Visualization of membrane loss during the shrinkage of giant vesicles under electropulsation

Synthesis Method and High Salt Concentration Can Affect Electrodeformation of GUVs under Strong Pulsed DC Fields

The study focuses on two important issues in the electrodeformation of giant unilamellar vesicles (GUVs) as biomimic objects, with regard to their electroporation. The results are presented with respect to the ratio of the conductivities of the inner and outer fluids of the GUV, (β = Λen/Λsus), and the concentration of salt in the enclosed medium (C en). In this work, low and high salt concentration regimes are referred to as C en ≤ 0.3 mM and C en ≥ 25 mM respectively. First, responses of GUVs under strong pulsed DC fields are observed to be sensitive to the synthesis methods (electroporation or gel-assisted method) for both β = 1 and β < 1 in the low salt concentration regime. This might be caused by a higher initial membrane tension (in the case of electroformed GUVs) or possibly a greater membrane edge tension (in the case of GUVs prepared by the gel-assisted method). Second, the effect of salt concentration on the electrohydrodynamic behavior of GUVs under strong pulsed DC fields indicates that the extent of poration and pore growth and, correspondingly, the shape deformation can be qualitatively different at different salt concentrations. This suggests the possibility of higher edge tension in GUVs as well as faster "electrical shorting" of the membrane due to the abundance of ions and thereby lower pore growth in high-salt GUVs. The study shows that the extrapolation of results obtained in GUV electroporation to biological cells should be done with caution.

SMARTINI3 parametrization of multi-scale membrane models via unsupervised learning methods

In this study, we utilize genetic algorithms to develop a realistic implicit solvent ultra-coarse-grained (ultra-CG) membrane model comprising only three interaction sites. The key philosophy of the ultra-CG membrane model SMARTINI3 is its compatibility with realistic membrane proteins, for example, modeled within the Martini coarse-grained (CG) model, as well as with the widely used GROMACS software for molecular simulations. Our objective is to parameterize this ultra-CG model to accurately reproduce the experimentally observed structural and thermodynamic properties of Phosphatidylcholine (PC) membranes in real units, including properties such as area per lipid, area compressibility, bending modulus, line tension, phase transition temperature, density profile, and radial distribution function. In our example, we specifically focus on the properties of a POPC membrane, although the developed membrane model could be perceived as a generic model of lipid membranes. To optimize the performance of the model (the fitness), we conduct a series of evolutionary runs with diverse random initial population sizes (ranging from 96 to 384). We demonstrate that the ultra-CG membrane model we developed exhibits authentic lipid membrane behaviors, including self-assembly into bilayers, vesicle formation, membrane fusion, and gel phase formation. Moreover, we demonstrate compatibility with the Martini coarse-grained model by successfully reproducing the behavior of a transmembrane domain embedded within a lipid bilayer. This facilitates the simulation of realistic membrane proteins within an ultra-CG bilayer membrane, enhancing the accuracy and applicability of our model in biophysical studies.

Probability and kinetics of rupture and electrofusion in giant unilamellar vesicles under various frequencies of direct current pulses

Irreversible electroporation induces permanent permeabilization of lipid membranes of vesicles, resulting in vesicle rupture upon the application of a pulsed electric field. Electrofusion is a phenomenon wherein neighboring vesicles can be induced to fuse by exposing them to a pulsed electric field. We focus how the frequency of direct current (DC) pulses of electric field impacts rupture and electrofusion in cell-sized giant unilamellar vesicles (GUVs) prepared in a physiological buffer. The average time, probability, and kinetics of rupture and electrofusion in GUVs have been explored at frequency 500, 800, 1050, and 1250 Hz. The average time of rupture of many 'single GUVs' decreases with the increase in frequency, whereas electrofusion shows the opposite trend. At 500 Hz, the rupture probability stands at 0.45 ± 0.02, while the electrofusion probability is 0.71 ± 0.01. However, at 1250 Hz, the rupture probability increases to 0.69 ± 0.03, whereas the electrofusion probability decreases to 0.46 ± 0.03. Furthermore, when considering kinetics, at 500 Hz, the rate constant of rupture is (0.8 ± 0.1)×10-2 s-1, and the rate constant of fusion is (2.4 ± 0.1)×10-2 s-1. In contrast, at 1250 Hz, the rate constant of rupture is (2.3 ± 0.8)×10-2 s-1, and the rate constant of electrofusion is (1.0 ± 0.1)×10-2 s-1. These results are discussed by considering the electrical model of the lipid bilayer and the energy barrier of a prepore.

Assessing membrane material properties from the response of giant unilamellar vesicles to electric fields

Knowledge of the material properties of membranes is crucial to understanding cell viability and physiology. A number of methods have been developed to probe membranes in vitro, utilizing the response of minimal biomimetic membrane models to an external perturbation. In this review, we focus on techniques employing giant unilamellar vesicles (GUVs), model membrane systems, often referred to as minimal artificial cells because of the potential they offer to mimick certain cellular features. When exposed to electric fields, GUV deformation, dynamic response and poration can be used to deduce properties such as bending rigidity, pore edge tension, membrane capacitance, surface shear viscosity, excess area and membrane stability. We present a succinct overview of these techniques, which require only simple instrumentation, available in many labs, as well as reasonably facile experimental implementation and analysis.

