RIPK2 NODs to XIAP and IBD

Expression of Truncated Products at the 5'-Terminal Region of RIPK2 and Evolutive Aspects that Support Their Biological Importance

Alternative splicing is the process of generating different mRNAs from the same primary transcript, which contributes to increase the transcriptome and proteome diversity. Abnormal splicing has been associated with the development of several diseases including cancer. Given that mutations and abnormal levels of the RIPK2 transcript and RIP-2 protein are frequent in tumors, and that RIP-2 modulates immune and inflammatory responses, we investigated alternative splicing events that result in partial deletions of the kinase domain at the N-terminus of RIP-2. We also investigated the structure and expression of the RIPK2 truncated variants and isoforms in different environments. In addition, we searched data throughout Supraprimates evolution that could support the biological importance of RIPK2 alternatively spliced products. We observed that human variants and isoforms were differentially regulated following temperature stress, and that the truncated transcript was more expressed than the long transcript in tumor samples. The inverse was found for the longer protein isoform. The truncated variant was also detected in chimpanzee, gorilla, hare, pika, mouse, rat, and tree shrew. The fact that the same variant has been preserved in mammals with divergence times up to 70 million years raises the hypothesis that it may have a functional significance.

Enterococcus faecalis Extracellular Vesicles Promote Apical Periodontitis

Enterococcus faecalis is an important contributor to the persistence of chronic apical periodontitis. However, the mechanism by which E. faecalis infection in the root canals and dentinal tubules affects periapical tissue remains unclear. Bacterial extracellular vesicles (EVs) act as natural carriers of microbe-associated molecular patterns (MAMPs) and have recently attracted considerable attention. In this study, we investigated the role of EVs derived from E. faecalis in the pathogenesis of apical periodontitis. We observed that E. faecalis EVs can induce inflammatory bone destruction in the periapical areas of mice. Double-labeling immunofluorescence indicated that M1 macrophage infiltration was increased by E. faecalis EVs in apical lesions. Moreover, in vitro experiments demonstrated the internalization of E. faecalis EVs into macrophages. Macrophages tended to polarize toward the M1 profile after treatment with E. faecalis EVs. Pattern recognition receptors (PRRs) can recognize MAMPs of bacterial EVs and, in turn, trigger inflammatory responses. Thus, we performed further mechanistic exploration, which showed that E. faecalis EVs considerably increased the expression of NOD2, a cytoplasmic PRR, and that inhibition of NOD2 markedly reduced macrophage M1 polarization induced by E. faecalis EVs. RIPK2 ubiquitination is a major downstream of NOD2. We also observed increased RIPK2 ubiquitination in macrophages treated with E. faecalis EVs, and E. faecalis EV-induced macrophage M1 polarization was notably alleviated by the RIPK2 ubiquitination inhibitor. Our study revealed the potential for EVs to be considered a virulence factor of E. faecalis and found that E. faecalis EVs can promote macrophage M1 polarization via NOD2/RIPK2 signaling. To our knowledge, this is the first report to investigate apical periodontitis development from the perspective of bacterial vesicles and demonstrate the role and mechanism of E. faecalis EVs in macrophage polarization. This study expands our understanding of the pathogenic mechanism of E. faecalis and provides novel insights into the pathogenesis of apical periodontitis.

Simultaneous substrate and ubiquitin modification recognition by bispecific antibodies enables detection of ubiquitinated RIP1 and RIP2

Ubiquitination is a posttranslational modification that is crucial for the dynamic regulation of diverse signaling pathways. To enhance our understanding of ubiquitination-mediated signaling, we generated a new class of bispecific antibodies that combine recognition of ubiquitination substrates and specific polyubiquitin linkages. RIP1-K63 and RIP1-linear (Lin) linkage polyubiquitin bispecific antibodies detected linkage-specific ubiquitination of the proinflammatory kinase RIP1 in cells and in tissues and revealed RIP1 ubiquitination by immunofluorescence. Similarly, ubiquitination of the RIP1-related kinase RIP2 with K63 or linear linkages was specifically detected with the RIP2-K63 and RIP2-Lin bispecific antibodies, respectively. Furthermore, using the RIP2-K63 and RIP2-Lin bispecific antibodies, we found prominent K63-linked and linear RIP2 ubiquitination in samples from patients with ulcerative colitis and Crohn's disease. We also developed a bispecific antibody (K63-Lin) that simultaneously recognizes K63-linked and linear ubiquitination of components of various signaling pathways. Together, these bispecific antibodies represent a new class of reagents with the potential to be developed for the detection of inflammatory biomarkers.

