A protein kinase signal-responsive gene carrier modified RGD peptide

We have previously reported artificial gene-regulation systems responding to cyclic AMP-dependent protein kinase (PKA) using a cationic polymer. However, this polymer alone cannot deliver any gene into living cells. In the present work, we modified the signal-responsive polymer to the RGD peptide for the introduction of a polymer/DNA complex into living cells and succeeded in regulating the gene expression responding to intracellular PKA activation.     

Surface-tethered DNA complexes for enhanced gene delivery

Overcoming the barriers to efficient gene transfer is a fundamental goal of biotechnology. A versatile approach to enhance the delivery of nonviral DNA involves complexation with cationic polymers, which can be designed to overcome the barriers to effective gene transfer. More recently, DNA release from a polymer substrate or scaffold has been shown to enhance gene transfer, likely by increasing DNA concentrations in the cell microenvironment. We propose a novel approach that combines these two strategies in which cationic polymer/DNA complexes are tethered to a substrate that supports cell adhesion. The cationic polymers package the DNA for efficient internalization and the surface tethering functions to maintain elevated concentrations in the cell microenvironment for cells adhered to the substrate. The cationic polymer polylysine (degree of polymerization equal to 19 or 150) was modified with biotin groups, which was confirmed by mass spectrometry and biochemical analysis. Complex formation of DNA with biotinylated-polylysine, or mixtures of biotinylated and nonbiotinylated polylysines, was confirmed by gel electrophoresis. Plasmid DNA encoding for the reporter gene beta-galactosidase was complexed with different mixtures of biotinylated and nonbiotinylated polylysine and incubated on neutravidin (nonglycosylated avidin)-coated surfaces. DNA surface densities ranging from 0.1 to 4.3 microg/cm2 were observed and found to be a function of the number of biotin groups, the molecular weight of the polylysine, and the amount of DNA. HEK293T or NIH/3T3 cells were then seeded onto the DNA-modified surfaces, and transfection was quantified at 48 and 96 h. Transfection by the DNA surfaces was observed with both cell lines, and expression levels up to 100 fold greater than bulk delivery of the complexes was obtained. Transfection was found to be a function of the surface DNA quantities and the number of tethers on the complex. Transfected cells were observed only in the region in which DNA complexes were tethered, suggesting that the location of transfected cells can be specifically controlled. Surface tethering of DNA represents a promising approach to enhancing gene transfer and spatially controlling gene delivery, which may have applications to a multitude of fields ranging from tissue engineering to functional genomics.     

Intracellular signal-responsive gene carrier for cell-specific gene expression

We designed a peptide-polymer conjugate (CPCCtat) as a novel gene carrier that could control gene expression responding to the intracellular caspase-3 signal. This carrier consists of an uncharged main polymer chain and a cationic peptide side chain, which includes the substrate sequence of caspase-3 and the protein transduction domain sequence of HIV-1 Tat. In the present study, CPCCtat formed a tight complex with DNA through an electrostatic interaction, and in this state the gene expression was totally suppressed. In contrast, the complex disintegrated in the presence of caspase-3 due to cleavage of the cationic portion from CPCCtat. This event led to an activation of gene expression. Our results also indicate that the complex can be delivered into living cells due to the cell-permeable peptide side chain of CPCCtat. This intracellular signal-responsive system with CPCCtat will be useful for the cell-specific gene expression system.     

Physicochemical properties of polymers: An important system to overcome the cell barriers in gene transfection

