PEGylated Lipoplexes: Preparation Protocols Affecting DNA Condensation and Cell Transfection Efficiency

Transgene expression in mice of the Opa1 mitochondrial transmembrane protein through bicontinuous cubic lipoplexes containing gemini imidazolium surfactants

BACKGROUND

Lipoplexes are non-viral vectors based on cationic lipids used to deliver DNA into cells, also known as lipofection. The positively charge of the hydrophilic head-group provides the cationic lipids the ability to condensate the negatively charged DNA into structured complexes. The polar head can carry a large variety of chemical groups including amines as well as guanidino or imidazole groups. In particular, gemini cationic lipids consist of two positive polar heads linked by a spacer with different length. As for the hydrophobic aliphatic chains, they can be unsaturated or saturated and are connected to the polar head-groups. Many other chemical components can be included in the formulation of lipoplexes to improve their transfection efficiency, which often relies on their structural features. Varying these components can drastically change the arrangement of DNA molecules within the lamellar, hexagonal or cubic phases that are provided by the lipid matrix. Lipofection is widely used to deliver genetic material in cell culture experiments but the simpler formulations exhibit major drawbacks related to low transfection, low specificity, low circulation half-life and toxicity when scaled up to in vivo experiments.

RESULTS

So far, we have explored in cell cultures the transfection ability of lipoplexes based on gemini cationic lipids that consist of two C16 alkyl chains and two imidazolium polar head-groups linked with a polyoxyethylene spacer, (C16Im)2(C4O). Here, PEGylated lipids have been introduced to the lipoplex formulation and the transgene expression of the Opa1 mitochondrial transmembrane protein in mice was assessed. The addition of PEG on the surface of the lipid mixed resulted in the formation of Ia3d bicontinuous cubic phases as determined by small angle X-ray scattering. After a single intramuscular administration, the cubic lipoplexes were accumulated in tissues with tight endothelial barriers such as brain, heart, and lungs for at least 48 h. The transgene expression of Opa1 in those organs was identified by western blotting or RNA expression analysis through quantitative polymerase chain reaction.

CONCLUSIONS

The expression reported here is sufficient in magnitude, duration and toxicity to consolidate the bicontinuous cubic structures formed by (C16Im)2(C4O)-based lipoplexes as valuable therapeutic agents in the field of gene delivery.

Double-strand cleavage of DNA by a polyamide-phenazine-di-N-oxide conjugate

Phenazine and its derivatives have been widely applied as nucleic acid cleavage agents due to active oxygen activating the C-H bond of the substrate. However, diffusion of oxygen radicals limits their potential applications in the DNA-targeted metal-free drug. Introduction of groove binder moiety such as polyamide enhanced the regional stability of radical molecules and reduced cytotoxicity of the drugs. In this work, we described the design and synthesis of a polyamide-modified phenazine-di-N-oxide as a DNA double-strand cleavage agent. The gel assays showed the hybrid conjugates can effectively break DNA double strands in a non-random manner under physiological conditions. The probable binding mode to DNA was investigated by sufficient spectral experiments, revealing weak interaction between hybrid ligand and nucleic acid molecules. The results of our study have implications on the design of groove-binding hybrid molecules as new artificial nucleases and may provide a strategy for developing efficient and safe DNA cleavage reagents.

Effect of ligand sequence-specific modification on DNA hybrid catalysis

We report a sequence-specific catalytic ligand as a chemical modification strategy to achieve DNA-based asymmetric reactions with sequence-dependent enantioselectivity.

