Glycosaminoglycans mediate cell surface oligomerization of chemokines

The CXCL12/CXCR7 signaling axis, isoforms, circadian rhythms, and tumor cellular composition dictate gradients in tissue

Chemokine CXCL12 gradients drive chemotaxis in a CXCR4-dependent mechanism and have been implicated in cancer metastasis. While CXCL12 gradients are typically studied in organized, defined environments, the tumor microenvironment is disorganized. In vivo, CXCL12 gradients depend on many factors: the number and arrangement of cells secreting and degrading CXCL12, isoform-dependent binding to the extracellular matrix, diffusion, and circadian fluctuations. We developed a computational model of the tumor microenvironment to simulate CXCL12 gradient dynamics in disorganized tissue. There are four major findings from the model. First, CXCL12-β and -γ form higher magnitude (steeper) gradients compared to CXCL12-α. Second, endothelial CXCR7+ cells regulate CXCL12 gradient direction by controlling concentrations near but not far from the vasculature. Third, the magnitude and direction of CXCL12 gradients are dependent on the local composition of secreting and scavenging cells within the tumor. We theorize that "micro-regions" of cellular heterogeneity within the tumor are responsible for forming strong gradients directed into the blood. Fourth, CXCL12 circadian fluctuations influence gradient magnitude but not direction. Our simulations provide predictions for future experiments in animal models. Understanding the generation of CXCL12 gradients is crucial to inhibiting cancer metastasis.

Syndecan-1 regulates dendritic cell migration in cutaneous hypersensitivity to haptens

In human dendritic cells (DCs), we previously demonstrated in vitro that syndecan-1 (SDC1) is downregulated during maturation correlating with enhanced motility. We investigated the effects of SDC1 on DC migration in vivo during TNCB(2,4,6-trinitro-1-chlorobenzene)-induced cutaneous hypersensitivity reaction (CHS) in mice. We show that DC in SDC1-deficient mice migrated faster and at a higher rate to lymph nodes draining the hapten-painted skin. Adoptive transfer of SDC1-deficient hapten- and fluorochrome-labelled DC into wild-type (WT) mice led to increased and faster migration of DC to paracortical lymph nodes, and to a stronger CHS compared to WT DC. In SDC1-/- mice, CCR7 remains longer on the DC surface within the first 15-minutes maturation (after LPS-induced maturation). In addition, a time-dependent upregulation of CCL2, CCL3, VCAM1 and talin was found during maturation in SDC1-/- DC. However, no difference in T-cell-stimulating capacity of SDC1-deficient DC was found compared to WT DC. Mechanistically, SDC1-deficient DC showed enhanced migration towards CCL21 and CCL19. This may result from functional overexpression of CCR7 in SDC1-/- DC. Increased and accelerated migration of otherwise functionally intact SDC1-deficient DC leads to an exacerbated CHS. Based on our results, we conclude that SDC1 on DC negatively regulates DC migration.

Platelet-Derived Chemokine CXCL7 Dimer Preferentially Exists in the Glycosaminoglycan-Bound Form: Implications for Neutrophil-Platelet Crosstalk

Platelet-derived chemokine CXCL7 (also known as NAP-2) plays a crucial role in orchestrating neutrophil recruitment in response to vascular injury. CXCL7 exerts its function by activating the CXC chemokine receptor 2 (CXCR2) receptor and binding sulfated glycosaminoglycans (GAGs) that regulate receptor activity. CXCL7 exists as monomers, dimers, and tetramers, and previous studies have shown that the monomer dominates at lower and the tetramer at higher concentrations. These observations then raise the question: what, if any, is the role of the dimer? In this study, we make a compelling observation that the dimer is actually the favored form in the GAG-bound state. Further, we successfully characterized the structural basis of dimer binding to GAG heparin using solution nuclear magnetic resonance (NMR) spectroscopy. The chemical shift assignments were obtained by exploiting heparin binding-induced NMR spectral changes in the WT monomer and dimer and also using a disulfide-linked obligate dimer. We observe that the receptor interactions of the dimer are similar to the monomer and that heparin-bound dimer is occluded from receptor interactions. Cellular assays also show that the heparin-bound CXCL7 is impaired for CXCR2 activity. We conclude that the dimer–GAG interactions play an important role in neutrophil–platelet crosstalk, and that these interactions regulate gradient formation and the availability of the free monomer for CXCR2 activation and intrathrombus neutrophil migration to the injury site.

Chemokines from a Structural Perspective

Chemokines are a family of small, highly conserved cytokines that mediate various biological processes, including chemotaxis, hematopoiesis, and angiogenesis, and that function by interacting with cell surface G-Protein Coupled Receptors (GPCRs). Because of their significant involvement in various biological functions and pathologies, chemokines and their receptors have been the focus of therapeutic discovery for clinical intervention. There are several sub-families of chemokines (e.g., CXC, CC, C, and CX3C) defined by the positions of sequentially conserved cysteine residues. Even though all chemokines also have a highly conserved, three-stranded β-sheet/α-helix tertiary structural fold, their quarternary structures vary significantly with their sub-family. Moreover, their conserved tertiary structures allow for subunit swapping within and between sub-family members, thus promoting the concept of a “chemokine interactome”. This review is focused on structural aspects of CXC and CC chemokines, their functional synergy and ability to form heterodimers within the chemokine interactome, and some recent developments in structure-based chemokine-targeted drug discovery.

Glycosaminoglycan Interactions with Chemokines Add Complexity to a Complex System

Chemokines have two types of interactions that function cooperatively to control cell migration. Chemokine receptors on migrating cells integrate signals initiated upon chemokine binding to promote cell movement. Interactions with glycosaminoglycans (GAGs) localize chemokines on and near cell surfaces and the extracellular matrix to provide direction to the cell movement. The matrix of interacting chemokine–receptor partners has been known for some time, precise signaling and trafficking properties of many chemokine–receptor pairs have been characterized, and recent structural information has revealed atomic level detail on chemokine–receptor recognition and activation. However, precise knowledge of the interactions of chemokines with GAGs has lagged far behind such that a single paradigm of GAG presentation on surfaces is generally applied to all chemokines. This review summarizes accumulating evidence which suggests that there is a great deal of diversity and specificity in these interactions, that GAG interactions help fine-tune the function of chemokines, and that GAGs have other roles in chemokine biology beyond localization and surface presentation. This suggests that chemokine–GAG interactions add complexity to the already complex functions of the receptors and ligands.

Heparan sulfate proteoglycans as key regulators of the mesenchymal niche of hematopoietic stem cells

The complex microenvironment that surrounds hematopoietic stem cells (HSCs) in the bone marrow niche involves different coordinated signaling pathways. The stem cells establish permanent interactions with distinct cell types such as mesenchymal stromal cells, osteoblasts, osteoclasts or endothelial cells and with secreted regulators such as growth factors, cytokines, chemokines and their receptors. These interactions are mediated through adhesion to extracellular matrix compounds also. All these signaling pathways are important for stem cell fates such as self-renewal, proliferation or differentiation, homing and mobilization, as well as for remodeling of the niche. Among these complex molecular cues, this review focuses on heparan sulfate (HS) structures and functions and on the role of enzymes involved in their biosynthesis and turnover. HS associated to core protein, constitute the superfamily of heparan sulfate proteoglycans (HSPGs) present on the cell surface and in the extracellular matrix of all tissues. The key regulatory effects of major medullar HSPGs are described, focusing on their roles in the interactions between hematopoietic stem cells and their endosteal niche, and on their ability to interact with Heparin Binding Proteins (HBPs). Finally, according to the relevance of HS moieties effects on this complex medullar niche, we describe recent data that identify HS mimetics or sulfated HS signatures as new glycanic tools and targets, respectively, for hematopoietic and mesenchymal stem cell based therapeutic applications.

