Electrical breakdown, electropermeabilization and electrofusion

Spectrofluorometric assay for the quantitation of cell-tissue electrofusion

Fluorometric assay for quantitating electrofusion, FAQE, was developed to measure the number of cells fused to intact tissue. The fluorescent vital dye hydroethidine is used in this method. The fluorescent intensity detected in the cell tissue hybrids is proportional to the number of individual cells fused. The number of cells fused was determined after fusion by lysing the epithelial layer with 0.2% sodium dodecyl sulfate and the supernatant fluids were measured in a spectrofluorometer and compared to established standard curves. The mean number of cells fused, in five separate experiments, was determined to be approximately 5000. All the experimental corneas had approximately the same number of fused cells with less than 10% variation. In addition, the technique was used to demonstrate an increase in the number of cells fused when multiple fusions were applied to the cell-tissue electrofusion system. These results demonstrate that FAQE can be utilized to quantitatively analyze the fusion yields.

The role of myosin I and II in cell motility

It has been recognized since the turn of the century that cell motility by non-muscle cells requires virtually continuous restructuring of the cytoskeleton (see refs [1-4]). It is also clear that cell motility requires a mechanism for converting chemical energy into mechanical work. The proteins actin and myosin, two important constituents of the cytoskeleton, have been postulated to act as the chemicomechanical transducer in motile cells. Central to their role as a force generating mechanism in motile cells is the ability of myosin (a) to hydrolyze ATP when it interacts with actin and (b) to form filaments. Recent studies on mammalian cells and on the cellular slime mold Dictyostelium discoideum have shed light and at the same time raised questions regarding the involvement of myosin in cell motility. Moreover, they have demonstrated the presence of two types of myosins, called myosin II and myosin I, that have unique biochemical and regulatory properties and that may play different roles in mediating cell motility. In this chapter we will discuss the properties of these two myosins and then describe what is known about their involvement in Dictyostelium and mammalian cell motility.

Electrically induced protoplast fusion for ergosterol-producing yeast strain improvement

Electrofusion was employed for hybrid construction in ergosterol-producing yeast strains. Some fusion products proved to be hybrid with respect to ergosterol content and to remain stable over several generations.

Selection of hybrid plants obtained by electrofusion of vacuolated x evacuolated plant protoplasts in hypo-osmolar solution

Vacuolated and evacuolated tobacco mesophyll protoplasts were electrically fused in hypo-osmolar media by using an alternating field of modulated amplitude for alignment. The vacuolated fusion partner was isolated from Nicotiana tabaccum L. cv Xanthi and the evacuolated one from the streptomycin-resistant strain Nicotiana tabaccum L. cv Petit Havana SR1. The field and osmolarity conditions used ensured relatively high yields of heterologous fusion products despite the differences in density and size of the parental cells. After removal of the evacuolated, streptomycin-resistant fused and unfused protoplasts by flotation of vacuole-containing cells on iso-osmolar sucrose medium, the cybrids and hybrids were cultured in 25 microliters drops of agarose. During the first 5 weeks the non-fused Xanthi-protoplasts were used as a nurse culture. After addition of streptomycin to the growth media, cybrids and hybrids were successfully selected whereas fused and unfused vacuole-containing protoplasts died within 6 days. Only the streptomycin-resistant cybrids and hybrids developed into whole plants. On average a yield of 0.025% of streptomycin-resistant plants (referred to the total number of parental cells) was obtained. Polyacrylamide gel electrophoresis of leaf extracts of these plants showed that at least 50% of the streptomycin-resistant plants had a hybrid-esterase isoenzyme pattern. The protocol can be generalised by fusion of iodoacetamide-inactivated vacuolated protoplasts with meristematic (or evacuolized) protoplasts carrying no genetic marker. Use of evacolated protoplasts for electrofusion with vacuole-containing protoplasts therefore offers a way of overcoming the lack of suitable genetic markers for hybrid selection.

