PAD4 Promotes the Tau Citrullination and Pathology in Alzheimer's Disease

Alzheimer's disease (AD) is a progressive, devastating form of dementia that affects not only patients but their caregivers and worsens the quality of life for everyone affected. Although a number of therapeutic strategies are in clinical trials, there is no cure for the disease. Tau pathology remains one of the hallmarks of AD and entails accumulation of tau aggregates, neurofibrillary tangles, cognitive decline and neuronal death. Post-translational modifications (PTMs) are recognized to contribute to pathological tau, phosphorylation being the most well-studied. We uncovered a novel PTM of tau, known as citrullination (cit), caused by the enzyme peptidylarginine deiminase (PAD). Protein citrullination remains particularly understudied in neurodegenerative diseases such as AD. Structurally, this conversion irreversible alters the target protein via charge distribution and protein-folding dynamics. Importantly, we identified several seminal findings that offer new biology, new pathways and therapeutic targets for anti-tau approaches. We found several novel citrullination sites on tau via mass spectrometry and confirmed these epitope modifications in AD brain tissue, animal models of tauopathy, and cell culture systems. We hypothesize that components of the tau phenotype promote aberrant PAD4 activity, which induces tau citrullination (cit-tau). We do not know exactly how citrullination of tau impact its biology and tau fate which provide the premise for this application. However, preliminary work indicates that citrullination of tau prevents fibrillization and sequesters tau in an oligomeric state. Our goal (Aim 1) is to test how PAD4-induced citrullination alters tau aggregation kinetics, metabolism, structure (morphology) and clearance. Specifically, we will measure oligomerization and degradation of cit-tau by 20S proteasome using novel cellular models and recombinant cell-free systems. Our second goal (Aim 2) will test how PAD4 overexpression impacts oligomerization and tangle formation in animal model of tauopathy. We will also measure components of the tau phenotype (hallmarks comprised of tau effects). Success in this application will provide several gaps in knowledge regarding the role of PAD4 in AD and a new understanding of tau citrullination. Importantly, it may provide new entry points for anti-tau approaches that exploit the citrullination pathway.