Gadolinium modifies the cell membrane to inhibit permeabilization by nanosecond electric pulses

Real-time imaging of individual electropores proves their longevity in cells

With over 50 years of electroporation research, the nature of cell membrane permeabilization remains elusive. The lifetime of electropores in molecular models is limited to nano- or microseconds, whereas the permeabilization of electroporated cells can last minutes. This study aimed at resolving a longstanding debate on whether the prolonged permeabilization is due to the formation of long-lived pores in cells. We developed a method for dynamic monitoring and conductance measurements of individual electropores. This was accomplished by time-lapse total internal reflection fluorescence (TIRF) imaging in HEK cells loaded with CAL-520 dye and placed on an indium tin oxide (ITO) surface. Applying a 1-ms, 0 to -400 mV pulse between the patch pipette and ITO evoked focal Ca2+ transients that identified individual electropores. Some transients disappeared in milliseconds but others persisted for over a minute. Persistent transients ("Ca2+ plumes") faded over time to a stable or a randomly fluctuating level that could include periods of full quiescence. Single pore conductance, measured by 0 to -50 mV, 50 ms steps at 30 and 60 s after the electroporation, ranged from 80 to 200 pS. These experiments proved electropore longevity in cells, in stark contrast to molecular simulations and many findings in lipid bilayers.

Silencing of ATP1A1 attenuates cell membrane disruption by nanosecond electric pulses

This study explored the role of the Na/K-ATPase (NKA) in membrane permeabilization induced by nanosecond electric pulses. Using CRISPR/Cas9 and shRNA, we silenced the ATP1A1 gene, which encodes α1 NKA subunit in U937 human monocytes. Silencing reduced the rate and the cumulative uptake of YoPro-1 dye after electroporation by 300-ns, 7-10 kV/cm pulses, while ouabain, a specific NKA inhibitor, enhanced YoPro-1 entry. We conclude that the α1 subunit supports the electropermeabilized membrane state, by forming or stabilizing electropores or by hindering repair mechanisms, and this role is independent of NKA's ion pump function.

Rapid and precise tracking of water influx and efflux across cell membranes induced by a pulsed electric field

Quantitative measurements of water content within a single cell are notoriously difficult. In this work, we introduce a single-shot optical method for tracking the intracellular water content, by mass and volume, of a single cell at video rate. We utilize quantitative phase imaging and a priori knowledge of a spherical cellular geometry, leveraging a two-component mixture model to compute the intracellular water content. We apply this technique to study CHO-K1 cells responding to a pulsed electric field, which induces membrane permeabilization and rapid water influx or efflux depending upon the osmotic environment. The effects of mercury and gadolinium on water uptake in Jurkat cells following electropermeabilization are also examined.

Next generation CANCAN focusing for remote stimulation by nanosecond electric pulses

Focusing electric pulse effects away from electrodes is a challenge because the electric field weakens with distance. Previously we introduced a remote focusing method based on bipolar cancellation, a phenomenon of low efficiency of bipolar nanosecond electric pulses (nsEP). Superpositioning two bipolar nsEP into a unipolar pulse canceled bipolar cancellation ("CANCAN" effect), enhancing bioeffects at a distance despite the electric field weakening. Here, we introduce the next generation (NG) CANCAN focusing with unipolar nsEP packets designed to produce bipolar waveforms near electrodes (suppressing electroporation) but not at the remote target. NG-CANCAN was tested in CHO cell monolayers using a quadrupole electrode array and labeling electroporated cells with YO-PRO-1 dye. We routinely achieved 1.5-2 times stronger electroporation in the center of the quadrupole than near electrodes, despite a 3-4-fold field attenuation. With the array lifted 1-2 mm above the monolayer (imitating a 3D treatment), the remote effect was enhanced up to 6-fold. We analyzed the role of nsEP number, amplitude, rotation, and inter-pulse delay, and showed how remote focusing is enhanced when re-created bipolar waveforms exhibit stronger cancellation. Advantages of NG-CANCAN include the exceptional versatility of designing pulse packets and easy remote focusing using an off-the-shelf 4-channel nsEP generator.

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.

