A 3D in vitro spheroid model as a way to study the mechanisms of electroporation

Development of 3D melanoma cultures on a hyaluronic acid-based scaffold with synthetic self-assembling peptides: Electroporation enhancement

Electrochemotherapy (ECT) with bleomycin is an effective antitumor treatment. Still, researchers are investigating new drugs and electroporation conditions to improve its efficacy. To this aim, in vivo assays are accurate but expensive and ethically questionable. Conversely, in vitro assays, although cheaper and straightforward, do not reflect the architecture of the biological tissue because they lack a tridimensional (3D) structure (as in the case of two-dimensional [2D] in vitro assays) or do not include all the extracellular matrix components (as in the case of 3D in vitro scaffolds). To address this issue, 3D in vitro models have been proposed, including spheroids and hydrogel-based cultures, which require a suitable low-conductive medium to allow cell membrane electroporation. In this study, a synthetic scaffold based on hyaluronic acid (HA) and self-assembling peptides (SAPs; EAbuK), condensed with a Laminin-derived adhesive sequence (IKVAV), is proposed as a reliable alternative. We compare SKMEL28 cells cultured in the HA-EAbuK-IKVAV scaffold to the control (HA only scaffold). Three days after seeding, the culture on the HA-EAbuK-IKVAV scaffold showed collagen production. SKMEL28 cells cultured on the HA-EAbuK-IKVAV scaffold started to be electroporated at 400 V/cm, whereas, at the same electric field intensity, those cultured on HA were not. As a reference, 2D experiments showed that electroporation of SKMEL28 cells starts at 600 V/cm using an electroporation buffer and at 800 V/cm in a culture medium, but with very low efficiency (<50 % of cells electroporated). 3D cultures on HA-EAbuK-IKVAV allowed the simulation of a more reliable microenvironment and may represent a valuable tool for studying electroporation conditions. Using Finite Element Analysis (FEA) to compute the transmembrane potential, we detected the influence of inhomogeneity of the extracellular matrix on electroporation effect. Our 3D cell culture electroporation simulations showed that the transmembrane potential increased when collagen surrounded the cells. Of note, in the collagen-enriched HA-EAbuK-IKVAV scaffold, EP was already improved at lower electric field intensities. This study shows the influence of the extracellular matrix on electric conductivity and electric field distribution on cell membrane electroporation and supports the adoption of more reliable 3D scaffolds in experimental electroporation studies.

In situ transient transfection of 3D cell cultures and tissues, a promising tool for tissue engineering and gene therapy

Various research fields use the transfection of mammalian cells with genetic material to induce the expression of a target transgene or gene silencing. It is a tool widely used in biological research, bioproduction, and therapy. Current transfection protocols are usually performed on 2D adherent cells or suspension cultures. The important rise of new gene therapies and regenerative medicine in the last decade raises the need for new tools to empower the in situ transfection of tissues and 3D cell cultures. This review will present novel in situ transfection methods based on a chemical or physical non-viral transfection of cells in tissues and 3D cultures, discuss the advantages and remaining gaps, and propose future developments and applications.

Gene Electrotransfer Efficiency in 2D and 3D Cancer Cell Models Using Different Electroporation Protocols: A Comparative Study

Electroporation, a method relying on a pulsed electric field to induce transient cell membrane permeabilization, can be used as a non-viral method to transfer genes in vitro and in vivo. Such transfer holds great promise for cancer treatment, as it can induce or replace missing or non-functioning genes. Yet, while efficient in vitro, gene-electrotherapy remains challenging in tumors. To assess the differences of gene electrotransfer in respect to applied pulses in multi-dimensional (2D, 3D) cellular organizations, we herein compared pulsed electric field protocols applicable to electrochemotherapy and gene electrotherapy and different "High Voltage-Low Voltage" pulses. Our results show that all protocols can result in efficient permeabilization of 2D- and 3D-grown cells. However, their efficiency for gene delivery varies. The gene-electrotherapy protocol is the most efficient in cell suspensions, with a transfection rate of about 50%. Conversely, despite homogenous permeabilization of the entire 3D structure, none of the tested protocols allowed gene delivery beyond the rims of multicellular spheroids. Taken together, our findings highlight the importance of electric field intensity and the occurrence of cell permeabilization, and underline the significance of pulses' duration, impacting plasmids' electrophoretic drag. The latter is sterically hindered in 3D structures and prevents the delivery of genes into spheroids' core.

