Differential regulation of CXCR4 and CCR5 endocytosis

Dendrite-selective redistribution of the chemokine receptor CXCR4 following agonist stimulation

The chemokine SDF-1 is a secreted protein that plays a critical role in several aspects of neuron development through interaction with its unique receptor CXCR4. A key mechanism that controls neuron responsiveness to extracellular signals during neuronal growth is receptor endocytosis. Since we previously reported that SDF-1 regulates axon development without affecting the other neurites, we asked whether this could correlate with a compartment-selective trafficking of CXCR4. We thus studied CXCR4 behavior upon SDF-1 exposure in rat hippocampus slices and in transfected neuron cultures. A massive agonist-induced redistribution of CXCR4 in endosomes was observed in dendrites whereas no modification was evidenced in axons. Our data suggest that CXCR4 trafficking may play a role in mediating selective effects of SDF-1 on distinct neuronal membrane subdomains.

Opposite fate of endocytosed CCR7 and its ligands: recycling versus degradation

The chemokine receptor CCR7 and its ligands CCL19 and CCL21 play a crucial role for the homing of lymphocytes and dendritic cells to secondary lymphoid tissues. Nevertheless, how CCR7 senses the gradient of chemokines and how migration is terminated are poorly understood. In this study, we demonstrate that CCR7(-GFP) is endocytosed into early endosomes containing transferrin receptor upon CCL19 binding, but less upon CCL21 triggering. Internalization of CCR7 was independent of lipid rafts but relied on dynamin and Eps15 and was inhibited by hypertonic sucrose, suggesting clathrin-dependent endocytosis. After chemokine removal, internalized CCR7 recycled back to the plasma membrane and was able to mediate migration again. In contrast, internalized CCL19 was sorted to lysosomes for degradation, showing opposite fate for endocytosed CCR7 and its ligand.

Expression of Nef downregulates CXCR4, the major coreceptor of human immunodeficiency virus, from the surfaces of target cells and thereby enhances resistance to superinfection

Lentiviral Nef proteins are key factors for pathogenesis and are known to downregulate functionally important molecules, including CD4 and major histocompatibility complex class I (MHC-I), from the surfaces of infected cells. Recently, we demonstrated that Nef reduces cell surface levels of the human immunodeficiency virus type 1 (HIV-1) entry coreceptor CCR5 (N. Michel, I. Allespach, S. Venzke, O. T. Fackler, and O. T. Keppler, Curr. Biol. 15:714-723, 2005). Here, we report that Nef downregulates the second major HIV-1 coreceptor, CXCR4, from the surfaces of HIV-infected primary CD4 T lymphocytes with efficiencies comparable to those of the natural CXCR4 ligand, stromal cell-derived factor-1 alpha. Analysis of a panel of mutants of HIV-1(SF2) Nef revealed that the viral protein utilized the same signature motifs for downmodulation of CXCR4 and MHC-I, including the proline-rich motif P(73)P(76)P(79)P(82) and the acidic cluster motif E(66)E(67)E(68)E(69.) Expression of wild-type Nef, but not of specific Nef mutants, resulted in a perinuclear accumulation of the coreceptor. Remarkably, the carboxy terminus of CXCR4, which harbors the classical motifs critical for basal and ligand-induced receptor endocytosis, was dispensable for the Nef-mediated reduction of surface exposure. Functionally, the ability of Nef to simultaneously downmodulate CXCR4 and CD4 correlated with maximum-level protection of Nef-expressing target cells from fusion with cells exposing X4 HIV-1 envelopes. Furthermore, the Nef-mediated downregulation of CXCR4 alone on target T lymphocytes was sufficient to diminish cells' susceptibility to X4 HIV-1 virions at the entry step. The downregulation of chemokine coreceptors is a conserved activity of Nef to modulate infected cells, an important functional consequence of which is an enhanced resistance to HIV superinfection.

Genetic variation in the CCL18-CCL3-CCL4 chemokine gene cluster influences HIV Type 1 transmission and AIDS disease progression

CCL3 (MIP-1 alpha), CCL4 (MIP-1 beta), and CCL18 (DC-CK1/PARC/AMAC-1) are potent chemoattractants produced by macrophages, natural killer cells, fibroblasts, mast cells, CD4(+) T cells, and CD8(+) T cells. CCL3 and CCL4 are natural ligands for the primary human immunodeficiency virus type 1 (HIV-1) coreceptor CCR5 and are also known to activate and enhance the cytotoxicity of natural killer cells. Genomic DNAs from >3,000 participants enrolled in five United States-based natural-history cohorts with acquired immunodeficiency syndrome (AIDS) were genotyped for 21 single-nucleotide polymorphisms (SNPs) in a 47-kb interval on chromosome 17q12 containing the genes CCL3, CCL4, and CCL18. All 21 SNPs were polymorphic in African Americans (AAs), whereas 7 of the 21 had minor-allele frequencies <0.01 in European Americans (EAs). Substantial linkage disequilibrium was observed in a 37-kb interval containing 17 SNPs where many pairwise D' values exceeded 0.70 in both racial groups, but particularly in EAs. Four and three haplotype blocks were observed in AAs and EAs, respectively. Blocks were strongly correlated with each other, and common haplotype diversity within blocks was limited. Two significant associations are reported that replicate an earlier study. First, among AA members of the AIDS Link to the Intravenous Experience cohort of injection drug users, frequencies of three correlated SNPs covering 2,231 bp in CCL3 were significantly elevated among highly exposed, persistently HIV-1-uninfected individuals compared with HIV-1-infected seroconvertors (P = .02-.03). Second, seven highly correlated SNPs spanning 36 kb and containing all three genes were significantly associated with more-rapid disease progression among EAs enrolled in the Multicenter AIDS Cohort Study cohort (P = .01-.02). These results reiterate the importance of chemokine gene variation in HIV-1/AIDS pathogenesis and emphasize that localized linkage disequilibrium makes the identification of causal mutations difficult.

The Nef protein of human immunodeficiency virus is a broad-spectrum modulator of chemokine receptor cell surface levels that acts independently of classical motifs for receptor endocytosis and Galphai signaling

Chemokine receptors (CKRs) are important physiological mediators of immune defense, inflammatory responses, and angiogenesis, and they have also been implicated in a number of viral disease processes. Here, we report that the Nef protein of human immunodeficiency virus (HIV) reduces cell surface levels of eight different members of the CC- and CXC-family of CKRs by up to 92%. This broad-range activity required specific elements in HIV(SF2) Nef, including the proline-rich motif P73P76P79P82 as well as the acidic cluster motif E66E67E68E69, and Nef expression induced a marked perinuclear accumulation of CKRs. Surprisingly, receptor mutagenesis demonstrated that the cytoplasmic tail of CCR5 and CXCR4, which is critical for basal and ligand-mediated endocytosis, was completely dispensable for this Nef activity. In contrast, triple-mutation of the highly conserved DRY motif in the second intracellular CKR loop abolished the Nef-mediated down-regulation of CXCR4 independently of this motif's role in CKR binding to heterotrimeric G proteins and signaling via the Galphai subunit. Thus, we identify the lentiviral pathogenicity factor Nef as a unique and broad-range modulator of CKR cell surface levels. Nef uses a mechanism that is distinct from well-established pathways orchestrating CKR metabolism and offers an interesting tool to study the multifaceted biology of CKRs.

Lower expression of CXCR4 in lymph node metastases than in primary breast cancers: potential regulation by ligand-dependent degradation and HIF-1alpha

Stromal-derived factor-1 (SDF-1) is a unique ligand of the CXC chemokine receptor 4 (CXCR4), which is critically involved in the metastasis of breast cancer. High levels of SDF-1 in the common destination organs of metastasis, such as the lymph nodes, lungs, liver, and bones, attract CXCR4-positive tumor cells. The interaction between SDF-1 and CXCR4 leads to the activation of specific signaling pathways, allowing for homing and metastatic progression. However, regulation of CXCR4 expression at the metastatic organ site is not well-documented. We detected the expression of CXCR4 and hypoxia inducible factor (HIF)-1alpha in breast tumor tissues by immunohistochemical staining and analyzed SDF-1 in primary tumors and lymph nodes using real-time RT-PCR. Compared to the corresponding metastasized tumors in the lymph nodes, primary invasive carcinomas showed more intense staining for CXCR4, particularly on the cellular membrane. Both primary tumors and lymph node metastases exhibited higher levels of CXCR4 expression compared to non-neoplastic breast tissues. Therefore, we hypothesized that the tumor environment in the lymph nodes may cause the reduction of CXCR4 levels in the metastatic tumor cells because of: (1) high SDF-1 levels and (2) lower levels of HIF-1alpha. Our in vitro data demonstrated that high levels of SDF-1 can induce the internalization and degradation of CXCR4 through the lysosome pathway. In addition, lower levels of HIF-1alpha in the lymph node metastases, probably induced by the less hypoxic environment, further lowered CXCR4 levels. These results indicate that ligand-dependent degradation and lower HIF-1alpha levels may be potential causes of lowered levels of CXCR4 in the lymph nodes compared to the primary tumors. Our study suggests that CXCR4 levels in tumor cells are regulated by its microenvironment. These findings may enhance our ability to understand the biological behavior of breast cancers.

