Selectivity and antagonism of chemokine receptors

The Chemerin Receptor CMKLR1 Requires Full-Length Chemerin for High Affinity in Contrast to GPR1 as Demonstrated by a New Nanoluciferase-Based Binding Assay

To study the binding mode of the adipokine chemerin as well as the short peptide agonist chemerin-9 (C9) to its two receptors chemokine-like receptor 1 (CMKLR1) and G protein-coupled receptor 1 (GPR1), we generated 5-carboxytetramethylrhodamine (TAMRA) modified variants of both ligands. In addition, we labeled GPR1 and CMKLR1 with a nanoluciferase at the N-terminus to perform NanoBRET binding assays. For GPR1, both ligands show high affinity and comparable binding. Significant differences were found for CMKLR1, whereby only full-length chemerin binds with high affinity in saturation and displacement assays. For TAMRA-C9 a biphasic binding consisting of two binding states has been found and no displacement studies could be performed. Thus, we conclude that CMKLR1 requires full-length chemerin for stable binding in contrast to GPR1. This work demonstrates the NanoBRET binding assay as a new tool for binding studies at chemerin receptors and it enables deeper insights into the ligand binding parameters.

Prognostic significance of SDF-1 chemokine and its receptors CXCR4 and CXCR7 involved in EMT of prostate cancer

Tendency to conversion from state of chronic inflammation to malignancy is a tumor characteristic trait, which encourages progression to its metastatic stage.. The inflammatory cells maintaining in the tumor inaugurate a communication with cancer cells and become tumor-fostering cells. Epithelial-mesenchymal transition (EMT) is a program supporting malignant cells during switch phenotype into metastatic form, providing looseness of cell-cell adherence and strengthens migratory or invasive features. EMT-undergone tumor cells become more aggressive and resistant to apoptosis. Additionally, malignant cells can be stimulated to manufacture proinflammatory factors throughout EMT program. Chronic inflammation is responsible for EMT induction in malignancies. Developed tumors induce inflammatory response through excretion of cytokines, chemokines and growth factors, which recruit populations of infiltrating immune cells straight to the tumor microenvironment. The inflammatory reaction potentially exerts tumor control, but instead it can be intercepted by the tumor to stimulate its own development in direction to metastatic form. Our study confirmed that SDF-1 chemokine and its receptors, CXCR4 and CXCR7 may participate in initiation of metastases formation and EMT process.

Functional anatomy of the full-length CXCR4-CXCL12 complex systematically dissected by quantitative model-guided mutagenesis

Because of their prominent roles in development, cancer, and HIV, the chemokine receptor CXCR4 and its ligand CXCL12 have been the subject of numerous structural and functional studies, but the determinants of ligand binding, selectivity, and signaling are still poorly understood. Here, building on our latest structural model, we used a systematic mutagenesis strategy to dissect the functional anatomy of the CXCR4-CXCL12 complex. Key charge swap mutagenesis experiments provided evidence for pairwise interactions between oppositely charged residues in the receptor and chemokine, confirming the accuracy of the predicted orientation of the chemokine relative to the receptor and providing insight into ligand selectivity. Progressive deletion of N-terminal residues revealed an unexpected contribution of the receptor N terminus to chemokine signaling. This finding challenges a longstanding "two-site" hypothesis about the essential features of the receptor-chemokine interaction in which the N terminus contributes only to binding affinity. Our results suggest that although the interaction of the chemokine N terminus with the receptor-binding pocket is the key driver of signaling, the signaling amplitude depends on the extent to which the receptor N terminus binds the chemokine. Together with systematic characterization of other epitopes, these data enable us to propose an experimentally consistent structural model for how CXCL12 binds CXCR4 and initiates signal transmission through the receptor transmembrane domain.

Hypoxia-induced microRNA-141 regulates trophoblast apoptosis, invasion, and vascularization by blocking CXCL12β/CXCR2/4 signal transduction

BACKGROUND

An impaired trophoblast invasion ability contributes to the development of pre-eclampsia (PE), and can be induced by the altered expression of various microRNAs (miRs). MiR-141 and CXCL12β (C-X-C motif chemokine ligand 12) signaling regulate trophoblast invasion and vascularization capabilities during PE pathogenesis; however, their interactions and underlying mechanisms of action remain unclear. We investigated how miR-141 modulates trophoblast invasion, with a focus on its interaction with CXCL12β signaling.

METHODS

A PE model was established by using HTR-8/SVneo cells, which were first cultured with 2% O₂ for 48 h, and then with 5% O₂. The expression of miR-141 in human villous trophoblast HTR-8/SVneo cells was modulated with mimics or an inhibitor, and analyzed by quantitative RT-PCR. CXCL12β levels were determined by ELISA. Cell apoptosis was determined by flow cytometry, and the invasion and vascularization capabilities of trophoblasts were evaluated by Transwell and tube formation assays, respectively. Binding of miR-141 with CXCL12β mRNA was verified by the dual luciferase assay. Protein levels were estimated by western blotting.

RESULTS

MiR-141 expression was significantly induced by hypoxia in HTR-8/SVneo cells. MiR-141 was found to promote apoptosis and inhibit the invasion and vascularization abilities of HTR-8/SVneo cells under conditions of hypoxia. MiR-141 could directly bind with the 3'UTR region of CXCL12β mRNA and inhibit its translation. In addition, we proved that miR-141 could inhibit the invasion and vascularization abilities, and promote the apoptosis of HTR-8/SVneo cells by targeting CXCL12β under hypoxic conditions. Furthermore, we demonstrated that arachidonic acid could reverse the invasion and apoptosis abilities of HTR-8/SVneo cells mediated by CXCL12β during hypoxia. In terms of mechanism, MiR-141 could downregulate MMP2, p62, and LC3B expression, and upregulate ROCK1 and RhoA expression in HTR-8/SVneo cells by targeting the CXCL12β gene during hypoxia. The effects of CXCL12βon HTR-8/SVneo cells could be reversed by arachidonic acid (ARA).

CONCLUSION

Induction of miR-141 by hypoxia promotes apoptosis, and inhibits the invasion and vascularization capabilities of HTR-8/SVneo cells by suppressing the CXCL12β and CXCR2/4 signaling pathways.

