The last major cell adhesion molecule family to discuss is the selectins. Selectins bind heterophilically to oligosaccharide moieties on glycoproteins. In fact the name of the family is based on lectin, a generic term for proteins that bind sugars. The selectins, like cadherins and IgSF molecules are modular glycoproteins that pass through the membrane once. From C-terminus to N-terminus, the selectins are composed of a relatively short cytoplasmic domain, a single transmembrane domain, a series of CR (consensus repeat) or structural domains (from 2 in L-selectin to 9 in P-selectin), an EGF (epidermal growth factor) -like domain, and a lectin-like domain. The lectin-like domain binds a fairly specific oligosaccharide composed of sialic acid, galactose, GlcNAc, and fucose, of which the sialic acid and fucose are most important for recognition. Selectin-mediated adhesion is a Ca2+ dependent process.
Figure 16. Selectins (L-, E-, and P-) are composed of between 2 and 9 short consensus repeat domains (purple), an EGF-like domain (orange), and the lectin-like binding domain (teal).
L-selectin, which is found on leukocytes, was the first discovered, by virtue of an interesting phenomenon called lymphocyte homing, in which lymphocytes were removed from various peripheral lymph nodes, labeled, and then injected back into the animal.
These lymphocytes would migrate back to the tissues from which they were derived without regard for the site of re-injection. The other two known selectins are E-selectin, which is expressed primarily on endothelial cells, and P-selectin, which is expressed on platelets and endothelial cells. The best-characterized ligand for selectins is PSGL-1, creatively named P-selectin glycoprotein ligand -1. Both E- and P-selectin are an important part of the inflammatory response, mediating the invasion of neutrophils from the bloodstream into the area of injury (fig. 17).
Figure 17. Neutrophil activation and invasion in the inflammatory response. First, endothelial cells in the blood vessel walls are activated by nearby damaged cells. These endothelial cells then begin to express E- and P- selectins, which bind onto neutrophils that are tumbling by in the bloodstream. This interaction slows the neutrophils down so that they are just rolling more slowly in partial contact with the endothelial cells. The endothelials release platelet activating factor, which activates α1β2 and αmβ2 integrins on neutrophils. Those integrins can bind onto the ICAM (and IgSF molecule) on the endothelial cell surface, stopping their movement. Finally the neutrophil binds to ICAMs on two adjacent cells, then remodels its cytoskeleton as it squeezes between the two to exit the bloodstream and move to the lesion site.