Itchy Mice: The Identification of a New Pathway for the Development of Autoimmunity

E3-ubiquitin ligases and recent progress in osteoimmunology

Ubiquitin-mediated proteasomal degradation is a post-transcriptional protein modification that is comprised of various components including the 76-amino acid protein ubiquitin (Ub), Ub-activating enzyme (E1), Ub-conjugating enzyme (E2), ubiquitin ligase (E3), deubiquitinating enzyme (DUB) and proteasome. We and others have recently provided genetic evidence showing that E3-ubiquitin ligases are associated with bone metabolism, the immune system and inflammation through ubiquitylation and subsequent degradation of their substrates. Dysregulation of the E3-ubiquitin ligase RNF146-mediated degradation of the adaptor protein 3BP2 (SH3 domain-binding protein 2) causes cherubism, an autosomal dominant disorder associated with severe inflammatory craniofacial dysmorphia syndrome in children. In this review, on the basis of our discoveries in cherubism, we summarize new insights into the roles of E3-ubiquitin ligases in the development of human disorders caused by an abnormal osteoimmune system by highlighting recent genetic evidence obtained in both human and animal model studies.

The E3 ubiquitin ligase Itch deficiency promotes antigen-driven B-cell responses in mice

Deficiency of Itch, an E3 ubiquitin ligase, usually induced severe systemic and progressive autoimmune disease. The Itch function is well studied in T cells but not in B cells. We hypothesize that B-cell-specific Itch deficiency promoted antigen-induced B-cell activation and antibody-expressing plasma cell (PC) production. We found that unlike Itch KO, Itch cKO (CD19^cre Itch^f/f ) mice did not demonstrated a significant increase in the sizes of spleens and LNs, antibody level, and base mutation of antibody gene. However, in line with the fact that Itch expression decreased in GC B cells, PCs, and plasmablast (PB)-like SP 2/0 cells, Itch deficiency promoted B-cell activation and antibody production induced by antigens including lipopolysaccharide (LPS) and sheep red blood cells (SRBCs). Mechanistically, we found that Itch deficiency promotes antigen-induced cytokine production because Itch controls the proteins (e.g., eIF3a, eIF3c, eIF3h) with translation initiation factor activity. Altogether, our data suggest that Itch deficiency promotes antigen-driven B-cell response. This may provide hints for Itch-targeted treatment of patients with autoimmune disease.

The Itch-Scratch Cycle: A Review of the Mechanisms

BACKGROUND

Despite being one of the most common presenting dermatological symptoms, itching continues to perplex health care professionals because it is notoriously difficult to control.

OBJECTIVE

This review gathers evidence to answer the 2-part question, "Why do we itch and scratch?" by exploring the history of itchy disease, the neurobiology of itch, and the 4 different clinical origins of itch: pruritogenic, neurological, neuropathic, and psychological.

RESULTS

The automated scratching reflex and its biological and psychological reasons for existence are complicated and poorly understood. Currently, there are a myriad of treatments available for individuals suffering from this condition; however, many remain symptomatic.

CONCLUSIONS

The itch-scratch cycle is a complex pain-like sensation with a reflex-like response. In the future, continued exploration into the mechanisms behind itch and scratch may open the doors for new therapeutic interventions.

Notch1 primes CD4 T cells for T helper type I differentiation through its early effects on miR-29

The transmembrane receptor, Notch1 plays an important role during the differentiation of CD4 T cells into T helper (Th) subsets in the presence of appropriate cytokines, including differentiation into Th1 cells. MicroRNAs have also been shown to be important regulators of immune responses, including negatively regulating cytokine production by Th1 cells. The miR-29 family of microRNAs can act to inhibit tbx21 and ifng transcription, two important pro-inflammatory genes that are abundantly expressed in Th1 cells. Here we show that Notch1 may prime CD4 T cells to be responsive to Th1-polarizing cues through its early repressive effects on the miR-29 family of microRNAs. Using a combination of cell lines and primary cells, we demonstrate that Notch1 can repress miR-29a, miR-29b, and miR-29c transcription through a mechanism that is independent of NF-κB. We further show that this repression is mediated by canonical Notch signaling and requires active Mastermind like (MAML) 1, but this process is superseded by positive regulation of miR-29 in response to IFNγ at later stages of CD4 T cell activation and differentiation. Collectively, our data suggest an additional mechanism by which Notch1 signaling may fine-tune Th1 cell differentiation.

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.

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.

