Mechanism of ligand binding to E- and P-selectin analyzed using selectin/mannose-binding protein chimeras

Quantitative Characterization of E-selectin Interaction with Native CD44 and P-selectin Glycoprotein Ligand-1 (PSGL-1) Using a Real Time Immunoprecipitation-based Binding Assay

Selectins (E-, P-, and L-selectins) interact with glycoprotein ligands to mediate the essential tethering/rolling step in cell transport and delivery that captures migrating cells from the circulating flow. In this work, we developed a real time immunoprecipitation assay on a surface plasmon resonance chip that captures native glycoforms of two well known E-selectin ligands (CD44/hematopoietic cell E-/L-selectin ligand and P-selectin glycoprotein ligand-1) from hematopoietic cell extracts. Here we present a comprehensive characterization of their binding to E-selectin. We show that both ligands bind recombinant monomeric E-selectin transiently with fast on- and fast off-rates, whereas they bind dimeric E-selectin with remarkably slow on- and off-rates. This binding requires the sialyl Lewis x sugar moiety to be placed on both O- and N-glycans, and its association, but not dissociation, is sensitive to the salt concentration. Our results suggest a mechanism through which monomeric selectins mediate initial fast on and fast off kinetics to help capture cells out of the circulating shear flow; subsequently, tight binding by dimeric/oligomeric selectins is enabled to significantly slow rolling.

Effects of an anti-inflammatory VAP-1/SSAO inhibitor, PXS-4728A, on pulmonary neutrophil migration

Background and purpose

The persistent influx of neutrophils into the lung and subsequent tissue damage are characteristics of COPD, cystic fibrosis and acute lung inflammation. VAP-1/SSAO is an endothelial bound adhesion molecule with amine oxidase activity that is reported to be involved in neutrophil egress from the microvasculature during inflammation. This study explored the role of VAP-1/SSAO in neutrophilic lung mediated diseases and examined the therapeutic potential of the selective inhibitor PXS-4728A.

Methods

Mice treated with PXS-4728A underwent intra-vital microscopy visualization of the cremaster muscle upon CXCL1/KC stimulation. LPS inflammation, Klebsiella pneumoniae infection, cecal ligation and puncture as well as rhinovirus exacerbated asthma models were also assessed using PXS-4728A.

Results

Selective VAP-1/SSAO inhibition by PXS-4728A diminished leukocyte rolling and adherence induced by CXCL1/KC. Inhibition of VAP-1/SSAO also dampened the migration of neutrophils to the lungs in response to LPS, Klebsiella pneumoniae lung infection and CLP induced sepsis; whilst still allowing for normal neutrophil defense function, resulting in increased survival. The functional effects of this inhibition were demonstrated in the RV exacerbated asthma model, with a reduction in cellular infiltrate correlating with a reduction in airways hyperractivity.

Conclusions and implications

This study demonstrates that the endothelial cell ligand VAP-1/SSAO contributes to the migration of neutrophils during acute lung inflammation, pulmonary infection and airway hyperractivity. These results highlight the potential of inhibiting of VAP-1/SSAO enzymatic function, by PXS-4728A, as a novel therapeutic approach in lung diseases that are characterized by neutrophilic pattern of inflammation.

Use of naturally occurring halloysite nanotubes for enhanced capture of flowing cells

The development of individualized treatments for cancer can be facilitated by more efficient methods for separating cancer cells from patient blood in such a way that they remain viable for live cell assays. We have previously shown that immobilized P-selectin protein can be used on the inner surface of a microscale flow system to induce leukemic cells and leukocytes to roll at different velocities and relative fluxes, thereby creating a means for rapid cell fractionation without inflicting cellular damage. In this study, we explore a method to more efficiently capture leukemic and epithelial cancer cells from flow by altering the nanoscale topography of the inner surface of P-selectin-coated microtubes. This functionalized topography is achieved by attaching naturally occurring halloysite nanotubes to the microtube surface via a monolayer of poly-L-lysine), followed by functionalization with recombinant human selectin protein. We have found that halloysite nanotube coatings promote increased capture of leukemic cells and have determined the key parameters for controlling cell capture under flow: halloysite content and selectin density. Ultimately, selectin-functionalized nanotube coatings should provide a means for enhanced cancer cell isolation from whole blood and other mixtures of cells.

