TRIM56-mediated monoubiquitination of cGAS for cytosolic DNA sensing

Mammalian and Avian Host Cell Influenza A Restriction Factors

The threat of a new influenza pandemic is real. With past pandemics claiming millions of lives, finding new ways to combat this virus is essential. Host cells have developed a multi-modular system to detect incoming pathogens, a phenomenon called sensing. The signaling cascade triggered by sensing subsequently induces protection for themselves and their surrounding neighbors, termed interferon (IFN) response. This response induces the upregulation of hundreds of interferon-stimulated genes (ISGs), including antiviral effectors, establishing an antiviral state. As well as the antiviral proteins induced through the IFN system, cells also possess a so-called intrinsic immunity, constituted of antiviral proteins that are constitutively expressed, creating a first barrier preceding the induction of the interferon system. All these combined antiviral effectors inhibit the virus at various stages of the viral lifecycle, using a wide array of mechanisms. Here, we provide a review of mammalian and avian influenza A restriction factors, detailing their mechanism of action and in vivo relevance, when known. Understanding their mode of action might help pave the way for the development of new influenza treatments, which are absolutely required if we want to be prepared to face a new pandemic.

RNF111-facilitated neddylation potentiates cGAS-mediated antiviral innate immune response

The cytosolic DNA sensor cyclic GMP-AMP (cGAMP) synthetase (cGAS) has emerged as a fundamental component fueling the anti-pathogen immunity. Because of its pivotal role in initiating innate immune response, the activity of cGAS must be tightly fine-tuned to maintain immune homeostasis in antiviral response. Here, we reported that neddylation modification was indispensable for appropriate cGAS-STING signaling activation. Blocking neddylation pathway using neddylation inhibitor MLN4924 substantially impaired the induction of type I interferon and proinflammatory cytokines, which was selectively dependent on Nedd8 E2 enzyme Ube2m. We further found that deficiency of the Nedd8 E3 ligase Rnf111 greatly attenuated DNA-triggered cGAS activation while not affecting cGAMP induced activation of STING, demonstrating that Rnf111 was the Nedd8 E3 ligase of cGAS. By performing mass spectrometry, we identified Lys231 and Lys421 as essential neddylation sites in human cGAS. Mechanistically, Rnf111 interacted with and polyneddylated cGAS, which in turn promoted its dimerization and enhanced the DNA-binding ability, leading to proper cGAS-STING pathway activation. In the same line, the Ube2m or Rnf111 deficiency mice exhibited severe defects in innate immune response and were susceptible to HSV-1 infection. Collectively, our study uncovered a vital role of the Ube2m-Rnf111 neddylation axis in promoting the activity of the cGAS-STING pathway and highlighted the importance of neddylation modification in antiviral defense.

Arginine methyltransferase PRMT5 negatively regulates cGAS-mediated antiviral immune response

Cyclic GMP-AMP synthase (cGAS) functions as an essential DNA sensor, which senses the cytoplasmic double-stranded DNA and activates the antiviral response. However, the posttranslational modification of cGAS remains to be fully understood and whether it has arginine methylation modification remains unknown. Here, we identified protein arginine methyltransferase 5 (PRMT5) as a direct binding partner of cGAS, and it catalyzed the arginine symmetrical dimethylation of cGAS at the Arg¹²⁴ residue. Further investigation demonstrated that methylation of cGAS by PRMT5 attenuated cGAS-mediated antiviral immune response by blocking the DNA binding ability of cGAS. Oral administration of PRMT5 inhibitors significantly protected mice from HSV-1 infection and prolonged the survival time of these infected mice. Therefore, our findings revealed an essential regulatory effect of PRMT5 on cGAS-mediated antiviral immune response and provided a promising potential antiviral strategy by modulating PRMT5.

TRIMming Type I Interferon-Mediated Innate Immune Response in Antiviral and Antitumor Defense

The tripartite motif (TRIM) family comprises at least 80 members in humans, with most having ubiquitin or SUMO E3 ligase activity conferred by their N-terminal RING domain. TRIMs regulate a wide range of processes in ubiquitination- or sumoylation-dependent manners in most cases, and fewer as adaptors. Their roles in the regulation of viral infections, autophagy, cell cycle progression, DNA damage and other stress responses, and carcinogenesis are being increasingly appreciated, and their E3 ligase activities are attractive targets for developing specific immunotherapeutic strategies for immune diseases and cancers. Given their importance in antiviral immune response, viruses have evolved sophisticated immune escape strategies to subvert TRIM-mediated mechanisms. In this review, we focus on their regulation of IFN-I-mediated innate immune response, which plays key roles in antiviral and antitumor defense.

Exploiting Ubiquitin Ligases for Induced Target Degradation as an Antiviral Strategy

Posttranslational modifications of targeted substrates alter their cellular fate. Ubiquitin is a highly conserved and ubiquitous covalent modifier protein that tags substrates with a single molecule or with a polyubiquitin chain. Monoubiquitination affects trafficking and signaling patterns of modified proteins. In contrast, polyubiquitination, particularly K48-linked polyubiquitination, targets the protein for degradation by the Ubiquitin-Proteasome System (UPS) resulting in a committed fate through irreversible inactivation of substrate. Given the diversity of cellular functions impacted by ubiquitination, it is no surprise that the wily pathogenic viruses have co-opted the UPS in myriad ways to ensure their survival. In this review, I describe viral exploitation of nondegradative ubiquitin signaling pathways to effect entry, replication, and egress. Additionally, viruses also harness the UPS to degrade antiviral cellular host factors. Finally, I describe how we can exploit the same proteolytic machinery to enable PROTACs (Proteolysis-Targeting Chimeras) to degrade essential viral proteins. Successful implementation of this modality will add to the arsenal of emerging antiviral therapies.

The Functional Deubiquitinating Enzymes in Control of Innate Antiviral Immunity

Innate antiviral immunity is the first line of host defense against invading viral pathogens. Immunity activation primarily relies on the recognition of pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs). Viral proteins or nucleic acids mainly engage three classes of PRRs: Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), and DNA sensor cyclic GMP-AMP (cGAMP) synthase (cGAS). These receptors initiate a series of signaling cascades that lead to the production of proinflammatory cytokines and type I interferon (IFN-I) in response to viral infection. This system requires precise regulation to avoid aberrant activation. Emerging evidence has unveiled the crucial roles that the ubiquitin system, especially deubiquitinating enzymes (DUBs), play in controlling immune responses. In this review, an overview of the most current findings on the function of DUBs in the innate antiviral immune pathways is provided. Insights into the role of viral DUBs in counteracting host immune responses are also provided. Furthermore, the prospects and challenges of utilizing DUBs as therapeutic targets for infectious diseases are discussed.

