Mechanisms that regulate the function of the selectins and their ligands

Selectins are a family of three cell adhesion molecules (L-, E-, and P-sele ctin) specialized in capturing leukocytes from the bloodstream to the blood vessel wall. This initial cell contact is followed by the selectin-mediate d rolling of leukocytes on the endothelial cell surface. This represents th e first step in a cascade of molecular interactions that lead to leukocyte extravasation, enabling the processes of lymphocyte recirculation and leuko cyte migration into inflamed tissue. The central importance of the selectin s in these processes has been well documented in vivo by the use of adhesio n-blocking antibodies as well as by studies on selectin gene-deficient mice . This review focuses on the molecular mechanisms that regulate expression and function(s) of the selectins and their ligands. Cell-surface expression of the selectins is regulated by a variety of different mechanisms. The se lectins bind to carbohydrate structures on glycoproteins, glycolipids, and proteoglycans. Glycoproteins are the most likely candidates for physiologic ally relevant ligands. Only a few glycoproteins are appropriately glycosyla ted to allow strong binding to the selectins. Recently, more knowledge abou t the structure and the regulated expression of some of the carbohydrates o n these Ligands necessary for selectin binding has been accumulated. For at least one of these Ligands, the physiological function is now well establi shed. A novel and exciting aspect is the signaling function of the selectin s and their ligands. Especially in the last two years, convincing data have been published supporting the idea that selectins and glycoprotein ligands of the selectins participate in the activation of leukocyte integrins.