Control by electrical parameters of short- and long-term cell death resulting from electropermeabilization of Chinese hamster ovary cells

Long-term changes in transmembrane voltage after electroporation are governed by the interplay between nonselective leak current and ion channel activation

Electroporation causes a temporal increase in cell membrane permeability and leads to prolonged changes in transmembrane voltage (TMV) in both excitable and non-excitable cells. However, the mechanisms of these TMV changes remain to be fully elucidated. To this end, we monitored TMV over 30 min after exposing two different cell lines to a single 100 µs electroporation pulse using the FLIPR Membrane Potential dye. In CHO-K1 cells, which express very low levels of endogenous ion channels, membrane depolarization following pulse exposure could be explained by nonselective leak current, which persists until the membrane reseals, enabling the cells to recover their resting TMV. In U-87 MG cells, which express many different ion channels, we unexpectedly observed membrane hyperpolarization following the initial depolarization phase, but only at 33 °C and not at 25 °C. We developed a theoretical model, supported by experiments with ion channel inhibitors, which indicated that hyperpolarization could largely be attributed to the activation of calcium-activated potassium channels. Ion channel activation, coupled with changes in TMV and intracellular calcium, participates in various physiological processes, including cell proliferation, differentiation, migration, and apoptosis. Therefore, our study suggests that ion channels could present a potential target for influencing the biological response after electroporation.

In-Cell Structural Biology by NMR: The Benefits of the Atomic Scale

In-cell structural biology aims at extracting structural information about proteins or nucleic acids in their native, cellular environment. This emerging field holds great promise and is already providing new facts and outlooks of interest at both fundamental and applied levels. NMR spectroscopy has important contributions on this stage: It brings information on a broad variety of nuclei at the atomic scale, which ensures its great versatility and uniqueness. Here, we detail the methods, the fundamental knowledge, and the applications in biomedical engineering related to in-cell structural biology by NMR. We finally propose a brief overview of the main other techniques in the field (EPR, smFRET, cryo-ET, etc.) to draw some advisable developments for in-cell NMR. In the era of large-scale screenings and deep learning, both accurate and qualitative experimental evidence are as essential as ever to understand the interior life of cells. In-cell structural biology by NMR spectroscopy can generate such a knowledge, and it does so at the atomic scale. This review is meant to deliver comprehensive but accessible information, with advanced technical details and reflections on the methods, the nature of the results, and the future of the field.

Impedimetric Detection and Electromediated Apoptosis of Vascular Smooth Muscle Using Microfabricated Biosensors for Diagnosis and Therapeutic Intervention in Cardiovascular Diseases

Cardiovascular diseases remain a significant global burden with 1-in-3 of all deaths attributable to the consequences of the disease. The main cause is blocked arteries which often remain undetected. Implantable medical devices (IMDs) such as stents and grafts are often used to reopen vessels but over time these too will re-block. A vascular biosensor is developed that can report on cellularity and is amenable to being mounted on a stent or graft for remote reporting. Moreover, the device is designed to also receive currents that can induce a controlled form of cell death, apoptosis. A combined diagnostic and therapeutic biosensor would be transformational for the treatment of vascular diseases such as atherosclerosis and central line access. In this work, a cell sensing and cell apoptosing system based on the same interdigitated electrodes (IDEs) is developed. It is shown that the device is scalable and that by miniaturizing the IDEs, the detection sensitivity is increased. Apoptosis of vascular smooth muscle cells is monitored using continuous impedance measurements at a frequency of 10 kHz and rates of cell death are tracked using fluorescent dyes and live cell imaging.

Physiological changes may dominate the electrical properties of liver during reversible electroporation: Measurements and modelling

This study presents electrical measurements (both conductivity during the pulses and impedance spectroscopy before and after) performed in liver tissue of mice during electroporation with classical electrochemotherapy conditions (8 pulses of 100 µs duration). A four-needle electrode arrangement inserted in the tissue was used for the measurements. The undesirable effects of the four-electrode geometry, notably concerning its sensitivity, were quantified and discussed showing how the electrode geometry chosen for the measurements can impact the results. Numerical modelling was applied to the information collected during the pulse, and to the impedance spectra acquired before and after the pulses sequence. Our results show that the numerical results were not consistent, suggesting that other collateral phenomena not considered in the model are at work during electroporation in vivo. We show how the modification in the volume of the intra and extra cellular media, likely caused by the vascular lock effect, could at least partially explain the recorded impedance evolution. In the present study we demonstrate the significant impact that physiological effects have on impedance changes following electroporation at the tissue scale and the potential need of introducing them into the numerical models. The code for the numerical model is publicly available at https://gitlab.inria.fr/poignard/4-electrode-system.

Impact of the number of electric pulses on cell electrochemotherapy in vitro: Limits of linearity and saturation

In this study the evolution in the efficiency of electrochemotherapy (reversible electroporation) with pulse number was assessed in vitro. Experiments were performed using 100 μs pulses at different electric field intensities and the chemotherapeutic agent bleomycin. Additionally, electrical impedance spectroscopy measurements were used as a different method to study in real time the changes produced on cells with pulse number during trains of consecutive pulses. Our results show that the relation between pulse number and the observed outcome is complex and difficult to fully characterize. This relation can display a highly linear behaviour up to a certain number of pulses and/or field intensity applied. However, the relation between the number of pulses and the observed outcome always evolves to a saturation or at least a reduction in the electric field effects that is displayed when either electric field intensity or pulse number are increased. An exponential model was found to best describe this relation within the range of experimental conditions considered. Electrical impedance measurements confirmed the results and gave a more precise quantification of this dependence. The study highlights the importance that pulse number has in the electrochemotherapy protocols and establishes some limits in the use of this parameter.