Recent progress in gene therapy to deliver nucleic acids with multivalent cationic vectors

Due to the potential use as transfecting agents of nucleic acids (DNA or RNA), multivalent cationic non-viral vectors have received special attention in the last decade. Much effort has been addressed to synthesize more efficient and biocompatible gene vectors able to transport nucleic acids into the cells without provoking an immune response. Among them, the mostly explored to compact and transfect nucleic acids are: (a) gemini and multivalent cationic lipids, mixed with a helper lipid, by forming lipoplexes; and (b) cationic polymers, polycations, and polyrotaxanes, by forming polyplexes. This review is focused on the progress and recent advances experimented in this area, mainly during the present decade, devoting special attention to the lipoplexes and polyplexes, as follows: (a) to its biophysical characterization (mainly electrostatics, structure, size and morphology) using a wide variety of experimental methods; and (b) to its biological activity (transfection efficacy and cytotoxicity) addressed to confirm the optimum formulations and viability of these complexes as very promising gene vectors of nucleic acids in nanomedicine.

Structural, biocomplexation and gene delivery properties of hydroxyethylated gemini surfactants with varied spacer length

Gemini surfactants with hexadecyl tails and hydroxyethylated head groups bridged with tetramethylene (G4), hexamethylene (G6) and dodecamethylene (G12) spacers were shown to self-assemble at the lower critical micelle concentration compared to their conventional m-s-m analogs. The lipoplex formation and the plasmid DNA transfer into different kinds of host cells were studied. In the case of eukaryotic cells, high transfection efficacy has been demonstrated for DNA-gemini complexes, which increased as follows: G6<G4<G12. Different activity series, i.e., G6>G4>G12 has been obtained in the case of transformation of bacterial cells with plasmid DNA-gemini complexes, mediated by electroporation technique. Solely G6 shows transformation efficacy exceeding the control result (uncomplexed DNA), while the inhibitory effect occurs for G4 and G12. Analysis of physico-chemical features of single surfactants and lipoplexes shows that compaction and condensation effects change as follows: G6<G4 ≤ G12, i.e., agree with the order of transfection efficacy, which is supported by membrane tropic properties of G12. On the other hand, gel retardation assay and docking study testify low electrostatic affinity in G12/DNA pair, thereby indicating that hydrophobic effect probably plays important role in the lipoplex formation. Two factors are assumed to be responsible for the inhibition effect of gemini in the case of transformation of bacterial cells. They are (i) an unfavorable influence of cationic surfactants on the electroporation procedure due to depressing the electrophoretic effect; and (ii) antibacterial activity of cationic surfactants that may cause the disruption of integrity of cell membranes.

A flow cytometric approach to study the mechanism of gene delivery to cells by gemini-lipid nanoparticles: an implication for cell membrane nanoporation

BACKGROUND

Gemini-lipid nanoparticles have been received major attention recently as non-viral delivery systems due to their successful non-invasive gene delivery through tough barriers such as eye and skin. The aim of this study was to evaluate non-viral gene delivery by a series of dicationic gemini surfactant-phospholipid nanoparticles (GL-NPs) and to explore their mechanism of interaction with cellular membranes of murine PAM212 epidermal keratinocytes.

METHODS

NPs containing pCMV-tdTomato plasmid encoding red fluorescent protein (RFP) were prepared using 12 different gemini surfactants (m-s-m, with m = 12, 16 and 18C alkyl tail and s = 3 and 7C polymethylene spacer group and 7C substituted spacers with 7NH and 7NCH3) and dioleoylphosphatidylethanolamine helper lipid. RFP gene expression and cell viability status were evaluated using flow cytometry. MitoTracker Deep Red mitochondrial stain and the cell impermeable Sytox red nuclear stain were used as indicators of cell viability and cell membrane integrity, respectively.

RESULTS

No significant viability loss was detected in cells transfected with 18-3-18, 18-7-18, 18-7NH-18, and 18-7NCH3-18 NPs, whereas a significant reduction of viability was detected in cells treated with 12-3-12, 12-7-12, 12-7NH-12, 16-7NH-16, or 16-7NCH3-16 GL-NPs. Compared to Lipofectamine Plus, 18-3-18 GL-NPs showed higher transfection efficiency and comparable viability profile by evaluation using MitoTracker Deep Red in PAM212 cells. Flow cytometric analysis of PAM212 cells stained with Sytox red revealed two cell populations with low and high fluorescent intensity, representing cells with partially-porated and highly-porated membranes, respectively. Additional combined staining with MitoTracker and ethidium homodimer showed that that 18-3-18 GL-NPs disturbed cell membrane integrity, while cells were still alive and had mitochondrial activity.

