Structural basis of chemokine recognition by the class A3 tick evasin EVA-ACA1001.

Ticks produce chemokine-binding proteins, known as evasins, in their saliva to subvert the host's immune response. Evasins bind to chemokines and thereby inhibit the activation of their cognate chemokine receptors, thus suppressing leukocyte recruitment and inflammation. We recently described subclass A3 evasins, which, like other class A evasins, exclusively target CC chemokines but appear to use a different binding site architecture to control target selectivity among CC chemokines. We now describe the structural basis of chemokine recognition by the class A3 evasin EVA-ACA1001. EVA-ACA1001 binds to almost all human CC chemokines and inhibits receptor activation. Truncation mutants of EVA-ACA1001 showed that, unlike class A1 evasins, both the N- and C-termini of EVA-ACA1001 play minimal roles in chemokine binding. To understand the structural basis of its broad chemokine recognition, we determined the crystal structure of EVA-ACA1001 in complex with the human chemokine CCL16. EVA-ACA1001 forms backbone-backbone interactions with the CC motif of CCL16, a conserved feature of all class A evasin-chemokine complexes. A hydrophobic pocket in EVA-ACA1001, formed by several aromatic side chains and the unique disulfide bond of class A3 evasins, accommodates the residue immediately following the CC motif (the "CC + 1 residue") of CCL16. This interaction is shared with EVA-AAM1001, the only other class A3 evasins characterized to date, suggesting it may represent a common mechanism that accounts for the broad recognition of CC chemokines by class A3 evasins.

Engineering broad-spectrum inhibitors of inflammatory chemokines from subclass A3 tick evasins.

Chemokines are key regulators of leukocyte trafficking and attractive targets for anti-inflammatory therapy. Evasins are chemokine-binding proteins from tick saliva, whose application as anti-inflammatory therapeutics will require manipulation of their chemokine target selectivity. Here we describe subclass A3 evasins, which are unique to the tick genus Amblyomma and distinguished from "classical" class A1 evasins by an additional disulfide bond near the chemokine recognition interface. The A3 evasin EVA-AAM1001 (EVA-A) bound to CC chemokines and inhibited their receptor activation. Unlike A1 evasins, EVA-A was not highly dependent on N- and C-terminal regions to differentiate chemokine targets. Structures of chemokine-bound EVA-A revealed a deep hydrophobic pocket, unique to A3 evasins, that interacts with the residue immediately following the CC motif of the chemokine. Mutations to this pocket altered the chemokine selectivity of EVA-A. Thus, class A3 evasins provide a suitable platform for engineering proteins with applications in research, diagnosis or anti-inflammatory therapy.

Unravelling G protein-coupled receptor signalling networks using global phosphoproteomics.

G protein-coupled receptor (GPCR) activation initiates signalling via a complex network of intracellular effectors that combine to produce diverse cellular and tissue responses. Although we have an advanced understanding of the proximal events following receptor stimulation, the molecular detail of GPCR signalling further downstream often remains obscure. Unravelling these GPCR-mediated signalling networks has important implications for receptor biology and drug discovery. In this context, phosphoproteomics has emerged as a powerful approach for investigating global GPCR signal transduction. Here, we provide a brief overview of the phosphoproteomic workflow and discuss current limitations and future directions for this technology. By highlighting some of the novel insights into GPCR signalling networks gained using phosphoproteomics, we demonstrate the utility of global phosphoproteomics to dissect GPCR signalling networks and to accelerate discovery of new targets for therapeutic development.

Swapping N-terminal regions among tick evasins reveals cooperative interactions influencing chemokine binding and selectivity.

Class A tick evasins are natural chemokine-binding proteins that block the signaling of multiple chemokines from the CC subfamily through their cognate receptors, thus suppressing leukocyte recruitment and inflammation. Development of tick evasins as chemokine-targeted anti-inflammatory therapeutics requires an understanding of the factors controlling their chemokine recognition and selectivity. To investigate the role of the evasin N-terminal region for chemokine recognition, we prepared chimeric evasins by interchanging the N-terminal regions of four class A evasins, including a newly identified evasin, EVA-RPU02. We show through chemokine binding analysis of the parental and chimeric evasins that the N-terminal region is critical for chemokine binding affinity and selectivity. Notably, we found some chimeras were unable to bind certain cognate chemokine ligands of both parental evasins. Moreover, unlike any natural evasins characterized to date, some chimeras exhibited specific binding to a single chemokine. These results indicate that the evasin N terminus interacts cooperatively with the "body" of the evasin to enable optimum chemokine recognition. Furthermore, the altered chemokine selectivity of the chimeras validates the approach of engineering the N termini of evasins to yield unique chemokine recognition profiles.

