Quantitative assessment of sterol traffic in living cells by dual labeling with dehydroergosterol and BODIPY-cholesterol

Actin cytoskeleton differently regulates cell surface organization of GPI-anchored proteins in polarized epithelial cells and fibroblasts

The spatiotemporal compartmentalization of membrane-associated glycosylphosphatidylinositol-anchored proteins (GPI-APs) on the cell surface regulates their biological activities. These GPI-APs occupy distinct cellular functions such as enzymes, receptors, and adhesion molecules, and they are implicated in several vital cellular processes. Thus, unraveling the mechanisms and regulators of their membrane organization is essential. In polarized epithelial cells, GPI-APs are enriched at the apical surface, where they form small cholesterol-independent homoclusters and larger heteroclusters accommodating multiple GPI-AP species, all confined within areas of approximately 65-70 nm in diameter. Notably, GPI-AP homoclustering occurs in the Golgi apparatus through a cholesterol- and calcium-dependent mechanism that drives their apical sorting. Despite the critical role of Golgi GPI-AP clustering in their cell surface organization and the importance of cholesterol in heterocluster formation, the regulatory mechanisms governing GPI-AP surface organization, particularly in the context of epithelial polarity, remain elusive. Given that the actin cytoskeleton undergoes substantial remodeling during polarity establishment, this study explores whether the actin cytoskeleton regulates the spatiotemporal apical organization of GPI-APs in MDCK cells. Utilizing various imaging techniques (number and brightness, FRET/FLIM, and dSTORM coupled to pair correlation analysis), we demonstrate that the apical organization of GPI-APs, at different scales, does not rely on the actin cytoskeleton, unlike in fibroblastic cells. Interestingly, calcium chelation disrupts the organization of GPI-APs at the apical surface by impairing Golgi GPI-AP clustering, emphasizing the existence of an interplay among Golgi clustering, apical sorting, and surface organization in epithelial cells. In summary, our findings unveil distinct mechanisms regulating the organization of GPI-APs in cell types of different origins, plausibly allowing them to adapt to different external signals and different cellular environments in order to achieve specialized functions.

Elder (Sambucus nigra), identified by high-content screening, counteracts foam cell formation without promoting hepatic lipogenesis

Cholesterol deposition in intimal macrophages leads to foam cell formation and atherosclerosis. Reverse cholesterol transport (RCT), initiated by efflux of excess cholesterol from foam cells, counteracts atherosclerosis. However, targeting RCT by enhancing cholesterol efflux was so far accompanied by adverse hepatic lipogenesis. Here, we aimed to identify novel natural enhancers of macrophage cholesterol efflux suitable for the prevention of atherosclerosis. Plant extracts of an open-access library were screened for their capacity to increase cholesterol efflux in RAW264.7 macrophages trace-labeled with fluorescent BODIPY-cholesterol. Incremental functional validation of hits yielded two final extracts, elder (Sambucus nigra) and bitter orange (Citrus aurantium L.) that induced ATP binding cassette transporter A1 (ABCA1) expression and reduced cholesteryl ester accumulation in aggregated LDL-induced foam cells. Aqueous elder extracts were subsequently prepared in-house and both, flower and leaf extracts increased ABCA1 mRNA and protein expression in human THP-1 macrophages, while lipogenic gene expression in hepatocyte-derived cells was not induced. Chlorogenic acid isomers and the quercetin glycoside rutin were identified as the main polyphenols in elder extracts with putative biological action. In summary, elder flower and leaf extracts increase macrophage ABCA1 expression and reduce foam cell formation without adversely affecting hepatic lipogenesis.

Enzyme-Free Liposome Active Motion via Asymmetrical Lipid Efflux

As a class of biocompatible, water-dispersed colloids, liposomes have found widespread applications ranging from food to drug delivery. Adding mobility to these colloids, i.e., liposome micromotors, represents an attractive approach to next-generation liposome carriers with enhanced functionality and effectiveness. Currently, it remains unclear as to the scope of material features useful for building liposome micromotors or how they may differ functionally from their inorganic/polymer counterparts. In this work, we demonstrate liposome active motion taking advantage of mainly a pair of intrinsic material properties associated with these assemblies: lipid phase separation and extraction. We show that global phase separation of ternary lipid systems (such as DPPC/DOPC/cholesterol) within individual liposomes yields stable Janus particles with two distinctive liquid domains. While these anisotropic liposomes undergo pure Brownian diffusion in water, similar to their homogeneous analogues, adding extracting agents, cyclodextrins, to the system triggers asymmetrical cholesterol efflux about the liposomes, setting the latter into active motion. We present detailed analyses of liposome movement and cholesterol extraction kinetics to establish their correlation. We explore various experimental parameters as well as mechanistic details to account for such motion. Our results highlight the rich possibility to hierarchically design lipid-based artificial motors, from individual lipids, to their organization, surface chemistry, and interfacial mechanics.

