Review Selectin Ligands

Proc. Nati. Acad. Sci. USA Vol. 91, pp. 7390-7397, August 1994 Review Selectin ligands Ajit Varki Gfro bg Program, Cancer Cente, and Division of Cellular and Molecular Medicine, Univert of California, San Diego, La Jolla, CA 92093 ABSTRACT The selectins initiate many critical interactions among blood cells. The volume of information and di- versity of opinions on the nature of the biologically relevant ligands for selectins is remarkable. This review analyzes the mat- ter and suggests the hypothesis that at least some of the specificity may involve recognition of "clustered saccharide patches." Five years ago, the cloning of three vascular proteins resulted in identification of the selectin family of cell adhesion molecules (1-9). A shared N-terminal carbohydrate-recognition domain homologous | L-SELECTIN yE-SELECTIN P-SELECTIN to other Ca2+-dependent (C-type) mammalian lectins (10) strengthened predic- FIG. 1. Location and topology of the selectin family of cell adhesion molecules and their tions from functional studies (for review, carbohydrate ligands. Expression of the selectins and their ligands may be constitutive or see ref. 1) that the cognate ligands for induced, depending upon the cell type, the tissue, and the biological circumstance. The selectins these receptors would be cell-surface can also be shed from cell surfaces by proteolysis. carbohydrates (Fig. 1). Several groups then reported that previously known lectin came from the inhibitory effects of (15, 18, 55-57). Taken together, these mammalian oligosaccharides bearing Fuc phosphorylated monosaccharides and a data indicated that specific carbohydrate and sialic acid (Sia) are recognized in a phosphorylated yeast mannan on the in- ligands are recognized by selecting. Ca2+-dependent manner by the selectins teraction of lymphocytes with high- Many Diverse Carbohydrates Are Rec- (11-24). Evidence for the function of se- endothelial venules (HEV) in lymph ognized by Selectins. A plethora ofsimple lectins in leukocyte trafficking, thrombo- nodes (1). These studies were inspired by and complex carbohydrates have been sis and inflammation, as well as the pos- work on a different class of mammalian reported as recognized by the selecting, sibility for therapeutic intervention in lectins, the mannose 6-phosphate recep- on the basis of either direct binding ex- reperfusion injury, inflammation, al- tors (52). However, sialidase treatment periments or inhibitory properties (see lergy, autoimmunity, and cancer (for re- of lymph node sections abolished lym- Table 1). Most, but not all, carry sialy- view, see refs. 1-9) attracted additional phocyte-HEV interaction, indicating a lated, sulfated, and/or fucosylated se- investigators (>500 articles in the last 5 critical role for Sia (53). Subsequently, quences normally found at the nonreduc- yr).* More recently, specific macromo- the selective inhibitory properties of var- ing termini of N-linked or O-linked oli- lecular ligands for the selectins have been ious phosphorylated and sulfated saccha- gosaccharides, or on glycosphingolipids. reported (25-38). Molecular cloning of rides (1) confirmed a lectin-like interac- Such sequences are presented in Table 2 the polypeptide backbones of some of tion. It is now evident that the selectin and contrasted with related sequences these molecules indicates that ligand for- ligands are not themselves phosphory- recognized by two other mammalian mation is indeed glycosylation-depen- lated (nor always sulfated) and that the blood cell adhesion molecules, CD22 and dent (28, 33, 34, 37, 39, 40). Some studies inhibitors worked either because of high sialoadhesin. The common feature of suggest that small sialylated, fucosylated most ofthose selectins is a oligosaccharides such as sialyl-Lewisx concentrations or high charge density. In recognized by (SLex) and sialyl-Lewisa (SLea) (11, 12, parallel, others (3) described monoclonal lactosamine backbone of either type 1 20, 22-24, 41-44) and/or their sulfated antibodies (mAbs) that recognized spe- (Galp1-3GlcNAc) or type 2 (Gal,81- equivalents (45-47) are the ligands, cific "addressins" involved in lympho- 4GlcNAc). When present, Sia is usually whereas others indicate that complex- cyte trafficking. The peripheral node ad- in a2-3 linkage, Fuc is in either al-3 or clustered saccharide motifs on specific dressin recognized by mAb MECA-79 al-4 linkage, and the location of the Fuc glycoconjugates are required for biolog- (54) is a family of glycoproteins (gps), residue relative to the Sia residue (deter- ically relevant recognition (28, 31). Some some ofwhich interact with L-selectin in mined by fucosyltransferase and cell have even reported selectin ligands that a Sia-dependent manner (25). Likewise, type) is important (43, 58-60) (see Table