Lipid Composition-Dependent Membrane Fragmentation and Pore-Forming Mechanisms of Membrane Disruption by Pexiganan (MSI-78)

Abstract

The potency and selectivity of many antimicrobial peptides (AMPs) are correlated with their ability to interact with and disrupt the bacterial cell membrane. In vitro experiments using model membranes have been used to determine the mechanism of membrane disruption of AMPs. Since the mechanism of action of an AMP depends on the ability of the model membrane to accurately mimic the cell membrane, it is important to understand the effect of membrane composition. Anionic lipids which are present in the outer membrane of prokaryotes but are less common in eukaryotic membranes are usually considered key for the bacterial selectivity of AMPs. We show by fluorescence measurements of peptide-induced membrane permeabilization that the presence of anionic lipids at high concentrations can actually inhibit membrane disruption by the AMP MSI-78 (pexiganan), a representative of a large class of highly cationic AMPs. Paramagnetic quenching studies suggest MSI-78 is in a surface-associated inactive mode in anionic SDS micelles, but is in a deeply buried and presumably more active mode in zwitterionic DPC micelles. Furthermore, a switch in mechanism occurs with lipid composition. Membrane fragmentation with MSI-78 is observable in mixed vesicles containing both anionic and zwitterionic lipids but not in vesicles composed of a single lipid of either type. These findings suggest membrane affinity and membrane permeabilization are not always correlated, and additional effects can be seen as the complexity of the model membranes is increased that may be more reflective of the actual cellular environment.

A 17-mer Membrane-Active MSI-78 Derivative with Improved Selectivity toward Bacterial Cells

“…In the case of MSI-78(4−20), its higher hydrophobicity (higher H and μH) can favor a deeper insertion within the bacterial lipid bilayer, which may explain its ability to exert a bacterial effect despite its lower ability to form α-helices. 41 These results are in line with the published MSI-78 mechanism of interaction with bacterial membrane models, involving induction of substancial changes in lipid bilayers via positive curvature strain and toroidal pore formation. 42 We have also found that MSI-78(4−20) is less hemolytic than MSI-78, which represents a major advantage of the 17-mer AMP as compared to the lead MSI-78.…”

Membrane disruptive antimicrobial activities of human β-defensin-3 analogs

“…For example, the roles of lipid charge, lipid bilayer thickness, and lipid bilayer composition can be better understood by suitably combining experimental measurements with MD simulations. In fact, the differences in the membrane composition between bacterial and mammalian membranes are key in differentiating the function of AMPs [39–41]. Studies have shown that the presence of anionic lipids enhances the membrane binding affinity of AMPs to bacterial membranes, while the presence of cholesterol in mammalian membranes enhances the selectivity of AMPs by inhibiting the AMP-induced membrane disruption [42].…”

Sequence-activity relationship, and mechanism of action of mastoparan analogues against extended-drug resistant Acinetobacter baumannii

“…There are several mechanisms of action for the antimicrobial peptides, although the global positive charge of most of the peptides leads to a mechanism of action involving the membrane of the bacteria [16]. AMPs has the ability to defeat bacteria creating pores into the membrane [17], also acting as detergents [18], or by the carpet mechanism [19]. We have previously reported the activity of different peptides against colistin-susceptible and colistin-resistant A. baumannii clinical isolates, showing that mastoparan, a wasp generated peptide (H-INLKALAALAKKIL-NH 2 ), has good in vitro activity against both colistin-susceptible and colistin-resistant A. baumannii [20].…”