Two glycosaminoglycan-binding domains of the mouse cytomegalovirus-encoded chemokine MCK-2 are critical for oligomerization of the full-length protein

Chemokines are essential for antimicrobial host defenses and tissue repair. Herpesviruses and poxviruses also encode chemokines, copied from their hosts and repurposed for multiple functions, including immune evasion. The CC chemokine MCK-2 encoded by mouse CMV (MCMV) has an atypical structure consisting of a classic chemokine domain N-terminal to a second unique domain, resulting from the splicing of MCMV ORFs m131 and m129 MCK-2 is essential for full MCMV infectivity in macrophages and for persistent infection in the salivary gland. However, information about its mechanism of action and specific biochemical roles for the two domains has been lacking. Here, using genetic, chemical, and enzymatic analyses of multiple mouse cell lines as well as primary mouse fibroblasts from salivary gland and lung, we demonstrate that MCK-2 binds glycosaminoglycans (GAGs) with affinities in the following order: heparin > heparan sulfate > chondroitin sulfate = dermatan sulfate. Both MCK-2 domains bound these GAGs independently, and computational analysis together with site-directed mutagenesis identified five basic residues distributed across the N terminus and the 30s and 50s loops of the chemokine domain that are important GAG binding determinants. Both domains were required for GAG-dependent oligomerization of full-length MCK-2. Thus, MCK-2 is an atypical viral chemokine consisting of a CC chemokine domain and a unique non-chemokine domain, both of which bind GAGs and are critical for GAG-dependent oligomerization of the full-length protein.

Antibody Neutralization of CXCL10 in Vivo Is Dependent on Binding to Free and Not Endothelial-bound Chemokine: IMPLICATIONS FOR THE DESIGN OF A NEW GENERATION OF ANTI-CHEMOKINE THERAPEUTIC ANTIBODIES

To improve our understanding of properties that confer successful inhibition of chemokines in vivo, we analyzed anti-murine CXCL10 monoclonal antibodies (mAb) having different characteristics. 1B6 displayed potent inhibition of cell recruitment in vitro with an IC50 of 0.5 nm but demonstrated little efficacy in various animal models of human disease. On the contrary, 1F11 showed efficacy in several models of inflammation yet was less potent at inhibiting chemotaxis in vitro with an IC50 of 21 nm Furthermore, we observed that 1B6 displayed a rapid dose-dependent clearance (t½ 10-60 h) in contrast to 1F11, which presented a dose-proportional pharmacokinetic profile and a half-life of 12 days. Moreover, 1B6 recognized glycosaminoglycan (GAG)-bound CXCL10, resulting in target-mediated clearance, which was corroborated using CXCL10-deficient mice. In contrast to 1B6, 1F11 inhibited the interaction of CXCL10 with GAGs, did not recognize GAG-bound CXCL10, and did not display target-mediated drug disposition. Confirming previous animal studies, 1B6 was poor at reversing glycemia in a model of type 1 diabetes, whereas 1F11 induced early and prolonged control of diabetes. Furthermore, when using 1A4, a subsequently generated anti-mCXCL10 mAb that shares the property with 1F11 of being unable to recognize CXCL10 immobilized on GAG, we observed a similar superior control of diabetes as compared with 1B6. We therefore concluded that targeting chemokines with antibodies such as 1B6 that recognize the more abundant GAG-bound form of the chemokine may not be the optimal strategy to achieve disease control.

A three-dimensional in vitro model to demonstrate the haptotactic effect of monocyte chemoattractant protein-1 on atherosclerosis-associated monocyte migration

Monocyte transendothelial migration is a multi-step process critical for the initiation and development of atherosclerosis. The chemokine monocyte chemoattractant protein-1 (MCP-1) is overexpressed during atheroma and its concentration gradients in the extracellular matrix (ECM) is critical for the transendothelial recruitment of monocytes. Based on prior observations, we hypothesize that both free and bound gradients of MCP-1 within the ECM are involved in directing monocyte migration. The interaction between a three-dimensional (3D), cell-free, collagen matrix and MCP-1; and its effect on monocyte migration was measured in this study. Our results showed such an interaction existed between MCP-1 and collagen, as 26% of the total MCP-1 added to the collagen matrix was bound to the matrix after extensive washes. We also characterized the collagen-MCP-1 interaction using biophysical techniques. The treatment of the collagen matrix with MCP-1 lead to increased monocyte migration, and this phenotype was abrogated by treating the matrix with an anti-MCP-1 antibody. Thus, our results indicate a binding interaction between MCP-1 and the collagen matrix, which could elicit a haptotactic effect on monocyte migration. A better understanding of such mechanisms controlling monocyte migration will help identify target cytokines and lead to the development of better anti-inflammatory therapeutic strategies.

Enrichment of Two Isomeric Heparin Oligosaccharides Exhibiting Different Affinities toward Monocyte Chemoattractant Protein-1

Chemokine-GAG interactions are crucial to facilitate chemokine immobilization, resulting in the formation of chemokine gradients that guide cell migration. Here we demonstrate chromatographic isolation and purification of two heparin hexasaccharide isomers that interact with the oligomeric chemokine Monocyte Chemoattractant Protein-1 (MCP-1)/CCL2 with different binding affinities. The sequences of these two hexasaccharides were deduced from unique MS/MS product ions and HPLC compositional analysis. Ion mobility mass spectrometry (IM-MS) showed that the two isolated oligosaccharides have different conformations and both displayed preferential binding for one of the two distinct conformations known for MCP-1 dimers. A significant shift in arrival time distribution of close to 70 Ų was observed, indicating a more compact protein:hexasaccharide conformation. Clear differences in the MS spectra between bound and unbound protein allowed calculation of K_d values from the resulting data. The structural difference between the two hexasaccharides was defined as the differential location of a single sulfate at either C-6 of glucosamine or C-2 of uronic acid in the reducing disaccharide, resulting in a 200-fold difference in binding affinity for MCP-1. These data indicate sequence specificity for high affinity binding, supporting the view that sulfate position, and not simply the number of sulfates, is important for heparan sulfate protein binding.