Ubiquitylation as a Rheostat for TCR Signaling: From Targeted Approaches Toward Global Profiling

T cell receptor (TCR) signaling must be precisely tuned to limit collateral damage and prevent reactivity to self, while still allowing robust protective immune responses that control pathogen invasion. One process that can be used to promote, modify, or terminate TCR signaling is ubiquitylation. During ubiquitylation, ubiquitin is covalently attached to target proteins through a multistep process, in which E3 ubiquitin ligases promote the formation of ubiquitin chains on selected substrates. Ubiquitylation can facilitate protein–protein interactions, direct a protein to a specific subcellular location, or initiate protein destruction. Like phosphorylation, ubiquitylation is a reversible process – deubiquitylating enzymes counteract ligase function by removing ubiquitin chains. This reversibility also allows for ubiquitin chain “editing.” Based on an emerging wealth of information from genetic loss-of-function studies showing that deregulation of ubiquitylation pathways leads to immune dysfunction, it has become increasingly apparent that the dynamic process of ubiquitylation is critical for normal immune cell function. In this review, we will describe how ubiquitylation acts as a key modulator and integrator of signaling downstream of TCR engagement. Specifically, we highlight the known roles of the substrate-specific E3 ligases and deubiquitylating enzymes in TCR signaling and T cell activation. While it is clear that ubiquitin enzymes tune T cell signaling and T cell function, elucidating the molecular mechanisms by which these proteins modulate T cells has met with significant challenges. Identifying substrates of these enzymes has been a particular challenge, and thus substrates of many E3 ligases and deubiquitylating enzymes remain largely unknown. To that end, we discuss the promise, and some practical considerations, of using proteomics-based techniques for unbiased identification of putative substrates of ubiquitin cascade proteins within primary T cells. These methods provide an exciting opportunity for further defining how TCR signals are regulated and for identifying new targets for therapeutic modulation.

Female mice with loss-of-function ITCH display an altered reproductive phenotype

Major progress in deciphering the role of the E3 ligase, ITCH, in animal physiology has come from the generation and identification of Itch loss-of-function mutant mice (itchy). Mutant mice display an autoimmune-like phenotype characterized by chronic dermatitis, which has been attributed to increased levels of ITCH target proteins (e.g. transcription factors JUNB and CJUN) in T cells. Autoimmune disorders also exist in humans with Itch frameshift mutations resulting in loss of functional ITCH protein. Recent phenotypic analysis of male itchy mice revealed reduced sperm production, although cross breeding experiments showed no difference in litter size when male itchy mice were bred to wild type females. However, a reduction in litter sizes did occur when itchy females were bred to wild type males. Based on these results, characterization of female reproductive function in itchy mice was performed. Developmental analysis of fetuses at gestational day 18.5, cytological evaluation of estrous cyclicity, histopathological analysis of ovaries, and protein analysis were used to investigate the itchy reproductive phenotype. Gross skeletal and soft tissue analysis of gestational day 18.5 itchy fetuses indicated no gross developmental deformities. Itchy females had reduced implantation sites, decreased corpora lutea, and increased estrous cycle length due to increased number of days in estrus compared to controls. Alterations in the expression of prototypical ITCH targets in the ovaries were not indicated, suggesting that an alteration in an as yet defined ovary-specific ITCH substrate or interaction with the altered immune system likely accounts for the disruption of female reproduction. This report indicates the importance of the E3 ligase, ITCH, in female reproduction.

An IκB Kinase-Regulated Feedforward Circuit Prolongs Inflammation

Loss of NF-κB signaling causes immunodeficiency, whereas inhibition of NF-κB can be efficacious in treating chronic inflammatory disease. Inflammatory NF-κB signaling must therefore be tightly regulated, and although many mechanisms to downregulate NF-κB have been elucidated, there have only been limited studies demonstrating positive feedforward regulation of NF-κB signaling. In this work, we use a bioinformatic and proteomic approach to discover that the IKK family of proteins can phosphorylate the E3 ubiquitin ligase ITCH, a critical downregulator of TNF-mediated NF-κB activation. Phosphorylation of ITCH by IKKs leads to impaired ITCH E3 ubiquitin ligase activity and prolongs NF-κB signaling and pro-inflammatory cytokine release. Since genetic loss of ITCH mirrors IKK-induced ITCH phosphorylation, we further show that the ITCH(-/-) mouse's spontaneous lung inflammation and subsequent death can be delayed when TNF signaling is genetically deleted. This work identifies a new positive feedforward regulation of NF-κB activation that drives inflammatory disease.

