Selectins and other mammalian sialic acid-binding lectins

Prolonged circulation of activated platelets following plasmapheresis

The extent and duration of in vivo platelet activation were determined in 12 volunteer donors undergoing automated plasmapheresis. Expression of P-selectin, activated GpIIb/IIIa, and platelet microparticle formation were measured by flow cytometry on peripheral blood samples obtained immediately before and after plasmapheresis and at 24 hour intervals thereafter for up to 3 days. Although no adverse effects were noted in any donor, immediately after apheresis 3-87% of circulating platelets expressed P-selectin; by 48 hours, 0.5-50% expressed P-selectin; and by 72 hours, all donors studied had fewer than 5% P-selectin expression on circulating platelets. Results were similar for the expression of the activated conformation of GpIIb/IIIa. There was a positive correlation with in vitro P-selectin expression in response to ADP in the pre-apheresis sample and the number of platelet microparticles detected in the donor following plasmapheresis. In addition, the percent expression of P-selectin and activated GpIIb/IIIa in response to ADP was reproducible in each individual studied on five separate occasions (CV < or = 8%). Platelets activated during plasmapheresis using an automated device may circulate for at least 48 hours, and pre-plasmapheresis response of platelets to the agonist ADP correlated with platelet microparticle formation post-plasmapheresis.

Conformation of the oligosaccharide receptor for E-selectin

A tetrasaccharide related to the blood group oligosaccharides, known as sialyl LewisX, has been proposed as the receptor for the lectin responsible for leukocyte adhesion named alternatively as E-selectin or ELAM-1. The 13C- and 1H-nmr spectra have been completely assigned for a tetrasaccharide model of this receptor, Neu5Ac alpha-(2-->3)-Gal beta-(1-->4)-[Fuc alpha-(1-->3)-]GlcNAc beta-NHAc. Quantitative nuclear Overhauser data (NOESY) have been recorded and analyzed by a complete spin matrix simulation method. Conformational space was exhaustively searched and all conformational models whose simulated NOESY spectra matched the experiment were found. Molecular mechanics and molecular dynamics calculations were carried out to test whether the experimental conformations are low energy and thus likely to represent true single conformations for the tetrasaccharide. It was concluded that while the LewisX trisaccharide portion of the compound adopts a single conformation, there is likely to be some flexibility about the Neu5Ac alpha-(2-->3)-linkage. A model featuring fast exchange between two different conformations of this linkage is found to be consistent with both the nmr experiments and the molecular dynamics simulations.

Expression cloning of a functional glycoprotein ligand for P-selectin

The initial adhesive interactions between circulating leukocytes and endothelia are mediated, in part, by P-selectin. We now report the expression cloning of a functional ligand for P-selectin from an HL-60 cDNA library. The predicted amino acid sequence reveals a novel mucin-like transmembrane protein. Significant binding of transfected COS cells to P-selectin requires coexpression of both the protein ligand and a fucosyltransferase. This binding is calcium dependent and can be inhibited by a neutralizing monoclonal antibody to P-selectin. Cotransfected COS cells express the ligand as a homodimer of 220 kd. A soluble ligand construct, when coexpressed with fucosyltransferase in COS cells, also mediates P-selectin binding and is immunocrossreactive with the major HL-60 glycoprotein that specifically binds P-selectin.

Analysis of carbohydrate-protein interactions with synthetic N-linked neoglycoconjugate probes

Recently we have describe a simple efficient chemical method of generating an asparagine side-chain linker with beta-stereochemistry at the anomeric position of neutral oligosaccharides. We now report the 1-N-glycyl beta-derivatization of sialylated saccharides. Several neoglycoconjugates formed using these N-linked inter-mediates were investigated for their usefulness in probing carbohydrate-protein interactions. First, biotinyl derivatives of two xylose/fucose class plant-type oligosaccharides purified from horseradish peroxidase were effective in demonstrating the carbohydrate specificity of polyclonal anti-(horseradish peroxidase) antibodies. Secondly, a fluorescein-labelled asialo- and digalactosylated biantennary complex sugar was synthesized and shown to bind to a Ricinus communis agglutinin column. This galactose-specific recognition was abolished by treating this fluorescein-labelled oligosaccharide with jack bean beta-galactosidase. Finally, two 1-N-glycyl beta-saccharide derivatives were modified with thiophosgene to form their corresponding isothiocyanate derivatives. Coupling of these isothiocyanate derivatives of sugars to BSA, amino-derivatized polystyrene plates and glass-fibre discs resulted in multiple sugar presentation. The binding of an anti-N-acetylglucosamine monoclonal antibody to N,N'-diacetylchitobiose residues presented on BSA and solid supports was shown by e.l.i.s.a. Similarly the binding of concanavalin A to asialo-, agalactosylated biantennary complex oligosaccharide residues attached to BSA was demonstrated by a competitive e.l.i.s.a. Our results demonstrate that N-linked neoglycoconjugates could be made readily available and they are valuable tools for the detailed analyses of carbohydrates and carbohydrate-binding proteins.

