Study of cellular responses and protein interactions of Abeta in yeast models

Our current understanding of Alzheimer’s disease (AD) is incomplete due to uncertainties in experimental models and the multi-faceted nature of Aβ. These difficulties have not prevented models being used to explore AD preventatives and therapeutics. As a result, a variety of hypotheses on the cause of AD and a range of assays to measure outcomes in different treatments have evolved. However, knowledge of the precise causes of AD is fundamental to develop rational preventative measures and treatment. My research has aimed at elucidating some of the biological effects and toxic mechanisms of intracellular Aβ, which is implicated in causing AD.

Genome-wide transcription in yeast producing intracellular Aβ fused to GFP (likened to a disease state due to Aβ) compared to yeast producing GFP (likened to a normal state) implied that intracellular Aβ may affect the cell cycle, intracellular copper availability, and stress responses like oxidative stress response and heat shock response, probably via the MAPK signalling pathway. Signs of protein misfolding, oxidative stress, proteasomal activity and ubiquitination were observed in the functional analysis of differentially expressed genes. In vivo yeast assays revealed that Aβ species, Aβ42 and Aβ40, are toxic enough to living cells to induce a heat shock response in the cells. Truncated Aβ species, Aβ28 and Aβ16, produced only basal stress levels similar to that seen in GFP producing cells. Though it is not clear whether Aβ caused GFP to misfold or the fusion to oligomerise, aberrant protein folding, oxidative stress and hydrophobicity of the residues in Aβ were noted as factors that influence the toxicity of Aβ. These factors may have the potential to convert normal Aβ to pathological plaques. HSR is a protective response that either reduces aggregation or facilitates proper folding of proteins. Consistent with this idea, it was observed that sustaining HSR could protect against Aβ toxicity. By increasing stress defence mechanisms, protein turn-over may be increased, thereby reducing the cell damage. Analysis of the in vivo interactions of Aβ with metals and clioquinol (CQ), a metal-chelator that has been suggested to be an AD chemopreventative, suggested that the toxicity of Aβ involved metals. The toxicity of Aβ could also be modulated by CQ. Further studies on CQ revealed that Cu2+, and not Fe2+ or Zn2+, could revert the growth inhibitory effects of CQ when added in molar excess.

Taken together, these studies affect our understanding of Aβ biology in the context of AD. It is notable that an enhanced HSR enabled better folding of Aβ. The yeast HSR model presented in this study and data from the literature reinforces the development of HSR as an ideal AD therapeutic target as it confers protection against the multiple effects of the pleiotrophic AD proteins. Furthermore, the yeast HSR assay was established to be a robust and reliable assay as it could accurately reproduce the increased and decreased toxicity of known mutants. These studies indicate that yeast may be a new tractable model system for the screening for inhibitors of the stress caused by Aβ