Age-dependent alterations in spermatogenesis in itchy mice

Spermatogenesis is an intricate process in which spermatogonial stem cells divide and differentiate to produce mature sperm. This process strongly depends on protein turnover both in the developing germ cells and the supportive Sertoli cells, and recent evidence has demonstrated the role of the ubiquitin-proteasome system in this protein turnover in the testis. Itch, an E3 ligase important in the immune system, has been implicated in regulating the blood testis barrier. Although the specific role of Itch during spermatogenesis is not yet well understood, its ubiquitous expression and wide array of functional targets suggest multiple and tissue-specific roles. Here the testes of mice that lack Itch protein are evaluated at two developmental time points: peri-pubertal postnatal day (PND) 28 and adult PND 56. Itchy mice demonstrate an increased germ cell apoptotic index compared with wild type C57BL/6J mice at both PND 28 and PND 56. A corresponding 27% reduction in the total number of spermatid heads produced in PND 56 itchy mice was also evident. A histological evaluation of itchy mice revealed a delay in spermatogenesis at PND 28 and disorganization of late stage spermatids at PND 56. An analysis of several apoptotic markers revealed an age-dependent change in cleaved caspase 9, an intrinsic apoptosis mediator. The breeding success of the itchy mice was also significantly decreased, possibly due to a developmental defect. Taken together, these findings indicate that Itch is required for functional spermatogenesis, and that it may play differing cellular roles during development.

The role of E3 ligases in the ubiquitin-dependent regulation of spermatogenesis

The ubiquitination of proteins is a post-translational modification that was first described as a means to target misfolded or unwanted proteins for degradation by the proteasome. It is now appreciated that the ubiquitination of proteins also serves as a mechanism to modify protein function and cellular functions such as protein trafficking, cell signaling, DNA repair, chromatin modifications, cell-cycle progression and cell death. The ubiquitination of proteins occurs through the hierarchal transfer of ubiquitin from an E1 ubiquitin-activating enzyme to an E2 ubiquitin-conjugating enzyme and finally to an E3 ubiquitin ligase that transfers the ubiquitin to its target protein. It is the final E3 ubiquitin ligase that confers the substrate specificity for ubiquitination and is the focus of this review. Spermatogenesis is a complex and highly regulated process by which spermatogonial stem cells undergo mitotic proliferation and expansion of the diploid spermatogonial population, differentiate into spermatocytes and progress through two meiotic divisions to produce haploid spermatids that proceed through a final morphogenesis to generate mature spermatozoa. The ubiquitination of proteins in the cells of the testis occurs in many of the processes required for the progression of mature spermatozoa. Since it is the E3 ubiquitin ligase that recognizes the target protein and provides the specificity and selectivity for ubiquitination, this review highlights known examples of E3 ligases in the testis and the differing roles that they play in maintaining functional spermatogenesis.

Recognition mechanism of p63 by the E3 ligase Itch: novel strategy in the study and inhibition of this interaction

The HECT-containing E3 ubiquitin ligase Itch mediates the degradation of several proteins, including p63 and p73, involved in cell specification and fate. Itch contains four WW domains, which are essential for recognition on the target substrate, which contains a short proline-rich sequence. Several signaling complexes containing these domains have been associated with human diseases such as muscular dystrophy, Alzheimer's or Huntington's diseases. To gain further insight into the structural determinants of the Itch-WW2 domain, we investigated its interaction with p63. We assigned, by 3D heteronuclear NMR experiments, the backbone and side chains of the uniformly (13)C-(15)N-labeled Itch-WW2. In vitro interaction of Itch-WW2 domain with p63 was studied using its interactive p63 peptide, pep63. Pep63 is an 18-mer peptide corresponding to the region from 534-551 residue of p63, encompassing the PPxY motif that interacts with the Itch-WW domains, and we identified the residues involved in this molecular recognition. Moreover, here, a strategy of stabilization of the conformation of the PPxY peptide has been adopted, increasing the WW-ligand binding. We demonstrated that cyclization of pep63 leads to an increase of both the biological stability of the peptide and of the WW-ligand complex. Stable metal-binding complexes of the pep63 have been also obtained, and localized oxidative damage on Itch-WW2 domain has been induced, demonstrating the possibility of use of metal-pep63 complexes as models for the design of metal drugs to inhibit the Itch-WW-p63 recognition in vivo. Thus, our data suggest a novel strategy to study and inhibit the recognition mechanism of Itch E3-ligase.

