Monoclonal anti-myelin associated glycoprotein (MAG) antibodies in patients with mixed sensory-motor polyneuropathies occurring in association with IgM gammopathy (paraproteinemia) are all directed toward carbohydrate epitopes in MAG and cross react with other glycoproteins of PNS myelin including PO and PMP-22 as well as with the glycosphingolipid, sulfate-3-glucuronyl paragloboside (SGPG). In order to gain insights about the pathogenic mechanisms involved, we are biochemically analyzing an animal model of this disease involving passive transfer to chickens by infusion of human anti-MAG antibodies and which exhibits pathohistological similarities to the human disease. Preliminary results show that all myelin proteins are decreased, but small doses produce a greater effect on MAG than other potential glycoprotein targets. Monoclonal antibodies that are MAG/SGPG-negative in patients with gammopathy and neuropathy frequently react with ganglioside antigens in nerve. Immunohistological experiments on a patient with mixed axonal and demyelinating symptoms and a monoclonal IgA lambda antibody are in progress to establish the relationship of a complex mixture of immunoreactivities to the pathology. In addition to the monoclonal antibody, this patient has polyclonal reactivity to the major LM1 ganglioside of peripheral myelin that is restricted to the IgA class and a low level of IgM reactivity of the MAG/SGPG type. In order to elucidate the functions of acidic glycoproteins in Schwann cell differentiation and myelination as well as to understand mechanisms by which the human anti- glycolipid antibodies may perturb function, we continued our studies of glycolipids in cultured Schwann cells. Basement membrane is well known to be required for Schwann cells to form myelin. We have shown that replacing the usual polylysine substratum with basement membrane (Matrigel), laminin or type IV collagen causes a specific increase in the incorporation of radioactive galactose into the major GM3 ganglioside of the cells and some complex neutral glycosphingolipids, without a similar increase in the synthesis of other lipids, proteins or glycoproteins. The enhanced GM3 synthesis is associated with increased Schwann cell proliferation, and an inhibitor of synthesis of GM3 and the other glycolipids (PDMP) also inhibits basement membrane induced proliferation, suggesting that the increased synthesis may be functionally involved in the proliferative response. The results also show that the Schwann cell response to basement membrane alone is quite different from that which occurs in the presence of neurons, in which case the cells will differentiate and myelinate.
