Enhanced T cell responsiveness to citrulline-containing myelin basic protein in multiple sclerosis patients

Citrullinated isomer of myelin basic protein can induce inflammatory responses in astrocytes

Purpose

During the course of demyelinating inflammatory diseases, myelin-derived proteins, including myelin basic protein(MBP), are secreted into extracellular space. MBP shows extensive post-translational modifications, including deimination/citrullination. Deiminated MBP is structurally less ordered, susceptible to proteolytic attack, and more immunogenic than unmodified MBP. This study investigated the effect of the deiminated/citrullinated isomer of MBP(C8) and the unmodified isomer of MBP(C1) on cultured primary astrocytes.

Methods

MBP charge isomers were isolated/purified from bovine brain. Primary astrocyte cultures were prepared from the 2-day-old Wistar rats. For evaluation of glutamate release/uptake a Fluorimetric glutamate assay was used. Expression of peroxisome proliferator-activated receptor-gamma(PPAR-γ), excitatory amino acid transporter 2(EAAT2), the inhibitor of the nuclear factor kappa-B(ikB) and high mobility group-B1(HMGB1) protein were assayed by Western blot analysis. IL-17A expression was determined in cell medium by ELISA.

Results

We found that MBP(C8) and MBP(C1) acted differently on the uptake/release of glutamate in astrocytes: C1 increased glutamate uptake and did not change its release, whereas C8 decreased glutamate release but did not change its uptake. Both isomers increased the expression of PPAR-γ and EAAT2 to the same degree. Western blots of cell lysates revealed decreased expression of ikB and increased expression of HMGB1 proteins after treatment of astrocytes by C8. Moreover, C8-treated cells released more nitric oxide and proinflammatory IL-17A than C1-treated cells.

Conclusions

These data suggest that the most immunogenic deiminated isomer C8, in parallel to the decreases in glutamate release, elicits an inflammatory response and enhances the secretion of proinflammatory molecules via activation of nuclear factor kappa B(NF-kB).

Summary statement

The most modified-citrullinated myelin basic protein charge isomer decreases glutamate release, elicits an inflammatory response and enhances the secretion of proinflammatory molecules via activation of nuclear factor kappa B in astrocytes.

Citrullination of myelin basic protein induces a Th17-cell response in healthy individuals and enhances the presentation of MBP85-99 in patients with multiple sclerosis

The post-translational modification citrullination has been proposed to play a role in the pathogenesis of multiple sclerosis (MS). Myelin basic protein (MBP) is a candidate autoantigen which is citrullinated to a minor extent under physiological conditions and hypercitrullinated in MS. We examined immune cell responses elicited by hypercitrullinated MBP (citMBP) in cultures of mononuclear cells from 18 patients with MS and 42 healthy donors (HDs). The immunodominant peptide of MBP, MBP85-99, containing citrulline in position 99, outcompeted the binding of native MBP85-99 to HLA-DR15, which is strongly linked to MS. Moreover, using the monoclonal antibody MK16 as probe, we observed that B cells and monocytes from HLA-DR15+ patients with MS presented MBP85-99 more efficiently after challenge with citMBP than with native MBP. Both citMBP and native MBP induced proliferation of CD4+ T cells from patients with MS as well as TNF-α production by their B cells and CD4+ T cells, and citrullination of MBP tended to enhance TNF-α secretion by CD4+ T cells from HLA-DR15+ patients. Unlike native MBP, citMBP induced differentiation into Th17 cells in cultures from HDs, while neither form of MBP induced Th17-cell differentiation in cultures from patients with MS. These data suggest a role for citrullination in the breach of tolerance to MBP in healthy individuals and in maintenance of the autoimmune response to MBP in patients with MS.

Autoimmune susceptible HLA class II motifs facilitate the presentation of modified neoepitopes to potentially autoreactive T cells

Rheumatoid arthritis (RA), multiple sclerosis (MS), type 1 diabetes (T1D), and celiac disease (CD), are strongly associated with susceptible HLA class II haplotypes. The peptide-binding pockets of these molecules are polymorphic, thus each HLA class II protein presents a distinct set of peptides to CD4⁺ T cells. Peptide diversity is increased through post-translational modifications, generating non-templated sequences that enhance HLA binding and/or T cell recognition. The high-risk HLA-DR alleles that confer susceptibility to RA are notable for their ability to accommodate citrulline, promoting responses to citrullinated self-antigens. Likewise, HLA-DQ alleles associated with T1D and CD favor the binding of deamidated peptides. In this review, we discuss structural features that promote modified self-epitope presentation, provide evidence supporting the relevance of T cell recognition of such antigens in disease processes, and make a case that interrupting the pathways that generate such epitopes and reprogramming neoepitope-specific T cells are key strategies for effective therapeutic intervention.

In Vivo Assessment of the Ameliorative Impact of Some Medicinal Plant Extracts on Lipopolysaccharide-Induced Multiple Sclerosis in Wistar Rats

