The Effect of Membrane Lipid Composition on Endosomal Cholesterol Transport

Abstract

Cholesterol is an important building block of cell membranes. The biosynthesis of cholesterol is very complicated, and therefore cholesterol is recycled by the cell. One of the key compartments in the cholesterol metabolism is the endosome. Endosomes are membrane-bound organelles that take care of waste recycling in cells. In addition to the limiting membrane, endosomes have multiple internal vesicles. In endosomes most cholesterol is in the membranes of internal vesicles. It is well known how cholesterol ends up in endosomes, but the mechanisms of cholesterol transport from endosomes to other compartments of a cell are poorly understood. NPC2 is a small soluble protein that stimulates cholesterol clearance from the internal membranes of endosomes. It can bind cholesterol and transport it between membranes. The lipid composition of these membranes affects the NPC2-mediated cholesterol transport rate. The endosomal lipid bis(monoacylglycero)phosphate (BMP) increases and sphingomyelin decreases the cholesterol transport rate, but it is not known how. Cholesterol can also be transported spontaneously in the absence of NPC2. The membrane lipid composition affects also the spontaneous cholesterol transport. In this study, the affinity of cholesterol to different membranes was studied using atomistic molecular dynamics simulations. Five model membranes with different lipid compositions were built, and the free energy difference of transferring a cholesterol molecule from one membrane to another was measured using the free energy perturbation method. The studied lipids in order of cholesterol affinity from highest to lowest were: sphingomyelin, DPPC, ceramide, POPC, and BMP. The results suggest that the affinity of cholesterol to these lipids at least partly explains their effect on NPC2-mediated cholesterol transport. The increased hydrocarbon chain order of lipids seems to increase cholesterol affinity, but it does not alone explain the observed affinities. Specific interactions between sphingomyelin and cholesterol increase the affinity of cholesterol to sphingomyelin-rich membranes.