Intracellular localisation of the Ro 52kD auto-antigen in HeLa cells visualised with green fluorescent protein chimeras

TRIM21 Promotes Innate Immune Response to RNA Viral Infection through Lys27-Linked Polyubiquitination of MAVS

Human innate immunity responds to viral infection by activating the production of interferons (IFNs) and proinflammatory cytokines. The mitochondrial adaptor molecule MAVS plays a critical role in innate immune response to viral infection. In this study, we show that TRIM21 (tripartite motif-containing protein 21) interacts with MAVS to positively regulate innate immunity. Under viral infection, TRIM21 is upregulated through the IFN/JAK/STAT signaling pathway. Knockdown of TRIM21 dramatically impairs innate immune response to viral infection. Moreover, TRIM21 interacts with MAVS and catalyzes its K27-linked polyubiquitination, thereby promoting the recruitment of TBK1 to MAVS. Specifically, the PRY-SPRY domain of TRIM21 is the key domain for its interaction with MAVS, while the RING domain of TRIM21 facilitates the polyubiquitination chains of MAVS. In addition, the MAVS-mediated innate immune response is enhanced by both the PRY-SPRY and RING domains of TRIM21. Mutation analyses of all the lysine residues of MAVS further revealed that Lys325 of MAVS is catalyzed by TRIM21 for the K27-linked polyubiquitination. Overall, this study reveals a novel mechanism by which TRIM21 promotes the K27-linked polyubiquitination of MAVS to positively regulate innate immune response, thereby inhibiting viral infection.IMPORTANCE Activation of innate immunity is essential for host cells to restrict the spread of invading viruses and other pathogens. MAVS plays a critical role in innate immune response to RNA viral infection. In this study, we demonstrated that TRIM21 targets MAVS to positively regulate innate immunity. Notably, TRIM21 targets and catalyzes K27-linked polyubiquitination of MAVS and then promotes the recruitment of TBK1 to MAVS, leading to upregulation of innate immunity. Our study outlines a novel mechanism by which the IFN signaling pathway blocks RNA virus to escape immune elimination.

Autoantigen TRIM21/Ro52 as a Possible Target for Treatment of Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a chronic, systemic, and autoimmune disease, whose etiology is still unknown. Although there has been progress in the treatment of SLE through the use of glucocorticoid and immunosuppressive drugs, these drugs have limited efficacy and pose significant risks of toxicity. Moreover, prognosis of patients with SLE has remained difficult to assess. TRIM21/Ro52/SS-A1, a 52-kDa protein, is an autoantigen recognized by antibodies in sera of patients with SLE and Sjögren's syndrome (SS), another systemic autoimmune disease, and anti-TRIM21 antibodies have been used as a diagnostic marker for decades. TRIM21 belongs to the tripartite motif-containing (TRIM) super family, which has been found to play important roles in innate and acquired immunity. Recently, TRIM21 has been shown to be involved in both physiological immune responses and pathological autoimmune processes. For example, TRIM21 ubiquitylates proteins of the interferon-regulatory factor (IRF) family and regulates type I interferon and proinflammatory cytokines. In this paper, we summarize molecular features of TRIM21 revealed so far and discuss its potential as an attractive therapeutic target for SLE.

SSA/Ro52 autoantigen interacts with Dcp2 to enhance its decapping activity

We identified human decapping enzyme 2 (hDCP2) as a binding protein with Ro52, being colocalized in processing bodies (p-bodies). We also showed that the N-terminus and C-terminus of Ro52 bound to hDCP2. Moreover, Ro52 enhanced decapping activity of hDCP2 in a dose-dependent manner. Our data support the novel notion of the association between Ro52 with hDCP2 protein in cytoplasmic p-bodies, playing a role in mRNA metabolism in response to cellular stimulation.

