Principles: receptor theory in pharmacology

Modeling activated states of GPCRs: the rhodopsin template

Activation of G Protein-Coupled Receptors (GPCRs) is an allosteric mechanism triggered by ligand binding and resulting in conformational changes transduced by the transmembrane domain. Models of the activated forms of GPCRs have become increasingly necessary for the development of a clear understanding of signal propagation into the cell. Experimental evidence points to a multiplicity of conformations related to the activation of the receptor, rendered important physiologically by the suggestion that different conformations may be responsible for coupling to different signaling pathways. In contrast to the inactive state of rhodopsin (RHO) for which several high quality X-ray structures are available, the structure-related information for the active states of rhodopsin and all other GPCRs is indirect. We have collected and stored such information in a repository we maintain for activation-specific structural data available for rhodopsin-like GPCRs, http://www.physiology.med.cornell.edu/GPCRactivation/gpcrindex.html . Using these data as structural constraints, we have applied Simulated Annealing Molecular Dynamics to construct a number of different active state models of RHO starting from the known inactive structure. The common features of the models indicate that TM3 and TM5 play an important role in activation, in addition to the well-established rearrangement of TM6. Some of the structural changes observed in these models occur in regions that were not involved in the constraints, and have not been previously tested experimentally; they emerge as interesting candidates for further experimental exploration of the conformational space of activated GPCRs. We show that none of the normal modes calculated from the inactive structure has a dominant contribution along the path of conformational rearrangement from inactive to the active forms of RHO in the models. This result may differentiate rhodopsin from other GPCRs, and the reasons for this difference are discussed in the context of the structural properties and the physiological function of the protein.

Molecular basis of partial agonism at the neurotransmitter alpha2A-adrenergic receptor and Gi-protein heterotrimer

To characterize the mechanism by which heterotrimeric G-proteins interpret the signals coming from various neurotransmitters of diverse efficacies (agonists and partial agonists) acting on alpha(2A)-adrenergic receptors, we used a fluorescent resonance energy transfer-based approach to study the effects of these partial agonists on the activation process of both the alpha(2A)-adrenergic receptor and its cognate G(i)-protein. We show that ligands of different efficacies switch the receptor into distinct conformational states, which in turn set the speed and extent of the G(i)-protein signaling. Thus, in cells the efficacy by which a receptor responds to diverse ligands is caused by the ability of the G-protein to differentiate between distinct receptor conformations. The data provide a new key characteristic underlying the mechanism of partial agonism at G-protein-coupled receptors.

Constitutive activity of muscarinic acetylcholine receptors

We review the literature describing constitutive activity of the five muscarinic acetylcholine receptors in native and recombinant systems and discuss the effect of constitutive activity on muscarinic pharmacology in the context of modern models of receptor activation. We include a summary of mutations found to cause constitutive activity and discuss the implications of these data for the structure, function, and activation mechanism of muscarinic receptors. Finally, we discuss the possible physiological significance of constitutive activity of muscarinic receptors, incorporating information provided by targeted deletion of each of the muscarinic subtypes.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Function-specific blockage of M(1) and M(3) muscarinic acetylcholine receptors by VX and echothiophate

Certain organophosphate (OP) cholinesterase inhibitors (ChEIs) are also known to bind to the muscarinic acetylcholine receptor (mAChR). The functional consequences of such binding were investigated here using the following OP compounds: VX, echothiophate, sarin, and soman. VX (charged at physiological pH) and echothiophate (formally charged) inhibited a specific signal transduction pathway in CHO cells expressing either the M(1) or M(3) mAChR. Hence, they blocked carbamylcholine (CCh)-induced cyclic adenosine monophosphate (cAMP) synthesis (muM) and had almost no effect on CCh-induced phosphoinositide (PI) hydrolysis. These substances were inactive on forskolin-induced cAMP inhibition signaling in CHO cells expressing M(2) mAChR. In binding studies, using [(3)H]-N-methyl scopolamine ([(3)H]NMS) as the competitor ligand, the ChEIs, VX and echothiophate exhibited binding to rat cortical mAChR with K(i) values in the muM range. The non-charged compounds, sarin and soman, were inert in modulating both cAMP metabolism and PI hydrolysis in CHO cells expressing M(1), M(2), and M(3) mAChRs, and no binding was observed in presence of [(3)H]NMS. These data suggest that VX and echothiophate act as function-specific blockers via a non-classical path of antagonistic activity, implying the involvement of allosteric/ectopic-binding site in M(1) and M(3) mAChRs. The functionally selective antagonistic behavior of echothiophate and VX makes them potential tools for dissecting the interactions of the mAChR with different G proteins.

Starting and stopping mechanisms of absence epileptic seizures are revealed by hazard functions

We show that the hazard function provides useful information about the starting and the stopping mechanisms of absence epileptic seizures. The hazard function quantifies changes in the probability that an event (respectively, the starting and the stopping of a seizure) occurs in some small time interval given that it has not occurred yet. It informs us about changes in the concentration of endogenous substances that modulate the neuronal signalling properties of (parts of) the brain. In a pharmacological experiment, we used the hazard function to study the effect of a GABA-transaminase inhibitor (vigabatrin) on the starting and the stopping mechanisms of absence epileptic seizures in a genetic rat model of absence epilepsy (the WAG/Rij rat). This experiment showed that a high GABA level changed the stopping mechanism of the absence epileptic seizures, creating much better conditions for very long seizures to develop. With respect to the starting mechanism, it was found that both with a high and a low GABA level, there was evidence for a recovery mechanism that decreases the probability that a new seizure starts. Initially, this probability is larger with a high GABA level, but gradually it converges to the same constant baseline probability as in the condition with a low GABA level.

