Asymmetric Patterns of Small Molecule Transport After Nanosecond and Microsecond Electropermeabilization

Calcium electroporation induces stress response through upregulation of HSP27, HSP70, aspartate β-hydroxylase, and CD133 in human colon cancer cells

BACKGROUND

Electroporation (EP) leverages electric pulses to permeabilize cell membranes, enabling the delivery of therapeutic agents like calcium in cancer treatment. Calcium electroporation (CaEP) induces a rapid influx of calcium ions, disrupting cellular calcium homeostasis and triggering cell death pathways. This study aims to compare the cellular responses between microsecond (µsEP) and nanosecond (nsEP) electroporation, particularly in terms of oxidative stress, immune response activation, and cancer stem cell (CSC) viability in drug-resistant (LoVo Dx) and non-resistant (LoVo) colorectal cancer cell lines.

RESULTS

Both µsEP and nsEP, particularly when combined with Ca2+, significantly reduced the viability of cancer cells, with nsEP showing greater efficacy. Reactive oxygen species (ROS) levels increased 5-fold in malignant cells following nsEP, correlating with decreased ATP production and mitochondrial dysfunction. Nanosecond CaEP (nsCaEP) also induced significant expression of aspartate-β-hydroxylase (ASPH), a protein linked to calcium homeostasis and tumor progression. Moreover, nsEP led to heightened expression of heat shock proteins (HSP27/70), indicating potential immune activation. Interestingly, nsEP without calcium drastically reduced the expression of CD133, a marker for CSCs, while the addition of Ca2+ preserved CD133 expression. The expression of death effector domain-containing DNA binding protein (DEDD), associated with apoptosis, was significantly elevated in treated cancer cells, especially in the nucleus after nsCaEP.

CONCLUSIONS

The study confirms that nsEP is more effective than µsEP in disrupting cancer cell viability, enhancing oxidative stress, and triggering immune responses, likely through HSP overexpression and ROS generation. nsEP also appears to reduce CSC viability, offering a promising therapeutic approach. However, preserving CD133 expression in the presence of calcium suggests complex interactions that require further investigation. These findings highlight the potential of nsCaEP as an innovative strategy for targeting both cancer cells and CSCs, potentially improving treatment outcomes in colorectal cancer. Further studies are needed to explore the exact cell death mechanisms and optimize protocols for clinical applications.

Interleaflet Translocation of Second-Harmonic-Generation-Active Dye Molecules in Phospholipid Bilayers with Transmembrane Pores

Second harmonic generation (SHG) measurements using SHG-active dye molecules have recently attracted attention as a method to detect the formation of pores in phospholipid bilayers. The bilayers, in which the dye molecules are embedded in the outer leaflet, exhibit a noncentrosymmetric structure, generating SHG signals. However, when pores form, these dye molecules translocate through the pores into the inner leaflet, leading to a more centrosymmetric structure and the subsequent loss of the SHG signals. A decrease in the SHG signals has been experimentally observed in membranes subjected to electrical stimuli. However, the characteristics of the interleaflet translocation of SHG-active dye molecules through pores remain unclear, hindering quantitative estimation of the membrane conditions, such as the pore size and density, based on the SHG signal reduction. In this study, we investigated the interleaflet translocation characteristics of Ap3, an SHG-active dye molecule, using molecular dynamics (MD) simulations and two-dimensional random-walk (RW) simulations. The MD simulations revealed that Ap3 molecules only translocate between the leaflets along the pore sidewalls. We determined the lateral diffusion coefficient of Ap3 within the membrane plane and its propensity for interleaflet movement at the pore wall. Based on these movement characteristics, the RW model successfully reproduced the characteristic time scale of the interleaflet translocation observed in the MD simulations. By varying the pore size and density in the RW simulations, we estimated that the characteristic time scale of interleaflet translocation depends on the -0.31 power of the pore radius and the -1.13 power of the pore density. Using these findings, we estimated the number of pores that probably formed in membranes during previous electroporation experiments. These results indicate the potential of optical measurement of the dye molecule movement for the indirect quantitative estimation of the pore size and number, which are challenging to measure optically.

