Sterol, protein and lipid trafficking in Chinese hamster ovary cells with Niemann-Pick type C1 defect

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.

Lipophagy and Lipolysis Status in Lipid Storage and Lipid Metabolism Diseases

This review discusses how lipophagy and cytosolic lipolysis degrade cellular lipids, as well as how these pathway ys communicate, how they affect lipid metabolism and energy homeostasis in cells and how their dysfunction affects the pathogenesis of lipid storage and lipid metabolism diseases. Answers to these questions will likely uncover novel strategies for the treatment of aforementioned human diseases, but, above all, will avoid destructive effects of high concentrations of lipids-referred to as lipotoxicity-resulting in cellular dysfunction and cell death.

2-Hydroxypropyl-gamma-cyclodextrin overcomes NPC1 deficiency by enhancing lysosome-ER association and autophagy

Niemann-Pick type C (NPC) disease is a fatal neurodegenerative disorder caused by mutations in NPC1 and NPC2 genes that result in an accumulation of cholesterol in lysosomes. The majority of children with NPC die in adolescence. Currently, no FDA-approved therapies exist for NPC and the mechanisms of NPC disease are not fully understood. Our recent study and the reports from other laboratories showed that 2-hydroxypropyl-γ-cyclodextrin (HPγCD) alleviates cholesterol accumulation in NPC1-deficient cells in spite of its low binding affinity for cholesterol. In this study, we explored the cellular changes that are induced upon HPγCD treatment in NPC1 patient-derived fibroblasts. We show that HPγCD treatment increases lysosome-ER association and enhances autophagic activity. Our study indicates that HPγCD induces an activation of the transcription factor EB (TFEB), a master regulator of lysosomal functions and autophagy. Lysosome-ER association could potentially function as conduits for cholesterol transport from lysosomes to the ER. Accumulating evidence suggests a role for autophagy in rescuing the cholesterol accumulation in NPC and other degenerative diseases. Collectively, our findings suggest that HPγCD restores cellular homeostasis in NPC1-deficient cells via enhancing lysosomal dynamics and functions. Understanding the mechanisms of HPγCD-induced cellular pathways could contribute to effective NPC therapies.

Stable reduction of STARD4 alters cholesterol regulation and lipid homeostasis

STARD4, a member of the evolutionarily conserved START gene family, is a soluble sterol transport protein implicated in cholesterol sensing and maintenance of cellular homeostasis. STARD4 is widely expressed and has been shown to transfer sterol between liposomes as well as organelles in cells. However, STARD4 knockout mice lack an obvious phenotype, so the overall role of STARD4 is unclear. To model long term depletion of STARD4 in cells, we use short hairpin RNA technology to stably decrease STARD4 expression in human U2OS osteosarcoma cells (STARD4-KD). We show that STARD4-KD cells display increased total cholesterol, slower cholesterol trafficking between the plasma membrane and the endocytic recycling compartment, and increased plasma membrane fluidity. These effects can all be rescued by transient expression of a short hairpin RNA-resistant STARD4 construct. Some of the cholesterol increase was due to excess storage in late endosomes or lysosomes. To understand the effects of reduced STARD4, we carried out transcriptional and lipidomic profiling of control and STARD4-KD cells. Reduction of STARD4 activates compensatory mechanisms that alter membrane composition and lipid homeostasis. Based on these observations, we propose that STARD4 functions as a critical sterol transport protein involved in sterol sensing and maintaining lipid homeostasis.

Niemann-Pick type C disease: cellular pathology and pharmacotherapy

Niemann-Pick type C disease (NPCD) was first described in 1914 and affects approximately 1 in 150 000 live births. It is characterized clinically by diverse symptoms affecting liver, spleen, motor control, and brain; premature death invariably results. Its molecular origins were traced, as late as 1997, to a protein of late endosomes and lysosomes which was named NPC1. Mutation or absence of this protein leads to accumulation of cholesterol in these organelles. In this review, we focus on the intracellular events that drive the pathology of this disease. We first introduce endocytosis, a much-studied area of dysfunction in NPCD cells, and survey the various ways in which this process malfunctions. We briefly consider autophagy before attempting to map the more complex pathways by which lysosomal cholesterol storage leads to protein misregulation, mitochondrial dysfunction, and cell death. We then briefly introduce the metabolic pathways of sphingolipids (as these emerge as key species for treatment) and critically examine the various treatment approaches that have been attempted to date.

Lipid⁻Protein Interactions in Niemann⁻Pick Type C Disease: Insights from Molecular Modeling

The accumulation of lipids in the late endosomes and lysosomes of Niemann⁻Pick type C disease (NPCD) cells is a consequence of the dysfunction of one protein (usually NPC1) but induces dysfunction in many proteins. We used molecular docking to propose (a) that NPC1 exports not just cholesterol, but also sphingosine, (b) that the cholesterol sensitivity of big potassium channel (BK) can be traced to a previously unappreciated site on the channel's voltage sensor, (c) that transient receptor potential mucolipin 1 (TRPML1) inhibition by sphingomyelin is likely an indirect effect, and (d) that phosphoinositides are responsible for both the mislocalization of annexin A2 (AnxA2) and a soluble NSF (N-ethylmaleimide Sensitive Fusion) protein attachment receptor (SNARE) recycling defect. These results are set in the context of existing knowledge of NPCD to sketch an account of the endolysosomal pathology key to this disease.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

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.

Role of STARD4 in sterol transport between the endocytic recycling compartment and the plasma membrane

The kinetics of sterol transport between the plasma membrane and the endocytic recycling compartment is measured using fluorescence microscopy. STARD4, a small, soluble sterol transport protein, is responsible for 25% of the total transport and 33% of nonvesicular transport. Elevated cholesterol dramatically increases sterol transport rate constants.