Regulation of NF-κB by deubiquitinases

The nuclear factor-κB (NF-κB) pathway is a critical regulator of innate and adaptive immunity. Noncanonical K63-linked polyubiquitination plays a key regulatory role in NF-κB signaling pathways by functioning as a scaffold to recruit kinase complexes containing ubiquitin-binding domains. Ubiquitination is balanced by deubiquitinases that cleave polyubiquitin chains and oppose the function of E3 ubiquitin ligases. Deubiquitinases therefore play an important role in the termination of NF-κB signaling and the resolution of inflammation. In this review, we focus on NF-κB regulation by deubiquitinases with an emphasis on A20 and CYLD. Deubiquitinases and the ubiquitin/proteasome components that regulate NF-κB may serve as novel therapeutic targets for inflammatory diseases and cancer.

Notch signaling and cardiac repair

Notch signaling is critical for proper heart development and recently has been reported to participate in adult cardiac repair. Notch resides at the cell surface as a single pass transmembrane receptor, transits through the cytoplasm following activation, and acts as a transcription factor upon entering the nucleus. This dynamic and widespread cellular distribution allows for potential interactions with many signaling and binding partners. Notch displays temporal as well as spatial versatility, acting as a strong developmental signal, controlling cell fate determination and lineage commitment, and playing a pivotal role in embryonic and adult stem cell proliferation and differentiation. This review serves as an update of recent literature addressing Notch signaling in the heart, with attention to findings from noncardiac research that provide clues for further interpretation of how the Notch pathway influences cardiac biology. Specific areas of focus include Notch signaling in adult myocardium following pathologic injury, the role of Notch in cardiac progenitor cells with respect to differentiation and cardiac repair, crosstalk between Notch and other cardiac signaling pathways, and emerging aspects of noncanonical Notch signaling in heart.

Regulation of NF-κB signaling by the A20 deubiquitinase

The NF-κB transcription factor is a central mediator of inflammatory and innate immune signaling pathways. Activation of NF-κB is achieved by K63-linked polyubiquitination of key signaling molecules which recruit kinase complexes that in turn activate the IκB kinase (IKK). Ubiquitination is a highly dynamic process and is balanced by deubiquitinases that cleave polyubiquitin chains and terminate downstream signaling events. The A20 deubiquitinase is a critical negative regulator of NF-κB and inflammation, since A20-deficient mice develop uncontrolled and spontaneous multi-organ inflammation. Furthermore, specific polymorphisms in the A20 genomic locus predispose humans to autoimmune disease. Recent studies also indicate that A20 is an important tumor suppressor that is inactivated in B-cell lymphomas. Therefore, targeting A20 may form the basis of novel therapies for autoimmune disease and lymphomas.

Genetic disorders with immune dysregulation

We summarize the clinical presentation and molecular basis of a unique group of congenital immunodeficiency disorders in which defects in immune tolerance mechanisms result in severe autoimmunity. Patients with severe, familial forms of multi-organ autoimmunity have been recognized and clinically described for more than 40 years (Clin Exp Immunol 1: 119-128, 1966; Clin Exp Immunol 2: 19-30, 1967). Some are characterized primarily by autoimmunity and others by autoimmunity combined with susceptibility to specific infectious organisms. The first mechanistic understanding of these disorders began to emerge approximately 10 years ago with the initial identification of causative genes. As a result, our understanding of how immune tolerance is established and maintained in humans has expanded dramatically. Data generated over the last 3-4 years including identification of additional gene defects and functional characterization of each identified gene product in human and animal models have added clarity. This, in turn, has improved our ability to diagnose and effectively treat these severe, life-threatening disorders. Inherited disorders characterized by immune dysregulation have dramatically expanded our understanding of immune tolerance mechanisms in humans. Recognition and diagnosis of these disorders in the clinic allows timely initiation of life-saving therapies that may prevent death or irreversible damage to vital organs.

Peptidoglycan: a critical activator of the mammalian immune system during infection and homeostasis

Peptidoglycan is a conserved structural component of the bacterial cell wall with molecular motifs unique to bacteria. The mammalian immune system takes advantage of these properties and has evolved to recognize this microbial associated molecular pattern. Mammals have four secreted peptidoglycan recognition proteins, PGLYRP-1-4, as well as two intracellular sensors of peptidoglycan, Nod1 and Nod2. Recognition of peptidoglycan is important in initiating and shaping the immune response under both homeostatic and infection conditions. During infection, peptidoglycan recognition drives both cell-autonomous and whole-organism defense responses. Here, we examine recent advances in the understanding of how peptidoglycan recognition shapes mammalian immune responses in these diverse contexts.

