Electric pulse induced membrane permeabilization. Spatial orientation and kinetics of solute efflux in freely suspended and dielectrophoretically aligned plant mesophyll protoplasts

Asymmetric Patterns of Small Molecule Transport After Nanosecond and Microsecond Electropermeabilization

Imaging of fluorescent small molecule transport into electropermeabilized cells reveals polarized patterns of entry, which must reflect in some way the mechanisms of the migration of these molecules across the compromised membrane barrier. In some reports, transport occurs primarily across the areas of the membrane nearest the positive electrode (anode), but in others cathode-facing entry dominates. Here we compare YO-PRO-1, propidium, and calcein uptake into U-937 cells after nanosecond (6 ns) and microsecond (220 µs) electric pulse exposures. Each of the three dyes exhibits a different pattern. Calcein shows no preference for anode- or cathode-facing entry that is detectable with our measurement system. Immediately after a microsecond pulse, YO-PRO-1 and propidium enter the cell roughly equally from the positive and negative poles, but transport through the cathode-facing side dominates in less than 1 s. After nanosecond pulse permeabilization, YO-PRO-1 and propidium enter primarily on the anode-facing side of the cell.

Asymmetric gas mixture transport in composite membranes

The asymmetry effects in gas and electrolyte transport through composite membranes are considered. The interrelation between the kinetic theory and non-equilibrium thermodynamics description of gas mixture transport in channels is discussed. The kinetic expressions for transport and slip coefficients are given. The effect of surface forces on gas transport is discussed. A set of general equations related to gas mixture flows in capillaries and porous media is deduced. The nano-size effects in gas flows are outlined. The theoretical analysis of one-way flow effect and asymmetric separation properties of a two-layer porous membrane is given.

Transmembrane potential measurements on plant cells using the voltage-sensitive dye ANNINE-6

The charging of the plasma membrane is a necessary condition for the generation of an electric-field-induced permeability increase of the plasmalemma, which is usually explained by the creation and the growth of aqueous pores. For cells suspended in physiological buffers, the time domain of membrane charging is in the submicrosecond range. Systematic measurements using Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) protoplasts stained with the fast voltage-sensitive fluorescence dye ANNINE-6 have been performed using a pulsed laser fluorescence microscopy setup with a time resolution of 5 ns. A clear saturation of the membrane voltage could be measured, caused by a strong membrane permeability increase, commonly explained by enhanced pore formation, which prevents further membrane charging by external electric field exposure. The field strength dependence of the protoplast's transmembrane potential V (M) shows strong asymmetric saturation characteristics due to the high resting potential of the plants plasmalemma. At the pole of the hyperpolarized hemisphere of the cell, saturation starts at an external field strength of 0.3 kV/cm, resulting in a measured transmembrane voltage shift of ∆V(M) = -150 mV, while on the cathodic (depolarized) cell pole, the threshold for enhanced pore formation is reached at a field strength of approximately 1.0 kV/cm and ∆V(M) = 450 mV, respectively. From this asymmetry of the measured maximum membrane voltage shifts, the resting potential of BY-2 protoplasts at the given experimental conditions can be determined to V(R) = -150 mV. Consequently, a strong membrane permeability increase occurs when the membrane voltage diverges |V(M)| = 300 mV from the resting potential of the protoplast. The largest membrane voltage change at a given external electric field occurs at the cell poles. The azimuthal dependence of the transmembrane potential, measured in angular intervals of 10° along the circumference of the cell, shows a flattening and a slight decrease at higher fields at the pole region due to enhanced pore formation. Additionally, at the hyperpolarized cell pole, a polarization reversal could be observed at an external field range around 1.0 kV/cm. This behavior might be attributed to a fast charge transfer through the membrane at the hyperpolarized pole, e.g., by voltage-gated channels.