Regulation of Chemokine Activity - A Focus on the Role of Dipeptidyl Peptidase IV/CD26

Chemokines are small, chemotactic proteins that play a crucial role in leukocyte migration and are, therefore, essential for proper functioning of the immune system. Chemokines exert their chemotactic effect by activation of chemokine receptors, which are G protein-coupled receptors (GPCRs), and interaction with glycosaminoglycans (GAGs). Furthermore, the exact chemokine function is modulated at the level of posttranslational modifications. Among the different types of posttranslational modifications that were found to occur in vitro and in vivo, i.e., proteolysis, citrullination, glycosylation, and nitration, NH₂-terminal proteolysis of chemokines has been described most intensively. Since the NH₂-terminal chemokine domain mediates receptor interaction, NH₂-terminal modification by limited proteolysis or amino acid side chain modification can drastically affect their biological activity. An enzyme that has been shown to provoke NH₂-terminal proteolysis of various chemokines is dipeptidyl peptidase IV or CD26. This multifunctional protein is a serine protease that preferably cleaves dipeptides from the NH₂-terminal region of peptides and proteins with a proline or alanine residue in the penultimate position. Various chemokines possess such a proline or alanine residue, and CD26-truncated forms of these chemokines have been identified in cell culture supernatant as well as in body fluids. The effects of CD26-mediated proteolysis in the context of chemokines turned out to be highly complex. Depending on the chemokine ligand, loss of these two NH₂-terminal amino acids can result in either an increased or a decreased biological activity, enhanced receptor specificity, inactivation of the chemokine ligand, or generation of receptor antagonists. Since chemokines direct leukocyte migration in homeostatic as well as pathophysiologic conditions, CD26-mediated proteolytic processing of these chemotactic proteins may have significant consequences for appropriate functioning of the immune system. After introducing the chemokine family together with the GPCRs and GAGs, as main interaction partners of chemokines, and discussing the different forms of posttranslational modifications, this review will focus on the intriguing relationship of chemokines with the serine protease CD26.

Chemokine CXCL1 mediated neutrophil recruitment: Role of glycosaminoglycan interactions

The chemokine CXCL1/MGSA plays a pivotal role in the host immune response by recruiting and activating neutrophils for microbial killing at the tissue site. CXCL1 exists reversibly as monomers and dimers, and mediates its function by binding glycosaminoglycans (GAG) and CXCR2 receptor. We recently showed that both monomers and dimers are potent CXCR2 agonists, the dimer is the high-affinity GAG ligand, lysine and arginine residues located in two non-overlapping domains mediate GAG interactions, and there is extensive overlap between GAG and receptor-binding domains. To understand how these structural properties influence in vivo function, we characterized peritoneal neutrophil recruitment of a trapped monomer and trapped dimer and a panel of WT lysine/arginine to alanine mutants. Monomers and dimers were active, but WT was more active indicating synergistic interactions promote recruitment. Mutants from both domains showed reduced GAG heparin binding affinities and reduced neutrophil recruitment, providing compelling evidence that both GAG-binding domains mediate in vivo trafficking. Further, mutant of a residue that is involved in both GAG binding and receptor signaling showed the highest reduction in recruitment. We conclude that GAG interactions and receptor activity of CXCL1 monomers and dimers are fine-tuned to regulate neutrophil trafficking for successful resolution of tissue injury.

Modulation of Chemokine Responses: Synergy and Cooperativity

Chemokine biology is mediated by more complex interactions than simple monomolecular ligand–receptor interactions, as chemokines can form higher order quaternary structures, which can also be formed after binding to glycosaminoglycans (GAGs) on endothelial cells, and their receptors are found as dimers and/or oligomers at the cell surface. Due to the complexity of the chemokine binding and signaling system, several mechanisms have been proposed to provide an explanation for the synergy observed between chemokines in leukocyte migration. Pioneering studies on interactions between different chemokines have revealed that they can act as antagonists, or synergize with other chemokines. The synergism can occur at different levels, involving either two chemokine receptors triggered simultaneously or sequentially exposed to their agonists, or the activation of one type of chemokine receptor triggered by chemokine heterocomplexes. In addition to the several chemokines that, by forming a heterocomplex with chemokine receptor agonists, act as enhancers of molecules of the same family, we have recently identified HMGB1, an endogenous damage-associated molecular patterns (DAMPs) molecule, as an enhancer of the activity of CXCL12. It is now evident that synergism between chemokines is crucial at the very early stage of inflammation. In addition, the low-affinity interaction with GAGs has recently been shown to induce cooperativity allowing synergy or inhibition of activity by displacement of other ligands.

The Anti-inflammatory Protein TSG-6 Regulates Chemokine Function by Inhibiting Chemokine/Glycosaminoglycan Interactions

TNF-stimulated gene-6 (TSG-6) is a multifunctional protein secreted in response to pro-inflammatory stimuli by a wide range of cells, including neutrophils, monocytes, and endothelial cells. It has been shown to mediate anti-inflammatory and protective effects when administered in disease models, in part, by reducing neutrophil infiltration. Human TSG-6 inhibits neutrophil migration by binding CXCL8 through its Link module (Link_TSG6) and interfering with the presentation of CXCL8 on cell-surface glycosaminoglycans (GAGs), an interaction that is vital for the function of many chemokines. TSG-6 was also found to interact with chemokines CXCL11 and CCL5, suggesting the possibility that it may function as a broad specificity chemokine-binding protein, functionally similar to those encoded by viruses. This study was therefore undertaken to explore the ability of TSG-6 to regulate the function of other chemokines. Herein, we demonstrate that Link_TSG6 binds chemokines from both the CXC and CC families, including CXCL4, CXCL12, CCL2, CCL5, CCL7, CCL19, CCL21, and CCL27. We also show that the Link_TSG6-binding sites on chemokines overlap with chemokine GAG-binding sites, and that the affinities of Link_TSG6 for these chemokines (K_D values 1–85 nm) broadly correlate with chemokine-GAG affinities. Link_TSG6 also inhibits chemokine presentation on endothelial cells not only through a direct interaction with chemokines but also by binding and therefore masking the availability of GAGs. Along with previous work, these findings suggest that TSG-6 functions as a pluripotent regulator of chemokines by modulating chemokine/GAG interactions, which may be a major mechanism by which TSG-6 produces its anti-inflammatory effects in vivo.

Examination of Glycosaminoglycan Binding Sites on the XCL1 Dimer

Known for its distinct metamorphic behavior, XCL1 interconverts between a canonical chemokine folded monomer (XCL1mon) that interacts with the receptor, XCR1, and a unique dimer (XCL1dim) that interacts with glycosaminoglycans and inhibits HIV-1 activity. This study presents the first detailed analysis of the GAG binding properties of XCL1dim. Basic residues within a conformationally selective dimeric variant of XCL1 (W55D) were mutated and analyzed for their effects on heparin binding. Mutation of Arg23 and Arg43 greatly diminished the level of heparin binding in both heparin Sepharose chromatography and surface plasmon resonance assays. To assess the contributions of different GAG structures to XCL1 binding, we developed a solution fluorescence polarization assay and correlated affinity with the length and level of sulfation of heparan sulfate oligosaccharides. It was recently demonstrated that the XCL1 GAG binding form, XCL1dim, is responsible for preventing HIV-1 infection through interactions with gp120. This study defines a GAG binding surface on XCL1dim that includes residues that are important for HIV-1 inhibition.