Deubiquitinases in the regulation of NF-κB signaling

Nuclear factor-kappa B (NF-κB) is a critical regulator of multiple biological functions including innate and adaptive immunity and cell survival. Activation of NF-κB is tightly regulated to preclude chronic signaling that may lead to persistent inflammation and cancer. Ubiquitination of key signaling molecules by E3 ubiquitin ligases has emerged as an important regulatory mechanism for NF-κB signaling. Deubiquitinases (DUBs) counteract E3 ligases and therefore play a prominent role in the downregulation of NF-κB signaling and homeostasis. Understanding the mechanisms of NF-κB downregulation by specific DUBs such as A20 and CYLD may provide therapeutic opportunities for the treatment of chronic inflammatory diseases and cancer.

Herpes simplex virus UL56 interacts with and regulates the Nedd4-family ubiquitin ligase Itch

Background

Herpes simplex virus type 2 (HSV-2) is one of many viruses that exploits and modifies the cellular ubiquitin system. HSV-2 expresses the tegument protein UL56 that has been implicated in cytoplasmic transport and/or release of virions, and is a putative regulatory protein of Nedd4 ubiquitin ligase. In order to elucidate the biological function of UL56, this study examined the interaction of UL56 with the Nedd4-family ubiquitin ligase Itch and its role in the regulation of Itch. Additionally, we assessed the similarity between UL56 and regulatory proteins of Itch and Nedd4, Nedd4-family-interactins proteins (Ndfip).

Results

UL56 interacted with Itch, independent of additional viral proteins, and mediated more striking degradation of Itch, compared to Nedd4. Moreover, it was suggested that the lysosome pathway as well as the proteasome pathway was involved in the degradation of Itch. Other HSV-2 proteins with PY motifs, such as VP5 and VP16, did not mediate the degradation of endogenous Itch. Ndfip1 and Ndfip2 were similar in subcellular distribution patterns to UL56 and colocalized with UL56 in co-transfected cells.

Conclusions

We believe that this is the first report demonstrating the interaction of a HSV-specific protein and Itch. Thus, UL56 could function as a regulatory protein of Itch. The mechanism, function and significance of regulating Itch in HSV-2 infection remain unclear and warrant further investigation.

Human ITCH E3 ubiquitin ligase deficiency causes syndromic multisystem autoimmune disease

Ubiquitin ligases play an important role in the regulation of the immune system. Absence of Itch E3 ubiquitin ligase in mice has been shown to cause severe autoimmune disease. Using autozygosity mapping in a large Amish kindred, we identified a linkage region on chromosome 20 and selected candidate genes for screening. We describe, in ten patients, identification of a mutation resulting in truncation of ITCH. These patients represent the first reported human phenotype associated with ITCH deficiency. These patients not only have multisystem autoimmune disease but also display morphologic and developmental abnormalities. This disorder underscores the importance of ITCH ubiquitin ligase in many cellular processes.

ITCH K63-ubiquitinates the NOD2 binding protein, RIP2, to influence inflammatory signaling pathways

BACKGROUND

The inability to coordinate the signaling pathways that lead to proper cytokine responses characterizes the pathogenesis of inflammatory diseases such as Crohn's disease. The Crohn's disease susceptibility protein, NOD2, helps coordinate cytokine responses upon intracellular exposure to bacteria, and this signal coordination by NOD2 is accomplished, in part, through K63-linked polyubiquitin chains that create binding surfaces for the scaffolding of signaling complexes.

RESULTS

In this work, we show that the NOD2 signaling partner, RIP2, is directly K63-polyubiquitinated by ITCH, an E3 ubiquitin ligase that when lost genetically causes widespread inflammatory disease at mucosal surfaces. We show that ITCH is responsible for RIP2 polyubiquitination in response to infection with listeria monocytogenes. We also show that NOD2 can bind polyubiquitinated RIP2 and that whereas ITCH E3 ligase activity is required for optimal NOD2:RIP2-induced p38 and JNK activation, ITCH inhibits NOD2:RIP2-induced nuclear factor kappa B (NFkappaB) activation. This effect can be seen independently at the whole-genome level by microarray analysis of muramyl dipeptide (MDP)-treated Itch(-/-) primary macrophages.

CONCLUSIONS

These findings suggest that ITCH helps regulate NOD2-dependent signal transduction pathways and, as such, may be involved in the pathogenesis of NOD2-mediated inflammatory disease.