Scanning electroporation of selected areas of adherent cell cultures

Impedance analysis of adherent cells after in situ electroporation-mediated delivery of bioactive proteins, DNA and nanoparticles in µL-volumes

Specific intracellular manipulation of animal cells is a persistent goal in experimental cell biology. Such manipulations allow precise and targeted interference with signaling cascades, metabolic pathways, or bi-molecular interactions for subsequent tracking of functional consequences. However, most biomolecules capable of molecular recognition are membrane impermeable. The ability to introduce these molecules into the cytoplasm and then to apply appropriate readouts to monitor the corresponding cell response could prove to be an important research tool. This study describes such an experimental approach combining in situ electroporation (ISE) as a means to efficiently deliver biomolecules to the cytoplasm with an impedance-based, time-resolved analysis of cell status using electric cell-substrate impedance sensing (ECIS). In this approach, gold-film electrodes, deposited on the bottom of regular culture dishes, are used for both electroporation and monitoring. The design of the electrode layout and measurement chamber allows working with sample volumes as small as 10 µL. A miniaturized setup for combined electroporation and impedance sensing (µISE-ECIS) was applied to load different adherent cells with bioactive macromolecules including enzymes, antibodies, nucleic acids and quantum dot nanoparticles. The cell response after loading the cytoplasm with RNase A or cytochrome c (in the presence or absence of caspase inhibitors) was tracked by non-invasive impedance readings in real-time.

Impact of pulse duration on localized single-cell nano-electroporation

We introduce a localized single-cell membrane nano-electroporation with controllable sequential molecular delivery by millisecond to nanosecond electrical pulses. An intense electrical field was generated by a pair of transparent indium tin oxide (ITO)-based nano-electrodes, which was confined to a narrow region of the single-cell membrane surface near the nano-electrode edges (approximately 2 μm × 50 nm area), whereas the remaining area of the membrane was unaffected. Moreover, a 250 nm SiO2 passivation layer on top of the nano-electrode reduced not only the thermal effect on the cell membrane surface, but it also avoided the generation of ions during the experiment, resulting in the reduction of cell toxicity and a significant enhancement of cell viability. Our approach precisely delivers dyes, Quantum Dots (QDs) and plasmids, through a localized region of single HeLa cells by considerably enhanced electrophoresis and diffusion effects with different duration of the pulsing process. The smaller molecules took less time to deliver into a single cell with a single pulse, whereas larger biomolecules took longer time even for multiple numbers of long lasting pulses. The system not only generates sequential well-controlled nano-pores allowing for the rapid recovery of cell membranes, but it also provides spatial, temporal and qualitative dosage control to deliver biomolecules into localized single-cell levels, which can be potentially beneficial for single cell studies and therapeutic applications.

Electroosmotic perfusion of tissue: sampling the extracellular space and quantitative assessment of membrane-bound enzyme activity in organotypic hippocampal slice cultures

This review covers recent advances in sampling fluid from the extracellular space of brain tissue by electroosmosis (EO). Two techniques, EO sampling with a single fused-silica capillary and EO push-pull perfusion, have been developed. These tools were used to investigate the function of membrane-bound enzymes with outward-facing active sites, or ectoenzymes, in modulating the activity of the neuropeptides leu-enkephalin and galanin in organotypic-hippocampal-slice cultures (OHSCs). In addition, the approach was used to determine the endogenous concentration of a thiol, cysteamine, in OHSCs. We have also investigated the degradation of coenzyme A in the extracellular space. The approach provides information on ectoenzyme activity, including Michaelis constants, in tissue, which, as far as we are aware, has not been done before. On the basis of computational evidence, EO push-pull perfusion can distinguish ectoenzyme activity with a ~100 μm spatial resolution, which is important for studies of enzyme kinetics in adjacent regions of the rat hippocampus.

