CXCL1/MGSA Is a Novel Glycosaminoglycan (GAG)-binding Chemokine: STRUCTURAL EVIDENCE FOR TWO DISTINCT NON-OVERLAPPING BINDING DOMAINS

Chemokine Cxcl1-Cxcl2 heterodimer is a potent neutrophil chemoattractant

Microbial infection is characterized by release of multiple proinflammatory chemokines that direct neutrophils to the insult site. How collective function of these chemokines orchestrates neutrophil recruitment is not known. Here, we characterized the role for heterodimer and show that the Cxcl1-Cxcl2 heterodimer is a potent neutrophil chemoattractant in mice and can recruit more neutrophils than the individual chemokines. Chemokine-mediated neutrophil recruitment is determined by Cxcr2 receptor signaling, Cxcr2 endocytosis, and binding to glycosaminoglycans. We have now determined heterodimer's Cxcr2 activity using cellular assays and Cxcr2 density in blood and recruited neutrophils in heterodimer-treated mice. We have shown that the heterodimer binds glycosaminoglycans with higher affinity and more efficiently than Cxcl1 or Cxcl2. These data collectively indicate that optimal glycosaminoglycan interactions and dampened receptor activity acting in concert in a dynamic fashion promote heterodimer-mediated robust neutrophil recruitment. We propose that this could play a critical role in combating infection.

Chemokine Heteromers and Their Impact on Cellular Function-A Conceptual Framework

Chemoattractant cytokines or chemokines are proteins involved in numerous biological activities. Their essential role consists of the formation of gradient and (immune) cell recruitment. Chemokine biology and its related signaling system is more complex than simple ligand-receptor interactions. Beside interactions with their cognate and/or atypical chemokine receptors, and glycosaminoglycans (GAGs), chemokines form complexes with themselves as homo-oligomers, heteromers and also with other soluble effector proteins, including the atypical chemokine MIF, carbohydrate-binding proteins (galectins), damage-associated molecular patterns (DAMPs) or with chemokine-binding proteins such as evasins. Likewise, nucleic acids have been described as binding targets for the tetrameric form of CXCL4. The dynamic balance between monomeric and dimeric structures, as well as interactions with GAGs, modulate the concentrations of free chemokines available along with the nature of the gradient. Dimerization of chemokines changes the canonical monomeric fold into two main dimeric structures, namely CC- and CXC-type dimers. Recent studies highlighted that chemokine dimer formation is a frequent event that could occur under pathophysiological conditions. The structural changes dictated by chemokine dimerization confer additional biological activities, e.g., biased signaling. The present review will provide a short overview of the known functionality of chemokines together with the consequences of the interactions engaged by the chemokines with other proteins. Finally, we will present potential therapeutic tools targeting the chemokine multimeric structures that could modulate their biological functions.

CXCL5: A coachman to drive cancer progression

Chemokines are a class of pro-inflammatory cytokines that can recruit and activate chemotactic cells. C‐X‐C motif chemokine ligand 5 (CXCL5) is a member of the chemokine family binding CXCR2 (C-X-C Motif Chemokine Receptor 2), a G-protein coupled receptor. Accumulated evidence has shown that dysregulated CXCL5 participates in tumor metastasis and angiogenesis in human malignant tumors. In this review, we summarized the advances in research on CXCL5, including its dysregulation in different tumors and the mechanism associated with tumor behavior (formation of the immunosuppressive microenvironment, promotion of tumor angiogenesis, and metastasis). We also summarized and discussed the perspective about the potential application of CXCL5 in tumor therapy targeting the tumor inflammatory microenvironment.

CXCL1: Gene, Promoter, Regulation of Expression, mRNA Stability, Regulation of Activity in the Intercellular Space

CXCL1 is one of the most important chemokines, part of a group of chemotactic cytokines involved in the development of many inflammatory diseases. It activates CXCR2 and, at high levels, CXCR1. The expression of CXCL1 is elevated in inflammatory reactions and also has important functions in physiology, including the induction of angiogenesis and recruitment of neutrophils. Due to a lack of reviews that precisely describe the regulation of CXCL1 expression and function, in this paper, we present the mechanisms of CXCL1 expression regulation with a special focus on cancer. We concentrate on the regulation of CXCL1 expression through the regulation of CXCL1 transcription and mRNA stability, including the involvement of NF-κB, p53, the effect of miRNAs and cytokines such as IFN-γ, IL-1β, IL-17, TGF-β and TNF-α. We also describe the mechanisms regulating CXCL1 activity in the extracellular space, including proteolytic processing, CXCL1 dimerization and the influence of the ACKR1/DARC receptor on CXCL1 localization. Finally, we explain the role of CXCL1 in cancer and possible therapeutic approaches directed against this chemokine.

Aiduqing formula suppresses breast cancer metastasis via inhibiting CXCL1-mediated autophagy

BACKGROUND

Metastasis is the most common lethal cause of breast cancer-related death. Recent studies have implied that autophagy is closely implicated in cancer metastasis. Therefore, it is of great significance to explore autophagy-related molecular targets involved in breast cancer metastasis and to develop therapeutic drugs.

PURPOSE

This study was designed to investigate the anti-metastatic effects and autophagy regulatory mechanisms of Aiduqing (ADQ) formula on breast cancer.

STUDY DESIGN/METHODS

Multiple cellular and molecular experiments were conducted to investigate the inhibitory effects of ADQ formula on autophagy and metastasis of breast cancer cells in vitro. Meanwhile, autophagic activator/inhibitor as well as CXCL1 overexpression or interference plasmids were used to investigate the underlying mechanisms of ADQ formula in modulating autophagy-mediated metastasis. Furthermore, the zebrafish xenotransplantation model and mouse xenografts were applied to validate the inhibitory effect of ADQ formula on autophagy-mediated metastasis in breast cancer in vivo.

