Fluorescence evidence for cholesterol regular distribution in phosphatidylcholine and in sphingomyelin lipid bilayers

Polyene Antibiotics Physical Chemistry and Their Effect on Lipid Membranes; Impacting Biological Processes and Medical Applications

This review examined a collection of studies regarding the molecular properties of some polyene antibiotic molecules as well as their properties in solution and in particular environmental conditions. We also looked into the proposed mechanism of action of polyenes, where membrane properties play a crucial role. Given the interest in polyene antibiotics as therapeutic agents, we looked into alternative ways of reducing their collateral toxicity, including semi-synthesis of derivatives and new formulations. We follow with studies on the role of membrane structure and, finally, recent developments regarding the most important clinical applications of these compounds.

Enhanced Concanavalin A Binding to Preorganized Mannose Nanoarrays in Glycodendrimersomes Revealed Multivalent Interactions

The effect of the two-dimensional glycan display on glycan-lectin recognition remains poorly understood despite the importance of these interactions in a plethora of cellular processes, in (patho)physiology, as well as its potential for advanced therapeutics. Faced with this challenge we utilized glycodendrimersomes, a type of synthetic vesicles whose membrane mimics the surface of a cell and offers a means to probe the carbohydrate biological activity. These single-component vesicles were formed by the self-assembly of sequence-defined mannose-Janus dendrimers, which serve as surrogates for glycolipids. Using atomic force microscopy and molecular modeling we demonstrated that even mannose, a monosaccharide, was capable of organizing the sugar moieties into periodic nanoarrays without the need of the formation of liquid-ordered phases as assumed necessary for rafts. Kinetics studies of Concanavalin A binding revealed that those nanoarrays resulted in a new effective ligand yielding a ten-fold increase in the kinetic and thermodynamic constant of association.

Volumetric Properties, Compressibilities and Volume Fluctuations in Phospholipid-Cholesterol Bilayers

We conducted detailed measurements of the apparent specific volume of dipalmitoylphosphatidylcholine (DPPC)–cholesterol mixtures in excess water as a function of pressure up to 70MPa (700bar) at 20, 38 and 50°C. The volumetric properties and the isothermal compressibility κ T of the lipid vesicles were determined at cholesterol concentrations, χchol, ranging up to 50 mol. The thermodynamic data are compared with other physico-chemical properties of phospholipid–cholesterol mixtures. Furthermore, the thermodynamic properties of the system are discussed in the light of the various T, χchol–phase diagrams and computer simulation studies published in the literature.

Fluorescence techniques using dehydroergosterol to study cholesterol trafficking

Cholesterol itself has very few structural/chemical features suitable for real-time imaging in living cells. Thus, the advent of dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol, DHE] the fluorescent sterol most structurally and functionally similar to cholesterol to date, has proven to be a major asset for real-time probing/elucidating the sterol environment and intracellular sterol trafficking in living organisms. DHE is a naturally occurring, fluorescent sterol analog that faithfully mimics many of the properties of cholesterol. Because these properties are very sensitive to sterol structure and degradation, such studies require the use of extremely pure (>98%) quantities of fluorescent sterol. DHE is readily bound by cholesterol-binding proteins, is incorporated into lipoproteins (from the diet of animals or by exchange in vitro), and for real-time imaging studies is easily incorporated into cultured cells where it co-distributes with endogenous sterol. Incorporation from an ethanolic stock solution to cell culture media is effective, but this process forms an aqueous dispersion of DHE crystals which can result in endocytic cellular uptake and distribution into lysosomes which is problematic in imaging DHE at the plasma membrane of living cells. In contrast, monomeric DHE can be incorporated from unilamellar vesicles by exchange/fusion with the plasma membrane or from DHE-methyl-beta-cyclodextrin (DHE-MbetaCD) complexes by exchange with the plasma membrane. Both of the latter techniques can deliver large quantities of monomeric DHE with significant distribution into the plasma membrane. The properties and behavior of DHE in protein-binding, lipoproteins, model membranes, biological membranes, lipid rafts/caveolae, and real-time imaging in living cells indicate that this naturally occurring fluorescent sterol is a useful mimic for probing the properties of cholesterol in these systems.

