Lysolipid exchange with lipid vesicle membranes

Phospholipase-catalyzed degradation drives domain morphology and rheology transitions in model lung surfactant monolayers

Lung surfactant is inactivated in acute respiratory distress syndrome (ARDS) by a mechanism that remains unclear. Phospholipase (PLA2) plays an essential role in the normal lipid recycling processes, but is present in elevated levels in ARDS, suggesting it plays a role in ARDS pathophysiology. PLA2 hydrolyzes lipids such as DPPC-the primary component of lung surfactant-into palmitic acid (PA) and lyso-PC (LPC). Because PA co-crystallizes with DPPC to form rigid, elastic domains, we hypothesize that PLA2-catalyzed degradation establishes a stiff, heterogeneous rheology in the monolayer, and suggests a potential mechanical role in disrupting lung surfactant function during ARDS. Here we study the morphological and rheological changes of DPPC monolayers as they are degraded by PLA2 using interfacial microbutton microrheometry coupled with fluorescence microscopy. While degrading, domain morphology passes through qualitatively distinct transitions: compactification, coarsening, solidification, aggregation, network percolation, network erosion, and nucleation of PLA2-rich domains. Initially, condensed domains relax to more compact shapes, and coarsen via Ostwald ripening and coalescence up until the domains solidify, marked by a distinct roughening of domain boundaries that does not relax. Domains aggregate and eventually form a percolated network, whose elements then erode and whose connections are broken as degradation continues. The relative enzymatic activity of PLA2, set by the age of the sample, impacts the order and the duration of morphology transitions. The fresher the PLA2, the faster the overall degradation, and the earlier the onset of domain solidification: domains solidify before aggregating with fresh PLA2 samples, but aggregate and percolate before solidification with aged PLA2. Irrespective of the activity of the PLA2, all measured linear viscoelastic surface shear moduli obey the same dependence on condensed phase area fraction (log|G*| ∝ ϕ) throughout monolayer degradation. Moreover, the onset of domain solidification coincides with the time when the relative surface elasticity begins to increase.

Manipulation of encapsulated artificial phospholipid membranes using sub-micellar lysolipid concentrations

Droplet Interface Bilayers (DIBs) constitute a commonly used model of artificial membranes for synthetic biology research applications. However, their practical use is often limited by their requirement to be surrounded by oil. Here we demonstrate in-situ bilayer manipulation of submillimeter, hydrogel-encapsulated droplet interface bilayers (eDIBs). Monolithic, Cyclic Olefin Copolymer/Nylon 3D-printed microfluidic devices facilitated the eDIB formation through high-order emulsification. By exposing the eDIB capsules to varying lysophosphatidylcholine (LPC) concentrations, we investigated the interaction of lysolipids with three-dimensional DIB networks. Micellar LPC concentrations triggered the bursting of encapsulated droplet networks, while at lower concentrations the droplet network endured structural changes, precisely affecting the membrane dimensions. This chemically-mediated manipulation of enclosed, 3D-orchestrated membrane mimics, facilitates the exploration of readily accessible compartmentalized artificial cellular machinery. Collectively, the droplet-based construct can pose as a chemically responsive soft material for studying membrane mechanics, and drug delivery, by controlling the cargo release from artificial cell chassis.

Surfactant-Mediated Structural Modulations to Planar, Amphiphilic Multilamellar Stacks

The hydrophobic effect, a ubiquitous process in biology, is a primary thermodynamic driver of amphiphilic self-assembly. It leads to the formation of unique morphologies including two highly important classes of lamellar and micellar mesophases. The interactions between these two types of structures and their involved components have garnered significant interest because of their importance in key biochemical technologies related to the isolation, purification, and reconstitution of membrane proteins. This work investigates the structural organization of mixtures of the lamellar-forming phospholipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and two zwitterionic micelle-forming surfactants, being n-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (Zwittergent 3-12 or DDAPS) and 1-oleoyl-2-hydroxy-sn-glycero-3-phosphocholine (O-Lyso-PC), when assembled by water vapor hydration with X-ray diffraction measurements, brightfield optical microscopy, wide-field fluorescence microscopy, and atomic force microscopy. The results reveal that multilamellar mesophases of these mixtures can be assembled across a wide range of POPC to surfactant (POPC:surfactant) concentration ratios, including ratios far surpassing the classical detergent-saturation limit of POPC bilayers without significant morphological disruptions to the lamellar motif. The mixed mesophases generally decreased in lamellar spacing (D) and headgroup-to-headgroup distance (Dhh) with a higher concentration of the doped surfactant, but trends in water layer thickness (Dw) between each bilayer in the stack are highly variable. Further structural characteristics including mesophase topography, bilayer thickness, and lamellar rupture force were revealed by atomic force microscopy (AFM), exhibiting homogeneous multilamellar stacks with no significant physical differences with changes in the surfactant concentration within the mesophases. Taken together, the outcomes present the assembly of unanticipated and highly unique mixed mesophases with varied structural trends from the involved surfactant and lipidic components. Modulations in their structural properties can be attributed to the surfactant's chemical specificity in relation to POPC, such as the headgroup hydration and the hydrophobic chain tail mismatch. Taken together, our results illustrate how specific chemical complexities of surfactant-lipid interactions can alter the morphologies of mixed mesophases and thereby alter the kinetic pathways by which surfactants dissolve lipid mesophases in bulk aqueous solutions.