RESULTS

ADQ formula significantly inhibited the proliferation, migration, invasion and autophagy but induced apoptosis of high-metastatic breast cancer cells in vitro. Similar results were also observed in starvation-induced breast cancer cells which exhibited elevated metastatic ability and autophagy activity. Mechanism investigations further approved that either CXCL1 overexpression or autophagic activator rapamycin can significantly abrogated the anti-metastatic effects of ADQ formula, suggesting that CXCL1-mediated autophagy may be the crucial pathway of ADQ formula in suppressing breast cancer metastasis. More importantly, ADQ formula suppressed breast cancer growth, autophagy, and metastasis in both the zebrafish xenotransplantation model and the mouse xenografts.

CONCLUSION

Our study not only revealed the novel function of CXCL1 in mediating autophagy-mediated metastasis but also suggested ADQ formula as a candidate drug for the treatment of metastatic breast cancer.

The Role of Post-Translational Modifications of Chemokines by CD26 in Cancer

Chemokines are a large family of small chemotactic cytokines that fulfill a central function in cancer. Both tumor-promoting and -impeding roles have been ascribed to chemokines, which they exert in a direct or indirect manner. An important post-translational modification that regulates chemokine activity is the NH₂-terminal truncation by peptidases. CD26 is a dipeptidyl peptidase (DPPIV), which typically clips a NH₂-terminal dipeptide from the chemokine. With a certain degree of selectivity in terms of chemokine substrate, CD26 only recognizes chemokines with a penultimate proline or alanine. Chemokines can be protected against CD26 recognition by specific amino acid residues within the chemokine structure, by oligomerization or by binding to cellular glycosaminoglycans (GAGs). Upon truncation, the binding affinity for receptors and GAGs is altered, which influences chemokine function. The consequences of CD26-mediated clipping vary, as unchanged, enhanced, and reduced activities are reported. In tumors, CD26 most likely has the most profound effect on CXCL12 and the interferon (IFN)-inducible CXCR3 ligands, which are converted into receptor antagonists upon truncation. Depending on the tumor type, expression of CD26 is upregulated or downregulated and often results in the preferential generation of the chemokine isoform most favorable for tumor progression. Considering the tight relationship between chemokine sequence and chemokine binding specificity, molecules with the appropriate characteristics can be chemically engineered to provide innovative therapeutic strategies in a cancer setting.

Neutrophil recruitment by chemokines Cxcl1/KC and Cxcl2/MIP2: Role of Cxcr2 activation and glycosaminoglycan interactions

Chemokines play a crucial role in combating microbial infection by recruiting blood neutrophils to infected tissue. In mice, the chemokines Cxcl1/KC and Cxcl2/MIP2 fulfill this role. Cxcl1 and Cxcl2 exist as monomers and dimers, and exert their function by activating the Cxcr2 receptor and binding glycosaminoglycans (GAGs). Here, we characterized Cxcr2 G protein and β-arrestin activities, and GAG heparan sulfate (HS) interactions of Cxcl1 and Cxcl2 and of the trapped dimeric variants. To understand how Cxcr2 and GAG interactions impact in vivo function, we characterized their neutrophil recruitment activity to the peritoneum, Cxcr2 and CD11b levels on peritoneal and blood neutrophils, and transport profiles out of the peritoneum. Cxcl2 variants compared with Cxcl1 variants were more potent for Cxcr2 activity. Native Cxcl1 compared with native Cxcl2 and dimers compared with native proteins bound HS with higher affinity. Interestingly, recruitment activity between native Cxcl1 and Cxcl2, between dimers, and between the native protein and the dimer could be similar or very different depending on the dose or the time point. These data indicate that peritoneal neutrophil recruitment cannot be solely attributed to Cxcr2 or GAG interactions, and that the relationship between recruited neutrophils, Cxcr2 activation, GAG interactions, and chemokine levels is complex and highly context dependent. We propose that the ability of Cxcl1 and Cxcl2 to reversibly exist as monomers and dimers and differences in their Cxcr2 activity and GAG interactions coordinate neutrophil recruitment and activation, which play a critical role for successful resolution of inflammation.

Neutrophil transendothelial migration hotspots - mechanisms and implications

During inflammation, leukocytes circulating in the blood stream exit the vasculature in a process called leukocyte transendothelial migration (TEM). The current paradigm of this process comprises several well-established steps, including rolling, adhesion, crawling, diapedesis and sub-endothelial crawling. Nowadays, the role of the endothelium in transmigration is increasingly appreciated. It has been established that leukocyte exit sites on the endothelium and in the pericyte layer are in fact not random but instead may be specifically recognized by migrating leukocytes. Here, we review the concept of transmigration hotspots, specific sites in the endothelial and pericyte layer where most transmigration events take place. Chemokine cues, adhesion molecules and membrane protrusions as well as physical factors, such as endothelial junction stability, substrate stiffness, the presence of pericytes and basement membrane composition, may all contribute to local hotspot formation to facilitate leukocytes exiting the vasculature. In this Review, we discuss the biological relevance of such hotspots and put forward multiple mechanisms and factors that determine a functional TEM hotspot.

