The Role of Electrophoresis in Gene Electrotransfer

Gene electrotransfer is an established method for gene delivery which uses high-voltage pulses to increase the permeability of a cell membrane and enables transfer of genes. Poor plasmid mobility in tissues is one of the major barriers for the successful use of gene electrotransfer in gene therapy. Therefore, we analyzed the effect of electrophoresis on increasing gene electrotransfer efficiency using different combinations of high-voltage (HV) and low-voltage (LV) pulses in vitro on CHO cells. We designed a special prototype of electroporator, which enabled us to use only HV pulses or combinations of LV + HV and HV + LV pulses. We used optimal plasmid concentrations used in in vitro conditions as well as lower suboptimal concentrations in order to mimic in vivo conditions. Only for the lowest plasmid concentration did the electrophoretic force of the LV pulse added to the HV pulse increase the transfection efficiency compared to using only HV. The effect of the LV pulse was more pronounced for HV + LV, while for the reversed sequence, LV + HV, there was only a minor effect of the LV pulse. For the highest plasmid concentrations no added effect of LV pulses were observed. Our results suggest that there are different contributing effects of LV pulses: electrophoretically increased contact of DNA with the membrane and increased insertion of DNA into permeabilized cell membrane and/or translocation due to electrophoretic force, which appears to be the dominant effect.     

Importance of association between permeabilization and electrophoretic forces for intramuscular DNA electrotransfer

Gene transfer using electrical pulses is a rapidly expanding field. Many studies have been performed in vitro to elucidate the mechanism of DNA electrotransfer. In vivo, the use of efficient procedures for DNA electrotransfer in tissues is recent, and the question of the implied mechanisms is largely open. We have evaluated the effects of various combinations of square wave electric pulses of variable field strength and duration, on cell permeabilization and on DNA transfection in the skeletal muscle in vivo. One high voltage pulse of 800 V/cm, 0.1 ms duration (short high pulse) or a series of four low voltage pulses of 80 V/cm, 83 ms duration (long low pulses) slightly amplified transfection efficacy, while no significant permeabilization was detected using the (51)Cr-EDTA uptake test. By contrast, the combination of one short high pulse followed by four long low pulses led to optimal gene transfer efficiency, while inducing muscle fibers permeabilization. These results are consistent with additive effects of electropermeabilization and DNA electrophoresis on electrotransfer efficiency. Finally, the described new combination, as compared to the previously reported use of repeated identical pulses of intermediate voltage, leads to similar gene transfer efficiency, while causing less permeabilization and thus being likely less deleterious. Thus, combination of pulses of various strengths and durations is a new procedure for skeletal muscle gene transfer that may represents a clear improvement in view of further clinical development.     

Influence of Plasmid Concentration on DNA Electrotransfer In Vitro Using High-Voltage and Low-Voltage Pulses

DNA electrotransfer in vivo for gene therapy is a promising method. For further clinical developments, the efficiency of the method should be increased. It has been shown previously that high efficiency of gene electrotransfer in vivo can be achieved using high-voltage (HV) and low-voltage (LV) pulses. In this study we evaluated whether HV and LV pulses could be optimized in vitro for efficient DNA electrotransfer. Experiments were performed using Chinese hamster ovary (CHO) cells. To evaluate the efficiency of DNA electrotransfer, two different plasmids coding for GFP and luciferase were used. For DNA electrotransfer experiments 50 μl of CHO cell suspension containing 100, 10 or 1 μg/ml of the plasmid were placed between plate electrodes and subjected to various combinations of HV and LV pulses. The results showed that at 100 μg/ml plasmid concentration LV pulse delivered after HV pulse increased neither the percentage of transfected cells nor the total transfection efficiency (luciferase activity). The contribution of the LV pulse was evident only at reduced concentration (10 and 1 μg/ml) of the plasmid. In comparison to HV (1,200 V/cm, 100 μs) pulse, addition of LV (100 V/cm, 100 ms) pulse increased transfection efficiency severalfold at 10 μg/ml and fivefold at 1 μg/ml. At 10 μg/ml concentration of plasmid, application of four LV pulses after HV pulse increased transfection efficiency by almost 10-fold. Thus, these results show that contribution of electrophoretic forces to DNA electrotransfer can be investigated in vitro using HV and LV pulses.     

