The N-Terminus of Tau in Filament Formation and the Regulation of Axonal Transport: A Perspective on Tau Assembly and ToxicityPublic Deposited
In several neurodegenerative diseases, the microtubule-associated protein tau self-aggregates to form filaments that accumulate in neurons and/or glia, although the relationship between tau aggregation and cell death is a subject of debate. The amino terminus of tau is involved in conformational changes that appear critical for filament formation, hinting at a regulatory role for this part of the protein in aggregation. The amino terminus may also be important to the study of physiological tau function, as it is implicated in a number of interactions between tau and various other cellular proteins. In this dissertation, we hypothesize that the amino terminus plays a critical role in the regulation of tau filament formation and toxicity. We examine the regulatory role of the amino terminus in filament formation using an in vitro polymerization assay, and address its involvement in filament toxicity in isolated axoplasm. We demonstrate that polymerization of full-length tau is inhibited when an excess of amino terminus is added to the reaction mixture, either in the form of recombinant N-terminal constructs or as the short, naturally occurring 6D and 6P isoforms. Our results are consistent with a model of soluble tau in which the amino terminus and the carboxy terminus are in close proximity. We also demonstrate that the amino terminus triggers a signaling cascade resulting in the inhibition of kinesin-dependent axonal transport. This effect is present in soluble 6D and 6P tau isoforms, which lack the C-terminal half of canonical tau, but is absent from full-length monomer. The folded conformation described above may explain why soluble full-length tau has no effect on transport in this system. In contrast, full-length tau triggers transport inhibition when in filamentous form, suggesting that filament formation locks canonical tau in a conformation in which the N-terminus is more accessible. Collectively, this dissertation proposes that the toxic effects of tau filaments result when the amino terminus is repositioned and "unmasked" during polymerization. Our work not only reveals a novel role for the amino terminus in the regulation of filament formation, but highlights the cell biological consequences of tau aggregation.