Structural basis for the interaction of CCR5 with a small molecule, functionally selective CCR5 agonist

Colorectal Cancer: The Contribution of CXCL12 and Its Receptors CXCR4 and CXCR7

Colorectal cancer is one of the most common cancers, and diagnosis at late metastatic stages is the main cause of death related to this cancer. This progression to metastasis is complex and involves different molecules such as the chemokine CXCL12 and its two receptors CXCR4 and CXCR7. The high expression of receptors in CRC is often associated with a poor prognosis and aggressiveness of the tumor. The interaction of CXCL12 and its receptors activates signaling pathways that induce chemotaxis, proliferation, migration, and cell invasion. To this end, receptor inhibitors were developed, and their use in preclinical and clinical studies is ongoing. This review provides an overview of studies involving CXCR4 and CXCR7 in CRC with an update on their targeting in anti-cancer therapies.

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

CCR5 structural plasticity shapes HIV-1 phenotypic properties

CCR5 plays immune functions and is the coreceptor for R5 HIV-1 strains. It exists in diverse conformations and oligomerization states. We interrogated the significance of the CCR5 structural diversity on HIV-1 infection. We show that envelope glycoproteins (gp120s) from different HIV-1 strains exhibit divergent binding levels to CCR5 on cell lines and primary cells, but not to CD4 or the CD4i monoclonal antibody E51. This owed to differential binding of the gp120s to different CCR5 populations, which exist in varying quantities at the cell surface and are differentially expressed between different cell types. Some, but not all, of these populations are antigenically distinct conformations of the coreceptor. The different binding levels of gp120s also correspond to differences in their capacity to bind CCR5 dimers/oligomers. Mutating the CCR5 dimerization interface changed conformation of the CCR5 homodimers and modulated differentially the binding of distinct gp120s. Env-pseudotyped viruses also use particular CCR5 conformations for entry, which may differ between different viruses and represent a subset of those binding gp120s. In particular, even if gp120s can bind both CCR5 monomers and oligomers, impairment of CCR5 oligomerization improved viral entry, suggesting that HIV-1 prefers monomers for entry. From a functional standpoint, we illustrate that the nature of the CCR5 molecules to which gp120/HIV-1 binds shapes sensitivity to inhibition by CCR5 ligands and cellular tropism. Differences exist in the CCR5 populations between T-cells and macrophages, and this is associated with differential capacity to bind gp120s and to support viral entry. In macrophages, CCR5 structural plasticity is critical for entry of blood-derived R5 isolates, which, in contrast to prototypical M-tropic strains from brain tissues, cannot benefit from enhanced affinity for CD4. Collectively, our results support a role for CCR5 heterogeneity in diversifying the phenotypic properties of HIV-1 isolates and provide new clues for development of CCR5-targeting drugs.

Structural Analysis of Chemokine Receptor-Ligand Interactions

This review focuses on the construction and application of structural chemokine receptor models for the elucidation of molecular determinants of chemokine receptor modulation and the structure-based discovery and design of chemokine receptor ligands. A comparative analysis of ligand binding pockets in chemokine receptors is presented, including a detailed description of the CXCR4, CCR2, CCR5, CCR9, and US28 X-ray structures, and their implication for modeling molecular interactions of chemokine receptors with small-molecule ligands, peptide ligands, and large antibodies and chemokines. These studies demonstrate how the integration of new structural information on chemokine receptors with extensive structure–activity relationship and site-directed mutagenesis data facilitates the prediction of the structure of chemokine receptor–ligand complexes that have not been crystallized. Finally, a review of structure-based ligand discovery and design studies based on chemokine receptor crystal structures and homology models illustrates the possibilities and challenges to find novel ligands for chemokine receptors.

