Identification and characterization of autoantibodies against the nuclear envelope lamin B receptor from patients with primary biliary cirrhosis

PBC68: a nuclear pore complex protein that associates reversibly with the mitotic spindle

Using autoimmune antibodies from a patient with primary biliary cirrhosis we have identified a 68 kDa nuclear envelope protein, termed PBC68. This protein is co-precipitated with a 98 kDa and a 250 kDa polypeptide and is distinct from the nuclear lamins. Immunostaining of digitonin-permeabilized cells indicates that PBC68 is restricted to the inner (nucleoplasmic) face of the nuclear envelope, while indirect immunofluorescence and immunoelectron microscopy show that PBC68 is located on fibrillar structures emanating from the nuclear pore complex. The autoantigen is modified at early prophase and disassembles at prometaphase concurrently with the breakdown of the nuclear envelope. The disassembled material, instead of diffusing throughout the cytoplasm as other nucleoporins, is targeted to the mitotic spindle and remains stably bound to it until anaphase. At telophase PBC68 is released from the mitotic apparatus and reassembles late, after incorporation of LAP2B and B-type lamins, onto the reforming nuclear envelope. The partitioning of PBC68 in dividing cells supports the notion that subsets of nuclear envelope proteins are actively sorted during mitosis by transiently anchoring to spindle microtubules. Furthermore, the data suggest that specific constituents of pore complex are released in a stepwise fashion from their anchorage sites before becoming available for nuclear reassembly.

Caspase-dependent proteolysis of integral and peripheral proteins of nuclear membranes and nuclear pore complex proteins during apoptosis

We have studied the fate of the nuclear envelope (NE) in different human cells committed to apoptosis by different chemical agents. Using a battery of antibodies against marker proteins of the three domains of the nuclear envelope, namely lamin B (LB) for the lamina, transmembrane proteins LBR and LAP2 for the inner nuclear membrane, and nucleoporins p62, Nup153 and gp210 for the nuclear pore complexes (NPCs), we observed a selective and conserved cleavage of LB, LAP2 and Nup153. In lymphoid cells, the rate of cleavage of these markers was independent of the apoptosis inducing agent, actinomycin D or etoposide, and more rapid than in attached epithelial cells. While lamin B is cleaved by caspase 6, the protease responsible for the cleavage of LAP2 and Nup153 was probably caspase 3, since (1) cleavage of both proteins was specifically prevented by in vivo addition of caspase 3 inhibitor Ac-DEVD-CHO and (2) consensus sites for these caspases are present in both proteins. As LB, LAP2 and Nup153 are exposed at the inner face of the nuclear envelope and all interact with chromatin, we suggest that their cleavage allows both the detachment of NE from chromatin and the clustering of NPCs in the plane of the membrane, two conserved morphological features of apoptosis observed in this study.

Autoantibodies in human chronic graft-versus-host disease after hematopoietic cell transplantation

Primary biliary cirrhosis (PBC) and graft-versus-host disease (GVHD) are thought to have common immunopathologic features and previous studies have reported that 5.2 to 81% of patients with chronic GVHD after allogeneic hematopoietic cell transplant have antimitochondrial antibodies (AMA). We studied a total of 89 patients with chronic GVHD and 60 controls for AMA reactivity by ELISA and immunoblotting using recombinant PDC-E2, BCOADC-E2, and OGDC-E2, immunoblotting of beef heart mitochondrial proteins, and reactivity to nuclei, smooth muscle (ASMA), ribonucleoprotein JO-1, extractable nuclear antigen, nuclear proteins SSA/ SSB, ribonucleic P proteinase III, cardiolipin (ACA), liver kidney microsomal, thyroid microsomal, myeloperoxidase, and the reactivity of rheumatoid factor. A subset of 60 chronic GVHD sera were tested for reactivity to gp210 and LBR. Finally, liver tissue from patients with chronic GVHD and PBC was studied by immunohistochemistry to determine whether there was comparable abnormal apical staining of biliary epithelial cells using PDC-E2-specific monoclonal antibodies. Surprisingly, there were no AMA found in the sera from the 89 patients with chronic GVHD. Review of published data on AMA in GVHD suggests that previous results were primarily false positives. In contrast, sera from the patients with GVHD did have a variety of other autoantibodies and, in particular, 20/89 (22.4%) positive ANA, 23/89 (25.8%) positive ASMA, and 9/89 (10.1%) positive ACA. The other autoantibodies assayed were not statistically different from controls. Finally, abnormal biliary epithelial luminal staining of bile ducts was found, as expected, in liver tissue of patients with PBC but was absent in chronic GVHD.

