja nigricollis alpha-neurotoxin. These data sufficed to locate the reactive thiolate in a "croissant-shaped" volume comprised between the first two loops of the toxin. The volume was further restrained by taking into account the absence or partial coupling of the other derivatives. Altogether, the data suggest that alphaCys(192) and/or alphaCys(193), at the alphagamma interface of a muscular-type acetylcholine receptor, is (are) located in a volume located between 11.5 and 15.5 A from the alpha-carbons at positions 10 and 33 of the toxin, under the tip of the toxin first loop and close to the second one.

Redox modulation of synaptic responses and plasticity in rat CA1 hippocampal neurons

Effects of redox reagents on excitatory and inhibitory synaptic responses as well as on the bidrectional plasticity of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses were studied in CA1 pyramidal neurons in rat hippocampal slices. The oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM) did not affect AMPA, GABAA or GABAB receptor-mediated synaptic responses or the activation of presynaptic metabotropic receptors. However, DTNB irreversibly decreased (by approximately 50%) currents evoked by focal application of NMDA. DTNB also decreased the NMDA component of the EPSC. The reversal potential of NMDA currents and the Mg2+ block were not modified. In the presence of physiological concentrations of Mg2+ (1.3 mM), DTNB did not affect the NMDA receptor-dependent induction of long-term potentiation (LTP) or long-term depression (LTD) expressed by AMPA receptors. In contrast, DTNB fully prevented LTP and LTD induced and expressed by NMDA receptors. Plasticity of NMDA receptor-mediated synaptic responses could be reinstated by the reducing agent tris-(2-carboxyethyl) phosphine (TCEP, 200 microM). These results suggest that persistent, bidirectional changes in synaptic currents mediated by NMDA receptors cannot be evoked when these receptors are in an oxidized state, whereas NMDA-dependent LTP and LTD are still expressed by AMPA receptors. Our observations raise the possibility of developing therapeutic agents that would prevent persistent excitotoxic enhancement of NMDA receptor-mediated events without blocking longterm modifications of AMPA receptor-mediated synaptic responses, thought to underlie memory processes.

Roles of sulfhydryl and disulfide groups in the binding of CP-55,940 to rat brain cannabinoid receptor

The roles of sulfhydryl and disulfide groups in the specific binding of synthetic cannabinoid CP-55,940 to the cannabinoid receptor in membrane preparations from the rat cerebral cortex have been examined. Various sulfhydryl blocking reagents including p-chloromercuribenzoic acid (p-CMB), N-ethylmaleimide (NEM), o-iodosobenzoic acid (o-ISB), and methyl methanethiosulfonate (MMTS) inhibited the specific binding of [3H]CP-55,940 to the cannabinoid receptor in a dose-dependent manner. About 80-95% inhibition was obtained at a 0.1 mM concentration of these reagents. Scatchard analysis of saturation experiments indicates that most of these sulfhydryl modifying reagents reduce both the binding affinity (Kd) and capacity (Bmax). On the other hand, DL-dithiothreitol (DTT), a disulfide reducing agent, also irreversibly inhibited the specific binding of [3H]CP-55,940 to the receptor and about 50% inhibition was obtained at a 5 mM concentration. Furthermore, 5 mM DTT was abelt to dissociate 50% of the bound ligand from the ligand-receptor complex. The marked inhibition of [3H]CP-55,940 binding by sulfhydryl reagents suggests that at least one free sulfhydryl group is essential to the binding of the ligand to the receptor. In addition, the inhibition of the binding by DTT implies that besides free sulfhydryl group(s), the integrity of a disulfide bridge is also important for [3H]CP-55,940 binding to the cannabinoid receptor.

