Divalent cations, phospholipid asymmetry and osmotic swelling in electrically-induced lysis, cell fusion and giant cell formation with human erythrocytes

Effect of static electric fields in root cells of Vicia faba (Fabaceae)

Serial electron microscopic sections were prepared from half-ripened meristematic root cells of Vicia faba (Fabaceae) which had been exposed gradually to 700, 1000, 2500, 3500, and 5000 V/m static electric fields during seven days with and without Zn and Cd electrodes. At the end of five weeks, wall loosenings and very small nuclei were observed in those root cells which were exposed to static electric currents from the lower side of the medium without electrodes, while abnormalities in cell formation, e.g., two cells with one nucleus, and GER occurrence were present in an electrolytic (Cd upward and Zn downward) medium. The cells exposed to a static current from the upper side of the medium had small nuclei and abnormal cell divisions in the electrolyte, but in a non-electrolyte very large nuclei and thicker cell walls were observed, the cytoplasm was dense with GER, pinocytosis was seen filled with mitochondria, and protoplast formation with big nuclei was seen in exocytosis.

Agents facilitating the electric field-induced fusion of intact rabbit erythrocytes

In the recent years, the role of specific membrane active agents in the electrofusion process has started to draw attention, and it has been found that the presence of various substances in the cell medium can affect the fusion process either in a positive or negative way. In this work, the effect of several proteins, bivalent cations and antibiotics was tested with respect to their ability to protect intact erythrocytes from hemolysis and facilitate the fusion process. The effect of different sugars was also studied. Among the different proteins, pronase and proteinase were found to be the most effective. With respect to bivalent cations, Ca2+ and Mn2+ were more effective while Mg2+ was less important. From the antibiotics, penicillin caused a negative effect while streptomycin acted positively. Finally, glucose medium was found to be the most effective compared to all sugars tested. The results indicated that there are strong differentiations of the induced effects caused by each substance, and some possible mechanisms of action of these agents on the erythrocyte membrane were discussed.

Phosphatidylserine-dependent adhesion of T cells to endothelial cells

Phosphatidylserine (PS) was exposed at the surface of human umbilical vein endothelial cells (HUVECs) and cultured cell lines by agonists that increase cytosolic Ca(2+), and factors governing the adhesion of T cells to the treated cells were investigated. Thrombin, ionophore A23187 and the Ca(2+)-ATPase inhibitor 2, 5-di-tert-butyl-1,4-benzohydroquinone each induced a PS-dependent adhesion of Jurkat T cells. A23187, which was the most effective agonist in releasing PS-bearing microvesicles, was the least effective in inducing the PS-dependent adhesion of Jurkat cells. Treatment of ECV304 and EA.hy926 cells with EGTA, followed by a return to normal medium, resulted in an influx of Ca(2+) and an increase in adhering Jurkat cells. Oxidised low-density lipoprotein induced a procoagulant response in cultured ECV304 cells and increased the number of adhering Jurkat cells, but adhesion was not inhibited by pretreating ECV304 cells with annexin V. PS was not significantly exposed on untreated Jurkat cells, as determined by flow cytometry with annexin V-FITC. However, after adhesion to thrombin-treated ECV304 cells for 10 min followed by detachment in 1 mM EDTA, there was a marked exposure of PS on the Jurkat cells. Binding of annexin V-FITC to the detached cells was inhibited by pretreating them with unlabelled annexin V. Contact with thrombin-treated ECV304 cells thus induced the exposure of PS on Jurkat cells and, as Jurkat cells were unable to adhere to thrombin-treated ECV304 cells in the presence of EGTA, the adhesion of the two cell types may involve a Ca(2+) bridge between PS on both cell surfaces. The number of T cells from normal, human peripheral blood that adhered to ECV304 cells was not increased by treating the latter with thrombin. However, findings made with several T cell lines were generally, but not completely, consistent with the possibility that adhesion to surface PS on endothelial cells may be a feature of T cells that express both CD4(+) and CD8(+) antigens. Possible implications for PS-dependent adhesion of T cells to endothelial cells in metastasis, and early in atherogenesis, are discussed.