Regulation and inhibition of the DNA sensor cGAS

Cell-autonomous sensing of nucleic acids is essential for host defence against invading pathogens by inducing antiviral and inflammatory cytokines. cGAS has emerged in recent years as a non-redundant DNA sensor important for detection of many viruses and bacteria. Upon binding to DNA, cGAS synthesises the cyclic dinucleotide 2'3'-cGAMP that binds to the adaptor protein STING and thereby triggers IRF3- and NFκB-dependent transcription. In addition to infection, the pathophysiology of an ever-increasing number of sterile inflammatory conditions in humans involves the recognition of DNA through cGAS. Consequently, the cGAS/STING signalling axis has emerged as an attractive target for pharmacological modulation. However, the development of cGAS and STING inhibitors has just begun and a need for specific and effective compounds persists. In this review, we focus on cGAS and explore how its activation by immunostimulatory DNA is regulated by cellular mechanisms, viral immune modulators and small molecules. We further use our knowledge of cGAS modulation by cells and viruses to conceptualise potential new ways of pharmacological cGAS targeting.

Interrogating Host Antiviral Environments Driven by Nuclear DNA Sensing: A Multiomic Perspective

Nuclear DNA sensors are critical components of the mammalian innate immune system, recognizing the presence of pathogens and initiating immune signaling. These proteins act in the nuclei of infected cells by binding to foreign DNA, such as the viral genomes of nuclear-replicating DNA viruses herpes simplex virus type 1 (HSV-1) and human cytomegalovirus (HCMV). Upon binding to pathogenic DNA, the nuclear DNA sensors were shown to initiate antiviral cytokines, as well as to suppress viral gene expression. These host defense responses involve complex signaling processes that, through protein–protein interactions (PPIs) and post-translational modifications (PTMs), drive extensive remodeling of the cellular transcriptome, proteome, and secretome to generate an antiviral environment. As such, a holistic understanding of these changes is required to understand the mechanisms through which nuclear DNA sensors act. The advent of omics techniques has revolutionized the speed and scale at which biological research is conducted and has been used to make great strides in uncovering the molecular underpinnings of DNA sensing. Here, we review the contribution of proteomics approaches to characterizing nuclear DNA sensors via the discovery of functional PPIs and PTMs, as well as proteome and secretome changes that define a host antiviral environment. We also highlight the value of and future need for integrative multiomic efforts to gain a systems-level understanding of DNA sensors and their influence on epigenetic and transcriptomic alterations during infection.

The Race between Host Antiviral Innate Immunity and the Immune Evasion Strategies of Herpes Simplex Virus 1

Herpes simplex virus 1 (HSV-1) is very successful in establishing acute and latent infections in humans by counteracting host antiviral innate immune responses. HSV-1 has evolved various strategies to evade host antiviral innate immunity and some cellular survival-associated pathways. Since there is still no vaccine available for HSV-1, a continuous update of information regarding the interaction between HSV-1 infection and the host antiviral innate immunity will provide novel insights to develop new therapeutic strategies for HSV-1 infection and its associated diseases. Here, we update recent studies about how HSV-1 evades the host antiviral innate immunity, specifically how HSV-1 proteins directly or indirectly target the adaptors in the antiviral innate immunity signaling pathways to downregulate the signal transduction. Additionally, some classical intracellular stress responses, which also play important roles in defense of viral invasion, will be discussed here. With a comprehensive review of evasion mechanisms of antiviral innate immunity by HSV-1, we will be able to develop potential new targets for therapies and a possible vaccine against HSV-1 infections.

RNF115 plays dual roles in innate antiviral responses by catalyzing distinct ubiquitination of MAVS and MITA

MAVS and MITA are essential adaptor proteins mediating innate antiviral immune responses against RNA and DNA viruses, respectively. Here we show that RNF115 plays dual roles in response to RNA or DNA virus infections by catalyzing distinct types of ubiquitination of MAVS and MITA at different phases of viral infection. RNF115 constitutively interacts with and induces K48-linked ubiquitination and proteasomal degradation of homeostatic MAVS in uninfected cells, whereas associates with and catalyzes K63-linked ubiquitination of MITA after HSV-1 infection. Consistently, the protein levels of MAVS are substantially increased in Rnf115^−/− organs or cells without viral infection, and HSV-1-induced aggregation of MITA is impaired in Rnf115^−/− cells compared to the wild-type counterparts. Consequently, the Rnf115^−/− mice exhibit hypo- and hyper-sensitivity to EMCV and HSV-1 infection, respectively. These findings highlight dual regulation of cellular antiviral responses by RNF115-mediated ubiquitination of MAVS and MITA and contribute to our understanding of innate immune signaling.

MAVS and MITA are adapter proteins that play distinct roles in the context of the host response to RNA and DNA viruses, respectively. Here the authors implicate RNF115 in dual temporal and spatial mechanisms of interacting and catalyzing distinct ubiquitination of MAVS and MITA to modulate RNA and DNA antiviral immune responses.

Role of Post-Translational Modifications of cGAS in Innate Immunity

Cyclic GMP–AMP synthase (cGAS) is the synthase that generates the second messenger cyclic GMP–AMP (cGAMP) upon DNA binding. cGAS was first discovered as the cytosolic DNA sensor that detects DNA exposed in the cytoplasm either from pathogens or cellular damage. Activated cGAS instigates the signaling cascades to activate type I interferon (IFN) expression, critical for host defense and autoimmune diseases. In addition, cGAS plays a role in senescence, DNA repair, apoptosis, and tumorigenesis. Recently, various post-translational modifications (PTMs) of cGAS have been reported, such as phosphorylation, ubiquitination, acetylation, glutamylation, and sumoylation. These PTMs profoundly affect cGAS functions. Thus, here we review the recent reported PTMs of cGAS and how these PTMs regulate cGAS enzymatic activity, DNA binding, and protein stability, and discuss the potential future directions.

When STING Meets Viruses: Sensing, Trafficking and Response

To effectively defend against microbial pathogens, the host cells mount antiviral innate immune responses by producing interferons (IFNs), and hundreds of IFN-stimulated genes (ISGs). Upon recognition of cytoplasmic viral or bacterial DNAs and abnormal endogenous DNAs, the DNA sensor cGAS synthesizes 2',3'-cGAMP that induces STING (stimulator of interferon genes) undergoing conformational changes, cellular trafficking, and the activation of downstream factors. Therefore, STING plays a pivotal role in preventing microbial pathogen infection by sensing DNAs during pathogen invasion. This review is dedicated to the recent advances in the dynamic regulations of STING activation, intracellular trafficking, and post-translational modifications (PTMs) by the host and microbial proteins.

