Leucine zipper domain of 52 kDa SS-A/Ro promotes protein dimer formation and inhibits in vitro transcription activity

Anti-Ro52 Autoantibody Is Common in Systemic Autoimmune Rheumatic Diseases and Correlating with Worse Outcome when Associated with interstitial lung disease in Systemic Sclerosis and Autoimmune Myositis

This review highlights the 30 plus years research progress since the discovery of autoantibody to Ro52/TRIM21 in patients with systemic lupus erythematosus (SLE) and Sjögren's syndrome (SjS). After the initial expression cloning of the Ro52 cDNA, it has taken many years to the current understanding in the interesting biological function of Ro52 as an E3 ubiquitin ligase and its role in innate immune clearance of intracellular IgG-bound complex. Early observations show that anti-Ro52, mostly associated with anti-SS-A/Ro60 and/or anti-SS-B/La, is commonly found in SLE (40-70%), SjS (70-90%), neonatal lupus erythematosus (NLE, 75-90%), and subacute cutaneous lupus erythematosus (50-60%). Anti-Ro52 has long been postulated to play a direct pathogenic role in congenital heart block in NLE as well as in the QT interval prolongation in some adults. The widespread availability of the anti-Ro52 assay has led to the detection of anti-Ro52 in other diseases including autoimmune hepatitis (20-40%), systemic sclerosis (10-30%), and autoimmune myositis (20-40%). More than ten studies have pointed to an association of anti-Ro52 with interstitial lung disease and, more importantly, correlating with poor outcome and worse survival. Other studies are implicating an interesting role for anti-Ro52 in the diagnosis of certain cancers. Future studies are needed to examine the mechanism in the pathogenesis of anti-Ro52 and carefully documenting its causal relationships in different disease conditions.

HDAC6 regulates antibody-dependent intracellular neutralization of viruses via deacetylation of TRIM21

Tripartite motif-containing protein 21 (TRIM21) is a cytosolic antibody receptor that targets the internalized virus-antibody complex to the proteasome for degradation. However, the precise mechanism regulating TRIM21 activity is unknown. Here we show that TRIM21 is a substrate of histone deacetylase 6 (HDAC6) and that its function is regulated by acetylation. HDAC6 interacts with TRIM21 through its PRYSPRY motif and deacetylates TRIM21 at lysine 385 and lysine 387, thus promoting its homodimerization. Inhibiting HDAC6 activity increases TRIM21 acetylation, and hyperacetylation blocks TRIM21 dimerization and ubiquitination, preventing its binding to the virus-antibody complex and its degradation via the ubiquitin-proteasome pathway. HDAC6 depletion or inhibition increases virus accumulation in cells, indicative of an impaired capacity for antibody-dependent intracellular neutralization of viruses, whereas TRIM21 acetylation-deficient K385/387R mutant rescues HDAC6 depletion-caused ADIN impairment. These findings provide evidence for HDAC6 as a novel regulator of TRIM21-mediated intracellular innate immunity.

Precocious detection on amphibian oocyte lampbrush chromosomes of subtle changes in the cellular localisation of the Ro52 protein induced by in vitro culture

Subterminal lampbrush loops of one of the 12 bivalents of the oocyte karyotype of Pleurodeles waltl (Amphibian, Urodele) underwent prominent morphological changes upon in vitro culture. These loops exhibited a fine ribonucleoprotein (RNP) granular matrix, which evolved during culture into huge structures that we have named 'chaussons' (slippers). This phenomenon involved progressive accumulation of proteins in the RNP matrix without protein neosynthesis. One of these proteins, which translocated into the nucleus during the culture, was identified as a homolog of the human Ro52 E3 ubiquitin ligase. RNA polymerase III was also found to accumulate on the same loops. These results suggest that the subterminal loops of bivalent XII act as a storage site for the components of a nuclear machinery involved in the quality control of RNA synthesis and maturation in response to cellular stress. They also emphasise the considerable value of the lampbrush chromosome system for a direct visualisation of modifications in gene expression and open the question of a nuclear accumulation of Ro52 in human or animal oocytes cultured in vitro for assisted reproductive technologies (ART).