Multiple sclerosis is a chronic autoimmune disorder that leads to the demyelination of nerve fibers, which is the major cause of non-traumatic disability all around the world. Herbal plants Nepeta hindustana L., Vitex negundo L., and Argemone albiflora L., in addition to anti-inflammatory and anti-oxidative effects, have shown great potential as neuroprotective agents. The study was aimed to develop a neuroprotective model to study the effectiveness of herbal plants (N. hindustana, V. negundo, and A. albiflora) against multiple sclerosis. The in vivo neuroprotective effects of ethanolic extracts isolated from N. hindustana, V. negundo, and A. albiflora were evaluated in lipopolysaccharides (LPS) induced multiple sclerosis Wistar rat model. The rat models were categorized into seven groups including group A as normal, B as LPS induced diseased group, while C, D, E, F, and G were designed as treatment groups. Histopathological evaluation and biochemical markers including stress and inflammatory (MMP-6, MDA, TNF-α, AOPPs, AGEs, NO, IL-17 and IL-2), antioxidant (SOD, GSH, CAT, GPx), DNA damage (Isop-2α, 8OHdG) as well as molecular biomarkers (RAGE, Caspase-8, p38) along with glutamate, homocysteine, acetylcholinesterase, and myelin binding protein (MBP) were investigated. The obtained data were analyzed using SPSS version 21 and GraphPad Prism 8.0. The different extract treated groups (C, D, E, F, G) displayed a substantial neuroprotective effect regarding remyelination of axonal terminals and oligodendrocytes migration, reduced lymphocytic infiltrations, and reduced necrosis of Purkinje cells. The levels of stress, inflammatory, and DNA damage markers were observed high in the diseased group B, which were reduced after treatments with plant extracts. The antioxidant activity was significantly reduced in diseased induced group B, however, their levels were raised after treatment with plant extract. Group F (a mélange of all the extracts) showed the most significant change among all other treatment groups (C, D, E, G). The communal dose of selected plant extracts regulates neurodegeneration at the cellular level resulting in restoration and remyelination of axonal neurons. Moreover, 400 mg/kg dose of three plants in conjugation (Group F) were found to be more effective in restoring the normal activities of all measured parameters than independent doses (Group C, D, E) and is comparable with standard drug nimodipine (Group G) clinically used for the treatment of multiple sclerosis. The present study, for the first time, reported the clinical evidence of N. hindustana, V. negundo, and A. albiflora against multiple sclerosis and concludes that all three plants showed remyelination as well neuroprotective effects which may be used as a potential natural neurotherapeutic agent against multiple sclerosis.

Overview of myelin, major myelin lipids, and myelin-associated proteins

Myelin is a modified cell membrane that forms a multilayer sheath around the axon. It retains the main characteristics of biological membranes, such as lipid bilayer, but differs from them in several important respects. In this review, we focus on aspects of myelin composition that are peculiar to this structure and differentiate it from the more conventional cell membranes, with special attention to its constituent lipid components and several of the most common and important myelin proteins: myelin basic protein, proteolipid protein, and myelin protein zero. We also discuss the many-fold functions of myelin, which include reliable electrical insulation of axons to ensure rapid propagation of nerve impulses, provision of trophic support along the axon and organization of the unmyelinated nodes of Ranvier, as well as the relationship between myelin biology and neurologic disease such as multiple sclerosis. We conclude with a brief history of discovery in the field and outline questions for future research.

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.

Effect of Cholesterol and Myelin Basic Protein (MBP) Content on Lipid Monolayers Mimicking the Cytoplasmic Membrane of Myelin

Myelin basic protein (MBP) is located in the insulating covers of nerve cells in the brain and spinal cord. By interacting with lipid membranes, it is responsible for compaction of the myelin sheath in the central nervous system, which is weakened in demyelinating diseases. The lipid composition of the myelin leaflet has a high impact on the interaction between the membrane and MBP. Cholesterol is present in the cytoplasmic leaflet with a rather high amount of 44% (mol%). In this study, the focus is on the effect of cholesterol, mainly by varying its content, on the interaction of MBP with a lipid monolayer. Therefore, Langmuir lipid monolayers mimicking the cytoplasmic membrane of myelin and monolayers with variations of cholesterol content between 0% and 100% were measured at the air/water interface with additional imaging by fluorescence microscopy. All experiments were performed with and without bovine MBP to study the dependence of the interaction of the protein with the monolayers on the cholesterol content. The native amount of 44% cholesterol in the monolayer combines optima in the order of the monolayer (presumably correlating to compaction and thermodynamic stability) and protein interaction and shows unique features in comparison to lower or higher cholesterol contents.

Brain Citrullination Patterns and T Cell Reactivity of Cerebrospinal Fluid-Derived CD4+ T Cells in Multiple Sclerosis

Immune responses to citrullinated peptides have been described in autoimmune diseases like rheumatoid arthritis (RA) and multiple sclerosis (MS). We investigated the post-translational modification (PTM), arginine to citrulline, in brain tissue of MS patients and controls (C) by proteomics and subsequently the cellular immune response of cerebrospinal fluid (CSF)-infiltrating T cells to citrullinated and unmodified peptides of myelin basic protein (MBP). Using specifically adapted tissue extraction- and combined data interpretation protocols we could establish a map of citrullinated proteins by identifying more than 80 proteins with two or more citrullinated peptides in human brain tissue. We report many of them for the first time. For the already described citrullinated proteins MBP, GFAP, and vimentin, we could identify additional citrullinated sites. The number of modified proteins in MS white matter was higher than control tissue. Citrullinated peptides are considered neoepitopes that may trigger autoreactivity. We used newly identified epitopes and previously reported immunodominant myelin peptides in their citrullinated and non-citrullinated form to address the recognition of CSF-infiltrating CD4⁺ T cells from 22 MS patients by measuring proliferation and IFN-γ secretion. We did not detect marked responses to citrullinated peptides, but slightly more strongly to the non-modified version. Based on these data, we conclude that citrullination does not appear to be an important activating factor of a T cell response, but could be the consequence of an immune- or inflammatory response. Our approach allowed us to perform a deep proteome analysis and opens new technical possibilities to analyze complex PTM patterns on minute quantities of rare tissue samples.

Epigenetic research in multiple sclerosis: progress, challenges, and opportunities

Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the central nervous system. MS likely results from a complex interplay between predisposing causal gene variants (the strongest influence coming from HLA class II locus) and environmental risk factors such as smoking, infectious mononucleosis, and lack of sun exposure/vitamin D. However, little is known about the mechanisms underlying MS development and progression. Moreover, the clinical heterogeneity and variable response to treatment represent additional challenges to a comprehensive understanding and efficient treatment of disease. Epigenetic processes, such as DNA methylation and histone posttranslational modifications, integrate influences from the genes and the environment to regulate gene expression accordingly. Studying epigenetic modifications, which are stable and reversible, may provide an alternative approach to better understand and manage disease. We here aim to review findings from epigenetic studies in MS and further discuss the challenges and clinical opportunities arising from epigenetic research, many of which apply to other diseases with similar complex etiology. A growing body of evidence supports a role of epigenetic processes in the mechanisms underlying immune pathogenesis and nervous system dysfunction in MS. However, disparities between studies shed light on the need to consider possible confounders and methodological limitations for a better interpretation of the data. Nevertheless, translational use of epigenetics might offer new opportunities in epigenetic-based diagnostics and therapeutic tools for a personalized care of MS patients.

