Chemokine receptor dimerization: two are better than one

Chemokine receptors in the central nervous system: role in brain inflammation and neurodegenerative diseases

Chemokines were originally described as chemotactic cytokines involved in leukocyte trafficking. Research over the last decade, however, has shown that chemokine receptors are not restricted to leukocytes. In the brain, chemokine receptors are not only found in microglia (a brain macrophage), but also in astrocytes, oligodendrocytes and neurons. In this review, we describe the spatial and cellular distribution of chemokine receptors in the brain, distinguishing between constitutively and inducibly expressed receptors. We then discuss possible physiological functions, including neuronal migration, cell proliferation and synaptic activity. Evidence is emerging that chemokine receptors are also involved in neuronal death and hence neurodegenerative diseases. Chemokines may induce neuronal death either indirectly (e.g. through activation of microglia killing mechanisms) or directly through activation of neuronal chemokine receptors. Disease processes in which chemokines and their receptors are likely to be involved include multiple sclerosis (MS), Alzheimer's disease (AD), HIV-associated dementia (HAD) and cerebral ischemic disease. The study of chemokines and their receptors in the central nervous system (CNS) is not only relevant for the understanding of brain physiology and pathophysiology, but may also lead to the development of targeted treatments for neurodegenerative diseases.

KSHV-GPCR and CXCR2 transforming capacity and angiogenic responses are mediated through a JAK2-STAT3-dependent pathway

The Kaposi's sarcoma herpesvirus encodes a G-protein-coupled chemokine receptor termed KSHV-GPCR. Expression of this constitutively active GPCR leads to cell transformation and vascular overgrowth characteristic of Kaposi's sarcoma. Previously, we have shown that CXCR2, the closest human homolog, is similarly able to transform cells if continuously stimulated or constitutively activated by amino-acid exchange D138V of the DRY sequence. Here, we demonstrate that STAT3 activation is a prerequisite for transformation in KSHV-GPCR and CXCR2 transfected NIH 3T3 cells. In KSHV-GPCR and D138V transfected cells, STAT-3 is constitutively phosphorylated on Tyr705. In CXCR2 transfected NIH 3T3 cells and human microvascular endothelial cells (HMEC), which express the CXCR2 constitutively, STAT3 is phosphorylated upon stimulation with IL-8 (CXCL8). Focus formation in NIH 3T3 cells transfected with the KSHV-GPCR, CXCR2, or the D138V mutant, was blocked by the specific JAK2 inhibitor AG490. Typical functions of the CXCR2 including actin stress fiber formation, haptotaxis, and the angiogenic response in HMEC shown by tube formation in Matrigel were blocked by AG490. These data suggest that the transforming capacity and migratory responses that are involved in tumor development, metastasis, and angiogenesis in KSHV or CXCR2-expressing cells is at least partially mediated through a JAK2-STAT3 dependent pathway.

The chemokine system -- a major regulator of angiogenesis in health and disease

The chemokine system controls leukocyte trafficking during homeostasis as well as during inflammation and is necessary for the linkage between innate and adaptive immunity. Tissue regulation outside the hematopoietic compartment, for instance, angiogenesis, organogenesis and tumor development, growth and metastasis, is another important function of the chemokine system. The chemokine-mediated regulation of angiogenesis is highly sophisticated and fine tuned, and involves pro-angiogenic chemokines, for instance, CXCL8/IL8 interacting with the CXCR2 receptor, and anti-angiogenic (i.e. angiostatic) chemokines, for instance, CXCL10/IP10 interacting with the CXCR3 receptor. Chemokines also regulate angiogenesis in a receptor-independent manner by means of a perturbation of bFGF and VEGF function. The current review focuses on the influence of the chemokine system in angiogenesis. Examples of the delicate angiogenesis regulation by the chemokine system in, for instance, wound healing and of the dysregulation in, for instance, tumor development are provided along with the interesting phenomenon of molecular piracy of host-encoded genes within the chemokine system. This phenomenon is a general strategy to circumvent and exploit the immune system -- and thereby improve survival -- for many viruses. Yet, a certain group of herpesviruses -- the gamma2-herpesviruses -- encode a functional CXCR2 receptor homolog that is activated by angiogenic chemokines and antagonized by angiostatic chemokines, and this particular gene seems to cause the development of a vascular tumor -- Kaposi's sarcoma -- in the host.