Streptavidin Promotes DNA Binding and Activation of cGAS to Enhance Innate Immunity

cGAS/STING signaling plays an essential role in sensing cytosolic DNA. cGAS activity is regulated by posttranslational modifications and binding partners. cGAS interactome largely includes mammalian or viral proteins. Whether and how bacterial proteins bind cGAS to modulate innate immunity remain elusive. Here, we found streptavidin, a secreted bacterial protein, selectively bound cGAS to promote DNA-induced cGAS activation and interferon-β production. Mechanistically, streptavidin enhanced DNA binding and cGAS phase separation, therefore facilitating cGAS activation. Using an HSV-1-infected mouse model, we found streptavidin nanoparticles facilitated HSV-1 clearance through improving innate immunity. Considering the clinical usage of streptavidin as an immune stimulant and drug delivery vehicle and its biotechnological usage for biotin-labeled protein purification and detection, our studies not only provide an example for a bacterial protein regulating cGAS activity but also suggest caution needs to be taken when using streptavidin in various applications given to its ability to induce innate immunity.

cGAS/STING: novel perspectives of the classic pathway

Cyclic GMP-AMP (cGAMP) synthase (cGAS) is a cytosolic DNA sensor and innate immune response initiator. Binding with exogenous or endogenous nucleic acids, cGAS activates its downstream adaptor, stimulator of interferon genes (STING). STING then triggers protective immune to enable the elimination of the pathogens and the clearance of cancerous cells. Apparently, aberrantly activated by self-DNA, cGAS/STING pathway is threatening to cause autoimmune and inflammatory diseases. The effects of cGAS/STING in defenses against infection and autoimmune diseases have been well studied, still it is worthwhile to discuss the roles of cGAS/STING pathway beyond the “classical” realm of innate immunity. Recent studies have revealed its involvement in non-canonical inflammasome formation, calcium hemostasis regulation, endoplasmic reticulum (ER) stress response, perception of leaking mitochondrial DNA (mtDNA), autophagy induction, cellular senescence and senescence-associated secretory phenotype (SASP) production, providing an exciting area for future exploration. Previous studies generally focused on the function of cGAS/STING pathway in cytoplasm and immune response. In this review, we summarize the latest research of this pathway on the regulation of other physiological process and STING independent reactions to DNA in micronuclei and nuclei. Together, these studies provide a new perspective of cGAS/STING pathway in human diseases.

TRIM Proteins and Their Roles in the Influenza Virus Life Cycle

The ubiquitin-proteasome system (UPS) has been recognized for regulating fundamental cellular processes, followed by induction of proteasomal degradation of target proteins, and triggers multiple signaling pathways that are crucial for numerous aspects of cellular physiology. Especially tripartite motif (TRIM) proteins, well-known E3 ubiquitin ligases, emerge as having critical roles in several antiviral signaling pathways against varying viral infections. Here we highlight recent advances in the study of antiviral roles of TRIM proteins toward influenza virus infection in terms of the modulation of pathogen recognition receptor (PRR)-mediated innate immune sensing, direct obstruction of influenza viral propagation, and participation in virus-induced autophagy.

Structural basis for sequestration and autoinhibition of cGAS by chromatin

Cyclic GMP-AMP synthase (cGAS) is an innate immune sensor for cytosolic microbial DNA¹. After binding DNA, cGAS synthesizes the messenger 2'3'-cyclic GMP-AMP (cGAMP)^2-4, which triggers cell-autonomous defence and the production of type I interferons and pro-inflammatory cytokines via the activation of STING⁵. In addition to responding to cytosolic microbial DNA, cGAS also recognizes mislocalized cytosolic self-DNA and has been implicated in autoimmunity and sterile inflammation^6,7. Specificity towards pathogen- or damage-associated DNA was thought to be caused by cytosolic confinement. However, recent findings place cGAS robustly in the nucleus^8-10, where tight tethering of chromatin is important to prevent autoreactivity to self-DNA⁸. Here we show how cGAS is sequestered and inhibited by chromatin. We provide a cryo-electron microscopy structure of the cGAS catalytic domain bound to a nucleosome, which shows that cGAS does not interact with the nucleosomal DNA, but instead interacts with histone 2A-histone 2B, and is tightly anchored to the 'acidic patch'. The interaction buries the cGAS DNA-binding site B, and blocks the formation of active cGAS dimers. The acidic patch robustly outcompetes agonistic DNA for binding to cGAS, which suggests that nucleosome sequestration can efficiently inhibit cGAS, even when accessible DNA is nearby, such as in actively transcribed genomic regions. Our results show how nuclear cGAS is sequestered by chromatin and provides a mechanism for preventing autoreactivity to nuclear self-DNA.

Systematic review of the antiviral properties of TRIM56: a potential therapeutic intervention for COVID-19

INTRODUCTION

The tripartite motif (TRIM) plays various roles in pathological and physiological functions, including neurological diseases, genetic disorders, carcinogenesis, innate immune signaling, and antiviral activity. TRIM56 is a cytoplasmic protein whose expression is stimulated by type I interferon and may function as an antiviral agent. Here, the authors conducted a systematic search on papers that reported antiviral effects of TRIM56.

AREAS COVERED

The authors conducted a comprehensive search of the PubMed database without time or language limitation, after using the Medical Subject Headings (MeSH) Database terms. Initially, a structured search and full article review yielded 31 papers. Relevant original and review articles on TRIM56 were included. The reference lists were then reviewed, and the cited articles were added. Expert opinion: TRIM56 has been shown to have direct antiviral actions against positive-sense single-stranded RNA viruses from the families Flaviviridae, Coronaviridae, and Retroviridae. Moreover, it may be effective against negative-sense single-strand RNA viruses from the families Paramyxoviridae and Orthomyxoviridae, as well as a DNA virus, Herpes simplex virus 1 (HSV-1). These studies could suggest the potential of a TRIM56-based antiviral against COVID-19 from the family Coronaviridae, containing single-stranded positive-sense RNA genome. However, its efficacy and antiviral mechanisms need to be further examined.

The antiviral activities of TRIM proteins

Tripartite motif (TRIM) proteins are a highly versatile family of host-cell factors that play an integral role in the mammalian defense against pathogens. TRIM proteins regulate either transcription-dependent antiviral responses such as pro-inflammatory cytokine induction, or they modulate other important cell-intrinsic defense pathways like autophagy. Additionally, TRIM proteins exert direct antiviral activity whereby they antagonize specific viral components through diverse mechanisms. Here, we summarize the latest discoveries on the molecular mechanisms of antiviral TRIM proteins and also discuss current and future trends in this fast-evolving field.

TRIM Proteins in Host Defense and Viral Pathogenesis

Purpose of Review

Tripartite motif (TRIM) proteins are a large group of E3 ubiquitin ligases involved in different cellular functions. Of special interest are their roles in innate immunity, inflammation, and virus replication. We discuss novel roles of TRIM proteins during virus infections that lead to increased pathogenicity.

Recent Findings

TRIM proteins regulate different antiviral and inflammatory signaling pathways, mostly by promoting ubiquitination of important factors including pattern recognition receptors, adaptor proteins, kinases, and transcription factors that are involved in type I interferon and NF-κB pathways. Therefore, viruses have developed mechanisms to target TRIMs for immune evasion. New evidence is emerging indicating that viruses have the ability to directly use TRIMs and the ubiquitination process to enhance the viral replication cycle and cause increased pathogenesis. A new report on TRIM7 also highlights the potential pro-viral role of TRIMs via ubiquitination of viral proteins and suggests a novel mechanism by which ubiquitination of virus envelope protein may provide determinants of tissue and species tropism.

