Mechanism of Cholesterol Transfer from the Niemann-Pick Type C2 Protein to Model Membranes Supports a Role in Lysosomal Cholesterol Transport

Molecular determinants of phospholipid treatment to reduce intracellular cholesterol accumulation in NPC1 deficiency

Niemann-Pick type C (NPC) disease, caused by mutations in the NPC1 or NPC2 genes, leads to abnormal intracellular cholesterol accumulation in late endosomes/lysosomes (LE/LY). Exogenous enrichment with lysobisphosphatidic acid (LBPA), also known as bis-monoacylglycerol phosphate or BMP, either directly or via the LBPA precursor phosphatidylglycerol (PG), has been investigated as a therapeutic intervention to reduce cholesterol accumulation in NPC disease. Here we report the effects of stereoisomer configuration and acyl chain composition of LBPA on cholesterol clearance in NPC1-deficient cells. We find that S,R, S,S, and S,R LBPA stereoisomers behaved similarly, with all 3 compounds leading to comparable reductions in filipin staining in two NPC1-deficient human fibroblast cell lines. Examination of several LBPA molecular species containing one or two mono- or polyunsaturated acyl chains showed that all LBPA species containing one 18:1 chain significantly reduced cholesterol accumulation, whereas the shorter chain species di-14:0 LBPA had little effect on cholesterol clearance in NPC1 deficient cells. Since cholesterol accumulation in NPC1 deficient cells can also be cleared by PG incubation, we used non-hydrolyzable PG analogues to determine whether conversion to LBPA is required for sterol clearance, or whether PG itself is effective. The results showed that non-hydrolyzable PG species were not appreciably converted to LBPA and showed virtually no cholesterol clearance efficacy in NPC1 deficient cells, supporting the notion that LBPA is the active agent promoting LE/LY cholesterol clearance. Overall these studies are helping to define the molecular requirements for potential therapeutic use of LBPA as an option for addressing NPC disease.

NPC Intracellular Cholesterol Transporter 1 Regulates Ovarian Maturation and Molting in Female Macrobrachium nipponense

NPC intracellular cholesterol transporter 1 (NPC1) plays an important role in sterol metabolism and transport processes and has been studied in many vertebrates and some insects, but rarely in crustaceans. In this study, we characterized NPC1 from Macrobrachium nipponense (Mn-NPC1) and evaluated its functions. Its total cDNA length was 4283 bp, encoding for 1344 amino acids. It contained three conserved domains typical of the NPC family (NPC1_N, SSD, and PTC). In contrast to its role in insects, Mn-NPC1 was mainly expressed in the adult female hepatopancreas, with moderate expression in the ovary and heart. No expression was found in the embryo (stages CS-ZS) and only weak expression in the larval stages from hatching to the post-larval stage (L1-PL15). Mn-NPC1 expression was positively correlated with ovarian maturation. In situ hybridization showed that it was mainly located in the cytoplasmic membrane and nucleus of oocytes. A 25-day RNA interference experiment was employed to illustrate the Mn-NPC1 function in ovary maturation. Experimental knockdown of Mn-NPC1 using dsRNA resulted in a marked reduction in the gonadosomatic index and ecdysone content of M. nipponense females. The experimental group showed a significant delay in ovarian maturation and a reduction in the frequency of molting. These results expand our understanding of NPC1 in crustaceans and of the regulatory mechanism of ovarian maturation in M. nipponense.

Caveolin-3 loss linked with the P104L LGMD-1C mutation modulates skeletal muscle mTORC1 signalling and cholesterol homeostasis

BACKGROUND

Caveolins are the principal structural components of plasma membrane caveolae. Dominant pathogenic mutations in the muscle-specific caveolin-3 (Cav3) gene isoform, such as the limb girdle muscular dystrophy type 1C (LGMD-1C) P104L mutation, result in dramatic loss of the Cav3 protein and pathophysiological muscle weakness/wasting. We hypothesize that such muscle degeneration may be linked to disturbances in signalling events that impact protein turnover. Herein, we report studies assessing the effects of Cav3 deficiency on mammalian or mechanistic target of rapamycin complex 1 (mTORC1) signalling in skeletal muscle cells.

METHODS

L6 myoblasts were stably transfected with Cav3P104L or expression of native Cav3 was abolished by CRISPR/Cas9 genome editing (Cav3 knockout [Cav3KO]) prior to performing subcellular fractionation and immunoblotting, analysis of real-time mitochondrial respiration or fixed cell immunocytochemistry. Skeletal muscle from wild-type and Cav3-/- mice was processed for immunoblot analysis of downstream mTORC1 substrate phosphorylation.

RESULTS

Cav3 was detected in lysosomal-enriched membranes isolated from L6 myoblasts and observed by confocal microscopy to co-localize with lysosomal-specific markers. Cav3P104L expression, which results in significant (~95%) loss of native Cav3, or CRISPR/Cas9-mediated Cav3KO, reduced amino acid-dependent mTORC1 activation. The decline in mTORC1-directed signalling was detected by immunoblot analysis of L6 muscle cells and gastrocnemius Cav3-/- mouse muscle as judged by reduced phosphorylation of mTORC1 substrates that play key roles in the initiation of protein synthesis (4EBP1S65 and S6K1T389 ). S6K1T389 and 4EBP1S65 phosphorylation reduced by over 75% and 80% in Cav3KO muscle cells and by over 90% and 30% in Cav3-/- mouse skeletal muscle, respectively. The reduction in protein synthetic capacity in L6 muscle cells was confirmed by analysis of puromycylated peptides using the SUnSET assay. Cav3 loss was also associated with a 26% increase in lysosomal cholesterol, and pharmacological manipulation of lysosomal cholesterol was effective in replicating the reduction in mTORC1 activity observed in Cav3KO cells. Notably, re-expression of Cav3 in Cav3KO myoblasts normalized lysosomal cholesterol content, which coincided with a recovery in protein translation and an associated increase in mTORC1-directed phosphorylation of downstream targets.

CONCLUSIONS

Our findings indicate that Cav3 can localize on lysosomal membranes and is a novel regulator of mTORC1 signalling in muscle. Cav3 deficiency associated with the Cav3P104L mutation impairs mTORC1 activation and protein synthetic capacity in skeletal muscle cells, which may be linked to disturbances in lysosomal cholesterol trafficking and contribute to the pathology of LGMD-1C.

