Fluorescent sterols as tools in membrane biophysics and cell biology

Exploring lipid signaling in plant physiology: From cellular membranes to environmental adaptation

Lipids have evolved as versatile signaling molecules that regulate a variety of physiological processes in plants. Convincing evidence highlights their critical role as mediators in a wide range of plant processes required for survival, growth, development, and responses to environmental conditions such as water availability, temperature changes, salt, pests, and diseases. Understanding lipid signaling as a critical process has helped us expand our understanding of plant biology by explaining how plants sense and respond to environmental cues. Lipid signaling pathways constitute a complex network of lipids, enzymes, and receptors that coordinate important cellular responses and stressing plant biology's changing and adaptable traits. Plant lipid signaling involves a wide range of lipid classes, including phospholipids, sphingolipids, oxylipins, and sterols, each of which contributes differently to cellular communication and control. These lipids function not only as structural components, but also as bioactive molecules that transfer signals. The mechanisms entail the production of lipid mediators and their detection by particular receptors, which frequently trigger downstream cascades that affect gene expression, cellular functions, and overall plant growth. This review looks into lipid signaling in plant physiology, giving an in-depth look and emphasizing its critical function as a master regulator of vital activities.

Alkyne-tagged imidazolium-based membrane cholesterol analogs for Raman imaging applications

Cholesterol is an important lipid playing a crucial role in mediating essential cellular processes as well as maintaining the basic structural integrity of biological membranes. Given its vast biological importance, there is an unabated need for sophisticated strategies to investigate cholesterol-mediated biological processes. Raman-tagged sterol analogs offer the advantage of being visualizable without the need for a bulky dye that potentially affects natural membrane integration and cellular interactions as it is the case for many conventionally used fluorescent analogs. Herein, we report a series of alkyne-tagged imidazolium-based cholesterol analogs (CHIMs) with large Raman scattering cross-sections that readily integrate into HEK cells and primary monocyte-derived macrophages and allow (multiplexed) cellular Raman imaging. We envision Raman-tagged CHIM analogs to be a powerful platform for the investigation of cholesterol-mediated cellular processes complementary to other established methods, such as the use of fluorescent analogs.

Endogenous and fluorescent sterols reveal the molecular basis for ligand selectivity of human sterol transporters

Sterol transport proteins (STPs) play a pivotal role in cholesterol homeostasis and therefore are essential for healthy human physiology. Despite recent advances in dissecting functions of STPs in the human cell, there is still a significant knowledge gap regarding their specific biological functions and a lack of suitable selective probes for their study. Here, we profile fluorescent steroid-based probes across ten STPs, uncovering substantial differences in their selectivity, aiding the retrospective and prospective interpretation of biological results generated with those probes. These results guided the establishment of an STP screening panel combining diverse biophysical assays, enabling the evaluation of 41 steroid-based natural products and derivatives. Combining this with a thorough structural analysis revealed the molecular basis for STP specific selectivity profiles, leading to the uncovering of several new potent and selective Aster-B inhibitors, and supporting the role of this protein in steroidogenesis.

Duo-Chol: A Photoconvertible Live Cell Imaging Tool for Tracking Cholesterol

Investigating cholesterol trafficking pathways continues to be of significant scientific interest owing to its homeostasis being associated with several debilitating cardiovascular and neurodegenerative diseases including atherosclerosis, Niemann-Pick's disease, Alzheimer's disease, and Parkinson's disease. To further our understanding of cholesterol trafficking, it is imperative to develop new fluorescent probes that possess improved photostability, low efflux, and high spatial and temporal resolution for live-cell imaging. In this study, we developed a photoconvertible fluorescent cholesterol analog, Duo-Chol, enabling the improved spatiotemporal fluorescence imaging of the dynamic localization of cholesterol in live cells. This tool provides a unique and powerful approach to interrogating cholesterol dynamics, addressing the limitations of existing methods, and expanding our ability to probe the biological role of sterols in living cells.