A novel mechanism for JAK2 activation by a G protein-coupled receptor, the CCK2R: implication of this signaling pathway in pancreatic tumor models

To date very few G protein-coupled receptors (GPCRs) have been shown to be connected to the Janus kinase (JAK)/STAT pathway. Thus our understanding of the mechanisms involved in the activation of this signaling pathway by GPCRs remains limited. In addition, little is known about the role of the JAK pathway in the physiological or pathophysiological functions of GPCRs. Here, we described a new mechanism of JAK activation that involves Galpha(q) proteins. Indeed, transfection of a constitutively activated mutant of Galpha(q) (Q209L) in COS-7 cells demonstrated that Galpha(q) is able to associate and activate JAK2. In addition, we showed that this mechanism is used to activate JAK2 by a GPCR principally coupled to G(q), the CCK2 receptor (CCK2R), and involves a highly conserved sequence in GPCRs, the NPXXY motif. In a pancreatic tumor cell line expressing the endogenous CCK2R, we demonstrated the activation of the JAK2/STAT3 pathway by this receptor and the involvement of this signaling pathway in the proliferative effects of the CCK2R. In addition, we showed in vivo that the targeted CCK2R expression in pancreas of Elas-CCK2 mice leads to the activation of JAK2 and STAT3. This process may contribute to the increase of pancreas growth as well as the formation of preneoplastic lesions leading to pancreatic tumor development observed in these transgenic animals.

Identification of the cytoplasmic domains of CXCR4 involved in Jak2 and STAT3 phosphorylation

The chemokine SDF-1alpha transduces G(i)-dependent and -independent signals through CXCR4. Activation of Jak2/STAT3, a G(i)-independent signaling pathway, which plays a major role in survival signals, is known to be activated after SDF-1alpha binding to CXCR4 but the domains of CXCR4 involved in this signaling remain unexplored. Using human embryonic kidney HEK-293 cells stably expressing wild-type or mutated forms of CXCR4, we demonstrated that STAT3 phosphorylation requires the N-terminal part of the third intracellular loop (ICL3) and the tyrosine 157 present at the end of the second intracellular loop (ICL2) of CXCR4. In contrast, neither the conserved Tyr(135) in the DRY motif at the N terminus of ICL2 nor the Tyr(65) and Tyr(76) in the first intracellular loop (ICL1) are involved in this activation. ICL3, which does not contain any tyrosine residues, is needed to activate Jak2. These results demonstrate that two separate domains of CXCR4 are involved in Jak2/STAT3 signaling. The N-terminal part of ICL3 is needed to activate Jak2 after SDF-1alpha binding to CXCR4, leading to phosphorylation of only one cytoplasmic Tyr, present at the C terminus of ICL2, which triggers STAT3 activation. This work has profound implications for the understanding of CXCR4-transduced signaling.

CCL3, acting via the chemokine receptor CCR5, leads to independent activation of Janus kinase 2 (JAK2) and Gi proteins

The interaction of the chemokine receptor, CCR5, expressed in recombinant cells, with different G proteins was investigated and CCR5 was found to interact with Gi, Go and Gq species. Interaction with Gi leads to G protein activation, whereas Gq does not seem to be activated. Additionally, CCR5 activation also leads to phosphorylation of Janus kinase 2 (JAK2). Activation of JAK2 is independent of Gi or Gq activation. Gi protein activation was not prevented by inhibition of JAK, showing that heterotrimeric G protein activation and activation of the JAK/signal transducer and activator of transcription (STAT) pathway are independent of each other.

