Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation

Pharmacological inhibition of TBK1/IKKε blunts immunopathology in a murine model of SARS-CoV-2 infection

TANK-binding kinase 1 (TBK1) is a key signalling component in the production of type-I interferons, which have essential antiviral activities, including against SARS-CoV-2. TBK1, and its homologue IκB kinase-ε (IKKε), can also induce pro-inflammatory responses that contribute to pathogen clearance. While initially protective, sustained engagement of type-I interferons is associated with damaging hyper-inflammation found in severe COVID-19 patients. The contribution of TBK1/IKKε signalling to these responses is unknown. Here we find that the small molecule idronoxil inhibits TBK1/IKKε signalling through destabilisation of TBK1/IKKε protein complexes. Treatment with idronoxil, or the small molecule inhibitor MRT67307, suppresses TBK1/IKKε signalling and attenuates cellular and molecular lung inflammation in SARS-CoV-2-challenged mice. Our findings additionally demonstrate that engagement of STING is not the major driver of these inflammatory responses and establish a critical role for TBK1/IKKε signalling in SARS-CoV-2 hyper-inflammation.

SARS-CoV-2 Nsp15 suppresses type I interferon production by inhibiting IRF3 phosphorylation and nuclear translocation

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes 2019 coronavirus disease (COVID-19), poses a significant threat to global public health security. Like other coronaviruses, SARS-CoV-2 has developed various strategies to inhibit the production of interferon (IFN). Here, we have discovered that SARS-CoV-2 Nsp15 obviously reduces the expression of IFN-β and IFN-stimulated genes (ISG56, CXCL10), and also inhibits IRF3 phosphorylation and nuclear translocation by antagonizing the RLR-mediated antiviral signaling pathway. Mechanically, we found that the poly-U-specific endonuclease domain (EndoU) of Nsp15 directly associates with the kinase domain (KD) of TBK1 to interfere TBK1 interacting with IRF3 and the flowing TBK1-mediated IRF3 phosphorylation. Furthermore, Nsp15 also prevented nuclear translocation of phosphorylated IRF3 via binding to the nuclear import adaptor karyopherin α1 (KPNA1) and promoting it autophagy-dependent degradation. These findings collectively reveal a novel mechanism by which Nsp15 antagonizes host's innate immune response.

cGAS-STING signaling pathway in intestinal homeostasis and diseases

The intestinal mucosa is constantly exposed to commensal microbes, opportunistic pathogens, toxins, luminal components and other environmental stimuli. The intestinal mucosa consists of multiple differentiated cellular and extracellular components that form a critical barrier, but is also equipped for efficient absorption of nutrients. Combination of genetic susceptibility and environmental factors are known as critical components involved in the pathogenesis of intestinal diseases. The innate immune system plays a critical role in the recognition and elimination of potential threats by detecting pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). This host defense is facilitated by pattern recognition receptors (PRRs), in which the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has gained attention due to its role in sensing host and foreign double-stranded DNA (dsDNA) as well as cyclic dinucleotides (CDNs) produced by bacteria. Upon binding with dsDNA, cGAS converts ATP and GTP to cyclic GMP-AMP (cGAMP), which binds to STING and activates TANK binding kinase 1 (TBK1) and interferon regulatory factor 3 (IRF3), inducing type I interferon (IFN) and nuclear factor kappa B (NF-κB)-mediated pro-inflammatory cytokines, which have diverse effects on innate and adaptive immune cells and intestinal epithelial cells (IECs). However, opposite perspectives exist regarding the role of the cGAS-STING pathway in different intestinal diseases. Activation of cGAS-STING signaling is associated with worse clinical outcomes in inflammation-associated diseases, while it also plays a critical role in protection against tumorigenesis and certain infections. Therefore, understanding the context-dependent mechanisms of the cGAS-STING pathway in the physiopathology of the intestinal mucosa is crucial for developing therapeutic strategies targeting the cGAS-STING pathway. This review aims to provide insight into recent findings of the protective and detrimental roles of the cGAS-STING pathway in intestinal diseases.

Endothelial type I interferon response and brain diseases: identifying STING as a therapeutic target

The endothelium layer lining the inner surface of blood vessels serves relevant physiological functions in all body systems, including the exchanges between blood and extravascular space. However, endothelial cells also participate in innate and adaptive immune response that contribute to the pathophysiology of inflammatory disorders. Type I Interferon (IFN) signaling is an inflammatory response triggered by a variety of pathogens, but it can also be induced by misplaced DNA in the cytosol caused by cell stress or gene mutations. Type I IFN produced by blood leukocytes or by the endothelium itself is well-known to activate the interferon receptor (IFNAR) in endothelial cells. Here, we discuss the induction of type I IFN secretion and signaling in the endothelium, specifically in the brain microvasculature where endothelial cells participate in the tight blood-brain barrier (BBB). This barrier is targeted during neuroinflammatory disorders such as infection, multiple sclerosis, Alzheimer's disease and traumatic brain injury. We focus on type I IFN induction through the cGAS-STING activation pathway in endothelial cells in context of autoinflammatory type I interferonopathies, inflammation and infection. By comparing the pathophysiology of two separate infectious diseases-cerebral malaria induced by Plasmodium infection and COVID-19 caused by SARS-CoV-2 infection-we emphasize the relevance of type I IFN and STING-induced vasculopathy in organ dysfunction. Investigating the role of endothelial cells as active type I IFN producers and responders in disease pathogenesis could lead to new therapeutic targets. Namely, endothelial dysfunction and brain inflammation may be avoided with strategies that target excessive STING activation in endothelial cells.

AhR diminishes the efficacy of chemotherapy via suppressing STING dependent type-I interferon in bladder cancer

The induction of type-I interferons (IFN-Is) is important for the efficacy of chemotherapy. By investigating the role of amino acids in regulation of IFN-I production under chemo-drug treatment in bladder cancer (BC) cells, we find an inherent AhR-dependent negative feedback to restrain STING signaling and IFN-I production. Mechanistically, in a ligand dependent manner, AhR bridges STING and CUL4B/RBX1 E3 ligase complex, facilitating STING degradation through ubiquitin-proteasome pathway. Inhibition of AhR increases STING levels and reduces tumor growth under cisplatin or STING agonist treatment. Endogenous AhR ligands are mainly consisted of tryptophan (Trp) metabolites; dietary Trp restriction, blocking the key Trp metabolism rate-limiting enzyme IDO1 or inhibition of cellular Trp importation also show similar effect as AhR inhibition. Clinically, BC patients with higher intratumoral expression of AhR or stronger intratumoral Trp metabolism (higher IDO1 or Kyn levels) that lead to higher AhR activation show worse response rate to neoadjuvant chemotherapy (NAC).

Role of the cGAS-STING pathway in radiotherapy for non-small cell lung cancer

One of the most important therapeutic interventions for non-small cell lung cancer is radiotherapy. Ionizing radiation (IR) is classified by traditional radiobiology principles as a direct cytocidal therapeutic agent against cancer, although there is growing recognition of other antitumor immunological responses induced by this modality. The most effective therapeutic combinations to harness radiation-generated antitumor immunity and enhance treatment results for malignancies resistant to existing radiotherapy regimens could be determined by a more sophisticated understanding of the immunological pathways created by radiation. Innate immune signaling is triggered by the activation of cGAS-STING, and this promotes adaptive immune responses to help fight cancer. This identifies a molecular mechanism radiation can use to trigger antitumor immune responses by bridging the DNA-damaging ability of IR with the activation of CD8 + cytotoxic T cell-mediated killing of tumors. We also discuss radiotherapy-related parameters that affect cGAS-STING signaling, negative consequences of cGAS-STING activation, and intriguing treatment options being tested in conjunction with IR to support immune activation by activating STING-signaling. Improved therapeutic outcomes will result from a better understanding of how IR promotes cGAS-STING signaling in immune-based treatment regimens that maximize radiotherapy's anticancer effectiveness.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

Protein kinase TBK1/IKKε inhibitor Amlexanox improves cardiac function after acute myocardial infarction in rats

BACKGROUND

The aim of this study was to study the effect of protein kinase TBK1 and IKKε inhibitor Amlexanox on cardiac function after acute myocardial infarction (AMI) in rats.

