Study of mechanisms of electric field-induced DNA transfection. IV. Effects of DNA topology on cell uptake and transfection efficiency

The impact of impaired DNA mobility on gene electrotransfer efficiency: analysis in 3D model

Background

Gene electrotransfer is an established method that enables transfer of DNA into cells with electric pulses. Several studies analyzed and optimized different parameters of gene electrotransfer, however, one of main obstacles toward efficient electrotransfection in vivo is relatively poor DNA mobility in tissues. Our aim was to analyze the effect of impaired mobility on gene electrotransfer efficiency experimentally and theoretically. We applied electric pulses with different durations on plated cells, cells grown on collagen layer and cells embedded in collagen gel (3D model) and analyzed gene electrotransfer efficiency. In order to analyze the effect of impaired mobility on gene electrotransfer efficiency, we applied electric pulses with different durations on plated cells, cells grown on collagen layer and cells embedded in collagen gel (3D model) and analyzed gene electrotransfer efficiency.

Results

We obtained the highest transfection in plated cells, while transfection efficiency of embedded cells in 3D model was lowest, similarly as in in vivo. To further analyze DNA diffusion in 3D model, we applied DNA on top or injected it into 3D model and showed, that for the former gene electrotransfer efficiency was similarly as in in vivo. The experimental results are explained with theoretical analysis of DNA diffusion and electromobility.

Conclusion

We show, empirically and theoretically that DNA has impaired electromobility and especially diffusion in collagen environment, where the latter crucially limits electrotransfection. Our model enables optimization of gene electrotransfer in in vitro conditions.

PASSPORT-seq: A Novel High-Throughput Bioassay to Functionally Test Polymorphisms in Micro-RNA Target Sites

Next-generation sequencing (NGS) studies have identified large numbers of genetic variants that are predicted to alter miRNA–mRNA interactions. We developed a novel high-throughput bioassay, PASSPORT-seq, that can functionally test in parallel 100s of these variants in miRNA binding sites (mirSNPs). The results are highly reproducible across both technical and biological replicates. The utility of the bioassay was demonstrated by testing 100 mirSNPs in HEK293, HepG2, and HeLa cells. The results of several of the variants were validated in all three cell lines using traditional individual luciferase assays. Fifty-five mirSNPs were functional in at least one of three cell lines (FDR ≤ 0.05); 11, 36, and 27 of them were functional in HEK293, HepG2, and HeLa cells, respectively. Only four of the variants were functional in all three cell lines, which demonstrates the cell-type specific effects of mirSNPs and the importance of testing the mirSNPs in multiple cell lines. Using PASSPORT-seq, we functionally tested 111 variants in the 3′ UTR of 17 pharmacogenes that are predicted to alter miRNA regulation. Thirty-three of the variants tested were functional in at least one cell line.

Development of the Multiple Gene Knockout System with One-Step PCR in Thermoacidophilic Crenarchaeon Sulfolobus acidocaldarius

Multiple gene knockout systems developed in the thermoacidophilic crenarchaeon Sulfolobus acidocaldarius are powerful genetic tools. However, plasmid construction typically requires several steps. Alternatively, PCR tailing for high-throughput gene disruption was also developed in S. acidocaldarius, but repeated gene knockout based on PCR tailing has been limited due to lack of a genetic marker system. In this study, we demonstrated efficient homologous recombination frequency (2.8 × 10⁴ ± 6.9 × 10³ colonies/μg DNA) by optimizing the transformation conditions. This optimized protocol allowed to develop reliable gene knockout via double crossover using short homologous arms and to establish the multiple gene knockout system with one-step PCR (MONSTER). In the MONSTER, a multiple gene knockout cassette was simply and rapidly constructed by one-step PCR without plasmid construction, and the PCR product can be immediately used for target gene deletion. As an example of the applications of this strategy, we successfully made a DNA photolyase- (phr-) and arginine decarboxylase- (argD-) deficient strain of S. acidocaldarius. In addition, an agmatine selection system consisting of an agmatine-auxotrophic strain and argD marker was also established. The MONSTER provides an alternative strategy that enables the very simple construction of multiple gene knockout cassettes for genetic studies in S. acidocaldarius.

Gene Electrotransfer: A Mechanistic Perspective

Gene electrotransfer is a powerful method of DNA delivery offering several medical applications, among the most promising of which are DNA vaccination and gene therapy for cancer treatment. Electroporation entails the application of electric fields to cells which then experience a local and transient change of membrane permeability. Although gene electrotransfer has been extensively studied in in vitro and in vivo environments, the mechanisms by which DNA enters and navigates through cells are not fully understood. Here we present a comprehensive review of the body of knowledge concerning gene electrotransfer that has been accumulated over the last three decades. For that purpose, after briefly reviewing the medical applications that gene electrotransfer can provide, we outline membrane electropermeabilization, a key process for the delivery of DNA and smaller molecules. Since gene electrotransfer is a multipart process, we proceed our review in describing step by step our current understanding, with particular emphasis on DNA internalization and intracellular trafficking. Finally, we turn our attention to in vivo testing and methodology for gene electrotransfer.

