Non-viral gene delivery vectors are widely used for the delivery of genetic materials into mammalian cells. Currently, there is a need to develop cheap and efficient transfection agents for use in production of recombinant proteins, such as monoclonal antibodies, via transient gene expression (TGE). There are several barriers that non-viral vectors must overcome for successful transfection, such as cellular internalisation, escape of the endosome, protection and transportation of DNA through the cytosol, delivery of DNA into the nucleus, and finally be able to release the DNA to allow protein expression to occur. The ability to escape the endosome and gain entry to the nucleus are the two primary barriers to successful transfection. The processes involved in the pathways for cellular uptake, intracellular trafficking, and nuclear entry are still not fully understood. More detailed understanding of the pathways involved in transfection is needed in order to develop highly efficient transfection agents.
This thesis investigates the use of three series of cationic diblock copolymers as transfection agents for the production of small and large recombinant proteins, as well as examining the pathways the polymers used to deliver the DNA into the nucleus. The diblock copolymers were synthesised using 'living' radical polymerization techniques, with each series using the same first block poly(2-dimethylaminoethyl acrylate) (PDMAEA). The second block consists of N-(3-(1H-imidazol-1-yl)propyl) acrylamide (ImPAA) or butyl acrylate (BA) or a combination of both. The copolymer with both ImPAA and BA units were used to mimic the influenza virus mechanism for endosomal escape. The three series of polymers were tested in both Chinese Hamster Ovary (CHOS) and Human Embryonic Kidney (HEK293) cell lines. Polymer A-C3, with the second block copolymer of the ImPAA and BA not only showed the best protection against DNase I with a timed-release mechanism between 24-48 h, but also achieved the highest level of transfection efficiency in both cell lines tested. Using a GFP reported gene up to 50% of CHOS cells and 95% of HEK293 cells tested positive for gene expression. When transfections were performed in the presence of chloroquine, a chemical known to swell and burst endosomes, no increase in transfection efficiency was seen, suggesting that the polymer A-C3 is efficient at endosome escape.
The A-C3 series was shown to be the most efficient at mediating transfection in both CHOS and HEK293 cells. Due to the high transfection efficiency of the A-C3 polymer in HEK293 cells, this polymer was thus chosen to investigate internalization and nuclear entry pathways. Uptake of polymer/pDNA polyplexes was investigated through the use of specific inhibitors to block endocytosis pathways (chlorpromazine, filipin III, dynasore and amiloride). Our results indicate that the main endocytosis pathway used is clathrin-mediated endocytosis. Nuclear entry was tested next. The pathway for plasmid DNA (pDNA), either complexed or alone, is thought to enter the nucleus either through the nuclear pores or during mitosis when the nuclear membrane is temporarily disintegrated. Through the use of wheat germ agglutinin that blocks nuclear pores it was demonstrated that entry occurs primarily though the nuclear pores, most likely via active transport due to the large size of pDNA. The relative pDNA copy number was determined for HEK293 cells transfected with A-C3 and PEI Max over a 48 h time period, and it was found that the amount of pDNA within the nucleus of cells transfected with A-C3 across all time points was higher than PEI Max, with the A-C3 polymer able to deliver 7 times the amount of pDNA than PEI Max.
The production of a large recombinant protein was the final challenge for the three series of polymers. Monoclonal antibody (mAb) production was performed in both CHOS and HEK293 cell lines using optimum conditions for transfection, based on transfection using GFP as a reporter gene. Out of the polymers tested polymer A-C3 was once again the best performer, producing similar levels of mAb titre at day 4 to commercially available transfection agents PEI Max and Freestyle Max in HEK293 cells. However by day 8 the mAb titre for the A-C3 polymer was lower at 25mg/L compared to mAb titres of PEI Max and Freestyle Max which were both ~30 mg/L. This difference in titre is thought to be due to toxicity caused by the A-C3 polymer during transfection.
The results presented in this thesis attempts to improve our understanding of the pathways involved in the successful delivery of pDNA, both inside the cell and the nucleus. The ability to rationally design cationic polymers for use as gene delivery vectors could result in the next generation of highly efficient transfection agents used in transient gene expression systems.