Sex-Dependent Attentional Impairments in a Subchronic Ketamine Mouse Model for Schizophrenia.

Background: The development of more effective treatments for schizophrenia targeting cognitive and negative symptoms has been limited, partly due to a disconnect between rodent models and human illness. Ketamine administration is widely used to model symptoms of schizophrenia in both humans and rodents. In mice, subchronic ketamine treatment reproduces key dopamine and glutamate dysfunction; however, it is unclear how this translates into behavioral changes reflecting positive, negative, and cognitive symptoms. Methods: In male and female mice treated with either subchronic ketamine or saline, we assessed spontaneous and amphetamine-induced locomotor activity to measure behaviors relevant to positive symptoms, and used a touchscreen-based progressive ratio task of motivation and the rodent continuous performance test of attention to capture specific negative and cognitive symptoms, respectively. To explore neuronal changes underlying the behavioral effects of subchronic ketamine treatment, we quantified expression of the immediate early gene product, c-Fos, in key corticostriatal regions using immunofluorescence. Results: We showed that spontaneous locomotor activity was unchanged in male and female subchronic ketamine-treated animals, and amphetamine-induced locomotor response was reduced. Subchronic ketamine treatment did not alter motivation in either male or female mice. In contrast, we identified a sex-specific effect of subchronic ketamine on attentional processing wherein female mice performed worse than control mice due to increased nonselective responding. Finally, we showed that subchronic ketamine treatment increased c-Fos expression in prefrontal cortical and striatal regions, consistent with a mechanism of widespread disinhibition of neuronal activity. Conclusions: Our results highlight that the subchronic ketamine mouse model reproduces a subset of behavioral symptoms that are relevant for schizophrenia.

Gpr88 Deletion Impacts Motivational Control Without Overt Disruptions to Striatal Dopamine.

Background: Disrupted motivational control is a common-but poorly treated-feature of psychiatric disorders, arising via aberrant mesolimbic dopaminergic signaling. GPR88 is an orphan G protein-coupled receptor that is highly expressed in the striatum and therefore well placed to modulate disrupted signaling. While the phenotype of Gpr88 knockout mice suggests a role in motivational pathways, it is unclear whether GPR88 is involved in reward valuation and/or effort-based decision making in a sex-dependent manner and whether this involves altered dopamine function. Methods: In male and female Gpr88 knockout mice, we used touchscreen-based progressive ratio, with and without reward devaluation, and effort-related choice tasks to assess motivation and cost/benefit decision making, respectively. To explore whether these motivational behaviors were related to alterations in the striatal dopamine system, we quantified expression of dopamine-related genes and/or proteins and used [18F]DOPA positron emission tomography and GTPγ[35S] binding to assess presynaptic and postsynaptic dopamine function, respectively. Results: We showed that male and female Gpr88 knockout mice displayed greater motivational drive than wild-type mice, which was maintained following reward devaluation. Furthermore, we showed that cost/benefit decision making was impaired in male, but not female, Gpr88 knockout mice. Surprisingly, we found that Gpr88 deletion had no effect on striatal dopamine by any of the measures assessed. Conclusions: Our results highlight that GPR88 regulates motivational control but that disruption of such behaviors following Gpr88 deletion occurs independently of gross perturbations to striatal dopamine at a gene, protein, or functional level. This work provides further insights into GPR88 as a drug target for motivational disorders.

Molecular and structural insights into the 5-HT2C receptor as a therapeutic target for substance use disorders.

Substance use disorder (SUD) is a chronic condition, with maintained abuse of a substance leading to physiological and psychological alterations and often changes in cognitive and social behaviours. Current therapies include psychotherapy coupled with medication; however, high relapse rates reveal the shortcomings of these therapies. The signalling, expression profile, and neurological function of the serotonin 2C receptor (5-HT2C receptor) make it a candidate of interest for the treatment of SUD. Recently, psychedelics, which broadly act at 5-HT2 receptors, have indicated potential for the treatment of SUD, implicating the 5-HT2C receptor. The modern psychedelic movement has rekindled interest in the 5-HT2C receptor, resulting in many new studies, especially structural analyses. This review explores the structural, molecular and cellular mechanisms governing 5-HT2C receptor function in the context of SUD. This provides the basis of the preclinical and clinical evidence for their role in SUD and highlights the potential for future exploration.

