Effects of Cholesterol Depletion and Increased Lipid Unsaturation on the Properties of Endocytic Membranes

Lipid analogs with dialkylindocarbocyanine (DiI) head groups and short or unsaturated hydrocarbon chains ( e.g. DiIC 12 and FAST DiI) enter the endocytic recycling compartment efficiently, whereas lipid analogs with long, saturated tails ( e.g. DiIC 16 and DiIC 18 ) are sorted out of this pathway and targeted to the late endosomes/lysosomes (Mukherjee, S., Soe, T. T., and Maxfield, F. R. (1999) J. Cell Biol . 144, 1271-1284). This differential trafficking of lipid analogs with the same polar head group was interpreted to result from differential partitioning to different types of domains with varying membrane order and/or curvature. Here we investigate the system further by monitoring the trafficking behavior of these lipid analogs under conditions that alter domain properties. There was a marked effect of cholesterol depletion on the cell-surface distribution and degree of internalization of the lipid probes. Furthermore, instead of going to the late endosomes/lysosomes as in control cells, long chain DiI analogs, such as DiIC 16 , were sorted to the recycling pathway in cholesterol-depleted cells. We confirmed that this difference was due to a change in overall membrane properties, and not cholesterol levels per se , by utilizing a Chinese hamster ovary cell line that overexpressed transfected stearoyl-CoA desaturase 1, a rate-limiting enzyme in the production of monounsaturated fatty acids. These cells have a decrease in membrane order because they contain a much larger fraction of unsaturated fatty acids. These cells showed alteration of DiI trafficking very similar to cholesterol-depleted cells. By using cold Triton X-100 extractability of different lipids as a criterion to determine the membrane properties of intracellular organelles, we found that the endocytic recycling compartment has abundant detergent-resistant membranes, in contrast to the late endosomes and lysosomes.