Rapid, nongenomic responses to ecdysteroids and catecholamines mediated by a novel Drosophila G-protein-coupled receptor

Nongenomic response pathways mediate many of the rapid actions of steroid hormones, but the mechanisms underlying such responses remain controversial. In some cases, cell-surface expression of classical nuclear steroid receptors has been suggested to mediate these effects, but, in a few instances, specific G-protein-coupled receptors (GPCRs) have been reported to be responsible. Here, we describe the activation of a novel, neuronally expressed Drosophila GPCR by the insect ecdysteroids ecdysone (E) and 20-hydroxyecdysone (20E). This is the first report of an identified insect GPCR interacting with steroids. The Drosophila melanogaster dopamine/ecdysteroid receptor (DmDopEcR) shows sequence homology with vertebrate beta-adrenergic receptors and is activated by dopamine (DA) to increase cAMP levels and to activate the phosphoinositide 3-kinase pathway. Conversely, E and 20E show high affinity for the receptor in binding studies and can inhibit the effects of DA, as well as coupling the receptor to a rapid activation of the mitogen-activated protein kinase pathway. The receptor may thus represent the Drosophila homolog of the vertebrate "gamma-adrenergic receptors," which are responsible for the modulation of various activities in brain, blood vessels, and pancreas. Thus, DmDopEcR can function as a cell-surface GPCR that may be responsible for some of the rapid, nongenomic actions of ecdysteroids, during both development and signaling in the mature adult nervous system.

Some implications of receptor theory for in vivo assessment of agonists, antagonists and inverse agonists

Drug effects can be classified into three major phenotypes: agonist, antagonist and inverse agonist. Agonist and inverse agonist effects are associated with receptor activation and inactivation, respectively, whereas antagonism implies that a drug produces no effect when administered alone but blocks the effects of agonists and inverse agonists. Attention has only recently begun to focus on the theoretical and clinical implications of inverse agonists, and studies of inverse agonism have also stimulated revisions in receptor theory. This commentary addresses two specific issues related to the application of receptor theory to studies of inverse agonists in vivo. First, principles of receptor theory suggest that increasing drug doses produce a graded pharmacological stimulus that is transduced by receptor-containing tissue into a biological response. However, assays vary in their ability to detect those responses, and any given assay provides only a narrow window on the full range of underlying drug effects. Consequently, in vivo assessment of inverse agonists will benefit from development of assays sensitive to graded inverse agonist effects. Second, detection of inverse agonist effects requires some preexisting level of receptor activity (or tone). This tone can result from at least two sources: (a) endogenous ligands for the receptor, or (b) constitutive receptor activity. Strategies for discriminating these two sources of tone will also contribute to the in vivo assessment of inverse agonist effects. Studies with intermediate efficacy ligands may be especially helpful in this regard, because their effects are differentially influenced by endogenous agonist tone versus constitutive receptor tone.

Exposure to AC and DC magnetic fields induces changes in 5-HT1B receptor binding parameters in rat brain membranes

The binding properties of the G-protein coupled receptor (GPCR) serotonin 5-HT1B receptor were studied under exposure to AC (50 and 400 Hz) and DC magnetic fields (MF) in rat brain membranes. This was an attempt at replicating the positive findings of Massot et al. In saturation experiments using [3H]5-HT, 1-h exposures at 1.1 mT(rms) 50 Hz caused statistically significant increases in both the K(D) and B(max) binding parameters, from 1.74 +/- 0.3 to 4.51 +/- 0.86 nM and from 1428 +/- 205 to 2137 +/- 399 CPM, respectively, in good agreement with previous results. Exposure of the membranes at 400 Hz 0.675 mT(rms) did not elicit a larger increase in K(D) in spite of a much larger induced current density. DC fields (1.1 and 11 mT) had a lesser effect compared to AC fields at low values of K(Dsham), but decreased the affinity at higher values of K(Dsham). Modeling of the receptor-ligand-G protein interactions using the extended ternary complex model yielded good fits for all our data and that of Massot et al., showing that the AC field may act by decreasing the ability of the G-protein to alter the ligand-receptor affinity. The hypothesis is that the bipolar nature of the AC field explains the different nature of the effects observed with AC and DC exposures. These findings constitute one of the few documented pieces of evidence for cell-free effects of DC and extremely low frequency (ELF) AC MFs in the mT range.

Neutrophil chemorepulsion in defined interleukin-8 gradients in vitro and in vivo

We report for the first time that primary human neutrophils can undergo persistent, directionally biased movement away from a chemokine in vitro and in vivo, termed chemorepulsion or fugetaxis. Robust neutrophil chemorepulsion in microfluidic gradients of interleukin-8 (IL-8; CXC chemokine ligand 8) was dependent on the absolute concentration of chemokine, CXC chemokine receptor 2 (CXCR2), and was associated with polarization of cytoskeletal elements and signaling molecules involved in chemotaxis and leading edge formation. Like chemoattraction, chemorepulsion was pertussis toxin-sensitive and dependent on phosphoinositide-3 kinase, RhoGTPases, and associated proteins. Perturbation of neutrophil intracytoplasmic cyclic adenosine monophosphate concentrations and the activity of protein kinase C isoforms modulated directional bias and persistence of motility and could convert a chemorepellent to a chemoattractant response. Neutrophil chemorepulsion to an IL-8 ortholog was also demonstrated and quantified in a rat model of inflammation. The finding that neutrophils undergo chemorepulsion in response to continuous chemokine gradients expands the paradigm by which neutrophil migration is understood and may reveal a novel approach to our understanding of the homeostatic regulation of inflammation.

