Multiple Targets for Oxysterols in Their Regulation of the Immune System

Oxysterols, or cholesterol oxidation products, are naturally occurring lipids which regulate the physiology of cells, including those of the immune system. In contrast to effects that are mediated through nuclear receptors or by epigenetic mechanism, which take tens of minutes to occur, changes in the activities of cell-surface receptors caused by oxysterols can be extremely rapid, often taking place within subsecond timescales. Such cell-surface receptor effects of oxysterols allow for the regulation of fast cellular processes, such as motility, secretion and endocytosis. These cellular processes play critical roles in both the innate and adaptive immune systems. This review will survey the two broad classes of cell-surface receptors for oxysterols (G-protein coupled receptors (GPCRs) and ion channels), the mechanisms by which cholesterol oxidation products act on them, and their presence and functions in the different cell types of the immune system. Overall, this review will highlight the potential of oxysterols, synthetic derivatives and their receptors for physiological and therapeutic modulation of the immune system.

Neutrophil recruitment by chemokines Cxcl1/KC and Cxcl2/MIP2: Role of Cxcr2 activation and glycosaminoglycan interactions

Chemokines play a crucial role in combating microbial infection by recruiting blood neutrophils to infected tissue. In mice, the chemokines Cxcl1/KC and Cxcl2/MIP2 fulfill this role. Cxcl1 and Cxcl2 exist as monomers and dimers, and exert their function by activating the Cxcr2 receptor and binding glycosaminoglycans (GAGs). Here, we characterized Cxcr2 G protein and β-arrestin activities, and GAG heparan sulfate (HS) interactions of Cxcl1 and Cxcl2 and of the trapped dimeric variants. To understand how Cxcr2 and GAG interactions impact in vivo function, we characterized their neutrophil recruitment activity to the peritoneum, Cxcr2 and CD11b levels on peritoneal and blood neutrophils, and transport profiles out of the peritoneum. Cxcl2 variants compared with Cxcl1 variants were more potent for Cxcr2 activity. Native Cxcl1 compared with native Cxcl2 and dimers compared with native proteins bound HS with higher affinity. Interestingly, recruitment activity between native Cxcl1 and Cxcl2, between dimers, and between the native protein and the dimer could be similar or very different depending on the dose or the time point. These data indicate that peritoneal neutrophil recruitment cannot be solely attributed to Cxcr2 or GAG interactions, and that the relationship between recruited neutrophils, Cxcr2 activation, GAG interactions, and chemokine levels is complex and highly context dependent. We propose that the ability of Cxcl1 and Cxcl2 to reversibly exist as monomers and dimers and differences in their Cxcr2 activity and GAG interactions coordinate neutrophil recruitment and activation, which play a critical role for successful resolution of inflammation.

CXCL5 Modified Nanoparticle Surface Improves CXCR2+ Cell Selective Internalization

Driving nanomaterials to specific cell populations is still a major challenge for different biomedical applications. Several strategies to improve cell binding and uptake have been tried thus far by intrinsic material modifications or decoration with active molecules onto their surface. In the present work, we covalently bound the chemokine CXCL5 on fluorescently labeled amino-functionalized SiO₂ nanoparticles to precisely targeting CXCR2⁺ immune cells. We synthesized and precisely characterized the physicochemical features of the modified particles. The presence of CXCL5 on the surface was detected by z-potential variation and CXCL5-specific electron microscopy immunogold labeling. CXCL5-amino SiO₂ nanoparticle cell binding and internalization performances were analyzed in CXCR2⁺ THP-1 cells by flow cytometry and confocal microscopy. We showed improved internalization of the chemokine modified particles in the absence or the presence of serum. This internalization was reduced by cell pre-treatment with free CXCL5. Furthermore, we demonstrated CXCR2⁺ cell preferential targeting by comparing particle uptake in THP-1 vs. low-CXCR2 expressing HeLa cells. Our results provide the proof of principle that chemokine decorated nanomaterials enhance uptake and allow precise cell subset localization. The possibility to aim at selective chemokine receptor-expressing cells can be beneficial for the diverse pathological conditions involving immune reactions.

Molecular characterization and expression analysis of the large yellow croaker (Larimichthys crocea) chemokine receptors CXCR2, CXCR3, and CXCR4 after bacterial and poly I:C challenge

The large yellow croaker (Larimichthys crocea) has a well-developed innate immune system. We studied a component of this system, chemokine receptor CXCR family. In this study, we report the full-length open reading frames, as well as the identification and characterization of the chemokine receptor genes CXCR2 (LycCXCR2), CXCR3 (LycCXCR3), and CXCR4 (LycCXCR4) of large yellow croaker. We report that LycCXCR3 and LycCXCR4 are evolving neutrally according to PAML analyses. Quantitative real-time PCR analysis revealed that CXCR transcripts were expressed in all examined tissues. The expression of chemokine receptors LycCXCR2, LycCXCR3, and LycCXCR4 was elevated in the kidney, spleen, and particularly the liver of the large yellow croaker after challenge with Vibrio anguillarum and polyinosinic:polycytidylic acid (poly I:C). These results suggest that LycCXCR2, LycCXCR3, and LycCXCR4 may be important immune-related genes, playing crucial roles in immune defence against bacterial infection.

