Influence of cholesterol on electroporation of bilayer lipid membranes: chronopotentiometric studies

A molecular dynamics study of phospholipid membrane electroporation induced by bipolar pulses with different intervals

The mechanism of changes in cell electroporation (EP) during the intervals of bipolar pulses is still unclear, and few studies have investigated the effect of the intervals at the molecular level. In this study, EP induced by bipolar pulses (BP) with different intervals was investigated using all-atom molecular dynamics simulations. Firstly, EP was formed during the positive pulses of 2 ns and 0.5 V nm^-1, then the effects of various intervals of 0, 1, 5, and 10 ns on EP evolution were investigated, and the dynamic changes of different degrees of EP induced by the following negative pulses of 2 ns and 0.5 V nm^-1 were analyzed. The elimination effect of intervals was determined and it was related to the degrees of EP and the time of intervals. At the last moment of the intervals the phospholipid membrane was classified and quantitatively defined in three states according to the degrees of EP, namely, Resealing, Destabilizing and Retaining states. These states appeared due to the combined effect of both the positive pulse and the interval, and the states represent the degrees of EP which had different responses after applying the negative pulse. These results can improve our understanding of the fundamental mechanism of BP-induced EP.

Modeling and simulation of current-clamp electroporation

Current-Clamp electroporation refers to the application of a constant current across a membrane which results in voltage fluctuations due to the creation of electropores. This method allows for the measurement of electroporation across a long timescale (minutes) and facilitates the comparison between experimental and theoretical studies. Of particular interest is the claim in the literature that current-clamp electroporation results in the creation of a single pore. We simulated current-clamp electroporation using the Smoluchowski and Langevin equations and identified two possible mechanisms to explain the observed voltage fluctuations. The voltage fluctuations may be due to a single pore or a few pores growing and shrinking via a negative feedback mechanism or the opening and closing of pores in a larger population of pores. Our results suggest that current-clamp conditions do not necessarily result in the creation of a single pore. Additionally, we showed that the Langevin model is more accurate than the Smoluchowski model under conditions where there are only a few pores.

Nanosecond Pulsed Electric Field (nsPEF): Opening the Biotechnological Pandora’s Box

Nanosecond Pulsed Electric Field (nsPEF) is an electrostimulation technique first developed in 1995; nsPEF requires the delivery of a series of pulses of high electric fields in the order of nanoseconds into biological tissues or cells. They primary effects in cells is the formation of membrane nanopores and the activation of ionic channels, leading to an incremental increase in cytoplasmic Ca2+ concentration, which triggers a signaling cascade producing a variety of effects: from apoptosis up to cell differentiation and proliferation. Further, nsPEF may affect organelles, making nsPEF a unique tool to manipulate and study cells. This technique is exploited in a broad spectrum of applications, such as: sterilization in the food industry, seed germination, anti-parasitic effects, wound healing, increased immune response, activation of neurons and myocites, cell proliferation, cellular phenotype manipulation, modulation of gene expression, and as a novel cancer treatment. This review thoroughly explores both nsPEF’s history and applications, with emphasis on the cellular effects from a biophysics perspective, highlighting the role of ionic channels as a mechanistic driver of the increase in cytoplasmic Ca2+ concentration.

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

In this paper, we applied all-atom molecular dynamics (MD) simulations to study the effects of phospholipids and cholesterol content on bilayer membrane electroporation.

Effect of the cholesterol on electroporation of planar lipid bilayer

Electroporation threshold depends on the membrane composition, with cholesterol being one of its key components already studied in the past, but the results were inconclusive. The aim of our study was to determine behaviour of planar lipid bilayers with varying cholesterol concentrations under electric field. This would give us a better insight into cholesterol effect on membrane properties during electroporation process, since cholesterol is one of the major components of biological membranes and plays a crucial role in membrane organisation, dynamics, and function. Planar lipid bilayers were prepared from phosphatidylcholine lipids with 0, 20, 30, 50 and 80 mol% cholesterol. Capacitance was measured using the discharge method. Results show no statistical difference of c_BLM between the cholesterol concentrations. Breakdown voltage U_br of planar lipid bilayers was measured by means of linear rising voltage with seven different slopes. Obtained results were fitted to a strength-duration curve, where parameter U_brmin represents minimal breakdown voltage, and parameter τ_RC represents the inclination of the strength-duration curve. Adding cholesterol to planar lipid bilayer gradually increased its U_brmin until 50 mol% cholesterol concentration. Afterwards at 80 mol% U_brmin does not further increase, in fact it reduces by 20% of the U_brmin at 50 mol% cholesterol concentration.

