Multi-component peptide-based carriers for gene delivery

The feasibility of most gene therapy strategies depends on the efficient delivery of DNA to target cells and tissues. Current gene delivery carriers can be divided into two classes: viral and non-viral delivery systems. Although the viral carriers are highly efficient due to their invasive nature, safety concerns may restrict their application in clinical settings. Synthetic non-viral carriers attract increasing attention because they are less toxic and allow readily modification. Non-viral carrier mediated gene delivery involves several processes. They must condense DNA into small particles, allow membrane penetration and protect DNA from extracellular and intracellular degradative enzymes. In the present study, a small library of carriers containing various combinations of cell penetrating peptide TAT, SV40 large T protein nuclear localisation signal (NLS) and cationic dendrimer of 7 lysine residues (DEN) was synthesised and tested for their ability to deliver DNA to mammalian cells. We evaluated the contribution of each component as well as the combination of the components on DNA condensation, uptake and gene expression. It was found that all carriers condensed DNA and protected DNA from DNase degradation. We showed that the TAT peptide was essential, but not sufficient, for uptake of exogenous DNA. The addition of either NLS or DEN significantly enhanced uptake. The most efficient carrier contained all three components (DEN-NLS-TAT). The carriers were able to deliver DNA in the presence of serum and were non-toxic to cells at up to 30 μM. However, for those peptides that facilitated DNA uptake, the complexes were targeted to intracellular compartments that required a fusogenic agent, such as chloroquine, before gene expression was observed. Modifications were introduced to the initial carrier library in order to circumvent the chloroquine dependence. The addition of cell penetrating peptide penetratin, virus derived fusogenic peptide or lipoamino acid C12 enhanced either DNA uptake or endosomal release. However, none of the modified carriers were able to produce high level transgene expression in the absence of chloroquine. We also found that the carriers containing lipid components were able to deliver DNA to T-lymphocytes derived cells, which are usually resistant to transfection. However, the toxicity of the lipid-based carriers needs to be reduced before further application. We also evaluated the function of chloroquine as a gene expression enhancer. We demonstrated that chloroquine did not enhance expression solely by promoting endosomal release. This was supported by the fact that fusogenic peptide and endosomal disruptive reagents (bafilomycin A1 and monensin) did not improve gene expression. Other properties of chloroquine, such as DNA protection and transcription enhancement, may also contribute to gene expression. We characterised the uptake mechanism of DEN-NLS-TAT in HeLa cell lines. We found that the uptake of DEN-NLS-TAT/DNA complex in HeLa cell line was mainly via receptor-mediated endocytosis and caveolae endocytosis. Moreover, various intracellular processes, such as intact cytoskeleton and microtubule network, tyrosine and PI 3 kinase activity, and membrane cholesterol were also required for the uptake of the carrier/DNA complex. In conclusion, the results from the present study demonstrated that multi-component peptide-based carriers are versatile carriers for the delivery of plasmid DNA in human cells. The results have improved our understanding of the role of chloroquine as a widely used gene expression enhancer which may be useful in the future improvement of non-viral gene delivery carriers. A strategy to overcome the dependence on chloroquine for gene expression or reduce the toxicity of chloroquine will be necessary for further in vivo applications. The current carrier library may also be used to delivery other cargos such as siRNA or protein to human cells