Altering the biochemical state of individual cultured cells and organelles with ultramicroelectrodes

Electrically assisted sampling across membranes with electrophoresis in nanometer inner diameter capillaries

A nondestructive method for sampling from ultrasmall environments has been developed utilizing electrophoresis in nanometer inner diameter capillaries and etched electrochemical detection. The desire to study increasingly smaller biological environments such as mammalian cells has led to the need for capillary electrophoresis techniques with subpicoliter volume sampling capabilities. This sampling technique involves the fabrication of a microinjector at the tip of a 770-nm-inner diameter capillary and the use of electroporation for insertion through the membrane. Separations of catecholamines sampled from the interior of intact liposomes have been achieved. A separation of a cytoplasmic sample taken from an intact mammalian cell has also been obtained.

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.

Microfluidic electroporation of robust 10-μm vesicles for manipulation of picoliter volumes

We present a new way to transport and handle picoliter volumes of analytes in a microfluidic context through electrically monitored electroporation of 10-25 microm vesicles. In this method, giant vesicles are used to isolate analytes in a microfluidic environment. Once encapsulated inside a vesicle, contents will not diffuse and become diluted when exposed to pressure-driven flow. Two vesicle compositions have been developed that are robust enough to withstand electrical and mechanical manipulation in a microfluidic context. These vesicles can be guided and trapped, with controllable transfer of material into or out of their confined environment. Through electroporation, vesicles can serve as containers that can be opened when mixing and diffusion are desired, and closed during transport and analysis. Both vesicle compositions contain lecithin, an ethoxylated phospholipid, and a polyelectrolyte. Their performance is compared using a prototype microfluidic device and a simple circuit model. It was observed that the energy density threshold required to induce breakdown was statistically equivalent between compositions, 10.2+/-5.0 mJ/m2 for the first composition and 10.5+/-1.8 mJ/m2 for the second. This work demonstrates the feasibility of using giant, robust vesicles with microfluidic electroporation technology to manipulate picoliter volumes on-chip.

Model of a confined spherical cell in uniform and heterogeneous applied electric fields

Cells exposed to electric fields are often confined to a small volume within a solid tissue or within or near a device. Here we report on an approach to describing the frequency and time domain electrical responses of a spatially confined spherical cell by using a transport lattice system model. Two cases are considered: (1) a uniform applied field created by parallel plane electrodes, and (2) a heterogeneous applied field created by a planar electrode and a sharp microelectrode. Here fixed conductivities and dielectric permittivities of the extra- and intracellular media and of the membrane are used to create local transport models that are interconnected to create the system model. Consistent with traditional analytical solutions for spherical cells in an electrolyte of infinite extent, in the frequency domain the field amplification, G(m) (f) is large at low frequencies, f<1 MHz. G(m) (f) gradually decreases above 1 MHz and reaches a lower plateau at about 300 MHz, with the cell becoming almost "electrically invisible". In the time domain the application of a field pulse can result in altered localized transmembrane voltage changes due to a single microelectrode. The transport lattice approach provides modular, multiscale modeling capability that here ranges from cell membranes (5 nm scale) to the cell confinement volume ( approximately 40 microm scale).

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.

Electrical and thermal characterization of nanochannels between a cell and a silicon based micro-pore

Micro and nano fabrication techniques have facilitated the production of new devices for manipulation of single cells on a chip, such as the planar micro-pore electroporation technology. To characterize this technology we have studied the seal that forms at the interface between an individual cell and the micro-pore, in which the cell normally resides, as a function of an electrical field applied across the cell and temperature. Mathematical analysis of non-electroporative electrical fields in experiments with Madin-Darby canine kidney (MDCK) cells suggests that nanoscale channels form between the exterior of the cell and the pore wall. The results indicate that the electrical currents through these channels need to be considered when using planar micro-pores in general and performing micro-pore electroporation in particular. Our results show that the size of these channels is strongly temperature dependent and the cell to pore wall distance can increase by as much as 60% when the temperature of the system is lowered from 35 to 0( composite function)C. Temperature appears to be an important factor in the use of devices for cells on a chip and our results suggest that physiological temperatures should yield better seal formation, thus improved feedback sensitivity, than the traditional use of room temperature in planar micro-pore electroporation devices.

Electroporation of neurons and growth cones in Aplysia californica

Specific labeling of individual neurons and neuronal processes is virtually an everyday task for neuroscientists. Many traditional ways for delivery of intracellular dyes have limitations in terms of speed, efficiency and reproducibility. Electroporation is a fast, reliable and efficient method to deliver microscopic amounts of polar and charged molecules into neurons and their compartments such as individual neurites and growth cones. Here, we present a simple and highly effective procedure for intracellular labeling of individual Aplysia neurons both in intact ganglia and in cell culture. Pleural mechanoreceptor neurons have been used as illustrative examples to demonstrate applicability of direct and local labeling of the smallest individual neurites (< 2 microm) and single growth cones. Specifically, a 3-s train of 1.0 V hyperpolarizing pulses at 50 Hz effectively filled discrete neurites in contact with the tip of the micropipette with no dye transfer visible to other, non-contacted neurites. Application of this localized dye labeling technique to single neurites reveals a surprisingly complex morphology for patterns of axonal branching in culture. The protocol can be easily applied to a variety of models in neuroscience including accessible nervous systems of invertebrate animals.