Recent developments in the kinetics of ruptures of giant vesicles under constant tension

External tension in membranes plays a vital role in numerous physiological and physicochemical phenomena. In this review, recent developments in the constant electric- and mechanical-tension-induced rupture of giant unilamellar vesicles (GUVs) are considered. We summarize the results relating to the kinetics of GUV rupture as a function of membrane surface charge, ions in the bathing solution, lipid composition, cholesterol content in the membrane, and osmotic pressure. The mechanical stability and line tension of the membrane under these conditions are discussed. The membrane tension due to osmotic pressure and the critical tension of rupture for various membrane compositions are also discussed. The results and their analysis provide a biophysical description of the kinetics of rupture, along with insight into biological processes. Future directions and possible developments in this research area are included.

Effects of osmotic pressure on the irreversible electroporation in giant lipid vesicles

Irreversible electroporation (IRE) is a nonthermal tumor/cell ablation technique in which a series of high-voltage short pulses are used. As a new approach, we aimed to investigate the rupture of giant unilamellar vesicles (GUVs) using the IRE technique under different osmotic pressures (Π), and estimated the membrane tension due to Π. Two categories of GUVs were used in this study. One was prepared with a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol (chol) for obtaining more biological relevance while other with a mixture of DOPG and DOPC, with specific molar ratios. We determined the rate constant (kp) of rupture of DOPG/DOPC/chol (46/39/15)-GUVs and DOPG/DOPC (40/60)-GUVs induced by constant electric tension (σc) under different Π. The σc dependent kp values were fitted with a theoretical equation, and the corresponding membrane tension (σoseq) at swelling equilibrium under Π was estimated. The estimated membrane tension agreed well with the theoretical calculation within the experimental error. Interestingly, the values of σoseq were almost same for both types of synthesized GUVs under same osmotic pressure. We also examined the sucrose leakage, due to large osmotic pressure-induced pore formation, from the inside of DOPG/DOPC/chol(46/39/15)-GUVs. The estimated membrane tension due to large Π at which sucrose leaked out was very similar to the electric tension at which GUVs were ruptured without Π. We explained the σc and Π induced pore formation in the lipid membranes of GUVs.

Tuning the transdermal transport by application of external continuous electric field: a coarse-grained molecular dynamics study

The control of skin permeability to specific substances (e.g., medications, vitamins, and nutrients) through stratum corneum is a challenge. Iontophoresis is an option in spite of the lack of a detailed understanding of the underlying molecular mechanism. In the present work, the simulations concerning application of an external continuous electric field to stratum corneum, in a range of low intensity (0-24 mV nm^-1), were carried out using the coarse-grained molecular dynamics approach. Using a set of random seed replicas of the starting configuration, we observed that in the range of electric field intensity of 22-23 mV nm^-1, water-rich lipid vesicles were formed in 20% of cases. Pores appeared in the remaining 80%. We argue that lipids undergo fast re-orientations under electric field inducing mechanical instability, which originates the pores. We presented a simple electrostatic model to interpret the results where the mismatch between electrical permittivities of the membrane and external media and the gradient of the local electric field in the membrane surface, govern the time scales and electric fields for vesicle formation. Our results indicate that just 10% difference between electrical permittivities of the membrane and external media decreases 1/6 the minimal time required for vesicle formation. The minimal electric field required decreases 10 times. The control and tunning of formation of biologically compatible vesicles, capable of transporting substances under low-intensity electric fields, has a promising application in fields such as drug therapy and dermo-cosmetics allowing the use of hydrophilic substances in dermal applications.