Delivery of the macromolecules including DNA, miRNA, and antisense oligonucleotides is typically mediated by carriers due to the large size and negative charge. Different physical (e.g., gene gun or electroporation), and chemical (e.g., cationic polymer or lipid) vectors have been already used to improve the efficiency of gene transfer. Polymer‐based DNA delivery systems have attracted special interest, in particular via intravenous injection with many intra‐ and extracellular barriers. The recent progress has shown that stimuli‐responsive polymers entitled as multifunctional nucleic acid vehicles can act to target specific cells. These nonviral carriers are classified by the type of stimulus including reduction potential, pH, and temperature. Generally, the physicochemical characterization of DNA‐polymer complexes is critical to enhance the transfection potency via protection of DNA from nuclease digestion, endosomal escape, and nuclear localization. The successful clinical applications will depend on an exact insight of barriers in gene delivery and development of carriers overcoming these barriers. Consequently, improvement of novel cationic polymers with low toxicity and effective for biomedical use has attracted a great attention in gene therapy. This article summarizes the main physicochemical and biological properties of polyplexes describing their gene transfection behavior, in vitro and in vivo. In this line, the relative efficiencies of various cationic polymers are compared. © 2015 Wiley Periodicals, Inc. Biopolymers 103: 363–375, 2015.     

Non-viral gene delivery systems

Non-viral gene delivery systems have the potential to create viable pharmaceuticals from nucleic acids. These DNA delivery systems contain lipids and/or cationic polymers. In order for these systems to be developed into commercial products, several barriers must be overcome. These include obstacles in manufacturing, formulation and stability. In vivo, problems of extracellular non-specific interactions and intracellular trafficking to the nucleus are also encountered. Recent progress has been made in overcoming these issues.     

Linear cationic click polymers/DNA nanoparticles: in vitro structure-activity relationship and in vivo evaluation for gene delivery

The aim of this work was to explore the structure--activity relationships (SAR) of a series of novel linear cationic click polymers with various structures for in vitro gene delivery and in vivo gene transfer. The experimental results revealed that the minimal structure variation could result in a crucial effect on DNA-binding ability, buffering capacity, and the cellular delivery capacity of polymer, all of which brought about the obvious effects on their transfection efficiencies. The polymer synthesized from diazide monomer containing bis-ethylenediamine unit and dialykene monomer containing bis-ethylene glycol unit (B(2)) could effectively condense DNA into complex nanoparticles (B(2)Ns), which showed the highest in vitro transfection efficiency. The biodistribution and transfection efficiency of B(2)Ns in nude mice bearing tumor demonstrated the ability of effectively delivering DNA into tumor tissue. These results implied that this gene vector based on linear cationic click polymer could be a promising gene delivery system for tumor gene therapy.     

Tumor-targeted gene delivery using molecularly engineered hybrid polymers functionalized with a tumor-homing peptide

Before gene therapy can be used in clinical settings, safe and efficient DNA delivery systems must be developed to overcome a range of extra- and intracellular transport barriers. As a step toward the development of a modular, multifunctional gene delivery system to overcome these diverse barriers, we have developed a family of linear-dendritic "hybrid" polymers which contain functionalities for tissue targeting, minimization of nonspecific interactions, endosomal buffering, and DNA binding. Here, we demonstrate the rapid three-step, room-temperature, aqueous synthesis of hybrid polymers, as well as the functionalization of these polymers with a peptide targeting ligand that specifically binds to glucose-regulated protein-78 kDa (GRP-78), a clinically relevant tumor antigen identified in human cancer patients. These polymer systems can condense plasmid DNA into small nanoparticle structures (<210 nm) and transfect cells expressing GRP-78 with efficiencies that exceed that of branched polyethylenimine (bPEI), one of the best commercially available polymers for in vitro transfections. The synthetic approach described here may be useful for the rapid synthesis and optimization of polymer gene delivery systems bearing a range of diverse functional domains, and the specific GRP-78-targeted systems developed in this study may potentially have clinical applications in cancer gene therapy.     

