Another hallmark of AD is the abnormal phosphorylation, mislocalization, and aggregation of the tau protein. Beta-plated sheets of hyperphosphorylated tau (pTau) leads to the disruption of neuronal microtubule assemblies and neurodegeneration [19]. To examine control and fAD organoids for the presence of tau pathology (tauopathy), we conducted immunohistochemistry in organoid sections using antibodies against pTau (Ser396 or Thr181). At 90d, the fAD organoids exhibit significantly greater pTau immunoreactivity than did control sections (Fig 1E and 1F and S4C Fig). Interestingly, there was no significant difference in pTau levels between fAD and control organoids at 60 days (Fig 1F). This is in contrast to the amyloid phenotype, in which more Aβ aggregates were observed at both 60d and 90d in the fAD organoids (Fig 1D). The Thioflavin-S dye binds aggregates of β-pleated sheets and is used as an indicator of tau pathology in human brain and mouse models [69]. We observed a greater total area covered by Thioflavin-S dye labeling in fAD organoids compared to controls, as well as a higher number of Thioflavin-S positive particles (S4D and S4E Fig). Together, these various measures demonstrate that the AD patient-derived organoids have the power to recapitulate both amyloid and pTau phenotypes.
