Novel properties of cholesterol–dioleoylphosphatidylcholine mixtures

Several common methods of making vesicles (except an emulsion method) capture intended lipid ratios

Researchers choose different methods of making giant unilamellar vesicles in order to satisfy different constraints of their experimental designs. A challenge of using a variety of methods is that each may produce vesicles of different lipid compositions, even if all vesicles are made from a common stock mixture. Here, we use mass spectrometry to investigate ratios of lipids in vesicles made by five common methods: electroformation on indium tin oxide slides, electroformation on platinum wires, gentle hydration, emulsion transfer, and extrusion. We made vesicles from either 5-component or binary mixtures of lipids chosen to span a wide range of physical properties: di(18:1)PC, di(16:0)PC, di(18:1)PG, di(12:0)PE, and cholesterol. For a mixture of all five of these lipids, ITO electroformation, Pt electroformation, gentle hydration, and extrusion methods result in only minor shifts (≤ 5 mol%) in lipid ratios of vesicles relative to a common stock solution. In contrast, emulsion transfer results in ∼80% less cholesterol than expected from the stock solution, which is counterbalanced by a surprising overabundance of saturated PC-lipid relative to all other phospholipids. Experiments using binary mixtures of some of the lipids largely support results from the 5-component mixture. Exact values of lipid ratios variations likely depend on the details of each method, so a broader conclusion is that experiments that increment lipid ratios in small steps will be highly sensitive to the method of lipid formation and to sample-to-sample variations, which are low (roughly ±2 mol% in the 5-component mixture and either scale proportionally with increasing mole fraction or remain low). Experiments that increment lipid ratios in larger steps or that seek to explain general trends or new phenomena will be less sensitive to the method used.

SIGNIFICANCE STATEMENT

Small changes to the amounts and types of lipids in membranes can drastically affect the membrane's behavior. Unfortunately, it is unknown whether (or to what extent) different methods of making vesicles alter the ratios of lipids in membranes, even when identical stock solutions are used. This presents challenges for researchers when comparing data with colleagues who use different methods. Here, we measure ratios of lipid types in vesicle membranes produced by five methods. We assess each method's reproducibility and compare resulting vesicle compositions across methods. In doing so, we provide a quantitative basis that the scientific community can use to estimate whether differences between their results can be simply attributed to differences between methods or to sample-to-sample variations.

Anti- c-myc cholesterol based lipoplexes as onco-nanotherapeutic agents in vitro

Background: Strategies aimed at inhibiting the expression of the c-myc oncogene could provide the basis for alternative cancer treatment. In this regard, silencing c-myc expression using small interfering RNA (siRNA) is an attractive option. However, the development of a clinically viable, siRNA-based, c-myc silencing system is largely dependent upon the design of an appropriate siRNA carrier that can be easily prepared. Nanostructures formed by the electrostatic association of siRNA and cationic lipid vesicles represent uncomplicated siRNA delivery systems. Methods: This study has focused on cationic liposomes prepared with equimolar quantities of the cytofectin, N,N-dimethylaminopropylamido-succinylcholesteryl-formylhydrazide (MS09), and cholesterol (Chol) for the development of a simple, but effective anti- c-myc onco-nanotherapeutic agent. Liposomes formulated with dioleoylphosphatidylethanolamine (DOPE) in place of Chol as the co-lipid were included for comparative purposes. Results: Liposomes successfully bound siRNA forming lipoplexes of less than 150 nm in size, which assumed bilamellar aggregrates. The liposome formulations were well tolerated in the human breast adenocarcinoma (MCF-7) and colon carcinoma (HT-29) cells, which overexpress c-myc. Lipoplexes directed against the c-myc transcript mediated a dramatic reduction in c-myc mRNA and protein levels. Moreover, oncogene knockdown and anti-cancer effects were superior to that of Lipofectamine™ 3000. Conclusion: This anti- c-myc MS09:Chol lipoplex exemplifies a simple anticancer agent with enhanced c-myc gene silencing potential in vitro.

Formation of cholesterol Bilayer Domains Precedes Formation of Cholesterol Crystals in Membranes Made of the Major Phospholipids of Human Eye Lens Fiber Cell Plasma Membranes

Purpose/Aim: The goal of this study is to reveal how age-related changes in phospholipid (PL) composition in the fiber cell plasma membranes of the human eye lens affect the cholesterol (Chol) content at which Chol bilayer domains (CBDs) and Chol crystals start to form.Materials and Methods: Saturation-recovery electron paramagnetic resonance with spin-labeled cholesterol analogs and differential scanning calorimetry were used to determine the Chol contents at which CBDs and cholesterol crystals, respectively, start to form in in membranes made of the major PL constituents of the plasma membrane of the human eye lens fiber cells. To preserve compositional homogeneity throughout the membrane suspension, the lipid multilamellar dispersions investigated in this work were prepared using a rapid solvent exchange method. The cholesterol content changed from 0 to 75 mol%.Results: The saturation recovery electron paramagnetic resonance results show that CBDs start to form at 33, 50, 46, and 48 mol% Chol in the phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and sphingomyelin bilayers, respectively. The differential scanning calorimetry results show that Chol crystals start to form at 50, 66, 70, and 66 mol% Chol in the phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, and sphingomyelin bilayers, respectively.Conclusions: These results, as well those of our previous studies, indicate that the formation of CBDs precedes the formation of Chol crystals in all of the studied systems, and the appearance of each depends on the type of PL forming the bilayer. These findings contribute to a better understanding of the molecular mechanisms involved in the regulation of Chol-dependent processes in eye lens fiber cell membranes.

