Inverse agonists: tools to reveal ligand-specific conformations of G protein-coupled receptors

Extracellular adenosine induces hypersecretion of IL-17A by T-helper 17 cells through the adenosine A2a receptor

Extracellular adenosine, produced from ATP secreted by neuronal or immune cells, may play a role in endogenous regulation of inflammatory responses. Studies show that adenosine induces hypersecretion of IL-17A by CD4+ T cells upon treatment with an A2aR agonist (PSB0777), and that adenosine-mediated IL-17A hypersecretion is suppressed by the A2aR antagonist (Istradefylline) in humans. However, it is unclear whether A2aR downstream signaling is involved in IL-17A hypersecretion. Here, we show that inhibitors of adenyl cyclase (AC), protein kinase A (PKA), and cAMP response element binding protein (CREB) (which are signaling molecules downstream of the Gs protein coupled to the A2aR), suppress IL-17A production, suggesting that activation of A2aR signaling induces IL-17A production by CD4+ T cells. Furthermore, immune subset studies revealed that adenosine induces hypersecretion of IL-17A by T-helper (Th)17 cells. These results indicate that adenosine is an endogenous modulator of neutrophilic inflammation. Administration of an A2aR antagonist to mice with experimental autoimmune encephalomyelitis led to marked amelioration of symptoms. Thus, inhibitors of the novel A2aR-AC-cAMP-PKA-CREB signaling pathway for IL-17A hypersecretion by TCR-activated Th17 cells suppresses adenosine-mediated IL-17A production, suggesting that it may be an effective treatment for Th17-related autoimmune diseases.

Tannic acid acts as an agonist of the dopamine D2L receptor, regulates immune responses, and ameliorates experimentally induced colitis in mice

Tannic acid (TA) is an herbal polyphenol containing a galloyl group that has been prescribed to treat gastroenteritis, diarrhea, and irritable bowel syndrome. TA has anti-inflammatory, anti-cancer, and anti-viral properties; however, the molecular mechanisms of these potential therapeutic effects are still largely unknown. Here, we examined the ability of TA to induce anti-inflammatory responses. TA was found to be an agonist of the dopamine D2L receptor. TA reduced interferon (IFN)-γ and interleukin (IL)-1β secretion but upregulated tumor necrosis factor α and IL-10 secretion from lipopolysaccharide (LPS)-stimulated mouse splenocytes. TA also reduced IFN-γ secretion but enhanced IL-10 secretion from anti-cluster of differentiation (CD) 3/CD28 antibody-stimulated splenocytes. An immune subset study confirmed that TA regulated cytokine secretion by various types of immune cells in the context of stimulation with LPS or anti-CD3/CD28 antibodies. Administration of TA to mice with experimentally induced colitis strikingly suppressed weight loss, colon shrinkage, and IL-17 secretion from mesenteric lymph node lymphocytes in response to CD3/CD28 stimulation. These data suggest that TA suppresses inflammatory responses in colitis by regulating cytokine secretion by immune cells in the colon.

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Tannic acid is an agonist of the dopamine D2L receptor.

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Tannic acid suppresses IFN-γ secretion by LPS-stimulated splenocytes.

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Tannic acid modulates anti-CD3/CD28 antibody-stimulated cytokine levels in CD4⁺ T cells.

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Tannic acid ameliorates dextran sodium salt (DSS)-induced colitis in C57BL/6 mice.

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Tannic acid reduces production of IL-17 in DSS-induced colitis.

High-dose naloxone, an experimental tool uncovering latent sensitisation: pharmacokinetics in humans

BACKGROUND

Naloxone, an opioid receptor antagonist, is used as a pharmacological tool to detect tonic endogenous activation of opioid receptors in experimental pain models. We describe a pharmacokinetic model linking naloxone pharmacokinetics to its main metabolite after high-dose naloxone infusion.

METHODS

Eight healthy volunteers received a three-stage stepwise high-dose i.v. naloxone infusion (total dose 3.25 mg kg^-1). Naloxone and naloxone-3-glucuronide (N3G) plasma concentrations were sampled from infusion onset to 334 min after infusion discontinuation. Pharmacokinetic analysis was performed using non-linear mixed effect models (NONMEM). The predictive performances of Dowling's and Yassen's models were evaluated, and target-controlled infusion simulations were performed.

