RNF90 negatively regulates cellular antiviral responses by targeting MITA for degradation

STING and metabolism-related diseases: Roles, mechanisms, and applications

The stimulator of interferon genes (STING) pathway plays a critical role in innate immunity, acting as a central mediator that links cytosolic DNA sensing to inflammatory signaling. STING not only responds to cellular metabolic states but also actively regulates key metabolic processes, including glycolysis, lipid metabolism, and redox balance. This bidirectional interaction underscores the existence of a dynamic feedback mechanism between STING signaling and metabolic pathways, which is essential for maintaining cellular homeostasis. This review provides a comprehensive analysis, beginning with an in-depth overview of the classical STING signaling pathway, followed by a detailed examination of its reciprocal regulation of various metabolic pathways. Additionally, it explores the role and mechanisms of STING signaling in metabolic disorders, including obesity, diabetes, and atherosclerosis. By integrating these insights into the mutual regulation between STING and its metabolism, novel therapeutic strategies targeting this pathway in metabolic diseases have been proposed.

Trim7 does not have a role in the restriction of murine norovirus infection in vivo

Trim7 is an E3 ubiquitin ligase that was recently identified as a central regulator of host-viral interactions with both pro-viral and anti-viral activity in cell culture. As an inhibitor, Trim7 overexpression ubiquitinates viral proteins by recognizing C-terminal glutamines that are hallmarks of 3C-like protease cleavage events. Here, we sought to determine the physiological impact of Trim7 in resolving murine norovirus (MNV) infection of mice, as MNV is potently inhibited by Trim7 in vitro. Utilizing two independently derived Trim7-deficient mouse lines, we found no changes in the viral burden or tissue distribution of MNV in both an acute and persistent model of infection. Additionally, no changes in cytokine responses were observed after acute MNV infection of Trim7-deficient mice. Furthermore, removal of potentially confounding innate immune responses such as STING and STAT1 did not reveal any role for Trim7 in regulating MNV replication. Taken together, our data fail to find a physiological role for Trim7 in regulating MNV infection outcomes in mice and serve as a caution for defining Trim7 as a broad-acting antiviral.IMPORTANCEIntrinsic antiviral molecules that restrict viral replication are important drivers of viral evolution and viral tropism. Recently, Trim7 was shown to provide cell-intrinsic protection against RNA viruses, including murine norovirus. Biochemically, Trim7 recognizes the cleavage product of viral proteases, suggesting a novel and broad mechanism to restrict viral replication. Here, we tested whether Trim7 had a physiological role in restricting murine norovirus replication in mice. Unexpectedly, we found no impact of viral replication or innate immune responses during murine norovirus infection. Our findings urge caution in defining Trim7 as a broad antiviral factor in the absence of in vivo evidence.

The E3 ubiquitin ligase RNF182 regulates the induction of innate immune response against GCRV by mediating the ubiquitination of RIG-I in grass carp (Ctenopharyngodon idella) and rare minnow (Gobiocypris rarus)

Innate immunity is the first line of antiviral or antimicrobial defence for the host. A cytoplasmic viral RNA sensor, which is known as retinoic acid-inducible gene 1 (RIG-I), makes a vital impact on the production of type I interferons (IFN) and eliminating RNA virus. This study indicated that E3 ubiquitin ligase RING finger protein 182 (RNF182) inhibited the antiviral activity of type I IFN in grass carp reovirus (GCRV)-infected cells by directly interplaying with RIG-I. The CiE3RNF182 cDNA encode a polypeptide of 158 amino acids. Cellular distribution analysis results suggested that cytoplasm was the main site of CiE3RNF182 location. Real-time quantitative PCR showed universal expression of CiE3RNF182 in all investigated tissues, with extremely high expression in liver. During virus infection, the CiE3RNF182 associates with the CiRIG-I and then induces the Lys-33-linked ubiquitin to the Lys33 residues of CiRIG-I to trigger its degradation, causing the inhibition of CiRIG-I downstream signalling. Furthermore, we obtained CRISPR/Cas9-mediated generation of E3RNF182-null rare minnows, finding that E3RNF182 deletion facilitates the survival ratio of GCRV-infected rare minnows. Additionally, the E3RNF182-/- rare minnows exhibited significantly lower relative copy number of GCRV compared to the wild-type group. In summary, our findings demonstrate the function of E3 ligase in controlling the anti-GCRV innate immunity through RIG-I in fish.

A brief overview of the E3 ubiquitin ligase: TRIM7

TRIM7, a member of the E3 ubiquitin ligase family, has garnered significant attentions in different research fields since its discovery. This enzyme plays indispensable roles in various pathophysiological processes through ubiquitination-mediated degradation of diverse protein substrates. This review systematically summarizes the current knowledge on the protein structure and biological functions of TRIM7. Structurally, TRIM7 features a conserved RBCC motif (RING, B-box, and coiled-coil domains) coupled with a variable C-terminal region that dictates the substrate specificity. In infectious contexts, TRIM7 is required for the pathogen-specific regulation, and exerts paradoxical effects by either promoting host defense or facilitating viral pathogenesis depending on pathogen type. Within oncology, TRIM7 manifests tumor-suppressive properties through regulating metastasis, apoptosis, and tumor immunology. In addition, it might serve as a reliable biomarker for monitoring the progression of idiopathic pulmonary fibrosis and also inhibits the progression of atherosclerosis. In summary, TRIM7 plays critical roles in different pathophysiological processes, and it might be a predictive and therapeutic target in certain human diseases.

DNA-RNA hybrids in inflammation: sources, immune response, and therapeutic implications

Cytoplasmic DNA-RNA hybrids are emerging as important immunogenic nucleic acids, that were previously underappreciated. DNA-RNA hybrids, formed during cellular processes like transcription and replication, or by exogenous pathogens, are recognized by pattern recognition receptors (PRRs), including cGAS, DDX41, and TLR9, which trigger immune responses. Post-translational modifications (PTMs) including ubiquitination, phosphorylation, acetylation, and palmitoylation regulate the activity of PRRs and downstream signaling molecules, fine-tuning the immune response. Targeting enzymes involved in DNA-RNA hybrid metabolism and PTMs regulation offers therapeutic potential for inflammatory diseases. Herein, we discuss the sources, immune response, and therapeutic implications of DNA-RNA hybrids in inflammation, highlighting the significance of DNA-RNA hybrids as potential targets for the treatment of inflammation.

E3 Ubiquitin Ligase TRIM7 Alleviates Lipopolysaccharide-Induced Acute Lung Injury via Inhibiting NLRP3 Inflammasome Activation

Acute lung injury (ALI) is a common clinical disease with high mortality, characterized by tissue damage caused by excessive activation of inflammation. TRIM7 is an E3 ligase that plays an important role in regulating viral infection, tumor progression, and innate immune response. However, its function in ALI is unclear. In this study, lipopolysaccharide (LPS) was used to stimulate C57BL/6j mice and HULEC-5a cells to establish ALI models in vivo and in vitro. TRIM7 expression was down-regulated during ALI. Furthermore, overexpressing TRIM7 in HULEC-5a cells relieved cell damage and inflammatory activation induced by LPS stimulation. TRIM7 knockdown had the opposite effect. Trim7-overexpressing mice were established by endotracheal injection of adeno-associated virus 6-Trim7 virus in vivo; the ALI model was then induced by LPS stimulation. Overexpression of TRIM7 could alleviate lung tissue injury, pulmonary interstitial hemorrhage, increased alveolar and vascular permeability, inflammatory cell infiltration, and secretion of inflammatory factors induced by LPS stimulation. Mechanistically, TRIM7 inhibited the expression of NOD-, LRR- and pyrin domain-containing 3 (NLRP3) and the activation of the NLRP3 inflammasome. The regulatory effect of TRIM7 on ALI depended on the NLRP3 inflammasome. This investigation, for the first time, showed the inhibitory effect of TRIM7 on ALI and activation of the NLRP3 inflammasome, providing new targets and ideas for the research on the mechanism and treatment of ALI.