Monocyte chemoattractant protein-1 (MCP-1/CCL2) in diabetic retinopathy: latest evidence and clinical considerations

Diabetic retinopathy (DR) is considered as a diabetes-related complication that can render severe visual impairments and is also a risk factor for acquired blindness in both developed as well as developing countries. Through fibrovascular epiretinal membranes (ERMs), this condition can similarly lead to tractional retinal detachment. Laboratory efforts evaluating the DR pathogenesis can be provided by ocular vitreous fluid and ERMs resulting from vitrectomy. The clinical stages of DR are significantly associated with expression levels of certain chemokines, including monocyte chemotactic protein-1 (MCP-1) in the intraocular fluid. The MCP-1 is also a known potent chemotactic factor for monocytes and macrophages that can stimulate them to produce superoxide and other mediators. Following hyperglycemia, retinal pigmented epithelial (RPE) cells, endothelial cells, and Müller's glial cells are of utmost importance for MCP-1 production, and vitreous MCP-1 levels rise in patients with DR. Increased expression of the MCP-1 in the eyes can also play a significant role in the pathogenesis of DR. In this review, current clinical and laboratory progress achieved on the MCP-1 and the DR concerning neovascularization and inflammatory responses in vitreous and/or aqueous humor of DR patients was summarized. It was suggested that further exploration of the MCP-1/CCR2 axis association between clinical stages of DR and expression levels of inflammatory and angiogenic cytokines and chemokines, principally the MCP-1 might lead to potential therapies aiming at neutralizing antibodies and viral vectors.

Rapid and low-cost multiplex synthesis of chemokine analogs

Peptides represent a promising source of new medicines, but improved technologies are needed to facilitate discovery and optimization campaigns. In particular, longer peptides with multiple disulfide bridges are challenging to produce, and producing large numbers of structurally related variants is dissuasively costly and time-consuming. The principal cost and time drivers are the multiple column chromatography purification steps that are used during the multistep chemical synthesis procedure, which involves both ligation and oxidative refolding steps. In this study, we developed a method for multiplex parallel synthesis of complex peptide analogs in which the structurally variant region of the molecule is produced as a small peptide on a 384-well synthesizer with subsequent ligation to the longer, structurally invariant region and oxidative refolding carried out in-well without any column purification steps. To test the method, we used a panel of 96 analogs of the chemokine RANTES (regulated on activation normal T cell expressed and secreted)/CCL5 (69 residues, two disulfide bridges), which had been synthesized using standard approaches and characterized pharmacologically in an earlier study. Although, as expected, the multiplex method generated chemokine analogs of lower purity than those produced in the original study, it was nonetheless possible to closely match the pharmacological attributes (anti-HIV potency, capacity to elicit G protein signaling, and capacity to elicit intracellular receptor sequestration) of each chemokine analog to reference data from the earlier study. This rapid, low-cost approach has the potential to support discovery and optimization campaigns based on analogs of other chemokines as well as those of other complex peptide and small protein targets of a similar size.

Oligomerization State of CXCL4 Chemokines Regulates G Protein-Coupled Receptor Activation

CXCL4 chemokines have antiangiogenic properties, mediated by different mechanisms, including CXCR3 receptor activation. Chemokines have distinct oligomerization states that are correlated with their biological functions. CXCL4 exists as a stable tetramer under physiological conditions. It is unclear whether the oligomerization state impacts CXCL4-receptor interaction. We found that the CXCL4 tetramer is sensitive to pH and salt concentration. Residues Glu28 and Lys50 were important for tetramer formation, and the first β-strand and the C-terminal helix are critical for dimerization. By mutating the critical residues responsible for oligomerization, we generated CXCL4 mutants that behave as dimers or monomers under neutral/physiological conditions. The CXCL4 monomer acts as the minimal active unit for interacting CXCR3A, and sulfation of N-terminal tyrosine residues on the receptor is important for binding. Noticeably, CXCL4L1, a CXCL4 variant that differs by three residues in the C-terminal helix, could activate CXCR3A. CXCL4L1 showed a higher tendency to dissociate into monomers, but native CXCL4 did not. This result indicates that monomeric CXCL4 behaves like CXCL4L1. Thus, in this chemokine family, being in the monomeric state seems critical for interaction with CXCR3A.

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

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

Molecular basis for the antagonistic activity of an anti-CXCR4 antibody

Antagonistic antibodies targeting the G-protein C-X-C chemokine receptor 4 (CXCR4) hold promising therapeutic potential in various diseases. We report for the first time the detailed mechanism of action at a molecular level of a potent anti-CXCR4 antagonistic antibody (MEDI3185). We characterized the MEDI3185 paratope using alanine scanning on all 6 complementary-determining regions (CDRs). We also mapped its epitope using CXCR4 mutagenesis to assess the relative importance of the CXCR4 N-terminal peptide, extracellular loops (ECL) and ligand-binding pocket. We show that the interaction between MEDI3185 and CXCR4 is mediated mostly by CDR3H in MEDI3185 and ECL2 in CXCR4. The MEDI3185 epitope comprises the entire ECL2 sequence, lacks any so-called ‘hot-spot’ and is remarkably resistant to mutations. The structure of MEDI3185 variable domains was modeled, and suggested a β-strand/β-strand interaction between MEDI3185 CDR3H and CXCR4 ECL2, resulting in direct steric hindrance with CXCR4 ligand SDF-1. These findings may have important implications for designing antibody therapies against CXCR4.

Diversity and Inter-Connections in the CXCR4 Chemokine Receptor/Ligand Family: Molecular Perspectives

CXCR4 and its ligand CXCL12 mediate the homing of progenitor cells in the bone marrow and their recruitment to sites of injury, as well as affect processes such as cell arrest, survival, and angiogenesis. CXCL12 was long thought to be the sole CXCR4 ligand, but more recently the atypical chemokine macrophage migration inhibitory factor (MIF) was identified as an alternative, non-cognate ligand for CXCR4 and shown to mediate chemotaxis and arrest of CXCR4-expressing T-cells. This has complicated the understanding of CXCR4-mediated signaling and associated biological processes. Compared to CXCL12/CXCR4-induced signaling, only few details are known on MIF/CXCR4-mediated signaling and it remains unclear to which extent MIF and CXCL12 reciprocally influence CXCR4 binding and signaling. Furthermore, the atypical chemokine receptor 3 (ACKR3) (previously CXCR7) has added to the complexity of CXCR4 signaling due to its ability to bind CXCL12 and MIF, and to evoke CXCL12- and MIF-triggered signaling independently of CXCR4. Also, extracellular ubiquitin (eUb) and the viral protein gp120 (HIV) have been reported as CXCR4 ligands, whereas viral chemokine vMIP-II (Herpesvirus) and human β3-defensin (HBD-3) have been identified as CXCR4 antagonists. This review will provide insight into the diversity and inter-connections in the CXCR4 receptor/ligand family. We will discuss signaling pathways initiated by binding of CXCL12 vs. MIF to CXCR4, elaborate on how ACKR3 affects CXCR4 signaling, and summarize biological functions of CXCR4 signaling mediated by CXCL12 or MIF. Also, we will discuss eUb and gp120 as alternative ligands for CXCR4, and describe vMIP-II and HBD-3 as antagonists for CXCR4. Detailed insight into biological effects of CXCR4 signaling und underlying mechanisms, including diversity of CXCR4 ligands and inter-connections with other (chemokine) receptors, is clinically important, as the CXCR4 antagonist AMD3100 has been approved as stem cell mobilizer in specific disease settings.