RESULTS

Three- and two-compartment disposition models with linear elimination kinetics described the naloxone and N3G concentration time-courses, respectively. Two covariate models were developed: simple (weight proportional) and complex (with the shallow peripheral volume of distribution linearly increasing with body weight). The median prediction error (MDPE) and wobble for Dowling's model were -32.5% and 33.4%, respectively. For Yassen's model, the MDPE and wobble were 1.2% and 19.9%, respectively.

CONCLUSIONS

A parent-metabolite pharmacokinetic model was developed for naloxone and N3G after high-dose naloxone infusion. No saturable pharmacokinetics were observed. Whereas Dowling's model was inaccurate and over-predicted naloxone concentrations, Yassen's model accurately predicted naloxone pharmacokinetics. The newly developed covariate models may be used for high-dose TCI-naloxone for experimental and clinical practice.

CLINICAL TRIALS REGISTRATION

NCT01992146.

Differential signalling in human cannabinoid CB1 receptors and their splice variants in autaptic hippocampal neurones

BACKGROUND AND PURPOSE

Cannabinoids such as Δ(9) - tetrahydrocannabinol, the major psychoactive component of marijuana and hashish, primarily act via cannabinoid CB(1) and CB(2) receptors to produce characteristic behavioural effects in humans. Due to the tractability of rodent models for electrophysiological and behavioural studies, most of the studies of cannabinoid receptor action have used rodent cannabinoid receptors. While CB(1) receptors are relatively well-conserved among mammals, human CB(1) (hCB(1) ) differs from rCB(1) and mCB(1) receptors at 13 residues, which may result in differential signalling. In addition, two hCB(1) splice variants (hCB(1a) and hCB(1b) ) have been reported, diverging in their amino-termini relative to hCB(1) receptors. In this study, we have examined hCB(1) signalling in neurones.

EXPERIMENTAL APPROACH

hCB(1) , hCB(1a) hCB(1b) or rCB(1) receptors were expressed in autaptic cultured hippocampal neurones from CB(1) (-/-) mice. Such cells express a complete endogenous cannabinoid signalling system. Electrophysiological techniques were used to assess CB(1) receptor-mediated signalling. KEY RESULTS Expressed in autaptic hippocampal neurones cultured from CB(1) (-/-) mice, hCB(1) , hCB(1a) and hCB(1b) signal differentially from one another and from rodent CB(1) receptors. Specifically, hCB(1) receptors inhibit synaptic transmission less effectively than rCB(1) receptors.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that cannabinoid receptor signalling in humans is quantitatively very different from that in rodents. As the problems of marijuana and hashish abuse occur in humans, our results highlight the importance of studying hCB(1) receptors. They also suggest further study of the distribution and function of hCB(1) receptor splice variants, given their differential signalling and potential impact on human health.

LINKED ARTICLES

This article is part of a themed section on Cannabinoids in Biology and Medicine. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.165.issue-8. To view Part I of Cannabinoids in Biology and Medicine visit http://dx.doi.org/10.1111/bph.2011.163.issue-7.

Fitting the complexity of GPCRs modulation into simple hypotheses of ligand design

G-protein coupled receptors (GPCR(s)) are a large family of membrane-bound receptors that mediate a wide range of physiologic responses to hormones, neurotransmitters and dietary lipids, which represent an important class of drug targets. Significant chemical space regions have been explored both in the academia and by pharmaceutical companies, in the quest for new GPCR modulators as potential therapeutic agents. This accumulated body of evidence provides new opportunities to evaluate potential features of GPCR agonists and antagonists, and how to distinguish them. In this study, the chemical space covered within the WOMBAT database by GPCRs modulators was investigated with the aim of identifying specific molecular determinants that distinguish GPCR agonists from antagonists. While instrumental to get insights into the design strategies of GPCRs modulators, the results of this study provide novel clues on the molecular mechanisms that underlie the complexity of GPCR modulation.

