Niemann-Pick C1 (NPC1) is a lysosomal membrane protein that exports cholesterol derived from receptor-mediated uptake of LDL, and it also mediates cellular entry of Ebola virus. Cholesterol export is inhibited by nanomolar concentrations of U18666A, a cationic sterol. To identify the target of U18666A, we synthesized U-X, a U18666A derivative with a benzophenone that permits ultraviolet-induced crosslinking. When added to CHO cells, U-X crosslinked to NPC1. Crosslinking was blocked by U18666A derivatives that block cholesterol export, but not derivatives lacking blocking activity. Crosslinking was prevented by point mutation in the sterol-sensing domain (SSD) of NPC1, but not by point mutation in the N-terminal domain (NTD). These data suggest that the SSD contains a U18666A-inhibitable site required for cholesterol export distinct from the cholesterol-binding site in the NTD. Inasmuch as inhibition of Ebola requires 100-fold higher concentrations of U18666A, the high affinity U16888A-binding site is likely not required for virus entry.DOI: http://dx.doi.org/10.7554/eLife.12177.001

Cancer specific inhibitors reflective of unique metabolic needs, are rare. We describe a novel small molecule, Gboxin, that specifically inhibits primary mouse and human glioblastoma (GBM) cell growth but not mouse embryo fibroblasts or neonatal astrocytes. Gboxin rapidly and irreversibly compromises GBM oxygen consumption. Reliant on its positive charge, Gboxin associates with mitochondrial oxidative phosphorylation complexes in a proton gradient dependent manner and inhibits F0F1 ATP synthase activity. Gboxin resistant cells require a functional mitochondrial permeability transition pore that regulates pH impeding matrix accumulation. Administration of a pharmacologically stable Gboxin analog inhibits GBM allografts and patient derived xenografts. Gboxin toxicity extends to established human cancer cell lines of diverse organ origin and exposes the elevated proton gradient pH in cancer cell mitochondria as a new mode of action for antitumor reagent development.

Uridine, a pyrimidine nucleoside present at high levels in the plasma of rodents and humans, is critical for RNA synthesis, glycogen deposition, and many other essential cellular processes. It also contributes to systemic metabolism, but the underlying mechanisms remain unclear. We found that plasma uridine levels are regulated by fasting and refeeding in mice, rats, and humans. Fasting increases plasma uridine levels, and this increase relies largely on adipocytes. In contrast, refeeding reduces plasma uridine levels through biliary clearance. Elevation of plasma uridine is required for the drop in body temperature that occurs during fasting. Further, feeding-induced clearance of plasma uridine improves glucose metabolism. We also present findings that implicate leptin signaling in uridine homeostasis and consequent metabolic control and thermoregulation. Our results indicate that plasma uridine governs energy homeostasis and thermoregulation in a mechanism involving adipocyte-dependent uridine biosynthesis and leptin signaling.

Niemann-Pick C1 (NPC1), a membrane protein of lysosomes, is required for the export of cholesterol derived from receptor-mediated endocytosis of LDL. Lysosomal cholesterol export is reportedly inhibited by itraconazole, a triazole that is used as an antifungal drug [Xu et al. (2010) Proc Natl Acad Sci USA 107:4764-4769]. Here we show that posaconazole, another triazole, also blocks cholesterol export from lysosomes. We prepared P-X, a photoactivatable cross-linking derivative of posaconazole. P-X cross-linked to NPC1 when added to intact cells. Cross-linking was inhibited by itraconazole but not by ketoconazole, an imidazole that does not block cholesterol export. Cross-linking of P-X was also blocked by U18666A, a compound that has been shown to bind to NPC1 and inhibit cholesterol export. P-X also crosslinked to purified NPC1 that was incorporated into lipid bilayer nanodiscs. In this in vitro system, cross-linking of P-X was inhibited by itraconazole, but not by U18666A. P-X cross-linking was not prevented by deletion of the N-terminal domain of NPC1, which contains the initial binding site for cholesterol. In contrast, P-X cross-linking was reduced when NPC1 contained a point mutation (P691S) in its putative sterolsensing domain. We hypothesize that the sterol-sensing domain has a binding site that can accommodate structurally different ligands.Niemann-Pick C disease | cholesterol transport | sterol-sensing domain | lipid nanodiscs | photoactivatable cross-linking

Niemann-Pick C1 (NPC1), a lysosomal protein of 13 transmembrane helices (TMs) and three lumenal domains, exports low-density-lipoprotein (LDL)-derived cholesterol from lysosomes. TMs 3-7 of NPC1 comprise the Sterol-Sensing Domain (SSD). Previous studies suggest that mutation of the NPC1-SSD or the addition of the anti-fungal drug itraconazole abolishes NPC1 activity in cells. However, the itraconazole binding site and the mechanism of NPC1-mediated cholesterol transport remain unknown. Here, we report a cryo-EM structure of human NPC1 bound to itraconazole, which reveals how this binding site in the center of NPC1 blocks a putative lumenal tunnel linked to the SSD. Functional assays confirm that blocking this tunnel abolishes NPC1-mediated cholesterol egress. Intriguingly, the palmitate anchor of Hedgehog occupies a similar site in the homologous tunnel of Patched, suggesting a conserved mechanism for sterol transport in this family of proteins and establishing a central function of their SSDs.