Meta-Analysis of the Relationship between CXCR4 Expression and Metastasis in Prostate Cancer

PURPOSE

Experimental studies have suggested that the stromal-derived factor-1 (SDF-1)/CXCR4 axis is associated with tumor aggressiveness and metastasis in several malignancies. We performed a meta-analysis to elucidate the relationship between CXCR4 expression and the clinicopathological features of prostate cancer.

MATERIALS AND METHODS

Data were collected from studies comparing Gleason score, T stage, and the presence of metastasis with CXCR4 levels in human prostate cancer samples. The studies were pooled, and the odds ratio (OR) of CXCR4 expression for clinical and pathological variables was calculated.

RESULTS

Five articles were eligible for the current meta-analysis. We found no relationship between CXCR4 expression and Gleason score (<7 vs. ≥7). The forest plot using the fixed-effects model indicated an OR of 1.585 (95% confidence interval [CI]: 0.793~3.171; p=0.193). Further, CXCR4 expression was not associated with the T stage (<T3 vs. ≥T3), and the relevant meta-analysis showed OR=1.803 (95% CI: 0.756~4.297, p=0.183). However, increased CXCR4 expression was strongly associated with metastatic disease with a fixed-effects pooled OR of 7.459 (95% CI: 2.665~20.878, p<0.001).

CONCLUSIONS

Our meta-analysis showed that the higher CXCR4 protein expression in prostate cancer specimens is significantly associated with the presence of metastatic disease. This supports previous experimental data supporting the role played by the SDF-1/CXCR4 axis in metastasis.

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

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

Dynamic conformational switching in the chemokine ligand is essential for G-protein-coupled receptor activation

Chemokines mediate diverse functions from organogenesis to mobilizing leucocytes, and are unusual agonists for class-A GPCRs (G-protein-coupled receptors) because of their large size and multi-domain structure. The current model for receptor activation, which involves interactions between chemokine N-loop and receptor N-terminal residues (Site-I) and between chemokine N-terminal and receptor extracellular loop/transmembrane residues (Site-II), fails to describe differences in ligand/receptor selectivity and the activation of multiple signalling pathways. In the present study, we show in neutrophil-activating chemokine CXCL8 that the highly conserved GP (glycine-proline) motif located distal to both N-terminal and N-loop residues couples Site-I and Site-II interactions. GP mutants showed large differences from native-like to complete loss of function that could not be correlated with the specific mutation, receptor affinity or subtype, or a specific signalling pathway. NMR studies indicated that the GP motif does not influence Site-I interactions, but molecular dynamics simulations suggested that this motif dictates substates of the CXCL8 conformational ensemble. We conclude that the GP motif enables diverse receptor functions by controlling cross-talk between Site-I and Site-II, and further propose that the repertoire of chemokine functions is best described by a conformational ensemble model in which a network of long-range coupled indirect interactions mediate receptor activity.

Bioinformatic analysis of HIV-1 entry and pathogenesis

The evolution of human immunodeficiency virus type 1 (HIV-1) with respect to co-receptor utilization has been shown to be relevant to HIV-1 pathogenesis and disease. The CCR5-utilizing (R5) virus has been shown to be important in the very early stages of transmission and highly prevalent during asymptomatic infection and chronic disease. In addition, the R5 virus has been proposed to be involved in neuroinvasion and central nervous system (CNS) disease. In contrast, the CXCR4-utilizing (X4) virus is more prevalent during the course of disease progression and concurrent with the loss of CD4(+) T cells. The dual-tropic virus is able to utilize both co-receptors (CXCR4 and CCR5) and has been thought to represent an intermediate transitional virus that possesses properties of both X4 and R5 viruses that can be encountered at many stages of disease. The use of computational tools and bioinformatic approaches in the prediction of HIV-1 co-receptor usage has been growing in importance with respect to understanding HIV-1 pathogenesis and disease, developing diagnostic tools, and improving the efficacy of therapeutic strategies focused on blocking viral entry. Current strategies have enhanced the sensitivity, specificity, and reproducibility relative to the prediction of co-receptor use; however, these technologies need to be improved with respect to their efficient and accurate use across the HIV-1 subtypes. The most effective approach may center on the combined use of different algorithms involving sequences within and outside of the env-V3 loop. This review focuses on the HIV-1 entry process and on co-receptor utilization, including bioinformatic tools utilized in the prediction of co-receptor usage. It also provides novel preliminary analyses for enabling identification of linkages between amino acids in V3 with other components of the HIV-1 genome and demonstrates that these linkages are different between X4 and R5 viruses.

Inhibition of tumor growth and histopathological changes following treatment with a chemokine receptor CXCR4 antagonist in a prostate cancer xenograft model

The stromal derived factor-1 (SDF-1)/CXCR4 axis is associated with tumor aggressiveness and metastasis in prostate cancer. The present study aimed to explore the potential therapeutic effects of a CXCR4 antagonist in prostate cancer. The effect of SDF-1 and a CXCR4-specific antagonist, AMD3100, on human prostate cancer PC-3 cell proliferation and protein kinase B (Akt) signaling was assessed. Moreover, a PC-3 tumor xenograft model was used to evaluate the effect of AMD3100 on tumor growth and to identify the histopathological changes and immunohistochemical differences between AMD3100-treated and untreated groups. Cell proliferation was not significantly affected by SDF-1 or AMD3100 treatment in vitro. Western blot analysis revealed that SDF-1 stimulation enhanced the expression of phosphorylated Akt in the PC-3 cells, but that the SDF-1-induced expression of phosphorylated Akt was abrogated in the AMD3100-treated PC-3 cells. In the PC-3 tumor xenograft model, AMD3100 significantly inhibited tumor growth, while AMD3100-treated PC-3 tumors had lower levels of microvessel formation and a lower immunoreactivity for the proliferation marker Ki-67 and the anti-apoptotic marker Bcl-2 compared to control tumors in vivo. The CXCR4-specific antagonist inhibits SDF-1-induced CXCR4/Akt signal transduction, and effectively suppresses tumor growth in the PC-3 xenograft model. The present study indicates that CXCR4 targeting may represent a novel strategy for the treatment of castration-resistant prostate cancer (CRPC).