NMR Characterization of the Interactions Between Glycosaminoglycans and Proteins

Glycosaminoglycans (GAGs) constitute a considerable fraction of the glycoconjugates found on cellular membranes and in the extracellular matrix of virtually all mammalian tissues. The essential role of GAG-protein interactions in the regulation of physiological processes has been recognized for decades. However, the underlying molecular basis of these interactions has only emerged since 1990s. The binding specificity of GAGs is encoded in their primary structures, but ultimately depends on how their functional groups are presented to a protein in the three-dimensional space. This review focuses on the application of NMR spectroscopy on the characterization of the GAG-protein interactions. Examples of interpretation of the complex mechanism and characterization of structural motifs involved in the GAG-protein interactions are given. Selected families of GAG-binding proteins investigated using NMR are also described.

Structural basis of a chemokine heterodimer binding to glycosaminoglycans

Chemokines Cxcl1/KC and Cxcl2/MIP2 play a crucial role in coordinating neutrophil migration to the insult site. Chemokines' recruitment activity is regulated by monomer-dimer equilibrium and binding to glycosaminoglycans (GAGs). GAG chains exist as covalently linked to core proteins of proteoglycans (PGs) and also as free chains due to cleavage by heparanases during the inflammatory response. Compared with free GAGs, binding to GAGs in a PG is influenced by their fixed directionality due to covalent linkage and restricted mobility. GAG interactions impact chemokine monomer/dimer levels, chemotactic and haptotactic gradients, life time, and presentation for receptor binding. Here, we show that Cxcl1 and Cxcl2 also form heterodimers. Using a disulfide-trapped Cxcl1-Cxcl2 heterodimer, we characterized its binding to free heparin using nuclear magnetic resonance and isothermal titration calorimetry, and to immobilized heparin and heparan sulfate using surface plasmon resonance. These data, in conjunction with molecular docking, indicate that the binding characteristics such as geometry and stoichiometry of the heterodimer are different between free and immobilized GAGs and are also distinctly different from those of the homodimers. We propose that the intrinsic asymmetry of the heterodimer structure, along with differences in its binding to PG GAGs and free GAGs, regulate chemokine function.

Role of inflammatory chemokines in hypertension

Hypertension is associated with immune cells activation and their migration into the kidney, vasculature, heart and brain. These inflammatory mechanisms are critical for blood pressure regulation and mediate target organ damage, creating unique novel targets for pharmacological modulation. In response to angiotensin II and other pro-hypertensive stimuli, the expression of several inflammatory chemokines and their receptors is increased in the target organs, mediating homing of immune cells. In this review, we summarize the contribution of key inflammatory chemokines and their receptors to increased accumulation of immune cells in target organs and effects on vascular dysfunction, remodeling, oxidative stress and fibrosis, all of which contribute to blood pressure elevation. In particular, the role of CCL2, CCL5, CXCL8, CXCL9, CXCL10, CXCL11, CXCL16, CXCL1, CX3CL1, XCL1 and their receptors in the context of hypertension is discussed. Recent studies have tested the efficacy of pharmacological or genetic targeting of chemokines and their receptors on the development of hypertension. Promising results indicate that some of these pathways may serve as future therapeutic targets to improve blood pressure control and prevent target organ consequences including kidney failure, heart failure, atherosclerosis or cognitive impairment.

Structural Insights Into How Proteoglycans Determine Chemokine-CXCR1/CXCR2 Interactions: Progress and Challenges

Proteoglycans (PGs), present in diverse environments, such as the cell membrane surface, extracellular milieu, and intracellular granules, are fundamental to life. Sulfated glycosaminoglycans (GAGs) are covalently attached to the core protein of proteoglycans. PGs are complex structures, and are diverse in terms of amino acid sequence, size, shape, and in the nature and number of attached GAG chains, and this diversity is further compounded by the phenomenal diversity in GAG structures. Chemokines play vital roles in human pathophysiology, from combating infection and cancer to leukocyte trafficking, immune surveillance, and neurobiology. Chemokines mediate their function by activating receptors that belong to the GPCR class, and receptor interactions are regulated by how, when, and where chemokines bind GAGs. GAGs fine-tune chemokine function by regulating monomer/dimer levels and chemotactic/haptotactic gradients, which are also coupled to how they are presented to their receptors. Despite their small size and similar structures, chemokines show a range of GAG-binding geometries, affinities, and specificities, indicating that chemokines have evolved to exploit the repertoire of chemical and structural features of GAGs. In this review, we summarize the current status of research on how GAG interactions regulate ELR-chemokine activation of CXCR1 and CXCR2 receptors, and discuss knowledge gaps that must be overcome to establish causal relationships governing the impact of GAG interactions on chemokine function in human health and disease.

Targeting Chemokine-Glycosaminoglycan Interactions to Inhibit Inflammation

Leukocyte migration into tissues depends on the activity of chemokines that form concentration gradients to guide leukocytes to a specific site. Interaction of chemokines with their specific G protein-coupled receptors (GPCRs) on leukocytes induces leukocyte adhesion to the endothelial cells, followed by extravasation of the leukocytes and subsequent directed migration along the chemotactic gradient. Interaction of chemokines with glycosaminoglycans (GAGs) is crucial for extravasation in vivo. Chemokines need to interact with GAGs on endothelial cells and in the extracellular matrix in tissues in order to be presented on the endothelium of blood vessels and to create a concentration gradient. Local chemokine retention establishes a chemokine gradient and prevents diffusion and degradation. During the last two decades, research aiming at reducing chemokine activity mainly focused on the identification of inhibitors of the interaction between chemokines and their cognate GPCRs. This approach only resulted in limited success. However, an alternative strategy, targeting chemokine-GAG interactions, may be a promising approach to inhibit chemokine activity and inflammation. On this line, proteins derived from viruses and parasites that bind chemokines or GAGs may have the potential to interfere with chemokine-GAG interactions. Alternatively, chemokine mimetics, including truncated chemokines and mutant chemokines, can compete with chemokines for binding to GAGs. Such truncated or mutated chemokines are characterized by a strong binding affinity for GAGs and abrogated binding to their chemokine receptors. Finally, Spiegelmers that mask the GAG-binding site on chemokines, thereby preventing chemokine-GAG interactions, were developed. In this review, the importance of GAGs for chemokine activity in vivo and strategies that could be employed to target chemokine-GAG interactions will be discussed in the context of inflammation.

