The subcellular localization of the Niemann-Pick Type C proteins depends on the adaptor complex AP-3

Retinal cells derived from patients with DRAM2-dependent CORD21 dystrophy exhibit key lysosomal enzyme deficiency and lysosomal content accumulation

Biallelic mutations in DRAM2 lead to an autosomal recessive cone-rod dystrophy known as CORD21, which typically presents between the third and sixth decades of life. Although DRAM2 localizes to the lysosomes of photoreceptor and retinal pigment epithelium (RPE) cells, its specific role in retinal degeneration has not been fully elucidated. In this study, we generated and characterized retinal organoids (ROs) and RPE cells from induced pluripotent stem cells (iPSCs) derived from two CORD21 patients. Our investigation revealed that CORD21-ROs and RPE cells exhibit abnormalities in lipid metabolism, defects in autophagic flux, accumulation of aberrant lysosomal content, and reduced lysosomal enzyme activity. We identified potential interactions of DRAM2 with vesicular trafficking proteins, suggesting its involvement in this cellular process. These findings collectively suggest that DRAM2 plays a crucial role in maintaining the integrity of photoreceptors and RPE cells by regulating lysosomal function, autophagy, and potentially vesicular trafficking.

A lysosomal biogenesis map reveals the cargo spectrum of yeast vacuolar protein targeting pathways

The lysosome is the major catabolic organelle in the cell that has been established as a key metabolic signaling center. Mutations in many lysosomal proteins have catastrophic effects and cause neurodegeneration, cancer, and age-related diseases. The vacuole is the lysosomal analog of Saccharomyces cerevisiae that harbors many evolutionary conserved proteins. Proteins reach vacuoles via the Vps10-dependent endosomal vacuolar protein sorting pathway, via the alkaline phosphatase (ALP or AP-3) pathway, and via the cytosol-to-vacuole transport (CVT) pathway. A systematic understanding of the cargo spectrum of each pathway is completely lacking. Here, we use quantitative proteomics of purified vacuoles to generate the yeast lysosomal biogenesis map. This dataset harbors information on the cargo-receptor relationship of almost all vacuolar proteins. We map binding motifs of Vps10 and the AP-3 complex and identify a novel cargo of the CVT pathway under nutrient-rich conditions. Our data show how organelle purification and quantitative proteomics can uncover fundamental insights into organelle biogenesis.

Sterol uptake by the NPC system in eukaryotes: a Saccharomyces cerevisiae perspective

Sterols are an essential component of membranes in all eukaryotic cells and the precursor of multiple indispensable cellular metabolites. After endocytotic uptake, sterols are integrated into the lysosomal membrane by the Niemann-Pick type C (NPC) system before redistribution to other membranes. The process is driven by two proteins that, together, compose the NPC system: the lysosomal sterol shuttle protein NPC2 and the membrane protein NPC1 (named NCR1 in fungi), which integrates sterols into the lysosomal membrane. The Saccharomyces cerevisiae NPC system provides a compelling model to study the molecular mechanism of sterol integration into membranes and sterol homeostasis. This review summarizes recent advances in the field, and by interpreting available structural data, we propose a unifying conceptual model for sterol loading, transfer and transport by NPC proteins.

RNF13 Dileucine Motif Variants L311S and L312P Interfere with Endosomal Localization and AP-3 Complex Association

Developmental and epileptic encephalopathies (DEE) are rare and serious neurological disorders characterized by severe epilepsy with refractory seizures and a significant developmental delay. Recently, DEE73 was linked to genetic alterations of the RNF13 gene, which convert positions 311 or 312 in the RNF13 protein from leucine to serine or proline, respectively (L311S and L312P). Using a fluorescence microscopy approach to investigate the molecular and cellular mechanisms affected by RNF13 protein variants, the current study shows that wild-type RNF13 localizes extensively with endosomes and lysosomes, while L311S and L312P do not extensively colocalize with the lysosomal marker Lamp1. Our results show that RNF13 L311S and L312P proteins affect the size of endosomal vesicles along with the temporal and spatial progression of fluorescently labeled epidermal growth factor, but not transferrin, in the endolysosomal system. Furthermore, GST-pulldown and co-immunoprecipitation show that RNF13 variants disrupt association with AP-3 complex. Knockdown of AP-3 complex subunit AP3D1 alters the lysosomal localization of wild-type RNF13 and similarly affects the size of endosomal vesicles. Importantly, our study provides a first step toward understanding the cellular and molecular mechanism altered by DEE73-associated genetic variations of RNF13.

