Role for peptidylarginine deiminase enzymes in disease and female reproduction

A programmable CRISPR/dCas9-based epigenetic editing system enabling loci-targeted histone citrullination and precise transcription regulation

Histone citrullination, an important post-translational modification mediated by peptidyl arginine deiminases, is essential for many physiological processes and epigenetic regulation. However, the causal relationship between histone citrullination and specific gene regulation remains unresolved. In this study, we develop a programmable epigenetic editor by fusing the peptidyl arginine deiminase PPAD from Porphyromonas gingivalis with dCas9. With the assistance of gRNA, PPAD-dCas9 can recruit peptidyl arginine deiminases to specific genomic loci, enabling direct manipulation of the epigenetic landscape and regulation of gene expression. Our citrullination editor allows for site-specific manipulation of histone H3R2,8,17 and 26 at target human gene loci, resulting in the activation or suppression of different genes in a locus-specific manner. Moreover, the epigenetic effects of the citrullination editor are specific and sustained. This epigenetic editor offers an accurate and efficient tool for exploring gene regulation of histone citrullination.

The influence of estrogen on myocardial post-translational modifications and cardiac function in women

The lifetime risk of heart failure (HF) is comparable in men and women; nevertheless, disparities exist in our understanding of how HF differs between sexes. Several differences in cardiac physiology exist between men and women including the propensity to develop specific HF phenotypes. Men are more likely to be diagnosed with HF failure with reduced ejection fraction, while women have a greater propensity to develop HF with preserved ejection fraction. The mechanisms responsible for these differences remain unclear. Post-translational modifications (PTMs) of myofilament proteins likely contribute to these sex-specific propensities. The role of PTMs in heart disease is an expanding field with immense potential therapeutic targets. However, numerous PTMs remain underexplored, particularly in the context of the female heart. Estrogen, a key gonadal hormone, cardioprotective in pre-menopausal women and its loss with menopause likely contributes to disease in aging women. However, how estrogen regulates PTMs to contribute to HF development is not fully clear. This review outlines key sex differences in HF along with characterizing the contributions of novel myocardial PTMs in cardiac physiology and their regulation by estrogen. Collectively, we highlight the necessity for further investigation into women's heart health and the distinctive mechanisms distinguishing women from men.

PADI6: What we know about the elusive fifth member of the peptidyl arginine deiminase family

Peptidyl arginine deiminase 6 (PADI6) is a maternal factor that is vital for early embryonic development. Deletion and mutations of its encoding gene in female mice or women lead to early embryonic developmental arrest, female infertility, maternal imprinting defects and hyperproliferation of the trophoblast. PADI6 is the fifth and least well-characterized member of the peptidyl arginine deiminases (PADIs), which catalyse the post-translational conversion of arginine to citrulline. It is less conserved than the other PADIs, and currently has no reported catalytic activity. While there are many suggested functions of PADI6 in the early mouse embryo, including in embryonic genome activation, cytoplasmic lattice formation, maternal mRNA and ribosome regulation, and organelle distribution, the molecular mechanisms of its function remain unknown. In this review, we discuss what is known about the function of PADI6 and highlight key outstanding questions that must be answered if we are to understand the crucial role it plays in early embryo development and female fertility. This article is part of the Theo Murphy meeting issue 'The virtues and vices of protein citrullination'.

Genetic variants underlying developmental arrests in human preimplantation embryos

Developmental arrest in preimplantation embryos is one of the major causes of assisted reproduction failure. It is briefly defined as a delay or a failure of embryonic development in producing viable embryos during ART cycles. Permanent or partial developmental arrest can be observed in the human embryos from one-cell to blastocyst stages. These arrests mainly arise from different molecular biological defects, including epigenetic disturbances, ART processes, and genetic variants. Embryonic arrests were found to be associated with a number of variants in the genes playing key roles in embryonic genome activation, mitotic divisions, subcortical maternal complex formation, maternal mRNA clearance, repairing DNA damage, transcriptional, and translational controls. In this review, the biological impacts of these variants are comprehensively evaluated in the light of existing studies. The creation of diagnostic gene panels and potential ways of preventing developmental arrests to obtain competent embryos are also discussed.

Altered ureido protein modification profiles in seminal plasma extracellular vesicles of non-normozoospermic men

INTRODUCTION

Extracellular vesicles (EVs) have been recognized as key players in numerous physiological functions. These vesicles alter their compositions attuned to the health and disease states of the organism. In men, significant changes in the proteomic composition(s) of seminal plasma EVs (sEVs) have already been found to be related to infertility.

METHODS

Methods: In this study, we analyze the posttranslational configuration of sEV proteomes from normozoospermic (NZ) men and non-normozoospermic (non-NZ) men diagnosed with teratozoospermia and/or asthenozoospermia by unbiased, discovery-driven proteomics and advanced bioinformatics, specifically focusing on citrulline (Cit) and homocitrulline (hCit) posttranscriptional residues, both considered product of ureido protein modifications.

RESULTS AND DISCUSSION

Significant increase in the proteome-wide cumulative presence of hCit together with downregulation of Cit in specific proteins related to decisive molecular functions have been encountered in sEVs of non-NZ subjects. These findings identify novel culprits with a higher chance of affecting fundamental aspects of sperm functional quality and define potential specific diagnostic and prognostic non-invasive markers for male infertility.

PADs and NETs in digestive system: From physiology to pathology

Peptidylarginine deiminases (PADs) are the only enzyme class known to deiminate arginine residues into citrulline in proteins, a process known as citrullination. This is an important post-translational modification that functions in several physiological and pathological processes. Neutrophil extracellular traps (NETs) are generated by NETosis, a novel cell death in neutrophils and a double-edged sword in inflammation. Excessive activation of PADs and NETs is critically implicated in their transformation from a physiological to a pathological state. Herein, we review the physiological and pathological functions of PADs and NETs, in particular, the involvement of PAD2 and PAD4 in the digestive system, from inflammatory to oncological diseases, along with related therapeutic prospects.

Peptidylarginine deiminase enzymes and citrullinated proteins in female reproductive physiology and associated diseases†

Citrullination, the post-translational modification of arginine residues, is catalyzed by the four catalytically active peptidylarginine deiminase (PAD or PADI) isozymes and alters charge to affect target protein structure and function. PADs were initially characterized in rodent uteri and, since then, have been described in other female tissues including ovaries, breast, and the lactotrope and gonadotrope cells of the anterior pituitary gland. In these tissues and cells, estrogen robustly stimulates PAD expression resulting in changes in levels over the course of the female reproductive cycle. The best-characterized targets for PADs are arginine residues in histone tails, which, when citrullinated, alter chromatin structure and gene expression. Methodological advances have allowed for the identification of tissue-specific citrullinomes, which reveal that PADs citrullinate a wide range of enzymes and structural proteins to alter cell function. In contrast to their important physiological roles, PADs and citrullinated proteins are also involved in several female-specific diseases including autoimmune disorders and reproductive cancers. Herein, we review current knowledge regarding PAD expression and function and highlight the role of protein citrullination in both normal female reproductive tissues and associated diseases.