METHODS

AMI model was established in rats. Experimental grouping: sham + dimethyl sulfoxide (DMSO) group, sham + Amlexanox group, AMI + DMSO group, AMI + Amlexanox group. 12 h after surgery, rats in the sham + Amlexanox group and AMI + Amlexanox group were given an intraperitoneal injection of Amlexanox at a dose of 25 mg/kg once a day for 7 consecutive days. The sham + DMSO group and the AMI + DMSO group were given the same amount of DMSO as a control. Ultrasound was used to detect changes in cardiac function in rats for 3 and 7 days after continuous administration, and real-time polymerase chain reaction was used to detect the transcription levels of ISGs and apoptosis in myocardial tissue. Hematoxylin-eosin and immunohistochemical staining were used to observe the inflammatory cell infiltration level and MOMA2 expression in myocardial tissue. Western blot was used to examine the TBK1 signaling pathway and its downstream protein expression.

RESULTS

Amlexanox can improve left ventricular ejection fraction (LVEF) and short-axis shortening rate (FS) after AMI in rats, reduce remodeling of cardiomyopathy, and reduce inflammatory cell infiltration, thus reducing myocardial apoptosis.

CONCLUSIONS

The protein kinases TBK1 and IKKε inhibitor Amlexanox can improve cardiac function in rats after AMI, reduce myocardial inflammatory response, reduce myocardial apoptosis, and then exert myocardial protection in vivo.

The Oxysterol Receptor EBI2 Links Innate and Adaptive Immunity to Limit IFN Response and Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with abnormal activation of the immune system. Recent attention is increasing about how aberrant lipid and cholesterol metabolism is linked together with type I interferon (IFN-I) signaling in the regulation of the pathogenesis of SLE. Here, a metabonomic analysis is performed and increased plasma concentrations of oxysterols, especially 7α, 25-dihydroxycholesterol (7α, 25-OHC), are identified in SLE patients. The authors find that 7α, 25-OHC binding to its receptor Epstein-Barr virus-induced gene 2 (EBI2) in macrophages can suppress STAT activation and the production of IFN-β, chemokines, and cytokines. Importantly, monocytes/macrophages from SLE patients and mice show significantly reduced EBI2 expression, which can be triggered by IFN-γ produced in activated T cells. Previous findings suggest that EBI2 enhances immune cell migration. Opposite to this effect, the authors demonstrate that EBI2-deficient macrophages produce higher levels of chemokines and cytokines, which recruits and activates myeloid cells,T and B lymphocytes to exacerbate tetramethylpentadecane-induced SLE. Together, via sensing the oxysterol 7α, 25-OHC, EBI2 in macrophages can modulate innate and adaptive immune responses, which may be used as a potential diagnostic marker and therapeutic target for SLE.

Antagonism between viral infection and innate immunity at the single-cell level

When infected with a virus, cells may secrete interferons (IFNs) that prompt nearby cells to prepare for upcoming infection. Reciprocally, viral proteins often interfere with IFN synthesis and IFN-induced signaling. We modeled the crosstalk between the propagating virus and the innate immune response using an agent-based stochastic approach. By analyzing immunofluorescence microscopy images we observed that the mutual antagonism between the respiratory syncytial virus (RSV) and infected A549 cells leads to dichotomous responses at the single-cell level and complex spatial patterns of cell signaling states. Our analysis indicates that RSV blocks innate responses at three levels: by inhibition of IRF3 activation, inhibition of IFN synthesis, and inhibition of STAT1/2 activation. In turn, proteins coded by IFN-stimulated (STAT1/2-activated) genes inhibit the synthesis of viral RNA and viral proteins. The striking consequence of these inhibitions is a lack of coincidence of viral proteins and IFN expression within single cells. The model enables investigation of the impact of immunostimulatory defective viral particles and signaling network perturbations that could potentially facilitate containment or clearance of the viral infection.

MEF2A suppresses stress responses that trigger DDX41-dependent IFN production

Cellular stress in the form of disrupted transcription, loss of organelle integrity, or damage to nucleic acids can elicit inflammatory responses by activating signaling cascades canonically tasked with controlling pathogen infections. These stressors must be kept in check to prevent unscheduled activation of interferon, which contributes to autoinflammation. This study examines the role of the transcription factor myocyte enhancing factor 2A (MEF2A) in setting the threshold of transcriptional stress responses to prevent R-loop accumulation. Increases in R-loops lead to the induction of interferon and inflammatory responses in a DEAD-box helicase 41 (DDX41)-, cyclic GMP-AMP synthase (cGAS)-, and stimulator of interferon genes (STING)-dependent manner. The loss of MEF2A results in the activation of ATM and RAD3-related (ATR) kinase, which is also necessary for the activation of STING. This study identifies the role of MEF2A in sustaining transcriptional homeostasis and highlights the role of ATR in positively regulating R-loop-associated inflammatory responses.

Prognostic significance of STING expression in solid tumor: a systematic review and meta-analysis

Objective

Stimulator of interferon genes (STING) is a key regulator in initiating innate immune response from sensing cytosolic DNA. Recent studies have revealed that the cGAS-STING signaling pathway has a crucial role in tumor development and progression across cancer types. Herein, we conducted a meta-analysis to explore the relationship between the immunoexpression of STING and the survival outcome of patients in various solid tumors. Studies relevant to the subject were searched from PubMed, Embase, and Web of Science.

Results

Eleven studies including 2,345 patients were eligible for the analysis. STING expression in tumor cells was related to improved disease-free survival/recurrence-free survival (DFS/RFS) (HR = 0.656, 95% CI = 0.455-0.946, p = 0.024) but not with overall survival (OS) (HR = 0.779, 95% CI = 0.534-1.136, p = 0.194). STING expression in stromal cells, however, did not show significant correlation with DFS/RFS and OS (HR = 0.979, 95% CI = 0.565-1.697, p-value = 0.940 and HR = 1.295, 95% CI = 0.845-1.985, p = 0.235, respectively). In a subgroup analysis, STING expression in tumor cells was associated with better DFS (HR = 0.622, 95% CI = 0.428-0.903, p = 0.012). In tumor cells, favorable DFS/RFS were also related to studies from univariate analysis and the gastrointestinal system (HR = 0.667, 95% CI = 0.482-0.923, p = 0.015 and HR = 0.566, 95% CI = 0.330-0.971, p = 0.039).

Conclusions

STING expression in tumor cells is associated with favorable outcome in solid tumors.

Systematic review registration

https://www.crd.york.ac.uk/prospero/, registration number: CRD42023427027.