Membrane Electroporation and Electropermeabilization: Mechanisms and Models

Exposure of biological cells to high-voltage, short-duration electric pulses causes a transient increase in their plasma membrane permeability, allowing transmembrane transport of otherwise impermeant molecules. In recent years, large steps were made in the understanding of underlying events. Formation of aqueous pores in the lipid bilayer is now a widely recognized mechanism, but evidence is growing that changes to individual membrane lipids and proteins also contribute, substantiating the need for terminological distinction between electroporation and electropermeabilization. We first revisit experimental evidence for electrically induced membrane permeability, its correlation with transmembrane voltage, and continuum models of electropermeabilization that disregard the molecular-level structure and events. We then present insights from molecular-level modeling, particularly atomistic simulations that enhance understanding of pore formation, and evidence of chemical modifications of membrane lipids and functional modulation of membrane proteins affecting membrane permeability. Finally, we discuss the remaining challenges to our full understanding of electroporation and electropermeabilization.

Increasing Uptake of Silica Nanoparticles with Electroporation: From Cellular Characterization to Potential Applications

In the fields of biology and medicine, nanoproducts such as nanoparticles (NPs) are specifically interesting as theranostic tools, since they offer the double capacity to locally deliver active drugs and to image exactly where the product is delivered. Among the many described possibilities, silica nanoparticles (SiNPs) represent a good choice because of their ease of synthesis, the possibility of their vast functionalization, and their good biocompatibility. However, SiNPs' passive cell internalization by endocytosis only distributes NPs into the cell cytoplasm and is unable to target the nucleus if SiNPs are larger than a few nanometers. In this study, we demonstrate that the cell penetration of SiNPs of 28⁻30 nm in diameter can be strongly enhanced using a physical method, called electroporation or electropermeabilization (EP). The uptake of fluorescently labelled silica nanoparticles was improved in two different cancer cell lines, namely, HCT-116 (human colon cancer) cells and RL (B-lymphoma) cells. First, we studied cells' capability for the regular passive uptake of SiNPs in vitro. Then, we set EP parameters in order to induce a more efficient and rapid cell loading, also comprising the nuclear compartment, while preserving the cell viability. In the final approach, we performed in vivo experiments, and evidenced that the labeling was long-lasting, as confirmed by fluorescence imaging of labeled tumors, which enabled a 30-day follow-up. This kind of SiNPs delivery, achieved by EP, could be employed to load extensive amounts of active ingredients into the cell nucleus, and concomitantly allow the monitoring of the long-term fate of nanoparticles.

Enhanced Raman Investigation of Cell Membrane and Intracellular Compounds by 3D Plasmonic Nanoelectrode Arrays

3D nanostructures are widely exploited in cell cultures for many purposes such as controlled drug delivery, transfection, intracellular sampling, and electrical recording. However, little is known about the interaction of the cells with these substrates, and even less about the effects of electroporation on the cellular membrane and the nuclear envelope. This work exploits 3D plasmonic nanoelectrodes to study, by surface-enhanced Raman scattering (SERS), the cell membrane dynamics on the nanostructured substrate before, during, and after electroporation. In vitro cultured cells tightly adhere on 3D plasmonic nanoelectrodes precisely in the plasmonic hot spots, making this kind of investigation possible. After electroporation, the cell membrane dynamics are studied by recording the Raman time traces of biomolecules in contact or next to the 3D plasmonic nanoelectrode. During this process, the 3D plasmonic nanoelectrodes are intracellularly coupled, thus enabling the monitoring of different molecular species, including lipids, proteins, and nucleic acids. Scanning electron microscopy cross-section analysis evidences the possibility of nuclear membrane poration compatible with the reported Raman spectra. These findings may open a new route toward controlled intracellular sampling and intranuclear delivery of genic materials. They also show the possibility of nuclear envelope disruption which may lead to negative side effects.

Control by Low Levels of Calcium of Mammalian Cell Membrane Electropermeabilization

Electric pulses, when applied to a cell suspension, induce a reversible permeabilization of the plasma membrane. This permeabilized state is a long-lived process (minutes). The biophysical molecular mechanisms supporting the membrane reorganization associated to its permeabilization remain poorly understood. Modeling the transmembrane structures as toroidal lipidic pores cannot explain why they are long-lived and why their resealing is under the control of the ATP level. Our results describe the effect of the level of free Calcium ions. Permeabilization induces a Ca²⁺ burst as previously shown by imaging of cells loaded with Fluo-3. But this sharp increase is reversible even when Calcium is present at a millimolar concentration. Viability is preserved to a larger extent when submillimolar concentrations are used. The effect of calcium ions is occurring during the resealing step not during the creation of permeabilization as the same effect is observed if Ca²⁺ is added in the few seconds following the pulses. The resealing time is faster when Ca²⁺ is present in a dose-dependent manner. Mg²⁺ is observed to play a competitive role. These observations suggest that Ca²⁺ is acting not on the external leaflet of the plasma membrane but due to its increased concentration in the cytoplasm. Exocytosis will be enhanced by this Ca²⁺ burst (but hindered by Mg²⁺) and occurs in the electropermeabilized part of the cell surface. This description is supported by previous theoretical and experimental results. The associated fusion of vesicles will be the support of resealing.

Soft electroporation for delivering molecules into tightly adherent mammalian cells through 3D hollow nanoelectrodes

Electroporation of in-vitro cultured cells is widely used in biological and medical areas to deliver molecules of interest inside cells. Since very high electric fields are required to electroporate the plasma membrane, depending on the geometry of the electrodes the required voltages can be very high and often critical to cell viability. Furthermore, in traditional electroporation configuration based on planar electrodes there is no a priori certain feedback about which cell has been targeted and delivered and the addition of fluorophores may be needed to gain this information. In this study we present a nanofabricated platform able to perform intracellular delivery of membrane-impermeable molecules by opening transient nanopores into the lipid membrane of adherent cells with high spatial precision and with the application of low voltages (1.5–2 V). This result is obtained by exploiting the tight seal that the cells present with 3D fluidic hollow gold-coated nanostructures that act as nanochannels and nanoelectrodes at the same time. The final soft-electroporation platform provides an accessible approach for controlled and selective drug delivery on ordered arrangements of cells.