Structure-guided engineering of tick evasins for targeting chemokines in inflammatory diseases.

As natural chemokine inhibitors, evasin proteins produced in tick saliva are potential therapeutic agents for numerous inflammatory diseases. Engineering evasins to block the desired chemokines and avoid off-target side effects requires structural understanding of their target selectivity. Structures of the class A evasin EVA-P974 bound to human CC chemokine ligands 7 and 17 (CCL7 and CCL17) and to a CCL8-CCL7 chimera reveal that the specificity of class A evasins for chemokines of the CC subfamily is defined by conserved, rigid backbone-backbone interactions, whereas the preference for a subset of CC chemokines is controlled by side-chain interactions at four hotspots in flexible structural elements. Hotspot mutations alter target preference, enabling inhibition of selected chemokines. The structure of an engineered EVA-P974 bound to CCL2 reveals an underlying molecular mechanism of EVA-P974 target preference. These results provide a structure-based framework for engineering evasins as targeted antiinflammatory therapeutics.

Glycosylation Regulates N-Terminal Proteolysis and Activity of the Chemokine CCL14.

Chemokines are secreted proteins that regulate leukocyte migration during inflammatory responses by signaling through chemokine receptors. Full length CC chemokine ligand 14, CCL14(1-74), is a weak agonist for the chemokine receptor CCR1, but its activity is substantially enhanced upon proteolytic cleavage to CCL14(9-74). CCL14 is O-glycosylated at Ser7, adjacent to the site of proteolytic activation. To determine whether glycosylation regulates the activity of CCL14, we used native chemical ligation to prepare four homogeneously glycosylated variants of CCL14(1-74). Each protein was assembled from three synthetic peptide fragments in "one-pot" using two sequential ligation reactions. We show that while glycosylation of CCL14(1-74) did not affect CCR1 binding affinity or potency of activation, sialylated variants of CCL14(1-74) exhibited reduced activity after treatment with plasmin compared to nonsialylated forms. These data indicate that glycosylation may influence the biological activity of CCL14 by regulating its conversion from the full-length to the truncated, activated form.

Systematic Assessment of Chemokine Signaling at Chemokine Receptors CCR4, CCR7 and CCR10.

Chemokines interact with chemokine receptors in a promiscuous network, such that each receptor can be activated by multiple chemokines. Moreover, different chemokines have been reported to preferentially activate different signalling pathways via the same receptor, a phenomenon known as biased agonism. The human CC chemokine receptors (CCRs) CCR4, CCR7 and CCR10 play important roles in T cell trafficking and have been reported to display biased agonism. To systematically characterize these effects, we analysed G protein- and ß-arrestin-mediated signal transduction resulting from stimulation of these receptors by each of their cognate chemokine ligands within the same cellular background. Although the chemokines did not elicit ligand-biased agonism, the three receptors exhibited different arrays of signaling outcomes. Stimulation of CCR4 by either CC chemokine ligand 17 (CCL17) or CCL22 induced ß-arrestin recruitment but not G protein-mediated signaling, suggesting that CCR4 has the potential to act as a scavenger receptor. At CCR7, both CCL19 and CCL21 stimulated G protein signaling and ß-arrestin recruitment, with CCL19 consistently displaying higher potency. At CCR10, CCL27 and CCL28(4-108) stimulated both G protein signaling and ß-arrestin recruitment, whereas CCL28(1-108) was inactive, suggesting that CCL28(4-108) is the biologically relevant form of this chemokine. These comparisons emphasize the intrinsic abilities of different receptors to couple with different downstream signaling pathways. Comparison of these results with previous studies indicates that differential agonism at these receptors may be highly dependent on the cellular context.

Semisynthesis of an evasin from tick saliva reveals a critical role of tyrosine sulfation for chemokine binding and inhibition.

