Control of the release of freely diffusing molecules in single-cell electroporation

Connecting the in vitro and in vivo experiments in electrochemotherapy - a feasibility study modeling cisplatin transport in mouse melanoma using the dual-porosity model

In electrochemotherapy two conditions have to be met to be successful - the electric field of sufficient amplitude and sufficient uptake of chemotherapeutics in the tumor. Current treatment plans only take into account critical electric field to achieve cell membrane permeabilization. However, permeabilization alone does not guarantee uptake of chemotherapeutics and consequently successful treatment. We performed a feasibility study to determine whether the transport of cisplatin in vivo could be calculated based on experiments performed in vitro. In vitro, a spectrum of parameters can be explored without ethical issues. Mouse melanoma B16-F1 cell suspension and inoculated B16-F10 tumors were exposed to electric pulses in the presence of chemotherapeutic cisplatin. The uptake of cisplatin was measured by inductively coupled plasma mass spectrometry. We modeled the transport of cisplatin with the dual-porosity model, which is based on the diffusion equation, connects pore formation with membrane permeability, and includes transport between several compartments. In our case, there were three compartments - tumor cells, interstitial fraction and peritumoral region. Our hypothesis was that in vitro permeability coefficient could be introduced in vivo, as long as tumor physiology was taken into account. Our hypothesis was confirmed as the connection of in vitro and in vivo experiments was possible by introducing a transformation coefficient which took into account the in vivo characteristics, i.e., smaller available area of the plasma membrane for transport due to cell density, presence of cell-matrix in vivo, and reduced drug mobility. We thus show that it is possible to connect in vitro and in vivo experiments of electrochemotherapy. However, more experimental work is required for model validation.

Low-Voltage Flow-Through Electroporation in Gold-Microtube Membranes

Electroporation is used to create pores within the membrane of living cells in order to deliver a substance, for example, a gene, into the cytoplasm. To achieve the high electric field gradients required to porate the membrane, current electroporation devices deliver voltage pulses in the kV range to the cell medium. We describe a new device based on gold-microtube membranes that can accomplish electroporation with voltage pulses that are orders of magnitude smaller, ≤5 V. This is possible because the voltage pulses are applied to the gold microtubes resulting in large electric field gradients down the length of the tubes. We used COMSOL simulations to calculate the electric field gradients, and these theoretical results were compared with known experimental values required to electroporate Escherichia coli. We developed two fluorescence-based methods to demonstrate successful electroporation of E. coli. The percentages of electroporated bacteria were found to be more than an order of magnitude higher than obtained with a commercial electroporator, although the voltage employed was 500 times lower. Furthermore, this microtube membrane device is flow through and is therefore capable of continuous, as opposed to batch-wise, electroporation and cell analysis. Cell throughput of >30 million cells per min, higher than any previously reported device, were obtained.

DERIVING AN ELECTRIC CIRCUIT EQUIVALENT MODEL OF CELL MEMBRANE PORES IN ELECTROPORATION

Electroporation is the formation of reversible pores in cell membranes under a brief pulse of high electric field. Dynamics of pore formation during electroporation suggests that the transmembrane potential would settle approximately at the threshold transmembrane potential and the transmembrane resistance would decrease significantly from the state of relaxation. The current electric circuit equivalent models for electroporation containing time-invariant, static and passive components are unable to capture the pore dynamics. A biophysically-inspired electric circuit equivalent model containing dynamic components for membrane pores has been derived using biological parameters. The model contains a voltage-controlled resistor driven by a two-stage cascaded integrator that is activated through a voltage-gated switch. Simulation results with the derived model showed higher accuracy compared to a commonly used model, where the transmembrane resistance decreased million-fold at the onset of electroporation and the transmembrane potential settled at 99.5% of the critical transmembrane potential, thus enabling improved dynamic behavior modeling ability of the pores in electroporation. The derived model allows fast and reliable analysis of this biophysical phenomenon and potentially aids in optimization of various parameters involved in electroporation.

Low-frequency ac electroporation shows strong frequency dependence and yields comparable transfection results to dc electroporation

Conventional electroporation has been conducted by employing short direct current (dc) pulses for delivery of macromolecules such as DNA into cells. The use of alternating current (ac) field for electroporation has mostly been explored in the frequency range of 10kHz-1MHz. Based on Schwan equation, it was thought that with low ac frequencies (10Hz-10kHz), the transmembrane potential does not vary with the frequency. In this report, we utilized a flow-through electroporation technique that employed continuous 10Hz-10kHz ac field (based on either sine waves or square waves) for electroporation of cells with defined duration and intensity. Our results reveal that electropermeabilization becomes weaker with increased frequency in this range. In contrast, transfection efficiency with DNA reaches its maximum at medium frequencies (100-1000Hz) in the range. We postulate that the relationship between the transfection efficiency and the ac frequency is determined by combined effects from electrophoretic movement of DNA in the ac field, dependence of the DNA/membrane interaction on the ac frequency, and variation of transfection under different electropermeabilization intensities. The fact that ac electroporation in this frequency range yields high efficiency for transfection (up to ~71% for Chinese hamster ovary cells) and permeabilization suggests its potential for gene delivery.

Kinetics of NF-κB nucleocytoplasmic transport probed by single-cell screening without imaging

Transport of protein and RNA cargoes between the nucleus and cytoplasm (nucleocytoplasmic transport) is vital for a variety of cellular functions. The studies of kinetics involved in such processes have been hindered by the lack of quantitative tools for measurement of the nuclear and cytosolic fractions of an intracellular protein at the single cell level for a cell population. In this report, we describe using a novel method, microfluidic electroporative flow cytometry, to study kinetics of nucleocytoplasmic transport of an important transcription factor NF-κB. With data collected from single cells, we quantitatively characterize the population-averaged kinetic parameters such as the rate constants and apparent activation barrier for NF-κB transport. Our data demonstrate that NF-κB nucleocytoplasmic transport fits first-order kinetics very well and is a fairly reversible process governed by equilibrium thermodynamics.

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.