Autoantibodies against a serine tRNA-protein complex implicated in cotranslational selenocysteine insertion

Incorporating the Molecular Mimicry of Environmental Antigens into the Causality of Autoimmune Hepatitis

Molecular mimicry between foreign and self-antigens has been implicated as a cause of autoimmune hepatitis in experimental models and cross-reacting antibodies in patients. This review describes the experimental and clinical evidence for molecular mimicry as a cause of autoimmune hepatitis, indicates the limitations and uncertainties of this premise, and encourages investigations that assess diverse environmental antigens as sources of disease-relevant molecular mimics. Pertinent articles were identified in PubMed using multiple search phrases. Several pathogens have linear or conformational epitopes that mimic the self-antigens of autoimmune hepatitis. The occurrence of an acute immune-mediated hepatitis after vaccination for severe acute respiratory syndrome (SARS)-associated coronavirus 2 (SARS-CoV-2) has suggested that vaccine-induced peptides may mimic disease-relevant tissue antigens. The intestinal microbiome is an under-evaluated source of gut-derived antigens that could also engage in molecular mimicry. Chaperone molecules may enhance the pathogenicity of molecular mimics, and they warrant investigation. Molecular mimics of immune dominant epitopes within cytochrome P450 IID6, the autoantigen most closely associated with autoimmune hepatitis, should be sought in diverse environmental antigens and assessed for pathogenicity. Avoidance strategies, dietary adjustments, vaccine improvement, and targeted manipulation of the intestinal microbiota may emerge as therapeutic possibilities. In conclusion, molecular mimicry may be a missing causality of autoimmune hepatitis. Molecular mimics of key immune dominant epitopes of disease-specific antigens must be sought in diverse environmental antigens. The ubiquity of molecular mimicry compels rigorous assessments of peptide mimics for immunogenicity and pathogenicity in experimental models. Molecular mimicry may complement epigenetic modifications as causative mechanisms of autoimmune hepatitis.

Selenocysteine Machinery Primarily Supports TXNRD1 and GPX4 Functions and Together They Are Functionally Linked with SCD and PRDX6

The human genome has 25 genes coding for selenocysteine (Sec)-containing proteins, whose synthesis is supported by specialized Sec machinery proteins. Here, we carried out an analysis of the co-essentiality network to identify functional partners of selenoproteins and Sec machinery. One outstanding cluster included all seven known Sec machinery proteins and two critical selenoproteins, GPX4 and TXNRD1. Additionally, these nine genes were further positively associated with PRDX6 and negatively with SCD, linking the latter two genes to the essential role of selenium. We analyzed the essentiality scores of gene knockouts in this cluster across one thousand cancer cell lines and found that Sec metabolism genes are strongly selective for a subset of primary tissues, suggesting that certain cancer cell lineages are particularly dependent on selenium. A separate outstanding cluster included selenophosphate synthetase SEPHS1, which was linked to a group of transcription factors, whereas the remaining selenoproteins were linked neither to these clusters nor among themselves. The data suggest that key components of Sec machinery have already been identified and that their primary role is to support the functions of GPX4 and TXNRD1, with further functional links to PRDX6 and SCD.

Aminoacyl-tRNA Synthetases: On Anti-Synthetase Syndrome and Beyond

Anti-synthetase syndrome (ASSD) is an autoimmune disease characterized by the presence of autoantibodies targeting one of several aminoacyl t-RNA synthetases (aaRSs) along with clinical features including interstitial lung disease, myositis, Raynaud’s phenomenon, arthritis, mechanic’s hands, and fever. The family of aaRSs consists of highly conserved cytoplasmic and mitochondrial enzymes, one for each amino acid, which are essential for the RNA translation machinery and protein synthesis. Along with their main functions, aaRSs are involved in the development of immune responses, regulation of transcription, and gene-specific silencing of translation. During the last decade, these proteins have been associated with cancer, neurological disorders, infectious responses, and autoimmune diseases including ASSD. To date, several aaRSs have been described to be possible autoantigens in different diseases. The most commonly described are histidyl (HisRS), threonyl (ThrRS), alanyl (AlaRS), glycyl (GlyRS), isoleucyl (IleRS), asparaginyl (AsnRS), phenylalanyl (PheRS), tyrosyl (TyrRS), lysyl (LysRS), glutaminyl (GlnRS), tryptophanyl (TrpRS), and seryl (SerRS) tRNA synthetases. Autoantibodies against the first eight autoantigens listed above have been associated with ASSD while the rest have been associated with other diseases. This review will address what is known about the function of the aaRSs with a focus on their autoantigenic properties. We will also describe the anti-aaRSs autoantibodies and their association to specific clinical manifestations, and discuss their potential contribution to the pathogenesis of ASSD.

Autoimmune Hepatitis: Serum Autoantibodies in Clinical Practice

Circulating autoantibodies are a key diagnostic tool in autoimmune hepatitis (AIH), being positive in 95% of the cases if tested according to dedicated guidelines issued by the International Autoimmune Hepatitis Group. They also allow the distinction between type 1 AIH, characterized by positive anti-nuclear and/or anti-smooth muscle antibody, and type 2 AIH, characterized by positive anti-liver kidney microsomal type 1 and/or anti-liver cytosol type 1 antibody. Anti-soluble liver antigen is the only AIH-specific autoantibody, and is found in 20-30% of both type 1 and type 2 AIH. Anti-neutrophil cytoplasmic antibody is frequently positive in type 1 AIH, being associated also with inflammatory bowel disease and with primary/autoimmune sclerosing cholangitis. The reference method for autoantibody testing remains indirect immunofluorescence on triple tissue (rodent liver, kidney and stomach), allowing both the detection of the majority of liver-relevant reactivities, including those autoantibodies whose molecular target antigens are unknown. Of note, the current knowledge of the clinical significance of autoantibodies relies on studies based on this technique. However, immunofluorescence requires trained laboratory personnel, is observer-dependent, and lacks standardization, leading to ongoing attempts at replacing this method with automated assays, the sensitivity, and specificity of which, however, require further studies before they can be used as a reliable alternative to immunofluorescence; currently, they may be used as complementary to immunofluorescence.

Human Disorders Affecting the Selenocysteine Incorporation Pathway Cause Systemic Selenoprotein Deficiency

Significance: Generalized selenoprotein deficiency has been associated with mutations in SECISBP2, SEPSECS, and TRU-TCA1-1, 3 factors that are crucial for incorporation of the amino acid selenocysteine (Sec) into at least 25 human selenoproteins. SECISBP2 and TRU-TCA1-1 defects are characterized by a multisystem phenotype due to deficiencies of antioxidant and tissue-specific selenoproteins, together with abnormal thyroid hormone levels reflecting impaired hormone metabolism by deiodinase selenoenzymes. SEPSECS mutations are associated with a predominantly neurological phenotype with progressive cerebello-cerebral atrophy. Recent Advances: The recent identification of individuals with defects in genes encoding components of the selenocysteine insertion pathway has delineated complex and multisystem disorders, reflecting a lack of selenoproteins in specific tissues, oxidative damage due to lack of oxidoreductase-active selenoproteins and other pathways whose nature is unclear. Critical Issues: Abnormal thyroid hormone metabolism in patients can be corrected by triiodothyronine (T3) treatment. No specific therapies for other phenotypes (muscular dystrophy, male infertility, hearing loss, neurodegeneration) exist as yet, but their severity often requires supportive medical intervention. Future Directions: These disorders provide unique insights into the role of selenoproteins in humans. The long-term consequences of reduced cellular antioxidant capacity remain unknown, and future surveillance of patients may reveal time-dependent phenotypes (e.g., neoplasia, aging) or consequences of deficiency of selenoproteins whose function remains to be elucidated. The role of antioxidant therapies requires evaluation. Antioxid. Redox Signal. 33, 481-497.

