Effect of osmotic pressure on pore formation in lipid bilayers by the antimicrobial peptide magainin 2

Osmotic pressure (pi) induces membrane tension in cells and lipid vesicles, which may affect the activity of antimicrobial peptides (AMPs) by an unknown mechanism. We recently quantitated the membrane tension of giant unilamellar vesicles (GUVs) due to pi under physiological conditions. Here, we applied this method to examine the effect of pi on the interaction of the AMP magainin 2 (Mag) with single GUVs. Under low pi values, Mag induced the formation of nanometer-scale pores, through which water-soluble fluorescent probe AF488 permeates across the membrane. The rate constant for Mag-induced pore formation (k(p)) increased with increasing pi. It has been proposed that the membrane tension in the GUV inner leaflet (sigma(in)) caused by Mag binding to the outer leaflet plays a vital role in Mag-induced pore formation. During the interactions between Mag and GUVs under pi, the sigma(in) increases due to pi, thereby increasing k(p). The relationship between the k(p) and the total sigma(in) due to pi and Mag agreed with that without pi. In contrast, Mag induced rupture of a subset of GUVs under higher pi. Using fluorescence microscopy with a high-speed camera, the GUV rupture process was revealed. First, a small micrometer-scale pore was observed in individual GUVs. Then, the pore radius increased within similar to 100 ms without changing the GUV diameter and concomitantly the thickness of the membrane at the pore rim increased, and finally the GUV transformed into a membrane aggregate. Based on these results, we discussed the effect of pi on Mag-induced damage of GUV membranes.