SLC15A4 controls endolysosomal TLR7-9 responses by recruiting the innate immune adaptor TASL

Endolysosomal Toll-like receptors (TLRs) play crucial roles in immune responses to pathogens, while aberrant activation of these pathways is associated with autoimmune diseases, including systemic lupus erythematosus (SLE). The endolysosomal solute carrier family 15 member 4 (SLC15A4) is required for TLR7/8/9-induced responses and disease development in SLE models. SLC15A4 has been proposed to affect TLR7-9 activation through its transport activity, as well as by assembling an IRF5-activating complex with TASL, but the relative contribution of these functions remains unclear. Here, we show that the essential role of SLC15A4 is to recruit TASL to endolysosomes, while its transport activity is dispensable when TASL is tethered to this compartment. Endolysosomal-localized TASL rescues TLR7-9-induced IRF5 activation as well as interferon β and cytokine production in SLC15A4-deficient cells. SLC15A4 acts as signaling scaffold, and this function is essential to control TLR7-9-mediated inflammatory responses. These findings support targeting the SLC15A4-TASL complex as a potential therapeutic strategy for SLE and related diseases.

Castanea crenata honey reduces influenza infection by activating the innate immune response

Influenza is an acute respiratory disorder caused by the influenza virus and is associated with prolonged hospitalization and high mortality rates in older individuals and chronically ill patients. Vaccination is the most effective preventive strategy for ameliorating seasonal influenza. However, the vaccine is not fully effective in cases of antigenic mismatch with the viral strains circulating in the community. The emergence of resistance to antiviral drugs aggravates the situation. Therefore, developing new vaccines and antiviral drugs is essential. Castanea crenata honey (CH) is an extensively cultivated food worldwide and has been used as a nutritional supplement or herbal medicine. However, the potential anti-influenza properties of CH remain unexplored. In this study, the in vitro and in vivo antiviral effects of CH were assessed. CH significantly prevented influenza virus infection in mouse Raw264.7 macrophages. CH pretreatment inhibited the expression of the viral proteins M2, PA, and PB1 and enhanced the secretion of proinflammatory cytokines and type-I interferon (IFN)-related proteins in vitro. CH increased the expression of RIG-1, mitochondrial antiviral signaling (MAVS) protein, and IFN-inducible transmembrane protein, which interferes with virus replication. CH reduced body weight loss by 20.9%, increased survival by 60%, and decreased viral replication and inflammatory response in the lungs of influenza A virus-infected mice. Therefore, CH stimulates an antiviral response in murine macrophages and mice by preventing viral infection through the RIG-1-mediated MAVS pathway. Further investigation is warranted to understand the molecular mechanisms involved in the protective effects of CH on influenza virus infection.

Conventional DNA-Damaging Cancer Therapies and Emerging cGAS-STING Activation: A Review and Perspectives Regarding Immunotherapeutic Potential

Many traditional cancer treatments such as radiation and chemotherapy are known to induce cellular DNA damage as part of their cytotoxic activity. The cGAS-STING signaling axis, a key member of the DNA damage response that acts as a sensor of foreign or aberrant cytosolic DNA, is helping to rationalize the DNA-damaging activity of these treatments and their emerging immunostimulatory capacity. Moreover, cGAS-STING, which is attracting considerable attention for its ability to promote antitumor immune responses, may fundamentally be able to address many of the barriers limiting the success of cancer immunotherapy strategies, including the immunosuppressive tumor microenvironment. Herein, we review the traditional cancer therapies that have been linked with cGAS-STING activation, highlighting their targets with respect to their role and function in the DNA damage response. As part of the review, an emerging "chemoimmunotherapy" concept whereby DNA-damaging agents are used for the indirect activation of STING is discussed as an alternative to the direct molecular agonism strategies that are in development, but have yet to achieve clinical approval. The potential of this approach to address some of the inherent and emerging limitations of cGAS-STING signaling in cancer immunotherapy is also discussed. Ultimately, it is becoming clear that in order to successfully employ the immunotherapeutic potential of the cGAS-STING axis, a balance between its contrasting antitumor and protumor/inflammatory activities will need to be achieved.

WDR77 inhibits prion-like aggregation of MAVS to limit antiviral innate immune response

RIG-I-MAVS signaling pathway plays a crucial role in defending against pathogen infection and maintaining immune balance. Upon detecting viral RNA, RIG-I triggers the formation of prion-like aggregates of the adaptor protein MAVS, which then activates the innate antiviral immune response. However, the mechanisms that regulate the aggregation of MAVS are not yet fully understood. Here, we identified WDR77 as a MAVS-associated protein, which negatively regulates MAVS aggregation. WDR77 binds to MAVS proline-rich region through its WD2-WD3-WD4 domain and inhibits the formation of prion-like filament of recombinant MAVS in vitro. In response to virus infection, WDR77 is recruited to MAVS to prevent the formation of its prion-like aggregates and thus downregulate RIG-I-MAVS signaling in cells. WDR77 deficiency significantly potentiates the induction of antiviral genes upon negative-strand RNA virus infections, and myeloid-specific Wdr77-deficient mice are more resistant to RNA virus infection. Our findings reveal that WDR77 acts as a negative regulator of the RIG-I-MAVS signaling pathway by inhibiting the prion-like aggregation of MAVS to prevent harmful inflammation.

The nucleotide receptor STING translocates to the phagosomes to negatively regulate anti-fungal immunity

STING (stimulator of interferon genes) exerts protective cellular responses to viral infection via induction of interferon production and autophagy. Here, we report the role of STING in modulating the immune responses toward fungal infection. Upon Candida albicans stimulation, STING transited alongside the endoplasmic reticulum (ER) to the phagosomes. In phagosomes, STING directly bound with Src via the N-terminal 18 amino acids of STING, and this binding prevented Src from recruiting and phosphorylating Syk. Consistently, Syk-associated signaling and production of pro-inflammatory cytokines and chemokines were increased in mouse BMDCs (bone-marrow-derived dendritic cells) lacking STING with fungal treatment. STING deficiency improved anti-fungal immunity in systemic C. albicans infection. Importantly, administration of the N-terminal 18-aa (amino acid) peptide of STING improved host outcomes in disseminated fungal infection. Overall, our study identifies a previously unrecognized function of STING in negatively regulating anti-fungal immune responses and offers a potential therapeutic strategy for controlling C. albicans infection.

STING is a cell-intrinsic metabolic checkpoint restricting aerobic glycolysis by targeting HK2

Evasion of antitumour immunity is a hallmark of cancer. STING, a putative innate immune signalling adaptor, has a pivotal role in mounting antitumour immunity by coordinating innate sensing and adaptive immune surveillance in myeloid cells. STING is markedly silenced in various human malignancies and acts as a cell-intrinsic tumour suppressor. How STING exerts intrinsic antitumour activity remains unclear. Here, we report that STING restricts aerobic glycolysis independent of its innate immune function. Mechanistically, STING targets hexokinase II (HK2) to block its hexokinase activity. As such, STING inhibits HK2 to restrict tumour aerobic glycolysis and promote antitumour immunity in vivo. In human colorectal carcinoma samples, lactate, which can be used as a surrogate for aerobic glycolysis, is negatively correlated with STING expression level and antitumour immunity. Taken together, this study reveals that STING functions as a cell-intrinsic metabolic checkpoint that restricts aerobic glycolysis to promote antitumour immunity. These findings have important implications for the development of STING-based therapeutic modalities to improve antitumour immunotherapy.

