Citrullination of TDP-43 is a key post-translation modification associated with structural and functional changes and progressive pathology in TDP-43 mouse models and human proteinopathies

Abstract

TAR DNA-binding protein 43 (TDP-43) pathology is associated with a spectrum of clinical dementias including limbic-predominant age-related TDP-43 encephalopathy neuropathological changes (LATE-NC). Post-translational modifications (PTM) are linked to TDP-43 toxic gain-of-function and cytoplasmic aggregation1-3. Phosphorylation remains the most investigated PTM and a standard criterion for determining pathology progression and clinical subclassification in TDP-43 proteinopathies4-7. However, full spectrum of PTMs on TDP-43 structure and biology remain unknown. Utilizing mass-spectrometry analysis we identified citrullination as a novel and irreversible "bona-fide" PTM of TDP-43 protein. We recognized peptidyl arginine deiminase 2 and 4 (PAD2 and PAD4) to mediate the conversion of arginine (R) to citrulline (citR) in vitro and demonstrated increased PAD2 and PAD4 expression and TDP-43 citrullination in a human wildtype TDP-43 mouse model (Tg (Thy1-TARDBP4). Transmission electron microscopy imaging analysis revealed citrullination induced vast structural changes while ThT analysis demonstrated altered aggregation kinetics of citrullinated (citR) TDP-43 protein. We further provided mechanistic evidence on reduced electrostatic and pi-pi interactions of citR TDP-43 Low Complexity Domain (LCD) with RNA, favoring liquid-solid phase separation and condensate formation. Generation and validation of several citR TDP-43 specific antibodies against several TDP-43 epitopes revealed epitope and domain-specific effects of citrullination on TDP-43 solubility in vivo. Importantly, we found distinct reactivities of citR TDP-43 antibodies shedding light into the contribution of epitope-specific properties of human citR TDP-43 to novel pathological citR TDP-43 assemblies in human brain tissue from LATE-NC, with or without comorbid Alzheimers disease neuropathologic changes (ADNC). These findings provided a unique look into the temporal citR TDP-43 signatures, and the potential clinical relevance associated with progression of pure LATE-NC and comorbid ADNC + LATE-NC. Collectively, these data reveal the existence of irreversible TDP-43 citrullination at targeted sites via induced PAD2/PAD4 activities, presenting a critical step in TDP-43 proteinopathy.