Lipid and cholesterol trafficking in NPC

Functional analysis of a NPC1 gene from the whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae)

Niemann-Pick C (NPC) disease is a neurodegenerative disorder related to cellular sterol trafficking and mutation of NPC1 gene is the main cause for this disease. The function of NPC1 have been reported in a few insects but rarely studied in hemipterans. In the present study, we investigate the function of NPC1 in a hemipteran pest, the whitefly Bemisia tabaci. It was found that B. tabaci had only one NPC1 homolog (BtNPC1), in contrast to two homologs in many other insects. BtNPC1 was ubiquitously expressed at all developmental stages and body parts of whiteflies, with the highest level in adult abdomen, and the expression of BtNPC1 was induced by cholesterol feeding. To further investigate the function of BtNPC1, leaf-mediated RNA interference experiments were carried out. Results showed that knockdown of BtNPC1 led to reduced survival of whiteflies, as well as reduced fecundity. Moreover, knockdown of BtNPC1 affected the development and metamorphosis of whitefly nymphs. Taken these together, we conclude that BtNPC1 played a crucial role in sterol-related biological processes of B. tabaci and might be used as an insecticide target for development of novel pest management approaches.

Regulation of Endosomal Trafficking by Rab7 and Its Effectors in Neurons: Clues from Charcot-Marie-Tooth 2B Disease

Intracellular endosomal trafficking controls the balance between protein degradation and synthesis, i.e., proteostasis, but also many of the cellular signaling pathways that emanate from activated growth factor receptors after endocytosis. Endosomal trafficking, sorting, and motility are coordinated by the activity of small GTPases, including Rab proteins, whose function as molecular switches direct activity at endosomal membranes through effector proteins. Rab7 is particularly important in the coordination of the degradative functions of the pathway. Rab7 effectors control endosomal maturation and the properties of late endosomal and lysosomal compartments, such as coordination of recycling, motility, and fusion with downstream compartments. The spatiotemporal regulation of endosomal receptor trafficking is particularly challenging in neurons because of their enormous size, their distinct intracellular domains with unique requirements (dendrites vs. axons), and their long lifespans as postmitotic, differentiated cells. In Charcot-Marie-Tooth 2B disease (CMT2B), familial missense mutations in Rab7 cause alterations in GTPase cycling and trafficking, leading to an ulcero-mutilating peripheral neuropathy. The prevailing hypothesis to account for CMT2B pathologies is that CMT2B-associated Rab7 alleles alter endocytic trafficking of the neurotrophin NGF and its receptor TrkA and, thereby, disrupt normal trophic signaling in the peripheral nervous system, but other Rab7-dependent pathways are also impacted. Here, using TrkA as a prototypical endocytic cargo, we review physiologic Rab7 effector interactions and control in neurons. Since neurons are among the largest cells in the body, we place particular emphasis on the temporal and spatial regulation of endosomal sorting and trafficking in neuronal processes. We further discuss the current findings in CMT2B mutant Rab7 models, the impact of mutations on effector interactions or balance, and how this dysregulation may confer disease.

Barriers to exocytotic vesicle discharge

Regulated exocytosis, a universal process of eukaryotic cells, involves the merging between the vesicle membrane and the plasma membrane, plays a key role in cell-to-cell communication, particularly in the release of hormones and neurotransmitters. There are a number of barriers a vesicle needs to pass to discharge vesicle content to the extracellular space. At the pre-fusion site vesicles need to be transported to the sites on the plasma membrane where the merger may begin. Classically cytoskeleton was considered an important barrier for vesicle translocation and was thought to be disintegrated to allow vesicle access to the plasma membrane [1]. However, it was considered later that cytoskeletal elements may also play a role at the post-fusion stage, promoting the vesicle merger with the plasma membrane and fusion pore expansion [4,22,23]. In this Special Issue of Cell Calcium entitled "Regulated Exocytosis", the authors address outstanding issues related to vesicle chemical messenger release by regulated exocytosis, including that related to the question whether vesicle content discharge is complete or only partial upon the merging of the vesicle membrane with the plasma membrane triggered by Ca²⁺. Among processes that limit vesicle discharge at the post-fusion stage is the accumulation of cholesterol in some vesicles [19], a process that has recently been associated with cell aging [20].

ER as master regulator of membrane trafficking and organelle function

The endoplasmic reticulum (ER), which occupies a large portion of the cytoplasm, is the cell’s main site for the biosynthesis of lipids and carbohydrate conjugates, and it is essential for folding, assembly, and biosynthetic transport of secreted proteins and integral membrane proteins. The discovery of abundant membrane contact sites (MCSs) between the ER and other membrane compartments has revealed that, in addition to its biosynthetic and secretory functions, the ER plays key roles in the regulation of organelle dynamics and functions. In this review, we will discuss how the ER regulates endosomes, lysosomes, autophagosomes, mitochondria, peroxisomes, and the Golgi apparatus via MCSs. Such regulation occurs via lipid and Ca2+ transfer and also via control of in trans dephosphorylation reactions and organelle motility, positioning, fusion, and fission. The diverse controls of other organelles via MCSs manifest the ER as master regulator of organelle biology.

A novel sterol-binding protein reveals heterogeneous cholesterol distribution in neurite outgrowth and in late endosomes/lysosomes

We identified a mushroom-derived protein, maistero-2 that specifically binds 3-hydroxy sterol including cholesterol (Chol). Maistero-2 bound lipid mixture in Chol-dependent manner with a binding threshold of around 30%. Changing lipid composition did not significantly affect the threshold concentration. EGFP-maistero-2 labeled cell surface and intracellular organelle Chol with higher sensitivity than that of well-established Chol probe, D4 fragment of perfringolysin O. EGFP-maistero-2 revealed increase of cell surface Chol during neurite outgrowth and heterogeneous Chol distribution between CD63-positive and LAMP1-positive late endosomes/lysosomes. The absence of strictly conserved Thr-Leu pair present in Chol-dependent cytolysins suggests a distinct Chol-binding mechanism for maistero-2.

Identification of novel mutations among Iranian NPC1 patients: a bioinformatics approach to predict pathogenic mutations

Background

Niemann-Pick disease type C (NPC) is a rare lysosomal neurovisceral storage disease caused by mutations in the NPC 1 (95%) or NPC2 (5%) genes. The products of NPC1 and NPC2 genes play considerable roles in glycolipid and cholesterol trafficking, which could consequently lead to NPC disease with variable phenotypes displaying a broad spectrum of symptoms.

Materials

In the present study 35 Iranian NPC unrelated patients were enrolled. These patients were first analysed by the Filipin Staining test of cholesterol deposits in cells for NPC diagnostics. Genomic DNA was extracted from the samples of peripheral blood leukocytes in EDTA following the manufacturer's protocol. All exon–intron boundaries and coding exons of the NPC1gene were amplified by polymerase chain reaction (PCR) using appropriate sets of primers. Thereafter, the products of PCR were sequenced and analysed using the NCBI database (https://blast.ncbi.nlm.nih.gov/Blast.cgi). The variants were reviewed by some databases including the Human Gene Mutation Database (HGMD) (http://www.hgmd.cf.ac.uk/ac/index.php) and ClinVar (https://www.ncbi.nlm.nih.gov/clinvar (. Moreover, all the variants were manually classified in terms of the American College of Medical Genetics and Genomics (ACMG) guideline.