Atypical antineutrophil cytoplasmic antibodies with perinuclear fluorescence in chronic inflammatory bowel diseases and hepatobiliary disorders colocalize with nuclear lamina proteins

Antineutrophil cytoplasmic antibodies (ANCA) are frequently associated with chronic inflammatory bowel diseases (IBD) and hepatobiliary disorders. However, their target antigens have not been identified yet. Recently, we observed an atypical perinuclear ANCA fluorescence (p-ANCA) together with an intranuclear staining using ANCA-positive sera from patients with IBD and hepatobiliary disorders. This observation suggests that the target antigens are localized within the nucleus of neutrophilic granulocytes. To further investigate this hypothesis, we examined sera from patients with ulcerative colitis, primary sclerosing cholangitis, autoimmune hepatitis or systemic vasculitis on ethanol or formaldehyde-fixed neutrophils using confocal laser scanning microscopy and immunoelectron microscopy. Counterstaining with propidium iodide, a DNA-specific dye, showed that ANCA-positive sera in IBD and heptobiliary disorders react with intranuclear antigens at the nuclear periphery of the neutrophils. Double immunolabeling techniques revealed that nuclear lamina proteins, lamins A, C and B1, and lamin B receptor were colocalized with the antigen(s) recognized by atypical p-ANCA. No colocalization was observed with classical p-ANCA and antibodies against histones (H1-H4). Our study showed that atypical p-ANCA are antinuclear antibodies reactive with granulocyte-specific antigens present in the nuclear lamina.

LBR, a chromatin and lamin binding protein from the inner nuclear membrane, is proteolyzed at late stages of apoptosis

Chromatin condensation and apposition to the nuclear envelope is an important feature of the execution phase of apoptosis. During this process, lamin proteins that are located between the inner nuclear membrane and heterochromatin are proteolyzed by the apoptosis-specific protease caspase 6. We have investigated the fate of nuclear membranes during apoptosis by studying the lamin B receptor (LBR), a transmembrane protein of the inner nuclear membrane. LBR interacts through its nucleoplasmic amino-terminal domain with both heterochromatin and B-type lamins, and is phosphorylated throughout the cell cycle, but on different sites in interphase and mitosis. We report here that: (i) the amino-terminal domain of LBR is specifically cleaved during apoptosis to generate an approximately 20 kDa soluble fragment; (ii) the cleavage of LBR is a late event of apoptosis and occurs subsequent to lamin B cleavage; (iii) the phosphorylation of LBR during apoptosis is similar to that occurring in interphase. As the association of condensed chromatin with the inner nuclear membrane persists until the late stages of apoptosis, we suggest that the chromatin binding protein LBR plays a major role in maintaining this association.

Autoantibodies against a 210 kDa glycoprotein of the nuclear pore complex as a prognostic marker in patients with primary biliary cirrhosis

It has been reported that the presence of anti-nuclear antibody against a 210 kDa glycoprotein of nuclear pore complex (anti-gp210) is highly specific for primary biliary cirrhosis (PBC). The aim of the present study was to investigate the significance of anti-gp210, especially as a prognostic marker. The presence of anti-gp210 was ascertained in 113 patients with PBC and 162 controls by indirect immunofluorescence assay using HepG2 cells and immunoblotting analysis using nuclear extracts from HeLa cells. Anti-gp210 was detected in 25 of the 113 (22.1%) patients. None of the 162 controls was positive for anti-gp210. The appearance and titre of anti-gp210 in the patients with PBC did not vary from the time of diagnosis and through their clinical course. Anti-mitochondrial antibodies (AMA), including antibodies against pyruvate dehydrogenase complex, branched chain alpha-ketoacid dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex, were not detected by enzyme-linked immunosorbent assay in five of the 113 (4.4%) patients with PBC. However, anti-gp210 alone was positive in one of these five patients. The difference in prognosis was statistically significant; patients with PBC positive for anti-gp210 died from hepatic failure more frequently than those who were negative (P < 0.01), although there were no statistically significant differences in the frequency of jaundice and the histological stage at the time of diagnosis between the two groups. We suggest that the presence of anti-gp210 is one of the independent prognostic markers able to predict, at the time of diagnosis, a poor outcome in patients with PBC.

