Ubiquitination system and autoimmunity: The bridge towards the modulation of the immune response

The E3 ubiquitin ligase Nedd4 fosters developmental myelination in the mouse central and peripheral nervous system

Ubiquitination is a major post-translational regulatory mechanism that tunes numerous aspects of ubiquitinated target proteins, including localization, stability, and function. During differentiation and myelination, Oligodendrocytes (OLs) in the central nervous system and Schwann cells (SCs) in the peripheral nervous system undergo major cellular changes, including the tightly controlled production of large and accurate amounts of proteins and lipids. Such processes have been implied to depend on regulatory aspects of ubiquitination, with E3 ubiquitin ligases being generally involved in the selection of specific ubiquitination substrates by ligating ubiquitin to targets and granting target selectivity. In this study, we have used multiple transgenic mouse lines to investigate the functional impact of the E3 ubiquitin ligase Nedd4 in the OL- and SC-lineages. Our findings in the developing spinal cord indicate that Nedd4 is required for the correct accumulation of differentiated OLs and ensures proper myelination, supporting and further expanding previously suggested conceptual models. In sciatic nerves, we found that Nedd4 is required for timely radial sorting of axons by SCs as a pre-requirement for correct onset of myelination. Moreover, Nedd4 ensures correct myelin thickness in both SCs and spinal cord OLs.

The circular RNA hsa_circ_0045800 serves as a favorable biomarker in pathogenesis of sjögren's syndrome

BACKGROUND

Circular RNAs (circRNAs) play various roles in the development of many autoimmune diseases. However, their expression profiles and specific function in Sjögren's Syndrome remains largely unknown.

OBJECTIVES

We aimed to investigate circRNAs potential diagnostic value in primary Sjögren's syndrome (pSS) and contribution to the pathogenesis of pSS.

METHODS

This study included 102 subjects, 51 pSS patients and 51 healthy controls. The concentration of hsa_circ_0045800 was analyzed in peripheral blood mononuclear cells obtained from 51 pSS patients and 51 healthy controls by qRT-PCR. We established a receiver operating characteristic curve (ROC) to assess the biological diagnostic value of hsa_circ_0045800 for pSS. In addition, we analyzed the correlation between hsa_circ_0045800 and disease activity in Sjogren's syndrome. A differential analysis was also conducted on the concentration of hsa_circ_0045800 in patients in pSS patients before and after treatment. We studied the downstream mechanism of hsa_circ_0045800 through bioinformatics analysis and confirmed it using luciferase reporter gene assay.

RESULTS

We confirmed that the concentration of hsa_circ_0045800 was elevated 10.4-fold in peripheral blood mononuclear cells of pSS patients than in healthy controls (p = 0.00). In the pSS active disease group, the concentration of hsa_circ_0045800 is 2.5-fold higher compared to the pSS non-active disease group (p = 0.04). The concentration of hsa_circ_0045800 after treatment was decreased by 80% compared with that before treatment (p = 0.037), suggesting its utility as a potential marker for monitoring treatment efficacy. ROC curve analysis showed that the diagnostic value of hsa_circ_0045800 in pSS patients was significantly higher than that in healthy controls, with an area under the curve of 0.865, a sensitivity of 74%, and a specificity of 92%. The concentration of hsa_circ_0045800 is correlated with various clinical factors: the concentration of hsa_circ_0045800 is positively associated with age (r = 0.328, P = 0.019), oral dryness (r = 0.331, P = 0.017), while it is negatively correlated with HGB (r = -0.435, P = 0.001) and and hypothyroidism (r = -0.318, P = 0.023). Bioinformatics predictions and luciferase assays indicated that hsa_circ_0045800 acts as a molecular sponge for miR-1247-5p, with SMAD2 being a target gene of miR-1247-5p.

CONCLUSION

Our study results show that hsa_circ_0045800 potentially contributes to the development and progression of pSS via the miR-1247-5p/SMAD2 pathway. Peripheral blood mononuclear cells are directly involved in the pathogenesis of pSS, and the discovery of hsa_circ_0045800 in peripheral blood mononuclear cells highlights its potential as a novel biomarker for disease activity and diagnosis in patients with pSS. Key Points • The concentration of hsa_circ_0045800 was higher in peripheral blood mononuclear cells of pSS patients. • Hsa_circ_0045800 promoted pSS progression through miR-1247-5p-SMAD2 axis. • Hsa_circ_0045800 is a potential biomarker for pSS.