Examining pathogenic concepts of autoimmune hepatitis for cues to future investigations and interventions

BACKGROUND

Multiple pathogenic mechanisms have been implicated in autoimmune hepatitis, but they have not fully explained susceptibility, triggering events, and maintenance or escalation of the disease. Furthermore, they have not identified a critical defect that can be targeted. The goals of this review are to examine the diverse pathogenic mechanisms that have been considered in autoimmune hepatitis, indicate investigational opportunities to validate their contribution, and suggest interventions that might evolve to modify their impact. English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed. Genetic and epigenetic factors can affect susceptibility by influencing the expression of immune regulatory genes. Thymic dysfunction, possibly related to deficient production of programmed cell death protein-1, can allow autoreactive T cells to escape deletion, and alterations in the intestinal microbiome may help overcome immune tolerance and affect gender bias. Environmental factors may trigger the disease or induce epigenetic changes in gene function. Molecular mimicry, epitope spread, bystander activation, neo-antigen production, lymphocytic polyspecificity, and disturbances in immune inhibitory mechanisms may maintain or escalate the disease. Interventions that modify epigenetic effects on gene expression, alter intestinal dysbiosis, eliminate deleterious environmental factors, and target critical pathogenic mechanisms are therapeutic possibilities that might reduce risk, individualize management, and improve outcome. In conclusion, diverse pathogenic mechanisms have been implicated in autoimmune hepatitis, and they may identify a critical factor or sequence that can be validated and used to direct future management and preventive strategies.

Anti-OJ autoantibodies: Rare or underdetected?

Anti-OJ autoantibodies are rare myositis-specific autoantibodies that have been described to target isoleucyl-tRNA synthetase. Routinely used multiplex assays perform poorly in detection of anti-OJ antibodies. In this manuscript, we review the existing literature on critical issues in detection of anti-OJ and the clinical features associated with anti-OJ. The challenging detection with line/blot immunoassays and ELISAs is most likely related to the characteristics of the autoantigen involved, which is part of a multi-enzyme synthetase complex. Anti-OJ autoantibodies might therefore be more aptly termed anti-OJ complex autoantibodies. Anti-OJ autoantibodies are associated with the anti-synthetase syndrome, with interstitial lung disease (ILD) frequently being the sole manifestation. Myositis, present in the majority of patients with anti-OJ antibodies, is more severe than in patients with other anti-aminoacyl-tRNA synthetases. Most patients respond to glucocorticoid therapy. As detection of anti-OJ is relevant for treatment, reliable and practical detection is needed. Meanwhile, clinicians need to be aware of the possibility of anti-OJ in patients with ILD, isolated or in combination with myositis.

Autoantibodies in Autoimmune Liver Disease-Clinical and Diagnostic Relevance

Testing for liver-related autoantibodies should be included in the workup of patients with hepatitis or cholestasis of unknown origin. Although most of these autoantibodies are not disease specific, their determination is a prerequisite to diagnose autoimmune hepatitis (AIH) and primary biliary cholangitis (PBC), and they are components of the diagnostic scoring system in these diseases. In primary sclerosing cholangitis (PSC), on the other hand, autoantibodies are frequently present but play a minor role in establishing the diagnosis. In PSC, however, data on antibodies suggest a link between disease pathogenesis and the intestinal microbiota. This review will focus on practical aspects of antibody testing in the three major autoimmune liver diseases AIH, PBC, and PSC.

Bench to bedside review of myositis autoantibodies

Idiopathic inflammatory myopathies represent a heterogeneous group of autoimmune diseases with systemic involvement. Even though numerous specific autoantibodies have been recognized, they have not been included, with the only exception of anti-Jo-1, into the 2017 Classification Criteria, thus perpetuating a clinical-serologic gap. The lack of homogeneous grouping based on the antibody profile deeply impacts the diagnostic approach, therapeutic choices and prognostic stratification of these patients. This review is intended to highlight the comprehensive scenario regarding myositis-related autoantibodies, from the molecular characterization and biological significance to target antigens, from the detection tools, with a special focus on immunofluorescence patterns on HEp-2 cells, to their relative prevalence and ethnic diversity, from the clinical presentation to prognosis. If, on the one hand, a notable body of literature is present, on the other data are fragmented, retrospectively based and collected from small case series, so that they do not sufficiently support the decision-making process (i.e. therapeutic approach) into the clinics.

Serology in autoimmune hepatitis: A clinical-practice approach

Serology is key to the diagnosis of autoimmune hepatitis (AIH). Clinicians need to be aware of which tests to request, how to interpret the laboratory reports, and be familiar with the laboratory methodology. If correctly tested, >95% of AIH patients show some serological reactivity. Indirect immunofluorescence on triple rodent tissue is recommended as first screening step, since it allows the detection of all liver-relevant autoantibodies, except for anti-soluble liver antigen (SLA) antibody, which needs to be detected by molecular based assays. The threshold of immunofluorescence positivity is a titer equal or exceeding 1/40, but for patients younger than 18years even lower titers are clinically significant. Anti-nuclear antibody (ANA) and/or anti-smooth muscle (SMA) antibody characterize type 1 AIH. ANA in AIH typically shows a homogeneous staining pattern on HEp2 cells, without any specific target antigen. Anti-SMA displays different staining patterns on indirect immunofluorescence: the vascular/glomerular (VG) and the vascular/glomerular/tubular (VGT) patterns are considered specific for AIH, whilst the V pattern can be found in a variety of diseases. Type 2 AIH, which is rare and affects mostly children/adolescents, is characterized by anti-liver kidney microsomal 1 and/or anti-liver cytosol 1 antibodies. The presence of anti-neutrophil cytoplasmic antibody (ANCA), particularly atypical p-ANCA (pANNA), points to the diagnosis of AIH, especially in absence of other autoantibodies. Since it is associated with sclerosing cholangitis and inflammatory bowel disease, these conditions have to be ruled out. The only antibody specific for AIH is anti-SLA, which is associated with a more severe disease course.

Global Disparities and Their Implications in the Occurrence and Outcome of Autoimmune Hepatitis

Autoimmune hepatitis has a variable occurrence, clinical phenotype, and outcome, and the factors contributing to this variability are uncertain. The goals of this review are to examine the global disparities in the occurrence and outcome of autoimmune hepatitis, suggest bases for these disparities, and encourage investigations that extend beyond single-center experiences. Disparities in the incidence and prevalence of autoimmune hepatitis in different age groups, genders, ethnicities, and geographical regions suggest that factors other than genetic predisposition are involved. Age- and gender-related antigen exposures from the external (infections, toxins, and medications) and internal (intestinal microbiome) environment may affect the incidence of the disease, and the timeliness and nature of treatment may influence its prevalence. The increasing incidence of autoimmune hepatitis in Spain, Denmark, and the Netherlands suggests that a new etiological trigger has been introduced or that the susceptible population has changed. Variations in mortality between Western and Asian-Pacific countries may result from differences in disease detection or management, and variations in gender predilection, peak age of onset, frequency of concurrent immune diseases, and serological profile may reflect gender-biased and age-related antigen exposures and genetic predispositions. Global collaborations, population-based epidemiological studies that identify case clustering, and controlled interview-based surveys are mechanisms by which to understand these disparities and improve management. In conclusion, autoimmune hepatitis has a rising incidence in some countries and variable occurrence, phenotype, and outcome between countries and subgroups within countries. These disparities suggest that unrecognized population-based environmental, infectious, or socioeconomic factors are affecting its character.

