Human Ro ribonucleoprotein particles: characterization of native structure and stable association with the La polypeptide

Ro60-Roles in RNA Processing, Inflammation, and Rheumatic Autoimmune Diseases

The Ro60/SSA2 autoantigen is an RNA-binding protein and a core component of nucleocytoplasmic ribonucleoprotein (RNP) complexes. Ro60 is essential in RNA metabolism, cell stress response pathways, and cellular homeostasis. It stabilises and mediates the quality control and cellular distribution of small RNAs, including YRNAs (for the 'y' in 'cytoplasmic'), retroelement transcripts, and misfolded RNAs. Ro60 transcriptional dysregulation or loss of function can result in the generation and release of RNA fragments from YRNAs and other small RNAs. Small RNA fragments can instigate an inflammatory cascade through endosomal toll-like receptors (TLRs) and cytoplasmic RNA sensors, which typically sense pathogen-associated molecular patterns, and mount the first line of defence against invading pathogens. However, the recognition of host-originating RNA moieties from Ro60 RNP complexes can activate inflammatory response pathways and compromise self-tolerance. Autoreactive B cells may produce antibodies targeting extracellular Ro60 RNP complexes. Ro60 autoantibodies serve as diagnostic markers for various autoimmune diseases, including Sjögren's disease (SjD) and systemic lupus erythematosus (SLE), and they may also act as predictive markers for anti-drug antibody responses among rheumatic patients. Understanding Ro60's structure, function, and role in self-tolerance can enhance our understanding of the underlying molecular mechanisms of autoimmune conditions.

Prognostic value of Sjögren's syndrome autoantibodies

Sjögren's syndrome is in part considered an autoimmune disease because patient sera contain antibodies binding self-structures. In fact, in addition to anti-Ro (or SSA) and anti-La (or SSB), which are included in the classification criteria, there are a wide variety of autoantibodies found among these patients. We reviewed English-language MEDLINE sources. Anti-Ro and anti-La found among healthy individuals, including mothers giving birth to infants with neonatal lupus, predicts future connective tissue disease. Those with Sjögren's syndrome can be divided into two groups; patients with only exocrine gland involvement and those with systemic disease. The presence of anti-Ro/La is associated with systemic, extraglandular disease. Rheumatoid factor is also associated with extraglandular disease while anti-cyclic citrullinated peptide (CCP) is likely associated with inflammatory arthritis and progression to rheumatoid arthritis. Anti-mitochondrial antibodies are uncommon but predict progression to primary biliary cirrhosis. Cryoglobulinemia is found in excess among those with non-Hodgkin's lymphoma. Determination of autoantibodies on the sera of Sjögren's syndrome patients has prognostic implications for Sjögren's syndrome itself as well as associated diseases.

Coherent somatic mutation in autoimmune disease

BACKGROUND

Many aspects of autoimmune disease are not well understood, including the specificities of autoimmune targets, and patterns of co-morbidity and cross-heritability across diseases. Prior work has provided evidence that somatic mutation caused by gene conversion and deletion at segmentally duplicated loci is relevant to several diseases. Simple tandem repeat (STR) sequence is highly mutable, both somatically and in the germ-line, and somatic STR mutations are observed under inflammation.

RESULTS

Protein-coding genes spanning STRs having markers of mutability, including germ-line variability, high total length, repeat count and/or repeat similarity, are evaluated in the context of autoimmunity. For the initiation of autoimmune disease, antigens whose autoantibodies are the first observed in a disease, termed primary autoantigens, are informative. Three primary autoantigens, thyroid peroxidase (TPO), phogrin (PTPRN2) and filaggrin (FLG), include STRs that are among the eleven longest STRs spanned by protein-coding genes. This association of primary autoantigens with long STR sequence is highly significant (p<3.0x10(-7)). Long STRs occur within twenty genes that are associated with sixteen common autoimmune diseases and atherosclerosis. The repeat within the TTC34 gene is an outlier in terms of length and a link with systemic lupus erythematosus is proposed.

CONCLUSIONS

The results support the hypothesis that many autoimmune diseases are triggered by immune responses to proteins whose DNA sequence mutates somatically in a coherent, consistent fashion. Other autoimmune diseases may be caused by coherent somatic mutations in immune cells. The coherent somatic mutation hypothesis has the potential to be a comprehensive explanation for the initiation of many autoimmune diseases.

Putative sequences on Ro60 three-dimensional structure accessible for 4-hydroxy-2-nonenal (HNE) modification compared to in vitro HNE modification of Ro60 sequences

We have previously reported accelerated acquisition of new autoreactivity upon immunization with 4-hydroxy-2-nonenal (HNE)-modified Ro60, as well as differential induction of lupus or Sjögren's syndrome by immunization with Ro60 containing varying amounts of HNE. Since the number of HNE molecules on Ro60 appears to be important, we hypothesized that specific sequences on Ro60 are targets for HNE-modification. Using surface plasmon resonance (SPR) we have also shown intramolecular protein-protein interaction between Ro60 and Ro multiple antigenic peptides (MAPs). We also hypothesized that intramolecular protein-protein interaction would be abolished by HNE-modification. To test this hypotheses we investigated (a) the epitopes of Ro60, using 19 Ro MAPs in an in vitro assay (involving HNE-modification of MAPs following immobilization on ELISA plates) to identify targets of HNE modification on Ro60 and (b) the protein-protein interaction between unmodified Ro60 MAPs, immobilized on the sensor surface of BIAcore, and unmodified Ro60 or HNE-modified Ro60 using SPR. New data obtained with SPR strengthens our earlier observation that immunization with HNE-Ro60 induces a stronger response. Unmodified Ro60 bound to several Ro60 MAPs through protein-protein interaction analyzed using SPR. This interaction was totally abrogated using HNE-modified Ro60 suggesting that sequences on Ro had become modified with HNE. When 19 Ro60 MAPs were modified in vitro with HNE, it was found that 10/19 MAPs significantly bound HNE covalently (p<0.001 compared to MAPs binding HNE poorly). The amino acid sequences 126-137, 166-272 and 401-495 on Ro60 were strongly HNE modified. Using computational model system based on the recently published crystal structure for Ro60 enabled us to identify regions on the Ro60 molecule represented by the HNE-modified Ro MAPs, which are part of the exposed tertiary structure of the Ro60 protein.

Ribonucleoprotein particles containing non-coding Y RNAs, Ro60, La and nucleolin are not required for Y RNA function in DNA replication

BACKGROUND

Ro ribonucleoprotein particles (Ro RNPs) consist of a non-coding Y RNA bound by Ro60, La and possibly other proteins. The physiological function of Ro RNPs is controversial as divergent functions have been reported for its different constituents. We have recently shown that Y RNAs are essential for the initiation of mammalian chromosomal DNA replication, whereas Ro RNPs are implicated in RNA stability and RNA quality control. Therefore, we investigate here the functional consequences of RNP formation between Ro60, La and nucleolin proteins with hY RNAs for human chromosomal DNA replication.

METHODOLOGY/PRINCIPAL FINDINGS

We first immunoprecipitated Ro60, La and nucleolin together with associated hY RNAs from HeLa cytosolic cell extract, and analysed the protein and RNA compositions of these precipitated RNPs by Western blotting and quantitative RT-PCR. We found that Y RNAs exist in several RNP complexes. One RNP comprises Ro60, La and hY RNA, and a different RNP comprises nucleolin and hY RNA. In addition about 50% of the Y RNAs in the extract are present outside of these two RNPs. Next, we immunodepleted these RNP complexes from the cytosolic extract and tested the ability of the depleted extracts to reconstitute DNA replication in a human cell-free system. We found that depletion of these RNP complexes from the cytosolic extract does not inhibit DNA replication in vitro. Finally, we tested if an excess of recombinant pure Ro or La protein inhibits Y RNA-dependent DNA replication in this cell-free system. We found that Ro60 and La proteins do not inhibit DNA replication in vitro.

CONCLUSIONS/SIGNIFICANCE

We conclude that RNPs containing hY RNAs and Ro60, La or nucleolin are not required for the function of hY RNAs in chromosomal DNA replication in a human cell-free system, which can be mediated by Y RNAs outside of these RNPs. These data suggest that Y RNAs can support different cellular functions depending on associated proteins.