Expression and regulation of the orphan receptor RDC1 and its putative ligand in human dendritic and B cells

Based on phylogenetic analysis and chromosomal mapping, the orphan receptor RDC1 was proposed to be a chemokine receptor. In this study we examined the expression of RDC1 on leukocytes by measuring mRNA levels and receptor expression using a new specific mAb. Both mRNA and protein levels were high in monocytes and B cells, relatively low on immature dendritic cells (DC), and up-regulated during final stages of maturation. Strikingly, in mature plasmacytoid DC the mRNA was up-regulated, but did not correlate with protein surface expression. We indeed report that CpG-activated plasmacytoid DC produce a putative ligand for RDC1, which selectively down-regulates RDC1, but not CXCR4 on primary human B cells. RDC1 expression was found to be tightly regulated during B cell development and differentiation. In blood-derived switch memory B cells, the expression of RDC1 appeared to correlate with the ability to differentiate into plasma cells upon activation, suggesting that RDC1 is a marker for memory B cells, which are competent to become Ab-secreting cells.

Sporadic case of warts, hypogammaglobulinemia, immunodeficiency, and myelokathexis syndrome

The term WHIM syndrome (WHIMS) is an acronym describing a rare primary immunodeficiency disorder characterized by warts, hypogammaglobulinemia, immunodeficiency, and myelokathexis, the unusual association of neutropenia with bone marrow myeloid hypercellularity. WHIMS was recently associated with mutations in the gene encoding the chemokine receptor CXCR4 and as such is the first disease ascribed to abnormalities of chemokine signaling. We report a sporadic case of WHIMS in a woman presenting with recurrent infections and human papilloma virus-related genital dysplasia.

G protein-coupled receptor kinases promote phosphorylation and beta-arrestin-mediated internalization of CCR5 homo- and hetero-oligomers

Expression levels of the chemokine receptor, CC chemokine receptor 5 (CCR5), at the cell surface determine cell susceptibility to HIV entry and infection. Cellular activation by CCR5 itself, but also by unrelated receptors leads to cross-phosphorylation and cross-internalization of CCR5. This study addresses the underlying molecular mechanisms of homologous and heterologous CCR5 regulation. As shown by bioluminescence resonance energy transfer experiments, CCR5 formed constitutive homo- as well as heterooligomeric complexes together with C5aR but not with the unrelated AT(1a)R in living cells. Stimulation with CCL5 of RBL cells, which co-expressed CCR5 together with an N-terminally truncated CCR5-DeltaNT mutant, resulted in both protein kinase C (PKC)- and G protein-coupled receptor (GPCR) kinase (GRK)-mediated cross-phosphorylation of the mutant unligated receptor, as determined by phosphosite-specific monoclonal antibody. Similarly, both PKC and GRK cross-phosphorylated CCR5 in a heterologous manner after C5a stimulation of RBL-CCR5/C5aR cells, whereas AT(1a)R stimulation resulted only in classical PKC-mediated CCR5 phosphorylation. Co-expression of CCR5-DeltaNT together with a phosphorylation-deficient CCR5 mutant that neither binds beta-arrestin nor undergoes internalization partially restored the CCL5-induced association of beta-arrestin with the homo-oligomeric receptor complex and augmented cellular uptake of (125)I-CCL5. Co-expression of C5aR, but not of AT(1a)R, promoted CCR5 co-internalization upon agonist stimulation by a mechanism independent of CCR5 phosphorylation. Co-internalization of phosphorylated CCR5 was also observed in C5a-stimulated macrophages. Finally, co-expression of a constitutively internalized C5aR-US28(CT) mutant led to intracellular accumulation of CCR5 in the absence of ligand stimulation. These results show that GRKs and beta-arrestin are involved in heterologous receptor regulation by cross-phosphorylating and co-internalizing unligated receptors within homo- or hetero-oligomeric protein complexes.

Mechanisms of HIV receptor and co-receptor down-regulation by prostratin: role of conventional and novel PKC isoforms

Prostratin is an unusual non-tumour promoting phorbol ester with potential as an inductive adjuvant therapy for highly active antiretroviral therapy (HAART) due to its ability to up-regulate viral expression from latent provirus. In addition, prostratin is also able to inhibit de novo HIV infection most probably because it induces down-regulation of HIV receptors from the surface of target cells. In this study, we investigate the mechanisms by which prostratin down-regulates HIV receptor and co-receptor surface expression in lymphocytic and monocytic cell lines. Our results indicate that prostratin induces down-regulation of surface expression of CD4 and CXCR4, but not CCR5, in various cell lines. Down-regulation of CD4 and CXCR4 by prostratin is achieved by internalization through receptor-mediated endocytosis and/or macropinocytosis, which is then followed by degradation of these molecules. Because prostratin is a protein kinase C (PKC) activator, we next examined the potential contribution of distinct PKC isoforms to down-regulate CD4 and CXCR4 in response to prostratin stimulation. Although exposure of cells to prostratin or phorbol-myristate-acetate (PMA) induces the translocation of several PKC isoforms to the plasma membrane, the use of specific PKC inhibitors revealed that novel PKCs are the main mediators of the prostratin-induced CD4 down-regulation, whereas both conventional and novel PKCs contribute to CXCR4 down-regulation. Altogether these results showed that prostratin, through the activation of conventional and/or novel PKC isoforms, rapidly reduces cell surface expression of CD4 and CXCR4, but not CCR5, by inducing their internalization and degradation.

CD8+ T cell dynamics during primary simian immunodeficiency virus infection in macaques: relationship of effector cell differentiation with the extent of viral replication

Immunological and virological events that occur during the earliest stages of HIV-1 infection are now considered to have a major impact on subsequent disease progression. We observed changes in the frequencies of CD8(bright) T cells expressing different chemokine receptors in the peripheral blood and lymph nodes of rhesus macaques during the acute phase of the pathogenic SIVmac251 infection; the frequency of CD8(bright) T cells expressing CXCR4 decreased, while the frequency of those expressing CCR5 increased. These reciprocal changes in chemokine receptor expression were associated with changes in the proportion of cycling (Ki67(+)) CD8(bright) T cells, and with the pattern of CD8(bright) T cell differentiation as defined by expression of CCR7 and CD45RA. In contrast, during the primary phase of the attenuated SIVmac251Deltanef infection, no major change was observed. Whereas during the acute phase of the infection with pathogenic SIV (2 wk postinfection) no correlate of disease protection was identified, once the viral load set points were established (2 mo postinfection), we found that the levels of cycling and of CCR5- and CXCR4-positive CD8(bright) T cells were correlated with the extent of viral replication and therefore with SIV-infection outcome. Our data reveal that, during primary SIV infection, despite intense CD8 T cell activation and an increase in CCR5 expression, which are considered as essential for optimal effector function of CD8(+) T cells, these changes are associated with a poor prognosis for disease progression to AIDS.

Chemokine receptor CCR5: insights into structure, function, and regulation

CC chemokine receptor 5 (CCR5) is a seven-transmembrane, G protein-coupled receptor (GPCR) which regulates trafficking and effector functions of memory/effector T-lymphocytes, macrophages, and immature dendritic cells. It also serves as the main coreceptor for the entry of R5 strains of human immunodeficiency virus (HIV-1, HIV-2). Chemokine binding to CCR5 leads to cellular activation through pertussis toxin-sensitive heterotrimeric G proteins as well as G protein-independent signalling pathways. Like many other GPCR, CCR5 is regulated by agonist-dependent processes which involve G protein coupled receptor kinase (GRK)-dependent phosphorylation, beta-arrestin-mediated desensitization and internalization. This review discusses recent advances in the elucidation of the structure and function of CCR5, as well as the complex mechanisms that regulate CCR5 signalling and cell surface expression.