Lipid Driven Nanodomains in Giant Lipid Vesicles are Fluid and Disordered

It is a fundamental question in cell biology and biophysics whether sphingomyelin (SM)- and cholesterol (Chol)- driven nanodomains exist in living cells and in model membranes. Biophysical studies on model membranes revealed SM and Chol driven micrometer-sized liquid-ordered domains. Although the existence of such microdomains has not been proven for the plasma membrane, such lipid mixtures have been often used as a model system for ‘rafts’. On the other hand, recent super resolution and single molecule results indicate that the plasma membrane might organize into nanocompartments. However, due to the limited resolution of those techniques their unambiguous characterization is still missing. In this work, a novel combination of Förster resonance energy transfer and Monte Carlo simulations (MC-FRET) identifies directly 10 nm large nanodomains in liquid-disordered model membranes composed of lipid mixtures containing SM and Chol. Combining MC-FRET with solid-state wide-line and high resolution magic angle spinning NMR as well as with fluorescence correlation spectroscopy we demonstrate that these nanodomains containing hundreds of lipid molecules are fluid and disordered. In terms of their size, fluidity, order and lifetime these nanodomains may represent a relevant model system for cellular membranes and are closely related to nanocompartments suggested to exist in cellular membranes.

Probing Microscopic Orientation in Membranes by Linear Dichroism

The cell membrane is an ordered environment, which anisotropically affects the structure and interactions of all of its molecules. Monitoring membrane orientation at a local level is rather challenging but could reward crucial information on protein conformation and interactions in the lipid bilayer. We monitored local lipid ordering changes upon varying the cholesterol concentration using polarized light spectroscopy and pyrene as a membrane probe. Pyrene, with a shape intermediate between a disc and a rod, can detect microscopic orientation variations at the level of its size. The global membrane orientation was determined using curcumin, a probe with nonoverlapping absorption relative to that of pyrene. While the macroscopic orientation of a liquid-phase bilayer decreases with increasing cholesterol concentration, the local orientation is improved. Pyrene is found to be sensitive to the local effects induced by cholesterol and temperature on the bilayer. Disentangling local and global orientation effects in membranes could provide new insights into functionally significant interactions of membrane proteins.

Thermally-induced aggregation and fusion of protein-free lipid vesicles

Membrane fusion is an important phenomenon in cell biology and pathology. This phenomenon can be modeled using vesicles of defined size and lipid composition. Up to now fusion models typically required the use of chemical (polyethyleneglycol, cations) or enzymatic catalysts (phospholipases). We present here a model of lipid vesicle fusion induced by heat. Large unilamellar vesicles consisting of a phospholipid (dioleoylphosphatidylcholine), cholesterol and diacylglycerol in a 43:57:3 mol ratio were employed. In this simple system, fusion was the result of thermal fluctuations, above 60 °C. A similar system containing phospholipid and cholesterol but no diacylglycerol was observed to aggregate at and above 60 °C, in the absence of fusion. Vesicle fusion occurred under our experimental conditions only when (31)P NMR and cryo-transmission electron microscopy of the lipid mixtures used in vesicle preparation showed non-lamellar lipid phase formation (hexagonal and cubic). Non-lamellar structures are probably the result of lipid reassembly of the products of individual fusion events, or of fusion intermediates. A temperature-triggered mechanism of lipid reassembly might have occurred at various stages of protocellular evolution.

Formation of cholesterol bilayer domains precedes formation of cholesterol crystals in cholesterol/dimyristoylphosphatidylcholine membranes: EPR and DSC studies

Saturation-recovery EPR along with DSC were used to determine the cholesterol content at which pure cholesterol bilayer domains (CBDs) and cholesterol crystals begin to form in dimyristoylphosphatidylcholine (DMPC) membranes. To preserve compositional homogeneity throughout the membrane suspension, lipid multilamellar dispersions were prepared using a rapid solvent exchange method. The cholesterol content increased from 0 to 75 mol %. With spin-labeled cholesterol analogues, it was shown that the CBDs begin to form at ~50 mol % cholesterol. It was confirmed by DSC that the cholesterol solubility threshold for DMPC membranes is detected at ~66 mol % cholesterol. At levels above this cholesterol content, monohydrate cholesterol crystals start to form. The major finding is that the formation of CBDs precedes formation of cholesterol crystals. The region of the phase diagram for cholesterol contents between 50 and 66 mol % is described as a structured one-phase region in which CBDs have to be supported by the surrounding DMPC bilayer saturated with cholesterol. Thus, the phase boundary located at 66 mol % cholesterol separates the structured one-phase region (liquid-ordered phase of DMPC with CBDs) from the two-phase region where the structured liquid-ordered phase of DMPC coexists with cholesterol crystals. It is likely that CBDs are precursors of monohydrate cholesterol crystals.

Ligands located within a cholesterol domain enhance gene delivery to the target tissue

Targeted gene delivery provides enormous potential for clinical treatment of many incurable diseases. Liposomes formulated with targeting ligands have been tested extensively both in vitro and in vivo, and many studies have strived to identify more efficacious ligands. However, the environment of the ligand within the delivery vehicle is generally not considered, and this study assesses the effect of ligand microenvironment by utilizing a lipoplex possessing a cholesterol domain. Our recent work has shown that the presence of the targeting ligand within the cholesterol domain promotes more productive transfection in cultured cells. In the present study, lipoplexes having the identical lipid composition were formulated with different conjugates of the folate ligand such that the ligand was included in, or excluded from, the cholesterol domain. The effect of locating the ligand within the cholesterol domain was then tested in a xenograft tumor model in mice. Lipoplexes that included the ligand within the cholesterol domain showed significantly higher luciferase expression and plasmid accumulation in tumors as compared to lipoplexes in which the ligand was excluded from the domain. These results demonstrate that the microenvironment of the ligand can affect gene delivery to tumors, and show that ligand-mediated delivery can be enhanced by locating targeting ligands within a cholesterol domain.

