Distinct modes of action of IAPP oligomers on membranes

Abstract

Islet Amyloid Polypeptide (IAPP, also known as amylin) is a peptide hormone which is co-secreted with insulin by pancreatic {beta}-cells and forms amyloid aggregates in type II diabetes. Various lines of evidence indicate that oligomers of this peptide may induce toxicity by disrupting or forming pores in cell membranes but the structures of these pores are unknown. Here we create models of pores for both helical and {beta}-structured peptides using implicit membrane modeling and test their stability using multimicrosecond all-atom simulations. We find that the helical peptides behave similarly to antimicrobial peptides; they remain stably inserted in a highly tilted or partially unfolded configuration creating a narrow water channel. Parallel helix orientation creates a somewhat larger pore. An octameric {beta} barrel of parallel {beta}-hairpins is highly stable in the membrane, whereas the corresponding barrel made of antiparallel hairpins is not. We propose that certain experiments probe the helical pore state while others probe the {beta}-structured pore state; this provides a possible explanation for lack of correlation that is sometimes observed between in vivo toxicity and in vitro liposome permeabilization experiments.