Involvement of peptidylarginine deiminase-mediated post-translational citrullination in pathogenesis of sporadic Creutzfeldt-Jakob disease

PAD2 dysregulation and aberrant protein citrullination feature prominently in reactive astrogliosis and myelin protein aggregation in sporadic ALS

Alteration in protein citrullination (PC), a common posttranslational modification (PTM), contributes to pathogenesis in various inflammatory disorders. We previously reported that PC and protein arginine deiminase 2 (PAD2), the predominant enzyme isoform that catalyzes this PTM in the central nervous system (CNS), are altered in mouse models of amyotrophic lateral sclerosis (ALS). We now demonstrate that PAD2 expression and PC are altered in human postmortem ALS spinal cord and motor cortex compared to controls, increasing in astrocytes while trending lower in neurons. Furthermore, PC is enriched in protein aggregates that contain the myelin proteins PLP and MBP in ALS. These results confirm our findings in ALS mouse models and suggest that altered PAD2 and PC contribute to neurodegeneration in ALS.

Selective inhibition of peptidyl-arginine deiminase (PAD): can it control multiple inflammatory disorders as a promising therapeutic strategy?

Peptidyl arginine deiminases (PADs) are a family of post-translational modification enzymes that irreversibly citrullinate (deiminate) arginine residues of protein and convert them to a non-classical amino acid citrulline in the presence of calcium ions. It has five isotypes, such as PAD1, PAD2, PAD3, PAD4, and PAD6, found in mammalian species. It has been suggested that increased PAD expression in various tissues contributes to the development of multiple inflammatory diseases, including rheumatoid arthritis (RA), cancer, diabetes, and neurological disorders. Elevation of PAD enzyme expression depends on several factors like rising intracellular Ca²⁺ levels, oxidative stress, and proinflammatory cytokines. PAD inhibitors originating from natural or synthetic sources can be used as a novel therapeutic approach concerning inflammatory disorders. Here, we review the pathological role of PAD in several inflammatory disorders, factors that trigger PAD expression, epigenetic role and finally, decipher the therapeutic approach of PAD inhibitors in multiple inflammatory disorders.

Protein citrullination marks myelin protein aggregation and disease progression in mouse ALS models

Increased protein citrullination (PC) and dysregulated protein arginine deiminase (PAD) activity have been observed in several neurodegenerative diseases. PC is a posttranslational modification catalyzed by the PADs. PC converts peptidyl-arginine to peptidyl-citrulline, thereby reducing the positive charges and altering structure and function of proteins. Of the five PADs, PAD2 is the dominant isoform in the central nervous system (CNS). Abnormal PC and PAD dysregulation are associated with numerous pathological conditions, including inflammatory diseases and neurodegeneration. Animal model studies have shown therapeutic efficacy from inhibition of PADs, thus suggesting a role of PC in pathogenesis. To determine whether PC contribute to amyotrophic lateral sclerosis (ALS), a deadly neurodegenerative disease characterized by loss of motor neurons, paralysis, and eventual death, we investigated alterations of PC and PAD2 in two different transgenic mouse models of ALS expressing human mutant SOD1^G93A and PFN1^C71G, respectively. PC and PAD2 expression are altered dynamically in the spinal cord during disease progression in both models. PC and PAD2 increase progressively in astrocytes with the development of reactive astrogliosis, while decreasing in neurons. Importantly, in the spinal cord white matter, PC accumulates in protein aggregates that contain the myelin proteins PLP and MBP. PC also accumulates progressively in insoluble protein fractions during disease progression. Finally, increased PC and PAD2 expression spatially correlate with areas of the CNS with the most severe motor neuron degeneration. These results suggest that altered PC is an integral part of the neurodegenerative process and potential biomarkers for disease progression in ALS. Moreover, increased PC may contribute to disease-associated processes such as myelin protein aggregation, myelin degeneration, and astrogliosis.

Pediatric Brain Tumors: Signatures from the Intact Proteome

The present investigation aimed to explore the intact proteome of tissues of pediatric brain tumors of different WHO grades and localizations, including medulloblastoma, pilocytic astrocytoma, and glioblastoma, in comparison with the available data on ependymoma, to contribute to the understanding of the molecular mechanisms underlying the onset and progression of these pathologies. Tissues have been homogenized in acidic water−acetonitrile solutions containing proteases inhibitors and analyzed by LC−high resolution MS for proteomic characterization and label-free relative quantitation. Tandem MS spectra have been analyzed by either manual inspection or software elaboration, followed by experimental/theoretical MS fragmentation data comparison by bioinformatic tools. Statistically significant differences in protein/peptide levels between the different tumor histotypes have been evaluated by ANOVA test and Tukey’s post-hoc test, considering a p-value > 0.05 as significant. Together with intact protein and peptide chains, in the range of molecular mass of 1.3−22.8 kDa, several naturally occurring fragments from major proteins, peptides, and proteoforms have been also identified, some exhibiting proper biological activities. Protein and peptide sequencing allowed for the identification of different post-translational modifications, with acetylations, oxidations, citrullinations, deamidations, and C-terminal truncations being the most frequently characterized. C-terminal truncations, lacking from two to four amino acid residues, particularly characterizing the β-thymosin peptides and ubiquitin, showed a different modulation in the diverse tumors studied. With respect to the other tumors, medulloblastoma, the most frequent malignant brain tumor of the pediatric age, was characterized by higher levels of thymosin β4 and β10 peptides, the latter and its des-IS form particularly marking this histotype. The distribution pattern of the C-terminal truncated forms was also different in glioblastoma, particularly underlying gender differences, according to the definition of male and female glioblastoma as biologically distinct diseases. Glioblastoma was also distinguished for the peculiar identification of the truncated form of the α-hemoglobin chain, lacking the C-terminal arginine, and exhibiting oxygen-binding and vasoconstrictive properties different from the intact form. The proteomic characterization of the undigested proteome, following the top-down approach, was challenging to originally investigate the post-translational events that differently characterize pediatric brain tumors. This study provides a contribution to elucidate the molecular profiles of the solid tumors most frequently affecting the pediatric age, and which are characterized by different grades of aggressiveness and localization.