The chemokine SDF-1 differentially regulates axonal elongation and branching in hippocampal neurons

Recent data have shown that the chemokine SDF-1 plays a critical role in several aspects of brain development such as cell migration and axon pathfinding. However, its potential function in the generation of axons and dendrites is poorly characterized. In order to better understand the role of SDF-1 in the development of central neurons, we studied the cellular distribution of the SDF-1 receptor CXCR4 by immunocytochemistry of developing hippocampal neurons and tested the effect of SDF-1 in process patterning at the early stages of neuronal development. We found that CXCR4 immunoreactivity undergoes a striking redistribution during development. At the early stages, from day 2 to day 4 in culture, CXCR4 is particularly concentrated at the leading edge of growing neurites. As the cells mature, staining declines at the tip of the processes and becomes more broadly distributed along axons and, to a lesser extent, dendrites. SDF-1 stimulation of neurons at day 1-2 in culture triggers several effects on neuronal morphogenesis. SDF-1 reduces growth cone number and axonal outgrowth but stimulates axonal branching. These latter two effects are not observed in other neurites. This study unravels a new role for SDF-1/CXCR4 in specifying hippocampal neuron morphology by regulating axonal patterning at an early stage of neuronal development.

Protecting from R5-tropic HIV: individual and combined effectiveness of a hammerhead ribozyme and a single-chain Fv antibody that targets CCR5

The CCR5 chemokine receptor is important for most clinical strains of HIV to establish infection. Individuals with naturally occurring polymorphisms in the CCR5 gene who have reduced or absent CCR5 are apparently otherwise healthy, but are resistant to HIV infection. With the goal of reducing CCR5 and protecting CCR5+ cells from R5-tropic HIV, we used Tag-deleted SV40-derived vectors to deliver several anti-CCR5 transgenes: 2C7, a single-chain Fv (SFv) antibody; VCKA1, a hammerhead ribozyme; and two natural CCR5 ligands, MIP-1alpha and MIP-1beta, modified to direct these chemokines, and hence their receptor to the endoplasmic reticulum. These transgenes were delivered using recombinant, Tag-deleted SV40-derived vectors to human CCR5+ cell lines and primary cells: monocyte-derived macrophages and brain microglia. All transgenes except MIP-1alpha decreased CCR5, as assayed by immunostaining, Northern blotting, and cytofluorimetry (FACS). Individually, all transgenes except MIP-1alpha protected from low challenge doses of HIV. At higher dose HIV challenges, protection provided by all transgenes diminished, the SFv and the ribozyme being most potent. Vectors carrying these two transgenes were used sequentially to deliver combination anti-CCR5 genetic therapy. This approach gave approximately additive reduction in CCR5, as measured by FACS and protected from higher dose HIV challenges. Reducing cell membrane CCR5 using anti-CCR5 transgenes, alone or in combinations, may therefore provide a degree of protection from R5-tropic strains of HIV.

Multiphoton-FLIM quantification of the EGFP-mRFP1 FRET pair for localization of membrane receptor-kinase interactions

We present an improved monomeric form of the red fluorescent protein, mRFP1, as the acceptor in biological fluorescence resonance energy transfer (FRET) experiments using the enhanced green fluorescent protein as donor. We find particular advantage in using this fluorophore pair for quantitative measurements of FRET using multiphoton fluorescence lifetime imaging microscopy (FLIM). The technique was exploited to demonstrate a novel receptor-kinase interaction between the chemokine receptor (CXCR4) and protein kinase C (PKC) alpha in carcinoma cells for both live- and fixed-cell experiments. The CXCR4-EGFP: PKCalpha-mRFP1 complex was found to be localized precisely to intracellular vesicles and cell protrusions when imaged by multiphoton fluorescence-FLIM. A comparison of the FRET efficiencies obtained using mRFP1-tagged regulatory domain or full-length PKCalpha as the acceptor revealed that PKCalpha, in the closed (inactive) form, is restrained from associating with the cytoplasmic portion of CXCR4. Live-cell FLIM experiments show that the assembly of this receptor:kinase complex is concomitant with the endocytosis process. This is confirmed by experimental evidence suggesting that the recycling of the CXCR4 receptor is increased on stimulation with phorbol ester and blocked on inhibition of PKC by bisindolylmaleimide. The EGFP-mRFP1 couple should be widely applicable, particularly to live-cell quantitative FRET assays.

CXCR4 mutations in WHIM syndrome: a misguided immune system?

Chemokines and their receptors are key molecules in the development and function of immune cell populations and the organization of lymphoid organs. Despite their central role in immunologic function, genetic studies exploring the intersection of chemokines or their receptors and human health have revealed few associations of unambiguous significance. The best-characterized examples have revealed striking selective advantage conferred by loss of receptors used as portals of entry by pathogens. Recently, mutations in the CXCR4 chemokine receptor gene were identified in a dominantly inherited immunodeficiency disease, WHIM syndrome. Genetic and biochemical evidences suggest that the loss of the receptor cytoplasmic tail domain results in aberrant signaling. Analyses of mutant cell responses to the receptor ligand CXCL12 have revealed enhanced chemotaxis, confirming the gain-of-function effect of the truncation mutations. The clinical features and potential mechanism of immunodeficiency in WHIM syndrome patients are discussed in this review.

Agonist-induced endocytosis of CC chemokine receptor 5 is clathrin dependent

The signaling activity of several chemokine receptors, including CC chemokine receptor 5 (CCR5), is in part controlled by their internalization, recycling, and/or degradation. For CCR5, agonists such as the chemokine CCL5 induce internalization into early endosomes containing the transferrin receptor, a marker for clathrin-dependent endocytosis, but it has been suggested that CCR5 may also follow clathrin-independent routes of internalization. Here, we present a detailed analysis of the role of clathrin in chemokine-induced CCR5 internalization. Using CCR5-transfected cell lines, immunofluorescence, and electron microscopy, we demonstrate that CCL5 causes the rapid redistribution of scattered cell surface CCR5 into large clusters that are associated with flat clathrin lattices. Invaginated clathrin-coated pits could be seen at the edge of these lattices and, in CCL5-treated cells, these pits contain CCR5. Receptors internalized via clathrin-coated vesicles follow the clathrin-mediated endocytic pathway, and depletion of clathrin with small interfering RNAs inhibits CCL5-induced CCR5 internalization. We found no evidence for CCR5 association with caveolae during agonist-induced internalization. However, sequestration of cholesterol with filipin interferes with agonist binding to CCR5, suggesting that cholesterol and/or lipid raft domains play some role in the events required for CCR5 activation before internalization.

Inhibition of human immunodeficiency virus replication by a dual CCR5/CXCR4 antagonist

Here we report that the N-pyridinylmethyl cyclam analog AMD3451 has antiviral activity against a wide variety of R5, R5/X4, and X4 strains of human immunodeficiency virus type 1 (HIV-1) and HIV-2 (50% inhibitory concentration [IC(50)] ranging from 1.2 to 26.5 microM) in various T-cell lines, CCR5- or CXCR4-transfected cells, peripheral blood mononuclear cells (PBMCs), and monocytes/macrophages. AMD3451 also inhibited R5, R5/X4, and X4 HIV-1 primary clinical isolates in PBMCs (IC(50), 1.8 to 7.3 microM). A PCR-based viral entry assay revealed that AMD3451 blocks R5 and X4 HIV-1 infection at the virus entry stage. AMD3451 dose-dependently inhibited the intracellular Ca(2+) signaling induced by the CXCR4 ligand CXCL12 in T-lymphocytic cells and in CXCR4-transfected cells, as well as the Ca(2+) flux induced by the CCR5 ligands CCL5, CCL3, and CCL4 in CCR5-transfected cells. The compound did not interfere with chemokine-induced Ca(2+) signaling through CCR1, CCR2, CCR3, CCR4, CCR6, CCR9, or CXCR3 and did not induce intracellular Ca(2+) signaling by itself at concentrations up to 400 microM. In freshly isolated monocytes, AMD3451 inhibited the Ca(2+) flux induced by CXCL12 and CCL4 but not that induced by CCL2, CCL3, CCL5, and CCL7. The CXCL12- and CCL3-induced chemotaxis was also dose-dependently inhibited by AMD3451. Furthermore, AMD3451 inhibited CXCL12- and CCL3L1-induced endocytosis in CXCR4- and CCR5-transfected cells. AMD3451, in contrast to the specific CXCR4 antagonist AMD3100, did not inhibit but enhanced the binding of several anti-CXCR4 monoclonal antibodies (such as clone 12G5) at the cell surface, pointing to a different interaction with CXCR4. AMD3451 is the first low-molecular-weight anti-HIV agent with selective HIV coreceptor, CCR5 and CXCR4, interaction.

