Recognition of cytosolic DNA by cGAS and other STING-dependent sensors

Abstract

The presence of DNA in the cytoplasm of mammalian cells is perceived as a danger signal, alerting the host to the presence of microbial infection. In response to the detection of cytoplasmic DNA, the immune system mounts a programed response that involves the transcription of anti-viral genes such as type I interferons and production of inflammatory cytokines such as IL-1β. The recent discovery of the cGAS-cGAMP second messenger pathway as well as IFI16 and additional sensors collectively provide critical insights into the molecular basis behind the sensing of cytoplasmic DNA. The insights obtained from these important discoveries could unveil new avenues to understand host-immunity, improve vaccine adjuvancy, and allow development of new treatments for inflammatory diseases associated with abberrant sensing of DNA.

Keywords: Immune regulation; Molecular biology; Molecular immunology.

Figure 1. cGAS and other receptors of cytosolic DNA activate IFNβ transcription via activation of STING

Cytosolic dsDNA binds cGAS and generates 2′3′cGAMP from the substrates ATP and GTP. 2′3′cGAMP in turn binds to and activates the STING dimer. This leads to phosphorylation of TBK-1, IRF3 which are translocated to the nucleus and activate IFN-β transcription. In addition, other receptors, such as, DDX41 and IFI16 also bind to cytosolic dsDNA and lead to IFN-β induction via the common STING pathway. IFN-β in turn induces expression of ISGs including IFI16. Different cytosolic receptors of dsDNA might be cell-type specific or specific to the stage of IFN-β induction.

Figure 2. Mechanism of cytosolic dsDNA sensing by cGAS

Cytosolic dsDNA binds cGAS via the basic surface of the bilobal scaffold of cGAS and the zinc thumb. Binding results in a conformational change in the NTase domain, allowing entry and binding of ATP and GTP to the catalytic core. Substrate binding coordinates two Mg²⁺ ions (depicted by purple spheres) to a specific tyrosine residue in the catalytic core causing a flip-over (shown by a curved arrow) in the linear intermediate of the enzymatic reaction between ATP and GTP, resulting in 2′3′cGAMP formation. 2′3′cGAMP binds to the inactive STING dimer at the dimer interface and triggers a conformational change which activates STING and leads to subsequent phosphorylation of TBK-1 and IRF3 thus activating transcription of IFN-β.