Conserved and divergent features of the structure and function of La and La-related proteins (LARPs)

Genuine La proteins contain two RNA binding motifs, a La motif (LAM) followed by a RNA recognition motif (RRM), arranged in a unique way to bind RNA. These proteins interact with an extensive variety of cellular RNAs and exhibit activities in two broad categories: i) to promote the metabolism of nascent pol III transcripts, including precursor-tRNAs, by binding to their common, UUU-3'OH containing ends, and ii) to modulate the translation of certain mRNAs involving an unknown binding mechanism. Characterization of several La-RNA crystal structures as well as biochemical studies reveal insight into their unique two-motif domain architecture and how the LAM recognizes UUU-3'OH while the RRM binds other parts of a pre-tRNA. Recent studies of members of distinct families of conserved La-related proteins (LARPs) indicate that some of these harbor activity related to genuine La proteins, suggesting that their UUU-3'OH binding mode has been appropriated for the assembly and regulation of a specific snRNP (e.g., 7SK snRNP assembly by hLARP7/PIP7S). Analyses of other LARP family members suggest more diverged RNA binding modes and specialization for cytoplasmic mRNA-related functions. Thus it appears that while genuine La proteins exhibit broad general involvement in both snRNA-related and mRNA-related functions, different LARP families may have evolved specialized activities in either snRNA or mRNA-related functions. In this review, we summarize recent progress that has led to greater understanding of the structure and function of La proteins and their roles in tRNA processing and RNP assembly dynamics, as well as progress on the different LARPs.

Neonatal lupus syndromes

Neonatal lupus has become an important model of passively acquired autoimmunity since the seminal observation in the late 1970s that sera from nearly all mothers of children with isolated congenital heart block (CHB) contain specific autoantibodies.

Crystal structure of Rsr, an ortholog of the antigenic Ro protein, links conformational flexibility to RNA binding activity

Ro ribonucleoproteins are a class of antigenic ribonucleoproteins associated with rheumatic autoimmune diseases like systemic lupus erythematosus and Sjögrens syndrome in humans. Ro ribonucleoproteins are mostly composed of the 60-kDa Ro protein and small cytoplasmic RNAs, called Y RNAs, of unknown function. In eukaryotes, where Ro has been found to associate with damaged or mutant RNAs, it has been suggested that Ro may play a role in RNA quality control. In the radiation-resistant bacterium Deinococcus radiodurans and some eukaryotes, Ro has also been implicated in cell survival following UV damage. Here we present the first high resolution structure of a prokaryotic Ro ortholog, Rsr from D. radiodurans. The structure has been solved to 1.9 A resolution and shows distinct differences when compared with the eukaryotic apo- and RNA-bound Ro structures. Rsr is composed of two domains: a helical RNA binding domain and a mixed "von Willebrand factor A-like" domain containing a divalent metal binding site. Although the individual domains of Rsr are similar to the eukaryotic Ro, significantly large differences are seen at the interface of the two domains. Since this interface communicates with the conserved central cavity of Ro, which is implicated in RNA binding, changes at this interface could potentially influence RNA binding by Ro. Although the apo-Rsr protein is monomeric, Rsr binds Y RNA to form multimers of approximately 12 molecules of a 1:1 Rsr-Y RNA complex. Rsr binds D. radiodurans Y RNA with low nanomolar affinity, comparable with previously characterized eukaryotic Ro orthologs.

Familial aggregation and linkage analysis of autoantibody traits in pedigrees multiplex for systemic lupus erythematosus

Autoantibodies are clinically relevant biomarkers for numerous autoimmune disorders. The genetic basis of autoantibody production in systemic lupus erythematosus (SLE) and other autoimmune diseases is poorly understood. In this study, we characterized autoantibody profiles in 1,506 individuals from 229 multiplex SLE pedigrees. There was strong familial aggregation of antinuclear antibodies (ANAs), anti-double-stranded DNA (dsDNA), anti-La/SSB, anti-Ro/SSA, anti-Sm, anti-nRNP (nuclear ribonucleoprotein), IgM antiphospholipid (aPL) antibodies (Abs) and rheumatoid factor (RF) across these families enriched for lupus. We performed genome-wide linkage analyses in an effort to map genes that contribute to the production of the following autoantibodies: Ro/SSA, La/SSB, nRNP, Sm, dsDNA, RF, nuclear and phospholipids. Using an approach to minimize false positives and adjust for multiple comparisons, evidence for linkage was found to anti-La/SSB Abs on chromosome 3q21 (adjusted P=1.9 x 10(-6)), to anti-nRNP and/or anti-Sm Abs on chromosome 3q27 (adjusted P=3.5 x 10(-6)), to anti-Ro/SSA and/or anti-La/SSB Abs on chromosome 4q34-q35 (adjusted P=3.4 x 10(-4)) and to anti-IgM aPL Abs on chromosome 13q14 (adjusted P=2.3 x 10(-4)). These results support the hypothesis that autoantibody production is a genetically complex trait. Identification of the causative alleles will advance our understanding of critical molecular mechanisms that underlie SLE and perhaps other autoimmune diseases.

Retropseudogenes derived from the human Ro/SS-A autoantigen-associated hY RNAs

We report the characterization in the human genome of 966 pseudogenes derived from the four human Y (hY) RNAs, components of the Ro/SS-A autoantigen. About 95% of the Y RNA pseudogenes are found in corresponding locations on the chimpanzee and human chromosomes. On the contrary, Y pseudogenes in mice are both infrequent and found in different genomic regions. In addition to this rodent/primate discrepancy, the conservation of hY pseudogenes relative to hY genes suggests that they occurred after rodent/primate divergence. Flanking regions of hY pseudogenes contain convincing evidence for involvement of the L1 retrotransposition machinery. Although Alu elements are found in close proximity to most hY pseudogenes, these are not chimeric retrogenes. Point mutations in hY RNA transcripts specifically affecting binding of Ro60 protein likely contributed to their selection for direct trans retrotransposition. This represents a novel requirement for the selection of specific RNAs for their genomic integration by the L1 retrotransposition machinery. Over 40% of the hY pseudogenes are found in intronic regions of protein-coding genes. Considering the functions of proteins known to bind subsets of hY RNAs, hY pseudogenes constitute a new class of L1-dependent non-autonomous retroelements, potentially involved in post-transcriptional regulation of gene expression.

Autoimmune-associated congenital heart block: dissecting the cascade from immunologic insult to relentless fibrosis

Few diseases exemplify the integration of research from bench to bedside as well as neonatal lupus, often described as a model of passively acquired autoimmunity. The signature histologic lesion of autoimmune congenital heart block (CHB) is fibrosis of the conducting tissue and, in some cases, the surrounding myocardium. Although anti-SSA/Ro-SSB/La antibodies are detected in > 85% of mothers whose fetuses are identified with conduction abnormalities in a structurally normal heart, the risk for a woman with the candidate antibodies to have a child with CHB is 2%. The mechanism by which maternal anti-SSA/Ro-SSB/La antibodies initiate and finally eventuate in atrioventricular nodal scarring is not yet defined, but it is clear that the antibodies alone are insufficient to cause disease and fetal factors are likely contributory. Previous in vitro and in vivo studies suggest that the pathologic cascade is initiated via apoptosis, resulting in translocation of SSA/Ro-SSB/La antigens to the cell surface where they are bound by maternal autoantibodies. Subsequently, the Fc portion of the bound immunoglobulin engages Fcgamma receptors on tissue macrophages, resulting in release of TGF-beta at a threshold favoring a profibrotic milieu and irreversible scarring. This cascade also involves tissue-specific activation of TGF-beta, which promotes the modulation of fibroblasts into myofibroblasts, a scarring phenotype. Recent findings point to genetic polymorphisms that promote high production of TGF-beta as possible fetal risk factors for CHB. Further elucidation of maternal and fetal contributory factors should provide insight into the pathogenesis of CHB and the rarity of irreversible injury.

Anti-Ro/SSA and anti-La/SSB autoantibodies in the tear fluid of patients with Sjögren's syndrome

PURPOSE

To investigate the presence of anti-Ro/SSA and anti-La/SSB antibodies in the tear fluid and serum of patients with Sjögren's syndrome and to evaluate the association of these autoantibodies with the severity of keratoconjunctivitis sicca.

