Identification of a hinge residue controlling islet amyloid polypeptide self-assembly and cytotoxicity

The islet amyloid polypeptide (IAPP) is a 37-residue peptide hormone whose deposition as amyloid fibrils in the pancreatic islets is associated with type 2 diabetes. Previous studies have suggested that residue Asn-21 plays a critical role in the in vitro self-assembly of IAPP. Herein, we studied structure-self-assembly relationships focusing on position 21 to gain detailed insights into the molecular mechanisms of IAPP self-assembly and to probe the conformational nature of the toxic assemblies associated with β-cell death. Thioflavin T (ThT) fluorescence, CD spectroscopy, and transmission EM analysis revealed that the Asn-21 amide side chain is not required for IAPP nucleation and amyloid elongation, as N21A and N21G variants assembled into prototypical fibrils. In contrast, Asn-21 substitution with the conformationally constrained and turn-inducing residue Pro accelerated IAPP self-assembly. Successive substitutions with hydrophobic residues led to the formation of ThT-negative β-sheet-rich aggregates having high surface hydrophobicity. Cell-based assays revealed no direct correlation between the in vitro amyloidogenicity of these variants and their toxicity. In contrast, leakage of anionic lipid vesicles disclosed that membrane disruption is closely associated with cytotoxicity. We observed that the N21F variant self-assembles into worm-like aggregates, causing loss of lipid membrane structural integrity and inducing β-cell apoptosis. These results indicate that specific intra- and intermolecular interactions involving Asn-21 promote IAPP primary nucleation events by modulating the conformational conversion of the oligomeric intermediates into amyloid fibrils. Our study identifies position 21 as a hinge residue that modulates IAPP amyloidogenicity and cytotoxicity.