PAD enzymes in rheumatoid arthritis: pathogenic effectors and autoimmune targets

Peptidylarginine deiminases (PADs) have an important role in the pathogenesis of rheumatoid arthritis (RA) owing to their ability to generate citrullinated proteins - the hallmark autoantigens of RA. Of the five PAD enzyme isoforms, PAD2 and PAD4 are the most strongly implicated in RA at both genetic and cellular levels, and PAD inhibitors have shown therapeutic efficacy in mouse models of inflammatory arthritis. PAD2 and PAD4 are additionally targeted by autoantibodies in distinct clinical subsets of patients with RA, suggesting anti-PAD antibodies as possible biomarkers for RA diagnosis and prognosis. This Review weighs the evidence that supports a pathogenic role for PAD enzymes in RA as both promoters and targets of the autoimmune response, as well as discussing the mechanistic and therapeutic implications of these findings in the wider context of RA pathogenesis. Understanding the origin and consequences of dysregulated PAD enzyme activity and immune responses against PAD enzymes will be important to fully comprehend the pathogenic mechanisms involved in this disease and for the development of novel strategies to treat and prevent RA.

Peptidylarginine deiminases and deiminated proteins at the epidermal barrier

Deimination or citrullination is a post-translational modification catalysed by a family of calcium-dependent enzymes called peptidylarginine deiminases (PADs). It corresponds to the transformation of arginine residues within a peptide sequence into citrulline residues. Deimination induces a decreased net charge of targeted proteins; therefore, it alters their folding and changes intra- and intermolecular ionic interactions. Deimination is involved in several physiological processes (inflammation, gene regulation, etc.) and human diseases (rheumatoid arthritis, neurodegenerative diseases, cancer, etc.). Here, we describe the PADs expressed in the epidermis and their known substrates, focusing on their role in the epidermal barrier function.

Necroptosis: Modules and molecular switches with therapeutic implications

Among the various programmed cell death (PCD) pathways, "Necroptosis" has gained much importance as a novel paradigm of cell death. This pathway has emerged as a backup mechanism when physiologically conserved PCD (apoptosis) is non-functional either genetically or pathogenically. The expanding spectrum of necroptosis from physiological development to diverse patho-physiological disorders, including xenobiotics-mediated toxicity has now grabbed the attention worldwide. The efficient role of necroptosis regulators in disease development and management are under constant examination. In fact, few regulators (e.g. MLKL) have already paved their way towards clinical trials and others are in queue. In this review, emphasis has been paid to the various contributing factors and molecular switches that can regulate necroptosis. Here we linked the overview of current knowledge of this enigmatic signaling with magnitude of therapeutics that may underpin the opportunities for novel therapeutic approaches to suppress the pathogenesis of necroptosis-driven disorders.

Induction of peptidylarginine deiminase 2 and 3 by dibutyryl cAMP via cAMP-PKA signaling in human astrocytoma U-251MG cells

Peptidylarginine deiminases (PADs) are posttranslational modification enzymes that citrullinate (deiminate) protein arginine residues in a calcium-dependent manner, yielding citrulline residues. Enzymatic citrullination abolishes positive charges of native protein molecules, inevitably causing significant alterations in their structure and function. Previously, we reported the abnormal accumulation of citrullinated proteins and an increase of PAD2 content in hippocampi of patients with Alzheimer disease. In this study, we investigated PAD expression by using dibutyryl cAMP (dbcAMP) in human astrocytoma U-251MG cells. Under normal culture conditions, PAD2 and PAD3 mRNA expression is detectable with quantitative PCR in U-251MG cells. The addition of dbcAMP in a dose-dependent manner significantly increased this mRNA expression and protein levels. Moreover, PAD enzyme activity also increased significantly and dose-dependently. Furthermore, the expression of PAD2 and PAD3 mRNA was inhibited by the cAMP-dependent PKA inhibitor KT5720, suggesting that such expression of dbcAMP-induced PAD2 and PAD3 mRNA is mediated by the cAMP-PKA signaling pathway in U-251MG cells. This is the first report to document the PAD2 and PAD3 mRNA expression induced by dbcAMP and to attribute the induction of these genes to mediation by the cAMP-PKA signaling pathway in U-251MG cells. © 2016 Wiley Periodicals, Inc.