The role of opioid antagonist efficacy and constitutive opioid receptor activity in the opioid withdrawal syndrome in mice

On the basis of efficacy, opioid antagonists are classified as inverse opioid agonists (e.g. naltrexone) or neutral opioid antagonists (e.g. 6β-naltrexol). This study examined the interaction between naltrexone and 6β-naltrexol in the precipitated opioid withdrawal syndrome in morphine dependent mice. Furthermore, the possible contribution of constitutive opioid receptor activity to precipitated withdrawal was evaluated using increasing levels of morphine dependence. In the first experiment, low doses of 6β-naltrexol antagonized naltrexone precipitated withdrawal while high doses acted additively. All doses of naltrexone increased 6β-naltrexol's potency to precipitate withdrawal. The next experiment examined changes in antagonist potency to precipitate withdrawal with increasing morphine dependence. Mice were exposed to morphine for 1-6 days and then withdrawal was precipitated. Naltrexone was more potent than 6β-naltrexol at all the time points. The ED(50) of both drugs decreased at the same rate suggesting that increased dependence produced no change in constitutive opioid receptor activity. Taken together these results indicate that the functional efficacy of 6β-naltrexol is dose-dependent and that constitutive opioid receptor activity did not change as opioid dependence increased from 1 to 6 days.

Distinct receptor-mediated activities in macrophages for natural ceramide-1-phosphate (C1P) and for phospho-ceramide analogue-1 (PCERA-1)

Ceramide-1-phosphate (C1P) is known as a second messenger regulating a multitude of processes including cell growth, apoptosis and inflammation. Exciting recent findings now suggest that C1P can stimulate macrophages migration in an extra-cellular manner via a G protein-coupled receptor (GPCR). Interestingly, a synthetic C1P analog, named phospho-ceramide analogue-1 (PCERA-1), was recently described as a potent in-vivo anti-inflammatory agent, and was suggested to act on macrophages in an extra-cellular manner via a GPCR. Here we summarize and compare the receptor-mediated as well as receptor-independent activities of natural C1P and its synthetic analog. We also provide experimental data in support of distinct C1P and PCERA-1 receptors.

Cannabinoid receptor type 1- and 2-mediated increase in cyclic AMP inhibits T cell receptor-triggered signaling

The aim of this study was to characterize inhibitory mechanisms on T cell receptor signaling mediated by the cannabinoid receptors CB1 and CB2. Both receptors are coupled to G(i/o) proteins, which are associated with inhibition of cyclic AMP formation. In human primary and Jurkat T lymphocytes, activation of CB1 by R(+)-methanandamide, CB2 by JWH015, and both by Delta9-tetrahydrocannabinol induced a short decrease in cyclic AMP lasting less than 1 h. However, this decrease was followed by a massive (up to 10-fold) and sustained (at least up to 48 h) increase in cyclic AMP. Mediated by the cyclic AMP-activated protein kinase A and C-terminal Src kinase, the cannabinoids induced a stable phosphorylation of the inhibitory Tyr-505 of the leukocyte-specific protein tyrosine kinase (Lck). By thus arresting Lck in its inhibited form, the cannabinoids prevented the dephosphorylation of Lck at Tyr-505 in response to T cell receptor activation, which is necessary for the subsequent initiation of T cell receptor signaling. In this way the cannabinoids inhibited the T cell receptor-triggered signaling, i.e. the activation of the zeta-chain-associated protein kinase of 70 kDa, the linker for activation of T cells, MAPK, the induction of interleukin-2, and T cell proliferation. All of the effects of the cannabinoids were blocked by the CB1 and CB2 antagonists AM281 and AM630. These findings help to better understand the immunosuppressive effects of cannabinoids and explain the beneficial effects of these drugs in the treatment of T cell-mediated autoimmune disorders like multiple sclerosis.