Results

The sequence analysis revealed 20 different variations, 10 of which are new, including one nonsense mutation (c.406C > T); three small deletions, (c.3126delC, c.2920_2923delCCTG, and c.2037delG); and six likely pathogenic missense mutations, (c.542C > A, c.1970G > A, c.1993C > G, c.2821 T > C, c.2872C > G, and c.3632 T > A). Finally, the pathogenicity of these new variants was determined using the ACMG guidelines.

Conclusion

The present study aimed to facilitate the prenatal diagnosis of NPC patients in the future. In this regard, we identified 10 novel mutations, and verified that the majority of them occurred in six NPC1 exons (5, 8, 9, 13, 19, and 21), that should be considered with a high priority for Iranian patients' cost-effective evaluation.

Vesicle cholesterol controls exocytotic fusion pore

In some lysosomal storage diseases (LSD) cholesterol accumulates in vesicles. Whether increased vesicle cholesterol affects vesicle fusion with the plasmalemma, where the fusion pore, a channel between the vesicle lumen and the extracellular space, is formed, is unknown. Super-resolution microscopy revealed that after stimulation of exocytosis, pituitary lactotroph vesicles discharge cholesterol which transfers to the plasmalemma. Cholesterol depletion in lactotrophs and astrocytes, both exhibiting Ca²⁺-dependent exocytosis regulated by distinct Ca²⁺sources, evokes vesicle secretion. Although this treatment enhanced cytosolic levels of Ca²⁺ in lactotrophs but decreased it in astrocytes, this indicates that cholesterol may well directly define the fusion pore. In an attempt to explain this mechanism, a new model of cholesterol-dependent fusion pore regulation is proposed. High-resolution membrane capacitance measurements, used to monitor fusion pore conductance, a parameter related to fusion pore diameter, confirm that at resting conditions reducing cholesterol increases, while enrichment with cholesterol decreases the conductance of the fusion pore. In resting fibroblasts, lacking the Npc1 protein, a cellular model of LSD in which cholesterol accumulates in vesicles, the fusion pore conductance is smaller than in controls, showing that vesicle cholesterol controls fusion pore and is relevant for pathophysiology of LSD.

Late endosomal/lysosomal accumulation of a neurotransmitter receptor in a cellular model of Smith-Lemli-Opitz syndrome

Smith-Lemli-Opitz syndrome (SLOS) is a congenital and developmental malformation syndrome associated with defective cholesterol biosynthesis. It is characterized by accumulation of 7-dehydrocholesterol (the immediate biosynthetic precursor of cholesterol in the Kandutsch-Russell pathway) and an altered cholesterol to total sterol ratio. Because SLOS is associated with neurological malfunction, exploring the function and trafficking of neuronal receptors and their interaction with membrane lipids under these conditions assume significance. In this work, we generated a cellular model of SLOS in HEK-293 cells stably expressing the human serotonin_1A receptor (an important neurotransmitter G-protein coupled receptor) using AY 9944, an inhibitor for the enzyme 3β-hydroxy-steroid-∆⁷ -reductase (7-DHCR). Using a quantitative flow cytometry based assay, we show that the plasma membrane population of serotonin_1A receptors was considerably reduced under these conditions without any change in total cellular expression of the receptor. Interestingly, the receptors were trafficked to sterol-enriched LysoTracker positive compartments, which accumulated under these conditions. To the best of our knowledge, our results constitute one of the first reports demonstrating intracellular accumulation and misregulated traffic of a neurotransmitter GPCR in SLOS-like conditions. We believe these results assume relevance in our overall understanding of the molecular basis underlying the functional relevance of neurotransmitter receptors in SLOS.

Secretory NPC2 Protein-Mediated Free Cholesterol Levels Were Correlated with the Sorafenib Response in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is the most common primary malignant tumor in the world. Sorafenib is the first-line drug for patients with advanced HCC. However, long-term treatment with sorafenib often results in reduced sensitivity of tumor cells to the drug, leading to acquired resistance. Identifying biomarkers which can predict the response to sorafenib treatment may represent a clinical challenge in the personalized treatment era. Niemann-Pick type C2 (NPC2), a secretory glycoprotein, plays an important role in regulating intracellular free cholesterol homeostasis. In HCC patients, downregulation of hepatic NPC2 is correlated with poor clinical pathological features through regulating mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) activation. This study aimed to investigate the roles of secretory NPC2-mediated free cholesterol levels as biomarkers when undergoing sorafenib treatment and evaluate its impact on acquired sorafenib resistance in HCC cells. Herein, we showed that NPC2 downregulation and free cholesterol accumulation weakened sorafenib’s efficacy through enhancing MAPK/AKT signaling in HCC cells. Meanwhile, NPC2 overexpression slightly enhanced the sorafenib-induced cytotoxic effect. Compared to normal diet feeding, mice fed a high-cholesterol diet had much higher tumor growth rates, whereas treatment with the free cholesterol-lowering agent, hydroxypropyl-β-cyclodextrin, enhanced sorafenib’s tumor-inhibiting ability. In addition, sorafenib treatment induced higher NPC2 secretion, which was mediated by inhibition of the Ras/Raf/MAPK kinase (MEK)/ERK signaling pathway in HCC cells. In both acquired sorafenib-resistant cell and xenograft models, NPC2 and free cholesterol secretion were increased in culture supernatant and serum samples. In conclusion, NPC2-mediated free cholesterol secretion may represent a candidate biomarker for the likelihood of HCC cells developing resistance to sorafenib.

Brain-specific lipoprotein receptors interact with astrocyte derived apolipoprotein and mediate neuron-glia lipid shuttling

Lipid shuttling between neurons and glia contributes to the development, function, and stress responses of the nervous system. To understand how a neuron acquires its lipid supply from specific lipoproteins and their receptors, we perform combined genetic, transcriptome, and biochemical analyses in the developing Drosophila larval brain. Here we report, the astrocyte-derived secreted lipocalin Glial Lazarillo (GLaz), a homolog of human Apolipoprotein D (APOD), and its neuronal receptor, the brain-specific short isoforms of Drosophila lipophorin receptor 1 (LpR1-short), cooperatively mediate neuron-glia lipid shuttling and support dendrite morphogenesis. The isoform specificity of LpR1 defines its distribution, binding partners, and ability to support proper dendrite growth and synaptic connectivity. By demonstrating physical and functional interactions between GLaz/APOD and LpR1, we elucidate molecular pathways mediating lipid trafficking in the fly brain, and provide in vivo evidence indicating isoform-specific expression of lipoprotein receptors as a key mechanism for regulating cell-type specific lipid recruitment.

Exploiting the Potential of Drosophila Models in Lysosomal Storage Disorders: Pathological Mechanisms and Drug Discovery

Lysosomal storage disorders (LSDs) represent a complex and heterogeneous group of rare genetic diseases due to mutations in genes coding for lysosomal enzymes, membrane proteins or transporters. This leads to the accumulation of undegraded materials within lysosomes and a broad range of severe clinical features, often including the impairment of central nervous system (CNS). When available, enzyme replacement therapy slows the disease progression although it is not curative; also, most recombinant enzymes cannot cross the blood-brain barrier, leaving the CNS untreated. The inefficient degradative capability of the lysosomes has a negative impact on the flux through the endolysosomal and autophagic pathways; therefore, dysregulation of these pathways is increasingly emerging as a relevant disease mechanism in LSDs. In the last twenty years, different LSD Drosophila models have been generated, mainly for diseases presenting with neurological involvement. The fruit fly provides a large selection of tools to investigate lysosomes, autophagy and endocytic pathways in vivo, as well as to analyse neuronal and glial cells. The possibility to use Drosophila in drug repurposing and discovery makes it an attractive model for LSDs lacking effective therapies. Here, ee describe the major cellular pathways implicated in LSDs pathogenesis, the approaches available for their study and the Drosophila models developed for these diseases. Finally, we highlight a possible use of LSDs Drosophila models for drug screening studies.

