MAPK family members differentially regulate pThr175 tau-mediated pathogenicity

The phosphorylation of tau is a critical determinant of both its physiological function and the induction of pathological misfolding and aggregation. We have previously provided evidence that tau phosphorylation at Thr175 results in the exposure of the N-terminal phosphatase-activating domain (PAD) leading to the subsequent phosphorylation of Thr231, and formation of tau oligomers. A number of tauopathies, including chronic traumatic encephalopathy (CTE), amyotrophic lateral sclerosis with cognitive impairment (ALSci), and experimental traumatic brain injury (TBI) have been proposed to be associated with this cascade of events. However, the cellular mechanism by which Thr175 tau is phosphorylated remains unclear. In this study we identified ERK2, JNK1, and p38 as candidate kinases through molecular and histological analyses in a rodent model of TBI, where increased kinase activity and protein interaction were associated with pThr175 tau. We confirmed that both ERK2 and JNK1 are capable of phosphorylating Thr175 tau in vitro, but only ERK2-mediated phosphorylation of Thr175 tau induced the pathological cascade characterized by PAD exposure and the generation of oligomeric, truncated and neurofibrillary tau. Thr175 phosphorylation was also associated with an altered interaction between tau and the molecular chaperone protein DnaJC7, which regulates tau misfolding. Additionally, we observed that pThr175 and pThr231 tau were increased by oxidative stress, which was associated with the activation of the MAPK signaling pathways. These findings further clarify the mechanisms leading to Thr175 tau phosphorylation and its role in pathological tau formation by identifying ERK1 and JNK2 as important cellular mediators.