Comparative biochemical pharmacology of central nervous system dopamine D1 and D2 receptors

The biochemical properties of central nervous system (CNS) dopamine (DA) D1 and D2 receptors were examined using the specific antagonists [3H]SCH23390 and [3H]raclopride, respectively. There is a different participation of sulfhydryl (-SH) and disulfide (-SS-) groups in the binding site and/or coupling to second messenger systems of D1 and D2 receptors. The ionic studies with [3H]SCH23390 showed slight agonist and antagonist affinity shifts for the D1 receptor. On the other hand, the D2 receptor is very sensitive to cations; even if lithium and sodium influence specific [3H]raclopride binding in a similar manner, there appear to be quantitative differences between these two ions that cannot be explained by surface charge mechanisms. The distribution of D1 and D2 receptors was heterogenous in both species, with the greatest densities in the neostriatum, where the highest concentrations of DA and metabolites were measured. Regions with low endogenous DA content (cerebral cortex and hippocampus) had lower densities of DA receptors. Furthermore, these binding sites were differentially localized within the various regions, and there were substantially more D1 than D2 receptors. The functional significance and heterogeneities in the distribution of D1 and D2 receptors can be related to dopaminergic innervation and turnover.

Changes in channel properties of acetylcholine receptors during the time course of thiol chemical modifications

The changes occurring during chemical modification of thiol groups in single acetylcholine receptor (AChR) channels of BC3H-11 cells were examined by the patch-clamp technique in the "cell-attached" configuration. Treatment with either 1 mM or 5 mM dithiothreitol or with 5 mM N-ethylmaleimide (NEM) does not cause significant changes in the conductance and mean open time of the channels. However, reduction with dithiothreitol followed by alkylation with NEM produces modifications of AChRs. Under these conditions, channels activated by 2 microM acetylcholine show decreased open times (about 15-fold shorter for the most-modified AChRs) and a slight reduction in single-channel current. Both changes are dependent on the time of exposure and concentration of NEM. The rate of occurrence of openings, however, changes little during NEM treatment. When reduced and alkylated AChRs are activated by 100 microM acetylcholine, clusters of short openings separated by silent periods of about 1 s are observed. The channel-open probability, determined for openings within a cluster, is decreased by about 10-fold when compared with control receptors. The observations at high agonist concentration indicate that the modified AChR is still able to undergo desensitization.

Desensitization of the nicotinic acetylcholine receptor: molecular mechanisms and effect of modulators

1. Loss of response after prolonged or repeated application of stimulus is generally termed desensitization. A wide variety of phenomena occurring in living organisms falls under this general definition of desensitization. There are two main types of desensitization processes: specific and non-specific. 2. Desensitization of the nicotinic acetylcholine receptor is triggered by prolonged or repeated exposure to agonists and results in inactivation of its ion channel. It is a case of specific desensitization and is an intrinsic molecular property of the receptor. 3. Desensitization of the nicotinic acetylcholine receptor at the neuromuscular junction was first reported by Katz and Thesleff in 1957. Desensitization of the receptor has been demonstrated by rapid kinetic techniques and also by the characteristic "burst kinetics" obtained from single-channel recordings of receptor activity in native as well as in reconstituted membranes. In spite of a number of studies, the detailed molecular mechanism of the nicotinic acetylcholine receptor desensitization is not known with certainty. The progress of desensitization is accompanied by an increase in affinity of the receptor for its agonist. This change in affinity is attributed to a conformational change of the receptor, as detected by spectroscopic and kinetic studies. A four-state general model is consistent with the major experimental observations. 4. Desensitization of the nicotinic acetylcholine receptor can be potentially modulated by exogenous and endogenous substances and by covalent modifications of the receptor structure. Modulators include the noncompetitive blockers, calcium, the thymic hormone peptides (thymopoietin and thymopentin), substance P, the calcitonin gene-related peptide, and receptor phosphorylation. Phosphorylation is an important posttranslational covalent modification that is correlated with the regulation and desensitization of the receptor through various protein kinases. 5. Although the physiological significance of desensitization of the nicotinic receptor is not yet fully understood, desensitization of receptors probably plays a significant role in the operation of the neuronal networks associated in memory and learning processes. Desensitization of the nicotinic receptor could also possibly be related to the neuromuscular disease, myasthenia gravis.

