DNA-induced 2'3'-cGAMP enhances haplotype-specific human STING cleavage by dengue protease

Loops Mediate Agonist-Induced Activation of the Stimulator of Interferon Genes Protein

The stimulator of interferon genes (STING) is an important therapeutic target for cancer diseases. The activated STING recruits downstream tank-binding kinase 1 (TBK1) to trigger several important immune responses. However, the molecular mechanism of how agonist molecules mediate the STING-TBK1 interactions remains elusive. Here, we performed molecular dynamics simulations to capture the conformational changes of STING and TBK1 upon agonist binding. Our simulations revealed that multiple helices (α5-α7) and especially three loops (loop 6, loop 8, and C-terminal tail) of STING participated in the allosteric mediation of the STING-TBK1 interactions. Consistent results were also observed in the simulations of the constitutive activating mutant of STING (R284S). We further identified α5 as a key region in this agonist-induced activation mechanism of STING. Free-energy perturbation calculations of multiple STING agonists demonstrated that an alkynyl group targeting α5 is a determinant for agonist activities. These results not only offer deeper insights into the agonist-induced allosteric mediation of STING-TKB1 interactions but also provide a guidance for future drug development of this important therapeutic target.

SIRT2 negatively regulates the cGAS-STING pathway by deacetylating G3BP1

SIRT2, a cytoplasmic member of the Sirtuin family, has important roles in immunity and inflammation. However, its function in regulating the response to DNA virus infection remains elusive. Here, we find that SIRT2 is a unique regulator among the Sirtuin family that negatively modulates the cGAS-STING-signaling pathway. SIRT2 is down-regulated after Herpes simplex virus-1 (HSV-1) infection, and SIRT2 deficiency markedly elevates the expression levels of type I interferon (IFN). SIRT2 inhibits the DNA binding ability and droplet formation of cGAS by interacting with and deacetylating G3BP1 at K257, K276, and K376, leading to the disassembly of the cGAS-G3BP1 complex, which is critical for cGAS activation. Administration of AGK2, a selective SIRT2 inhibitor, protects mice from HSV-1 infection and increases the expression of IFN and IFN-stimulated genes. Our study shows that SIRT2 negatively regulates cGAS activation through G3BP1 deacetylation, suggesting a potential antiviral strategy by modulating SIRT2 activity.

Chemical Biology Perspectives on STING Agonists as Tumor Immunotherapy

Stimulator of interferon genes (STING) is a crucial adaptor protein in the innate immune response. STING activation triggers cytokine secretion, including type I interferon and initiates T cell-mediated adaptive immunity. The activated immune system converts "cold tumors" into "hot tumors" that are highly responsive to T cells by recruiting them to the tumor microenvironment, ultimately leading to potent and long-lasting antitumor effects. Unlike most immune checkpoint inhibitors, STING agonists represent a groundbreaking class of innate immune agonists that hold great potential for effectively targeting various cancer populations and are poised to become a blockbuster in tumor immunotherapy. This review will focus on the correlation between the STING signaling pathway and tumor immunity, as well as explore the impact of STING activation on other biological processes. Ultimately, we will summarize the development and optimization of STING agonists from a medicinal chemistry perspective, evaluate their potential in cancer therapy, and identify possible challenges for future advancement.

Flavonoids derived from licorice suppress LPS-induced acute lung injury in mice by inhibiting the cGAS-STING signaling pathway

In recent years, we have found that the dysregulation of the cyclic-GMP-AMP synthase (cGAS)‒stimulator of interferon genes (STING) pathway leads to the development of immune and inflammatory diseases, therefore, finding compounds that can specifically regulate this pathway is essential for effective regulation of the immune pathway for addressing inflammatory diseases. Licorice flavonoids (LFs), are active ingredients extracted from the Chinese herb licorice, which has been reported to have strong anti-inflammatory activity in previous studies. Here, we report that LFs inhibit the activation of the cGAS-STING pathway evidenced by the inhibition of the expression of type I interferons and related downstream genes such as interferon-stimulated gene 15 (ISG15) and C-X-C motif chemokine ligand 10 (CXCL10), as well as inflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α). Notably, LFs markedly improve the LPS-induced acute lung injury by inhibiting the excessive activation of cGAS-STING signaling pathway. Mechanistically, LFs treatment leads to the blocking of 2'3'-cyclic GMP-AMP (cGAMP) synthesis resulting in an inhibition of the activation of the cGAS-STING pathway. Our results indicate that LFs is a specific inhibitor of the cGAS-STING pathway, which is suggested to be a potential candidate for the treatment of cGAS-STING pathway-mediated inflammatory diseases.

