Cholesterol distribution in living cells: fluorescence imaging using dehydroergosterol as a fluorescent cholesterol analog

Exploring the Enigma: The Role of the Epithelial Protein Lost in Neoplasm in Normal Physiology and Cancer Pathogenesis

The cytoskeleton plays a pivotal role in maintaining the epithelial phenotype and is vital to several hallmark processes of cancer. Over the past decades, researchers have identified the epithelial protein lost in neoplasm (EPLIN, also known as LIMA1) as a key regulator of cytoskeletal dynamics, cytoskeletal organization, motility, as well as cell growth and metabolism. Dysregulation of EPLIN is implicated in various aspects of cancer progression, such as tumor growth, invasion, metastasis, and therapeutic resistance. Its altered expression levels or activity can disrupt cytoskeletal dynamics, leading to aberrant cell motility and invasiveness characteristic of malignant cells. Moreover, the involvement of EPLIN in cell growth and metabolism underscores its significance in orchestrating key processes essential for cancer cell survival and proliferation. This review provides a comprehensive exploration of the intricate roles of EPLIN across diverse cellular processes in both normal physiology and cancer pathogenesis. Additionally, this review discusses the possibility of EPLIN as a potential target for anticancer therapy in future studies.

Dynamic Mode Decomposition of Multiphoton and Stimulated Emission Depletion Microscopy Data for Analysis of Fluorescent Probes in Cellular Membranes

An analysis of the membrane organization and intracellular trafficking of lipids often relies on multiphoton (MP) and super-resolution microscopy of fluorescent lipid probes. A disadvantage of particularly intrinsically fluorescent lipid probes, such as the cholesterol and ergosterol analogue, dehydroergosterol (DHE), is their low MP absorption cross-section, resulting in a low signal-to-noise ratio (SNR) in live-cell imaging. Stimulated emission depletion (STED) microscopy of membrane probes like Nile Red enables one to resolve membrane features beyond the diffraction limit but exposes the sample to a lot of excitation light and suffers from a low SNR and photobleaching. Here, dynamic mode decomposition (DMD) and its variant, higher-order DMD (HoDMD), are applied to efficiently reconstruct and denoise the MP and STED microscopy data of lipid probes, allowing for an improved visualization of the membranes in cells. HoDMD also allows us to decompose and reconstruct two-photon polarimetry images of TopFluor-cholesterol in model and cellular membranes. Finally, DMD is shown to not only reconstruct and denoise 3D-STED image stacks of Nile Red-labeled cells but also to predict unseen image frames, thereby allowing for interpolation images along the optical axis. This important feature of DMD can be used to reduce the number of image acquisitions, thereby minimizing the light exposure of biological samples without compromising image quality. Thus, DMD as a computational tool enables gentler live-cell imaging of fluorescent probes in cellular membranes by MP and STED microscopy.

A cholesterol switch controls phospholipid scrambling by G protein-coupled receptors

Class A G protein-coupled receptors (GPCRs), a superfamily of cell membrane signaling receptors, moonlight as constitutively active phospholipid scramblases. The plasma membrane of metazoan cells is replete with GPCRs yet has a strong resting trans-bilayer phospholipid asymmetry, with the signaling lipid phosphatidylserine confined to the cytoplasmic leaflet. To account for the persistence of this lipid asymmetry in the presence of GPCR scramblases, we hypothesized that GPCR-mediated lipid scrambling is regulated by cholesterol, a major constituent of the plasma membrane. We now present a technique whereby synthetic vesicles reconstituted with GPCRs can be supplemented with cholesterol to a level similar to that of the plasma membrane and show that the scramblase activity of two prototypical GPCRs, opsin and the β1-adrenergic receptor, is impaired upon cholesterol loading. Our data suggest that cholesterol acts as a switch, inhibiting scrambling above a receptor-specific threshold concentration to disable GPCR scramblases at the plasma membrane.

Bile acids-mediated intracellular cholesterol transport promotes intestinal cholesterol absorption and NPC1L1 recycling

Niemann-Pick C1-like 1 (NPC1L1) is essential for intestinal cholesterol absorption. Together with the cholesterol-rich and Flotillin-positive membrane microdomain, NPC1L1 is internalized via clathrin-mediated endocytosis and transported to endocytic recycling compartment (ERC). When ERC cholesterol level decreases, NPC1L1 interacts with LIMA1 and moves back to plasma membrane. However, how cholesterol leaves ERC is unknown. Here, we find that, in male mice, intracellular bile acids facilitate cholesterol transport to other organelles, such as endoplasmic reticulum, in a non-micellar fashion. When cholesterol level in ERC is decreased by bile acids, the NPC1L1 carboxyl terminus that previously interacts with the cholesterol-rich membranes via the A1272LAL residues dissociates from membrane, exposing the Q1277KR motif for LIMA1 recruitment. Then NPC1L1 moves back to plasma membrane. This study demonstrates an intracellular cholesterol transport function of bile acids and explains how the substantial amount of cholesterol in NPC1L1-positive compartments is unloaded in enterocytes during cholesterol absorption.

Intracellular Cholesterol Trafficking

Cholesterol is an essential lipid species of mammalian cells. Cells acquire it through synthesis in the endoplasmic reticulum (ER) and uptake from lipoprotein particles. Newly synthesized cholesterol is efficiently distributed from the ER to other organelles via lipid-binding/transfer proteins concentrated at membrane contact sites (MCSs) to reach the trans-Golgi network, endosomes, and plasma membrane. Lipoprotein-derived cholesterol is exported from the plasma membrane and endosomal compartments via a combination of vesicle/tubule-mediated membrane transport and transfer through MCSs. In this review, we provide an overview of intracellular cholesterol trafficking pathways, including cholesterol flux from the ER to other membranes, cholesterol uptake from lipoprotein donors and transport from the plasma membrane to the ER, cellular cholesterol efflux to lipoprotein acceptors, as well as lipoprotein cholesterol secretion from enterocytes, hepatocytes, and astrocytes. We also briefly discuss human diseases caused by defects in these processes and therapeutic strategies available in such conditions.

Activity and Structural Dynamics of Human ABCA1 in a Lipid Membrane

The human ATP-binding cassette (ABC) transporter ABCA1 plays a critical role in lipid homeostasis as it extracts sterols and phospholipids from the plasma membrane for excretion to the extracellular apolipoprotein A-I and subsequent formation of high-density lipoprotein (HDL) particles. Deleterious mutations of ABCA1 lead to sterol accumulation and are associated with atherosclerosis, poor cardiovascular outcomes, cancer, and Alzheimer's disease. The mechanism by which ABCA1 drives lipid movement is poorly understood, and a unified platform to produce active ABCA1 protein for both functional and structural studies has been missing. In this work, we established a stable expression system for both a human cell-based sterol export assay and protein purification for in vitro biochemical and structural studies. ABCA1 produced in this system was active in sterol export and displayed enhanced ATPase activity after reconstitution into a lipid bilayer. Our single-particle cryo-EM study of ABCA1 in nanodiscs showed protein induced membrane curvature, revealed multiple distinct conformations, and generated a structure of nanodisc-embedded ABCA1 at 4.0-Å resolution representing a previously unknown conformation. Comparison of different ABCA1 structures and molecular dynamics simulations demonstrates both concerted domain movements and conformational variations within each domain. Taken together, our platform for producing and characterizing ABCA1 in a lipid membrane enabled us to gain important mechanistic and structural insights and paves the way for investigating modulators that target the functions of ABCA1.

