Exchange of monooleoylphosphatidylcholine with single egg phosphatidylcholine vesicle membranes

Asymmetric desorption of lipid oxidation products induces membrane bending

Lipid oxidation, detected in metabolic processes, is induced in excess when the cellular membrane suffers extra oxidative stress. Lipid oxidation can compromise biomembrane function in part through perturbations of lipid packing, membrane permeability, and morphology. Two major types of oxidation products, one with a partially truncated lipid tail with a hydrophilic group at the tail-end, and secondly, a lysolipid (with one of the chains completely truncated) can disturb the membrane bilayer packing significantly. However, they also have an increased tendency to desorb from the membrane. In this study we investigated desorption kinetics of two characteristic lipid oxidation products (PAzePC and 18 : 1 LysoPC) from a model membrane system, and we evaluated the consequences of this process on membrane shape transitions. Using a microfluidic chamber coupled with micropipette aspiration, we observed the incorporation of the two lipids into the membrane of a giant unilamellar vesicle (GUV) and further determined their desorption rates, association rates and flip-flop rates. For both lipids, the desorption is on the time scale of seconds, one to two orders of magnitude faster than their flipping rates. Dilution of the outer solution of the GUVs allowed asymmetric desorption of these two lipids from the GUVs. This process induced lipid number asymmetry and charge asymmetry, specifically for PAzePC containing GUVs, and caused membrane tubulation. Our results indicate that the desorption of lipid oxidation products can alter the local structure of biomembranes and result in morphological changes that may relate to membrane function.

The real reason for having a meibomian lipid layer covering the outer surface of the tear film - A review

This review critically evaluates a broad range of literature in order to show the relationship between meibum, tear lipids and the tear film lipid layer (TFLL). The relationship of meibum composition to dry eye syndrome is briefly discussed. The review also explores the interactions between aqueous and the TFLL by examining the correlations between meibomian lipids and lipids extracted from whole tears, and by considering protein adsorption to the TFLL from the aqueous. Although it is clear to the authors that a normal tear film resists evaporation, an emerging idea from the literature is that the main purpose of the TFLL is to allow the spread of the tear film and to prevent its collapse onto the ocular surface, rather than to be an evaporative blanket. Current models on the possible structure of the TFLL are also examined.

Materials Science and Engineering of the Low Temperature Sensitive Liposome (LTSL): Composition-Structure-Property Relationships That Underlie its Design and Performance

This chapter presents the material science and materials engineering concepts that went into the design and testing of the Low Temperature-Sensitive Liposome (LTSL), including: the roles of each of the components that make up the composite membrane; how the molecular and nanostructures that they form might influence the already anomalous permeability at the phase transition of the bilayer; and how this thermally sensitive “Smart Drug Delivery System” leads to ultrafast release of a loaded doxorubicin drug, triggered and controlled in the micro-vasculature of tumors by applied mild hyperthermia. This formulation approach, as ThermoDox®, has been used in a completed 700-patient Phase III human clinical trial in liver cancer (HEAT study), is in a Phase II trial in chest wall recurrence of cancer (DIGNITY study) and has been used in a Phase I trial of patients with colorectal liver metastases (ABLATE study). With additional research and preclinical studies underway, and a range of other drugs, imaging agents and biological modifiers poised for encapsulation, the LTSL could provide a new paradigm for drug and agent delivery for the treatment of localized tumors: rapid triggered drug release in the tumor bloodstream and deep penetration of drug into the tumor tissue.

Growth and shape transformations of giant phospholipid vesicles upon interaction with an aqueous oleic acid suspension

The interaction of two types of vesicle systems was investigated: micrometer-sized, giant unilamellar vesicles (GUVs) formed from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and submicrometer-sized, large unilamellar vesicles (LUVs) formed from oleic acid and oleate, both in a buffered aqueous solution (pH 8.8). Individual POPC GUVs were transferred with a micropipette into a suspension of oleic acid/oleate LUVs, and the shape changes of the GUVs were monitored using optical microscopy. The behavior of POPC GUVs upon transfer into a 0.8mM suspension of oleic acid, in which oleic acid/oleate forms vesicular bilayer structures, was qualitatively different from the behavior upon transfer into a 0.3mM suspension of oleic acid/oleate, in which oleic acid/oleate is predominantly present in the form of monomers and possibly non-vesicular aggregates. In both cases, changes in vesicle morphology were observed within tens of seconds after the transfer. After an initial increase of the vesicle cross-section, the vesicle started to evaginate, spawning dozens of satellite vesicles connected to the mother vesicle with narrow necks or tethers. In 60% of the cases of transfer into a 0.8mM oleic acid suspension, the evagination process reversed and proceeded to the point where the membrane formed invaginations. In some of these cases, several consecutive transitions between invaginated and evaginated shapes were observed. In the remaining 40% of the cases of transfer into the 0.8mM oleic acid suspension and in all cases of vesicle transfer into the 0.3mM oleic acid suspension, no invaginations nor subsequent evaginations were observed. An interpretation of the observed vesicle shape transformation on the basis of the bilayer-couple model is proposed, which takes into account uptake of oleic acid/oleate molecules by the POPC vesicles, oleic acid flip-flop processes and transient pore formation.