Molecular insights into orphan G protein-coupled receptors relevant to schizophrenia.

Schizophrenia remains a sizable socio-economic burden that continues to be treated with therapeutics based on 70-year old science. All currently approved therapeutics primarily target the dopamine D2 receptor to achieve their efficacy. Whilst dopaminergic dysregulation is a key feature in this disorder, the targeting of dopaminergic machinery has yielded limited efficacy and an appreciable side effect burden. Over the recent decades, numerous drugs that engage non-dopaminergic G protein-coupled receptors (GPCRs) have yielded a promise of efficacy without the deleterious side effect profile, yet none have successfully completed clinical studies and progressed to the market. More recently, there has been increased attention around non-dopaminergic GPCR-targeting drugs, which demonstrated efficacy in some schizophrenia symptom domains. This provides renewed hope that effective schizophrenia treatment may lie outside of the dopaminergic space. Despite the potential for muscarinic receptor- (and other well-characterised GPCR families) targeting drugs to treat schizophrenia, they are often plagued with complications such as lack of receptor subtype selectivity and peripheral on-target side effects. Orphan GPCR studies have opened a new avenue of exploration with many demonstrating schizophrenia-relevant mechanisms and a favourable expression profile, thus offering potential for novel drug development. This review discusses centrally expressed orphan GPCRs: GPR3, GPR6, GPR12, GPR52, GPR85, GPR88 and GPR139 and their relationship to schizophrenia. We review their expression, signalling mechanisms and cellular function, in conjunction with small molecule development and structural insights. We seek to provide a snapshot of the growing evidence and development potential of new classes of schizophrenia therapeutics.

Targeting appetite and satiety in diabetes and obesity, via G protein-coupled receptors.

Type 2 diabetes and obesity have reached pandemic proportions throughout the world, so much so that the World Health Organisation coined the term "Globesity" to help encapsulate the magnitude of the problem. G protein-coupled receptors (GPCRs) are highly tractable drug targets due to their wide involvement in all aspects of physiology and pathophysiology, indeed, GPCRs are the targets of approximately 30% of the currently approved drugs. GPCRs are also broadly involved in key physiologies that underlie type 2 diabetes and obesity including feeding reward, appetite and satiety, regulation of blood glucose levels, energy homeostasis and adipose function. Despite this, only two GPCRs are the target of approved pharmaceuticals for treatment of type 2 diabetes and obesity. In this review we discuss the role of these, and select other candidate GPCRs, involved in various facets of type 2 diabetic or obese pathophysiology, how they might be targeted and the potential reasons why pharmaceuticals against these targets have not progressed to clinical use. Finally, we provide a perspective on the current development pipeline of anti-obesity drugs that target GPCRs.

Beyond antipsychotics: a twenty-first century update for preclinical development of schizophrenia therapeutics.

Despite 50+ years of drug discovery, current antipsychotics have limited efficacy against negative and cognitive symptoms of schizophrenia, and are ineffective-with the exception of clozapine-against any symptom domain for patients who are treatment resistant. Novel therapeutics with diverse non-dopamine D2 receptor targets have been explored extensively in clinical trials, yet often fail due to a lack of efficacy despite showing promise in preclinical development. This lack of translation between preclinical and clinical efficacy suggests a systematic failure in current methods that determine efficacy in preclinical rodent models. In this review, we critically evaluate rodent models and behavioural tests used to determine preclinical efficacy, and look to clinical research to provide a roadmap for developing improved translational measures. We highlight the dependence of preclinical models and tests on dopamine-centric theories of dysfunction and how this has contributed towards a self-reinforcing loop away from clinically meaningful predictions of efficacy. We review recent clinical findings of distinct dopamine-mediated dysfunction of corticostriatal circuits in patients with treatment-resistant vs. non-treatment-resistant schizophrenia and suggest criteria for establishing rodent models to reflect such differences, with a focus on objective, translational measures. Finally, we review current schizophrenia drug discovery and propose a framework where preclinical models are validated against objective, clinically informed measures and preclinical tests of efficacy map onto those used clinically.