Role of STAR and SCP2/SCPx in the Transport of Cholesterol and Other Lipids

Cholesterol is a lipid molecule essential for several key cellular processes including steroidogenesis. As such, the trafficking and distribution of cholesterol is tightly regulated by various pathways that include vesicular and non-vesicular mechanisms. One non-vesicular mechanism is the binding of cholesterol to cholesterol transport proteins, which facilitate the movement of cholesterol between cellular membranes. Classic examples of cholesterol transport proteins are the steroidogenic acute regulatory protein (STAR; STARD1), which facilitates cholesterol transport for acute steroidogenesis in mitochondria, and sterol carrier protein 2/sterol carrier protein-x (SCP2/SCPx), which are non-specific lipid transfer proteins involved in the transport and metabolism of many lipids including cholesterol between several cellular compartments. This review discusses the roles of STAR and SCP2/SCPx in cholesterol transport as model cholesterol transport proteins, as well as more recent findings that support the role of these proteins in the transport and/or metabolism of other lipids.

Cholesterol and Its Derivatives: Multifaceted Players in Breast Cancer Progression

Cholesterol is an essential lipid primarily synthesized in the liver through the mevalonate pathway. Besides being a precursor of steroid hormones, bile acid, and vitamin D, it is an essential structural component of cell membranes, is enriched in membrane lipid rafts, and plays a key role in intracellular signal transduction. The lipid homeostasis is finely regulated end appears to be impaired in several types of tumors, including breast cancer. In this review, we will analyse the multifaceted roles of cholesterol and its derivatives in breast cancer progression. As an example of the bivalent role of cholesterol in the cell membrane of cancer cells, on the one hand, it reduces membrane fluidity, which has been associated with a more aggressive tumor phenotype in terms of cell motility and migration, leading to metastasis formation. On the other hand, it makes the membrane less permeable to small water-soluble molecules that would otherwise freely cross, resulting in a loss of chemotherapeutics permeability. Regarding cholesterol derivatives, a lower vitamin D is associated with an increased risk of breast cancer, while steroid hormones, coupled with the overexpression of their receptors, play a crucial role in breast cancer progression. Despite the role of cholesterol and derivatives molecules in breast cancer development is still controversial, the use of cholesterol targeting drugs like statins and zoledronic acid appears as a challenging promising tool for breast cancer treatment.

Rhenium(I) conjugates as tools for tracking cholesterol in cells

Cholesterol is vital to control membrane integrity and fluidity, but is also a precursor to produce steroid hormones, bile acids, and vitamin D. Consequently, altered cholesterol biology has been linked to many diseases, including metabolic syndromes and cancer. Defining the intracellular pools of cholesterol and its trafficking within cells is essential to understand both normal cell physiology and mechanisms of pathogenesis. We have synthesized a new cholesterol mimic (ReTEGCholestanol), comprising a luminescent rhenium metal complex and a cholestanol targeting unit, linked using a tetraethylene glycol (TEG) spacer. ReTEGCholestanol demonstrated favourable imaging properties and improved water solubility when compared to a cholesterol derivative, and structurally related probes lacking the TEG linker. A non-malignant and three malignant prostate cell lines were used to characterize the uptake and intracellular distribution of ReTEGCholestanol. The ReTEGCholestanol complex was effectively internalized and mainly localized to late endosomes/lysosomes in non-malignant PNT1a cells, while in prostate cancer cells it also accumulated in early endosomes and multivesicular bodies, suggesting disturbed cholesterol biology in the malignant cells. The ReTEGCholestanol is a novel imaging agent for visualizing endosomal uptake and trafficking, which may be used to define cholesterol related biology including membrane integration and altered lipid trafficking/processing.

Molecular mechanisms for ABCA1-mediated cholesterol efflux

The maintenance of cellular cholesterol homeostasis is essential for normal cell function and viability. Excessive cholesterol accumulation is detrimental to cells and serves as the molecular basis of many diseases, such as atherosclerosis, Alzheimer's disease, and diabetes mellitus. The peripheral cells do not have the ability to degrade cholesterol. Cholesterol efflux is therefore the only pathway to eliminate excessive cholesterol from these cells. This process is predominantly mediated by ATP-binding cassette transporter A1 (ABCA1), an integral membrane protein. ABCA1 is known to transfer intracellular free cholesterol and phospholipids to apolipoprotein A-I (apoA-I) for generating nascent high-density lipoprotein (nHDL) particles. nHDL can accept more free cholesterol from peripheral cells. Free cholesterol is then converted to cholesteryl ester by lecithin:cholesterol acyltransferase to form mature HDL. HDL-bound cholesterol enters the liver for biliary secretion and fecal excretion. Although how cholesterol is transported by ABCA1 to apoA-I remains incompletely understood, nine models have been proposed to explain this effect. In this review, we focus on the current view of the mechanisms underlying ABCA1-mediated cholesterol efflux to provide an important framework for future investigation and lipid-lowering therapy.

Sterolight as imaging tool to study sterol uptake, trafficking and efflux in living cells

Information about cholesterol subcellular localization and transport pathways inside cells is essential for understanding and treatment of cholesterol-related diseases. However, there is a lack of reliable tools to monitor it. This work follows the fate of Sterolight, a BODIPY-labelled sterol, within the cell and demonstrates it as a suitable probe for visualization of sterol/lipid trafficking. Sterolight enters cells through an energy-independent process and knockdown experiments suggest caveolin-1 as its potential cellular carrier. Intracellular transport of Sterolight is a rapid process, and transfer from ER and mitochondria to lysosomes and later to lipid droplets requires the participation of active microtubules, as it can be inhibited by the microtubule disruptor nocodazole. Excess of the probe is actively exported from cells, in addition to being stored in lipid droplets, to re-establish the sterol balance. Efflux occurs through a mechanism requiring energy and may be selectively poisoned with verapamil or blocked in cells with mutated cholesterol transporter NPC1. Sterolight is efficiently transferred within and between different cell populations, making it suitable for monitoring numerous aspects of sterol biology, including the live tracking and visualization of intracellular and intercellular transport.