OTUD1 ameliorates cerebral ischemic injury through inhibiting inflammation by disrupting K63-linked deubiquitination of RIP2

BACKGROUND

Inflammatory response triggered by innate immunity plays a pivotal element in the progress of ischemic stroke. Receptor-interacting kinase 2 (RIP2) is implicated in maintaining immunity homeostasis and regulating inflammatory response. However, the underlying mechanism of RIP2 in ischemic stroke is still not well understood. Hence, the study investigated the role and the ubiquitination regulatory mechanism of RIP2 in ischemic stroke.

METHODS

Focal cerebral ischemia was introduced by middle cerebral artery occlusion (MCAO) in wild-type (WT) and OTUD1-deficient (OTUD1-/-) mice, oxygen glucose deprivation and reoxygenation (OGD/R) models in BV2 cells and primary cultured astrocytes were performed for monitoring of experimental stroke. GSK2983559 (GSK559), a RIP2 inhibitor was intraventricularly administered 30 min before MCAO. Mice brain tissues were collected for TTC staining and histopathology. Protein expression of RIP2, OTUD1, p-NF-κB-p65 and IκBα was determined by western blot. Localization of RIP2 and OTUD1 was examined by immunofluorescence. The change of IL-1β, IL-6 and TNF-α was detected by ELISA assay and quantitative real-time polymerase chain reaction. Immunoprecipitation and confocal microscopy were used to study the interaction of RIP2 and OTUD1. The activity of NF-κB was examined by dual-luciferase assay.

RESULTS

Our results showed upregulated protein levels of RIP2 and OTUD1 in microglia and astrocytes in mice subjected to focal cerebral ischemia. Inhibition of RIP2 by GSK559 ameliorated the cerebral ischemic outcome by repressing the NF-κB activity and the inflammatory response. Mechanistically, OTUD1 interacted with RIP2 and sequentially removed the K63-linked polyubiquitin chains of RIP2, thereby inhibiting NF-κB activation. Furthermore, OTUD1 deficiency exacerbated cerebral ischemic injury in response to inflammation induced by RIP2 ubiquitination.

CONCLUSIONS

These findings suggested that RIP2 mediated cerebral ischemic lesion via stimulating inflammatory response, and OTUD1 ameliorated brain injury after ischemia through inhibiting RIP2-induced NF-κB activation by specifically cleaving K63-linked ubiquitination of RIP2.

Environmental pollutants induce NLRP3 inflammasome activation and pyroptosis: Roles and mechanisms in various diseases

Environmental pollution is changing with economic development. Most environmental pollutants are characterized by stable chemical properties, strong migration, potential toxicity, and multiple exposure routes. Harmful substances are discharged excessively, and large quantities of unknown new compounds are emerging, being transmitted and amplifying in the food chain. The increasingly severe problems of environmental pollution have forced people to re-examine the relationship between environmental pollution and health. Pyroptosis and activation of the NLRP3 inflammasome are critical in maintaining the immune balance and regulating the inflammatory process. Numerous diseases caused by environmental pollutants are closely related to NLRP3 inflammasome activation and pyroptosis. We intend to systematically explain the steps and important events that are common in life but easily overlooked by which environmental pollutants activate the NLRP3 inflammasome and pyroptosis pathways. This comprehensive review also discusses the interaction network between environmental pollutants, the NLRP3 inflammasome, pyroptosis, and diseases. Thus, research progress on the impact of decreasing oxidative stress levels to inhibit the NLRP3 inflammasome and pyroptosis, thereby repairing homeostasis and reshaping health, is systematically examined. This review aims to deepen the understanding of the impact of environmental pollutants on life and health and provide a theoretical basis and potential programs for the development of corresponding treatment strategies.