Lysolipid incorporation in dipalmitoylphosphatidylcholine bilayer membranes enhances the ion permeability and drug release rates at the membrane phase transition

The enhanced permeability of lipid bilayer membranes at their gel-to-liquid phase transition has been explained using a "bilayer lipid heterogeneity" model, postulating leaky interfacial regions between still solid and melting liquid phases. The addition of lysolipid to dipalmitoylphosphatidylcholine bilayers dramatically enhances the amount of, and speed at which, encapsulated markers or drugs are released at this, already leaky, phase transition through these interfacial regions. To characterize and attempt to determine the mechanism behind lysolipid-generated permeability enhancement, dithionite permeability and doxorubicin release were measured for lysolipid and non-lysolipid, containing membranes. Rapid release of contents from lysolipid-containing membranes appears to occur through lysolipid-stabilized pores rather than a simple enhancement due to increased drug solubility in the bilayer. A dramatic enhancement in the permeability rate constant begins about two degrees below the calorimetric peak of the thermal transition, and extends several degrees past it. The maximum permeability rate constant coincides exactly with this calorimetric peak. Although some lysolipid desorption from liquid state membranes cannot be dismissed, dialyzation above T(m) and mass spectrometry analysis indicate lysolipid must, and can, remain in the membrane for the permeability enhancement, presumably as lysolipid stabilized pores in the grain boundary regions of the partially melted solid phase.

Pro-apoptotic Bid induces membrane perturbation by inserting selected lysolipids into the bilayer

Bid is a BH3-only member of the Bcl-2 family that regulates cell death at the level of mitochondrial membranes. Bid appears to link the mitochondrial pathway with the death receptor-mediated pathway of cell death. It is generally assumed that the f.l. (full-length) protein becomes activated after proteolytic cleavage, especially by apical caspases like caspase 8. The cleaved protein then relocates to mitochondria and promotes membrane permeabilization, presumably by interaction with mitochondrial lipids and other Bcl-2 proteins that facilitate the release of apoptogenic proteins like cytochrome c. Although the major action may reside in the C-terminus part, tBid (cleaved Bid), un-cleaved Bid also has pro-apoptotic potential when ectopically expressed in cells or in vitro. This pro-apoptotic action of f.l. Bid has remained unexplained, especially at the biochemical level. In the present study, we show that f.l. (full-length) Bid can insert specific lysolipids into the membrane surface, thereby priming mitochondria for the release of apoptogenic factors. This is most effective for lysophosphatidylcholine species that we report to accumulate in mitochondria during apoptosis induction. A Bid mutant that is not pro-apoptotic in vivo is defective in lysophosphatidylcholine-mediated membrane perturbation in vitro. Our results thus provide a biochemical explanation for the pro-apoptotic action of f.l. Bid.

Fractional occurrence of defects in membranes and mechanically driven interleaflet phospholipid transport

The picture of biological membranes as uniform, homogeneous bileaflet structures has been revised in recent times due to the growing recognition that these structures can undergo significant fluctuations both in local curvature and in thickness. In particular, evidence has been obtained that a temporary, localized disordering of the lipid bilayer structure (defects) may serve as a principal pathway for movement of lipid molecules from one leaflet of the membrane to the other. How frequently these defects occur and how long they remain open are important unresolved questions. In this report, we calculate the rate of molecular transport through a transient defect in the membrane and compare this result to measurements of the net transbilayer flux of lipid molecules measured in an experiment in which the lipid flux is driven by differences between the mechanical stress in the two leaflets of the membrane bilayer. Based on this comparison, we estimate the frequency of defect occurrence in the membrane. The occurrence of defects is rare: the probability of finding a defect in 1.0 microm2 of a lecithin membrane is estimated to be approximately 6.0x10(-6). Based on this fractional occurrence of defects, the free energy of defect formation is estimated to be approximately 1.0x10(-19) J. The calculations provide support for a model in which interleaflet transport in membranes is accelerated by mechanically driven lipid flow.