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

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

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

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

Plasma membrane voltage changes during nanosecond pulsed electric field exposure

The change in the membrane potential of Jurkat cells in response to nanosecond pulsed electric fields was studied for pulses with a duration of 60 ns and maximum field strengths of approximately 100 kV/cm (100 V/cell diameter). Membranes of Jurkat cells were stained with a fast voltage-sensitive dye, ANNINE-6, which has a subnanosecond voltage response time. A temporal resolution of 5 ns was achieved by the excitation of this dye with a tunable laser pulse. The laser pulse was synchronized with the applied electric field to record images at times before, during, and after exposure. When exposing the Jurkat cells to a pulse, the voltage across the membrane at the anodic pole of the cell reached values of 1.6 V after 15 ns, almost twice the voltage level generally required for electroporation. Voltages across the membrane on the side facing the cathode reached values of only 0.6 V in the same time period, indicating a strong asymmetry in conduction mechanisms in the membranes of the two opposite cell hemispheres. This small voltage drop of 0.6-1.6 V across the plasma membrane demonstrates that nearly the entire imposed electric field of 10 V/mum penetrates into the interior of the cell and every organelle.

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

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

New insights in the visualization of membrane permeabilization and DNA/membrane interaction of cells submitted to electric pulses

Electropermeabilization designates the use of electric pulses to overcome the barrier of the cell membrane. This physical method is used to transfer anticancer drugs or genes into living cells. Its mechanism remains to be elucidated. A position-dependent modulation of the membrane potential difference is induced, leading to a transient and reversible local membrane alteration. Electropermeabilization allows a fast exchange of small hydrophilic molecules across the membrane. It occurs at the positions of the cell facing the two electrodes on an asymmetrical way. In the case of DNA transfer, a complex process is present, involving a key step of electrophoretically driven association of DNA only with the destabilized membrane facing the cathode. We report here at the membrane level, by using fluorescence microscopy, the visualization of the effect of the polarity and the orientation of electric pulses on membrane permeabilization and gene transfer. Membrane permeabilization depends on electric field orientation. Moreover, at a given electric field orientation, it becomes symmetrical for pulses of reversed polarities. The area of cell membrane where DNA interacts is increased by applying electric pulses with different orientations and polarities, leading to an increase in gene expression. Interestingly, under reversed polarity conditions, part of the DNA associated with the membrane can be removed, showing some evidence for two states of DNA in interaction with the membrane: DNA reversibly associated and DNA irreversibly inserted.

Asymmetric pore distribution and loss of membrane lipid in electroporated DOPC vesicles

An externally applied electric field across vesicles leads to transient perforation of the membrane. The distribution and lifetime of these pores was examined using 1,2-di-oleoyl-sn-glycero-3-phosphocholine (DOPC) phospholipid vesicles using a standard fluorescent microscope. The vesicle membrane was stained with a fluorescent membrane dye, and upon field application, a single membrane pore as large as approximately 7 microm in diameter was observed at the vesicle membrane facing the negative electrode. At the anode-facing hemisphere, large and visible pores are seldom found, but formation of many small pores is implicated by the data. Analysis of pre- and post-field fluorescent vesicle images, as well as images from negatively stained electron micrographs, indicate that pore formation is associated with a partial loss of the phospholipid bilayer from the vesicle membrane. Up to approximately 14% of the membrane surface could be lost due to pore formation. Interestingly, despite a clear difference in the size distribution of the pores observed, the effective porous areas at both hemispheres was approximately equal. Ca(2+) influx measurements into perforated vesicles further showed that pores are essentially resealed within approximately 165 ms after the pulse. The pore distribution found in this study is in line with an earlier hypothesis (E. Tekle, R. D. Astumian, and P. B. Chock, 1994, Proc. Natl. Acad. Sci. U.S.A. 91:11512--11516) of asymmetric pore distribution based on selective transport of various fluorescent markers across electroporated membranes.

Early stage shape change of human erythrocytes after application of electric field pulses

Erythrocytes which receive electric field pulses are subject to poration, fusion and shape changes due to electrodynamic forces, aminophospholipid perturbation and influences on the normal flip-flop process. The shape change characteristics of cells suspended in different media were analysed after application of rectangular electric field pulses from t=11-44 micros and from E=4-8 kV/cm. Albumin is shown to decelerate the echinocyte shape change within the first few seconds after pulse application. The addition of fluoride and vanadate accelerates the shape change due to their inhibiting influence on the aminophospholipid translocase. For both the duration of the field pulse and its field strength, there exist lower threshold values under which no early stage shape change is observable. The activation energy calculated from the dissipative influence of the electric field alone is smaller than expected, indicating the electrodynamic influence on the flip-flop process. Cell shapes were additionally analysed by contour tracing to focus on the echinocyte spicule distribution after pulse application. This image analysis revealed that, with an increase of both pulse duration and field strength, the shape change velocity and the shape change intensity increase.