Image segmentation and separation of spectrally similar dyes in fluorescence microscopy by dynamic mode decomposition of photobleaching kinetics

Background

Image segmentation in fluorescence microscopy is often based on spectral separation of fluorescent probes (color-based segmentation) or on significant intensity differences in individual image regions (intensity-based segmentation). These approaches fail, if dye fluorescence shows large spectral overlap with other employed probes or with strong cellular autofluorescence.

Results

Here, a novel model-free approach is presented which determines bleaching characteristics based on dynamic mode decomposition (DMD) and uses the inferred photobleaching kinetics to distinguish different probes or dye molecules from autofluorescence. DMD is a data-driven computational method for detecting and quantifying dynamic events in complex spatiotemporal data. Here, DMD is first used on synthetic image data and thereafter used to determine photobleaching characteristics of a fluorescent sterol probe, dehydroergosterol (DHE), compared to that of cellular autofluorescence in the nematode Caenorhabditis elegans. It is shown that decomposition of those dynamic modes allows for separating probe from autofluorescence without invoking a particular model for the bleaching process. In a second application, DMD of dye-specific photobleaching is used to separate two green-fluorescent dyes, an NBD-tagged sphingolipid and Alexa488-transferrin, thereby assigning them to different cellular compartments.

Conclusions

Data-based decomposition of dynamic modes can be employed to analyze spatially varying photobleaching of fluorescent probes in cells and tissues for spatial and temporal image segmentation, discrimination of probe from autofluorescence and image denoising. The new method should find wide application in analysis of dynamic fluorescence imaging data.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12859-022-04881-x.

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.

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.

Cellular cholesterol and how to find it

Cholesterol is an essential component of eukaryotic cellular membranes. Information about its subcellular localization and transport pathways inside cells are key for the understanding and treatment of cholesterol-related diseases. In this review we give an overview over the most commonly used methods that contributed to our current understanding of subcellular cholesterol localization and transport routes. First, we discuss methods that provide insights into cholesterol metabolism based on readouts of downstream effects such as esterification. Subsequently, we focus on the use of cholesterol-binding molecules as probes that facilitate visualization and quantification of sterols inside of cells. Finally, we explore different analogues of cholesterol which, when taken up by living cells, are integrated and transported in a similar fashion as endogenous sterols. Taken together, we highlight the challenges and advantages of each method such that researchers studying aspects of cholesterol transport may choose the most pertinent approach for their problem.

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.

Endocytosis in cellular uptake of drug delivery vectors: Molecular aspects in drug development

Drug delivery vectors are widely applied to increase drug efficacy while reducing the side effects and potential toxicity of a drug. They allow for patient-tailored therapy, dose titration, and therapeutic drug monitoring. A major part of drug delivery systems makes use of large nanocarriers: liposomes or virus-like particles (VLPs). These systems allow for a relatively large amount of cargo with good stability of vectors, and they offer multiple options for targeting vectors in vivo. Here we discuss endocytic pathways that are available for drug delivery by large nanocarriers. We focus on molecular aspects of the process, including an overview of potential molecular targets for studies of drug delivery vectors and for future solutions allowing targeted drug delivery.

Cholesterol Handling in Lysosomes and Beyond

Lysosomes are of major importance for the regulation of cellular cholesterol homeostasis. Food-derived cholesterol and cholesterol esters contained within lipoproteins are delivered to lysosomes by endocytosis. From the lysosomal lumen, cholesterol is transported to the inner surface of the lysosomal membrane through the glycocalyx; this shuttling requires Niemann-Pick C (NPC) 1 and NPC2 proteins. The lysosomal membrane proteins lysosomal-associated membrane protein (LAMP)-2 and lysosomal integral membrane protein (LIMP)-2/SCARB2 also bind cholesterol. LAMP-2 may serve as a cholesterol reservoir, whereas LIMP-2, like NPC1, is able to transport cholesterol through a transglycocalyx tunnel. Contact sites and fusion events between lysosomes and other organelles mediate the distribution of cholesterol. Lysosomal cholesterol content is sensed thereby regulating mammalian target of rapamycin complex (mTORC)-dependent signaling. This review summarizes our understanding of the major steps in cholesterol handling from the moment it enters the lysosome until it leaves this compartment.

