Functional activation of beta-adrenergic receptors by thiols in the presence or absence of agonists

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.

Delineation of ligand binding and receptor signaling activities of purified P2Y receptors reconstituted with heterotrimeric G proteins

P2Y receptors are G protein coupled receptors that respond to extracellular nucleotides to promote a multitude of signaling events. Our laboratory has purified several P2Y receptors with the goal of providing molecular insight into their: (1) ligand binding properties, (2) G protein signaling selectivities, and (3) regulation by RGS proteins and other signaling cohorts. The human P2Y₁ receptor and the human P2Y₁₂ receptor, both of which are intimately involved in ADP-mediated platelet aggregation, were purified to near homogeneity and studied in detail. After high-level expression from recombinant baculovirus infection of Sf9 insect cells, approximately 50% of the receptors were successfully extracted with digitonin. Purification of nearly homogeneous epitope-tagged P2Y receptor was achieved using metal-affinity chromatography followed by other traditional chromatographic steps. Yields of purified P2Y receptors range from 10 to 100 μg/l of infected cells. Once purified, the receptors were reconstituted in model lipid vesicles along with their cognate G proteins to assess receptor function. Agonist-promoted increases in steady-state GTPase assays demonstrated the functional activity of the reconstituted purified receptor. We have utilized this reconstitution system to assess the action of various nucleotide agonists and antagonists, the relative G protein selectivity, and the influence of other proteins, such as phospholipase C, on P2Y receptor-promoted signaling. Furthermore, we have identified the RGS expression profile of platelets and have begun to assess the action of these RGS proteins in a reconstituted P2Y receptor/G protein platelet model.

Analysis of adenosine A₂a receptor stability: effects of ligands and disulfide bonds

G protein-coupled receptors (GPCRs) constitute the largest family of integral membrane proteins present in all eukaryotic cells, yet relatively little information about their structure, folding, and stability has been published. In this work, we describe several approaches to characterizing the conformational stability of the human adenosine A(2)a receptor (hA(2)aR). Thermal denaturation and chemical denaturation were not reversible, yet clear differences in the unfolding behavior were observed upon ligand binding via circular dichroism and fluorescence spectrometry. We found that the stability of hA(2)aR was increased upon incubation with the agonist N(6)-cyclohexyladenosine or the antagonist theophylline. When extracellular disulfide bonds were reduced with a chemical reducing agent, the ligand binding activity decreased by ~40%, but reduction of these bonds did not compromise the unfolding transition observed via urea denaturation. Overall, these approaches offer a general strategy for characterizing the effect of surfactant and ligand effects on the stability of GPCRs.

Neutral antagonist activity of naltrexone and 6beta-naltrexol in naïve and opioid-dependent C6 cells expressing a mu-opioid receptor

BACKGROUND AND PURPOSE

Adenylyl cyclase sensitization occurs on chronic agonist activation of mu-opioid receptors and is manifested by an increase in cAMP levels (overshoot) on challenge with antagonist. It has been proposed that a long lasting constitutively active receptor is formed on chronic mu-opioid exposure and that antagonists with inverse agonist activity rapidly return the receptor to a basal state causing a cAMP overshoot and a more severe withdrawal response in vivo. This hypothesis depends on an accurate characterization of neutral and inverse agonist properties of opioid antagonists.

EXPERIMENTAL APPROACH

C6 glioma and HEK293 cells expressing mu-opioid receptors were used. Opioid antagonists were examined for their ability to induce a cAMP overshoot following chronic treatment with the agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Glyol(5)]-enkephalin). The compounds were also characterized as agonists, inverse agonists or neutral antagonists by using assays for competitive binding, [(35)S]GTPgammaS (guanosine-5'-O-(3-[(35)S]thio)triphosphate) binding and changes in cell surface receptor expression.

KEY RESULTS

Naltrexone, 6beta-naltrexol and naloxone were indistinguishable to the mu-opioid receptor in the opioid-naïve or dependent state and acted as neutral antagonists. The delta-opioid receptor inverse agonist RTI-5989-25 [(+)-N-[trans-4'-(2-methylphenyl)-2'-butenyl]-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine], a 3,4-dimethyl-4-(3-hydroxyphenyl)-piperidine, was an inverse agonist at the mu-opioid receptor, and the peptide antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) showed variable, assay-dependent properties. All the antagonists precipitated the same degree of cAMP overshoot in opioid-dependent cells.

CONCLUSIONS AND IMPLICATIONS

Antagonists at the mu-opioid receptor may be neutral or show inverse agonist activity. Formation of a constitutively active mu-opioid receptor is not a requirement for the development or expression of adenylyl cyclase sensitization.

Agonist-stimulated reactive oxygen species formation regulates beta2-adrenergic receptor signal transduction

Generation of reactive oxygen species (ROS) can occur upon agonist stimulation of surface receptors to modulate downstream signaling processes. Here, we show that activation of the beta2 adrenergic receptor (beta2AR) by stimulation with the agonist isoproterenol leads to generation of ROS that is required for beta2AR signal transduction. Specifically, we show that inhibition of NADPH oxidase with diphenyliodonium chloride, inhibition of the small GTPase Rac1 with NSC23766, and inhibition of formed ROS with the antioxidant N-acetyl-L-cysteine decreases beta2AR-mediated cAMP formation, protein kinase A activation, and receptor phosphorylation and internalization, but does not impact ligand binding. The results also show that inhibition of ROS attenuates active beta2AR-mediated binding of GTP to alpha subunits of heterotrimeric G proteins. Based on these results, we propose that agonist-dependent ROS formation is needed for beta2AR signal transduction, perhaps through stabilization of active receptor conformers by redox-mediated modification of receptor and/or Galpha proteins cysteine residues.

