Effect of Pressure on Islet Amyloid Polypeptide Aggregation: Revealing the Polymorphic Nature of the Fibrillation Process

Abstract

Type II diabetes mellitus is a disease which is characterized by peripheral insulin resistance coupled with a progressive loss of insulin secretion that is associated with a decrease in pancreatic islet beta-cell mass and the deposition of amyloid in the extracellular matrix of beta-cells, which lead to islet cell death. The principal component of the islet amyloid is a pancreatic hormone called islet amyloid polypeptide (IAPP). High-pressure coupled with FT-IR spectroscopic and AFM studies were carried out to elucidate further information about the aggregation pathway as well as the aggregate structures of IAPP. To this end, a comparative fibrillation study of IAPP fragments was carried out as well. As high hydrostatic pressure (HHP) is acting to weaken or even prevent hydrophobic self-organization and electrostatic interactions, application of HHP has been used as a measure to reveal the importance of these interactions in the fibrillation process of IAPP and its fragments. IAPP preformed fibrils exhibit a strong polymorphism with heterogeneous structures, a large population of which are rather sensitive to high hydrostatic pressure, thus indicating a high percentage of ionic and hydrophobic interactions and loose packing of these species. Conversely, fragments 1-19 and 1-29 are resistant to pressure treatment, suggesting more densely packed aggregate structures with less void volume and strong cooperative hydrogen bonding. Furthermore, the FT-IR data indicate that fragment 1-29 has intermolecular beta-sheet conformational properties different from those of fragment 1-19, the latter exhibiting polymorphic behavior with more disordered structures and less strongly hydrogen bonded fibrillar assemblies. The data also suggest that hydrophobic interactions and/or less efficient packing of amino acids 30-37 region leads to the marked pressure sensitivity observed for full-length IAPP.

Interaction of hIAPP with Model Raft Membranes and Pancreatic β‐Cells: Cytotoxicity of hIAPP Oligomers

“…[53] In fact, hydrophobic interactions are also considered to be a major driving force in hIAPP fibril formation in the absence of membranes. [54] To diminish energetic costs, initial incorporation of the peptides at the rim of the domains of the heterogeneous membrane, where the volume and area fluctuations are most prominent, would be most likely, leading to a favorable decrease of the associated line energy. [42] The increase in membrane defects upon incorporation of the peptide will facilitate further peptide penetration, which leads to an increase in local peptide concentration in the membrane's core region, thus allowing condensation of oligomeric particles and fibril growth.…”

Fibril Structure of Human Islet Amyloid Polypeptide

“…S1, A and B). Seventeen spin labels were added to every fourth peptides at residues 12,13,14,15,16,17,18,19,24,27,28,29,30,31,32,35, and 36, beginning from the first peptide (supplemental Fig. S1C).…”

“…However, the three-dimensional structure of hIAPP fibrils or other misfolded forms remains elusive. FT-IR spectroscopy and circular dichroism show that the fibrils contain ␤-sheet structure (14,15), and x-ray and electron diffraction indicate that the cross-␤-strands are 4.7 Å apart and perpendicular to the fibril axis (16). Site-directed spin labeling and electron paramagnetic resonance (EPR) of hIAPP fibrils show these strands are arranged in a parallel, in-register structure (17).…”

Membrane permeabilization by Islet Amyloid Polypeptide

“…In vitro produced fibrils of hIAPP or hIAPP fragments, even from the same sample, display various cross- structures (Madine, et al 2008) and morphologies (Goldsbury, et al 1997;Sumner Makin, et al 2004;Radovan, et al 2008), for example coiled fibrils and ribbon-like fibrils (Fig. 1b).…”