Interaction of synthetic HA2 influenza fusion peptide analog with model membranes

The interaction of the synthetic 21 amino acid peptide (AcE4K) with 1-oleoyl-2-[caproyl-7-NBD]-sn-glycero-3-phosphocholine membranes is used as a model system for the pH-sensitive binding of fusion peptides to membranes. The sequence of AcE4K (Ac-GLFEAIAGFIENGWEGMIDGK) is based on the sequence of the hemagglutinin HA2 fusion peptide and has similar partitioning into phosphatidylcholine membranes as the viral peptide. pH-dependent partitioning in the membrane, circular dichroism, tryptophan fluorescence, change of membrane area, and membrane strength, are measured to characterize various key aspects of the peptide-membrane interaction. The experimental results show that the partitioning of AcE4K in the membrane is pH dependent. The bound peptide inserts in the membrane, which increases the overall membrane area in a pH-dependent manner, however the depth of insertion of the peptide in the membrane is independent of pH. This result suggests that the binding of the peptide to the membrane is driven by the protonation of its three glutamatic acids and the aspartic acid, which results in an increase of the number of bound molecules as the pH decreases from pH 7 to 4.5. The transition between the bound state and the free state is characterized by the Gibbs energy for peptide binding. This Gibbs energy for pH 5 is equal to -30.2 kJ/mol (-7.2 kcal/mol). Most of the change of the Gibbs energy during the binding of AcE4K is due to the enthalpy of binding -27.3 kJ/mol (-6.5 kcal/mol), while the entropy change is relatively small and is on the order of 6.4 J/mol.K (2.3 cal/mol.K). The energy barrier separating the bound and the free state, is characterized by the Gibbs energy of the transition state for peptide adsorption. This Gibbs energy is equal to 51.3 kJ/mol (12.3 kcal/mol). The insertion of the peptide into the membrane is coupled with work for creation of a vacancy for the peptide in the membrane. This work is calculated from the measured area occupied by a single peptide molecule (220 A(2)) and the membrane elasticity (190 mN/m), and is equal to 15.5 kJ/mol (3.7 kcal/mol). The comparison of the work for creating a vacancy and the Gibbs energy of the transition state shows that the work for creating a vacancy may have significant effect on the rate of peptide insertion and therefore plays an important role in peptide binding. Because the work for creating a vacancy depends on membrane elasticity and the elasticity of the membrane is dependent on membrane composition, this provides a tool for modulating the pH for membrane instability by changing membrane composition. The insertion of the peptide in the membrane does not affect the membrane permeability for water, which shows that the peptide does not perturb substantially the packing of the hydrocarbon region. However, the ability of the membrane to retain solutes in the presence of peptide is compromised, suggesting that the inserted peptide promotes formation of short living pores. The integrity of the membrane is substantially compromised below pH 4.8 (threshold pH), when large pores are formed and the membrane breaks down. The binding of the peptide in the pore region is reversible, and the pore size varies on the experimental conditions, which suggests that the peptide in the pore region does not form oligomers.

PEG-covered lipid surfaces: bilayers and monolayers

In this review paper we survey the ways in which various micropipet techniques have been used to study the mechanochemical and interactive features of lipid bilayer vesicles and monolayer-coated gas bubbles. Special emphasis will be made on characterizing the barrier properties of grafted PEG layers and how a hierarchical approach that uses a short barrier and extended ligand allows us to start to mimic nature's own solution to the problem of ubiquitous repulsion and specific attraction. The information gained from such studies not only characterizes the membrane and other lipid surfaces and their intersurface interactions from a fundamental materials science perspective, but also provides essential materials property data that are required for the successful design and deployment of lipid-based carriers and other capsules in applications involving this so-called 'stealthy' surface.