Gene electrotransfer: from biophysical mechanisms to in vivo applications: Part 1- Biophysical mechanisms

Electropulsation is one of the nonviral methods successfully used to deliver genes into living cells in vitro and in vivo. This approach shows promise in the field of gene and cellular therapies. The present review focuses on the processes supporting gene electrotransfer in vitro. In the first part, we will report the events occurring before, during, and after pulse application in the specific field of plasmid DNA electrotransfer at the cell level. A critical discussion of the present theoretical considerations about membrane electropermeabilization and the transient structures involved in the plasmid uptake follows in a second part.     

In vivo gene electrotransfer into skeletal muscle: effects of plasmid DNA on the occurrence and extent of muscle damage

BACKGROUND: Understanding the mechanisms underlying gene electrotransfer muscle damage can help to design more effective gene electrotransfer strategies for physiological and therapeutical applications. The present study investigates the factors involved in gene electrotransfer associated muscle damage. METHODS: Histochemical analyses were used to determine the extent of transfection efficiency and muscle damage in the Tibialis anterior muscles of Sprague-Dawley male rats after gene electrotransfer. RESULTS: Five days after gene electrotransfer, features of muscle degeneration and regeneration were consistently observed, thus limiting the extent of transfection efficiency. Signs of muscle degeneration/regeneration were no longer evident 21 days after gene electrotransfer except for the presence of central myonuclei. Neither the application of electrical pulses per se nor the extracellular presence of plasmid DNA per se contributed significantly to muscle damage (2.9 +/- 1.0 and 2.1 +/- 0.7% of the whole muscle cross-sectional area, respectively). Gene electrotransfer of a plasmid DNA, which does not support gene expression, increased significantly muscle damage (8.7 +/- 1.2%). When plasmid DNA expression was permitted (gene electrotransfer of pCMV-beta-galactosidase), muscle damage was further increased to 19.7 +/- 4.5%. Optimization of cumulated pulse duration and current intensity dramatically reduced gene electrotransfer associated muscle damage. Finally, mathematical modeling of gene electrotransfer associated muscle damage as a function of the number of electrons delivered to the tissue indicated that pulse length critically determined the extent of muscle damage. CONCLUSION: Our data suggest that neither the extracellular presence of plasmid DNA per se nor the application of electric pulses per se contributes significantly to muscle damage. Gene electrotransfer associated muscle damage mainly arises from the intracellular presence and expression of plasmid DNA.     

Electroporator with automatic change of electric field direction improves gene electrotransfer <Emphasis Type="Italic">in-vitro</Emphasis>

Background  

Gene electrotransfer is a non-viral method used to transfer genes into living cells by means of high-voltage electric pulses. An exposure of a cell to an adequate amplitude and duration of electric pulses leads to a temporary increase of cell membrane permeability. This phenomenon, termed electroporation or electropermeabilization, allows various otherwise non-permeant molecules, including DNA, to cross the membrane and enter the cell. The aim of our research was to develop and test a new system and protocol that would improve gene electrotransfer by automatic change of electric field direction between electrical pulses.     

Nucleic Acids Electrotransfer-Based Gene Therapy (Electrogenetherapy): Past,Current, and Future

About 25 years after the publication of the first report on gene transfer in vitro in cultured cells by the means of electric pulses delivery, reversible cell electroporation for gene transfer and gene therapy (DNA electrotransfer) is at a cross in its development. Present knowledge on the effects of cell exposure to appropriate electric field pulses, particularly at the level of the cell membrane, is reported here. The importance of the models of electric field distribution in tissues and of the correct choice of electrodes and applied voltages is highlighted. The mechanisms involved in DNA electrotransfer, which include cell electropermeabilization and DNA electrophoresis, are also surveyed. This knowledge has allowed developing new nucleic acids electrotransfer conditions using combinations of permeabilizing pulses of high voltage and short duration, and of electrophoretic pulses of low voltage and long duration, which are very efficient and safer. Feasibility of electric pulses delivery for gene transfer in humans is discussed taking into account that electric pulses delivery is already regularly used for localized drug delivery in the treatment of cutaneous and subcutaneous solid tumors by electrochemotherapy. Because recent technological developments made DNA electrotransfer more and more efficient and safer, this non-viral gene therapy approach is now ready to reach the clinical stage. A good understanding of DNA electrotransfer principles and the respect of safe procedures will be key elements for a successful future transfer DNA electrotransfer into the clinics.     