Molecular dynamics of a biophysical model for β2-adrenergic and G protein-coupled receptor activation

This study analyzes 16 molecular dynamic simulations of a biophysical model for beta(2)-adrenergic (B2AR) and G protein-coupled receptor (GPCR) activation. In this model, a highly conserved cysteine residue, C106 (C3.25 or CysIII:01), provides a free sulfhydryl or thiol group in an acid-base equilibrium between uncharged (RSH) and charged (RS(-)) states that functions as an electrostatic molecular switch for receptor activation. The transition of C106 in the B2AR between acid and base states significantly changes the helical/transmembrane (TM) domain interactions and the electrostatic interaction energy differences (DeltaDeltaE(EL)). The DeltaDeltaE(EL) changes correlate well with the experimentally observed ligand efficacies. The TM interaction energies display patterns compatible with those previously recognized as responsible for GPCR activation. Key differences between the agonist, epinephrine, and the antagonist, pindolol, are seen for the TM3 x 6, TM3 x 4, TM6 x 7 and TM1 x 7 interaction energies. Pindolol also produces a weaker DeltaDeltaE(EL) interaction and less TM interaction energy changes, which are important differences between the agonist and antagonist ligands. The D115E mutant with pindolol displays a greater DeltaDeltaE(EL) and TM interactions than for the wild-type B2AR with pindolol. This explains the higher activity of pindolol in the D115E mutant. The constitutively active D130A mutant displays TM interaction patterns similar to those for the activating ligands implying a common pattern for receptor activation. These findings support the broad concept of protean agonism and demonstrate the potential for allosteric modulation. They also demonstrate that this two-state model agrees with many previous experimental and theoretical observations of GPCRs.

Historical review: Negative efficacy and the constitutive activity of G-protein-coupled receptors

The idea that a receptor can produce signalling without agonist intervention and that several antagonists can be 'active' in repressing such spontaneous activity is contained in the concept of ligand-induced conformational changes. Yet, this idea was neglected by pharmacologists for many years. In this article, we review the events that brought inverse agonism and constitutive activity to general attention and made this phenomenon a topic of current research. We also suggest a classification of antagonists based on the cooperativity that links their primary site of interaction with other functional domains of the receptor.

Alkylation of sulfhydryl groups on Galpha(s/olf) subunits by N-ethylmaleimide: regulation by guanine nucleotides

In rat striatum A(2A) adenosine receptors activate adenylyl cyclase through coupling to G(s)-like proteins, mainly G(olf) that is expressed at high levels in this brain region. In this study we report that the sulfhydryl alkylating reagent, N-ethylmaleimide (NEM), causes a concentration- and time-dependent inhibition of [3H] 2-p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamido adenosine ([3H]CGS21680) binding to rat striatal membranes. Membrane treatment with [14C]N-ethylmaleimide ([14C]NEM) labels numerous proteins while addition of 5'-guanylylimidodiphosphate (Gpp(NH)p) reduces labeling of only three protein bands that migrate in SDS-polyacrylamide gel electrophoresis with apparent molecular masses of approximately 52, 45 and 39 kDa, respectively. The 52- and 45-kDa labeled bands show electrophoretic motilities as Galpha(s)-long and Galpha(s)-short/Galpha(olf) subunits. An anti-Galpha(s/olf) antiserum immunoprecipitates two 14C labeled bands of 44 and 39 kDa. The band density decreases by 21-26% when membranes are treated with NEM in the presence of Gpp(NH)p. An anti-A(2A) receptor antibody also immunoprecipitates two 14C labeled bands of 40 and 38 kDa, respectively. However, such protein bands do not show any decrease of their density upon membrane treatment with NEM plus Gpp(NH)p. These results indicate that in rat striatal membranes NEM alkylates sulfhydryl groups of both Galpha(s/olf) subunits and A(2A) adenosine receptors. In addition, cysteine residues of Galpha(s/olf) are easily accessible to modification when the subunit is in the GDP-bound form. The 39- and 38-kDa labeled proteins may represent proteolytic fragments of Galpha(s/olf) and A(2A) adenosine receptor, respectively.

Dependence of the coupling of dopamine receptors to G proteins on the protein redox state in the neural plasma membranes of pond snail

Binding analysis using [3H]dopamine has shown that reduction of protein thiol groups with dithiothreitol (DTT) led to a dual effect on the receptors. First, the amount of dopamine-binding sites on the membranes and their affinity to the ligand were decreased. Second, the affinity of the receptors to [3H]dopamine was enhanced in the presence of GDP. Binding of D(1) antagonist [3H]SCH23390 to dopamine receptors increased following DTT treatment, opposite to the case with D(1) agonist [3H]SKF38393. The displacement of [3H]GDP by GTPgammaS was depressed by dopamine. Stimulation of [3H]GDP binding by dopamine was potentiated after incubation with DTT. Membrane nitrosylation eliminated the reciprocal dependence of GDP and dopamine binding to the membranes. It is suggested that binding of dopamine to the receptors can lead to both stimulation and inhibition of G protein activity, and the ratio of these effects depends on the reduction and oxidation of sulfhydryl groups of membrane proteins. Thiol reduction potentiated inhibitory action of dopamine receptors on coupled G proteins, and nitrosylation led to their uncoupling.