Differences in Sulfotyrosine Binding amongst CXCR1 and CXCR2 Chemokine Ligands

Tyrosine sulfation, a post-translational modification found on many chemokine receptors, typically increases receptor affinity for the chemokine ligand. A previous bioinformatics analysis suggested that a sulfotyrosine (sY)-binding site on the surface of the chemokine CXCL12 may be conserved throughout the chemokine family. However, the extent to which receptor tyrosine sulfation contributes to chemokine binding has been examined in only a few instances. Computational solvent mapping correctly identified the conserved sulfotyrosine-binding sites on CXCL12 and CCL21 detected by nuclear magnetic resonance (NMR) spectroscopy, demonstrating its utility for hot spot analysis in the chemokine family. In this study, we analyzed five chemokines that bind to CXCR2, a subset of which also bind to CXCR1, to identify hot spots that could participate in receptor binding. A cleft containing the predicted sulfotyrosine-binding pocket was identified as a principal hot spot for ligand binding on the structures of CXCL1, CXCL2, CXCL7, and CXCL8, but not CXCL5. Sulfotyrosine titrations monitored via NMR spectroscopy showed specific binding to CXCL8, but not to CXCL5, which is consistent with the predictions from the computational solvent mapping. The lack of CXCL5–sulfotyrosine interaction and the presence of CXCL8–sulfotyrosine binding suggests a role for receptor post-translational modifications regulating ligand selectivity.

Structural Basis of Native CXCL7 Monomer Binding to CXCR2 Receptor N-Domain and Glycosaminoglycan Heparin

CXCL7, a chemokine highly expressed in platelets, orchestrates neutrophil recruitment during thrombosis and related pathophysiological processes by interacting with CXCR2 receptor and sulfated glycosaminoglycans (GAG). CXCL7 exists as monomers and dimers, and dimerization (~50 μM) and CXCR2 binding (~10 nM) constants indicate that CXCL7 is a potent agonist as a monomer. Currently, nothing is known regarding the structural basis by which receptor and GAG interactions mediate CXCL7 function. Using solution nuclear magnetic resonance (NMR) spectroscopy, we characterized the binding of CXCL7 monomer to the CXCR2 N-terminal domain (CXCR2Nd) that constitutes a critical docking site and to GAG heparin. We found that CXCR2Nd binds a hydrophobic groove and that ionic interactions also play a role in mediating binding. Heparin binds a set of contiguous basic residues indicating a prominent role for ionic interactions. Modeling studies reveal that the binding interface is dynamic and that GAG adopts different binding geometries. Most importantly, several residues involved in GAG binding are also involved in receptor interactions, suggesting that GAG-bound monomer cannot activate the receptor. Further, this is the first study that describes the structural basis of receptor and GAG interactions of a native monomer of the neutrophil-activating chemokine family.

Boronic acid-containing CXCR1/2 antagonists: Optimization of metabolic stability, in vivo evaluation, and a proposed receptor binding model

Blockade of undesired neutrophil migration to sites of inflammation remains an area of substantial pharmaceutical interest. To effect this blockade, a validated therapeutic target is antagonism of the chemokine receptor CXCR2. Herein we report the discovery of 6-(2-boronic acid-5-trifluoromethoxy-benzylsulfanyl)-N-(4-fluoro-phenyl)-nicotinamide 6, an antagonist with activity at both CXCR1 and CXCR2 receptors (IC50 values 31 and 21 nM, respectively). Compound 6 exhibited potent inhibition of neutrophil influx in a rat model of pulmonary inflammation, and is hypothesized to interact with a unique intracellular binding site on CXCR2. Compound 6 (SX-576) is undergoing further investigation as a potential therapy for pulmonary inflammation.

Molecular aspects, genomic arrangement and immune responsive mRNA expression profiles of two CXC chemokine receptor homologs (CXCR1 and CXCR2) from rock bream, Oplegnathus fasciatus

