Scutellarin ameliorates ischemia/reperfusion injury‑induced cardiomyocyte apoptosis and cardiac dysfunction via inhibition of the cGAS‑STING pathway

Ischemic heart disease is a common cardiovascular disease. Scutellarin (SCU) exhibits protective effects in ischemic cardiomyocytes; however, to the best of our knowledge, the protective mechanism of SCU remains unclear. The present study was performed to investigate the protective effect of SCU on cardiomyocytes after ischemia/reperfusion (I/R) injury and the underlying mechanism. Mice were intraperitoneally injected with SCU (20 mg/kg) for 7 days before establishing the heart I/R injury model. Cardiac function was detected using small animal echocardiography, apoptotic cells were visualized using TUNEL staining, the myocardial infarct area was assessed by 2,3,5-triphenyltetrazolium chloride staining, and the protein levels of cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), Bcl-2, Bax and cleaved Caspase-3 were detected by western blotting. In in vitro experiments, H9c2 cells were pretreated with SCU, RU.521 (cGAS inhibitor) and H-151 (STING inhibitor), before cell hypoxia/reoxygenation (H/R) injury. The viability of H9c2 cells was detected using a Cell Counting Kit-8 assay, the rate of apoptosis was determined by flow cytometry, and the protein expression levels of cGAS, STING, Bcl-2, Bax and cleaved Caspase-3 were detected by western blotting. It was revealed that SCU ameliorated cardiac dysfunction and apoptosis, and inhibited the activation of the cGAS-STING and Bcl-2/Bax/Caspase-3 signaling pathways in I/R-injured mice. It was also observed that SCU significantly increased cell viability and decreased apoptosis in H/R-induced H9c2 cells. Furthermore, H/R increased the expression levels of cGAS, STING and cleaved Caspase-3, and decreased the ratio of Bcl-2/Bax, which could be reversed by treatment with SCU, RU.521 and H-151. The present study demonstrated that the cGAS-STING signaling pathway may be involved in the regulation of the activation of the Bcl-2/Bax/Caspase-3 signaling pathway to mediate I/R-induced cardiomyocyte apoptosis and cardiac dysfunction, which could be ameliorated by SCU treatment.

Discovery and characterization of a novel cGAS covalent inhibitor for the treatment of inflammatory bowel disease

Cyclic GMP-AMP synthase (cGAS), a cytosolic DNA sensor, acts as a nucleotidyl transferase that catalyzes ATP and GTP to form cyclic GMP-AMP (cGAMP) and plays a critical role in innate immunity. Hyperactivation of cGAS-STING signaling contributes to hyperinflammatory responses. Therefore, cGAS is considered a promising target for the treatment of inflammatory diseases. Herein, we report the discovery and identification of several novel types of cGAS inhibitors by pyrophosphatase (PPiase)-coupled activity assays. Among these inhibitors, 1-(1-phenyl-3,4-dihydro-1H-pyrrolo[1,2-a]pyrazin-2-yl)prop-2-yn-1-one (compound 3) displayed the highest potency and selectivity at the cellular level. Compound 3 exhibited better inhibitory activity and pathway selectivity than RU.521, which is a selective cGAS inhibitor with anti-inflammatory effects in vitro and in vivo. Thermostability analysis, nuclear magnetic resonance and isothermal titration calorimetry assays confirmed that compound 3 directly binds to the cGAS protein. Mass spectrometry and mutation analysis revealed that compound 3 covalently binds to Cys419 of cGAS. Notably, compound 3 demonstrated promising therapeutic efficacy in a dextran sulfate sodium (DSS)-induced mouse colitis model. These results collectively suggest that compound 3 will be useful for understanding the biological function of cGAS and has the potential to be further developed for inflammatory disease therapies.

Pharmacological boosting of cGAS activation sensitizes chemotherapy by enhancing antitumor immunity

Enhancing chemosensitivity is one of the largest unmet medical needs in cancer therapy. Cyclic GMP-AMP synthase (cGAS) connects genome instability caused by platinum-based chemotherapeutics to type I interferon (IFN) response. Here, by using a high-throughput small-molecule microarray-based screening of cGAS interacting compounds, we identify brivanib, known as a dual inhibitor of vascular endothelial growth factor receptor and fibroblast growth factor receptor, as a cGAS modulator. Brivanib markedly enhances cGAS-mediated type I IFN response in tumor cells treated with platinum. Mechanistically, brivanib directly targets cGAS and enhances its DNA binding affinity. Importantly, brivanib synergizes with cisplatin in tumor control by boosting CD8⁺ T cell response in a tumor-intrinsic cGAS-dependent manner, which is further validated by a patient-derived tumor-like cell clusters model. Taken together, our findings identify cGAS as an unprecedented target of brivanib and provide a rationale for the combination of brivanib with platinum-based chemotherapeutics in cancer treatment.

