Self-assembly and seeding capabilities of an Alzheimer’s disease associated fragment of tau

In Alzheimer’s disease (AD) and other tauopathies, soluble tau protein self-assembles into insoluble aggregates. In AD, tau is deposited as neurofibrillary tangles (NFTs) composed mainly of paired helical filaments (PHFs). The structural core of PHFs is made up of two protofilaments with C-shaped subunits. There is a need for a relevant model of tau aggregation that does not require non-physiological inducers of aggregation, to permit the study of inhibition of tau aggregation and tau seeding in a physiological environment. The tau protein fragment, dGAE (amino acids 297-391) maps onto the proteolytically-stable PHF-core tau sequence and represents a model system with which to understand the cellular effects of misfolded tau. In this thesis, dGAE self-assembly was characterised in vitro, to investigate changes to dGAE size and morphology over the course of assembly and the role of disulphide bonds in this process. Here, we show that dGAE assembles into filaments that resemble PHFs from AD brain, but this process does not depend on Cys-322. The effect of the small molecule, MT, on dGAE self-assembly was investigated, showing that MT must be reduced to LMT to efficiently inhibit dGAE self-assembly and forms species with an alternative conformation. The effect of exogenously-applied dGAE on cell viability, internalisation and seeding capabilities were examined in differentiated SH-SY5Y human neuroblastoma cells, which do not overexpress aggregation prone mutant tau or tau fragments. These cells can internalise dGAE in a soluble form and accumulates within the cytoplasm. Exposure to exogenous dGAE induces changes to endogenously expressed tau, namely a redistribution and increase in phosphorylated endogenous tau. The results presented in this thesis characterise the self-assembly of the PHF core under physiological conditions and aids in understanding the process leading to PHF formation. This approach will provide a useful tool to facilitate the study of tau aggregation and ultimately to study the potential of tau aggregation inhibitors to target tau seeding activity