The Effect of Nanosecond, High-Voltage Electric Pulses on the Shape and Permeability of Polymersome GUVs

Polymersomes, vesicles composed of block copolymers, are promising candidates as membrane alternatives and functional containers, e.g., as potential carriers for functional molecules because of their stability and tunable membrane properties. In the scope of possible use for membrane protein delivery to cells by electrofusion, we investigated the cytotoxicity of such polymersomes as well as the effects of nanosecond electric pulses with variable repetition rate on the shape and permeability of polymersomes in buffers with different conductivities. The polymersomes did not show cytotoxic effects to CHO and B16-F1 cells in vitro in concentrations up to 250 µg/mL (for 48 h) or 1.35 mg/mL (for 60 min), which renders them suitable for interacting with living cells. We observed a significant effect of the pulse repetition rate on electrodeformation of the polymersomes. The electrodeformation was most pronounced in low conductivity buffer, which is favorable for performing electrofusion with cells. However, despite more pronounced deformation at higher pulse repetition rate, the electroporation performance of polymersomes was unaffected and remained in similar ranges both at 10 Hz and 10 kHz. This phenomenon is possibly due to the higher stability and rigidity of polymer vesicles, compared to liposomes, and can serve as an advantage (or disadvantage) depending on the aim in employing polymersomes such as stable membrane alternative architectures or drug vehicles.

Demonstration of the Protein Involvement in Cell Electropermeabilization using Confocal Raman Microspectroscopy

Confocal Raman microspectroscopy was used to study the interaction between pulsed electric fields and live cells from a molecular point of view in a non-invasive and label-free manner. Raman signatures of live human adipose-derived mesenchymal stem cells exposed or not to pulsed electric fields (8 pulses, 1 000 V/cm, 100 μs, 1 Hz) were acquired at two cellular locations (nucleus and cytoplasm) and two spectral bands (600–1 800 cm⁻¹ and 2 800–3 100 cm⁻¹). Vibrational modes of proteins (phenylalanine and amide I) and lipids were found to be modified by the electropermeabilization process with a statistically significant difference. The relative magnitude of four phenylalanine peaks decreased in the spectra of the pulsed group. On the contrary, the relative magnitude of the amide I band at 1658 cm⁻¹ increased by 40% when comparing pulsed and control group. No difference was found between the control and the pulsed group in the high wavenumber spectral band. Our results reveal the modification of proteins in living cells exposed to pulsed electric fields by means of confocal Raman microspectroscopy.

Dielectrophoresis study of temporal change in internal conductivity of single CHO cells after electroporation by pulsed electric fields

Applying sufficiently strong pulsed electric fields to a cell can permeabilize the membrane and subsequently affect its dielectric properties. In this study, we employ a microfluidic dielectrophoresis cytometry technique to simultaneously electroporate and measure the time-dependent dielectric response of single Chinese hamster ovary cells. Using experimental measurements along with numerical simulations, we present quantitative results for the changes in the cytoplasm conductivity of single cells within seconds after exposure to 100 μs duration pulsed electric fields with various intensities. It is shown that, for electroporation in a medium with conductivity lower than that of the cell's cytoplasm, the internal conductivity of the cell decreases after the electroporation on a time scale of seconds and stronger pulses cause a larger and more rapid decrease. We also observe that, after the electroporation, the cell's internal conductivity is constrained to a threshold. This implies that the cell prevents some of the ions in its cytoplasm from diffusing through the created pores to the external medium. The temporal change in the dielectric response of each individual cell is continuously monitored over minutes after exposure to pulsed electric fields. A time constant associated with the cell's internal conductivity change is observed, which ranges from seconds to tens of seconds depending on the applied pulse intensity. This experimental observation supports the results of numerical models reported in the literature.

Impact of external medium conductivity on cell membrane electropermeabilization by microsecond and nanosecond electric pulses

The impact of external medium conductivity on the efficiency of the reversible permeabilisation caused by pulsed electric fields was investigated. Pulses of 12 ns, 102 ns or 100 μs were investigated. Whenever permeabilisation could be detected after the delivery of one single pulse, media of lower conductivity induced more efficient reversible permeabilisation and thus independently of the medium composition. Effect of medium conductivity can however be hidden by some saturation effects, for example when pulses are cumulated (use of trains of 8 pulses) or when the detection method is not sensitive enough. This explains the contradicting results that can be found in the literature. The new data are complementary to those of one of our previous study in which an opposite effect of the conductivity was highlighted. It stresses that the conductivity of the medium influences the reversible permeabilization by several ways. Moreover, these results clearly indicate that electropermeabilisation does not linearly depend on the energy delivered to the cells.

Transport, resealing, and re-poration dynamics of two-pulse electroporation-mediated molecular delivery

Electroporation is of interest for many drug-delivery and gene-therapy applications. Prior studies have shown that a two-pulse-electroporation protocol consisting of a short-duration, high-voltage first pulse followed by a longer, low-voltage second pulse can increase delivery efficiency and preserve viability. In this work the effects of the field strength of the first and second pulses and the inter-pulse delay time on the delivery of two different-sized Fluorescein-Dextran (FD) conjugates are investigated. A series of two-pulse-electroporation experiments were performed on 3T3-mouse fibroblast cells, with an alternating-current first pulse to permeabilize the cell, followed by a direct-current second pulse. The protocols were rationally designed to best separate the mechanisms of permeabilization and electrophoretic transport. The results showed that the delivery of FD varied strongly with the strength of the first pulse and the size of the target molecule. The delivered FD concentration also decreased linearly with the logarithm of the inter-pulse delay. The data indicate that membrane resealing after electropermeabilization occurs rapidly, but that a non-negligible fraction of the pores can be reopened by the second pulse for delay times on the order of hundreds of seconds. The role of the second pulse is hypothesized to be more than just electrophoresis, with a minimum threshold field strength required to reopen nano-sized pores or defects remaining from the first pulse. These results suggest that membrane electroporation, sealing, and re-poration is a complex process that has both short-term and long-term components, which may in part explain the wide variation in membrane-resealing times reported in the literature.