CONCLUSION

Taken together, this study demonstrated that 18-3-18 GL-NPs have higher transfection efficiency and comparable viability profile to the commercial Lipofectamine Plus, and the interaction of 18-3-18 GL-NPs with PAM212 cell membranes involves a permeability increase, possibly through the formation of nanoscale pores, which could explain efficient gene delivery. This novel nanoconstruct appears to be a promising delivery system for further skin gene therapy studies in vivo.

A delocalizable cationic headgroup together with an oligo-oxyethylene spacer in gemini cationic lipids improves their biological activity as vectors of plasmid DNA

Lipoplexes of plasmid DNA and mixed liposomes, with a gemini cationic lipid of the 1,2-bis(hexadecyl imidazolium) oxyethylene series, improves their biological activity.

Effective and reversible DNA condensation induced by a simple cyclic/rigid polyamine containing carbonyl moiety

The transfection of DNA in gene therapy largely depends on the possibility of obtaining its condensation. The details of nanoparticle formation are essential for functioning, as mediated by the diverse elements containing molecular structure, ionic strength in mediums, and condensing motivator. Here, we report two kinds of DNA condensing agents based on simple cyclic/rigid polyamine molecules, having evaluated their structural effect on nanoparticle formation. The reversible condensation-dissociation process was achieved by ion-switching, attributing to a possible condensing mechanism-competitive building of external hydrogen bonds. Using poly[(dA-dT)2] and poly[(dG-dC)2] as substrates, respectively, circular dichroism (CD) signals clearly presented dissimilar interactions between polyamines and both rich sequences, implying potential preference for G-C sequence. The presence of divalent ion Zn(2+) as an efficient motivator accelerated the achievement of DNA condensation, and an accessible schematic model was depicted to explain the promotion in detail. In addition, by comparison with the behaviors of linear polyamines, differences between condensation and aggregation were explicitly elucidated in aspects of morphology and surface charges, as well as induced condition. The present work may have the potential to reveal the precise mechanism of DNA nanoparticle formation and, in particular, be applied to gene delivery as an efficient nonviral vector.

Physicochemical properties of transferrin-associated lipopolyplexes and their role in biological activity

The combination of polyethylenimine (PEI), as a plasmid DNA pre-condensing agent, and cationic lipids has been reported to result in a synergistic effect on transfection. Recently, we have explored this effect by associating low-molecular weight PEIs with transferrin-associated lipoplexes using different cationic liposome formulations. The resulting lipopolyplexes that have shown to be the most efficient in mediating transfection were those prepared from cationic liposomes composed of DOTAP:Chol (associated or not with transferrin) and from a pH-sensitive liposome formulation (DOTAP:Chol:DOPE:CHEMS). In the present work, the physicochemical properties of these lipopolyplexes were studied aiming at establishing a correlation with their transfection efficiency. For this purpose, the lipopolyplexes were characterized in terms of their morphology by performing ultrastructural studies using cryo-TEM microscopy, investigating inner DNA structure using circular dichroism and characterizing particle size by photon correlation spectroscopy. A correlation between efficiency of transfection and more compact inner DNA structure and smaller particle sizes (around 250nm) was found. In addition, the visualization of liposomes and lipopolyplexes at the ultrastructural level revealed that the particles presenting enhanced transfection efficiencies are associated with higher electron density. Recently, PEI-based lipopolyplexes were reported to gain entry into the cell through the caveolae-mediated pathway. Based on the present finding that DOTAP:Chol liposomes exhibit the ability to form hexagonal structures when prepared at high concentrations, we propose that the lipopolyplexes containing DOTAP:Chol take advantage of such capacity to escape from the endocytotic vesicles, which will contribute to the observed high transfection efficiencies.