Duck cGAS inhibits DNA and RNA virus replication by activating IFNs and antiviral ISGs

Cyclic GMP-AMP Synthase (cGAS) is a pivotal adaptor of the signaling pathways involving the pattern recognition receptors and plays an important role in apoptosis and immune regulation. The cGAS function in mammals has been investigated extensively; however, the function of duck cGAS (du-cGAS) in response to viral infections is still unclear. This study aimed to clone the mallard (Anas platyrhynchos) cGAS homolog to investigate the function of duck cGAS (du-cGAS) in host antiviral innate immunity. The results showed that the open reading frame (ORF) region of the du-cGAS gene was 1296 bp, encoding 432 amino acids (aa) and exhibiting similar functional domains with its chicken counterpart. Knockdown of the endogenous du-cGAS by specific sgRNA strongly increased the replication of DNA viruses, including duck adenovirus B2 (DAdV B2) and duck short beak and dwarfism syndrome virus (SBDSV). However, the knockout did not impair the replication of novel duck reovirus (NDRV), an RNA virus. Furthermore, the mRNA expressions of type I interferon (IFNs) and vital interferon-stimulated genes (ISGs) were remarkably reduced in the du-cGAS knockout DEF cell line. Inversely, du-cGAS overexpression greatly activated the transcription of IFN-α, IFN-β, and vital ISGs, and impaired the replication of DAdV B2, SBDSV, and NDRV in the DEF cell line. Importantly, we found that a deletion of 68 aa in the N terminus didn't impair the antiviral function of du-cGAS. Overexpressing NTase Core, C-Domain (Mab21), or Zinc-Ribbon domain independently had no antiviral effects. Generally, these results reveal that du-cGAS is a vital component of the innate immune system of ducks, with a universal antiviral activity, and provides a useful strategy for the control of waterfowl viral diseases.

Compressive force regulates orthodontic tooth movement via activating the NLRP3 inflammasome

Mechanical stress regulates various cellular functions like cell inflammation, immune responses, proliferation, and differentiation to maintain tissue homeostasis. However, the impact of mechanical signals on macrophages and the underlying mechanisms by which mechanical force regulates bone remodeling during orthodontic tooth movement remain unclear. NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome has been reported to promote osteoclastic differentiation to regulate alveolar bone resorption. But the relationship between the compressive force and NLRP3 inflammasome in macrophages remains unknown. In this study, immunohistochemical staining results showed elevated expression of NLRP3 and interleukin-1β, as well as an increased number of macrophages expressing NLRP3, on the compression side of the periodontal tissues, after force application for 7 days. Furthermore, the number of tartrate-resistant acid phosphatase-positive osteoclasts, and the mRNA and protein expression levels of osteoclast-related genes in the periodontal tissue decreased in the Nlrp3^-/- mice compared to the WT mice group after orthodontic movement. In vitro mechanical force activates the NLRP3 inflammasome and inhibits autophagy. Intraperitoneal injection of the autophagy inhibitor 3-methyladenine in Nlrp3^-/- mice promoted orthodontic tooth movement. This result indicates that the absence of NLRP3 inflammasome activation can be partially compensated for by autophagy inhibitors. Mechanistically, force-induced activation of the NLRP3 inflammasome in macrophages via the cGAS/P2X7R axis. In conclusion, compressive force regulates orthodontic tooth movement via activating the NLRP3 inflammasome.

cGAS Regulates the Radioresistance of Human Head and Neck Squamous Cell Carcinoma Cells