Calcium phosphate embedded PLGA nanoparticles: a promising gene delivery vector with high gene loading and transfection efficiency

In the purpose of increasing incorporation efficiency and improving the release kinetics of plasmid DNA (pDNA) from poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles, a facile method for the fabrication of calcium phosphate (CaPi) embedded PLGA nanoparticles (CaPi-pDNA-PLGA-NPs) was developed. The effect of several preparation factors on the particle size, incorporation efficiency, pDNA release and transfection efficiency in vitro was studied by Single Factor Screening Method. These preparation factors included the molecular weight (MW), hydrolysis degree (HD) of polyvinyl alcohol (PVA), sonication power and time, composition of organic phase, initial concentration of calcium phosphate and calcium (Ca) to phosphate ion (P) ratio (Ca/P ratio), etc. The CaPi-pDNA-PLGA-NPs made according to the optimal formulation were spherical in shape observed by transmission electron microscopy (TEM) with a mean particle size of 207±5 nm and an entrapment efficiency of 95.7±0.8%. Differential scanning calorimetry (DSC) suggested that there existed interaction between the DNA-calcium-phosphate (CaPi-pDNA) complexes and the polymeric matrices of PLGA. X-ray diffractometry (XRD) further proved the conclusion and indicated that the CaPi-pDNA was in weak crystallization form inside the nanoparticles. The Brunauer-Emmett-Teller (BET) surface area measurement demonstrated that the CaPi-pDNA-PLGA-NPs are mesoporous with specific surface area of 57.5m(2)/g and an average pore size of 96.5 Å. The transfection efficiency of the CaPi-pDNA-PLGA-NPs on human embryonic kidney 293 (HEK 293) cells in vitro was 22.4±1.2%, which was much higher than those of both the pDNA loaded PLGA nanoparticles (pDNA-PLGA-NPs) and the CaPi-pDNA embedded PLGA microparticles (CaPi-pDNA-PLGA-MPs). The CaPi-pDNA-PLGA-NPs are promising vectors for gene delivery.

Changing the direction and orientation of electric field during electric pulses application improves plasmid gene transfer in vitro

Gene electrotransfer is a physical method used to deliver genes into the cells by application of short and intense electric pulses, which cause destabilization of cell membrane, making it permeable to small molecules and allows transfer of large molecules such as DNA. It represents an alternative to viral vectors, due to its safety, efficacy and ease of application. For gene electrotransfer different electric pulse protocols are used in order to achieve maximum gene transfection, one of them is changing the electric field direction and orientation during the pulse delivery. Changing electric field direction and orientation increase the membrane area competent for DNA entry into the cell. In this video, we demonstrate the difference in gene electrotransfer efficacy when all pulses are delivered in the same direction and when pulses are delivered by changing alternatively the electric field direction and orientation. For this purpose tip with integrated electrodes and high-voltage prototype generator, which allows changing of electric field in different directions during electric pulse application, were used. Gene electrotransfer efficacy is determined 24h after pulse application as the number of cells expressing green fluorescent protein divided with the number of all cells. The results show that gene transfection is increased when the electric field orientation during electric pulse delivery is changed.

Transformation of Kluyveromyces lactis by Electroporation

The physical and biological parameters involved in efficient transformation of Kluyveromyces lactis by electroporation have been analyzed. By using an optimum voltage and a constant volume of cell suspension in a cuvette, the efficiency of transformation increased with increases in cell numbers and plasmid concentration. However, the most important parameter was the time of the pulse. Changes of 1 ms decreased the efficiency of transformation more than 70 to 80%. Under our best conditions, between 10 and 10 transformants per mug of plasmid DNA could be obtained. Under certain conditions, the size of the plasmid also affected electroporation efficiency. In any case, we did not obtain integrative transformation with an autonomously replicating plasmid.

