DNA-induced endocytosis upon local microinjection to giant unilamellar cationic vesicles

Generation of interconnected vesicles in a liposomal cell model

We introduce an experimental method based upon a glass micropipette microinjection technique for generating a multitude of interconnected vesicles (IVs) in the interior of a single giant unilamellar phospholipid vesicle (GUV) serving as a cell model system. The GUV membrane, consisting of a mixture of soybean polar lipid extract and anionic phosphatidylserine, is adhered to a multilamellar lipid vesicle that functions as a lipid reservoir. Continuous IV formation was achieved by bringing a micropipette in direct contact with the outer GUV surface and subjecting it to a localized stream of a Ca²⁺ solution from the micropipette tip. IVs are rapidly and sequentially generated and inserted into the GUV interior and encapsulate portions of the micropipette fluid content. The IVs remain connected to the GUV membrane and are interlinked by short lipid nanotubes and resemble beads on a string. The vesicle chain-growth from the GUV membrane is maintained for as long as there is the supply of membrane material and Ca²⁺ solution, and the size of the individual IVs is controlled by the diameter of the micropipette tip. We also demonstrate that the IVs can be co-loaded with high concentrations of neurotransmitter and protein molecules and displaying a steep calcium ion concentration gradient across the membrane. These characteristics are analogous to native secretory vesicles and could, therefore, serve as a model system for studying secretory mechanisms in biological systems.

Inward Budding and Endocytosis of Membranes Regulated by de Novo Designed Peptides

Protein-mediated endocytosis of membrane is a key event in biological system. The mechanism, however, is still not clear. Using a de novo designed bola-type peptide KKKLLLLLLLLKKK (K₃L₈K₃) as a protein mimic, we studied how it induced giant unilamellar vesicle (GUV) to form inward buds or endocytosis at varying conditions. Results show that the inward budding is initiated as the charged lipids are neutralized by K₃L₈K₃, which results in a negative spontaneous curvature. If the charged lipids have unsaturated tails, the buddings are slim fibrils, which can further wrap into a spherical structure. In the case of saturated charged lipids, the buddings are rigid tubules, stable in the studied time period. The unsaturated lipid to saturated lipid ratio in the mother membrane is another key parameter governing the shape and dynamics of the buds. A complete endocytosis is observed when K₃L₈K₃ is attached with a hydrophobic moiety, suggesting that hydrophobic interaction helps the buds to detach from the mother membrane. The molecules in the surrounding medium, such as negatively charged oligonucleotides, are engulfed into the GUV via endocytosis pathway induced by K₃L₈K₃. Our study provides a novel strategy for illustrating the endocytosis mechanism by using peptides of simple sequence.

pH sensing by lipids in membranes: The fundamentals of pH-driven migration, polarization and deformations of lipid bilayer assemblies

Most biological molecules contain acido-basic groups that modulate their structure and interactions. A consequence is that pH gradients, local heterogeneities and dynamic variations are used by cells and organisms to drive or regulate specific biological functions including energetic metabolism, vesicular traffic, migration and spatial patterning of tissues in development. While the direct or regulatory role of pH in protein function is well documented, the role of hydrogen and hydroxyl ions in modulating the properties of lipid assemblies such as bilayer membranes is only beginning to be understood. Here, we review approaches using artificial lipid vesicles that have been instrumental in providing an understanding of the influence of pH gradients and local variations on membrane vectorial motional processes: migration, membrane curvature effects promoting global or local deformations, crowding generation by segregative polarization processes. In the case of pH induced local deformations, an extensive theoretical framework is given and an application to a specific biological issue, namely the structure and stability of mitochondrial cristae, is described. This article is part of a Special Issue entitled: Emergence of Complex Behavior in Biomembranes edited by Marjorie Longo.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

Production of Isolated Giant Unilamellar Vesicles under High Salt Concentrations

The cell membrane forms a dynamic and complex barrier between the living cell and its environment. However, its in vivo studies are difficult because it consists of a high variety of lipids and proteins and is continuously reorganized by the cell. Therefore, membrane model systems with precisely controlled composition are used to investigate fundamental interactions of membrane components under well-defined conditions. Giant unilamellar vesicles (GUVs) offer a powerful model system for the cell membrane, but many previous studies have been performed in unphysiologically low ionic strength solutions which might lead to altered membrane properties, protein stability and lipid-protein interaction. In the present work, we give an overview of the existing methods for GUV production and present our efforts on forming single, free floating vesicles up to several tens of μm in diameter and at high yield in various buffer solutions with physiological ionic strength and pH.

