Critical factors for detection of biphasic changes in membrane properties at specific sterol mole fractions for maximal superlattice formation

Gramicidin Lateral Distribution in Phospholipid Membranes: Fluorescence Phasor Plots and Statistical Mechanical Model

When using small mole fraction increments to study gramicidins in phospholipid membranes, we found that the phasor dots of intrinsic fluorescence of gramicidin D and gramicidin A in dimyristoyl-sn-glycero-3-phosphocholine (DMPC) unilamellar and multilamellar vesicles exhibit a biphasic change with peptide content at 0.143 gramicidin mole fraction. To understand this phenomenon, we developed a statistical mechanical model of gramicidin/DMPC mixtures. Our model assumes a sludge-like mixture of fluid phase and aggregates of rigid clusters. In the fluid phase, gramicidin monomers are randomly distributed. A rigid cluster is formed by a gramicidin dimer and DMPC molecules that are condensed to the dimer, following particular stoichiometries (critical gramicidin mole fractions, Xcr including 0.143). Rigid clusters form aggregates in which gramicidin dimers are regularly distributed, in some cases, even to superlattices. At Xcr, the size of cluster aggregates and regular distributions reach a local maximum. Before a similar model was developed for cholesterol/DMPC mixtures (Sugar and Chong (2012) J. Am. Chem. Soc. 134, 1164⁻1171) and here the similarities and differences are discussed between these two models.

Series of concentration-induced phase transitions in cholesterol/phosphatidylcholine mixtures

In lipid membranes, temperature-induced transition from gel-to-fluid phase increases the lateral diffusion of the lipid molecules by three orders of magnitude. In cell membranes, a similar phase change may trigger the communication between the membrane components. Here concentration-induced phase transition properties of our recently developed statistical mechanical model of cholesterol/phospholipid mixtures are investigated. A slight (<1%) decrease in the model parameter values, controlling the lateral interaction energies, reveals the existence of a series of first- or second-order phase transitions. By weakening the lateral interactions first, the proportion of the ordered (i.e., superlattice) phase (Areg) is slightly and continuously decreasing at every cholesterol mole fraction. Then sudden decreases in Areg appear at the 0.18-0.26 range of cholesterol mole fractions. We point out that the sudden changes in Areg represent first- or second-order concentration-induced phase transitions from fluid to superlattice and from superlattice to fluid phase. Sudden changes like these were detected in our previous experiments at 0.2, 0.222, and 0.25 sterol mole fractions in ergosterol/DMPC mixtures. By further decreasing the lateral interactions, the fluid phase will dominate throughout the 0.18-0.26 interval, whereas outside this interval sudden increases in Areg may appear. Lipid composition-induced phase transitions as specified here should have far more important biological implications than temperature- or pressure-induced phase transitions. This is the case because temperature and pressure in cell membranes are largely invariant under physiological conditions.

Cholesterol superlattice modulates CA4P release from liposomes and CA4P cytotoxicity on mammary cancer cells

Liposomal drugs are a useful alternative to conventional drugs and hold great promise for targeted delivery in the treatment of many diseases. Most of the liposomal drugs on the market or under clinical trials include cholesterol as a membrane stabilizing agent. Here, we used liposomal CA4P, an antivascular drug, to demonstrate that cholesterol content can actually modulate the release and cytotoxicity of liposomal drugs in a delicate and predictable manner. We found that both the rate of the CA4P release from the interior aqueous compartment of the liposomes to the bulk aqueous phase and the extent of the drug's cytotoxicity undergo a biphasic variation, as large as 50%, with liposomal cholesterol content at the theoretically predicted C(r), e.g., 22.0, 22.2, 25.0, 33.3, 40.0, and 50.0 mol % cholesterol for maximal superlattice formation. It appears that at C(r), CA4P can be released from the liposomes more readily than at non-C(r), probably due to the increased domain boundaries between superlattice and nonsuperlattice regions, which consequently results in increased cytotoxicity. The idea that the increased domain boundaries at C(r) would facilitate the escape of molecules from membranes was further supported by the data of dehydroergosterol transfer from liposomes to MβCD. These results together show that the functional importance of sterol superlattice formation in liposomes can be propagated to distal targeted cells and reveal a new, to our knowledge, mechanism for how sterol content and membrane lateral organization can control the release of entrapped or embedded molecules in membranes.

