Endosomal cholesterol trafficking: protein factors at a glance

Lactose-Appended Hydroxypropyl-β-Cyclodextrin Lowers Cholesterol Accumulation and Alleviates Motor Dysfunction in Niemann-Pick Type C Disease Model Mice

Niemann-Pick disease type C (NPC) is characterized by the accumulation of glycolipids such as free cholesterol, sphingomyelin, and gangliosides in late endosomes/lysosomes (endolysosomes) due to abnormalities in the membrane proteins NPC1 or NPC2. The main symptoms of NPC caused by free cholesterol accumulation in various tissues vary depending on the time of onset, but hepatosplenomegaly and neurological symptoms accompanied by decreased motor, cognitive, and mental functions are observed in all age groups. However, the efficacy of NPC treatment remains limited. Herein, we have fabricated lactose-appended hydroxypropyl-β-cyclodextrin (Lac-HPβCD) and evaluated its lowering effects on cholesterol accumulation in NPC model mice. We reveal that Lac-HPβCD lowers cholesterol accumulation in the liver and spleen by reducing the amount of free cholesterol. Moreover, Lac-HPβCD reduces the amount of free cholesterol in the cerebrum and slightly alleviates motor dysfunction. These results suggest that Lac-HPβCD has potential for the treatment of NPC.

Novel Indel Variation of NPC1 Gene Associates With Risk of Sudden Cardiac Death

Background and Aims: Sudden cardiac death (SCD) was defined as an unexpected death from cardiac causes during a very short duration. It has been reported that Niemann-Pick type C1 (NPC1) gene mutations might be related to cardiovascular diseases. The purpose of the study is to investigate whether common genetic variants of NPC1 is involved in SCD susceptibility.

Methods: Based on a candidate-gene-based approach and systematic screening strategy, this study analyzed an 8-bp insertion/deletion polymorphism (rs150703258) within downstream of NPC1 for the association with SCD risk in Chinese populations using 158 SCD cases and 524 controls. The association of rs150703258 and SCD susceptibility was analyzed using logistic regression. Genotype-phenotype correlation analysis was performed using public database including 1000G, expression quantitative trait loci (eQTL), and further validated by human heart tissues using PCR. Dual-luciferase assay was used to explore the potential regulatory role of rs150703258. Gene expression profiling interactive analysis and transcription factors prediction were performed.

Results: Logistic regression analysis exhibited that the deletion allele of rs150703258 significantly increased the risk of SCD [odds ratio (OR) = 1.329; 95% confidence interval (95%CI):1.03–1.72; p = 0.0289]. Genotype-phenotype correlation analysis showed that the risk allele was significantly associated with higher expression of NPC1 at mRNA and protein expressions level in human heart tissues. eQTL analysis showed NPC1 and C18orf8 (an adjacent gene to NPC1) are both related to rs150703258 and have higher expression level in the samples with deletion allele. Dual-luciferase activity assays indicate a significant regulatory role for rs150703258. Gene expression profiling interactive analysis revealed that NPC1 and C18orf8 seemed to be co-regulated in human blood, arteries and heart tissues. In silico analysis showed that the rs150703258 deletion variant may create transcription factor binding sites. In addition, a rare 12-bp allele (4-bp longer than the insertion allele) of rs150703258 was discovered in the current cohort.

Conclusion: In summary, our study revealed that rs150703258 might contribute to SCD susceptibility by regulating NPC1 and C18orf8 expression. This indel may be a potential marker for risk stratification and molecular diagnosis of SCD. Validations in different ethnic groups with larger sample size and mechanism explorations are warranted to confirm our findings.

Oxysterols and mesenchymal stem cell biology

Mesenchymal stem cells have the ability to differentiate into several cell types when exposed to determined substances, including oxysterols. Oxysterols are cholesterol products derived from its auto-oxidation by reactive species or from enzymatic action. They are present in the body in low quantities under physiological conditions and exhibit several physiological and pharmacological actions according to both the types of oxysterol and tissue. Some of them are cytotoxic while others have been shown to promote cell differentiation through the action on several different receptors, such as nuclear LXR receptors and Smoothened receptor ligands. Here, we review the main pathways by which oxysterols have been associated with cell differentiation and death of mesenchymal stem cells.

Inside the Cell: Assembly of Filoviruses

This chapter reviews our current knowledge about the spatiotemporal assembly of filoviral particles. We will follow particles from nucleocapsid entry into the cytoplasm until the nucleocapsids are enveloped at the plasma membrane. We will also highlight the currently open scientific questions surrounding filovirus assembly.

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.

