STING cyclic dinucleotide sensing originated in bacteria

Short prokaryotic Argonaute systems trigger cell death upon detection of invading DNA

Argonaute proteins use single-stranded RNA or DNA guides to target complementary nucleic acids. This allows eukaryotic Argonaute proteins to mediate RNA interference and long prokaryotic Argonaute proteins to interfere with invading nucleic acids. The function and mechanisms of the phylogenetically distinct short prokaryotic Argonaute proteins remain poorly understood. We demonstrate that short prokaryotic Argonaute and the associated TIR-APAZ (SPARTA) proteins form heterodimeric complexes. Upon guide RNA-mediated target DNA binding, four SPARTA heterodimers form oligomers in which TIR domain-mediated NAD(P)ase activity is unleashed. When expressed in Escherichia coli, SPARTA is activated in the presence of highly transcribed multicopy plasmid DNA, which causes cell death through NAD(P)+ depletion. This results in the removal of plasmid-invaded cells from bacterial cultures. Furthermore, we show that SPARTA can be repurposed for the programmable detection of DNA sequences. In conclusion, our work identifies SPARTA as a prokaryotic immune system that reduces cell viability upon RNA-guided detection of invading DNA.

Molecular innovations in plant TIR-based immunity signaling

A protein domain (Toll and Interleukin-1 receptor [TIR]-like) with homology to animal TIRs mediates immune signaling in prokaryotes and eukaryotes. Here, we present an overview of TIR evolution and the molecular versatility of TIR domains in different protein architectures for host protection against microbial attack. Plant TIR-based signaling emerges as being central to the potentiation and effectiveness of host defenses triggered by intracellular and cell-surface immune receptors. Equally relevant for plant fitness are mechanisms that limit potent TIR signaling in healthy tissues but maintain preparedness for infection. We propose that seed plants evolved a specialized protein module to selectively translate TIR enzymatic activities to defense outputs, overlaying a more general function of TIRs.

Bacterial origins of human cell-autonomous innate immune mechanisms

The cell-autonomous innate immune system enables animal cells to resist viral infection. This system comprises an array of sensors that, after detecting viral molecules, activate the expression of antiviral proteins and the interferon response. The repertoire of immune sensors and antiviral proteins has long been considered to be derived from extensive evolutionary innovation in vertebrates, but new data challenge this dogma. Recent studies show that central components of the cell-autonomous innate immune system have ancient evolutionary roots in prokaryotic genes that protect bacteria from phages. These include the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, Toll/IL-1 receptor (TIR) domain-containing pathogen receptors, the viperin family of antiviral proteins, SAMHD1-like nucleotide-depletion enzymes, gasdermin proteins and key components of the RNA interference pathway. This Perspective details current knowledge of the elements of antiviral immunity that are conserved from bacteria to humans, and presents possible evolutionary scenarios to explain the observed conservation.

Lipotoxicity-induced mtDNA release promotes diabetic cardiomyopathy by activating the cGAS-STING pathway in obesity-related diabetes

Diabetic cardiomyopathy (DCM) is characterized by lipid accumulation, mitochondrial dysfunction, and aseptic inflammatory activation. Mitochondria-derived cytosolic DNA has been reported to induce inflammation by activating cyclic GMP-AMP synthase (cGAS)/the stimulator of interferon genes (STING) pathway in the adipose, liver, and kidney tissues. However, the role of cytosolic mtDNA in the progression of DCM is unclear. In this study, with an obesity-related DCM mouse model established by feeding db/db mice with a high-fat diet (HFD), we observed increased mtDNA in the cytosol and activated cGAS-STING signaling pathway during DCM, as well as the downstream targets, IRF3, NF-κB, IL-18, and IL-1β. In a further study with a palmitic acid (PA)-induced lipotoxic cell model established in H9C2 cells, we revealed that the cytosolic mtDNA was the result of PA-induced overproduction of mitochondrial ROS, which also led to the activation of the cGAS/STING system and its downstream targets. Notably, treatment of extracted mtDNA alone was sufficient to activate the cGAS-STING signaling pathway in cultured H9C2 cells. Besides, both knockdown of STING in PA-induced H9C2 cells and inhibition of STING by C-176 injection in the DCM mouse model could remarkably block the inflammation and apoptosis of cardiomyocytes. In conclusion, our study elucidated the critical role of cytosolic mtDNA-induced cGAS-STING activation in the pathogenesis of obesity-related DCM and provided preclinical validation for using a STING inhibitor as a new potential therapeutic strategy for the treatment of DCM.

A Glimpse of Programmed Cell Death Among Bacteria, Animals, and Plants

Programmed cell death (PCD) in animals mainly refers to lytic and non-lytic forms. Disruption and integrity of the plasma membrane are considered as hallmarks of lytic and apoptotic cell death, respectively. These lytic cell death programs can prevent the hosts from microbial pathogens. The key to our understanding of these cases is pattern recognition receptors, such as TLRs in animals and LRR-RLKs in plants, and nod-like receptors (NLRs). Herein, we emphatically discuss the biochemical and structural studies that have clarified the anti-apoptotic and pro-apoptotic functions of Bcl-2 family proteins during intrinsic apoptosis and how caspase-8 among apoptosis, necroptosis, and pyroptosis sets the switchable threshold and integrates innate immune signaling, and that have compared the similarity and distinctness of the apoptosome, necroptosome, and inflammasome. We recapitulate that the necroptotic MLKL pore, pyroptotic gasdermin pore, HR-inducing resistosome, and mitochondrial Bcl-2 family all can form ion channels, which all directly boost membrane disruption. Comparing the conservation and unique aspects of PCD including ferrroptosis among bacteria, animals, and plants, the commonly shared immune domains including TIR-like, gasdermin-like, caspase-like, and MLKL/CC-like domains act as arsenal modules to restructure the diverse architecture to commit PCD suicide upon stresses/stimuli for host community.

cGAS-like receptor-mediated immunity: the insect perspective

The cGAS-STING pathway plays a central role in the detection of DNA in the cytosol of mammalian cells and activation of immunity. Although the early evolutionary origin of this pathway in animals has been noted, its ancestral functions have remained elusive so far. We review here new findings in invertebrates establishing a role in sensing and signaling infection, triggering potent transcriptional responses, in addition to autophagy. Results from flies and moths/butterflies point to the importance of STING signaling in antiviral immunity in insects. The recent characterization of cGAS-like receptors in Drosophila reveals the plasticity of this family of pattern-recognition receptors, able to accommodate ligands different from DNA and to produce cyclic dinucleotides beyond 2'3'-cGAMP.

CBASS phage defense and evolution of antiviral nucleotide signaling

Cyclic oligonucleotide-based antiphage signaling system (CBASS) immunity is a widespread form of antiphage defense in bacteria and archaea. Each CBASS operon encodes a cGAS/DncV-like Nucleotidyltransferase (CD-NTase) enzyme that synthesizes a nucleotide second messenger in response to viral infection. An associated Cap effector protein then binds the nucleotide signal and executes cell death to destroy the host cell and block phage propagation. Here we build upon recent advances to establish rules controlling each step of CBASS activation and antiphage defense. Comparative analysis of CBASS, CRISPR, Pycsar, and cGAS-STING immunity provides insight into the evolution of phage defense and animal innate immunity and highlights new questions emerging in the role of nucleotide second messenger signaling in host-virus interactions.