Clinical and serological features of patients with autoantibodies to GW/P bodies

GW bodies (GWBs) are unique cytoplasmic structures involved in messenger RNA (mRNA) processing and RNA interference (RNAi). GWBs contain mRNA, components of the RNA-induced silencing complex (RISC), microRNA (miRNA), Argonaute proteins, the Ge-1/Hedls protein and other enzymes involving mRNA degradation. The objective of this study was to identify the target GWB autoantigens reactive with 55 sera from patients with anti-GWB autoantibodies and to identify clinical features associated with these antibodies. Analysis by addressable laser bead immunoassay (ALBIA) and immunoprecipitation of recombinant proteins indicated that autoantibodies in this cohort of anti-GWB sera were directed against Ge-1/Hedls (58%), GW182 (40%) and Ago2 (16%). GWB autoantibodies targeted epitopes that included the N-terminus of Ago2 and the nuclear localization signal (NLS) containing region of Ge-1/Hedls. Clinical data were available on 42 patients of which 39 were female and the mean age was 61 years. The most common clinical presentations were neurological symptoms (i.e. ataxia, motor and sensory neuropathy) (33%), Sjögren's syndrome (SjS) (31%) and the remainder had a variety of other diagnoses that included systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and primary biliary cirrhosis (PBC). Moreover, 44% of patients with anti-GWB antibodies had reactivity to Ro52. These studies indicate that Ge-1 is a common target of anti-GWB sera and the majority of patients in a GWB cohort had SjS and neurological disease.

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.

The Sjögren’s Syndrome-Associated Autoantigen Ro52 Is an E3 Ligase That Regulates Proliferation and Cell Death

Patients affected by Sjögren's syndrome and systemic lupus erythematosus (SLE) carry autoantibodies to an intracellular protein denoted Ro52. Although the serologic presence of Ro52 autoantibodies is used clinically for diagnostic purposes, the function of the protein or why it is targeted as an autoantigen in several rheumatic conditions has not been elucidated. In this study, we show that the expression of Ro52 is significantly increased in PBMC of patients with Sjögren's syndrome and SLE, and demonstrate that Ro52 is a RING-dependent E3 ligase involved in ubiquitination. Overexpression of Ro52, but not of Ro52 lacking the RING domain, in a mouse B cell line lead to decreased growth in steady state and increased cell death after activation via the CD40 pathway. The role of Ro52 in activation-mediated cell death was further confirmed as a reduction in Ro52 expression restored cell viability. These findings suggest that the increased expression of the Ro52 autoantigen in patients may be directly involved in the reduced cellular proliferation and increased apoptotic cell death observed in Sjögren's syndrome and SLE, and may thus contribute to the autoantigenic load and induction of autoimmune B and T cell responses observed in rheumatic patients.

Nuclear-targeting autoantibodies induced nuclear PARP cleavage accompanied by more pronounced decrease of peripheral white blood cells than Ro/SSA and La/SSB antigen-targeting autoantibodies

Autoantibody production and leukocytopenia may be linked in patients with lupus erythematosus (LE). Unclear is the ability of different autoantibody species to induce apoptosis and cell loss. Laboratory routine analyses (white blood cell counts, autoantibody detection), and flow cytometry (annexin V, CD3, CD4, CD8) have been performed in 126 consecutive LE-patients. Nuclei of PBMC were investigated flow cytometrically for the presence of the 85 kDa poly-(ADP-ribose)-polymerase (PARP) fragment. Peripheral total white blood cells (WBC), lymphocytes, T-cells, CD3+ CD4+, and CD3+ CD8+ cells were significantly decreased in patients with LE (P from 1.2 x 10(-14) to P < .0008). In the presence of either antinuclear (P from 1.2 x 10(-14) to P < .0008) or anti-dsDNA antibodies (P from 2.9 x 10(-12) to P < .007) were significantly diminished. Differences in cell numbers in LE patients with versus without anti-Ro/SSA were less pronounced: significant differences could be only obtained in lymphocytes and T-cells (P < .02). Anti-La/SSB antibodies were accompanied by significant increased leukocytes (P < .02). PARP cleavage (85 kDa) in nuclei was preferentially observed in cases with nuclear targeting autoantibodies. These results indicate that nuclear targeting autoantibodies are associated to lower peripheral blood cells counts than Ro/SSA, and La/SSB cytoplasmic targeting autoantibodies. This provides an explanation for the pathogenesis of cytopenias associated with SLE.

Subcellular redistribution and surface exposure of the Ro52, Ro60 and La48 autoantigens during apoptosis in human ductal epithelial cells: a possible mechanism in the pathogenesis of Sjögren's syndrome

