C-terminal inhibition of tau assembly in vitro and in Alzheimer's disease

Alzheimer's disease (AD) is, in part, defined by the polymerization of tau into paired helical and straight filaments (PHF/SFs) which together compris e the fibrillar pathology in degenerating brain regions. Much of the tau in these filaments is modified by phosphorylation, Additionally, a subset als o appears to be proteolytically truncated, resulting in the removal of its C terminus. Antibodies that recognize tau phosphorylated at S-396/404 or tr uncated at E-391 do not stain control brains but do stain brain sections ve ry early in the disease process. We modeled these phosphorylation and trunc ation events by creating pseudo-phosphorylation and deletion mutants derive d from a full-length recombinant human tau protein isoform (ht40) that cont ains N-terminal exons 2 and 3 and all four microtubule-binding repeats. In vitro assembly experiments demonstrate that both modifications greatly enha nce the rates of tau filament formation and that truncation increases the m ass of polymer formed, as well. :Removal of as few as 12 or as many as 121 amino acids from the C terminus of tau greatly increases the rate and exten t of tao polymerization, However, deletion of an :additional 7 amino acids, (DLSKVTS320)-D-314, from the third microtubule-binding repeat results in t he loss of tau's :ability to form filaments in vitro. These results suggest that only part of the microtubde-binding domain (repeats 1, 2 :and a small portion of 3) is crucial for tau polymerization, Moreover, the C terminus of tau clearly inhibits the :assembly process; this inhibition can be parti ally reversed by site-specific phosphorylation and completely removed by tr uncation events at various sites from S-320 to the end of the molecule.