Mixture toxicity of 6PPD-quinone and polystyrene nanoplastics in zebrafish

Plastic pollution, including micro- and nanoplastics, is a growing concern. Tyre-wear particles (TWPs) are the second largest source of microplastics in the ocean following abrasion of synthetic fibres. In addition to the particles themselves, TWPs contain many harmful chemicals, including 6PPD. This chemical reacts with atmospheric ozone and forms the toxic compound 6PPD-quinone (6PPDq), which poses a danger to aquatic life. There is a knowledge gap in understanding risks associated with the combined toxicity of nanoplastics (NPs) and 6PPDq. The present study aimed to investigate the toxicity of NPs and 6PPDq on adult zebrafish using phenotypic (behaviour, histology) and transcriptomic endpoints. Zebrafish were exposed to four treatments: control (contaminant-free), 50 μg/L 6PPDq, 3 mg/L polystyrene (PS)-NPs, and a combination of 50 μg/L 6PPDq and 3 mg/L PS-NPs. We did not observe locomotory dysregulation in zebrafish exposed to NPs. However, we found significant hyperlocomotion in zebrafish exposed to 6PPDq and this effect was even more substantial after co-exposure with PS-NPs. This study explores the molecular mechanisms behind these effects, identifying genes associated with neurotransmitters and fatty acid metabolism that were dysregulated by the co-exposure. Transcriptomic analysis further showed that both 6PPDq and PS-NPs impacted cellular processes associated with sterol biosynthesis, cholesterol metabolism, and muscle tissue development. The effects on these mechanisms were stronger in co-exposed zebrafish, indicating a heightened risk to cellular integrity and mitochondrial dysfunction. These results highlight the significance of mixture toxicity when studying the effects of NPs and associated chemicals like 6PPDq.

Photochemistry and Photophysics of Cholesta-5,7,9(11)-trien-3β-ol in Ethanol

Cholesta-5,7,9(11)-trien-3β-ol (9,11-dehydroprovitamin D₃, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. We recently described the photochemistry and photophysics of CTL in degassed and air-saturated tetrahydrofuran (THF) solution, an aprotic solvent. The zwitterionic nature of the singlet excited state, ¹CTL* is revealed in ethanol, a protic solvent. In ethanol, the products observed in THF are accompanied by ether photoadducts and by photoreduction of the triene moiety to four dienes, including provitamin D₃. The major diene retains the conjugated s-trans-diene chromophore and the minor is unconjugated, involving 1,4-addition of H at the 7 and 11 positions. In the presence of air, peroxide formation is a major reaction channel as in THF. X-ray crystallography confirmed the identification of two of the new diene products as well as of a peroxide rearrangement product.

Cell Uptake of Steroid-BODIPY Conjugates and Their Internalization Mechanisms: Cancer Theranostic Dyes

Estradiol-BODIPY linked via an 8-carbon spacer chain and 19-nortestosterone- and testosterone-BODIPY linked via an ethynyl spacer group were evaluated for cell uptake in the breast cancer cell lines MCF-7 and MDA-MB-231 and prostate cancer cell lines PC-3 and LNCaP, as well as in normal dermal fibroblasts, using fluorescence microscopy. The highest level of internalization was observed with 11β-OMe-estradiol-BODIPY 2 and 7α-Me-19-nortestosterone-BODIPY 4 towards cells expressing their specific receptors. Blocking experiments showed changes in non-specific cell uptake in the cancer and normal cells, which likely reflect differences in the lipophilicity of the conjugates. The internalization of the conjugates was shown to be an energy-dependent process that is likely mediated by clathrin- and caveolae-endocytosis. Studies using 2D co-cultures of cancer cells and normal fibroblasts showed that the conjugates are more selective towards cancer cells. Cell viability assays showed that the conjugates are non-toxic for cancer and/or normal cells. Visible light irradiation of cells incubated with estradiol-BODIPYs 1 and 2 and 7α-Me-19-nortestosterone-BODIPY 4 induced cell death, suggesting their potential for use as PDT agents.