Synthesis and in vitro evaluation of novel star-shaped block copolymers (blocked star vectors) for efficient gene delivery

Novel 4-branched diblock copolymers consisting of cationic chains as an inner domain and nonionic chains as an outer domain were prepared by iniferter-based living radial polymerization and evaluated as a polymeric transfectant. The cationic polymerization of 3-(N,N-dimethylamino)propyl acrylamide (DMAPAAm) using 1,2,4,5-tetrakis( N,N-diethyldithiocarbamylmethyl)benzene as a 4-functional iniferter followed by the nonionic block polymerization of N,N-dimethylacrylamide (DMAAm) afforded 4-branched diblock copolymers with controlled compositions. By changing the solution or irradiation conditions, 4-branched PDMAPAAms with molecular weights of 10,000, 20,000, and 50,000 were synthesized. In addition, by graft polymerization, PDMAPAAm-PDMAAm blocked copolymers with copolymer composition (unit ratio of DMAAm/DMAPAAm) ranging from 0.18 to 1.0 for each cationic polymer were synthesized. All polymers were shown to interact with and condense plasmid DNA to yield polymer/DNA complexes (polyplexes). A transfection study on COS-1 cells showed that the polyplexes from block copolymers with cationic chain length of approximately 50,000 and a nonionic chain length of 30,000, which were approximately 200 nm in diameter and very stable in aqueous media, had the most efficient luciferase activity with minimal cellular cytotoxicity under a charge ratio of 20 (vector/pDNA). The PDMAPAAm-PDMAAm-blocked, star-shaped polymers are an attractive novel class of nonviral gene delivery systems.     

Low charge density cationic polymers for gene delivery: exploring the influence of structural elements on in vitro transfection

A series of end-functionalized poly(trimethylene carbonate) DNA carriers, characterized by low cationic charge density and pronounced hydrophobicity, is used to study structural effects on in vitro gene delivery. As the DNA-binding moieties are identical in all polymer structures, the differences observed between the different polymers are directly related to the functionality and length of the polymer backbone. The transfection efficiency and cytotoxicity of the polymer/DNA complexes are thus found to be dependent on a combination of polymer charge density and functionality, highlighting the importance of such structural considerations in the development of materials for efficient gene delivery.     

Poly[Lys-(AEDTP)]: a cationic polymer that allows dissociation of pDNA/cationic polymer complexes in a reductive medium and enhances polyfection.

Polyplexes of high stability resulting from the condensation of a plasmid DNA by a cationic polymer are widely used to develop polymer-based gene delivery systems. However, the plasmid must be released from its vector once inside the cells for an efficient expression of the exogenous gene in the cell nucleus. We have designed a disulfide-containing cationic polymer termed poly[Lys-(AEDTP)] which allowed for the formation of polyplexes and the release of the plasmid in a reductive medium. The amino groups of polylysine were substituted with 3-(2-aminoethyldithio)propionyl residues in order to have each amino group of poly[Lys-(AEDTP)] interacting with a phosphate DNA linked to the polymer backbone via a disulfide bond. As evidenced by agarose gel electrophoresis and ethidium bromide/pDNA fluorescence restoration, poly[Lys-(AEDTP)] polyplexes were decondensed and the plasmid released upon treatment with either dithiothreitol, glutathione in the presence of glutathione reductase, or the thioredoxin reductase. Electron microscopy showed that polyplexes exhibiting spherical particles of a mean size at about 100 nm were decondensed in the presence of glutathione and exhibited filamentous aggregates. Finally, we found that the transfection of 293T7 and HepG2 cells was 10- and 50-fold more efficient with poly[Lys-(AEDTP)] polyplexes, respectively, than with poly[Lys] polyplexes. These results indicate that disulfide-containing cationic polymers must be borne in mind for developing polymer-base gene delivery systems.     

Synthesis and characterization of poly (amino ester) for slow biodegradable gene delivery vector

Many therapeutic carrier materials were exploited for human gene therapy from viral to polymeric vectors. This research describes the evaluation of two biodegradable ester-bonded polymers synthesized by double-monomer polycondensation for a non-viral cationic polymer-based gene delivery system. The backbone was constructed to include inner tertiary amines and outer primary amines. Self-assembly with DNA resulted in the production of regularly nano-sized spherical polyplexes with good transfection efficiency, especially in the presence of serum. The polymers showed a relatively slow degradability for an amine-containing ester polymer, as they maintained DNA/polymer complex for 7 days in physiological buffer conditions. Finally, the low toxicity and slow degradation concluded these polymers reliable for long-term therapeutic applications.     