pH-Independent Endocytic Cycling of the Chemokine Receptor CCR5

Following agonist activation, the chemokine receptor CCR5 is internalised through clathrin-coated pits and delivered to recycling endosomes. Subsequently, ligand- free and resensitised receptors are recycled to the cell surface. Currently little is known of the mechanisms regulating resensitisation and recycling of this G-protein coupled receptor. Here we show that raising the pH of endocytic compartments, using bafilomycin A, monensin or NH(4)Cl, does not significantly affect CCR5 endocytosis, recycling or dephosphorylation. By contrast, these reagents inhibited recycling of another well-characterised G protein coupled receptor, the beta(2)-adrenergic receptor, following agonist-induced internalisation. CCR5-bound RANTES (CCL5) and MIP-1beta (CCL4) only exhibit pH-dependent dissociation at pH < 4.0, below the values normally found in endocytic organelles. Although receptor-agonist dissociation is not dependent on low pH, the subsequent degradation of released chemokine is inhibited in the presence of reagents that raise endosomal pH. Our data show that exposure to low pH is not required for RANTES or MIP-1beta dissociation from CCR5, or for recycling of internalised CCR5 to the cell surface.

Intracellular localization and constitutive endocytosis of CXCR4 in human CD34+ hematopoietic progenitor cells

CXCR4, the stromal cell-derived factor-1 receptor, plays an important role in the migration of hematopoietic progenitor/stem cells. The surface and cytoplasmic expression of CXCR4 on human hematopoietic CD34(+) cells was investigated. We show that its surface expression is low, whereas a large part of CXCR4 protein is sequestered intracellularly. Using confocal microscopy, we demonstrated that CXCR4 is colocalized with EEA-1, Rab5, Rab4, and Rab11, which are localized in early and recycling endosomes. No significant colocalization of CXCR4 with lysosomal markers CD63 and Lamp-1 was detected. Using antibody feeding experiments, we report a role for CXCR4 constitutive endocytosis in subcellular localization in stably transduced UT7-CXCR4-GFP and CD34(+) cells. Agonist-independent endocytosis of CXCR4 occurs through clathrin-coated vesicles. These data implicate a constitutive endocytosis in the regulation of CXCR4 membrane expression and suggest that constitutive endocytosis may be involved in the regulation of trafficking the human hematopoietic progenitor/stem cells to and in the bone marrow microenvironment.

Molecular mechanism of AMD3100 antagonism in the CXCR4 receptor: transfer of binding site to the CXCR3 receptor

AMD3100 is a symmetric bicyclam, prototype non-peptide antagonist of the CXCR4 chemokine receptor. Mutational substitutions at 16 positions located in TM-III, -IV, -V, -VI, and -VII lining the main ligand-binding pocket of the CXCR4 receptor identified three acid residues: Asp(171) (AspIV:20), Asp(262) (AspVI:23), and Glu(288) (GluVII:06) as the main interaction points for AMD3100. Molecular modeling suggests that one cyclam ring of AMD3100 interacts with Asp(171) in TM-IV, whereas the other ring is sandwiched between the carboxylic acid groups of Asp(262) and Glu(288) from TM-VI and -VII, respectively. Metal ion binding in the cyclam rings of AMD3100 increased its dependence on Asp(262) and provided a tighter molecular map of the binding site, where borderline mutational hits became clear hits for the Zn(II)-loaded analog. The proposed binding site for AMD3100 was confirmed by a gradual build-up in the rather distinct CXCR3 receptor, for which the compound normally had no effect. Introduction of only a Glu at position VII:06 and the removal of a neutralizing Lys residue at position VII:02 resulted in a 1000-fold increase in affinity of AMD3100 to within 10-fold of its affinity in CXCR4. We conclude that AMD3100 binds through interactions with essentially only three acidic anchor-point residues, two of which are located at one end and the third at the opposite end of the main ligand-binding pocket of the CXCR4 receptor. We suggest that non-peptide antagonists with, for example, improved oral bioavailability can be designed to mimic this interaction and thereby efficiently and selectively block the CXCR4 receptor.

Targeting CCR5 with siRNAs: Using Recombinant SV40-Derived Vectors to Protect Macrophages and Microglia from R5-Tropic HIV

Transducing macrophages and other phagocytic cells has been problematic because these cells are largely nondividing and can phagocytose and degrade viral gene delivery vectors. Because of their carriage of the CCR5 chemokine receptor that functions as a coreceptor for most clinical strains of HIV, these cells are also key targets in early HIV infection and dissemination. We describe here a strategy to transduce these phagocytes, reduce cell membrane CCR5, and protect from infection with R5-tropic HIV. Recombinant Tag-deleted SV40 vectors were used to transduce unselected CCR5-bearing cell lines and primary cells with >98% efficiency. rSV40s were designed to express two different anti-CCR5 small interfering RNAs (siRNAs), driven by the adenoviral VA1 polymerase III (pol III) promoter, which localizes the transcripts in the cytoplasm. Transduction with both siRNAs substantially reduced CCR5 mRNA, which in turn decreased detectable cell membrane CCR5. Both CCR5+ cell lines and primary cells were used: SupT1/CCR5 cells, monocyte-derived macrophages (MDM), and primary human brain microglia. In addition, one siRNA, siRNA R5 #5, was designed to recognize conserved sequences in both murine and human CCR5 mRNA and effectively reduced CCR5 transcript in cells of both species. These siRNAs largely protected CCR5+ cell lines and primary human macrophages and brain microglia from challenge with R5-tropic HIV. Therefore, strategies to target CCR5 using rSV40-delivered, VA promoter-driven siRNAs may be useful therapeutic options for treating HIV infection.

Predominant Intracellular Expression of CXCR4 and CCR5 in Purified Primary Trophoblast Cells from First Trimester and Term Human Placentae

PROBLEM

The aim of the present study was to define the expression of CXCR4 and CCR5 on non-cultured non-stimulated primary human trophoblast cells (TCs) immediately after their immunopurification.

METHOD OF STUDY

We have evaluated by flow cytometric analysis and immunofluorescence, highly purified primary TCs prepared from first trimester (8.2 +/- 0.3 weeks, n = 15) and term (Caesarean section, n = 10) placentae for the cell surface and intracellular expression of CXCR4 and CCR5.

RESULTS

There was a high level of individual variability for CXCR4 and CCR5 expression between trophoblast batches. In first trimester and term placentae TCs, we found a greater number of TCs preparations expressing intracellular CXCR4 than CCR5 (P < 0.05). Both receptors were predominantly localized in the intracellular compartment of TCs, whatever if isolated from first trimester or term placentae.

CONCLUSIONS

The functional consequences of the predominance of CXCR4 expression and of cellular addressing are briefly discussed.

Distinct mechanisms of agonist-induced endocytosis for human chemokine receptors CCR5 and CXCR4

Desensitization of the chemokine receptors, a large class of G protein-coupled receptors, is mediated in part by agonist-driven receptor endocytosis. However, the exact pathways have not been fully defined. Here we demonstrate that the rate of ligand-induced endocytosis of CCR5 in leukocytes and expression systems is significantly slower than that of CXCR4 and requires prolonged agonist treatment, suggesting that these two receptors use distinct mechanisms. We show that the C-terminal domain of CCR5 is the determinant of its slow endocytosis phenotype. When the C-tail of CXCR4 was exchanged for that of CCR5, the resulting CXCR4-CCR5 (X4-R5) chimera displayed a CCR5-like trafficking phenotype. We found that the palmitoylated cysteine residues in this domain anchor CCR5 to plasma membrane rafts. CXCR4 and a C-terminally truncated CCR5 mutant (CCR5-KRFX) lacking these cysteines are not raft associated and are endocytosed by a clathrin-dependent pathway. Genetic inhibition of clathrin-mediated endocytosis demonstrated that a significant fraction of ligand-occupied CCR5 trafficked by clathrin-independent routes into caveolin-containing vesicular structures. Thus, the palmitoylated C-tail of CCR5 is the major determinant of its raft association and endocytic itineraries, differentiating it from CXCR4 and other chemokine receptors. This novel feature of CCR5 may modulate its signaling potential and could explain its preferential use by HIV for person-to-person transmission of disease.

