Electrotransformation pathways of procaryotic and eucaryotic cells: recent developments

The open reading frame 02797 from Candida tropicalis encodes a novel NADH-dependent aldehyde reductase

Owing to its high-temperature tolerance, robustness, and wide use of carbon sources, Candida tropicalis is considered a good candidate microorganism for bioconversion of lignocellulose to ethanol. It also has the intrinsic ability to in situ detoxify aldehydes derived from lignocellulosic hydrolysis. However, the aldehyde reductases that catalyze this bioconversion in C. tropicalis remain unknown. Herein, we found that the uncharacterized open reading frame (ORF), CTRG_02797, from C. tropicalis encodes a novel and broad substrate-specificity aldehyde reductase that reduces at least seven aldehydes. This enzyme strictly depended on NADH rather than NADPH as the co-factor for catalyzing the reduction reaction. Its highest affinity (K_m), maximum velocity (Vmax), catalytic rate constant (Kcat), and catalytic efficiency (Kcat/Km) were observed when reducing acetaldehyde (AA) and its enzyme activity was influenced by different concentrations of salts, metal ions, and chemical protective additives. Protein localization assay demonstrated that Ctrg_02797p was localized in the cytoplasm in C. tropicalis cells, which ensures an effective enzymatic reaction. Finally, Ctrg_02797p was grouped into the cinnamyl alcohol dehydrogenase (CADH) subfamily of the medium-chain dehydrogenase/reductase family. This research provides guidelines for exploring more uncharacterized genes with reduction activity for detoxifying aldehydes.

Ultrastructural Analysis of Vesicular Transport in Electrotransfection

Emerging evidence from various studies indicates that plasmid DNA (pDNA) is internalized by cells through an endocytosis-like process when it is used for electrotransfection. To provide morphological evidence of the process, we investigated ultrastructures in cells that were associated with the electrotransfected pDNA, using immunoelectron microscopy. The results demonstrate that four endocytic pathways are involved in the uptake of the pDNA, including caveolae- and clathrin-mediated endocytosis, macropinocytosis, and the clathrin-independent carrier/glycosylphosphatidylinositol-anchored protein-enriched early endosomal compartment (CLIC/GEEC) pathway. Among them, macropinocytosis is the most common pathway utilized by cells having various pDNA uptake capacities, and the CLIC/GEEC pathway is observed primarily in human umbilical vein endothelial cells. Quantitatively, the endocytic pathways are more active in easy-to-transfect cells than in hard-to-transfect ones. Taken together, our data provide ultrastructural evidence showing that endocytosis plays an important role in cellular uptake and intracellular transport of electrotransfected pDNA.

Modular Serial Flow Through device for pulsed electric field treatment of the liquid samples

In biotechnology, medicine, and food processing, simple and reliable methods for cell membrane permeabilization are required for drug/gene delivery into the cells or for the inactivation of undesired microorganisms. Pulsed electric field treatment is among the most promising methods enabling both aims. The drawback in current technology is controllable large volume operation. To address this challenge, we have developed an experimental setup for flow through electroporation with online regulation of the flow rate with feedback control. We have designed a modular serial flow-through co-linear chamber with a smooth inner surface, the uniform cross-section geometry through the majority of the system’s length, and the mesh in contact with the electrodes, which provides uniform electric field distribution and fluid velocity equilibration. The cylindrical cross-section of the chamber prevents arching at the active treatment region. We used mathematical modeling for the evaluation of electric field distribution and the flow profile in the active region. The system was tested for the inactivation of Escherichia coli. We compared two flow-through chambers and used a static chamber as a reference. The experiments were performed under identical experimental condition (product and similar process parameters). The data were analyzed in terms of inactivation efficiency and specific energy consumption.

