Synthetic Vesicles As a Predictive Tool for Permeabilizing Activity of Peptides in Biological Membranes

To better understand the basis for selective membrane-permeabilizing activity by cationic, amphipathic peptides, the bee venom lytic peptide melittin and various antimicrobial peptides were assayed for their ability to cause leakage of entrapped contents from synthetic vesicles of varying lipid compositions. The vesicle compositions were selected on the basis of cell type and consist of common membrane models for eukaryotic cells as well as Gram-negative and Gram-positive bacteria. Membrane active peptides are shown to display a variety of membrane composition, sequence, and concentration-dependent leakage-inducing activity. The various membrane models are good predictors of antimicrobial and cytolytic potency. In order to differentiate between leakage mechanisms that conserve vesicle structural integrity and activity due to vesicle aggregation and fusion, a common artifact of membrane-active peptides, 5 mol percent DOPE-PEG(2000) lipids were introduced into each membrane model to inhibit aggregation. Vesicles with POPE rather than POPC as the primary zwitterionic lipid were the most impacted by the addition of PEGylated lipids, showing an increase in stability as well as the largest increases in peptide concentration required to induce 50% leakage of entrapped contents (LIC50). In POPC vesicle, the melittin derived peptides showed the largest increases in LIC50, while the inverse was true of POPE vesicles with library derived antimicrobial peptides being inactive at peptide to lipid ratios lower than 1:25 in vesicles with PEGylated lipid. The results show both the utility and the limitations of synthetic vesicles as predictors of cytolytic activity as well as the impact of PEGylated lipids beyond limiting aggregation.