Cholesterol is an essential constituent of membranes in mammalian cells. The plasma membrane and the endocytic recycling compartment (ERC) are both highly enriched in cholesterol. The abundance and distribution of cholesterol among organelles are tightly controlled by a combination of mechanisms involving vesicular and nonvesicular sterol transport processes. Using the fluorescent cholesterol analogue dehydroergosterol, we examined sterol transport between the plasma membrane and the ERC using fluorescence recovery after photobleaching and a novel sterol efflux assay. We found that sterol transport between these organelles in a U2OS cell line has a t_1/2 =12–15 min. Approximately 70% of sterol transport is ATP independent and therefore is nonvesicular. Increasing cellular cholesterol levels dramatically increases bidirectional transport rate constants, but decreases in cholesterol levels have only a modest effect. A soluble sterol transport protein, STARD4, accounts for ∼25% of total sterol transport and ∼33% of nonvesicular sterol transport between the plasma membrane and ERC. This study shows that nonvesicular sterol transport mechanisms and STARD4 in particular account for a large fraction of sterol transport between the plasma membrane and the ERC.

Histone deacetylase inhibitors correct the cholesterol storage defect in most Niemann-Pick C1 mutant cells

Niemann-Pick C (NPC) disease is an autosomal recessive disorder that leads to excessive storage of cholesterol and other lipids in late endosomes and lysosomes. The large majority of NPC disease is caused by mutations in NPC1, a large polytopic membrane protein that functions in late endosomes. There are many disease-associated mutations in NPC1, and most patients are compound heterozygotes. The most common mutation, NPC1^I1061T, has been shown to cause endoplasmic reticulum-associated degradation of the NPC1 protein. Treatment of patient-derived NPC1^I1061T fibroblasts with histone deacetylase inhibitors (HDACis) vorinostat or panobinostat increases expression of the mutant NPC1 protein and leads to correction of the cholesterol storage. Here, we show that several other human NPC1 mutant fibroblast cell lines can also be corrected by vorinostat or panobinostat and that treatment with vorinostat extends the lifetime of the NPC1^I1061T protein. To test effects of HDACi on a large number of NPC1 mutants, we engineered a U2OS cell line to suppress NPC1 expression by shRNA and then transiently transfected these cells with 60 different NPC1 mutant constructs. The mutant NPC1 did not significantly reduce cholesterol accumulation, but approximately 85% of the mutants showed reduced cholesterol accumulation when treated with vorinostat or panobinostat.

One-Step Selective Exoenzymatic Labeling (SEEL) Strategy for the Biotinylation and Identification of Glycoproteins of Living Cells

Technologies that can visualize, capture, and identify subsets of biomolecules that are not encoded by the genome in the context of healthy and diseased cells will offer unique opportunities to uncover the molecular mechanism of a multitude of physiological and disease processes. We describe here a chemical reporter strategy for labeling of cell surface glycoconjugates that takes advantage of recombinant glycosyltransferases and a corresponding sugar nucleotide functionalized by biotin. The exceptional efficiency of this method, termed one-step selective exoenzymatic labeling, or SEEL, greatly improved the ability to enrich and identify large numbers of tagged glycoproteins by LC-MS/MS. We further demonstrated that this labeling method resulted in far superior enrichment and detection of glycoproteins at the plasma membrane compared to a sulfo-NHS-activated biotinylation or two-step SEEL. This new methodology will make it possible to profile cell surface glycoproteomes with unprecedented sensitivity in the context of physiological and disease states.

Role of STARD4 and NPC1 in intracellular sterol transport

Cholesterol plays an important role in determining the biophysical properties of membranes in mammalian cells, and the concentration of cholesterol in membranes is tightly regulated. Cholesterol moves among membrane organelles by a combination of vesicular and nonvesicular transport pathways, but the details of these transport pathways are not well understood. In this review, we discuss the mechanisms for nonvesicular sterol transport with an emphasis on the role of STARD4, a small, soluble, cytoplasmic sterol transport protein. STARD4 can rapidly equilibrate sterol between membranes, especially membranes with anionic lipid headgroups. We also discuss the sterol transport in late endosomes and lysosomes, which is mediated by a soluble protein, NPC2, and a membrane protein, NPC1. Homozygous mutations in these proteins lead to a lysosomal lipid storage disorder, Niemann-Pick disease type C. Many of the disease-causing mutations in NPC1 are associated with degradation of the mutant NPC1 proteins in the endoplasmic reticulum. Several histone deacetylase inhibitors have been found to rescue the premature degradation of the mutant NPC1 proteins, and one of these is now in a small clinical trial.

A comparative study on fluorescent cholesterol analogs as versatile cellular reporters

Cholesterol (Chol) is a crucial component of cellular membranes, but knowledge of its intracellular dynamics is scarce. Thus, it is of utmost interest to develop tools for visualization of Chol organization and dynamics in cells and tissues. For this purpose, many studies make use of fluorescently labeled Chol analogs. Unfortunately, the introduction of the label may influence the characteristics of the analog, such as its localization, interaction, and trafficking in cells; hence, it is important to get knowledge of such bias. In this report, we compared different fluorescent lipid analogs for their performance in cellular assays: 1) plasma membrane incorporation, specifically the preference for more ordered membrane environments in phase-separated giant unilamellar vesicles and giant plasma membrane vesicles; 2) cellular trafficking, specifically subcellular localization in Niemann-Pick type C disease cells; and 3) applicability in fluorescence correlation spectroscopy (FCS)-based and super-resolution stimulated emission depletion-FCS-based measurements of membrane diffusion dynamics. The analogs exhibited strong differences, with some indicating positive performance in the membrane-based experiments and others in the intracellular trafficking assay. However, none showed positive performance in all assays. Our results constitute a concise guide for the careful use of fluorescent Chol analogs in visualizing cellular Chol dynamics.