Chemokine CXCL1 dimer is a potent agonist for the CXCR2 receptor

The CXCL1/CXCR2 axis plays a crucial role in recruiting neutrophils in response to microbial infection and tissue injury, and dysfunction in this process has been implicated in various inflammatory diseases. Chemokines exist as monomers and dimers, and compelling evidence now exists that both forms regulate in vivo function. Therefore, knowledge of the receptor activities of both CXCL1 monomer and dimer is essential to describe the molecular mechanisms by which they orchestrate neutrophil function. The monomer-dimer equilibrium constant (~20 μm) and the CXCR2 binding constant (1 nm) indicate that WT CXCL1 is active as a monomer. To characterize dimer activity, we generated a trapped dimer by introducing a disulfide across the dimer interface. This disulfide-linked CXCL1 dimer binds CXCR2 with nanomolar affinity and shows potent agonist activity in various cellular assays. We also compared the receptor binding mechanism of this dimer with that of a CXCL1 monomer, generated by deleting the C-terminal residues that stabilize the dimer interface. We observe that the binding interactions of the dimer and monomer to the CXCR2 N-terminal domain, which plays an important role in determining affinity and activity, are essentially conserved. The potent activity of the CXCL1 dimer is novel: dimers of the CC chemokines CCL2 and CCL4 are inactive, and the dimer of the CXC chemokine CXCL8 (which is closely related to CXCL1) is marginally active for CXCR1 but shows variable activity for CXCR2. We conclude that large differences in dimer activity among different chemokine-receptor pairs have evolved for fine-tuned leukocyte function.

Chemokines: structure, receptors and functions. A new target for inflammation and asthma therapy?

Five to 10% of the human population have a disorder of the respiratory tract called 'asthma'. It has been known as a potentially dangerous disease for over 2000 years, as it was already described by Hippocrates and recognized as a disease entity by Egyptian and Hebrew physicians. At the beginning of this decade, there has been a fundamental change in asthma management. The emphasis has shifted from symptom relief with bronchodilator therapies (e.g. beta(2)-agonists) to a much earlier introduction of anti-inflammatory treatment (e.g. corticosteroids). Asthma is now recognized to be a chronic inflammatory disease of the airways, involving various inflammatory cells and their mediators. Although asthma has been the subject of many investigations, the exact role of the different inflammatory cells has not been elucidated completely. Many suggestions have been made and several cells have been implicated in the pathogenesis of asthma, such as the eosinophils, the mast cells, the basophils and the lymphocytes. To date, however, the relative importance of these cells is not completely understood. The cell type predominantly found in the asthmatic lung is the eosinophil and the recruitment of these eosinophils can be seen as a characteristic of asthma. In recent years much attention is given to the role of the newly identified chemokines in asthma pathology. Chemokines are structurally and functionally related 8-10 kDa peptides that are the products of distinct genes clustered on human chromosomes 4 and 17 and can be found at sites of inflammation. They form a superfamily of proinflammatory mediators that promote the recruitment of various kinds of leukocytes and lymphocytes. The chemokine superfamily can be divided into three subgroups based on overall sequence homology. Although the chemokines have highly conserved amino acid sequences, each of the chemokines binds to and induces the chemotaxis of particular classes of white blood cells. Certain chemokines stimulate the recruitment of multiple cell types including monocytes, lymphocytes, basophils, and eosinophils, which are important cells in asthma. Intervention in this process, by the development of chemokine antagonists, might be the key to new therapy. In this review we present an overview of recent developments in the field of chemokines and their role in inflammations as reported in literature.

CXCL12/CXCR4 axis triggers the activation of EGF receptor and ERK signaling pathway in CsA-induced proliferation of human trophoblast cells

Introduction

Our previous study has demonstrated Cyclosporin A (CsA) promotes the proliferation of human trophoblast cells. Therefore, we further investigate the intracellular signaling pathway involved in the CsA-induced proliferation of human trophoblast cells.

Methods

Enzyme-linked immunosorbent assay (ELISA) was performed to evaluate the regulation of CsA on CXCL12 secretion in human trophoblast cells. Immunofluorescence analysis and western blotting analysis were used to investigate the role of CXCL12/CXCR4 axis in the CsA-induced epidermal growth factor receptor (EGFR) phosphorylation in human trophoblast cells. 5-bromo-2′-deoxyuridine (BrdU) cell proliferation assay was performed to analyze the involvement of EGFR and its downstream extracellular signal-regulated protein kinase (ERK) signaling pathway in the CsA-induced proliferation of human trophoblast cells.

Results

Low concentration of CsA promoted the secretion of CXCL12, and recombinant human CXCL12 promoted the phosphorylation of EGFR in primary human trophoblast cells and choriocarcinoma cell line JEG-3. The inhibition of CXCL12 or CXCR4 by either neutralizing antibodies or small interfering RNA (siRNA) could completely block the CsA-induced EGFR phosphorylation. The CsA-induced proliferation of human trophoblast cells was effectively abrogated by the EGFR inhibitor AG1478 as well as the ERK inhibitor U0126, but not by the PI3K/PKB inhibitor LY294002. CsA promoted the activation of ERK in JEG-3 cells, which was markedly abrogated in the presence of CXCL12 siRNA, or CXCR4 siRNA, or AG1478.

Conclusions

CsA may promote EGFR activation via CXCL12/CXCR4 axis, and EGFR downstream ERK signaling pathway may be involved in the CsA-induced proliferation of human trophoblast cells.

Design and receptor interactions of obligate dimeric mutant of chemokine monocyte chemoattractant protein-1 (MCP-1)

Chemokine-receptor interactions regulate leukocyte trafficking during inflammation. CC chemokines exist in equilibrium between monomeric and dimeric forms. Although the monomers can activate chemokine receptors, dimerization is required for leukocyte recruitment in vivo, and it remains controversial whether dimeric CC chemokines can bind and activate their receptors. We have developed an obligate dimeric mutant of the chemokine monocyte chemoattractant protein-1 (MCP-1) by substituting Thr(10) at the dimer interface with Cys. Biophysical analysis showed that MCP-1(T10C) forms a covalent dimer with similar structure to the wild type MCP-1 dimer. Initial cell-based assays indicated that MCP-1(T10C) could activate chemokine receptor CCR2 with potency reduced 1 to 2 orders of magnitude relative to wild type MCP-1. However, analysis of size exclusion chromatography fractions demonstrated that the observed activity was due to a small proportion of MCP-1(T10C) being monomeric and highly potent, whereas the majority dimeric form could neither bind nor activate CCR2 at concentrations up to 1 μM. These observations help to reconcile previous conflicting results and indicate that dimeric CC chemokines do not bind to their receptors with affinities approaching those of the corresponding monomeric chemokines.

