Chemerin-activated functions of CMKLR1 are regulated by G protein-coupled receptor kinase 6 (GRK6) and β-arrestin 2 in inflammatory macrophages

Chemerin receptor (CMKLR1) is a G protein-coupled receptor (GPCR) implicated in macrophage-mediated inflammation and in several forms of human arthritis. Analogous to other GPCR, CMKLR1 is likely regulated by G protein-coupled receptor kinase (GRK) phosphorylation of intracellular domains in an activation-dependent manner, which leads to recruitment and termination of intracellular signaling via desensitization and internalization of the receptor. The ubiquitously expressed GRK family members include GRK2, GRK3, GRK5, and GRK6, but it is unknown which GRK regulates CMKLR1 cellular and signaling functions. Our data show that activation of CMKLR1 by chemerin in primary macrophages leads to signaling and functional outcomes that are regulated by GRK6 and β-arrestin 2. We show that arrestin recruitment to CMKLR1 following chemerin stimulation is enhanced with co-expression of GRK6. Further, internalization of endogenous CMKLR1, following the addition of chemerin, is decreased in inflammatory macrophages from GRK6- and β-arrestin 2-deficient mice. These GRK6- and β-arrestin 2-deficient macrophages display increased migration toward chemerin and altered AKT and Extracellular-signal Related Kinase (ERK) signaling. Our findings show that chemerin-activated CMKLR1 regulation in inflammatory macrophages is largely GRK6 and β-arrestin mediated, which may impact innate immunity and have therapeutic implications in rheumatic disease.

Prosocial effects of an oxytocin metabolite, but not synthetic oxytocin receptor agonists, in a mouse model of autism

Currently, there are no established pharmaceutical strategies that effectively treat social deficits in autism spectrum disorder (ASD). Oxytocin, a neurohormone that plays a role in multiple types of social behaviors, has been proposed as a possible therapeutic against social impairment and other symptoms in ASD. However, from the standpoint of pharmacotherapy, oxytocin has several liabilities as a standard clinical treatment, including rapid metabolism, low brain penetrance, and activity at the vasopressin (antidiuretic hormone) receptors. The present studies describe findings from a preclinical screening program to evaluate oxytocin receptor (OXTR) agonists and oxytocin metabolites for potential clinical use as more optimal treatments. We first investigated two synthetic oxytocin analogs, TC-OT-39 and carbetocin, using in vitro cell-based assays for pharmacological characterization and behavioral tests in the BALB/cByJ mouse model of ASD-like social deficits. Although both TC-OT-39 and carbetocin selectively activate the OXTR, neither synthetic agonist had prosocial efficacy in the BALB/cByJ model. We next evaluated two oxytocin metabolites: OT(4-9) and OT(5-9). While OT(5-9) failed to affect social deficits, the metabolite OT(4-9) led to significant social preference in the BALB/cByJ model, in a dose-dependent manner. The increased sociability was observed at both 24 h and 12 days following the end of a subchronic regimen with OT(4-9) (2.0 mg/kg). Overall, these results suggest that the prosocial effects of oxytocin could be mediated by downstream activity of oxytocin metabolites, raising the possibility of new pathways to target for drug discovery relevant to ASD.

Selectivity Challenges in Docking Screens for GPCR Targets and Antitargets

To investigate large library docking's ability to find molecules with joint activity against on-targets and selectivity versus antitargets, the dopamine D₂ and serotonin 5-HT_2A receptors were targeted, seeking selectivity against the histamine H₁ receptor. In a second campaign, κ-opioid receptor ligands were sought with selectivity versus the μ-opioid receptor. While hit rates ranged from 40% to 63% against the on-targets, they were just as good against the antitargets, even though the molecules were selected for their putative lack of binding to the off-targets. Affinities, too, were often as good or better for the off-targets. Even though it was occasionally possible to find selective molecules, such as a mid-nanomolar D₂/5-HT_2A ligand with 21-fold selectivity versus the H₁ receptor, this was the exception. Whereas false-negatives are tolerable in docking screens against on-targets, they are intolerable against antitargets; addressing this problem may demand new strategies in the field.