Chemokines in innate and adaptive host defense: basic chemokinese grammar for immune cells

Chemokines compose a sophisticated communication system used by all our cell types, including immune cells. Chemokine messages are decoded by specific receptors that initiate signal transduction events leading to a multitude of cellular responses, leukocyte chemotaxis and adhesion in particular. Critical determinants of the in vivo activities of chemokines in the immune system include their presentation by endothelial cells and extracellular matrix molecules, as well as their cellular uptake via "silent" chemokine receptors (interceptors) leading either to their transcytosis or to degradation. These regulatory mechanisms of chemokine histotopography, as well as the promiscuous and overlapping receptor specificities of inflammation-induced chemokines, shape innate responses to infections and tissue damage. Conversely, the specific patterns of homeostatic chemokines, where each chemokine is perceived by a single receptor, are charting lymphocyte navigation routes for immune surveillance. This review presents our current understanding of the mechanisms that regulate the cellular perception and pathophysiologic meaning of chemokines.

Dual signaling and effector pathways mediate human eosinophil activation by platelet-activating factor

Platelet-activating factor (PAF) induces various cellular functions in eosinophils including chemotaxis, adhesion, superoxide anion (O2-) production, and degranulation. While PAF shares many biological effects with other chemotactic factors such as N-formyl-methionyl-leucyl-phenylalanine, complement fragments, and lipid mediators, PAF is unique in that its action is relatively resistant to pertussis toxin (PTX), and in activating eosinophils more strongly than neutrophils. In this review we consider how PAF might activate human eosinophils in preference to neutrophils, and discuss possible mechanisms of PAF-induced activation of human eosinophils via two distinct signaling and effector pathways. Recently we analyzed O2- production by eosinophils using a sensitive, real-time chemiluminescence method. Our results showed that in human eosinophils PAF activates two distinct signaling and effector pathways coupled to the PAF receptor: one linked to PTX-sensitive G protein(s) and another to PTX-resistant G protein(s), phosphatidylinositol 3-kinase, and cellular adhesion. This activation of two different G proteins by the eosinophil PAF receptor may explain the strong and diverse biological responses of human eosinophils to PAF.

Purification and biochemical characterization of the D6 chemokine receptor

There is much interest in chemokine receptors as therapeutic targets in diseases such as AIDS, autoimmune and inflammatory disorders, and cancer. Hampering such studies is the lack of accurate three-dimensional structural models of these molecules. The CC-chemokine receptor D6 is expressed at exceptionally high levels in heterologous transfectants. Here we report the purification and biochemical characterization of milligram quantities of D6 protein from relatively small cultures of transfected mammalian cells. Importantly, purified D6 retains full functional activity, shown by displaceable binding of 125I-labelled MIP-1beta (macrophage inflammatory protein-1beta) and by complete binding of the receptor to a MIP-1alpha affinity column. In addition, we show that D6 is decorated on the N-terminus by N-linked glycosylation. Mutational analysis reveals that this glycosylation is dispensable for ligand binding and high expression in transfected cells. Metabolic labelling has revealed the receptor to also be sulphated and phosphorylated. Phosphorylation is ligand independent and is not enhanced by ligand binding and internalization, suggesting similarities with the viral chemokine receptor homologue US28. Like US28, an analysis of the full cellular complement of D6 in transfected cells indicates that >80% is found associated with intracellular vesicular structures. This may account for the high quantities of D6 that can be synthesized in these cells. These unusual properties of D6, and the biochemical characterization described here, leads the way towards work aimed at generating the three-dimensional structure of this seven-transmembrane-spanning receptor.

Regulation of CXCR4 receptor dimerization by the chemokine SDF-1alpha and the HIV-1 coat protein gp120: a fluorescence resonance energy transfer (FRET) study