Distinct effects of endosomal escape and inhibition of endosomal trafficking on gene delivery via electrotransfection

A recent theory suggests that endocytosis is involved in uptake and intracellular transport of electrotransfected plasmid DNA (pDNA). The goal of the current study was to understand if approaches used previously to improve endocytosis of gene delivery vectors could be applied to enhancing electrotransfection efficiency (eTE). Results from the study showed that photochemically induced endosomal escape, which could increase poly-L-lysine (PLL)-mediated gene delivery, decreased eTE. The decrease could not be blocked by treatment of cells with endonuclease inhibitors (aurintricarboxylic acid and zinc ion) or antioxidants (L-glutamine and ascorbic acid). Chemical treatment of cells with an endosomal trafficking inhibitor that blocks endosome progression, bafilomycin A1, resulted in a significant decrease in eTE. However, treatment of cells with lysosomotropic agents (chloroquine and ammonium chloride) had little effects on eTE. These data suggested that endosomes played important roles in protecting and intracellular trafficking of electrotransfected pDNA.

Improvement in Electrotransfection of Cells Using Carbon-Based Electrodes

Electrotransfection has been widely used as a versatile, non-viral method for gene delivery. However, electrotransfection efficiency (eTE) is still low and unstable, compared to viral methods. To understand potential mechanisms of the unstable eTE, we investigated effects of electrode materials on eTE and viability of mammalian cells. Data from the study showed that commonly used metal electrodes generated a significant amount of particles during application of pulsed electric field, which could cause precipitation of plasmid DNA from solutions, thereby reducing eTE. For aluminum electrodes, the particles were composed of aluminum hydroxide and/or aluminum oxide, and their median sizes were 300 to 400 nm after the buffer being pulsed 4 to 8 times at 400 V cm^-1, 5 ms duration and 1 Hz frequency. The precipitation could be prevented by using carbon (graphite) electrodes in electrotransfection experiments. The use of carbon electrodes also increased cell viability. Taken together, the study suggested that electrodes made of inner materials were desirable for electrotransfection of cells in vitro.

Membrane binding of plasmid DNA and endocytic pathways are involved in electrotransfection of mammalian cells

Electric field mediated gene delivery or electrotransfection is a widely used method in various studies ranging from basic cell biology research to clinical gene therapy. Yet, mechanisms of electrotransfection are still controversial. To this end, we investigated the dependence of electrotransfection efficiency (eTE) on binding of plasmid DNA (pDNA) to plasma membrane and how treatment of cells with three endocytic inhibitors (chlorpromazine, genistein, dynasore) or silencing of dynamin expression with specific, small interfering RNA (siRNA) would affect the eTE. Our data demonstrated that the presence of divalent cations (Ca(2+) and Mg(2+)) in electrotransfection buffer enhanced pDNA adsorption to cell membrane and consequently, this enhanced adsorption led to an increase in eTE, up to a certain threshold concentration for each cation. Trypsin treatment of cells at 10 min post electrotransfection stripped off membrane-bound pDNA and resulted in a significant reduction in eTE, indicating that the time period for complete cellular uptake of pDNA (between 10 and 40 min) far exceeded the lifetime of electric field-induced transient pores (∼10 msec) in the cell membrane. Furthermore, treatment of cells with the siRNA and all three pharmacological inhibitors yielded substantial and statistically significant reductions in the eTE. These findings suggest that electrotransfection depends on two mechanisms: (i) binding of pDNA to cell membrane and (ii) endocytosis of membrane-bound pDNA.

The actin cytoskeleton has an active role in the electrotransfer of plasmid DNA in mammalian cells

Electrotransfer of molecules is a well established technique which finds extensive use for gene transfer and holds great promise for anticancer treatment. Despite its widespread application, the mechanisms governing the entry of DNA into the cell and its intracellular trafficking are not yet known. The aim of this study is to unravel the role of the actin cytoskeleton during gene electrotransfer in cells. We performed single-cell level approaches to observe the organization of the actin cytoskeleton in Chinese hamster ovary (CHO) cells. In addition, we performed experiments at the multiple-cell level to evaluate the efficiency of DNA transfer after alteration of the actin cytoskeleton using the drug latrunculin B. Actin patches colocalizing with the DNA at the plasma membrane were observed with additional characteristics similar to those of the DNA aggregates in terms of time, number, and size. The disruption of the microfilaments reduces the DNA accumulation at the plasma membrane and the gene expression. This is the first direct experimental evidence of the participation of the actin cytoskeleton in DNA electrotransfer.