Cyclic GMP-AMP synthase (cGAS) plays an important role in biological responses to pathogens. The activation of the cGAS pathway in immune cells is known to induce antitumor effects, but the role of cGAS in cancer cells remains poorly understood. In silico analysis using public databases suggested that high cGAS expression in head and neck squamous cell carcinoma (HNSCC) is indicative of a poor prognosis for HNSCC patients. We therefore investigated the role of cGAS in malignancies and the cellular radiation response of human HNSCC cells (SAS and Ca9-22) in vitro, because radiotherapy is one of the treatments most commonly used for HNSCC. Although cGAS knockdown failed to suppress the proliferation of non-irradiated HNSCC cells, it enhanced the radiosensitivity of HNSCC cells. The administration of the cGAS agonist increased the radioresistance of HNSCC cells. cGAS knockdown increased radiation-induced mitotic catastrophe, apoptosis, or cellular senescence, depending on the cell line, and this cell line-dependent response might be due to different responses of p21 after irradiation. Collectively, our findings indicate that the cGAS pathway regulates the radioresistance of HNSCC cells.

Cytoplasmic PARP1 links the genome instability to the inhibition of antiviral immunity through PARylating cGAS

Virus infection modulates both host immunity and host genomic stability. Poly(ADP-ribose) polymerase 1 (PARP1) is a key nuclear sensor of DNA damage, which maintains genomic integrity, and the successful application of PARP1 inhibitors for clinical anti-cancer therapy has lasted for decades. However, precisely how PARP1 gains access to cytoplasm and regulates antiviral immunity remains unknown. Here, we report that DNA virus induces a reactive nitrogen species (RNS)-dependent DNA damage and activates DNA-dependent protein kinase (DNA-PK). Activated DNA-PK phosphorylates PARP1 on Thr⁵⁹⁴, thus facilitating the cytoplasmic translocation of PARP1 to inhibit the antiviral immunity both in vitro and in vivo. Mechanistically, cytoplasmic PARP1 interacts with and directly PARylates cyclic GMP-AMP synthase (cGAS) on Asp¹⁹¹ to inhibit its DNA-binding ability. Together, our findings uncover an essential role of PARP1 in linking virus-induced genome instability with inhibition of host immunity, which is of relevance to cancer, autoinflammation, and other diseases.

Nuclear cGAS: guard or prisoner?

cGAS, an innate immune sensor of cellular stress, recognizes double-stranded DNA mislocalized in the cytosol upon infection, mitochondrial stress, DNA damage, or malignancy. Early models suggested that cytosolic localization of cGAS prevents autoreactivity to nuclear and mitochondrial self-DNA, but this paradigm has shifted in light of recent findings of cGAS as a predominantly nuclear protein tightly bound to chromatin. This has raised the question how nuclear cGAS is kept inactive while being surrounded by chromatin, and what function nuclear localization of cGAS may serve in the first place? Cryo-EM structures have revealed that cGAS interacts with nucleosomes, the minimal units of chromatin, mainly via histones H2A/H2B, and that these protein-protein interactions block cGAS from DNA binding and thus prevent autoreactivity. Here, we discuss the biological implications of nuclear cGAS and its interaction with chromatin, including various mechanisms for nuclear cGAS inhibition, release of chromatin-bound cGAS, regulation of different cGAS pools in the cell, and chromatin structure/chromatin protein effects on cGAS activation leading to cGAS-induced autoimmunity.

The Cytosolic DNA-Sensing cGAS-STING Pathway in Liver Diseases

Inflammation is regulated by the host and is a protective response activated by the evolutionarily conserved immune system in response to harmful stimuli, such as dead cells or pathogens. cGAS-STING pathway is a vital natural sensor of host immunity that can defend various tissues and organs against pathogenic infection, metabolic syndrome, cellular stress and cancer metastasis. The potential impact of cGAS-STING pathway in hepatic ischemia reperfusion (I/R) injury, alcoholic/non-alcoholic steatohepatitis (ASH), hepatic B virus infection, and other liver diseases has recently attracted widespread attention. In this review, the relationship between cGAS-STING pathway and the pathophysiological mechanisms and progression of liver diseases is summarized. Additionally, we discuss various pharmacological agonists and antagonists of cGAS-STING signaling as novel therapeutics for the treatment of liver diseases. A detailed understanding of mechanisms and biology of this pathway will lay a foundation for the development and clinical application of therapies for related liver diseases.