Inflammation, Autoinflammation and Autoimmunity in Inflammatory Bowel Diseases

In this review, the role of innate and adaptive immunity in the pathogenesis of inflammatory bowel diseases (IBD) is reported. In IBD, an altered innate immunity is often found, with increased Th17 and decreased Treg cells infiltrating the intestinal mucosa. An associated increase in inflammatory cytokines, such as IL-1 and TNF-α, and a decrease in anti-inflammatory cytokines, such as IL-10, concur in favoring the persistent inflammation of the gut mucosa. Autoinflammation is highlighted with insights in the role of inflammasomes, which activation by exogenous or endogenous triggers might be favored by mutations of NOD and NLRP proteins. Autoimmunity mechanisms also take place in IBD pathogenesis and in this context of a persistent immune stimulation by bacterial antigens and antigens derived from intestinal cells degradation, the adaptive immune response takes place and results in antibodies and autoantibodies production, a frequent finding in these diseases. Inflammation, autoinflammation and autoimmunity concur in altering the mucus layer and enhancing intestinal permeability, which sustains the vicious cycle of further mucosal inflammation.

Discovery of a novel RIPK2 inhibitor for the treatment of inflammatory bowel disease

Nucleotide-binding oligomerization domain-containing protein 1 and 2 (NOD 1/2) are important cytosolic pattern recognition receptors that initiate host immune response. The dysregulation of NOD signaling is highly associated with inflammatory bowel disease (IBD) that needs novel treatment options. Receptor-interacting protein kinase 2 (RIPK2) is a critical mediator of NOD signaling and considered a promising therapeutic target for IBD treatment. However, there are currently no RIPK2 inhibitors available for clinical use. Here, we report the discovery and characterization of Zharp2-1 as a novel and potent RIPK2 inhibitor that effectively blocks RIPK2 kinase function and NOD-mediated NF-κB/MAPK activation in both human and mouse cell lines. Zharp2-1 exhibits significantly superior solubility compared to the non-prodrug form of the advanced RIPK2 inhibitor prodrug GSK2983559. The improved solubility combined with favorable in vitro metabolic stability translated to excellent in vivo pharmacokinetic profiles for Zharp2-1. In addition, Zharp2-1 demonstrates better effects than GSK2983559 in inhibiting the muramyl dipeptide (MDP)-induced production of pro-inflammatory cytokines in human peripheral blood mononuclear cells (PBMCs) and MDP-induced peritonitis in mice. Furthermore, Zharp2-1 markedly reduces Listeria monocytogenes infection-induced cytokines release in both human and mouse cells. Importantly, Zharp2-1 significantly ameliorates DNBS-induced colitis in rats and suppressed pro-inflammatory cytokine release in intestinal specimens from IBD patients. Collectively, our findings indicate that Zharp2-1 is a promising RIPK2 inhibitor with the potential to be further developed for IBD therapy.

RIPK2: a promising target for cancer treatment

As an essential mediator of inflammation and innate immunity, the receptor-interacting serine/threonine-protein kinase-2 (RIPK2) is responsible for transducing signaling downstream of the intracellular peptidoglycan sensors nucleotide oligomerization domain (NOD)-like receptors 1 and 2 (NOD1/2), which will further activate nuclear factor kappa-B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways, leading to the transcription activation of pro-inflammatory cytokines and productive inflammatory response. Thus, the NOD2-RIPK2 signaling pathway has attracted extensive attention due to its significant role in numerous autoimmune diseases, making pharmacologic RIPK2 inhibition a promising strategy, but little is known about its role outside the immune system. Recently, RIPK2 has been related to tumorigenesis and malignant progression for which there is an urgent need for targeted therapies. Herein, we would like to evaluate the feasibility of RIPK2 being the anti-tumor drug target and summarize the research progress of RIPK2 inhibitors. More importantly, following the above contents, we will analyze the possibility of applying small molecule RIPK2 inhibitors to anti-tumor therapy.

