Formation of bioactive sphingoid molecules from exogenous sphingomyelin in primary cultures of neurons and astrocytes.

Exogenous sphingomyelin, radiolabelled at the sphingosine moiety, was administered to primary cultures of cerebellar granule cells and astrocytes for different pulse times (20 min-2 h) and the fate of the radioactivity was followed. Ceramide was the main metabolic product in both cells, whereas sphingosine, glucosyl-ceramide and gangliosides GM3 and GD3 were produced only in astrocytes. When endocytosis was prevented and the lysosomal apparatus inactivated, ceramide formation was reduced slightly in granule cells and almost completely blocked in astrocytes, with disappearance of sphingosine, glucosyl-ceramide, GM3 and GD3. These data indicate that (a) ceramide is rapidly produced in cerebellar granule cells and astrocytes, presumably at the level of the plasma membrane in the first cell type, and of the lysosomes in the second one; (b) sphingosine is produced in cerebellar astrocytes by lysosomal sphingomyelin degradation and is partly reused for glucosyl-ceramide and ganglioside biosynthesis.