Regulation and function of the cGAS-MITA/STING axis in health and disease

The innate immune systems detect pathogens via pattern-recognition receptors including nucleic acid sensors and non-nucleic acid sensors. Cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS, also known as MB21D1) is a cytosolic DNA sensor that recognizes double-stranded DNA (dsDNA) and catalyzes the synthesis of 2',3'-cGAMP. Subsequently, 2',3'-cGAMP binds to the adaptor protein mediator of IRF3 activation (MITA, also known as STING, MPYS, ERIS, and TMEM173) to activate downstream signaling cascades. The cGAS-MITA/STING signaling critically mediates immune responses against DNA viruses, retroviruses, bacteria, and protozoan parasites. In addition, recent discoveries have extended our understanding of the roles of the cGAS-MITA/STING pathway in autoimmune diseases and cancers. Here, we summarize the identification and activation of cGAS and MITA/STING, present the updated functions and regulatory mechanisms of cGAS-MITA/STING signaling and provide a comprehensive understanding of the cGAS-MITA/STING axis in autoimmune diseases and cancers.

Bacteria elicit a phage tolerance response subsequent to infection of their neighbors

Appearance of plaques on a bacterial lawn is a sign of successive rounds of bacteriophage infection. Yet, mechanisms evolved by bacteria to limit plaque spread have been hardly explored. Here, we investigated the dynamics of plaque development by lytic phages infecting the bacterium Bacillus subtilis. We report that plaque expansion is followed by a constriction phase owing to bacterial growth into the plaque zone. This phenomenon exposed an adaptive process, herein termed "phage tolerance response", elicited by non-infected bacteria upon sensing infection of their neighbors. The temporary phage tolerance is executed by the stress-response RNA polymerase sigma factor σX (SigX). Artificial expression of SigX prior to phage attack largely eliminates infection. SigX tolerance is primarily conferred by activation of the dlt operon, encoding enzymes that catalyze D-alanylation of cell wall teichoic acid polymers, the major attachment sites for phages infecting Gram-positive bacteria. D-alanylation impedes phage binding and hence infection, thus enabling the uninfected bacteria to form a protective shield opposing phage spread.

The cGAS-STING Pathway in Bacterial Infection and Bacterial Immunity

Cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS), along with the adaptor stimulator of interferon genes (STING), are crucial components of the innate immune system, and their study has become a research hotspot in recent years. Many biochemical and structural studies that have collectively elucidated the mechanism of activation of the cGAS-STING pathway with atomic resolution have provided insights into the roles of the cGAS-STING pathway in innate immunity and clues to the origin and evolution of the modern cGAS-STING signaling pathway. The cGAS-STING pathway has been identified to protect the host against viral infection. After detecting viral dsDNA, cGAS synthesizes a second messenger to activate STING, eliciting antiviral immune responses by promoting the expression of interferons (IFNs) and hundreds of IFN-stimulated genes (ISGs). Recently, the cGAS-STING pathway has also been found to be involved in response to bacterial infections, including bacterial pneumonia, melioidosis, tuberculosis, and sepsis. However, compared with its functions in viral infection, the cGAS-STING signaling pathway in bacterial infection is more complex and diverse since the protective and detrimental effects of type I IFN (IFN-I) on the host depend on the bacterial species and infection mode. Besides, STING activation can also affect infection prognosis through other mechanisms in different bacterial infections, independent of the IFN-I response. Interestingly, the core protein components of the mammalian cGAS-STING signaling pathway have been found in the bacterial defense system, suggesting that this widespread signaling pathway may have originated in bacteria. Here, we review recent findings related to the structures of major molecules involved in the cGAS-STING pathway and the effects of the cGAS-STING pathway in various bacterial infections and bacterial immunity, which may pave the way for the development of new antibacterial drugs that specifically kill bacteria without harmful effects on the host.

Lung Inflammation in STING-Associated Vasculopathy with Onset in Infancy (SAVI)

STING-associated vasculopathy with onset in infancy (SAVI) is a type I interferonopathy caused by gain-of-function mutations in STING1 encoding stimulator of interferon genes (STING) protein. SAVI is characterized by severe inflammatory lung disease, a feature not observed in previously described type I interferonopathies i.e., Mendelian autoinflammatory disorders defined by constitutive activation of the type I interferon (IFN) pathway. Molecular defects in nucleic acid metabolism or sensing are central to the pathophysiology of these diseases, with such defects occurring at any step of the tightly regulated pathway of type I IFN production and signaling (e.g., exonuclease loss of function, RNA-DNA hybrid accumulation, constitutive activation of adaptor proteins such as STING). Among over 30 genotypes, SAVI and COPA syndrome, whose pathophysiology was recently linked to a constitutive activation of STING signaling, are the only type I interferonopathies presenting with predominant lung involvement. Lung disease is the leading cause of morbidity and mortality in these two disorders which do not respond to conventional immunosuppressive therapies and only partially to JAK1/2 inhibitors. In human silicosis, STING-dependent sensing of self-DNA following cell death triggered by silica exposure has been found to drive lung inflammation in mice and human models. These recent findings support a key role for STING and nucleic acid sensing in the homeostasis of intrinsic pulmonary inflammation. However, mechanisms by which monogenic defects in the STING pathway lead to pulmonary damages are not yet fully elucidated, and an improved understanding of such mechanisms is fundamental to improved future patient management. Here, we review the recent insights into the pathophysiology of SAVI and outline our current understanding of self-nucleic acid-mediated lung inflammation in humans.

Immunological Diversity Is a Cornerstone of Organismal Defense and Allorecognition across Metazoa

The ongoing arms race between hosts and microbes has fueled the evolution of novel strategies for diversifying the molecules involved in immune responses. Characterization of immune systems from an ever-broadening phylogenetic range of organisms reveals that there are many mechanisms by which this diversity can be generated and maintained. Diversification strategies operate at the level of populations, genomes, genes, and even individual transcripts. Lineage-specific innovations have been cataloged within the immune systems of both invertebrates and vertebrates. Furthermore, somatic diversification of immune receptor genes has now been described in jawless vertebrates and some invertebrate species. In addition to pathogen detection, immunological diversity plays important roles in several distinct allorecognition systems. In this Brief Review, we highlight some of the evolutionary innovations employed by a variety of metazoan species to generate the molecular diversity required to detect a vast array of molecules in the context of both immune response and self/nonself-recognition.

