Kinetics of pore resealing in cell membranes after electroporation

HiViPore: a highly viable in-flow compression for a one-step cell mechanoporation in microfluidics to induce a free delivery of nano- macro-cargoes

BACKGROUND

Among mechanoporation techniques for intracellular delivery, microfluidic approaches succeed in high delivery efficiency and throughput. However, especially the entry of large cargoes (e.g. DNA origami, mRNAs, organic/inorganic nanoparticles) is currently impaired since it requires large cell membrane pores with the need to apply multi-step processes and high forces, dramatically reducing cell viability.

RESULTS

Here, HiViPore presents as a microfluidic viscoelastic contactless compression for one-step cell mechanoporation to produce large pores while preserving high cell viability. Inducing an increase of curvature at the equatorial region of cells, formation of a pore with a size of ~ 1 μm is obtained. The poration is coupled to an increase of membrane tension, measured as a raised fluorescence lifetime of 12% of a planarizable push-pull fluorescent probe (Flipper-TR) labelling the cell plasma membrane. Importantly, the local disruptions of cell membrane are transient and non-invasive, with a complete recovery of cell integrity and functions in ~ 10 min. As result, HiViPore guarantees cell viability as high as ~ 90%. In such conditions, an endocytic-free diffusion of large nanoparticles is obtained with typical size up to 500 nm and with a delivery efficiency up to 12 times higher than not-treated cells.

CONCLUSIONS

The proposed one-step contactless mechanoporation results in an efficient and safe approach for advancing intracellular delivery strategies. In detail, HiViPore solves the issues of low cell viability when multiple steps of poration are required to obtain large pores across the cell plasma membrane. Moreover, the compression uses a versatile, low-cost, biocompatible viscoelastic fluid, thus also optimizing the operational costs. With HiViPore, we aim to propose an easy-to-use microfluidic device to a wide range of users, involved in biomedical research, imaging techniques and nanotechnology for intracellular delivery applications in cell engineering.

The estimation of pore size distribution of electroporated MCF-7 cell membrane

The size of the pores created by external electrical pulses is important for molecule delivery into the cell. The size of pores and their distribution on the cell membrane determine the efficiency of molecule transport into the cell. There are very few studies visualizing the presence of electropores. In this study, we aimed to investigate the size distribution of electropores that were created by high intensity and short duration electrical pulses on MCF-7 cell membrane. Scanning Electron Microscopy (SEM) was used to visualize and characterize the membrane pores created by the external electric field. Structural changes on the surface of the electroporated cell membrane was observed by Atomic Force Microscopy (AFM). The size distribution of pore sizes was obtained by measuring the radius of 500 electropores. SEM imaging showed non-uniform patterning. The average radius of the electropores was 12 nm, 51.60% of pores were distributed within the range of 5 to 10 nm, and 81% of pores had radius below 15 nm. These results showed that microsecond (µs) high intensity electrical pulses cause the creation of heterogeneous nanopores on the cell membrane.

Recent Advancements in Electroporation Technologies: From Bench to Clinic

Over the past decade, the increased adoption of electroporation-based technologies has led to an expansion of clinical research initiatives. Electroporation has been utilized in molecular biology for mammalian and bacterial transfection; for food sanitation; and in therapeutic settings to increase drug uptake, for gene therapy, and to eliminate cancerous tissues. We begin this article by discussing the biophysics required for understanding the concepts behind the cell permeation phenomenon that is electroporation. We then review nano- and microscale single-cell electroporation technologies before scaling up to emerging in vivo applications.

Threshold Microsecond Pulsed Electric Field Exposures for Change in Spinach Quality

Pulsed electric fields (PEFs) are often used to pretreat foods to enhance subsequent processes, such as drying, where maintaining food product quality is important for consumer satisfaction. This study aims to establish a threshold PEF exposure to determine the doses at which electroporation is viable for use on spinach leaves, wherein integrity is maintained postexposure. Three numbers of consecutive pulses (1, 5, 50) and two pulse durations (10 and 100 μs) have been examined herein at a constant pulse repetition of 10 Hz and 1.4 kV/cm field strength. The data indicate that pore formation in itself is not a cause for loss of spinach leaf food quality, i.e., significant changes in color and water content. Rather, cell death, or the rupture of the cell membrane from a high-intensity treatment, is necessary to significantly alter the exterior integrity of the plant tissue. PEF exposures thus can be used on leafy greens up until the point of inactivation before consumers would see any alterations, making reversible electroporation a viable treatment for consumer-intended products. These results open up future opportunities to use emerging technologies based on PEF exposures and provide useful information in setting parameters to avoid food quality diminishment.