RNF144A inhibits autophagy by targeting BECN1 for degradation during L. monocytogenes infection

Listeria monocytogenes (L. monocytogenes, Lm) is widely used in the laboratory as an infection model for the research on pathogenesis and host defense against gram-positive intracellular bacteria. Macroautophagy (called simply "autophagy" hereafter), is important in the host defense against pathogens, such as bacteria, viruses, and parasites. BECN1 plays a pivotal role in the initiation of autophagy and accumulating evidence indicates that post-translational modifications of BECN1 provide multiple strategies for autophagy regulation. In this study, we demonstrated that the RING1-IBR-RING2 (RBR) family member RNF144A (ring finger protein 144A), which was induced by Lm infection, promoted Lm infection in an autophagy-dependent but STING1-independent pattern. rnf144a deficiency in mice protected mice from Lm infection with inhibited innate immune responses. Interestingly, RNF144A decreased Lm-induced autophagosome accumulation. Mechanistic investigation indicated that RNF144A interacted with BECN1 and promoted its K48-linked ubiquitination, leading to the subsequent proteasome-dependent degradation of BECN1 and reduced autophagosome accumulation. Further study demonstrated that RNF144A promoted the ubiquitination of BECN1 at K117 and K427, and these two ubiquitination sites were essential to the role of BECN1 in autophagy and Lm infection. Thus, our findings suggested a new regulator in intracellular bacterial infection and autophagy, which may contribute to our understanding of host defense against intracellular bacterial infection via autophagy.Abbreviations: ATG3: autophagy related 3; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG10: autophagy related 10; ATG12: autophagy related 12; ATG16L1: autophagy related 16 like 1; Baf A1: bafilomycin A1; BECN1: beclin 1; BMDC: bone marrow-derived dendritic cell; BMDM: bone marrow-derived macrophage; CFUs: colony-forming units; CHX: cycloheximide; CQ: chloroquine; CXCL10/IP-10: C-X-C motif chemokine ligand 10; EBSS: Earle's balanced salt solution; ELISA: enzyme-linked immunosorbent assay; IFIT1/ISG56: interferon induced protein with tetratricopeptide repeats 1; IFNB/IFN-β: interferon beta; IL6: interleukin 6; IRF3, interferon regulatory factor 3; Lm: L. monocytogenes; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; MOI: multiplicity of infection; PLA: proximity ligation assay; PMA: phorbol myristate acetate; PMA-THP1, PMA-differentiated THP1; PMs: peritoneal macrophages; PTMs: posttranslational modifications; RBR: RING1-IBR-RING2; RNF144A: ring finger protein 144A; STING1, stimulator of interferon response cGAMP interactor 1; TBK1, TANK binding kinase 1; TNF/TNF-α: tumor necrosis factor.

HnRNPC triggers the degradation of MITA to suppress the interferon-mediated antiviral response

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a group of 34-120 kDa nuclear proteins that have recently been reported to participate in virus replication. The hnRNP family contains approximately 20 members, including hnRNP A1, hnRNP A2, hnRNP A2B1, hnRNPC, hnRNPD and hnRNPK. HnRNPC plays important roles in RNA biology, including expression, stability, mRNA splicing, nonspecific sequence export and 3'-end processing; however, the mechanisms underlying hnRNPC regulatory roles are not fully understood. Here, we found that zebrafish hnRNPC promoted spring viraemia of carp virus (SVCV) replication by increasing the stability of SVCV phosphoprotein while inhibiting the K48-linked ubiquitination of virus phosphoprotein, thereby suppressing the type I interferon (IFN) response. Mechanistically, hnRNPC could interact with the mediator of IFN regulatory factor 3 activation (MITA) to activate K48-linked ubiquitination for MITA degradation through the C-terminal domain of hnRNPC. We also showed that human hnRNPC could interact with MITA and that the overexpression of human hnRNPC decreased MITA protein in HEK293 cells, suggesting that the negative regulatory effects of hnRNPC on the type I IFN response are evolutionarily conserved. Collectively, our data indicate that hnRNPC promotes virus replication by suppressing IFN production activated by MITA and increasing the availability of viral proteins. Our work reveals an evolutionarily conserved mechanism that controls the IFN-mediated antiviral response by a member of the hnRNP family in vertebrates.

The STING signaling pathways and bacterial infection

As antibiotic-resistant bacteria continue to emerge frequently, bacterial infections have become a significant and pressing challenge to global public health. Innate immunity triggers the activation of host responses by sensing "non-self" components through various pattern recognition receptors (PRRs), serving as the first line of antibacterial defense. Stimulator of interferon genes (STING) is a PRR that binds with cyclic dinucleotides (CDN) to exert effects against bacteria, viruses, and cancer by inducing the production of type I interferon and inflammatory cytokines, and facilitating regulated cell death. Currently, drugs targeting the STING signaling pathway are predominantly applied in the fields of modulating host immune defense against cancer and viral infections, with relatively limited application in treating bacterial infections. Given the significant immunomodulatory functions of STING in the interaction between bacteria and hosts, this review summarizes the research progress on STING signaling pathways and their roles in bacterial infection, as well as the novel functions of STING modulators, aiming to offer insights for the development of antibacterial drugs.

Regulation of viral replication by host restriction factors

Viral infectious diseases, caused by numerous viruses including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), influenza A virus (IAV), enterovirus (EV), human immunodeficiency virus (HIV), hepatitis B virus (HBV), and human papillomavirus (HPV), pose a continuous threat to global health. As obligate parasites, viruses rely on host cells to replicate, and host cells have developed numerous defense mechanisms to counteract viral infection. Host restriction factors (HRFs) are critical components of the early antiviral response. These cellular proteins inhibit viral replication and spread by impeding essential steps in the viral life cycle, such as viral entry, genome transcription and replication, protein translation, viral particle assembly, and release. This review summarizes the current understanding of how host restriction factors inhibit viral replication, with a primary focus on their diverse antiviral mechanisms against a range of viruses, including SARS-CoV-2, influenza A virus, enteroviruses, human immunodeficiency virus, hepatitis B virus, and human papillomavirus. In addition, we highlight the crucial role of these factors in shaping the host-virus interactions and discuss their potential as targets for antiviral drug development.

RING finger protein 5 is a key anti-FMDV host factor through inhibition of virion assembly

Foot-and-mouth disease virus (FMDV) are small, icosahedral viruses that cause serious clinical symptoms in livestock. The FMDV VP1 protein is a key structural component, facilitating virus entry. Here, we find that the E3 ligase RNF5 interacts with VP1 and targets it for degradation through ubiquitination at the lys200 of VP1, ultimately inhibiting virus replication. Mutations at this lysine site have been found to increase the replication of FMDV in mice. Importantly, the RNF5 pharmacological activator Analog-1 alleviates disease development in a mouse infection model. Furthermore, RNF5 recognizes the VP1 protein from several picornaviruses, suggesting that targeting RNF5 may be a broad-spectrum antiviral strategy. These findings shed light on the role of the ubiquitin-proteasome system in controlling virus replication, offering potential new strategies for treating viral infections.

Early host defense against virus infections

Early host defense eliminates many viruses before infections are established while clearing others so they remain subclinical or cause only mild disease. The field of immunology has been shaped by broad concepts, including the pattern recognition theory that currently dominates innate immunology. Focusing on early host responses to virus infections, we analyze the literature to build a working hypothesis for the principles that govern the early line of cellular antiviral defense. Aiming to ultimately arrive at a criteria-based theory with strong explanatory power, we propose that both controlling infection and limiting inflammation are key drivers for the early cellular antiviral response. This response, which we suggest is exerted by a set of "microbe- and inflammation-restricting mechanisms," directly restrict viral replication while also counteracting inflammation. Exploring the mechanisms and physiological importance of the early layer of cellular antiviral defense may open further lines of research in immunology.