METHODS

Tear fluid and serum were obtained from 28 patients with Sjögren's syndrome and 17 age matched normal control subjects. Evaluation of tear fluid and sera anti-Ro/SSA and anti-La/SSB levels was done by using a quantitative enzyme linked immunosorbent assay kit designed for the quantitative measurement of IgG class autoantibodies directed against highly purified SSA and SSB antigens. Tear function and ocular surface were evaluated by Schirmer I test, tear break up time, and rose bengal staining. Dry eye symptom scores were recorded.

RESULTS

Increased levels of anti-Ro/SSA and anti-La/SSB antibodies were detected in sera of 57.1% and 50% of SS patients, respectively. Six patients had increased levels of anti-Ro/SSA in the tear fluid, in one case anti-Ro/SSA being detected in tear fluid when it was negative in serum. Ten patients had positive anti-La/SSB titres in tear fluid and in four of these patients, anti-La/SSB titres were not elevated in serum. A positive correlation was observed between serum and tear fluid titres of anti-Ro/SSA (r = 0.43, p = 0.02), but not of anti-La/SSB. Serum anti-Ro/SSA and anti-La/SSB concentrations correlated positively with dry eye symptom scores (r = 0.42, p = 0.02 and r = 0.48, p = 0.01, respectively) and negatively correlated with Schirmer I test scores (r = -0.39, p = 0.04 and r = -0.40, p = 0.03, respectively). Significant correlations were found between tear anti-La/SSB concentrations and dry eye symptom score (r = 0.56, p = 0.02) and also rose bengal staining scores of the ocular surface (r = 0.44, p = 0.02).

CONCLUSION

This study shows that autoantibodies against Ro/SSA and La/SSB antigens are present in the tear fluid of some patients with SS and their presence in serum or tear fluid is associated with the severity of keratoconjunctivitis sicca. Additional measurement of tear fluid levels of anti-Ro/SSA and anti-La/SSB may serve as a valuable diagnostic indicator of SS.

A new conformational epitope generated by the binding of recombinant 70-kd protein and U1 RNA to anti-U1 RNP autoantibodies in sera from patients with mixed connective tissue disease

OBJECTIVE

To establish an enzyme-linked immunosorbent assay (ELISA) using a complex of in vitro-transcribed U1 RNA and recombinant 70-kd, A, and C proteins (C-ELISA) to detect anti-U1 RNP antibodies reactive in double immunodiffusion (DID), but not in ELISA using the proteins alone (P-ELISA).

METHODS

Sera from 196 patients with mixed connective tissue disease were used to test reactivity in P- and C-ELISAs, and the specificity of the sera was also tested by DID and immunoprecipitation (IP).

RESULTS

In P-ELISA, 15 of 196 sera positive for anti-U1 RNP in DID did not react, while all sera reacted in C-ELISA. The reactivity of 15 sera to the U1 RNA was tested by IP and ELISA, and only 3 sera reacted with the U1 RNA. These results indicated that the increased reactivity in C-ELISA was not due to the U1 RNA itself. We confirmed that the 70-kd and A proteins were bound directly to the U1 RNA by IP using antibodies to His-tag, and we tested the reactivity of the sera to the U1 RNA-70-kd protein complex and the U1 RNA-A protein complex by IP. All sera reacted with the U1 RNA-70-kd protein complex, and 1 sample reacted with the U1 RNA-A protein complex.

CONCLUSION

These results suggest that some anti-U1 RNP-positive sera specifically recognize the conformational structure altered by the binding of U1 RNA to the proteins, and the ELISA using U1 RNA and recombinant proteins is as useful as the DID method for detecting anti-U1 RNP antibodies.

Autoimmune responses to proliferating cell nuclear antigen multiprotein complexes involved in cell proliferation are strongly associated with their structure and biologic function in patients with systemic lupus erythematosus

OBJECTIVE

To analyze the reaction of lupus sera with proliferating cell nuclear antigen (PCNA) multiprotein complexes (PCNA complexes), which are part of the protein machinery involved in cell proliferation.

METHODS

PCNA complexes were purified from rabbit thymus extract by affinity chromatography using anti-PCNA monoclonal antibodies (TOB7, TO17, and TO30); monomeric and trimeric PCNA forms (AK-PCNA) were purified using anti-PCNA serum AK. The reactions to these antigens of 10 anti-PCNA-positive and 40 anti-PCNA-negative sera selected from 560 lupus patients were tested by immunoblotting, immunoprecipitation, and enzyme-linked immunosorbent assays (ELISAs).

RESULTS

With one exception (serum OK), anti-PCNA-positive sera reacted exclusively with only the 34-kd polypeptide. In contrast, 14 of 40 anti-PCNA-negative sera reacted with multiple proteins within PCNA complexes. Most anti-PCNA-positive sera probably recognize as epitopes the binding sites for other proteins on PCNA, which are likely hidden when PCNA is complexed with other proteins. As a consequence, only serum OK reacted with the PCNA complex in a series of ELISAs. Using AK-PCNA as a competitive inhibitor, it was determined that serum OK reacts with both the 58-kd polypeptide and the 34-kd PCNA within complexes. Together with the results of a longitudinal analysis, these results suggest that the immune system of patient OK likely recognized the complexed PCNA protein, after which the autoimmune response spread to other elements of the complexes.

CONCLUSION

Intermolecular-intrastructural help, leading to the spread of autoimmune response from PCNA to other proteins associated with its biologic function, plays a crucial role in the induction of the autoimmune response seen in lupus patients.

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.

Nucleolin associates with a subset of the human Ro ribonucleoprotein complexes

Ro RNPs are evolutionarily conserved, small cytoplasmic RNA-protein complexes with an unknown function. In human cells, Ro RNPs consist of one of the four hY RNAs and two core proteins: Ro60 and La. Recently, the association of hnRNP I and hnRNP K with particles containing hY1 and hY3 RNAs has been described. The association of three other proteins, namely calreticulin, Ro52 and RoBPI, with (subsets of) the Ro RNPs is still controversial. To gain more insight into the composition and function of the Ro RNPs, we have immunopurified these particles from HeLa cell extracts using monoclonal antibodies against Ro60 and La. Using this approach, we have identified the RNA-binding protein nucleolin as a novel subunit of Ro RNP particles containing hY1 or hY3 RNA, but not hY4 and hY5 RNA. Using an in vitro hY RNA-binding assay we established that the internal pyrimidine-rich loop of hY1 and hY3 RNA is essential for the association of nucleolin. The binding is critically dependent on the presence of all four RNP motifs of nucleolin, but not of the C-terminal RGG-box. Moreover, we demonstrate that, in contrast to nucleolin and hnRNP K, nucleolin and hnRNP I can bind simultaneously to the internal pyrimidine-rich loop of hY1 RNA. We postulate that nucleolin functions in the biogenesis and/or trafficking of hY1 and hY3 RNPs through the nucleolus and subsequent transport to the cytoplasm.

Immune responses to small nuclear ribonucleoproteins: antigen-dependent distinct B cell epitope spreading patterns in mice immunized with recombinant polypeptides of small nuclear ribonucleoproteins

Complex patterns of autoantibody reactivities with the small nuclear ribonucleoproteins (snRNPs) are observed in systemic lupus erythematosus. To investigate the role of individual snRNP components in the initiation and diversification of anti-snRNP Ab responses, we immunized A/J mice with recombinant Smith D (SmD), Smith B (SmB), and A ribonucleoprotein (A-RNP) with alum as adjuvant. Sera at different time points after initial immunizations were analyzed by Western blot and immunoprecipitation assays. In SmD-immunized mice, specific Abs to A-RNP and SmB were generated by 2 mo postimmunization, in addition to the detection of cross-reactive Abs between the immunogen and other snRNPs. Whereas Abs reactive with the immunogen decreased by 5 mo, Abs capable of immunoprecipitating A-RNP and SmB increased. In SmB-immunized mice, specific Abs to A-RNP were readily detectable, in addition to cross-reactive Abs. In contrast, A-RNP-immunized mice had only cross-reactive Abs to SmB without detectable Abs to SmD. However, in these mice, specific Abs to the 70-kDa protein were generated. Abs, which precipitated the native snRNP particle, were generated in all three groups of the immunized mice. Our results show that different initiating Ags from the same multiprotein antigenic complex induce distinct patterns of epitope spreading to proteins within that complex. These data have significant implications for the mechanisms of autoantibody diversification in systemic lupus erythematosus.