Studies on a glucose-binding lectin from peripheral blood lymphocytes

Lectin-carbohydrate interactions have been found to be important in many of the steps of lymphocyte recirculation and inflammatory responses. A D-glucose-specific lectin was isolated from goat peripheral blood lymphocytes by affinity chromatography on N-acetyl-D-glucosamine agarose and gave a single band corresponding to 112 kDa in SDS-PAGE, irrespective of treatment with 2-mercaptoethanol. The lymphocyte lectin agglutinated rabbit and human ABO erythrocytes, the hemagglutinating activity being Ca2+ dependent. It appears to be a member of type C animal lectins.

Gel-immobilized heparin-binding lectin as sensitive sensor for certain groups of charge-bearing carbohydrates

The specificity of lectins to carbohydrate moieties in principle enables them to serve as sensors for sugars with ligand properties. However, experimental systems and parameters to measure this interaction need to be defined. On the basis of knowledge about temperature-sensitive volume changes of gels, composed of acrylamide derivatives, and about the influence of presence of charge-bearing groups within the gel on this behavior, we covalently immobilized a human heparin-binding lectin into a gel matrix. Besides the lectin-carrying derivative N-isopropylacrylamide and N,N'-methylenebisacrylamide are the monomeric constituents of the polymer. The lectin has been attached to divinyl sulfone-activated N-hydroxymethylacrylamide. Several anionic sugar moieties are added to the solution, covering the gel pieces, and the mechanical response of the individual gel slices in dependence to stepwise temperature increases is automatically recorded with an electronic transducer at a sensitivity of 5 mV/microns. Only carboxyl group-containing sugar moieties like glucuronic acid notably reduce the extent of the temperature-dependent gel shrinking as indicator for a protein-carbohydrate interaction. The individual slices are reuseable, emphasizing practical applications. This sensitive and automated assay concept with the covalently immobilized heparin-binding protein is supposed to be adaptable to other groups of lectins with specificity to anionic sugars like sialic acid-binding proteins.

Distribution of saccharides in pig lymph-node high-endothelial venules and associated lymphocytes visualized using fluorescent lectins and confocal microscopy

The distribution of saccharides in pig lymph nodes, particularly on high-endothelial venule (HEV) endothelium and on lymphocytes in these vessels, was studied by examining the binding of fluorescent conjugates of 18 different lectins. Eight of the lectins, particularly with glycan specificity restricted to mannose and polyacetyllactosamine determinants, were found to bind with a high affinity to these structures. Competitive inhibition experiments revealed that polylactosamine-containing glycans were present on endothelia and lymphocytes using lectins from Lycopersicon esculentum and Solanum tuberosum, the latter lectin reacting with lymphocytes only when apparently adherent to the luminal endothelium. The The absence on pig endothelium of the Ulex europaeus binding, shown by human endothelia due to the presence of certain fucose epitopes, was confirmed. Pig lymph-node endothelium, however, bound the fucose-specific lectin of Tetragonolobus purpureas, indicating the presence of fucose on pig endothelia in a different conformation to that seen on human endothelia. The results suggested that pig lymph-node HEV endothelium expressed a core fucosylated tri- or tetra-antennary complex glycan with polylactosamine extensions and expressing an Ley determinant.

