Kinetics of ultrastructural changes during electrically induced fusion of human erythrocytes

Electropermeabilization of the cell membrane

Membrane electropermeabilization is the observation that the permeability of a cell membrane can be transiently increased when a micro-millisecond external electric field pulse is applied on a cell suspension or on a tissue. Applicative aspects for the transfer of foreign molecules (macromolecules) into the cytoplasm are routinely used. But only a limited knowledge about what is really occurring in the cell and its membranes at the molecular levels is available. This chapter is a critical attempt to report the present state of the art and to point out some of the still open problems. The experimental facts associated to membrane electropermeabilization are firstly reported. They are valid on biological and model systems. Secondly, soft matter approaches give access to the bioelectrochemical description of the thermodynamical constraints supporting the destabilization of simplified models of the biological membrane. It is indeed described as a thin dielectric leaflet, where a molecular transport takes place by electrophoresis and then diffusion. This naïve approach is due to the lack of details on the structural aspects affecting the living systems as shown in a third part. Membranes are part of the cell machinery. The critical property of cells as being an open system from the thermodynamical point of view is almost never present. Computer simulations are now contributing to our knowledge on electropermeabilization. The last part of this chapter is a (very) critical report of all the efforts that have been performed. The final conclusion remains that we still do not know all the details on the reversible structural and dynamical alterations of the cell membrane (and cytoplasm) supporting its electropermeabilization. We have a long way in basic and translational researches to reach a pertinent description.

An experimental system for controlled exposure of biological samples to electrostatic discharges

Electrostatic discharges occur naturally as lightning strokes, and artificially in light sources and in materials processing. When an electrostatic discharge interacts with living matter, the basic physical effects can be accompanied by biophysical and biochemical phenomena, including cell excitation, electroporation, and electrofusion. To study these phenomena, we developed an experimental system that provides easy sample insertion and removal, protection from airborne particles, observability during the experiment, accurate discharge origin positioning, discharge delivery into the sample either through an electric arc with adjustable air gap width or through direct contact, and reliable electrical insulation where required. We tested the system by assessing irreversible electroporation of Escherichia coli bacteria (15 mm discharge arc, 100 A peak current, 0.1 μs zero-to-peak time, 0.2 μs peak-to-halving time), and gene electrotransfer into CHO cells (7 mm discharge arc, 14 A peak current, 0.5 μs zero-to-peak time, 1.0 μs peak-to-halving time). Exposures to natural lightning stroke can also be studied with this system, as due to radial current dissipation, the conditions achieved by a stroke at a particular distance from its entry are also achieved by an artificial discharge with electric current downscaled in magnitude, but similar in time course, correspondingly closer to its entry.

Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer

Phylogenetic studies show that horizontal gene transfer (HGT) is a significant contributor to genetic variability of prokaryotes, and was perhaps even more abundant during the early evolution. Hitherto, research of natural HGT has mainly focused on three mechanisms of DNA transfer: conjugation, natural competence, and viral transduction. This paper discusses the feasibility of a fourth such mechanism--cell electroporation and/or electrofusion triggered by atmospheric electrostatic discharges (lightnings). A description of electroporation as a phenomenon is followed by a review of experimental evidence that electroporation of prokaryotes in aqueous environments can result in release of non-denatured DNA, as well as uptake of DNA from the surroundings and transformation. Similarly, a description of electrofusion is followed by a review of experiments showing that prokaryotes devoid of cell wall can electrofuse into hybrids expressing the genes of their both precursors. Under sufficiently fine-tuned conditions, electroporation and electrofusion are efficient tools for artificial transformation and hybridization, respectively, but the quantitative analysis developed here shows that conditions for electroporation-based DNA release, DNA uptake and transformation, as well as for electrofusion are also present in many natural aqueous environments exposed to lightnings. Electroporation is thus a plausible contributor to natural HGT among prokaryotes, and could have been particularly important during the early evolution, when the other mechanisms might have been scarcer or nonexistent. In modern prokaryotes, natural absence of the cell wall is rare, but it is reasonable to assume that the wall has formed during a certain stage of evolution, and at least prior to this, electrofusion could also have contributed to natural HGT. The concluding section outlines several guidelines for assessment of the feasibility of lightning-triggered HGT.

Electron microscopic demonstration and evaluation of irreversible electroporation-induced nanopores on hepatocyte membranes

PURPOSE

To demonstrate, evaluate, and verify the existence of irreversible electroporation (IRE)-ablation induced nanopores on the plasma membrane of hepatocytes.

MATERIALS AND METHODS

On animal research committee approval, four New Zealand rabbits and two Yorkshire swine underwent IRE ablation of the liver (90 pulses, 100 μs per pulse at 2,500 V), and selected ablated liver tissues were harvested, fixed, and air-dried according to the electron microscopy (EM) protocol. A scanning electron microscope (SEM; Nova 230 NanoSEM [FEI, Hillsboro, Oregon] with 80 picoamperes and 10-kV acceleration) was used to visualize and verify IRE-created nanopores. Using NIH image (Bethesda, Maryland) and ImageScope (Aperio Inc., Vista, California), 21 ablated tissues (16 rabbit, 5 swine) were evaluated. Corresponding hematoxylin and eosin (H&E) evaluation was performed to verify IRE-induced cell death.

RESULTS

In all 21 IRE-ablated tissues, the SEM was able to show numerous, well-circumscribed, round, and concave-shaped pore defects disturbing the hepatocyte plasma membranes. These pores were not seen in normal liver. The size of the nanopores ranged from 80-490 nm with the greatest frequency of pores in bimodal distribution. The highest frequency of pore size was noted at the size range 340-360 nm.

CONCLUSIONS

IRE induces nanopores on hepatocyte membranes, as shown by SEM. The pore diameters are larger than nanopores created by reversible electroporation (RE), which may have implications for irreversibility or permanency.

