Regulation of divalent metal ions to the aggregation and membrane damage of human islet amyloid polypeptide oligomers

The accumulation of human islet amyloid polypeptide (hIAPP) on the surface of pancreatic β cells is closely related to the death of the cells. Divalent metal ions play a significant role in the cytotoxicity of hIAPP. In this study, we examined the roles played by the divalent metal ions of zinc, copper and calcium in the aggregation of both hIAPP18-27 fragment and full-length hIAPP and the ability of their oligomers to damage the membrane of POPC/POPG 4 : 1 LUVs using the ThT fluorescence, TEM, AFM, CD, ANS binding fluorescence and dye leakage experiments. We prepared metal-free and metal-associated oligomers that are similar in size and aggregate slowly using the short peptide and confirmed that the ability of the peptide oligomers to damage the lipid membrane is reduced by the binding to the metal ions, which is closely linked to the reducing hydrophobic exposure of the metal-associated oligomers. The study on the full-length hIAPP showed that the observed membrane damage induced by hIAPP oligomers is either mitigated at a peptide-to-metal ratio of 1 : 0.33 or aggravated at a peptide-to-metal ratio of 1 : 1 in the presence of Zn(ii) and Cu(ii), while the surface hydrophobicity of hIAPP oligomers was reduced at both peptide-to-metal ratios. The observed results of the membrane damage were attributed to the counteraction between a decrease in the disruptive ability of metal-associated oligomer species and an increase in the quantity of oligomers promoted by the binding of the metal ions to hIAPP oligomers. The former could play a predominant role in reducing the membrane damage at a peptide-to-metal ratio of 1 : 0.33, while the latter could play a predominant role in enhancing the membrane damage at a peptide-to-metal ratio of 1 : 1. This study shows that an enhanced membrane damage could be caused by the oligomer species with a decreased instead of an increased disruptive ability, given that the abundance of the oligomer species is high enough.