Biosynthesis of Neutral Glucocerebroside Homologues in the Absence of Myelin Assembly After Nerve Transection

The biosynthesis of myelin‐associated glycolipids during various stages of myelination was studied by in vitro incorporation of [ 3 H]Gal, [ 3 H]Glc, or [ 35 S]sulfate into the endoneurium of rat sciatic nerve. In the normal adult nerve, where the level of myelin assembly is substantially reduced and Schwann cells are principally involved in maintaining the existing myelin membrane, [ 3 H]Gal was primarily incorporated into monogalactosyl diacylglycerol (MGDG) and the galactocerebrosides (GalCe) with lower levels of incorporation into the sulfatides. Such incorporation was enhanced 35 days after crush injury of the adult rat sciatic nerve, which is characterized by active myelin assembly. In contrast, at 35 days after permanent nerve transection where there is no axonal regeneration or myelin assembly, the incorporation of [ 3 H]Gal or [ 3 H]Glc into GalCe was nearly undetected whereas the incorporation of [ 3 H]Gal into MGDG was completely inhibited. Instead, the 3 H‐labeled glycolipids in transected nerve were identified as the glucocerebrosides (GlcCe) and oligohexosylceramide derivatives with tetrahexosylceramide being a major product. In contrast, [ 35 S]sulfate was incorporated into endo‐neurial sulfatides in the transected nerve, which suggests that endogenous GalCe rather than newly synthesized GalCe served as the substrate for the sulfotransferase reaction. The GlcCe homologues are not considered as constituents of the myelin membrane but are likely plasma membrane components synthesized in the absence of myelin assembly. It is likely that the cells responsible for GlcCe biosynthesis are Schwann cells, since they comprise 90% of the total endoneurial cell area in the distal nerve segment at 35 days after transection. The biosynthesis of these GlcCe homologues, therefore, may represent the phenotypic expression of Schwann cells in the absence of myelin assembly and axonal contact.