Inhibition of PARP1 Dampens Pseudorabies Virus Infection through DNA Damage-Induced Antiviral Innate Immunity

Pseudorabies virus (PRV) is the causative pathogen of Aujeszky's disease in pigs. Although vaccination is currently applied to prevent the morbidity of PRV infection, new applications are urgently needed to control this infectious disease. Poly(ADP-ribose) polymerase 1 (PARP1) functions in DNA damage repair. We report here that pharmacological and genetic inhibition of PARP1 significantly influenced PRV replication. Moreover, we demonstrate that inhibition of PARP1 induced DNA damage response and antiviral innate immunity. Mechanistically, PARP1 inhibition-induced DNA damage response resulted in the release of double-stranded DNA (dsDNA) into the cytosol, where dsDNA interacted with cyclic GMP-AMP (cGAMP) synthase (cGAS). cGAS subsequently catalyzed cGAMP production to activate the STING/TBK1/IRF3 innate immune signaling pathway. Furthermore, challenge of mice with PARP1 inhibitor stimulated antiviral innate immunity and protected mice from PRV infection in vivo. Our results demonstrate that PARP1 inhibitors may be used as a new strategy to prevent Aujeszky's disease in pigs. IMPORTANCE Aujeszky's disease is a notifiable infectious disease of pigs and causes economic losses worldwide in the pig industry. The causative pathogen is PRV, which is a member of the subfamily Alphaherpesvirinae of the family Herpesviridae. PRV has a wide range of hosts, such as ruminants, carnivores, and rodents. More seriously, recent reports suggest that PRV can cause human endophthalmitis and encephalitis, which indicates that PRV may be a potential zoonotic pathogen. Although vaccination is currently the major strategy used to control the disease, new applications are also urgently needed for the pig industry and public health. We report here that inhibition of PARP1 induces DNA damage-induced antiviral innate immunity through the cGAS-STING signaling pathway. Therefore, PARP1 is a therapeutic target for PRV infection as well as alphaherpesvirus infection.

Chicken cGAS Senses Fowlpox Virus Infection and Regulates Macrophage Effector Functions

The anti-viral immune response is dependent on the ability of infected cells to sense foreign nucleic acids. In multiple species, the pattern recognition receptor (PRR) cyclic GMP-AMP synthase (cGAS) senses viral DNA as an essential component of the innate response. cGAS initiates a range of signaling outputs that are dependent on generation of the second messenger cGAMP that binds to the adaptor protein stimulator of interferon genes (STING). Here we show that in chicken macrophages, the cGAS/STING pathway is essential not only for the production of type-I interferons in response to intracellular DNA stimulation, but also for regulation of macrophage effector functions including the expression of MHC-II and co-stimulatory molecules. In the context of fowlpox, an avian DNA virus infection, the cGAS/STING pathway was found to be responsible for type-I interferon production and MHC-II transcription. The sensing of fowlpox virus DNA is therefore essential for mounting an anti-viral response in chicken cells and for regulation of a specific set of macrophage effector functions.

[Effect of cyclic GMP-AMP synthase on EMT in breast cancer cells]

OBJECTIVE

To investigate the effects of cyclic GMP-AMP synthase (cGAS) on the proliferation, migration and epithelial to mesenchymal transition (EMT) of breast cancer cells.

METHODS

The cGAS lentiviral vector and control fluorescence vector were transfected into breast cancer MCF7 cells and were divided into negative group (NC) and MCF7-cGAS group. The effect of cGAS on proliferation in the MCF7 cells was detected by MTT. The effect of cGAS on cell migration was detected by Transwell assay. The expressions of EMT related proteins were analyzed by Western blot.

RESULTS

After over-expressed with cGAS, the proliferation and migration of MCF7 cells were increased (P＜0.05). The expression level of the epithelial markers E-cadherin was decreased, while the expression level of the mesenchymal markers N-cadherin was increased(P＜0.05).

CONCLUSION

The over-expression of cGAS increased the proliferation and migration of breast cancer cells and induced EMT in breast cancer cells.