TRIM25-mediated ubiquitination of G3BP1 regulates the proliferation and migration of human neuroblastoma cells

Neuroblastoma is one of the most severe malignant tumors and accounts for substantial cancer-related mortality in children. Ras-GTPase-activating protein SH3 domain-binding protein 1 (G3BP1) is highly expressed in various cancers and acts as an important biomarker of poor prognosis. The ablation of G3BP1 inhibited the proliferation and migration of human SHSY5Y cells. Because of its important role in neuroblastoma, the regulation of G3BP1 protein homeostasis was probed. TRIM25, which belongs to the tripartite motif (TRIM) family of proteins, was identified as an interacting partner for G3BP1 using the yeast two-hybrid (Y2H) method. TRIM25 mediates the ubiquitination of G3BP1 at multiple sites and stabilizes its protein level. Then, our study found that TRIM25 knockdown also inhibited the proliferation and migration of neuroblastoma cells. The TRIM25 and G3BP1 double knockdown SHSY5Y cell line was generated, and double knockdown cells exhibited lower proliferation and migration ability than cells with only TRIM25 or G3BP1 knockdown. Further study demonstrated that TRIM25 promotes the proliferation and migration of neuroblastoma cells in a G3BP1-dependent manner. Tumor xenograft assays indicated that the ablation of TRIM25 and G3BP1 synergistically suppressed the tumorigenicity of neuroblastoma cells in nude mice, and TRIM25 promoted the tumorigenicity of G3BP1 intact SHSY5Y cells but not G3BP1 knockout cells. Thus, TRIM25 and G3BP1, two oncogenic genes, are suggested as potential therapeutic targets for neuroblastoma.

Protein ubiquitination in T cell development

As an important form of posttranslational modification, protein ubiquitination regulates a wide variety of biological processes, including different aspects of T cell development and differentiation. During T cell development, thymic seeding progenitor cells (TSPs) in the thymus undergo multistep maturation programs and checkpoints, which are critical to build a functional and tolerant immune system. Currently, a tremendous amount of research has focused on the transcriptional regulation of thymocyte development. However, in the past few years, compelling evidence has revealed that the ubiquitination system also plays a crucial role in the regulation of thymocyte developmental programs. In this review, we summarize recent findings on the molecular mechanisms and cellular pathways that regulate thymocyte ubiquitination and discuss the roles of E3 ligases and deubiquitinating enzymes (DUBs) involved in these processes. Understanding how T cell development is regulated by ubiquitination and deubiquitination will not only enhance our understanding of cell fate determination via gene regulatory networks but also provide potential novel therapeutic strategies for treating autoimmune diseases and cancer.

Ubiquitin Activating Enzyme UBA6 Regulates Th1 and Tc1 Cell Differentiation

Ubiquitination is a crucial mechanism in regulating the immune response, setting the balance between immunity and tolerance. Here, we investigated the function of a poorly understood alternative branch of the ubiquitin-activating E1 enzyme UBA6 in activating immune cells. UBA6 expression levels were elevated in T cells by toll-like receptor agonists and anti-CD3/28 antibody stimulation, but not in dendritic cells, macrophages, B cells, and natural killer cells. Additionally, we generated T cell-specific UBA6-deficient mice and found that UBA6-deficient CD4 and CD8 T cells elevated the production of interferon-gamma (IFN-γ). Moreover, the transfer of UBA6-deficient CD4 and CD8 T cells in RAG1-knockout mice exacerbated the development of multi-organ inflammation compared with control CD4 and CD8 T cell transfer. In human peripheral blood CD4 and CD8 T cells, basal levels of UBA6 in lupus patients presented much lower than those in healthy controls. Moreover, the IFN-γ production efficiency of CD4 and CD8 T cells was negatively correlated to UBA6 levels in patients with lupus. Finally, we found that the function of UBA6 was mediated by destabilization of IκBα degradation, thereby increasing NF-κB p65 activation in the T cells. Our study identifies UBA6 as a critical regulator of IFN-γ production in T cells by modulating the NF-κB p65 activation pathway.

Exploring the Roles of HERC2 and the NEDD4L HECT E3 Ubiquitin Ligase Subfamily in p53 Signaling and the DNA Damage Response

Cellular homeostasis is governed by the precise expression of genes that control the translation, localization, and termination of proteins. Oftentimes, environmental and biological factors can introduce mutations into the genetic framework of cells during their growth and division, and these genetic abnormalities can result in malignant transformations caused by protein malfunction. For example, p53 is a prominent tumor suppressor protein that is capable of undergoing more than 300 posttranslational modifications (PTMs) and is involved with controlling apoptotic signaling, transcription, and the DNA damage response (DDR). In this review, we focus on the molecular mechanisms and interactions that occur between p53, the HECT E3 ubiquitin ligases WWP1, SMURF1, HECW1 and HERC2, and other oncogenic proteins in the cell to explore how irregular HECT-p53 interactions can induce tumorigenesis.