Crystal structure and functional implication of bacterial STING

Mammalian innate immune sensor STING (STimulator of INterferon Gene) was recently found to originate from bacteria. During phage infection, bacterial STING sense c-di-GMP generated by the CD-NTase (cGAS/DncV-like nucleotidyltransferase) encoded in the same operon and signal suicide commitment as a defense strategy that restricts phage propagation. However, the precise binding mode of c-di-GMP to bacterial STING and the specific recognition mechanism are still elusive. Here, we determine two complex crystal structures of bacterial STING/c-di-GMP, which provide a clear picture of how c-di-GMP is distinguished from other cyclic dinucleotides. The protein-protein interactions further reveal the driving force behind filament formation of bacterial STING. Finally, we group the bacterial STING into two classes based on the conserved motif in β-strand lid, which dictate their ligand specificity and oligomerization mechanism, and propose an evolution-based model that describes the transition from c-di-GMP-dependent signaling in bacteria to 2’3’-cGAMP-dependent signaling in eukaryotes.

The bacterial Cyclic-oligonucleotide-Based Anti-phage Signaling System (CBASS) contains a CD-NTase that synthesizes cyclic di- and tri-nucleotides, and bacterial STING proteins recognize c-di-GMP generated by CD-NTase during phage infection and signal the infected bacteria to commit suicide. Here, the authors provide insights into the molecular basis for c-di-GMP recognition of bacterial STING proteins by determining two STING protein crystal structures with bound c-di-GMP from Prevotella corporis and Myroides sp. ZB35.

STING orchestrates the crosstalk between polyunsaturated fatty acid metabolism and inflammatory responses

Concerted alteration of immune and metabolic homeostasis underlies several inflammation-related pathologies, ranging from metabolic syndrome to infectious diseases. Here, we explored the coordination of nucleic acid-dependent inflammatory responses and metabolic homeostasis. We reveal that the STING (stimulator of interferon genes) protein regulates metabolic homeostasis through inhibition of the fatty acid desaturase 2 (FADS2) rate-limiting enzyme in polyunsaturated fatty acid (PUFA) desaturation. STING ablation and agonist-mediated degradation increased FADS2-associated desaturase activity and led to accumulation of PUFA derivatives that drive thermogenesis. STING agonists directly activated FADS2-dependent desaturation, promoting metabolic alterations. PUFAs in turn inhibited STING, thereby regulating antiviral responses and contributing to resolving STING-associated inflammation. Thus, we have unveiled a negative regulatory feedback loop between STING and FADS2 that fine-tunes inflammatory responses. Our results highlight the role of metabolic alterations in human pathologies associated with aberrant STING activation and STING-targeting therapies.

Evidence for cGAS-STING signaling in the female genital tract resistance to Chlamydia trachomatis infection

Sexually transmitted Chlamydia trachomatis can ascend to the upper genital tract due to its resistance to innate immunity in the lower genital tract. C. trachomatis can activate cGAS-STING signaling pathway in cultured cells via either cGAS or STING. The current study was designed to evaluate the role of the cGAS-STING pathway in innate immunity against C. trachomatis in the mouse genital tract. Following intravaginal inoculation, C. trachomatis significantly declined by day 5 following a peak infection on day 3 while the mouse-adapted C. muridarum continued to rise for >1 week, indicating that C. trachomatis is susceptible to the innate immunity in the female mouse genital tract. This conclusion was supported by the observation of a similar shedding course in mice deficient in adaptive immunity. Thus, C. trachomatis can be used to evaluate innate immunity in the female genital tract. It was found that mice deficient in either cGAS or STING significantly increased the yields of live C. trachomatis on day 5, indicating an essential role of the cGAS-STING signaling pathway in innate immunity of the mouse genital tract. Comparison of live C. trachomatis recovered from different genital tissues revealed that the cGAS-STING-dependent immunity against C. trachomatis was restricted to the mouse lower genital tract regardless of whether C. trachomatis was inoculated intravaginally or transcervically. Thus, we have demonstrated an essential role of the cGAS-STING signaling pathway in innate immunity against chlamydial infection, laying a foundation for further illuminating the mechanisms of the innate immunity in the female lower genital tract.

Discovery of coordinately regulated pathways that provide innate protection against interbacterial antagonism

Bacterial survival is fraught with antagonism, including that deriving from viruses and competing bacterial cells. It is now appreciated that bacteria mount complex antiviral responses; however, whether a coordinated defense against bacterial threats is undertaken is not well understood. Previously, we showed that Pseudomonas aeruginosa possess a danger-sensing pathway that is a critical fitness determinant during competition against other bacteria. Here, we conducted genome-wide screens in P. aeruginosa that reveal three conserved and widespread interbacterial antagonism resistance clusters (arc1-3). We find that although arc1-3 are coordinately activated by the Gac/Rsm danger-sensing system, they function independently and provide idiosyncratic defense capabilities, distinguishing them from general stress response pathways. Our findings demonstrate that Arc3 family proteins provide specific protection against phospholipase toxins by preventing the accumulation of lysophospholipids in a manner distinct from previously characterized membrane repair systems. These findings liken the response of P. aeruginosa to bacterial threats to that of eukaryotic innate immunity, wherein threat detection leads to the activation of specialized defense systems.

Cryo-EM structure of an active bacterial TIR-STING filament complex

Stimulator of interferon genes (STING) is an antiviral signalling protein that is broadly conserved in both innate immunity in animals and phage defence in prokaryotes^1-4. Activation of STING requires its assembly into an oligomeric filament structure through binding of a cyclic dinucleotide^4-13, but the molecular basis of STING filament assembly and extension remains unknown. Here we use cryogenic electron microscopy to determine the structure of the active Toll/interleukin-1 receptor (TIR)-STING filament complex from a Sphingobacterium faecium cyclic-oligonucleotide-based antiphage signalling system (CBASS) defence operon. Bacterial TIR-STING filament formation is driven by STING interfaces that become exposed on high-affinity recognition of the cognate cyclic dinucleotide signal c-di-GMP. Repeating dimeric STING units stack laterally head-to-head through surface interfaces, which are also essential for human STING tetramer formation and downstream immune signalling in mammals⁵. The active bacterial TIR-STING structure reveals further cross-filament contacts that brace the assembly and coordinate packing of the associated TIR NADase effector domains at the base of the filament to drive NAD⁺ hydrolysis. STING interface and cross-filament contacts are essential for cell growth arrest in vivo and reveal a stepwise mechanism of activation whereby STING filament assembly is required for subsequent effector activation. Our results define the structural basis of STING filament formation in prokaryotic antiviral signalling.

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

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

Global proteomics of fibroblast cells treated with bacterial cyclic dinucleotides, c-di-GMP and c-di-AMP

BACKGROUND

Constant exposure of human gingival fibroblasts (HGFs) to oral pathogens trigger selective immune responses. Recently, the activation of immune response to cyclic dinucleotides (CDNs) via STING has come to the forefront. Reports show that other proteins outside the STING-TBK1-IRF3 axis respond to CDNs but a global view of impacted proteome in diverse cells is lacking. HGFs are constantly exposed to bacterial-derived cyclic-di-adenosine monophosphate (c-di-AMP) and cyclic-di-guanosine monophosphate (c-di-GMP).

AIM

To understand the response of HGFs to bacterial-derived CDNs, we carried out a global proteomics analysis of HGFs treated with c-di-AMP or c-di-GMP.