Both the chemokine SDF-1alpha and the human immunodeficiency virus-1 (HIV-1) coat protein gp120 can bind to CXCR4 chemokine receptors but with different signaling consequences. To understand the molecular basis for these differences, we tagged the rat CXCR4 receptor with enhanced cyan (ECFP) and yellow (EYFP) derivatives of the green fluorescent protein and investigated CXCR4 receptor dimerization in human embryonic kidney (HEK)-tsA201 cells using fluorescence resonance energy transfer (FRET). Elevated FRET was detected under basal conditions from EYFP-CXCR4 and ECFP-CXCR4 receptor-transfected cells indicating a high level of CXCR4 receptor dimerization. In comparison, EYFP-CXCR4 and ECFP-mu-opioid receptor-cotransfected cells displayed a much lower FRET signal. The FRET signal resulting from EYFP-CXCR4- and ECFP-CXCR4-expressing cells could be attenuated by coexpressing nontagged CXCR4 receptors suggesting competition with fluorophore-tagged receptors in the membrane. Nontagged mu-opioid, kappa-opioid, and muscarinic receptors also decreased the FRET between the tagged CXCR4 receptor pairs but to a lesser extent. Application of the CXCR4 receptor agonist SDF-1alpha (50 nM) further increased the FRET signal from tagged CXCR4 receptors, an effect that was inhibited by the CXCR4 antagonist AMD3100. SDF-1alpha had no effect when EYFP-CXCR4 and ECFP-mu-opioid receptors were coexpressed. The effect of gp120IIIB on CXCR4 FRET was dependent on the coexpression of human CD4 (hCD4) when it increased the FRET signal, and this was decreased by AMD3100 pretreatment. FRET analysis of tagged hCD4 constructs demonstrated that there was significant association of hCD4 and CXCR4, as well as hCD4 dimerization. These data suggest that CXCR4 dimerization is involved in SDF-1alpha- and gp120-induced signaling events.

Blocking HIV-1 infection via CCR5 and CXCR4 receptors by acting in trans on the CCR2 chemokine receptor

The identification of chemokine receptors as HIV-1 coreceptors has focused research on developing strategies to prevent HIV-1 infection. We generated CCR2-01, a CCR2 receptor-specific monoclonal antibody that neither competes with the chemokine CCL2 for binding nor triggers signaling, but nonetheless blocks replication of monotropic (R5) and T-tropic (X4) HIV-1 strains. This effect is explained by the ability of CCR2-01 to induce oligomerization of CCR2 with the CCR5 or CXCR4 viral coreceptors. HIV-1 infection through CCR5 and CXCR4 receptors can thus be prevented in the absence of steric hindrance or receptor downregulation by acting in trans on a receptor that is rarely used by the virus to infect cells.

Identification of amino acid residues crucial for chemokine receptor dimerization

Chemokines coordinate leukocyte trafficking by promoting oligomerization and signaling by G protein-coupled receptors; however, it is not known which amino acid residues of the receptors participate in this process. Bioinformatic analysis predicted that Ile52 in transmembrane region-1 (TM1) and Val150 in TM4 of the chemokine receptor CCR5 are key residues in the interaction surface between CCR5 molecules. Mutation of these residues generated nonfunctional receptors that could not dimerize or trigger signaling. In vitro and in vivo studies in human cell lines and primary T cells showed that synthetic peptides containing these residues blocked responses induced by the CCR5 ligand CCL5. Fluorescence resonance energy transfer showed the presence of preformed, ligand-stabilized chemokine receptor oligomers. This is the first description of the residues involved in chemokine receptor dimerization, and indicates a potential target for the modification of chemokine responses.

Tools for anti-inflammatory drug design:In vitro models of leukocyte migration

Inhibiting leukocyte recruitment is now a major focus in the design of novel anti-inflammatory drugs. Following the identification of lead compounds from conventional high-throughput screens using appropriate receptors or enzymes, it is important to validate the action of the compounds in a suitable in vitro model of leukocyte migration. Here, we review a range of different experimental approaches to modelling leukocyte migration, and identify the multi-well filter migration assay as the best compromise between the amount of resources required to screen multiple compounds and the amount of information gained about the effects of the compounds on cell movement behavior. However, there are pitfalls in the interpretation of data obtained using the multi-well filter migration assay, which arise from the imperfect correlation between the number of cells undergoing migration and the inhibitory activity of the test substances. We examine a number of such pitfalls and provide practical approaches to mitigate these problems as far as possible. We recommend a general strategy for screening inhibitors of cell migration using in vitro functional assays. While being more resource intensive than surrogate measures such as calcium flux, functional approaches nevertheless provide superior correlations with anti-inflammatory activity in vivo.