RIPK2 as a promising druggable target for autoimmune diseases

Receptor Interacting Serine/Threonine Kinase 2 (RIPK2) is an essential regulator of the inflammatory process and immune response. In innate immunity, the NOD-RIPK2 signaling axis is an important pathway that directly mediates inflammation and immune response. In adaptive immunity, RIPK2 may affect T cell proliferation, differentiation and cellular homeostasis thereby involving T cell-driven autoimmunity, but the exact mechanism remains unclear. Recent advances suggest a key role of RIPK2 in diverse autoimmune diseases (ADs) such as inflammatory bowel diseases, rheumatoid arthritis, multiple sclerosis, systemic lupus erythematosus, and Behcet's disease. This review aims to provide valuable therapeutic direction for ADs by focusing on the function and modulation of RIPK2 in innate and adaptive immunity, its involvement with various ADs and the application of RIPK2-related drugs in ADs. We raise the notion that drug targeting RIPK2 could be a promising therapeutic strategy for the treatment of ADs, though much work remains to be done for clinical application.

The ubiquitination of RIPK2 is mediated by Peli3 and negatively regulates the onset of infectious osteomyelitis

Osteomyelitis is the infection and destruction of bone. Until now, there is no universal protocol for its treatment. Receptor-interacting serine/threonine-protein kinase 2 (RIPK2) was implicated in the development of osteomyelitis. However, its detailed mechanism remains unknown. 6-8 weeks old wild-type or Pellino E3 Ubiquitin Protein Ligase Family Member 3 (Peli3) deficiency mice were injected with S. aureus to induce osteomyelitis. RAW264.7 cells or bone marrow-derived macrophages (BMDMs) isolated from mice, were treated with lipopolysaccharides (LPS). Knocking down Peli3 in RAW264.7 cells increased the expressions of inflammatory cytokines after the stimulation of LPS, including interleukin-1β, interleukin-6 and tumor necrosis factor-α. Inflammation was also activated in S. aureus-induced Peli3 deficiency mice. Moreover, Peli3 deficiency mice also displayed more severe symptoms of osteomyelitis in S. aureus-infected mice. Moreover, Peli3 targeted and degraded RIPK2 through K48-linked ubiquitination. Peli3 negatively modulates osteomyelitis by degrading RIPK2. Our data further expand current understanding of RIPK2 on osteomyelitis, which suggests that RIPK2 might serve as novel therapeutic target for treating osteomyelitis.

XIAP deletion sensitizes mice to TNF-induced and RIP1-mediated death

XIAP is a caspase-inhibitory protein that blocks several cell death pathways, and mediates proper activation of inflammatory NOD2-RIP2 signaling. XIAP deficiency in patients with inflammatory diseases such as Crohn's disease, or those needing allogeneic hematopoietic cell transplantation, is associated with a worse prognosis. In this study, we show that XIAP absence sensitizes cells and mice to LPS- and TNF-mediated cell death without affecting LPS- or TNF-induced NF-κB and MAPK signaling. In XIAP deficient mice, RIP1 inhibition effectively blocks TNF-stimulated cell death, hypothermia, lethality, cytokine/chemokine release, intestinal tissue damage and granulocyte migration. By contrast, inhibition of the related kinase RIP2 does not affect TNF-stimulated events, suggesting a lack of involvement for the RIP2-NOD2 signaling pathway. Overall, our data indicate that in XIAP's absence RIP1 is a critical component of TNF-mediated inflammation, suggesting that RIP1 inhibition could be an attractive option for patients with XIAP deficiency.

DBDPE upregulates NOD-like receptor signaling to induce NLRP3 inflammasome-mediated HAECs pyroptosis

Decabromodiphenyl ethane (DBDPE), a typical new brominated flame retardant (BFR), is a widespread new pollutant in the environment. Several studies and our previous studies have found that DBDPE can cause aortic endothelial injury and aortic endothelial cell pyroptosis, whereas the molecular mechanism involved has not been elucidated. In this study, we exposed human aortic endothelial cells (HAECs) to 25 μmol/L of DBDPE and analyzed the gene expression profiles by Affymetrix PrimeView™ Human Gene Expression Chip. The results showed that 886 genes were differentially expressed in the DBDPE exposure group. Enrichment analyses revealed that differentially expressed genes were mainly enriched in the inflammatory response and NOD-like receptor signal pathway. Gene-gene functional interaction analyses and crossover genes and pathways analyses found that the NOD-like receptor signal pathway may be involved in regulating NLRP3 and IL-18. We found that NOD2 cannot interact with NLRP3 directly through an immunoprecipitation experiment. Thus, we construct the RIPK2 knockdown HAECs cell line to repress the NOD-like receptor signaling and further study the mechanism of DBDPE-activated NLRP3 inflammasome to induce HAECs pyroptosis. The results showed that RIPK2 knockdown could repress DBDPE-induced NOD-like receptor signaling pathway upregulation, inhibit NLRP3 inflammasome activation, and decrease HAECs pyroptosis. In addition, RIPK2 knockdown decreased the ROS generation in HAECs induced by DBDPE. And NAC pretreated HAECs inhibited DBDPE-induced NLRP3 inflammasome activation and HAECs pyroptosis. These results demonstrated that DBDPE upregulated NOD-like receptor signaling to induce ROS generation and, in turn, activated NLRP3 inflammasome, leading to HAECs pyroptosis.