Mechanisms of Regulation of the Chemokine-Receptor Network

The interactions of chemokines with their G protein-coupled receptors promote the migration of leukocytes during normal immune function and as a key aspect of the inflammatory response to tissue injury or infection. This review summarizes the major cellular and biochemical mechanisms by which the interactions of chemokines with chemokine receptors are regulated, including: selective and competitive binding interactions; genetic polymorphisms; mRNA splice variation; variation of expression, degradation and localization; down-regulation by atypical (decoy) receptors; interactions with cell-surface glycosaminoglycans; post-translational modifications; oligomerization; alternative signaling responses; and binding to natural or pharmacological inhibitors.

Biased small-molecule ligands for selective inhibition of HIV-1 cell entry via CCR5

Since the discovery of HIV's use of CCR5 as the primary coreceptor in fusion, the focus on developing small-molecule receptor antagonists for inhibition hereof has only resulted in one single drug, Maraviroc. We therefore investigated the possibility of using small-molecule CCR5 agonists as HIV-1 fusion inhibitors. A virus-free cell-based fusion reporter assay, based on mixing "effector cells" (expressing HIV Env and luciferase activator) with "target cells" (expressing CD4, CCR5 wild type or a selection of well-described mutations, and luciferase reporter), was used as fusion readout. Receptor expression was evaluated by ELISA and fluorescence microscopy. On CCR5 WT, Maraviroc and Aplaviroc inhibited fusion with high potencies (EC 50 values of 91 and 501 nM, respectively), whereas removal of key residues for both antagonists (Glu283Ala) or Maraviroc alone (Tyr251Ala) prevented fusion inhibition, establishing this assay as suitable for screening of HIV entry inhibitors. Both ligands inhibited HIV fusion on signaling-deficient CCR5 mutations (Tyr244Ala and Trp248Ala). Moreover, the steric hindrance CCR5 mutation (Gly286Phe) impaired fusion, presumably by a direct hindrance of gp120 interaction. Finally, the efficacy switch mutation (Leu203Phe) - converting small-molecule antagonists/inverse agonists to full agonists biased toward G-protein activation - uncovered that also small-molecule agonists can function as direct HIV-1 cell entry inhibitors. Importantly, no agonist-induced receptor internalization was observed for this mutation. Our studies of the pharmacodynamic requirements for HIV-1 fusion inhibitors highlight the possibility of future development of biased ligands with selective targeting of the HIV-CCR5 interaction without interfering with the normal functionality of CCR5.

Partial Agonist and Biased Signaling Properties of the Synthetic Enantiomers J113863/UCB35625 at Chemokine Receptors CCR2 and CCR5

Biased agonism at G protein-coupled receptors constitutes a promising area of research for the identification of new therapeutic molecules. In this study we identified two novel biased ligands for the chemokine receptors CCR2 and CCR5 and characterized their functional properties. We showed that J113863 and its enantiomer UCB35625, initially identified as high affinity antagonists for CCR1 and CCR3, also bind with low affinity to the closely related receptors CCR2 and CCR5. Binding of J113863 and UCB35625 to CCR2 or CCR5 resulted in the full or partial activation of the three G_i proteins and the two G_o isoforms. Unlike chemokines, the compounds did not activate G₁₂ Binding of J113863 to CCR2 or CCR5 also induced the recruitment of β-arrestin 2, whereas UCB35625 did not. UCB35625 induced the chemotaxis of L1.2 cells expressing CCR2 or CCR5. In contrast, J113863 induced the migration of L1.2-CCR2 cells but antagonized the chemokine-induced migration of L1.2-CCR5 cells. We also showed that replacing the phenylalanine 3.33 in CCR5 TM3 by the corresponding histidine of CCR2 converts J113863 from an antagonist for cell migration and a partial agonist in other assays to a full agonist in all assays. Further analyses indicated that F3.33H substitution strongly increased the activation of G proteins and β-arrestin 2 by J113863. These results highlight the biased nature of the J113863 and UCB35625 that act either as antagonist, partial agonist, or full agonist according to the receptor, the enantiomer, and the signaling pathway investigated.