Characterisation of the binding interaction between poly(L-lysine) and DNA using the fluorescamine assay in the preparation of non-viral gene delivery vectors

A major factor limiting the development of non-viral gene delivery systems is the poor characterisation of polyelectrolyte complexes formed between cationic polymers and DNA. The present study uses the fluorescamine reagent to improve characterisation of poly(L-lysine) (pLL)/DNA complexes post-modified with a multivalent hydrophilic polymer by determining the availability of free amino groups. The results show that the fluorescamine reagent can be used to monitor the self-assembly reaction between pLL and DNA and the degree of surface modification of the resultant complexes with a hydrophilic polymer. This experimental approach should enable the preparation of fully defined complexes whose properties can be better related to their biological activity.     

An NLS peptide covalently linked to linear DNA does not enhance transfection efficiency of cationic polymer based gene delivery systems

BACKGROUND: Transfection with non-viral gene delivery vectors, such as cationic polymers, generally results in low transgene expression in vivo. This is likely due to poor cytoplasmic transport and intra-nuclear DNA delivery. METHODS: In this study two strategies to improve nuclear import were investigated. Linear DNA constructs with or without an NLS peptide were prepared by PCR. Alternatively, linear DNA obtained by enzymatic cleavage followed by capping of both ends with DNA-hairpins was used. An NLS peptide was attached to one of the capped ends of the linear DNA. Both biodegradable (pDMAEAppz) and non-degradable polymers (PEI or pDMAEMA) were used to complex the DNA. Several cell types, dividing and non-dividing, were transfected with the linear DNA constructs containing a SV40-derived NLS peptide. Nuclear import of the DNA constructs was studied using digitonin-permeabilized cells. RESULTS: Linear DNA prepared by PCR proved not useful as it was degraded from the 3'end. Linear DNA capped with hairpins was more successful with regard to stability. However, Cells transfected with linear DNA constructs by electroporation or by using cationic polymers with linear DNA containing a NLS peptide, failed to show significantly higher luciferase expression levels when compared to cells transfected with plasmid DNA or linear DNA without an NLS peptide attached. No nuclear localization was observed in digitonin-permeabilized cells. CONCLUSION: Taken together, these data demonstrate that this nuclear localisation signal when attached to DNA is neither able to improve transfection efficiency of cationic polymers nor the nuclear import of the DNA constructs.     

Preliminary characterization of novel amino acid based polymeric vesicles as gene and drug delivery agents

The amino acid homopolymers, poly-L-lysine and poly-L-ornithine, have been modified by the covalent attachment of palmitoyl and methoxypoly(ethylene glycol) (mPEG) residues to produce a new class of amphiphilic polymers-PLP and POP, respectively. These amphiphilic amino acid based polymers have been found to assemble into polymeric vesicles in the presence of cholesterol. Representatives of this new class of polymeric vesicles have been evaluated in vitro as nonviral gene delivery systems with a view to finding delivery systems that combine effective gene expression with low toxicity in vivo. In addition, the drug-carrying capacity of these polymeric vesicles was evaluated with the model drug doxorubicin. Chemical characterization of the modified polymers was carried out using (1)H NMR spectroscopy and the trinitrobenzene sulfonic acid (TNBS) assay for amino groups. The amphiphilic polymers were found to have an unreacted amino acid, palmitoyl, mPEG ratio of 11:5:1, and polymeric vesicle formation was confirmed by freeze-fracture electron microscopy and drug encapsulation studies. The resulting polymeric vesicles, by virtue of the mPEG groups, bear a near neutral zeta-potential. In vitro biological testing revealed that POP and PLP vesicle-DNA complexes are about one to 2 orders of magnitude less cytotoxic than the parent polymer-DNA complexes although more haemolytic than the parent polymer-DNA complexes. The polymeric vesicles condense DNA at a polymer:DNA weight ratio of 5:1 or greater and the polymeric vesicle-DNA complexes improved gene transfer to human tumor cell lines in comparison to the parent homopolymers despite the absence of receptor specific ligands and lysosomotropic agents such as chloroquine.     