Defective Mitochondrial Quality Control during Dengue Infection Contributes to Disease Pathogenesis

Mitochondrial fitness is governed by mitochondrial quality control pathways comprising mitochondrial dynamics and mitochondrial-selective autophagy (mitophagy). Disruption of these processes has been implicated in many human diseases, including viral infections. Here, we report a comprehensive analysis of the effect of dengue infection on host mitochondrial homeostasis and its significance in dengue disease pathogenesis. Despite severe mitochondrial stress and injury, we observed that the pathways of mitochondrial quality control and mitochondrial biogenesis are paradoxically downregulated in dengue-infected human liver cells. This leads to the disruption of mitochondrial homeostasis and the onset of cellular injury and necrotic death in the infected cells. Interestingly, dengue promotes global autophagy but selectively disrupts mitochondrial-selective autophagy (mitophagy). Dengue downregulates the expression of PINK1 and Parkin, the two major proteins involved in tagging the damaged mitochondria for elimination through mitophagy. Mitophagy flux assays also suggest that Parkin-independent pathways of mitophagy are also inactive during dengue infection. Dengue infection also disrupts mitochondrial biogenesis by downregulating the master regulators PPARγ and PGC1α. Dengue-infected cells release mitochondrial damage-associated molecular patterns (mtDAMPs) such as mitochondrial DNA into the cytosol and extracellular milieu. Furthermore, the challenge of naive immune cells with culture supernatants from dengue-infected liver cells was sufficient to trigger proinflammatory signaling. In correlation with our in vitro observations, dengue patients have high levels of cell-free mitochondrial DNA in their blood in proportion to the degree of thrombocytopenia. Overall, our study shows how defective mitochondrial homeostasis in dengue-infected liver cells can drive dengue disease pathogenesis. IMPORTANCE Many viruses target host cell mitochondria to create a microenvironment conducive to viral dissemination. Dengue virus also exploits host cell mitochondria to facilitate its viral life cycle. Dengue infection of liver cells leads to severe mitochondrial injury and inhibition of proteins that regulate mitochondrial quality control and biogenesis, thereby disrupting mitochondrial homeostasis. A defect in mitochondrial quality control leads to the accumulation of damaged mitochondria and promotes cellular injury. This leads to the release of mitochondrial damage-associated molecular patterns (mt-DAMPs) into the cell cytoplasm and extracellular milieu. These mt-DAMPs activate the naive immune cells and trigger proinflammatory signaling, leading to the release of cytokines and chemokines, which may trigger systemic inflammation and contribute to dengue disease pathogenesis. In correlation with this, we observed high levels of cell-free mitochondrial DNA in dengue patient blood. This study provides insight into how the disruption of mitochondrial quality control in dengue-infected cells can trigger inflammation and drive dengue disease pathogenesis.

Flaviviridae Nonstructural Proteins: The Role in Molecular Mechanisms of Triggering Inflammation

Members of the Flaviviridae family are posing a significant threat to human health worldwide. Many flaviviruses are capable of inducing severe inflammation in humans. Flaviviridae nonstructural proteins, apart from their canonical roles in viral replication, have noncanonical functions strongly affecting antiviral innate immunity. Among these functions, antagonism of type I IFN is the most investigated; meanwhile, more data are accumulated on their role in the other pathways of innate response. This review systematizes the last known data on the role of Flaviviridae nonstructural proteins in molecular mechanisms of triggering inflammation, with an emphasis on their interactions with TLRs and RLRs, interference with NF-κB and cGAS-STING signaling, and activation of inflammasomes.

Glycosylation and S-palmitoylation regulate SARS-CoV-2 spike protein intracellular trafficking

Post-translational modifications (PTMs), such as glycosylation and palmitoylation, are critical to protein folding, stability, intracellular trafficking, and function. Understanding regulation of PTMs of SARS-CoV-2 spike (S) protein could help the therapeutic drug design. Herein, the VSV vector was used to produce SARS-CoV-2 S pseudoviruses to examine the roles of the ⁶¹¹LYQD⁶¹⁴ and cysteine-rich motifs in S protein maturation and virus infectivity. Our results show that ⁶¹¹LY⁶¹² mutation alters S protein intracellular trafficking and reduces cell surface expression level. It also changes S protein glycosylation pattern and decreases pseudovirus infectivity. The S protein contains four cysteine-rich clusters with clusters I and II as the main palmitoylation sites. Mutations of clusters I and II disrupt S protein trafficking from ER-to-Golgi, suppress pseudovirus production, and reduce spike-mediated membrane fusion activity. Taken together, glycosylation and palmitoylation orchestrate the S protein maturation processing and are critical for S protein-mediated membrane fusion and infection.