The CXCR1 and CXCR2 are the prototypical receptors and are the only known receptors for mammalian ELR+ (Glu-Leu-Arg) CXC chemokines, including CXCL8 (interleukin 8). These receptors transduce the ELR+ chemokine signals and operate the downstream signaling pathways in inflammation and innate immunity. In this study, we report the identification and characterization of CXCR1 and CXCR2 genes from rock bream fish (OfCXCR1 and OfCXCR2) at the molecular level. The cDNA and genomic DNA sequences of the OfCXCR1 and OfCXCR2 were identified from a transcriptome library and a custom-constructed BAC library, respectively. Both OfCXCR genes consisted of two exons, separated by an intron. The 5'-flanking regions of OfCXCR genes possessed multiple putative transcription factor binding sites related to immune response. The coding sequences of OfCXCR1 and OfCXCR2 encoded putative peptides of 355 and 360 amino acids (aa), respectively. The deduced aa sequences of OfCXCR1 and OfCXCR2 comprised of a G-protein coupled receptors (GPCR) family 1 profile with a GPCR signature and a DRY motif. In addition, seven conserved transmembrane regions were predicted in both OfCXCRs. While our multiple alignment study revealed the functionally significant conserved elements of the OfCXCR1 and OfCXCR2, phylogeny analyses further confirmed their position in teleost sub clade, in which they manifested an evolutionary relatedness with other fish counterparts. Based on comparative analyses, teleost CXC chemokine receptors appear to be distinct from their non-fish orthologs in terms of evolution (both CXCR1 and CXCR2) and genomic organization (CXCR2). Quantitative real-time PCR (qPCR) detected the transcripts of OfCXCR1 and OfCXCR2 in eleven examined tissues, with higher levels in head kidney, kidney and spleen highlighting their crucial importance in immunity. In vitro stimulation of peripheral blood leukocytes (PBLs) with concanavalin A (Con A) resulted in modulation of OfCXCR2 transcription, but not that of OfCXCR1. In addition, the magnitude of the OfCXCR1 and OfCXCR2 transcripts in head kidney and spleen was differentially increased after the in vivo administration of immune stimulants, LPS and poly I:C and in the infection models injected with rock bream irido virus, Edwardsiella tarda and Streptococcus iniae. These lines of evidence suggest that these receptors may play an important role(s) in immune responsive signaling during pathogenesis of rock bream.

Multiple acquisition/multiple observation separated local field/chemical shift correlation solid-state magic angle spinning NMR spectroscopy

Multiple acquisition spectroscopy (MACSY) experiments that enable multiple free induction decays to be recorded during individual experiments are demonstrated. In particular, the experiments incorporate separated local field spectroscopy into homonuclear and heteronuclear correlation spectroscopy. The measured heteronuclear dipolar couplings are valuable in structure determination as well as in enhancing resolution by providing an additional frequency axis. In one example four different three-dimensional spectra are obtained in a single experiment, demonstrating that substantial potential saving in experimental time is available when multiple multi-dimensional spectra are required as part of solid-state NMR studies.

Crystal structure of the NHERF1 PDZ2 domain in complex with the chemokine receptor CXCR2 reveals probable modes of PDZ2 dimerization

The formation of CXCR2-NHERF1-PLCβ2 macromolecular complex in neutrophils regulates CXCR2 signaling and plays a key role in neutrophil chemotaxis and transepithelial neutrophilic migration. However, NHERF1 by itself, with only two PDZ domains, has a limited capacity in scaffolding the multiprotein-complex formation. Here we report the crystal structure of the NHERF1 PDZ2 domain in complex with the C-terminal CXCR2 sequence. The structure reveals that the PDZ2-CXCR2 binding specificity is achieved by numerous hydrogen bonds and hydrophobic contacts with the last four CXCR2 residues contributing to specific interactions. The structure also reveals two probable modes of PDZ2 dimerization where the two canonical ligand-binding pockets are well separated and orientated in a unique parallel fashion. This study provides not only the structural basis for the PDZ-mediated NHERF1-CXCR2 interaction, but also an additional example of how PDZ domains may dimerize, which both could prove valuable in understanding NHERF1 complex-scaffolding function in neutrophils.

New conformational state of NHERF1-CXCR2 signaling complex captured by crystal lattice trapping

NHERF1 is a PDZ adaptor protein that scaffolds the assembly of diverse signaling complexes and has been implicated in many cancers. However, little is known about the mechanism responsible for its scaffolding promiscuity or its ability to bind to multiple targets. Computational studies have indicated that PDZ promiscuity may be attributed to its conformational dynamics, but experimental evidence for this relationship remains very limited. Here we examine the conformational flexibility of the NHERF1 PDZ1 domain using crystal lattice trapping via solving PDZ1 structure of a new crystal form. The structure, together with prior PDZ1 structures of a different space group, reveals that 4 of 11 ligand-interacting residues undergo significant crystal packing-induced structural changes. Most of these residues correspond to the residues involved in allosteric transition when a peptide ligand binds. In addition, a subtle difference in ligand conformations causes the same peptide to bind in slightly different modes in different crystal forms. These findings indicate that substantial structural flexibility is present in the PDZ1 peptide-binding pocket, and the structural substate trapped in the present crystal form can be utilized to represent the conformational space accessible to the protein. Such knowledge will be critical for drug design against the NHERF1 PDZ1 domain, highlighting the continued need for experimentally determined PDZ1-ligand complexes.

Identification and profiling of novel α1A-adrenoceptor-CXC chemokine receptor 2 heteromer