Histone citrullination: a new target for tumors

As the main protein components of chromatin, histones play central roles in gene regulation as spools of winding DNA. Histones are subject to various modifications, including phosphorylation, acetylation, glycosylation, methylation, ubiquitination and citrullination, which affect gene transcription. Histone citrullination, a posttranscriptional modification catalyzed by peptidyl arginine deiminase (PAD) enzymes, is involved in human carcinogenesis. In this study, we highlighted the functions of histone citrullination in physiological regulation and tumors. Additionally, because histone citrullination involves forming neutrophil extracellular traps (NETs), the relationship between NETs and tumors was illustrated. Finally, the clinical application of histone citrullination and PAD inhibitors was discussed.

Citrullination and PAD Enzyme Biology in Type 1 Diabetes - Regulators of Inflammation, Autoimmunity, and Pathology

The generation of post-translational modifications (PTMs) in human proteins is a physiological process leading to structural and immunologic variety in proteins, with potentially altered biological functions. PTMs often arise through normal responses to cellular stress, including general oxidative changes in the tissue microenvironment and intracellular stress to the endoplasmic reticulum or immune-mediated inflammatory stresses. Many studies have now illustrated the presence of 'neoepitopes' consisting of PTM self-proteins that induce robust autoimmune responses. These pathways of inflammatory neoepitope generation are commonly observed in many autoimmune diseases including systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes (T1D), among others. This review will focus on one specific PTM to self-proteins known as citrullination. Citrullination is mediated by calcium-dependent peptidylarginine deiminase (PAD) enzymes, which catalyze deimination, the conversion of arginine into the non-classical amino acid citrulline. PADs and citrullinated peptides have been associated with different autoimmune diseases, notably with a prominent role in the diagnosis and pathology of rheumatoid arthritis. More recently, an important role for PADs and citrullinated self-proteins has emerged in T1D. In this review we will provide a comprehensive overview on the pathogenic role for PADs and citrullination in inflammation and autoimmunity, with specific focus on evidence for their role in T1D. The general role of PADs in epigenetic and transcriptional processes, as well as their crucial role in histone citrullination, neutrophil biology and neutrophil extracellular trap (NET) formation will be discussed. The latter is important in view of increasing evidence for a role of neutrophils and NETosis in the pathogenesis of T1D. Further, we will discuss the underlying processes leading to citrullination, the genetic susceptibility factors for increased recognition of citrullinated epitopes by T1D HLA-susceptibility types and provide an overview of reported autoreactive responses against citrullinated epitopes, both of T cells and autoantibodies in T1D patients. Finally, we will discuss recent observations obtained in NOD mice, pointing to prevention of diabetes development through PAD inhibition, and the potential role of PAD inhibitors as novel therapeutic strategy in autoimmunity and in T1D in particular.

Post-Translational Protein Deimination Signatures in Plasma and Plasma EVs of Reindeer (Rangifer tarandus)

Simple Summary

Reindeer are an important wild and domesticated species of the Arctic, Northern Europe, Siberia and North America. As reindeer have developed various strategies to adapt to extreme environments, this makes them an interesting species for studies into diversity of immune and metabolic functions in the animal kingdom. Importantly, while reindeer carry natural infections caused by viruses (including coronaviruses), bacteria and parasites, they can also act as carriers for transmitting such diseases to other animals and humans, so called zoonosis. Reindeer are also affected by chronic wasting disease, a neuronal disease caused by prions, similar to scrapie in sheep, mad cows disease in cattle and Creutzfeldt-Jakob disease in humans. The current study assessed a specific protein modification called deimination/citrullination, which can change how proteins function and allow them to take on different roles in health and disease processes. Profiling of deiminated proteins in reindeer showed that many important pathways for immune defenses, prion diseases and metabolism are enriched in deiminated proteins, both in plasma, as well as in plasma extracellular vesicles. This study provides a platform for the development of novel biomarkers to assess wild life health status and factors relating to zoonotic disease.

Abstract

The reindeer (caribou) Rangifer tarandus is a Cervidae in the order Artiodactyla. Reindeer are sedentary and migratory populations with circumpolar distribution in the Arctic, Northern Europe, Siberia and North America. Reindeer are an important wild and domesticated species, and have developed various adaptive strategies to extreme environments. Importantly, deer have also been identified to be putative zoonotic carriers, including for parasites, prions and coronavirus. Therefore, novel insights into immune-related markers are of considerable interest. Peptidylarginine deiminases (PADs) are a phylogenetically conserved enzyme family which causes post-translational protein deimination by converting arginine into citrulline in target proteins. This affects protein function in health and disease. Extracellular vesicles (EVs) participate in cellular communication, in physiological and pathological processes, via transfer of cargo material, and their release is partly regulated by PADs. This study assessed deiminated protein and EV profile signatures in plasma from sixteen healthy wild female reindeer, collected in Iceland during screening for parasites and chronic wasting disease. Reindeer plasma EV profiles showed a poly-dispersed distribution from 30 to 400 nm and were positive for phylogenetically conserved EV-specific markers. Deiminated proteins were isolated from whole plasma and plasma EVs, identified by proteomic analysis and protein interaction networks assessed by KEGG and GO analysis. This revealed a large number of deimination-enriched pathways for immunity and metabolism, with some differences between whole plasma and EVs. While shared KEGG pathways for whole plasma and plasma EVs included complement and coagulation pathways, KEGG pathways specific for EVs were for protein digestion and absorption, platelet activation, amoebiasis, the AGE–RAGE signaling pathway in diabetic complications, ECM receptor interaction, the relaxin signaling pathway and the estrogen signaling pathway. KEGG pathways specific for whole plasma were pertussis, ferroptosis, SLE, thyroid hormone synthesis, phagosome, Staphylococcus aureus infection, vitamin digestion and absorption, and prion disease. Further differences were also found between molecular function and biological processes GO pathways when comparing functional STRING networks for deiminated proteins in EVs, compared with deiminated proteins in whole plasma. This study highlights deiminated proteins and EVs as candidate biomarkers for reindeer health and may provide information on regulation of immune pathways in physiological and pathological processes, including neurodegenerative (prion) disease and zoonosis.

