Autophagy induced by STING, an unnoticed and primordial function of cGAS

Insights into embryonic chromosomal instability: mechanisms of DNA elimination during mammalian preimplantation development

Mammalian preimplantation embryos often contend with aneuploidy that arose either by the inheritance of meiotic errors from the gametes, or from mitotic mis-segregation events that occurred following fertilization. Regardless of the origin, mis-segregated chromosomes become encapsulated in micronuclei (MN) that are spatially isolated from the main nucleus. Much of our knowledge of MN formation comes from dividing somatic cells during tumorigenesis, but the error-prone cleavage-stage of early embryogenesis is fundamentally different. One unique aspect is that cellular fragmentation (CF), whereby small subcellular bodies pinch off embryonic blastomeres, is frequently observed. CF has been detected in both in vitro and in vivo-derived embryos and likely represents a response to chromosome mis-segregation since it only appears after MN formation. There are multiple fates for MN, including sequestration into CFs, but the molecular mechanism(s) by which this occurs remains unclear. Due to nuclear envelope rupture, the chromosomal material contained within MN and CFs becomes susceptible to double stranded-DNA breaks. Despite this damage, embryos may still progress to the blastocyst stage and exclude chromosome-containing CFs, as well as non-dividing aneuploid blastomeres, from participating in further development. Whether these are attempts to rectify MN formation or eliminate embryos with poor implantation potential is unknown and this review will discuss the potential implications of DNA removal by CF/blastomere exclusion. We will also extrapolate what is known about the intracellular pathways mediating MN formation and rupture in somatic cells to preimplantation embryogenesis and how nuclear budding and DNA release into the cytoplasm may impact overall development.

Plausible Role of Mitochondrial DNA Copy Number in Neurodegeneration-a Need for Therapeutic Approach in Parkinson's Disease (PD)

Parkinson's disease (PD) is an advancing age-associated progressive brain disorder which has various diverse factors, among them mitochondrial dysfunction involves in dopaminergic (DA) degeneration. Aging causes a rise in mitochondrial abnormalities which leads to structural and functional modifications in neuronal activity and cell death in PD. This ends in deterioration of mitochondrial function, mitochondrial alterations, mitochondrial DNA copy number (mtDNA CN) and oxidative phosphorylation (OXPHOS) capacity. mtDNA levels or mtDNA CN in PD have reported that mtDNA depletion would be a predisposing factor in PD pathogenesis. To maintain the mtDNA levels, therapeutic approaches have been focused on mitochondrial biogenesis in PD. The depletion of mtDNA levels in PD can be influenced by autophagic dysregulation, apoptosis, neuroinflammation, oxidative stress, sirtuins, and calcium homeostasis. The current review describes the regulation of mtDNA levels and discusses the plausible molecular pathways in mtDNA CN depletion in PD pathogenesis. We conclude by suggesting further research on mtDNA depletion which might show a promising effect in predicting and diagnosing PD.

The STING-IRF3 Signaling Pathway, Mediated by Endoplasmic Reticulum Stress, Contributes to Impaired Myocardial Autophagic Flux After Ischemia/Reperfusion

ABSTRACT

This study aimed to determine whether endoplasmic reticulum (ER) stress is involved in impaired autophagy after myocardial ischemia/reperfusion (M-I/R) and elucidate the underlying mechanisms. The expression levels of stimulator of interferon gene (STING) and interferon regulatory transcription factor 3 (IRF3) phosphorylation increased in M-I/R heart tissues and hypoxia-treated/reoxygenation-treated H9c2 cells. The ER stress inhibitor 4-phenylbutyric acid (4-PBA) significantly suppressed the stimulation of STING-IRF3 transcription and alleviated cardiac dysfunction caused by M-I/R injury. In addition, 4-PBA reversed ischemia-induced/reperfusion-induced autophagic flux dysfunction, as demonstrated by a decrease in p 62 and LC3 levels. Similarly, the protective effect of STING deficiency on myocardial cell damage was achieved by the recovery of autophagic flux. Conversely, the protective effect of 4-PBA against hypoxia/reoxygenation injury in cardiomyocytes was offset by STING overexpression, wherein the activated STING-IRF3 pathway promoted the expression of Rubicon (a negatively-regulated autophagic molecule) by binding to the Rubicon promoter. Rubicon ablation effectively counteracts the adverse effects of STING overexpression in cardiomyocytes. The data showed that STING-IRF3 signaling of ER stress receptors is particularly important in the progression of physiological M-I/R caused by the inhibition of autophagic flow in vivo and in vitro.