A hemifused complex is the hub in a network of pathways to membrane fusion

Membrane fusion is a critical step for many essential processes, from neurotransmission to fertilization. For over 40 years, protein-free fusion driven by calcium or other cationic species has provided a simplified model of biological fusion, but the mechanisms remain poorly understood. Cation-mediated membrane fusion and permeation are essential in their own right to drug delivery strategies based on cell-penetrating peptides or cation-bearing lipid nanoparticles. Experimental studies suggest calcium drives anionic membranes to a hemifused intermediate that constitutes a hub in a network of pathways, but the pathway selection mechanism is unknown. Here we develop a mathematical model that identifies the network hub as a highly dynamic hemifusion complex. Multivalent cations drive expansion of this high-tension hemifusion interface between interacting vesicles during a brief transient. The fate of this interface determines the outcome, either fusion, dead-end hemifusion, or vesicle lysis. The model reproduces the unexplained finding that calcium-driven fusion of vesicles with planar membranes typically stalls at hemifusion, and we show the equilibrated hemifused state is a novel lens-shaped complex. Thus, membrane fusion kinetics follow a stochastic trajectory within a network of pathways, with outcome weightings set by a hemifused complex intermediate.

Surfactant-Mediated Solubilization of Myelin Figures: A Multistep Morphological Cascade

Lamellar mesophases of insoluble lipids are readily solubilized by the micellar mesophases of soluble surfactants. This simple process underscores a broad array of biochemical methodologies, including purification, reconstitution, and crystallization of membrane proteins, as well as the isolation of detergent-resistant membrane fractions. Although much is now known about the thermodynamic driving forces of membrane solubilization, the kinetic pathways by which the surfactant alters vesicular mesophases are only beginning to be appreciated. Little is known about how these interactions affect the solubilization of more complex, multilamellar mesophases. Here, we investigate how a common zwitterionic detergent affects the solubilization of a smectic, multilamellar, cylindrical mesophase of lipids, called the myelin figure. Our results reveal that myelin solubilization occurs in a multistep manner, producing a well-defined sequence of morphologically distinct intermediates en route to complete solubilization. The kinetic processes producing these intermediates include (1) coiling, which encompasses the formation, propagation, and tightening of extended helices; (2) thinning, which reflects the unbinding of lamellae in the smectic stacks; and (3) detachment or retraction, which either dissociates the myelinic protrusion from the source lipid mass or returns the myelinic protrusion to the source lipid mass─all in transit toward complete solubilization. These occasionally overlapping steps are most pronounced in single-lipid component myelins, while compositionally graded multicomponent myelins inhibit the coiling step and detach more frequently. Taken together, the appearance of these intermediates during the solubilization of myelins suggests a complex free-energy landscape characterizing myelin solubilization populated by metastable, morphological intermediates correlated with locally minimized changes in energy dependent upon the mesophase's composition. This then predicts the accessibility of structurally distinct, kinetic intermediates─such as loose and tight coiled helices, peeled myelins, retracted tubes, and detached protrusions─before reaching the stable ground state corresponding to a dissolved suspension of mixed surfactant-lipid micelles.

Peptide lipidation in lysophospholipid micelles and lysophospholipid-enriched membranes

Acyl transfer from lipids to membrane-associated peptides is a well-documented process, leading to the generation of a lipidated peptide and a lysolipid. In this article, we demonstrate that acyl transfer from lysophosphatidylcholines (lysoPCs) to the peptide melittin also occurs, both in micelles of pure lysolipid and in lipid/lysolipid mixtures. In the case of bilayers containing lysolipids, acyl transfer from the lysolipid is marginally favoured over transfer from the lipid. In pure bilayers of saturated lipids, the introduction of even small amounts of lysolipid appears to significantly increase the reactivity towards lipidation.

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.

Cyanine Dyes for Photo-Thermal Therapy: A Comparison of Synthetic Liposomes and Natural Erythrocyte-Based Carriers

Cyanine fluorescent dyes are attractive diagnostic or therapeutic agents due to their excellent optical properties. However, in free form, their use in biological applications is limited due to the short circulation time, instability, and toxicity. Therefore, their encapsulation into nano-carriers might help overcome the above-mentioned issues. In addition to indocyanine green (ICG), which is clinically approved and therefore the most widely used fluorescent dye, we tested the structurally similar and cheaper alternative called IR-820. Both dyes were encapsulated into liposomes. However, due to the synthetic origin of liposomes, they can induce an immunogenic response. To address this challenge, we proposed to use erythrocyte membrane vesicles (EMVs) as “new era” nano-carriers for cyanine dyes. The optical properties of both dyes were investigated in different biological relevant media. Then, the temperature stability and photo-stability of dyes in free form and encapsulated into liposomes and EMVs were evaluated. Nano-carriers efficiently protected dyes from thermal degradation, as well as from photo-induced degradation. Finally, a hemotoxicity study revealed that EMVs seem less hemotoxic dye carriers than clinically approved liposomes. Herein, we showed that EMVs exhibit great potential as nano-carriers for dyes with improved stability and hemocompatibility without losing excellent optical properties.

PIP2 Reshapes Membranes through Asymmetric Desorption

Phosphatidylinositol-4,5-bisphosphate (PIP2) is an important signaling lipid in eukaryotic cell plasma membranes, playing an essential role in diverse cellular processes. The headgroup of PIP2 is highly negatively charged, and this lipid displays a high critical micellar concentration compared to housekeeping phospholipid analogs. Given the crucial role of PIP2, it is imperative to study its localization, interaction with proteins, and membrane-shaping properties. Biomimetic membranes have served extensively to elucidate structural and functional aspects of cell membranes including protein-lipid and lipid-lipid interactions, as well as membrane mechanics. Incorporation of PIP2 into biomimetic membranes, however, has at times resulted in discrepant findings described in the literature. With the goal to elucidate the mechanical consequences of PIP2 incorporation, we studied the desorption of PIP2 from biomimetic giant unilamellar vesicles by means of a fluorescent marker. A decrease in fluorescence intensity with the age of the vesicles suggested that PIP2 lipids were being desorbed from the outer leaflet of the membrane. To evaluate whether this desorption was asymmetric, the vesicles were systematically diluted. This resulted in an increase in the number of internally tubulated vesicles within minutes after dilution, suggesting that the desorption was asymmetric and also generated membrane curvature. By means of a saturated chain homolog of PIP2, we showed that the fast desorption of PIP2 is facilitated by presence of an arachidonic lipid tail and is possibly due to its oxidation. Through measurements of the pulling force of membrane tethers, we quantified the effect of this asymmetric desorption on the spontaneous membrane curvature. Furthermore, we found that the spontaneous curvature could be modulated by externally increasing the concentration of PIP2 micelles. Given that the local concentration of PIP2 in biological membranes is variable, spontaneous curvature generated by PIP2 may affect the formation of highly curved structures that can serve as initiators for signaling events.