Endosomal escape of polymeric gene delivery complexes is not always enhanced by polymers buffering at low pH

One of the crucial steps in gene delivery with cationic polymers is the escape of the polymer/DNA complexes ("polyplexes") from the endosome. A possible way to enhance endosomal escape is the use of cationic polymers with a pKa around or slightly below physiological pH ("proton sponge"). We synthesized a new polymer with two tertiary amine groups in each monomeric unit [poly(2-methyl-acrylic acid 2-[(2-(dimethylamino)-ethyl)-methyl-amino]-ethyl ester), abbreviated as pDAMA]. One pKa of the monomer is approximately 9, providing cationic charge at physiological pH, and thus DNA binding properties, the other is approximately 5 and provides endosomal buffering capacity. Using dynamic light scattering and zeta potential measurements, it was shown that pDAMA is able to condense DNA in small particles with a surface charge depending on the polymer/DNA ratio. pDAMA has a substantial lower toxicity than other polymeric transfectants, but in vitro, the transfection activity of the pDAMA-based polyplexes was very low. The addition of a membrane disruptive peptide to pDAMA-based polyplexes considerably increased the transfection efficiency without adversely affecting the cytotoxicity of the system. This indicates that the pDAMA-based polyplexes alone are not able to mediate escape from the endosomes via the proton sponge mechanism. Our observations imply that the proton sponge hypothesis is not generally applicable for polymers with buffering capacity at low pH and gives rise to a reconsideration of this hypothesis.     

Effect of thiol pendant conjugates on plasmid DNA binding, release, and stability of polymeric delivery vectors

Polymers have attracted much attention as potential gene delivery vectors due to their chemical and structural versatility. However, several challenges associated with polymeric carriers, including low transfection efficiencies, insufficient cargo release, and high cytotoxicity levels have prevented clinical implementation. Strong electrostatic interactions between polymeric carriers and DNA cargo can prohibit complete cargo release within the cell. As a result, cargo DNA never reaches the cell's nucleus where gene expression takes place. In addition, highly charged cationic polymers have been correlated with high cytotoxicity levels, making them unsuitable carriers in vivo. Using poly(allylamine) (PAA) as a model, we investigated how pH-sensitive disulfide cross-linked polymer networks can improve the delivery potential of cationic polymer carriers. To accomplish this, we conjugated thiol-terminated pendant chains onto the primary amines of PAA using 2-iminothiolane, developing three new polymer vectors with 5, 13, or 20% thiol modification. Unmodified PAA and thiol-conjugated polymers were tested for their ability to bind and release plasmid DNA, their capacity to protect genetic cargo from enzymatic degradation, and their potential for endolysosomal escape. Our results demonstrate that polymer-plasmid complexes (polyplexes) formed by the 13% thiolated polymer demonstrate the greatest delivery potential. At high N/P ratios, all thiolated polymers (but not unmodified counterparts) were able to resist decomplexation in the presence of heparin, a negatively charged polysaccharide used to mimic in vivo polyplex-protein interactions. Further, all thiolated polymers exhibited higher buffering capacities than unmodified PAA and, therefore, have a greater potential for endolysosomal escape. However, 5 and 20% thiolated polymers exhibited poor DNA binding-release kinetics, making them unsuitable carriers for gene delivery. The 13% thiolated polymers, on the other hand, displayed high DNA binding efficiency and pH-sensitive release.     