Chemokine-receptor interactions: GPCRs, glycosaminoglycans and viral chemokine binding proteins

A key feature of the immune system is the migration of leukocytes throughout the organism in an effort to patrol for infectious pathogens, tissue damage, and other physiological insults. This remarkable surveillance system is controlled by a family of proteins called chemokines (chemoattractant cytokines), and their respective receptors. Originally discovered because of their role in cell recruitment during inflammation, it is now well recognized that chemokines are also involved in other diverse processes including lymphocyte development and homing, organogenesis, and neuronal communication. While chemokines have evolved largely for host protection, their ability to induce cell damage and inappropriate cell recruitment, can lead to disease. Thus, there is considerable interest in developing antagonists. In this review we emphasize what is known about the structural biology of chemokines, chemokine receptors, and interactions with cell surface glycosaminoglycans. We also briefly describe their role in certain diseases and strategies for interfering with chemokine function that have emerged from mechanistic and structural understanding of their function. Finally we discuss viral mechanisms for sabotaging or manipulating the chemokine system, in part to illustrate the level of molecular mimicry that viruses have achieved and the evolutionary pressure imposed on the immune system by these pathogens.

Inhibition of chemokine receptor function by membrane cholesterol oxidation

Membrane cholesterol is required to maintain chemokine receptor conformation and function for CXCR4 and CCR5. We previously demonstrated that chemokines preferentially bind to receptors within lipid rafts, which are cholesterol- and sphingolipid-rich membrane microdomains. To further elucidate the role of cholesterol in chemokine receptor function, we examined the effects of membrane cholesterol oxidation by cholesterol oxidase (CO), which enzymatically converts cholesterol to 4-cholesten-3-one. Here, we demonstrate that CO treatment (0.25-2.0 U/ml) of human T cells inhibits CXCL12 (SDF-1alpha) and CCL4 (MIP-1beta) binding to cell surface CXCR4 and CCR5, respectively, resulting in the inhibition of chemokine-mediated intracellular calcium mobilization and chemotaxis. The effects were significantly enhanced by cotreatment with low-dose sphingomyelinase (SMase) (0.125 mU/ml), which produced little inhibitory effect by itself. CO and SMase treatment also inhibited HIV-1 infection through CXCR4, but not virus replication. Similar to the removal of membrane cholesterol, CO/SMase treatment induced conformation changes in the chemokine receptors as detected by differential loss in binding of epitope-specific monoclonal antibodies. We conclude that the native form of cholesterol with the hydroxyl group at C3 is critical to CXCR4 and CCR5 conformation and function.

The Chemokine Connection: Hormonal and Embryonic Regulation at the Human Maternal-Embryonic Interface—A Review

Chemokines are small polypeptides that attract specific leukocyte subsets by binding to cell-surface receptors. In reproductive biology, they have been implicated in ovulation, menstruation, and embryo implantation, and pathological processes such as preterm delivery, HIV infection, and endometriosis. It is known that successful implantation requires a functionally normal embryo at the blastocyst stage and a receptive endometrium that is adequately communicated through the implantation process. This crosstalk is highly regulated, with numerous molecules taking part. Accumulated evidence suggests that chemokines produced and received by the endometrial epithelium and the human blastocyst are implicated in this molecular network. Here, we present updated information on the presence and hormonal and embryonic regulation of chemokines and their receptors during human implantation.

