Enhancing Electrotransfection Efficiency through Improvement in Nuclear Entry of Plasmid DNA

Internalization of transferrin-tagged Myxococcus xanthus encapsulins into mesenchymal stem cells

Currently, various functionalized nanocarrier systems are extensively studied for targeted delivery of drugs, peptides, and nucleic acids. Joining the approaches of genetic and chemical engineering may produce novel carriers for precise targeting different cellular proteins, which is important for both therapy and diagnosis of various pathologies. Here we present the novel nanocontainers based on vectorized genetically encoded Myxococcus xanthus (Mx) encapsulin, confining a fluorescent photoactivatable mCherry (PAmCherry) protein. The shells of such encapsulins were modified using chemical conjugation of human transferrin (Tf) prelabeled with a fluorescein-6 (FAM) maleimide acting as a vector. We demonstrate that the vectorized encapsulin specifically binds to transferrin receptors (TfRs) on the membranes of mesenchymal stromal/stem cells (MSCs) followed by internalization into cells. Two spectrally separated fluorescent signals from Tf-FAM and PAmCherry are clearly distinguishable and co-localized. It is shown that Tf-tagged Mx encapsulins are internalized by MSCs much more efficiently than by fibroblasts. It has been also found that unlabeled Tf effectively competes with the conjugated Mx-Tf-FAM formulations. That indicates the conjugate internalization into cells by Tf-TfR endocytosis pathway. The developed nanoplatform can be used as an alternative to conventional nanocarriers for targeted delivery of, e.g., genetic material to MSCs.

Nanocarrier-based gene delivery for immune cell engineering

Clinical advances in genetically modified immune cell therapies, such as chimeric antigen receptor T cell therapies, have raised hope for cancer treatment. The majority of these biotechnologies are based on viral methods for ex vivo genetic modification of the immune cells, while the non-viral methods are still in the developmental phase. Nanocarriers have been emerging as materials of choice for gene delivery to immune cells. This is due to their versatile physicochemical properties such as large surface area and size that can be optimized to overcome several practical barriers to successful gene delivery. The in vivo nanocarrier-based gene delivery can revolutionize cell-based cancer immunotherapies by replacing the current expensive autologous cell manufacturing with an off-the-shelf biomaterial-based platform. The aim of this research is to review current advances and strategies to overcome the challenges in nanoparticle-based gene delivery and their impact on the efficiency, safety, and specificity of the process. The main focus is on polymeric and lipid-based nanocarriers, and their recent preclinical applications for cancer immunotherapy.

Knockout of the lysosomal membrane protein, LAMP2C, improves transient gene expression in HEK293 cells via increased intracellular plasmid availability

Plasmid-based transfection can be used in many applications such as transient gene expression (TGE)-based therapeutic protein production. These applications preferentially require maximization of intracellular plasmid availability. Here, we applied a lysosome engineering approach to alleviate lysosome-mediated nucleic acid degradation and enhance the TGE in mammalian cells. By knocking out the lysosomal membrane protein LAMP2C, which is known to be the main player in RNautophagy/DNautophagy (RDA), we significantly improved transient fluorescent protein expression in HEK293 cells by improving the retention rate of transfected plasmids; however, this effect was not observed in CHO cells. Additional knockout of a lysosomal membrane transporter and another RDA player, SIDT2, was ineffective, regardless of the presence of LAMP2C. LAMP2C knockout enhanced TGE-based mAb production in HEK293 cells by up to 2.82-fold increase in specific mAb productivity. Taken together, these results demonstrate that HEK293 cells can be engineered to improve the usage of the transfected plasmid via knockout of the lysosomal membrane protein LAMP2C and provide efficient host cells in TGE systems for therapeutic protein production.

Nuclear Entry of DNA and Transgene Expression in Dividing and Non-dividing Cells

Introduction

Plasmid DNA (pDNA) must be delivered into the nucleus for transgene expression in mammalian cells. The entry may happen passively during the nuclear envelope breakdown and reformation in dividing cells or actively through the nuclear pore complexes. The goal of this study was to investigate the relative importance of these two pathways for pDNA nuclear entry and subsequent gene expression.