Effect of Mg ions on efficiency of gene electrotransfer and on cell electropermeabilization

Gene electrotransfer is a promising nonviral method that enables DNA to be transferred into living cells with electric pulses. However, there are many parameters that determine gene electrotransfer efficiency. One of the steps involved in gene electrotransfer is interaction of DNA with the cell membrane. Divalent cations in the electroporative media can influence the anchoring of DNA to the cell membrane and by that gene electrotransfer efficiency. Here we report the effect of different concentrations of Mg2+ on electropermeabilization for small molecule (propidium iodide), gene electrotransfer and viability of the cells. We also used TOTO-1 dye to visualize DNA-cell membrane interaction for different [Mg]. For this purpose, we used different electroporative media with increasing [Mg]. Our study shows that higher [Mg] lead to higher electropermeabilization for propidium iodide and higher viability, while causing lower gene electrotransfer efficiency. Because we observed higher TOTO-1 labeled DNA at cell surface when using higher [Mg], we suggest that Mg2+ ions can bind DNA at cell surface at such strength that cannot pass into the cell during application of electric pulses, which can lead to lower gene transfection. There may also be other mechanisms involved, since there are many steps of gene electrotransfer on which Mg2+ ions can have an effect on. Our results also imply that membrane permeability changes are not sufficient for an efficient gene electrotransfer.

Formulating poly(lactide-co-glycolide) particles for plasmid DNA delivery

Biodegradable poly(lactide-co-glycolide) (PLGA) particles have shown significant potential for sustained and targeted delivery of several pharmaceutical agents, including plasmid DNA (pDNA). Here, we survey current approaches to PLGA particle preparation for pDNA delivery and discuss recent progress on optimizing formulation development.

Separation of topological forms of plasmid DNA by anion-exchange HPLC: Shifts in elution order of linear DNA

We sought to establish a single anion-exchange HPLC method for the separation of linear, open circular and supercoiled plasmid topoisomers using purified topoisomeric forms of three plasmids (3.0, 5.5 and 7.6 kb). However, finding one condition proved elusive as the topoisomer elution order was determined to depend on salt gradient slope. The observed change in selectivity increased with plasmid size and was most pronounced for the linear form. Indeed, the elution order of the linear 7.6 kb plasmid was reversed relative to the supercoiled form. This observation may have implications for methods used in quality control of plasmid DNA.

Versatility of biodegradable poly(d,l-lactic-co-glycolic acid) microspheres for plasmid DNA delivery

In this study, we have optimized different formulations of DNA encapsulated into PLGA microspheres by correlating the protocol of preparation and the molecular weight and composition of the polymer, with the main characteristics of these systems in order to design an efficient non-viral gene delivery vector. For that, we prepared poly(D,L-lactic-co-glycolic acid) (PLGA) microparticles with an optimized water-oil-water double emulsion process, by using several types of polymers (RG502, RG503, RG504, RG502H and RG752), and characterized in terms of size, zeta potential, encapsulation efficiency (EE%), morphology, DNA conformation, release kinetics, plasmid integrity and erosion. The size of the particles ranged between 0.7 and 5.7 microm depending on the protocol of formulation and the molecular mass of the polymer used. The microspheres prepared by using in their formulation polymers of high molecular weight (RG503 and RG504) were bigger in size than in the case of using a lower molecular weight polymer (RG502). The EE (%) of plasmid DNA increased with increasing the molecular mass of the polymer and by using the most hydrophilic polymer RG502H, which contains terminal acidic groups in its structure. The plasmid could be encapsulated without compromising its structural and functional integrity. Also a protective effect of PLGA on endonuclease digestion is observed. Plasmid DNA release from microspheres composed of low molecular weight or hydrophilic polymers, like RG502H, was faster than from particles containing high molecular weight or hydrophobic polymers. These PLGA microspheres could be an alternative to the viral vectors used in gene therapy, given that may be used to deliver genes and other bioactive molecules, either very rapidly or in a controlled manner.

Characterization of DNA release from composites of oligo(poly(ethylene glycol) fumarate) and cationized gelatin microspheres in vitro

This research investigates the release of plasmid DNA from novel hydrogel composites of oligo(poly(ethylene glycol) fumarate) (OPF) and cationized gelatin microspheres (CGMS), as well as the swelling and degradation of these materials in vitro. The release of total DNA and of double-stranded DNA was measured fluorescently, and the swelling properties and polymer mass loss of the hydrogels were assessed. Further, the structural integrity of the released DNA was determined through electrophoresis. It was found that plasmid DNA can be released in a sustained fashion over the course of up to 49-140 days in vitro from hydrogels of OPF synthesized from poly(ethylene glycol) of nominal molecular weights of 10 kDa and 3 kDa, respectively, with the release kinetics depending upon the material composition and the method of DNA loading. Released DNA was predominately double-stranded DNA (dsDNA) in structure and of the open-circular conformation. The results suggest that DNA release from hydrogel composites of OPF and CGMS is dominated by the degradation of the OPF component of the gels. Electrophoresis results indicate that the released DNA retains suitable conformation for potential bioactivity over the course of at least 63 days of release. Thus, these studies demonstrate the potential of composites of OPF and CGMS in controlled gene delivery applications.