Nanosecond pulsed electric field suppresses growth and reduces multi-drug resistance effect in pancreatic cancer

Nanosecond pulsed electric fields (nsPEF) have been shown to exert anticancer effects; however, little is known about the mechanisms triggered in cancer cells by nanosecond-length pulses, especially when low, sub-permeabilization voltage is used. In this study, three human pancreatic cancer cell lines were treated with nsPEF and molecular changes at the cellular level were analyzed. Further, we assessed the efficacy of paclitaxel chemotherapy following nsPEF treatment and correlated that with the changes in the expression of multi-drug resistance (MDR) proteins. Finally, we examined the influence of nsPEF on the adhesive properties of cancer cells as well as the formation and growth of pancreatic cancer spheroids. Cell line response differed with the application of a 200 ns, 100 pulses, 8 kV/cm, 10 kHz PEF treatment. PEF treatment led to (1) the release of microvesicles (MV) in EPP85-181RDB cells, (2) electropermeabilization in EPP85-181RNOV cells and (3) cell shrinkage in EPP85-181P cells. The release of MV's in EPP85-181RDB cells reduced the membrane content of P-gp and LRP, leading to a transient increase in vulnerability of the cells towards paclitaxel. In all cell lines we observed an initial reduction in size of the cancer spheroids after the nsPEF treatment. Cell line EPP85-181RNOV exhibited a permanent reduction in the spheroid size after nsPEF. We propose a mechanism in which the surface tension of the membrane, regulated by the organization of actin fibers, modulates the response of cancer cells towards nsPEF. When a membrane's surface tension remains low, we observed some cells form protrusions and release MVs containing MDR proteins. In contrast, when cell surface tension remains high, the cell membrane is being electroporated. The latter effect may be responsible for the reduced tumor growth following nsPEF treatment.

How to alleviate cardiac injury from electric shocks at the cellular level

Electric shocks, the only effective therapy for ventricular fibrillation, also electroporate cardiac cells and contribute to the high-mortality post-cardiac arrest syndrome. Copolymers such as Poloxamer 188 (P188) are known to preserve the membrane integrity and viability of electroporated cells, but their utility against cardiac injury from cardiopulmonary resuscitation (CPR) remains to be established. We studied the time course of cell killing, mechanisms of cell death, and protection with P188 in AC16 human cardiomyocytes exposed to micro- or nanosecond pulsed electric field (μsPEF and nsPEF) shocks. A 3D printer was customized with an electrode holder to precisely position electrodes orthogonal to a cell monolayer in a nanofiber multiwell plate. Trains of nsPEF shocks (200, 300-ns pulses at 1.74 kV) or μsPEF shocks (20, 100-μs pulses at 300 V) produced a non-uniform electric field enabling efficient measurements of the lethal effect in a wide range of the electric field strength. Cell viability and caspase 3/7 expression were measured by fluorescent microscopy 2-24 h after the treatment. nsPEF shocks caused little or no caspase 3/7 activation; most of the lethally injured cells were permeable to propidium dye already at 2 h after the exposure. In contrast, μsPEF shocks caused strong activation of caspase 3/7 at 2 h and the number of dead cells grew up to 24 h, indicating the prevalence of the apoptotic death pathway. P188 at 0.2-1% reduced cell death, suggesting its potential utility in vivo to alleviate electric injury from defibrillation.

Nanosecond pulsed electric field (nsPEF) and vaccines: a novel technique for the inactivation of SARS-CoV-2 and other viruses?