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.

Membrane electroporation: a molecular dynamics simulation

We present results of molecular dynamics simulations of lipid bilayers under a high transverse electrical field aimed at investigating their electroporation. Several systems are studied, namely 1), a bare bilayer, 2), a bilayer containing a peptide nanotube channel, and 3), a system with a peripheral DNA double strand. In all systems, the applied transmembrane electric fields (0.5 V.nm(-1) and 1.0 V.nm(-1)) induce an electroporation of the lipid bilayer manifested by the formation of water wires and water channels across the membrane. The internal structures of the peptide nanotube assembly and that of the DNA strand are hardly modified under field. For system 2, no perturbation of the membrane is witnessed at the vicinity of the channel, which indicates that the interactions of the peptide with the nearby lipids stabilize the bilayer. For system 3, the DNA strand migrates to the interior of the membrane only after electroporation. Interestingly enough, switching of the external transmembrane potential in cases 1 and 2 for few nanoseconds is enough to allow for complete resealing and reconstitution of the bilayer. We provide evidence that the electric field induces a significant lateral stress on the bilayer, manifested by surface tensions of magnitudes in the order of 1 mN.m(-1). This study is believed to capture the essence of several dynamical phenomena observed experimentally and provides a framework for further developments and for new applications.

Utilization of Cell-Sized Lipid Containers for Nanostructure and Macromolecule Handling in Microfabricated Devices

We propose an original approach to handle submicrometer-sized biological or inorganic materials in microfabricated devices for micro total analysis applications. Cell-sized liposomes were utilized as containers for nanoparticles, green fluorescent proteins, or DNA and handled within a microfluidic chip. Due to the micrometer size of these liposomes, their detection could be achieved by conventional optical systems. Moreover, liposomes are hardly sensitive to Brownian motion; their trapping or transportation is thereby made easy with electrostatic-based techniques, for instance, developed the past few years for cells and particles. Encapsulated materials were confined for long durations with respect to the diffusive scale time, and the liposome membrane provided excellent protection from the outside environment, inhibiting undesirable interactions. A microfluidic device consisting of a flow cell covering an array of asymmetric electrodes allowed us to convey readily liposomes by the AC electroosmosis effect. We also assessed the electrofusion of liposomes between micromachined electrodes, opening up controlled initiation of reaction inside these containers; it was exemplified by fusing differently colored liposomes. We observed that a large fraction of the liposomes fused for electric field intensity around 6 kV/cm. Applications ranging from ultrasmall biomimetic reactors to large-scale drug delivery or cell labeling can be envisaged.

Controlled Initiation of Enzymatic Reactions in Micrometer-Sized Biomimetic Compartments

We present a technique to initiate chemical reactions involving few reactants inside micrometer-scale biomimetic vesicles (10(-12) to 10(-15) L) integral to three-dimensional surfactant networks. The shape of these networks is under dynamic control, allowing for transfer and mixing of two or several reactants at will. Specifically, two nanotube-connected vesicles were filled with reactants (substrate and enzyme, respectively) by microinjection. Initially, the vesicles are far apart and any diffusive mixing (on relevant experimental time scales) between the contents of the separated vesicles is hindered because of the narrow diameter and long axial extension of the nanotube. To initiate a reaction, the vesicles were brought close together, the nanotube was consumed by the vesicles and at a critical distance, the nanotube-vesicle junctions were dilated leading to formation of one spherical reactor, and hence mixing of the contents. We demonstrate the concept using a model enzymatic reaction, which yields a fluorescent product (two-step hydrolysis of fluorescein diphosphate by alkaline phosphatase), where product formation was measured as a function of time using a FRAP fluorescence microscopy protocol. By comparing the enzymatic activity with bulk measurements, the enzyme concentration inside the vesicle could be determined. Reactions could be followed for systems having as few as approximately 15 enzyme molecules confined to a reactor vesicle. To describe the experiments we use a simple diffusion-controlled reaction model and solve it using a survival probability approach. The agreement with experiment is qualitative, but the model describes the trends well. It is shown that the model correctly predicts (i) single-exponential decay after a few seconds, and (ii) that the substrate decay constant depends on the number of enzymes and geometry of reaction container. The numerical correction factor Lambda is introduced in order to ensure semiquantitative agreement between experiment and theory. It was shown that this numerical factor depends weakly on vesicle radius and number of enzymes, thus it is sufficient to determine this factor only once in a single calibration measurement.

