Developmental and age-related changes of peptidylarginine deiminase 2 in the mouse brain

Peptidylarginine deiminases (PADs) are a group of posttranslational modification enzymes that citrullinate (deiminate) protein arginine residues in a Ca(2+)-dependent manner. Enzymatic citrullination abolishes positive charges of native protein molecules, inevitably causing significant alterations in their structure and functions. Among the five isoforms of PADs, PAD2 and PAD4 are proved occupants of the central nervous system (CNS), and especially PAD2 is a main PAD enzyme expressed in the CNS. We previously reported that abnormal protein citrullination by PAD2 has been closely associated with the pathogenesis of neurodegenerative disorders such as Alzheimer's disease and prion disease. Protein citrullination in these patients is thought to play a role during the initiation and/or progression of disease. However, the contribution of changes in PAD2 levels, and consequent citrullination, during developmental and aging processes remained unclear. Therefore, we used quantitative real-time RT-PCR, Western blot analysis, and immunohistochemical methods to measure PAD2 expression and localization in the brain during those processes. PAD2 mRNA expression was detected in the brains of mice as early as embryonic day 15, and its expression in cerebral cortex, hippocampus, and cerebellum increased significantly as the animals aged from 3 to 30 months old. No citrullinated proteins were detected during that period. Moreover, we found here, for the first time, that PAD2 localized specifically in the neuronal cells of the cerebral cortex and Purkinje cells of the cerebellum. These findings indicate that, despite PAD2's normally inactive status, it becomes active and citrullinates cellular proteins, but only when the intracellular Ca(2+) balance is upset during neurodegenerative changes.