Pulsed Electric Field (PEF) Treatment Results in Growth Promotion, Main Flavonoids Extraction, and Phytochemical Profile Modulation of Scutellaria baicalensis Georgi Roots

This study aims to explore the effect of pulsed electric field (PEF) treatment as a method very likely to result in reversible electroporation of Scutellaria baicalensis Georgi underground organs, resulting in increased mass transfer and secondary metabolites leakage. PEF treatment with previously established empirically tailored parameters [E = 0.3 kV/cm (U = 3 kV, d = 10 cm), t = 50 µs, N = 33 f = 1 Hz] was applied 1-3 times to S. baicalensis roots submerged in four different Natural Deep Eutectic Solvents (NADES) media (1-choline chloride/xylose (1:2) + 30% water, 2-choline chloride/glucose (1:2) + 30% water, 3-choline chloride/ethylene glycol (1:2), and 4-tap water (EC = 0.7 mS/cm). Confocal microscopy was utilized to visualize the impact of PEF treatment on the root cells in situ. As a result of plant cell membrane permeabilization, an extract containing major active metabolites was successfully acquired in most media, achieving the best results using medium 1 and repeating the PEF treatment twice (baicalein Collapse

Key Words

• electroporation
• eutectic solvents
• flavonoids
• hydroponics

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MESH Headings

• Plant Roots/growth & development
• Plant Roots/chemistry
• Plant Roots/metabolism
• Scutellaria baicalensis/growth & development
• Scutellaria baicalensis/chemistry
• Scutellaria baicalensis/metabolism
• Flavonoids/isolation & purification
• Flavonoids/chemistry
• Phytochemicals/isolation & purification
• Phytochemicals/chemistry
• Plant Extracts/chemistry
• Plant Extracts/pharmacology
• Electricity
• Electroporation/methods

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Grants

• UMO-2020/39/D/NZ9/01402 National Science Centre Poland

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Affiliation(s)

Kajetan Grzelka Department of Pharmaceutical Biology and Biotechnology, Division of Pharmaceutical Biology and Botany, Wrocław Medical University, 50-367 Wrocław, Poland; (K.G.); (A.M.)
Adam Matkowski Department of Pharmaceutical Biology and Biotechnology, Division of Pharmaceutical Biology and Botany, Wrocław Medical University, 50-367 Wrocław, Poland; (K.G.); (A.M.) Botanical Garden of Medicinal Plants, Wrocław Medical University, 50-367 Wrocław, Poland; (J.J.); (A.P.-B.)
Grzegorz Chodaczek Bioimaging Laboratory at Łukasiewicz Research Network—PORT Polish Center for Technology Development, 54-066 Wrocław, Poland;
Joanna Jaśpińska Botanical Garden of Medicinal Plants, Wrocław Medical University, 50-367 Wrocław, Poland; (J.J.); (A.P.-B.)
Anna Pawlikowska-Bartosz Botanical Garden of Medicinal Plants, Wrocław Medical University, 50-367 Wrocław, Poland; (J.J.); (A.P.-B.)
Wojciech Słupski Department of Pharmacology, Wrocław Medical University, 50-367 Wrocław, Poland;
Dorota Lechniak Department of Genetics and Animal Breeding, Poznań University of Life Sciences, Wołyńska 33, 60-637 Poznań, Poland;
Małgorzata Szumacher-Strabel Department of Animal Nutrition, Poznań University of Life Sciences, Poznań, Wołyńska 33, 60-637 Poznań, Poland; (M.S.-S.); (S.O.)
Segun Olorunlowu Department of Animal Nutrition, Poznań University of Life Sciences, Poznań, Wołyńska 33, 60-637 Poznań, Poland; (M.S.-S.); (S.O.)
Karolina Szulc Department of Animal Breeding and Product Quality Assessment, Poznań University of Life Sciences, Zlotniki, ul. Słoneczna 1, 62-002 Suchy Las, Poland;
Adam Cieślak Department of Animal Nutrition, Poznań University of Life Sciences, Poznań, Wołyńska 33, 60-637 Poznań, Poland; (M.S.-S.); (S.O.)
Sylwester Ślusarczyk Department of Pharmaceutical Biology and Biotechnology, Division of Pharmaceutical Biology and Botany, Wrocław Medical University, 50-367 Wrocław, Poland; (K.G.); (A.M.)