All-atom molecular dynamics simulations of the combined effects of different phospholipids and cholesterol content on electroporation

The electroporation mechanism could be related to the composition of the plasma membrane, and the combined effect of different phospholipid molecules and cholesterol content on electroporation has rarely been studied nor conclusions drawn. In this paper, we applied all-atom molecular dynamics (MD) simulations to study the effects of phospholipids and cholesterol content on bilayer membrane electroporation. The palmitoyloleoylphosphatidylcholine (POPC) model, palmitoyloleoylphosphatidylethanolamine (POPE) model, and a 1 : 1 mixed model of POPC and POPE called PEPC, were the three basic models used. An electric field of 0.45 V nm-1 was applied to nine models, which were the three basic models, each with three different cholesterol content values of 0%, 24%, and 40%. The interfacial water molecules moved under the electric field and, once the first water bridge formed, the rest of the water molecules would dramatically flood into the membrane. The simulation showed that a rapid rise in the Z-component of the average dipole moment of the interfacial water molecules (Z-DM) indicated the occurrence of electroporation, and the same increment of Z-DM represented a similar change in the size of the water bridge. With the same cholesterol content, the formation of the first water bridge was the most rapid in the POPC model, regarding the average electroporation time (t ep), and the average t ep of the PEPC model was close to that of the POPE model. We speculate that the differences in membrane thickness and initial number of hydrogen bonds of the interfacial water molecules affect the average t ep for different membrane compositions. Our results reveal the influence of membrane composition on the electroporation mechanism at the molecular level.

Juglone Inactivates Pseudomonas aeruginosa through Cell Membrane Damage, Biofilm Blockage, and Inhibition of Gene Expression

Due to the strong drug resistance of Pseudomonas aeruginosa (P. aeruginosa), the inhibition effects of conventional disinfectants and antibiotics are not obvious. Juglone extracted from discarded walnut husk, as a kind of plant-derived antimicrobial agent, has the advantages of naturalness, high efficiency, and low residue, with a potential role in the inhibition of P. aeruginosa. This study elucidated the inhibitory effect of juglone on the growth of plankton and the formation of P. aeruginosa biofilm. The results showed that juglone (35 μg/mL) had an irreversible inhibitory effect on P. aeruginosa colony formation (about 10⁷ CFU/mL). The integrity and permeability of the cell membrane were effectively destroyed, accompanied by disorder of the membrane permeability, mass leakage of the cytoplasm, and ATP consumption. Further studies manifested that juglone could induce the abnormal accumulation of ROS in cells and block the formation of the cell membrane. In addition, RT-qPCR showed that juglone could effectively block the expression of five virulence genes and two genes involved in the production of extracellular polymers, thereby reducing the toxicity and infection of P. aeruginosa and preventing the production of extracellular polymers. This study can provide support for the innovation of antibacterial technology toward P. aeruginosa in food.

Pulsed Electric Fields Promote Liposome Buddings

Background: Liposomes have been a useful tool to analyze membrane behavior. Various studies have attempted to induce biological activities, for example, buddings, divisions, and endocytosis, on liposomes, focusing on lipid rafts that move along electric fields. Materials and Methods: Liposomes consisting of soybean lecithin, phosphatidylcholine, and cholesterol were prepared, with inner and outer liquid conductivities of 0.595 and 1.564 S/m, respectively. Results: We tried to induce buddings by pulsed electric fields (PEFs) on liposomes. Results demonstrated that 1.248 kV/cm, 400 μs PEF promoted postpulse liposome buddings, which were preceded by a membrane relaxation. Although a transient thick area (a lipid raft-like area) on the membrane just after PEF application preceded buddings, it was not the sufficient factor for buddings. Conclusion: We established a brief model as follows: 1.248 kV/cm, 400 μs PEF induced the lipid membrane relaxation without electroporation to trigger buddings. The current results could be a new frontier in bioelectrics.

Actin networks regulate the cell membrane permeability during electroporation

Transient physical disruption of cell membranes by electric pulses (or electroporation) has significance in biomedical and biological applications requiring the delivery of exogenous (bio)molecules to living cells. We demonstrate that actin networks regulate the cell membrane permeability during electroporation. Disruption of actin networks increases the uptake of membrane-impermeable molecules such as propidium iodide during electroporation. Our experiments at different temperatures ranging from 11 °C to 37 °C show that molecular uptake during electroporation increases with temperature. Furthermore, by examining the temperature-dependent kinetics of propidium iodide uptake, we infer that the activation energy barrier of electroporation is lowered when the actin networks are disrupted. Our numerical calculations of transmembrane voltage show that the reduced activation energy barrier for the cells with disrupted actin is not a consequence of the changes in transmembrane voltage associated with changes in the cell shape due to the disruption of actin, indicating that this could be due to changes in membrane mechanical properties. Our results suggest that the current theoretical models of electroporation should be advanced further by including the contributions of the cytoskeletal networks on the cell membrane permeability during the delivery of exogenous materials.