Mechanisms of ligand binding to the parathyroid hormone (PTH)/PTH-related protein receptor: selectivity of a modified PTH(1-15) radioligand for GalphaS-coupled receptor conformations

Mechanisms of ligand binding to the PTH/PTHrP receptor (PTHR) were explored using PTH fragment analogs as radioligands in binding assays. In particular, the modified amino-terminal fragment analog, (125)I-[Aib(1,3),Nle8,Gln10,homoarginine11,Ala12,Trp14,Tyr15]rPTH(1-15)NH2, (125)I-[Aib(1,3),M]PTH(1-15), was used as a radioligand that we hypothesized to bind solely to the juxtamembrane (J) portion of the PTHR containing the extracellular loops and transmembrane helices. We also employed (125)I-PTH(1-34) as a radioligand that binds to both the amino-terminal extracellular (N) and J domains of the PTHR. Binding was examined in membranes derived from cells expressing either wild-type or mutant PTHRs. We found that the binding of (125)I-[Aib(1,3),M]PTH(1-15) to the wild-type PTHR was strongly (approximately 90%) inhibited by guanosine 5'-O-(3-thio)triphosphate (GTPgammaS), whereas the binding of (125)I-PTH(1-34) was only mildly (approximately 25%) inhibited by GTPgammaS. Of these two radioligands, only (125)I-[Aib(1,3),M]PTH(1-15) bound to PTHR-delNt, which lacks most of the receptor's N domain, and again this binding was strongly inhibited by GTPgammaS. Binding of (125)I-[Aib(1,3),M]PTH(1-15) to the constitutively active receptor, PTHR-H223R, was only mildly (approximately 20%) inhibited by GTPgammaS, as was the binding of (125)I-PTH(1-34). In membranes prepared from cells lacking Galpha(S) via knockout mutation of Gnas, no binding of (125)I-[Aib(1,3),M]PTH(1-15) was observed, but binding of (125)I-[Aib(1,3),M]PTH(1-15) was recovered by virally transducing the cells to heterologously express Galpha(S). (125)I-PTH(1-34) bound to the membranes with or without Galpha(S). The overall findings confirm the hypothesis that (125)I-[Aib(1,3),M]PTH(1-15) binds solely to the J domain of the PTHR. They further show that this binding is strongly dependent on coupling of the receptor to Galpha(S)-containing heterotrimeric G proteins, whereas the binding of (125)I-PTH(1-34) can occur in the absence of such coupling. Thus, (125)I-[Aib(1,3),M]PTH(1-15) appears to function as a selective probe of Galpha(S)-coupled, active-state PTHR conformations.

Conformational analysis of the C-terminal Gly-Leu-Met-NH2 tripeptide of substance P bound to the NK-1 receptor

We examined the effect of simultaneously incorporating proline or proline-amino acid chimeras in positions 9, 10, and/or 11 of substance P, on the affinity for the two NK-1 binding sites and on second-messenger activation. Because these 3-substituted prolines constrain not only the (phi,psi) values of the peptide backbone, but also the chi space of the amino acid side chain, we were able to gather data on the structural requirements for high-affinity binding to the NK-1 receptor. We were able to confirm that this C-terminal component is crucial and that it should adopt an extended conformation close to a polyproline II structure when bound to the receptor. The partial additivity of these constraints, more specifically, for the NK-1M site, suggests that the peptide backbone flexibility around the hinge-point residue Gly9 is essential to subtly position crucial side chains.

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.

Pharmacological characterization of novel adenosine ligands in recombinant and native human A2B receptors

The present study was designed to evaluate the effects of novel and recognised compounds at human recombinant A(2B) adenosine receptors expressed in Chinese hamster ovary (hA(2B)CHO), in human embryonic kidney 293 (hA(2B)HEK-293) and at endogenous A(2B) receptors in human mast cells (HMC-1). Saturation binding experiments performed using the new high affinity A(2B) adenosine radioligand [(3)H]-N-benzo[1,3]dioxol-5-yl-2-[5-(2,6-dioxo-1,3-dipropyl-2,3,6,7-tetra hydro-1H-purin-8-yl)-1-methyl-1H-pyrazol-3-yloxy]-acetamide ([(3)H]-MRE 2029F20) revealed a single class of binding sites in hA(2B)CHO, hA(2B)HEK-293 and HMC-1 cells with K(D) (nM) of 1.65+/-0.18, 2.83+/-0.34, 2.62+/-0.27 and B(max) (fmol/mg protein) of 36+/-4, 475+/-50 and 128+/-15, respectively. The pharmacological profile of new compounds, determined in inhibition binding experiments in hA(2B)HEK-293 cells using [(3)H]-MRE 2029F20, showed a rank order of potency typical of the A(2B) receptors with K(i) values in the range 3.2-28nM. In functional assays, recognised agonists and antagonists were studied by evaluating their capability to modulate the cAMP production in hA(2B)CHO and in HMC-1 cells. Novel compounds were able to decrease NECA-stimulated cAMP production in hA(2B)CHO and in HMC-1 cells showing a high potency. New compounds were also able to inhibit cAMP levels in the absence of NECA and in the presence of forskolin stimulation in hA(2B)CHO and in HMC-1 cells. In HEK-293 cells MRE 2029F20 reduced cAMP basal levels with an IC(50) value of 2.9+/-0.3nM. These results suggest that novel compounds are antagonists with an inverse agonist activity in recombinant and native human A(2B) receptors.

Enhanced 5-HT2C receptor signaling is associated with haloperidol-induced "early onset" vacuous chewing in rats: implications for antipsychotic drug therapy

RATIONALE

Haloperidol is a representative of typical antipsychotics that are still in clinical use and which can lead to abnormal motor activity following repeated administration. The mechanisms underlying antipsychotic-induced dyskinesias are not well understood but are widely held to be related to excessive loss of dopamine function. In several models of dopamine hypofunction, serotonin 5-HT2C receptors have been shown to mediate vacuous chewing movements (VCM), a form of abnormal motor activity. It is well established that repeated haloperidol administration also elicits VCM, but there is no information on how repeated haloperidol administration affects 5-HT2C receptor signaling.

