Amino terminus of the interleukin-8 receptor is a major determinant of receptor subtype specificity

Bioinformatic Analysis of the CXCR2 Ligands in Cancer Processes

Human CXCR2 has seven ligands, i.e., CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, CXCL7, and CXCL8/IL-8-chemokines with nearly identical properties. However, no available study has compared the contribution of all CXCR2 ligands to cancer progression. That is why, in this study, we conducted a bioinformatic analysis using the GEPIA, UALCAN, and TIMER2.0 databases to investigate the role of CXCR2 ligands in 31 different types of cancer, including glioblastoma, melanoma, and colon, esophageal, gastric, kidney, liver, lung, ovarian, pancreatic, and prostate cancer. We focused on the differences in the regulation of expression (using the Tfsitescan and miRDB databases) and analyzed mutation types in CXCR2 ligand genes in cancers (using the cBioPortal). The data showed that the effect of CXCR2 ligands on prognosis depends on the type of cancer. CXCR2 ligands were associated with EMT, angiogenesis, recruiting neutrophils to the tumor microenvironment, and the count of M1 macrophages. The regulation of the expression of each CXCR2 ligand was different and, thus, each analyzed chemokine may have a different function in cancer processes. Our findings suggest that each type of cancer has a unique pattern of CXCR2 ligand involvement in cancer progression, with each ligand having a unique regulation of expression.

CXCR2 Receptor: Regulation of Expression, Signal Transduction, and Involvement in Cancer

Chemokines are a group of about 50 chemotactic cytokines crucial for the migration of immune system cells and tumor cells, as well as for metastasis. One of the 20 chemokine receptors identified to date is CXCR2, a G-protein-coupled receptor (GPCR) whose most known ligands are CXCL8 (IL-8) and CXCL1 (GRO-α). In this article we present a comprehensive review of literature concerning the role of CXCR2 in cancer. We start with regulation of its expression at the transcriptional level and how this regulation involves microRNAs. We show the mechanism of CXCR2 signal transduction, in particular the action of heterotrimeric G proteins, phosphorylation, internalization, intracellular trafficking, sequestration, recycling, and degradation of CXCR2. We discuss in detail the mechanism of the effects of activated CXCR2 on the actin cytoskeleton. Finally, we describe the involvement of CXCR2 in cancer. We focused on the importance of CXCR2 in tumor processes such as proliferation, migration, and invasion of tumor cells as well as the effects of CXCR2 activation on angiogenesis, lymphangiogenesis, and cellular senescence. We also discuss the importance of CXCR2 in cell recruitment to the tumor niche including tumor-associated neutrophils (TAN), tumor-associated macrophages (TAM), myeloid-derived suppressor cells (MDSC), and regulatory T (T_reg) cells.

Selection of a picomolar antibody that targets CXCR2-mediated neutrophil activation and alleviates EAE symptoms

Receptors and their ligands are important therapeutic targets for about one third of marketed drugs. Here, we describe an epitope-guided approach for selection of antibodies that modulate cellular signaling of targeted receptors. We chose CXC chemokine receptor 2 (CXCR2) in the G-protein coupled receptor superfamily as receptor and a CXCR2 N-terminal peptide for antibody selection. We obtain a highly selective, tight-binding antibody from a 10¹¹-member antibody library using combinatorial enrichment. Structural and Hydrogen-Deuterium-Exchange mass spectrometry analyses demonstrate antibody interaction with an N-terminal region of CXCR2 that is part of the IL-8 epitope. The antibody strongly inhibits IL-8-induced and CXCR2-mediated neutrophil chemotaxis in vitro and alleviates hCXCR2-dependent experimental autoimmune encephalomyelitis symptoms in mice. As inappropriate neutrophil migration accompanies many diseases including inflammatory bowel disease, glomerulonephritis, allergic asthma, chronic obstructive pulmonary disease, and cancer, this antibody has potential for development as a therapeutic agent, akin to anti-TNF antibodies. However, an important difference here is that the antibody targets the chemokine receptor and competes with natural ligand, rather than targeting the ligand itself.

CXCR2 is central to neutrophil chemotaxis and hence to some inflammatory diseases. Here the authors demonstrate the value of an epitope-guided antibody panning method to develop a tight binding anti-hCXCR2 antibody, along with crystal structures of this antibody and antigen, that can block neutrophil chemotaxis and protect mice in an EAE model.

A Requirement for Neutrophil Glycosaminoglycans in Chemokine:Receptor Interactions Is Revealed by the Streptococcal Protease SpyCEP

SpyCEP-cleaved CXCL8 is unable to bind and activate CXCL8 receptors.

Neutrophil glycosaminoglycans are required for migration along a CXCL8 gradient.

To evade the immune system, the lethal human pathogen Streptococcus pyogenes produces SpyCEP, an enzyme that cleaves the C-terminal α-helix of CXCL8, resulting in markedly impaired recruitment of neutrophils to sites of invasive infection. The basis for chemokine inactivation by SpyCEP is, however, poorly understood, as the core domain of CXCL8 known to interact with CXCL8 receptors is unaffected by enzymatic cleavage. We examined the in vitro migration of human neutrophils and observed that their ability to efficiently navigate a CXCL8 gradient was compromised following CXCL8 cleavage by SpyCEP. SpyCEP-mediated cleavage of CXCL8 also impaired CXCL8-induced migration of transfectants expressing the human chemokine receptors CXCR1 or CXCR2. Despite possessing an intact N terminus and preserved disulfide bonds, SpyCEP-cleaved CXCL8 had impaired binding to both CXCR1 and CXCR2, pointing to a requirement for the C-terminal α-helix. SpyCEP-cleaved CXCL8 had similarly impaired binding to the glycosaminoglycan heparin. Enzymatic removal of neutrophil glycosaminoglycans was observed to ablate neutrophil navigation of a CXCL8 gradient, whereas navigation of an fMLF gradient remained largely intact. We conclude, therefore, that SpyCEP cleavage of CXCL8 results in chemokine inactivation because of a requirement for glycosaminoglycan binding in productive chemokine:receptor interactions. This may inform strategies to inhibit the activity of SpyCEP, but may also influence future approaches to inhibit unwanted chemokine-induced inflammation.

Fine-Tuning of GPCR-Chemokine Interactions. Design and Identification of Chemokine Analogues as Receptor Agonists, Biased Agonists, and Antagonists

Chemokines play important roles in immune defense by directing migration of leukocytes and serve as key promoters of tumorigenesis and metastasis. This study explores the molecular mechanisms of recognition and activation of two homologous chemokine receptors, CXCR1 and CXCR2, using CXCL8 analogues with residue substitutions in the conserved Glu4Leu5Arg6 (ELR) triad. Analysis of the binding of CXCL8 analogues to CXCR1 is consistent with the two-site model for signal recognition of CXCR1, whereas analysis of the binding of CXCL8 analogues to CXCR2 supported a single-site model for signal recognition of CXCR2. The CXCL8-Arg6His analogue stimulated calcium release, phosphorylation of ERK1/2, and chemotaxis in cells expressing CXCR1. However, CXCL8-Arg6His failed to stimulate calcium release and chemotaxis in cells expressing CXCR2, although it stimulated phosphorylation of ERK1/2, indicating that CXCL8-Arg6His operated as a classical CXCR2 biased agonist. The CXCL8-Glu4AlaLeu5AlaArg6His analogue was inactive in cells expressing CXCR1 and CXCR2. These findings suggest that the Glu4Leu5 motif in CXCL8 is essential for activation of CXCR1 and CXCR2. Importantly, CXCL8-Glu4AlaLeu5AlaArg6His blocked specifically the calcium release and chemotaxis of cells expressing CXCR1 but not of cells expressing CXCR2. CXCL8-Glu4AlaLeu5AlaArg6His was identified as the first specific CXCR1 antagonist. The binding of CXCL8-ELR6H to CXCR1 created a Zn²⁺ coordination site at the receptor activation domain responsible for calcium release, as ZnCl₂ specifically blocked CXCL8-Arg6His-induced calcium release without affecting CXCL8-induced calcium release. This work provides the basis for further exploration of the activation mechanisms of chemokine receptors and will assist in the design of the next generation of modulators of CXCR1 and CXCR2.