Redox-dependent oligomerization through a leucine zipper motif is essential for MG53-mediated cell membrane repair

We recently discovered that MG53, a muscle-specific tripartite motif (TRIM) family protein, functions as a sensor of oxidation to nucleate the assembly of cell membrane repair machinery. Our data showed that disulfide bond formation mediated by Cys242 is critical for MG53-mediated translocation of intracellular vesicles toward the injury sites. Here we test the hypothesis that leucine zipper motifs in the coiled-coil domain of MG53 constitute an additional mechanism that facilitates oligomerization of MG53 during cell membrane repair. Two leucine zipper motifs in the coiled-coil domain of MG53 (LZ1 - L176/L183/L190/V197 and LZ2 - L205/L212/L219/L226) are highly conserved across the different animal species. Chemical cross-linking studies show that LZ1 is critical for MG53 homodimerization, whereas LZ2 is not. Mutations of the conserved leucines into alanines in LZ1, not in LZ2, diminish the redox-dependent oligomerization of MG53. Live cell imaging studies demonstrate that the movement of green fluorescent protein (GFP)-tagged MG53 mutants (GFP-LA1 and GFP-LA2) is partially compromised in response to mechanical damage of the cell membrane, and the GFP-LA1/2 double mutant is completely ineffective in translocation toward the injury sites. In addition to the leucine zipper-mediated intermolecular interaction, redox-dependent cross talk between MG53 appears to be an obligatory step for cell membrane repair, since in vivo modification of cysteine residues with alkylating reagents can prevent the movement of MG53 toward the injury sites. Our data show that oxidation of the thiol group of Cys242 and leucine zipper-mediated interaction among the MG53 molecules both contribute to the nucleation process for MG53-mediated cell membrane repair.

Autoimmune response and target autoantigens in Sjogren's syndrome

BACKGROUND

Primary Sjogren's syndrome (pSS) is characterized by the presence of autoantibodies targeting mainly the Ro/La ribonucleoprotein complex. It is now appreciated that the production of autoantibodies is an antigen-driven immune response.

DESIGN

In this review, candidate mechanisms for autoantigen presentation and perpetuation of the autoimmune response within the autoimmune tissue lesion of pSS are discussed.

RESULTS

Several studies have shown that the epithelial cell in labial salivary glands of patients with Sjogren's syndrome is activated, bearing characteristics of an antigen-presenting cell, as suggested by inappropriate expression of class II HLA and co-stimulatory molecules. Other studies have confirmed that in salivary glands, there is an increased autoantigen presentation via apoptotic blebs and bodies, exosomes and heat shock protein-mediated cross-priming. There is also an increased expression of interferon (IFN)-induced genes, such as the autoantigen Ro52, which provide negative feedback regulation in inflammation. Ro60 and La autoantigens also appear to play a major role in the local autoimmune response in Sjogren's syndrome. In this regard, La and Ro60 the messenger RNA (mRNA) expression is upregulated in the affected salivary glands with different isoforms of La autoantigen mRNA to be expressed in patients with pSS. At the protein level, La/SSB in pSS salivary glands is found to be post-translationally modified.

CONCLUSIONS

 Autoantigen alterations in a microenvironment of local inflammation with increased in situ apoptosis, Toll-like receptor (TLR) signalling and antigen presentation may drive the autoimmune response and local autoantibody production in pSS.

Maternal MHC regulates generation of pathogenic antibodies and fetal MHC-encoded genes determine susceptibility in congenital heart block

Congenital heart block develops in fetuses of anti-Ro52 Ab-positive women. A recurrence rate of 20%, despite the persistence of maternal autoantibodies, indicates that there are additional, yet unidentified, factors critical for development of congenital heart block. In this study, we demonstrate that besides the maternal MHC controlling Ab specificity, fetal MHC-encoded genes influence fetal susceptibility to congenital heart block. Using MHC congenic rat strains, we show that heart block develops in rat pups of three strains carrying MHC haplotype RT1(av1) (DA, PVG.AV1, and LEW.AV1) after maternal Ro52 immunization, but not in LEW rats (RT1(l)). Different anti-Ro52 Ab fine specificities were generated in RT1(av1) versus RT1(l) animals. Maternal and fetal influence was determined in an F(2) cross between LEW.AV1 and LEW strains, which revealed higher susceptibility in RT1(l) than RT1(av1) pups once pathogenic Ro52 Abs were present. This was further confirmed in that RT1(l) pups more frequently developed heart block than RT1(av1) pups after passive transfer of RT1(av1) anti-Ro52 sera. Our findings show that generation of pathogenic Ro52 Abs is restricted by maternal MHC, whereas the fetal MHC locus regulates susceptibility and determines the fetal disease outcome in anti-Ro52-positive pregnancies.