Fluorescence sensors for imaging membrane lipid domains and cholesterol

Lipid membrane domains are supramolecular lateral heterogeneities of biological membranes. Of nanoscopic dimensions, they constitute specialized hubs used by the cell as transient signaling platforms for a great variety of biologically important mechanisms. Their property to form and dissolve in the bulk lipid bilayer endow them with the ability to engage in highly dynamic processes, and temporarily recruit subpopulations of membrane proteins in reduced nanometric compartments that can coalesce to form larger mesoscale assemblies. Cholesterol is an essential component of these lipid domains; its unique molecular structure is suitable for interacting intricately with crevices and cavities of transmembrane protein surfaces through its rough β face while "talking" to fatty acid acyl chains of glycerophospholipids and sphingolipids via its smooth α face. Progress in the field of membrane domains has been closely associated with innovative improvements in fluorescence microscopy and new fluorescence sensors. These advances enabled the exploration of the biophysical properties of lipids and their supramolecular platforms. Here I review the rationale behind the use of biosensors over the last few decades and their contributions towards elucidation of the in-plane and transbilayer topography of cholesterol-enriched lipid domains and their molecular constituents. The challenges introduced by super-resolution optical microscopy are discussed, as well as possible scenarios for future developments in the field, including virtual ("no staining") staining.

The role of cholesterol recognition (CARC/CRAC) mirror codes in the allosterism of the human organic cation transporter 2 (OCT2, SLC22A2)

The human organic cation transporter 2 (OCT2) is a multispecific transporter with cholesterol-dependent allosteric features. The present work elucidates the role of evolutionarily conserved cholesterol recognition/interaction amino acid consensus sequences (CRAC and CARC) in the allosteric binding to 1-methyl-4-phenylpyridinium (MPP⁺) in human embryonic kidney 293 cells stably or transiently expressing OCT2. Molecular blind simulations docked two mirroring cholesterol molecules in the 5th putative transmembrane domain, where a CARC and a CRAC sequence lie. The impact of the conserved amino acids that may constitute the CARC/CRAC mirror code was studied by alanine-scanning mutagenesis. At a saturating extracellular concentration of substrate, at which the impact of cholesterol depletion is maximal, five mutants transported MPP⁺ at a significantly lower rate than the wild-type OCT2 (WT), resembling the behavior of the WT upon cholesterol depletion. MPP⁺ influx rate as a function of the extracellular concentration of substrate was measured for the mutants R234A, R235A, L252A and R263A. R234A kinetic behavior was similar to that of the WT, whereas R235A, L252A and R263A activity shifted from allosteric to one-binding site kinetics, very much like the WT upon cholesterol depletion. The impact of cholesterol on protein thermal stability was assessed for WT, R234A and R263A. While the thermal stability of WT and R234A was improved by the supplementation with cholesterol, R263A was not sensitive to the presence of cholesterol. To conclude, the disruption of the CARC/CRAC mirror code in the 5th putative transmembrane domain is sufficient to abolish the allosteric interaction between OCT2 and MPP⁺.

The impact of TiO2 nanoparticle exposure on transmembrane cholesterol transport and enhanced bacterial infectivity in HeLa cells

We have previously shown that exposure to TiO₂ nanoparticles (NPs) reduces the resistance of HeLa cells to bacterial infection. Here we demonstrate that the increased infectivity is associated with enhanced asymmetry in the cholesterol distribution. We applied a live cell imaging method which uses tunable orthogonal cholesterol sensors to visualize and quantify in-situ cholesterol distribution between the two leaflets of the plasma membrane (PM). In the control culture, we found marked transbilayer asymmetry of cholesterol, with the concentration in the outer plasma membrane (OPM) being 13 ± 2-fold higher than that in the inner plasma membrane (IPM). Exposure of the culture to 0.1 mg/mL of rutile TiO₂ NPs increased the asymmetry such that the concentration in the OPM was 51 ± 10 times higher, while the total cholesterol content increased only 21 ± 2%. This change in cholesterol gradient may explain the increase in bacterial infectivity in HeLa cells exposed to TiO₂ NPs since many pathogens, including Staphylococcus aureus used in the present study, require cholesterol for proper membrane attachment and virulence. RT-PCR indicated that exposure to TiO₂ was responsible for upregulation of the ABCA1 and ABCG1 mRNAs, which are responsible for the production of the cholesterol transporter proteins that facilitate cholesterol transport across cellular membranes. This was confirmed by the observation of an overall decrease in bacterial infection in ABCA1 knockout or methyl-β-cyclodextrin-treated HeLa cells, as regardless of TiO₂ NP exposure. Hence rather than preventing bacterial infection, TiO₂ nanoparticles upregulate genes associated with membrane cholesterol production and distribution, hence increasing infectivity. STATEMENT OF SIGNIFICANCE: A great deal of work has been done regarding the toxicology of the particles, especially focusing on detrimental outcomes associated with reactive oxygen species (ROS) production. In this paper we show unambiguously a very surprising result, namely the ability of these particles to enhance bacterial infection even at very small exposure levels, where none of the deleterious effects of ROS products can yet be detected. Using a new imaging technique, we are able to demonstrate, in operando, the effect of the particles on cholesterol generation and distribution in live HeLa cells. This paper also represents the first in a series where we explore other consequences of increased membrane cholesterol, due to particle exposure, which are known to have multiple other consequences on human tissue function and development.

Synthesis and biological evaluation of cationic TopFluor cholesterol analogues

Cholesterol is not only a major component of the cell membrane, but also plays an important role in a wide range of biological processes and pathologies. It is therefore crucial to develop appropriate tools for visualizing intracellular cholesterol transport. Here, we describe new cationic analogues of BODIPY-Cholesterol (TopFluor-Cholesterol, TF-Chol), which combine a positive charge on the sterol side chain and a BODIPY group connected via a C-4 linker. In contrast to TF-Chol, the new analogues TF-1 and TF-3 possessing acetyl groups on the A ring (C-3 position on steroid) internalized much faster and displayed slightly different levels of intracellular localization. Their applicability for cholesterol monitoring was indicated by the fact that they strongly label compartments with accumulated cholesterol in cells carrying a mutation of the Niemann-Pick disease-associated cholesterol transporter, NPC1.

Lack of Environmental Sensitivity of a Naturally Occurring Fluorescent Analog of Cholesterol

Dehydroergosterol (DHE, Δ^5,7,9(11),22-ergostatetraen-3β-ol) is a naturally occurring fluorescent analog of cholesterol found in yeast. Since DHE has been shown to faithfully mimic cholesterol in a large number of biophysical, biochemical, and cell biological studies, it is widely used to explore cholesterol organization, dynamics and trafficking in model and biological membranes. In this work, we show that DHE, in spite of its localization at the membrane interface, does not exhibit red edge excitation shift (REES) in model membranes, irrespective of the membrane phase. These results are reinforced by semi-empirical quantum chemical calculations of dipole moment changes of DHE in ground and excited states, which show a very small change in the dipole moment of DHE upon excitation. We conclude that DHE fluorescence exhibits lack of environmental sensitivity, despite its usefulness in monitoring cholesterol organization, dynamics and traffic in model and biological membranes.