Specificity of autoantibodies to SS-A/Ro on a transfected and overexpressed human 60 kDa Ro autoantigen substrate

The objective of this study was to analyze apparently discrepant results that arose during the use of an indirect immunofluorescence (IIF) assay using transfected HEp-2 cells to detect anti-SS-A/Ro autoantibodies in human sera. Fourteen sera that had SS-A/Ro antibodies as detected on this commercial substrate, but did not have antibodies to SS-A/Ro as determined by double immunodiffusion (ID) or enzyme-linked immunosorbent assay (ELISA), were studied by immunoprecipitation (IP) of radiolabeled cell extracts and full-length recombinant SS-A/Ro. A multi-antigen strip immunoblotting (IB) assay containing both the 52- and 60-kDa antigens was included in the analysis. We confirmed that 12 of 14 of the sera under study had antibodies to SS-A/Ro protein antigens as determined by at least one other immunoassay. One serum had antibodies to hyRNA but no detectable reactivity with the 52- or 60-kDa antigens. One serum remained negative in all assays for SS-A/Ro autoantibodies.

Human Ro60 (SSA2) genomic organization and sequence alterations, examined in cutaneous lupus erythematosus

BACKGROUND

The Ro 60 kDa protein (Ro60 or SSA2) is the major component of the Ro ribonucleoprotein (Ro RNP) complex, to which an immune response is a specific feature of several autoimmune diseases. The genomic organization and any sequence variation within the DNA encoding Ro60 are unknown.

OBJECTIVES

To characterize the Ro60 gene structure and to assess whether any sequence alterations might be associated with serum anti-Ro antibody in subacute cutaneous lupus erythematosus (SCLE), thus potentially providing new insight into disease pathogenesis.

METHODS

The cDNA sequence for Ro60 was obtained from the NCBI database and used for a BLAST search for a clone containing the entire genomic sequence. The intron-exon borders were confirmed by designing intronic primer pairs to flank each exon, which were then used to amplify genomic DNA for automated sequencing from 36 caucasian patients with SCLE (anti-Ro positive) and 49 with discoid LE (DLE, anti-Ro negative), in addition to 36 healthy caucasian controls.

RESULTS

Heteroduplex analysis of polymerase chain reaction (PCR) products from patients and controls spanning all Ro60 exons (1-8) revealed a common bandshift in the PCR products spanning exon 7. Sequencing of the corresponding PCR products demonstrated an A > G substitution at nucleotide position 1318-7, within the consensus acceptor splice site of exon 7 (GenBank XM001901). The allele frequencies were major allele A (0.71) and minor allele G (0.29) in 72 control chromosomes, with no significant differences found between SCLE patients, DLE patients and controls.

CONCLUSIONS

The genomic organization of the DNA encoding the Ro60 protein is described, including a common polymorphism within the consensus acceptor splice site of exon 7. Our delineation of a strategy for the genomic amplification of Ro60 forms a basis for further examination of the pathological functions of the Ro RNP in autoimmune disease.

Heterogeneity of human Ro ribonucleoproteins (RNPS): nuclear retention of Ro RNPS containing the human hY5 RNA in human and mouse cells

Ro ribonucleoproteins (RNPs) are autoantigens that result from the association of a 60-kDa protein (Ro60) with a small RNA (hY1, hY3, hY4 or hY5 in humans, mY1 or mY3 in mice). Previous studies localized Ro RNPs to the cytoplasm. Because Ro RNPs containing hY5 RNA (Ro(hY5) RNPs) have unique biochemical and immunological properties, their intracellular localization was reassessed. Subcellular distribution of mouse and human Ro RNPs in intact and hY-RNA transfected cells was assessed by immunoprecipitation and Northern hybridization. Human Ro(hY1--4) RNPs as well as murine Ro(mY1, mY3) RNPs are exclusively cytoplasmic. Ro RNPs containing an intact hY5 RNA, but not those containing a mutated form of hY5 RNA, are found in the nuclear fractions of human and mouse cells. Ro(hY5) RNPs are stably associated with transcriptionally active La protein and are known to associate with RoBPI, a nuclear autoantigen. Our results demonstrate that Ro(hY5) RNPs are specifically present in the nucleus of cultured human and murine cells. The signal for nuclear localization of Ro(hY5) RNPs appears to reside within the hY5 sequence itself. In conclusion, we suggest that the unique localization and interactions of primate-specific Ro(hY5) RNPs reflect functions that are distinct from the predicted cytoplasmic function(s) of more conserved Ro RNPs.

The heterogeneous nuclear ribonucleoproteins I and K interact with a subset of the ro ribonucleoprotein-associated Y RNAs in vitro and in vivo

The hY RNAs are a group of four small cytoplasmic RNAs of unknown function that are stably associated with at least two proteins, Ro60 and La, to form Ro ribonucleoprotein complexes. Here we show that the heterogeneous nuclear ribonucleoproteins (hnRNP) I and K are able to associate with a subset of hY RNAs in vitro and demonstrate these interactions to occur also in vivo in a yeast three-hybrid system. Experiments performed in vitro and in vivo with deletion mutants of hY1 RNA revealed its pyrimidine-rich central loop to be involved in interactions with both hnRNP I and K and clearly showed their binding sites to be different from the Ro60 binding site. Both hY1 and hY3 RNAs coprecipitated with hnRNP I in immunoprecipitation experiments performed with HeLa S100 extracts and cell extracts from COS-1 cells transiently transfected with VSV-G-tagged hnRNP-I, respectively. Furthermore, both anti-Ro60 and anti-La antibodies coprecipitated hnRNP I, whereas coprecipitation of hnRNP K was not observed. Taken together, these data strongly suggest that hnRNP I is a stable component of a subpopulation of Ro RNPs, whereas hnRNP K may be transiently bound or interact only with (rare) Y RNAs that are devoid of Ro60 and La. Given that functions related to translation regulation have been assigned to both proteins and also to La, our findings may provide novel clues toward understanding the role of Y RNAs and their respective RNP complexes.

[Evaluation of an immunoblotting technique for the detection of anti-SSA and anti-SSB antibodies]

Immunoblotting and ELISA methods are now employed for the detection of anti Ro/SSA and anti La/SSB antibodies sometimes more often than double immunodiffusion or counter-immunoelectrophoresis. We have developed an immunoblotting technique for the detection of anti Ro/SSA and anti-La/SSB antibodies. This assay named IB-Hela, which uses a Hela cell extract, was compared with the double immunodiffusion technique like two other commercial immunoassays (Elisa test from Sanofi-diagnostic Pasteur and Line immunoassay from Innogenetics). This evaluation shows a good agreement especially in the case of Ro/SSA antibodies detection. IB-Hela seems to be useful in the La/SSB detection with a better sensitivity than the double immunodiffusion technique. We conclude that the IB Hela is helpful for the search for anti-Ro/SSA and anti-La/SSB autoantibodies and that it can be used as an alternative method to the classical techniques of immunodiffusion and counter immunoelectrophoresis.

Analysis of the molecular composition of Ro ribonucleoprotein complexes. Identification of novel Y RNA-binding proteins

Human Ro ribonucleoproteins (RNPs) are composed of one of the four small Y RNAs and at least two proteins, Ro60 and La; association of additional proteins including the Ro52 protein and calreticulin has been suggested, but clear-cut evidence is still lacking. Partial purification of Ro RNPs from HeLa S100 extracts allowed characterization of several subpopulations of Ro RNPs with estimated molecular masses of between 150 and 550 kDa. The majority of these complexes contained Ro60 and La, whereas only a small proportion of Ro52 appeared to be associated with Ro RNPs. To identify novel Y RNA-associated proteins in vitro, binding of cytoplasmic proteins to biotinylated Y RNAs was investigated. In these reconstitution experiments, several proteins with estimated molecular masses of 80, 68, 65, 62, 60 and 53 kDa, the latter two being immunologically distinct from Ro60 and Ro52, respectively, appeared to bind specifically to Y RNAs. Furthermore, autoantibodies to these proteins were found in sera from patients with systemic lupus erythematosus. The proteins bound preferentially to Y1 and Y3 RNA but, with the exception of the 53-kDa protein, only weakly to Y4 RNA and not at all to Y5 RNA. Coprecipitation of the 80, 68, 65, and 53-kDa proteins by antibodies to Ro60 and La was observed, suggesting that at least a proportion of the novel proteins may reside on the same particles as La and/or Ro60. Finally, the binding sites for these proteins on Y1 RNA were clearly distinct from the Ro60-binding site involving a portion of the large central loop 2, which was found to be indispensable for binding of the 80, 68, 65 and 53-kDa proteins, as well as the stem 3-loop 3 and stem 2-loop 1 regions. Interestingly, truncation of the La-binding site resulted in decreased binding of the novel proteins (but not of Ro60), indicating La to be required for efficient association. Taken together, these results suggest the existence of further subpopulations of Ro RNPs or Y RNPs, consistent with the heterogeneous characteristics observed for these particles in the biochemical fractionation experiments.