Molecular Mechanism of Action for Allosteric Modulators and Agonists in CC-chemokine Receptor 5 (CCR5)

The small molecule metal ion chelators bipyridine and terpyridine complexed with Zn²⁺ (ZnBip and ZnTerp) act as CCR5 agonists and strong positive allosteric modulators of CCL3 binding to CCR5, weak modulators of CCL4 binding, and competitors for CCL5 binding. Here we describe their binding site using computational modeling, binding, and functional studies on WT and mutated CCR5. The metal ion Zn²⁺ is anchored to the chemokine receptor-conserved Glu-283^VII:06/7.39 Both chelators interact with aromatic residues in the transmembrane receptor domain. The additional pyridine ring of ZnTerp binds deeply in the major binding pocket and, in contrast to ZnBip, interacts directly with the Trp-248^VI:13/6.48 microswitch, contributing to its 8-fold higher potency. The impact of Trp-248 was further confirmed by ZnClTerp, a chloro-substituted version of ZnTerp that showed no inherent agonism but maintained positive allosteric modulation of CCL3 binding. Despite a similar overall binding mode of all three metal ion chelator complexes, the pyridine ring of ZnClTerp blocks the conformational switch of Trp-248 required for receptor activation, thereby explaining its lack of activity. Importantly, ZnClTerp becomes agonist to the same extent as ZnTerp upon Ala mutation of Ile-116^III:16/3.40, a residue that constrains the Trp-248 microswitch in its inactive conformation. Binding studies with ¹²⁵I-CCL3 revealed an allosteric interface between the chemokine and the small molecule binding site, including residues Tyr-37^I:07/1.39, Trp-86^II:20/2.60, and Phe-109^III:09/3.33 The small molecules and CCL3 approach this interface from opposite directions, with some residues being mutually exploited. This study provides new insight into the molecular mechanism of CCR5 activation and paves the way for future allosteric drugs for chemokine receptors.

Evolution of coreceptor utilization to escape CCR5 antagonist therapy

The HIV-1 envelope interacts with coreceptors CCR5 and CXCR4 in a dynamic, multi-step process, its molecular details not clearly delineated. Use of CCR5 antagonists results in tropism shift and therapeutic failure. Here we describe a novel approach using full-length patient-derived gp160 quasispecies libraries cloned into HIV-1 molecular clones, their separation based on phenotypic tropism in vitro, and deep sequencing of the resultant variants for structure-function analyses. Analysis of functionally validated envelope sequences from patients who failed CCR5 antagonist therapy revealed determinants strongly associated with coreceptor specificity, especially at the gp120-gp41 and gp41-gp41 interaction surfaces that invite future research on the roles of subunit interaction and envelope trimer stability in coreceptor usage. This study identifies important structure-function relationships in HIV-1 envelope, and demonstrates proof of concept for a new integrated analysis method that facilitates laboratory discovery of resistant mutants to aid in development of other therapeutic agents.

Biased signaling at chemokine receptors

The ability of G protein-coupled receptors (GPCRs) to activate selective signaling pathways according to the conformation stabilized by bound ligands (signaling bias) is a challenging concept in the GPCR field. Signaling bias has been documented for several GPCRs, including chemokine receptors. However, most of these studies examined the global signaling bias between G protein- and arrestin-dependent pathways, leaving unaddressed the potential bias between particular G protein subtypes. Here, we investigated the coupling selectivity of chemokine receptors CCR2, CCR5, and CCR7 in response to various ligands with G protein subtypes by using bioluminescence resonance energy transfer biosensors monitoring directly the activation of G proteins. We also compared data obtained with the G protein biosensors with those obtained with other functional readouts, such as β-arrestin-2 recruitment, cAMP accumulation, and calcium mobilization assays. We showed that the binding of chemokines to CCR2, CCR5, and CCR7 activated the three Gαi subtypes (Gαi1, Gαi2, and Gαi3) and the two Gαo isoforms (Gαoa and Gαob) with potencies that generally correlate to their binding affinities. In addition, we showed that the binding of chemokines to CCR5 and CCR2 also activated Gα12, but not Gα13. For each receptor, we showed that the relative potency of various agonist chemokines was not identical in all assays, supporting the notion that signaling bias exists at chemokine receptors.