Ultrathin glass fiber microprobe for electroporation of arbitrary selected cell groups

A new type of ultrathin fiber microprobe for selective electroporation is reported. The microprobe is 10 cm long and has a diameter of 350 µm. This microprobe is a low cost tool, which allows electroporation of an arbitrary selected single cell or groups of cells among population with use of a standard microscope and cell culture plates. The microprobe in its basic form contains two metal microelectrodes made of a silver-copper alloy, running along the fiber, each with a diameter of 23 µm. The probe was tested in vitro on a population of normal and cancer cells. Successful targeted electroporation was observed by means of accumulation of trypan blue (TB) dye marker in the cell. The electroporation phenomenon was also verified with propidium iodide and AnnexinV in fluorescent microscopy.

Liposome destruction by hydrodynamic cavitation in comparison to chemical, physical and mechanical treatments

Liposomes are widely applied in research, diagnostics, medicine and in industry. In this study we show for the first time the effect of hydrodynamic cavitation on liposome stability and compare it to the effect of well described chemical, physical and mechanical treatments. Fluorescein loaded giant 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipid vesicles were treated with hydrodynamic cavitation as promising method in inactivation of biological samples. Hydrodynamic treatment was compared to various chemical, physical and mechanical stressors such as ionic strength and osmolarity agents (glucose, Na⁺, Ca²⁺, and Fe³⁺), free radicals, shear stresses (pipetting, vortex mixing, rotational shear stress), high pressure, electroporation, centrifugation, surface active agents (Triton X-100, ethanol), microwave irradiation, heating, freezing-thawing, ultrasound (ultrasonic bath, sonotrode). The fluorescence intensity of individual fluorescein loaded lipid vesicles was measured with confocal laser microscopy. The distribution of lipid vesicle size, vesicle fluorescence intensity, and the number of fluorescein loaded vesicles was determined before and after treatment with different stressors. The different environmental stressors were ranked in order of their relative effect on liposome fluorescein release. Of all tested chemical, physical and mechanical treatments for stability of lipid vesicles, the most detrimental effect on vesicles stability had hydrodynamic cavitation, vortex mixing with glass beads and ultrasound. Here we showed, for the first time that hydrodynamic cavitation was among the most effective physico-chemical treatments in destroying lipid vesicles. This work provides a benchmark for lipid vesicle robustness to a variety of different physico-chemical and mechanical parameters important in lipid vesicle preparation and application.

DNA translocation to giant unilamellar vesicles during electroporation is independent of DNA size

Delivery of naked DNA molecules into living cells via physical disruption of the membrane under electric pulses has potential biomedical applications ranging from gene electro-transfer, electro-chemotherapy, to gene therapy, yet the mechanisms involved in DNA transport remain vague. To investigate the mechanism of DNA translocation across the cell membrane, giant unilamellar vesicles (GUVs) were electroporated in the presence of DNA molecules keeping the size of the DNA molecules as a variable parameter. We experimentally determined the translocation efficiency for each size of the DNA molecule, to compare the results with the existing and conflicting theories of the translocation mechanism i.e. stochastic threading and bulk electrophoresis. We observed that the translocation efficiency is independent of DNA size (ranging from 25-20 000 bp, bp = base pairs), implying that DNA molecules translocate freely across the electro-pores in the lipid membrane in their native polymer conformation, as opposed to unravelling and threading through the electro-pore. Bulk electrophoretic mobility determines the relationship between translocation efficiency and the size of the DNA molecule. This research provides experimental evidence of the mechanistic understanding of DNA translocation across lipid membranes which is essential for devising efficient and predictable protocols for electric field mediated naked DNA delivery.

Ejection of Large Particulate Materials from Giant Unilamellar Vesicles Induced by Electropulsation

Electroporation or electropermealization is a technique to open pores in the lipid bilayer membrane of cells and vesicles transiently to increase its permeability to otherwise impermeable molecules. However, the upper size limit of the materials permeable through this operation has not been studied in the past. Here, we investigate the size of the material that can be released (ejected) from giant unilamellar vesicles (GUVs) upon electrical pulsation. We confirm that the volume of GUV shrinks in a stepwise manner upon periodical pulsation, in accordance with previous studies. When the same operation is applied to GUVs that encapsulate microbeads, we find that beads as large as 20 μm can be ejected across the membrane without rupturing the whole GUV structure. We also demonstrate that functional bioactive particulate materials, such as gel balls, vesicles, and cells can be encapsulated in and ejected from GUVs. We foresee that this phenomenon can be applied to precisely regulate the time and location of release of these particulate materials in the microenvironment.