A systematic approach to identify recycling endocytic cargo depending on the GARP complex

Proteins and lipids of the plasma membrane underlie constant remodeling via a combination of the secretory- and the endocytic pathway. In the yeast endocytic pathway, cargo is sorted for recycling to the plasma membrane or degradation in vacuoles. Previously we have shown a role for the GARP complex in sphingolipid sorting and homeostasis (Fröhlich et al. 2015). However, the majority of cargo sorted in a GARP dependent process remain largely unknown. Here we use auxin induced degradation of GARP combined with mass spectrometry based vacuolar proteomics and lipidomics to show that recycling of two specific groups of proteins, the amino-phospholipid flippases and cell wall synthesis proteins depends on a functional GARP complex. Our results suggest that mis-sorting of flippases and remodeling of the lipid composition are the first occurring defects in GARP mutants. Our assay can be adapted to systematically map cargo of the entire endocytic pathway.

Mechanisms of Filovirus Entry

Filovirus entry into cells is complex, perhaps as complex as any viral entry mechanism identified to date. However, over the past 10 years, the important events required for filoviruses to enter into the endosomal compartment and fuse with vesicular membranes have been elucidated (Fig. 1). Here, we highlight the important steps that are required for productive entry of filoviruses into mammalian cells.

The AP-3 adaptor complex mediates sorting of yeast and mammalian PQ-loop-family basic amino acid transporters to the vacuolar/lysosomal membrane

The limiting membrane of lysosomes in animal cells and that of the vacuole in yeast include a wide variety of transporters, but little is known about how these proteins reach their destination membrane. The mammalian PQLC2 protein catalyzes efflux of basic amino acids from the lysosome, and the similar Ypq1, −2, and −3 proteins of yeast perform an equivalent function at the vacuole. We here show that the Ypq proteins are delivered to the vacuolar membrane via the alkaline phosphatase (ALP) trafficking pathway, which requires the AP-3 adaptor complex. When traffic via this pathway is deficient, the Ypq proteins pass through endosomes from where Ypq1 and Ypq2 properly reach the vacuolar membrane whereas Ypq3 is missorted to the vacuolar lumen via the multivesicular body pathway. When produced in yeast, PQLC2 also reaches the vacuolar membrane via the ALP pathway, but tends to sort to the vacuolar lumen if AP-3 is defective. Finally, in HeLa cells, inhibiting the synthesis of an AP-3 subunit also impairs sorting of PQLC2 to lysosomes. Our results suggest the existence of a conserved AP-3-dependent trafficking pathway for proper delivery of basic amino acid exporters to the yeast vacuole and to lysosomes of human cells.

Lysosomal Targeting of Cystinosin Requires AP-3

Cystinosin is a lysosomal cystine transporter defective in cystinosis, an autosomal recessive lysosomal storage disorder. It is composed of seven transmembrane (TM) domains and contains two lysosomal targeting motifs: a tyrosine-based signal (GYDQL) in its C-terminal tail and a non-classical motif in its fifth inter-TM loop. Using the yeast two-hybrid system, we showed that the GYDQL motif specifically interacted with the μ subunit of the adaptor protein complex 3 (AP-3). Moreover, cell surface biotinylation and total internal reflection fluorescence microscopy revealed that cystinosin was partially mislocalized to the plasma membrane (PM) in AP-3-depleted cells. We generated a chimeric CD63 protein to specifically analyze the function of the GYDQL motif. This chimeric protein was targeted to lysosomes in a manner similar to cystinosin and was partially mislocalized to the PM in AP-3 knockdown cells where it also accumulated in the trans-Golgi network and early endosomes. Together with the fact that the surface levels of cystinosin and of the CD63-GYDQL chimeric protein were not increased when clathrin-mediated endocytosis was impaired, our data show that the tyrosine-based motif of cystinosin is a 'strong' AP-3 interacting motif responsible for lysosomal targeting of cystinosin by a direct intracellular pathway.

