Designing Mimics of Membrane Active Proteins

Abstract

CONSPECTUS As a semi-permeable 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, it has been possible to generate scaffolds with biological potency superior to the natural analogs. In one case, a compound has progressed through a phase II clinical trial for pan)staph infections. Modern biophysical assays highlighted the interplay between the synthetic scaffold and lipid composition leading to negative Gaussian curvature, a requirement for both pore formation and endosomal escape. The complexity of this interplay between lipids, bilayer components, and the scaffolds remains to be better resolved, but significant new insight has been provided. It is worthwhile to consider the various aspects of permeation and how these are related to ‘pore formation.’ More recently, our efforts have expanded toward protein transduction domains, or cell penetrating peptide, mimics. The combination of unique molecular scaffolds and guanidinium) rich side chains has produced an array of polymers with robust transduction (and delivery) activity. Being a new area, the fundamental interactions between these new scaffolds and the plasma membrane are just beginning to be understood. Negative Gaussian curvature is important but the detailed relationships between molecular structure, self)assembly with lipids, and translocation require more investigation. It has become clear that the combination of molecular design, biophysical models, and biological evaluation provide a robust approach to the generation and study of novel proteinomimetics.

Viral and Other Cell-Penetrating Peptides as Vectors of Therapeutic Agents in Medicine

“…This step is followed by the translocation of the Tat peptides that diffuse on the pore surface (Herce and Garcia, 2007). The pore formation model occurs through disruption of the lipid bilayer initiated by interactions between the side chains of basic amino acids such as Arg9 and phospholipid phosphate groups of cell membranes (Sgolastra et al, 2013). In this model, hydrophobic residues are near the lipid chains, and hydrophilic residues help in formation of the central pore.…”

Membrane-Bound Conformations of Antimicrobial Agents and Their Modes of Action

“…The use of such naturally occurring AMPs for a rigorous study of relationship between the helical structure and antimicrobial activity is thus limited [76]. Recent studies, however, opened up the possibility of studying the helical structureactivity relationship more thoroughly through development of mimics of naturally occurring AMPs using both biotic and abiotic backbones [76][77][78][79][80][81]. Such oligomers, along with the ability to form a diverse range of proteomimetic structures, are also considerably less susceptible to proteolysis [77,82].…”

“…Structure-activity studies using such agents have not indicated any appreciable correlation between propensity for helix formation and antimicrobial activity [80]. The importance of other structural attributes such as amphiphilicity, charge-to-hydrophobic ratio, and oligomer size has been observed to be critical for such agents too [81,86,87]. Enhancement of hydrophobicity, generally resulting in greater stability of ordered helical conformations, has often been observed to adversely affect hemolytic activity of AMPs [82,85,87].…”

“…While the above-mentioned studies strongly suggest the importance of amphiphilicity over stability of secondary helical structures for antimicrobial activity, it is not clear whether the ability to adopt a helical (or any other well-defined secondary structural) conformation at the lipid interface is necessary for antimicrobial action [81]. Insight into structural requirement, as well as tunability of parameters such as charge, hydrophobicity, and molecular weight, was gained by the use of polymers and oligomers using synthetic (abiotic) backbones.…”

“…Diverse classes of synthetic oligomers and polymers have been designed, which vary widely from peptides, yet demonstrate antimicrobial potency as well as selectivity against toxicity to host cells [20,24,26,29,81,98]. Design principles have often involved rigid backbones with amphiphilicity built into the molecular structure (phenylene ethynylenes [27,28,81], arylamides [24,25,81], polynorbornenes [26,81]) so as to mimic the functional structures of folded naturally occurring AMPs. The design strategies have also been extended to more flexible polymeric structures such as polymethacrylate copolymers [20] and nylon derivatives [29,30].…”