Induction of yeast prions by reactive oxygen species

Creutzfeldt-Jakob Disease (CJD) is an unusual infectious disease of the human brain that leads to dementia and death. The most baffling fact about this and related diseases such as sheep scrapie and bovine spongiform encephalopathy (BSE, Mad Cow Disease) is that they appear to be triggered by a unique class of infectious agent known as a prion (protein-only infectious agent). Although CJD is a comparatively rare disease in humans (approximately 1 in a million people will contract the disease in any one year), sufferers show many of the pathological features seen in individuals suffering from other more common, non-infectious diseases of the brain such as Alzheimer's Disease and Parkinson's Disease. In spite of its infectious nature, the majority of cases of human CJD (~80%) are known to occur 'sporadically' i.e. appear spontaneously, without any evidence of the diseased individual acquiring an infectious prion from a third party, or through a mutation in the individual's genes. Prions are remarkable infectious agents because they consist only of a single protein that is a structurally altered form of a protein (called PrP) normally found in the brain. Yet we know remarkably little about how prions form spontaneously to cause sporadic CJD or for that matter what will trigger their formation. To help us address these questions we are proposing to study prions that are found in Baker's yeast (Saccharomyces cerevisiae). Prions were first described in this fungus over 15 years ago and the many subsequent studies on yeast prions have revealed fascinating often surprising new aspects of prion biology not least the fact that their presence can be of benefit to the host they infect rather than detrimental. We are proposing to use two different yeast prions - called [PSI+] and [PIN+] - to identify what triggers the spontaneous formation of a prion in the cell. The approach we are taking is based on our recent discovery that certain potentially dangerous forms of oxygen known as 'reactive oxygen species' (or ROS for short) can trigger the spontaneous formation of the [PSI+] prion in yeast. In yeast cells lacking a defence system known to prevent such oxidative damage, the [PSI+] prion formed spontaneously at a remarkably high frequency. Because it is already known that ROS can also trigger the formation of the human prion that causes CJD then we believe that by using modern methods of genetics and biochemistry, we will be able to establish the significance of oxidative damage in triggering prion formation. This study will also be extended to include other human disease associated proteins whose misfolding also leads to brain degeneration.

What triggers the formation of both infectious and non-infectious amyloids in vivo represents a significant gap in our understanding of the etiology of a major family of human neurodegenerative diseases. Using the yeast Saccharomyces cerevisiae we and others have shown that the frequency with which the yeast [PSI+] prion form of the Sup35 protein arises de novo is controlled by a number of genetic and environmental factors. Most significantly we have very recently shown that in yeast cells lacking the peroxiredoxin proteins Tsa1 and Tsa2, the frequency of de novo formation of [PSI+] is greatly elevated. Peroxiredoxins (Prxs) play multiple roles in protecting cells against stress, including acting as antioxidants and suppressing potential harmful oxidative damage to proteins following oxidative stress. Using two different yeast prions, [PSI+]/Sup35 and [PIN+]/Rnq1, we will dissect the mechanism by which oxidative stress induces yeast prion formation de novo in both wild-type cells and mutants lacking the Tsa1/Tsa2 Prxs. We will use high-throughput assays for de novo formation of the [PSI+] and [PIN+] prions to establish whether prion formation is a common response to diverse oxidative stress conditions and extend that to other amyloid-forming human proteins such as alpha-synuclein. The functional interplay between Tsa1/Tsa2, the target prion proteins and their interactions with ribosomes will be investigated. This will enable us to define the ribosome-associated functions of the Prxs and determine whether protection against de novo prion formation occurs on ribosomes at the level of the nascent polypeptide chain. Our over-arching hypothesis is that the direct oxidation of Sup35 and Rnq1 leads to structural transitions favouring conversion to the transmissible amyloid-like form. We will therefore establish why a lack of Prx activity leads to the induction of de novo prion formation and establish whether this can be extrapolated to non-transmissible amyloids.

Sporadic Creutzfeldt - Jakob disease (CJD) accounts for over 80% of the verified cases of this fatal neurodegenerative disease in the UK, yet we know remarkably little about the underlying mechanism or triggers for disease development. The emergence of the disease correlates with the appearance of PrPSc, a novel conformational form of the cellular PrP protein. This entity, known as a prion, replicates through a novel cycle of seeded polymerisation and fragmentation, and it is assumed that certain genetic or environmental factors can trigger the conformational change in the absence of any pre-existing PrPSc 'seeds'. We have recently discovered an enzyme-mediated mechanism that prevents or suppresses oxidative damage to proteins in eukaryotes but also suppresses the de novo formation of the [PSI+] prion in yeast. [PSI+] is the prion form of the Sup35 protein and the antioxidant enzymes in question are the peroxiredoxins Tsa1 and Tsa2. The [PSI+] prion has been used widely as a model to demonstrate the protein-only nature of the prion propagation cycle and to identify some of the genetic and environmental factors that control the de novo formation of a prion. The major impact of our finding is clear: the accumulation of oxidative damage in certain proteins such as Sup35 can lead to the formation of a heritable amyloid-like form of the protein in the absence of any underlying genetic change. The same may also apply to the much more prevalent common 'protein misfolding' diseases that lead to dementia and death and which are increasing in proportion to the increased life span of the human, namely Alzheimer's Disease, Huntington's Disease and Parkinson's Disease. What will emerge from our study will be a detailed understanding of how oxidative damage acts as a trigger and the role cellular factors play in suppressing the potentially fatal consequences of such damage. The major impacts of this research nationally and internationally will therefore be on: (a) the bioscience research community with specific interests in the fields of protein synthesis and folding, (b) research aimed at identifying the causes of protein misfolding that lead to disease in humans and/or farmed animals, and (c) the biopharmaceutical sector.