Summary

TRIM proteins have important functions in promoting host defense against virus infection; however, viruses have adapted to evade TRIM-mediated immune responses and can hijack TRIMs to ultimately increase virus pathogenesis. Only by understanding specific TRIM-virus interactions and by using more in vivo approaches can we learn how to harness TRIM function to develop therapeutic approaches to reduce virus pathogenesis.

HDAC3 inhibition ameliorates ischemia/reperfusion-induced brain injury by regulating the microglial cGAS-STING pathway

Rationale: It is known that neuroinflammation plays a critical and detrimental role in the development of cerebral ischemia/reperfusion (I/R), but the regulation of the cyclic GMP-AMP synthase (cGAS)-mediated innate immune response in I/R-induced neuroinflammation is largely unexplored. This study aimed to investigate the function and regulatory mechanism of cGAS in I/R-induced neuroinflammation and brain injury, and to identify possible strategies for the treatment of ischemic stroke.

Methods: To demonstrate that microglial histone deacetylase 3 (HDAC3) regulates the microglial cGAS-stimulator of interferon genes (cGAS-STING) pathway and is involved in I/R-induced neuroinflammation and brain injury, a series of cell biological, molecular, and biochemical approaches were utilized. These approaches include transient middle cerebral artery occlusion (tMCAO), real-time polymerase chain reaction (PCR), RNA sequencing, western blot, co-immunoprecipitation, chromosome-immunoprecipitation, enzyme-linked immunosorbent assay (ELISA), dual-luciferase reporter assay, immunohistochemistry, and confocal imaging.

Results: The microglial cGAS- STING pathway was activated by mitochondrial DNA, which promoted the formation of a pro-inflammatory microenvironment. In addition, we revealed that HDAC3 transcriptionally promoted the expression of cGAS and potentiated the activation of the cGAS-STING pathway by regulating the acetylation and nuclear localization of p65 in microglia. Our in vivo results indicated that deletion of cGAS or HDAC3 in microglia attenuated I/R-induced neuroinflammation and brain injury.

Conclusion: Collectively, we elucidated that the HDAC3-p65-cGAS-STING pathway is involved in the development of I/R-induced neuroinflammation, identifying a new therapeutic avenue for the treatment of ischemic stroke.

Herpes Simplex Virus Type 1 Interactions with the Interferon System

The interferon (IFN) system is one of the first lines of defense activated against invading viral pathogens. Upon secretion, IFNs activate a signaling cascade resulting in the production of several interferon stimulated genes (ISGs), which work to limit viral replication and establish an overall anti-viral state. Herpes simplex virus type 1 is a ubiquitous human pathogen that has evolved to downregulate the IFN response and establish lifelong latent infection in sensory neurons of the host. This review will focus on the mechanisms by which the host innate immune system detects invading HSV-1 virions, the subsequent IFN response generated to limit viral infection, and the evasion strategies developed by HSV-1 to evade the immune system and establish latency in the host.

HSV1 VP1-2 deubiquitinates STING to block type I interferon expression and promote brain infection

Herpes simplex virus (HSV) is the main cause of viral encephalitis in the Western world, and the type I interferon (IFN) system is important for antiviral control in the brain. Here, we have compared Ifnb induction in mixed murine brain cell cultures by a panel of HSV1 mutants, each devoid of one mechanism to counteract the IFN-stimulating cGAS-STING pathway. We found that a mutant lacking the deubiquitinase (DUB) activity of the VP1-2 protein induced particularly strong expression of Ifnb and IFN-stimulated genes. HSV1 ΔDUB also induced elevated IFN expression in murine and human microglia and exhibited reduced viral replication in the brain. This was associated with increased ubiquitination of STING and elevated phosphorylation of STING, TBK1, and IRF3. VP1-2 associated directly with STING, leading to its deubiquitination. Recruitment of VP1-2 to STING was dependent on K150 of STING, which was ubiquitinated by TRIM32. Thus, the DUB activity of HSV1 VP1-2 is a major viral immune-evasion mechanism in the brain.

Quantitative Temporal Proteomic Analysis of Vaccinia Virus Infection Reveals Regulation of Histone Deacetylases by an Interferon Antagonist

Vaccinia virus (VACV) has numerous immune evasion strategies, including multiple mechanisms of inhibition of interferon regulatory factor 3 (IRF-3), nuclear factor κB (NF-κB), and type I interferon (IFN) signaling. Here, we use highly multiplexed proteomics to quantify ∼9,000 cellular proteins and ∼80% of viral proteins at seven time points throughout VACV infection. A total of 265 cellular proteins are downregulated >2-fold by VACV, including putative natural killer cell ligands and IFN-stimulated genes. Two-thirds of these viral targets, including class II histone deacetylase 5 (HDAC5), are degraded proteolytically during infection. In follow-up analysis, we demonstrate that HDAC5 restricts replication of both VACV and herpes simplex virus type 1. By generating a protein-based temporal classification of VACV gene expression, we identify protein C6, a multifunctional IFN antagonist, as being necessary and sufficient for proteasomal degradation of HDAC5. Our approach thus identifies both a host antiviral factor and a viral mechanism of innate immune evasion.

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Temporal proteomic analysis quantifies host and viral dynamics during vaccinia infection

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Host protein families are proteasomally degraded over the course of vaccinia infection

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Vaccinia protein C6 targets HDAC5 for proteasomal degradation

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HDAC5 is a host antiviral factor that restricts different families of DNA viruses

The DNA Sensor cGAS is Decorated by Acetylation and Phosphorylation Modifications in the Context of Immune Signaling

The cyclic GMP-AMP synthase (cGAS) protein is a pattern-recognition receptor of the mammalian innate immune system that is recognized as a main cytosolic sensor of pathogenic or damaged DNA. cGAS DNA binding initiates catalytic production of the second messenger, cyclic GMP-AMP, which activates the STING-TBK1-IRF3 signaling axis to induce cytokine expression. Post-translational modification (PTM) has started to be recognized as a critical component of cGAS regulation, yet the extent of these modifications remains unclear. Here, we report the identification and functional analysis of cGAS phosphorylations and acetylations in several cell types under basal and immune-stimulated conditions. cGAS was enriched by immunoaffinity purification from human primary fibroblasts prior to and after infection with herpes simplex virus type 1 (HSV-1), as well as from immune-stimulated STING-HEK293T cells. Six phosphorylations and eight acetylations were detected, of which eight PTMs were not previously documented. PTMs were validated by parallel reaction monitoring (PRM) mass spectrometry in fibroblasts, HEK293T cells, and THP-1 macrophage-like cells. Primary sequence and structural analysis of cGAS highlighted a subset of PTM sites with elevated surface accessibility and high evolutionary sequence conservation. To assess the functional relevance of each PTM, we generated a series of single-point cGAS mutations. Stable cell lines were constructed to express cGAS with amino acid substitutions that prevented phosphorylation (Ser-to-Ala) and acetylation (Lys-to-Arg) or that mimicked the modification state (Ser-to-Asp and Lys-to-Gln). cGAS-dependent apoptotic and immune signaling activities were then assessed for each mutation. Our results show that acetyl-mimic mutations at Lys384 and Lys414 inhibit the ability of cGAS to induce apoptosis. In contrast, the Lys198 acetyl-mimic mutation increased cGAS-dependent interferon signaling when compared with the unmodified charge-mimic. Moreover, targeted PRM quantification showed that Lys198 acetylation is decreased upon infections with two herpesviruses-HSV-1 and human cytomegalovirus (HCMV), highlighting this residue as a regulatory point during virus infection.