The relative potency of inverse opioid agonists and a neutral opioid antagonist in precipitated withdrawal and antagonism of analgesia and toxicity

Opioid antagonists can be classified as inverse agonists and neutral antagonists. In the opioid-dependent state, neutral antagonists are significantly less potent in precipitating withdrawal than inverse agonists. Consequently, neutral opioid antagonists may offer advantages over inverse agonists in the management of opioid overdose. In this study, the relative potency of three opioid antagonists to block opioid analgesia and toxicity and precipitate withdrawal was examined. First, the potency of two opioid inverse agonists (naltrexone and naloxone) and a neutral antagonist (6beta-naltrexol) to antagonize fentanyl-induced analgesia and lethality was determined. The order of potency to block analgesia was naltrexone > naloxone > 6beta-naltrexol (17, 4, 1), which was similar to that to block lethality (13, 2, 1). Next, the antagonists were compared using withdrawal jumping in fentanyl-dependent mice. The order of potency to precipitate withdrawal jumping was naltrexone > naloxone 6beta-naltrexol (1107, 415, 1). The relative potencies to precipitate withdrawal for the inverse agonists compared with the neutral antagonist were dramatically different from that for antagonism of analgesia and lethality. Finally, the effect of 6beta-naltrexol pretreatment on naloxone-precipitated jumping was determined in morphine and fentanyl-dependent mice. 6beta-Naltrexol pretreatment decreased naloxone precipitated withdrawal, indicating that 6beta-naltrexol is a neutral antagonist. These data demonstrate that inverse agonists and neutral antagonists have generally comparable potencies to block opioid analgesia and lethality, whereas the neutral opioid antagonist is substantially less potent in precipitating opioid withdrawal. These results support suggestions that neutral antagonists may have advantages over inverse agonists in the management of opioid overdose.

Multiplexed Assays by High-Content Imaging for Assessment of GPCR Activity

G-protein-coupled receptors (GPCR) participate in many disease pathways and represent the largest family of therapeutic targets. Thus, great investments are made to discover drugs modulating GPCR-mediated events. Among functional assays for screening GPCRs, the Transfluor®imaging assay is based on redistribution of cytosolic β-arrestin to an activated GPCR and has become widely used in high-content screening. However, assessing Transfluor®alone has limitations: relying on a single mechanistic step of β-arrestin redistribution during GPCR activation, providing no information on the stimulated GPCR's intracellular fate, and using only a single fluorescent color (green fluorescent protein). Taking full advantage of high-content imaging to screen approximately 2000 compounds, the authors multifplexed the Transfluor®assay with an immunofluorescence-based quantification of GPCR internalization. This approach identified and classified 377 compounds interfering with agonist-induced activation of the Transfluor®assay, receptor internalization, or both. In addition, a subset of compounds was analyzed for their performance across imaging, cell-based calcium release (fluorometric imaging plate reader [FLIPR]), and biochemical receptor binding assays (scintillation proximity assay). This indicated that the imaging assays have even better predictive power for direct inhibition of receptor binding than the FLIPR assay. In conclusion, compounds inducing unique responses can suggest novel mechanisms of action and be used as tools to study GPCR activation and internalization. ( Journal of Biomolecular Screening 2008:449-455)

Mu-opioid receptor up-regulation and functional supersensitivity are independent of antagonist efficacy

Chronic opioid antagonist treatment up-regulates opioid receptors and produces functional supersensitivity. Although opioid antagonists vary from neutral to inverse, the role of antagonist efficacy in mediating the chronic effects of opioid antagonists is not known. In this study, the effects of two putative inverse agonists (naltrexone, naloxone) and a putative neutral antagonist (6beta-naltrexol) were examined. Initially, peak effect (40 min, naltrexone and naloxone; 70 min, 6beta-naltrexol) and relative potency to antagonize morphine analgesia were determined (relative potencies = 1, 2, and 16, 6beta-naltrexol, naloxone, and naltrexone, respectively). Next, mice were infused for 7 days with naloxone (0.1-10 mg/kg/day), naltrexone (10 or 15 mg s.c. pellet), or 6beta-naltrexol (0.2-20 mg/kg/day), and spinal micro-opioid receptor density was examined, or morphine analgesia dose-response studies were conducted. All antagonists up-regulated mu-opioid receptors (60-122%) and induced supersensitivity (1.8-2.0-fold increase in morphine potency). There were no differences in antagonist potency to produce up-regulation or supersensitivity. These data suggest that opioid antagonist-induced mu-opioid receptor up-regulation and supersensitivity require occupancy of the receptor and that antagonist efficacy is not critical. Finally, the ED(50) to precipitate withdrawal jumping was examined in morphine-dependent mice. Naltrexone, naloxone, and 6beta-naltrexol produced withdrawal jumping, although potencies relative to 6beta-naltrexol were 211, 96, and 1, respectively. Thus, antagonist potency to precipitate opioid withdrawal was related to inverse agonist efficacy. Overall, the estimated relative potency of the opioid antagonists was a function of the outcome measured, and inverse agonist activity was not required for mu-opioid receptor up-regulation and supersensitivity.