Computational modeling of carbohydrate-recognition process in E-selectin complex: structural mapping of sialyl Lewis X onto ab initio QM/MM free energy surface

To advance our knowledge of carbohydrate recognition by lectins, we propose a systematic computational modeling strategy to identify complex sugar-chain conformations on the reduced free energy surface (FES). We selected the complex of E-selectin with sialyl Lewis X (denoted E-selectin/SLe(x) complex) as a first target molecule. First, we introduced the reduced 2D-FES that characterizes conformational changes in carbohydrate structure as well as the degree of solvation stability of the carbohydrate ligand, and evaluated the overall free energy profile by classical molecular dynamics simulation combined with ab initio QM/MM energy corrections. Second, we mapped flexible carbohydrate structures onto the reduced QM/MM 2D-FES, and identified the details of molecular interactions between each monosaccharide component and the amino acid residues at the carbohydrate-recognition domain. Finally, we confirmed the validity of our modeling strategy by evaluating the chemical shielding tensor by ab initio QM/MM-GIAO computations for several QM/MM-refined geometries sampled from the minimum free energy region in the 2D-FES, and compared this theoretical averaging data with the experimental 1D-NMR profile. The model clearly shows that the binding geometries of the E-selectin/SLe(x) complex are determined not by one single, rigid carbohydrate structure but rather by the sum of averaged conformations fluctuating around the minimum free energy region. For the E-selectin/SLe(x) complex, the major molecular interactions are hydrogen bonds between Fuc and the Ca(2+) binding site in the carbohydrate-recognition domain, and Gal is important in determining the ligand specificity.

Distinctive properties of the hyaluronan-binding domain in the lymphatic endothelial receptor Lyve-1 and their implications for receptor function

The lymphatic endothelial hyaluronan (HA) receptor Lyve-1 is a member of the Link protein superfamily most similar to the leukocyte HA receptor CD44. However, the structure of Lyve-1 and the nature of its interaction with ligand are obscure. Here we present new evidence that Lyve-1 is functionally distinct from CD44. Using truncation mutagenesis we confirm that Lyve-1 in common with CD44 contains an extended HA-binding unit, comprising elements flanking the N and C termini of the consensus lectin-like Link module, bridged by a third conserved disulfide linkage that is critical for HA binding. In addition, we identify six essential residues Tyr-87, Ile-97, Arg-99, Asn-103, Lys-105, and Lys-108 that define a compact HA-binding surface on Lyve-1, encompassing the epitope for an adhesion-blocking monoclonal antibody 3A, in an analogous position to the HA-binding surface in CD44. The overtly electrostatic character of HA binding in Lyve-1 and its sensitivity to ionic strength (IC(50) of 150 mm NaCl) contrast markedly with CD44 (IC(50) > 2 m NaCl) in which HA binding is mediated by hydrogen bonding and hydrophobic interactions. In addition, unlike the extended Link module in CD44, which binds HA efficiently when expressed as a soluble monomer (K(d) = 65.7 mum), that of Lyve-1 requires artificial dimerization, although the full ectodomain is active as a monomer (K(d) = 35.6 mum). Finally, full-length Lyve-1 did not form stable dimers in binding-competent 293T transfectants when assessed using bioluminescent resonance energy transfer. These results reveal that elements additional to the extended Link module are required to stabilize HA binding in Lyve-1 and indicate important structural and functional differences with CD44.

