Probing tau citrullination in Alzheimer’s disease brains and mouse models of tauopathy

Alzheimer's disease (AD) includes a defining hallmark that correlates most closely to cognitive decline, namely misfolded tau protein. However, the "upstream" etiology and downstream clinical manifestations of tauopathies are quite diverse. Tau deposition elicits different pathological phenotypes and outcomes depending on the tau strain, proteoforms, and regional susceptibility. Posttranslational modifications (PTM) can alter tau structure, function, networks, and its pathological sequelae. We uncovered tau citrullination on multiple epitopes caused by peptidyl arginine deiminase (PAD) enzymes. PAD-induced citrullination irreversibly converts arginine residues to citrulline, producing a net loss of positive charge, elimination of pi-pi interactions, and increased hydrophobicity. We observed increased PAD2 and PAD4 in Alzheimer's disease (AD) brain and that they both can citrullinate tau. Tau can become citrullinated by PADs at all 14 arginine residues throughout the N-terminal domain (N-term), proline-rich domain (PR), microtubule-binding repeats domain (MBR), and C-terminal domain (C-term) on full-length tau (2N4R). Citrullination of tau impacts fibrillization and oligomerization rates in aggregation assays. Utilizing a panel of novel citrullinated tau (citR tau) antibodies, we identified citrullination of tau in vitro, several animal models of tauopathies, and Alzheimer's disease (AD). CitR tau increased with Braak stage and was enriched in AD brains with higher pathological tau burden. This work provides a new area of tau biology that signifies further consideration in the emerging spectrum of tauopathies and its clinical understanding.