Fluorescence-Detected Mid-Infrared Photothermal Microscopy

Mid-infrared photothermal microscopy is a new chemical imaging technology in which a visible beam senses the photothermal effect induced by a pulsed infrared laser. This technology provides infrared spectroscopic information at submicrometer spatial resolution and enables infrared spectroscopy and imaging of living cells and organisms. Yet, current mid-infrared photothermal imaging sensitivity suffers from a weak dependence of scattering on the temperature, and the image quality is vulnerable to the speckles caused by scattering. Here, we present a novel version of mid-infrared photothermal microscopy in which thermosensitive fluorescent probes are harnessed to sense the mid-infrared photothermal effect. The fluorescence intensity can be modulated at the level of 1% per Kelvin, which is 100 times larger than the modulation of scattering intensity. In addition, fluorescence emission is free of interference, thus much improving the image quality. Moreover, fluorophores can target specific organelles or biomolecules, thus augmenting the specificity of photothermal imaging. Spectral fidelity is confirmed through fingerprinting a single bacterium. Finally, the photobleaching issue is successfully addressed through the development of a wide-field fluorescence-detected mid-infrared photothermal microscope which allows video rate bond-selective imaging of biological specimens.

Coronaviruses, cholesterol and statins: Involvement and application for Covid-19

The infectious power of coronaviruses is dependent on cholesterol present in the membranes of their target cells. Indeed, the virus enters the infected cell either by fusion or by endocytosis, in both cases involving cholesterol-enriched membrane microdomains. These membrane domains can be disorganized in-vitro by various cholesterol-altering agents, including statins that inhibit cell cholesterol biosynthesis. As a consequence, numerous cell physiology processes, such as signaling cascades, can be compromised. Also, some examples of anti-bacterial and anti-viral effects of statins have been observed for infectious agents known to be cholesterol dependent. In-vivo, besides their widely-reported hypocholesterolemic effect, statins display various pleiotropic effects mediated, at least partially, by perturbation of membrane microdomains as a consequence of the alteration of endogenous cholesterol synthesis. It should thus be worth considering a high, but clinically well-tolerated, dose of statin to treat Covid-19 patients, in the early phase of infection, to inhibit virus entry into the target cells, in order to control the viral charge and hence avoid severe clinical complications. Based on its efficacy and favorable biodisposition, an option would be considering Atorvastatin, but randomized controlled clinical trials are required to test this hypothesis. This new therapeutic proposal takes benefit from being a drug repurposing, applied to a widely-used drug presenting a high efficiency-to-toxicity ratio. Additionally, this therapeutic strategy avoids any risk of drug resistance by viral mutation since it is host-targeted. Noteworthy, the same pharmacological approach could also be proposed to address different animal coronavirus endemic infections that are responsible for heavy economic losses.

Photophysical and Structural Characterization of Intrinsically Fluorescent Sterol Aggregates

Self-association of cholesterol into aggregates and crystals is a hallmark of developing atherosclerosis. Intrinsically fluorescent sterols, such as dehydroergosterol (DHE), can be used to study sterol aggregation by fluorescence spectroscopy and microscopy, but a thorough understanding of DHE's photophysical and structural properties in the aggregated state is missing. Here, we show that DHE forms submicron fluorescent aggregates when evaporated from an ethanol solution. Using atomic force microscopy, we find that DHE, like cholesterol, forms compact oblate-shape aggregates of <100 nm in diameter. DHE's fluorescence is lowered in the aggregate compared to the monomeric form, and characteristic spectral changes accompany the aggregation process. Electronic structure calculations of DHE dimers in water indicate that Frenkel-type exciton coupling contributes to the lowered DHE fluorescence in the aggregates. Using molecular dynamics (MD) simulations, we show that DHE forms compact aggregates on the nanosecond scale and with strong intermolecular attraction, in which a broad range of orientations, and therefore electronic couplings, will take place. Tight packing of DHE in aggregates also lowers the apparent absorption cross section, further reducing the molecular brightness of the aggregates. Our results pave the way for systematic solubility studies of intrinsically fluorescent analogues of biologically relevant sterols.

Quantitative imaging of membrane contact sites for sterol transfer between endo-lysosomes and mitochondria in living cells

Mitochondria receive cholesterol from late endosomes and lysosomes (LE/LYSs) or from the plasma membrane for production of oxysterols and steroid hormones. This process depends on the endo-lysosomal sterol transfer protein Niemann Pick C2 (NPC2). Using the intrinsically fluorescent cholesterol analog, cholestatrienol, we directly observe sterol transport to mitochondria in fibroblasts upon treating NPC2 deficient human fibroblasts with NPC2 protein. Soft X-ray tomography reveals the ultrastructure of mitochondria and discloses close contact to endosome-like organelles. Using fluorescence microscopy, we localize endo-lysosomes containing NPC2 relative to mitochondria based on the Euclidian distance transform and use statistical inference to show that about 30% of such LE/LYSs are in contact to mitochondria in human fibroblasts. Using Markov Chain Monte Carlo image simulations, we show that interaction between both organelle types, a defining feature of membrane contact sites (MCSs) can give rise to the observed spatial organelle distribution. We devise a protocol to determine the surface fraction of endo-lysosomes in contact with mitochondria and show that this fraction does not depend on functional NPC1 or NPC2 proteins. Finally, we localize MCSs between LE/LYSs containing NPC2 and mitochondria in time-lapse image sequences and show that they either form transiently or remain stable for tens of seconds. Lasting MCSs between endo-lysosomes containing NPC2 and mitochondria move by slow anomalous sub-diffusion, providing location and time for sterol transport between both organelles. Our quantitative imaging strategy will be of high value for characterizing the dynamics and function of MCSs between various organelles in living cells.

Analysis of the cyp51 genes contribution to azole resistance in Aspergillus section Nigri with the CRISPR-Cas9 technique

Cyp51 contribution to azole resistance has been broadly studied and characterized in Aspergillus fumigatus, whereas it remains poorly investigated in other clinically relevant species of the genus, such as those of section Nigri In this work, we aimed to analyze the impact of cyp51 genes (cyp51A and cyp51B) on the voriconazole (VRC) response and resistance of Aspergillus niger and Aspergillus tubingensis We generated CRISPR-Cas9 cyp51A and cyp51B knock-out mutants from strains with different genetic backgrounds and diverse patterns of azole susceptibility. Single gene deletions of cyp51 genes resulted in 2 to 16-fold decrease of the VRC Minimum Inhibitory Concentration (MIC) values, which were below the VRC Epidemiological Cutoff Value (ECV) established by the Clinical and Laboratory Standards Institute (CLSI) irrespective of their parental strains susceptibilities. Gene expression studies in the tested species confirmed that cyp51A participates more actively than cyp51B in the transcriptional response of azole stress. However, ergosterol quantification revealed that both enzymes comparably impact the total ergosterol content within the cell, as basal and VRC-induced changes to ergosterol content was similar in all cases. These data contribute to our understanding on Aspergillus azole resistance, especially in non-fumigatus species.

