Transport of plasma membrane-derived cholesterol and the function of Niemann-Pick C1 Protein

To visualize the intracellular transport of plasma membrane-derived cholesterol under physiological and pathophysiological conditions, a novel fluorescent cholesterol analog, 6-dansyl cholestanol (DChol), has been synthesized. We present several lines of evidence that DChol mimics cholesterol. The cholesterol probe could be efficiently incorporated into the plasma membrane via cyclodextrin-donor complexes. The itinerary of DChol from the plasma membrane to the cell was studied to determine its dependence on the function of Niemann-Pick C1 (NPC) protein. In all cells, DChol moved from the plasma membrane to the endoplasmic reticulum. Its further transport to the Golgi complex was observed but with marked differences among various cell lines. DChol was finally transported to small (approximately 0.5 microm diameter) lipid droplets, a process that required functional acyl-CoA:cholesterol acyltransferase. In human NPC fibroblasts, NPC-like cells, or in cells mimicking the NPC phenotype, DChol was found in enlarged (>1 microm diameter) droplets. When the NPC-phenotype was corrected by transfection with NPC1, DChol was again found in small-sized droplets. Our data show that NPC1 has an essential role in the distribution of plasma membrane-derived cholesterol by maintaining the small size of cholesterol-containing lipid droplets in the cell.