Partial Immunoblotting of 2D-Gels: A Novel Method to Identify Post-Translationally Modified Proteins Exemplified for the Myelin Acetylome

Post-translational modifications (PTMs) play a key role in regulating protein function, yet their identification is technically demanding. Here, we present a straightforward workflow to systematically identify post-translationally modified proteins based on two-dimensional gel electrophoresis. Upon colloidal Coomassie staining the proteins are partially transferred, and the investigated PTMs are immunodetected. This strategy allows tracking back the immunopositive antigens to the corresponding spots on the original gel, from which they are excised and mass spectrometrically identified. Candidate proteins are validated on the same membrane by immunodetection using a second fluorescence channel. We exemplify the power of partial immunoblotting with the identification of lysine-acetylated proteins in myelin, the oligodendroglial membrane that insulates neuronal axons. The excellent consistency of the detected fluorescence signals at all levels allows the differential comparison of PTMs across multiple conditions. Beyond PTM screening, our multi-level workflow can be readily adapted to clinical applications such as identifying auto-immune antigens or host-pathogen interactions.

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.

Immune recognition of citrullinated epitopes

Conversion of arginine into citrulline is a post-translational modification that is observed in normal physiological processes. However, abnormal citrullination can provoke autoimmunity by generating altered self-epitopes that are specifically targeted by autoantibodies and T cells. In this review we discuss the recognition of citrullinated antigens in human autoimmune diseases and the role that this modification plays in increasing antigenic diversity and circumventing tolerance mechanisms. Early published work demonstrated that citrullinated proteins are specifically targeted by autoantibodies in rheumatoid arthritis and that citrullinated peptides are more readily presented to T cells by arthritis-susceptible HLA class II 'shared epitope' proteins. Emerging data support the relevance of citrullinated epitopes in other autoimmune diseases, including type 1 diabetes and multiple sclerosis, whose susceptible HLA haplotypes also preferentially present citrullinated peptides. In these settings, autoimmune patients have been shown to have elevated responses to citrullinated epitopes derived from tissue-specific antigens. Contrasting evidence implicates autophagy or perforin and complement-mediated membrane attack as inducers of ectopic citrullination. In either case, the peptidyl deiminases responsible for citrullination are activated in response to inflammation or insult, providing a mechanistic link between this post-translational modification and interactions with the environment and infection. As such, it is likely that immune recognition of citrullinated epitopes also plays a role in pathogen clearance. Indeed, our recent data suggest that responses to citrullinated peptides facilitate recognition of novel influenza strains. Therefore, increased understanding of responses to citrullinated epitopes may provide important insights about the initiation of autoimmunity and recognition of heterologous viruses.

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.

Epigenetics of multiple sclerosis: an updated review

Multiple sclerosis (MS) is an inflammatory and neurodegenerative disease characterized with autoimmune response against myelin proteins and progressive axonal loss. The heterogeneity of the clinical course and low concordance rates in monozygotic twins have indicated the involvement of complex heritable and environmental factors in MS pathogenesis. MS is more often transmitted to the next generation by mothers than fathers suggesting an epigenetic influence. One of the possible reasons of this parent-of-origin effect might be the human leukocyte antigen-DRB1*15 allele, which is the major risk factor for MS and regulated by epigenetic mechanisms such as DNA methylation and histone deacetylation. Moreover, major environmental risk factors for MS, vitamin D deficiency, smoking and Ebstein-Barr virus are all known to exert epigenetic changes. In the last few decades, compelling evidence implicating the role of epigenetics in MS has accumulated. Increased or decreased acetylation, methylation and citrullination of genes regulating the expression of inflammation and myelination factors appear to be particularly involved in the epigenetics of MS. Although much less is known about epigenetic factors causing neurodegeneration, epigenetic mechanisms regulating axonal loss, apoptosis and mitochondrial dysfunction in MS are in the process of identification. Additionally, expression levels of several microRNAs (miRNAs) (e.g., miR-155 and miR-326) are increased in MS brains and potential mechanisms by which these factors might influence MS pathogenesis have been described. Certain miRNAs may also be potentially used as diagnostic biomarkers in MS. Several reagents, especially histone deacetylase inhibitors have been shown to ameliorate the symptoms of experimental allergic encephalomyelitis. Ongoing efforts in this field are expected to result in characterization of epigenetic factors that can be used in prediction of treatment responsive MS patients, diagnostic screening panels and treatment methods with specific mechanism of action.

Toward biomarkers in multiple sclerosis: new advances

Multiple sclerosis is an autoimmune disease that commonly affects young adults. If initially characterized by acute relapses, it is later followed by only incomplete remission. Over years, progressive disability and irreversible deficit lead to chronic neurological deficits in the majority of patients. The clinical course is protracted and unpredictable, and no biological marker is useful in predicting the evolution of autoaggression and disability. It is difficult to diagnose and to monitor disease progression after the initial symptoms or even during the major clinical manifestations, and it is difficult to treat. In this review, the authors report recent advances in the field, focusing on the search of new antigens as a marker of the disease, in their relevance to the pathophysiology and diagnosis of the disease.