Specific [3H]SCH23390 binding to dopamine D1 receptors in cerebral cortex and neostriatum: role of disulfide and sulfhydryl groups

Receptor binding studies were performed in cerebral cortex (CTX) and neostriatum (CPU; caudate-putamen) using the dopamine D1 antagonist [3H]SCH23390. Because receptors are of protein nature, we examined the role of disulfide bonds (--SS--) and sulfhydryl groups (--SH) in the specific binding of [3H]SCH23390. Furthermore, membrane preparations contain a certain amount of lipid, so that treatments with --SH and --SS-- reagents could determine whether the fixation of the radioligand was to protein or to the lipid moiety. Pretreatment of CTX and CPU membranes with dithioerythritol, L-dithiothreitol, or 5,5'-dithiobis(2-nitrobenzoic acid), as well as with the alkylating agent N-ethylmaleimide, produced dose-dependent decreases of specific [3H]SCH23390 binding in membrane preparations from both tissues. These changes were not reversible after up to two washes, but could be prevented in part if the treatments were performed in the presence of dopamine. Additional protection experiments were conducted with (+)- and (-)-butaclamol, as well as with (+)- and (-)-SKF38393. A series of saturation experiments (with pretreated membranes in the absence of reactives) demonstrated that the alkylation of --SS-- groups reduced specific [3H]SCH23390 binding mainly through an affinity change, but L-dithiothreitol and 5,5-dithiobis(2-nitrobenzoic acid) decreased the number of binding sites. The affinity of the receptor to agonists was examined with the two enantiomers of SKF38393; the inhibition curves showed that residual binding was not affected and stereospecificity was conserved. The present results provide evidence for the participation of both --SS-- and --SH groups in the recognition site of the dopamine D1 receptor in both the CTX and the CPU.

Alpha-1 and alpha-2 adrenoceptor binding in cerebral cortex: role of disulfide and sulfhydryl groups

The tritiated adrenergic antagonists Prazosin ([3H]PRZ) and Idazoxan ([3H]IDA, or RX-781094) bind specifically and with high affinity to alpha 1 and alpha 2-adrenoceptors respectively, in membrane preparations from cerebral cortex. Saturation experiments performed to determine the density of receptors and the dissociation constant (Kd) were analyzed by the methods of Eadie Hofstee, iterative modelling, and the procedure of Hill, while the specificity of the labelling was verified by displacement experiments. Since receptors are proteins, we examined the role of disulfide (-SS-) bridges and sulfhydryl (-SH) groups in the specific combination of [3H]PRZ and [3H]IDA to the alpha 1 and alpha 2 adrenoceptors. Pretreatment of the membranes with the -SS- reactive DL-dithiothreitol (DTT) or the alkylating agent N-ethylmaleimide (NEM), alone or in combination, decreased specific binding of both ligands, with only minor changes in the non-specific counts. The [3H]IDA binding (alpha 2-sites) was more sensitive to both DTT and NEM than the [3H]PRZ sites (alpha 1-adrenoceptors), and the initial changes induced by alkylation of the alpha 2-site were due to an important decrease in the affinity for [3H]IDA, as judged by the increase in the Kd. This modulation in the affinity caused by alkylation of a thiol group could explain the higher potency of the blocking agent tetramine disulfide benextramine at the alpha 2-site. The results provide evidence for the participation of -SS- and -SH groups in the binding site of alpha 1- and alpha 2-adrenoceptors in the cerebral cortex.

Redox status and pulmonary vascular reactivity

Oxygen radicals are produced during oxidative metabolism in proportion to the tissue oxygen tension. The studies reported here have shown that oxygen radicals or the sulfhydryl oxidant, diamide, caused pulmonary vasodilatation in the isolated perfused rat lung. Could oxygen radicals play a role in the physiologic control of pulmonary vascular smooth muscle tone?

