Structural and functional implications of tau hyperphosphorylation: Information from phosphorylation-mimicking mutated tau proteins

Abnormal tau-immunoreactive filaments are a hallmark of tauopathies, includ ing Alzheimer's disease (AD). A higher phosphorylation ("hyperphosphorylati on") state of tau protein may represent a critical event. To determine the potential role of tau hyperphosphorylation in these disorders, mutated tau proteins were produced where serine/threonine residues known to be highly p hosphorylated in tau filaments isolated from AD patients were substituted f or glutamate to simulate a paired helical filament (PHF)-like tau hyperphos phorylation. We demonstrate that, like hyperphosphorylation, glutamate subs titutions induce compact structure elements and SDS-resistant conformationa l domains in tau protein. Hyperphosphorylation-mimicking glutamate-mutated tau proteins display a complete functional loss in its ability to promote m icrotubule nucleation which can partially be overcome by addition of the os molyte trimethylamine N-oxide (TMAO), which is similar to phosphorylated ta u. In addition, glutamate-mutated tau proteins fail to interact with the do minant brain protein phosphatase 2A isoform AB alphaC, and exhibit a reduce d ability to assemble into filaments. Interestingly, wild-type tau and phos phorylation-mimicking tau similarly bind to microtubules when added alone, but the mutated tau is almost completely displaced from the microtubule sur face by equimolar concentrations of wild-type tau. The data indicate that g lutamate-mutated tau proteins provide a useful model for analyzing the func tional consequences of tau hyperphosphorylation. They suggest that several mechanisms contribute to the abnormal tau accumulation observed during tauo pathies, in particular a selective displacement of hyperphosphorylated tau from microtubules, a functional loss in promoting microtubule nucleation, a nd a failure to interact with phosphatases.