Propidium uptake and ATP release in A549 cells share similar transport mechanisms

The lipid bilayer of eukaryotic cells' plasma membrane is almost impermeable to small ions and large polar molecules, but its miniscule basal permeability in intact cells is poorly characterized. This report describes the intrinsic membrane permeability of A549 cells toward the charged molecules propidium (Pr²⁺) and ATP^4-. Under isotonic conditions, we detected with quantitative fluorescence microscopy, a continuous low-rate uptake of Pr (∼150 × 10^-21 moles (zmol)/h/cell, [Pr]_o = 150 μM, 32°C). It was stimulated transiently but strongly by 66% hypotonic cell swelling reaching an influx amplitude of ∼1500 (zmol/h)/cell. The progressive Pr uptake with increasing [Pr]_o (30, 150, and 750 μM) suggested a permeation mechanism by simple diffusion. We quantified separately ATP release with custom wide-field-of-view chemiluminescence imaging. The strong proportionality between ATP efflux and Pr²⁺ influx during hypotonic challenge, and the absence of stimulation of transmembrane transport following 300% hypertonic shock, indicated that ATP and Pr travel the same conductive pathway. The fluorescence images revealed a homogeneously distributed intracellular uptake of Pr not consistent with high-conductance channels expressed at low density on the plasma membrane. We hypothesized that the pathway consists of transiently formed water pores evenly spread across the plasma membrane. The abolition of cell swelling-induced Pr uptake with 500 μM gadolinium, a known modulator of membrane fluidity, supported the involvement of water pores whose formation depends on the membrane fluidity. Our study suggests an alternative model of a direct permeation of ATP (and other molecules) through the phospholipid bilayer, which may have important physiological implications.

Electroporation and cell killing by milli- to nanosecond pulses and avoiding neuromuscular stimulation in cancer ablation

Ablation therapies aim at eradication of tumors with minimal impact on surrounding healthy tissues. Conventional pulsed electric field (PEF) treatments cause pain and muscle contractions far beyond the ablation area. The ongoing quest is to identify PEF parameters efficient at ablation but not at stimulation. We measured electroporation and cell killing thresholds for 150 ns–1 ms PEF, uni- and bipolar, delivered in 10- to 300-pulse trains at up to 1 MHz rates. Monolayers of murine colon carcinoma cells exposed to PEF were stained with YO-PRO-1 dye to detect electroporation. In 2–4 h, dead cells were labeled with propidium. Electroporation and cell death thresholds determined by matching the stained areas to the electric field intensity were compared to nerve excitation thresholds (Kim et al. in Int J Mol Sci 22(13):7051, 2021). The minimum fourfold ratio of cell killing and stimulation thresholds was achieved with bipolar nanosecond PEF (nsPEF), a sheer benefit over a 500-fold ratio for conventional 100-µs PEF. Increasing the bipolar nsPEF frequency up to 100 kHz within 10-pulse bursts increased ablation thresholds by < 20%. Restricting such bursts to the refractory period after nerve excitation will minimize the number of neuromuscular reactions while maintaining the ablation efficiency and avoiding heating.

Ca2+ dependence and kinetics of cell membrane repair after electropermeabilization

Electroporation, in particular with nanosecond pulses, is an efficient technique to generate nanometer-size membrane lesions without the use of toxins or other chemicals. The restoration of the membrane integrity takes minutes and is only partially dependent on [Ca²⁺]. We explored the impact of Ca²⁺ on the kinetics of membrane resealing by monitoring the entry of a YO-PRO-1 dye (YP) in BPAE and HEK cells. Ca²⁺ was promptly removed or added after the electric pulse (EP) by a fast-step perfusion. YP entry increased sharply after the EP and gradually slowed down following either a single- or a double-exponential function. In BPAE cells permeabilized by a single 300- or 600-ns EP at 14 kV/cm in a Ca²⁺-free medium, perfusion with 2 mM of external Ca²⁺ advanced the 90% resealing and reduced the dye uptake about twofold. Membrane restoration was accomplished by a combination of fast, Ca²⁺-independent resealing (τ = 13-15 s) and slow, Ca²⁺-dependent processes (τ ~70 s with Ca²⁺ and ~ 110 s or more without it). These time constants did not change when the membrane damage was doubled by increasing EP duration from 300 to 600 ns. However, injury by microsecond-range EP (300 and 600 μs) took longer to recover even when the membrane initially was less damaged, presumably because of the larger size of pores made in the membrane. Full membrane recovery was not prevented by blocking both extra- and intracellular Ca²⁺ (by loading cells with BAPTA or after Ca²⁺ depletion from the reticulum), suggesting the recruitment of unknown Ca²⁺-independent repair mechanisms.

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.