The Ro52, Ro60 and La48 autoantigens are associated with Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE). The mechanisms behind tolerance breakdown of these self-peptides remain unclear; however, apoptosis has been proposed to cause their presentation to the immune system. We have examined the localization of transiently expressed enhanced green fluorescent protein (EGFP)-tagged Ro52, Ro60 and La48 autoantigens in a human salivary gland (HSG) cell line by laser confocal microscopy under normal growth conditions and during apoptosis. Surface exposure of Ro52, Ro60 and La48 was demonstrated on nonfixed apoptotic cells with monoclonal antibodies (MoAbs) or with primary SS patient antisera. Laser scanning cytometry determined the apoptotic frequency. EGFP alone was studied as control. We found that Ro52 mainly is cytoplasmic, Ro60 both nuclear and cytoplasmic, while La48 only resides in the nucleus under normal conditions. During early apoptosis, La48 is dramatically redistributed to the cytoplasm, while the localization of Ro52 and Ro60 is maintained. All three autoantigens filled apoptotic blebs and covered TUNEL (terminal-deoxynucleotidyl-transferase-mediated dUTP-digoxigenin nick end labelling)-positive apoptotic bodies. Identical results were obtained in COS-7 cells. We have developed a transfection system to study the intracellular localization of the three autoantigens Ro52, Ro60 and La48, without antibody detection. During apoptosis, there is an intracellular redistribution of endogenous and EGFP-tagged Ro52, Ro60 and La48, leading to surface exposure. These findings may indicate a role for apoptosis in the induction and facilitation of humoral responses to Ro52, Ro60 and La48 in the autoimmune exocrinopathy of SS.

The 52 000 MW Ro/SS-A autoantigen in Sjögren's syndrome/systemic lupus erythematosus (Ro52) is an interferon-gamma inducible tripartite motif protein associated with membrane proximal structures

The 52 000 MW Ro/SS-A (Ro52) protein is a major target of autoantibodies in autoimmune conditions such as systemic lupus erythematosus and Sjögren's syndrome. Recent genomic and bioinformatic studies have shown that Ro52 belongs to a large family of related RING/Bbox/coiled-coil (RBCC) tripartite motif proteins sharing overall domain structure and 40-50% identity at the amino acid level. Ro52 also has a B30.2 domain at the C-terminus. Using the human genome draft sequence, the genomic organization of the Ro52 gene on human chromosome 11p15.5 has been deduced and related to the protein domain structure. We show that the steady-state levels of Ro52 mRNA are normally very low but are induced by cell activation with interferon-gamma. In transient transfection of HeLa cells, epitope-tagged Ro52 protein was localized to unidentified membrane proximal rod-like structures. Using in vitro coupled transcription/translation followed by immunoprecipitation, the autoimmune response to Ro52 protein was investigated and two distinct interactions were resolved. The Ro52 C-terminal B30.2 domain interacts with human immunoglobulin independently of antibody specificities. Sera derived from patients with Sjögren's syndrome and systemic lupus erythematosus, in addition, contained specific autoantibodies directed towards the rest of the Ro52 molecule. The majority of these autoimmune sera also immunoprecipitated the Ro52-related molecule RNF15. A possible role for Ro52 protein in alterations of plasma membranes during cellular activation or apoptosis is discussed.

Oxidative stress mediates cell surface expression of SS-A/Ro antigen on keratinocytes

Exposure to ultraviolet radiation exacerbates the skin lesions of autoimmune diseases, and is known to induce cell surface expression of SS-A/Ro antigen on keratinocytes in vitro. Following up on recent reports on ultraviolet-B (UVB)-induced oxidative stress, we examined the role of oxidative stress in the surface expression of SS-A/Ro antigen on human keratinocytes. First, the exclusive induction by UVB irradiation of the 52-kDa protein (Ro52) but not of the 60-kDa protein (Ro60) of SS-A/Ro antigen was demonstrated by means of indirect immunofluorescence. The surface expression of Ro52 induced by UVB irradiation was concentration-dependently inhibited by N-acetyl-L-cysteine, an antioxidant. Furthermore, surface expression of Ro52 was similarly induced by diamide, a chemical oxidant. We next used Hoechst 33342 staining and the TUNEL assay to demonstrate that a low dose (20 mJ/cm(2)) of UVB did not induce apoptosis but induced the surface expression of Ro52. Moreover, zVAD-fmk, a pan-caspase inhibitor, did not inhibit UVB-induced surface expression of Ro52 even at a high dose (200 mJ/cm(2)) of UVB, which was sufficient to induce apoptosis in keratinocytes in the absence of zVAD-fmk. Taken together, we concluded that UVB-induced surface expression of Ro52 on keratinocytes is mediated by oxidative stress through a pathway other than apoptosis.

