A Baseline Cellular Antiviral State Is Maintained by cGAS and Its Most Frequent Naturally Occurring Variant rs610913

Upon recognition of aberrantly located DNA, the innate immune sensor cyclic GMP-AMP synthase (cGAS) activates stimulator of IFN genes (STING)/IFN regulatory factor (IRF)3-driven antiviral responses. In this study, we characterized the ability of a specific variant of the human cGAS-encoding gene MB21D1, rs610913, to alter cGAS-mediated DNA sensing and viral infection. rs610913 is a frequent G>T polymorphism resulting in a P261H exchange in the cGAS protein. Data from the International Collaboration for the Genomics of HIV suggested that rs610913 nominally associates with HIV-1 acquisition in vivo. Molecular modeling of cGAS(P261H) hinted toward the possibility for an additional binding site for a potential cellular cofactor in cGAS dimers. However, cGAS(wild-type [WT]) or cGAS(P261H)-reconstituted THP-1 cGAS knockout cells shared steady-state expression of IFN-stimulated genes, as opposed to cells expressing the enzymatically inactive cGAS(G212A/S(213)A). Accordingly, cGAS(WT) and cGAS(P261H) cells were less susceptible to lentiviral transduction and infection with HIV-1, HSV-1, and Chikungunya virus as compared with cGAS knockout or cGAS(G212A/S(213)A) cells. Upon DNA challenge, innate immune activation appeared to be mildly reduced upon expression of cGAS(P261H) compared with cGAS(WT). Finally, DNA challenge of PBMCs from donors homozygously expressing rs610913 provoked a trend toward a slightly reduced type I IFN response as compared with PBMCs from GG donors. Taken together, the steady-state activity of cGAS maintains a baseline antiviral state rendering cells more refractory to IFN-stimulated gene-sensitive viral infections. rs610913 failed to grossly differ phenotypically from the WT gene, suggesting that cGAS(P261H) and WT cGAS share a similar ability to sense viral infections in vivo.

Intracellular bacteria engage a STING-TBK1-MVB12b pathway to enable paracrine cGAS-STING signalling

Andrea Ablasser, Ensieh Farahani, Alix Martin, Bing Zhang

The innate immune system is crucial for eventual control of infections, but may also contribute to pathology. Listeria monocytogenes is an intracellular Gram-positive bacteria and a major cause of food-borne disease. However, important knowledge on the interactions between L. monocytogenes and the immune system is still missing. Here, we report that Listeria DNA is sorted into extracellular vesicles (EVs) in infected cells and delivered to bystander cells to stimulate the cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) pathway. This was also observed during infections with Francisella tularensis and Legionella pneumophila. We identify the multivesicular body protein MVB12b as a target for TANK-binding kinase 1 phosphorylation, which is essential for the sorting of DNA into EVs and stimulation of bystander cells. EVs from Listeria-infected cells inhibited T-cell proliferation, and primed T cells for apoptosis. Collectively, we describe a pathway for EV-mediated delivery of foreign DNA to bystander cells, and suggest that intracellular bacteria exploit this pathway to impair antibacterial defence.

NATURE PUBLISHING GROUP2019

cGAS regulation and activity during genotoxic stress

Sélène-Véronique Glück

The innate immune system has evolved to detect pathogens through germlineencoded pattern recognition receptors (PRRs). Nucleic acid sensors, a defined class of PRRs, bind to microbial and viral nucleic acids and trigger the production of type I interferons and cytokines which mediate the first line of host defense. In the past, these sensors have been primarily studied in the context of infections. Yet, the DNA sensor cyclic GMP-AMP synthase (cGAS) has been recently described to detect not only microbial and viral derived dsDNA but also self-DNA in a sequenceindependent manner. At steady state, the organization of nuclear self-DNA is tightly controlled by a complex network of multiple factors that can become imbalanced during genotoxic stress. Whether and how this stress can be sensed by cGAS in the absence of infection are the main questions of this thesis. Observing the fate of cGAS in space and time in the presence of several genotoxic insults including chemotherapeutics allowed me to gain novel insights into processes regulating the localization and activation of cGAS. In this thesis, I demonstrate that cGAS localizes to both nuclear and cytosolic compartments of the cell. We show that cGAS is inactive in the nucleus due to binding to the acidic patch on H2A-H2B heterodimers. Upon genotoxic stress, cGAS is removed from the nucleus in a manner that is dependent on histones and remains inactive. Moreover, with time cGAS transcription is repressed resulting in a gradual decrease of cGAS levels. On the other hand, cGAS-activating DNA substrates such as cytosolic chromatin fragments accumulate in the cytosol of senescent cells deficient in Lamin B1 when cGAS levels are already decreased. Along with the occurrence of these chromatin fragments, cGAS mediates an inflammatory response dependent on STING (Stimulator of interferon genes), referred to as the senescence-associated secretory phenotype (SASP). Overall, we conclude that acute genotoxic stress does not result in cGAS-STING signaling, whereas the chronic DNA damage response, senescence, activates the cGAS-STING pathway. Moreover, the results presented in this work suggest a new role for cGAS-STING signaling during senescence beyond the recognition of pathogens.

EPFL2020

Structural mechanism of cytosolic DNA sensing by cGAS

Andrea Ablasser

Cytosolic DNA arising from intracellular bacterial or viral infections is a powerful pathogen-associated molecular pattern (PAMP) that leads to innate immune host defence by the production of type I interferon and inflammatory cytokines. Recognition of cytosolic DNA by the recently discovered cyclic-GMP-AMP (cGAMP) synthase (cGAS) induces the production of cGAMP to activate the stimulator of interferon genes (STING). Here we report the crystal structure of cGAS alone and in complex with DNA, ATP and GTP along with functional studies. Our results explain the broad DNA sensing specificity of cGAS, show how cGAS catalyses dinucleotide formation and indicate activation by a DNA-induced structural switch. cGAS possesses a remarkable structural similarity to the antiviral cytosolic double-stranded RNA sensor 2'-5'oligoadenylate synthase (OAS1), but contains a unique zinc thumb that recognizes B-form double-stranded DNA. Our results mechanistically unify dsRNA and dsDNA innate immune sensing by OAS1 and cGAS nucleotidyl transferases.

2013

cGAS regulation and activity during genotoxic stress

Sélène-Véronique Glück

EPFL2020