In vivo release of plasmid DNA from composites of oligo(poly(ethylene glycol)fumarate) and cationized gelatin microspheres

Composites of cationized gelatin microspheres (CGMS), crosslinked with either 3 mM or 6 mM glutaraldehyde solution, and a novel hydrogel material, oligo(poly(ethylene glycol)fumarate) (OPF) were fabricated and investigated toward prolonging the release of plasmid DNA in vivo relative to the constituent materials. The composites and constituent materials were investigated in a subcutaneous murine model to assess the release of 125I-labeled plasmid DNA and 125I-labeled cationized gelatin in vivo. The time profiles of the radioactivity remaining were employed to compare the profiles of DNA release and cationized gelatin degradation. Both composite formulations (incorporating either 3 mM or 6 mM CGMS) prolonged the bioavailability of plasmid DNA relative to both injected plasmid DNA solution and the respective non-embedded cationized gelatin microspheres. Injected plasmid DNA solution persisted in the subject for only 7-10 days, whereas the persistence of DNA from composites of OPF and either 3 mM or 6 mM CGMS extended to at least day 42. The 3 mM and 6 mM CGMS each increased the persistence of DNA slightly, relative to injection of DNA solution, to between 28 and 35 days. Interestingly, the release profile of plasmid DNA from composites was not significantly different from the release of DNA from OPF alone. The release of plasmid DNA from the composites was in accord with the degradation of the microspheres within the OPF. These results show that composites of OPF and cationized gelatin microspheres are able to prolong the availability of plasmid DNA in vivo relative to cationized gelatin microspheres alone and provide a promising candidate material for the sustained, controlled release of plasmid DNA.

Transfection of a mouse dendritic cell line by plasmid DNA-loaded PLGA microparticles in vitro

Targeting of DC for DNA vaccination may be achieved by DNA-loaded poly(lactide-co-glycolide) (PLGA) biodegradable microparticles, since DC efficiently capture these microparticles in vitro and in vivo. DNA was encapsulated in PLGA microparticles by spray-drying. Various additives were tested and process parameters adjusted in order to prevent degradation of the DNA during encapsulation. The highest degree of supercoiled DNA was maintained by adding a strong buffering agent, such as PBS or NaHCO(3), whereas the cryoprotective lactose did not show a significant protective effect. DNA-containing PLGA microparticles were administered to a mouse DC line. Transfection efficacy was compared with commonly employed cationic transfectants and was visually assessed by green fluorescent protein expression. Transfection rate was very low in DC for all microparticle formulations and was comparable with commonly used cationic transfectants. It is concluded that the transfection of DC using PLGA microparticles is feasible, but efforts need to be undertaken to improve transfection efficiency in vitro, which may in addition lead to improved immune responses in vivo.

Effect of electric field vectoriality on electrically mediated gene delivery in mammalian cells

Electropermeabilization is a nonviral method used to transfer genes into living cells. Up to now, the mechanism is still to be elucidated. Since cell permeabilization, a prerequired for gene transfection, is triggerred by electric field, its characteristics should depend on its vectorial properties. The present investigation addresses the effect of pulse polarity and orientation on membrane permeabilization and gene delivery by electric pulses applied to cultured mammalian cells. This has been directly observed at the single-cell level by using digitized fluorescence microscopy. While cell permeabilization is only slightly affected by reversing the polarity of the electric pulses or by changing the orientation of pulses, transfection level increases are observed. These last effects are due to an increase in the cell membrane area where DNA interacts. Fluorescently labelled plasmids only interact with the electropermeabilized side of the cell facing the cathode. The plasmid interaction with the electropermeabilized cell surface is stable and is not affected by pulses of reversed polarities. Under such conditions, DNA interacts with the two sites of the cell facing the two electrodes. When changing both the pulse polarity and their direction, DNA interacts with the whole membrane cell surface. This is associated with a huge increase in gene expression. This present study demonstrates the relationship between the DNA/membrane surface interaction and the gene transfer efficiency, and it allows to define the experimental conditions to optimize the yield of transfection of mammalian cells.

An automated microplate-based method for monitoring DNA strand breaks in plasmids and bacterial artificial chromosomes

A method is described for high-throughput monitoring of DNA backbone integrity in plasmids and artificial chromosomes in solution. The method is based on the denaturation properties of double-stranded DNA in alkaline conditions and uses PicoGreen fluorochrome to monitor denaturation. In the present method, fluorescence enhancement of PicoGreen at pH 12.4 is normalised by its value at pH 8 to give a ratio that is proportional to the average backbone integrity of the DNA molecules in the sample. A good regression fit (r2 > 0.98) was obtained when results derived from the present method and those derived from agarose gel electrophoresis were compared. Spiking experiments indicated that the method is sensitive enough to detect a proportion of 6% (v/v) molecules with an average of less than two breaks per molecule. Under manual operation, validation parameters such as inter-assay and intra-assay variation gave values of <5% coefficient of variation. Automation of the method showed equivalence to the manual procedure with high reproducibility and low variability within wells. The method described requires as little as 0.5 ng of DNA per well and a 96-well microplate can be analysed in 12 min providing an attractive option for analysis of high molecular weight vectors. A preparation of a 116 kb bacterial artificial chromosome was subjected to chemical and shear degradation and DNA integrity was tested using the method. Good correlation was obtained between time of chemical degradation and shear rate with fluorescence response. Results obtained from pulsed- field electrophoresis of sheared samples were in agreement with those obtained using the microplate-based method.