Quantitative analysis of surface micro-roughness alterations in human spermatozoa using atomic force microscopy

A new male contraceptive given the name RISUG (an acronym for Reversible Inhibition of Sperm Under Guidance) has been developed by our research group. RISUG is a bioactive polymer and is injected into the lumen of the vas deferens using a no-scalpel approach. The polyelectrolytic nature of this contraceptive induces a surface charge imbalance on sperm membrane system leading to its destabilization. Complete disintegration of the plasma membrane with subsequent rupture and dispersion of the acrosomal contents is observed on RISUG treatment. In the present study, micro-structural properties of human spermatozoa exposed to RISUG in vitro have been quantitatively analysed using atomic force microscopy. The parameters used to quantify these morphological changes include amplitude (peak-valley height difference, arithmetic roughness, root mean square roughness) and spatial roughness. Factor loadings (Varimax rotation) have been used to determine the parameters displaying maximum variation. Further, sperm cells have been classified in various principal-component planes using principal-component analysis. The periodic structural features of the atomic force microscopy images have also been obtained using power spectral analysis.

Cell hybridization by electrofusion on filters

Electric field pulses induce permeabilization and associated fusogenicity in cell membranes. Electrofusion of cells is usually performed in two steps: the first is the creation of close intercellular contacts; the second is an application of electric pulses that induces membrane fusion. Very large cell contacts can be obtained by a filter aspiration method. A cell monolayer is created by controlled suction on biocompatible filter. No spontaneous fusion results. Just after filtration, electrofusion is obtained by field pulses applied parallel to the filter. Cell viability is not strongly affected and cells recover their spherical shape in the minute time range after filtration. The electrical parameters, the cell density, and the flow rate control fusion. Fusion is obtained with cells of different origins with very different adhesion properties. Hybrid cells are easily formed. This approach appears to be a very efficient method for cell hybridization with an easy-to-use protocol.

Human tumour and dendritic cell hybrids generated by electrofusion: potential for cancer vaccines

Hybrid cells created by fusion of antigen presenting and tumour cells have been shown to induce potent protective and curative anti-tumour immunity in rodent cancer models. The application of hybrid cell vaccines for human tumour therapy and the timely intervention in disease control are limited by the requirement to derive sufficient autologous cells to preserve homologous tumour antigen presentation. In this study, the efficiency of various methods of electrofusion in generating hybrid human cells have been investigated with a variety of human haemopoietic, breast and prostate cell lines. Cell fusion using an electrical pulse is enhanced by a variety of stimuli to align cells electrically or bring cells into contact. Centrifugation of cells after an exponential pulse from a Gene Pulser electroporation apparatus provided the highest yield of mixed cell hybrids by FACS analysis. An extensive fusogenic condition generated in human cells after an electrical pulse contradicts the presumption that prior cell contact is necessary for cell fusion. Alignment of cells in a concurrent direct current charge and osmotic expansion of cells in polyethylene glycol also generated high levels of cell fusion. Waxing of one electrode of the electroporation cuvette served to polarize the fusion chamber and increase cell fusion 5-fold. Optimisation of a direct current charge in combination with a fusogenic pulse in which fusion of a range of human cells approached or exceeded 30% of the total pulsed cells. The yield of hybrid prostate and breast cancer cells with dendritic cells was similar to the homologous cell fusion efficiencies indicating that dendritic cells were highly amenable to fusion with human tumour cells under similar electrical parameters. Elimination of unfused cells by density gradient and culture is possible to further increase the quantity of hybrid cells. The generation and purification of quantities of hybrid cells sufficient for human vaccination raises the possibility of rapid, autologous tumour antigen presenting vaccines for trial with common human tumours.

Multiple-pulse-mediated electrofusion of intact erythrocyte onto human term placental amnion

The creation of surface modified human term placental amnion by electrofusing human cells onto its surface has been thought of. A multiple-pulse electrofusion protocol with 10 square pulses of 10-micros pulse length, and electric field of 0.2 kV cm(-1), can make erythrocyte-amnion tissue electrofusion possible. The protocol devised merge the cell-tissue-adherence steps with fusogenic pulse. The finding opens up a new avenue of cell electrofusion onto human tissue with minimal procedural complexities.

Time-dependent ultrastructural changes to porcine stratum corneum following an electric pulse

The morphological changes to heat-stripped porcine stratum corneum following an electroporating pulse were studied by time-resolved freeze fracture electron microscopy. Pulses at a supra-electroporation threshold of 80 volts and 300 microseconds were applied across the stratum corneum with a pair of copper plate electrodes, which also served as cooling contacts. Multilamellar vesicles of 0.1-5.5 mm in diameter in the intercellular lipid bilayers of the stratum corneum appeared in less than milliseconds after pulsing. Pulsed samples exhibited aggregations of vesicles, whereas only occasional single vesicles were seen in the unpulsed samples. Aggregates form in less than a millisecond and disappear within minutes after the pulse. Their size ranged from 0.3 to 700 mm2. The size of individual vesicles, aggregate density, and size were analyzed as functions of postpulse time. These aggregate formations seem to be a secondary reaction to the pulse-induced skin permeabilization, determined by the resistance drop and recovery after the pulse. Heating the samples to 65 degrees C also caused vesicle aggregates of similar appearance to form, suggesting that these aggregations are related to the heating effect of the pulse. Hydration is thought to play an important role in aggregate formation.

Membrane microextension: a possible mechanism for establishing molecular contact in electrofusion

True cell membrane contact is an essential condition for electro-pulsed cell fusion, but initial morphological perturbation leading to true contact is still not clear. Dielectrophoresis mediated compression and fusogenic pulse induced compaction of cells led to rapid merger of tight membranes, and deprived direct microscopic view of surface membrane perturbation. Freely suspending cells with large and different cell-cell gaps may proceed to electrofusion with perturbed membrane and initiates fusion events at different time. These pulsed exposed cells can be used for capturing changes in the membrane surface and early electrofusion events. Early stage of fusion of freely suspended intact human erythrocytes exposed to single exponential decay pulse was studied by scanning electron microscopy (SEM). Field pulse induces small membrane bumps. Interaction of bumps on adjacent membranes lead to true membrane contact and form bridges between the membranes as microextension, combining both membranes into a topologically single structure. Some fusion products showed expanded fusion zones, which suggest indication of open lumen at contact area.

