Emergence of the A20/ABIN-mediated inhibition of NF-κB signaling via modifying the ubiquitinated proteins in a basal chordate

A20: a jack of all trades

Mutations and polymorphisms in A20/TNFAIP3 have been linked to various inflammatory disorders. However, in addition to its well-known role in inflammation, A20 also controls EDAR- and receptor activator of NF-κB (RANK)-induced NF-κB signaling, regulating the development of epidermal skin appendages and bone, respectively. Furthermore, A20 regulates synapse remodeling through a mechanism dependent on NF-κB.

Current research into A20 mediation of allergic respiratory diseases and its potential usefulness as a therapeutic target

Allergic airway diseases are characterized by excessive and prolonged type 2 immune responses to inhaled allergens. Nuclear factor κB (NF-κB) is a master regulator of the immune and inflammatory response, which has been implicated to play a prominent role in the pathogenesis of allergic airway diseases. The potent anti-inflammatory protein A20, termed tumor necrosis factor-α-inducible protein 3 (TNFAIP3), exerts its effects by inhibiting NF-κB signaling. The ubiquitin editing abilities of A20 have attracted much attention, resulting in its identification as a susceptibility gene in various autoimmune and inflammatory disorders. According to the results of genome-wide association studies, several TNFAIP3 gene locus nucleotide polymorphisms have been correlated to allergic airway diseases. In addition, A20 has been found to play a pivotal role in immune regulation in childhood asthma, particularly in the protection against environmentally mediated allergic diseases. The protective effects of A20 against allergy were observed in conditional A20-knockout mice in which A20 was depleted in the lung epithelial cells, dendritic cells, or mast cells. Furthermore, A20 administration significantly decreased inflammatory responses in mouse models of allergic airway diseases. Here, we review emerging findings elucidating the cellular and molecular mechanisms by which A20 regulates inflammatory signaling in allergic airway diseases, as well as discuss its potential as a therapeutic target.

RNF31 mediated ubiquitination of A20 aggravates inflammation and hepatocyte apoptosis through the TLR4/MyD88/NF-κB signaling pathway

Inflammatory cytokine storm is one of the main pathogenesis of acute liver injury, and accumulating evidence suggests that the E3 ubiquitin ligase ring finger protein 31 (RNF31) plays an important regulatory role in the activation of inflammatory pathways. We found that RNF31 expression was up-regulated in lipopolysaccharide (LPS)-treated HL-7702 cells. Western blotting results showed decreased expression of RNF31 and total ubiquitinated proteins after transfection of si-RNF31. The results of MTT assay indicated that cell viability was enhanced. Flow cytometry analysis showed that cell apoptosis and ROS content was decreased, and ELISA assay results exhibited that the inflammatory factors secretion was reduced. Interestingly, A20 protein expression was inhibited as RNF31 expression was upregulated. On this basis, we performed co-immunoprecipitation assays and found that RNF31 could interact with A20. Actinomycin tracing and proteasome inhibition experiments showed that RNF31 degrades A20 through the proteasome pathway. Furthermore, overexpression of A20 enhanced cell viability, reduced apoptosis, and inhibited ROS generation and inflammatory factor secretion. Mechanistic studies revealed that RNF31 was able to degrade A20, which affected the inflammatory response and hepatocyte apoptosis mediated by the toll like receptor 4 (TLR4)/myeloid differentiation factor88 (MyD88)/nuclear transcription factor-κB (NF-κB) signaling pathway. Moreover, knockdown of RNF31 attenuated the inflammatory response induced by d-Gal/LPS in mice with acute liver injury. In conclusion, RNF31 degrades A20 by ubiquitination and activates the TLR4/MyD88/NF-κB signaling pathway to aggravate acute liver injury.