A recursive vesicle-based model protocell with a primitive model cell cycle

Self-organized lipid structures (protocells) have been proposed as an intermediate between nonliving material and cellular life. Synthetic production of model protocells can demonstrate the potential processes by which living cells first arose. While we have previously described a giant vesicle (GV)-based model protocell in which amplification of DNA was linked to self-reproduction, the ability of a protocell to recursively self-proliferate for multiple generations has not been demonstrated. Here we show that newborn daughter GVs can be restored to the status of their parental GVs by pH-induced vesicular fusion of daughter GVs with conveyer GVs filled with depleted substrates. We describe a primitive model cell cycle comprising four discrete phases (ingestion, replication, maturity and division), each of which is selectively activated by a specific external stimulus. The production of recursive self-proliferating model protocells represents a step towards eventual production of model protocells that are able to mimic evolution.

Optical stretching as a tool to investigate the mechanical properties of lipid bilayers

Measurements of lipid bilayer bending modulus by various techniques produce widely divergent results. We attempt to resolve some of this ambiguity by measuring bending modulus in a system that can rapidly process large numbers of samples, yielding population statistics. This system is based on optical stretching of giant unilamellar vesicles (GUVs) in a microfluidic dual-beam optical trap (DBOT). The microfluidic DBOT system is used here to measure three populations of GUVs with distinct lipid compositions. We find that gel-phase membranes are significantly stiffer than liquid-phase membranes, consistent with previous reports. We also find that the addition of cholesterol does not alter the bending modulus of membranes composed of a monounsaturated phospholipid.

Membrane properties revealed by spatiotemporal response to a local inhomogeneity

We study theoretically the spatiotemporal response of a lipid membrane submitted to a local chemical change of its environment, taking into account the time-dependent profile of the reagent concentration due to diffusion in the solution above the membrane. We show that the effect of the evolution of the reagent concentration profile becomes negligible after some time. It then becomes possible to extract interesting properties of the membrane response to the chemical modification. We find that a local density asymmetry between the two monolayers relaxes by spreading diffusively in the whole membrane. This behavior is driven by intermonolayer friction. Moreover, we show how the ratio of the spontaneous curvature change to the equilibrium density change induced by the chemical modification can be extracted from the dynamics of the local membrane deformation. Such information cannot be obtained by analyzing the equilibrium vesicle shapes that exist in different membrane environments in light of the area-difference elasticity model.

Towards the mechanisms for efficient gene transfer into cells and tissues by means of cell electroporation

INTRODUCTION

Intracellular gene electrotransfer by means of electroporation has been on the increase during the past decade. Significant progress has been achieved both in characterizing mechanisms of gene electrotransfer and in optimizing the protocol in many preclinical trials. Recently this has led to initiation of clinical trials of gene electrotransfer to treat metastatic melanomas. Further progress with the method in various clinical trials requires better understanding of mechanisms of gene electrotransfer.

AREAS COVERED

A summary of recent progress in understanding mechanisms of gene electrotransfer, imparting general knowledge of cell electroporation and intracellular molecule electrotransfer.

EXPERT OPINION

Gene electrotransfer into cells and tissues is a complex process involving multiple steps that lead to plasmid DNA passage from the extracellular region to the cell nucleus crossing the barriers of the plasma membrane, cytoplasm and nucleus membrane. Electrical parameters of pulses used for gene electrotransfer affect the initial steps of DNA translocation through the plasma membrane and play a crucial role in determining the transfection efficiency. When considering gene electrotransfer into tissues it becomes clear that other nonelectrical conditions are also of primary importance.

Natural and artificial ion channels for biosensing platforms

The single-molecule selectivity and specificity of the binding process together with the expected intrinsic gain factor obtained when utilizing flow through a channel have attracted the attention of analytical chemists for two decades. Sensitive and selective ion channel biosensors for high-throughput screening are having an increasing impact on modern medical care, drug screening, environmental monitoring, food safety, and biowarefare control. Even virus antigens can be detected by ion channel biosensors. The study of ion channels and other transmembrane proteins is expected to lead to the development of new medications and therapies for a wide range of illnesses. From the first attempts to use membrane proteins as the receptive part of a sensor, ion channels have been engineered as chemical sensors. Several other types of peptidic or nonpeptidic channels have been investigated. Various gating mechanisms have been implemented in their pores. Three technical problems had to be solved to achieve practical biosensors based on ion channels: the fabrication of stable lipid bilayer membranes, the incorporation of a receptor into such a structure, and the marriage of the modified membrane to a transducer. The current status of these three areas of research, together with typical applications of ion-channel biosensors, are discussed in this review.

Artificial membrane-like environments for in vitro studies of purified G-protein coupled receptors

Functional reconstitution of transmembrane proteins remains a significant barrier to their biochemical, biophysical, and structural characterization. Studies of seven-transmembrane G-protein coupled receptors (GPCRs) in vitro are particularly challenging because, ideally, they require access to the receptor on both sides of the membrane as well as within the plane of the membrane. However, understanding the structure and function of these receptors at the molecular level within a native-like environment will have a large impact both on basic knowledge of cell signaling and on pharmacological research. The goal of this article is to review the main classes of membrane mimics that have been, or could be, used for functional reconstitution of GPCRs. These include the use of micelles, bicelles, lipid vesicles, nanodiscs, lipidic cubic phases, and planar lipid membranes. Each of these approaches is evaluated with respect to its fundamental advantages and limitations and its applications in the field of GPCR research. This article is part of a Special Issue entitled: Membrane protein structure and function.