A novel intrinsically fluorescent probe for study of uptake and trafficking of 25-hydroxycholesterol

Cholesterol homeostasis is regulated not only by cholesterol, but also by oxygenated cholesterol species, referred to as oxysterols. Side-chain oxysterols, such as 25-hydroxycholesterol (25-HC), regulate cholesterol homeostasis through feedback inhibition and feed-forward activation of transcriptional pathways that govern cholesterol synthesis, uptake, and elimination, as well as through direct nongenomic actions that modulate cholesterol accessibility in membranes. Elucidating the cellular distribution of 25-HC is required to understand its biological activity at the molecular level. However, studying oxysterol distribution and behavior within cells has proven difficult due to the lack of fluorescent analogs of 25-HC that retain its chemical and physical properties. To address this, we synthesized a novel intrinsically fluorescent 25-HC mimetic, 25-hydroxycholestatrienol (25-HCTL). We show that 25-HCTL modulates sterol homeostatic responses in a similar manner as 25-HC. 25-HCTL associates with lipoproteins in media and is taken up by cells through LDL-mediated endocytosis. In cultured cells, 25-HCTL redistributes among cellular membranes and, at steady state, has a similar distribution as cholesterol, being enriched in both the endocytic recycling compartment as well as the plasma membrane. Our findings indicate that 25-HCTL is a faithful fluorescent 25-HC mimetic that can be used to investigate the mechanisms through which 25-HC regulates sterol homeostatic pathways.

Cholesterol-mediated activation of acid sphingomyelinase disrupts autophagy in the retinal pigment epithelium

How autophagy is regulated in the postmitotic retinal pigment epithelium (RPE) is unclear. Visual cycle metabolites and cholesterol that accumulate in the RPE inhibit autophagic flux by activating acid sphingomyelinase (ASMase). Increased ceramide promotes tubulin acetylation, which prevents autophagosome traffic. ASMase inhibition restores RPE autophagy.

Autophagy is an essential mechanism for clearing damaged organelles and proteins within the cell. As with neurodegenerative diseases, dysfunctional autophagy could contribute to blinding diseases such as macular degeneration. However, precisely how inefficient autophagy promotes retinal damage is unclear. In this study, we investigate innate mechanisms that modulate autophagy in the retinal pigment epithelium (RPE), a key site of insult in macular degeneration. High-speed live imaging of polarized adult primary RPE cells and data from a mouse model of early-onset macular degeneration identify a mechanism by which lipofuscin bisretinoids, visual cycle metabolites that progressively accumulate in the RPE, disrupt autophagy. We demonstrate that bisretinoids trap cholesterol and bis(monoacylglycero)phosphate, an acid sphingomyelinase (ASMase) cofactor, within the RPE. ASMase activation increases cellular ceramide, which promotes tubulin acetylation on stabilized microtubules. Live-imaging data show that autophagosome traffic and autophagic flux are inhibited in RPE with acetylated microtubules. Drugs that remove excess cholesterol or inhibit ASMase reverse this cascade of events and restore autophagosome motility and autophagic flux in the RPE. Because accumulation of lipofuscin bisretinoids and abnormal cholesterol homeostasis are implicated in macular degeneration, our studies suggest that ASMase could be a potential therapeutic target to ensure the efficient autophagy that maintains RPE health.

Niemann-Pick C1 affects the gene delivery efficacy of degradable polymeric nanoparticles

Despite intensive research effort, the rational design of improved nanoparticulate drug carriers remains challenging, in part due to a limited understanding of the determinants of nanoparticle entry and transport in target cells. Recent studies have shown that Niemann-Pick C1 (NPC1), the lysosome membrane protein that mediates trafficking of cholesterol in cells, is involved in the endosomal escape and subsequent infection caused by filoviruses, and that its absence promotes the retention and efficacy of lipid nanoparticles encapsulating siRNA. Here, we report that NPC1 deficiency results in dramatic reduction in internalization and transfection efficiency mediated by degradable cationic gene delivery polymers, poly(β-amino ester)s (PBAEs). PBAEs utilized cholesterol and dynamin-dependent endocytosis pathways, and these were found to be heavily compromised in NPC1-deficient cells. In contrast, the absence of NPC1 had minor effects on DNA uptake mediated by polyethylenimine or Lipofectamine 2000. Strikingly, stable overexpression of human NPC1 in chinese hamster ovary cells was associated with enhanced gene uptake (3-fold) and transfection (10-fold) by PBAEs. These findings reveal a role of NPC1 in the regulation of endocytic mechanisms affecting nanoparticle trafficking. We hypothesize that in-depth understanding sites of entry and endosomal escape may lead to highly efficient nanotechnologies for drug delivery.

Tracking the subcellular fate of 20(s)-hydroxycholesterol with click chemistry reveals a transport pathway to the Golgi

Oxysterols, oxidized metabolites of cholesterol, are endogenous small molecules that regulate lipid metabolism, immune function, and developmental signaling. Although the cell biology of cholesterol has been intensively studied, fundamental questions about oxysterols, such as their subcellular distribution and trafficking pathways, remain unanswered. We have therefore developed a useful method to image intracellular 20(S)-hydroxycholesterol with both high sensitivity and spatial resolution using click chemistry and fluorescence microscopy. The metabolic labeling of cells with an alkynyl derivative of 20(S)-hydroxycholesterol has allowed us to directly visualize this oxysterol by attaching an azide fluorophore through cyclo-addition. Unexpectedly, we found that this oxysterol selectively accumulates in the Golgi membrane using a pathway that is sensitive to ATP levels, temperature, and lysosome function. Although previous models have proposed nonvesicular pathways for the rapid equilibration of oxysterols between membranes, direct imaging of oxysterols suggests that a vesicular pathway is responsible for differential accumulation of oxysterols in organelle membranes. More broadly, clickable alkynyl sterols may represent useful tools for sterol cell biology, both to investigate the functions of these important lipids and to decipher the pathways that determine their cellular itineraries.