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Collaborators Collapse

New insights into the mechanism of electrotransfer of small nucleic acids

RNA interference (RNAi) is a powerful and rapidly developing technology that enables precise silencing of genes of interest. However, the clinical development of RNAi is hampered by the limited cellular uptake and stability of the transferred molecules. Electroporation (EP) is an efficient and versatile technique for the transfer of both RNA and DNA. Although the mechanism of electrotransfer of small nucleic acids has been studied previously, too little is known about the potential effects of significantly larger pDNA on this process. Here we present a fundamental study of the mechanism of electrotransfer of oligonucleotides and siRNA that occur independently and simultaneously with pDNA by employing confocal fluorescence microscopy. In contrast to the conditional understanding of the mechanism, we have shown that the electrotransfer of oligonucleotides and siRNA is driven by both electrophoretic forces and diffusion after EP, followed by subsequent entry into the nucleus within 5 min after treatment. The study also revealed that the efficiency of siRNA electrotransfer decreases in response to an increase in pDNA concentration. Overall, the study provides new insights into the mechanism of electrotransfer of small nucleic acids which may have broader implications for the future application of RNAi-based strategies.

An experimental and theoretical study on cell swelling for osmotic imbalance induced by electroporation

The cellular membrane serves as a pivotal barrier in regulating intra- and extracellular matter exchange. Disruption of this barrier through pulsed electric fields (PEFs) induces the transmembrane transport of ions and molecules, creating a concentration gradient that subsequently results in the imbalance of cellular osmolality. In this study, a multiphysics model was developed to simulate the electromechanical response of cells exposed to microsecond pulsed electric fields (μsPEFs). Within the proposed model, the diffusion coefficient of the cellular membrane for various ions was adjusted based on electropore density. Cellular osmolality was governed and described using Van't Hoff theory, subsequently converted to loop stress to dynamically represent the cell swelling process. Validation of the model was conducted through a hypotonic experiment and simulation at 200 mOsm/kg, revealing a 14.2% increase in the cell's equivalent radius, thereby confirming the feasibility of the cell mechanical model. With the transmembrane transport of ions induced by the applied μsPEF, the hoop stress acting on the cellular membrane reached 179.95 Pa, and the cell equivalent radius increased by 11.0% when the extra-cellular medium was supplied with normal saline. The multiphysics model established in this study accurately predicts the dynamic changes in cell volume resulting from osmotic imbalance induced by PEF action. This model holds theoretical significance, offering valuable references for research on drug delivery and tumor microenvironment modulation.

Characterization of Experimentally Observed Complex Interplay between Pulse Duration, Electrical Field Strength, and Cell Orientation on Electroporation Outcome Using a Time-Dependent Nonlinear Numerical Model

Electroporation is a biophysical phenomenon involving an increase in cell membrane permeability to molecules after a high-pulsed electric field is applied to the tissue. Currently, electroporation is being developed for non-thermal ablation of cardiac tissue to treat arrhythmias. Cardiomyocytes have been shown to be more affected by electroporation when oriented with their long axis parallel to the applied electric field. However, recent studies demonstrate that the preferentially affected orientation depends on the pulse parameters. To gain better insight into the influence of cell orientation on electroporation with different pulse parameters, we developed a time-dependent nonlinear numerical model where we calculated the induced transmembrane voltage and pores creation in the membrane due to electroporation. The numerical results show that the onset of electroporation is observed at lower electric field strengths for cells oriented parallel to the electric field for pulse durations ≥10 µs, and cells oriented perpendicular for pulse durations ~100 ns. For pulses of ~1 µs duration, electroporation is not very sensitive to cell orientation. Interestingly, as the electric field strength increases beyond the onset of electroporation, perpendicular cells become more affected irrespective of pulse duration. The results obtained using the developed time-dependent nonlinear model are corroborated by in vitro experimental measurements. Our study will contribute to the process of further development and optimization of pulsed-field ablation and gene therapy in cardiac treatments.

Enhanced Drug Uptake on Application of Electroporation in a Single-Cell Model

Electroporation method is a useful tool for delivering drugs into various diseased tissues in the human body. As a result of an applied electric field, drug particles enter the intracellular compartment through the temporarily permeabilized cell membrane. Consequently, electroporation method allows better penetration of the drug into the diseased tissue and improves treatment clinically. In this study, a more generalized model of drug transport in a single cell is proposed. The model is able to capture non-homogeneous drug transport in the cell due to non-uniform cell membrane permeabilization. Several numerical experiments are conducted to understand the effects of electric field and drug permeability on drug uptake into the cell. Through investigation, the appropriate electric field and drug permeability are identified, which lead to sufficient drug uptake into the cell. This model can be used by experimentalists to get information prior to conduct any experiment, and it may help reduce the number of actual experiments that might be conducted otherwise.