Structure of monomeric Interleukin-8 and its interactions with the N-terminal Binding Site-I of CXCR1 by solution NMR spectroscopy

The structure of monomeric human chemokine IL-8 (residues 1-66) was determined in aqueous solution by NMR spectroscopy. The structure of the monomer is similar to that of each subunit in the dimeric full-length protein (residues 1-72), with the main differences being the location of the N-loop (residues 10-22) relative to the C-terminal α-helix and the position of the side chain of phenylalanine 65 near the truncated dimerization interface (residues 67-72). NMR was used to analyze the interactions of monomeric IL-8 (1-66) with ND-CXCR1 (residues 1-38), a soluble polypeptide corresponding to the N-terminal portion of the ligand binding site (Binding Site-I) of the chemokine receptor CXCR1 in aqueous solution, and with 1TM-CXCR1 (residues 1-72), a membrane-associated polypeptide that includes the same N-terminal portion of the binding site, the first trans-membrane helix, and the first intracellular loop of the receptor in nanodiscs. The presence of neither the first transmembrane helix of the receptor nor the lipid bilayer significantly affected the interactions of IL-8 with Binding Site-I of CXCR1.

Solution NMR characterization of WT CXCL8 monomer and dimer binding to CXCR1 N-terminal domain

Chemokine CXCL8 and its receptor CXCR1 are key mediators in combating infection and have also been implicated in the pathophysiology of various diseases including chronic obstructive pulmonary disease (COPD) and cancer. CXCL8 exists as monomers and dimers but monomer alone binds CXCR1 with high affinity. CXCL8 function involves binding two distinct CXCR1 sites - the N-terminal domain (Site-I) and the extracellular/transmembrane domain (Site-II). Therefore, higher monomer affinity could be due to stronger binding at Site-I or Site-II or both. We have now characterized the binding of a human CXCR1 N-terminal domain peptide (hCXCR1Ndp) to WT CXCL8 under conditions where it exists as both monomers and dimers. We show that the WT monomer binds the CXCR1 N-domain with much higher affinity and that binding is coupled to dimer dissociation. We also characterized the binding of two CXCL8 monomer variants and a trapped dimer to two different hCXCR1Ndp constructs, and observe that the monomer binds with ∼10- to 100-fold higher affinity than the dimer. Our studies also show that the binding constants of monomer and dimer to the receptor peptides, and the dimer dissociation constant, can vary significantly as a function of pH and buffer, and so the ability to observe WT monomer peaks is critically dependent on NMR experimental conditions. We conclude that the monomer is the high affinity CXCR1 agonist, that Site-I interactions play a dominant role in determining monomer vs. dimer affinity, and that the dimer plays an indirect role in regulating monomer function.

Function, diversity and therapeutic potential of the N-terminal domain of human chemokine receptors

Chemokines and their receptors play fundamental roles in many physiological and pathological processes such as leukocyte trafficking, inflammation, cancer and HIV-1 infection. Chemokine-receptor interactions are particularly intricate and therefore require precise orchestration. The flexible N-terminal domain of human chemokine receptors has regularly been demonstrated to hold a crucial role in the initial recognition and selective binding of the receptor ligands. The length and the amino acid sequences of the N-termini vary considerably among different receptors but they all show a high content of negatively charged residues and are subject to post-translational modifications such as O-sulfation and N- or O-glycosylation. In addition, a conserved cysteine that is most likely engaged in a receptor-stabilizing disulfide bond delimits two functionally distinct parts in the N-terminus, characterized by specific molecular signatures. Structural analyses have shown that the N-terminus of chemokine receptors recognizes a groove on the chemokine surface and that this interaction is stabilized by high-affinity binding to a conserved sulfotyrosine-binding pocket. Altogether, these data provide new insights on the chemokine-receptor molecular interplay and identify the receptor N-terminus-binding site as a new target for the development of therapeutic molecules. This review presents and discusses the diversity and function of human chemokine receptor N-terminal domains and provides a comprehensive annotated inventory of their sequences, laying special emphasis on the presence of post-translational modifications and functional features. Finally, it identifies new molecular signatures and proposes a computational model for the positioning and the conformation of the CXCR4 N-terminus grafted on the first chemokine receptor X-ray structure.

Synergistic interactions between chemokine receptor elements in recognition of interleukin-8 by soluble receptor mimics

Interleukin-8 (IL-8 or CXCL8), the archetypal member of the CXC chemokine subfamily, stimulates neutrophil chemotaxis by activating receptors CXCR1/IL8RA and CXCR2/IL8RB. Previous mutational studies have implicated both the N-terminal and third extracellular loop (E3) regions of these receptors in binding to IL-8. To investigate the interactions of these receptor elements with IL-8, we have constructed soluble proteins in which the N-terminal and E3 elements of either CXCR1 or CXCR2 are juxtaposed on a soluble scaffold protein; these are termed CROSS-N(X1)E3(X1) and CROSS-N(X2)E3(X2), respectively. Isothermal titration calorimetry and nuclear magnetic resonance spectroscopy were used to compare the IL-8 binding properties of the receptor mimics to those of control proteins containing only the N-terminal or E3 receptor element. CROSS-N(X2)E3(X2) bound to monomeric IL-8 with the same affinity and induced the same chemical shift changes as the control protein containing only the N-terminal element of CXCR2, indicating that the E3 element of CXCR2 did not contribute to IL-8 binding. In contrast, CROSS-N(X1)E3(X1) bound to IL-8 with ~10-fold increased affinity and induced different chemical shift changes compared to the control protein containing only the N-terminal element of CXCR1, suggesting that the E3 region of CXCR1 was interacting with IL-8. However, a chimeric protein containing the N-terminal region of CXCR1 and the E3 region of CXCR2 (CROSS-N(X1)E3(X2)) bound to IL-8 with thermodynamic properties and induced chemical shift changes indistinguishable from those of CROSS-N(X1)E3(X1) and substantially different from those of CROSS-N(X2)E3(X2). These results indicate that the N-terminal and E3 regions of CXCR1 interact synergistically to achieve optimal binding interactions with IL-8.

Diverging mechanisms of activation of chemokine receptors revealed by novel chemokine agonists

CXCL8/interleukin-8 is a pro-inflammatory chemokine that triggers pleiotropic responses, including inflammation, angiogenesis, wound healing and tumorigenesis. We engineered the first selective CXCR1 agonists on the basis of residue substitutions in the conserved ELR triad and CXC motif of CXCL8. Our data reveal that the molecular mechanisms of activation of CXCR1 and CXCR2 are distinct: the N-loop of CXCL8 is the major determinant for CXCR1 activation, whereas the N-terminus of CXCL8 (ELR and CXC) is essential for CXCR2 activation. We also found that activation of CXCR1 cross-desensitized CXCR2 responses in human neutrophils co-expressing both receptors, indicating that these novel CXCR1 agonists represent a new class of anti-inflammatory agents. Further, these selective CXCR1 agonists will aid at elucidating the functional significance of CXCR1 in vivo under pathophysiological conditions.