Redox-dependent gating of G protein-coupled inwardly rectifying K+ channels

G protein-coupled inwardly rectifying K(+) channels (GIRK) play a major role in inhibitory signaling in excitable and endocrine tissues. The gating mechanism of these channels is mediated by a direct interaction of the Gbetagamma subunits of G protein, which are released upon inhibitory neurotransmitter receptor activation. This gating mechanism is further manifested by intracellular factors such as anionic phospholipids and Na(+) and Mg(2+) ions. In addition to the essential role of these components for channel function, phosphorylation events can also modulate channel activity. In this study we explored the involvement of redox modulation on GIRK channel function. Extracellular application of the reducing agent dithiothreitol (DTT), but not reduced glutathione, activated GIRK channels without affecting their permeation or rectification properties. The DTT-dependent activation was found to mimic receptor activation and to act directly on the channel in a membrane delimited fashion. A critical cysteine residue located in the N-terminal cytoplasmic domain was found to be essential for DTT-dependent activation in hetero- and homotetrameric contexts. Interestingly, when mutating this cysteine residue, DTT-dependent activation was abolished, but receptor-mediated channel activation was not affected. These results suggest that intracellular redox potential can play a major role in tuning GIRK channel activity in a receptor-independent manner. This sort of redox modulation can be part of an important cellular protective mechanism against ischemic or hypoxic insults.

NMDA receptors are involved in dithiothreitol-induced hypothermia

Sulfhydryl-reducing agents, such as dithiothreitol, modulate glutamate N-methyl-D-aspartate (NMDA) receptors. Since these receptors are involved in thermoregulatory processes, we studied the effects of their positive modulation, through a dithiothreitol-induced reduction of the receptor redox site, on thermoregulation in rats maintained at an ambient temperature of 20-22 degrees C. Given intraperitoneally at the dose of 25 and 50 mg x kg(-1), dithiothreitol induced dose-dependent hypothermia. The prior administration of 0.5 mg x kg(-1) of (+/-)-dizocilpine maleate (MK801), a non-competitive glutamate NMDA receptor antagonist, blocked most of the dithiothreitol-induced hypothermia. MK801 given alone was followed by slight transient hyperthermia. This confirms the involvement of NMDA receptors in thermoregulation and suggests that they might be under redox modulation.

The full-length, cytoplasmic C-terminus of the beta 2-adrenergic receptor expressed in E. coli acts as a substrate for phosphorylation by protein kinase A, insulin receptor tyrosine kinase, GRK2, but not protein kinase C and suppresses desensitization when expressed in vivo

The ability of the cytoplasmic, full-length C-terminus of the beta 2-adrenergic receptor (BAC1) expressed in Escherichia coli to act as a functional domain and substrate for protein phosphorylation was tested. BAC1 was expressed at high-levels, purified, and examined in solution as a substrate for protein phosphorylation. The mobility of BAC1 on SDS-PAGE mimics that of the native receptor itself, displaying decreased mobility upon chemical reduction of disulfide bonds. Importantly, the C-terminal, cytoplasmic domain of the receptor expressed in E. coli was determined to be a substrate for phosphorylation by several candidate protein kinases known to regulate G-protein-linked receptors. Mapping was performed by proteolytic degradation and matrix-assisted laser desorption ionization, time-of-flight mass spectrometry. Purified BAC1 is phosphorylated readily by protein kinase A, the phosphorylation occurring within the predicted motif RRSSSK. The kinetic properties of the phosphorylation by protein kinase A displayed cooperative character. The activated insulin receptor tyrosine kinase, which phosphorylates the beta-adrenergic receptor in vivo, phosphorylates BAC1. The Y364 residue of BAC1 was predominantly phosphorylated by the insulin receptor kinase. GRK2 catalyzed modest phosphorylation of BAC1. Phosphorylation of the human analog of BAC1 in which Cys341 and Cys378 were mutated to minimize disulfide bonding constraints, displayed robust phosphorylation following thermal activation, suggesting under standard conditions that the population of BAC1 molecules capable of assuming the "activated" conformer required by GRKs is low. BAC1 was not a substrate for protein kinase C, suggesting that the canonical site in the second cytoplasmic loop of the intact receptor is preferred. The functional nature of BAC1 was tested additionally by expression of BAC1 protein in human epidermoid carcinoma A431 cells. BAC1 was found to act as a dominant-negative, blocking agonist-induced desensitization of the beta-adrenergic receptor when expressed in mammalian cells. Thus, the C-terminal, cytoplasmic tail of this G-protein-linked receptor expressed in E. coli acts as a functional domain, displaying fidelity with regard to protein kinase action in vivo and acting as a dominant-negative with respect to agonist-induced desensitization.

Reversible inactivation of AT(2) angiotensin II receptor from cysteine-disulfide bond exchange

Dithiothreitol (DTT) treatment of angiotensin II (Ang II) type 2 (AT(2)) receptor potentiates ligand binding, but the underlying mechanism is not known. Two disulfide bonds proposed in the extracellular domain were examined in this report. Based on the analysis of ligand affinity of cysteine (Cys, C) to alanine (Ala, A) substitution mutants, we provide evidence that Cys(35)-Cys(290) and Cys(117)-Cys(195) disulfide bonds are formed in the wild-type AT(2) receptor. Disruption of the highly conserved Cys(117)-Cys(195) disulfide bond linking the second and third extracellular segments leads to inactivation of the receptor. The Cys(35)-Cys(290) bond is highly sensitive to DTT. Its breakage results in an increased binding affinity for both Ang II and the AT(2) receptor-specific antagonist PD123319. Surprisingly, in the single Cys mutants, C35A and C290A, a labile population of receptors is produced which can be re-folded to high-affinity state by DTT treatment. These results suggest that the free -SH group of Cys(35) or Cys(290) competes with the disulfide bond formation between Cys(117) and Cys(195). This Cys-disulfide bond exchange results in production of the inactive population of the mutant receptors through formation of a non-native disulfide bond.