Molecular and Structural Basis of DNA Sensors in Antiviral Innate Immunity

DNA viruses are a source of great morbidity and mortality throughout the world by causing many diseases; thus, we need substantial knowledge regarding viral pathogenesis and the host’s antiviral immune responses to devise better preventive and therapeutic strategies. The innate immune system utilizes numerous germ-line encoded receptors called pattern-recognition receptors (PRRs) to detect various pathogen-associated molecular patterns (PAMPs) such as viral nucleic acids, ultimately resulting in antiviral immune responses in the form of proinflammatory cytokines and type I interferons. The immune-stimulatory role of DNA is known for a long time; however, DNA sensing ability of the innate immune system was unraveled only recently. At present, multiple DNA sensors have been proposed, and most of them use STING as a key adaptor protein to exert antiviral immune responses. In this review, we aim to provide molecular and structural underpinnings on endosomal DNA sensor Toll-like receptor 9 (TLR9) and multiple cytosolic DNA sensors including cyclic GMP-AMP synthase (cGAS), interferon-gamma inducible 16 (IFI16), absent in melanoma 2 (AIM2), and DNA-dependent activator of IRFs (DAI) to provide new insights on their signaling mechanisms and physiological relevance. We have also addressed less well-understood DNA sensors such as DEAD-box helicase DDX41, RNA polymerase III (RNA pol III), DNA-dependent protein kinase (DNA-PK), and meiotic recombination 11 homolog A (MRE11). By comprehensive understanding of molecular and structural aspects of DNA-sensing antiviral innate immune signaling pathways, potential new targets for viral and autoimmune diseases can be identified.

Cutibacterium acnes Infection Induces Type I Interferon Synthesis Through the cGAS-STING Pathway

Cutibacterium (previously Propionibacterium) acnes is an anaerobic, Gram-positive commensal of the human body. The bacterium has been associated with a variety of diseases, including acne vulgaris, prosthetic joint infections, prostate cancer, and sarcoidosis. The accumulation of C. acnes in diseases such as acne and prostate cancer has been shown to correlate with enhanced inflammation. While the C. acnes-induced proinflammatory axis, via NF-κB and MAPK signaling and inflammasome activation, has been investigated over the last few decades, the potential role of C. acnes in triggering the type I interferon (IFN-I) pathway has not been addressed. Our results show that C. acnes induces the IFN-I signaling axis in human macrophages by triggering the cGAS-STING pathway. In addition, IFN-I signaling induced by C. acnes strongly depends on the adapter protein TRIF in a non-canonical manner; these signaling events occurred in the absence of any detectable intracellular replication of the bacterium. Collectively, our results provide important insight into C. acnes-induced intracellular signaling cascades in human macrophages and suggest IFN-I as a factor in the etiology of C. acnes-induced diseases. This knowledge may be valuable for developing novel therapies targeting C. acnes in diseases where the accumulation of the bacterium leads to an inflammatory pathology.

MYSM1 Represses Innate Immunity and Autoimmunity through Suppressing the cGAS-STING Pathway

The immune system is not only required for preventing threats exerted by pathogens but also essential for developing immune tolerance to avoid tissue damage. This study identifies a distinct mechanism by which MYSM1 suppresses innate immunity and autoimmunity. The expression of MYSM1 is induced upon DNA virus infection and by intracellular DNA stimulation. MYSM1 subsequently interacts with STING and cleaves STING K63-linked ubiquitination to suppress cGAS-STING signaling. Notably, Mysm1-deficient mice exhibit a hyper-inflammatory response, acute tissue damage, and high mortality upon virus infection. Moreover, in the PBMCs of patients with systemic lupus erythematosus (SLE), MYSM1 production decreases, while type I interferons and pro-inflammatory cytokine expressions increase. Importantly, MYSM1 treatment represses the production of IFNs and pro-inflammatory cytokines in the PBMCs of SLE patients. Thus, MYSM1 is a critical repressor of innate immunity and autoimmunity and is thus a potential therapeutic agent for infectious, inflammatory, and autoimmune diseases.

cGAS-STING pathway in oncogenesis and cancer therapeutics

The host innate immunity offers the first line of defense against infection. However, recent evidence shows that the host innate immunity is also critical in sensing the presence of cytoplasmic DNA derived from genomic instability events, such as DNA damage and defective cell cycle progression. This is achieved through the cyclic GMP-AMP synthase (cGAS)/Stimulator of interferon (IFN) genes (STING) pathway. Here we discuss recent insights into the regulation of this pathway in cancer immunosurveillance, and the downstream signaling cascades that coordinate immune cell recruitment to the tumor microenvironment to destroy transformed cells through cellular senescence or cell death programs. Its central role in immunosurveillance positions the cGAS-STING pathway as an attractive anti-cancer immunotherapeutic drug target for chemical agonists or vaccine adjuvants and suggests a key node to be targeted in a synthetic lethal approach. We also discuss adaptive mechanisms used by cancer cells to circumvent cGAS-STING signaling and present evidence linking chronic cGAS-STING activation to inflammation-induced carcinogenesis, cautioning against the use of activating the cGAS-STING pathway as an anti-tumor immunotherapy. A deeper mechanistic understanding of the cGAS-STING pathway will aid in the identification of potentially efficacious anti-cancer therapeutic targets.