OBJECTIVES

In the present study, we tested the hypothesis that repeated daily administration of haloperidol leads to enhanced serotonin 5-HT2C receptor signaling that is associated with increased 5-HT2C-mediated VCM.

METHODS

Rats were treated by subcutaneous injection once daily for 21 days with either vehicle, a low dose of haloperidol (0.1 mg kg(-1) day(-1)), or a high dose of haloperidol (1.0 mg kg(-1) day(-1)). Following 1-day withdrawal, rats were either used for behavioral scoring of VCM or sacrificed for biochemical assessment of 5-HT2 receptor-mediated phospholipase C activity and radioligand binding. VCM were scored following two successive "drug" challenges. The first challenge was an injection of vehicle (0.9% saline), and the second challenge was an injection of the 5-HT2C agonist meta-chlorophenylpiperazine (1.0 mg/kg). In this manner, a measure of "spontaneous" and "5-HT2C-elicited" orofacial activity could be made while minimizing animal use.

RESULTS

Following 21-day haloperidol treatment at either dose, there was an increase in expression of meta-chlorophenylpiperazine-induced VCM. In a separate experiment, meta-chlorophenylpiperazine-induced VCM were shown to be mediated through 5-HT2C receptors. Striatal 5-HT2C receptor-mediated phospholipase C (PLC) activity and high-affinity agonist-labeled 5-HT2C receptors were also increased following either dose of haloperidol as compared to vehicle treatment. GTP-stimulated PLC activity and striatal Gq proteins were unchanged by haloperidol suggesting that enhanced signaling could be accounted for by alterations at the level of the receptor and not at downstream mechanisms.

CONCLUSIONS

Repeated daily administration of haloperidol leads to an adaptive increase in 5-HT2C signaling which may contribute to abnormal motor function associated with antipsychotic use.

Fugetaxis: active movement of leukocytes away from a chemokinetic agent

Chemotaxis or active movement of leukocytes toward a stimulus has been shown to occur in response to chemokinetic agents including members of the recently identified superfamily of proteins called chemokines. Leukocyte chemotaxis is thought to play a central role in a wide range of physiological and pathological processes including the homing of immune cells to lymph nodes and the accumulation of these cells at sites of tissue injury and pathogen or antigen challenge. We have recently identified a novel biological mechanism, which we term fugetaxis (fugere, to flee from; taxis, movement) or chemorepulsion, which describes the active movement of leukocytes away from chemokinetic agents including the chemokine, stromal cell derived factor-1, and the HIV-1 envelope protein, gp120. In this article, we review the evidence that supports the observation that leukocyte fugetaxis occurs in vitro and in vivo and suggestions that this novel mechanism can be exploited to modulate the immune response. We propose that leukocyte fugetaxis plays a critical role in both physiological and pathological processes in which leukocytes are either excluded or actively repelled from specific sites in vivo including thymic emigration, the establishment of immune privileged sites and immune evasion by viruses and cancer. We believe that current data support the thesis that a greater understanding of leukocyte fugetaxis will lead to the development of novel therapeutic approaches for a wide range of human diseases.

The differential effects of the gonadotropin receptors on aromatase expression in primary cultures of immature rat granulosa cells are highly dependent on the density of receptors expressed and the activation of the inositol phosphate cascade

Signaling pathways mediating the divergent effects of FSH and LH on aromatase in immature rat granulosa cells were studied by infecting cells with increasing amounts of adenoviral vectors for the human LH receptor (hLHR) or FSH receptor (hFSHR). Increasing amounts of Ad-hLHR, used at a multiplicity of infection (MOI) of 20 or 200 viable viral particles/cell, increased human chorionic gonadotropin (hCG) binding and hCG-induced cAMP and Akt phosphorylation, but inositol phosphates only increased in response to hCG in cells infected with 200 MOI Ad-hLHR. In contrast, hCG increased aromatase expression in cells infected with 20, but not in cells infected with 200, MOI Ad-hLHR. Cells infected with 20 or 200 MOI Ad-hFSHR showed increased hFSH binding and hFSH-induced Akt phosphorylation, but the hFSH-induced cAMP response was unchanged relative to control cells. However, hFSH was able to stimulate the inositol phosphate cascade in the Ad-hFSHR-infected cells, and the hFSH induction of aromatase was abolished. We also found that activation of C kinase or expression of a constitutively active form of Galphaq inhibited the induction of aromatase by hFSH or 8Br-cAMP. We conclude that the differential effects of FSH and LH on aromatase in immature granulosa cells are highly dependent on gonadotropin receptor density and on the signaling pathways activated. We propose that aromatase is induced by common signals generated by activation of the FSHR and LHR (possibly cAMP and Akt) and that the activation of the inositol phosphate cascade in cells expressing a high density of LHR or FSHR antagonizes this induction.

Palatability-dependent appetite and benzodiazepines: new directions from the pharmacology of GABA(A) receptor subtypes

This paper updates an early review on benzodiazepine-enhanced food intake, published in the first issue of Appetite, and describes the considerable advances since then in the pharmacology of benzodiazepines, their sites and mechanisms of action, and in understanding the psychological processes leading to the increase in food consumption. A great diversity of benzodiazepine receptor ligands have been developed, many of which affect food intake. Agonists can be divided into full agonists (which produce the full spectrum of benzodiazepine effects) and partial agonists (which are more selective in their effects). In addition, inverse agonists have been identified, with high affinity for benzodiazepine receptors but having negative efficacy: these drugs exhibit anorectic properties. Benzodiazepine receptors are part of GABA(A) receptor complexes, and ligands thereby modulate inhibitory neurotransmission in the brain. Molecular approaches have identified a palette of receptor subunits from which GABA(A) receptors are assembled. In all likelihood, benzodiazepine-induced hyperphagia is mediated by the alpha2/alpha3 subtype not the alpha1 subtype. Novel alpha2/alpha3 selective compounds will test this hypothesis. A probable site of action in the caudal brainstem for benzodiazepines is the parabrachial nucleus. Behavioural evidence strongly indicates that a primary action of benzodiazepines is to enhance the positive hedonic evaluation (palatability) of tastes and foodstuffs. This generates the increased food intake and instrumental responding for food rewards. Therapeutic applications may derive from the actions of benzodiazepine agonists and inverse agonists on food procurement and ingestion.