Cholesterol Binds the Amphipathic Helix of IFITM3 and Regulates Antiviral Activity

The interferon-induced transmembrane (IFITM) proteins broadly inhibit the entry of diverse pathogenic viruses, including Influenza A virus (IAV), Zika virus, HIV-1, and SARS coronaviruses by inhibiting virus-cell membrane fusion. IFITM3 was previously shown to disrupt cholesterol trafficking, but the functional relationship between IFITM3 and cholesterol remains unclear. We previously showed that inhibition of IAV entry by IFITM3 is associated with its ability to promote cellular membrane rigidity, and these activities are functionally linked by a shared requirement for the amphipathic helix (AH) found in the intramembrane domain (IMD) of IFITM3. Furthermore, it has been shown that the AH of IFITM3 alters lipid membranes in vitro in a cholesterol-dependent manner. Therefore, we aimed to elucidate the relationship between IFITM3 and cholesterol in more detail. Using a fluorescence-based in vitro binding assay, we found that a peptide derived from the AH of IFITM3 directly interacted with the cholesterol analog, NBD-cholesterol, while other regions of the IFITM3 IMD did not, and native cholesterol competed with this interaction. In addition, recombinant full-length IFITM3 protein also exhibited NBD-cholesterol binding activity. Importantly, previously characterized mutations within the AH of IFITM3 that strongly inhibit antiviral function (F63Q and F67Q) disrupted AH structure in solution, inhibited cholesterol binding in vitro, and restricted bilayer insertion in silico. Our data suggest that direct interactions with cholesterol may contribute to the inhibition of membrane fusion pore formation by IFITM3. These findings may facilitate the design of therapeutic peptides for use in broad-spectrum antiviral 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.

Photochemistry and photophysics of cholesta-5,7,9(11)-trien-3β-ol: a fluorescent analogue of cholesterol

Cholesta-5,7,9(11)-trien-3β-ol (9,11-dehydroprovitamin D₃, CTL) is used as a fluorescent probe to track the presence and migration of cholesterol in vivo. CTL is known to be photochemically active, but little consideration has been given to the formation efficiency and possible toxicity of its photoproducts. In degassed tetrahydrofuran (THF) solution, we isolated the photoproduct of CTL and of its 25-hydroxy derivative (HOCTL), and X-ray crystal structures were obtained for HOCTL and the photorearrangement product. The X-ray crystal structure and its ¹H NMR spectrum confirm the product structure as a pentacyclic HOCTL isomer. In the presence of air in THF, endoperoxide formation via [2+4] addition of ¹O₂* across the B ring of CTL or HOCTL becomes the dominant photoreaction. The UV spectrum and decay kinetics of the triplet state of HOCTL, the precursor of ¹O₂*, are determined by transient absorption spectroscopy. We confirm the proposed structure of the endoperoxide by X-ray crystallography. Kinetics analysis of quantum yields provides rate constants for photophysical and photochemical events.

Therapeutic potential of melatonin in colorectal cancer: Focus on lipid metabolism and gut microbiota

Colorectal cancer (CRC) is one of the most common gastrointestinal malignancies. The occurrence and development of CRC are complicated processes. Obesity and dysbacteriosis have been increasingly regarded as the main risk factors for CRC. Understanding the etiology of CRC from multiple perspectives is conducive to screening for some potential drugs or new treatment strategies to limit the serious side effects of conventional treatment and prolong the survival of CRC patients. Melatonin, a natural indoleamine, is mainly produced by the pineal gland, but it is also abundant in other tissues, including the gastrointestinal tract, retina, testes, lymphocytes, and Harder's glands. Melatonin could participate in lipid metabolism by regulating adipogenesis and lipolysis. Additionally, many studies have focused on the potential beneficial effects of melatonin in CRC, such as promotion of apoptosis; inhibition of cell proliferation, migration, and invasion; antioxidant activity; and immune regulation. Meaningfully, gut microbiota is the main determinant of all aspects of health and disease (including obesity and tumorigenesis). The gut microbiota is of great significance for understanding the relationship between obesity and increased risk of CRC. Although the current understanding of how the melatonin-mediated gut microbiota coordinates a variety of physiological and pathological activities is fairly comprehensive, there are still many unknown topics to be explored in the face of a complex nutritional status and a changeable microbiota. This review summarizes the potential links among melatonin, lipid metabolism, gut microbiota, and CRC to promote the development of melatonin as a preventive and therapeutic agent for CRC.

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, Δ5,7,9(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.