Bendamustine-rituximab elicits dual tumoricidal and immunomodulatory responses via cGAS-STING activation in diffuse large B-cell lymphoma

BACKGROUND

Bendamustine-rituximab (BR) therapy stands out as a promising alternative for elderly patients with diffuse large B-cell lymphoma (DLBCL), demonstrating notable efficacy when conventional regimens pose challenges. Despite its clinical success, the intricate mechanisms underlying BR therapy have remained elusive.

METHODS

DLBCL cell lines were used to investigate the mechanism of BR therapy in vitro. RNA-seq and Western blot were used to explore the target pathways of BR therapy. STING was knocked out using Crispr-cas9 and inhibited using H-151 to investigate its role in BR therapy. Bulk RNA-seq and single-cell RNA-seq data from patients were analyzed to investigate the association between STING and pyroptosis pathways, validated using STING downregulated cells. Flow cytometry, transwell experiments and co-culture experiments were performed to investigate the inflammatory phenotype of DLBCL cells after BR treatment and its effect on T-cell recruitment and activation.

RESULTS

This study elucidates that BR elicits direct tumoricidal effects by promoting apoptosis and inducing cell cycle arrest. The synergistic impact with rituximab is further potentiated by complement addition, demonstrating the pivotal role of in vivo antibody-dependent cellular cytotoxicity. Moreover, our investigation reveals that, through a cGAS-STING-dependent pathway, prolonged exposure to BR induces pyroptosis in DLBCL cells. Activation of the cGAS-STING pathway by BR therapy triggers the release of inflammatory factors and upregulates major histocompatibility complex molecules, shaping an immunologically hot tumor microenvironment.

CONCLUSIONS

This unique dual influence not only directly targets DLBCL cells but also engages the patient's immune system, paving the way for innovative combination therapies. The study provides comprehensive insights into the multifaceted actions of BR in DLBCL, offering a foundation for refined and personalized treatment strategies in elderly patients.

TRIM Proteins: Key Regulators of Immunity to Herpesvirus Infection

Herpesviruses are ubiquitous DNA viruses that can establish latency and cause a range of mild to life-threatening diseases in humans. Upon infection, herpesviruses trigger the activation of several host antiviral defense programs that play critical roles in curbing virus replication and dissemination. Recent work from many groups has integrated our understanding of TRIM (tripartite motif) proteins, a specific group of E3 ligase enzymes, as pivotal orchestrators of mammalian antiviral immunity. In this review, we summarize recent advances in the modulation of innate immune signaling by TRIM proteins during herpesvirus infection, with a focus on the detection of herpes simplex virus type 1 (HSV-1, a prototype herpesvirus) by cGAS-STING, RIG-I-like receptors, and Toll-like receptors. We also review the latest progress in understanding the intricate relationship between herpesvirus replication and TRIM protein-regulated autophagy and apoptosis. Finally, we discuss the maneuvers used by HSV-1 and other herpesviruses to overcome TRIM protein-mediated virus restriction.

Trim7 does not have a role in the restriction of murine norovirus infection in vivo

Trim7 is an E3 ubiquitin ligase that was recently identified as a central regulator of host-viral interactions with both pro-viral and anti-viral activity in cell culture. As an inhibitor, Trim7 overexpression ubiquitinates viral proteins by recognizing C-terminal glutamines that are hallmarks of 3C-like protease cleavage events. Here we sought to determine the physiological impact of Trim7 in resolving murine norovirus (MNV) infection of mice as MNV is potently inhibited by Trim7 in vitro. Utilizing two independently derived Trim7 deficient mouse lines we found no changes in the viral burden or tissue distribution of MNV in both an acute and persistent model of infection. Additionally, no changes in cytokine responses were observed after acute MNV infection of Trim7-deficient mice. Furthermore, removal of potentially confounding innate immune responses such as STING and STAT1 did not reveal any role for Trim7 in regulating MNV replication. Taken together, our data fails to find a physiological role for Trim7 in regulating MNV infection outcomes in mice and serves as a caution for defining Trim7 as a broad acting antiviral.

The Dual Role of TRIM7 in Viral Infections

The E3 ubiquitin ligase TRIM7 is known to have dual roles during viral infections. Like other TRIM proteins, TRIM7 can regulate the IFN pathway via the regulation of the cytosolic receptors RIG-I or MDA-5, which promote the production of type I interferons (IFN-I) and antiviral immune responses. Alternatively, under certain infectious conditions, TRIM7 can negatively regulate IFN-I signaling, resulting in increased virus replication. A growing body of evidence has also shown that TRIM7 can, in some cases, ubiquitinate viral proteins to promote viral replication and pathogenesis, while in other cases it can promote degradation of viral proteins through the proteasome, reducing virus infection. TRIM7 can also regulate the host inflammatory response and modulate the production of inflammatory cytokines, which can lead to detrimental inflammation. TRIM7 can also protect the host during infection by reducing cellular apoptosis. Here, we discuss the multiple functions of TRIM7 during viral infections and its potential as a therapeutic target.

Pseudorabies virus tegument protein US2 antagonizes antiviral innate immunity by targeting cGAS-STING signaling pathway

Background

The cGAS-STING axis-mediated type I interferon pathway is a crucial strategy for host defense against DNA virus infection. Numerous evasion strategies developed by the pseudorabies virus (PRV) counteract host antiviral immunity. To what extent PRV-encoded proteins evade the cGAS-STING signaling pathway is unknown.

Methods

Using US2 stably expressing cell lines and US2-deficient PRV model, we revealed that the PRV tegument protein US2 reduces STING protein stability and downregulates STING-mediated antiviral signaling.

Results

To promote K48-linked ubiquitination and STING degradation, US2 interacts with the LBD structural domain of STING and recruits the E3 ligase TRIM21. TRIM21 deficiency consistently strengthens the host antiviral immune response brought on by PRV infection. Additionally, US2-deficient PRV is less harmful in mice.

Conclusions

Our study implies that PRV US2 inhibits IFN signaling by a new mechanism that selectively targets STING while successfully evading the host antiviral response. As a result, the present study reveals a novel strategy by which PRV evades host defense and offers explanations for why the Bartha-K61 classical vaccine strain failed to offer effective defense against PRV variant strains in China, indicating that US2 may be a key target for developing gene-deficient PRV vaccines.

Emerging roles of tripartite motif family proteins (TRIMs) in breast cancer

Breast cancer (BC) is the most common malignant tumor worldwide. Despite enormous progress made in the past decades, the underlying mechanisms of BC remain further illustrated. Recently, TRIM family proteins proved to be engaged in BC progression through regulating various aspects. Here we reviewed the structures and basic functions of TRIM family members and first classified them into three groups according to canonical polyubiquitination forms that they could mediate: K48- only, K63- only, and both K48- and K63-linked ubiquitination. Afterwards, we focused on the specific biological functions and mechanisms of TRIMs in BCs, including tumorigenesis and invasiveness, drug sensitivity, tumor immune microenvironment (TIME), cell cycle, and metabolic reprogramming. We also explored the potential of TRIMs as novel biomarkers for predicting prognosis and future therapeutic targets in BC.

Porcine deltacoronavirus nonstructural protein 2 inhibits type I and III IFN production by targeting STING for degradation

Porcine deltacoronavirus (PDCoV) is an enteropathogenic coronavirus that has been reported to use various strategies to counter the host antiviral innate immune response. The cGAS-STING signalling pathway plays an important role in antiviral innate immunity. However, it remains unclear whether PDCoV achieves immune evasion by regulating the cGAS-STING pathway. Here, we demonstrated that the nonstructural protein 2 (nsp2) encoded by PDCoV inhibits cGAS-STING-mediated type I and III interferon (IFN) responses via the regulation of porcine STING (pSTING) stability. Mechanistically, ectopically expressed PDCoV nsp2 was found to interact with the N-terminal region of pSTING. Consequently, pSTING was degraded through K48-linked ubiquitination and the proteasomal pathway, leading to the disruption of cGAS-STING signalling. Furthermore, K150 and K236 of pSTING were identified as crucial residues for nsp2-mediated ubiquitination and degradation. In summary, our findings provide a basis for elucidating the immune evasion mechanism of PDCoV and will contribute to the development of targets for anti-coronavirus drugs.