Dynamics of oscillating erythrocyte doublets after electrofusion

Erythrocytes were electrofused with multiple rectangular voltage pulses to show an oscillatory movement, divided into swell phases and pump events. During each swell phase, which lasted from 0.5 s to more than 180 s, the fused cells' (doublets') volume increased by colloid osmotic swelling, and the membrane area was expanded until rupture. Membrane rupture initiated the pump event, where the doublets' volume and membrane area decreased with an almost exponential time course and time constants between 2 ms and 8 ms. Simultaneously, a portion of cytosolic hemoglobin solution was ejected into extracellular space ("jet"). Pump event time constants and swell phase durations decreased with rising chamber temperature, indicating that both parts of the oscillatory movements were determined by physical properties of membrane and liquids. Relative volume change developments express a gradual loss of membrane elasticity during the oscillation, decreasing the elastic forces stored in the membrane. Evidence is given that the first rupture causes a weakening of the membrane at the rupture site. Heat treatment up to 45 degrees C had a negligible effect on swell times, pump time constants, and relative volume changes. A heat treatment of 50 degrees C prevented oscillatory movements. The rupture location accorded with theories of potential induced membrane electropermeabilization.

Modeling electroporation in a single cell. I. Effects Of field strength and rest potential

This study develops a model for a single cell electroporated by an external electric field and uses it to investigate the effects of shock strength and rest potential on the transmembrane potential V(m) and pore density N around the cell. As compared to the induced potential predicted by resistive-capacitive theory, the model of electroporation predicts a smaller magnitude of V(m) throughout the cell. Both V(m) and N are symmetric about the equator with the same value at both poles of the cell. Larger shocks do not increase the maximum magnitude of V(m) because more pores form to shunt the excess stimulus current across the membrane. In addition, the value of the rest potential does not affect V(m) around the cell because the electroporation current is several orders of magnitude larger than the ionic current that supports the rest potential. Once the field is removed, the shock-induced V(m) discharges within 2 micros, but the pores persist in the membrane for several seconds. Complete resealing to preshock conditions requires approximately 20 s. These results agree qualitatively and quantitatively with the experimental data reported by Kinosita and coworkers for unfertilized sea urchin eggs exposed to large electric fields.

Modeling electroporation in a single cell. II. Effects Of ionic concentrations

This study expands a previously developed model of a single cell electroporated by an external electric field by explicitly accounting for the ionic composition of the electroporation current. The previous model with non-specific electroporation current predicts that both the transmembrane potential V(m) and the pore density N are symmetric about the equator, with the same values at either end of the cell. The new, ion-specific case predicts that V(m) is symmetric and almost identical to the profile from the non-specific case, but N has a profound asymmetry with the pore density at the hyperpolarized end of the cell twice the value at the depolarized end. These modeling results agree with the experimentally observed preferential uptake of marker molecules at the hyperpolarized end of the cell as reported in the literature. This study also investigates the changes in intracellular ionic concentrations induced around an electroporated single cell. For all ion species, the concentrations near the membrane vary significantly, which may explain the electrical disturbances observed experimentally after large electric shocks are delivered to excitable cells and tissues.

Effect of medium conductivity and composition on the uptake of propidium iodide into electropermeabilized myeloma cells

The effects of ionic composition and conductivity of the medium on electropermeabilization of the plasma membrane of mammalian cells were studied. Temporal and spatial uptake of propidium iodide (PI) into field-treated cells was measured by means of flow cytometry, spectrofluorimetry and confocal laser scanning microscopy. Murine myeloma cells were electropulsed in iso-osmolar solutions. These contained 10-100 micrograms ml-1 PI at different conductivities (0.8-14 mS cm-1) and ionic strengths, adjusted by varying concentrations of K+, Na+, Cl- and SO4(2-). Field-induced incorporation of PI into reversibly permeabilized cells was (almost) independent of ionic composition and strength (at a fixed medium conductivity), but increased dramatically with decreasing medium conductivity at a fixed field strength. The time-course of PI uptake (which apparently reflected the resealing process of the membrane) could be fitted by single-exponential curve (relaxation time about 2 min in the absence of Ca2+) and was independent of medium conductivity and composition. These and other data suggested that the expansion of the 'electroleaks' during the breakdown process is field-controlled and depends, therefore, on the (conductivity-dependent) discharging process of the permeabilized membrane. The threshold field strength for dye uptake increased with increasing K+ concentration (about 0.6 kV cm-1 in K(+)-free, NaCl-containing medium and about 0.9 kV cm-1 in 30 mM KCl-containing medium). Also, the spatial uptake pattern of PI shifted from an asymmetric permeation through the cell hemisphere facing the anode to a more symmetric uptake through both hemispheres. These results suggested that the generated potential is superimposed on the (K(+)-dependent) resting membrane potential. Taking this into account, the breakdown voltage of the membrane was estimated to be about 1 V.