Two-Color Visualization of Cholesterol Fluctuation in Plasma Membranes by Spatial Distribution-Controllable Single Fluorescent Probes

Visualizing cholesterol (CL) fluctuation in plasma membranes is a crucially important yet challenging task in cell biology. Here, we proposed a new imaging strategy based on permeability changes of plasma membranes triggered by different CL contents to result in controllable spatial distribution of single fluorescent probes (SF-probes) in subcellular organelles. Three spatial distribution-controllable SF-probes (PMM-Me, PMM-Et, and PMM-Bu) for imaging CL fluctuation in plasma membranes were rationally developed. These SF-probes target plasma membranes and mitochondria at normal CL levels, while they display solely staining in plasma membranes and mitochondria at increased and decreased CL levels, respectively. These polarity-sensitive probes also show distinct emission colors with fluorescence peaks of 575 and 620 nm in plasma membranes and mitochondria, respectively. Thus, the CL fluctuation in plasma membranes can be clearly visualized by means of the spatially distributed and two-color emissive SF-probes.

CHIMs are versatile cholesterol analogs mimicking and visualizing cholesterol behavior in lipid bilayers and cells

Cholesterol is an essential component of cellular membranes regulating the structural integrity and fluidity of biological bilayers and cellular processes such as signal transduction and membrane trafficking. However, tools to investigate the role and dynamics of cholesterol in live cells are still scarce and often show limited applicability. To address this, we previously developed a class of imidazolium-based cholesterol analogs, CHIMs. Here we confirm that CHIM membrane integration characteristics largely mimic those of cholesterol. Computational studies in simulated phospholipid bilayers and biophysical analyses of model membranes reveal that in biologically relevant systems CHIMs behave similarly to natural cholesterol. Importantly, the analogs can functionally replace cholesterol in membranes, can be readily labeled by click chemistry and follow trafficking pathways of cholesterol in live cells. Thus, CHIMs represent chemically versatile cholesterol analogs that can serve as a flexible toolbox to study cholesterol behavior and function in live cells and organisms.

Cholesterol interaction attenuates scramblase activity of SCRM-1 in the artificial membrane

Phospholipid (PL) scramblases are single-pass transmembrane protein mediating bidirectional PL translocation. Previously in silico analysis of human PL scramblases, predicted the presence of an uncharacterized cholesterol-binding domain spanning partly in the transmembrane helix as well as in the adjacent extracellular coil. This domain was found to be universally conserved in diverse organisms like Caenorhabditis elegans. In this study, we investigated the saturable cholesterol-binding domain of SCRM-1 using fluorescence sterol binding assay, Stern-Volmer quenching, Förster resonance energy transfer, and CD spectroscopy. We observed high-affinity interaction between cholesterol and SCRM-1. Our results support a previous report, which showed that the cholesterol ordering effect reduced the scramblase activity of hPLSCR1. Considering the presence of a high-affinity binding sequence, we propose that the reduction in activity could partly be due to the cholesterol binding. To validate this, we generated a C-terminal helix (CTH) deletion construct (∆CTH SCRM-1) and a point mutation in the putative cholesterol-binding domain I273D SCRM-1. Deletion construct greatly reduced cholesterol affinity along with loss of scramblase activity. In contrast to this, I273D SCRM-1 retained scrambling activity in proteoliposomes containing ~30 mol% cholesterol but lost sterol binding ability. These results suggest that C-terminal helix is crucial for membrane insertion and in the lipid bilayer the scrambling activity of SCRM-1 is modulated through its interaction with cholesterol.

New 20-hydroxycholesterol-like compounds with fluorescent NBD or alkyne labels: Synthesis, in silico interactions with proteins and uptake by yeast cells

20-hydroxycholesterol is a signaling oxysterol with immunomodulating functions and, thus, structural analogues with reporter capabilities could be useful for studying and modulating the cellular processes concerned. We have synthesized three new 20-hydroxycholesterol-like pregn-5-en-3β-ol derivatives with fluorescent 7-nitrobenzofurazan (NBD) or Raman-sensitive alkyne labels in their side-chains. In silico computations demonstrated the compounds possess good membrane permeability and can bind within active sites of known 20-hydroxycholesterol targets (e.g. Smoothened and yeast Osh4) and some other sterol-binding proteins (human LXRβ and STARD1; yeast START-kins Lam4S2 and Lam2S2). Having found good predicted membrane permeability and binding to some yeast proteins, we tested the compounds on microorganisms. Fluorescent microscopy indicated the uptake of the steroids by both Saccharomyces cerevisiae and Yarrowia lipolytica, whereas only S. cerevisiae demonstrated conversion of the compounds into 3-O-acetates, likely because 3-O-acetyltransferase Atf2p is present only in its genome. The new compounds provide new options to study the uptake, intracellular distribution and metabolism of sterols in yeast cells as well as might be used as ligands for sterol-binding proteins.

