Characterization of single-cell electroporation by using patch-clamp and fluorescence microscopy

Calcium indicator loading of neurons using single-cell electroporation

Studies of subcellular Ca(2+) signaling rely on methods for labeling cells with fluorescent Ca(2+) indicator dyes. In this study, we demonstrate the use of single-cell electroporation for Ca(2+) indicator loading of individual neurons and small neuronal networks in rat neocortex in vitro and in vivo. Brief voltage pulses were delivered through glass pipettes positioned close to target cells. This approach resulted in reliable and rapid (within seconds) loading of somata and subsequent complete labeling of dendritic and axonal arborizations. By using simultaneous whole-cell recordings in brain slices, we directly addressed the effect of electroporation on neurons. Cell viability was high (about 85%) with recovery from the membrane permeabilization occurring within a minute. Electrical properties of recovered cells were indistinguishable before and after electroporation. In addition, Ca(2+) transients with normal appearance could be evoked in dendrites, spines, and axonal boutons of electroporated cells. Using negative-stains of somata, targeted single-cell electroporation was equally applicable in vivo. We conclude that electroporation is a simple approach that permits Ca(2+) indicator loading of multiple cells with low background staining within a short amount of time, which makes it especially well suited for functional imaging of subcellular Ca(2+) dynamics in small neuronal networks.

Self-electroporation as a model for fusion pore formation

The creation of a small opening called the fusion pore is a necessary prerequisite for neurotransmitter release from synaptic vesicles. It is known that high intensity electric fields can create pores in vesicles by a process called electroporation. Due to the presence of charged phosphatidylserine (PS) molecules on the inner leaflet of the cell membrane, an electric field that is strong enough to cause electroporation of a synaptic vesicle might be present. It was shown by K. Rosenheck [K. Rosenheck. Biophys J 75, 1237-1243 (1998)] that in a planar geometry, fields sufficient to cause electroporation can occur at intermembrane separations of less than approximately 3 nm. It is frequently found, however, that the cell membrane is not planar but caves inward at the locations where a vesicle is close to it. Indentation of the cell membrane in the fusion region was modelled as a hemisphere and a theoretical study of the electric field in the vicinity of the cell membrane taking into account the screening effect of dissolved ions in the cytoplasm was performed. It was discovered that fields crossing the electroporation threshold occurred at a distance of 2 nm or less, supporting the claim that electroporation could be a possible mechanism for fusion pore formation.

High-throughput and real-time study of single cell electroporation using microfluidics: effects of medium osmolarity

Electroporation has been widely accepted as an important tool for the delivery of exogenous molecules into cells. Previous mechanistic studies have been carried out by observing either the average behavior from a large population of cells or the response from a small number of single cells. In this study, we demonstrated a novel microfluidic method with high throughput (up to 30 Hz) for real-time studies of single cell electroporation events. Electroporation occurred when cells flowed through a section of a microfluidic channel defined by special geometry. A CCD camera was used to monitor the response of cells starting from the onset of the electroporation. We studied the swelling of Chinese hamster ovary cells and the rupture of cell membrane during electroporation using this technique. We applied buffers with different osmolarities to investigate the effects of medium osmolarity, based on results from a population of single cells. We were able to establish the distributions of the rates of swelling and membrane rupture in the cell population. We also explored establishing the correlation between the property (the cell diameter) and the behavior (the swelling rate) of single cells. Our results indicated that the processes of swelling and rupture occurred more rapidly in the hypotonic or hypertonic buffers than in the isotonic buffer. Statistical analysis did not reveal strong linear correlation between the cell size and the swelling rate. These proof-of-concept studies reveal the potential of applying microfluidics to study electroporation of a cell population at single cell level in real time with high throughput. The limitations associated with this approach were also addressed.

Simultaneous maximization of cell permeabilization and viability in single-cell electroporation using an electrolyte-filled capillary

A549 cells were briefly exposed to Thioglo-1, which converts thiols to fluorescent adducts. The fluorescent cells were exposed to short (50-300 ms) electric field pulses (500 V across a 15 cm capillary) created at the tip of an electrolyte-filled capillary. Fluorescence microscopy revealed varying degrees of cell permeabilization depending on the conditions. Longer pulses and a shorter cell-capillary tip distance led to a greater decrease in the cell's fluorescence. Live/dead (calcein AM and propidium iodide) testing revealed that a certain fraction of cells died. Longer pulses and shorter cell-capillary tip distances were more deadly. An optimum condition exists at a cell-capillary tip distance of 3.5-4.5 microm and a pulse duration of 120-150 ms. At these conditions, >90% of the cells are permeabilized and 80-90% survive.