Modification of cardiac beta-adrenoceptor mechanisms by H2O2

From the role of oxidative stress in cardiac dysfunction, we investigated the effect of H2O2, an activated species of oxygen, on beta-adrenoceptors, G proteins, and adenylyl cyclase activities. Rat heart membranes were incubated with different concentrations of H2O2 before the biochemical parameters were measured. Both the affinity and density of beta 1-adrenoceptors were decreased, whereas the density of the beta 2-adrenoceptors was decreased and the affinity was increased by 1 mM H2O2. Time- and concentration-dependent biphasic changes in adenylyl cyclase activities in the absence or presence of isoproterenol were observed when membranes were incubated with H2O2; however, activation of the enzyme by isoproterenol was increased or unaltered. The adenylyl cyclase activities in the absence or presence of forskolin, NaF, and Gpp(NH)p were depressed by H2O2. Catalase alone or in combination with mannitol was able to significantly decrease the magnitude of alterations due to H2O2. The cholera toxin-stimulated adenylyl cyclase activity and ADP ribose labeling of Gs proteins were decreased by treatment with 1 mM H2O2, whereas Gi protein activities, as reflected by pertussis toxin-stimulation of adenylyl cyclase and ADP ribosylation, were unaltered. The Gs and Gi protein immunoreactivities, estimated by labeling with respective antibodies, indicate a decrease in binding to the 45-kDa band of Gs protein, whereas no change in the binding of antibodies to the 52-kDa band of Gs protein or the 40-kDa subunit of Gi protein was evident when the membranes were treated with 1 mM H2O2. These results suggest that H2O2 in high concentrations may attenuate the beta-adrenoceptor-linked signal transduction in the heart by changing the functions of Gs proteins and the catalytic subunit of the adenylyl cyclase enzyme.

Alternations in beta-Adrenoceptor Mechanisms in Hearts Perfused With Xanthine Plus Xanthine Oxidase

BACKGROUND: Although beta-adrenoceptors and adenylyl cyclase are known to be affected upon exposing cardiac membranes to some oxyradical generating systems, the results are conflicting. Furthermore, functional significance of alterations in the beta-adrenoceptor-adenylyl cyclase systems in terms of changes in the inotropic responses to catecholamines is not clear. METHODS AND RESULTS: The positive inotropic effect of isoproterenol was augmented on perfusing the isolated rat hearts with xanthine (X) plus xanthine oxidase (XO) for 5 minutes but was attenuated by perfusion for 15 minutes. The isoproterenol-stimulated adenylyl cyclase activity in cardiac membranes showed an increase at 10 minutes and a decrease at 30 minutes perfusion of hearts with X plus XO. The density of beta-adrenoceptors in cardiac membraners was reduced after 10 minutes and 30 minutes of perfusion with X plus XO, whereas the affinity of beta-adrenoceptors was increased after 10 minutes and reduced after 30 minutes. Although beta-adrenoceptors was increased after 10 minutes and reduced after 30 minutes. Although beta-adrenoceptors were unaltered by 10 minutes of perfusion with X plus XO, their affinity was increased and density was decreased by 30 minutes of perfusion. The agonist competition curves using isoproterenol indicated an increase in the number of coupled receptors in the high affinity state on 10 minutes of perfusion and an increase in the low affinity state of coupled receptor due to 30 minutes of perfusion with X plus XO. The basal as well as forskolin-, NaF- and Gpp(NH)p-stimulated adenylyl cyclase activities in cardiac membranes exhibited an increase after 10 minutes and decrease after 30 minutes of perfusion with X plus XO. Although the presence of superoxide dismutase plus catalase in the perfusion medium prevented most of the alterations due to X plus XO, it did not alter the increased affinity of the beta-adrenoceptor upon perfusing hearts for 10 minutes with X plus XO. CONCLUSIONS: The results in this study suggest the biphasic nature of the oxyradical-induced alterations in both the inotropic responses to catecholamines and the beta-adrenoceptor-mediated signal transduction mechanism in the heart.

Carboxyl-terminal fragments of phospholipase C-beta1 with intrinsic Gq GTPase-activating protein (GAP) activity

Fragments of the approximately 50 kDa COOH-terminal region of phospholipase C-beta1 (PLC-beta1(1)), ranging in size from 14 to 38 kDa, were expressed in Escherichia coli, purified, and tested for their regulatory activities. As expected, none of the fragments had phospholipase activity. Several fragments, referred to as PLC tails, displayed GTPase-activating protein (GAP) activity for Gq, the G protein class that stimulates the PLC-betas in response to receptors. Gq GAP activity is characteristic of intact PLC-betas. In reconstituted phospholipid vesicles that contained purified Gq and m1 muscarinic cholinergic receptors, the most active tails increased agonist-stimulated, steady-state GTPase activity over 4-fold. Stimulation of steady-state GTPase by the tails depended on receptors for facilitation of GDP-GTP exchange, suggesting that the tails act by accelerating hydrolysis of bound GTP. In addition to intrinsic GAP activity, one tail with high GAP activity and others with low or minimal activity potentiated the GAP activity of intact PLC-beta1. Other tails inhibited PLC-beta1s GAP effect. Both intrinsic GAP activity and potentiation of the PLC-beta1 GAP effect were often biphasic, with maxima as low as 100 nM tail and declining activities at higher concentrations. Several tails inhibited either the phospholipase activity of PLC-beta1, its stimulation by Gq, or both. The tails thus define the region of PLC-beta1 that has Gq GAP activity and suggest a mechanism of action in which the COOH terminus of PLC-betas can interact with Gq and with other PLC-beta1 molecules.

Towards a standard method to demonstrate adenylate cyclase activity at the electron microscopical level

The ventral epidermis of the frog Rana fuscigula is a typical tight epithelium which acts as a functional syncytium in the active transepithelial transport of sodium ions. Transport across this epithelium is regulated by cyclic adenosine monophosphate (cAMP). This study was undertaken to formulate an optimal protocol for the localization, within this epithelium, of adenylate cyclase; the enzyme involved in cAMP synthesis. The ventral epithelium of R. fuscigula was collagenase treated and processed using five different fixation/incubation protocols. The components of a basal incubating medium were modified by changing the localizing agent, adding adenylate cyclase stimulators and inhibitors of other enzymes. Control incubations undertaken included a) leaving the substrate out, b) prior heat inactivation of the enzyme, c) specific blockers and d) incubation for alkaline phosphatase as an alternative enzyme. The samples were then processed for electron microscopy. Localization of adenylate cyclase was best obtained, when fixing the tissue after incubation for 30 min at 37 degrees C. The medium that gave the best and most consistent localization contained magnesium chloride; as a required ion, theophylline, dithiothreitol, ouabain, levamisole; as enzyme inhibitors, forskolin; as a stimulator of adenylate cyclase, lead nitrate; as the capture agent and column purified adenylyl imidodiphosphate; as the substrate.