Interaction between 52 kDa SSA/Ro and deubiquitinating enzyme UnpEL: a clue to function

The detection of isolated heart block in utero strongly predicts the presence of maternal autoantibodies reactive with 52 kDa SSA/Ro. The mechanisms that underlie this observation may be elucidated by defining the function of the target antigen. The initial approach was to identify proteins interactive with 52Ro using transcriptional activity in the yeast 2-hybrid system. A cDNA library was constructed using RNA isolated from human fetal hearts (12-23 weeks) and cloned into the HybriZAP vector encoding the activation domain of GAL4(AD) as target. Approximately 7 x 10(6) cDNAs were cotransformed with the bait into YRG-2. Plasmids from five interactive colonies were sequenced and three identified as the specific human deubiquitinating enzyme, UnpEL. UnpEL did not interact with bait plasmid encoding 52 beta, an alternative leucine zipper-minus form of 52 kDa SSA/Ro which is maximally expressed in fetal life. The mammalian 2-hybrid assay confirmed the interaction between full-length 52Ro and UnpEL. Further support for a biologic interaction was the marked redistribution in cellular localization of UnpEL following cotransfection of the two proteins into cultured human cardiocytes, human renal carcinoma cells (293 cells), and monkey kidney fibroblasts (COS-1). In conclusion, the interaction of full-length 52Ro and UnpEL implies that the former may also be involved in the ubiquitin pathway, an observation of particular interest since 52Ro contains a RING finger domain, a motif common to several recently reported proteins involved in modulating ubiquitination. The absence of an interaction with 52 beta raises the consideration that regulation of protein ubiquitination might differ in fetal life.

The lin-41 RBCC gene acts in the C. elegans heterochronic pathway between the let-7 regulatory RNA and the LIN-29 transcription factor

Null mutations in the C. elegans heterochronic gene lin-41 cause precocious expression of adult fates at larval stages. Increased lin-41 activity causes the opposite phenotype, reiteration of larval fates. let-7 mutations cause similar reiterated heterochronic phenotypes that are suppressed by lin-41 mutations, showing that lin-41 is negatively regulated by let-7. lin-41 negatively regulates the timing of LIN-29 adult specification transcription factor expression. lin-41 encodes an RBCC protein, and two elements in the lin-413'UTR are complementary to the 21 nucleotide let-7 regulatory RNA. A lin-41::GFP fusion gene is downregulated in the tissues affected by lin-41 at the time that the let-7 regulatory RNA is upregulated. We suggest that late larval activation of let-7 RNA expression downregulates LIN-41 to relieve inhibition of lin-29.

Immunoglobulin variable genes and epitope recognition of human monoclonal anti-Ro 52-kd in primary Sjögren's syndrome

OBJECTIVE

To clone and characterize human anti-Ro/SSA autoantibodies from a patient with primary Sjögren's syndrome (pSS).

METHODS

Monoclonal antibodies (mAb) were raised from the peripheral blood of a patient with pSS using Epstein-Barr virus transformation and a hybridoma technique. Specificity was determined using cell extracts, recombinant Ro 52-kd, Ro 60-kd, and La proteins as well as Ro 52-kd peptides in enzyme-linked immunosorbent assay (ELISA) and Western blot. The immunofluorescence pattern was analyzed using cultured human and mouse cell lines. Complementary DNA was amplified by polymerase chain reaction, and Ig variable (V)-region genes were directly sequenced.

RESULTS

Two human anti-Ro 52-kd mAb of IgM isotype, denoted SG1 and SG3, were cloned from the peripheral blood of a patient with pSS. The 2 mAb reacted with the Ro 52-kd antigen in cell extracts of human cell lines and mouse cell lines, and with purified human recombinant Ro 52-kd protein in ELISA and Western blot. SG1 reacted specifically with 1 peptide, amino acids 136-156, of the Ro 52-kd protein, and SG3 was mapped to react with a recombinant fragment representing amino acids 136-292. Immunofluorescence studies revealed cytoplasmic staining with both mAb. Both were encoded by V(H)3-family genes. SG1 was highly homologous to the DP-77 germ-line gene, with 2 replacement mutations and 1 silent. It utilized the DPL-11 germ-line gene from the Vlambda2-family gene, with 1 silent mutation. SG3 was 100% homologous to the DP-47 germ-line gene, combined with a Vkappa1-family gene that was 100% homologous to the A30 germ-line gene.

CONCLUSION

Two human mAb were demonstrated to be specific for the Ro 52-kd protein and to be directed against 2 different epitopes, 1 linear and 1 conformation-dependent, within a region previously described to be immunodominant. Somatic hypermutation appeared to be of minor importance in generating these 2 specificities.