Blood-feeding arthropods produce antiinflammatory salivary proteins called evasins that function through inhibition of chemokine-receptor signaling in the host. Herein, we show that the evasin ACA-01 from the Amblyomma cajennense tick can be posttranslationally sulfated at two tyrosine residues, albeit as a mixture of sulfated variants. Homogenously sulfated variants of the proteins were efficiently assembled via a semisynthetic native chemical ligation strategy. Sulfation significantly improved the binding affinity of ACA-01 for a range of proinflammatory chemokines and enhanced the ability of ACA-01 to inhibit chemokine signaling through cognate receptors. Comparisons of evasin sequences and structural data suggest that tyrosine sulfation serves as a receptor mimetic strategy for recognizing and suppressing the proinflammatory activity of a wide variety of mammalian chemokines. As such, the incorporation of this posttranslational modification (PTM) or mimics thereof into evasins may provide a strategy to optimize tick salivary proteins for antiinflammatory applications.

Discovery of Potent Cyclic Sulfopeptide Chemokine Inhibitors via Reprogrammed Genetic Code mRNA Display.

Targeting chemokine signaling is an attractive avenue for the treatment of inflammatory disorders. Tyrosine sulfation is an important post-translational modification (PTM) that enhances chemokine-receptor binding and is also utilized by a number of pathogenic organisms to improve the binding affinity of immune-suppressive chemokine binding proteins (CKBPs). Here we report the display selection of tyrosine-sulfated cyclic peptides using a reprogrammed genetic code to discover high-affinity ligands for the chemokine CCL11 (eotaxin-1). The selected cyclic sulfopeptides possess high affinity for the target chemokine (as well as one or more of the related family members CCL2, CCL7 and CCL24) and inhibit CCL11 activation of CC chemokine receptor 3 (CCR3). This work demonstrates the utility of exploiting native PTMs as binding motifs for the generation of new leads for medicinal chemistry.

Phosphoproteomic characterization of the signaling network resulting from activation of the chemokine receptor CCR2.

Leukocyte recruitment is a universal feature of tissue inflammation and regulated by the interactions of chemokines with their G protein-coupled receptors. Activation of CC chemokine receptor 2 (CCR2) by its cognate chemokine ligands, including CC chemokine ligand 2 (CCL2), plays a central role in recruitment of monocytes in several inflammatory diseases. In this study, we used phosphoproteomics to conduct an unbiased characterization of the signaling network resulting from CCL2 activation of CCR2. Using data-independent acquisition MS analysis, we quantified both the proteome and phosphoproteome in FlpIn-HEK293T cells stably expressing CCR2 at six time points after activation with CCL2. Differential expression analysis identified 699 significantly regulated phosphorylation sites on 441 proteins. As expected, many of these proteins are known to participate in canonical signal transduction pathways and in the regulation of actin cytoskeleton dynamics, including numerous guanine nucleotide exchange factors and GTPase-activating proteins. Moreover, we identified regulated phosphorylation sites in numerous proteins that function in the nucleus, including several constituents of the nuclear pore complex. The results of this study provide an unprecedented level of detail of CCR2 signaling and identify potential targets for regulation of CCR2 function.

Structural basis of chemokine and receptor interactions: Key regulators of leukocyte recruitment in inflammatory responses.

In response to infection or injury, the body mounts an inflammatory immune response in order to neutralize pathogens and promote tissue repair. The key effector cells for these responses are the leukocytes (white blood cells), which are specifically recruited to the site of injury. However, dysregulation of the inflammatory response, characterized by the excessive migration of leukocytes to the affected tissues, can also lead to chronic inflammatory diseases. Leukocyte recruitment is regulated by inflammatory mediators, including an important family of small secreted chemokines and their corresponding G protein-coupled receptors expressed in leukocytes. Unsurprisingly, due to their central role in the leukocyte inflammatory response, chemokines and their receptors have been intensely investigated and represent attractive drug targets. Nonetheless, the full therapeutic potential of chemokine receptors has not been realized, largely due to the complexities in the chemokine system. The determination of chemokine-receptor structures in recent years has dramatically shaped our understanding of the molecular mechanisms that underpin chemokine signaling. In this review, we summarize the contemporary structural view of chemokine-receptor recognition, and describe the various binding modes of peptide and small-molecule ligands to chemokine receptors. We also provide some perspectives on the implications of these data for future research and therapeutic development. IMPORTANCE STATEMENT: Given their central role in the leukocyte inflammatory response, chemokines and their receptors are considered as important regulators of physiology and viable therapeutic targets. In this review, we provide a summary of the current understanding of chemokine: chemokine-receptor interactions that have been gained from structural studies, as well as their implications for future drug discovery efforts.