Diagnostic autoantibodies for autoimmune liver diseases

Autoimmune liver diseases are conditions of low prevalence that comprise the triad of autoimmune hepatitis, primary biliary cholangitis (cirrhosis) and primary sclerosing cholangitis and their poorly characterised overlapping syndromes. Diagnostic autoantibodies are associated with autoimmune hepatitis and primary biliary cholangitis but not with primary sclerosing cholangitis. Autoantibodies are useful disease markers that facilitate early diagnosis of autoimmune hepatitis and primary biliary cholangitis and allow for therapeutic intervention to prevent progression to liver cirrhosis and associated complications. Adult onset type 1 autoimmune hepatitis is associated with F-actin reactive smooth muscle autoantibody, antinuclear autoantibody in 60% of patients, and autoantibody to SLA/LP in 15–20%. Juvenile onset type 2 autoimmune hepatitis is associated with LKM-1 and LC-1 autoantibodies. Primary biliary cholangitis is associated with a mitochondria-associated autoantibody designated M2 in >90% of patients and with disease-specific antinuclear autoantibodies in 50% that bind to antigens in the nuclear core complex and in multiple nuclear dots. Autoantibodies to the nuclear core complex target gp210, nucleoporin p62 and nuclear lamin B receptor. Autoantibodies to multiple nuclear dots target Sp100 and PML antigens. Liver autoantibodies in asymptomatic patients with normal liver function may precede the subsequent development of overt autoimmune liver disease. For routine diagnostic immunology laboratories, initial screening for liver autoantibodies by immunofluorescence remains the method of choice with confirmation for reactivity with their target antigen by enzyme-linked immunosorbent assay (ELISA) or line blot when required.

Autoantibodies in Autoimmune Liver Diseases-Methods of Detection and Interpretation: An Update for the Reporting Pathologist

Autoimmune liver disease (AILD) is a type of chronic liver disease with autoimmune etiology. The diagnosis of the disease is multipronged and detection of autoantibodies in AILDs is an important diagnostic tool and it also helps in the classification of the disease. There are multiple autoantibodies that are detected in AILDs but none is diagnostic. Moreover, these autoantibodies are detected in many other pathological and nonpathological conditions. So the significance of seropositivity for these autoantibodies should be known by both the pathologists as well as the clinicians. In addition, there is prognostic significance associated with some of the antibodies and they also sometimes help in the disease monitoring. The whole array of antibodies detected in AILDs is discussed in detail in this review along with their clinical significance and interpretation.

Prediction of short- and long-term outcome in patients with autoimmune hepatitis

Autoimmune hepatitis (AIH) is a chronic inflammatory disease characterized by a loss of tolerance toward the hepatocellular epithelium. Liver transplantation (LT) represents the ultimate therapeutic option for a fulminant course or end-stage liver disease. The aim of this study was to elucidate the clinical, serological, and genetic features of remission, relapse, and overall and LT-free survival. Between 2000 and 2014, 354 AIH patients from Hannover Medical School were included. Clinical, laboratory, and histological reports were analyzed. DRB1 allele analyses were performed in 264 AIH and 399 non-AIH patients. Cox's regression analysis was performed to identify factors significantly associated with survival. Patients diagnosed in childhood were at higher risk for relapses (P=0.003), requirement for LTs (P=0.014, log rank), and had a reduced life expectancy (P<0.001, log rank). Detection of soluble liver antigen/liver pancreas antigen (SLA/LP) antibodies was significantly associated with reduced overall and LT-free survival (P=0.037; P=0.021). Cirrhosis, which was evident in 25% at first diagnosis, was found to be a predictor of poor survival and requirement for LT (P=0.003; P=0.009). DRB1*04:01-positive phenotype was associated with a higher rate of complete remissions and with a lower frequency of cirrhosis and LTs. There were no significant differences for subsequent relapses or survival in patients achieving either partial or complete remission.

CONCLUSION

Diagnosis<18 years, histological cirrhosis at first diagnosis and SLA/LP antibodies are major risk factors for a poor short- and long-term outcome. These patients are in need of high surveillance. Separating patients with positive SLA/LP antibodies into a third group may be reconsidered. DRB1*04:01 positivity has been identified in association with a favorable clinical outcome.

Expression of Selenoproteins Is Maintained in Mice Carrying Mutations in SECp43, the tRNA Selenocysteine 1 Associated Protein (Trnau1ap)

Selenocysteine tRNA 1 associated protein (Trnau1ap) has been characterized as a tRNA^[Ser]Sec-binding protein of 43 kDa, hence initially named SECp43. Previous studies reported its presence in complexes containing tRNA^[Ser]Sec implying a role of SECp43 as a co-factor in selenoprotein expression. We generated two conditionally mutant mouse models targeting exons 3+4 and exons 7+8 eliminating parts of the first RNA recognition motif or of the tyrosine-rich domain, respectively. Constitutive inactivation of exons 3+4 of SECp43 apparently did not affect the mice or selenoprotein expression in several organs. Constitutive deletion of exons 7+8 was embryonic lethal. We therefore generated hepatocyte-specific Secp43 knockout mice and characterized selenoprotein expression in livers of mutant mice. We found no significant changes in the levels of ⁷⁵Se-labelled hepatic proteins, selenoprotein levels as determined by Western blot analysis, enzymatic activity or selenoprotein mRNA abundance. The methylation pattern of tRNA^[Ser]Sec remained unchanged. Truncated Secp43 ^Δ7,8mRNA increased in Secp43-mutant livers suggesting auto-regulation of Secp43 mRNA abundance. We found no signs of liver damage in Secp433-mutant mice, but neuron-specific deletion of exons 7+8 impaired motor performance, while not affecting cerebral selenoprotein expression or cerebellar development. These findings suggest that the targeted domains in the SECp43 protein are not essential for selenoprotein biosynthesis in hepatocytes and neurons. Whether the remaining second RNA recognition motif plays a role in selenoprotein biosynthesis and which other cellular process depends on SECp43 remains to be determined.

Meta-analysis: diagnostic accuracy of antinuclear antibodies, smooth muscle antibodies and antibodies to a soluble liver antigen/liver pancreas in autoimmune hepatitis

Background

Antinuclear antibodies (ANA), smooth muscle antibodies (SMA) and antibodies to a soluble liver antigen/liver pancreas (anti-SLA/LP) are useful markers that can help clinicians to diagnose and classify autoimmune hepatitis (AIH).

Objectives

To determine whether ANA, SMA and anti-SLA/LP help to accurately diagnose patients with AIH.

Search strategy

The PubMed, CNKI, WANFANG, and SinoMed databases were accessed to retrieve studies published in English and Chinese. Studies published up to October 2013 were reviewed.

Selection criteria

Studies on the diagnostic value of ANA, SMA or anti-SLA/LP in the diagnosis of known or suspected AIH were included.

Data collection and analysis

Two authors evaluated studies independently and rated their methodological quality using quality assessment of diagnostic accuracy studies (QUADAS) tools; relevant data were abstracted. The random-effects method was used to summarize sensitivities, specificities, positive and negative likelihood ratios, and diagnostic odds ratios (DORs) from all 29 studies.