Theoretical aspects of single pulse induced modulation of cell-cell interactions favoring iso-electrofusion

A single pulse-mediated electrofusion of freely suspending widely separated cells could reduce steps in the protocol. Prediction of such electrofusion is still unsolved. In pre-pulse condition, quantitative estimation with three-layered model cell surface reveals electrostatic repulsion (due to negative surface charge) is the major hindrance to membrane-to-membrane contact. On theoretical grounds we predict that designed single high voltage pulsing on widely separated model cell surface can counteract the non-specific repulsive interaction and favor approach of two apposed membranes. But hydrodynamic modulation of pulse exposed cell surface interaction can hamper approach of membranes contact when cells are held at a gap of more than approximately 450 A.

Electropermeabilization of mammalian cells to macromolecules: control by pulse duration

Membrane electropermeabilization to small molecules depends on several physical parameters (pulse intensity, number, and duration). In agreement with a previous study quantifying this phenomenon in terms of flow (Rols and Teissié, Biophys. J. 58:1089-1098, 1990), we report here that electric field intensity is the deciding parameter inducing membrane permeabilization and controls the extent of the cell surface where the transfer can take place. An increase in the number of pulses enhances the rate of permeabilization. The pulse duration parameter is shown to be crucial for the penetration of macromolecules into Chinese hamster ovary cells under conditions where cell viability is preserved. Cumulative effects are observed when repeated pulses are applied. At a constant number of pulses/pulse duration product, transfer of molecules is strongly affected by the time between pulses. The resealing process appears to be first-order with a decay time linearly related to the pulse duration. Transfer of macromolecules to the cytoplasm can take place only if they are present during the pulse. No direct transfer is observed with a postpulse addition. The mechanism of transfer of macromolecules into cells by electric field treatment is much more complex than the simple diffusion of small molecules through the electropermeabilized plasma membrane.

Correlation between electric field pulse induced long-lived permeabilization and fusogenicity in cell membranes

Electric field pulses have been reported to induce long-lived permeabilization and fusogenicity on cell membranes. The two membrane property alterations are under the control of the field strength, the pulse duration, and the number of pulses. Experiments on mammalian cells pulsed by square wave form pulses and then brought into contact randomly through centrifugation revealed an even stronger analogy between the two processes. Permeabilization was known to affect well-defined regions of the cell surface. Fusion can be obtained only when permeabilized surfaces on the two partners were brought into contact. Permeabilization was under the control of the pulse duration and of the number of pulses. A similar relationship was observed as far as fusion is concerned. But a critical level of local permeabilization must be present for fusion to take place when contacts are created. The same conclusions are obtained from previous experiments on ghosts subjected to exponentially decaying field pulses and then brought into contact by dielectrophoresis. These observations are in agreement with a model of membrane fusion in which the merging of local random defects occurs when the two membranes are brought into contact. The local defects are considered part of the structural membrane reorganization induced by the external field. Their density is dependent on the pulse duration and number of pulses. They support the long-lived permeabilization. Their number must be very large to support the occurrence of membrane fusion.

Truncation of the COOH-terminal region of the paramyxovirus SV5 fusion protein leads to hemifusion but not complete fusion

The role of the simian virus 5 (SV5) fusion (F) protein 20 residue COOH-terminal region, thought to represent the cytoplasmic tail, in fusion activity was examined by constructing a series of COOH-terminal truncation mutants. When the altered F proteins were expressed in eukaryotic cells, by using the vaccinia virus-T7 transient expression system, all the F proteins exhibited similar intracellular transport properties and all were expressed abundantly on the cell surface. Quantitative and qualitative cell fusion assays indicated that all of the F protein COOH-terminal truncation mutants mediated lipid mixing with similar kinetics and efficiency as that of wild-type F protein. However, the cytoplasmic content mixing activity decreased in parallel with the extent of the deletion in the F protein COOH-terminal truncation mutants. These data indicate that it is possible to separate the presumptive early step in the fusion reaction, hemifusion, and the final stage of fusion, content mixing, and that the presence of the F protein COOH-terminal region is important for the final steps of fusion.

GPI-anchored influenza hemagglutinin induces hemifusion to both red blood cell and planar bilayer membranes

Under fusogenic conditions, fluorescent dye redistributed from the outer monolayer leaflet of red blood cells (RBCs) to cells expressing glycophosphatidylinositol-anchored influenza virus hemagglutinin (GPI-HA) without transfer of aqueous dye. This suggests that hemifusion, but not full fusion, occurred (Kemble, G. W., T. Danieli, and J. M. White. 1994. Cell. 76:383-391). We extended the evidence for hemifusion by labeling the inner monolayer leaflets of RBCs with FM4-64 and observing that these inner leaflets did not become continuous with GPI-HA-expressing cells. The region of hemifusion-separated aqueous contents, the hemifusion diaphragm, appeared to be extended and was long-lived. But when RBCs hemifused to GPI-HA-expressing cells were osmotically swollen, some diaphragms were disrupted, and spread of both inner leaflet and aqueous dyes was observed. This was characteristic of full fusion: inner leaflet and aqueous probes spread to cells expressing wild-type HA (wt-HA). By simultaneous video fluorescence microscopy and time-resolved electrical admittance measurements, we rigorously demonstrated that GPI-HA-expressing cells hemifuse to planar bilayer membranes: lipid continuity was established without formation of fusion pores. The hemifusion area became large. In contrast, for cells expressing wt-HA, before lipid dye spread, fusion pores were always observed, establishing that full fusion occurred. We present an elastic coupling model in which the ectodomain of wt-HA induces hemifusion and the transmembrane domain, absent in the GPI-HA-expressing cells, mediates full fusion.