Robert Feulgen Lecture 1993. L-selectin and its biological ligands

This review considers the leukocyte adhesive receptor known as L-selectin. This protein, belonging to the selectin family of cell-cell adhesion receptors, mediates adhesion by virtue of a C-type lectin domain at its amino terminus. The protein was discovered as a lymphocyte homing receptor involved in the attachment of lymphocytes to high endothelial venules (HEV) of lymph nodes. Its widespread distribution on all leukocyte populations underlies a more general role in a variety of leukocyte-endothelial interactions. In the HEV interaction, cognate HEV ligands for L-selectin have been identified as two sulfated, sialylated, and fucosylated glycoproteins, known as GlyCAM-1 and Sgp90. These ligands have mucin-like domains which confer important properties for their proposed adhesive function. The carbohydrate features of these ligands, essential for their interaction with L-selectin, are reviewed. The existence of extralymphoid ligands for L-selectin is also discussed.

Leukocyte rolling and extravasation are severely compromised in P selectin-deficient mice

P selectin, expressed on surfaces of activated endothelial cells and platelets, is an adhesion receptor for leukocytes. We report that P selectin-deficient mice, generated by gene targeting in embryonic stem cells, exhibit a number of defects in leukocyte behavior, including elevated numbers of circulating neutrophils, virtually total absence of leukocyte rolling in mesenteric venules, and delayed recruitment of neutrophils to the peritoneal cavity upon experimentally induced inflammation. These results clearly demonstrate a role for P selectin in leukocyte interactions with the vessel wall and in the early steps of leukocyte recruitment at sites of inflammation. These mutant mice should prove useful in deciphering the contributions of P selectin in various inflammatory responses as well as in platelet functions.

Inflammatory roles of P-selectin

Polymorphonuclear leukocytes (PMNs) bind rapidly and reversibly to endothelial cells induced to express P-selectin, a glycoprotein that mediates adhesive intercellular interactions. In addition, PMNs adherent to endothelium expressing P-selectin demonstrate an intracellular Ca2+ transient, functionally up-regulate beta-2-integrins (CD11/CD18 glycoproteins), become polarized in shape, and are primed for enhanced degranulation when subsequently stimulated with chemotactic factors. However, P-selectin induces none of these responses directly when used alone, when incorporated into model membranes, or when expressed by transfected cells. The absence of direct activation of the PMNs is not due to competing antiinflammatory effects of P-selectin; instead, purified P-selectin and P-selectin in membranes support agonist-stimulated PMN responses. Furthermore, tethering of PMNs to endothelial surfaces by P-selectin is required for priming to occur efficiently, as shown by experiments with blocking monoclonal antibodies. The priming event is directly mediated by the signaling molecule, platelet-activating factor (PAF), and is inhibited by blocking the PAF receptor on PMNs. Thus, P-selectin and PAF are components of an adhesion and activation cascade, but have distinct roles: P-selectin tethers and captures the PMN, whereas PAF mediates juxtacrine activation. In vivo, selectins may facilitate interaction of target cells with membrane-bound molecules that send intercellular signals, in addition to mediating rolling of leukocytes and other adhesive functions.

Adhesion of human lymphoid cell lines to immobilized carbohydrates and to bone-marrow stromal cell layers by surface sugar receptors

Cell-surface sugar receptors may participate in interactions of lymphoid cells that influence their adhesive properties and proliferation. Their expression on cells of the pre-B line BLIN-I, the B-lymphoblastoid line Croco II, the myeloma line RPMI 8226 and the T-lymphoblastoid line CCRF-CEM was monitored with a panel of 14 types of chemically glycosylated E. coli beta-galactosidase at a non-saturating ligand concentration. Quantitative differences were determined for the capacity of the different cell types to bind constituents of the carbohydrate part of glycoconjugates. They were corroborated by analyses of binding for lactose-, beta-N-acetylgalactosamine-, beta-N-acetylglucosamine- and fucose-exposing neoglycoenzymes up to saturation levels. Values of dissociation constants of the tetrameric enzyme were in the range of 3-300 nM. Several types of sugar receptor led to carbohydrate-inhibitable adhesion of cells to 6 types of nitrocellulose-immobilized neoglycoprotein, their effectiveness being most obvious for the myeloma cells. Analyses of the carbohydrate-ligand-mediated adhesion of the other cell types revealed a comparatively decreased response. Only a few carbohydrates among the 7 types tested were effective in reducing cell adhesion to a far more complex ligand-bearing matrix than immobilized neoglycoproteins, namely bone-marrow stromal cell layers: sialic acid and N-acetylgalactosamine for B-lymphoblastoid cells and rhamnose for pre-B cells. These cellular interactions may encompass sugar receptors on the stromal cells and other types of molecular recognition in addition to the detected activities on the lymphoid cells.