Padi2/3 Deficiency Alters the Epigenomic Landscape and Causes Premature Differentiation of Mouse Trophoblast Stem Cells

Histone citrullination is a relatively poorly studied epigenetic modification that involves the irreversible conversion of arginine residues into citrulline. It is conferred by a small family of enzymes known as protein arginine deiminases (PADIs). PADI function supports the pluripotent state of embryonic stem cells, but in other contexts, also promotes efficient cellular differentiation. In the current study, we sought to gain deeper insights into the possible roles of PADIs in mouse trophoblast stem cells (TSCs). We show that Padi2 and Padi3 are the most highly expressed PADI family members in TSCs and are rapidly down-regulated upon differentiation. Padi2/3 double knockout (DKO) TSCs express lower levels of stem cell transcription factors CDX2 and SOX2 and are prone to differentiate into extremely large trophoblast giant cells, an effect that may be mediated by centrosome duplication defects. Interestingly, Padi2/3 DKO TSCs display alterations to their epigenomic landscape, with fewer H3K9me3-marked chromocentric foci and globally reduced 5-methylcytosine levels. DNA methylation profiling identifies that this effect is specifically evident at CpG islands of critical trophoblast genes, such as Gata3, Peg3, Socs3 and Hand1. As a consequence of the hypomethylated state, these factors are up-regulated in Padi2/3 DKO TSCs, driving their premature differentiation. Our data uncover a critical epigenetic role for PADI2/3 in safeguarding the stem cell state of TSCs by modulating the DNA methylation landscape to restrict precocious trophoblast differentiation.

A Pilot Study on Peptidylarginine Deiminases and Protein Deimination in Animal Cancers across Vertebrate Species

PADs are a group of calcium-dependent enzymes that play key roles in inflammatory pathologies and have diverse roles in cancers. PADs cause irreversible post-translational modification of arginine to citrulline, leading to changes in protein function in different cellular compartments. PAD isozyme diversity differs throughout phylogeny in chordates, with five PAD isozymes in mammals, three in birds, and one in fish. While the roles for PADs in various human cancers are mounting (both in regards to cancer progression and epigenetic regulation), investigations into animal cancers are scarce. The current pilot-study therefore aimed at assessing PAD isozymes in a range of animal cancers across the phylogeny tree. In addition, the tissue samples were assessed for total protein deimination and histone H3 deimination (CitH3), which is strongly associated with human cancers and also indicative of gene regulatory changes and neutrophil extracellular trap formation (NETosis). Cancers were selected from a range of vertebrate species: horse, cow, reindeer, sheep, pig, dog, cat, rabbit, mink, hamster, parrot, and duck. The cancers chosen included lymphoma, kidney, lung, testicular, neuroendocrine, anaplastic, papilloma, and granulosa cell tumour. Immunohistochemical analysis revealed that CitH3 was strongly detected in all of the cancers assessed, while pan-deimination detection was overall low. Both PAD2 and PAD3 were the most predominantly expressed PADs across all of the cancers assessed, while PAD1, PAD4, and PAD6 were overall expressed at lower, albeit varying, levels. The findings from this pilot study provide novel insights into PAD-mediated roles in different cancers across a range of vertebrate species and may aid in the understanding of cancer heterogeneity and cancer evolution.

Molecular Mechanism of Protein Arginine Deiminase 2: A Study Involving Multiple Microsecond Long Molecular Dynamics Simulations

Peptidylarginine deiminase 2 (PAD2) is a Ca²⁺-dependent enzyme that catalyzes the conversion of protein arginine residues to citrulline. This kind of structural modification in histone molecules may affect gene regulation, leading to effects that may trigger several diseases, including breast cancer, which makes PAD2 an attractive target for anticancer drug development. To design new effective inhibitors to control activation of PAD2, improving our understanding of the molecular mechanisms of PAD2 using up-to-date computational techniques is essential. We have designed five different PAD2-substrate complex systems based on varying protonation states of the active site residues. To search the conformational space broadly, multiple independent molecular dynamics simulations of the complexes have been performed. In total, 50 replica simulations have been performed, each of 1 μs, yielding a total simulation time of 50 μs. Our findings identify that the protonation states of Cys647, Asp473, and His471 are critical for the binding and localization of the N-α-benzoyl-l-arginine ethyl ester substrate within the active site. A novel mechanism for enzyme activation is proposed according to near attack conformers. This represents an important step in understanding the mechanism of citrullination and developing PAD2-inhibiting drugs for the treatment of breast cancer.

The virtues and vices of protein citrullination

The post-translational modification of proteins expands the regulatory scope of the proteome far beyond what is achievable through genome regulation. The field of protein citrullination has seen significant progress in the last two decades. The small family of peptidylarginine deiminase (PADI or PAD) enzymes, which catalyse citrullination, have been implicated in virtually all facets of molecular and cell biology, from gene transcription and epigenetics to cell signalling and metabolism. We have learned about their association with a remarkable array of disease states and we are beginning to understand how they mediate normal physiological functions. However, while the biochemistry of PADI activation has been worked out in exquisite detail in vitro, we still lack a clear mechanistic understanding of the processes that regulate PADIs within cells, under physiological and pathophysiological conditions. This review summarizes and discusses the current knowledge, highlights some of the unanswered questions of immediate importance and gives a perspective on the outlook of the citrullination field.