Assessing membrane material properties from the response of giant unilamellar vesicles to electric fields

Knowledge of the material properties of membranes is crucial to understanding cell viability and physiology. A number of methods have been developed to probe membranes in vitro, utilizing the response of minimal biomimetic membrane models to an external perturbation. In this review, we focus on techniques employing giant unilamellar vesicles (GUVs), model membrane systems, often referred to as minimal artificial cells because of the potential they offer to mimick certain cellular features. When exposed to electric fields, GUV deformation, dynamic response and poration can be used to deduce properties such as bending rigidity, pore edge tension, membrane capacitance, surface shear viscosity, excess area and membrane stability. We present a succinct overview of these techniques, which require only simple instrumentation, available in many labs, as well as reasonably facile experimental implementation and analysis.

Recent advances in microfluidic-based electroporation techniques for cell membranes

Electroporation is a fundamental technique for applications in biotechnology. To date, the ongoing research on cell membrane electroporation has explored its mechanism, principles and potential applications. Therefore, in this review, we first discuss the primary electroporation mechanism to help establish a clear framework. Within the context of its principles, several critical terms are highlighted to present a better understanding of the theory of aqueous pores. Different degrees of electroporation can be used in different applications. Thus, we discuss the electric factors (shock strength, shock duration, and shock frequency) responsible for the degree of electroporation. In addition, finding an effective electroporation detection method is of great significance to optimize electroporation experiments. Accordingly, we summarize several primary electroporation detection methods in the following sections. Finally, given the development of micro- and nano-technology has greatly promoted the innovation of microfluidic-based electroporation devices, we also present the recent advances in microfluidic-based electroporation devices. Also, the challenges and outlook of the electroporation technique for cell membrane electroporation are presented.

Identification of electroporation sites in the complex lipid organization of the plasma membrane

The plasma membrane of a biological cell is a complex assembly of lipids and membrane proteins, which tightly regulate transmembrane transport. When a cell is exposed to strong electric field, the membrane integrity becomes transiently disrupted by formation of transmembrane pores. This phenomenon termed electroporation is already utilized in many rapidly developing applications in medicine including gene therapy, cancer treatment, and treatment of cardiac arrhythmias. However, the molecular mechanisms of electroporation are not yet sufficiently well understood; in particular, it is unclear where exactly pores form in the complex organization of the plasma membrane. In this study, we combine coarse-grained molecular dynamics simulations, machine learning methods, and Bayesian survival analysis to identify how formation of pores depends on the local lipid organization. We show that pores do not form homogeneously across the membrane, but colocalize with domains that have specific features, the most important being high density of polyunsaturated lipids. We further show that knowing the lipid organization is sufficient to reliably predict poration sites with machine learning. Additionally, by analysing poration kinetics with Bayesian survival analysis we show that poration does not depend solely on local lipid arrangement, but also on membrane mechanical properties and the polarity of the electric field. Finally, we discuss how the combination of atomistic and coarse-grained molecular dynamics simulations, machine learning methods, and Bayesian survival analysis can guide the design of future experiments and help us to develop an accurate description of plasma membrane electroporation on the whole-cell level. Achieving this will allow us to shift the optimization of electroporation applications from blind trial-and-error approaches to mechanistic-driven design.

Cytotoxicity of a Cell Culture Medium Treated with a High-Voltage Pulse Using Stainless Steel Electrodes and the Role of Iron Ions

High-voltage pulses applied to a cell suspension cause not only cell membrane permeabilization, but a variety of electrolysis reactions to also occur at the electrode–solution interfaces. Here, the cytotoxicity of a culture medium treated by a single electric pulse and the role of the iron ions in this cytotoxicity were studied in vitro. The experiments were carried out on mouse hepatoma MH-22A, rat glioma C6, and Chinese hamster ovary cells. The cell culture medium treated with a high-voltage pulse was highly cytotoxic. All cells died in the medium treated by a single electric pulse with a duration of 2 ms and an amplitude of just 0.2 kV/cm. The medium treated with a shorter pulse was less cytotoxic. The cell viability was inversely proportional to the amount of electric charge that flowed through the solution. The amount of iron ions released from the stainless steel anode (>0.5 mM) was enough to reduce cell viability. However, iron ions were not the sole reason of cell death. To kill all MH-22A and CHO cells, the concentration of Fe³⁺ ions in a medium of more than 2 mM was required.