HIV coreceptors: role of structure, posttranslational modifications, and internalization in viral-cell fusion and as targets for entry inhibitors

The human immunodeficiency virus (HIV) envelope glycoprotein forms trimers on the virion surface, with each monomer consisting of two subunits, gp120 and gp41. The gp120 envelope component binds to CD4 on target cells and undergoes conformational changes that allow gp120 to interact with certain G-protein-coupled receptors (GPCRs) on the same target membranes. The GPCRs that function as HIV coreceptors were found to be chemokine receptors. The primary coreceptors are CCR5 and CXCR4, but several other chemokine receptors were identified as "minor coreceptors", indicating their ability support entry of some HIV strains in tissue cultures. Formation of the tri-molecular complexes stabilizes virus binding and triggers a series of conformational changes in gp41 that facilitate membrane fusion and viral cell entry. Concerted efforts are underway to decipher the specific interactions between gp120/CD4, gp120/coreceptors, and their contributions to the subsequent membrane fusion process. It is hoped that some of the transient conformational intermediates in gp120 and gp41 would serve as targets for entry inhibitors. In addition, the CD4 and coreceptors are primary targets for several classes of inhibitors currently under testing. Our review summarizes the current knowledge on the interactions of HIV gp120 with its receptor and coreceptors, and the important properties of the chemokine receptors and their regulation in primary target cells. We also summarize the classes of coreceptor inhibitors under development.

Positive regulation of CXCR4 expression and signaling by interleukin-7 in CD4+ mature thymocytes correlates with their capacity to favor human immunodeficiency X4 virus replication

The emergence of X4 human immunodeficiency virus type 1 (HIV-1) variants in infected individuals is associated with poor prognosis. One of the possible causes of this emergence might be the selection of X4 variants in some specific tissue compartment. We demonstrate that the thymic microenvironment favors the replication of X4 variants by positively modulating the expression and signaling of CXCR4 in mature CD4(+) CD8(-) CD3(+) thymocytes. Here, we show that the interaction of thymic epithelial cells (TEC) with these thymocytes in culture induces an upregulation of CXCR4 expression. The cytokine secreted by TEC, interleukin-7 (IL-7), increases cell surface expression of CXCR4 and efficiently overcomes the downregulation induced by SDF-1 alpha, also produced by TEC. IL-7 also potentiates CXCR4 signaling, leading to actin polymerization, a process necessary for virus entry. In contrast, in intermediate CD4(+) CD8(-) CD3(-) thymocytes, the other subpopulation known to allow virus replication, TEC or IL-7 has little or no effect on CXCR4 expression and signaling. CCR5 is expressed at similarly low levels in the two thymocyte subpopulations, and neither its expression nor its signaling was modified by the cytokines tested. This positive regulation of CXCR4 by IL-7 in mature CD4(+) thymocytes correlates with their high capacity to favor X4 virus replication compared with intermediate thymocytes or peripheral blood mononuclear cells. Indeed, we observed an enrichment of X4 viruses after replication in thymocytes initially infected with a mixture of X4 (NL4-3) and R5 (NLAD8) HIV strains and after the emergence of X4 variants from an R5 primary isolate during culture in mature thymocytes.

Mutations in the chemokine receptor gene CXCR4 are associated with WHIM syndrome, a combined immunodeficiency disease

WHIM syndrome is an immunodeficiency disease characterized by neutropenia, hypogammaglobulinemia and extensive human papillomavirus (HPV) infection. Despite the peripheral neutropenia, bone marrow aspirates from affected individuals contain abundant mature myeloid cells, a condition termed myelokathexis. The susceptibility to HPV is disproportionate compared with other immunodeficiency conditions, suggesting that the product of the affected gene may be important in the natural control of this infection. We describe here the localization of the gene associated with WHIM syndrome to a region of roughly 12 cM on chromosome 2q21 and the identification of truncating mutations in the cytoplasmic tail domain of the gene encoding chemokine receptor 4 (CXCR4). Haplotype and mutation analyses in a pedigree transmitting myelokathexis as an apparently autosomal recessive trait support genetic heterogeneity for this aspect of the WHIM syndrome phenotype. Lymphoblastoid cell lines carrying a 19-residue truncation mutation show significantly greater calcium flux relative to control cell lines in response to the CXCR4 ligand, SDF-1, consistent with dysregulated signaling by the mutant receptor. The identification of mutations in CXCR4 in individuals with WHIM syndrome represents the first example of aberrant chemokine receptor function causing human disease and suggests that the receptor may be important in cell-mediated immunity to HPV infection.

Localization of CD4 and CCR5 in living cells

The events preceding human immunodeficiency virus fusion and entry are influenced by the concentration and distribution of receptor and coreceptor molecules on the cell surface. However, the extent to which these proteins colocalize with one another in the cell membrane remains unclear. Using high-resolution deconvolution fluorescent microscopy of living cells, we found that both CD4 and CCR5 accumulate in protruding membrane structures containing actin and ezrin. Although CD4 and CCR5 extensively colocalize in these structures, they do not exist in a stable complex.

Cell-cell fusion and internalization of the CNS-based, HIV-1 co-receptor, APJ

APJ, a member of the human G protein-coupled seven-transmembrane receptor family, has been shown to serve as a coreceptor for the entry of human immunodeficiency virus type I (HIV-1) and simian immunodeficiency virus (SIV), and it is dramatically expressed in central nervous system (CNS)-based cells. In this study, expression of APJ tagged with the green fluorescent protein (GFP) and a fluorescent peptide, 5-carboxyfluorescein (5-CF) conjugated Apelin-13, were utilized for studying receptor internalization and recycling, in stably expressing indicator cells, human neurons, primary CNS microvascular endothelial cells (MVECs), and astrocytes. Fusion of the C-terminus of APJ to the N-terminus of GFP did not alter receptor ligand binding and functions, including signaling and internalization. Using 293 cells stably expressing APJ-GFP, we demonstrated that rapid internalization of the APJ receptor was induced by stimulation with Apelin-36 and Apelin-13, in a dose-dependent manner. Furthermore, investigations showed that the internalized APJ was colocalized with transferrin receptors, suggesting that the internalization of APJ induced by Apelin is likely to be via clathrin-coated pits. Interestingly, we found that the internalized APJ molecules were recycled to the cell surface within 60 min after removal of Apelin-13, but most of the internalized APJ still remained in the cytoplasm, even 2 h after washout of Apelin-36. The intact cytoplasmic C-terminal domain was found to be required for ligand-induced APJ internalization. Human neurons were dramatically stained by the APJ-binding fluorescent peptides. Primary human fetal astrocytes were less strongly labeled with 5-CF-Apelin-13, and in primary human CNS MVECs only weak distribution of green fluorescence specific for APJ in the cytoplasm was observed. Apelin-36 blocked cell membrane fusion mostly due to steric interference, with only a very modest effect on receptor internalization. The CNS represents a unique reservoir site for HIV-1. As such, molecular therapeutics and small molecular inhibitors of HIV-1 entry via this unique CNS receptor are now able to be rationally designed.

Metal ion enhanced binding of AMD3100 to Asp262 in the CXCR4 receptor

The affinity of AMD3100, a symmetrical nonpeptide antagonist composed of two 1,4,8,11-tetraazacyclotetradecane (cyclam) rings connected through a 1,4-dimethylene(phenylene) linker to the CXCR4 chemokine receptor was increased 7, 36, and 50-fold, respectively, by incorporation of the following: Cu(2+), Zn(2+), or Ni(2+) into the cyclam rings of the compound. The rank order of the transition metal ions correlated with the calculated binding energy between free acetate and the metal ions coordinated in a cyclam ring. Construction of AMD3100 substituted with only a single Cu(2+) or Ni(2+) ion demonstrated that the increase in binding affinity of the metal ion substituted bicyclam is achieved through an enhanced interaction of just one of the ring systems. Mutational analysis of potential metal ion binding residues in the main ligand binding crevice of the CXCR4 receptor showed that although binding of the bicyclam is dependent on both Asp(171) and Asp(262), the enhancing effect of the metal ion was selectively eliminated by substitution of Asp(262) located at the extracellular end of TM-VI. It is concluded that the increased binding affinity of the metal ion substituted AMD3100 is obtained through enhanced interaction of one of the cyclam ring systems with the carboxylate group of Asp(262). It is suggested that this occurs through a strong concomitant interaction of one of the oxygen's directly with the metal ion and the other oxygen to one of the nitrogens of the cyclam ring through a hydrogen bond.