Acute Hypoxia Alters Extracellular Vesicle Signatures and the Brain Citrullinome of Naked Mole-Rats (Heterocephalus glaber)

Peptidylarginine deiminases (PADs) and extracellular vesicles (EVs) may be indicative biomarkers of physiological and pathological status and adaptive responses, including to diseases and disorders of the central nervous system (CNS) and related to hypoxia. While these markers have been studied in hypoxia-intolerant mammals, in vivo investigations in hypoxia-tolerant species are lacking. Naked mole-rats (NMR) are among the most hypoxia-tolerant mammals and are thus a good model organism for understanding natural and beneficial adaptations to hypoxia. Thus, we aimed to reveal CNS related roles for PADs in hypoxia tolerance and identify whether circulating EV signatures may reveal a fingerprint for adaptive whole-body hypoxia responses in this species. We found that following in vivo acute hypoxia, NMR: (1) plasma-EVs were remodelled, (2) whole proteome EV cargo contained more protein hits (including citrullinated proteins) and a higher number of associated KEGG pathways relating to the total proteome of plasma-EVs Also, (3) brains had a trend for elevation in PAD1, PAD3 and PAD6 protein expression, while PAD2 and PAD4 were reduced, while (4) the brain citrullinome had a considerable increase in deiminated protein hits with hypoxia (1222 vs. 852 hits in normoxia). Our findings indicate that circulating EV signatures are modified and proteomic content is reduced in hypoxic conditions in naked mole-rats, including the circulating EV citrullinome, while the brain citrullinome is elevated and modulated in response to hypoxia. This was further reflected in elevation of some PADs in the brain tissue following acute hypoxia treatment. These findings indicate a possible selective role for PAD-isozymes in hypoxia response and tolerance.

Novel Homozygous PADI6 Variants in Infertile Females with Early Embryonic Arrest

Early embryonic arrest denotes premature termination of development in preimplantation embryos, which is one of the major phenotypes of recurrent assisted reproduction failure. Padi6 is proven to be a member of the subcortical maternal complex (SCMC) in mice, which is essential in oocyte maturation and embryogenesis. We and other groups previously found that biallelic mutations in PADI6 caused female infertility manifesting as early embryonic arrest. In this study, we identified two novel homozygous variants (p.Cys163Arg, and p. Trp475*) of PADI6 in two infertile patients from a cohort of 75 females with the phenotype of early embryonic arrest. An in vitro expression study indicated severe decrease of PADI6, which might destruct the stability of SCMC. Our study expands the mutational spectrum of PADI6 and further supports the causality between PADI6 mutations and female infertility.

Citrullination in the pathology of inflammatory and autoimmune disorders: recent advances and future perspectives

Numerous post-translational modifications (PTMs) govern the collective metabolism of a cell through altering the structure and functions of proteins. The action of the most prevalent PTMs, encompassing phosphorylation, methylation, acylations, ubiquitination and glycosylation is well documented. A less explored protein PTM, conversion of peptidylarginine to citrulline, is the subject of this review. The process of citrullination is catalysed by peptidylarginine deiminases (PADs), a family of conserved enzymes expressed in a variety of human tissues. Accumulating evidence suggest that citrullination plays a significant role in regulating cellular metabolism and gene expression by affecting a multitude of pathways and modulating the chromatin status. Here, we will discuss the biochemical nature of arginine citrullination, the enzymatic machinery behind it and also provide information on the pathological consequences of citrullination in the development of inflammatory diseases (rheumatoid arthritis, multiple sclerosis, psoriasis, systemic lupus erythematosus, periodontitis and COVID-19), cancer and thromboembolism. Finally, developments on inhibitors against protein citrullination and recent clinical trials providing a promising therapeutic approach to inflammatory disease by targeting citrullination are discussed.

Novel Approaches in the Immunotherapy of Multiple Sclerosis: Cyclization of Myelin Epitope Peptides and Conjugation with Mannan

Multiple Sclerosis (MS) is a serious autoimmune disease. The patient in an advanced state of the disease has restrained mobility and remains handicapped. It is therefore understandable that there is a great need for novel drugs and vaccines for the treatment of MS. Herein we summarise two major approaches applied for the treatment of the disease using peptide molecules alone or conjugated with mannan. The first approach focuses on selective myelin epitope peptide or peptide mimetic therapy alone or conjugated with mannan, and the second on immune-therapy by preventing or controlling disease through the release of appropriate cytokines. In both approaches the use of cyclic peptides offers the advantage of increased stability from proteolytic enzymes. In these approaches, the synthesis of myelin epitope peptides conjugated to mannan is of particular interest as this was found to protect mice against experimental autoimmune encephalomyelitis, an animal model of MS, in prophylactic and therapeutic protocols. Protection was peptide-specific and associated with reduced antigen-specific T cell proliferation. The aim of the studies of these peptide epitope analogs is to understand their molecular basis of interactions with human autoimmune T-cell receptor and a MS-associated human leucocyte antigen (HLA)-DR2b. This knowledge will lead the rational design to new beneficial non-peptide mimetic analogs for the treatment of MS. Some issues of the use of nanotechnology will also be addressed as a future trend to tackle the disease. We highlight novel immunomodulation and vaccine-based research against MS based on myelin epitope peptides and strategies developed in our laboratories.

Peptidylarginine Deiminase and Alzheimer's Disease

Peptidylarginine deiminases (PADs) are indispensable enzymes for post-translational modification of proteins, which can convert Arg residues on the surface of proteins to citrulline residues. The PAD family has five isozymes, PAD1, 2, 3, 4, and 6, which have been found in multiple tissues and organs. PAD2 and PAD4 were detected in cerebral cortex and hippocampus from human and rodent brain. In the central nervous system, abnormal expression and activation of PADs are involved in the pathological changes and pathogenesis of Alzheimer's disease (AD). This article reviews the classification, distribution, and function of PADs, with an emphasis on the relationship between the abnormal activation of PADs and AD pathogenesis, diagnosis, and the therapeutic potential of PADs as drug targets for AD.

The Role of Citrullination in Inflammatory Bowel Disease: A Neglected Player in Triggering Inflammation and Fibrosis?

Citrullination is a posttranslational modification of proteins mediated by a specific family of enzymes called peptidylarginine deiminases (PAD). Dysregulation of these enzymes is involved in the etiology of various diseases, from cancer to autoimmune disorders. In inflammatory bowel disease (IBD), data for a role of citrullination in the disease process are starting to accumulate at different experimental levels including gene expression analyses, RNA, and protein quantifications. Most data have been generated in ulcerative colitis, but data in Crohn disease are lacking so far. In addition, the citrullination of histones is the fundamental process promoting inflammation through the formation of neutrophil extracellular traps (NETs). Interestingly, NETs have also been shown to activate fibroblasts into myofibroblasts in fibrotic interstitial lung disease. Therefore, citrullination merits more thorough study in the bowel to determine its role in driving disease complications such as fibrosis. In this review we describe the process of citrullination and the different players in this pathway, the role of citrullination in autoimmunity with a special focus on IBD, the emerging role for citrullination and NETs in triggering fibrosis, and, finally, how this process could be therapeutically targeted.

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.