The inhibition of gadolinium ion (Gd3+) on the mitochondrial F1FO-ATPase is linked to the modulation of the mitochondrial permeability transition pore

The mitochondrial permeability transition pore (PTP), which drives regulated cell death when Ca²⁺ concentration suddenly increases in mitochondria, was related to changes in the Ca²⁺-activated F₁F_O-ATPase. The effects of the gadolinium cation (Gd³⁺), widely used for diagnosis and therapy, and reported as PTP blocker, were evaluated on the F₁F_O-ATPase activated by Mg²⁺ or Ca²⁺ and on the PTP. Gd³⁺ more effectively inhibits the Ca²⁺-activated F₁F_O-ATPase than the Mg²⁺-activated F₁F_O-ATPase by a mixed-type inhibition on the former and by uncompetitive mechanism on the latter. Most likely Gd³⁺ binding to F₁, is favoured by Ca²⁺ insertion. The maximal inactivation rates (k_inact) of pseudo-first order inactivation are similar either when the F₁F_O-ATPase is activated by Ca²⁺ or by Mg²⁺. The half-maximal inactivator concentrations (K_I) are 2.35 ± 0.35 mM and 0.72 ± 0.11 mM, respectively. The potency of a mechanism-based inhibitor (k_inact/K_I) also highlights a higher inhibition efficiency of Gd³⁺ on the Ca²⁺-activated F₁F_O-ATPase (0.59 ± 0.09 mM^-1∙s^-1) than on the Mg²⁺-activated F₁F_O-ATPase (0.13 ± 0.02 mM^-1∙s^-1). Consistently, the PTP is desensitized in presence of Gd³⁺. The Gd³⁺ inhibition on both the mitochondrial Ca²⁺-activated F₁F_O-ATPase and the PTP strengthens the link between the PTP and the F₁F_O-ATPase when activated by Ca²⁺ and provides insights on the biological effects of Gd³⁺.

Benefits and Detriments of Gadolinium from Medical Advances to Health and Ecological Risks

Gadolinium (Gd)-containing chelates have been established as diagnostics tools. However, extensive use in magnetic resonance imaging has led to increased Gd levels in industrialized parts of the world, adding to natural occurrence and causing environmental and health concerns. A vast amount of data shows that metal may accumulate in the human body and its deposition has been detected in organs such as brain and liver. Moreover, the disease nephrogenic systemic fibrosis has been linked to increased Gd³⁺ levels. Investigation of Gd³⁺ effects at the cellular and molecular levels mostly revolves around calcium-dependent proteins, since Gd³⁺ competes with calcium due to their similar size; other reports focus on interaction of Gd³⁺ with nucleic acids and carbohydrates. However, little is known about Gd³⁺ effects on membranes; yet some results suggest that Gd³⁺ interacts strongly with biologically-relevant lipids (e.g., brain membrane constituents) and causes serious structural changes including enhanced membrane rigidity and propensity for lipid fusion and aggregation at much lower concentrations than other ions, both toxic and essential. This review surveys the impact of the anthropogenic use of Gd emphasizing health risks and discussing debilitating effects of Gd³⁺ on cell membrane organization that may lead to deleterious health consequences.

Nanosecond pulsed electric fields modulate the expression of the astaxanthin biosynthesis genes psy, crtR-b and bkt 1 in Haematococcus pluvialis

Nanosecond pulsed electric fields (nsPEFs) have been extensively studied with respect to cellular responses. Whether nsPEFs can regulate gene expression and to modulate the synthesis of valuable compounds, has so far been only tested in the context of apoptosis in cancer cells. We used the unicellular algae Haematococcus pluvialis as system to test, whether nsPEFs could alter gene expression and to promote the biosynthesis of astaxanthin. We find that nsPEFs induce a mild, but significant increase of mortality up to about 20%, accompanied by a moderate increase of astaxanthin accumulation. Steady-state transcript levels of three key genes psy, crtR-b and bkt 1 were seen to increase with a maximum at 3 d after PEF treatment at 50 ns. Pulsing at 25 ns reduce the transcripts of psy, crtR-b from around day 2 after the pulse, while those of bkt 1 remain unchanged. By blocking the membrane-located NADPH oxidase RboH, diphenylene iodonium by itself increased both, the levels of astaxanthin and transcripts of all three biosynthetic genes, and this increase was added up to that produced by nsPEFs. Artificial calcium influx by an ionophore did not induce major changes in the accumulation of astaxanthin, nor in the transcript levels, but amplified the response of crtR-b to nsPEFs at 25 ns, while decreased in 50 ns treatment. When Ca²⁺ influx was inhibited by GdCl₃, the transcript of psy and bkt 1 were decreased for both 25 ns and 50 ns treatments, while crtR-b exhibited an obvious increase for the 25 ns treatment. We interpret these data in a working model, where nsPEFs permeabilise plasma and chloroplast membrane depending on pulse duration leading to a differential release of plastid retrograde signaling to the nucleus.

