Roles of the adaptor protein tumor necrosis factor receptor type 1-associated death domain protein (TRADD) in human diseases.

Cells communication in response to extracellular or biophysical stimulus relies on elaborated systems of signal transduction. In the course of most signal pathway, the cascades involve signal protein complexes, which are often assembled by adaptor proteins. Tumor necrosis factor receptor type 1-associated death domain protein (TRADD) is an adaptor molecule involved in various signal pathways and mediating multiple biological activities, including cell survival, cell proliferation, cell differentiation, apoptosis, necroptosis and inflammation. TRADD contains an N terminal tumor necrosis factor receptor-associated factor 2 (TRAF2) binding domain and a C terminal death domain (DD) for interacting with multiple DD-containing proteins. Following activation of specific receptors, such as tumor necrosis factor receptor 1 (TNFR1), death receptor 3 (DR3), tumor necrosis factor-related apoptosis-inducing ligand receptor 1 (TRAILR1, DR4), TRAILR1 (DR5), DR6 and p75 neurotrophin receptor (p75NTR)，TRADD can bind to the receptors, serving as a platform for the recruitment of the downstream molecules for signal propagating and thus mediating various physiological and pathological processes. In this review, we provide a brief overview of the current knowledge on TRADD and discuss the roles of TRADD in infectious and inflammatory diseases, cardiovascular diseases, central nervous system diseases, cancer, endometriosis, hepatocyte proliferation, preterm birth and perinatal development.

Downregulation of TNFR2 decreases survival gene expression, promotes apoptosis and affects the cell cycle of gastric cancer cells.

BACKGROUND: Chronic inflammation due to Helicobacter pylori (H. pylori) infection promotes gastric carcinogenesis. Tumour necrosis factor-α (TNF-α), a key mediator of inflammation, induces cell survival or apoptosis by binding to two receptors (TNFR1 and TNFR2). TNFR1 can induce both survival and apoptosis, while TNFR2 results only in cell survival. The dysregulation of these processes may contribute to carcinogenesis. AIM: To evaluate the effects of TNFR1 and TNFR2 downregulation in AGS cells treated with H. pylori extract on the TNF-α pathway. METHODS: AGS cell lines containing TNFR1 and TNFR2 receptors downregulated by specific shRNAs and nonsilenced AGS cells were treated with H. pylori extract for 6 h. Subsequently, quantitative polymerase chain reaction with TaqMan® assays was used for the relative quantification of the mRNAs (TNFA, TNFR1, TNFR2, TRADD, TRAF2, CFLIP, NFKB1, NFKB2, CASP8, CASP3) and miRNAs (miR-19a, miR-34a, miR-103a, miR-130a, miR-181c) related to the TNF-α signalling pathway. Flow cytometry was employed for cell cycle analysis and apoptosis assays. RESULTS: In nonsilenced AGS cells, H. pylori extract treatment increased the expression of genes involved in cell survival and inhibited both apoptosis (NFKB1, NFKB2 and CFLIP) and the TNFR1 receptor. TNFR1 downregulation significantly decreased the expression of the TRADD and CFLIP genes, although no change was observed in the cellular process or miRNA expression. In contrast, TNFR2 downregulation decreased the expression of the TRADD and TRAF2 genes, which are both important downstream mediators of the TNFR1-mediated pathway, as well as that of the NFKB1 and CFLIP genes, while upregulating the expression of miR-19a and miR-34a. Consequently, a reduction in the number of cells in the G0/G1 phase and an increase in the number of cells in the S phase were observed, as well as the promotion of early apoptosis. CONCLUSION: Our findings mainly highlight the important role of TNFR2 in the TNF-α pathway in gastric cancer, indicating that silencing it can reduce the expression of survival and anti-apoptotic genes.

Structural basis of NF-κB signaling by the p75 neurotrophin receptor interaction with adaptor protein TRADD through their respective death domains.