Biochemical characterization of a recombinant TRIM5alpha protein that restricts human immunodeficiency virus type 1 replication

The rhesus monkey intrinsic immunity factor TRIM5alpha(rh) recognizes incoming capsids from a variety of retroviruses, including human immunodeficiency virus type 1 (HIV-1) and equine infectious anemia virus (EIAV), and inhibits the accumulation of viral reverse transcripts. However, direct interactions between restricting TRIM5alpha proteins and retroviral capsids have not previously been demonstrated using pure recombinant proteins. To facilitate structural and mechanistic studies of retroviral restriction, we have developed methods for expressing and purifying an active chimeric TRIM5alpha(rh) protein containing the RING domain from the related human TRIM21 protein. This recombinant TRIM5-21R protein was expressed in SF-21 insect cells and purified through three chromatographic steps. Two distinct TRIM5-21R species were purified and shown to correspond to monomers and dimers, as analyzed by analytical ultracentrifugation. Chemically cross-linked recombinant TRIM5-21R dimers and mammalian-expressed TRIM5-21R and TRIM5alpha proteins exhibited similar sodium dodecyl sulfate-polyacrylamide gel electrophoresis mobilities, indicating that mammalian TRIM5alpha proteins are predominantly dimeric. Purified TRIM5-21R had ubiquitin ligase activity and could autoubquitylate with different E2 ubiquitin conjugating enzymes in vitro. TRIM5-21R bound directly to synthetic capsids composed of recombinant HIV-1 CA-NC proteins and to authentic EIAV core particles. HIV-1 CA-NC assemblies bound dimeric TRIM5-21R better than either monomeric TRIM5-21R or TRIM5-21R constructs that lacked the SPRY domain or its V1 loop. Thus, our studies indicate that TRIM5alpha proteins are dimeric ubiquitin E3 ligases that recognize retroviral capsids through direct interactions mediated by the SPRY domain and demonstrate that these activities can be recapitulated in vitro using pure recombinant proteins.

Is SS-A/Ro52 a hydrogen peroxide-sensitive signaling molecule?

SS-A/Ro52 (Ro52) protein is one of the targets of autoantibodies in Sjogren's syndrome and systemic lupus erythematosus. Ro52 structurally belongs to the RING-B-box/coiled-coil family, which appears to carry out diverse functions, but the physiological function of Ro52 remains largely unknown. Here, the authors demonstrate that hydrogen peroxide but not other oxidative stressors induced translocation of Ro52 protein from the cytoplasm to the nucleus and this phenomenon was attenuated by inhibition of MAP kinases, ERK in particular. These findings raise the possibility that SS-A/Ro52 may function as a hydrogen peroxide-selective, oxidative stress-sensitive signaling molecule that is activated via the MAP kinase pathway.

Autoantibodies to intracellular autoantigens and their B-cell epitopes: molecular probes to study the autoimmune response

A common laboratory finding in systemic autoimmune diseases is the presence of autoantibodies against intracellular autoantigens. Although their pathogenesis is not fully understood, autoantibodies are important tools for establishing diagnosis, classification, and prognosis of autoimmune diseases. Autoantibodies mainly target multicomponent complexes containing both protein antigens and (ribo)-nucleic acid(s), such as the spliceosome or Ro/La RNPs. In this review, we address the main characteristics and the clinical value of the main autoantibody types with respect to their disease association, and we describe the corresponding autoantigens, their biologic function, and their B-cell antigenic determinants (epitopes). The structural characteristics and clinical associations of these epitopes, and their utility as tools to investigate the autoimmune response, are discussed in detail. New insights into the pathogenetic role of epitopes in systemic autoimmunity are also examined. In this regard, using the defined structures of the B-cell antigenic epitopes, complementary epitopes can be designed according to the "molecular recognition" theory. These complementary epitopes can be used as probes to study pathogenetic and immunoregulatory aspects of the anti-idiotypic response. The origin of humoral autoimmunity and the spreading of the epitopes in systemic lupus erythematosus are also discussed. Finally, the ability of post-translational modifications to induce autoreactive immune attack via the generation of neo-epitopes is summarized.