A20 Inhibits LPS-Induced Inflammation by Regulating TRAF6 Polyubiquitination in Rainbow Trout

The ubiquitin-editing enzyme A20 is known to inhibit the NF-κB transcription factor in the Toll-like receptor (TLR) pathways, thereby negatively regulating inflammation. However, its role in the TLR signaling pathway in fish is still largely unknown. Here, we identified a gene encoding A20 (OmA20) in rainbow trout, Oncorhynchus mykiss, and investigated its role in TLR response regulation. The deduced amino acid sequence of OmA20 contained a conserved N-terminal ovarian tumor (OTU) domain and seven C-terminal zinc-finger (ZnF) domains. Lipopolysaccharide (LPS) stimulation increased OmA20 expression in RTH-149 cells. In LPS-stimulated RTH-149 cells, gain- and loss-of-function experiments revealed that OmA20 inhibited MAPK and NF-κB activation, as well as the expression of pro-inflammatory cytokines. OmA20 interacted with TRAF6, a key molecule involved in the activation of TLR-mediated NF-κB signaling pathways. LPS treatment increased the K63-linked polyubiquitination of TRAF6 in RTH-149 cells, which was suppressed when OmA20 was forced expression. Furthermore, mutations in the OTU domain significantly decreased deubiquitination of the K63-linked ubiquitin chain on TRAF6, indicating that deubiquitinase activity is dependent on the OTU domain. These findings suggest that OmA20, like those of mammals, reduces LPS-induced inflammation in rainbow trout, most likely by regulating K63-linked ubiquitination of TRAF6.

Advances in immunological research of amphioxus

Amphioxus, one of the most closely related invertebrates to vertebrates, is an important animal model for studying the origin and evolution of vertebrate immunity, especially the transition from innate immunity to adaptive immunity. The current research progresses of amphioxus in the field of immune organs, immune cells, complement system, cytokines, nuclear factor kappa B, immune-related lectins and enzymes are summarized, and some issues that remain to be understood or are in need of further clarification are highlighted. We hope to provide references for more in-depth study of the amphioxus immune system and lay a solid foundation for the construction of three-dimensional immune network in amphioxus from ontogeny to phylogeny.

Expression and role of ABIN1 in sepsis: In vitro and in vivo studies

In this research, we attempted to explain the effect and the related molecular mechanisms of ABIN1 in lipopolysaccharide (LPS)-induced septic mice or RAW264.7 macrophages. LPS was adopted to treat RAW264.7 macrophages for 4 h, and the levels of inflammatory factors were assessed by ELISA. Besides, ABIN1 expression was measured by quantitative reverse transcription polymerase chain reaction. Apparently, LPS enhanced immunoreaction, suggested by increased expression of IL-1β, tumor necrosis factor (TNF)-α, and IL-6. ABIN1 levels were obviously reduced compared to the control. Furthermore, we evaluated the roles of ABIN1-plasmid in immunoreaction and nuclear factor-κB (NF-κB) pathway. We found that ABIN1-plasmid significantly reduced the expression of IL-1β, TNF-α, and IL-6 in LPS-treated cells and inhibited NF-κB pathway activation. Meanwhile, a septic mouse mode was conducted to validate the role of ABIN1 in inflammatory response and organ damage in vivo. These data suggested that ABIN1-plasmid significantly inhibited the secretion of inflammatory cytokines and Cr, BUN, AST, and ALT levels in the serum of LPS-stimulated mice compared to LPS + control-plasmid group, reflecting the relieved inflammation and organ injury. In summary, the present findings indicated that ABIN1 alleviated sepsis by repressing inflammatory response through NF-κB signaling pathway, emphasizing the potential value of ABIN1 as therapeutic strategy for sepsis.

A zebrafish functional genomics model to investigate the role of human A20 variants in vivo

Germline loss-of-function variation in TNFAIP3, encoding A20, has been implicated in a wide variety of autoinflammatory and autoimmune conditions, with acquired somatic missense mutations linked to cancer progression. Furthermore, human sequence data reveals that the A20 locus contains ~ 400 non-synonymous coding variants, which are largely uncharacterised. The growing number of A20 coding variants with unknown function, but potential clinical impact, poses a challenge to traditional mouse-based approaches. Here we report the development of a novel functional genomics approach that utilizes a new A20-deficient zebrafish (Danio rerio) model to investigate the impact of TNFAIP3 genetic variants in vivo. A20-deficient zebrafish are hyper-responsive to microbial immune activation and exhibit spontaneous early lethality. Ectopic addition of human A20 rescued A20-null zebrafish from lethality, while missense mutations at two conserved A20 residues, S381A and C243Y, reversed this protective effect. Ser381 represents a phosphorylation site important for enhancing A20 activity that is abrogated by its mutation to alanine, or by a causal C243Y mutation that triggers human autoimmune disease. These data reveal an evolutionarily conserved role for TNFAIP3 in limiting inflammation in the vertebrate linage and show how this function is controlled by phosphorylation. They also demonstrate how a zebrafish functional genomics pipeline can be utilized to investigate the in vivo significance of medically relevant human TNFAIP3 gene variants.