Immunological functions of hyaluronan and its receptors in the lymphatics

The lymphatic system is best known for draining interstitial fluid from the tissues and returning it to the blood circulation. However, the lymphatic system also provides the means for immune surveillance in the immune system, acting as conduits that convey soluble antigens and antigen-presenting cells from the tissues to the lymph nodes, where primary lymphocyte responses are generated. One macromolecule that potentially unites these two functions is the large extracellular matrix glycosaminoglycan hyaluronan (HA), a chemically simple copolymer of GlcNAc and GlcUA that fulfills a diversity of functions from danger signal to adhesive substratum, depending upon chain length and particular interaction with its many different binding proteins and a small but important group of receptors. The two most abundant of these receptors are CD44, which is expressed on leukocytes that traffic through the lymphatics, and LYVE-1, which is expressed almost exclusively on lymphatic endothelium. Curiously, much of the HA within the tissues is turned over and degraded in lymph nodes, by a poorly understood process that occurs in the medullary sinuses. Indeed there are several mysterious aspects to HA in the lymphatics. Here we cover some of these by reviewing recent findings in the biology of lymphatic endothelial cells and their possible roles in HA homeostasis together with fresh insights into the complex and enigmatic nature of LYVE-1, its regulation of HA binding by sialylation and self-association, and its potential function in leukocyte trafficking.

Diverse sugar-binding specificities of marine invertebrate C-type lectins

The sugar-binding specificities of C-type lectins isolated from marine invertebrates were investigated by frontal affinity chromatography (FAC) using 100 oligosaccharides. The lectins included BRA-2 and BRA-3, multiple lectins from the hemolymph of the acorn barnacle, Megabalanus rosa, and BRL from the acorn barnacle, Balanus rostatus. The diverse sugar-binding specificities of the C-type lectins were determined by FAC analysis. BRA-2 recognized alpha2-6 sialylation but not alpha2-3 sialylation on glycans. On the other hand, BRA-3 showed high affinity for oligosaccharides with alpha-linked non-reducing terminal galactose, but not for sialylated forms, and BRL showed enhanced recognition activity towards Lewis(x) and Lewis(a) epitopes.

Calcium is not required for immulectin-2 binding, but protects the protein from proteinase digestion

Mammalian C-type lectins are calcium-dependent carbohydrate-binding proteins. They serve as cell adhesion molecules in cell-cell interactions, or function as pattern-recognition receptors in innate immunity. Calcium is a direct ligand for carbohydrate binding in mammalian C-type lectins such as mannose-binding proteins and macrophage mannose receptor. In the tobacco hornworm Manduca sexta, a group of lectins named immulectins have been discovered. Each immulectin contains dual carbohydrate-recognition domains. Previously, we showed that immulectin-2 (IML-2) binds to a bacterial lipopolysaccharide, and agglutination of Escherichia coli cells by IML-2 is calcium dependent. In this study, we demonstrated that IML-2 bound to bacterial lipid A, smooth and rough mutants of lipopolysaccharide, lipoteichoic acid and peptidoglycan, as well as to fungal mannan and beta-1, 3-glucan (laminarin and curdlan). Binding of IML-2 to microbial components was calcium independent, and was increased by addition of spermine, a polyamine. In addition, plasma IML-2 bound to mannan-agarose independent of calcium. But trypsin digestion of IML-2 was inhibited in the presence of calcium. Our results suggest that calcium is not required for IML-2 binding but protects IML-2 from trypsin digestion.

Structure of the Ly49 family of natural killer (NK) cell receptors and their interaction with MHC class I molecules

Natural killer (NK) cells are an essential component of the innate immunity toward tumors and virally infected cells. The function of NK cells is regulated by a precise balance between inhibitory and activating signals. These signals are mediated by NK cell receptors that bind either classical MHC class I molecules or their structural relatives such as MICA, ULBP, RAE-1, and H-60. Two separate families of NK cell receptors have been identified: the immunoglobulin-like family (KIR, LIR) and C-type lectin-like family (Ly49, NKG2D, and CD94/NKG2). Here we summarize the structure of Ly49 C-type lectin-like proteins hitherto solved (Ly49A, Ly49C and Ly49I) and their interaction with MHC class I molecules as determined by the co-crystal structure of Ly49A/H-2Dd and Ly49C/H-2Kb.