Function and subcellular location of Ro52β

Autoantigen Ro52alpha was recently identified as an E3 ubiquitin ligase. Its splicing variant Ro52beta, which lacks a leucine zipper, has not been characterized yet. We therefore characterized Ro52beta in contrast to Ro52alpha. Our biochemical assays revealed that both Ro52alpha and Ro52beta function as E3 ubiquitin ligases and self-ubiquitinate in cooperation with UbcH5B in vitro. In addition, both Ro52alpha and Ro52beta are ubiquitinated when overexpressed with ubiquitin in HEK293T cells, suggesting that both function as E3 ligases and self-ubiquitinate in vivo. However, cytological studies revealed that Ro52alpha mainly localizes to the cytoplasmic rod-like structures, whereas Ro52beta diffusely localizes to both the cytoplasm and the nucleus. Since the leucine zipper plays a role in the homodimerization and heterodimerization of Ro52alpha, the dimerization might be required for the localization of Ro52alpha to the rod-like structures. On the basis of these results, Ro52alpha and Ro52beta appear to ubiquitinate their particular substrates at different locations.

Structural, functional and immunologic characterization of folded subdomains in the Ro52 protein targeted in Sjögren's syndrome

Ro52, one of the major autoantigens in the rheumatic disease Sjögren's syndrome (SS), belongs to the tripartite motif (TRIM) or RING-B-box-coiled-coil (RBCC) protein family, thus comprising an N-terminal RING, followed by a B-box and a coiled-coil region. Several different proteomic functions have been suggested for Ro52, including DNA binding, protein interactions and Zn(2+)-binding. To analyze the presence and/or absence of these functions and, in particular, map those to different subregions, the modular composition of the Ro52 protein was experimentally characterized. Two structured parts of Ro52 were identified, corresponding to the RING-B-box and the coiled-coil regions, respectively. Secondary structure analysis by circular dichroism (CD) spectroscopy indicated that the two subregions are independently structured. The entire RING-B-box region displayed Zn(2+)-dependent stabilization against proteolysis in the presence of Zn2+, indicating functional Zn(2+)-binding sites in both the RING and the B-box. However, no stabilization with DNA was detected, irrespective of Zn(2+), thus suggesting that the RING-B-box region does not bind DNA. Oligomerization of the coiled-coil was investigated by analytical ultracentrifugation and in a mammalian two-hybrid system. Both methods show weak homodimer affinity, in parity with other coiled-coil domains involved in regulatory interactions. The C-terminal B30.2 region was rapidly degraded both during cellular expression and refolding, indicating a less stable structure. Immunologic analysis of the stable protein regions with sera from patients with Sjögren's syndrome shows that immunodominant epitopes to a large extent are localized in the structurally stable parts of Ro52. The results form a basis for further Ro52 functional studies on the proteome level.

Structural Organization and Zn2+-dependent Subdomain Interactions Involving Autoantigenic Epitopes in the Ring-B-box-Coiled-coil (RBCC) Region of Ro52

Ro52 is one of the major autoantigens targeted in the autoimmune disease Sjögren syndrome. By sequence similarity, Ro52 belongs to the RING-B-box-coiled-coil (RBCC) protein family. Disease-related antibodies bind Ro52 in a conformation-dependent way both in the coiled-coil region and in the Zn2+-binding Ring-B-box region. Primarily associated with Sjögren syndrome, Ro52 autoantibodies directed to a specific, partially structured epitope in the coiled-coil region may also induce a congenital heart block in the fetus of pregnant Ro52-positive mothers. To improve our understanding of the pathogenic effects of autoantibody binding to the Zn2+-binding region, a multianalytical mapping of its structural, biophysical, and antigenic properties is presented. Structure content and ligand binding of subregions, dissected by peptide synthesis and subcloning, were analyzed by fluorescence and circular dichroism spectroscopy. A novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry strategy for time-resolved proteolysis experiments of large protein domains was developed to facilitate analysis and to help resolve the tertiary arrangement of the entire RBCC subregion. The linker region between the RING and B-box motifs is crucial for full folding, and Zn2+ affinity of the RING-B-box region is further protected in the entire RBCC region and appears to interact with the coiled-coil region. Murine monoclonal antibodies raised toward the RING-B-box region were primarily directed toward the linker, further supporting a highly functional role for the linker in a cellular environment. Taken together with our previous analysis of autoantigenic epitopes in the coiled-coil region, localization of autoantigenic epitopes in Ro52 appears closely related to molecular functionalities.

