Three-dimensional imaging of lipid gene-carriers: Membrane charge density controls universal transfection behavior in lamellar cationic liposome-DNA complexes

Cationic liposomes; (CLs) are used worldwide as gene vectors (carriers) in nonviral clinical applications of gene delivery, albeit with unacceptably low transfection efficiencies (TE). We present three-dimensional laser scanning confocal microscopy studies revealing distinct interactions between CL-DNA complexes, for both lamellar L-alpha(C) and inverted hexagonal H-II(C) nanostructures, and mouse fibroblast cells. Confocal images of L complexes in cells identified two regimes. For low membrane charge density (sigma(M)), DNA remained trapped in CL-vectors. By contrast, for high sigma(M), released DNA was observed in the cytoplasm, indicative of escape from endosomes; through fusion. Remarkably, firefly luciferase reporter gene studies in the highly complex L-alpha(C)-mammalian cell system revealed an unexpected simplicity where, at a constant cationic to anionic charge ratio, TE data for univalent and multivalent cationic lipids; merged into a single curve as a function of sigma(M), identifying it as a key universal parameter. The universal curve for transfection by L-alpha(C) complexes climbs exponentially over approximate to four decades with increasing sigma(M) below an optimal charge density (sigma(M)*), and saturates for sigma(M) > sigma(M)* at a value rivaling the high transfection efficiency of H-II(C), complexes. In contrast, the transfection efficiency of H-II(C), complexes is independent of sigma(M). The exponential dependence of TE on sigma(M) for L-alpha(C) complexes, suggests the existence of a kinetic barrier against endosomal fusion, where an increase in sigma(M) lowers the barrier. In the saturated TE regime, for both L-alpha(C) complexes and H-II(C), confocal microscopy reveals the dissociation of lipid and DNA. However, the lipid-released DNA is observed to be in a condensed state, most likely with oppositely charged macro-ion condensing agents from the cytoplasm, which remain to be identified. Much of the observed bulk of condensed DNA may be transcriptionally inactive and may determine the current limiting factor to transfection by cationic lipid gene vectors.