Electrofusion: a biophysical modification of cell membrane and a mechanism in exocytosis

The molecular bases of the exocytosis process remain poorly known. Many proteins have been recognized to play key roles in the machinery. Their functions are well characterized in the specificity of the docking processes. Forces involved in the merging of the two partners must take into account the physics of membrane interfaces. The target membrane and the vesicle are both electrically charged interfaces. Strong electrostatic fields are triggered when they are brought in close neighborhood. These fields are high enough to induce an electropermeabilisation process. It is now well known that when applied on a cell, an external field induces a modulation of the transmembrane potential difference. When high enough the transmembrane potential may induce a membrane destabilisation. This results in a free exchange of polar molecules across well defined parts of the cell surface. Furthermore, when permeabilization is present on two cells, if those parts of the cell surfaces are brought in close contact, membrane merging occurs spontaneously. Cell fusion results from this membrane coalescence. The similarity with what is taking place in exocytosis is striking. The present review describes the state-of-the-art in the knowledge on electrofusion. It is emphasized that it results from electropermeabilisation and not from a direct effect of the external field. A local destabilisation of the vesicle membrane results from electrostatic interactions while keeping unaffected its viability. Such processes appear relevant for what takes place during exocytosis.

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.

Electropermeabilization of cell membranes

A position dependent modulation of the membrane potential difference is induced when an electric field is applied to a cell. When cells are submitted to short lived electric field pulses with an overcritical intensity, a local membrane alteration is induced, which may reseal. Its molecular definition remains unknown. A free exchange of hydrophilic molecules takes place across the membrane. A leakage of cytosolic metabolites is present. However, a loading of polar drugs into the cytoplasm is obtained. A short description of the processes affecting the cell membrane organization is given. Lipids appear as the primary target of the field effect as in the case of liposomes. Nevertheless membrane proteins appear to be affected by a direct or by a back effect. The permeabilized state is long lived. The cell metabolism plays indeed a critical role in the recovery. The cell viability can be nevertheless preserved.

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.

The effect of lipid molecular packing stress on cationic liposome-induced rabbit erythrocyte fusion

The effect of curvature stress on the efficiency of cationic liposome-induced fusion between rabbit erythrocytes was studied. Multilamellar cationic liposomes containing 1,2-dioleoyl-3-trimethylammoniumpropane (DOTAP) and different PEs (1,2-dilnoleoyl-sn-glycero-3-phosphoethanolamine (dilin-PE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), and lysophosphatidylethanolamine, egg (lyso-PE)) were used to induce cell-cell fusion. It was found that high cell-cell fusion yield (FY) of about 50% could be achieved in sucrose solution by using cationic liposomes containing 50% DOTAP. Cell-cell fusion was assayed by shape criterion and was verified by fluorescence microscopy, using a membrane dye mixing method. The curvature stress, as a result of mixing unsaturated PEs in cationic liposomes, had a significant effect on cell-cell FY which increased with the degree of acyl chain unsaturation, in the order dilin-PE > DOPE > POPE > lyso-PE. Replacement of dilin-PE, DOPE, or POPE by lyso-PE gradually in cationic liposomes lowered the cell-cell FY from 50% to 1%. Furthermore, cationic liposome induced cell lysis, and fusion between cationic liposomes and cells, as assayed by the N-(lissamine rhodamine B sulfonyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt and N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2- dihexadecanoyl-sn-glycero-3-phosphoethanolamine, triethylammonium salt (Rh-PE/NBD-PE) energy transfer method, followed the same order as that for cell-cell fusion. Fusion between the negatively charged PS liposomes and cationic liposomes also followed the same order. The electric double layer screening by electrolytes in NaCl-containing solution and phosphate buffered saline (PBS) was found to reduce the cell-cell FY and cell lysis. These findings suggest that liposome-induced cell-cell fusion was achieved by cationic liposomes serving as fusion-bridges among cells.