Experimental evidence of improved transthoracic defibrillation with electroporation-enhancing pulses

There is considerable work on defibrillation wave form optimization. This paper determines the impedance changes during defibrillation, then uses that information to derive the optimum defibrillation wave form.

METHODS PART I

Twelve guinea pigs and six swine were used to measure the current wave form for square voltage pulses of a strength which would defibrillate about 50% of the time. In guinea pigs, electrodes were placed thoracically, abdominally and subcutaneously using two electrode materials (zinc and steel) and two electrode pastes (Core-gel and metallic paste).

RESULTS PART I

The measured current wave form indicated an exponentially increasing conductance over the first 3 ms, consistent with enhanced electroporation or another mechanism of time-dependent conductance. We fit this current with a parallel conductance composed of a time-independent component (g0 = 1.22 +/- 0.28 mS) and a time-dependent component described by g delta (1-e(-t/tau)), where g delta = 0.95 +/- 0.20 mS and tau = 0.82 +/- 0.17 ms in guinea pigs using zinc and Cor-gel. Different electrode placements and materials had no significant effect on this fit. From our fit, we determined the stimulating wave form that would theoretically charge the myocardial membrane to a given threshold using the least energy from the defibrillator. The solution was a very short, high voltage pulse followed immediately by a truncated ascending exponential tail.

METHODS PART II

The optimized wave forms and similar nonoptimized wave forms were tested for efficacy in 25 additional guinea pigs and six additional swine using methods similar to Part I.

RESULTS PART II

Optimized wave forms were significantly more efficacious than similar nonoptimized wave forms. In swine, a wave form with the short pulse was 41% effective while the same wave form without the short pulse was 8.3% effective (p < 0.03) despite there being only a small difference in energy (111 J versus 116 CONCLUSIONS: We conclude that a short pulse preceding a defibrillation pulse significantly improves efficacy, perhaps by enhancing electroporation.

Electroporation of cells in microfluidic devices: a review

In recent years, several publications on microfluidic devices have focused on the process of electroporation, which results in the poration of the biological cell membrane. The devices involved are designed for cell analysis, transfection or pasteurization. The high electric field strengths needed are induced by placing the electrodes in close proximity or by creating a constriction between the electrodes, which focuses the electric field. Detection is usually achieved through fluorescent labeling or by measuring impedance. So far, most of these devices have only concerned themselves solely with the electroporation process, but integration with separation and detection processes is expected in the near future. In particular, single-cell content analysis is expected to add further value to the concept of the microfluidic chip. Furthermore, if advanced pulse schemes are employed, such microdevices can also enhance research into intracellular electroporation.

Single-cell analysis by intracellular immuno-reaction and capillary electrophoresis with laser-induced fluorescence detection

A novel method for single-cell analysis was developed by combining electroporation for intracellular immuno-reaction and capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection. Human interferon-gamma (IFN-gamma) in natural killer (NK) cells was chosen as the test antigen. Two forms of IFN-gamma in single cells could be well separated and detected with a limit of detection of zeptomole. In this assay, the anti-IFN-gamma monoclonal antibody labeled with fluorescein isothiocyanate (Ab*) was introduced into NK cells by electrophoration for intracellular immuno-reaction. After completion of the intracellular immuno-reaction, the NK cells were chemically pre-perforated with digitonin to lyse easily. Then, one NK cell containing the complexes of IFN-gamma isoantigens with Ab* was electrokinetically injected into the capillary. The cell adsorbed on the tip of capillary was lysed by ultrasonication. Finally, the complexes of the different forms of IFN-gamma in the cell were separated and detected by CE-LIF detection.

Analysis of Mammalian Cell Cytoplasm with Electrophoresis in Nanometer Inner Diameter Capillaries

Capillary electrophoresis in 770 nanometer inner diameter capillaries coupled to electrochemical detection with an etched electrode matching an etched capillary (etched electrochemical detection) has been used with ultrasmall sampling to inject subcellular samples from intact single mammalian cells. Separations of cytoplasmic samples taken from rat pheochromocytoma cells have been achieved. As little as 8% of the total volume of a single cell has been sampled and analyzed. Dopamine has been identified and quantified in these PC12 cells using this technique. The average cytoplasmic level of dopamine in rat pheochromocytoma cells has been determined to be 240 ± 60 μM. The use of electrophoresis in 770 nanometer inner diameter capillaries with electrochemical detection to monitor cytoplasmic neurotransmitters at the single cell level can provide information about complex cellular functions such as neurotransmitter storage and synthesis.

