Tau phosphorylation impedes functionality of protective tau envelopes

Tau, an axonal microtubule-associated protein, is a critical regulator of microtubule function and stability. Tau interaction with microtubules is regulated by tau phosphorylation. Tau hyperphosphorylation is implicated in microtubule destabilization related to neurodegenerative disorders. How tau phosphorylation leads to microtubule destabilization is however unknown. Recently, it was shown that tau molecules on microtubules cooperatively assemble into cohesive layers termed envelopes. Tau envelopes protect microtubules against degradation by microtubule-severing enzymes, suggesting a functional link between envelopes and microtubule stability. Here we show that tau phosphorylation has deleterious effects on the microtubule-protective function of tau envelopes. Using reconstitution and live-cell experiments, we found that tau phosphorylation destabilizes tau envelopes and decreases their integrity, leading to reduced microtubule protection against microtubule-severing enzymes. Our data suggest that a perturbation of microtubule homeostasis linked to tau hyperphosphorylation in neurodegeneration, could be explained by the disassembly and impaired functionality of the tau envelopes.