The influence of membrane curvature on insulin aggregation.

Insulin can form fibrils at the injection site in diabetic patients and is often used as a model for understanding protein aggregation properties, particularly the role of membranes. Here, we employ a range of spectroscopies and molecular dynamics simulations to understand differences in aggregation speed and structure of fibrils formed in the absence and presence of lipid vesicles, and influence of membrane curvature. We show that human insulin aggregation is accelerated in the presence of phosphatidylcholine vesicles, with small unilamellar vesicles accelerating aggregation more than large vesicles. Insulin fibrils formed in the presence of lipid vesicles have a more ordered β-sheet structure than those formed from bulk solution, which is not influenced by vesicle curvature as shown by vibrational spectroscopy. Changes in hydrogen bonding of the lipid phosphate and glycerol bonds within the lipid vesicles indicate that native insulin becomes embedded within the lipid headgroups. Molecular dynamics simulations confirm an increased residence time of insulin with increasing number of lipid packing defects and reveal the presence of three insulin adsorption states with different orientations. Our results suggest that a reduction in membrane damage can reduce insulin amyloid formation.