Dynamical membrane curvature instability controlled by intermonolayer friction

We study a dynamical curvature instability caused by a local chemical modification of a phospholipid membrane. In our experiments, a basic solution is microinjected close to a giant unilamellar vesicle, which induces a local chemical modification of some lipids in the external monolayer of the membrane. This modification causes a local deformation of the vesicle, which then relaxes. We present a theoretical description of this instability, taking into account both the change of the equilibrium lipid density and the change of the spontaneous membrane curvature induced by the chemical modification. We show that these two types of changes of the membrane properties yield different dynamics. In contrast, it is impossible to distinguish them when studying the equilibrium shape of a vesicle subjected to a global modification. In our model, the longest relaxation timescale is related to the intermonolayer friction, which plays an important part when there is a change in the equilibrium density in one monolayer. We compare our experimental results to the predictions of our model by fitting the measured time evolution of the deformation height to the solution of our dynamical equations. We obtain good agreement between theory and experiments. Our fits enable us to estimate the intermonolayer friction coefficient, yielding values that are consistent with previous measurements.

Mechanism of inhibition of enveloped virus membrane fusion by the antiviral drug arbidol

The broad-spectrum antiviral arbidol (Arb) inhibits cell entry of enveloped viruses by blocking viral fusion with host cell membrane. To better understand Arb mechanism of action, we investigated its interactions with phospholipids and membrane peptides. We demonstrate that Arb associates with phospholipids in the micromolar range. NMR reveals that Arb interacts with the polar head-group of phospholipid at the membrane interface. Fluorescence studies of interactions between Arb and either tryptophan derivatives or membrane peptides reconstituted into liposomes show that Arb interacts with tryptophan in the micromolar range. Interestingly, apparent binding affinities between lipids and tryptophan residues are comparable with those of Arb IC50 of the hepatitis C virus (HCV) membrane fusion. Since tryptophan residues of membrane proteins are known to bind preferentially at the membrane interface, these data suggest that Arb could increase the strength of virus glycoprotein's interactions with the membrane, due to a dual binding mode involving aromatic residues and phospholipids. The resulting complexation would inhibit the expected viral glycoprotein conformational changes required during the fusion process. Our findings pave the way towards the design of new drugs exhibiting Arb-like interfacial membrane binding properties to inhibit early steps of virus entry, i.e., attractive targets to combat viral infection.

Bax activates endophilin B1 oligomerization and lipid membrane vesiculation

Endophilins participate in membrane scission events that occur during endocytosis and intracellular organelle biogenesis through the combined activity of an N-terminal BAR domain that interacts with membranes and a C-terminal SH3 domain that mediates protein binding. Endophilin B1 (Endo B1) was identified to bind Bax, a Bcl-2 family member that promotes apoptosis, through yeast two-hybrid protein screens. Although Endo B1 does not bind Bax in healthy cells, during apoptosis, Endo B1 interacts transiently with Bax and promotes cytochrome c release from mitochondria. To explore the molecular mechanism of action of Endo B1, we have analyzed its interaction with Bax in cell-free systems. Purified recombinant Endo B1 in solution displays a Stokes radius indicating a tetrameric quarternary structure. However, when incubated with purified Bax, it assembles into oligomers more than 4-fold greater in molecular weight. Although Endo B1 oligomerization is induced by Bax, Bax does not stably associate with the high molecular weight Endo B1 complex. Endo B1 oligomerization requires its C-terminal Src homology 3 domain and is not induced by Bcl-xL. Endo B1 combined with Bax reduces the size and changes the morphology of giant unilamellar vesicles by inducing massive vesiculation of liposomes. This activity of purified Bax protein to induce cell-free assembly of Endo B1 may reflect its activity in cells that regulates apoptosis and/or mitochondrial fusion.

Rate of permeabilization of giant vesicles by amphiphilic polyacrylates compared to the adsorption of these polymers onto large vesicles and tethered lipid bilayers

We examined by fluorescence microscopy the permeabilization of giant vesicles by hydrophobically modified polyacrylates (called amphipols). Amphipols trigger permeabilization to FITC-dextran of egg-PC/DPPA vesicles with no breakage of the lipid bilayers. The polyanionic amphipols were passing through bilayers as shown by permeabilization of multilamellar vesicles. Remarkably, the vesicles were not simultaneously permeable but became leaky one after the other. Altogether, our observations suggest a random formation of pores having diameters above a few nanometers. Decreasing pH and increasing ionic strength and polymer concentration were increasing the rate of permeabilization. The rate and efficiency of permeabilization was compared to the rate and density of adsorption of amphipols onto lipid membranes (as estimated by titration calorimetry onto large unilamellar vesicles and neutron reflectivity measurements on tethered bilayers). The polymer adsorption layer is built up in a few minutes. We conclude that the rate-limiting step for permeabilization is not the adsorption from the bulk solution but relates to slow intramembrane reorganizations.