Peptidyl arginine deiminase 4 and its potential role in Alzheimer's disease

Alzheimer's disease (AD) is the main cause of dementia, and its pathogenesis is still not clear. Peptidyl arginine deiminases 4(PAD4) as one of the important members of PAD family, is the only protein with nuclear transfer function, it can regulate the expression of many proteins through citrullinating histone. PAD4 can also interact with many transcription factors, involved in regulating gene expression. PAD4 expression is closely related to the inflammatory factors secreted, cell autophagy, tumorigenesis and other neurodegenerative diseases. More importantly, PAD4 and its citrullinated protein were found in cortical and hippocampal neurons of AD patients. To study the expression and regulatory pathway of PAD4 in vivo and in vitro experiments on AD may be of helpful to elucidate the pathogenesis of AD. Meanwhile, detection of anti-citrullinated antibody will have potential value as novel biomarkers of AD.

PADs in cancer: Current and future

Protein arginine deiminases (PADs), is a group of calcium-dependent enzymes, which play crucial roles in citrullination, and can catalyze arginine residues into citrulline. This chemical reaction induces citrullinated proteins formation with altered structure and function, leading to numerous pathological diseases, including inflammation and autoimmune diseases. To date, multiple studies have provided solid evidence that PADs are implicated in cancer progression. Nevertheless, the findings on PADs functions in tumors are too complex to understand due to its involvements in variable signaling pathways. The increasing interest in PADs has heightened the need for a comprehensive description for its role in cancer. The present study aims to identify the gaps in present knowledge, including its structures, biological substrates and tissue distribution. Since several irreversible inhibitors for PADs with good potency and selectivity have been explored, the mechanisms on the dysregulation in tumors remain poorly understood. The present study discusses the relationship between PADs and tumor apoptosis, EMT formation and metastasis as well as the implication of neutrophil extracellular traps (NETs) in tumorigenesis. In addition, the potential uses of citrullinated antigens for immunotherapy were proposed.

Deimination, Intermediate Filaments and Associated Proteins

Deimination (or citrullination) is a post-translational modification catalyzed by a calcium-dependent enzyme family of five peptidylarginine deiminases (PADs). Deimination is involved in physiological processes (cell differentiation, embryogenesis, innate and adaptive immunity, etc.) and in autoimmune diseases (rheumatoid arthritis, multiple sclerosis and lupus), cancers and neurodegenerative diseases. Intermediate filaments (IF) and associated proteins (IFAP) are major substrates of PADs. Here, we focus on the effects of deimination on the polymerization and solubility properties of IF proteins and on the proteolysis and cross-linking of IFAP, to finally expose some features of interest and some limitations of citrullinomes.

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.

Protein Deimination Signatures in Plasma and Plasma-EVs and Protein Deimination in the Brain Vasculature in a Rat Model of Pre-Motor Parkinson's Disease

The identification of biomarkers for early diagnosis of Parkinson’s disease (PD) is of pivotal importance for improving approaches for clinical intervention. The use of translatable animal models of pre-motor PD therefore offers optimal opportunities for novel biomarker discovery in vivo. Peptidylarginine deiminases (PADs) are a family of calcium-activated enzymes that contribute to protein misfolding through post-translational deimination of arginine to citrulline. Furthermore, PADs are an active regulator of extracellular vesicle (EV) release. Both protein deimination and extracellular vesicles (EVs) are gaining increased attention in relation to neurodegenerative diseases, including in PD, while roles in pre-motor PD have yet to be investigated. The current study aimed at identifying protein candidates of deimination in plasma and plasma-EVs in a rat model of pre-motor PD, to assess putative contributions of such post-translational changes in the early stages of disease. EV-cargo was further assessed for deiminated proteins as well as three key micro-RNAs known to contribute to inflammation and hypoxia (miR21, miR155, and miR210) and also associated with PD. Overall, there was a significant increase in circulating plasma EVs in the PD model compared with sham animals and inflammatory and hypoxia related microRNAs were significantly increased in plasma-EVs of the pre-motor PD model. A significantly higher number of protein candidates were deiminated in the pre-motor PD model plasma and plasma-EVs, compared with those in the sham animals. KEGG (Kyoto encyclopedia of genes and genomes) pathways identified for deiminated proteins in the pre-motor PD model were linked to “Alzheimer’s disease”, “PD”, “Huntington’s disease”, “prion diseases”, as well as for “oxidative phosphorylation”, “thermogenesis”, “metabolic pathways”, “Staphylococcus aureus infection”, gap junction, “platelet activation”, “apelin signalling”, “retrograde endocannabinoid signalling”, “systemic lupus erythematosus”, and “non-alcoholic fatty liver disease”. Furthermore, PD brains showed significantly increased staining for total deiminated proteins in the brain vasculature in cortex and hippocampus, as well as increased immunodetection of deiminated histone H3 in dentate gyrus and cortex. Our findings identify EVs and post-translational protein deimination as novel biomarkers in early pre-motor stages of PD.