Effect of polymer ionization on the interaction with DNA in nonviral gene delivery systems

The optimization of DNA-cationic polymer complexation is crucial for nonviral gene delivery. Although physicochemical characterization of the interaction between DNA and cationic polymers has recently attracted more attention in the nonviral DNA delivery field, the literature on the effect of varying polycation charge density on DNA-cationic polymer complexation is still scarce. Thus, the aim of this study was to systematically assess the influence of the degree of ionization of a weak cationic polyelectrolyte (poly[2-(dimethylamino)ethyl methacrylate] or DMAEMA homopolymer) on its ability to form complexes with DNA. This was achieved by varying the solution pH from 4.0 to 8.0 and analyzing the resulting effects on the binding affinity, thermodynamic properties, complex size, and morphology. Lowering the solution pH led to higher degrees of ionization for the cationic polymer and hence greater binding affinities with DNA, as judged by the increased propensity of the former to displace ethidium bromide from DNA and also by relatively low monomer:nucleotide molar ratio (0.8:1) required to retard the migration of free DNA. Isothermal titration microcalorimetry studies further confirmed that a stronger interaction occurred at low pH than at high pH. By decreasing the pH from 8.0 to 6.6, K(obs) increased from 7.8 x 10(5) to 20.4 x 10(5) M(-1). More efficient condensation at low pH was demonstrated by the reduction of ethidium bromide fluorescence in the loading wells from gel electrophoresis, decreased complex sizes without agglomeration occurring at high polymer/DNA ratios, together with discrete and dense spherical complexes observed in TEM studies. This may be attributed to the presence of electrostatic stabilization from excess cationic polymer chains, which provide a repulsive shell around the polymer/DNA complex. The physicochemical data indicate that the increased degree of ionization for the DMAEMA homopolymer at lower pH results in higher binding affinity, smaller and more compact complexes, and more efficient condensation. These findings therefore highlight the importance of the degree of ionization on DNA complex formation for weak cationic polyelectrolytes.     

Design of imidazole-containing endosomolytic biopolymers for gene delivery

The development of safe and effective gene delivery agents poses a great challenge in the quest to make human gene therapy a reality. Cationic polymers represent one important class of materials for gene delivery, but to date they have shown only moderate efficiency. Improving the efficiency will require the design of new polymers incorporating optimized gene delivery properties. For example, inefficient release of the DNA/polymer complex from endocytic vesicles into the cytoplasm is one of the primary causes of poor gene delivery. Here we report the synthesis of a biocompatible, imidazole-containing polymer designed to overcome this obstacle. DNA/polymer polyplexes incorporating this polymer were shown to have desirable physico-chemical properties for gene delivery and are essentially nontoxic. Using this system, mammalian cells in vitro were transfected in the absence of any exogenous endosomolytic agent such as chloroquine.     

Biodegradable cross-linked poly(amino alcohol esters) based on LMW PEI for gene delivery

Polyethylenimine (PEI, especially with M(w) of 25,000) has been known as an efficient gene carrier and a gold standard of gene transfection due to its high transfection efficiency (TE). However, high concomitant cytotoxicity limited the application of PEI. In this report, several cationic polymers derived from low molecular weight (LMW) PEI (M(w) 600) linked with diglycidyl adipate (DA-PEI) or its analogs (diglycidyl succinate, DS-PEI and diglycidyl oxalate, DO-PEI; D-PEIs for all 3 polymers) were prepared and characterized. GPC gave M(w)s of DA-PEI, DS-PEI and DO-PEI as 6861, 16,015 and 35,281, respectively. Moreover, degradation of the ester-containing DS-PEI was also confirmed by GPC. In addition, hydroxyls in these polymers could improve their water solubility. These polymers exhibited good ability to condense plasmid DNA into nanoparticles with the size of 120-250 nm. ζ-potentials of the polyplexes were found to be around +10-20 mV under weight ratios (polymer/DNA) from 0.5 to 32. Agarose gel retardation showed that DNA could be released from the polyplexes after being pre-incubated for 30 h. In vitro experiments were carried out and it was found that DS-PEI showed about 5 times of TE compared to that of the PEI/DNA polyplex under a weight ratio of 1 in A549 cells. Meanwhile, the cytotoxicity of D-PEIs assayed by MTT is lower than that of 25 kDa PEI in HEK293 cells. These results suggested that this series of PEI derivatives would be promising non-viral biodegradable vectors for gene delivery.