Acetylation Mimetic and Null Mutations Within the Filament Core of <scp>P301L</scp> Tau Have Varied Effects on Susceptibility to Seeding and Aggregation

The two most prominent post-translational modifications of pathologic tau are Ser/Thr/Tyr phosphorylation and Lys acetylation. Whether acetylation impacts the susceptibility of tau to templated seeding in diseases like Alzheimer's disease (AD) and Progressive Supranuclear Palsy (PSP) is largely uncharacterized. Towards this, we examined how acetylation mimicking or nullifying mutations on five sites of tau (K311, K353, K369, K370, K375), located within the tau filament core, influenced the susceptibility of P301L (PL) tau to seeds from AD (AD-tau) or PSP (PSP-tau) brain donors in HEK293T cells. Acetyl-mimicking substitutions of individual Lys sites to Glutamine as well as mutation of all 5 sites together (PL+5K-Q and PL+5K-R) had inconsistent effects on tau seeding by AD-tau or PSP-tau seeds. Unexpectedly, mutating all 5 sites to Alanine (PL+5K-A) resulted in a tau variant that spontaneously aggregated. These aggregates were amorphous and yet able to propagate to naïve cells expressing P301L tau but not wild-type tau. Previously, we reported that a phospho-mimetic S305E mutation in PL tau abrogated seeding by AD-tau but not PSP-tau seeds in the HEK293T cells. To assess how changes in acetylation and phosphorylation together could influence seeding, we combined the S305E and the 5K-Q mutations in PL tau, creating a variant that retained specificity for PSP-tau seeds over AD-tau seeds. Our findings indicate that phosphorylation of tau at Ser305 is a strong determinant of disease-specific tau templating, even in the presence of hyperacetylation within the fibril core domain. Overall, our findings suggest that acetylation in the tau filament core domain has limited effects on tau seeding.