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.

Integrated platform for culture, observation, and parallelized electroporation of spheroids

Reversible electroporation is a method to introduce molecules into cells by increasing the permeability of their membranes, thanks to the application of pulsed electric fields. One of its main biomedical applications is electro-chemotherapy, where electroporation is used to deliver anticancer drugs into tumor tissues. To improve our understanding of the electroporation effect on tissues and select efficient treatments, in vitro tumor models are needed. Cell spheroids are relevant models as they can reproduce tumor microenvironment and cell-cell interactions better than 2D cell cultures. Various methods offering a relatively simple workflow are now available for their production. However, electroporation protocols usually require handling steps that may damage spheroids and result in random spacing, inducing variations in electric field distribution around spheroids and non-reproducible electroporation conditions. In addition, only a few microsystems allow the production and electroporation of spheroids, and the spheroids produced lack reproducibility in size and location. To overcome these issues, we developed a unique device enabling culture, monitoring, and electroporation of hundreds of regular spheroids in parallel, with a design ensuring that all spheroids are submitted to the same electric field conditions. It is comprised of a microfluidic chamber encompassing a micro-structured agarose gel, allowing easy medium exchange while avoiding spheroid handling. It also enables optical imaging of spheroids in situ, thanks to transparent electrodes. In this paper, we describe the fabrication and characterization of the developed microsystem and demonstrate its applicability to electroporation of a network of spheroids. We present a first successful application as an anticancer drug testing platform, by evaluating the bleomycin effect on HT29 colorectal cancer cell spheroids. This work opens new perspectives in the development of in vitro assays for the preclinical evaluation of electroporation-based treatment.

Revisiting the role of pulsed electric fields in overcoming the barriers to in vivo gene electrotransfer

Gene therapies are revolutionizing medicine by providing a way to cure hitherto incurable diseases. The scientific and technological advances have enabled the first gene therapies to become clinically approved. In addition, with the ongoing COVID-19 pandemic, we are witnessing record speeds in the development and distribution of gene-based vaccines. For gene therapy to take effect, the therapeutic nucleic acids (RNA or DNA) need to overcome several barriers before they can execute their function of producing a protein or silencing a defective or overexpressing gene. This includes the barriers of the interstitium, the cell membrane, the cytoplasmic barriers and (in case of DNA) the nuclear envelope. Gene electrotransfer (GET), i.e., transfection by means of pulsed electric fields, is a non-viral technique that can overcome these barriers in a safe and effective manner. GET has reached the clinical stage of investigations where it is currently being evaluated for its therapeutic benefits across a wide variety of indications. In this review, we formalize our current understanding of GET from a biophysical perspective and critically discuss the mechanisms by which electric field can aid in overcoming the barriers. We also identify the gaps in knowledge that are hindering optimization of GET in vivo.

Gene electrotransfer of FGF2 enhances collagen scaffold biocompatibility

Tendon injuries are a common athletic injury that have been increasing in prevalence. While there are current clinical treatments for tendon injuries, they have relatively long recovery times and often do not restore native function of the tendon. In the current study, gene electrotransfer (GET) parameters for delivery to the skin were optimized with monophasic and biphasic pulses with reporter and effector genes towards optimizing underlying tendon healing. Tissue twitching and damage, as well as gene expression and distribution were evaluated. Bioprinted collagen scaffolds, mimicking healthy tendon structure were then implanted subcutaneously for biocompatibility and angiogenesis analyses when combined with GET to accelerate healing. GET of human fibroblast FGF2 significantly increased angiogenesis and biocompatibility of the bioprinted implants when compared to implant only sites. The combination of bioprinted collagen fibers and angiogenic GET therapy may lead to better graft biocompatibility in tendon repair.