New insights into human endometrial aminopeptidases in both implantation and menstruation

The endometrium cycle involves proliferation of endometrial epithelial cells in preparation for implantation of fertilized ovum. With ovulation, the endometrium secretes nutrients such as peptides and amino acids into the endometrial cavity. The histological evidence of ovulation in normal menstrual cycle includes subnuclear glycogen vacuoles surrounded by placental leucine aminopeptidase (P-LAP) in endometrial epithelial cells. P-LAP is an essentially involved in intracellular trafficking of glucose transporter (GLUT) 4 which is primarily important for glucose uptake in skeletal muscles and fat tissues. On the other hand, glucose influx from blood into endometrial epithelial cells is not mainly mediated by GLUTs, but by coincident appearing progesterone just after ovulation. Progesterone increases permeability of not only plasma membranes, but also lysosomal membranes, and this may be primarily involved in glucose influx. Progesterone also expands the exocytosis in the endometrium after ovulation, and endometrial secretion after ovulation is possibly apocrine and holocrine, which is augmented and exaggerated exocytosis of the lysosomal contents. The endometrial spiral arteries/arterioles are surrounded by endometrial stromal cells which are differentiated into decidual/pre-decidual cells. Decidual cells are devoid of aminopeptidase A (APA), possibly leading to enhancement of Angiotensin-II action in decidual cell area due to loss of its degradation by APA. Angiotensin-II is thought to exert growth-factor-like effects in post-implantation embryos in decidual cells, thereby contributing to implantation. Without implantation, angiotensin-II constricts the endometrial spiral arteries/arterioles to promote menstruation. Thus, P-LAP and APA may be involved in homeostasis in uterus via regulating glucose transport and vasoconstrictive peptides.

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.

Preventive Effects of Dairy Products on Dementia and the Underlying Mechanisms

Alongside the rapid population aging occurring worldwide, the prevention of age-related memory decline and dementia has become a high priority. Dairy products have many physiological effects owing to their contents of lactic acid bacteria and the fatty acids and peptides generated during their fermentation. In particular, several recent studies have elucidated the effects of fermented dairy products on cognitive function. Epidemiological and clinical evidence has indicated that fermented dairy products have preventive effects against dementia, including Alzheimer’s disease. Recent preclinical studies have identified individual molecules generated during fermentation that are responsible for those preventive effects. Oleamide and dehydroergosterol have been identified as the agents responsible for reducing microglial inflammatory responses and neurotoxicity. In this review, the protective effects of fermented dairy products and their components on cognitive function, the mechanisms underlying those effects, and the prospects for their future clinical development will be discussed.

Analysis of the Lipid Profile in Patients with Colorectal Cancer in Advanced Stages

Backgrounds: Colorectal (CRC) is one of the main cause of cancer worldwide. The search for noninvasive markers for diagnosis and monitoring as the use of analytical technologies such as mass spectrometry (MS), which allowed the search for lipid metabolites as candidates for probable biomarkers are needed. Objective and Methods: The objective was to establish the lipid profile of patients with locally advanced, unresectable or metastatic CRC. Peripheral blood was collected from patients with CRC and controls with normal colonoscopy. After lipid extraction, the samples were processed and analyzed in the MALDI TOF / TOF equipment. From the data matrix, the statistical analyzes were performed by the principal component analysis methods and the least squares discriminant analysis. The importance of the variable in the projection was used to identify the ions that had the greatest discriminatory effect between the groups. Results: Eight lipids were identified as potential biomarkers and a multiple logistic regression model was proposed to calculate the performance of the test where we observed values of AUC 0.87, sensitivity 88.33% and specificity 83.78% and for a validation test with 1,000 permutations a p <0.001. The classes of lipids found were sphingolipids, glycerophospholipids and policetidis. The strength of the association between the peak intensities of these lipids and the presence of CRC make these metabolites candidates for possible biomarkers. The sphingolipid (m / z = 742.98869) could be a biomarker in monitoring patients with CRC. In the survival analysis, three lipids showed a prognostic value for colorectal cancer, sphingolipid (m / z = 857.11525) and policetidis (m / z = 876.20796) and glycerophospholipid (m / z = 1031.54773).