Spatially and temporally controlled gene transfer by electroporation into adherent cells on plasmid DNA-loaded electrodes

Functional characterization of human genes is one of the most challenging tasks in current genomics. Owing to a large number of newly discovered genes, high-throughput methodologies are greatly needed to express in parallel each gene in living cells. To develop a method that allows efficient transfection of plasmids into adherent cells in spatial- and temporal-specific manners, we studied electric pulse-triggered gene transfer using a plasmid-loaded electrode. A plasmid was loaded on a gold electrode surface having an adsorbed layer of poly(ethyleneimine), and cells were then plated directly onto this modified surface. The plasmid was detached from the electrode by applying a short electric pulse and introduced into the cells cultured on the electrode, resulting in efficient gene expression, even in primary cultured cells. The location of transfected cells could be restricted within a small area on a micropatterned electrode, showing the versatility of the method for spatially controlled transfection. Plasmid transfection could also be performed in a temporally controlled manner without a marked loss of the efficiency when an electric pulse was applied within 3 days after cell plating. The method described here will provide an efficient means to transfer multiple genes, in parallel, into cultured mammalian cells for high-throughput reverse genetics research.

Electrotransfection of Mammalian Cells Using Microchannel-Type Electroporation Chip

Transfection of DNA molecules into mammalian cells with electric pulsations, which is so-called electroporation, is a powerful and widely used method that can be directly applied to gene therapy. However, very little is known about the basic mechanisms of DNA transfer and cell response to the electric pulse. We developed a microelectroporation chip with poly(dimethylsiloxane) (PDMS) to investigate the mechanism of electroporation as a first step of DNA transfer and to introduce the benefits of miniaturization into the genetic manipulation. The microelectroporation chip has a microchannel with a height of 20 microm and a length of 2 cm. Owing to the transparency of PDMS, we could in situ observe the uptake process of propidium iodide (PI) into SK-OV-3 cells, which shows promise in visualization of gene delivery in living cells. We also noticed the geometric effect on the degree of electroporation in microchannels with diverse channel width. This experimental result shows that the geometry can be another parameter to be considered for the electroporation when it is performed in microchannels with an exponential decaying pulse generator. Cell culturing is possible within the microelectroporation chip, and we also successfully transfected SK-OV-3 cells with enhanced green fluorescent protein genes, which demonstrates the feasibility of the microelectroporation chip in genetic manipulation.

Electroporation loading of calcium-sensitive dyes into the CNS

Calcium imaging of neural network function has been limited by the extent of tissue labeled or the time taken for labeling. We now describe the use of electroporation-an established technique for transfecting cells with genes-to load neurons with calcium-sensitive dyes in the isolated spinal cord of the neonatal mouse in vitro. The dyes were injected subdurally, intravascularly, or into the central canal. This technique results in rapid and extensive labeling of neurons and their processes at all depths of the spinal cord, over a rostrocaudal extent determined by the position and size of the electrodes. Our results suggest that vascular distribution of the dye is involved in all three types of injections. Electroporation disrupts local reflex and network function only transiently (approximately 1 h), after which time they recover. We describe applications of the method to image activity of neuronal populations and individual neurons during antidromic, reflex, and locomotor-like behaviors. We show that these different motor behaviors are characterized by distinct patterns of activation among the labeled populations of cells.

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.

An approach to electrical modeling of single and multiple cells

Previous theoretical approaches to understanding effects of electric fields on cells have used partial differential equations such as Laplace's equation and cell models with simple shapes. Here we describe a transport lattice method illustrated by a didactic multicellular system model with irregular shapes. Each elementary membrane region includes local models for passive membrane resistance and capacitance, nonlinear active sources of the resting potential, and a hysteretic model of electroporation. Field amplification through current or voltage concentration changes with frequency, exhibiting significant spatial heterogeneity until the microwave range is reached, where cellular structure becomes almost "electrically invisible." In the time domain, membrane electroporation exhibits significant heterogeneity but occurs mostly at invaginations and cell layers with tight junctions. Such results involve emergent behavior and emphasize the importance of using multicellular models for understanding tissue-level electric field effects in higher organisms.

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.