Results

The pooled sensitivity, specificity, positive and negative likelihood ratios, and DOR for ANA were 0.650 (95% confidence interval [CI], 0.619 to 0.680), 0.751 (95%CI, 0.737 to 0.764), 3.030 (95%CI, 2.349 to 3.910), 0.464 (95%CI, 0.356 to 0.604), and 7.380 (95%CI, 4.344 to 12.539), respectively. For SMA, the values were 0.593 (95%CI, 0.564 to 0.621), 0.926 (95%CI, 0.917 to 0.934), 11.740 (95%CI, 7.379 to 18.678), 0.449 (95%CI, 0.367 to 0.549), and 31.553 (95%CI, 17.147 to 58.060), respectively. Finally, for anti-SLA/LP, the values were 0.194 (95%CI, 0.168 to 0.222), 0.989 (95%CI, 0.985 to 0.993), 11.089 (95%CI, 7.601 to 16.177), 0.839 (95%CI, 0.777 to 0.905), and 16.867 (95%CI, 10.956 to 25.967), respectively.

Authors’ conclusions

ANA provided moderate sensitivity and specificity, while SMA gave moderate sensitivity and high specificity, and anti-SLA/LP exhibited low sensitivity and high specificity. All three antibodies were limited by their unsatisfactory sensitivities and lack of consistency.

Evolutionary conservation and expression of human RNA-binding proteins and their role in human genetic disease

RNA-binding proteins (RBPs) are effectors and regulators of posttranscriptional gene regulation (PTGR). RBPs regulate stability, maturation, and turnover of all RNAs, often binding thousands of targets at many sites. The importance of RBPs is underscored by their dysregulation or mutations causing a variety of developmental and neurological diseases. This chapter globally discusses human RBPs and provides a brief introduction to their identification and RNA targets. We review RBPs based on common structural RNA-binding domains, study their evolutionary conservation and expression, and summarize disease associations of different RBP classes.

Liver immunology

The liver is the largest organ in the body and is generally regarded by nonimmunologists as having little or no lymphoid function. However, such is far from accurate. This review highlights the importance of the liver as a lymphoid organ. Firstly, we discuss experimental data surrounding the role of liver as a lymphoid organ. The liver facilitates tolerance rather than immunoreactivity, which protects the host from antigenic overload of dietary components and drugs derived from the gut and it is instrumental to fetal immune tolerance. Loss of liver tolerance leads to autoaggressive phenomena, which if not controlled by regulatory lymphoid populations, may lead to the induction of autoimmune liver diseases. Liver-related lymphoid subpopulations also act as critical antigen-presenting cells. The study of the immunological properties of liver and delineation of the microenvironment of the intrahepatic milieu in normal and diseased livers provides a platform to understand the hierarchy of a series of detrimental events that lead to immune-mediated destruction of the liver and the rejection of liver allografts. The majority of emphasis within this review will be on the normal mononuclear cell composition of the liver. However, within this context, we will discuss selected, but not all, immune-mediated liver disease and attempt to place these data in the context of human autoimmunity.

Structural mimicry between SLA/LP and Rickettsia surface antigens as a driver of autoimmune hepatitis: insights from an in silico study

BACKGROUND

Autoimmune hepatitis (AIH) is a chronic, progressive liver disease, characterized by continuing hepatocellular inflammation and necrosis. A subgroup of AIH patients presents specific autoantibodies to soluble liver antigen/liver-pancreas (SLA/LP) protein, which is regarded as a highly specific diagnostic marker. Autoantigenic SLA/LP peptides are targeted by CD4+ T cells, and restricted by the allele HLA-DRB1*03:01, which confers disease susceptibility in Europeans and Americans. A positively charged residue at position 71 has been indicated as critical for AIH susceptibility in all of the HLA alleles identified to date. Though the exact molecular mechanisms underlying pathogenesis of AIH are not clear, molecular mimicry between SLA/LP and viral/bacterial antigens has been invoked.

METHODS

The immunodominant region of SLA/LP was used as query in databank searches to identify statistically significant similarities with viral/bacterial peptides. Homology modeling and docking was used to investigate the potential interaction of HLA-DRB1*03:01 with the identified peptides. By molecular mechanics means, the interactions and energy of binding at the HLA binding site was also scrutinized.

RESULTS

A statistically significant structural similarity between the immunodominant regions of SLA/LP and a region of the surface antigen PS 120 from Rickettsia spp. has been detected. The interaction of the SLA/LP autoepitope and the corresponding Rickettsia sequence with the allele HLA-DRB1*03:01 has been simulated. The obtained results predict for both peptides a similar binding mode and affinity to HLA-DRB1*03:01. A "hot spot" of interaction between HLA-DRB1*03:01 and PS 120 is located at the P4 binding pocket, and is represented by a salt bridge involving Lys at position 71 of the HLA protein, and Glu 795 of PS120 peptide.

CONCLUSIONS

These findings strongly support the notion that a molecular mimicry mechanism can trigger AIH onset. CD4+ T cells recognizing peptides of SLA/LP could indeed cross-react with foreign Rickettsia spp. antigens. Finally, the same analysis suggests a molecular explanation for the importance of position 71 in conferring the susceptibility of the allele HLA-DRB1*03:01 to AIH. The lack of a positive charge at such position could prevent HLA alleles from binding the foreign peptides and triggering the molecular mimicry event.

Development of a recombinant cell-based indirect immunofluorescence assay for the determination of autoantibodies against soluble liver antigen in autoimmune hepatitis

Autoantibodies against soluble liver antigen (SLA) are specific markers for autoimmune hepatitis (AIH) type 1. In contrast to the determination of other AIH-associated autoantibodies by indirect immunofluorescence assay (IFA), detection of anti-SLA relied up to now on ELISA or immunoblot based on bacterially expressed recombinant protein. In order to develop a complementary IFA substrate, SLA isoform 1 was recombinantly produced in the human cell line HEK293 and controlled by a rabbit hyperimmune serum against SLA. The recombinant cells were used in IFA (RC-IFA) to analyze sera from 20 AIH patients with anti-SLA positivity predetermined by ELISA together with 80 controls (20 anti-SLA negative AIH, 15 primary biliary cirrhosis, 15 HCV, and 30 healthy blood donors). Using RC-IFA, anti-SLA was detected in all ELISA positive AIH sera but in none of the controls. Furthermore, a cytosolic fraction of HEK293 containing SLA was able to neutralize the autoantibodies in all positive sera in a dose-dependent manner. HEK293 cells expressing SLA are a valid substrate for the serodiagnosis of AIH relevant autoantibodies by IFA. In concert with cryosections of primate liver, rat kidney, rat liver, rat stomach, and HEp-2 cells, they enable the parallel determination of all autoantibodies associated with autoimmune liver diseases.