Human ZBP1 induces cell death-independent inflammatory signaling via RIPK3 and RIPK1

ZBP1 is an interferon-induced cytosolic nucleic acid sensor that facilitates antiviral responses via RIPK3. Although ZBP1-mediated programmed cell death is widely described, whether and how it promotes inflammatory signaling is unclear. Here, we report a ZBP1-induced inflammatory signaling pathway mediated by K63- and M1-linked ubiquitin chains, which depends on RIPK1 and RIPK3 as scaffolds independently of cell death. In human HT29 cells, ZBP1 associated with RIPK1 and RIPK3 as well as ubiquitin ligases cIAP1 and LUBAC. ZBP1-induced K63- and M1-linked ubiquitination of RIPK1 and ZBP1 to promote TAK1- and IKK-mediated inflammatory signaling and cytokine production. Inhibition of caspase activity suppressed ZBP1-induced cell death but enhanced cytokine production in a RIPK1- and RIPK3 kinase activity-dependent manner. Lastly, we provide evidence that ZBP1 signaling contributes to SARS-CoV-2-induced cytokine production. Taken together, we describe a ZBP1-RIPK3-RIPK1-mediated inflammatory signaling pathway relayed by the scaffolding role of RIPKs and regulated by caspases, which may induce inflammation when ZBP1 is activated below the threshold needed to trigger a cell death response.

Design, synthesis, and structure-activity relationship of novel RIPK2 inhibitors

The NOD1/2 (nucleotide-binding oligomerization domain-containing protein 1/2) signaling pathways are involved in innate immune control and host defense. NOD dysfunction can result in a variety of autoimmune disorders. NOD-induced generation of inflammatory cytokines is mediated by receptor-interacting protein kinase 2 (RIPK2), which has been considered as a promising therapeutic target. Herein, we disclose the design, synthesis, and SAR study of a series of RIPK2 inhibitors. The lead compound 17 displayed a high affinity for RIPK2 (K_d = 5.9 nM) and was capable of inhibiting RIPK2 kinase function in an ADP-Glo assay. In vitro DMPK studies showed that compound 17 had good metabolic stability and no CYP inhibition. Compound 17 effectively suppressed inflammatory cytokine production in both cells and animal model.

Application and prospect of targeting innate immune sensors in the treatment of autoimmune diseases

Dysregulation of auto-reactive T cells and autoantibody-producing B cells and excessive inflammation are responsible for the occurrence and development of autoimmune diseases. The suppression of autoreactive T cell activation and autoantibody production, as well as inhibition of inflammatory cytokine production have been utilized to ameliorate autoimmune disease symptoms. However, the existing treatment strategies are not sufficient to cure autoimmune diseases since patients can quickly suffer a relapse following the end of treatments. Pattern recognition receptors (PRRs), including Toll-like receptors (TLRs), Nod-like receptors (NLRs), RIG-I like receptors (RLRs), C-type lectin receptors (CLRs) and various nucleic acid sensors, are expressed in both innate and adaptive immune cells and are involved in the development of autoimmune diseases. Here, we have summarized advances of PRRs signaling pathways, association between PRRs and autoimmune diseases, application of inhibitors targeting PRRs and the corresponding signaling molecules relevant to strategies targeting autoimmune diseases. This review emphasizes the roles of different PRRs in activating both innate and adaptive immunity, which can coordinate to trigger autoimmune responses. The review may also prompt the formulation of novel ideas for developing therapeutic strategies against autoimmune diseases by targeting PRRs-related signals.

