Regulation of myelination: axons not required for the biosynthesis of basal levels of the major myelin glycoprotein by Schwann cells in denervated distal segments of the adult cat sciatic nerve.
The adult cat sciatic nerve was examined for Schwann cell biosynthesis of the major myelin glycoprotein (P0) in the distal segments after permanent nerve transection, where there is no axonal regeneration or myelin assembly. Endoneurial slices (intrafascicular tissue) from the distal segment of the desheathed cat sciatic nerves at 10 wk after transection and from normal adult desheathed brachial nerves were incubated with radioactive mannose; [3H]mannose incorporation into P0 was observed by fluorography after sodium dodecyl sulphate-pore gradient electrophoresis (SDS-PGE). Analysis of immune precipitates by SDS-PGE after incubation of an aliquot of an endoneurial fraction with rabbit antichick P0 gamma globulin verified that the [3H]mannose-labeled glycoprotein was P0. The level of incorporation of [3H]mannose into P0 and into other endoneurial glycoproteins in the normal brachial nerve from the adult cat was at substantially reduced levels compared with the transected nerve. Such incorporation was detectable by fluorography only after prolonged exposure to X-ray film (15 days). As a result, the level of biosynthesis of P0 in the normal adult cat is substantially reduced, suggesting that the extent of active myelination in the adult cat nerve is at a low level. Furthermore, Schwann cells are capable of continued synthesis of P0 in the adult, permanently transected nerve in the absence of axonal influence, suggesting that axonal association is not an absolute requirement for specifying myelin protein synthesis.