Electrostatic effects on the electrical tension-induced irreversible pore formation in giant unilamellar vesicles

Irreversible electroporation (IRE) is a new technique in which a series of short pulses with high frequency electrical energy is applied on the targeted regions of cells or vesicles for their destruction or rupture formation. IRE induces lateral tension in the membranes of vesicles. We have investigated the electrostatic interaction effects on the constant electrical tension-induced rate constant of irreversible pore formation in the membranes of giant unilamellar vesicles (GUVs). The electrostatic interaction has been varied by changing the salt concentration in buffer and the surface charge density of membranes. The membranes of GUVs are synthesized by a mixture of negatively charged lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. The rate constant of pore formation increases with the decrease of salt concentration in buffer along with the increase of surface charge density of membranes. The tension dependent probability of pore formation and the rate constant of pore formation are fitted to the theoretical equation, and obtained the line tension of membranes. The decrease in energy barrier of a prepore due to electrostatic interaction is the key factor causing an increase of rate constant of pore formation.

Influence of cholesterol on electroporation in lipid membranes of giant vesicles

Irreversible electroporation (IRE) is primarily a nonthermal ablative technology that uses a series of high-voltage and ultra-short pulses with high-frequency electrical energy to induce cell death. This paper presents the influence of cholesterol on the IRE-induced probability of pore formation and the rate constant of pore formation in giant unilamellar vesicles (GUVs). The GUVs are prepared by a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol using the natural swelling method. An IRE signal of frequency 1.1 kHz is applied to the membranes of GUVs. The probability of pore formation and the rate constant of pore formation events are obtained using statistical analysis from several single GUVs. The time-dependent fraction of intact GUVs among all those examined is fitted to a single exponential decay function from where the rate constant of pore formation is calculated. The probability of pore formation and the rate constant of pore formation decreases with an increase in cholesterol content in the membranes of GUVs. Theoretical equations are fitted to the tension-dependent rate constant of pore formation and to the probability of pore formation, which allows us to obtain the line tension of membranes. The obtained line tension increases with an increase in cholesterol in the membranes. The increase in the energy barrier of the prepore state, due to the increase of cholesterol in membranes, is the main factor explaining the decrease in the rate constant of pore formation.

Extracellular-Ca2+-Induced Decrease in Small Molecule Electrotransfer Efficiency: Comparison between Microsecond and Nanosecond Electric Pulses

Electroporation—a transient electric-field-induced increase in cell membrane permeability—can be used to facilitate the delivery of anticancer drugs for antitumour electrochemotherapy. In recent years, Ca²⁺ electroporation has emerged as an alternative modality to electrochemotherapy. The antitumor effect of calcium electroporation is achieved as a result of the introduction of supraphysiological calcium doses. However, calcium is also known to play a key role in membrane resealing, potentially altering the pore dynamics and molecular delivery during electroporation. To elucidate the role of calcium for the electrotransfer of small charged molecule into cell we have performed experiments using nano- and micro-second electric pulses. The results demonstrate that extracellular calcium ions inhibit the electrotransfer of small charged molecules. Experiments revealed that this effect is related to an increased rate of membrane resealing. We also employed mathematical modelling methods in order to explain the differences between the CaCl₂ effects after the application of nano- and micro-second duration electric pulses. Simulation showed that these differences occur due to the changes in transmembrane voltage generation in response to the increase in specific conductivity when CaCl₂ concentration is increased.