The cytosolic adaptor AP-1A is essential for the trafficking and function of Niemann-Pick type C proteins

Niemann-Pick type C (NPC) disease is a fatal neurodegenerative disorder characterized by over-accumulation of low-density lipoprotein-derived cholesterol and glycosphingolipids in late endosomes/lysosomes (LE/L) throughout the body. Human mutations in either NPC1 or NPC2 genes have been directly associated with impaired cholesterol efflux from LE/L. Independent from its role in cholesterol homeostasis and its NPC2 partner, NPC1 was unexpectedly identified as a critical player controlling intracellular entry of filoviruses such as Ebola. In this study, a yeast three-hybrid system revealed that the NPC1 cytoplasmic tail directly interacts with the clathrin adaptor protein AP-1 via its acidic/di-leucine motif. Consequently, a nonfunctional AP-1A cytosolic complex resulted in a typical NPC-like phenotype mainly due to a direct impairment of NPC1 trafficking to LE/L and a partial secretion of NPC2. Furthermore, the mislocalization of NPC1 was not due to cholesterol accumulation in LE/L, as it was not rescued upon treatment with Mβ-cyclodextrin, which almost completely eliminated intracellular free cholesterol. Our cumulative data demonstrate that the cytosolic clathrin adaptor AP-1A is essential for the lysosomal targeting and function of NPC1 and NPC2.

Defective HIV-1 particle assembly in AP-3-deficient cells derived from patients with Hermansky-Pudlak syndrome type 2

Adaptor protein complex 3 (AP-3) is a heterotetramer that is involved in signal-mediated protein sorting to endosomal-lysosomal organelles. AP-3 deficiency in humans, induced by mutations in the AP3B1 gene, which encodes the β3A subunit of the AP-3 complex, results in Hermansky-Pudlak syndrome 2 (HPS2), which is a rare genetic disorder with defective lysosome-related organelles. In a previous study, we identified the AP-3 complex as an important contributor to HIV-1 assembly and release. We hypothesized that cells from patients affected by HPS2 should demonstrate abnormalities of HIV-1 assembly. Here we report that HIV-1 particle assembly and release are indeed diminished in HPS2 fibroblast cultures. Transient or stable expression of the full-length wild-type β3A subunit in HPS2 fibroblasts restored the impaired virus assembly and release. In contrast, virus-like particle release mediated by MA-deficient Gag mutants lacking the AP-3 binding site was not altered in HPS2 cells, indicating that the MA domain serves as the major viral determinant required for the recruitment of the AP-3 complex. AP-3 deficiency decreased HIV-1 Gag localization at the plasma membrane and late endosomes and increased the accumulation of HIV-1 Gag at an intermediate step between early and late endosomes. Blockage of the clathrin-mediated endocytic pathway in HPS2 cells did not reverse the inhibited virus assembly and release imposed by the AP-3 deficiency. These results demonstrate that the intact and stable AP-3 complex is required for HIV-1 assembly and release, and the involvement of the AP-3 complex in late stages of the HIV-1 replication cycle is independent of clathrin-mediated endocytosis.

AP-3 regulates PAR1 ubiquitin-independent MVB/lysosomal sorting via an ALIX-mediated pathway

A GPCR ubiquitin-independent MVB/lysosomal sorting pathway is regulated by the adaptor protein complex-3 (AP-3) and ALIX, a noncanonical ESCRT component. AP-3 binds to a PAR1 C-tail–localized, tyrosine-based motif and mediates PAR1 lysosomal degradation. AP-3 also facilitates PAR1 interaction with ALIX, suggesting that AP-3 functions before PAR1 engagement of ALIX and MVB/lysosomal sorting.

The sorting of signaling receptors within the endocytic system is important for appropriate cellular responses. After activation, receptors are trafficked to early endosomes and either recycled or sorted to lysosomes and degraded. Most receptors trafficked to lysosomes are modified with ubiquitin and recruited into an endosomal subdomain enriched in hepatocyte growth factor–regulated tyrosine kinase substrate (HRS), a ubiquitin-binding component of the endosomal-sorting complex required for transport (ESCRT) machinery, and then sorted into intraluminal vesicles (ILVs) of multivesicular bodies (MVBs)/lysosomes. However, not all receptors use ubiquitin or the canonical ESCRT machinery to sort to MVBs/lysosomes. This is exemplified by protease-activated receptor-1 (PAR1), a G protein–coupled receptor for thrombin, which sorts to lysosomes independent of ubiquitination and HRS. We recently showed that the adaptor protein ALIX binds to PAR1, recruits ESCRT-III, and mediates receptor sorting to ILVs of MVBs. However, the mechanism that initiates PAR1 sorting at the early endosome is not known. We now report that the adaptor protein complex-3 (AP-3) regulates PAR1 ubiquitin-independent sorting to MVBs through an ALIX-dependent pathway. AP-3 binds to a PAR1 cytoplasmic tail–localized tyrosine-based motif and mediates PAR1 lysosomal degradation independent of ubiquitination. Moreover, AP-3 facilitates PAR1 interaction with ALIX, suggesting that AP-3 functions before PAR1 engagement of ALIX and MVB/lysosomal sorting.