We have provided the first evidence for specific heteromerization between the α(1A)-adrenoceptor (α(1A)AR) and CXC chemokine receptor 2 (CXCR2) in live cells. α(1A)AR and CXCR2 are both expressed in areas such as the stromal smooth muscle layer of the prostate. By utilizing the G protein-coupled receptor (GPCR) heteromer identification technology on the live cell-based bioluminescence resonance energy transfer (BRET) assay platform, our studies in human embryonic kidney 293 cells have identified norepinephrine-dependent β-arrestin recruitment that was in turn dependent upon co-expression of α(1A)AR with CXCR2. These findings have been supported by co-localization observed using confocal microscopy. This norepinephrine-dependent β-arrestin recruitment was inhibited not only by the α(1)AR antagonist Terazosin but also by the CXCR2-specific allosteric inverse agonist SB265610. Furthermore, Labetalol, which is marketed for hypertension as a nonselective β-adrenoceptor antagonist with α(1)AR antagonist properties, was identified as a heteromer-specific-biased agonist exhibiting partial agonism for inositol phosphate production but essentially full agonism for β-arrestin recruitment at the α(1A)AR-CXCR2 heteromer. Finally, bioluminescence resonance energy transfer studies with both receptors tagged suggest that α(1A)AR-CXCR2 heteromerization occurs constitutively and is not modulated by ligand. These findings support the concept of GPCR heteromer complexes exhibiting distinct pharmacology, thereby providing additional mechanisms through which GPCRs can potentially achieve their diverse biological functions. This has important implications for the use and future development of pharmaceuticals targeting these receptors.

CXCR2--the receptor to hit?

Emigration of leukocytes from the microcirculation into inflammatory tissues is one of the hallmarks of our immune system. However, excessive leukocyte recruitment can disturb the integrity of the organism and aggravate acute and chronic inflammatory diseases. Chemokines are chemoattractant peptides that are used as messengers to direct leukocytes to sites of inflammation. They mediate their effects through G-protein-coupled chemokine receptors on the surface of leukocytes and other, nonhematopoietic cells, where they induce a variety of cell responses including cell activation and transmigration. CXC chemokine receptor 2 (CXCR2) has been implicated in numerous inflammatory disorders. In many models of acute and chronic inflammatory diseases, blockade of CXCR2 substantially reduces leukocyte recruitment, tissue damage and mortality. The physiological importance of CXCR2 has led to the development of selective CXCR2 inhibitors that are now being tested in clinical trials. This review will summarize current knowledge about CXCR2 in inflammatory diseases and discuss its potential as a pharmaceutical target.

The IL sequence in the LLKIL motif in CXCR2 is required for full ligand-induced activation of Erk, Akt, and chemotaxis in HL60 cells

The chemotaxis of differentiated HL60 cells stably expressing CXCR2 was examined in a microfluidic gradient device where the steepness of the CXCL8 chemokine gradient was varied from 2 pg/ml/mum (0-1 ng/ml over a width of 500 microm) to 50 pg/ml/microm (0-25 ng/ml over 500 microm). The differentiated HL60 cells stably expressing CXCR2 exhibited little chemotaxis in response to a 0-1 ng/ml gradient, but displayed an increasing chemotactic response as the gradient steepness increased from 0 to 5, 0 to 10, and 0 to 25 ng/ml, demonstrating that steepness of gradient is a major determinant of the relative ability of cells to persistently migrate up a chemotactic gradient. When HL60 cells expressed CXCR2 mutated in the C terminus LLKIL motif (IL to AA), ligand-induced internalization of receptors was reduced 50%, whereas cell migration along the gradient of CXCL8 was completely lost. Although both mutant and wild-type receptors could mediate Akt and Erk activation in response to CXCL8, the level of activation of these two kinases was much lower in the cell line expressing the mutant receptors. These data imply that the IL amino acid residues in the LLKIL motif are very important for activation of the signal transduction cascade, which is necessary for cells to sense the chemokine gradient and respond with chemotaxis. Moreover, because mutation of the IL residues in the LLKIL motif resulted in only 50% reduction in receptor internalization, and a 50% reduction in Akt and Erk phosphorylation, but a complete loss of chemotactic response, the data imply that IL amino acid residues in the LLKIL motif are key either for amplification or oscillation of crucial signaling events or for establishment of a threshold for signals required for chemotaxis.

A homolog of the human chemokine receptor CXCR1 is expressed in the mouse

Two distinct genes are present in the human genome encoding receptors for human interleukin-8 (hCXCL8), referred to as hCXCR1 and hCXCR2. While it seems clear that orthologous genes are present in the genomes of several mammals, the existence of a gene encoding an ortholog of hCXCR1 in the mouse has thus far been controversial. We have isolated a cDNA that is highly similar to the cDNAs of hCXCR1 and hCXCR2, but is clearly distinct from the cDNA encoding mouse CXCR2 (mCXCR2). The encoded protein, designated mouse CXCR1-like (mCXCR1-like), shares 64, 57, 57, and 89% identical amino acids with hCXCR1, hCXCR2, mCXCR2, and rCXCR1-like, respectively. The gene encoding mCXCR1-like was mapped to mouse chromosome 1 and its genomic organization was determined to be very similar to the organization of the gene encoding hCXCR1. Like hCXCR1, mCXCR1-like was found to be expressed at the mRNA level in neutrophils. In addition, mRNA encoding mCXCR1-like was detected in liver, kidney, and spleen. In spleen, mCXCR1-like transcripts were predominantly found in CD4+ T cells. In liver, mCXCR1-like transcripts were identified in residual CD3+ T cells and macrophages, suggesting that mCXCR1-like may regulate inflammatory and immunological processes in the liver. When expressed as a recombinant protein, mCXCR1-like was not activated by a large panel of known CXC chemokines of human and murine origin. These findings suggest that a homolog or ortholog of hCXCR1 is expressed in the mouse to be activated by a hitherto unknown CXC chemokine of the mouse.