Characterization of PEG-mediated electrofusion of human erythrocytes

Polyethylene glycol (PEG) and electrofusion were applied together in a simple and highly efficient cell fusion method. PEG (8000 M(r)) was used to bring human erythrocytes into contact, and a single 4.4 kV/cm, 80 microseconds duration pulse was applied to cell suspensions. The fusion yield (FY) is PEG concentration-dependent. A maximum FY (50%) was found at about 10% PEG. Higher PEG concentrations (> 10%) suppressed FY caused by colloid osmotic shrinkage. Morphological changes, such as colloidal osmotic swelling and shrinking, and the expanding and contraction of fusion lumen, when suspension media were changed from PBS to isotonic 15% dextran solutions, was examined by microscopy. FY was found to depend on both simple osmotic and colloidal-osmotic swelling. From the swelling behavior, we propose two types of electropores: the pre-fusion sites between cell pairs, and electropores on each individual cell connecting intracellular and extracellular space. The latter type is responsible for the colloidal osmotic swelling and shrinking of cell which, together with simple osmotic swelling, is responsible for expanding the pre-fusion sites into fusion lumens. Resealing of electropores resulted in reducing FY, but the FY can be restored by simple osmotic shock. Apparently, PEG plays two opposite roles in this fusion method; one is to promote pre-pulse and post-pulse cell-cell contact, protecting pre-fusion sites, and the other suppresses FY by colloid osmotic shrinkage of cells after pulsing, especially when high PEG concentration is used. 10% PEG 8000 represents the optimal combination of these properties.

Stages leading to and following fusion of sperm and egg plasma membranes

The site of gamete interaction of electrophysiologically recorded Lytechinus variegatus eggs, fixed with osmium tetroxide (OsO4) and/or glutaraldehyde (GTA) at varying intervals after the onset of the increase in membrane conductance induced by an attached sperm, has been examined by high-voltage and conventional transmission electron microscopy. Although GTA and a GTA-OsO4 mixture induced different electrical responses, specimens prepared with the two fixatives were ultrastructurally similar. In specimens observed within 5 s of the change in conductance, the acrosomal process projected through the vitelline layer and abutted the egg plasma membrane. A conspicuous layer of bindin surrounded the acrosomal process and connected the sperm to the egg's vitelline layer. In a fortuitous specimen fixed within 4 s following the change in conductance, the area of contact between the gamete plasma membranes possessed a trilaminar structure that separated the egg's and sperm's cytoplasms. The morphology of this area of contact was consistent with previously proposed intermediates of membrane fusion. Five to six seconds after the change in conductance, the sperm was connected to the egg via a narrow cytoplasmic bridge that consisted of the former acrosomal process and a projection of the egg cortex. The region of the bridge midway between the fused gametes was encircled by dense material that marked the site of sperm-egg fusion. Gamete interactions in which the activation potential was recorded (unclamped egg) were comparable in time and ultrastructure to events taking place in voltage-clamped eggs except for one major difference. Intact cortical granules (one to three) were observed beneath the tip of the incorporating sperm in unclamped eggs fixed following the onset of the activation potential, whereas all cortical granules dehisced in clamped eggs.

Electrofusion of cell-size liposomes

Cell size liposomes of egg phosphatidylcholine (PC), trans-acylated egg phosphatidylethanolamine (PE), bovine brain phosphatidylserine (PS) and egg phosphatidylglycerol, suspended in 2.5% of polyethylene glycol (M(r) 8000), Ficoll (M(r) 400,000) or Dextran (M(r) 71,200) were aligned by dielectrophoresis and fused by applying a 1.7 kV/cm pulse of varying duration. Because the internal and external media of liposome suspension can be controlled, and the surface charge is known, the results can be described mathematically. The fusion yields (FY) at different pulse length were measured by microscopy. The FY curves were sigmoidal, with a common minimally required pulse length of 19 microseconds, and shifted to longer pulse length with increasing external media conductivity or vesicle surface charge. The types of polymer in solutions had little effect. The minimal required pulse length was interpreted to be the minimum rise time for the smallest vesicle capable of reaching the bilayer breakdown potential induced by the pulse, and the sigmoidal shape FY curves represented the cumulative vesicle size distribution curve. The shifting of FY curves upon changing media conductivity or vesicle surface charge were quantitatively accounted for by the balance of pulse-induced dipole-dipole attraction and electrostatic repulsion. Deviation from sigmoidal shape FY curves in the cases of charged liposomes was explained by increasing electrostatic repulsion due to vesicle deformation under pulse-induced dipole pressure. The data confirm the hypothesis that membrane potential breakdown is a pre-requisite or minimum requirement for electrofusion, and support our earlier proposition that pulse-induced dipole force plays an important role in the electrofusion process, and that electrostatic repulsion posts additional barrier to electrofusion.

