Ye Wang: Islet Amyloid Polypeptide: Interaction with Amyloid Beta, Alpha-Synuclein and BRICHOS

Abstract

Amyloid, the congophilic deposits of misfolded protein, are pathological hallmarks of many common diseases, among others, Alzheimer’s disease (AD), Parkinson’s disease (PD), and type 2 diabetes (T2D). Amyloid beta (Aβ) forms senile plaques in AD, alpha-synuclein (aSyn) forms Lewy bodies and Lewy neurites in PD, islet amyloid polypeptide (IAPP) forms islet amyloid in T2D. Amyloid fibril formation is a nucleation-dependent process, which can be accelerated via the addition of preformed seeds. This mechanism is referred to as seeding or cross-seeding, depending on whether homologous or heterologous seeds are added. The present thesis has investigated the interaction between IAPP and Aβ, IAPP and aSyn, and IAPP and anti-amyloid chaperone Bri2 BRICHOS. The possibility of using luminescent conjugated oligothiophenes (LCOs) to detect islet amyloid formation has also been examined.

IAPP and Aβ interacted at molecular level in living cells and preferred parallel binding over anti-parallel binding. Interaction of IAPP and Aβ led to the formation of intracellular amyloid, increased lysosomal area, increased superoxide production, and increased susceptibility to cell death. In addition, co-expression of IAPP and Aβ in brain of Drosophila melanogaster resulted in co-deposition of proteins and reduced fly lifespan. These results provide a molecular link between AD and T2D.

ASyn expressed in pancreatic β cells, co-localized with IAPP but was absent in extracellular islet amyloid. Preformed aSyn seeds triggered IAPP fibril formation, but not vice versa. Neither knockdown nor overexpression of aSyn affected β cell viability. These results suggest that the proximity of aSyn and IAPP does not guarantee functional interference.

Bri2 BRICHOS expressed in pancreatic β cells and co-localized with both intracellular IAPP and extracellular islet amyloid. Bri2 BRICHOS inhibited IAPP fibril formation and reduced IAPP-induced cytotoxicity. Knockdown endogenous Bri2 increased the susceptibility of β cells death under stressed conditions, but overexpression of BRICHOS rescued β cells. These results suggest BRICHOS be a potential endogenous chaperone in preventing islet amyloid formation in β cells.

The LCO pFTAA-CN detected islet amyloid in fixed islets, unfixed frozen islets, and living islets and could be used to monitor islet amyloid formation in real-time without interfering with the formation of islet amyloid.

In conclusion, the interaction between IAPP and Aβ results in increased toxicity on cells, while the impact of the interaction between IAPP and aSyn is still an open question, Bri2 BRICHOS is an endogenous inhibitor of IAPP fibrillation and toxicity. PFTAA-CN is suitable for detecting islet amyloid under various conditions.