Design, synthesis, and functionalization of dimeric peptides targeting chemokine receptor CXCR4

The chemokine receptor CXCR4 is a critical regulator of inflammation and immune surveillance, and it is specifically implicated in cancer metastasis and HIV-1 infection. On the basis of the observation that several of the known antagonists remarkably share a C(2) symmetry element, we constructed symmetric dimers with excellent antagonistic activity using a derivative of a cyclic pentapeptide as monomer. To optimize the binding affinity, we investigated the influence of the distance between the monomers and the pharmacophoric sites in the synthesized constructs. The affinity studies in combination with docking computations support a two-site binding model. In a final step, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was introduced as chelator for (radio-)metals, thus allowing to exploit these compounds as a new group of CXCR4-binding peptidic probes for molecular imaging and endoradiotherapeutic purposes. Both the DOTA conjugates and some of their corresponding metal complexes retain good CXCR4 affinity, and one (68)Ga labeled compound was studied as PET tracer.

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

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

Chemokine oligomerization and interactions with receptors and glycosaminoglycans: the role of structural dynamics in function

The first chemokine structure, that of IL-8/CXCL8, was determined in 1990. Since then, many chemokine structures have emerged. To the initial disappointment of structural biologists, the tertiary structures of these small proteins were found to be highly conserved. However, they have since proven to be much more interesting and diverse than originally expected. Somewhat like lego blocks, many chemokines oligomerize and there is significant diversity in their oligomeric forms and propensity to oligomerize. Chemokines not only interact with receptors where different oligomeric forms can induce different signaling responses, they also interact with glycosaminoglycans which can stabilize oligomers and other structures that would not otherwise form in solution. Although chemokine monomers and dimers yielded quickly to structure determination, structural information about larger chemokine oligomers, chemokines receptors, and complexes of chemokines with glycosaminoglycans and receptors has been more difficult to obtain, but recent breakthroughs suggest that this information will be forthcoming, especially with receptor structures. Equally important and challenging, will be efforts to correlate the structural information with function.

Structures of the CXCR4 chemokine GPCR with small-molecule and cyclic peptide antagonists

Chemokine receptors are critical regulators of cell migration in the context of immune surveillance, inflammation, and development. The G protein-coupled chemokine receptor CXCR4 is specifically implicated in cancer metastasis and HIV-1 infection. Here we report five independent crystal structures of CXCR4 bound to an antagonist small molecule IT1t and a cyclic peptide CVX15 at 2.5 to 3.2 angstrom resolution. All structures reveal a consistent homodimer with an interface including helices V and VI that may be involved in regulating signaling. The location and shape of the ligand-binding sites differ from other G protein-coupled receptors and are closer to the extracellular surface. These structures provide new clues about the interactions between CXCR4 and its natural ligand CXCL12, and with the HIV-1 glycoprotein gp120.

Induction of interferon-gamma-inducible protein 10 by SARS-CoV infection, interferon alfacon 1 and interferon inducer in human bronchial epithelial Calu-3 cells and BALB/c mice

BACKGROUND

The pathogenesis of severe acute respiratory syndrome coronavirus (SARS-CoV) is poorly understood. Several mechanisms involving both direct effects on target cells and indirect effects via the immune system might exist. SARS-CoV has been shown in vitro to induce changes of cytokines and chemokines in various human and animal cells. We previously reported that interferon (IFN) alfacon-1 was more active against SARS-CoV infection in human bronchial epithelial Calu-3 cells than in African green monkey kidney epithelial cells on day 3 post-infection.

METHODS

In the current study, we first evaluated the efficacy of IFN-alfacon 1 in Calu-3 cells during the first 7 days of virus infection. We then used the two-antibody sandwich ELISA method to detect IFN-gamma-inducible protein 10 (IP-10). We further evaluated the efficacy of antivirals directed against SARS-CoV infection in BALB/c mice.

RESULTS

A potent, prolonged inhibition of SARS-CoV replication in Calu-3 cells with IFN-alfacon 1 was observed. Furthermore, IP-10, an IFN-inducible leukocyte chemoattractant, was detected in Calu-3 cells after SARS-CoV infection. Interestingly, IP-10 expression was shown to be significantly increased when SARS-CoV-infected Calu-3 cells were treated with IFN alfacon-1. IP-10 expression was detected in the lungs of SARS-CoV-infected BALB/c mice. Significantly high levels of mouse IP-10 in BALB/c mice was also detected when SARS-CoV-infected mice were treated with the interferon inducer, polyriboinosinic-polyribocytidylic acid stabilized with poly-L-lysine and carboxymethyl cellulose (poly IC:LC). Treatment with poly IC:LC by intranasal route were effective in protecting mice against a lethal infection with mouse-adapted SARS-CoV and reduced the viral lung titres.

CONCLUSIONS

Our data might provide an important insight into the mechanism of pathogenesis of SARS-CoV and these properties might be therapeutically advantageous.

Monocyte chemoattractant protein-1 (MCP-1): an overview

Chemokines constitute a family of chemoattractant cytokines and are subdivided into four families on the basis of the number and spacing of the conserved cysteine residues in the N-terminus of the protein. Chemokines play a major role in selectively recruiting monocytes, neutrophils, and lymphocytes, as well as in inducing chemotaxis through the activation of G-protein-coupled receptors. Monocyte chemoattractant protein-1 (MCP-1/CCL2) is one of the key chemokines that regulate migration and infiltration of monocytes/macrophages. Both CCL2 and its receptor CCR2 have been demonstrated to be induced and involved in various diseases. Migration of monocytes from the blood stream across the vascular endothelium is required for routine immunological surveillance of tissues, as well as in response to inflammation. This review will discuss these biological processes and the structure and function of CCL2.

Monocyte chemoattractant protein-1 (MCP-1): an overview

Chemokines constitute a family of chemoattractant cytokines and are subdivided into four families on the basis of the number and spacing of the conserved cysteine residues in the N-terminus of the protein. Chemokines play a major role in selectively recruiting monocytes, neutrophils, and lymphocytes, as well as in inducing chemotaxis through the activation of G-protein-coupled receptors. Monocyte chemoattractant protein-1 (MCP-1/CCL2) is one of the key chemokines that regulate migration and infiltration of monocytes/macrophages. Both CCL2 and its receptor CCR2 have been demonstrated to be induced and involved in various diseases. Migration of monocytes from the blood stream across the vascular endothelium is required for routine immunological surveillance of tissues, as well as in response to inflammation. This review will discuss these biological processes and the structure and function of CCL2.