C(X)CR in silico: Computer-aided prediction of chemokine receptor-ligand interactions

This review will focus on the construction, refinement, and validation of chemokine receptor models for the purpose of structure-based virtual screening and ligand design. The review will present a comparative analysis of ligand binding pockets in chemokine receptors, including a review of the recently released CXCR4 X-ray structures, and their implication on chemokine receptor (homology) modeling. The recommended protein-ligand modeling procedure as well as the use of experimental anchors to steer the modeling procedure is discussed and an overview of several successful structure-based ligand discovery and design studies is provided. This review shows that receptor models, despite structural inaccuracies, can be efficiently used to find novel ligands for chemokine receptors.:

Bias in chemokine receptor signalling

Chemokine receptors are widely expressed on a variety of immune cells and play a crucial role in normal physiology as well as in inflammatory and infectious diseases. The existence of 23 chemokine receptors and 48 chemokine ligands guarantees a tight control and fine-tuning of the immune system. Here, we discuss the multiple regulatory mechanisms of chemokine signalling at a systemic, cellular, and molecular level. In particular, we focus on the impact of biased signalling at the receptor level; an emerging concept in molecular pharmacology. An improved understanding of these mechanisms may provide a framework for more effective drug discovery and development at a target class that is so relevant for immune function.

Pharmacological characterization of a small-molecule agonist for the chemokine receptor CXCR3

BACKGROUND AND PURPOSE

The chemokine receptor CXCR3 is a GPCR found predominantly on activated T cells. CXCR3 is activated by three endogenous peptides; CXCL9, CXCL10 and CXCL11. Recently, a small-molecule agonist, VUF10661, has been reported in the literature and synthesized in our laboratory. The aim of the present study was to provide a detailed pharmacological characterization of VUF10661 by comparing its effects with those of CXCL11.

EXPERIMENTAL APPROACH

Agonistic properties of VUF10661 were assessed in a chemotaxis assay with murine L1.2 cells transiently transfected with cDNA encoding the human CXCR3 receptor and in binding studies, with [(125)I]-CXCL10 and [(125)I]-CXCL11, on membrane preparations from HEK293 cells stably expressing CXCR3. [(35)S]-GTPγS binding was used to determine its potency to induce CXCR3-mediated G protein activation and BRET-based assays to investigate its effects on intracellular cAMP levels and β-arrestin recruitment.

KEY RESULTS

VUF10661 acted as a partial agonist in CXCR3-mediated chemotaxis, bound to CXCR3 in an allosteric fashion in ligand binding assays and activated G(i) proteins with the same efficacy as CXCL11 in the [(35)S]-GTPγS binding and cAMP assay, while it recruited more β-arrestin1 and β-arrestin2 to CXCR3 receptors than the chemokine.

CONCLUSIONS AND IMPLICATIONS

VUF10661, like CXCL11, activates both G protein-dependent and -independent signalling via the CXCR3 receptor, but probably exerts its effects from an allosteric binding site that is different from that for CXCL11. It could stabilize different receptor and/or β-arrestin conformations leading to differences in functional output. Such ligand-biased signalling might offer interesting options for the therapeutic use of CXCR3 agonists.

