Amylin (20-29) (human)

Many unrelated proteins and peptides can assemble into amyloid or amyloid-like nanostructures, all of which share the cross-beta motif of repeat arrays of beta-strands hydrogen-bonded along the fibril axis. Yet, paradoxically, structurally polymorphic fibrils may derive from the same initial polypeptide sequence. Here, solid-state nuclear magnetic resonance (SSNMR) analysis of amyloid-like fibrils of the peptide hIAPP 20-29, corresponding to the region S (20)NNFGAILSS (29) of the human islet amyloid polypeptide amylin, reveals that the peptide assembles into two amyloid-like forms, (1) and (2), which have distinct structures at the molecular level. Rotational resonance SSNMR measurements of (13)C dipolar couplings between backbone F23 and I26 of hIAPP 20-29 fibrils are consistent with form (1) having parallel beta-strands and form (2) having antiparallel strands within the beta-sheet layers of the protofilament units. Seeding hIAPP 20-29 with structurally homogeneous fibrils from a 30-residue amylin fragment (hIAPP 8-37) produces morphologically homogeneous fibrils with similar NMR properties to form (1). A model for the architecture of the seeded fibrils is presented, based on the analysis of X-ray fiber diffraction data, combined with an extensive range of SSNMR constraints including chemical shifts, torsional angles, and interatomic distances. The model features a cross-beta spine comprising two beta-sheets with an interface defined by residues F23, A25, and L27, which form a hydrophobic zipper. We suggest that the energies of formation for fibril form containing antiparallel and parallel beta-strands are similar when both configurations can be stabilized by a core of hydrophobic contacts, which has implications for the relationship between amino acid sequence and amyloid polymorphism in general.

The human Islet amyloid polypeptide (20-29) (hIAPP20-29) is considered to be the core fibrillating fragment of hIAPP, which is associated with the pathogenesis of Type-II diabetes mellitus. A current challenge is the discovery of an efficient way to modulate amyloid aggregation and inhibit the toxicity of its aggregates. In this work, photoexcited porphyrins are successfully used to inhibit the fibrillation of hIAPP20-29. Insights on the inhibitory mechanism are explored by the analysis of the secondary structure, the morphology and the mechanical properties of amyloid aggregates. In addition, photoexcited porphyrins displayed a retained inhibitory effect on hIAPP20-29aggregation without irradiation. These findings may establish a new avenue to inhibit the aggregation of amyloid peptide hIAPP and enrich the current selection of modulators.

The misfolding and subsequent assembly of proteins and peptides into insoluble amyloid structures play important roles in the development of numerous diseases. The dynamics of self-assembly and the morphology of the resulting aggregates critically depend on various environmental factors and especially on the presence of interfaces. Here, we show in detail how the presence of surfaces with different physicochemical properties influences the assembly dynamics and especially the aggregate morphology of hIAPP(20-29), an amyloidogenic fragment of the peptide hormone human islet amyloid polypeptide (hIAPP), which is involved in the development of type 2 diabetes. Time-lapse atomic force microscopy is employed to study the assembly dynamics of hIAPP(20-29) and the morphology of the resulting aggregates in bulk solution as well as at hydrophilic and hydrophobic model surfaces. We find that the presence of hydrophilic mica surfaces promotes fibrillation when compared with the assembly in bulk solution and results in a more pronounced polymorphism. Three fibrillar species are found to coexist on the mica surface, that is, straight, coiled, and ribbon-like fibrils, whereas only the straight and coiled fibrils are observed in bulk solution after comparable incubation times. In addition, the straight and coiled fibrils assembled at the mica surface have significantly different dimensions compared with those assembled in bulk solution. The three fibrillar species found on the mica surface most likely form independently by lateral association of arbitrary numbers of protofibrils with about 2 nm height. On hydrophobic hydrocarbon surfaces, fibrillation is retarded but not completely suppressed, in contrast to previous observations for full-length hIAPP(1-37). Our results show that peptide-surface interactions may induce diverse, peptide-specific alterations of amyloid assembly dynamics and fibrillar polymorphism. They may therefore contribute to a deeper understanding of the molecular processes that govern amyloid aggregation at different surfaces.

The highly amyloidogenic peptide sequence of amylin(20-29) was transformed into its corresponding peptoid and retropeptoid sequences to design a novel class of beta-sheet breaker peptides as amyloid inhibitors. This report describes the synthesis of the chiral peptoid building block of L-isoleucine, the solid phase synthesis of the peptoid and retropeptoid sequences of amylin(20-29), and the structural analysis of these amylin derivatives in solution by infrared spectroscopy, circular dichroism, and transmission electron microscopy. It was found that the peptoid sequence did not form amyloid fibrils or any other secondary structures and was able to inhibit amyloid formation of native amylin(20-29). Although the retropeptoid did not form amyloid fibrils it had only modest amyloid inhibitor properties since supramolecular tapes were formed.