Analysis and Comparison of Electrical Pulse Parameters for Gene Electrotransfer of Two Different Cell Lines

Knowledge of the parameters which influence the efficiency of gene electrotransfer has importance for practical implementation of electrotransfection for gene therapy as well as for better understanding of the underlying mechanism. The focus of this study was to analyze the differences in gene electrotransfer and membrane electropermeabilization between plated cells and cells in a suspension in two different cell lines (CHO and B16F1). Furthermore, we determined the viability and critical induced transmembrane voltage (ITV_c) for both cell lines. In plated cells we obtained relatively little difference in electropermeabilization and gene electrotransfection between CHO and B16F1 cells. However, significant differences between the two cell lines were observed in a suspension. CHO cells exhibited a much higher gene electrotransfection rate compared to B16F1 cells, whereas B16F1 cells reached maximum electropermeabilization at lower electric fields than CHO cells. Both in a suspension and on plated cells, CHO cells had a slightly better survival rate at higher electric fields than B16F1 cells. Calculation of ITV_c in a suspension showed that, for both electropermeabilization and gene electrotransfection, CHO cells have lower ITV_c than B16F1 cells. In all cases, ITV_c for electropermeabilization was lower than ITV_c for gene electrotransfer, which is in agreement with other studies. Our results show that there is a marked difference in the efficiency of gene electrotransfer between suspended and plated cells.     

Nanosecond Electric Pulse Effects on Gene Expression

Gene electrotransfection using micro- or millisecond electric pulses is a well-established method for safe gene transfer. For efficient transfection, plasmid DNA has to reach the nucleus. Shorter, high-intensity nanosecond electric pulses (nsEPs) affect internal cell membranes and may contribute to an increased uptake of plasmid by the nucleus. In our study, nsEPs were applied to Chinese hamster ovary (CHO) cells after classical gene electrotransfer, using micro- or millisecond pulses with a plasmid coding the green fluorescent protein (GFP). Time gaps between classical gene electrotransfer and nsEPs were varied (0.5, 2, 6 and 24 h) and three different nsEP parameters were used: 18 ns-10 kV/cm, 10 ns-40 kV/cm and 15 ns-60 kV/cm. Results analyzed by either fluorescence microscopy or flow cytometry showed that neither the percentage of electrotransfected cells nor the amount of GFP expressed was increased by nsEP. All nsEP parameters also had no effects on GFP fluorescence intensity of human colorectal tumor cells (HCT-116) with constitutive expression of GFP. We thus conclude that nsEPs have no major contribution to gene electrotransfer in CHO cells and no effect on constitutive GFP expression in HCT-116 cells.     

In vivo cell electrofusion

In vitro electrofusion of cells brought into contact and exposed to electric pulses is an established procedure. Here we report for the first time the occurrence of fusion of cells within a tissue exposed in vivo to permeabilizing electric pulses. The dependence of electrofusion on the ratio of applied voltage to distance between the electrodes, and thus on the achievement of in vivo cell electropermeabilization (electroporation) is demonstrated in the metastasizing B16 melanoma tumor model. The kinetics of the morphological changes induced by cell electrofusion (appearance of syncytial areas or formation of giant cells) are also described, as well as the kinetics of mitosis and cell death occurrence. Finally, tissue dependence of in vivo cell electrofusion is reported and discussed, since electrofusion has been observed neither in liver nor in another tumor type. Particular microenvironmental conditions, such as the existence of reduced extracellular matrices, could be necessary for electrofusion achievement. Since biomedical applications of in vivo cell electropermeabilization are rapidly developing, we also discuss the influence of cell electrofusion on the efficacy of DNA electrotransfer for gene therapy and of antitumor electrochemotherapy, in which electrofusion could be an interesting advantage to treat metastasizing tumors.     