METHODS

The expression levels of several proteins modulated by CDNs were examined.

RESULTS

Interferon signaling proteins such as Ubiquitin-like protein ISG15 (ISG15), Interferon-induced GTP-binding protein Mx1 (MX1), Interferon-induced protein with tetratricopeptide repeats (IFIT) 1 (IFIT1), and (IFIT3) were significantly upregulated. Interestingly, other pathways not fully characterized to be regulated by CDNs, such as necroptosis signaling, iron homeostasis signaling, protein ubiquitination, EIF2 signaling, sumoylation and nucleotide excision repair pathways were also modulated by the bacterial-derived CDNs.

CONCLUSION

This study has added to the increasing appreciation that beyond the regulation of cytokine production via STING, cyclic dinucleotides also broadly affect many critical processes in human cells.

The cyclic dinucleotide 2'3'-cGAMP induces a broad antibacterial and antiviral response in the sea anemone Nematostella vectensis

In mammals, cyclic dinucleotides (CDNs) bind and activate STING to initiate an antiviral type I interferon response. CDNs and STING originated in bacteria and are present in most animals. By contrast, interferons are believed to have emerged in vertebrates; thus, the function of CDN signaling in invertebrates is unclear. Here, we use a CDN, 2'3' cyclic guanosine monophosphate-adenosine monophosphate (2'3'-cGAMP), to activate immune responses in a model cnidarian invertebrate, the starlet sea anemone Nematostella vectensis Using RNA sequencing, we found that 2'3'-cGAMP induces robust transcription of both antiviral and antibacterial genes in N. vectensis Many of the antiviral genes induced by 2'3'-cGAMP are homologs of vertebrate interferon-stimulated genes, implying that the interferon response predates the evolution of interferons. Knockdown experiments identified a role for NF-κB in specifically inducing antibacterial genes downstream of 2'3'-cGAMP. Some of these putative antibacterial genes were also found to be induced during Pseudomonas aeruginosa infection. We characterized the protein product of one of the putative antibacterial genes, the N. vectensis homolog of Dae4, and found that it has conserved antibacterial activity. This work suggests that a broad antibacterial and antiviral transcriptional response is an evolutionarily ancestral output of 2'3'-cGAMP signaling in animals.

Roles of Emerging RNA-Binding Activity of cGAS in Innate Antiviral Response

cGAS, a DNA sensor in mammalian cells, catalyzes the generation of 2'-3'-cyclic AMP-GMP (cGAMP) once activated by the binding of free DNA. cGAMP can bind to STING, activating downstream TBK1-IRF-3 signaling to initiate the expression of type I interferons. Although cGAS has been considered a traditional DNA-binding protein, several lines of evidence suggest that cGAS is a potential RNA-binding protein (RBP), which is mainly supported by its interactions with RNAs, RBP partners, RNA/cGAS-phase-separations as well as its structural similarity with the dsRNA recognition receptor 2'-5' oligoadenylate synthase. Moreover, two influential studies reported that the cGAS-like receptors (cGLRs) of fly Drosophila melanogaster sense RNA and control 3'-2'-cGAMP signaling. In this review, we summarize and discuss in depth recent studies that identified or implied cGAS as an RBP. We also comprehensively summarized current experimental methods and computational tools that can identify or predict RNAs that bind to cGAS. Based on these discussions, we appeal that the RNA-binding activity of cGAS cannot be ignored in the cGAS-mediated innate antiviral response. It will be important to identify RNAs that can bind and regulate the activity of cGAS in cells with or without virus infection. Our review provides novel insight into the regulation of cGAS by its RNA-binding activity and extends beyond its DNA-binding activity. Our review would be significant for understanding the precise modulation of cGAS activity, providing the foundation for the future development of drugs against cGAS-triggering autoimmune diseases such as Aicardi-Gourtières syndrome.

Enzymatic Synthesis of 3'-5', 3'-5' Cyclic Dinucleotides, Their Binding Properties to the Stimulator of Interferon Genes Adaptor Protein, and Structure/Activity Correlations

The 3'-5', 3'-5' cyclic dinucleotides (3'3'CDNs) are bacterial second messengers that can also bind to the stimulator of interferon genes (STING) adaptor protein in vertebrates and activate the host innate immunity. Here, we profiled the substrate specificity of four bacterial dinucleotide synthases from Vibrio cholerae (DncV), Bacillus thuringiensis (btDisA), Escherichia coli (dgcZ), and Thermotoga maritima (tDGC) using a library of 33 nucleoside-5'-triphosphate analogues and then employed these enzymes to synthesize 24 3'3'CDNs. The STING affinity of CDNs was evaluated in cell-based and biochemical assays, and their ability to induce cytokines was determined by employing human peripheral blood mononuclear cells. Interestingly, the prepared heterodimeric 3'3'CDNs bound to the STING much better than their homodimeric counterparts and showed similar or better potency than bacterial 3'3'CDNs. We also rationalized the experimental findings by in-depth STING-CDN structure-activity correlations by dissecting computed interaction free energies into a set of well-defined and intuitive terms. To this aim, we employed state-of-the-art methods of computational chemistry, such as quantum mechanics/molecular mechanics (QM/MM) calculations, and complemented the computed results with the {STING:3'3'c-di-ara-AMP} X-ray crystallographic structure. QM/MM identified three outliers (mostly homodimers) for which we have no clear explanation of their impaired binding with respect to their heterodimeric counterparts, whereas the R² = 0.7 correlation between the computed ΔG'_{int_rel} and experimental ΔT_m's for the remaining ligands has been very encouraging.

Antiviral activity of bacterial TIR domains via immune signalling molecules

The Toll/interleukin-1 receptor (TIR) domain is a canonical component of animal and plant immune systems^1,2. In plants, intracellular pathogen sensing by immune receptors triggers their TIR domains to generate a molecule that is a variant of cyclic ADP-ribose^3,4. This molecule is hypothesized to mediate plant cell death through a pathway that has yet to be resolved⁵. TIR domains have also been shown to be involved in a bacterial anti-phage defence system called Thoeris⁶, but the mechanism of Thoeris defence remained unknown. Here we show that phage infection triggers Thoeris TIR-domain proteins to produce an isomer of cyclic ADP-ribose. This molecular signal activates a second protein, ThsA, which then depletes the cell of the essential molecule nicotinamide adenine dinucleotide (NAD) and leads to abortive infection and cell death. We also show that, similar to eukaryotic innate immune systems, bacterial TIR-domain proteins determine the immunological specificity to the invading pathogen. Our results describe an antiviral signalling pathway in bacteria, and suggest that the generation of intracellular signalling molecules is an ancient immunological function of TIR domains that is conserved in both plant and bacterial immunity.