High-Resolution Secondary Ion Mass Spectrometry Analysis of Cell Membranes

This Feature describes the use a Cameca NanoSIMS instrument for directly imaging specific lipid and protein species in the plasma membranes of mammalian cells with approximately 100 nm-lateral resolution and discusses how these analyses have already begun to transform fundamental concepts in the field of membrane biology. Secondary ion generation is discussed with emphasis on the constraints that affect the detection and identification of membrane components, and then the sample preparation methodologies and data analysis strategies that address these constraints are described.

Computational Characterization of a Cholesterol-Based Molecular Rotor in Lipid Membranes

Biophysical properties of cellular membranes critically depend on their content of cholesterol and its interaction with various other lipid species. Cholesterol-dependent friction at the nanoscale can be studied with molecular rotors, whose quantum yield depends on rotational dynamics of functional groups during their excited state lifetime. Here, we present a detailed computational analysis of a phenyl-BODIPY-linked cholesterol based molecular rotor in direct comparison with the well-known TopFluor-cholesterol. We describe a new parametrization strategy of force field parameters for the BODIPY moiety and carry out extensive molecular dynamics simulations of the probe in membranes in the absence or presence of cholesterol. Our study quantifies the extent of membrane perturbation by these probes, analyzes their tilting resistance in the bilayer and derives dynamic properties directly related to the rotor propensity. We show that phenyl-BODIPY-cholesterol bears potential as a cholesterol-dependent molecular rotor to report about microviscosity of sterol-containing model and cell membranes.

Visualizing Cholesterol Uptake by Self-Assembling Rhodamine B-Labeled Polymer Inside Living Cells via FLIM-FRET Microscopy

Atherosclerosis is a widespread and hazardous disease characterized by the formation of arterial plaques mostly composed of fat, cholesterol, and calcium ions. The direct solubilization of cholesterol represents a promising, atheroprotective strategy to subside lipid blood levels and reverse atherosclerosis. This study deals with the in-depth analysis of polymer-mediated cholesterol dissolution inside living human cells. To this end, a recently described multifunctional block-polymer is labeled with Rhodamine B (RhoB) to investigate its interaction with cells via fluorescence microscopy. This gives insight into the cellular internalization process of the polymer, which appears to be clathrin- and caveolae/raft-dependent endocytosis. In cell single particle tracking reveals an active transport of RhoB polymer including structures. Förster resonance energy transfer (FRET) measurements of cells treated with a fluorophore-tagged cholesterol derivative and the RhoB polymer indicates the uptake of cholesterol by the polymeric particles. Hence, these results present a first step toward possible applications of cholesterol-absorbing polymers for treating atherosclerosis.

Synthesis and characterization of diazirine alkyne probes for the study of intracellular cholesterol trafficking

Cholesterol is an essential structural component of cellular membranes and precursor molecule for oxysterol, bile acid, and hormone synthesis. The study of intracellular cholesterol trafficking pathways has been limited in part due to a lack of suitable cholesterol analogues. Herein, we developed three novel diazirine alkyne cholesterol probes: LKM38, KK174, and KK175. We evaluated these probes as well as a previously described diazirine alkyne cholesterol analogue, trans-sterol, for their fidelity as cholesterol mimics and for study of cholesterol trafficking. LKM38 emerged as a promising cholesterol mimic because it both sustained the growth of cholesterol-auxotrophic cells and appropriately regulated key cholesterol homeostatic pathways. When presented as an ester in lipoprotein particles, LKM38 initially localized to the lysosome and subsequently trafficked to the plasma membrane and endoplasmic reticulum. LKM38 bound to diverse, established cholesterol binding proteins. Through a detailed characterization of the cellular behavior of a panel of diazirine alkyne probes using cell biological, biochemical trafficking assays and immunofluorescence approaches, we conclude that LKM38 can serve as a powerful tool for the study of cholesterol protein interactions and trafficking.