What is (still not) known of the mechanism by which electroporation mediates gene transfer and expression in cells and tissues

Cell membranes can be transiently permeabilized under application of electric pulses. This treatment allows hydrophilic therapeutic molecules, such as anticancer drugs and DNA, to enter into cells and tissues. This process, called electropermeabilization or electroporation, has been rapidly developed over the last decade to deliver genes to tissues and organs, but there is a general agreement that very little is known about what is really occurring during membrane electropermeabilization. It is well accepted that the entry of small molecules, such as anticancer drugs, occurs mostly through simple diffusion after the pulse while the entry of macromolecules, such as DNA, occurs through a multistep mechanism involving the electrophoretically driven interaction of the DNA molecule with the destabilized membrane during the pulse and then its passage across the membrane. Therefore, successful DNA electrotransfer into cells depends not only on cell permeabilization but also on the way plasmid DNA interacts with the plasma membrane and, once into the cytoplasm, migrates towards the nucleus. The focus of this review is to describe the different aspects of what is known of the mechanism of membrane permeabilization and associated gene transfer and, by doing so, what are the actual limits of the DNA delivery into cells.

Membrane deformation under local pH gradient: mimicking mitochondrial cristae dynamics

Mitochondria are cell substructures (organelles) critical for cell life, because biological fuel production, the ATP synthesis by oxidative phosphorylation, occurs in them driven by acidity (pH) gradients. Mitochondria play a key role as well in the cell death and in various fatigue and exercise intolerance syndromes. It is clear now that mitochondria present an astonishing variety of inner membrane morphologies, dynamically correlated with their functional state, coupled with the rate of the ATP synthesis, and characteristic for normal as well as for pathological cases. Our work offers some original insights into the factors that determine the dynamical tubular structures of the inner membrane cristae. We show the possibility to induce, by localized proton flow, a macroscopic cristae-like shape remodeling of an only-lipid membrane. We designed a minimal membrane system (GUV) and experimentally showed that the directional modulation of local pH gradient at membrane level of cardiolipin-containing vesicles induces dynamic cristae-like membrane invaginations. We propose a mechanism and theoretical model to explain the observed tubular membrane morphology and suggest the underlying role of cardiolipin. Our results support the hypothesis of localized bioenergetic transduction and contribute to showing the inherent capacity of cristae morphology to become self-maintaining and to optimize the ATP synthesis.

Mechanism by which electroporation mediates DNA migration and entry into cells and targeted tissues

Cell membranes can be transiently permeabilized under application of electric pulses that allow hydrophilic therapeutic molecules, such as anticancer drugs and DNA, to enter into cells and tissues. This process, called electropermeabilization or electroporation, has been rapidly developed over the last decade to deliver genes to tissues and organs, but there is a general agreement that very little is known about what is really occurring during membrane electropermeabilization. It is well accepted that the entry of small molecules, such as anticancer drugs, occurs through simple diffusion while the entry of macromolecules, such as DNA, occurs through a multistep mechanism involving the electrophoretically driven association of the DNA molecule with the destabilized membrane and then its passage across the membrane. Therefore, successful DNA electrotransfer into cells depends not only on cell permeabilization but also on the way plasmid DNA interacts with the plasma membrane and, once into the cell, migrates toward the nuclei.

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.

Interaction between a rodlike inclusion and a supported bilayer membrane

The interactions between a rodlike inclusion and a supported copolymer bilayer membrane are investigated by using the self-consistent field theory. For different system parameters, physical observables, such as the interaction free energy, entropy, and translocation energy barrier, are obtained. Particular emphasis is put on the closely energetic and entropic analyses of the interaction. It shows that the interfacial energy provides a qualitative trend and dominates the basic shape of the interaction free energy curve; the combination of chemical potential energy and total entropy contribution is responsible for the translocation energy barrier and the weak attraction in the vicinity of upper monolayer surface. We also specify the nature, height, and shape of the energy barrier to translocation. Particularly, the height is roughly proportional to the rod radius.

Visualization of membrane rafts using a perylene monoimide derivative and fluorescence lifetime imaging

A new membrane probe, based on the perylene imide chromophore, with excellent photophysical properties (high absorption coefficient, quantum yield (QY) approximately 1, high photostability) and excited in the visible domain is proposed for the study of membrane rafts. Visualization of separation between the liquid-ordered (Lo) and the liquid-disordered (Ld) phases can be achieved in artificial membranes by fluorescence lifetime imaging due to the different decay times of the membrane probe in the two phases. Rafts on micrometer-scale in cell membranes due to cellular activation can also be observed by this method. The decay time of the dye in the Lo phase is higher than in organic solvents where its QY is 1. This allows proposing a (possible general) mechanism for the decay time increase in the Lo phase, based on the local field effects of the surrounding molecules. For other fluorophores with QY<1, the suggested mechanism could also contribute, in addition to effects reducing the nonradiative decay pathways, to an increase of the fluorescence decay time in the Lo phase.