Ebola virus entry requires the host-programmed recognition of an intracellular receptor

Ebola and Marburg filoviruses cause deadly outbreaks of haemorrhagic fever. Despite considerable efforts, no essential cellular receptors for filovirus entry have been identified. We showed previously that Niemann-Pick C1 (NPC1), a lysosomal cholesterol transporter, is required for filovirus entry. Here, we demonstrate that NPC1 is a critical filovirus receptor. Human NPC1 fulfills a cardinal property of viral receptors: it confers susceptibility to filovirus infection when expressed in non-permissive reptilian cells. The second luminal domain of NPC1 binds directly and specifically to the viral glycoprotein, GP, and a synthetic single-pass membrane protein containing this domain has viral receptor activity. Purified NPC1 binds only to a cleaved form of GP that is generated within cells during entry, and only viruses containing cleaved GP can utilize a receptor retargeted to the cell surface. Our findings support a model in which GP cleavage by endosomal cysteine proteases unmasks the binding site for NPC1, and GP-NPC1 engagement within lysosomes promotes a late step in entry proximal to viral escape into the host cytoplasm. NPC1 is the first known viral receptor that recognizes its ligand within an intracellular compartment and not at the plasma membrane.

Characterization of the Niemann-Pick C pathway in alveolar type II cells and lamellar bodies of the lung

The Niemann-Pick C (NPC) pathway plays an essential role in the intracellular trafficking of cholesterol by facilitating the release of lipoprotein-derived sterol from the lumen of lysosomes. Regulation of cellular cholesterol homeostasis is of particular importance to lung alveolar type II cells because of the need for production of surfactant with an appropriate lipid composition. We performed microscopic and biochemical analysis of NPC proteins in isolated rat type II pneumocytes. NPC1 and NPC2 proteins were present in the lung, isolated type II cells in culture, and alveolar macrophages. The glycosylated and nonglycosylated forms of NPC1 were prominent in the lung and the lamellar body organelles. Immunocytochemical analysis of isolated type II pneumocytes showed localization of NPC1 to the limiting membrane of lamellar bodies. NPC2 and lysosomal acid lipase were found within these organelles, as confirmed by z-stack analysis of confocal images. All three proteins also were identified in small, lysosome-like vesicles. In the presence of serum, pharmacological inhibition of the NPC pathway with compound U18666A resulted in doubling of the cholesterol content of the type II cells. Filipin staining revealed a striking accumulation of cholesterol within lamellar bodies. Thus the NPC pathway functions to control cholesterol accumulation in lamellar bodies of type II pneumocytes and, thereby, may play a role in the regulation of surfactant cholesterol content.

Lack of efficacy of curcumin on neurodegeneration in the mouse model of Niemann-Pick C1

In order to determine the efficacy of curcumin in ameliorating symptoms of neurodegeneration in the mouse model of Niemann-Pick C1, a variety of formulations and dosages of curcumin, one comparable to one previously reported as efficacious, were provided orally to Npc1(-/-)mice. Plasma levels of curcumin, survival, tests of motor performance, and memory (in some cases) were performed. We found variable, but mild, increases in survival (1.5% to 18%). The greatest increased survival occurred with the highest dose (which was unformulated) while the control for the lipidated formulation (containing phosphatidylcholine and stearic acid) had an equivalent impact and other formulations, while not significantly increased, are also not statistically different in effect from the highest dose. We conclude that curcumin is not a highly efficacious treatment for neurodegeneration in Npc1(-/-) mice. Phosphatidylcholine and stearic acid should be studied further.

Endosome maturation

Being deeply connected to signalling, cell dynamics, growth, regulation, and defence, endocytic processes are linked to almost all aspects of cell life and disease. In this review, we focus on endosomes in the classical endocytic pathway, and on the programme of changes that lead to the formation and maturation of late endosomes/multivesicular bodies. The maturation programme entails a dramatic transformation of these dynamic organelles disconnecting them functionally and spatially from early endosomes and preparing them for their unidirectional role as a feeder pathway to lysosomes.

