Human ITCH E3 ubiquitin ligase deficiency causes syndromic multisystem autoimmune disease

ITCH as a potential therapeutic target in human cancers

The ITCH/AIP4 ubiquitin E3 ligase was discovered independently by two groups searching for atrophin-1 interacting proteins and studying the genetics of mouse coat color alteration, respectively. ITCH is classified as a NEDD4 family E3 ligase featured with the C-terminal HECT domain for E3 ligase function and WW domains for substrate recruiting. ITCH deficiency in the mouse causes severe multi-organ autoimmune disease. Its roles in maintaining a balanced immune response have been extensively characterized over the past two and a half decades. A wealth of reports demonstrate a multifaceted role of ITCH in human cancers. Given the versatility of ITCH in catalyzing both proteolytic and non-proteolytic ubiquitination of its over fifty substrates, ITCH's role in malignancies is believed to be context-dependent. In this review, we summarize the downstream substrates of ITCH, the functions of ITCH in both tumor cells and the immune system, as well as the implications of such functions in human cancers. Moreover, we describe the upstream regulatory mechanisms of ITCH and the efforts have been made to target ITCH using small molecule inhibitors.

North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition Position Paper on the Evaluation and Management for Patients With Very Early-onset Inflammatory Bowel Disease

The rate of pediatric inflammatory bowel disease (IBD) has been increasing over the last decade and this increase has occurred most rapidly in the youngest children diagnosed <6 years, known as very early-onset inflammatory bowel disease (VEO-IBD). These children can present with more extensive and severe disease than older children and adults. The contribution of host genetics in this population is underscored by the young age of onset and the distinct, aggressive phenotype. In fact, monogenic defects, often involving primary immunodeficiency genes, have been identified in children with VEO-IBD and have led to targeted and life-saving therapy. This position paper will discuss the phenotype of VEO-IBD and outline the approach and evaluation for these children and what factors should trigger concern for an underlying immunodeficiency. We will then review the immunological assays and genetic studies that can facilitate the identification of the underlying diagnosis in patients with VEO-IBD and how this evaluation may lead to directed therapies. The position paper will also aid the pediatric gastroenterologist in recognizing when a patient should be referred to a center specializing in the care of these patients. These guidelines are intended for pediatricians, allied health professionals caring for children, pediatric gastroenterologists, pediatric pathologists, and immunologists.

Gastrointestinal Disorders Associated with Primary Immunodeficiency Diseases

There are now 354 inborn errors of immunity (primary immunodeficiency diseases (PIDDs)) with 344 distinct molecular etiologies reported according to the International Union of Immunological Sciences (IUIS) (Clin Gastroenterol Hepatol 11: p. 1050-63, 2013, Semin Gastrointest Dis 8: p. 22-32, 1997, J Clin Immunol 38: p. 96-128, 2018). Using the IUIS document as a reference and cross-checking PubMed ( www.ncbi.nlm.nih.pubmed.gov ), we found that approximately one third of the 354 diseases of impaired immunity have a gastrointestinal component [J Clin Immunol 38: p. 96-128, 2018]. Often, the gastrointestinal symptomatology and pathology is the heralding sign of a PIDD; therefore, it is important to recognize patterns of disease which may manifest along the gastrointestinal tract as a more global derangement of immune function. As such, holistic consideration of immunity is warranted in patients with clinically significant gastrointestinal disease. Here, we discuss the manifold presentations and GI-specific complications of PIDDs which could lead patients to seek advice from a variety of clinician specialists. Often, patients with these medical problems will engage general pediatricians, surgeons, gastroenterologists, rheumatologists, and clinical immunologists among others. Following delineation of the presenting concern, accurate and often molecular diagnosis is imperative and a multi-disciplinary approach warranted for optimal management. In this review, we will summarize the current state of understanding of PIDD gastrointestinal disease involvement. We will do so by focusing upon gastrointestinal disease categories (i.e., inflammatory, diarrhea, nodular lymphoid hyperplasia, liver/biliary tract, structural disease, and oncologic disease) with an intent to aid the healthcare provider who may encounter a patient with an as-yet undiagnosed PIDD who presents initially with a gastrointestinal symptom, sign, or problem.

The Ubiquitin Ligase Adaptor NDFIP1 Selectively Enforces a CD8+ T Cell Tolerance Checkpoint to High-Dose Antigen

Escape from peripheral tolerance checkpoints that control cytotoxic CD8⁺ T cells is important for cancer immunotherapy and autoimmunity, but pathways enforcing these checkpoints are mostly uncharted. We reveal that the HECT-type ubiquitin ligase activator, NDFIP1, enforces a cell-intrinsic CD8⁺ T cell checkpoint that desensitizes TCR signaling during in vivo exposure to high antigen levels. Ndfip1-deficient OT-I CD8⁺ T cells responding to high exogenous tolerogenic antigen doses that normally induce anergy aberrantly expanded and differentiated into effector cells that could precipitate autoimmune diabetes in RIP-OVA^hi mice. In contrast, NDFIP1 was dispensable for peripheral deletion to low-dose exogenous or pancreatic islet-derived antigen and had little impact upon effector responses to Listeria or acute LCMV infection. These data provide evidence that NDFIP1 mediates a CD8⁺ T cell tolerance checkpoint, with a different mechanism to CD4⁺ T cells, and indicates that CD8⁺ T cell deletion and anergy are molecularly separable checkpoints.

Flow Cytometry Contributions for the Diagnosis and Immunopathological Characterization of Primary Immunodeficiency Diseases With Immune Dysregulation

Almost 70 years after establishing the concept of primary immunodeficiency disorders (PIDs), more than 320 monogenic inborn errors of immunity have been identified thanks to the remarkable contribution of high-throughput genetic screening in the last decade. Approximately 40 of these PIDs present with autoimmune or auto-inflammatory symptoms as the primary clinical manifestation instead of infections. These PIDs are now recognized as diseases of immune dysregulation. Loss-of function mutations in genes such as FOXP3, CD25, LRBA, IL-10, IL10RA, and IL10RB, as well as heterozygous gain-of-function mutations in JAK1 and STAT3 have been reported as causative of these disorders. Identifying these syndromes has considerably contributed to expanding our knowledge on the mechanisms of immune regulation and tolerance. Although whole exome and whole genome sequencing have been extremely useful in identifying novel causative genes underlying new phenotypes, these approaches are time-consuming and expensive. Patients with monogenic syndromes associated with autoimmunity require faster diagnostic tools to delineate therapeutic strategies and avoid organ damage. Since these PIDs present with severe life-threatening phenotypes, the need for a precise diagnosis in order to initiate appropriate patient management is necessary. More traditional approaches such as flow cytometry are therefore a valid option. Here, we review the application of flow cytometry and discuss the relevance of this powerful technique in diagnosing patients with PIDs presenting with immune dysregulation. In addition, flow cytometry represents a fast, robust, and sensitive approach that efficiently uncovers new immunopathological mechanisms underlying monogenic PIDs.

