Selective enhancement of histamine H1-receptor responses in guinea-pig ileal smooth muscle by 1,4-dithiothreitol

Serotonin 2A receptor disulfide bridge integrity is crucial for ligand binding to different signalling states but not for its homodimerization

The serotonin 2_A (5-HT_2A) receptor is a G-protein coupled receptor (GPCR) with a conserved disulfide bridge formed by Cys¹⁴⁸ (transmembrane helix 3, TM3) and Cys²²⁷ (extracellular loop 2, ECL-2). We hypothesized that disulfide bridges may determine serotonin 5-HT_2A receptor functions such as receptor activation, functional selectivity and ligand recognition. We used the reducing agent dithiothreitol (DTT) to determine how the reduction of disulfide bridges affects radioligand binding, second messenger mobilization and receptor dimerization. A DTT-induced decrease in the number of binding sites (1190 ± 63.55 fmol/mg protein for control cells compared with 921.2 ± 60.84 fmol/mg protein for DTT-treated cells) as well as in the efficacy of both signalling pathways characterized was observed, although the affinity and potency were unchanged. Bioluminiscence resonance energy transfer (BRET) assays revealed the DTT treatment did not modify the homodimeric nature of serotonin 5-HT_2A receptors. In molecular dynamic simulations, the ECL-2 of the receptor with a broken cysteine bond adopts a wider variety of conformations, some of which protrude deeper into the receptor orthosteric binding pocket leading to collapse of the pocket. A shrunken binding pocket would be incapable of accommodating lysergic acid diethylamide (LSD). Our findings suggest that the decrease of efficacy may be due to disruption of disulfide bridge between TM3 and ECL-2. This reveals the integrity of the ECL-2 epitope, which should be explored in the development of novel ligands acting as allosteric modulators of serotonin 5-HT_2A receptors.

Decline of the calcium response on successive stimulation of a rat brain endothelial cell P2U purinoceptor

A microfluorimetric method using Fura-2 as calcium indicator was used to study the mechanism of desensitization of the calcium response evoked by activation of a brain endothelial cell P2U receptor. The study was mainly carried out on an immortalized rat brain endothelial cell line (RBE4), with some additional experiments on primary cultured rat brain microvascular endothelial cells. As previously described (Nobles et al. 1995), ATP (100 microM, 20 s) caused a transient increase in intracellular calcium levels ([Ca2+]i). This effect was dependent on the rate of filling of intracellular calcium stores, since a large inhibition of the ATP-mediated response was seen in the presence of cyclopiazonic acid, an inhibitor of the store Ca(2+)-ATPase. Application of repeated pulses of extracellular ATP led to a desensitization of the response, as measured by a decline in the release of intracellular calcium (Nobles et al. 1995). This desensitization was partially reversed after 300 s of incubation in agonist-free medium. Extracellular phosphorylation of the purinergic receptor appeared not to be involved in the desensitization process, since a similar rate of desensitization was obtained with the non-hydrolysable ATP analogue ATP gammaS. Oxidation of the purinergic receptor cannot account for the desensitization, since the decline of the ATP-mediated response was unchanged in the presence of 3 mM dithiothreitol. In the presence of ATP together with UTP, two equally potent activators of the P2U receptor, the desensitization was less than in the presence of only one of the agonists. The desensitization was greater when ATP was applied for longer (150 s) periods. Although these results do not exclude the participation of post-receptor events in the desensitization process, they suggest that desensitization is governed at least in part by agonist-receptor interaction.

Metabotropic glutamate receptor 5 is a disulfide-linked dimer

The sequences of the metabotropic glutamate receptors (mGluRs) show little homology with other members of the G protein-coupled receptor family and exhibit several distinctive features, including a large N-terminal extracellular domain with 17 cysteines in conserved positions. Here we demonstrate that mGluR5, as well as other mGluRs, behave as species approximately twice as large as expected from their sequence, but reducing conditions cause a decrease to the predicted molecular mass. Co-immunoprecipitation experiments using wild type and epitope-tagged receptors demonstrate that this is due to specific, disulfide-dependent dimerization of the receptor. The intermolecular disulfide that mediates dimerization occurs in the extracellular domain, within about 17 kDa from the N terminus.

Potentiation by 1,4-dithiothreitol of histamine-induced inositol phosphate formation in rat cerebral cortex and human HeLa cells

Dithiothreitol (1 mM) caused a 340-fold shift of the concentration-response curve for histamine-induced [3H]inositol monophosphate ([3H]IP1) accumulation in slices of rat cerebral cortex prelabelled with [3H]inositol. Dithiothreitol had no significant effect on carbachol-induced [3H]IP1 accumulation. The effect of dithiothreitol appeared to be at the level of the H1-receptor, since curves of histamine inhibition of [3H]mepyramine binding to a membrane fraction from rat cerebral cortex were shifted to lower concentrations by dithiothreitol, with a similar concentration-dependence for dithiothreitol as observed for potentiation of [3H]IP1 accumulation. The small shift of the curve of histamine inhibition of [3H]mepyramine binding by a stable GTP analogue, 5'-guanylylimidodiphosphate, was not increased in the presence of dithiothreitol. Histamine-induced [3H]IP1 accumulation in human HeLa cells was also potentiated by dithiothreitol, but the decrease in the EC50 was only 3-fold. It is suggested that differences in the magnitude of the effect of dithiothreitol on histamine H1-receptor mediated responses between tissues may reflect in part the position of the concentration-response curve in the absence of dithiothreitol.