Prevalence, kinetics, and therapeutic modulation of autoantibodies against Sp100 and promyelocytic leukemia protein in a large cohort of patients with primary biliary cirrhosis

Antinuclear antibodies (ANA) staining nuclear dot structures predominantly occur in primary biliary cirrhosis (PBC) patients and recognize the Sp100 and promyelocytic leukemia protein (PML). From retrospective analysis of sera from a clinically well-defined Canadian series of 170 PBC patients included into a 24-month therapeutic trial of ursodeoxycholic acid (UDCA), we report the prevalence of these ANA and their dynamics in the course of the disease. Using an enzyme-linked immunosorbent assay (ELISA), anti-Sp100 autoantibodies were shown in 35 (21%) patients. Thirty-three patients (19%) had autoantibodies against PML as determined by indirect immunostaining of cells overexpressing PML. Altogether, anti-nuclear dot autoantibodies were present in 25% of the 170 PBC patients. Their occurrence correlated with an unfavorable disease course, because these patients progressed significantly more frequently from early stages (I/II) to late stages (III/IV) within the 24-month observation period (P < .05). During the course of the disease, the autoantibody levels against the Sp100 full-length protein remained nearly constant in all 35 positive patients. However, 9 patients showed remarkable changes in Sp100 epitope recognition as revealed by ELISA and immunoblotting. When the occurrence of these changes and the treatment of the patients were compared retrospectively, it became evident that 8 of the 9 patients had received UDCA (42% of all Sp100-positive patients treated with UDCA). These findings indicate subtle changes of the Sp100 epitope recognition pattern during the natural course of the disease and its induction or acceleration by UDCA treatment. This implies that UDCA can modulate immunoglobulin (Ig) expression not only quantitatively, but also qualitatively.

Molecular biological xxxmethods in diagnosis and treatment of liver diseases

Molecular biology is making a tremendous impact on the diagnosis and treatment of liver diseases. Methods such as the polymerase chain reaction are changing the way physicians diagnose and monitor patients with viral hepatitis. Assays based on recombinant protein antigens allow for detection of specific autoantibodies in diseases such as primary biliary cirrhosis. The diagnosis of inherited metabolic diseases, such as hemochromatosis and Wilson disease, is being revolutionized by discovery of the defective genes involved and the development of methods to rapidly sequence DNA and identify mutations. Treatments and preventive measures are now possible with use of drugs and vaccines produced by recombinant DNA technology. Gene therapy and nucleic acid-based therapeutics are also realistic future treatment options for individuals with liver diseases.

The lamin B receptor (LBR) provides essential chromatin docking sites at the nuclear envelope

Morphological studies have established that peripheral heterochromatin is closely associated with the nuclear envelope. The tight coupling of the two structures has been attributed to nuclear lamins and lamin-associated proteins; however, it remains to be determined which of these elements are essential and which play an auxiliary role in nuclear envelope-chromatin interactions. To address this question, we have used as a model system in vitro reconstituted vesicles assembled from octyl glucoside-solubilized nuclear envelopes. Comparing the chromosome binding properties of normal, immunodepleted and chemically extracted vesicles, we have arrived at the conclusion that the principal chromatin anchorage site at the nuclear envelope is the lamin B receptor (LBR), a ubiquitous integral protein of the inner nuclear membrane. Consistent with this interpretation, purified LBR binds directly to chromatin fragments and decorates the surface of chromosomes in a distinctive banding pattern.

Targeting of membranes to sea urchin sperm chromatin is mediated by a lamin B receptor-like integral membrane protein

We have identified an integral membrane protein of sea urchin gametes with an apparent molecular mass of 56 kD that cross-reacts with an antibody against the nucleoplasmic NH2-terminal domain of human lamin B receptor (LBR). In mature sperm, p56 is located at the tip and base of the nucleus from where it is removed by egg cytosol in vitro. In the egg, p56 is present in a subset of cytoplasmic membranes (MV2 beta) which contributes the bulk of the nuclear envelope during male pronuclear formation. p56-containing vesicles are required for nuclear envelope assembly and have a chromatin-binding capacity that is mediated by p56. Lamin B is not present in these vesicles and is imported into the nucleus from a soluble pool at a later stage of pronuclear formation. Lamin B incorporation and addition of new membranes are necessary for pronuclear swelling and nuclear envelope growth. We suggest that p56 is a sea urchin LBR homologue that targets membranes to chromatin and later anchors the membrane to the lamina.