Automatic system for electroporation of adherent cells growing in standard multi-well plates

In this study an automatic system is presented to perform electroporation, also known as electropermeabilization, on adherent cells. It is an intention of this system to apply electric field pulses directly to cells growing in standard multi-well plates as a step forward to include this technique in standard laboratory protocols. An interdigitated microelectrode assembly constructed with Printed Circuit Board (PCB) is placed closely above the cell monolayer, and in order to avoid direct contact with cells, small micro-separators were included in the structure. Additionally, distribution of current density was modified by filling the gap between adjacent electrodes with a non conductive material as predicted by electric field simulations. This modification helps to concentrate the electric field intensity in the region where cells are present. The device was tested using C2C12 cell line growing adhered in 24 multi-well plates and fluorescent labeled dextran FD20S as the molecule to be delivered. Successful transfection was observed with minimal invasiveness of the operation reducing the stress caused to cells.

Nanofountain probe electroporation (NFP-E) of single cells

The ability to precisely deliver molecules into single cells is of great interest to biotechnology researchers for advancing applications in therapeutics, diagnostics, and drug delivery toward the promise of personalized medicine. The use of bulk electroporation techniques for cell transfection has increased significantly in the past decade, but the technique is nonspecific and requires high voltage, resulting in variable efficiency and low cell viability. We have developed a new tool for electroporation using nanofountain probe (NFP) technology, which can deliver molecules into cells in a manner that is highly efficient and gentler to cells than bulk electroporation or microinjection. Here we demonstrate NFP electroporation (NFP-E) of single HeLa cells within a population by transfecting them with fluorescently labeled dextran and imaging the cells to evaluate the transfection efficiency and cell viability. Our theoretical analysis of the mechanism of NFP-E reveals that application of the voltage creates a localized electric field between the NFP cantilever tip and the region of the cell membrane in contact with the tip. Therefore, NFP-E can deliver molecules to a target cell with minimal effect of the electric potential on the cell. Our experiments on HeLa cells confirm that NFP-E offers single cell selectivity, high transfection efficiency (>95%), qualitative dosage control, and very high viability (92%) of transfected cells.

Method for electric parametric characterization and optimization of electroporation on a chip

We have developed a rapid method to optimize the electric parameters of cell electroporation. In our design, a pair of ring-dot formatted electrodes was used to generate a radial distribution of electric field from the center to the periphery. Varied electric field intensity was acquired in different annulus when an electric pulse was applied. Cells were cultured on the microchips for adherent cell electroporation and in situ observation. The electroporation parameters of electric field intensity were explored and evaluated in terms of cell viability and transfection efficiency. The optimization was performed in consideration of both cell viability, which was investigated to decrease as electric field increases, and the transfection rate, which normally increases at stronger electric field. The electroporation characteristics HEK-293A and Hela cells were investigated, and the optimum parameters were obtained. Verified by a commercial electroporation system as well as self-made microchips endowed the optimization with wider meaning. At last, as applications, we acquired the optimal electroporation pulse intensity of Neuro-2A cells and a type of primary cell (human umbilical vein endothelial cell, HUVEC) by one time electroporation using the proposed method.

Micro-/nanofluidics based cell electroporation

Non-viral gene delivery has been extensively explored as the replacement for viral systems. Among various non-viral approaches, electroporation has gained increasing attention because of its easy operation and no restrictions on probe or cell type. Several effective systems are now available on the market with reasonably good gene delivery performance. To facilitate broader biological and medical applications, micro-/nanofluidics based technologies were introduced in cell electroporation during the past two decades and their advances are summarized in this perspective. Compared to the commercially available bulk electroporation systems, they offer several advantages, namely, (1) sufficiently high pulse strength generated by a very low potential difference, (2) conveniently concentrating, trapping, and regulating the position and concentration of cells and probes, (3) real-time monitoring the intracellular trafficking at single cell level, and (4) flexibility on cells to be transfected (from single cell to large scale cell population). Some of the micro-devices focus on cell lysis or fusion as well as the analysis of cellular properties or intracellular contents, while others are designed for gene transfection. The uptake of small molecules (e.g., dyes), DNA plasmids, interfering RNAs, and nanoparticles has been broadly examined on different types of mammalian cells, yeast, and bacteria. A great deal of progress has been made with a variety of new micro-/nanofluidic designs to address challenges such as electrochemical reactions including water electrolysis, gas bubble formation, waste of expensive reagents, poor cell viability, low transfection efficacy, higher throughput, and control of transfection dosage and uniformity. Future research needs required to advance micro-/nanofluidics based cell electroporation for broad life science and medical applications are discussed.

