O2‐09‐01: Endoplasmic Reticulum (ER) Stress and Abnormal Tau Converge in a Ubiquitin‐related Vicious Cycle: Early Events in Neuronal Damage

A recent genome-wide association study linked a single nucleotide polymorphism (SNP) with increased risk for progressive supranuclear palsy (PSP). As in Alzheimer's disease, PSP is pathologically characterized by the abnormal intraneuronal aggregation of tau. The SNP is on the EIF2AK3 gene, which codes for the ER-stress sensing protein PERK. Previous reports indicate that pre-tangle neurons in PSP and Alzheimer's patients show elevated levels of activated PERK. However, the impact of pPERK in disease and its relationship with tau has not been studied. We investigated the possible connection between these two proteins. Tau transgenic mice and tau inducible-HEK cells were utilized. Assessment of protein levels and localization were performed using immunoblots and immuno-fluorescent confocal microscopy, respectively. An ex vivo functional assay method was employed: brains of mice were extracted and maintained alive in artificial CSF for incubation with chemical manipulators of the proteasome and apoptosis. We assessed whether tau induced ER stress in tau transgenic brains and tau-overexpressing cell cultures. Active PERK and ER-resident chaperones were significantly increased; in fact, these ER stress markers were directly proportional to tau levels. ER-associated degradation (ERAD) is an ubiquitin-assisted clearance mechanism by which the ER removes proteins that cannot be folded/refolded. Alterations to ERAD elicit ER stress, so we assessed whether changes in tau affected ubiquitin: levels were increased in vivo and in vitro. Furthermore, while excess ubiquitin co-localized with the ER, tau did not. We performed ex vivo functional assays and established that ubiquitinylated proteins accumulate in tau mice. However, inhibitors of proteasomal function caused potent upregulation of ubiquitinylated species in control brains, while further ubiquitinylation was impaired in tau brains. Since chronic ERAD initiates apoptosis, we explored for apoptosis markers and found that tau brains have elevated cleaved-PARP. Similarly, further chemical induction of apoptosis did not manifest in tau brains. Our data suggest that tau impairs protein ubiquitinylation eliciting ER stress. Consequent initiation of apoptosis is also evident in tau-bearing brains. This work adds critical information to the mechanism by which tau induces cell damage. Further elucidation of this process may reveal potential therapeutic targets for tauopathies.