Selenium. Role of the essential metalloid in health

Selenium is an essential micronutrient in mammals, but is also recognized as toxic in excess. It is a non-metal with properties that are intermediate between the chalcogen elements sulfur and tellurium. Selenium exerts its biological functions through selenoproteins. Selenoproteins contain selenium in the form of the 21st amino acid, selenocysteine (Sec), which is an analog of cysteine with the sulfur-containing side chain replaced by a Se-containing side chain. Sec is encoded by the codon UGA, which is one of three termination codons for mRNA translation in non-selenoprotein genes. Recognition of the UGA codon as a Sec insertion site instead of stop requires a Sec insertion sequence (SECIS) element in selenoprotein mRNAs and a unique selenocysteyl-tRNA, both of which are recognized by specialized protein factors. Unlike the 20 standard amino acids, Sec is biosynthesized from serine on its tRNA. Twenty-five selenoproteins are encoded in the human genome. Most of the selenoprotein genes were discovered by bioinformatics approaches, searching for SECIS elements downstream of in-frame UGA codons. Sec has been described as having stronger nucleophilic and electrophilic properties than cysteine, and Sec is present in the catalytic site of all selenoenzymes. Most selenoproteins, whose functions are known, are involved in redox systems and signaling pathways. However, several selenoproteins are not well characterized in terms of their function. The selenium field has grown dramatically in the last few decades, and research on selenium biology is providing extensive new information regarding its importance for human health.

Advances in the current treatment of autoimmune hepatitis

Current treatment strategies for autoimmune hepatitis are complicated by frequent relapse after drug withdrawal, medication intolerance, and refractory disease. The objective of this review is to describe advances that have improved treatment outcomes by defining the optimum objectives of initial therapy, managing relapse more effectively, identifying problematic patients early, and incorporating the new pharmacological interventions that have emerged as frontline and salvage therapies. Initial corticosteroid treatment should be continued until serum aminotransferase, γ-globulin, and immunoglobulin G levels are normal, and maintenance of this improvement for 3-8 months before liver tissue assessment. Improvement to normal liver tissue is the ideal histological result that justifies drug withdrawal, but it is achievable in only 22 % of patients. Minimum portal hepatitis, inactive cirrhosis, or minimally active cirrhosis is the most common treatment end point. Relapse after drug withdrawal warrants institution of a long-term maintenance regimen, preferably with azathioprine. Mathematical models can identify problematic adult patients early, as also can clinical phenotype (age ≤ 30 years and HLA DRB1 03), rapidity of treatment response (≤ 24 months), presence of antibodies to soluble liver antigen, and non-white ethnicity. The calcineurin inhibitors (cyclosporine and tacrolimus) can be effective in steroid-refractory disease; mycophenolate mofetil can be corticosteroid-sparing and effective for azathioprine intolerance; budesonide combined with azathioprine can be effective for treatment-naïve, non-cirrhotic patients. Standard treatment regimens for autoimmune hepatitis can be upgraded without adjustments that require major new expertise.

Prognostic implications of antibodies to Ro/SSA and soluble liver antigen in type 1 autoimmune hepatitis

BACKGROUND

Antibodies to soluble liver antigen are frequently co-expressed with antibodies to ribonucleoprotein/Sjögren's syndrome A (Ro/SSA) in autoimmune hepatitis.

AIMS

Our goals were to evaluate the prognostic implications of antibodies to Ro/SSA in type 1 autoimmune hepatitis and to determine their independence from antibodies to soluble liver antigen.

METHODS

Three hundred and seventy-six serum samples from 170 patients were tested by enzyme immunoassays.

RESULTS

Sixty-five patients (38%) had antibodies to Ro52; 11 patients (6%) had antibodies to Ro60; and 27 patients had antibodies to soluble liver antigen (16%). Twenty-six patients with antibodies to Ro52 had antibodies to soluble liver antigen (40%), and 26 patients with antibodies to soluble liver antigen had antibodies to Ro52 (96%). Patients with antibodies to Ro52 and antibodies to soluble liver antigen had a higher frequency of human leucocyte antigen (HLA) DRB1(*) 03 (78 vs 50%, P=0.05) and lower occurrence of HLA DRB1(*) 04 (22 vs 57%, P=0.01) than patients with antibodies to Ro52 alone. Antibodies to Ro52 alone [hazard ratio (HR), 2.90; 95% confidence interval (CI), 1.18-7.14, P=0.02] and antibodies to Ro52 in conjunction with antibodies to soluble liver antigen (HR, 2.98; 95% CI, 1.07-8.43, P=0.04) were independently associated with the development of cirrhosis and hepatic death or liver transplantation.

CONCLUSIONS

Antibodies to Ro52 alone and antibodies to Ro52 in conjunction with antibodies to soluble liver antigen are independently associated with a poor prognosis in type 1 autoimmune hepatitis. The prognostic implications ascribed to antibodies to soluble liver antigen may reflect their almost invariable concurrence with antibodies to Ro52.

Therapy of autoimmune hepatitis

Autoimmune hepatitis was the first chronic liver disease with a favourable response to drug therapy and a dismal prognosis when left untreated. Since its original description in 1950 and first treatment studies the basic therapeutic strategy of inducing remission with steroids and azathioprine has not been modified in principle. A timely diagnosis before cirrhosis develops, the avoidance of immunosuppressant side effect, non-responders to standard induction therapy, and adherence to therapy are the greatest challenges. Alternative immunosuppressive drugs have been tested in small series and include transplant immunosuppressants. A recent large multicenter prospective treatment trial suggests that budesonide may offer an alternative in non-cirrhotic AIH patients capable of minimizing unwanted steroid effects. The ultimate treatment approach upon drug treatment failure is liver transplantation. Only four percent of transplant candidates are transplanted for AIH but the risk for graft loss because of recurrence has to be considered and recurrent AIH treated after transplantation.

Identification of T cell epitopes on soluble liver antigen in Chinese patients with auto-immune hepatitis

AIM

To identify soluble liver antigen (SLA)-specific dominant epitopes and analyse the correlation between SLA-specific T cell response and the status of the disease.

METHODS

A cross-sectional analysis of SLA-specific T cell responses to 54 overlapping peptides covering the entire SLA sequence was performed using an interferon (IFN)-γ ELISpot assay in 31 patients with auto-immune hepatitis (AIH)-1, 15 patients with primary biliary cirrhosis, 16 hepatitis B virus, seven hepatitis C virus infection and 10 healthy subjects, in order to assess the correlation between SLA-specific T cell responses and the clinical outcome.

RESULTS

Soluble liver antigen-specific IFN-γ responses in AIH were significantly more frequent in AIH patients (58.1%) than those in controls (6.7% in PBC, P=0.001; 4.3% in hepatitis B/C, P<0.001 and 0% in healthy subjects, P=0.0015). Among 31 AIH patients, the frequency of recognition and the magnitude of response to SLA peptides in anti-SLA antibody-positive patients were higher and stronger than those negative for anti-SLA antibodies (P=0.02 and 0.037 respectively). We further analysed T-cell restriction and found that six individual SLA peptides (4, 9, 11, 12, 41 and 44) were recognized by CD4 T cells, and the most frequently recognized peptides were peptides 12 (61.1% of participants), followed by peptide 4 and peptide 44 (55.6 and 38.9% respectively). Moreover, a positive association was found between the breadth of recognition of SLA peptides and the indices of liver damage.

CONCLUSION

T cell response to SLA in Chinese patients with AIH is broad and associated with hepatocyte damage.