Probing Nanoelectroporation and Resealing of the Cell Membrane by the Entry of Ca2+ and Ba2+ Ions

The principal bioeffect of the nanosecond pulsed electric field (nsPEF) is a lasting cell membrane permeabilization, which is often attributed to the formation of nanometer-sized pores. Such pores may be too small for detection by the uptake of fluorescent dyes. We tested if Ca²⁺, Cd²⁺, Zn²⁺, and Ba²⁺ ions can be used as nanoporation markers. Time-lapse imaging was performed in CHO, BPAE, and HEK cells loaded with Fluo-4, Calbryte, or Fluo-8 dyes. Ca²⁺ and Ba²⁺ did not change fluorescence in intact cells, whereas their entry after nsPEF increased fluorescence within <1 ms. The threshold for one 300-ns pulse was at 1.5–2 kV/cm, much lower than >7 kV/cm for the formation of larger pores that admitted YO-PRO-1, TO-PRO-3, or propidium dye into the cells. Ba²⁺ entry caused a gradual emission rise, which reached a stable level in 2 min or, with more intense nsPEF, kept rising steadily for at least 30 min. Ca²⁺ entry could elicit calcium-induced calcium release (CICR) followed by Ca²⁺ removal from the cytosol, which markedly affected the time course, polarity, amplitude, and the dose-dependence of fluorescence change. Both Ca²⁺ and Ba²⁺ proved as sensitive nanoporation markers, with Ba²⁺ being more reliable for monitoring membrane damage and resealing.

Orally administered gadolinium orthovanadate GdVO4:Eu3+ nanoparticles do not affect the hydrophobic region of cell membranes of leukocytes

AIM

To assess the phospholipid bilayer of white blood cells (WBCs) and the ability of leukocytes to generate reactive oxygen species (ROS) in rats orally exposed to GdVO₄:Eu³⁺ nanoparticle (VNP) solution for 2 weeks by fluorescent probes-ortho-hydroxy derivatives of 2,5-diaryl‑1,3‑oxazole.

METHODS

Steady-state fluorescence spectroscopy, i.e., a study by the environment-sensitive fluorescent probes 2‑(2'-OH-phenyl)-5-(4'-phenyl-phenyl)-1,3-oxazole (probe O6O) and 2‑(2'-OH-phenyl)-phenanthro[9,10]-1,3-oxazole (probe PH7), and flow cytometry, i.e., analysis of 2',7'-dichlorofluorescein (DCF), a product of a dye 2',7'-dichlorodihydrofluorescein diacetate (H₂DCFDA), fluorescence in CD45⁺/7-aminoactinomycin D (7-AAD)^- cells, were used to evaluate the state of cell membranes and reactive oxygen species (ROS) generation in leukocytes of rats orally exposed to gadolinium orthovanadate nanoparticles(VNPs).

RESULTS

No significant changes were detected in the spectra of the fluorescent probes bound to the WBCs from the rats orally exposed to nanoparticles in comparison with the corresponding spectra of the probes bound to the cells from the control group of animals. This indicates that in the case of the rats orally exposed to nanoparticles, no noticeable changes in physicochemical properties (i.e., in the polarity and the proton-donor ability) are observed in the lipid membranes of WBCs in the region where the probes locate. There was no statistically significant difference in the amount of ROS^high viable leukocytes in rats treated with VNPs and control samples.

CONCLUSION

Neither changes in the physical and chemical properties of the leukocyte membranes nor in ROS generation by WBCs are detected in the rats orally exposed to VNP solution for 2 weeks.

Cancellation of nerve excitation by the reversal of nanosecond stimulus polarity and its relevance to the gating time of sodium channels

The initiation of action potentials (APs) by membrane depolarization occurs after a brief vulnerability period, during which excitation can be abolished by the reversal of the stimulus polarity. This vulnerability period is determined by the time needed for gating of voltage-gated sodium channels (VGSC). We compared nerve excitation by ultra-short uni- and bipolar stimuli to define the time frame of bipolar cancellation and of AP initiation. Propagating APs in isolated frog sciatic nerve were elicited by cathodic pulses (200 ns-300 µs), followed by an anodic (canceling) pulse of the same duration after a 0-200-µs delay. We found that the earliest and the latest boundaries for opening the critical number of VGSC needed to initiate AP are, respectively, between 11 and 20 µs and between 100 and 200 µs after the onset of depolarization. Stronger depolarization accelerated AP initiation, apparently due to faster VGSC opening, but not beyond the 11-µs limit. Bipolar cancellation was augmented by reducing pulse duration, shortening the delay between pulses, decreasing the amplitude of the cathodic pulse, and increasing the amplitude of the anodic one. Some of these characteristics contrasted the bipolar cancellation of cell membrane electroporation (Pakhomov et al. in Bioelectrochemistry 122:123-133, 2018; Gianulis et al. in Bioelectrochemistry 119:10-19, 2017), suggesting different mechanisms. The ratio of nerve excitation thresholds for a unipolar cathodic pulse and a symmetrical bipolar pulse increased as a power function as the pulse duration decreased, in remarkable agreement with the predictions of SENN model of nerve excitation (Reilly and Diamant in Health Phys 83(3):356-365, 2002).

