The Aggregation Mechanism Of Human Islet Amyloid Polypeptide

The process of protein aggregation is considered as an important causal factor for over20diseases. The misfolding of human islet amyloid polypeptide (hIAPP) is regarded as an important factor that leads to type2diabetes and islet transplantation failure. A better understanding on the mechanism of hIAPP aggregation is important not only for understanding the pathological process of many amyloid diseases like type2diabetes, but is also critical for the development of novel anti type2diabetes drugs that based on inhibiting the aggregation of hIAPP.Under physiological conditions, the porcine islet amyloid polypeptide is unable to aggregate. Sequence comparison showed that seven out often variation sites between sequences of amyloid-resistant pIAPP and hIAPP, locate within residues17-29.To investigate why pIAPP is resistant to aggregation, and the role of each residue on aggregation, we chemically synthesized twenty six IAPP (17-29) or IAPP (20-29) variants, and their secondary structures, properties of aggregation and cytotoxicity were studied. Our results suggested that pIAPP fragments show much lower aggregation and toxicity propensities than the control hIAPP fragments. In addition, among all human to porcine substitutions, S20R caused the most prolonged lag time and significantly attenuated cytotoxicity, indicating the20S of hIAPP aggregation plays an important role during the amyloid formation of hIAPP. Further size exclusion chromatography (SEC) analysis showed that pIAPP (20-29) forms a stable dimer in solution, whereas hIAPP (20-29) exists as a mixture of monomers and trimers, which may be the molecular basis of the process of different aggregation behaviors.A number of cellular factors affect the amyloid formation of hIAPP in the pancreatic β cells, including acidic pH, metal ions and the bio-membrane. The only histidine in hIAPP, His18, has been suggested to be an important site for these interactions. In order to study the important role of the imidazole ring in the hIAPP aggregation, we used DEPC modification to obtain a modified hIAPP compound with acylated ethyl. ThT fluorescence, fluorescein leakage, circular dichroism spectroscopy, plasma coupled mass spectrometry and in silico simulation were further applied to study the role of imidazole ring plays in the aggregation process of hIAPP, as well as its interaction with metal ions, biomembranes and different pH conditions.