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

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

Porcine cGAS-STING signaling induced autophagy inhibits STING downstream IFN and apoptosis

The innate immune DNA sensing cGAS-STING signaling pathway has been widely recognized for inducing interferons (IFNs) and subsequent antiviral state. In addition to IFN, the cGAS-STING pathway also elicits other cell autonomous immunity events including autophagy and apoptosis. However, the downstream signaling events of this DNA sensing pathway in livestock have not been well defined. Here, we systematically analyzed the porcine STING (pSTING) induced IFN, autophagy and apoptosis, revealed the distinct dynamics of three STING downstream events, and established the IFN independent inductions of autophagy and apoptosis. Further, we investigated the regulation of autophagy on pSTING induced IFN and apoptosis. Following TBK1-IRF3-IFN activation, STING induced Atg5/Atg16L1 dependent autophagy through LIR motifs. In turn, the autophagy likely promoted the pSTING degradation, inhibited both IFN production and apoptosis, and thus restored the cell homeostasis. Therefore, this study sheds lights on the molecular mechanisms of innate immunity in pigs.

Inhibition of cGAS in Paraventricular Nucleus Attenuates Hypertensive Heart Injury Via Regulating Microglial Autophagy

Neuroinflammation in the cardiovascular center plays a critical role in the progression of hypertensive heart disease. And microglial autophagy is involved in the regulation of neuroinflammation. Cyclic GMP-AMP synthase (cGAS), a cytosolic DNA sensor, senses mitochondrial DNA (mtDNA) and regulates autophagy. The detailed mechanisms of central cGAS affects neuroinflammatory response in hypertensive heart disease via regulating autophagy remain unknown. Angiotensin II (Ang II, 1.5 mg·kg⁻¹·12 h⁻¹, 2 weeks) was intraperitoneally injected to induce hypertension in mice. The cGAS-STING pathway was activated in the paraventricular nucleus (PVN) of Ang II-induced hypertensive mice. The contractile dysfunction of heart was alleviated in Ang II-induced hypertensive cGAS^−/− mice. To observe the central effects of cGAS on regulating hypertensive heart disease, the RU.521 (a cGAS inhibitor) was intracisternally infused in hypertensive mice. Intracisternal infusion of the RU.521-alleviated myocardial interstitial fibrosis, cardiomyocyte hypertrophy, and the contractile dysfunction in Ang II-induced hypertensive mice. Intracisternal infusion of RU.521 attenuated the microglial activation, neuroinflammation, sympathetic/parasympathetic activity ratio, and lowered blood pressure. The autophagic flux in the PVN cells was blocked, while intracisternal infusion of RU.521 alleviated this effect in the Ang II-induced hypertensive mice. In vitro, it was found that cGAS-STING activation-induced autophagic flux blockage, while when the impaired autophagic flux was facilitated by rapamycin, an autophagy inducer, the microglial M1 polarization was decreased correspondingly. In conclusion, cGAS induces the inflammatory phenotype of microglia via impairing autophagic flux, thereby participating in neuroinflammation, which leads to sympathetic overactivation in hypertension and further caused hypertensive myocardial injury.

Regulation of innate immune signaling pathways by autophagy in dengue virus infection

Autophagy is not only an intracellular recycling degradation system that maintains cellular homeostasis but is also a component of innate immunity that contributes to host defense against viral infection. The viral components as well as viral particles trapped in autophagosomes can be delivered to lysosomes for degradation. Abundant evidence indicates that dengue virus (DENV) has evolved the potent ability to hijack or subvert autophagy process for escaping host immunity and promoting viral replication. Moreover, autophagy is often required to deliver viral components to pattern recognition receptors signaling for interferon (IFN)-mediated viral elimination. Hence, this review summarizes DENV-induced autophagy, which exhibits dual effects on proviral activity of promoting replication and antiviral activity to eliminating viral particles.