Large yellow croaker (Larimichthys crocea) mitofusin 2 inhibits type I IFN responses by degrading MAVS via enhanced K48-linked ubiquitination

In mammals, mitofusin 2 (MFN2) is involved in mitochondrial fusion, and suppresses the virus-induced RIG-I-like receptor (RLR) signaling pathway. However, little is known about the function of MFN2 in non-mammalian species. In the present study, we cloned an MFN2 ortholog (LcMFN2) in large yellow croaker (Larimichthys crocea). Phylogenetic analysis showed that MFN2 emerged after the divergence of amphioxus and vertebrates. The protein sequences of MFN2 were well conserved from fish to mammals. LcMFN2 was expressed in all the tissues/organs examined at different levels, and its expression was upregulated in response to poly(I:C) stimulation. Overexpression of LcMFN2 inhibited MAVS-induced type I interferon (IFN) promoter activation and antiviral gene expression. In contrast, knockdown of endogenous LcMFN2 enhanced poly(I:C) induced production of type I IFNs. Additionally, LcMFN2 enhanced K48-linked polyubiquitination of MAVS, promoting its degradation. Also, overexpression of LcMFN2 impaired the cellular antiviral response, as evidenced by the increased expression of viral genes and more severe cytopathic effects (CPE) in cells infected with spring viremia of carp virus (SVCV). These results indicated that LcMFN2 inhibited type I IFN response by degrading MAVS, suggesting its negative regulatory role in cellular antiviral response. Therefore, our study sheds a new light on the regulatory mechanisms of the cellular antiviral response in teleosts.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-023-00189-8.

Herpes simplex virus infected cell protein 8 is required for viral inhibition of the cGAS pathway

DNA virus infection triggers an antiviral type I interferon (IFN) response in cells that suppresses infection of surrounding cells. Consequently, viruses have evolved mechanisms to inhibit the IFN response for efficient replication. The cellular cGAS protein binds to double-stranded DNA and synthesizes the small molecule cGAMP to initiate DNA-dependent type I IFN production. We showed previously that cGAMP production is relatively low during HSV-1 infection compared to plasmid DNA transfection. Therefore, we hypothesized that HSV-1 produces antagonists of the cGAS DNA sensing pathway. In this study, we found that the HSV-1 ICP8 protein is required for viral inhibition of the cGAS pathway by reducing cGAMP levels stimulated by double-stranded DNA transfection. ICP8 alone inhibited the cGAMP response and may inhibit cGAS action by direct interaction with DNA, cGAS, or other infected cell proteins. Our results reveal another cGAS antiviral pathway inhibitor and highlight the importance of countering IFN for efficient viral replication.

cGAS-STING drives ageing-related inflammation and neurodegeneration

Low-grade inflammation is a hallmark of old age and a central driver of ageing-associated impairment and disease1. Multiple factors can contribute to ageing-associated inflammation2; however, the molecular pathways that transduce aberrant inflammatory signalling and their impact in natural ageing remain unclear. Here we show that the cGAS-STING signalling pathway, which mediates immune sensing of DNA3, is a critical driver of chronic inflammation and functional decline during ageing. Blockade of STING suppresses the inflammatory phenotypes of senescent human cells and tissues, attenuates ageing-related inflammation in multiple peripheral organs and the brain in mice, and leads to an improvement in tissue function. Focusing on the ageing brain, we reveal that activation of STING triggers reactive microglial transcriptional states, neurodegeneration and cognitive decline. Cytosolic DNA released from perturbed mitochondria elicits cGAS activity in old microglia, defining a mechanism by which cGAS-STING signalling is engaged in the ageing brain. Single-nucleus RNA-sequencing analysis of microglia and hippocampi of a cGAS gain-of-function mouse model demonstrates that engagement of cGAS in microglia is sufficient to direct ageing-associated transcriptional microglial states leading to bystander cell inflammation, neurotoxicity and impaired memory capacity. Our findings establish the cGAS-STING pathway as a driver of ageing-related inflammation in peripheral organs and the brain, and reveal blockade of cGAS-STING signalling as a potential strategy to halt neurodegenerative processes during old age.

Beyond natural biology: rewiring cellular networks to study innate immunity

Within immune cells, microbial and self-ligands trigger pattern recognition receptors (PRRs) to nucleate and activate the signaling organelles of the immune system. Much work in this area has derived from observational biology of natural innate immune signaling. More recently, synthetic biology approaches have been used to rewire and study innate immune networks. By utilizing controllable chemical or optogenetic inputs, rearranging protein building blocks, or engineering signal recording circuits, synthetic biology-based techniques complement and inform studies of natural immune pathway operation. In this review, we describe recent synthetic biology-based approaches that have uncovered new insights into PRR signaling, virus-host interactions, and systemic cytokine responses.

An HBV susceptibility variant of KNG1 modulates the therapeutic effects of interferons α and λ1 in HBV infection by promoting MAVS lysosomal degradation

BACKGROUND

Hepatitis B virus (HBV) infection is one of the main causes of hepatocellular carcinoma (HCC). The relationship between HBV infection and the host genome as well as their underlying mechanisms remain largely unknown.

METHODS

In this study, we performed a whole-genome exon sequencing analysis of 300 sib-pairs of Chinese HBV-infected families with the goal of identifying variants and genes involved in HBV infection. A site-direct mutant plasmid was used to investigate the function of SNP rs76438938 in KNG1. The functional and mechanical studies of KNG1 were conducted with in vitro liver cell lines and a hydrodynamic injection model in vivo. The impact of KNG1 on HBV infection therapy was determined in hepatocytes treated with IFN-α/λ1.

FINDINGS

Our whole-exon association study of 300 families with hepatitis B infection found that SNP rs76438938 in KNG1 significantly increased the risk for HBV infection, and the rs76438938-T allele was found to promote HBV replication by increasing the stability of KNG1 mRNA. By competitively binding HSP90A with MAVS, KNG1 can inhibit the expression of types I and III IFNs by promoting MAVS lysosomal degradation. Such suppression of IFN expression and promotion of HBV replication by Kng1 were further demonstrated with an animal model in vivo. Lastly, we showed that the rs76438938-C allele can improve the therapeutic effect of IFN-α and -λ1 in HBV infection.

INTERPRETATION

This study identified a SNP, rs76438938, in a newly discovered host gene, KNG1, for its involvement in HBV infection and treatment effect through modulating the cellular antiviral process.

FUNDING

This study was supported in part by the Independent Task of State Key Laboratory for Diagnosis and Treatment of Infectious Diseases of the First Affiliated Hospital of Zhejiang University, the China Precision Medicine Initiative (2016YFC0906300), and the Research Center for Air Pollution and Health of Zhejiang University.

Mitochondrial Dysfunction Associated with mtDNA in Metabolic Syndrome and Obesity

Metabolic syndrome (MetS) is a precursor to the major health diseases associated with high mortality in industrialized countries: cardiovascular disease and diabetes. An important component of the pathogenesis of the metabolic syndrome is mitochondrial dysfunction, which is associated with tissue hypoxia, disruption of mitochondrial integrity, increased production of reactive oxygen species, and a decrease in ATP, leading to a chronic inflammatory state that affects tissues and organ systems. The mitochondrial AAA + protease Lon (Lonp1) has a broad spectrum of activities. In addition to its classical function (degradation of misfolded or damaged proteins), enzymatic activity (proteolysis, chaperone activity, mitochondrial DNA (mtDNA)binding) has been demonstrated. At the same time, the spectrum of Lonp1 activity extends to the regulation of cellular processes inside mitochondria, as well as outside mitochondria (nuclear localization). This mitochondrial protease with enzymatic activity may be a promising molecular target for the development of targeted therapy for MetS and its components. The aim of this review is to elucidate the role of mtDNA in the pathogenesis of metabolic syndrome and its components as a key component of mitochondrial dysfunction and to describe the promising and little-studied AAA + LonP1 protease as a potential target in metabolic disorders.