Identification of Cosalane as an Inhibitor of Human and Murine CC-Chemokine Receptor 7 Signaling via a High-Throughput Screen

CC-chemokine receptor 7 (CCR7) is a G protein-coupled receptor expressed on a variety of immune cells. CCR7 plays a critical role in the migration of lymphocytes into secondary lymphoid tissues. CCR7 expression, however, has been linked to numerous disease states. Due to its therapeutic relevance and absence of available CCR7 inhibitors, we undertook a high-throughput screen (HTS) to identify small-molecule antagonists of the receptor. Here, we describe a robust HTS approach using a commercially available β-galactosidase enzyme fragment complementation system and confirmatory transwell chemotaxis assays. This work resulted in the identification of several compounds with activity against CCR7. The most potent of these was subsequently determined to be cosalane, a cholesterol derivative previously designed as a therapeutic for human immunodeficiency virus. Cosalane inhibited both human and murine CCR7 in response to both CCL19 and CCL21 agonists at physiologic concentrations. Furthermore, cosalane produced durable inhibition of the receptor following a cellular incubation period with subsequent washout. Overall, our work describes the development of an HTS-compatible assay, completion of a large HTS campaign, and demonstration for the first time that cosalane is a validated CCR7 antagonist. These efforts could pave the way for new approaches to address CCR7-associated disease processes.

Structure-based discovery of opioid analgesics with reduced side effects

Morphine is an alkaloid from the opium poppy used to treat pain. The potentially lethal side effects of morphine and related opioids-which include fatal respiratory depression-are thought to be mediated by μ-opioid-receptor (μOR) signalling through the β-arrestin pathway or by actions at other receptors. Conversely, G-protein μOR signalling is thought to confer analgesia. Here we computationally dock over 3 million molecules against the μOR structure and identify new scaffolds unrelated to known opioids. Structure-based optimization yields PZM21-a potent G_i activator with exceptional selectivity for μOR and minimal β-arrestin-2 recruitment. Unlike morphine, PZM21 is more efficacious for the affective component of analgesia versus the reflexive component and is devoid of both respiratory depression and morphine-like reinforcing activity in mice at equi-analgesic doses. PZM21 thus serves as both a probe to disentangle μOR signalling and a therapeutic lead that is devoid of many of the side effects of current opioids.

G Protein Coupled Receptor Kinase 3 Regulates Breast Cancer Migration, Invasion, and Metastasis

Triple negative breast cancer (TNBC) is a heterogeneous disease that has a poor prognosis and limited treatment options. Chemokine receptor interactions are important modulators of breast cancer metastasis; however, it is now recognized that quantitative surface expression of one important chemokine receptor, CXCR4, may not directly correlate with metastasis and that its functional activity in breast cancer may better inform tumor pathogenicity. G protein coupled receptor kinase 3 (GRK3) is a negative regulator of CXCR4 activity, and we show that GRK expression correlates with tumorigenicity, molecular subtype, and metastatic potential in human tumor microarray analysis. Using established human breast cancer cell lines and an immunocompetent in vivo mouse model, we further demonstrate that alterations in GRK3 expression levels in tumor cells directly affect migration and invasion in vitro and the establishment of distant metastasis in vivo. The effects of GRK3 modulation appear to be specific to chemokine-mediated migration behaviors without influencing tumor cell proliferation or survival. These data demonstrate that GRK3 dysregulation may play an important part in TNBC metastasis.

Single Amino Acid Variation Underlies Species-Specific Sensitivity to Amphibian Skin-Derived Opioid-like Peptides

It has been suggested that the evolution of vertebrate opioid receptors (ORs) follow a vector of increased functionality. Here, we test this idea by comparing human and frog ORs. Interestingly, some of the most potent opioid peptides known have been isolated from amphibian skin secretions. Here we show that such peptides (dermorphin and deltorphin) are highly potent in the human receptors and inactive in frog ORs. The molecular basis for the insensitivity of the frog ORs to these peptides was studied using chimeras and molecular modeling. The insensitivity of the delta OR (DOR) to deltorphin was due to variation of a single amino acid, Trp7.35, which is a leucine in mammalian DORs. Notably, Trp7.35 is completely conserved in all known DOR sequences from lamprey, fish, and amphibians. The deltorphin-insensitive phenotype was verified in fish. Our results provide a molecular explanation for the species selectivity of skin-derived opioid peptides.