Lateral mobility of tetramethylrhodamine (TMR) labelled G protein alpha and beta gamma subunits in NG 108-15 cells

Multi-step signal transducing events, such as those mediated by G proteins, have been difficult to study in intact cells. We prepared fluorescently labelled G protein subunits, tetramethylrhodamine-alpha o (TMR-alpha o) and TMR-beta gamma, in order to study their subcellular distribution and lateral mobility. Heterotrimeric G proteins labelled in the alpha (TMR-alpha o/beta gamma) or beta (TMR-beta gamma/alpha o) subunit were reconstituted into lipid vesicles and fused to NG-108-15 cells using polyethylene glycol (PEG). Vesicles fused completely to the cells as determined by dequenching of a fluorescent lipid probe, octadecyl rhodamine B. The orientation of G protein beta gamma subunits after fusion followed the expected random distribution; the quenching of surface fluorescence with anti-fluorescein antibodies showed that about 50% of the label was accessible extracellularly. G proteins incorporated by the fusion method were able to couple to endogenous alpha 2 adrenergic receptors based on the restoration of high affinity agonist binding to pertussis toxin-treated cells. The subcellular localization of TMR-alpha o and TMR-beta gamma determined by differential centrifugation and confocal microscopy indicated that TMR-alpha o was present in the plasma membrane and in intracellular membranes, whereas TMR-beta gamma was mainly localized in the plasma membrane. The lateral mobility of TMR-alpha o and TMR-beta gamma measured using fluorescence recovery after photobleaching (FRAP) demonstrated low mobile fractions of 0.34 +/- 0.03 and 0.16 +/- 0.03, respectively. The translational diffusion coefficients of the mobile components were similar, 4.0 x 10(-9) and 2.0 x 10(-9) cm2/s, for alpha and beta gamma respectively. Neither activation of Gi-linked receptors nor cytoskeletal disruption with nocodozole or cytochalasin D changed the mobile fraction or diffusion coefficient of the alpha or beta gamma subunits. The FRAP data combined with the localization of fluorescent subunits by confocal microscopy suggest that the beta gamma subunits are highly constrained to localized regions of the plasma membrane while the alpha subunit may diffuse in intracellular regions to transmit signals from receptors to effector proteins.

Reconstitution of the myometrial oxytocin receptor into proteoliposomes. Dependence of oxytocin binding on cholesterol

The requirements for regaining high-affinity binding of the myometrial oxytocin receptor after detergent solubilization were investigated by reconstitution experiments. Large unilamellar liposomes were prepared by reverse-phase evaporation from different mixtures of phospholipids, cholesterol and cholesteryl hemisuccinate. In the presence of the oxytocin receptor solubilized from myometrial membranes from pregnant guinea pig uterus, liposomes were treated with 3-[(3-cholamidopropyl)-dimethylammonio]-2-hydroxy-1-propanesulfonate (Chapso) throughout the range of detergent concentrations that cause the transformation of lamellar structures to mixed micelles. Detergent removal was achieved using bio-beads SM-2 as adsorbent. The presence of cholesterol was a prerequisite for regaining high-affinity binding of [3H]oxytocin and 125I-oxytocin antagonist to reconstituted proteoliposomes. Binding of [3H]oxytocin but not of the antagonist was dependent on the presence of Mn2+ ions. Reconstitution after lectin chromatography and photoaffinity labeling of reconstituted vesicles resulted in the exclusive labeling of the oxytocin receptor with a molecular mass of 68-80 kDa.

Modulation of neutrophil migration by captopril

Because neutrophils might be involved in some of the therapeutic effects of captopril we studied the effect of captopril on neutrophil migration. Captopril strongly enhances migration by rabbit peritoneal neutrophils. Stimulation by captopril was maximal at a concentration of about 400 microM; at higher concentrations stimulation decreased again. The stimulatory effect is partly chemokinetic, and partly chemotactic. Captopril disulfide also stimulates migration, though the stimulating effect is less than that of captopril. Captopril-induced stimulation of migration was strongly inhibited by pretreatment of neutrophils with pertussis toxin, indicating that the stimulation is mediated by a pertussis toxin-sensitive G protein. Intact sulfhydryl groups on the cell surface are required for stimulation because inactivation of these groups with the non-penetrating sulfhydryl reagent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) completely abolishes the stimulating effect of captopril and of captopril disulfide. Both captopril and captopril disulfide cause an enhancement of cyclic GMP level in the neutrophil. The enhancement of both the cGMP level and that of migration is blocked by methylene blue and by LY-83583, suggesting that the stimulating effect of captopril is mediated by cGMP. Inhibitors of NO generation, such as NG-monomethyl-L-arginine, NG-nitro-L-arginine and canavanine have no effect on captopril-induced stimulation of migration, indicating that NO is not involved in the stimulating effect of captopril.