Cylindrical inclusions in a copolymer membrane

The membrane-mediated interaction between two parallel, cylindrical inclusions is investigated by using the self-consistent field theory (SCFT). The rodlike inclusions are located within the interior of the bilayer and are enveloped by two monolayers. They may exhibit one of the two basic types of behaviors involving pinching two monolayers together and swelling them outward. For different parameters, we calculate the density profile of the deformation membrane, the associated interaction free energy, as well as the conformational entropy of polymer chains. The similarity of the two types of interaction potentials is the qualitative characteristics. An energy barrier separates an attractive from a repulsive region; the repulsive region is preceded by a weak attraction at a large distance. The difference between them, which is due to the different contact environments around the rods, lies in the appearance of a small barrier at a short distance in the pinching structure. Particular emphasis is put on the closely energetic and entropic analyses of the interaction potential. We show that the chemical potential energy has provided a qualitative trend and roughly dominated the basic shape of the interaction potential; the amphiphile entropy in the swelling structure and the solvent entropy in the pinching structure, combined with the corresponding chemical potential energy, are responsible for the repulsive barrier at an intermediate distance and for the weak attraction at a large distance, respectively. The influence of inclusion hydrophobicity on the interaction potential is taken into account. In particular, the pinching and swelling structures can appear and can transform into each other in a system at intermediate hydrophobicity.

To see or not to see: Lateral organization of biological membranes and fluorescence microscopy

In the last few years several experimental strategies based on epi-, confocal and two photon excitation fluorescence microscopy techniques have been employed to study the lateral structure of membranes using giant vesicles as model systems. This review article discusses the methodological aspects of the aforementioned experimental approaches, particularly stressing the information obtained by the use of UV excited fluorescent probes using two-photon excitation fluorescence microscopy. Additionally, the advantages of utilizing visual information, to correlate the lateral structure of compositionally simple membranes with complex situations, i.e., biological membranes, will be addressed.

siRNA-containing liposomes modified with polyarginine effectively silence the targeted gene

Development of RNA interference (RNAi) technology utilizing the short interfering RNA sequences (siRNA) based ‘targeted’ therapeutics has focused on creating methods for delivering siRNAs to cells and for enhancing siRNA stability in vitro and in vivo. Here, we describe a novel approach for siRNA cellular delivery using siRNA encapsulated into liposomes additionally bearing arginine octamer (R8) molecules attached to their outer surface (R8-liposomes). The R8-liposomal human double minute gene 2 (HDM2)-siRNA demonstrated a significant stability against degradation in the blood serum (siRNA-loaded R8-liposomes remained intact even after 24-h incubation), and higher transfection efficiency into all three tested lung tumor cell lines. siRNA delivery successfully proceeds in the presence of plasma proteins, and R8-liposomes demonstrate low non-specific toxicity. The mechanism of action of R8-liposome-encapsulated siRNA is associated with the RNAi-mediated degradation of the target mRNA. siRNA in R8-liposomes effectively inhibited the targeted gene and significantly reduced the proliferation of cancer cells. The approach offers the potential for siRNA delivery for various in vitro and in vivo applications.

Electropermeabilization, a physical method for the delivery of therapeutic molecules into cells

Electropermeabilization designates the use of short high-voltage pulses to overcome the barrier of the cell membrane. A position-dependent reversible local membrane permeabilization is induced leading to an exchange of hydrophilic molecules across the membrane. This permeabilized state can be used to load cells with therapeutic molecules. In the case of small molecules, such as anticancer drugs, transfer occurs through simple diffusion. In the case of DNA, transfer occurs through a multi-step mechanism, a process that involves the electrophoretically driven association of the DNA molecule with the destabilised membrane and then its passage.

Polyelectrolyte-induced peeling of charged multilamellar vesicles

We study mixtures of charged surfactants, which alone in solution form uni- and multilamellar vesicles, and oppositely charged polyelectrolytes (PEs). The phase behavior is investigated at fixed surfactant concentration as a function of the PE-to-surfactant charge ratio, x. We find that, for x > 0, aggregates form. Light microscopy and X-ray scattering experiments show that the isoelectric point plays a crucial role, since the morphology and the microscopic structure of the aggregates are different before (x < or = 1) and after the isoelectric point (x > 1). To better understand the dynamics for the formation of PE/surfactant complexes, we perform light microscopy experiments where we follow in real time the effect of a PE solution on one multilamellar vesicle (MLV). We find that the PE induces a peeling of the bilayers of the MLV one by one. The peeling is accompanied by strong shape fluctuations of the MLV and leads ultimately to a pile of small aggregates. This novel phenomenon is analyzed in detail and discussed in terms of PE-induced tension and pore formation and growth in a surfactant bilayer.