Response of an actin network in vesicles under electric pulses

We study the role of a biomimetic actin network during the application of electric pulses that induce electroporation or electropermeabilization, using giant unilamellar vesicles (GUVs) as a model system. The actin cortex, a subjacently attached interconnected network of actin filaments, regulates the shape and mechanical properties of the plasma membrane of mammalian cells, and is a major factor influencing the mechanical response of the cell to external physical cues. We demonstrate that the presence of an actin shell inhibits the formation of macropores in the electroporated GUVs. Additionally, experiments on the uptake of dye molecules after electroporation show that the actin network slows down the resealing process of the permeabilized membrane. We further analyze the stability of the actin network inside the GUVs exposed to high electric pulses. We find disruption of the actin layer that is likely due to the electrophoretic forces acting on the actin filaments during the permeabilization of the GUVs. Our findings on the GUVs containing a biomimetic network provide a step towards understanding the discrepancies between the electroporation mechanism of a living cell and its simplified model of the empty GUV.

Membrane Electroporation and Electropermeabilization: Mechanisms and Models

Exposure of biological cells to high-voltage, short-duration electric pulses causes a transient increase in their plasma membrane permeability, allowing transmembrane transport of otherwise impermeant molecules. In recent years, large steps were made in the understanding of underlying events. Formation of aqueous pores in the lipid bilayer is now a widely recognized mechanism, but evidence is growing that changes to individual membrane lipids and proteins also contribute, substantiating the need for terminological distinction between electroporation and electropermeabilization. We first revisit experimental evidence for electrically induced membrane permeability, its correlation with transmembrane voltage, and continuum models of electropermeabilization that disregard the molecular-level structure and events. We then present insights from molecular-level modeling, particularly atomistic simulations that enhance understanding of pore formation, and evidence of chemical modifications of membrane lipids and functional modulation of membrane proteins affecting membrane permeability. Finally, we discuss the remaining challenges to our full understanding of electroporation and electropermeabilization.

How Imaging Membrane and Cell Processes Involved in Electropermeabilization Can Improve Its Development in Cell Biology and in Clinics

Cell membranes can be transiently permeabilized under the application of electric pulses. This process, called electropermeabilization or electroporation, allows hydrophilic molecules, such as anticancer drugs and DNA, to enter into cells and tissues. The method is nowadays used in clinics to treat cancers. Vaccination and gene therapy are other fields of application of DNA electrotransfer. A description of the mechanisms can be assayed by using different complementary systems with increasing complexities (models of membranes, cells cultivated in 2D and 3D culture named spheroids, and tissues in living mice) and different microscopy tools to visualize the processes from single molecules to entire animals. Single-cell imaging experiments revealed that the uptake of molecules (nucleic acids, antitumor drugs) takes place in well-defined membrane regions and depends on their chemical and physical properties (size, charge). If small molecules freely cross the electropermeabilized membrane and have a free access to the cytoplasm, larger molecules, such as plasmid DNA, face physical barriers (plasma membrane, cytoplasm crowding, nuclear envelope) which reduce transfection efficiency and engender a complex mechanism of transfer. Gene electrotransfer indeed involves different steps that include the initial interaction with the membrane, its crossing, transport within the cytoplasm, and finally gene expression. In vivo, additional very important effects of electric pulses are present such as blood flow modifications. The full knowledge on the way molecules are transported across the electropermeabilized membranes and within tissues is mandatory to improve the efficacy and the safety of the electropermeabilization process both in cell biology and in clinics.

The role of gel-phase domains in electroporation of vesicles

Transient permeabilisation of the cell membrane is a critical step to introduce drugs or DNA into living cells, yet challenging for both biological research and therapeutic applications. To achieve this, electroporation (or electropermeabilisation) has become a widely used method due to its simplicity to deliver almost any biomolecule to any cell type. Although this method demonstrates promise in the field of drug/gene delivery, the underlying physical mechanisms of the response of the heterogeneous cell membrane to strong electric pulses is still unknown. In this study, we have investigated the role of gel-phase lipids in the electroporation of binary giant unilamellar vesicles (GUVs), composed from DPPC (gel-phase) and DPhPC (fluid-phase) lipids (molar ratio 8:2 and 2:8). We have observed that the exposure to electric pulses leads to expel of fluid-phase lipids and concomitant decrease in GUV size, whereas the gel-phase domains become buckled. Based on experiments on pure fluid-phase and gel-phase GUVs, we have found that fluid-phase lipids can be expelled by electrical forces and the highly viscous gel-phase lipids cannot. Moreover, our analyses suggest that pore formation occurs primarily in fluid-phase domains and that the pore size is similar in all GUVs containing fluid-phase lipids, irrespective of the gel-phase percentage.

