Structural and morphological dynamics of "on-path" and "off-path" oligomers of human islet amyloid polypeptide.

The deposition of cytotoxic human islet amyloid polypeptide (IAPP) aggregates is a hallmark feature of Type 2 Diabetes. However, the structural evolution and cytotoxicity of IAPP aggregate species remain poorly understood. This study combines kinetics, biophysical and cell assays to resolve the morphological dynamics of IAPP aggregation. Using atomic force microscopy (AFM) and atomic force microscopy Infrared (AFM-IR) spectroscopy, we observed two distinctly different types of oligomers, donut-like (DO) and round oligomers (RO), formed at the early stages of protein aggregation. DO were dominated by parallel β-sheet secondary structure. Their evanescence is linked to the formation of IAPP fibrils, which also had parallel β-sheet secondary structure. In contrast, RO had primarily disordered secondary structure and persisted throughout the course of fibril formation. This structural and kinetic analyses showed that RO were "off-path", while DO were "on-path" protein aggregates. Cell toxicity assays indicated that structural evolution of IAPP amyloids as well as persistent "off-path" oligomeric species both contribute to high cytotoxicity in pancreatic β cells. These results revealed a complex mechanism of IAPP aggregation which is highly important in the context of the prevention of pathological protein aggregation.