Mulation through homeostatic ER mechanisms by signaling cholesterol excess [15]. Lipidosis, and intracellular accumulation of phospholipids, is a side effect of certain cationic amphiphilic drugs, including quinacrine, desipramine, imipramineLysosomal Stability Is Regulated by CholesterolFigure 1. Cholesterol modulation in human fibroblasts is associated with alterations of the lysosomal compartment. Human wt fibroblasts were treated with U18666A or quinacrine to induce cholesterol accumulation, and NPC1-mutant fibroblasts were treated with methyl-bcyclodextrin (MbCD) or 25-hydroxy cholesterol (25-HC) to revert cholesterol storage. A) Measurement of unesterified cholesterol (n = 4) and B) representative images of filipin staining (scale bar 10 mm). C and D) Representative 374913-63-0 histogram from flow cytometric analysis of Lysotracker fluorescence staining. M1 gate denotes the highly fluorescent population. E and F) Quantification of peak channel in the M1 population (seen in C and D; n = 4). Data are presented as the mean 6 SD, * p#0.05. doi:10.1371/journal.pone.0050262.gand amiodarone, used to treat e.g., depression and arrhythmias [16,17]. The exact mechanism of action of these small lysosomotropic compounds remains poorly understood, but their amphiphilic nature allows them to accumulate in membranes and might disrupt the activity of Pentagastrin site membrane proteins like NPC1 [18]. In addition, the compound U18666A has been extensively used to mimic the NPC phenotype by impairing the intracellular transport of LDL-derived cholesterol from lysosomes [19], thus resulting in cholesterol accumulation in this compartment.The way in which increased lysosomal cholesterol contributes to NPC is unknown, but it has been suggested that both lipid storage and a concomitant inflammatory response, involving macrophages in peripheral organs and activated glia in the central nervous system, converge to produce the pathological lesions that characterize the disease [10]. Recently, we reported that enhanced lysosomal cholesterol content protects cells from LMP-dependent apoptosis [20]. Although this finding, which has also been confirmed by others [21], may seem counterintuitive, it is possibleLysosomal Stability Is Regulated by CholesterolFigure 2. Manipulation of lysosomal cholesterol content modulates the cellular sensitivity to apoptosis. Cholesterol content of human fibroblasts was modulated using U18666A, quinacrine, methyl-b-cyclodextrin (MbCD) or 25-hydroxy cholesterol (25-HC) before apoptosis was induced using O-methyl-serine dodecylamide hydrochloride (MSDH; 24 h). Phase contrast images of A) 
wt and B) NPC1-mutant fibroblasts (NPC1mut). Scale bar 20 mm. C) Viability of cultures in A and B, respectively, assessed by the MTT assay (n = 4). Viability is expressed as percentage of untreated cultures. D) Caspase-3 like activity (n = 4?). E) Representative curve of increase in green fluorescence during photo-oxidation of acridine orange. F) Quantification of lag time (as presented in E; n = 5?). Data are presented as the mean 6 SD, * p#0.05. doi:10.1371/journal.pone.0050262.gthat cholesterol preserves the integrity of the lysosomal membrane and thus promotes neuronal survival upon acute cellular stress. Importantly, in both NPC1-mutant cells and U18666A treated cells, cholesterol accumulation is associated with storage of severalother lipids [9,22], which might influence lysosomal stability. In addition, the expression of LAMP-2 was increased by U18666A treatment [20].Mulation through homeostatic ER mechanisms by signaling cholesterol excess [15]. Lipidosis, and intracellular accumulation of phospholipids, is a side effect of certain cationic amphiphilic drugs, including quinacrine, desipramine, imipramineLysosomal Stability Is Regulated by CholesterolFigure 1. Cholesterol modulation in human fibroblasts is associated with alterations of the lysosomal compartment. Human wt fibroblasts were treated with U18666A or quinacrine to induce cholesterol accumulation, and NPC1-mutant fibroblasts were treated with methyl-bcyclodextrin (MbCD) or 25-hydroxy cholesterol (25-HC) to revert cholesterol storage. A) Measurement of unesterified cholesterol (n = 4) and B) representative images of filipin staining (scale bar 10 mm). C and D) Representative histogram from flow cytometric analysis of Lysotracker fluorescence staining. M1 gate denotes the highly fluorescent population. E and F) Quantification of peak channel in the M1 population (seen in C and D; n = 4). Data are presented as the mean 6 SD, * p#0.05. doi:10.1371/journal.pone.0050262.gand amiodarone, used to treat e.g., depression and arrhythmias [16,17]. The exact mechanism of action of these small lysosomotropic compounds remains poorly understood, but their amphiphilic nature allows them to accumulate in membranes and might disrupt the activity of membrane proteins like NPC1 [18]. In addition, the compound U18666A has been extensively used to mimic the NPC phenotype by impairing the intracellular transport of LDL-derived cholesterol from lysosomes [19], thus resulting in cholesterol accumulation in this compartment.The way in which increased lysosomal cholesterol contributes to NPC is unknown, but it has been suggested that both lipid storage and a concomitant inflammatory response, involving macrophages in peripheral organs and activated glia in the central nervous system, converge to produce the pathological lesions that characterize the disease [10]. Recently, we reported that enhanced lysosomal cholesterol content protects cells from LMP-dependent apoptosis [20]. Although this finding, which has also been confirmed by others [21], may seem counterintuitive, it is possibleLysosomal Stability Is Regulated by CholesterolFigure 2. Manipulation of lysosomal cholesterol content modulates the cellular sensitivity to apoptosis. Cholesterol content of human fibroblasts was modulated using U18666A, quinacrine, methyl-b-cyclodextrin (MbCD) or 25-hydroxy cholesterol (25-HC) before apoptosis was induced using O-methyl-serine dodecylamide hydrochloride (MSDH; 24 h). Phase contrast images of A) wt and B) NPC1-mutant fibroblasts (NPC1mut). Scale bar 20 mm. C) Viability of cultures in A and B, respectively, assessed by the MTT assay (n = 4). Viability is expressed as percentage of untreated cultures. D) Caspase-3 like activity (n = 4?). E) Representative curve of increase in green fluorescence during photo-oxidation of acridine orange. F) Quantification of lag time (as presented in E; n = 5?). Data are presented as the mean 6 SD, * p#0.05. doi:10.1371/journal.pone.0050262.gthat cholesterol preserves the integrity of the lysosomal membrane and thus promotes neuronal survival upon acute cellular stress. Importantly, in both NPC1-mutant cells and U18666A treated cells, cholesterol accumulation is associated with storage of severalother lipids [9,22], which might influence lysosomal stability. In addition, the expression of LAMP-2 was increased by 
U18666A treatment [20].