Methods

To measure nuclear entry of pDNA encoding enhanced green florescence protein (EGFP) in electrotransfected cells, we developed a sensitive technique for quantitative analysis of pDNA in the nuclei, based on a hybridization probe for pDNA detection at the single molecule level and automatic image analysis. In matched experiments, we used an mRNA targeted hybridization probe to quantify reporter mRNA expression per cell, and flow cytometry to quantify expression of EGFP.

Results

We discovered two distinct patterns of pDNA distribution in the nuclei: punctate and diffuse, which were dominant in arrested and unarrested cells, respectively. The cell cycle arrest decreased diffuse pDNA and increased punctate pDNA. Its net effect was a decrease in the total intranuclear pDNA. Additionally, the cell cycle arrest increased the reporter mRNA synthesis but had no substantial impact on reporter protein expression.

Conclusion

Results from the study demonstrated that the efficient nuclear entry of pDNA during cell division did not necessarily lead to a high level of transgene expression. They also suggested that the punctate pDNA was more transcriptionally active than diffuse pDNA in the nuclei. These data will be useful in future studies for understanding mechanisms of nonviral gene delivery.

Copy number of naked DNA delivered into nucleus of mammalian cells by electrotransfection

Electrotransfection is a non-viral method for delivery of nucleic acids into cells. In our previous study, we have determined the minimal copy number of plasmid DNA (pDNA) per cell required for transgene expression post electrotransfection, and developed a statistical framework to predict the pDNA copy number in the nucleus. To experimentally verify the prediction, the current study was designed to quantify the average copy number of pDNA per nucleus post electrotransfection. To achieve it, we developed a novel approach to effectively obtain isolated nuclei with minimal contamination by extranuclear pDNA. This sample preparation method enabled us to accurately measure intranuclear pDNA using quantitative real-time PCR. The data showed that the copy number of pDNA per nucleus was dependent on the period of cell culture post pulsing and the pDNA dose for electrotransfection. Additionally, the data were used to improve the statistical framework for understanding kinetics of pDNA transport in cells, and predicting how the kinetics depended on different factors. It is expected that the framework and the methodology developed in the current study will be useful for evaluating factors that may affect kinetics and mechanisms of pDNA transport in cells.

Effect of Experimental Electrical and Biological Parameters on Gene Transfer by Electroporation: A Systematic Review and Meta-Analysis

The exact mechanisms of nucleic acid (NA) delivery with gene electrotransfer (GET) are still unknown, which represents a limitation for its broader use. Further, not knowing the effects that different experimental electrical and biological parameters have on GET additionally hinders GET optimization, resulting in the majority of research being performed using a trial-and-error approach. To explore the current state of knowledge, we conducted a systematic literature review of GET papers in in vitro conditions and performed meta-analyses of the reported GET efficiency. For now, there is no universal GET strategy that would be appropriate for all experimental aims. Apart from the availability of the required electroporation device and electrodes, the choice of an optimal GET approach depends on parameters such as the electroporation medium; type and origin of cells; and the size, concentration, promoter, and type of the NA to be transfected. Equally important are appropriate controls and the measurement or evaluation of the output pulses to allow a fair and unbiased evaluation of the experimental results. Since many experimental electrical and biological parameters can affect GET, it is important that all used parameters are adequately reported to enable the comparison of results, as well as potentially faster and more efficient experiment planning and optimization.

"Genetic scissors" CRISPR/Cas9 genome editing cutting-edge biocarrier technology for bone and cartilage repair

CRISPR/Cas9 is a revolutionary genome editing technology with the tremendous advantages such as precisely targeting/shearing ability, low cost and convenient operation, becoming an efficient and indispensable tool in biological research. As a disruptive technique, CRISPR/Cas9 genome editing has a great potential to realize a future breakthrough in the clinical bone and cartilage repairing as well. This review highlights the research status of CRISPR/Cas9 system in bone and cartilage repair, illustrates its mechanism for promoting osteogenesis and chondrogenesis, and explores the development tendency of CRISPR/Cas9 in bone and cartilage repair to overcome the current limitations.