Efficient hybridization of mouse-human cell lines by means of hypo-osmolar electrofusion

The fusion of a mouse-human heteromyeloma with a mouse hybridoma is used as a model to define parameters to generate human hybridomas. Electrofusion of these cells in 300 mosM and 75 mosM solutions showed that strong hypo-osmolar conditions resulted in a dramatic increase in the efficiency of hybridoma formation. In contrast to iso-osmolar electrofusion, a high hybrid yield could be obtained by injection of only a single field pulse. The field strength was adjusted in proportion to the increased size of the cells in hypo-osmolar solutions. Hypo-osmolar electrofusion allowed the generation of approximately 0.45% hybrids at a suspension density of 1.75 X 10(5) mouse-human cells/ml corresponding to an input number of 3.5 X 10(4) mouse-human cells. A further increase in the efficiency of hybridoma formation to about 0.6% was achieved by cell alignment in an alternating field of modulated field strength. Experiments in which the total cell number per fusion chamber was decreased at constant optimum suspension density showed that a further increase in the efficiency of hybridoma formation in hypo-osmolar solution was not possible because of the increasing influence of the heterogeneity of the cell lines with decreasing cell number. The results allow the conclusion that hypo-osmolar electrofusion is a potential tool to enhance successful immortalisation of human B lymphocytes.

Electro-permeabilization of cell membranes: effect of the resting membrane potential

Electric field induced permeabilization of cell membranes is an important technique for gene transfection and cell hybridization. Mechanistic studies of this process revealed that the uptake of fluorescent indicator by plant protoplasts occurs predominantly on the hemisphere facing the positive electrode, while in erythrocyte ghosts the probes exit through the hemisphere facing the negative electrode. To reconcile these observations symmetrical pore formation and a mechanism of molecular exchange by electroosmosis has been proposed. In light of these controversial observations, we conducted a systematic study of electroporation of NIH3T3 cells with varying electric field strength, waveform and frequency. Our data revealed that (i) symmetrical permeabilization of the cell membrane occurs only with bipolar a.c. fields. (ii) When a critical membrane breakdown potential, Vc, is applied using either an unipolar a.c. fields or a single d.c. square pulse, the cell membrane becomes permeabilized only at the hemisphere facing the positive electrode. (iii) When the pulse-induced membrane potential, Vm, is approximately equal to or larger than the intrinsic membrane potential (i.e. using d.c. or unipolar a.c. field), asymmetric permeabilization was observed with the hemisphere facing the positive electrode being most permeable. (iv) The rate of fluorescent indicator uptake is dependent on the concentration of the indicator. These results indicate that electro-permeabilization of cell membranes is affected by its resting potential and that electroosmosis is not the dominant mechanism for the cellular uptake of foreign molecules in electroporation.

Biphasic voltage relaxation pattern observed in cells of Eremosphaera viridis after injection of charge-pulses of short duration: detection of tip clogging of intracellular microelectrodes by charge-pulse technique

Charge pulse experiments performed on the peat-bog alga Eremosphaera viridis revealed an unusual voltage relaxation behaviour. Injection of charge pulses of 1 microseconds duration resulted in an immediate charging of the membranes (time constant of the order of 40 ns). Nevertheless, the potential-measuring microelectrode recorded an exponential increase in membrane voltage with a time constant of about 1.3 ms. The maximum voltage value was recorded after about 3 ms, followed by an exponential decay with a time constant of about 9.6 ms. This biphasic time course was independent of the amplitude of the injected charge and of the location of the impaled microelectrodes in the vacuole. Centrifuged cells in which the chloroplasts and the other organelles were pelleted in one part of the cells showed the same electrical response. Electrical breakdown of the cell membranes resulted in the disappearance of the biphasic voltage response. In this case only the decaying relaxation process could be recorded with a time constant of 3 ms. After resealing of the membranes the original biphasic relaxation response was restored. Increasing concentrations of KCl in the bathing medium reduced both time constants almost correspondingly. The experimental findings were evaluated with an electrical equivalent circuit. Theoretical analysis with reference to the experimental data suggested that the delayed voltage response of the potential-recording electrode resulted from a membrane seal across the tip of this electrode. The resistance of this seal was calculated to be about 400 M omega. The specific resistances and capacitances of tonoplast and plasmalemma membranes were calculated from the decaying part of the biphasic relaxation curves. The average values were found to be 2.58 omega.m2 and 5 mF.m-2. The investigations reported here suggest that charge pulse experiments can be generally used for the detection of membrane and cytoplasmic material clogging of the tip of intracellular microelectrodes, a problem with which most electrophysiologists are faced when interpreting data obtained from impaled microelectrodes.