RING finger protein 122-like (RNF122L) negatively regulates antiviral immune response by targeting STING in common carp (Cyprinus carpio L.)

Stimulator of interferon genes (STING), as an imperative adaptor protein in innate immune, responds to nucleic acid from invading pathogens to build antiviral responses in host cells. Aberrant activation of STING may trigger tissue damage and autoimmune diseases. Given the decisive role in initiating innate immune response, the activity of STING is intricately governed by several posttranslational modifications, including phosphorylation and ubiquitination. Here, we cloned and characterized a novel RNF122 homolog from common carp (named CcRNF122L). Expression analysis disclosed that the expression of CcRNF122L is up-regulated under spring viremia of carp virus (SVCV) stimulation in vivo and in vitro. Overexpression of CcRNF122L hampers SVCV- or poly(I:C)-mediated the expression of IFN-1 and ISGs in a dose-dependent way. Mechanistically, CcRNF122L interacts with STING and promotes the polyubiquitylation of STING. This polyubiquitylation event inhibits the aggregation of STING and the subsequent recruitment of TBK1 and IRF3 to the signaling complex. Additionally, the deletion of the TM domain abolishes the negative regulatory function of CcRNF122L. Collectively, our discoveries unveil a mechanism that governs the STING function and the precise adjustment of the innate immune response in teleost.

Autophagy mediated degradation of MITA/TBK1/IRF3 by a hnRNP family member attenuates interferon production in fish

HnRNP A/B belongs to the heterogeneous nuclear ribonucleoprotein (hnRNP) family and plays an important role in regulating viral protein translation and genome replication. Here, we found that overexpression of hnRNP A/B promoted spring viremia of carp virus (SVCV) and cyprinid herpesvirus 3 (CyHV3) replication. Further, hnRNP A/B was shown to act as a negative regulator of type I interferon (IFN) response. Mechanistically, hnRNP A/B interacted with MITA, TBK1 and IRF3 to initiate their degradation. In addition, hnRNP A/B bound to the kinase domain of TBK1, the C terminal domain of MITA and IAD domain of IRF3, and the RRM1 domain of hnRNP A/B bound to TBK1, RRM2 domain bound to IRF3 and MITA. Our study provides novel insights into the functions of hnRNP A/B in regulating host antiviral response.

Small molecule induced STING degradation facilitated by the HECT ligase HERC4

Stimulator of interferon genes (STING) is a central component of the cytosolic nucleic acids sensing pathway and as such master regulator of the type I interferon response. Due to its critical role in physiology and its' involvement in a variety of diseases, STING has been a focus for drug discovery. Targeted protein degradation (TPD) has emerged as a promising pharmacology for targeting previously considered undruggable proteins by hijacking the cellular ubiquitin proteasome system (UPS) with small molecules. Here, we identify AK59 as a STING degrader leveraging HERC4, a HECT-domain E3 ligase. Additionally, our data reveals that AK59 is effective on the common pathological STING mutations, suggesting a potential clinical application of this mechanism. Thus, these findings introduce HERC4 to the fields of TPD and of compound-induced degradation of STING, suggesting potential therapeutic applications.

ISGylation by HERCs facilitates STING activation

Optimal activation of stimulator of interferon genes (STING) protein is crucial for host defenses against pathogens and avoiding detrimental effects. Various post-translational modifications control STING activity. However, the function of interferon (IFN)-stimulated gene (ISG) 15 modification (ISGylation) in controlling STING stability and activation is unclear. Here, we show that the E3 ISGylation ligases HECT domain- and RCC1-like domain-containing proteins (HERCs; HERC5 in humans and HERC6 in mice) facilitate STING activation by mediating ISGylation of STING at K150, preventing its K48-linked ubiquitination and degradation. Concordantly, Herc6 deficiency suppresses herpes simplex virus 1 infection-induced type I IFN responses and facilitates viral replication both in vitro and in vivo. Notably, severe acute respiratory syndrome coronavirus 2 protein papain-like protease cleaves HERC5-mediated ISGylation of STING, suppressing host antiviral responses. These data identify a mechanism by which HERCs-mediated ISGylation controls STING stability and activation and uncover the correlations and interactions of ISGylation and ubiquitination during STING activation.

Microtubule disruption synergizes with STING signaling to show potent and broad-spectrum antiviral activity

The activation of stimulator of interferon genes (STING) signaling induces the production of type I interferons (IFNs), which play critical roles in protective innate immunity for the host to defend against viral infections. Therefore, achieving sustained or enhanced STING activation could become an antiviral immune strategy with potential broad-spectrum activities. Here, we discovered that various clinically used microtubule-destabilizing agents (MDAs) for the treatment of cancer showed a synergistic effect with the activation of STING signaling in innate immune response. The combination of a STING agonist cGAMP and a microtubule depolymerizer MMAE boosted the activation of STING innate immune response and showed broad-spectrum antiviral activity against multiple families of viruses. Mechanistically, MMAE not only disrupted the microtubule network, but also switched the cGAMP-mediated STING trafficking pattern and changed the distribution of Golgi apparatus and STING puncta. The combination of cGAMP and MMAE promoted the oligomerization of STING and downstream signaling cascades. Importantly, the cGAMP plus MMAE treatment increased STING-mediated production of IFNs and other antiviral cytokines to inhibit viral propagation in vitro and in vivo. This study revealed a novel role of the microtubule destabilizer in antiviral immune responses and provides a previously unexploited strategy based on STING-induced innate antiviral immunity.

Impaired STING Activation Due to a Variant in the E3 Ubiquitin Ligase AMFR in a Patient with Severe VZV Infection and Hemophagocytic Lymphohistiocytosis

Varicella zoster virus (VZV) is a neurotropic alphaherpesvirus exclusively infecting humans, causing two distinct pathologies: varicella (chickenpox) upon primary infection and herpes zoster (shingles) following reactivation. In susceptible individuals, VZV can give rise to more severe clinical manifestations, including disseminated infection, pneumonitis, encephalitis, and vasculopathy with stroke. Here, we describe a 3-year-old boy in whom varicella followed a complicated course with thrombocytopenia, hemorrhagic and necrotic lesions, pneumonitis, and intermittent encephalopathy. Hemophagocytic lymphohistiocytosis (HLH) was strongly suspected and as the condition deteriorated, HLH therapy was initiated. Although the clinical condition improved, longstanding hemophagocytosis followed despite therapy. We found that the patient carries a rare monoallelic variant in autocrine motility factor receptor (AMFR), encoding a ubiquitin ligase involved in innate cytosolic DNA sensing and interferon (IFN) production through the cyclic GMP-AMP synthase-stimulator of IFN genes (cGAS-STING) pathway. Peripheral blood mononuclear cells (PBMCs) from the patient exhibited impaired signaling downstream of STING in response dsDNA and 2'3'-cGAMP, agonists of cGAS and STING, respectively, and fibroblasts from the patient showed impaired type I IFN responses and significantly increased VZV replication. Overexpression of the variant AMFR R594C resulted in decreased K27-linked STING ubiquitination compared to WT AMFR. Moreover, ImageStream technology revealed reduced STING trafficking from ER to Golgi in cells expressing the patient AMFR R594C variant. This was supported by a dose-dependent dominant negative effect of expression of the patient AMFR variant as measured by IFN-β reporter gene assay. Finally, lentiviral transduction with WT AMFR partially reconstituted 2'3'-cGAMP-induced STING-mediated signaling and ISG expression in patient PBMCs. This work links defective AMFR-STING signaling to severe VZV disease and hyperinflammation and suggests a direct role for cGAS-STING in the control of viral infections in humans. In conclusion, we describe a novel genetic etiology of severe VZV disease in childhood, also representing the first inborn error of immunity related to a defect in the cGAS-STING pathway.