CRISPR/Cas9 mediated deletion of the adenosine A2A receptor enhances CAR T cell efficacy.

Adenosine is an immunosuppressive factor that limits anti-tumor immunity through the suppression of multiple immune subsets including T cells via activation of the adenosine A2A receptor (A2AR). Using both murine and human chimeric antigen receptor (CAR) T cells, here we show that targeting A2AR with a clinically relevant CRISPR/Cas9 strategy significantly enhances their in vivo efficacy, leading to improved survival of mice. Effects evoked by CRISPR/Cas9 mediated gene deletion of A2AR are superior to shRNA mediated knockdown or pharmacological blockade of A2AR. Mechanistically, human A2AR-edited CAR T cells are significantly resistant to adenosine-mediated transcriptional changes, resulting in enhanced production of cytokines including IFNγ and TNF, and increased expression of JAK-STAT signaling pathway associated genes. A2AR deficient CAR T cells are well tolerated and do not induce overt pathologies in mice, supporting the use of CRISPR/Cas9 to target A2AR for the improvement of CAR T cell function in the clinic.

Translation-Focused Approaches to GPCR Drug Discovery for Cognitive Impairments Associated with Schizophrenia.

There are no effective therapeutics for cognitive impairments associated with schizophrenia (CIAS), which includes deficits in executive functions (working memory and cognitive flexibility) and episodic memory. Compounds that have entered clinical trials are inadequate in terms of efficacy and/or tolerability, highlighting a clear translational bottleneck and a need for a cohesive preclinical drug development strategy. In this review we propose hippocampal-prefrontal-cortical (HPC-PFC) circuitry underlying CIAS-relevant cognitive processes across mammalian species as a target source to guide the translation-focused discovery and development of novel, procognitive agents. We highlight several G protein-coupled receptors (GPCRs) enriched within HPC-PFC circuitry as therapeutic targets of interest, including noncanonical approaches (biased agonism and allosteric modulation) to conventional clinical targets, such as dopamine and muscarinic acetylcholine receptors, along with prospective novel targets, including the orphan receptors GPR52 and GPR139. We also describe the translational limitations of popular preclinical cognition tests and suggest touchscreen-based assays that probe cognitive functions reliant on HPC-PFC circuitry and reflect tests used in the clinic, as tests of greater translational relevance. Combining pharmacological and behavioral testing strategies based in HPC-PFC circuit function creates a cohesive, translation-focused approach to preclinical drug development that may improve the translational bottleneck currently hindering the development of treatments for CIAS.

Differential GLP-1R Binding and Activation by Peptide and Non-peptide Agonists.

Peptide drugs targeting class B1 G-protein-coupled receptors (GPCRs) can treat multiple diseases; however, there remains substantial interest in the development of orally delivered non-peptide drugs. Here, we reveal unexpected overlap between signaling and regulation of the glucagon-like peptide-1 (GLP-1) receptor by the non-peptide agonist PF 06882961 and GLP-1 that was not observed for another compound, CHU-128. Compounds from these patent series, including PF 06882961, are currently in clinical trials for treatment of type 2 diabetes. High-resolution cryoelectron microscopy (cryo-EM) structures reveal that the binding sites for PF 06882961 and GLP-1 substantially overlap, whereas CHU-128 adopts a unique binding mode with a more open receptor conformation at the extracellular face. Structural differences involving extensive water-mediated hydrogen bond networks could be correlated to functional data to understand how PF 06882961, but not CHU-128, can closely mimic the pharmacological properties of GLP-1. These findings will facilitate rational structure-based discovery of non-peptide agonists targeting class B GPCRs.

In the Loop: Extrastriatal Regulation of Spiny Projection Neurons by GPR52.

