| Nerve cells depend on specific interactions with glial cells for proper function. Myelinating glial cells associate with neuronal axons, in part, via the cell-surface adhesion protein, myelin-associated glycoprotein (MAG). MAG is also a significant inhibitor of neurite outgrowth (axon regeneration) in the adult central nervous system. Primary structure and in vitro function place MAG in an immunoglobulin-related family of sialic acid-binding lectins. We propose that a limited set of structurally related gangliosides, sialic acid-binding glycoconjugates expressed on myelinated neurons in vivo, are the functional ligands for MAG.;When major brain gangliosides were adsorbed in artificial membranes on plastic microwells, two structurally related species, GT1b and GD1a, supported cell adhesion of MAG-transfected COS-1 cells. These gangliosides are prominent on axons, supporting their potential role as endogenous MAG ligands. Furthermore, a quantitatively minor ganglioside expressed on cholinergic neurons, GQ1b;To study the molecular mechanisms underlying such ganglioside-mediated interactions, recombinant DNA techniques were used to express lectins in an otherwise unreactive mammalian cell line (COS-1). Methods were developed to detect and isolate lectin-transfected COS cells based on their adhesion to immobilized glycosphingolipids. Glycolipid- and lectin-specific cell adhesion were readily and rapidly detected with high sensitivity, facilitating the screening of cDNAs for those encoding ganglioside receptors as well as the screening of glycosphingolipids for those bound by known lectins. |