RNF144A promotes antiviral responses by modulating STING ubiquitination

Stimulator of interferon (IFN) genes (STING, also named MITA, ERIS, MPYS, or TMEM173) plays an essential role in DNA virus- or cytosolic DNA-triggered innate immune responses. Here, we demonstrate that the RING-in-between RING (RBR) E3 ubiquitin ligase family member RING-finger protein (RNF) 144A interacts with STING and promotes its K6-linked ubiquitination at K236, thereby enhancing STING translocation from the ER to the Golgi and downstream signaling pathways. The K236R mutant of STING displays reduced activity in promoting innate immune signal transduction. Overexpression of RNF144A upregulates HSV-1- or cytosolic DNA-induced immune responses, while knockdown of RNF144A expression has the opposite effect. In addition, Rnf144a-deficient cells exhibit impaired DNA virus- or cytosolic DNA-triggered signaling, and RNF144A protects mice from DNA virus infection. In contrast, RNF144A does not affect RNA virus- or cytosolic RNA-triggered innate immune responses. Taken together, our findings identify a new positive regulator of DNA virus- or cytosolic DNA-triggered signaling pathways and a critical ubiquitination site important for fully functional STING during antiviral responses.

Singapore grouper iridovirus VP122 targets grouper STING to evade the interferon immune response

Singapore grouper iridovirus (SGIV) is a highly pathogenic Iridoviridae that causes hemorrhage and spleen enlargement in grouper. Despite previous genome annotation efforts, many open reading frames (ORFs) in SGIV remain uncharacterized, with largely unknown functions. In this study, we identified the protein encoded by SGIV ORF122, now referred to as VP122. Notably, overexpression of VP122 promoted SGIV replication. Moreover, VP122 exhibited antagonistic effects on the natural antiviral immune response through the cGAS-STING signaling pathway. It specifically inhibited the cGAS-STING-triggered transcription of various immune-related genes, including IFN1, IFN2, ISG15, ISG56, PKR, and TNF-α in GS cells. Additionally, VP122 significantly inhibited the activation of the ISRE promoter mediated by EccGAS and EcSTING but had no effect on EccGAS or EcSTING alone. Immunoprecipitation and Western blotting experiments revealed that VP122 specifically interacts with EcSTING but not EccGAS. Notably, this interaction between VP122 and EcSTING was independent of any specific domain of EcSTING. Furthermore, VP122 inhibited the self-interaction of EcSTING. Interestingly, VP122 did not affect the recruitment of EcTBK1 and EcIRF3 to the EcSTING complex. Collectively, our results demonstrate that SGIV VP122 targets EcSTING to evade the type I interferon immune response, revealing a crucial role for VP122 in modulating the host-virus interaction.

Interplay between TRIM7 and antiviral immunity

TRIM7 has been demonstrated to have significant roles in promoting host defense against viral infections and regulating immune signaling pathways. As an E3 ubiquitin ligase, it catalyzes the ubiquitination of various substrates, including adaptor proteins (MAVS and STING) and transcription factors (NF-κB and IRF3), thereby exerting positive or negative regulation on immune signal pathways. However, viruses have developed immune evasion mechanisms to counteract TRIM7. Some viruses can inhibit TRIM7 function by targeting it for degradation or sequestering it away from its targets. Moreover, TRIM7 may even facilitate viral infection by ubiquitinating viral proteins, including envelope proteins that are critical for tissue and species tropism. A comprehensive understanding of the interaction between TRIM7 and antiviral immunity is crucial for the development of innovative treatments for viral diseases.

TRIM7/RNF90 promotes autophagy via regulation of ATG7 ubiquitination during L. monocytogenes infection

L. monocytogenes is a widely used infection model for the research on pathogenesis and host defense against gram-positive intracellular bacteria. Emerging evidence indicates that posttranslational modifications play a critical role in the regulation of macroautophagy/autophagy. However, little is known about the posttranslational modifications of ATG7, the essential protein in the autophagy process. In this study, we demonstrated that the RING-type E3 ligase TRIM7/RNF90 positively regulated autophagosome accumulation by promoting the ubiquitination of ATG7 at K413, thereby affecting L. monocytogenes infection. TRIM7 expression was induced by a variety range of conditions, including starvation, rapamycin stimulation, and L. monocytogenes infection. TRIM7 deficiency in mice or cells resulted in elevated innate immune responses and increased L. monocytogenes infection. ATG7 was associated with TRIM7 and the positive regulatory role of TRIM7 in L. monocytogenes infection-, starvation- or rapamycin-induced autophagosome accumulation was suggested by TRIM7 deficiency, TRIM7 overexpression, and TRIM7 knockdown. Further mechanistic investigation indicated that TRIM7 promoted the K63-linked ubiquitination of ATG7 at K413 and ubiquitination at this site was required for the function of ATG7 in autophagy and L. monocytogenes infection. Thus, our findings suggested a new regulator in intracellular bacterial infection and autophagy, with a novel posttranslational modification targeting ATG7. This research may expand our understanding of host anti-bacterial defense and the role of autophagy in intracellular bacterial infection.Abbreviations: ATG3: autophagy related 3; ATG5: autophagy related 5; ATG7: autophagy related 7; ATG10: autophagy related 10; ATG12: autophagy related 12; ATG16L1: autophagy related 16 like 1; Baf A1: bafilomycin A1; CQ: chloroquine; BMDC: bone marrow-derived dendritic cell; BMDM: bone marrow-derived macrophage; CFUs: colony-forming units; CXCL10/IP-10: C-X-C motif chemokine ligand 10; EBSS: Earle's balanced salt solution; ELISA: enzyme-linked immunosorbent assay; IFIT1/ISG56: interferon induced protein with tetratricopeptide repeats 1; IFNB/IFN-β: interferon beta; IL6: interleukin 6; IRF3, interferon regulatory factor 3; Lm: L. monocytogenes; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MEF: mouse embryonic fibroblast; MOI: multiplicity of infection; PLA: proximity ligation assay; PMA: phorbol myristate acetate; PMA-THP1, PMA-differentiated THP1; PMs: peritoneal macrophages; PTMs: posttranslational modifications; STING1, stimulator of interferon response cGAMP interactor 1; TBK1, TANK binding kinase 1; TNF/TNF-α: tumor necrosis factor; TRIM7/RNF90: tripartite motif containing; Hainan Provincial Natural Science Foundation of China.

The Many Ways to Deal with STING

The stimulator of interferon genes (STING) is an adaptor protein involved in the activation of IFN-β and many other genes associated with the immune response activation in vertebrates. STING induction has gained attention from different angles such as the potential to trigger an early immune response against different signs of infection and cell damage, or to be used as an adjuvant in cancer immune treatments. Pharmacological control of aberrant STING activation can be used to mitigate the pathology of some autoimmune diseases. The STING structure has a well-defined ligand binding site that can harbor natural ligands such as specific purine cyclic di-nucleotides (CDN). In addition to a canonical stimulation by CDNs, other non-canonical stimuli have also been described, whose exact mechanism has not been well defined. Understanding the molecular insights underlying the activation of STING is important to realize the different angles that need to be considered when designing new STING-binding molecules as therapeutic drugs since STING acts as a versatile platform for immune modulators. This review analyzes the different determinants of STING regulation from the structural, molecular, and cell biology points of view.