A new approach for investigating the response of lipid membranes to electrocompression by coupling droplet mechanics and membrane biophysics

A new method for quantifying lipid-lipid interactions within biomimetic membranes undergoing electrocompression is demonstrated by coupling droplet mechanics and membrane biophysics. The membrane properties are varied by altering the lipid packing through the introduction of cholesterol. Pendant drop tensiometry is used to measure the lipid monolayer tension at an oil-water interface. Next, two lipid-coated aqueous droplets are manipulated into contact to form a bilayer membrane at their adhered interface. The droplet geometries are captured from two angles to provide accurate measurements of both the membrane area and the contact angle between the adhered droplets. Combining the monolayer tension and contact angle measurements enables estimations of the membrane tension with respect to lipid composition. Then, the membrane is electromechanically compressed using a transmembrane voltage. Electrostatic pressure, membrane tension and the work necessary for bilayer thinning are tracked, and a model is proposed to capture the mechanics of membrane compression. The results highlight that a previously unaccounted for energetic term is produced during compression, potentially reflecting changes in the lateral membrane structure. This residual energy is eliminated in cases with cholesterol mole fractions of 0.2 and higher, suggesting that cholesterol diminishes these adjustments.

Activity of the Gramicidin A Ion Channel in a Lipid Membrane with Switchable Physical Properties

Lipid bilayer membranes formed from the artificial 1,3-diamidophospholipid Pad-PC-Pad have the remarkable property that their hydrophobic thickness can be modified in situ: the particular arrangement of the fatty acid chains in Pad-PC-Pad allows them to fully interdigitate below 37 °C, substantially thinning the membrane with respect to the noninterdigitated state. Two stimuli, traversing the main phase transition temperature of the lipid or addition of cholesterol, have previously been shown to disable the interdigitated state. Both manipulations cause an increase in hydrophobic thickness of about 25 Å due to enhanced conformational entropy of the lipids. Here, we characterize the interdigitated state using electrophysiological recordings from free-standing lipid-membranes formed on micro structured electrode cavity arrays. Compared to standard membranes made from 1,2-diphytanoyl-sn-glycero-3-phosphocholin (DPhPC), pure Pad-PC-Pad membranes at room temperature had lowered electroporation threshold and higher capacitance. Ion channel formation by the peptide Gramicidin A was clearly facilitated in pure Pad-PC-Pad membranes at room temperature, with activity occurring at significantly lower peptide concentrations and channel dwell times increased by 2 orders of magnitude with respect to DPhPC-membranes. Both elevation of temperature beyond the phase transition and addition of cholesterol reduced channel dwell times, as expected if the reduced membrane thickness stabilized channel formation due to decreased hydrophobic mismatch.

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.

Electroporation Using Dissipative Particle Dynamics with a Novel Protocol for Applying Electric Field

In molecular dynamics simulations of membrane electroporation, the bilayer is subjected to an electric field E either by direct addition of a force f = qE on the charge-bearing species or by imposing an ion imbalance in the salt solutions on the two sides of the bilayer. The former is believed to mimic electroporation with high fields over nanosecond pulse period, during which the membrane is almost uncharged, especially in the low salt limit. Conversely, the ion imbalance method elucidates a low electric field-induced poration over a longer period of micro- to milliseconds with a fully charged membrane. Both these methods of applying electric field have disadvantages while investigating electroporation using dissipative particle dynamics (DPD) simulations. The method involving direct addition of force fails to address the presence of a nonuniform dielectric background for ions embedded in nonpolarizable DPD water and those found in the core of the bilayer. The ion imbalance method in DPD simulations suffers from its unavoidable use of a wall potential to prevent the movement of ions across the periodic boundaries. To address the above issues, we propose a simple method for imposing a desired transmembrane potential (TMV) by placing oppositely but uniformly charged plates on either side of the bilayer. Our DPD simulations demonstrate that the profiles for bead density, mechanical stress, electrical potential, as well as the transient responses in the dipole moment and species fluxes obtained from the proposed method utilizing charged plates are quite similar to those obtained using the ion imbalance method. The proposed protocol is free from the aforementioned drawbacks of the direct force addition and ion imbalance methods.