Hepatic metabolic response to restricted copper intake in a Niemann–Pick C murine model

Niemann–Pick C disease (NPC) is a vesicular trafficking disorder primarily caused by mutations in theNpc1gene and characterized by liver dysfunction and neuropathology.

Galectin-3 guides intracellular trafficking of some human serotransferrin glycoforms

Transferrin internalization via clathrin-mediated endocytosis and subsequent recycling after iron delivery has been extensively studied. Here we demonstrate a previously unrecognized parameter regulating this recycling, the binding of galectin-3 to particular glycoforms of transferrin. Two fractions of transferrin, separated by affinity chromatography based on their binding or not to galectin-3, are targeted to kinetically different endocytic pathways in HFL-1 cells expressing galectin-3 but not in SKBR3 cells lacking galectin-3; the SKBR3 cells, however, can acquire the ability to target these transferrin glycoforms differently after preloading with exogenously added galectin-3. In all, this study provides the first evidence of a functional role for transferrin glycans, in intracellular trafficking after uptake. Moreover, the galectin-3-bound glycoform increased in cancer, suggesting a pathophysiological regulation. These are novel aspects of transferrin cell biology, which has previously considered only a degree of iron loading, but not other forms of heterogeneity.

Insights into the mechanisms of sterol transport between organelles

In cells, the levels of sterol vary greatly among organelles. This uneven distribution depends largely on non-vesicular routes of transfer, which are mediated by soluble carriers called lipid-transfer proteins (LTPs). These proteins have a domain with a hydrophobic cavity that accommodates one sterol molecule. However, a demonstration of their role in sterol transport in cells remains difficult. Numerous LTPs also contain membrane-binding elements, but it is not clear how these LTPs couple their ability to target organelles with lipid transport activity. This issue appears critical, since many sterol transporters are thought to act at contact sites between two membrane-bound compartments. Here, we emphasize that biochemical and structural studies provide precious insights into the mode of action of sterol-binding proteins. Recent studies on START, Osh/ORP and NPC proteins suggest models on how these proteins could transport sterol between organelles and, thereby, influence cellular functions.

Abnormal accumulation and recycling of glycoproteins visualized in Niemann-Pick type C cells using the chemical reporter strategy

Niemann-Pick type C (NPC) disease is characterized by impaired cholesterol efflux from late endosomes and lysosomes and secondary accumulation of lipids. Although impaired trafficking of individual glycoproteins and glycolipids has been noted in NPC cells and other storage disorders, there is currently no effective way to monitor their localization and movement en masse. Using a chemical reporter strategy in combination with pharmacologic treatments, we demonstrate a disease-specific and previously unrecognized accumulation of a diverse set of glycoconjugates in NPC1-null and NPC2-deficient fibroblasts within endocytic compartments. These labeled vesicles do not colocalize with the cholesterol-laden compartments of NPC cells. Experiments using the endocytic uptake marker dextran show that the endosomal accumulation of sialylated molecules can be largely attributed to impaired recycling as opposed to altered fusion of vesicles. Treatment of either NPC1-null or NPC2-deficient cells with cyclodextrin was effective in reducing cholesterol storage as well as the endocytic accumulation of sialoglycoproteins, demonstrating a direct link between cholesterol storage and abnormal recycling. Our data further demonstrate that this accumulation is largely glycoproteins, given that inhibitors of O-glycan initiation or N-glycan processing led to a significant reduction in staining intensity. Taken together, our results provide a unique perspective on the trafficking defects in NPC cells, and highlight the utility of this methodology in analyzing cells with altered recycling and turnover of glycoproteins.

Cholesterol-depletion corrects APP and BACE1 misstrafficking in NPC1-deficient cells

Cholesterol accumulation in Niemann-Pick type C disease (NPC) causes increased levels of the amyloid-precursor-protein C-terminal fragments (APP-CTFs) and intracellular amyloid-β peptide (Aβ), the two central molecules in Alzheimer's disease (AD) pathogenesis. We previously reported that cholesterol accumulation in NPC-cells leads to cholesterol-dependent increased APP processing by β-secretase (BACE1) and decreased APP expression at the cell surface (Malnar et al. Biochim Biophys Acta. 1802 (2010) 682-691.). We hypothesized that increased formation of APP-CTFs and Aβ in NPC disease is due to cholesterol-mediated altered endocytic trafficking of APP and/or BACE1. Here, we show that APP endocytosis is prerequisite for enhanced Aβ levels in NPC-cells. Moreover, we observed that NPC cells show cholesterol dependent sequestration and colocalization of APP and BACE1 within enlarged early/recycling endosomes which can lead to increased β-secretase processing of APP. We demonstrated that increased endocytic localization of APP in NPC-cells is likely due to both its increased internalization and its decreased recycling to the cell surface. Our findings suggest that increased cholesterol levels, such as in NPC disease and sporadic AD, may be the upstream effector that drives amyloidogenic APP processing characteristic for Alzheimer's disease by altering endocytic trafficking of APP and BACE1.