Study of sonoporation dynamics affected by ultrasound duty cycle

Sonoporation is the ultrasound-induced membrane porosity and has been investigated as a means for intracellular drug delivery and nonviral gene transfection. The dynamic characteristics of sonoporation, such as formation, duration and resealing of the pores in the cell membrane, determine the process of intracellular uptake of molecules or agents of interest that are otherwise obstructed by the cell membrane barrier. Sonoporation dynamics is also important for postultrasound cell survival. In this study, we investigated the effects of ultrasound duty cycle on sonoporation dynamics using Xenopus oocyte as a model system. Transducer with a center frequency of 0.96 MHz was used to generate pulsed ultrasound of desired duty cycle (5%, 10% and 15%) at a pulse repetition frequency of 1 Hz and an acoustic pressure of 0.4 MPa in our experiments. Employing voltage clamp techniques, we measured the transmembrane current as the direct result of decreased membrane resistance due to pore formation induced by ultrasound application. We characterized the sonoporation dynamics from these time-resolved recordings of transmembrane current to indicate cell membrane status, including pore formation, extension and resealing. We observed that the transmembrane current amplitude increased with increasing duty cycle, while the recovering process of membrane pores and cell survival rate decreased at higher duty cycles.

Effect of cell electroporation on the conductivity of a cell suspension

An increased permeability of a cell membrane during the application of high-voltage pulses results in increased transmembrane transport of molecules that otherwise cannot enter the cell. Increased permeability of a cell membrane is accompanied by increased membrane conductivity; thus, by measuring electric conductivity the extent of permeabilized tissue could be monitored in real time. In this article the effect of cell electroporation caused by high-voltage pulses on the conductivity of a cell suspension was studied by current-voltage measurements during and impedance measurement before and after the pulse application. At the same time the percentage of permeabilized and survived cells was determined and the extent of osmotic swelling measured. For a train of eight pulses a transient increase in conductivity of a cell suspension was obtained above permeabilization threshold in low- and high-conductive medium with complete relaxation in <1 s. Total conductivity changes and impedance measurements showed substantial changes in conductivity due to the ion efflux in low-conductive medium and colloid-osmotic swelling in both media. Our results show that by measuring electric conductivity during the pulses we can detect limit permeabilization threshold but not directly permeabilization level, whereas impedance measurements in seconds after the pulse application are not suitable.

BIOMIMETIC NANOSCALE REACTORS AND NETWORKS

Methods based on self-assembly, self-organization, and forced shape transformations to form synthetic or semisynthetic enclosed lipid bilayer structures with several properties similar to biological nanocompartments are reviewed. The procedures offer unconventional micro- and nanofabrication routes to yield complex soft-matter devices for a variety of applications for example, in physical chemistry and nanotechnology. In particular, we describe novel micromanipulation methods for producing fluid-state lipid bilayer networks of nanotubes and surface-immobilized vesicles with controlled geometry, topology, membrane composition, and interior contents. Mass transport in nanotubes and materials exchange, for example, between conjugated containers, can be controlled by creating a surface tension gradient that gives rise to a moving boundary or by induced shape transformations. The network devices can operate with extremely small volume elements and low mass, to the limit of single molecules and particles at a length scale where a continuum mechanics approximation may break down. Thus, we also describe some concepts of anomalous fluctuation-dominated kinetics and anomalous diffusive behaviours, including hindered transport, as they might become important in studying chemistry and transport phenomena in these confined systems. The networks are suitable for initiating and controlling chemical reactions in confined biomimetic compartments for rationalizing, for example, enzyme behaviors, as well as for applications in nanofluidics, bioanalytical devices, and to construct computational and complex sensor systems with operations building on chemical kinetics, coupled reactions and controlled mass transport.

