Fusion of mammalian cells by unilamellar lipid vesicles: Influence of lipid surface charge, fluidity and cholesterol

The fusion of mammalian cells cultured in vitro by unilamellar vesicles prepared from individual or mixed phospholipid species is described. Maximum cell fusion in both monolayer and suspension cell cultures occurred at a dose of approx. 10⁴ vesicles per cell. No significant cell damage was observed in the treated cells. Variation of the chemical composition of the vesicles has permitted an evaluation of the importance of such membrane parameters as surface charge, fluidity and the presence of cholesterol. The results were as follows. 1. 1. Extensive cell fusion occurred in cell cultures treated with vesicles prepared from negatively charged phosphatidylserine molecules or a mixture of 10% phosphatidylserine and 90% phosphatidylcholine. Neutral phosphatidylcholine vesicles did not induce cell fusion. Incorporation of lysophosphatidylcholine into phosphatidylserine or mixed phosphatidylserine-phosphatidylcholine vesicles produced a small increase in their cell fusion capacity but was accompanied by marked cytotoxicity and cytolysis. 2. 2. The treatment of cells at 37°C with vesicles containing lipids that were in the liquid-crystalline state at 37°C (10% phosphatidylglycerol-90% phosphatidylcholine) or were in an intermediate fluid condition at this temperature (10% dipalmitoylphosphatidylglycerol-90% dipalmitoylphosphatidylcholine) induced significantly greater cell fusion than vesicles composed of lipids that were in the solid phase at 37°C (10% distearylphosphatidylglycerol-90% distearoylphosphatidylcholine). 3. 3. The incorporation of equimolar amounts of cholesterol into dipalmitoyl-phosphatidylglycerol-dipalmitoylphosphatidylcholine vesicles reduced significantly their ability to fuse cells but similar amounts of cholesterol had no significant effect on the cell fusion potential of phosphatidylserine-phosphatidylcholine vesicles. 4. 4. The influence of surface charge, "fluidity" and the presence of cholesterol on the cell fusion capacity of vesicles is discussed with reference to possible events occurring in the fusion of natural membranes.