lack Sia or Fuc (36, 43, 48-51). Because the mAb HECA-452 recognizes Sia- 2). Conformational analyses suggest that of recent excellent reviews on other as- dependent epitopes of a proposed cuta- a specific face of the SLeX tetrasaccha- pects of selectin biology (3-9), this dis- neous lymphocyte antigen, which medi- cussion will focus exclusively upon the ates recognition of skin-seeking lympho- Abbreviations: SLex, sialyl-Lewisx; SLea, sialyl-Lewisa; HEV, high-endothelial venule(s); nature of the selectin ligands. cytes by E-selectin (27). Others defined mAbs, monoclonal antibodies; Sia, sialic acid; Selectin Ligands: Historical Back- the ligands operationally, showing loss of gp, glycoprotein. ground. Early evidence for Ca2+-depen- selectin-mediated binding upon sialidase *Because of space constraints the bibliogra- dent carbohydrate recognition by L-se- or protease treatment of the target cells phy presented is necessarily incomplete. 7390 Downloaded by guest on September 29, 2021 Review: Varki Proc. Natl. Acad. Sci. USA 91 (1994) 7391 ride involved in recognition is shared in an in vitro assay and what does actually to reveal biologically relevant binding of with the isomer SLea (22, 24, 61). Signif- bind in vivo. specific molecules. Likewise, cell inter- icant differences in binding can be gen- Importance ofAssay Conditions Used to action studies incorporating biologically erated by modifications ofthe GlcNAc or Identify Potential Ligands. In vitro, cer- relevant shear forces (e.g., in flow cham- Sia N-acetyl groups (24, 62) or specific tain assay conditions can show selectin bers or on rocker platforms) are more OH groups on the Fuc or Gal residues binding that may not be biologically rel- likely to reveal specific binding. (63). In contrast, truncation of the Sia evant. For example, the use of TLC to When Is Recognition by Selectins Bio- side chain has little effect upon recogni- detect potential ligands has the advan- logically Relevant? Selectin ligands must tion (24, 64). tage that natural or artificial glycolipids have both the opportunity (be located at Such data suggest a high specificity for can be separated into distinct molecular the right place at the right time), as well the interaction with tetrasaccharides species, all of which can be probed si- as adequate affinity or avidity (bind such as SLeX. However, sulfate esters or multaneously, with a visual readout. strongly enough to mediate specific bio- uronic acids can substitute for Sia resi- However, apotential disadvantage is that logical recognition under natural circum- dues, and some neutral fucosylated the oligosaccharides might be presented stances). The selectins may not normally chains can be recognized, at high densi- in an unnaturally high density, giving have the opportunity in vivo to recognize ties or under specific experimental con- avid binding that might not occur if the all structures listed in Table 1. Thus, ditions (13, 20, 43, 65). Also, some se- same molecules were presented as minor some candidate oligosaccharides (e.g., quences that lack Fuc residues are rec- components of a natural lipid bilayer. SLea) have not been found on any of the ognized (see Table 2), and P-selectin may Likewise, the use of highly multivalent relevant vascular cells (although type 1 recognize Sia residues in a2-6 linkage arrays of either selectins (e.g., as immu- chains could generate pathological li- (63, 66). Thus, there now appears to be a nocomplexes) or of potential oligosac- gands in carcinomas). Regarding binding wide and diverse range of candidate oli- charide ligands (e.g., as neoglycopro- strength, most studies of monomeric li- gosaccharide ligands. However, selec- teins) can give a positive result that may gands (Table 1) report IC50 values (rough tive inhibition ofN- or O-linked glycosy- not be relevant to the natural situation. indicators of binding constants) in the lation (38, 66-68) suggests that the bio- The same is true ofthe use oftransfected PM-mM range (20, 51, 62, 63, 69, 70). logically relevant ligands may not be cell lines expressing very high densities One reported exception (71) might be carried on all glycoconjugate classes. A of selectins. Also, static cell adhesion explained by a 60g force used to induce further difficulty is that if all three selec- assays with long interaction times can initial cell binding. In contrast, the ability tins recognized the same carbohydrate show in vitro binding that may not be to use stringent techniques such as prob- structure (e.g., SLex), the situation could relevant in the dynamic flow situations ing of blots, flow cytometry analysis, be impractical because selectin and li- encountered in vivo. In contrast, assays precipitation, and affinity chromatogra- gand

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