Cholesterol-dependent molecular mechanisms contribute to cationic amphiphilic drugs' prevention of silica-induced inflammation

Silicosis is considered an irreversible chronic inflammatory disease caused by the inhalation of crystalline silica (cSiO2). The cycle of inflammation that drives silicosis and other particle-caused respiratory diseases is mediated by NLRP3 inflammasome activity in macrophages resulting in the release of IL-1β. Lysosomal membrane permeability (LMP) initiated by inhaled particles is the key regulatory step in leading to NLRP3 activity. In addition to its role in LMP, the lysosome is crucial to cellular cholesterol trafficking. Lysosomal cholesterol has been demonstrated to regulate LMP while cationic amphiphilic drugs (CADs) reduce cholesterol trafficking from lysosomes and promote endolysosomal cholesterol accumulation as seen in Niemann Pick disease. Using a bone marrow derived macrophage (BMdM) model, four CADs were examined for their potential to reduce cSiO2-induced inflammation. Here we found that FDA-approved CAD drugs imipramine, hydroxychloroquine, fluvoxamine, and fluoxetine contributed to reduced LMP and IL-1β release in cSiO2 treated BMdM. These drugs inhibited lysosomal enzymatic activity of acid sphingomyelinase, decreased lysosomal proteolytic function, and increased lysosomal pH. CADs also demonstrated a significant increase in lysosomal-associated free cholesterol. Increased lysosomal cholesterol was associated with a significant reduction in cSiO2 induced LMP and IL-1β release. In contrast, reduced lysosomal cholesterol significantly exacerbated cSiO2-induced IL-1β release and reduced the protective effect of CADs on IL-1β release following cSiO2 exposure. Taken together, these results suggest that CAD modification of lysosomal cholesterol may be used to reduce LMP and cSiO2-induced inflammation and could prove an effective therapeutic for silicosis and other particle-caused respiratory diseases.

The Niemann-Pick type diseases – A synopsis of inborn errors in sphingolipid and cholesterol metabolism

Disturbances of lipid homeostasis in cells provoke human diseases. The elucidation of the underlying mechanisms and the development of efficient therapies represent formidable challenges for biomedical research. Exemplary cases are two rare, autosomal recessive, and ultimately fatal lysosomal diseases historically named "Niemann-Pick" honoring the physicians, whose pioneering observations led to their discovery. Acid sphingomyelinase deficiency (ASMD) and Niemann-Pick type C disease (NPCD) are caused by specific variants of the sphingomyelin phosphodiesterase 1 (SMPD1) and NPC intracellular cholesterol transporter 1 (NPC1) or NPC intracellular cholesterol transporter 2 (NPC2) genes that perturb homeostasis of two key membrane components, sphingomyelin and cholesterol, respectively. Patients with severe forms of these diseases present visceral and neurologic symptoms and succumb to premature death. This synopsis traces the tortuous discovery of the Niemann-Pick diseases, highlights important advances with respect to genetic culprits and cellular mechanisms, and exposes efforts to improve diagnosis and to explore new therapeutic approaches.

Alterations in Cholesterol and Phosphoinositides Levels in the Intracellular Cholesterol Trafficking Disorder NPC

Lipid mistrafficking is a biochemical hallmark of Niemann-Pick Type C (NPC) disease and is classically characterized with endo/lysosomal accumulation of unesterified cholesterol due to genetic mutations in the cholesterol transporter proteins NPC1 and NPC2. Storage of this essential signaling lipid leads to a sequence of downstream events, including oxidative stress, calcium imbalance, neuroinflammation, and progressive neurodegeneration, another hallmark of NPC disease. These observations have been validated in a growing number of studies ranging from NPC cell cultures and animal models to patient specimens. In recent reports, alterations in the levels of another class of critical signaling lipids, namely phosphoinositides, have been described in NPC disease. Focusing on cholesterol and phosphoinositides, the chapter begins by reviewing the interactions of NPC proteins with cholesterol and their role in cholesterol transport. It then continues to describe the modulation of cholesterol efflux in NPC disease. The chapter concludes with a summary of findings related to the functional consequences of perturbations in phosphoinositides in this fatal disease.

Acat1/Soat1 knockout extends the mutant Npc1 mouse lifespan and ameliorates functional deficiencies in multiple organelles of mutant cells

Multiple membrane organelles require cholesterol for proper function within cells. The Niemann-Pick type C (NPC) proteins export cholesterol from endosomes to other membrane compartments, including the endoplasmic reticulum (ER), plasma membrane (PM), trans-Golgi network (TGN), and mitochondria, to meet their cholesterol requirements. Defects in NPC cause malfunctions in multiple membrane organelles and lead to an incurable neurological disorder. Acyl-coenzyme A:cholesterol acyltransferase 1 (ACAT1), a resident enzyme in the ER, converts cholesterol to cholesteryl esters for storage. In mutant NPC cells, cholesterol storage still occurs in an NPC-independent manner. Here we report the interesting finding that in a mutant Npc1 mouse (Npc1nmf), Acat1 gene (Soat1) knockout delayed the onset of weight loss, motor impairment, and Purkinje neuron death. It also improved hepatosplenic pathology and prolonged lifespan by 34%. In mutant NPC1 fibroblasts, ACAT1 blockade (A1B) increased cholesterol content associated with TGN-rich membranes and mitochondria, while decreased cholesterol content associated with late endosomes. A1B also restored proper localization of syntaxin 6 and golgin 97 (key proteins in membrane trafficking at TGN) and improved the levels of cathepsin D (a key protease in lysosome and requires Golgi/endosome transport for maturation) and ABCA1 (a key protein controlling cholesterol release at PM). This work supports the hypothesis that diverting cholesterol from storage can benefit multiple diseases that involve cholesterol deficiencies in cell membranes.

Enrichment of NPC1-deficient cells with the lipid LBPA stimulates autophagy, improves lysosomal function, and reduces cholesterol storage

Niemann-Pick C (NPC) is an autosomal recessive disorder characterized by mutations in the NPC1 or NPC2 genes encoding endolysosomal lipid transport proteins, leading to cholesterol accumulation and autophagy dysfunction. We have previously shown that enrichment of NPC1-deficient cells with the anionic lipid lysobisphosphatidic acid (LBPA; also called bis(monoacylglycerol)phosphate) via treatment with its precursor phosphatidylglycerol (PG) results in a dramatic decrease in cholesterol storage. However, the mechanisms underlying this reduction are unknown. In the present study, we showed using biochemical and imaging approaches in both NPC1-deficient cellular models and an NPC1 mouse model that PG incubation/LBPA enrichment significantly improved the compromised autophagic flux associated with NPC1 disease, providing a route for NPC1-independent endolysosomal cholesterol mobilization. PG/LBPA enrichment specifically enhanced the late stages of autophagy, and effects were mediated by activation of the lysosomal enzyme acid sphingomyelinase. PG incubation also led to robust and specific increases in LBPA species with polyunsaturated acyl chains, potentially increasing the propensity for membrane fusion events, which are critical for late-stage autophagy progression. Finally, we demonstrated that PG/LBPA treatment efficiently cleared cholesterol and toxic protein aggregates in Purkinje neurons of the NPC1I1061T mouse model. Collectively, these findings provide a mechanistic basis supporting cellular LBPA as a potential new target for therapeutic intervention in NPC disease.