Platelet chemokines and chemokine receptors: linking hemostasis, inflammation, and host defense

Blood platelets play critical roles in hemostasis, providing rapid essential protection against bleeding and catalyzing the important slower formation of stable blood clots via the coagulation cascade. They are also involved in protection from infection by phagocytosis of pathogens and by secreting chemokines that attract leukocytes. Platelet function usually is activated by primary agonists such as adenosine diphosphate (ADP), thrombin, and collagen, whereas secondary agonists like adrenalin do not induce aggregation on their own but become highly effective in the presence of low levels of primary agonists. Current research has revealed that chemokines represent an important additional class of agonists capable of causing significant activation of platelet function. Early work on platelet alpha-granule proteins suggested that platelet factor 4, now known as CXCL4, modulated aggregation and secretion induced by low agonist levels. Subsequent reports revealed the presence in platelets of messenger RNA for several additional chemokines and chemokine receptors. Three chemokines in particular, CXCL12 (SDF-1), CCL17 (TARC), and CCL22 (MDC), recently have been shown to be strong and rapid activators of platelet aggregation and adhesion after their binding to platelet CXCR4 or CCR4, when acting in combination with low levels of primary agonists. CXCL12 can be secreted by endothelial cells and is present in atherosclerotic plaques, whereas CCL17 and CCL22 are secreted by monocytes and macrophages. Platelet activation leads to the release of alpha-granule chemokines, including CCL3 (MIP-1alpha), CCL5 (RANTES), CCL7 (MCP-3), CCL17, CXCL1 (growth-regulated oncogene-alpha), CXCL5 (ENA-78), and CXCL8 (IL-8), which attract leukocytes and further activate other platelets. These findings help to provide a direct linkage between hemostasis, infection, and inflammation and the development of atherosclerosis.

Chemokines: roles in leukocyte development, trafficking, and effector function

Chemokines, representing a large superfamily of 8- to 15-kd proteins, were originally discovered through their ability to recruit various cell types into sites of inflammation. It is now clear that these molecules play a much wider role in immune homeostasis, playing key roles in driving the maturation, homing, and activation of leukocytes. In this review we analyze the roles chemokines play in the development, recruitment, and activation of leukocytes. Because signaling from the receptors drives these processes, signal transduction from chemokine receptors will also be reviewed. Taken together, we highlight the various points at which chemokines contribute to allergic inflammation and at which their targeting might contribute to new therapies for type I hypersensitivity reactions.

Membrane incorporation of 22-hydroxycholesterol inhibits chemokine receptor activity

Cell membrane exposure to oxysterols, such as 22-hydroxycholesterol (22-OHC), has previously been shown to induce a suppressive effect on lymphocyte activation. Based on our previous findings that chemokine binding was significantly inhibited by the extraction of membrane cholesterol, we sought to assess the effects of 22-OHC treatment on chemokine ligand-binding and receptor activity. Our results revealed that 22-OHC, but not nonoxidized cholesterol, significantly reduced the binding of both SDF-1alpha and MIP-1beta to human T-cell lines and PBMCs within 1 h of treatment. Incubating the treated cells at 37 degrees C for 1 h reversed a majority of the inhibitory effects on chemokine binding. 22-OHC also inhibited intracellular calcium mobilization and cell migration in response to SDF-1alpha treatment. Interestingly, while the presence of oxysterols in cell membranes significantly inhibits chemokine receptor function, this inhibitory effect does not involve alterations in receptor conformation, expression, or a direct antagonism of chemokine binding. We propose here a novel mechanism for oxysterol-mediated inhibition of chemokine receptor function and the implications for the presence of oxysterols on immune cells.

Characterization of the molecular interactions of interleukin-8 (CXCL8), growth related oncogen alpha (CXCL1) and a non-peptide antagonist (SB 225002) with the human CXCR2

Neutrophil recruitment to inflammatory sites is mediated by two related receptors: CXC chemokine receptors 1 (CXCR1) and 2 (CXCR2). Both receptors share two ligands, interleukin-8 (CXCL8) and GCP-2 (CXCL6), whereas several chemokines, including growth related oncogen alpha (CXCL1) and a non-peptide antagonist (SB 225002) are specific for CXCR2. The objective of this study was to map the different amino acids involved in the binding and activation/inhibition of human CXCR2. This was performed by exchanging non-conserved amino acids of CXCR2 with their counterparts in CXCR1. The mutants generated showed that: (a) for CXCL8 binding, the N-terminus of CXCR1 and the second extra-cellular loop of CXCR2 are determinant, the N-terminus of CXCR2 is not sufficient and the transmembrane domain seven is probably involved; (b) for CXCL1, the N-terminus of CXCR2 is necessary but not sufficient for binding. The activation study indicated that amino acids critical for activation are not necessarily involved in binding process. Finally, the mechanism of binding of a non-peptide antagonist on CXCR2 was investigated: it occurred through epitopes (a) which were disseminated within the receptor, (b) which differed according to the use of CXCL8 or CXCL1 as a competitor and (c) which did not necessarily overlap with agonist binding sites. We also showed that inhibition of binding and inhibition of activation involved different amino acids.

