Insights into CC Chemokine Ligand 2/Chemokine Receptor 2 Molecular Recognition: A Step Forward toward Antichemotactic Agents

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.

Gallate Moiety of Catechin Is Essential for Inhibiting CCL2 Chemokine-Mediated Monocyte Recruitment

Leukocyte recruitment witnesses an orchestrated complex formation between the chemokines and their molecular partners. CCL2 chemokine that regulates monocyte trafficking is a worthwhile system from the pharmaceutical perspective. In the current study, four major catechins (EC/EGC/ECG/EGCG) were assessed for their inhibitory potential against CCL2-regulated monocyte/macrophage recruitment. Interestingly, catechins with the gallate moiety (ECG/EGCG) could only attenuate the CCL2-induced macrophage migration. These molecules specifically bound to CCL2 on a pocket comprising the N-terminal, β₀-sheets, and β₃-sheets, and the binding affinity of ECGC (K_d = 22 ± 4 μM) is ∼4 times higher than that of the ECG complex (K_d = 85 ± 6 μM). MD simulation analysis evidenced that the molecular specificity/stability of CCL2-catechin complexes is regulated by multiple factors, including stereospecificity, number of hydroxyl groups on the annular ring-B, the positioning of the carbonyl group, and the methylation of the galloyl ring. Further, a significant overlap on the binding surface of CCL2 for EGCG/ECG and receptor interactions as evidenced from NMR data provided the rationale for the observed inhibition of macrophage migration in response to EGCG/ECG binding. In summary, these galloylated epicatechins can be considered as potent protein-protein interaction (PPI) inhibitors that regulate CCL2-directed leukocyte recruitment for resolving inflammatory/immunomodulatory disorders.

The molecular link between obesity and genomic instability in cancer development

Obesity has been known to be a major risk factor for various types of cancers for several decades. More recently, the relationship between dysregulated adipokines and cancer development has been the focus of much research. Adipose tissue is an important endocrine organ that secretes adipokines that affect both autocrine and paracrine signaling. These adipokines modulate inflammation, induce insulin resistance, and regulate their own behavior and production. Adipokine-production dysregulation is due to physiological changes in adipose tissue that prompt molecular modifications, including low-grade inflammation and the stimulatory production of reactive oxygen species. Additionally, studies have linked DNA damage response, genomic instability, and the innate immune response to tumorigenesis. Further investigation of adipokines and their role in the promotion of genomic instability may clarify the link between obesity and cancer, as well as elucidate potential pharmaceutical targets. In this review, we discuss the progress of recent literature, focusing on the impact of adipokines, genomic instability, and the innate immune response on increasing the risk of cancer.

Investigation of synovial fluid lubricants and inflammatory cytokines in the horse: a comparison of recombinant equine interleukin 1 beta-induced synovitis and joint lavage models

BACKGROUND

Lameness is a debilitating condition in equine athletes that leads to more performance limitation and loss of use than any other medical condition. There are a limited number of non-terminal experimental models that can be used to study early inflammatory and synovial fluid biophysical changes that occur in the equine joint. Here, we compare the well-established carpal IL-1β-induced synovitis model to a tarsal intra-articular lavage model, focusing on serial changes in synovial fluid inflammatory cytokines/chemokines and the synovial fluid lubricating molecules lubricin/proteoglycan 4 and hyaluronic acid. The objectives of this study were to evaluate clinical signs; synovial membrane and synovial fluid inflammation; and synovial fluid lubricants and biophysical properties in response to carpal IL-1β synovitis and tarsal intra-articular lavage.

RESULTS

Hyaluronic acid (HA) concentrations, especially high molecular weight HA, and synovial fluid viscosity decreased after both synovitis and lavage interventions. Synovial fluid lubricin concentrations increased 17-20-fold for both synovitis and lavage models, with similar changes in both affected and contralateral joints, suggesting that repeated arthrocentesis alone resulted in elevated synovial fluid lubricin concentrations. Synovitis resulted in a more severe inflammatory response based on clinical signs (temperature, heart rate, respiratory rate, lameness and joint effusion) and clinicopathological and biochemical parameters (white blood cell count, total protein, prostaglandin E2, sulfated glycosaminoglycans, tumor necrosis factor-α and CC chemokine ligands - 2, - 3, - 5 and - 11) as compared to lavage.

CONCLUSIONS

Synovial fluid lubricin increased in response to IL-1β synovitis and joint lavage but also as a result of repeated arthrocentesis. Frequent repeated arthrocentesis is associated with inflammatory changes, including increased sulfated glycosaminoglycan concentrations and decreased hyaluronic acid concentrations. Synovitis results in more significant inflammatory changes than joint lavage. Our data suggests that synovial fluid lubricin, TNF-α, CCL2, CCL3, CCL5, CCL11 and sGAG may be useful biomarkers for synovitis and post-lavage joint inflammation. Caution should be exercised when performing repeated arthrocentesis clinically or in experimental studies due to the inflammatory response and loss of HA and synovial fluid viscosity.