The roles of PAD2- and PAD4-mediated protein citrullination catalysis in cancers

Peptidylarginine deiminases (PADs) catalyze the conversion of arginine residues to citrulline residues on target proteins in the presence of calcium ions. This elaborate type of posttranslational modification is termed citrullination. PADs may regulate gene transcriptional activity via histone citrullination. There has been an increasing appreciation for the roles of PADs in a wide variety of biological processes. In this review article, we summarize recent evidence indicating that PADs and citrullinated proteins are involved in several physiological and pathological processes related to cancer. Of particular interest is that PAD2 and PAD4 exhibit characteristic expression levels, activities and specific biological effects in diverse types of cancer. We also list several PAD inhibitors, propose the possible mechanisms underlying the biological actions of PAD-mediated protein citrullination in experimental models and discuss the potential therapeutic value of PADs and their inhibitors for disease diagnosis and treatment.

Identification and Characterization of the Lactating Mouse Mammary Gland Citrullinome

Citrullination is a post-translational modification (PTM) in which positively charged peptidyl-arginine is converted into neutral peptidyl-citrulline by peptidylarginine deiminase (PAD or PADI) enzymes. The full protein citrullinome in many tissues is unknown. Herein, we used mass spectrometry and identified 107 citrullinated proteins in the lactation day 9 (L9) mouse mammary gland including histone H2A, α-tubulin, and β-casein. Given the importance of prolactin to lactation, we next tested if it stimulates PAD-catalyzed citrullination using mouse mammary epithelial CID-9 cells. Stimulation of CID-9 cells with 5 µg/mL prolactin for 10 min induced a 2-fold increase in histone H2A citrullination and a 4.5-fold increase in α-tubulin citrullination. We next investigated if prolactin-induced citrullination regulates the expression of lactation genes β-casein (Csn2) and butyrophilin (Btn1a1). Prolactin treatment for 12 h increased β-casein and butyrophilin mRNA expression; however, this increase was significantly inhibited by the pan-PAD inhibitor, BB-Cl-amidine (BB-ClA). We also examined the effect of tubulin citrullination on the overall polymerization rate of microtubules. Our results show that citrullinated tubulin had a higher maximum overall polymerization rate. Our work suggests that protein citrullination is an important PTM that regulates gene expression and microtubule dynamics in mammary epithelial cells.

Citrullinated Histone H3 as a Therapeutic Target for Endotoxic Shock in Mice

Sepsis results in millions of deaths every year, with acute lung injury (ALI) being one of the leading causes of mortality in septic patients. As neutrophil extracellular traps (NETs) are abundant in sepsis, neutralizing components of NETs may be a useful strategy to improve outcomes of sepsis. Citrullinated histone H3 (CitH3) has been recently shown to be involved in the NET formation. In this study, we demonstrate that CitH3 damages human umbilical vein endothelial cells (HUVECs) and potentiates NET formation through a positive feedback mechanism. We developed a novel CitH3 monoclonal antibody to target peptidylarginine deiminase (PAD) 2 and PAD 4 generated CitH3. In a mouse model of lethal lipopolysaccharide (LPS) induced shock, neutralizing CitH3 with the newly developed anti-CitH3 monoclonal antibody attenuates inflammatory responses, ameliorates ALI, and improves survival. Our study suggests that effectively blocking circulating CitH3 might be a potential therapeutic method for the treatment of endotoxemia.

An interplay of structure and intrinsic disorder in the functionality of peptidylarginine deiminases, a family of key autoimmunity-related enzymes

Citrullination is a post-translation modification of proteins, where the proteinaceous arginine residues are converted to non-coded citrulline residues. The immune tolerance to such citrullinated protein can be lost, leading to inflammatory and autoimmune diseases. Citrullination is a chemical reaction mediated by peptidylarginine deiminase enzymes (PADs), which are a family of calcium-dependent cysteine hydrolase enzymes that includes five isotypes: PAD1, PAD2, PAD3, PAD4, and PAD6. Each PAD has specific substrates and tissue distribution, where it modifies the arginine to produce a citrullinated protein with altered structure and function. All mammalian PADs have a sequence similarity of about 70-95%, whereas in humans, they are 50-55% homologous in their structure and amino acid sequences. Being calcium-dependent hydrolases, PADs are inactive under the physiological level of calcium, but could be activated due to distortions in calcium homeostasis, or when the cellular calcium levels are increased. In this article, we analyze some of the currently available data on the structural properties of human PADs, the mechanisms of their calcium-induced activation, and show that these proteins contain functionally important regions of intrinsic disorder. Citrullination represents an important trigger of multiple physiological and pathological processes, and as a result, PADs are recognized to play a number of important roles in autoimmune diseases, cancer, and neurodegeneration. Therefore, we also review the current state of the art in the development of PAD inhibitors with good potency and selectivity.

Histone Citrullination Represses MicroRNA Expression, Resulting in Increased Oncogene mRNAs in Somatolactotrope Cells

Peptidylarginine deiminase (PAD) enzymes convert histone arginine residues into citrulline to modulate chromatin organization and gene expression. Although PADs are expressed in anterior pituitary gland cells, their functional role and expression in pituitary adenomas are unknown. To begin to address these issues, we first examined normal human pituitaries and pituitary adenomas and found that PAD2, PAD4, and citrullinated histones are highest in prolactinomas and somatoprolactinomas. In the somatoprolactinoma-derived GH3 cell line, PADs citrullinate histone H3, which is attenuated by a pan-PAD inhibitor. RNA sequencing and chromatin immunoprecipitation (ChIP) studies show that the expression of microRNAs (miRNAs) let-7c-2, 23b, and 29c is suppressed by histone citrullination. Our studies demonstrate that these miRNAs directly target the mRNA of the oncogenes encoding HMGA, insulin-like growth factor 1 (IGF-1), and N-MYC, which are highly implicated in human prolactinoma/somatoprolactinoma pathogenesis. Our results are the first to define a direct role for PAD-catalyzed histone citrullination in miRNA expression, which may underlie the etiology of prolactinoma and somatoprolactinoma tumors through regulation of oncogene expression.

BB-Cl-Amidine as a novel therapeutic for canine and feline mammary cancer via activation of the endoplasmic reticulum stress pathway

Background

Mammary cancer is highly prevalent in dogs and cats and results in a poor prognosis due to critically lacking viable treatment options. Recent human and mouse studies have suggested that inhibiting peptidyl arginine deiminase enzymes (PAD) may be a novel breast cancer therapy. Based on the similarities between human breast cancer and mammary cancer in dogs and cats, we hypothesized that PAD inhibitors would also be an effective treatment for mammary cancer in these animals.