Since the beginning of 2020, worldwide attention has been being focussed on SARS-CoV-2, the second strain of the severe acute respiratory syndrome virus. Although advances in vaccine technology have been made, particularly considering the advent of mRNA vaccines, up to date, no single antigen design can ensure optimal immune response. Therefore, new technologies must be tested as to their ability to further improve vaccines. Nanosecond Pulsed Electric Field (nsPEF) is one such method showing great promise in different biomedical and industrial fields, including the fight against COVID-19. Of note, available research shows that nsPEF directly damages the cell's DNA, so it is critical to determine if this technology could be able to fragment either viral DNA or RNA so as to be used as a novel technology to produce inactivated pathogenic agents that may, in turn, be used for the production of vaccines. Considering the available evidence, we propose that nsPEF may be used to produce inactivated SARS-CoV-2 viruses that may in turn be used to produce novel vaccines, as another tool to address 20 the current COVID-19 pandemic.Key MessagesViral inactivation by using pulsed electric fields in the nanosecond frequency.DNA fragmentation by a Nanosecond Pulsed Electric Field (nsPEF).Opportunity to apply new technologies in vaccine development.

The role of catechin in electroporation of pancreatic cancer cells - Effects on pore formation and multidrug resistance proteins

Catechin is a bioflavonoid known for its anti-cancer properties. In the present study, we combined theoretical and experimental approaches to reveal the potential of catechin application in the electroporation (EP) or electrochemotherapy (ECT) of pancreatic cancer cells. The molecular dynamics simulations were implemented to examine the interactions of catechin with a model of a membrane, its influence on the membrane's thickness, and the impact of the catechin-membrane interaction on the pore formation. The data were confronted with experimental measurement of the threshold electric field required for permeabilization of pancreatic cancer cells to a fluorescent dye YO-PRO-1. Further, we examined the influence of catechin on cell viability following electroporation with cisplatin or calcium ions. Finally, we investigated the catechin impact on four proteins associated with multidrug resistance: P-glycoprotein, MRP1, BCRP, and LRP. We demonstrated that catechin may boost the effects of electroporation through various mechanisms: i) increasing the cell permeability prior to electroporation ii) increasing the electroporation threshold iii) sensitization of cells to chemotherapeutic compounds. We showed that catechin incubation influences mRNA levels and mitigates the immunoreactivity of Pgp, MRP1, BCRP, and LRP but these changes did not translate to the efficacy of electrochemotherapy.

Does the shape of the electric pulse matter in electroporation?

Electric pulses are widely used in biology, medicine, industry, and food processing. Numerous studies indicate that electroporation (EP) is a pulse-dependent process, and the electric pulse shape and duration strongly determine permeabilization efficacy. EP protocols are precisely planned in terms of the size and charge of the molecules, which will be delivered to the cell. In reversible and irreversible EP applications, rectangular or sine, polar or bipolar pulses are commonly used. The usage of pulses of the asymmetric shape is still limited to high voltage and low voltage (HV/LV) sequences in the context of gene delivery, while EP-based applications of ultra-short asymmetric pulses are just starting to emerge. This review emphasizes the importance and role of the pulse shape for membrane permeabilization by EP.

GM1 asymmetry in the membrane stabilizes pores

Cell membranes are highly asymmetric and their stability against poration is crucial for survival. We investigated the influence of membrane asymmetry on electroporation of giant unilamellar vesicles with membranes doped with GM1, a ganglioside asymmetrically enriched in the outer leaflet of neuronal cell membranes. Compared with symmetric membranes, the lifetimes of micronsized pores are about an order of magnitude longer suggesting that pores are stabilized by GM1. Internal membrane nanotubes caused by the GM1 asymmetry, obstruct and additionally slow down pore closure, effectively reducing pore edge tension and leading to leaky membranes. Our results point to the drastic effects this ganglioside can have on pore resealing in biotechnology applications based on poration as well as on membrane repair processes.

Chemotherapy and Physical Therapeutics Modulate Antigens on Cancer Cells

Cancer cells possess specific properties, such as multidrug resistance or unlimited proliferation potential, due to the presence of specific proteins on their cell membranes. The release of proliferation-related proteins from the membrane can evoke a loss of adaptive ability in cancer cells and thus enhance the effects of anticancer therapy. The upregulation of cancer-specific membrane antigens results in a better outcome of immunotherapy. Moreover, cytotoxic T-cells may also become more effective when stimulated ex-vivo toward the anticancer response. Therefore, the modulation of membrane proteins may serve as an interesting attempt in anticancer therapy. The presence of membrane antigens relies on various physical factors such as temperature, exposure to radiation, or drugs. Therefore, changing the tumor microenvironment conditions may lead to cancer cells becoming sensitized to subsequent therapy. This paper focuses on the therapeutic approaches modulating membrane antigens and enzymes in anticancer therapy. It aims to analyze the possible methods for modulating the antigens, such as pharmacological treatment, electric field treatment, photodynamic reaction, treatment with magnetic field or X-ray radiation. Besides, an overview of the effects of chemotherapy and immunotherapy on the immunophenotype of cancer cells is presented. Finally, the authors review the clinical trials that involved the modulation of cell immunophenotype in anticancer therapy.