Cholesterol Organization in Phosphatidylcholine Liposomes: A Surface Plasmon Resonance Study

Models for the organization of sterols into regular arrays within phospholipid bilayers have been proposed previously. The existence of such arrays in real systems has been supported by the fact that concentration-dependent sterol properties show discontinuities at the cholesterol mole fractions corresponding to regular lattice arrangements. Experimental results presented here are based on a surface plasmon resonance assay that was used to analyze rates of cyclodextrin-mediated removal of cholesterol from adsorbed liposomes at cholesterol mole fractions up to χ_C = 0.55. Two kinetic pools of cholesterol were detected; there was a fast pool present at χ_C > 0.25, and a slow pool, with a removal rate that was dependent on the initial χ_C but that did not vary as χ_C decreased during the course of one experiment. The cholesterol activity therefore seems to be affected by sample history as well as local concentration, which could be explained in terms of the formation of superlattices that are stable for relatively long times. We also describe a variation on the traditional lattice models, with phosphatidylcholine (PC) being treated as an arrangement of hexagonal tiles; the cholesterol is then introduced at any vertex point, without increasing the total area occupied by all the lipid molecules. This model is consistent with Langmuir trough measurements of total lipid area and provides a simple explanation for the maximum solubility of cholesterol in the PC bilayer.

Large conductance, calcium- and voltage-gated potassium (BK) channels: regulation by cholesterol

Cholesterol (CLR) is an essential component of eukaryotic plasma membranes. CLR regulates the membrane physical state, microdomain formation and the activity of membrane-spanning proteins, including ion channels. Large conductance, voltage- and Ca²⁺-gated K⁺ (BK) channels link membrane potential to cell Ca²⁺ homeostasis. Thus, they control many physiological processes and participate in pathophysiological mechanisms leading to human disease. Because plasmalemma BK channels cluster in CLR-rich membrane microdomains, a major driving force for studying BK channel-CLR interactions is determining how membrane CLR controls the BK current phenotype, including its pharmacology, channel sorting, distribution, and role in cell physiology. Since both BK channels and CLR tissue levels play a pathophysiological role in human disease, identifying functional and structural aspects of the CLR-BK channel interaction may open new avenues for therapeutic intervention. Here, we review the studies documenting membrane CLR-BK channel interactions, dissecting out the many factors that determine the final BK current response to changes in membrane CLR content. We also summarize work in reductionist systems where recombinant BK protein is studied in artificial lipid bilayers, which documents a direct inhibition of BK channel activity by CLR and builds a strong case for a direct interaction between CLR and the BK channel-forming protein. Bilayer lipid-mediated mechanisms in CLR action are also discussed. Finally, we review studies of BK channel function during hypercholesterolemia, and underscore the many consequences that the CLR-BK channel interaction brings to cell physiology and human disease.

A statistical mechanical model of cholesterol/phospholipid mixtures: linking condensed complexes, superlattices, and the phase diagram

Despite extensive studies for nearly three decades, lateral distribution of molecules in cholesterol/phospholipid bilayers remains elusive. Here we present a statistical mechanical model of cholesterol/phospholipid mixtures that is able to rationalize almost every critical mole fraction (X(cr)) value previously reported for sterol superlattice formation as well as the observed biphasic changes in membrane properties at X(cr). This model is able to explain how cholesterol superlattices and cholesterol/phospholipid condensed complexes are interrelated. It gives a more detailed characterization of the LG(I)region (a broader region than the liquid disordered-liquid ordered mixed-phase region), which is considered to be a sludgelike mixture of fluid phase and aggregates of rigid clusters. A rigid cluster is formed by a cholesterol molecule and phospholipid molecules that are condensed to the cholesterol. Rigid clusters of similar size tend to form aggregates, in which cholesterol molecules are regularly distributed into superlattices. According to this model, the extent and type of sterol superlattices, thus the lateral distribution of the entire membrane, should vary with cholesterol mole fraction in a delicate, predictable, and nonmonotonic manner, which should have profound functional implications.

Acyl-chain mismatch driven superlattice arrangements in DPPC/DLPC/cholesterol bilayers

