Juyang Huang

Maximum solubility of cholesterol in phosphatidylcholine and phosphatidylethanolamine bilayers

In any lipid bilayer membrane, there is an upper limit on the cholesterol concentration that can be accommodated within the bilayer structure; excess cholesterol will precipitate as crystals of pure cholesterol monohydrate. This cholesterol solubility limit is a well-defined quantity. It is a first-order phase boundary in the phospholipid/cholesterol phase diagram. There are many different solubility limits in the literature, but no clear picture has emerged that can unify the disparate results. We have studied the effects that different sample preparation methods can have on the apparent experimental solubility limit. We find that artifactual demixing of cholesterol can occur during conventional sample preparation and that this demixed cholesterol may produce artifactual cholesterol crystals. Therefore, phospholipid/cholesterol suspensions which are prepared by conventional methods may manifest variable, falsely low cholesterol solubility limits. We have developed two novel preparative methods which are specifically designed to prevent demixing during sample preparation. For detection of the cholesterol crystals, X-ray diffraction has proven to be quantitative and highly sensitive. Experiments based on these methods yield reproducible and precise cholesterol solubility limits: 66 mol% for phosphatidylcholine (PC) bilayers and 51 mol% for phosphatidylethanolamine (PE) bilayers. We present evidence that these are true, equilibrium values. In contrast to the dramatic headgroup effect (PC vs. PE), acyl chain variations had no effect on the cholesterol solubility limit in four different PC/cholesterol mixtures.

Assess the nature of cholesterol-lipid interactions through the chemical potential of cholesterol in phosphatidylcholine bilayers.

Cholesterol plays a vital role in determining the physiochemical properties of cell membranes. However, the detailed nature of cholesterol–lipid interactions is a subject of ongoing debate. Existing conceptual models, including the Condensed Complex Model, the Superlattice Model, and the Umbrella Model, identify different molecular mechanisms as the key to cholesterol–lipid interactions. In this work, the compositional dependence of the chemical potential of cholesterol in cholesterol/phosphatidylcholine mixtures was systematically measured at high resolution at 37°C by using an improved cholesterol oxidase (COD) activity assay. The chemical potential of cholesterol was found to be much higher in di18:1-PC bilayers than in di16:0-PC bilayers, indicating a more favorable interaction between cholesterol and saturated chains. More significantly, in 16:0,18:1-PC and di18:1-PC bilayers, the COD initial-reaction rate displays a series of distinct jumps near the cholesterol mole fractions (χC) of 0.15, 0.25, 0.40, 0.50, and 0.57 and a peak at the cholesterol maximum solubility limit of 0.67. These jumps have been identified as the thermodynamic signatures of stable cholesterol regular distributions. In contrast, no such jumps were evident in di16:0-PC bilayers below χC of 0.57. The observed chemical potential profile is in excellent agreement with previous Monte Carlo simulations based on the Umbrella Model but not with the predictions from the other models. The data further indicate that the cholesterol regular distribution domains (superlattices) are not the hypothesized condensed complexes. Those complexes were mainly implicated from studies on lipid monolayer that may not be relevant to the lipid bilayer in cell membranes.

A Molecular View of the Cholesterol Condensing Effect in DOPC Lipid Bilayers

The condensing effect of cholesterol in dioleoylphosphatidylcholine (DOPC) lipid bilayers was systematically investigated via atomistic molecular dynamics (MD) simulation. Fourteen independent 200 ns simulations, spanning the entire range of cholesterol mole fraction (x(c)) in DOPC bilayers (i.e., from x(c) = 0 to 0.66), were performed at 323 K. The molecular areas occupied by DOPC and cholesterol at different distances from the bilayer center were analyzed using a slicing method based on the VDW radii of atoms. Curiously, while the average area per lipid and the cholesterol tilt angle, with respect to the bilayer normal, both show monotonic decreases as x(c) increases, the average bilayer height shows a significant decrease for x(c) > 0.35, following an initial increase. The calculated partial-specific areas of lipids clearly show the condensing effect of cholesterol. The VDW areal analysis showed that the condensing effect is limited only to the cholesterol sterol ring region, where the acyl chains of DOPC are severely compressed by cholesterol. As x(c) increases, the headgroups of DOPC gradually expand along the bilayer-aqueous interface to occupy more lateral area. Thus, it confirmed a key prediction of the umbrella model. At high cholesterol mole fractions, the calculated area per DOPC and area per cholesterol using some existing methods showed an inconsistent result: both increase while the overall area per lipid decreases. The inconsistency stems from the problematic assumption that cholesterol and DOPC have cylindrical shape and the same height. Our results showed that the total area of a PC/cholesterol bilayer is primarily determined by the molecular packing in the cholesterol sterol ring region. An alternative analysis of area per molecule within this region is proposed, which takes into account the cholesterol tilt angle and the practical incompressibility of cholesterol sterol rings. The new calculation shows that the majority of the area lost due to the cholesterol condensing effect is taken from PC molecules.