Electroporation-mediated gene delivery

Electroporation has been used extensively to transfer DNA to bacteria, yeast, and mammalian cells in culture for the past 30 years. Over this time, numerous advances have been made, from using fields to facilitate cell fusion, delivery of chemotherapeutic drugs to cells and tissues, and most importantly, gene and drug delivery in living tissues from rodents to man. Electroporation uses electrical fields to transiently destabilize the membrane allowing the entry of normally impermeable macromolecules into the cytoplasm. Surprisingly, at the appropriate field strengths, the application of these fields to tissues results in little, if any, damage or trauma. Indeed, electroporation has even been used successfully in human trials for gene delivery for the treatment of tumors and for vaccine development. Electroporation can lead to between 100 and 1000-fold increases in gene delivery and expression and can also increase both the distribution of cells taking up and expressing the DNA as well as the absolute amount of gene product per cell (likely due to increased delivery of plasmids into each cell). Effective electroporation depends on electric field parameters, electrode design, the tissues and cells being targeted, and the plasmids that are being transferred themselves. Most importantly, there is no single combination of these variables that leads to greatest efficacy in every situation; optimization is required in every new setting. Electroporation-mediated in vivo gene delivery has proven highly effective in vaccine production, transgene expression, enzyme replacement, and control of a variety of cancers. Almost any tissue can be targeted with electroporation, including muscle, skin, heart, liver, lung, and vasculature. This chapter will provide an overview of the theory of electroporation for the delivery of DNA both in individual cells and in tissues and its application for in vivo gene delivery in a number of animal models.

Scaling relationship and optimization of double-pulse electroporation

The efficacy of electroporation is known to vary significantly across a wide variety of biological research and clinical applications, but as of this writing, a generalized approach to simultaneously improve efficiency and maintain viability has not been available in the literature. To address that discrepancy, we here outline an approach that is based on the mapping of the scaling relationships among electroporation-mediated molecular delivery, cellular viability, and electric pulse parameters. The delivery of Fluorescein-Dextran into 3T3 mouse fibroblast cells was used as a model system. The pulse was rationally split into two sequential phases: a first precursor for permeabilization, followed by a second one for molecular delivery. Extensive data in the parameter space of the second pulse strength and duration were collected and analyzed with flow cytometry. The fluorescence intensity correlated linearly with the second pulse duration, confirming the dominant role of electrophoresis in delivery. The delivery efficiency exhibited a characteristic sigmoidal dependence on the field strength. An examination of short-term cell death using 7-Aminoactinomycin D demonstrated a convincing linear correlation with respect to the electrical energy. Based on these scaling relationships, an optimal field strength becomes identifiable. A model study was also performed, and the results were compared with the experimental data to elucidate underlying mechanisms. The comparison reveals the existence of a critical transmembrane potential above which delivery with the second pulse becomes effective. Together, these efforts establish a general route to enhance the functionality of electroporation.

Harnessing DNA-induced immune responses for improving cancer vaccines

DNA vaccines have emerged as an attractive strategy to promote protective cellular and humoral immunity against the encoded antigen. DNA vaccines are easy to generate, inexpensive to produce and purify at large-scale, highly stable and safe. In addition, plasmids used for DNA vaccines act as powerful "danger signals" by stimulating several DNA-sensing innate immune receptors that promote the induction of protective adaptive immunity. The induction of tumor-specific immune responses represents a major challenge for DNA vaccines because most of tumor-associated antigens are normal non-mutated self-antigens. As a consequence, induction of potentially self-reactive T cell responses against such poorly immunogenic antigens is controlled by mechanisms of central and peripheral tolerance as well as tumor-induced immunosuppression. Although several DNA vaccines against cancer have reached clinical testing, disappointing results have been observed. Therefore, the development of new adjuvants that strongly stimulate the induction of antitumor T cell immunity and counteract immune-suppressive regulation is an attractive approach to enhance the potency of DNA vaccines and overcome tumor-associated tolerance. Understanding the DNA-sensing signaling pathways of innate immunity that mediate the induction of T cell responses elicited by DNA vaccines represents a unique opportunity to develop novel adjuvants that enhance vaccine potency. The advance of DNA adjuvants needs to be complemented with the development of potent delivery systems, in order to step toward successful clinical application. Here, we briefly discuss recent evidence showing how to harness DNA-induced immune response to improve the potency of cancer vaccines and counteract tumor-associated tolerance.

Chemically modified oligonucleotide-increased stability negatively correlates with its efficacy despite efficient electrotransfer

Despite great potential for disease treatment, small interfering RNA (siRNA) development has been hampered due to its poor stability and the lack of efficient delivery method. To overcome the sensitivity, new generations of chemically modified oligonucleotides have been developed such as the locked nucleic acid (LNA). LNA substitution in an siRNA sequence (siLNA) is supposed to increase its stability and its affinity for its complementary sequence. The purpose of this study was to evaluate the potential benefit of an anti-GFP siLNA using the biophysical delivery method electropermeabilization. We used two types of electrical conditions: electrochemotherapy (ECT), a condition for efficient transfer of small molecules in clinics, and electrogenotherapy (EGT), a condition for efficient transfer of macromolecules. We first confirmed that siLNA was indeed more stable in mouse serum than unmodified siRNA. After determining the ECT and EGT optimal electrical parameters for a human colorectal carcinoma cell line (HCT-116) expressing eGFP, we showed that modifications of siRNA do not interfere with electrotransfer efficiency. However, despite its higher stability and its high electrotransfer efficacy, siLNA was less efficient for eGFP silencing compared to the electrotransferred, unmodified siRNA regardless of the electrical conditions used. Our study highlighted the care that is needed when designing chemically modified oligonucleotides.