Role of two dileucine-like motifs in insulin receptor anchoring to microvilli

In the absence of ligand, the insulin receptor is maintained on microvilli on the cell surface. A dileucine motif (LL(986-987)) is necessary but not sufficient for this anchoring, which also required the presence of additional sequence(s) downstream of position 1000. The aim of the present study was to identify this (these) additional sequence(s). First, exons 16 or 17 were fused to the extracellular and transmembrane domains of complement receptor 1 and stably expressed in Chinese hamster ovary cells. Results obtained indicate that exon 17 is sufficient for anchoring to microvilli. Second, analysis of insulin receptor mutants truncated within exon 17 demonstrated that whereas receptors truncated at position 1000 showed no preferential association with microvilli, receptors truncated at position 1012 displayed a level of association identical to that of the full-length insulin receptor. Third, mutation of a diisoleucine motif (II(1006-1007)) present within this 12-amino acid stretch abrogated the preferential association of the receptor with microvilli. These results indicate that the domain required for association of insulin receptor with microvilli is contained within the region encoded by exon 17 and that, within this sequence, two dileucine-like motifs (LL(986-987) and II(1006-1007)) play a crucial role.

Natural truncation of the chemokine MIP-1 beta /CCL4 affects receptor specificity but not anti-HIV-1 activity

Activated lymphocytes synthesize and secrete substantial amounts of the beta-chemokines macrophage inflammatory protein (MIP)-1 alpha/CCL3 and MIP-1 beta/CCL4, both of which inhibit infection of cells with human immunodeficiency virus type 1 (HIV-1). The native form of MIP-1 beta secreted by activated human peripheral blood lymphocytes (MIP-1 beta(3-69)) lacks the two NH(2)-terminal amino acids of the full-length protein. This truncated form of MIP-1 beta has now been affinity-purified from the culture supernatant of such cells, and its structure has been confirmed by mass spectrometry. Functional studies of the purified protein revealed that MIP-1 beta(3-69) retains the abilities to induce down-modulation of surface expression of the chemokine receptor CCR5 and to inhibit the CCR5-mediated entry of HIV-1 in T cells. Characterization of the chemokine receptor specificity of MIP-1 beta(3-69) showed that the truncated protein not only shares the ability of intact MIP-1 beta to induce Ca(2+) signaling through CCR5, but unlike the full-length protein, it also triggers a Ca(2+) response via CCR1 and CCR2b. These results demonstrate that NH(2)-terminally truncated MIP-1 beta functions as a chemokine agonist with expanded receptor reactivity, which may represent an important mechanism for regulation of immune cell recruitment during inflammatory and antiviral responses.

Ocsyn, a novel syntaxin-interacting protein enriched in the subapical region of inner hair cells

Sensory (hair) cells of the inner ear contain two specialized areas of membrane delivery. The first, located at the cell base, is the afferent synapse where rapid delivery of synaptic vesicles is required to convey information about auditory signals with exceedingly high temporal precision. The second area is at the apex. To accommodate the continuous movement of stereocilia and facilitate their repair, recycling of membrane components is required. Intense vesicular traffic is restricted to a narrow band of cytoplasm around the cuticular plate, which anchors stereocilia. Our previous analyses showed that SNARE proteins (syntaxin 1A/SNAP25/VAMP1) are concentrated at both poles of hair cells, consistent with their involvement in membrane delivery at both locations. To investigate further the molecules involved in membrane delivery at these two sites, we constructed a two-hybrid library of the organ of Corti and probed it with syntaxin 1A. Here we report the cloning of a novel syntaxin-binding protein that is concentrated in a previously uncharacterized organelle at the apex of inner hair cells.

Role of tyrosine phosphorylation in ligand-independent sequestration of CXCR4 in human primary monocytes-macrophages

The chemokine stromal cell-derived factor (SDF)-1 and its receptor, CXCR4, play important roles in human immunodeficiency virus type 1 (HIV-1) pathophysiology, leukocyte trafficking, inflammation, hematopoiesis, embryogenesis, angiogenesis, and cancer metastasis. The effects of cytokines on the regulation of CXCR4 function were investigated in human primary monocytes-macrophages. The expression of functional CXCR4 on the cell surface was demonstrated by the detection of ligand-induced Ca(2+) mobilization, chemotaxis, and ligand-induced receptor endocytosis. Surface CXCR4 expression was down-regulated by cytokines interleukin-4 (IL-4), IL-13, and granulocyte-macrophage colony-stimulating factor (GM-CSF) and up-regulated by IL-10 and transforming growth factor-beta 1. Down-regulation was mediated post-translationally, in the absence of protein degradation, through an endocytotic mechanism. In contrast to SDF-1 alpha-induced CXCR4 endocytosis, cytokine-induced endocytosis of this receptor was independent of actin filament polymerization. GM-CSF increased the expression of G protein-coupled receptor kinase 3 (GRK3), beta-arrestin-1, Pyk2, and focal adhesion kinase (FAK). Cytokine treatment also increased the total and tyrosine-specific phosphorylation of CXCR4 as well as the phosphorylation of FAK on tyrosine 397. It also induced the formation of GRK3.CXCR4 or FAK.CXCR4 complexes. Infection of macrophages by primary R5X4 and X4 isolates of HIV-1 was inhibited by IL-4, IL-13, and GM-CSF, an effect that was associated with down-regulation of surface CXCR4 expression. These data indicate that ligand-dependent and ligand-independent endocytoses of CXCR4 are mediated by different mechanisms. Cytokine-induced endocytosis of chemokine receptors may be of therapeutic value in HIV-1 infection, inflammation, tumor metastasis, and defective hematopoiesis.

CXCR3 internalization following T cell-endothelial cell contact: preferential role of IFN-inducible T cell alpha chemoattractant (CXCL11)

Chemokine receptors are rapidly desensitized and internalized following ligand binding, a process that attenuates receptor-mediated responses. However, the physiological settings in which this process occurs are not clear. Therefore, we examined the fate of CXCR3, a chemokine receptor preferentially expressed on activated T cells following contact with endothelial cells. By immunofluorescence microscopy and flow cytometry, we found that CXCR3 was rapidly internalized when T cells were incubated with IFN-gamma-activated human saphenous vein endothelial cells (HSVEC), but not with resting HSVEC. Similar results were obtained using human CXCR3-transfected murine 300-19 B cells. CXCR3 down-regulation was significantly more pronounced when T cells were in contact with HSVEC than with their supernatants, suggesting that CXCR3 ligands were efficiently displayed on the surface of HSVEC. Using neutralizing mAbs to IFN-induced protein-10 (CXCL10), monokine induced by IFN-gamma (CXCL9), and IFN-inducible T cell alpha chemoattractant (I-TAC; CXCL11), we found that even though I-TAC was secreted from IFN-gamma-activated HSVEC to lower levels than IFN-induced protein-10 or the monokine induced by IFN-gamma, it was the principal chemokine responsible for CXCR3 internalization. This correlated with studies using recombinant chemokines, which revealed that I-TAC was the most potent inducer of CXCR3 down-regulation and of transendothelial migration. Known inhibitors of chemokine-induced chemotaxis, such as pertussis toxin or wortmannin, did not reduce ligand-induced internalization, suggesting that a distinct signal transduction pathway mediates internalization. Our data demonstrate that I-TAC is the physiological inducer of CXCR3 internalization and suggest that chemokine receptor internalization occurs in physiological settings, such as leukocyte contact with an activated endothelium.

Evidence for a post-entry barrier to R5 HIV-1 infection of CD4 memory T cells

BACKGROUND

HIV-1 strains R5 and X4 can infect CD4 memory T cells in vivo. Anti-CD3/28 stimulation induces beta-chemokines and CCR5 down-regulation and renders these cells resistant to R5 HIV-1 infection. Here we describe an additional cellular mechanism that blocks productive R5 HIV-1 infection of CD4 memory T cells.

METHODS

Blood-derived CD4 memory T cells and CD4 T-cell clones were infected with primary R5 and X4 HIV-1 strains. Virus replication was correlated with CCR5 expression and beta-chemokine production. Virus entry and infectivity were measured by PCR for early and late products of HIV reverse transcription respectively.

