Trends in Immunology
ReviewSpecial Issue: Innate Sensing Across KingdomsEvolutionary Origins of cGAS-STING Signaling
Section snippets
Innate Immune Detection of Nucleic Acids
Sensing of nucleic acids is a central strategy by which the innate immune system detects many pathogens [1]. Nucleic acid sensors appear to be especially critical for the innate detection of viruses, as this class of pathogen is synthesized from host cell components, and therefore lacks many pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharide, flagellin or peptidoglycan that can be used to detect bacteria. Several nucleic acid sensors have been described, including TLR9,
The cGAS-STING Pathway in Vertebrates
Cyclic GMP–AMP synthase (cGAS), and its homologues in the OAS family, are atypical among innate immune sensors in being both receptors and biosynthetic enzymes 15, 16. Upon binding to dsDNA via its zinc-ribbon domain, the active site of cGAS rearranges and becomes competent to synthesize a second messenger, cyclic GMP–AMP (cGAMP), an unusual member of a small family of second messengers called cyclic dinucleotides (CDNs) 15, 17, 18, 19, 20 (Figure 1). CDNs were first described in bacteria [21],
STING Binding to CDNs Predates the Evolution of Animals
Bioinformatic analyses have revealed the presence of STING homologs in most animal phyla and even in unicellular choanoflagellates 61, 62, 63, 64 (Figure 2). Remarkably, the ancient origin of STING is not only evident in its genetic sequence, but also in its molecular function. Indeed, analysis of STING homologs from diverse metazoans revealed a deeply ancestral ability to bind CDNs [52]. The most divergent CDN-binding STING homolog identified was in the starlet sea anemone, Nematostella
The CTT of STING May Have Emerged in Vertebrates
Despite extensive conservation of STING across animal phyla, the CTT of STING is notably only apparently present in vertebrates (Figure 2). The CTT (residues 341–379 of human STING) is critical for TBK1 recruitment and IRF3 phosphorylation in response to STING activation, and therefore for type I IFN signaling 32, 71. The CTT also appears critical for IFN induction by zebrafish STING [72]. As expected from its lack of a CTT, nvSTING is unable to induce an IFN-β reporter in mammalian cells in
Evolutionary Loss and Diversification of the cGAS–STING Pathway
While STING is clearly an ancient protein, it is not universally conserved (Figure 2). For example, STING is absent from the genomes of all Nematoda that have been searched [61]. Interestingly, most components of vertebrate immune signaling (e.g., NF-κB) are also absent from nematodes [64]. Although insect genomes (e.g., those of flies, bumblebees, wasps, and silkworms) do contain STING homologs, there is at present no evidence that these homologs bind CDNs [52]. While these negative results do
Ancestral Origins and Function of cGAS
Like STING, putative cGAS homologs have been identified in a variety of metazoan lineages 61, 62, 63. However, whether these homologs exhibit a conserved ability to sense DNA and synthesize CDNs is difficult to predict, as homology is inferred based on the presence of a Mab-21 domain that exists in numerous other nucleotidyltransferase (NTase) family proteins. Many of these Mab-21 family members are required during embryonic development and are not known to produce CDNs 80, 81, 82. Thus,
Evolution of dsDNA Binding by cGAS
While functional cGAS-like enzymes are present in highly divergent species, it is currently believed that the key ability to bind and detect dsDNA is a vertebrate innovation. Vertebrate cGAS contains a unique zinc-ribbon domain that is required for DNA binding and cGAMP synthesis in response to cytosolic DNA 15, 17, 18, 19, 20 (Figure 2). As expected due to the absence of this domain, nv-cGAS does not bind, nor is it activated by, dsDNA in vitro [52]. DncV, a bacterial cGAS-like enzyme from
Evolution of Signaling Components Downstream of STING
Although the major antiviral output of STING signaling in mammals is the transcription of type I IFN genes, these genes have only been identified in vertebrates 89, 90, 91 (Figure 2). IRF3, the transcription factor downstream of STING activation that leads to type I IFN transcription, is also only present in this lineage [61]. Thus, many of the STING pathway components required for antiviral immunity are unique to vertebrates.
Two other, less well understood downstream consequences of cGAS-STING
Concluding Remarks and Future Perspectives
Type I IFN induction downstream of the cGAS-STING pathway is the major innate mammalian defense against DNA viruses. However, the antiviral response to cytosolic DNA can be detrimental when the source of that DNA is of self or bacterial origin. For this reason, and given that cGAS is blind as to the source of its DNA ligands, it is interesting to consider how the cGAS-STING pathway could have evolved to provide beneficial responses to viruses while ameliorating the risk of autoimmunity or
Acknowledgments
We thank all former lab members who worked on the STING project, especially Dara Burdette, Elie Diner, Kate Monroe, Michael Raulet, JD Sauer, and Katia Sotelo-Troha. We also thank our colleagues Jennifer Doudna, Ming Hammond, Yoshihiro Hayakawa, and especially Philip Kranzusch for their collaborative support and discussions.
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