Micro-Surface and -Interfacial Tensions Measured Using the Micropipette Technique: Applications in Ultrasound-Microbubbles, Oil-Recovery, Lung-Surfactants, Nanoprecipitation, and Microfluidics

This review presents a series of measurements of the surface and interfacial tensions we have been able to make using the micropipette technique. These include: equilibrium tensions at the air-water surface and oil-water interface, as well as equilibrium and dynamic adsorption of water-soluble surfactants and water-insoluble and lipids. At its essence, the micropipette technique is one of capillary-action, glass-wetting, and applied pressure. A micropipette, as a parallel or tapered shaft, is mounted horizontally in a microchamber and viewed in an inverted microscope. When filled with air or oil, and inserted into an aqueous-filled chamber, the position of the surface or interface meniscus is controlled by applied micropipette pressure. The position and hence radius of curvature of the meniscus can be moved in a controlled fashion from dimensions associated with the capillary tip (~5–10 μm), to back down the micropipette that can taper out to 450 μm. All measurements are therefore actually made at the microscale. Following the Young–Laplace equation and geometry of the capillary, the surface or interfacial tension value is simply obtained from the radius of the meniscus in the tapered pipette and the applied pressure to keep it there. Motivated by Franklin’s early experiments that demonstrated molecularity and monolayer formation, we also give a brief potted-historical perspective that includes fundamental surfactancy driven by margarine, the first use of a micropipette to circuitously measure bilayer membrane tensions and free energies of formation, and its basis for revolutionising the study and applications of membrane ion-channels in Droplet Interface Bilayers. Finally, we give five examples of where our measurements have had an impact on applications in micro-surfaces and microfluidics, including gas microbubbles for ultrasound contrast; interfacial tensions for micro-oil droplets in oil recovery; surface tensions and tensions-in-the surface for natural and synthetic lung surfactants; interfacial tension in nanoprecipitation; and micro-surface tensions in microfluidics.

Measuring bilayer surface energy and curvature in asymmetric droplet interface bilayers

For the past decade, droplet interface bilayers (DIBs) have had an increased prevalence in biomolecular and biophysical literature. However, much of the underlying physics of these platforms is poorly characterized. To further our understanding of these structures, lipid membrane tension on DIB membranes is measured by analysing the equilibrium shape of asymmetric DIBs. To this end, the morphology of DIBs is explored for the first time using confocal laser scanning fluorescence microscopy. The experimental results confirm that, in accordance with theory, the bilayer interface of a volume-asymmetric DIB is curved towards the smaller droplet and a lipid-asymmetric DIB is curved towards the droplet with the higher monolayer surface tension. Moreover, the DIB shape can be exploited to measure complex bilayer surface energies. In this study, the bilayer surface energy of DIBs composed of lipid mixtures of phosphatidylgylcerol (PG) and phosphatidylcholine are shown to increase linearly with PG concentrations up to 25%. The assumption that DIB bilayer area can be geometrically approximated as a spherical cap base is also tested, and it is discovered that the bilayer curvature is negligible for most practical symmetric or asymmetric DIB systems with respect to bilayer area.

Mechanism of Initial Stage of Pore Formation Induced by Antimicrobial Peptide Magainin 2

Antimicrobial peptide magainin 2 forms pores in lipid bilayers, a property that is considered the main cause of its bactericidal activity. Recent data suggest that tension or stretching of the inner monolayer plays an important role in magainin 2-induced pore formation in lipid bilayers. Here, to elucidate the mechanism of magainin 2-induced pore formation, we investigated the effect on pore formation of asymmetric lipid distribution in two monolayers. First, we developed a method to prepare giant unilamellar vesicles (GUVs) composed of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC), and lyso-PC (LPC) in the inner monolayer and of DOPG/DOPC in the outer monolayer. We consider that in these GUVs, the lipid packing in the inner monolayer was larger than that in the outer monolayer. Next, we investigated the interaction of magainin 2 with these GUVs with an asymmetric distribution of LPC using the single GUV method, and found that the rate constant of magainin 2-induced pore formation, k_p, decreased with increasing LPC concentration in the inner monolayer. We constructed a quantitative model of magainin 2-induced pore formation, whereby the binding of magainin 2 to the outer monolayer of a GUV induces stretching of the inner monolayer, causing pore formation. A theoretical equation defining k_p as a function of magainin 2 surface concentration, X, reasonably explains the experimental relationship between k_p and X. This model quantitatively explains the effect on k_p of the LPC concentration in the inner monolayer. On the basis of these results, we discuss the mechanism of the initial stage of magainin 2-induced pore formation.

Inside-outside self-assembly of light-activated fast-release liposomes

Building additional functionality into both the membrane and the internal compartments of biocompatible liposomes by self-assembly can provide ways of enhancing colloidal stability and spatial and temporal control of contents release. An interdigitation-fusion process is used to encapsulate near infrared light absorbing copper sulfide nanoparticles in the interior compartments of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol liposomes. Once formed, the liposome membrane is modified to include lysolipids and polyethylene glycol lipids by partitioning from lysolipid and PEG-lipid micelles in solution. This results in sterically stable, thermosensitive liposomes with a permeability transition near physiological temperature that can be triggered by NIR light irradiation. Rapid changes in local concentration can be induced with spatial and temporal control using NIR laser light.