Role for peptidylarginine deiminase enzymes in disease and female reproduction

The peptidylarginine deiminases (PADs) are a family of calcium-dependent enzymes that post-translationally convert positively charged arginine residues to neutrally charged citrulline in a process called citrullination. There are five PAD family members (PAD1-4 and 6), each with unique tissue distribution patterns and functional roles including: cellular differentiation, nerve growth, apoptosis, inflammation, gene regulation, and early embryonic development. Previous review articles have focused on the expression and function of PADs and on their catalytic activity, citrullination, while other, more recent reviews have addressed the role of these enzymes in disease [1-3]. What has not been previously reviewed in any level of detail is the role that PAD proteins play in female reproduction. Given that: (1) PAD family members are highly represented in female reproductive tissues, (2) that some of the earlier PAD literature suggests that PADs play a critical role in female reproduction, and (3) that our studies have demonstrated that oocyte and early embryo restricted PAD6 is essential for female reproduction, we felt that a more comprehensive review of this topic was warranted.

Epigenetics and autoimmune diseases

Epigenetics is defined as the study of all inheritable and potentially reversible changes in genome function that do not alter the nucleotide sequence within the DNA. Epigenetic mechanisms such as DNA methylation, histone modification, nucleosome positioning, and microRNAs (miRNAs) are essential to carry out key functions in the regulation of gene expression. Therefore, the epigenetic mechanisms are a window to understanding the possible mechanisms involved in the pathogenesis of complex diseases such as autoimmune diseases. It is noteworthy that autoimmune diseases do not have the same epidemiology, pathology, or symptoms but do have a common origin that can be explained by the sharing of immunogenetic mechanisms. Currently, epigenetic research is looking for disruption in one or more epigenetic mechanisms to provide new insights into autoimmune diseases. The identification of cell-specific targets of epigenetic deregulation will serve us as clinical markers for diagnosis, disease progression, and therapy approaches.

Human T cell expansion and experimental autoimmune encephalomyelitis inhibited by Lenaldekar, a small molecule discovered in a zebrafish screen

Immune-mediated diseases [multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE)] are driven by proliferating, highly activated autoreactive T-cells that are unresponsive to in vivo immunoregulatory mechanisms. The compound Lenaldekar (LDK) was identified in a zebrafish screen by inhibiting T-cell expansion. By monitoring mitogen- and antigen-driven proliferation, we found that LDK inhibited human and murine T-cell expansion in a non-cytolytic manner. This suppressive activity directly correlated with the degree of activation/proliferation of the T-cells. In testing LDK in an EAE model of MS, exacerbations were suppressed in treated animals. Therefore, LDK represents a novel therapeutic approach to T-cell-mediated autoimmune diseases.

Methylation-dependent PAD2 upregulation in multiple sclerosis peripheral blood

BACKGROUND

Peptidylarginine deiminase 2 (PAD2) and peptidylarginine deiminase 4 (PAD4) are two members of PAD family which are over-expressed in the multiple sclerosis (MS) brain. Through its enzymatic activity PAD2 converts myelin basic protein (MBP) arginines into citrullines - an event that may favour autoimmunity - while peptidylarginine deiminase 4 (PAD4) is involved in chromatin remodelling.

OBJECTIVES

Our aim was to verify whether an altered epigenetic control of PAD2, as already shown in the MS brain, can be observed in peripheral blood mononuclear cells (PBMCs) of patients with MS since some of these cells also synthesize MBP.

METHODS

The expression of most suitable reference genes and of PAD2 and PAD4 was assessed by qPCR. Analysis of DNA methylation was performed by bisulfite method.

RESULTS

The comparison of PAD2 expression level in PBMCs from patients with MS vs. healthy donors showed that, as well as in the white matter of MS patients, the enzyme is significantly upregulated in affected subjects. Methylation pattern analysis of a CpG island located in the PAD2 promoter showed that over-expression is associated with promoter demethylation.

CONCLUSION

Defective regulation of PAD2 in the periphery, without the immunological shelter of the blood-brain barrier, may contribute to the development of the autoimmune responses in MS.

Modification of citrulline residues with 2,3-butanedione facilitates their detection by liquid chromatography/mass spectrometry

Citrullination is a post-translational modification (PTM) that results from the deimination of the amino acid arginine into citrulline by Peptidyl Arginine Deiminase enzymes and occurs in a wide range of proteins in health and disease. This modification causes a 1 Da mass shift, which can be used to identify citrullination sites in proteins by the use of mass spectrometry. However, other PTMs, such as deamidation from asparagine to aspartic acid or from glutamine to glutamic acid, can also cause a 1 Da mass shift, making correct interpretation of the data more difficult. We developed a chemical tagging strategy which, combined with an open source search application, allowed us to selectively pinpoint citrullinated peptides in a complex mixture after liquid chromatography/mass spectrometry (LC/MS) analysis. After incubation of a peptide mixture with 2,3 butanedione, citrulline residues were covalently modified which resulted in a 50 Da shift in singly charged mass. By comparison of the peptide mass fingerprint from a modified and an unmodified version of the same sample, our in-house search application was able to identify the citrullinated peptides in the mixture. This strategy was optimized on synthetic peptides and validated on a digest of in vitro citrullinated fibrinogen, where different proteolytic enzymes were used to augment the protein coverage. This new method results in easy detection of citrullinated residues, without the need for complex mass spectrometry equipment.

The experimental autoimmune encephalomyelitis model for proteomic biomarker studies: from rat to human

Multiple sclerosis (MScl) is defined by central nervous system (CNS) inflammation, demyelination and axonal damage. Some of the disease mechanisms are known but the cause of this complex disorder stays an enigma. Experimental autoimmune encephalomyelitis (EAE) is an animal model mimicking many aspects of MScl. This review aims to provide an overview over proteomic biomarker studies in the EAE model emphasizing the translational aspects with respect to MScl in humans.