High-efficiency gene electrotransfer into skeletal muscle: description and physiological applicability of a new pulse generator

Efficiency and reproducibility of gene electrotransfer depend on the electrical specifications provided by the pulse generator, such as pulse duration, pulse number, pulse frequency, pulse combination, and current intensity. Here, we describe the performances of GET42, a pulse generator specifically designed for gene electrotransfer into skeletal muscle. Expression of beta-galactosidase in the Tibialis anterior muscle of Sprague-Dawley male rats was increased 250-fold by GET42 compared to DNA injection alone. Combination of high and low current intensity pulses further increased transfection efficiency (400-fold compared to DNA injection without electrotransfer). Varying degrees of muscle necrosis were observed after gene electrotransfer. Nevertheless, muscle necrosis was dramatically reduced after optimization of cumulated pulse duration without significant reduction in transfection efficiency. Physiological applicability was illustrated by the analysis of cytochrome c promoter transactivation. In conclusion, GET42 has proven to be a reliable and efficient pulse generator for gene electrotransfer experiments, and provides a powerful mean to study in vivo the regulation of gene expression.     

Long-term,high level in vivo gene expression after electric pulse-mediated gene transfer into skeletal muscle

Gene delivery to skeletal muscle is a promising strategy for the treatment of muscle disorders and for the local or systemic secretion of therapeutic proteins. However, current DNA delivery technologies have to be improved. We report very efficient luciferase gene transfer into muscle fibres obtained through the delivery of squarewave electric pulses of moderate field strength (100–200 V/cm) and of long duration (20 ms) to muscle previously injected with plasmid DNA. This intramuscular ‘electrotransfer’ method increases reporter gene expression by more than 100 times. It is noteworthy that this expression remains high and stable for at least 9 months. Moreover, intramuscular electrotransfer strongly decreases the interindividual variability usually observed after plasmid DNA injection into muscle fibres. Therefore, DNA electrotransfer in muscle possesses broad potential applications in gene therapy and for physiological, pharmacological and developmental studies.     

Calcium influx determines the muscular response to electrotransfer

Cell membrane permeabilization by electric pulses (electropermeabilization), results in free exchange of ions across the cell membrane. The role of electrotransfer-mediated Ca(2+)-influx on muscle signaling pathways involved in degeneration (β-actin and MurF), inflammation (IL-6 and TNF-α), and regeneration (MyoD1, myogenin, and Myf5) was investigated, using pulse parameters of both electrochemotherapy (8 HV) and DNA delivery (HVLV). Three pulsing conditions were used: 8 high-voltage pulses (8 HV), resulting in large permeabilization and ion flux, and a combination of one high-voltage pulse and one low-voltage pulse (HVLV), either alone or in combination with injection of DNA. Mice and rats were anesthetized before pulsing. At the times given, animals were killed, and intact tibialis cranialis muscles were excised for analysis. Uptake of Ca(2+) was assessed using (45)Ca as a tracer. Using gene expression analyses and histology, we showed a clear association between Ca(2+) influx and muscular response. Moderate Ca(2+) influx induced by HVLV pulses results in activation of pathways involved in immediate repair and hypertrophy. This response could be attenuated by intramuscular injection of EGTA reducing Ca(2+) influx. Larger Ca(2+) influx as induced by 8-HV pulses leads to muscle damage and muscle fiber regeneration through recruitment of satellite cells. The extent of Ca(2+) influx determines the muscular response to electrotransfer and, thus, the success of a given application. In the case of electrochemotherapy, in which the objective is cell death, a large influx of Ca(2+) may be beneficial, whereas for DNA electrotransfer, muscle recovery should occur without myofiber loss to ensure preservation of plasmid DNA.     