Microtrap array on a chip for localized electroporation and electro-gene transfection

We developed a localized single-cell electroporation chip to deliver exogenous biomolecules with high efficiency while maintaining high cell viability. In our microfluidic device, the cells are trapped in a microtrap array by flow, after which target molecules are supplied to the device and electrotransferred to the cells under electric pulses. The system provides the ability to monitor the electrotransfer of exogenous biomolecules in real time. We reveal through numerical simulations that localized electroporation is the mechanism of permeabilization in the microtrap array electroporation device. We demonstrate the simplicity and accuracy of this microtrap technology for electroporation by delivery of both small molecules using propidium iodide and large molecules using plasmid DNA for gene expression, illustrating the potential of this minimally invasive method to be widely used for precise intracellular delivery purposes (from bioprocess engineering to therapeutic applications).

Recent Progress in Recombinant Influenza Vaccine Development Toward Heterosubtypic Immune Response

Flu, a viral infection caused by the influenza virus, is still a global public health concern with potential to cause seasonal epidemics and pandemics. Vaccination is considered the most effective protective strategy against the infection. However, given the high plasticity of the virus and the suboptimal immunogenicity of existing influenza vaccines, scientists are moving toward the development of universal vaccines. An important property of universal vaccines is their ability to induce heterosubtypic immunity, i.e., a wide immune response coverage toward different influenza subtypes. With the increasing number of studies and mounting evidence on the safety and efficacy of recombinant influenza vaccines (RIVs), they have been proposed as promising platforms for the development of universal vaccines. This review highlights the current progress and advances in the development of RIVs in the context of heterosubtypic immunity induction toward universal vaccine production. In particular, this review discussed existing knowledge on influenza and vaccine development, current hemagglutinin-based RIVs in the market and in the pipeline, other potential vaccine targets for RIVs (neuraminidase, matrix 1 and 2, nucleoprotein, polymerase acidic, and basic 1 and 2 antigens), and deantigenization process. This review also provided discussion points and future perspectives in looking at RIVs as potential universal vaccine candidates for influenza.

Compact High-Voltage AC Generator with Pulse Transformer for High-Frequency Irreversible Electroporation (H-FIRE)

This paper is focused on a design of a high-voltage (HV) generator, which is proposed for a high-frequency irreversible electroporation (H-FIRE). The generator produces bursts of bipolar symmetrical pulses. Most HV sources used for cell electroporation are based on a controlled discharge of a capacitor into a resistive load. This solution is very simple, but it is associated with a certain risk of an uncontrolled discharge of the capacitor. We present a different type of the generator, where a DC-AC inverter with pulse transformer is used and where the mentioned risk is eliminated. Our generator is able to deliver bursts with variable length from 50 to 150 μs and a gap between bursts can be set from 0.5 to 1.5 s. Pulse frequency can be varied from 65 to 470 kHz and the output voltage is controlled in two ranges from 0 to 1.3 kV or from 0 to 2.5 kV. Results are presented with resistive load and with tissue impedance load.

Electroporation-Based Therapy for Brain Tumors: A Review

Electroporation-based therapy (EBT), as a high-voltage-pulse technology has been prevalent with favorable clinical outcomes in the treatment of various solid tumors. This review paper aims to promote the clinical translation of EBT for brain tumors. First, we briefly introduced the mechanism of pore formation in a cell membrane activated by external electric fields using a single cell model. Then, we summarized and discussed the current in vitro and in vivo preclinical studies, in terms of (1) the safety and effectiveness of EBT for brain tumors in animal models, and (2) the blood-brain barrier (BBB) disruption induced by EBT. Two therapeutic effects could be achieved in EBT for brain tumors simultaneously, i.e., the tumor ablation induced by irreversible electroporation (IRE) and transient BBB disruption induced by reversible electroporation (RE). The BBB disruption could potentially improve the uptake of antitumor drugs thereby enhancing brain tumor treatment. The challenges that hinder the application of EBT in the treatment of human brain tumors are discussed in the review paper as well.