Transfer of small interfering RNA by electropermeabilization in tumor spheroids

The ability to modulate deregulated genes by RNAi provides treatment perspectives in certain diseases including cancers. Electrotransfer of oligonucleotides was studied in vitro, showing a direct transfer of negatively charged siRNA across the plasma membrane into the cytoplasm. In vivo, the feasibility of siRNA electrotransfer was demonstrated in different studies and tissues. While effective, electrotransfer of siRNA into 3D tissues still needs to be understood. Here, we evaluated the efficiency of siRNA electrotransfer and assessed its effect in 3D spheroids made of HCT116-GFP cells by confocal fluorescence microscopy and flow cytometry. Our results indicate that siRNA uptake was not uniform across 3D multicellular spheroids. The electrophoretic migration of nucleic acids upon delivery of unipolar electric field pulses could explain the asymmetry of siRNA uptake. Moreover, a gradient was observed from external layers toward the center, leading to siRNA silencing of GFP positive cells located in the outer rim. While siRNA delivery experiments on spheroids may differ from intratumoral injections, the levels of transfection in spheroids are comparable to levels observed in published studies in vivo. Taken together, our results provide fundamental information about siRNA 3D distribution during electrotransfer, indicating that multicellular spheroids remain a relevant alternative to animal experimentation.

A nanosecond pulsed electric field (nsPEF) can affect membrane permeabilization and cellular viability in a 3D spheroids tumor model

Three-dimensional (3D) cellular models represent more realistically the complexity of in vivo tumors compared to 2D cultures. While 3D models were largely used in classical electroporation, the effects of nanosecond pulsed electric field (nsPEF) have been poorly investigated. In this study, we evaluated the biological effects induced by nsPEF on spheroid tumor model derived from the HCT-116 human colorectal carcinoma cell line. By varying the number of pulses (from 1 to 500) and the polarity (unipolar and bipolar), the response of nsPEF exposure (10 ns duration, 50 kV/cm) was assessed either immediately after the application of the pulses or over a period lasting up to 6 days. Membrane permeabilization and cellular death occurred following the application of at least 100 pulses. The extent of the response increased with the number of pulses, with a significant decrease of viability, 24 h post-exposure, when 250 and 500 pulses were applied. The effects were highly reduced when an equivalent number of bipolar pulses were delivered. This reduction was eliminated when a 100 ns interphase interval was introduced into the bipolar pulses. Altogether, our results show that nsPEF effects, previously observed at the single cell level, also occur in more realistic 3D tumor spheroids models.

Growth environment influences B16.F10 mouse melanoma cell response to gene electrotransfer

We developed and characterized a 3D collagen hydrogel model for B16.F10 melanoma tumors. Cells in this 3D environment exhibited lower proliferation than cells in the conventional 2D culture environment. Interestingly, the basal expression levels of several genes varied when compared to conventionally grown cells. In each growth environment, a significant number of melanoma cells were transfected by plasmid electroporation (electrotransfer), although expression could only be ascertained on the surface of the 3D constructs. Cellular responses to plasmid entry as demonstrated by pro-inflammatory cytokine and chemokine upregulation varied based on the growth environment, as did the mRNA levels of several putative DNA-specific pattern recognition receptors (DNA sensors). Unexpectedly, when plasmid DNA was delivered while cells where attached in the 2D or 3D environments, the mRNAs of the DNA sensor p204 and the inflammatory mediator TNFα were regulated in cells receiving pulses only. However, we were unable to confirm coordinate upregulation of TNFα and p204 proteins. This study confirms that cell responses differ significantly based on their environment, and demonstrates the difficulty of extending experimental observations between cell environments.