Inhibition of herpes simplex virus 1 by cepharanthine via promoting cellular autophagy through up-regulation of STING/TBK1/P62 pathway

Cepharanthine (CEP), a naturally occurring isoquinoline alkaloid extracted from the genus CEP of the Tetrandrine family, was reported to possess many biological activities such as anti-inflammatory, antitumor, antiviral, and immune-enhancing effects. Nevertheless, the underlying mechanisms of CEP against herpes simplex virus type 1 (HSV-1) are still elusive. In this study, we explored the anti-HSV effects and mechanisms of CEP in vitro. The results showed that CEP possessed a strong inhibitory effect against HSV-1 infection with the TC50 of 5.4 μg/mL, the IC50 of 0.835 μg/mL, and the TI of 6.47. Most importantly, CEP could promote the phosphorylation of STING, TBK1, and P62 and the expression of LC3II without induction of interferon by directly targeting the STING/TBK1/P62 signaling pathways. Electron microscopy showed that autophagy induced by CEP could degrade viral particles and cellular components. RT-PCR results revealed that a sharp reduction of large numbers of virus gene transcription in 16 h after CEP treatment. Furthermore, CEP also reduced the HSV-1 gB and gC transcription. In conclusion, one of the effects of CEP was to promote interferon-independent autophagy through STING mediated signaling.

cGAS-STING Pathway Does Not Promote Autoimmunity in Murine Models of SLE

Detection of DNA is an important determinant of host-defense but also a driver of autoinflammatory and autoimmune diseases. Failure to degrade self-DNA in DNAseII or III(TREX1)-deficient mice results in activation of the cGAS-STING pathway. Deficiency of cGAS or STING in these models ameliorates disease manifestations. However, the contribution of the cGAS-STING pathway, relative to endosomal TLRs, in systemic lupus erythematosus (SLE) is controversial. In fact, STING deficiency failed to rescue, and actually exacerbated, disease manifestations in Fas-deficient SLE-prone mice. We have now extended these observations to a chronic model of SLE induced by the i.p. injection of TMPD (pristane). We found that both cGAS- and STING-deficiency not only failed to rescue mice from TMPD-induced SLE, but resulted in increased autoantibody production and higher proteinuria levels compared to cGAS STING sufficient mice. Further, we generated cGAS^KOFas^lpr mice on a pure MRL/Fas^lpr background using Crispr/Cas9 and found slightly exacerbated, and not attenuated, disease. We hypothesized that the cGAS-STING pathway constrains TLR activation, and thereby limits autoimmune manifestations in these two models. Consistent with this premise, mice lacking cGAS and Unc93B1 or STING and Unc93B1 developed minimal systemic autoimmunity as compared to cGAS or STING single knock out animals. Nevertheless, TMPD-driven lupus in B6 mice was abrogated upon AAV-delivery of DNAse I, implicating a DNA trigger. Overall, this study demonstrated that the cGAS-STING pathway does not promote systemic autoimmunity in murine models of SLE. These data have important implications for cGAS-STING-directed therapies being developed for the treatment of systemic autoimmunity.

CGAS is a micronucleophagy receptor for the clearance of micronuclei

Micronuclei are constantly considered as a marker of genome instability and very recently found to be a trigger of innate immune responses. An increased frequency of micronuclei is associated with many diseases, but the mechanism underlying the regulation of micronuclei homeostasis remains largely unknown. Here, we report that CGAS (cyclic GMP-AMP synthase), a known regulator of DNA sensing and DNA repair, reduces the abundance of micronuclei under genotoxic stress in an autophagy-dependent manner. CGAS accumulates in the autophagic machinery and directly interacts with MAP1LC3B/LC3B in a manner dependent upon its MAP1LC3-interacting region (LIR). Importantly, the interaction is essential for MAP1LC3 recruitment to micronuclei and subsequent clearance of micronuclei via autophagy (micronucleophagy) in response to genotoxic stress. Moreover, in contrast to its DNA sensing function to activate micronuclei-driven inflammation, CGAS-mediated micronucleophagy blunts the production of cyclic GMP-AMP (cGAMP) induced by genotoxic stress. We therefore conclude that CGAS is a receptor for the selective autophagic clearance of micronuclei and uncovered an unprecedented role of CGAS in micronuclei homeostasis to dampen innate immune surveillance.