The CXCR1 and CXCR2 Receptors Form Constitutive Homo- and Heterodimers Selectively and with Equal Apparent Affinities

Both homo- and heterodimeric interactions between the CXCR1 and CXCR2 chemokine receptors were observed following co-expression of forms of these receptors in HEK293 cells using assays, including co-immunoprecipitation, single cell imaging of fluorescence resonance energy transfer, cell surface time-resolved fluorescence resonance energy transfer, and bioluminescence resonance energy transfer. These interactions were constitutive and unaffected by the presence of the agonist interleukin 8 and selective as no significant interactions were noted between either the CXCR1 or CXCR2 receptor and the alpha(1A)-adrenoreceptor. Saturation bioluminescence resonance energy transfer indicated that heteromeric interactions between CXCR1 and CXCR2 were of similar affinity as the corresponding homomeric interactions. A novel endoplasmic reticulum trapping strategy demonstrated that these interactions were initiated during protein synthesis and maturation and prior to cell surface delivery. These studies indicated that CXCR1-CXCR2 heterodimers are as likely to form in cells co-expressing these two chemokine receptors as the corresponding homodimers and stand in contrast to previous studies indicating an inability of the CXCR1 receptor to homodimerize or to interact with the CXCR2 receptor (Trettel, F., Di Bartolomeo, S., Lauro, C., Catalano, M., Ciotti, M. T., and Limatola, C. (2003) J. Biol. Chem. 278, 40980-40988).

The CXCR1 tail mediates beta1 integrin-dependent cell migration via MAP kinase signaling

In this study, we examined how IL-8 induces leukocyte migration on major beta1 integrin ligands derived from the extracellular matrix protein fibronectin. We assessed individual contributions of signaling by IL-8 receptors by transfection of CXCR1 and CXCR2 into rat basophilic leukemia (RBL) cells and human monocytic THP-1 cells. CXCR1 expressing cells migrated on the fibronectin ligands for alpha4beta1 and alpha5beta1 integrins in response to IL-8, whereas CXCR2 expressing cells did not. RBL cells expressing the chimeric CXCR1 receptor containing the cytoplasmic tail of CXCR2 had greatly blunted migration, while cells expressing the CXCR2 chimera with the tail of CXCR1 had augmented migration. Last, inhibitors of p38 and JNK MAP kinases blocked IL-8-induced migration in CXCR1+ cells. We conclude that IL-8 stimulated beta1 integrin-mediated leukocyte migration on fibronectin through CXCR1 is dependent on the C-terminal cytoplasmic domain of CXCR1 and subsequent p38 and JNK MAPK signaling.

Reversing systemic inflammatory response syndrome with chemokine receptor pepducins

We describe a new therapeutic approach for the treatment of lethal sepsis using cell-penetrating lipopeptides-termed pepducins-that target either individual or multiple chemokine receptors. Interleukin-8 (IL-8), a ligand for the CXCR1 and CXCR2 receptors, is the most potent endogenous proinflammatory chemokine in sepsis. IL-8 levels rise in blood and lung fluids to activate neutrophils and other cells, and correlate with shock, lung injury and high mortality. We show that pepducins derived from either the i1 or i3 intracellular loops of CXCR1 and CXCR2 prevent the IL-8 response of both receptors and reverse the lethal sequelae of sepsis, including disseminated intravascular coagulation and multi-organ failure in mice. Conversely, pepducins selective for CXCR4 cause a massive leukocytosis that does not affect survival. CXCR1 and CXCR2 pepducins conferred nearly 100% survival even when treatment was postponed, suggesting that our approach might be beneficial in the setting of advanced disease.

Platelet Factor 4 and Interleukin-8 CXC Chemokine Heterodimer Formation Modulates Function at the Quaternary Structural Level

The apparent complexity of biology increases as more biomolecular interactions that mediate function become known. We have used NMR spectroscopy and molecular modeling to provide direct evidence that tetrameric platelet factor-4 (PF4) and dimeric interleukin-8 (IL8), two members of the CXC chemokine family, readily interact by exchanging subunits and forming heterodimers via extension of their antiparallel beta-sheet domains. We further demonstrate using functional assays that PF4/IL8 heterodimerization has a direct and significant consequence on the biological activity of both chemokines. Formation of heterodimers enhances the anti-proliferative effect of PF4 on endothelial cells in culture, as well as the IL8-induced migration of CXCR2 vector-transfected Baf3 cells. These results suggest that CXC chemokine biology, and perhaps cytokine biology in general, may be functionally modulated at the molecular level by formation of heterodimers. This concept, in turn, has implications for designing chemokine/cytokine variants with modified biological properties.