Atomistic Simulations of Electroporation of Model Cell Membranes

Electroporation is a phenomenon that modifies the fundamental function of the cell since it perturbs transiently or permanently the integrity of its membrane. Today, this technique is applied in fields ranging from biology and biotechnology to medicine, e.g., for drug and gene delivery into cells, tumor therapy, etc., in which it made it to preclinical and clinical treatments. Experimentally, due to the complexity and heterogeneity of cell membranes, it is difficult to provide a description of the electroporation phenomenon in terms of atomically resolved structural and dynamical processes, a prerequisite to optimize its use. Atomistic modeling in general and molecular dynamics (MD) simulations in particular have proven to be an effective approach for providing such a level of detail. This chapter provides the reader with a comprehensive account of recent advances in using such a technique to complement conventional experimental approaches in characterizing several aspects of cell membranes electroporation.

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.

Selective susceptibility to nanosecond pulsed electric field (nsPEF) across different human cell types

Tumor ablation by nanosecond pulsed electric fields (nsPEF) is an emerging therapeutic modality. We compared nsPEF cytotoxicity for human cell lines of cancerous (IMR-32, Hep G2, HT-1080, and HPAF-II) and non-cancerous origin (BJ and MRC-5) under strictly controlled and identical conditions. Adherent cells were uniformly treated by 300-ns PEF (0-2000 pulses, 1.8 kV/cm, 50 Hz) on indium tin oxide-covered glass coverslips, using the same media and serum. Cell survival plotted against the number of pulses displayed three distinct regions (initial resistivity, logarithmic survival decline, and residual resistivity) for all tested cell types, but with differences in LD₅₀ spanning as much as nearly 80-fold. The non-cancerous cells were less sensitive than IMR-32 neuroblastoma cells but more vulnerable than the other cancers tested. The cytotoxic efficiency showed no apparent correlation with cell or nuclear size, cell morphology, metabolism level, or the extent of membrane disruption by nsPEF. Increasing pulse duration to 9 µs (0.75 kV/cm, 5 Hz) produced a different selectivity pattern, suggesting that manipulation of PEF parameters can, at least for certain cancers, overcome their resistance to nsPEF ablation. Identifying mechanisms and cell markers of differential nsPEF susceptibility will critically contribute to the proper choice and outcome of nsPEF ablation therapies.

A molecular insight into the electro-transfer of small molecules through electropores driven by electric fields

The transport of chemical compounds across the plasma membrane into the cell is relevant for several biological and medical applications. One of the most efficient techniques to enhance this uptake is reversible electroporation. Nevertheless, the detailed molecular mechanism of transport of chemical species (dyes, drugs, genetic materials, …) following the application of electric pulses is not yet fully elucidated. In the past decade, molecular dynamics (MD) simulations have been conducted to model the effect of pulsed electric fields on membranes, describing several aspects of this phenomenon. Here, we first present a comprehensive review of the results obtained so far modeling the electroporation of lipid membranes, then we extend these findings to study the electrotransfer across lipid bilayers subject to microsecond pulsed electric fields of Tat11, a small hydrophilic charged peptide, and of siRNA. We use in particular a MD simulation protocol that allows to characterize the transport of charged species through stable pores. Unexpectedly, our results show that for an electroporated bilayer subject to transmembrane voltages in the order of 500mV, i.e. consistent with experimental conditions, both Tat11 and siRNA can translocate through nanoelectropores within tens of ns. We discuss these results in comparison to experiments in order to rationalize the mechanism of drug uptake by cells. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.

Properties of lipid electropores I: Molecular dynamics simulations of stabilized pores by constant charge imbalance

Molecular dynamics (MD) simulations have become a powerful tool to study electroporation (EP) in atomic detail. In the last decade, numerous MD studies have been conducted to model the effect of pulsed electric fields on membranes, providing molecular models of the EP process of lipid bilayers. Here we extend these investigations by modeling for the first time conditions comparable to experiments using long (μs-ms) low intensity (~kV/cm) pulses, by studying the characteristics of pores formed in lipid bilayers maintained at a constant surface tension and subject to constant charge imbalance. This enables the evaluation of structural (size) and electrical (conductance) properties of the pores formed, providing information hardly accessible directly by experiments. Extensive simulations of EP of simple phosphatidylcholine bilayers in 1M NaCl show that hydrophilic pores with stable radii (1-2.5 nm) form under transmembrane voltages between 420 and 630 mV, allowing for ionic conductance in the range of 6.4-29.5 nS. We discuss in particular these findings and characterize both convergence and size effects in the MD simulations. We further extend these studies in a follow-up paper (Rems et al., Bioelectrochemistry, Submitted), by proposing an improved continuum model of pore conductance consistent with the results from the MD simulations.