Interaction cloning and characterization of RoBPI, a novel protein binding to human Ro ribonucleoproteins

Human Ro ribonucleoproteins (RNPs) are autoantigenic particles of unknown function(s) that consist of a 60-kDa protein (Ro60) associated with one hY RNA (hY1-5). Using a modified yeast three-hybrid system, named RNP interaction trap assay (RITA), we cloned a novel Ro RNP-binding protein (RoBPI), based on its property to interact in vivo in yeast with an RNP complex made of recombinant Ro60 (rRo60) protein and hY5 (rhY5) RNA. RoBPI cDNA contains three conserved RNA recognition motifs (RRM) and is present as a family of isoforms differing slightly at their 5' end. The 2.0-kb RoBPI mRNA was detected in all human tissues tested. Highly homologous cDNA sequences were found in banks of expressed sequence tags (ESTs) from mice. Two-hybrid, three-hybrid, and RITA experiments respectively established that 60 kDa RoBPI did not interact in yeast with rRo60 alone, with rhY5 RNA alone, or with bait RNPs consisting of rRo60 and recombinant hY1, hY3, or hY4 RNAs. RoBPI coimmunoprecipitated with Ro RNPs from HeLa cell extracts and partially colocalized with Ro60 in nuclei of cultured cells. Because hY5 RNA and RohY5 RNPs are recent evolutionary additions seen only in primates, but RoBPI seems more conserved, their interaction may represent a gain of function for Ro RNPs. Alternatively, interaction of RohY5 RNPs with RoBPI may have no functional bearing, but may underlie some of the unique biochemical and immunological properties of these RNPs.

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.

Protein-protein interaction of the Ro-ribonucleoprotein particle using multiple antigenic peptides

Protein protein interactions play a significant role in maintaining the structural and functional integrity of the cell. We used multiple antigen peptides (MAPs) to analyze such interactions within the Ro (or SSA) ribonucleoprotein complex. Our data showed that 60 kD Ro and La colocalize in the nucleus of the cell. Previous data have indicated that 60 kD Ro and La co-exist via interactions with the hYRNAs. We were interested to see whether 60 kD Ro and La interact with each other through protein protein interactions. MAPs were produced with sequences derived from the autoepitopes of 60 kD Ro. When used in agarose immunodiffusion certain MAPs formed precipitin lines specifically with Ro and La antigens. Used in affinity chromatography the Ro MAPs purified the Ro ribonucleoprotein particle from lymphocyte extract. Solid phase immunoassay and surface plasmon resonance (SPR) confirmed the observations obtained with agarose diffusion. Using SPR, kinetic analyses gave an apparent affinity constant of about 1 x 10(7) M(-1) for Ro-MAP-60 kD Ro interactions. The autoantigens Ro and La are specific targets in autoimmune diseases, particularly systemic lupus erythematosus (SLE) and Sjögren's syndrome, and are known to exist together as a complex with hYRNAs. The present data indicate that there are protein-protein interactions between Ro and La.

Immune responses to Ro60 and its peptides in mice. I. The nature of the immunogen and endogenous autoantigen determine the specificities of the induced autoantibodies

Anti-Ro60 autoantibodies are found in a variety of autoimmune disorders including systemic lupus erythematosus (SLE), Sjögren's syndrome, primary biliary cirrhosis, and active hepatitis. They are the most prevalent autoantibodies in normal individuals and in asymptomatic mothers of infants afflicted with neonatal lupus. In the present study, immune responses to recombinant human Ro60 (rhRo60) and recombinant mouse Ro60 (rmRo60) and selected Ro60 peptides in non-SLE-prone mice were investigated. Multiple T and B cell epitopes were identified in Ro60. Immunizations with either xenogeneic or autologous Ro60 induced autoantibodies to a diverse group of autoantigens. In addition to La and Ro52, proteins in the small nuclear ribonucleoprotein (snRNP) particles such as SmA, SmB, SmD, and 70-kD U1-RNP were unexpectedly identified as targeted antigens. In the studies involving synthetic Ro60 peptides, both human and mouse Ro60316-335 peptides, which differ in three amino acids, were found to contain dominant cross-reactive T cell determinants. Immunizations with these peptides induced autoantibodies to Ro60, La, SmD, and 70-kD U1-RNP without autoantibodies to Ro52, SmA, or SmB. With human Ro60316-335 as the immunogen, additional autoantibodies reactive with the Golgi complex were found. In contrast to the immunodominance of both human and mouse Ro60316-335 peptides, the T cell determinant in human Ro60441-465 was dominant, whereas that in the mouse peptide was cryptic. Immunization with human Ro60441-465 induced primarily anti-peptide Abs. Mouse Ro60441-465 failed to induce an antibody response. These results show that both the nature of the immunogen and the immunogenicity of the related endogenous antigen are important in determining the specificities of the autoantibodies generated. They have significant implications for proposed mechanisms on the generation of complex patterns of autoantibodies to a diverse group of autoantigens in SLE patients.

mRNA and protein expression of SSA/Ro and SSB/La in human fetal cardiac myocytes cultured using a novel application of the Langendorff procedure

Irreversible congenital heart block (CHB) and the transient rash of neonatal lupus are strongly associated with maternal antibodies to SSA/Ro and SSB/La proteins; however, the precise mechanism by which these antibodies mediate organ-specific injury is not yet defined. Culturing of keratinocytes has provided critical insights. Accordingly, successful culturing of human fetal cardiac myocytes at high yield would constitute a powerful tool to directly examine conditions that promote expression of the target autoantigens. To accomplish this aim, fetal cardiac myocytes from 18- to 22-wk abortuses were established in culture using a novel technique in which cells were isolated after perfusion of the aorta with collagenase in a Langendorff apparatus. After preplating to decrease fibroblast contamination, cardiocytes were grown in flasks and slide chambers. Staining with monoclonal anti-sarcomeric alpha-actinin revealed the expected striations typical of cardiac myocytes in 70-90% of the cells after 4 d in culture. Furthermore, the cells were observed to beat at rates varying between 25-75 beats per minute (bpm) after the addition of 1.8 mM CaCl2. An average yield of 45-60 x 10(6) cells was obtained from a 3- to 5-g heart. Cellular localization of SSA/Ro and SSB/La by indirect immunofluorescence and demonstration of mRNA expression by reverse transcriptase polymerase chain reaction supports the feasibility of cultured cardiac myocytes for the study of congenital heart block. In contrast to the increased expression of SSA/Ro reported for keratinocytes, incubation of cultured human cardiac myocytes with either 17beta-estradiol or progesterone did not alter mRNA expression or cellular localization of 48 kD SSB/La, 52 kD SSA/Ro, or 60 kD SSA/Ro. In summary, we describe a novel method to successfully culture human fetal cardiac myocytes that should provide a valuable resource for investigation of the molecular mechanism(s) contributing to the development of congenital heart block. Differential constitutive and estradiol-induced expression of 52 and 60 kD SSA/Ro in human cardiac myocytes compared with keratinocytes may be a factor contributing to the marked discordance of clinically detectable injury in these two target tissues.