Matching models to data: a receptor pharmacologist's guide

In this review we discuss a number of topics related to fitting mechanistic mathematical models to experimental data. These can be divided broadly into issues to consider before beginning an experiment or fitting a model and the advantages of direct fitting of a model over other methods of analysis (e.g. null methods). We have sought to address some commonplace issues of receptor pharmacology with some real-life examples: How should dilutions be distributed along a concentration-response curve? How do problems with dilutions manifest in assay results? What assumptions are made when certain analysis models are applied? What is global data fitting and how does it work? How can it be applied to improve data analysis? How can models such as one-site and two-site fits be compared? What is the principal behind statistical comparison of data fits? It is our hope that after reading this review you will have a greater appreciation of assay planning and subsequent data analysis and interpretation which will improve the quality of the data that you generate.

LINKED ARTICLES

This article is part of a themed section on Analytical Receptor Pharmacology in Drug Discovery. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2010.161.issue-6.

MIF-chemokine receptor interactions in atherogenesis are dependent on an N-loop-based 2-site binding mechanism

Macrophage migration inhibitory factor (MIF) is a cytokine that mediates inflammatory diseases. MIF promotes atherogenic leukocyte recruitment through a promiscuous, yet highly affine, interaction with CXCR2 and CXCR4. Binding to CXCR2 is dependent on a pseudo-(E)LR motif in MIF, but a second interaction site has been elusive. Here we identified an N-like loop in MIF, suggesting that MIF binding to CXCR2 follows the 2-site binding mode of bona fide chemokines. For MIF, the model predicts interactions between the N-like loop and the CXCR2 N domain (site 1) and pseudo-(E)LR and extracellular loops (ELs) of CXCR2 (site 2). Applying biophysical and peptide array analysis, we demonstrated an interaction between MIF and the CXCR2 N domain, which was pseudo-(E)LR independent. Peptide array analysis also indicated that the pseudo-(E)LR motif is responsible for MIF binding to EL2 and 3. Notably, peptides MIF-(40-49) and MIF-(47-56), representing N-like-loop-derived peptides, but not a scrambled control peptide, significantly blocked MIF/CXCR2 binding, MIF-mediated monocyte arrest under flow on aortic endothelial cells in vitro (IC(50): 1.24×10(-6) M), and MIF-dependent monocyte adhesion to atherosclerotic mouse carotid arteries in vivo. Thus, the N-like loop in MIF is critical for MIF's noncognate interaction with CXCR2 and proatherogenic functions. The 2-site binding model that explains chemokine receptor activation also applies to MIF.

Structural basis for differential binding of the interleukin-8 monomer and dimer to the CXCR1 N-domain: role of coupled interactions and dynamics

Interleukin-8 (IL-8 or CXCL8) plays a critical role in orchestrating the immune response by binding and activating the receptor CXCR1 that belongs to the GPCR class. IL-8 exists as both monomers and dimers, and both bind CXCR1 but with differential affinities. It is well established that the monomer is the high-affinity ligand and that the interactions between the ligand N-loop and receptor N-domain play a critical role in determining binding affinity. In order to characterize the structural basis of differential binding of the IL-8 monomer and dimer to the CXCR1 N-domain, we analyzed binding-induced NMR chemical shift and peak intensity changes and show that they are exquisitely sensitive and can provide detailed insights into the binding process. We used three IL-8 variants, a designed monomer, a trapped disulfide-linked dimer, and WT at dimeric concentrations. NMR data for the monomer show that nonsequential residues that span the entire N-loop are involved in the binding process and that the binding is mediated by a network of extensive direct and indirect coupled interactions. Interestingly, in the case of WT, binding induces dissociation of the dimer-receptor complex to the monomer-receptor complex, and in the case of the trapped dimer, binding results in increased global conformational flexibility. Increased dynamics is evidence of unfavorable interactions, indicating that binding of the WT dimer triggers conformational changes that disrupt dimer-interface interactions, resulting in its dissociation. These results together provide evidence that binding is not a localized event but results in extensive coupled interactions within the monomer and across the dimer interface and that these interactions play a fundamental role in determining binding affinity.

Adenovirus mediated expression "in vivo" of the chemokine receptor CXCR1

A major hurdle in the structural analysis of membrane proteins is the expression of a functional and homogeneous form of the protein. Except for rhodopsin, most G protein-coupled receptors (GPCRs) are endogenously expressed at very low levels. Heterologous expression of GPCRs in bacteria, yeast, insect cells or mammalian cell lines often yields proteins with large amounts of misfolded proteins and heterogeneous posttranslational modifications. Here, we report a novel mammalian "in vivo" system for the expression of the chemokine receptor CXCR1. This receptor was expressed in liver of mice infected with adenovirus encoding CXCR1. Liver plasma membranes from infected mice displayed high-levels of (125)I-labeled human interleukin-8 (IL-8) binding. The pharmacological profile of the recombinant CXCR1 expressed "in vivo" was similar to those expressed in neutrophils. We found that the incorporation of the detergent solubilized CXCR1 into phospholipid vesicles in the presence of Gi/Go proteins is required for the reconstitution of (125)I-IL-8 binding. On the basis of the presence of the several endogenous His residues and glycosylation moieties in CXCR1 we fractionated the detergent-solubilized plasma membranes by employing Ni- and Concanavalin A-based chromatography. Fractions enriched with CXCR1 were monitored by (125)I-IL-8-bound to the receptor and Western blots with anti-CXCR1 antibodies. This robust expression system could be readily applied for the expression of GPCRs and other eukaryotic membrane proteins.

Identification of a putative intracellular allosteric antagonist binding-site in the CXC chemokine receptors 1 and 2

The chemokine receptors CXCR1 and CXCR2 are G-protein-coupled receptors (GPCRs) implicated in mediating cellular functions associated with the inflammatory response. Potent CXCR2 receptor antagonists have been discovered, some of which have recently entered clinical development. The aim of this study was to identify key amino acid residue differences between CXCR1 and CXCR2 that influence the relative antagonism by two compounds that have markedly different chemical structures. By investigating the effects of domain switching and point mutations, we found that the second extracellular loop, which contained significant amino acid sequence diversity, was not important for compound antagonism. We were surprised to find that switching the intracellular C-terminal 60 amino acid domains of CXCR1 and CXCR2 caused an apparent reversal of antagonism at these two receptors. Further investigation showed that a single amino acid residue, lysine 320 in CXCR2 and asparagine 311 in CXCR1, plays a predominant role in describing the relative antagonism of the two compounds. Homology modeling studies based on the structure of bovine rhodopsin indicated a potential intracellular antagonist binding pocket involving lysine 320. We conclude that residue 320 in CXCR2 forms part of a potential allosteric binding pocket on the intracellular side of the receptor, a site that is distal to the orthosteric site commonly assumed to be the location of antagonist binding to GPCRs. The existence of a common intracellular allosteric binding site at GPCRs related to CXCR2 may be of value in the design of novel antagonists for therapeutic intervention.