Facilitated electrofusion of vacuolated x evacuolated oat mesophyll protoplasts in hypo-osmolar media after alignment with an alternating field of modulated strength

Electrofusion of evacuolated and vacuolated oat leaf protoplasts is difficult because of the different size and density of these cells which results in separation of the two fusion partners during dielectrophoresis. The fusion yield of this cell system was considerably enhanced by electrofusion in hypo-osmolar media containing 0.4 M mannitol, 0.1 mM calcium acetate and 0.1% bovine serum albumin. This increase in yield was only achieved if the dielectrophoretically induced membrane contact between the two fusion partners was enhanced by an initial short 'burst' of higher field strength (500 V/cm, peak to peak, for 5 s followed by a reduction of to 90 V/cm, peak to peak, for 20 s, frequency 1 MHz). Due to the high field strength of the alternating field at the beginning of cell chain formation separation of fusion partners of different size and density was mainly avoided. Simultaneously, the short duration of this high field 'burst' avoided the generation of lethal effects in the cell membranes. The subsequent low field strength of the alternating field was sufficient to keep the aligned cells in position. Optimum fusion was induced by a single square pulse of 750 V/cm and 30 musec duration. The time required for rounding up of the heterologous fusion products decreased with decreasing osmolarity. Fusion resulted in a 5.7 +/- 1.2% yield of heterologous fusion products (compared to 0.7% using the conventional electrofusion protocol) as determined by flow cytometric assay. About 50% of the vacuolated oat protoplasts and 20-50% of the heterologous fusion products regenerated their cell walls within 5 days after hypo-osmolar treatment, but no cell divisions could be observed. Evacuolated oat protoplasts died after 2-3 days in culture without any detectable cell wall regeneration.

Differential effects of electrofusion and electropermeabilization parameters on the membrane integrity of plant protoplasts

Ethane was used as an indicator of lipid peroxidation in order to characterize the membrane damage induced by electrical pulses during the processes of electrofusion and electropermeabilization. The increase of ethane in fused protoplasts ofVicia faba L. was found to be correlated with the intensity of field strength and pulse number, which also affected the yield of hybrids. The degree of membrane damage is postulated to depend on the accumulation of lipid free-radicals, which can be increased by light, by longer storage time of protoplasts and by higher field strength and pulse number. As a result, the conditions for electropermeabilization lead to greater membrane damage compared with those for electrofusion. The measurement of ethane production may prove to be useful for characterization of the membrane integrity, viability and regeneration ability of protoplasts.

Permeabilizing cells: some methods and applications for the study of intracellular processes

The techniques described allow controlled permeabilization of plasma membranes from different types of cells for gaining access to the cell interior and enables one to control intracellular events. Most common techniques are electropermeabilization, permeabilization with mild non-ionic detergents such as saponin and digitonin and by pore-forming toxins, such as alpha-toxin and streptolysin O. Whereas electropermeabilization and alpha-toxin create small pores of approximately 2 nm, digitonin, saponin, and streptolysin O form bigger holes and therefore also allow the introduction of large molecules, such as enzymes and immunoglobulins. A disadvantage of the latter methods is the loss of cytosolic constituents which might be necessary for signal-transduction pathways in the cell. In secretory cells the main requirement for exocytosis appears to be Ca2+, which brings about the full response comparable to hormone effects in some cells (platelets), adrenal medullary cells, but not in all cells (pancreatic acinar cells). The nucleotide, anion, and cation requirements are different for different cell types and are probably intimately related to the cell-specific mechanisms involved in exocytosis such as regulation of ion channels and ion carriers, or the involvement of nucleotide-binding proteins. Since permeabilized cells are preparations intermediate between intact cells and isolated organelles, they offer great opportunities for the advancement of our understanding of the mechanisms involved in stimulus-response coupling.