GPR52 is a Gαs-coupled orphan receptor identified as a putative target for the treatment of schizophrenia. The unique expression and signaling profile of GPR52 in key areas of dopamine and glutamate dysregulation suggests its activation may resolve both cortical and striatal dysfunction in the disorder. GPR52 mRNA is enriched in the striatum, almost exclusively on dopamine D2-expressing medium spiny neurons (MSNs), and to a lesser extent in the cortex, predominantly on D1-expressing pyramidal neurons. Synthetic, small molecule GPR52 agonists are effective in preclinical models of psychosis; however, the relative contribution of cortical and striatal GPR52 is unknown. Here we show that the GPR52 agonist, 3-BTBZ, inhibits phencyclidine-induced hyperlocomotor activity to a greater degree than amphetamine-induced motor effects, suggesting a mechanism beyond functional antagonism of striatal dopamine D2 receptor signaling. Using DARPP-32 phosphorylation and electrophysiological recordings in either striatopallidal or striatonigral MSNs, we were surprised to find no significant effect of 3-BTBZ in striatopallidal MSNs, but GPR52-mediated effects in striatonigral MSNs, where its mRNA is absent. 3-BTBZ increases phosphorylation of T75 on DARPP-32 in striatonigral MSNs, an effect that was dependent on cortical inputs. A similar role for GPR52 in regulating extrastriatal glutamatergic drive onto striatonigral MSNs was also evident in recordings of spontaneous excitatory postsynaptic currents and was shown to be dependent on the metabotropic glutamate (mGlu) receptor subtype 1. Our results demonstrate that GPR52-mediated regulation of striatal function depends heavily on extrastriatal inputs, which may further support its utility as a novel target for the treatment of schizophrenia.

ß-Arrestin-2-Dependent Mechanism of GPR52 Signaling in Frontal Cortical Neurons.

The orphan Gαs-coupled receptor GPR52 is expressed exclusively in the brain, predominantly in circuitry relating to symptoms of neuropsychiatric and cognitive disorders such as schizophrenia. While GPR52 agonists have displayed antipsychotic and procognitive efficacy in murine models, there remains limited evidence delineating the molecular mechanisms of these effects. Indeed, previous studies have solely reported canonical cAMP signaling and CREB phosphorylation downstream of GPR52 activation. In the present study, we demonstrated that the synthetic GPR52 agonist, 3-BTBZ, equipotently induces cAMP accumulation, ERK1/2 phosphorylation, and ß-arrestin-1 and -2 recruitment in transfected HEK293T cells. In cultured frontal cortical neurons, however, 3-BTBZ-induced ERK1/2 phosphorylation was significantly more potent than cAMP signaling, with a more prolonged signaling profile than that in HEK293T cells. Furthermore, knock down of ß-arrestin-2 in frontal cortical neurons abolished 3-BTBZ-induced ERK1/2 phosphorylation, but not cAMP accumulation. These results suggest a ß-arrestin-2-dependent mechanism for GPR52-mediated ERK1/2 signaling, which may link to cognitive function in vivo. Finally, these findings highlight the context-dependence of GPCR signaling in recombinant cells and neurons, offering new insights into translationally relevant GPR52 signaling mechanisms.

A pH-responsive nanoparticle targets the neurokinin 1 receptor in endosomes to prevent chronic pain.

Nanoparticle-mediated drug delivery is especially useful for targets within endosomes because of the endosomal transport mechanisms of many nanomedicines within cells. Here, we report the design of a pH-responsive, soft polymeric nanoparticle for the targeting of acidified endosomes to precisely inhibit endosomal signalling events leading to chronic pain. In chronic pain, the substance P (SP) neurokinin 1 receptor (NK1R) redistributes from the plasma membrane to acidified endosomes, where it signals to maintain pain. Therefore, the NK1R in endosomes provides an important target for pain relief. The pH-responsive nanoparticles enter cells by clathrin- and dynamin-dependent endocytosis and accumulate in NK1R-containing endosomes. Following intrathecal injection into rodents, the nanoparticles, containing the FDA-approved NK1R antagonist aprepitant, inhibit SP-induced activation of spinal neurons and thus prevent pain transmission. Treatment with the nanoparticles leads to complete and persistent relief from nociceptive, inflammatory and neuropathic nociception and offers a much-needed non-opioid treatment option for chronic pain.

Targeting Adenosine Receptor Signaling in Cancer Immunotherapy.