The epigenetics of autoimmunity

The etiology of autoimmune diseases remains largely unknown. Concordance rates in monozygotic twins are lower than 50% while genome-wide association studies propose numerous significant associations representing only a minority of patients. These lines of evidence strongly support other complementary mechanisms involved in the regulation of genes expression ultimately causing overt autoimmunity. Alterations in the post-translational modification of histones and DNA methylation are the two major epigenetic mechanisms that may potentially cause a breakdown of immune tolerance and the perpetuation of autoimmune diseases. In recent years, several studies both in clinical settings and experimental models proposed that the epigenome may hold the key to a better understanding of autoimmunity initiation and perpetuation. More specifically, data support the impact of epigenetic changes in systemic lupus erythematosus, rheumatoid arthritis, multiple sclerosis and other autoimmune diseases, in some cases based on mechanistical observations. We herein discuss what we currently know and what we expect will come in the next future. Ultimately, epigenetic treatments already being used in oncology may soon prove beneficial also in autoimmune diseases.

Antigen processing and presentation in multiple sclerosis

CD4(+) T cells play a central role in the pathogenesis of multiple sclerosis (MS). Generation, activation and effector function of these cells crucially depends on their interaction with MHC II-peptide complexes displayed by antigen presenting cells (APC). Processing and presentation of self antigens by different APC therefore influences the disease course at all stages. Selection by thymic APC leads to the generation of autoreactive T cells, which can be activated by peripheral APC. Reactivation by central nervous system APC leads to the initiation of the inflammatory response resulting in demyelination. In this review we will focus on how MHC class II antigenic epitopes are created by different APC from the thymus, the periphery and from the brain, and will discuss the relevance of the balance between creation and destruction of such epitopes in the context of MS. A solid understanding of these processes offers the possibility for designing future therapeutic strategies.

Hepatitis B virus and Homo sapiens proteome-wide analysis: A profusion of viral peptide overlaps in neuron-specific human proteins

The primary amino acid sequence of the hepatitis B virus (HBV) proteome was searched for identity spots in the human proteome by using the Protein Information Resource database. We find that the HBV polyprotein shares sixty-five heptapeptides, one octapeptide, and one nonapeptide with the human proteins. The viral matches are disseminated among fundamental human proteins such as adhesion molecules, leukocyte differentiation antigens, enzymes, proteins associated with spermatogenesis, and transcription factors. As a datum of special interest, a number of peptide motifs are shared between the virus- and brain-specific antigens involved in neuronal protection. This study may help to evaluate the potential cross reactions and side effects of HBV antigen-based vaccines.

Contribution of myelin autoantigen citrullination to T cell autoaggression in the central nervous system

Breakdown in immunological self tolerance, leading to autoimmune diseases such as multiple sclerosis, might arise from immune recognition of self proteins that have undergone heightened posttranslational modification under pathophysiological conditions. A posttranslational modification of particular interest is the deimination of Arg to citrulline, catalyzed by peptidylarginyl deiminase (PAD) enzymes. As a CD4(+) T cell-driven model of multiple sclerosis, we used experimental autoimmune encephalomyelitis (EAE) induced with the immunodominant 35-55 peptide of myelin oligodendrocyte glycoprotein (pMOG) in C57BL/6 mice to test whether citrullination of a T cell epitope can contribute to disease etiopathology. Immunization with an altered peptide ligand (APL) of pMOG with an Arg-->citrulline conversion at a TCR contact (residue 41) led to the activation of two populations of APL-responsive T cells that either did, or did not cross-react with the native pMOG peptide. This APL could induce EAE. However, this reflected the activation of T cells that cross-reacted with the native pMOG epitope, because prior tolerization of these T cells using pMOG prevented APL-induced EAE. Using a passive transfer model, we found that T cells that responded specifically to the citrullinated form of pMOG were neither necessary, nor sufficient to initiate the EAE lesion. Nevertheless, these cells could provoke exacerbation of pathology if transferred into mice with ongoing EAE. The PAD2 and PAD4 enzymes were markedly upregulated in the inflamed CNS. Therefore, once inflammation is established, citrullination of target autoantigens can allow an expanded repertoire of T cells to contribute to CNS pathology.

Epigenetic mechanisms facilitating oligodendrocyte development, maturation, and aging

The process of oligodendrocyte differentiation is regulated by a dynamic interaction between a genetic and an epigenetic program. Recent studies, addressing nucleosomal histone modifications have considerably increased our knowledge regarding epigenetic regulation of gene expression during oligodendrocyte development and aging. These results have generated new hypotheses regarding the mechanisms underlying the decreased efficiency of endogenous remyelination in response to demyelinating injuries with increasing age. In this review, we present an overview of the epigenetic mechanisms regulating gene expression at specific stages of oligodendrocyte differentiation and maturation as well as the changes that occur with aging.

Peptidylarginine deiminase 4 and citrullination in health and disease

Deimination is catalyzed by a family of calcium binding enzymes, called peptidylarginine deiminases (PADs). Among these, the PAD4 isoform has been more extensively studied for its role in some autoimmune diseases. PAD4 is localized in the cytoplasm of monocytes, T and B cells, neutrophils, eosinophils and NK cells and can move to the nucleus upon cell activation. PAD4 plays a physiological role in gene regulation via citrullination of histones. In rheumatoid arthritis (RA), PAD4 contributes to the generation of ACPA specific substrates and is itself a target of autoantibodies; alleles of the PADI4 gene confer susceptibility to RA in Asians but not in Caucasians. In multiple sclerosis, extensive deimination of brain proteins is observed in active lesions, but no role for the PADI4 gene in susceptibility to MS has been so far described.

Assessment of citrullinated myelin by 1H-MR spectroscopy in early-onset multiple sclerosis

BACKGROUND AND PURPOSE

Myelin instability and citrullinated myelin basic protein have been demonstrated in the brains of patients with chronic and fulminating forms of multiple sclerosis (MS). Our aim was to trace citrulline in the brains of patients with early-onset MS by using proton MR spectroscopy ((1)H-MR spectroscopy).