Pharmacological modulation of chemokine receptor function

G protein-coupled chemokine receptors and their peptidergic ligands are interesting therapeutic targets due to their involvement in various immune-related diseases, including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease, chronic obstructive pulmonary disease, HIV-1 infection and cancer. To tackle these diseases, a lot of effort has been focused on discovery and development of small-molecule chemokine receptor antagonists. This has been rewarded by the market approval of two novel chemokine receptor inhibitors, AMD3100 (CXCR4) and Maraviroc (CCR5) for stem cell mobilization and treatment of HIV-1 infection respectively. The recent GPCR crystal structures together with mutagenesis and pharmacological studies have aided in understanding how small-molecule ligands interact with chemokine receptors. Many of these ligands display behaviour deviating from simple competition and do not interact with the chemokine binding site, providing evidence for an allosteric mode of action. This review aims to give an overview of the evidence supporting modulation of this intriguing receptor family by a range of ligands, including small molecules, peptides and antibodies. Moreover, the computer-assisted modelling of chemokine receptor-ligand interactions is discussed in view of GPCR crystal structures. Finally, the implications of concepts such as functional selectivity and chemokine receptor dimerization are considered.

An inducible cell-cell fusion system with integrated ability to measure the efficiency and specificity of HIV-1 entry inhibitors

HIV-1 envelope glycoproteins (Envs) mediate virus entry by fusing the viral and target cell membranes, a multi-step process that represents an attractive target for inhibition. Entry inhibitors with broad-range activity against diverse isolates of HIV-1 may be extremely useful as lead compounds for the development of therapies or prophylactic microbicides. To facilitate the identification of such inhibitors, we have constructed a cell-cell fusion system capable of simultaneously monitoring inhibition efficiency and specificity. In this system, effector cells stably express a tetracycline-controlled transactivator (tTA) that enables tightly inducible expression of both HIV-1 Env and the Renilla luciferase (R-Luc) reporter protein. Target cells express the HIV-1 receptors, CD4 and CCR5, and carry the firefly luciferase (F-Luc) reporter gene under the control of a tTA-responsive promoter. Thus, Env-mediated fusion of these two cell types allows the tTA to diffuse to the target cell and activate the expression of the F-Luc protein. The efficiency with which an inhibitor blocks cell-cell fusion is measured by a decrease in the F-Luc activity, while the specificity of the inhibitor is evaluated by its effect on the R-Luc activity. The system exhibited a high dynamic range and high Z'-factor values. The assay was validated with a reference panel of inhibitors that target different steps in HIV-1 entry, yielding inhibitory concentrations comparable to published virus inhibition data. Our system is suitable for large-scale screening of chemical libraries and can also be used for detailed characterization of inhibitory and cytotoxic properties of known entry inhibitors.

Allosteric and orthosteric sites in CC chemokine receptor (CCR5), a chimeric receptor approach

Chemokine receptors play a major role in immune system regulation and have consequently been targets for drug development leading to the discovery of several small molecule antagonists. Given the large size and predominantly extracellular receptor interaction of endogenous chemokines, small molecules often act more deeply in an allosteric mode. However, opposed to the well described molecular interaction of allosteric modulators in class C 7-transmembrane helix (7TM) receptors, the interaction in class A, to which the chemokine receptors belong, is more sparsely described. Using the CCR5 chemokine receptor as a model system, we studied the molecular interaction and conformational interchange required for proper action of various orthosteric chemokines and allosteric small molecules, including the well known CCR5 antagonists TAK-779, SCH-C, and aplaviroc, and four novel CCR5 ago-allosteric molecules. A chimera was successfully constructed between CCR5 and the closely related CCR2 by transferring all extracellular regions of CCR2 to CCR5, i.e. a Trojan horse that resembles CCR2 extracellularly but signals through a CCR5 transmembrane unit. The chimera bound CCR2 (CCL2 and CCL7), but not CCR5 chemokines (CCL3 and CCL5), with CCR2-like high affinities and potencies throughout the CCR5 signaling unit. Concomitantly, high affinity binding of small molecule CCR5 agonists and antagonists was retained in the transmembrane region. Importantly, whereas the agonistic and antagonistic properties were preserved, the allosteric enhancement of chemokine binding was disrupted. In summary, the Trojan horse chimera revealed that orthosteric and allosteric sites could be structurally separated and still act together with transmission of agonism and antagonism across the different receptor units.