Selective potentiation of histamine H1-receptor stimulated calcium responses by 1,4-dithiothreitol in DDT1MF-2 cells

The effect of 1,4-dithiothreitol (DTT) on agonist-stimulated increases in intracellular free calcium concentration ([Ca2+]i) has been investigated in the smooth muscle cell line, DDT1MF-2, derived from hamster vas deferens. Pretreatment with DTT (1 mM) produced a large leftward parallel shift in concentration-response curve for histamine H1-receptor mediated increases in [Ca2+]i. The EC50 values for H1-receptor stimulated increases in [Ca2+]i in the absence and presence of DTT were 11.3 +/- 1.5 microM (N = 6) and 0.52 +/- 0.15 microM (N = 6), respectively. DTT had no significant effect on the maximum Ca2+ response elicited by histamine (100 microM). In the presence of DTT the partial H1-receptor agonist 2-pyridylethylamine (100 microM) increased [Ca2+]i from 112 +/- 14 nM to 237 +/- 24 nM (N = 10). In control cells 2-pyridylethylamine (100 microM) did not elicit a Ca2+ response. DTT had no significant effect on the maximum Ca2+ response elicited by 1 mM 2-pyridylethylamine. The enhancement of histamine H1-receptor Ca2+ responses by DTT was reversed by the sulphydryl oxidizing agent dithiobis-(2-nitrobenzoic acid). DTT had no significant effect on adenosine A1-, bradykinin and ATP-receptor stimulated increases in [Ca2+]i. [3H]mepyramine binding experiments confirmed that DTT increased agonist affinity. DTT produced a small, but significant, leftward shift in concentration-response curve for histamine displacement of [3H]mepyramine binding. These data suggest that DTT potentiates H1-receptor mediated Ca2+ responses by increasing agonist affinity.

Desensitization of histamine H1 receptor-mediated inositol phosphate accumulation in guinea pig cerebral cortex slices

1. Histamine stimulated the production of [3H]-inositol phosphates in untreated (control) guinea-pig cerebral cortex slices with a best-fit EC50 of 17 +/- 4 microM, and a best-fit maximum response of 385 +/- 23% over basal accumulation. 2. Histamine pretreatment desensitized guinea-pig cortex slices to a subsequent challenge with histamine, which was observed as a reduction in the best-fit maximum response to 182 +/- 32% over basal accumulation. 3. The time-course for the histamine-induced production of [3H]-inositol phosphates was approximately linear over 90 min of stimulation in both control and histamine pretreated slices. The rate of production in pretreated slices was significantly slowed compared to control, such that by 90 min of histamine stimulation the desensitized slices produced 2.8 times the basal [3H]-inositol phosphate accumulation compared to 5.3 fold the basal [3H]-inositol phosphate accumulation in the control slices. 4. Displacement of [3H]-mepyramine binding to homogenates of guinea-pig cerebral cortex by mepyramine and histamine revealed that histamine pretreatment did not alter the apparent affinity of the H1 receptor for histamine (control Kd = 6.3 +/- 0.7 microM, desensitized Kd = 7.9 +/- 1.6 microM) or mepyramine (control Kd = 3.4 +/- 0.8 nM, desensitized Kd = 3.4 +/- 1.3 nM), nor was there any reduction in the calculated maximum number of [3H]-mepyramine binding sites (control Bmax = 192 +/- 31 fmol mg-1 protein, desensitized Bmax = 220 +/- 50 fmol mg-1 protein). 5. The histamine-mediated desensitization of response in guinea-pig slices was mediated by the HI receptor subtype, since the attentuated maximum histamine-stimulated [3H]-inositol phosphate accumulation could not be prevented by inclusion of an H2- (ranitidine) and an H3- (thioperamide) receptor antagonist during the pretreatment period.6. The desensitized histamine-stimulated [3H]-inositol phosphate accumulation recovered to 90% of control levels over a period of 150 min after the removal of the conditioning dose of histamine, with a half-time of recovery of about 95 min.