Ultrasound-induced cell membrane porosity

Recent studies of ultrasound (US) methods for targeted drug delivery and nonviral gene transfection revealed new, advantageous possibilities. These studies utilized US contrast agents, commonly stabilized microbubbles, to facilitate delivery and suggested that US delivery resulted from cell sonoporation, the formation of temporary pores in the cell membrane induced by US. Using voltage clamp techniques, we obtained real-time measurements of sonoporation of single Xenopus oocyte in the presence of Optison trade mark, an agent consisting of albumin-shelled C(3)F(8) gas bubbles (mean diameter 3.2 microm). Ultrasound increased the transmembrane current as a direct result of decreased membrane resistance due to pore formation. We observed a distinct delay of sonoporation following US activation and characteristic stepwise increases of transmembrane current throughout US duration. We discovered that the resealing of cell membrane following US exposure required Ca(2+) entering the cell through US-induced pores.

Effective conductivity of a suspension of permeabilized cells: a theoretical analysis

During the electroporation cell membrane undergoes structural changes, which increase the membrane conductivity and consequently lead to a change in effective conductivity of a cell suspension. To correlate microscopic membrane changes to macroscopic changes in conductivity of a suspension, we analyzed the effective conductivity theoretically, using two different approaches: numerically, using the finite elements method; and analytically, by using the equivalence principle. We derived the equation, which connects membrane conductivity with effective conductivity of the cell suspension. The changes in effective conductivity were analyzed for different parameters: cell volume fraction, membrane and medium conductivity, critical transmembrane potential, and cell orientation. In our analysis we used a tensor form of the effective conductivity, thus taking into account the anisotropic nature of the cell electropermeabilization and rotation of the cells. To determine the effect of cell rotation, as questioned by some authors, the difference between conductivity of a cell suspension with normally distributed orientations and parallel orientation was also calculated, and determined to be <10%. The presented theory provides a theoretical basis for the analysis of measurements of the effective conductivity during electroporation.

Fast Electrical Lysis of Cells for Capillary Electrophoresis

In the past decade, capillary electrophoresis has demonstrated increasing utility for the quantitative analysis of single cells. New applications for the analysis of dynamic cellular properties demand sampling methods with sufficient temporal resolution to accurately measure these processes. In particular, intracellular signaling pathways involving many enzymes can be modulated on subsecond time scales. We have developed a technique to rapidly lyse an adherent mammalian cell using a single electrical pulse followed by efficient loading of the cellular contents into a capillary. Microfabricated electrodes were designed to create a maximum voltage drop across the flattened cell's plasma membrane at a minimum interelectrode voltage. The influence of the interelectrode distance, pulse duration, and pulse strength on the rate of cell lysis was determined. The ability to rapidly lyse a cell and collect and separate the cellular contents was demonstrated by loading cells with Oregon Green and two isomers of carboxyfluorescein. All three fluorophores were detected with a separation efficiency comparable to that of standards. Parallel comparison of electrical lysis to that produced by a laser-based lysis system revealed that the sampling efficiencies of the two techniques were comparable. Rapid cell lysis by an electrical pulse may increase the application of capillary electrophoresis to the study of cellular dynamics requiring fast sampling times.

Single-cell electroporation

Electroporation is a widely used method for the introduction of polar and charged agents such as dyes, drugs, DNA, RNA, proteins, peptides, and amino acids into cells. Traditionally, electroporation is performed with large electrodes in a batch mode for treatment of a large number of cells in suspension. Recently, microelectrodes that can produce extremely localized electric fields, such as solid carbon fiber microelectrodes, electrolyte-filled capillaries and micropipettes as well as chip-based microfabricated electrode arrays, have proven useful to electroporate single cells and subcellular structures. Single-cell electroporation opens up a new window of opportunities in manipulating the genetic, metabolic, and synthetic contents of single targeted cells in tissue slices, cell cultures, in microfluidic channels or at specific loci on a chip-based device.

Nanotube-vesicle networks with functionalized membranes and interiors

We describe nanotube-vesicle networks with reconstituted membrane protein from cells and with interior activity defined by an injection of microparticles or molecular probes. The functionality of a membrane protein after reconstitution was verified by single-channel ion conductance measurements in excised inside-out patches from the vesicle membranes. The distribution of protein, determined by fluorescence detection, in the network membrane was homogeneous and could diffuse via a nanotube connecting two vesicles. We also show how injecting small unilamellar protein-containing vesicles can differentiate the contents of individual containers in a network. The combination of membrane activity and interior activity was demonstrated by ionophore-assisted accumulation, and internal Calcium Green-mediated detection, of Ca2+ within a single network container. This system can model a variety of biological functions and complex biological multicompartment structures and might serve as a platform for constructing complex sensor and computational devices.