The immune system plays a major role in the surveillance and control of malignant cells, with the presence of tumor infiltrating lymphocytes (TILs) correlating with better patient prognosis in multiple tumor types. The development of 'checkpoint blockade' and adoptive cellular therapy has revolutionized the landscape of cancer treatment and highlights the potential of utilizing the patient's own immune system to eradicate cancer. One mechanism of tumor-mediated immunosuppression that has gained attention as a potential therapeutic target is the purinergic signaling axis, whereby the production of the purine nucleoside adenosine in the tumor microenvironment can potently suppress T and NK cell function. The production of extracellular adenosine is mediated by the cell surface ectoenzymes CD73, CD39, and CD38 and therapeutic agents have been developed to target these as well as the downstream adenosine receptors (A1R, A2AR, A2BR, A3R) to enhance anti-tumor immune responses. This review will discuss the role of adenosine and adenosine receptor signaling in tumor and immune cells with a focus on their cell-specific function and their potential as targets in cancer immunotherapy.

Allosteric interactions between GABAB1 subunits control orthosteric binding sites occupancy within GABAB oligomers.

The GABAB receptor was the first G protein-coupled receptor identified as an obligate heterodimer. It is composed of two subunits, GABAB1 containing the agonist binding site and GABAB2 responsible for G protein activation. The GABAB receptor was found to associate into larger complexes through GABAB1-GABAB1 interactions, both in transfected cells and in brain membranes. Here we assessed the possible allosteric interactions between GABAB heterodimers by analyzing the effect of mutations located at the putative interface between the extracellular binding domains. These mutations decrease, but do not suppress, the Förster resonance energy transfer (FRET) signal measured between GABAB1 subunits. Further analysis of one of these mutations revealed an increase in G protein-coupling efficacy and in the maximal antagonist binding by approximately two-fold. Hypothesizing that a tetramer is an elementary unit within oligomers, additional FRET data using fluorescent ligands and tagged subunits suggest that adjacent binding sites within the GABAB oligomers are not simultaneously occupied. Our data show a strong negative effect between GABAB1 binding sites within GABAB oligomers. Accordingly, GABAB receptor assembly appears to limit receptor signaling to G proteins, a property that may offer novel regulatory mechanism for this important neuronal receptor. This article is part of the "Special Issue Dedicated to Norman G. Bowery".

Biased allosteric agonism and modulation of metabotropic glutamate receptor 5: Implications for optimizing preclinical neuroscience drug discovery.

Allosteric modulators, that exhibit no intrinsic agonist activity, offer the advantage of spatial and temporal fine-tuning of endogenous agonist activity, allowing the potential for increased selectivity, reduced adverse effects and improved clinical outcomes. Some allosteric ligands can differentially activate and/or modulate distinct signaling pathways arising from the same receptor, phenomena referred to as 'biased agonism' and 'biased modulation'. Emerging evidence for CNS disorders with glutamatergic dysfunction suggests the metabotropic glutamate receptor subtype 5 (mGlu5) is a promising target. Current mGlu5 allosteric modulators have largely been classified based on modulation of intracellular calcium (iCa2+) responses to orthosteric agonists alone. We assessed eight mGlu5 allosteric modulators previously classified as mGlu5 PAMs or PAM-agonists representing four distinct chemotypes across multiple measures of receptor activity, to explore their potential for engendering biased agonism and/or modulation. Relative to the reference orthosteric agonist, DHPG, the eight allosteric ligands exhibited distinct biased agonism fingerprints for iCa2+ mobilization, IP1 accumulation and ERK1/2 phosphorylation in HEK293A cells stably expressing mGlu5 and in cortical neuron cultures. VU0424465, DPFE and VU0409551 displayed the most disparate biased signaling fingerprints in both HEK293A cells and cortical neurons that may account for the marked differences observed previously for these ligands in vivo. Select mGlu5 allosteric ligands also showed 'probe dependence' with respect to their cooperativity with different orthosteric agonists, as well as biased modulation for the magnitude of positive cooperativity observed. Unappreciated biased agonism and modulation may contribute to unanticipated effects (both therapeutic and adverse) when translating from recombinant systems to preclinical models. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Murine GPRC6A Mediates Cellular Responses to L-Amino Acids, but Not Osteocalcin Variants.