In vivo expression of peptidylarginine deiminase in Drosophila melanogaster

Peptidylarginine deiminase (PAD) modifies peptidylarginine and converts it to peptidylcitrulline in the presence of elevated calcium. Protein modification can lead to severe changes in protein structure and function, and aberrant PAD activity is linked to human pathologies. While PAD homologs have been discovered in vertebrates-as well as in protozoa, fungi, and bacteria-none have been identified in Drosophila melanogaster, a simple and widely used animal model for human diseases. Here, we describe the development of a human PAD overexpression model in Drosophila. We established fly lines harboring human PAD2 or PAD4 transgenes for ectopic expression under control of the GAL4/UAS system. We show that ubiquitous or nervous system expression of PAD2 or PAD4 have minimal impact on fly lifespan, fecundity, and the response to acute heat stress. Although we did not detect citrullinated proteins in fly homogenates, fly-expressed PAD4-but not PAD2-was active in vitro upon Ca2+ supplementation. The transgenic fly lines may be valuable in future efforts to develop animal models of PAD-related disorders and for investigating the biochemistry and regulation of PAD function.

Neurochemical regulation of the expression and function of glial fibrillary acidic protein in astrocytes

Glial fibrillary acidic protein (GFAP), a type III intermediate filament, is a marker of mature astrocytes. The expression of GFAP gene is regulated by many transcription factors (TFs), mainly Janus kinase-2/signal transducer and activator of transcription 3 cascade and nuclear factor κ-light-chain-enhancer of activated B cell signaling. GFAP expression is also modulated by protein kinase and other signaling molecules that are elicited by neuronal activity and hormones. Abnormal expression of GFAP proteins occurs in neuroinflammation, neurodegeneration, brain edema-eliciting diseases, traumatic brain injury, psychiatric disorders and others. GFAP, mainly in α-isoform, is the major component of cytoskeleton and the scaffold of astrocytes, which is essential for the maintenance of astrocytic structure and shape. GFAP also has highly morphological plasticity because of its quick changes in assembling and polymerizing states in response to environmental challenges. This plasticity and its corresponding cellular morphological changes endow astrocytes the functions of physical barrier between adjacent neurons and stabilizer of extracellular environment. Moreover, GFAP colocalizes and even molecularly associates with many functional molecules. This feature allows GFAP to function as a platform for direct interactions between different molecules. Last, GFAP involves transportation and localization of other functional proteins and thus serves as a protein transport guide in astrocytes. This guiding role of GFAP involves an elastic retraction and extension cytoskeletal network that couples with GFAP reassembling, transporting, and membrane protein recycling machinery. This paper reviews our current understanding of the expression and functions of GFAP as well as their regulation.

Peptidylarginine Deiminases-Roles in Cancer and Neurodegeneration and Possible Avenues for Therapeutic Intervention via Modulation of Exosome and Microvesicle (EMV) Release?

Exosomes and microvesicles (EMVs) are lipid bilayer-enclosed structures released from cells and participate in cell-to-cell communication via transport of biological molecules. EMVs play important roles in various pathologies, including cancer and neurodegeneration. The regulation of EMV biogenesis is thus of great importance and novel ways for manipulating their release from cells have recently been highlighted. One of the pathways involved in EMV shedding is driven by peptidylarginine deiminase (PAD) mediated post-translational protein deimination, which is calcium-dependent and affects cytoskeletal rearrangement amongst other things. Increased PAD expression is observed in various cancers and neurodegeneration and may contribute to increased EMV shedding and disease progression. Here, we review the roles of PADs and EMVs in cancer and neurodegeneration.

Myelin Basic Protein Citrullination, a Hallmark of Central Nervous System Demyelination, Assessed by Novel Monoclonal Antibodies in Prion Diseases

Myelin basic protein (MBP) citrullination by peptidylarginine deiminase (PAD) enzymes leads to incomplete protein-lipid bilayer interactions and vulnerability to proteolytic enzymes, resulting in disorganization of the myelin sheath in the central nervous system. Therefore, citrullinated MBP (citMBP) has been suggested as a hallmark of demyelination, but how citMBP is implicated in prion diseases remains unknown. For the first time, we developed mouse monoclonal anti-citMBP IgG1 (clones 1B8, 1H1, and 3C6) and IgM (clone 3G5) antibodies that recognize human citMBP at its R25, R122, and R130 residues and at its C-terminal region (or the corresponding sites in mouse MBP), respectively. Using a biochemical, immunohistochemical, and immunogold-silver staining for electron microscopy techniques, we found that MBP residue R23 (corresponding to human R25) was specifically citrullinated, was stained as intense punctae in the corpus callosum, the striatum, and the cerebellar white matter, and was predominantly localized in disorganized myelin in the brains of scrapie-infected mice. In the brains of Creutzfeldt-Jakob disease (CJD) patients, MBP residues R25, R122, and R130 were markedly citrullinated and were stained as fibrils and punctae. In particular, white matter regions, such as the midbrain and the medulla, exhibited high levels of citMBP compared to other regions. However, the high levels of citMBP were not correlated with PAD2 expression. The clone 3G5 recognized significantly increased expression of the 18.5 kDa and/or 21.5 kDa variants of MBP in prion disease. Our findings suggest that significantly increased levels of citMBP may reflect demyelinating neuropathology, and that these newly developed antibodies may be useful for identifying demyelination.

Upregulation of Connexin 43 Expression Via C-Jun N-Terminal Kinase Signaling in Prion Disease

Prion infection leads to neuronal cell death, glial cell activation, and the accumulation of misfolded prion proteins. However, the altered cellular environments in animals with prion diseases are poorly understood. In the central nervous system, cells connect the cytoplasm of adjacent cells via connexin (Cx)-assembled gap junction channels to allow the direct exchange of small molecules, including ions, neurotransmitters, and signaling molecules, which regulate the activities of the connected cells. Here, we investigate the role of Cx43 in the pathogenesis of prion diseases. Upregulated Cx43 expression, which was dependent on c-Jun N-Terminal Kinase (JNK)/c-Jun signaling cascades, was found in prion-affected brain tissues and hippocampal neuronal cells. Scrapie infection-induced Cx43 formed aggregated plaques within the cytoplasmic compartments at the cell-cell interfaces. The ethidium bromide (EtBr) uptake assay and scrape-loading dye transfer assay demonstrated that increased Cx43 has functional consequences for the activity of Cx43 hemichannels. Interestingly, blockade of PrPSc accumulation reduced Cx43 expression through the inhibition of JNK signaling, indicating that PrPSc accumulation may be directly involved in JNK activation-mediated Cx43 upregulation. Overall, our findings describe a scrapie infection-mediated novel regulatory signaling pathway of Cx43 expression and may suggest a role for Cx43 in the pathogenesis of prion diseases.