Peptidylarginine Deiminases as Drug Targets in Neonatal Hypoxic-Ischemic Encephalopathy

Oxygen deprivation and infection are major causes of perinatal brain injury leading to cerebral palsy and other neurological disabilities. The identification of novel key factors mediating white and gray matter damage are crucial to allow better understanding of the specific contribution of different cell types to the injury processes and pathways for clinical intervention. Recent studies in the Rice-Vannucci mouse model of neonatal hypoxic ischemia (HI) have highlighted novel roles for calcium-regulated peptidylarginine deiminases (PADs) and demonstrated neuroprotective effects of pharmacological PAD inhibition following HI and synergistic infection mimicked by lipopolysaccharide stimulation.

Potential protein targets of the peptidylarginine deiminase 2 and peptidylarginine deiminase 4 enzymes in rheumatoid synovial tissue and its possible meaning

OBJECTIVE

The molecular mechanism of citrullination involves the calcium-dependent peptidylarginine deiminase (PAD) family of enzymes. These enzymes induce a stereochemical modification of normal proteins and transform them into autoantigens, which in rheumatoid arthritis trigger a complex cascade of joint inflammatory events followed by chronic synovitis, pannus formation, and finally, cartilage destruction. By hypothesizing that PAD2 and PAD4 enzymes produce autoantigens, we investigated five possible synovial protein targets of PAD enzymes.

MATERIAL AND METHODS

We measured PAD2, PAD4, and citrullinated proteins in 10 rheumatoid and 10 osteoarthritis synovial biopsies and then assessed the post-translational modifications of fibrinogen, cytokeratin, tubulin, IgG, and vimentin proteins using a double-fluorescence assay with specific antibodies and an affinity-purified anti-citrullinated peptide (CCP) antibody. The degree of co-localization was analyzed, and statistical significance was determined by ANOVA, Fisher's exact test, and regression analysis.

RESULTS

The principal results of this study demonstrated that citrullinated proteins, such as fibrinogen, IgG, and other probed proteins, were targets of PAD2 and PAD4 activity in rheumatoid synovial biopsies, whereas osteoarthritis biopsies were negative for this enzyme (p<0.0001). An analysis of citrullination sites using the UniProtKB/Swiss-Prot data bank predicts that the secondary structure of the analyzed proteins displays most of the sites for citrullination; a discussion regarding its possible meaning in terms of pathogenesis is made.

CONCLUSION

Our results support the conclusion that the synovial citrullination of proteins is PAD2 and PAD4 dependent. Furthermore, there is a collection of candidate proteins that can be citrullinated.

Peptidylarginine Deiminase 3 (PAD3) Is Upregulated by Prolactin Stimulation of CID-9 Cells and Expressed in the Lactating Mouse Mammary Gland

Peptidylarginine deiminases (PADs) post-translationally convert arginine into neutral citrulline residues. Our past work shows that PADs are expressed in the canine and murine mammary glands; however, the mechanisms regulating PAD expression and the function of citrullination in the normal mammary gland are unclear. Therefore, the first objective herein was to investigate regulation of PAD expression in mammary epithelial cells. We first examined PAD levels in CID-9 cells, which were derived from the mammary gland of mid-pregnant mice. PAD3 expression is significantly higher than all other PAD isoforms and mediates protein citrullination in CID-9 cells. We next hypothesized that prolactin regulates PAD3 expression. To test this, CID-9 cells were stimulated with 5 μg/mL of prolactin for 48 hours which significantly increases PAD3 mRNA and protein expression. Use of a JAK2 inhibitor and a dominant negative (DN)-STAT5 adenovirus indicate that prolactin stimulation of PAD3 expression is mediated by the JAK2/STAT5 signaling pathway in CID-9 cells. In addition, the human PAD3 gene promoter is prolactin responsive in CID-9 cells. Our second objective was to investigate the expression and activity of PAD3 in the lactating mouse mammary gland. PAD3 expression in the mammary gland is highest on lactation day 9 and coincident with citrullinated proteins such as histones. Use of the PAD3 specific inhibitor, Cl4-amidine, indicates that PAD3, in part, can citrullinate proteins in L9 mammary glands. Collectively, our results show that upregulation of PAD3 is mediated by prolactin induction of the JAK2/STAT5 signaling pathway, and that PAD3 appears to citrullinate proteins during lactation.