Z-analysis: a new approach to analyze stimulation curves with intrinsic basal stimulation

In the study of receptor biology it is of considerable importance to describe the stimulatory properties of an agonist according to mathematically defined models. However, the presently used models are insufficient if the experimental preparation contains an intrinsic basal stimulation. We have developed a novel approach, tentatively named Z-analysis. In this approach, the concentration of endogenous agonist is calculated by extending the stimulation curve to zero effect. The concentration of endogenous agonist is then combined with the concentration of added agonist to estimate the true EC(50) value. We developed a new model, the Z-model, specifically for this purpose, but in addition, we describe how Z-analysis can be applied to the traditional E(0)-model. Models were applied to computer-generated curves with different Hill coefficients, using iterative curve fitting procedures. In addition to applying the models to ideal cases, we also used Monte Carlo-simulated data. Specific transformations were used to enable comparisons between parameters determined from these models. Both models were able to provide estimates of all eight parameters analyzed, both using ideal data and on Monte Carlo-simulated data. The Z-model was found to provide better estimates of the concentration of endogenous agonist, the EC(50) values, and the Hill value, in curves with Hill coefficient deviating from one. In conclusion, Z-analysis was suitable both to determine the concentration of endogenous agonists and to determine true EC(50) values. We found several advantages with the Z-model compared to traditional E(0)-model for analysis of stimulation curves that contain basic intrinsic stimulation.

The place of partial agonism in psychiatry: recent developments

Drugs used to treat psychiatric disorders, although effective, are often restricted by adverse events. The use of partial agonists for treating hypertension was found to limit some of the side-effects in some patients. This led to the investigation of partial agonists as a treatment modality in psychiatric disorders. Partial agonists have a lower intrinsic efficacy than full agonists leading to reduced maximum response. They can act as antagonists by competing for receptor binding with full agonists. The level of activity depends on the level of endogenous receptor activity. Buprenorphine, a partial agonist at the mu-opioid receptor, is used to treat patients with addiction and decreases the symptoms of withdrawal and risks of overdose and intoxication. The anxiolytic buspirone shows partial agonism at 5-HT(1A) receptors, and this seems to provide anxioselective effects, without inducing extrapyramidal side-effects, convulsions, tolerance or withdrawal reactions. In schizophrenia, partial dopamine agonism results in antagonistic effects at sites activated by high concentrations of dopamine and agonistic effects at sites activated by low concentrations of dopamine. This stabilizes the dopamine system to effect antipsychotic action without inducing adverse motor or hormonal events. Aripiprazole is the first 'dopamine system stabilizer', and the data are promising, with efficacy at least equivalent to that with current atypical antipsychotics but fewer of the troublesome side-effects. Partial agonists seem to provide a way to fine-tune the treatment of psychiatric disorders by maximizing the treatment effect while minimizing undesirable adverse events.

Dimer-based model for heptaspanning membrane receptors

The existence of intramembrane receptor-receptor interactions for heptaspanning membrane receptors is now fully accepted, but a model considering dimers as the basic unit that binds to two ligand molecules is lacking. Here, we propose a two-state-dimer model in which the ligand-induced conformational changes from one component of the dimer are communicated to the other. Our model predicts cooperativity in binding, which is relevant because the other current models fail to address this phenomenon satisfactorily. Our two-state-dimer model also predicts the variety of responses elicited by full or partial agonists, neutral antagonists and inverse agonists. This model can aid our understanding of the operation of heptaspanning receptors and receptor channels, and, potentially, be important for improving the treatment of cardiovascular, neurological and neuropsychyatric diseases.

Inverse agonism: more than reverting constitutively active receptor signaling

Seven-transmembrane receptors constitute one of the major families of proteins encoded by the genome. This type of receptor is one of the most important targets of the pharmaceutical industry, and many of the drugs with significant therapeutic action have been shown to be inverse agonists. Concepts regarding the mechanisms by which ligands activate and inactivate receptors are thought to be far more complex that a simple on-off switch. For both drug design and pharmacology principles, it is important to understand the mechanisms by which these drugs achieve their effects. Recent studies have demonstrated intriguing actions of inverse agonists. They have been shown not only to block constitutive responses of receptors but also to activate and regulate seven-transmembrane receptor signaling and trafficking. The activation of pathways by inverse agonists was shown to occur mainly via G-protein-independent mechanisms. These findings emphasize the importance of inverse agonism as a principle of receptor regulation. In this paper, we will review the evidence supporting inverse agonist promoted signaling and trafficking.

Evaluation of Histamine H1-, H2-, and H3-Receptor Ligands at the Human Histamine H4 Receptor: Identification of 4-Methylhistamine as the First Potent and Selective H4 Receptor Agonist

The histamine H(4) receptor (H(4)R) is involved in the chemotaxis of leukocytes and mast cells to sites of inflammation and is suggested to be a potential drug target for asthma and allergy. So far, selective H(4)R agonists have not been identified. In the present study, we therefore evaluated the human H(4)R (hH(4)R) for its interaction with various known histaminergic ligands. Almost all of the tested H(1)R and H(2)R antagonists, including several important therapeutics, displaced less than 30% of specific [(3)H]histamine binding to the hH(4)R at concentrations up to 10 microM. Most of the tested H(2)R agonists and imidazole-based H(3)R ligands show micromolar-to-nanomolar range hH(4)R affinity, and these ligands exert different intrinsic hH(4)R activities, ranging from full agonists to inverse agonists. Interestingly, we identified 4-methylhistamine as a high-affinity H(4)R ligand (K(i) = 50 nM) that has a >100-fold selectivity for the hH(4)R over the other histamine receptor subtypes. Moreover, 4-methylhistamine potently activated the hH(4)R (pEC(50) = 7.4 +/- 0.1; alpha = 1), and this response was competitively antagonized by the selective H(4)R antagonist JNJ 7777120 [1-[(5-chloro-1H-indol-2-yl)-carbonyl]-4-methylpiperazine] (pA(2) = 7.8). The identification of 4-methylhistamine as a potent H(4)R agonist is of major importance for future studies to unravel the physiological roles of the H(4)R.