Receptor- and store-operated mechanisms of calcium entry during the nanosecond electric pulse-induced cellular response

Nanosecond electric pulses have been shown to open nanopores in the cell plasma membrane by fluorescent imaging of calcium uptake and fluorescent dyes, including propidium (Pr) iodide and YO-PRO-1 (YP₁). Recently, we demonstrated that nsEPs also induce the phosphoinositide intracellular signaling cascade by phosphatidylinositol-4,5-bisphosphate (PIP₂) depletion resulting in physiological responses similar to those observed following stimulation of G_q11-coupled receptors. In this paper, we explore the role of receptor- and store-operated calcium entry (ROCE/SOCE) mechanisms in the observed response of cells to nsEP. We show that addition of the ROCE/SOCE and transient receptor potential channel (TRPC) blocker gadolinium (Gd³⁺, 300 μM) slows PIP2 depletion following 1 and 20 nsEP exposures. Lipid rafts, regions of the plasma membrane rich in PIP₂ and TRPC, are also disrupted by nsEP exposure suggesting that ROCE/SOCE mechanisms are likely impacted. Reducing the expression of stromal interaction molecule 1 (STIM1) protein, a key protein in ROCE and SOCE, in cells exposure to nsEP resulted in a reduction in induced intracellular calcium rise. Additionally, after exposure to 1 and 20 nsEPs (16.2 kV/cm, 5 Hz), intracellular calcium rises were significantly reduced by the addition of GD³⁺ and SKF-96365 (1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl) propoxy] ethyl-1H-imidazole hydrochloride, 100 μM), a blocker of STIM1 interaction. However, using similar nsEP exposure parameters, SKF-96365 was less effective at reducing YP₁ uptake compared to Gd³⁺. Thus, it is possible that SKF-96365 could block STIM1 interactions within the cell, while Gd³⁺ could acts on TRPC/nanopores from outside of the cell. Our results present evidence of nsEP induces ROCE and SOCE mechanisms and demonstrate that YP₁ and Ca²⁺ cannot be used solely as markers of nsEP-induced nanoporation.

Expression of voltage-gated calcium channels augments cell susceptibility to membrane disruption by nanosecond pulsed electric field

We compared membrane permeabilization by nanosecond pulsed electric field (nsPEF) in HEK293 cells with and without assembled CaV1.3 L-type voltage-gated calcium channel (VGCC). Individual cells were subjected to one 300-ns pulse at 0 (sham exposure); 1.4; 1.8; or 2.3 kV/cm, and membrane permeabilization was evaluated by measuring whole-cell currents and by optical monitoring of cytosolic Ca²⁺. nsPEF had either no effect (0 and 1.4 kV/cm), or caused a lasting (>80 s) increase in the membrane conductance in about 50% of cells (1.8 kV/cm), or in all cells (2.3 kV/cm). The conductance pathway opened by nsPEF showed strong inward rectification, with maximum conductance increase for the inward current at the most negative membrane potentials. Although these potentials were below the depolarization threshold for VGCC activation, the increase in conductance in cells which expressed VGCC (VGCC+ cells) was about twofold greater than in cells which did not (VGCC- cells). Among VGCC+ cells, the nsPEF-induced increase in membrane conductance showed a positive correlation with the amplitude of VGCC current measured in the same cells prior to nsPEF exposure. These findings demonstrate that the expression of VGCC makes cells more susceptible to membrane permeabilization by nsPEF. Time-lapse imaging of nsPEF-induced Ca²⁺ transients confirmed permeabilization by a single 300-ns pulse at 1.8 or 2.3 kV/cm, but not at 1.4 kV/cm, and the transients were expectedly larger in VGCC+ cells. However, it remains to be established whether larger transients reflected additional Ca²⁺ entry through VGCC, or were a result of more severe electropermeabilization of VGCC+ cells.