Imaging potassium-flux through individual electropores in droplet interface bilayers

Using total internal reflection fluorescence microscopy of droplet interface bilayers containing the potassium-sensitive fluorophore APG-4, we imaged the ionic flux through individual electropores. We are able to monitor up to 30 individual pores in parallel and show voltage dependent responses in fluorescence that corresponds to the measured ionic current. These experiments help quantify the scope and current limitations of optical single channel recordings of potassium flux. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Line tension of multicomponent bilayer membranes

The line tension or edge energy of bilayer membranes self-assembled from binary amphiphilic molecules is studied using self-consistent-field theory (SCFT). Specifically, solutions of the SCFT equations corresponding to an infinite membrane with a circular pore, or an open membrane, are obtained for a coarse-grained model in which the amphiphilic species and hydrophilic solvents are represented by ABandED diblock copolymers and C homopolymers, respectively. The edge energy of the membrane is extracted from the free energy of the open membranes. Results for membranes composed of mixtures of symmetric and cone- or inverse cone-shaped amphiphilic molecules with neutral and/or repulsive interactions are obtained and analyzed. It is observed that an increase in the concentration of the cone-shaped species leads to a decrease of the line tension. In contrast, adding inverse cone-shaped copolymers results in an increase of the line tension. Furthermore, the density profile of the copolymers reveals that the line tension is regulated by the distribution of the amphiphiles at the bilayer edge.

Visualization of the membrane engineering concept: evidence for the specific orientation of electroinserted antibodies and selective binding of target analytes

Membrane engineering is a generic methodology for increasing the selectivity of a cell biosensor against a target molecule, by electroinserting target-specific receptor-like molecules on the cell surface. Previous studies have elucidated the biochemical aspects of the interaction between various analytes (including viruses) and their homologous membrane-engineered cells. In the present study, purified anti-biotin antibodies from a rabbit antiserum along with in-house prepared biotinylated bovine serum albumin (BSA) were used as a model antibody-antigen pair of molecules for facilitating membrane engineering experiments. It was proven, with the aid of fluorescence microscopy, that (i) membrane-engineered cells incorporated the specific antibodies in the correct orientation and that (ii) the inserted antibodies are selectively interacting with the homologous target molecules. This is the first time the actual working concept of membrane engineering has been visualized, thus providing a final proof of the concept behind this innovative process. In addition, the fluorescence microscopy measurements were highly correlated with bioelectric measurements done with the aid of a bioelectric recognition assay.

Effects of dimethyl sulfoxide on lipid membrane electroporation

Pores can be generated in lipid membranes by the application of an external electric field or by the addition of particular chemicals such as dimethyl sulfoxide (DMSO). Molecular dynamics (MD) has been shown to be a useful tool for unveiling many aspects of pore formation in lipid membranes in both situations. By means of MD simulations, we address the formation of electropores in cholesterol-containing lipid bilayers under the influence of DMSO. We show how a combination of physical and chemical mechanisms leads to more favorable conditions for generating membrane pores and, in particular, how the addition of DMSO to the medium significantly reduces the minimum electric field required to electroporate a lipid membrane. The strong alteration of membrane transversal properties and the energetic stabilization of the hydrophobic pore stage by DMSO provide the physicochemical mechanisms that explain this effect.

Pore formation in lipid bilayer membranes made of phosphatidylcholine and cholesterol followed by means of constant current

This paper describes the application of chronopotentiometry to lipid bilayer research. The experiments were performed on bilayer lipid membranes composed of phosphatidylcholine and cholesterol and formed using the painting technique. Chronopotentiometric (U = f(t)) measurements were used to determine the membrane capacitance, resistance, and breakdown voltage as well as pore conductance and diameter.