Autoantibodies to nuclear envelope antigens in chronic fatigue syndrome

We have identified and partially characterized the autoantibodies in sera of 60 patients with chronic fatigue syndrome. Approximately 52% of the sera were found to react with nuclear envelope antigens. The combination of nuclear rim staining observed in immunofluorescence microscopy and immunoblot analysis of highly purified nuclear envelope proteins provided initial characterization of these autoantibodies. Further characterization showed that some sera immunoprecipitated the in vitro transcription and translation product of a human cDNA clone encoding the nuclear envelope protein lamin B1. The autoantibodies were of the IgG isotype. The occurrence of autoantibodies to a conserved intracellular protein like lamin B1 provides new laboratory evidence for an autoimmune component in chronic fatigue syndrome.

Interaction between an integral protein of the nuclear envelope inner membrane and human chromodomain proteins homologous to Drosophila HP1

At the nuclear envelope in higher eukaryotic cells, the nuclear lamina and the heterochromatin are adjacent to the inner nuclear membrane, and their attachment is presumably mediated by integral membrane proteins. In a yeast two-hybrid screen, the nucleoplasmic domain of lamin B receptor (LBR), an integral protein of the inner nuclear membrane, associated with two human polypeptides homologous to Drosophila HP1, a heterochromatin protein involved in position-effect variegation. LBR fusion proteins bound to HP1 proteins synthesized by in vitro translation and present in cell lysates. Antibodies against LBR also co-immunoprecipitated HP1 proteins from cell extracts. LBR can interact with chromodomain proteins that are highly conserved in eukaryotic species and may function in the attachment of heterochromatin to the inner nuclear membrane in cells.

Primary biliary cirrhosis sera recognize not only gp210 but also proteins of the p62 complex bearing N-acetylglucosamine residues from rat liver nuclear envelope. Anti-p62 complex antibody in PBC

We have recently observed reactivity of primary biliary cirrhosis (PBC) sera with several proteins bearing N-acetylglucosamine residues from rat liver nuclear envelopes. The aim of this study was to characterize the reactive antigens. Sera from 31 patients with PBC, 30 with rheumatoid arthritis (RA) and 30 with Sjögren's syndrome (SS) were examined. Rim-like immunofluorescence staining was observed in 15 of 31 (48%) sera from patients with PBC, in 1 of 30 with RA and in 1 of 30 with SS. Upon immunoblotting using preparations of whole rat liver nuclear envelopes and their Triton X 100-KCl extract as antigen sources, a 200 kDa protein band was observed in 9 of sera with PBC. Furthermore, upon immunoblotting using the wheat germ aggulutinin-bound fraction of rat liver envelope as antigen, 62, 60 and 54 kDa protein bands corresponding to components of the p62 complex in the nuclear pore complex (Kita et al. Biochem. 113, 377-382) were observed in 7, 5 and 6 samples respectively, of the 31 PBC sera. Our data suggest that PBC sera recognize not only the 210 kDa protein but also the p62 complex proteins.

Signals and structural features involved in integral membrane protein targeting to the inner nuclear membrane

We have examined transfected cells by immunofluorescence microscopy to determine the signals and structural features required for the targeting of integral membrane proteins to the inner nuclear membrane. Lamin B receptor (LBR) is a resident protein of the nuclear envelope inner membrane that has a nucleoplasmic, amino-terminal domain and a carboxyl-terminal domain with eight putative transmembrane segments. The amino-terminal domain of LBR can target both a cytosolic protein to the nucleus and a type II integral protein to the inner nuclear membrane. Neither a nuclear localization signal (NLS) of a soluble protein, nor full-length histone H1, can target an integral protein to the inner nuclear membrane although they can target cytosolic proteins to the nucleus. The addition of an NLS to a protein normally located in the inner nuclear membrane, however, does not inhibit its targeting. When the amino-terminal domain of LBR is increased in size from approximately 22.5 to approximately 70 kD, the chimeric protein cannot reach the inner nuclear membrane. The carboxyl-terminal domain of LBR, separated from the amino-terminal domain, also concentrates in the inner nuclear membrane, demonstrating two nonoverlapping targeting signals in this protein. Signals and structural features required for the inner nuclear membrane targeting of proteins are distinct from those involved in targeting soluble polypeptides to the nucleoplasm. The structure of the nucleocytoplasmic domain of an inner nuclear membrane protein also influences targeting, possibly because of size constraints dictated by the lateral channels of the nuclear pore complexes.

