Study On The Structures Of HIAPP_18-27 Oligomers And Their Interactions With Membranes

Alzheimer’s disease(AD), Parkinsons disease(PD), type II diabetes mellitus(T2MD), and several other age-related neurodegenerative and systemic disorders are protein-misfolding diseases characterized by the accumulation of insoluble protein deposits. The main constituents of these amyloid deposits are Aβ, α-synuclein, IAPP, respectively. These protein deposits are composed of β-sheet-rich fibrils. h IAPP has drawn wide attention of scientists because of its close correlation with type-2 diabetes mellitus. The human islet amyloid polypeptide(IAPP) includes 37 amino acid residues and co-secretes with insulin from pancreatic islet β-cells to maintain the balance blood sugar. The pathology of type II diabetes mellitus is characterized by an extracellular accumulation of amyloid plaques that are composed mainly of islet amyloid polypeptide(IAPP) near pancreatic β-cells. Increasing evidences from numerous structural determinations and biophysical experiments have suggested that soluble oligomeric intermediates rather than mature fibers of amyloid proteins are likely to be the cytotoxic species. Prefibrillar amyloid aggregates of proteins are known as cytotoxic species and a common pathogenesis of many degenerative diseases. The mechanism underlying the formation and cytotoxicity of prefibrillar aggregates is hypothesized to be independent of the actual nature of the amyloid protein.Short peptide fragments derived from amyloid peptides have been widely studied to explore the refolding mechanism of amyloid peptides. The so-called amyloid genic sequence(residues 22-27) in the core region is confirmed to be the minimal amyloid-forming sequence of h IAPP and plays a crucial role in fibrillation of h IAPP. The variation in the protonation state of His18 at different p H conditions affects the fibrillar structure, the aggregation rate and membrane damage of h IAPP significantly. In this study, we constructed soluble micelle-like oligomers using the human islet amyloid peptide fragment(h IAPP18-27), The peptide aggregated as micelle-like oligomers with the inner core comprised of the hydrophobic C-terminal part of residues and the outer shell comprised of the polar N-terminal part of residues in aqueous solution. We examined the disruptive effect of the peptide oligomers on the liposomes and determined the toxic species of the peptide oligomers to membrane by fluorescence probe, dynamic light scattering(DLS), transmission electron microscope(TEM), atomic force microscope(AFM) and proton nuclear magnetic resonance(NMR) measurements.We found that the short peptide first aggregates as a spherical micelle with a diameter of ~8 nm in aqueous solution and then transfers into a lamellar micelle with a length of ~20-60 nm, a width of ~5-10 nm and a thickness of ~1.5 nm in average by a fusion of a number of spherical oligomers. The lamellar micelle species of the peptide oligomers was found to be most disruptive to the lipid membrane of POPC/POPG(4:1) by micellization process. The lamellar species of the peptide could grow linearly in several days without changing lamel structure. The findings in this study give first evidence that the formation of a lamellar micelle-like species of the peptide might be crucial to the membrane damage by a micellization process. This study showed that the use of short peptides may be a choice in the study of toxic mechanism of amyloid peptides mediated by the oligomeric species because of an elongated lifetime of the intermediates of the short peptides.The results in our work may provide relating information for a better understanding of h IAPP pathogenic mechanism, and are valuable for understanding the structures and morphologies of amyloid peptide oligomers.