Lipid Composition and Raft Domain Formation on the IAPP-Membrane Interaction: The Role of Cholesterol on the Inhibition of IAPP (Amylin) Fibrilization and the Reduction of Membrane Disruption in Model Liposomes and Mouse Pancreatic Islets

Abstract

The misfolding and aggregation of islet amyloid polypeptide (IAPP) is known to exhibit an important role in the etiology of type-two diabetes mellitus. While it has been believed that the peptide becomes toxic in its large fibrous form, recent studies suggest that pre-fibril oligomers are in fact the predominantly toxic species. Permiabilization of IAPP on membranes is thus biphasic in nature. The latter phase is correlated with fiber formation and reaches maximal leakage. The degree of leakage, and hence membrane disruption, of the initial phase is dependent on the peptide to lipid ratio and percent anionic lipid in the membrane. Studies of IAPP toxicity have focused on POPC/POPS(G) vesicle systems. The homogeneity and anionic nature of the membranes have been shown to increase aggregation and toxicity. However, cellular membranes are vastly more complex. In this study, we investigate the in vitro interaction between IAPP and non-homogenous, cholesterol-containing model membranes; extend previous observations of the role of anionic phospholipids in membrane-catalyzed IAPP fibrilogenesis; and show that the presence of cholesterol dramatically inhibits IAPP fribrilogenesis and decreases membrane disruption in large unilamellar vesicles. Additionally, we demonstrate that human IAPP strongly permiablilizes raft type membranes. Fluorescence microscopy of islet cells shows increased IAPP toxicity after the removal of cholesterol from the membrane. These findings demonstrate that the mode of IAPP membrane binding and permiablization is highly dependent on the fluidity and phase of the membrane. The differences in this behavior may have significant implications in the development of type-two diabetes, as the change in membrane composition is dependent on a number of factors also related to the disease.