Phenotyping of Gprc6a KO mice has shown that this promiscuous class C G protein coupled receptor is variously involved in regulation of metabolism, inflammation and endocrine function. Such effects are described as mediated by extracellular calcium, L-amino acids, the bone-derived peptide osteocalcin (OCN) and the male hormone testosterone, introducing the concept of a bone-energy-metabolism-reproduction functional crosstalk mediated by GPRC6A. However, whilst the calcium and L-amino acid-sensing properties of GPRC6A are well established, verification of activity of osteocalcin at both human and mouse GPRC6A in vitro has proven somewhat elusive. This study characterises the in vitro pharmacology of mouse GPRC6A in response to its putative ligands in both recombinant and endogenous GPRC6A-expressing cells. Using cell signalling, and glucagon-like peptide (GLP)-1 and insulin release assays, our results confirm that basic L-amino acids act as agonists of the murine GPRC6A receptor in both recombinant cells and immortalised entero-endocrine and pancreatic ß-cells. In contrast, our studies do not support a role for OCN as a direct ligand for mouse GPRC6A, suggesting that the reported in vivo effects of OCN that require GPRC6A may be indirect, rather than via direct activation of the receptor.

Prediction of functionally selective allosteric interactions at an M3 muscarinic acetylcholine receptor mutant using Saccharomyces cerevisiae.

Saccharomyces cerevisiae is a tractable yeast species for expression and coupling of heterologous G protein-coupled receptors with the endogenous pheromone response pathway. Although this platform has been used for ligand screening, no studies have probed its ability to predict novel pharmacology and functional selectivity of allosteric ligands. As a proof of concept, we expressed a rat M(3) muscarinic acetylcholine receptor (mAChR) bearing a mutation (K(7.32)E) recently identified to confer positive cooperativity between acetylcholine and the allosteric modulator brucine in various strains of S. cerevisiae, each expressing a different human Galpha/yeast Gpa1 protein chimera, and probed for G protein-biased allosteric modulation. Subsequent assays performed in this system revealed that brucine was a partial allosteric agonist and positive modulator of carbachol when coupled to Gpa1/G(q) proteins, a positive modulator (no agonism) when coupled to Gpa1/G(12) proteins, and a neutral modulator when coupled to Gpa1/G(i) proteins. It is noteworthy that these results were validated at the human M(3)K(7.32)E mAChR expressed in a mammalian (Chinese hamster ovary) cell background by determination of calcium mobilization and membrane ruffling as surrogate measures of G(q) and G(12) protein activation, respectively. Furthermore, the combination of this functionally selective allosteric modulator with G protein-biased yeast screens allowed us to ascribe a potential G protein candidate (G(12)) as a key mediator for allosteric modulation of M(3)K(7.32)E mAChR-mediated ERK1/2 phosphorylation, which was confirmed by small interfering RNA knockdown experiments. These results highlight how the yeast platform can be used to identify functional selectivity of allosteric ligands and to facilitate dissection of convergent signaling pathways.

Detection of novel functional selectivity at M3 muscarinic acetylcholine receptors using a Saccharomyces cerevisiae platform.

"Functional selectivity", although new to many chemists and biologists only a few years ago, has now become a dominant theme in drug discovery. This concept posits that different ligands engender unique receptor conformations such that only a subset of signaling pathways linked to a given receptor are recruited. However, successful exploitation of the phenomenon to achieve pathway-based selectivity requires the ability to routinely detect it when assessing ligand behavior. We have utilized different strains of the yeast S. cerevisiae, each expressing a specific human Galpha/yeast Gpa1 protein chimera coupled to a MAP kinase-linked reporter gene readout, to investigate the signaling of the M(3) muscarinic receptor, a G protein-coupled receptor (GPCR) for which various antagonists are used clinically. Using this novel platform, we found that the "antagonists", atropine, N-methylscopolamine, and pirenzepine, were inverse agonists for Gpa1/Galpha(q) but low efficacy agonists for Gpa1/Galpha(12.) Subsequent studies with atropine performed in mammalian 3T3 cells validated these findings by demonstrating inverse agonism for G(q/11)-mediated calcium mobilization but positive agonism for G(12)-mediated membrane ruffling. This is the first study to utilize a yeast platform to discover pathway-biased functional selectivity in a GPCR. In addition to the likely applicability of this approach for identifying biased signaling by novel chemical entities, our findings also suggest that currently marketed medications may exhibit hitherto unappreciated functional selectivity.