Probing the role of nuclear-envelope invaginations in the nuclear-entry route of lipofected DNA by multi-channel 3D confocal microscopy

Nuclear breakdown was found to be the dominant route for DNA entry into the nucleus in actively dividing cells. The possibility that alternative routes contribute to DNA entry into the nucleus, however, cannot be ruled out. Here we address the process of lipofection by monitoring the localization of fluorescently-labelled DNA plasmids at the single-cell level by confocal imaging in living interphase cells. As test formulation we choose the cationic 3β-[N-(N,N-dimethylaminoethane)-carbamoyl] cholesterol (DC-Chol) and the zwitterionic helper lipid dioleoylphosphatidylethanolamine (DOPE) with plasmidic DNA pre-condensed by means of protamine. By exploiting the spectral shift of the fluorescent dye FM4-64 (N-(3-triethylammoniumpropyl)-4-(p-diethylaminophenylhexatrienyl)-pyridinium 2Br) we monitor the position of the nuclear envelope (NE), while concomitantly imaging the whole nucleus (by Hoechst) and the DNA (by Cy3 fluorophore) in a multi-channel 3D confocal imaging experiment. Reported results show that DNA clusters are typically associated with the NE membrane in the form of tubular invaginations spanning the nuclear environment, but not completely trapped within the NE invaginations, i.e. the DNA may use these NE regions as entry-points towards the nucleus. These observations pave the way to investigating the molecular details of the postulated processes for a better exploitation of gene-delivery vectors, particularly for applications in non-dividing cells.

Evaluation of a Novel Plasmid for Simultaneous Gene Electrotransfer-Mediated Silencing of CD105 and CD146 in Combination with Irradiation

Targeting tumor vasculature through specific endothelial cell markers represents a promising approach for cancer treatment. Here our aim was to construct an antibiotic resistance gene-free plasmid encoding shRNAs to simultaneously target two endothelial cell markers, CD105 and CD146, and to test its functionality and therapeutic potential in vitro when delivered by gene electrotransfer (GET) and combined with irradiation (IR). Functionality of the plasmid was evaluated by determining the silencing of the targeted genes using qRT-PCR. Antiproliferative and antiangiogenic effects were determined by the cytotoxicity assay tube formation assay and wound healing assay in murine endothelial cells 2H-11. The functionality of the plasmid construct was also evaluated in malignant melanoma tumor cell line B16F10. Additionally, potential activation of immune response was measured by induction of DNA sensor STING and proinflammatory cytokines by qRT-PCR in endothelial cells 2H-11. We demonstrated that the plasmid construction was successful and can efficiently silence the expression of the two targeted genes. As a consequence of silencing, reduced migration rate and angiogenic potential was confirmed in 2H-11 endothelial cells. Furthermore, induction of DNA sensor STING and proinflammatory cytokines were determined, which could add to the therapeutic effectiveness when used in vivo. To conclude, we successfully constructed a novel plasmid DNA with two shRNAs, which holds a great promise for further in vivo testing.

Mechanistic studies of gene delivery into mammalian cells by electrical short-circuiting via an aqueous droplet in dielectric oil

We have developed a novel methodology for the delivery of cell-impermeable molecules, based on electrical short-circuiting via a water droplet in dielectric oil. When a cell suspension droplet is placed between a pair of electrodes with an intense DC electric field, droplet bouncing and droplet deformation, which results in an instantaneous short-circuit, can be induced, depending on the electric field strength. We have demonstrated successful transfection of various mammalian cells using the short-circuiting; however, the molecular mechanism remains to be elucidated. In this study, flow cytometric assays were performed with Jurkat cells. An aqueous droplet containing Jurkat cells and plasmids carrying fluorescent proteins was treated with droplet bouncing or short-circuiting. The short-circuiting resulted in sufficient cell viability and fluorescent protein expression after 24 hours’ incubation. In contrast, droplet bouncing did not result in successful gene transfection. Transient membrane pore formation was investigated by uptake of a cell-impermeable fluorescence dye YO-PRO-1 and the influx of calcium ions. As a result, short-circuiting increased YO-PRO-1 fluorescence intensity and intracellular calcium ion concentration, but droplet bouncing did not. We also investigated the contribution of endocytosis to the transfection. The pre-treatment of cells with endocytosis inhibitors decreased the efficiency of gene transfection in a concentration-dependent manner. Besides, the use of pH-sensitive dye conjugates indicated the formation of an acidic environment in the endosomes after the short-circuiting. Endocytosis is a possible mechanism for the intracellular delivery of exogenous DNA.