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.

Myelin Basic Protein Citrullination in Multiple Sclerosis: A Potential Therapeutic Target for the Pathology

Multiple sclerosis (MS) is a multifactorial demyelinating disease characterized by neurodegenerative events and autoimmune response against myelin component. Citrullination or deimination, a post-translational modification of protein-bound arginine into citrulline, catalyzed by Ca(2+) dependent peptidylarginine deiminase enzyme (PAD), plays an essential role in physiological processes include gene expression regulation, apoptosis and the plasticity of the central nervous system, while aberrant citrullination can generate new epitopes, thus involving in the initiation and/or progression of autoimmune disorder like MS. Myelin basic protein (MBP) is the major myelin protein and is generally considered to maintain the stability of the myelin sheath. This review describes the MBP citrullination and its consequence, as well as offering further support for the "inside-out" hypothesis that MS is primarily a neurodegenerative disease with secondary inflammatory demyelination. In addition, it discusses the role of MBP citrullination in the immune inflammation and explores the potential of inhibition of PAD enzymes as a therapeutic strategy for the disease.

Protein Citrullination: A Proposed Mechanism for Pathology in Traumatic Brain Injury

Protein citrullination is a calcium-driven post-translational modification proposed to play a causative role in the neurodegenerative disorders of Alzheimer’s disease, multiple sclerosis (MS), and prion disease. Citrullination can result in the formation of antigenic epitopes that underlie pathogenic autoimmune responses. This phenomenon, which is best understood in rheumatoid arthritis, may play a role in the chronic dysfunction following traumatic brain injury (TBI). Despite substantial evidence of aberrations in calcium signaling following TBI, there is little understanding of how TBI alters citrullination in the brain. The present investigation addressed this gap by examining the effects of TBI on the distribution of protein citrullination and on the specific cell types involved. Immunofluorescence revealed that controlled cortical impact in rats profoundly up-­regulated protein citrullination in the cerebral cortex, external capsule, and hippocampus. This response was exclusively seen in astrocytes; no such effects were observed on the status of protein citrullination in neurons, oligodendrocytes or microglia. Further, proteomic analyses demonstrated that the effects of TBI on citrullination were confined to a relatively small subset of neural proteins. Proteins most notably affected were those also reported to be citrullinated in other disorders, including prion disease and MS. In vivo findings were extended in an in vitro model of simulated TBI employing normal human astrocytes. Pharmacologically induced calcium excitotoxicity was shown to activate the citrullination and breakdown of glial fibrillary acidic protein, producing a novel candidate TBI biomarker and potential target for autoimmune recognition. In summary, these findings demonstrate that the effects of TBI on protein citrullination are selective with respect to brain region, cell type, and proteins modified, and may contribute to a role for autoimmune dysfunction in chronic pathology following TBI.

2-Chloroacetamidine, a novel immunomodulator, suppresses antigen-induced mouse airway inflammation

BACKGROUND

Citrullination is a presently under-recognized posttranslational protein modification catalyzed by PAD enzymes. Immune responses to citrullinated neo-epitopes are identified in a growing number of inflammatory and autoimmune diseases. However, the involvement of hypercitrullination in the pathogenesis of bronchial asthma is still unknown.

METHODS

As main experimental tool, we examined the effect of 2-chloroacetamidine (2CA), a PAD enzyme inhibitor, on OVA-immunized and airway-challenged BALB/c mice; a commonly used model of allergic airway inflammation. We also measured the effect of 2CA on ex vivo lymphocytes and cell lines.

RESULTS

In vivo, 2CA dramatically suppressed lung tissue hypercitrullination, inflammatory cell recruitment, and airway-Th2 cytokine secretion. 2CA also suppressed systemic OVA-specific and total IgE production dramatically, effectively preventing de novo and diminishing established disease without measurably impacting general immunocompetence. In vitro, 2CA markedly inhibited the proliferation of mouse and human T cells with cell cycle block and apoptosis during a limited, postactivation phase.

CONCLUSIONS

2CA acts as narrow-spectrum immunosuppressant that selectively targets lymphocyte populations involved in active inflammatory tissue lesions. If hypercitrullination is generated in patients with asthma, 2CA may represent a novel disease modulator for human asthmatics/allergic diseases.

Design, synthesis, and biological evaluation of tetrazole analogs of Cl-amidine as protein arginine deiminase inhibitors

Protein arginine deiminases (PADs) catalyze the post-translational hydrolysis of arginine residues to form citrulline. This once obscure modification is now known to play a key role in the etiology of multiple autoimmune diseases (e.g., rheumatoid arthritis, multiple sclerosis, lupus, and ulcerative colitis) and in some forms of cancer. Among the five human PADs (PAD1, -2, -3, -4, and -6), it is unclear which isozyme contributes to disease pathogenesis. Toward the identification of potent, selective, and bioavailable PAD inhibitors that can be used to elucidate the specific roles of each isozyme, we describe tetrazole analogs as suitable backbone amide bond bioisosteres for the parent pan PAD inhibitor Cl-amidine. These tetrazole based analogs are highly potent and show selectivity toward particular isozymes. Importantly, one of the compounds, biphenyl tetrazole tert-butyl Cl-amidine (compound 13), exhibits enhanced cell killing in a PAD4 expressing osteosarcoma bone marrow (U2OS) cell line and can also block the formation of neutrophil extracellular traps. These bioisosteres represent an important step in our efforts to develop stable, bioavailable, and selective inhibitors for the PADs.