Fluorescence and infrared spectroscopy and cholesterol oxidase activity were employed to investigate the effect of phosphatidylcholine (PC) acyl chain length mismatch on the lateral organizations of lipids in liquid-ordered dipalmitoyl-PC/dilauroyl-PC/cholesterol (DPPC/DLPC/CHOL) bilayers. Plots of steady-state fluorescence emission anisotropy of diphenylhexatriene (DPH) labeled PC (DPH-PC) embedded in the DPPC/DLPC/CHOL bilayers revealed significant peaks at several DPPC mole fractions (Y(DPPC)) when the cholesterol mole fraction (X(CHOL)) was fixed to particular values. Analogously, the DPH-PC anisotropy peaked at several critical X(CHOL)'s when Y(DPPC) was fixed. Acyl chain C-H and C horizontal lineO vibrational peak frequencies of native PC as well as the activity of cholesterol oxidase also revealed dips and peaks at similar Y(DPPC)'s. Importantly, most of the observed peaks/dips coincide with the critical mole fractions predicted by the Superlattice (SL) model. A three-dimensional map of DPH-PC anisotropy versus composition in the range 0.32 <or= X(CHOL) <or= 0.50; 0.54 <or= Y(DPPC) <or= 0.72 revealed a prominent peak at (X(CHOL), Y(DPPC)) approximately (0.42, 0.64). This suggests a simultaneous presence of two different types of superlattices, one where cholesterol is the quest molecule in a PC host lattice and another where DPPC is the guest in the DLPC host lattice. Time-resolved measurements of DPH-PC fluorescence indicated the existence of an ordered, rotationally hindered environment of acyl chains at that "critical" composition consistent with the existence of SL arrangements. We propose that beside CHOL/PC superlattices, DPPC, and DLPC as well tend to adopt regular SL-like lateral distributions relative to each other, presumably because the less hydrophobic DLPC molecule is slightly displaced toward the aqueous phase, thus allowing more room and mobility for the head groups of both DPPC and DLPC as well as for the acyl chain tails of DPPC. The parallel presence of two kinds of superlattices, that is, CHOL/PC-SL and DPPC/DLPC-SL as demonstrated here, has intriguing implications regarding lipid homeostasis of eukaryote membranes.

Cholesterol-dependent nanomechanical stability of phase-segregated multicomponent lipid bilayers

Cholesterol is involved in endocytosis, exocytosis, and the assembly of sphingolipid/cholesterol-enriched domains, as has been demonstrated in both model membranes and living cells. In this work, we explored the influence of different cholesterol levels (5-40 mol%) on the morphology and nanomechanical stability of phase-segregated lipid bilayers consisting of dioleoylphosphatidylcholine/sphingomyelin/cholesterol (DOPC/SM/Chol) by means of atomic force microscopy (AFM) imaging and force mapping. Breakthrough forces were consistently higher in the SM/Chol-enriched liquid-ordered domains (Lo) than in the DOPC-enriched fluid-disordered phase (Ld) at a series of loading rates. We also report the activation energies (DeltaEa) for the formation of an AFM-tip-induced fracture, calculated by a model for the rupture of molecular thin films. The obtained DeltaEa values agree remarkably well with reported values for fusion-related processes using other techniques. Furthermore, we observed that within the Chol range studied, the lateral organization of bilayers can be categorized into three distinct groups. The results are rationalized by fracture nanomechanics of a ternary phospholipid/sphingolipid/cholesterol mixture using correlated AFM-based imaging and force mapping, which demonstrates the influence of a wide range of cholesterol content on the morphology and nanomechanical stability of model bilayers. This provides fundamental insights into the role of cholesterol in the formation and stability of sphingolipid/cholesterol-enriched domains, as well as in membrane fusion.

Cholesterol modulates the interaction of beta-amyloid peptide with lipid bilayers

The interaction of an amphiphilic, 40-amino acid beta-amyloid (Abeta) peptide with liposomal membranes as a function of sterol mole fraction (X(sterol)) was studied based on the fluorescence anisotropy of a site-specific membrane sterol probe, dehydroergosterol (DHE), and fluorescence resonance energy transfer (FRET) from the native Tyr-10 residue of Abeta to DHE. Without Abeta, peaks or kinks in the DHE anisotropy versus X(sterol) plot were detected at X(sterol) approximately 0.25, 0.33, and 0.53. Monomeric Abeta preserved these peaks/kinks, but oligomeric Abeta suppressed them and created a new DHE anisotropy peak at X(sterol) approximately 0.38. The above critical X(sterol) values coincide favorably with the superlattice compositions predicted by the cholesterol superlattice model, suggesting that membrane cholesterol tends to adopt a regular lateral arrangement, or domain formation, in the lipid bilayers. For FRET, a peak was also detected at X(sterol) approximately 0.38 for both monomeric and oligomeric Abeta, implying increased penetration of Abeta into the lipid bilayer at this sterol mole fraction. We conclude that the interaction of Abeta with membranes is affected by the lateral organization of cholesterol, and hypothesize that the formation of an oligomeric Abeta/cholesterol domain complex may be linked to the toxicity of Abeta in neuronal membranes.