A new method for the reconstitution of membrane proteins into giant unilamellar vesicles

In this work, we have investigated a new and general method for the reconstitution of membrane proteins into giant unilamellar vesicles (GUVs). We have analyzed systematically the reconstitution of two radically different membrane proteins, the sarcoplasmic reticulum Ca(2+)-ATPase and the H(+) pump bacteriorhodopsin. In a first step, our method involved a detergent-mediated reconstitution of solubilized membrane proteins into proteoliposomes of 0.1-0.2 microm in size. In a second step, these preformed proteoliposomes were partially dried under controlled humidity followed, in a third step, by electroswelling of the partially dried film to give GUVs. The physical characteristics of GUVs were analyzed in terms of morphology, size, and lamellarity using phase-contrast and differential interference contrast microscopy. The reconstitution process was further characterized by analyzing protein incorporation and biological activity. Both membrane proteins could be homogeneously incorporated into GUVs at lipid/protein ratios ranging from 5 to 40 (w/w). After reconstitution, both proteins retained their biological activity as demonstrated by H(+) or Ca(2+) pumping driven by bacteriorhodopsin or Ca(2+)-ATPase, respectively. This constitutes an efficient new method of reconstitution, leading to the production of large unilamellar membrane protein-containing vesicles of more than 20 microm in diameter, which should prove useful for functional and structural studies through the use of optical microscopy, optical tweezers, microelectrodes, or atomic force microscopy.

Dynamics of nonequilibrium membrane bud formation

The dynamical response of a lipid membrane to a local perturbation of its molecular symmetry is investigated theoretically. A density asymmetry between the two membrane leaflets is predominantly released by in-plane lipid diffusion or membrane curvature, depending upon the spatial extent of the perturbation. It may result in the formation of nonequilibrium structures (buds), for which a dynamical size selection is observed. A preferred size in the microm range is predicted, as a signature of the crossover between membrane and solvent dominated dynamical membrane response.

Interactions of Histone H1 with Phospholipids and Comparison of Its Binding to Giant Liposomes and Human Leukemic T Cells†

Due to its net positive charge histone H1 readily associates with liposomes containing acidic phospholipids, such as phosphatidylserine (PS). Interestingly, circular dichroism reveals that while histone H1 in aqueous solutions appears as a random coil, its binding to liposomes containing PS is associated with a pronounced increase in alpha-helicity and beta-sheet content, estimated at 7% and 24%, respectively. This interaction further results in vesicle aggregation and lipid mixing. Fluorescence microscopy revealed rapid binding of Texas Red-labeled H1 (TR-H1) to giant liposomes composed of phosphatidylcholine and PS (SOPC/brain PS, 9/1 molar ratio), followed by lateral segregation and subsequent translocation of the membrane-bound H1 into the giant liposome. The above processes in giant liposomes did depend on the presence of the negatively charged PS. Comparison of the behavior of H1 in giant liposomes to that in cultured leukemic T cells demonstrated very similar patterns. More specifically, fluorescence microscopy revealed binding of TR-H1 to the plasma membrane as lateral segregated microdomains, followed by translocation into the cell. H1 also triggered membrane blebbing and fragmentation of the nuclei of these cells, thus suggesting induction of apoptosis. Our findings indicate that histone H1 and acidic phospholipids form supramolecular aggregates in the plasma membrane of T cells, subsequently resulting in major rearrangements of cellular membranes. Our results allow us to conclude that the minimal requirement for the interaction of histone H1 with the leukemia cell plasma membrane is reproduced by giant liposomes composed of unsaturated phosphatidylcholine and phosphatidylserine, the latter being mandatory for the observed changes in the secondary structure of H1 as well as the macroscopic consequences of the H1-PS interactions.

An experimental approach for direct observation of the interaction of polyanions with sphingosine-containing giant vesicles

A new approach for direct optical microscopy observation of polyanion interactions with bilayers of giant cationic liposomes (GUVs) was suggested. Polyanions as DNA, dextran sulfate (DS), heparin (H) and polyacrylic acids (PA) were locally delivered by a micropipette to a part of a giant unilamellar vesicle membrane. The phenomena were directly observed under optical microscope. GUVs, about 100 micro m in diameter, formed of phosphatidylcholines and up to 33 mol% of the natural bioactive cationic amphiphile sphingosine (Sph), were prepared by electroformation. The effects of water-soluble molecules with high negative linear charge density as dextran sulfate (DS), heparin (H) polyacrylic acids (PA) and adenosine-5'-triphosphoric acid (ATP) were compared with those of DNAs. The resulting membrane topology transformations were monitored in phase contrast, while the DNA distribution was followed in fluorescence. DNA-induced endocytosis-like membrane morphology transformation due to the DNA/lipid membrane local interactions was observed. The DS, H and PA induced membrane topology transformations similar to those of the DNAs, while ATP did not cause any detectable ones. The endocytosis mechanism involves the formation of ordered domains in the GUV membrane where some surface and charge asymmetries between the two membrane monolayers were created. The sizes of created polyanionic/cationic membrane domains depend on the form, length and elasticity of the adsorbed highly charged molecules. Endosome-including capacities of polyanionic molecules depend heavily on the high linear negative charge at a certain length. An original method for direct studying of the DNA/membrane interactions in autoadaptable giant liposome system imitating biological membrane interactions was forwarded. The model observations could also help for understanding events associated with cationic liposome/DNA complex formation in gene transfer processes.