Dissecting the differential structural and dynamics features of CCL2 chemokine orthologs

Chemokines are a sub-group of cytokines that regulate the leukocyte migration. Monocyte chemoattractant protein-1 (MCP/CCL2) is one of the essential CC chemokine that regulates the migration of monocytes into inflamed tissues. It has been observed that the primary sequences of CCL2 orthologs among rodents and primates vary significantly at the C-terminal region. However, no structural details are available for the rodentia family CCL2 proteins. The current study unravelled the structural, dynamics and in-silico functional characteristics of murine CCL2 chemokine using a comprehensive set of NMR spectroscopy techniques and evolutionary approaches. The study unravelled that the N-terminal portion of the murine CCL2 forms a canonical CC chemokine dimer similar to that of human CCL2. However, unlike human CCL2, the murine ortholog exhibits extensive dynamics in the μs-ms timescales. The presence of C-terminal region of the murine CCL2 protein/rodentia family is highly glycosylated, completely disordered, and inhibits the folding of the structured CCL2 regions. Further, it has been observed that the glycosaminoglycan binding surfaces of these orthologs proteins are greatly differed. In a nut shell, this comparative study provided the role of molecular evolution in generating orthologous proteins with differential structural and dynamics characteristics to engage them in specific molecular interactions.

Crosslinking-guided geometry of a complete CXC receptor-chemokine complex and the basis of chemokine subfamily selectivity

Chemokines and their receptors are orchestrators of cell migration in humans. Because dysregulation of the receptor-chemokine system leads to inflammation and cancer, both chemokines and receptors are highly sought therapeutic targets. Yet one of the barriers for their therapeutic targeting is the limited understanding of the structural principles behind receptor-chemokine recognition and selectivity. The existing structures do not include CXC subfamily complexes and lack information about the receptor distal N-termini, despite the importance of the latter in signaling, regulation, and bias. Here, we report the discovery of the geometry of the complex between full-length CXCR4, a prototypical CXC receptor and driver of cancer metastasis, and its endogenous ligand CXCL12. By comprehensive disulfide cross-linking, we establish the existence and the structure of a novel interface between the CXCR4 distal N-terminus and CXCL12 β1-strand, while also recapitulating earlier findings from nuclear magnetic resonance, modeling and crystallography of homologous receptors. A cross-linking-informed high-resolution model of the CXCR4-CXCL12 complex pinpoints the interaction determinants and reveals the occupancy of the receptor major subpocket by the CXCL12 proximal N terminus. This newly found positioning of the chemokine proximal N-terminus provides a structural explanation of CXC receptor-chemokine selectivity against other subfamilies. Our findings challenge the traditional two-site understanding of receptor-chemokine recognition, suggest the possibility of new affinity and signaling determinants, and fill a critical void on the structural map of an important class of therapeutic targets. These results will aid the rational design of selective chemokine-receptor targeting small molecules and biologics with novel pharmacology.

Dynamics-Derived Insights into Complex Formation between the CXCL8 Monomer and CXCR1 N-Terminal Domain: An NMR Study

Interleukin-8 (CXCL8), a potent neutrophil-activating chemokine, exerts its function by activating the CXCR1 receptor that belongs to class A G protein-coupled receptors (GPCRs). Receptor activation involves interactions between the CXCL8 N-terminal loop and CXCR1 N-terminal domain (N-domain) residues (Site-I) and between the CXCL8 N-terminal and CXCR1 extracellular/transmembrane residues (Site-II). CXCL8 exists in equilibrium between monomers and dimers, and it is known that the monomer binds CXCR1 with much higher affinity and that Site-I interactions are largely responsible for the differences in monomer vs. dimer affinity. Here, using backbone 15N-relaxation nuclear magnetic resonance (NMR) data, we characterized the dynamic properties of the CXCL8 monomer and the CXCR1 N-domain in the free and bound states. The main chain of CXCL8 appears largely rigid on the picosecond time scale as evident from high order parameters (S²). However, on average, S² are higher in the bound state. Interestingly, several residues show millisecond-microsecond (ms-μs) dynamics only in the bound state. The CXCR1 N-domain is unstructured in the free state but structured with significant dynamics in the bound state. Isothermal titration calorimetry (ITC) data indicate that both enthalpic and entropic factors contribute to affinity, suggesting that increased slow dynamics in the bound state contribute to affinity. In sum, our data indicate a critical and complex role for dynamics in driving CXCL8 monomer-CXCR1 Site-I interactions.