Mitochondrial Targeting Involving Cholesterol-Rich Lipid Rafts in the Mechanism of Action of the Antitumor Ether Lipid and Alkylphospholipid Analog Edelfosine

The ether lipid edelfosine induces apoptosis selectively in tumor cells and is the prototypic molecule of a family of synthetic antitumor compounds collectively known as alkylphospholipid analogs. Cumulative evidence shows that edelfosine interacts with cholesterol-rich lipid rafts, endoplasmic reticulum (ER) and mitochondria. Edelfosine induces apoptosis in a number of hematological cancer cells by recruiting death receptors and downstream apoptotic signaling into lipid rafts, whereas it promotes apoptosis in solid tumor cells through an ER stress response. Edelfosine-induced apoptosis, mediated by lipid rafts and/or ER, requires the involvement of a mitochondrial-dependent step to eventually elicit cell death, leading to the loss of mitochondrial membrane potential, cytochrome c release and the triggering of cell death. The overexpression of Bcl-2 or Bcl-xL blocks edelfosine-induced apoptosis. Edelfosine induces the redistribution of lipid rafts from the plasma membrane to the mitochondria. The pro-apoptotic action of edelfosine on cancer cells is associated with the recruitment of F1FO-ATP synthase into cholesterol-rich lipid rafts. Specific inhibition of the FO sector of the F1FO-ATP synthase, which contains the membrane-embedded c-subunit ring that constitutes the mitochondrial permeability transcription pore, hinders edelfosine-induced cell death. Taking together, the evidence shown here suggests that the ether lipid edelfosine could modulate cell death in cancer cells by direct interaction with mitochondria, and the reorganization of raft-located mitochondrial proteins that critically modulate cell death or survival. Here, we summarize and discuss the involvement of mitochondria in the antitumor action of the ether lipid edelfosine, pointing out the mitochondrial targeting of this drug as a major therapeutic approach, which can be extrapolated to other alkylphospholipid analogs. We also discuss the involvement of cholesterol transport and cholesterol-rich lipid rafts in the interactions between the organelles as well as in the role of mitochondria in the regulation of apoptosis in cancer cells and cancer therapy.

Niemann Pick C2 protein enables cholesterol transfer from endo-lysosomes to the plasma membrane for efflux by shedding of extracellular vesicles

The Niemann-Pick C2 protein (NPC2) is a sterol transfer protein in the lumen of late endosomes and lysosomes (LE/LYSs). Absence of functional NPC2 leads to endo-lysosomal buildup of cholesterol and other lipids. How NPC2's known capacity to transport cholesterol between model membranes is linked to its function in living cells is not known. Using quantitative live-cell imaging combined with modeling of the efflux kinetics, we show that NPC2-deficient human fibroblasts can export the cholesterol analog dehydroergosterol (DHE) from LE/LYSs. Internalized NPC2 accelerated sterol efflux extensively, accompanied by reallocation of LE/LYSs containing fluorescent NPC2 and DHE to the cell periphery. Using quantitative fluorescence loss in photobleaching of TopFluor-cholesterol (TF-Chol), we estimate a residence time for a rapidly exchanging sterol pool in LE/LYSs localized in close proximity to the plasma membrane (PM), of less than one min and observed non-vesicular sterol exchange between LE/LYSs and the PM. Excess sterol was released from the PM by shedding of cholesterol-rich vesicles. The ultrastructure of such vesicles was analyzed by combined fluorescence and cryo soft X-ray tomography (SXT), revealing that they can contain lysosomal cargo and intraluminal vesicles. Treating cells with apoprotein A1 and with nuclear receptor liver X-receptor (LXR) agonists to upregulate expression of ABC transporters enhanced cholesterol efflux from the PM, at least partly by accelerating vesicle release. We conclude that NPC2 inside LE/LYSs facilitates non-vesicular sterol exchange with the PM for subsequent sterol efflux to acceptor proteins and for shedding of sterol-rich vesicles from the cell surface.

NPC intracellular cholesterol transporter 1 (NPC1)-mediated cholesterol export from lysosomes

Low-density lipoprotein particles are taken up by cells and delivered to the lysosome where their cholesterol esters are cleaved off by acid lipase. The released, free cholesterol is then exported from lysosomes for cellular needs or storage. This article summarizes recent advances in our understanding of the molecular basis of cholesterol export from lysosomes. Cholesterol export requires NPC intracellular cholesterol transporter 1 (NPC1) and NPC2, genetic mutations of which can cause Niemann-Pick type C disease, a disorder characterized by massive lysosomal accumulation of cholesterol and glycosphingolipids. Analysis of the NPC1 and NPC2 structures and biochemical properties, together with new structures of the related Patched (PTCH) protein, provides new clues to the mechanisms by which NPC proteins may function.

CCDC115-CDG: A new rare and misleading inherited cause of liver disease

Congenital disorders of glycosylation (CDG) linked to defects in Golgi apparatus homeostasis constitute an increasing part of these rare inherited diseases. Among them, COG-CDG, ATP6V0A2-CDG, TMEM199-CDG and CCDC115-CDG have been shown to disturb Golgi vesicular trafficking and/or lumen pH acidification. Here, we report 3 new unrelated cases of CCDC115-CDG with emphasis on diagnosis difficulties related to strong phenotypic similarities with mitochondriopathies, Niemann-Pick disease C and Wilson Disease. Indeed, while two individuals clinically presented with early and severe liver fibrosis and cirrhosis associated with neurological symptoms, the other one "only" showed isolated and late severe liver involvement. Biological results were similar to previously described patients, including hypercholesterolemia, elevated alkaline phosphatases and defects in copper metabolism. CDG screening and glycosylation study finally led to the molecular diagnosis of CCDC115-CDG. Besides pointing to the importance of CDG screening in patients with unexplained and severe liver disease, these reports expand the clinical and molecular phenotypes of CCDC115-CDG. The hepatic involvement is particularly addressed. Furthermore, hypothesis concerning the pathogenesis of the liver disease and of major biological abnormalities are proposed.