Observation of topical catalysis by sphingomyelinase coupled to microspheres

Sphingomyelinase, SMase (EC 3.1.4.12), was coupled onto amino-derivatized acrylate microspheres and was shown to retain its catalytic activity. The immobilized enzyme allows one to carry out topical enzymatic reaction in a controlled manner. Accordingly, these spheres were held with a micropipet and using micromanipulator brought into contact with a giant liposome membrane composed of phosphatidylcholine and sphingomyelin (SOPC/C16:0-SM, 0.75:0.25, molar ratio), representing the substrate for the immobilized enzyme. The macroscopic consequences of the enzyme reaction were visualized using fluorescence microscopy as well as differential interference contrast microscopy. The surface contact of the giant vesicle and immobilized enzyme causes membrane microdomain formation and domain clustering (capping) in the membrane and subsequent shedding of small vesicles from the membrane into the interior of the giant liposome. The method described represents a novel approach to study enzymatic reactions and allows manipulating giant vesicles as well as cultured cells in a spatially controlled manner.

Synergy between cationic lipid and co-lipid determines the macroscopic structure and transfection activity of lipoplexes

The large number of cytofectin and co-lipid combinations currently used for lipoplex-mediated gene delivery reflects the fact that the optimal cytofectin/co-lipid combination varies with the application. The effects of structural changes in both cytofectin and co-lipid were systematically examined to identify structure-activity relationships. Specifically, alkyl chain length, degree of unsaturation and the head group to which the alkyl side chain was attached were examined to determine their effect on lipoplex structure and biological activity. The macroscopic lipoplex structure was assessed using a dye-binding assay and the biological activity was examined using in vitro transfection in three diverse cell lines. Lipoplexes were formulated in three different vehicles currently in use for in vivo delivery of naked plasmid DNA (pDNA) and lipoplex formulations. The changes in dye accessibility were consistent with structural changes in the lipoplex, which correlated with alterations in the formulation. In contrast, transfection activity of different lipoplexes was cell type and vehicle dependent and did not correlate with dye accessibility. Overall, the results show a correlation between transfection and enhanced membrane fluidity in both the lipoplex and cellular membranes.

Comparison of the membrane association of two antimicrobial peptides, magainin 2 and indolicidin

Interactions of two antimicrobial peptides, magainin 2 and indolicidin, with three different model biomembranes, namely, monolayers, large unilamellar vesicles (LUVs), and giant liposomes, were studied. Insertion of both peptides into lipid monolayers was progressively enhanced when the content of an acidic phospholipid, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a film of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC) was increased. Indolicidin and magainin 2 penetrated also into lipid monolayers containing cholesterol (mole fraction, X = 0.1). Membrane association of magainin 2 attenuated lipid lateral diffusion in POPG-containing LUVs as revealed by the decrease in the excimer/monomer fluorescence ratio I(e)/I(m) for the pyrene fatty-acid-containing phospholipid derivative 1-palmitoyl-2-[10-(pyren-1-yl) decanoyl]-sn-glycero-3-phospho-rac-glycerol (PPDPG). Likewise, an increase in steady-state fluorescence anisotropy of the membrane-incorporated diphenylhexatriene (DPH) was observed, revealing magainin 2 to increase acyl chain order and induce segregation of acidic phospholipids. Similar effects were observed for indolicidin. The topological effects of magainin 2 and indolicidin on phospholipid membranes were investigated using optical microscopy of giant vesicles. Magainin 2 had essentially no influence on either SOPC or SOPC:cholesterol (X = 0.1) giant liposomes. However, effective vesiculation was observed when acidic phospholipid (X(PG) = 0.1) was included in the giant vesicles. Indolicidin caused only a minor shrinkage of giant SOPC vesicles whereas the formation of endocytotic vesicles was observed when the giant liposome contained POPG (X(PG) = 0.1). Interestingly, for indolicidin, vesiculation was also observed for giant vesicles composed of SOPC/cholesterol (X(chol) = 0.1). Possible mechanisms of membrane transformation induced by these two peptides are discussed.

