Incorporation of axonally transported substances into myelin lipids.

Abstract

Abstract— The possibility that axonally transported lipids and/or proteins might undergo transaxonal migration and become incorporated into surrounding myelin lamellae was studied by isolating myelin from optic tracts of myelinating rabbits at various times following intraocular injection of [3‐14C]‐serine and [2‐3H]glycerol. Myelin isolated by a procedure employing ethylene glycol‐bis(β‐aminoethyl ether)‐.N,N'‐tetraacetic acid had relatively constant specific radioactivity with respect to both isotopes over a 21 day period. Myelin lipids showed a gradual increase in 14C specific radioactivity, attributed to reutilization of [14C]serine from the axon by a compartment of the oligodendrocyte. Free serine is postulated to arise in the axon from catabolism of axonally transported proteins (and possibly lipids) and to migrate transaxonally into the neighboring oligodendroglia. This reutilization mechanism resulted in synthesis of myelin cerebrosides, sphingomyelin, ethanolamine phosphoglycerides and possibly sulfatides, but not gangliosides or serine phosphoglycerides. The data for choline‐ and inositol‐phosphoglycerides are inconclusive. [3H]Glycerol‐labeled myelin lipids decreased slowly in 3H specific radioactivity with time, indicating either that [2‐3H]glycerol does not participate in the reutilization pathway or that the label is lost in the process. Evidence is presented that 3H‐ and 14C‐labeled lipids are true myelin constituents. Lipids from the myelin, axolemma‐ and axon‐enriched fractions tended to converge in specific radioactivity over the 21 days, especially the former two fractions. These results together with isotope ratio changes point to an equilibration process whereby lipids are able to transfer. (or exchange) between the 3 compartments. Protein radioactivity in isolated myelin was suggested to arise from residual axon/axolemma contamination, and no evidence was found for transaxonal migration of protein into myelin. The 2 mechanisms elucidated here are believed to account for a quantitatively small portion of myelin lipid and are considered to represent a form of axon‐glia interaction.