Cys2,7EtalphaCGRP is a potent agonist for CGRP1 receptors in SK-N-MC cells

The present study reveals that cystein2,7 ethyl-amidealphaCGRP (Cys2,7EtalphaCGRP), an advertised calcitonin gene-related peptide 2 (CGRP2) receptor subtype-selective agonist, is also a potent agonist for the calcitonin gene-related peptide 1 (CGRP1) receptors natively expressed in the SK-N-MC human neuroblastoma cell line. Cys2,7EtalphaCGRP and alpha calcitonin gene-related peptide (alphaCGRP) promote cyclic AMP accumulation in intact SK-N-MC cells to the same extent with EC50 of 1.6+/-0.2 and 0.4+/-0.08 nM, respectively. The antagonist alpha calcitonin gene-related peptide-8-37 (alphaCGRP-(8-37)) produces a concentration-dependent rightward shift of the alphaCGRP- and Cys2,7EtalphaCGRP concentration-response curves with KB-values (71+/-33 and 47+/-21 nM, respectively). The competitive antagonism by alphaCGRP-(8-37) and the similar KB-values suggests that alphaCGRP and Cys2,7EtalphaCGRP stimulate the same receptor. In competition binding studies with [125I]-alphaCGRP on SK-N-MC cell membranes, Cys2,7EtalphaCGRP and alphaCGRP-(8-37) display high affinity for the majority of the binding sites with Ki-values of 0.030+/-0.013 and 0.60+/-0.013 nM, respectively. The present findings are at odds with the proclaimed utilization of Cys2,7EtalphaCGRP as a CGRP2 receptor-selective pharmacological tool. Differences between the agonistic profile of this ligand in this and other experimental systems might be species--or even cell type--dependent.

Functional selectivity of G protein signaling by agonist peptides and thrombin for the protease-activated receptor-1

Thrombin activates protease-activated receptor-1 (PAR-1) by cleavage of the amino terminus to unmask a tethered ligand. Although peptide analogs can activate PAR-1, we show that the functional responses mediated via PAR-1 differ between the agonists. Thrombin caused endothelial monolayer permeability and mobilized intracellular calcium with EC(50) values of 0.1 and 1.7 nm, respectively. The opposite order of activation was observed for agonist peptide (SFLLRN-CONH(2) or TFLLRNKPDK) activation. The addition of inactivated thrombin did not affect agonist peptide signaling, suggesting that the differences in activation mechanisms are intramolecular in origin. Although activation of PAR-1 or PAR-2 by agonist peptides induced calcium mobilization, only PAR-1 activation affected barrier function. Induced barrier permeability is likely to be Galpha(12/13)-mediated as chelation of Galpha(q)-mediated intracellular calcium with BAPTA-AM, pertussis toxin inhibition of Galpha(i/o), or GM6001 inhibition of matrix metalloproteinase had no effect, whereas Y-27632 inhibition of the Galpha(12/13)-mediated Rho kinase abrogated the response. Similarly, calcium mobilization is Galpha(q)-mediated and independent of Galpha(i/o) and Galpha(12/13) because pertussis toxin Y-27632 and had no effect, whereas U-73122 inhibition of phospholipase C-beta blocked the response. It is therefore likely that changes in permeability reflect Galpha(12/13) activation, and changes in calcium reflect Galpha(q) activation, implying that the pharmacological differences between agonists are likely caused by the ability of the receptor to activate Galpha(12/13) or Galpha(q). This functional selectivity was characterized quantitatively by a mathematical model describing each step leading to Rho activation and/or calcium mobilization. This model provides an estimate that peptide activation alters receptor/G protein binding to favor Galpha(q) activation over Galpha(12/13) by approximately 800-fold.

Identification of indole derivatives exclusively interfering with a G protein-independent signaling pathway of the prostaglandin D2 receptor CRTH2

The anti-inflammatory drugs indomethacin and ramatroban, the latter showing clinical efficacy in treating allergic asthma, have been shown to act as a classic agonist and antagonist, respectively, of the G protein-coupled chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2 receptor). Here, we report the identification of two indole derivatives 1-(4-ethoxyphenyl)-5-methoxy-2-methylindole-3-carboxylic acid and N(alpha)-tosyltryptophan (hereafter referred to as 1 and 2, respectively), which are structurally related to indomethacin and ramatroban but which selectively interfere with a specific G protein-independent signaling pathway of CRTH2. In whole-cell saturation-binding assays, 1 and 2 both increase the number of [(3)H]prostaglandin D2 (PGD2)-recognizing CRTH2 sites and the affinity of PGD2 for CRTH2. Enzyme-linked immunosorbent assays show that they do not alter the total number of CRTH2 receptors on the cell surface. Analysis of their binding mode indicates that unlike indomethacin or ramatroban, 1 and 2 can occupy CRTH2 simultaneously with PGD2. On a functional level, however, 1 and 2 do not interfere with PGD2-mediated activation of heterotrimeric G proteins by CRTH2. In contrast, both compounds inhibit PGD2-mediated arrestin translocation via a G protein-independent mechanism. In human eosinophils endogenously expressing CRTH2, 1 selectively decreases the efficacy but not the potency of PGD2-induced shape change, unlike ramatroban, which displays competitive antagonistic behavior. These data show for the first time that "antagonists" can cause markedly dissimilar degrees of inhibition for different effector pathways and suggest that it may be possible to develop novel classes of specific signal-inhibiting drugs distinct from conventional antagonists.