Cell Synchronization Enhances Nuclear Transformation and Genome Editing via Cas9 Enabling Homologous Recombination in Chlamydomonas reinhardtii

In Chlamydomonas reinhardtii, the model organism for eukaryotic green algae and plants, the processes of nuclear transformation and genome editing in particular are still marked by a low level of efficiency, and so intensive work is required in order to create and identify mutants for the investigation of basic physiological processes, as well as the implementation of biotechnological applications. In this work, we show that cell synchronization during the stages of the cell cycle, obtained from long-term cultivation under specific growth conditions, greatly enhances the efficiency of transformation and allows the identification of DNA repair mechanisms that occur preferentially at different stages of the cell cycle. We demonstrate that the transformation of synchronized cells at different times was differentially associated with nonhomologous end joining (NHEJ) and/or homologous recombination (HR), and makes it possible to knock-in specific foreign DNA at the genomic nuclear location desired by exploiting HR. This optimization greatly reduces the overall complexity of the genome editing procedure and creates new opportunities for altering genes and their products.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Redirecting Vesicular Transport to Improve Nonviral Delivery of Molecular Cargo

Cell engineering relies heavily on viral vectors for the delivery of molecular cargo into cells due to their superior efficiency compared to nonviral ones. However, viruses are immunogenic and expensive to manufacture, and have limited delivery capacity. Nonviral delivery approaches avoid these limitations but are currently inefficient for clinical applications. This work demonstrates that the efficiency of nonviral delivery of plasmid DNA, mRNA, Sleeping Beauty transposon, and ribonucleoprotein can be significantly enhanced through pretreatment of cells with the nondegradable sugars (NDS), such as sucrose, trehalose, and raffinose. The enhancement is mediated by the incorporation of the NDS into cell membranes, causing enlargement of lysosomes and formation of large (>500 nm) amphisome-like bodies (ALBs). The changes in subcellular structures redirect transport of cargo to ALBs rather than to lysosomes, reducing cargo degradation in cells. The data indicate that pretreatment of cells with NDS is a promising approach to improve nonviral cargo delivery in biomedical applications.

Compact Square-Wave Pulse Electroporator with Controlled Electroporation Efficiency and Cell Viability

The design and development of a compact square-wave pulse generator for the electroporation of biological cells is presented. This electroporator can generate square-wave pulses with durations from 3 μs up to 10 ms, voltage amplitudes up to 3500 V, and currents up to 250 A. The quantity of the accumulated energy is optimized by means of a variable capacitor bank. The pulse forming unit design uses a crowbar circuit, which gives better control of the pulse form and its duration, independent of the load impedance. In such cases, the square-wave pulse form ensures better control of electroporation efficiency by choosing parameters determined in advance. The device has an integrated graphic LCD screen and measurement modules for the visualization of the current pulse, allowing for express control of the electroporation quality and does not require an external oscilloscope for current pulse recording. This electroporator was tested on suspensions of Saccharomyces cerevisiae yeast cells, during which, it was demonstrated that the application of such square-wave pulses ensured better control of the electroporation efficiency and cell viability after treatment using the pulsed electric field (PEF).

Episomal vectors based on S/MAR and the β-globin Replicator, encoding a synthetic transcriptional activator, mediate efficient γ-globin activation in haematopoietic cells

We report the development of episomal vectors for the specific γ-globin transcription activation in its native position by activator Zif-VP64, based on the Scaffold/Matrix Attachment Region (S/MAR) for episomal retention and the β-globin Replicator, the DNA replication-Initiation Region from the β-globin locus. Vector Zif-VP64-Ep1 containing transcription cassettes CMV- Zif-VP64 and CMV-eGFP-S/MAR transfected a)K562 cells; b)murine β-YAC bone marrow cells (BMC); c)human haematopoietic progenitor CD34⁺ cells, with transfection efficiencies of 46.3 ± 5.2%, 23.0 ± 2.1% and 24.2 ± 2.4% respectively. K562 transfections generated stable cell lines running for 28 weeks with and without selection, with increased levels of γ-globin mRNA by 3.3 ± 0.13, of γ-globin protein by 6.75 ± 3.25 and HbF protein by 2 ± 0.2 fold, while the vector remained episomal and non integrated. In murine β-YAC BMCs the vector mediated the activation of the silent human γ-globin gene and in CD34⁺ cells, increased γ-globin mRNA, albeit only transiently. A second vector Zif-VP64-Ep2, with both transcription cassettes carrying promoter SFFV instead of CMV and the addition of β-globin Replicator, transferred into CD34⁺ cells, produced CD34⁺ eGFP⁺ cells, that generated colonies in colony forming cell cultures. Importantly, these were 100% fluorescent, with 2.11 ± 0.13 fold increased γ-globin mRNA, compared to non-transfected cells. We consider these episomal vectors valid, safer alternatives to viral vectors.