Ubiquitin Ligases Involved in the Regulation of Wnt, TGF-β, and Notch Signaling Pathways and Their Roles in Mouse Development and Homeostasis

The Wnt, TGF-β, and Notch signaling pathways are essential for the regulation of cellular polarity, differentiation, proliferation, and migration. Differential activation and mutual crosstalk of these pathways during animal development are crucial instructive forces in the initiation of the body axis and the development of organs and tissues. Due to the ability to initiate cell proliferation, these pathways are vulnerable to somatic mutations selectively producing cells, which ultimately slip through cellular and organismal checkpoints and develop into cancer. The architecture of the Wnt, TGF-β, and Notch signaling pathways is simple. The transmembrane receptor, activated by the extracellular stimulus, induces nuclear translocation of the transcription factor, which subsequently changes the expression of target genes. Nevertheless, these pathways are regulated by a myriad of factors involved in various feedback mechanisms or crosstalk. The most prominent group of regulators is the ubiquitin-proteasome system (UPS). To open the door to UPS-based therapeutic manipulations, a thorough understanding of these regulations at a molecular level and rigorous confirmation in vivo are required. In this quest, mouse models are exceptional and, thanks to the progress in genetic engineering, also an accessible tool. Here, we reviewed the current understanding of how the UPS regulates the Wnt, TGF-β, and Notch pathways and we summarized the knowledge gained from related mouse models.

The E3 ubiquitin ligase Itch restricts antigen-driven B cell responses

The E3 ubiquitin ligase Itch regulates antibody levels and prevents autoimmune disease in humans and mice, yet how Itch regulates B cell fate or function is unknown. We now show that Itch directly limits B cell activity. While Itch-deficient mice displayed normal numbers of preimmune B cell populations, they showed elevated numbers of antigen-experienced B cells. Mixed bone marrow chimeras revealed that Itch acts within B cells to limit naive and, to a greater extent, germinal center (GC) B cell numbers. B cells lacking Itch exhibited increased proliferation, glycolytic capacity, and mTORC1 activation. Moreover, stimulation of these cells in vivo by WT T cells resulted in elevated numbers of GC B cells, PCs, and serum IgG. These results support a novel role for Itch in limiting B cell metabolism and proliferation to suppress antigen-driven B cell responses.

Dominant TOM1 mutation associated with combined immunodeficiency and autoimmune disease

Mutations in several proteins functioning as endolysosomal components cause monogenic autoimmune diseases, of which pathogenesis is linked to increased endoplasmic reticulum stress, inefficient autophagy, and defective recycling of immune receptors. We report here a heterozygous TOM1 p.G307D missense mutation, detected by whole-exome sequencing, in two related patients presenting with early-onset autoimmunity, antibody deficiency, and features of combined immunodeficiency. The index patient suffered from recurrent respiratory tract infections and oligoarthritis since early teens, and later developed persistent low-copy EBV-viremia, as well as an antibody deficiency. Her infant son developed hypogammaglobulinemia, autoimmune enteropathy, interstitial lung disease, profound growth failure, and treatment-resistant psoriasis vulgaris. Consistent with previous knowledge on TOM1 protein function, we detected impaired autophagy and enhanced susceptibility to apoptosis in patient-derived cells. In addition, we noted diminished STAT and ERK1/2 signaling in patient fibroblasts, as well as poor IFN-γ and IL-17 secretion in T cells. The mutant TOM1 failed to interact with TOLLIP, a protein required for IL-1 recycling, PAMP signaling and autophagosome maturation, further strengthening the link between the candidate mutation and patient pathophysiology. In sum, we report here an identification of a novel gene, TOM1, associating with early-onset autoimmunity, antibody deficiency, and features of combined immunodeficiency. Other patient cases from unrelated families are needed to firmly establish a causal relationship between the genotype and the phenotype.

Primary Immunodeficiency and the Gut

This article presents the most common gastrointestinal, hepatic, and pancreatic manifestations of the primary immunodeficiency diseases, including the appropriate laboratory testing, endoscopic evaluation, and recommendations for further management.

Biallelic human ITCH variants causing a multisystem disease with dysmorphic features: A second report

We report a 23 year old female with biallelic truncating variants in the ITCH (Itchy E3 Ubiquitin protein ligase, mouse homolog of; OMIM60649) gene associated with marked short stature, severe early onset chronic lung disease resembling asthma, dysmorphic facial features, and symmetrical camptodactyly of the fingers but normal intellect. The condition has only been reported once previously (Lohr et al., American Journal of Human Genetics, 2010, 86, 447-453) in 10 children from an Old Order Amish family found to have a homozygous frameshift truncating variant in association with failure to thrive, chronic lung disease, motor and cognitive delay, and variable autoimmune diseases including autoimmune hepatitis, enteropathy, hypothyroidism, and diabetes. The condition is listed in OMIM as Autoimmune disease, Multisystem with Facial Dysmorphism (OMIM613385). The clinical course as well as the dysmorphic facial and limb features overlap closely with our patient. We believe the triad of marked syndromic short stature, chronic lung disease, and dysmorphism (with or without cognitive impairment and wider autoimmune involvement) is distinctive.

A rare case of autoimmune polyglandular syndrome with Sjögren’s syndrome and primary hypoparathyroidism

A 31-year-old woman experienced tetany and was diagnosed with Hashimoto thyroiditis and hypoparathyroidism. At 33 years of age, her renal function gradually decreased. At 39 years of age, she moved to our hospital and was diagnosed with tubulointerstitial nephritis by renal biopsy. Simultaneously, she was diagnosed with Sjögren’s syndrome by autoantibodies and salivary gland biopsy. At 40 years of age, based on hypoglycaemia and eosinophilia, she was suspected of adrenal insufficiency, and was diagnosed with primary adrenal insufficiency by both corticotropin stimulation and corticotropin-releasing hormone stimulation test. She was diagnosed as autoimmune polyglandular syndrome (APS) (Hashimoto thyroiditis and possible primary adrenal insufficiency) as well as primary hypoparathyroidism and Sjögren’s syndrome, which are very rarely complicated in APS-2. Therefore, in this patient, it was helpful to pay attention for new onset of other autoimmune diseases.