Induction of Antibodies Reactive with SSA/Ro-SSB/La and Development of Congenital Heart Block in a Murine Model

To correlate the arrhythmogenic effects of maternal autoantibodies with the genesis of congenital heart block, female BALB/c mice were immunized with human recombinant 48-kDa SSB/La, 60-kDa SSA/Ro, 52-kDa SSA/Ro (52α), and 52β (amino acids 169–245 deleted) as well as with murine recombinant 52-kDa SSA/Ro. Control animals received β-galactosidase or a polypeptide encoded by pET-28 alone. Following primary immunization and two boosters, high titer responses to the respective Ags were established by ELISA, immunoblotting, and immunoprecipitation. Sera from mice immunized with either human 52α or 52β immunoprecipitated murine 52Ro. mRNA and protein expression of 52Ro was demonstrated in the newborn murine heart. A spectrum of atrioventricular nodal conduction abnormalities was identified by electrocardiogram. First-degree block was detected in 7% of 27 pups born to mothers immunized with 48La, 20% of 54 pups born to 60Ro-immunized mothers, 6% of 56 pups born to 52α-immunized mothers, 7% of 86 pups born to 52β-immunized mothers, and 9% of 22 pups born to mothers immunized with murine 52Ro. Advanced conduction abnormalities were only identified in offspring of 52α- or 52β-immunized mice. In the 52α group, one pup had complete block and another had second-degree block (Wenckebach type); in the 52β group, five pups had complete block. Maternal Abs to the primary immunogens were detected in the pups. No control had any conduction abnormalities. This Ab-specific animal model provides strong evidence for a pathogenic role of anti-SSA/Ro-SSB/La Abs, particularly 52Ro, in the development of congenital heart block. The range and frequency of conduction defects suggest that additional factors promote disease expression.

Detection of anti-Ro/SSA and anti-La/SSB autoantibody-producing cells in salivary glands from patients with Sjögren's syndrome

OBJECTIVE

To investigate and identify the presence of cells producing anti-Ro/SSA and anti-La/SSB autoantibodies in salivary glands from patients with Sjögren's syndrome (SS).

METHODS

Submucosal salivary gland biopsy samples from 10 SS patients (8 with and 2 without circulating Ro and La autoantibodies) and 14 control subjects were evaluated. Frozen tissue sections were immunostained by an avidin-biotin complex technique, using biotinylated recombinant Ro and La proteins as detection reagents. Autoantibody levels in SS patient sera were analyzed by enzyme-linked immunosorbent assay.

RESULTS

Cells producing autoantibodies to the Ro 52-kd, Ro 60-kd, and La proteins were recorded in 8, 6, and 7 of the 10 SS patient biopsy samples, respectively. Samples from the 2 SS patients without circulating Ro and La autoantibodies were negative for these autoantibody-producing cells, as were all control biopsy samples. A strong positive correlation between the presence of autoantibodies in sera and the presence of autoantibody-producing cells in glandular biopsy tissues was evident. The number of autoantibody-producing cells and the serum autoantibody levels were also correlated (r(s)=0.94, P < 0.0001).

CONCLUSION

Using a novel technique, we have demonstrated the presence of Ro and La autoantibody-producing cells in salivary gland biopsy tissues from patients with SS. These findings indicate that anti-Ro/ SSA and anti-La/SSB autoantibodies are produced and are present at sites of inflammation and indicate their potential involvement in the autoimmune exocrinopathy of this disease.

Accessibility of SSA/Ro and SSB/La Antigens to Maternal Autoantibodies in Apoptotic Human Fetal Cardiac Myocytes

Access of intracellular Ags SSA/Ro and SSB/La to cognate maternal autoantibodies is unexplained despite their strong association with congenital heart block. To investigate the hypothesis that apoptosis facilitates surface accessibility of these Ags, human fetal cardiac myocytes from 16- to 22-wk abortuses were established in culture using a novel technique in which cells were isolated after perfusing the aorta with collagenase. Confirmation of cardiac myocytes included positive staining with antisarcomeric α-actinin and contractility induced by 1.8 mM calcium. Incubation with 0.5 μM staurosporine or 0.3 mM 2,3-dimethoxy-1,4-naphthoquinone induced the characteristic morphologic and biochemical changes of apoptosis. The cellular topology of Ro and La was evaluated with confocal microscopy and determined in nonapoptotic and apoptotic cardiocytes by indirect immunofluorescence. In permeabilized nonapoptotic cardiocytes, Ro and La were predominantly nuclear, and propidium iodide (PI) stained the nucleus. In early apoptotic cardiocytes, condensation of the PI- and Ro- or La-stained nucleus was observed, accompanied by Ro/La fluorescence around the cell periphery. In later stages of apoptosis, nuclear Ro and La staining became weaker, and PI demonstrated nuclear fragmentation. Ro/La-stained blebs emerged from the cell membrane, a finding observed in nonpermeabilized cells, supporting an Ab-Ag interaction at the cell surface. In summary, induction of apoptosis in cultured cardiocytes results in surface translocation of Ro/La and recognition by Abs. Although apoptotic cells are programmed to die and do not characteristically evoke inflammation, binding of maternal Abs and subsequent influx of leukocytes could damage surrounding healthy fetal cardiocytes.

Serum and immunoglobulin G from the mother of a child with congenital heart block induce conduction abnormalities and inhibit L-type calcium channels in a rat heart model

Although a strong clinical association exists between congenital heart block (CHB) and an immune response to SSA/Ro and SSB/La proteins, a causative role of these antibodies in the pathogenesis is just emerging. In a preliminary report, we have demonstrated that IgG fractions isolated from the sera of mothers whose children have CHB are arrhythmogenic in the human fetal heart. To more precisely define the arrhythmogenic effect of anti-SSA/Ro-SSB/La antibodies, we used the readily available rat heart model to record: 1) ECGs from Langendorff beating hearts; 2) action potentials from atrioventricular (AV) nodal preparations; 3) L-type Ca currents, I(Ca) at the whole-cell and single channel levels; and 4) other currents such as the transient outward K+ current, I(to), the inward rectifier K+ current, I(K1), and the Na+ current, I(Na). Perfusion of hearts with purified IgG (800 microg/mL), isolated from the serum of a mother with SSA/Ro and SSB/La antibodies whose child had CHB, resulted in bradycardia associated with 2:1 AV block. Simultaneous action potentials were recorded from dissected atrial and AV nodal areas of the rat heart. Superfusion of these preparations with the same mother's IgG fraction resulted in 2:1 AV block followed by complete inhibition of AV nodal action potential. Because AV nodal electrogenesis is largely dependent on I(Ca), the effect of these antibodies on I(Ca) was subsequently determined. Superfusion of myocytes with whole serum or purified IgG (80 microg/mL) from the same mother consistently inhibited whole cell I(Ca), ensemble average Ba2+ currents (I(Ba)) and open state probability, p(o), without affecting the channel conductance. IgG had no significant effect on I(to), I(K1), or I(Na). Whole sera and IgG fractions from a healthy mother with no detectable anti-SSA/Ro or SSB/La antibodies did not inhibit I(Ca) or I(Ba). These results demonstrate that IgG containing anti-SSA/Ro and -SSB/La antibodies induces complete AV block in beating hearts and in multicellular preparations, thus implicating a preferential interaction of these autoantibodies with Ca channels and/or associated regulatory proteins. This is consistent with the observed inhibition of Ca channels that may be a critical factor contributing to the pathogenesis of CHB.

Determination of antibodies to extractable nuclear antigens by commercial kits: a multicenter study

Several enzyme immunoassays for serum antibodies to extractable nuclear antigen have recently become available. The aim of this study was to evaluate the results obtained with: (1) the same kit under different conditions; (2) different enzyme immunoassays; (3) Western blot and enzyme immunoassays. Twenty-five sera from patients with autoimmune disorders were tested in five different laboratories by one Western blot and four enzyme immunoassay commercial kits. The different methods produced comparable qualitative results. However, semiquantitative evaluation, based on a cut-off value (index), yielded different results due both to laboratory conditions and to the kits employed. Standardization of commercial products and methods should be improved so that the results of different laboratories can be compared and large-scale and follow-up studies conducted. Western blot analysis could also be useful to analyze complex reactivities, although greater experience is necessary to interpret these results correctly.