High expression of RIPK2 is associated with Taxol resistance in serous ovarian cancer

Background

Taxol resistance in serous ovarian cancer is responsible for its poor prognosis, yet the underlying mechanism is still poorly understood. Thus, we probed the mechanism of Taxol resistance in serous ovarian cancer with multiple bioinformatic methods to provide novel insights into potential therapies.

Methods

The differentially expressed genes (DEGs) in Taxol-sensitive and Taxol-resistant cell lines and their relationship with the overall survival (OS) and progression-free interval (PFI) of ovarian cancer patients were analyzed using gene expression datasets from the Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO). The role of receptor interacting serine/threonine kinase 2 (RIPK2) was validated via identification of its coexpressed genes, functional analysis and generation of a protein-protein interaction (PPI) network. The single sample gene set enrichment analysis (ssGSEA) was used to explore immune infiltration, and genomic alterations of RIPK2 were also analyzed via cBio Cancer Genomics Portal (cBioProtal).

Results

RIPK2 was highly expressed in Taxol resistant ovarian cancer cell lines, and its high expression was also linked with shorter OS and PFI in serous ovarian cancer patients. The PPI network analysis and pathway analysis demonstrated that RIPK2 might participate in the positive regulation of NF-κB transcription factor activity. RIPK2 expression was related to tumor microenvironment alterations, which might participate in the formation of Taxol resistance.

Conclusions

Our studies suggested that high expression of RIPK2 is related to Taxol resistance in serous ovarian cancer, and that RIPK2 induces Taxol resistance through NOD1/RIPK2/NF-κB inflammatory pathway activation and tumor microenvironment changes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13048-022-00986-2.

Ubiquitin-modifying enzymes as regulators of colitis

Inflammatory bowel disease (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), is a chronic inflammatory disorder of the gastrointestinal tract. Although the pathophysiology of IBD is multifaceted, ubiquitination, a post-translational modification, has been shown to have essential roles in its pathogenesis and development. Ubiquitin-modifying enzymes (UMEs) work in synergy to orchestrate the optimal ubiquitination of target proteins, thereby maintaining intestinal homeostasis. Genome-wide association studies (GWAS) have identified multiple UME genes as IBD susceptibility loci, implying the importance of UMEs in IBD. Furthermore, accumulative evidence demonstrates that UMEs affect intestinal inflammation by regulating various aspects, such as intestinal barrier functions and immune responses. Considering the significant functions of UMEs in IBD, targeting UMEs could become a favorable therapeutic approach for IBD.

Assaying RIPK2 Activation by Complex Formation

The receptor-interacting serine/threonine-protein kinase-2 (RIPK2, RIP2) is a key player in downstream signaling of nuclear oligomerization domain (NOD)-like receptor (NLR)-mediated innate immune response against bacterial infections. RIPK2 is recruited following activation of the pattern recognition receptors (PRRs) NOD1 and NOD2 by sensing bacterial peptidoglycans leading to activation of NF-κB and MAPK pathways and the production of pro-inflammatory cytokines. Upon NOD1/2 activation, RIPK2 forms complexes in the cytoplasm of human cells, also called RIPosomes. These can be induced by Shigella flexneri or by the inhibition of RIPK2 by small compounds, such as GSK583 and gefitinib.In this chapter, we describe fluorescent light microscopic and Western blot approaches to analyze the cytoplasmic aggregation of RIPK2 upon infection with the invasive, Gram-negative bacterial pathogen Shigella flexneri, or by the treatment with RIPK2 inhibitors. This method is based on HeLa cells stably expressing eGFP-tagged RIPK2 and describes a protocol to induce and visualize RIPosome formation. The described method is useful to study the deposition of RIPK2 in speck-like structures, also in living cells, using live cell imaging and can be adopted for the study of other inhibitory proteins or to further analyze the process of RIPosome structure assembly.

The Role of E3 Ubiquitin Ligases and Deubiquitinases in Inflammatory Bowel Disease: Friend or Foe?