Lateral Distribution of Cholesterol in Dioleoylphosphatidylcholine Lipid Bilayers: Cholesterol-Phospholipid Interactions at High Cholesterol Limit

Lateral organization of cholesterol in dioleoyl-phosphatidylcholine (DOPC) lipid bilayers at high cholesterol concentration (>45 mol%) was investigated using steady-state fluorescence anisotropy and fluorescent resonance energy transfer techniques. The recently devised Low Temperature Trap method was used to prepare compositionally uniform cholesterol/DOPC liposomes to avoid the problem of lipid demixing. The fluorescence anisotropy of diphenylhexatrience chain-labeled phosphatidylcholine (DPH-PC) in these liposomes exhibited local maxima at cholesterol mol fractions of 0.50 and 0.57, and a sharp drop at 0.67. For the liposomes labeled with both dehydroergosterol and DPH-PC, the fluorescent resonance energy transfer efficiency from dehydroergosterol to DPH-PC displayed a steep jump at cholesterol mol fraction of 0.5, and dips at 0.57 and 0.68. These results indicate the presence of highly ordered cholesterol regular distribution domains at those observed critical compositions. The observed critical mol fraction at 0.67 agreed favorably with the solubility limit of cholesterol in DOPC bilayers as independently measured by light scattering and optical microscopy. The regular distribution at 0.57 was previously predicted from a Monte Carlo simulation based on the Umbrella model. The results strongly support the hypothesis that the primary requirement for cholesterol-phospholipid mixing is that the polar phospholipid headgroups need to cover the nonpolar body of cholesterol to avoid the exposure of cholesterol to water.

Exploration of Molecular Interactions in Cholesterol Superlattices: Effect of Multibody Interactions

Experimental evidences have indicated that cholesterol may adapt highly regular lateral distributions (i.e., superlattices) in a phospholipid bilayer. We investigated the formations of superlattices at cholesterol mole fraction of 0.154, 0.25, 0.40, and 0.5 using Monte Carlo simulation. We found that in general, conventional pairwise-additive interactions cannot produce superlattices. Instead, a multibody (nonpairwise) interaction is required. Cholesterol superlattice formation reveals that although the overall interaction between cholesterol and phospholipids is favorable, it contains two large opposing components: an interaction favoring cholesterol-phospholipid mixing and an unfavorable acyl chain multibody interaction that increases nonlinearly with the number of cholesterol contacts. The magnitudes of interactions are in the order of kT. The physical origins of these interactions can be explained by our umbrella model. They most likely come from the requirement for polar phospholipid headgroups to cover the nonpolar cholesterol to avoid the exposure of cholesterol to water and from the sharp decreasing of acyl chain conformation entropy due to cholesterol contact. This study together with our previous work demonstrate that the driving force of cholesterol-phospholipid mixing is a hydrophobic interaction, and multibody interactions dominate others over a wide range of cholesterol concentration.

Monte Carlo simulation of lipid mixtures: finding phase separation

The nonideal mixing of phosphatidylserine (PS) and phosphatidylcholine (PC) binary lipid mixtures was studied by computer simulation based on a model wherein the excess energy of mixing is divided between an electrostatic term and one adjustable term delta Em that includes all other nonideal interactions. The lateral distribution of the lipids and the energy of the mixtures were obtained by using Kawasaki relaxation in a canonical ensemble. The Gibbs free energies were calculated by Kirkwoods coupling parameter method. The simulation results are strongly dependent on simulation size for sizes smaller than about 1000 lipids. Nonideal interaction between lipids can result in large scale separation of lipid phases of different composition at reasonable delta Em values as well as clustering of like lipids. In plots of total Gibbs free energy of mixing versus PS mole fraction in PS/PC, the boundaries of the two phase region could be accurately determined. The electrostatic interaction influences cluster size and shape, and also the composition of phases in the two-phase region.

Cholesterol Modulates the Interaction of β-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.

Preparation of giant unilamellar vesicles from damp lipid film for better lipid compositional uniformity.