The p75 neurotrophin receptor (p75NTR) is a critical mediator of neuronal death and tissue remodeling and has been implicated in various neurodegenerative diseases and cancers. The death domain (DD) of p75NTR is an intracellular signaling hub and has been shown to interact with diverse adaptor proteins. In breast cancer cells, binding of the adaptor protein TRADD to p75NTR depends on nerve growth factor and promotes cell survival. However, the structural mechanism and functional significance of TRADD recruitment in neuronal p75NTR signaling remain poorly understood. Here we report an NMR structure of the p75NTR-DD and TRADD-DD complex and reveal the mechanism of specific recognition of the TRADD-DD by the p75NTR-DD mainly through electrostatic interactions. Furthermore, we identified spatiotemporal overlap of p75NTR and TRADD expression in developing cerebellar granule neurons (CGNs) at early postnatal stages and discover the physiological relevance of the interaction between TRADD and p75NTR in the regulation of canonical NF-κB signaling and cell survival in CGNs. Our results provide a new structural framework for understanding how the recruitment of TRADD to p75NTR through DD interactions creates a membrane-proximal platform, which can be efficiently regulated by various neurotrophic factors through extracellular domains of p75NTR, to propagate downstream signaling in developing neurons.

Bacterial death and TRADD-N domains help define novel apoptosis and immunity mechanisms shared by prokaryotes and metazoans.

Several homologous domains are shared by eukaryotic immunity and programmed cell-death systems and poorly understood bacterial proteins. Recent studies show these to be components of a network of highly regulated systems connecting apoptotic processes to counter-invader immunity, in prokaryotes with a multicellular habit. However, the provenance of key adaptor domains, namely those of the Death-like and TRADD-N superfamilies, a quintessential feature of metazoan apoptotic systems, remained murky. Here, we use sensitive sequence analysis and comparative genomics methods to identify unambiguous bacterial homologs of the Death-like and TRADD-N superfamilies. We show the former to have arisen as part of a radiation of effector-associated α-helical adaptor domains that likely mediate homotypic interactions bringing together diverse effector and signaling domains in predicted bacterial apoptosis- and counter-invader systems. Similarly, we show that the TRADD-N domain defines a key, widespread signaling bridge that links effector deployment to invader-sensing in multicellular bacterial and metazoan counter-invader systems. TRADD-N domains are expanded in aggregating marine invertebrates and point to distinctive diversifying immune strategies probably directed both at RNA and retroviruses and cellular pathogens that might infect such communities. These TRADD-N and Death-like domains helped identify several new bacterial and metazoan counter-invader systems featuring underappreciated, common functional principles: the use of intracellular invader-sensing lectin-like (NPCBM and FGS), transcription elongation GreA/B-C, glycosyltransferase-4 family, inactive NTPase (serving as nucleic acid receptors), and invader-sensing GTPase switch domains. Finally, these findings point to the possibility of multicellular bacteria-stem metazoan symbiosis in the emergence of the immune/apoptotic systems of the latter.

The Odd Faces of Oligomers: The Case of TRAF2-C, A Trimeric C-Terminal Domain of TNF Receptor-Associated Factor.

TNF Receptor Associated Factor 2 (TRAF2) is a trimeric protein that belongs to the TNF receptor associated factor family (TRAFs). The TRAF2 oligomeric state is crucial for receptor binding and for its interaction with other proteins involved in the TNFR signaling. The monomer-trimer equilibrium of a C- terminal domain truncated form of TRAF2 (TRAF2-C), plays also a relevant role in binding the membrane, causing inward vesiculation. In this study, we have investigated the conformational dynamics of TRAF2-C through circular dichroism, fluorescence, and dynamic light scattering, performing temperature-dependent measurements. The data indicate that the protein retains its oligomeric state and most of its secondary structure, while displaying a significative increase in the heterogeneity of the tyrosines signal, increasing the temperature from ≈15 to ≈35 °C. The peculiar crowding of tyrosine residues (12 out of 18) at the three subunit interfaces and the strong dependence on the trimer concentration indicate that such conformational changes mainly involve the contact areas between each pair of monomers, affecting the oligomeric state. Molecular dynamic simulations in this temperature range suggest that the interfaces heterogeneity is an intrinsic property of the trimer that arises from the continuous, asymmetric approaching and distancing of its subunits. Such dynamics affect the results of molecular docking on the external protein surface using receptor peptides, indicating that the TRAF2-receptor interaction in the solution might not involve three subunits at the same time, as suggested by the static analysis obtainable from the crystal structure. These findings shed new light on the role that the TRAF2 oligomeric state might have in regulating the protein binding activity in vivo.