Lysosomal membrane protein composition, acidic pH and sterol content are regulated via a light-dependent pathway in metazoan cells

In metazoans, lysosomes are characterized by a unique tubular morphology, acidic pH, and specific membrane protein (LAMP) and lipid (cholesterol) composition as well as a soluble protein (hydrolases) composition. Here we show that perturbation to the eye-color gene, light, results in impaired lysosomal acidification, sterol accumulation, altered endosomal morphology as well as compromised lysosomal degradation. We find that Drosophila homologue of Vps41, Light, regulates the fusion of a specific subset of biosynthetic carriers containing characteristic endolysosomal membrane proteins, LAMP1, V0-ATPase and the cholesterol transport protein, NPC1, with the endolysosomal system, and is then required for the morphological progression of the multivesicular endosome. Inhibition of Light results in accumulation of biosynthetic transport intermediates that contain these membrane cargoes, whereas under similar conditions, endosomal delivery of soluble hydrolases, previously shown to be mediated by Dor, the Drosophila homologue of Vps18, is not affected. Unlike Dor, Light is recruited to endosomes in a PI3P-sensitive fashion wherein it facilitates fusion of these biosynthetic cargoes with the endosomes. Depletion of the mammalian counterpart of Light, hVps41, in a human cell line also inhibits delivery of hLAMP to endosomes, suggesting an evolutionarily conserved pathway in metazoa.

Role of AP1 and Gadkin in the traffic of secretory endo-lysosomes

Whereas lysosome-related organelles (LRO) of specialized cells display both exocytic and endocytic features, lysosomes in nonspecialized cells can also acquire the property to fuse with the plasma membrane upon an acute rise in cytosolic calcium. Here, we characterize this unconventional secretory pathway in fibroblast-like cells, by monitoring the appearance of Lamp1 on the plasma membrane and the release of lysosomal enzymes into the medium. After sequential ablation of endocytic compartments in living cells, we find that donor membranes primarily derive from a late compartment, but that an early compartment is also involved. Strikingly, this endo-secretory process is not affected by treatments that inhibit endosome dynamics (microtubule depolymerization, cholesterol accumulation, overexpression of Rab7 or its effector Rab-interacting lysosomal protein [RILP], overexpression of Rab5 mutants), but depends on Rab27a, a GTPase involved in LRO secretion, and is controlled by F-actin. Moreover, we find that this unconventional endo-secretory pathway requires the adaptor protein complexes AP1, Gadkin (which recruits AP1 by binding to the γ1 subunit), and AP2, but not AP3. We conclude that a specific fraction of the AP2-derived endocytic pathway is dedicated to secretory purposes under the control of AP1 and Gadkin.

Npc1 is involved in sterol trafficking in the filamentous fungus Fusarium graminearum

The ortholog of the human gene NPC1 was identified in the plant pathogenic, filamentous fungus Fusarium graminearum by shared amino acid sequence, protein domain structure and cellular localization of the mature fungal protein. The FusariumNpc1 gene shares 34% amino acid sequence identity and 51% similarity to the human gene, has similar domain structure and is constitutively expressed, although up-regulated in ungerminated macroconidia and ascospores. GFP-tagged Npc1p localizes to the fungal vacuolar membrane. Cultures derived from a Δnpc1 mutant strain contain significantly more ergosterol than cultures of the wildtype. Staining with the fluorescent, sterol binding dye filipin, shows that ergosterol accumulates in vacuoles of the Δnpc1 mutant but not the wildtype strain. The Δnpc1 mutant has a temperature dependent reduction in growth and greater sensitivity to the ergosterol synthesis inhibiting fungicide tebuconazole compared with the wildtype strain or the mutant complemented with wildtype Npc1. The mutant also is significantly reduced in pathogenicity to wheat. Our results are consistent with the interpretation that Npc1p is important for normal transport of ergosterol from the vacuole and is essential for proper membrane function under particular environmental conditions.

Pdro, a protein associated with late endosomes and lysosomes and implicated in cellular cholesterol homeostasis

BACKGROUND

Cellular cholesterol is a vital component of the cell membrane. Its concentration is tightly controlled by mechanisms that remain only partially characterized. In this study, we describe a late endosome/lysosomes-associated protein whose expression level affects cellular free cholesterol content.