The second phase of bipolar, nanosecond-range electric pulses determines the electroporation efficiency

Bipolar cancellation refers to a phenomenon when applying a second electric pulse reduces ("cancels") cell membrane damage by a preceding electric pulse of the opposite polarity. Bipolar cancellation is a reason why bipolar nanosecond electric pulses (nsEP) cause weaker electroporation than just a single unipolar phase of the same pulse. This study was undertaken to explore the dependence of bipolar cancellation on nsEP parameters, with emphasis on the amplitude ratio of two opposite polarity phases of a bipolar pulse. Individual cells (CHO, U937, or adult mouse ventricular cardiomyocytes (VCM)) were exposed to either uni- or bipolar trapezoidal nsEP, or to nanosecond electric field oscillations (NEFO). The membrane injury was evaluated by time-lapse confocal imaging of the uptake of propidium (Pr) or YO-PRO-1 (YP) dyes and by phosphatidylserine (PS) externalization. Within studied limits, bipolar cancellation showed little or no dependence on the electric field intensity, pulse repetition rate, chosen endpoint, or cell type. However, cancellation could increase for larger pulse numbers and/or for longer pulses. The sole most critical parameter which determines bipolar cancellation was the phase ratio: maximum cancellation was observed with the 2nd phase of about 50% of the first one, whereas a larger 2nd phase could add a damaging effect of its own. "Swapping" the two phases, i.e., delivering the smaller phase before the larger one, reduced or eliminated cancellation. These findings are discussed in the context of hypothetical mechanisms of bipolar cancellation and electroporation by nsEP.

Electropermeabilization of cells by closely spaced paired nanosecond-range pulses

Decreasing the time gap between two identical electric pulses is expected to render bioeffects similar to those of a single pulse of equivalent total duration. In this study, we show that it is not necessarily true, and that the effects vary for different permeabilization markers. We exposed individual CHO or NG108 cells to one 300-ns pulse (3.7-11.6 kV/cm), or a pair of such pulses (0.4-1000 μs interval), or to a single 600-ns pulse of the same amplitude. Electropermeabilization was evaluated (a) by the uptake of YO-PRO-1 (YP) dye; (b) by the amplitude of elicited Ca²⁺ transients, and (c) by the entry of Tl⁺ ions. For YP uptake, applying a 600-ns pulse or a pair of 300-ns pulses doubled the effect of a single 300-ns pulse; this additive effect did not depend on the time interval between pulses or the electric field, indicating that already permeabilized cells are as susceptible to electropermeabilization as naïve cells. In contrast, Ca²⁺ transients and Tl⁺ uptake increased in a supra-additive fashion when two pulses were delivered instead of one. Paired pulses at 3.7 kV/cm with minimal separation (0.4 and 1 μs) elicited 50-100% larger Ca²⁺ transients than either a single 600-ns pulse or paired pulses with longer separation (10-1000 μs). This paradoxically high efficiency of the closest spaced pulses was emphasized when Ca²⁺ transients were elicited in a Ca²⁺-free solution (when the endoplasmic reticulum (ER) was the sole significant source of Ca²⁺), but was eliminated by Ca²⁺ depletion from the ER and was not observed for Tl⁺ entry through the electropermeabilized membrane. We conclude that closely spaced paired pulses specifically target ER, by either permeabilizing it to a greater extent than a single double-duration pulse thus causing more Ca²⁺ leak, or by amplifying Ca²⁺-induced Ca²⁺ release by an unknown mechanism.

Delayed hypersensitivity to nanosecond pulsed electric field in electroporated cells

We demonstrate that conditioning of mammalian cells by electroporation with nanosecond pulsed electric field (nsPEF) facilitates their response to the next nsPEF treatment. The experiments were designed to unambiguously separate the electroporation-induced sensitization and desensitization effects. Electroporation was achieved by bursts of 300-ns, 9 kV/cm pulses (50 Hz, n = 3-100) and quantified by propidium dye uptake within 11 min after the nsPEF exposure. We observed either sensitization to nsPEF or no change (when the conditioning was either too weak or too intense, or when the wait time after conditioning was too short). Within studied limits, conditioning never caused desensitization. With settings optimal for sensitization, the second nsPEF treatment became 2.5 times (25 °C) or even 6 times (37 °C) more effective than the same nsPEF treatment delivered without conditioning. The minimum wait time required for sensitization development was 30 s, with still longer delays increasing the effect. We show that the delayed hypersensitivity was not mediated by either cell swelling or oxidative effect of the conditioning treatment; biological mechanisms underlying the delayed electrosensitization remain to be elucidated. Optimizing nsPEF delivery protocols to induce sensitization can reduce the dose and adverse side effects of diverse medical treatments which require multiple pulse applications.