Studies of cell pellets: II. Osmotic properties, electroporation, and related phenomena: membrane interactions

Using the relations between pellet structure and electric properties derived from the preceding paper, the responses of rabbit erythrocyte pellets to osmotic or colloidal-osmotic effects from exchanged supernatants and from electroporation were investigated. Changing the ionic strength of the supernatant, or replacing it with dextran or poly(ethylene glycol) solutions, caused changes of Rp according to the osmotic behavior of the pellet. Rp was high and ohmic before electroporation, but dropped abruptly in the first few microseconds once the transmembrane voltage exceeded the membrane breakdown potential. After the initial drop, Rp increased as a result of the reduction of intercellular space. Rp increased regardless of whether the pellets were formed before or immediately after the pulse, indicating that porated cells experienced a slow colloidal-osmotic swelling. The intercellular or intermembrane distances between cells in a pellet, as a function of osmotic, colloidal-osmotic, and centrifugal pressures used to compress rabbit erythrocyte pellets, were deduced from the Rp measurement. This offered a unique opportunity to measure the intermembrane repulsive force in a disordered system including living cells. Electrohemolysis of pelleted cells was reduced because of limited swelling by the compactness of the pellet. Electrofusion was observed when the applied voltage per pellet membrane exceeded the breakdown voltage. The fusion yield was independent of pulse length greater than 10 microseconds, because after the breakdown of membrane resistance, voltage drop across the pellet became insignificant. Replacing the supernatant with poly(ethylene glycol) or dextran solutions, or coating pellets with unporated cell layers reduced the colloidal-osmotic swelling and hemolysis, but also reduced the electrofusion yield. These manipulations can be explored to increase electroloading and electrofusion efficiencies.

Control by pulse parameters of electric field-mediated gene transfer in mammalian cells

Electric field-mediated gene transfer in mammalian cells (electrotransformation) depends on the pulsing conditions (field intensity, pulse duration, number of pulses). The effect of these parameters was systematically investigated using the transient expression of the chloramphenicol acetyltransferase and the beta-galactosidase activities in Chinese hamster ovary cells. Pulsing conditions inducing reversible permeabilization of the cell plasma membrane are not sufficient to induce gene transfer. The plasmid must be present during the electric pulse if it is to be transferred across the membrane into the cytoplasm. Only the localized part of the cell membrane brought to the permeabilized state by the external field is competent. Pulse duration plays a key role in the magnitude of the transfer. The field induces a complex reaction between the membrane and the plasmid that is accumulated at the cell interface by electrophoretic forces. This leads to an insertion of the plasmid, which can then cross the membrane.

Nucleic acid transfer through cell membranes: towards the underlying mechanisms

Various cases of DNA (RNA) transfer through membranes of living cells are reviewed. They are classified into two major categories: those which occur in Nature (natural transfer) and those imposed by various physical and chemical treatments of cells (induced transfer). Among the examples of natural transfer surveyed are the transfer during bacterial conjugation, genetic transformation, viral infection of bacteria, and nuclear membrane trafficking. Consideration of the induced transfer is focused on the two methods most widely used at present to introduce foreign genetic information into pro- and eukaryotic cells: Ca2+ (and some other divalent cations)-induced and calcium phosphate-induced transfer, and transfer during electroporation of cells. Emphasis is made on the underlying mechanisms of transfer, or rather on what is currently known about them. Energetic aspects of transfer are also discussed and different tentative models of transfer are presented.

Electrical properties of cell pellets and cell electrofusion in a centrifuge

A new approach is proposed for studying cell deformability by centrifugal force, electrical properties of cell membranes in a high electric field, and for performing efficient cell electrofusion. Suspensions of cells (L929 and four other cell types examined) are centrifuged in special chambers, thus forming compact cell pellets in the gap between the electrodes. The setup allows measurement of the pellet resistance and also the high-voltage pulse application during centrifugation. The pellet resistance increases sharply with the centripetal acceleration, which correlates with reduction of the cell pellet porosity due to cell compression and deformation. Experiments with cells pretreated with cytochalasin B or colcemid showed that cell deformability depends significantly on the state of cytoskeleton. When the voltage applied to the cell pellet exceeds a 'critical' value, electrical breakdown (poration) of cell membranes occurs. This is seen as a deflection in the I(V) curve for the cell pellet. The electropores formed during the breakdown reseal in several stages: the fastest takes 0.5-1 ms while the whole process completes in minutes. A novel effect of colloid-osmotic compression of cell pellets after electric cell permeabilization is described. Supercritical pulse application to the cell pellet during intensive centrifugation leads to massive cell fusion. The fusion index grows with the increase of centripetal acceleration, and drops drastically when the pulse is applied after the centrifuge is stopped. The colloid-osmotic pellet compression enhances the fusion efficiency. No fusion occurs when cells are brought in contact after the pulse treatment. The data suggest that tight intermembrane contact formed prior to pulse application is a prerequisite condition for efficient cell electrofusion. The capacities of the technique proposed and the mechanism of membrane electrofusion are discussed.

An experimental evaluation of the critical potential difference inducing cell membrane electropermeabilization

When applied on intact cell suspension, electric field pulses are known to induce membrane permeabilization (electropermeabilization) and fusion (electrofusion). These effects are triggered through a modulation of the membrane potential difference. Due to the vectorial character of the electric field effects, this modulation, which is superimposed on the resting membrane potential difference, is position-dependent on the cell surface. This explains the difference between the experimentally observed critical field strengths requested to trigger the processes of permeabilization and fusion. The critical membrane potential difference which induces membrane permeabilization can be calculated from these experimental observations. It is observed that its value is always about 200 mV for many different cell systems as we previously reported in the case of pure lipid vesicles. This is much less than assumed in most previous studies.

Kinetics and mechanism of cell membrane electrofusion

A new quantitative approach to study cell membrane electrofusion has been developed. Erythrocyte ghosts were brought into close contact using dielectrophoresis and then treated with one square or even exponentially decaying fusogenic pulse. Individual fusion events were followed by lateral diffusion of the fluorescent lipid analogue 1,1'-dihexadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) from originally labeled to unlabeled adjacent ghosts. It was found that ghost fusion can be described as a first-order rate process with corresponding rate constants; a true fusion rate constant, k(f), for the square waveform pulse and an effective fusion rate constant, k(ef), for the exponential pulse. Compared with the fusion yield, the fusion rate constants are more fundamental characteristics of the fusion process and have implications for its mechanisms. Values of k(f) for rabbit and human erythrocyte ghosts were obtained at different electric field strength and temperatures. Arrhenius k(f) plots revealed that the activation energy of ghost electrofusion is in the range of 6-10 kT. Measurements were also made with the rabbit erythrocyte ghosts exposed to 42 degrees C for 10 min (to disrupt the spectrin network) or 0.1-1.0 mM uranyl acetate (to stabilize the bilayer lipid matrix of membranes). A correlation between the dependence of the fusion and previously published pore-formation rate constants for all experimental conditions suggests that the cell membrane electrofusion process involve pores formed during reversible electrical breakdown. A statistical analysis of fusion products (a) further supports the idea that electrofusion is a stochastic process and (b) shows that the probability of ghost electrofusion is independent of the presence of Dil as a label as well as the number of fused ghosts.