Antiviral activity of bacterial TIR domains via immune signalling molecules

The Toll/interleukin-1 receptor (TIR) domain is a canonical component of animal and plant immune systems^1,2. In plants, intracellular pathogen sensing by immune receptors triggers their TIR domains to generate a molecule that is a variant of cyclic ADP-ribose^3,4. This molecule is hypothesized to mediate plant cell death through a pathway that has yet to be resolved⁵. TIR domains have also been shown to be involved in a bacterial anti-phage defence system called Thoeris⁶, but the mechanism of Thoeris defence remained unknown. Here we show that phage infection triggers Thoeris TIR-domain proteins to produce an isomer of cyclic ADP-ribose. This molecular signal activates a second protein, ThsA, which then depletes the cell of the essential molecule nicotinamide adenine dinucleotide (NAD) and leads to abortive infection and cell death. We also show that, similar to eukaryotic innate immune systems, bacterial TIR-domain proteins determine the immunological specificity to the invading pathogen. Our results describe an antiviral signalling pathway in bacteria, and suggest that the generation of intracellular signalling molecules is an ancient immunological function of TIR domains that is conserved in both plant and bacterial immunity.

Cyclic Nucleotide (cNMP) Analogues: Past, Present and Future

Cyclic nucleotides are important second messengers involved in cellular events, and analogues of this type of molecules are promising drug candidates. Some cyclic nucleotide analogues have become standard tools for the investigation of biochemical and physiological signal transduction pathways, such as the Rp-diastereomers of adenosine and guanosine 3′,5′-cyclic monophosphorothioate, which are competitive inhibitors of cAMP- and cGMP-dependent protein kinases. Next generation analogues exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity, or are caged or photoactivatable for fast and/or targeted cellular imaging. Novel specific nucleotide analogues activating or inhibiting cyclic nucleotide-dependent ion channels, EPAC/GEF proteins, and bacterial target molecules have been developed, opening new avenues for basic and applied research. This review provides an overview of the current state of the field, what can be expected in the future and some practical considerations for the use of cyclic nucleotide analogues in biological systems.

2',3'-Cyclic GMP-AMP Dinucleotides for STING-Mediated Immune Modulation: Principles, Immunotherapeutic Potential, and Synthesis

The cGAS-STING pathway discovered ten years ago is an important component of the innate immune system. Activation of cGAS-STING triggers downstream signalling, such as TBK1-IRF3, NF-κB and autophagy, which in turn leads to antipathogen responses, durable antitumour immunity or autoimmune diseases. 2',3'-Cyclic GMP-AMP dinucleotides (2',3'-cGAMP), the key second messengers produced by cGAS, play a pivotal role in cGAS-STING signalling by binding and activating STING. Thus, 2',3'-cGAMP has immunotherapeutic potential, which in turn has stimulated research on the design and synthesis of 2',3'-cGAMP analogues for clinical applications over the past ten years. This review presents the discovery, metabolism, and function of 2',3'-cGAMP in the cGAS-STING innate immune signalling axis. The enzymatic and chemical syntheses of 2',3'-cGAMP analogues as STING-targeting therapeutics are also summarized.

Yin and Yang of Biofilm Formation and Cyclic di-GMP Signaling of the Gastrointestinal Pathogen Salmonella enterica Serovar Typhimurium

Within the last 60 years, microbiological research has challenged many dogmas such as bacteria being unicellular microorganisms directed by nutrient sources; these investigations produced new dogmas such as cyclic diguanylate monophosphate (cyclic di-GMP) second messenger signaling as a ubiquitous regulator of the fundamental sessility/motility lifestyle switch on the single-cell level. Successive investigations have not yet challenged this view; however, the complexity of cyclic di-GMP as an intracellular bacterial signal, and, less explored, as an extracellular signaling molecule in combination with the conformational flexibility of the molecule, provides endless opportunities for cross-kingdom interactions. Cyclic di-GMP-directed microbial biofilms commonly stimulate the immune system on a lower level, whereas host-sensed cyclic di-GMP broadly stimulates the innate and adaptive immune responses. Furthermore, while the intracellular second messenger cyclic di-GMP signaling promotes bacterial biofilm formation and chronic infections, oppositely, Salmonella Typhimurium cellulose biofilm inside immune cells is not endorsed. These observations only touch on the complexity of the interaction of biofilm microbial cells with its host. In this review, we describe the Yin and Yang interactive concepts of biofilm formation and cyclic di-GMP signaling using S. Typhimurium as an example.

Cyclic pyrimidines jump on the anti-phage bandwagon

Cyclic pyrimidines cCMP and cUMP were known to be present in a variety of organisms and cell types, but their biological roles remained mysterious. Tal et al. show that bacteria use cCMP and cUMP as second messengers that function in anti-phage defense.

Cyclic CMP and cyclic UMP mediate bacterial immunity against phages

The cyclic pyrimidines 3',5'-cyclic cytidine monophosphate (cCMP) and 3',5'-cyclic uridine monophosphate (cUMP) have been reported in multiple organisms and cell types. As opposed to the cyclic nucleotides 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP), which are second messenger molecules with well-established regulatory roles across all domains of life, the biological role of cyclic pyrimidines has remained unclear. Here we report that cCMP and cUMP are second messengers functioning in bacterial immunity against viruses. We discovered a family of bacterial pyrimidine cyclase enzymes that specifically synthesize cCMP and cUMP following phage infection and demonstrate that these molecules activate immune effectors that execute an antiviral response. A crystal structure of a uridylate cyclase enzyme from this family explains the molecular mechanism of selectivity for pyrimidines as cyclization substrates. Defense systems encoding pyrimidine cyclases, denoted here Pycsar (pyrimidine cyclase system for antiphage resistance), are widespread in prokaryotes. Our results assign clear biological function to cCMP and cUMP as immunity signaling molecules in bacteria.

Effector-mediated membrane disruption controls cell death in CBASS antiphage defense

Cyclic oligonucleotide-based antiphage signaling systems (CBASS) are antiviral defense operons that protect bacteria from phage replication. Here, we discover a widespread class of CBASS transmembrane (TM) effector proteins that respond to antiviral nucleotide signals and limit phage propagation through direct membrane disruption. Crystal structures of the Yersinia TM effector Cap15 reveal a compact 8-stranded β-barrel scaffold that forms a cyclic dinucleotide receptor domain that oligomerizes upon activation. We demonstrate that activated Cap15 relocalizes throughout the cell and specifically induces rupture of the inner membrane. Screening for active effectors, we identify the function of distinct families of CBASS TM effectors and demonstrate that cell death via disruption of inner-membrane integrity is a common mechanism of defense. Our results reveal the function of the most prominent class of effector protein in CBASS immunity and define disruption of the inner membrane as a widespread strategy of abortive infection in bacterial phage defense.

Molecular mechanisms of the CdnG-Cap5 antiphage defense system employing 3',2'-cGAMP as the second messenger

Cyclic-oligonucleotide-based antiphage signaling systems (CBASS) are diverse and abundant in bacteria. Here, we present the biochemical and structural characterization of two CBASS systems, composed of CdnG and Cap5, from Asticcacaulis sp. and Lactococcus lactis. We show that CdnG from Asticcacaulis sp. synthesizes 3′,2′-cGAMP in vitro, and 3′,2′-cGAMP is the biological signaling molecule that activates Cap5 for DNA degradation. Crystal structures of Cap5, together with the SAVED domain in complex with 3′,2′-cGAMP, provide insight into the architecture of Cap5 as well as molecular recognition of 3′,2′-cGAMP by the SAVED domain of Cap5. Amino acid conservation of the SAVED domain of Cap5, together with mutational studies, led us to propose a mechanism of Back-to-Front stacking of two SAVED domains, mediated by 3′,2′-cGAMP, to activate HNH nuclease domain for DNA degradation. This study of the most abundant CBASS system provides insights into the mechanisms employed by bacteria in their conflicts against phage.