Biosynthesis of selenocysteine, the 21st amino acid in the genetic code, and a novel pathway for cysteine biosynthesis

The biosynthetic pathway for selenocysteine (Sec), the 21st amino acid in the genetic code whose codeword is UGA, was recently determined in eukaryotes and archaea. Sec tRNA, designated tRNA([Ser]Sec), is initially aminoacylated with serine by seryl-tRNA synthetase and the resulting seryl moiety is converted to phosphoserine by O-phosphoseryl-tRNA kinase to form O-phosphoseryl-tRNA([Ser]Sec). Sec synthase (SecS) then uses O-phosphoseryl-tRNA([Ser]Sec) and the active donor of selenium, selenophosphate, to form Sec-tRNA([Ser]Sec). Selenophosphate is synthesized from selenide and ATP by selenophosphate synthetase 2 (SPS2). Sec was the last protein amino acid in eukaryotes whose biosynthesis had not been established and the only known amino acid in eukaryotes whose biosynthesis occurs on its tRNA. Interestingly, sulfide can replace selenide to form thiophosphate in the SPS2-catalyzed reaction that can then react with O-phosphoseryl-tRNA([Ser]Sec) in the presence of SecS to form cysteine-(Cys-)tRNA([Ser]Sec). This novel pathway of Cys biosynthesis results in Cys being decoded by UGA and replacing Sec in normally selenium-containing proteins (selenoproteins). The selenoprotein, thioredoxin reductase 1 (TR1), was isolated from cells in culture and from mouse liver for analysis of Cys/Sec replacement by MS. The level of Cys/Sec replacement in TR1 was proportional to the level of selenium in the diet of the mice. Elucidation of the biosynthesis of Sec and Sec/Cys replacement provides novel ways of regulating selenoprotein functions and ultimately better understanding of the biological roles of dietary selenium.

Human SepSecS or SLA/LP: selenocysteine formation and autoimmune hepatitis

Selenocysteine, the 21st genetically encoded amino acid, is the major form of the antioxidant trace element selenium in the human body. In eukaryotes and archaea its synthesis proceeds through a phosphorylated intermediate in a tRNA-dependent fashion. The final step of selenocysteine formation is catalyzed by O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS) that converts phosphoseryl-tRNA(Sec) to selenocysteinyl-tRNA(Sec). The human SepSecS protein is also known as soluble liver antigen/liver pancreas (SLA/LP), which represents one of the antigens of autoimmune hepatitis. Here we review the discovery of human SepSecS and the current understanding of the immunogenicity of SLA/LP in autoimmune hepatitis.

Current topics in autoimmune hepatitis

Autoimmune hepatitis is a chronic liver disease of unknown aetiology characterized by interface hepatitis, hypergammaglobulinaemia and circulating autoantibodies. In the last decade a number of advancements have been made in the field of clinical and basic research: the simplified diagnostic criteria, the complete response defined as normalization of transaminase levels, the molecular identification of the antigenic targets of anti-liver cytosol antibody type 1 and anti-soluble liver antigen, the detection of anti-actin antibodies, the description of de novo autoimmune hepatitis after liver transplantation for non-autoimmune liver diseases, the characterization of autoimmune hepatitis with overlapping features of primary biliary cirrhosis or primary sclerosing cholangitis, the preliminary experience with novel treatment strategies based on cyclosporine, mycophenolate mofetil and budesonide, the role played by "impaired" regulatory T cells and the development of novel animal models of autoimmune hepatitis.

Autoimmune hepatitis

Autoimmune hepatitis is a chronic inflammatory disease of the liver with a dismal prognosis when left untreated. Key for the improvement of prognosis is a timely diagnosis before cirrhosis has developed. This is reached by the exclusion of other causes of hepatitis, elevated immunoglobulin G, autoantibody profile and histological assessment. Treatment achieves remission rates in 80% of individuals and consists of immunosuppression with corticosteroids and azathioprine. A recent randomised controlled multicenter trial has added budesonide to the effective treatment options in non-cirrhotic patients and leads to a reduction of unwanted steroid side effects. Autoimmune hepatitis is an autoimmune disease of unknown aetiology. Association studies of major histocompatibility complex and other genes demonstrate an influence of immunogenetics. However, apart from the autoimmune polyglandular syndrome type 1, in which 10% of patients suffer from an autoantibody-positive autoimmune hepatitis linked to mutations of the autoimmune regulator gene, there is no clear evidence for a hereditary aetiology of this disease.

Autoantibodies as prognostic markers in autoimmune liver disease

Certain autoantibodies in autoimmune liver disease have prognostic implications that are under-utilized and under-developed. The goals of this review are to indicate progress in characterizing the autoantibodies with prognostic connotations and to indicate the feasibility and importance of discovering other markers. Prime source and review articles in English were selected by a Medline search through 2010. Antibodies to soluble liver antigen, actin, liver cytosol type 1, asialoglycoprotein receptor, chromatin, cyclic citrullinated peptide, and uridine glucuronosyltransferases have been associated with the occurrence, severity, and progression of autoimmune hepatitis, and antibodies to Sp100, gp210, and centromere have had similar implications in primary biliary cirrhosis. Antibodies to soluble liver antigen have shown the most promise in autoimmune hepatitis as they have been associated with severe histological changes, long durations of treatment, relapse after drug withdrawal, and high frequency of liver failure. Antibodies to the nuclear rim pore protein, gp210, have shown the most promise in primary biliary cirrhosis as they have been associated with severe interface hepatitis, lobular inflammation, and progression to liver failure. The major limitations of the autoantibodies have been their lack of standardized assays, low negative predictabilities, and fluctuating levels. Performance parameters will improve as critical pathogenic pathways, comprehensive testing batteries, and standardized assays through international exchange workshops are developed. Progress has been made in identifying the serological markers of prognosis in autoimmune liver disease, and they promise to reflect critical disease mechanisms and enhance patient management.

Advances in the diagnosis, pathogenesis, and management of autoimmune hepatitis

Autoimmune hepatitis (AIH) is characterized by chronic inflammation of the liver, interface hepatitis (based on histologic examination), hypergammaglobulinemia, and production of autoantibodies. Many clinical and basic science studies have provided important insights into the pathogenesis and treatment of AIH. Transgenic mice that express human antigens and develop autoantibodies, liver-infiltrating CD4(+) T cells, liver inflammation, and fibrosis have been developed as models of AIH. AIH has been associated with autoantibodies against members of the cytochrome P450 superfamily of enzymes, transfer RNA selenocysteine synthase, formiminotransferase cyclodeaminase, and the uridine diphosphate glucuronosyltransferases, whereas alleles such as DRB1*0301 and DRB1*0401 are genetic risk factors in white North American and northern European populations. Deficiencies in the number and function of CD4(+)CD25(+) (regulatory) T cells disrupt immune homeostasis and might be corrected as a therapeutic strategy. Treatment can be improved by continuing corticosteroid therapy until normal liver test results and normal liver tissue are within normal limits, instituting ancillary therapies to prevent drug-related side effects, identifying problematic patients early, and providing long-term maintenance therapy after patients experience a first relapse. Calcineurin inhibitors and mycophenolate mofetil are potential salvage therapies, and reagents such as recombinant interleukin-10, abatacept, and CD3-specific antibodies are feasible as therapeutics. Liver transplantation is an effective salvage therapy, even in the elderly, and AIH must be considered in all patients with graft dysfunction after liver transplantation. Identification of the key defects in immune homeostasis and antigen targets will direct new therapies.

