The N terminus of cGAS de-oligomerizes the cGAS:DNA complex and lifts the DNA size restriction of core-cGAS activity

The Dengue virus protease NS2B3 cleaves cyclic GMP-AMP synthase to suppress cGAS activation

Dengue virus (DENV) is one of the most prevalent mosquito-transmitted human viruses that causes significant morbidity and mortality worldwide. To persist in the cell and consequently cause disease, DENV is evolved with mechanisms to suppress the induction of type I interferons by antagonizing cGAS-STING signaling. Using recombinant proteins and in vitro cleavage assays, we have shown that the DENV protease NS2B3 is capable of cleaving cGAS in the N-terminal region without disrupting the C-terminal catalytic center. This generates two major cleavage products: cleavage product N-terminal (CP-N) and cleavage product C-terminal (CP-C). We observed reduction in DNA-binding affinity of CP-C as compared to full-length cGAS. Reduction in DNA-binding affinity is also correlated with the decrease in enzymatic activity of CP-C. CP-N, on the other hand, has almost comparable DNA-binding ability as that of the full-length cGAS. In fact, CP-N competitively inhibits cyclic GMP-AMP production by both full-length cGAS and CP-C. We hypothesize that high DNA-binding affinity of CP-N enables it to sequester the DNA from CP-C and noncleaved full-length cGAS and thus reduces the rate of enzyme activation and cyclic GMP-AMP synthesis. Furthermore, we found that NS2B3 physically interacts with full-length cGAS and CP-C, laying the basis for their shuttling to and eventual degradation in the autophagosome. Overall, our study highlights a multifaceted and effective strategy by which an RNA virus antagonizes cGAS-STING signaling which may be useful for the design of antivirals targeting viral proteases.

Mechanisms of phase-separation-mediated cGAS activation revealed by dcFCCS

Cyclic GMP-AMP synthase (cGAS), as a DNA sensor, plays an important role in cGAS-STING pathway, which further induces expression of type I interferon as the innate immune response. Previous studies reported that liquid-liquid phase separation (LLPS) driven by cGAS and long DNA is essential to promote catalytic activity of cGAS to produce a second messenger, cyclic GMP-AMP (cGAMP). However, the molecular mechanism of LLPS promoting cGAS activity is still unclear. Here, we applied dual-color fluorescence cross-correlation spectroscopy (dcFCCS), a highly sensitive and quantitative method, to characterize phase separation driven by cGAS and DNA from miscible individual molecule to micronscale. Thus, we captured nanoscale condensates formed by cGAS at close-to-physiological concentration and quantified their sizes, molecular compositions and binding affinities within condensates. Our results pinpointed that interactions between DNA and cGAS at DNA binding sites A, B, and C and the dimerization of cGAS are the fundamental molecular basis to fully activate cGAS in vitro. Due to weak binding constants of these sites, endogenous cGAS cannot form stable interactions at these sites, leading to no activity in the absence of LLPS. Phase separation of cGAS and DNA enriches cGAS and DNA by 2 to 3 orders of magnitude to facilitate these interactions among cGAS and DNA and to promote cGAS activity as an on/off switch. Our discoveries not only shed lights on the molecular mechanisms of phase-separation-mediated cGAS activation, but also guided us to engineer a cGAS fusion, which can be activated by 15 bp short DNA without LLPS.

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.

Mn2+ Directly Activates cGAS and Structural Analysis Suggests Mn2+ Induces a Noncanonical Catalytic Synthesis of 2'3'-cGAMP

DNA binding allosterically activates the cytosolic DNA sensor cGAS (cyclic GMP-AMP [cGAMP] synthase) to synthesize 2'3'-cGAMP, using Mg²⁺ as the metal cofactor that catalyzes two nucleotidyl-transferring reactions. We previously found that Mn²⁺ potentiates cGAS activation, but the underlying mechanism remains unclear. Here, we report that Mn²⁺ directly activates cGAS. Structural analysis reveals that Mn²⁺-activated cGAS undergoes globally similar conformational changes to DNA-activated cGAS but forms a unique η1 helix to widen the catalytic pocket, allowing substrate entry and cGAMP synthesis. Strikingly, in Mn²⁺-activated cGAS, the linear intermediates pppGpG and pGpA take an inverted orientation in the active pocket, suggesting a noncanonical but accelerated cGAMP cyclization without substrate flip-over. Moreover, unlike the octahedral coordination around Mg²⁺, the two catalytic Mn²⁺ are coordinated by triphosphate moiety of the inverted substrate, independent of the catalytic triad residues. Our findings thus uncover Mn²⁺ as a cGAS activator that initiates noncanonical 2'3'-cGAMP synthesis.