METHODOLOGY/PRINCIPAL FINDINGS

Using a restricted proteomic analysis of detergent-resistant membranes (DRMs), we have identified a protein encoded by gene C11orf59. It is mainly localized to late endosome/lysosome (LE/LY) compartment through N-terminal myristoylation and palmitoylation. We named it Pdro for protein associated with DRMs and endosomes. Very recently, three studies have reported on the same protein under two other names: the human p27RF-Rho that regulates RhoA activation and actin dynamics, and its rodent orthologue p18 that controls both LE/LY dynamics through the MERK-ERK pathway and the lysosomal activation of mammalian target of rapamycin complex 1 by amino acids. We found that, consistent with the presence of sterol-responsive element consensus sequences in the promoter region of C11orf59, Pdro mRNA and protein expression levels are regulated positively by cellular cholesterol depletion and negatively by cellular cholesterol loading. Conversely, Pdro is involved in the regulation of cholesterol homeostasis, since its depletion by siRNA increases cellular free cholesterol content that is accompanied by an increased cholesterol efflux from cells. On the other hand, cells stably overexpressing Pdro display reduced cellular free cholesterol content. Pdro depletion-mediated excess cholesterol results, at least in part, from a stimulated low-density lipoprotein (LDL) uptake and an increased cholesterol egress from LE/LY.

CONCLUSIONS/SIGNIFICANCE

LDL-derived cholesterol release involves LE/LY motility that is linked to actin dynamics. Because Pdro regulates these two processes, we propose that modulation of Pdro expression in response to sterol levels regulates LDL-derived cholesterol through both LDL uptake and LE/LY dynamics, to ultimately control free cholesterol homeostasis.

Relationship between phosphatidylinositol 4-phosphate synthesis, membrane organization, and lateral diffusion of PI4KIIalpha at the trans-Golgi network

Type II phosphatidylinositol 4-kinase IIalpha (PI4KIIalpha) is the dominant phosphatidylinositol kinase activity measured in mammalian cells and has important functions in intracellular vesicular trafficking. Recently PI4KIIalpha has been shown to have important roles in neuronal survival and tumorigenesis. This study focuses on the relationship between membrane cholesterol levels, phosphatidylinositol 4-phosphate (PI4P) synthesis, and PI4KIIalpha mobility. Enzyme kinetic measurements, sterol substitution studies, and membrane fragmentation analyses all revealed that cholesterol regulates PI4KIIalpha activity indirectly through effects on membrane structure. In particular, we found that cholesterol levels determined the distribution of PI4KIIalpha to biophysically distinct membrane domains. Imaging studies on cells expressing enhanced green fluorescent protein (eGFP)-tagged PI4KIIalpha demonstrated that cholesterol depletion resulted in morphological changes to the juxtanuclear membrane pool of the enzyme. Lateral membrane diffusion of eGFP-PI4KIIalpha was assessed by fluorescence recovery after photobleaching (FRAP) experiments, which revealed the existence of both mobile and immobile pools of the enzyme. Sterol depletion decreased the size of the mobile pool of PI4KIIalpha. Further measurements revealed that the reduction in the mobile fraction of PI4KIIalpha correlated with a loss of trans-Golgi network (TGN) membrane connectivity. We conclude that cholesterol modulates PI4P synthesis through effects on membrane organization and enzyme diffusion.

Cholesterol in Niemann-Pick Type C disease

Niemann-Pick Type C (NPC) disease is associated with accumulation of cholesterol and other lipids in late endosomes/lysosomes in virtually every organ; however, neurodegeneration represents the fatal cause for the disease. Genetic analysis has identified loss-of-function mutations in NPC1 and NPC2 genes as the molecular triggers for the disease. Although the precise function of these proteins has not yet been clarified, recent research suggests that they orchestrate cholesterol efflux from late endosomes/lysosomes. NPC protein deficits result in impairment in intracellular cholesterol trafficking and dysregulation of cholesterol biosynthesis. Disruption of cholesterol homeostasis is also associated with deregulation of autophagic activity and early-onset neuroinflammation, which may contribute to the pathogenesis of NPC disease. This chapter reviews recent achievements in the investigation of disruption of cholesterol homeostasis-induced neurodegeneration in NPC disease, and provides new insight for developing a potential therapeutic strategy for this disorder.

Nogo-B receptor stabilizes Niemann-Pick type C2 protein and regulates intracellular cholesterol trafficking

The Nogo-B receptor (NgBR) is a recently identified receptor for the N terminus of reticulon 4B/Nogo-B. Other than its role in binding Nogo-B, little is known about the biology of NgBR. To elucidate a basic cellular role for NgBR, we performed a yeast two-hybrid screen for interacting proteins, using the C-terminal domain as bait, and identified Niemann-Pick type C2 protein (NPC2) as an NgBR-interacting protein. NPC2 protein levels are increased in the presence of NgBR, and NgBR enhances NPC2 protein stability. NgBR localizes primarily to the endoplasmic reticulum (ER) and regulates the stability of nascent NPC2. RNAi-mediated disruption of NgBR or genetic deficiency in NgBR lead to a decrease in NPC2 levels, increased intracellular cholesterol accumulation, and a loss of sterol sensing, all hallmarks of an NPC2 mutation. These data identify NgBR as an NPC2-interacting protein and provide evidence of a role for NgBR in intracellular cholesterol trafficking.

