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

Altering model cell membranes by means of localized magnetic heating

Isolated iron oxide magnetic nanoparticles (MNPs), 12 nm in diameter, coated with oleic acid molecules as capping agents have been deposited by the Langmuir-Blodgett (LB) method onto a model cell membrane incorporating 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and Cholesterol (Chol) in the 1:1 ratio, which was also fabricated by the LB technique. Atomic Force Microscopy (AFM) experiments showed that the application of an alternating magnetic field results in the embedding of the MNPs through the phospholipidic layer. These experimental results reveal that the heating of individual MNPs may induce a local increase in the fluidity of the film with a large control of the spatial and temporal specificity.

Ultra-high capacity microfluidic trapping of giant vesicles for high-throughput membrane studies

Biomimetic systems such as model lipid membranes are vital to many research fields including synthetic biology, drug discovery and membrane biophysics. One of the most commonly used are giant unilamellar vesicles (GUVs) due to their size similarity with biological cells and their ease of production. Typical methods for handling such delicate objects are low-throughput and do not allow solution exchange or long-term observations, all of which limits the experimental options. Herein, we present a new device designed to confine large assemblies of GUVs in microfluidic traps but is still able to perform precise and fast solution exchanges. An optimised design allows efficient filling with as many as 114 GUVs per trap and over 23 000 GUVs per device. This allows high-throughput dataset acquisitions which we demonstrate with two proof-of-concept experiments: (i) end-point measurements of vesicle interior pH and (ii) membrane transport kinetics. Moreover, we show that the design is able to selectively trap sub-populations of specific vesicle sizes and assemble them in different layers. The device can easily be applied to other high-throughput membrane studies and will pave the way for future applications using vesicle assemblies to model cellular tissues or even prototissues.

Lipid Unsaturation Properties Govern the Sensitivity of Membranes to Photoinduced Oxidative Stress

Unsaturated lipid oxidation is a fundamental process involved in different aspects of cellular bioenergetics; dysregulation of lipid oxidation is often associated with cell aging and death. To study how lipid oxidation affects membrane biophysics, we used a chlorin photosensitizer to oxidize vesicles of various lipid compositions and degrees of unsaturation in a controlled manner. We observed different shape transitions that can be interpreted as an increase in the area of the targeted membrane followed by a decrease. These area modifications induced by the chemical modification of the membrane upon oxidation were followed in situ by Raman tweezers microspectroscopy. We found that the membrane area increase corresponds to the lipids' peroxidation and is initiated by the delocalization of the targeted double bonds in the tails of the lipids. The subsequent decrease of membrane area can be explained by the formation of cleaved secondary products. As a result of these area changes, we observe vesicle permeabilization after a time lag that is characterized in relation with the level of unsaturation. The evolution of photosensitized vesicle radius was measured and yields an estimation of the mechanical changes of the membrane over oxidation time. The membrane is both weakened and permeabilized by the oxidation. Interestingly, the effect of unsaturation level on the dynamics of vesicles undergoing photooxidation is not trivial and thus carefully discussed. Our findings shed light on the fundamental dynamic mechanisms underlying the oxidation of lipid membranes and highlight the role of unsaturations on their physical and chemical properties.

Comparison of the Effects of Daptomycin on Bacterial and Model Membranes

Daptomycin is a phosphatidylglycerol specific, calcium-dependent membrane-active antibiotic that has been approved for the treatment of Gram-positive infections. A recent Bacillus subtilis study found that daptomycin clustered into fluid lipid domains of bacterial membranes and the membrane binding was correlated with dislocation of peripheral membrane proteins and depolarization of membrane potential. In particular, the study disproved the existence of daptomycin ion channels. Our purpose here is to study how daptomycin interacts with lipid bilayers to understand the observed phenomena on bacterial membranes. We performed new types of experiments using aspirated giant vesicles with an ion leakage indicator, making comparisons between daptomycin and ionomycin, performing vesicle-vesicle transfers, and measuring daptomycin binding to fluid phase versus gel phase bilayers and bilayers including cholesterol. Our findings are entirely consistent with the observations for bacterial membranes. In addition, daptomycin is found to cause ion leakage through the membrane only if its concentration in the membrane is over a certain threshold. The ion leakage caused by daptomycin is transient. It occurs only when daptomycin binds the membrane for the first time; afterward, they cease to induce ion leakage. The ion leakage effect of daptomycin cannot be transferred from one membrane to another. The level of membrane binding of daptomycin is reduced in the gel phase versus the fluid phase. Cholesterol also weakens the membrane binding of daptomycin. The combination of membrane concentration threshold and differential binding is significant. This could be a reason why daptomycin discriminates between eukaryotic and prokaryotic cell membranes.