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.

Cell-geometry-dependent changes in plasma membrane order direct stem cell signalling and fate

Cell size and shape affect cellular processes such as cell survival, growth and differentiation^1-4, thus establishing cell geometry as a fundamental regulator of cell physiology. The contributions of the cytoskeleton, specifically actomyosin tension, to these effects have been described, but the exact biophysical mechanisms that translate changes in cell geometry to changes in cell behaviour remain mostly unresolved. Using a variety of innovative materials techniques, we demonstrate that the nanostructure and lipid assembly within the cell plasma membrane are regulated by cell geometry in a ligand-independent manner. These biophysical changes trigger signalling events involving the serine/threonine kinase Akt/protein kinase B (PKB) that direct cell-geometry-dependent mesenchymal stem cell differentiation. Our study defines a central regulatory role by plasma membrane ordered lipid raft microdomains in modulating stem cell differentiation with potential translational applications.

Lipidome in colorectal cancer

Colorectal cancer (CRC) is the second leading cause of cancer-related deaths. Understanding its pathophysiology is essential for developing efficient strategies to treat this disease. Lipidome, the sum of total lipids, related enzymes, receptors and signaling pathways, plays crucial roles in multiple cellular processes, such as metabolism, energy storage, proliferation and apoptosis. Dysregulation of lipid metabolism and function contributes to the development of CRC, and can be used towards the evaluation of prognosis. The strategies targeting lipidome have been applied in clinical trails and showed promising results. Here we discuss recent advances in abnormal lipid metabolism in CRC, the mechanisms by which the lipidome regulates tumorigenesis and tumor progression, and suggest potential therapeutic targets for clinical trials.

Changes in membrane biophysical properties induced by the Budesonide/Hydroxypropyl-β-cyclodextrin complex

Budesonide (BUD), a poorly soluble anti-inflammatory drug, is used to treat patients suffering from asthma and COPD (Chronic Obstructive Pulmonary Disease). Hydroxypropyl-β-cyclodextrin (HPβCD), a biocompatible cyclodextrin known to interact with cholesterol, is used as a drug-solubilizing agent in pharmaceutical formulations. Budesonide administered as an inclusion complex within HPβCD (BUD:HPβCD) required a quarter of the nominal dose of the suspension formulation and significantly reduced neutrophil-induced inflammation in a COPD mouse model exceeding the effect of each molecule administered individually. This suggests the role of lipid domains enriched in cholesterol for inflammatory signaling activation. In this context, we investigated the effect of BUD:HPβCD on the biophysical properties of membrane lipids. On cellular models (A549, lung epithelial cells), BUD:HPβCD extracted cholesterol similarly to HPβCD. On large unilamellar vesicles (LUVs), by using the fluorescent probes diphenylhexatriene (DPH) and calcein, we demonstrated an increase in membrane fluidity and permeability induced by BUD:HPβCD in vesicles containing cholesterol. On giant unilamellar vesicles (GUVs) and lipid monolayers, BUD:HPβCD induced the disruption of cholesterol-enriched raft-like liquid ordered domains as well as changes in lipid packing and lipid desorption from the cholesterol monolayers, respectively. Except for membrane fluidity, all these effects were enhanced when HPβCD was complexed with budesonide as compared with HPβCD. Since cholesterol-enriched domains have been linked to membrane signaling including pathways involved in inflammation processes, we hypothesized the effects of BUD:HPβCD could be partly mediated by changes in the biophysical properties of cholesterol-enriched domains.