Peptidylarginine deiminase inhibition impairs Toll-like receptor agonist-induced functional maturation of dendritic cells, resulting in the loss of T cell-proliferative capacity: a partial mechanism with therapeutic potential in inflammatory settings

Cl-amidine, which is a small-molecule inhibitor of PAD, has therapeutic potential for inflammation-mediated diseases. However, little is known regarding the manner by which PAD inhibition by Cl-amidine regulates inflammatory conditions. Here, we investigated the effects of PAD inhibition by Cl-amidine on the functioning of DCs, which are pivotal immune cells that mediate inflammatory diseases. When DC maturation was induced by TLR agonists, reduced cytokine levels (IL-6, IL-1β, and IL-12p70) were observed in Cl-amidine-treated DCs. Cl-amidine-treated, LPS-activated DCs exhibited alterations in their mature and functional statuses with up-regulated antigen uptake, down-regulated CD80, and MHC molecules. In addition, Cl-amidine-treated DCs dysregulated peptide-MHC class formations. Interestingly, the decreased cytokines were independent of MAPK/NF-κB signaling pathways and transcription levels, indicating that PAD inhibition by Cl-amidine may be involved in post-transcriptional steps of cytokine production. Transmission electron microscopy revealed morphotypical changes with reduced dendrites in the Cl-amidine-treated DCs, along with altered cellular compartments, including fragmented ERs and the formation of foamy vesicles. Furthermore, in vitro and in vivo Cl-amidine treatments impaired the proliferation of naïve CD4(+) and CD8(+) T cells. Overall, our findings suggest that Cl-amidine has therapeutic potential for treating inflammation-mediated diseases.

Proteomics analysis of amyloid and nonamyloid prion disease phenotypes reveals both common and divergent mechanisms of neuropathogenesis

Prion diseases are a heterogeneous group of neurodegenerative disorders affecting various mammals including humans. Prion diseases are characterized by a misfolding of the host-encoded prion protein (PrP^C) into a pathological isoform termed PrP^Sc. In wild-type mice, PrP^C is attached to the plasma membrane by a glycosylphosphatidylinositol (GPI) anchor and PrP^Sc typically accumulates in diffuse nonamyloid deposits with gray matter spongiosis. By contrast, when mice lacking the GPI anchor are infected with the same prion inoculum, PrP^Sc accumulates in dense perivascular amyloid plaques with little or no gray matter spongiosis. In order to evaluate whether different host biochemical pathways were implicated in these two phenotypically distinct prion disease models, we utilized a proteomics approach. In both models, infected mice displayed evidence of a neuroinflammatory response and complement activation. Proteins involved in cell death and calcium homeostasis were also identified in both phenotypes. However, mitochondrial pathways of apoptosis were implicated only in the nonamyloid form, whereas metal binding and synaptic vesicle transport were more disrupted in the amyloid phenotype. Thus, following infection with a single prion strain, PrP^C anchoring to the plasma membrane correlated not only with the type of PrP^Sc deposition but also with unique biochemical pathways associated with pathogenesis.

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.

Role of proteomics in understanding prion infection

Transmissible spongiform encephalopathies or prion diseases are fatal neurodegenerative pathologies characterized by the autocatalytic misfolding and polymerization of a cellular glycoprotein (cellular prion protein [PrP(C)]) that accumulates in the CNS and leads to neurodegeneration. The detailed mechanics of PrP(C) conversion to its pathological isoform (PrP(TSE)) are unclear but one or more exogenous factors are likely involved in the process of PrP misfolding. In the last 20 years, proteomic investigations have identified several endogenous proteins that interact with PrP(C), PrP(TSE) or both, which are possibly involved in the prion pathogenetic process. However, current approaches have not yet produced convincing conclusions on the biological value of such PrP interactors. Future advancements in the comprehension of the molecular pathogenesis of prion diseases, in experimental techniques and in data analysis procedures, together with a boost in more productive international collaborations, are therefore needed to improve the understanding on the role of PrP interactors. Finally, the advancement of 'omics' techniques in prion diseases will contribute to the development of novel diagnostic tests and effective drugs.

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.

Interrogation of the Active Sites of Protein Arginine Deiminases (PAD1, -2, and -4) Using Designer Probes

Protein arginine deiminases (PADs) are involved in a number of cellular pathways, and they catalyze the transformation of peptidyl arginine residue into a citrulline as part of post-translational modifications. To understand ligand preferences, a group of probe molecules were investigated against PAD1, PAD2, and PAD4. These probe molecules carried a well-known covalent modifier of the catalytic cysteine residue, 2-chloroacetamidine moiety, which was tethered to an α-amino acid via a carbon linker. The chain length for the linker varied from 0 to 4. Time-dependent assays indicated that 2-chloroacetamidine (2CA) with no linker inhibited all PAD enzymes with a similar trend in the second-order rate constants, although with poor affinity. Among the other three probe molecules, compound 3 with a three-carbon linker exhibited the best second-order rate constants for optimal ligand reactivity with the binding site. These analyses provide insights into the relative patterns of covalent inactivation of PAD isozymes and the design of novel inhibitors targeting PAD enzymes as potential therapeutic targets.

