Amyloidogenic Propensities and Conformational Properties of ProIAPP and IAPP in the Presence of Lipid Bilayer Membranes

Abstract

The islet amyloid polypeptide (IAPP), which is considered as the primary culprit for β-cell degenration in type 2 diabetes mellitus patients, is synthesized in β-cells of pancreas from its precursor, pro-islet amyloid polypeptide (proIAPP). It has been proposed that proIAPP may be important in early intracellular amyloid formation, and misprocessing or abnormal interactions of the peptide may trigger the amyloid deposition. Here, we compare the amyloidogenic and conformational propensities of rhproIAPP and hIAPP in the presence of negatively charged lipid membranes, which have been discussed as loci of initiation of the fibrillation reaction. CD studies verify the initial secondary structures of the peptides to be predominantly unordered with small amounts of ordered secondary structure elements, and exhibit only minor differences between the peptides. Using ATR-FTIR and Thioflavin T fluorescence spectroscopy as well as AFM we show that in the presence of negatively charged membranes, proIAPP exhibits a much higher amyloidogenic propensity than in the absence of the membrane. Compared to IAPP, it is still much less amyloidogenic, however, probably due to the increased net charge on proIAPP. Differences in secondary structure content of the aggregated species of hIAPP and proIAPP at the membranous interface are also reflected in morphological changes. Unlike hIAPP, proIAPP forms small oligomeric-like disordered structures at the lipid interface, having heights of ~3.5 nm. This difference in morphological structure of proIAPP may be rationalized by the presence of the pro-region. We have also shown that the addition of rhproIAPP to hIAPP effectively decreases the rate of mature hIAPP fibril formation, probably by complex formation. Thus, it appears reasonable to speculate that the pro-region of the proIAPP could serve to protect the amyloidogenic peptide hIAPP against fibrillogenesis.