NMR determination of protein partitioning into membrane domains with different curvatures and application to the influenza M2 peptide

The M2 protein of the influenza A virus acts both as a drug-sensitive proton channel and mediates virus budding through membrane scission. The segment responsible for causing membrane curvature is an amphipathic helix in the cytoplasmic domain of the protein. Here, we use (31)P and (13)C solid-state NMR to examine M2-induced membrane curvature. M2(22-46), which includes only the transmembrane (TM) helix, and M2(21-61), which contains an additional amphipathic helix, are studied. (31)P chemical shift lineshapes indicate that M2(21-61) causes a high-curvature isotropic phase to both cholesterol-rich virus-mimetic membranes and 1,2-dimyristoyl-sn-glycero-3-phosphocholine bilayers, whereas M2(22-46) has minimal effect. The lamellar and isotropic domains have distinct (31)P isotropic chemical shifts, indicating perturbation of the lipid headgroup conformation by the amphipathic helix. (31)P- and (13)C-detected (1)H T(2) relaxation and two-dimensional peptide-lipid correlation spectra show that M2(21-61) preferentially binds to the high-curvature domain. (31)P linewidths indicate that the isotropic vesicles induced by M2(21-61) are 10-35 nm in diameter, and the virus-mimetic vesicles are smaller than the 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles. A strong correlation is found between high membrane curvature and weak drug-binding ability of the TM helix. Thus, the M2 amphipathic helix causes membrane curvature, which in turn perturbs the TM helix conformation, abolishing drug binding. These NMR experiments are applicable to other curvature-inducing membrane proteins such as fusion proteins and antimicrobial peptides.

Using spin-label electron paramagnetic resonance (EPR) to discriminate and characterize the cholesterol bilayer domain

Electron paramagnetic resonance (EPR) spin-labeling methods make it possible not only to discriminate the cholesterol bilayer domain (CBD) but also to obtain information about the organization and dynamics of cholesterol molecules in the CBD. The abilities of spin-label EPR were demonstrated for Chol/POPC (cholesterol/1-palmitoyl-2-oleoylphosphatidylcholine) membranes, with a Chol/POPC mixing ratio that changed from 0 to 3. Using the saturation-recovery (SR) EPR approach with cholesterol analogue spin labels, ASL and CSL, and oxygen or NiEDDA relaxation agents, it was confirmed that the CBD was present in all membrane suspensions when the mixing ratio exceeded the cholesterol solubility threshold (CST). Conventional EPR spectra of ASL and CSL in the CBD were similar to those in the surrounding POPC bilayer (which is saturated with cholesterol), indicating that in both domains, cholesterol exists in the lipid-bilayer-like structures. The behavior of ASL and CSL (and, thus, the behavior of cholesterol molecules) in the CBD was compared with that in the surrounding POPC-cholesterol domain (PCD). In the CBD, ASL and CSL molecules are better ordered than in the surrounding PCD. This difference is small and can be compared to that induced in the surrounding domain by an ~10°C decrease in temperature. Thus, cholesterol molecules are unexpectedly dynamic in the CBD, which should enhance their interaction with the surrounding domain. The polarity of the water/membrane interface of the CBD is significantly greater than that of the surrounding PCD, which significantly enhances penetration of the water-soluble relaxation agent, NiEDDA, into that region. Hydrophobicity measured in the centers of both domains is similar. The oxygen transport parameter (oxygen diffusion-concentration product) measured in the center of the CBD is about ten times smaller than that measured in the center of the surrounding domain. Thus, the CBD can form a significant barrier to oxygen transport. The results presented here point out similarities between the organization and dynamics of cholesterol molecules in the CBD and in the surrounding PCD, as well as significant differences between CBDs and cholesterol crystals.

Nucleic acid-lipid membrane interactions studied by DSC

The interactions of nucleic acids with lipid membranes are of great importance for biological mechanisms as well as for biotechnological applications in gene delivery and drug carriers. The optimization of liposomal vectors for clinical use is absolutely dependent upon the formation mechanisms, the morphology, and the molecular organization of the lipoplexes, that is, the complexes of lipid membranes with DNA. Differential scanning calorimetry (DSC) has emerged as an efficient and relatively easy-to-operate experimental technique that can straightforwardly provide data related to the thermodynamics and the kinetics of the DNA-lipid complexation and especially to the lipid organization and phase transitions within the membrane. In this review, we summarize DSC studies considering nucleic acid-membrane systems, accentuating DSC capabilities, and data analysis. Published work involving cationic, anionic, and zwitterionic lipids as well as lipid mixtures interacting with RNA and DNA of different sizes and conformations are included. It is shown that despite limitations, issues such as DNA- or RNA-induced phase separation and microdomain lipid segregation, liposomal aggregation and fusion, alterations of the lipid long-range molecular order, as well as membrane-induced structural changes of the nucleic acids can be efficiently treated by systematic high-sensitivity DSC studies.

Differential scanning calorimetry: An invaluable tool for a detailed thermodynamic characterization of macromolecules and their interactions

Differential Scanning Calorimetry (DSC) is a highly sensitive technique to study the thermotropic properties of many different biological macromolecules and extracts. Since its early development, DSC has been applied to the pharmaceutical field with excipient studies and DNA drugs. In recent times, more attention has been applied to lipid-based drug delivery systems and drug interactions with biomimetic membranes. Highly reproducible phase transitions have been used to determine values, such as, the type of binding interaction, purity, stability, and release from a drug delivery mechanism. This review focuses on the use of DSC for biochemical and pharmaceutical applications.