Isolation and characterization of the LGT1 gene encoding a low-affinity glucose transporter from Torulaspora delbrueckii

Torulaspora delbrueckii PYCC 5321 displayed a mediated glucose transport activity best fitted assuming a biphasic Michaelis-Menten kinetics with a low- and a high-affinity component. A genomic library of this yeast strain was used to transform a mutant of Saccharomyces cerevisiae deficient in glucose transport. Sequence analysis of a DNA fragment cloned, revealed the presence of a 1704 bp length ORF. This ORF, named LGT1, displayed a high homology to yeast glucose transporter genes. Functional characterization of the LGT1 gene product in S. cerevisiae revealed that it encodes a low-affinity transporter, able to mediate the uptake of glucose and fructose. In consonance with this, expression of LGT1 in S. cerevisiae was high in media containing 4% of glucose and almost undetectable in galactose as sole carbon source. In the absence of glucose, repression of LGT1 expression required the transcription factor Rgt1p. However, a functional Rgt1p does not appear to be required for a full induction of LGT1 at high glucose levels. Deletion of the gene coding for the general repressor Mig1p had no effect on LGT1 expression, but additional disruption of MIG2 in a mig1 background indicated that Mig2p or both Mig1p and Mig2p in a redundant way, act as repressors of LGT1 expression at high glucose concentrations. The GeneBank Accession No. for LGT1 is AY598344.

Structurally derived mutations define congenital heart block-related epitopes within the 200-239 amino acid stretch of the Ro52 protein

Congenital heart block is a passively transferred autoimmune condition, which affects the children of mothers with Ro/SSA autoantibodies. During pregnancy, the antibodies are transported across the placenta and affect the fetus. We have previously demonstrated that antibodies directed to the 200-239 amino acid (aa) stretch of the Ro52 component of the Ro/SSA antigen correlate with the development of congenital heart block. In this report, we investigated the antibody-antigen interaction of this target epitope in detail at a molecular and structural level. Peptides representing aa 200-239 (p200) with structurally derived mutations were synthesized to define the epitopes recognized by two Ro52 human monoclonal antibodies, S3A8 and M4H1, isolated from patient-derived phage display libraries. Analyses by ELISA, circular dichroism and MALDI-TOF-MS demonstrate that the antibody recognition is dependent on a partly alpha-helical fold within the putative leucine zipper of the 200-239 aa stretch and that the two human anti-p200 monoclonal antibodies, M4H1 and S3A8, recognize different epitopic structures within the p200 peptide. In addition, we investigated the representation of each fine specificity within the sera of mothers with children born with congenital heart block, and in such sera, antibodies of the S3A8 idiotype were more commonly detected and at higher levels than M4H1-like antibodies.

Cloning and characterization of two human Ro52-specific monoclonal autoantibodies directed towards a domain associated with congenital heart block

Autoantibodies against amino acid 200-239 (p200) in the predicted leucine zipper region of the Ro52 protein are associated with congenital heart block, a potentially fatal condition that may affect fetuses of women with Ro52 autoantibodies. To allow detailed studies of the antibodies associated with congenital heart block, B-cell derived combinatorial antibody libraries from patients were screened for Ro52 and p200 specific antibody clones. Two human monoclonal anti-p200 antibody fragments, S3A8 and M4H1, were isolated and analysed with regard to VHand VL gene utilization, somatic mutations and binding properties. Both identified clones recognized recombinant and native intact Ro52, and reacted only with p200 in a set of related Ro52 peptides. The specificity and affinity was confirmed by biosensor measurements. Structural analysis of overlapping peptides revealed increased helicity in the p200 peptide compared to non-recognized peptides, indicating epitope conformation as essential for antibody binding. Both monoclonals produced punctate nuclear and diffuse cytoplasmic staining in human and mouse cell lines. The identified antibodies, which react specifically with the leucine zipper structure of Ro52, will be valuable in further exploration of the mechanisms operating during development of Ro52 antibody-associated congenital heart block.

SS-A/Ro52, an autoantigen involved in CD28-mediated IL-2 production

An autoantibody against SS-A/Ro52 (Ro52) is most frequently found in the sera of patients with Sjögren's syndrome, systemic lupus erythematosus, and congenital heart block from anti-Ro52 Ab-positive mother. However, the physiological function of the autoantigen SS-A/Ro52 has not yet been elucidated. In this study, we describe the role of Ro52 protein in T cell activation. Overexpression of SS-A/Ro52 in Jurkat T cell resulted in enhanced IL-2 production following CD28 stimulation. Furthermore, transfection of anti-Ro52-specific small RNA duplexes partially blocked the expression of native and overexpressed Ro52 in Jurkat T cell, resulting in decreased IL-2 production via CD28 pathway in these cells. Finally, intracellular localization of Ro52 dramatically changed following CD28 stimulation. Our data reveal a novel function of Ro52 in CD28-mediated pathway, which eventually contributes to cytokine production and expression of the T cell biological programs.