Evaluation of the stability of polymer-based plasmid DNA delivery systems after ultrasound exposure

Under ultrasound exposure, the stability of plasmid DNA protected by polymer-based gene delivery system is an important factor for achieving optimal transfection into cells. We have evaluated the effectiveness of various polymer-based plasmid DNA delivery systems, which are interactive polymers and cationic polymers, to avoid shear degradation induced by ultrasound exposure. Alternatively, it is shown that sonication of plasmid DNA for exposure time as low as 10s resulted in total DNA fragmentation and the loss of transfection potency in NIH/3T3 cells. Among these polymer-based plasmid DNA delivery systems, only cationic polymers had the ability to provide the protection of plasmid DNA from ultrasonic degradation as indicated by the reservation in supercoiled circular (SC) and open circular (OC) forms of plasmid DNA on the agarose gel electrophoresis. The DNA stability protected by cationic polymers decreased after ultrasound exposure in 1M sodium chloride solution. Also, higher molecular weight of cationic polymers and sufficient cationic polymer/DNA weight ratios are essential to prevent DNA from degradation under ultrasound exposure in aqueous or salt solution. These results suggest that the protective mechanism by cationic polymers is due to the attractive bonding between cationic polymer and negative plasmid DNA. Whereas, DNA condensation alone provoked by the addition of polyethylene glycols was not sufficient to resist the DNA fragmentation induced by ultrasound exposure.

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.

Interference of serum with lipoplex-cell interaction: modulation of intracellular processing

We have investigated the mechanism of lipoplex-mediated transfection, employing a dialkyl pyridinium surfactant (SAINT-2), and using serum as a modulator of complex stability and processing. Particle size and stability determine lipoplex internalization, the kinetics of intracellular processing, and transfection efficiency. Clustered SAINT-2 lipoplexes are obtained in the absence of serum (-FBS lipoplexes), but not in its presence (+FBS lipoplexes), or when serum was present during lipoplex formation [FBS], conditions that mimic potential penetration of serum proteins. The topology of DNA in [FBS] lipoplexes shifts from a supercoiled, as in -FBS lipoplexes, to a predominantly open-circular conformation, and is more prone to digestion by DNase. Consistently, atomic force microscopy revealed complexes with tubular extensions, reflecting DNA that protrudes from the lipoplex surface. Interestingly, the internalization of [FBS] lipoplexes is approximately three-fold higher than that of -FBS and +FBS lipoplexes, yet their transfection efficiency is approximately five-fold lower. Moreover, in contrast to -FBS and +FBS complexes, [FBS] complexes were rapidly processed into the late endosomal/lysosomal degradation pathway. Intriguingly, transfection by [FBS] complexes is greatly improved by osmotic rupture of endocytic compartments. Our data imply that constraints in size and morphology govern the complex' ability to interact with and perturb cellular membranes, required for gene release. By extrapolation, we propose that serum may regulate these parameters in an amphiphile-dependent manner, by complex 'penetration' and modulation of DNA conformation.

Functional pleiotropy of an intramolecular triplex-forming fragment from the 3'-UTR of the rat Pigr gene

A microsatellite-containing 359-bp restriction fragment, isolated from the rat Pigr gene (murine polymeric immunoglobulin receptor gene) 3'-untranslated region (3'-UTR) and inserted into 3'-UTR or 3' flanking positions in transcription units of supercoiled plasmids, attenuates luciferase reporter gene expression in orientation- and position-dependent ways following transient transfection of human 293 cells. The same fragment stimulates orientation-dependent gene expression in a 5' flanking position. Plasmid linearization abrogates both orientation- and position-dependent responses. Cell-free translation reveals that 5' and 3' flanking expression responses are proportional to increased and decreased luciferase mRNA levels, whereas 3'-UTR expression is associated with control mRNA levels. Hypersensitivity to nucleases S1 and P1, gel mobility differences between supercoiled plasmids carrying opposing microsatellite orientations, and anomalous melting profiles of this fragment are also observed. These results suggest that functional pleiotropy of this fragment depends on the DNA context of its purine-rich microsatellite strand and on DNA supercoiling. Intramolecular triplexes stabilized by supercoiling and secondary structures of purine repeat-rich mRNAs may also confer regulatory properties to similar genomic elements.

