Murine Retinal Citrullination Declines With Age and is Mainly Dependent on Peptidyl Arginine Deiminase 4 (PAD4)

Investigating the cell membrane localization of PADI4 in breast cancer cells and inhibition of anti-PADI4 monoclonal antibody

BACKGROUND

Peptidyl arginine deiminase 4 (PADI4) is a post-translational modification enzymecan that converts arginine in protein into citrulline in the presence of calcium ions, which is called citrullination. PADI4 has been reported to be expressed in the cytoplasm and nucleus in a variety of malignant tumors. Based on the GeneCards database and our previous research, it is speculated that PADI4 may also be expressed on the cell membrane. This study aimed to confirm the membrane expression of PADI4 and the effect of anti-PADI4 antibodies on cell membrane PADI4. This may be another mechanism of action of anti-PADI4 monoclonal antibodies in the treatment of breast cancer.

METHODS

The subcellular localizations of PADI4 in MDA-MB-231 and MCF-7 breast cancer cells were determined by immunofluorescence, immunoelectron microscopy, and Western blot analysis. The tumor cells were treated with PADI4 antibody, and cell proliferation, migration, colony formation, apoptosis, glycolysis, and epithelial-mesenchymal transition (EMT) were measured as well as the expression of some essential tumor genes.

RESULTS

PADI4 was not only localized in the nucleus and cytoplasm of breast cancer cells but was also detected on the cell membrane. Following PADI4 antibody treatment, cell proliferation, migration, colony formation, EMT, and ATP production through glycolysis were decreased, and the mRNA expression of MYC proto-oncogene (MYC), FAT atypical cadherin 1 (FAT1), nuclear factor kappa B subunit 1 (NFκB), and tumor necrosis factor (TNF-α) in breast cancer cells was downregulated, while the mRNA expression of tumor protein p63 (TP63) was upregulated.

CONCLUSIONS

PADI4 is expressed on the cell membrane in breast cancer cells. Anti-PADI4 antibodies can affect the biological functions of cell membrane PADI4, including proliferation, migration, apoptosis, and glycolysis, thereby inhibiting tumor progression.

Role of Peptidylarginine Deiminase 4 in Central Nervous System Diseases

The deimination or citrullination of arginine residues in the polypeptide chain by peptidylarginine deiminase 4 alters the charge state of the polypeptide chain and affects the function of proteins. It is one of the main ways of protein post-translational modifications to regulate its function. Peptidylarginine deiminase 4 is widely expressed in multiple tissues and organs of the body, especially the central nervous system, and regulates the normal development of organisms. The abnormal expression and activation of peptidylarginine deiminase 4 is an important pathological mechanism for the occurrence and development of central nervous system diseases such as multiple sclerosis, Alzheimer's disease, cerebral ischemia reperfusion injury, and glioblastoma.

A corneo-retinal hypercitrullination axis underlies ocular injury to nitrogen mustard

The vesicant sulfur mustard (SM) is a chemical warfare agent that causes acute and chronic injury to the cornea and proximal anterior segment structures. Despite clinical evidence of SM-exposure causing unexplained retinal deficits, there have been no animal studies conducted to examine the retinal toxicity of this vesciant. The cardinal hallmark of retinal response to stressors or injury is the activation of reactive gliosis, a cellular process largely governed by Müller glia. Previously we showed that corneal exposure to sodium hydroxide elicits rapid induction of reactive gliosis and results in retinal degeneration in a dose-related manner. Based on this evidence, we hypothesized that the vesicant nitrogen mustard (NM), an analog of SM, may also elicit reactive gliosis. To test this idea, we developed a mouse model of NM ocular injury and investigated corneal and retinal effects focusing on citrullination, a posttranslational modification (PTM) of proteins. This PTM was recently linked to alkali injury and has also been shown to occur in retinal degenerative conditions. Here, we demonstrate that corneal exposure to 1% NM causes a synchronous activation of citrullination in both the cornea and retina with hypercitrullination becoming apparent temporally and manifesting with altered cellular expression characteristics. A key finding is that ocular citrullination occurs acutely as early as 1-h post-injury in both the cornea and retina, which underscores a need for expeditious interception of this acute corneal and retinal response. Moreover, exploiting dose response and temporal studies, we uncoupled NM-induced retinal citrullination from its induction of retinal gliosis. Our findings demonstrate that hypercitrullination is a common corneo-retinal mechanism that sensitizes the eye to NM injury and suggests that counteracting hypercitrullination may provide a suitable countermeasure to vesicant injury.