Molecular mechanisms of mitochondrial DNA release and activation of the cGAS-STING pathway

In addition to constituting the genetic material of an organism, DNA is a tracer for the recognition of foreign pathogens and a trigger of the innate immune system. cGAS functions as a sensor of double-stranded DNA fragments and initiates an immune response via the adaptor protein STING. The cGAS-STING pathway not only defends cells against various DNA-containing pathogens but also modulates many pathological processes caused by the immune response to the ectopic localization of self-DNA, such as cytosolic mitochondrial DNA (mtDNA) and extranuclear chromatin. In addition, macrophages can cause inflammation by forming a class of protein complexes called inflammasomes, and the activation of the NLRP3 inflammasome requires the release of oxidized mtDNA. In innate immunity related to inflammasomes, mtDNA release is mediated by macropores that are formed on the outer membrane of mitochondria via VDAC oligomerization. These macropores are specifically formed in response to mitochondrial stress and tissue damage, and the inhibition of VDAC oligomerization mitigates this inflammatory response. The rapidly expanding area of research on the mechanisms by which mtDNA is released and triggers inflammation has revealed new treatment strategies not only for inflammation but also, surprisingly, for neurodegenerative diseases such as amyotrophic lateral sclerosis.

Sirt1 Negatively Regulates Cellular Antiviral Responses by Preventing the Cytoplasmic Translocation of Interferon-Inducible Protein 16 in Human Cells

Interferon-inducible protein 16 (IFI16) plays a critical role in antiviral innate immune responses against DNA viruses. Although the acetylation of IFI16 is crucial to its cytoplasmic translocation and downstream signal transduction, the regulation of IFI16 acetylation remains unclear. In this study, we demonstrated that the NAD-dependent deacetylase silent information regulatory 1 (Sirtuin1, Sirt1) interacted with IFI16 and decreased the acetylation of IFI16, resulting in the inhibition of IFI16 cytoplasmic localization and antiviral responses against DNA virus and viral DNA in human cells. Meantime, Sirt1 could not inhibit RNA virus-triggered signal transduction. Interestingly, even p204, the murine ortholog of human IFI16, barely interacted with Sirt1. Thus, Sirt1 could not negatively regulate the acetylation of p204 and subsequent signal transduction upon herpes simplex virus 1 (HSV-1) infection in mouse cells. Taken together, our research work showed a new mechanism by which Sirt1 manipulated IFI16-mediated host defense. Our study also demonstrated a difference in the regulation of antiviral host defense between humans and mice, which might be considered in preclinical studies for antiviral treatment. IMPORTANCE DNA viruses, such as hepatitis B virus (HBV), human papillomavirus (HPV), human cytomegalovirus (HCMV), Epstein-Barr virus (EBV), and herpes simplex virus (HSV), can cause a wide range of diseases and are considered a global threat to human health. Interferon-inducible protein 16 (IFI16) binds virus DNA and triggers antiviral innate immune responses to restrict viral infection. In this study, we identified that silent information regulatory 1 (Sirtuin1, Sirt1) interacted with IFI16 and regulated IFI16-mediated innate host defense. Therefore, the activator or inhibitor of Sirt1 may have the potential to be used as a novel strategy to treat DNA virus-associated diseases. We also found that Sirt1 barely interacted with p204, the murine ortholog of human IFI16, and could not negatively regulate innate immune responses upon HSV-1 infection in mouse cells. This difference between humans and mice in the regulation of antiviral host defense might be considered in preclinical studies for antiviral treatment.

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.

Current understanding of the cGAS-STING signaling pathway: Structure, regulatory mechanisms, and related diseases

The innate immune system protects the host from external pathogens and internal damage in various ways. The cGAS-STING signaling pathway, comprised of cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), and downstream signaling adaptors, plays an essential role in protective immune defense against microbial DNA and internal damaged-associated DNA and is responsible for various immune-related diseases. After binding with DNA, cytosolic cGAS undergoes conformational change and DNA-linked liquid-liquid phase separation to produce 2'3'-cGAMP for the activation of endoplasmic reticulum (ER)-localized STING. However, further studies revealed that cGAS is predominantly expressed in the nucleus and strictly tethered to chromatin to prevent binding with nuclear DNA, and functions differently from cytosolic-localized cGAS. Detailed delineation of this pathway, including its structure, signaling, and regulatory mechanisms, is of great significance to fully understand the diversity of cGAS-STING activation and signaling and will be of benefit for the treatment of inflammatory diseases and cancer. Here, we review recent progress on the above-mentioned perspectives of the cGAS-STING signaling pathway and discuss new avenues for further study.

Duck TRIM29 negatively regulates type I IFN production by targeting MAVS

The innate immune response is a host defense mechanism that induces type I interferon and proinflammatory cytokines. Tripartite motif (TRIM) family proteins have recently emerged as pivotal regulators of type I interferon production in mammals. Here, we first identified duck TRIM29, which encodes 571 amino acids and shows high sequence homology with other bird TRIM29 proteins. DuTRIM29 inhibited IFN-β and IRF7 promoter activation in a dose-dependent manner and downregulated the mRNA expression of IFN-β, IRF7, Mx and IL-6 mediated by duRIG-I. Moreover, duTRIM29 interacted and colocalized with duMAVS in the cytoplasm. DuTRIM29 interacted with duMAVS via its C-terminal domains. In addition, duTRIM29 inhibited IFN-β and IRF7 promoter activation and significantly downregulated IFN-β and immune-related gene expression mediated by duMAVS in ducks. Furthermore, duTRIM29 induced K29-linked polyubiquitination and degradation of duMAVS to suppress the expression of IFN-β. Overall, our results demonstrate that duTRIM29 negatively regulates type I IFN production by targeting duMAVS in ducks. This study will contribute to a better understanding of the molecular mechanism regulating the innate immune response by TRIM proteins in ducks.

E3 Ubiquitin Ligases: The Operators of the Ubiquitin Code That Regulates the RLR and cGAS-STING Pathways

The outbreaks caused by RNA and DNA viruses, such as SARS-CoV-2 and monkeypox, pose serious threats to human health. The RLR and cGAS-STING pathways contain major cytoplasmic sensors and signaling transduction axes for host innate antiviral immunity. In physiological and virus-induced pathological states, the activation and inactivation of these signal axes are tightly controlled, especially post-translational modifications (PTMs). E3 ubiquitin ligases (E3s) are the direct manipulator of ubiquitin codons and determine the type and modification type of substrate proteins. Therefore, members of the E3s family are involved in balancing the host's innate antiviral immune responses, and their functions have been extensively studied over recent decades. In this study, we overviewed the mechanisms of different members of three E3s families that mediate the RLR and cGAS-STING axes and analyzed them as potential molecular targets for the prevention and treatment of virus-related diseases.

A C-terminal glutamine recognition mechanism revealed by E3 ligase TRIM7 structures

The E3 ligase TRIM7 has emerged as a critical player in viral infection and pathogenesis. However, the mechanism governing the TRIM7-substrate association remains to be defined. Here we report the crystal structures of TRIM7 in complex with 2C peptides of human enterovirus. Structure-guided studies reveal the C-terminal glutamine residue of 2C as the primary determinant for TRIM7 binding. Leveraged by this finding, we identify norovirus and SARS-CoV-2 proteins, and physiological proteins, as new TRIM7 substrates. Crystal structures of TRIM7 in complex with multiple peptides derived from SARS-CoV-2 proteins display the same glutamine-end recognition mode. Furthermore, TRIM7 could trigger the ubiquitination and degradation of these substrates, possibly representing a new Gln/C-degron pathway. Together, these findings unveil a common recognition mode by TRIM7, providing the foundation for further mechanistic characterization of antiviral and cellular functions of TRIM7.