Discovery of CC chemokine receptor-3 (CCR3) antagonists with picomolar potency

Starting with our previously described(20) class of CC chemokine receptor-3 (CCR3) antagonist, we improved the potency by replacing the phenyl linker of 1 with a cyclohexyl linker and by replacing the 4-benzylpiperidine with a 3-benzylpiperidine. The resulting compound, 32, is a potent and selective antagonist of CCR3. SAR studies showed that the 3-acetylphenyl urea of 32 could be replaced with heterocyclic ureas or heterocyclic-substituted phenyl ureas and still maintain the potency (inhibition of eotaxin-induced chemotaxis) of this class of compounds in the low-picomolar range (IC(50) = 10-60 pM), representing some of the most potent CCR3 antagonists reported to date. The potency of 32 for mouse CCR3 (chemotaxis IC(50) = 41 nM) and its oral bioavailability in mice (20% F ) were adequate to assess the efficacy in animal models of allergic airway inflammation. Oral administration of 32 reduced eosinophil recruitment into the lungs in a dose-dependent manner in these animal models. On the basis of its overall potency, selectivity, efficacy, and safety profile, the benzenesulfonate salt of 32, designated DPC168, entered phase I clinical trials.

A simple mathematical model of cooperativity in receptor mosaics based on the “symmetry rule”

The phenomenon of receptor-receptor interactions was hypothesized about 20 years ago. It has been demonstrated by now that receptor-receptor interactions between G-protein coupled receptors (GPCRs) occur at plasma membrane level and result in the reciprocal modulation of their binding characteristics (i.e., cooperativity). One of the most important feature of this phenomenon is the concept of cluster of receptors, or receptor mosaic (RM). However, no proper mathematical approach has still been available to characterize RMs as far as their receptor composition, receptor topography and order of receptor activation inside the RM. This paper tries to fill the gap. A simple mathematical approach to the cooperativity in RMs formed by dimers of identical receptors and/or by iso-receptors is proposed. To this aim the so-called "symmetry rule" has been considered. This approach allows to describe by means of a simple energy function the effects of receptor composition (number of dimers), spatial organisation (respective location of the dimers) and order of activation (order according to which the single receptors are ligated) on the integrative cooperativity (index) of the RMs.

How receptor mosaics decode transmitter signals. Possible relevance of cooperativity

It has been demonstrated that receptor-receptor interactions between G-protein-coupled receptors (GPCRs) occur at the plasma-membrane level. It has also been shown that clustering of GPCRs in aggregates or receptor mosaics (RMs) results in the reciprocal modulation of their binding and decoding characteristics. It is hypothesized that cooperativity plays an important part in the decoding of signals processed by RMs of GPCRs. Thus, the binding of the ligand at one receptor alters the likelihood of the same ligand binding at the next site, in the case of RMs, formed by identical receptors and/or by iso-receptors (receptors that bind the same ligand).

Oligomerization of G protein-coupled receptors: past, present, and future

G protein-coupled receptor (GPCR)-mediated signal transduction has been studied for more than a century. Despite the intense focus on this class of proteins, a molecular understanding of what constitutes the functional form of the receptor is still uncertain. GPCRs have traditionally been conceptualized as monomeric proteins, and this view has changed little over the years until relatively recently. Recent biochemical and biophysical studies have challenged this traditional concept, and point instead to a mechanistic view of signal transduction wherein the receptor functions as an oligomer. Cooperative interactions within such an oligomeric array may be critical for the propagation of an external signal across the cell membrane and to the G protein, and may therefore underlie the mechanistic basis of signaling.

Basal opioid receptor activity, neutral antagonists, and therapeutic opportunities

The mu opioid receptor (MOR, OPRM)--the principal receptor involved in narcotic addiction--has been shown to display basal (spontaneous, constitutive) signaling activity. Interaction with other signaling proteins, such as calmodulin, regulates basal MOR activity. Providing a mechanism for long-lasting regulation, basal MOR activity potentially plays a key role in addiction, in combination with gene regulation and synaptic remodeling. Recent results support a link to physical dependence--one of the main manifestations of addiction to drugs of abuse. The prototypical opioid antagonists, naloxone and naltrexone, were shown to act as inverse agonists in the morphine-dependent state (i.e., they suppress basal MOR signaling) and thereby appear to elicit or contribute to precipitated withdrawal. This affords the opportunity to explore therapeutic applications for neutral antagonists (blocking agonists at MOR without affecting basal activity) with reduced adverse effects. Neutral antagonists are promising drug candidates in the treatment of addiction and overdose, and of peripheral adverse effects of narcotic analgesics.

The two neutrophil members of the formylpeptide receptor family activate the NADPH-oxidase through signals that differ in sensitivity to a gelsolin derived phosphoinositide-binding peptide

Background

The formylpeptide receptor family members FPR and FPRL1, expressed in myeloid phagocytes, belong to the G-protein coupled seven transmembrane receptor family (GPCRs). They share a high degree of sequence similarity, particularly in the cytoplasmic domains involved in intracellular signaling. The established model of cell activation through GPCRs states that the receptors isomerize from an inactive to an active state upon ligand binding, and this receptor transformation subsequently activates the signal transducing G-protein. Accordingly, the activation of human neutrophil FPR and FPRL1 induces identical, pertussis toxin-sensitive functional responses and a transient increase in intracellular calcium is followed by a secretory response leading to mobilization of receptors from intracellular stores, as well as a release of reactive oxygen metabolites.