Electro-mediated gene transfer and expression are controlled by the life-time of DNA/membrane complex formation

BACKGROUND

Electroporation is a physical method used to transfer molecules into cells and tissues. Clinical applications have been developed for antitumor drug delivery. Clinical trials of gene electrotransfer are under investigation. However, knowledge about how DNA enters cells is not complete. By contrast to small molecules that have direct access to the cytoplasm, DNA forms a long lived complex with the plasma membrane and is transferred into the cytoplasm with a considerable delay.

METHODS

To increase our understanding of the key step of DNA/membrane complex formation, we investigated the dependence of DNA/membrane interaction and gene expression on electric pulse polarity and repetition frequency.

RESULTS

We observed that both are affected by reversing the polarity and by increasing the repetition frequency of pulses. The results obtained in the present study reveal the existence of two classes of DNA/membrane interaction: (i) a metastable DNA/membrane complex from which DNA can leave and return to external medium and (ii) a stable DNA/membrane complex, where DNA cannot be removed, even by applying electric pulses of reversed polarity. Only DNA belonging to the second class leads to effective gene expression.

CONCLUSIONS

The life-time of DNA/membrane complex formation is of the order of 1 s and has to be taken into account to improve protocols of electro-mediated gene delivery.

Gene electrotransfer: from biophysical mechanisms to in vivo applications : Part 1- Biophysical mechanisms

Electropulsation is one of the nonviral methods successfully used to deliver genes into living cells in vitro and in vivo. This approach shows promise in the field of gene and cellular therapies. The present review focuses on the processes supporting gene electrotransfer in vitro. In the first part, we will report the events occurring before, during, and after pulse application in the specific field of plasmid DNA electrotransfer at the cell level. A critical discussion of the present theoretical considerations about membrane electropermeabilization and the transient structures involved in the plasmid uptake follows in a second part.

Metabolic state and cell cycle as determinants of facilitated uptake of genetic information by yeast Saccharomyces cerevisiae

The transformation efficiency of yeast cells during exponential growth might be characterised as undulatory. The aim of the study was to investigate the reason for the fluctuation in transformation efficiency of yeast Saccharomyces cerevisiae p63-DC5 cells during exponential growth. The heightened response to exogenous DNA was observed with the growing yeast culture when budded cells were predominant. To confirm this phenomenon we carried out synchronization of yeast cells with 10 mM hydroxyurea. Results showed that synchronous yeast cells in the S-phase of cell cycle have enhanced transformation efficiency. Furthermore, S. cerevisiae p63-DC5 cells in the S-phase were successfully transformed with plasmid pl13 in the absence of lithium acetate. We indicated that the permeability of yeast cells in the S-phase to tetraphenylphosphonium (TPP) cations was significantly higher than in asynchronous culture. The results of our study showed that the fluctuation in transformation efficiency was strictly dependent on the metabolic state of yeast cells and the capacity of the yeast cells to become competent was related to the S-phase of cell cycle.

Nucleofection-BasedEx VivoNonviral Gene Delivery to Human Stem Cells as a Platform for Tissue Regeneration

There are several gene therapy approaches to tissue regeneration. Although usually efficient, virusbased approaches may elicit an immune response against the viral proteins. An alternative approach, nonviral transfer, is safer, and can be controlled and reproduced. We hypothesized that in vivo bone formation could be achieved using human mesenchymal stem cells (hMSCs) nonvirally transfected with the human bone morphogenetic protein-2 (hBMP-2) or -9 (hBMP-9) gene. Human MSCs were transfected using nucleofection, a unique electropermeabilization-based technique. Postnucleofection, cell viability was 53.6 +/- 2.5% and gene delivery efficiency was 51% to 88% (mean 68.2 +/- 4.1%), as demonstrated by flow cytometry in enhanced green fluorescent protein (EGFP)-nucleofected hMSCs. Transgene expression lasted longer than 14 days and was very low 21 days postnucleofection. Both hBMP-2- and hBMP-9-nucleofected hMSCs in culture demonstrated a significant increase in calcium deposition compared with EGFP-nucleofected hMSCs. Human BMP-2- and hBMP-9-nucleofected hMSCs transplanted in ectopic sites in NOD/SCID mice induced bone formation 4 weeks postinjection. We conclude that in vivo bone formation can be achieved by using nonvirally nucleofected hMSCs. This could lead to a breakthrough in the field of regenerative medicine, in which safer, nonviral therapeutic strategies present a very attractive alternative.