Ultralong-Time Recovery and Low-Voltage Electroporation for Biological Cell Monitoring Enabled by a Microsized Multipulse Framework

Long-term nondestructive monitoring of cells is of significant importance for understanding cell proliferation, cell signaling, cell death, and other processes. However, traditional monitoring methods are limited to a certain range of testing conditions and may reduce cell viability. Here, we present a microgap, multishot electroporation (M2E) system for monitoring cell recovery for up to ∼2 h using ∼5 V pulses and with excellent cell viability using a medium cell population. Electric field simulations reveal the bias-voltage- and gap-size-dependent electric field intensities in the M2E system. In addition to excellent transparency with low cell toxicity, the M2E system does not require specialized components, expensive materials, complicated fabrication processes, or cell manipulations; it just consists of a micrometer-sized pattern and a low-voltage square-wave generator. Ultimately, the M2E system can offer a long-term and nontoxic method of cell monitoring.

Evaluation of electroporated area using 2,3,5-triphenyltetrazolium chloride in a potato model

Irreversible electroporation (IRE) is a tissue ablation method, uses short high electric pulses and results in cell death in target tissue by irreversibly permeabilizing the cell membrane. Potato is commonly used as a tissue model for electroporation experiments. The blackened area that forms 12 h after electric pulsing is regarded as an IRE-ablated area caused by melanin accumulation. Here, the 2,3,5-triphenyltetrazolium chloride (TTC) was used as a dye to assess the IRE-ablated area 3 h after potato model ablation. Comparison between the blackened area and TTC-unstained white area in various voltage conditions showed that TTC staining well delineated the IRE-ablated area. Moreover, whether the ablated area was consistent over time and at different staining times was investigated. In addition, the presumed reversible electroporation (RE) area was formed surrounding the IRE-ablated area. Overall, TTC staining can provide a more rapid and accurate electroporated area evaluation.

Cell death due to electroporation - A review

Exposure of cells to high voltage electric pulses increases transiently membrane permeability through membrane electroporation. Electroporation can be reversible and is used in gene transfer and enhanced drug delivery but can also lead to cell death. Electroporation resulting in cell death (termed as irreversible electroporation) has been successfully used as a new non-thermal ablation method of soft tissue such as tumours or arrhythmogenic heart tissue. Even though the mechanisms of cell death can influence the outcome of electroporation-based treatments due to use of different electric pulse parameters and conditions, these are not elucidated yet. We review the mechanisms of cell death after electroporation reported in literature, cell injuries that may lead to cell death after electroporation and membrane repair mechanisms involved. The knowledge of membrane repair and cell death mechanisms after cell exposure to electric pulses, targets of electric field in cells need to be identified to optimize existing and develop of new electroporation-based techniques used in medicine, biotechnology, and food technology.

Digital holographic microscopy evaluation of dynamic cell response to electroporation

Phase-derived parameters and time autocorrelation functions were used to analyze the behavior of murine B16 cells exposed to different amplitudes of electroporation pulses. Cells were observed using an off-axis digital holographic microscope equipped with a fast camera. Series of quantitative phase images of cells were reconstructed and further processed using MATLAB codes. Projected area, dry mass density, and entropy proved to be predictors for permeabilized cells that swell or collapse. Autocorrelation functions of phase fluctuations in different regions of the cell showed a good correlation with the local effectiveness of permeabilization.