Creatine kinase activity in the Torpedo electrocyte and in the nonreceptor, peripheral v proteins from acetylcholine receptor-rich membranes

The nonreceptor, peripheral v proteins (Mr 43,000 proteins) are conspicuous components of the acetylcholine receptor-rich membranes and the Torpedo electrocyte, so far devoid of any known enzymatic function. Creatine kinase (adenosine 5'-triphosphate:creatine N-phosphotransferase, EC 2.7.3.2) is identified in distinct polypeptides belonging to the family of v proteins. Embryonic (70- to 90-mm embryos), neonatal, and adult electric organs of Torpedo marmorata contain two isoenzymes of creatine kinase: the BB (brain) and the MM (muscle) forms. The proportion of the two isoenzymes does not appear to change in the course of ontogenic and postnatal development. Only the BB isoenzyme appears to be associated with the acetylcholine-rich membranes in adult Torpedo. The creatine kinase can be purified to homogeneity by chromatographic procedures that exploit the richness in free sulfhydryl groups of the enzyme. Specific activities of 150 units/mg are obtained from electric tissue. The enzyme subunits identified by two-dimensional gel electrophoresis and immunoblotting techniques have pI values in the 6.0-6.5 region and apparent molecular weights in the 40,000-43,000 range, the latter values depending on redox conditions.

Receptor--ion channel interactions in Torpedo electric organ: influence of thiol group modification

The influence of sulfhydryl and disulfide reagents on nicotinic cholinergic receptors and ion channels and their interactions was investigated using specific probes for receptor and channel binding sites in electric organs from Torpedo californica. N-ethylmaleimide, a sulfhydryl alkylating agent, did not alter receptor or ion channel binding, or receptor-mediated ion channel binding activation or desensitization. Alteration of receptor--ion channel coupling produced by treatment with an organic sulfhydryl, dithiothreitol, could be accounted for on the basis of decreases in receptor affinity for agonists. These changes were reversed by exposure to an oxidizing agent. Following reduction by dithiothreitol, N-ethylmaleimide treatment produced large decreases in the extent and affinity of both receptor and channel binding. No evidence for a direct role of thiol groups in ion channel function was obtained.

Evidence that the porcine thyrotropin (TSH) receptor contains an essential disulphide bridge

The effects of reducing agents on membrane bound and detergent solubilised porcine TSH receptors have been investigated. Both 2-mercaptoethanol and dithiothreitol appeared to inhibit the TSH-binding activity by a direct effect on the TSH receptor itself and Scatchard analysis suggested that this was primarily due to an alteration in TSH-binding capacity. In addition, some binding activity could be recovered by reoxidation of reduced receptor preparations. These results suggest therefore that the porcine TSH receptor contains an essential disulphide bridge.

Oligomeric forms of the membrane-bound acetylcholine receptor disclosed upon extraction of the Mr 43,000 nonreceptor peptide

Oligomeric forms of the acetylcholine receptor are directly visualized by electron microscopy in receptor-rich membranes from torpedo marmorata. The receptor structures are quantitatively correlated with the molecular species so far identified only after detergent solubilization, and further related to the polypeptide composition of the membranes and changes thereof. The structural identification is made possibly by the increased fragility of the membranes after extraction of nonreceptor peptides and their subsequent disruption upon drying onto hydrophilic carbon supports. Receptor particles in native membranes depleted of nonreceptor peptides appear as single units of 7-8 nm, and double and multiple aggregates thereof. Particle doublets having a main-axis diameter of 19 +/- 3 nm predominate in these membranes. Linear aggregates of particles similar to those observed in rotary replicas of quick-frozen fresh electrolytes (Heuser, J.E. and S. R. Salpeter. 1979, J. Cell Biol. 82: 150-173) are also present in the alkaline-extracted membranes. Chemical modifications of the thiol groups shift the distribution of structural species. Dithiothreitol reduction, which renders almost exclusively the 9S, monomeric receptor form, results in the observation of the 7-8 nm particle in isolated form. The proportion of doublets increases in membranes alkylated with N-ethylmaleimide. Treatment with 5,5'-dithiobis-(nitrobenzoic acid) increases the proportion of higher oligomeric species, and particle aggregates (n=oligo) predominate. The nonreceptor v-peptide (doublet of M(r) 43,000) appears to play a role in the receptor monomer-polymer equilibria. Receptor protein and v-peptide co-aggregate upon reduction and reoxidation of native membranes. In membranes protected ab initio with N- ethylmaleimide, only the receptor appears to self-aggregate. The v-peptide cannot be extracted from these alkylated membranes, though it is easily released from normal, subsequently alkylated or reduced membranes. A stabilization of the dimeric species by the nonreceptor v-peptide is suggested by these experiments. Monospecific antibodies against the v-peptide are used in conjunction with rhodamine- labeled anti-bodies in an indirect immunoflourescence assay to map the vectorial exposure of the v-peptide. When intact membranes, v-peptide depleted and "holey" native membranes (treated with 0.3 percent saponin) are compared, maximal labeling is obtained with the latter type of membranes, suggesting a predominantly cytoplasmic exposure of the antigenic determinants of the v-peptide in the membrane. The influence of the v-peptide in the thiol-dependent interconversions of the receptor protein and the putative topography of the peptide are analyzed in the light of the present results.