Sterol uptake analysis in Saccharomyces and non-Saccharomyces wine yeast species

Sterols are essential components of the yeast membrane and their synthesis requires oxygen. Yet, Saccharomyces cerevisiae has developed the ability to take up sterols from the medium under anaerobiosis. Here we investigated sterol uptake efficiency and the expression of genes related to sterol import in Saccharomyces and non-Saccharomyces wine yeast species fermenting under anaerobic conditions. The sterol uptake efficiency of 39 strains was evaluated by flow cytometry (with 25-NBD Cholesterol, a fluorescent cholesterol probe introduced in the medium) and we found an important discrepancy between Saccharomyces and non-Saccharomyces wine yeast species that we correlated to a lower final cell population and a lower fermentation rate. A high uptake of sterol was observed in the various Saccharomyces strains. Spot tests performed on 13 of these strains confirmed the differences between Saccharomyces and non-Saccharomyces strains, suggesting that the presence of the sterol uptake transporters AUS1 and PDR11 could cause these discrepancies. Indeed, we could not find any homologue to these genes in the genome of Hanseniaspora uvarum, H. guillermondii, Lachancea thermotolerans, Torulaspora delbreueckii, Metschnikowia pulcherrima, or Starmarella bacillaris species. The specialization of sterol import function for post genome-duplication species may have favored growth under anaerobiosis.

Design, Synthesis, and Evaluation of a Luminescent Cholesterol Mimic

The development of new chemical tools with improved properties is essential to chemical and cell biology. Of particular interest is the development of mimics of small molecules with important cellular function that allow the direct observation of their trafficking in a cell. To this end, a novel 15-azasterol has been designed and synthesized as a luminescent cholesterol mimic for the monitoring of cholesterol trafficking. The brightness of this probe, which is ∼32-times greater than the widely used dehydroergosterol probe, is combined with resistance to photobleaching in solution and in human fibroblasts and an exceptionally large Stokes-like shift of ∼150-200 nm. The photophysical properties of the probe have been studied experimentally and computationally, suggesting an intersystem crossing to the triplet excited state with subsequent phosphorescent decay. Molecular dynamics simulations show a similar binding mode of cholesterol and the azasterol probe to NPC proteins, demonstrating the structural similarity of the probe to cholesterol.

Compartmentalized GPCR Signaling from Intracellular Membranes

G protein-coupled receptors (GPCRs) are integral membrane proteins that transduce a wide array of inputs including light, ions, hormones, and neurotransmitters into intracellular signaling responses which underlie complex processes ranging from vision to learning and memory. Although traditionally thought to signal primarily from the cell surface, GPCRs are increasingly being recognized as capable of signaling from intracellular membrane compartments, including endosomes, the Golgi apparatus, and nuclear membranes. Remarkably, GPCR signaling from these membranes produces functional effects that are distinct from signaling from the plasma membrane, even though often the same G protein effectors and second messengers are activated. In this review, we will discuss the emerging idea of a "spatial bias" in signaling. We will present the evidence for GPCR signaling through G protein effectors from intracellular membranes, and the ways in which this signaling differs from canonical plasma membrane signaling with important implications for physiology and pharmacology. We also highlight the potential mechanisms underlying spatial bias of GPCR signaling, including how intracellular membranes and their associated lipids and proteins affect GPCR activity and signaling.

Cholesterol-binding protein TSPO2 coordinates maturation and proliferation of terminally differentiating erythroblasts

TSPO2 (translocator protein 2) is a transmembrane protein specifically expressed in late erythroblasts and has been postulated to mediate intracellular redistribution of cholesterol. We identified TSPO2 as the causative gene for the HK (high-K⁺) trait with immature red cell phenotypes in dogs and investigated the effects of the TSPO2 defects on erythropoiesis in HK dogs with the TSPO2 mutation and Tspo2 knockout (Tspo2^−/−) mouse models. Bone marrow–derived erythroblasts from HK dogs showed increased binucleated and apoptotic cells at various stages of maturation and shed large nuclei with incomplete condensation when cultured in the presence of erythropoietin, indicating impaired maturation and cytokinesis. The canine TSPO2 induces cholesterol accumulation in the endoplasmic reticulum and could thereby regulate cholesterol availability by changing intracellular cholesterol distribution in erythroblasts. Tspo2^−/− mice consistently showed impaired cytokinesis with increased binucleated erythroblasts, resulting in compensated anemia, and their red cell membranes had increased Na,K-ATPase, resembling the HK phenotype in dogs. Tspo2-deficient mouse embryonic stem cell–derived erythroid progenitor (MEDEP) cells exhibited similar morphological defects associated with a cell-cycle arrest at the G₂/M phase, resulting in decreased cell proliferation and had a depletion in intracellular unesterified and esterified cholesterol. When the terminal maturation was induced, Tspo2^−/− MEDEP cells showed delays in hemoglobinization; maturation-associated phenotypic changes in CD44, CD71, and TER119 expression; and cell-cycle progression. Taken together, these findings imply that TSPO2 is essential for coordination of maturation and proliferation of erythroblasts during normal erythropoiesis.

Plasma membrane asymmetry of lipid organization: fluorescence lifetime microscopy and correlation spectroscopy analysis

A fundamental feature of the eukaryotic cell membrane is the asymmetric arrangement of lipids in its two leaflets. A cell invests significant energy to maintain this asymmetry and uses it to regulate important biological processes, such as apoptosis and vesiculation. The dynamic coupling of the inner or cytoplasmic and outer or exofacial leaflets is a challenging open question in membrane biology. Here, we combined fluorescence lifetime imaging microscopy (FLIM) with imaging total internal reflection fluorescence correlation spectroscopy (ITIR-FCS) to differentiate the dynamics and organization of the two leaflets of live mammalian cells. We characterized the biophysical properties of fluorescent analogs of phosphatidylcholine, sphingomyelin, and phosphatidylserine in the plasma membrane of two mammalian cell lines (CHO-K1 and RBL-2H3). Because of their specific transverse membrane distribution, these probes allowed leaflet-specific investigation of the plasma membrane. We compared the results of the two methods having different temporal and spatial resolution. Fluorescence lifetimes of fluorescent lipid analogs were in ranges characteristic for the liquid ordered phase in the outer leaflet and for the liquid disordered phase in the inner leaflet. The observation of a more fluid inner leaflet was supported by free diffusion in the inner leaflet, with high average diffusion coefficients. The liquid ordered phase in the outer leaflet was accompanied by slower diffusion and diffusion with intermittent transient trapping. Our results show that the combination of FLIM and ITIR-FCS with specific fluorescent lipid analogs is a powerful tool for investigating lateral and transbilayer characteristics of plasma membrane in live cell lines.