Many bacterial cyclic-oligonucleotide-based antiphage signaling systems (CBASS) employ effectors with SAVED domains. Here, the authors present a biochemical and structural characterization of two such CBASS, providing mechanistic insights into bacterial antiphage defense.

STING mediates immune responses in the closest living relatives of animals

Animals have evolved unique repertoires of innate immune genes and pathways that provide their first line of defense against pathogens. To reconstruct the ancestry of animal innate immunity, we have developed the choanoflagellate Monosiga brevicollis, one of the closest living relatives of animals, as a model for studying mechanisms underlying pathogen recognition and immune response. We found that M. brevicollis is killed by exposure to Pseudomonas aeruginosa bacteria. Moreover, M. brevicollis expresses STING, which, in animals, activates innate immune pathways in response to cyclic dinucleotides during pathogen sensing. M. brevicollis STING increases the susceptibility of M. brevicollis to P. aeruginosa-induced cell death and is required for responding to the cyclic dinucleotide 2'3' cGAMP. Furthermore, similar to animals, autophagic signaling in M. brevicollis is induced by 2'3' cGAMP in a STING-dependent manner. This study provides evidence for a pre-animal role for STING in antibacterial immunity and establishes M. brevicollis as a model system for the study of immune responses.

STING inhibition accelerates the bone healing process while enhancing type H vessel formation

The stimulator of interferon genes (STING), one of the critical factors of innate immunity, is indicated to be closely related to angiogenesis. This study examined STING's role in angiogenesis and the formation of type H vessels, a specific subtype of bone vessels that regulates bone healing. Different concentrations of 2',3'-cGAMP, and H-151 or C-176 were applied to activate or inhibit STING, respectively. Human umbilical vein endothelial cells were used to examine the effect of STING on angiogenesis in vitro; cell viability, cell migration, and quantitative real-time polymerase chain reactions were performed. Also, the metatarsal experiment was applied as ex vivo proof. Bone fracture or defect mice models were used to examine the effect of STING in vivo; the bone healing process was evaluated by radiography weekly and by μCT on the 14th day after surgery. The formation of type H vessels (CD31^hi Emcn^hi endothelial cells) and osteogenesis (OCN-positive cells) was assessed using the cryosection and paraffin section. STING activation inhibited angiogenesis both in vitro and ex vivo and slowed down the bone healing process in vivo. Histological analysis showed an increased callus formation, fewer type H vessels, and almost no callus mineralization in the STING activation group compared to the control group. By contrast, H-151 (a hsSTING inhibitor) promoted angiogenesis at a low dose. Moreover, inhibition of mmSTING by C-176 enhanced type H vessels' formation, implying osteogenesis promotion in bone healing (higher bone volume density and more OCN-positive cells). Our data suggested that STING inhibition accelerates the bone healing process while enhancing type H vessel formation.

cGAS-like receptors: ancient catchers of viral nucleic acids

Living organisms have evolved conserved 'catchers' for detecting and handling molecular patterns from invading viruses. Slavik et al. and Holleufer et al. recently identified cyclic GMP-AMP (cGAMP) synthase (cGAS)-like receptors that detect double-stranded viral RNA in Drosophila. cGAS-like receptors constitute a new expanding family of pattern recognition receptors (PRRs) for viral nucleic acids.

The induction of inflammation by the cGAS-STING pathway in human dental pulp cells: A laboratory investigation

AIM

To explore the presence of the cGAS-STING inflammatory pathway in human pulp tissue and human dental pulp cells (HDPCs).

METHODOLOGY

Pulp tissue was collected from freshly extracted human healthy third molars or third molars with irreversible pulpitis. Quantitative real-time polymerase chain reaction (qRT-PCR) and enzyme-linked immunoassay (ELISA) were performed to assess IFN-β, TNF and IL-6. Human dental pulp cells prepared from healthy human pulp tissues were transfected with interferon stimulatory DNA (ISD), bacterial genomic DNA, bacterial cyclic dinucleotides c-di-AMP, c-di-GMP or host cyclic dinucleotide cGAMP. SiRNA was used to knock down the endogenous cGAS or STING. G140 and H-151 were used to inhibit cGAS and STING respectively. Amlexanox and BAY 11-7082 were used to inhibit TBK1 and NF-κB respectively. qRT-PCR and ELISA were performed to detect the level of IFN-β, TNF and IL-6. Western blot was performed to evaluate the TBK1, IRF3 and p65 phosphorylation. The Student's t-test and one-way anova were used for statistical analysis.

RESULTS

IFN-β, TNF and IL-6 were up-regulated in the inflamed human dental pulp tissue. CGAS and STING mRNA were increased in the inflamed human dental pulp tissue and detected in HDPCs prepared from healthy human pulp tissues. ISD transfection induced TBK1, IRF3 and p65 phosphorylation as well as IFN-β, TNF and IL-6 production. IFN-β, TNF and IL-6 production were also induced by transfection of bacterial and host cyclic dinucleotides or bacteria DNA. ISD or bacteria DNA transfection elevated the intracellular levels of cGAMP. Knock-down of cGAS or STING, as well as using cGAS inhibitor G140 or STING inhibitor H-151 abolished the IFN-β, TNF and IL-6 production induced by ISD transfection. Knock-down of STING or using STING inhibitor H-151 abolished the IFN-β, TNF and IL-6 induction by transfection of bacterial and host cyclic dinucleotides. Both Amlexanox and BAY 11-7082 inhibited IFN-β, TNF and IL-6 production triggered by ISD and cyclic dinucleotides transfection.

CONCLUSIONS

Human dental pulp cells expressed an intact cGAS-STING signalling axis. The cGAS-STING signalling axis may play an important role in pulp inflammation and immune defence.

The SARM1 TIR NADase: Mechanistic Similarities to Bacterial Phage Defense and Toxin-Antitoxin Systems

The Toll/interleukin-1 receptor (TIR) domain is the signature signalling motif of innate immunity, with essential roles in innate immune signalling in bacteria, plants, and animals. TIR domains canonically function as scaffolds, with stimulus-dependent multimerization generating binding sites for signalling molecules such as kinases and ligases that activate downstream immune mechanisms. Recent studies have dramatically expanded our understanding of the TIR domain, demonstrating that the primordial function of the TIR domain is to metabolize NAD+. Mammalian SARM1, the central executioner of pathological axon degeneration, is the founding member of the TIR-domain class of NAD+ hydrolases. This unexpected NADase activity of TIR domains is evolutionarily conserved, with archaeal, bacterial, and plant TIR domains all sharing this catalytic function. Moreover, this enzymatic activity is essential for the innate immune function of these proteins. These evolutionary relationships suggest a link between SARM1 and ancient self-defense mechanisms that has only been strengthened by the recent discovery of the SARM1 activation mechanism which, we will argue, is strikingly similar to bacterial toxin-antitoxin systems. In this brief review we will describe the regulation and function of SARM1 in programmed axon self-destruction, and highlight the parallels between the SARM1 axon degeneration pathway and bacterial innate immune mechanisms.