Design, synthesis, biological evaluation and structure-activity relationship study of quinazolin-4(3H)-one derivatives as novel USP7 inhibitors

Recent research has indicated that the abnormal expression of the deubiquitinase USP7 induces tumorigenesis via multiple cell pathways, and in particular, the p53-MDM2-USP7 pathway is well understood. USP7 is emerging as a promising target for cancer therapy. However, there are limited reports on USP7 inhibitors. Here we report design, synthesis and biological evaluation of novel quinazolin-4(3H)-one derivatives as potent USP7 inhibitors. Our results indicated that the compounds C9 and C19 exhibited the greatest potency against the USP7 catalytic domain, with IC₅₀ values of 4.86 μM and 1.537 μM, respectively. Ub-AMC assays further confirmed IC₅₀ values of 5.048 μM for C9 and 0.595 μM for C19. MTT assays indicated that gastric cancer MGC-803 cells were more sensitive to these compounds than BGC-823 cells. Flow cytometry analysis revealed that C9 restricted cancer cell growth at the G0/G1 and S phases and inhibited the proliferation and clone formation of MGC-803 cells. Further biochemical experiments indicated that C9 decreased the MDM2 protein level and increased the levels of the tumour suppressors p53 and p21 in a dose-dependent manner. Docking studies predicted that solvent exposure of the side chains of C9 and C19 would uniquely form hydrogen bonds with Met407 of USP7. Additionally, C9 exhibited a remarkable anticancer effect in a zebrafish gastric cancer MGC-803 cell model. Our results demonstrated that quinazolin-4(3H)-one derivatives were suitable as leads for the development of novel USP7 inhibitors and especially for anti-gastric cancer drugs.

Conformational changes in myeloperoxidase induced by ubiquitin and NETs containing free ISG15 from systemic lupus erythematosus patients promote a pro-inflammatory cytokine response in CD4+ T cells

Background

Neutrophil extracellular traps (NETs) from patients with systemic lupus erythematosus (SLE) are characterized by lower ubiquitylation and myeloperoxidase (MPO) as a substrate. The structural and functional effect of such modification and if there are additional post-translational modifications (PTMs) are unknown.

Methods

To assess the expression and functional role of PTMs in NETs of patients with SLE; reactivation, proliferation and cytokine production was evaluated by flow cytometry using co-cultures with dendritic cells (DC) and CD4⁺ from SLE patients and healthy controls. The impact of ubiquitylation on MPO was assessed by molecular dynamics. The expression of ISG15 in NETs was evaluated by immunofluorescence and Western Blot.

Results

Fifteen patients with SLE and ten healthy controls were included. In the co-cultures of CD4⁺ lymphocytes with DC stimulated with ubiquitylated MPO or recombinant MPO, a higher expression of IFNγ and IL-17A was found in CD4⁺ from SLE patients (p < 0.05). Furthermore, with DC stimulated with ubiquitylated MPO a trend towards increased expression of CD25 and Ki67 was found in lupus CD4⁺ lymphocytes, while the opposite was documented in controls (p < 0.05). Through molecular dynamics we found the K129-K488-K505 residues of MPO as susceptible to ubiquitylation. Ubiquitylation affects the hydration status of the HEME group depending on the residue to which it is conjugated. R239 was found near by the HEME group when the ubiquitin was in K488-K505. In addition, we found greater expression of ISG15 in the SLE NETs vs controls (p < 0.05), colocalization with H2B (r = 0.81) only in SLE samples and increased production of IFNγ in PBMCs stimulated with lupus NETs compared to healthy controls NETs.

Conclusion

The ubiquitylated MPO has a differential effect on the induction of reactivation of CD4⁺ lymphocytes in patients with SLE, which may be related to structural changes by ubiquitylation at the catalytic site of MPO. Besides a lower ubiquitylation pattern, NETs of patients with SLE are characterized by the expression of ISG15, and the induction of IFNγ by Th1 cells.

HRD1, an Important Player in Pancreatic β-Cell Failure and Therapeutic Target for Type 2 Diabetic Mice

Inadequate insulin secretion in response to glucose is an important factor for β-cell failure in type 2 diabetes (T2D). Although HMG-CoA reductase degradation 1 (HRD1), a subunit of the endoplasmic reticulum-associated degradation complex, plays a pivotal role in β-cell function, HRD1 elevation in a diabetic setting contributes to β-cell dysfunction. We report in this study the excessive HRD1 expression in islets from humans with T2D and T2D mice. Functional studies reveal that β-cell-specific HRD1 overexpression triggers impaired insulin secretion that will ultimately lead to severe hyperglycemia; by contrast, HRD1 knockdown improves glucose control and response in diabetic models. Proteomic analysis results reveal a large HRD1 interactome, which includes v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA), a master regulator of genes implicated in the maintenance of β-cell function. Furthermore, mechanistic assay results indicate that HRD1 is a novel E3 ubiquitin ligase that targets MafA for ubiquitination and degradation in diabetic β-cells, resulting in cytoplasmic accumulation of MafA and in the reduction of its biological function in the nucleus. Our results not only reveal the pathological importance of excessive HRD1 in β-cell dysfunction but also establish the therapeutic importance of targeting HRD1 in order to prevent MafA loss and suppress the development of T2D.