Investigating the Mechanism of Cyclodextrins in the Treatment of Niemann-Pick Disease Type C Using Crosslinked 2-Hydroxypropyl-β-cyclodextrin

Niemann-Pick disease type C (NPC) is a severe disorder that is characterized by intracellular transport abnormalities leading to cytoplasmic accumulation of lipids such as cholesterol and sphingolipids. The compound 2-hydroxypropyl-β-cyclodextrin (HPβCD) has high cholesterol complexation capacity and is currently under clinical investigation for the NPC treatment. However, due to its short blood half-life, high doses are required to produce a therapeutic effect. In this work, stable polymerized HPβCD is generated to investigate their in vitro mechanisms of action and in vivo effects. Crosslinked CDs (8-312 kDa) display a ninefold greater cholesterol complexation capacity than monomeric HPβCD but are taken up to a lower extent, resulting in an overall comparable in vitro effect. In vivo, the 19.3 kDa HPβCD exhibits a longer half-life than the monomeric HPβCD but it does not increase the life span of Npc1 mice, possibly due to reduced brain penetration. This is circumvented by the application of magnetic resonance imaging-guided low intensity-pulsed focused ultrasound (MRIg-FUS), which increases the brain penetration of the CD. In conclusion, stable polymerized HPβCDs can elucidate CDs' mechanism of action while the use of MRIg-FUS warrants further investigation, as it may be key to harnessing CDs full therapeutic potential in the NPC treatment.

Insect Sterol Nutrition: Physiological Mechanisms, Ecology, and Applications

Insects, like all eukaryotes, require sterols for structural and metabolic purposes. However, insects, like all arthropods, cannot make sterols. Cholesterol is the dominant tissue sterol for most insects; insect herbivores produce cholesterol by metabolizing phytosterols, but not always with high efficiency. Many insects grow on a mixed-sterol diet, but this ability varies depending on the types and ratio of dietary sterols. Dietary sterol uptake, transport, and metabolism are regulated by several proteins and processes that are relatively conserved across eukaryotes. Sterol requirements also impact insect ecology and behavior. There is potential to exploit insect sterol requirements to (a) control insect pests in agricultural systems and (b) better understand sterol biology, including in humans. We suggest that future studies focus on the genetic mechanism of sterol metabolism and reverse transportation, characterizing sterol distribution and function at the cellular level, the role of bacterial symbionts in sterol metabolism, and interrupting sterol trafficking for pest control.

The role of dopamine in the pathogenesis of GBA1-linked Parkinson's disease

Our understanding of the molecular mechanisms underlying differential vulnerability of substantia nigra dopamine neurons in Parkinson's disease (PD) remains limited, and previous therapeutic efforts targeting rodent nigral neurons have not been successfully translated to humans. However, recent emergence of induced pluripotent stem cell technology has highlighted some fundamental differences between human and rodent midbrain dopamine neurons that may at least in part explain relative resistance of rodent neurons to degeneration in genetic models of PD. Using GBA1-linked PD as an example, we discuss cellular pathways that may predispose human neurons to degeneration in PD, including mitochondrial oxidant stress, elevated intracellular calcium, altered synaptic vesicle endocytosis, accumulation of oxidized dopamine and neuromelanin. Recent studies have suggested that a combination of mitochondrial oxidant stress and accumulation of oxidized dopamine contribute to dysfunction of nigral neurons in various genetic and sporadic forms of PD. We also briefly summarize the development of targeted therapies for GBA1-associated synucleinopathies and highlight that modulation of wild-type GCase activity serves as an important target for the treatment of genetic and idiopathic forms of PD and dementia with Lewy bodies.

Genome and transcriptome analyses providing insight into the immune response of pearl oysters after allograft and xenograft transplantations

The immune response after allograft or xenograft transplantation in the pearl oyster is a major factor that cause its nucleus rejection and death. To determine the mechanism underlying the immune response after allograft and xenograft transplantations in the pearl oyster Pinctada fucata martensii, we constructed two sets of transcriptomes of hemocytes at different times (6 and 12 h; 1, 3, 6, 12, and 30 d) after allograft and xenograft transplantations, in which the xenografted mantle tissue was from Pinctada maxima. The transcriptomic analysis reveals many genes are involved in the immune response to transplantation, such as transient receptor potential cation channel (TRP), calmodulin (CaM), DNA replication-related genes, and sugar and lipid metabolism-related genes. The expression of these identified genes was higher in the host pearl oyster transplanted with xenograft than that by allograft. The histological analysis of the pearl sac also confirmed that many hemocytes were still gathered around the transplanted nucleus, and no pearl sac was formed in the host pearl oysters at 30 d after xenograft transplantation. The genomic analysis indicated that pearl oysters evolved many copies of genes, such as TRP, CaM, and GST, to sense and cope with the immune response after transplantation. "Ribosome" and "Cytosolic DNA-sensing pathway" were specifically induced in the xenograft group, whereas "Notch signaling pathway" specifically responded to the allograft transplantation. These results can improve our understanding of the mechanism underlying the immune response of pearl oysters after allograft and xenograft transplantations.

The Late Endosomal Pathway Regulates the Ciliary Targeting of Tetraspanin Protein Peripherin 2

Peripherin 2 (PRPH2) is a tetraspanin protein concentrated in the light-sensing cilium (called the outer segment) of the vertebrate photoreceptor. The mechanism underlying the ciliary targeting of PRPH2 and the etiology of cone dystrophy caused by PRPH2 mutations remain elusive. Here we show that the late endosome (LE) is the main waystation that critically sorts newly synthesized PRPH2 to the cilium. PRPH2 is expressed in the luminal membrane of the LE. We delineate multiple C-terminal motifs of PRPH2 that distinctively regulate its LE and ciliary targeting through ubiquitination and binding to ESCRT (Endosomal Sorting Complexes Required for Transport) component Hrs. Using the newly developed TetOn-inducible system in transfected male and female mouse cones in vivo, we show that the entry of nascent PRPH2 into the cone outer segment can be blocked by either cone dystrophy-causing C-terminal mutations of PRPH2, or by short-term perturbation of the LE or recycling endosomal traffic. These findings open new avenues of research to explore the biological role of the LE in the biosynthetic pathway and the etiology of cone dystrophy caused by PRPH2 mutations and/or malfunctions of the LE.SIGNIFICANCE STATEMENT Peripherin 2 (PRPH2) is a tetraspanin protein abundantly expressed in the light-sensing cilium, the outer segment, of the vertebrate photoreceptor. The mechanism underlying the ciliary transport of PRPH2 is unclear. The present study reveals a novel ciliary targeting pathway, in which the newly synthesized PRPH2 is first targeted to the lumen of the late endosome (LE) en route to the cilia. We deciphered the protein motifs and the machinery that regulates the LE trafficking of PRPH2. Using a novel TetOn-inducible system in transfected mouse cones, we showed that the LE pathway of PRPH2 is critical for its outer segment expression. A cone dystrophy-causing mutation impairs the LE and ciliary targeting of PRPH2, implicating the relevance of LE to cone/macular degenerative diseases.