Evasins: Tick Salivary Proteins that Inhibit Mammalian Chemokines.

Ticks are hematophagous arachnids that parasitize mammals and other hosts, feeding on their blood. Ticks secrete numerous salivary factors that enhance host blood flow or suppress the host inflammatory response. The recruitment of leukocytes, a hallmark of inflammation, is regulated by chemokines, which activate chemokine receptors on the leukocytes. Ticks target this process by secreting glycoproteins called Evasins, which bind to chemokines and prevent leukocyte recruitment. This review describes the recent discovery of numerous Evasins produced by ticks, their classification into two structural and functional classes, and the efficacy of Evasins in animal models of inflammatory diseases. The review also proposes a standard nomenclature system for Evasins and discusses the potential of repurposing or engineering Evasins as therapeutic anti-inflammatory agents.

Influence of Chemokine N-Terminal Modification on Biased Agonism at the Chemokine Receptor CCR1.

Leukocyte migration, a hallmark of the inflammatory response, is stimulated by the interactions between chemokines, which are expressed in injured or infected tissues, and chemokine receptors, which are G protein-coupled receptors (GPCRs) expressed in the leukocyte plasma membrane. One mechanism for the regulation of chemokine receptor signaling is biased agonism, the ability of different chemokine ligands to preferentially activate different intracellular signaling pathways via the same receptor. To identify features of chemokines that give rise to biased agonism, we studied the activation of the receptor CCR1 by the chemokines CCL7, CCL8, and CCL15(Δ26). We found that, compared to CCL15(Δ26), CCL7 and CCL8 exhibited biased agonism towards cAMP inhibition and away from ß-Arrestin 2 recruitment. Moreover, N-terminal substitution of the CCL15(Δ26) N-terminus with that of CCL7 resulted in a chimera with similar biased agonism to CCL7. Similarly, N-terminal truncation of CCL15(Δ26) also resulted in signaling bias between cAMP inhibition and ß-Arrestin 2 recruitment signals. These results show that the interactions of the chemokine N-terminal region with the receptor transmembrane region play a key role in selecting receptor conformations coupled to specific signaling pathways.

Evaluation and extension of the two-site, two-step model for binding and activation of the chemokine receptor CCR1.

Interactions between secreted immune proteins called chemokines and their cognate G protein-coupled receptors regulate the trafficking of leukocytes in inflammatory responses. The two-site, two-step model describes these interactions. It involves initial binding of the chemokine N-loop/ß3 region to the receptor's N-terminal region and subsequent insertion of the chemokine N-terminal region into the transmembrane helical bundle of the receptor concurrent with receptor activation. Here, we test aspects of this model with C-C motif chemokine receptor 1 (CCR1) and several chemokine ligands. First, we compared the chemokine-binding affinities of CCR1 with those of peptides corresponding to the CCR1 N-terminal region. Relatively low affinities of the peptides and poor correlations between CCR1 and peptide affinities indicated that other regions of the receptor may contribute to binding affinity. Second, we evaluated the contributions of the two CCR1-interacting regions of the cognate chemokine ligand CCL7 (formerly monocyte chemoattractant protein-3 (MCP-3)) using chimeras between CCL7 and the non-cognate ligand CCL2 (formerly MCP-1). The results revealed that the chemokine N-terminal region contributes significantly to binding affinity but that differences in binding affinity do not completely account for differences in receptor activation. On the basis of these observations, we propose an elaboration of the two-site, two-step model-the "three-step" model-in which initial interactions of the first site result in low-affinity, nonspecific binding; rate-limiting engagement of the second site enables high-affinity, specific binding; and subsequent conformational rearrangement gives rise to receptor activation.

Proteomic Identification of Interferon-Induced Proteins with Tetratricopeptide Repeats as Markers of M1 Macrophage Polarization.