Niemann-Pick C2 protein regulates sterol transport between plasma membrane and late endosomes in human fibroblasts

Niemann-Pick disease type C2 is a lipid storage disorder in which mutations in the NPC2 protein cause accumulation of lipoprotein-derived cholesterol in late endosomes and lysosomes (LE/LYSs). Whether cholesterol delivered by other means to NPC2 deficient cells also accumulates in LE/LYSs is currently unknown. We show that the close cholesterol analog dehydroergosterol (DHE), when delivered to the plasma membrane (PM) accumulates in LE/LYSs of human fibroblasts lacking functional NPC2. We measured two different time scales of sterol diffusion; while DHE rich LE/LYSs moved by slow anomalous diffusion in disease cells (D ∼ 4.6∙10^-4 μm²/sec; α∼0.76), a small pool of sterol could exchange rapidly with D ∼ 3 μm²/s between LE/LYSs, as shown by fluorescence recovery after photobleaching (FRAP). By quantitative lipid mass spectrometry we found that esterification of ¹³C-labeled cholesterol but not of DHE is reduced 10-fold in disease fibroblasts compared to control cells. Internalized NPC2 rescued the sterol storage phenotype and strongly expanded the dynamic sterol pool seen in FRAP experiments. Together, our study shows that cholesterol esterification and trafficking of sterols between the PM and LE/LYSs depends on a functional NPC2 protein. NPC2 likely acts inside LE/LYSs from where it increases non-vesicular sterol exchange with other organelles.

The new obesity-associated protein, neuronal growth regulator 1 (NEGR1), is implicated in Niemann-Pick disease Type C (NPC2)-mediated cholesterol trafficking

Neuronal growth regulator 1 (NEGR1) is a newly identified raft-associated protein, which has recently been spotlighted as a new locus related to human obesity. Niemann-Pick disease Type C2 (NPC2) protein functions as a key player in the intracellular cholesterol trafficking, and its defect is linked to a fatal human neurodegenerative disease, NPC. In this study, we identified that NEGR1 interacts with NPC2 and increases its protein stability. Ectopically expressed NEGR1 proteins relieved an abnormal cholesterol accumulation in endosomal compartments. Importantly, NEGR1-defective mouse embryonic fibroblast cells exhibit increased cholesterol levels and triglyceride contents. These findings provide the first insight into the role of NEGR1 in intracellular cholesterol homeostasis, possibly explaining the missing link between NEGR1 with human obesity.

Clues to the mechanism of cholesterol transfer from the structure of NPC1 middle lumenal domain bound to NPC2

Export of LDL-derived cholesterol from lysosomes requires the cooperation of the integral membrane protein Niemann-Pick C1 (NPC1) and a soluble protein, Niemann-Pick C2 (NPC2). Mutations in the genes encoding these proteins lead to Niemann-Pick disease type C (NPC). NPC2 binds to NPC1's second (middle), lumenally oriented domain (MLD) and transfers cholesterol to NPC1's N-terminal domain (NTD). Here, we report the 2.4-Å resolution crystal structure of a complex of human NPC1-MLD and NPC2 bearing bound cholesterol-3-O-sulfate. NPC1-MLD uses two protruding loops to bind NPC2, analogous to its interaction with the primed Ebola virus glycoprotein. Docking of the NPC1-NPC2 complex onto the full-length NPC1 structure reveals a direct cholesterol transfer tunnel between NPC2 and NTD cholesterol binding pockets, supporting the "hydrophobic hand-off" cholesterol transfer model.

α/β Hydrolase Domain-containing 6 (ABHD6) Degrades the Late Endosomal/Lysosomal Lipid Bis(monoacylglycero)phosphate

α/β Hydrolase domain-containing 6 (ABHD6) can act as monoacylglycerol hydrolase and is believed to play a role in endocannabinoid signaling as well as in the pathogenesis of obesity and liver steatosis. However, the mechanistic link between gene function and disease is incompletely understood. Here we aimed to further characterize the role of ABHD6 in lipid metabolism. We show that mouse and human ABHD6 degrade bis(monoacylglycero)phosphate (BMP) with high specific activity. BMP, also known as lysobisphosphatidic acid, is enriched in late endosomes/lysosomes, where it plays a key role in the formation of intraluminal vesicles and in lipid sorting. Up to now, little has been known about the catabolism of this lipid. Our data demonstrate that ABHD6 is responsible for ∼90% of the BMP hydrolase activity detected in the liver and that knockdown of ABHD6 increases hepatic BMP levels. Tissue fractionation and live-cell imaging experiments revealed that ABHD6 co-localizes with late endosomes/lysosomes. The enzyme is active at cytosolic pH and lacks acid hydrolase activity, implying that it degrades BMP exported from acidic organelles or de novo-formed BMP. In conclusion, our data suggest that ABHD6 controls BMP catabolism and is therefore part of the late endosomal/lysosomal lipid-sorting machinery.

Multiple Surface Regions on the Niemann-Pick C2 Protein Facilitate Intracellular Cholesterol Transport

The cholesterol storage disorder Niemann-Pick type C (NPC) disease is caused by defects in either of two late endosomal/lysosomal proteins, NPC1 and NPC2. NPC2 is a 16-kDa soluble protein that binds cholesterol in a 1:1 stoichiometry and can transfer cholesterol between membranes by a mechanism that involves protein-membrane interactions. To examine the structural basis of NPC2 function in cholesterol trafficking, a series of point mutations were generated across the surface of the protein. Several NPC2 mutants exhibited deficient sterol transport properties in a set of fluorescence-based assays. Notably, these mutants were also unable to promote egress of accumulated intracellular cholesterol from npc2(-/-) fibroblasts. The mutations mapped to several regions on the protein surface, suggesting that NPC2 can bind to more than one membrane simultaneously. Indeed, we have previously demonstrated that WT NPC2 promotes vesicle-vesicle interactions. These interactions were abrogated, however, by mutations causing defective sterol transfer properties. Molecular modeling shows that NPC2 is highly plastic, with several intense positively charged regions across the surface that could interact favorably with negatively charged membrane phospholipids. The point mutations generated in this study caused changes in NPC2 surface charge distribution with minimal conformational changes. The plasticity, coupled with membrane flexibility, probably allows for multiple cholesterol transfer routes. Thus, we hypothesize that, in part, NPC2 rapidly traffics cholesterol between closely appositioned membranes within the multilamellar interior of late endosomal/lysosomal proteins, ultimately effecting cholesterol egress from this compartment.

