Interaction between lipids and antimicrobial oligomers studied by solid-state NMR

Bicontinuous inverted cubic phase stabilization as an index of antimicrobial and membrane fusion peptide activity

Some antimicrobial peptides (AMPs) and membrane fusion-catalyzing peptides (FPs) stabilize bicontinuous inverted cubic (QII) phases. Previous authors proposed a topological rationale: since AMP-induced pores, fusion intermediates, and QII phases all have negative Gaussian curvature (NGC), peptides which produce NGC in one structure also do it in another. This assumes that peptides change the curvature energy of the lipid membranes. Here I test this with a Helfrich curvature energy model. First, experimentally, I show that lipid systems often used to study peptide NGC have NGC without peptides at higher temperatures. To determine the net effect of an AMP on NGC, the equilibrium phase behavior of the host lipids must be determined. Second, the model shows that AMPs must make large changes in the curvature energy to stabilize AMP-induced pores. Peptide-induced changes in elastic constants affect pores and QII phase differently. Changes in spontaneous curvature affect them in opposite ways. The observed correlation between QII phase stabilization and AMP activity doesn't show that AMPs act by lowering pore curvature energy. A different rationale is proposed. In theory, AMPs could simultaneously stabilize QII phase and pores by drastically changing two particular elastic constants. This could be tested by measuring AMP effects on the individual constants. I propose experiments to do that. Unlike AMPs, FPs must make only small changes in the curvature energy to catalyze fusion. It they act in this way, their fusion activity should correlate with their ability to stabilize QII phases.

Designing mimics of membrane active proteins

As a semipermeable barrier that controls the flux of biomolecules in and out the cell, the plasma membrane is critical in cell function and survival. Many proteins interact with the plasma membrane and modulate its physiology. Within this large landscape of membrane-active molecules, researchers have focused significant attention on two specific classes of peptides, antimicrobial peptides (AMPs) and cell penetrating peptides (CPPs), because of their unique properties. In this Account, we describe our efforts over the last decade to build and understand synthetic mimics of antimicrobial peptides (SMAMPs). These endeavors represent one specific example of a much larger effort to understand how synthetic molecules interact with and manipulate the plasma membrane. Using both defined molecular weight oligomers and easier to produce, but heterogeneous, polymers, we have generated scaffolds with biological potency exceeding that of the natural analogues. One of these compounds has progressed through a phase II clinical trial for pan-staph infections. Modern biophysical assays have highlighted the interplay between the synthetic scaffold and lipid composition: a negative Gaussian curvature is required both for pore formation and for the initiation of endosome creation. Although work remains to better resolve the complexity of this interplay between lipids, other bilayer components, and the scaffolds, significant new insights have been discovered. These results point to the importance of considering the various aspects of permeation and how these are related to "pore formation". More recently, our efforts have expanded toward protein transduction domains, or mimics of cell penetrating peptides. Using a combination of unique molecular scaffolds and guanidinium-rich side chains, we have produced an array of polymers with robust membrane (and delivery) activity. In this new area, researchers are just beginning to understand the fundamental interactions between these new scaffolds and the plasma membrane. Negative Gaussian curvature is also important in these systems, but the detailed relationships between molecular structure, self-assembly with lipids, and translocation will require more investigation. It has become clear that the combination of molecular design, biophysical models, and biological evaluation provides a robust approach to the generation and study of novel proteinomimetics.