MATERIALS AND METHODS

A short-echo single-voxel (1)H-MR spectroscopy by using the point-resolved proton spectroscopy sequence was performed in 27 patients with MS and 23 healthy subjects. Voxels of interest were chronic demyelinating lesions (CDLs, n = 25) and normal-appearing white matter (NAWM, n = 25) on T2-weighted imaging, and when available in patients with MS, enhancing demyelinating lesions (EDLs, n = 8). Frontal white matter (WM) was studied in control subjects. N-acetylaspartate, choline, and myo-inositol (mIns)-creatine (Cr) ratios and the presence of a citrulline peak were noted.

RESULTS

Citrulline peaks were more frequently observed in patients with MS than in control subjects (P = .035), located in the NAWM in 8/25 (32%), in CDLs in 7/25 (28%), and in EDLs of 1/8 (12.5%) patients with MS. The presence of citrulline and measured metabolite/Cr ratios was not related to age at imaging, age at disease onset, duration of disease, or number of relapses. There was no significant metabolic difference between the NAWM of patients with MS and the WM of the control subjects. mIns/Cr was significantly greater in CDLs compared with the NAWM of patients with MS and the WM of healthy subjects.

CONCLUSIONS

Citrulline was more frequently identified in the brains of patients with early-onset MS than in healthy subjects by (1)H-MR spectroscopy, suggesting an association of increased citrullination of myelin proteins with demyelinating diseases.

Evaluating epigenetic landmarks in the brain of multiple sclerosis patients: a contribution to the current debate on disease pathogenesis

The evidence suggesting a role of epigenetics in the definition of complex trait diseases is rapidly increasing. The gender prevalence of multiple sclerosis, the low level concordance in homozygous twins and the linkage to several genetic loci, suggest an epigenetic component to the definition of this demyelinating disorder. While the immune etio-pathogenetic mechanism of disease progression has been well characterized, still relatively little is known about the initial events contributing to onset and progression of the demyelinating lesion. This article addresses the challenging question of whether loss of the mechanisms of epigenetic regulation of gene expression in the myelinating cells may contribute to the pathogenesis of multiple sclerosis, by affecting the repair process and by modulating the levels of enzymes involved in neo-epitope formation. The role of altered post-translational modifications of nucleosomal histones and DNA methylation in white matter oligodendroglial cells are presented in terms of pathogenetic concepts and the relevance to therapeutic intervention is then discussed.

Blood CD8+ T cell responses against myelin determinants in multiple sclerosis and healthy individuals

Patients with multiple sclerosis (MS) display significant peripheral blood CD8(+) T cell receptor biases, suggesting clonal selection. Our objective was to identify relevant myelin-derived peptides capable of eliciting responses of fresh blood CD8+ T cells in MS patients. We focused our analysis on the HLA supertypes (HLA-A3, -A2, -B7, -B27, -B44) predominant in a patient cohort. Three myelin protein (MBP, PLP and MOG) sequences were screened for HLA binding motifs and peptides were tested for their binding to HLA molecules. The cellular responses of 27 MS patients and 19 age- and sex-matched healthy controls (HC) were tested in IFN-gamma ELISPOT assays only detecting pre-committed CD8+ T cells. Sixty-nine new epitopes elicited positive responses, with MOG-derived peptides being the most immunogenic and peptides binding to HLA-A3 being the most frequent. However, MS patients and HC displayed the same frequency of autoreactive cells. The epitopes inducing the strongest responses were not those with the highest HLA binding, suggesting an effective thymic selection in MS patients. Our data extend the concept that the frequency of myelin-reactive T cells in MS patient blood is not increased compared to HC. The description of this set of myelin-derived peptides (MHC class I restricted, recognized by CD8+ T cells) offers new tools to explore the CD8+ cell role in MS.

Malondialdehyde modification of myelin oligodendrocyte glycoprotein leads to increased immunogenicity and encephalitogenicity

Self proteins may become autoantigenic through structural modification. We studied malondialdehydation of recombinant rat (rr) myelin oligodendrocyte glycoprotein (MOG), an autoantigen in multiple sclerosis. Malondialdehyde (MDA) modification changed protein weight and charge, the location of these adducts being mapped by Fourier transform ion cyclotron resonance. Molecular modelling revealed significant differences in the MDA-rrMOG three-dimensional structure. DBA/1 mice immunised with MDA-rrMOG developed greater proliferative responses and more severe experimental autoimmune encephalomyelitis than mice immunised with unmodified rrMOG. MDA-rrMOG was taken up more effectively by antigen-presenting cells (APC), at least partially through scavenger receptors. Exposure to MDA-rrMOG led to increased expression of IL-23, IL-12 and IL-12R, indicating a role not only for increased antigen uptake but also for activation of APC. We thus provide biochemical, structural, immunological and clinical data that suggest that the post-translationally modified form of this myelin autoantigen is a more relevant form of the molecule.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Celiac disease--sandwiched between innate and adaptive immunity

Celiac disease (CD) patients are intolerant to gluten, proteins in wheat, and related cereals. Virtually all patients are human leukocyte antigen (HLA)-DQ2 or HLA-DQ8 positive and several studies have demonstrated that CD4 T cells specific for (modified) gluten peptides bound to these HLA-DQ molecules are found in patients but not in control subjects. These T cell responses are therefore thought to be responsible for disease development. Many immunogenic gluten peptides which may relate to the disease-inducing properties of gluten have now been identified. In addition, gluten can stimulate IL-15 production that ultimately leads to NKG2D-mediated epithelial cell killing. However, CD develops in only a minority of HLA-DQ2 and HLA-DQ8 individuals. This may be attributed to the default setting of the intestinal immune system: induction and maintenance of tolerance to dietary components and commensal flora. Although at present it is unknown why tolerance in CD is not established or broken, both environmental and genetic factors have been implicated. There is strong evidence for the existence of genes or gene variants on chromosomes 5, 6, and 19 that predispose to CD. In addition, type I interferons have been implicated in development of several autoimmune disorders, including CD. Thus, viral infection and/or tissue damage in the intestine may cause inflammation and induce protective Th1-mediated immunity leading to loss of tolerance for gluten. Once tolerance is broken, a broad gluten-reactive T cell repertoire may develop through determinant spreading. This may be a critical step toward full-blown disease.