Budding Yeast: An Ideal Backdrop for In vivo Lipid Biochemistry

Biological membranes are non-covalent assembly of lipids and proteins. Lipids play critical role in determining membrane physical properties and regulate the function of membrane associated proteins. Budding yeast Saccharomyces cerevisiae offers an exceptional advantage to understand the lipid-protein interactions since lipid metabolism and homeostasis are relatively simple and well characterized as compared to other eukaryotes. In addition, a vast array of genetic and cell biological tools are available to determine and understand the role of a particular lipid in various lipid metabolic disorders. Budding yeast has been instrumental in delineating mechanisms related to lipid metabolism, trafficking and their localization in different subcellular compartments at various cell cycle stages. Further, availability of tools and enormous potential for the development of useful reagents and novel technologies to localize a particular lipid in different subcellular compartments in yeast makes it a formidable system to carry out lipid biology. Taken together, yeast provides an outstanding backdrop to characterize lipid metabolic changes under various physiological conditions.

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.

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.

Functionalized lipids and surfactants for specific applications

Synthetic lipids and surfactants that do not exist in biological systems have been used for the last few decades in both basic and applied science. The most notable applications for synthetic lipids and surfactants are drug delivery, gene transfection, as reporting molecules, and as support for structural lipid biology. In this review, we describe the potential of the synergistic combination of computational and experimental methodologies to study the behavior of synthetic lipids and surfactants embedded in lipid membranes and liposomes. We focused on select cases in which molecular dynamics simulations were used to complement experimental studies aiming to understand the structure and properties of new compounds at the atomistic level. We also describe cases in which molecular dynamics simulations were used to design new synthetic lipids and surfactants, as well as emerging fields for the application of these compounds. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Cholesterol-based tethers and markers for model membranes investigation

A series of new bifunctional cholesterol compounds for tethered bilayer membrane (tBLM) systems were synthesized and tested. The compounds containing cyclic disulfide group may be used as molecular anchors for phospholipid bilayers. Anchoring occurs through the insertion of the cholesterol moiety into the hydrophobic slab of the phospholipid layer, while the surface density of anchor molecules may be adjusted using disulfides terminated spacers. Five ethylene oxide segments between the disulfide group and the cholesteryl provide hydration of the layer separating solid support and model membrane. Another group of cholesterol derivatives described in this work contains either fluorescence probe or electroactive functional groups. We demonstrated the practical utility of these compounds for visualization of cholesterol extraction from and loading to tBLMs. We demonstrated that electroactive group containing cholesterol derivatives can be reconstituted either into vesicles or tBLMs. In both cases an electrochemical signal can be generated on electrodes from these probes. In general, the newly synthesized compound may be utilized in a variety of applications involving tethered bilayer systems and vesicles.

Recent progress on lipid lateral heterogeneity in plasma membranes: From rafts to submicrometric domains

The concept of transient nanometric domains known as lipid rafts has brought interest to reassess the validity of the Singer-Nicolson model of a fluid bilayer for cell membranes. However, this new view is still insufficient to explain the cellular control of surface lipid diversity or membrane deformability. During the past decades, the hypothesis that some lipids form large (submicrometric/mesoscale vs nanometric rafts) and stable (>min vs s) membrane domains has emerged, largely based on indirect methods. Morphological evidence for stable submicrometric lipid domains, well-accepted for artificial and highly specialized biological membranes, was further reported for a variety of living cells from prokaryot es to yeast and mammalian cells. However, results remained questioned based on limitations of available fluorescent tools, use of poor lipid fixatives, and imaging artifacts due to non-resolved membrane projections. In this review, we will discuss recent evidence generated using powerful and innovative approaches such as lipid-specific toxin fragments that support the existence of submicrometric domains. We will integrate documented mechanisms involved in the formation and maintenance of these domains, and provide a perspective on their relevance on membrane deformability and regulation of membrane protein distribution.

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.