Mitochondrial antigens, molecular mimicry and autoimmune disease

The immune system is normally tolerant to mitochondrial self-antigens, but responsive against bacteria. Low-titre anti-mitochondrial antibodies (AMA) might be involved in this discrimination. Tolerance is broken in diseases characterised by high titre AMA. Some of these AMA, against cardiolipin, cross-react with DNA. The best studied AMA are those characterising primary biliary cirrhosis (PBC). These are directed against E2 subunits of the oxo-acid dehydrogenase complexes, and also against subunits E1 alpha, E1 beta and X of the pyruvate dehydrogenase complex. AMA of PBC patients also react with bacterial E2s. Reactivities are primarily peptide-specific but with cross-reactivity between mitochondrial and microbial antigens and between E2s of respective complexes. Immunodominant epitopes, for anti E2 AMA, include the conserved sequence flanking the site of lipoyl attachment. It is proposed that the initial stimulus for antibody production is chronic urinary tract infection. AMA themselves are not pathogenic, but CD4+ T-cells would be primed, recognising the lipoyl domain epitope in association with class II HLA. Inappropriate expression of class II antigens on bile duct epithelia, (as found in PBC), might lead to presentation of a particular fragment of HLA-DR alpha, known to be a major MHC presented self-peptide in the mouse. That sequence strongly mimics the lipoyl domain and might be recognised by primed T-cells, initiating the autoimmune cascade. In the mouse, a peptide of ND1 of Complex I is presented in association with class I MHC. Cells exhibiting somatic mutation of such a peptide might thus be subject to attack by CD8+ T-cells. If such peptides were presented by class II HLA, autoimmune diseases might arise, related to mimicry between such peptides and microbial sequences and/or self-antigens. These considerations might apply in Leber's disease and in age-related pathology.

Autoantibodies in the diagnosis of systemic rheumatic diseases

Distinct profiles of autoantibodies directed to intracellular antigens can be detected in the systemic connective tissue diseases. They aid in establishing the correct diagnosis and are included in many sets of diagnostic criteria, such as the ones developed for systemic lupus erythematosus (anti-Smith antigen and anti-double-strand DNA antibodies), mixed connective tissue disease (anti-U1-nuclear ribonucleoprotein antibodies), and Sjögren's syndrome (SS) (anti-SS-A/Ro and anti-SS-B/La antibodies). They are useful prognostic markers in some situations and facilitate clinical and treatment follow-up. Autoantibodies have also been used as probes to gain insights into cell biology, helping to isolate and purify intracellular proteins involved in key cellular functions. We give detailed information on two of the most useful techniques for the detection of autoantibodies in the clinical and research laboratory settings, indirect immunofluorescence and immunoblotting. We also discuss several of the antigen-autoantibody systems found in systemic lupus erythematosus (Smith antigen, U1-nuclear ribonucleoprotein, SS-A/Ro, SS-B/La, proliferating cell nuclear antigen ribosomal ribonucleoprotein, double-strand DNA, histones, antiphospholipids, Ku, Ki/SL), systemic sclerosis (centromere, topo I, RNA polymerases, fibrillarin, polymyositis-Scl, Th/To), polymyositis/dermatomyositis (transferRNA synthetases, signal recognition particle, and others), and SS (SS-A/Ro, SS-B/La, nucleolar organizing region-90, p80-coilin), addressing their clinical significance, common detection methods, immunogenetic associations, and the molecular and cellular biology of the cognate antigens.

Autoantibodies in hepatitis delta

Hepatitis delta virus infection is associated with a wide range of different autoantibodies. The humoral immune response in chronic hepatitis D is directed against the cytoskeleton, the nucleus, the nuclear lamina and the endoplasmic reticulum. Smooth muscle antibodies (SMA), basal cell layer antibodies (BCLA), stellate epithelial cell antibodies (SECA), thymic reticular cell antibodies (TRA), perithymocytic cell antibodies (PTA), and anti-calmodulin antibodies are reactive with constituents of the cytoskeleton. Antinuclear antibodies (ANA) and anti-lamin C antibodies recognize antigens of the nucleus and the nuclear lamina, respectively. Autoantibodies directed against antigens of the endodoplasmic reticulum (LKM) are also common in chronic hepatitis D. Recently, the major molecular target of LKM-3 autoantibodies was described as an epitope on UDP-glucuronosyltransferases of family 1. In view of the important role of UGTs in drug metabolism, LKM-3 autoantibodies represent a new model to study virus induced autoimmunity in man. Future studies should focus on the role of the host immune response and the clinical relevance of these autoantibodies in chronic hepatitis D.