Knockdown of circROBO2 attenuates acute myocardial infarction through regulating the miR-1184/TRADD axis.

BACKGROUND: Studies have found that circular RNAs (circRNAs) play key roles in cardiovascular diseases. However, the function of circROBO2 in acute myocardial infarction (AMI) is unclear. This study aimed to investigate the pathogenesis of circROBO2 in AMI. METHODS: qRT-PCR and Western blot were used to determine the expression levels of circROBO2, miR-1184, and TRADD in AMI and sham-operated mouse models at mRNA and protein level, respectively. The relationship among miR-1184, circROBO2 and TRADD was evaluated by RNA immunoprecipitation (RIP) analysis and luciferase reporter gene analysis. The roles of circROBO2, miR-1184, and TRADD in myocardial cell apoptosis were evaluated using flow cytometry. Ultrasound echocardiography, serum creatine kinase MB (CK-MB) and lactate dehydrogenase (LDH), myocardial infarction area, and myocardial cell apoptosis were measured to examine the effects of circROBO2 on myocardial injury. RESULTS: The expression levels of miR-1184 were significantly reduced, and the expression levels of circROBO2 and TRADD were significantly increased in MI group. CircROBO2 acted as a sponge for miR-1184 by upregulating the expression of TRADD. In addition, overexpression of miR-1184 enhanced the protective effect of knockdown of circROBO2 by partially inhibiting the expression of TRADD in vivo and in vitro. CONCLUSION: Knockdown of circROBO2 reduced the apoptosis of cardiomyocytes by increasing the expression levels of miR-1184, which in turn decreased the expression levels of TRADD in the myocardium post-MI.

Roles of TNF receptor-associated and Fas-associated death domain proteins in the apoptosis of Eimeria tenella host cells.

The present study aimed to investigate the effects of death receptor adapter proteins, namely, TNF receptor-associated death domain (TRADD) and Fas-associated death domain (FADD) proteins, on Eimeria tenella-induced host cell apoptosis. Gene silencing, culture technique for primary chick embryo cecal epithelial cells, enzyme-linked immunosorbent assay, Hoechst-Annexin V/PI apoptosis staining, fluorescence quantitative PCR, and flow cytometry were used to detect the E. tenella host cell apoptotic rate, RIP1 and FADD protein expression levels, and caspase-8 activity of the TRADD siRNA-treated and FADD siRNA-treated groups. Results showed that the apoptotic rate in the TRADD siRNA group was significantly higher than that in the NC siRNA group at 4 h post-infection with E. tenella (P < 0.05). The RIP1 protein expression level in the TRADD siRNA group was significantly lower than that in the NC siRNA group at 4-24 h (P < 0.05). The FADD expression and apoptotic rates in the TRADD siRNA group were significantly lower than those in the NC siRNA group at 24-120 h (P < 0.05). The caspase-8 activity and apoptotic rates in the FADD siRNA group were significantly lower than those in the NC siRNA group (P < 0.05) at 24-120 h. These findings indicated that E. tenella inhibited the host cell apoptosis through the TRADD-RIP1 pathway at the early developmental stage and promoted host cell apoptosis via the TRADD-FADD-caspase-8 apoptotic pathway at the middle and late developmental stages.

Ameliorative effects of alginate oligosaccharide on tumour necrosis factor-α-induced intestinal epithelial cell injury.

