Showing papers on "Galectin published in 1990"

Carbohydrate Ligands of the LEC Cell Adhesion Molecules Minirwiew

Abstract: Brian K. Brandley,” Stuart J. Swiedler: and Phillips W. Robbinst * Glycomed, Inc. 880 Atlantic Avenue Alameda, California 94501 *Center for Cancer Research Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Scientists have long postulated that cell surface carbohy- drates are involved in specific cell-cell recognition. Sud- denly this theory has found exciting new life with the iden- tification of the oligosaccharides that interact with LEC-CAMS (or selectins), a family of cell adhesion mole- cules (CAMS) that have been implicated in the interaction of leukocytes with platelets or vascular endothelium (Stool- man, 1989; Osborn, 1990). Adhesion is an early event in endothelial cell extravasation, leading to leukocyte recir- culation, thrombosis, and the inflammatory response. Based on their cDNA sequences, the three identified LEC-CAMS contain a single domain similar to that de- scribed in calcium-dependent lectins (C-lectins; Drick- amer, 1988) one epidermal growth factor-like domain, and several complement binding protein-like domains (all of which give rise to the LEC acronym). The first LEC- CAM, the Mel-14 antigen (gp90MEL, LAM-l, Leu-8, Ly-22) is constitutively expressed on a majority of leukocytes (Stoolman, 1989). ELAMl is transiently expressed on vas- cular endothelial cells in response to either interleukin-1 or tumor necrosis factor treatment (Bevilacqua et al., 1987). PADGEM (GMP140) is found in platelet a-granules as well as endothelial cell Weibel-Palade bodies and is expressed at the cell surface following degranulation (Lar- sen et al., 1990, and references therein). The nomencla- ture for these receptors has been confusing owing to their independent identification in several laboratories. An in- formal poll of over sixty researchers in the field suggested that gp90MEL and its human equivalent should be re- named LECAMl (leukocyte-endothelial cell adhesion molecule 1, or lectin adhesion molecule 1). We suggest extending this convention to the other two LEC-CAMS, based on the publication dates that established their LEC structural motifs. Therefore, we will refer to ELAMl as LECAMP, and PADGEM (GMP140) as LECAM3. Many cell surface, carbohydrate binding proteins (lec- tins) mediate cell recognition and adhesion in vitro (Brand- ley and Schnaar, 1986). However, with the notable excep- tion of sperm-egg binding, well-defined biological functions of such mammalian lectin-carbohydrate pairs have been elusive. The LEC-CAMS are unique, in that these recep- tors were initially defined by function, and later found to be calcium-dependent lectins. Recently, several laborato- ries have confirmed the lectin function of the LEC-CAMS and used a variety of approaches to identify their carbo- hydrate ligands. Here we will briefly describe these ap- proaches and the considerable amount of information ob- tained about the LEC-CAM carbohydrate ligands. Approaches for Defining Carbohydrate Ligands of EC-CAMS Experimental methods to define the carbohydrate ligands for the LEC-CAMS have utilized blocking antibodies, enzy- matic modifications carbohydrates, soluble inhibitors, and direct binding of receptors to endogenous ligands. Many monoclonal antibodies have been generated that bind to carbohydrate antigens with a relatively high de- gree of structural specificity and affinity (Peizi, 1985). Al- though providing a rapid method for implicating role carbohydrates in cell-cell interactions, these antibodies may cross-react with undefined oligosaccharides. In addi- tion, antibodies binding to the cell surface may nonspecif- ically interfere with neighboring molecules, alter cell sur- face topography (by cross-linking surface structures), be internalized, or even agglutinate cells, thereby interfering with adhesion without actually binding to the ligand. Therefore, ligand identification using antibodies should be considered presumptive until confirmation is obtained by additional methods. Exoglycosidases or glycosyltransferases that alter oligo- saccharide structures have been used to define the re- quirement for a terminal sugar on native and artificial ligands. Expression of novel combinations of glycosyl- transferases by transfection with plasmids encoding these enzymes has been used to generate cells with unique arrays of oligosaccharide structures (Paulson and Colley, 1989). These methodologies are most effectively interpreted in conjunction with structural analysis of the enzymatically altered oligosaccharides. The use of carbohydrates as soluble inhibitors of cell-cell adhesion has also been a valuable approach. Because monovalent, soluble oligosaccharides often have a relatively low affinity for their receptors, substantial quantities of the purified oligosaccharides may be needed to either directly inhibit adhesion or construct multivalent high affinity ligands (Lee, 1989). Unfortunately, relatively few complex carbohydrates are readily available in the pu- rity and quantity required for these experiments. Conclu- sions reached are only as good the structural charac- terization and purity of the inhibitors used. The most direct approach-measurement of the inter- action of the cells or receptors with purified endogenous oligosaccharide ligands-requires extraction and purifi- cation of potential ligands from the native source in quanti- ties sufficient to allow structural analysis. These ligands can be immobilized to prevent internalization by the receptor-bearing cells, which has the added advantage of presenting the oligosaccharides a multivalent array (thereby increasing the likelihood of a high affinity interac- tion with the receptors). Direct binding of receptor-bearing cells with immobilized glycoconjugates has been used to define the carbohydrate binding specificity of prokaryotic (Hansson et al., 1985) and eukaryotic cells (Schnaar