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.

Cholesterol accumulation, lipid droplet formation, and steroid production in Leydig cells: Role of translocator protein (18-kDa)

BACKGROUND

Cholesterol import into the mitochondria of steroid-producing cells is the rate-determining step in steroidogenesis. Numerous studies have provided evidence that the cholesterol-binding translocator protein (18 kDa TSPO) plays an important role in cholesterol translocation into mitochondria and that it also might act on cholesterol homeostasis. Several TSPO-specific ligands have been shown to increase steroid production in vitro and in vivo.

OBJECTIVES

The present study assessed the effects of the TSPO drug ligand FGIN-1-27 on cholesterol accumulation and lipid droplet formation in relationship to steroid formation.

MATERIALS AND METHODS

Using MA-10 and primary Leydig cells, immunocytochemical and molecular methods were used to examine cholesterol accumulation, the formation of lipid droplets, and steroid formation in response to LH and FGIN-1-27. Additionally, we determined the effects of Tspo knockout by CRISPR/Cas9, and of siRNA knockdowns of Tspo and Plin2 (Perilipin 2; also known as adipose differentiation-related protein, ADFP) on LH- and FGIN-1-27-induced steroidogenesis.

RESULTS

In response to LH and FGIN-1-27, cultured MA-10 cells and primary Leydig cells increased steroid formation, cholesterol accumulation, and lipid droplet formation. Cholesterol accumulation in the lipid droplets also was increased in Tspo knockout cells. Knockout of Tspo or its knockdown in MA-10 cells resulted in reduced progesterone formation in response to both LH and FGIN-1-27, as did knockdown of Plin2. Steroid production also was inhibited by the cholesteryl ester hydrolase inhibitor diethylumbelliferyl phosphate.

DISCUSSION AND CONCLUSION

These results support the conclusion that FGIN-1-27 stimulates steroid formation by increasing TSPO-mediated cholesterol translocation into the inner mitochondria for steroidogenesis, as well as into the cytosol for lipid droplet formation. FGIN-1-27 also increased steroid formation at least in part by inducing the conversion of cholesteryl ester located in lipid droplets to cholesterol, thus making available more substrate for steroid formation.

FRET Detects the Size of Nanodomains for Coexisting Liquid-Disordered and Liquid-Ordered Phases

Biomembranes with as few as three lipid components can form coexisting liquid-disordered (Ld) and liquid-ordered (Lo) phases. In the coexistence region of Ld and Lo phases, the lipid mixtures 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)/1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/chol or brain sphingomyelin (bSM)/DOPC/chol form micron-scale domains that are easily visualized with light microscopy. Although large domains are not observed in the mixtures DSPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/chol and bSM/POPC/chol, lateral heterogeneity is nevertheless detected using techniques with nanometer-scale spatial resolution. We propose a simple and accessible method to measure domain sizes below optical resolution (∼200 nm). We measured nanodomain size for the latter two mixtures by combining experimental Förster resonance energy transfer data with a Monte-Carlo-based analysis. We found a domain radius of 7.5-10 nm for DSPC/POPC/chol, similar to values obtained previously by neutron scattering, and ∼5 nm for bSM/POPC/chol, slightly smaller than measurable by neutron scattering. These analyses also detect the domain-size transition that is observed by fluorescence microscopy in the four-component lipid mixture bSM/DOPC/POPC/chol. Accurate measurements of fluorescent-probe partition coefficients are especially important for the analysis; therefore, we exploit three different methods to measure the partition coefficient of fluorescent molecules between Ld and Lo phases.

An Increase in Membrane Cholesterol by Graphene Oxide Disrupts Calcium Homeostasis in Primary Astrocytes

The use of graphene nanomaterials (GNMs) for biomedical applications targeted to the central nervous system is exponentially increasing, although precise information on their effects on brain cells is lacking. In this work, the molecular changes induced in cortical astrocytes by few-layer graphene (FLG) and graphene oxide (GO) flakes are addressed. The results show that exposure to FLG/GO does not affect cell viability or proliferation. However, proteomic and lipidomic analyses unveil alterations in several cellular processes, including intracellular Ca²⁺ ([Ca²⁺ ]_i ) homeostasis and cholesterol metabolism, which are particularly intense in cells exposed to GO. Indeed, GO exposure impairs spontaneous and evoked astrocyte [Ca²⁺ ]_i signals and induces a marked increase in membrane cholesterol levels. Importantly, cholesterol depletion fully rescues [Ca²⁺ ]_i dynamics in GO-treated cells, indicating a causal relationship between these GO-mediated effects. The results indicate that exposure to GNMs alters intracellular signaling in astrocytes and may impact astrocyte-neuron interactions.

Cholesterol-dependent Modulation of Stem Cell Biomechanics: Application to Adipogenesis

Cell mechanics has been shown to regulate stem cell differentiation. We have previously reported that altered cell stiffness of mesenchymal stem cells can delay or facilitate biochemically directed differentiation. One of the factors that can affect the cell stiffness is cholesterol. However, the effect of cholesterol on differentiation of human mesenchymal stem cells (hMSCs) remains elusive. In this paper, we demonstrate that cholesterol is involved in the modulation of the cell stiffness and subsequent adipogenic differentiation. Rapid cytoskeletal actin reorganization was evident and correlated with the cell's Young's modulus measured using atomic force microscopy (AFM). In addition, the level of membrane-bound cholesterol was found to increase during adipogenic differentiation and inversely varied with the cell stiffness. Furthermore, cholesterol played a key role in the regulation of the cell morphology and biomechanics, suggesting its crucial involvement in mechanotransduction. To better understand the underlying mechanisms, we investigated the effect of cholesterol on the membrane-cytoskeleton linker proteins (ezrin and moesin). Cholesterol depletion was found to up-regulate the ezrin expression which promoted cell spreading, increased Young's modulus, and hindered adipogenesis. In contrast, cholesterol enrichment increased the moesin expression, decreased Young's modulus, and induced cell rounding and facilitated adipogenesis. Taken together, cholesterol appears to regulate the stem cell mechanics and adipogenesis through the membrane-associated linker proteins.