Endoplasmic reticulum stress in autoimmune diseases: Can altered protein quality control and/or unfolded protein response contribute to autoimmunity? A critical review on Sjögren's syndrome

For many years, researchers in the field of autoimmunity have focused on the role of the immune components in the etiopathogenesis of autoimmune diseases. However, some studies have demonstrated the importance of target tissues in their pathogenesis and the breach of immune tolerance. The immune system as well as target tissue cells (plasmatic, β-pancreatic, fibroblast-like synoviocytes, thyroid follicular and epithelial cells of the lachrymal glands, salivary glands, intestine, bronchioles and renal tubules) share the characteristic of secretory cells with an extended endoplasmic reticulum (ER). The function of these cells depends considerably on a normal ER function and calcium homeostasis, so they can produce and secrete their main components, which include glycoproteins involved in antigenic presentation such as major histocompatibility complex (MHC) class I and II. All these proteins are synthesized and modified in the ER, and for this reason disturbances in the normal functions of this organelle such as protein folding, protein quality control, calcium homeostasis and redox balance, promote accumulation of unfolded or misfolded proteins, a condition known as ER stress. Autoimmune diseases are characterized by inflammation, which has been associated with an ER stress condition. Interestingly, patients with these diseases contain circulating auto-antibodies against chaperone proteins (such as Calnexin and GRP94), thus affecting the folding and assembly of MHC class I and II glycoproteins and their loading with peptide. The main purpose of this article is to review the involvement of the protein quality control and unfolded protein response (UPR) in the ER protein homeostasis (proteostasis) and their alterations in autoimmune diseases. In addition, we describe the interaction between ER stress and inflammation and evidences are shown of how autoimmune diseases are associated with an ER stress condition, with a special emphasis on the second most prevalent autoimmune rheumatic disease, Sjögren's syndrome.

Differential ubiquitination in NETs regulates macrophage responses in systemic lupus erythematosus

OBJECTIVES

To assess if ubiquitinated proteins potentially present in neutrophil extracellular traps (NETs) can modify cellular responses and induce inflammatory mechanisms in patients with systemic lupus erythematosus (SLE) and healthy subjects.

MATERIALS AND METHODS

We studied 74 subjects with SLE and 77 healthy controls. Neutrophils and low-density granulocytes were isolated, and NETs were induced. Ubiquitin content was quantified in NETs by western blot analysis, ELISA and immunofluorescence microscopy, while ubiquitination of NET proteins was assessed by immunoprecipitation. Monocyte-derived macrophages from SLE and controls were isolated and stimulated with NETs or ubiquitin. Calcium flux and cytokine synthesis were measured following these stimuli.

RESULTS

NETs contain ubiquitinated proteins, with a lower expression of polyubiquitinated proteins in subjects with SLE than in controls. Myeloperoxidase (MPO) is present in ubiquitinated form in NETs. Patients with SLE develop antiubiquitinated MPO antibodies, and titres positively correlate with Systemic Lupus Erythematosus Disease Activity Index (SLEDAI) score (P<0.01), and negatively correlate with complement components (P<0.01). Stimulation of monocyte-derived macrophages with NETs or with ubiquitin led to enhanced calcium flux. In addition, stimulation with NETs led to enhanced cytokine (tumour necrosis factor-α and interleukin-10) production in macrophages from patients with SLE when compared with controls, which was hampered by inhibition of NET internalisation by macrophages.

CONCLUSION

This is the first study to find ubiquitinated proteins in NETs, and evidence for adaptive immune responses directed towards ubiquitinated NET proteins in SLE. The distinct differences in ubiquitin species profile in NETs compared with healthy controls may contribute to dampened anti-inflammatory responses observed in SLE. These results also support a role for extracellular ubiquitin in inflammation in SLE.

Activation of CD4 and CD8 T cell receptors and regulatory T cells in response to human proteins

This study assessed in detail the influence of four different human proteins on the activation of CD4+ and CD8+ T lymphocytes and on the formation of regulatory T cells. Human whole-blood samples were incubated with four different human proteins. The effects of these proteins on the downstream immune-system response, on the expression of extracellular activation markers on and intracellular cytokines in T lymphocytes, and on the number of regulatory T cells (T-reg cells) were investigated via flow cytometry. Incubation with β-actin or glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which are cytoplasmic proteins, increased the expression of both extracellular activation markers (CD69 and HLA-DR) and intracellular cytokines but did not significantly affect the number of T-reg cells. In contrast, incubation with human albumin or insulin, which are serum proteins, reduced both extracellular activation markers and intracellular cytokine expression and subsequently increased the number of T-reg cells. These findings may help to explain the etiological basis of autoimmune diseases.