Coronin 1C promotes triple-negative breast cancer invasiveness through regulation of MT1-MMP traffic and invadopodia function

Membrane type 1-matrix metalloproteinase (MT1-MMP), a membrane-tethered protease, is key for matrix breakdown during cancer invasion and metastasis. Assembly of branched actin networks by the Arp2/3 complex is required for MT1-MMP traffic and formation of matrix-degradative invadopodia. Contrasting with the well-established role of actin filament branching factor cortactin in invadopodia function during cancer cell invasion, the contribution of coronin-family debranching factors to invadopodia-based matrix remodeling is not known. Here, we investigated the contribution of coronin 1C to the invasive potential of breast cancer cells. We report that expression of coronin 1C is elevated in invasive human breast cancers, correlates positively with MT1-MMP expression in relation with increased metastatic risk and is a new independent prognostic factor in breast cancer. We provide evidence that, akin to cortactin, coronin 1C is required for invadopodia formation and matrix degradation by breast cancer cells lines and for 3D collagen invasion by multicellular spheroids. Using intravital imaging of orthotopic human breast tumor xenografts, we find that coronin 1C accumulates in structures forming in association with collagen fibrils in the tumor microenvironment. Moreover, we establish the role of coronin 1C in the regulation of positioning and trafficking of MT1-MMP-positive endolysosomes. These results identify coronin 1C as a novel player of the multi-faceted mechanism responsible for invadopodia formation, MT1-MMP surface exposure and invasiveness in breast cancer cells.

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.

Hepatitis C Virus Replication Depends on Endosomal Cholesterol Homeostasis

Similar to other positive-strand RNA viruses, hepatitis C virus (HCV) causes massive rearrangements of intracellular membranes, resulting in a membranous web (MW) composed of predominantly double-membrane vesicles (DMVs), the presumed sites of RNA replication. DMVs are enriched for cholesterol, but mechanistic details on the source and recruitment of cholesterol to the viral replication organelle are only partially known. Here we focused on selected lipid transfer proteins implicated in direct lipid transfer at various endoplasmic reticulum (ER)-membrane contact sites. RNA interference (RNAi)-mediated knockdown identified several hitherto unknown HCV dependency factors, such as steroidogenic acute regulatory protein-related lipid transfer domain protein 3 (STARD3), oxysterol-binding protein-related protein 1A and -B (OSBPL1A and -B), and Niemann-Pick-type C1 (NPC1), all residing at late endosome and lysosome membranes and required for efficient HCV RNA replication but not for replication of the closely related dengue virus. Focusing on NPC1, we found that knockdown or pharmacological inhibition caused cholesterol entrapment in lysosomal vesicles concomitant with decreased cholesterol abundance at sites containing the viral replicase factor NS5A. In untreated HCV-infected cells, unesterified cholesterol accumulated at the perinuclear region, partially colocalizing with NS5A at DMVs, arguing for NPC1-mediated endosomal cholesterol transport to the viral replication organelle. Consistent with cholesterol being an important structural component of DMVs, reducing NPC1-dependent endosomal cholesterol transport impaired MW integrity. This suggests that HCV usurps lipid transfer proteins, such as NPC1, at ER-late endosome/lysosome membrane contact sites to recruit cholesterol to the viral replication organelle, where it contributes to MW functionality.IMPORTANCE A key feature of the replication of positive-strand RNA viruses is the rearrangement of the host cell endomembrane system to produce a membranous replication organelle (RO). The underlying mechanisms are far from being elucidated fully. In this report, we provide evidence that HCV RNA replication depends on functional lipid transport along the endosomal-lysosomal pathway that is mediated by several lipid transfer proteins, such as the Niemann-Pick type C1 (NPC1) protein. Pharmacological inhibition of NPC1 function reduced viral replication, impaired the transport of cholesterol to the viral replication organelle, and altered organelle morphology. Besides NPC1, our study reports the importance of additional endosomal and lysosomal lipid transfer proteins required for viral replication, thus contributing to our understanding of how HCV manipulates their function in order to generate a membranous replication organelle. These results might have implications for the biogenesis of replication organelles of other positive-strand RNA viruses.

Lipid metabolism fattens up hedgehog signaling

Signaling pathways direct organogenesis, often through concentration-dependent effects on cells. The hedgehog pathway enables cells to sense and respond to hedgehog ligands, of which the best studied is sonic hedgehog. Hedgehog signaling is essential for development, proliferation, and stem cell maintenance, and it is a driver of certain cancers. Lipid metabolism has a profound influence on both hedgehog signal transduction and the properties of the ligands themselves, leading to changes in the strength of hedgehog signaling and cellular functions. Here we review the evolving understanding of the relationship between lipids and hedgehog signaling.

Psychosine enhances the shedding of membrane microvesicles: Implications in demyelination in Krabbe's disease

In prior studies, our laboratory showed that psychosine accumulates and disrupts lipid rafts in brain membranes of Krabbe’s disease. A model of lipid raft disruption helped explaining psychosine’s effects on several signaling pathways important for oligodendrocyte survival and differentiation but provided more limited insight in how this sphingolipid caused demyelination. Here, we have studied how this cationic inverted coned lipid affects the fluidity, stability and structure of myelin and plasma membranes. Using a combination of cutting-edge imaging techniques in non-myelinating (red blood cell), and myelinating (oligodendrocyte) cell models, we show that psychosine is sufficient to disrupt sphingomyelin-enriched domains, increases the rigidity of localized areas in the plasma membrane, and promotes the shedding of membranous microvesicles. The same physicochemical and structural changes were measured in myelin membranes purified from the mutant mouse Twitcher, a model for Krabbe’s disease. Areas of higher rigidity were measured in Twitcher myelin and correlated with higher levels of psychosine and of myelin microvesiculation. These results expand our previous analyses and support, for the first time a pathogenic mechanism where psychosine’s toxicity in Krabbe disease involves deregulation of cell signaling not only by disruption of membrane rafts, but also by direct local destabilization and fragmentation of the membrane through microvesiculation. This model of membrane disruption may be fundamental to introduce focal weak points in the myelin sheath, and consequent diffuse demyelination in this leukodystrophy, with possible commonality to other demyelinating disorders.