Capripoxvirus G-protein-coupled chemokine receptor: a host-range gene suitable for virus animal origin discrimination

The genus Capripoxvirus within the family Poxviridae comprises three closely related viruses, namely goat pox, sheep pox and lumpy skin disease viruses. This nomenclature is based on the animal species from which the virus was first isolated, respectively, goat, sheep and cattle. Since capripoxviruses are serologically identical, their specific identification relies exclusively on the use of molecular tools. We describe here the suitability of the G-protein-coupled chemokine receptor (GPCR) gene for use in host-range grouping of capripoxviruses. The analysis of 58 capripoxviruses showed three tight genetic clusters consisting of goat pox, sheep pox and lumpy skin disease viruses. However, a few discrepancies exist with the classical virus-host origin nomenclature: a virus isolated from sheep is grouped in the goat poxvirus clade and vice versa. Intra-group diversity was further observed for the goat pox and lumpy skin disease virus isolates. Despite the presence of nine vaccine strains, no genetic determinants of virulence were identified on the GPCR gene. For sheep poxviruses, the addition or deletion of 21 nucleic acids (7 aa) was consistently observed in the 5' terminal part of the gene. Specific signatures for each cluster were also identified. Prediction of the capripoxvirus GPCR topology, and its comparison with other known mammalian GPCRs and viral homologues, revealed not only a classical GPCR profile in the last three-quarters of the protein but also unique features such as a longer N-terminal end with a proximal hydrophobic alpha-helix and a shorter serine-rich C-tail.

Structural and functional characterization of CC chemokine CCL14

CC chemokine ligand 14, CCL14, is a human CC chemokine that is of recent interest because of its natural ability, upon proteolytic processing of the first eight NH2-terminal residues, to bind to and signal through the human immunodeficiency virus type-1 (HIV-1) co-receptor, CC chemokine receptor 5 (CCR5). We report X-ray crystallographic structures of both full-length CCL14 and signaling-active, truncated CCL14 [9-74] determined at 2.23 and 1.8 A, respectively. Although CCL14 and CCL14 [9-74] differ in their ability to bind CCR5 for biological signaling, we find that the NH2-terminal eight amino acids (residues 1 through 8) are completely disordered in CCL14 and both show the identical mode of the dimeric assembly characteristic of the CC type chemokine structures. However, analytical ultracentrifugation studies reveal that the CCL14 is stable as a dimer at a concentration as low as 100 nM, whereas CCL14 [9-74] is fully monomeric at the same concentration. By the same method, the equilibrium between monomers of CCL14 [9-74] and higher order oligomers is estimated to be of EC1,4 = 4.98 microM for monomer-tetramer conversion. The relative instability of CCL14 [9-74] oligomers as compared to CCL14 is also reflected in the Kd's that are estimated by the surface plasmon resonance method to be approximately 9.84 and 667 nM for CCL14 and CCL14 [9-74], respectively. This approximately 60-fold difference in stability at a physiologically relevant concentration can potentially account for their different signaling ability. Functional data from the activity assays by intracellular calcium flux and inhibition of CCR5-mediated HIV-1 entry show that only CCL14 [9-74] is fully active at these near-physiological concentrations where CCL14 [9-74] is monomeric and CCL14 is dimeric. These results together suggest that the ability of CCL14 [9-74] to monomerize can play a role for cellular activation.

Cytokine gene expression profiles in human lymphocytes induced by a formula of traditional Chinese medicine, vigconic VI-28

VI-28 is a formula of traditional Chinese medicine (TCM) that has been used in aged individuals to improve health and, recently, to treat patients with chronic human immunodeficiency virus (HIV) and hepatitis B virus (HBV) infections. The mechanism underlying its clinical effect is, however, largely unknown. In the current study, we used a transwell culture system that mimics the in vivo situation and applied microarray technology to explore the effect of VI-28 on gene expression in human lymphocytes. The VI-28 treatment induced expression of a number of proinflammatory cytokines/chemokines in both peripheral blood mononuclear cells (PBMC) and spleen cells, including interleukin-1 (IL-1), growth-related protein-beta (GRO-beta) and epithelial cellderived neutrophil-activating peptide (ENA-78 [CXCL5]). Furthermore, a specific upregulation of interferon- gamma (IFN-gamma), monokine induced by gamma interferon (MIG [CXCL9]) and interleukin-2 receptor alpha (IL-2Ralpha) in spleen cells was noted, whereas tissue inhibitor of metalloproteinase-3 (TIMP-3) and disabled-2 (DAB2) were downregulated. VI-28 might, thus, enhance both innate and acquired immunity, in particular, T cell function. In addition, genes with no obvious immunologic function, such as insulin-like growth factor-2 (IGF- 1) and CD9, were also differentially affected. Further analysis of individual and combination of ingredients of VI-28 may shed light on the role of this herbal medicine in combating different diseases.

Structural and functional basis of CXCL12 (stromal cell-derived factor-1 alpha) binding to heparin

CXCL12 (SDF-1alpha) and CXCR4 are critical for embryonic development and cellular migration in adults. These proteins are involved in HIV-1 infection, cancer metastasis, and WHIM disease. Sequestration and presentation of CXCL12 to CXCR4 by glycosaminoglycans (GAGs) is proposed to be important for receptor activation. Mutagenesis has identified CXCL12 residues that bind to heparin. However, the molecular details of this interaction have not yet been determined. Here we demonstrate that soluble heparin and heparan sulfate negatively affect CXCL12-mediated in vitro chemotaxis. We also show that a cluster of basic residues in the dimer interface is required for chemotaxis and is a target for inhibition by heparin. We present structural evidence for binding of an unsaturated heparin disaccharide to CXCL12 attained through solution NMR spectroscopy and x-ray crystallography. Increasing concentrations of the disaccharide altered the two-dimensional (1)H-(15)N-HSQC spectra of CXCL12, which identified two clusters of residues. One cluster corresponds to beta-strands in the dimer interface. The second includes the amino-terminal loop and the alpha-helix. In the x-ray structure two unsaturated disaccharides are present. One is in the dimer interface with direct contacts between residues His(25), Lys(27), and Arg(41) of CXCL12 and the heparin disaccharide. The second disaccharide contacts Ala(20), Arg(21), Asn(30), and Lys(64). This is the first x-ray structure of a CXC class chemokine in complex with glycosaminoglycans. Based on the observation of two heparin binding sites, we propose a mechanism in which GAGs bind around CXCL12 dimers as they sequester and present CXCL12 to CXCR4.

