Revisiting the complexation between DNA and polyethylenimine - when and where -S-S- linked PEI is cleaved inside the...

As a non-viral gene vector, long PEI chains are more effective but also more cytotoxic. To solve this problem, people have tried to use disulfide (-S-S-) to link short PEI chains into a long one to generate highly efficient and less cytotoxic gene vectors because -S-S- is degradable inside the cell. In order to investigate when and where -S-S- is cleaved during intracellular trafficking, we designed and synthesized rhodamine B labeled linear PEI chains (M_n≈ 3 kg mol^-1) with one end modified with a mercapto-group so that they can be coupled together via one disulfide bond in the middle and one rhodamine B molecule on each side of the disulfide bond, where fluorescence is self-quenched because two rhodamine B molecules are closely linked together. The cleavage of the -S-S- bond separates the two rhodamine B molecules and enhances their fluorescent intensity. In addition, plasmid DNA was also modified with bodipy, a FRET donor of rhodamine B. Using this specially prepared PEI, we studied the intracellular trafficking of the PEI/DNA polyplexes by using flow cytometry and confocal laser scanning microscopy. Our results reveal that (1) DNA is gradually dissociated from the polyplex before the disulfide bond's cleavage; (2) some of polyplexes escaped from endosomes before reaching lysosomes; and (3) the disulfide bonds are mainly cleaved inside lysosomes at ∼5 h post-transfection.