The Specificity and Molecular Basis of α1-Adrenoceptor and CXCR Chemokine Receptor Dimerization

It is now well established that rhodopsin-like, family-A G protein-coupled receptors (GPCRs) can exist within homo- and heterodimeric/oligomeric complexes. However, limited information is currently available on the molecular basis of these interactions or their selectivity. Using the alpha1-adrenoceptor family as a model, this has been examined using assays including coimmunoprecipitation, saturation bioluminescence resonance energy transfer (BRET), time-resolved fluorescence resonance energy transfer (FRET), and bimolecular fluorescence complementation. We demonstrate key roles for transmembrane helices I and IV in homodimeric/oligomeric interactions of the alpha1b-adrenoceptor and suggest that other interactions indicate that this GPCR can exist as a higher-order oligomeric complex. Literature reports on heterodimerization between chemokine receptor family members and the effects or otherwise of agonist ligands are complex. It was recently indicated that although the CXCR2 receptor is able to homodimerize, this is not the case for the closely related CXCR1 receptor and that these two GPCRs do not heterodimerize. We have reinvestigated these issues using combinations of coimmunoprecipitation, saturation BRET, and a novel endoplasmic reticulum-trapping strategy. Unlike the previous report, we demonstrate that CXCR1 is able to both homodimerize and heterodimerize with the CXCR2 receptor and that the relative affinity of these interactions suggests that with coexpression of these two GPCRs a random mixture of homo- and heterodimers will be present.

The C-terminal domain LLKIL motif of CXCR2 is required for ligand-mediated polarization of early signals during chemotaxis

HEK293 cells expressing wild-type CXCR2 recruit PH-Akt-GFP to the leading edge of the cell in response to chemokine. However, in cells expressing mutant CXCR2 defective in AP-2 and HIP binding, i.e. with a mutation in the LLKIL motif, PH-Akt-GFP does not localize to the leading edge in response to ligand. Inhibition of Akt/PKB by transfection of HEK 293 cells with a dominant negative (kinase defective) Akt/PKB inhibits CXCR2 mediated chemotaxis. FRET analysis reveals that membrane-bound activated Cdc42 and Rac1 localize to the leading edge of cells expressing wild-type CXCR2 receptor, but not in cells expressing mutant CXCR2. By contrast, when the activation of Cdc42 and Rac1 are monitored by affinity precipitation assay, cells expressing either wild-type or LLKIL mutant receptors show equivalent ligand induction. Altogether, these data suggest that restricted localized activation of Akt/PKB, Rac1 and Cdc42 is crucial for chemotactic responses and that events mediated by the LLKIL motif are crucial for chemotaxis.

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.

Contrasting use of CCR5 structural determinants by R5 and R5X4 variants within a human immunodeficiency virus type 1 primary isolate quasispecies

Macrophagetropic R5 human immunodeficiency virus type 1 (HIV-1) isolates often evolve into dualtropic R5X4 variants during disease progression. The structural basis for CCR5 coreceptor function has been studied in a limited number of prototype strains and suggests that R5 and R5X4 Envs interact differently with CCR5. However, differences between unrelated viruses may reflect strain-specific factors and do not necessarily represent changes resulting from R5 to R5X4 evolution of a virus in vivo. Here we addressed CCR5 domains involved in fusion for a large set of closely related yet functionally distinct variants within a primary isolate swarm, employing R5 and R5X4 Envs derived from the HIV-1 89.6(PI) quasispecies. R5 variants of 89.6(PI) could fuse using either N-terminal or extracellular loop CCR5 sequences in the context of CCR5/CXCR2 chimeras, similar to the unrelated R5 strain JRFL, but R5X4 variants of 89.6(PI) were highly dependent on the CCR5 N terminus. Similarly, R5 89.6(PI) variants and isolate JRFL tolerated N-terminal CCR5 deletions, but fusion by most R5X4 variants was markedly impaired. R5 89.6(PI) Envs also tolerated multiple extracellular domain substitutions, while R5X4 variants did not. In contrast to CCR5 use, fusion by R5X4 variants of 89.6(PI) was largely independent of the CXCR4 N-terminal region. Thus, R5 and R5X4 species from a single swarm differ in how they interact with CCR5. These results suggest that R5 Envs possess a highly plastic capacity to interact with multiple CCR5 regions and support the concept that viral evolution in vivo results from the emergence of R5X4 variants with the capacity to use the CXCR4 extracellular loops but demonstrate less-flexible interactions with CCR5 that are strongly dependent on the N-terminal region.

Ligand-independent CXCR2 dimerization

Homo- and hetero-oligomerization have been reported for several G protein-coupled receptors (GPCRs). The CXCR2 is a GPCR that is activated, among the others, by the chemokines CXCL8 (interleukin-8) and CXCL2 (growth-related gene product beta) to induce cell chemotaxis. We have investigated the oligomerization of CXCR2 receptors expressed in human embryonic kidney cells and generated a series of truncated mutants to determine whether they could negatively regulate the wild-type (wt) receptor functions. CXCR2 receptor oligomerization was also studied by coimmunoprecipitation of green fluorescent protein- and V5-tagged CXCR2. Truncated CXCR2 receptors retained their ability to form oligomers only if the region between the amino acids Ala-106 and Lys-163 was present. In contrast, all of the deletion mutants analyzed were able to form heterodimers with the wt CXCR2 receptor, albeit with different efficiency, competing for wt/wt dimer formation. The truncated CXCR2 mutants were not functional and, when coexpressed with wt CXCR2, interfered with receptor functions, impairing cell signaling and chemotaxis. When CXCR2 was expressed with the AMPA-type glutamate receptor GluR1, CXCR2 dimerization was again impaired in a dose-dependent way, and receptor functions were prejudiced. In contrast, CXCR1, a chemokine receptor that shares many similarities with CXCR2, did not dimerize alone or with CXCR2 and when coexpressed with CXCR2 did not impair receptor signaling and chemotaxis. The formation of CXCR2 dimers was also confirmed in cerebellar neuron cells. Taken together, we conclude from these studies that CXCR2 functions as a dimer and that truncated receptors negatively modulate receptor activities competing for the formation of wt/wt dimers.