Macrophages, which accumulate in tissues during inflammation, may be polarized toward pro-inflammatory (M1) or tissue reparative (M2) phenotypes. The balance between these phenotypes can have a substantial influence on the outcome of inflammatory diseases such as atherosclerosis. Improved biomarkers of M1 and M2 macrophages would be beneficial for research, diagnosis, and monitoring the effects of trial therapeutics in such diseases. To identify novel biomarkers, we have characterized the global proteomes of THP-1 macrophages polarized to M1 and M2 states in comparison with unpolarized (M0) macrophages. M1 polarization resulted in increased expression of numerous pro-inflammatory proteins including the products of 31 genes under the transcriptional control of interferon regulatory factor 1 (IRF-1). In contrast, M2 polarization identified proteins regulated by components of the transcription factor AP-1. Among the most highly upregulated proteins under M1 conditions were the three interferon-induced proteins with tetratricopeptide repeats (IFITs: IFIT1, IFIT2, and IFIT3), which function in antiviral defense. Moreover, IFIT1, IFIT2, and IFIT3 mRNA were strongly upregulated in M1 polarized human primary macrophages and IFIT1 was also expressed in a subset of macrophages in aortic sinus and brachiocephalic artery sections from atherosclerotic ApoE-/- mice. On the basis of these results, we propose that IFITs may serve as useful markers of atherosclerosis and potentially other inflammatory diseases.

Regulation of Chemokine-Receptor Interactions and Functions.

Inflammation is the body's response to injury or infection. As early as 2000 years ago, the Roman encyclopaedist Aulus Cornelius Celsus recognised four cardinal signs of this response-redness, heat, swelling and pain; a fifth sign is loss of function.[...].

Ticks from diverse genera encode chemokine-inhibitory evasin proteins.

To prolong residence on their hosts, ticks secrete many salivary factors that target host defense molecules. In particular, the tick Rhipicephalus sanguineus has been shown to produce three salivary glycoproteins named "evasins," which bind to host chemokines, thereby inhibiting the recruitment of leukocytes to the location of the tick bite. Using sequence similarity searches, we have identified 257 new putative evasin sequences encoded by the genomes or salivary or visceral transcriptomes of numerous hard ticks, spanning the genera Rhipicephalus, Amblyomma, and Ixodes of the Ixodidae family. Nine representative sequences were successfully expressed in Escherichia coli, and eight of the nine candidates exhibited high-affinity binding to human chemokines. Sequence alignments enabled classification of the evasins into two subfamilies: C8 evasins share a conserved set of eight Cys residues (four disulfide bonds), whereas C6 evasins have only three of these disulfide bonds. Most of the identified sequences contain predicted secretion leader sequences, N-linked glycosylation sites, and a putative site of tyrosine sulfation. We conclude that chemokine-binding evasin proteins are widely expressed among tick species of the Ixodidae family, are likely to play important roles in subverting host defenses, and constitute a valuable pool of anti-inflammatory proteins for potential future therapeutic applications.

Key determinants of selective binding and activation by the monocyte chemoattractant proteins at the chemokine receptor CCR2.

Chemokines and their receptors collectively orchestrate the trafficking of leukocytes in normal immune function and inflammatory diseases. Different chemokines can induce distinct responses at the same receptor. In comparison to monocyte chemoattractant protein-1 (MCP-1; also known as CCL2), the chemokines MCP-2 (CCL8) and MCP-3 (CCL7) are partial agonists of their shared receptor CCR2, a key regulator of the trafficking of monocytes and macrophages that contribute to the pathology of atherosclerosis, obesity, and type 2 diabetes. Through experiments with chimeras of MCP-1 and MCP-3, we identified the chemokine amino-terminal region as being the primary determinant of both the binding and signaling selectivity of these two chemokines at CCR2. Analysis of CCR2 mutants showed that the chemokine amino terminus interacts with the major subpocket in the transmembrane helical bundle of CCR2, which is distinct from the interactions of some other chemokines with the minor subpockets of their receptors. These results suggest the major subpocket as a target for the development of small-molecule inhibitors of CCR2.

Sulfation of the Human Cytomegalovirus Protein UL22A Enhances Binding to the Chemokine RANTES.

UL22A is an 83 amino acid chemokine-binding protein produced by human cytomegalovirus that likely assists the virus in dampening the host antiviral response. We proposed that UL22A is sulfated on two tyrosine residues and tested this hypothesis through the chemical synthesis of a small library of differentially sulfated protein variants. The (sulfo)proteins were efficiently prepared using a novel ß-selenoleucine motif to facilitate one-pot ligation-deselenization chemistry. Tyrosine sulfation of UL22A proved critical for RANTES binding, with the doubly sulfated variant exhibiting an improvement in binding of 2.5 orders of magnitude compared to the unmodified protein.