Niemann-Pick type C2 protein mediating chemical communication in the worker ant

Ants are eusocial insects that are found in most regions of the world. Within its caste, worker ants are responsible for various tasks that are required for colony maintenance. In their chemical communication, α-helical carrier proteins, odorant-binding proteins, and chemosensory proteins, which accumulate in the sensillum lymph in the antennae, play essential roles in transferring hydrophobic semiochemicals to chemosensory receptors. It has been hypothesized that semiochemicals are recognized by α-helical carrier proteins. The number of these proteins, however, is not sufficient to interact with a large number of semiochemicals estimated from chemosensory receptor genes. Here we shed light on this conundrum by identifying a Niemann-Pick type C2 (NPC2) protein from the antenna of the worker Japanese carpenter ant, Camponotus japonicus (CjapNPC2). CjapNPC2 accumulated in the sensillum cavity in the basiconic sensillum. The ligand-binding pocket of CjapNPC2 was composed of a flexible β-structure that allowed it to bind to a wide range of potential semiochemicals. Some of the semiochemicals elicited electrophysiolgical responses in the worker antenna. In vertebrates, NPC2 acts as an essential carrier protein for cholesterol from late endosomes and lysosomes to other cellular organelles. However, the ants have evolved an NPC2 with a malleable ligand-binding pocket as a moderately selective carrier protein in the sensillum cavity of the basiconic sensillum. CjapNPC2 might be able to deliver various hydrophobic semiochemicals to chemosensory receptor neurons and plays crucial roles in chemical communication required to perform the worker ant tasks.

Alkylphospholipids deregulate cholesterol metabolism and induce cell-cycle arrest and autophagy in U-87 MG glioblastoma cells

Glioblastoma is the most common malignant primary brain tumour in adults and one of the most lethal of all cancers. Growing evidence suggests that human tumours undergo abnormal lipid metabolism, characterised by an alteration in the mechanisms that regulate cholesterol homeostasis. We have investigated the effect that different antitumoural alkylphospholipids (APLs) exert upon cholesterol metabolism in the U-87 MG glioblastoma cell line. APLs altered cholesterol homeostasis by interfering with its transport from the plasma membrane to the endoplasmic reticulum (ER), thus hindering its esterification. At the same time they stimulated the synthesis of cholesterol from radiolabelled acetate and its internalisation from low-density lipoproteins (LDLs), inducing both 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) and LDL receptor (LDLR) genes. Fluorescent microscopy revealed that these effects promoted the accumulation of intracellular cholesterol. Filipin staining demonstrated that this accumulation was not confined to the late endosome/lysosome (LE/LY) compartment since it did not colocalise with LAMP2 lysosomal marker. Furthermore, APLs inhibited cell growth, producing arrest at the G2/M phase. We also used transmission electron microscopy (TEM) to investigate ultrastructural alterations induced by APLs and found an abundant presence of autophagic vesicles and autolysosomes in treated cells, indicating the induction of autophagy. Thus our findings clearly demonstrate that antitumoural APLs interfere with the proliferation of the glioblastoma cell line via a complex mechanism involving cholesterol metabolism, cell-cycle arrest or autophagy. Knowledge of the interrelationship between these processes is fundamental to our understanding of tumoural response and may facilitate the development of novel therapeutics to improve treatment of glioblastoma and other types of cancer.

Genetic dissection of a cell-autonomous neurodegenerative disorder: lessons learned from mouse models of Niemann-Pick disease type C

Understanding neurodegenerative disease progression and its treatment requires the systematic characterization and manipulation of relevant cell types and molecular pathways. The neurodegenerative lysosomal storage disorder Niemann-Pick disease type C (NPC) is highly amenable to genetic approaches that allow exploration of the disease biology at the organismal, cellular and molecular level. Although NPC is a rare disease, genetic analysis of the associated neuropathology promises to provide insight into the logic of disease neural circuitry, selective neuron vulnerability and neural-glial interactions. The ability to control the disorder cell-autonomously and in naturally occurring spontaneous animal models that recapitulate many aspects of the human disease allows for an unparalleled dissection of the disease neurobiology in vivo. Here, we review progress in mouse-model-based studies of NPC disease, specifically focusing on the subtype that is caused by a deficiency in NPC1, a sterol-binding late endosomal membrane protein involved in lipid trafficking. We also discuss recent findings and future directions in NPC disease research that are pertinent to understanding the cellular and molecular mechanisms underlying neurodegeneration in general.

Study of cnidarian-algal symbiosis in the "omics" age

The symbiotic associations between cnidarians and dinoflagellate algae (Symbiodinium) support productive and diverse ecosystems in coral reefs. Many aspects of this association, including the mechanistic basis of host-symbiont recognition and metabolic interaction, remain poorly understood. The first completed genome sequence for a symbiotic anthozoan is now available (the coral Acropora digitifera), and extensive expressed sequence tag resources are available for a variety of other symbiotic corals and anemones. These resources make it possible to profile gene expression, protein abundance, and protein localization associated with the symbiotic state. Here we review the history of "omics" studies of cnidarian-algal symbiosis and the current availability of sequence resources for corals and anemones, identifying genes putatively involved in symbiosis across 10 anthozoan species. The public availability of candidate symbiosis-associated genes leaves the field of cnidarian-algal symbiosis poised for in-depth comparative studies of sequence diversity and gene expression and for targeted functional studies of genes associated with symbiosis. Reviewing the progress to date suggests directions for future investigations of cnidarian-algal symbiosis that include (i) sequencing of Symbiodinium, (ii) proteomic analysis of the symbiosome membrane complex, (iii) glycomic analysis of Symbiodinium cell surfaces, and (iv) expression profiling of the gastrodermal cells hosting Symbiodinium.

Niemann-Pick type C 1 function requires lumenal domain residues that mediate cholesterol-dependent NPC2 binding

Niemann-Pick type C1 (NPC1) protein is needed for cellular utilization of low-density lipoprotein-derived cholesterol that has been delivered to lysosomes. The protein has 13 transmembrane domains, three large lumenal domains, and a cytoplasmic tail. NPC1's lumenally oriented, N-terminal domain binds cholesterol and has been proposed to receive cholesterol from NPC2 protein as part of the process by which cholesterol is exported from lysosomes into the cytosol. Using surface plasmon resonance and affinity chromatography, we show here that the second lumenal domain of NPC1 binds directly to NPC2 protein. For these experiments, a soluble NPC1 lumenal domain 2 was engineered by replacing adjacent transmembrane domains with antiparallel coiled-coil sequences. Interaction of NPC2 with NPC1 lumenal domain 2 is only detected at acidic pH, conditions that are optimal for cholesterol binding to NPC2 and transfer to NPC1; the pH is also appropriate for the acidic environment where binding would take place. Binding to NPC1 domain 2 requires the presence of cholesterol on NPC2 protein, a finding that supports directional transfer of cholesterol from NPC2 onto NPC1's N-terminal domain. Finally, human disease-causing mutations in NPC1 domain 2 decrease NPC2 binding, suggesting that NPC2 binding is necessary for NPC1 function in humans. These data support a model in which NPC1 domain 2 holds NPC2 in position to facilitate directional cholesterol transfer from NPC2 onto NPC1 protein for export from lysosomes.