Heterocyclic sterol probes for live monitoring of sterol trafficking and lysosomal storage disorders

The monitoring of intracellular cholesterol homeostasis and trafficking is of great importance because their imbalance leads to many pathologies. Reliable tools for cholesterol detection are in demand. This study presents the design and synthesis of fluorescent probes for cholesterol recognition and demonstrates their selectivity by a variety of methods. The construction of dedicated library of 14 probes was based on heterocyclic (pyridine)-sterol derivatives with various attached fluorophores. The most promising probe, a P1-BODIPY conjugate FP-5, was analysed in detail and showed an intensive labelling of cellular membranes followed by intracellular redistribution into various cholesterol rich organelles and vesicles. FP-5 displayed a stronger signal, with faster kinetics, than the commercial TF-Chol probe. In addition, cells with pharmacologically disrupted cholesterol transport, or with a genetic mutation of cholesterol transporting protein NPC1, exhibited strong and fast FP-5 signal in the endo/lysosomal compartment, co-localizing with filipin staining of cholesterol. Hence, FP-5 has high potential as a new probe for monitoring cholesterol trafficking and its disorders.

Transverse distribution of plasma membrane bilayer cholesterol: Picking sides

The transverse asymmetry (sidedness) of phospholipids in plasma membrane bilayers is well characterized, distinctive, actively maintained and functionally important. In contrast, numerous studies using a variety of techniques have concluded that plasma membrane bilayer cholesterol is either mostly in the outer leaflet or the inner leaflet or is fairly evenly distributed. Sterols might simply partition according to their differing affinities for the asymmetrically disposed phospholipids, but some studies have proposed that it is actively transported to the outer leaflet. Other work suggests that the sterol is enriched in the inner leaflet, driven by either positive interactions with the phosphatidylethanolamine on that side or by its exclusion from the outer leaflet by the long chain sphingomyelin molecules therein. This uncertainty raises three questions: is plasma membrane cholesterol sidedness fixed in a given cell or cell type; is it generally the same among mammalian species; and does it serve specific physiological functions? This review grapples with these issues.

In vitro exploration of ACAT contributions to lipid droplet formation during adipogenesis

As adipose tissue is the major cholesterol storage organ and most of the intracellular cholesterol is distributed to lipid droplets (LDs), cholesterol homeostasis may have a role in the regulation of adipocyte size and function. ACATs catalyze the formation of cholesteryl ester (CE) from free cholesterol to modulate the cholesterol balance. Despite the well-documented role of ACATs in hypercholesterolemia, their role in LD development during adipogenesis remains elusive. Here, we identify ACATs as regulators of de novo lipogenesis and LD formation in murine 3T3-L1 adipocytes. Pharmacological inhibition of ACAT activity suppressed intracellular cholesterol and CE levels, and reduced expression of genes involved in cholesterol uptake and efflux. ACAT inhibition resulted in decreased de novo lipogenesis, as demonstrated by reduced maturation of SREBP1 and SREBP1-downstream lipogenic gene expression. Consistent with this observation, knockdown of either ACAT isoform reduced total adipocyte lipid content by approximately 40%. These results demonstrate that ACATs are required for storage ability of lipids and cholesterol in adipocytes.

Loss of Calreticulin Uncovers a Critical Role for Calcium in Regulating Cellular Lipid Homeostasis

A direct link between Ca²⁺ and lipid homeostasis has not been definitively demonstrated. In this study, we show that manipulation of ER Ca²⁺ causes the re-distribution of a portion of the intracellular unesterified cholesterol to a pool that is not available to the SCAP-SREBP complex. The SREBP processing pathway in ER Ca²⁺ depleted cells remained fully functional and responsive to changes in cellular cholesterol status but differed unexpectedly in basal activity. These findings establish the role of Ca²⁺ in determining the reference set-point for controlling cellular lipid homeostasis. We propose that ER Ca²⁺ status is an important determinant of the basal sensitivity of the sterol sensing mechanism inherent to the SREBP processing pathway.

Cellular Cholesterol Facilitates the Postentry Replication Cycle of Herpes Simplex Virus 1

Cholesterol is an essential component of cell membranes and is required for herpes simplex virus 1 (HSV-1) entry (1-3). Treatment of HSV-1-infected Vero cells with methyl beta-cyclodextrin from 2 to 9 h postentry reduced plaque numbers. Transport of incoming viral capsids to the nuclear periphery was unaffected by the cholesterol reduction, suggesting that cell cholesterol is important for the HSV-1 replicative cycle at a stage(s) beyond entry, after the arrival of capsids at the nucleus. The synthesis and release of infectious HSV-1 and cell-to-cell spread of infection were all impaired in cholesterol-reduced cells. Propagation of HSV-1 on DHCR24^-/- fibroblasts, which lack the desmosterol-to-cholesterol conversion enzyme, resulted in the generation of infectious extracellular virions (HSV^des) that lack cholesterol and likely contain desmosterol. The specific infectivities (PFU per viral genome) of HSV^chol and HSV^des were similar, suggesting cholesterol and desmosterol in the HSV envelope support similar levels of infectivity. However, infected DHCR24^-/- fibroblasts released ∼1 log less infectious HSV^des and ∼1.5 log fewer particles than release of cholesterol-containing particles (HSV^chol) from parental fibroblasts, suggesting that the hydrocarbon tail of cholesterol facilitates viral synthesis. Together, the results suggest multiple roles for cholesterol in the HSV-1 replicative cycle.IMPORTANCE HSV-1 infections are associated with a wide range of clinical manifestations that are of public health importance. Cholesterol is a key player in the complex interaction between viral and cellular factors that allows HSV-1 to enter host cells and establish infection. Previous reports have demonstrated a role for cellular cholesterol in the entry of HSV-1 into target cells. Here, we employed both chemical treatment and cells that were genetically defined to synthesize only desmosterol to demonstrate that cholesterol is important at stages following the initial entry and transport of viral capsids to the nucleus. Viral protein expression, encapsidation of the viral genome, and the release of mature virions were impacted by the reduction of cellular cholesterol. Cholesterol was also critical for cell-to-cell spread of infection. These findings provide new insights into the cholesterol dependence of HSV-1 replication.

Structure of Yeast OSBP-Related Protein Osh1 Reveals Key Determinants for Lipid Transport and Protein Targeting at the Nucleus-Vacuole Junction

Yeast Osh1 belongs to the oxysterol-binding protein (OSBP) family of proteins and contains multiple targeting modules optimized for lipid transport at the nucleus-vacuole junction (NVJ). The key determinants for NVJ targeting and the role of Osh1 at NVJs have remained elusive because of unknown lipid specificities. In this study, we determined the structures of the ankyrin repeat domain (ANK), and OSBP-related domain (ORD) of Osh1, in complex with Nvj1 and ergosterol, respectively. The Osh1 ANK forms a unique bi-lobed structure that recognizes a cytosolic helical segment of Nvj1. We discovered that Osh1 ORD binds ergosterol and phosphatidylinositol 4-phosphate PI(4)P in a competitive manner, suggesting counter-transport function of the two lipids. Ergosterol is bound to the hydrophobic pocket in a head-down orientation, and the structure of the PI(4)P-binding site in Osh1 is well conserved. Our results suggest that Osh1 performs non-vesicular transport of ergosterol and PI(4)P at the NVJ.