Biology and clinical relevance of chemokines and chemokine receptors CXCR4 and CCR5 in human diseases

Chemokines and their receptors are implicated in a wide range of human diseases, including acquired immune deficiency syndrome (AIDS). The entry of human immunodeficiency virus type 1 (HIV-1) into a cell is initiated by the interaction of the virus's surface envelope proteins with two cell surface components of the target cell, namely CD4 and a chemokine co-receptor, usually CXCR4 or CCR5. Typical anti-HIV-1 agents include protease and reverse transcriptase inhibitors, but the targets of these agents tend to show rapid mutation rates. As such, strategies based on HIV-1 co-receptors have appeal because they target invariant host determinants. Chemokines and their receptors are also of general interest since they play important roles in numerous physiological and pathological processes in addition to AIDS. Therefore, intensive basic and translational research is ongoing for the dissection of their structure - function relationships in an effort to understand the molecular mechanism of chemokine - receptor interactions and signal transductions across cellular membranes. This paper reviews and discusses recent advances and the translation of new knowledge and discoveries into novel interventional strategies for clinical application.

Elucidation of CXCR7-mediated signaling events and inhibition of CXCR4-mediated tumor cell transendothelial migration by CXCR7 ligands

CXCR7 binds chemokines CXCL11 (I-TAC) and CXCL12 (SDF-1) but does not act as a classical chemoattractant receptor. Using CCX771, a novel small molecule with high affinity and selectivity for CXCR7, we found that, although CXCR7 is dispensable for "bare filter" in vitro chemotaxis, CXCR7 plays an essential role in the CXCL12/CXCR4-mediated transendothelial migration (TEM) of CXCR4(+)CXCR7(+) human tumor cells. Importantly, although CXCL11 is unable to stimulate directly the migration of these cells, it acts as a potent antagonist of their CXCL12-induced TEM. Furthermore, even though this TEM is driven by CXCR4, the CXCR7 ligand CCX771 is substantially more potent at inhibiting it than the CXCR4 antagonist AMD3100, which is more than 100 times weaker at inhibiting TEM when compared with its ability to block bare filter chemotaxis. Far from being a "silent" receptor, we show that CXCR7 displays early hallmark events associated with intracellular signaling. Upon cognate chemokine binding, CXCR7 associates with beta-arrestin2, an interaction that can be blocked by CXCR7-specific mAbs. Remarkably, the synthetic CXCR7 ligand CCX771 also potently stimulates beta-arrestin2 recruitment to CXCR7, with greater potency and efficacy than the endogenous chemokine ligands. These results indicate that CXCR7 can regulate CXCL12-mediated migratory cues, and thus may play a critical role in driving CXCR4(+)CXCR7(+) tumor cell metastasis and tissue invasion. CXCR7 ligands, such as the chemokine CXCL11 and the newly described synthetic molecule CCX771, may represent novel therapeutic opportunities for the control of such cells.

Species selectivity of small-molecular antagonists for the CCR5 chemokine receptor

The species selectivity of four structurally different compounds, SCH-351125, E-913, TAK-779 and UK-427857 has been examined using cloned human, rhesus, and mouse CCR5 receptors. SCH-351125 and E-913 potently inhibited the binding of [125I]-CCL3 to human CCR5, but their inhibitory activities against rhesus CCR5 were more than 10-fold weaker. In contrast, TAK-779 and UK-427857 inhibited binding to human and rhesus CCR5 with similar potency. The inhibitory activities of all four compounds against mice CCR5 receptors were weak. The inhibitory activities of the CCR5 antagonists in the [125I]-CCL3 binding assay agreed well with those induced by CCL3 in the intracellular calcium ([Ca(2+)]i) elevation assay. Mutational analysis of the human CCR5 receptor showed that its Ile198 component plays a critical role in the inhibitory activities of both SCH-351125 and E-913, but not that of TAK-779 or UK-427857. These results provide a structural basis for understanding how specific antagonists interact with CCR5, and will aid the process of creating new, improved CCR5 antagonists.

