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

Abstract

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.

Glycosaminoglycans as Multifunctional Anti-Elastase and Anti-Inflammatory Drugs in Cystic Fibrosis Lung Disease

“…Heparan sulfate (HS) is expressed widely on many cell types as a proteoglycan. HS proteoglycans regulate inflammation by binding to ELR (Glu Leu Arg)- CXC chemokines at conserved His, Lys, Arg residues, controlling chemotactic gradients in the extracellular and pericellular matrices ( Rajarathnam KaD, 2020 ). However, in the CF lung, endogenous HS proteoglycans have pro-inflammatory properties ( Reeves et al, 2011 ); HS stabilizes cytokine and chemokine ligands, preventing protease digestion, thus increasing CXCL ligation to CXCR1 and 2 to upregulate inflammation ( Rajarathnam KaD, 2020 ).…”

“…HS proteoglycans regulate inflammation by binding to ELR (Glu Leu Arg)- CXC chemokines at conserved His, Lys, Arg residues, controlling chemotactic gradients in the extracellular and pericellular matrices ( Rajarathnam KaD, 2020 ). However, in the CF lung, endogenous HS proteoglycans have pro-inflammatory properties ( Reeves et al, 2011 ); HS stabilizes cytokine and chemokine ligands, preventing protease digestion, thus increasing CXCL ligation to CXCR1 and 2 to upregulate inflammation ( Rajarathnam KaD, 2020 ). HS enables RAGE hexamer formation for more efficient intracellular signaling ( Xu et al, 2013 ), and binds L-selectin to promote neutrophil slowing and diapedesis across endothelial cells into tissues ( Farrugia et al, 2018 ).…”

“…Heparin/ODSH oligosaccharides bind to P- and L-selectins and block neutrophil adhesion and chemotaxis ( Nelson et al, 1993 ; Rao et al, 2010 ). Heparin inhibits CXCL8/IL-8 and other ELR (Glu Leu Arg)-CXC chemokines from binding to G-protein coupled receptors CXCR1 and CXCR2 ( Rajarathnam KaD, 2020 ). Heparin competes with HSPG for binding to microbial proteins which prevents bacterial or viral-epithelial adhesion and invasion ( Rostand and Esko, 1997 ).…”

Long-range coupled motions underlie ligand recognition by a chemokine receptor

“…Chemokines, in turn, show a range of receptor specificity and activity. Diversity of interactions is further compounded by chemokines and chemokine receptors existing as monomers and dimers, biased G protein vs. barrestin signaling, interacting with glycosaminoglycans, post-translational modification, and binding to atypical chemokine receptors (Bhusal et al, 2020;Rajagopal et al, 2013;Rajarathnam and Desai, 2020).…”

“…Whereas all ELR-chemokines are potent CXCR2 agonists, CXCL8 monomer alone functions as a potent CXCR1 agonist (Nasser et al, 2009). CXCR1 and CXCR2 are expressed in neutrophils and in non-immune cells such as hepatocytes and neuronal cells, and these receptors play fundamental roles in combating infection, regulating pain, and in the trafficking and proliferation of cancer cells (Ha et al, 2017;Rajarathnam et al, 2019). CXCR1 and CXCR2 sequences show large differences for the N-domain indicating a prominent role for Site-I interactions in determining specificity and affinity.…”