The Efficiency of Gene Electrotransfer in Breast-Cancer Cell Lines Cultured on a Novel Collagen-Free 3D Scaffold

Gene Electro-Transfer (GET) is a powerful method of DNA delivery with great potential for medical applications. Although GET has been extensively studied in vitro and in vivo, the optimal parameters remain controversial. 2D cell cultures have been widely used to investigate GET protocols, but have intrinsic limitations, whereas 3D cultures may represent a more reliable model thanks to the capacity of reproducing the tumor architecture. Here we applied two GET protocols, using a plate or linear electrode, on 3D-cultured HCC1954 and MDA-MB231 breast cancer cell lines grown on a novel collagen-free 3D scaffold and compared results with conventional 2D cultures. To evaluate the electrotransfer efficiency, we used the plasmid pEGFP-C3 encoding the enhanced green fluorescent protein (EGFP) reporter gene. The novel 3D scaffold promoted extracellular matrix deposition, which particularly influences cell behavior in both in vitro cell cultures and in vivo tumor tissue. While the transfection efficiency was similar in the 2D-cultures, we observed significant differences in the 3D-model. The transfection efficiency in the 3D vs 2D model was 44% versus 15% (p < 0.01) and 24% versus 17% (p < 0.01) in HCC1954 and MDA-MB231 cell cultures, respectively. These findings suggest that the novel 3D scaffold allows reproducing, at least partially, the peculiar morphology of the original tumor tissues, thus allowing us to detect meaningful differences between the two cell lines. Following GET with plate electrodes, cell viability was higher in 3D-cultured HCC1954 (66%) and MDA-MB231 (96%) cell lines compared to their 2D counterpart (53% and 63%, respectively, p < 0.001). Based on these results, we propose the novel 3D scaffold as a reliable support for the preparation of cell cultures in GET studies. It may increase the reliability of in vitro assays and allow the optimization of GET parameters of in vivo protocols.

A Novel 3D Scaffold for Cell Growth to Asses Electroporation Efficacy

Tumor electroporation (EP) refers to the permeabilization of the cell membrane by means of short electric pulses thus allowing the potentiation of chemotherapeutic drugs. Standard plate adhesion 2D cell cultures can simulate the in vivo environment only partially due to lack of cell–cell interaction and extracellular matrix (ECM). In this study, we assessed a novel 3D scaffold for cell cultures based on hyaluronic acid and ionic-complementary self-assembling peptides (SAPs), by studying the growth patterns of two different breast carcinoma cell lines (HCC1569 and MDA-MB231). This 3D scaffold modulates cell shape and induces extracellular matrix deposit around cells. In the MDA-MB 231 cell line, it allows three-dimensional growth of structures known as spheroids, while in HCC1569 it achieves a cell organization similar to that observed in vivo. Interestingly, we were able to visualize the electroporation effect on the cells seeded in the new scaffold by means of standard propidium iodide assay and fluorescence microscopy. Thanks to the presence of cell–cell and cell–ECM interactions, the new 3D scaffold may represent a more reliable support for EP studies than 2D cancer cell cultures and may be used to test new EP-delivered drugs and novel EP protocols.

Direct transfection of clonal organoids in Matrigel microbeads: a promising approach toward organoid-based genetic screens

Organoid cultures in 3D matrices are relevant models to mimic the complex in vivo environment that supports cell physiological and pathological behaviors. For instance, 3D epithelial organoids recapitulate numerous features of glandular tissues including the development of fully differentiated acini that maintain apico-basal polarity with hollow lumen. Effective genetic engineering in organoids would bring new insights in organogenesis and carcinogenesis. However, direct 3D transfection on already formed organoids remains challenging. One limitation is that organoids are embedded in extracellular matrix and grow into compact structures that hinder transfection using traditional techniques. To address this issue, we developed an innovative approach for transgene expression in 3D organoids by combining single-cell encapsulation in Matrigel microbeads using a microfluidic device and electroporation. We demonstrate that direct electroporation of encapsulated organoids reaches up to 80% of transfection efficiency. Using this technique and a morphological read-out that recapitulate the different stages of tumor development, we further validate the role of p63 and PTEN as key genes in acinar development in breast and prostate tissues. We believe that the combination of controlled organoid generation and efficient 3D transfection developed here opens new perspectives for flow-based high-throughput genetic screening and functional genomic applications.