Effect of oxidative stress on receptors and signal transmission

Reactive oxygen metabolites affect binding of ligands to membrane receptors and also coupling of receptors to G-proteins and effector enzymes. Peroxidation of membrane lipids may lead to a lowered receptor density and also will alter the viscosity of the plasma membrane, which affects receptor coupling. Reactive oxygen species may also interact with thiol/disulfide moieties on receptor proteins or on other factors in the receptor system, which is responsible for alterations in receptor binding or coupling. Moreover, lipid peroxidation is associated with the phospholipase A2 pathway, which might indirectly affect receptor function. Moreover, oxidative stress may lead to a disturbance in cellular Ca(2+)-homeostasis. This might be related to an effect on Ca(2+)-mobilizing receptors, but there is also evidence for a decreased Ca(2+)-sequestration by ATPases. In addition, peroxidation of membrane lipids increases membrane permeability to Ca2+. Finally, reactive oxygen species interfere with actions of nitric oxide, thus affecting another pharmacological messenger system.

Evolving concepts of partial agonism

The exact mechanism of receptor activation at the molecular level are still not known, nor do we completely understand the precise factors that distinguish agonist- and partial agonist-induced activation. Nevertheless, recent years have brought forth an explosion of new information regarding beta-adrenergic receptor structure and ligand-induced activation. Partial agonists are likely intermediate in their ability to interact with crucial serine residues (Ser204 and Ser207) on the beta-adrenergic receptor; these interactions allow either incomplete stimulation of the entire receptor population, or full stimulation of only a portion of the entire receptor population. From the work presented by Tota and Schimerlik for the muscarinic cholinergic receptor (another G-protein coupled receptor), it is likely that partial agonists induce or stabilize receptor conformations that have a lower affinity for their G protein compared to receptors stimulated by a full agonist. Molecular cloning of beta-adrenergic receptors and analyses of mutated and chimeric receptors expressed in transfected systems have indicated that domains of the receptor that bind agonists may be different from those with which antagonists interact. Thus, the ability of a partial agonist to interact with these two different domains may be a determinant of efficacy. Agonists alter the sulfhydryl redox status of the beta-adrenergic receptors in the presence of Gs. Disulfide rearrangement has been postulated to provide a structural constraint which biases G-protein-linked receptors in the "ground state" and may be important for stabilizing the active state of the receptor and holding the agonist/receptor/Gs ternary complex in the high-affinity state. Partial agonists induce this state less efficaciously or are less capable of holding the receptor in the active conformation to allow disulfide exchange to take place. The extent of receptor stimulation may dictate which G proteins are activated by a particular receptor, and thus which cellular effectors are stimulated. Alternatively, the level of activation of a receptor may translate into varying states of activation of a particular G protein (stabilized in part by disulfide bonds). Techniques such as fluorescence energy transfer in reconstitution systems or nuclear magnetic resonance spectroscopy should prove useful in distinguishing among these possible mechanisms. Ultimately, as a long-term goal, X-ray crystallography of unoccupied receptors and receptors liganded by partial or full agonists may provide definitive insights. Although definitive answers are not yet possible, the rapid progress in understanding aspects of receptor structure allows a reformulation of ideas regarding the molecular basis of efficacy and partial agonism.(ABSTRACT TRUNCATED AT 400 WORDS)

A review of sweet taste potentiation brought about by divalent oxygen and sulfur incorporation

The plethora of high-potency sweetener research has allowed the construction of important structure-taste relationships. In light of new structure-taste relationships, it is instructive to review sweet taste potentiation brought about by divalent oxygen and sulfur incorporation. The taste of sulfur-containing organic compounds was reviewed in Japanese by Yasuo Ariyoshi in 1977. Several new representative examples of sweet taste potentiation and taste dichotomy (sweet and bitter) found within similar classes of oxygen- and sulfur-containing organic compound: amides, dipeptides, ureas, sulfamates, sulfonamides, oximes, sugars, dihydroisocoumarins, and others are reviewed. Special attention is given to the thioethers and thioureas in sulfamates, dipeptides, aryl ureas, and hybrid dipeptide ureas. The most notable contributions have arisen from the work of Nofre and Tinti at Université Claude Bernard in Lyons, France. A common trend emerges with certain sweeteners when a carbon atom is strategically replaced by sulfur or oxygen atoms. The net result is an increase in the sweetness two- to tenfold. With saccharins, the usual bitter, metallic taste is removed. Sweet taste receptor models that have been published are mainly based on the original Shallenberger and Acree model of the glucophores AH-B with contributions from Kier (AH-B-X). AH is a proton donor group, B is a proton acceptor group, and X is some hydrophobic group. All of the models have overlooked the contributions of divalent sulfur (often in place of oxygen) in bringing about sweetness potentiation. There is no precedence for localizing the energy-minimized structures of sulfur-containing sweeteners in a binding mode that includes sulfur. These sulfur potentiation loci are analyzed and illustrated in a computer-generated sweetener model to show the specific region in which sulfur is being "recognized" as a potentiating feature.

Immunological approaches for probing receptor structure and function

Molecular cloning has revealed the primary sequence of numerous membrane receptors, and this information catalysed two important efforts: modeling of receptor structure by hydropathy analysis and generating sequence-specific immunological probes with which these models can be tested experimentally. Craig Malbon and his colleagues outline the recent advances that illustrate how anti-peptide antibodies raised to synthetic sequences of membrane receptor have generated new information on the topology, functional domains and cellular localization of transmembrane signaling elements. They focus on two examples, the G protein-linked beta-adrenoceptor, and the nicotinic acetylcholine receptor, an intrinsic ion channel receptor. These two classes of receptor provide templates for the analysis of topographical models of membrane proteins with immunological probes, especially anti-peptide antibodies, and demonstrate how these results complement those obtained from molecular, biochemical and biophysical techniques. Although this powerful strategy is not without faults, it is likely to continue to be applied successfully to the analysis of the structure and function of receptors, ion channels and other membrane proteins.