Interaction of fluorescent phospholipids with cyclodextrins

Fluorescent analogs of phospholipids are often employed to investigate the structure and dynamics of lipids in membranes. Some of those studies have used cyclodextrins e.g., to modulate the lipid phase. However, the role of the fluorescence moiety of analogs for the interaction between cyclodextrins and fluorescent lipids has not been investigated so far in detail. Therefore, in the present study the interaction of various fluorescent phospholipid analogs with methylated α-, β- and γ- cyclodextrins was investigated. The analogs differed in their structure, in the length of the fatty acyl chain, in the position of the fluorescence group, and in the attached fluorescence moiety (7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) or dipyrrometheneboron difluoride (BODIPY)). In aqueous buffer, cyclodextrins bind fluorescent lipids disturbing the organization of the analogs. When incorporated into lipid vesicles, analogs are selectively extracted from the membrane upon addition of cyclodextrins. The results show that the interaction of cyclodextrins with fluorescent phospholipids depends on the cyclodextrin species, the fluorescence moiety and the phospholipid structure. The presented data should be of interest for studies using fluorescent phospholipids and cyclodextrins, since the interaction between the fluorescence group and the cyclodextrin may interfere with the process(es) under study.

Evaluation of sterol transport from the endoplasmic reticulum to mitochondria using mitochondrially targeted bacterial sterol acyltransferase in Saccharomyces cerevisiae

To elucidate the mechanism of interorganelle sterol transport, a system to evaluate sterol transport from the endoplasmic reticulum (ER) to the mitochondria was constructed. A bacterial glycerophospholipid: cholesterol acyltransferase fused with a mitochondria-targeting sequence and a membrane-spanning domain of the mitochondrial inner membrane protein Pet100 and enhanced green fluorescent protein was expressed in a Saccharomyces cerevisiae mutant deleted for ARE1 and ARE2 encoding acyl-CoA:sterol acyltransferases. Microscopic observation and subcellular fractionation suggested that this fusion protein, which was named mito-SatA-EGFP, was localized in the mitochondria. Steryl esters were synthesized in the mutant expressing mito-SatA-EGFP. This system will be applicable for evaluations of sterol transport from the ER to the mitochondria in yeast by examining sterol esterification in the mitochondria.

Specific cellular incorporation of a pyrene-labelled cholesterol: lipoprotein-mediated delivery toward ordered intracellular membranes

In the aim of testing tools for tracing cell trafficking of exogenous cholesterol, two fluorescent derivatives of cholesterol, 22-nitrobenzoxadiazole-cholesterol (NBD-Chol) and 21-methylpyrenyl-cholesterol (Pyr-met-Chol), with distinctive chemico-physical characteristics, have been compared for their cell incorporation properties, using two cell models differently handling cholesterol, with two incorporation routes. In the Caco-2 cell model, the cholesterol probes were delivered in bile salt micelles, as a model of intestinal absorption. The two probes displayed contrasting behaviors for cell uptake characteristics, cell staining, and efflux kinetics. In particular, Pyr-met-Chol cell incorporation involved SR-BI, while that of NBD-Chol appeared purely passive. In the PC-3 cell model, which overexpresses lipoprotein receptors, the cholesterol probes were delivered via the serum components, as a model of systemic delivery. We showed that Pyr-met-Chol-labelled purified LDL or HDL were able to specifically deliver Pyr-met-Chol to the PC-3 cells, while NBD-Chol incorporation was independent of lipoproteins. Observations by fluorescence microscopy evidenced that, while NBD-Chol readily stained the cytosolic lipid droplets, Pyr-met-Chol labelling led to the intense staining of intracellular structures of membranous nature, in agreement with the absence of detectable esterification of Pyr-met-Chol. A 48 h incubation of PC-3 cells with either Pyr-met-Chol-labelled LDL or HDL gave same staining patterns, mainly colocalizing with Lamp1, caveolin-1 and CD63. These data indicated convergent trafficking downwards their respective receptors, LDL-R and SR-BI, toward the cholesterol-rich internal membrane compartments, late endosomes and multivesicular bodies. Interestingly, Pyr-met-Chol staining of these structures exhibited a high excimer fluorescence emission, revealing their ordered membrane environment, and indicating that Pyr-met-Chol behaves as a fair cholesterol tracer regarding its preferential incorporation into cholesterol-rich domains. We conclude that, while NBD-Chol is a valuable marker of cholesterol esterification, Pyr-met-Chol is a reliable new lipoprotein fluorescent marker which allows to probe specific intracellular trafficking of cholesterol-rich membranes.