Small-Molecule Inhibitors of Islet Amyloid Polypeptide Fibril Formation

Recent Advances in the Discovery of Therapeutics to Curtail Islet Amyloid Polypeptide Aggregation for Type 2 Diabetes Treatment

In humans with type 2 diabetes, at least 70% of patients exhibit islet amyloid plaques formed by misfolding islet amyloid polypeptides (IAPP). The oligomeric conformation and accumulation of the IAPP plaques lead to a panoply of cytotoxic effects on the islet β-cells. Currently, no marketed therapies for the prevention or elimination of these amyloid deposits exist, and therefore significant efforts are required to address this gap. To date, most of the experimental treatments are limited to only in vitro stages of testing. In general, the proposed therapeutics use various targeting strategies, such as binding to the N-terminal region of islet amyloid polypeptide on residues 1-19 or the hydrophobic region of IAPP. Other strategies include targeting the peptide self-assembly through π-stacking. These methods are realized by using several different families of compounds, four of which are highlighted in this review: naturally occurring products, small molecules, organometallic compounds, and nanoparticles. Each of these categories holds immense potential to optimize and develop inhibitor(s) of pancreatic amyloidosis in the near future.

Structural Dissection of the First Events Following Membrane Binding of the Islet Amyloid Polypeptide

The islet amyloid polypeptide (IAPP) is the main constituent of the amyloid fibrils found in the pancreas of type 2 diabetes patients. The aggregation of IAPP is known to cause cell death, where the cell membrane plays a dual role: being a catalyst of IAPP aggregation and being the target of IAPP toxicity. Using ATR-FTIR spectroscopy, transmission electron microscopy, and molecular dynamics simulations we investigate the very first molecular steps following IAPP binding to a lipid membrane. In particular, we assess the combined effects of the charge state of amino-acid residue 18 and the IAPP-membrane interactions on the structures of monomeric and aggregated IAPP. Distinct IAPP-membrane interaction modes for the various IAPP variants are revealed. Membrane binding causes IAPP to fold into an amphipathic α-helix, which in the case of H18K-, and H18R-IAPP readily moves beyond the headgroup region. For all IAPP variants but H18E-IAPP, the membrane-bound helix is an intermediate on the way to amyloid aggregation, while H18E-IAPP remains in a stable helical conformation. The fibrillar aggregates of wild-type IAPP and H18K-IAPP are dominated by an antiparallel β-sheet conformation, while H18R- and H18A-IAPP exhibit both antiparallel and parallel β-sheets as well as amorphous aggregates. Our results emphasize the decisive role of residue 18 for the structure and membrane interaction of IAPP. This residue is thus a good therapeutic target for destabilizing membrane-bound IAPP fibrils to inhibit their toxic actions.

Simultaneous IR-Spectroscopic Observation of α-Synuclein, Lipids, and Solvent Reveals an Alternative Membrane-Induced Oligomerization Pathway

The intrinsically disordered protein α-synuclein (αS), a known pathogenic factor for Parkinson's disease, can adopt defined secondary structures when interacting with membranes or during fibrillation. The αS-lipid interaction and the implications of this process for aggregation and damage to membranes are still poorly understood. Therefore, we established a label-free infrared (IR) spectroscopic approach to allow simultaneous monitoring of αS conformation and membrane integrity. IR showed its unique sensitivity for identifying distinct β-structured aggregates. A comparative study of wild-type αS and the naturally occurring splicing variant αS Δexon3 yielded new insights into the membrane's capability for altering aggregation pathways.

Inhibitory Mechanism of Epigallocatechin Gallate on Fibrillation and Aggregation of Amidated Human Islet Amyloid Polypeptide

The abnormal fibrillation of human islet amyloid polypeptide (hIAPP) is associated with development of type II diabetes mellitus (T2DM). (-)-Epigallocatechin gallate (EGCG) can bind amyloid proteins to inhibit the fibrillation of these proteins. However, the mechanic detail of EGCG inhibiting amyloid formation is still unclear at the molecular level. In the present work, we sought to investigate the effect of EGCG on amidated hIAPP (hIAPP-NH₂ ) fibrillation and aggregation by using spectroscopic and microscopic techniques, and also sought to gain insights into the interaction of EGCG and hIAPP_22-27 by using spectroscopic experiments and quantum chemical calculations. ThT fluorescence, real-time NMR, and TEM studies demonstrated that EGCG inhibits the formation of hIAPP-NH₂ fibrils, while promoting the formation of hIAPP-NH₂ amorphous aggregates. Phenylalanine intrinsic fluorescence and NMR studies of the EGCG/hIAPP_22-27 complex revealed three important binding sites including the A ring of EGCG, residue Phe23, and residue Ile26. DFT calculations identified the dominant binding structures of EGCG/Phe23 and EGCG/Ile26 complexes, named structure I and structure II, respectively. Our study demonstrates the inhibitory mechanism of EGCG on fibrillation and aggregation of hIAPP-NH₂ in which EGCG interacts with hIAPP-NH₂ through hydrogen bonding and π-π interactions between the A ring and residue Phe23 as well as hydrophobic interactions between the A ring and residue Ile26, which can thus inhibit the interpeptide interaction between hIAPP-NH₂ monomers and finally inhibit fibrillation of hIAPP-NH₂ . This study agrees with and reinforces previous studies and offers an intuitive explanation at both the atomic and molecular levels. Our findings may provide an invaluable reference for the future development of new drugs in the management of diabetes.

Synthesis of 2,5-diaryl-substituted thiophenes as helical mimetics: towards the modulation of islet amyloid polypeptide (IAPP) amyloid fibril formation and cytotoxicity

A range of 2,5-diarylated thiophenes were synthesised as small molecule mimetics of the α-helix to modulate the amyloidogenesis and cytotoxic effect of islet amyloid polypeptide (IAPP). 3-Substituted thiophene-2-carboxylic acids were used as key intermediates and functionalised by palladium decarboxylative cross-coupling and direct C-H activation successively with overall yields ranging from 23 to 95 %. The effect of the ligands on IAPP amyloid fibril formation was evaluated with the thioflavin T (ThT) fluorescence-based assay. Furthermore, the capacity of these compounds to inhibit the cytotoxic effect of IAPP was assessed using β-pancreatic cells.

Inhibiting islet amyloid polypeptide fibril formation by the red wine compound resveratrol

GRAPES FOR AMYLOIDS: The red wine compound resveratrol can effectively inhibit the formation of IAPP amyloid that is found in type II diabetes. Our in vitro inhibition results do not depend on the antioxidant activity of resveratrol. Further, the markedly enhanced cell survival in the presence of resveratrol also indicates that the small oligomeric structures that are observed during beta-sheet formation are not toxic and could be off-pathway assembly products.