RESULTS

R5 strains were up to 1000-fold less infectious than X4 viruses for CD4 memory T cells. This resistance was independent of CCR5 levels and of the Delta-32 mutation and the CCR2-V64I/CCR5-59653T linked mutations. Blocking endogenous beta-chemokines relieved minimally this restriction. At the single cell level, CD4 memory cells were either permissive or non-permissive for R5 HIV-1 infection. R5 HIV titre was up to 10-fold lower than X4 virus titre even in a permissive clone. However, R5 viruses replicated as efficiently as X4 viruses in the permissive clone when neutralizing anti-beta chemokine antibodies were added. Non-permissive cells blocked a post-entry step of the virus life-cycle and expressed early but not late HIV transcripts. Neutralizing anti-beta chemokine antibodies promoted R5 virus replication marginally in the non-permissive clone.

CONCLUSION

Some blood memory CD4 T cells retard R5 HIV-1 replication via endogenous beta-chemokines whereas others block productive R5 HIV-1 infection by an additional mechanism that interferes with a post-entry step of the virus life cycle. These natural barriers might contribute to lower pathogenicity of R5 HIV-1 strains for CD4 memory T cells than X4 viruses that emerge late in disease.

Analysis of CC chemokine and chemokine receptor expression in solid ovarian tumours

To understand the chemokine network in a tissue, both chemokine and chemokine receptor expression should be studied. Human epithelial ovarian tumours express a range of chemokines but little is known about the expression and localisation of chemokine receptors. With the aim of understanding chemokine action in this cancer, we investigated receptors for CC-chemokines and their ligands in 25 biopsies of human ovarian cancer. CC-chemokine receptor mRNA was generally absent from solid tumours, the exception being CCR1 which was detected in samples from 75% of patients. CCR1 mRNA localised to macrophages and lymphocytes and there was a correlation between numbers of CD8(+) and CCR1 expressing cells (P = 0.031). mRNA for 6 CC-chemokines was expressed in a majority of tumour samples. In a monocytic cell line in vitro, we found that CCR1 mRNA expression was increased 5-fold by hypoxia. We suggest that the CC-chemokine network in ovarian cancer is controlled at the level of CC-chemokine receptors and this may account for the phenotypes of infiltrating cells found in these tumours. The leukocyte infiltrate may contribute to tumour growth and spread by providing growth survival factors and matrix metalloproteases. Thus, CCR1 may be a novel therapeutic target in ovarian cancer.

Human immunodeficiency virus type-1 and chemokines: beyond competition for common cellular receptors

The chemokines and their receptors have been receiving exceptional attention in recent years following the discoveries that some chemokines could specifically block human immunodeficiency virus type 1 (HIV-1) infection and that certain chemokine receptors were the long-sought coreceptors which, along with CD4, are required for the productive entry of HIV-1 and HIV-2 isolates. Several chemokine receptors or orphan chemokine receptor-like molecules can support the entry of various viral strains, but the clinical significance of the CXCR4 and CCR5 coreceptors appear to overshadow a critical role for any of the other coreceptors and all HIV-1 and HIV-2 strains best employ one or both of these coreceptors. Binding of the HIV-1 envelope glycoprotein gp120 subunit to CD4 and/or an appropriate chemokine receptor triggers conformational changes in the envelope glycoprotein oligomer that allow it to facilitate the fusion of the viral and host cell membranes. During these interactions, gp120 appears to be capable of inducing a variety of signaling events, all of which are still not defined in detail. In addition, the more recently observed dichotomous effects, of both inhibition and enhancement, that chemokines and their receptor signaling events elicit on the HIV-1 entry and replication processes has once again highlighted the intricate and complex balance of factors that govern the pathogenic process. Here, we will review and discuss these new observations summarizing the potential significance these processes may have in HIV-1 infection. Understanding the complexities and significance of the signaling processes that the chemokines and viral products induce may substantially enhance our understanding of HIV-1 pathogenesis, and perhaps facilitate the discovery of new ways for the prevention and treatment of HIV-1 disease.

Molecular interactions of cyclam and bicyclam non-peptide antagonists with the CXCR4 chemokine receptor

The non-peptide CXCR4 receptor antagonist AMD3100, which is a potent blocker of human immunodeficiency virus cell entry, is a symmetrical bicyclam composed of two identical 1,4,8,11-tetraazacyclotetradecane (cyclam) moieties connected by a relatively rigid phenylenebismethylene linker. Based on the known strong propensity of the cyclam moiety to bind carboxylic acid groups, receptor mutagenesis identified Asp(171) and Asp(262), located in transmembrane domain (TM) IV and TM-VI, respectively, at each end of the main ligand-binding crevice of the CXCR4 receptor, as being essential for the ability of AMD3100 to block the binding of the chemokine ligand stromal cell-derived factor (SDF)-1alpha as well as the binding of the receptor antibody 12G5. The free cyclam moiety had no effect on 12G5 binding, but blocked SDF-1alpha binding with an affinity of 3 microm through interaction with Asp(171). The effect on SDF-1alpha binding of a series of bicyclam analogs with variable chemical linkers was found to rely either only on Asp(171), i.e. the bicyclams acted as the isolated cyclam, or on both Asp(171) and Asp(262), i.e. they acted as AMD3100, depending on the length and the chemical nature of the linker between the two cyclam moieties. A positive correlation was found between the dependence of these compounds on Asp(262) for binding and their potency as anti-human immunodeficiency virus agents. It is concluded that AMD3100 acts on the CXCR4 receptor through binding to Asp(171) in TM-IV and Asp(262) in TM-VI with each of its cyclam moieties, and it is suggested that part of its function is associated with a conformational constraint imposed upon the receptor by the connecting phenylenebismethylene linker.

Coregulation of CXC chemokine receptor and CD4 expression on T lymphocytes during allogeneic activation

Upon activation, naive T cells alter their migratory patterns, acquiring the ability to move through peripheral tissues as well as the general lymphoid circulation. Although the mechanisms responsible for these alterations are not well understood, changes in chemokine receptor expression may play a critical role. To investigate these changes, the expression patterns of two chemokine receptors, CXCR3 and CXCR4, were compared on CD4(+) T cells following activation in the MLR. By day 9 of activation, expression of the inflammatory chemokine receptor CXCR3 was up-regulated, while expression of the homeostatic chemokine receptor CXCR4 was down-regulated. Alterations in receptor expression occurred almost exclusively on a subpopulation of T cells that expressed higher levels of CD4. These CD4(high) T cells demonstrated many characteristics of activated T cells and had undergone division in the MLR. By day 9 of culture, the majority of CXCR3(+) and CXCR4(-) cells had divided and had acquired an activated/memory phenotype (CD45RA(-) CD45RO(+) CD69(+) CD25(+)). The levels of transcripts for both CXCR3 and CXCR4 were increased upon allo-activation. The discrepancy between levels of CXCR4 mRNA and surface protein was not due to sequestration of the receptor in intracellular compartments, as CXCR4 was not detectable intracellularly. However, intracellular CXCR3 was readily detectable. Finally, cells from allogeneic cultures demonstrated enhanced migration toward IFN-inducible T cell alpha chemoattractant and reduced migration toward stromal cell-derived factor-1 compared with syngeneic controls, thus suggesting that the observed switch in receptor expression may at least partly contribute to the differential patterns of migration displayed by naive and memory T cells.

Differential level in co-down-modulation of CD4 and CXCR4 primed by HIV-1 gp120 in response to phorbol ester, PMA, among HIV-1 isolates

HIV-1 enters cells through interacting with cell surface molecules such as CD4 and chemokine receptors. We generated recombinant soluble gp120s derived from T-cell line-tropic (T-tropic) and macrophage-tropic (M-tropic) HIV-1 strains using a baculovirus expression system and investigated the association of CD4-gp120 complex with the chemokine receptor and/or other surface molecule(s). For monitoring the co-down-modulations of the CD4-gp120 complex, a cytoplasmic domain deletion mutant (tailless CD4), which is not capable of undergoing down-modulation by itself in response to phorbol ester PMA, was used. Our studies revealed both cell-type and HIV-1 strain-specific differences. We found that T-tropic gp120s were capable of priming co-down-modulation with tailless CD4 by interacting with CXCR4, whereas M-tropic SF162 gp120 could not after PMA treatment even in the presence of CCR5. Among the T-tropic HIV-1 envelopes, IIIB gp120 was the most potent. Furthermore, the ability of gp120 to prime the PMA induced co-down-modulation of tailless CD4 appeared to be dependent on the concentration of the principal coreceptor CXCR4. Nevertheless, the observation that IIIB gp120 strongly primed tailless CD4 co-down-modulation on human osteosarcoma HOS cells that express undetectable levels of surface CXCR4 raised the possibility that membrane component(s) other than those recently identified can be involved in down-modulation of the CD4/gp120 complexes.