Cell synchronization effect on mammalian cell permeabilization and gene delivery by electric field

Electropermeabilization is a promising nonviral method for gene therapy. However, despite the fact that it is widely used to transfer genes into living cells, the steps that limit DNA transfer remain to be determined. Here, we report the effect of cell synchronization on membrane permeabilization and gene delivery by electric fields. Chinese hamster ovary (CHO) cells were synchronized by aphidicolin or butyrate treatment. Electro-mediated transfection of these cells was evaluated under electric field conditions leading to the same level of membrane permeabilization. Aphidicolin cell synchronization in G2/M phase leads to a slight increase in plasma membrane permeabilization but to a three-fold increase in percentage of transfected cells and to an eight-fold increase in gene expression. This increase in cell transfection is specifically due to the G2/M synchronization process. Indeed, cell synchronization in G1 phase by sodium butyrate has no effect on cell permeabilization and transfection. Our results suggest that the enhanced transfection level in G2/M phase is not simply due to enhanced permeabilization, but reinforce the statement that the melting of the nuclear membrane facilitates direct access of plasmid DNA to the nucleus.

Direct visualization at the single-cell level of electrically mediated gene delivery

Electropermeabilization is one of the nonviral methods successfully used to transfer genes into living cells in vitro and in vivo. Although this approach shows promise in the field of gene therapy, very little is known about the basic processes supporting DNA transfer. The present investigation studies this process at the single-cell level by using digitized fluorescence microscopy. Permeabilization is a prerequisite for gene transfer. Its assay by propidium-iodide (PI) penetration shows that it occurs at the sides of the cell membrane facing the two electrodes, whereas fluorescently labeled plasmids only interact with the electropermeabilized side of the cell facing the cathode. The plasmid interaction with the electropermeabilized part of the cell surface results in the formation of localized aggregates. These membrane-associated spots are formed only when pulses with a longer duration than a critical value are applied. These complexes are formed within 1 s after the pulses and cannot be destroyed by pulses of reversed polarities. They remain at the membrane level up to 10 min after pulsing. Although freely accessible to DNA dye (TOTO-1) 1 min after the pulses, they are fully protected when the addition takes place 10 min after. They diffuse in the cytoplasm 30 min after pulses and are present around the nucleus 24 h later.

Therapeutic perspectives of in vivo cell electropermeabilization

Cell electropermeabilization (also termed cell electroporation) is nowadays a routine technique used in biochemical and pharmacological studies for the in vitro introduction of nonpermeant molecules into living cells. But electric pulses can be used as well in vivo for the delivery of drugs or DNA into cells of tissues. This review then gives an updated overview of the therapeutic perspectives of cell electropermeabilization in vivo, in particular of the antitumour electrochemotherapy (i.e., the combination of a cytotoxic nonpermeant drug with permeabilizing electric pulses delivered to the tumours) and of in vivo DNA electrotransfer for gene therapy. After a short summary of the present knowledge on cell electropermeabilization (particularly in vivo), the basis, the present achievements, and the challenges of electrochemotherapy are described and discussed, which includes an overview of still open questions and an update on recent clinical trials. DNA electrotransfer for gene therapy is an emerging field in which results are rapidly accumulating. Present knowledge on DNA electrotransfer mechanisms, as wel as the potentialities of DNA electrotransfer to become an efficient non-viral approach for gene therapy, are reviewed.