Experimental autoimmune encephalomyelitis induction in peptidylarginine deiminase 2 knockout mice

During the development of multiple sclerosis the destruction of the myelin sheath surrounding the neurites is accompanied by citrullination of several central nervous system (CNS) proteins, including myelin basic protein and glial fibrillary acidic protein. In experimental autoimmune encephalomyelitis (EAE), a disease induced in animals by immunization with proteins or peptides from the CNS, the animals develop symptoms similar to multiple sclerosis (MS). The increased levels of citrullinated CNS proteins associated with MS are also observed during the development of EAE. To study the role of CNS protein citrullination in EAE development, we induced EAE with a peptide derived from myelin oligodendrocyte glycoprotein (MOG(35-55)) in mice lacking the peptidylarginine deiminase 2 (PAD2) protein, because this enzyme was the most likely candidate to be involved in catalyzing CNS protein citrullination in the diseased state. Even though the PAD2 knockout mice displayed a dramatic reduction in the amount of citrullination present in the CNS, indicating that PAD2 is indeed responsible for the majority of detectable citrullination observed in EAE, the development of EAE was not impaired by genetic deletion of PAD2, suggesting that PAD2 catalyzed citrullination is not essential to the development of EAE.

Post-translational modifications of naturally processed MHC-binding epitopes

A variety of different post-translational modifications of peptides displayed by class I and II MHC molecules have now been described. Some modifications promote the binding of peptides to MHC molecules, and might also influence the ability of the peptide to be produced by antigen processing pathways. In some instances, the antigen processing components themselves are actually responsible for generating post-translational modifications. Finally, evidence is accumulating that modifications can be altered as a consequence of inflammation, transformation, apoptosis and aging. This leads to altered repertories of MHC-associated peptides, which may be important in immune responses associated with autoimmune diseases, infection and cancer.

Posttranslational modifications of self-antigens

Although the immune system has developed mechanisms to distinguish "self" from "non-self," the presence of autoimmune diseases demonstrates that these mechanisms can be bypassed. The posttranslational modification of self-antigens is one way in which "new" antigens are created for which immune tolerance does not exist. We review some of the posttranslationally modified self-antigens associated with autoimmune diseases, how they arise, and how they break immune tolerance.

Post-translational modifications of self antigens: implications for autoimmunity

Alterations in amino acid sequence can generate neo-epitopes from self proteins, causing autoaggressive immune attack. There is a range of possible post-translational modifications (PTMs) of mammalian proteins that can allow immune recognition of neo-self epitopes. These effects can vary from overt increase in affinity of MHC or T-cell receptor binding, to more subtle effects on the activity of proteolytic enzymes involved in antigen processing. Furthermore, intriguing insights into how the complex interactions between inflammation, enzyme activity and protein modification can direct self recognition are beginning to be unearthed.

Expression of citrullinated proteins in murine experimental autoimmune encephalomyelitis

In this study, we demonstrate for the first time the immunohistochemical expression of citrullinated proteins in the central nervous system (CNS) of mice with myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). By using an established monoclonal antibody (F95) against natural and synthetic citrullinated proteins (Nicholas and Whitaker [2002] Glia 37:328-336), numerous, small, previously unrecognized "patches" of citrullinated proteins were discovered throughout EAE brains, whereas EAE spinal cords showed similar but much larger lesions. On dual color immunofluorescence, these lesions were found to contain citrullinated myelin basic protein (MBP) and were surrounded by astrocytes immunoreactive for both glial fibrillary acidic protein (GFAP) and F95. These lesions became evident about the time when EAE mice became symptomatic and increased in size and number with increasing disease severity. In some sections of spinal cord but not brains of severely debilitated EAE mice, a widespread gliotic response was seen, with astrocytes containing citrullinated GFAP spread throughout the gray and white matter. Western blot analysis of acidic proteins from the brains and spinal cords of EAE mice had higher levels of multiple citrullinated GFAP isoforms compared with controls, with more F95-positive bands in the EAE brains vs. spinal cords. These results raise the possibility that citrullination of both GFAP and MBP may contribute to the pathophysiology of EAE and that the brains of EAE mice may contain more pathology than previously realized.

Delayed neuropathology after carbon monoxide poisoning is immune-mediated

The neuropathological sequelae of carbon monoxide (CO) poisoning cannot be explained by hypoxic stress alone. CO poisoning also causes adduct formation between myelin basic protein (MBP) and malonylaldehyde, a reactive product of lipid peroxidation, resulting in an immunological cascade. MBP loses its normal cationic characteristics, and antibody recognition of MBP is altered. Immunohistochemical evidence of degraded MBP occurs in brain over days, along with influx of macrophages and CD-4 lymphocytes. Lymphocytes from CO-poisoned rats subsequently exhibit an auto-reactive proliferative response to MBP, and there is a significant increase in the number of activated microglia in brain. Rats rendered immunologically tolerant to MBP before CO poisoning exhibit acute biochemical changes in MBP but no lymphocyte proliferative response or brain microglial activation. CO poisoning causes a decrement in learning that is not observed in immunologically tolerant rats. These results demonstrate that delayed CO-mediated neuropathology is linked to an adaptive immunological response to chemically modified MBP.