The immiscible cholesterol bilayer domain exists as an integral part of phospholipid bilayer membranes

Electron paramagnetic resonance (EPR) spin-labeling methods were used to study the organization of cholesterol and phospholipids in membranes formed from Chol/POPS (cholesterol/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine) mixtures, with mixing ratios from 0 to 3. It was confirmed using the discrimination by oxygen transport and polar relaxation agent accessibility methods that the immiscible cholesterol bilayer domain (CBD) was present in all of the suspensions when the mixing ratio exceeded the cholesterol solubility threshold (CST) in the POPS membrane. The behavior of phospholipid molecules was monitored with phospholipid analogue spin labels (n-PCs), and the behavior of cholesterol was monitored with the cholesterol analogue spin labels CSL and ASL. Results indicated that phospholipid and cholesterol mixtures can form a membrane suspension up to a mixing ratio of ~2. Additionally, EPR spectra for n-PC, ASL, and CSL indicated that both phospholipids and cholesterol exist in these suspensions in the lipid-bilayer-like structures. EPR spectral characteristics of n-PCs (spin labels located in the phospholipid cholesterol bilayer, outside the CBD) change with increase in the cholesterol content up to and beyond the CST. These results present strong evidence that the CBD forms an integral part of the phospholipid bilayer when formed from a Chol/POPS mixture up to a mixing ratio of ~2. Interestingly, CSL in cholesterol alone (without phospholipids) when suspended in buffer does not detect formation of bilayer-like structures. A broad, single-line EPR signal is given, similar to that obtained for the dry film of cholesterol before addition of the buffer. This broad, single-line signal is also observed in suspensions formed for Chol/POPS mixtures (as a background signal) when the Chol/POPS ratio is much greater than 3. It is suggested that the EPR spin-labeling approach can discriminate and characterize the fraction of cholesterol that forms the CBD within the phospholipid bilayer.

Solubility Limits of Cholesterol, Lanosterol, Ergosterol, Stigmasterol, and β-Sitosterol in Electroformed Lipid Vesicles

Here we use nuclear magnetic resonance to measure the solubility limit of several biologically relevant sterols in electroformed giant unilamellar vesicle membranes containing phosphatidylcholine (PC) lipids in ratios of 1:1:X DOPC:DPPC:sterol. We find solubility limits of cholesterol, lanosterol, ergosterol, stigmasterol, and β-sitosterol to be 65-70%, ~35%, 30-35%, 20-25%, and ~40%, respectively. The low solubilities of stigmasterol and β-sitosterol, which differ from cholesterol only in their alkyl tails, show that subtle differences in tail structure can strongly affect sterol solubility. Below the solubility limits, the fraction of sterol to PC-lipid in electroformed vesicles linearly reflects the fraction in the original stock solutions used in the electroformation process.

Cationic liposome-nucleic acid complexes for gene delivery and silencing: pathways and mechanisms for plasmid DNA and siRNA

Motivated by the promises of gene therapy, there is great interest in developing non-viral lipid-based vectors for therapeutic applications due to their low immunogenicity, low toxicity, ease of production, and the potential of transferring large pieces of DNA into cells. In fact, cationic liposome (CL) based vectors are among the prevalent synthetic carriers of nucleic acids (NAs) currently used in gene therapy clinical trials worldwide. These vectors are studied both for gene delivery with CL-DNA complexes and gene silencing with CL-siRNA (short interfering RNA) complexes. However, their transfection efficiencies and silencing efficiencies remain low compared to those of engineered viral vectors. This reflects the currently poor understanding of transfection-related mechanisms at the molecular and self-assembled levels, including a lack of knowledge about interactions between membranes and double stranded NAs and between CL-NA complexes and cellular components. In this review we describe our recent efforts to improve the mechanistic understanding of transfection by CL-NA complexes, which will help to design optimal lipid-based carriers of DNA and siRNA for therapeutic gene delivery and gene silencing.

Physical properties of the lipid bilayer membrane made of cortical and nuclear bovine lens lipids: EPR spin-labeling studies

The physical properties of membranes derived from the total lipids extracted from the lens cortex and nucleus of a 2-year-old cow were investigated using EPR spin-labeling methods. Conventional EPR spectra and saturation-recovery curves show that spin labels detect a single homogenous environment in membranes made from cortical lipids. Properties of these membranes are very similar to those reported by us for membranes made of the total lipid extract of 6-month-old calf lenses (J. Widomska, M. Raguz, J. Dillon, E. R. Gaillard, W. K. Subczynski, Biochim. Biophys. Acta 1768 (2007) 1454-1465). However, in membranes made from nuclear lipids, two domains were detected by the EPR discrimination by oxygen transport method using the cholesterol analogue spin label and were assigned to the bulk phospholipid-cholesterol domain (PCD) and the immiscible cholesterol crystalline domain (CCD), respectively. Profiles of the order parameter, hydrophobicity, and the oxygen transport parameter are practically identical in the bulk PCD when measured for either the cortical or nuclear lipid membranes. In both membranes, lipids in the bulk PCD are strongly immobilized at all depths. Hydrophobicity and oxygen transport parameter profiles have a rectangular shape with an abrupt change between the C9 and C10 positions, which is approximately where the steroid ring structure of cholesterol reaches into the membrane. The permeability coefficient for oxygen, estimated at 35 degrees C, across the bulk PCD in both membranes is slightly lower than across the water layer of the same thickness. However, the evaluated upper limit of the permeability coefficient for oxygen across the CCD (34.4 cm/s) is significantly lower than across the water layer of the same thickness (85.9 cm/s), indicating that the CCD can significantly reduce oxygen transport in the lens nucleus.

Membrane fusion intermediates and the effect of cholesterol: an in-house X-ray scattering study

We have developed an X-ray scattering setup which allows to study membrane fusion intermediates or other nonlamellar lipid mesophases by laboratory-scale X-ray sources alone, thus taking advantage of unrestricted beamtime compared to synchrotron sources. We report results of a study of pure lipid bilayers and phospholipid/cholesterol binary mixtures. Stalks, putative intermediate structures occurring during the membrane fusion process, can clearly be identified from reconstructed electron density maps. Phase diagrams of the lyotropic phase behavior of DOPC/cholesterol and DPhPC/cholesterol samples are presented. If cholesterol is present in moderate concentrations, it can substantially promote the formation of stalks at higher degree of hydration. In addition, a possibly new phase in DOPC/cholesterol is found at high cholesterol content in the low humidity range.