Determination of adenosine A1 receptor agonist and antagonist pharmacology using Saccharomyces cerevisiae: implications for ligand screening and functional selectivity.

The budding yeast, Saccharomyces cerevisiae, is a convenient system for coupling heterologous G protein-coupled receptors (GPCRs) to the pheromone response pathway to facilitate empirical ligand screening and/or GPCR mutagenesis studies. However, few studies have applied this system to define GPCR-G protein-coupling preferences and furnish information on ligand affinities, efficacies, and functional selectivity. We thus used different S. cerevisiae strains, each expressing a specific human Galpha/yeast Gpa1 protein chimera, and determined the pharmacology of various ligands of the coexpressed human adenosine A(1) receptor. These assays, in conjunction with the application of quantitative models of agonism and antagonism, revealed that (-)-N(6)-(2-phenylisopropyl)adenosine was a high-efficacy agonist that selectively coupled to Gpa/1Galpha(o), Gpa1/Galpha(i1/2), and Gpa1/Galpha(i3), whereas the novel compound, 5'-deoxy-N(6)-(endo-norborn-2-yl)-5'-(2-fluorophenylthio)adenosine (VCP-189), was a lower-efficacy agonist that selectively coupled to Gpa1/Galpha(i) proteins; the latter finding suggested that VCP-189 might be functionally selective. The affinity of the antagonist, 8-cyclopentyl-1,3-dipropylxanthine, was also determined at the various strains. Subsequent experiments performed in mammalian Chinese hamster ovary cells monitoring cAMP formation/inhibition, intracellular calcium mobilization, phosphorylation of extracellular signal-regulated kinase 1 and 2 or (35)S-labeled guanosine 5'-(gamma-thio)triphosphate binding, were in general agreement with the yeast data regarding agonist efficacy estimation and antagonist affinity estimation, but revealed that the apparent functional selectivity of VCP-189 could be explained by differences in stimulus-response coupling between yeast and mammalian cells. Our results suggest that this yeast system is a useful tool for quantifying ligand affinity and relative efficacy, but it may lack the sensitivity required to detect functional selectivity of low-efficacy agonists.

Pharmacology of 5HT(2C) receptor-mediated ERK1/2 phosphorylation: agonist-specific activation pathways and the impact of RNA editing.

We have previously characterized a mechanism of 5HT-stimulated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation via the non-RNA-edited isoform of the serotonin 5HT(2C) receptor (5HT(2C)R-INI) in a CHO cell line. We have now used CV1 cells, which endogenously express epidermal growth factor receptors (EGFRs), to investigate whether the mechanisms underlying ERK1/2 activation by the 5HT(2C)R change in a time-, agonist-, and cell background-dependent manner. Interrogation of the CV1 5HT(2C)R-INI ERK1/2 signaling pathway, using a variety of pathway-selective inhibitors, revealed a clear time-dependence in the involvement of specific pathway components such as phosphatidylinositol 3-kinase, EGFR, matrix metalloproteases and protein kinase C. The contribution of these components to the overall response also varied with the agonist used to stimulate the receptor, providing further evidence for the ability of 5HT(2C)R-INI to signal in an agonist-specific manner. We also investigated the impact of 5HT(2C)R RNA editing on this phenomenon. Although we found no alteration in antagonist pharmacology, the partially edited VSV and fully edited VGV isoforms of the 5HT(2C)R exhibited altered temporal and pharmacological characteristics, including the degree of dependence on specific effectors, in signaling to ERK1/2 in comparison to the 5HT(2C)R-INI. In conclusion, we provide evidence for remarkable flexibility in 5HT(2C)R-mediated ERK1/2 signaling that can be pharmacologically and mechanistically distinct depending on the agonist or edited isoform involved and on the duration of receptor activation.