Hit-to-lead studies: the discovery of potent, orally bioavailable triazolethiol CXCR2 receptor antagonists

A Hit-to-Lead optimisation programme was carried out on the high throughput screening hit, the triazolethiol 1, resulting in the discovery of the potent, orally bioavailable triazolethiol CXCR2 receptor antagonist 45.

Reactivation of formyl peptide receptors triggers the neutrophil NADPH-oxidase but not a transient rise in intracellular calcium

In neutrophils, coupling of chemoattractants to their cell surface receptor at low temperature (<or=15 degrees C) leads to receptor deactivation/desensitization without any triggering of the superoxide anion-generating NADPH-oxidase. We show that the deactivated formyl peptide receptors (FPRs) can be reactivated/resensitized by the cytoskeleton-disrupting drug cytochalasin B. Such cytoskeleton-dependent receptor reactivation occurs also with the closely related receptors FPR-like-1 and C5aR but not with the receptors for interleukin-8 and platelet-activating factor. The reactivation state was further characterized with FPR as a model. The signals generated by receptor reactivation induced superoxide production that was terminated in 5-8 min, after which the neutrophils entered a new state of homologous deactivation. FPR antagonists were potent inhibitors of the superoxide production induced by the reactivated receptors, suggesting that the occupied receptors turn into an actively signaling state when the cytoskeleton is disrupted. The signals generated by the reactivated receptor were pertussis toxin-sensitive, indicating involvement of a G-protein. However, no transient elevation of intracellular Ca2+ accompanies the NADPH-oxidase activation. This was not due to a general down-regulation of phospholipase C/Ca2+ signaling, and despite the fact that no intracellular Ca2+ transient was generated, protein kinase C still appeared to be involved in the response. Further, phosphatidylinositol 3-kinase, mitogen-activated protein kinase, and MEK all participated in the generation of second messengers from the reactivated receptors.

A G protein-coupled receptor encoded by rhesus rhadinovirus is similar to ORF74 of Kaposi's sarcoma-associated herpesvirus

Rhesus rhadinovirus (RRV) is a gamma-2 herpesvirus and is the rhesus macaque homologue of human herpesvirus 8 (HHV-8), also known as Kaposi's sarcoma-associated herpesvirus. DNA sequence analysis of RRV indicates that it shares numerous open reading frames (ORFs) with HHV-8, including one (ORF74) encoding a seven-transmembrane-spanning G protein-coupled receptor (GPCR) with similarity to cellular chemokine receptors. Examination of the predicted amino acid sequence of RRV ORF74 reveals that it encodes a seven-transmembrane-spanning GPCR sharing 40.8% amino acid sequence identity with HHV-8 ORF74 and 24.1% amino acid sequence identity with rhesus macaque CXCR2. In addition, immunofluorescence studies indicate that an epitope-tagged version of RRV ORF74 is expressed on the surfaces of transfected cells, suggesting that this protein is in fact a membrane receptor. In in vitro cell culture assays, RRV ORF74 possesses transforming potential, as NIH 3T3 clones stably expressing the receptor demonstrate an increased ability to grow in soft agarose and to induce tumor formation in nude mice. Further analysis of RRV ORF74 indicates that expression of the receptor in NIH 3T3 cells causes an increased secretion of vascular endothelial growth factor and activation of the ERK1/2 (p44/42) mitogen-activated protein kinase signaling pathway. The results of these studies suggest that RRV ORF74 encodes a GPCR with properties similar to those of its homologue in HHV-8 and that this gene may play a role in RRV-associated pathogenesis.

Intracellular trafficking of human CXCR1 and CXCR2: regulation by receptor domains and actin-related kinases

In this study we investigated the regulation of CXCR1 and CXCR2 intracellular trafficking. First, we produced a chimeric CXCR2 receptor that contained the internalization motifs of both CXCR2 and CXCR1 (CXCR2: LLKIL sequence; CXCR1: C-terminal phosphorylation sites). Elevated levels of internalization were induced by different ELR-expressing CXC chemokines on the chimeric receptor, as compared to wild-type CXCR2. Analysis of inter-relationships between CXCR1 and CXCR2 during internalization indicated that the exposure of cells that expressed both CXCR1 and CXCR2 to CXCL8 or CXCL6 resulted in decreased levels of CXCR1 internalization as compared to those in cells that expressed only CXCR1. To characterize the role of actin-related components in CXCR1 and CXCR2 trafficking, wortmannin, a potent inhibitor of phosphatidylinositol kinases, was used. The presence of wortmannin during receptor recycling inhibited CXCR1 and CXCR2 re-expression following CXCL8-induced internalization, and resulted in a marked disruption of the proper organization of actin filaments. The kinase-dependent recycling process required CXCR2 C-terminal phosphorylation sites. Our results suggest that actin-related kinases are required for the proper functionality of actin filaments, which are the instrumental factors needed for receptor recycling. In all, CXCR1 and CXCR2 internalization and recycling are tightly regulated by receptor domains and by actin-related kinases.