Tick Saliva Protein Evasin-3 Allows for Visualization of Inflammation in Arteries through Interactions with CXC-Type Chemokines Deposited on Activated Endothelium

Atherosclerosis is one of the leading causes of mortality in developed and developing countries. The onset of atherosclerosis development is accompanied by overexpression of several inflammatory chemokines. Neutralization of these chemokines by chemokine-binding agents attenuates atherosclerosis progression. Here, we studied structural binding features of the tick protein Evasin-3 to chemokine (C-X-C motif) ligand 1 (CXCL1). We showed that Evasin-3-bound CXCL1 is unable to activate the CXCR2 receptor, but retains affinity to glycosaminoglycans. This observation was exploited to detect inflammation by visualizing a group of closely related CXC-type chemokines deposited on cell walls in human endothelial cells and murine carotid arteries by a fluorescent Evasin-3 conjugate. This work highlights the applicability of tick-derived chemokine-binding conjugates as a platform for the development of new agents for inflammation imaging.

Anti-α4β1 Integrin Antibodies Attenuated Brain Inflammatory Changes in a Mouse Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is associated with age-associated central nervous system degeneration and dementia. This decline in the function correlates with deposition of Aβ peptide containing plaques and associated reactive gliosis. The inflammatory phenotype of microglia, in particular, is often considered detrimental to cognitive function in AD. In addition to the changes in the CNS, altered immune changes in the periphery have recently been observed in AD suggesting a critical immune- related communication between the periphery and the brain.

OBJECTIVE

We hypothesized that modulating the peripheral immune system may alter the proinflammatory gliosis associated with AD. Therapeutic antibodies against the α4β1 integrin receptor have been used clinically to attenuate the ability of various immune cells to adhere to endothelium and migrate into target tissues such as the intestines (Crohn's disease) or brain (multiple sclerosis). We hypothesized that a similar peripheral antibody-based therapy would attenuate gliosis by altering immune cell infiltration or phenotype in peripheral organs and the brain using an APP/PS1 mouse model of Alzheimer's disease.

METHOD

Littermate control wild-type and APP/PS1 mice were tail vein injected with either saline, isotype control (IgG2b), or an antibody recognizing α4-integrin, anti-CD49d, once a week for 4 consecutive weeks. To understand CNS and peripheral immune changes, brains and spleen were used.

RESULTS/CONCLUSION

Our data suggests that the antibody therapy was able to reduce microgliosis, astrogliosis, and synaptic changes in the APP/PS1 mice compared to isotype control injections without changing amyloid-β plaque load. Interestingly, both isotype control and antibody therapy also reduced the number of proinflammatory cytokines in the spleen although changes in the brain were less robust. The anti-CD49d and isotype control treatments also reduced CD4 immunoreactivity in the brains, suggesting a possible mechanism for attenuation of inflammation in the brain. This data suggests that it is indeed feasible to alter the immune component of AD brain changes using a clinically feasible strategy of delivering a particular subtype of IgG or epitope selective antibodies that target infiltration of the peripheral immune system.

Structural basis of chemokine interactions with heparan sulfate, chondroitin sulfate, and dermatan sulfate

Chemokines play diverse roles in human pathophysiology, ranging from trafficking leukocytes and immunosurveillance to the regulation of metabolism and neural function. Chemokine function is intimately coupled to binding tissue glycosaminoglycans (GAGs), heparan sulfate (HS), chondroitin sulfate (CS), and dermatan sulfate (DS). Currently, very little is known about how the structural features and sequences of a given chemokine, the structure and sulfation pattern of a given GAG, and structural differences among GAGs and among chemokines impact binding interactions. In this study, we used solution NMR spectroscopy to characterize the binding interactions of two related neutrophil-activating chemokines, CXCL1 and CXCL5, with HS, CS, and DS. For both chemokines, the dimer bound all three GAGs with higher affinity than did the monomer, and affinities of the chemokines for CS and DS were lower than for HS. NMR-based structural models reveal diverse binding geometries and show that the binding surfaces for each of the three GAGs were different between the two chemokines. However, a given chemokine had similar binding interactions with CS and DS that were different from HS. Considering the fact that CXCL1 and CXCL5 activate the same CXCR2 receptor, we conclude that GAG interactions play a role in determining the nature of chemokine gradients, levels of free chemokine available for receptor activation, how chemokines bind their receptors, and that differences in these interactions determine chemokine-specific function.

Distribution of bioactive factors in human milk samples

Background

Breast milk provides nutrition for infants and also contains a variety of bioactive factors that influence the development of the newborn. Human milk is a complex biological fluid that can be separated into different layers (water phase and lipid phase with its component water and lipid fractions). It can affect the developing human body along the whole length of the gastrointestinal tract, and through the circulation, its factors may reach every organ.

