Neurofibrillary tangles (NFTs) made of abnormally hyperphosphorylated tau are a hallmark of Alzheimer’s disease (AD) and related tauopathies. Regional distribution of NFTs is associated with the progression of the disease and has been proposed to be a result of prion-like propagation of misfolded tau. Tau in AD brain is heterogenous and presents in various forms. In the present study, we prepared different tau fractions by sedimentation combined with sarkosyl solubility from AD brains and analyzed their biochemical and pathological properties. We found that tau in oligomeric fraction (O-tau), sarkosyl-insoluble fractions 1 and 2 (SI1-tau and SI2-tau) and monomeric heat-stable fraction (HS-tau) showed differences in truncation, hyperphosphorylation, and resistance to proteinase K. O-tau, SI1-tau, and SI2-tau, but not HS-tau, were hyperphosphorylated at multiple sites and contained SDS- and β-mercaptoethanol–resistant high molecular weight aggregates, which lacked the N-terminal portion of tau. O-tau and SI2-tau displayed more truncation and less hyperphosphorylation than SI1-tau. Resistance to proteinase K was increased from O-tau to SI1-tau to SI2-tau. O-tau and SI1-tau, but not SI2-tau or HS-tau, captured tau from cell lysates and seeded tau aggregation in cultured cells. Heat treatment could not kill the prion-like activity of O-tau to capture normal tau. Hippocampal injection of O-tau into 18-month-old FVB mice induced significant tau aggregation in both ipsilateral and contralateral hippocampi, but SI1-tau only induced tau pathology in the ipsilateral hippocampus, and SI2-tau and HS-tau failed to induce any detectable tau aggregation. These findings suggest that O-tau and SI1-tau have prion-like activities and may serve as seeds to recruit tau and template tau to aggregate, resulting in the propagation of tau pathology. Heterogeneity of tau pathology within AD brain results in different fractions with different biological and prion-like properties, which may pose a major challenge in targeting tau for development of effective therapeutic treatments.

Background: Neurofibrillary pathology of abnormally hyperphosphorylated tau spreads along neuroanatomical connections, underlying the progression of Alzheimer’s disease (AD). The propagation of tau pathology to axonally connected brain regions inevitably involves trafficking of seeding-competent tau within the axonal compartment of the neuron. Objective: To determine the seeding activity of tau in cerebral gray and white matters of AD. Methods: Levels of total tau, hyperphosphorylation of tau, and SDS- and β-mercaptoethanol–resistant high molecular weight tau (HMW-tau) in crude extracts from gray and white matters of AD frontal lobes were analyzed by immuno-blots. Tau seeding activity was quantitatively assessed by measuring RIPA-buffer–insoluble tau in HEK-293FT/Tau151 - 391 cells treated with brain extracts. Results: We found a comparable level of soluble tau in gray matter versus white matter of control brains, but a higher level of soluble tau in gray matter than white matter of AD brains. In AD brains, tau is hyperphosphorylated in both gray and white matters, with a higher level in the former. The extracts of both gray and white matters of AD brains seeded tau aggregation in HEK-293FT/tau151–391 cells but the white matter showed less potency. Seeding activity of tau in brain extracts was positively correlated with the levels of tau hyperphosphorylation and HMW-tau. RIPA-insoluble tau, but not RIPA-soluble tau, was hyperphosphorylated tau at multiple sites. Conclusion: Both gray and white matters of AD brain contain seeding-competent tau that can template aggregation of hyperphosphorylated tau, but the seeding potency is markedly higher in gray matter than in white matter.

Trans‐active response DNA‐binding protein of 43 kDa (TDP‐43) promotes tau mRNA instability and tau exon 10 inclusion. Aggregation of phosphorylated TDP‐43 is associated with amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration. Casein kinase 1ε (CK1ε) phosphorylates TDP‐43 at multiple sites, enhances its cytoplasmic aggregation, and modulates its function in tau mRNA processing. To determine roles of TDP‐43 site‐specific phosphorylation in its localization, aggregation, and function in tau mRNA processing, TDP‐43 was mutated to alanine or aspartic acid at Ser379, Ser403/404, or Ser409/410 to block or mimic phosphorylation. Site‐specific phosphorylation of TDP‐43 and its mutants by CK1ε was studied in vitro and in cultured cells. Cytoplasmic and nuclear TDP‐43 and phospho‐TDP‐43 were analyzed by western blots. Aggregation of TDP‐43 was assessed by immunostaining and level of radioimmunoprecipitation assay buffer‐insoluble TDP‐43. Green florescent protein tailed with tau 3′‐untranslated region and mini‐tau gene pCI/SI9‐LI10 were used to study tau mRNA stability and alternative splicing of tau exon 10. We found that phospho‐blocking mutations of TDP‐43 at Ser379, Ser403/404, or Ser409/410 were not effectively phosphorylated by CK1ε. Compared with TDP‐43, higher level of phosphorylated TDP‐43 in the cytoplasm was observed. Phospho‐mimicking mutations at these sites enhanced cytoplasmic aggregation of TDP‐43. Green florescent protein expression was not inhibited by phospho‐blocking mutants of TDP‐43, but tau exon 10 inclusion was further enhanced by phospho‐blocking mutations at Ser379 and Ser403/404. Phosphorylation of TDP‐43 at Ser379, Ser403/404, or Ser409/410 primes its phosphorylation by CK1ε, promotes TDP‐43 cytoplasmic aggregation, and modulates its function in tau mRNA processing in site‐specific manner.