Relationships between the surface exposure of acidic phospholipids and cell fusion in erythrocytes subjected to electrical breakdown

The procoagulant activity of human erythrocytes, which provides a measure of the translocation of acidic phospholipids from the inner to the outer monolayer of the plasma membrane, has been compared with the percentage cell fusion in experiments on the effects of electrical breakdown pulses under differing experimental conditions. After treatment with breakdown pulses of 20 microseconds or longer (5 kV cm-1), the plasma membranes of erythrocytes in 250 mM sucrose exhibited an almost complete loss of asymmetry with respect to acidic phospholipids. As the breakdown voltage was increased from 2 to 5 kV cm-1 (with breakdown pulses of 99 microseconds), the surface exposure of acidic phospholipids and cell fusion increased approximately in parallel. Furthermore, with 99 microseconds pulses and a voltage of 3 kV cm-1, a decrease in the osmolarity from 250 to 150 mM of the sucrose medium was accompanied by an increase in both the surface exposure of acidic phospholipids and the extent of cell fusion. Breakdown pulses of 2-5 microseconds were sufficient to cause a marked loss of asymmetry, but no cell fusion was observed unless the pulse length was at least 20 microseconds. Kinetic experiments indicated that exposure of the acidic phospholipids at the cell surface was more likely to be due to a direct effect of the electric field pulses on plasma membrane structure than to secondary effects, such as the action of endogenous proteinases on the membrane skeleton. It seems possible that a localised, surface exposure of acidic phospholipids may contribute to the 'long-lived fusogenic state' (Sowers, A.E. (1986) J. Cell Biol. 102, 1358-1362) and the 'transient permeant structures' (Teissié, J. and Rols, M.P. (1986) Biochem. Biophys. Res. Commun. 140, 258-266) that enable cell fusion to occur when contact between cells is established after they have been subjected to field pulses. Our observations also provide circumstantial support for the concept that changes in the phospholipid asymmetry of membranes may be important in physiologically-occurring instances of biomembrane fusion.

Escherichia coli membranes during electrotransformation: an electron microscopy study

Structural changes undergone by Escherichia coli cell envelope membranes under the conditions of electrically induced gene (DNA) transfer (exponential pulse of about 13 kV/cm, tau = 5 ms) were studied by freeze-fracture electron microscopy. Special device similar to that of Stenger and Hui [1986) J. Membr. Biol. 93, 43-53), that allowed cryofixation of samples almost simultaneously with application of electric pulse, was employed to examine the cells within a short time (less than or equal to 1 s) after the pulse. Extensive blebbing of cells was observed immediately after the pulse. At later times (30-40 s after the pulse) blebbing was not detected, instead infrequent cellular membrane fusion and formation of large membrane 'opening' or pores were observed. An attempt to relate the observed membrane changes with cellular viability and permeability to exogenous DNA failed. Challenge of cells with a plasmid DNA 10 s after the pulse application resulted in a dramatic loss (at least four orders of magnitude) of the number of transformants compared to cells pulsed in the presence of DNA. On the other hand the results on additional pulsing of cell prior to the main electrotransformation procedure suggested that the life-time of membrane defects is at least no less than 2 min. Possible ways to reconcile the results are suggested.

Membrane fusion without cytoplasmic fusion (hemi-fusion) in erythrocytes that are subjected to electrical breakdown

There are many reports of hemi-fusion in phospholipid vesicles but few published studies on hemi-fusion in cells. We report evidence from both fluorescence microscopy and freeze-fracture electron microscopy for hemi-fusion in the electrofusion of human erythrocytes. We have also characterised the conditions that favour hemi-fusion as opposed to complete fusion, and discuss the possibility that hemi-fusion might precede complete electrically-induced cell fusion. A membrane probe (DiIC16) and a cytoplasmic probe (6-carboxyfluorescein) were used to investigate the behaviour of doubly-labelled human erythrocytes which were aligned in chains by dielectrophoresis and then exposed to high voltage breakdown pulses. Some of the cells were fused by the pulses, as shown by diffusion of both membrane and cytoplasmic probes from labelled to unlabelled cells. With other cells, the membrane probe diffused into unlabelled cells after the breakdown pulses, without the cytoplasmic probe diffusing into unlabelled cells or leaking into the medium. Membrane fusion (hemi-fusion) thus occurred without cytoplasmic fusion in these erythrocytes. Such cells were irreversibly, but fragilely, attached to their neighbours by the breakdown pulses. There was an inverse relationship between conditions that permit complete fusion and those that favour hemi-fusion, with respect to breakdown pulse length, breakdown voltage and, in particular, osmolarity and temperature. The incidence of hemi-fusion in 250 mM erythritol was twice that in 150 mM erythritol, and hemi-fusion was 5-fold greater at 25 degrees C than at 20 degrees C. Hemi-fused erythrocytes occasionally fused completely on heating to 50 degrees C, demonstrating that hemi-fusion can proceed to complete cell fusion. Freeze fracture electron micrographs of preparations of hemi-fused cells revealed long-lived, complementary depressions and protrusions on the E- and P-fracture faces, respectively, of tightly apposed cells that may mediate hemi-fusion. The possibility that the fusion of closely adjacent human erythrocytes by electrical breakdown pulses may involve an intermediate, shared bilayer structure, which is stable in certain conditions but which can be ruptured by osmotic swelling of the permeabilised cells, is discussed.