Peptidylarginine deiminases: novel drug targets for prevention of neuronal damage following hypoxic ischemic insult (HI) in neonates

Neonatal hypoxic ischaemic (HI) injury frequently causes neural impairment in surviving infants. Our knowledge of the underlying molecular mechanisms is still limited. Protein deimination is a post-translational modification caused by Ca⁺²-regulated peptidylarginine deiminases (PADs), a group of five isozymes that display tissue-specific expression and different preference for target proteins. Protein deimination results in altered protein conformation and function of target proteins, and is associated with neurodegenerative diseases, gene regulation and autoimmunity. In this study, we used the neonatal HI and HI/infection [lipopolysaccharide (LPS) stimulation] murine models to investigate changes in protein deimination. Brains showed increases in deiminated proteins, cell death, activated microglia and neuronal loss in affected brain areas at 48 h after hypoxic ischaemic insult. Upon treatment with the pan-PAD inhibitor Cl-amidine, a significant reduction was seen in microglial activation, cell death and infarct size compared with control saline or LPS-treated animals. Deimination of histone 3, a target protein of the PAD4 isozyme, was increased in hippocampus and cortex specifically upon LPS stimulation and markedly reduced following Cl-amidine treatment. Here, we demonstrate a novel role for PAD enzymes in neural impairment in neonatal HI Encephalopathy, highlighting their role as promising new candidates for drug-directed intervention in neurotrauma.

Hypoxic Ischaemic Insult (HI) results in activation of peptidylarginine deiminases (PADs) because of calcium dysregulation. Target proteins undergo irreversible changes of protein bound arginine to citrulline, resulting in protein misfolding. Infection in synergy with HI causes up-regulation of TNFα, nuclear translocation of PAD4 and change in gene regulation as a result of histone deimination. Pharmacological PAD inhibition significantly reduced HI brain damage.

Modulation of calcium-induced cell death in human neural stem cells by the novel peptidylarginine deiminase-AIF pathway

PADs (peptidylarginine deiminases) are calcium-dependent enzymes that change protein-bound arginine to citrulline (citrullination/deimination) affecting protein conformation and function. PAD up-regulation following chick spinal cord injury has been linked to extensive tissue damage and loss of regenerative capability. Having found that human neural stem cells (hNSCs) expressed PAD2 and PAD3, we studied PAD function in these cells and investigated PAD3 as a potential target for neuroprotection by mimicking calcium-induced secondary injury responses. We show that PAD3, rather than PAD2 is a modulator of cell growth/death and that PAD activity is not associated with caspase-3-dependent cell death, but is required for AIF (apoptosis inducing factor)-mediated apoptosis. PAD inhibition prevents association of PAD3 with AIF and AIF cleavage required for its translocation to the nucleus. Finally, PAD inhibition also hinders calcium-induced cytoskeleton disassembly and association of PAD3 with vimentin, that we show to be associated also with AIF; together this suggests that PAD-dependent cytoskeleton disassembly may play a role in AIF translocation to the nucleus. This is the first study highlighting a role of PAD activity in balancing hNSC survival/death, identifying PAD3 as an important upstream regulator of calcium-induced apoptosis, which could be targeted to reduce neural loss, and shedding light on the mechanisms involved.