High vapor pressure perfluorocarbons cause vesicle fusion and changes in membrane packing

Perfluorocarbons (PFCs) hold great promise for biomedical applications. However, relatively little is known about the impact of these chemicals on membranes. We used unilamellar vesicles to explore the effects of PFCs on membrane packing and vesicle stability. Four clinically relevant PFCs with varying vapor pressures (PP1, 294 mbar; PP2, 141 mbar; PP4, 9.6 mbar; and PP9, 2.9 mbar) were examined. Microscopy imaging and spectroscopic measurements suggest that PFCs, especially those with high vapor pressures, lead to vesicle fusion within hours. Upon exposure to PP1 and PP2 for 72 h, vesicles retained a spherical shape, but the size changed from approximately 200 nm to approximately 20-40 mum. In addition, membrane packing underwent marked changes during this timeframe. A significant decrease in water content in the lipid polar headgroup regions occurred during the first 1-2-h exposure to PFCs, followed by a steady increase in water content over time. Possible mechanisms were proposed to explain these dramatic structural changes. The finding that chemically inert PFCs exhibited fusogenic activity and marked changes in membrane surface packing is novel, and should be considered when using PFCs for biomedical applications.

Use of fluorescence to determine the effects of cholesterol on lipid behavior in sphingomyelin liposomes and erythrocyte membranes

The purpose of this study was to generate the equivalent of a cholesterol/temperature phase map for a biological membrane using fluorescence spectroscopy. The pseudo-phase map was created using human erythrocytes treated with various concentrations of methyl-beta-cyclodextrin to remove defined amounts of cholesterol and a trio of fluorescent probes that assess different membrane properties (laurdan, diphenylhexatriene, and merocyanine 540). Parallel experiments with two-photon microscopy suggested that changes in cellular cholesterol content affected the entire membrane rather than being localized to specific macroscopic domains. The various regions of the composite erythrocyte pseudo-phase map were interpreted using analogous data acquired from multilamellar vesicles that served as simplified models of cholesterol-dependent phases. The vesicles consisted of various concentrations of cholesterol (0 to 50 mol%) with either palmitoyl sphingomyelin, 1:1 dipalmitoylphosphatidylcholine and dioleoylphosphatidylcholine, or phospholipid mixtures intended to simulate either the inner or outer leaflet of erythrocyte membranes. Four distinguishable regions were observed in sphingomyelin phase maps corresponding to the traditional solid-ordered and liquid-disordered phases and two types of liquid-ordered behavior. Physical properties were less diverse in the mixed phospholipid vesicles, as expected, based on previous studies. Erythrocytes displayed five regions of different combinations of membrane properties along the phase map. Some of the observations identified similarities between the cells and liquid-ordered behavior observed in the various types of liposomes as well as some interesting differences.

Effect of cholesterol and ergosterol on the compressibility and volume fluctuations of phospholipid-sterol bilayers in the critical point region: a molecular acoustic and calorimetric study

Although sterol-phospholipid interactions have been of interest for many years now, a complete thermodynamic profile of these systems is still missing. To contribute to a better understanding of the thermodynamic functions of these systems, we determined isothermal compressibility coefficient data for dipalmitoylphosphocholine (DPPC) and DPPC-containing cholesterol and ergosterol vesicles by means of molecular acoustics (ultrasound velocimetry and densimetry) and differential scanning and pressure perturbation calorimetric techniques. A particular focus was on the influence of the differential structural properties of the two sterols on the thermodynamic properties of lipid bilayers, and on the nature of the critical point region of phospholipid-sterol systems by determining thermodynamic fluctuation parameters. Contrary to significant changes in conformational and dynamical properties of the DPPC-sterol membranes, no marked differences were found in the various thermodynamic properties studied, including the adiabatic (beta(S)(lipid)) and isothermal (beta(T)(lipid)) compressibility, as well as the volume fluctuations. Differences in beta(T)(lipid) and beta(S)(lipid) become dramatic in the gel-fluid transition region only, due to a significant degree of slow relaxational processes in the microsecond time range in the transition region. Our data show no evidence for the existence of a typical critical point phenomenon in the concentration and temperature range where a critical point in the DPPC-sterol phase diagram is expected to appear. Hence, on a macroscopic level, it seems more appropriate to describe the sterol-phospholipid binary mixtures in the liquid-ordered/liquid-disordered coexistence region as a phase region consisting essentially of small nanodomains only. Such small-domain dimensions, with a series of particular properties such as increased line energy, spontaneous curvature, and limited lifetime, seem also to be typical of raftlike domains in cell membranes.

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.