Identification and characterization of a new interleukin-8 receptor in bovine species

Mastitis is an inflammation of the mammary gland, most of the time caused by invading pathogens. Phagocytosis by neutrophils is a crucial defense of the mammary gland and the prompt recruitment of these phagocytes from blood to milk compartments is essential for the outcome of the infection. ELR+ CXC chemokines, ligands of the two interleukin-8 receptors (IL-8R), CXCR1 and CXCR2, are likely to be involved in the initiation of the inflammatory response and also in the migration of neutrophils. Recently, the polymorphism of bovine CXCR2 has been associated with resistance to mastitis. However, as the bovine IL-8R are not functionally defined, their contribution to the recruitment of neutrophils remains undetermined. In this study, the RNA ligase-mediated (RLM)-RACE method was used to clone a novel bovine interleukin-8 receptor (nIL-8R) of the bovine species. We showed that both bovine IL-8R (nIL-8R and the published CXCR2) are functional since bovine IL-8 induced migration of HEK-293 cells expressing either IL-8R. In addition, comparisons of full-length sequences suggested that the published CXCR2 sequence was improperly annotated and that the sequences of the nIL-8R and the published CXCR2 are homologous to human CXCR2 and CXCR1, respectively. This was confirmed by binding assays with labeled IL-8 and GRO-beta and calcium (Ca) flux responses of transfected cells. Moreover, the C-terminal of both bovine IL-8R showed 100% identity, whereas they differ in most other species, suggesting that the two bovine IL-8R initiate similar signal transduction. These results constitute a basis to improve our understanding of the molecular mechanisms implicated in the recruitment of bovine neutrophils.

Structural basis of chemokine receptor function--a model for binding affinity and ligand selectivity

Chemokine receptors play fundamental roles in human physiology from embryogenesis to inflammatory response. The receptors belong to the G-protein coupled receptor class, and are activated by chemokine ligands with a range of specificities and affinities that result in a complicated network of interactions. The molecular basis for function is largely a black box, and can be directly attributed to the lack of structural information on the receptors. Studies to date indicate that function can be best described by a two-site model, that involves interactions between the receptor N-domain and ligand N-terminal loop residues (site-I), and between receptor extracellular loop and the ligand N-terminal residues (site-II). In this review, we describe how the two-site model could modulate binding affinity and ligand selectivity, and also highlight some of the unique chemokine receptor features, and their role in function.

Dimerization of chemokine receptors and its functional consequences

It became clear over the recent years that most, if not all, G protein-coupled receptors (GPCR) are able to form dimers or higher order oligomers. Chemokine receptors make no exception to this new rule and both homo- and heterodimerization were demonstrated for CC and CXC receptors. Functional analyses demonstrated negative binding cooperativity between the two subunits of a dimer. The consequence is that only one chemokine can bind with high affinity onto a receptor dimer. In the context of receptor activation, this implies that the motions of helical domains triggered by the binding of agonists induce correlated changes in the other protomer. The impact of the chemokine dimerization process in terms of co-receptor function and drug development is discussed.

Chemokine receptor antagonists: novel therapeutics for inflammation and AIDS

To date almost 40 human chemokines have been described, modulating a complex and overlapping set of biological activities important for basal leukocyte trafficking, as well as the extravasation and tissue infiltration of leukocytes in response to inciting agents. Chemokines appear to be key mediators in a number of inflammatory pathologies as a result of their chemotactic effects on almost every leukocyte type, including neutrophils, eosinophils, basophils, monocytes, dendritic cells, NK cells, B-cells and T-cells, together with their ability to stimulate effector functions, such as granule release and superoxide radical production from these cells. With the recent discovery that some chemokine receptors function as HIV-1 co-receptors and with accumulating evidence that chemokines may also regulate certain aspects of haematopoiesis, angiogenesis and apoptosis, the chemokines and their receptors have emerged as a new set of drug discovery targets for the pharmaceutical and biotechnology industry.

Rabbit neutrophil chemotactic protein (NCP) activates both CXCR1 and CXCR2 and is the functional homologue for human CXCL6

Neutrophil chemotactic protein (NCP) is a rabbit CXC chemokine with activating and chemotactic properties on neutrophilic granulocytes. Although its selective activity on neutrophils is demonstrated, its interactions with specific chemokine receptors are not defined. For further functional characterization, NCP was chemically synthesized and was found to be equipotent as natural NCP in neutrophil chemotaxis. To identify its human homologue, we separately expressed two potential rabbit NCP receptors (CXCR1 and CXCR2) in Jurkat cells. Pure synthetic NCP was equally efficient to promote chemotaxis through either rabbit CXCR1 or CXCR2. Moreover, chemotaxis assays on rabbit CXCR1 and CXCR2 transfectants showed that NCP uses the same receptors as interleukin-8 (IL-8), a major rabbit CXC chemokine, but not rabbit GROalpha, which only recognized CXCR2. In addition, specific inhibitors for CXCR1 or CXCR2 reduced rabbit neutrophil chemotaxis induced by NCP and rabbit IL-8. Furthermore, NCP and the structurally related human CXCR1/CXCR2 agonist CXCL6/GCP-2 (granulocyte chemotactic protein-2) cross-desensitized each other in intracellular calcium release assays on human neutrophils, further indicating that both chemokines share the same receptors. The inflammatory role of NCP was also evidenced by its potent granulocytosis inducing capacity in rabbits upon systemic administration. This study provides in vitro and in vivo evidences that NCP is the functional rabbit homologue for human CXCL6/GCP-2 rather than the most related CXCR2 agonist CXCL5/ENA-78 (epithelial cell-derived neutrophil activating peptide-78). It is concluded that the rabbit is a better model to study human neutrophil activation compared to mice, which lack CXCL8/IL-8.

Ligand Selectivity and Affinity of Chemokine Receptor CXCR1

Glu-Leu-Arg ("ELR") CXC chemokines interleukin-8 (IL-8) and melanoma growth stimulatory activity (MGSA) recruit neutrophils by binding and activating two receptors, CXCR1 and CXCR2. CXCR1 is specific, binding only IL-8 with nanomolar affinity, whereas CXCR2 is promiscuous, binding all ELRCXC chemokines with high affinity. Receptor signaling consists of two events: interactions between the ligand N-terminal loop (N-loop) and receptor N-terminal domain (N-domain) residues (site I), and between the ligand N-terminal ELR and the receptor juxtamembrane domain (J-domain) residues (site II). It is not known how these interactions mediate ligand affinity and selectivity, and whether binding at one site influences binding and function at the other. Sequence analysis and structure-function studies have suggested that the receptor N-domain plays an important role in ligand selectivity. Here, we report ligand-binding properties and structural characteristics of the CXCR1 N-domain in solution and in detergent micelles that mimic the native membrane environment. We find that IL-8 binds the N-domain with significantly higher affinity in micelles than in solution (approximately 1 microM versus approximately 20 microM) and that MGSA does not bind the N-domain in solution but does in micelles with appreciable affinity (approximately 3 microM). We find that the N-domain is structured in micelles and that the entire N-domain interacts with the micelle in an extended fashion. We conclude that the micellar environment constrains the N-domain, and this conformational restraint influences its ligand-binding properties. Most importantly, our data suggest that for both ligands, site I interaction provides similar affinity and that differential coupling between site I and II interactions is responsible for the observed differences in affinity.