The GTP-insensitive component of high-affinity [3H]8-hydroxy-2-(di-n-propylamino)tetralin binding in the rat hippocampus corresponds to an oxidized state of the 5-hydroxytryptamine1A receptor

Previous studies on central 5-hydroxytryptamine1A (5-HT1A) receptors have consistently shown the existence of a GTP-insensitive component of agonist binding, i.e., binding of [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) that persists in the presence of 0.1 mM GTP or guanylylimidodiphosphate (GppNHp). The molecular basis for this apparent heterogeneity was investigated pharmacologically and biochemically in the present study. The GppNHp-insensitive component of [3H]8-OH-DPAT binding increased spontaneously by exposure of rat hippocampal membranes or their 3-[3-(cholamidopropyl)dimethylammonio]-1-propane sulfonate-soluble extracts to air; it was reduced by preincubation of solubilized 5-HT1A binding sites in the presence of dithiothreitol and, in contrast, reversibly increased by preincubation in the presence of various oxidizing reagents like sodium tetrathionate or hydrogen peroxide. In addition, exposure of hippocampal soluble extracts to short-cross-linking reagents specific for thiols produced an irreversible increase in the proportion of GppNHp-insensitive over total [3H]8-OH-DPAT binding. The pharmacological properties of this GppNHp-insensitive component of [3H]8-OH-DPAT binding were similar to those of 5-HT1A sites in the absence of nucleotide. Sucrose gradient sedimentation of solubilized 5-HT1A binding sites treated by dithiothreitol or sodium tetrathionate showed that oxidation prevented the dissociation by GTP of the complex formed by the 5-HT1A receptor binding subunit (R[5-HT1A]) and a guanine nucleotide-binding protein (G protein). Moreover, the oxidation of -SH groups by sodium tetrathionate did not prevent the inactivation of [3H]8-OH-DPAT specific binding by N-ethylmaleimide, in contrast to that expected from an interaction of both reagents with the same -SH groups on the R[5-HT1A]-G protein complex. These data suggest that the appearance of GTP-insensitive [3H]8-OH-DPAT specific binding occurs as a result of the (spontaneous) oxidation of essential -SH groups (different from those preferentially inactivated by N-ethylmaleimide) on the R[5-HT1A]-G protein complex.

The biology of beta-adrenergic receptors: analysis in human epidermoid carcinoma A431 cells

1. G-protein-linked transmembrane signaling has emerged as a major pathway for information transduction across the cell membrane. 2. In addition to photopigments that propagate the signal from light, cell-surface receptors for hormones, neurotransmitters, and autacoids propagate signals from ligand binding to membrane-bound effector units via G-proteins. 3. Biochemical and molecular features of one prominent member of these receptors, the beta-adrenergic receptor, will be highlighted in the present article. 4. The role of the human epidermoid carcinoma A431 cells as a model for the study of the structure and biology of beta-adrenergic receptors will be emphasized. 5. A model for receptor regulation, gleaned from recent advances in the biochemistry, cell and molecular biology of beta-adrenergic receptors, is discussed.

Further evidence against the coupling of dopamine receptors to phosphoinositide hydrolysis in rat striatum

The effects of D1 and D2 dopamine receptor agonists on phosphoinositide hydrolysis were studied by measuring the accumulation of radioactive inositol phosphates in slices of rat corpus striatum prelabelled with [3H]inositol. All assays were performed in the presence of lithium. Neither the D1 receptor agonist SKF 38393 nor the D2 receptor agonist quinpirole, alone or in combination, had an effect on basal accumulation of inositol phosphates. The muscarinic receptor agonist carbachol produced a robust increase in the accumulation of inositol monophosphate and a smaller increase in the accumulation of inositol bisphosphate. These effects were not altered by the presence of quinpirole. Additionally, quinpirole also had no effect when assays were conducted in the presence of the muscarinic receptor antagonist scopolamine, the glutamic acid receptor antagonist kynurenic acid, and the antioxidant glutathione. These results are discussed in relation to recent contradictory reports and lend support to the position that D2 dopamine receptors are not coupled to phosphoinositide hydrolysis in rat striatum.

Role of extracellular disulfide-bonded cysteines in the ligand binding function of the beta 2-adrenergic receptor

Evidence is presented for a role of disulfide bridging in forming the ligand binding site of the beta 2-adrenergic receptor (beta AR). The presence of disulfide bonds at the ligand binding site is indicated by "competitive" inhibition by dithiothreitol (DTT) in radioligand binding assays, by specific protection by beta-adrenergic ligands of these effects, and by the requirement of disulfide reduction for limit proteolysis of affinity ligand labeled receptor. The kinetics of binding inhibition by DTT suggest at least two pairs of disulfide-bonded cysteines essential for normal binding. Through site-directed mutagenesis, we indeed were able to identify four cysteines which are critical for normal ligand binding affinities and for the proper expression of functional beta AR at the cell surface. Unexpectedly, the four cysteines required for normal ligand binding are not those located within the hydrophobic transmembrane domains of the receptor (where ligand binding is presumed to occur) but lie in the extracellular hydrophilic loops connecting these transmembrane segments. These findings indicate that, in addition to the well-documented involvement of the membrane-spanning domains of the receptor in ligand binding, there is an important and previously unsuspected role of the hydrophilic extracellular domains in forming the ligand binding site.

Agonist-selective protection of the opioid receptor-coupled G proteins from inactivation by 5'-p-fluorosulphonylbenzoyl guanosine

The guanine nucleotide analogue, 5'-p-fluorosulphonylbenzoyl guanosine (FSBG), can react covalently with GTP-binding proteins (G proteins). In rat brain membranes, FSBG causes a time-dependent loss of beta,gamma-imido[8-3H]guanosine 5'-triphosphate binding sites. Using 1 mM FSBG, the guanyl nucleotide modulation of opioid agonist binding is abolished, whereas the guanyl nucleotide sensitivity of neurotensin binding is retained. The action of FSBG can be prevented by the presence of opioid agonists, but not the antagonist naloxone. Iodoacetamide treatment of membranes in the presence of agonist, but not antagonist, can attenuate the action of FSBG in blocking guanyl nucleotide modulation of opioid agonist binding. These results suggest that FSBG covalently modifies essential thiol groups, whose exposure to the reagent is modified by agonist occupancy of the receptor, on a species of G protein linked to opioid receptors, but not on a species of G protein linked to neurotensin receptors. Thus, FSBG may have selectivity for the forms of Gi or Go, proteins associated with opioid receptors.