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

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

The N-terminal domain of CCL21 reconstitutes high affinity binding, G protein activation, and chemotactic activity, to the C-terminal domain of CCL19

CC chemokine receptor 7 (CCR7), which regulates the trafficking of leucocytes to the secondary lymphoid organs, has two endogenous chemokine ligands: CCL19 and CCL21. Although both ligands possess similar affinities for the receptor and similar abilities to promote G protein activation and chemotaxis, they share only 25% sequence identity. Here, we show that substituting N-terminal six amino acids of CCL21 (SDGGAQ) for the corresponding N-terminal domain of CCL19 (GTNDAE) results in a chimeric chemokine that exhibits high affinity binding and G protein activation of CCR7. These data demonstrate that despite dissimilar sequences, the amino terminal hexapeptide of these two chemokines is capable of performing similar roles resulting in receptor activation.

SMM-chemokines: a class of unnatural synthetic molecules as chemical probes of chemokine receptor biology and leads for therapeutic development

Chemokines and their receptors play important roles in numerous physiological and pathological processes. To develop natural chemokines into receptor probes and inhibitors of pathological processes, the lack of chemokine-receptor selectivity must be overcome. Here, we apply chemical synthesis and the concept of modular modifications to generate unnatural synthetically and modularly modified (SMM)-chemokines that have high receptor selectivity and affinity, and reduced toxicity. A proof of the concept was shown by transforming the nonselective viral macrophage inflammatory protein-II into new analogs with enhanced selectivity and potency for CXCR4 or CCR5, two principal coreceptors for human immunodeficiency virus (HIV)-1 entry. These new analogs provided insights into receptor binding and signaling mechanisms and acted as potent HIV-1 inhibitors. These results support the concept of SMM-chemokines for studying and controlling the function of other chemokine receptors.

The Amino Terminus and the Third Extracellular Loop of CX3CR1 Contain Determinants Critical for Distinct Receptor Functions

The G protein-coupled receptor CX3CR1 is a specific receptor for the CX3C chemokine fractalkine (CX3CL1 according to the new chemokine nomenclature). The aim of this study was to identify receptor elements that contribute independently to agonist binding and receptor activation. Targeted mutation of selected acidic amino acid residues demonstrated that the binding activity of CX3CR1 was critically dependent on the two negatively charged residues Asp25 and Glu254 located on the N-terminal domain and third extracellular loop, respectively. In addition, mutation of the uncharged polar residue Tyr14 in the amino terminus caused a reduction in the ligand binding affinity. In contrast, the three acidic residues Glu13, Asp16, and Asp266 did not contribute to ligand binding but were crucial for receptor activation. The mutant receptors E13A, D16A, and D266A bound fractalkine with high affinity but were unable to induce signaling events necessary to support chemotaxis. These acidic residues may engage in electrostatic interactions with basic residues on fractalkine that are necessary for receptor function but not for binding. Our data are consistent with a model of chemokine receptor activation consisting of a multi-step mechanism. Step one mediates the high-affinity fractalkine binding involving Tyr14, Asp25, and Glu254. The initial interaction then triggers the engagement of Glu13, Asp16, and Asp266, which are necessary for CX3CR1 activation.

Innate and adaptive immunity in female genital tract: cellular responses and interactions

The mucosal immune system in the female reproductive tract (FRT) has evolved to meet the unique requirements of dealing with sexually transmitted bacterial and viral pathogens, allogeneic spermatozoa, and the immunologically distinct fetus. Analysis of the FRT indicates that the key cells of the innate and adaptive immune systems are present and functionally responsive to antigens. Acting through Toll-like receptors in the Fallopian tubes, uterus, cervix, and in the vagina, epithelial cells, macrophages, natural killer cells, and neutrophils confer protection through the production of chemokines and cytokines, which recruit and activate immune cells, as well as bactericidal and virucidal agents, which confer protection at times when adaptive immunity is downregulated by sex hormones to meet the constraints of procreation. The overall goal of this paper is to define the innate immune system in the FRT and, where possible, to define the regulatory influences that occur during the menstrual cycle that contribute to protection from and susceptibility to potential pathogens. By understanding the nature of this protection and the ways in which innate and adaptive immunity interact, these studies provide the opportunity to contribute to the foundation of information essential for ensuring reproductive health.

Transient expression of CC chemokine TECK in the ovary during ovulation: its potential role in ovulation

PROBLEM

Chemokine thymus-expressed chemokine (TECK), which is expressed exclusively in the thymus and small intestine, plays a critical role in T-cell development. Our previous study revealed its expression in the ovary also. This study investigated its ovarian expression during ovulatory process.

METHOD OF STUDY

Super-ovulation was induced in young female CD1 mice by equine chorionic gonadotropin (eCG) and human chorionic gonadotropic (hCG). Ovarian TECK expression during ovulation was determined by: (1) reverse transcriptase-polymerase chain reaction (RT-PCR) at mRNA level, (2) Western blot and immunohistology at the protein level, and (3) leukocyte infiltration assay at the bioactive level.

RESULTS

A transient, high-level expression of TECK in murine ovaries at the mRNA level during hCG-induced ovulation was detected. Sequencing of directly cloned PCR product confirmed the ovarian expression of TECK. The peak expression of TECK was observed at 10-12 hr post-hCG injection; real-time PCR revealed an 800-fold increase during its expression peak over 0 hr. The expressed ovarian TECK protein was readily detectable by Western blot. Immunohistochemistry localized TECK expression to the ovarian interstitial tissue surrounding, or in the theca layer of the mature follicles undergoing ovulatory process. Expression of TECK receptor, the CC chemokine receptor (CCR9) was also detected in the ovulating ovaries. Using in vitro leukocyte infiltration assay, we first demonstrated that ovaries undergoing the ovulatory process were able to selectively chemoattract mononuclear cells. Importantly, neutralization of TECK by the antibody resulted in a 85% reduction in the chemotactic activities of the ovaries.

CONCLUSION

This study suggested that ovarian expression of TECK is under a tight hormonal regulation, and expressed TECK may be responsible for recruitment of mononuclear cells into the ovary to participate in the ovulatory process.