Inflammatory bowel disease (IBD), which include Crohn’s disease (CD) and ulcerative colitis (UC), exhibits a complex multifactorial pathogenesis involving genetic susceptibility, imbalance of gut microbiota, mucosal immune disorder and environmental factors. Recent studies reported associations between ubiquitination and deubiquitination and the occurrence and development of inflammatory bowel disease. Ubiquitination modification, one of the most important types of post-translational modifications, is a multi-step enzymatic process involved in the regulation of various physiological processes of cells, including cell cycle progression, cell differentiation, apoptosis, and innate and adaptive immune responses. Alterations in ubiquitination and deubiquitination can lead to various diseases, including IBD. Here, we review the role of E3 ubiquitin ligases and deubiquitinases (DUBs) and their mediated ubiquitination and deubiquitination modifications in the pathogenesis of IBD. We highlight the importance of this type of posttranslational modification in the development of inflammation, and provide guidance for the future development of targeted therapeutics in IBD.

Phenotype, genotype, treatment, and survival outcomes in patients with X-linked inhibitor of apoptosis deficiency

BACKGROUND

X-linked inhibitor of apoptosis (XIAP) deficiency is a rare, primary immunodeficiency disease caused by XIAP gene mutations. A broad range of phenotype, severity, and age of onset present challenges for patient management.

OBJECTIVE

To characterize the phenotype, treatment, and survival outcomes of XIAP deficiency and assess parameters influencing prognosis.

METHODS

Data published from 2006-2020 were retrospectively analyzed.

RESULTS

167 patients from 117 families with XIAP deficiency were reported with 90 different mutations. A wide spectrum of clinical features were seen, of which hemophagocytic lymphohistiocytosis (HLH) and inflammatory bowel disease (IBD) were the most common. Patients frequently developed multiple features with no clear genotype-phenotype correlation. 117 patients were managed conservatively and 50 underwent hematopoietic stem cell transplantation (HSCT), with respective overall survival probabilities of 90% and 53% at age 16 years. The predominant indication for HSCT was early-onset HLH. Active HLH and myeloablative conditioning regimens increased HSCT-related mortality, although HSCT outcome was much better after 2015 than before. For conservatively managed patients reaching adulthood, survival probabilities were 86% at age 30 years and 37% by age 52 years, with worse outcomes for patients developing the disease before the age of 5 years or with new disease features in adulthood. 9 asymptomatic mutation carriers were identified with a median age of 13.5 years.

CONCLUSIONS

Our study demonstrates the variable nature of XIAP deficiency which evolves over life for individual patients. Better therapeutic strategies and prospective studies are required to reduce morbidity and mortality and improve decision-making and long-term outcomes for patients with XIAP deficiency.

Regulation of CYLD activity and specificity by phosphorylation and ubiquitin-binding CAP-Gly domains

Non-degradative ubiquitin chains and phosphorylation events govern signaling responses by innate immune receptors. The deubiquitinase CYLD in complex with SPATA2 is recruited to receptor signaling complexes by the ubiquitin ligase LUBAC and regulates Met1- and Lys63-linked polyubiquitin and receptor signaling outcomes. Here, we investigate the molecular determinants of CYLD activity. We reveal that two CAP-Gly domains in CYLD are ubiquitin-binding domains and demonstrate a requirement of CAP-Gly3 for CYLD activity and regulation of immune receptor signaling. Moreover, we identify a phosphorylation switch outside of the catalytic USP domain, which activates CYLD toward Lys63-linked polyubiquitin. The phosphorylated residue Ser568 is a novel tumor necrosis factor (TNF)-regulated phosphorylation site in CYLD and works in concert with Ser418 to enable CYLD-mediated deubiquitination and immune receptor signaling. We propose that phosphorylated CYLD, together with SPATA2 and LUBAC, functions as a ubiquitin-editing complex that balances Lys63- and Met1-linked polyubiquitin at receptor signaling complexes to promote LUBAC signaling.