Reversible Monolayer-Bilayer Transition in Supported Phospholipid LB Films under the Presence of Water: Morphological and Nanomechanical Behavior

Mixed monolayer Langmuir-Blodgett (LB) films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol (Chol) in the 1:1 ratio have been prepared onto solid mica substrates. Upon immersion in water or in an aqueous HEPES solution (pH 7.4) the monolayer LB films were spontaneously converted into well-organized bilayers leaving free mica areas. The process has been demonstrated to be reversible upon removal of the aqueous solution, resulting in remarkably free of defects monolayers that are homogeneously distributed onto the mica. In addition, the nanomechanical properties exhibited by the as-formed bilayers have been determined by means of AFM breakthrough force studies. The bilayers formed by immersion of the monolayer in an aqueous media exhibit nanomechanical properties and stability under compression analogous to those of DPPC:Chol supported bilayers obtained by other methods previously described in the literature. Consequently, the hydration of a monolayer LB film has been revealed as an easy method to produce well-ordered bilayers that mimic the cell membrane and that could be used as model cell membranes.

Action of Antimicrobial Peptides on Bacterial and Lipid Membranes: A Direct Comparison

The bacterial membrane represents an attractive target for the design of new antibiotics to combat widespread bacterial resistance. Understanding how antimicrobial peptides (AMPs) and other membrane-active agents attack membranes could facilitate the design of new, effective antimicrobials. Despite intense study of AMPs on model membranes, we do not know how well the mechanism of attack translates to real biological membranes. To that end, we have characterized the attack of AMPs on Escherichia coli cytoplasmic membranes and directly compared this action to model membranes. AMPs induce membrane permeability in E. coli spheroplasts or giant unilamellar vesicles (GUVs) under well-defined concentrations of AMPs and fluorescent molecules. The action of AMPs on spheroplasts is unique in producing an intracellular fluorescence intensity time curve that increases in a sigmoidal fashion to a steady state. This regular pattern is reproducible by melittin, LL37, and alamethicin but not by CCCP or daptomycin, agents known to cause ion leakage. Remarkably, a similar pattern was also reproduced in GUVs. Indeed the steady-state membrane permeability induced by AMPs is quantitatively the same in spheroplasts and GUVs. There are, however, interesting dissimilarities in details that reveal differences between bacterial and lipid membranes. Spheroplast membranes are permeabilized by a wide range of AMP concentrations to the same steady-state membrane permeability. In contrast, only a narrow range of AMP concentrations permeabilized GUVs to a steady state. Tension in GUVs also influences the action of AMPs, whereas the spheroplast membranes are tensionless. Despite these differences, our results provide a strong support for using model membranes to study the molecular interactions of AMPs with bacterial membranes. As far as we know, this is the first time the actions of AMPs, on bacterial membranes and on model membranes, have been directly and quantitatively compared.

Adhesion and merging of lipid bilayers: a method for measuring the free energy of adhesion and hemifusion

Lipid bilayers can be induced to adhere to each other by molecular mediators, and, depending on the lipid composition, such adhesion can lead to merging of the contacting monolayers in a process known as hemifusion. Such bilayer-bilayer reactions have never been systematically studied. In the course of our studies of membrane-active molecules, we encountered such reactions. We believe that they need to be understood whenever bilayer-bilayer interactions take place, such as during membrane fusion. For illustration, we discuss three examples: spontaneous adhesion between phospholipid bilayers induced by low pH, polymer-induced osmotic depletion attraction between lipid bilayers, and anionic lipid bilayers cross-bridged by multicationic peptides. Our purpose here is to describe a general method for studying such interactions. We used giant unilamellar vesicles, each of which was aspirated in a micropipette so that we could monitor the tension of the membrane and the membrane area changes during the bilayer-bilayer interaction. We devised a general method for measuring the free energy of adhesion or hemifusion. The results show that the energies of adhesion or hemifusion of lipid bilayers could vary over 2 orders of magnitude from -1 to -50 × 10(-5) J/m(2) in these examples alone. Our method can be used to measure the energy of transition in each step of lipid transformation during membrane fusion. This is relevant for current research on membrane fusion, which focuses on how fusion proteins induce lipid transformations.

Asymmetric oxidation of giant vesicles triggers curvature-associated shape transition and permeabilization

Oxidation of unsaturated lipids is a fundamental process involved in cell bioenergetics as well as in cell death. Using giant unilamellar vesicles and a chlorin photosensitizer, we asymmetrically oxidized the outer or inner monolayers of lipid membranes. We observed different shape transitions such as oblate to prolate and budding, which are typical of membrane curvature modifications. The asymmetry of the shape transitions is in accordance with a lowered effective spontaneous curvature of the leaflet being targeted. We interpret this effect as a decrease in the preferred area of the targeted leaflet compared to the other, due to the secondary products of oxidation (cleaved-lipids). Permeabilization of giant vesicles by light-induced oxidation is observed after a lag and is characterized in relation with the photosensitizer concentration. We interpret permeabilization as the opening of a pore above a critical membrane tension, resulting from the budding of vesicles. The evolution of photosensitized giant vesicle lysis tension was measured and yields an estimation of the effective spontaneous curvature at lysis. Additionally photo-oxidation was shown to be fusogenic.