Regulation of autoimmune disease by the E3 ubiquitin ligase Itch

Itch is a HECT type E3 ubiquitin ligase that is required to prevent the development of autoimmune disease in both mice and humans. Itch is expressed in most mammalian cell types, and, based on published data, it regulates many cellular pathways ranging from T cell differentiation to liver tumorigenesis. Since 1998, when Itch was first discovered, hundreds of publications have described mechanisms through which Itch controls various biologic activities in both immune and non-immune cells. Other studies have provided insight into how Itch catalytic activity is regulated. However, while autoimmunity is the primary clinical feature that occurs in both mice and humans lacking Itch, and Itch control of immune cell function has been well-studied, it remains unclear how Itch prevents the emergence of autoimmune disease. In this review, we explore recent discoveries that advance our understanding of how Itch regulates immune cell biology, and the extent to which these clarify how Itch prevents autoimmune disease. Additionally, we discuss how molecular regulators of Itch impact its ability to control these processes, as this may provide clues on how to therapeutically target Itch to treat patients with autoimmune disease.

Ubiquitin Ligases in Cancer Immunotherapy - Balancing Antitumor and Autoimmunity

Considerable progress has been made in understanding the contribution of E3 ubiquitin ligases to health and disease, including the pathogenesis of immunological disorders. Ubiquitin ligases exert exquisite spatial and temporal control over protein stability and function, and are thus crucial for the regulation of both innate and adaptive immunity. Given that immune responses can be both detrimental (autoimmunity) and beneficial (antitumor immunity), it is vital to understand how ubiquitin ligases maintain immunological homeostasis. Such knowledge could reveal novel mechanisms underlying immune regulation and identify new therapeutic approaches to enhance antitumor immunity and safeguard against autoimmunity.

Mutation in ITCH Gene Can Cause Syndromic Multisystem Autoimmune Disease With Acute Liver Failure

Pediatric intractable autoimmune hepatitis is rare and may be responsible for acute liver failure. Mutations in the itchy E3 ubiquitin protein ligase (ITCH) gene (located on chromosome 20q11.22) can lead to a deficiency of the encoded protein, resulting in increased T-cell activity with lack of immune tolerance and manifestation of a complex systemic autoimmune disease. A 1-year-old girl of consanguineous parents received a liver transplant (LT) because of acute liver failure attributed to a drug-induced hypereosinophilic syndrome with positive liver-kidney-mikrosome-2 antibodies. Notable findings were syndromic features, dystrophy, short stature, psychomotor retardation, and muscular hypotonia. Later, we saw corticosteroid-sensitive rejections as well as a systemic autoimmune disease with detection of specific antibodies (de novo autoimmune hepatitis, thyroiditis with exophthalmos, diabetes mellitus type 1, and immune neutropenia). Histologically, liver cirrhosis with lobular inflammatory infiltrates, giant-cell hepatitis, and ductopenia was verified in chronic cholestasis. Shortly after a second LT, a comparable liver histology could be detected, and viral, bacterial, and mycotic infections deteriorated the general health condition. Because of refractory pancytopenia related to portal hypertension and hypersplenism, a posttransplant lymphoproliferative disorder was excluded. One year after the second LT, epidural and subdural bleeding occurred. Three months afterward, the girl died of sepsis. Postmortem, whole-exome sequencing revealed a homozygous mutation in the ITCH gene. A biallelic mutation in ITCH can cause a severe syndromic multisystem autoimmune disease with the above phenotypic characteristics and acute liver failure because of autoimmune hepatitis. This case reveals the importance of ubiquitin pathways for regulation of the immune system.

Early-onset inflammatory bowel disease as a model disease to identify key regulators of immune homeostasis mechanisms

Rare, monogenetic diseases present unique models to dissect gene functions and biological pathways, concomitantly enhancing our understanding of the etiology of complex (and often more common) traits. Although inflammatory bowel disease (IBD) is a generally prototypic complex disease, it can also manifest in an early-onset, monogenic fashion, often following Mendelian modes of inheritance. Recent advances in genomic technologies have spurred the identification of genetic defects underlying rare, very early-onset IBD (VEO-IBD) as a disease subgroup driven by strong genetic influence, pinpointing key players in the delicate homeostasis of the immune system in the gut and illustrating the intimate relationships between bowel inflammation, systemic immune dysregulation, and primary immunodeficiency with increased susceptibility to infections. As for other human diseases, it is likely that adult-onset diseases may represent complex diseases integrating the effects of host genetic susceptibility and environmental triggers. Comparison of adult-onset IBD and VEO-IBD thus provides beautiful models to investigate the relationship between monogenic and multifactorial/polygenic diseases. This review discusses the present and novel findings regarding monogenic IBD as well as key questions and future directions of IBD research.

Allogeneic Hematopoietic Stem Cell Transplantation for Congenital Immune Dysregulatory Disorders

Primary immunodeficiency disorders that predominantly affect immune regulation and mechanisms of self-tolerance have come into the limelight, because at least for a subgroup of monogenetic disorders, a targeted therapy has become available. Nevertheless, their management often involves the treatment of severely compromising, refractory, multi-organ autoimmunity, leading to further increased susceptibility to infections and complications of long-term immune suppressive treatment, including the risk of malignancy. While evidence for allogeneic hematopoietic stem cell transplantation (alloHSCT) as a curative treatment option for severely affected patients by this disease category accumulates, clear indications, and guidelines for alloHSCT are lacking. Predictive and stratification-relevant tools such as disease activity scores are largely missing and often there is not a consistent genotype-phenotype correlation within the same family to facilitate the decision whether to transplant or not. In this review, we provide a literature-based update on indications and outcomes of alloHSCT for congenital immune dysregulative inborn errors of immunity according to the IUIS classification 2017.

Immune regulation by protein ubiquitination: roles of the E3 ligases VHL and Itch

Protein ubiquitination is an important means of post-translational modification which plays an essential role in the regulation of various aspects of leukocyte development and function. The specificity of ubiquitin tagging to a protein substrate is determined by E3 ubiquitin ligases via defined E3-substrate interactions. In this review, we will focus on two E3 ligases, VHL and Itch, to discuss the latest progress in understanding their roles in the differentiation and function of CD4⁺ T helper cell subsets, the stability of regulatory T cells, effector function of CD8⁺ T cells, as well as the development and maturation of innate lymphoid cells. The biological implications of these E3 ubiquitin ligases will be highlighted in the context of normal and dysregulated immune responses including the control of homeostasis, inflammation, auto-immune responses and anti-tumor immunity. Further elucidation of the ubiquitin system in immune cells will help in the design of new therapeutic interventions for human immunological diseases and cancer.

Monogenic polyautoimmunity in primary immunodeficiency diseases

Primary immunodeficiency diseases (PIDs) consist of a large group of genetic disorders that affect distinct components of the immune system. PID patients are susceptible to infection and non-infectious complications, particularly autoimmunity. A specific group of monogenic PIDs are due to mutations in genes that are critical for the regulation of immunological tolerance and immune responses. This group of monogenic PIDs is at high risk of developing polyautoimmunity (i.e., the presence of more than one autoimmune disease in a single patient) because of their impaired immunity. In this review, we discuss the mechanisms of autoimmunity in PIDs and the characteristics of polyautoimmunity in the following PIDs: IPEX; monogenic IPEX-like syndrome; LRBA deficiency; CTLA4 deficiency; APECED; ALPS; and PKCδ deficiency.