Lys63-polyubiquitination by the E3 ligase casitas B-lineage lymphoma-b (Cbl-b) modulates peripheral regulatory T cell tolerance in patients with systemic lupus erythematosus

T cells from systemic lupus erythematosus (SLE) patients display a wide array of anomalies in peripheral immune tolerance mechanisms. The role of ubiquitin ligases such as Cbl-b has been described recently in these phenomena. However, its role in resistance to suppression phenotype in SLE has not been characterized, which was the aim of the present study. Thirty SLE patients (20 with active disease and 10 with complete remission) and 30 age- and sex-matched healthy controls were recruited. Effector (CD4⁺ CD25^- ) and regulatory (CD4⁺ CD25⁺ ) T cells (T_regs ) were purified from peripheral blood mononuclear cells (PBMCs) by magnetic selection. Suppression assays were performed in autologous and allogeneic co-cultures and analysed by a flow cytometry assay. Cbl-b expression and lysine-63 (K63)-specific polyubiquitination profile were assessed by Western blotting. We found a defective Cbl-b expression in T_regs from lupus patients in contrast to healthy controls (1·1 ± 0·9 versus 2·5 ± 1·8, P = 0·003), which was related with resistance to suppression (r = 0·633, P = 0·039). Moreover, this feature was associated with deficient K63 polyubiquitination substrates and enhanced expression of phosphorylated signal transducer and activation of transcription 3 (pSTAT-3) in T_regs from lupus patients. Our findings support that Cbl-b modulates resistance to suppression by regulating the K63 polyubiquitination profile in lupus T_regs . In addition, defective K63 polyubiquitination of STAT-3 is related to increased pSTAT-3 expression, and might promote the loss of suppressive capacity of T_regs in lupus patients.

Post-Translational Modification Profiling-Functional Proteomics for the Analysis of Immune Regulation

Posttranslational modifications (PTMs) of proteins are an integral part of major cellular regulatory mechanisms dictating protein function, localization, and stability. The capacity to screen PTMs using protein microarrays has advanced our ability to identify their targets and regulatory role. This chapter discusses a unique procedure that combines functional extract-based activity assay with large-scale screening utilities of protein microarrays. This "PTM-profiling" system offers advantages in quantitatively identifying modifications in an unbiased manner in the context of specific cellular conditions. While the possibilities of studying PTMs in different settings are enormous, the immune system presents an attractive model for studying the effects of perturbations in PTMs, and specifically the ubiquitin system, as these were already implicated in both immune function and dysfunction. This chapter discusses the significance of PTM profiling in addressing basic questions in immunology. We describe detailed protocols for the preparation of functional cell extracts from immune cell cultures, following differentiation or induced signals, and screening PTMs on protein arrays, as well as basic guidelines for data analysis and interpretation.

Rituximab in the Treatment of Jo1 Antibody–associated Antisynthetase Syndrome: Anti-Ro52 Positivity as a Marker for Severity and Treatment Response

Objective.

Rituximab (RTX) has been used successfully for the treatment of severe Jo1 antibody-associated antisynthetase syndrome. The aim of this retrospective study was to evaluate the effect of RTX in severe Jo1 antisynthetase syndrome and determine predictive factors for response.

Methods.

There were 61 patients with Jo1 antisynthetase syndrome identified; 18 of these received RTX. One patient was lost to followup. The remaining 17 patients and 30 out of 43 patients who were treated with conventional immunosuppressive (IS) drugs were followed for a mean of 35 months and 84 months, respectively.

Results.

Polymyositis/dermatomyositis (95%) and interstitial lung disease (ILD; 66%) were the dominant clinical manifestations. Detection of anti-Ro52 antibodies (43%) was significantly associated with acute-onset ILD (p = 0.016) with O2 dependency, and patients with high concentrations of anti-Ro52 (20%) had the highest risk (p = 0.0005). Sixteen out of 18 patients (89%) showed a fast and marked response to RTX. Among those patients who were highly positive for anti-Ro52, response to RTX was seen in 7 out of 7 cases (100%), but no response to cyclophosphamide (n = 4), cyclosporine A (n = 3), azathioprine (n = 9), methotrexate (n = 5), or leflunomide (n = 2) was observed. One patient treated with RTX died of pneumonia.

Conclusion.