Golgi Apparatus: An Emerging Platform for Innate Immunity

The Golgi apparatus serves as a receiving station where proteins from the endoplasmic reticulum (ER) are further processed before being sent to other cellular compartments. In addition to its well-appreciated roles in vesicular trafficking and protein/lipid secretion, recent studies have demonstrated that the Golgi acts as a signaling platform to facilitate multiple innate immune pathways. Moreover, the membranous networks that connect the Golgi with the ER, mitochondria, endosomes, and autophagosomes provide convenient access to innate immune signal transduction and subsequent effector responses. Here, we review the emerging knowledge about the roles of the Golgi in the initiation and activation of innate immune signaling. Moreover, microbial hijacking strategies that inhibit Golgi-associated innate immune responses will also be discussed.

Lipopolysaccharide enhances DNA-induced IFN-β expression and autophagy by upregulating cGAS expression in A549 cells

Cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) is a newly identified cytosolic DNA sensor, but its function in lung epithelial cells is relatively unknown. In the present study, the effects of lipopolysaccharide (LPS) on the expression and function of cGAS in the A549 lung epithelial cell line was investigated. The cells were treated with LPS at different concentrations (e.g., 100, 200, 400 and 800 ng/ml), and the cGAS expression levels were examined via western blot analysis and reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The cells were pretreated with LPS, followed by E. coli DNA transfection using Lipofectamine^® 3000. After 24 h, interferon (IFN)-β production was measured using ELISA and the expression of the autophagic markers, microtubule-associated proteins 1A/1B light chain 3 and sequestosome-1, were determined using western blot analysis. The cells were also pretreated with either a toll-like receptor (TLR) 4 inhibitor, a serine/threonine-protein kinase TBK1 (TBK1) inhibitor or an nuclear factor (NF)-κB inhibitor, followed by LPS treatment, and the cGAS expression levels were examined via western blot analysis and RT-qPCR. The result showed that LPS treatment upregulated cGAS expression in a dose-dependent manner. E. coli DNA treatment could induce IFN-β production and autophagy via cGAS, which was enhanced by LPS pretreatment. The effect of LPS on cGAS expression was suppressed by treatment with a TLR4 inhibitor, a TBK1 inhibitor and an NF-κB inhibitor. In conclusion, LPS enhances DNA-induced IFN-β production and autophagy by upregulating cGAS expression through the myeloid differentiation primary response protein MyD88-independent TLR4 signaling pathway in A549 cells.

Cellular sensing by phase separation: Using the process, not just the products

Phase separation creates two distinct liquid phases from a single mixed liquid phase, like oil droplets separating from water. Considerable attention has focused on how the products of phase separation-the resulting condensates-might act as biological compartments, bioreactors, filters, and membraneless organelles in cells. Here, we expand this perspective, reviewing recent results showing how cells instead use the process of phase separation to sense intracellular and extracellular changes. We review case studies in phase separation-based sensing and discuss key features, such as extraordinary sensitivity, which make the process of phase separation ideally suited to meet a range of sensory challenges cells encounter.

Induction of neutrophil extracellular traps during tissue injury: Involvement of STING and Toll-like receptor 9 pathways

Objectives

Neutrophils are thought to release neutrophil extracellular traps (NETs) to form in response to exogenous bacteria, viruses and other pathogens. However, the mechanisms underlying NET formation during sterile inflammation are still unclear. In this study, we would like to identify neutrophil extracellular traps formation during sterile inflammation and tissue injury and associated pathways and its mechanism.

Materials and methods

We identified different injuries such as chemical‐induced and trauma‐induced formation of NETs and investigated mechanism of the formation of NETs in vitro and in vivo during the treatment of mtDNA.

Results

Here, we find the release of mitochondrial DNA (mtDNA) and oxidized mtDNA in acute peripheral tissue trauma models or other chemically induced lung injury, and moreover, endogenous mtDNA and oxidized mtDNA induce the formation of NETs and sterile inflammation. Oxidized mtDNA is a more potent inducer of NETs. Mitochondrial DNA activates neutrophils via cyclic GMP‐AMP synthase (cGAS)‐STING and the Toll‐like receptor 9 (TLR9) pathways and increases the production of neutrophil elastase and extracellular neutrophil‐derived DNA in NETs. Mitochondrial DNA also increases the production of reactive oxygen species (ROS) and expression of the NET‐associated proteins Rac 2 and peptidylarginine deiminase 4 (PAD4).