Aminooxypentane addition to the chemokine macrophage inflammatory protein-1alpha P increases receptor affinities and HIV inhibition

To enter its target cells, human immunodeficiency virus (HIV) must interact with CD4 and one of a family of chemokine receptors. CCR5 is widely used by the virus in this context, and its ligands can prevent HIV entry. Amino-terminal modified chemokine variants, in particular AOP-RANTES (aminooxypentane-linked regulated on activation normal T cell expressed and secreted), exhibit enhanced HIV entry inhibition. We have previously demonstrated that a non-allelic isoform of macrophage inflammatory protein (MIP)-1alpha, termed MIP-1alphaP, is the most active naturally occurring inhibitor of HIV entry known. Here we report the properties of a variant of MIP-1alphaP with an AOP group on the amino terminus. We show that, like RANTES, the addition of AOP to MIP-1alphaP enhances its interactions with CCR1 and CCR5, allows more effective internalization of CCR5, and increases the ligand's potency as an inhibitor of HIV entry through CCR5. Importantly, AOP-MIP-1alphaP is about 10-fold more active than AOP-RANTES at inhibiting HIV entry, making it the most effective chemokine-based inhibitor of HIV entry through CCR5 described to date. Surprisingly, the enhanced receptor interactions of AOP-MIP-1alphaP do not translate into increased chemotaxis or coupling to calcium ion fluxes, suggesting that this protein should be viewed as a partial, rather than a full, agonist for CCR1 and CCR5.

Involvement of protein kinase C in HIV-1 gp120-induced apoptosis in primary endothelium

We previously showed that HIV-1 gp120-induced apoptosis in primary human umbilical vein endothelial cell cultures (HUVEC), through CCR5 and CXCR4. Here, we have found that agonists of protein kinase C (PKC), basic fibroblast growth factor (bFGF), and short exposure to low concentrations of phorbol esters were found to block gp120-induced apoptosis in HUVEC cultures. PKC antagonists, sphingosine, H7, and extended exposure of cultures to high concentrations of phorbol esters were also found to block gp120-induced apoptosis in HUVEC cultures. A significant increase in the total amount of cellular PKC enzymatic activity was observed on exposure of HUVEC to gp120. No increase in total PKC activity was observed on exposure of HUVECs to the natural ligands SDF-1alpha, or regulated-on-activation normal T-expressed and secreted (RANTES) cells, and gp120-induced PKC induction was found to be totally blocked by CXCR4 antibodies and partially blocked by the caspase 3 inhibitor, DEVD-CHO. Alternatively, CXCR4 antibodies and DEVD-CHO totally blocked apoptosis. Finally, gp120-induced effects were found to be insensitive to pertussis toxin. Accumulated evidence suggests PKC involvement at multiple points in the gp120-induced apoptotic pathway; also suggests involvement of the CXCR4 receptor internalization pathway, and potentially suggests different downstream effects of gp120-receptor interactions and natural ligand-receptor interactions.

Endocytosis and recycling of the HIV coreceptor CCR5

The chemokine receptor CCR5 is a cofactor for the entry of R5 tropic strains of human immunodeficiency viruses (HIV)-1 and -2 and simian immunodeficiency virus. Cells susceptible to infection by these viruses can be protected by treatment with the CCR5 ligands regulated on activation, normal T cell expressed and secreted (RANTES), MIP-1alpha, and MIP-1beta. A major component of the mechanism through which chemokines protect cells from HIV infection is by inducing endocytosis of the chemokine receptor. Aminooxypentane (AOP)-RANTES, an NH(2)-terminal modified form of RANTES, is a potent inhibitor of infection by R5 HIV strains. AOP-RANTES efficiently downmodulates the cell surface expression of CCR5 and, in contrast with RANTES, appears to prevent recycling of CCR5 to the cell surface. Here, we investigate the cellular basis of this effect. Using CHO cells expressing human CCR5, we show that both RANTES and AOP-RANTES induce rapid internalization of CCR5. In the absence of ligand, CCR5 shows constitutive turnover with a half-time of 6-9 h. Addition of RANTES or AOP-RANTES has little effect on the rate of CCR5 turnover. Immunofluorescence and immunoelectron microscopy show that most of the CCR5 internalized after RANTES or AOP-RANTES treatment accumulates in small membrane-bound vesicles and tubules clustered in the perinuclear region of the cell. Colocalization with transferrin receptors in the same clusters of vesicles indicates that CCR5 accumulates in recycling endosomes. After the removal of RANTES, internalized CCR5 recycles to the cell surface and is sensitive to further rounds of RANTES-induced endocytosis. In contrast, after the removal of AOP-RANTES, most CCR5 remains intracellular. We show that these CCR5 molecules do recycle to the cell surface, with kinetics equivalent to those of receptors in RANTES-treated cells. However, these recycled CCR5 molecules are rapidly reinternalized. Our results indicate that AOP-RANTES-induced changes in CCR5 alter the steady-state distribution of the receptor and provide the first evidence for G protein-coupled receptor trafficking through the recycling endosome compartment.

Characterization of binding sites for chemokines MCP-1 and MIP-1alpha on human brain microvessels

The presence of binding sites for the beta chemokines monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1alpha (MIP-1alpha) has recently been identified on human brain microvessels. We extend these findings in this report to reveal that such sites exemplify characteristics of the recognized major receptors for MCP-1 and MIP-1alpha: CCR2, and CCR1 and CCR5, respectively. Specifically, labeled MCP-1 binding to isolated brain microvessels was inhibited by unlabeled MCP-1 and MCP-3, the latter another CCR2 ligand, but not by MIP-1alpha. Inhibition of labeled MIP-1alpha binding was achieved with unlabeled MIP-1alpha and RANTES, the latter a beta chemokine that binds to both CCR1 and CCR5, but not by MCP-1. Labeled MIP-1alpha binding was also antagonized by unlabeled MCP-3, which is also recognized by CCR1, and MIP-1beta, which is a ligand for CCR5. Labeled MCP-1 and MIP-1alpha were further observed to be internalized within the endothelial cells of brain microvessels, following their binding to the microvascular surface at 37 degrees C. Additionally, exposure of microvessels to unlabeled MCP-1 or MIP-1alpha was accompanied by the initial loss and subsequent recovery of surface binding sites for these chemokines, which occurred on a time scale consistent with ligand-induced endocytosis and recycling. These collective features bear striking similarity to those that characterize interactions of MCP-1 and MIP-1alpha with their receptors on leukocytes and underscore the concept of cognate chemokine receptors on brain microvascular endothelium.

The primate lentiviral receptor Bonzo/STRL33 is coordinately regulated with CCR5 and its expression pattern is conserved between human and mouse

Chemokines play necessary and important roles in regulating the trafficking of lymphocytes to intra- or interlymphoid tissues as well as to sites of inflammation. The complex migratory patterns of lymphoid lineage cells is governed by subset-specific expression of chemokine receptors and their access to specific ligands. Several chemokine receptors and chemokine receptor-like orphan receptors also serve, in conjunction with CD4, as coreceptors for infection by human and simian immunodeficiency viruses (HIV and SIV). Here we show that the expression pattern of Bonzo/STRL33, an orphan SIV/HIV coreceptor, is highly restricted to the memory subset of T cells and is up-regulated upon stimulation of these cells with IL-2 or IL-15. Both the pattern and the regulation of Bonzo expression closely paralleled that of CC family chemokine receptors CCR5 or CCR6 and inversely correlated with CXCR4 expression. However, in striking contrast to CCR5, Bonzo expression was not down-modulated by PMA or mitogen stimulation of T cells. Targeted replacement of the Bonzo gene with a gene encoding green fluorescent protein in mice revealed that the expression and cytokine regulation of mouse Bonzo are comparable to those of its human counterpart. The similar expression and regulation patterns of Bonzo and the HIV coreceptor CCR5 may have implications for understanding the role of HIV/SIV receptors in viral evolution and pathogenesis.