Colocalization of vertebrate lamin B and lamin B receptor (LBR) in nuclear envelopes and in LBR-induced membrane stacks of the yeast Saccharomyces cerevisiae

We have expressed human lamin B and the chicken lamin B receptor (LBR), either separately or together, in yeast and have monitored the subcellular location of the expressed proteins by immunofluorescence microscopy, immunoelectron microscopy, and cell fractionation. At the light microscopic level, the heterologous lamin B localized to the yeast nuclear rim and at electron microscopic resolution was found subjacent to the yeast inner nuclear membrane. These data indicate that vertebrate lamin B was correctly targeted in yeast. Expression of the heterologous LBR, either alone or together with the heterologous lamin B, resulted in the formation of membrane stacks primarily adjacent to the nuclear envelope, but also projecting from the nuclear envelope into the cytoplasm or under the plasma membrane. Double immunoelectron microscopy showed colocalization of the heterologous lamin B and LBR in the yeast nuclear envelope and in the LBR-induced membrane stacks. Cell fractionation showed the presence of the heterologous lamin B and LBR in a subnuclear fraction enriched in nuclear envelopes. The heterologous lamin B was extracted at 8 M urea, but not at 4 M urea, thus behaving as a peripheral membrane protein and indistinguishable from assembled lamins. The heterologous LBR was not extracted by 8 M urea, indicating that it was integrated into the membrane. The observed colocalization and cofractionation are consistent with previously reported in vitro binding data and suggest that heterologous lamin B and LBR interact with each other when coexpressed in yeast.

Autoantibodies against integral membrane proteins of the nuclear envelope in patients with primary biliary cirrhosis

BACKGROUND/AIMS

Autoantibodies against nuclear membrane proteins have been identified in patients with primary biliary cirrhosis (PBC). The aim of the present study was to determine the incidence of these autoantibodies in patients with PBC and examine their significance.

METHODS

An assay using recombinant polypeptides was designed to unequivocally detect autoantibodies against gp210 and the lamin B receptor, integral proteins of the nuclear membranes.

RESULTS

Autoantibodies against gp210 were detected in 15 of 159 patients with PBC and 0 of 46 controls. Autoantibodies against lamin B receptor were detected in 2 patients with PBC and 0 controls. The presence of these autoantibodies had a sensitivity of 11% and specificity of 100% for the diagnosis of PBC. Autoantibodies against gp210 were present in 4 of 19 (21%) patients with PBC who did not have detectable antimitochondrial antibodies. Patients with PBC and gp210 autoantibodies had a higher incidence of associated arthritis.

CONCLUSIONS

Autoantibodies against gp210 and the lamin B receptor are present in approximately 10% of patients with PBC. These autoantibodies are highly specific for the diagnosis of PBC and may be useful in diagnosing individuals without antimitochondrial antibodies and in identifying a subgroup of patients with an increased incidence of associated arthritis.

Autoantibodies from patients with primary biliary cirrhosis recognize a restricted region within the cytoplasmic tail of nuclear pore membrane glycoprotein Gp210

Patients with primary biliary cirrhosis (PBC) frequently have autoantibodies against a 210-kD integral glycoprotein of the nuclear envelope pore membrane. This protein, termed gp210, has a 1,783-amino acid amino-terminal domain located in the perinuclear space, a 20-amino acid transmembrane segment, and a 58-amino acid cytoplasmic carboxy-terminal tail. We now demonstrate that autoantibodies from 25 patients with PBC that recognize gp210 react with the cytoplasmic carboxy-terminal tail while none react with unmodified linear epitopes in the amino-terminal domain. The epitope(s) recognized by autoantibodies from all 25 patients is contained within a stretch of 15 amino acids. The recognized amino acid sequence is homologous to the protein products of the Escherichia coli mutY gene and Salmonella typhimurium mutB gene with an exact identity of six consecutive amino acids, suggesting that anti-gp210 antibodies may arise by molecular mimicry of bacterial antigenic determinants.

Stepwise reassembly of the nuclear envelope at the end of mitosis

The nuclear envelope consists of three distinct membrane domains: the outer membrane with the bound ribosomes, the inner membrane with the bound lamina, and the pore membrane with the bound pore complexes. Using biochemical and morphological methods, we observed that the nuclear membranes of HeLa cells undergoing mitosis are disassembled in a domain-specific manner, i.e., integral membrane proteins representing the inner nuclear membrane (the lamin B receptor) and the nuclear pore membrane (gp210) are segregated into different populations of mitotic vesicles. At the completion of mitosis, the inner nuclear membrane-derived vesicles associate with chromatin first, beginning in anaphase, whereas the pore membranes and the lamina assemble later, during telophase and cytokinesis. Our data suggest that the ordered reassembly of the nuclear envelope is triggered by the early attachment of inner nuclear membrane-derived vesicles to the chromatin.