In vitro pharmacological characterization of AM1241: a protean agonist at the cannabinoid CB2 receptor?

BACKGROUND AND PURPOSE

The CB2 receptor has been proposed as a novel target for the treatment of pain, and CB2 receptor agonists defined in in vitro assays have demonstrated analgesic activity in animal models. Based on its in vivo analgesic efficacy, AM1241 has been classified as a CB2-selective agonist. However, in vitro characterization of AM1241 in functional assays has not been reported.

EXPERIMENTAL APPROACH

In this study, AM1241 was characterized across multiple in vitro assays employing heterologous recombinant receptor expression systems to assess its binding potencies at the human CB2 and CB1 receptors and its functional efficacies at the human CB2 receptor.

KEY RESULTS

AM1241 exhibited distinct functional properties depending on the assay conditions employed, a unique profile in contrast to those of the agonist CP 55,940 and the inverse agonist SR144528. AM1241 displayed neutral antagonist activities in FLIPR and cyclase assays. However, when cyclase assays were performed using lower forskolin concentrations for stimulation, AM1241 exhibited partial agonist efficacy. In addition, it behaved as a partial agonist in ERK (or MAP) kinase assays.

CONCLUSIONS AND IMPLICATIONS

The unusual phenomenon of inconsistent functional efficacies suggests that AM1241 is a protean agonist at the CB2 receptor. We postulate that functional efficacies displayed by protean agonists in various assay systems may depend on the levels of receptor constitutive activities exhibited in the assay systems, and therefore, efficacies observed in in vitro assays may not predict in vivo activities.

Use of an inverse agonist radioligand [3H]A-317920 reveals distinct pharmacological profiles of the rat histamine H3 receptor

Selective radioligands for histamine H(3) receptors have been used to characterize H(3) receptor pharmacology by radioligand binding assays and to determine H(3) receptor distribution by tissue autoradiography. Here we report the synthesis and receptor binding characterization of [(3)H]A-317920 (furan-2-carboxylic acid(2-[4-[3-([3,5-(3)H]4-cyclopropanecarbonyl-phenoxy)-propyl]-piperazin-1-yl]-1-methyl-2-oxo-ethyl)-amide), a high affinity inverse agonist radioligand for the rat H(3) receptor. The binding of [(3)H]A-317920 to rat cortical and cloned H(3) receptors revealed fast on- and slower off-rate kinetics with calculated K(d) values in agreement with those determined in saturation binding assays (0.2 nM for both receptors). Further, we compared [(3)H]A-317920 with the agonist [(3)H](N)-alpha-methylhistamine ([(3)H]NalphaMH) as radioligand tools to study receptor pharmacology. Agonists and antagonists displaced [(3)H]NalphaMH with one-site binding characteristics and Hill slopes approached unity. In contrast, although antagonists exhibited one-site binding, [(3)H]A-317920 displacement by agonists was best fit by two-site binding models, and the potencies of the high affinity, GDP-sensitive sites correlated with the potencies defined in [(3)H]NalphaMH binding. Unlike [(125)I]iodoproxyfan, [(3)H]A-317920 exhibits potent and selective binding to rat H(3) receptors with low binding to non-H(3) sites, including cytochrome P450. These findings show that [(3)H]A-317920 is a potent rat H(3) receptor antagonist radioligand and has utility for studying H(3) receptor pharmacology.