The role of cholesterol and structurally related molecules in enhancing transfection of cationic liposome-DNA complexes

Motivated by its important role in gene delivery, we have studied the effect of cholesterol and analogs on the transfection efficiency (TE) of lamellar cationic liposome-DNA (CL-DNA) complexes in vitro. Addition of cholesterol to low-transfecting DOTAP/DOPC-DNA complexes increases TE, with 15 mol % cholesterol already yielding 10-fold improvement. Steroids lacking the alkyl tail only modestly enhance TE, while molecules retaining it strongly enhance TE. All steroid-containing CL-DNA complexes exhibit the lamellar structure. The increase in experimentally determined membrane charge density (a universal parameter governing the TE of lamellar CL-DNA complexes) with cholesterol content alone cannot account for the rapid increase of TE. Instead, the reduction of the hydration repulsion layer of the membrane, caused by replacement of DOPC by cholesterol, promotes fusion between cationic membranes of CL-DNA complexes and anionic endosomal membranes, thus facilitating release of complexes and enhancing TE.

Large changes in the CRAC segment of gp41 of HIV do not destroy fusion activity if the segment interacts with cholesterol

The membrane-proximal external region (MPER) of the gp41 fusion protein of HIV is highly conserved among isolates of this virus and is considered a target for vaccine development. This region also appears to play a role in membrane fusion as well as localization of the virus to cholesterol-rich domains in membranes. The carboxyl terminus of MPER has the sequence LWYIK and appears to have an important role in cholesterol interactions. We have tested how amino acid substitutions that would affect the conformational flexibility of this segment could alter its interaction with cholesterol. We studied a family of peptides (all peptides as N-acetyl-peptide amides) with P, G, or A substituting for W and I of the LWYIK sequence. The peptide having the greatest effect on cholesterol distribution in membranes was the most flexible one, LGYGK. The corresponding mutation in gp41 resulted in a protein retaining 72% of the fusion activity of the wild-type protein. Two other peptides were synthesized, also containing two Gly residues, GWGIK and LWGIG, and did not have the ability to sequester cholesterol as efficiently as LGYGK did. Making the corresponding mutants of gp41 showed that these other two double Gly substitutions resulted in proteins that were much less fusogenic, although they were equally well expressed at the cell surface. The study demonstrates that drastic changes can be made in the LWYIK segment with the retention of a significant fraction of the fusogenic activity, as long as the mutant proteins interact with cholesterol.

Characterization of lipid domains in reconstituted porcine lens membranes using EPR spin-labeling approaches

The physical properties of membranes derived from the total lipid extract of porcine lenses before and after the addition of cholesterol were investigated using EPR spin-labeling methods. Conventional EPR spectra and saturation-recovery curves indicate that the spin labels detect a single homogenous environment in membranes before the addition of cholesterol. After the addition of cholesterol (when cholesterol-to-phospholipid mole to mole ratio of 1.55-1.80 was achieved), two domains were detected by the discrimination by oxygen transport method using a cholesterol analogue spin label. The domains were assigned to a bulk phospholipid-cholesterol bilayer made of the total lipid mixture and to a cholesterol crystalline domain. Because the phospholipid analogue spin labels cannot partition into the pure cholesterol crystalline domain, they monitor properties of the phospholipid-cholesterol domain outside the pure cholesterol crystalline domain. Profiles of the order parameter, hydrophobicity, and oxygen transport parameter are identical within experimental error in this domain when measured in the absence and presence of a cholesterol crystalline domain. This indicates that both domains, the phospholipid-cholesterol bilayer and the pure cholesterol crystalline domain, can be treated as independent, weakly interacting membrane regions. The upper limit of the oxygen permeability coefficient across the cholesterol crystalline domain at 35 degrees C had a calculated value of 42.5 cm/s, indicating that the cholesterol crystalline domain can significantly reduce oxygen transport to the lens center. This work was undertaken to better elucidate the major factors that determine membrane resistance to oxygen transport across the lens lipid membrane, with special attention paid to the cholesterol crystalline domain.

Insights into morphological nature of precipitation of cholesterol

Additional effects on the previously reported procedure of precipitation of narrowly dispersed and well-defined, brick-shaped cholesterol particles, including non-solvent addition rate, temperature, solvent purity, aging treatments, ultrasound agitation and fine mechanical effects were investigated. Based on the presented results, significant morphological sensitivity of cholesterol precipitation processes upon variations from the standard established procedure of crystallization is induced. However, the tendency of cholesterol to crystallize in the form of biaxially grown particles was evidenced as dominating the precipitation processes, irrespective of any modifications of experimental parameters involved in the preparation procedure investigated hereby. Prolonged aging time and temperature effects lead to "face-to-face" aggregation of particles, promoted by the discrepancy in surface charges between particle sides and faces. In light of these observations, the mechanism of precipitation of cholesterol is further discussed.

In vivo comparative study of lipid/DNA complexes with different in vitro serum stability: effects on biodistribution and tumor accumulation

To evaluate the in vivo biodistribution and expression of DOTAP-Chol/DNA complexes (lipoplexes) with different in vitro serum stability, quantitative real-time PCR, in vitro luciferase expression and whole body luminescence imaging were used. In general, less tissue biodistribution, lower luciferase expression and whole body luminescence were observed for DOTAP:Chol (mol/mol 1:4)/DNA lipoplexes which had higher in vitro serum stability as compared to DOTAP:Chol (mol/mol 1:1)/DNA lipoplexes. Plasmid DNA biodistribution and expression were mainly confined to the lungs, and the results suggest that in vitro serum stability may serve as a predictor of transfection in the lung. No correlation between plasmid DNA tissue biodistribution and gene expression was observed by simultaneous determination of the level of plasmid DNA tissue biodistribution and gene expression. While high doses of the formulation possessing increased in vitro serum stability did exhibit reduced entrapment in the lung, no corresponding increase in the plasmid levels of other tissues was observed. However, this formulation did show increased accumulation in tumors that was not further enhanced by PEGylation.