Determining the membrane topology of peptides by fluorescence quenching

Determination of the topology of peptides in membranes is important for characterizing and understanding the interactions of peptides with membranes. We describe a method that uses fluorescence quenching arising from resonance energy transfer ("FRET") for determining the topology of the tryptophan residues of peptides partitioned into phospholipid bilayer vesicles. This is accomplished through the use of a novel lyso-phospholipid quencher (lysoMC), N-(7-hydroxyl-4-methylcoumarin-3-acetyl)-1-palmitoyl-2-hydroxy-sn-gly cero-3-phosphoethanolamine. The design principle was to anchor the methylcoumarin quencher in the membrane interface by attaching it to the headgroup of lyso-phosphoethanolamine. We show that lysoMC can be incorporated readily into large unilamellar phospholipid vesicles to yield either symmetrically (both leaflets) or asymmetrically (outer leaflet only) labeled bilayers. LysoMC quenches the fluorescence of membrane-bound tryptophan by the Förster mechanism with an apparent R(0) that is comparable to the thickness of the hydrocarbon core of a lipid bilayer (approximately 25 A). Consequently, the methylcoumarin acceptor predominantly quenches tryptophans that reside in the same monolayer as the probe. The topology of a peptide's tryptophan in membranes can be determined by comparing the quenching in symmetric and asymmetric lysoMC-labeled vesicles. Because it is essential to know that asymmetrically incorporated lysoMC remains so under all conditions, we also developed a second type of FRET experiment for assessing the rate of transbilayer diffusion (flip-flop) of lysoMC. Except in the presence of pore-forming peptides, there was no measurable flip-flop of lysoMC, indicating that asymmetric distributions of quencher are stable. We used these methods to show that N-acetyl-tryptophan-octylamide and tryptophan-octylester rapidly equilibrate across phosphatidylcholine (POPC) and phosphatidylglycerol (POPG) bilayers, while four amphipathic model peptides remain exclusively on the outer monolayer. The topology of the amphipathic peptide melittin bound to POPC could not be determined because it induced rapid flip-flop of lysoMC. Interestingly, melittin did not induce lysoMC flip-flop in POPG vesicles and was found to remain stably on the external monolayer.

The effects of gramicidin on electroporation of lipid bilayers

The effects of the channel-forming peptide gramicidin D (gD) on the conductance and electroporation thresholds of planar bilayer lipid membranes, made of the synthetic lipid 1-palmitoyl 2-oleoyl phosphatidylcholine (POPC), was studied. High-amplitude ( approximately 200-900 mV) rectangular voltage pulses of 15 ms duration were used to perturb the bilayers and monitor the transmembrane conductance. Electroporation voltage thresholds were found, and conductance was recorded before and after electroporation. Gramicidin was added to the system in peptide/lipid ratios of 1:10, 000, 1:500, and 1:15. The addition of gD in a ratio of 1:10,000 had no effect on electroporation, but ratios of 1:500 and 1:15 significantly increased the thresholds by 16% (p < 0.0001) and 40% (p < 0.0001), respectively. Membrane conductance before electroporation was measurable only after the addition of gD and increased monotonically as the peptide/lipid ratio increased. The effect of gD on the membrane area expansivity modulus (K) was tested using giant unilamellar vesicles (GUVs). When gD was incorporated into the vesicles in a 1:15 ratio, K increased by 110%, consistent with the increase in thresholds predicted by an electromechanical model. These findings suggest that the presence of membrane proteins may affect the electroporation of lipid bilayers by changing their mechanical properties.

Dehydration of model membranes induced by lectins from Ricinus communis and Viscum album

The effects of ribosome-inactivating proteins (RIPs) from Ricinus communis and from Viscum album on the water permeability, Pf, and the surface dielectric constant, epsilon, of model membranes were studied. Pf was calculated from microelectrode measurements of the ion concentration distribution in the immediate vicinity of a planar membrane, and epsilon was obtained from the fluorescence of dansyl phosphatidylethanolamine incorporated into unilamellar vesicles. Pf and epsilon of fully saturated phosphatidylcholine membranes were affected only in the presence of a lectin receptor (monosialoganglioside, GM1) in the bilayer. It is suggested that the membrane area occupied by clustered lectin-receptor complexes is markedly less permeable to water. Protein binding to the receptor was not a prelude for hydrophobic lipid-protein interactions when the membranes were formed from a mixture of natural phospholipids with a high content of unsaturated fatty acids. These membranes, characterized by a high initial water permeability, were found to interact with the RIPs unspecifically. From a decrease of both Pf and epsilon it was concluded that not only water partitioning but also protein adsorption correlates with looser packing of polyunsaturated lipids at the lipid-water interface.