Elevated Cholesterol in the Coxiella burnetii Intracellular Niche Is Bacteriolytic

Coxiella burnetii is an intracellular bacterial pathogen and a significant cause of culture-negative endocarditis in the United States. Upon infection, the nascent Coxiella phagosome fuses with the host endocytic pathway to form a large lysosome-like vacuole called the parasitophorous vacuole (PV). The PV membrane is rich in sterols, and drugs perturbing host cell cholesterol homeostasis inhibit PV formation and bacterial growth. Using cholesterol supplementation of a cholesterol-free cell model system, we found smaller PVs and reduced Coxiella growth as cellular cholesterol concentration increased. Further, we observed in cells with cholesterol a significant number of nonfusogenic PVs that contained degraded bacteria, a phenotype not observed in cholesterol-free cells. Cholesterol had no effect on axenic Coxiella cultures, indicating that only intracellular bacteria are sensitive to cholesterol. Live-cell microscopy revealed that both plasma membrane-derived cholesterol and the exogenous cholesterol carrier protein low-density lipoprotein (LDL) traffic to the PV. To test the possibility that increasing PV cholesterol levels affects bacterial survival, infected cells were treated with U18666A, a drug that traps cholesterol in lysosomes and PVs. U18666A treatment led to PVs containing degraded bacteria and a significant loss in bacterial viability. The PV pH was significantly more acidic in cells with cholesterol or cells treated with U18666A, and the vacuolar ATPase inhibitor bafilomycin blocked cholesterol-induced PV acidification and bacterial death. Additionally, treatment of infected HeLa cells with several FDA-approved cholesterol-altering drugs led to a loss of bacterial viability, a phenotype also rescued by bafilomycin. Collectively, these data suggest that increasing PV cholesterol further acidifies the PV, leading to Coxiella death.

The intracellular Gram-negative bacterium Coxiella burnetii is a significant cause of culture-negative infectious endocarditis, which can be fatal if untreated. The existing treatment strategy requires prolonged antibiotic treatment, with a 10-year mortality rate of 19% in treated patients. Therefore, new clinical therapies are needed and can be achieved by better understanding C. burnetii pathogenesis. Upon infection of host cells, C. burnetii grows within a specialized replication niche, the parasitophorous vacuole (PV). Recent data have linked cholesterol to intracellular C. burnetii growth and PV formation, leading us to further decipher the role of cholesterol during C. burnetii-host interaction. We observed that increasing PV cholesterol concentration leads to increased acidification of the PV and bacterial death. Further, treatment with FDA-approved drugs that alter host cholesterol homeostasis also killed C. burnetii through PV acidification. Our findings suggest that targeting host cholesterol metabolism might prove clinically efficacious in controlling C. burnetii infection.

Lipids regulate the hydrolysis of membrane bound glucosylceramide by lysosomal β-glucocerebrosidase

Glucosylceramide (GlcCer) is the primary storage lipid in the lysosomes of Gaucher patients and a secondary one in Niemann-Pick disease types A, B, and C. The regulatory roles of lipids on the hydrolysis of membrane bound GlcCer by lysosomal β-glucocerebrosidase (GBA1) was probed using a detergent-free liposomal assay. The degradation rarely occurs at uncharged liposomal surfaces in the absence of saposin (Sap) C. However, anionic lipids stimulate GlcCer hydrolysis at low pH by up to 1,000-fold depending on the nature and position of the negative charges in their head groups while cationic lipids inhibit the degradation, thus showing the importance of electrostatic interactions between the polycationic GBA1 and the negatively charged vesicle surfaces at low pH. Ceramide, fatty acids, monoacylglycerol, and diacylglycerol also stimulate GlcCer hydrolysis while SM, sphingosine, and sphinganine play strong inhibitory roles, thereby explaining the secondary storage of GlcCer in Niemann-Pick diseases. Surprisingly, cholesterol stimulates GlcCer degradation in the presence of bis(monoacylglycero)phosphate (BMP). Sap C strongly stimulates GlcCer hydrolysis even in the absence of BMP and the regulatory roles of the intraendolysosomal lipids on its activity is discussed. Our data suggest that these strong modifiers of GlcCer hydrolysis affect the genotype-phenotype correlation in several cases of Gaucher patients independent of the types.

Quantitative Co-Localization and Pattern Analysis of Endo-Lysosomal Cargo in Subcellular Image Cytometry and Validation on Synthetic Image Sets

Late endosomes and lysosomes (LE/LYSs) play a central role in trafficking of endocytic cargo, secretion of exosomes, and hydrolysis of ingested proteins and lipids. Failure in such processes can lead to lysosomal storage disorders in which a particular metabolite accumulates within LE/LYSs. Analysis of endocytic trafficking relies heavily on quantitative fluorescence microscopy, but evaluation of the huge image data sets is challenging and demands computer-assisted statistical tools. Here, we describe how to use SpatTrack ( www.sdu.dk/bmb/spattrack ), an imaging toolbox, which we developed for quantification of the distribution and dynamics of endo-lysosomal cargo from fluorescence images of living cells. First, we explain how to analyze experimental images of endocytic processes in Niemann Pick C2 disease fibroblasts using SpatTrack. We demonstrate how to quantify the location of the sterol-binding protein NPC2 in LE/LYSs relative to cholesterol -rich lysosomal storage organelles (LSOs) stained with filipin. Second, we show how to simulate realistic vesicle patterns in the cell geometry using Markov Chain Monte Carlo and suitable inter-vesicle and cell-vesicle interaction potentials. Finally, we use such synthetic vesicle patterns as "ground truth" for validation of two-channel analysis tools in SpatTrack, revealing their high reliability. An improved version of SpatTrack for microscopy-based quantification of cargo transport through the endo-lysosomal system accompanies this protocol.

Disorders in the initial steps of steroid hormone synthesis

Steroidogenesis begins with cellular internalization of low-density lipoprotein particles and subsequent intracellular processing of cholesterol. Disorders in these steps include Adrenoleukodystrophy, Wolman Disease and its milder variant Cholesterol Ester Storage Disease, and Niemann-Pick Type C Disease, all of which may present with adrenal insufficiency. The means by which cholesterol is directed to steroidogenic mitochondria remains incompletely understood. Once cholesterol reaches the outer mitochondrial membrane, its delivery to the inner mitochondrial membrane is regulated by the steroidogenic acute regulatory protein (StAR). Severe StAR mutations cause classic congenital lipoid adrenal hyperplasia, characterized by lipid accumulation in the adrenal, adrenal insufficiency, and disordered sexual development in 46,XY individuals. The lipoid CAH phenotype, including spontaneous puberty in 46,XX females, is explained by a two-hit model. StAR mutations that retain partial function cause a milder, non-classic disease characterized by glucocorticoid deficiency, with lesser disorders of mineralocorticoid and sex steroid synthesis. Once inside the mitochondria, cholesterol is converted to pregnenolone by the cholesterol side-chain cleavage enzyme, P450scc, encoded by the CYP11A1 gene. Rare patients with mutations of P450scc are clinically and hormonally indistinguishable from those with lipoid CAH, and may also present as milder non-classic disease. Patients with P450scc defects do not have the massive adrenal hyperplasia that characterizes lipoid CAH, but adrenal imaging may occasionally fail to distinguish these, necessitating DNA sequencing.