Optimization of a 96-Well Electroporation Assay for Postnatal Rat CNS Neurons Suitable for Cost-Effective Medium-Throughput Screening of Genes that Promote Neurite Outgrowth

Following an injury, central nervous system (CNS) neurons show a very limited regenerative response which results in their failure to successfully form functional connections with their original target. This is due in part to the reduced intrinsic growth state of CNS neurons, which is characterized by their failure to express key regeneration-associated genes (RAGs) and by the presence of growth inhibitory molecules in CNS environment that form a molecular and physical barrier to regeneration. Here we have optimized a 96-well electroporation and neurite outgrowth assay for postnatal rat cerebellar granule neurons (CGNs) cultured upon an inhibitory cellular substrate expressing myelin-associated glycoprotein or a mixture of growth inhibitory chondroitin sulfate proteoglycans. Optimal electroporation parameters resulted in 28% transfection efficiency and 51% viability for postnatal rat CGNs. The neurite outgrowth of transduced neurons was quantitatively measured using a semi-automated image capture and analysis system. The neurite outgrowth was significantly reduced by the inhibitory substrates which we demonstrated could be partially reversed using a Rho Kinase inhibitor. We are now using this assay to screen large sets of RAGs for their ability to increase neurite outgrowth on a variety of growth inhibitory and permissive substrates.

Muscle gene electrotransfer is increased by the antioxidant tempol in mice

Electropermeabilization (EP) is an effective method of gene transfer into different tissues. During EP, reactive oxygen species (ROS) are formed, which could affect transfection efficiency. The role of generated ROS and the role of antioxidants in electrotransfer in myoblasts in vitro and in Musculus tibialis cranialis in mice were, therefore, investigated. We demonstrate in the study that during EP of C2C12 myoblasts, ROS are generated on the surface of the cells, which do not induce long-term genomic DNA damage. Plasmid DNA for transfection (pEGFP-N1), which is present outside the cells during EP, neutralizes the generated ROS. The ROS generation is proportional to the amplitude of the electric pulses and can be scavenged by antioxidants, such as vitamin C or tempol. When antioxidants were used during gene electrotransfer, the transfection efficiency of C2C12 myoblasts was statistically significantly increased 1.6-fold with tempol. Also in vivo, the transfection efficiency of M. tibialis cranialis in mice was statistically significantly increased 1.4-fold by tempol. The study indicates that ROS are generated on cells during EP and can be scavenged by antioxidants. Specifically, tempol can be used to improve gene electrotransfer into the muscle and possibly also to other tissues.

Direct visualization at the single-cell level of siRNA electrotransfer into cancer cells

The RNA interference-mediated gene silencing approach is promising for therapies based on the targeted inhibition of disease-relevant genes. Electropermeabilization is one of the nonviral methods successfully used to transfer siRNA into living cells in vitro and in vivo. Although this approach is effective in the field of gene silencing by RNA interference, very little is known about the basic processes supporting siRNA transfer. In this study, we investigated, by direct visualization at the single-cell level, the delivery of Alexa Fluor 546-labeled siRNA into murine melanoma cells stably expressing the enhanced green fluorescent protein (EGFP) as a target gene. The electrotransfer of siRNA was quantified by time lapse fluorescence microscopy and was correlated with the silencing of egfp expression. A direct transfer into the cell cytoplasm of the negatively charged siRNA was observed across the plasma membrane exclusively on the side facing the cathode. When added after electropulsation, the siRNA was inefficient for gene silencing because it did not penetrate the cells. Therefore, we report that an electric field acts on both the permeabilization of the cell plasma membrane and on the electrophoretic drag of the negatively charged siRNA molecules from the bulk phase into the cytoplasm. The transfer kinetics of siRNA are compatible with the creation of nanopores, which are described with the technique of synthetic nanopores. The mechanism involved was clearly specific for the physico-chemical properties of the electrotransferred molecule and was different from that observed with small molecules or plasmid DNA.

Control of the release of freely diffusing molecules in single-cell electroporation

Single-cell electroporation using an electrolyte-filled capillary is an emerging technique for transient pore formation in adherent cells. Because adherent cells do not have a simple and consistent shape and because the electric field emanating from the tip of the capillary is inhomogeneous, the Schwan equation based on spherical cells in homogeneous electrical fields does not apply. We sought to determine experimental and cell parameters that influence the outcome of a single-cell electroporation experiment. A549 cells were exposed to the thiol-reactive dye Thioglo-1, leading to green fluorescence from intracellular thiol adducts. Electroporation causes a decrease with time of the intracellular fluorescence intensity of Thioglo-1-loaded cells from diffusive loss of thiol adducts. The transient curves thus obtained are well-described by a simple model originally developed by Puc et al. We find that the final fluorescence following electroporation is related to the capillary tip-to-cell distance and cell size (specifically, 2(A/pi)(1/2) where A is the area of the cell's image in pixels. This quantity is the diameter if the image is a circle). In separate experiments, the relationship obtained can be used to control the final fluorescence following electroporation by adjusting the tip-to-cell distance based on cell size. The relationship was applied successfully to A549 as well as DU 145 and PC-3 cells. Finally, F-tests show that the variability in the final fluorescence (following electroporation) is decreased when the tip-to-cell distance is controlled according to the derived relationship in comparison to experiments in which the tip-cell distance is a constant irrespective of cell size.