The Zn2+ binding domain of the human Ro 52 kDa protein is a target for conformation-dependent autoantibodies

The Ro 52 kDa protein was originally identified using autoimmune sera directed against the Ro/SS-A antigen, also known as Ro ribonucleoprotein particles (Ro RNPs). In most human cells these consist of one of four small human cytoplasmic (hY) RNA molecules complexed with the Ro 60 kDa protein. This protein interacts directly with the hY RNAs, whereas the Ro 52 kDa protein is thought to associate via protein-protein interactions. The Ro RNPs are present in all mammalian cells, but their intracellular location and function remain unclear. The primary structure shows that the Ro 52 kDa protein is a member of a small family of proteins sharing two features, a leucine zipper region and an aminoterminal cysteine-histidine rich region with two different putative zinc finger motifs. To study if the Ro 52 kDa protein actually binds Zn2+, both the full-length Ro 52 kDa clone and various subclones were expressed as recombinant proteins and assayed for Zn2+ binding in vitro. A fragment containing the first cysteine-histidine cluster at residues 14-54 and other larger overlapping fragments were found to bind Zn2+. In this report we also demonstrate that the Zn2+ binding domain is a target for conformation-dependent anti-Ro 52 kDa autoantibodies. The antigenicity is dramatically increased by the same reducing conditions that promote Zn2+ binding. This is in contrast to the previously described immunodominant Ro 52 kDa domain.

Characterization of human telomerase complex

Telomerase, a ribonucleoprotein complex, adds hexameric repeats called "telomeres" to the growing ends of chromosomal DNA. Characterization of mammalian telomerase has been elusive because of its low level of expression. We describe a bioinformatics approach to enrich and characterize the human telomerase complex. Using local sequence homology search methods, we detected similarity of the Tetrahymena p80 subunit of telomerase with the autoantigen Ro60. Antibodies to Ro60 immunoprecipitated the telomerase activity. Ro60 and p80 proteins were cross-recognizable by antibodies to either protein. Telomerase activity and the RNA component of telomerase complex were localized to a doublet in a native gel from the Ro60 antibody-precipitated material. The enriched material showed specific binding to a TTA GGG probe in vitro in an RNA template-dependent manner. Polyclonal antibodies to the doublet also immunoprecipitated the telomerase activity. These results suggest an evolutionary conservation of the telomerase proteins.

The 52-kd protein as a target of intermolecular spreading of the immune response to components of the SS-A/Ro-SS-B/La complex

OBJECTIVE

To determine whether immunization of healthy non-autoimmune mice with 52-kd SS-A/Ro induces a secondary antibody response to other components of the 48-kd SS-B/La-60-kd SS-A/Ro RNP complex and vice versa, since anti-52-kd antibodies have been invariably linked to these antigens in patients with Sjogren's syndrome and in mothers whose children have neonatal lupus.

METHODS

Female BALB/c mice were immunized with 100 microg of 6xHis recombinant human 48-kd SS-B/La, 52-kd SS-A/Ro, or 60-kd SS-A/Ro proteins, or the 6xHis polypeptide control, each purified by Ni2+ affinity chromatography. Mice subsequently received booster injections with 50 microg of the same antigen every 10-21 days. Immune responses were measured by enzyme-linked immunosorbent assay (ELISA), immunoblotting of recombinant antigens, and immunoprecipitation of 35S-methionine-labeled in vitro translation products.

RESULTS

Immunization with 48-kd SS-B/La resulted in anti-48-kd SS-B/La antibodies within 45 days, followed 10 days later by a secondary response to 52-kd SS-A/Ro, as measured by ELISA. Antibody spreading to 60-kd SS-A/Ro was not detected. Immunization with 52-kd SS-A/Ro resulted in rapid high-titer anti-52-kd SS-A/Ro responses within 27 days. Spreading to 48-kd SS-B/La occurred in only 1 mouse and 60-kd SS-A/Ro was detected in a minority of the mice after prolonged antigen exposure. Immunization with 60-kd SS-A/Ro led to anti-60-kd SS-A/Ro responses within 37 days, followed 3 months later by low-titer anti-48-kd SS-B/La and anti-52-kd SS-A/Ro antibodies. All primary immune responses were confirmed by immunoblotting and immunoprecipitation. While immunoblotting of the recombinant proteins revealed reciprocal intermolecular spreading in the majority of mice, immunoprecipitation clearly demonstrated that predominant spreading was generated after immunization with 48-kd SS-B/La, which consistently resulted in antibodies to 52-kd SS-A/Ro.

CONCLUSION

The murine responses observed in the present study, demonstrating reciprocal intermolecular spreading to 48-kd SS-B/La, 52-kd SS-A/Ro, and 60-kd SS-A/Ro, support the linkage of 52-kd SS-A/Ro with the other proteins, despite their as-yet-undetected association in vivo. The marked recruitment of anti-52-kd SS-A/Ro responses elicited by 48-kd SS-B/La may provide a lead to exploring the physical interaction, direct or indirect, of 52-kd SS-A/Ro with the SS-A/Ro-SS-B/La RNP particle and its presentation to the immune system. These data should facilitate the establishment of a murine model of neonatal lupus.

The ultrastructural localization of 60-kDa Ro protein and human cytoplasmic RNAs: association with novel electron-dense bodies

The 60-kDa Ro ribonucleoprotein is an important target of humoral autoimmune responses. However, the ultrastructural locations of the 60-kDa Ro protein and its associated small cytoplasmic RNAs (Y RNAs) have not been previously determined, and the functions of the Ro protein and RNAs are not known. In this study, the cellular locations of the 60-kDa Ro protein and the Ro Y1 and Y4 RNAs are determined by immunoelectron microscopy and in situ hybridization electron microscopy, respectively. Both Ro protein and Y RNAs are concentrated in discrete areas of the nucleoplasm, nucleolus, and cytoplasm of cultured cells and human skin sections. The 60-kDa Ro protein and Y RNAs are also present diffusely in the cytoplasm, where they occur in ribosome-rich regions, and in the nucleus. The presence of Ro ribonucleoprotein components in nucleoli and in ribosome-rich cytoplasmic areas suggests a potential for the involvement of Y RNAs and/or 60-kDa Ro protein in ribosome synthesis, assembly, or transport. Double labeling experiments show that Ro protein and Y RNAs colocalize in the nucleoplasm, nucleolus, and cytoplasm. In addition, aggregates of Y RNA occur unassociated with 60-kDa Ro protein, and aggregates of 60-kDa Ro protein occur unassociated with Y RNA. Aggregates of both Ro protein and Y RNAs label previously unreported nuclear and cytoplasmic electron-dense bodies. We propose that these distinctive Ro-associated electron-dense bodies may represent structure(s) important for cellular transport and/or Ro function.

Production of anti-Ro/SS-A and anti-La/SS-B autoantibodies is closely coordinated in systemic lupus erythematosus and independent of anti-dsDNA production

Autoimmune diseases such as Sjögren's Syndrome and systemic lupus erythematosus (SLE) are characterized by serum autoantibodies against protein components of small cytoplasmic ribonucleoproteins (scRNPs). The origin and regulation of these anti-Ro/SS-A and anti-La/SS-B antibodies is not well understood. Regular co-occurrence of these two autoantibodies in humans together with murine studies on antibody responses against scRNPs after immunization suggest a role for scRNPs as common antigen. We sought additional evidence for this hypothesis in a longitudinal serological study of patients with SLE. Quantitative measurement of the antibody responses against Ro/SS-A and La/SS-B proteins and for comparison to dsDNA in 852 serum samples was performed. These samples were collected from nine patients during an average observation period of more than 10 years. A significant and strong correlation between the two anti-scRNP responses emerged during 90% of follow-up. In contrast, correlation of anti-scRNP with anti-dsDNA responses was remarkably absent in the same patients. Our results confirm the unique relationship between anti-Ro/SS-A and anti-La/ SS-B responses and could thus be interpreted as support for a model wherein induction and perpetuation of autoantibody production is dependent on scRNPs containing both proteins as antigen.

Neonatal lupus syndromes

Congenital heart block is considered to be a model of passively acquired autoimmunity, whereby immune abnormalities in the mother lead to the production of autoantibodies that cross the placenta and presumably injure the otherwise normally developing fetus. The major targets of the maternal immune response are the SSA/Ro and SSB/La ribonucleoproteins. Other neonatal abnormalities affecting the skin, liver, and blood elements have also been reported to be associated with anti-SSA/Ro-SSB/La antibodies in the maternal and fetal circulation and are now grouped along with congenital heart block under the heading of the neonatal lupus syndromes. This review covers the histopathology, SSA/Ro-SSB/La antigen-antibody systems, immunogenetics, clinical manifestations, and diagnosis and management strategies of these syndromes.

