Lipid membrane composition modulates the structural evolution of early-stage human islet amyloid polypeptide amyloids

Progressive β cell death resulting from the aggregation of human islet amyloid polypeptide (hIAPP) in the pancreas is a molecular driver of type 2 diabetes (T2D). Recent studies from our group have shown that hIAPP forms two distinct amyloid oligomers: on-path donut oligomers (DOs) and off-path round oligomers (ROs). Previous studies have also shown lipids, one of the most abundant classes of biomolecules within cells, alter the kinetics of hIAPP amyloid formation and template structurally distinct hIAPP fibrils. However, the influence of the specific lipid environment on the evolution of early-stage hIAPP amyloids remains unknown. In this study, we employ atomic force microscopy (AFM) and atomic force microscopy Infrared (AFM-IR) spectroscopy to resolve the morphological and structural dynamics of early-stage hIAPP DOs, ROs, and fibrils (FBs) formed in the presence of phosphatidylcholine (PC) or phosphatidylserine (PS). PC-templated DOs with predominantly parallel β-sheet structure, while PS-templated DOs were predominantly antiparallel β-sheet. These DOs were present only during the early stages of aggregation. Conversely, ROs were persistent and their structural evolution was more stochastic irrespective of the lipid environment. The secondary structure of lipid-templated DOs correlated precisely with that of early-stage FBs, highlighting their role as on-path oligomers in hIAPP aggregation. Finally, the lipid environment present during aggregation uniquely altered the structural evolution of FBs, with PC favoring a progressive increase in antiparallel β-sheet FBs and PS favoring an increase in parallel β-sheet. These results provide crucial insight into the mechanisms of hIAPP aggregation in the lipid-rich cellular environment, simultaneously solidifying lipids as critical players in the early stages of hIAPP aggregation.