The transport of low density lipoprotein-derived cholesterol to the plasma membrane is defective in NPC1 cells

Abstract

Niemann-Pick disease type C (NPC) is characterized by lysosomal storage of cholesterol and gangliosides, which results from defects in intracellular lipid trafficking. Most studies of NPC1 have focused on its role in intracellular cholesterol movement. Our hypothesis is that NPC1 facilitates the egress of cholesterol from late endosomes, which are where active NPC1 is located. When NPC1 is defective, cholesterol does not exit late endosomes; instead, it is carried along to lysosomal storage bodies, where it accumulates. In this study, we addressed whether cholesterol is transported from endosomes to the plasma membrane before reaching NPC1-containing late endosomes. Our study was conducted in Chinese hamster ovary cell lines that display the classical NPC biochemical phenotype and belong to the NPC1 complementation group. We used three approaches to test whether low density lipoprotein (LDL)-derived cholesterol en route to NPC1-containing organelles passes through the plasma membrane. First, we used cyclodextrins to measure the arrival of LDL cholesterol at the plasma membrane and found that the arrival of LDL cholesterol in a cyclodextrin-accessible pool was significantly delayed in NPC1 cells. Second, the movement of LDL cholesterol to NPC1-containing late endosomes was assessed and found to be normal in Chinese hamster ovary mutant 3-6, which exhibits defective movement of plasma membrane cholesterol to intracellular membranes. Third, we examined the movement of plasma membrane cholesterol to the endoplasmic reticulum and found that this pathway is intact in NPC1 cells, i.e. it does not pass through NPC1-containing late endosomes. Our data suggest that in NPC1 cells LDL cholesterol traffics directly through endosomes to lysosomes, bypassing the plasma membrane, and is trapped there because of dysfunctional NPC1.