Parallel synthesis of glycomimetic libraries: targeting a C-type lectin

We have developed methods for the parallel synthesis of two libraries of non-carbohydrate-based analogues of mannose on a solid support. The natural product shikimic acid was used as a key building block. The ability of the compounds to block the binding of the C-type lectin MBP-A to a mannosylated surface was assessed in a high-throughput assay. Ten library members with inhibitory activities equivalent to that of alpha-methyl mannopyranoside were identified. [reaction: see text]

Minimal requirements for the binding of selectin ligands to a C-type carbohydrate-recognition domain

The C-type carbohydrate-recognition domains of E-selectin and rat serum mannose-binding protein have similar structures. Selectin/mannose-binding protein chimeras created by transfer of key sequences from E-selectin into mannose-binding protein have previously been shown to bind the selectin ligand sialyl-Lewis(X) through a Ca(2+)-dependent subsite, common to many C-type lectins, and an accessory site containing positively charged amino acid residues. Further characterization of these chimeras as well as analysis of novel constructs containing additional regions of E-selectin demonstrate that selectin-like interaction with sialyl-Lewis(X) can be faithfully reproduced even though structural evidence indicates that the mechanisms of binding to E-selectin and the chimeras are different. Selectin-like binding to the nonfucosylated sulfatide and sulfoglucuronyl glycolipids can also be reproduced with selectin/mannose-binding protein chimeras that contain the two subsites involved in sialyl-Lewis(X) binding. These results indicate that binding of structurally distinct anionic glycans to C-type carbohydrate-recognition domains can be mediated by the Ca(2+)-dependent subsite in combination with a positively charged region that forms an ionic strength-sensitive subsite.

Development of potent non-carbohydrate imidazole-based small molecule selectin inhibitors with antiinflammatory activity

A novel series of non-carbohydrate imidazole-based selectin inhibitors has been discovered via high-throughput screening using a P-selectin ELISA-based assay system. The initial lead 1 had an IC(50) of 17 microM in the P-selectin ELISA; this potency was significantly improved via an extensive SAR exploration. One of the current lead compounds (29) has an IC(50) of 300 nM in a P-selectin ELISA; it also has good activity in P- and E-selectin cell adhesion assays and shows efficacy in vivo. These compounds represent a novel series of sLe(X) mimetics with antiinflammatory activity. Their unique profile supports our interest in their further evaluation as drug candidates for the treatment of inflammation. Herein we describe the syntheses, optimization, and SAR of this series of novel potent selectin antagonists.

Binding of glycosulfopeptides to P-selectin requires stereospecific contributions of individual tyrosine sulfate and sugar residues