A lipid-mediated quality control process in the Golgi apparatus in yeast

When heme biosynthesis is disrupted, the yeast Saccharomyces cerevisiae becomes unable to synthesize its major sterol, ergosterol, and desaturate fatty acids. We took advantage of this physiological peculiarity to evaluate the consequences of ergosterol and/or unsaturated fatty acid (UFA) depletions on the biogenesis of a model polytopic plasma membrane protein, the uracil permease Fur4p. We show that under UFA shortage, which results in low amounts of diunsaturated phospholipid species, and under ergosterol depletion, Fur4p is prematurely routed from the Golgi apparatus to the vacuolar lumen in a process that requires the ubiquitin ligase Rsp5p. Interestingly, this diversion is not correlated to Fur4p exclusion from detergent-resistant membranes. In an independent set of experiments, we show that Fur4p targeting to the plasma membrane depends on phosphatidylethanolamine amounts and more specifically on the propensity of this phospholipid to form a hexagonal phase. In light of recent literature, we propose a model in which ergosterol and diunsaturated phospholipid species maintain optimal membrane curvature for Fur4p to evade the Golgi quality control process and to be properly delivered to its normal destination.

Uniform particles of pure and silica-coated cholesterol

Uniform crystalline colloidal cholesterol particles of narrow size distribution were obtained by precipitation. The method consisted of adding a miscible non-solvent (water) into cholesterol solutions of different alcohols and acetone, without any additives. The properties of the resulting particles depended in a sensitive way on the concentration of all reactants, temperature, pH, ionic strength, and aging time. The major observed effects were due to the solubility of cholesterol, which was strongly affected by the solvent mixture and temperature. Precipitation in 1-propanol/water system yielded stable dispersions of well-defined particles, which were used to evaluate the effects of different experimental parameters on their properties. Aging of stable dispersions resulted in multi-layered aggregation of the primary platelets, the degree and rate of which process was strongly affected by temperature. Finally, it was shown that the colloidal cholesterol particles could be coated with homogeneous silica layers in order to alter their surface characteristics.

Cubic phases in phosphatidylcholine-cholesterol mixtures: cholesterol as membrane "fusogen"

X-ray diffraction reveals that mixtures of some unsaturated phosphatidylcholines (PCs) with cholesterol (Chol) readily form inverted bicontinuous cubic phases that are stable under physiological conditions. This effect was studied in most detail for dioleoyl PC/Chol mixtures with molar ratios of 1:1 and 3:7. Facile formation of Im3m and Pn3m phases with lattice constants of 30-50 nm and 25-30 nm, respectively, took place in phosphate-buffered saline, in sucrose solution, and in water near the temperature of the Lalpha-HII transition of the mixtures, as well as during cooling of the HII phase. Once formed, the cubic phases displayed an ability to supercool and replace the initial Lalpha phase over a broad range of physiological temperatures. Conversion into stable cubic phases was also observed for mixtures of Chol with dilinoleoyl PC but not for mixtures with palmitoyl-linoleoyl PC or palmitoyl-oleoyl PC, for which only transient cubic traces were recorded at elevated temperatures. A saturated, branched-chain PC, diphytanoyl PC, also displayed a cubic phase in mixture with Chol. Unlike the PEs, the membrane PCs are intrinsically nonfusogenic lipids: in excess water they only form lamellar phases and not any of the inverted phases on their own. Thus, the finding that Chol induces cubic phases in mixtures with unsaturated PCs may have important implications for its role in fusion. In ternary mixtures, saturated PCs and sphingomyelin are known to separate into liquid-ordered domains along with Chol. Our results thus suggest that unsaturated PCs, which are excluded from these domains, could form fusogenic domains with Chol. Such a dual role of Chol may explain the seemingly paradoxical ability of cell membranes to simultaneously form rigid, low-curvature raft-like patches while still being able to undergo facile membrane fusion.

Lipid lateral diffusion in bilayers with phosphatidylcholine, sphingomyelin and cholesterol

Pulsed field gradient (pfg)-NMR measurements of the lipid lateral diffusion coefficients in several macroscopically aligned bilayer systems were summarized from previous and new studies. The aim was to carry out a comparison of the translational dynamics for bilayers with various mixtures of l,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), l,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and chicken egg yolk sphingomyelin (eSM), with or without cholesterol. New useful information was obtained on the dynamics in these lipid bilayers that has not been previously appreciated. Thus, we were able to propose that the driving force behind the phase separation into l(d)and l(o)phases evolves from the increasing difficulty to incorpotate DOPC into a highly ordered phase. Our results suggest that DOPC has a preference to be located in a disordered phase, while DPPC and eSM prefer the ordered phase. Quite unexpectedly, CHOL seems to partition into both phases to roughly the same extent, indicating that CHOL has no particular preference for any of the l(d)or l(o) phases, and there are no specific interactions between CHOL and saturated lipids.

Detecting the presence of membrane domains using DSC

Both biological and model liposomal membranes have unequal distribution of molecular components in the plane of the membrane. There is increasing interest to determine the composition and properties of membrane domains enriched with specific molecular components. Several methodologies have been applied to study this. Each has its own advantages and provides a particular kind of information. In the present article, we will focus on the application of differential scanning calorimetry to the determination of the distribution of molecules into membrane domains with particular emphasis on protein and peptide-induced domain formation.