Alginate oligosaccharide (AOS), produced by the depolymerisation of alginate (a polysaccharide naturally present in certain species of brown algae), has been shown to have versatile biological functions. In the present study, the porcine small intestinal epithelial cell line IPEC-J2 was used to assess the ameliorative effects of AOS on tumour necrosis factor-α (TNF-α)-induced intestinal epithelial cell injury. IPEC-J2 cells were pre-treated with or without AOS (600 µg/mL) in the presence or absence of TNF-α (50 ng/mL) for 24 h. AOS pre-treatment increased (P < 0.05) the occludin protein abundance and decreased (P < 0.05) the cytokine (interleukin-6 and TNF-α) concentrations, apoptosis rate and cysteinyl aspartate-specific protease-3 (caspase-3) and caspase-8 activities in TNF-α-treated IPEC-J2 cells. In addition, AOS pre-treatment increased (P < 0.05) the content of cellular inhibitor of apoptosis protein 2 and decreased (P < 0.05) the expression levels of TNF receptor 1 (TNFR1), TNFR-associated death domain protein and Fas-associated death domain protein in TNF-α-treated IPEC-J2 cells. These results demonstrate that AOS can reduce TNFR1-mediated apoptosis, thereby alleviating TNF-α-induced inflammatory injury in intestinal epithelial cells.

Modulating TRADD to restore cellular homeostasis and inhibit apoptosis.

Cell death in human diseases is often a consequence of disrupted cellular homeostasis. If cell death is prevented without restoring cellular homeostasis, it may lead to a persistent dysfunctional and pathological state. Although mechanisms of cell death have been thoroughly investigated1-3, it remains unclear how homeostasis can be restored after inhibition of cell death. Here we identify TRADD4-6, an adaptor protein, as a direct regulator of both cellular homeostasis and apoptosis. TRADD modulates cellular homeostasis by inhibiting K63-linked ubiquitination of beclin 1 mediated by TRAF2, cIAP1 and cIAP2, thereby reducing autophagy. TRADD deficiency inhibits RIPK1-dependent extrinsic apoptosis and proteasomal stress-induced intrinsic apoptosis. We also show that the small molecules ICCB-19 and Apt-1 bind to a pocket on the N-terminal TRAF2-binding domain of TRADD (TRADD-N), which interacts with the C-terminal domain (TRADD-C) and TRAF2 to modulate the ubiquitination of RIPK1 and beclin 1. Inhibition of TRADD by ICCB-19 or Apt-1 blocks apoptosis and restores cellular homeostasis by activating autophagy in cells with accumulated mutant tau, α-synuclein, or huntingtin. Treatment with Apt-1 restored proteostasis and inhibited cell death in a mouse model of proteinopathy induced by mutant tau(P301S). We conclude that pharmacological targeting of TRADD may represent a promising strategy for inhibiting cell death and restoring homeostasis to treat human diseases.

Copper Induces Oxidative Stress and Apoptosis in the Mouse Liver.

Copper (Cu) is an essential trace element involved in the normal physiological processes of animals. However, excessive exposure to Cu can produce numerous detrimental impacts. The aim of this study was to investigate the effects of Cu on oxidative stress and apoptosis as well as their relationship in the mouse liver. Four-week-old ICR mice (n = 240) were randomly assigned to different Cu (Cu2+-CuSO4) treatment groups (0, 4, 8, and 16 mg/kg) for periods of 21 and 42 days. The high doses of Cu exposure could induce oxidative stress, by increasing the levels of reactive oxygen species (ROS) and protein carbonyls (PC) and decreasing the activities of antisuperoxide anion (ASA) and antihydroxyl radical (AHR) and content of glutathione (GSH), as well as activities and mRNA expression levels of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px). Moreover, high doses of Cu exposure induced hepatic apoptosis via the mitochondrial apoptotic pathway, as characterized by the depolarization of mitochondrial membrane potential (MMP); significantly increased mRNA and protein expression levels of cytosolic cytochrome (Cyt c), apoptosis-inducing factor (AIF), endonuclease G (Endo G), apoptosis protease-activating factor-1 (Apaf-1), cleaved caspase-9, cleaved caspase-3, cleaved PARP, Bcl-2 antagonist killer (Bak), Bcl-2-associated X protein (Bax), and Bcl-2-interacting mediator of cell death (Bim); and decreased mRNA and protein expression levels of B-cell lymphoma-2 (Bcl-2) and Bcl-extra-large (Bcl-xL). Furthermore, the activation of the tumor necrosis factor receptor-1 (TNF-R1) signaling pathway was involved in Cu-induced apoptosis, as characterized by the significantly increased mRNA and protein expression levels of TNF-R1, Fas-associated death domain (FADD), TNFR-associated death domain (TRADD), and cleaved caspase-8. These results indicated that exposure to excess Cu could cause oxidative stress triggered by ROS overproduction and diminished antioxidant function, which in turn promoted hepatic apoptosis via mitochondrial apoptosis and that the TNF-R1 signaling pathway was also involved in the Cu-induced apoptosis.