The neutrophil-recruiting chemokine GCP-2/CXCL6 is expressed in cystic fibrosis airways and retains its functional properties after binding to extracellular DNA

Infections in cystic fibrosis (CF), often involving Pseudomonas aeruginosa, result from a dysregulated airway immunity where one hallmark is the accumulation of necrotic and apoptotic immune cells, in particular neutrophils. In addition, neutrophils actively release DNA, forming neutrophil extracellular traps (NETs) that contain antimicrobial proteins. Altogether, free DNA in complex with actin accumulates in the airway lumen, resulting in highly viscous sputum that provides an anionic matrix, binding cationic antimicrobial proteins. In this study, granulocyte chemotactic protein 2 (GCP-2)/CXCL6, a neutrophil-activating chemokine with bactericidal properties, was detected in the airway epithelium of CF patients and was also present in azurophilic and specific granules of neutrophils. Elastase of neutrophils, but not of P. aeruginosa, completely degraded CXCL6 (chemokine (C-X-C motif) ligand 6). In addition, CXCL6 colocalized with extracellular DNA in both CF sputa and in in vitro-formed NETs. In vitro, CXCL6 bound DNA with a KD of 2,500 nM. Interestingly, both the bactericidal and the receptor-activating properties of CXCL6 (against neutrophils) remained largely unaffected in the presence of DNA. However, the chemotactic properties of CXCL6 were reduced by the presence of DNA. Taken together, CXCL6 is expressed in CF, retaining its functional properties even after binding to the anionic scaffold that extracellular DNA provides in CF.

The dependence of chemokine-glycosaminoglycan interactions on chemokine oligomerization

Both chemokine oligomerization and binding to glycosaminoglycans (GAGs) are required for their function in cell recruitment. Interactions with GAGs facilitate the formation of chemokine gradients, which provide directional cues for migrating cells. In contrast, chemokine oligomerization is thought to contribute to the affinity of GAG interactions by providing a more extensive binding surface than single subunits alone. However, the importance of chemokine oligomerization to GAG binding has not been extensively quantified. Additionally, the ability of chemokines to form different oligomers has been suggested to impart specificity to GAG interactions, but most studies have been limited to heparin. In this study, several differentially oligomerizing chemokines (CCL2, CCL3, CCL5, CCL7, CXCL4, CXCL8, CXCL11 and CXCL12) and select oligomerization-deficient mutants were systematically characterized by surface plasmon resonance to determine their relative affinities for heparin, heparan sulfate (HS) and chondroitin sulfate-A (CS-A). Wild-type chemokines demonstrated a hierarchy of binding affinities for heparin and HS that was markedly dependent on oligomerization. These results were corroborated by their relative propensity to accumulate on cells and the critical role of oligomerization in cell presentation. CS-A was found to exhibit greater chemokine selectivity than heparin or HS, as it only bound a subset of chemokines; moreover, binding to CS-A was ablated with oligomerization-deficient mutants. Overall, this study definitively demonstrates the importance of oligomerization for chemokine-GAG interactions, and demonstrates diversity in the affinity and specificity of different chemokines for GAGs. These data support the idea that GAG interactions provide a mechanism for fine-tuning chemokine function.

Preparation and Characterization of Glycosaminoglycan Chemokine Coreceptors

Interactions between chemokines and glycosaminoglycans (GAGs) are crucial for the physiological and pathophysiological activities of chemokines. GAGs are therefore commonly designated as chemokine coreceptors which are deeply involved in the chemokine-signaling network. Studying the interaction of chemokines with GAGs is therefore a major prerequisite to fully understand the biological function of chemokines. GAGs are, however, a very complex class of biomacromolecules which cannot be produced by conventional recombinant methods and which, if purchased from commercial suppliers, are often not subjected to rigorous quality control and therefore frequently differ in batch characteristics. This naturally impacts chemokine-GAG interaction studies. In order to standardize the quality of our GAG ligands, we have therefore established protocols for the preparation and characterization of GAGs from various cells and tissues, for which we give practical examples relating to the major GAG classes heparin, heparan sulfate, and chondroitin sulfate. We will also outline robust and sensitive protocols for chemokine-GAG interaction studies. By this means, a better and more common understanding of the involvement of GAGs in chemokine-signaling networks can be envisaged.

Lactobacillus plantarum displaying CCL3 chemokine in fusion with HIV-1 Gag derived antigen causes increased recruitment of T cells

Background

Chemokines are attractive candidates for vaccine adjuvants due to their ability to recruit the immune cells. Lactic acid bacteria (LAB)-based delivery vehicles have potential to be used as a cheap and safe option for vaccination. Chemokine produced on the surface of LAB may potentially enhance the immune response to an antigen and this approach can be considered in development of future mucosal vaccines.

Results

We have constructed strains of Lactobacillusplantarum displaying a chemokine on their surface. L. plantarum was genetically engineered to express and anchor to the surface a protein called CCL3Gag. CCL3Gag is a fusion protein comprising of truncated HIV-1 Gag antigen and the murine chemokine CCL3, also known as MIP-1α. Various surface anchoring strategies were explored: (1) a lipobox-based covalent membrane anchor, (2) sortase-mediated covalent cell wall anchoring, (3) LysM-based non-covalent cell wall anchoring, and (4) an N-terminal signal peptide-based transmembrane anchor. Protein production and correct localization were confirmed using Western blotting, flow cytometry and immunofluorescence microscopy. Using a chemotaxis assay, we demonstrated that CCL3Gag-producing L. plantarum strains are able to recruit immune cells in vitro.

Conclusions

The results show the ability of engineered L. plantarum to produce a functional chemotactic protein immobilized on the bacterial surface. We observed that the activity of surface-displayed CCL3Gag differed depending on the type of anchor used. The chemokine which is a part of the bacteria-based vaccine may increase the recruitment of immune cells and, thereby, enhance the reaction of the immune system to the vaccine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0360-z) contains supplementary material, which is available to authorized users.

The glycocalyx--linking albuminuria with renal and cardiovascular disease

Albuminuria is commonly used as a marker of kidney disease progression, but some evidence suggests that albuminuria also contributes to disease progression by inducing renal injury in specific disease conditions. Studies have confirmed that in patients with cardiovascular risk factors, such as diabetes and hypertension, endothelial damage drives progression of kidney disease and cardiovascular disease. A key mechanism that contributes to this process is the loss of the glycocalyx--a polysaccharide gel that lines the luminal endothelial surface and that normally acts as a barrier against albumin filtration. Degradation of the glycocalyx in response to endothelial activation can lead to albuminuria and subsequent renal and vascular inflammation, thus providing a pathophysiological framework for the clinical association of albuminuria with renal and cardiovascular disease progression. In this Review, we examine the likely mechanisms by which glycocalyx dysfunction contributes to kidney injury and explains the link between cardiovascular disease and albuminuria. Evidence suggests that glycocalyx dysfunction is reversible, suggesting that these mechanisms could be considered as therapeutic targets to prevent the progression of renal and cardiovascular disease. This possibility enables the use of existing drugs in new ways, provides an opportunity to develop novel therapies, and indicates that albuminuria should be reconsidered as an end point in clinical trials.

Solution NMR characterization of chemokine CXCL8/IL-8 monomer and dimer binding to glycosaminoglycans: structural plasticity mediates differential binding interactions

Structural plasticity plays a major role in determining differential binding of CXCL8 monomer and dimer to glycosaminoglycans (GAGs) and that dimer is the high-affinity GAG ligand. We propose that these properties play important roles in orchestrating in vivo chemokine-mediated neutrophil function.

