Basic Science and Pathogenesis

Intracellular neurofibrillary tangles (NFTs) consisting of aggregated and hyperphosphorylated tau is a major pathological hallmark of Alzheimer's disease and related tauopathies. In AD brain, tau pathology spreads through prion-like propagation and follows a stereotypical pattern: it originates from the locus coeruleus and transentorhinal cortex (Braak stages I-II), then spreads to the limbic system (Braak stages III-IV), and eventually to the neocortex (Braak stages V-VI). Normally, the acidic N- and C-termini of tau prefers to fold over the basic microtubule binding repeats (MTBR) to form a paperclip-like conformation, preventing the protein from self-aggregation. In AD brain, the extreme N-terminus of tau is lost from NFTs and C-terminal truncations are found in PHFs as well as mature and ghost tangles. Loss of the N- and C-termini of tau may facilitate self-aggregation and drive tau pathogenesis.

A truncated form of tau, tau_151-391 was generated by deleting the acidic N- and C-termini of tau and overexpressed in HEK-293T cells. RIPA buffer insoluble tau_151-391 aggregates were obtained by ultracentrifugation. Its phosphorylation and proteopathic properties were determined in vitro and in vivo.

We found that tau_151-391, but not full-length tau (tau_FL), can form aggregates in cultured cells and in vivo in mouse brain. Aggregated tau_151-391 is hyperphosphorylated at multiple sites, including Thr181, Ser199, AT8, Thr212, Ser214, Thr217, and Thr231, and is partially resistant to hydrolysis by proteinase K. Tau_151-391 aggregates potently induce tau aggregation and site-specific hyperphosphorylation in cultured cells and in mouse brain. By repeatedly inducing aggregation in cultured cells, aggregated tau increases exponentially. The seeding activity of tau_151-391 aggregates is sensitive to repeated freezing and thawing. 3R-tau_151-391, which contains three MTBRs, and 4R-tau_151-391, which contains four MTBRs, can be induced to aggregate by each other's aggregates.

Deletion of the acidic N- and C-termini facilitates tau aggregation and acquires proteopathic properties. Tau_151-391 aggregates effectively induce tau aggregation and site-specific hyperphosphorylation in cultured cells and in vivo in mouse brain.