3D-printing enabled micro-assembly of a microfluidic electroporation system for 3D tissue engineering

Electro-transfection is an essential workhorse tool for regulating cellular responses and engineering cellular materials in tissue engineering. However, most of the existing approaches are only focused on cell suspensions in vitro, which fails to mimic an in vivo tissue microenvironment regarding the 3D electric field distribution and mass transport in a biological matrix. However, building a 3D electro-transfection system that is compatible with 3D cell culture for mimicking the in vivo tissue microenvironment is challenging, due to the substantial difficulties in control of the 3D electric field distribution as well as the cellular growth. To address such challenges, we introduce a novel 3D micro-assembly strategy assisted by 3D printing, which enables the molding of 3D microstructures as LEGO® parts from 3D-printed molds. The molded PDMS LEGO® bricks are then assembled into a 3D-cell culture chamber interconnected with vertical and horizontal perfusion microchannels as a 3D channel network. Such a 3D perfusion microchannel network is unattainable by direct 3D printing or other microfabrication approaches, which can facilitate the highly-efficient exchange of nutrition and waste for 3D cell growth. Four flat electrodes are mounted into the 3D culture chamber via a 3D-printed holder and controlled by a programmable power sequencer for multi-directional electric frequency scanning (3D μ-electro-transfection). This multi-directional scanning not only can create transient pores all over the cell membrane, but also can generate local oscillation for enhancing mass transport and improving cell transfection efficiency. As a proof-of-concept, we electro-delivered the pAcGFP1-C1 vector to 3D cultured HeLa cells within peptide hydrogel scaffolding. The expressed GFP level from transfected HeLa cells reflects the transfection efficiency. We found two key parameters including electric field strength and plasmid concentration playing more important roles than the pulse duration and duty cycles. The results showed an effective transfection efficiency of ∼15% with ∼85% cell viability, which is 3-fold higher compared to that of the conventional benchtop 3D cell electro-transfection. This 3D μ-electrotransfection system was further used for genetically editing 3D-cultured Hek-293 cells via direct delivery of CRISPR/Cas9 plasmid which showed successful transfection with GFP expressed in the cytoplasm as the reporter. The 3D-printing enabled micro-assembly allows facile creation of a novel 3D culture system for electro-transfection, which can be employed for versatile gene delivery and cellular engineering, as well as building in vivo like tissue models for fundamentally studying cellular regulation mechanisms at the molecular level.

Upregulation of DNA Sensors in B16.F10 Melanoma Spheroid Cells After Electrotransfer of pDNA

Increased expression of cytosolic DNA sensors, a category of pattern recognition receptor, after control plasmid DNA electrotransfer was observed in our previous studies on B16.F10 murine melanoma cells. This expression was correlated with the upregulation of proinflammatory cytokines and chemokines and was associated with cell death. Here, we expanded our research to include the influence of features of cells in a 3-dimensional environment, which better represents the tumors' organization in vivo. Our results show that lower number of cells were transfected in spheroids compared to 2-dimensional cultures, that growth was delayed after electroporation alone or after electrotransfer of plasmid DNA, and that DNA sensors DDX60, DAI/ZBP1, and p204 were upregulated 4 hours and 24 hours after electrotransfer of plasmid DNA. Moreover, the cytokines interferon β and tumor necrosis factor α were also upregulated but only 4 hours after electrotransfer of plasmid DNA. Thus, our results confirm the results obtained in 2-dimensional cell cultures demonstrating that electrotransfer of plasmid DNA to tumor cells in spheroids also upregulated cytosolic DNA sensors and cytokines.

Recellularized human dermis for testing gene electrotransfer ex vivo

Gene electrotransfer (GET) is a proven and valuable tool for in vivo gene delivery to a variety of tissues such as skin, cardiac muscle, skeletal muscle, and tumors, with controllable gene delivery and expression levels. Optimizing gene expression is a challenging hurdle in preclinical studies, particularly for skin indications, due to differences in electrical conductivity of animal compared to human dermis. Therefore, the goal of this study was to develop an ex vivo model for GET using recellularized human dermis to more closely mimic human skin. Decellularized human dermis (DermACELL(®)) was cultured with human dermal fibroblasts and keratinocytes for 4 weeks. After one week of fibroblast culture, fibroblasts infiltrated and dispersed throughout the dermis. Air-liquid interface culture led to epithelial cell proliferation, stratification and terminal differentiation with distinct basal, spinous, granular and cornified strata. Firefly luciferase expression kinetics were evaluated after GET of recellularized constructs for testing gene delivery parameters to skin in vitro. Elevated luciferase expression persisted up to a week following GET compared to controls without electrotransfer. In summary, recellularized dermis structurally and functionally resembled native human skin in tissue histological organization and homeostasis, proving an effective 3D human skin model for preclinical gene delivery studies.