Characterization of fluorescent sterol binding to purified human NPC1

Mutations in the NPC1 gene cause Niemann-Pick type C disease, which appears to result from a defect in intracellular cholesterol trafficking. NPC1 is a member of the resistance-nodulation-cell division (RND) permease superfamily and contains a sterol-sensing domain, yet its cellular function and the identity of its substrates remain unknown. FLAG-tagged human NPC1 was purified from NPC1-expressing Chinese hamster ovary cells by solubilization in 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS), followed by affinity chromatography. Purified NPC1 in detergent solution appeared to be oligomeric as determined by gel filtration fast protein liquid chromatography and was photolabeled by an azido-cholesterol derivative. Fluorescent cholesterol analogs, including dehydroergosterol, cholestatrienol, and 22-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)-23,24-bisnor-5-cholen-3beta-ol (NBD-cholesterol), displayed enhanced fluorescence upon binding to NPC1 and also resulted in saturable, concentration-dependent quenching of NPC1 intrinsic Trp fluorescence. The apparent binding affinity for these three sterols was in the 0.5-6 microm range. Binding of NBD-cholesterol to NPC1 at low detergent concentration (2 mm CHAPS) was of high apparent affinity (0.5-0.6 microm) and occurred rapidly (<1 min). However, binding of a BODIPY-labeled cholesterol derivative was very slow, requiring approximately 3 h to reach equilibrium. The apparent NBD-cholesterol binding affinity was greatly reduced at higher detergent concentration. The stoichiometry of NBD-cholesterol binding to NPC1 was approximately 1. Various sterols, including native cholesterol and 25-hydroxycholesterol, inhibited NBD-cholesterol binding, suggesting that they compete for binding to the protein. Dynamic quenching studies showed that bound NBD-cholesterol was almost completely shielded from the aqueous medium, suggesting that it is buried in a deep hydrophobic pocket in NPC1. The use of fluorescent cholesterol analogs provides novel information on the molecular properties of the sterol-binding site in the full-length NPC1 protein.

Transport of LDL-derived cholesterol from the NPC1 compartment to the ER involves the trans-Golgi network and the SNARE protein complex

Mammalian cells acquire cholesterol mainly from LDL. LDL enter the endosomes, allowing cholesteryl esters to be hydrolyzed by acid lipase. The hydrolyzed cholesterol (LDL-CHOL) enters the Niemann-Pick type C1 (NPC1)-containing endosomal compartment en route to various destinations. Whether the Golgi is involved in LDL-CHOL transport downstream of the NPC1 compartment has not been demonstrated. Using subcellular fractionation and immunoadsorption to enrich for specific membrane fractions, here we show that, when parental Chinese hamster ovary (CHO) cells are briefly exposed to (3)H-cholesteryl linoleate (CL) labeled-LDL, newly liberated (3)H-LDL-CHOL appears in membranes rich in trans-Golgi network (TGN) long before it becomes available for re-esterification at the endoplasmic reticulum (ER) or for efflux at the plasma membrane. In mutant cells lacking NPC1, the appearance of newly liberated (3)H-LDL-CHOL in the TGN-rich fractions is much reduced. We next report a reconstituted transport system that recapitulates the transport of LDL-CHOL to the TGN and to the ER. The transport system requires ATP and cytosolic factors and depends on functionality of NPC1. We demonstrate that knockdown by RNAi of 3 TGN-specific SNAREs (VAMP4, syntaxin 6, and syntaxin 16) reduces >/=50% of the LDL-CHOL transport in intact cells and in vitro. These results show that vesicular trafficking is involved in transporting a significant portion of LDL-CHOL from the NPC1-containing endosomal compartment to the TGN before its arrival at the ER.

Selective decrease of bis(monoacylglycero)phosphate content in macrophages by high supplementation with docosahexaenoic acid

Bis(monoacylglycero)phosphate (BMP) is a unique phospholipid (PL) preferentially found in late endosomal membranes, where it forms specialized lipid domains. Recently, using cultured macrophages treated with anti-BMP antibody, we showed that BMP-rich domains are involved in cholesterol homeostasis. We had previously stressed the high propensity of BMP to accumulate docosahexaenoic acid (DHA), compared with other PUFAs. Because phosphatidylglycerol (PG) was reported as a precursor for BMP synthesis in RAW macrophages, we examined the effects of PG supplementation on both FA composition and amount of BMP in this cell line. Supplementation with dioleoyl-PG (18:1/18:1-PG) induced BMP accumulation, together with an increase of oleate proportion. Supplementation with high concentrations of didocosahexaenoyl-PG (22:6/22:6-PG) led to a marked enrichment of DHA in BMP, resulting in the formation of diDHA molecular species. However, the amount of BMP was selectively decreased. Similar effects were observed after supplementation with high concentrations of nonesterified DHA. Addition of vitamin E prevented the decrease of BMP and further increased its DHA content. Supplementation with 22:6/22:6-PG promoted BMP accumulation with an enhanced proportion of 22:6/22:6-BMP. DHA-rich BMP was significantly degraded after cell exposure to oxidant conditions, in contrast to oleic acid-rich BMP, which was not affected. Using a cell-free system, we showed that 22:6/22:6-BMP is highly oxidizable and partially protects cholesterol oxidation, compared with 18:1/18:1-BMP. Our data suggest that high DHA content in BMP led to specific degradation of this PL, possibly through the diDHA molecular species, which is very prone to peroxidation and, as such, a potential antioxidant in its immediate vicinity.

NPC2 facilitates bidirectional transfer of cholesterol between NPC1 and lipid bilayers, a step in cholesterol egress from lysosomes

Egress of lipoprotein-derived cholesterol from lysosomes requires two lysosomal proteins, polytopic membrane-bound Niemann-Pick C1 (NPC1) and soluble Niemann-Pick C2 (NPC2). The reason for this dual requirement is unknown. Previously, we showed that the soluble luminal N-terminal domain (NTD) of NPC1 (amino acids 25-264) binds cholesterol. This NTD is designated NPC1(NTD). We and others showed that soluble NPC2 also binds cholesterol. Here, we establish an in vitro assay to measure transfer of [(3)H]cholesterol between these two proteins and phosphatidylcholine liposomes. Whereas NPC2 rapidly donates or accepts cholesterol from liposomes, NPC1(NTD) acts much more slowly. Bidirectional transfer of cholesterol between NPC1(NTD) and liposomes is accelerated >100-fold by NPC2. A naturally occurring human mutant of NPC2 (Pro120Ser) fails to bind cholesterol and fails to stimulate cholesterol transfer from NPC1(NTD) to liposomes. NPC2 may be essential to deliver or remove cholesterol from NPC1, an interaction that links both proteins to the cholesterol egress process from lysosomes. These findings may explain how mutations in either protein can produce a similar clinical phenotype.