SLA/LP/tRNP((Ser)Sec) antigen in autoimmune hepatitis: identification of the native protein in human hepatic cell extract

A diagnostic subgroup of AIH type 1 is characterized by specific serum antibodies against soluble liver protein. The respective autoantigen was named SLA/LP/tRNP((Ser)Sec), after three homologous recombinant polypeptides were isolated from expression gene libraries. We analyzed human cultured liver cells for the human homologue of recombinant SLA/LP/tRNP((Ser)Sec) by antigen purification. In addition, a monoclonal antibody was generated against recombinant SLA-p35, a truncated recombinant SLA-reactive polypeptide. With a positive patient serum, immune affinity chromatography was performed on the 52 kD-SLA main antigenic determinant pre-enriched by ion exchange chromatography. By mass spectrometry, the 52 kD-SLA/LP/tRNP ((Ser)Sec) autoantigen was unambiguously identified in the purification product. The identity of the recombinant SLA-p35 and its human homologue was further confirmed by a specific signal of the anti SLA-p35 monoclonal antibody with purified human SLA/LP/tRNP((Ser)Sec). The 48 kD-SLA species frequently comigrating in SLA-immunoblotting however was not identified by either approach. We conclude that the native counterpart of recombinant tRNP((Ser)(Sec)) indeed is detectable with a molecular weight of 52 kD in soluble liver extract of human cells as the major antigenic component of SLA/LP/tRNP((Ser)Sec).

The human SepSecS-tRNASec complex reveals the mechanism of selenocysteine formation

Selenocysteine is the only genetically encoded amino acid in humans whose biosynthesis occurs on its cognate transfer RNA (tRNA). O-Phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS) catalyzes the final step of selenocysteine formation by a poorly understood tRNA-dependent mechanism. The crystal structure of human tRNA(Sec) in complex with SepSecS, phosphoserine, and thiophosphate, together with in vivo and in vitro enzyme assays, supports a pyridoxal phosphate-dependent mechanism of Sec-tRNA(Sec) formation. Two tRNA(Sec) molecules, with a fold distinct from other canonical tRNAs, bind to each SepSecS tetramer through their 13-base pair acceptor-TPsiC arm (where Psi indicates pseudouridine). The tRNA binding is likely to induce a conformational change in the enzyme's active site that allows a phosphoserine covalently attached to tRNA(Sec), but not free phosphoserine, to be oriented properly for the reaction to occur.

Selenoproteins in Archaea and Gram-positive bacteria

Selenium is an essential trace element for many organisms by serving important catalytic roles in the form of the 21st co-translationally inserted amino acid selenocysteine. It is mostly found in redox-active proteins in members of all three domains of life and analysis of the ever-increasing number of genome sequences has facilitated identification of the encoded selenoproteins. Available data from biochemical, sequence, and structure analyses indicate that Gram-positive bacteria synthesize and incorporate selenocysteine via the same pathway as enterobacteria. However, recent in vivo studies indicate that selenocysteine-decoding is much less stringent in Gram-positive bacteria than in Escherichia coli. For years, knowledge about the pathway of selenocysteine synthesis in Archaea and Eukarya was only fragmentary, but genetic and biochemical studies guided by analysis of genome sequences of Sec-encoding archaea has not only led to the characterization of the pathways but has also shown that they are principally identical. This review summarizes current knowledge about the metabolic pathways of Archaea and Gram-positive bacteria where selenium is involved, about the known selenoproteins, and about the respective pathways employed in selenoprotein synthesis.

How an obscure archaeal gene inspired the discovery of selenocysteine biosynthesis in humans

Selenocysteine (Sec) is the 21st genetically encoded amino acid found in organisms from all three domains of life. Sec biosynthesis is unique in that it always proceeds from an aminoacyl-tRNA precursor. Even though Sec biosynthesis in bacteria was established almost two decades ago, only recently the pathway was elucidated in archaea and eukaryotes. While other aspects of Sec biology have been reviewed previously (Allmang and Krol, Biochimie 2006;88:1561-1571, Hatfield et al., Prog Nucleic Acid Res Mol Biol 2006;81:97-142, Squires and Berry, IUBMB Life 2008;60:232-235), here we review the biochemistry and evolution of Sec biosynthesis and coding and show how the knowledge of an archaeal cysteine biosynthesis pathway helped to uncover the route to Sec formation in archaea and eukaryotes.

The selenium to selenoprotein pathway in eukaryotes: more molecular partners than anticipated

The amino acid selenocysteine (Sec) is the major biological form of the trace element selenium. Sec is co-translationally incorporated in selenoproteins. There are 25 selenoprotein genes in humans, and Sec was found in the active site of those that have been attributed a function. This review will discuss how selenocysteine is synthesized and incorporated into selenoproteins in eukaryotes. Sec biosynthesis from serine on the tRNA(Sec) requires four enzymes. Incorporation of Sec in response to an in-frame UGA codon, otherwise signaling termination of translation, is achieved by a complex recoding machinery to inform the ribosomes not to stop at this position on the mRNA. A number of the molecular partners acting in this machinery have been identified but their detailed mechanism of action has not been deciphered yet. Here we provide an overview of the literature in the field. Particularly striking is the higher than originally envisaged number of factors necessary to synthesize Sec and selenoproteins. Clearly, selenoprotein synthesis is an exciting and very active field of research.

[Antibodies in systemic autoimmune diseases. Special mention to systemic lupus erythematosus]

Autoantibodies are the expression of humoral response to self-antigens and they may be diagnostic of autoimmune diseases. Studies in systemic lupus erythematosus (SLE) have shown that autoantibodies react with macromolecular structures such as the nucleosome or the spliceosome. These self-antigens are complexes of protein-DNA or protein-RNA like those recognized by anti-dsDNA or anti-RNPs (U1, Sm, Ro, La) antibodies respectively. Recent knowledge on innate immunity has shed more light on the pathological role of these autoantibodies. The antigen-antibody complexes formed as the result of an increase of sel-antigens in the blood as a consequence of an increase in apoptosis, attach to dendritic FcγII or B cell receptors. Through the attachment to the receptor, the macrocomplex is internalized within the cell and recognized in the endosomic membranes by receptors of the innate immune system named TLR (Toll-like receptor). There are at least 13 TLRs localized either in the cellular or the endosomic membranes. Of the latter group, TLR-7 is specific for ssRNA, and TLR-9 is specific for CpG DNA. The reaction of the immunocomplexes with the receptor triggers a kinase cascade that leads to IFNα production. The IFNα is a molecule of the innate and adaptative immune system responsible for the immune deregulation and pathological signs in the SLE. It plays an important role in antigen presentation to the autoreactive quiescent autoreactive T cells and in increasing the life span of dendritic and B cells. In addition, the increase in self-antigens released by greater apoptosis enhances the production of autoantibodies and their effect on the increase of IFNα production.

Structural insights into RNA-dependent eukaryal and archaeal selenocysteine formation

The micronutrient selenium is present in proteins as selenocysteine (Sec). In eukaryotes and archaea, Sec is formed in a tRNA-dependent conversion of O-phosphoserine (Sep) by O-phosphoseryl-tRNA:selenocysteinyl-tRNA synthase (SepSecS). Here, we present the crystal structure of Methanococcus maripaludis SepSecS complexed with PLP at 2.5 Å resolution. SepSecS, a member of the Fold Type I PLP enzyme family, forms an (α₂)₂ homotetramer through its N-terminal extension. The active site lies on the dimer interface with each monomer contributing essential residues. In contrast to other Fold Type I PLP enzymes, Asn247 in SepSecS replaces the conserved Asp in binding the pyridinium nitrogen of PLP. A structural comparison with Escherichia coli selenocysteine lyase allowed construction of a model of Sep binding to the SepSecS catalytic site. Mutations of three conserved active site arginines (Arg72, Arg94, Arg307), protruding from the neighboring subunit, led to loss of in vivo and in vitro activity. The lack of active site cysteines demonstrates that a perselenide is not involved in SepSecS-catalyzed Sec formation; instead, the conserved arginines may facilitate the selenation reaction. Structural phylogeny shows that SepSecS evolved early in the history of PLP enzymes, and indicates that tRNA-dependent Sec formation is a primordial process.