Changes in neuromuscular junction endplate current time constants produced by sulfhydryl reagents

The acetylcholine receptor is a protein that contains certain critical disulfide bonds. Experiments were designed to determine the role such bonds might play in the physiological activity of the receptor. Modification of the receptor with sodium bisulfite and diamide produced an increase in the time constants of the miniature endplate current without changes in the single-channel properties of the receptor. Controls were done to determine that this change in the miniature endplate current was not due to an effect on acetylcholinesterase at the endplate. These data are interpreted to mean that the reagents increase the time acetylcholine is bound to the receptor before the channel opens and is most probably due to a change in receptor affinity brought about by chemical modification of the receptor protein.

Rotational molecular dynamics of the membrane-bound acetylcholine receptor revealed by phosphorescence spectroscopy

The rotational mobility of the membrane-bound acetylcholine receptor from Torpedo marmorata has been studied by phosphorescence anisotropy techniques using eosin-5'-isothiocyanate and eosin-5'-iodoacetamide derivatives of the alpha-neurotoxin of Bungarus multicinctus (alpha-bungarotoxin). Normal membranes, those depleted of nonreceptor peripheral peptides by alkaline treatment, as well as membranes subjected to various chemical modifications of the thiol groups, have been characterized. Rotational correlation times (10--26 mus) compatible with the motion of individual 9-S monomeric receptor species of Mr 250 000 were observed upon reduction of the membranes with dithiothreitol or by raising the temperature to 39 degrees C in the case of alkaline-treated membranes. Membranes prepared throughout in N-ethylmaleimide, which yield upon detergent solubilization a predominant 13-S dimeric species and which are not depleted of the nonreceptor v-peptide (Mr 43 000) by alkaline treatment, are relatively more 'rigid' than normal membranes and do not show the anisotropy components characteristic of the monomeric receptor. The influence of the v-peptide on the structural stability of the receptor oligomeric forms and the thiol-dependent interconversions are thus reflected in the time-dependent spectroscopic measurements.

Altered physical states of the membrane-bound acetylcholine receptor after affinity labelling

The study of the interacstion of the bifunctional cholinergic ligand alpha-bromoacetylcholine with the membrane-bound acetylcholine receptor has allowed the identification of physically altered states of the receptor following affinity labelling. Depending on the integrity of a disulphide bond in the receptor, the ligand can either trigger the normal conversion of affinity states in unmodified membranes (apparent Kd's of approximately 0.5-1 microM and 5-10 nM in the low and high affinity states, respectively) or reversibly lock the reduced receptor in an agonist-insensitive state. Antagonists like d-tubocurarine release the receptor from this state, in accordance with in vivo observations. The integrity of a disulphide bond available for affinity acylation after reduction of the receptor appears to be essential for correct ligand discrimination and for the occurrence of ligand-induced state transitions of the membrane-bound receptor in vitro.