Chemokine CXCL8/interleukin-8 (IL-8) plays a crucial role in directing neutrophils and oligodendrocytes to combat infection/injury and tumour cells in metastasis development. CXCL8 exists as monomers and dimers and interaction of both forms with glycosaminoglycans (GAGs) mediate these diverse cellular processes. However, very little is known regarding the structural basis underlying CXCL8–GAG interactions. There are conflicting reports on the affinities, geometry and whether the monomer or dimer is the high-affinity GAG ligand. To resolve these issues, we characterized the binding of a series of heparin-derived oligosaccharides [heparin disaccharide (dp2), heparin tetrasaccharide (dp4), heparin octasaccharide (dp8) and heparin 14-mer (dp14)] to the wild-type (WT) dimer and a designed monomer using solution NMR spectroscopy. The pattern and extent of binding-induced chemical shift perturbation (CSP) varied between dimer and monomer and between longer and shorter oligosaccharides. NMR-based structural models show that different interaction modes coexist and that the nature of interactions varied between monomer and dimer and oligosaccharide length. MD simulations indicate that the binding interface is structurally plastic and provided residue-specific details of the dynamic nature of the binding interface. Binding studies carried out under conditions at which WT CXCL8 exists as monomers and dimers provide unambiguous evidence that the dimer is the high-affinity GAG ligand. Together, our data indicate that a set of core residues function as the major recognition/binding site, a set of peripheral residues define the various binding geometries and that the structural plasticity of the binding interface allows multiplicity of binding interactions. We conclude that structural plasticity most probably regulates in vivo CXCL8 monomer/dimer–GAG interactions and function.

Chemokine Transfer by Liver Sinusoidal Endothelial Cells Contributes to the Recruitment of CD4+ T Cells into the Murine Liver

Leukocyte adhesion and transmigration are central features governing immune surveillance and inflammatory reactions in body tissues. Within the liver sinusoids, chemokines initiate the first crucial step of T-cell migration into the hepatic tissue. We studied molecular mechanisms involved in endothelial chemokine supply during hepatic immune surveillance and liver inflammation and their impact on the recruitment of CD4⁺ T cells into the liver. In the murine model of Concanavalin A-induced T cell-mediated hepatitis, we showed that hepatic expression of the inflammatory CXC chemokine ligands (CXCL)9 and CXCL10 strongly increased whereas homeostatic CXCL12 significantly decreased. Consistently, CD4⁺ T cells expressing the CXC chemokine receptor (CXCR)3 accumulated within the inflamed liver tissue. In histology, CXCL9 was associated with liver sinusoidal endothelial cells (LSEC) which represent the first contact site for T-cell immigration into the liver. LSEC actively transferred basolaterally internalized CXCL12, CXCL9 and CXCL10 via clathrin-coated vesicles to CD4⁺ T cells leading to enhanced transmigration of CXCR4⁺ total CD4⁺ T cells and CXCR3⁺ effector/memory CD4⁺ T cells, respectively in vitro. LSEC-expressed CXCR4 mediated CXCL12 transport and blockage of endothelial CXCR4 inhibited CXCL12-dependent CD4⁺ T-cell transmigration. In contrast, CXCR3 was not involved in the endothelial transport of its ligands CXCL9 and CXCL10. The clathrin-specific inhibitor chlorpromazine blocked endothelial chemokine internalization and CD4⁺ T-cell transmigration in vitro as well as migration of CD4⁺ T cells into the inflamed liver in vivo. Moreover, hepatic accumulation of CXCR3⁺ CD4⁺ T cells during T cell-mediated hepatitis was strongly reduced after administration of chlorpromazine. These data demonstrate that LSEC actively provide perivascularly expressed homeostatic and inflammatory chemokines by CXCR4- and clathrin-dependent intracellular transport mechanisms thereby contributing to the hepatic recruitment of CD4⁺ T-cell populations during immune surveillance and liver inflammation.

Chemokines and microRNAs in atherosclerosis

The crucial role of chemokines in the initiation and progression of atherosclerosis has been widely recognized. Through essential functions in leukocyte recruitment, chemokines govern the infiltration with mononuclear cells and macrophage accumulation in atherosclerotic lesions. Beyond recruitment, chemokines also provide homeostatic functions supporting cell survival and mediating the mobilization and homing of progenitor cells. As a new regulatory layer, several microRNAs (miRNAs) have been found to modulate the function of endothelial cells (ECs), smooth muscle cells and macrophages by controlling the expression levels of chemokines and thereby affecting different stages in the progression of atherosclerosis. For instance, the expression of CXCL1 can be down-regulated by miR-181b, which inhibits nuclear factor-κB activation in atherosclerotic endothelium, thus attenuating the adhesive properties of ECs and exerting early atheroprotective effects. Conversely, CXCL12 expression can be induced by miR-126 in ECs through an auto-amplifying feedback loop to facilitate endothelial regeneration, thus limiting atherosclerosis and mediating plaque stabilization. In contrast, miR-155 plays a pro-atherogenic role by promoting the expression of CCL2 in M1-type macrophages, thereby enhancing vascular inflammation. Herein, we will review novel aspects of chemokines and their regulation by miRNAs during atherogenesis. Understanding the complex cross-talk of miRNAs controlling chemokine expression may open novel therapeutic options to treat atherosclerosis.

Interactions of the Chemokine CCL5/RANTES with Medium-Sized Chondroitin Sulfate Ligands

Interactions of the chemokine CCL5 (RANTES) with glycosaminoglycans (GAGs) are crucial to the CCL5-mediated inflammation process. However, structural information on interactions between CCL5 and longer GAG fragments is lacking. In this study, the interactions between oligosaccharides derived from chondroitin sulfate and a dimeric variant of CCL5 were investigated using solution nuclear magnetic resonance. The data indicate that, in addition to the BBXB motif in the 40s loop, GAGs also contact residues in the N loop in a manner similar to interactions between chemokine and the receptor N terminus, leading to possible stabilization of the dimer. Using 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl-tagged hexasaccharides, the binding orientation of the hexasaccharides was shown to be highly dependent on the sulfation pattern of the N-acetyl galactosamine groups. Finally, a model of the CCL5 dimer complexed to chondroitin sulfate hexasaccharides was constructed using paramagnetic relaxation enhancement and intra- and intermolecular nuclear Overhauser effect constraints.

Heparin/Heparan sulfate proteoglycans glycomic interactome in angiogenesis: biological implications and therapeutical use

Angiogenesis, the process of formation of new blood vessel from pre-existing ones, is involved in various intertwined pathological processes including virus infection, inflammation and oncogenesis, making it a promising target for the development of novel strategies for various interventions. To induce angiogenesis, angiogenic growth factors (AGFs) must interact with pro-angiogenic receptors to induce proliferation, protease production and migration of endothelial cells (ECs). The action of AGFs is counteracted by antiangiogenic modulators whose main mechanism of action is to bind (thus sequestering or masking) AGFs or their receptors. Many sugars, either free or associated to proteins, are involved in these interactions, thus exerting a tight regulation of the neovascularization process. Heparin and heparan sulfate proteoglycans undoubtedly play a pivotal role in this context since they bind to almost all the known AGFs, to several pro-angiogenic receptors and even to angiogenic inhibitors, originating an intricate network of interaction, the so called "angiogenesis glycomic interactome". The decoding of the angiogenesis glycomic interactome, achievable by a systematic study of the interactions occurring among angiogenic modulators and sugars, may help to design novel antiangiogenic therapies with implications in the cure of angiogenesis-dependent diseases.