Role of the PADI family in inflammatory autoimmune diseases and cancers: A systematic review

The peptidyl arginine deiminase (PADI) family is a calcium ion-dependent group of isozymes with sequence similarity that catalyze the citrullination of proteins. Histones can serve as the target substrate of PADI family isozymes, and therefore, the PADI family is involved in NETosis and the secretion of inflammatory cytokines. Thus, the PADI family is associated with the development of inflammatory autoimmune diseases and cancer, reproductive development, and other related diseases. In this review, we systematically discuss the role of the PADI family in the pathogenesis of various diseases based on studies from the past decade to provide a reference for future research.

The double-edged sword of inflammation in inherited retinal degenerations: Clinical and preclinical evidence for mechanistically and prognostically impactful but treatable complications

We present retrospective data from our clinical research efforts of the past several years alongside a review of past and current clinical and preclinical data independently by several investigators supporting our clinical evidence for the importance of inflammation in inherited retinal degenerations (IRDs). We show how inflammation is a complicating factor in IRDs but, if recognized and managed, also a great opportunity to mitigate disease severity immediately, improve patient prognosis and quality of life, extend the treatment windows for gene-specific and agnostic therapeutic approaches, mitigate the impact of inflammatory complications on the accurate estimate of vision changes in IRD natural history studies, improve the chances of safer outcomes following cataract surgery, and potentially reduce the likelihood of inflammatory adverse events and augment the efficacy of viral vector-based treatment approaches to IRDs. Manuscript contribution to the field. Inflammation has been suspected to be at play in IRDs since the beginning of the 1900s and became a research focus through the early 1990s but was then largely abandoned in favor of genetic-focused research. Thanks to regained cognizance, better research tools, and a more holistic approach to IRDs, the recent reappraisal of the role of inflammation in IRDs has brought back to the surface its importance. A potential confounder in natural history studies and a limiting factor in clinical trials if not accounted for, inflammation can be managed and often offers an opportunity for immediately improved prognosis and outcomes for IRD patients. We present our retrospective clinical evidence for connections with a measurable secondary autoimmune component that can develop in IRDs and contribute to vision loss but is at least in part treatable. We also present ample lines of evidence from the literature corroborating our clinical observations at the preclinical level.

Post-translational protein deimination signatures and extracellular vesicles (EVs) in the Atlantic horseshoe crab (Limulus polyphemus)

The horseshoe crab is a living fossil and a species of marine arthropod with unusual immune system properties which are also exploited commercially. Given its ancient status dating to the Ordovician period (450 million years ago), its standing in phylogeny and unusual immunological characteristics, the horseshoe crab may hold valuable information for comparative immunology studies. Peptidylarginine deiminases (PADs) are calcium dependent enzymes that are phylogenetically conserved and cause protein deimination via conversion of arginine to citrulline. This post-translational modification can lead to structural and functional protein changes contributing to protein moonlighting in health and disease. PAD-mediated regulation of extracellular vesicle (EV) release, a critical component of cellular communication, has furthermore been identified to be a phylogenetically conserved mechanism. PADs, protein deimination and EVs have hitherto not been studied in the horseshoe crab and were assessed in the current study. Horseshoe crab haemolymph serum-EVs were found to be a poly-dispersed population in the 20-400 nm size range, with the majority of EVs falling within 40-123 nm. Key immune proteins were identified to be post-translationally deiminated in horseshoe crab haemolymph serum, providing insights into protein moonlighting function of Limulus and phylogenetically conserved immune proteins. KEGG (Kyoto encyclopaedia of genes and genomes) and GO (gene ontology) enrichment analysis of deiminated proteins identified in Limulus revealed KEGG pathways relating to complement and coagulation pathways, Staphylococcus aureus infection, glycolysis/gluconeogenesis and carbon metabolism, while GO pathways of biological and molecular pathways related to a range of immune and metabolic functions, as well as developmental processes. The characterisation of EVs, and post-translational deimination signatures, revealed here in horseshoe crab, contributes to current understanding of protein moonlighting functions and EV-mediated communication in this ancient arthropod and throughout phylogeny.