Bovine olfactory cilia preparation: thiol-modulated odorant-sensitive adenylyl cyclase

We have characterized the adenylyl cyclase activity in a newly developed preparation of isolated olfactory cilia from the bovine chemosensory neuroepithelium. Like its counterparts from frog and rat, the ciliary enzyme was stimulated by guanine nucleotides, by forskolin, and by a variety of odorants in the presence of GTP. The main difference between the bovine olfactory cilia preparation and the frog and rat olfactory cilia preparation is that odorant stimulation of the bovine olfactory adenylyl cyclase is strongly inhibited by submillimolar concentrations of dithiothreitol. This inhibition is a consequence of a concomitant increase in the GTP-stimulated level and the decrease of the odorant stimulation of the enzyme. Nasal respiratory cilia have a much lower level of adenylyl cyclase activity and show no odorant stimulation. Owing to the large quantities of material available, the bovine olfactory cilia preparation is advantageous for studies of the proteins involved in chemosensory transduction.

Evidence for the presence of disulfide bridges in opioid receptors essential for ligand binding. Possible role in receptor activation

The mobility of purified mu opioid binding protein in SDS-polyacrylamide gek electrophoresis is sensitive to the presence of reducing agents. In the presence of increasing concentrations of DTT the apparent molecular weight increases in a stepwise fashion from 53 kDa to 65 kDa. This reduction in mobility is attributed to the successive breakage of disulfide bridges, resulting in an increasingly asymmetric molecule. Treatment of cell membranes from various brain areas with reducing agents, such as DTT, produced a concentration-dependent inhibition of opioid binding. Sensitivity to DTT inhibition varied between receptor types, mu greater than delta much greater than kappa. For mu receptors, agonist binding was considerably more sensitive to DTT than antagonist binding. Inhibition by DTT is readily reversible and is unaffected by Na+ and/or Mg2+ ions. Reversibility may be partially prevented by the inclusion of a low concentration of a reducing reagent such as glutathione which does not inhibit binding but blocks reformation of disulfide bonds. Scatchard analysis of saturation data shows that DTT causes a pronounced decrease in binding affinity with little effect on receptor number. It is suggested that disulfide bonds are essential for ligand binding and that cleavage of one or more of these bonds may play a role in opioid receptor activation by agonists.

Modification of ornithine decarboxylase activity by adrenergic stimulation in cultured chicken spleen cells

1. In vivo, adrenergic agonists promote an increase of ornithine decarboxylase activity (ODC) in chicken spleen, as opposed to a decrease in thymus and bursa of Fabricius. The increase is not due to the cell fraction separated on Lymphoprep, i.e. the spleen cells, but it could be due to the macrophages. 2. With spleen cells in culture, a marked increase of ODC activity is observed during the first 3 hr, followed by a decrease. 3. cAMP drastically decreases after 10 min in culture. 4. Adrenergic agonists promote a decrease of activity, both alpha and beta receptors being involved in these modifications. TPA promotes partial desensitization. 5. Selenite, which in vivo has the same effect as epinephrine, enhances ODC activity in culture. Propranolol partially counteracts this effect, while prazosin has a synergistic effect. TPA partially desensitizes spleen cells to selenite.

Is the autoxidation of catecholamines involved in ischemia-reperfusion injury?

The autoxidation of catecholamines has been proposed to be a source of oxygen radicals in ischemia-reperfusion injury. However, this autoxidation per se is extremely slow at physiological pH and therefore is unlikely to be a primary source of oxygen radicals in ischemia-reperfusion injury. On the other hand, oxygen radicals from catecholamines are more likely to arise through catalyzed oxidations involving enzymatic systems and/or metal ions. It is these latter reactions that may be of interest with respect to damage associated with ischemia-reperfusion injury.

Effect of thiols on beta 2-adrenoceptors in human mononuclear leucocytes

The effect of the disulfide reducing agent dithiothreitol (DTT) and other thiols on binding of the beta-adrenoceptor antagonist (-)-125iodocyanopindolol (125ICYP) to human mononuclear leucocytes (MNL) was investigated. Saturation experiments and dissociation kinetics revealed two classes of specific 125ICYP binding sites, one of high and the other of low affinity, respectively. In intact MNL DTT caused a decrease in specific binding. This was due almost selectively to a decrease in the affinity of high affinity binding sites, which decreased gradually in a concentration-dependent manner to the affinity of low affinity binding sites. In MNL membranes DTT decreased not only the affinity but also the number of high affinity binding sites. The DTT effect was completely reversible by simple reoxidation on air. The structural isomers (+/-)-DTT. (-)-DTT and dithioerythritol revealed identical effects on specific binding, whereas the monothiols mercaptoethanol and alpha-monothioglycerol, having a lower redox potential, were considerably less effective. In the same concentration range that influenced specific binding. DTT stimulated intracellular cAMP production. These results suggest functionally important disulfide bridges which regulate the affinity of beta-adrenoceptor binding sites in human MNL. They stabilize the receptor in a high affinity state; their reduction causes the conversion of the high affinity state into a low affinity state in a process associated with stimulation of adenylate cyclase. Available evidence indicates that a similar transformation is made by beta-adrenoceptor agonists. Consequently low affinity 125ICYP binding sites preexistent in untreated cells could represent a reduced receptor state resulting from agonist-receptor interaction in vivo.