P-selectin glycoprotein ligand-1 (PSGL-1) is a mucin on leukocytes that binds to selectins. P-selectin binds to an N-terminal region of PSGL-1 that requires sulfation of at least one of three clustered tyrosines (TyrSO(3)) and an adjacent core-2-based O-glycan expressing sialyl Lewis x (C2-O-sLe(x)). We synthesized glycosulfopeptides (GSPs) modeled after this region of PSGL-1 to explore the roles of individual TyrSO(3) residues, the placement of C2-O-sLe(x) relative to TyrSO(3), the relative contributions of fucose and sialic acid on C2-O-sLe(x), and the function of the peptide sequence for binding to P-selectin. Binding of GSPs to P-selectin was measured by affinity chromatography and equilibrium gel filtration. 2-GSP-6, which has C2-O-sLe(x) at Thr-57 and TyrSO(3) at residues 46, 48, and 51, bound to P-selectin with high affinity (K(d) approximately 650 nm), whereas an isomeric trisulfated GSP containing C2-O-sLe(x) at Thr-44 bound much less well. Non-sulfated glycopeptide (2-GP-6) containing C2-O-sLe(x) at Thr-57 bound to P-selectin with approximately 40-fold lower affinity (K(d) approximately 25 microm). Proteolysis of 2-GP-6 abolished detectable binding of the residual C2-O-sLe(x)-Thr to P-selectin, demonstrating that the peptide backbone contributes to binding. Monosulfated and disulfated GSPs bound significantly better than non-sulfated 2-GP-6, but sulfation of Tyr-48 enhanced affinity (K(d) approximately 6 microm) more than sulfation of Tyr-46 or Tyr-51. 2-GSP-6 lacking sialic acid bound to P-selectin at approximately 10% that of the level of the parent 2-GSP-6, whereas 2-GSP-6 lacking fucose did not detectably bind; thus, fucose contributes more than sialic acid to binding. Reducing NaCl from 150 to 50 mm markedly enhanced binding of 2-GSP-6 to P-selectin (K(d) approximately 75 nm), demonstrating the charge dependence of the interaction. These results reveal a stereospecific interaction of P-selectin with PSGL-1 that includes distinct contributions of each of the three TyrSO(3) residues, adjacent peptide determinants, and fucose/sialic acid on an optimally positioned core-2 O-glycan.

Recognition of Class I MHC by NK Receptor Ly-49C: Identification of Critical Residues

The Ly-49 family of inhibitory receptors plays a major role in regulating mouse NK cell cytotoxicity. Two of its members, Ly-49C and I, are recognized by the mAb 5E6, which also defines a subset of NK cells involved in the hybrid resistance phenomenon. Previous studies have shown that Ly-49C binds to a broad spectrum of class I MHC molecules, while Ly-49I apparently does not bind to any class I MHC molecules tested. In the present investigation we have defined the amino acid residues of Ly-49C that are critical for determining its ligand specificities. First, using quantitative COS cell adhesion assays, we demonstrated that Ly-49CB6 bound to Dd, Db, Kb, or Kk as well as to murine leukemic cell lines GM979 (H-2s) and IC-21 (H-2b). In contrast, COS cells expressing Ly-49IB6 did not significantly bind to any of the class I MHC tested. To determine which amino acid residues of Ly-49C are critical for their specific binding to class I MHC, a series of chimeric and mutant Ly-49C and I were generated and tested. Exchanging the critical residues between Ly-49C and I significantly affected their binding specificities. Finally, we identified the epitopes on Ly-49C recognized by mAbs 5E6 and 4LO3311 that functionally inhibit Ly-49C recognition of its ligands. These results further define the class I specificities of Ly-49C and provide insight into the structural basis for how class I MHC is recognized by the Ly-49 family of NK receptors.

The C-type lectin superfamily in the immune system

Protein-carbohydrate interactions serve multiple functions in the immune system. Many animal lectins (sugar-binding proteins) mediate both pathogen recognition and cell-cell interactions using structurally related Ca(2+)-dependent carbohydrate-recognition domains (C-type CRDs). Pathogen recognition by soluble collections such as serum mannose-binding protein and pulmonary surfactant proteins, and also the macrophage cell-surface mannose receptor, is effected by binding of terminal monosaccharide residues characteristic of bacterial and fungal cell surfaces. The broad selectivity of the monosaccharide-binding site and the geometrical arrangement of multiple CRDs in the intact lectins explains the ability of the proteins to mediate discrimination between self and non-self. In contrast, the much narrower binding specificity of selectin cell adhesion molecules results from an extended binding site within a single CRD. Other proteins, particularly receptors on the surface of natural killer cells, contain C-type lectin-like domains (CTLDs) that are evolutionarily divergent from the C-type lectins and which would be predicted to function through different mechanisms.