Dipole interactions in electrofusion. Contributions of membrane potential and effective dipole interaction pressures

The contributions of pulse-induced dipole-dipole interaction to the total pressure acting normal to the membranes of closely positioned pronase treated human erythrocytes during electrofusion was calculated. The total pressure was modeled as the sum of pressures arising from membrane potential and dipole-dipole attraction opposed by interbilayer repulsion. The dipole-dipole interaction was derived from the experimentally obtained cell polarizability. The threshold electric field amplitude necessary for fusion of pronase-treated human erythrocytes was experimentally obtained at various combinations of pulse duration, frequency, and the conductivity of the external medium. The theoretical values of the critical electric field amplitude compared favorably to the experimentally obtained threshold field amplitudes. Fusion by dc pulses may be primarily attributed to attainment of sufficiently high membrane potentials. However, with decreasing external conductivity and increasing sinusoidal pulse frequency (100 kHz-2.5 MHz), the induced dipole-dipole interactions provide the principal driving force for membrane failure leading to fusion.

Study of mechanisms of electric field-induced DNA transfection. II. Transfection by low-amplitude, low-frequency alternating electric fields

Electroporation for DNA transfection generally uses short intense electric pulses (direct current of kilovolts per centimeter, microseconds to milliseconds), or intense dc shifted radio-frequency oscillating fields. These methods, while remarkably effective, often cause death of certain cell populations. Previously it was shown that a completely reversible, high ionic permeation state of membranes could be induced by a low-frequency alternating electric field (ac) with a strength one-tenth, or less, of the critical breakdown voltage of the cell membrane (Teissie, J., and T. Y. Tsong. 1981. J. Physiol. (Paris). 77:1043-1053). We report the transfection of E. coli (JM105) by plasmid PUC18 DNA, which carries an ampicillin-resistance gene, using low-amplitude, low-frequency ac fields. E. coli transformants confer the ampicillin resistance and the efficiency of the transfection can be conveniently assayed by counting colonies in a selection medium containing ampicillin. For the range of ac fields employed (peak-to-peak amplitude 50-200 V/cm, frequency 0.1 Hz-1 MHz, duration 1-100 s), 100% of the E. coli survived the electric field treatment. Transfection efficiencies varied with field strength and frequency, and as high as 1 x 10(5)/micrograms DNA was obtained with a 200 V/cm square wave, 1 Hz ac field, 30 s exposure time, when the DNA/cell ratio was 50-75. Control samples gave a background transfection of much less than 10/micrograms DNA. With a square wave ac field, the transfection efficiency showed a frequency window: the optimal frequency was 1 Hz with a 200 V/cm field, and was approximately 0.1 Hz with a 50 V/cm field. Transfection efficiency varied with the waveform: square wave > sine wave > triangle wave. If the DNA was added after the ac field was turned off, transfection efficiency was reduced to the background level within 1 min. The field intensity used in this study was low and insufficient to cause electric breakdown of cell membranes. Thus, DNA transfection was not caused by electroporation of the cell membranes. Other possible mechanisms will be considered.

Changes in membrane structure induced by electroporation as revealed by rapid-freezing electron microscopy

Cells can be transiently permeabilized by exposing them briefly to an intense electric field (a process called "electroporation"), but it is not clear what structural changes the electric field induces in the cell membrane. To determine whether membrane pores are actually created in the electropermeabilized cells, rapid-freezing electron microscopy was used to examine human red blood cells which were exposed to a radio-frequency electric field. Volcano-shaped membrane openings appeared in the freeze-fracture faces of electropermeabilized cell membranes at intervals as short as 3 ms after the electrical pulse. We suggest that these openings represent the membrane pathways which allow entry of macromolecules (such as DNA) during electroporation. The pore structures rapidly expand to 20-120 nm in diameter during the first 20 ms of electroporation, and after several seconds begin to shrink and reseal. The distribution of pore sizes and pore dynamics suggests that interactions between the membrane and the submembrane cytoskeleton may have an important role in the formation and resealing of pores.

A mechanism of formation of protein-free regions in the red cell membrane: the rupture of the membrane skeleton

The process of rupture and redistribution of the red cell membrane skeleton is analyzed theoretically. Following the emergence of the rupture the spectrin-actin network is redistributed on the cytoplasmic surface of the membrane bilayer. Due to the interaction of the membrane skeleton and integral proteins the redistribution of the spectrin-actin network leads to the release of purely lipid regions of the membrane. The scale of the protein redistribution caused by the rupture of the membrane skeleton and the size of the lipid domains produced depend on the shape of the membrane and the value of the electrical interaction of the membrane proteins. The lipid domains occurring as a result of the rupture and relaxation of the spectrinactin network can spontaneously increase or decrease its area. The criteria determining the conditions which result in the system's evolutions leading to the domain growth have been obtained. The character of the evolution is determined by the shape of the membrane region in which the rupture occurs as well as the relation between the effective linear tension of the rupture boundary and the modulus of elasticity of the spectrin-actin network.

Cell poration and cell fusion using an oscillating electric field

It has been shown in previous studies that cell poration (i.e., reversible permeabilization of cell membrane) and cell fusion can be induced by applying a pulse (or pulses) of high-intensity DC (direct current) electric field. Recently we suggested that such electro-poration or electro-fusion can also be accomplished by using an oscillating electric field. The DC field relies solely on the dielectric breakdown of the cell membrane to induce cell fusion. The oscillating field, on the other hand, can produce not only a dielectric breakdown, but also a sonicating motion in the membrane that could result in a structural fatigue. Thus, a combination of a DC field and an oscillating field is expected to enhance the efficiency of cell poration and cell fusion. This study is an experimental test of such an idea. Here, pulses of high-intensity, DC-shifted RF (radio frequency) electric field were used to induce cell poration and cell fusion. The fusion experiments were done on human red blood cells. The poration experiments were done on a fibroblast cell line using a molecular probe (which is a DNA plasmid containing the marker gene chloramphenicol acetyltransferase, CAT) and assayed by a gene transfection technique. It was found that the pulsed RF field is highly efficient in both cell fusion and cell poration. Also, in comparison with electro-poration using a DC field, the RF field results in a higher percentage of cells surviving the exposure to the electric field.

