Acid sphingomyelinase (ASM), a member of the saposin like protein (SAPLIP) family, is a lysosomal hydrolase that converts sphingomyelin to ceramide. The deficient activity of ASM causes a variant form (i.e., type A/B) of the inherited disorder Niemann-Pick disease. The lysosomal targeting mechanism of ASM has not been conclusively identified. Previous studies suggested that ASM could use another membrane-associated receptor as well as M6P receptor to target lysosomes. Sortilin, a type I transmembrane glycoprotein, belongs to a novel family of receptor proteins. Both the luminal domain and the cytoplasmic domain of sortilin show structural features typical of receptors involved in lysosomal or vacuolar targeting. Using a dominant-negative sortilin construct lacking the cytoplasmic tail, I proved that sortilin was involved in the lysosomal targeting of ASM. Confocal microscopy revealed that truncated sortilin partially inhibited the lysosomal targeting of ASM in COS 7 cells and completely abolished the lysosomal targeting of ASM in I-cells. Pulse-chase experiments also suggested that sortilin was involved in normal sorting of newly synthesized ASM. Over-expression of truncated sortilin accelerated and enhanced the secretion of ASM from COS 7 cells and I-cells. Co-immunoprecipitation assays further confirmed the interaction between sortilin and ASM. I also observed that the lysosomal transport of ASM was reduced to some extent in I-cell disease fibroblasts as compared to normal fibroblasts. In conclusion, both the M6P receptor and sortilin mediate lysosomal targeting of ASM.

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