Methods

Canine and feline mammary cancer cell lines were treated with BB-Cl-Amidine (BB-CLA) and evaluated for viability and tumorigenicity. Endoplasmic reticulum stress was tested by western blot, immunofluorescence, and quantitative reverse transcriptase polymerase chain reaction (qRT-PCR). Canine and feline mammary cancer xenograft models were created using NOD scid gamma (NSG) mice, and were treated with BB-CLA for two weeks.

Results

We found that BB-CLA reduced viability and tumorigenicity of canine and feline mammary cancer cell lines in vitro. Additionally, we demonstrated that BB-CLA activates the endoplasmic reticulum stress pathway in these cells by downregulating 78 kDa Glucose-regulated Protein (GRP78), a potential target in breast cancer for molecular therapy, and upregulating the downstream target gene DNA Damage Inducible Transcript 3 (DDIT3). Finally, we established a mouse xenograft model of both canine and feline mammary cancer in which we preliminarily tested the effects of BB-CLA in vivo.

Conclusion

We propose that our established mouse xenograft models will be useful for the study of mammary cancer in dogs and cats, and furthermore, that BB-CLA has potential as a novel therapeutic for mammary cancer in these species.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4323-8) contains supplementary material, which is available to authorized users.

Multifaceted Targeting of the Chromatin Mediates Gonadotropin-Releasing Hormone Effects on Gene Expression in the Gonadotrope

Gonadotropin-releasing hormone (GnRH) stimulates the expression of multiple genes in the pituitary gonadotropes, most notably to induce synthesis of the gonadotropins, luteinizing hormone (LH), and follicle-stimulating hormone (FSH), but also to ensure the appropriate functioning of these cells at the center of the mammalian reproductive endocrine axis. Aside from the activation of gene-specific transcription factors, GnRH stimulates through its membrane-bound receptor, alterations in the chromatin that facilitate transcription of its target genes. These include changes in the histone and DNA modifications, nucleosome positioning, and chromatin packaging at the regulatory regions of each gene. The requirements for each of these events vary according to the DNA sequence which determines the basal chromatin packaging at the regulatory regions. Despite considerable progress in this field in recent years, we are only beginning to understand some of the complexities involved in the role and regulation of this chromatin structure, including new modifications, extensive cross talk, histone variants, and the actions of distal enhancers and non-coding RNAs. This short review aims to integrate the latest findings on GnRH-induced alterations in the chromatin of its target genes, which indicate multiple and diverse actions. Understanding these processes is illuminating not only in the context of the activation of these hormones during the reproductive life span but may also reveal how aberrant epigenetic regulation of these genes leads to sub-fertility.

PAD2 overexpression in transgenic mice augments malignancy and tumor-associated inflammation in chemically initiated skin tumors

We previously found that transgenic mice overexpressing MMTV-FLAG-hPAD2 (PAD2OE) developed spontaneous skin lesions, with a subset of these lesions progressing to invasive squamous cell carcinoma (SCC). The goal of this report was to better understand the potential mechanisms by which PAD2 overexpression promotes skin cancer. Here, PAD2OE mice were treated with the carcinogen, 9,10-dimethyl-1,2-benzanthracene and with O-tetradecanoylphorbol-13-acetate and then scored for papilloma formation. Additionally, tumor sections were evaluated for evidence of tumor cell invasion and inflammation. We found that the total number of papillomas was significantly increased in PAD2OE mice compared to controls. Histopathologic analysis of the lesions found that in PAD2OE skin tumors progressed to invasive SCC more frequently than controls. Additionally, we found that PAD2OE lesions were highly inflamed, with a dense inflammatory cell infiltrate and an associated increase in nuclear phospho-STAT3 (signal transducer and activator of transcription 3) in the transgenic tumors. These data suggest that overexpression of the hPAD2 transgene in the epidermis increases the malignant conversion rate of benign tumors by promoting an inflammatory microenvironment.

Role of peptidylarginine deiminase 2 (PAD2) in mammary carcinoma cell migration

BACKGROUND

Penetration of the mammary gland basement membrane by cancer cells is a crucial first step in tumor invasion. Using a mouse model of ductal carcinoma in situ, we previously found that inhibition of peptidylarginine deiminase 2 (PAD2, aka PADI2) activity appears to maintain basement membrane integrity in xenograft tumors. The goal of this investigation was to gain insight into the mechanisms by which PAD2 mediates this process.

METHODS

For our study, we modulated PAD2 activity in mammary ductal carcinoma cells by lentiviral shRNA-mediated depletion, lentiviral-mediated PAD2 overexpression, or PAD inhibition and explored the effects of these treatments on changes in cell migration and cell morphology. We also used these PAD2-modulated cells to test whether PAD2 may be required for EGF-induced cell migration. To determine how PAD2 might promote tumor cell migration in vivo, we tested the effects of PAD2 inhibition on the expression of several cell migration mediators in MCF10DCIS.com xenograft tumors. In addition, we tested the effect of PAD2 inhibition on EGF-induced ductal invasion and elongation in primary mouse mammary organoids. Lastly, using a transgenic mouse model, we investigated the effects of PAD2 overexpression on mammary gland development.

RESULTS

Our results indicate that PAD2 depletion or inhibition suppresses cell migration and alters the morphology of MCF10DCIS.com cells. In addition, we found that PAD2 depletion suppresses the expression of the cytoskeletal regulatory proteins RhoA, Rac1, and Cdc42 and also promotes a mesenchymal to epithelial-like transition in tumor cells with an associated increase in the cell adhesion marker, E-cadherin. Our mammary gland organoid study found that inhibition of PAD2 activity suppresses EGF-induced ductal invasion. In vivo, we found that PAD2 overexpression causes hyperbranching in the developing mammary gland.

CONCLUSION

Together, these results suggest that PAD2 plays a critical role in breast cancer cell migration. Our findings that EGF treatment increases protein citrullination and that PAD2 inhibition blocks EGF-induced cell migration suggest that PAD2 likely functions within the EGF signaling pathway to mediate cell migration.