Functional screening of intracellular proteins in single cells and in patterned cell arrays using electroporation

A tool for detection and characterization of intracellular enzyme-substrate and receptor-ligand interactions inside the cytoplasm of single targeted cells or small confined groups of cells is presented. Fluorogenic enzyme substrates and receptor ligands were rapidly delivered by electroosmosis and internalized by electroporation in cells using an electrolyte-filled capillary (EFC) biased at a high voltage. Specifically, alkaline phosphatase and proteases were detected in single NG108-15 cells using fluorescein diphosphate and casein BODIPY FL, respectively. The intracellular 1,4,5-inositol triphosphate (IP3) and ryanodine receptors were detected after EFC introduction of the selective receptor agonists IP3 and cyclic adenosine diphosphate ribose (cADPr), respectively. Receptor activation in both cases resulted in increased cytosolic concentrations of free calcium ions that were measured using the calcium-ion-selective probe, fluo-3. The effect of cADPr could be blocked by coadministration of the ryanodine receptor antagonist ruthenium red. Furthermore, electroporation of a plurality of cells grown in microwell structures (100 x 100 x 45 microm) molded in PDMS is demonstrated. The methods and systems described using an EFC for electroporation and delivery of protein markers, ligands, and substrates might be useful in high-throughput screening of intracellular targets, with applications in proteomics and phenotype profiling, as well as in drug discovery.

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.

Selective introduction of antisense oligonucleotides into single adult CNS progenitor cells using electroporation demonstrates the requirement of STAT3 activation for CNTF-induced gliogenesis

We have developed a novel method in which antisense DNA is selectively electroporated into individual adult neural progenitor cells. By electroporation of antisense oligonucleotides against signal transducer and activator of transcription 3 (STAT3) we demonstrate that ciliary neurotrophic factor (CNTF) is an instructive signal for astroglial type 2 cell fate specifically mediated via activation of STAT3. Activation of the mitogen-activated protein kinase (MAPK) signaling pathway induced only a transient increase in glial fibrillary acidic protein (GFAP) expression, and inhibition of this signaling pathway did not block the induction by CNTF of glial differentiation in progenitor cells. In addition we show that microelectroporation is a new powerful method for introducing antisense agents into single cells in complex cellular networks.

Nanoengineered structures for holding and manipulating liposomes and cells

We describe the fabrication of nanoengineered holding pipets with concave seating surfaces and fine pressure control. These pipets were shown to exhibit exceptional stability in capturing, transporting, and releasing single cells and liposomes 1-12 microm in diameter, which opens previously inaccessible avenues of research. Three specific examples demonstrated the utility and versatility of this manipulation system. In the first, carboxyrhodamine was selectively incorporated into individual cells by electroporation, after which nearly all the medium (hundreds of microliters) surrounding the docked and tagged cells was rapidly exchanged (in seconds) and the cells were subsequently probed by laser-induced fluorescence (LIF). In the second study, a single liposome containing carboxyrhodamine was transported to a dye-free solution using a transfer pipet, docked to a holding pipet, and held firmly during physical agitation and interrogation by LIF. In the third study, pairs of liposomes were positioned between two microelectrodes, held in contact, and selectively electrofused and the resulting liposomes undocked intact.

Electroinjection of colloid particles and biopolymers into single unilamellar liposomes and cells for bioanalytical applications

A combined electroporation and pressure-driven microinjection method for efficient loading of biopolymers and colloidal particles into single-cell-sized unilamellar liposomes was developed. Single liposomes were positioned between a approximately 2-microm tip diameter solute-filled glass micropipet, equipped with a Pt electrode, and a 5-microm-diameter carbon fiber electrode. A transient, 1-10 ms, rectangular waveform dc voltage pulse (10-40 V/cm) was applied between the electrodes, thus focusing the electric field over the liposome. Dielectric membrane breakdown induced by the applied voltage pulse caused the micropipet tip to enter the liposome and a small volume (typically 50-500 x 10(-15) L) of fluorescein, YOYO-intercalated T7-phage DNA, 100-nm-diameter unilamellar liposomes, or fluorescent latex spheres could be injected into the intraliposomal compartment. We also demonstrate initiation of a chemical intercalation reaction between T2-phage DNA and YOYO-1 by dual injection into a single giant unilamellar liposome. The method was also successfully applied for loading of single cultured cells.

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

Electroporation of single NG108-15 cells with carbon-fiber microelectrodes was characterized by patch-clamp recordings and fluorescence microscopy. To minimize adverse capacitive charging effects, the patch-clamp pipette was sealed on the cell at a 90(o) angle with respect to the microelectrodes where the applied potential reaches a minimum. From transmembrane current responses, we determined the electric field strengths necessary for ion-permeable pore formation and investigated the kinetics of pore opening and closing as well as pore open times. From both patch-clamp and fluorescence microscopy experiments, the threshold transmembrane potentials for dielectric breakdown of NG108-15 cells, using 1-ms rectangular waveform pulses, was approximately 250 mV. The electroporation pulse preceded pore formation, and analyte entry into the cells was dictated by concentration, and membrane resting potential driving forces. By stepwise moving a cell out of the focused field while measuring the transmembrane current response during a supramaximal pulse, we show that cells at a distance of approximately 30 microm from the focused field were not permeabilized.