Methods

In the present study, we analyzed milk samples collected monthly for 6 months from 16 mothers from the 4^th week postpartum between 2014 and 2016 in Baranya County, Hungary. The 96 samples provided us information about the fluctuation of certain bioactive factors during the first 6 months of lactation. We investigated with Luminex technology the concentrations of several cytokines (CD40, Flt-3L), chemokines (MCP-1, RANTES, GRO, MIP-1ß, MDC, eotaxin, fractalkine), and epidermal growth factor (EGF). Paired t-tests and one-way ANOVA followed by Bonferroni post-hoc tests were used to compare the data.

Results

We detected the presence of each bioactive factor in every layer of the milk samples during the first 6 months of breastfeeding in widespread concentration ranges. In the case of GRO, MIP-1ß, MDC, Flt-3L, fractalkine, and eotaxin, the concentrations were constant during the first 6 months of lactation. The water phase of human milk contained higher factor concentrations compared to both fractions of the lipid phase for most factors (except eotaxin and MIP-1ß). The concentrations of CD40, EGF, MCP-1, and RANTES in the first 3 months were significantly different compared to the values detected between 4^th and 6^th months. In the water phase, the level of MCP-1 was significantly decreased, while all of the other factors increased during the 4^th through 6^th months. We found significantly higher EGF, GRO, and RANTES levels in the water fraction compared to the lipid fraction of the lipid phase.

Conclusions

The novel findings of this investigation were the presence of Flt-3L and MDC in all layers of breast milk, and nearly all bioactive factors in the lipid phase. Due to their widespread physiological effects these factors may have an essential role in organogenesis.

How do chemokines navigate neutrophils to the target site: Dissecting the structural mechanisms and signaling pathways

Chemokines play crucial roles in combating microbial infection and initiating tissue repair by recruiting neutrophils in a timely and coordinated manner. In humans, no less than seven chemokines (CXCL1, CXCL2, CXCL3, CXCL5, CXCL6, CXCL7, and CXCL8) and two receptors (CXCR1 and CXCR2) mediate neutrophil functions but in a context dependent manner. Neutrophil-activating chemokines reversibly exist as monomers and dimers, and their receptor binding triggers conformational changes that are coupled to G-protein and β-arrestin signaling pathways. G-protein signaling activates a variety of effectors including Ca2+ channels and phospholipase C. β-arrestin serves as a multifunctional adaptor and is coupled to several signaling hubs including MAP kinase and tyrosine kinase pathways. Both G-protein and β-arrestin signaling pathways play important non-overlapping roles in neutrophil trafficking and activation. Functional studies have established many similarities but distinct differences for a given chemokine and between chemokines at the level of monomer vs. dimer, CXCR1 vs. CXCR2 activation, and G-protein vs. β-arrestin pathways. We propose that two forms of the ligand binding two receptors and activating two signaling pathways enables fine-tuned neutrophil function compared to a single form, a single receptor, or a single pathway. We summarize the current knowledge on the molecular mechanisms by which chemokine monomers/dimers activate CXCR1/CXCR2 and how these interactions trigger G-protein/β-arrestin-coupled signaling pathways. We also discuss current challenges and knowledge gaps, and likely advances in the near future that will lead to a better understanding of the relationship between the chemokine-CXCR1/CXCR2-G-protein/β-arrestin axis and neutrophil function.

Distinct Differences in Structural States of Conserved Histidines in Two Related Proteins: NMR Studies of the Chemokines CXCL1 and CXCL8 in the Free Form and Macromolecular Complexes

Hydrogen-bonding and ionic interactions play fundamental roles in macromolecular recognition and function. In contrast to lysines and arginines, how histidines mediate these interactions is less well-understood due to the unique properties of its side chain imidazole that include an aromatic ring with two titratable nitrogens, a p K_a that can vary significantly, and the ability to exist in three distinct forms: protonated imidazolium and two tautomeric neutral (N^δ1 and N^ε2) states. Here, we characterized the structural features of histidines in the chemokines CXCL8 and CXCL1 in the free, GAG heparin-bound, and CXCR2 receptor N-terminal domain-bound states using solution NMR spectroscopy. CXCL8 and CXCL1 share two conserved histidines, one in the N-loop and the other in the 30s loop. In CXCL8, both histidines exist in the N^ε2 tautomeric state in the free, GAG-bound, and receptor-bound forms. On the other hand, in unliganded CXCL1, each of the two histidines exists in two states, as the neutral N^ε2 tautomer and charged imidazolium. Further, both histidines exclusively exist as the imidazolium in the GAG-bound and as the N^ε2 tautomer in the receptor-bound forms. The N-loop histidine alone in both chemokines is involved in direct GAG and receptor interactions, indicating the role of the 30s loop varies between the chemokines. Our observation that the structural features of conserved histidines and their functional role in two related proteins can be quite different is novel. We further propose that directly probing the imidazole structural features is essential to fully appreciate the molecular basis of histidine function.

Structural basis, stoichiometry, and thermodynamics of binding of the chemokines KC and MIP2 to the glycosaminoglycan heparin

Keratinocyte-derived chemokine (KC or mCXCL1) and macrophage inflammatory protein 2 (MIP2 or mCXCL2) play nonredundant roles in trafficking blood neutrophils to sites of infection and injury. The functional responses of KC and MIP2 are intimately coupled to their interactions with glycosaminoglycans (GAGs). GAG interactions orchestrate chemokine concentration gradients and modulate receptor activity, which together regulate neutrophil trafficking. Here, using NMR, molecular dynamics (MD) simulations, and isothermal titration calorimetry (ITC), we characterized the molecular basis of KC and MIP2 binding to the GAG heparin. Both chemokines reversibly exist as monomers and dimers, and the NMR analysis indicates that the dimer binds heparin with higher affinity. The ITC experiments indicate a stoichiometry of two GAGs per KC or MIP2 dimer and that the enthalpic and entropic contributions vary significantly between the two chemokine-heparin complexes. NMR-based structural models of heparin-KC and heparin-MIP2 complexes reveal that different combinations of residues from the N-loop, 40s turn, β₃-strand, and C-terminal helix form a binding surface within a monomer and that both conserved residues and residues unique to a particular chemokine mediate the binding interactions. MD simulations indicate significant residue-specific differences in their contribution to binding and affinity for a given chemokine and between chemokines. On the basis of our observations that KC and MIP2 bind to GAG via distinct molecular interactions, we propose that the differences in these GAG interactions lead to differences in neutrophil recruitment and play nonoverlapping roles in resolution of inflammation.