Giant unilamellar vesicles (GUVs) containing cholesterol often have a wide distribution in lipid composition. In this study, GUVs of 1,2-dioleoyl-sn-glycero-3-phosphocholine(DOPC)/1,2-distearoyl-sn-glycero-3-phosphocholine(DSPC)/cholesterol and 1,2-diphytanoyl-sn-glycero-3-phosphocholine(diPhyPC)/1,2-dipalmitoyl-sn-glycero-3-phosphocholine(DPPC)/cholesterol were prepared from dry lipid films using the standard electroformation method as well as a modified method from damp lipid films, which are made from compositional uniform liposomes prepared using the Rapid Solvent Exchange (RSE) method. We quantified the lipid compositional distributions of GUV by measuring the miscibility transition temperature of GUVs using fluorescence microscopy, since a narrower distribution in the transition temperature should correspond to a more uniform distribution in GUV lipid composition. Cholesterol molecules can demix from other lipids in dry state and form cholesterol crystals. Using optical microscopy, micron-sized crystals were observed in some dry lipid films. Thus, a major cause of GUV lipid compositional heterogeneity is the demixing of lipids in the dry film state. By avoiding the dry film state, GUVs prepared from damp lipid films have a better uniformity in lipid composition, and the standard deviations of miscibility transition temperature are about 2.5 times smaller than that of GUVs prepared from dry lipid films. Comparing the two ternary systems, diPhyPC/DPPC/cholesterol GUVs has a larger cholesterol compositional heterogeneity, which directly correlates with the low maximum solubility of cholesterol in diPhyPC lipid bilayers (40.2±0.5mol%) measured by light scattering. Our data indicate that cholesterol interacts far less favorably with diPhyPC than it does with other PCs. The damp lipid film method also has a potential of preparing GUVs from cell membranes containing native proteins without going through a dry state.

Molecular Dynamics Simulation of Diacylglycerols in Phosphatidycholine Lipid Bilayers

In this study, atomistic MD simulations were performed to investigate the interactions between diacylglycerols (i.e. DPG, POG, or DOG) and phosphatidylcholine (i.e. POPC or DOPC) bilayers. Our results show that diacylglycerols (DAG) increase acyl chain order, headgroup spacing and bilayer thickness, and reduce area-per-lipid. In a lipid bilayer, in order to avoid the unfavorable exposure of DAG hydrophobic parts to water, neighboring phospholipid (PC) headgroups move toward DAG to provide cover. This interaction between DAG and phospholipid is explained by the Umbrella Model. Comparing the three types of DAG in POPC and DOPC bilayers, DOG is located closer to the bilayer/aqueous interfaces than DPG and POG and it requires more coverage according to our umbrella index calculation, likely due to its longer and unsaturated chains. The potentials of mean force were calculated for POPC in its bilayers containing DPG, POG, or DOG. Our results show that POPC flip-flop induces pores in pure PC bilayer, while adding diacylglycerol prevents the pore formation during flip-flop. System that contains POG has a higher free energy of desorption and lower free energy barriers for flip-flop, compared with systems that contain DOG or DPG. This indicates that POG is more favorable in POPC bilayer than the other DAGs, since its acyl chains are similar to POPC.

Not All Hybrid Lipids are Linactants

Hybrid lipids are thought to be able to perform the function of linactants at membrane domain interfaces: They can reduce line tension and stabilize nanoscopic lipid raft domains in biomembranes. Hybrid lipids are lipids with one saturated chain and one unsaturated chain. Here we provide evidences that only certain hybrid lipids behave like linactants. In this study, we compared three hybrid lipids (i.e., 16:0-181PC (POPC), 16:0-18:2PC, and 16:0-22:4PC) in their abilities to reduce lipid domain size and shift phase boundary. The Lo-Ld phase boundaries of hybrid-lipid/di18:0PC(DSPC)/cholesterol systems were determined from giant unilamellar vesicles (GUV) using fluorescence microscopy. We found that 16:0-22:4PC behaves similarly to a fluid-phase lipid: The Lo and Ld lipid domains in 16:0-22:4PC/DSPC/CHOL mixtures are macroscopic and the phase coexisting region is very wide. On the other hand, 16:0-18:2PC/DSPC/CHOL system has a much narrower Lo-Ld phase coexisting region; however, the lipid domains are still macroscopic. Only POPC/DSPC/CHOL system contains nanoscopic lipid domains. These results were compared with Monte Carlo simulations. Based on the magnitudes of interaction energies, it appears that only mono-unsaturated hybrid lipids behave like linactants, while poly-unsaturated hybrid lipids behave more or less like fluid-phase lipids.