Cloning and Pharmacological Characterization of CXCR1 and CXCR2 fromMacaca fascicularis

Two genes with high sequence homology to human CXCR1 (hCXCR1) and CXCR2 (hCXCR2) were cloned from blood of cynomolgus monkey (Macaca fascicularis). Comparison of the expression pattern of these receptors in different species demonstrated that, like in humans, cynomolgus CXCR1 (cCXCR1) and CXCR2 (cCXCR2) are highly expressed in blood. Membranes from transfected BaF3 cells expressing cCXCR1 bind interleukin (IL)-8 with an affinity similar to hCXCR1 (Kd values, 170 +/- 87 and 103 +/- 37 pM, respectively) and show low binding affinity to Gro-alpha. Cynomolgus CXCR2 also binds hIL-8 but with somewhat higher affinity than the hCXCR2 (46 +/- 28 and 220 +/- 14 pM, respectively). Surprisingly, cCXCR2 has a reduced binding affinity to hGro-alpha (3.7 +/- 2.2 nM), a specific ligand of hCXCR2 (540 +/- 140 pM). Furthermore, the CXCR2-specific antagonist SB225002 [N-(2-hydroxy-4-nitrophenyl)-N'-(2-bromophenyl)urea] is 10-fold more potent in inhibiting IL-8 binding to hCXCR2 than to cCXCR2, suggesting that some of the observed differences in the amino acid sequences of the human and monkey receptor affect ligand binding sites or the conformation of the receptor. Both cynomolgus receptors were functionally active in inducing guanosine 5'-O-(3-thio)triphosphate exchange on membranes in response to IL-8 and Gro-alpha and in mediating chemotactic activity of recombinant BA/F3 cells in response to IL-8 and Gro-alpha. These results identify the products of the novel cynomolgus genes as functional homologs of hCXCR1 and hCXCR2.

Chemotaxis Receptors and Signaling

Chemotaxis is an important cellular response common in biology. In many chemotaxing cells the signal that regulates movement is initiated by G protein-coupled receptors on the cell surface that bind specific chemoattractants. These receptors share important structural similarities with other G protein-coupled receptors, including rhodopsin, which currently serves as the best starting point for modeling their structures. However, the chemotaxis receptors also share a number of relatively unique structural features that are less common in other GPCRs. The chemoattractant ligands of chemotaxis receptors exhibit a broad variety of sizes and chemical properties, ranging from small molecules and peptides to protein ligands. As a result, different chemotaxis receptors have evolved specialized mechanisms for the early steps of ligand binding and receptor activation. The mechanism of transmembrane signaling is currently under intensive study and several alternate mechanisms proposing different conformational rearrangements of the transmembrane helices have been proposed. Some chemotaxis receptors are proposed to form dimers, and in certain cases dimer formation is proposed to play a role in transmembrane signaling. In principle the structural and dynamical changes that occur during transmembrane signaling could be specialized for different receptors, or could be broadly conserved. Extensive mutagenesis studies have been carried out, and have begun to identify critical residues involved in ligand binding, receptor activation, and transmembrane signaling.

Two Novel Fully Functional Isoforms of CX3CR1 Are Potent HIV Coreceptors

We identified two novel isoforms of the human chemokine receptor CX3CR1, produced by alternative splicing and with N-terminal regions extended by 7 and 32 aa. Expression of the messengers coding these isoforms, compared with that of previously described V28 messengers, is lower in monocytes and NK cells, but higher in CD4(+) T lymphocytes. CX3CR1 and its extended isoforms were expressed in HEK-293 cells and compared for expression, ligand binding, and cellular responses. In steady state experiments, all three CX3CR1 isoforms bound CX3CL1 with similar affinity. In kinetic binding studies, however, k(on) and k(off) were significantly greater for the extended CX3CR1 isoforms, thereby suggesting that the N-terminal extensions may alter the functions induced by CX3CL1. In signaling studies, all three CX3CR1 isoforms mediated agonist-dependent calcium mobilization, but the EC(50) was lower for the extended than for the standard isoforms. In addition, chemotactic responses for these extended isoforms shifted left, also indicating a more sensitive response. Finally, the longer variants appeared to be more potent HIV coreceptors when tested in fusion and infection assays. In conclusion, we identified and characterized functionally two novel isoforms of CX3CR1 that respond more sensitively to CX3CL1 and HIV viral envelopes. These data reveal new complexity in CX3CR1 cell activation and confirm the critical role of the N-terminal domain of the chemokine receptors in ligand recognition and cellular response.

Accumulation of microglial cells expressing ELR motif-positive CXC chemokines and their receptor CXCR2 in monkey hippocampus after ischemia-reperfusion

ELR(+) CXC chemokines including IL-8 are known to be involved in the ischemia-reperfusion injuries in various organs including rodent brain. However, the roles of these chemokines during the ischemia-reperfusion injuries of the primate brain still remain unknown. Here, we studied expressions of CXC chemokines and their receptor CXCR2 in monkey hippocampus known to develop total CA1 neuronal loss on day 5 after 20-min ischemia and reperfusion. ELR(+) chemokines and their receptor CXCR2 were not detected in the hippocampus of non-ischemic monkeys. On the contrary, at 30-60 min after the start of reperfusion, CD68-positive microglial cells increased significantly in the hippocampal CA1 sector, but there was negligible infiltration of neutrophils. These microglial cells expressed simultaneously growth regulated oncogene (Gro)-alpha and other ELR(+) CXC chemokines. Moreover, CD68-positive microglial cells also expressed the receptor for ELR(+) CXC chemokines. On day 4, capillary endothelial cells were significantly increased in the CA1 sector. Considering that ELR(+) CXC chemokines have potent angiogenic activities, the coordinate expression of ELR(+) CXC chemokines and their receptor CXCR2 in microglial cells may be related not only to the ischemic brain injuries but also to the microglial and capillary proliferation in the monkey hippocampus.

Identification of a signal transduction switch in the chemokine receptor CXCR1

Chemokine receptors belong to the superfamily of G protein-coupled receptors, which regulate the trafficking and activation of leukocytes, and operate as coreceptors in the entry of HIV-1. To investigate the early steps in the signal transmission from the chemokine-binding site to the G protein-coupling region we engineered metal ion-binding sites at putative extracellular sites in the chemokine receptor CXCR1. We introduced histidines into sites located in the second and third putative extracellular loops of CXCR1, creating single, double, and triple mutant receptors: R199H, R203H, D265H, R199H/R203H, R199H/D265H, R203H/D265H, R203H/H207Q, and R199H/R203H/D265H. Cells expressing the double mutants R199H/D265H and R203H/D265H and the triple mutant R199H/R203H/D265H failed to trigger interleukin 8-dependent calcium responses. Interestingly, calcium responses mediated by the single mutant R203H and the double mutants R199H/R203H and R203H/H207Q were blocked by Zn(II), indicating the creation of a functional metal ion-binding site. On the other hand, cells expressing all single, double, or triple histidine-substituted CXCR1 demonstrated high affinity binding to interleukin 8 in the presence and absence of metal ions. These findings indicate that occupation of the engineered metal-binding site uncouples the chemokine-binding site from the activation mechanism in CXCR1. Most importantly, we identify for the first time elements of an early signal transduction switch of chemokine receptors before the activation of cytoplasmic G proteins.

Chemokine receptors

Although chemokines were originally defined as host defense proteins it is now clear that their repertoire of functions extend well beyond this role. For example chemokines such as MGSA have growth regulatory properties while members of the CXC chemokine family can be mediators or inhibitors of angiogenesis and may be important targets for oncology. Recent work shows that the chemokine receptor CXCR4 and its cognate ligand SDF play important roles in the development of the immune, circulatory and central nervous systems. In addition, chemokine receptors play an important role in the pathogenesis of the AIDS virus, HIV-1. Taken together these findings expand the biological importance of chemokines from that of simple immune modulators to a much broader biological role than was at first appreciated and these and other properties of the chemokine receptor family are discussed in detail in this review.