Molecular genetic analysis of orf virus: a poxvirus that has adapted to skin

Orf virus is the type species of the Parapoxvirus genus of the family Poxviridae. It induces acute pustular skin lesions in sheep and goats and is transmissible to humans. The genome is G+C rich, 138 kbp and encodes 132 genes. It shares many essential genes with vaccinia virus that are required for survival but encodes a number of unique factors that allow it to replicate in the highly specific immune environment of skin. Phylogenetic analysis suggests that both viral interleukin-10 and vascular endothelial growth factor genes have been "captured" from their host during the evolution of the parapoxviruses. Genes such as a chemokine binding protein and a protein that binds granulocyte-macrophage colony-stimulating factor and interleukin-2 appear to have evolved from a common poxvirus ancestral gene while three parapoxvirus nuclear factor (NF)-κB signalling pathway inhibitors have no homology to other known NF-κB inhibitors. A homologue of an anaphase-promoting complex subunit that is believed to manipulate the cell cycle and enhance viral DNA synthesis appears to be a specific adaptation for viral-replication in keratinocytes. The review focuses on the unique genes of orf virus, discusses their evolutionary origins and their role in allowing viral-replication in the skin epidermis.

Oligomerized, filamentous surface presentation of RANTES/CCL5 on vascular endothelial cells

Vascular endothelial cells present luminal chemokines that arrest rolling leukocytes by activating integrins. It appears that several chemokines must form higher-order oligomers to elicit proper in vivo effects, as mutants restricted to forming dimers have lost the ability to recruit leukocytes to sites of inflammation. Here, we show for the first time that the chemokine RANTES/CCL5 binds to the surface of human endothelial cells in a regular filamentous pattern. Furthermore, the filaments bound to the surface in a heparan sulfate-dependent manner. By electron microscopy we observed labeling for RANTES on membrane projections as well as on the remaining plasma membrane. Mutant constructs of RANTES restricted either in binding to heparin, or in forming dimers or tetramers, appeared either in a granular, non-filamentous pattern or were not detectable on the cell surface. The RANTES filaments were also present after exposure to flow, suggesting that they can be present in vivo. Taken together with the lacking in vivo or in vitro effects of RANTES mutants, we suggest that the filamentous structures of RANTES may be of physiological importance in leukocyte recruitment.

Targeting chemokines: Pathogens can, why can't we?

Chemoattractant cytokines, or chemokines, are the largest sub-family of cytokines. About 50 distinct chemokines have been identified in humans. Their principal role is to stimulate the directional migration of leukocytes, which they achieve through activation of their receptors, following immobilization on cell surface glycosaminoglycans (GAGs). Chemokine receptors belong to the G protein-coupled 7-transmembrane receptor family, and hence their identification brought great promise to the pharmaceutical industry, since this receptor class is the target for a large percentage of marketed drugs. Unfortunately, the development of potent and efficacious inhibitors of chemokine receptors has not lived up to the early expectations. Several approaches to targeting this system will be described here, which have been instrumental in establishing paradigms in chemokine biology. Whilst drug discovery programs have not yet elucidated how to make successful drugs targeting the chemokine system, it is now known that certain parasites have evolved anti-chemokine strategies in order to remain undetected by their hosts. What can we learn from them?

Chemokine-Releasing Nanoparticles for Manipulation of Lymph Node Microenvironment

Chemokines (CKs) secreted by the host cells into surrounding tissue establish concentration gradients directing the migration of leukocytes. We propose an in vivo CK gradient remodeling approach based on sustained release of CKs by the crosslinked poly(N-isopropylacrylamide) hydrogel open meshwork nano-particles (NPs) containing internal crosslinked dye affinity baits for a reversible CK binding and release. The sustained release is based on a new principle of affinity off-rate tuning. The NPs with Cibacron Blue F3G-A and Reactive Blue-4 baits demonstrated a low-micromolar affinity binding to IL-8, MIP-2, and MCP-1 with a half-life of several hours at 37°C. The capacity of NPs loaded with IL-8 and MIP-1α to increase neutrophil recruitment to lymph nodes (LNs) was tested in mice after footpad injection. Fluorescently-labeled NPs used as tracers indicated the delivery into the sub-capsular compartment of draining LNs. The animals administered the CK-loaded NPs demonstrated a widening of the sub-capsular space and a strong lymph node influx of leukocytes, while mice injected with control NPs without CKs or bolus doses of soluble CKs alone showed only a marginal neutrophil response. This technology provides a new means therapeutically direct or restore immune cell traffic, and can also be employed for simultaneous therapy delivery.

Chemokine and chemokine receptor structure and interactions: implications for therapeutic strategies

The control of cell migration by chemokines involves interactions with two types of receptors: seven transmembrane chemokine-type G protein-coupled receptors and cell surface or extracellular matrix-associated glycosaminoglycans. Coordinated interaction of chemokines with both types of receptors is required for directional migration of cells in numerous physiological and pathological processes. Accumulated structural information, culminating most recently in the structure of a chemokine receptor in complex with a chemokine, has led to a view where chemokine oligomers bind to glycosaminoglycans through epitopes formed when chemokine subunits come together, while chemokine monomers bind to receptors in a pseudo two-step mechanism of receptor activation. Exploitation of this structural knowledge has and will continue to provide important information for therapeutic strategies, as described in this review.

Sulfated glycans in inflammation

Sulfated glycans such as glycosaminoglycans on proteoglycans are key players in both molecular and cellular events of inflammation. They participate in leukocyte rolling along the endothelial surface of inflamed sites; chemokine regulation and its consequential functions in leukocyte guidance, migration and activation; leukocyte transendothelial migration; and structural assembly of the subendothelial basement membrane responsible to control tissue entry of cells. Due to these and other functions, exogenous sulfated glycans of various structures and origins can be used to interventionally down-regulate inflammation processes. In this review article, discussion is given primarily on the anti-inflammatory functions of mammalian heparins, heparan sulfate, chondroitin sulfate, dermatan sulfate and related compounds as well as the holothurian fucosylated chondroitin sulfate and the brown algal fucoidans. Understanding the underlying mechanisms of action of these sulfated glycans in inflammation, helps research programs involved in developing new carbohydrate-based drugs aimed to combat acute and chronic inflammatory disorders.

Rational design of a peptide capture agent for CXCL8 based on a model of the CXCL8:CXCR1 complex

A CXCL8-binding peptide designed from the interaction sites of CXCR1 with CXCL8 serves as a capture agent and inhibits neutrophil migration.

Stromal infrastructure of the lymph node and coordination of immunity

The initiation of adaptive immune responses depends upon the careful maneuvering of lymphocytes and antigen into and within strategically placed lymph nodes (LNs). Non-hematopoietic stromal cells form the cellular infrastructure that directs this process. Once regarded as merely structural features of lymphoid tissues, these cells are now appreciated as essential regulators of immune cell trafficking, fluid flow, and LN homeostasis. Recent advances in the identification and in vivo targeting of specific stromal populations have resulted in striking new insights to the function of stromal cells and reveal a level of complexity previously unrealized. We discuss here recent discoveries that highlight the pivotal role that stromal cells play in orchestrating immune cell homeostasis and adaptive immunity.