Single-cell electroporation using a multifunctional pipette

We present here a novel platform combination, using a multifunctional pipette to individually electroporate single-cells and to locally deliver an analyte, while in their culture environment. We demonstrate a method to fabricate low-resistance metallic electrodes into a PDMS pipette, followed by characterization of its effectiveness, benefits and limits in comparison with an external carbon microelectrode.

Direct access and control of the intracellular solution environment in single cells

Methods that can control and vary the solution environment around single cells are abundant. In contrast, methods that offer direct access to the intracellular proteome and genome in single cells with the control, flexibility, and convenience given by microfluidic methods are both scarce and in great demand. Here, we present such a method based on using a microfluidic device mounted on a programmable scanning stage and cells on-chip permeabilized by the pore-forming glycoside digitonin. We characterized the on-chip digitonin poration, as well as the solution exchange within cells. Intracellular solution exchange times vary with the dose of exposure to digitonin from less than a second to tens of seconds. Also, the degree of permeabilization obtained for cells treated with the same dose varies considerably, especially for low doses of digitonin exposure and low permeabilities. With the use of the presented setup, the degree of permeabilization can be measured during the permeabilization process, which allows for "on-line" optimization of the digitonin exposure time. Using this calibrated permeabilization method, we demonstrate the generation of intracellular oscillations, intracellular gradients, and the delivery of substrate to initiate enzymatic reactions in situ. This method holds the potential to screen and titrate intracellular receptors or enzymes or to generate intracellular oscillations, useful in the study of signaling pathways and oscillation decoding among other applications.

Single-cell electroporation

Single-cell electroporation (SCEP) is a relatively new technique that has emerged in the last decade or so for single-cell studies. When a large enough electric field is applied to a single cell, transient nano-pores form in the cell membrane allowing molecules to be transported into and out of the cell. Unlike bulk electroporation, in which a homogenous electric field is applied to a suspension of cells, in SCEP an electric field is created locally near a single cell. Today, single-cell-level studies are at the frontier of biochemical research, and SCEP is a promising tool in such studies. In this review, we discuss pore formation based on theoretical and experimental approaches. Current SCEP techniques using microelectrodes, micropipettes, electrolyte-filled capillaries, and microfabricated devices are all thoroughly discussed for adherent and suspended cells. SCEP has been applied in in-vivo and in-vitro studies for delivery of cell-impermeant molecules such as drugs, DNA, and siRNA, and for morphological observations.

Determination of zeta-potential and tortuosity in rat organotypic hippocampal cultures from electroosmotic velocity measurements under feedback control

Extracellular translational motion in the brain is generally considered to be governed by diffusion and tortuosity. However, the brain as a whole has a significant zeta-potential, thus translational motion is also governed by electrokinetic effects under a naturally occurring or applied electric field. We have previously measured zeta-potential and tortuosity in intact brain tissue; however, the method was tedious. In this work, we use a four-electrode potentiostat to control the potential difference between two microreference electrodes in the tissue, creating a constant electric field. Additionally, some alterations have been made to simplify our previous procedure. The method entails simultaneously injecting two 70 kDa dextran conjugated fluorophores into rat organotypic hippocampal cultures and observing their mobility using fluorescence microscopy. We further present two methods of data analysis: regression and two-probe analysis. Statistical comparisons are made between the previous and current methods as well as between the two data analysis methods. In comparison to the previous method, the current, simpler method with data analysis by regression gives statistically indistinguishable mean values of zeta-potential and tortuosity, with a similar variability for zeta-potential, -21.3 +/- 2.8 mV, and a larger variability for the tortuosity, 1.98 +/- 0.12. On the other hand, we find that the current method combined with the two-probe analysis produces accurate and more precise results, with a zeta-potential of -22.8 +/- 0.8 mV and a tortuosity of 2.24 +/- 0.10.