The CC chemokine eotaxin (CCL11) is a partial agonist of CC chemokine receptor 2b

Despite sharing considerable homology with the members of the monocyte chemoattractant protein (MCP) family, the CC chemokine eotaxin (CCL11) has previously been reported to signal exclusively via the receptor CC chemokine receptor 3 (CCR3). Using the monocyte cell line THP-1, we investigated the relative abilities of eotaxin and MCPs 1-4 to induce CCR2 signaling, employing assays of directed cell migration and intracellular calcium flux. Surprisingly, 1 microm concentrations of eotaxin were able to recruit THP-1 cells in chemotaxis assays, and this migration was sensitive to antagonism of CCR2 but not CCR3. Radiolabeled eotaxin binding assays performed on transfectants bearing CCR2b or CCR3 confirmed eotaxin binding to CCR2 with a K(d) of 7.50 +/- 3.30 nm, compared with a K(d) of 1.68 +/- 0.91 nm at CCR3. In addition, whereas 1 microm concentrations of eotaxin were able to recruit CCR2b transfectants, substimulatory concentrations of eotaxin inhibited MCP-1-induced chemotaxis of CCR2b transfectants and also inhibited MCP-1-induced intracellular calcium flux of THP-1 cells. Collectively, these findings suggest that eotaxin is a partial agonist of the CCR2b receptor. A greater understanding of the interaction of CCR2 with all of its ligands, both full and partial agonists, may aid the rational design of specific antagonists that hold great promise as future therapeutic treatments for a variety of inflammatory disorders.

Molecular determinants of receptor binding and signaling by the CX3C chemokine fractalkine

Fractalkine/CX3CL1 is a membrane-tethered chemokine that functions as a chemoattractant and adhesion protein by interacting with the receptor CX3CR1. To understand the molecular basis for the interaction, an extensive mutagenesis study of fractalkine's chemokine domain was undertaken. The results reveal a cluster of basic residues (Lys-8, Lys-15, Lys-37, Arg-45, and Arg-48) and one aromatic (Phe-50) that are critical for binding and/or signaling. The mutant R48A could bind but not induce chemotaxis, demonstrating that Arg-48 is a signaling trigger. This result also shows that signaling residues are not confined to chemokine N termini, as generally thought. F50A showed no detectable binding, underscoring its importance to the stability of the complex. K15A displayed unique signaling characteristics, eliciting a wild-type calcium flux but minimal chemotaxis, suggesting that this mutant can activate some, but not all, pathways required for migration. Fractalkine also binds the human cytomegalovirus receptor US28, and analysis of the mutants indicates that US28 recognizes many of the same epitopes of fractalkine as CX3CR1. Comparison of the binding surfaces of fractalkine and the CC chemokine MCP-1 reveals structural details that may account for their dual recognition by US28 and their selective recognition by host receptors.

Orphan G protein-coupled receptor, GPR41, induces apoptosis via a p53/Bax pathway during ischemic hypoxia and reoxygenation

Orphan receptors that couple to G protein without known ligands are considered to relate directly to drug discovery. Here, we examine the expression of various orphan receptors in H9c2 cells during ischemic hypoxia and reoxygenation. Among orphan receptors examined, the level of G protein-coupled receptor 41 (GPR41) mRNA increases significantly, with a peak at 2 h after reoxygenation, and recovers to the control level by 3 h after reoxygenation. The level of glyceraldehyde-3-phosphate dehydrogenase mRNA used as an internal control remains almost constant. The levels of c-fos and c-jun mRNA increase significantly with ischemic hypoxia and reoxygenation. The transfection of GPR41 into H9c2 cells results in a significant decrease in cell number, with DNA fragmentation observed by in vitro and in situ assay. The amount of p53 protein increases significantly in the nuclei of cells expressing GPR41, accompanying an increase in the transcriptional activity of p53. Consistent with the activation of p53, the level of bax mRNA is significantly increased, which leads to an increase in Bax protein. Furthermore, the expression of a deletion mutant of a GPR41, which lacks the G protein binding site and shows an attenuation of intracellular phosphorylation signals to H9c2 cells, inhibits cell death and the increase in p53 protein within 24 h after reoxygenation. These observations demonstrate that GPR41 is a novel receptor that activates p53 leading to apoptosis during reoxygenation after ischemic hypoxia in H9c2 cells. We have designated GPR41 as the hypoxia-induced apoptosis receptor, HIA-R.

The TXP motif in the second transmembrane helix of CCR5. A structural determinant of chemokine-induced activation

CCR5 is a G-protein-coupled receptor activated by the chemokines RANTES (regulated on activation normal T cell expressed and secreted), macrophage inflammatory protein 1alpha and 1beta, and monocyte chemotactic protein 2 and is the main co-receptor for the macrophage-tropic human immunodeficiency virus strains. We have identified a sequence motif (TXP) in the second transmembrane helix of chemokine receptors and investigated its role by theoretical and experimental approaches. Molecular dynamics simulations of model alpha-helices in a nonpolar environment were used to show that a TXP motif strongly bends these helices, due to the coordinated action of the proline, which kinks the helix, and of the threonine, which further accentuates this structural deformation. Site-directed mutagenesis of the corresponding Pro and Thr residues in CCR5 allowed us to probe the consequences of these structural findings in the context of the whole receptor. The P84A mutation leads to a decreased binding affinity for chemokines and nearly abolishes the functional response of the receptor. In contrast, mutation of Thr-82(2.56) into Val, Ala, Cys, or Ser does not affect chemokine binding. However, the functional response was found to depend strongly on the nature of the substituted side chain. The rank order of impairment of receptor activation is P84A > T82V > T82A > T82C > T82S. This ranking of impairment parallels the bending of the alpha-helix observed in the molecular simulation study.

Dual structural requirements for multilineage hematopoietic-suppressive activity of chemokine-derived peptides

Many chemokines have direct suppressive activity in vitro and in vivo on primitive hematopoietic cells. However, few chemokine-derived peptides have shown a significant activity in inhibiting hematopoiesis. Interestingly, a peptide derived from the 34-58 sequence of the CXC chemokine platelet factor 4 (PF4) produced a 30-40% inhibition of proliferation of murine hematopoietic progenitors (CFU-MK, CFU-GM, and BFU-E) in vitro, at concentrations of 30-60-fold lower than PF4. The aim of the present work was to define the structural parameters and motifs involved in conferring biological activity to the peptide PF4(34-58). Both structural predictions and determinations revealed a new helical motif that was further localized between residues 38 and 46. This helix was necessary for binding of the peptide and for permitting the functional DLQ motif at position 54-56 to activate the putative receptor site. Peptides lacking either the helical or the DLQ motif were devoid of inhibitory activity on the hematopoietic progenitors in vitro. However, among inactive peptides, only those having the helical motif counteracted the inhibition induced by the active peptide PF4(34-58). This suggested that the helix might be required for peptide interactions with a putative receptor site, whereas the DLQ motif would be implicated in the activation of this receptor. These results identify for the first time the dual requirements for the design of chemokine-derived peptides with high suppressive activity on hematopoiesis, as well as for the design of molecules with antagonistic action.

Human T-cell lymphotropic virus type-I tax gene induces interleukin-8 secretion by autocrine mechanism and has No effect on interleukin-16 in transfected Jurkat cells

Human T-cell lymphotropic virus I (HTLV-I) Tax has been shown to transactivate several cellular genes. In this study, we show that interleukin-8 (IL-8) is expressed and secreted in tax-transfected Jurkat cells that were further augmented by mitogen stimulation. Expression of high-affinity IL-8-R (Type A) mRNA in these cells suggests an autocrine role for this chemokine in HTLV-I-infected T-cells. However, interleukin-16 (IL-16) mRNA expression or protein secretion was not significantly modulated either constitutively or even upon mitogen stimulation in these tax-transfected cells.