Bacteria-Catalyzed Arginine Glycosylation in Pathogens and Host.

In recent years, protein glycosylation in pathogenic bacteria has attracted more and more attention, and accumulating evidence indicated that this type of posttranslational modification is involved in many physiological processes. The NleB from several enteropathogenic bacteria species as well as SseK from Salmonella enterica are type III secretion system effectors, which have an atypical N-acetylglucosamine (N-GlcNAc) transferase activity that specifically modified a conserved arginine in TRADD, FADD, and RIPK1. NleB/SseKs GlcNAcylation of death domain proteins abrogates homotypic and heterotypic death receptors/adaptors interactions, thereby blocking an important antimicrobial host response. Interestingly, NleB/SseKs could also GlcNAcylate themselves, and self-GlcNAcylation of NleB, SseK1, and SseK3 are crucial for their biological activity during infection. In addition, EarP (EF-P specific arginine rhamnosyl transferase for Posttranslational activation) catalyzes arginine rhamnosylation of translation elongation factor P (EF-P). Importantly, this kind of N-linked protein glycosylation is not only important for EF-P dependent rescue of polyproline stalled ribosomes but also for pathogenicity in Pseudomonas aeruginosa and other clinically relevant bacteria. Glycosylation of arginine is unique because the guanidine group of arginine has a high acid dissociation constant value and representing an extremely poor nucleophile. Recently, the crystal structures of NleB, SseKs, EarP, arginine GlcNAcylated death domain-containing proteins, NleB/FADD-DD, and EarP/EF-P/dTDP-ß-L-rhamnose were solved by our group and other groups, revealing the unique catalytic mechanisms. In this review, we provide detailed information about the currently known arginine glycosyltransferases and their potential catalytic mechanisms.

Coronin 1B regulates the TNFα-induced apoptosis of HUVECs by mediating the interaction between TRADD and FADD.

Coronin 1B is an actin-binding protein that plays important roles in actin-dependent cellular processes. We previously reported that coronin 1B is involved in vascular endothelial cell growth factor-induced migration of human umbilical vein endothelial cells (HUVECs). However, the role of coronin 1B in tumor necrosis factor alpha (TNFα)-induced endothelial cell apoptosis remained unknown. In this study, we investigated whether coronin 1B affects TNFα-induced HUVEC apoptosis and sought to elucidate the mechanism by which coronin 1B regulates this cellular process. Depletion of coronin 1B by siRNA transfection decreased TNFα-induced apoptosis of HUVECs, as determined by MTT, terminal deoxynucleotidyl transferase dUTP nick end labeling and caspase-3 activity assays. Coronin 1B depletion also decreased caspase-8 cleavage via a JNK-independent pathway. Coronin 1B interacted with Fas-associated death domain protein (FADD) in both a plasmid overexpression system in HEK293T cells and at the endogenous protein level in TNFα-stimulated HUVECs. Immunoprecipitation and in situ proximity ligation assays showed that coronin 1B depletion diminished the interaction between TNFα-induced TNF receptor-1-associated death domain protein (TRADD) and FADD, suggesting that coronin 1B is required for the TNFα-induced TRADD and FADD interaction and subsequent caspase-8/caspase-3 cascade activation, ultimately leading to apoptosis.

SNHG9, delivered by adipocyte-derived exosomes, alleviates inflammation and apoptosis of endothelial cells through suppressing TRADD expression.