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

CC and CX3C chemokines differentially interact with the N terminus of the human cytomegalovirus-encoded US28 receptor

Human cytomegalovirus (HCMV) is the causative agent of life-threatening systemic diseases in immunocompromised patients as well as a risk factor for vascular pathologies, like atherosclerosis, in immunocompetent individuals. HCMV encodes a G-protein-coupled receptor (GPCR), referred to as US28, that displays homology to the human chemokine receptor CCR1 and binds several chemokines of the CC family as well as the CX3C chemokine fractalkine with high affinity. Most importantly, following HCMV infection, US28 activates several intracellular pathways, either constitutively or in a chemokine-dependent manner. In this study, our goal was to understand the molecular interactions between chemokines and the HCMV-encoded US28 receptor. To achieve this goal, a double approach has been used, consisting in the analysis of both receptor and ligand mutants. This approach has led us to identify several amino acids located in the N terminus of US28 that differentially contribute to the high affinity binding of CC versus CX3C chemokines. Additionally, our results highlight the importance of secondary modifications occurring at US28, such as sulfation, for ligand recognition. Finally, the effects of chemokine dimerization and interaction with glycosaminoglycans (GAGs) on chemokine binding and activation of US28 were investigated as well using CCL4 as model ligand. In line with the two-state model describing chemokine/receptor interaction, we show that an aromatic residue in the N-loop region of CCL4 promotes tight binding to US28, whereas receptor activation depends on the presence of the N terminus of CCL4, as shown previously for CCR5.

Functional analysis of chemically synthesized derivatives of the human CC chemokine CCL15/HCC-2, a high affinity CCR1 ligand

The CCL15 is a human CC chemokine that activates the receptors, CCR1 and CCR3. Unlike other chemokines, it contains an unusually long N-terminal domain of 31 amino acids preceding the first cysteine residue and a third disulfide bond. To elucidate the functional role of distinct structural determinants, a series of sequential amino-terminal truncated and point-mutated CCL15 derivatives as well as mutants lacking the third disulfide bond and the carboxy-terminal alpha-helix were synthesized using 9-fluorenylmethoxycarbonyl (Fmoc) chemistry. We demonstrate that a truncation of 24 amino acid residues (delta24-CCL15) converts the slightly active 92-residue delta0-CCL15 into a potent agonist of CC chemokine receptor 1 (CCR1) and a weak agonist of CCR3 in cell-based assays. The biological activity decreases from delta24-CCL15 to delta29-CCL15, and re-increases from delta29-CCL15 to delta30-CCL15. Thus, an exocyclic N-terminal region of only one amino acid residue is sufficient for efficient CCR1 activation. As none of the peptides investigated except for delta24-CCL15 activates CCR3, we suggest that CCR1 is the major receptor for CCL15 in vivo. Further we demonstrate that the third disulfide bond of CCL15 and an exchange of tyrosine in position 70 by a leucine residue, which is conserved in CXC chemokines, do not alter the interaction with CCR1. In contrast, a CCL15 derivative lacking the carboxy-terminal alpha-helix exhibits a complete loss of tertiary structure and hence loss of CCR1 agonistic and binding activity. This study demonstrates that specific protein residues in chemokines, which contribute to receptor-ligand interaction, vary significantly between chemokines and cannot be extrapolated using data from functionally related chemokines.

Identification of CC chemokine receptor 7 residues important for receptor activation

The binding pocket of family A GPCRs that bind small biogenic amines is well characterized. In this study we identify residues on CC chemokine receptor 7 (CCR-7) that are involved in agonist-mediated receptor activation but not in high affinity ligand binding. The mutations also affect the ability of the ligands to induce chemotaxis. Two of the residues, Lys3.33(137) and Gln5.42(227), are consistent with the binding pocket described for biogenic amines, while Lys3.26(130) and Asn7.32(305), are found at, or close to, the cell surface. Our observations are in agreement with findings from other peptide and chemokine receptors, which indicate that receptors that bind larger ligands contain contact sites closer to the cell surface in addition to the conventional transmembrane binding pocket. These findings also support the theory that chemokine receptors require different sets of interactions for high affinity ligand binding and receptor activation.

Determinants of high-affinity binding and receptor activation in the N-terminus of CCL-19 (MIP-3 beta)

CC chemokine receptor 7 (CCR-7) is expressed on mature dendritic cells and T-cells. Its ligands, CCL-19 (MIP-3beta) and CCL-21 (SLC), play an important role in the migration of these cells to secondary lymphoid organs where they are predominantly expressed. For most chemokines, the N-terminal domain preceding the first two conserved cysteines is involved in stabilizing the active conformation of its cognate receptors. We have chemically synthesized N-terminal analogues of CCL-19 with the aid of a native chemical ligation method to investigate structure function requirements of this ligand domain by performing ligand binding, GTP-gammaS binding, and chemotaxis assays. Successive truncations of the N-terminus of CCL-19 reduced the affinity of the receptor for the ligand in a size-dependent manner. Furthermore, Ala substitutions of Asn(3), Asp(4), and Asp(7) show that the side chains of these residues are important for high-affinity binding of CCL-19 to CCR-7. The effects observed were mirrored in both GTP-gammaS binding and chemotaxis assays, highlighting the functional importance of this ligand domain. We also describe two partial agonists of CCR-7 ([Nle(72)]CCL-19(6-77) and Ac-[Nle(72)]CCL-19(7-77)), and identify the first analogue of CCL-19 (Ac-[Nle(72)]CCL-19(8-77)) that acts as a functional antagonist in vitro (K(B) approximately 350 nM for GTP-gammaS binding assays). As mutations of both Glu(6) and Asp(7) to Ala did not dissociate effects on ligand binding from receptor activation, it is likely that the backbone of these two residues is crucial for agonist activity.

Eotaxin and monocyte chemotactic protein-3 use different modes of action

Eotaxin selectively binds CC chemokine receptor (CCR) 3, whereas monocyte chemotactic protein (MCP)-3 binds CCR1, CCR2, and CCR3. To identify the functional determinants of the chemokines, we generated four reciprocal chimeric chemokines-M10E9, M22E21, E8M11, and E20M23-by shuffling the N-terminus and N-loop of eotaxin and MCP-3. M22E21 and E8M11, which shared the N-loop from MCP-3, bound to monocytes with high affinity, and activated monocytes. In contrast, M10E9 and E20M23, which lacked the N-loop, failed to bind and transduce monocyte responses, identifying the N-loop of MCP-3 as the selectivity determinant for CCR1/CCR2. A BIAcore assay with an N-terminal peptide of CCR3 (residues 1-35) revealed that all chimeras except E20M23 exhibited varying degrees of binding affinity with commensurate chemotaxis activity of eosinophils. Surprisingly, E20M23 could neither bind the CCR3 peptide nor activate eosinophils, despite having both N-terminal motifs from eotaxin. These results suggest that the two N-terminal motifs of eotaxin must cooperate with other regions to successfully bind and activate CCR3.