AP-3-dependent trafficking and disease: the first decade

The adaptor protein (AP)-3 complex defines a pathway for the intracellular trafficking of membrane-associated proteins in most eukaryotic cells. Ten years ago, genetic defects in AP-3 were linked to a human Mendelian disease, named Hermansky-Pudlak syndrome, characterized by abnormal biogenesis and function of lysosome-related organelles such as melanosomes and platelet dense granules. During recent years, research on this trafficking pathway has significantly expanded its horizons to include evolutionarily divergent eukaryotic models and to embrace functional genomics and proteomics approaches. These studies have brought into focus ideas about the specific roles of this pathway in protein trafficking and organelle biogenesis within the endosomal-lysosomal system.

Deficiency of niemann-pick type C-1 protein impairs release of human immunodeficiency virus type 1 and results in Gag accumulation in late endosomal/lysosomal compartments

Human immunodeficiency virus type 1 (HIV-1) relies on cholesterol-laden lipid raft membrane microdomains for entry into and egress out of susceptible cells. In the present study, we examine the need for intracellular cholesterol trafficking pathways with respect to HIV-1 biogenesis using Niemann-Pick type C-1 (NPC1)-deficient (NPCD) cells, wherein these pathways are severely compromised, causing massive accumulation of cholesterol in late endosomal/lysosomal (LE/L) compartments. We have found that induction of an NPC disease-like phenotype through treatment of various cell types with the commonly used hydrophobic amine drug U18666A resulted in profound suppression of HIV-1 release. Further, NPCD Epstein-Barr virus-transformed B lymphocytes and fibroblasts from patients with NPC disease infected with a CD4-independent strain of HIV-1 or transfected with an HIV-1 proviral clone, respectively, replicated HIV-1 poorly compared to normal cells. Infection of the NPCD fibroblasts with a vesicular stomatitis virus G-pseudotyped strain of HIV-1 produced similar results, suggesting a postentry block to HIV-1 replication in these cells. Examination of these cells using confocal microscopy showed an accumulation and stabilization of Gag in LE/L compartments. Additionally, normal HIV-1 production could be restored in NPCD cells upon expression of a functional NPC1 protein, and overexpression of NPC1 increased HIV-1 release. Taken together, our findings demonstrate that intact intracellular cholesterol trafficking pathways mediated by NPC1 are needed for efficient HIV-1 production.

Niemann-Pick C2 (NPC2) and intracellular cholesterol trafficking

Cholesterol is an important precursor for numerous biologically active molecules, and it plays a major role in membrane structure and function. Cholesterol can be endogenously synthesized or exogenously taken up via the endocytic vesicle system and subsequently delivered to post-endo/lysosomal sites including the plasma membrane and the endoplasmic reticulum. Niemann-Pick C (NPC) disease results in the accumulation of exogenously-derived cholesterol, as well as other lipids, in late endosomes and lysosomes (LE/LY). Identification of the two genes that underlie NPC disease, NPC1 and NPC2, has focused attention on the mechanisms by which lipids, in particular cholesterol, are transported out of the LE/LY compartment. This review discusses the role of the NPC2 protein in cholesterol transport, and the potential for concerted action of NPC1 and NPC2 in regulating normal intracellular cholesterol homeostasis.

Dual sorting of the Saccharomyces cerevisiae vacuolar protein Sna4p

Sna4p, a vacuolar membrane protein, belongs to a small family of proteins conserved in plants and fungi. It is transported to the vacuolar membrane via the alkaline phosphatase (ALP) pathway, which bypasses the multivesicular bodies (MVBs). Here, we show that transfer of Sna4p by the ALP route involves the AP-3 adaptor protein complex, which binds to an acidic dileucine sorting signal in the cytoplasmic region of Sna4p. In addition, Sna4p can use the MVB pathway by using a PPPY motif, which is involved in the interaction with ubiquitin ligase Rsp5p. Deletion or mutation of the Sna4p PPPY motif or a low level of Rsp5p inhibits the entrance of Sna4p into MVBs. Sna4p is polyubiquitylated on its only lysine, and Sna4p lacking this lysine shows defective MVB sorting. These data indicate that Sna4p has two functional motifs, one for interaction with the AP-3 complex, followed by entry into the ALP pathway, and one for binding Rsp5p, which directs the protein to the MVB pathway. The presence of these two motifs allows Sna4p to localize to both the vacuolar membrane and the lumen.