Experimental investigation on neural cell survival after dielectrophoretic trapping

Negative dielectrophoretic forces can effectively be used to trap cortical rat neurons. The creation of dielectrophoretic forces requires electric fields of high non-uniformity. High electric field strengths, however, can cause excessive membrane potentials by which cells may unrecoverably be changed or it may lead to cell death. In a previous study it was found that cells trapped at 3 Vtt/14 MHz did not change morphologically as compared to cells that were not exposed to the electric field. This study investigates the viability of fetal cortical rat neurons after being trapped by negative dielectrophoretic forces at frequencies up to 1 MHz. A planar quadrupole micro-electrode structure was used for the creation of a non-uniform electric field. The sinusoidal input signal was varied in amplitude (3 and 5 Vtt) and frequency (10 kHz-1 MHz). The results presented in this paper show that the viability of dielectrophoretically trapped postnatal cortical rat cells was greatly frequency dependent. To preserve viability frequencies above 100 kHz (at 3 Vtt) or 1 MHz (5 Vtt) must be used.

Single-cell imaging of the Ca2+ influx into bovine endothelial cells occurring in response to an alternating electric stimulus

The electric control of cellular functions via Ca2+ was formerly suggested. From this viewpoint, the involvement of a Ca2+ channel was studied using bovine fetal arterial endothelial (BFAE) cells in which P2X4, an ATP-operated and fluid shear stress sensitive Ca2+ channel, exists predominantly. An electric stimulus (sine wave, 10 Hz, 10 VPP, 30 s) caused a marked influx of Ca2+ into BFAE cells from an extracellular solution. The magnitude of the [Ca2+]i change increased with a decrease in the frequency in the range from 100 Hz to 5 Hz. Regarding the pathway of this Ca2+ influx, single-cell imaging and an ATP depletion experiment strongly suggested the involvement of a pathway different from P2X4. This pathway was thought to be a non-specific one, because typical Ca2+ channel blockers, such as verapamil, Gd3+, and Co2+, could not inhibit the Ca2+ influx.

Investigating membrane breakdown of neuronal cells exposed to nonuniform electric fields by finite-element modeling and experiments

High electric field strengths may induce high cell membrane potentials. At a certain breakdown level the membrane potential becomes constant due to the transition from an insulating state into a high conductivity and high permeability state. Pores are thought to be created through which molecules may be transported into and out of the cell interior. Membrane rupture may follow due to the expansion of pores or the creation of many small pores across a certain part of the membrane surface. In nonuniform electric fields, it is difficult to predict the electroporated membrane area. Therefore, in this study the induced membrane potential and the membrane area where this potential exceeds the breakdown level is investigated by finite-element modeling. Results from experiments in which the collapse of neuronal cells was detected were combined with the computed field strengths in order to investigate membrane breakdown and membrane rupture. It was found that in nonuniform fields membrane rupture is position dependent, especially at higher breakdown levels. This indicates that the size of the membrane site that is affected by electroporation determines rupture.

Electroporation of single cells and tissues with an electrolyte-filled capillary

We show how an electrolyte-filled capillary (EFC) coupled to a high-voltage power supply can be used as a versatile electroporation tool for the delivery of dyes, drugs, and biomolecules to the cytoplasm of single cells and cells in tissues. A large-voltage pulse applied across the EFC (fused silica, 30 cm long, 375-microm o.d., 30-microm i.d.) gives rise to a small electric field outside the terminus of the EFC, which causes pore formation in cell membranes and induces an electroosmotic flow of electrolyte. When the EFC contains cell-loading agents, then the electroosmotic flow delivers the agents at the site of pore formation. The combination of pore formation and delivery enables loading of materials into the cytoplasm. By patch-clamp and fluorescence microscopy, formation of pores was observed at estimated transmembrane voltages of <85 mV with half-maximum values around 206 mV. The electroporation protocol was demonstrated by introduction of fluorogenic dyes into single NG108-15 cells, cellular processes, and small populations of cells in organotypic hippocampal cultures. Preliminary results are shown in which this protocol was employed for in vivo electroporation of ventral mesencephalon in rat brains. The technique was also used to access organelle-based detection systems inside cells. As a demonstration, 1,4,5-inositoltriphosphate was added to the electrolyte and detected by intracellular organelles in electroporated cells.