Mitochondrial cholesterol import

All animal subcellular membranes require cholesterol, which influences membrane fluidity and permeability, fission and fusion processes, and membrane protein function. The distribution of cholesterol among subcellular membranes is highly heterogeneous and the cholesterol content of each membrane must be carefully regulated. Compared to other subcellular membranes, mitochondrial membranes are cholesterol-poor, particularly the inner mitochondrial membrane (IMM). As a result, steroidogenesis can be controlled through the delivery of cholesterol to the IMM, where it is converted to pregnenolone. The low basal levels of cholesterol also make mitochondria sensitive to changes in cholesterol content, which can have a relatively large impact on the biophysical and functional characteristics of mitochondrial membranes. Increased mitochondrial cholesterol levels have been observed in diverse pathological conditions including cancer, steatohepatitis, Alzheimer disease and Niemann-Pick Type C1-deficiency, and are associated with increased oxidative stress, impaired oxidative phosphorylation, and changes in the susceptibility to apoptosis, among other alterations in mitochondrial function. Mitochondria are not included in the vesicular trafficking network; therefore, cholesterol transport to mitochondria is mostly achieved through the activity of lipid transfer proteins at membrane contact sites or by cytosolic, diffusible lipid transfer proteins. Here we will give an overview of the main mechanisms involved in mitochondrial cholesterol import, focusing on the steroidogenic acute regulatory protein StAR/STARD1 and other members of the StAR-related lipid transfer (START) domain protein family, and we will discuss how changes in mitochondrial cholesterol levels can arise and affect mitochondrial function. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Niemann-Pick Type C2 Protein Mediates Hepatic Stellate Cells Activation by Regulating Free Cholesterol Accumulation

In chronic liver diseases, regardless of their etiology, the development of fibrosis is the first step toward the progression to cirrhosis, portal hypertension, and hepatocellular carcinoma. Hepatic stellate cells (HSCs) are the main profibrogenic cells that promote the pathogenesis of liver fibrosis, and so it is important to identify the molecules that regulate HSCs activation and liver fibrosis. Niemann-Pick type C2 (NPC2) protein plays an important role in the regulation of intracellular cholesterol homeostasis by directly binding with free cholesterol. However, the roles of NPC2 in HSCs activation and liver fibrosis have not been explored in detail. Since a high-cholesterol diet exacerbates liver fibrosis progression in both rodents and humans, we propose that the expression of NPC2 affects free cholesterol metabolism and regulates HSCs activation. In this study, we found that NPC2 is decreased in both thioacetamide- and carbon tetrachloride-induced liver fibrosis tissues. In addition, NPC2 is expressed in quiescent HSCs, but its activation status is down-regulated. Knockdown of NPC2 in HSC-T6 cells resulted in marked increases in transforming growth factor-β1 (TGF-β1)-induced collagen type 1 α1 (Col1a1), α-smooth muscle actin (α-SMA) expression, and Smad2 phosphorylation. In contrast, NPC2 overexpression decreased TGF-β1-induced HSCs activation. We further demonstrated that NPC2 deficiency significantly increased the accumulation of free cholesterol in HSCs, increasing Col1a1 and α-SMA expression and activating Smad2, and leading to sensitization of HSCs to TGF-β1 activation. In contrast, overexpression of NPC2 decreased U18666A-induced free cholesterol accumulation and inhibited the subsequent HSCs activation. In conclusion, our study has demonstrated that NPC2 plays an important role in HSCs activation by regulating the accumulation of free cholesterol. NPC2 overexpression may thus represent a new treatment strategy for liver fibrosis.

Fluorescent Sterols and Cholesteryl Esters as Probes for Intracellular Cholesterol Transport

Cholesterol transport between cellular organelles comprised vesicular trafficking and nonvesicular exchange; these processes are often studied by quantitative fluorescence microscopy. A major challenge for using this approach is producing analogs of cholesterol with suitable brightness and structural and chemical properties comparable with those of cholesterol. This review surveys currently used fluorescent sterols with respect to their behavior in model membranes, their photophysical properties, as well as their transport and metabolism in cells. In the first part, several intrinsically fluorescent sterols, such as dehydroergosterol or cholestatrienol, are discussed. These polyene sterols (P-sterols) contain three conjugated double bonds in the steroid ring system, giving them slight fluorescence in ultraviolet light. We discuss the properties of P-sterols relative to cholesterol, outline their chemical synthesis, and explain how to image them in living cells and organisms. In particular, we show that P-sterol esters inserted into low-density lipoprotein can be tracked in the fibroblasts of Niemann–Pick disease using high-resolution deconvolution microscopy. We also describe fluorophore-tagged cholesterol probes, such as BODIPY-, NBD-, Dansyl-, or Pyrene-tagged cholesterol, and eventual esters of these analogs. Finally, we survey the latest developments in the synthesis and use of alkyne cholesterol analogs to be labeled with fluorophores by click chemistry and discuss the potential of all approaches for future applications.

Antioxidant and renoprotective activity of 2-hydroxypropyl-β-cyclodextrin in nephrectomized rats

OBJECTIVES

Oxidative stress is known to be involved in the pathogenesis of chronic renal failure (CRF). In this study, the effect of cyclodextrins (CDs) on oxidative stress and CRF was investigated using 5/6 nephrectomized rats as model animals.

METHODS

CRF model rats were divided into five groups and treated for 8 weeks as follows: control, α-CD, β-CD, γ-CD and 2-hydroxypropyl-β-CD (HP-β-CD). Blood was collected from the rats after 4 and 8 weeks for an analysis of renal function and oxidative stress tests were carried out.

KEY FINDINGS

An oral administration of HP-β-CD over an 8-week period resulted in a significant decrease in serum indoxyl sulphate, creatinine and urea nitrogen levels, compared with the other CDs. The ingestion of HP-β-CD also resulted in an increase in antioxidant potential, compared with the other CDs. In in vitro studies, the interaction of HP-β-CD with a uremic toxin, indole molecule, was much higher than that for the other CDs, as evidenced by Proton nuclear magnetic resonance ((1) H NMR) measurements.

CONCLUSIONS

These results suggest that the ingestion of HP-β-CD might result in a significant reduction in the levels of pro-oxidants in the gastrointestinal tract, such as uremic toxins, thereby inhibiting the subsequent development of oxidative stress in the systemic circulation.

ER-endosome contact sites: molecular compositions and functions

Recent studies have revealed the existence of numerous contact sites between the endoplasmic reticulum (ER) and endosomes in mammalian cells. Such contacts increase during endosome maturation and play key roles in cholesterol transfer, endosome positioning, receptor dephosphorylation, and endosome fission. At least 7 distinct contact sites between the ER and endosomes have been identified to date, which have diverse molecular compositions. Common to these contact sites is that they impose a close apposition between the ER and endosome membranes, which excludes membrane fusion while allowing the flow of molecular signals between the two membranes, in the form of enzymatic modifications, or ion, lipid, or protein transfer. Thus, ER-endosome contact sites ensure coordination of molecular activities between the two compartments while keeping their general compositions intact. Here, we review the molecular architectures and cellular functions of known ER-endosome contact sites and discuss their implications for human health.

Niemann-Pick type C2 protein regulates liver cancer progression via modulating ERK1/2 pathway: Clinicopathological correlations and therapeutical implications

Primary hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide and the third leading cause of cancer-related death. It is important to identify new targets for early diagnosis and treatment of HCC. Niemann-Pick type C2 (NPC2) plays an important role in the regulation of intracellular cholesterol homeostasis via direct binding with free cholesterol. However, little is known about the significance of NPC2 in HCC tumorigenesis. In this study, we showed that NPC2 is abundantly expressed in normal liver, but is downregulated in human HCC tissues. The patients with NPC2 downregulation expressed much higher α-fetoprotein, multiple tumor type, vascular invasion, later pathological stage and shorter survival rate. Knockdown NPC2 in liver cancer cell lines promote cell proliferation, migration and xenograft tumorigenesis. In contrast, NPC2 overexpression inhibits HuH7 promoted tumor growth. Furthermore, administration of hepatotropic adeno-associated virus 8 (AAV8) delivered NPC2 decreased the inflammatory infiltration, the expression of two early HCC markers-glypican 3 and survivin and suppressed the spontaneous HCC development in mice. To identify the NPC2-dependent mechanism, we emphasized on the status of MAPK/ERK signaling. MEK1/2 inhibitor treatment demonstrated that the expression of NPC2 affected the activation of ERK1/2 but not MEK1/2. In addition, cholesterol trafficking inhibitor treatment did not alter the cell proliferation and the activation of MEK/ERK. In conclusion, our study demonstrates that NPC2 may play an important role in negatively regulate cell proliferation and ERK1/2 activation that were independent of cholesterol accumulation. AAV-NPC2 may thus represent a new treatment strategy for liver cancer.