RTX is effective in the treatment of severe forms of Jo1 antisynthetase syndrome. In our retrospective study, the presence of high anti-Ro52 antibody concentrations predicts severe acute-onset ILD and nonresponse to IS drugs. In contrast to conventional IS, RTX is equally effective in patients with Jo1 antisynthetase syndrome, independent of their anti-Ro52 antibody status.

Immune response to chemically modified proteome

Both enzymatic and nonenzymatic PTMs of proteins involve chemical modifications. Some of these modifications are prerequisite for the normal functioning of cell, while other chemical modifications render the proteins as "neo-self" antigens, which are recognized as "non-self" leading to aberrant cellular and humoral immune responses. However, these modifications could be a secondary effect of autoimmune diseases, as in the case of type I diabetes, hyperglycemia leads to protein glycation. The enigma of chemical modifications and immune response is akin to the "chick-and-egg" paradox. Nevertheless, chemical modifications regulate immune response. In some of the well-known autoimmune diseases such as rheumatoid arthritis, systemic lupus erythematosus, and multiple sclerosis, chemically modified proteins act as autoantigens forming immune complexes. In some instances, chemical modifications are also involved in regulating immune response during pathogen infection. Further, the usefulness of proteomic analysis of immune complexes is briefly discussed.

Ubiquitin in the immune system

Ubiquitination is a post-translational modification process that has been implicated in the regulation of innate and adaptive immune responses. There is increasing evidence that both ubiquitination and its reversal, deubiquitination, play crucial roles not only during the development of the immune system but also in the orchestration of an immune response by ensuring the proper functioning of the different cell types that constitute the immune system. Here, we provide an overview of the latest discoveries in this field and discuss how they impact our understanding of the ubiquitin system in host defence mechanisms as well as self-tolerance.

Molecular mechanisms of T-cell anergy

Anergy is a long-term stable state of T-lymphocyte unresponsiveness to antigenic stimulation associated with the blockade of IL-2 production and proliferation. Anergy is a pathway of peripheral tolerance formation. In this review, mechanisms underlying T-cell tolerization are considered in a classical in vitro model of clonal anergy, and these mechanisms are compared with different pathways of anergy induction in vivo. Special attention is given to regulatory T-lymphocytes because, on one hand, anergy is a specific feature of these cells, and on the other hand anergy is also a mechanism of their action on target cells - effector T-lymphocytes. The role of this phenomenon in the differentiation of regulatory T-cells and also in the development of activation-induced apoptosis in effector T-lymphocytes is discussed.

Pharmacological targets in the ubiquitin system offer new ways of treating cancer, neurodegenerative disorders and infectious diseases

Recent advances in the development and discovery of pharmacological interventions within the ubiquitin-proteasome system (UPS) have uncovered an enormous potential for possible novel treatments of neurodegenerative disease, cancer, immunological disorder and microbial infection. Interference with proteasome activity, although initially considered unlikely to be exploitable clinically, has already proved to be very effective against haematological malignancies, and more specific derivatives that target subsets of proteasomes are emerging. Recent small-molecule screens have revealed inhibitors against ubiquitin-conjugating and -deconjugating enzymes, many of which have been evaluated for their potential use as therapeutics, either as single agents or in synergy with other drugs. Here, we discuss recent advances in the characterisation of novel UPS modulators (in particular, inhibitors of ubiquitin-conjugating and -deconjugating enzymes) and how they pave the way towards new therapeutic approaches for the treatment of proteotoxic disease, cancer and microbial infection.

Ligand of Numb proteins LNX1p80 and LNX2 interact with the human glycoprotein CD8α and promote its ubiquitylation and endocytosis

E3 ubiquitin ligases give specificity to the ubiquitylation process by selectively binding substrates. Recently, their function has emerged as a crucial modulator of T-cell tolerance and immunity. However, substrates, partners and mechanism of action for most E3 ligases remain largely unknown. In this study, we identified the human T-cell co-receptor CD8 α-chain as binding partner of the ligand of Numb proteins X1 (LNX1p80 isoform) and X2 (LNX2). Both LNX mRNAs were found expressed in T cells purified from human blood, and both proteins interacted with CD8α in human HPB-ALL T cells. By using an in vitro assay and a heterologous expression system we showed that the interaction is mediated by the PDZ (PSD95-DlgA-ZO-1) domains of LNX proteins and the cytosolic C-terminal valine motif of CD8α. Moreover, CD8α redistributed LNX1 or LNX2 from the cytosol to the plasma membrane, whereas, remarkably, LNX1 or LNX2 promoted CD8α ubiquitylation, downregulation from the plasma membrane, transport to the lysosomes, and degradation. Our findings highlight the function of LNX proteins as E3 ligases and suggest a mechanism of regulation for CD8α localization at the plasma membrane by ubiquitylation and endocytosis.