Slip and leak in mitochondrial oxidative phosphorylation

During oxidative phosphorylation by mammalian mitochondria part of the free energy stored in reduced substrates is dissipated and energy is released as heat. Here I review the mechanisms and the physiological significance of this phenomenon.

The number of molecules taken up by electroporated cells: quantitative determination

Fluorescent and fluorescent-labeled molecules were used with calibrated flow cytometric fluorescence measurements of electrically pulsed cells (intact yeast: Saccharomyces cerevisiae) to demonstrate a method for determining the net number of molecules transported into electroporated cells. For the conditions used, a single pulse of width 50 microseconds and magnitude 8.0 +/- 0.5 kV/cm resulted in an average net molecular uptake which is large, n = 1.4 x 10(5) molecules of 70 kDa FITC-dextran (supplied extracellular concentration of 500 microM), and n = 1.0 x 10(8) molecules of 660 Da propidium iodide (PI; 80 microM). Both molecules were present in pulsed cells at less than equilibrium values, consistent with a transient uptake mechanism. Intracellular FITC-dextran is present in soluble form, while PI is predominantly bound to nucleic acids. A broad, statistically significant distribution of molecular uptake was also observed. Such quantitative determinations should be important for guiding applications of electroporation, and for testing models of electroporation mechanisms. Further, the use of PI, which is well established as a membrane exclusion dye, provides additional support for the interpretation that both PI and FITC-dextran were internalized as a result of an electrical pulse.

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.

Electrofusion parameters for mouse two-cell embryos

We studied electrofusion of mouse two-cell embryos in order to define parameters which would result in a high yield of fused embryos. Various cell alignment times (from <10 to >60 s) and alternating current percentages (2 to 100%) were examined. The fusion parameters tested were the number of fusion pulses (1-9), pulse length (30-90 mus) and pulse strength (0.50-1.79 kV/cm). Furthermore different combinations of these three parameters were tested. In addition the influence of several embryo culture media on the fusion rates was examined. The results show that the fusion rate of the embryos increases with shorter alignment and higher percentages of the alternating current. The highest fusion rate (95%) was obtained by use of one pulse with a duration of 70 mus and a field strength of 0.60-0.79 kV/cm. The survival rate of the embryos was best if Whitten Medium was used before and after the fusion pulses. The fusion of two-cell stages results in tetraploid embryos which can serve as models for studies in polyploid cells.

Electrofusion of individual animal cells directly to intact corneal epithelial tissue

An electromechanical process was developed to electrofuse human and nonhuman cultured cells directly to rabbit corneal epithelial tissue in vitro and in situ. This new process was utilized successfully to incorporate functional gonococcal membrane attachment receptors from human lymphoma cells into superficial rabbit corneal epithelium. Thus, cell-tissue electrofusion biotechnology may be employed to establish unique and novel animal models for investigating receptor-mediated processes in vivo.

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.

Electric field-induced cell fusion and human monoclonal antibodies

A persistent problem in the generation of antigen-specific human monoclonal antibodies is the rarity of appropriate B cells in human blood or splenic tissues. In order to immortalize the rare antigen-specific cells that are available, an electric field-induced cell fusion technique has been shown to markedly increase the fusion efficiency in comparison to polyethylene glycol-induced cell fusion using Epstein-Barr virus (EBV) or pokeweed mitogen activated B cells. Fusion efficiency of 10(-3)-10(-4) has been achieved by this process with as low as 1 X 10(6) input EBV-activated B cells. A panel of human monoclonal antibodies to human cytomegalovirus has subsequently been produced using this technique. This improvement should enable wider therapeutic and diagnostic applications of human monoclonal antibodies.

Magnetically controlled targeted micro-carrier systems

Magnetically controlled targeted drug delivery systems are aimed at concentrating drugs at a defined target site, with the aid of a magnetic field. This technique has been developed specifically for directing drugs away from the reticuloendothelial system (RES). Literature on this topic suggests that these delivery systems are capable of altering the distribution of chemotherapeutic agents in the body. Hence these delivery devices offer the possibility of improving the therapeutic efficacy of the associated drugs. This paper reviews the work done to date towards the development and evaluation of biodegradable and non-biodegradable magnetic targeted drug delivery systems and outlines their future prospects and limitations in cancer chemotherapy.