Mechanistic analysis of electroporation-induced cellular uptake of macromolecules

Pulsed electric field has been widely used as a nonviral gene delivery platform. The delivery efficiency can be improved through quantitative analysis of pore dynamics and intracellular transport of plasmid DNA. To this end, we investigated mechanisms of cellular uptake of macromolecules during electroporation. In the study, fluorescein isothiocyanate-labeled dextran (FD) with molecular weight of 4,000 (FD-4) or 2,000,000 (FD-2000) was added into suspensions of a murine mammary carcinoma cell (4T1) either before or at different time points (ie, 1, 2, or 10 sec) after the application of different pulsed electric fields (in high-voltage mode: 1.2-2.0 kV in amplitude, 99 microsec in duration, and 1-5 pulses; in low-voltage mode: 100-300 V in amplitude, 5-20 msec in duration, and 1-5 pulses). The intracellular concentrations of FD were quantified using a confocal microscopy technique. To understand transport mechanisms, a mathematical model was developed for numerical simulation of cellular uptake. We observed that the maximum intracellular concentration of FD-2000 was less than 3% of that in the pulsing medium. The intracellular concentrations increased linearly with pulse number and amplitude. In addition, the intracellular concentration of FD-2000 was approximately 40% lower than that of FD-4 under identical pulsing conditions. The numerical simulations predicted that the pores larger than FD-4 lasted <10 msec after the application of pulsed fields if the simulated concentrations were on the same order of magnitude as the experimental data. In addition, the simulation results indicated that diffusion was negligible for cellular uptake of FD molecules. Taken together, the data suggested that large pores induced in the membrane by pulsed electric fields disappeared rapidly after pulse application and convection was likely to be the dominant mode of transport for cellular uptake of uncharged macromolecules.

A study of the immunological response to tumor ablation with irreversible electroporation

Immune cell recruitment during the treatment of sarcoma tumors in mice with irreversible electroporation was studied by immunohistochemistry. Irreversible electroporation is a non-thermal tissue ablation technique in which certain short duration electrical fields are used to permanently permeabilize the cell membrane, presumably through the formation of nanoscale defects in the membrane. Employing irreversible electroporation parameters known to completely ablate the tumors without thermal effects we did not find infiltration of immune cells probably because of the destruction of infiltration routes. We confirm here that immune response is not instrumental in irreversible electroporation efficacy, and we propose that irreversible electroporation may be, therefore, a treatment modality of interest to immunodepressed cancer patients.

A multi-channel electroporation microchip for gene transfection in mammalian cells

We developed a multi-channel electroporation microchip made of polydimethylsiloxane (PDMS) and glass for gene transfer in mammalian cells. This chip produces multiple electric field gradients in a single microchip by varying the lengths of the microchannels from 2 to 4 cm. Electric fields of 0.65, 0.57, 0.49, 0.41, and 0.33 kV/cm were simultaneously produced in a single chip when the voltage of 1.3 kV was applied. We transferred enhanced green fluorescent protein genes (pEGFP) into HEK-293 and CHO cells, which were cultured within the microchannels. The feasibility of our device was demonstrated because it was able to produce five different transfection rates and survival rates at different electric fields produced in a single microchip. This system is expected to optimize the experimental conditions in gene transfection research more easily and faster than conventional electroporation methods.

Effect of cell size and shape on single-cell electroporation

Single-cell electroporation was performed using electrolyte-filled capillaries on fluorescently labeled A549 cells. Cells were exposed to brief pulses (50-300 ms) at various cell-capillary tip distances. Cell viability and electroporation success were measured. In order to understand the variability in single-cell electroporation, logistic regression was used to determine whether the probabilities of cell survival and electroporation depend on experimental conditions and cell properties. Both experimental conditions and cell properties (size and shape) have a significant effect on the outcome. Finite element simulations were used to compare bulk electroporation to single-cell electroporation in terms of cell size and shape. Cells are more readily permeabilized and are more likely to survive if they are large and hemispherical as opposed to small and ellipsoidal with a high aspect ratio. The dependence of the maximum transmembrane potential across the cell membrane on cell size is much weaker than it is for bulk electroporation. Observed survival probabilities are related to the calculated fraction of the cell's surface area that is electroporated. Observed success of electroporation is related to the maximum transmembrane potential achieved.

Determination of cell electroporation from the release of intracellular potassium ions

When cells are exposed to a strong enough external electric field, transient aqueous pores are formed in the membrane. The fraction of electroporated cells can be determined by measuring the release of intracellular potassium ions. The current work is the first study where such a method was employed successfully not only with cells suspended in the medium with a rather high concentration of potassium (4-5 mM) but also with cells that release some part of intracellular potassium responding, in this way, to the stress caused by manipulation procedures during the preparation of the cell suspension. Experiments were carried out on mouse hepatoma MH-A22 cells exposed to a square-wave electric pulse. The curves showing the dependence of the fraction of the cells that have become permeable to bleomycin, a membrane-impermeable cytotoxic drug, are close to the ones showing the release of intracellular potassium ions.

Simultaneous maximization of cell permeabilization and viability in single-cell electroporation using an electrolyte-filled capillary

A549 cells were briefly exposed to Thioglo-1, which converts thiols to fluorescent adducts. The fluorescent cells were exposed to short (50-300 ms) electric field pulses (500 V across a 15 cm capillary) created at the tip of an electrolyte-filled capillary. Fluorescence microscopy revealed varying degrees of cell permeabilization depending on the conditions. Longer pulses and a shorter cell-capillary tip distance led to a greater decrease in the cell's fluorescence. Live/dead (calcein AM and propidium iodide) testing revealed that a certain fraction of cells died. Longer pulses and shorter cell-capillary tip distances were more deadly. An optimum condition exists at a cell-capillary tip distance of 3.5-4.5 microm and a pulse duration of 120-150 ms. At these conditions, >90% of the cells are permeabilized and 80-90% survive.

