Maturation of the axonal plasma membrane requires upregulation of sphingomyelin synthesis and formation of protein-lipid complexes

Neuronal maturation is a gradual process; first axons and dendrites are est ablished as distinct morphological entities; next the different intracellul ar organization of these processes occurs; and finally the specialized plas ma membrane domains of these two compartments are formed. Only when this ha s been accomplished does proper neuronal function take place. In this work we present evidence that the correct distribution of a class of axonal memb rane proteins requires a mechanism which involves formation of protein-lipi d (sphingomyelin/cholesterol) detergent-insoluble complexes (DIGs), Using b iochemistry and immunofluorescence microscopy we now show that in developin g neurons the randomly distributed Thy-1 does not interact with lipids into DIGs (in fully developed neurons the formation of such complexes is essent ial for the correct axonal targeting of this protein). Using lipid mass spe ctrometry and thin layer chromatography we show that the DIG lipid missing in the developing neurons is sphingomyelin, but not cholesterol or glucosyl ceramide. Finally, by increasing the intracellular levels of sphingomyelin in the young neurons the formation of Thy-l/DIGs was induced and, consisten t with a role in sorting, proper axonal distribution was facilitated. These results emphasize the role of sphingomyelin in axonal, and therefore, neur onal maturation.