Electroporation Knows No Boundaries: The Use of Electrostimulation for siRNA Delivery in Cells and Tissues

The discovery of RNA interference (RNAi) has enabled several breakthrough discoveries in the area of functional genomics. The RNAi technology has emerged as one of the major tools for drug target identification and has been steadily improved to allow gene manipulation in cell lines, tissues, and whole organisms. One of the major hurdles for the use of RNAi in high-throughput screening has been delivery to cells and tissues. Some cell types are refractory to high-efficiency transfection with standard methods such as lipofection or calcium phosphate precipitation and require different means. Electroporation is a powerful and versatile method for delivery of RNA, DNA, peptides, and small molecules into cell lines and primary cells, as well as whole tissues and organisms. Of particular interest is the use of electroporation for delivery of small interfering RNA oligonucleotides and clustered regularly interspaced short palindromic repeats/Cas9 plasmid vectors in high-throughput screening and for therapeutic applications. Here, we will review the use of electroporation in high-throughput screening in cell lines and tissues.

Cell-cell proximity effects in multi-cell electroporation

We report a fundamental study of how the electropermeabilization of a cell is affected by nearby cells. Previous researchers studying electroporation of dense suspensions of cells have observed, both theoretically and experimentally, that such samples cannot be treated simply as collections of independent cells. However, the complexity of those systems makes quantitative modeling difficult. We studied the change in the minimum applied electric field, the threshold field, required to affect electropermeabilization of a cell due to the presence of a second cell. Experimentally, we used optical tweezers to accurately position two cells in a custom fluidic electroporation device and measured the threshold field for electropermeabilization. We also captured video of the process. In parallel, finite element simulations of the electrostatic potential distributions in our systems were generated using the 3-layer model and the contact resistance methods. Reasonably good agreement with measurements was found assuming a model in which changes in a cell's threshold field were predicted from the calculated changes in the maximum voltage across the cell's membrane induced by the presence of a second cell. The threshold field required to electroporate a cell is changed ∼5%-10% by a nearby, nearly touching second cell. Cells aligned parallel to the porating field shield one another. Those oriented perpendicular to the field enhance the applied field's effect. In addition, we found that the dynamics of the electropermeabilization process are important in explaining observations for even our simple two-cell system.

Focused ultrasound mediated drug delivery from temperature-sensitive liposomes: in-vitro characterization and validation

Nanomedicine-based delivery with non-invasive techniques is a promising approach to increase local drug concentration and to reduce systemic side effects. Focused ultrasound (FUS) has become a promising strategy for non-invasive local drug delivery by mild hyperthermia. In this study, traditional temperature-sensitive liposomes (TTSLs) encapsulating doxorubicin (DOX) were evaluated for FUS-mediated drug delivery with an in-vitro FUS setup. In-vitro studies showed quantitative release of the DOX from the lumen of the temperature-sensitive liposomes when heated to 42 °C with FUS using 1 MHz sinusoidal waves at 1.75 MPa for 10 min. No release was observed when heated at 37 °C. Moreover, we showed that DOX released from TTSLs by FUS is as efficiently internalized by glioblastoma cells as free DOX at 37 °C. In-vitro therapeutic evaluation showed that exposure of a cell monolayer to FUS-activated TTSLs induced a 60% and a 50% decrease in cell viability compared to cell medium and to TTSLs preheated at 37 °C, respectively. Using an in-vitro 3D cell culture model, the results showed that after FUS-mediated hyperthermia, preheated liposomes induced a 1.7-fold decrease in U-87 MG spheroid growth in comparison to the preheated liposomes at 37 °C. In conclusion, our results show that in-vitro FUS allows the evaluation of TTSLs and does not modify the cellular uptake of the released DOX nor its cytotoxic activity.