Drosophila STING protein has a role in lipid metabolism

Stimulator of interferon genes (STING) plays an important role in innate immunity by controlling type I interferon response against invaded pathogens. In this work, we describe a previously unknown role of STING in lipid metabolism in Drosophila. Flies with STING deletion are sensitive to starvation and oxidative stress, have reduced lipid storage, and downregulated expression of lipid metabolism genes. We found that Drosophila STING interacts with lipid synthesizing enzymes acetyl-CoA carboxylase (ACC) and fatty acid synthase (FASN). ACC and FASN also interact with each other, indicating that all three proteins may be components of a large multi-enzyme complex. The deletion of Drosophila STING leads to disturbed ACC localization and decreased FASN enzyme activity. Together, our results demonstrate a previously undescribed role of STING in lipid metabolism in Drosophila.

cGAS-STING pathway: post-translational modifications and functions in sterile inflammatory diseases

Cytoplasmic microbial and host aberrant DNAs act as danger signals and trigger host immune responses. Upon recognition, the cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) catalyzes the production of a second messenger 2'3'-cGAMP, which activates endoplasmic reticulum (ER)-associated stimulator of interferon (IFN) genes (STING) and ultimately leads to the induction of type I IFNs and inflammatory genes that collectively initiate host immune defense against microbial invasion. Inappropriate activation or suppression of this signaling pathway has been implicated in the development of some autoimmune diseases, sterile inflammation, and cancers. In this review, we describe how the activity of cGAS and STING is regulated by host post-translational modifications and summarize the recent advances of cell-specific cGAS-STING activation and its association in sterile inflammatory diseases. We also discuss key outstanding questions in the field, including how our knowledge of cGAS-STING pathway could be translated into clinical applications.

Two cGAS-like receptors induce antiviral immunity in Drosophila

In mammals, cyclic GMP-AMP (cGAMP) synthase (cGAS) produces the cyclic dinucleotide 2'3'-cGAMP in response to cytosolic DNA and this triggers an antiviral immune response. cGAS belongs to a large family of cGAS/DncV-like nucleotidyltransferases that is present in both prokaryotes¹ and eukaryotes^2-5. In bacteria, these enzymes synthesize a range of cyclic oligonucleotides and have recently emerged as important regulators of phage infections^6-8. Here we identify two cGAS-like receptors (cGLRs) in the insect Drosophila melanogaster. We show that cGLR1 and cGLR2 activate Sting- and NF-κB-dependent antiviral immunity in response to infection with RNA or DNA viruses. cGLR1 is activated by double-stranded RNA to produce the cyclic dinucleotide 3'2'-cGAMP, whereas cGLR2 produces a combination of 2'3'-cGAMP and 3'2'-cGAMP in response to an as-yet-unidentified stimulus. Our data establish cGAS as the founding member of a family of receptors that sense different types of nucleic acids and trigger immunity through the production of cyclic dinucleotides beyond 2'3'-cGAMP.

Microbial Arsenal of Antiviral Defenses. Part II

Bacteriophages or phages are viruses that infect bacterial cells (for the scope of this review we will also consider viruses that infect Archaea). The constant threat of phage infection is a major force that shapes evolution of microbial genomes. To withstand infection, bacteria had evolved numerous strategies to avoid recognition by phages or to directly interfere with phage propagation inside the cell. Classical molecular biology and genetic engineering had been deeply intertwined with the study of phages and host defenses. Nowadays, owing to the rise of phage therapy, broad application of CRISPR-Cas technologies, and development of bioinformatics approaches that facilitate discovery of new systems, phage biology experiences a revival. This review describes variety of strategies employed by microbes to counter phage infection. In the first part defense associated with cell surface, roles of small molecules, and innate immunity systems relying on DNA modification were discussed. The second part focuses on adaptive immunity systems, abortive infection mechanisms, defenses associated with mobile genetic elements, and novel systems discovered in recent years through metagenomic mining.

NAD+-targeting by bacteria: an emerging weapon in pathogenesis

Nicotinamide adenine dinucleotide (NAD+) is a major cofactor in redox reactions in all lifeforms. A stable level of NAD+ is vital to ensure cellular homeostasis. Some pathogens can modulate NAD+ metabolism to their advantage and even utilize or cleave NAD+ from the host using specialized effectors known as ADP-ribosyltransferase toxins and NADases, leading to energy store depletion, immune evasion, or even cell death. This review explores recent advances in the field of bacterial NAD+-targeting toxins, highlighting the relevance of NAD+ modulation as an emerging pathogenesis strategy. In addition, we discuss the role of specific NAD+-targeting toxins in niche colonization and bacterial lifestyle as components of Toxin/Antitoxin systems and key players in inter-bacterial competition. Understanding the mechanisms of toxicity, regulation, and secretion of these toxins will provide interesting leads in the search for new antimicrobial treatments in the fight against infectious diseases.

cGAS-STING: insight on the evolution of a primordial antiviral signaling cassette

Stimulator of interferon genes (STING) functions in the cytosolic DNA-sensing pathway of innate immunity in mammals. It is activated upon binding the cyclic dinucleotide 2′3′-cGAMP, a second messenger produced by the enzyme cyclic guanosine monophosphate–adenosine monophosphate synthase (cGAS), which acts as the receptor for DNA in this pathway, and triggers the expression of interferons and other viral stress-induced genes. The ancient origin of STING in the evolution of animals had been noted, but its primitive function was speculative. We review here recent advances in the remarkable history of cGAS-STING signaling, which establish that cGAS is a member of the family of cGAS/DncV-like nucleotidyltransferases (CD-NTases). In bacteria, CD-NTases synthesize a wide range of cyclic oligonucleotide second messengers in response to bacteriophage infections, which in turn activate a variety of effector proteins to abort phage infection. Among these effectors, some are related to STING, revealing an ancestral function for the cGAS-STING cassette in antiviral host defense. Study of STING signaling in invertebrate animals is consistent with an early acquisition in the history of metazoans of CD-NTase- and STING-encoding genes to counter the universal threat of viruses. In particular, STING-dependent immunity appears to play a previously unsuspected important role in some insects. These discoveries open up interesting perspectives for the use of model organisms to decipher emerging aspects of cGAS-STING biology in mammals, such as the activation of interferon-independent responses or the function and regulation of cGAS in the nucleus.