Effects of neurosteroids on a model membrane including cholesterol: A micropipette aspiration study

Amphiphilic molecules supposed to affect membrane protein activity could strongly interact also with the lipid component of the membrane itself. Neurosteroids are amphiphilic molecules that bind to plasma membrane receptors of cells in the central nervous system but their effect on membrane is still under debate. For this reason it is interesting to investigate their effects on pure lipid bilayers as model systems. Using the micropipette aspiration technique (MAT), here we studied the effects of a neurosteroid, allopregnanolone (3α,5α-tetrahydroprogesterone or Allo) and of one of its isoforms, isoallopregnanolone (3β,5α-tetrahydroprogesterone or isoAllo), on the physical properties of pure lipid bilayers composed by DOPC/bSM/chol. Allo is a well-known positive allosteric modulator of GABAA receptor activity while isoAllo acts as a non-competitive functional antagonist of Allo modulation. We found that Allo, when applied at nanomolar concentrations (50-200 nM) to a lipid bilayer model system including cholesterol, induces an increase of the lipid bilayer area and a decrease of the mechanical parameters. Conversely, isoAllo, decreases the lipid bilayer area and, when applied, at the same nanomolar concentrations, it does not affect significantly its mechanical parameters. We characterized the kinetics of Allo uptake by the lipid bilayer and we also discussed its aspects in relation to the slow kinetics of Allo gating effects on GABAA receptors. The overall results presented here show that a correlation exists between the modulation of Allo and isoAllo of GABAA receptor activity and their effects on a lipid bilayer model system containing cholesterol.

Phosphatidylserine-mediated cellular signaling

Phosphatidylserine (PS), a phospholipid with a negatively charged head group, is an important constituent of eukaryotic membranes. Rather than being a passive component of cellular membranes, PS plays an important role in a number of signaling pathways. Signaling is mediated by proteins that are recruited and/or activated by PS in one of two ways: via domains that stereospecifically recognize the head group, or by electrostatic interactions with membranes that are rich in PS and therefore display negative surface charge. Such interactions are key to both intracellular and extracellular signaling cascades. PS, exposed extracellularly, is instrumental in triggering blood clotting and also serves as an "eat me" signal for the clearance of apoptotic cells. Inside the cell, a number of pathways depend of PS; these include kinases, small GTPases and fusogenic proteins. This review will discuss the generation and distribution of PS, current methods of phospholipid visualization within live cells, as well as the current understanding of the role of PS in both extracellular and intracellular signaling events.

Interactions between non-phospholipid liposomes containing cetylpyridinium chloride and biofilms of Streptococcus mutans: modulation of the adhesion and of the biodistribution

Cetylpyridinium chloride (CPC) is a surfactant that binds strongly to bacteria and bacterial biofilms. In this study, fluorescence-based techniques were used to determine the penetration and adhesion of CPC when it was introduced in liposomes. In spite of a reduced adhesion as compared to pure CPC micelles, CPC-containing liposomes adhered significantly to the biofilms of Streptococcus mutans. In contrast, no binding was observed for liposomes that were composed of phosphatidylcholine-cholesterol. The influence of the charge of the liposome on its adhesion to biofilms was studied using cholesterol (Chol) and cholesterol sulfate (Schol). In spite of similar binding to the biofilms, positively charged CPC/Chol liposomes were located mainly in the core of the biofilm microcolonies, whereas the negatively charged CPC/Schol liposomes were mainly concentrated at their periphery. This effect may be attributed to the different availability of the CPC head group. In summary, this work demonstrates the high potential for tailoring drug nanovectors by modulating sterol selection in order to selectively target and bind biofilms.

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.

Lipids, curvature, and nano-medicine

The physical properties of the lamellar lipid-bilayer component of biological membranes are controlled by a host of thermodynamic forces leading to overall tensionless bilayers with a conspicuous lateral pressure profile and build-in curvature-stress instabilities that may be released locally or globally in terms of morphological changes. In particular, the average molecular shape and the propensity of the different lipid and protein species for forming non-lamellar and curved structures are a source of structural transitions and control of biological function. The effects of different lipids, sterols, and proteins on membrane structure are discussed and it is shown how one can take advantage of the curvature-stress modulations brought about by specific molecular agents, such as fatty acids, lysolipids, and other amphiphilic solutes, to construct intelligent drug-delivery systems that function by enzymatic triggering via curvature.Practical applications: The simple concept of lipid molecular shape and how it impacts on the structure of lipid aggregates, in particular the curvature and curvature stress in lipid bilayers and liposomes, can be exploited to construct liposome-based drug-delivery systems, e.g., for use as nano-medicine in cancer therapy. Non-lamellar-forming lysolipids and fatty acids, some of which may be designed to be prodrugs, can be created by phospholipase action in diseased tissues thereby providing for targeted drug release and proliferation of molecular entities with conical shape that break down the permeability barrier of the target cells and may hence enhance efficacy.

Elastic moderation of intrinsically applied tension in lipid membranes

Tension in lipid membranes is often controlled externally, by pulling on the boundary of the membrane or changing osmotic pressure across a curved membrane. But modifications of the tension can also be induced in an internal fashion, for instance as a byproduct of changing a membrane's electric potential or, as observed experimentally, by activity of membrane proteins. Here we develop a theory that demonstrates how such internal contributions to the tension are moderated through elastic stretching of the membrane when the membrane is initially in a low-tension floppy state.