Cholesterol shuttling is important for RNA replication of coxsackievirus B3 and encephalomyocarditis virus

Picornaviruses are a family of positive-strand RNA viruses that includes important human and animal pathogens. Upon infection, picornaviruses induce an extensive remodelling of host cell membranes into replication organelles (ROs), which is critical for replication. Membrane lipids and lipid remodelling processes are at the base of RO formation, yet their involvement remains largely obscure. Recently, phosphatidylinositol-4-phosphate was the first lipid discovered to be important for the replication of a number of picornaviruses. Here, we investigate the role of the lipid cholesterol in picornavirus replication. We show that two picornaviruses from distinct genera that rely on different host factors for replication, namely the enterovirus coxsackievirus B3 (CVB3) and the cardiovirus encephalomyocarditis virus (EMCV), both recruited cholesterol to their ROs. Although CVB3 and EMCV both required cholesterol for efficient genome replication, the viruses appeared to rely on different cellular cholesterol pools. Treatments that altered the distribution of endosomal cholesterol inhibited replication of both CVB3 and EMCV, showing the importance of endosomal cholesterol shuttling for the replication of these viruses. Summarizing, we here demonstrate the importance of cholesterol homeostasis for efficient replication of CVB3 and EMCV.

Changes in caries risk and activity of a 9-year-old patient with niemann-pick disease type C

Objective. This case report describes the changes in caries risk and activity and dental treatment of a 9-year-old patient who presented with signs and symptoms of Niemann-Pick disease type C (NPC). Treatment. The preventive dental treatment included instructions to caregivers for oral hygiene and diet. A calcium hydroxide pulpotomy and restorative dental treatments were performed in a dental office with desensitization techniques and behavioral management. The patient was attended every 3 months for the control of dental plaque biofilm, for topical fluoride application, and for observing the pulpotomized tooth. Results. The bacterial plaque biofilm was being adequately controlled by the caregiver. After 2 years, the clinical and radiographic examination of the pulpotomized tooth showed the absence of internal root resorption and bone rarefaction, and clinical examination showed tooth sensitivity, dental pain, and gingival swelling. Conclusion. The pulpotomy prevented clinical and radiographic success. Dentists must be aware of and be able to identify systemic and local aspects associated with caries risk of children with NPC disease. Furthermore, dentists must employ stringent preventive measures and provide instructions to caregivers to reduce caries risk.

Developmental ethanol exposure leads to dysregulation of lipid metabolism and oxidative stress in Drosophila

Ethanol exposure during development causes an array of developmental abnormalities, both physiological and behavioral. In mammals, these abnormalities are collectively known as fetal alcohol effects (FAE) or fetal alcohol spectrum disorder (FASD). We have established a Drosophila melanogaster model of FASD and have previously shown that developmental ethanol exposure in flies leads to reduced expression of insulin-like peptides (dILPs) and their receptor. In this work, we link that observation to dysregulation of fatty acid metabolism and lipid accumulation. Further, we show that developmental ethanol exposure in Drosophila causes oxidative stress, that this stress is a primary cause of the developmental lethality and delay associated with ethanol exposure, and, finally, that one of the mechanisms by which ethanol increases oxidative stress is through abnormal fatty acid metabolism. These data suggest a previously uncharacterized mechanism by which ethanol causes the symptoms associated with FASD.

Exon-skipping antisense oligonucleotides to correct missplicing in neurogenetic diseases

Alternative splicing is an important regulator of the transcriptome. However, mutations may cause alteration of splicing patterns, which in turn leads to disease. During the past 10 years, exon skipping has been looked upon as a powerful tool for correction of missplicing in disease and progress has been made towards clinical trials. In this review, we discuss the use of antisense oligonucleotides to correct splicing defects through exon skipping, with a special focus on diseases affecting the nervous system, and the latest stage achieved in its progress.

IFITM3 restricts influenza A virus entry by blocking the formation of fusion pores following virus-endosome hemifusion

Interferon-induced transmembrane proteins (IFITMs) inhibit infection of diverse enveloped viruses, including the influenza A virus (IAV) which is thought to enter from late endosomes. Recent evidence suggests that IFITMs block virus hemifusion (lipid mixing in the absence of viral content release) by altering the properties of cell membranes. Consistent with this mechanism, excess cholesterol in late endosomes of IFITM-expressing cells has been reported to inhibit IAV entry. Here, we examined IAV restriction by IFITM3 protein using direct virus-cell fusion assay and single virus imaging in live cells. IFITM3 over-expression did not inhibit lipid mixing, but abrogated the release of viral content into the cytoplasm. Although late endosomes of IFITM3-expressing cells accumulated cholesterol, other interventions leading to aberrantly high levels of this lipid did not inhibit virus fusion. These results imply that excess cholesterol in late endosomes is not the mechanism by which IFITM3 inhibits the transition from hemifusion to full fusion. The IFITM3's ability to block fusion pore formation at a post-hemifusion stage shows that this protein stabilizes the cytoplasmic leaflet of endosomal membranes without adversely affecting the lumenal leaflet. We propose that IFITM3 interferes with pore formation either directly, through partitioning into the cytoplasmic leaflet of a hemifusion intermediate, or indirectly, by modulating the lipid/protein composition of this leaflet. Alternatively, IFITM3 may redirect IAV fusion to a non-productive pathway, perhaps by promoting fusion with intralumenal vesicles within multivesicular bodies/late endosomes.

Interferon-induced transmembrane proteins (IFITMs) block infection of many enveloped viruses, including the influenza A virus (IAV) that enters from late endosomes. IFITMs are thought to prevent virus hemifusion (merger of contacting leaflets without formation of a fusion pore) by altering the properties of cell membranes. Here we performed single IAV imaging and found that IFITM3 did not interfere with hemifusion, but prevented complete fusion. Also, contrary to a current view that excess cholesterol in late endosomes of IFITM3-expressing cells inhibits IAV entry, we show that cholesterol-laden endosomes are permissive for virus fusion. The ability of IFITM3 to block the formation of fusion pores implies that this protein stabilizes the cytoplasmic leaflet of endosomal membranes, either directly or indirectly, through altering its physical properties. IFITM3 may also redirect IAV to a non-productive pathway by promoting fusion with intralumenal vesicles of late endosomes instead of their limiting membrane.