Formulation of poly(D,L-lactic-co-glycolic acid) microparticles for rapid plasmid DNA delivery

An optimised water-in-oil-in-water double emulsion process for the microencapsulation of plasmid DNA in poly(D,L-lactic-co-glycolic acid) (PLGA) was used to prepare microparticles from a range of different PLGA formulations. This process has been developed using pharmaceutically accepted solvents and is potentially scaleable. Incorporation of plasmid DNA in the microparticles of up to 11 microg/mg was obtained and the retention of plasmid DNA integrity was considerably greater than previously reported. Microparticle structure was determined, by scanning electron microscopy, to be hollow and size distribution characteristics were found to be independent of polymer formulation. The ability to vary the plasmid DNA release profile by changing the PLGA formulation and polymer concentration used in the encapsulation process was also demonstrated. This ability to control the release profile of the microparticles was shown to be especially important as the physical integrity of the encapsulated plasmid DNA was found to deteriorate with extended release times in vitro.

Microencapsulation of DNA using poly(DL-lactide-co-glycolide): stability issues and release characteristics

The design of DNA vaccination delivery systems for the targeting of professional antigen presenting cells could be an interesting approach to elicit cytotoxic T-cell responses to fight viral infections and in cancer therapy. Stability studies with linear high and low molecular DNA and supercoiled plasmid DNA were performed in order to check their ability to withstand stress conditions applied during formulation processes. DNA was tested for integrity by the PicoGreen assay and transfectivity was assessed in cell culture transfection experiments. Double-stranded DNA is extremely stable under physiological conditions in vitro but is rapidly degraded under acidic conditions and high shear forces. Thereby, different stress factors resulted in distinct degradation patterns such as fragmentation and strand separation possibly followed by further decomposition of single-stranded DNA. DNA containing PLGA microparticles as a potential delivery system was prepared by spray-drying. Encapsulation efficiency, DNA stability and burst release varied significantly depending on the different parameters explored in this study. The microencapsulation process was altered to achieve maximal stability of encapsulated DNA by reducing exposure to shear forces and by the addition of NaHCO(3) which acts as a buffering agent and furthermore stabilizes dsDNA against mechanical degradation. Stability of DNA is maintained during the burst release phase, but massive degradation occurred during the second release phase possibly due to acidic catalyzed decomposition. In summary, we feel that microencapsulation of DNA vaccines by spray-drying offers manifold possibilities to design suitable delivery systems in terms of optimizing phagocytosis by APCs and maintaining stability of DNA in phagosomes.

Effect of DNA topology on the transfection efficiency of poly((2-dimethylamino)ethyl methacrylate)-plasmid complexes

In this paper the effect of the topology of plasmid DNA (supercoiled, open-circular and linear) on its binding characteristics with the polymeric transfectant poly((2-dimethylamino)ethyl methacrylate) was studied. The formed polyplexes were also evaluated for their transfection properties in vitro in two different cell lines. Anion-exchange chromatography was used for the separation of supercoiled and open-circular plasmid from a plasmid stock solution. Linear plasmids were prepared by endonucleases that cleaved the plasmid either in the promoter region or in a region not specific for expression (ampicillin resistance region). Plasmid DNA was also heat-denatured for 6 h at 70 degrees C, resulting in DNA mainly in the open-circular and oligomeric forms. The transfection of two different cell lines was dependent on the topology of the DNA in the order supercoiled>open-circular approximately heat-denatured>linear DNA prepared by cleaving in the nonspecific region>linear DNA prepared by cleaving in the promoter region. No differences in the size of the complexes or in the quenching of the DNA-intercalating fluorophore acridine orange were found as function of the topology. However, circular dichroism spectroscopy revealed differences between the topological plasmid species, both in the free form and in the presence of excess of cationic polymer.

Calcium-mediated DNA adsorption to yeast cells and kinetics of cell transformation by electroporation

