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Type 2 diabetes is one of the most prevalent metabolic diseases worldwide, affecting hundreds of millions of people. A hallmark of the disease is the accumulation of amyloid fibrils formed by the islet amyloid polypeptide, hIAPP, in the pancreatic islets, contributing to β-cell dysfunction and death. Despite decades of research, the structural determinants of hIAPP aggregation and their implications for disease remain poorly understood. This thesis makes use of cryo-electron microscopy and biophysical characterization to investigate the structural diversity of hIAPP fibrils and leverage this knowledge toward the development of new therapeutic strategies.

We first investigate the effect of solution conditions on hIAPP polymorphism and cross-aggregation with rat IAPP. Our results reveal that buffer composition, co-solvents and peptide ratios determine the fibril structures formed, and that rat IAPP can switch from inhibitor to co-aggregator depending on the aggregation environment, highlighting the importance of solution conditions in aggregation studies.

Building on these findings, we solved the cryo-EM structures of three proline mutants of hIAPP inspired by the non-amyloidogenic rat sequence. Each mutant gives rise to distinct fibril polymorphs, revealing that proline substitutions reshape the amyloidogenic core of hIAPP. Across all structures, conserved structural motifs emerge, such as the central role of Phe23 in hydrophobic core stabilization. These recurring features were used as targets for a structure-based design, yielding two new peptide sequences with reduced amyloidogenicity. Most strikingly, the F23R-A25P double mutant showed complete resistance to fibril formation under all conditions tested, including physiologically relevant and seeded conditions. In addition, it fully abolished hIAPP-associated cytotoxicity in pancreatic β-cell assays, demonstrating the power of rational, structure-based design for the development of therapeutic candidates against type 2 diabetes.

Finally, we determined the cryo-EM structure of proIAPP(1-48) fibrils and found that it closely resembles a polymorph exclusively associated with ex vivo seeded hIAPP fibrils. Molecular dynamics simulations further revealed transient interactions between the disordered N-terminal extension and His18, suggesting that proIAPP acts as a structural template that initiates disease-relevant amyloid formation in the pancreatic islets, positioning precursor misprocessing as an early and potentially targetable event in islet amyloidosis.