Glycosynapses: microdomains controlling carbohydrate-dependent cell adhesion and signaling.

The concept of microdomains in plasma membranes was developed over two decades, following observation of polarity of membrane based on clustering of specific membrane components. Microdomains involved in carbohydrate-dependent cell adhesion with concurrent signal transduction that affect cellular phenotype are termed "glycosynapse". Three types of glycosynapse have been distinguished: "type 1" having glycosphingolipid associated with signal transducers (small G-proteins, cSrc, Src family kinases) and proteolipids; "type 2" having O-linked mucin-type glycoprotein associated with Src family kinases; and "type 3" having N-linked integrin receptor complexed with tetraspanin and ganglioside. Different cell types are characterized by presence of specific types of glycosynapse or their combinations, whose adhesion induces signal transduction to either facilitate or inhibit signaling. E.g., signaling through type 3 glycosynapse inhibits cell motility and differentiation. Glycosynapses are distinct from classically-known microdomains termed "caveolae", "caveolar membrane", or more recently "lipid raft", which are not involved in carbohydrate-dependent cell adhesion. Type 1 and type 3 glycosynapses are resistant to cholesterol-binding reagents, whereas structure and function of "caveolar membrane" or "lipid raft" are disrupted by these reagents. Various data indicate a functional role of glycosynapses during differentiation, development, and oncogenic transformation.