ATP-binding cassette transporter-2 (ABCA2) as a therapeutic target

The ATP binding cassette transporter ABCA2 is primarily an endolysosomal membrane protein that demonstrates pleiotropic functionalities, coalescing around the maintenance of homeostasis of sterols, sphingolipids and cholesterol. It is most highly expressed in brain tissue and ABCA2 knockout mice express neurological defects consistent with aberrant myelination. Increased expression of the transporter has been linked with resistance to cancer drugs, particularly those possessing a steroid backbone and gene expression (in concert with other genes involved in cholesterol metabolism) was found to be regulated by sterols. Moreover, in macrophages ABCA2 is influenced by sterols and has a role in regulating cholesterol sequestration, potentially important in cardiovascular disease. Accumulating data indicate the critical importance of ABCA2 in mediating movement of sphingolipids within cellular compartments and these have been implicated in various aspects of cholesterol trafficking. Perhaps because the functions of ABCA2 are linked with membrane building blocks, there are reports linking it with human pathologies, including, cholesterolemias and cardiovascular disease, Alzheimer's and cancer. The present review addresses whether there is now sufficient information to consider ABCA2 as a plausible therapeutic target.

Thoughts on interactions between PGRMC1 and diverse attested and potential hydrophobic ligands

Progesterone Receptor Membrane Component 1 (PGRMC1) is located in many different subcellular locations with many different attested and probably location-specific functions. PGRMC1 was recently identified in the mitochondrial outer membrane where it interacts with ferrochelatase, the last enzyme in the heme synthetic pathway. It has been proposed that PGRMC1 may act as a chaperone to shuttle newly synthesized heme from the mitochondrion to cytochrome P450 (cyP450) enzymes. Here we consider potential roles that PGRMC1 may play in transferring heme, and other small hydrophobic ligands such as cholesterol and steroids, between the hydrophobic compartment of the membrane lipid bilayer interior to aqueous proteins, and perhaps to the membranes of other organelles. We review the synthesis and roles of especially PGRMC1- and cyP450-bound heme, the sources and transport of cholesterol, the involvement of PGRMC1 in cholesterol regulation, and the production of the first progestogen pregnenolone from cholesterol. We also show by clustering by inferred models of evolution (CLIME) analysis that PGRMC1 and related proteins exhibit co-evolution with a series of cyP450 enzymes, as well as a group of mitochondrial proteins lacking in several parasitic protist groups. Altogether, PGRMC1 is implicated with important roles in sterol synthesis and energy regulation that are dispensable in certain parasites. Some novel hypothetical models for PGRMC1 function are proposed to direct future investigative research.

Intracellular cholesterol transport proteins: roles in health and disease

Effective cholesterol homoeostasis is essential in maintaining cellular function, and this is achieved by a network of lipid-responsive nuclear transcription factors, and enzymes, receptors and transporters subject to post-transcriptional and post-translational regulation, whereas loss of these elegant, tightly regulated homoeostatic responses is integral to disease pathologies. Recent data suggest that sterol-binding sensors, exchangers and transporters contribute to regulation of cellular cholesterol homoeostasis and that genetic overexpression or deletion, or mutations, in a number of these proteins are linked with diseases, including atherosclerosis, dyslipidaemia, diabetes, congenital lipoid adrenal hyperplasia, cancer, autosomal dominant hearing loss and male infertility. This review focuses on current evidence exploring the function of members of the ‘START’ (steroidogenic acute regulatory protein-related lipid transfer) and ‘ORP’ (oxysterol-binding protein-related proteins) families of sterol-binding proteins in sterol homoeostasis in eukaryotic cells, and the evidence that they represent valid therapeutic targets to alleviate human disease.

The formation and function of ER-endosome membrane contact sites

Recent advances in membrane contact site (MCS) biology have revealed key roles for MCSs in inter-organellar exchange, the importance of which is becoming increasingly apparent. Roles for MCSs in many essential physiological processes including lipid transfer, calcium exchange, receptor tyrosine kinase signalling, lipid droplet formation, autophagosome formation, organelle dynamics and neurite outgrowth have been reported. The ER forms an extensive and dynamic network of MCSs with a diverse range of functionally distinct organelles. MCSs between the ER and endocytic pathway are particularly abundant, suggesting important physiological roles. Here, our current knowledge of the formation and function of ER contact sites with endocytic organelles from studies in mammalian systems is reviewed. Their relatively poorly defined molecular composition and recently identified functions are discussed. In addition, likely, but yet to be established, roles for these contacts in lipid transfer and calcium signalling are considered. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

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.

Transcriptional control of steroid biosynthesis genes in the Drosophila prothoracic gland by ventral veins lacking and knirps