The amino-terminal domain of the lamin B receptor is a nuclear envelope targeting signal

The lamin B receptor (LBR) is a polytopic protein of the inner nuclear membrane. It is synthesized without a cleavable amino-terminal signal sequence and composed of a nucleoplasmic amino-terminal domain of 204 amino acids followed by a hydrophobic domain with eight putative transmembrane segments. To identify a nuclear envelope targeting signal, we have examined the cellular localization by immunofluorescence microscopy of chicken LBR, its amino-terminal domain and chimeric proteins transiently expressed in transfected COS-7. Full-length LBR was targeted to the nuclear envelope. The amino-terminal domain, without any transmembrane segments, was transported to the nucleus but excluded from the nucleolus. When the amino-terminal domain of LBR was fused to the amino-terminal side of a transmembrane segment of a type II integral membrane protein of the ER/plasma membrane, the chimeric protein was targeted to the nuclear envelope, likely the inner nuclear membrane. When the amino-terminal domain was deleted from LBR and replaced by alpha-globin, the chimeric protein was retained in the ER. These findings demonstrate that the amino-terminal domain of LBR is targeted to the nucleus after synthesis in the cytoplasm and that this polypeptide can function as a nuclear envelope targeting signal when located at the amino terminus of a type II integral membrane protein synthesized on the ER.

The first membrane spanning region of the lamin B receptor is sufficient for sorting to the inner nuclear membrane

The lamin B receptor (LBR) is a polytopic integral membrane protein localized exclusively in the inner nuclear membrane domain of the nuclear envelope. Its cDNA deduced primary structure consists of a highly charged amino-terminal domain of 205 residues that faces the nucleoplasm followed by a hydrophobic domain with eight potential transmembrane segments. To identify determinants that sort LBR from its site of integration (RER and outer nuclear membrane) to the inner nuclear membrane, we prepared full-length, truncated, and chimeric cDNA constructs of chick LBR, transfected these into mammalian cells and detected the expressed protein by immunofluorescence microscopy using appropriate antibodies. Surprisingly, we found that the determinants for sorting of LBR to the inner nuclear membrane reside in a region comprising its first transmembrane sequence plus flanking residues on either side. The other transmembrane regions as well as the nucleoplasmic domain are not required for sorting. We propose that the first transmembrane segment of LBR interacts specifically with another transmembrane segment and consider several mechanisms by which such specific interaction could result in sorting to the inner nuclear membrane.

Autoantibodies to the nuclear Sp100 protein in primary biliary cirrhosis and associated diseases: epitope specificity and immunoglobulin class distribution

Sp100, a protein with a dot-like intranuclear localization in immunofluorescence microscopy, is a major target for patient autoantibodies in primary biliary cirrhosis (PBC) and occasionally in rheumatic disorders. The human Sp100 cDNA has recently been cloned, and the deduced amino acid sequence was found to contain sequence similarities with an MHC class I domain and several transacting regulatory proteins, including HIV-1 nef proteins. In this study, recombinant Sp100 fusion proteins were used to differentiate the immunoglobulin isotypes and to map the epitopes involved in the anti-Sp100 autoimmune response. PBC patients developed IgG as well as IgM and/or IgA class anti-Sp100 autoantibodies whereas most patients with rheumatic diseases developed IgG class autoantibodies only. For epitope mapping, truncated versions of the Sp100 protein were probed for immunoreactivity in ELISA and immunoblotting. With 55 sera, 17 different reaction patterns were obtained, and at least three non-overlapping major autoantigenic domains were recognized by the majority of sera. One domain, which contains the sequence similarity with HIV nef proteins, was recognized by all anti-Sp100 sera and harbours multiple, in part discontinuous, epitopes. These data demonstrate a heterogeneous and patient-specific anti-Sp100 autoimmune response which is antigen-driven and, at least in terms of isotype composition, different in PBC and non-PBC patients.