Structure and catalytic mechanism of eukaryotic selenocysteine synthase

In eukaryotes and Archaea, selenocysteine synthase (SecS) converts O-phospho-L-seryl-tRNA [Ser]Sec into selenocysteyl-tRNA [Ser]Sec using selenophosphate as the selenium donor compound. The molecular mechanisms underlying SecS activity are presently unknown. We have delineated a 450-residue core of mouse SecS, which retained full selenocysteyl-tRNA [Ser]Sec synthesis activity, and determined its crystal structure at 1.65 A resolution. SecS exhibits three domains that place it in the fold type I family of pyridoxal phosphate (PLP)-dependent enzymes. Two SecS monomers interact intimately and together build up two identical active sites around PLP in a Schiff-base linkage with lysine 284. Two SecS dimers further associate to form a homotetramer. The N terminus, which mediates tetramer formation, and a large insertion that remodels the active site set SecS aside from other members of the family. The active site insertion contributes to PLP binding and positions a glutamate next to the PLP, where it could repel substrates with a free alpha-carboxyl group, suggesting why SecS does not act on free O-phospho-l-serine. Upon soaking crystals in phosphate buffer, a previously disordered loop within the active site insertion contracted to form a phosphate binding site. Residues that are strictly conserved in SecS orthologs but variant in related enzymes coordinate the phosphate and upon mutation corrupt SecS activity. Modeling suggested that the phosphate loop accommodates the gamma-phosphate moiety of O-phospho-l-seryl-tRNA [Ser]Sec and, after phosphate elimination, binds selenophosphate to initiate attack on the proposed aminoacrylyl-tRNA [Ser]Sec intermediate. Based on these results and on the activity profiles of mechanism-based inhibitors, we offer a detailed reaction mechanism for the enzyme.

New developments in selenium biochemistry: selenocysteine biosynthesis in eukaryotes and archaea

We used comparative genomics and experimental analyses to show that (1) eukaryotes and archaea, which possess the selenocysteine (Sec) protein insertion machinery contain an enzyme, O-phosphoseryl-transfer RNA (tRNA) [Ser]Sec kinase (designated PSTK), which phosphorylates seryl-tRNA [Ser]Sec to form O-phosphoseryl-tRNA [Ser]Sec and (2) the Sec synthase (SecS) in mammals is a pyridoxal phosphate-containing protein previously described as the soluble liver antigen (SLA). SecS uses the product of PSTK, O-phosphoseryl-tRNA[Ser]Sec, and selenophosphate as substrates to generate selenocysteyl-tRNA [Ser]Sec. Sec could be synthesized on tRNA [Ser]Sec from selenide, adenosine triphosphate (ATP), and serine using tRNA[Ser]Sec, seryl-tRNA synthetase, PSTK, selenophosphate synthetase, and SecS. The enzyme that synthesizes monoselenophosphate is a previously identified selenoprotein, selenophosphate synthetase 2 (SPS2), whereas the previously identified mammalian selenophosphate synthetase 1 did not serve this function. Monoselenophosphate also served directly in the reaction replacing ATP, selenide, and SPS2, demonstrating that this compound was the active selenium donor. Conservation of the overall pathway of Sec biosynthesis suggests that this pathway is also active in other eukaryotes and archaea that contain selenoproteins.

From selenium to selenoproteins: synthesis, identity, and their role in human health

The requirement of the trace element selenium for life and its beneficial role in human health has been known for several decades. This is attributed to low molecular weight selenium compounds, as well as to its presence within at least 25 proteins, named selenoproteins, in the form of the amino acid selenocysteine (Sec). Incorporation of Sec into selenoproteins employs a unique mechanism that involves decoding of the UGA codon. This process requires multiple features such as the selenocysteine insertion sequence (SECIS) element and several protein factors including a specific elongation factor EFSec and the SECIS binding protein 2, SBP2. The function of most selenoproteins is currently unknown; however, thioredoxin reductases (TrxR), glutathione peroxidases (GPx) and thyroid hormone deiodinases (DIO) are well characterised selenoproteins involved in redox regulation of intracellular signalling, redox homeostasis and thyroid hormone metabolism. Recent evidence points to a role for selenium compounds as well as selenoproteins in the prevention of some forms of cancer. A number of clinical trials are either underway or being planned to examine the effects of selenium on cancer incidence. In this review we describe some of the recent progress in our understanding of the mechanism of selenoprotein synthesis, the role of selenoproteins in human health and disease and the therapeutic potential of some of these proteins.

Biosynthesis of selenocysteine on its tRNA in eukaryotes

Selenocysteine (Sec) is cotranslationally inserted into protein in response to UGA codons and is the 21st amino acid in the genetic code. However, the means by which Sec is synthesized in eukaryotes is not known. Herein, comparative genomics and experimental analyses revealed that the mammalian Sec synthase (SecS) is the previously identified pyridoxal phosphate-containing protein known as the soluble liver antigen. SecS required selenophosphate and O-phosphoseryl-tRNA^[Ser]Sec as substrates to generate selenocysteyl-tRNA^[Ser]Sec. Moreover, it was found that Sec was synthesized on the tRNA scaffold from selenide, ATP, and serine using tRNA^[Ser]Sec, seryl-tRNA synthetase, O-phosphoseryl-tRNA^[Ser]Sec kinase, selenophosphate synthetase, and SecS. By identifying the pathway of Sec biosynthesis in mammals, this study not only functionally characterized SecS but also assigned the function of the O-phosphoseryl-tRNA^[Ser]Sec kinase. In addition, we found that selenophosphate synthetase 2 could synthesize monoselenophosphate in vitro but selenophosphate synthetase 1 could not. Conservation of the overall pathway of Sec biosynthesis suggests that this pathway is also active in other eukaryotes and archaea that synthesize selenoproteins.

Sec synthase, a conserved protein responsible for the biosynthesis of the rare 21st amino acid, selenocysteine, is identified in eukaryotes, and the underlying biochemical pathway is characterized.

Biosynthesis of the 20 canonical amino acids is well established in eukaryotes. However, many eukaryotes also have a rare selenium-containing amino acid, selenocysteine, which is the 21st amino acid in the genetic code. Selenium is essential for human health, and its health benefits, including preventing cancer and heart disease and delaying aging, have been attributed to the presence of selenocysteine in protein. How selenocysteine is made in eukaryotes has not been established. To gain insight into its biosynthesis, we used computational analyses to search completely sequenced genomes for proteins that occur exclusively in organisms that utilize selenocysteine. This approach revealed a putative selenocysteine synthase, which had been previously identified as a pyridoxal phosphate–containing protein dubbed soluble liver antigen. We were able to characterize the activity of this synthase using selenophosphate and a tRNA aminoacylated with phosphoserine as substrates to generate selenocysteine. Moreover, identification of selenocysteine synthase allowed us to delineate the entire pathway of selenocysteine biosynthesis in mammals. Interestingly, selenocysteine synthase is present only in those archaea and eukaryotes that make selenoproteins, indicating that the newly defined pathway of selenocysteine biosynthesis is active in these domains of life.