Four Channel 6.5 kV, 65 A, 100 ns–100 µs Generator with Advanced Control of Pulse and Burst Protocols for Biomedical and Biotechnological Applications

Pulsed electric fields in the sub-microsecond range are being increasingly used in biomedical and biotechnology applications, where the demand for high-voltage and high-frequency pulse generators with enhanced performance and pulse flexibility is pushing the limits of pulse power solid state technology. In the scope of this article, a new pulsed generator, which includes four independent MOSFET based Marx modulators, operating individually or combined, controlled from a computer user interface, is described. The generator is capable of applying different pulse shapes, from unipolar to bipolar pulses into biological loads, in symmetric and asymmetric modes, with voltages up to 6.5 kV and currents up to 65 A, in pulse widths from 100 ns to 100 µs, including short-circuit protection, current and voltage monitoring. This new scientific tool can open new research possibility due to the flexibility it provides in pulse generation, particularly in adjusting pulse width, polarity, and amplitude from pulse-to-pulse. It also permits operating in burst mode up to 5 MHz in four independent channels, for example in the application of synchronized asymmetric bipolar pulses, which is shown together with other characteristics of the generator.

Pulse Duration Dependent Asymmetry in Molecular Transmembrane Transport Due to Electroporation in H9c2 Rat Cardiac Myoblast Cells In Vitro

Electroporation (EP) is one of the successful physical methods for intracellular drug delivery, which temporarily permeabilizes plasma membrane by exposing cells to electric pulses. Orientation of cells in electric field is important for electroporation and, consequently, for transport of molecules through permeabilized plasma membrane. Uptake of molecules after electroporation are the greatest at poles of cells facing electrodes and is often asymmetrical. However, asymmetry reported was inconsistent and inconclusive-in different reports it was either preferentially anodal or cathodal. We investigated the asymmetry of polar uptake of calcium ions after electroporation with electric pulses of different durations, as the orientation of elongated cells affects electroporation to a different extent when using electric pulses of different durations in the range of 100 ns to 100 µs. The results show that with 1, 10, and 100 µs pulses, the uptake of calcium ions is greater at the pole closer to the cathode than at the pole closer to the anode. With shorter 100 ns pulses, the asymmetry is not observed. A different extent of electroporation at different parts of elongated cells, such as muscle or cardiac cells, may have an impact on electroporation-based treatments such as drug delivery, pulse-field ablation, and gene electrotransfection.

Interference targeting of bipolar nanosecond electric pulses for spatially focused electroporation, electrostimulation, and tissue ablation

Stimulation and electroporation by nanosecond electric pulses (nsEP) are distinguished by a phenomenon of bipolar cancellation, which stands for a reduced efficiency of bipolar pulses compared to unipolar ones. When two pairs of stimulating electrodes are arrayed in a quadrupole, bipolar cancellation inhibits nsEP effects near the electrodes, where the electric field is the strongest. Two properly shaped and synchronized bipolar nsEP overlay into a unipolar pulse towards the center of the electrode array, thus canceling the bipolar cancellation (a "CANCAN effect"). High efficiency of the re-created unipolar nsEP outweighs the weakening of the electric field with distance and focuses nsEP effects to the center. In monolayers of CHO, BPAE, and HEK cells, CANCAN effect achieved by the interference of two bipolar nsEP enhanced electroporation up to tenfold, with a peak at the quadrupole center. Introducing a time interval between bipolar nsEP prevented the formation of a unipolar pulse and eliminated the CANCAN effect. Strong electroporation by CANCAN stimuli killed cells over the entire area encompassed by the electrodes, whereas the time-separated pulses caused ablation only in the strongest electric field near the electrodes. The CANCAN approach is promising for uniform tumor ablation and stimulation targeting away from electrodes.

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.

Single-cell electroporation with high-frequency nanosecond pulse bursts: Simulation considering the irreversible electroporation effect and experimental validation

To study the electroporation characteristics of cells under high-frequency nanosecond pulse bursts (HFnsPBs), the original electroporation mathematical model was improved. By setting a threshold value for irreversible electroporation (IRE) and considering the effect of an electric field on the surface tension of a cell membrane, a mathematical model of electroporation considering the effect of IRE is proposed for the first time. A typical two-dimensional cell system was discretized into nodes using MATLAB, and a mesh transport network method (MTNM) model was established for simulation. The dynamic processes of single-cell electroporation and molecular transport under the application of 50 unipolar HFnsPBs with field intensities of 9 kV cm^-1 and different frequencies (10 kHz, 100 kHz and 500 kHz) to the target system was simulated with a 300 s simulation time. The IRE characteristics and molecular transport were evaluated. In addition, a PI fluorescent dye assay was designed to verify the correctness of the model by providing time-domain and spatial results that were compared with the simulation results. The simulation achieved IRE and demonstrated the cumulative effects of multipulse bursts and intraburst frequency on irreversible pores. The model can also reflect the cumulative effect of multipulse bursts on reversible pores by introducing an assumption of stable reversible pores. The experimental results agreed qualitatively with the simulation results. A relative calibration of the fluorescence data gave time-domain molecular transport results that were quantitatively similar to the simulation results. This article reveals the cell electroporation characteristics under HFnsPBs from a mechanism perspective and has important guidance for fields involving the IRE of cells.