MAVS Antagonizes Human Stem Cell Senescence as a Mitochondrial Stabilizer

Mitochondrial dysfunction is a hallmark feature of cellular senescence and organ aging. Here, we asked whether the mitochondrial antiviral signaling protein (MAVS), which is essential for driving antiviral response, also regulates human stem cell senescence. To answer this question, we used CRISPR/Cas9-mediated gene editing and directed differentiation techniques to generate various MAVS-knockout human stem cell models. We found that human mesenchymal stem cells (hMSCs) were sensitive to MAVS deficiency, as manifested by accelerated senescence phenotypes. We uncovered that the role of MAVS in maintaining mitochondrial structural integrity and functional homeostasis depends on its interaction with the guanosine triphosphatase optic atrophy type 1 (OPA1). Depletion of MAVS or OPA1 led to the dysfunction of mitochondria and cellular senescence, whereas replenishment of MAVS or OPA1 in MAVS-knockout hMSCs alleviated mitochondrial defects and premature senescence phenotypes. Taken together, our data underscore an uncanonical role of MAVS in safeguarding mitochondrial homeostasis and antagonizing human stem cell senescence.

ROP16 of Toxoplasma gondii Inhibits Innate Immunity by Triggering cGAS-STING Pathway Inactivity through the Polyubiquitination of STING

cGAS-STING signaling is a major pathway in inducing type Ⅰ IFN, which plays a crucial role in the defense against T. gondii infection. In contrast, T. gondii develops multiple strategies to counteract the host defense, causing serious diseases in a wide range of hosts. Here, we demonstrate that T. gondii rhoptry protein 16 (ROP16) dampens type I interferon signaling via the inhibition of the cGAS (cyclic GMP-AMP synthase) pathway through the polyubiquitination of STING. Mechanistically, ROP16 interacts with STING through the SignalP domain and inhibits the K63-linked ubiquitination of STING in an NLS (nuclear localization signal)-domain-dependent manner. Consequently, knocking out the ROP16 in PRU tachyzoites promotes the STING-mediated production of type I IFNs and limits the replication of T. gondii. Together, these findings describe a distinct pathway where T. gondii exploits the ubiquitination of STING to evade host anti-parasite immunity, revealing new insights into the interaction between the host and parasites.

Role of neuroinflammation in neurodegeneration development

Studies in neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Amyotrophic lateral sclerosis, Huntington's disease, and so on, have suggested that inflammation is not only a result of neurodegeneration but also a crucial player in this process. Protein aggregates which are very common pathological phenomenon in neurodegeneration can induce neuroinflammation which further aggravates protein aggregation and neurodegeneration. Actually, inflammation even happens earlier than protein aggregation. Neuroinflammation induced by genetic variations in CNS cells or by peripheral immune cells may induce protein deposition in some susceptible population. Numerous signaling pathways and a range of CNS cells have been suggested to be involved in the pathogenesis of neurodegeneration, although they are still far from being completely understood. Due to the limited success of traditional treatment methods, blocking or enhancing inflammatory signaling pathways involved in neurodegeneration are considered to be promising strategies for the therapy of neurodegenerative diseases, and many of them have got exciting results in animal models or clinical trials. Some of them, although very few, have been approved by FDA for clinical usage. Here we comprehensively review the factors affecting neuroinflammation and the major inflammatory signaling pathways involved in the pathogenicity of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. We also summarize the current strategies, both in animal models and in the clinic, for the treatment of neurodegenerative diseases.

Exposure to microbial products followed by loss of Tet2 promotes myelodysplastic syndrome via remodeling HSCs

Aberrant innate immune signaling in myelodysplastic syndrome (MDS) hematopoietic stem/progenitor cells (HSPCs) has been implicated as a driver of the development of MDS. We herein demonstrated that a prior stimulation with bacterial and viral products followed by loss of the Tet2 gene facilitated the development of MDS via up-regulating the target genes of the Elf1 transcription factor and remodeling the epigenome in hematopoietic stem cells (HSCs) in a manner that was dependent on Polo-like kinases (Plk) downstream of Tlr3/4-Trif signaling but did not increase genomic mutations. The pharmacological inhibition of Plk function or the knockdown of Elf1 expression was sufficient to prevent the epigenetic remodeling in HSCs and diminish the enhanced clonogenicity and the impaired erythropoiesis. Moreover, this Elf1-target signature was significantly enriched in MDS HSPCs in humans. Therefore, prior infection stress and the acquisition of a driver mutation remodeled the transcriptional and epigenetic landscapes and cellular functions in HSCs via the Trif-Plk-Elf1 axis, which promoted the development of MDS.

Regulation of cGAS and STING signaling during inflammation and infection

Stimulator of interferon genes (STING) is a sensor of cyclic dinucleotides including cyclic GMP-AMP, which is produced by cyclic GMP-AMP synthase (cGAS) in response to cytosolic DNA. The cGAS-STING signaling pathway regulates both innate and adaptive immune responses, as well as fundamental cellular functions such as autophagy, senescence, and apoptosis. Mutations leading to constitutive activation of STING cause devastating human diseases. Thus, the cGAS-STING pathway is of great interest because of its role in diverse cellular processes and because of the potential therapeutic implications of targeting cGAS and STING. Here, we review molecular and cellular mechanisms of STING signaling, and we propose a framework for understanding the immunological and other cellular functions of STING in the context of disease.

Mitochondrial OGG1 expression reduces age-associated neuroinflammation by regulating cytosolic mitochondrial DNA

Aging is accompanied by a decline in DNA repair efficiency, which leads to the accumulation of different types of DNA damage. Age-associated chronic inflammation and generation of reactive oxygen species exacerbate the aging process and age-related chronic disorders. These inflammatory processes establish conditions that favor accumulation of DNA base damage, especially 8-oxo-7,8 di-hydroguanine (8-oxoG), which in turn contributes to various age associated diseases. 8-oxoG is repaired by 8-oxoG glycosylase1 (OGG1) through the base excision repair (BER) pathway. OGG1 is present in both the cell nucleus and in mitochondria. Mitochondrial OGG1 has been implicated in mitochondrial DNA repair and increased mitochondrial function. Using transgenic mouse models and cell lines that have been engineered to have enhanced expression of mitochondria-targeted OGG1 (mtOGG1), we show that elevated levels of mtOGG1 in mitochondria can reverse aging-associated inflammation and improve functions. Old male mtOGG1Tg mice show decreased inflammation response, decreased TNFα levels and multiple pro-inflammatory cytokines. Moreover, we observe that male mtOGG1Tg mice show resistance to STING activation. Interestingly, female mtOGG1Tg mice did not respond to mtOGG1 overexpression. Further, HMC3 cells expressing mtOGG1 display decreased release of mtDNA into the cytoplasm after lipopolysacchride induction and regulate inflammation through the pSTING pathway. Also, increased mtOGG1 expression reduced LPS-induced loss of mitochondrial functions. These results suggest that mtOGG1 regulates age-associated inflammation by controlling release of mtDNA into the cytoplasm.

ISG15: A link between innate immune signaling, DNA replication, and genome stability

Interferon stimulated gene 15 (ISG15) encodes a ubiquitin-like protein that is highly induced upon activation of interferon signaling and cytoplasmic DNA sensing pathways. As part of the innate immune system ISG15 acts to inhibit viral replication and particle release via the covalent conjugation to both viral and host proteins. Unlike ubiquitin, unconjugated ISG15 also functions as an intracellular and extra-cellular signaling molecule to modulate the immune response. Several recent studies have shown ISG15 to also function in a diverse array of cellular processes and pathways outside of the innate immune response. This review explores the role of ISG15 in maintaining genome stability, particularly during DNA replication, and how this relates to cancer biology. It puts forth the hypothesis that ISG15, along with DNA sensors, function within a DNA replication fork surveillance pathway to help maintain genome stability.