The E3 ligases Itch and WWP2 cooperate to limit TH2 differentiation by enhancing signaling through the TCR

The mechanisms by which the sensitivity of naive CD4⁺ T cells to stimulation by the cognate antigen via the T cell antigen receptor (TCR) determines their differentiation into distinct helper T cell subsets remain elusive. Here we demonstrate functional collaboration of the ubiquitin E3 ligases Itch and WWP2 in regulating the strength of the TCR signal. Mice lacking both Itch and WWP2 in T cells showed spontaneous autoimmunity and lung inflammation. CD4⁺ T cells deficient in Itch and WWP2 exhibited hypo-responsiveness to TCR stimulation and a bias toward differentiation into the T_H2 subset of helper T cells. Itch and WWP2 formed a complex and cooperated to enhance TCR-proximal signaling by catalyzing the conjugation of atypical ubiquitin chains to the phosphatase SHP-1 and reducing the association of SHP-1 with the tyrosine kinase Lck. These findings indicate that targeted ubiquitination regulates the strength of the TCR signal and differentiation toward the T_H2 lineage.

Molecular Classification of Primary Immunodeficiencies of T Lymphocytes

Proper regulation of the immune system is required for protection against pathogens and preventing autoimmune disorders. Inborn errors of the immune system due to inherited or de novo germline mutations can lead to the loss of protective immunity, aberrant immune homeostasis, and the development of autoimmune disease, or combinations of these. Forward genetic screens involving clinical material from patients with primary immunodeficiencies (PIDs) can vary in severity from life-threatening disease affecting multiple cell types and organs to relatively mild disease with susceptibility to a limited range of pathogens or mild autoimmune conditions. As central mediators of innate and adaptive immune responses, T cells are critical orchestrators and effectors of the immune response. As such, several PIDs result from loss of or altered T cell function. PID-associated functional defects range from complete absence of T cell development to uncontrolled effector cell activation. Furthermore, the gene products of known PID causal genes are involved in diverse molecular pathways ranging from T cell receptor signaling to regulators of protein glycosylation. Identification of the molecular and biochemical cause of PIDs can not only guide the course of treatment for patients, but also inform our understanding of the basic biology behind T cell function. In this chapter, we review PIDs with known genetic causes that intrinsically affect T cell function with particular focus on perturbations of biochemical pathways.

Polyautoimmunity in Patients with LPS-Responsive Beige-Like Anchor (LRBA) Deficiency

BACKGROUND

Polyautoimmunity is defined as the presence of more than one autoimmune disorder in a single patient. Lipopolysaccharide (LPS)-responsive beige-like anchor (LRBA) deficiency is one of the monogenic causes of polyautoimmunity. The aim of this study was to report the characteristics of polyautoimmunity in patients with LRBA deficiency.

METHODS

A total of 14 LRBA deficiency patients with confirmed autoimmunity were enrolled in this study. For those patients with polyautoimmunity, demographic information, clinical records, laboratory, and molecular data were collected. We also compared our results with the currently reported patients with LRBA deficiency associated with polyautoimmunity.

RESULTS

In 64.2% (9 out of 14) of patients, autoimmunity presented as polyautoimmunity. In these patients, autoimmune cytopenias were the most frequent complication, observed in seven patients. Three patients presented with four different types of autoimmune conditions. The review of the literature showed that 41 of 72 reported LRBA deficient patients (74.5%) had also polyautoimmunity, with a wide spectrum of autoimmune diseases described. Hematopoietic stem cell transplantation is increasingly used as the treatment for patients with severe polyautoimmunity associated to LRBA deficiency.

CONCLUSIONS

Mutation in LRBA gene is one of the causes of monogenic polyautoimmunity. Awareness of this association is important in order to make an early diagnosis and prompt treatment.

IL-17-driven intestinal fibrosis is inhibited by Itch-mediated ubiquitination of HIC-5

Intestinal fibrosis is a major complication in inflammatory bowel diseases, but the regulatory mechanism that inhibits fibrosis remains unclear. Here we demonstrate that Itch^-/-myofibroblasts express increased amounts of profibrotic collagen type I and α-SMA in response to IL-17. Mechanistically, we demonstrate that Itch directly binds to HIC-5 and targets it for K63-linked ubiquitination to inhibit IL-17-driven intestinal fibrosis. Reconstitution of Itch^-/- myofibroblasts with wild-type Itch but not the Itch-C830A mutant normalized the expression of profibrotic genes. Similarly, shRNA-mediated inhibition of HIC-5 normalized the expression of profibrotic gene expression. Thus, we have uncovered a novel mechanism by which Itch negatively regulates intestinal fibrosis.

The ubiquitin ligase ITCH coordinates small intestinal epithelial homeostasis by modulating cell proliferation, differentiation, and migration

Maintenance of the intestinal mucosa is driven by local signals that coordinate epithelial proliferation, differentiation, and turnover in order to separate antigenic luminal contents from the host's immune system. Breaches in this barrier promote gastrointestinal pathologies ranging from inflammatory bowel disease to cancer. The ubiquitin ligase ITCH is known to regulate immune responses, and loss of function of ITCH has been associated with gastrointestinal inflammatory disorders, particularly in the colon. However, the small intestine appears to be spared from this pathology. Here we explored the physiological mechanism that underlies the preservation of mucosal homeostasis in the small intestine in mice lacking ITCH (Itch^a18H/a18H). Histological analysis of the small intestines from young adult mice revealed architectural changes in animals deficient for ITCH, including villus blunting with cell crowding, crypt expansion, and thickening of the muscularis propria relative to age-matched mice sufficient for ITCH. These differences were more prominent in the distal part of the small intestine and were not dependent upon lymphoid cells. Underlying the observed changes in the epithelium were expansion of the Ki67⁺ proliferating transit amplifying progenitor population and increased numbers of terminally differentiated mucus-secreting goblet and anti-microbial producing Paneth cells, which are both important in controlling local inflammation in the small intestine and are known to be dysregulated in inflammatory bowel disease. Homeostasis in the small intestine of Itch^a18H/a18H animals was maintained by increased cell turnover, including accelerated migration of epithelial cells along the crypt-villus axis and increased apoptosis of epithelial cells at the crypt-villus junction. Consistent with this enhanced turnover, Itch^a18H/a18H mice carrying the Min mutation (Itch^a18H/a18H; Apc^Min/+) displayed a 76% reduction in tumor burden as compared to Apc^Min/+ littermates with normal levels of ITCH. These findings highlight the role of ITCH as an important modulator of intestinal epithelial homeostasis.