Colony, hanging drop, and methylcellulose three dimensional hypoxic growth optimization of renal cell carcinoma cell lines

Renal cell carcinoma (RCC) is the most lethal of the common urologic malignancies, comprising 3% of all human neoplasms, and the incidence of kidney cancer is rising annually. We need new approaches to target tumor cells that are resistant to current therapies and that give rise to recurrence and treatment failure. In this study, we focused on low oxygen tension and three-dimensional (3D) cell culture incorporation to develop a new RCC growth model. We used the hanging drop and colony formation methods, which are common in 3D culture, as well as a unique methylcellulose (MC) method. For the experiments, we used human primary RCC cell lines, metastatic RCC cell lines, human kidney cancer stem cells, and human healthy epithelial cells. In the hanging drop assay, we verified the potential of various cell lines to create solid aggregates in hypoxic and normoxic conditions. With the semi-soft agar method, we also determined the ability of various cell lines to create colonies under different oxygen conditions. Different cell behavior observed in the MC method versus the hanging drop and colony formation assays suggests that these three assays may be useful to test various cell properties. However, MC seems to be a particularly valuable alternative for 3D cell culture, as its higher efficiency of aggregate formation and serum independency are of interest in different areas of cancer biology.

Human Deiminases: Isoforms, Substrate Specificities, Kinetics, and Detection

Peptidylarginine deiminase (PAD) enzymes are of enormous interest in biomedicine. They catalyze the conversion of a positively-charged guanidinium at an arginine side chain into a neutral ureido group. As a result of this conversion, proteins acquire the non-ribosomally encoded amino acid "citrulline". This imposes critical influences on the structure and function of the target molecules. In multiple sclerosis, myelin hyper-citrullination promotes demyelination by reducing its compaction and triggers auto-antibody production. Immune responses to citrulline-containing proteins play a central role in the pathogenesis of autoimmune diseases. Moreover, auto-antibodies, specific to citrullinated proteins, such as collagen type I and II and filaggrin, are early detectable in rheumatoid arthritis, serving as diagnostic markers of the disease. Despite their significance, little is understood about the role in demyelinating disorders, diversified cancers, and auto-immune diseases. To impart their biological and pathological effects, it is crucial to better understand the reaction mechanism, kinetic properties, substrate selection, and specificities of peptidylarginine deiminase isoforms.Many aspects of PAD biochemistry and physiology have been ignored in past, but, herein is presented a comprehensive survey to improve our current understandings of the underlying mechanism and regulation of PAD enzymes.

Cyclic citrullinated MBP87-99 peptide stimulates T cell responses: Implications in triggering disease

Amino acid mutations to agonist peptide epitopes of myelin proteins have been used to modulate immune responses and experimental autoimmune encephalomyelitis (EAE, animal model of multiple sclerosis). Such amino acid alteration are termed, altered peptide ligands (APL). We have shown that the agonist myelin basic protein (MBP) 87-99 epitope (MBP_87-99) with crucial T cell receptor (TCR) substitutions at positions 91 and 96 (K⁹¹,P⁹⁶ (TCR contact residues) to R⁹¹,A⁹⁶; [R⁹¹,A⁹⁶]MBP_87-99) results in altered T cell responses and inhibits EAE symptoms. In this study, the role of citrullination of arginines in [R⁹¹,A⁹⁶]MBP_87-99 peptide analog was determined using in vivo experiments in combination with computational studies. The immunogenicity of linear [Cit⁹¹,A⁹⁶,Cit⁹⁷]MBP_87-99 and its cyclic analog - cyclo(87-99)[Cit⁹¹,A⁹⁶,Cit⁹⁷]MBP_87-99 when conjugated to the carrier mannan (polysaccharide) were studied in SJL/J mice. It was found that mannosylated cyclo(87-99)[Cit⁹¹,A⁹⁶,Cit⁹⁷]MBP_87-99 peptide induced strong T cell proliferative responses and IFN-gamma cytokine secretion compared with the linear one. Moreover, the interaction of linear and cyclic peptide analogs with the major histocompatibility complex (MHC II, H2-IA^s) and TCR was analyzed using molecular dynamics simulations at the receptor level, in order to gain a better understanding of the molecular recognition mechanisms that underly the different immunological profiles of citrullinated peptides compared to its agonist native counterpart MBP_87-99 epitope. The results demonstrate that the citrullination of arginine in combination with the backbone conformation of mutated linear and cyclic analogs are significant elements for the immune response triggering the induction of pro-inflammatory cytokines.

Citrullination regulates the expression of insulin-like growth factor-binding protein 1 (IGFBP1) in ovine uterine luminal epithelial cells

There are five peptidylarginine deiminase (PAD) isozymes designated as PADs 1, 2, 3, 4 and 6, and many are expressed in female reproductive tissues. These enzymes post-translationally convert positively charged arginine amino acids into neutral citrulline residues. Targets for PAD-catalyzed citrullination include arginine residues on histone tails, which results in chromatin decondensation and changes in gene expression. Some of the first studies examining PADs found that they are localized to rodent uterine epithelial cells. Despite these findings, the function of PAD-catalyzed citrullination in uterine epithelial cells is still unknown. To address this, we first examined PAD expression in uterine cross-sections from pregnant ewes on gestation day 25 (d25). Immunohistochemistry revealed that the levels of PADs 2 and 4 are robust in luminal and glandular epithelia compared with those of PADs 1 and 3. As PADs 2 and 4 have well-characterized roles in histone citrullination, we next hypothesized that PADs citrullinate histones in these uterine cells. Examination of caruncle lysates from pregnant ewes on gestation d25 and an ovine luminal epithelial (OLE) cell line shows that histone H3 arginine residues 2, 8, 17 and 26 are citrullinated, but histone H4 arginine 3 is not. Using a pan-PAD inhibitor, we next attenuated histone citrullination in OLE cells, which resulted in a significant decrease in the expression of insulin-like growth factor-binding protein 1 (IGFBP1) mRNA. As IGFBP1 is important for the migration and attachment of the trophectoderm to uterine endometrium, our results suggest that PAD-catalyzed citrullination may be an important post-translational mechanism for the establishment of pregnancy in ewes.

GnRH Stimulates Peptidylarginine Deiminase Catalyzed Histone Citrullination in Gonadotrope Cells

Peptidylarginine deiminase (PAD) enzymes convert histone tail arginine residues to citrulline resulting in chromatin decondensation. Our previous work found that PAD isoforms are expressed in female reproductive tissues in an estrous cycle-dependent fashion, but their role in the anterior pituitary gland is unknown. Thus, we investigated PAD expression and function in gonadotrope cells. The gonadotrope-derived LβT2 cell line strongly expresses PAD2 at the protein level compared with other PAD isoforms. Consistent with this, PAD2 protein expression is highest during the estrous phase of the estrous cycle and colocalizes with the LH β-subunit in the mouse pituitary. Using the GnRH agonist buserelin (GnRHa), studies in LβT2 and mouse primary gonadotrope cells revealed that 30 minutes of stimulation caused distinct puncta of PAD2 to localize in the nucleus. Once in the nucleus, GnRHa stimulated PAD2 citrullinates histone H3 tail arginine residues at sites 2, 8, and 17 within 30 minutes; however, this effect and PAD2 nuclear localization was blunted by incubation of the cells with the pan-PAD inhibitor, biphenyl-benzimidazole-Cl-amidine. Given that PAD2 citrullinates histones in gonadotropes, we next analyzed the functional consequence of PAD2 inhibition on gene expression. Our results show that GnRHa stimulates an increase in LHβ and FSHβ mRNA and that this response is significantly reduced in the presence of the PAD inhibitor biphenyl-benzimidazole-Cl-amidine. Overall, our data suggest that GnRHa stimulates PAD2-catalyzed histone citrullination in gonadotropes to epigenetically regulate gonadotropin gene expression.