To TRIM or not to TRIM: the balance of host-virus interactions mediated by the ubiquitin system

The innate immune system responds rapidly to protect against viral infections, but an overactive response can cause harmful damage. To avoid this, the response is tightly regulated by post-translational modifications (PTMs). The ubiquitin system represents a powerful PTM machinery that allows for the reversible linkage of ubiquitin to activate and deactivate a target's function. A precise enzymatic cascade of ubiquitin-activating, conjugating and ligating enzymes facilitates ubiquitination. Viruses have evolved to take advantage of the ubiquitin pathway either by targeting factors to dampen the antiviral response or by hijacking the system to enhance their replication. The tripartite motif (TRIM) family of E3 ubiquitin ligases has garnered attention as a major contributor to innate immunity. Many TRIM family members limit viruses either indirectly as components in innate immune signalling, or directly by targeting viral proteins for degradation. In spite of this, TRIMs and other ubiquitin ligases can be appropriated by viruses and repurposed as valuable tools in viral replication. This duality of function suggests a new frontier of research for TRIMs and raises new challenges for discerning the subtleties of these pro-viral mechanisms. Here, we review current findings regarding the involvement of TRIMs in host-virus interactions. We examine ongoing developments in the field, including novel roles for unanchored ubiquitin in innate immunity, the direct involvement of ubiquitin ligases in promoting viral replication, recent controversies on the role of ubiquitin and TRIM25 in activation of the pattern recognition receptor RIG-I, and we discuss the implications these studies have on future research directions.

The interactions between cGAS-STING pathway and pathogens

Cytosolic DNA is an indicator of pathogen invasion or DNA damage. The cytosolic DNA sensor cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) detects DNA and then mediates downstream immune responses through the molecule stimulator of interferon genes (STING, also known as MITA, MPYS, ERIS and TMEM173). Recent studies focusing on the roles of the cGAS-STING pathway in evolutionary distant species have partly sketched how the mammalian cGAS-STING pathways are shaped and have revealed its evolutionarily conserved mechanism in combating pathogens. Both this pathway and pathogens have developed sophisticated strategies to counteract each other for their survival. Here, we summarise current knowledge on the interactions between the cGAS-STING pathway and pathogens from both evolutionary and mechanistic perspectives. Deeper insight into these interactions might enable us to clarify the pathogenesis of certain infectious diseases and better harness the cGAS-STING pathway for antimicrobial methods.

Specificity in Ubiquitination Triggered by Virus Infection

Ubiquitination is a prominent posttranslational modification, in which the ubiquitin moiety is covalently attached to a target protein to influence protein stability, interaction partner and biological function. All seven lysine residues of ubiquitin, along with the N-terminal methionine, can each serve as a substrate for further ubiquitination, which effectuates a diverse combination of mono- or poly-ubiquitinated proteins with linear or branched ubiquitin chains. The intricately composed ubiquitin codes are then recognized by a large variety of ubiquitin binding domain (UBD)-containing proteins to participate in the regulation of various pathways to modulate the cell behavior. Viruses, as obligate parasites, involve many aspects of the cell pathways to overcome host defenses and subjugate cellular machineries. In the virus-host interactions, both the virus and the host tap into the rich source of versatile ubiquitination code in order to compete, combat, and co-evolve. Here, we review the recent literature to discuss the role of ubiquitin system as the infection progresses in virus life cycle and the importance of ubiquitin specificity in the regulation of virus-host relation.

USP29 maintains the stability of cGAS and promotes cellular antiviral responses and autoimmunity

Cyclic GMP-AMP synthase (cGAS) is an essential sensor of cytosolic DNA and critically mediates innate immune responses and autoimmunity. Modulating the activity and stability of cGAS provides potential strategies for treating viral or autoimmune diseases. Here, we report that ubiquitin-specific protease 29 (USP29) deubiquitinates and stabilizes cGAS and promotes cellular antiviral responses and autoimmunity. Knockdown or knockout of USP29 severely impairs Herpes simplex virus 1 (HSV-1)- or cytosolic DNA-induced expression of type I interferons (IFNs) and proinflammatory cytokines. Consistently, Usp29^m/m mice produce decreased type I IFNs and proinflammatory cytokines after HSV-1 infection and are hypersensitive to HSV-1 infection compared to the wild-type littermates. In addition, genetic ablation of USP29 in Trex1^−/− mice eliminated the detectable pathological and molecular autoimmune phenotypes. Mechanistically, USP29 constitutively interacts with cGAS, deconjugates K48-linked polyubiquitin chains from cGAS and stabilizes cGAS in uninfected cells or after HSV-1 infection. Reconstitution of cGAS into Usp29^−/− cells fully rescues type I IFN induction and cellular antiviral responses after HSV-1 infection. Our findings thus reveal a critical role of USP29 in the innate antiviral responses against DNA viruses and autoimmune diseases and provide insight into the regulation of cGAS.

Small molecule inhibition of human cGAS reduces total cGAMP output and cytokine expression in cells

The cGAS-STING pathway is a major mechanism that mammalian cells utilize to detect cytoplasmic dsDNA from incoming viruses, bacteria, or self. CYCLIC GMP-AMP SYNTHASE (cGAS) is the sensor protein that directly binds dsDNAs. cGAS synthesizes cyclic GMP-AMP (cGAMP), which binds to the adaptor STIMULATOR OF INTERFERON GENES (STING), activating an INTERFERON REGULATORY FACTOR 3 (IRF3)-mediated immune response. Constitutive activation can result in interferonopathies such as Aicardi-Goutieres Syndrome (AGS) or other lupus-like autoimmune disorders. While inhibitors targeting mouse or human cGAS have been reported, the identification of a small molecule that targets both homologs of cGAS has been challenging. Here, we show that RU.521 is capable of potently and selectively inhibiting mouse and human cGAS in cell lines and human primary cells. This inhibitory activity requires the presence of cGAS, but it cannot suppress an immune response in cells activated by RNA, Toll-like receptor ligands, cGAMP, or recombinant interferon. Importantly, when RU.521 is applied to cells, the production of dsDNA-induced intracellular cGAMP is suppressed in a dose-dependent manner. Our work validates the use of RU.521 for probing DNA-induced innate immune responses and underscores its potential as an ideal scaffold towards pre-clinical development, given its potency against human and mouse cGAS.