Specialized endocrine cells produce and release steroid hormones that govern development, metabolism and reproduction. In order to synthesize steroids, all the genes in the biosynthetic pathway must be coordinately turned on in steroidogenic cells. In Drosophila, the steroid producing endocrine cells are located in the prothoracic gland (PG) that releases the steroid hormone ecdysone. The transcriptional regulatory network that specifies the unique PG specific expression pattern of the ecdysone biosynthetic genes remains unknown. Here, we show that two transcription factors, the POU-domain Ventral veins lacking (Vvl) and the nuclear receptor Knirps (Kni), have essential roles in the PG during larval development. Vvl is highly expressed in the PG during embryogenesis and is enriched in the gland during larval development, suggesting that Vvl might function as a master transcriptional regulator in this tissue. Vvl and Kni bind to PG specific cis-regulatory elements that are required for expression of the ecdysone biosynthetic genes. Knock down of either vvl or kni in the PG results in a larval developmental arrest due to failure in ecdysone production. Furthermore, Vvl and Kni are also required for maintenance of TOR/S6K and prothoracicotropic hormone (PTTH) signaling in the PG, two major pathways that control ecdysone biosynthesis and PG cell growth. We also show that the transcriptional regulator, Molting defective (Mld), controls early biosynthetic pathway steps. Our data show that Vvl and Kni directly regulate ecdysone biosynthesis by transcriptional control of biosynthetic gene expression and indirectly by affecting PTTH and TOR/S6K signaling. This provides new insight into the regulatory network of transcription factors involved in the coordinated regulation of steroidogenic cell specific transcription, and identifies a new function of Vvl and Knirps in endocrine cells during post-embryonic development.

Hrs- and CD63-dependent competing mechanisms make different sized endosomal intraluminal vesicles

Multivesicular endosomes/bodies (MVBs) contain intraluminal vesicles (ILVs) that bud away from the cytoplasm. Multiple mechanisms of ILV formation have been identified, but the relationship between different populations of ILVs and MVBs remains unclear. Here, we show in HeLa cells that different ILV subpopulations can be distinguished by size. EGF stimulation promotes the formation of large ESCRT-dependent ILVs, whereas depletion of the ESCRT-0 component, Hrs, promotes the formation of a uniformly sized population of small ILVs, the formation of which requires CD63. CD63 has previously been implicated in ESCRT-independent sorting of PMEL in MVBs and transfected PMEL is present on the small ILVs that form on Hrs depletion. Upregulation of CD63-dependent ILV formation by Hrs depletion indicates that Hrs and CD63 regulate competing machineries required for the generation of distinct ILV subpopulations. Taken together our results indicate that ILV size is influenced by their cargo and mechanism of formation and suggest a competitive relationship between ESCRT-dependent and -independent mechanisms of ILV formation within single MVBs.

A close-up view of membrane contact sites between the endoplasmic reticulum and the endolysosomal system: from yeast to man

Maintenance of organelle identity is crucial for the functionality of eukaryotic cells. Hence, transfer reactions between different compartments must be highly efficient and tightly regulated at the same time. Membrane contact sites (MCSs) represent an important route for inter-organelle transport and communication independent of vesicular trafficking. Due to extensive research, the mechanistic understanding of these sites increases constantly. However, how the formation and the versatile functions of MCSs are regulated is mainly unclear. Within this review, we focus on one well-known MCS, the nucleus-vacuole junction in yeast and discuss its analogy to endoplasmic reticulum-late endosome contacts in metazoan. Formation of the junction in yeast requires Vac8, a protein that is involved in various cellular processes at the yeast vacuole and a target of multiple posttranslational modifications. We discuss the possibility that dual functionality of proteins involved in contact formation is a common principle to coordinate inter-organelle transfer with organellar biogenesis.

Imaging lipid order changes in endosome membranes of live cells by using a Nile Red-based membrane probe

Changes in the composition of endosome membranes during endocytosis can be imaged in live cells with the NR12S membrane probe.

Adenovirus RIDα uncovers a novel pathway requiring ORP1L for lipid droplet formation independent of NPC1

Expression of the adenovirus protein RIDα rescues the cholesterol storage phenotype in NPC1-deficient cells by inducing formation of lipid droplets. The function of RIDα is independent of NPC1 but dependent on NPC2 and the oxysterol-binding protein ORP1L. This study provides the first evidence that ORP1L plays a role in sterol transport and LD formation.

Niemann–Pick disease type C (NPC) is caused by mutations in NPC1 or NPC2, which coordinate egress of low-density-lipoprotein (LDL)-cholesterol from late endosomes. We previously reported that the adenovirus-encoded protein RIDα rescues the cholesterol storage phenotype in NPC1-mutant fibroblasts. We show here that RIDα reconstitutes deficient endosome-to-endoplasmic reticulum (ER) transport, allowing excess LDL-cholesterol to be esterified by acyl-CoA:cholesterol acyltransferase and stored in lipid droplets (LDs) in NPC1-deficient cells. Furthermore, the RIDα pathway is regulated by the oxysterol-binding protein ORP1L. Studies have classified ORP1L as a sterol sensor involved in LE positioning downstream of GTP-Rab7. Our data, however, suggest that ORP1L may play a role in transport of LDL-cholesterol to a specific ER pool designated for LD formation. In contrast to NPC1, which is dispensable, the RIDα/ORP1L-dependent route requires functional NPC2. Although NPC1/NPC2 constitutes the major pathway, therapies that amplify minor egress routes for LDL-cholesterol could significantly improve clinical management of patients with loss-of-function NPC1 mutations. The molecular identity of putative alternative pathways, however, is poorly characterized. We propose RIDα as a model system for understanding physiological egress routes that use ORP1L to activate ER feedback responses involved in LD formation.