Unravelling the mechanisms underpinning chemokine receptor activation and blockade by small molecules: a fine line between agonism and antagonism?

Chemokines are a family of small basic proteins which induce the directed migration of cells, notably leucocytes, by binding to specific GPCRs (G-protein-coupled receptors). Both chemokines and their receptors have been implicated in a host of clinically important diseases, leading to the notion that antagonism of the chemokine–chemokine receptor network may be therapeutically advantageous. Consequently, considerable effort has been put into the development of small-molecule antagonists of chemokine receptors and several such compounds have been described in the literature. One curious by-product of this activity has been the description of several small-molecule agonists of the receptors, which are typically discovered following the optimization of lead antagonists. In this review we discuss these findings and conclude that these small-molecule agonists might be exploited to further our understanding of the molecular mechanisms by which chemokine receptors are activated.

Small molecule receptor agonists and antagonists of CCR3 provide insight into mechanisms of chemokine receptor activation

Chemokine receptor CCR3 is highly expressed by eosinophils and signals in response to binding of the eotaxin family of chemokines, which are up-regulated in allergic disorders. Consequently, CCR3 blockade is of interest as a possible therapeutic approach for the treatment of allergic disease. We have described previously a bispecific antagonist of CCR1 and CCR3 named UCB35625 that was proposed to interact with the transmembrane residues Tyr-41, Tyr-113, and Glu-287 of CCR1, all of which are conserved in CCR3. Here, we show that cells expressing the CCR3 constructs Y113A and E287Q are insensitive to antagonism by UCB35625 and also exhibit impaired chemotaxis in response to CCL11/eotaxin, suggesting that these residues are important for antagonist binding and also receptor activation. Furthermore, mutation of the residue Tyr-113 to alanine was found to turn the antagonist UCB35625 into a CCR3 agonist. Screens of small molecule libraries identified a novel specific agonist of CCR3 named CH0076989. This was able to activate eosinophils and transfectants expressing both wild-type CCR3 and a CCR1-CCR3 chimeric receptor lacking the CCR3 amino terminus, indicating that this region of CCR3 is not required for CH0076989 binding. A direct interaction with the transmembrane helices of CCR3 was supported by mutation of the residues Tyr-41, Tyr-113, and Glu-287 that resulted in complete loss of CH0076989 activity, suggesting that the compound mimics activation by CCL11. We conclude that both agonists and antagonists of CCR3 appear to occupy overlapping sites within the transmembrane helical bundle, suggesting a fine line between agonism and antagonism of chemokine receptors.

Identification of Nonpeptide CCR5 Receptor Agonists by Structure-based Virtual Screening

A three-dimensional model of the chemokine receptor CCR5 has been built to fulfill structural peculiarities of its alpha-helix bundle and to distinguish known CCR5 antagonists from randomly chosen drug-like decoys. In silico screening of a library of 1.6 million commercially available compounds against the CCR5 model by sequential filters (drug-likeness, 2-D pharmacophore, 3-D docking, scaffold clustering) yielded a hit list of 59 compounds, out of which 10 exhibited a detectable binding affinity to the CCR5 receptor. Unexpectedly, most binders tested in a functional assay were shown to be agonists of the CCR5 receptor. A follow-up database query based on similarity to the most potent binders identified three new CCR5 agonists. Despite a moderate affinity of all nonpeptide ligands for the CCR5 receptor, one of the agonists was shown to promote efficient receptor internalization, which is a process therapeutically favorable for protection against HIV-1 infection.