SAMHD1 impairs type I interferon induction through the MAVS, IKKε, and IRF7 signaling axis during viral infection

Sterile alpha motif and HD domain-containing protein 1 (SAMHD1) restricts human immunodeficiency virus type 1 (HIV-1) infection by reducing the intracellular dNTP pool. We have shown that SAMHD1 suppresses nuclear factor kappa-B activation and type I interferon (IFN-I) induction by viral infection and inflammatory stimuli. However, the mechanism by which SAMHD1 inhibits IFN-I remains unclear. Here, we show that SAMHD1 inhibits IFN-I activation induced by the mitochondrial antiviral-signaling protein (MAVS). SAMHD1 interacted with MAVS and suppressed MAVS aggregation in response to Sendai virus infection in human monocytic THP-1 cells. This resulted in increased phosphorylation of TANK binding kinase 1 (TBK1), inhibitor of nuclear factor kappa-B kinase epsilon (IKKε), and IFN regulatory factor 3 (IRF3). SAMHD1 suppressed IFN-I activation induced by IKKε and prevented IRF7 binding to the kinase domain of IKKε. We found that SAMHD1 interaction with the inhibitory domain (ID) of IRF7 (IRF7-ID) was necessary and sufficient for SAMHD1 suppression of IRF7-mediated IFN-I activation in HEK293T cells. Computational docking and molecular dynamics simulations revealed possible binding sites between IRF7-ID and full-length SAMHD1. Individual substitution of F411, E416, or V460 in IRF7-ID significantly reduced IRF7 transactivation activity and SAMHD1 binding. Furthermore, we investigated the role of SAMHD1 inhibition of IRF7-mediated IFN-I induction during HIV-1 infection. We found that THP-1 cells lacking IRF7 expression had reduced HIV-1 infection and viral transcription compared to control cells, indicating a positive role of IRF7 in HIV-1 infection. Our findings suggest that SAMHD1 suppresses IFN-I induction through the MAVS, IKKε, and IRF7 signaling axis.

Viral recognition and the antiviral interferon response

Interferons (IFNs) are antiviral cytokines that play a key role in the innate immune response to viral infections. In response to viral stimuli, cells produce and release interferons, which then act on neighboring cells to induce the transcription of hundreds of genes. Many of these gene products either combat the viral infection directly, e.g., by interfering with viral replication, or help shape the following immune response. Here, we review how viral recognition leads to the production of different types of IFNs and how this production differs in spatial and temporal manners. We then continue to describe how these IFNs play different roles in the ensuing immune response depending on when and where they are produced or act during an infection.

A Golgi-resident GPR108 cooperates with E3 ubiquitin ligase Smurf1 to suppress antiviral innate immunity

The regulation of antiviral immunity is crucial in maintaining host immune homeostasis, a process that involves dynamic modulations of host organelles. The Golgi apparatus is increasingly perceived as a host organelle functioning as a critical platform for innate immunity, but the detailed mechanism by which it regulates antiviral immunity remains elusive. Here, we identify the Golgi-localized G protein-coupled receptor 108 (GPR108) as a regulator of type Ι interferon responses by targeting interferon regulatory factor 3 (IRF3). Mechanistically, GPR108 enhances the ubiquitin ligase Smad ubiquitylation regulatory factor 1 (Smurf1)-mediated K63-linked polyubiquitination of phosphorylated IRF3 for nuclear dot 10 protein 52 (NDP52)-dependent autophagic degradation, leading to suppression of antiviral immune responses against DNA or RNA viruses. Taken together, our study provides insight into the crosstalk between the Golgi apparatus and antiviral immunity via a dynamic and spatiotemporal regulation of GPR108-Smurf1 axis, thereby indicating a potential target for treating viral infection.

Regulation of PKR-dependent RNA translation inhibition by TRIM21 upon virus infection or other stress

The host always employs various ways to defend against viral infection and spread. However, viruses have evolved their own effective strategies, such as inhibition of RNA translation of the antiviral effectors, to destroy the host's defense barriers. Protein synthesis, commonly controlled by the α-subunit of eukaryotic translation initiation factor 2 (eIF2α), is a basic cellular biological process among all species. In response to viral infection, in addition to inducing the transcription of antiviral cytokines by innate immunity, infected cells also inhibit the RNA translation of antiviral factors by activating the protein kinase R (PKR)-eIF2α signaling pathway. Regulation of innate immunity has been well studied; however, regulation of the PKR-eIF2α signaling pathway remains unclear. In this study, we found that the E3 ligase TRIM21 negatively regulates the PKR-eIF2α signaling pathway. Mechanistically, TRIM21 interacts with the PKR phosphatase PP1α and promotes K6-linked polyubiquitination of PP1α. Ubiquitinated PP1α augments its interaction with PKR, causing PKR dephosphorylation and subsequent translational inhibition release. Furthermore, TRIM21 can constitutively restrict viral infection by reversing PKR-dependent translational inhibition of various previously known and unknown antiviral factors. Our study highlights a previously undiscovered role of TRIM21 in regulating translation, which will provide new insights into the host antiviral response and novel targets for the treatment of translation-associated diseases in the clinic.

Photochemically controlled activation of STING by CAIX-targeting photocaged agonists to suppress tumor cell growth

Controllable activation of the innate immune adapter protein - stimulator of interferon genes (STING) pathway is a critical challenge for the clinical development of STING agonists due to the potential "on-target off-tumor" toxicity caused by systematic activation of STING. Herein, we designed and synthesized a photo-caged STING agonist 2 with a tumor cell-targeting carbonic anhydrase inhibitor warhead, which could be readily uncaged by blue light to release the active STING agonist leading to remarkable activation of STING signaling. Furthermore, compound 2 was found to preferentially target tumor cells, stimulate the STING signaling in zebrafish embryo upon photo-uncaging and to induce proliferation of macrophages and upregulation of the mRNA expression of STING as well as its downstream NF-kB and cytokines, thus leading to significant suppression of tumor cell growth in a photo-dependent manner with reduced systemic toxicity. This photo-caged agonist not only provides a powerful tool to precisely trigger STING signalling, but also represents a novel controllable STING activation strategy for safer cancer immunotherapy.

Vinorelbine causes a neuropathic pain-like state in mice via STING and MNK1 signaling associated with type I interferon induction

Type I interferons (IFNs) increase the excitability of dorsal root ganglion (DRG) neurons via activation of MNK-eIF4E translation signaling to promote pain sensitization in mice. Activation of STING signaling is a key component of type I IFN induction. Manipulation of STING signaling is an active area of investigation in cancer and other therapeutic areas. Vinorelbine is a chemotherapeutic that activates STING and has been shown to cause pain and neuropathy in oncology clinical trials in patients. There are conflicting reports on whether STING signaling promotes or inhibits pain in mice. We hypothesized that vinorelbine would cause a neuropathic pain-like state in mice via STING and signaling pathways in DRG neurons associated with type I IFN induction. Vinorelbine (10 mg/kg, i.v.) induced tactile allodynia and grimacing in WT male and female mice and increased p-IRF3 and type I IFN protein in peripheral nerves. In support of our hypothesis, vinorelbine-mediated pain was absent in male and female StingGt/Gt mice. Vinorelbine also failed to induce IRF3 and type I IFN signaling in these mice. Since type I IFNs engage translational control via MNK1-eIF4E in DRG nociceptors, we assessed vinorelbine-mediated p-eIF4E changes. Vinorelbine increased p-eIF4E in DRG in WT animals but not in StingGt/Gt or Mknk1-/- (MNK1 KO) mice. Consistent with these biochemical findings, vinorelbine had an attenuated pro-nociceptive effect in male and female MNK1 KO mice. Our findings support the conclusion that activation of STING signaling in the peripheral nervous system causes a neuropathic pain-like state that is mediated by type I IFN signaling to DRG nociceptors.

