Defining mechanisms of neurodegeneration associated with protein misfolding diseases

Abstract

Protein misfolding diseases (PMDs) are a broad group of disorders including Alzheimer’s, Parkinson’s and prion diseases. They are characterised by the presence of aggregated, misfolded host proteins which are thought to cause cell death. Prion diseases are associated with misfolded prion protein (PrPSc), which has a tendency to form fibrillar aggregates. By contrast, Alzheimer’s disease (AD) is associated with misfolded amyloid beta (Aβ), which aggregates to form characteristic Aβ plaques. A feature which is common across PMDs is that small assemblies (oligomers) of the misfolded proteins are thought to be the important neurotoxic species, and it has been proposed that there may be a shared mechanism leading to cell death across PMDs caused by oligomers. In this study, the toxicity of different misfolded forms of recombinant PrP (recPrP) and recombinant Aβ (recAβ) and the mechanisms leading to cell death were investigated using a primary cell culture model. In addition, the importance of the disulphide bond in recPrP in relation to oligomer formation was explored using size exclusion chromatography and mass spectrometry, the toxicity of the different resulting oligomer populations were also investigated. Both recPrP oligomers and fibrils were shown to cause toxicity to mouse primary cortical neurons. Interestingly, oligomers were shown to cause apoptotic cell death, while the fibrils did not, suggesting the activation of different pathways. By contrast, recAβ fibrils were shown to be non-toxic to cortical neurons, Aβ oligomers, however, were shown to cause toxicity. Similar to recPrP, my data showed that it is likely that recAβ 1-42 oligomers also cause apoptosis. However, by contrast this seemed to be caused by excitotoxicity, which was not found to be the case for recPrP. Additionally, I have shown that the presence or absence of the disulphide bond in PrP has a profound effect on the size of oligomers which form. RecPrP lacking a disulphide bond leads to the formation of larger oligomers which are highly toxic to primary neurons. Findings from this study suggest that structural properties such as the disulphide bond in PrP can affect the size and toxicity of oligomers, furthermore, whilst oligomers have been shown to be important in both AD and prion diseases, they may not trigger the same pathways leading to cell death.