Flow Cytometric Analysis To Evaluate Morphological Changes in Giant Liposomes As Observed in Electrofusion Experiments

Liposome fusion is a way of supplying additional components for in-liposome biochemical reactions. Electrofusion is a method that does not require the addition of fusogens, which often alter the liposome dispersion, and is therefore useful for repetitive liposome fusion. However, the details of electrofusion have not been elucidated because of the limitations surrounding observing liposomes using a microscope. Therefore, we introduced fluorescent markers and high-throughput flow cytometry to analyze the morphological changes that occur in liposome electrofusion. (i) The content mixing was evaluated by a calcein-Co²⁺-EDTA system, in which green fluorescence from dequenched free calcein is detected when the quenched calcein-Co²⁺ complex and EDTA are mixed together. (ii) Liposome destruction was evaluated from the decrease in the total membrane volume of giant liposomes. (iii) Liposome fission was evaluated from the increase in the number of giant liposomes. By applying the flow cytometric analysis, we investigated the effect of three parameters (DC pulse, AC field, and lipid composition) on liposome electrofusion. The larger numbers or higher voltages of DC pulses induced liposome fusion and destruction with higher probability. The longer application time of the AC field induced liposome fusion, fission, and destruction with higher probability. Higher content of negatively charged POPG (≥19%) strongly inhibited liposome electrofusion.

High power electromagnetic pulse applicators for evaluation of biological effects induced by electromagnetic radiation waves

Micro applicators for real-time observation of electromagnetic radiation waves effects on giant unilamellar vesicles and mammalian cells.

Lipid vesicles in pulsed electric fields: Fundamental principles of the membrane response and its biomedical applications

The present review focuses on the effects of pulsed electric fields on lipid vesicles ranging from giant unilamellar vesicles (GUVs) to small unilamellar vesicles (SUVs), from both fundamental and applicative perspectives. Lipid vesicles are the most popular model membrane systems for studying biophysical and biological processes in living cells. Furthermore, as vesicles are made from biocompatible and biodegradable materials, they provide a strategy to create safe and functionalized drug delivery systems in health-care applications. Exposure of lipid vesicles to pulsed electric fields is a common physical method to transiently increase the permeability of the lipid membrane. This method, termed electroporation, has shown many advantages for delivering exogenous molecules including drugs and genetic material into vesicles and living cells. In addition, electroporation can be applied to induce fusion between vesicles and/or cells. First, we discuss in detail how research on cell-size GUVs as model cell systems has provided novel insight into the basic mechanisms of cell electroporation and associated phenomena. Afterwards, we continue with a thorough overview how electroporation and electrofusion have been used as versatile methods to manipulate vesicles of all sizes in different biomedical applications. We conclude by summarizing the open questions in the field of electroporation and possible future directions for vesicles in the biomedical field.

A model of lipid rearrangements during pore formation in the DPPC lipid bilayer

CONTEXT

The molecular bases of pore formation in the lipid bilayer remain unclear, as do the exact characteristics of their sizes and distributions. To understand this process, numerous studies have been performed on model lipid membranes including cell-sized giant unilamellar vesicles (GUV). The effect of an electric field on DPPC GUV depends on the lipid membrane state: in the liquid crystalline phase the created pores have a cylinder-like shape, whereas in the gel phase a crack has been observed.

OBJECTIVE

The aim of the study was to investigate the geometry of pores created in a lipid bilayer in gel and liquid crystalline phases in reference to literature experimental data.

METHODS

A mathematical model of the pore in a DPPC lipid bilayer developed based on the law of conservation of mass and the assumption of constant volume of lipid molecules, independent of their conformation, allows for analysis of pore shape and accompanying molecular rearrangements.

RESULTS

The membrane area occupied by the pore of a cylinder-like shape is greater than the membrane area occupied by lipid molecules creating the pore structure (before pore appearance). Creation of such pores requires more space, which can be achieved by conformational changes of lipid chains toward a more compact state. This process is impossible for a membrane in the most compact, gel phase.

DISCUSSION AND CONCLUSIONS

We show that the geometry of the pores formed in the lipid bilayer in the gel phase must be different from the cylinder shape formed in the lipid bilayer in a liquid crystalline state, confirming experimental studies. Furthermore, we characterize the occurrence of the 'buffer' zone surrounding pores in the liquid crystalline phase as a mechanism of separation of neighbouring pores.

Antifungal activity of the cationic antimicrobial polymer-polyhexamethylene guanidine hydrochloride and its mode of action

The antifungal activity of polyhexamethylene guanidine hydrochloride (PHMGH) was studied against various pathogenic fungi. PHMGH had more potent antifungal activity than amphotericin B, which is a commonly used antifungal drug, and also showed no hemolytic and lactate dehydrogenase release activities in the range of 1.25-40.0 μg mL^-1. PHMGH is a cationic polymer containing an amino group and a polymeric guanidine group. Based on its characteristics such as the cationic charge and hydrophobicity, the antifungal mechanism of PHMGH was investigated using Candida albicans, as a model organism. Flow cytometric contour-plot analysis and microscopy showed changes in the size and granularity of the cells after treatment with PHMGH. A membrane study using 1,6-diphenyl-1,3,5-hexatriene labelling indicated a great loss of phospholipid area in the plasma membrane following PHMGH treatment. To investigate the extent of the damage, fluorescein isothiocyanate-labelled dextran leakage from large unilamellar vesicles was observed, indicating that PHMGH acts on the fungal membranes by inducing pore formation, with the majority of pore size being between 2.3 and 3.3 nm. This mechanism was confirmed with ion transition assays using 3,3'-dipropylthiacarbocyanine iodide and an ion-selective electrode meter, which indicated that membrane depolarization involving K⁺ leakage was induced. Taken together, these results show that PHMGH exerts its fungicidal effect by forming pores in the cell membrane.

