Poly(ethylenimine)-mediated transfection: A new paradigm for gene delivery

Poly(ethylenimine) (PEI) is a synthetic polycation that has been used succe ssfully for gene delivery both in vitro and in vivo due, in theory, to a fo rm of protection that is afforded to the carried plasmids. In this study th e stability of PEI/DNA complexes was demonstrated using deoxyribonuclease ( DNase) 1 and DNase 2, various levels of pH, and increasing exposure times. DNA that was complexed with PEI was not degraded when exposed to at least 2 5 Units of either enzyme for 24 h while uncomplexed forms of the same plasm id were digested when exposed to 0.010 Units of DNase 1 for 0.05 h or 0.003 Units of DNase 2 for 1 h. For further comparison, the stability of complex es made with poly(l-lysine) (PLL) and DNA was examined and found to be lowe r than that of PEI/DNA complexes; PLL-complexed DNA was digested on exposur e to 1.25 Units of DNase 1 for 3 min. Cells were transfected with PEI/DNA c omplexes and, by using a pH indicator and optical recording techniques, it was found that the normal lysosomal pH value of 5.0 was not altered, bringi ng into question PEI's hypothesized lysosomal entry. Confocal microscopy sh owed that PEI/DNA complexes and lysosomes do not merge during transfection (although PLL/DNA complexes do). The lack of lysosomal involvement in PEI-m ediated transfection is surprising because it goes against the conventional wisdom that has attempted to explain how PEI functions during transfection . PEI forms a stable complex with DNA, which moves from endocytosis to nucl ear entry without significant cellular obstacles. (C) 2000 John Wiley & Son s, Inc.