Autocrine growth effect of IL-8 and GROalpha on a human pancreatic cancer cell line, Capan-1

A human pancreatic cancer cell line, Capan-1, secretes the chemokines interleukin-8 (IL-8) and growth-related oncogene alpha (GROalpha). Capan-1 cells also express the chemokine receptor 2 (CXCR2), which is a Gialpha-protein coupled receptor. Growth of Capan-1 cells was inhibited when anti-IL-8 or anti-GROalpha monoclonal antibody was added into the culture medium. Pertussis toxin, which blocks Gialpha also demonstrated a growth-inhibitory effect on Capan-1 cells. These results indicated that IL-8 and GROalpha act on Capan-1 cells as growth factors in an autocrine manner through CXCR2.

The isolation and sequence of canine interleukin-8 receptor

A cDNA corresponding to canine IL-8 receptor has been cloned and sequenced. The cDNA was synthesized using RT-PCR, with oligonucleotide primers designed from conserved regions of published IL-8 receptors. The 5'-end was cloned by 5'-RACE and the 3'-end was cloned by 3'-RACE. The cDNA encodes a predicted full length IL-8r protein of 356 amino acids. At the nucleic acid level, the canine cDNA shows 83.9%, 82.4%/78.8%, 81.5%/78%, 81.4%/77.7%, 77.8% and 77.3%/71.9% identity to published sequences of bovine, human, gorilla, rabbit, mouse and rat IL8RB/IL8RA, respectively. The derived protein from the cDNA sequences shows 75.3%/70.3%, 75.3%/70.1%, 74.8%/69.4%, 70%/59%, and 69.7% identity to that of human, rabbit, gorilla, rat and mouse IL8RB/IL8RA homolog.

Multiple charged and aromatic residues in CCR5 amino-terminal domain are involved in high affinity binding of both chemokines and HIV-1 Env protein

CCR5 is a functional receptor for MIP-1alpha, MIP-1beta, RANTES (regulated on activation normal T cell expressed), MCP-2, and MCP-4 and constitutes the main coreceptor for macrophage tropic human and simian immunodeficiency viruses. By using CCR5-CCR2b chimeras, we have shown previously that the second extracellular loop of CCR5 is the major determinant for chemokine binding specificity, whereas the amino-terminal domain plays a major role for human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus coreceptor function. In the present work, by using a panel of truncation and alanine-scanning mutants, we investigated the role of specific residues in the CCR5 amino-terminal domain for chemokine binding, functional response to chemokines, HIV-1 gp120 binding, and coreceptor function. Truncation of the amino-terminal domain resulted in a progressive decrease of the binding affinity for chemokines, which correlated with a similar drop in functional responsiveness. Mutants lacking residues 2-13 exhibited fairly weak responses to high concentrations (500 nM) of RANTES or MIP-1beta. Truncated mutants also exhibited a reduction in the binding affinity for R5 Env proteins and coreceptor activity. Deletion of 4 or 12 residues resulted in a 50 or 80% decrease in coreceptor function, respectively. Alanine-scanning mutagenesis identified several charged and aromatic residues (Asp-2, Tyr-3, Tyr-10, Asp-11, and Glu-18) that played an important role in both chemokine and Env high affinity binding. The overlapping binding site of chemokines and gp120 on the CCR5 amino terminus, as well as the involvement of these residues in the epitopes of monoclonal antibodies, suggests that these regions are particularly exposed at the receptor surface.

Role of the first extracellular loop in the functional activation of CCR2. The first extracellular loop contains distinct domains necessary for both agonist binding and transmembrane signaling

The physiological cellular responses to monocyte chemoattractant protein-1 (MCP-1), a potent chemotactic and activating factor for mononuclear leukocytes, are mediated by specific binding to CCR2. The aim of this investigation is to identify receptor microdomains that are involved in high affinity agonist binding and receptor activation. The results from our functional studies in which we utilized neutralizing antisera against CCR2 are consistent with a multidomain binding model, previously proposed by others. The first extracellular loop was of particular interest, because in addition to a ligand-binding domain it contained also information for receptor activation, crucial for transmembrane signaling. Replacement of the first extracellular loop of CCR2 with the corresponding region of CCR1 decreased the MCP-1 binding affinity about 10-fold and prevented transmembrane signaling. A more detailed analysis by site-directed mutagenesis revealed that this receptor segment contains two distinct microdomains. The amino acid residues Asn(104) and Glu(105) are essential for high affinity agonist binding but are not involved in receptor activation. In contrast, the charged amino acid residue His(100) does not contribute to ligand binding but is vital for receptor activation and initiation of transmembrane signaling. We hypothesize that the interaction of agonist with this residue initiates the conformational switch that allows the formation of the functional CCR2-G protein complex.

Structure-activity relationships of G protein-coupled receptors

The primary function of cell-surface receptors is to discriminate the specific signaling molecule or ligand from a large array of chemically diverse extracellular substances and to activate an effector signaling cascade that triggers an intracellular response and eventually a biological effect. G protein-coupled cell-surface receptors (GPCRs) mediate their intracellular actions through the activation of guanine nucleotide-binding signal-transducing proteins (G proteins), which form a diverse family of regulatory GTPases that, in the GTP-bound state, bind and activate downstream membrane-localized effectors. Hundreds of GPCRs signal through one or more of these G proteins in response to a large variety of stimuli including photons, neurotransmitters, and hormones of variable molecular structure. The mechanisms by which these ligands provoke activation of the receptor/G-protein system are highly complex and multifactorial. Knowledge and mapping of the structural determinants and requirements for optimal GPCR function are of paramount importance, not only for a better and more detailed understanding of the molecular basis of ligand action and receptor function in normal and abnormal conditions, but also for a rational design of early diagnostic and therapeutic tools that may allow exogenous regulation of receptor and G protein function in disease processes.

Identification of residues in the monocyte chemotactic protein-1 that contact the MCP-1 receptor, CCR2

The CC chemokine, MCP-1, has been identified as a major chemoattractant for T cells and monocytes, and plays a significant role in the pathology of inflammatory diseases. To identify the regions of MCP-1 that contact its receptor, CCR2, we substituted all surface-exposed residues with alanine. Some residues were also mutated to other amino acids to identify the importance of charge, hydrophobicity, or aromaticity at specific positions. The binding affinity of each mutant for CCR2 was assayed with THP-1 and CCR2-transfected CHL cells. The majority of point mutations had no effect. Residues at the N-terminus of the protein, known to be crucial for signaling, contribute less than a factor of 10 to the binding affinity. However, two clusters of primarily basic residues (R24, K35, K38, K49, and Y13), separated by a 35 A hydrophobic groove, reduced the level of binding by 15-100-fold. A peptide fragment encompassing residues 13-35 recapitulated some of the mutational data derived from the intact protein. It exhibited modest binding as a linear peptide and dramatically improved affinity when the region which adopts a single turn of a 3(10)-helix in the protein, which includes R24, was constrained by a disulfide bond. Additional constraints at the ends of the peptide, corresponding to the disulfide between the first and third cysteines in MCP-1, yielded further improvements in affinity. Together, these data suggest a model in which a large surface area of MCP-1 contacts the receptor, and the accumulation of a number of weak interactions results in the 35 pM affinity observed for the wild-type (WT) protein. The receptor binding site of MCP-1 also is significantly different from the binding sites of RANTES and IL-8, providing insight into the issue of receptor specificity. It was previously shown that the N-terminus of CCR2 is critical for binding MCP-1 [Monteclaro, F. S., and Charo, I. F. (1996) J. Biol. Chem. 271, 19084-92; Monteclaro, F. S., and Charo, I. F. (1997) J. Biol. Chem. 272, 23186-90]. Point mutations of six acidic residues in this region of the receptor were made to test their role in ligand binding. This identified D25 and D27 of the DYDY motif as being important. On the basis of our data, we propose a model in which the receptor N-terminus lies along the hydrophobic groove in an extended fashion, placing the DYDY motif near the basic cluster involving R24 and K49 of MCP-1. This in turn orients the signaling residues (Y13 and the N-terminus) for productive interaction with the receptor.