Research Advances in How the cGAS-STING Pathway Controls the Cellular Inflammatory Response

Double-stranded DNA (dsDNA) sensor cyclic-GMP-AMP synthase (cGAS) along with the downstream stimulator of interferon genes (STING) acting as essential immune-surveillance mediators have become hot topics of research. The intrinsic function of the cGAS-STING pathway facilitates type-I interferon (IFN) inflammatory signaling responses and other cellular processes such as autophagy, cell survival, senescence. cGAS-STING pathway interplays with other innate immune pathways, by which it participates in regulating infection, inflammatory disease, and cancer. The therapeutic approaches targeting this pathway show promise for future translation into clinical applications. Here, we present a review of the important previous works and recent advances regarding the cGAS-STING pathway, and provide a comprehensive understanding of the modulatory pattern of the cGAS-STING pathway under multifarious pathologic states.

Inflammatory microenvironment remodelling by tumour cells after radiotherapy

The development of immune checkpoint inhibitors (ICIs) is revolutionizing the way we think about cancer treatment. Even so, for most types of cancer, only a minority of patients currently benefit from ICI therapies. Intrinsic and acquired resistance to ICIs has focused research towards new combination therapy approaches that seek to increase response rates, the depth of remission and the durability of benefit. In this Review, we describe how radiotherapy, through its immunomodulating effects, represents a promising combination partner with ICIs. We describe how recent research on DNA damage response (DDR) inhibitors in combination with radiotherapy may be used to augment this approach. Radiotherapy can kill cancer cells while simultaneously triggering the release of pro-inflammatory mediators and increasing tumour-infiltrating immune cells - phenomena often described colloquially as turning immunologically 'cold' tumours 'hot'. Here, we focus on new developments illustrating the key role of tumour cell-autonomous signalling after radiotherapy. Radiotherapy-induced tumour cell micronuclei activate cytosolic nucleic acid sensor pathways, such as cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING), and propagation of the resulting inflammatory signals remodels the immune contexture of the tumour microenvironment. In parallel, radiation can impact immunosurveillance by modulating neoantigen expression. Finally, we highlight how tumour cell-autonomous mechanisms might be exploited by combining DDR inhibitors, ICIs and radiotherapy.

The E3 Ubiquitin Ligase TRIM21 Promotes HBV DNA Polymerase Degradation

The tripartite motif (TRIM) protein family is an E3 ubiquitin ligase family. Recent reports have indicated that some TRIM proteins have antiviral functions, especially against retroviruses. However, most studies mainly focus on the relationship between TRIM21 and interferon or other antiviral effectors. The effect of TRIM21 on virus-encoded proteins remains unclear. In this study, we screened candidate interacting proteins of HBV DNA polymerase (Pol) by FLAG affinity purification and mass spectrometry assay and identified TRIM21 as its regulator. We used a coimmunoprecipitation (co-IP) assay to demonstrate that TRIM21 interacted with the TP domain of HBV DNA Pol. In addition, TRIM21 promoted the ubiquitination and degradation of HBV DNA Pol using its RING domain, which has E3 ubiquitin ligase activity. Lys260 and Lys283 of HBV DNA Pol were identified as targets for ubiquitination mediated by TRIM21. Finally, we uncovered that TRIM21 degrades HBV DNA Pol to restrict HBV DNA replication, and its SPRY domain is critical for this activity. Taken together, our results indicate that TRIM21 suppresses HBV DNA replication mainly by promoting the ubiquitination of HBV DNA Pol, which may provide a new potential target for the treatment of HBV.

Consensus guidelines for the definition, detection and interpretation of immunogenic cell death

Cells succumbing to stress via regulated cell death (RCD) can initiate an adaptive immune response associated with immunological memory, provided they display sufficient antigenicity and adjuvanticity. Moreover, multiple intracellular and microenvironmental features determine the propensity of RCD to drive adaptive immunity. Here, we provide an updated operational definition of immunogenic cell death (ICD), discuss the key factors that dictate the ability of dying cells to drive an adaptive immune response, summarize experimental assays that are currently available for the assessment of ICD in vitro and in vivo, and formulate guidelines for their interpretation.

Regulation of cGAS-Mediated Immune Responses and Immunotherapy

Early detection of infectious nucleic acids released from invading pathogens by the innate immune system is critical for immune defense. Detection of these nucleic acids by host immune sensors and regulation of DNA sensing pathways have been significant interests in the past years. Here, current understandings of evolutionarily conserved DNA sensing cyclic GMP-AMP (cGAMP) synthase (cGAS) are highlighted. Precise activation and tight regulation of cGAS are vital in appropriate innate immune responses, senescence, tumorigenesis and immunotherapy, and autoimmunity. Hence, substantial insights into cytosolic DNA sensing and immunotherapy of indispensable cytosolic sensors have been detailed to extend limited knowledge available thus far. This Review offers a critical, in-depth understanding of cGAS regulation, cytosolic DNA sensing, and currently established therapeutic approaches of essential cytosolic immune agents for improved human health.

Emerging Roles and Research Tools of Atypical Ubiquitination

Ubiquitination is a posttranslational modification characterized by the covalent attachment of ubiquitin molecules to protein substrates. The ubiquitination modification process is reversible, dynamic, and involved in the regulation of various biological processes, such as autophagy, inflammatory responses, and DNA damage responses. The forms of ubiquitin modification are very diverse, incorporating either a single ubiquitin molecule or a complicated ubiquitin polymer, and different types of ubiquitination usually elicit corresponding cellular responses. The development of research tools and strategies has afforded more detailed insight into atypical ubiquitin signaling pathways that were previously poorly understood. Here, an update on the understanding of atypical ubiquitin chain signaling pathways is provided and the recent development of representative research tools for ubiquitin systems is discussed. In addition, the future challenges in ubiquitin research are reflected on and summarized.

STING pathway agonism as a cancer therapeutic

The fact that a subset of human cancers showed evidence for a spontaneous adaptive immune response as reflected by the T cell-inflamed tumor microenvironment phenotype led to the search for candidate innate immune pathways that might be driving such endogenous responses. Preclinical studies indicated a major role for the host STING pathway, a cytosolic DNA sensing pathway, as a proximal event required for optimal type I interferon production, dendritic cell activation, and priming of CD8⁺ T cells against tumor-associated antigens. STING agonists are therefore being developed as a novel cancer therapeutic, and a greater understanding of STING pathway regulation is leading to a broadened list of candidate immune regulatory targets. Early phase clinical trials of intratumoral STING agonists are already showing promise, alone and in combination with checkpoint blockade. Further advancement will derive from a deeper understanding of STING pathway biology as well as mechanisms of response vs resistance in individual cancer patients.

To TRIM the Immunity: From Innate to Adaptive Immunity

The tripartite motif (TRIM) proteins have been intensively studied as essential modulators in various biological processes, especially in regulating a wide range of signaling pathways involved in immune responses. Most TRIM proteins have E3 ubiquitin ligase activity, mediating polyubiquitination of target proteins. Emerging evidence demonstrates that TRIM proteins play important roles in innate immunity by regulating pattern recognition receptors, vital adaptor proteins, kinases, and transcription factors in innate immune signaling pathways. Additionally, the critical roles of TRIM proteins in adaptive immunity, especially in T cell development and activation, are increasingly appreciated. In this review, we aim to summarize the studies on TRIMs in both innate and adaptive immunity, focusing on their E3 ubiquitin ligase functions in pattern recognition receptor signaling pathways and T cell functions, shedding light on the developing new strategies for modulating innate and adaptive immune responses against invading pathogens and avoiding autoimmunity.