Functional Variation of IL-1R-Associated Kinases in the Conserved MyD88-TRAF6 Pathway during Evolution

IL-1R-associated kinases (IRAK) are important regulators in the TLR/IL-1R pathways, but their function appears inconsistent between Drosophila, bony fishes, and vertebrates. This causes a difficulty to understand the IRAK functions. As a step to reveal the evolution of IRAKs, in this study, we performed comparative and functional analysis of IRAKs by exploiting the amphioxus, a pivotal taxon connecting invertebrates and vertebrates. Sequence and phylogenetic analysis indicated three major IRAK lineages: IRAK1/2/3 is a vertebrate-specific lineage, IRAK4 is an ancient lineage conserved between invertebrate and vertebrates, and Pelle is another ancient lineage that is preserved in protostomes and invertebrate deuterostomes but lost in vertebrate deuterostomes. Pelle is closer neither to IRAK4 nor to IRAK1/2/3, hence suggesting no clear functional analogs to IRAK1/2/3 in nonvertebrates. Functional analysis showed that both amphioxus IRAK4 and Pelle could suppress NF-κB activation induced by MyD88 and TRAF6, which are unlike mammalian and Drosophila IRAKs, but, surprisingly, similar to bony fish IRAK4. Also unlike Drosophila IRAKs, no interaction was detected between amphioxus IRAK4 and Pelle, although both of them were shown capable of binding MyD88. These findings, together with previous reports, show that unlike other signal transducers in the TLR/IL-1R pathways, such as MyD88 and TRAF6, the functions of IRAKs are highly variable during evolution and very specialized in different major animal taxa. Indeed, we suggest that the functional variability of IRAKs might confer plasticity to the signal transduction of the TLR/IL-1R pathways, which in return helps the species to evolve against the pathogens.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

Interleukin-17 signaling triggers degradation of the constitutive NF-κB inhibitor ABIN-1

IL-17 activates NF-κB and inducing expression of proinflammatory genes. IL-17 drives disease in autoimmune conditions, and anti-IL-17 antibodies have shown impressive success in the clinic. Although produced by lymphocytes, IL-17 predominantly signals in fibroblasts and epithelial cells. IL-17-driven inflammation is kept in check by negative feedback signaling molecules, including the ubiquitin editing enzyme A20, whose gene TNFΑIP3 is and similarly linked to autoimmune disease susceptibility. Accordingly, we hypothesized that ABIN-1 might play a role in negatively regulating IL-17 signaling activity. Indeed, ABIN-1 enhanced both tonic and IL-17-dependent NF-κB signaling in IL-17-responsive fibroblast cells. Interestingly, the inhibitory activities of ABIN-1 on IL-17 signaling were independent of A20. ABIN-1 is a known NF-κB target gene, and we found that IL-17-induced activation of NF-κB led to enhanced ABIN-1 mRNA expression and promoter activity. Surprisingly, however, the ABIN-1 protein was inducibly degraded following IL-17 signaling in a proteasome-dependent manner. Thus, ABIN-1, acting independently of A20, restricts both baseline and IL-17-induced inflammatory gene expression. We conclude that IL-17-induced signals lead to degradation of ABIN-1, thereby releasing a constitutive cellular brake on NF-κB activation.

Immune regulator ABIN1 suppresses HIV-1 transcription by negatively regulating the ubiquitination of Tat

BACKGROUND

A20-binding inhibitor of NF-κB activation (ABIN1), an important immune regulator, was previously shown to be involved in HIV-1 replication. However, the reported studies done with overexpressed ABIN1 provided controversial results.

RESULTS

Here we identified ABIN1 as a suppressor of HIV-1 transcription since transient knockdown of ABIN1 led to increased HIV-1 replication both in transformed Jurkat T cell line and in primary human CD4+ T lymphocytes. Depletion of ABIN1 specifically enhanced the HIV-1 transcription from the integrated genome during viral life cycle, but not the earlier steps such as reverse transcription or integration. Immunoprecipitation assays revealed that ABIN1 specifically inhibits the proto-oncogene HDM2 catalyzed K63-linked polyubiquitination of Tat at Lys71, which is critical for the transactivation activity of Tat. The ubiquitin chain binding activity of ABIN1 carried by UBAN domain turned out to be essential for the inhibitory role of ABIN1. The results of immunofluorescence localization experiments suggested that ABIN1 may obstruct Tat ubiquitination by redistributing some of HDM2 from the nucleus to the cytoplasm.