Clinical significance of anti-Ro52 (TRIM21) antibodies non-associated with anti-SSA 60kDa antibodies: results of a multicentric study

Ro52 antigen has recently been identified as TRIM21 protein, but the clinical significance of anti-Ro52/TRIM21 antibodies remains controversial. The aim of this multicentric study was to investigate the significance of anti-Ro52 antibodies without anti-SSA/Ro60 antibodies in various connective diseases. Sera were selected by each laboratory using its own method (ELISA, immunodot or Luminex technology), and then performed with ANA Screen BioPlex™ reagent (BIO-RAD). Among the 247 screened sera, 155/247 (63%) were confirmed as anti-Ro52 positive and anti-SSA/Ro60 negative. These sera were analyzed for the detection of other antibodies in relation with clinical settings. Isolated anti-Ro52 antibodies were detected in 89/155 (57%) sera. For the remaining sera (66/155), the main antibodies associations were Sm/SmRNP or Chromatin (n=38; 57%), Jo1 (n=17; 26%) and CenpB (n=9; 14%). Clinical data from the 155 patients showed high prevalence in autoimmune diseases (73%) including myositis or dermatomyositis (n=30), lupus (n=23); Sjögren and/or sicca syndrome (n=27); CREST or Systemic sclerosis (n=11) and autoimmune hepatitis (n=11). We found that pulmonary manifestations were often associated with the presence of anti-Ro52 antibodies (n=34, 22%), in addition with anti-tRNA synthetases, anti-SRP or anti-Ku antibodies (18/34) or isolated in half of cases (16/34). Separate detection of anti-Ro52 antibodies might be useful in related antisynthetase syndrome diagnosis. The presence of anti-Ro52 antibodies should probably precede development of autoimmune disease and must induce sequential follow-up of positive patients, particularly in interstitial lung disease progression.

Contribution of lysine 11-linked ubiquitination to MIR2-mediated major histocompatibility complex class I internalization

The polyubiquitin chain is generated by the sequential addition of ubiquitin moieties to target molecules, a reaction between specific lysine residues that is catalyzed by E3 ubiquitin ligase. The Lys(48)-linked and Lys(63)-linked polyubiquitin chains are well established inducers of proteasome-dependent degradation and signal transduction, respectively. The concept has recently emerged that polyubiquitin chain-mediated regulation is even more complex because various types of atypical polyubiquitin chains have been discovered in vivo. Here, we demonstrate that a novel complex ubiquitin chain functions as an internalization signal for major histocompatibility complex class I (MHC I) membrane proteins in vivo. Using a tetracycline-inducible expression system and quantitative mass spectrometry, we show that the polyubiquitin chain generated by the viral E3 ubiquitin ligase of Kaposi sarcoma-associated herpesvirus, MIR2, is a Lys(11) and Lys(63) mixed-linkage chain. This novel ubiquitin chain can function as an internalization signal for MHC I through its association with epsin1, an adaptor molecule containing ubiquitin-interacting motifs.

Neuropilins and semaphorins - from angiogenesis to autoimmunity

Angiogenesis, the growth of new blood vessels from preexisting ones, is an important process in health and disease. The persistence of neovascularization in inflammatory diseases, such as rheumatoid arthritis (RA), might facilitate the entrance of inflammatory cells into the synovium and stimulate pannus formation. Several potent pro-angiogenic cytokines have been implicated in inflammatory angiogenesis. Of these, vascular endothelial growth factor (VEGF) and its receptors (VEGFRs) have been demonstrated to play a central role in RA, systemic lupus erythematosus (SLE) and multiple sclerosis (MS). Increased serum levels of VEGF were found to correlate with disease activity and severity of these diseases whereas, remission was associated with decreased levels. In the last few years, other molecules, initially found in neurodevelopment, were found to be involved in angiogenesis and recently also in the immune system and autoimmunity. Neuropilins (NPs) are VEGF receptors, while some of the semaphorins (SEMAs) are neuropilins' ligands. Their involvement in the development of autoimmune diseases and the various mechanisms by which they may induce autoimmunity will be discussed in this review.

Regulation of T-lymphocyte physiology by the Chat-H/CasL adapter complex

The Cas family of proteins consists of at least four members implicated in the regulation of diverse cellular processes such as cell proliferation, adhesion, motility, and cancer cell metastasis. Cas family members have conserved C-termini that mediate constitutive heterotypic interactions with members of a different group of proteins, the NSP family. Both the Cas and NSP proteins have conserved domains that mediate protein-protein interactions with other cytoplasmic intermediates. Signaling modules assembled by these proteins in turn regulate signal transduction downstream of a variety of receptors including integrin, chemokine, and antigen receptors. T lymphocytes express the NSP protein NSP3/Chat-H and the Cas protein Hef1/CasL, which are found in a constitutive complex in naive T cells. We recently showed that Chat-H and Hef1/CasL regulate integrin-mediated adhesion and promote T-cell migration and trafficking downstream of activated chemokine receptors. It is currently unclear if the Chat-H/CasL module also plays a role in antigen receptor signaling. Here we review our current knowledge of how Chat-H and Hef1/CasL regulate T-cell physiology and whether this protein complex plays a functional role downstream of T-cell receptor activation.