Phosphorylation of the E3 ubiquitin protein ligase ITCH diminishes binding to its cognate E2 ubiquitin ligase

Heightened and extended inflammation underlies the pathogenesis of many disorders, including inflammatory bowel disease, sepsis, and inflammatory arthritis. Ubiquitin networks help dictate the strength and duration of inflammatory signaling. In innate immunity, the itchy E3 ubiquitin protein ligase (ITCH)-A20 ubiquitin-editing complex inhibits receptor-interacting Ser/Thr kinase (RIPK) activation by removing Lys-63-linked polyubiquitinated chains from key proteins in the nuclear factor kappa B (NF-κB) signaling pathway. The complex then attaches polyubiquitinated chains to these proteins to target them for lysosomal or proteasomal destruction. ITCH is phosphorylated and thereby inhibited by inhibitor of nuclear factor kappa B kinase subunit beta (IKKβ) to fine-tune the inflammatory response to the strength of the offending signal. However, the biochemical mechanism by which E3 ubiquitination is impaired by IKK-driven phosphorylation remains unclear. Here, we report that this phosphorylation impedes ITCH binding to its cognate E2 ubiquitin ligase, UbcH7. Using CRISPR-Cas9 genetic knockout to mimic the ITCH-UbcH7-inhibited state, we further show that genetic UbcH7 deficiency phenocopies ITCH phosphorylation in regulating RIPK2 ubiquitination. We conclude that phosphorylation can disrupt the binding of an E3 ubiquitin ligase to an E2-conjugating enzyme, leading to prolonged inflammatory signaling. To our knowledge, this is the first report of E3 ubiquitin ligase phosphorylation inhibiting E3 ligase activity by impairing E2-E3 complex formation.

Signal transducer and activator of transcription gain-of-function primary immunodeficiency/immunodysregulation disorders

PURPOSE OF REVIEW

To describe primary immunodeficiencies caused by gain-of-function (GOF) mutations of signal transducer and activator of transcription (STAT) genes, a group of genetically determined disorders characterized by susceptibility to infections and, in many cases, autoimmune manifestations.

RECENT FINDINGS

GOF mutations affecting STAT1 result in increased STAT tyrosine phosphorylation and secondarily increased response to STAT1-signaling cytokines, such as interferons. In contrast, STAT3 hyperactivity is not usually related to hyperphosphorylation but rather to increased STAT3-mediated transcriptional activity. In both cases, heterozygous STAT1 and STAT3 GOF mutations trigger a distinct set of genes in target cells that lead to abnormal functioning of antimicrobial response and/or autoimmunity and result in autosomal dominant diseases.

SUMMARY

Clinical manifestations of patients with STAT1 GOF are characterized by mucocutaneous candidiasis and recurrent lower tract respiratory infections. In addition, many patients have thyroiditis, type 1 diabetes mellitus, autoimmune cytopenias, cancer or aneurysms. Patients with germline STAT3 GOF mutations have an increased frequency of early-onset multiorgan autoimmunity (i.e. autoimmune enteropathy, type 1 diabetes mellitus, autoimmune interstitial lung disease and autoimmune cytopenias), lymphoproliferation, short stature and, less frequently, severe recurrent infections. Treatment options range from antimicrobial therapy, intravenous or subcutaneous immunoglobulin and immunosuppressive drugs. Some patients with STAT1 GOF disorder have undergone hematopoietic stem cell transplantation, although these have been difficult because of the underlying proinflammatory milieu from the mutation.

De novo PHIP-predicted deleterious variants are associated with developmental delay, intellectual disability, obesity, and dysmorphic features

Using whole-exome sequencing, we have identified novel de novo heterozygous pleckstrin homology domain-interacting protein (PHIP) variants that are predicted to be deleterious, including a frameshift deletion, in two unrelated patients with common clinical features of developmental delay, intellectual disability, anxiety, hypotonia, poor balance, obesity, and dysmorphic features. A nonsense mutation in PHIP has previously been associated with similar clinical features. Patients with microdeletions of 6q14.1, including PHIP, have a similar phenotype of developmental delay, intellectual disability, hypotonia, and obesity, suggesting that the phenotype of our patients is a result of loss-of-function mutations. PHIP produces multiple protein products, such as PHIP1 (also known as DCAF14), PHIP, and NDRP. PHIP1 is one of the multiple substrate receptors of the proteolytic CUL4-DDB1 ubiquitin ligase complex. CUL4B deficiency has been associated with intellectual disability, central obesity, muscle wasting, and dysmorphic features. The overlapping phenotype associated with CUL4B deficiency suggests that PHIP mutations cause disease through disruption of the ubiquitin ligase pathway.

New molecular targets for the treatment of sarcoidosis

PURPOSE OF REVIEW

Sarcoidosis is a chronic granulomatous disease typically affecting the lung, lymph nodes, and other organ systems. Evidence suggests that the morbidity and mortality rates for sarcoidosis in the USA are rising, despite widespread use of anti-inflammatory therapies. In this review, we survey new therapies that target specific inflammatory pathways in other diseases (such as rheumatoid arthritis, Crohn's disease, and psoriasis) that are similar to pathways relevant to sarcoidosis immunopathogenesis, and therefore, represent potentially new sarcoidosis therapies.

RECENT FINDINGS

Immunopathogenesis of sarcoidosis has been well elucidated over the past few years. There is abundant evidence for T-cell activation in sarcoidosis leading to activation of both Th1 and Th17 inflammatory cascades. Therapies targeting T-cell activation, Th1 pathways (such as the interleukin-6 inhibitors), Th17 pathway mediators, and others have been Food and Drug Administration approved or under investigation to treat a variety of autoimmune inflammatory diseases, but have not been studied in sarcoidosis. Targeting the p38 mitogen-activated protein kinases and the ubiquitine proteasome system with new agents may also represent a novel therapeutic option for patients with sarcoidosis.

SUMMARY

Rising morbidity and mortality rates for patients with sarcoidosis strongly support the need to develop more effective anti-inflammatory therapies to treat chronic disease.

Mechanism-Based Strategies for the Management of Autoimmunity and Immune Dysregulation in Primary Immunodeficiencies

A broad spectrum of autoimmunity is now well described in patients with primary immunodeficiencies (PIDs). Management of autoimmune disease in the background of PID is particularly challenging given the seemingly discordant goals of immune support and immune suppression. Our growing ability to define the molecular underpinnings of immune dysregulation has facilitated novel targeted therapeutics. This review focuses on mechanism-based treatment strategies for the most common autoimmune and inflammatory complications of PID including autoimmune cytopenias, rheumatologic disease, and gastrointestinal disease. We aim to provide guidance regarding the rational use of these agents in the complex PID patient population.

DOCK8 Deficiency Presenting as an IPEX-Like Disorder

PURPOSE

The dedicator of cytokinesis 8 (DOCK8) deficiency is an autosomal recessive-combined immunodeficiency whose clinical spectra include recurrent infections, autoimmunity, malignancies, elevated serum IgE, eczema, and food allergies. Here, we report on patients with loss of function DOCK8 mutations with profound immune dysregulation suggestive of an immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX)-like disorder.

