Ceramide inhibits axonal growth and nerve growth factor uptake without compromising the viability of sympathetic neurons

Ceramide inhibits the axonal growth of cultured rat sympathetic neurons whe n the ceramide content of distal axons, but not cell bodies, is increased ( Posse de Chaves, E. I., Bussiere, M., Vance, D. E., Campenot, R. B., and Va nce, J. E. (1997) J. Biol. Chem. 272, 3028-3035). We now report that inhibi tion of growth does not result from cell death since although ceramide is a known apoptotic agent, C.-ceramide given to the neurons for 24 h did not c ause cell death but instead protected the neurons from death induced by dep rivation of nerve growth factor (NGF). We also find that a pool of ceramide generated from sphingomyelin in distal axons, but not cell bodies, inhibit s axonal growth. Analysis of endogenous sphingomyelinase activities demonst rated that distal axons are rich in neutral sphingomyelinase activity but c ontain almost no acidic sphingomyelinase, which is concentrated in cell bod ies/proximal axons. Together, these observations are consistent with the id ea that generation of ceramide from sphingomyelin by a neutral sphingomyeli nase in axons inhibits axonal growth. Furthermore, we demonstrate that trea tment of distal axons with ceramide inhibits the uptake of NGF and low dens ity lipoproteins by distal axons by similar to 70 and 40%, respectively, su ggesting that the inhibition of axonal growth by ceramide might be due, at least in part, to impaired endocytosis of NGF. However, inhibition of endoc ytosis of NGF by ceramide could not be ascribed to decreased phosphorylatio n of TrkA.