The reduction in electroporation voltages by the addition of a surfactant to planar lipid bilayers

The effects of a nonionic surfactant, octaethyleneglycol mono n-dodecyl ether (C12E8), on the electroporation of planar bilayer lipid membranes made of the synthetic lipid 1-pamitoyl 2-oleoyl phosphatidylcholine (POPC), was studied. High-amplitude ( approximately 100-450 mV) rectangular voltage pulses were used to electroporate the bilayers, followed by a prolonged, low-amplitude ( approximately 65 mV) voltage clamp to monitor the ensuing changes in transmembrane conductance. The electroporation thresholds of the membranes were found for rectangular voltage pulses of given durations. The strength-duration relationship was determined over a range from 10 micros to 10 s. The addition of C12E8 at concentrations of 0.1, 1, and 10 microM to the bath surrounding the membranes decreased the electroporation threshold monotonically with concentration for all durations (p < 0.0001). The decrease from control values ranged from 10% to 40%, depending on surfactant concentration and pulse duration. For a 10-micros pulse, the transmembrane conductance 150 micros after electroporation (G150) increased monotonically with the surfactant concentration (p = 0.007 for 10 microM C12E8). These findings suggest that C12E8 incorporates into POPC bilayers, allowing electroporation at lower intensities and/or shorter durations, and demonstrate that surfactants can be used to manipulate the electroporation threshold of lipid bilayers.

Material property characteristics for lipid bilayers containing lysolipid

The apparent area expansion modulus and tensile strength of egg phosphatidylcholine (EPC) membranes are measured in the presence of monooleoylphosphatidylcholine (MOPC). The apparent area expansion modulus decreases from 171 mN m-1 for pure EPC membrane to 82 mN m-1 for a membrane containing 30 mol % MOPC. This significant decrease of the apparent area expansion modulus is attributed to the change of the membrane area due to the tension-dependent exchange of MOPC between the bathing solution and the membrane. Similar to the apparent area expansion modulus, the tensile strength of the membrane decreases with the increase of the molar concentration of MOPC in the membrane. The tensile strength of pure EPC membrane is 9.4 mN m-1 whereas that for a membrane containing 30 mol % MOPC is only 1.8 mN m-1, and for a membrane containing 50 mol % MOPC it is even smaller, on the order of 0.07 mN m-1. The decrease of the tensile strength is coupled with a decrease of the work for membrane breakdown, which changes from 4.3 x 10(-2) kT for pure EPC membrane to 2 x 10(-6) kT for a membrane with 50 mol % MOPC. Overall, these results show that the decrease of the apparent area expansion modulus in the presence of exchangeable molecules is a fundamental property for all membranes and depends on the area occupied by these molecules. The method presented here provides a unique tool for measuring the area occupied by an exchangeable molecule in the bilayer membrane.

Mechanical properties of model membranes studied from shape transformations of giant vesicles

Membrane deformations occur frequently in cell functioning. From the physical point of view, the understanding of such shape changes requires the introduction of mechanical parameters like bending elasticity. In this article it is shown how this physical property can be obtained from the analysis of small or large shape transformations from giant vesicles. Then it is demonstrated that the bending modulus is strongly dependent on the membrane composition and environmental conditions. This is the case for one-component bilayers (dilauroylphosphatidylcholine (DLPC), dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine and stearoyloleoyl-phosphatidylcholine (SOPC) and for two-component lipid mixtures (DMPC/cholesterol, DLPC/dilauroylphosphatidic acid). Further it is shown that the bending elasticity of natural lipid extracts (egg phosphatidylcholine, digalactosyl diglyceride and red blood cell lipid extracts) is generally smaller than that of comparable synthetic model membranes. The role of transmembrane proteins is examined by measuring the bending elasticity of SOPC/gramicidin mixtures. Finally, larger scale shape transformations of giant vesicles under an alternative electric field are discussed.

Exchange of monooleoylphosphatidylcholine as monomer and micelle with membranes containing poly(ethylene glycol)-lipid

Surface-grafted polymers, such as poly(ethylene glycol) (PEG), provide an effective steric barrier against surface-surface and surface-macromolecule interactions. In the present work, we have studied the exchange of monooleoylphosphatidylcholine (MOPC) with vesicle membranes containing 750 mol wt surface-grafted PEG (incorporated as PEG-lipid) from 0 to 20 mol % and have analyzed the experimental results in terms of thermodynamic and stationary equilibrium models. Micropipette manipulation was used to expose a single lipid vesicle to a flow of MOPC solution (0.025 microM to 500 microM). MOPC uptake was measured by a direct measure of the vesicle area change. The presence of PEG(750) lipid in the vesicle membrane inhibited the partitioning of MOPC micelles (and to some extent microaggregates) into the membrane, while even up to 20 mol % PEG-lipid, it did not affect the exchange of MOPC monomers both into and out of the membrane. The experimental data and theoretical models show that grafted PEG acts as a very effective molecular scale "filter" and prevents micelle-membrane contact, substantially decreasing the apparent rate and amount of MOPC taken up by the membrane, thereby stabilizing the membrane in a solution of MOPC that would otherwise dissolve it.