Results

We report that a cell permeable ten amino acid peptide (PBP10) derived from the phosphatidylinositol 4,5-bisphosphate (PIP₂) binding region of gelsolin (an uncapper of actin filaments) blocks granule mobilization as well as secretion of oxygen radicals. The inhibitory effect of PBP10 is, however, receptor specific and affects the FPRL1-, but not the FPR-, induced cellular response. The transient rise in intracellular calcium induced by the active receptors is not affected by PBP10, suggesting that the blockage occurs in a parallel, novel signaling pathway used by FPRL1 to induce oxygen radical production and secretion. Also the FPR can activate neutrophils through a PBP10-sensitive signaling pathway, but this signal is normally blocked by the cytoskeleton.

Conclusions

This study demonstrates that the two very closely related chemoattractant receptors, FPR and FPRL1, use distinct signaling pathways in activation of human neutrophils. The PIP₂-binding peptide PBP10 selectively inhibits FPRL1-mediated superoxide production and granule mobilization. Furthermore, the activity of this novel PBP10 sensitive pathway in neutrophils is modulated by the actin cytoskeleton network.

A database for G proteins and their interaction with GPCRs

Background

G protein-coupled receptors (GPCRs) transduce signals from extracellular space into the cell, through their interaction with G proteins, which act as switches forming hetero-trimers composed of different subunits (α,β,γ). The α subunit of the G protein is responsible for the recognition of a given GPCR. Whereas specialised resources for GPCRs, and other groups of receptors, are already available, currently, there is no publicly available database focusing on G Proteins and containing information about their coupling specificity with their respective receptors.

Description

gpDB is a publicly accessible G proteins/GPCRs relational database. Including species homologs, the database contains detailed information for 418 G protein monomers (272 Gα, 87 Gβ and 59 Gγ) and 2782 GPCRs sequences belonging to families with known coupling to G proteins. The GPCRs and the G proteins are classified according to a hierarchy of different classes, families and sub-families, based on extensive literature searchs. The main innovation besides the classification of both G proteins and GPCRs is the relational model of the database, describing the known coupling specificity of the GPCRs to their respective α subunit of G proteins, a unique feature not available in any other database. There is full sequence information with cross-references to publicly available databases, references to the literature concerning the coupling specificity and the dimerization of GPCRs and the user may submit advanced queries for text search. Furthermore, we provide a pattern search tool, an interface for running BLAST against the database and interconnectivity with PRED-TMR, PRED-GPCR and TMRPres2D.

Conclusions

The database will be very useful, for both experimentalists and bioinformaticians, for the study of G protein/GPCR interactions and for future development of predictive algorithms. It is available for academics, via a web browser at the URL:

Modulation of serotonergic receptor signaling and cross-talk by prion protein

The inducible serotonergic 1C115-HT cell line expresses a defined set of serotonergic receptors of the 5-HT2B, 5-HT1B/D, and 5-HT2A subtypes, which sustain a regulation of serotonergic associated functions through G-protein-dependent signaling. 1C115-HT cells have been instrumental to assign a signaling function to the cellular prion protein PrPC. Here, we establish that antibody-mediated ligation of PrPC concomitant to agonist stimulation of 5-HT receptors modulates the couplings of all three serotonergic receptors present on 1C115-HT cells. Specific impacts of PrP antibodies were monitored depending on the receptor and pathway considered. PrPC ligation selectively cancels the 5-HT2A-PLC response, decreases the 5-HT1B/D negative coupling to adenylate cyclase, and potentiates the 5-HT2B-PLA2 coupling. As a result, PrPC ligation disturbs the functional interactions occurring between the signaling pathways of the three receptor subtypes. In 1C115-HT cells, antagonizing cross-talks arising from 5-HT2B and 5-HT2A receptors control the 5-HT1B/D function. PrPC ligation reinforces the negative regulation exerted by 5-HT2B on 5-HT1B/D receptors. On the other hand it abrogates the blocking action of 5-HT2A on the regulatory loop linking 5-HT1B/D receptors. We propose that the ligation of PrPC affects the potency or dynamics of G-protein activation by agonist-bound serotonergic receptors. Finally, the PrPC-dependent modulation of 5-HT receptor couplings is restricted to 1C115-HT cells expressing a complete serotonergic phenotype. It critically involves a PrPC-caveolin platform implemented on the neurites of 1C115-HT cells during differentiation. Our findings define PrPC as a modulator of 5-HT receptor coupling to G-proteins and thereby as a protagonist contributing to the homeostasis of serotonergic neurons. They provide a foundation for uncovering the impact of prion infection on serotonergic functions.

A cell biologist’s perspective on physiological adaptation to opiate drugs

Opiate drugs such as morphine and heroin are among the most effective analgesics known but are also highly addictive. The clinical utility of opiates is limited by adaptive changes in the nervous system occurring after prolonged or repeated drug administration. These adaptations are believed to play an important role in the development of physiological tolerance and dependence to opiates, and to contribute to additional changes underlying the complex neurobehavioral syndrome of drug addiction. All of these adaptive changes are initiated by the binding of opiate drugs to a subfamily of G protein-coupled receptors that are also activated by endogenously produced opioid neuropeptides. It is increasingly evident that opiate-induced adaptations occur at multiple levels in the nervous system, beginning with regulation of opioid receptors themselves and extending to a complex network of direct and indirect modifications of "downstream" signaling machinery. Efforts in my laboratory are directed at understanding the biochemical and cell biological basis of opiate adaptations. So far, we have focused primarily on adaptations occurring at the level of opioid receptors themselves. These studies have contributed to defining a set of membrane trafficking mechanisms by which the number and functional activity of opioid receptors are controlled. The role of these mechanisms in affecting adaptation of "downstream" neurobiological substrates, and in mediating opiate-induced changes in whole-animal physiology and behavior, are exciting questions that are only beginning to be explored.