Enterovirus 3C Protease Cleaves TRIM7 To Dampen Its Antiviral Activity

Mammalian TRIM7 is an antiviral protein that inhibits multiple human enteroviruses by degrading the viral 2BC protein. Whether TRIM7 is reciprocally targeted by enteroviruses is not known. Here, we report that the 3C protease (3Cpro) from two enteroviruses, coxsackievirus B3 (CVB3) and poliovirus, targets TRIM7 for cleavage. CVB3 3Cpro cleaves TRIM7 at glutamine 24 (Q24), resulting in a truncated TRIM7 that fails to inhibit CVB3 due to dampened E3 ubiquitin ligase activity. TRIM7 Q24 is highly conserved across mammals, except in marsupials, which instead have a naturally occurring histidine (H24) that is not subject to 3Cpro cleavage. Marsupials also express two isoforms of TRIM7, and the two proteins from koalas have distinct antiviral activities. The longer isoform contains an additional exon due to alternate splice site usage. This additional exon contains a unique 3Cpro cleavage site, suggesting that certain enteroviruses may have evolved to target marsupial TRIM7 even if the canonical Q24 is missing. Combined with computational analyses indicating that TRIM7 is rapidly evolving, our data raise the possibility that TRIM7 may be targeted by enterovirus evasion strategies and that evolution of TRIM7 across mammals may have conferred unique antiviral properties. IMPORTANCE Enteroviruses are significant human pathogens that cause viral myocarditis, pancreatitis, and meningitis. Knowing how the host controls these viruses and how the viruses may evade host restriction is important for understanding fundamental concepts in antiviral immunity and for informing potential therapeutic interventions. In this study, we demonstrate that coxsackievirus B3 uses its virally encoded protease to target the host antiviral protein TRIM7 for cleavage, suggesting a potential mechanism of viral immune evasion. We additionally show that TRIM7 has evolved in certain mammalian lineages to express protein variants with distinct antiviral activities and susceptibilities to viral protease-mediated cleavage.

Post-Translational Modifications of cGAS-STING: A Critical Switch for Immune Regulation

Innate immune mechanisms initiate immune responses via pattern-recognition receptors (PRRs). Cyclic GMP-AMP synthase (cGAS), a member of the PRRs, senses diverse pathogenic or endogenous DNA and activates innate immune signaling pathways, including the expression of stimulator of interferon genes (STING), type I interferon, and other inflammatory cytokines, which, in turn, instructs the adaptive immune response development. This groundbreaking discovery has rapidly advanced research on host defense, cancer biology, and autoimmune disorders. Since cGAS/STING has enormous potential in eliciting an innate immune response, understanding its functional regulation is critical. As the most widespread and efficient regulatory mode of the cGAS-STING pathway, post-translational modifications (PTMs), such as the covalent linkage of functional groups to amino acid chains, are generally considered a regulatory mechanism for protein destruction or renewal. In this review, we discuss cGAS-STING signaling transduction and its mechanism in related diseases and focus on the current different regulatory modalities of PTMs in the control of the cGAS-STING-triggered innate immune and inflammatory responses.

Regulation of cGAS/STING signaling and corresponding immune escape strategies of viruses

Innate immunity is the first line of defense against invading external pathogens, and pattern recognition receptors (PRRs) are the key receptors that mediate the innate immune response. Nowadays, there are various PRRs in cells that can activate the innate immune response by recognizing pathogen-related molecular patterns (PAMPs). The DNA sensor cGAS, which belongs to the PRRs, plays a crucial role in innate immunity. cGAS detects both foreign and host DNA and generates a second-messenger cGAMP to mediate stimulator of interferon gene (STING)-dependent antiviral responses, thereby exerting an antiviral immune response. However, the process of cGAS/STING signaling is regulated by a wide range of factors. Multiple studies have shown that viruses directly target signal transduction proteins in the cGAS/STING signaling through viral surface proteins to impede innate immunity. It is noteworthy that the virus utilizes these cGAS/STING signaling regulators to evade immune surveillance. Thus, this paper mainly summarized the regulatory mechanism of the cGAS/STING signaling pathway and the immune escape mechanism of the corresponding virus, intending to provide targeted immunotherapy ideas for dealing with specific viral infections in the future.

Selective Polyprotein Processing Determines Norovirus Sensitivity to Trim7

Noroviruses are a leading cause of gastroenteritis worldwide, yet the molecular mechanisms of how host antiviral factors restrict norovirus infection are poorly understood. Here, we present a CRISPR activation screen that identifies mouse genes which inhibit murine norovirus (MNV) replication. Detailed analysis of the major hit Trim7 demonstrates a potent inhibition of the early stages of MNV replication. Leveraging in vitro evolution, we identified MNV mutants that escape Trim7 restriction by altering the cleavage of the viral NS6-7 polyprotein precursor. NS6, but not the NS6-7 precursor, directly binds the substrate-binding domain of Trim7. Surprisingly, the selective polyprotein processing that enables Trim7 evasion inflicts a significant evolutionary burden, as viruses with decreased NS6-7 cleavage are strongly attenuated in viral replication and pathogenesis. Our data provide an unappreciated mechanism of viral evasion of cellular antiviral factors through selective polyprotein processing and highlight the evolutionary tradeoffs in acquiring resistance to host restriction factors. IMPORTANCE To maximize a limited genetic capacity, viruses encode polyproteins that can be subsequently separated into individual components by viral proteases. While classically viewed as a means of economy, recent findings have indicated that polyprotein processing can spatially and temporally coordinate the distinct phases of the viral life cycle. Here, we present a function for alternative polyprotein processing centered on immune defense. We discovered that selective polyprotein processing of the murine norovirus polyprotein shields MNV from restriction by the host antiviral protein Trim7. Trim7 can bind the viral protein NS6 but not the viral precursor protein NS6-7. Our findings provide insight into the evolutionary pressures that define patterns of viral polyprotein processing and uncover a trade-off between viral replication and immune evasion.

E3 ligase RNF5 inhibits type I interferon response in herpes simplex virus keratitis through the STING/IRF3 signaling pathway

Herpes simplex keratitis (HSK), caused by the herpes simplex virus 1 (HSV-1), is a major blinding disease in developed countries. HSV-1 can remain latent in the host for life and cannot be eradicated. The infection causes the secretion of various cytokines and aggregation of inflammatory cells. In the early stage of inflammation, mainly neutrophils infiltrate the cornea, and CD4⁺ T cells mediate the immunopathological changes in herpetic stromal keratitis in the subsequent progression. The STING/IRF3-mediated type I interferon (IFN) response can effectively inhibit viral replication and control infection, but the activity of STING is affected by various ubiquitination modifications. In this study, we found that the expression of RNF5 was elevated in corneal tissues and corneal epithelial cells after infection with HSV-1. Immunofluorescence staining confirmed that RNF5 was mainly expressed in the corneal epithelial layer. We silenced and overexpressed RNF5 expression in corneal epithelial cells and then inoculated them with HSV-1. We found that the expressions of STING, p-IRF3, p-TBK1, and IFN-β mRNA increased after RNF5 silencing. The opposite results were obtained after RNF5 overexpression. We also used siRNA to silence RNF5 in the mouse cornea and then established the HSK model. Compared with the siRNA-control group, the siRNA-RNF5 group showed significantly improved corneal inflammation, reduced clinical scores and tear virus titers, and significantly increased corneal IFN-β expression. In addition, the expressions of the proinflammatory cytokines IL-6 and TNF-α in the corneal tissue were significantly decreased, indicating that RNF5 silencing could effectively promote IFN-I expression, inhibit virus replication, alleviate inflammation, and reduce corneal inflammatory damage. In summary, our results suggest that RNF5 limits the type I IFN antiviral response in HSV corneal epithelitis by inhibiting STING/IRF3 signaling.

UHRF1 modulates breast cancer cell growth via estrogen signaling

The ubiquitination process, which involves that binding of an ubiquitin protein to certain substrates, regulates several human biological processes and human cancers. Several studies report that the abnormal expression of quite a few E3 ubiquitin ligases could play critical role in carcinogenic process and cancer progression. In our current study, we identify UHRF1 (Ubiquitin Like with PHD And Ring Finger Domain 1) is an important regulator for breast cancer growth. UHRF1 depletion significantly decreases breast cancer growth in vitro and in vivo. Clinical data analysis reveals that UHRF1 is dramatically elevated in breast cancer, compared to normal breast tissue. UHRF1 correlates with poor survival in luminal type of breast cancer patients, but not in ER-negative groups. The molecular biological studies show that UHRF1 localizes in the nuclear and interact with ERα via its SRA domain, which subsequently inhibits K48-linked ubiquitination of ERα and enhances ERα stability. Our study provides a novel function of UHRF1 in regulation estrogen signaling in breast cancer and a promising target for breast cancer therapeutics.