Sterol transfer between cyclodextrin and membranes: similar but not identical mechanism to NPC2-mediated cholesterol transfer

Niemann--Pick C disease is an inherited disorder in which cholesterol and other lipids accumulate in the late endosomal/lysosomal compartment. Recently, cyclodextrins (CD) have been shown to reduce symptoms and extend lifespan in animal models of the disease. In the present studies we examined the mechanism of sterol transport by CD using in vitro model systems and fluorescence spectroscopy and NPC2-deficient fibroblasts. We demonstrate that cholesterol transport from the lysosomal cholesterol-binding protein NPC2 to CD occurs via aqueous diffusional transfer and is very slow; the rate-limiting step appears to be dissociation of cholesterol from NPC2, suggesting that specific interactions between NPC2 and CD do not occur. In contrast, the transfer rate of the fluorescent cholesterol analogue dehydroergosterol (DHE) from CD to phospholipid membranes is very rapid and is directly proportional to the acceptor membrane concentration, as is DHE transfer from membranes to CD. Moreover, CD dramatically increases the rate of sterol transfer between membranes, with rates that can approach those mediated by NPC2. The results suggest that sterol transfer from CD to membranes occurs by a collisional transfer mechanism involving direct interaction of CD with membranes, similar to that shown previously for NPC2. For CD, however, absolute rates are slower compared to NPC2 for a given concentration, and the lysosomal phospholipid lysobisphosphatidic acid (LBPA) does not stimulate rates of sterol transfer between membranes and CD. As expected from the apparent absence of interaction between CD and NPC2, the addition of CD to NPC2-deficient fibroblasts rapidly rescued the cholesterol accumulation phenotype. Thus, the recent observations of CD efficacy in mouse models of NPC disease are likely the result of CD enhancement of cholesterol transport between membranes, with rapid sterol transfer occurring during CD--membrane interactions.

Potential of ultraviolet wide-field imaging and multiphoton microscopy for analysis of dehydroergosterol in cellular membranes

Dehydroergosterol (DHE) is an intrinsically fluorescent sterol with absorption/emission in the ultraviolet (UV) region and biophysical properties similar to those of cholesterol. We compared the potential of UV-sensitive low-light-level wide-field (UV-WF) imaging with that of multiphoton (MP) excitation microscopy to monitor DHE in living cells. Significantly reduced photobleaching in MP microscopy of DHE enabled us to acquire three-dimensional z-stacks of DHE-stained cells and to obtain high-resolution maps of DHE in surface ruffles, nanotubes, and the apical membrane of epithelial cells. We found that the lateral resolution of MP microscopy is ∼1.5-fold higher than that of UV-WF deconvolution microscopy, allowing for improved spatiotemporal analysis of plasma membrane sterol distribution. Surface intensity patterns of DHE with a diameter of 0.2 μm persisting over several minutes could be resolved by MP time-lapse microscopy. Diffusion coefficients of 0.25-μm-diameter endocytic vesicles containing DHE were determined by MP spatiotemporal image correlation spectroscopy. The requirement of extremely high laser power for visualization of DHE by MP microscopy made this method less potent for multicolor applications with organelle markers like green fluorescent protein-tagged proteins. The signal-to-noise ratio obtainable by UV-WF imaging could be significantly improved by pixelwise bleach rate fitting and calculation of an amplitude image from the decay model and by frame averaging after pixelwise bleaching correction of the image stacks. We conclude that UV-WF imaging and MP microscopy of DHE provide complementary information regarding membrane distribution and intracellular targeting of sterols.

Probes for studying cholesterol binding and cell biology

Cholesterol is a multifunctional lipid in eukaryotic cells. It regulates the physical state of the phospholipid bilayer, is crucially involved in the formation of membrane microdomains, affects the activity of many membrane proteins, and is the precursor for steroid hormones and bile acids. Thus, cholesterol plays a profound role in the physiology and pathophysiology of eukaryotic cells. The cholesterol molecule has achieved evolutionary perfection to fulfill its different functions in membrane organization. Here, we review basic approaches to explore the interaction of cholesterol with proteins, with a particular focus on the high diversity of fluorescent and photoreactive cholesterol probes available today.

Niemann-Pick type C disease: molecular mechanisms and potential therapeutic approaches

Cholesterol is an important lipid of mammalian cells. Its unique physicochemical properties modulate membrane behavior and it serves as the precursor for steroid hormones, oxysterols and vitamin D. Cholesterol is effluxed from the late endosomes/lysosomes via the concerted action of at least two distinct proteins: Niemann-Pick C (NPC)1 and NPC2. Mutations in these two proteins manifest as NPC disease - a very rare, usually fatal, autosomal, recessive, neurovisceral, lysosomal storage disorder. In this review, we discuss the possible mechanisms of action for NPC1 and NPC2 in mediating cholesterol efflux, as well as the different therapeutic approaches being pursued for the treatment of this lipid storage disorder.

Cholesterol pathways affected by small molecules that decrease sterol levels in Niemann-Pick type C mutant cells

Background

Niemann-Pick type C (NPC) disease is a genetically inherited multi-lipid storage disorder with impaired efflux of cholesterol from lysosomal storage organelles.

Methodology/Principal Findings

The effect of screen-selected cholesterol lowering compounds on the major sterol pathways was studied in CT60 mutant CHO cells lacking NPC1 protein. Each of the selected chemicals decreases cholesterol in the lysosomal storage organelles of NPC1 mutant cells through one or more of the following mechanisms: increased cholesterol efflux from the cell, decreased uptake of low-density lipoproteins, and/or increased levels of cholesteryl esters. Several chemicals promote efflux of cholesterol to extracellular acceptors in both non-NPC and NPC1 mutant cells. The uptake of low-density lipoprotein-derived cholesterol is inhibited by some of the studied compounds.

Conclusions/Significance

Results herein provide the information for prioritized further studies in identifying molecular targets of the chemicals. This approach proved successful in the identification of seven chemicals as novel inhibitors of lysosomal acid lipase (Rosenbaum et al, Biochim. Biophys. Acta. 2009, 1791:1155–1165).

ARF6-mediated endocytic recycling impacts cell movement, cell division and lipid homeostasis

A wide range of cellular activities depends upon endocytic recycling. ARF6, a small molecular weight GTPase, regulates the processes of endocytosis and endocytic recycling in concert with various effector molecules and other small GTPases. This review highlights three critical processes that involve ARF6-mediated endosomal membrane trafficking-cell motility, cytokinesis, and cholesterol homeostasis. In each case, the function of ARF6-mediated trafficking varies-including localization of specific protein and lipid cargo, regulation of bulk membrane movement, and modulation of intracellular signaling. As described in this review, mis-regulation of endocytic traffic can result in human disease when it compromises the cell's ability to regulate cell movement and invasion, cell division, and lipid homeostasis.