Electrophysiological and ultrastructural studies on reversible neural conduction disturbance after high voltage discharge

High-voltage condenser discharges exerting a field strength of up to 1000 V/cm (discharge time constant 0.24-8 msec) applied to isolated sciatic frog nerve lead to disturbances of the propagation of action potentials including transient complete block of conduction. Such conduction disturbances are normally reversible within minutes. Inhibition of the activity of the membrane-bound Na+-K+ATPase prevents the recovery from conduction block. Withdrawal of external Ca2+ also prevents recovery, whereas blockade of protein synthesis by cycloheximide has no influence. The velocity of recovery depends on the temperature, with temperature coefficients (Q10) from 1.31 to 1.84 between 2 degrees and 30 degrees C. Transmission electron microscopy of nerves subjected to strong discharges shows alterations of the myelin sheath (splitting and cleft formation) which are, however, not specific for this mechanism of injury. No alterations are seen in the region of the free axoplasmic membrane of the node of Ranvier or in organelles. The results suggest a breakdown of the transmembrane ionic gradient causing the conduction disturbance.

High-speed electro-fusion and electro-transfection of plant protoplasts by a continuous flow electro-manipulator

A continuous flow electro-manipulator available both for mass production of fused and of transfected plant protoplasts was devised using a flow chamber with gold-coated glass panel electrodes. Up to 100 ml of protoplasts suspension were treated within 20 min at the rate of approximately 1×10(6) protoplasts / min. The yield of diheterokaryons between tobacco mesophyll and carrot root protoplasts reached approximately 10 % of total protoplasts by flow electro-fusion. More than 95 % of tobacco and cowpea mesophyll protoplasts became infected with tobacco mosaic virus RNA by flow electro-transfection.

Nuclear membrane fusion in electrofused mammalian cells

Fusion of nuclei was studied in electrofused cells using staining procedures and DNA flow cytometry. Homogeneous and heterogeneous electrofusion of Ehrlich ascites tumor cells. Muntjac cells and V79-S181 cells were performed in balanced-salt solutions at low temperature. Incubation of the cells subjected to electrofusion in fusion media for about 2 h was required to complete cell fusion and, in particular, nuclear membrane fusion. Under optimum electrofusion conditions it was found that fusion of nuclei is a very frequent event. Half of the fused cells (about 30 to 50% of the field-exposed cells) underwent nuclear membrane fusion. It is shown that the high frequency of nuclear membrane fusion in electrofused, unsynchronised cells resulted from intracellular dielectrophoresis occurring during cell alignment. In accordance with theory, maximum nuclear membrane fusion was observed using alignment fields of between 1 and 4 MHz (depending on the cell species), that is above the frequencies at which the plasmalemma capacity no longer shielded the cell interior from participation in the conduction process. In this frequency range a potential difference can be built up across the nuclear membrane leading to repositioning of the nuclei into the contact zone of the plasmalemmas of two attached cells. This intracellular dielectrophoresis apparently facilitated fusion of nuclei once intermingling of the plasma membranes had occurred. It was further demonstrated that exponentially growing cells showed higher cell fusion rates than cells taken from the unfed plateau phase. One, but not the only reason, might be the higher ATP content of exponentially growing cells compared to cells of the plateau phase. Addition of external ATP to plateau phase cells during electrofusion resulted, in accordance with this assumption, in an increase of fusion frequency, whereas ATP had apparently no effect on the fusion yield of exponentially growing cells. G1 cells obtained by mitotic selection after nocodazole-induced blockage in metaphase also showed higher cellular and nuclear membrane fusion yields than exponentially growing cells. Most importantly, it could be demonstrated both experimentally and theoretically that electrofusion of cells in a dielectrophoretically aligned chain is controlled by a simple law of probability resulting predominantly in fusion of two cells independent of the number of cells in the chain. The likelihood of fusion of various numbers of cells in a chain is given by the appropriate power of the probability of two-cell fusion.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased efficiency of transfection of murine hybridoma cells with DNA by electropermeabilization