Post-Translational Modifications of STING: A Potential Therapeutic Target

Stimulator of interferon genes (STING) is an endoplasmic-reticulum resident protein, playing essential roles in immune responses against microbial infections. However, over-activation of STING is accompanied by excessive inflammation and results in various diseases, including autoinflammatory diseases and cancers. Therefore, precise regulation of STING activities is critical for adequate immune protection while limiting abnormal tissue damage. Numerous mechanisms regulate STING to maintain homeostasis, including protein-protein interaction and molecular modification. Among these, post-translational modifications (PTMs) are key to accurately orchestrating the activation and degradation of STING by temporarily changing the structure of STING. In this review, we focus on the emerging roles of PTMs that regulate activation and inhibition of STING, and provide insights into the roles of the PTMs of STING in disease pathogenesis and as potential targeted therapy.

DDIT3 antagonizes innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway

DNA damage-inducible transcript 3 (DDIT3), a transcription factor, is typically involved in virus replication control. We are the first to report that DDIT3 promotes the replication of bovine viral diarrhea virus, an RNA virus, by inhibiting innate immunity. However, whether the DDIT3 gene participates in DNA virus replication by regulating innate immunity remains unclear. This study reported that DDIT3 suppressed the innate immune response caused by DNA viruses to promote bovine herpesvirus 1 (BoHV-1) replication. After BoHV-1 infection of Madin-Darby bovine kidney (MDBK) cells, upregulated expression of DDIT3 induced SQSTM1-mediated autophagy and promoted STING degradation. Overexpression of the SQSTM1 protein effectively reduced STING protein levels, whereas SQSTM1 knockdown increased STING protein levels. Coimmunoprecipitation experiments and confocal laser scanning microscopy revealed that the SQSTM1 protein interacts with and colocalizes with STING. Knockdown of SQSTM1 expression in DDIT3-overexpressing cell lines restored STING protein levels. Moreover, a dual-luciferase reporter assay revealed that DDIT3 directly binds to the bovine SQSTM1 promoter and induces SQSTM1 transcription. Overexpression of SQSTM1 promoted BoHV-1 replication by inhibiting IFN-β and IFN-stimulated genes (ISGs) production; silencing of SQSTM1 promoted the expression of IFN-β and ISGs to inhibit BoHV-1 replication. In conclusion, DDIT3 targets STING via SQSTM1-mediated autophagy to promote BoHV-1 replication. These results suggest a novel mechanism by which DDIT3 regulates DNA virus replication by targeting innate immunity. DDIT3 antagonizes the innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway.

When RING Finger Family Proteins meet SARS-CoV-2

The pandemic coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is currently the most formidable challenge to humans. Understanding the complicated virus-host interplay is crucial for fighting against viral infection. A growing number of studies point to the critical roles of RING (really interesting new gene) finger (RNF) proteins during SARS-CoV-2 infection. RNF proteins exert direct antiviral activity by targeting genome and envelope glycoproteins of SARS-CoV-2. Additionally, some RNF members serve as potent regulators for antiviral innate immunity and antibody-dependent neutralization of SARS-CoV-2. Notably, SARS-CoV-2 also hijacks the RNF proteins-mediated ubiquitination process to evade host antiviral innate immunity and enhance viral replication. In this mini-review, we discuss the diverse antiviral mechanisms of RNF proteins and viral immune evasion in an RNF proteins-dependent manner. Understanding the crosstalk between RNF proteins and SARS-CoV-2 infection would help design potential novel targets for COVID-19 treatment. This article is protected by copyright. All rights reserved.

RNF19a inhibits antiviral immune response to RNA viruses through degradation of TBK1

TANK-binding kinase 1 (TBK1) plays a pivotal role in antiviral innate immunity. TBK1 mediates the activation of interferon regulatory factor (IRF) 3, leading to the induction of type I IFNs (IFN-α/β) and of NF-κB signal transduction following viral infections. TBK1 must be tightly regulated to effectively control viral infections and maintain immune homeostasis. Here, we found that E3 ubiquitin ligase RNF19a mediated K48-linked ubiquitination and proteasomal degradation of TBK1. Specifically, the silence of RNF19a enhanced the production of type I interferons and suppressed RNA viral replication. Our results uncover that RNF19a acts as a negative mediator in the RIG-I signaling pathway to attenuate antiviral immune responses and suggest RNF19a as a potential therapy target in clinical infectious and inflammatory diseases.

Gain-of-function genetic screening identifies the antiviral function of TMEM120A via STING activation

Zika virus (ZIKV) infection can be associated with neurological pathologies, such as microcephaly in newborns and Guillain-Barre syndrome in adults. Effective therapeutics are currently not available. As such, a comprehensive understanding of virus-host interactions may guide the development of medications for ZIKV. Here we report a human genome-wide overexpression screen to identify host factors that regulate ZIKV infection and find TMEM120A as a ZIKV restriction factor. TMEM120A overexpression significantly inhibits ZIKV replication, while TMEM120A knockdown increases ZIKV infection in cell lines. Moreover, Tmem120a knockout in mice facilitates ZIKV infection in primary mouse embryonic fibroblasts (MEF) cells. Mechanistically, the antiviral activity of TMEM120A is dependent on STING, as TMEM120A interacts with STING, promotes the translocation of STING from the endoplasmic reticulum (ER) to ER-Golgi intermediate compartment (ERGIC) and enhances the phosphorylation of downstream TBK1 and IRF3, resulting in the expression of multiple antiviral cytokines and interferon-stimulated genes. In summary, our gain-of-function screening identifies TMEM120A as a key activator of the antiviral signaling of STING.

Understanding the interplay between host and viral factors during infection is essential for the interactome of infection. Here the authors perform a gain-of-function screen to identify factors involved during Zika virus infection and identify TMEM120A as a key factor in the STING mediated immune responses.

The Regulatory Network of Cyclic GMP-AMP Synthase-Stimulator of Interferon Genes Pathway in Viral Evasion

Virus infection has been consistently threatening public health. The cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway is a critical defender to sense various pathogens and trigger innate immunity of mammalian cells. cGAS recognizes the pathogenic DNA in the cytosol and then synthesizes 2'3'-cyclic GMP-AMP (2'3'cGAMP). As the second messenger, cGAMP activates STING and induces the following cascade to produce type I interferon (IFN-I) to protect against infections. However, viruses have evolved numerous strategies to hinder the cGAS-STING signal transduction, promoting their immune evasion. Here we outline the current status of the viral evasion mechanism underlying the regulation of the cGAS-STING pathway, focusing on how post-transcriptional modifications, viral proteins, and non-coding RNAs involve innate immunity during viral infection, attempting to inspire new targets discovery and uncover potential clinical antiviral treatments.

Regulation of antiviral innate immune signaling and viral evasion following viral genome sensing

A harmonized balance between positive and negative regulation of pattern recognition receptor (PRR)-initiated immune responses is required to achieve the most favorable outcome for the host. This balance is crucial because it must not only ensure activation of the first line of defense against viral infection but also prevent inappropriate immune activation, which results in autoimmune diseases. Recent studies have shown how signal transduction pathways initiated by PRRs are positively and negatively regulated by diverse modulators to maintain host immune homeostasis. However, viruses have developed strategies to subvert the host antiviral response and establish infection. Viruses have evolved numerous genes encoding immunomodulatory proteins that antagonize the host immune system. This review focuses on the current state of knowledge regarding key host factors that regulate innate immune signaling molecules upon viral infection and discusses evidence showing how specific viral proteins counteract antiviral responses via immunomodulatory strategies.