Spontaneous entrapment of polynucleotides upon electrostatic interaction with ethanol-destabilized cationic liposomes

This study describes the effect of ethanol and the presence of poly(ethylene) glycol (PEG) lipids on the interaction of nucleotide-based polyelectrolytes with cationic liposomes. It is shown that preformed large unilamellar vesicles (LUVs) containing a cationic lipid and a PEG coating can be induced to entrap polynucleotides such as antisense oligonucleotides and plasmid DNA in the presence of ethanol. The interaction of the cationic liposomes with the polynucleotides leads to the formation of multilamellar liposomes ranging in size from 70 to 120 nm, only slightly bigger than the parent LUVs from which they originated. The degree of lamellarity as well as the size and polydispersity of the liposomes formed increases with increasing polynucleotide-to-lipid ratio. A direct correlation between the entrapment efficiency and the membrane-destabilizing effect of ethanol was observed. Although the morphology of the liposomes is still preserved at the ethanol concentrations used for entrapment (25-40%, v/v), entrapped low-molecular-weight solutes leak rapidly. In addition, lipids can flip-flop across the membrane and exchange rapidly between liposomes. Furthermore, there are indications that the interaction of the polynucleotides with the cationic liposomes in ethanol leads to formation of polynucleotide-cationic lipid domains, which act as adhesion points between liposomes. It is suggested that the spreading of this contact area leads to expulsion of PEG-ceramide and triggers processes that result in the formation of multilamellar systems with internalized polynucleotides. The high entrapment efficiencies achieved at high polyelectrolyte-to-lipid ratios and the small size and neutral character of these novel liposomal systems are of utility for liposomal delivery of macromolecular drugs.

A correlation between lipid domain shape and binary phospholipid mixture composition in free standing bilayers: A two-photon fluorescence microscopy study

Giant unilamellar vesicles (GUVs) composed of different phospholipid binary mixtures were studied at different temperatures, by a method combining the sectioning capability of the two-photon excitation fluorescence microscope and the partition and spectral properties of 6-dodecanoyl-2-dimethylamino-naphthalene (Laurdan) and Lissamine rhodamine B 1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (N-Rh-DPPE). We analyzed and compared fluorescence images of GUVs composed of 1,2-dilauroyl-sn-glycero-3-phosphocholine/1, 2-dipalmitoyl-sn-glycero-3-phosphocholine (DLPC/DPPC), 1, 2-dilauroyl-sn-glycero-3-phosphocholine/1, 2-distearoyl-sn-glycero-3-phosphocholine (DLPC/DSPC), 1, 2-dilauroyl-sn-glycero-3-phosphocholine/1, 2-diarachidoyl-sn-glycero-3-phosphocholine (DLPC/DAPC), 1, 2-dimyristoyl-sn-glycero-3-phosphocholine/1, 2-distearoyl-sn-glycero-3-phosphocholine (DMPC/DSPC) (1:1 mol/mol in all cases), and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine/1, 2-dimyristoyl-sn-glycero-3-phosphocholine (DMPE/DMPC) (7:3 mol/mol) at temperatures corresponding to the fluid phase and the fluid-solid phase coexistence. In addition, we studied the solid-solid temperature regime for the DMPC/DSPC and DMPE/DMPC mixtures. From the Laurdan intensity images the generalized polarization function (GP) was calculated at different temperatures to characterize the phase state of the lipid domains. We found a homogeneous fluorescence distribution in the GUV images at temperatures corresponding to the fluid region for all of the lipid mixtures. At temperatures corresponding to phase coexistence we observed concurrent fluid and solid domains in the GUVs independent of the lipid mixture. In all cases the lipid solid domains expanded and migrated around the vesicle surface as we decreased the temperature. The migration of the solid domains decreased dramatically at temperatures close to the solid-fluid-->solid phase transition. For the DLPC-containing mixtures, the solid domains showed line, quasicircular, and dendritic shapes as the difference in the hydrophobic chain length between the components of the binary mixture increases. In addition, for the saturated PC-containing mixtures, we found a linear relationship between the GP values for the fluid and solid domains and the difference between the hydrophobic chain length of the binary mixture components. Specifically, at the phase coexistence temperature region the difference in the GP values, associated with the fluid and solid domains, increases as the difference in the chain length of the binary mixture component increases. This last finding suggests that in the solid-phase domains, the local concentration of the low melting temperature phospholipid component increases as the hydrophobic mismatch decreases. At the phase coexistence temperature regime and based on the Laurdan GP data, we observe that when the hydrophobic mismatch is 8 (DLPC/DAPC), the concentration of the low melting temperature phospholipid component in the solid domains is negligible. This last observation extends to the saturated PE/PC mixtures at the phase coexistence temperature range. For the DMPC/DSPC we found that the nonfluorescent solid regions gradually disappear in the solid temperature regime of the phase diagram, suggesting lipid miscibility. This last result is in contrast with that found for DMPE/DMPC mixtures, where the solid domains remain on the GUV surface at temperatures corresponding to that of the solid region. In all cases the solid domains span the inner and outer leaflets of the membrane, suggesting a strong coupling between the inner and outer monolayers of the lipid membrane. This last finding extends previous observations of GUVs composed of DPPE/DPPC and DLPC/DPPC mixtures (, Biophys. J. 78:290-305).