ON THE MECHANISM OF THE TAU R406W MUTATION

Abstract

The tau-R406W mutation is observed to have a causative effective in neurodegeneration in human tauopathies, and is widely used to induce neurodegeneration in mouse models of Alzheimer's disease. Despite its clinical importance and wide use in research, the mechanism by which the R406W mutation leads to tau aggregation and neurodegeneration is not understood. We tested a series of hypotheses for potential mechanisms of functional effects of tau-R406W mutation: (1) increased hydrophobicity; (2) increased tau phosphorylation via increased activity by the kinases cdk5, GSK-3b, and/or PKA; (3) increased tau phosphorylation through decreased phosphorylation by the phosphatases PP2A or PP2B; (4) conformational change via increased cis amide bond formation due to a cis-proline-aromatic interaction. A series of peptides derived from the tau C-terminal domain was synthesized, in non-phosphorylated form and with phosphorylation at Ser396, Ser400, Thr403, Ser404, and/or Ser409. In addition, peptides were synthesized with R406 changed to Trp or other aromatic amino acids. Peptides were characterized by NMR spectroscopy. The R406W mutation did not increase phosphorylation by any kinase tested. Surprisingly, R406W led to increased dephosphorylation by PP2A, contrary to expectations. In contrast, the R406W mutation led to a substantial increase in cis amide bond at Ser404-Pro405, via a cis-proline-aromatic C-H/Π interaction that significantly stabilizes the cis proline amide bond. A significant increase in cis amide bond was also observed upon Ser404 phosphorylation, with the effects further modulated by phosphorylation at other sites. Additional structural changes dependent on phosphorylation and/or tauR406W were observed and characterized by NMR. The phosphorylation-dependent proline isomerism Pin1 has been strongly implicated in tau aggregation and Alzheimer's disease. These data suggest that tau phosphorylation is not directly increased by the R406W mutation. Instead, the data suggest induction of proline-405 cis amide bond formation and subsequent structural changes as a primary mechanism for the functional effects of the R406W mutation.