The three members of the selectin family of cell adhesion molecules, E-, P-, and L-selectin, comprise a family because they share a common structural domain organization and similar carbohydrate-recognition properties. The selectins participate, along with integrins and immunoglobulin-like cell adhesion molecules, in leukocyte adhesion to endothelial cells. The temporal expression of these adhesion molecules and their cell-specific binding properties orchestrate the recruitment of leukocytes into tissue sites to combat infections. This article will focus on recent papers describing the structure, carbohydrate-binding properties, and functions of E-selectin.
Platelet/endothelial cell adhesion molecule-1 (PECAM-1, CD31, and henceforth in this review referred to as “PECAM”) is a member of the immunoglobulin gene superfamily. This 130kDa glycoprotein is concentrated in the junctions between vascular endothelial cells and expressed diffusely on the surfaces of platelets and many types of circulating leukocytes. In addition to being expressed on multiple cell types that interact within the bone marrow and vascular system, it is capable of mediating several different adhesion reactions. It can bind like cells to each other (homotypic adhesion), such as at the endothelial junction. It can bind different cells together (heterotypic adhesion), such as leukocyte to endothelium. On a molecular level, PECAM is capable of homophilic (PECAM binding to PECAM) as well as heterophilic adhesion, (for which certain sulfated glycosaminoglycans may be ligands), depending upon the experimental conditions. Moreover, on leukocytes, ligation of PECAM has been shown to activate adhesion molecules of the integrin family on the same cell. PECAM is thus in a position to play many different roles in the physiology and pathophysiology of the vascular system.
Some specific glycosyltransferases are now known to be expressed on the cell surface, in addition to their traditional intracellular locations. The best studied cell surface glycosyl transferase is β1, 4-galactosyltransferase, which functions as a cell adhesion molecule during fertilization and embryonic development by binding to specific glycoside ligands in the extracellular matrix. The recent cloning of β1, 4-galactosyltransferase has provided a molecular hypothesis to account for its expression on the cell surface. This has enabled one to selectively manipulate its expression on the cell surface and examine the consequences on selected cellular interactions.
Cholera toxin (CT) and the type I heat-labile enterotoxin (LT-I) of E. coli, the causative agents in cholera and traveler's diarrhea, are structurally and functionally similar. Both have homopentameric B subunits which bind to specific cell surface receptors and monomeric A subunits which persistantly activate adenylylcyclase in target cells. In the human intestinal cell, the rise in cyclic AMP mediates the pathophysiological effects of the toxins. Both toxins bind with the A subunit facing away from the membrane, and during a characteristic lag phase, the holotoxins are internalized and undergo inter-cellular processing which leads to release of the A1 peptide. The latter is an ADP-ribosyltransferase which modifies the subunit of stimulatory G protein (GSα), keeping adenylylcyclase in an activated state. Exposure of cells to brefeldin A, a disruptor of the Golgi apparatus, blocks the release of the A1 peptide and thus prevents the activation of adenylylcyclase. This suggests that an intact Golgi apparatus is required for processing and activation of the toxins. The ganglioside GM1 has been identified as the only natural, functional receptor for cholera toxin and also can serve as a functional receptor for LT-I. Intestinal cells of several species including man have additional receptors for LT-I which have been identified as galactoproteins with polylactosaminyl-glycan determinants required for toxin binding. Paradoxically, both toxins can bind to neoganglio-proteins containing GM1-oligosacharride, which have been generated on the surface of GM1-deficient cells, but are unable to activate adenylylcyclase. Activation can occur in the presence of chloroquine but with a delayed lag phase. This suggests that the toxins, when bound to the neoganglioproteins, are entering a different intracellular pathway then when bound to their natural receptors. The influence of receptor structure on toxin action is further indicated by a series of neogangliolipids synthesized from GM1-oligosacharride and different lipids. When taken up by GM1-deficient cells, they support toxin binding but mediate varying toxin activity. For one series of neoglycolipids, the latter is inversely related to the distance between the oligosaccharide and the lipid moiety.