Calcium-dependent heparin-like ligands for L-selectin in nonlymphoid endothelial cells

L-Selectin is a calcium-dependent mammalian lectin that mediates lymphocyte trafficking by recognizing sialylated ligands on high endothelial venules in lymph nodes. Although L-selectin probably mediates neutrophil extravasation into nonlymphoid tissues, no corresponding ligand has been characterized. Staining of cultured endothelial cells with an L-selectin chimera (LS-Rg) showed an internal pool of ligands. Metabolic labeling with sulfur-35-labeled sulfate revealed heparin lyase-sensitive ligands that bound LS-Rg in a calcium-dependent, sialic acid-independent manner. A fraction of commercial heparin bound to LS-Rg and LS-Rg bound to heparin-agarose, both in a calcium-dependent manner. Thus, L-selectin recognizes endothelial heparin-like chains, which could be physiological ligands mediating leucocyte trafficking.

P- and E-selectin use common sites for carbohydrate ligand recognition and cell adhesion

The selectins are a family of three calcium-dependent lectins that mediate adhesive interactions between leukocytes and the endothelium during normal and abnormal inflammatory episodes. Previous work has implicated the carbohydrate sialyl Lewis(x) (sLe(x); sialic acid alpha 2-3 galactose beta 1-4 [Fucose alpha 1-3] N-acetyl glucosamine) as a component of the ligand recognized by E- and P-selectin. In the case of P-selectin, other components of the cell surface, including 2'6-linked sialic acid and sulfatide (galactose-4-sulfate ceramide), have also been proposed for adhesion mediated by this selectin. We have recently defined a region of the E-selectin lectin domain that appears to be directly involved with carbohydrate recognition and cell adhesion (Erbe, D. V., B. A. Wolitzky, L. G. Presta, C. R. Norton, R. J. Ramos, D. K. Burns, R. M. Rumberger, B. N. N. Rao, C. Foxall, B. K. Brandley, and L. A. Lasky. 1992. J. Cell Biol. 119:215-227). Here we describe a similar analysis of the P-selectin lectin domain which demonstrates that a homologous region of this glycoprotein's lectin motif is involved with carbohydrate recognition and cell binding. In addition, we present evidence that is inconsistent with a biological role for either 2'6-linked sialic acid or sulfatide in P-selectin-mediated adhesion. These results suggest that a common region of the E- and P-selectin lectin domains appears to mediate carbohydrate recognition and cell adhesion.

Selectins

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Enhanced interaction of L-selectin with the high endothelial venule ligand via selectively oxidized sialic acids

The selectins are adhesion receptors that play key roles in leukocyte trafficking. Each has an N-terminal C-type lectin domain that binds to specific carbohydrates in a calcium-dependent manner. L-selectin recognizes sulfated, sialylated ligands on lymph node high endothelial venules. This recognition is abolished by strong periodate oxidation under conditions that destroy oligosaccharides. In contrast, mild periodate oxidation, which selectively oxidizes the side chain of sialic acid residues without affecting the underlying oligosaccharide, markedly enhances this interaction. The enhancement is calcium dependent, indicating that lectin recognition is maintained. Reduction of the sialic acid aldehydes generated by mild periodate to alcohol groups abolishes this effect. Covalent cross-linking of the oxidized ligand to L-selectin can be demonstrated, suggesting Schiff base formation between lysine residues of the selectin and the newly formed aldehydes. Such selectively oxidized sialylated ligands could be used to probe the lectin domains of the selectins and to identify lysine residues near the binding site. Also, this approach could be used to design drugs for disrupting leukocyte-endothelial interactions leading to pathological inflammation.

Leukocyte-endothelial cell interactions

The movement of leukocytes from the blood circulation into organized lymphoid tissues or sites of inflammation requires cooperative interactions between signaling and adhesion molecules. Selectins mediate the initial rolling contacts of leukocytes with the endothelium. Following leukocyte activation, integrins strengthen adhesion and then direct migration beneath the endothelium. Unique combinations of signaling and adhesion molecules may regulate the subsets of leukocytes that are recruited into specific tissues.