Exosomes are membrane-derived vesicles and play a critical role in cell signaling by transferring RNAs and proteins to target cells through fusion with the cell membrane. Long non-coding RNA-small nucleolar RNA host gene 9 (lncRNA-SNHG9) was proven to be an important element in lncRNA-mRNA interaction networks during adipocyte differentiation, suggesting its potential involvement in the development of obesity, an important risk factor of cardiovascular and cerebrovascular endothelial dysfunction. However, the role of lncRNA-SNHG9 within the exosome in endothelial dysfunction of obese patients is largely unknown. In this study, we proved that adipocytes-derived exosomal SNHG9 were downregulated in obese persons and further decreased in obese individuals with endothelial dysfunction. Functional experimentations demonstrated that adipocytes-derived exosomal SNHG9 alleviated inflammation and apoptosis in endothelial cells. Bioinformatic analysis revealed that there was a potential interaction between SNHG9 and the TNF receptor type 1-associated death domain protein (TRADD) mRNA. Then, RNA-binding protein immunoprecipitation assay based on Ago2 antibody and ribonuclease protection assay demonstrated that exosomal SNHG9 directly bound to a specific region in TRADD mRNA sequence and formed an RNA dimeric inducible silencing complex. Moreover, knockdown of TRADD markedly inhibited inflammation and apoptosis in human umbilical vein endothelial cells (HUVECs), whereas overexpression of TRADD dramatically neutralized the protective effect of exosomal SNHG9 on epithelial dysfunction. Therefore, SNHG9 could prevent endothelial dysfunction in obese patients by suppressing inflammation and apoptosis, indicating that SNHG9 may be a potential therapeutic target for obese patients with endothelial dysfunction.

miR-149* Suppresses Liver Cancer Progression by Down-Regulating Tumor Necrosis Factor Receptor 1-Associated Death Domain Protein Expression.

Liver cancer is the third leading cause of cancer-related death worldwide. Herein, we show that miR-149* serves as a novel tumor suppressor for liver tumorigenesis. Mice with genetic deletion of miR-149* (miR-149*-/- mice), which caused loss of both miR-149 and miR-149*, were considerably more susceptible to acute liver injury and hepatic carcinogenesis induced by diethylnitrosamine than wild-type mice, accompanied by increased compensatory proliferation and up-regulated gene expression of certain inflammatory cytokines. miR-149* mimics dramatically impaired liver cancer cell proliferation and migration in vitro and blocked liver cancer progression in a xenograft model. Furthermore, miR-149* strongly suppressed NF-κB signaling and repressed tumor necrosis factor receptor type 1-associated death domain protein expression in the NF-κB signaling pathway. These results reveal that miR-149*, as a novel liver tumor suppressor, may serve as a potential therapeutic target for liver cancer treatment.

MiR-199a-5p regulates rat liver regeneration and hepatocyte proliferation by targeting TNF-α TNFR1/TRADD/CASPASE8/CASPASE3 signalling pathway.

Abnormally expressed miR-199a-5p (miR-199a) has been frequently reported in multiple types of malignancies. Nevertheless, its effect in liver regeneration (LR) is largely still unclear. Herein, we investigated the function of miR-199a in hepatocyte proliferation during LR. As a result, miR-199a expression was significantly increased 12-30 h, in rat hepatic tissue, after partial hepatectomy (PH). The down-regulated expression of miR-199a inhibited proliferation as well as promoted cell apoptosis of BRL-3A. Additionally, TNF-α was found as a target of miR-199a. The administration of TNF-α siRNA regulated the effects of miR-199a on hepatocyte proliferation as well as miR-199a-modulated TNF-α/TNFR1/TRADD/CASPASE8/CASPASE3 signalling pathways. Taken together, these present findings suggested that miR-199a promoted hepatocyte proliferation as well as LR via targeting TNF-α/TNFR1/TRADD/CASPASE8/CASPASE3.

Grouper TRADD Mediates Innate Antiviral Immune Responses and Apoptosis Induced by Singapore Grouper Iridovirus (SGIV) Infection.

