Emerging Role of Tau Citrullination During Alzheimer's Disease and Tauopathies

Human tauopathies are a group of neurological disorders, including Alzheimer’s disease (AD), for which no disease modifying therapies exist. Tau proteinopathic phenotypes show a spectrum of pathological hallmarks and clinical syndromes. There is increasing recognition that post-translational modifications (PTMs) promote strain or conformational assemblies that contribute to tauopathies, signifying a need to understand how novel PTMs contribute to tauopathy risk. Protein citrullination (citR), induced by peptidyl arginine deiminases (PADs), leads to an irreversible PTM that alters protein structure/ function by neutralizing positively charged arginine residues within proteins. We discovered that aberrant tau activates PAD4 in neurons (protein PAD4, gene PADi4); tau is client of PAD4 and is citrullinated at 14 arginine residues and those changes are associated with reduced tau fibrillization; citR prevents tau phosphorylation of key epitopes and impacts tau clearance; and tau citR is elevated in AD and in animal models of tauopathy. Our central hypothesis argues that increased PAD4-induced tau citR prevents higher-order fibrillization, which decreases tangle formation, alters tau metabolism, impacts structure/ function, and reduces seeding activity. Assumptions of this model will be tested in vivo and in vitro by neuronal PADi4 overexpression and conditional knockout mice, multiple biochemical techniques, electrophysiologic, and behavioral approaches. We will establish how neuronal PADi4 expression impacts tau neuropathologic changes. Aim 1 will advance our understanding for PADi4 gain/ loss of function to establish the foundation for citR tau biology and the impact on behavior in different models of tauopathy. We will establish how citR regulates tau biochemistry and biology in cellular models. Aim 2 will mechanistically determine how PAD4-induced citR tau alters tau clearance, aggregation, seeding and microtubule binding. We will establish the spatio-temporal signature of citR tau in common human tauopathies. Aim 3 will provide the qualitative framework and seeding activity of citR tau in common human tauopathies. The proposed work provides the necessary foundation in not only better characterizing the role of citR in tauopathies, but also further establishing the first mechanistic cause for loss and gain of function of PAD4 during tau pathogenesis. Overall, our application describes a new tau PTM and provide a therapeutic strategy for a new pathway in tau biology and in AD.