A REGULATORY ROLE FOR SPHINGOLIPIDS IN NEURONAL GROWTH - INHIBITION OF SPHINGOLIPID SYNTHESIS AND DEGRADATION HAVE OPPOSITE EFFECTS ON AXONAL BRANCHING

Sphingolipids, particularly gangliosides, are enriched in neuronal mem branes where they have been implicated as mediators of various regulat ory events, We recently provided evidence that sphingolipid synthesis is necessary to maintain neuronal growth by demonstrating that in hipp ocampal neurons, inhibition of ceramide synthesis by Fumonisin B-1 (FB 1) disrupted axonal outgrowth (Harel, R, and Futerman, A, H, (1993) J, Biol, Chem, 268, 14476-14481), We now analyze further the relationshi p between neuronal growth and sphingolipid metabolism by examining the effect of an inhibitor of glucosylceramide synthesis, o-1-phenyl-2-de canoylamino-3-morpholino-1-propanol (PDMP) and by examining the effect s of both FB1 and PDMP at various stages of neuronal development, No e ffects of FB1 or PDMP were observed during the first 2 days in culture , but by day 3 axonal morphology was significantly altered, irrespecti ve of the time of addition of the inhibitors to the cultures, Cells in cubated with FB1 or PDMP had a shorter axon plexus and less axonal bra nches, FB1 appeared to cause a retraction of axonal branches between d ays 2 and 3, although long term incubation had no apparent effect on n euronal morphology or on the segregation of axonal or dendritic protei ns, In contrast, incubation of neurons with conduritol B-epoxide, an i nhibitor of glucosylceramide degradation, caused an increase in the nu mber of axonal branches and a corresponding increase in the length of the axon plexus, A direct correlation was observed between the number of axonal branch points per cell and the extent of inhibition of eithe r sphingolipid synthesis or degradation, These results suggest that sp hingolipids play an important role in the formation or stabilization o f axonal branches.