siRNA delivery via electropulsation: a review of the basic processes

Due to their capacity for inducing strong and sequence specific gene silencing in cells, small interfering RNAs (siRNAs) are now recognized not only as powerful experimental tools for basic research in Molecular biology but with promising potentials in therapeutic development. Delivery is a bottleneck in many studies. There is a common opinion that full potential of siRNA as therapeutic agent will not be attained until better methodologies for its targeted intracellular delivery to cells and tissues are developed. Electropulsation (EP) is one of the physical methods successfully used to transfer siRNA into living cells in vitro and in vivo. This review will describe how siRNA electrotransfer obeys characterized biophysical processes (cell-size-dependent electropermeabilization, electrophoretic drag) with a strong control of a low loss of viability. Protocols can be easily adjusted by a proper setting of the electrical parameters and pulsing buffers. EP can be easily directly applied on animals. Preclinical studies showed that electropermeabilization brings a direct cytoplasmic distribution of siRNA and an efficient silencing of the targeted protein expression. EP appears as a promising tool for clinical applications of gene silencing. A panel of successful trials will be given.

3D spheroids' sensitivity to electric field pulses depends on their size

Dramatic differences of cells behavior exist between cells cultured under classical 2D monolayers and 3D models, the latter being closer to in vivo responses. Thus, many 3D cell culture models have been developed. Among them, multicellular tumor spheroid appears as a nice and easy-to-handle 3D model based on cell adhesion properties. It is composed of one or several cell types and is widely used to address carcinogenesis, or drugs screening. A few and recent publications report the use of spheroids to investigate electropermeabilization process. We studied the response of spheroids to electrical field pulses (EP) in terms of their age, diameter or formation technique. We found that small human HCT-116 colorectal spheroids are more sensitive to electric field pulses than larger ones. Indeed, the growth of spheroids with a diameter of 300 μm decreased by a factor 2 over 4 days when submitted to EP (8 pulses, lasting 100 μs at a 1,300 V/cm field intensity). Under those electrical conditions, 650 μm spheroids were not affected. These data were the same whatever the formation method (i.e. hanging drop and nonadherent techniques). These observations point out the fact that characteristics of 3D cell models have to be taken into account to avoid biased conclusions of experimental data.

Antitumor drug delivery in multicellular spheroids by electropermeabilization

Electrochemotherapy (ECT) is a physical technique that allows cytotoxic molecules to be efficiently released in tumor cells by inducing transient cell plasma membrane permeabilization. The main antitumoral drugs used in ECT are nonpermeant bleomycin and low permeant cisplatin. The method is nowadays applied in clinics as a palliative treatment. In order to improve it, we took advantage of a human 3D multicellular tumor spheroid as a model of tumor to visually and molecularly assess the effect of ECT. We used bleomycin and cisplatin to confirm its relevance and doxorubicin to show its potential to screen new antitumor drug candidates for ECT. Confocal microscopy was used to visualize the topological distribution of permeabilized cells in 3D spheroids subjected to electric pulses. Our results revealed that all cells were efficiently permeabilized, whatever their localization in the spheroid, even those in the core. The combination of antitumor drugs and electric pulses (ECT) led to changes in spheroid macroscopic morphology and cell cohesion, to tumor spheroid growth arrest and finally to its complete apoptosis-mediated dislocation, mimicking previously observed in vivo situations. Taken together, these results indicate that the spheroid model is relevant for the study and optimization of electromediated drug delivery protocols.

The use of an in vitro 3D melanoma model to predict in vivo plasmid transfection using electroporation

A large-scale in vitro 3D tumor model was generated to evaluate gene delivery procedures in vivo. This 3D tumor model consists of a "tissue-like" spheroid that provides a micro-environment supportive of melanoma proliferation, allowing cells to behave similarly to cells in vivo. This functional spheroid measures approximately 1 cm in diameter and can be used to effectively evaluate plasmid transfection when testing various electroporation (EP) electrode applicators. In this study, we identified EP conditions that efficiently transfect green fluorescent protein (GFP) and interleukin 15 (IL-15) plasmids into tumor cells residing in the 3D construct. We found that plasmids delivered using a 6-plate electrode applying 6 pulses with nominal electric field strength of 500 V/cm and pulse-length of 20 ms produced significant increase of GFP (7.3-fold) and IL-15 (3.0-fold) expression compared to controls. This in vitro 3D model demonstrates the predictability of cellular response toward delivery techniques, limits the numbers of animals employed for transfection studies, and may facilitate future developments of clinical trials for cancer therapies in vivo.

