Calcium-activated membrane interaction of the islet amyloid polypeptide: implications in the pathogenesis of type II diabetes mellitus.

The role played by Ca(2+) ions in the interaction of the human islet amyloid polypeptide (hIAPP) with model membranes has been investigated by differential scanning calorimetry (DSC) and circular dichroism (CD) experiments. In particular, the interaction of hIAPP and its rat isoform (rIAPP) with zwitterionic dipalmitoyl-phosphatidylcholine (DPPC), negatively charged dipalmitoyl-phosphatidylserine (DPPS) vesicles and with a 3:1 mixtures of them, has been studied in the presence of Ca(2+) ions. The experiments have evidenced that amorphous, soluble hIAPP assemblies interact with the hydrophobic core of DPPC bilayers. Conversely, the presence of Ca(2+) ions is necessary to activate a preferential interaction of hIAPP with the hydrophobic core of DPPS membranes. These findings support the hypothesis that an impaired cellular homeostasis of Ca(2+) ions may promote the insertion of hIAPP into the hydrophobic core of carrier vesicles which is thought to contribute to an eventual intracellular accumulation of beta-sheet rich hIAPP aggregates.