The superlattice model of lateral organization of membranes and its implications on membrane lipid homeostasis

Most biological membranes are extremely complex structures consisting of hundreds of different lipid and protein molecules. According to the famous fluid-mosaic model lipids and many proteins are free to diffuse very rapidly in the plane of the membrane. While such fast diffusion implies that different membrane lipids would be laterally randomly distributed, accumulating evidence indicates that in model and natural membranes the lipid components tend to adopt regular (superlattice-like) distributions. The superlattice model, put forward based on such evidence, is intriguing because it predicts that 1) there is a limited number of allowed compositions representing local minima in membrane free energy and 2) those energy minima could provide set-points for enzymes regulating membrane lipid compositions. Furthermore, the existence of a discrete number of allowed compositions could help to maintain organelle identity in the face of rapid inter-organelle membrane traffic.

Fluorescence detection of signs of sterol superlattice formation in lipid membranes

There is a significant amount of experimental data, obtained predominantly from fluorescence studies, showing that sterol-containing liposomes can exhibit multiple biphasic changes in membrane properties at specific critical mole fractions of sterol such as 20.0, 22.2, 25.0, 33.3, 40.0, and 50.0 mol%. This can be understood in terms of the sterol regular distribution (e.g., superlattice) model. Here, the authors use excitation generalized polarization of 6-lauroyl-2-dimethylamino-naphthalene fluorescence in fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/cholesterol unilamellar vesicles to illustrate the experimental procedures and conditions that are required to detect multiple biphasic changes at predicted sterol mole fraction values in liposomal membranes. For this detection, the use of small sterol increments over a wide sterol mole fraction range is essential. Lipid concentration, incubation time, thermal history, and degree of sterol oxidation of liposomal membranes are critical factors. The principles and methodologies described here can be extended to other probes or bioactive molecules, such as enzymes, and can be applied to study sterol lateral organization in multicomponent lipid membranes.

Fluorometric assay for detection of sterol oxidation in liposomal membranes

The authors have developed a fluorescence assay to measure the rate and extent of sterol oxidation in lipid bilayers. Dehydroergosterol (DHE), a fluorescent cholesterol analog, is used as a probe and at the same time as a membrane component. The assay can also be performed on bilayers containing a mixture of sterols including DHE and nonfluorescent sterols, such as cholesterol and ergosterol. The fluorescence intensity of DHE decreases on oxidation, so the rate and extent of free radical- or enzyme-induced sterol oxidation can be measured as a function of temperature and membrane composition. For the studies, two-component (e.g., phosphatidylcholine (PC)/DHE) and multicomponent (e.g., DHE/PC/bovine-brain sphingomyelin) large unilamellar vesicles were used, and sterol oxidation was initiated either by the peroxy radical generator 2,2'-azobis (2-amidinopropane) dihydrochloride or by the enzyme cholesterol oxidase. The data gathered from this assay may be used to examine the effects of water- and lipid-soluble antioxidants on membrane sterol oxidation produced by free radicals. This assay can be used to test the potency of antioxidants and pro-oxidants, and can be used to determine whether unknown substances demonstrate antioxidant activity against sterol oxidation. The assay can also be used as a tool to examine the effect of sterol lateral organization on sterol oxidation (in the presence or absence of antioxidants). In agreement with the sterol regular distribution model, it is found that both free radical- and enzyme-induced sterol oxidation vary with membrane sterol content in a well defined alternating manner.

Time-resolved fluorescence and fourier transform infrared spectroscopic investigations of lateral packing defects and superlattice domains in compositionally uniform cholesterol/phosphatidylcholine bilayers

