Membrane fusion due to dehydration by polyethylene glycol, dextran, or sucrose

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Abstract
To determine whether polyethylene glycol (PEG) causes growth of liposomes by affecting them directly or indirectly, vesicles composed of phosphatidylcholine were exposed to increasing concentrations of MW 15,000-20,000 PEG or MW 40,000 dextran either by direct mixing or across a dialysis membrane. After incubation at room temperature and dilution below at least 5% (w/w) polymer, the vesicles were monitored for fluorescence energy transfer and for absorbance at 400 nm. PEG induced the same levels of dequenching or lipid mixing and increased turbidity, regardless of whether the vesicles had been mixed directly or with dialyzed against PEG. These changes occurred within 5-15 min of polymer application. The increased lipid mixing and/or increased turbidity, indicating vesicle growth, resulted from an indirect effect of PEG on the vesicles-most likely dehydration. Dextran, in contrast to PEG, induced less dequenching and/or less turbidity increase when vesicles were directly mixed with, as opposed to dialyzed against, dextran. Although dextran not in contact with vesicles and with osmotic activity comparable to PEG was able to cause a degree of membrane fusion similar to that of PEG, therefore, the dehydrating effect of dextran could be mitigated if it were allowed to interact with vesicles. In further support of membrane dehydration as a precursor to membrane fusion, lipid mixing among sonicated and sonicated, frozen-thawed vesicles dialyzed against sucrose increased as a function of sucrose concentration. Vesicle morphology generally determined the maximal degree of membrane fusion inducible by the polymers. Sonicated vesicles gave maximal responses to 20-30% (w/w) polymer, which were generally greater than maximal responses to 30-40% (w/w) polymer given by sonicated vesicles frozen-thawed 3 times (i.e., medium sized and uni- to paucilamellar). Large, multilamellar vesicles, in turn, gave the lowest maximal responses and also to 30-40% (w/w) polymer.