Localization, induction, and cellular effects of tau phosphorylated at threonine 217¹

Abstract Introduction Tau phosphorylation at T217 is a promising Alzheimer's disease (AD) biomarker, but its functional consequences were unknown. Methods Human brain and cultured mouse neurons were analyzed by immunoblotting and immunofluorescence for total tau, tau pT217 , tau pT181 , tau pT231 , and tau pS396/pS404 . Direct stochastic optical reconstruction microscopy (dSTORM) super resolution microscopy was used to localize tau pT217 in cultured neurons. Enhanced green fluorescent protein (EGFP)‐tau was expressed in fibroblasts as wild type and T217E pseudo‐phosphorylated tau, and fluorescence recovery after photobleaching (FRAP) reported tau turnover rates on microtubules. Results In the brain, tau pT217 appears in neurons at Braak stages I and II, becomes more prevalent later, and co‐localizes partially with other phospho‐tau epitopes. In cultured neurons, tau pT217 is increased by extracellular tau oligomers (xcTauOs) and is associated with developing post‐synaptic sites. FRAP recovery was fastest for EGFP‐tau T217E . Conclusion Tau pT217 increases in the brain as AD progresses and is induced by xcTauOs. Post‐synaptic tau pT217 suggests a role for T217 phosphorylation in synapse impairment. T217 phosphorylation reduces tau's affinity for microtubules. Highlights Validation of anti‐tau phosphorylated at threonine‐217 (tau pT217 ) specificity is essential due to epitope redundancy. tau pT217 increases as Alzheimer's disease progresses and is found throughout diseased neurons. tau pT217 is associated with developing post‐synaptic sites in cultured neurons. Extracellular oligomers of tau, but not amyloid beta, increase intracellular tau pT217 . T217E pseudo‐phosphorylation reduces tau's affinity for microtubules.