The functional roles of poly(ethylene glycol)-lipid and lysolipid in the drug retention and release from lysolipid-containing thermosensitive liposomes in vitro and in vivo

Triggered release of liposomal contents following tumor accumulation and mild local heating is pursued as a means of improving the therapeutic index of chemotherapeutic drugs. Lysolipid-containing thermosensitive liposomes (LTSLs) are composed of dipalmitoylphosphatidylcholine (DPPC), the lysolipid monostearoylphosphatidylcholine (MSPC), and poly(ethylene glycol)-conjugated distearoylphosphatidylethanolamine (DSPE-PEG(2000)). We investigated the roles of DSPE-PEG(2000) and lysolipid in the functional performance of the LTSL-doxorubicin formulation. Varying PEG-lipid concentration (0-5 mol%) or bilayer orientation did not affect the release; however, lysolipid (0-10 mol%) had a concentration-dependent effect on drug release at 42 degrees C in vitro. Pharmacokinetics of various LTSL formulations were compared in mice with body temperature controlled at 37 degrees C. As expected, incorporation of the PEG-lipid increased doxorubicin plasma half-life; however, PEG-lipid orientation (bilayer vs. external leaflet) did not significantly improve circulation lifetime or drug retention in LTSL. Approximately 70% of lysolipid was lost within 1 h postinjection of LTSL, which could be due to interactions with the large membrane pool of the biological milieu. Considering that the present LTSL-doxorubicin formulation exhibits significant therapeutic activity when used in conjunction with mild heating, our current study provided critical insights into how the physicochemical properties of LTSL can be tailored to achieve better therapeutic activity.

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.

Formation of complex three-dimensional structures in supported lipid bilayers

Cell membranes are continually undergoing a wide range of shape transformations. Here, we demonstrate the formation of several structures in supported bilayers, including tubules, caps, and giant multivesicular structures. The key elements required for these transformations are osmotic pressure imbalances, insertion of lipids with positive curvature, and lipids whose curvature is dependent on the screening environment. With these elements, a wide variety of transformations can be achieved in the absence of protein.

Interactions of novel, nonhemolytic surfactants with phospholipid vesicles

PEG-12-acyloxystearates constitute a novel class of pharmaceutical solubilizers and are synthesized from polyethylene glycol and 12-hydroxystearic acid, which has been esterified with a second acyl chain. The hemolytic activity of these surfactants decreases drastically with increasing pendant acyloxy chain length, and surfactants with an acyloxy chain of 14 carbon atoms or more are essentially nonhemolytic. In this paper, the interactions of PEG-12-acyloxystearates (acyloxy chain lengths ranging from 8 to 16 carbon atoms) with phosphatidylcholine vesicles, used as a model system for erythrocyte membranes, were studied in search of an explanation for the large variations in hemolytic activity. Surfactant-induced alterations of membrane permeability were investigated by studying the leakage of vesicle-entrapped calcein. It was found that all of the surfactants within the series interact with the vesicle membranes and cause slow leakage at elevated surfactant concentrations, but with large variations in leakage kinetics. The initial leakage rate decreases rapidly with increasing pendant acyloxy chain length. After prolonged incubation, on the other hand, the leakage is not a simple function of acyloxy chain length. The effect of the surfactants on membrane integrity was also investigated by turbidity measurements and cryo-transmission electron microscopy. At a surfactant/lipid molar ratio of 0.4, the vesicle membranes are saturated with surfactant. When the surfactant/lipid molar ratio is further increased, the vesicle membranes are progressively solubilized into mixed micelles. The rate of this process decreases strongly with increasing acyloxy chain length. When comparing the results of the different experiments, it can be concluded that there is no membrane permeabilization below saturation of the vesicle membranes. The large variations in the kinetics suggest that several steps are involved in the mechanism of leakage induced by PEG-12-acyloxystearates and that their relative rates vary with acyloxy chain length. The slow kinetics may in part be explained by the low critical micelle concentrations (CMCs) exhibited by the surfactants. The CMCs were found to be in the range of 0.003-0.025 microM.

Surface tension induced by sphingomyelin to ceramide conversion in lipid membranes

We have investigated the effect of sphingomyelin (SM) to ceramide enzymatic conversion on lipid bilayers using Giant Unilamellar Vesicles (GUVs). Sphingomyelinase was added externally to GUVs containing various proportions of SM. In situ asymmetrical SM conversion to ceramide reduced the area of one leaflet. In the absence of equilibration of all the lipids between the two leaflets, a mismatch between the two monolayers was generated. The tension generated by this mismatch was sufficient to trigger the formation of membrane defects and total vesicle collapse at relatively low percentage of SM ( approximately 5% mol). The formation of nanometric size defects was visualised by AFM in supported bilayers. Vesicle rupture was prevented in two circumstances: (a) in GUVs containing a mixture of l(d) and l(o) domains and (b) in GUVs containing 5% lyso-phosphatidylcholine. In both cases, the accumulation of enough ceramide (at initial SM concentration of 10%) allowed the formation of ceramide-rich domains. The coupling between the two asymmetrical monolayers and the condensing effect produced by the newly formed ceramide generated a tension that could underlie the mechanism through which ceramide formation induces membrane modifications observed during the late stages of apoptosis.