Mouse monocyte chemoattractant protein 1 (MCP1) functions as a monomer

Monocyte chemoattractant protein 1 (MCP1) is an important chemoattractant for microglia. Rodent MCP1 carries a heavily glycosylated C-terminus, which has been predicted to increase local MCP1 concentration, promote MCP1 dimerization/oligomerization, and facilitate receptor engagement. Previous studies have shown that MCP1 mutant lacking the glycosylated C-terminus cannot dimerize/oligomerize, but has higher chemotactic potency than the wild-type (full-length) MCP1, suggesting that rodent MCP1 may function as a monomer. Although many groups support this hypothesis, there is no direct evidence on whether rodent MCP1 dimer is functional. In this paper, using forced recombinant dimeric MCP1 proteins we show that mouse MCP1 dimer is unable to activate Rac1, promote protrusion of lamellipodia, or induce microglial migration, although it can bind to CCR2 and mediate its internalization. These results support the idea that signaling events mediated by MCP1 require the presence of the monomeric form of this chemokine.

Unique properties of human β-defensin 6 (hBD6) and glycosaminoglycan complex: sandwich-like dimerization and competition with the chemokine receptor 2 (CCR2) binding site

Defensins are components of the innate immune system that promote the directional migration and activation of dendritic cells, thereby modulating the adaptive immune response. Because matrix glycosaminoglycan (GAG) is known to be important for these functions, we characterized the structural features of human β-defensin 6 (hBD6) and GAG interaction using a combination of structural and in silico analyses. Our results showed that GAG model compounds, a pentasaccharide (fondaparinux, FX) and an octasaccharide heparin derivative (dp8) bind to the α-helix and in the loops between the β2 and β3 strands, inducing the formation of a ternary complex with a 2:1 hBD6:FX stoichiometry. Competition experiments indicated an overlap of GAG and chemokine receptor CCR2 binding sites. An NMR-derived model of the ternary complex revealed that FX interacts with hBD6 along the dimerization interface, primarily contacting the α-helices and β2-β3 loops from each monomer. We further demonstrated that high-pressure NMR spectroscopy could capture an intermediate stage of hBD6-FX interaction, exhibiting features of a cooperative binding mechanism. Collectively, these data suggest a "sandwich-like" model in which two hBD6 molecules bind a single FX chain and provide novel structural insights into how defensin orchestrates leukocyte recruitment through GAG binding and G protein-coupled receptor activation. Despite the similarity to chemokines and hBD2, our data indicate different properties for the hBD6-GAG complex. This work adds significant information to the currently limited data available for the molecular structures and dynamics of defensin carbohydrate binding.

Chemokine receptor oligomerization and allostery

Oligomerization of chemokine receptors has been reported to influence many aspects of receptor function through allosteric communication between receptor protomers. Allosteric interactions within chemokine receptor hetero-oligomers have been shown to cause negative cooperativity in the binding of chemokines and to inhibit receptor activation in the case of some receptor pairs. Other receptor pairs can cause enhanced signaling and even activate entirely new, hetero-oligomer-specific signaling complexes and responses downstream of receptor activation. Many mechanisms contribute to these effects including direct allosteric coupling between the receptors, G protein-mediated allostery, G protein stealing, ligand sequestration, and recruitment of new intracellular proteins by exposing unique binding interfaces on the oligomerized receptors. These effects present both challenges as well as exciting opportunities for drug discovery. One of the most difficult challenges will involve determining if and when hetero-oligomers versus homomeric receptors are involved in specific disease states.

Endothelial and leukocyte heparan sulfates regulate the development of allergen-induced airway remodeling in a mouse model

Heparan sulfate (HS) proteoglycans (HSPGs) participate in several aspects of inflammation because of their ability to bind to growth factors, chemokines, interleukins and extracellular matrix proteins as well as promote inflammatory cell trafficking and migration. We investigated whether HSPGs play a role in the development of airway remodeling during chronic allergic asthma using mice deficient in endothelial- and leukocyte-expressed N-deacetylase/N-sulfotransferase-1 (Ndst1), an enzyme involved in modification reactions during HS biosynthesis. Ndst1-deficient and wild-type (WT) mice exposed to repetitive allergen (ovalbumin [OVA]) challenge were evaluated for the development of airway remodeling. Chronic OVA-challenged WT mice exhibited increased HS expression in the lungs along with airway eosinophilia, mucus hypersecretion, peribronchial fibrosis, increased airway epithelial thickness and smooth muscle mass. In OVA-challenged Ndst1-deficient mice, lung eosinophil and macrophage infiltration as well as airway mucus accumulation, peribronchial fibrosis and airway epithelial thickness were significantly lower than in allergen-challenged WT mice along with a trend toward decreased airway smooth muscle mass. Leukocyte and endothelial Ndst 1 deficiency also resulted in significantly decreased expression of IL-13 as well as remodeling-associated mediators such as VEGF, FGF-2 and TGF-β1 in the lung tissue. At a cellular level, exposure to eotaxin-1 failed to induce TGF-β1 expression by Ndst1-deficient eosinophils relative to WT eosinophils. These studies suggest that leukocyte and endothelial Ndst1-modified HS contribute to the development of allergen-induced airway remodeling by promoting recruitment of inflammatory cells as well as regulating expression of pro-remodeling factors such as IL-13, VEGF, TGF-β1 and FGF-2 in the lung.

Functional aspects of the interaction between interleukin-8 and sulfated glycosaminoglycans

During the immune response, the cytokine interleukin 8 (IL-8, CXCL8) functions as a strong chemoattractant for polymorphonuclear leukocytes helping to direct these cells to infected/injured sites. This review focuses on the interaction of IL-8 with sulfated glycosaminoglycans expressed on cell surfaces and the extracellular matrix. This interaction contributes to the recruitment of polymorphonuclear cells from blood, penetration of these cells through the vessel wall, and their directed migration to inflammatory sites. Regulatory aspects of the interplay between IL-8 and heparan sulfate, the most abundant glycosaminoglycan, are highlighted. In this field, the large natural heterogeneity of glycosaminoglycans represents a great challenge that impedes the modeling of IL-8 functions. The interaction of IL-8 with newly developed artificial sulfated hyaluronan derivatives is also considered as these artificial substrates are an important tool for development of new materials in regenerative medicine.

Chemokine cooperativity is caused by competitive glycosaminoglycan binding

Chemokines comprise a family of secreted proteins that activate G protein-coupled chemokine receptors and thereby control the migration of leukocytes during inflammation or immune surveillance. The positional information required for such migratory behavior is governed by the binding of chemokines to membrane-tethered glycosaminoglycans (GAGs), which establishes a chemokine concentration gradient. An often observed but incompletely understood behavior of chemokines is the ability of unrelated chemokines to enhance the potency with which another chemokine subtype can activate its cognate receptor. This phenomenon has been demonstrated to occur between many chemokine combinations and across several model systems and has been dubbed chemokine cooperativity. In this study, we have used GAG binding-deficient chemokine mutants and cell-based functional (migration) assays to demonstrate that chemokine cooperativity is caused by competitive binding of chemokines to GAGs. This mechanistic explanation of chemokine cooperativity provides insight into chemokine gradient formation in the context of inflammation, in which multiple chemokines are secreted simultaneously.