Regulation of histamine H1 receptor-mediated phosphoinositide hydrolysis by histamine and phorbol esters in DDT1 MF-2 cells

The regulation of histamine-stimulated phosphoinositide turnover by histamine and phorbol esters was examined in intact DDT1 MF-2 cells grown in suspension culture. Histamine increased the incorporation of 32P into phosphatidylinositol (PI) in these cells, and this stimulation was inhibited by the H1 antagonist diphenhydramine but not by the H2 antagonist cimetidine. Pretreatment of cells with histamine or with phorbol 12-myristate 13-acetate (PMA) or other activators of protein kinase C induced a marked decrease in the subsequent stimulation by histamine. PMA, but not histamine, also decreased the ability of epinephrine to stimulate PI labelling through alpha 1-adrenoceptors. Thus, histamine appears to induce homologous desensitization of histamine H1 receptor-mediated PI turnover, whereas direct activation of protein kinase C in the absence of receptor occupancy by agonist induces nonspecific heterologous desensitization of both histamine H1- and alpha 1-adrenoceptor-mediated responses.

1,4-Dithiothreitol-induced changes in histamine H1-agonist efficacy and affinity in the longitudinal smooth muscle of guinea-pig ileum

The effect of 1,4-dithiothreitol (DTT) on histamine H1-receptor agonist affinity and efficacy has been investigated in longitudinal muscle strips of guinea-pig ileum. Exposure of ileal smooth muscle to DTT significantly increased the maximal responses to the partial agonists SKF71473 and DE-2PEA, indicative of an increase in agonist efficacy. This effect was paralleled by a small decrease in EC50 values. In contrast, DTT produced a parallel displacement of the concentration-response curve to the full agonist histamine in the same muscle strips. Studies in which phenoxybenzamine and benzilylcholine mustard were used to reduce the maximum response to histamine suggested that DTT altered both agonist affinity and efficacy. The affinity constant for histamine, calculated by the method of Furchgott & Bursztyn (1967), increased by 2.7 fold in the presence of DTT. Furthermore, agonist efficacy also appeared to increase in the presence of DTT since the maximum response to histamine following phenoxybenzamine treatment increased on application of DTT. [3H]-mepyramine binding studies confirmed that DTT increased agonist affinity. DTT produced a significant parallel shift to the left of the displacement curves for histamine, 2-methylhistamine, 2-pyridylethylamine and 2-thiazolylethylamine. The results of this study therefore suggest that DTT potentiates H1-receptor-mediated contractile activity in guinea-pig ileum by increasing both agonist efficacy and affinity.

Enhancement of histamine H1-receptor agonist activity by 1,4-dithiothreitol in guinea-pig cerebellum and cerebral cortex

The disulphide bond-reducing agent 1,4-dithiothreitol (1 mM) produced a marked potentiation of histamine-stimulated accumulation of [3H]inositol phosphates in lithium-treated slices of guinea-pig cerebellum and cerebral cortex. This was seen as a parallel shift of the concentration-response curve for histamine to lower agonist concentrations, with no significant effect on the maximal response or Hill coefficient. Dithiothreitol similarly potentiated the augmentation of adenosine-stimulated cyclic AMP accumulation elicited by histamine in guinea-pig cerebral cortex. Studies with partial agonists suggested that this potentiating effect was associated with an increase in agonist efficacy rather than a change in agonist binding affinity. Thus, dithiothreitol increased the maximal accumulation of [3H]inositol phosphates produced by both 2-pyridylethylamine and 2-methylhistamine, which appeared to act as partial agonists in guinea-pig cerebral cortex. Dithiothreitol similarly increased the maximal extent of the augmentation of adenosine-stimulated accumulation of cyclic AMP produced by 2-methylhistamine. The site of action of dithiothreitol is not known; however, a comparison of the effect of dithiothreitol on muscarinic and histamine H1-receptor-mediated phosphoinositide responses in guinea-pig cerebral cortex suggests that it is before the stage at which the receptor-effector pathways are shared by these two receptor systems.

1,4-Dithiothreitol-induced alteration in histamine H1-agonist binding in guinea-pig cerebellum and cerebral cortex

The effect of the disulphide bond reducing agent 1,4-dithiothreitol (DTT) on the binding characteristics of the H1-selective ligand [3H]mepyramine has been examined in homogenates of guinea-pig cerebral cortex and cerebellum. DTT was found to be without effect on antagonist binding. This was evident from studies using different concentrations of [3H]mepyramine (0.5-10.0 nM), which showed no change in either the equilibrium dissociation constant (KD) or specific binding site capacity (Bmax). Furthermore, the IC50 values and slope parameters determined from the inhibition of the binding of 1 nM [3H]mepyramine by non-radioactive mepyramine were similarly insensitive to DTT in both cerebral cortex and cerebellum. In contrast, DTT shifted the inhibition curve for histamine to lower agonist concentrations and reduced the Hill coefficient in these two tissues. Analysis of these inhibition curves as double hyperbolae revealed two binding sites in the presence of DTT and only one low affinity site in the absence of DTT. Similar changes in the location (IC50) and slope (Hill coefficient) parameters were obtained with the H1-selective agonist 2-thiazolylethylamine in cerebellum and with 2-methylhistamine in both brain regions. The results of this study show that DTT affects agonist but not antagonist binding in guinea-pig cerebellum and cerebral cortex, and suggest that DTT stabilises a proportion of the agonist binding sites in a high affinity state.