Gene Electrotransfer: A Mechanistic Perspective

Gene electrotransfer is a powerful method of DNA delivery offering several medical applications, among the most promising of which are DNA vaccination and gene therapy for cancer treatment. Electroporation entails the application of electric fields to cells which then experience a local and transient change of membrane permeability. Although gene electrotransfer has been extensively studied in in vitro and in vivo environments, the mechanisms by which DNA enters and navigates through cells are not fully understood. Here we present a comprehensive review of the body of knowledge concerning gene electrotransfer that has been accumulated over the last three decades. For that purpose, after briefly reviewing the medical applications that gene electrotransfer can provide, we outline membrane electropermeabilization, a key process for the delivery of DNA and smaller molecules. Since gene electrotransfer is a multipart process, we proceed our review in describing step by step our current understanding, with particular emphasis on DNA internalization and intracellular trafficking. Finally, we turn our attention to in vivo testing and methodology for gene electrotransfer.

Electric Destabilization of Supramolecular Lipid Vesicles Subjected to Fast Electric Pulses

Biological membranes are weakly permeable to hydrophilic molecules and ions and electric pulses can induce their transient permeabilization, but this process is not well characterized. We directly assay the electropermeabilization process, on the minimum model of lipid vesicles, by using a highly sensitive fluorescence method based on manganese ion transport. The approach gives access, at the single-lipid self-assembly level, to the transmembrane potential needed to detect divalent ion permeabilization on supramolecular giant unilamellar lipid vesicles. The critical values are strongly dependent on the lipid composition and are observed to vary from 10 to 150 mV. These values appear to be much lower than those previously reported in the literature for cells and vesicles. The detection method appears to be a decisive parameter as it is controlled by the transport of the reporter dye. We also provide evidence that the electropermeabilization process is a transient transition of the lipid self-organization due to the loss of assembly cohesion induced by bioelectrochemical perturbations of the zwitterionic interface with the solution.

Voltage-morphology coupling in biomimetic membranes: dynamics of giant vesicles in applied electric fields

An electric potential difference across the plasma membrane is common to all living cells and is essential to physiological functions such as the generation of action potentials for cell-to-cell communication. While the basics of cell electrical activity are well established (e.g. the Hodgkin-Huxley model of the action potential), the reciprocal coupling of voltage and membrane deformation has received limited attention. In recent years, studies of biomimetic membranes in externally applied electric fields have revealed a plethora of intriguing dynamics (formation of edges, pearling, and phase separation) that challenge the current understanding of membrane electromechanics.

Electroporation-mediated gene delivery

Electroporation has been used extensively to transfer DNA to bacteria, yeast, and mammalian cells in culture for the past 30 years. Over this time, numerous advances have been made, from using fields to facilitate cell fusion, delivery of chemotherapeutic drugs to cells and tissues, and most importantly, gene and drug delivery in living tissues from rodents to man. Electroporation uses electrical fields to transiently destabilize the membrane allowing the entry of normally impermeable macromolecules into the cytoplasm. Surprisingly, at the appropriate field strengths, the application of these fields to tissues results in little, if any, damage or trauma. Indeed, electroporation has even been used successfully in human trials for gene delivery for the treatment of tumors and for vaccine development. Electroporation can lead to between 100 and 1000-fold increases in gene delivery and expression and can also increase both the distribution of cells taking up and expressing the DNA as well as the absolute amount of gene product per cell (likely due to increased delivery of plasmids into each cell). Effective electroporation depends on electric field parameters, electrode design, the tissues and cells being targeted, and the plasmids that are being transferred themselves. Most importantly, there is no single combination of these variables that leads to greatest efficacy in every situation; optimization is required in every new setting. Electroporation-mediated in vivo gene delivery has proven highly effective in vaccine production, transgene expression, enzyme replacement, and control of a variety of cancers. Almost any tissue can be targeted with electroporation, including muscle, skin, heart, liver, lung, and vasculature. This chapter will provide an overview of the theory of electroporation for the delivery of DNA both in individual cells and in tissues and its application for in vivo gene delivery in a number of animal models.

Synthesis of multifunctional lipid–polymer conjugates: application to the elaboration of bright far-red fluorescent lipid probes

Well-defined multifunctional lipid-polymer conjugates as new tools for the functionalization of lipid assemblies.