Molecular dynamics simulations of ion conductance in field-stabilized nanoscale lipid electropores

Molecular dynamics (MD) simulations of electrophoretic transport of monovalent ions through field-stabilized electropores in POPC lipid bilayers permit systematic characterization of the conductive properties of lipid nanopores. The radius of the electropore can be controlled by the magnitude of the applied sustaining external electric field, which also drives the transport of ions through the pore. We examined pore conductances for two monovalent salts, NaCl and KCl, at physiological concentrations. Na(+) conductance is significantly less than K(+) and Cl(-) conductance and is a nonlinear function of pore radius over the range of pore radii investigated. The single pore electrical conductance of KCl obtained from MD simulation is comparable to experimental values measured by chronopotentiometry.

Molecular-level characterization of lipid membrane electroporation using linearly rising current

We present experimental and theoretical results of electroporation of small patches of planar lipid bilayers by means of linearly rising current. The experiments were conducted on ~120-μm-diameter patches of planar phospholipid bilayers. The steadily increasing voltage across the bilayer imposed by linearly increasing current led to electroporation of the membrane for voltages above a few hundred millivolts. This method shows new molecular mechanisms of electroporation. We recorded small voltage drops preceding the breakdown of the bilayer due to irreversible electroporation. These voltage drops were often followed by a voltage re-rise within a fraction of a second. Modeling the observed phenomenon by equivalent electric circuits showed that these events relate to opening and closing of conducting pores through the bilayer. Molecular dynamics simulations performed under similar conditions indicate that each event is likely to correspond to the opening and closing of a single pore of about 5 nm in diameter, the conductance of which ranges in the 100-nS scale. This combined experimental and theoretical investigation provides a better quantitative characterization of the size, conductance and lifetime of pores created during lipid bilayer electroporation. Such a molecular insight should enable better control and tuning of electroporation parameters for a wide range of biomedical and biotechnological applications.

Chronopotentiometric studies of phosphatidylcholine bilayers modified by ergosterol

We have monitored the effect of ergosterol on electrical capacitance and electrical resistance of the phosphatidylcholine bilayer membranes using chronopotentiometry method. The chronopotentiometric characteristic of the bilayers depends on constant-current flow through the membranes. For low current values, no electroporation takes place and the membrane voltage rises exponentially to a constant value described by the Ohm's law. Based on these kinds of chronopotentiometric curves, a method of the membrane capacitance and the membrane resistance calculations is presented.

Carbon nanotubes activate store-operated calcium entry in human blood platelets

Carbon nanotubes (CNTs) are known to potentiate arterial thrombosis in animal models, which raises serious safety issues concerning environmental or occupational exposure to CNTs and their use in various biomedical applications. We have shown previously that different CNTs, but not fullerene (nC60), induce the aggregation of human blood platelets. To date, however, a mechanism of potentially thrombogenic CNT-induced platelet activation has not been elucidated. Here we show that pristine multiwalled CNTs (MWCNTs) penetrate platelet plasma membrane without any discernible damage but interact with the dense tubular system (DTS) causing depletion of platelet intracellular Ca(2+) stores. This process is accompanied by the clustering of stromal interaction molecule 1 (STIM1) colocalized with Orai1, indicating the activation of store-operated Ca(2+) entry (SOCE). Our findings reveal the molecular mechanism of CNT-induced platelet activation which is critical in the evaluation of the biocompatibility of carbon nanomaterials with blood.

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.

Chronopotentiometric technique as a method for electrical characterization of bilayer lipid membranes

The basic electrical parameters of bilayer lipid membranes are capacitance and resistance. This article describes the application of chronopotentiometry to the research of lipid bilayers. Membranes were made from egg yolk phosphatidylcholine. The chronopotentiometric characteristic of the membranes depends on the current value. For low current values, no electroporation takes place and the voltage rises exponentially to a constant value. Based on these kinds of chronopotentiometric curves, a method of the membrane capacitance and the membrane resistance calculations are presented.

