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A detour for stalled intracellular lipid traffic

JCI Journals

The NPC1 protein, which is defective in the glycosphingolipid (GSL) storage disease Niemann-Pick disease type C (NP-C), resides in the cell?s endosomes, Golgi stacks, and lysosomes. Defects in this protein cause free cholesterol, rather than cholesterol esters (CEs), to accumulate in these organelles and interfere with the normal routing of internalized GSLs to the Golgi apparatus. Pagano?s group has found that the normal Golgi trafficking of GSLs involves caveolae or some similar specialized membrane domain and differs from the better known mechanism involving clathrin-coated vesicles, which is used by other endocytosed lipids and proteins. How NPC1 defects affect this caveolar pathway is uncertain, but these authors have proposed that high intracellular cholesterol levels interfere with the normal flux of vesicles through the Golgi apparatus and the endosomes. The current work by Choudhury et al. demonstrates that a previously unsuspected detour can be found around this intracellular roadblock. These authors have studied the biochemical requirements for intracellular transport of fluorescent lipid analogs, and they have implicated two small GTP binding proteins, Rab7 and Rab9, in the clathrin-independent trafficking of GSLs. Overexpression of either protein not only restores their normal patterns of GSL trafficking in NP-C cells, but also reverses their abnormal accumulation of free cholesterol. These molecules thus appear to be limiting for caveolar endocytosis in the mutant cells, suggesting that treatments that increase their activity could reverse the cellular phenotype in NP-C.


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