Control by membrane order of voltage-induced permeabilization, loading and gene transfer in mammalian cells

Cells can be transiently permeabilized by application of electric pulses. A direct consequence of this treatment is to create a new state in the membrane leading to DNA and protein transfers. A key step, in the interaction between macromolecules and the electropermeabilized membrane, is involved. We previously reported that membrane and DNA associated hydration and undulation forces appeared to be involved in this process by studying the effects of osmotic pressure. Effects of ethanol (EtOH) and L-alpha-lysophosphatidylcholine (lyso-PC), molecules known to affect membrane order and therefore undulation forces, were investigated on Chinese hamster ovary (CHO) cells. We used millisecond square wave pulses, conditions giving high efficiency for gene transfer. No effect was observed on cell permeabilization for small sized molecules. Only little change on electroloading of proteins such as R-phycoerythrin was obtained in presence of EtOH. But, a decrease (increase) in electrotransfection was observed for cells treated with EtOH (lyso-PC). Under our conditions, no additional effects of the chemical treatment were observed on cell viability and on membrane resealing. These results tentatively explained in terms of the effect of membrane order on membrane organization and interaction between molecules and membrane supports the existence of the plasmid-membrane interaction in the mechanism of electrically mediated gene transfer.

Importance of association between permeabilization and electrophoretic forces for intramuscular DNA electrotransfer

Gene transfer using electrical pulses is a rapidly expanding field. Many studies have been performed in vitro to elucidate the mechanism of DNA electrotransfer. In vivo, the use of efficient procedures for DNA electrotransfer in tissues is recent, and the question of the implied mechanisms is largely open. We have evaluated the effects of various combinations of square wave electric pulses of variable field strength and duration, on cell permeabilization and on DNA transfection in the skeletal muscle in vivo. One high voltage pulse of 800 V/cm, 0.1 ms duration (short high pulse) or a series of four low voltage pulses of 80 V/cm, 83 ms duration (long low pulses) slightly amplified transfection efficacy, while no significant permeabilization was detected using the (51)Cr-EDTA uptake test. By contrast, the combination of one short high pulse followed by four long low pulses led to optimal gene transfer efficiency, while inducing muscle fibers permeabilization. These results are consistent with additive effects of electropermeabilization and DNA electrophoresis on electrotransfer efficiency. Finally, the described new combination, as compared to the previously reported use of repeated identical pulses of intermediate voltage, leads to similar gene transfer efficiency, while causing less permeabilization and thus being likely less deleterious. Thus, combination of pulses of various strengths and durations is a new procedure for skeletal muscle gene transfer that may represents a clear improvement in view of further clinical development.

The generation of reactive-oxygen species associated with long-lasting pulse-induced electropermeabilisation of mammalian cells is based on a non-destructive alteration of the plasma membrane

Chinese hamster ovary (CHO) cells in suspension were subjected to pulsed electric fields suitable for electrically mediated gene transfer (pulse duration longer than 1 ms). Using the chemiluminescence probe lucigenin, we showed that a generation of reactive-oxygen species (oxidative jump) was present when the cells were electropermeabilised using millisecond pulses. The oxidative jump yield was controlled by the extent of alterations allowing permeabilisation within the electrically affected cell area, but showed a saturating dependence on the pulse duration over 1 ms. Cell electropulsation induced reversible and irreversible alterations of the membrane assembly. The oxidative stress was only present when the membrane permeabilisation was reversible. Irreversible electrical membrane disruption inhibited the oxidative jump. The oxidative jump was not a simple feedback effect of membrane electropermeabilisation. It strongly controlled long-term cell survival. This had to be associated with the cell-damaging action of reactive-oxygen species. However, for millisecond-cumulated pulse duration, an accumulation of a large number of short pulses (microsecond) was extremely lethal for cells, while no correlation with an increased oxidative jump was found. Cell responses, such as the production of free radicals, were present during electropermeabilisation of living cells and controlled partially the long-term behaviour of the pulsed cell.