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.

Electrode Microchamber for Noninvasive Perturbation of Mammalian Cells With Nanosecond Pulsed Electric Fields

Nanosecond pulsed electric fields can pass through the external membrane of biological cells and disturb fast-responding intracellular structures and processes. To enable real-time imaging and investigation of these phenomena, a microchamber with integral electrodes and optical path for observing individual cells exposed to ultrashort electric pulses was designed and fabricated utilizing photolithographic and microelectronic methods. SU-8 photoresist was patterned to form straight sidewalls from 10 to 30 microm in height, with gold film deposited on the top and sidewalls for conductive, nonreactive electrodes and a uniform electric field. Channel dimensions (10-30 microm x 100 microm x 12 000 microm) are suitable for observations of mammalian cells during nanosecond, megavolt-per-meter pulsed electric field exposure. Experimental studies utilizing the electrode microchamber include live-cell imaging of nanoelectropulse-induced intracellular calcium bursts and membrane phospholipid translocation.

Mechanisms of cell membrane electropermeabilization: a minireview of our present (lack of ?) knowledge

Cell electropulsation is routinely used in cell Biology for protein, RNA or DNA transfer. Its clinical applications are under development for targeted drug delivery and gene therapy. Nevertheless, the molecular mechanisms supporting the induction of permeabilizing defects in the membrane assemblies remain poorly understood. This minireview describes the present state of the investigations concerning the different steps in the reversible electropermeabilization process. The different hypotheses, which were proposed to give a molecular description of the membrane events, are critically discussed. Other possibilities are then given. The need for more basic research on the associated loss of cohesion of the membrane appears as a conclusion.

Polynorbornene polycationic polymers as gene transfer agents. Influence of the counterion for in vitro transfection

Polycationic derivatives of polynorbornene with different non-cytotoxic counterions, have been prepared by organometallic polymerization of methyleneammonium norbornene and subsequent exchange of the counterion. In this paper the effect of the counterion on the polycationic polymer binding onto plasmid DNA was studied via different ethidium bromide assays, heparin displacement and protection against degradation by DNAse. According to the nature of the counterions and consequently the size of the polymer particles, their complexation with the DNA led to aggregates with variable binding affinity for the plasmid. The relative transfection efficiency of each polyplex was compared, on the basis of reporter gene expression, in cells in culture. The nature of the counterion was seen to affect gene delivery. The order of transfection efficiency of the counterions studied at equivalent charge ratios (NH3+/PO4-) is lactobionate, acetate, chloride. The results obtained with the polynorbornene methyleneammonium lactobionate and acetate are particularly encouraging.

Retroactive pathway involving mitochondria in electroloaded cytochrome c-induced apoptosis. Protective properties of Bcl-2 and Bcl-XL

Cytochrome c release is thought to play an important role in the initiation of apoptosis. The nature of the control exerted by Bcl-2 and Bcl-XL on such a pathway is not precisely known. We addressed this issue by square-wave pulse electroloading of exogenous cytochrome c into Jurkat cells. Three hours after cytochrome c loading into the cells, characteristic phenotypes of apoptosis were observed. However, a significant drop in the mitochondrial membrane potential (Deltapsim) was also observed, while cytochrome c was generally considered to act downstream from the mitochondria. Related to the Deltapsim drop, there was a release of proapoptotic proteins such as AIF and Smac from the mitochondria. This release, as well as NAD(P)H and cardiolipids oxidation, are linked to previous caspase activation. Cytochrome c-linked caspase activation also led to potassium efflux out of the cell. Overexpression of Bcl-2 and Bcl-XL or N-acetyl-DEVD-aldehyde treatment not only prevented the mitochondrial membrane potential decrease, but also protected cells from the apoptosis directly induced by cytochrome c electroloading. Bcl-2 and Bcl-XL protection is based on the inhibition of the caspase-dependent retroactive pathway affecting the mitochondrial compartment.

Factors controlling electropermeabilisation of cell membranes

Electric field pulses are a new approach for drug and gene delivery for cancer therapy. They induce a localized structural alteration of cell membranes. The associated physical mechanisms are well explained and can be safely controlled. A position dependent modulation of the membrane potential difference is induced when an electric field is applied to a cell. Electric field pulses with an overcritical intensity evoke a local membrane alteration. A free exchange of hydrophilic low molecular weight molecules takes place across the membrane. A leakage of cytosolic metabolites and a loading of polar drugs into the cytoplasm are obtained. The fraction of the cell surface which is competent for exchange is a function of the field intensity. The level of local exchange is strongly controlled by the pulse duration and the number of successive pulses. The permeabilised state is long lived. Its lifetime is under the control of the cumulated pulse duration. Cell viability can be preserved. Gene transfer is obtained but its mechanism is not a free diffusion. Plasmids are electrophoretically accumulated against the permeabilised cell surface and form aggregates due to the field effect. After the pulses, several steps follow: translocation to the cytoplasm, traffic to the nucleus and expression. Molecular structural and metabolic changes in cells remain mostly poorly understood. Nevertheless, while most studies were established on cells in culture (in vitro), recent experiments show that similar effects are obtained on tissue (in vivo). Transfer remains controlled by the physical parameters of the electrical treatment.

Elimination of free-living amoebae in fresh water with pulsed electric fields

This study investigates the effects of pulsed electric fields on the inactivation of trophozoite form of Naegleria lovaniensis Ar9M-1 in batch and flow processes, systematically examining the lethal effect of field strength, pulse duration, number of pulses, and pulse frequency. Our results show that amoebae eradication is modulated by pulse parameters, composition of the pulsing medium, and physiological state of the cells. Cell survival is not related to the energy delivered to the cell suspension during the electrical treatment. For a given energy a strong field applied for a short cumulative pulse duration affects viability more than a weak field with a long cumulative pulsation. We also determine the optimal electrical conditions to obtain an inactivation rate higher than 95% while using the least energy. Flow processes allow to treat large-scale volumes. Our results show that the most efficient flow process for amoeba eradication requires a field parallel to the flow. Pulsed electric fields are a new and attractive method for inactivating amoebae in large volumes of fresh water.