Microscale electroporation: challenges and perspectives for clinical applications

Microscale engineering plays a significant role in developing tools for biological applications by miniaturizing devices and providing controllable microenvironments for in vitro cell research. Miniaturized devices offer numerous benefits in comparison to their macroscale counterparts, such as lower use of expensive reagents, biomimetic environments, and the ability to manipulate single cells. Microscale electroporation is one of the main beneficiaries of microscale engineering as it provides spatial and temporal control of various electrical parameters. Microscale electroporation devices can be used to reduce limitations associated with the conventional electroporation approaches such as variations in the local pH, electric field distortion, sample contamination, and the difficulties in transfecting and maintaining the viability of desired cell types. Here, we present an overview of recent advances of the microscale electroporation methods and their applications in biology, as well as current challenges for its use for clinical applications. We categorize microscale electroporation into microchannel and microcapillary electroporation. Microchannel-based electroporation can be used for transfecting cells within microchannels under dynamic flow conditions in a controlled and high-throughput fashion. In contrast, microcapillary-based electroporation can be used for transfecting cells within controlled reaction chambers under static flow conditions. Using these categories we examine the use of microscale electroporation for clinical applications related to HIV-1, stem cells, cancer and other diseases and discuss the challenges in further advancing this technology for use in clinical medicine and biology.

In Vitro Electrochemistry of Biological Systems

This article reviews recent work involving electrochemical methods for in vitro analysis of biomolecules, with an emphasis on detection and manipulation at and of single cells and cultures of cells. The techniques discussed include constant potential amperometry, chronoamperometry, cellular electroporation, scanning electrochemical microscopy, and microfluidic platforms integrated with electrochemical detection. The principles of these methods are briefly described, followed in most cases with a short description of an analytical or biological application and its significance. The use of electrochemical methods to examine specific mechanistic issues in exocytosis is highlighted, as a great deal of recent work has been devoted to this application.

Determination of zeta-potential in rat organotypic hippocampal cultures

zeta-potentials of entities such as cells and synaptosomes have been determined, but zeta of brain tissue has never been measured. Electroosmotic flow, and the resulting transport of neuroactive substances, would result from naturally occurring and experimentally or clinically induced electric fields if zeta is significant. We have developed a simple method for determining zeta in tissue. An electric field applied across a rat organotypic hippocampal slice culture (OHSC) drives fluorescent molecules through the tissue by both electroosmotic flow and electrophoresis. Fluorescence microscopy is used to determine each molecule's velocity. Independently, capillary electrophoresis is used to measure the molecules' electrophoretic mobilities. The experiment yields zeta-potential and average tissue tortuosity. The zeta-potential of OHSCs is -22 +/- 2 mV, and the average tortuosity is 1.83 +/- 0.06. In a refined experiment, zeta-potential is measured in various subregions. The zeta-potentials of the CA1 stratum pyramidale, CA3 stratum pyramidal, and dentate gyrus are -25.1 +/- 1.6 mV, -20.3 +/- 1.7 mV, and -25.4 +/- 1.0 mV, respectively. Simple dimensional arguments show that electroosmotic flow is potentially as important as diffusion in molecular transport.

Gene transfer and protein dynamics in stem cells using single cell electroporation in a microfluidic device

There is great interest in genetic modification of bone marrow-derived mesenchymal stem cells (MSC), not only for research purposes but also for use in (autologous) patient-derived-patient-used transplantations. A major drawback of bulk methods for genetic modifications of (stem) cells, like bulk-electroporation, is its limited yield of DNA transfection (typically then 10%). This is even more limited when cells are present at very low numbers, as is the case for stem cells. Here we present an alternative technology to transfect cells with high efficiency (>75%), based on single cell electroporation in a microfluidic device. In a first experiment we show that we can successfully transport propidium iodide (PI) into single mouse myoblastic C2C12 cells. Subsequently, we show the use of this microfluidic device to perform successful electroporation of single mouse myoblastic C2C12 cells and single human MSC with vector DNA encoding a green fluorescent-erk1 fusion protein (EGFP-ERK1 (MAPK3)). Finally, we performed electroporation in combination with live imaging of protein expression and dynamics in response to extracellular stimuli, by fibroblast growth factor (FGF-2). We observed nuclear translocation of EGFP-ERK1 in both cell types within 15 min after FGF-2 stimulation. Due to the successful and promising results, we predict that microfluidic devices can be used for highly efficient small-scale 'genetic modification' of cells, and biological experimentation, offering possibilities to study cellular processes at the single cell level. Future applications might be small-scale production of cells for therapeutic application under controlled conditions.