Purified NPC1 protein: II. Localization of sterol binding to a 240-amino acid soluble luminal loop

Defects in Niemann-Pick, Type C-1 protein (NPC1) cause cholesterol, sphingolipids, phospholipids, and glycolipids to accumulate in lysosomes of liver, spleen, and brain. In cultured fibroblasts, NPC1 deficiency causes lysosomal retention of lipoprotein-derived cholesterol after uptake by receptor-mediated endocytosis. NPC1 contains 1278 amino acids that form 13 membrane-spanning helices and three large loops that project into the lumen of lysosomes. We showed earlier that NPC1 binds cholesterol and oxysterols. Here we localize the binding site to luminal loop-1, a 240-amino acid domain with 18 cysteines. When produced in cultured cells, luminal loop-1 was secreted as a soluble dimer. This loop bound [(3)H]cholesterol (K(d), 130 nM) and [(3)H]25-hydroxycholesterol (25-HC, K(d), 10 nM) with one sterol binding site per dimer. Binding of both sterols was competed by oxysterols (24-, 25-, and 27-HC). Unlabeled cholesterol competed strongly for binding of [(3)H]cholesterol, but weakly for [(3)H]25-HC binding. Binding of [(3)H]cholesterol but not [(3)H]25-HC was inhibited by detergents. We also studied NPC2, a soluble protein whose deficiency causes a similar disease phenotype. NPC2 bound cholesterol, but not oxysterols. Epicholesterol and cholesteryl sulfate competed for [(3)H]cholesterol binding to NPC2, but not NPC1. Glutamine 79 in luminal loop-1 of NPC-1 is important for sterol binding; a Q79A mutation abolished binding of [(3)H]cholesterol and [(3)H]25-HC to full-length NPC1. Nevertheless, the Q79A mutant restored cholesterol transport to NPC1-deficient Chinese hamster ovary cells. Thus, the sterol binding site on luminal loop-1 is not essential for NPC1 function in fibroblasts, but it may function in other cells where NPC1 deficiency produces more complicated lipid abnormalities.

Purified NPC1 protein. I. Binding of cholesterol and oxysterols to a 1278-amino acid membrane protein

The Niemann-Pick, Type C1 protein (NPC1) is required for the transport of lipoprotein-derived cholesterol from lysosomes to endoplasmic reticulum. The 1278-amino acid, polytopic membrane protein has not been purified, and its mechanism of action is unknown. Unexpectedly, we encountered NPC1 in a search for a membrane protein that binds 25-hydroxycholesterol (25-HC) and other oxysterols. A 25-HC-binding protein was purified more than 14,000-fold from rabbit liver membranes and identified as NPC1 by mass spectroscopy. We prepared recombinant human NPC1 and confirmed its ability to bind oxysterols, including those with a hydroxyl group on the 24, 25, or 27 positions. Hydroxyl groups on the 7, 19, or 20 positions failed to confer binding. Recombinant human NPC1 also bound [(3)H]cholesterol in a reaction inhibited by Nonidet P-40 above its critical micellar concentration. Low concentrations of unlabeled 25-HC abolished binding of [(3)H]cholesterol, but the converse was not true, i.e. unlabeled cholesterol, even at high concentrations, did not abolish binding of [(3)H]25-HC. NPC1 is not required for the known regulatory actions of oxysterols. Thus, in NPC1-deficient fibroblasts 25-HC blocked the processing of sterol regulatory element-binding proteins and activated acyl-CoA:cholesterol acyltransferase in a normal fashion. The availability of assays to measure NPC1 binding in vitro may further the understanding of ways in which oxysterols regulate intracellular lipid transport.

Drosophila Niemann-Pick type C-2 genes control sterol homeostasis and steroid biosynthesis: a model of human neurodegenerative disease

Mutations in either of the two human Niemann-Pick type C (NPC) genes, NPC1 and NPC2, cause a fatal neurodegenerative disease associated with abnormal cholesterol accumulation in cells. npc1a, the Drosophila NPC1 ortholog, regulates sterol homeostasis and is essential for molting hormone (20-hydroxyecdysone; 20E) biosynthesis. While only one npc2 gene is present in yeast, worm, mouse and human genomes, a family of eight npc2 genes (npc2a-h) exists in Drosophila. Among the encoded proteins, Npc2a has the broadest expression pattern and is most similar in sequence to vertebrate Npc2. Mutation of npc2a results in abnormal sterol distribution in many cells, as in Drosophila npc1a or mammalian NPC mutant cells. In contrast to the ecdysteroid-deficient, larval-lethal phenotype of npc1a mutants, npc2a mutants are viable and fertile with relatively normal ecdysteroid level. Mutants in npc2b, another npc2 gene, are also viable and fertile, with no significant sterol distribution abnormality. However, npc2a; npc2b double mutants are not viable but can be rescued by feeding the mutants with 20E or cholesterol, the basic precursor of 20E. We conclude that npc2a functions redundantly with npc2b in regulating sterol homeostasis and ecdysteroid biosynthesis, probably by controlling the availability of sterol substrate. Moreover, npc2a; npc2b double mutants undergo apoptotic neurodegeneration, thus constituting a new fly model of human neurodegenerative disease.

Receptor-mediated and bulk-phase endocytosis cause macrophage and cholesterol accumulation in Niemann-Pick C disease

These studies explored the roles of receptor-mediated and bulk-phase endocytosis as well as macrophage infiltration in the accumulation of cholesterol in the mouse with Niemann-Pick type C (NPC) disease. Uptake of LDL-cholesterol varied from 514 microg/day in the liver to zero in the central nervous system. In animals lacking LDL receptors, liver uptake remained about the same (411 microg/day), but more cholesterol was taken up in extrahepatic organs. This uptake was unaffected by the reductive methylation of LDL and consistent with bulk-phase endocytosis. All tissues accumulated cholesterol in mice lacking NPC1 function, but this accumulation was decreased in adrenal, unchanged in liver, and increased in organs like spleen and lung when LDL receptor function was also deleted. Over 56 days, the spleen and lung accumulated amounts of cholesterol greater than predicted, and these organs were heavily infiltrated with macrophages. This accumulation of both cholesterol and macrophages was increased by deleting LDL receptor function. These observations indicate that both receptor-mediated and bulk-phase endocytosis of lipoproteins, as well as macrophage infiltration, contribute to the cholesterol accumulation seen in NPC disease. These macrophages may also play a role in parenchymal cell death in this syndrome.