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⁶¹¹LY⁶¹² mutation alters the glycosylation pattern of the SARS-CoV-2 S protein

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⁶¹¹LY⁶¹² mutation reduces S protein surface expression level

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Palmitoylation targets mature S protein to the Golgi and plasma membrane

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Palmitoylation is required for pseudovirus and SARS-CoV-2 production

How Fragile We Are: Influence of Stimulator of Interferon Genes (STING) Variants on Pathogen Recognition and Immune Response Efficiency

The stimulator of interferon genes (STING) protein is a cornerstone of the human immune response. Its activation by cGAMP in the presence of cytosolic DNA stimulates the production of type I interferons and inflammatory cytokines. In the human population, several STING variants exist and exhibit dramatic differences in their activity, impacting the efficiency of the host defense against infections. Understanding the molecular mechanisms of these variants opens perspectives for personalized medicine treatments against diseases such as viral infections, cancers, or autoinflammatory diseases. Through microsecond-scale molecular modeling simulations, contact analyses, and machine learning techniques, we reveal the dynamic behavior of four STING variants (wild type, G230A, R293Q, and G230A/R293Q) and rationalize the variability of efficiency observed experimentally. Our results show that the decrease in STING activity is linked to a stiffening of key structural elements of the binding cavity together with changes in the interaction patterns within the protein.

Metformin inactivates the cGAS-STING pathway through autophagy and suppresses senescence in nucleus pulposus cells

Intervertebral disc degeneration (IVDD) is a complex process involving many factors, among which excessive senescence of nucleus pulposus cells (NPCs) is considered to be the main factor. Our previous study found that metformin may inhibit senescence in nucleus pulposus cells; however, its working mechanism is still largely unknown. In the current study, we found that metformin may inactivate cGAS-STING pathway during oxidative stress. Knock-down of STING may further suppress senescence, indicating metformin may exert its effect through cGAS-STING pathway. Damaged DNA is a major inducer of the activation of cGAS-STING pathway. Mechanistically, our study showed that DNA damage was reduced during metformin treatment; however, suppression of autophagy by 3-methyladenine (3MA) may compromise the effect of metformin on DNA damage. The in vivo study also showed that 3MA may recede the therapeutic effect of metformin on IVDD. Taken together, our results reveal that metformin may suppress senescence via inactivating the cGAS-STING pathway through autophagy, implying the new application of metformin in cGAS-STING pathway related diseases.

Generation and Characterization of Human-Mouse STING Chimeras That Allow DENV Replication in Mouse Cells

Our group was the first to describe direct antagonism of the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway by dengue virus (DENV) in human cells, and here, we report new findings on the characterization of the interaction between the DENV nonstructural protein 2B (NS2B)-NS3 (NS2B3) protease complex and STING. We demonstrate interactions between NS2B and the transmembrane domains of human STING and between NS3 and a portion of the cytoplasmic C-terminal domain of human STING. One significant obstacle we face today in the DENV field is the lack of small animal models available that can effectively recapitulate DENV pathogenesis in the early events of infection. The existing mouse models are either immunocompromised mice lacking interferon (IFN) receptors or "humanized" mice reconstituted with human stem cells. However, both approaches fail to capture important aspects of human pathogenesis because they lack critical innate immunity components or have deficiencies in immune cell development or maintenance. As an important step toward developing an immunocompetent mouse model for DENV, we have generated two chimeric human-mouse STING constructs that have promise in retaining both cleavability by NS2B3 and signaling capacity in the mouse. IMPORTANCE This article characterizes the interaction between human STING and DENV viral protease complex NS2B3 by constructing serial deletion mutants of STING. Our findings suggest that DENV nonstructural protein NS2B interacts with the transmembrane domains and NS3 with the C-terminal cyclic dinucleotide binding domain of human STING. Furthermore, as there exists no ideal immunocompetent murine model that can simultaneously support robust DENV replication and recapitulate the clinical manifestation of dengue disease observed in humans, we expressed and characterized two promising human-mouse chimeric STING constructs that can be used for developing a relevant transgenic mouse model to study dengue in the future. Both constructs can activate normal IFN responses in the overexpression system and be cleaved under infection conditions. We believe our findings offer a roadmap to the further development of a murine model that can greatly facilitate antiviral discoveries and vaccine research for DENV.

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.