Electrotransfer of human IL-1Ra into skeletal muscles reduces the incidence of murine collagen-induced arthritis

BACKGROUND: It has previously been demonstrated that high levels of gene expression in skeletal muscles can be achieved after direct in vivo electrotransfer of naked plasmid DNA. The purpose of this study is to examine the potential of in vivo electroporation of plasmid DNA encoding human IL-1Ra for the prevention of murine collagen-induced arthritis (CIA). METHODS: DBA/1 mice were injected in gastrocnemius muscles with plasmid DNA followed by in vivo electroporation. To uncover the optimum conditions of gene transfer, various electric field strengths and different amounts of plasmid DNA were applied. Calf muscles around the injected areas were investigated with histological methods for damage to muscle tissue. The levels of human IL-1Ra expression in the injected area and also in the serum were determined with ELISA for human IL-1Ra. Based on these data, the effects of electrotransfer of plasmid DNA were tested using the murine CIA model. DBA/1 mice were immunized with bovine collagen type II at the base of the tail. On day 21, mice were given a booster injection with the same antigen. Mice were divided into two groups on day 26. One group of mice received plasmid containing the IL-1Ra cDNA sequence, while control mice were given plasmid lacking the IL-1Ra coding sequence. The incidence of arthritis was evaluated by macroscopic analysis, histological analysis, and the levels of inflammatory cytokines. RESULTS: IL-1Ra expression increased as a function of the electrical field strength and the amount of DNA. 200 V/cm (eight pulses; 20 ms per pulse; 1 Hz) and 15 microg of plasmid DNA per mouse were found to be optimum for gene transfer. After in vivo electroporation, gene expression in both muscle and serum increased gradually, reaching a peak value on day 10. Significant levels of human IL-1Ra expression were maintained for 20 days. Macroscopic analysis showed that the onset of CIA was significantly inhibited by direct electrotransfer of plasmid DNA encoding human IL-1Ra. Histological analysis of knee joints showed that the incidence of arthritis in knee joints was also prevented. The levels of mouse IL-1beta and IL-12 in paws were significantly lower in the group treated with IL-1Ra than those in the control group. CONCLUSIONS: These results demonstrate that direct electrotransfer of plasmid containing the human IL-1Ra cDNA sequence to skeletal muscle can reduce the incidence of CIA in mice.     

In Vitro Targeted Gene Electrotransfer to Endothelial Cells with Plasmid DNA Containing Human Endothelin-1 Promoter

Development of recombinant DNA technologies has allowed us to create new delivery systems that target specific cell types and that can be used in gene therapy. One of these targets is vascular endothelium because of its important role in tumor angiogenesis. For tumor endothelium-specific targeting, we prepared plasmid DNA encoding green fluorescent protein under the control of human endothelin-1 promoter (pENDO-EGFP), which is specific for endothelial cells. First we determined gene electrotransfer parameters for improved transfection of endothelial cells evaluating different osmolarity of electroporation buffer, voltages of applied electric pulses, and addition of fetal bovine serum immediately after electroporation to the cells for improved transfection and survival. Transfection efficacy of pENDO-EGFP in different endothelial and nonendothelial cell lines was determined next. Gene electrotransfer efficacy was evaluated using three different methods: fluorescence microscopy, fluorescence microplate reader, and flow cytometry. Our results showed that transfection efficacy was higher when cells were prepared in hypoosmolar compared to isoosmolar electroporation buffer. Furthermore, immediate addition of fetal bovine serum to the cells after pulsing also improved gene electrotransfer into target cells. We proved expression of EGFP under the control of human endothelin-1 promoter in endothelial cells, which was also significantly higher compared to nonendothelial cells. Taken together, we successfully constructed pENDO-EGFP, which was specifically expressed in endothelial cells using improved gene electrotransfer parameters.     