Lysines and Arginines play non-redundant roles in mediating chemokine-glycosaminoglycan interactions

Glycosaminoglycans (GAGs) bind a large array of proteins and mediate fundamental and diverse roles in human physiology. Ion pair interactions between protein lysines/arginines and GAG sulfates/carboxylates mediate binding. Neutrophil-activating chemokines (NAC) are GAG-binding proteins, and their sequences reveal high selectivity for lysines over arginines indicating they are functionally not equivalent. NAC binding to GAGs impacts gradient formation, receptor functions, and endothelial activation, which together regulate different components of neutrophil migration. We characterized the consequence of mutating lysine to arginine in NAC CXCL8, a well-characterized GAG-binding protein. We chose three lysines — two highly conserved lysines (K20 and K64) and a CXCL8-specific lysine (K67). Interestingly, the double K64R/K20R and K64R/K67R mutants are highly impaired in recruiting neutrophils in a mouse model. Further, both the mutants bind GAG heparin with higher affinity but show similar receptor activity. NMR and MD studies indicate that the structures are essentially identical to the WT, but the mutations alter the network of intramolecular ion pair interactions. These observations collectively indicate that the reduced in vivo recruitment is due to altered GAG interactions, higher GAG binding affinity can be detrimental, and specificity of lysines fine-tunes in vivo GAG interactions and function.

Heparin-bound chemokine CXCL8 monomer and dimer are impaired for CXCR1 and CXCR2 activation: implications for gradients and neutrophil trafficking

Chemokine CXCL8 plays a pivotal role in host immune response by recruiting neutrophils to the infection site. CXCL8 exists as monomers and dimers, and mediates recruitment by interacting with glycosaminoglycans (GAGs) and activating CXCR1 and CXCR2 receptors. How CXCL8 monomer and dimer interactions with both receptors and GAGs mediate trafficking is poorly understood. In particular, both haptotactic (mediated by GAG-bound chemokine) and chemotactic (mediated by soluble chemokine) gradients have been implicated, and whether it is the free or the GAG-bound CXCL8 monomer and/or dimer that activates the receptor remains unknown. Using solution NMR spectroscopy, we have now characterized the binding of heparin-bound CXCL8 monomer and dimer to CXCR1 and CXCR2 receptor N-domains. Our data provide compelling evidence that heparin-bound monomers and dimers are unable to bind either of the receptors. Cellular assays also indicate that heparin-bound CXCL8 is impaired for receptor activity. Considering dimer binds GAGs with higher affinity, dimers will exist predominantly in the GAG-bound form and the monomer in the free form. We conclude that GAG interactions determine the levels of free CXCL8, and that it is the free, and not GAG-bound, CXCL8 that activates the receptors and mediates recruitment of blood neutrophils to the infected tissue.

Directed evolution of broadly crossreactive chemokine-blocking antibodies efficacious in arthritis

Chemokine receptors typically have multiple ligands. Consequently, treatment with a blocking antibody against a single chemokine is expected to be insufficient for efficacy. Here we show single-chain antibodies can be engineered for broad crossreactivity toward multiple human and mouse proinflammatory ELR⁺ CXC chemokines. The engineered molecules recognize functional epitopes of ELR⁺ CXC chemokines and inhibit neutrophil activation ex vivo. Furthermore, an albumin fusion of the most crossreactive single-chain antibody prevents and reverses inflammation in the K/BxN mouse model of arthritis. Thus, we report an approach for the molecular evolution and selection of broadly crossreactive antibodies towards a family of structurally related, yet sequence-diverse protein targets, with general implications for the development of novel therapeutics.

CXCR2 antagonism has been shown to be anti-arthritic, but anti-chemokine therapies usually fail in the clinic owing to redundancy in chemokine-receptor interactions. Here the authors develop single-chain antibodies with multiple chemokine specificities to achieve high affinity and broad specificity to mouse and human CXC chemokines with efficacy in a K/BxN serum transfer model of arthritis.

Glycosaminoglycan Interactions Fine-Tune Chemokine-Mediated Neutrophil Trafficking: Structural Insights and Molecular Mechanisms

Circulating neutrophils, rapidly recruited in response to microbial infection, form the first line in host defense. Humans express ~50 chemokines, of which a subset of seven chemokines, characterized by the conserved "Glu-Leu-Arg" motif, mediate neutrophil recruitment. Neutrophil-activating chemokines (NACs) share similar structures, exist as monomers and dimers, activate the CXCR2 receptor on neutrophils, and interact with tissue glycosaminoglycans (GAGs). Considering cellular assays have shown that NACs have similar CXCR2 activity, the question has been and remains, why do humans express so many NACs? In this review, we make the case that NACs are not redundant and that distinct GAG interactions determine chemokine-specific in vivo functions. Structural studies have shown that the GAG-binding interactions of NACs are distinctly different, and that conserved and specific residues in the context of structure determine geometries that could not have been predicted from sequences alone. Animal studies indicate recruitment profiles of monomers and dimers are distinctly different, monomer-dimer equilibrium regulates recruitment, and that recruitment profiles vary between chemokines and between tissues, providing evidence that GAG interactions orchestrate neutrophil recruitment. We propose in vivo GAG interactions impact several chemokine properties including gradients and lifetime, and that these interactions fine-tune and define the functional response of each chemokine that can vary between different cell and tissue types for successful resolution of inflammation.

