Hyperphosphorylation Renders Tau Prone to Aggregate and to Cause Cell Death

BackgroundAlzheimer’s disease (AD) is an irreversible neurodegenerative disease without a cure or prevention to date. The defining features of AD include neurofibrillary tangles (NFTs) of hyperphosphorylated tau fibrils, and the senile plaques of β amyloid (Aβ) aggregates. In addition to AD, aggregation of hyperphosphorylated tau underlies approximately 20 neurodegenerative disorders collectively known as tauopathies, suggesting a pathogenic role of tau fibrils. The majority of AD disease-modifying drug trials targeted Aβ control, which have yet to produce satisfactory outcomes. The interest of developing anti-NFT drugs has been increasing. However, an effective method that produces hyperphosphorylated tau with disease relevant characters to support drug discovery is lacking.MethodsWe previously reported the PIMAX system for the production of recombinant proteins bearing a desired post-translational modification. Here we use PIMAX to express hyperphosphorylated tau (p-tau) in E. coli, and subjected this p-tau to mass spectrometry mapping of phosphorylation, to biochemical and biophysical assays of fibrillogenesis, and to cell culture treatment for the effects of hyperphosphorylation on tau molecular characters.ResultsMass spectrometry mapped p-tau phosphorylation to phosphoepitopes linked to AD pathological progression. In stark contrast to the unmodified tau that required an inducer for efficient aggregation, and which had only mild effects on cell functions, p-tau formed fibrils in an inducer-free and redox-independent manner. Immediately after the purification, a disease-specific conformation recognized by the MC-1 monoclonal antibody was readily detectable, which continued to increase during the fibrillization reaction. When applied to cultured cells, p-tau triggered a spike of mitochondrial superoxide, induced apoptosis, and caused cell death at sub-micromolar concentrations. In both aggregation and cytotoxicity assays, p-tau exhibited seeding activities that converted the unmodified tau into a cytotoxic species with increased propensity for fibrillization, consistent with the emerging view that hyperphosphorylated tau spreads in a prion-like fashion.ConclusionsHyperphosphorylation potentiates tau fibrillization and cytotoxicity. These characters are consistent with the model that abnormally phosphorylated tau plays a direct role in neurodegeneration in tauopathies. We suggest that p-tau produced by PIMAX affords a feasible tool for drug discovery and disease mechanistic studies for Alzheimer's disease and other tauopathies.