Single Cell Forces after Electroporation

Exogenous high-voltage pulses increase cell membrane permeability through a phenomenon known as electroporation. This process may also disrupt the cell cytoskeleton causing changes in cell contractility; however, the contractile signature of cell force after electroporation remains unknown. Here, single-cell forces post-electroporation are measured using suspended extracellular matrix-mimicking nanofibers that act as force sensors. Ten, 100 μs pulses are delivered at three voltage magnitudes (500, 1000, and 1500 V) and two directions (parallel and perpendicular to cell orientation), exposing glioblastoma cells to electric fields between 441 V cm-1 and 1366 V cm-1. Cytoskeletal-driven force loss and recovery post-electroporation involves three distinct stages. Low electric field magnitudes do not cause disruption, but higher fields nearly eliminate contractility 2-10 min post-electroporation as cells round following calcium-mediated retraction (stage 1). Following rounding, a majority of analyzed cells enter an unusual and unexpected biphasic stage (stage 2) characterized by increased contractility tens of minutes post-electroporation, followed by force relaxation. The biphasic stage is concurrent with actin disruption-driven blebbing. Finally, cells elongate and regain their pre-electroporation morphology and contractility in 1-3 h (stage 3). With increasing voltages applied perpendicular to cell orientation, we observe a significant drop in cell viability. Experiments with multiple healthy and cancerous cell lines demonstrate that contractile force is a more dynamic and sensitive metric than cell shape to electroporation. A mechanobiological understanding of cell contractility post-electroporation will deepen our understanding of the mechanisms that drive recovery and may have implications for molecular medicine, genetic engineering, and cellular biophysics.

Changes in the packing of bilayer lipids triggered by electroporation: real-time measurements on cells in suspension

Electropermeabilization of the cell membrane is a technique used to facilitate penetration of impermeant molecules into cells. Although there are studies regarding the mechanism of processes occurring after electropermeabilization, the relationship between electropermeabilization and associated phenomena (e.g. generation of reactive oxygen species, endocytosis, lipid peroxidation, etc.) is yet to be elucidated. This work aimed to get information on the changes in the packing of the bilayer lipids and their peroxidation induced by application of electroporation pulses. We used a specially designed system of electrodes which allowed performing electropermeabilization of cells in suspension simultaneously with time-dependent measurements of fluorescence and temperature. The kinetics of membrane packing and production of reactive oxygen species were studied using various conductivity buffers (0.01, 0.04 and 0.14 S/m) and different number of 1 kV/cm bipolar pulses (1-50). Two categories of effects were observed: a thermal effect, consisting in an increased bilayer disorder (a deeper penetration of water into the hydrophobic core), and a nonthermal effect, leading to a higher degree of lipids packing, the latter being attributed to a peroxidation process. An analysis of the permeabilization conditions in which one of these two processes predominates was performed.

How Can a Punch Knock You Out?

Several hypotheses have been put forth over time to explain how consciousness can be so rapidly lost, and then spontaneously regained, following mechanical head trauma. The knockout punch in boxing is a relatively homogenous form of traumatic brain injury and can thus be used to test the predictions of these hypotheses. While none of the hypotheses put forth can be considered fully verified, pore formation following stretching of the axonal cell membrane, mechanoporation, is a strong contender. We here argue that the theoretical foundation of mechanoporation can be strengthened by a comparison with the experimental method electroporation.

Physical transfection technologies for macrophages and dendritic cells in immunotherapy

INTRODUCTION

Dendritic cells (DCs) and macrophages, two important antigen presenting cells (APCs) of the innate immune system, are being explored for the use in cell-based cancer immunotherapy. For this application, the therapeutic potential of patient-derived APCs is increased by delivering different types of functional macromolecules, such as mRNA and pDNA, into their cytosol. Compared to the use of viral and non-viral delivery vectors, physical intracellular delivery techniques are known to be more straightforward, more controllable, faster and generate high delivery efficiencies.

AREAS COVERED

This review starts with electroporation as the most traditional physical transfection method, before continuing with the more recent technologies such as sonoporation, nanowires and microfluidic cell squeezing. A description is provided of each of those intracellular delivery technologies with their strengths and weaknesses, especially paying attention to delivery efficiency and safety profile.

EXPERT OPINION

Given the common use of electroporation for the production of therapeutic APCs, it is recommended that more detailed studies are performed on the effect of electroporation on APC fitness, even down to the genetic level. Newer intracellular delivery technologies seem to have less impact on APC functionality but further work is needed to fully uncover their suitability to transfect APCs with different types of macromolecules.

Irreversible Electroporation for De-epithelialization of Murine Small Intestine

BACKGROUND

Nonthermal irreversible electroporation (NTIRE) has been shown to ablate the small intestinal epithelium while maintaining submucosal and muscularis propriae integrity. NTIRE is used here in a first-in-mouse study to eliminate the native intestinal stem cell population to understand optimal parameters and timeline of mucosal regeneration.