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.

Dye Transport through Bilayers Agrees with Lipid Electropore Molecular Dynamics

Although transport of molecules into cells via electroporation is a common biomedical procedure, its protocols are often based on trial and error. Despite a long history of theoretical effort, the underlying mechanisms of cell membrane electroporation are not sufficiently elucidated, in part, because of the number of independent fitting parameters needed to link theory to experiment. Here, we ask if the electroporation behavior of a reduced cell membrane is consistent with time-resolved, atomistic, molecular dynamics (MD) simulations of phospholipid bilayers responding to electric fields. To avoid solvent and tension effects, giant unilamellar vesicles (GUVs) were used, and transport kinetics were measured by the entry of the impermeant fluorescent dye calcein. Because the timescale of electrical pulses needed to restructure bilayers into pores is much shorter than the time resolution of current techniques for membrane transport kinetics measurements, the lifetimes of lipid bilayer electropores were measured using systematic variation of the initial MD simulation conditions, whereas GUV transport kinetics were detected in response to a nanosecond timescale variation in the applied electric pulse lifetimes and interpulse intervals. Molecular transport after GUV permeabilization induced by multiple pulses is additive for interpulse intervals as short as 50 ns but not 5-ns intervals, consistent with the 10-50-ns lifetimes of electropores in MD simulations. Although the results were mostly consistent between GUV and MD simulations, the kinetics of ultrashort, electric-field-induced permeabilization of GUVs were significantly different from published results in cells exposed to ultrashort (6 and 2 ns) electric fields, suggesting that cellular electroporation involves additional structures and processes.

From algal cells to autofluorescent ghost plasma membrane vesicles

Plasma membrane vesicles can be effective, non-toxic carriers for microscale material transport, provide a convenient model for probing membrane-related processes, since intracellular biochemical processes are eliminated. We describe here a fine-tuned protocol for isolating ghost plasma membrane vesicles from the unicellular alga Dunaliella tertiolecta, and preliminary characterization of their structural features and permeability properties, with comparisons to giant unilamellar phospholipid vesicles. The complexity of the algal ghost membrane vesicles reconstructed from the native membrane material released after hypoosmotic stress lies between that of phospholipid vesicles and cells. AFM structural characterization of reconstructed vesicles shows a thick envelope and a nearly empty vesicle interior. The surface of the envelope contains a heterogeneous distribution of densely packed, nanometer-scale globules and pore-like structures which may be derived from surface coat proteins. Confocal fluorescence imaging reveals the highly pigmented photosynthetic apparatus located within the thylakoid membrane and retained in the vesicle membrane. Transport of the fluorescent dye calcein into ghost and giant unilamellar vesicles reveals significant differences in permeability. Expanded knowledge of this unique membrane system will contribute to the design of marine bio-inspired carriers for advanced biotechnological applications.

Membrane Electroporation and Electropermeabilization: Mechanisms and Models

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

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.

Asymmetrical bipolar nanosecond electric pulse widths modify bipolar cancellation

A bipolar (BP) nanosecond electric pulse (nsEP) exposure generates reduced calcium influx compared to a unipolar (UP) nsEP. This attenuated physiological response from a BP nsEP exposure is termed "bipolar cancellation" (BPC). The predominant BP nsEP parameters that induce BPC consist of a positive polarity (↑) front pulse followed by the delivery of a negative polarity (↓) back pulse of equal voltage and width; thereby the duration is twice a UP nsEP exposure. We tested these BPC parameters, and discovered that a BP nsEP with symmetrical pulse widths is not required to generate BPC. For example, our data revealed the physiological response initiated by a ↑900 nsEP exposure can be cancelled by a second pulse that is a third of its duration.  However, we observed a complete loss of BPC from a ↑300 nsEP followed by a ↓900 nsEP exposure. Spatiotemporal analysis revealed these asymmetrical BP nsEP exposures generate distinct local YO-PRO®-1 uptake patterns across the plasma membrane. From these findings, we generated a conceptual model that suggests BPC is a phenomenon balanced by localized charging and discharging events across the membrane.