PRESTO-Tango as an open-source resource for interrogation of the druggable human GPCRome

G protein-coupled receptors (GPCRs) are essential mediators of cellular signaling and are important targets of drug action. Of the approximately 350 nonolfactory human GPCRs, more than 100 are still considered to be 'orphans' because their endogenous ligands remain unknown. Here, we describe a unique open-source resource that allows interrogation of the druggable human GPCRome via a G protein-independent β-arrestin-recruitment assay. We validate this unique platform at more than 120 nonorphan human GPCR targets, demonstrate its utility for discovering new ligands for orphan human GPCRs and describe a method (parallel receptorome expression and screening via transcriptional output, with transcriptional activation following arrestin translocation (PRESTO-Tango)) for the simultaneous and parallel interrogation of the entire human nonolfactory GPCRome.

PRESTO-Tango as an open-source resource for interrogation of the druggable human GPCRome

G protein-coupled receptors (GPCRs) are essential mediators of cellular signaling and are important targets of drug action. Of the approximately 350 nonolfactory human GPCRs, more than 100 are still considered to be 'orphans' because their endogenous ligands remain unknown. Here, we describe a unique open-source resource that allows interrogation of the druggable human GPCRome via a G protein-independent β-arrestin-recruitment assay. We validate this unique platform at more than 120 nonorphan human GPCR targets, demonstrate its utility for discovering new ligands for orphan human GPCRs and describe a method (parallel receptorome expression and screening via transcriptional output, with transcriptional activation following arrestin translocation (PRESTO-Tango)) for the simultaneous and parallel interrogation of the entire human nonolfactory GPCRome.

The TLQP-21 peptide activates the G-protein-coupled receptor C3aR1 via a folding-upon-binding mechanism

TLQP-21, a VGF-encoded peptide is emerging as a novel target for obesity-associated disorders. TLQP-21 is found in the sympathetic nerve terminals in the adipose tissue and targets the G-protein-coupled receptor complement-3a receptor1 (C3aR1). The mechanisms of TLQP-21-induced receptor activation remain unexplored. Here, we report that TLQP-21 is intrinsically disordered and undergoes a disorder-to-order transition, adopting an α-helical conformation upon targeting cells expressing the C3aR1. We determined that the hot spots for TLQP-21 are located at the C terminus, with mutations in the last four amino acids progressively reducing the bioactivity and, a single site mutation (R21A) or C-terminal amidation abolishing its function completely. Additionally, the human TLQP-21 sequence carrying a S20A substitution activates the human C3aR1 receptor with lower potency compared to the rodent sequence. These studies reveal the mechanism of action of TLQP-21 and provide molecular templates for designing agonists and antagonists to modulate C3aR1 functions.

HTS navigator: freely accessible cheminformatics software for analyzing high-throughput screening data

SUMMARY

We report on the development of the high-throughput screening (HTS) Navigator software to analyze and visualize the results of HTS of chemical libraries. The HTS Navigator processes output files from different plate readers' formats, computes the overall HTS matrix, automatically detects hits and has different types of baseline navigation and correction features. The software incorporates advanced cheminformatics capabilities such as chemical structure storage and visualization, fast similarity search and chemical neighborhood analysis for retrieved hits. The software is freely available for academic laboratories.

AVAILABILITY AND IMPLEMENTATION

http://fourches.web.unc.edu/

In silico molecular comparisons of C. elegans and mammalian pharmacology identify distinct targets that regulate feeding

This paper takes advantage of similarities between the C. elegans and human pharmacopeia to identify and validate pharmacological targets that regulate C. elegans feeding rates.