Dispase-treated murine hybridoma cells (SP2/0-Ag14) were transfected with the G418 resistance gene bearing plasmid pSV2-neo by electropermeabilization with a high degree of efficiency. The cells were subjected to intermittent multiple high-voltage short duration (5 microsecond) DC pulses at intervals of 1 min in a weakly conducting medium followed by selection in G418-containing medium. The transfection medium, temperature, pulse duration, and voltage were empirically determined by preliminary electropermeabilization experiments. Increasing the number of pulses resulted in a higher percentage of transfected cells, but a decrease in the number of viable cells, with the optimal transfectant yield resulting when five pulses of 10 kV/cm were administered. This method allows the rapid and efficient injection of DNA into mammalian cells, and permits the rapid production of stable, drug resistant hybridoma cell lines for use in subsequent fusion experiments.

The permeability of electroporated cells and protoplasts of sugar beet

A simple method has been developed to determine the changes in permeability of protoplasts and intact cells when electroporated. Cells and protoplasts of sugar beet, Beta vulgaris L., were subjected to electric pulse treatments of different field strengths, pulse number and pulse duration, and the ability to accumulate and retain the hydrophilic dye phenosafranine was determined spectrophotometrically. Results of timecourse studies of phenosafranine accumulation and retention indicated that pores are formed or enlarged rapidly in the plasmamembrane and remain permeable to phenosafranine for relatively long periods; the half-life of the 'pores" was temperaturedependent. Both cells and protoplasts retained the highest levels of phenosafranine when supplied with a series of five rectangular pulses of 50 μs duration and of field strength 2500 V·cm(-1). If these parameters were exceeded, The phenosafranine content was reduced, concomitant with a decline in viability as indicated by fluorescein-diacetate staining, indicating the loss of the integrity of the plasmamembrane. The pattern of accumulation and retention by protoplasts of radioactivity from [(3)H]pABD1, a modified bacterial plasmid, was similar to that of phenosafranine, but uptake of the plasmid by cells was not demonstrated. The mothod can be used to determine conditions for the optimum permeabilization of protoplasts for direct gene transfer.

Large scale transfection of mouse L-cells by electropermeabilization

Mouse L-cells were transfected by electropermeabilization using the selectable plasmid pSV2-neo which confers resistance to G-418 (Geneticin). The DNA concentration used was 1 microgram/ml, the field strength was 10 kV/cm, the duration of the pulse was 5 microseconds. Transfection yield was optimal at a temperature of 4 degrees C when using a time in between consecutive pulses of 1 minute compared to shorter (of the order of seconds) or longer (3 minutes) time intervals. A more detailed study of the relationship between the number of pulses applied (up to 10) and transfection yield showed it to be almost linear in this range at 4 degrees C. The yield of transfectants in response to 10 pulses was up to 1000 per 10(6) cells (using 3.3 pg DNA per cell). The influence of the growth phase of the cells on the transfection yield and/or the subpopulation of the mouse L-cell line used was shown. Furthermore the clone yield depended on the DNA per cell ratio within a very small range.

Transient gene expression in electroporated protoplasts and intact cells of sugar beet

Factors influencing the transient expression of introduced foreign DNA in electroporated protoplasts and intact cells of sugar beet were determined by assaying for the activity of chloramphenicol acetyltransferase (CAT), using a rectangular pulse generating system. Extractable CAT activity depended upon 1) applied plasmid DNA concentration, 2) protoplast density, 3) the interaction between pulse field strength, duration, number, time interval between pulses and the resultant effect on culture viability, and 4) the physiological state of the protoplasts. Mesophyll protoplasts were more susceptible to damage by electroporation, and were more specific in their requirement for electroporations which allowed CAT expression, than were protoplasts derived from suspension culture cells. CAT activity was also demonstrated, at low levels, after electroporation of intact suspension culture cells, and could be increased by pectinase treatment of the cells before electroporation.