Abbreviations: ATG: autophagy-related; CGAS: cyclic GMP-AMP synthase; CQ: chloroquine; GABARAP: GABA type A receptor-associated protein; GFP: green fluorescent protein; LAMP1: lysosomal associated membrane protein 1; LAMP2: lysosomal associated membrane protein 2; LIR, MAP1LC3-interacting region; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; NDZ: nocodazole; STING1: stimulator of interferon response cGAMP interactor 1

STING, a promising target for small molecular immune modulator: A review

Stimulator of interferon genes (STING) plays a crucial role in human innate immune system, which is gradually concerned following the emerging immunotherapy. Activated STING induces the production of type I interferons (IFNs) and proinflammatory cytokines through STING-TBK1-IRF3/NF-κB pathway, which could be applied into the treatment of infection, inflammation, and tumorigenesis. Here, we provided a detailed summary of STING from its structure, function and regulation. Especially, we illustrated the canonical or noncanonical cyclic dinucleotides (CDNs) and synthetic small molecules for STING activation or inhibition and their efficacy in related diseases. Importantly, we particularly emphasized the discovery, development and modification of STING agonist or antagonist, attempting to enlighten reader's mind for enriching small molecular modulator of STING. In addition, we summarized biological evaluation methods for the assessment of small molecules activity.

cGAS-STING signaling in cancer immunity and immunotherapy

Recent studies have shown that the innate immune cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway may play an important role in antitumor immunity. Additionally, the cGAS-STING pathway promotes the senescence of cancer cells, induces apoptosis of cancer cells, and increases the protective effect of cytotoxic T cells and natural killer cell-mediated cytotoxicity. We believe that the combination of the cGAS-STING signaling pathway with other therapeutic methods provides a new perspective from which to overcome obstacles in the application of this review. Further, we highlight the antitumor mechanism of the cGAS-STING signaling pathway and the latest advances in monotherapy and combination therapy with related agonists.

The cGAS-STING signaling pathway contributes to the inflammatory response and autophagy in Aspergillus fumigatus keratitis

Fungal keratitis is a serious corneal infection, which can lead to significant visual impairment and blindness. The cGAS-STING signaling pathway has emerged as a key player in innate immunity by sensing of invading pathogens. However, the role of the cGAS-STING pathway in Aspergillus fumigatus (A. fumigatus) keratitis is still unknown. In this study, we showed that the cGAS-STING signaling pathway was activated in human corneal epithelial cells (HCECs) and in mouse corneas infected with A. fumigatus. Knockdown of cGAS reduced A. fumigatus-induced production of pro-inflammatory cytokines, including TNF-α, IL-1β, IL-6, and IFN-β. However, reconstruction of cGAS activity restored the inflammatory response in HCECs infected with A. fumigatus. A specific cGAS inhibitor, RU.521, could also significantly inhibit A. fumigatus-induced inflammatory cytokine expression. In addition, we found that cGAS was indispensable for the autophagy flux evoked by A. fumigatus infection. Moreover, inhibition of cGAS using siRNA or RU.521 alleviated the severity of A. fumigatus keratitis in the mouse cornea. Therefore, the cGAS-STING signaling pathway contributes to the progression of A. fumigatus keratitis and targeting this pathway may provide therapeutic potential.

Role of cGAS-STING signaling pathway in colon cancer

The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon gene (STING) signaling pathway is an important immune response pathway in the cytoplasm, and it is an important mechanism that regulates innate immune and adaptive immune responses. As an important mechanism that detects and responds to pathogens, the cGAS-STING signaling pathway plays a key role in mediating immunity against DNA pathogens and the body's internal immunity against tumors. Clinically, STING activators are often used for tumor treatment. Also, cGAS can act as a tumor prognostic marker. At present, related agonists of cGAS and STING have been used in clinical treatment of colon cancer, but their effects in tumors from other tissues are not clear yet. Thus, their effectiveness and safety are still needed to be further studied.

When STING Meets Viruses: Sensing, Trafficking and Response

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

The STING pathway in response to chlamydial infection

The past decades have witnessed significant progress in discovery and characterize cytosolic DNA sensing and signaling, especially the understanding of the stimulator of interferon genes (STING). This pathway to foreign nucleic acids enables the initiation of robust anti-pathogenic responses to protect the host, and provides a new understanding for therapeutic intervention in a growing infectious disease, including chlamydial infection. Chlamydiae are obligate intracellular pathogenic bacterium causing widespread human diseases such as sexually transmitted infections and respiratory tract infections. Previous studies have shown that IFN production and autophagy are well recognized as being two critical processes induced by STING, and these two processes were also activated during chlamydial infection. In this review, we summarize the important characteristics of the STING activation pathway and recent snapshots about the role of STING in chlamydial infection. Studying the role of STING in chlamydial infection could provide valuable information to further understand the pathogenesis and treatment of chlamydial infection.