Rational design of novel fluorescent analogues of cholesterol: a “step-by-step” computational study

State-of-the-art quantum chemical and molecular dynamics simulations are used as guidelines in design of novel fluorescent analogues of cholesterol.

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.

Live-cell imaging of new polyene sterols for improved analysis of intracellular cholesterol transport

Analysis of intracellular cholesterol transport by fluorescence microscopy requires suitable fluorescent analogues of cholesterol. Most existing cholesterol analogues contain lipophilic dyes which can compromise the sterol properties in membranes. An alternative strategy is to introduce additional double bonds into the sterol ring system resulting in intrinsic fluorescence, while at the same time keeping the cholesterol-like properties of the analogues. Existing polyene sterols, such as dehydroergosterol (DHE) or cholestatrienol (CTL), however, contain only three double bonds and suffer from low brightness, significant photobleaching and excitation/emission in the ultraviolet region. Thus, special equipment is required to image such sterols. Here, we describe synthesis, characterization and intracellular imaging of new polyene sterols containing four conjugated double bonds in the sterol ring system. We show that such analogues have red-shifted excitation and emission by ∼20 nm compared to DHE or CTL. The red shift was even more pronounced when preventing keto-enol tautomer equilibration by protecting the 3'-hydroxy group with acetate. We show that the latter analogue can be imaged on a conventional wide field microscope with a DAPI/filipin filter cube. The new polyene sterols show reduced photobleaching compared to DHE or CTL allowing for improved deconvolution microscopy of sterol containing cellular membranes.

Cholesterol added prior to vitrification on the cryotolerance of immature and in vitro matured bovine oocytes

This study examines whether incorporating cholesterol-loaded methyl-β-cyclodextrin (CLC) in the bovine oocyte plasma membrane improves oocyte tolerance to vitrification. In vitro matured oocytes were incubated with 2 mg/ml BODIPY-labeled CLC for different time intervals in FCS or PVA supplemented medium or exposed to different CLC concentrations to examine the subcellular localization of cholesterol by confocal microscopy live-cell imaging. Subsequently, the effects of optimized CLC concentrations and incubation times prior to vitrification on early embryo development were assessed. Then, we evaluated the effects of pretreatment with 2 mg/ml CLC for 30 min before the vitrification of immature (GV) and in vitro matured (MII) oocytes on developmental competence and gene expression. Our results indicate a high plasma membrane labeling intensity after 30 min of incubation with 2 mg/ml CLC for 30 min, regardless of the holding medium used. When oocytes were incubated with 1 mg/ml, 2 mg/ml and 3 mg/ml of CLC, intense labeling was observed at the plasma membrane after 40, 30 and 20 min, respectively. CLC pre-treatment before the vitrification of bovine oocytes did not affect subsequent cleavage and embryo development rates irrespective of CLC concentrations, incubation times or meiotic stage. However, pretreatment seems to improve the quality of embryos derived from vitrified oocytes, mainly when oocytes were vitrified at the GV stage.

Synthesis and Live-Cell Imaging of Fluorescent Sterols for Analysis of Intracellular Cholesterol Transport

Cellular cholesterol homeostasis relies on precise control of the sterol content of organelle membranes. Obtaining insight into cholesterol trafficking pathways and kinetics by live-cell imaging relies on two conditions. First, one needs to develop suitable analogs that resemble cholesterol as closely as possible with respect to their biophysical and biochemical properties. Second, the cholesterol analogs should have good fluorescence properties. This interferes, however, often with the first requirement, such that the imaging instrumentation must be optimized to collect photons from suboptimal fluorophores, but good cholesterol mimics, such as the intrinsically fluorescent sterols, cholestatrienol (CTL) or dehydroergosterol (DHE). CTL differs from cholesterol only in having two additional double bonds in the ring system, which is why it is slightly fluorescent in the ultraviolet (UV). In the first part of this protocol, we describe how to synthesize and image CTL in living cells relative to caveolin, a structural component of caveolae. In the second part, we explain in detail how to perform time-lapse experiments of commercially available BODIPY-tagged cholesterol (TopFluor-cholesterol^®; TF-Chol) in comparison to DHE. Finally, using two-photon time-lapse imaging data of TF-Chol, we demonstrate how to use our imaging toolbox SpatTrack for tracking sterol rich vesicles in living cells over time.

