Macromolecular Crowding Stabilizes the Functional, Non-Toxic State of IAPP by Suppressing its Fibrillation

The interior of the biological cell is known to be a crowded milieu, which significantly influences protein association and aggregation. As several cell degenerative diseases, such as Parkinson¯s disease or type-2 diabetes mellitus, are related to the misfolding, self-association and subsequent fibrillation of amyloidogenic peptides, understanding of the impact of macromolecular crowding on these processes is of high biomedical importance. This study focuses on the properties of human islet amyloid polypeptide (hIAPP) in crowded environments of two different kinds: network-like structures formed by polysaccharides and high concentrations of inert globular proteins. Two distinct processes could be distinguished in these crowded solutions: The formation of stable globular off-pathway species, and the usual hIAPP aggregation pathway from a disordered monomeric structure via nuclei formation to fibril formation. To which extent the different pathways are populated is shown to depend markedly on the crowder concentration and the geometry of the confinement. Different to other amyloidogenic peptides, the latter process is retarded or even inhibited at high crowding concentrations, but unchanged on the mechanistic level. As hIAPP is related to type-2 diabetes mellitus and presumably responsible for the disease accompanying β-cell-membrane permeabilization and the final β-cell loss, hIAPP specific cytotoxicity assays were conducted as well. Conversely to the high cytotoxicity exhibited by the normal fibrillation pathway, the data reveal a non-toxic effect for the off-pathway species stabilized through the crowding agents. From these results it can be postulated that cellular crowding is able to stabilize the native, non-toxic and functional conformation of hIAPP inside the biological cell.