Activating an Adaptive Immune Response with a Telomerase-Mediated Telomere Targeting Therapeutic in Hepatocellular Carcinoma

A select group of patients with hepatocellular carcinomas (HCC) benefit from surgical, radiologic, and systemic therapies that include a combination of anti-angiogenic and immune-checkpoint inhibitors. However, because HCC is generally asymptomatic in its early stages, this not only leads to late diagnosis, but also to therapy resistance. The nucleoside analogue 6-thio-dG (THIO) is a first-in-class telomerase-mediated telomere-targeting anticancer agent. In telomerase expressing cancer cells, THIO is converted into the corresponding 5'-triphosphate, which is efficiently incorporated into telomeres by telomerase, activating telomere damage responses and apoptotic pathways. Here, we show how THIO is effective in controlling tumor growth and, when combined with immune checkpoint inhibitors, is even more effective in a T-cell-dependent manner. We also show telomere stress induced by THIO increases both innate sensing and adaptive antitumor immunity in HCC. Importantly, the extracellular high-mobility group box 1 protein acts as a prototypical endogenous DAMP (Damage Associated Molecular Pattern) in eliciting adaptive immunity by THIO. These results provide a strong rationale for combining telomere-targeted therapy with immunotherapy.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

Borrelia burgdorferi Engages Mammalian Type I IFN Responses via the cGAS-STING Pathway

Borrelia burgdorferi, the etiologic agent of Lyme disease, is a spirochete that modulates numerous host pathways to cause a chronic, multisystem inflammatory disease in humans. B. burgdorferi infection can lead to Lyme carditis, neurologic complications, and arthritis because of the ability of specific borrelial strains to disseminate, invade, and drive inflammation. B. burgdorferi elicits type I IFN (IFN-I) responses in mammalian cells and tissues that are associated with the development of severe arthritis or other Lyme-related complications. However, the innate immune sensors and signaling pathways controlling IFN-I induction remain unclear. In this study, we examined whether intracellular nucleic acid sensing is required for the induction of IFN-I to B. burgdorferi. Using fluorescence microscopy, we show that B. burgdorferi associates with mouse and human cells in culture, and we document that internalized spirochetes colocalize with the pattern recognition receptor cyclic GMP-AMP synthase (cGAS). Moreover, we report that IFN-I responses in mouse macrophages and murine embryonic fibroblasts are significantly attenuated in the absence of cGAS or its adaptor stimulator of IFN genes (STING), which function to sense and respond to intracellular DNA. Longitudinal in vivo tracking of bioluminescent B. burgdorferi revealed similar dissemination kinetics and borrelial load in C57BL/6J wild-type, cGAS-deficient, or STING-deficient mice. However, infection-associated tibiotarsal joint pathology and inflammation were modestly reduced in cGAS-deficient compared with wild-type mice. Collectively, these results indicate that the cGAS-STING pathway is a critical mediator of mammalian IFN-I signaling and innate immune responses to B. burgdorferi.

Orchestration of Mitochondrial Function and Remodeling by Post-Translational Modifications Provide Insight into Mechanisms of Viral Infection

The regulation of mitochondria structure and function is at the core of numerous viral infections. Acting in support of the host or of virus replication, mitochondria regulation facilitates control of energy metabolism, apoptosis, and immune signaling. Accumulating studies have pointed to post-translational modification (PTM) of mitochondrial proteins as a critical component of such regulatory mechanisms. Mitochondrial PTMs have been implicated in the pathology of several diseases and emerging evidence is starting to highlight essential roles in the context of viral infections. Here, we provide an overview of the growing arsenal of PTMs decorating mitochondrial proteins and their possible contribution to the infection-induced modulation of bioenergetics, apoptosis, and immune responses. We further consider links between PTM changes and mitochondrial structure remodeling, as well as the enzymatic and non-enzymatic mechanisms underlying mitochondrial PTM regulation. Finally, we highlight some of the methods, including mass spectrometry-based analyses, available for the identification, prioritization, and mechanistic interrogation of PTMs.

β-Glucan Induces Training Immunity to Promote Antiviral Activity by Activating TBK1

Many studies have shown that β-glucan induces a trained immune phenotype in innate immune cells to defend against bacterial and fungal infections. The specific mechanism involves cellular metabolism and epigenetic reprogramming. However, it is unclear whether β-glucan plays a role in antiviral infection. Therefore, this study investigated the role of trained immunity induced by Candida albicans and β-glucan in antiviral innate immunity. It showed that C. albicans and β-glucan promoted the expression of interferon-β (IFN-β) and interleukin-6 (IL-6) in mouse macrophages triggered by viral infection. In addition, β-glucan pretreatment attenuated the pathological damage induced by the virus in mouse lungs and promoted the expression of IFN-β. Mechanistically, β-glucan could promote the phosphorylation and ubiquitination of TANK Binding Kinase 1 (TBK1), a key protein of the innate immune pathway. These results suggest that β-glucan can promote innate antiviral immunity, and this bioactive material may be a potential therapeutic target for antiviral treatment.

The Impact of Tumor Cell-Intrinsic Expression of Cyclic GMP-AMP Synthase (cGAS)-Stimulator of Interferon Genes (STING) on the Infiltration of CD8+ T Cells and Clinical Outcomes in Mismatch Repair Proficient/Microsatellite Stable Colorectal Cancer

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway plays a crucial role in activating immune cells in the tumor microenvironment, thereby contributing to a more favorable response to immune checkpoint inhibitors (ICI) in colorectal cancer (CRC). However, the impact of the expression of cGAS-STING in tumor cells on the infiltration of CD8⁺ T cells and clinical outcomes in mismatch repair proficient/microsatellite stable (pMMR/MSS) CRC remains largely unknown. Our findings reveal that 56.8% of all pMMR CRC cases were cGAS-negative/STING-negative expressions (cGAS^-/STING^-) in tumor cells, whereas only 9.9% of all pMMR CRC showed cGAS-positive/STING-positive expression (cGAS⁺/STING⁺) in tumor cells. The frequency of cGAS⁺/STING⁺ cases was reduced in the advanced stages of pMMR/MSS CRC, and histone methylation might be involved in the down-regulation of STING expression in tumor cells. Since the expression level of cGAS-STING in tumor cells has been associated with the infiltration of CD8⁺ and/or CD4⁺ T cells and the frequency of recurrence in pMMR/MSS CRC, decreased expression of cGAS-STING in tumor cells might lead to poor immune cell infiltration and worse prognosis in most pMMR/MSS CRC patients. Our current findings provide a novel insight for the treatment of patients with pMMR/MSS CRC.

The Role of TLR4 in the Immunotherapy of Hepatocellular Carcinoma: Can We Teach an Old Dog New Tricks?