Strategies for chemokine antagonists as therapeutics

Chemokines are responsible for specific recruitment of leukocytes that are involved both in homing as well as in inflammation. Dysregulation of the system results in excessive recruitment to inflammatory sites and thus prevention of this recruitment is an effective anti-inflammatory strategy. Chemokine receptors are not limited only to cellular recruitment but are also the essential co-factor along with CD4 that enable HIV-1 viruses to infect cells. In this review we discuss the various points of intervention that can be addressed to provide anti-inflammatory and anti-HIV infectivity therapeutics. These include prevention of the receptor-ligand interaction, prevention of the chemokine-glycosaminoglycan interaction, interfering with the signaling pathways that are induced upon receptor activation, and modification of receptor trafficking pathways. We summarize the status of the approaches that have been undertaken to produce therapeutics that block chemokine action.

Analysis of ligand-stimulated CC chemokine receptor 5 (CCR5) phosphorylation in intact cells using phosphosite-specific antibodies

Human CC chemokine receptor 5 (CCR5), a member of the superfamily of G protein-coupled receptors, regulates the activation and directed migration of leukocytes and serves as the main coreceptor for the entry of R5 tropic strains of human immunodeficiency viruses. We have previously shown that RANTES/CCL5 binding to CCR5 induces GPCR kinase (GRK)- and protein kinase C (PKC)-mediated phosphorylation of four distinct C-terminal serine residues. To study these phosphorylation events in vivo, we have generated monoclonal antibodies, which specifically react only with either phosphorylated or nonphosphorylated CCR5. These phosphosite-specific antibodies reveal that following ligand stimulation of the receptor serine 337 is exclusively phosphorylated by a PKC-mediated mechanism, while GRKs phosphorylate serine 349. GRK-mediated receptor phosphorylation proceeds in a regular time-dependent manner (t(12) approximately 2 min) with an apparent EC(50) of 5 nm. In contrast, PKC phosphorylates serine 337 at 50-fold lower concentrations and in a very rapid, albeit transient manner. Protein phosphatases that are active at neutral pH and are inhibited by okadaic acid rapidly dephosphorylate phosphoserine 337, but less efficiently phosphoserine 349, in intact cells and in an in vitro assay. Immunofluorescence microscopy demonstrates that phosphorylated receptors accumulate in a perinuclear compartment, which resembles recycling endosomes. This study is the first to analyze in detail the spatial and temporal dynamics of GRK- versus PKC-mediated phosphorylation of a G protein-coupled receptor and its subsequent dephosphorylation on the level of individual phosphorylation sites.

Platelet-activating factor activates two distinct effector pathways in human eosinophils

In granulocytes, platelet-activating factor (PAF) shares many of its biological effects with other chemotactic factors, such as FMLP, complement fragments, and lipid mediators. Two unique effects are that PAF is relatively resistant to pertussis toxin (PTX) and that PAF activates the inflammatory functions of eosinophils more strongly than it activates those of neutrophils. To investigate the molecular mechanisms of the responses of eosinophils to PAF, we analyzed superoxide anion production by a chemiluminescence method that provides real-time kinetic data for the cellular responses. We found that PAF induced bimodal superoxide anion production in human eosinophils, consisting of an intense, but transient, first phase and a larger and sustained second phase. In contrast, PAF induced essentially a transient unimodal response in human neutrophils. The two phases of eosinophil response were mediated by distinct cellular mechanisms: the second phase was highly dependent on cellular adhesion and beta(2) integrins, but the first phase was independent of both adhesion and beta(2) integrins. The upstream signaling mechanisms were also different: the second phase was mediated by PTX-resistant G-protein(s) and through activation of phosphatidylinositol 3-kinase, while the first phase was mediated by PTX-sensitive G-protein(s). Furthermore, the second-phase response was approximately 100-fold more resistant to inhibition by a competitive PAF receptor antagonist than the first phase. Thus, eosinophils and neutrophils react differently to PAF, and PAF activates two separate and distinct effector pathways in human eosinophils. These two activation pathways may explain the eosinophils' strong and diverse biological responses to PAF.