Shock wave-mediated molecular delivery into cells

A single shock wave generated by a shock tube is able to effectively deliver macromolecules such as fluorescein isothiocyanate-dextran into the cytoplasm of living cells without causing cytotoxicity. We report on the effect of varying the molecular weight of the dextran and the number of shock waves on the efficiency of delivery into a cancer cell line. The fraction of cells permeabilized and the total fluorescence delivered were measured by flow cytometry, and the cellular viability by a tetrazolium assay on adherent cells and these values were compared to cell permeabilization using digitonin. Shock waves can deliver molecules of up to 2000000 molecular weight into the cytoplasm of cells without toxicity and may have applications in gene therapy.

The influence of medium conductivity on electropermeabilization and survival of cells in vitro

Electropermeabilization and cell death caused by the exposure to high voltage electric pulses depends on the parameters of pulses, as well as the composition of the extracellular medium. We studied the influence of extracellular conductivity on electropermeabilization and survival of cells in vitro. For this purpose, we used a physiological medium with a conductivity of 1.6 S/m and three artificial media with conductivities of 0.14, 0.005, and 0.001 S/m. Measurements of pH, osmolarity, and cell diameter were made to estimate possible side effects of the media on the cells. Our study shows that the percentage of surviving cells increases with the decreasing medium conductivity, while the percentage of electropermeabilized cells remains unaffected. Our results show that cell survival in experiments involving electropermeabilization can be improved by decreasing the medium conductivity. To provide an interpretation of experimental results, we have theoretically estimated the resting transmembrane voltage, the induced transmembrane voltage, the time constant of the voltage inducement, and heating of the cell suspension for each of the media used. These calculations imply that for accurate interpretation of experimental results, both the induced and the resting transmembrane voltage must be considered, taking into account the conductivity and the ionic composition of the extracellular medium.

Control by membrane order of voltage-induced permeabilization, loading and gene transfer in mammalian cells

Cells can be transiently permeabilized by application of electric pulses. A direct consequence of this treatment is to create a new state in the membrane leading to DNA and protein transfers. A key step, in the interaction between macromolecules and the electropermeabilized membrane, is involved. We previously reported that membrane and DNA associated hydration and undulation forces appeared to be involved in this process by studying the effects of osmotic pressure. Effects of ethanol (EtOH) and L-alpha-lysophosphatidylcholine (lyso-PC), molecules known to affect membrane order and therefore undulation forces, were investigated on Chinese hamster ovary (CHO) cells. We used millisecond square wave pulses, conditions giving high efficiency for gene transfer. No effect was observed on cell permeabilization for small sized molecules. Only little change on electroloading of proteins such as R-phycoerythrin was obtained in presence of EtOH. But, a decrease (increase) in electrotransfection was observed for cells treated with EtOH (lyso-PC). Under our conditions, no additional effects of the chemical treatment were observed on cell viability and on membrane resealing. These results tentatively explained in terms of the effect of membrane order on membrane organization and interaction between molecules and membrane supports the existence of the plasmid-membrane interaction in the mechanism of electrically mediated gene transfer.

Theoretical modeling of the effects of shock duration, frequency, and strength on the degree of electroporation

Electroporation is becoming an increasingly important tool for introducing biologically active compounds into living cells, yet the effectiveness of this technique can be low, particularly in vivo. One way to improve the success rate is to optimize the shock protocols, but experimental studies are costly, time consuming, and yield only an indirect measurement of pore creation. Alternatively, this study models electroporation in two geometries, a space-clamped membrane and a single cell, and investigates the effects of pulse duration, frequency, shape, and strength. The creation of pores is described by a first order differential equation derived from the Smoluchowski equation. Both the membrane and the cell are exposed to monophasic and biphasic shocks of varying duration (membrane, 10 micros-100 s; cell, 0.1 micros-200 ms) and to trains of monophasic and biphasic pulses of varying frequency (membrane, 50 Hz-4 kHz; cell, 200 kHz-6 MHz). The effectiveness of each shock is measured by the fractional pore area (FPA). The results indicate that FPA is sensitive to shock duration only in a very narrow range (membrane, approximately 1 ms; cell, approximately 0.25 micros). In contrast, FPA is sensitive to shock strength and frequency of the pulse train, increasing linearly with shock strength and decreasing slowly with frequency. In all cases, monophasic shocks were at least as effective as biphasic shocks, implying that varying the strength and frequency of a monophasic pulse train is the most effective way to control the creation of pores.

Volume changes of isolated human K562 leukemia cells induced by electric field pulses

Electropermeabilization of immobilized human leukemia K562 cells was studied by measuring changes in cell volume. Such changes reflect mass transfer between the cell and external medium. Electropermeabilization was carried out in an isosmotic water-sorbitol medium with a range of electric field strengths from 500 to 800 V. cm(-1), corresponding to low-energy levels. Electroporation of the K562 cell membrane was found to provoke an inflow of sorbitol and a corresponding osmotic inflow of water and/or an outflow of intracellular solutes due to Fick diffusion. Such flows were found to involve the shrinkage, swelling, or rupture of K562 cells, depending on the characteristics of the electric field and of the physiological state of cells. The behavior of immobilized cells was observed during their exposure to the electric field. The response in immobilized cell volume corresponded with the theoretical pore size and pore opening time, permitting an explanation of the behavior of cell suspensions subject to electrical fields.