METHODS

Immunophenotyping of lymphocyte subpopulations and analysis of DOCK8 protein expression were evaluated by flow cytometry. T regulatory (T_reg) cells were isolated by cell sorting, and their suppressive activity was analyzed by flow cytometry. Gene mutational analysis was performed by whole-exome and Sanger sequencing.

RESULTS

Patient 1 (P1) presented at 10 months of age with chronic severe diarrhea and active colitis in the absence of an infectious trigger, severe eczema with elevated serum IgE, and autoimmune hemolytic anemia, suggestive of an IPEX-related disorder. Whole-exome sequencing revealed a homozygous nonsense mutation in DOCK8 at the DOCK-homology region (DHR)-1 (c.1498C>T; p. R500X). Patient P2, a cousin of P1 who carries the same DOCK8 nonsense mutation, presented with eczema and recurrent ear infections in early infancy, and she developed persistent diarrhea by 3 years of age. Patient P3 presented with lymphoproliferation, severe eczema with allergic dysregulation, and chronic diarrhea with colitis. She harbored a homozygous loss of function DOCK8 mutation (c.2402 -1G→A). T_reg cell function was severely compromised by both DOCK8 mutations.

CONCLUSION

DOCK8 deficiency may present severe immune dysregulation with features that may overlap with those of IPEX and other IPEX-like disorders.

Ubiquitin-Modifying Enzymes and Regulation of the Inflammasome

Ubiquitin and ubiquitin-modifying enzymes play critical roles in a wide variety of intracellular signaling pathways. Inflammatory signaling cascades downstream of TNF, TLR agonists, antigen receptor cross-linking, and cytokine receptors, all rely on ubiquitination events to direct subsequent immune responses. In the past several years, inflammasome activation and subsequent signal transduction have emerged as an excellent example of how ubiquitin signals control inflammatory responses. Inflammasomes are multiprotein signaling complexes that ultimately lead to caspase activation and release of the interleukin-1 (IL-1) family members, IL-1β and IL-18. Inflammasome activation is critical for the host's defense against pathogens, but dysregulation of inflammasomes may contribute to the pathogenesis of multiple diseases. Ultimately, understanding how various ubiquitin interacting proteins control inflammatory signaling cascades could provide new pathways for therapeutic intervention. Here we review specific ubiquitin-modifying enzymes and ubiquitination events that orchestrate inflammatory responses, with an emphasis on the NLRP3 inflammasome.

Autoimmune enteropathies

Autoimmune enteropathy (AIE) is a rare condition characterized by intractable diarrhea and immune-mediated injury of the intestinal mucosa. As the clinical and histopathologic manifestations of this disease are highly variable, its diagnosis is challenging for both clinicians and pathologists. In fact, the term autoimmune enteropathies is likely more appropriate since the clinicopathologic manifestations are observed in association with a heterogeneous group of disorders. The pathophysiology of AIE has not been fully elucidated. It appears to result from dysregulation of intestinal immunity and particularly in children, often presents in association with immunodeficiency. The overarching histopathologic changes seen in AIE include mucosal inflammation and epithelial injury, although this can manifest in the form of different patterns. Recognition of the clinical settings and of the various histologic patterns can aid the pathologist in establishing the correct diagnosis.

High-Throughput Screening of HECT E3 Ubiquitin Ligases Using UbFluor

HECT E3 ubiquitin ligases are responsible for many human disease phenotypes and are promising drug targets; however, screening assays for HECT E3 inhibitors are inherently complex, requiring upstream E1 and E2 enzymes as well as ubiquitin, ATP, and detection reagents. Intermediate ubiquitin thioesters and a complex mixture of polyubiquitin products provide further opportunities for off-target inhibition and increase the complexity of the assay. UbFluor is a novel ubiquitin thioester that bypasses the E1 and E2 enzymes and undergoes direct transthiolation with HECT E3 ligases. The release of fluorophore upon transthiolation allows fluorescence polarization detection of HECT E3 activity. In the presence of inhibitors, HECT E3 activity is ablated, and thus no reaction and no change in FP are observed. This assay has been adapted for high-throughput screening of small molecules against HECT E3 ligases, and its utility has been proven in the discovery of HECT E3 ligase inhibitors. © 2017 by John Wiley & Sons, Inc.

Altered expression of Tumor Necrosis Factor Alpha -Induced Protein 3 correlates with disease severity in Ulcerative Colitis

Ulcerative colitis (UC), an inflammatory disorder of the colon arises from dysregulated immune response towards gut microbes. Transcription factor NFκB is a major regulatory component influencing mucosal inflammation. We evaluated expression of Tumor Necrosis Factor Alpha Induced Protein3 (TNFAIP3), the inhibitor of NFκB activation and its associated partners ITCH, RNF11 and Tax1BP1 in inflamed mucosa of UC patients. We found highly significant up-regulated mRNA expression of TNFAIP3 that negatively correlated with disease activity in UC. mRNA levels of ITCH, RNF11 and Tax1BP1 were significantly down-regulated. Significant positive correlation with disease activity was noted for Tax1BP1. All four genes showed significant down-regulation at protein level. mRNA levels of inducers of TNFAIP3 expression, NFκB p65 subunit and MAST3 was determined. There was significant increase in p65 mRNA expression and down-regulated MAST3 expression. This suggested that increase in NFκB expression regulates TNFAIP3 levels. Deficiency of TNFAIP3 expression resulted in significant up-regulation of NFκB p65 sub-unit as well as its downstream genes such as iNOS, an inflammatory marker, inhibitors of apoptosis like cIAP2 and XIAP and mediators of anti-apoptotic signals TRAF1 and TRAF2. Taken together, decreased expression of TNFAIP3 and its partners contribute to inflammation and up-regulation of apoptosis inhibitors that may create microenvironment for colorectal cancer.

Ubiquitin enzymes in the regulation of immune responses

Ubiquitination plays a central role in the regulation of various biological functions including immune responses. Ubiquitination is induced by a cascade of enzymatic reactions by E1 ubiquitin activating enzyme, E2 ubiquitin conjugating enzyme, and E3 ubiquitin ligase, and reversed by deubiquitinases. Depending on the enzymes, specific linkage types of ubiquitin chains are generated or hydrolyzed. Because different linkage types of ubiquitin chains control the fate of the substrate, understanding the regulatory mechanisms of ubiquitin enzymes is central. In this review, we highlight the most recent knowledge of ubiquitination in the immune signaling cascades including the T cell and B cell signaling cascades as well as the TNF signaling cascade regulated by various ubiquitin enzymes. Furthermore, we highlight the TRIM ubiquitin ligase family as one of the examples of critical E3 ubiquitin ligases in the regulation of immune responses.