Intracellular sensing of viral genomes and viral evasion

During viral infection, virus-derived cytosolic nucleic acids are recognized by host intracellular specific sensors. The efficacy of this recognition system is crucial for triggering innate host defenses, which then stimulate more specific adaptive immune responses against the virus. Recent studies show that signal transduction pathways activated by sensing proteins are positively or negatively regulated by many modulators to maintain host immune homeostasis. However, viruses have evolved several strategies to counteract/evade host immune reactions. These systems involve viral proteins that interact with host sensor proteins and prevent them from detecting the viral genome or from initiating immune signaling. In this review, we discuss key regulators of cytosolic sensor proteins and viral proteins based on experimental evidence.

Fine-tuning of antiviral innate immunity by ubiquitination

The innate immune system represents the first defense line of the host following viral infection. The infection triggers the recognition of pathogen-associated molecular patterns (PAMPs) from the viruses by pattern recognition receptors (PRRs) of the host cell. The interaction between viral PAMPs and PRRs evokes a sophisticated signal transduction system and eventually promotes the expression of type I interferons (IFNs) and proinflammatory cytokines. Ubiquitination plays an indispensable role in fine-tuning almost every single step of this signaling cascade given on its versatile functions. Ubiquitin ligases and deubiquitinases (DUBs), which cooperatively and accurately regulate the dynamic and reversible ubiquitination process, are the master regulators of antiviral signaling. In this review, we concentrate on summarizing the ubiquitin ligases and DUBs that modulate the central signaling molecules in antiviral innate immunity. Especially, we emphasize the ones that were identified by the immunologists from China.

cGAS and CD-NTase enzymes: structure, mechanism, and evolution

Cyclic GMP-AMP synthase (cGAS) is a signaling enzyme in human cells that controls immune-sensing of cytosolic DNA. The recent discoveries of diverse structural homologs of cGAS in animals and bacteria reveal that cGAS-like signaling is surprisingly ancient and widespread in biology. Together with the Vibrio cholerae protein dinucleotide cyclase in Vibrio (DncV), cGAS and DncV homologs comprise a family of cGAS/DncV-like nucleotidyltransferase (CD-NTase) enzymes that synthesize noncanonical RNA signals including cyclic dinucleotides, cyclic trinucleotides, and linear oligonucleotides. Structural and biochemical breakthroughs provide a framework to understand how CD-NTase signaling allows cells to respond to changing environmental conditions. The CD-NTase family also includes uncharacterized human genes like MB21D2 and Mab21L1, highlighting emerging functions of cGAS-like signaling beyond innate immunity.

Interrupting cyclic dinucleotide-cGAS-STING axis with small molecules

The cyclic dinucleotide-cGAS-STING axis plays important roles in host immunity. Activation of this signaling pathway, via cytosolic sensing of bacterial-derived c-di-GMP/c-di-AMP or host-derived cGAMP, leads to the production of inflammatory interferons and cytokines that help resolve infection. Small molecule activators of the cGAS-STING axis have the potential to augment immune response against various pathogens or cancer. The aberrant activation of this pathway, due to gain-of-function mutations in any of the proteins that are part of the signaling axis, could lead to various autoimmune diseases. Inhibiting various nodes of the cGAS-STING axis could provide relief to patients with autoimmune diseases. Many excellent reviews on the cGAS-STING axis have been published recently, and these have mainly focused on the molecular details of the cGAS-STING pathway. This review however focuses on small molecules that can be used to modulate various aspects of the cGAS-STING pathway, as well as other parallel inflammatory pathways.

STING-Mediated IFI16 Degradation Negatively Controls Type I Interferon Production

γ-interferon-inducible protein-16 (IFI16), a key DNA sensor, triggers downstream STING-dependent type I interferon (IFN-I) production and antiviral immunity. However, it is still unclear how to negatively regulate IFI16 to avoid excessive IFN-I production and autoimmunity. Here, we find that STING directly interacts with IFI16 and facilitates IFI16 degradation via the ubiquitin-proteasome pathway by recruiting the E3 ligase TRIM21. The 1-pyrin region of IFI16 is responsible for the IFI16-STING interaction, and the first three lysines in the N-terminal region of IFI16 are the key sites that lead to STING-mediated IFI16 ubiquitination and degradation. Compared to wild-type IFI16, a higher level of viral DNA triggered IFN-β and antiviral IFN-stimulated gene expression, and thus less HSV-1 infection, was observed in the cells transfected with IFI16-K3/4/6R, an IFI16 mutant that is resistant to degradation. STING-mediated negative feedback regulation of IFI16 restricts IFN-I overproduction during antiviral immunity to avoid autoimmune diseases.

Filament-like Assemblies of Intracellular Nucleic Acid Sensors: Commonalities and Differences

Self versus non-self discrimination by innate immune sensors is critical for mounting effective immune responses against pathogens while avoiding harmful auto-inflammatory reactions against the host. Foreign DNA and RNA sensors must discriminate between self versus non-self nucleic acids, despite their shared building blocks and similar physicochemical properties. Recent structural and biochemical studies suggest that multiple steps of filament-like assembly are required for the functions of several nucleic acid sensors. Here, we discuss ligand discrimination and oligomerization of RIG-I-like receptors, AIM2-like receptors, and cGAS. We discuss how filament-like assembly allows for robust and accurate discrimination of self versus non-self nucleic acids and how these assemblies enable sensing of multiple distinct features in foreign nucleic acids, including structure, length, and modifications. We also discuss how individual receptors differ in their assembly and disassembly mechanisms and how these differences contribute to the diversity in nucleic acid specificity and pathogen detection strategies.

Cutting Edge: USP27X Deubiquitinates and Stabilizes the DNA Sensor cGAS to Regulate Cytosolic DNA-Mediated Signaling

Cyclic GMP-AMP synthase (cGAS), a cytosolic DNA sensor, catalyzes the formation of the second messenger 2'3'-cGAMP that binds to STING and triggers the type I IFN signaling. Activation of cGAS can be modulated by several protein posttranslational modifications, including ubiquitination. However, the cGAS activation regulated by protein deubiquitination remains poorly understood. In this study, we identified that deubiquitinase USP27X could interact with cGAS and cleave K48-linked polyubiquitination chains from cGAS, leading to cGAS stabilization. Consistently, knockout of Usp27x in mice macrophages resulted in an accelerated turnover of cGAS, decreased cGAMP production, phosphorylation of TBK1 and IRF3, and IFN-β production. Furthermore, Usp27x knockout mice macrophages showed impaired innate antiviral responses against HSV type 1 infection. Our data suggest that USP27X is a novel regulator of the cGAS-STING cytosolic DNA sensing pathway.

