Regulation of Myelination: Biosynthesis of the Major Myelin Glycoprotein by Schwann Cells in the Presence and Absence of Myelin Assembly

Abstract: Schwann cell biosynthesis of the major myelin glycoprotein, P₀, was investigated in the crush‐injured adult rat sciatic nerve, where there is myelin assembly, and in the permanently transected nerve, where there is no myelin assembly. Endoneurial fractions from desheathed rat sciatic nerves distal to the crush were compared with similar fractions from the permanently transected nerves at 7, 14, 21, 28, and 35 days after injury. The Schwann cell expression of this asparagine‐linked glycoprotein was evaluated after sodium dodecyl sulfatepore gradient electrophoresis by Coomassie Blue and silver stain and by autoradiography after direct overlay of radioiodinated lectins [wheat germ agglutinin, gorse agglutinin, and concanavalin A (Con A)]. As evaluated by these parameters, the concentration of P₀ after crush decreased and subsequently increased as a function of time after injury, corresponding to the events of demyelination and remyelination. After permanent transection, the P_o concentration decreased following the same time course found after crush. At subsequent time points, P₀ could not be detected with Coomassie Blue stain, silver stain, or wheat germ agglutinin. Both gorse agglutinin and Con A, however, showed binding to P₀. Radioactive precursor incorporation studies with [³H]fucose or [³H] mannose into endoneurial slices at 35 days posttransection revealed active oligosaccharide processing of P₀ glycoprotein by Schwann cells in this permanent transection model. Compared with other Schwann cell glycoproteins in the transected nerve, the highest level of incorporation of [³H]mannose was found in P₀ which accounted for 42.7% of the incorporated label. In contrast, incorporation of [³H]mannose into endoneurial slices at 35 days after crush accounted for only 13.3% in P_o. In addition, higher levels of Con A binding were observed in P₀ in the transected nerve compared with the contralateral control or the crushed nerve. Both the [³H]fucose incorporation and gorse agglutinin binding to P₀ in the transected nerve suggest posttranslational processing of this glycoprotein in the Golgi apparatus; however, the absence of wheat germ agglutinin binding, the high level of mannose incorporation, and the high level of binding by Con A imply that additional processing steps are required prior to its assembly into myelin.