METHODS

Adult C57 background mice underwent laparotomy and electroporation of 1.5 cm of jejunum using a BTX 830 ECM electroporator and electrode calipers. Parameters were varied by voltage, pulse number, interval, and duration to determine optimal de-epithelialization. Electroporated segments were extracted 1 to 3 d after intervention with same-animal control segment. Cross sections were preserved, and measurements were taken to compare effects of parameters on villi height, crypt depth, crypt obliteration, and serosal thickness.

RESULTS

Morbidity was limited at a standard set of electroporation parameters (14%), and increased with higher voltage, longer interval, and shorter or longer pulses. Serosa/muscularis thickness was unaffected by varying interventions. Crypt depth and obliterated crypts were most affected by modulating pulse number, intervals, and duration. Villi height was most significantly shortened by altering pulse duration, with limited recovery by day 3, otherwise mucosal regeneration was observed in most cases by this point.

CONCLUSIONS

NTIRE is an effective method of denuding small intestinal epithelium in mice and temporarily ablating crypts while sparing the support scaffold for native regeneration. This first-in-mouse study of electroporation suggests it is a practical tool that can be utilized in a small mammalian system.

Pulsed electric field-assisted extraction of valuable compounds from microorganisms

Microorganisms (bacteria, yeast, and microalgae) are a promising resource for products of high value such as nutrients, pigments, and enzymes. The majority of these compounds of interest remain inside the cell, thus making it necessary to extract and purify them before use. This review presents the challenges and opportunities in the production of these compounds, the microbial structure and the location of target compounds in the cells, the different procedures proposed for improving extraction of these compounds, and pulsed electric field (PEF)-assisted extraction as alternative to these procedures. PEF is a nonthermal technology that produces a precise action on the cytoplasmic membrane improving the selective release of intracellular compounds while avoiding undesirable consequences of heating on the characteristics and purity of the extracts. PEF pretreatment with low energetic requirements allows for high extraction yields. However, PEF parameters should be tailored to each microbial cell, according to their structure, size, and other factors affecting efficiency. Furthermore, the recent discovery of the triggering effect of enzymatic activity during cell incubation after electroporation opens up the possibility of new implementations of PEF for the recovery of compounds that are bounded or assembled in structures. Similarly, PEF parameters and suspension storage conditions need to be optimized to reach the desired effect. PEF can be applied in continuous flow and is adaptable to industrial equipment, making it feasible for scale-up to large processing capacities.

Continuous nanosecond pulsed electric field treatments foster the upstream performance of Chlorella vulgaris-based biorefinery concepts

Nanosecond pulsed electric field treatment (nsPEF) is an innovative, technology-driven, and resource-efficient approach to foster the upstream performance of microalgae-based biorefinery concepts to transform microalgae into economic more viable raw materials for the biobased industry. A processing window applying three treatments of 100 ns, 5 Hz, and 10 kV cm^-1 to industrially relevant phototrophic Chlorella vulgaris in the early exponential growth phase significantly increased biomass yields by up to 17.53 ± 10.46% (p = 3.18 × 10^-5). Treatments had limited effects on the carbon and pigment contents, but the protein content was decreased. The longest possible pulse width (100 ns) resulted in the highest biomass yield indicating underlying working mechanisms of enhanced cell proliferation based on intracellular and plasma membrane-related effects. The applicability to eukaryotes and prokaryotes, such as C. vulgaris and cyanobacteria highlights the possible impacts of nsPEF across multiple domains of the biobased industry relying on single-cell-based value-chains.

Cellular Bioeffect Investigations on Low-Intensity Pulsed Ultrasound and Sonoporation: Platform Design and Flow Cytometry Protocol

At low-intensity levels, ultrasound can potentially generate therapeutic effects on living cells, and it can trigger sonoporation when microbubbles (MBs) are present to facilitate drug delivery. Yet, our foundational knowledge of low-intensity pulsed ultrasound (LIPUS) and sonoporation remains to be critically weak because the pertinent cellular bioeffects have not been rigorously studied. In this article, we present a population-based experimental protocol that can effectively foster investigations on the mechanistic bioeffects of LIPUS and sonoporation over a cell population. Walkthroughs of different methodological details are presented, including the fabrication of the ultrasound exposure platform and its calibration, as well as the design of a bioassay procedure that uses fluorescent tracers and flow cytometry to isolate sonicated cells with similar characteristics. An application example is also presented to illustrate how our protocol can be used to investigate the downstream cellular bioeffects of leukemia cells. We show that, with 1-MHz LIPUS exposure (with 29.1 J/cm² delivered acoustic energy density), variations in viability and morphology would be found among different types of sonicated leukemia cells (HL-60, Molt-4) in the absence and presence of MBs. Taken altogether, this article provides a reference on how cellular bioeffect experiments on LIPUS and sonoporation can be planned meticulously to acquire strong observations that are critical to establish the biological foundations for therapeutic applications.