Allosteric auto-inhibition and activation of the Nedd4 family E3 ligase Itch

The Nedd4 family E3 ligases are key regulators of cell growth and proliferation and are often misregulated in human cancers and other diseases. The ligase activities of Nedd4 E3s are tightly controlled via auto-inhibition. However, the molecular mechanism underlying Nedd4 E3 auto-inhibition and activation is poorly understood. Here, we show that the WW domains proceeding the catalytic HECT domain play an inhibitory role by binding directly to HECT in the Nedd4 E3 family member Itch. Our structural and biochemical analyses of Itch reveal that the WW2 domain and a following linker allosterically lock HECT in an inactive state inhibiting E2-E3 transthiolation. Binding of the Ndfip1 adaptor or JNK1-mediated phosphorylation relieves the auto-inhibition of Itch in a WW2-dependent manner. Aberrant activation of Itch leads to migration defects of cortical neurons during development. Our study provides a new mechanism governing the regulation of Itch.

E3 Ubiquitin Ligases Neurobiological Mechanisms: Development to Degeneration

Cells regularly synthesize new proteins to replace old or damaged proteins. Deposition of various aberrant proteins in specific brain regions leads to neurodegeneration and aging. The cellular protein quality control system develop various defense mechanisms against the accumulation of misfolded and aggregated proteins. The mechanisms underlying the selective recognition of specific crucial protein or misfolded proteins are majorly governed by quality control E3 ubiquitin ligases mediated through ubiquitin-proteasome system. Few known E3 ubiquitin ligases have shown prominent neurodevelopmental functions, but their interactions with different developmental proteins play critical roles in neurodevelopmental disorders. Several questions are yet to be understood properly. How E3 ubiquitin ligases determine the specificity and regulate degradation of a particular substrate involved in neuronal proliferation and differentiation is certainly the one, which needs detailed investigations. Another important question is how neurodevelopmental E3 ubiquitin ligases specifically differentiate between their versatile range of substrates and timing of their functional modulations during different phases of development. The premise of this article is to understand how few E3 ubiquitin ligases sense major molecular events, which are crucial for human brain development from its early embryonic stages to throughout adolescence period. A better understanding of these few E3 ubiquitin ligases and their interactions with other potential proteins will provide invaluable insight into disease mechanisms to approach toward therapeutic interventions.

Aberrant Th2 inflammation drives dysfunction of alveolar macrophages and susceptibility to bacterial pneumonia

The ubiquitin ligase, Itch, is required to prevent autoinflammatory disease in mice and humans. Itch-deficient mice develop lethal pulmonary inflammation characterized by the production of Th2 cytokines (for example, interleukin-4 (IL-4)); however, the contribution of Itch to immune defense against respiratory pathogens has not been determined. We found that Itch-deficient mice were highly susceptible to intranasal infection with the respiratory pathogen Klebsiella pneumoniae. Infected Itch-deficient mice exhibited increased immune cell infiltration, cytokine levels and bacterial burden in the respiratory tract compared with control mice. However, numbers of resident alveolar macrophages were reduced in the lungs from Itch-deficient mice both before and after infection. High levels of Th2 cytokines in the respiratory tract correlated with deceased alveolar macrophages, and genetic ablation of IL-4 restored alveolar macrophages and host defense to K. pneumoniae in Itch-deficient mice, suggesting that loss of alveolar macrophages occurred as a consequence of Th2 inflammation. Adoptive transfer of Itch^-/- CD4⁺ T cells into Rag^-/- mice was sufficient to drive reduction in numbers of Itch-replete alveolar macrophages. Finally, we found that Stat6 signaling downstream of the IL-4 receptor directly reduced fitness of alveolar macrophages when these cells were exposed to the Itch^-/- inflamed respiratory tract. These data suggest that Th2 inflammation directly impairs alveolar macrophage fitness in Itch^-/- mice, and elucidate a previously unappreciated link between Th2 cells, alveolar macrophages and susceptibility to bacterial infection.

UbFluor: A Fluorescent Thioester to Monitor HECT E3 Ligase Catalysis

HECT E3 ubiquitin ligases (∼28 are known) are associated with many phenotypes in eukaryotes and are important drug targets. However, assays used to screen for small molecule inhibitors of HECT E3s are complex and require ATP, Ub, E1, E2, and HECT E3 enzymes, producing three covalent thioester enzyme intermediates E1∼Ub, E2∼Ub, and HECT E3∼Ub (where ∼ indicates a thioester bond), and mixtures of polyubiquitin chains. To reduce the complexity of the assay, we developed a novel class of fluorescent probes, UbFluor, that act as mechanistically relevant pseudosubstrates of HECT E3s. These probes undergo a direct transthiolation reaction with the catalytic cysteine of HECT E3s, producing the catalytically active HECT E3∼Ub thioester accompanied by fluorophore release. Thus, a fluorescence polarization assay can continuously monitor UbFluor consumption by HECT E3s, and changes in UbFluor consumption rendered by biochemical point mutations or small molecule modulation of HECT E3 activity. © 2017 by John Wiley & Sons, Inc.

Ubiquitin E3 ligase Itch negatively regulates osteoblast function by promoting proteasome degradation of osteogenic proteins

OBJECTIVES

Ubiquitin E3 ligase-mediated protein degradation regulates osteoblast function. Itch, an E3 ligase, affects numerous cell functions by regulating ubiquitination and proteasomal degradation of related proteins. However, the Itch-related cellular and molecular mechanisms by which osteoblast differentiation and function are elevated during bone fracture repair are as yet unknown.

METHODS

We examined the expression levels of E3 ligases and NF-κB members in callus samples during bone fracture repair by quantitative polymerase chain reaction (qPCR) and the total amount of ubiquitinated proteins by Western blot analysis in wild-type (WT) mice. The expression levels of osteoblast-associated genes in fracture callus from Itch knockout (KO) mice and their WT littermates were examined by qPCR. The effect of NF-κB on Itch expression in C2C12 osteoblast cells was determined by a chromatin immunoprecipitation (ChIP) assay.

RESULTS

The expression levels of WW Domain Containing E3 Ubiquitin Protein Ligase 1 (Wwp1), SMAD Specific E3 Ubiquitin Protein Ligase 1 (Smurf1), SMAD Specific E3 Ubiquitin Protein Ligase 2 (Smurf2) and Itch were all significantly increased in the fracture callus of WT mice, which was associated with elevated expression of NF-κB members and total ubiquitinated proteins. Callus tissue isolated from Itch KO mice expressed higher levels of osteoblast-associated genes, including Runx2, a positive regulator of osteoblast differentiation, but osteoclast-associated genes were not increased. Both NF-κB RelA and RelB proteins were found to bind to the NF-κB binding site in the mouse Itch promoter.