Running With Scissors: Evolutionary Conflicts Between Viral Proteases and the Host Immune System

Many pathogens encode proteases that serve to antagonize the host immune system. In particular, viruses with a positive-sense single-stranded RNA genome [(+)ssRNA], including picornaviruses, flaviviruses, and coronaviruses, encode proteases that are not only required for processing viral polyproteins into functional units but also manipulate crucial host cellular processes through their proteolytic activity. Because these proteases must cleave numerous polyprotein sites as well as diverse host targets, evolution of these viral proteases is expected to be highly constrained. However, despite this strong evolutionary constraint, mounting evidence suggests that viral proteases such as picornavirus 3C, flavivirus NS3, and coronavirus 3CL, are engaged in molecular 'arms races' with their targeted host factors, resulting in host- and virus-specific determinants of protease cleavage. In cases where protease-mediated cleavage results in host immune inactivation, recurrent host gene evolution can result in avoidance of cleavage by viral proteases. In other cases, such as recently described examples in NLRP1 and CARD8, hosts have evolved 'tripwire' sequences that mimic protease cleavage sites and activate an immune response upon cleavage. In both cases, host evolution may be responsible for driving viral protease evolution, helping explain why viral proteases and polyprotein sites are divergent among related viruses despite such strong evolutionary constraint. Importantly, these evolutionary conflicts result in diverse protease-host interactions even within closely related host and viral species, thereby contributing to host range, zoonotic potential, and pathogenicity of viral infection. Such examples highlight the importance of examining viral protease-host interactions through an evolutionary lens.

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.

Recent Insights Into the Molecular Mechanism of Toll-Like Receptor Response to Dengue Virus Infection

Dengue is the most prevalent and rapidly spreading mosquito-borne viral disease caused by dengue virus (DENV). Recently, DENV has been affecting humans within an expanding geographic range due to the warming of the earth. Innate immune responses play a significant role in antiviral defense, and Toll-like receptors (TLRs) are key regulators of innate immunity. Therefore, a detailed understanding of TLR and DENV interactions is important for devising therapeutic and preventive strategies. Several studies have indicated the ability of DENV to modulate the TLR signaling pathway and host immune response. Vaccination is considered one of the most successful medical interventions for preventing viral infections. However, only a partially protective dengue vaccine, the first licensed dengue vaccine CYD-TDV, is available in some dengue-endemic countries to protect against DENV infection. Therefore, the development of a fully protective, durable, and safe DENV vaccine is a priority for global health. Here, we demonstrate the progress made in our understanding of the host response to DENV infection, with a particular focus on TLR response and how DENV avoids the response toward establishing infection. We also discuss dengue vaccine candidates in late-stage development and the issues that must be overcome to enable their success.

STING promotes senescence, apoptosis, and extracellular matrix degradation in osteoarthritis via the NF-κB signaling pathway

Damaged deoxyribonucleic acid (DNA) is a primary pathologic factor for osteoarthritis (OA); however, the mechanism by which DNA damage drives OA is unclear. Previous research demonstrated that the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) participates in DNA damage response. As a result, the current study aimed at exploring the role STING, which is the major effector in the cGAS-STING signaling casacde, in OA progress in vitro, as well as in vivo. In this study, the expression of STING was evaluated in the human and mouse OA tissues, and in chondrocytes exposed to interleukin-1 beta (IL-1β). The influences of STING on the metabolism of the extracellular matrix (ECM), apoptosis, and senescence, were assessed in STING overexpressing and knocking-down chondrocytes. Moreover, the NF-κB-signaling casacde and its role in the regulatory effects of STING on ECM metabolism, apoptosis, and senescence were explored. The STING knockdown lentivirus was intra-articularly injected to evaluate its therapeutic impact on OA in mice in vivo. The results showed that the expression of STING was remarkably elevated in the human and mouse OA tissues and in chondrocytes exposed to IL-1β. Overexpression of STING promoted the expression of MMP13, as well as ADAMTS5, but suppressed the expression of Aggrecan, as well as Collagen II; it also enhanced apoptosis and senescence in chondrocytes exposed to and those untreated with IL-1β. The mechanistic study showed that STING activated NF-κB signaling cascade, whereas the blockage of NF-κB signaling attenuated STING-induced apoptosis and senescence, and ameliorated STING-induced ECM metabolism imbalance. In in vivo study, it was demonstrated that STING knockdown alleviated destabilization of the medial meniscus-induced OA development in mice. In conclusion, STING promotes OA by activating the NF-κB signaling cascade, whereas suppression of STING may provide a novel approach for OA therapy.

Innate Immune DNA Sensing of Flaviviruses

Flaviviruses are arthropod-borne RNA viruses that have been used extensively to study host antiviral responses. Often selected just to represent standard single-stranded positive-sense RNA viruses in early studies, the Flavivirus genus over time has taught us how truly unique it is in its remarkable ability to target not just the RNA sensory pathways but also the cytosolic DNA sensing system for its successful replication inside the host cell. This review summarizes the main developments on the unexpected antagonistic strategies utilized by different flaviviruses, with RNA genomes, against the host cyclic GAMP synthase (cGAS)/stimulator of interferon genes (STING) cytosolic DNA sensing pathway in mammalian systems. On the basis of the recent advancements on this topic, we hypothesize that the mechanisms of viral sensing and innate immunity are much more fluid than what we had anticipated, and both viral and host factors will continue to be found as important factors contributing to the host innate immune system in the future.