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

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

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.

BODIPY–steroid conjugates: Syntheses and biological applications

BODIPY and aza-BODIPY fluorophores linked to steroids are being developed as multimodal-imaging agents to monitor the mechanism of action of biologically active components in living systems.

Imaging approaches for analysis of cholesterol distribution and dynamics in the plasma membrane

Cholesterol is an important lipid component of the plasma membrane (PM) of mammalian cells, where it is involved in control of many physiological processes, such as endocytosis, cell migration, cell signalling and surface ruffling. In an attempt to explain these functions of cholesterol, several models have been put forward about cholesterol's lateral and transbilayer organization in the PM. In this article, we review imaging techniques developed over the last two decades for assessing the distribution and dynamics of cholesterol in the PM of mammalian cells. Particular focus is on fluorescence techniques to study the lateral and inter-leaflet distribution of suitable cholesterol analogues in the PM of living cells. We describe also several methods for determining lateral cholesterol dynamics in the PM including fluorescence recovery after photobleaching (FRAP), fluorescence correlation spectroscopy (FCS), single particle tracking (SPT) and spot variation FCS coupled to stimulated emission depletion (STED) microscopy. For proper interpretation of such measurements, we provide some background in probe photophysics and diffusion phenomena occurring in cell membranes. In particular, we show the equivalence of the reaction-diffusion approach, as used in FRAP and FCS, and continuous time random walk (CTRW) models, as often invoked in SPT studies. We also discuss mass spectrometry (MS) based imaging of cholesterol in the PM of fixed cells and compare this method with fluorescence imaging of sterols. We conclude that evidence from many experimental techniques converges towards a model of a homogeneous distribution of cholesterol with largely free and unhindered diffusion in both leaflets of the PM.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

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.

Fluorescent mimics of cholesterol that rapidly bind surfaces of living mammalian cells

Mammalian cells acquire cholesterol, a critical membrane constituent, through multiple mechanisms. We synthesized mimics of cholesterol, fluorescent N-alkyl-3β-cholesterylamine-glutamic acids, that are rapidly incorporated into cellular plasma membranes compared with analogous cholesteryl amides, ethers, esters, carbamates, and a sitosterol analogue. This process was inhibited by ezetimibe, indicating a receptor-mediated uptake pathway.

Rapid, one-step fabrication and loading of nanoscale 1,2-distearoyl-sn-glycero-3-phosphocholine liposomes in a simple, double flow-focusing microfluidic device

Liposomes are currently well-established as biocompatible delivery vehicles for numerous compounds. However, conventional manufacturing tends to rely on time-consuming processes, costly equipment, unstable reaction parameters, and numerous pre- and post-processing steps. Herein, we demonstrate a microscope-slide-sized alternative: a double flow-focusing microfluidic geometry capable of sub-hour synthesis and controlled loading of tunable liposomes. Using phospholipid 1,2-distearoyl-sn-glycero-3-phosphocholine as the bilayer constituent, the effect of varying the dissolved lipid concentration and flow rate ratio on synthesized liposome diameters was investigated and the encapsulation of fluorescent hydrophobic drug model ergost-5,7,9(11),22-tetraen-3β-ol was performed to ascertain the potential of this device as a loading platform.

Photobleaching of fluorescein as a probe for oxidative stress in single cells

BACKGROUND

ROS are involved in the regulation of many physiological and pathological processes. Apoptosis and necrosis are processes that are induced by changes in concentrations of Reactive Oxygen Species (ROS). This study aims to detect and quantify the cellular response to changing ROS concentrations in the scope of apoptosis and necrosis.

METHODS

Photobleaching of the fluorescent substrate fluorescein is used as a probe to detect the response of individual Jurkat-T-lymphocytes and Prostate-Cancer-3(PC-3) cells to oxidative stress, induced by hydrogen peroxide (H₂O₂). A kinetic model is proposed to describe changes in intracellular dye quantities due to photobleaching, dye hydrolysis, influx and leakage, yielding a single time-dependent decaying exponent+constant.