CONCLUSIONS

Our findings indicate that Itch depletion may have a strong positive effect on osteoblast differentiation in fracture callus. Thus, ubiquitin E3 ligase Itch could be a potential target for enhancing bone fracture healing.Cite this article: J. Liu, X. Li, H. Zhang, R. Gu, Z. Wang, Z. Gao, L. Xing. Ubiquitin E3 ligase Itch negatively regulates osteoblast function by promoting proteasome degradation of osteogenic proteins. Bone Joint Res 2017;6:154-161. DOI: 10.1302/2046-3758.63.BJR-2016-0237.R1.

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.

Monogenic autoimmune diseases of the endocrine system

The most common endocrine diseases, type 1 diabetes, hyperthyroidism, and hypothyroidism, are the result of autoimmunity. Clustering of autoimmune endocrinopathies can result from polygenic predisposition, or more rarely, may present as part of a wider syndrome due to a mutation within one of seven genes. These monogenic autoimmune diseases show highly variable phenotypes both within and between families with the same mutations. The average age of onset of the monogenic forms of autoimmune endocrine disease is younger than that of the common polygenic forms, and this feature combined with the manifestation of other autoimmune diseases, specific hallmark features, or both, can inform clinicians as to the relevance of genetic testing. A genetic diagnosis can guide medical management, give an insight into prognosis, inform families of recurrence risk, and facilitate prenatal diagnoses.

Advantages of Extracellular Ubiquitin in Modulation of Immune Responses

T and B lymphocytes play a central role in protecting the human body from infectious pathogens but occasionally they can escape immune tolerance, become activated, and induce autoimmune diseases. All deregulated cellular processes are associated with improper functioning of the ubiquitin-proteasome system (UPS) in eukaryotic cells. The role of ubiquitin in regulation of immune responses and in autoimmune diseases is only beginning to emerge. Ubiquitin is found in intra- and extracellular fluids and is involved in regulation of numerous cellular processes. Extracellular ubiquitin ascribed a role in lymphocyte differentiation. It regulates differentiation and maturation of hematopoietic cell lines. Ubiquitination is involved in initiation, propagation, and termination of immune responses. Disrupted ubiquitination can lead to autoimmunity. Recent observations showed that it can suppress immune response and prevent inflammation. Exogenous ubiquitin may provide good potential as a new tool for targeted therapy for immune mediated disorders of various etiologies.

Itch inhibits IL-17-mediated colon inflammation and tumorigenesis by ROR-γt ubiquitination

Dysregulated expression of interleukin 17 (IL-17) in the colonic mucosa is associated with colonic inflammation and cancer. However, the cell-intrinsic molecular mechanisms by which IL-17 expression is regulated remain unclear. We found that deficiency in the ubiquitin ligase Itch led to spontaneous colitis and increased susceptibility to colon cancer. Itch deficiency in the TH17 subset of helper T cells, innate lymphoid cells and γδ T cells resulted in the production of elevated amounts of IL-17 in the colonic mucosa. Mechanistically, Itch bound to the transcription factor ROR-γt and targeted ROR-γt for ubiquitination. Inhibition or genetic inactivation of ROR-γt attenuated IL-17 expression and reduced spontaneous colonic inflammation in Itch(-/-) mice. Thus, we have identified a previously unknown role for Itch in regulating IL-17-mediated colonic inflammation and carcinogenesis.

The E3 ligase Itch in immune regulation and beyond

Itch or itchy E3 ubiquitin ligase was initially discovered by genetic studies on the mouse coat color changes, and its deletion results in an itchy phenotype with constant skin scratching and multi-organ inflammation. It is a member of the homologous to E6-associated protein C-terminus (HECT)-type family of E3 ligases, with the protein-interacting WW-domains for the recruitment of substrate and the HECT domain for the transfer of ubiquitin to the substrate. Since its discovery, numerous studies have demonstrated that Itch is involved in the control of many aspects of immune responses including T-cell activation and tolerance and T-helper cell differentiation. Itch is also implicated in other biological contexts such as tumorigenesis, development, and stress responses. Many signaling pathways are regulated by Itch-promoted ubiquitylation of diverse target proteins. Itch is also involved in human diseases. Here, we discuss the major progress in understanding the biological significance of Itch-promoted protein ubiquitylation in the immune and other systems and in Itch-mediated regulation of signal transduction.

Rare autoimmune disorders with Mendelian inheritance

Autoimmune diseases represent a heterogeneous group of common disorders defined by complex trait genetics and environmental effects. The genetic variants usually align in immune and metabolic pathways that affect cell survival or apoptosis and modulate leukocyte function. Nevertheless, the exact triggers of disease development remain poorly understood and the current therapeutic interventions only modify the disease course. Both the prevention and the cure of autoimmune disorders are beyond our present medical capabilities. In contrast, a growing number of single gene autoimmune disorders have also been identified and characterized in the last few decades. Mutations and other gene alterations exert significant effects in these conditions, and often affect genes involved in central or peripheral immunologic tolerance induction. Even though a single genetic abnormality may be the disease trigger, it usually upsets a number of interactions among immune cells, and the biological developments of these monogenic disorders are also complex. Nevertheless, identification of the triggering molecular abnormalities greatly contributes to our understanding of the pathogenesis of autoimmunity and facilitates the development of newer and more effective treatment strategies.

UbFluor: A Mechanism-Based Probe for HECT E3 Ligases

Homologous to E6AP Carboxyl Terminus E3 ubiquitin ligases (HECT, ~28 known) are genetically implicated in cancer, neurological, hypertensive, and autoimmune disorders, and are potential drug targets to treat these diseases. The major bottleneck in the field of HECT E3s is a lack of simple assays to quantify the enzymatic activity of these enzymes in the presence of small molecules. Typical assays require E1, E2, HECT E3, ubiquitin (Ub), ATP and additional reagents to detect the resulting free poly-ubiquitin chains. To address this need, we developed UbFluor, a fluorescent thioester conjugate between the C-terminus of Ub and fluorescein-thiol (Fluor-SH). UbFluor is a mechanism-based probe that undergoes a direct transthiolation reaction with the catalytic cysteine of the model HECT E3 ligase Rsp5, producing the catalytically active Rsp5~Ub (~ indicates thioester) accompanied by release of Fluor-SH. The kinetics of this two-component reaction can be easily monitored with real-time fluorescence polarization (FP) assays. Importantly, UbFluor eliminates the need to use SDS-PAGE, ATP, E1, E2 enzymes, and extra poly-ubiquitin chain detection reagents. Although the developed system lacks ATP, E1 and E2 enzymes, we show that UbFluor can recapitulate the native ubiquitination reaction by detecting and quantifying defects in transthiolation and isopeptide ligation of Rsp5 HECT E3 alanine mutants. Based on our findings, we show that UbFluor can be utilized to conduct high-throughput screens (HTS) of small molecules against HECT ligases.