Mechanisms of amyloid-beta toxicity in the yeast Saccharomyces cerevisiae

Alzheimer's disease (AD) is the most common form of dementia, and a devastating neurodegenerative disease in which there is no cure or effective treatments to delay disease onset. One of the major culprits of the disease is the protein amyloid-beta (A&beta), which accumulates as both soluble and insoluble aggregates in the brain of AD patients, leading to cellular toxicity and neuronal degeneration. To gain a better understanding of the toxic effects of A&beta, we used the yeast Saccharomyces cerevisiae as a model organism. The advantage of using yeast was that we could conduct an unbiased, high-throughput screen to identify loss-of-function genetic modifiers of A&beta toxicity. In addition, yeast is a simple, eukaryotic system to study the intracellular pathways that might be affected by A&beta accumulation. Genetic manipulation of the yeast genome is technically easy: genes of interest can be deleted, fusion proteins can be tagged to endogenous genes to track their localization within the cell, and expression of heterotrophic genes can be introduced. Furthermore, many tools are available to study physiological processes and pathways in yeast, such as intracellular trafficking, induction of stress responses, and growth rates measurements. Our studies show that the expression of A&beta in yeast causes cellular toxicity, reduced growth rate, and induction of the unfolded protein response. However, these effects are not seen in A&beta mutant peptides that have a reduced aggregation propensity. This suggests that the accumulation of aggregated A&beta is a key component of toxicity. In addition, we observed that degradation of A&beta is impaired, resulting in slow turnover of the protein in the cell. Fluorescence microscopy studies showed that A&beta localizes to the endoplasmic reticulum (ER), where misfolded or aggregated A&beta may be retained. A high-throughput screen identified FAB1 as a loss-of-function genetic enhancer of A&beta toxicity. Loss of FAB1 further retards A&beta degradation, which may enhance A&beta accumulation and toxicity; however, the precise mechanisms for A&beta toxicity are still unknown. Our model provides a potential tool to study the quality control mechanisms that maintain intracellular A&beta in a properly folded and non-toxic state