Inhibition of peptidyl-arginine deiminases reverses protein-hypercitrullination and disease in mouse models of multiple sclerosis

Multiple sclerosis (MS) is the most common CNS-demyelinating disease of humans, showing clinical and pathological heterogeneity and a general resistance to therapy. We first discovered that abnormal myelin hypercitrullination, even in normal-appearing white matter, by peptidylarginine deiminases (PADs) correlates strongly with disease severity and might have an important role in MS progression. Hypercitrullination is known to promote focal demyelination through reduced myelin compaction. Here we report that 2-chloroacetamidine (2CA), a small-molecule, PAD active-site inhibitor, dramatically attenuates disease at any stage in independent neurodegenerative as well as autoimmune MS mouse models. 2CA reduced PAD activity and protein citrullination to pre-disease status. In the autoimmune models, disease induction uniformly induced spontaneous hypercitrullination with citrulline+ epitopes targeted frequently. 2CA rapidly suppressed T cell autoreactivity, clearing brain and spinal cord infiltrates, through selective removal of newly activated T cells. 2CA essentially prevented disease when administered before disease onset or before autoimmune induction, making hypercitrullination, and specifically PAD enzymes, a therapeutic target in MS models and thus possibly in MS.

Peptidylarginine deiminase and protein citrullination in prion diseases: strong evidence of neurodegeneration

The post-translational citrullination (deimination) process is mediated by peptidylarginine deiminases (PADs), which convert peptidylarginine into peptidylcitrulline in the presence of high calcium concentrations. Over the past decade, PADs and protein citrullination have been commonly implicated as abnormal pathological features in neurodegeneration and inflammatory responses associated with diseases such as multiple sclerosis, Alzheimer disease and rheumatoid arthritis. Based on this evidence, we investigated the roles of PADs and citrullination in the pathogenesis of prion diseases. Prion diseases (also known as transmissible spongiform encephalopathies) are fatal neurodegenerative diseases that are pathologically well characterized as the accumulation of disease-associated misfolded prion proteins, spongiform changes, glial cell activation and neuronal loss. We previously demonstrated that the upregulation of PAD2, mainly found in reactive astrocytes of infected brains, leads to excessive citrullination, which is correlated with disease progression. Further, we demonstrated that various cytoskeletal and energy metabolism-associated proteins are particularly vulnerable to citrullination. Our recent in vivo and in vitro studies elicited altered functions of enolase as the result of citrullination; these altered functions included reduced enzyme activity, increased protease sensitivity and enhanced plasminogen-binding affinity. These findings suggest that PAD2 and citrullinated proteins may play a key role in the brain pathology of prion diseases. By extension, we believe that abnormal increases in protein citrullination may be strong evidence of neurodegeneration.

Two novel sandwich ELISAs identify PAD4 levels and PAD4 autoantibodies in patients with rheumatoid arthritis

OBJECTIVE

The peptidylarginine deiminase 4 (PAD4) gene and PAD4 autoantibodies have been associated with rheumatoid arthritis (RA) and its pathogenesis. Therefore, methods for accurately determining their levels in the peripheral blood of these patients would be a diagnostic asset. The objective of our study was to adapt the enzyme-linked immunosorbent assay (ELISA) method for evaluating PAD4 levels in human blood.

METHODS

We prepared recombinant human (h)PAD1, -2, -3, and -4 proteins to develop mouse monoclonal antibodies specific to hPAD4. We then generated six monoclonal antibodies against hPAD4 and developed two new sandwich ELISA methods for evaluating hPAD4 and PAD4 autoantibodies in the peripheral blood from 32 patients with RA, ten patients with osteoarthrosis, and 20 healthy individuals.

RESULTS

The distribution of hPAD4 in the patients' plasma was determined. Two populations were identified: one group with high hPAD4 levels (>0.57 ng/mL) and a second group with near-zero levels (<0.1 ng/mL). Most patients approximating zero hPAD4 levels had PAD4 autoantibodies. In contrast, most of those with higher plasma hPAD4 levels did not have detectable PAD4 autoantibodies.

CONCLUSION

The combination of these sandwich ELISA methods may be a potentially beneficial clinical tool for diagnosing RA.

Potential role for PAD2 in gene regulation in breast cancer cells

The peptidylarginine deiminase (PAD) family of enzymes post-translationally convert positively charged arginine residues in substrate proteins to the neutral, non-standard residue citrulline. PAD family members 1, 2, 3, and 6 have previously been localized to the cell cytoplasm and, thus, their potential to regulate gene activity has not been described. We recently demonstrated that PAD2 is expressed in the canine mammary gland epithelium and that levels of histone citrullination in this tissue correlate with PAD2 expression. Given these observations, we decided to test whether PAD2 might localize to the nuclear compartment of the human mammary epithelium and regulate gene activity in these cells. Here we show, for the first time, that PAD2 is specifically expressed in human mammary gland epithelial cells and that a portion of PAD2 associates with chromatin in MCF-7 breast cancer cells. We investigated a potential nuclear function for PAD2 by microarray, qPCR, and chromatin immunoprecipitation analysis. Results show that the expression of a unique subset of genes is disregulated following depletion of PAD2 from MCF-7 cells. Further, ChIP analysis of two of the most highly up- and down-regulated genes (PTN and MAGEA12, respectively) found that PAD2 binds directly to these gene promoters and that the likely mechanism by which PAD2 regulates expression of these genes is via citrullination of arginine residues 2-8-17 on histone H3 tails. Thus, our findings define a novel role for PAD2 in gene expression in human mammary epithelial cells.