Selective and asymmetric molecular transport across electroporated cell membranes

Transport of a divalent cation (Ca2+) and three DNA indicators [ethidium bromide (EB), propidium iodide (PI), and ethidium homodimer (EthD-1)] across electroporated membranes of several mammalian cell lines was found to be selective and asymmetrical. In low salt medium, Ca2+ and EB were preferentially transported across the anodefacing cell membrane while PI and EthD-1 predominately entered at the site facing the cathode. In high salt medium, the entry site for Ca2+ and EB was reversed to the cathode-facing hemisphere while it remained unchanged for PI and EthD-1. In all these experiments, the observed transport patterns remained unaffected whether the dyes (or ion) were present during or added after the electroporating pulse. The data suggest that asymmetric pores are created on both sides of the membrane facing the electrodes, with smaller pore size (but greater in number) on the anode side and larger pores (with a lower population) on the cathode side. Furthermore, the rate of resealing of the membrane pores is significantly enhanced in high ionic strength medium, thus affecting the entry site. The asymmetric transport pattern is neither caused by electrophoresis induced by the externally applied electric field nor due to one-sided membrane breakdown as previously believed.

Electric field-mediated glycophorin insertion in cell membrane is a localized event

Purified soluble glycophorin, an intrinsic protein, can be back 'electroinserted' in the membrane of Chinese hamster ovary cells by submitting the cell/protein mixture to short electric field pulses. Previous studies showed that this complex between pulsed cells and proteins, which is detected only when the cell membrane is electropermeabilized, was very stable. This strongly suggested that the protein was indeed inserted in the membrane. The basic processes involved in this phenomena are studied in the present work. The association is observed at the single cell level by means of videoimmunofluorescence. Electric field-mediated insertion occurs firstly in a limited patch of the cell surface, which size is in agreement with the prediction of Electropermeabilization theory. A free diffusion of the inserted proteins then follows on the cell surface. The diffusion coefficient is computed to be less than 10(-10) cm2/s as observed for transmembranous proteins. This slow process gives an homogeneous distribution of the inserted protein.

Cell-attached patch clamp study of the electropermeabilization of amphibian cardiac cells

Potential gradients imposed across cell or lipid membranes break down the insulating properties of these barriers if an intensity and time-dependent threshold is exceeded. Potential gradients of this magnitude may occur throughout the body, and in particular in cardiac tissue, during clinical defibrillation, ablation, and electrocution trauma. To study the dynamics of membrane electropermeabilization a cell-attached patch clamp technique was used to directly control the potential across membrane patches of single ventricular cells enzymatically isolated from frog (Rana pipiens) hearts. Ramp waveshapes were used to reveal rapid membrane conductance changes that may have otherwise been obscured using rectangular waveshapes. We observed a step increase (delta t less than 30 microseconds) or breakdown in membrane conductance at transmembrane potential thresholds of 0.6-1.1 V in response to 0.1-1.0 kV/s voltage ramps. Conductance kinetics on a sub-millisecond time scale indicate that breakdown is preceded by a period of instability during which the noise and amplitude of the membrane conductance begin to increase. In some cells membrane breakdown was observed to be fully reversible when using an intershock interval of 1 min (20-23 degrees C). These findings support energetic models of membrane electropermeabilization which describe the formation of membrane pores (or growth of existing pores) to a conducting state (instability), followed by a rapid expansion of these pores when the energy barrier for the formation of hydrophilic pores is overcome (breakdown).