Niemann-Pick type C cells show cholesterol dependent decrease of APP expression at the cell surface and its increased processing through the beta-secretase pathway

The link between cholesterol and Alzheimer's disease has recently been revealed in Niemann-Pick type C disease. We found that NPC1(-/-) cells show decreased expression of APP at the cell surface and increased processing of APP through the beta-secretase pathway resulting in increased C99, sAPPbeta and intracellular Abeta40 levels. This effect is dependent on increased cholesterol levels, since cholesterol depletion reversed cell surface APP expression and lowered Abeta/C99 levels in NPC1(-)(/)(-) cells to the levels observed in wt cells. Finding that overexpression of C99, a direct gamma-secretase substrate, does not lead to increased intracellular Abeta levels in NPC1(-)(/)(-) cells vs. CHOwt suggests that the effect on intracellular Abeta upon cholesterol accumulation in NPC1(-)(/)(-) cells is not due to increased APP cleavage by gamma-secretase. Our results indicate that cholesterol may modulate APP processing indirectly by modulating APP expression at the cell surface and thus its cleavage by beta-secretase.

Endocytosis of beta-cyclodextrins is responsible for cholesterol reduction in Niemann-Pick type C mutant cells

Niemann-Pick type C disease (NPC) is a lysosomal storage disorder causing accumulation of unesterified cholesterol in lysosomal storage organelles. Recent studies have shown that hydroxypropyl-beta-cyclodextrin injections in npc1(-/-) mice are partially effective in treating this disease. Using cultured fibroblasts, we have investigated the cellular mechanisms responsible for reduction of cholesterol accumulation. We show that decreased levels of cholesterol accumulation are maintained for several days after removal of cyclodextrin from the culture medium. This suggests that endocytosed cyclodextrin can reduce the cholesterol storage by acting from inside endocytic organelles rather than by removing cholesterol from the plasma membrane. To test this further, we incubated both NPC1 and NPC2 mutant cells with cholesterol-loaded cyclodextrin for 1 h, followed by chase in serum-containing medium. Although the cholesterol content of the treated cells increased after the 1-h incubation, the cholesterol levels in the storage organelles were later reduced significantly. We covalently coupled cyclodextrin to fluorescent dextran polymers. These cyclodextrin-dextran conjugates were delivered to cholesterol-enriched lysosomal storage organelles and were effective at reducing the cholesterol accumulation. We demonstrate that methyl-beta-cyclodextrin is more potent than hydroxypropyl-beta-cyclodextrin in reducing both cholesterol and bis(monoacylglycerol) phosphate accumulation in NPC mutant fibroblasts. Brief treatment of cells with cyclodextrins causes an increase in cholesterol esterification by acyl CoA:cholesterol acyl transferase, indicating increased cholesterol delivery to the endoplasmic reticulum. These findings suggest that cyclodextrin-mediated enhanced cholesterol transport from the endocytic system can reduce cholesterol accumulation in cells with defects in either NPC1 or NPC2.

Improvement in lipid and protein trafficking in Niemann-Pick C1 cells by correction of a secondary enzyme defect

Different primary lysosomal trafficking defects lead to common alterations in lipid trafficking, suggesting cooperative interactions among lysosomal lipids. However, cellular analysis of the functional consequences of this phenomenon is lacking. As a test case, we studied cells with defective Niemann-Pick C1 (NPC1) protein, a cholesterol trafficking protein whose defect gives rise to lysosomal accumulation of cholesterol and other lipids, leading to NPC disease. NPC1 cells also develop a secondary defect in acid sphingomyelinase (SMase) activity despite a normal acid SMase gene (SMPD1). When acid SMase activity was restored to normal levels in NPC1-deficient CHO cells through SMPD1 transfection, there was a dramatic reduction in lysosomal cholesterol. Two other defects, excess lysosomal bis-(monoacylglycerol) phosphate (BMP) and defective transferrin receptor (TfR) recycling, were also markedly improved. To test its relevance in human cells, the acid SMase activity defect in fibroblasts from NPC1 patients was corrected by SMPD1 transfection or acid SMase enzyme replacement. Both treatments resulted in a dramatic reduction in lysosomal cholesterol. These data show that correcting one aspect of a complex lysosomal lipid storage disease can reduce the cellular consequences even if the primary genetic defect is not corrected.

Cholesterol-protein interaction: methods and cholesterol reporter molecules

Cholesterol is a major constituent of the plasma membrane in eukaryotic cells. It regulates the physical state of the phospholipid bilayer and is crucially involved in the formation of membrane microdomains. Cholesterol also affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Here, methods are described that are used to explore the binding and/or interaction of proteins to cholesterol. For this purpose, a variety of cholesterol probes bearing radio-, spin-, photoaffinity- or fluorescent labels are currently available. Examples of proven cholesterol binding molecules are polyene compounds, cholesterol-dependent cytolysins, enzymes accepting cholesterol as substrate, and proteins with cholesterol binding motifs. Main topics of this report are the localization of candidate membrane proteins in cholesterol-rich microdomains, the issue of specificity of cholesterol- protein interactions, and applications of the various cholesterol probes for these studies.