Demystifying heparan sulfate-protein interactions

Numerous proteins, including cytokines and chemokines, enzymes and enzyme inhibitors, extracellular matrix proteins, and membrane receptors, bind heparin. Although they are traditionally classified as heparin-binding proteins, under normal physiological conditions these proteins actually interact with the heparan sulfate chains of one or more membrane or extracellular proteoglycans. Thus, they are more appropriately classified as heparan sulfate-binding proteins (HSBPs). This review provides an overview of the various modes of interaction between heparan sulfate and HSBPs, emphasizing biochemical and structural insights that improve our understanding of the many biological functions of heparan sulfate.

Monocyte chemoattractant protein-1 and the blood-brain barrier

The blood-brain barrier (BBB) is a dynamic structure that maintains the homeostasis of the brain and thus proper neurological functions. BBB compromise has been found in many pathological conditions, including neuroinflammation. Monocyte chemoattractant protein-1 (MCP1), a chemokine that is transiently and significantly up-regulated during inflammation, is able to disrupt the integrity of BBB and modulate the progression of various diseases, including excitotoxic injury and hemorrhage. In this review, we first introduce the biochemistry and biology of MCP1, and then summarize the effects of MCP1 on BBB integrity as well as individual BBB components.

HIV-1 matrix protein p17 promotes lymphangiogenesis and activates the endothelin-1/endothelin B receptor axis

OBJECTIVE

AIDS-related lymphomas are high grade and aggressively metastatic with poor prognosis. Lymphangiogenesis is essential in supporting proliferation and survival of lymphoma, as well as tumor dissemination. Data suggest that aberrant lymphangiogenesis relies on action of HIV-1 proteins rather than on a direct effect of the virus itself. HIV-1 matrix protein p17 was found to accumulate and persist in lymph nodes of patients even under highly active antiretroviral therapy. Because p17 was recently found to exert a potent proangiogenic activity by interacting with chemokine (C-X-C motif) receptors 1 and 2, we tested the prolymphangiogenic activity of the viral protein.

APPROACH AND RESULTS

Human primary lymph node-derived lymphatic endothelial cells were used to perform capillary-like structure formation, wound healing, spheroids, and Western blot assays after stimulation with or without p17. Here, we show that p17 promotes lymphangiogenesis by binding to chemokine (C-X-C motif) receptor-1 and chemokine (C-X-C motif) receptor-2 expressed on lymph node-derived lymphatic endothelial cells and activating the Akt/extracellular signal-regulated kinase signaling pathway. In particular, it was found to induce capillary-like structure formation, sprout formation from spheroids, and increase lymph node-derived lymphatic endothelial cells motility. The p17 lymphangiogenic activity was, in part, sustained by activation of the endothelin-1/endothelin receptor B axis. A Matrigel plug assay showed that p17 was able to promote the outgrowth of lymphatic vessels in vivo, demonstrating that p17 directly regulates lymphatic vessel formation.

CONCLUSIONS

Our results suggest that p17 may generate a prolymphangiogenic microenvironment and plays a role in predisposing the lymph node to lymphoma growth and metastasis. This finding offers new opportunities to identify treatment strategies in combating AIDS-related lymphomas.

Heparan sulfate: a ubiquitous glycosaminoglycan with multiple roles in immunity

Heparan sulfate (HS) is a highly acidic linear polysaccharide with a very variable structure. It is ubiquitously expressed on cell surfaces and in the extracellular matrix and basement membrane of mammalian tissues. Synthesized attached to various core proteins to form HS-proteoglycans, HS is capable of interacting with various polypeptides and exerting diverse functions. In fact, a bioinformatics analysis of mammalian proteins that express a heparin/HS-binding motif and are associated with the immune system identified 235 candidate proteins, the majority having an intracellular location. This simple analysis suggests that HS may, in fact, interact with many more components of the immune system than previously realized. Numerous studies have also directly demonstrated that HS plays multiple prominent functional roles in the immune system that are briefly reviewed in this article. In particular, the molecule has been shown to regulate leukocyte development, leukocyte migration, immune activation, and inflammatory processes.

Structural Evidence for the Tetrameric Assembly of Chemokine CCL11 and the Glycosaminoglycan Arixtra™

Understanding chemokine interactions with glycosaminoglycans (GAG) is critical as these interactions have been linked to a number of inflammatory medical conditions, such as arthritis and asthma. To better characterize in vivo protein function, comprehensive knowledge of multimeric species, formed by chemokines under native conditions, is necessary. Herein is the first report of a tetrameric assembly of the human chemokine CCL11, which was shown bound to the GAG Arixtra™. Isothermal titration calorimetry data indicated that CCL11 interacts with Arixtra, and ion mobility mass spectrometry (IM-MS) was used to identify ions corresponding to the CCL11 tetrameric species bound to Arixtra. Collisional cross sections (CCS) of the CCL11 tetramer-Arixtra noncovalent complex were compared to theoretical CCS values calculated using a preliminary structure of the complex deduced using X-ray crystallography. Experimental CCS values were in agreement with theoretical values, strengthening the IM-MS evidence for the formation of the noncovalent complex. Tandem mass spectrometry data of the complex indicated that the tetramer-GAG complex dissociates into a monomer and a trimer-GAG species, suggesting that two CC-like dimers are bridged by Arixtra. As development of chemokine inhibitors is of utmost importance to treatment of medical inflammatory conditions, these results provide vital insights into chemokine-GAG interactions.

Molecular basis of glycosaminoglycan heparin binding to the chemokine CXCL1 dimer

Glycosaminoglycan (GAG)-bound and soluble chemokine gradients in the vasculature and extracellular matrix mediate neutrophil recruitment to the site of microbial infection and sterile injury in the host tissue. However, the molecular principles by which chemokine-GAG interactions orchestrate these gradients are poorly understood. This, in part, can be directly attributed to the complex interrelationship between the chemokine monomer-dimer equilibrium and binding geometry and affinities that are also intimately linked to GAG length. To address some of this missing knowledge, we have characterized the structural basis of heparin binding to the murine CXCL1 dimer. CXCL1 is a neutrophil-activating chemokine and exists as both monomers and dimers (Kd = 36 μm). To avoid interference from monomer-GAG interactions, we designed a trapped dimer (dCXCL1) by introducing a disulfide bridge across the dimer interface. We characterized the binding of GAG heparin octasaccharide to dCXCL1 using solution NMR spectroscopy. Our studies show that octasaccharide binds orthogonally to the interhelical axis and spans the dimer interface and that heparin binding enhances the structural integrity of the C-terminal helical residues and stability of the dimer. We generated a quadruple mutant (H20A/K22A/K62A/K66A) on the basis of the binding data and observed that this mutant failed to bind heparin octasaccharide, validating our structural model. We propose that the stability enhancement of dimers upon GAG binding regulates in vivo neutrophil trafficking by increasing the lifetime of "active" chemokines, and that this structural knowledge could be exploited for designing inhibitors that disrupt chemokine-GAG interactions and neutrophil homing to the target tissue.