DNA-binding mechanisms of human and mouse cGAS: a comparative MD and MM/GBSA study

Cyclic GMP-AMP synthase (cGAS) can detect the presence of cytoplasmic DNA and activate the innate immune system via the cGAS-STING pathway. Although several structures of cGAS-DNA complexes were resolved recently, the molecular mechanism of cGAS in its recognition of DNA has not yet been fully understood. In order to reveal the subtle differences between human and mouse cGAS in terms of their DNA-binding mechanisms, four systems, both human and mouse cGAS in complex with two different DNA sequences of equal length, were studied by molecular dynamics simulations and molecular mechanics/generalized Born surface area analysis. Several residues, including ARG176/ARG161, ARG195/ARG180, ASN210/ASN196, LYS384/LYS372, CYM397/CYM385, LYS403/LYS391, LYS407/LYS395, and LYS411/LYS399, were identified to be the common key residues in the recognition of DNA for cGAS in both humans and mice. In addition, four residue pairs LYS173/ARG158, ASP177/LYS162, CYS199/LYS184, and GLU398/SER387 were suggested to be the major residues that make human cGAS and mouse cGAS different in terms of their binding to DNA. Besides the well-known zinc-thumb domain, two residues at the kink of the spine helix were also proposed for the first time to be the major binding motifs in cGAS-DNA interaction.

Protein Dynamics in Cytosolic DNA-Sensing Antiviral Innate Immune Signaling Pathways

Antiviral innate immunity works as the first line of host defense against viral infection. Pattern recognition receptors (PRRs) and adaptor proteins involved in the innate immune signaling pathways play critical roles in controlling viral infections via the induction of type I interferon and its downstream interferon-stimulated genes. Dynamic changes of adaptor proteins contribute to precise regulation of the activation and shut-off of signaling transduction, though numerous complex processes are involved in achieving dynamic changes to various proteins of the host and viruses. In this review, we will summarize recent progress on the trafficking patterns and conformational transitions of the adaptors that are involved in the antiviral innate immune signaling pathway during viral DNA sensing. Moreover, we aim to dissect the relationships between protein dynamics and DNA-sensing antiviral innate immune responses, which will reveal the underlying mechanisms controlling protein activity and maintaining cell homeostasis. By comprehensively revealing protein dynamics in cytosolic DNA-sensing antiviral innate immune signaling pathways, we will be able to identify potential new targets for the therapies of certain autoimmune diseases.

Rational Reprogramming of O-Methylation Regioselectivity for Combinatorial Biosynthetic Tailoring of Benzenediol Lactone Scaffolds

O-Methylation modulates the pharmacokinetic and pharmacodynamic (PK/PD) properties of small-molecule natural products, affecting their bioavailability, stability, and binding to targets. Diversity-oriented combinatorial biosynthesis of new chemical entities for drug discovery and optimization of known bioactive scaffolds during drug development both demand efficient O-methyltransferase (OMT) biocatalysts with considerable substrate promiscuity and tunable regioselectivity that can be deployed in a scalable and sustainable manner. Here we demonstrate efficient total biosynthetic and biocatalytic platforms that use a pair of fungal OMTs with orthogonal regiospecificity to produce unnatural O-methylated benzenediol lactone polyketides. We show that rational, structure-guided active-site cavity engineering can reprogram the regioselectivity of these enzymes. We also characterize the interplay of engineered regioselectivity with substrate plasticity. These findings will guide combinatorial biosynthetic tailoring of unnatural products toward the generation of diverse chemical matter for drug discovery and the PK/PD optimization of bioactive scaffolds for drug development.

