Bilayer mixing, fusion, and lysis following the interaction of populations of cationic and anionic phospholipid bilayer vesicles

Cationic, O-alkylphosphatidylcholines, recently developed as DNA transfection agents, form bilayers indistinguishable from those of natural phospholipids and undergo fusion with anionic bilayers. Membrane merging (lipid mixing), contents release, and contents mixing between populations of positive vesicles containing O-ethylphosphatidylcholine (EDOPC) and negative vesicles containing dioleolylphosphatidylglycerol (DOPG) have been determined with standard fluorometric vesicle-population assays. Surface-charge densities were varied from zero to full charge. All interactions depended critically on surface-charge density, as expected from the adhesion-condensation mechanism. Membrane mixing ranged from zero to 100%, with significant mixing (>10 <70%) occurring between cationic vesicles that were fully charged and anionic vesicles that had fractional surface charges as low as 0.1. Such mixing with membranes as weakly charged as cell membranes should be relevant to transfection with cationic lipids. Unexpectedly, lipid mixing was higher at high than at low ionic strength when one lipid dispersion was prepared from EDOPC plus DOPG (in different proportions), especially when the other vesicles were of EDOPC; this may somehow be a consequence of the ability of the former mixture to assume non-lamellar phases. Leakage of aqueous contents was also a strong function of charge, with fully charged vesicles releasing essentially all of their contents less than 1 min after mixing. EDOPC was more active in this regard than was DOPG, which probably reflects stronger intermolecular interactions of DOPG. Fusion, as measured by contents mixing, exhibited maximal values of 10% at intermediate surface charge. Reduced fusion at higher charge is attributed to multiple vesicle interactions leading to rupture. The existence of previously published data on individual interactions of vesicles of the same composition made it possible for the first time to compare pairwise with population interactions, confirming the likelihood of population studies to overestimate rupture and hemifusion and underestimate true vesicle fusion.