Cell and animal imaging of electrically mediated gene transfer

Electropermeabilization is a nonviral method successfully used to transfer genes into cells in vitro as in vivo. Although it shows promise in field of gene therapy, very little is known on the basic processes supporting the DNA transfer. The aim of the present investigation is to visualize gene electrotransfer and expression both in vitro and in vivo. In vitro studies have been performed by using digitized fluorescence microscopy. Membrane permeabilization occurs at the sides of the cell membrane facing the two electrodes. A free diffusion of propidium iodide across the membrane to the cytoplasm is observed in the seconds following electric pulses. Fluorescently labeled plasmids only interact with the electropermeabilized side of the cell facing the cathode. The plasmid interaction with the electropermeabilized cell surface is stable over a few minutes. Changing the polarity and the orientation of the pulses lead to an increase in gene expression. In vivo experiments have been performed in Tibialis Cranialis mice muscle. Electric field application lead to the in vivo expression of plasmid DNA. We directly visualize gene expression of the Green Fluorescent Protein (GFP) on live animals. GFP expression is shown to be increased by applying electric field pulses with different polarities and orientations.     

What is (Still not) Known of the Mechanism by Which Electroporation Mediates Gene Transfer and Expression in Cells and Tissues

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

Numerical optimization of gene electrotransfer into muscle tissue

Background  

Electroporation-based gene therapy and DNA vaccination are promising medical applications that depend on transfer of pDNA into target tissues with use of electric pulses. Gene electrotransfer efficiency depends on electrode configuration and electric pulse parameters, which determine the electric field distribution. Numerical modeling represents a fast and convenient method for optimization of gene electrotransfer parameters. We used numerical modeling, parameterization and numerical optimization to determine the optimum parameters for gene electrotransfer in muscle tissue.     

Changing the Direction and Orientation of Electric Field During Electric Pulses Application Improves Plasmid Gene Transfer in vitro

Gene electrotransfer is a physical method used to deliver genes into the cells by application of short and intense electric pulses, which cause destabilization of cell membrane, making it permeable to small molecules and allows transfer of large molecules such as DNA. It represents an alternative to viral vectors, due to its safety, efficacy and ease of application. For gene electrotransfer different electric pulse protocols are used in order to achieve maximum gene transfection, one of them is changing the electric field direction and orientation during the pulse delivery. Changing electric field direction and orientation increase the membrane area competent for DNA entry into the cell. In this video, we demonstrate the difference in gene electrotransfer efficacy when all pulses are delivered in the same direction and when pulses are delivered by changing alternatively the electric field direction and orientation. For this purpose tip with integrated electrodes and high-voltage prototype generator, which allows changing of electric field in different directions during electric pulse application, were used. Gene electrotransfer efficacy is determined 24h after pulse application as the number of cells expressing green fluorescent protein divided with the number of all cells. The results show that gene transfection is increased when the electric field orientation during electric pulse delivery is changed.Download video file.^{(27M, mov)}     

Plasmid DNA electrotransfer for intracellular and secreted proteins expression: new methodological developments and applications

In vivo electrotransfer is a physical method of gene delivery in various tissues and organs, relying on the injection of a plasmid DNA followed by electric pulse delivery. The importance of the association between cell permeabilization and DNA electrophoresis for electrotransfer efficiency has been highlighted. In vivo electrotransfer is of special interest since it is the most efficient non-viral strategy of gene delivery and also because of its low cost, easiness of realization and safety. The potentiality of this technique can be further improved by optimizing plasmid biodistribution in the targeted organ, plasmid structure, and the design of the encoded protein. In particular, we found that plasmids of smaller size were electrotransferred more efficiently than large plasmids. It is also of importance to study and understand kinetic expression of the transgene, which can be very variable, depending on many factors including cellular localization of the protein, physiological activity and regulation. The most widely targeted tissue is skeletal muscle, because this strategy is not only promising for the treatment of muscle disorders, but also for the systemic secretion of therapeutic proteins. Vaccination and oncology gene therapy are also major fields of application of electrotransfer, whereas application to other organs such as liver, brain and cornea are expanding. Many published studies have shown that plasmid electrotransfer can lead to long-lasting therapeutic effects in various pathologies such as cancer, blood disorders, rheumatoid arthritis or muscle ischemia. DNA electrotransfer is also a powerful laboratory tool to study gene function in a given tissue.