Detailed kinetic data suggest that the direct transfer of plasmid DNA (YEp 351, 5.6 kbp, supercoiled, Mr approximately 3.5 x 10(6)) by membrane electroporation of yeast cells (Saccharomyces cerevisiae, strain AH 215) is mainly due to electrodiffusive processes. The rate-limiting step for the cell transformation, however, is a bimolecular DNA-binding interaction in the cell interior. Both the adsorption of DNA, directly measured with [32P]dCTP DNA, and the number of transformants are collinearly enhanced with increasing total concentrations [Dt] and [Cat] of DNA and of calcium, respectively. At [Cat] = 1 mM, the half-saturation or equilibrium constant is KD = 15 +/- 1 nM at 293 K (20 degrees C). The optimal transformation frequency is TFopt = 4.1 +/- 0.4 X 10(-5) if a single exponential pulse of initial field strength E0 = 4 kV cm-1 and decay time constant tauE = 45 ms is applied at [Dt] = 2.7 nM and 10(8) cells in 0.1 ml. The dependence of TF on [Cat] yields the equilibrium constants KCazero = 1.8 +/- 0.2 mM (in the absence of DNA) and K'Ca (at 2.7 nM DNA), comparable with and derived from electrophoresis data. In yeast cells, too, the appearance of a DNA molecule in its whole length in the cell interior is clearly an after-field event. At Eo = 4.0 kV cm-1 and T = 293 K, the flow coefficient of DNA through the porous membrane patches is Kto = 7.0 +/- 0.7 x 10(3)S-1 and the electrodiffusion of DNA is approximately 10 times more effective than simple diffusion: D/D0 approximately 10.3. The mean radius of these pores is rp = 0.39 +/- 0.05 nm, and the mean number of pores per cell (of size ø approximately 5.5 microns) is Np = 2.2 +/- 0.2 x 10(4). The maximal membrane area that is involved in the electrodiffusive penetration of adsorbed DNA into the outer surface of the electroporated cell membrane patches is only 0.023% of the total cell surface. The surface penetration is followed either by additional electrodiffusive or by passive (after-field) diffusive translocation of the inserted DNA into the cell interior. For practical purposes of optimal transformation efficiency, 1 mM calcium is necessary for sufficient DNA binding and the relatively long pulse duration of 20-40 ms is required to achieve efficient electrodiffusive transport across the cell wall and into the outer surface of electroporated cell membrane patches.

Studies on electrotransfer of DNA into Escherichia coli: effect of molecular form of DNA

Effects of several molecular forms of DNA were examined on voltage-pulse-mediated transfection or transformation. Among circular DNAs, the single-stranded microvirid DNA was less infective than the double-stranded replicative form, whereas transfectivity of the relaxed or nicked molecule was nearly equal to or slightly lower than that of the supercoiled DNA. The linearized double-stranded DNA derived from phage or plasmid electrotransfects Escherichia coli, albeit at a reduced efficiency. Alkaline denaturation of the linearized DNA resulted in complete loss of the infectivity. Relationship between terminal structure of the linearized DNA and efficiency of the transfection was investigated. Host recombination function did not significantly affect the infectivity of the linearized DNA.

Electric field mediated loading of macromolecules in intact yeast cells is critically controlled at the wall level

The mechanism of electric field mediated macromolecule transfer inside an intact yeast cell was investigated by observing, under a microscope, the fluorescence associated to cells after pulsation in a buffer containing two different hydrophilic fluorescent dyes. In the case of a small probe such as propidium iodide, a long lived permeabilized state was induced by the field as classically observed on wall free systems. Penetration of a 70 kDa FITC dextran was obtained only by using drastic conditions and only a very limited number of yeast cells which took up macromolecules remained viable. Most dextrans were trapped in the wall. A dramatic improvement in transfer of dextrans was observed when the cells were treated by dithiothreitol before pulsation. A cytoplasmic protein leakage was detected after the electric treatment suggesting that an irreversible damage took place in the walls of many pulsed cells. Electroloading of macromolecules in intact yeast cells appears to be controlled by a field induced short lived alteration of the envelope organization.

Transformation of Escherichia coli with large DNA molecules by electroporation

We have examined bacterial electroporation with a specific interest in the transformation of large DNA, i.e. molecules > 100 kb. We have used DNA from bacterial artificial chromosomes (BACs) ranging from 7 to 240 kb, as well as BAC ligation mixes containing a range o different sized molecules. The efficiency of electroporation with large DNA is strongly dependent on the strain of Escherichia coli used; strains which offer comparable efficiencies for 7 kb molecules differ in their uptake of 240 kb DNA by as much as 30-fold. Even with a host strain that transforms relatively well with large DNA, transformation efficiency drops dramatically with increasing size of the DNA. Molecules of 240 kb transform approximately 30-fold less well, on a molar basis, than molecules of 80 kb. Maximum transformation of large DNA occurs with different voltage gradients and with different time constants than are optimal for smaller DNA. This provides the opportunity to increase the yield of transformants which have taken up large DNA relative to the number incorporating smaller molecules. We have demonstrated that conditions may be selected which increase the average size of BAC clones generated by electroporation and compare the overall efficiency of each of the conditions tested.

