The myelin-associated glycoprotein (MAG) is localized in the periaxonal membranes of PNS and CNS myelin sheaths where is appears to be involved in glia-axon interactions. MAG is a member of the immunoglobulin gene superfamily along with other neural adhesion proteins, and alternative splicing of its mRNA generates two developmentally regulated isoforms with differing C-terminal tails. The extracellular domains of the two forms of MAG are identical with 5 immunoglobulin-like domains and 8 potential sites for N-linked glycosylation. The carbohydrate consists of a mixture of oligosaccharides, many of which are sialylated and sulfated, and which are currently being isolated and characterized. A prominent neutral oligosaccharide of rat MAG has been identified as Man(alpha1)-6(Man alpha1-3)Man (alpha1)-6(Man alpha1-3)Man beta1-4GlcNAc beta1-4GlcNAc. It was also demonstrated that the adhesion-related HNK- 1/L2 carbohydrate epitope expressed on MAG is expressed on MAG is expressed on the recently cloned PMP-22 of peripheral myelin suggesting that PMP-22 may function in cell-cell interactions. The expression of MAG in cultured oligodendrocytes and Schwann cells is being studied with the ultimate objectives of identifying factors that control its synthesis and probing its function in cell-cell interactions. MAG is phosphorylated in cultured oligodendrocytes as it is in vivo, and the phosphorylation appears to be catalyzed at lease in part by protein kinase C and a calcium-activated kinase. Addition of exogenous GM3 ganglioside to the culture media stimulates the formation of oligodendroglial processes in a manner reminiscent of the well established neurotogenic effects of gangliosides.Biochemical analyses of the GM3-treated oligodendrocytes demonstrate that the ganglioside promotes differentiation in the direction of myelination. Although cultured primary Schwann cells do not normally express MAG in the absence of neurons, some immortalized Schwann cell lines generated in our laboratory express remarkably high levels of MAG, and the post- translational glycosylation, sulfation and phosphorylation in these cells is similar to that in vivo. The amount of MAG expressed by these cell lines is inversely proportional to their rates of growth and directly proportional to the number of processes on the cells. The phosphorylation of MAG is primarily on serine residues, and it is mediated in part by protein kinase C. These cell lines expressing high levels of MAG are being used to investigate the cell biology and function of this glycoprotein.
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