Electrofusion between heterogeneous-sized mammalian cells in a pellet: potential applications in drug delivery and hybridoma formation

High-efficiency electrofusion between cells of different sizes was achieved by application of fusing electric pulses to cells in centrifuged pellets. Larger target cells (Chinese hamster ovary or L1210 cells) were stacked among smaller human erythrocytes or erythrocyte ghosts by sequential centrifugation at 700 g to form five-tier pellets in a specially designed centrifugation-electrofusion chamber. The membranes of erythrocytes and ghost were labeled with fluorescent membrane dye (1,1' dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine (Dil)), and the contents of ghosts were loaded with water-soluble fluorescent dye (42-kDa fluorescein isothiocyanate dextran (FITC-dextran)), to monitor heterogeneous cell fusion. Fusion efficiency was assayed by the extent of either membrane dye mixing or contents (FITC-dextran) mixing with target cells. Four rectangular electric pulses at 300 V and 80 microseconds each were found to give the optimal fusion results of approximately 80% heterogeneous fusion by the content-mixing assay and approximately 95% by the membrane-dye-mixing assay. Cell viability remained greater than 80% after electrofusion. Because of the electric breakdown of cell membranes at the beginning of the pulse, the pellet resistance and hence the partial voltage across the pellet reduced rapidly during the remaining pulse time. This voltage redistribution favored the survival of fused cells. The limited colloidal-osmotic swelling of cells in pellets enhanced cell-cell contact and increased the pellet resistance after each pulse. As a result, the partial voltage across the pellet was restored when the next pulse was applied. This redistribution of pulse voltage in the pellet system permitted the breakdown of cell membranes at a lower applied voltage threshold than that required for electrofusion of cells in suspension or in dielectrophoretic cell chains. The cell viability and soluble dye retention within cells (FITC-dextran) remained at the same high levels for 3 h when the cells were incubated in respective culture media with serum at 37 degrees C. Viability and dye retention decreased significantly within 30 min when cells were incubated in phosphate-buffered saline without serum. The pellet technique was applied to form hybridomas by fusion of larger SP2/0 murine myelomas with smaller naive mouse lymphocytes. An optimum of 173 +/- 70 hypoxanthine aminopterin thymidine (HAT)-selected clones of the hybridomas was obtained from 40,000 SP2/0 cells and 1.5 x 10(6) lymphocytes used in each trial. This high-efficiency fusion technique may be adapted to mediate drug and gene transfer to target cells ex vivo as well as to form hybrid cells with limited cell sources.

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.

Interactions between metal ions and poly(ethylene glycol) in the fusion of human erythrocytes

Diffusion of the fluorescent membrane probe, Dil-C16 (3), from labelled to unlabelled human erythrocytes has been employed to monitor hemi-fusion (membrane fusion) in monolayers of cells exposed to poly(ethylene glycol) (PEG). Diffusion of the cytoplasmic probe, 6-carboxyfluorescein, was used similarly to monitor cell fusion (cytoplasmic mixing). Hemi-fusion, which is normally seen when erythrocytes are exposed to dehydrating concentrations of commercial PEG 6000, did not occur when the PEG was pretreated with Chelex 100 resin to remove metal ions. Cytoplasmic mixing, which is normally observed when the dehydrated erythrocytes are substantially rehydrated, also failed to occur when both PEG 6000 and the rehydrating buffer had been treated with Chelex 100. The re-addition to Chelex-treated PEG of components removed by the resin, and the addition of 10 mu mM concentrations of La3+ or Al3+, restored its ability to induce hemi-fusion and cell fusion. Higher concentrations of several other metals, including Ca2+, were also effective. These observations show that metal ions are required for hemi-fusion with erythrocytes in the presence of PEG, and that dehydration alone is insufficient to induce hemi-fusion. Phosphatidylserine was apparently not accessible in erythrocytes treated with PEG 6000 until the cells were rehydrated. This indicates that metal ions do not assist the hemi-fusion of erythrocytes by forming trans complexes with surface phosphatidylserine when the cells are dehydrated by PEG.(ABSTRACT TRUNCATED AT 250 WORDS)