Genome-wide analysis of AP-3-dependent protein transport in yeast

The evolutionarily conserved adaptor protein-3 (AP-3) complex mediates cargo-selective transport to lysosomes and lysosome-related organelles. To identify proteins that function in AP-3-mediated transport, we performed a genome-wide screen in Saccharomyces cerevisiae for defects in the vacuolar maturation of alkaline phosphatase (ALP), a cargo of the AP-3 pathway. Forty-nine gene deletion strains were identified that accumulated precursor ALP, many with established defects in vacuolar protein transport. Maturation of a vacuolar membrane protein delivered via a separate, clathrin-dependent pathway, was affected in all strains except those with deletions of YCK3, encoding a vacuolar type I casein kinase; SVP26, encoding an endoplasmic reticulum (ER) export receptor for ALP; and AP-3 subunit genes. Subcellular fractionation and fluorescence microscopy revealed ALP transport defects in yck3Delta cells. Characterization of svp26Delta cells revealed a role for Svp26p in ER export of only a subset of type II membrane proteins. Finally, ALP maturation kinetics in vac8Delta and vac17Delta cells suggests that vacuole inheritance is important for rapid generation of proteolytically active vacuolar compartments in daughter cells. We propose that the cargo-selective nature of the AP-3 pathway in yeast is achieved by AP-3 and Yck3p functioning in concert with machinery shared by other vacuolar transport pathways.

The yeast vacuolar membrane proteome

Transport of solutes between the cytosol and the vacuolar lumen is of crucial importance for various functions of vacuoles, including ion homeostasis; detoxification; storage of different molecules such as amino acids, phosphate, and calcium ions; and proteolysis. To identify proteins that catalyze solute transport across the vacuolar membrane, the membrane proteome of purified Saccharomyces cerevisiae vacuoles was analyzed. Subtractive proteomics was used to distinguish contaminants from true vacuolar proteins by comparing the relative abundances of proteins in pure and crude preparations. A robust statistical analysis combining enrichment ranking with the double boundary iterative group analysis revealed that 148 proteins were significantly enriched in the pure vacuolar preparations. Among these proteins were well characterized vacuolar proteins, such as the subunits of the vacuolar H(+)-ATPase, but also proteins that had not previously been assigned to a cellular location, many of which are likely novel vacuolar membrane transporters, e.g. for nucleosides and oligopeptides. Although the majority of contaminating proteins from other organelles were depleted from the pure vacuolar membranes, some proteins annotated to reside in other cellular locations were enriched along with the vacuolar proteins. In many cases the enrichment of these proteins is biologically relevant, and we discuss that a large group is involved in membrane fusion and protein trafficking to vacuoles and may have multiple localizations. Other proteins are degraded in vacuoles, and in some cases database annotations are likely to be incomplete or incorrect. Our work provides a wealth of information on vacuolar biology and a solid basis for further characterization of vacuolar functions.

Leishmania adaptor protein-1 subunits are required for normal lysosome traffic, flagellum biogenesis, lipid homeostasis, and adaptation to temperatures encountered in the mammalian host

The adaptor protein-1 (AP-1) complex is involved in membrane transport between the Golgi apparatus and endosomes. In the protozoan parasite Leishmania mexicana mexicana, the AP-1 mu1 and sigma1 subunits are not required for growth at 27 degrees C but are essential for infectivity in the mammalian host. In this study, we have investigated the function of these AP-1 subunits in order to understand the molecular basis for this loss of virulence. The mu1 and sigma1 subunits were localized to late Golgi and endosome membranes of the major parasite stages. Parasite mutants lacking either AP-1 subunit lacked obvious defects in Golgi structure, endocytosis, or exocytic transport. However, these mutants displayed reduced rates of endosome-to-lysosome transport and accumulated fragmented, sterol-rich lysosomes. Defects in flagellum biogenesis were also evident in nondividing promastigote stages, and this phenotype was exacerbated by inhibitors of sterol and sphingolipid biosynthesis. Furthermore, both AP-1 mutants were hypersensitive to elevated temperature and perturbations in membrane lipid composition. The pleiotropic requirements for AP-1 in membrane trafficking and temperature stress responses explain the loss of virulence of these mutants in the mammalian host.