Destabilization of fatty acid-containing liposomes by polyamidoamine dendrimers

Abstract

Polyamidoamine dendrimers have been shown to be very effective at transporting DNA across cell membranes in transfection experiments. To investigate the membrane interactions with dendrimers that could contribute to this efficacy, the ability of dendrimers to permeabilize lipid vesicles in suspension has been studied. Vesicles were prepared containing the self-quenching, membrane impermeant dye calcein, and were treated with dendrimers of different sizes. Increase in fluorescence was attributed to release of the dye. Membranes containing dioleoyl phosphatidylethanolamine and stearic or oleic acid, lipids with a preference for non-lamellar phases, were very susceptible to disruption by dendrimers, with larger dendrimers being more effective than smaller. However, membranes containing lipids with a preference for the lamellar phase (either pure phosphatidylcholine, or phosphatidylcholine:phosphatidylserine) were largely unaffected. The concentration dependence of the permeabilization strongly suggests an aggregation-mediated mechanism for membrane disruption. Requenching measurements using cobalt citrate showed that permeabilization did not occur uniformly among the vesicles, but rather was all-or-none, with a subpopulation of vesicles responsible for essentially all of the dye release. This is also suggestive of an aggregation-induced response. Lastly, although osmotic forces are thought to play an important role in dendrimer-mediated transfection, we observed no effect of osmotic pressure and membrane tension on the efficacy of dendrimers in solution. It is likely that, in cells, dendrimers traverse cell membranes via endosomes, and the entrapment of the dendrimer itself within the endosomal vesicle may be a key factor in its ability to cause rupture.