Novel biomarkers for the evaluation of aging-induced proteinopathies

Proteinopathies are characterized by aging related accumulation of misfolded protein aggregates. Irreversible covalent modifications of aging proteins may significantly affect the native three dimentional conformation of proteins, alter their function and lead to accumulation of misfolded protein as dysfunctional aggregates. Protein misfolding and accumulation of aberrant proteins are known to be associated with aging-induced proteinopathies such as amyloid ß and tau proteins in Alzheimer's disease, α-synuclein in Parkinson's disease and islet amyloid polypeptides in Type 2 diabetes mellitus. Protein oxidation processes such as S-nitrosylation, dityrosine formation and some of the newly elucidated processes such as carbamylation and citrullination recently drew the attention of researchers in the field of Gerontology. Studying over these processes and illuminating their relations between proteinopathies may help to diagnose early and even to treat age related disorders. Therefore, we have chosen to concentrate on aging-induced proteinopathic nature of these novel protein modifications in this review.

Innate and Autoimmunity in the Pathogenesis of Inherited Retinal Dystrophy

Inherited retinal dystrophies (RDs) are heterogenous in many aspects including genes involved, age of onset, rate of progression, and treatments. While RDs are caused by a plethora of different mutations, all result in the same outcome of blindness. While treatments, both gene therapy-based and drug-based, have been developed to slow or halt disease progression and prevent further blindness, only a small handful of the forms of RDs have treatments available, which are primarily for recessively inherited forms. Using immunohistochemical methods coupled with electroretinography, optical coherence tomography, and fluorescein angiography, we show that in rhodopsin mutant mice, the involvement of both the innate and the autoimmune systems could be a strong contributing factor in disease progression and pathogenesis. Herein, we show that monocytic phagocytosis and inflammatory cytokine release along with protein citrullination, a major player in forms of autoimmunity, work to enhance the progression of RD associated with a rhodopsin mutation.

Neutrophil extracellular traps (NETs) exacerbate severity of infant sepsis

Background

Neutrophil extracellular traps (NETs) are innate defense mechanisms that are also implicated in the pathogenesis of organ dysfunction. However, the role of NETs in pediatric sepsis is unknown.

Methods

Infant (2 weeks old) and adult (6 weeks old) mice were submitted to sepsis by intraperitoneal (i.p.) injection of bacteria suspension or lipopolysaccharide (LPS). Neutrophil infiltration, bacteremia, organ injury, and concentrations of cytokine, NETs, and DNase in the plasma were measured. Production of reactive oxygen and nitrogen species and release of NETs by neutrophils were also evaluated. To investigate the functional role of NETs, mice undergoing sepsis were treated with antibiotic plus rhDNase and the survival, organ injury, and levels of inflammatory markers and NETs were determined. Blood samples from pediatric and adult sepsis patients were collected and the concentrations of NETs measured.

Results

Infant C57BL/6 mice subjected to sepsis or LPS-induced endotoxemia produced significantly higher levels of NETs than the adult mice. Moreover, compared to that of the adult mice, this outcome was accompanied by increased organ injury and production of inflammatory cytokines. The increased NETs were associated with elevated expression of Padi4 and histone H3 citrullination in the neutrophils. Furthermore, treatment of infant septic mice with rhDNase or a PAD-4 inhibitor markedly attenuated sepsis. Importantly, pediatric septic patients had high levels of NETs, and the severity of pediatric sepsis was positively correlated with the level of NETs.

Conclusion

This study reveals a hitherto unrecognized mechanism of pediatric sepsis susceptibility and suggests that NETs represents a potential target to improve clinical outcomes of sepsis.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2407-8) contains supplementary material, which is available to authorized users.

Increased Protein Citrullination as a Trigger for Resident Immune System Activation, Intraretinal Inflammation, and Promotion of Anti-retinal Autoimmunity: Intersecting Paths in Retinal Degenerations of Potential Therapeutic Relevance

We present evidence that protein citrullination, a proinflammatory and immune system-activating posttranslational modification (PTM) of arginine residues mediated by peptidyl arginine deiminases (PADs), is elevated in mouse models of retinal degenerations. Together with the fact that the animal models that we investigated (and their human counterparts) exhibit also anti-retinal autoantibodies, we propose that retinal citrullination is an immunogenic trigger that activates the immune system both locally and systemically, contributing to disease pathogenesis. Consistent with this possibility, we show that PAD compromise reduces the severity of Mertk-related retinal degeneration. Thus, PAD inhibition may be as a potential treatment strategy for retinal degenerations.