Acetylation Blocks cGAS Activity and Inhibits Self-DNA-Induced Autoimmunity

The presence of DNA in the cytoplasm is normally a sign of microbial infections and is quickly detected by cyclic GMP-AMP synthase (cGAS) to elicit anti-infection immune responses. However, chronic activation of cGAS by self-DNA leads to severe autoimmune diseases for which no effective treatment is available yet. Here we report that acetylation inhibits cGAS activation and that the enforced acetylation of cGAS by aspirin robustly suppresses self-DNA-induced autoimmunity. We find that cGAS acetylation on either Lys384, Lys394, or Lys414 contributes to keeping cGAS inactive. cGAS is deacetylated in response to DNA challenges. Importantly, we show that aspirin can directly acetylate cGAS and efficiently inhibit cGAS-mediated immune responses. Finally, we demonstrate that aspirin can effectively suppress self-DNA-induced autoimmunity in Aicardi-Goutières syndrome (AGS) patient cells and in an AGS mouse model. Thus, our study reveals that acetylation contributes to cGAS activity regulation and provides a potential therapy for treating DNA-mediated autoimmune diseases.

The allosteric activation of cGAS underpins its dynamic signaling landscape

Cyclic G/AMP synthase (cGAS) initiates type-1 interferon responses against cytosolic double-stranded (ds)DNA, which range from antiviral gene expression to apoptosis. The mechanism by which cGAS shapes this diverse signaling landscape remains poorly defined. We find that substrate-binding and dsDNA length-dependent binding are coupled to the intrinsic dimerization equilibrium of cGAS, with its N-terminal domain potentiating dimerization. Notably, increasing the dimeric fraction by raising cGAS and substrate concentrations diminishes duplex length-dependent activation, but does not negate the requirement for dsDNA. These results demonstrate that reaction context dictates the duplex length dependence, reconciling competing claims on the role of dsDNA length in cGAS activation. Overall, our study reveals how ligand-mediated allostery positions cGAS in standby, ready to tune its signaling pathway in a switch-like fashion.

The human immune system protects the body from various threats such as damaged cells or invading microbes. Many of these threats can move molecules of DNA, which are usually only found within a central compartment in the cell known as the nucleus, to the surrounding area, the cytoplasm.

An enzyme called cGAS searches for DNA in the cytoplasm of human cells. When DNA binds to cGAS it activates the enzyme to convert certain molecules (referred to as ‘substrates’) into another molecule (the ‘signal’) that triggers various immune responses to protect the body against the threat. To produce the signal, two cGAS enzymes need to work together as a single unit called a dimer.

The length of DNA molecules in the cytoplasm of cells can vary widely. It was initially thought that DNA molecules of any length binding to cGAS could activate the enzyme to a similar degree, but later studies demonstrated that this is not the case. However, it remains unclear how the length of the DNA could affect the activity of the enzyme, or why some of the earlier studies reported different findings.

Hooy and Sohn used biochemical approaches to study the human cGAS enzyme. The experiments show that cGAS can form dimers even when no DNA is present. However, when DNA bound to cGAS, the enzyme was more likely to form dimers. Longer DNA molecules were better at promoting cGAS dimers to form than shorter DNA molecules. The binding of substrates to cGAS also made it more likely that the enzyme would form dimers. These findings suggest that inside cells cGAS is primed to trigger a switch-like response when it detects DNA in the cytoplasm.

The work of Hooy and Sohn establishes a simple set of rules to predict how cGAS might respond in a given situation. Such information may aid in designing and tailoring efforts to regulate immune responses in human patients, and may provide insight into why the body responds to biological threats in different ways.

Radiation-Induced Chromosomal Aberrations and Immunotherapy: Micronuclei, Cytosolic DNA, and Interferon-Production Pathway

Radiation-induced chromosomal aberrations represent an early marker of late effects, including cell killing and transformation. The measurement of cytogenetic damage in tissues, generally in blood lymphocytes, from patients treated with radiotherapy has been studied for many years to predict individual sensitivity and late morbidity. Acentric fragments are lost during mitosis and create micronuclei (MN), which are well correlated to cell killing. Immunotherapy is rapidly becoming a most promising new strategy for metastatic tumors, and combination with radiotherapy is explored in several pre-clinical studies and clinical trials. Recent evidence has shown that the presence of cytosolic DNA activates immune response via the cyclic GMP-AMP synthase/stimulator of interferon genes pathway, which induces type I interferon transcription. Cytosolic DNA can be found after exposure to ionizing radiation either as MN or as small fragments leaking through nuclear envelope ruptures. The study of the dependence of cytosolic DNA and MN on dose and radiation quality can guide the optimal combination of radiotherapy and immunotherapy. The role of densely ionizing charged particles is under active investigation to define their impact on the activation of the interferon pathway.