Cutting Edge: CXCR4-Lo: Molecular Cloning and Functional Expression of a Novel Human CXCR4 Splice Variant

Human CXCR4 is a specific receptor for the CXC chemokine stromal cell-derived factor-1 (SDF-1) and a coreceptor for T cell line tropic strains of HIV-1. Genetic knockouts of CXCR4 and SDF-1 have delineated their critical role during embryonic cardiogenesis, leukopoiesis, and vasculogenesis. Herein, we used bioinformatics and differential strategies like isoform-specific RT-PCR and Northern blots to identify and clone a novel unspliced isoform of human CXCR4, termed CXCR4-Lo. CXCR4-Lo corresponds to a larger ∼4.0-kb mRNA transcript and differs from the known human CXCR4 by the first 9 aa in the functionally important NH2-terminal extracellular domain of the receptor. CXCR4-Lo-transfected rat basophil leukemia-2H3 cells responded to SDF-1 with a transient rise of intracellular Ca2+ concentration and by undergoing chemotaxis. Expression of CXCR4-Lo is noteworthy, as it may have differential affinity as a coreceptor for HIV strains in comparison with CXCR4. Furthermore, CXCR4-Lo may also provide a functional backup to CXCR4 during embryogenesis.

Differential mechanisms of recognition and activation of interleukin-8 receptor subtypes

We have probed an epitope sequence (His18-Pro19-Lys20-Phe21) in interleukin-8 (IL-8) by site-directed mutagenesis. This work shows that single and double Ala substitutions of His18 and Phe21 in IL-8 reduced up to 77-fold the binding affinity to IL-8 receptor subtypes A (CXCR1) and B (CXCR2) and to the Duffy antigen. These Ala mutants triggered neutrophil degranulation and induced calcium responses mediated by CXCR1 and CXCR2. Single Asp or Ser substitutions, H18D, F21D, F21S, and double substitutions, H18A/F21D, H18A/F21S, and H18D/F21D, reduced up to 431-fold the binding affinity to CXCR1, CXCR2, and the Duffy antigen. Interestingly, double mutants with charged residue substitutions failed to trigger degranulation or to induce wild-type calcium responses mediated by CXCR1. Except for the H18A and F21A mutants, all other IL-8 mutants failed to induce superoxide production in neutrophils. This study demonstrates that IL-8 recognizes and activates CXCR1, CXCR2, and the Duffy antigen by distinct mechanisms.

Structure of a CXC chemokine-receptor fragment in complex with interleukin-8

BACKGROUND

Interactions between CXC chemokines (e.g. interleukin-8, IL-8) and their receptors (e.g. CXCR-1) have a key role in host defense and disease by attracting and upregulating neutrophils to sites of inflammation. The transmembrane nature of the receptor impedes structure-based understanding of ligand interactions. Linear peptides based on the N-terminal, extracellular portion of the receptor CXCR-1 do bind to IL-8, however, and inhibit the binding of IL-8 to the full-length receptor.

RESULTS

The NMR solution structure of the complex formed between IL-8 and one such receptor-based peptide indicates that a cleft between a loop and a beta hairpin constitute part of the receptor interaction surface on IL-8. Nine residues from the C terminus of the receptor peptide (corresponding to Pro21-Pro29 of CXCR-1) occupy the cleft in an extended fashion. Intermolecular contacts are mostly hydrophobic and sidechain mediated.

CONCLUSIONS

The results offer the first details at an atomic level of the interaction between a chemokine and its receptor. Consideration of other biochemical data allow extrapolation to a model for the interaction of IL-8 with the full-length receptor. In this model, the heparin-binding residues of IL-8 are exposed, thereby allowing presentation of the chemokine from endothelial cell-surface glycosaminoglycans. This first glimpse of how IL-8 binds to its receptor provides a foundation for the structure-based design of chemokine antagonists.

Solution structure and dynamics of the CX3C chemokine domain of fractalkine and its interaction with an N-terminal fragment of CX3CR1

Fractalkine, a novel CX3C chemokine, is unusual because of both its membrane-associated structure and its direct role in cell adhesion. We have solved the solution structure of the chemokine domain of fractalkine (residues 1-76) by heteronuclear NMR methods. The 20 lowest energy structures in the ensemble have an average backbone rmsd of 0.43 A, excluding the termini. In contrast to many other chemokines which form homodimers, fractalkine's chemokine module is monomeric. Comparison of the structure to CC and CXC chemokines reveals interesting differences which are likely to be relevant to receptor binding. These include a bulge formed by the CX3C motif, the relative orientation of the N-terminus and 30's loop (residues 30-38), and the conformation of the N-loop (residues 9-19). 15N backbone relaxation experiments indicate that these same regions of the protein are dynamic. We also titrated 15N-labeled protein with a peptide from the N-terminus of the receptor CX3CR1 and confirmed that this region of the receptor contacts the fractalkine chemokine domain. Interestingly, the binding site maps roughly to the regions of greatest flexibility and structural variability. Together, these data provide a first glimpse of how fractalkine interacts with its receptor and should help guide mutagenesis studies to further elucidate the molecular details of binding and signaling through CX3CR1.

The N-terminal extracellular segments of the chemokine receptors CCR1 and CCR3 are determinants for MIP-1alpha and eotaxin binding, respectively, but a second domain is essential for efficient receptor activation

CCR1 and CCR3 are seven-transmembrane domain G protein-coupled receptors specific for members of the CC chemokine subgroup of leukocyte chemoattractants. Both have been implicated in the inflammatory response, and CCR3, through its expression on eosinophils, basophils, and Th2 lymphocytes, may be especially important in allergic inflammation. CCR1 and CCR3 are 54% identical in amino acid sequence and share some ligands but not others. In particular, macrophage inflammatory protein 1alpha (MIP-1alpha) is a ligand for CCR1 but not CCR3, and eotaxin is a ligand for CCR3 but not CCR1. To map ligand selectivity determinants and to guide rational antagonist design, we analyzed CCR1:CCR3 chimeric receptors. When expressed in mouse pre-B cells, chimeras in which the N-terminal extracellular segments were switched were both able to bind both MIP-1alpha and eotaxin, but in each case, binding occurred via separate sites. Nevertheless, neither MIP-1alpha nor eotaxin were effective agonists at either chimeric receptor in either calcium flux or chemotaxis assays. These data are consistent with a multi-site model for chemokine-chemokine receptor interaction in which one or more subsites determine chemokine selectivity, but others are needed for receptor activation. Agents that bind to the N-terminal segments of CCR1 and CCR3 may be useful in blocking receptor function.