Bis(monoacylglycero)phosphate, a peculiar phospholipid to control the fate of cholesterol: Implications in pathology

Bis(monoacylglycero)phosphate (BMP) is a structural isomer of phosphatidylglycerol that exhibits an unusual sn1:sn1' stereoconfiguration, based on the position of the phosphate moiety on its two glycerol units. Early works have underlined the high concentration of BMP in the lysosomal compartment, especially during some lysosomal storage disorders and drug-induced phospholipidosis. Despite numerous studies, both biosynthetic and degradative pathways of BMP remained not completely elucidated. More recently, BMP has been localized in the internal membranes of late endosomes where it forms specialized lipid domains. Its involvement in both dynamics and lipid/protein sorting functions of late endosomes has started to be documented, especially in the control of cellular cholesterol distribution. BMP also plays an important role in the late endosomal/lysosomal degradative pathway. Another peculiarity of BMP is to be naturally enriched in docosahexaenoic acid and/or to specifically incorporate this fatty acid compared to other polyunsaturated fatty acids, which may confer specific biophysical and functional properties to this phospholipid. This review summarizes and updates our knowledge on BMP with an emphasis on its possible implication in human health and diseases, especially in relation to cholesterol homeostasis.

Chemical screen to reduce sterol accumulation in Niemann-Pick C disease cells identifies novel lysosomal acid lipase inhibitors

Niemann-Pick C disease (NPC) is a lysosomal storage disorder causing abnormal accumulation of unesterified free cholesterol in lysosomal storage organelles. High content phenotypic microscopy chemical screens in both human and hamster NPC-deficient cells have identified several compounds that partially revert the NPC phenotype. Cell biological and biochemical studies show that several of these molecules inhibit lysosomal acid lipase, the enzyme that hydrolyzes LDL-derived triacylglycerol and cholesteryl esters. The effects of reduced lysosomal acid lipase activity in lowering cholesterol accumulation in NPC mutant cells were verified by RNAi-mediated knockdown of lysosomal acid lipase in NPC1-deficient human fibroblasts. This work demonstrates the utility of phenotypic cellular screens as a means to identify molecular targets for altering a complex process such as intracellular cholesterol trafficking and metabolism.

ARF6-mediated endosome recycling reverses lipid accumulation defects in Niemann-Pick Type C disease

In human Niemann-Pick Type C (NPC) disease, endosomal trafficking defects lead to an accumulation of free cholesterol and other lipids in late endosome/lysosome (LE/LY) compartments, a subsequent block in cholesterol esterification and significantly reduced cholesterol efflux out of the cell. Here we report that nucleotide cycling or cellular knockdown of the small GTP-binding protein, ARF6, markedly impacts cholesterol homeostasis. Unregulated ARF6 activation attenuates the NPC phenotype at least in part by decreasing cholesterol accumulation and restoring normal sphingolipid trafficking. These effects depend on ARF6-stimulated cholesterol efflux out of the endosomal recycling compartment, a major cell repository for free cholesterol. We also show that fibroblasts derived from different NPC patients displayed varying levels of ARF6 that is GTP-bound, which correlate with their response to sustained ARF6 activation. These studies support emerging evidence that early endocytic defects impact NPC disease and suggest that such heterogeneity in NPC disease could result in diverse responses to therapeutic interventions aimed at modulating the trafficking of lipids.

Intracellular sterol dynamics

We review the cellular mechanisms implicated in cholesterol trafficking and distribution. Recent studies have provided new information about the distribution of sterols within cells, including analysis of its transbilayer distribution. The cholesterol interaction with other lipids and its engagement in various trafficking processes will determine its proper level in a specific membrane; making the cholesterol distribution uneven among the various intracellular organelles. The cholesterol content is important since cholesterol plays an essential role in membranes by controlling their physicochemical properties as well as key cellular events such as signal transduction and protein trafficking. Cholesterol movement between cellular organelles is highly dynamic, and can be achieved by vesicular and non-vesicular processes. Various studies have analyzed the proteins that play a significant role in these processes, giving us new information about the relative importance of these two trafficking pathways in cholesterol transport. Although still poorly characterized in many trafficking routes, several potential sterol transport proteins have been described in detail; as a result, molecular mechanisms for sterol transport among membranes start to be appreciated.

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

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

Cholesterol regulation of rab-mediated sphingolipid endocytosis

Despite a tight regulation of its intracellular content, cholesterol is found accumulated in pathological conditions such as sphingolipidosis as well as after cell treatment with drugs like hydrophobic amines. Furthermore, cellular cholesterol increases when cultured cells approach confluence. Under these conditions, the endocytic pathways of plasma membrane sphingolipids are differently affected. In this short review, we will summarize recent results from our laboratory as well as those of other groups, indicating that the intracellular accumulation of cholesterol inhibits the dissociation of rab GTPases from the target membranes, causing the alteration of rab-mediated membrane traffic.

Regulation of sterol transport between membranes and NPC2

Niemann-Pick disease type C (NPC) is caused by defects in either the NPC1 or NPC2 gene and is characterized by accumulation of cholesterol and glycolipids in the late endosome/lysosome compartment. NPC2 is an intralysosomal protein that binds cholesterol in vitro. Previous studies demonstrated rapid rates of cholesterol transfer from NPC2 to model membranes [Cheruku, S. R., et al. (2006) J. Biol. Chem. 281, 31594-31604]. To model the potential role of NPC2 as a lysosomal cholesterol export protein, in this study we used fluorescence spectroscopic approaches to examine cholesterol transfer from membranes to NPC2, assessing the rate, mechanism, and regulation of this transport step. In addition, we examined the effect of NPC2 on the rate and kinetic mechanism of intermembrane sterol transport, to model the movement of cholesterol from internal lysosomal membranes to the limiting lysosomal membrane. The results support the hypothesis that NPC2 plays an important role in endo/lysosomal cholesterol trafficking by markedly accelerating the rates of cholesterol transport. Rates of sterol transfer from and between membranes were increased by as much as 2 orders of magnitude by NPC2. The transfer studies indicate that the mechanism of NPC2 action involves direct interaction of the protein with membranes. Such interactions were observed directly using FTIR spectroscopy and protein tryptophan spectral shifts. Additionally, cholesterol transfer by NPC2 was found to be greatly enhanced by the unique lysosomal phospholipid lyso-bisphosphatidic acid (LBPA), suggesting an important role for LBPA in NPC2-mediated cholesterol trafficking.