On the electroporation thresholds of lipid bilayers: molecular dynamics simulation investigations

Electroporation relates to the cascade of events that follows the application of high electric fields and that leads to cell membrane permeabilization. Despite a wide range of applications, little is known about the electroporation threshold, which varies with membrane lipid composition. Here, using molecular dynamics simulations, we studied the response of dipalmitoyl-phosphatidylcholine, diphytanoyl-phosphocholine-ester and diphytanoyl-phosphocholine-ether lipid bilayers to an applied electric field. Comparing between lipids with acyl chains and methyl branched chains and between lipids with ether and ester linkages, which change drastically the membrane dipole potential, we found that in both cases the electroporation threshold differed substantially. We show, for the first time, that the electroporation threshold of a lipid bilayer depends not only on the "electrical" properties of the membrane, i.e., its dipole potential, but also on the properties of its component hydrophobic tails.

Destabilization induced by electropermeabilization analyzed by atomic force microscopy

Electropermeabilization is a physical method that uses electric field pulses to deliver molecules into cells and tissues. Despite its increasing interest in clinics, little is known about plasma membrane destabilization process occurring during electropermeabilization. In this work, we took advantage of atomic force microscopy to directly visualize the consequences of electropermeabilization in terms of membrane reorganization and to locally measure the membrane elasticity. We visualized transient rippling of membrane surface and measured a decrease in membrane elasticity by 40%. Our results obtained both on fixed and living CHO cells give evidence of an inner effect affecting the entire cell surface that may be related to cytoskeleton destabilization. Thus, AFM appears as a useful tool to investigate basic process of electroporation on living cells in absence of any staining or cell preparation.

Destabilizing giant vesicles with electric fields: an overview of current applications

This review presents an overview of the effects of electric fields on giant unilamellar vesicles. The application of electrical fields leads to three basic phenomena: shape changes, membrane breakdown, and uptake of molecules. We describe how some of these observations can be used to measure a variety of physical properties of lipid membranes or to advance our understanding of the phenomena of electropermeabilization. We also present results on how electropermeabilization and other liposome responses to applied fields are affected by lipid composition and by the presence of molecules of therapeutic interest in the surrounding solution.

Giant lipid vesicles under electric field pulses assessed by non invasive imaging

We present experimental results regarding the effects of electric pulses on giant unilamellar vesicles (GUVs). We have used phase contrast and coherent anti-Stokes Raman scattering (CARS) microscopy as relevant optical approaches to gain insight into membrane changes under electropermeabilization. No addition of exogenous molecules (lipid analogue, fluorescent dye) was needed. Therefore, experiments were performed on pure lipid systems avoiding possible artefacts linked to their use. Structural membrane changes were assessed by loss of contrast inside the GUVs due to sucrose and glucose mixing. Our observations, performed at the single vesicle level, indicate these changes are under the control of the number of pulses and field intensity. Larger number of pulses enhances membrane alterations. A threshold value of the field intensity must be applied to allow exchange of molecules between GUVs and the external medium. This threshold depends on the size of the vesicles, the larger GUVs being affected at lower electric field strengths than the smaller ones. Our experimental data are well described by a simple model in which molecule entry is driven by direct exchange. The CARS microscopic study of the effect of pulse duration confirms that pulses, in the ms time range, induce loss of lipids and membrane deformations facing the electrodes.

Vesicle deformation and poration under strong dc electric fields

When subject to applied electric pulses, a lipid membrane exhibits complex responses including electrodeformation and electroporation. In this work, the electrodeformation of giant unilamellar vesicles under strong dc electric fields was investigated. Specifically, the degree of deformation was quantified as a function of the applied field strength and the electrical conductivity ratio of the fluids inside and outside of the vesicles. The vesicles were made from L-α-phosphatidylcholine with diameters ranging from 14 to 30 μm. Experiments were performed with field strengths ranging from 0.9 to 2.0 kV/cm, and intra-to-extra-vesicular conductivity ratios varying between 1.92 and 53.0. With these parametric configurations, the vesicles exhibited prolate elongations along the direction of the electric field. The degree of deformation was, in general, significant. In some cases, the aspect ratio of a deformed vesicle exceeded 10, representing a strong-deformation regime previously not explored. The aspect ratio scaled quadratically with the field strength, and increased asymptotically to a maximum value at high conductivity ratios. Appreciable area and volumetric changes were observed both during and after pulsation, indicating the concurrence of electroporation. A theoretical model is developed to predict these large deformations in the strongly permeabilized limit, and the results are compared with the experimental data. Both agreements and discrepancies are found, and the model limitations and possible extensions are discussed.