Time-resolved fluorescence and Fourier transform infrared spectroscopies were used to investigate the lateral organization of lipids in compositionally uniform and fully equilibrated 1-palmitoyl-2-oleoyl-phosphatidylcholine/cholesterol (POPC/CHOL) liposomes prepared by a recently devised low-temperature trapping method. Independent fluorescence decay lifetime and rotational dynamics parameters of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) in these liposomes were recovered from the time-resolved fluorescence measurements as a function of cholesterol molar fraction (X(CHOL)) at 23 degrees C. The results indicate significantly greater lifetime heterogeneity, shorter average lifetime, rotational correlation time, and lower order parameter of the DPH moiety at X(CHOL) approximately 0.40 and 0.50 as compared to the adjacent cholesterol concentrations. Less prominent changes were also detected at, for example, X(CHOL) approximately 0.20 and 0.33. These X(CHOL)'s coincide with the "critical" X(CHOL)'s predicted by the previously proposed superlattice (SL) model, thus indicating that POPC and cholesterol molecules tend to form SL domains where the components tend to be regularly distributed. The data also support another prediction of the SL model, namely that lateral packing defects coexist with the ordered SL domains. It appears that unfavorable interaction of the DPH-moiety of DPH-PC with cholesterol results in a preferential partition of DPH-PC to the defect regions. Fourier transform infrared analysis of the native lipid O=P=O, C=O, and C-H vibrational bands of POPC/CHOL liposomes in the absence of DPH-PC revealed an increase in the conformational order of the acyl chains and a decrease in the conformational order (or increased hydration) of the interfacial and headgroup regions at or close to the predicted critical X(CHOL)'s. This provides additional but probe-independent evidence for SL domain formations in the POPC/CHOL bilayers. We propose that the defect regions surrounding the putative SL domains could play an important role in modulating the activity of various membrane-associated enzymes, e.g., those regulating the lipid compositions of cell membranes.

Fluorescence and multiphoton imaging resolve unique structural forms of sterol in membranes of living cells

Although cholesterol is an essential component of mammalian membranes, resolution of cholesterol organization in membranes and organelles (i.e. lysosomes) of living cells is hampered by the paucity of nondestructive, nonperturbing methods providing real time structural information. Advantage was taken of the fact that the emission maxima of a naturally occurring fluorescent sterol (dehydroergosterol) were resolvable into two structural forms, monomeric (356 and 375 nm) and crystalline (403 and 426 nm). Model membranes (sterol:phospholipid ratios in the physiological range, e.g. 0.5-1.0), subcellular membrane fractions (plasma membranes, lysosomal membranes, microsomes, and mitochondrial membranes), and lipid rafts/caveolae (plasma membrane cholesterol-rich microdomain purified by a nondetergent method) contained primarily monomeric sterol and only small quantities (i.e. 1-5%) of the crystalline form. In contrast, the majority of sterol in isolated lysosomes was crystalline. However, addition of sterol carrier protein-2 in vitro significantly reduced the proportion of crystalline dehydroergosterol in the isolated lysosomes. Multiphoton laser scanning microscopy (MPLSM) of living L-cell fibroblasts cultured with dehydroergosterol for the first time provided real time images showing the presence of monomeric sterol in plasma membranes, as well as other intracellular membrane structures of living cells. Furthermore, MPLSM confirmed that crystalline sterol colocalized in highest amounts with LysoTracker Green, a lysosomal marker dye. Although crystalline sterol was also detected in the cytoplasm, the extralysosomal crystalline sterol did not colocalize with BODIPY FL C(5)-ceramide, a Golgi marker, and crystals were not associated with the cell surface membrane. These noninvasive, nonperturbing methods demonstrated for the first time that multiple structural forms of sterol normally occurred within membranes, membrane microdomains (lipid rafts/caveolae), and intracellular organelles of living cells, both in vitro and visualized in real time by MPLSM.