Tumor necrosis factor (TNF) receptor type 1-associated DEATH domain protein (TRADD) is a TNFR1-associated signal transducer and an essential component of the TNFR1 complex that is involved in activating both apoptotic and nuclear factor (NF)-κB pathways as an adaptor. It also is required for TNFR-1-initiated neuronal apoptosis following in vitro infection with virus as an essential component of the antiviral response. To date, few studies have investigated the function of TRADD in lower vertebrates and its antiviral response to DNA virus infection. In the present study, a TRADD gene (named as EcTRADD) from the orange-spotted grouper (Epinephelus coioides) was cloned and characterized. The full-length cDNA of EcTRADD consists of 1,370 base pairs (bp) and contains a 44 bp 5'-terminal untranslated region (UTR), a 450 bp 3'-UTR including a poly (A) tail, and an 876 bp open reading frame encoding a putative 291 amino acid protein. EcTRADD has two conserved domains of N-terminal domain (TRADD-N) and a death domain (DD). EcTRADD was detected in all examined tissues. EcTRADD was up-regulated in the spleen after infection with Singapore grouper iridovirus (SGIV). Subcellular localization analysis revealed that EcTRADD and EcTRADD-DD exhibited a clear pattern of discrete and interconnecting cytoplasmic filaments resembling the death-effector filaments, while EcTRADD-N was observed in the cytoplasm. After infection with SGIV, EcTRADD, and EcTRADD-DD were transferred to the nucleus. Overexpression of EcTRADD and its domains inhibited replication of SGIV in vitro. Both EcTRADD and EcTRADD-DD induced the caspase-dependent apoptosis in control and infected cells, while EcTRADD-N inhibited the apoptosis. Additionally, EcTRADD and EcTRADD-DD significantly promoted activation of NF-κB and reporter gene p53, whereas EcTRADD-N had no significant effect on p53. The results may provide new insights into the role of fish TRADD in fish virus infection.

Herbs-partitioned moxibustion alleviates aberrant intestinal epithelial cell apoptosis by upregulating A20 expression in a mouse model of Crohn's disease.

BACKGROUND: A20 inhibits intestinal epithelial cell apoptosis in Crohn's disease, and herbs-partitioned moxibustion (HPM) has been demonstrated to be an effective treatment for Crohn's disease. However, the mechanism by which HPM reduces intestinal epithelial cell apoptosis in Crohn's disease has not been thoroughly elucidated to date. AIM: To elucidate whether HPM exerts its effects by upregulating A20 to affect intestinal epithelial cell apoptosis in a Crohn's disease mouse model. METHODS: In this study, mice with A20 deletion in intestinal epithelial cells (A20IEC-KO) were utilized to establish a Crohn's disease mouse model with 2,4,6-trinitrobenzene sulfonic acid (TNBS) administration, as well as wild-type mice. Mice were randomly divided into normal control (NC), model control (MC), mesalazine (MESA), and HPM groups. The morphology of the colonic mucosa was observed by hematoxylin-eosin staining, and serum endotoxin and apoptosis of epithelial cells were evaluated by enzyme-linked immunosorbent assay and terminal dUTP nick-end labeling assay accordingly. The protein expression levels of A20 and tumor necrosis factor receptor 1 (TNFR1)-related signaling molecules were evaluated by Western blot, and co-expression of A20 and TNFR1-associated death domain (TRADD) and co-expression of A20 and receptor-interacting protein 1 (RIP1) were observed by double immunofluorescence staining. RESULTS: The intestinal epithelial barrier was noted to have an improvement in the HPM group of wild-type (WT) mice compared with that in A20IEC-KO mice. Compared with A20 IEC-KO HPM mice, serum endotoxin levels and apoptosis percentages were decreased (P < 0.01), A20 expression levels were increased (P < 0.01), and expression of TNFR1, TRADDD, and RIP1 was decreased in the HPM group of WT mice (P TNFR1 < 0.05, P TRADD < 0.01, P RIP1 < 0.01). Both of the co-expression of A20/TRADD and A20/RIP1 showed a predominantly yellow fluorescence in the HPM group of WT mice, while a predominantly red fluorescence was noted in the HPM group of A20IEC-KO mice. CONCLUSION: Our findings suggest that HPM in treating Crohn's disease functions possibly via upregulation of the A20 expression level, resulting in downregulation of TNFR1, TRADD, and RIP1 to alleviate increased cell apoptosis in the intestinal epithelial barrier in Crohn's disease.