Signal transduction by membrane receptors in viable electropermeabilized cells: isoproterenol-stimulated cyclic AMP synthesis in C6 glioma cells

The activity of beta-adrenergic receptors at the plasma membrane level was investigated in viable, electropermeabilized C6 glioma cells. Electric field pulses were applied directly to the plated cells without any previous proteinase treatment. The affinity for isoproterenol and the density of the beta-adrenergic receptors, as judged from the number of [3H]CGP-12177 binding sites, were not affected by the electropermeabilization whereas the isoproterenol-stimulated cAMP accumulation was transiently impaired. This decrease in activity is due to an electropermeabilization-induced GTP leak. Normal activity could be obtained either by treating the cells by the electric field in a GTP-containing buffer, or by spontaneous recovery of the cells after the resealing of the plasma membrane, with a delay depending on the temperature. The activity of the receptors was not affected by the structural organization of the membrane associated to its electropermeabilization.

Cytoskeletal reorganization during electric-field-induced fusion of Chinese hamster ovary cells grown in monolayers

Mammalian cells were shown to fuse after direct electric pulsation of the plated cells in culture. The extent of fusion was controlled by the duration of the post-pulse incubation. Formation of polynucleated cells was slow, even at 37 degrees C. Pre-pulse incubation with colchicine increased the fusion yield slightly. Cytoskeletal organization during the post-pulse incubation was observed using immunofluorescence techniques. Microfilaments were unaffected, but microtubules disappeared during the first minutes following the pulse, and then reformed on subsequent incubation.

Effects of pH on cell fusion induced by electric fields

Electrofusion has recently become an important area of cell biology research. We studied the effects of pH of the cell medium on the electrofusion of human red blood cells. Cell fusion was monitored by observing the movement of a lipophylic dye between neighboring fused cells using a fluorescence microscope. The cells were first brought into close contact by dielectrophoresis. Fusion was then induced by three pulses of high-intensity electric field. Within minutes following the pulse application, many cells were observed to fuse together to form fusion chains of different lengths. We found that the optimal pH for cell fusion is around pH 7.5. At this pH, the fusion yield was highest (ranging from 57 to 81%) and the average number of cells within a fusion chain was also the largest. The dependence of cell fusion on pH is more sensitive at low than at high pH. The fusion yield was decreased by 40% when the pH was changed from 7.5 to 6.0, but there was only a 20% decrease in yield between pH 7.5 and 10.0. We suspect that the observed pH effects may be caused by a redistribution of fixed charges at the cell surface, or changes in amphipathicity of the surface proteins.

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

Asymmetric breakdown (occurring in only one hemisphere of the cell) was induced in freely suspended and dielectrophoretically aligned vacuole-containing or evacuolated plant protoplasts as well as in isolated vacuoles. In suspended cells breakdown was restricted to the hemisphere facing the anode and in isolated vacuoles to the opposite hemisphere. This difference in the orientation of the asymmetric breakdown can be explained by the opposite direction of the intrinsic membrane potentials of isolated vacuoles and of cells on which the generated potential difference is superimposed. The ensuing permeabilization of the membrane was microscopically monitored by dye uptake and by release of chloroplasts and of cytoplasmic and/or vacuolar solutes. The asymmetric release of intracellular substances (organic acids and/or amino acids) was detected by accumulation of chemotactic bacteria (Pseudomonas aeruginosa) close to the permeabilised membrane area of the cells or vacuoles. Maximum bacteria accumulation required about 5 min and subsequently disappeared after a further 20 min presumably because of the restoration of the original membrane impermeability. With vacuoles retention of the accumulated bacteria was shorter indicating that the resealing process of the tonoplast membrane was faster than that of the plasmalemma. From the kinetics of bacteria accumulation and retention it is therefore possible to deduce information about the life-span and the resealing properties of electropermeabilized membrane areas on the single-cell level. Symmetric breakdown in both hemispheres of the cells could be achieved by electric field-mediated cell rotation of about 180 degrees between two pulses of the same polarity or by application of two pulses of alternating polarity. In dielectrophoretically aligned protoplasts of comparable diameter, breakdown occurred in both hemispheres, even though the breakdown was still asymmetric. It could be demonstrated by the uptake of the vital dye neutral red that the size of the membrane area which was permeabilized was much larger in that hemisphere oriented to the anode than in the other one. The relevance of these observations for further improvement of electroinjection of macromolecules and of electrofusion is discussed. In particular, it is pointed out that positioning of differently sized cells in electric field-mediated hybridisation and the polarity of the breakdown pulse is of great importance with respect to hybrid yield.

Ionic-strength modulation of electrically induced permeabilization and associated fusion of mammalian cells

Application of a high electric field to cells in culture has been shown to make them both permeable and fusogenic. The molecular events involved in the phenomenon are still poorly understood. In this study we investigated the effects of the ionic strength of the pulsing buffer on the electropermeabilization and electrofusion of Chinese hamster ovary cells. Increasing the ionic strength of the pulsing medium results in an increase in sieving of transient permeant structures, but decreases the fusion index. Treatment of cells with trypsin or pronase before application of the pulses abolishes the ionic modulation of both electropermeabilization and electrofusion. A similar rate of expansion of permeabilization is obtained whatever the ionic content of the pulsing buffer, and cells fuse even at high ionic strength. This observation lends support to our hypothesis that membrane proteins play a role in electrofusion.