Conclusions

Altogether, these findings highlight that endogenous mitochondrial DNA inducted NETs formation and subsequent sterile inflammation and the mechanism associated with NET formation.

Cytosolic DNA-sensing immune response and viral infection

How host cells recognize many kinds of RNA and DNA viruses and initiate innate antiviral responses against them has not yet been fully elucidated. Over the past decade, investigations into the mechanisms underlying these antiviral responses have focused extensively on immune surveillance sensors that recognize virus‐derived components (such as lipids, sugars and nucleic acids). The findings of these studies have suggested that antiviral responses are mediated by cytosolic or intracellular compartment sensors and their adaptor molecules (e.g., TLR, myeloid differentiation primary response 88, retinoic acid inducible gene‐I, IFN‐β promoter stimulator‐1, cyclic GMP‐AMP synthase and stimulator of IFN genes axis) for the primary sensing of virus‐derived nucleic acids, leading to production of type I IFNs, pro‐inflammatory cytokines and chemokines by the host cells. Thus, host cells have evolved an elaborate host defense machinery to recognize and eliminate virus infections. In turn, to achieve sustained viral infection and induce pathogenesis, viruses have also evolved several counteracting strategies for achieving immune escape by targeting immune sensors, adaptor molecules, intracellular kinases and transcription factors. In this review, we discuss recent discoveries concerning the role of the cytosolic nucleic acid‐sensing immune response in viral recognition and control of viral infection. In addition, we consider the regulatory machinery of the cytosolic nucleic acid‐sensing immune response because these immune surveillance systems must be tightly regulated to prevent aberrant immune responses to self and non‐self‐nucleic acids.

STING-associated lung disease in mice relies on T cells but not type I interferon

BACKGROUND

Monogenic interferonopathies are thought to be mediated by type I interferon. For example, a gain-of-function mutation in stimulator of interferon genes (STING; N153S) upregulates type I interferon-stimulated genes and causes perivascular inflammatory lung disease in mice. The equivalent mutation in human subjects also causes lung disease, which is thought to require signaling through the cyclic GMP-AMP synthase (cGAS)-STING pathway and subsequent activation of interferon regulatory factors (IRFs) 3 and 7, type I interferon, and interferon-stimulated genes.

OBJECTIVE

We set out to define the roles of cGAS, IRF3, IRF7, the type I interferon receptor (IFN-α and IFN-β receptor subunit 1 [IFNAR1]), T cells, and B cells in spontaneous lung disease in STING N153S mice.

METHODS

STING N153S mice were crossed to animals lacking cGAS, IRF3/IRF7, IFNAR1, adaptive immunity, αβ T cells, and mature B cells. Mice were evaluated for spontaneous lung disease. Additionally, bone marrow chimeric mice were assessed for lung disease severity and survival.

RESULTS

Lung disease in STING N153S mice developed independently of cGAS, IRF3/IRF7, and IFNAR1. Bone marrow transplantation revealed that certain features of STING N153S-associated disease are intrinsic to the hematopoietic compartment. Recombination-activating gene 1 (Rag1)^-/- STING N153S mice that lack adaptive immunity had no lung disease, and T-cell receptor β chain (Tcrb)^-/- STING N153S animals only had mild disease. STING N153S led to a reduction in percentages and numbers of naive and regulatory T cells, as well as an increased frequency of cytokine-producing effector T cells.

CONCLUSION

Spontaneous lung disease in STING N153S mice develops independently of type I interferon signaling and cGAS. STING N153S relies primarily on T cells to promote lung disease in mice.

Human microglia and astrocytes express cGAS-STING viral sensing components

While microglia and astrocytes are known to produce key inflammatory and anti-viral mediators following infection with replicative DNA viruses, the mechanisms by which these cell types perceive such threats are poorly understood. Recently, cyclic GMP-AMP synthase (cGAS) has been identified as an important cytosolic sensor for DNA viruses and retroviruses in peripheral leukocytes. Here we confirm the ability of human microglial and astrocytic cell lines and primary human glia to respond to foreign intracellular double stranded DNA. Importantly, we provide the first demonstration that human microglia and astrocytes show robust levels of cGAS protein expression at rest and following activation. Furthermore, we show these cell types also constitutively express the critical downstream cGAS adaptor protein, stimulator of interferon genes (STING). The present finding that human glia express the principle components of the cGAS-STING pathway provides a foundation for future studies to investigate the relative importance of these molecules in clinically relevant viral CNS infections.

