Lysosomal delivery of the major myelin glycoprotein in the absence of myelin assembly: Posttranslational regulation of the level of expression by Schwann cells

The major myelin protein, P₀, has ben shown to have decreased levels of expression and altered oligosaccharide processing after the disruption of Schwann cell-axon interaction. We show here that lysosomal degradation of the glycoprotein shortly after its synthesis accounts for much of its decreased expression in the permanently transected adult rat sciatic nerve, a denervated preparation where there is no axonal regeneration or myelin assembly. If [³H]mannose incorporation into sciatic nerve endoneurial slices is examined in the presence of the lysosomotropic agent, NH₄Cl, a marked increase in the level of newly synthesized P₀ is seen. Pulse-chase analysis of [³H]mannose-labeled P₀ in the presence of NH₄Cl indicates that this increase is a consequence of inhibition of P₀ degradation that normally occurs 1-2 h after biosynthesis in the transected nerve. P₀ degradation can also be inhibited if lysosomal function is disturbed by dilation of secondary lysosomes with L-methionine methyl ester. The addition of deoxymannonojirimycin or swainsonine (SW), inhibitors of oligosaccharide-processing mannosidases I and II, respectively, also results in a decrease in P₀ degradation. This inhibition is presumably caused by a blockage of transport to the lysosomes due to altered processing of the glycoprotein, although the direct inhibition of lysosomal mannosidases cannot be excluded. In contrast to the transected nerve, addition of NH₄Cl or SW has no effect on P₀ levels in the crushed nerve, where myelin assembly occurs. The delivery of P₀ to the lysosomes of the transected nerve Schwann cells does not appear to be triggered by the mannose-6-phosphate transport system involved in acid hydrolase routing. The accumulation of a fucosylated species of P₀ in the presence of SW indicates that the glycoprotein has progressed at least as far as the site of GlcNAc transferase I without lysosomal delivery, and thus differs from the typical mannose-6-phosphate-containing glycoproteins. Furthermore, there is no evidence of P₀ phosphorylation either in the absence or presence of NH₄Cl or SW. These findings indicate that the amount of P₀ in the transected nerve is regulated at the posttranslational level as well as at the level of transcription.