Electropermeabilization by uni- or bipolar nanosecond electric pulses: The impact of extracellular conductivity

Cellular effects caused by nanosecond electric pulses (nsEP) can be reduced by an electric field reversal, a phenomenon known as bipolar cancellation. The reason for this cancellation effect remains unknown. We hypothesized that assisted membrane discharge is the mechanism for bipolar cancellation. CHO-K1 cells bathed in high (16.1mS/cm; HCS) or low (1.8mS/cm; LCS) conductivity solutions were exposed to either one unipolar (300-ns) or two opposite polarity (300+300-ns; bipolar) nsEP (4-40kV/cm) with increasing interpulse intervals (0.1-50μs). Time-lapse YO-PRO-1 (YP) uptake revealed enhanced membrane permeabilization in LCS compared to HCS at all tested voltages. The time-dependence of bipolar cancellation was similar in both solutions, using either identical (22kV/cm) or isoeffective nsEP treatments (12 and 32kV/cm for LCS and HCS, respectively). However, cancellation was significantly stronger in LCS when the bipolar nsEP had no, or very short (<1μs), interpulse intervals. Finally, bipolar cancellation was still present with interpulse intervals as long as 50μs, beyond the time expected for membrane discharge. Our findings do not support assisted membrane discharge as the mechanism for bipolar cancellation. Instead they exemplify the sustained action of nsEP that can be reversed long after the initial stimulus.

Neuronal excitation and permeabilization by 200-ns pulsed electric field: An optical membrane potential study with FluoVolt dye

Electric field pulses of nano- and picosecond duration are a novel modality for neurostimulation, activation of Ca²⁺ signaling, and tissue ablation. However it is not known how such brief pulses activate voltage-gated ion channels. We studied excitation and electroporation of hippocampal neurons by 200-ns pulsed electric field (nsPEF), by means of time-lapse imaging of the optical membrane potential (OMP) with FluoVolt dye. Electroporation abruptly shifted OMP to a more depolarized level, which was reached within <1ms. The OMP recovery started rapidly (τ=8-12ms) but gradually slowed down (to τ>10s), so cells remained above the resting OMP level for at least 20-30s. Activation of voltage-gated sodium channels (VGSC) enhanced the depolarizing effect of electroporation, resulting in an additional tetrodotoxin-sensitive OMP peak in 4-5ms after nsPEF. Omitting Ca²⁺ in the extracellular solution did not reduce the depolarization, suggesting no contribution of voltage-gated calcium channels (VGCC). In 40% of neurons, nsPEF triggered a single action potential (AP), with the median threshold of 3kV/cm (range: 1.9-4kV/cm); no APs could be evoked by stimuli below the electroporation threshold (1.5-1.9kV/cm). VGSC opening could already be detected in 0.5ms after nsPEF, which is too fast to be mediated by the depolarizing effect of electroporation. The overlap of electroporation and AP thresholds does not necessarily reflect the causal relation, but suggests a low potency of nsPEF, as compared to conventional electrostimulation, for VGSC activation and AP induction.

Selective susceptibility to nanosecond pulsed electric field (nsPEF) across different human cell types

Tumor ablation by nanosecond pulsed electric fields (nsPEF) is an emerging therapeutic modality. We compared nsPEF cytotoxicity for human cell lines of cancerous (IMR-32, Hep G2, HT-1080, and HPAF-II) and non-cancerous origin (BJ and MRC-5) under strictly controlled and identical conditions. Adherent cells were uniformly treated by 300-ns PEF (0-2000 pulses, 1.8 kV/cm, 50 Hz) on indium tin oxide-covered glass coverslips, using the same media and serum. Cell survival plotted against the number of pulses displayed three distinct regions (initial resistivity, logarithmic survival decline, and residual resistivity) for all tested cell types, but with differences in LD₅₀ spanning as much as nearly 80-fold. The non-cancerous cells were less sensitive than IMR-32 neuroblastoma cells but more vulnerable than the other cancers tested. The cytotoxic efficiency showed no apparent correlation with cell or nuclear size, cell morphology, metabolism level, or the extent of membrane disruption by nsPEF. Increasing pulse duration to 9 µs (0.75 kV/cm, 5 Hz) produced a different selectivity pattern, suggesting that manipulation of PEF parameters can, at least for certain cancers, overcome their resistance to nsPEF ablation. Identifying mechanisms and cell markers of differential nsPEF susceptibility will critically contribute to the proper choice and outcome of nsPEF ablation therapies.

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.

Electroporation by subnanosecond pulses

Electropermeabilization of cell membranes by micro- and nanosecond-duration stimuli has been studied extensively, whereas effects of picosecond electric pulses (psEP) remain essentially unexplored. We utilized whole-cell patch clamp and Di-8-ANEPPS voltage-sensitive dye measurements to characterize plasma membrane effects of 500 ps stimuli in rat hippocampal neurons (RHN), NG108, and CHO cells. Even a single 500-ps pulse at 190 kV/cm increased membrane conductance and depolarized cells. These effects were augmented by applying brief psEP bursts (5-125 pulses), whereas the rate of pulse delivery (8Hz - 1 kHz) played little role. psEP-treated cells displayed large inward current at negative membrane potentials but modest or no conductance changes at positive potentials. A 1-kHz burst of 25 pulses increased the whole-cell conductance in the range (-100) - (-60) mV to 22-26 nS in RHN and NG108 cells (from 3 and 0.7 nS, respectively), but only to 5 nS in CHO (from 0.3 nS). The conductance increase was reversible within about 2 min. Such pattern of cell permeabilization, with characteristic inward rectification and slow recovery, was similar to earlier reported effects of 60- and 600-ns pulses, pointing to the similarity of structural membrane rearrangements in spite of a different membrane charging mechanism.