Effect of Loading Method on a Peptide Substrate Reporter in Intact Cells

Studies of live cells often require loading of exogenous molecules through the cell membrane; however, effects of loading method on experimental results are poorly understood. Therefore, in this work, we compared three methods for loading a fluorescently labeled peptide into cells of the model organism Dictyostelium discoideum. We optimized loading by pinocytosis, electroporation, and myristoylation to maximize cell viability and characterized loading efficiency, localization, and uniformity. We also determined how the loading method affected measurements of enzyme activity on the peptide substrate reporter using capillary electrophoresis. Loading method had a strong effect on the stability and phosphorylation of the peptide. The half-life of the intact peptide in cells was 19 ± 2, 53 ± 15, and 12 ± 1 min, for pinocytosis, electroporation, and myristoylation, respectively. The peptide was phosphorylated only in cells loaded by electroporation. Fluorescence microscopy suggested that the differences between methods were likely due to differences in peptide localization.

Transport of charged small molecules after electropermeabilization - drift and diffusion

Background

Applications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment. A unified understanding of the underlying mechanisms of membrane electropermeabilization, however, has not been achieved. Protocols are empirical, and models are descriptive rather than predictive, which hampers the optimization and expansion of electroporation-based technologies. A common feature of existing models is the assumption that the permeabilized membrane is passive, and that transport through it is entirely diffusive. To demonstrate the necessity to go beyond that assumption, we present here a quantitative analysis of the post-permeabilization transport of three small molecules commonly used in electroporation research — YO-PRO-1, propidium, and calcein — after exposure of cells to minimally perturbing, 6 ns electric pulses.

Results

Influx of YO-PRO-1 from the external medium into the cell exceeds that of propidium, consistent with many published studies. Both are much greater than the influx of calcein. In contrast, the normalized molar efflux of calcein from pre-loaded cells into the medium after electropermeabilization is roughly equivalent to the influx of YO-PRO-1 and propidium. These relative transport rates are correlated not with molecular size or cross-section, but rather with molecular charge polarity.

Conclusions

This comparison of the kinetics of molecular transport of three small, charged molecules across electropermeabilized cell membranes reveals a component of the mechanism of electroporation that is customarily taken into account only for the time during electric pulse delivery. The large differences between the influx rates of propidium and YO-PRO-1 (cations) and calcein (anion), and between the influx and efflux of calcein, suggest a significant role for the post-pulse transmembrane potential in the migration of ions and charged small molecules across permeabilized cell membranes, which has been largely neglected in models of electroporation.

Electronic supplementary material

The online version of this article (10.1186/s13628-018-0044-2) contains supplementary material, which is available to authorized users.

Cell Electrosensitization Exists Only in Certain Electroporation Buffers

Electroporation-induced cell sensitization was described as the occurrence of a delayed hypersensitivity to electric pulses caused by pretreating cells with electric pulses. It was achieved by increasing the duration of the electroporation treatment at the same cumulative energy input. It could be exploited in electroporation-based treatments such as electrochemotherapy and tissue ablation with irreversible electroporation. The mechanisms responsible for cell sensitization, however, have not yet been identified. We investigated cell sensitization dynamics in five different electroporation buffers. We split a pulse train into two trains varying the delay between them and measured the propidium uptake by fluorescence microscopy. By fitting the first-order model to the experimental results, we determined the uptake due to each train (i.e. the first and the second) and the corresponding resealing constant. Cell sensitization was observed in the growth medium but not in other tested buffers. The effect of pulse repetition frequency, cell size change, cytoskeleton disruption and calcium influx do not adequately explain cell sensitization. Based on our results, we can conclude that cell sensitization is a sum of several processes and is buffer dependent. Further research is needed to determine its generality and to identify underlying mechanisms.