Cholesterol implications in plasmid DNA electrotransfer: Evidence for the involvement of endocytotic pathways

The delivery of therapeutic molecules such as plasmid DNA in cells and tissues by means of electric fields holds great promise for anticancer treatment. To allow for their therapeutic action, the molecules have first to traverse the cell membrane. The mechanisms by which the electrotransferred pDNA interacts with and crosses the plasma membrane are not yet fully explained. The aim of this study is to unravel the role of cholesterol during gene electrotransfer in cells. We performed cholesterol depletion experiments and measured its effects on various steps of the electroporation process. The first two steps consisting of electropermeabilization of the plasma membrane and of pDNA interaction with it were not affected by cholesterol depletion. In contrast, gene expression decreased. Colocalization studies with endocytotic markers showed that pDNA is endocytosed with concomitant clathrin- and caveolin/raft-mediated endocytosis. Cholesterol might be involved in the pDNA translocation through the plasma membrane. This is the first direct experimental evidence of the occurrence of endocytosis in gene electrotransfer.

Vesicle electrohydrodynamics

A small amplitude perturbation analysis is developed to describe the effect of a uniform electric field on the dynamics of a lipid bilayer vesicle in a simple shear flow. All media are treated as leaky dielectrics and fluid motion is described by the Stokes equations. The instantaneous vesicle shape is obtained by balancing electric, hydrodynamic, bending, and tension stresses exerted on the membrane. We find that in the absence of ambient shear flow, it is possible that an applied stepwise uniform dc electric field could cause the vesicle shape to evolve from oblate to prolate over time if the encapsulated fluid is less conducting than the suspending fluid. For a vesicle in ambient shear flow, the electric field damps the tumbling motion, leading to a stable tank-treading state.

A new method for measuring edge tensions and stability of lipid bilayers: effect of membrane composition

We report a novel and facile method for measuring edge tensions of lipid membranes. The approach is based on electroporation of giant unilamellar vesicles and analysis of the pore closure dynamics. We applied this method to evaluate the edge tension in membranes with four different compositions: egg phosphatidylcholine (eggPC), dioleoylphosphatidylcholine (DOPC), and mixtures of DOPC with cholesterol and dioleoylphosphatidylethanolamine. Our data confirm previous results for eggPC and DOPC. The addition of 17 mol % cholesterol to the DOPC membrane causes an increase in the membrane edge tension. On the contrary, when the same fraction of dioleoylphosphatidylethanolamine is added to the membrane, a decrease in the edge tension is observed, which is an unexpected result considering the inverted-cone shape geometry of the molecule. It is presumed that interlipid hydrogen bonding is the origin of this behavior. Furthermore, cholesterol was found to lower the lysis tension of DOPC bilayers. This behavior differs from that observed on bilayers made of stearoyloleoylphosphatidylcholine, suggesting that cholesterol influences the membrane mechanical stability in a lipid-specific manner.

Pre-treatment of cells with pluronic L64 increases DNA transfection mediated by electrotransfer

Gene transfer into muscle cells is a key issue in biomedical research. Indeed, it is important for the development of new therapy for many genetic disorders affecting this tissue and for the use of muscle tissue as a secretion platform of therapeutic proteins. Electrotransfer is a promising method to achieve gene expression in muscles. However, this method can lead to some tissue damage especially on pathologic muscles. Therefore there is a need for the development of new and less deleterious methods. Triblock copolymers as pluronic L64 are starting to be used to improve gene transfer mediated by several agents into muscle tissue. Their mechanism of action is still under investigation. The combination of electrotransfer and triblock copolymers, in allowing softening electric field conditions leading to efficient DNA transfection, could potentially represent a milder and more secure transfection method. In the present study, we addressed the possible synergy that could be obtained by combining the copolymer triblock L64 and electroporation. We have found that a pre-treatment of cells with L64 could improve the transfection efficiency. This pre-treatment was shown to increase cell viability and this is partly responsible for the improvement of transfection efficiency. We have then labelled the plasmid DNA and the pluronic L64 in order to gain some insights into the mechanism of transfection of the combined physical and chemical methods. These experiences allowed us to exclude an action of L64 either on membrane permeabilization or on DNA/membrane interaction. Using plasmids containing or not binding sequences for NF-κB and an inhibitor of NF-κB pathway activation we have shown that this beneficial effect was rather related to the NF-κB signalling pathway, as it is described for other pluronics. Finally we address here some mechanistic issues on electrically mediated transfection, L64 mediated membrane permeabilization and the combination of both for gene transfer.

Effects of PEO-PPO-PEO triblock copolymers on phospholipid membrane integrity under osmotic stress

The effects of PEO-PPO-PEO triblock copolymers, mainly Poloxamer 188, on phospholipid membrane integrity under osmotic gradients were explored using giant unilamellar vesicles (GUVs). Fluorescence leakage assays showed two opposing effects of P188 on the structural integrity of GUVs depending on the duration of their incubation time. A two-state transition mechanism of interaction between the triblock copolymers and the phospholipid membrane is proposed: an adsorption (I) and an insertion (II) state. While the triblock copolymer in state I acts to moderately retard the leakage, their insertion in state II perturbs the lipid packing, thus increasing the membrane permeability. Our results suggest that the biomedical application of PEO-PPO-PEO triblock copolymers, either as cell membrane resealing agents or as accelerators for drug delivery, is directed by the delicate balance between these two states.