Synthesis and characterization of a novel rhodamine labeled cholesterol reporter

We introduce the novel fluorescent cholesterol probe RChol in which a sulforhodamine group is linked to the sixth carbon atom of the steroid backbone of cholesterol. The same position has recently been selected to generate the fluorescent reporter 6-dansyl-cholestanol (DChol) and the photoreactive 6-azi-cholestanol. In comparison with DChol, RChol is brighter, much more photostable, and requires less energy for excitation, i.e. favorable conditions for microscopical imaging. RChol easily incorporates into methyl-β-cyclodextrin forming a water-soluble inclusion complex that acts as an efficient sterol donor for cells and membranes. Like cholesterol, RChol possesses a free 3'OH group, a prerequisite to undergo intracellular esterification. RChol was also able to support the growth of cholesterol auxotrophic cells and can therefore substitute for cholesterol as a major component of the plasma membrane. According to subcellular fractionation, slight amounts of RChol (~12%) were determined in low-density Triton-insoluble fractions whereas the majority of RChol was localized in non-rafts fractions. In phase-separated giant unilamellar vesicles, RChol preferentially partitions in liquid-disordered membrane domains. Intracellular RChol was transferred to extracellular sterol acceptors such as high density lipoproteins in a dose-dependent manner. Unlike DChol, RChol was not delivered to the cholesterol storage pathway. Instead, it translocated to endosomes/lysosomes with some transient contacts to peroxisomes. Thus, RChol is considered as a useful probe to study the endosomal/lysosomal pathway of cholesterol.

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.

Chemically-activatable alkyne-tagged probe for imaging microdomains in lipid bilayer membranes

A chemically-activatable alkynyl steroid analogue probe has been synthesized for visualizing the lipid raft membrane domains by Raman microscopy. The Raman probe, in which ring A of its steroid backbone is replaced with an alkynyl group, was designed to enable activation of the alkyne signal through the Eschenmoser-Tanabe fragmentation reaction of the oxidized cholesterol precursor in lipid bilayer membranes. The alkynyl steroid analogue was observed to form liquid-ordered raft-like domains on a model giant-liposome system in a similar manner as cholesterol, and the large alkyne signal of the accumulated probe at 2120 cm⁻¹ was mapped on the microdomains with a Raman microscope. The alkyne moiety of the probe was confirmed to be converted from the α,β-epoxy ketone group of its precursor by reaction with p-toluensulfonyl hydrazine under a mild condition. Through the reaction, the alkyne signal of the probe was activated on the lipid bilayer membrane of liposomes. Furthermore, the signal activation of the probe was also detected on living cells by stimulated Raman scattering microscopy. The ring-A-opened alkyne steroid analogue, thus, provides a first chemically-activatable Raman probe as a promising tool for potentially unravelling the intracellular formation and trafficking of cholesterol-rich microdomains.

Clinically used selective estrogen receptor modulators affect different steps of macrophage-specific reverse cholesterol transport

Selective estrogen receptor modulators (SERMs) are widely prescribed drugs that alter cellular and whole-body cholesterol homeostasis. Here we evaluate the effect of SERMs on the macrophage-specific reverse cholesterol transport (M-RCT) pathway, which is mediated by HDL. Treatment of human and mouse macrophages with tamoxifen, raloxifene or toremifene induced the accumulation of cytoplasmic vesicles of acetyl-LDL-derived free cholesterol. The SERMs impaired cholesterol efflux to apolipoprotein A-I and HDL, and lowered ABCA1 and ABCG1 expression. These effects were not altered by the antiestrogen ICI 182,780 nor were they reproduced by 17β-estradiol. The treatment of mice with tamoxifen or raloxifene accelerated HDL-cholesteryl ester catabolism, thereby reducing HDL-cholesterol concentrations in serum. When [³H]cholesterol-loaded macrophages were injected into mice intraperitoneally, tamoxifen, but not raloxifene, decreased the [³H]cholesterol levels in serum, liver and feces. Both SERMs downregulated liver ABCG5 and ABCG8 protein expression, but tamoxifen reduced the capacity of HDL and plasma to promote macrophage cholesterol efflux to a greater extent than raloxifene. We conclude that SERMs interfere with intracellular cholesterol trafficking and efflux from macrophages. Tamoxifen, but not raloxifene, impair M-RCT in vivo. This effect is primarily attributable to the tamoxifen-mediated reduction of the capacity of HDL to promote cholesterol mobilization from macrophages.

Graphene Oxide Nanosheets Disrupt Lipid Composition, Ca(2+) Homeostasis, and Synaptic Transmission in Primary Cortical Neurons

Graphene has the potential to make a very significant impact on society, with important applications in the biomedical field. The possibility to engineer graphene-based medical devices at the neuronal interface is of particular interest, making it imperative to determine the biocompatibility of graphene materials with neuronal cells. Here we conducted a comprehensive analysis of the effects of chronic and acute exposure of rat primary cortical neurons to few-layer pristine graphene (GR) and monolayer graphene oxide (GO) flakes. By combining a range of cell biology, microscopy, electrophysiology, and "omics" approaches we characterized the graphene-neuron interaction from the first steps of membrane contact and internalization to the long-term effects on cell viability, synaptic transmission, and cell metabolism. GR/GO flakes are found in contact with the neuronal membrane, free in the cytoplasm, and internalized through the endolysosomal pathway, with no significant impact on neuron viability. However, GO exposure selectively caused the inhibition of excitatory transmission, paralleled by a reduction in the number of excitatory synaptic contacts, and a concomitant enhancement of the inhibitory activity. This was accompanied by induction of autophagy, altered Ca(2+) dynamics, and a downregulation of some of the main players in the regulation of Ca(2+) homeostasis in both excitatory and inhibitory neurons. Our results show that, although graphene exposure does not impact neuron viability, it does nevertheless have important effects on neuronal transmission and network functionality, thus warranting caution when planning to employ this material for neurobiological applications.

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.

Three-dimensional imaging of cholesterol and sphingolipids within a Madin-Darby canine kidney cell

Metabolic stable isotope incorporation and secondary ion mass spectrometry (SIMS) depth profiling performed on a Cameca NanoSIMS 50 were used to image the (18)O-cholesterol and (15)N-sphingolipid distributions within a portion of a Madin-Darby canine kidney (MDCK) cell. Three-dimensional representations of the component-specific isotope distributions show clearly defined regions of (18)O-cholesterol and (15)N-sphingolipid enrichment that seem to be separate subcellular compartments. The low levels of nitrogen-containing secondary ions detected at the (18)O-enriched regions suggest that these (18)O-cholesterol-rich structures may be lipid droplets, which have a core consisting of cholesterol esters and triacylglycerides.