Dependence of Electroporation Detection Threshold on Cell Radius: An Explanation to Observations Non Compatible with Schwan's Equation Model

It is widely accepted that electroporation occurs when the cell transmembrane voltage induced by an external applied electric field reaches a threshold. Under this assumption, in order to trigger electroporation in a spherical cell, Schwan's equation leads to an inversely proportional relationship between the cell radius and the minimum magnitude of the applied electric field. And, indeed, several publications report experimental evidences of an inverse relationship between the cell size and the field required to achieve electroporation. However, this dependence is not always observed or is not as steep as predicted by Schwan's equation. The present numerical study attempts to explain these observations that do not fit Schwan's equation on the basis of the interplay between cell membrane conductivity, permeability, and transmembrane voltage. For that, a single cell in suspension was modeled and the electric field necessary to achieve electroporation with a single pulse was determined according to two effectiveness criteria: a specific permeabilization level, understood as the relative area occupied by the pores during the pulse, and a final intracellular concentration of a molecule due to uptake by diffusion after the pulse, during membrane resealing. The results indicate that plausible model parameters can lead to divergent dependencies of the electric field threshold on the cell radius. These divergent dependencies were obtained through both criteria and using two different permeabilization models. This suggests that the interplay between cell membrane conductivity, permeability, and transmembrane voltage might be the cause of results which are noncompatible with the Schwan's equation model.

Optimization of cerebellar purkinje neuron cultures and development of a plasmid-based method for purkinje neuron-specific, miRNA-mediated protein knockdown

We present a simple and efficient method to knock down proteins specifically in Purkinje neurons (PN) present in mixed mouse primary cerebellar cultures. This method utilizes the introduction via nucleofection of a plasmid encoding a specific miRNA downstream of the L7/Pcp2 promoter, which drives PN-specific expression. As proof-of-principle, we used this plasmid to knock down the motor protein myosin Va, which is required for the targeting of smooth endoplasmic reticulum (ER) into PN spines. Consistent with effective knockdown, transfected PNs robustly phenocopied PNs from dilute-lethal (myosin Va-null) mice with regard to the ER targeting defect. Importantly, our plasmid-based approach is less challenging technically and more specific to PNs than several alternative methods (e.g., biolistic- and lentiviral-based introduction of siRNAs). We also present a number of improvements for generating mixed cerebellar cultures that shorten the procedure and improve the total yield of PNs, and of transfected PNs, considerably. Finally, we present a method to rescue cerebellar cultures that develop large cell aggregates, a common problem that otherwise precludes the further use of the culture.

Electroporation induced by internal defibrillation shock with and without recovery in intact rabbit hearts

Defibrillation shocks from implantable cardioverter defibrillators can be lifesaving but can also damage cardiac tissues via electroporation. This study characterizes the spatial distribution and extent of defibrillation shock-induced electroporation with and without a 45-min postshock period for cell membranes to recover. Langendorff-perfused rabbit hearts (n = 31) with and without a chronic left ventricular (LV) myocardial infarction (MI) were studied. Mean defibrillation threshold (DFT) was determined to be 161.4 ± 17.1 V and 1.65 ± 0.44 J in MI hearts for internally delivered 8-ms monophasic truncated exponential (MTE) shocks during sustained ventricular fibrillation (>20 s, SVF). A single 300-V MTE shock (twice determined DFT voltage) was used to terminate SVF. Shock-induced electroporation was assessed by propidium iodide (PI) uptake. Ventricular PI staining was quantified by fluorescent imaging. Histological analysis was performed using Masson's Trichrome staining. Results showed PI staining concentrated near the shock electrode in all hearts. Without recovery, PI staining was similar between normal and MI groups around the shock electrode and over the whole ventricles. However, MI hearts had greater total PI uptake in anterior (P < 0.01) and posterior (P < 0.01) LV epicardial regions. Postrecovery, PI staining was reduced substantially, but residual staining remained significant with similar spacial distributions. PI staining under SVF was similar to previously studied paced hearts. In conclusion, electroporation was spatially correlated with the active region of the shock electrode. Additional electroporation occurred in the LV epicardium of MI hearts, in the infarct border zone. Recovery of membrane integrity postelectroporation is likely a prolonged process. Short periods of SVF did not affect electroporation injury.