Strategies for the identification of ubiquitin ligase inhibitors

Dysregulation of the UPS (ubiquitin-proteasome system) has been implicated in a wide range of pathologies including cancer, neurodegeneration and viral infection. Inhibiting the proteasome has been shown to be an effective therapeutic strategy in humans; however, toxicity with this target remains high. E3s (Ub-protein ligases) represent an alternative attractive therapeutic target in the UPS. In this paper, we will discuss current platforms that report on E3 ligase activity and can detect E3 inhibitors, and underline the advantages and disadvantages of each approach.

Virus-induced autoimmune diabetes in the LEW.1WR1 rat requires Iddm14 and a genetic locus proximal to the major histocompatibility complex

OBJECTIVE

To identify genes that confer susceptibility to autoimmune diabetes following viral infection in the LEW.1WR1 rat.

RESEARCH DESIGN AND METHODS

About 2% of LEW.1WR1 rats develop spontaneous autoimmune diabetes. Immunological perturbants including viral infection increase both the frequency and tempo of diabetes onset. To identify diabetes susceptibility genes (LEW.1WR1 x WF), F2 rats were infected with Kilham rat virus following brief pretreatment with polyinosinic:polycytidylic acid. This treatment induces diabetes in 100% of parental LEW.1WR1 rats and 0% of parental WF rats. Linkage to diabetes was analyzed by genome-wide scanning.

RESULTS

Among 182 F2 rats, 57 (31%) developed autoimmune diabetes after a mean latency of 16 days. All diabetic animals and approximately 20% of nondiabetic animals exhibited pancreatic insulitis. Genome-wide scanning revealed a requirement for the Iddm14 locus, long known to be required for diabetes in the BB rat. In addition, a new locus near the RT1 major histocompatibility complex (MHC) was found to be a major determinant of disease susceptibility. Interestingly, one gene linked to autoimmune diabetes in mouse and human, UBD, lies within this region.

CONCLUSIONS

The Iddm14 diabetes locus in the rat is a powerful determinant of disease penetrance in the LEW.1WR1 rat following viral infection. In addition, a locus near the MHC (Iddm37) conditions diabetes susceptibility in these animals. Other, as-yet-unidentified genes are required to convert latent susceptibility to overt diabetes. These data provide insight into the polygenic nature of autoimmune diabetes in the rat and the interplay of genetic and environmental factors underlying disease expression.

In vitro model for the activation of CD4 and CD8 T cell receptors

Previously, most models that sought to explain the misregulation of immune cell function assumed molecular similarities between the disease-causing pathogens and the host's proteins. In recent time several different models have been proposed and in this study, these concepts are compared to a new hypothesis proposing another explanation for this immune dysregulation: the possibility that the mislocalization of proteins may be responsible for autoimmune activity. Based on this hypothesis, proteins are recognized as self or non-self depending on where they appear in sufficiently high concentrations. To examine this new idea, the intracellular human proteins beta-actin, GAPDH, and hemoglobin as well as the extracellular human proteins insulin and albumin, were added to human whole blood samples. After an incubation period, the activation of whole-blood T lymphocytes in the samples was measured. The observed activation pattern of the T lymphocytes fit well with the proposed hypothesis. Therefore, these data suggest that protein mislocalization and/or errors within protein trafficking might be important in the development of autoimmune diseases.

Emerging roles for the ubiquitin-proteasome system and autophagy in pancreatic beta-cells

Protein degradation in eukaryotic cells is mediated primarily by the ubiquitin-proteasome system and autophagy. Turnover of protein aggregates and other cytoplasmic components, including organelles, is another function attributed to autophagy. The ubiquitin-proteasome system and autophagy are essential for normal cell function but under certain pathological conditions can be overwhelmed, which can lead to adverse effects in cells. In this review we will focus primarily on the insulin-producing pancreatic beta-cell. Pancreatic beta-cells respond to glucose levels by both producing and secreting insulin. The inability of beta-cells to secrete sufficient insulin is a major contributory factor in the development of type 2 diabetes. The aim of this review is to examine some of the crucial roles of the ubiquitin-proteasome system and autophagy in normal pancreatic beta-cell function and how these pathways may become dysfunctional under pathological conditions associated with metabolic syndromes.