RESULTS

Fluorescein photobleaching is controlled and used to detect intracellular ROS. An increase in the decay time of fluorescence of intracellular fluorescein (slow photobleaching) was measured from cells incubated with H₂O₂ at 50 μM. At higher H₂O₂ concentrations a decrease in the decay time was measured (fast photobleaching), in contrast to in vitro results with fluorescein and H₂O₂ in phosphate buffer saline (PBS), where the addition of H₂O₂ decreases the decay time, regardless of the irradiation dose used.

CONCLUSIONS

The anomalous, ROS-concentration dependent reduction of the photobleaching rate in cells, as opposed to solutions, might indicate on the regulation of the activity of intracellular oxidative-stress protective mechanisms, as seen earlier with other methods.

SIGNIFICANCE

Assessing photobleaching via the time decay of the fluorescence intensity of an ROS-sensitive fluorophore may be adapted to monitor oxidative stress or ROS-related processes in cells.

Synthesis of cholesterol analogues bearing BODIPY fluorophores by Suzuki or Liebeskind-Srogl cross-coupling and evaluation of their potential for visualization of cholesterol pools

We report a synthetic route to BODIPY-cholesterol conjugates in which the key steps were Suzuki or Liebeskind-Srogl cross-coupling of cholesterol phenyl moieties with structurally diverse BODIPY scaffolds. All conjugates feature single-bonded and hydrophobic linkages between the fluorophore and sterol that are devoid of heteroatoms. Using HeLa cells, we show that these BODIPY-cholesterol analogues can be used simultaneously with the parent BODIPY-cholesterol for cell imaging and flow cytometry. The BODIPY-cholesterol analogues exhibit similar cellular localization in HeLa cells and show similar cholesterol efflux properties from THP-1 cells to HDL acceptors. These results demonstrate that the red-shifted BODIPY-cholesterol analogues behave in a manner similar to unlabeled cholesterol and are useful probes for simultaneous visualization of intracellular cholesterol pools and for monitoring cholesterol efflux from cells to extracellular acceptors.

Photobleaching kinetics and time-integrated emission of fluorescent probes in cellular membranes

Since the pioneering work of Hirschfeld, it is known that time-integrated emission (TiEm) of a fluorophore is independent of fluorescence quantum yield and illumination intensity. Practical implementation of this important result for determining exact probe distribution in living cells is often hampered by the presence of autofluorescence. Using kinetic modelling of photobleaching combined with pixel-wise bleach rate fitting of decay models with an updated plugin to the ImageJ program, it is shown that the TiEm of a fluorophore in living cells can be determined exactly from the product of bleaching amplitude and time constant. This applies to mono-exponential bleaching from the first excited singlet and/or triplet state and to multi-exponential combinations of such processes. The TiEm can be used to correct for illumination shading and background autofluorescence without the need for fluorescent test layers or separate imaging of non-stained cells. We apply the method to simulated images and to images of cells, whose membranes were labelled with fluorescent sterols and sphingolipids. Our bleaching model can be extended to include a probability density function (PDF) of intrinsic bleach rate constants with a memory kernel. This approach results in a time-dependent bleach rate coefficient and is exemplified for fluorescent sterols in restricted intracellular environments, like lipid droplets. We show that for small deviations from the classical exponential bleaching, the TiEm of decay functions with rate coefficients remains largely independent of fluorescence lifetime and illumination, and thereby represents a faithful measure of probe distribution.

Membrane orientation and lateral diffusion of BODIPY-cholesterol as a function of probe structure

Cholesterol tagged with the BODIPY fluorophore via the central difluoroboron moiety of the dye (B-Chol) is a promising probe for studying intracellular cholesterol dynamics. We synthesized a new BODIPY-cholesterol probe (B-P-Chol) with the fluorophore attached via one of its pyrrole rings to carbon-24 of cholesterol (B-P-Chol). Using two-photon fluorescence polarimetry in giant unilamellar vesicles and in the plasma membrane (PM) of living intact and actin-disrupted cells, we show that the BODIPY-groups in B-Chol and B-P-Chol are oriented perpendicular and almost parallel to the bilayer normal, respectively. B-Chol is in all three membrane systems much stronger oriented than B-P-Chol. Interestingly, we found that the lateral diffusion in the PM was two times slower for B-Chol than for B-P-Chol, although we found no difference in lateral diffusion in model membranes. Stimulated emission depletion microscopy, performed for the first time, to our knowledge, with fluorescent sterols, revealed that the difference in lateral diffusion of the BODIPY-cholesterol probes was not caused by anomalous subdiffusion, because diffusion of both analogs in the PM was free but not hindered. Our combined measurements show that the position and orientation of the BODIPY moiety in cholesterol analogs have a severe influence on lateral diffusion specifically in the PM of living cells.