Hsc/Hsp70 interacting protein (hip) associates with CXCR2 and regulates the receptor signaling and trafficking

The ligand-induced trafficking of chemokine receptors plays a significant role in the regulation of inflammatory processes and human immunodeficiency infection. Although many chemokine receptors have been demonstrated to internalize through clathrin-coated vesicles, a process that involves the binding of arrestins to the receptors, accumulating evidence has suggested the possible existence of other regulators. In a yeast two-hybrid screening using the C-terminal domain of CXCR2 as a bait, the Hsc70-interacting protein (Hip) was identified to interact with CXCR2. Hip binds CXCR2 through its C-terminal domain binding to the C-terminal leucine-rich domain (KILAIHGLI) of CXCR2. Hip associates with CXCR2 or CXCR4 in intact cells, and agonist stimulation increases the association. Mutation of the Ile-Leu motif in the C-terminal domain of CXCR2 blocks the agonist-dependent association of the mutant receptor with Hip. Overexpression of a tetratricopeptide repeat (TPR) deletion mutant form of Hip (Delta TPR), which is unable to bind Hsc70 (Prapapanich, V., Chen, S., Nair, S. C., Rimerman, R. A., and Smith, D. F. (1996) Mol. Endocrinol. 10, 420-431), but retains the ability to bind CXCR2, does not affect CXCR2-mediated mitogen-activated protein kinase activation. However, overexpression of Delta TPR significantly attenuates the agonist-induced internalization of CXCR2 and CXCR4 and attenuates CXCR2-mediated chemotaxis. These findings open the possibility for regulation of chemokine receptor signaling and trafficking by protein chaperone molecules.

Phosphorylation-independent association of CXCR2 with the protein phosphatase 2A core enzyme

Protein phosphatase 2A (PP2A) is postulated to be involved in the dephosphorylation of G protein-coupled receptors. In the present study, we demonstrate that the carboxyl terminus of CXCR2 physically interacts with the PP2A core enzyme, a dimer formed by PP2Ac and PR65, but not with the PP2Ac monomer, suggesting direct interaction of the receptor with PR65. The integrity of a sequence motif in the C terminus of CXCR2, KFRHGL, which is conserved in all CC and CXC chemokine receptors, is required for the receptor binding to the PP2A core enzyme. CXCR2 co-immunoprecipitates with the PP2A core enzyme in HEK293 cells and in human neutrophils. Overexpression of dominant negative dynamin 1 (dynamin 1 K44A) in CXCR2-expressing cells blocks the receptor association with the PP2A core enzyme, and an internalization-deficient mutant form of CXCR2 (I323A,L324A) also exhibits impaired association with the PP2A core enzyme, suggesting that the receptor internalization is required for the receptor binding to PP2A. A phosphorylation-deficient mutant of CXCR2 (331T), which has previously been shown to undergo internalization in HEK293 cells, binds to an almost equal amount of the PP2A core enzyme in comparison with the wild-type CXCR2, suggesting that the interaction of the receptor with PP2A is phosphorylation-independent. The dephosphorylation of CXCR2 is reversed by treatment of the cells with okadaic acid. Moreover, pretreatment of the cells with okadaic acid increases basal phosphorylation of CXCR2 and attenuates CXCR2-mediated calcium mobilization and chemotaxis. Taken together, these data indicate that PP2A is involved in the dephosphorylation of CXCR2. We postulate that this interaction results from direct binding of the regulatory subunit A (PR65) of PP2A to the carboxyl terminus of CXCR2 after receptor sequestration and internalization.

Cloning and chromosomal mapping of an orphan chemokine receptor: mouse RDC1

Degenerate RT-PCR was used to identify a new seven-transmembrane-spanning receptor expressed in astrocytes. A receptor, termed RDC1, displaying the characteristic structural features of a chemokine receptor was cloned. The predicted 362-amino-acid sequence displayed 92% and 91% similarity to the human and dog orphan receptor RDC1, respectively. In addition, RDC1 shares 43% amino acid similarity to rabbit and mouse CXCR2. Transcripts of RDC1 were found in astrocytes, heart, kidney, the mesangial tumor line MES-13, spleen, and neutrophils by means of northern blot. Using linkage analysis of interspecies backcross mice, we localized to chromosome 1 the genes for mouse CXCR2, CXCR4, and RDC1. Mouse RDC1 is linked to and lies between the genes for the mouse CXC chemokine receptors CXCR2 and CXCR4. The combined data of chromosomal location and sequence similarity suggest that RDC1 is an orphan CXC chemokine receptor.