Cholesterol efflux via HDL resecretion occurs when cholesterol transport out of the lysosome is impaired

Recently, we showed that holo HDL particle uptake and resecretion occur in physiologically relevant cell lines and that HDL uptake is mediated by scavenger receptor class B type I (SR-BI). Furthermore, we established that HDL resecretion is accompanied by [(3)H]cholesterol efflux. This study shows that HDL uptake and resecretion occur even when LDL uptake and cholesterol trafficking are disturbed. First, we used a set of inhibitors that block cholesterol transport out of the lysosome: chloroquine, imipramine, U18666A, and monensin. In all cases, HDL retroendocytosis occurred and HDL resecretion mediated [(3)H]cholesterol efflux, although to a lesser extent. Second, cell lines carrying somatic mutations in intracellular cholesterol transport were used: CHO 2-2 and CHO 3-6 cells accumulated LDL-derived lipid in the lysosome but showed all components of HDL retroendocytosis. SR-BI overexpression increased HDL uptake and resecretion and [(3)H]cholesterol efflux in these mutant cells. Finally, we used Niemann-Pick type C (NPC) patient fibroblast cells, which carry a defect in cholesterol transfer out of the lysosome. NPC fibroblast cells accumulate cholesterol in the lysosome as a result of a mutation in the NPC1 gene. Despite disturbed intracellular cholesterol transfer, NPC fibroblast cells exhibited HDL retroendocytosis and [(3)H]cholesterol efflux via HDL resecretion, although to a lesser extent. Thus, [(3)H]cholesterol efflux via HDL resecretion is independent of the cholesterol uptake pathway via the LDL receptor and may be an alternative way to remove excess cholesterol.

Structural basis of sterol binding by NPC2, a lysosomal protein deficient in Niemann-Pick type C2 disease

NPC2 is a small lysosomal glycoprotein that binds cholesterol with submicromolar affinity. Deficiency in NPC2 is the cause of Niemann-Pick type C2 disease, a fatal neurovisceral disorder characterized by accumulation of cholesterol in lysosomes. Here we report the crystal structure of bovine NPC2 bound to cholesterol-3-O-sulfate, an analog that binds with greater apparent affinity than cholesterol. Structures of both apo-bound and sterol-bound NPC2 were observed within the same crystal lattice, with an asymmetric unit containing one molecule of apoNPC2 and two molecules of sterol-bound NPC2. As predicted from a previously determined structure of apoNPC2, the sterol binds in a deep hydrophobic pocket sandwiched between the two beta-sheets of NPC2, with only the sulfate substituent of the ligand exposed to solvent. In the two available structures of apoNPC2, the incipient ligand-binding pocket, which ranges from a loosely packed hydrophobic core to a small tunnel, is too small to accommodate cholesterol. In the presence of sterol, the pocket expands, facilitated by a slight separation of the beta-strands and substantial reorientation of some side chains, resulting in a perfect molding of the pocket around the hydrocarbon portion of cholesterol. A notable feature is the repositioning of two aromatic residues at the tunnel entrance that are essential for NPC2 function. The NPC2 structures provide evidence of a malleable binding site, consistent with the previously documented broad range of sterol ligand specificity.

Misassembled mutant DeltaF508 CFTR in the distal secretory pathway alters cellular lipid trafficking

Most patients with cystic fibrosis (CF) have a single codon deletion (DeltaF508) in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) that impairs assembly of the multidomain glycoprotein. The mutant protein escapes endoplasmic reticulum (ER) quality control at low temperature, but is rapidly cleared from the distal secretory pathway and degraded in lysosomes. CF cells accumulate free cholesterol similar to Niemann-Pick disease type C cells. We show that this lipid alteration is caused by the presence of misassembled mutant CFTR proteins, including DeltaF508, in the distal secretory pathway rather than the absence of functional CFTR. By contrast, cholesterol distribution is not changed by either D572N CFTR, which does not mature even at low temperature, or G551D, which is processed normally but is inactive. On expression of the DeltaF508 mutant, cholesterol and glycosphingolipids accumulate in punctate endosomal structures and cholesterol esters are reduced, indicating a block in the translocation of cholesterol to the ER for esterification. This is overcome by Rab9 overexpression, resulting in clearance of accumulating intracellular cholesterol. Similar but less pronounced alterations in intracellular cholesterol distribution are observed on expression of a temperature-rescued mutant variant of the related ATP-binding cassette (ABC) protein multidrug resistance-associated protein 1 (MRP1). Thus, on escape from ER quality control, misassembled mutants of CFTR and MRP1 impair lipid homeostasis in endocytic compartments.

NPC2, the Protein Deficient in Niemann-Pick C2 Disease, Consists of Multiple Glycoforms That Bind a Variety of Sterols

Niemann-Pick C disease is a fatal neurodegenerative disorder characterized by an endolysosomal accumulation of cholesterol and other lipids. One form of the disease is caused by a deficiency in NPC2, a soluble lysosomal glycoprotein that binds cholesterol. To better understand the biological function of NPC2 and how its deficiency results in disease, we have characterized the structural and functional properties of recombinant human protein. Highly purified NPC2 consists of a complex mixture of glycosylated isoforms, similar to that observed in human brain autopsy specimens. Mass spectrometric analysis revealed that of the three potential N-linked glycosylation sites present in the mature protein, Asn-19 is not utilized; Asn-39 is linked to an endoglycosidase H (Endo H)-sensitive oligosaccharide, and Asn-116 is variably utilized, either being unmodified or linked to Endo H-sensitive or Endo H-resistant oligosaccharides. All glycoforms are endocytosed and ameliorate the cholesterol storage phenotype of NPC2-deficient fibroblasts. In addition, the purified preparation contains a mixture of both free and lipid-bound protein. All glycoforms bind cholesterol, and sterol binding to NPC2 significantly alters its behavior upon cation-exchange chromatography. Based on this observation, we developed chromatography-based binding assays and determined that NPC2 forms an equimolar complex with the fluorescent cholesterol analog dehydroergosterol. In addition, we find that NPC2 binds a range of cholesterol-related molecules (cholesterol precursors, plant sterols, some oxysterols, cholesterol sulfate, cholesterol acetate, and 5-alpha-cholestan-3-one) and that 27-hydroxysterol accumulates in NPC2-deficient mouse liver. Binding was not detected for various glycolipids, phospholipids, or fatty acids. These biochemical properties support a direct and specialized function of NPC2 in lysosomal sterol transport.