Two-photon time-lapse microscopy of BODIPY-cholesterol reveals anomalous sterol diffusion in chinese hamster ovary cells

Background

Cholesterol is an important membrane component, but our knowledge about its transport in cells is sparse. Previous imaging studies using dehydroergosterol (DHE), an intrinsically fluorescent sterol from yeast, have established that vesicular and non-vesicular transport modes contribute to sterol trafficking from the plasma membrane. Significant photobleaching, however, limits the possibilities for in-depth analysis of sterol dynamics using DHE. Co-trafficking studies with DHE and the recently introduced fluorescent cholesterol analog BODIPY-cholesterol (BChol) suggested that the latter probe has utility for prolonged live-cell imaging of sterol transport.

Results

We found that BChol is very photostable under two-photon (2P)-excitation allowing the acquisition of several hundred frames without significant photobleaching. Therefore, long-term tracking and diffusion measurements are possible. Two-photon temporal image correlation spectroscopy (2P-TICS) provided evidence for spatially heterogeneous diffusion constants of BChol varying over two orders of magnitude from the cell interior towards the plasma membrane, where D ~ 1.3 μm²/s. Number and brightness (N&B) analysis together with stochastic simulations suggest that transient partitioning of BChol into convoluted membranes slows local sterol diffusion. We observed sterol endocytosis as well as fusion and fission of sterol-containing endocytic vesicles. The mobility of endocytic vesicles, as studied by particle tracking, is well described by a model for anomalous subdiffusion on short time scales with an anomalous exponent α ~ 0.63 and an anomalous diffusion constant of D_α = 1.95 x 10^-3 μm²/s^α. On a longer time scale (t > ~5 s), a transition to superdiffusion consistent with slow directed transport with an average velocity of v ~ 6 x 10^-3 μm/s was observed. We present an analytical model that bridges the two regimes and fit this model to vesicle trajectories from control cells and cells with disrupted microtubule or actin filaments. Both treatments reduced the anomalous diffusion constant and the velocity by ~40-50%.

Conclusions

The mobility of sterol-containing vesicles on the short time scale could reflect dynamic rearrangements of the cytoskeleton, while directed transport of sterol vesicles occurs likely along both, microtubules and actin filaments. Spatially varying anomalous diffusion could contribute to fine-tuning and local regulation of intracellular sterol transport.

Visualization of lipid metabolism in the zebrafish intestine reveals a relationship between NPC1L1-mediated cholesterol uptake and dietary fatty acid

The small intestine is the primary site of dietary lipid absorption in mammals. The balance of nutrients, microorganisms, bile, and mucus that determine intestinal luminal environment cannot be recapitulated ex vivo, thus complicating studies of lipid absorption. We show that fluorescently labeled lipids can be used to visualize and study lipid absorption in live zebrafish larvae. We demonstrate that the addition of a BODIPY-fatty acid to a diet high in atherogenic lipids enables imaging of enterocyte lipid droplet dynamics in real time. We find that a lipid-rich meal promotes BODIPY-cholesterol absorption into an endosomal compartment distinguishable from lipid droplets. We also show that dietary fatty acids promote intestinal cholesterol absorption by rapid re-localization of NPC1L1 to the intestinal brush border. These data illustrate the power of the zebrafish system to address longstanding questions in vertebrate digestive physiology.

Whole plant based treatment of hypercholesterolemia with Crataegus laevigata in a zebrafish model

Background

Consumers are increasingly turning to plant-based complementary and alternative medicines to treat hypercholesterolemia. Many of these treatments are untested and their efficacy is unknown. This multitude of potential remedies necessitates a model system amenable to testing large numbers of organisms that maintains similarity to humans in both mode of drug administration and overall physiology. Here we develop the larval zebrafish (4–30 days post fertilization) as a vertebrate model of dietary plant-based treatment of hypercholesterolemia and test the effects of Crataegus laevigata in this model.