Phenotypic screens can identify molecules that are at once penetrant and active on the integrated circuitry of a whole cell or organism. These advantages are offset by the need to identify the targets underlying the phenotypes. Additionally, logistical considerations limit screening for certain physiological and behavioral phenotypes to organisms such as zebrafish and C. elegans. This further raises the challenge of elucidating whether compound-target relationships found in model organisms are preserved in humans. To address these challenges we searched for compounds that affect feeding behavior in C. elegans and sought to identify their molecular mechanisms of action. Here, we applied predictive chemoinformatics to small molecules previously identified in a C. elegans phenotypic screen likely to be enriched for feeding regulatory compounds. Based on the predictions, 16 of these compounds were tested in vitro against 20 mammalian targets. Of these, nine were active, with affinities ranging from 9 nM to 10 µM. Four of these nine compounds were found to alter feeding. We then verified the in vitro findings in vivo through genetic knockdowns, the use of previously characterized compounds with high affinity for the four targets, and chemical genetic epistasis, which is the effect of combined chemical and genetic perturbations on a phenotype relative to that of each perturbation in isolation. Our findings reveal four previously unrecognized pathways that regulate feeding in C. elegans with strong parallels in mammals. Together, our study addresses three inherent challenges in phenotypic screening: the identification of the molecular targets from a phenotypic screen, the confirmation of the in vivo relevance of these targets, and the evolutionary conservation and relevance of these targets to their human orthologs.

Many beneficial pharmacological interventions were first discovered by observing the effects of perturbation of intact biological systems by small organic molecules without a priori knowledge of their targets. This forward pharmacological approach has the advantage of directly identifying new pharmacological agents that are active on complex biological processes. However, because of experimental feasibility, systematic application of this approach is generally limited to small animals such as the roundworm C. elegans and zebrafish, raising the question of whether use of these animals could identify compounds that act on ortholgous mammalian targets. A significant challenge in addressing this question is the determination of the molecular identities of the compounds' targets responsible for the desired phenotypic outcomes. Here we describe a computational approach for target identification based on structural similarities of newly identified compounds to known ligand interactions with mostly mammalian targets. For several of the compounds emerging from a C. elegans phenotypic screen, we predict and confirm mammalian targets using in vitro binding assays. Using genetic and pharmacological assays, we then demonstrate that a subset of these compounds alter C. elegans feeding rates through the C. elegans counterparts of the predicted mammalian targets.

Discovery of β2 Adrenergic Receptor Ligands Using Biosensor Fragment Screening of Tagged Wild-Type Receptor

G-protein coupled receptors (GPCRs) are the primary target class of currently marketed drugs, accounting for about a quarter of all drug targets of approved medicines. However, almost all the screening efforts for novel ligand discovery rely exclusively on cellular systems overexpressing the receptors. An alternative ligand discovery strategy is a fragment-based drug discovery, where low molecular weight compounds, known as fragments, are screened as initial starting points for optimization. However, the screening of fragment libraries usually employs biophysical screening methods, and as such, it has not been routinely applied to membrane proteins. We present here a surface plasmon resonance biosensor approach that enables, cell-free, label-free, fragment screening that directly measures fragment interactions with wild-type GPCRs. We exemplify the method by the discovery of novel, selective, high affinity antagonists of human β2 adrenoceptor.

Conformation guides molecular efficacy in docking screens of activated β-2 adrenergic G protein coupled receptor

A prospective, large library virtual screen against an activated β2-adrenergic receptor (β2AR) structure returned potent agonists to the exclusion of inverse-agonists, providing the first complement to the previous virtual screening campaigns against inverse-agonist-bound G protein coupled receptor (GPCR) structures, which predicted only inverse-agonists. In addition, two hits recapitulated the signaling profile of the co-crystal ligand with respect to the G protein and arrestin mediated signaling. This functional fidelity has important implications in drug design, as the ability to predict ligands with predefined signaling properties is highly desirable. However, the agonist-bound state provides an uncertain template for modeling the activated conformation of other GPCRs, as a dopamine D2 receptor (DRD2) activated model templated on the activated β2AR structure returned few hits of only marginal potency.