Systemic lupus erythematosus: clinical expression and anti-Ro/SS--a response in patients with and without lesions of subacute cutaneous lupus erythematosus

Patients with subacute cutaneous lupus erythematosus (SCLE) have recurrent annular and/or psoriasiphorm skin lesions, with or without systemic disease.

OBJECTIVE

To analyse the clinical expression and the Ro/SS-A response associated with SCLE in patients with systemic lupus erythematosus (SLE).

METHODS

128 consecutive patients with SLE were studied. Anti-Ro/SS-A antibodies were detected by ELISA, (anti-60 kD Ro/SSA antibodies), immunoblotting (anti-60 kD and anti-52 kD Ro/SS-A antibodies) and counterimmunoelectrophoresis (CIE).

RESULTS

Seventeen patients (13.2%) showed SCLE lesions. Photosensitivity was more frequent in patients with SCLE (82%) than in patients without these cutaneous lesions (45%) (OR: 5.6). Arthritis (OR: 6.3), Raynaud's phenomenon (OR: 4.9), pleuritis (OR: 7.6), central nervous system disorder (OR: 6.4), renal disease (OR: 6.3), anemia (OR: 7.9), hypocomplementemia (OR: 6.1) and anti-dsDNA antibodies (OR: 12.7) were significantly more frequent in patients without SCLE. Anti-Ro/SS-A antibodies were detected in 15 (88.2%) patients with SCLE and 62 (55.8%) patients without SCLE by ELISA, in 10 (58.8%) and 34 (30.6%) patients by immunoblotting, and in 13 (76.4%) and 34 (30.6%) by CIE, respectively. Anti-60 kD-Ro/SS-A and anti-La antibodies, but not anti-52 kD-Ro/SS-A, were significantly more frequent in patients with SCLE than in patients without SCLE.

CONCLUSIONS

The presence of SCLE lesions in patients with SLE is associated with a more favourable prognosis. The major anti-Ro/SS-A response is directed against the native 60 kD Ro/SS-A protein.

A subpopulation of WIL-2 cell calreticulin molecules is associated with RO/SS-A ribonucleoprotein particles

A subpopulation of human calreticulin (CR) molecules that is reactive with human Ro/SS-A autoimmune sera was identified in a nucleic acid- enriched Wil-2 cell fraction derived by anion exchange column chromatography. Further resolution of this fraction by gel filtration size separation demonstrated that the appearance of CR (true mol. weight 46 kD) coincided with the emergence of Ro/SS-A ribonucleoprotein (mol. weight > 250 kD) antigenic activity and increasing 260 nm ultraviolet absorbance. This high nucleic acid fraction could be further partitioned into four small RNA-containing Ro/SS-A antigenic subfractions by a second passage over the anion exchange column. CR was enriched in one subfraction and present in the other three subfractions as well. No CR was found in the RNA-free fraction of the repartition eluate. These results represent the first direct demonstration that CR, a high-affinity calcium binding protein, exists in a form that is directly associated with all four varieties of native, human Ro/SS-A ribonucleoprotein particles (hY1-5).

Gene encoding human Ro-associated autoantigen Y5 RNA

Ro ribonucleoproteins are composed of Y RNAs and the Ro 60 kDa protein. While the Ro 60 kDa protein is implicated in an RNA discard pathway that recognizes 3'-extended 5S rRNAs, the function of Y RNAs remains unknown [O'Brien,C.A. and Wolin,S.L. (1995) Genes Dev. 8,2891-2903]. Y5 RNA occupies a large fraction of Ro 60 kDa protein in human Ro RNPs, contains an atypical 3'-extension not found on other Y RNAs, and constitutes an RNA antigen in certain autoimmune patients [Boulanger et al. (1995) Clin. Exp. Immunol. 99, 29-36]. An overabundance of Y RNA retroposed pseudogenes has previously complicated the isolation of mammalian Y RNA genes. The source gene for Y5 RNA was isolated from human DNA as well as from Galago senegalis DNA. Authenticity of the hY5 RNA gene was demonstrated in vivo and its activity was compared with the hY4 RNA gene that also uses a type 3 promoter for RNA polymerase III. The hY5 RNA gene was subsequently found to reside within a few hundred thousand base pairs of other Y RNA genes and the linear order of the four human Y RNA genes on chromosome 7q36 was determined. Phylogenetic comparative analyses of promoter and RNA structure indicate that the Y5 RNA gene has been subjected to positive selection during primate evolution. Consistent with the proposal of O'Brien and Harley [O'Brian,C.A. and Wolin,S.L. (1992) Gene 116, 285-289], analysis of flanking sequences suggest that the hY5 RNA gene may have originated as a retroposon.

Significance of the anti-Ro (SS-A) antibody in evaluation of patients with cutaneous manifestations of a connective tissue disease

The anti-Ro(SS-A) antibody is arguably the most important antibody determination except for antinuclear antibodies in evaluation of patients suspected of having lupus erythematosus. During the past 25 years, studies have established the importance of this antibody in the evaluation of patients with atypical lupus erythematosus, who have a photosensitive dermatitis as the presenting sign. The purpose of this review is to demonstrate the utility and the necessity of this antibody determination in the evaluation of all patients with cutaneous manifestations who are suspected of having a connective tissue disease.

The interactions with Ro60 and La differentially affect nuclear export of hY1 RNA

Ro RNPs are evolutionarily conserved ribonucleoprotein particles that consist of a small RNA, known as Y RNA, associated with several proteins, such as La, Ro60, and Ro52. The Y RNAs (Y1-Y5), which are transcribed by RNA polymerase III, have been shown to reside almost exclusively in the cytoplasm as Ro RNPs. To obtain more insight into the nuclear export pathway of Y RNAs, hY1 RNA export was studied in Xenopus laevis oocytes. Injection of various hY1 RNA mutants showed that an intact Ro60 binding site is a prerequisite for nuclear export, whereas the presence of an intact La binding site resulted in strong nuclear retention of hY1 RNA. Competition studies with various classes of RNAs indicated that, in addition to Ro60, another titratable factor was necessary for nuclear export of hY1 RNA. This factor appears also to be involved in nuclear export of tRNA. Because export of hY1 RNA could not be blocked by a synthetic peptide containing the recently identified nuclear export signal of the HIV-1 Rev protein, nuclear export of hY1 RNA does not seem to be dependent on a Rev-like nuclear export signal.

Rapid and sensitive detection of anti-Ro (SS-A) antibodies by indirect immunofluorescence of 60kDa Ro HEp-2 transfectants

Anti-Ro (SS-A) antibodies are important diagnostic markers for primary Sjögren's syndrome and systemic lupus erythematosus, but their detection by indirect immunofluorescence (IF) in the diagnostic laboratory is hindered by the low cellular abundance of 60kDa Ro protein (Ro60). The approach we used to overcome this problem was to transfect and over-express the Ro60 gene into HEp-2 cells. In this study we have used a mixture of Ro60 transfectants and untransfected HEp-2 cells (HEp-Ro60) as a substrate for IF-antinuclear antibody (ANA) testing in a hospital laboratory. Screening of 240 routine serum specimens identified 14 Ro transfectant-positive sera which were confirmed by counterimmunoelectrophoresis (CIE); 3 of these sera were ANA-negative on untransfected cells and regular HEp-2. A comparison of HEp-Ro60 and regular HEp-2 showed strong concordance of the different ANA patterns between the 2 substrates. No increase in background staining was observed on the Ro transfectants when reacted with normal human sera. A comparison between HEp-Ro60 and CIE for 53 sera from patients with primary Sjögren's syndrome showed that HEp-Ro60 were a sensitive and specific substrate for detection of anti-Ro antibodies. Masking of positive Ro transfectants was observed rarely in sera containing multiple ANA specificities, but the Ro60 staining on these transfectants were unmasked at higher serum dilutions. We conclude that HEp-Ro60 are a suitable substrate for IF-ANA in the routine laboratory and that they have the additional advantage over regular HEp-2 slides of being able to detect anti-Ro in ANA-negative sera. HEp-RO60 are also a valuable confirmatory test for sera giving equivocal precipitin reactions or ELISA results.