Hepatocellular carcinoma (HCC) is the most common type of primary liver cancer and is a leading cause of cancer-related death worldwide. Immunotherapy has emerged as the mainstay treatment option for unresectable HCC. Toll-like receptor 4 (TLR4) plays a crucial role in the innate immune response by recognizing and responding primarily to bacterial lipopolysaccharides. In addition to its role in the innate immune system, TLR4 has also been implicated in adaptive immunity, including specific anti-tumor immune responses. In particular, the TLR4 signaling pathway seems to be involved in the regulation of several cancer hallmarks, such as the continuous activation of cellular pathways that promote cell division and growth, the inhibition of programmed cell death, the promotion of several invasion and metastatic mechanisms, epithelial-to-mesenchymal transition, angiogenesis, drug resistance, and epigenetic modifications. Emerging evidence further suggests that TLR4 signaling holds promise as a potential immunotherapeutic target in HCC. The aim of this review was to explore the multilayer aspects of the TLR4 signaling pathway, regarding its role in liver diseases and HCC, as well as its potential utilization as an immunotherapy target for HCC.

MAVS deSUMOylation by SENP1 inhibits its aggregation and antagonizes IRF3 activation

Mitochondrial antiviral signaling protein (MAVS) is an adapter that recruits and activates IRF3. However, the mechanisms underpinning the interplay between MAVS and IRF3 are largely unknown. Here we show that small ubiquitin-like modifier (SUMO)-specific protease 1 negatively regulates antiviral immunity by deSUMOylating MAVS. Upon virus infection, PIAS3-induced poly-SUMOylation promotes lysine 63-linked poly-ubiquitination and aggregation of MAVS. Notably, we observe that SUMO conjugation is required for MAVS to efficiently produce phase-separated droplets through association with a newly identified SUMO-interacting motif (SIM) in MAVS. We further identify a yet-unknown SIM in IRF3 that mediates its enrichment to the multivalent MAVS droplets. Conversely, IRF3 phosphorylation at crucial residues close to SIM rapidly disables SUMO-SIM interactions and releases activated IRF3 from MAVS. Our findings implicate SUMOylation in MAVS phase separation and suggest a thus far unknown regulatory process by which IRF3 can be efficiently recruited and released to facilitate timely activation of antiviral responses.

A tissue injury sensing and repair pathway distinct from host pathogen defense

Pathogen infection and tissue injury are universal insults that disrupt homeostasis. Innate immunity senses microbial infections and induces cytokines/chemokines to activate resistance mechanisms. Here, we show that, in contrast to most pathogen-induced cytokines, interleukin-24 (IL-24) is predominately induced by barrier epithelial progenitors after tissue injury and is independent of microbiome or adaptive immunity. Moreover, Il24 ablation in mice impedes not only epidermal proliferation and re-epithelialization but also capillary and fibroblast regeneration within the dermal wound bed. Conversely, ectopic IL-24 induction in the homeostatic epidermis triggers global epithelial-mesenchymal tissue repair responses. Mechanistically, Il24 expression depends upon both epithelial IL24-receptor/STAT3 signaling and hypoxia-stabilized HIF1α, which converge following injury to trigger autocrine and paracrine signaling involving IL-24-mediated receptor signaling and metabolic regulation. Thus, parallel to innate immune sensing of pathogens to resolve infections, epithelial stem cells sense injury signals to orchestrate IL-24-mediated tissue repair.

RIG-I-like receptors: Molecular mechanism of activation and signaling

During RNA viral infection, RIG-I-like receptors (RLRs) recognize the intracellular pathogenic RNA species derived from viral replication and activate antiviral innate immune response by stimulating type 1 interferon expression. Three RLR members, namely, RIG-I, MDA5, and LGP2 are homologous and belong to a subgroup of superfamily 2 Helicase/ATPase that is preferably activated by double-stranded RNA. RLRs are significantly different in gene architecture, RNA ligand preference, activation, and molecular functions. As switchable macromolecular sensors, RLRs' activities are tightly regulated by RNA ligands, ATP, posttranslational modifications, and cellular cofactors. We provide a comprehensive review of the structure and function of the RLRs and summarize the molecular understanding of sensing and signaling events during the RLR activation process. The key roles RLR signaling play in both anti-infection and immune disease conditions highlight the therapeutic potential in targeting this important molecular pathway.

Stimulator of interferon genes from Asian swamp eel (MaSTING) is involved in host defense against bacterial infection

Stimulator of interferon genes (STING) is an endoplasmic reticulum (ER)-associated protein that plays critical roles in innate immunity and pathogenesis of various diseases. To date, teleost STING against viral stimulation has been identified, whereas STING signaling events in fish against bacteria are not well understood. In the present study, the open reading frame (ORF) of STING from Asian swamp eel (Monopterus albus) was cloned (named MaSTING) and its roles in bacterial infection were investigated. Amino acid sequence alignment and phylogenetic analysis revealed that MaSTING had conserved structures with mammalian STING and shared the closest relationship with mandarin fish STING. Subcellular localization analysis showed that MaSTING distributed in the whole cytoplasm and mainly co-localized with ER. Expression pattern analysis found that MaSTING was constitutively expressed in all the examined tissues with the highest expression in the liver and spleen. Post stimulation with bacteria and various PAMPs, the expression of MaSTING was induced at indicated time points in the immune-related organs and isolated peripheral blood leucocytes. Furthermore, the mechanism underlying MaSTING against bacterial infection was further studied. The qPCR analysis showed that MaSTING overexpression promoted 2'3'-cGAMP induced the expression of IFN-1, ISG15, Viperin, Mx, IL-1β and TNF-α. Western blotting assay suggested that MaSTING significantly enhanced the phosphorylation of TANK-binding kinase 1 (TBK1) and p65. MaSTING also significantly increased the luciferase activity of IFN-1 and NF-κB promoters. Taken together, MaSTING is involved in host defense against bacterial infection by inducing the inflammatory response.

Termination of STING responses is mediated via ESCRT-dependent degradation

cGAS-STING signalling is induced by detection of foreign or mislocalised host double-stranded (ds)DNA within the cytosol. STING acts as the major signalling hub, where it controls production of type I interferons and inflammatory cytokines. Basally, STING resides on the ER membrane. Following activation STING traffics to the Golgi to initiate downstream signalling and subsequently to endolysosomal compartments for degradation and termination of signalling. While STING is known to be degraded within lysosomes, the mechanisms controlling its delivery remain poorly defined. Here we utilised a proteomics-based approach to assess phosphorylation changes in primary murine macrophages following STING activation. This identified numerous phosphorylation events in proteins involved in intracellular and vesicular transport. We utilised high-temporal microscopy to track STING vesicular transport in live macrophages. We subsequently identified that the endosomal complexes required for transport (ESCRT) pathway detects ubiquitinated STING on vesicles, which facilitates the degradation of STING in murine macrophages. Disruption of ESCRT functionality greatly enhanced STING signalling and cytokine production, thus characterising a mechanism controlling effective termination of STING signalling.

Role of cGAS-Sting Signaling in Alzheimer's Disease

There is mounting evidence that the development of Alzheimer's disease (AD) interacts extensively with immunological processes in the brain and extends beyond the neuronal compartment. Accumulation of misfolded proteins can activate an innate immune response that releases inflammatory mediators and increases the severity and course of the disease. It is widely known that type-I interferon-driven neuroinflammation in the central nervous system (CNS) accelerates the development of numerous acute and chronic CNS diseases. It is becoming better understood how the cyclic GMP-AMP synthase (cGAS) and its adaptor protein Stimulator of Interferon Genes (STING) triggers type-I IFN-mediated neuroinflammation. We discuss the principal elements of the cGAS-STING signaling pathway and the mechanisms underlying the association between cGAS-STING activity and various AD pathologies. The current understanding of beneficial and harmful cGAS-STING activity in AD and the current treatment pathways being explored will be discussed in this review. The cGAS-STING regulation offers a novel therapeutic opportunity to modulate inflammation in the CNS because it is an upstream regulator of type-I IFNs.