Peptidylarginine deiminase modulates the physiological roles of enolase via citrullination: links between altered multifunction of enolase and neurodegenerative diseases

The citrullination of enolase by PAD (peptidylarginine deiminase) has emerged as an important post-translational modification in human disorders; however, the physiological function of citrullination remains unknown. In the present study, we report that citrullination diversely regulates the biological functions of ENO1 (α-enolase) and NSE (neuron-specific enolase). We developed three mouse IgG1 monoclonal antibodies with specificity to the following: (i) citrullination of Arg9 of ENO1 [ENO1Cit9; anti-CE1 (citrullinated enolase 1) antibody]; (ii) citrullination of Arg9 in ENO1 and NSE (ENO1Cit9/NSECit9; anti-CE1/2 antibody); and (iii) citrullination of Arg429 of NSE (NSECit429; anti-CE2 antibody). Regardless of the total protein expression level, the levels of ENO1Cit9 and NSECit429 were elevated, and their immunoreactivities were also increased in cortical neuronal cells or around blood vessels in the frontal cortex of patients with sporadic Creutzfeldt-Jakob disease and Alzheimer's disease compared with controls. In a time- and dose-dependent manner, PAD negatively regulated enolase activity via citrullination, and enolase in diseased patients was more inactive than in controls. Interestingly, the citrullination of enolase effectively promoted its proteolytic degradation by Ca2+-dependent calpain-1, and leupeptin (calpain inhibitor I) abrogated this degradation. Surprisingly, using an affinity assay, the citrullination of enolase enhanced its plasminogen-binding affinity, which was blocked by the lysine analogue ϵ-aminocaproic acid. These findings suggest that PAD-mediated citrullination regulates the diverse physiological activities of enolase and that CE may be a candidate diagnostic/prognostic factor for degenerative diseases.

Neuronal PAD4 expression and protein citrullination: possible role in production of autoantibodies associated with neurodegenerative disease

Peptidyl arginine deiminases (PADs) catalyze a post-translational protein modification reaction called citrullination, where arginine is converted to citrulline. This modification has been linked to the pathogenesis of autoimmune diseases including rheumatoid arthritis (RA). More recently, several studies have suggested that Alzheimer's disease (AD), a devastating neurodegenerative disorder, may have an autoimmune component. In the present study, we have investigated the possibility that expression of PADs and protein citrullination plays a role in the production of brain-reactive autoantibodies that may contribute to Alzheimer's-related brain pathology. Here, we report the selective expression of the PAD isoforms, PAD2 and PAD4, in astrocytes and neurons, respectively, and the concomitant accumulation of citrullinated proteins within PAD4-expressing cells, including neurons of the hippocampus and cerebral cortex. Expression of PADs and citrullinated proteins is prominent in brain regions engaged in neurodegenerative changes typical for AD pathology. Furthermore, we also demonstrate that the pentatricopeptide repeat domain2 (PTCD2) protein, an antigen target of a prominent AD diagnostic autoantibody, is present in a citrullinated form in AD brains. Our results suggest that disease-associated neuronal loss results in the release of cellular contents, including citrullinated proteins, into the brain interstitium. We propose that these citrullinated proteins and their degradation fragments enter into the blood and lymphatic circulation, and some are capable of eliciting an immune response that results in the production of autoantibodies. The long-term and progressive nature of AD and other neurodegenerative diseases results in chronic exposure of the immune system to these citrullinated products and may drive the continual production of autoantibodies.

Citrullination of CNS proteins in the pathogenesis of multiple sclerosis

Multiple sclerosis is a chronic immune-mediated disease of the CNS. Although it is a predominantly T-cell mediated condition, B cells and autoreactive antibodies play an important role in its pathogenesis, with the presence of oligoclonal immunoglobulins in the cerebrospinal fluid being an important diagnostic indicator. The target of these immunoglobulins has not yet been fully characterized. However, post-translational modifications of CNS-specific proteins are thought to contribute to their production through the generation of novel epitopes. One post-translational modification in particular, the conversion of the amino acid arginine to the nonstandard amino acid, citrulline, has been increasingly described in the literature as a factor in the pathogenesis of this condition. In this article, we summarize and discuss the current knowledge on citrullination in multiple sclerosis, the importance of this in relation to its pathogenesis and, potentially, its diagnosis.

[Deimination or citrullination, a post-translational modification with many physiological and pathophysiological facets]

Deimination or citrullination, is a post-translational modification with many facets. It is involved in several basic cellular processes, including gene regulation, embryonic development and terminal differentiation, and also in various pathophysiological mechanisms linked to severe human diseases such as multiple sclerosis and rheumatoid arthritis. Deimination, the calcium-dependent enzymatic conversion of peptidyl-arginine to peptidyl-citrulline, induces a decrease in the charge of the modified proteins with major consequences on their conformation, stability and/or interactions, and therefore on their functions. Five isotypes of peptidylarginine deiminases (1-4 and 6), exist in humans with a variable tissue expression. These highly conserved enzymes are closely regulated at transcriptional and post-transcriptional levels, probably including auto-deimination.

Subcellular localization of peptidylarginine deiminase 2 and citrullinated proteins in brains of scrapie-infected mice: nuclear localization of PAD2 and membrane fraction-enriched citrullinated proteins

Peptidylarginine deiminase (PAD) and citrullinated proteins have emerged as key molecules in various human diseases, but detailed subcellular localizations of PAD2 and citrullinated proteins are poorly mapped in brain under normal and pathologic conditions. We performed subcellular fractionation and electron microscopic analysis using brains of normal and scrapie-infected mice. Peptidylarginine deiminase 2 was abundantly present in cytosol and weakly in microsomal and mitochondrial fractions and expression in these fractions was higher in brains of scrapie-infected mice. Despite relatively low PAD2 expression, in microsomal and mitochondrial fractions, citrullinated proteins were present at high levels in these fractions in scrapie-infected brains. Surprisingly, increased PAD2 expression and accumulated citrullinated proteins were also found in nuclear fractions in scrapie-infected brains. By electron microscopy, PAD2 and citrullinated proteins in scrapie-infected brains were widely distributed in most cellular compartments including mitochondria, endoplasmic reticulum, glial filaments, nuclei, and Golgi apparatus in astrocytes and hippocampal neurons. Taken together, we report for the first time the nuclear localization of PAD2 and the detailed subcellular localization of PAD2 and of citrullinated proteins in scrapie-infected brains. Our findings suggest that different subcellular compartmentalization of PAD2 and citrullinated proteins may have different physiological roles in normal and neurodegenerative conditions.