Nucleic acid delivery to mesenchymal stem cells: a review of nonviral methods and applications

Background

Mesenchymal stem cells (MSCs) are multipotent stem cells that can be isolated and expanded from many tissues, and are being investigated for use in cell therapies. Though MSC therapies have demonstrated some success, none have been FDA approved for clinical use. MSCs lose stemness ex vivo, decreasing therapeutic potential, and face additional barriers in vivo, decreasing therapeutic efficacy. Culture optimization and genetic modification of MSCs can overcome these barriers. Viral transduction is efficient, but limited by safety concerns related to mutagenicity of integrating viral vectors and potential immunogenicity of viral antigens. Nonviral delivery methods are safer, though limited by inefficiency and toxicity, and are flexible and scalable, making them attractive for engineering MSC therapies.

Main text

Transfection method and nucleic acid determine efficiency and expression profile in transfection of MSCs. Transfection methods include microinjection, electroporation, and nanocarrier delivery. Microinjection and electroporation are efficient, but are limited by throughput and toxicity. In contrast, a variety of nanocarriers have been demonstrated to transfer nucleic acids into cells, however nanocarrier delivery to MSCs has traditionally been inefficient. To improve efficiency, plasmid sequences can be optimized by choice of promoter, inclusion of DNA targeting sequences, and removal of bacterial elements. Instead of DNA, RNA can be delivered for rapid protein expression or regulation of endogenous gene expression. Beyond choice of nanocarrier and nucleic acid, transfection can be optimized by priming cells with media additives and cell culture surface modifications to modulate barriers of transfection. Media additives known to enhance MSC transfection include glucocorticoids and histone deacetylase inhibitors. Culture surface properties known to modulate MSC transfection include substrate stiffness and specific protein coating. If nonviral gene delivery to MSCs can be sufficiently improved, MSC therapies could be enhanced by transfection for guided differentiation and reprogramming, transplantation survival and directed homing, and secretion of therapeutics. We discuss utilized delivery methods and nucleic acids, and resulting efficiency and outcomes, in transfection of MSCs reported for such applications.

Conclusion

Recent developments in transfection methods, including nanocarrier and nucleic acid technologies, combined with chemical and physical priming of MSCs, may sufficiently improve transfection efficiency, enabling scalable genetic engineering of MSCs, potentially bringing effective MSC therapies to patients.

Current Progress in Electrotransfection as a Nonviral Method for Gene Delivery

Electrotransfection (ET) is a nonviral method for delivery of various types of molecules into cells both in vitro and in vivo. Close to 90 clinical trials that involve the use of ET have been performed, and approximately half of them are related to cancer treatment. Particularly, ET is an attractive technique for cancer immunogene therapy because treatment of cells with electric pulses alone can induce immune responses to solid tumors, and the responses can be further enhanced by ET of plasmid DNA (pDNA) encoding therapeutic genes. Compared to other gene delivery methods, ET has several unique advantages. It is relatively inexpensive, flexible, and safe in clinical applications, and introduces only naked pDNA into cells without the use of additional chemicals or viruses. However, the efficiency of ET is still low, partly because biological mechanisms of ET in cells remain elusive. In previous studies, it was believed that pDNA entered the cells through transient pores created by electric pulses. As a result, the technique is commonly referred to as electroporation. However, recent discoveries have suggested that endocytosis plays an important role in cellular uptake and intracellular transport of electrotransfected pDNA. This review will discuss current progresses in the study of biological mechanisms underlying ET and future directions of research in this area. Understanding the mechanisms of pDNA transport in cells is critical for the development of new strategies for improving the efficiency of gene delivery in tumors.