Seeing spots: Complex phase behavior in simple membranes

Liquid domains in model lipid bilayers are frequently studied as models of raft domains in cell plasma membranes. Micron-scale liquid domains are easily produced in vesicles composed of ternary mixtures of a high melting temperature lipid, a low melting temperature lipid, and cholesterol. Here, we describe the rich phase behavior observed in binary and ternary systems. We then discuss experimental challenges inherent in mapping phase diagrams of even simple lipid systems. For example, miscibility behavior varies with lipid type, lipid ratio, lipid oxidation, and level of impurity. Liquid domains are often circular, but can become noncircular when membranes are near critical points. Finally, we reflect on applications of phase diagrams in model systems to rafts in cell membranes.

Transmembrane peptides stabilize inverted cubic phases in a biphasic length-dependent manner: implications for protein-induced membrane fusion

WALP peptides consist of repeating alanine-leucine sequences of different lengths, flanked with tryptophan "anchors" at each end. They form membrane-spanning alpha-helices in lipid membranes, and mimic protein transmembrane domains. WALP peptides of increasing length, from 19 to 31 amino acids, were incorporated into N-monomethylated dioleoylphosphatidylethanolamine (DOPE-Me) at concentrations up to 0.5 mol % peptide. When pure DOPE-Me is heated slowly, the lamellar liquid crystalline (L(alpha)) phase first forms an inverted cubic (Q(II)) phase, and the inverted hexagonal (H(II)) phase at higher temperatures. Using time-resolved x-ray diffraction and slow temperature scans (1.5 degrees C/h), WALP peptides were shown to decrease the temperatures of Q(II) and H(II) phase formation (T(Q) and T(H), respectively) as a function of peptide concentration. The shortest and longest peptides reduced T(Q) the most, whereas intermediate lengths had weaker effects. These findings are relevant to membrane fusion because the first step in the L(alpha)/Q(II) phase transition is believed to be the formation of fusion pores between pure lipid membranes. These results imply that physiologically relevant concentrations of these peptides could increase the susceptibility of biomembrane lipids to fusion through an effect on lipid phase behavior, and may explain one role of the membrane-spanning domains in the proteins that mediate membrane fusion.

Phosphatidylcholine structure determines cholesterol solubility and lipid polymorphism

In the present work, we demonstrate that phosphatidylcholine with (16:1)9 acyl chains undergoes polymorphic rearrangements in mixtures with 0.6-0.8 mol fraction cholesterol. Studies were performed using differential scanning calorimetry, X-ray diffraction, cryo-electron microscopy, 31P NMR static powder patterns and 13C MAS/NMR. Mixtures of phosphatidylcholine with (16:1)9 acyl chains and 0.6 mol fraction cholesterol, after being heated to 100 degrees C, can form an ordered array with periodicity 14 nm which may be indicative of a cubic phase. Our results indicate that the formation of highly curved bilayer structures, such as those required for membrane fusion, can occur in mixtures of cholesterol with certain phosphatidylcholines that do not form non-lamellar structures in the absence of cholesterol. We also determine the polymorphic behavior of mixtures of symmetric phosphatidylcholines with cholesterol. Species of phosphatidylcholine with (20:1)11, (22:1)13 or (24:1)15 acyl chains in mixtures with 0.6-0.8 mol fraction cholesterol undergo a transition to the hexagonal phase at temperatures 70-80 degrees C. This is not the case for phosphatidylcholine with (18:1)6 acyl chains which remains in the lamellar phase up to 100 degrees C when mixed with as much as 0.8 mol fraction cholesterol. Thus, the polymorphic behavior of mixtures of phosphatidylcholine and cholesterol is not uncommon and is dependent on the intrinsic curvature of the phospholipid. Crystals of cholesterol can be detected in mixtures of all of these phosphatidylcholines at sufficiently high cholesterol mole fraction. What is unusual about the formation of these crystals in several cases is that cholesterol crystals are present in the monohydrate form in preference to the anhydrous form. Furthermore, after heating to 100 degrees C and recooling, the cholesterol crystals are again observed to be in the monohydrate form, although pure cholesterol crystals require many hours to rehydrate after being heated to 100 degrees C. Both the nature of the acyl chain as well as the mole fraction cholesterol determine whether cholesterol crystals in mixtures with the phospholipids will be in the monohydrate or in the anhydrous form.

Induction of raft-like domains by a myristoylated NAP-22 peptide and its Tyr mutant

The N-terminally myristoylated, 19-amino acid peptide, corresponding to the amino terminus of the neuronal protein NAP-22 (NAP-22 peptide) is a naturally occurring peptide that had been shown by fluorescence to cause the sequestering of a Bodipy-labeled PtdIns(4,5)P2 in a cholesterol-dependent manner. The present work, using differential scanning calorimetry (DSC), extends the observation that formation of a PtdIns(4,5)P2-rich domain is cholesterol dependent and shows that it also leads to the formation of a cholesterol-depleted domain. The PtdIns(4,5)P2 used in the present work is extracted from natural sources and does not contain any label and has the native acyl chain composition. Peptide-induced formation of a cholesterol-depleted domain is abolished when the sole aromatic amino acid, Tyr11 is replaced with a Leu. Despite this, the modified peptide can still sequester PtdIns(4,5)P2 into domains, probably because of the presence of a cluster of cationic residues in the peptide. Cholesterol and PtdIns(4,5)P2 also modulate the insertion of the peptide into the bilayer as revealed by 1H NOESY MAS/NMR. The intensity of cross peaks between the aromatic protons of the Tyr residue and the protons of the lipid indicate that in the presence of cholesterol there is a change in the nature of the interaction of the peptide with the membrane. These results have important implications for the mechanism by which NAP-22 affects actin reorganization in neurons.