Control by pulse parameters of electric field-mediated gene transfer in mammalian cells

Electric field-mediated gene transfer in mammalian cells (electrotransformation) depends on the pulsing conditions (field intensity, pulse duration, number of pulses). The effect of these parameters was systematically investigated using the transient expression of the chloramphenicol acetyltransferase and the beta-galactosidase activities in Chinese hamster ovary cells. Pulsing conditions inducing reversible permeabilization of the cell plasma membrane are not sufficient to induce gene transfer. The plasmid must be present during the electric pulse if it is to be transferred across the membrane into the cytoplasm. Only the localized part of the cell membrane brought to the permeabilized state by the external field is competent. Pulse duration plays a key role in the magnitude of the transfer. The field induces a complex reaction between the membrane and the plasmid that is accumulated at the cell interface by electrophoretic forces. This leads to an insertion of the plasmid, which can then cross the membrane.

Heat shock and osmotically dependent steps by DNA uptake after Escherichia coli electroporation

Currently, the major problem in the genetic transformation is to understand how such a large molecule as the plasmid DNA passes through the cell membrane. To solve this problem we used a simplified experimental model with Escherichia coli and the plasmid pBR322: the DNA-bacteria mixture was electroporated in a sucrose solution at 0 degree C and at fixed electrical parameters; the samples were then directly plated into agar. It was found that the electrically-induced bacterial transformation after pulsing is dependent on two factors: heat shock (delta T) and osmotic stress. Our results indicate that two mechanisms contribute to these effects: (i) thermodiffusion of the DNA across the membrane and (ii) osmotic flows of the medium, containing the DNA, into the interior of the cell.

Study of mechanisms of electric field-induced DNA transfection. V. Effects of DNA topology on surface binding, cell uptake, expression, and integration into host chromosomes of DNA in the mammalian cell

Neumann and coworkers (Neumann, E., M. Schaefer-Ridder, Y. Wang, and P. H. Hofschneider. 1982. EMBO J. 1:841-845) have shown that the efficiency of pulsed electric field (PEF)-induced DNA transfection of mouse L-cells by the thymidine kinase gene is several times higher for the linear DNA than for the closed circular DNA. Transfection of Escherichia coli bacteria by several plasmids indicates that the transfection efficiency was much higher for the closed circular/supercoiled (sc-) and circular/relaxed (cr-) DNA than for the linearized (In-) DNA (Xie, T. D., L. Sun, H. G. Zhao, J. A. Fuchs, and T. Y. Tsong. 1992. Biophys. J. 63:1026-1031). To resolve these conflicting observations, we have systematically examined electrotransfection of NIH3T3 mouse fibroblast by the plasmids, pRSVcat, pRSVneo, and pRSVgpt. Mg(2+)-facilitated surface binding of DNA before, and DNA uptake by 3T3 cells after treatment with PEF, were monitored by 3H-labeled plasmids. Transfection efficiency was evaluated by both the transient expression of chloramphenicol acetyltransferase (cat) activity 2-3 days after, and the permanent expression of neomycin phosphotransferase (neo) and xanthine-guanine phosphoribosyltransferase (gpt) genes in the transformants 2 weeks after the PEF treatment. Our results indicate that cell surface binding and PEF-induced cell uptake of DNA did not depend on the topology of DNA. However, both the transient and the permanent expression of the plasmids were three to five times more efficient for the cr-DNA and the sc-DNA than for the in-DNA. These results indicate that electrotransfection of cells involves several steps: the cation-dependent binding of DNA to the cell surface, the electric field-driven DNA entry into the cells, the transient expression of DNA, and the integration of DNA into the host chromosomes. For understanding mechanisms of electrotransfection, only the DNA binding to the cell surface and the electric field assisted membrane-crossing of DNA are relevant. Both the expression of the loaded DNA and the DNA integration into the host chromosomes depend more on the properties of the cell and its interactions with a foreign gene. Since these properties and interactions will be similar irrespective of the method chosen to facilitate DNA transfer, they are not relevant for the study of mechanisms of electrotransfection. Our results also support the idea that the PEF-induced cellular uptake of DNA is mainly by the electrophoresis of the surface bound DNA across the plasma membrane.

Gene transfer by electroporation into intact scutellum cells of wheat embryos

Gene transfer into intact cells was achieved by electroporating zygotic wheat embryos without any special pretreatment. Electroporation was tissue specific in so far as scutellum cells were found to be much more susceptible to gene transfer than other cell types of the embryo. The orientation of the embryos in the electroporation chamber also influenced the number of transformed scutellum cells; during electroporation, as in electrophoresis, the negatively charged plasmid DNA molecules seemed to move towards the positive electrode. Therefore, the embryos were arranged so that the scutella faced the negative electrode. The use of plasmids carrying either two chimeric anthocyanin regulatory genes or a chimeric gusA gene allowed clear identification of transformed cells in the scutellum. On some of the embryos, more than 100 transformed scutellum cells were found after electroporation with single electric pulses of 275 V/cm discharged from a 960-μF capacitor and with 100 μg DNA/ml electroporation buffer. Using the anthocyanin marker system, visibly transformed cells grew to produce red sectors.