Toward understanding amyloid aggregation

Identification of a penta- and hexapeptide of islet amyloid polypeptide (IAPP) with amyloidogenic and cytotoxic properties

Pancreatic amyloid is found in more than 95 % of type II diabetes patients. Pancreatic amyloid is formed by the aggregation of islet amyloid polypeptide (hIAPP or amylin), which is a 37-residue peptide. Because pancreatic amyloid is cytotoxic, it is believed that its formation is directly associated with the development of the disease. We recently showed that hIAPP amyloid formation follows the nucleation-dependent polymerization mechanism and proceeds via a conformational transition of soluble hIAPP into aggregated beta-sheets. Here, we report that the penta- and hexapeptide sequences, hIAPP(23-27) (FGAIL) and hIAPP(22-27) (NFGAIL) of hIAPP are sufficient for the formation of beta-sheet-containing amyloid fibrils. Although these two peptides differ by only one amino acid residue, they aggregate into completely different fibrillar assemblies. hIAPP(23-27) (FGAIL) fibrils self-assemble laterally into unusually broad ribbons, whereas hIAPP(22-27) (NFGAIL) fibrils coil around each other in a typical amyloid fibril morphology. hIAPP(20-27) (SNNFGAIL) also aggregates into beta-sheet-containing fibrils, whereas no amyloidogenicity is found for hIAPP(24-27) (GAIL), indicating that hIAPP(23-27) (FGAIL) is the shortest fibrillogenic sequence of hIAPP. Insoluble amyloid formation by the partial hIAPP sequences followed kinetics that were consistent with a nucleation-dependent polymerization mechanism. hIAPP(22-27) (NFGAIL), hIAPP(20-27) (SNNFGAIL), and also the known fibrillogenic sequence, hIAPP(20-29) (SNNFGAILSS) exhibited significantly lower kinetic and thermodynamic solubilities than the pentapeptide hIAPP(23-27) (FGAIL). Fibrils formed by all short peptide sequences and also by hIAPP(20-29) were cytotoxic towards the pancreatic cell line RIN5fm, whereas no cytotoxicity was observed for the soluble form of the peptides, a notion that is consistent with hIAPP cytotoxicity. Our results suggest that a penta- and hexapeptide sequence of an appropriate amino acid composition can be sufficient for beta-sheet and amyloid fibril formation and cytotoxicity and may assist in the rational design of inhibitors of pancreatic amyloid formation or other amyloidosis-related diseases.

Conformational transitions of islet amyloid polypeptide (IAPP) in amyloid formation in vitro

Amyloid aggregates have been recognized to be a pathological hallmark of several fatal diseases, including Alzheimer's disease, the prion-related diseases, and type II diabetes. Pancreatic amyloidosis is characterized by the deposition of amyloid consisting of islet amyloid polypeptide (IAPP). We followed the steps preceding IAPP insolubilization and amyloid formation in vitro using a variety of biochemical methods, including a filtration assay, far and near-UV circular dichroism (CD) spectropolarimetry, 1-anilino-8-naphthalenesulfonic acid (ANS) binding, and atomic force (AFM) and electron (EM) microscopy. IAPP insolubilization and amyloid formation followed kinetics that were consistent with the nucleation-dependent polymerization mechanism. Nucleation of IAPP amyloid formation with traces of preformed fibrils induced a rapid conformational transition into beta-sheets that subsequently aggregated into insoluble amyloid fibrils. Transition proceeded via a molten globule-like conformeric state with large contents of secondary structure, fluctuating tertiary and quaternary aromatic interactions, and strongly solvent-exposed hydrophobic patches. In the temperature denaturation pathway at 5 microM peptide, we found that this state was mostly populated at about 45 degrees C, and either aggregated rapidly into amyloid by prolonged exposure to this temperature, or melted into denaturated but still structured IAPP, when heated further to 65 degrees C. The state at 45 degrees C was also found to be populated at 4.25 M GdnHCl at 25 degrees C during GdnHCl-induced equilibrium denaturation, and was stable in solution for several hours before aggregating into amyloid fibrils. Our studies suggested that this amyloidogenic state was a self-associated form of an aggregation-prone, partially folded state of IAPP. We propose that this partially folded population and its self-associated forms are in a concentration-dependent equilibrium with a non-amyloidogenic IAPP conformer and may act as early, soluble precursors of beta-sheet and amyloid formation. Our findings on the molecular mechanism of IAPP amyloid formation in vitro should assist in gaining insight into the pathogenesis and inhibition of pancreatic amyloidosis and other amyloid-related diseases.