Biophysical interactions with model lipid membranes: applications in drug discovery and drug delivery

The transport of drugs or drug delivery systems across the cell membrane is a complex biological process, often difficult to understand because of its dynamic nature. In this regard, model lipid membranes, which mimic many aspects of cell-membrane lipids, have been very useful in helping investigators to discern the roles of lipids in cellular interactions. One can use drug-lipid interactions to predict pharmacokinetic properties of drugs, such as their transport, biodistribution, accumulation, and hence efficacy. These interactions can also be used to study the mechanisms of transport, based on the structure and hydrophilicity/hydrophobicity of drug molecules. In recent years, model lipid membranes have also been explored to understand their mechanisms of interactions with peptides, polymers, and nanocarriers. These interaction studies can be used to design and develop efficient drug delivery systems. Changes in the lipid composition of cells and tissue in certain disease conditions may alter biophysical interactions, which could be explored to develop target-specific drugs and drug delivery systems. In this review, we discuss different model membranes, drug-lipid interactions and their significance, studies of model membrane interactions with nanocarriers, and how biophysical interaction studies with lipid model membranes could play an important role in drug discovery and drug delivery.

Interaction of tea catechin (-)-epigallocatechin gallate with lipid bilayers

A major component of green tea extracts, catechin (-)-Epigallocatechin gallate (EGCg), has been reported to be biologically active and interacting with membranes. A recent study reported drastic effects of EGCg on giant unilamellar vesicles (GUVs). In particular, EGCg above 30 microM caused GUVs to burst. Here we investigated the effect of EGCg on single GUVs at lower concentrations, believing that its molecular mechanism would be more clearly revealed. We used the micropipette aspiration method, by which the changes of surface area and volume of a GUV could be measured as a result of interaction with EGCg. We also used x-ray diffraction to measure the membrane thinning effect by EGCg. To understand the property of EGCg, we compared its effect with other membrane-active molecules, including pore-forming peptide magainin, the turmeric (curry) extract curcumin, and detergent Triton X100. We found the effect of EGCg somewhat unique. Although EGCg readily binds to lipid bilayers, its membrane area expansion effect is one order of magnitude smaller than curcumin. EGCg also solubilizes lipid molecules from lipid bilayers without forming pores, but its effect is different from that of Triton X100.

Flexible macromolecules attached to lipid bilayers: impact on fluidity, curvature, permeability and stability of the membranes

This review summarizes recent investigations on the association of macromolecules on lipid bilayers. Hydrophilic and flexible polymers can form soft coronae tenuously adsorbed or anchored on the lipid membrane. Other synthetic macromolecules are embedded in the apolar region of the membrane. Recent experimental and theoretical works focus on the perturbation of lipid properties achieved depending on the nature and strength of binding. Of importance to biomimicry, to tethered model membranes, and drug carriers, the effects achievable include modulation of the lateral diffusivity of lipids, shape distortions, lateral segregations, formation of well-defined nanopores and ultimately the stimuli responsive disruption of the membrane.

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.

Transition from long- to short-lived transient pores in giant vesicles in an aqueous medium

We have observed large pores in the membrane of giant vesicles in an aqueous medium. The lifetime of the pores can reach 2 min and their size (a few micrometers) enables their visualization by fluorescence microscopy. These pores are obtained thanks to a destabilization of the membrane due to the synergistic action of a cone-shaped and nitrobenzodiazole (NBD) labeled phospholipid illuminated in the presence of dithionite. The opening of the pore occurs immediately after illumination starts so that it can be accurately triggered. A concomitant decrease of the vesicle radius is observed; we interpret it as a solubilization of the membrane. Depending on the rate of this solubilization, long- or short-lived pores were observed. At the transition between both regimes for a 30 microm vesicle, the solubilization rate was about 1/300 s{-1} . In order to interpret these observations, we have revisited the current model of pore opening to take into account this solubilization. This proposed model along with simulations enables us to prove that solubilization explains why the large long-lived pores are observed even in an aqueous medium. The model also predicts the solubilization rate at the transition between a single long-lived pore and a cascade of short-lived pores.

Electric pulses induce cylindrical deformations on giant vesicles in salt solutions

In this article, we report for the first time unusual shape changes of vesicles subjected to strong electric pulses in salt solutions of low concentration. The electric field is created by two parallel electrodes between which the vesicle solution is located. Surprisingly, the vesicles assume cylindrical shapes during the pulse. These deformations are short-lived (their lifetime is approximately 1 ms) and occur only in the presence of salt outside the vesicles, irrespective of their content. When the solution conductivities inside and outside are the same, vesicles with square cross section are observed. Using a fast digital camera, we were able to record these deformations and study the vesicle shape dynamics. The aims of this article are to report the new vesicle morphologies and their dynamics and to provoke theoretical work in this direction.