Colloidal aggregation causes inhibition of G protein-coupled receptors

Colloidal aggregation is the dominant mechanism for artifactual inhibition of soluble proteins, and controls against it are now widely deployed. Conversely, investigating this mechanism for membrane-bound receptors has proven difficult. Here we investigate the activity of four well-characterized aggregators against three G protein-coupled receptors (GPCRs) recognizing peptide and protein ligands. Each of the aggregators was active at micromolar concentrations against the three GPCRs in cell-based assays. This activity could be attenuated by either centrifugation of the inhibitor stock solution or by addition of Tween-80 detergent. In the absence of agonist, the aggregators acted as inverse agonists, consistent with a direct receptor interaction. Meanwhile, several literature GPCR ligands that resemble aggregators themselves formed colloids, by both physical and enzymological tests. These observations suggest that some GPCRs may be artifactually antagonized by colloidal aggregates, an effect that merits the attention of investigators in this field.

A pharmacological organization of G protein-coupled receptors

Protein classification typically uses structural, sequence, or functional similarity. Here we introduce an orthogonal method that organizes proteins by ligand similarity, focusing here on the class A G protein-coupled receptor (GPCR) protein family. Comparing a ligand-based dendogram to a sequence-based one, we sought examples of GPCRs that were distantly linked by sequence but neighbors by ligand similarity. Experimental testing of compounds predicted to link three of these new pairs confirmed the predicted association, with potencies ranging from the low-nanomolar to low-micromolar. We then identified hundreds of non-GPCRs closely related to GPCRs by ligand similarity, including the CXCR2 chemokine receptor to Casein kinase I, the cannabinoid receptors to epoxide hydrolase 2, and the α₂ adrenergic receptor to phospholipase D. These, too, were confirmed experimentally. Ligand similarities among these targets may reflect a chemical integration in the time domain of molecular signaling.

Automated design of ligands to polypharmacological profiles

The clinical efficacy and safety of a drug is determined by its activity profile across many proteins in the proteome. However, designing drugs with a specific multi-target profile is both complex and difficult. Therefore methods to design drugs rationally a priori against profiles of several proteins would have immense value in drug discovery. Here we describe a new approach for the automated design of ligands against profiles of multiple drug targets. The method is demonstrated by the evolution of an approved acetylcholinesterase inhibitor drug into brain-penetrable ligands with either specific polypharmacology or exquisite selectivity profiles for G-protein-coupled receptors. Overall, 800 ligand-target predictions of prospectively designed ligands were tested experimentally, of which 75% were confirmed to be correct. We also demonstrate target engagement in vivo. The approach can be a useful source of drug leads when multi-target profiles are required to achieve either selectivity over other drug targets or a desired polypharmacology.

Structure-functional selectivity relationship studies of β-arrestin-biased dopamine D₂ receptor agonists

Functionally selective G protein-coupled receptor (GPCR) ligands, which differentially modulate canonical and noncanonical signaling, are extremely useful for elucidating key signal transduction pathways essential for both the therapeutic actions and side effects of drugs. However, few such ligands have been created, and very little purposeful attention has been devoted to studying what we term: "structure-functional selectivity relationships" (SFSR). We recently disclosed the first β-arrestin-biased dopamine D(2) receptor (D(2)R) agonists UNC9975 (44) and UNC9994 (36), which have robust in vivo antipsychotic drug-like activities. Here we report the first comprehensive SFSR studies focused on exploring four regions of the aripiprazole scaffold, which resulted in the discovery of these β-arrestin-biased D(2)R agonists. These studies provide a successful proof-of-concept for how functionally selective ligands can be discovered.

Rational Drug Design Leading to the Identification of a Potent 5-HT(2C) Agonist Lacking 5-HT(2B) Activity

The 5-HT(2C) receptor is an attractive drug target in the quest for new therapeutics to treat a variety of human disorders. We have previously undertaken a structural optimization campaign that has led to some potent and moderately selective 5-HT(2C) receptor agonists. After expanding our structure-function library, we were able to combine our datasets so as to allow the design of compounds of improved selectivity and potency. We disclose herein the structural optimization of our previously reported 5-HT(2B)/5-HT(2C) agonists, which has led to the identification of a highly selective 5-HT(2C) agonist, (+)-trans-[2-(2-cyclopropylmethoxyphenyl)cyclopropyl]methylamine hydrochloride, with an EC(50) of 55 nM and no detectable agonism at the 5-HT(2B) receptor.