Direct detection of lysine side chain NH3+ in protein-heparin complexes using NMR spectroscopy

Two NMR observables, the N_ζH₃⁺ peak in the HISQC spectrum and N_ζ chemical shift difference between the free and heparin-bound forms, can identify binding-interface lysines in protein-heparin complexes. Unlike backbone chemical shifts, these direct probes are stringent and are less prone to either false positives or false negatives.

The Role of Heparan Sulfate in Inflammation, and the Development of Biomimetics as Anti-Inflammatory Strategies

Key events that occur during inflammation include the recruitment, adhesion, and transmigration of leukocytes from the circulation to the site of inflammation. These events are modulated by chemokines, integrins, and selectins and the interaction of these molecules with glycosaminoglycans, predominantly heparan sulfate (HS). The development of HS/heparin mimetics that interfere or inhibit the interactions that occur between glycosaminoglycans and modulators of inflammation holds great potential for use as anti-inflammatory therapeutics. This review will detail the role of HS in the events that occur during inflammation, their interaction and modulation of inflammatory mediators, and the current advances in the development of HS/heparin mimetics as anti-inflammatory biotherapeutics.

The role of the immune system in Alzheimer disease: Etiology and treatment

The immune system is now considered a major factor in Alzheimer Disease (AD). This review seeks to demonstrate how various aspects of the immune system, both in the brain and peripherally, interact to contribute to AD. We highlight classical nervous system immune components, such as complement and microglia, as well as novel aspects of the peripheral immune system that can influence disease, such as monocytes and lymphocytes. By detailing the roles of various immune cells in AD, we summarize an emerging perspective for disease etiology and future therapeutic targets.

Characterization of structural motifs for interactions between glycosaminoglycans and proteins

Glycosaminoglycans (GAGs) are a family of linear and anionic polysaccharides that play essential roles in many biological and physiological processes. Interactions between GAGs and proteins regulate function in many proteins and are related to many human diseases and disorders. The structural motifs and mechanisms for interactions between GAGs and proteins are not fully understood. Specific bindings, including minor but unique sequences sporadically distributed along the GAG chains or variably sulfated domains interspersed by undersulfated regions, may be specifically recognized by defined domains of a variety of proteins. Understanding the molecular basis of these interactions will provide a template for developing novel glycotherapeutic agents. The present article reviews recent methodologies and progress on the characterization of structural motifs in both GAGs and proteins involved in GAG-protein interactions. The analytical approaches are categorized into three groups: affinity-based methods; molecular docking, nuclear magnetic resonance (NMR) spectroscopy and X-ray crystallography; and mass spectrometry (MS) techniques. The advantages and limitations of each category of methods are discussed and are based on examples of using these techniques to investigate binding between GAGs and proteins.

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.

Differences in Sulfotyrosine Binding amongst CXCR1 and CXCR2 Chemokine Ligands

Tyrosine sulfation, a post-translational modification found on many chemokine receptors, typically increases receptor affinity for the chemokine ligand. A previous bioinformatics analysis suggested that a sulfotyrosine (sY)-binding site on the surface of the chemokine CXCL12 may be conserved throughout the chemokine family. However, the extent to which receptor tyrosine sulfation contributes to chemokine binding has been examined in only a few instances. Computational solvent mapping correctly identified the conserved sulfotyrosine-binding sites on CXCL12 and CCL21 detected by nuclear magnetic resonance (NMR) spectroscopy, demonstrating its utility for hot spot analysis in the chemokine family. In this study, we analyzed five chemokines that bind to CXCR2, a subset of which also bind to CXCR1, to identify hot spots that could participate in receptor binding. A cleft containing the predicted sulfotyrosine-binding pocket was identified as a principal hot spot for ligand binding on the structures of CXCL1, CXCL2, CXCL7, and CXCL8, but not CXCL5. Sulfotyrosine titrations monitored via NMR spectroscopy showed specific binding to CXCL8, but not to CXCL5, which is consistent with the predictions from the computational solvent mapping. The lack of CXCL5–sulfotyrosine interaction and the presence of CXCL8–sulfotyrosine binding suggests a role for receptor post-translational modifications regulating ligand selectivity.

Mechanistic insights into molecular evolution of species-specific differential glycosaminoglycan binding surfaces in growth-related oncogene chemokines

Chemokines are chemotactic cytokines involved in leucocyte trafficking to infected tissue. Growth-related oncogene (GRO) chemokines namely CXCL1, CXCL2 and CXCL3 are neutrophil activating chemokines sharing a conserved three-dimensional structure, but encompassing functional diversity due to gene duplication and evolutionary events. However, the evolutionary mechanisms including selection pressures involved in diversification of GRO genes have not yet been characterized. Here, we performed comprehensive evolutionary analysis of GRO genes among different mammalian species. Phylogenetic analysis illustrated a species-specific evolution pattern. Selection analysis evidenced that these genes have undergone concerted evolution. Seventeen positively selected sites were obtained, although the majority of the protein is under purifying selection. Interestingly, these positively selected sites are more concentrated on the C-terminal/glycosaminoglycan (GAG) binding and dimerization segment compared to receptor binding domain. Substitution rate analysis confirmed the C-terminal domain of GRO genes as the highest substituted segment. Further, structural analysis established that the nucleotide alterations in the GAG binding domain are the source of surface charge modulation, thus generating the differential GAG binding surfaces and multiple binding sites as per evolutionary pressure, although the helical surface is primordial for GAG binding. Indeed, such variable electrostatic surfaces are crucial to regulate chemokine gradient formation during a host's defence against pathogens and also explain the significance of chemokine promiscuity.

