Linking Type 2 Diabetes and Alzheimer's Disease: The Inhibition of the Cytotoxic IAPP and Aβ Proteins Self- and Co Aggregation by Small Molecule

  • Kenana Al Adem

Student thesis: Doctoral Thesis

Abstract

Alzheimer's disease (AD) and Type 2 diabetes (T2D) are highly prevalent protein aggregation diseases characterized by excessive neuronal and pancreatic cell loss due to the aggregation of β-amyloid (Aβ) peptide and Islet Amyloid Polypeptide (IAPP or Amylin), respectively. AD and T2D have been linked together where patients of T2D were found to be at higher risk of AD and vice versa. Increasing evidence, particularly the co-deposition of Aβ and IAPP in both brain and pancreatic tissues, suggests that the cross-interaction of the two peptides may be responsible for a pathological link between AD and T2D. Existing literature has mainly focused on the prevention of fibril formation of a single peptide only (i.e. IAPP alone or Aβ alone), while inhibiting the co-aggregation pathways of IAPP and Aβ are not yet addressed. This thesis aims to investigate disease modifying therapies to target AD, T2D and their clinically established crosstalk at the protein level, by utilizing natural small molecules for the inhibition of self-aggregation and co-aggregation pathways of the pathogenic IAPP and Aβ peptides. First, this thesis adopted the model protein, Lysozyme, to further understand the small-molecule-based inhibition strategy in preventing protein aggregation and its induced cytotoxicity. We showed that the selected aliphatic small molecules had differential inhibitory potencies under different in vitro aggregation conditions. In specific, the aliphatic organic acids were more efficient in inhibiting lysozyme aggregation and cellular toxicity at a physiological pH environment than at an acidic one. Next, this thesis studied the self-aggregation pathways of IAPP and Aβ by optimizing their in vitro preparation methods to ensure the homogeneity and monomeric conformations of each peptide that is required for performing the self-aggregation experiments. Then we evaluated, using molecular docking approaches, the interaction interfaces of homo- and hetero-dimeric complexes of IAPP and Aβ peptides to understand the bonding network formed along their interaction interfaces. Subsequently, we examined the inhibitory activity of two groups of natural small molecules (aromatic and aliphatic) on the two individual peptide systems, IAPP and Aβ, to identify the most effective candidate inhibitor(s) that we further tested on the IAPP-Aβ co-aggregation system. We found that aromatic small molecules, specifically EGCG and Myricetin, were most effective in inhibiting the self-aggregation of both peptide systems in comparison to the aliphatic ones. Importantly, this thesis examined the nature of IAPP-Aβ co-aggregation and its inhibition by small molecules. In specific, we characterized the kinetic profiles, morphologies, secondary structures and toxicities of IAPP-Aβ hetero-assemblies and compared them to those formed by their homo-assemblies. We demonstrated that monomeric IAPP and Aβ strongly co-interact to form hetero-species that form β-sheet rich fibrils or hetero-aggregates which are toxic (cell viability
Date of AwardJul 2021
Original languageAmerican English

Keywords

  • β-Amyloid (Aβ)
  • Islet Amyloid Polypeptide (IAPP)
  • Intrinsically Disordered Proteins
  • Hen Egg White Lysozyme (HEWL)
  • Amyloid Fibrils
  • Amorphous Aggregates
  • Co-aggregation
  • Cross-interaction
  • Small Molecules
  • Cellular Toxicity
  • and Epigallocatechin gallate (EGCG)

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