Determination of physical membrane properties of plant cell protoplasts via the electrofusion technique: prediction of optimal fusion yields and protoplast viability

By variation of physical parameters (field strength, pulse duration) which result in electrofusion and electroporation, properties of the plasma membrane of different types of plant cell protoplasts were analyzed. The lower threshold for that field pulse intensity at which membrane breakdown occurred (recorded as fusion event) depended on pulse duration, protoplast size, and protoplast type (tobacco, oat; vacuolated, evacuolated). This fusion characteristic of plant protoplasts can also be taken as a measure of the charging process of the membrane and allows thus a non-invasive determination of the time constant and the specific membrane capacitance. Although the fusion yield was comparable at pulse duration/field strength couples of, e.g., 10 μs/1.5 kV*cm(-1) and 200 μs/0.5 kV*cm(-1), hybrid viability was not. Rates of cell wall regeneration and cell division of tobacco mesophyll protoplasts were not affected but may have been increased at short pulse duration/high field strength. Plating efficiency, in contrast, was significantly decreased with longer pulse duration at low field strengths.

Membrane electroporation--fast molecular exchange by electroosmosis

Human and rabbit erythrocyte ghosts loaded with FITC-dextran (mol. mass = 10 kDa) and NBD-glucosamine (mol. mass = 342 Da) in buffers of different ionic strength and composition were subjected to electric pulses (intensity 0.7 kV/mm and decay half-time 1 ms) at 7-10 degrees C and 20-24 degrees C. The transfer of the fluorescent dyes from the interior of the ghosts through the electropores was observed by low light level video microscopy. The pulses caused the fluorescence to appear outside the membranes as a transient cylindrical cloud directed toward the negative electrode during the first video frame (17 ms). It was similar in both rabbit and human erythrocyte ghosts and at both temperatures but differs for the two dyes, the fluorescence cylinder is long and tall for the FITC-dextran and relatively short and thick for the NBD-glucosamine. The molecular exchange was 2-3 orders of magnitude faster within the first 17 ms after the pulse than the diffusional exchange. It decreased with increasing ionic strength. Formulae for the transfer of molecules by electroosmotic flow through the pores are in agreement with these observations. They allow estimation of the total area of pores with radii larger than that of the fluorescent dye during the pulse. The major conclusion is that electroosmosis is the dominating mechanism of molecular exchange in electroporation of erythrocyte ghosts.

Stable, resealable pores formed in sea urchin eggs by electric discharge (electroporation) permit substrate loading for assay of enzymes in vivo

We describe a simple electroporation procedure for loading suspensions of unfertilized sea urchin eggs with impermeant small molecules under conditions that allow close to 90% successful fertilization and development. Poration is carried out in a low-Ca2+ medium that mimicks the intracellular milieu. The induced pores remain open for several minutes in this medium, allowing loading of the cells; resealing is achieved by adding back millimolar calcium ions to the medium. While the pores are open, an influx of exogenous molecules and efflux of endogenous metabolites takes place, and the eggs can lose up to 40% of their ATP content and still survive. Introduced metabolites are utilized by the cells, e.g., introduced 3H-thymidine is incorporated into DNA. This procedure will be useful for loading impermeant substrates into eggs, permitting in vivo assessment of metabolism, and also for introducing other interesting impermeant molecules, such as inhibitors, fluorescent indicators, etc. Though the details may differ, the principle of electroporation in an intracellular-like medium may prove to be useful for loading other cell types with minimal loss of viability.

High yields of stable transformants by hypo-osmolar plasmid electroinjection

Electrotransfection of mouse L cells and macrophages suspended in strongly hypo-osmolar solutions gave high yields of stable transformants which significantly exceeded the clone number obtained under iso-osmolar conditions. The cells survived these extremely low osmolarities for 1 h without any apparent deterioration of cellular or membrane functions. Highest yields were obtained in buffered 75 mosmol solutions containing 30 mM KCl and an appropriate amount of inositol provided that the strength of the breakdown pulse was matched to the dramatic increase in cell volume at low osmolarity. The absolute clone number depended on the post-incubation time in the hypo-osmolar solution after application of a single breakdown pulse at 4 degrees C. The absolute number of transformants was maximum when post-incubation was restricted to 2 min. Towards longer incubation times the absolute number decreased even though the relative clone number was similar. This was because of a corresponding decrease of the number of viable cells. It is conceivable that enhanced DNA uptake in hypo-osmolar solutions is faciliated by an overall slight (and reversible) increase in membrane permeability generated by the osmotically created tension in the membrane of the swollen cells.