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.

Regulation of Chemokine Function: The Roles of GAG-Binding and Post-Translational Nitration

The primary function of chemokines is to direct the migration of leukocytes to the site of injury during inflammation. The effects of chemokines are modulated by several means, including binding to G-protein coupled receptors (GPCRs), binding to glycosaminoglycans (GAGs), and through post-translational modifications (PTMs). GAGs, present on cell surfaces, bind chemokines released in response to injury. Chemokines bind leukocytes via their GPCRs, which directs migration and contributes to local inflammation. Studies have shown that GAGs or GAG-binding peptides can be used to interfere with chemokine binding and reduce leukocyte recruitment. Post-translational modifications of chemokines, such as nitration, which occurs due to the production of reactive species during oxidative stress, can also alter their biological activity. This review describes the regulation of chemokine function by GAG-binding ability and by post-translational nitration. These are both aspects of chemokine biology that could be targeted if the therapeutic potential of chemokines, like CXCL8, to modulate inflammation is to be realised.

What Do Structures Tell Us About Chemokine Receptor Function and Antagonism?

Chemokines and their cell surface G protein-coupled receptors are critical for cell migration, not only in many fundamental biological processes but also in inflammatory diseases and cancer. Recent X-ray structures of two chemokines complexed with full-length receptors provided unprecedented insight into the atomic details of chemokine recognition and receptor activation, and computational modeling informed by new experiments leverages these insights to gain understanding of many more receptor:chemokine pairs. In parallel, chemokine receptor structures with small molecules reveal the complicated and diverse structural foundations of small molecule antagonism and allostery, highlight the inherent physicochemical challenges of receptor:chemokine interfaces, and suggest novel epitopes that can be exploited to overcome these challenges. The structures and models promote unique understanding of chemokine receptor biology, including the interpretation of two decades of experimental studies, and will undoubtedly assist future drug discovery endeavors.

Chemokine CXCL7 Heterodimers: Structural Insights, CXCR2 Receptor Function, and Glycosaminoglycan Interactions

Chemokines mediate diverse fundamental biological processes, including combating infection. Multiple chemokines are expressed at the site of infection; thus chemokine synergy by heterodimer formation may play a role in determining function. Chemokine function involves interactions with G-protein-coupled receptors and sulfated glycosaminoglycans (GAG). However, very little is known regarding heterodimer structural features and receptor and GAG interactions. Solution nuclear magnetic resonance (NMR) and molecular dynamics characterization of platelet-derived chemokine CXCL7 heterodimerization with chemokines CXCL1, CXCL4, and CXCL8 indicated that packing interactions promote CXCL7-CXCL1 and CXCL7-CXCL4 heterodimers, and electrostatic repulsive interactions disfavor the CXCL7-CXCL8 heterodimer. As characterizing the native heterodimer is challenging due to interference from monomers and homodimers, we engineered a “trapped” disulfide-linked CXCL7-CXCL1 heterodimer. NMR and modeling studies indicated that GAG heparin binding to the heterodimer is distinctly different from the CXCL7 monomer and that the GAG-bound heterodimer is unlikely to bind the receptor. Interestingly, the trapped heterodimer was highly active in a Ca²⁺ release assay. These data collectively suggest that GAG interactions play a prominent role in determining heterodimer function in vivo. Further, this study provides proof-of-concept that the disulfide trapping strategy can serve as a valuable tool for characterizing the structural and functional features of a chemokine heterodimer.

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.

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.

Molecular Basis of Chemokine CXCL5-Glycosaminoglycan Interactions

Chemokines, a large family of highly versatile small soluble proteins, play crucial roles in defining innate and adaptive immune responses by regulating the trafficking of leukocytes, and also play a key role in various aspects of human physiology. Chemokines share the characteristic feature of reversibly existing as monomers and dimers, and their functional response is intimately coupled to interaction with glycosaminoglycans (GAGs). Currently, nothing is known regarding the structural basis or molecular mechanisms underlying CXCL5-GAG interactions. To address this missing knowledge, we characterized the interaction of a panel of heparin oligosaccharides to CXCL5 using solution NMR, isothermal titration calorimetry, and molecular dynamics simulations. NMR studies indicated that the dimer is the high-affinity GAG binding ligand and that lysine residues from the N-loop, 40s turn, β3 strand, and C-terminal helix mediate binding. Isothermal titration calorimetry indicated a stoichiometry of two oligosaccharides per CXCL5 dimer. NMR-based structural models reveal that these residues form a contiguous surface within a monomer and, interestingly, that the GAG-binding domain overlaps with the receptor-binding domain, indicating that a GAG-bound chemokine cannot activate the receptor. Molecular dynamics simulations indicate that the roles of the individual lysines are not equivalent and that helical lysines play a more prominent role in determining binding geometry and affinity. Further, binding interactions and GAG geometry in CXCL5 are novel and distinctly different compared with the related chemokines CXCL1 and CXCL8. We conclude that a finely tuned balance between the GAG-bound dimer and free soluble monomer regulates CXCL5-mediated receptor signaling and function.