Interplay of cGAS with chromatin

Recognition of DNA is an evolutionarily highly conserved mechanism of immunity. In mammals, the cGAS-STING pathway plays a central role in coupling DNA sensing to the execution of innate immune mechanisms, both in contexts of infection as well as in noninfectious settings of cellular stress and injury. The indiscriminate ability of double-stranded DNA (dsDNA) to activate cGAS challenges our understanding on how engagement of this pathway is prevented on genomic self-DNA under homeostatic conditions. Here, we review recent discoveries on the regulation of cGAS on chromatin and we discuss implications for cGAS-dependent inflammatory phenotypes. We close by highlighting emerging developments on the role of nuclear cGAS and related open questions for future research.

Functional characterization of the cnidarian antiviral immune response reveals ancestral complexity

Animals evolved a broad repertoire of innate immune sensors and downstream effector cascades for defense against RNA viruses. Yet, this system varies greatly among different bilaterian animals, masking its ancestral state. In this study, we aimed to characterize the antiviral immune response of the cnidarian Nematostella vectensis and decipher the function of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) known to detect viral double-stranded RNA (dsRNA) in bilaterians but activate different antiviral pathways in vertebrates and nematodes. We show that polyinosinic:polycytidylic acid (poly(I:C)), a mimic of long viral dsRNA and a primary ligand for the vertebrate RLR melanoma differentiation-associated protein 5 (MDA5), triggers a complex antiviral immune response bearing features distinctive for both vertebrate and invertebrate systems. Importantly, a well-characterized agonist of the vertebrate RIG-I receptor does not induce a significant transcriptomic response that bears signature of the antiviral immune response, which experimentally supports the results of a phylogenetic analysis indicating clustering of the two N. vectensis RLR paralogs (NveRLRa and NveRLRb) with MDA5. Furthermore, the results of affinity assays reveal that NveRLRb binds poly(I:C) and long dsRNA and its knockdown impairs the expression of putative downstream effector genes including RNA interference components. Our study provides for the first time the functional evidence for the conserved role of RLRs in initiating immune response to dsRNA that originated before the cnidarian–bilaterian split and lay a strong foundation for future research on the evolution of the immune responses to RNA viruses.

Human SLC46A2 Is the Dominant cGAMP Importer in Extracellular cGAMP-Sensing Macrophages and Monocytes

Administration of exogenous CDNs to activate the cGAMP-STING pathway is a promising therapeutic strategy to unleash the full potential of cancer immunotherapy. This strategy mirrors the role of endogenous extracellular cGAMP, an immunotransmitter that is transferred from cancer cells to cGAMP-sensing cells in the host, promoting immunity. However, the CDN import mechanisms used by host cells within tumors remain unknown. Here we identified the protein SLC46A2 as the dominant cGAMP importer in primary human monocytes. Furthermore, we discovered that monocytes and M1-polarized macrophages directly sense tumor-derived extracellular cGAMP in murine tumors. Finally, we demonstrated that SLC46A2 is the dominant cGAMP importer in monocyte-derived macrophages. Together, we provide the first cellular and molecular mechanisms of cGAMP as an immunotransmitter, paving the way for effective STING pathway therapeutics.

The cGAS/STING-TBK1-IRF Regulatory Axis Orchestrates a Primitive Interferon-Like Antiviral Mechanism in Oyster

Interferon (IFN) system is considered as the first defense line against viral infection, and it has been extensively studied in vertebrates from fish to mammals. In invertebrates, Vagos from arthropod and IFN-like protein (CgIFNLP) from Crassostrea gigas appeared to function as IFN-like antiviral cytokines. In the present study, the CgIFNLP protein in hemocytes was observed to increase after Poly (I:C) stimulation. After CgIFNLP was knocked down by RNAi, the mRNA expression of IFN-stimulated genes (CgISGs) was significantly inhibited. Both cyclic GMP-AMP synthase (CgcGAS) and stimulator of interferon gene (CgSTING) identified from oyster were able to recognize the double-stranded nucleic acid [Poly (I:C) and dsDNA] and expressed at high level after Poly (I:C) stimulation. The expression of CgIFNLP and interferon regulatory factors (CgIRF1/8) and the nuclear translocation of CgIRF8 were all suppressed in CgcGAS-RNAi or CgSTING-RNAi oysters after Poly (I:C) stimulation. The expression level of CgSTING and TANK binding kinase1 (CgTBK1) did not decrease in CgcGAS-RNAi oysters. After CgSTING was knocked down, the high expression of CgTBK1 induced by Poly (I:C) was prevented significantly. These results indicated that there was a primitive IFN-like antiviral mechanism dependent on the cGAS/STING–TBK1–IRFs regulatory axis in mollusks, which was different from the classic cGAS–STING–TBK1 signal pathway in mammals.

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

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

Molecular basis of CD-NTase nucleotide selection in CBASS anti-phage defense

cGAS/DncV-like nucleotidyltransferase (CD-NTase) enzymes are signaling proteins that initiate antiviral immunity in animal cells and cyclic-oligonucleotide-based anti-phage signaling system (CBASS) phage defense in bacteria. Upon phage recognition, bacterial CD-NTases catalyze synthesis of cyclic-oligonucleotide signals, which activate downstream effectors and execute cell death. How CD-NTases control nucleotide selection to specifically induce defense remains poorly defined. Here, we combine structural and nucleotide-analog interference-mapping approaches to identify molecular rules controlling CD-NTase specificity. Structures of the cyclic trinucleotide synthase Enterobacter cloacae CdnD reveal coordinating nucleotide interactions and a possible role for inverted nucleobase positioning during product synthesis. We demonstrate that correct nucleotide selection in the CD-NTase donor pocket results in the formation of a thermostable-protein-nucleotide complex, and we extend our analysis to establish specific patterns governing selectivity for each of the major bacterial CD-NTase clades A-H. Our results explain CD-NTase specificity and enable predictions of nucleotide second-messenger signals within diverse antiviral systems.

Bacterial Cyclic Dinucleotides and the cGAS-cGAMP-STING Pathway: A Role in Periodontitis?

Host cells can recognize cytosolic double-stranded DNAs and endogenous second messengers as cyclic dinucleotides-including c-di-GMP, c-di-AMP, and cGAMP-of invading microbes via the critical and essential innate immune signaling adaptor molecule known as STING. This recognition activates the innate immune system and leads to the production of Type I interferons and proinflammatory cytokines. In this review, we (1) focus on the possible role of bacterial cyclic dinucleotides and the STING/TBK1/IRF3 pathway in the pathogenesis of periodontal disease and the regulation of periodontal immune response, and (2) review and discuss activators and inhibitors of the STING pathway as immune response regulators and their potential utility in the treatment of periodontitis. PubMed/Medline, Scopus, and Web of Science were searched with the terms "STING", "TBK 1", "IRF3", and "cGAS"-alone, or together with "periodontitis". Current studies produced evidence for using STING-pathway-targeting molecules as part of anticancer therapy, and as vaccine adjuvants against microbial infections; however, the role of the STING/TBK1/IRF3 pathway in periodontal disease pathogenesis is still undiscovered. Understanding the stimulation of the innate immune response by cyclic dinucleotides opens a new approach to host modulation therapies in periodontology.