Inborn Errors in the LRR Domain of Nod2 and Their Potential Consequences on the Function of the Receptor

The innate immune system plays a critical role in the early detection of pathogens, primarily by relying on pattern-recognition receptor (PRR) signaling molecules. Nucleotide-binding oligomerization domain 2 (NOD2) is a cytoplasmic receptor that recognizes invading molecules and danger signals inside the cells. Recent studies highlight the importance of NOD2′s function in maintaining the homeostasis of human body microbiota and innate immune responses, including induction of proinflammatory cytokines, regulation of autophagy, modulation of endoplasmic reticulum (ER) stress, etc. In addition, there is extensive cross-talk between NOD2 and the Toll-like receptors that are so important in the induction and tuning of adaptive immunity. Polymorphisms of NOD2′s encoding gene are associated with several pathological conditions, highlighting NOD2′s functional importance. In this study, we summarize NOD2′s role in cellular signaling pathways and take a look at the possible consequences of common NOD2 polymorphisms on the structure and function of this receptor.

Metabolic Host-Microbiota Interactions in Autophagy and the Pathogenesis of Inflammatory Bowel Disease (IBD)

Inflammatory bowel disease (IBD) is a family of conditions characterized by chronic, relapsing inflammation of the gastrointestinal tract. IBD afflicts over 3 million adults in the United States and shows increasing prevalence in the Westernized world. Current IBD treatments center on modulation of the damaging inflammatory response and carry risks such as immunosuppression, while the development of more effective treatments is hampered by our poor understanding of the molecular mechanisms of IBD pathogenesis. Previous genome-wide association studies (GWAS) have demonstrated that gene variants linked to the cellular response to microorganisms are most strongly associated with an increased risk of IBD. These studies are supported by mechanistic work demonstrating that IBD-associated polymorphisms compromise the intestine's anti-microbial defense. In this review, we summarize the current knowledge regarding IBD as a disease of defects in host-microbe interactions and discuss potential avenues for targeting this mechanism for future therapeutic development.

14-3-3 and erlin proteins differentially interact with RIPK2 complexes

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for signal transduction induced by the pattern recognition receptors NOD1 and NOD2 (referred to collectively as NOD1/2). Upon NOD1/2 activation, RIPK2 forms complexes in the cytoplasm of human cells. Here, we identified the molecular composition of these complexes. Infection with Shigella flexneri to activate NOD1-RIPK2 revealed that RIPK2 formed dynamic interactions with several cellular proteins, including A20 (also known as TNFAIP3), erlin-1, erlin-2 and 14-3-3. Whereas interaction of RIPK2 with 14-3-3 proteins was strongly reduced upon infection with Shigella, erlin-1 and erlin-2 (erlin-1/2) specifically bound to RIPK2 complexes. The interaction of these proteins with RIPK2 was validated using protein binding assays and immunofluorescence staining. Beside bacterial activation of NOD1/2, depletion of the E3 ubiquitin ligase XIAP and treatment with RIPK2 inhibitors also led to the formation of RIPK2 cytosolic complexes. Although erlin-1/2 were recruited to RIPK2 complexes following XIAP inhibition, these proteins did not associate with RIPK2 structures induced by RIPK2 inhibitors. While the specific recruitment of erlin-1/2 to RIPK2 suggests a role in innate immune signaling, the biological response regulated by the erlin-1/2-RIPK2 association remains to be determined.

Ubiquitination in the regulation of inflammatory cell death and cancer

The ubiquitin system is complex, multifaceted, and is crucial for the modulation of a vast number of cellular processes. Ubiquitination is tightly regulated at different levels by a range of enzymes including E1s, E2s, and E3s, and an array of DUBs. The UPS directs protein degradation through the proteasome, and regulates a wide array of cellular processes including transcription and epigenetic factors as well as key oncoproteins. Ubiquitination is key to the dynamic regulation of programmed cell death. Notably, the TNF signaling pathway is controlled by competing ubiquitin conjugation and deubiquitination, which governs both proteasomal degradation and signaling complex formation. In the inflammatory response, ubiquitination is capable of both activating and dampening inflammasome activation through the control of either protein stability, complex formation, or, in some cases, directly affecting receptor activity. In this review, we discuss the enzymes and targets in the ubiquitin system that regulate fundamental cellular processes regulating cell death, and inflammation, as well as disease consequences resulting from their dysregulation. Finally, we highlight several pre-clinical and clinical compounds that regulate ubiquitin system enzymes, with the aim of restoring homeostasis and ameliorating diseases.