Salmonella Effectors SseK1 and SseK3 Target Death Domain Proteins in the TNF and TRAIL Signaling Pathways.

Strains of Salmonella utilize two distinct type three secretion systems to deliver effector proteins directly into host cells. The Salmonella effectors SseK1 and SseK3 are arginine glycosyltransferases that modify mammalian death domain containing proteins with N-acetyl glucosamine (GlcNAc) when overexpressed ectopically or as recombinant protein fusions. Here, we combined Arg-GlcNAc glycopeptide immunoprecipitation and mass spectrometry to identify host proteins GlcNAcylated by endogenous levels of SseK1 and SseK3 during Salmonella infection. We observed that SseK1 modified the mammalian signaling protein TRADD, but not FADD as previously reported. Overexpression of SseK1 greatly broadened substrate specificity, whereas ectopic co-expression of SseK1 and TRADD increased the range of modified arginine residues within the death domain of TRADD. In contrast, endogenous levels of SseK3 resulted in modification of the death domains of receptors of the mammalian TNF superfamily, TNFR1 and TRAILR, at residues Arg376 and Arg293 respectively. Structural studies on SseK3 showed that the enzyme displays a classic GT-A glycosyltransferase fold and binds UDP-GlcNAc in a narrow and deep cleft with the GlcNAc facing the surface. Together our data suggest that salmonellae carrying sseK1 and sseK3 employ the glycosyltransferase effectors to antagonise different components of death receptor signaling.

Redundant and receptor-specific activities of TRADD, RIPK1 and FADD in death receptor signaling.

We evaluated redundant and receptor-specific activities of TRADD, RIPK1, and FADD in RIPK3-expressing HeLa cells lacking expression of these proteins or any combination of two of these factors. We confirmed the opposing role of FADD in TNF- and TRAIL-induced necroptosis and observed an anti-necroptotic function of TRADD. RIPK1 and TRADD act in a redundant manner in TNF- but not TRAIL-induced apoptosis. Complementary, FADD proved to be sufficient for TRAIL- but not for TNF-induced apoptosis. TRADD and RIPK1, however, redundantly mediated proinflammatory signaling in response to TNF and TRAIL. FADD deficiency sensitized more efficiently for TNFR1-mediated necroptosis than caspase-8 deficiency pointing to a caspase-8 independent inhibitory activity of FADD on TNF-induced necroptosis. Based on these characteristics, we propose a model in which the death receptor-specific activities of TRADD, RIPK1, and FADD are traced back to their hierarchically different position in TNFR1- and TRAIL death receptor signaling.

TRADD regulates perinatal development and adulthood survival in mice lacking RIPK1 and RIPK3.

TRADD is an adaptor for TNFR1-induced apoptosis and NFκB activation. However, TRADD-deficient mice undergo normal development and contain normal lymphoid populations, which contrasts with an embryonic defect in mice lacking FADD, the shared adaptor mediating apoptosis. Recent studies indicate FADD suppresses embryonic necroptosis mediated by RIPK1. TRADD was suggested to also mediate necroptosis. Here we report that targeting TRADD fails to rescue Fadd-/- embryos from necroptosis, and ablation of TRADD rescues Ripk1-/- mice from perinatal lethality when RIPK3-mediated necroptosis is disabled. The resulting Ripk1-/-Ripk3-/-Tradd-/- mice survive until early adulthood, but die thereafter. A single allele of Tradd is optimal for survival of Ripk1-/-Ripk3-/-Tradd+/- mice. We show that TRADD plays a more dominating role in NFκB-signaling than RIPK1. While RIPK1 protects thymocytes from TNFα-induced apoptosis, TRADD promotes this process. The data demonstrate that TRADD is critical in perinatal and adult mice lacking RIPK1 and RIPK3, which has not been appreciated in prior studies.