Mutations in PADI6 Cause Female Infertility Characterized by Early Embryonic Arrest

Early embryonic arrest is one of the major causes of female infertility. However, because of difficulties in phenotypic evaluation, genetic determinants of human early embryonic arrest are largely unknown. With the development of assisted reproductive technology, the phenotype of early human embryonic arrest can now be carefully evaluated. Here, we describe a consanguineous family with a recessive inheritance pattern of female infertility characterized by recurrent early embryonic arrest in cycles of in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI). We have identified a homozygous PADI6 nonsense mutation (c.1141C>T [p.Gln381(∗)]) that is responsible for the phenotype. Mutational analysis of PADI6 in a cohort of 36 individuals whose embryos displayed developmental arrest identified two affected individuals with compound-heterozygous mutations (c.2009_2010del [p.Glu670Glyfs(∗)48] and c.633T>A [p.His211Gln]; c.1618G>A [p.Gly540Arg] and c.970C>T [p.Gln324(∗)]). Immunostaining indicated a lack of PADI6 in affected individuals' oocytes. In addition, the amount of phosphorylated RNA polymerase II and expression levels of seven genes involved in zygotic genome activation were reduced in the affected individuals' embryos. This phenotype is consistent with Padi6 knockout mice. These findings deepen our understanding of the genetic basis of human early embryonic arrest, which has been a largely ignored Mendelian phenotype. Our findings lay the foundation for uncovering other genetic causes of infertility resulting from early embryonic arrest.

Rheumatoid arthritis onset in postmenopausal women: Does the ACPA seropositive subset result from genetic effects, estrogen deficiency, skewed profile of CD4(+) T-cells, and their interactions?

Rheumatoid arthritis (RA) incidence displays a differentiated age-dependent female-to-male ratio in which women outnumber men. Evidence that the peak incidence of RA in women coincides with menopause age, suggests a potential estrogenic role to disease etiology. Estrogens exert physiologically both stimulatory and inhibitory effects on the immune system. Epidemiologic and animal model studies with estrogen deprivation or supplementation suggested estrogens as to play, mainly, a protective role in RA immunopathology. In this review, we propose that some yet unidentified disturbances associated with estrogen circulating levels, differentiated by the menopausal status, play a major role in women's RA susceptibility. We focus on the interaction between estrogen deprivation and genetic risk alleles for anti-citrullinated protein antibodies (ACPA) seropositive RA, as a major driving force for increased immune reactivity and RA susceptibility, in postmenopausal women. This opens up new fields for research concerning the association among different irregular estrogenic conditions, the cytokine milieu, and age/menopausal status bias in RA.

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.

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.

Inflammation. Neutrophil extracellular traps license macrophages for cytokine production in atherosclerosis

Secretion of the cytokine interleukin-1β (IL-1β) by macrophages, a major driver of pathogenesis in atherosclerosis, requires two steps: Priming signals promote transcription of immature IL-1β, and then endogenous "danger" signals activate innate immune signaling complexes called inflammasomes to process IL-1β for secretion. Although cholesterol crystals are known to act as danger signals in atherosclerosis, what primes IL-1β transcription remains elusive. Using a murine model of atherosclerosis, we found that cholesterol crystals acted both as priming and danger signals for IL-1β production. Cholesterol crystals triggered neutrophils to release neutrophil extracellular traps (NETs). NETs primed macrophages for cytokine release, activating T helper 17 (TH17) cells that amplify immune cell recruitment in atherosclerotic plaques. Therefore, danger signals may drive sterile inflammation, such as that seen in atherosclerosis, through their interactions with neutrophils.

Utilization of the Soft Agar Colony Formation Assay to Identify Inhibitors of Tumorigenicity in Breast Cancer Cells

Given the inherent difficulties in investigating the mechanisms of tumor progression in vivo, cell-based assays such as the soft agar colony formation assay (hereafter called soft agar assay), which measures the ability of cells to proliferate in semi-solid matrices, remain a hallmark of cancer research. A key advantage of this technique over conventional 2D monolayer or 3D spheroid cell culture assays is the close mimicry of the 3D cellular environment to that seen in vivo. Importantly, the soft agar assay also provides an ideal tool to rigorously test the effects of novel compounds or treatment conditions on cell proliferation and migration. Additionally, this assay enables the quantitative assessment of cell transformation potential within the context of genetic perturbations. We recently identified peptidylarginine deiminase 2 (PADI2) as a potential breast cancer biomarker and therapeutic target. Here we highlight the utility of the soft agar assay for preclinical anti-cancer studies by testing the effects of the PADI inhibitor, BB-Cl-amidine (BB-CLA), on the tumorigenicity of human ductal carcinoma in situ (MCF10DCIS) cells.

Biological relevance of citrullinations: diagnostic, prognostic and therapeutic options

OBJECTIVE

Citrullination has become a hot topic within recent years due to its involvement in diseases such as rheumatoid arthritis (RA), multiple sclerosis and fibrosis. Citrullinations are the conversion of arginine to citrulline by peptidylarginine deiminase (PAD) enzymes, which affect protein properties. The aim of this review is to summarize the advances in citrullination research and further explore the potential of citrullination as a diagnostic tool as well as inhibition of PAD enzymes as a target for treatment.

METHOD

We reviewed current literature with emphasis on the role of citrullination in health and disease, the nature of enzymes responsible for citrullination, and the potential of applying citrullinations in diagnostics and pharmaceuticals.

CONCLUSION

Current literature suggests that increased levels of citrullinated proteins are found in several if not all inflammatory diseases. In RA measurement of anti-citrullinated protein antibodies (ACPA) against citrullinated protein fragments are widely used as a prognostic biomarker. More recently, it has been indicated that levels of selected citrullinated proteins carries additional potential as biomarkers. This includes citrullinated vimentin which provide prognostic information in diseases as fibrosis and ankylosing spondylitis. In addition, recent studies suggest that inhibition of PAD is a target for treatment of diseases such as RA and cancer where proteins that are citrullinated are believed to influence the disease activity.

