Localization, induction, and cellular effects of tau phosphorylated at threonine 217

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.