(-)-Epigallocatechin gallate causes internalization of the epidermal growth factor receptor in human colon cancer cells

We recently found that the inhibitory effect of (-)-epigallocatechin gallate (EGCG) on epidermal growth factor (EGF) binding to the epidermal growth factor receptor (EGFR) is associated with alterations in lipid organization in the plasma membrane of colon cancer cells. Since changes in lipid organizations are thought to play a role in the trafficking of several membrane proteins, in this study we examined the effects of EGCG on cellular localization of the EGFR in SW480 cells. Treatment of the cells for 30 min with as little as 1 microg/ml of EGCG caused a decrease in cell surface-associated EGFRs and this was associated with internalization of EGFRs into endosomal vesicles. Similar effects were seen with a green fluorescent protein (GFP)-EGFR fusion protein. As expected, the EGFR protein was phosphorylated at tyrosine residues, ubiquitinated and partially degraded when the cells were treated with EGF, but treatment with EGCG caused none of these effects. The loss of EGFRs from the cell surface induced by treating the cells with EGF for 30 min persisted for at least 2 h. However, the loss of EGFRs from the cell surface induced by temporary exposure to EGCG was partially restored within 1-2 h. These studies provide the first evidence that EGCG can induce internalization of EGFRs into endosomes, which can recycle back to the cell surface. This sequestrating of inactivated EGFRs into endosomes may explain, at least in part, the ability of EGCG to inhibit activation of the EGFR and thereby exert anticancer effects.

Kinetic imaging of NPC1L1 and sterol trafficking between plasma membrane and recycling endosomes in hepatoma cells

Niemann-Pick C1-like 1 (NPC1L1) is a recently identified protein that mediates intestinal cholesterol absorption and regulates biliary cholesterol excretion. The itineraries and kinetics of NPC1L1 trafficking remain uncertain. In this study, we have visualized movement of NPC1L1-enhanced green fluorescent protein (NPC1L1-EGFP) and cholesterol analogs in hepatoma cells. At steady state, about 42% of NPC1L1 resided in the transferrin (Tf)-positive, sterol-enriched endocytic recycling compartment (ERC), whereas time-lapse microscopy demonstrated NPC1L1 traffic between the plasma membrane and the ERC. Fluorescence recovery after photobleaching revealed rapid recovery (half-time approximately 2.5 min) of about 35% of NPC1L1 in the ERC, probably replenished from peripheral sorting endosomes. Acute cholesterol depletion blocked internalization of NPC1L1-EGFP and Tf and stimulated recycling of NPC1L1-EGFP from the ERC to the plasma membrane. NPC1L1-EGFP facilitated transport of fluorescent sterols from the plasma membrane to the ERC. Insulin induced translocation of vesicles containing NPC1L1 and fluorescent sterol from the ERC to the cell membrane. Upon polarization of hepatoma cells, NPC1L1 resided almost exclusively in the canalicular membrane, where the protein is highly mobile. Our study demonstrates dynamic trafficking of NPC1L1 between the cell surface and intracellular compartments and suggests that this transport is involved in NPC1L1-mediated cellular sterol uptake.

Cholesterol reporter molecules

Cholesterol is a major constituent of the membranes in most eukaryotic cells where it fulfills multiple functions. Cholesterol regulates the physical state of the phospholipid bilayer, affects the activity of several membrane proteins, and is the precursor for steroid hormones and bile acids. Cholesterol plays a crucial role in the formation of membrane microdomains such as "lipid rafts" and caveolae. However, our current understanding on the membrane organization, intracellular distribution and trafficking of cholesterol is rather poor. This is mainly due to inherent difficulties to label and track this small lipid. In this review, we describe different approaches to detect cholesterol in vitro and in vivo. Cholesterol reporter molecules can be classified in two groups: cholesterol binding molecules and cholesterol analogues. The enzyme cholesterol oxidase is used for the determination of cholesterol in serum and food. Susceptibility to cholesterol oxidase can provide information about localization, transfer kinetics, or transbilayer distribution of cholesterol in membranes and cells. The polyene filipin forms a fluorescent complex with cholesterol and is commonly used to visualize the cellular distribution of free cholesterol. Perfringolysin O, a cholesterol binding cytolysin, selectively recognizes cholesterol-rich structures. Photoreactive cholesterol probes are appropriate tools to analyze or to identify cholesterol binding proteins. Among the fluorescent cholesterol analogues one can distinguish probes with intrinsic fluorescence (e.g., dehydroergosterol) from those possessing an attached fluorophore group. We summarize and critically discuss the features of the different cholesterol reporter molecules with a special focus on recent imaging approaches.

Endocytic trafficking of sphingomyelin depends on its acyl chain length

To study the principles of endocytic lipid trafficking, we introduced pyrene sphingomyelins (PyrSMs) with varying acyl chain lengths and domain partitioning properties into human fibroblasts or HeLa cells. We found that a long-chain, ordered-domain preferring PyrSM was targeted Hrs and Tsg101 dependently to late endosomal compartments and recycled to the plasma membrane in an NPC1- and cholesterol-dependent manner. A short-chain, disordered domain preferring PyrSM recycled more effectively, by using Hrs-, Tsg101- and NPC1-independent routing that was insensitive to cholesterol loading. Similar chain length-dependent recycling was observed for unlabeled sphingomyelins (SMs). The findings 1) establish acyl chain length as an important determinant in the endocytic trafficking of SMs, 2) implicate ESCRT complex proteins and NPC1 in the endocytic recycling of ordered domain lipids to the plasma membrane, and 3) introduce long-chain PyrSM as the first fluorescent lipid tracing this pathway.