Cholesterol-induced modifications in lipid bilayers: a simulation study

We present analysis of new configurational bias Monte Carlo and molecular dynamics simulation data for bilayers of dipalmitoyl phosphatidyl choline and cholesterol for dipalmitoyl phosphatidyl choline:cholesterol ratios of 24:1, 47:3, 11.5:1, 8:1, 7:1, 4:1, 3:1, 2:1, and 1:1, using long molecular dynamics runs and interspersed configurational bias Monte Carlo to ensure equilibration and enhance sampling. In all cases with cholesterol concentrations above 12.5% the area per molecule of the heterogeneous membrane varied linearly with cholesterol fraction. By extrapolation to pure cholesterol, we find the cross-sectional area of cholesterol in these mixtures is approximately 22.3 A(2). From the slope of the area/molecule relationship, we also find that the phospholipid in these mixtures is in a liquid ordered state with an average cross-sectional area per lipid of 50.7 A(2), slightly above the molecular area of a pure phospholipid gel. For lower concentrations of cholesterol, the molecular area rises above the straight line, indicating the "melting" of at least some of the phospholipid into a fluid state. Analysis of the lateral distribution of cholesterol molecules in the leaflets reveals peaks in radial distributions of cholesterols at multiples of approximately 5 A. These peaks grow in size as the simulation progresses, suggesting a tendency for small subunits of one lipid plus one cholesterol, hydrogen bonded together, to act as one composite particle, and perhaps to aggregate with other composites. Our results are consistent with experimentally observed effects of cholesterol, including the condensation effect of cholesterol in phospholipid monolayers and the tendency of cholesterol-rich domains to form in cholesterol-lipid bilayers. We are continuing to analyze this tendency on longer timescales and for larger bilayer patches.

Fluorescence studies of lipid regular distribution in membranes

This article reviews the use of fluorescent lipids and free probes in the studies of lipid regular distribution in model membranes. The first part of this article summarizes the evidence and physical properties for lipid regular distribution in pyrene-labeled phosphatidylcholine (PC)/unlabeled PC binary mixtures as revealed by the fluorescence of pyrene-labeled PC. The original and the extended hexagonal superlattice model are discussed. The second part focuses on the fluorescence studies of sterol regular distributions in membranes. The experimental evidence for sterol superlattice formation obtained from the fluorescent sterol (i.e. dehydroergosterol) and non-sterol fluorescent probes (e.g. DPH and Laurdan) are evaluated. Prospects and concerns are given with regard to the sterol regular distribution. The third part deals briefly with the evidence for polar headgroup superlattices. The emphasis of this article is placed on the new concept that membrane properties and activities, including the activities of surface acting enzymes, drug partitioning, and membrane free volume, are fine-tuned by minute changes in the concentration of bulky lipids (e.g. sterols and pyrene-containing acyl chains) in the vicinities of the critical mole fractions for superlattice formation.

Lateral organisation of membrane lipids. The superlattice view

Most biological membranes are extremely complex structures consisting of hundreds or even thousands of different lipid and protein molecules. The prevailing view regarding the organisation of these membranes is based on the fluid-mosaic model proposed by Singer and Nicholson in 1972. According to this model, phospholipids together with some other lipids form a fluid bilayer in which these lipids are diffusing very rapidly laterally. The idea of rapid lateral diffusion implies that, in general, the different lipid species would be randomly distributed in the plain of the membrane. However, there are recent data indicating that the components tend to adopt regular (superlattice-like) distributions in fluid, mixed bilayers. Based on this, a superlattice model of membranes has been proposed. This superlattice model is intriguing because it allows only a limited certain number of 'critical' compositions. These critical compositions could play a key role in the regulation of the lipid compositions of biological membranes. Furthermore, such putative critical compositions could explain how compositionally distinct organelles can exist despite of rapid inter-organelle membrane traffic. In this review, these intriguing predictions are discussed along with the basic principles of the model and the evidence supporting it.

Small-scale lipid-membrane structure: simulation versus experiment

Recently, it has become obvious that the conventional picture of the fluid lipid-bilayer component of biological membranes being a fairly structureless 'fluid mosaic' solvent is far from correct. The lipid bilayer displays distinct static and dynamic structural organization on a small scale, for example in terms of differentiated lipid domains, and evidence is accumulating that these structures are of importance for the functioning of biological membranes, including the activity of membrane-bound enzymes and receptors and morphological changes at the cell surface. Insight into the relationship between this small-scale structure and biological functioning holds promise for a more rational approach to modulate function via manipulation of the lipid microenvironment and the lipid/protein interface in particular. Computer simulation has proved to be a useful tool in investigating membrane structure on a small scale-specifically the nanometer scale (1-100 nm), which is in between the molecular scale accessible by various spectroscopic techniques and molecular dynamics calculations, and the micrometer scale accessible by scattering and microscopy techniques.