Peptidyl arginine deiminase-4 activation exacerbates kidney ischemia-reperfusion injury

Peptidyl arginine deiminase (PAD)4 is a nuclear enzyme that catalyzes the posttranslational conversion of arginine residues to citrulline. Posttranslational protein citrullination has been implicated in several inflammatory autoimmune diseases, including rheumatoid arthritis, colitis, and multiple sclerosis. Here, we tested the hypothesis that PAD4 contributes to ischemic acute kidney injury (AKI) by exacerbating the inflammatory response after renal ischemia-reperfusion (I/R). Renal I/R injury in mice increased PAD4 activity as well as PAD4 expression in the mouse kidney. After 30 min of renal I/R, vehicle-treated mice developed severe AKI with large increases in plasma creatinine. In contrast, mice pretreated with PAD4 inhibitors (2-chloroamidine or streptonigrin) had significantly reduced renal I/R injury. Further supporting a critical role for PAD4 in generating ischemic AKI, mice pretreated with recombinant human PAD4 (rPAD4) protein and subjected to mild (20 min) renal I/R developed exacerbated ischemic AKI. Consistent with the hypothesis that PAD4 regulates renal tubular inflammation after I/R, mice treated with a PAD4 inhibitor had significantly reduced renal neutrophil chemotactic cytokine (macrophage inflammatory protein-2 and keratinocyte-derived cytokine) expression and had decreased neutrophil infiltration. Furthermore, mice treated with rPAD4 had significantly increased renal tubular macrophage inflammatory protein-2 and keratinocyte-derived cytokine expression as well as increased neutrophil infiltration and necrosis. Finally, cultured mouse kidney proximal tubules treated with rPAD4 had significantly increased proinflammatory chemokine expression compared with vehicle-treated cells. Taken together, our results suggest that PAD4 plays a critical role in renal I/R injury by increasing renal tubular inflammatory responses and neutrophil infiltration after renal I/R.

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.

Three isozymes of peptidylarginine deiminase in the chicken: molecular cloning, characterization, and tissue distribution

Peptidylarginine deiminase (PAD; EC 3.5.3.15) is a post-translational modification enzyme that catalyzes the conversion of protein-bound arginine to citrulline (deimination) in a calcium ion dependent manner. Although PADI genes are widely conserved among vertebrates, their function in the chicken is poorly understood. Here, we cloned and sequenced three chicken PADI cDNAs and analyzed the expression of their proteins in various tissues. Immunoblotting analysis showed that chicken PAD1 and PAD3 were present in cells of several central neuron system tissues including the retina; the chicken PAD2 protein was not detected in any tissue. We expressed recombinant chicken PADs in insect cells and characterized their enzymatic properties. The chicken PAD1 and PAD3 recombinant proteins required calcium ions as an essential cofactor for their catalytic activity. The two recombinant proteins showed similar substrate specificities toward synthetic arginine derivatives. By contrast to them, chicken PAD2 did not show any activity. We found that one of the conserved active centers in mammalian PADs had been altered in chicken PAD2; we prepared a reverse mutant but we did not detect an activity. We conclude that chicken PAD1 and PAD3 might play specific roles in the nervous system, but that chicken PAD2 might not be functional under normal physiological conditions.

Peptidylarginine deiminases in citrullination, gene regulation, health and pathogenesis

Peptidylarginine deiminases are a family of enzymes that mediate post-translational modifications of protein arginine residues by deimination or demethylimination to produce citrulline. In vitro, the activity of PADs is dependent on calcium and reductive reagents carrying a free sulfhydryl group. The discovery that PAD4 can target both arginine and methyl-arginine for citrullination about 10years ago renewed our interest in studying this family of enzymes in gene regulation and their physiological functions. The deregulation of PADs is involved in the etiology of multiple human diseases, including cancers and autoimmune disorders. There is a growing effort to develop isoform specific PAD inhibitors for disease treatment. However, the regulation of the activity of PADs in vivo remains largely elusive, and we expect that much will be learned about the role of these enzymes in a normal life cycle and under pathology conditions.

Posttranslational Protein Modification in the Salivary Glands of Sjögren's Syndrome Patients

The present study investigated posttranslational reactions in the salivary glands of patients with Sjögren's syndrome. We analysed the biopsies of primary Sjögren's patients using immunohistochemistry and a tag-purified anticyclic citrullinated protein (CCP) antibody to detect citrullinated peptides, and the presence of peptidylarginine deiminase 2 (PAD2) was assessed simultaneously. The present work demonstrated the weak presence of the PAD2 enzyme in some normal salivary glands, although PAD2 expression was increased considerably in Sjögren's patients. The presence of citrullinated proteins was also detected in the salivary tissues of Sjögren's patients, which strongly supports the in situ posttranslational modification of proteins in this setting. Furthermore, the mutual expression of CCP and PAD2 suggests that this posttranslational modification is enzyme dependent. In conclusion, patients with Sjögren's syndrome expressed the catalytic machinery to produce posttranslational reactions that may result in autoantigen triggering.

Potential role of peptidylarginine deiminase enzymes and protein citrullination in cancer pathogenesis

The peptidylarginine deiminases (PADs) are a family of posttranslational modification enzymes that catalyze the conversion of positively charged protein-bound arginine and methylarginine residues to the uncharged, nonstandard amino acid citrulline. This enzymatic activity is referred to as citrullination or, alternatively, deimination. Citrullination can significantly affect biochemical pathways by altering the structure and function of target proteins. Five mammalian PAD family members (PADs 1–4 and 6) have been described and show tissue-specific distribution. Recent reviews on PADs have focused on their role in autoimmune diseases. Here, we will discuss the potential role of PADs in tumor progression and tumor-associated inflammation. In the context of cancer, increasing clinical evidence suggests that PAD4 (and possibly PAD2) has important roles in tumor progression. The link between PADs and cancer is strengthened by recent findings showing that treatment of cell lines and mice with PAD inhibitors significantly suppresses tumor growth and, interestingly, inflammatory symptoms. At the molecular level, transcription factors, coregulators, and histones are functional targets for citrullination by PADs, and citrullination of these targets can affect gene expression in multiple tumor cell lines. Next generation isozyme-specific PAD inhibitors may have therapeutic potential to regulate both the inflammatory tumor microenvironment and tumor cell growth.