Development of targeted therapies for Parkinson's disease and related synucleinopathies

Therapeutic efforts in neurodegenerative diseases have been very challenging, particularly due to a lack of validated and mechanism-based therapeutic targets and biomarkers. The basic idea underlying the novel therapeutic approaches reviewed here is that by exploring the molecular basis of neurodegeneration in a rare lysosomal disease such as Gaucher's disease (GD), new molecular targets will be identified for therapeutic development in common synucleinopathies. Accumulation of α-synuclein plays a key role in the pathogenesis of Parkinson's disease (PD) and other synucleinopathies, suggesting that improved clearance of α-synuclein may be of therapeutic benefit. To achieve this goal, it is important to identify specific mechanisms and targets involved in the clearance of α-synuclein. Recent discovery of clinical, genetic, and pathological linkage between GD and PD offers a unique opportunity to examine lysosomal glucocerebrosidase, an enzyme mutated in GD, for development of targeted therapies in synucleinopathies. While modulation of glucocerebrosidase and glycolipid metabolism offers a viable approach to treating disorders associated with synuclein accumulation, the compounds described to date either lack the ability to penetrate the CNS or have off-target effects that may counteract or limit their capabilities to mediate the desired pharmacological action. However, recent emergence of selective inhibitors of glycosphingolipid biosynthesis and noninhibitory pharmacological chaperones of glycosphingolipid processing enzymes that gain access to the CNS provide a novel approach that may overcome some of the limitations of compounds reported to date. These new strategies may allow for development of targeted treatments for synucleinopathies that affect both children and adults.

2-Hydroxypropyl-β-cyclodextrin promotes transcription factor EB-mediated activation of autophagy: implications for therapy

2-Hydroxypropyl-β-cyclodextrin (HPβCD) is a Food and Drug Administration-approved excipient used to improve the stability and bioavailability of drugs. Despite its wide use as a drug delivery vehicle and the recent approval of a clinical trial to evaluate its potential for the treatment of a cholesterol storage disorder, the cellular pathways involved in the adaptive response that is activated upon exposure to HPβCD are still poorly defined. Here, we show that cell treatment with HPβCD results in the activation of the transcription factor EB, a master regulator of lysosomal function and autophagy, and in enhancement of the cellular autophagic clearance capacity. HPβCD administration promotes transcription factor EB-mediated clearance of proteolipid aggregates that accumulate due to inefficient activity of the lysosome-autophagy system in cells derived from a patient with a lysosomal storage disorder. Interestingly, HPβCD-mediated activation of autophagy was found not to be associated with activation of apoptotic pathways. This study provides a mechanistic understanding of the cellular response to HPβCD treatment, which will inform the development of safe HPβCD-based therapeutic modalities and may enable engineering HPβCD as a platform technology to reduce the accumulation of lysosomal storage material.

Small regulators, major consequences - Ca²⁺ and cholesterol at the endosome-ER interface

The ER is the largest cellular compartment and a major storage site for lipids and ions. In recent years, much attention has focused on contacts between the ER and other organelles, and one particularly intimate relationship is that between the ER and the endosomal system. ER-endosome contacts intensify when endosomes mature, and the ER participates in endosomal processes, such as the termination of surface receptor signaling, multi-vesicular body formation, and transport and fusion events. Cholesterol and Ca(2+) are transferred between the ER and endosomes, possibly acting as messengers for ER-endosome crosstalk. Here, we summarize different types of ER-endosomal communication and discuss membrane contact sites that might facilitate this crosstalk. We review the protein pairs that interact at the ER-endosome interface and find that many of these have a role in cholesterol exchange. We also summarize Ca(2+) exchange between the ER and endosomes, and hypothesize that ER-endosome contacts integrate several cellular functions to guide endosomal maturation. We post the hypothesis that failure in ER-endosome contacts is an unrecognized but important contributor to diseases, such as Niemann-Pick type C disease, Alzheimer's disease and amyotrophic lateral sclerosis.

Stuck in traffic: an emerging theme in diseases of the nervous system

The past decade has seen an explosion of DNA sequencing activities and many mutations and genetic variances underlying neurological and neurodegenerative diseases have been determined. This wealth of genetic data is now placed in molecular pathways revealing the nodes that underlie the disrupted processes. Many mutations in neurological diseases affect proteins controlling endosomal/lysosomal transport. Although the age of onset of these diseases range from juvenile [i.e., Niemann-Pick type C (NPC) and Charcot-Marie-Tooth (CMT) disease] to late onset (Parkinson's and Alzheimer's disease), deregulation of endosomal transport is a common theme. This review summarizes how elucidating the genetic basis for the various neurological diseases has advanced our understanding of the endo-lysosomal system and why the various mutations all translate into similar disease phenotypes.

Annexin A6-balanced late endosomal cholesterol controls influenza A replication and propagation

Influenza is caused by influenza A virus (IAV), an enveloped, negative-stranded RNA virus that derives its envelope lipids from the host cell plasma membrane. Here, we examined the functional role of cellular cholesterol in the IAV infection cycle. We show that shifting of cellular cholesterol pools via the Ca²⁺-regulated membrane-binding protein annexin A6 (AnxA6) affects the infectivity of progeny virus particles. Elevated levels of cellular AnxA6, which decrease plasma membrane and increase late endosomal cholesterol levels, impaired IAV replication and propagation, whereas RNA interference-mediated AnxA6 ablation increased viral progeny titers. Pharmacological accumulation of late endosomal cholesterol also diminished IAV virus propagation. Decreased IAV replication caused by upregulated AnxA6 expression could be restored either by exogenous replenishment of host cell cholesterol or by ectopic expression of the late endosomal cholesterol transporter Niemann-Pick C1 (NPC1). Virus released from AnxA6-overexpressing cells displayed significantly reduced cholesterol levels. Our results show that IAV replication depends on maintenance of the cellular cholesterol balance and identify AnxA6 as a critical factor in linking IAV to cellular cholesterol homeostasis.

Influenza A virus (IAV) is a major public health concern, and yet, major host-pathogen interactions regulating IAV replication still remain poorly understood. It is known that host cell cholesterol is a critical factor in the influenza virus life cycle. The viral envelope is derived from the host cell membrane during the process of budding and, hence, equips the virus with a special lipid-protein mixture which is high in cholesterol. However, the influence of host cell cholesterol homeostasis on IAV infection is largely unknown. We show that IAV infection success critically depends on host cell cholesterol distribution. Cholesterol sequestration in the endosomal compartment impairs progeny titer and infectivity and is associated with reduced cholesterol content in the viral envelope.

Lipoproteins and Hedgehog signalling--possible implications for the adrenal gland function

BACKGROUND

Metabolic syndrome is a common metabolic disorder that is associated with an increased risk of type 2 diabetes and cardiovascular diseases. Disturbances in adrenal steroid hormone production significantly contribute to the development of this disorder. Therefore, it is extremely important to fully understand the mechanisms governing adrenal gland function, both in physiological and pathological conditions.

RESULTS

Recently, Sonic hedgehog has emerged as an important regulator of adrenal development, with a possible role in adult gland homeostasis. Recent work of our group shows that lipoproteins are important regulators of Hedgehog signaling; they act as carriers for the spread of Hedgehog proteins, but also contain lipid(s) that inhibit the pathway.

CONCLUSIONS

We propose that lipoproteins may affect Sonic hedgehog signaling in the adult adrenal gland at multiple levels. Understanding the interplay between lipoprotein metabolism and adrenal Hedgehog signaling may improve our understanding of how adrenal gland disorders contribute to the metabolic syndrome.