CONCLUSIONS

Our findings have revealed ABIN1 as intrinsic suppressor of HIV-1 mRNA transcription by regulating the ubiquitination of Tat.

MALT1 is not alone after all: identification of novel paracaspases

Paracaspases and metacaspases are two families of caspase-like proteins identified in 2000. Up until now paracaspases were considered a single gene family with one known non-metazoan paracaspase in the slime mold Dictyostelium and a single animal paracaspase called MALT1. Human MALT1 is a critical signaling component in many innate and adaptive immunity pathways that drive inflammation, and when it is overly active, it can also cause certain forms of cancer. Here, we report the identification and functional analysis of two new vertebrate paracaspases, PCASP2 and PCASP3. Functional characterization indicates that both scaffold and protease functions are conserved across the three vertebrate paralogs. This redundancy might explain the loss of two of the paralogs in mammals and one in Xenopus. Several of the vertebrate paracaspases currently have incorrect or ambiguous annotations. We propose to annotate them accordingly as PCASP1, PCASP2, and PCASP3 similar to the caspase gene nomenclature. A comprehensive search in other metazoans and in non-metazoan species identified additional new paracaspases. We also discovered the first animal metacaspase in the sponge Amphimedon. Comparative analysis of the active site suggests that paracaspases constitute one of the several subclasses of metacaspases that have evolved several times independently.

Novel Toll/IL-1 Receptor Homologous Region Adaptors Act as Negative Regulators in Amphioxus TLR Signaling

Studies have shown that the basal chordate amphioxus possesses an extraordinarily complex TLR system, including 39 TLRs and at least 40 Toll/IL-1R homologous region (TIR) adaptors. Besides homologs to MyD88 and TIR domain-containing adaptor molecule (TICAM), most amphioxus TIR adaptors exhibit domain architectures that are not observed in other species. To reveal how these novel TIR adaptors function in amphioxus Branchiostoma belcheri tsingtauense (bbt), four representatives, bbtTIRA, bbtTIRB, bbtTIRC, and bbtTIRD, were selected for functional analyses. We found bbtTIRA to show a unique inhibitory role in amphioxus TICAM-mediated pathway by interacting with bbtTICAM and bbt receptor interacting protein 1b, whereas bbtTIRC specifically inhibits the amphioxus MyD88-dependent pathway by interacting with bbtMyD88 and depressing the polyubiquitination of bbt TNFR-associated factor 6. Although both bbtTIRB and bbtTIRD are located on endosomes, the TIR domain of bbtTIRB can interact with bbtMyD88 in the cytosol, whereas the TIR domain of bbtTIRD is enclosed in endosome, suggesting that bbtTIRD may be a redundant gene in amphioxus. This study indicated that most expanded TIR adaptors play nonredundant regulatory roles in amphioxus TLR signaling, adding a new layer to understanding the diversity and complexity of innate immunity at basal chordate.

Amphioxus as a model for investigating evolution of the vertebrate immune system

As the most basal chordate, the cephalochordate amphioxus has unique features that make it a valuable model for understanding the phylogeny of immunity. Vertebrate adaptive immunity (VAI) mediated by lymphocytes bearing variable receptors has been well-studied in mammals but not observed in invertebrates. However, the identification of lymphocyte-like cells in the gill along with genes related with lymphoid proliferation and differentiation indicates the presence of some basic components of VAI in amphioxus. Without VAI, amphioxus utilizes about 10% of its gene repertoires, and an ongoing domain reshuffling mechanism among these genes, for innate immunity, suggesting extraordinary innate complexity and diversity not observed in other species. Innate diversity may not be comparable to the somatic diversity of the VAI, but there is no doubt of the success of this immune system, since amphioxus has existed for over 500 million years. Studies of amphioxus immunity may provide information on the reduction of innate immune complexity and the conflict between microbiota and host shaped the evolution of adaptive immune systems (AIS) during chordate evolution.