High variability of peptidylarginine deiminase 4 (PADI4) in a healthy white population: characterization of six new variants of PADI4 exons 2?4 by a novel haplotype-specific sequencing-based approach

Seven single nucleotide polymorphisms (SNPs) of the peptidylarginine deiminase 4 (PADI4) gene have recently been reported to be strongly associated with rheumatoid arthritis in Japanese individuals. These SNPs are located in or close to exons 2-4 of PADI4 and are organized in at least four different haplotypes. However, a detailed sequencing-based characterization of the PADI4 gene in other populations is still lacking. We therefore analyzed exons 2-4 of the PADI4 gene in 102 healthy white Germans individuals by DNA sequencing and characterized new variants and haplotypes by a novel haplotype-specific sequencing-based approach. The haplotypes 2/3 (padi4_89*G, padi4_90*T, padi4_92*G, padi4_94*T, padi4_104*T, padi4_95*C, padi4_96*C), and haplotype 4 (padi4_89*G, padi4_90*T, padi4_92*G, padi4_94*T, padi4_104*C, padi4_95*G, padi4_96*T) conferring susceptibility to rheumatoid arthritis were detected at frequencies of 30.9% and 7.8%, respectively. In addition, three novel coding SNPs in exons 2, 3, and 4, and three SNPs in introns 2 and 3 located near the exon-intron boundaries were identified in 11 individuals (10.8%). The so-called nonsusceptibility haplotype 1 (padi4_89*A, padi4_90*C, padi4_92*C, padi4_94*C, padi4_104*C, padi4_95*G, padi4_96*T) occurred at a frequency of 58.3%. Additionally, we identified a closely related novel haplotype, haplotype 1B (2.9%), that differs from haplotype 1 only by padi4_92*G/padi4_96*C. This haplotype was not described in the Japanese population. Our results indicate that the PADI4 gene exhibits a remarkable variability and a rather complex haplotypic organization. Further studies on disease association of PADI4 should be performed by haplotype-specific sequencing-based approaches to identify the exact genotype of the PADI4 fragment of interest.

Multiple sclerosis: an important role for post-translational modifications of myelin basic protein in pathogenesis

Myelin basic protein (MBP) represents a candidate autoantigen in multiple sclerosis (MS). We isolated MBP from normal and MS human white matter and purified six components (charge isomers) to compare the post-translational modifications on each. The sites and extent of methylation, deimination, and phosphorylation were documented for all tryptic peptides by mass spectrometry. We found that mono and dimethylated arginine 107 was increased in MS samples; deimination of arginine occurred at a number of sites and was elevated in MS; phosphorylation was observed in 10 peptides in normal samples but was greatly reduced or absent in most peptides from MS samples. Data obtained with MBP isolated from fresh brain obtained from a spontaneously demyelinating mouse model supported the view that the changes observed in human brain were probably related to pathogenesis of demyelination, i.e. we found decreased phosphorylation and decreased amounts of glycogen synthesis kinase in brain homogenates using specific antibodies. This study represents the first to define post-translational modifications in demyelinating disease and suggest an important role in pathogenesis.

Modifications of arginines and their role in autoimmunity

Posttranslational modifications of proteins often perform a key role in the biological functioning of proteins. Some of these modifications also change the immunogenicity of proteins and peptides and create 'self'-antigens which might induce autoimmune responses. In particular modifications of arginines within a defined protein context can lead to a specific B-cell immune response. This review discusses the generation of such modifications and their relevance for autoimmunity.

cDNA cloning, gene organization and expression analysis of human peptidylarginine deiminase type I

Peptidylarginine deiminases (PADs) catalyse a post-translational modification of proteins through the conversion of arginine residues into citrullines. The existence of four isoforms of PAD (types I, II, III and IV) encoded by four different genes, which are distinct in their substrate specificities and tissue-specific expression, was reported in rodents. In the present study, starting from epidermis polyadenylated RNA, we cloned by reverse transcriptase-PCR a full-length cDNA encoding human PAD type I. The cDNA was 2711 bp in length and encoded a 663-amino-acid sequence. The predicted protein shares 75% identity with the rat PAD type I sequence, but displays only 50-57% identity with the three other known human isoforms. We have described the organization of the human PAD type I gene on chromosome 1p36. A recombinant PAD type I was produced in Escherichia coli and shown to be enzymically active. Human PAD type I mRNAs were detected by reverse transcriptase-PCR not only in the epidermis, but also in various organs, including prostate, testis, placenta, spleen and thymus. In human epidermis extracts analysed by Western blotting, PAD type I was detected as a 70 kDa polypeptide, in agreement with its predicted molecular mass. As shown by immunohistochemistry, the enzyme was expressed in all the living layers of human epidermis, with the labelling being increased in the granular layer. This is the first description of the human PAD type I gene and the first demonstration of its expression in epidermis.

Citrulline immunohistochemistry may not necessarily identify nitric oxide synthase activity: the pitfall of peptidylarginine deiminase

Nitric oxide synthase (NOS) converts L-arginine as a substrate to form nitric oxide and the "by-product" citrulline. To characterize NOS activity in the nervous tissue at the single-cell level, citrulline immunostaining is considered to be a suitable means of working on the principle that in brain tissue, due to the incomplete urea cycle, citrulline is produced exclusively by NOS. This assumption is correct for free citrulline but it does not consider the conversion of arginine to citrulline residues of proteins by the calcium-dependent peptidylarginine deiminase (PAD). Using a polyclonal antiserum against citrulline we observed in cerebellar cell cultures immunopositivity in a few, mostly NOS-positive, neurons, in activated microglia, and in oligodendroglia (which under control conditions are in doubt to be able to express NOS), but not in astroglia. Treatment with the excitotoxin kainate substantially enhanced the staining intensity for citrulline in neurons and glial cells. To distinguish between free (NOS-related) and protein-bound (PAD-related) citrulline we blocked NOS activity by 7-nitroindazole or L-N5-(1-iminoethyl)lysine. The results provide evidence that citrulline immunolabeling results partly from PAD-mediated protein citrullination, enhanced pathophysiologically under stimulated conditions by exposure to kainate. Our immunocytochemical observations were corroborated by Western blot analysis showing several bands of citrulline-positive proteins, whose number and staining intensity depended on kainate treatment and calcium ions.