The sialoadhesins - A family of sialic acid-dependent cellular recognition molecules within the immunoglobulin superfamily

For many years evidence has accumulated that sialic acids function in cellular interactions either by masking or as a recognition site. However, receptors or adhesion molecules mediating such functions between eukaryotic cells were unknown until about 5 years ago, when it was found that the members of the Selectin family mediate adhesion of leukocytes to specific endothelia through binding to sialylated glycans like sialyl Lewis(x). More recently, the Sialoadhesin family of sialic acid-dependent adhesion molecules was defined within the superfamily of immunoglobulin-like molecules. So far, it has been shown that sialoadhesin (Sn), CD22, CD33, the myelin-associated glycoprotein (MAG) and the Schwann cell myelin protein (SMP) belong to this family. In contrast to the Selectins, these proteins are associated with diverse biological processes, i.e. hemopoiesis, neuronal development and immunity. In this review their properties, carbohydrate specificities and potential biological functions are discussed. Finally, we provide perspectives with respect to the nature of ligands, implications of sialic acid modifications and future research.