Electrosensitization assists cell ablation by nanosecond pulsed electric field in 3D cultures

Previous studies reported a delayed increase of sensitivity to electroporation (termed “electrosensitization”) in mammalian cells that had been subjected to electroporation. Electrosensitization facilitated membrane permeabilization and reduced survival in cell suspensions when the electric pulse treatments were split in fractions. The present study was aimed to visualize the effect of sensitization and establish its utility for cell ablation. We used KLN 205 squamous carcinoma cells embedded in an agarose gel and cell spheroids in Matrigel. A local ablation was created by a train of 200 to 600 of 300-ns pulses (50 Hz, 300–600 V) delivered by a two-needle probe with 1-mm inter-electrode distance. In order to facilitate ablation by engaging electrosensitization, the train was split in two identical fractions applied with a 2- to 480-s interval. At 400–600 V (2.9–4.3 kV/cm), the split-dose treatments increased the ablation volume and cell death up to 2–3-fold compared to single-train treatments. Under the conditions tested, the maximum enhancement of ablation was achieved when two fractions were separated by 100 s. The results suggest that engaging electrosensitization may assist in vivo cancer ablation by reducing the voltage or number of pulses required, or by enabling larger inter-electrode distances without losing the ablation efficiency.

Electroporation of mammalian cells by nanosecond electric field oscillations and its inhibition by the electric field reversal

The present study compared electroporation efficiency of bipolar and unipolar nanosecond electric field oscillations (NEFO). Bipolar NEFO was a damped sine wave with 140 ns first phase duration at 50% height; the peak amplitude of phases 2–4 decreased to 35%, 12%, and 7% of the first phase. This waveform was rectified to produce unipolar NEFO by cutting off phases 2 and 4. Membrane permeabilization was quantified in CHO and GH3 cells by uptake of a membrane integrity marker dye YO-PRO-1 (YP) and by the membrane conductance increase measured by patch clamp. For treatments with 1–20 unipolar NEFO, at 9.6–24 kV/cm, 10 Hz, the rate and amount of YP uptake were consistently 2-3-fold higher than after bipolar NEFO treatments, despite delivering less energy. However, the threshold amplitude was about 7 kV/cm for both NEFO waveforms. A single 14.4 kV/cm unipolar NEFO caused a 1.5–2 times greater increase in membrane conductance (p < 0.05) than bipolar NEFO, along with a longer and less frequent recovery. The lower efficiency of bipolar NEFO was preserved in Ca²⁺-free conditions and thus cannot be explained by the reversal of electrophoretic flows of Ca²⁺. Instead, the data indicate that the electric field polarity reversals reduced the pore yield.

Diffuse, non-polar electropermeabilization and reduced propidium uptake distinguish the effect of nanosecond electric pulses

Ca2+ activation and membrane electroporation by 10-ns and 4-ms electric pulses (nsEP and msEP) were compared in rat embryonic cardiomyocytes. The lowest electric field which triggered Ca2+ transients was expectedly higher for nsEP (36 kV/cm) than for msEP (0.09 kV/cm) but the respective doses were similar (190 and 460 mJ/g). At higher intensities, both stimuli triggered prolonged firing in quiescent cells. An increase of basal Ca2+ level by >10 nM in cells with blocked voltage-gated Ca2+ channels and depleted Ca2+ depot occurred at 63 kV/cm (nsEP) or 0.14 kV/cm (msEP) and was regarded as electroporation threshold. These electric field values were at 150-230% of stimulation thresholds for both msEP and nsEP, notwithstanding a 400,000-fold difference in pulse duration. For comparable levels of electroporative Ca2+ uptake, msEP caused at least 10-fold greater uptake of propidium than nsEP, suggesting increased yield of larger pores. Electroporation by msEP started Ca2+ entry abruptly and locally at the electrode-facing poles of cell, followed by a slow diffusion to the center. In a stark contrast, nsEP evoked a "supra-electroporation" pattern of slower but spatially uniform Ca2+ entry. Thus nsEP and msEP had comparable dose efficiency, but differed profoundly in the size and localization of electropores.