Molecular basis of viral DNA sensing through the cGAS-STING pathway

Texas A&M Agrilife Research, College Station TX

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Abstract

The Molecular Basis of Viral DNA Sensing through the cGAS-STING Pathway The innate immune response is the first line of defense against bacterial or viral infections. Nucleic acids from the pathogens serve as danger signals to the host and induce potent innate immune responses. The cGAS- STING pathway plays a central role in sensing viral DNA. Upon DNA binding, cGAS is activated and catalyzes the synthesis of a second messenger cyclic-GMP-AMP (cGAMP). The engagement of cGAMP by STING initiates three lines of signaling events. First, the transcription factor IRF3 is activated and initiates the induction of type I interferon (IFN-I). Second, the transcription factor NF-kB is activated, which synergizes with IRF3 to promote the transcription of IFN-I genes and mediates the induction of proinflammatory cytokines. Third, the activation of STING induces noncanonical autophagy, which is also important for the clearance of intracellular pathogens. The molecular mechanism of the cGAS-STING pathway has been studied extensively over the last few years. The crystal structure of cGAS bound to dsDNA demonstrates that cGAS is activated by DNA induced oligomerization. The structures of STING bound to cGAMP and small molecule agonists reveal the mechanism of STING activation upon ligand binding. The crystal structures of TBK1 and IRF3 bound to peptides from STING C-terminal tail reveal the molecular bases of TBK1 and IRF3 recruitment and activation. The structures of phosphorylated IRF3 provide insight into the mechanism of IRF3 activation by TBK1. The cryo-EM structure of cGAS bound to the nucleosome core particle reveals the mechanism of cGAS inhibition under resting condition. These comprehensive structural and functional studies significantly advanced our understanding about the cGAS-STING pathway. Despite these extensive studies, many aspects of the cGAS- STING pathway remain poorly understood. As a diffusible second messenger, cGAMP can be exported from infected cells and imported by uninfected cells to induce the expression of IFNs. However, the structural basis of cGAMP export remains to be determined. The exact mechanism of IRF3 activation upon phosphorylation by TBK1 remains poorly defined. The activation of NF-kB plays a critical role in IFN-independent response mediated by STING. However, the mechanism of NF-kB activation via STING in remains to be determined. The proposed research will elucidate the molecular bases of several key events involved in the cGAS-STING pathway with the following aims: 1). Elucidate the structural basis of cGAMP export by ABCC1; 2). Determine the molecular basis of IRF3 activation upon phosphorylation; 3). Investigate the mechanism of NF-kB activation via STING. The proposed studies represent a rigorous and comprehensive investigation into the mechanisms of DNA sensing through the cGAS-STING pathway. It will dramatically advance our understanding of the molecular basis of innate immunity against viral DNA and provide a foundation for innovative approaches to treat viral disease, autoimmune disorder, and cancer.