Dual immunofluorescence study of citrullinated proteins in Alzheimer diseased frontal cortex

Deimination is a post-translational modification of proteins in which selected arginine amino acids are enzymatically converted to citrullines. Using dual-color immunofluorescence, the present study is the first to examine the frontal cortex of patients with Alzheimer's disease (AD) versus age-matched controls with an established monoclonal antibody (F95) against peptidyl-citrulline moieties. In AD specimens, a number of new findings were discovered, including evidence for deiminated proteins in extracellular plaques, the walls of large blood vessels, the nuclei of selective neurons immunoreactive for phosphorylated tau and numerous reactive astrocytes concentrated around extracellular plaques, ventricular surfaces and at the interface between the gray and white matter of the cortex. Although the identities of these citrullinated proteins remain largely unknown, the present study adds to the growing number of locations in which deiminated proteins may be found in the brains of patients with AD.

Protein deiminases: new players in the developmentally regulated loss of neural regenerative ability

Spinal cord regenerative ability is lost with development, but the mechanisms underlying this loss are still poorly understood. In chick embryos, effective regeneration does not occur after E13, when spinal cord injury induces extensive apoptotic response and tissue damage. As initial experiments showed that treatment with a calcium chelator after spinal cord injury reduced apoptosis and cavitation, we hypothesized that developmentally regulated mediators of calcium-dependent processes in secondary injury response may contribute to loss of regenerative ability. To this purpose we screened for such changes in chick spinal cords at stages of development permissive (E11) and non-permissive (E15) for regeneration. Among the developmentally regulated calcium-dependent proteins identified was PAD3, a member of the peptidylarginine deiminase (PAD) enzyme family that converts protein arginine residues to citrulline, a process known as deimination or citrullination. This post-translational modification has not been previously associated with response to injury. Following injury, PAD3 up-regulation was greater in spinal cords injured at E15 than at E11. Consistent with these differences in gene expression, deimination was more extensive at the non-regenerating stage, E15, both in the gray and white matter. As deimination paralleled the extent of apoptosis, we investigated the effect of blocking PAD activity on cell death and deiminated-histone 3, one of the PAD targets we identified by mass-spectrometry analysis of spinal cord deiminated proteins. Treatment with the PAD inhibitor, Cl-amidine, reduced the abundance of deiminated-histone 3, consistent with inhibition of PAD activity, and significantly reduced apoptosis and tissue loss following injury at E15. Altogether, our findings identify PADs and deimination as developmentally regulated modulators of secondary injury response, and suggest that PADs might be valuable therapeutic targets for spinal cord injury.

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.

Accumulation of citrullinated proteins by up-regulated peptidylarginine deiminase 2 in brains of scrapie-infected mice: a possible role in pathogenesis

Peptidylarginine deiminases (PADs), which are a group of posttranslational modification enzymes, are involved in protein citrullination (deimination) by the conversion of peptidylarginine to peptidylcitrulline in a calcium concentration-dependent manner. Among the PADs, PAD2 is widely distributed in various tissues and is the only type that is expressed in brain. To elucidate the involvement of protein citrullination by PAD2 in the pathogenesis of brain-specific prion diseases, we examined the profiles of citrullinated proteins using the brains of scrapie-infected mice as a prion disease model. We found that, compared with controls, increased levels of citrullinated proteins of various molecular weights were detected in different brain sections of scrapie-infected mice. In support of this data, expression levels of PAD2 protein as well as its enzyme activity were significantly increased in brain sections of scrapie-infected mice, including hippocampus, brain stem, and striatum. Additionally, the expression levels of PAD2 mRNA were increased during scrapie infection. Moreover, PAD2 immunoreactivity was increased in scrapie-infected brains, with staining detected primarily in reactive astrocytes. Using two-dimensional electrophoresis and matrix-assisted laser desorption/ionization-time of flight mass spectrometry, various citrullinated proteins were identified in the brains of scrapie-infected mice, including glial fibrillary acidic protein, myelin basic protein, enolases, and aldolases. This study suggests that accumulated citrullinated proteins and abnormal activation of PAD2 may function in the pathogenesis of prion diseases and serve as potential therapeutic targets.