[Effects of cytosolic bacteria on cyclic GMP-AMP synthase expression in human gingival tissues and periodontal ligament cells]

OBJECTIVE

This work aims to determine the effect of cytosolic bacteria on the expression of cyclic GMP-AMP synthase (cGAS) in human periodontal ligament cells (hPDLCs) and gingival tissues.

METHODS

The ability of Porphyromonas gingivalis (P. gingivalis) to invade hPDLCs was detected using laser scanning confocal microscope assay at a multiplicity of infection of 10. P. gingivalis-infected cells were sorted by fluorescence-activated cell sorting (FACS). Then, quantitative real time reverse transcription polymerase chain reaction (qRT-PCR) and Western blot were used to detect cGAS expression in infected cells. Finally, the location and expression of cGAS in inflammatory and normal gingival tissues were investigated by immunohistochemistry.

RESULTS

P. gingivalis actively invaded hPDLCs. Moreover, cGAS expression significantly increased in P. gingivalis-infected cells. Although cGAS was expressed in the epithelial and subepithelial cells of both inflamed and normal gingival tissues, cGAS expression significantly increased in inflamed gingival tissues.

CONCLUSIONS

Cytosolic bacteria can upregulate cGAS expression in infected cells. These data suggest that cGAS may act as pattern-recognition receptors and participate in recognizing cytosolic nucleic acid pathogen-associated molecular patterns.
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The Role of Interferons in Inflammation and Inflammasome Activation

Inflammation is an essential physiological process, which enables survival during infection and maintains tissue homeostasis. Interferons (IFNs) and pro- and anti-inflammatory cytokines are crucial for appropriate response to pathogens, damaged cells, or irritants in inflammatory response. The inflammasom is multiprotein complex, which initiates cleavage of pro-inflammatory cytokines IL-1β and IL-18 into active forms. In addition, inflammasomes initiate pyroptotic cell death. In the present review, I summarize and analyze recent findings regarding the cross talk of IFNs and inflammasomes.

Cytoplasmic isoforms of Kaposi sarcoma herpesvirus LANA recruit and antagonize the innate immune DNA sensor cGAS

The latency-associated nuclear antigen (LANA) of Kaposi sarcoma herpesvirus (KSHV) is mainly localized and functions in the nucleus of latently infected cells, playing a pivotal role in the replication and maintenance of latent viral episomal DNA. In addition, N-terminally truncated cytoplasmic isoforms of LANA, resulting from internal translation initiation, have been reported, but their function is unknown. Using coimmunoprecipitation and MS, we found the cGMP-AMP synthase (cGAS), an innate immune DNA sensor, to be a cellular interaction partner of cytoplasmic LANA isoforms. By directly binding to cGAS, LANA, and particularly, a cytoplasmic isoform, inhibit the cGAS-STING-dependent phosphorylation of TBK1 and IRF3 and thereby antagonize the cGAS-mediated restriction of KSHV lytic replication. We hypothesize that cytoplasmic forms of LANA, whose expression increases during lytic replication, inhibit cGAS to promote the reactivation of the KSHV from latency. This observation points to a novel function of the cytoplasmic isoforms of LANA during lytic replication and extends the function of LANA from its role during latency to the lytic replication cycle.

The interferon response to intracellular DNA: why so many receptors?

The detection of intracellular DNA has emerged to be a key event in the innate immune response to viruses and intracellular bacteria, and during conditions of sterile inflammation and autoimmunity. One of the consequences of the detection of DNA as a 'stranger' and a 'danger' signal is the production of type I interferons and pro-inflammatory cytokines. Much work has been dedicated to the elucidation of the signalling cascades that activate this DNA-induced gene expression programme. However, while many proteins have been proposed to act as sensors for intracellular DNA in recent years, none has been met with universal acceptance, and a theory linking all the recent observations is, as yet, lacking. This review presents the evidence for the various interferon-inducing DNA receptors proposed to date, and examines the hypotheses that might explain why so many different receptors appear to be involved in the innate immune recognition of intracellular DNA.