Analysis of the carbohydrate composition of axonally transported glycoconjugates in sciatic nerve

Glycosidase enzyme digestion in combination with postembedding lectin cytochemistry was used to study the carbohydrate composition of axonally transported glycoproteins. A cold block procedure for the interruption of axonal transport was employed to increase selectively the population of anterograde moving components on the proximal side of the transport block. Electron-microscopic observations revealed that a cold block applied to the sciatic nerve of an anesthetized rat produced an increase in axonal smooth membrane vesicles at a site directly proximal to the cold block. Postembedding lectin cytochemistry of the sciatic nerve demonstrated a substantial increase in concanavalin A (Con A), wheat germ agglutinin (WGA), and succinylated WGA binding sites in axons directly proximal to the cold block. Endoglycosidase H (endo H) digestion prior to lectin cytochemistry characterized a large population of the axonally transported Con A binding sites as polymannose and/or hybrid N-linked oligosaccharides (endo H-susceptible). A distinct population of neuraminidase-resistant WGA binding sites was also found in axons directly proximal to the transport block. The concomitant increase in smooth membrane vesicles and lectin binding sites in axons at the transport block supports the hypothesis that a system(s) of smooth membrane inside the axon is involved in the transport of glycoproteins from the cell soma to their cell surface destinations. Results of glycosidase digestions and lectin cytochemistry experiments suggest that many of the axonally transported glycoprotein carbohydrates are polymannose and/or hybrid N-linked oligosaccharides. This observation is especially interesting in relation to our previous reports, which indicated that most lectin binding sites on the neuronal cell surface are composed of complex oligosaccharides.(ABSTRACT TRUNCATED AT 250 WORDS)