Peroxidation of polyunsaturated phosphatidyl-choline lipids during electroformation

Giant unilamellar vesicles (GUVs) have been utilized both as model systems to study the physico-chemical properties of biomembranes and as host materials for investigating biological processes in microbioreactors. GUVs are commonly formed by an electroformation technique. However, there is a concern that the electric fields applied during electroformation can peroxidize lipid acyl chains, thereby altering the phospholipid composition and material properties of the synthesized vesicles. Here in this paper, we report the effect of electroformation on the extent of peroxidation of a number of polyunsaturated phosphatidyl-choline lipids (PULs). Specifically, we detected peroxidation byproducts (malonaldehydes and conjugated dienes) of the following lipids utilizing UV/Vis spectroscopy: dilinoleoyl phosphatidyl-choline (DLPC) (di-18:2 PC), dilinolenoyl phosphatidyl-choline (DNPC) (di-18:3 PC), diarachidonoyl phosphatidyl-choline (DAPC) (di-20:4 PC), and didocosaheexaenoyl phosphatidyl-choline (DHA) (di-22:6 PC). The results indicate that PC PULs lipids are prone to peroxidation, with increasing unsaturation levels leading to higher levels of peroxidation byproducts. The levels of peroxidation byproducts of DAPC were found to depend linearly on the strength of the electric field, indicating that the observed effects were due to the applied electric field. Lipid peroxidation can affect a number of important membrane properties, including domain formation and mechanical stability. Thus, alteration of the chemical composition of polyunsaturated lipids (PULs) by the electroformation technique can potentially complicate the interpretation of experimental studies that utilize GUVs composed of PULs.

Micropipet aspiration for measuring elastic properties of lipid bilayers

Micropipet aspiration of giant unilamellar vesicles can be used to determine the mechanical properties of area compressibility modulus, bending modulus, and lysis tension of lipid bilayers. In this technique, giant (approximately 25-microm diameter) single bilayered vesicles are aspirated into a pipet of inner diameter approximately 8 microm. The changes in projection length of each vesicle inside of the pipet in response to changes in aspiration (suction) pressure are used to determine mechanical moduli. The suction pressure of vesicle rupture (lysis) is used to determine the membrane tension of lysis (lysis tension). Micropipet aspiration of giant unilamellar vesicles is highly specialized, requiring custom laboratory fabrication of most components (e.g., micropipets, chambers, and manometer). Herein, methods for fabrication of each of these components and instructions for measurements are described.

Solution pH alters mechanical and electrical properties of phosphatidylcholine membranes: relation between interfacial electrostatics, intramembrane potential, and bending elasticity

Solution pH affects numerous biological processes and some biological membranes are exposed to extreme pH environments. We utilized micropipette aspiration of giant unilamellar vesicles composed of 1-stearoyl-2-oleoyl-phosphatidylcholine to characterize the effect of solution pH (2-9) on membrane mechanical properties. The elastic area compressibility modulus was unaffected between pH 3 and 9 but was reduced by approximately 30% at pH 2. Fluorescence experiments utilizing the phase-sensitive probe Laurdan confirmed gel-phase characteristics at pH 2, explaining the reduction of membrane elasticity. The membrane bending stiffness, kc, increased by approximately 40% at pH 4 and pH 9 over the control value at pH 6.5. Electrophoretic mobility measurements indicate that these changes are qualitatively consistent with theoretical models that predict the effect of membrane surface charge density and Debye length on kc, substantiating a coupling between the mechanical and interfacial electrical properties of the membrane. The effect of pH on intramembrane electrical properties was examined by studying the spectral shifts of the potentiometric probe di-8 ANEPPS. The intramembrane (dipole) potential (Psid) increased linearly as the solution pH decreased in a manner consistent with the partitioning of hydroxide ions into the membrane. However, changes in Psid did not correlate with changes in kc. These mechanical and electrical studies lead to the conclusion that the effect of pH on membrane bending stiffness results from alterations in interfacial, as opposed to intramembrane, electrostatics.

Indirect evidence of submicroscopic pores in giant unilamellar [correction of unilamelar] vesicles

Formation of pore-like structures in cell membranes could participate in exchange of matter between cell compartments and modify the lipid distribution between the leaflets of a bilayer. We present experiments on two model systems in which major lipid redistribution is attributed to few submicroscopic transient pores. The first kind of experiments consists in destabilizing the membrane of a giant unilamellar vesicle by inserting conic-shaped fluorescent lipids from the outer medium. The inserted lipids (10% of the vesicle lipids) should lead to membrane rupture if segregated on the outer leaflet. We show that a 5-nm diameter pore is sufficient to ease the stress on the membrane by redistributing the lipids. The second kind of experiments consists in forcing giant vesicles containing functionalized lipids to adhere. This adhesion leads to hemifusion (merging of the outer leaflets). In certain cases, the formation of pores in one of the vesicles is attested by contrast loss on this vesicle and redistribution of fluorescent labels between the leaflets. The kinetics of these phenomena is compatible with transient submicroscopic pores and long-lived membrane defects.

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.

Elasticity and phase behavior of DPPC membrane modulated by cholesterol, ergosterol, and ethanol

Giant vesicles formed of 1,2-dipalmitoylphosphatidylcholine (DPPC) and sterols (cholesterol or ergosterol) in water and water/ethanol solutions have been used to examine the effect of sterol composition and ethanol concentration on the area compressibility modulus (K(a)), overall mechanical behavior, vesicle morphology, and induction of lipid alkyl chain interdigitation. Our results from micropipette aspiration suggest that cholesterol and ergosterol impact the order and microstructure of the gel (L(beta)') phase DPPC membrane. At low concentration (10-15 mol%) these sterols disrupt the long-range lateral order and fluidize the membrane (K(a) approximately 300 mN/m). Then at 18 mol%, these sterols participate in the formation of a continuous cohesive liquid-ordered (L(o)) phase with a sterol-dependent membrane density (K(a) approximately 750 for DPPC/ergosterol and K(a) approximately 1100 mN/m for DPPC/cholesterol). Finally at approximately 40 mol% both cholesterol and ergosterol impart similar condensation to the membrane (K(a) approximately 1200 mN/m). Introduction of ethanol (5-25 vol%) results in drops in the magnitude of K(a), which can be substantial, and sometimes individual vesicles with lowered K(a) reveal two slopes of tension versus apparent area strain. We postulate that this behavior represents disruption of lipid-sterol intermolecular interactions and therefore the membrane becomes interdigitation prone. We find that for DPPC vesicles with sterol concentrations of 20-25 mol%, significantly more ethanol is required to induce interdigitation compared to pure DPPC vesicles; approximately 7 vol% more for ergosterol and approximately 10 vol% more for cholesterol. For lower sterol concentrations (10-15 mol%), interdigitation is offset, but by <5 vol%. These data support the idea that ergosterol and cholesterol do enhance survivability for cells exposed to high concentrations of ethanol and provide evidence that the appearance of the interdigitated (L(beta)I) phase bilayer is a major factor in the disruption of cellular activity, which typically occurs between approximately 12 and approximately 16 vol% ethanol in yeast fermentations. We summarize our findings by producing, for the first time, "elasticity/phase diagrams" over a wide range of sterol (cholesterol and ergosterol) and ethanol concentrations.