Upregulation of surface feline CXCR4 expression following ectopic expression of CCR5: implications for studies of the cell tropism of feline immunodeficiency virus

Feline CXCR4 and CCR5 were expressed in feline cells as fusion proteins with enhanced green fluorescent protein (EGFP). Expression of the EGFP fusion proteins was localized to the cell membrane, and surface expression of CXCR4 was confirmed by using a cross-species-reactive anti-CXCR4 monoclonal antibody. Ectopic expression of feline CCR5 enhanced expression of either endogenous feline CXCR4 or exogenous feline or human CXCR4 expressed from a retrovirus vector, indicating that experiments investigating the effect of CCR5 expression on feline immunodeficiency virus (FIV) infection must be interpreted with caution. Susceptibility to infection with cell culture-adapted strains of FIV or to syncytium formation following transfection with a eukaryotic vector expressing an env gene from a cell culture-adapted strain of virus correlated with expression of either human or feline CXCR4, whereas feline CCR5 had no effect. In contrast, neither CXCR4 nor CCR5 rendered cells permissive to either productive infection with primary strains of FIV or syncytium formation following transfection with primary env gene expression vectors. Screening a panel of Ghost cell lines expressing diverse human chemokine receptors confirmed that CXCR4 alone supported fusion mediated by the FIV Env from cell culture-adapted viruses. CXCR4 expression was upregulated in Ghost cells coexpressing CXCR4 and CCR5 or CXCR4, CCR5, and CCR3, and susceptibility to FIV infection could be correlated with the level of CXCR4 expression. The data suggest that beta-chemokine receptors may influence FIV infection by modulating the expression of CXCR4.

Functional inactivation of CXC chemokine receptor 4-mediated responses through SOCS3 up-regulation

Hematopoietic cell growth, differentiation, and chemotactic responses require coordinated action between cytokines and chemokines. Cytokines promote receptor oligomerization, followed by Janus kinase (JAK) kinase activation, signal transducers and transactivators of transcription (STAT) nuclear translocation, and transcription of cytokine-responsive genes. These include genes that encode a family of negative regulators of cytokine signaling, the suppressors of cytokine signaling (SOCS) proteins. After binding their specific receptors, chemokines trigger receptor dimerization and activate the JAK/STAT pathway. We show that SOCS3 overexpression or up-regulation, stimulated by a cytokine such as growth hormone, impairs the response to CXCL12, measured by Ca(2+) flux and chemotaxis in vitro and in vivo. This effect is mediated by SOCS3 binding to the CXC chemokine receptor 4 receptor, blocking JAK/STAT and Galpha(i) pathways, without interfering with cell surface chemokine receptor expression. The data provide clear evidence for signaling cross-talk between cytokine and chemokine responses in building a functional immune system.

Analysis of G-protein-coupled receptor dimerization following chemokine signaling

An abundance of information has been generated in recent decades on the signaling events triggered through G-protein-coupled receptors (GPCRs). Nonetheless, the structural changes at the cell surface that provoke receptor activation are only now beginning to be understood. It is becoming clear that receptors are not isolated entities that are activated following ligand binding, but that they interact with other molecules already present or recruited to the vicinity, which results in a wide variety of new signaling possibilities. Understanding receptor interactions with relatives and/or friends on the cell surface is thus critical. The most important point is to determine which of these interactions are "casual" and which give rise to functional consequences.