Effect of membrane characteristics on phase separation and domain formation in cholesterol-lipid mixtures

We examine, using an analytical mean-field model, the distribution of cholesterol in a lipid bilayer. The model accounts for the perturbation of lipid packing induced by the embedded cholesterol, in a manner similar to that of transmembrane proteins. We find that the membrane-induced interactions between embedded cholesterol molecules vary as a function of the cholesterol content. Thus, the effective lipid-cholesterol interaction is concentration-dependent. Moreover, it transitions from repulsive to attractive to repulsive as the cholesterol content increases. As the concentration of cholesterol in the bilayer exceeds a critical value, phase separation occurs. The coexistence between cholesterol-rich and cholesterol-poor domains is universal for any bilayer parameters, although the composition of the cholesterol-rich phase varies as a function of the lipid properties. Although we do not assume specific cholesterol-lipid interactions or the formation of a lipid-cholesterol cluster, we find that the composition of the cholesterol-rich domains is constant, independent of the cholesterol content in the bilayer.

Non-raft forming sphingomyelin–cholesterol mixtures

Sphingomyelin from biological membranes forms segregated domains with cholesterol in fluid bilayers. However, a synthetic form of sphingomyelin with an oleoyl chain linked to sphingosine is not incorporated into cholesterol-rich domains. We have studied the properties of mixtures of oleoyl-sphingomyelin and cholesterol as well as mixtures of oleoyl-sphingomyelin with 1-stearoyl-2-oleoyl-phosphatidylcholine by DSC and NMR. Cholesterol has a high miscibility with oleoyl-sphingomyelin and it does not separate in crystalline form until the mol fraction of cholesterol reaches a value above 0.6. A large fraction of the cholesterol crystals that are formed are in the monohydrate form. Furthermore, these crystals rehydrate relatively rapidly compared with pure cholesterol crystals in the absence of phospholipid. The environment of the carbonyl group of the phospholipid indicates that it is similar to other forms of sphingomyelin with saturated acyl chains. Also similar to other forms of sphingomyelin, the quaternary ammonium group of oleoyl-sphingomyelin is more rigid than that of phosphatidylcholines, as indicated by the strong resonance observed with cross-polarization/magic angle spinning. Additionally, oleoyl-sphingomyelin produces a larger alteration than egg sphingomyelin of the phase transition of 1-stearoyl-2-oleoyl-phosphatidylcholine. These studies indicate that oleoyl-sphingomyelin, unlike saturated forms of sphingomyelin, does not form segregated domains with cholesterol because of its greater miscibility with phosphatidylcholine.

The gaussian curvature elastic modulus of N-monomethylated dioleoylphosphatidylethanolamine: relevance to membrane fusion and lipid phase behavior

The energy of intermediates in fusion of phospholipid bilayers is sensitive to kappa(m), the saddle splay (Gaussian curvature) elastic modulus of the lipid monolayers. The value kappa(m) is also important in understanding the stability of inverted cubic (Q(II)) and rhombohedral (R) phases relative to the lamellar (L(alpha)) and inverted hexagonal (H(II)) phases in phospholipids. However, kappa(m) cannot be measured directly. It was previously measured by observing changes in Q(II) phase lattice dimensions as a function of water content. Here we use observations of the phase behavior of N-mono-methylated dioleoylphosphatidylethanolamine (DOPE-Me) to determine kappa(m). At the temperature of the L(alpha)/Q(II) phase transition, T(Q), the partial energies of the two phases are equal, and we can express kappa(m) in terms of known lipid monolayer parameters: the spontaneous curvature of DOPE-Me, the monolayer bending modulus kappa(m), and the distance of the monolayer neutral surface from the bilayer midplane, delta. The calculated ratio kappa(m)/kappa(m) is -0.83 +/- 0.08 at T(Q) approximately 55 degrees C. The uncertainty is due primarily to uncertainty in the value of delta for the L(alpha) phase. This value of kappa(m)/kappa(m) is in accord with theoretical expectations, including recent estimates of the value required to rationalize observations of rhombohedral (R) phase stability in phospholipids. The value kappa(m) substantially affects the free energy of formation of fusion intermediates: more energy (tens of k(B)T) is required to form stalks and fusion pores (ILAs) than estimated solely on the basis of the bending elastic energy. In particular, ILAs are much higher in energy than previously estimated. This rationalizes the action of fusion-catalyzing proteins in stabilizing nascent fusion pores in biomembranes; a function inferred from recent experiments in viral systems. These results change predictions of earlier work on ILA and Q(II) phase stability and L(alpha)/Q(II) phase transition mechanisms. To our knowledge, this is the first determination of the saddle splay (Gaussian) modulus in a lipid system consisting only of phospholipids.

Properties of polyunsaturated phosphatidylcholine membranes in the presence and absence of cholesterol

Mixtures of cholesterol with phosphatidylcholine species containing the polyunsaturated acyl chains arachidonoyl or docosahexaenoyl were studied by (13)C magic angle spinning (MAS) NMR using both cross-polarization and direct polarization, by (31)P NMR and by differential scanning calorimetry. Several unique features of these systems were observed. The separation of cholesterol in crystalline form occurred at much lower molar fractions than with other forms of phosphatidylcholine. The crystals that were formed were sensitive to the history of the sample. At cholesterol molar fractions below 0.5, they dissolved into the membrane by sequential heating and cooling scans. With higher molar fractions of cholesterol, larger amounts of anhydrous crystals were formed after the first heating. This was accompanied by the formation of non-lamellar phases. The cholesterol crystals that were formed generally were not observed by direct polarization (13)C MAS NMR, even with delay times of 100 s. This suggests that the cholesterol crystals are in a more rigid state in mixtures with these lipids. This is in contrast with the terminal methyl group of the acyl chains that is too mobile to allow cross-polarization using 1 ms contact times.