Catabolic Regulation of the Expression of the Major Myelin Glycoprotein by Schwann Cells in Culture

Previous studies have suggested that neonatal Schwann cell cultures deprived of axonal contact do not express components of the myelin membrane, including the major myelin glycoprotein, P₀. In contrast, Schwann cells from permanently transected, adult nerve exhibit continued biosynthesis of P₀ after culture, suggesting that the ability to express the myelin glycoprotein may depend on the degree of cellular differentiation. To examine further the ability of Schwann cell cultures to express P₀ as a function of age, we have performed precursor incorporation studies on endoneurial explants from 4‐ to 12‐day‐old rat sciatic nerves after 5 days in culture. The data reveal that explants from 12‐day‐old animals synthesize detectable levels of this integral myelin protein when assayed by [³H]mannose incorporation, even though there is no apparent myelin assembly in the cultures. Pulse–chase analysis of cultures from 12‐day‐old rats demonstrates that [³H]mannose‐labeled P₀ is substantially degraded within 3 h. This catabolism largely can be prevented by the addition of swainsonine, ammonium chloride, or L‐methionine methyl ester to the pulse‐chase media. The former agent alters oligosaccharide processing whereas the latter two compounds inhibit lysosomal function. The P₀ synthesized by the 12‐day explant cultures following the addition of swainsonine is readily fucosylated, implying that the protein has progressed at least as far as the medial Golgi before its exit and subsequent catabolism. If cultures from 4‐, 6‐, and 8‐day‐old animals are analyzed for P₀ biosynthesis by [³H]mannose incorporation in the presence of swainsonine, detectable levels of the glyoprotein are seen. The inability to demonstrate the presence of P₀ in earlier studies of neonatal cultures was presumably due in part to the rapid lysosomal catabolism of the protein after translation.