Allelic variation in the TNF-beta gene does not explain the low TNF-beta response in patients with primary biliary cirrhosis

Autoimmune disorders in humans are often associated with particular alleles of major histocompatibility genes. However, the chronic inflammatory liver disease primary biliary cirrhosis (PBC) has not been found to be correlated with certain haplotypes so far. Interestingly, an impaired production of tumour necrosis factor beta (TNF-beta) upon mitogen stimulation was observed for PBC patients, especially in the immunologically active stages of the disease. Furthermore, the identification of alleles of the TNF-beta gene which differ in one unique amino acid, and in the production of TNF-beta after phytohaemagglutinin stimulation, has prompted the idea of a possible linkage between the impaired TNF-beta response in PBC and the genetic prevalence of a certain TNF haplotype. We report here a rapid method for typing the TNFB*1 and TNFB*2 genes by a standard polymerase chain reaction. PBC patients (n = 60) as well as randomized healthy controls (n = 179) of the Munich area were studied for the occurrence of the TNF alleles. No deviation was found in the PBC collective (0.7) for the TNFB*2 distribution when compared with the control (0.67).

Primary structure of a human arginine-rich nuclear protein that colocalizes with spliceosome components

The cDNA for a 54-kDa nuclear protein (p54) has been cloned from a human hepatoma expression library. Contained within p54 is an arginine/serine-rich region similar to segments of several proteins that participate in pre-mRNA splicing including the 70-kDa component of U1 small nuclear and "suppressor-of-white-apricot" proteins. The arginine/serine-rich region is dominated by a series of 8-amino acid imperfect repetitive motifs (consensus sequence, Arg-Arg-Ser-Arg-Ser-Arg-Ser-Arg). Antibodies raised against synthetic peptides of p54 react with an approximately 70-kDa protein on immunoblots of HeLa cell and rat liver nuclear proteins. This apparent discrepancy in mass is also observed when p54 mRNA is translated in vitro. Indirect immunofluorescence studies in HeLa cells show that p54 is distributed throughout the nucleus in a speckled pattern, with an additional diffuse labeling of the nucleus excluding the nucleoli. Double immunofluorescence experiments indicate that these punctate regions are coincident with the speckles seen in cells stained with antibodies against several constituents of the pre-mRNA splicing machinery. Sedimentation analysis of HeLa cell extracts on sucrose gradients showed that p54 migrates at 4-6 S, indicating that the protein is not a tightly associated component of snRNPs. Although the function of p54 is not yet known, our structure and immunolocalization data suggest that this protein may have a role in pre-mRNA processing.

Human autoantibodies to lamin B receptor are also anti-idiotypic to certain anti-lamin B antibodies

Autoantibodies reactive with nuclear envelope proteins are mainly detected in human sera from patients with liver diseases. Some of these antibodies are directed to lamin B, lamins A and C, or to the lamin B receptor (LBR). We show here that the latter one are anti-idiotypic to certain anti-lamin B antibodies. Using an enzyme-linked immunosorbent assay specific for lamins we found that serum M containing anti-LBR antibodies inhibited the binding to lamins of anti-lamin B autoantibodies from three of five sera tested. Similar results were obtained using patient's M purified IgG. The binding of monoclonal IgM, lambda anti-lamin B antibodies produced by a lymphoblastoid cell line derived from the patient's blood lymphocytes was also inhibited. Absorption of serum M with nuclei abolished the inhibitory activity. No inhibition was recorded with normal sera or sera containing other antinuclear specificities. Anti-LBR antibodies did not alter the binding to lamins of sera containing anti-lamins A and C antibodies. Altogether these findings demonstrate that anti-LBR antibodies are also combining site related anti-idiotypic antibodies (Ab2) to certain anti-lamin B antibodies, provide further evidence for discrete specificities among anti-lamin B antibodies and suggest that the occurrence of autoantibodies to nuclear envelope antigens may be under idiotypic regulation.

Characterization of A 54-kD protein of the inner nuclear membrane: evidence for cell cycle-dependent interaction with the nuclear lamina

Using a mAb (R-7), we have characterized a 54-kD protein of the chicken nuclear envelope. Based on its biochemical properties and subnuclear distribution p54 is likely to be an integral membrane component specific to the inner nuclear membrane. Fractionation experiments indicate that p54 interacts, directly or indirectly, with the nuclear lamina, and analysis of p54 in cultured cells suggests that this interaction is controlled by cell cycle-dependent posttranslational modification, most likely phosphorylation. Modification of p54 results in a slightly reduced electrophoretic mobility, and it converts the protein from a detergent-resistant to a detergent-extractable form. Detergent solubilization of p54 can be induced in vivo by treating isolated nuclei or nuclear envelopes with highly purified cdc2 kinase, one of the most prominent kinases active in mitotic cells. These results suggest that mitotic phosphorylation of p54 might contribute to control nuclear envelope dynamics during mitosis in vivo.