DNA sensing by the cGAS-STING pathway in health and disease

The detection of pathogens through nucleic acid sensors is a defining principle of innate immunity. RNA-sensing and DNA-sensing receptors sample subcellular compartments for foreign nucleic acids and, upon recognition, trigger immune signalling pathways for host defence. Over the past decade, our understanding of how the recognition of nucleic acids is coupled to immune gene expression has advanced considerably, particularly for the DNA-sensing receptor cyclic GMP-AMP synthase (cGAS) and its downstream signalling effector stimulator of interferon genes (STING), as well as the molecular components and regulation of this pathway. Moreover, the ability of self-DNA to engage cGAS has emerged as an important mechanism fuelling the development of inflammation and implicating the cGAS-STING pathway in human inflammatory diseases and cancer. This detailed mechanistic and biological understanding is paving the way for the development and clinical application of pharmacological agonists and antagonists in the treatment of chronic inflammation and cancer.

mTOR Dysregulation by Vaccinia Virus F17 Controls Multiple Processes with Varying Roles in Infection

Despite producing enormous amounts of cytoplasmic DNA, poxviruses continue to replicate efficiently by deploying an armory of proteins that counter host antiviral responses at multiple levels. Among these, poxvirus protein F17 dysregulates the host kinase mammalian target of rapamycin (mTOR) to prevent the activation of stimulator of interferon genes (STING) expression and impair the production of interferon-stimulated genes (ISGs). However, the host DNA sensor(s) involved and their impact on infection in the absence of F17 remain unknown. Here, we show that cyclic-di-GMP-AMP (cGAMP) synthase (cGAS) is the primary sensor that mediates interferon response factor (IRF) activation and ISG responses to vaccinia virus lacking F17 in both macrophages and lung fibroblasts, although additional sensors also operate in the latter cell type. Despite this, ablation of ISG responses through cGAS or STING knockout did not rescue defects in late-viral-protein production, and the experimental data pointed to other functions of mTOR in this regard. mTOR adjusts both autophagic and protein-synthetic processes to cellular demands. No significant differences in autophagic responses to wild-type or F17 mutant viruses could be detected, with autophagic activity differing across cell types or states and exhibiting no correlations with defects in viral-protein accumulation. In contrast, results using transformed cells or altered growth conditions suggested that late-stage defects in protein accumulation reflect failure of the F17 mutant to deregulate mTOR and stimulate protein production. Finally, rescue approaches suggest that phosphorylation may partition F17's functions as a structural protein and mTOR regulator. Our findings reveal the complex multifunctionality of F17 during infection.IMPORTANCE Poxviruses are large, double-stranded DNA viruses that replicate entirely in the cytoplasm, an unusual act that activates pathogen sensors and innate antiviral responses. In order to replicate, poxviruses therefore encode a wide range of innate immune antagonists that include F17, a protein that dysregulates the kinase mammalian target of rapamycin (mTOR) to suppress interferon-stimulated gene (ISG) responses. However, the host sensor(s) that detects infection in the absence of F17 and its precise contribution to infection remains unknown. Here, we show that the cytosolic DNA sensor cGAS is primarily responsible for activating ISG responses in biologically relevant cell types infected with a poxvirus that does not express F17. However, in line with their expression of ∼100 proteins that act as immune response and ISG antagonists, while F17 helps suppress cGAS-mediated responses, we find that a critical function of its mTOR dysregulation activity is to enhance poxvirus protein production.

The Commensal Microbiota and Viral Infection: A Comprehensive Review

The human body is inhabited by a diverse microbial community that is collectively coined as commensal microbiota. Recent research has greatly advanced our understanding of how the commensal microbiota affects host health. Among the various kinds of pathogenic infections of the host, viral infections constitute one of the most serious public health problems worldwide. During the infection process, viruses may have substantial and intimate interactions with the commensal microbiota. A plethora of evidence suggests that the commensal microbiota regulates and is in turn regulated by invading viruses through diverse mechanisms, thereby having stimulatory or suppressive roles in viral infections. Furthermore, the integrity of the commensal microbiota can be disturbed by invading viruses, causing dysbiosis in the host and further influencing virus infectivity. In the present article, we discuss current insights into the regulation of viral infection by the commensal microbiota. We also draw attention to the disruption of microbiota homeostasis by several viruses.

The E3 ligase TRIM56 is a host restriction factor of Zika virus and depends on its RNA-binding activity but not miRNA regulation, for antiviral function

Infection by Zika virus (ZIKV) is linked to microcephaly and other neurological disorders, posing a significant health threat. Innate immunity is the first line of defense against invading pathogens, but relatively little is understood regarding host intrinsic mechanisms that guard against ZIKV. Here, we show that host tripartite motif-containing protein 56 (TRIM56) poses a barrier to ZIKV infection in cells of neural, epithelial and fibroblast origins. Overexpression of TRIM56, but not an E3 ligase-dead mutant or one lacking a short C-terminal portion, inhibited ZIKV RNA replication. Conversely, depletion of TRIM56 increased viral RNA levels. Although the C-terminal region of TRIM56 bears sequence homology to NHL repeat of TRIM-NHL proteins that regulate miRNA activity, knockout of Dicer, which abolishes production of miRNAs, had no demonstrable effect on ZIKV restriction imposed by TRIM56. Rather, we found that TRIM56 is an RNA-binding protein that associates with ZIKV RNA in infected cells. Moreover, a recombinant TRIM56 fragment comprising the C-terminal 392 residues captured ZIKV RNA in cell-free reactions, indicative of direct interaction. Remarkably, deletion of a short C-terminal tail portion abrogated the TRIM56-ZIKV RNA interaction, concomitant with a loss in antiviral activity. Altogether, our study reveals TRIM56 is an RNA binding protein that acts as a ZIKV restriction factor and provides new insights into the antiviral mechanism by which this E3 ligase tackles flavivirus infections.

Regulation of estrogen signaling and breast cancer proliferation by an ubiquitin ligase TRIM56

Breast cancer ranks no. 1 in women cancer worldwide, while 60–70% are estrogen receptor alpha positive. The estrogen selective modulators, such as tamoxifen, become the effective drugs for controlling ER alpha breast cancer progression. However, tamoxifen resistance will develop during long-time treatment and cancer progression. Thus, further understanding of ER alpha signaling becomes necessary for the improvement of breast cancer therapy. Here, we identify TRIM56 as a novel regulatory factor in ER alpha signaling. TRIM56 expression is positively correlated with ER alpha and PR in breast cancer samples and is related to poor prognosis in endocrine therapy patients. TRIM56 depletion significantly decreases ER alpha signaling activity and ER-alpha-positive breast cancer proliferation in vitro and in vivo. TRIM56 associates with AF1 domain of ER alpha via its WD40 domain in the cytoplasm. TRIM56 prolongs ER alpha protein stability, possibly through targeting ER alpha K63-linked ubiquitination. In conclusion, our study reveals an interesting posttranslational mechanism between TRIM56 and ER alpha in breast cancer progression. Targeting TRIM56 could be a promising approach for ER-alpha-positive breast cancer.