Rapid Transbilayer Movement of Ceramides in Phospholipid Vesicles and inHumanErythrocytes

The transbilayer diffusion of unlabeled ceramides with different acyl chains (C6-Cer, C10-Cer, and C16-Cer) was investigated in giant unilamellar vesicles (GUVs) and in human erythrocytes. Incorporation of a very small percentage of ceramides (approximately 0.1% of total lipids) to the external leaflet of egg phosphatidylcholine GUVs suffices to trigger a shape change from prolate to pear shape vesicle. By observing the reversibility of this shape change the transmembrane diffusion of lipids was inferred. We found a half-time for unlabeled ceramide flip-flop below 1 min at 37 degrees C. The rapid diffusion of ceramides in a phosphatidylcholine bilayer was confirmed by flip-flop experiments with a spin-labeled ceramide analogue incorporated into large unilamellar vesicles. Shape change experiments were also carried out with human erythrocytes to determine the trans-membrane diffusion of unlabeled ceramides into a biological membrane. Addition of exogenous ceramides to the external leaflet of human erythrocytes did not trigger echinocyte formation immediately as one would anticipate from an asymmetrical accumulation of new amphiphiles in the outer leaflet but only after approximately 15 min of incubation at 20 degrees C in the presence of an excess of ceramide. We interpret these data as being indicative of a rapid ceramide equilibration between both erythrocyte leaflets as indicated also by electron spin resonance spectroscopy with a spin-labeled ceramide. The late appearance of echinocytes could reveal a progressive trapping of a fraction of the ceramide molecules in the outer erythrocytes leaflet. Thus, we cannot exclude the trapping of ceramides into plasma membrane domains.

Effect of salicylate on the elasticity, bending stiffness, and strength of SOPC membranes

Salicylate is a small amphiphilic molecule which has diverse effects on membranes and membrane-mediated processes. We have utilized micropipette aspiration of giant unilamellar vesicles to determine salicylate's effects on lecithin membrane elasticity, bending rigidity, and strength. Salicylate effectively reduces the apparent area compressibility modulus and bending modulus of membranes in a dose-dependent manner at concentrations above 1 mM, but does not greatly alter the actual elastic compressibility modulus at the maximal tested concentration of 10 mM. The effect of salicylate on membrane strength was investigated using dynamic tension spectroscopy, which revealed that salicylate increases the frequency of spontaneous defect formation and lowers the energy barrier for unstable hole formation. The mechanical and dynamic tension experiments are consistent and support a picture in which salicylate disrupts membrane stability by decreasing membrane stiffness and membrane thickness. The tension-dependent partitioning of salicylate was utilized to calculate the molecular volume of salicylate in the membrane. The free energy of transfer for salicylate insertion into the membrane and the corresponding partition coefficient were also estimated, and indicated favorable salicylate-membrane interactions. The mechanical changes induced by salicylate may affect several biological processes, especially those associated with membrane curvature and permeability.

The influence of short-chain alcohols on interfacial tension, mechanical properties, area/molecule, and permeability of fluid lipid bilayers

We used micropipette aspiration to directly measure the area compressibility modulus, bending modulus, lysis tension, lysis strain, and area expansion of fluid phase 1-stearoyl, 2-oleoyl phosphatidylcholine (SOPC) lipid bilayers exposed to aqueous solutions of short-chain alcohols at alcohol concentrations ranging from 0.1 to 9.8 M. The order of effectiveness in decreasing mechanical properties and increasing area per molecule was butanol>propanol>ethanol>methanol, although the lysis strain was invariant to alcohol chain-length. Quantitatively, the trend in area compressibility modulus follows Traube's rule of interfacial tension reduction, i.e., for each additional alcohol CH(2) group, the concentration required to reach the same area compressibility modulus was reduced roughly by a factor of 3. We convert our area compressibility data into interfacial tension values to: confirm that Traube's rule is followed for bilayers; show that alcohols decrease the interfacial tension of bilayer-water interfaces less effectively than oil-water interfaces; determine the partition coefficients and standard Gibbs adsorption energy per CH(2) group for adsorption of alcohol into the lipid headgroup region; and predict the increase in area per headgroup as well as the critical radius and line tension of a membrane pore for each concentration and chain-length of alcohol. The area expansion predictions were confirmed by direct measurements of the area expansion of vesicles exposed to flowing alcohol solutions. These measurements were fitted to a membrane kinetic model to find membrane permeability coefficients of short-chain alcohols. Taken together, the evidence presented here supports a view that alcohol partitioning into the bilayer headgroup region, with enhanced partitioning as the chain-length of the alcohol increases, results in chain-length-dependent interfacial tension reduction with concomitant chain-length-dependent reduction in mechanical moduli and membrane thickness.