Methods

Larval zebrafish were fed high cholesterol diets infused with fluorescent sterols and phytomedicines. Plants were ground with mortar and pestle into a fine powder before addition to food. Fluorescent sterols were utilized to optically quantify relative difference in intravascular cholesterol levels between groups of fish. We utilized the Zeiss 7-Live Duo high-speed confocal platform in order to both quantify intravascular sterol fluorescence and to capture video of the heart beat for determination of cardiac output.

Results

In this investigation we developed and utilized a larval zebrafish model to investigate dietary plant-based intervention of the pathophysiology of hypercholesterolemia. We found BODIPY-cholesterol effectively labels diet-introduced intravascular cholesterol levels (P < 0.05, Student’s t-test). We also established that zebrafish cardiac output declines as cholesterol dose increases (difference between 0.1% and 8% (w/w) high cholesterol diet-treated cardiac output significant at P < 0.05, 1-way ANOVA). Using this model, we found hawthorn leaves and flowers significantly reduce intravascular cholesterol levels (P < 0.05, 1-way ANOVA) and interact with cholesterol to impact cardiac output in hypercholesterolemic fish (2-way ANOVA, P < 0.05 for interaction effect).

Conclusions

The results of this study demonstrate that the larval zebrafish has the potential to become a powerful model to test plant based dietary intervention of hypercholesterolemia. Using this model we have shown that hawthorn leaves and flowers have the potential to affect cardiac output as well as intravascular cholesterol levels. Further, our observation that hawthorn leaves and flowers interact with cholesterol to impact cardiac output indicates that the physiological effects of hawthorn may depend on diet.

Synthetic cell surface receptors for delivery of therapeutics and probes

Receptor-mediated endocytosis is a highly efficient mechanism for cellular uptake of membrane-impermeant ligands. Cells use this process to acquire nutrients, initiate signal transduction, promote development, regulate neurotransmission, and maintain homeostasis. Natural receptors that participate in receptor-mediated endocytosis are structurally diverse, ranging from large transmembrane proteins to small glycolipids embedded in the outer leaflet of cellular plasma membranes. Despite their vast structural differences, these receptors share common features of binding to extracellular ligands, clustering in dynamic membrane regions that pinch off to yield intracellular vesicles, and accumulation of receptor-ligand complexes in membrane-sealed endosomes. Receptors typically dissociate from ligands in endosomes and cycle back to the cell surface, whereas internalized ligands are usually delivered into lysosomes, where they are degraded, but some can escape and penetrate into the cytosol. Here, we review efforts to develop synthetic cell surface receptors, defined as nonnatural compounds, exemplified by mimics of cholesterol, that insert into plasma membranes, bind extracellular ligands including therapeutics, probes, and endogenous proteins, and engage endocytic membrane trafficking pathways. By mimicking natural mechanisms of receptor-mediated endocytosis, synthetic cell surface receptors have the potential to function as prosthetic molecules capable of seamlessly augmenting the endocytic uptake machinery of living mammalian cells.

Expanding the horizons of lipidomics. Towards fluxolipidomics

This short review takes into consideration the status of lipidomics as issued from almost a decade of development. Because of the huge number of molecular species analyzed, there is a trend in subdividing lipidomics according to subdomains, in particular relating to the function of molecules. It is also pointed out that lipid imaging without the use of exogenous probes will help making relationships between molecular structures and the topography of lipid assemblies, especially in cellular compartments. Finally, a fluxomics approach is proposed for lipid molecular species, both in terms of compartments and biochemical metabolism. The example of fluxolipidomics of essential fatty acids toward their enzyme-dependent oxygenated metabolites and further toward their degradation products is developed.

Analysis of cholesterol trafficking with fluorescent probes

Cholesterol plays an important role in determining the biophysical properties of biological membranes, and its concentration is tightly controlled by homeostatic processes. The intracellular transport of cholesterol among organelles is a key part of the homeostatic mechanism, but sterol transport processes are not well understood. Fluorescence microscopy is a valuable tool for studying intracellular transport processes, but this method can be challenging for lipid molecules because addition of a fluorophore may alter the properties of the molecule greatly. We discuss the use of fluorescent molecules that can bind to cholesterol to reveal its distribution in cells. We also discuss the use of intrinsically fluorescent sterols that closely mimic cholesterol, as well as some minimally modified fluorophore-labeled sterols. Methods for imaging these sterols by conventional fluorescence microscopy and by multiphoton microscopy are described. Some label-free methods for imaging cholesterol itself are also discussed briefly.