Pore formation induced by an antimicrobial peptide: electrostatic effects

We investigate the mode of action of Cateslytin, an antimicrobial peptide, on zwitterionic biomembranes by performing numerical simulations and electrophysiological measurements on membrane vesicles. Using this natural beta-sheet antimicrobial peptide secreted during stress as a model we show that a single peptide is able to form a stable membrane pore of 1 nm diameter of 0.25 nS conductance found both from calculation and electrical measurements. The resulting structure does not resemble the barrel-stave or carpet models earlier predicted, but is very close to that found in the simulation of alpha-helical peptides. Based on the simulation of a mutated peptide and the effects of small external electric fields, we conclude that electrostatic forces play a crucial role in the process of pore formation.

Modeling the induction of lipid membrane electropermeabilization

Experiments show significant effects of an electric field on lipid membrane, leading to a pore formation when a high intensity field is applied. The phenomenon of electroporation is preceded by the induction and expansion of defects, responsible for the pre-pore excitation. We examine the mechanism of the induction of the field-driven defects by Monte Carlo simulations. The study is based on the improved Pink's model, which includes explicit interactions between the polar heads and energy of interactions between the heads and the field. No anomalous deformation of the molecules is considered. The study, provided for bilayer dipalmitoyl-phosphatidylcholine (DPPC) membrane in the gel (300 K) and fluid (330 K) phases, shows dependence of the membrane conformational and energetical state on the value of the electric field. We observe that the electric field affects the number of molecules in the gel and in the fluid states. In the layer at the negative potential, when the transmembrane voltage is above U(c) approximately 280 mV, lipid heads abruptly reorient and the number of local spots with fluid conformation increases. The other layer slightly tends to tighten its structure, producing additional mechanical stress between layers. Lipids showed complete insensitivity to the electric field within physiological limits, U<70 mV.

Natural fluctuations of an electropore show fractional Lévy stable motion

Until now a stable long-lived electronanopore could be generated in a lipid membrane only under current-clamp conditions, and stochastic properties of a single nanopore have been studied by the chronopotentiometry. The current-clamp experiment introduces negative feedback, which could be responsible for the electropore fluctuations and observed 1/fB power spectrum. A new electroporation method, chronoamperometry after current clamp (CACC), prevents irreversible rupture of the membrane and eliminates the feedback by clamping the voltage after previous electroporation. The experiments show that the electropore size can also be stabilized under constant potential. The electropore fluctuations do not need feedback to appear. The fluctuations are self-similar with a short memory. CACC provides an effective tool for studying the natural dynamics of an electropore in various environments, which was tested with Na+ and Al3+ ions. Comparison between chronopotentiometry and CACC reveals that the feedback mainly shortens the memory of the stochastic fluctuations. Statistical analysis shows that the conductance fluctuations can be approximately modeled as a fractional Lévy stable motion for a small hydrophilic electropore, which tends to fractional Brownian motion when the electropore increases its size. A hypothesis is presented that this transition reflects a more regular shape of big nanopores.

Simulation of electroporated cell by chronopotentiometry

Chronopotentiometry on planar lipid bilayer (BLM) is proposed as a method for modeling the electrical phenomena in electroporated cell. Two techniques are discussed: constant-current and linear-current chronopotentiometry. It is proposed that the constant-current chronopotentiometry may provide basis for modeling the electroporated cell shortly after the removal of the electric field, when activity of cellular pumps counteracts ionic fluxes through the electropore and ionic channels. The linear-current method can be considered for modeling the cell in the later stage after electroporation, when energetical resources of the cell are gradually getting exhausted and the activity of pumps decreases. Based on this idea, it may be postulated that the electropore in the cell has fluctuating dynamics whose stochastic characteristics, similarly as biological channels, shows 1/f noise. The model implies that the fluctuations would disappear leaving the electropore with a constant resistance when efficiency of the pumps becomes very small. The results of chronopotentiometry also may suggest that opening time, conductivity and selectivity of the electropore can be controlled by the cell environment or membrane composition.