Drug delivery by electropulsation: Recent developments in oncology

Electro-permeabilisation allows the free access of polar compounds to the cytoplasm by a reversible alteration of the cell membrane. It is now used in clinics for the eradication of cutaneous solid tumors. New developments predict its future applications for other anti-cancer treatments.

Pipette tip with integrated electrodes for gene electrotransfer of cells in suspension: a feasibility study in CHO cells

Background

Gene electrotransfer is a non-viral gene delivery method that requires successful electroporation for DNA delivery into the cells. Changing the direction of the electric field during the pulse application improves the efficacy of gene delivery. In our study, we tested a pipette tip with integrated electrodes that enables changing the direction of the electric field for electroporation of cell suspension for gene electrotransfer.

Materials and methods

A new pipette tip consists of four cylindrical rod electrodes that allow the application of electric pulses in different electric field directions. The experiments were performed on cell suspension of CHO cells in phosphate buffer. Plasmid DNA encoding for green fluorescent protein (GFP) was used and the efficiency of gene electrotransfer was determined by counting cells expressing GFP 24 h after the experiment.

Results

Experimental results showed that the percentage of cells expressing GFP increased when the electric field orientation was changed during the application. The GFP expression was almost two times higher when the pulses were applied in orthogonal directions in comparison with single direction, while cell viability was not significantly affected.

Conclusions

We can conclude that results obtained with the described pipette tip are comparable to previously published results on gene electrotransfer using similar electrode geometry and electric pulse parameters. The tested pipette tip, however, allows work with small volumes/samples and requires less cell manipulation.

Use of collagen gel as a three-dimensional in vitro model to study electropermeabilization and gene electrotransfer

Gene electrotransfer is a promising nonviral method that enables transfer of plasmid DNA into cells with electric pulses. Although many in vitro and in vivo studies have been performed, the question of the implied gene electrotransfer mechanisms is largely open. The main obstacle toward efficient gene electrotransfer in vivo is relatively poor mobility of DNA in tissues. Since cells are mechanically coupled to their extracellular environment and act differently compared to standard in vitro conditions, we developed a three-dimensional (3-D) in vitro model of CHO cells embedded in collagen gel as an ex vivo model of tissue to study electropermeabilization and different parameters of gene electrotransfer. For this purpose, we first used propidium iodide to detect electropermeabilization of CHO cells embedded in collagen gel. Then, we analyzed the influence of different concentrations of plasmid DNA and pulse duration on gene electrotransfer efficiency. Our results revealed that even if cells in collagen gel can be efficiently electropermeabilized, gene expression is significantly lower. Gene electrotransfer efficiency in our 3-D in vitro model had similar dependence on concentration of plasmid DNA and pulse duration comparable to in vivo studies, where longer (millisecond) pulses were shown to be more optimal compared to shorter (microsecond) pulses. The presented results demonstrate that our 3-D in vitro model resembles the in vivo situation more closely than conventional 2-D cell cultures and, thus, provides an environment closer to in vivo conditions to study mechanisms of gene electrotransfer.

Gene electrotransfer: from biophysical mechanisms to in vivo applications : Part 2 - In vivo developments and present clinical applications

Gene electrotransfer can be obtained not just on single cells in diluted suspension. For more than 10 years, this is a quasi routine strategy in tissue on the living animal and a few clinical trials have now been approved. New problems have been brought by the close contacts of cells in tissue both on the local field distribution and on the access of DNA to target cells. They need to be solved to provide a further improvement in the efficacy and safety of protein expression. There is a competition between gene transfer and cell destruction. Nevertheless, present results are indicative that electrotransfer is a promising approach for gene therapy. High level and long-lived expression of proteins can be obtained in muscles. This is used for a successful method of electrovaccination.