Shape changes and vesicle fission of giant unilamellar vesicles of liquid-ordered phase membrane induced by lysophosphatidylcholine

Liquid-ordered phase (lo phase) of lipid membranes has properties that are intermediate between those of liquid-crystalline phase and those of gel phase and has attracted much attention in both biological and biophysical aspects. Rafts in the lo phase in biomembranes play important roles in cell function of mammalian cells such as signal transduction. In this report, we have prepared giant unilamellar vesicles (GUVs) of lipid membranes in the lo phase and investigated their physical properties using phase-contrast microscopy and fluorescence microscopy. GUVs of dipalmitoyl-phosphatidylcholine (DPPC)/cholesterol membranes and also GUVs of sphingomyelin (SM)/cholesterol membranes in the lo phase in water were formed at 20-37 degrees C successfully, when these membranes contained >/=30 mol % cholesterol. The diameters of GUVs of DPPC/cholesterol and SM/cholesterol membranes did not change from 50 to 28 degrees C, supporting that the membranes of these GUVs were in the lo phase. To elucidate the interaction of a substance with a long hydrocarbon chain with the lo phase membrane, we investigated the interaction of low concentrations (less than critical micelle concentration) of lysophosphatidylcholine (lyso-PC) with DPPC/cholesterol GUVs and SM/cholesterol GUVs in the lo phase. We found that lyso-PC induced several shape changes and vesicle fission of these GUVs above their threshold concentrations in water. The analysis of these shape changes indicates that lyso-PC can be partitioned into the external monolayer in the lo phase of the GUV from the aqueous solution. Threshold concentrations of lyso-PC in water to induce the shape changes and vesicle fission increased greatly with a decrease in chain length of lyso-PC. Thermodynamic analysis of this result indicates that shape changes and vesicle fission occur at threshold concentrations of lyso-PC in the membrane. These new findings on GUVs of the lo phase membranes indicate that substances with a long hydrocarbon chain such as lyso-PC can enter into the lo phase membrane and also the raft in the cell membrane. We have also proposed a mechanism for the lyso-PC-induced vesicle fission of GUVs.

Phospholipase A2 promotes raft budding and fission from giant liposomes

Cellular processes involving membrane vesiculation are related to cellular transport and membrane components trafficking. Endocytosis, formation of caveolae and caveosomes, as well as Golgi membranes traffic have been linked to the existence and dynamics of particular types of lipid/protein membrane domains, enriched in sphingolipids and cholesterol, called rafts [Nature 387 (1997) 569; Trends Cell Biol. 12 (2002) 296; Biochemistry 27 (1988) 6197]. In addition, the participation of phospholipases in the vesiculation of Golgi and other membranes has been already established [Traffic 1 (2000) 504] essentially in their role in the production of second messenger molecules. In this work we illustrate with raft-containing giant lipid vesicles a mechanism for raft-vesicle expulsion from the membrane due to the activity of a single enzyme-phospholipase A(2) (PLA(2)). This leads to the hypothesis that the PLA(2), apart from its role in second messenger generation, might play a direct and general role in the vesiculation processes underlying the intermembrane transport of rafts through purely physicochemical mechanisms. These mechanisms would be: enzyme adsorption leading to membrane curvature generation (budding), and enzyme activity modulation of the line tension at the raft boundaries, which induces vesicle fission.

Calcium modulates the mechanical properties of anionic phospholipid membranes

Using micropipette aspiration and fluorescence techniques, we have studied the material properties of charged lipid vesicles in calcium solutions. Vesicles were composed of phosphatidylglycerol (PG)/phosphatidylcholine (PC) or phosphatidic acid (PA)/PC mixtures. For the case of PG/PC membranes, we measure no effect of anionic lipid fraction on elasticity but a monotonic decrease up to 20% for tension required to induce membrane failure. Both of these observations are rationalized by a model we have developed to describe membrane electrostatic interactions in a two-component salt solution and the resulting changes in membrane properties. Critical tensions measured for PA/PC membranes, on the other hand, did not depend on anionic lipid fraction and were uniformly approximately 35% lower than PG/PC vesicles. This is likely due to a lateral phase separation in the membrane. By combining mechanical properties with fluorescence observations we propose that the PA-rich phase separates into small unconnected domains.

Material studies of lipid vesicles in the L(alpha) and L(alpha)-gel coexistence regimes

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L(alpha)) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L(alpha) and gel phases. We show that the elastic modulus, K, and critical tension, tau(c), of L(alpha) vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15 degrees C to 45 degrees C, we measure decreases in both K and tau(c) of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L(alpha) vesicles including critical tensions that are 20-40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.

Secreted phospholipase A(2) as a new enzymatic trigger mechanism for localised liposomal drug release and absorption in diseased tissue

Polymer-coated liposomes can act as versatile drug-delivery systems due to long vascular circulation time and passive targeting by leaky blood vessels in diseased tissue. We present an experimental model system illustrating a new principle for improved and programmable drug-delivery, which takes advantage of an elevated activity of secretory phospholipase A(2) (PLA(2)) at the diseased target tissue. The secretory PLA(2) hydrolyses a lipid-based proenhancer in the carrier liposome, producing lyso-phospholipids and free fatty acids, which are shown in a synergistic way to lead to enhanced liposome destabilization and drug release at the same time as the permeability of the target membrane is enhanced. Moreover, the proposed system can be made thermosensitive and offers a rational way for developing smart liposome-based drug delivery systems. This can be achieved by incorporating specific lipid-based proenhancers or prodestabilisers into the liposome carrier, which automatically becomes activated by PLA(2) only at the diseased target sites, such as inflamed or cancerous tissue.

Controlling and measuring local composition and properties in lipid bilayer membranes

Local composition, structure, morphology, and phase are interrelated in lipid bilayer membranes. This gives us the opportunity to control one or more of these properties by manipulating others. We investigate theserelationships with combinations of simultaneous two-color widefield fluorescence imaging, three-dimensional rendering of vesicle domains, andmanipulation of the vesicle morphology via optical trapping and micropipetteaspiration. We describe methods to modulate, to measure, and to probe thelocal structure of model membranes through control of membrane curvature inliposomes.