Structures and Mechanisms in the cGAS-STING Innate Immunity Pathway.

The cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) senses invasion of pathogenic DNA and stimulates inflammatory signaling, autophagy, and apoptosis. Organization of host DNA into nucleosomes was proposed to limit cGAS autoinduction, but the underlying mechanism was unknown. Here, we report the structural basis for this inhibition. In the cryo-electron microscopy structure of the human cGAS-nucleosome core particle (NCP) complex, two cGAS monomers bridge two NCPs by binding the acidic patch of the histone H2A-H2B dimer and nucleosomal DNA. In this configuration, all three known cGAS DNA binding sites, required for cGAS activation, are repurposed or become inaccessible, and cGAS dimerization, another prerequisite for activation, is inhibited. Mutating key residues linking cGAS and the acidic patch alleviates nucleosomal inhibition. This study establishes a structural framework for why cGAS is silenced on chromatinized self-DNA.

Structural basis for the inhibition of cGAS by nucleosomes

Chromosomal instability (CIN) drives cancer cell evolution, metastasis and therapy resistance, and is associated with poor prognosis1. CIN leads to micronuclei that release DNA into the cytoplasm after rupture, which triggers activation of inflammatory signalling mediated by cGAS and STING2,3. These two proteins are considered to be tumour suppressors as they promote apoptosis and immunosurveillance. However, cGAS and STING are rarely inactivated in cancer4, and, although they have been implicated in metastasis5, it is not known why loss-of-function mutations do not arise in primary tumours4. Here we show that inactivation of cGAS-STING signalling selectively impairs the survival of triple-negative breast cancer cells that display CIN. CIN triggers IL-6-STAT3-mediated signalling, which depends on the cGAS-STING pathway and the non-canonical NF-κB pathway. Blockade of IL-6 signalling by tocilizumab, a clinically used drug that targets the IL-6 receptor (IL-6R), selectively impairs the growth of cultured triple-negative breast cancer cells that exhibit CIN. Moreover, outgrowth of chromosomally instable tumours is significantly delayed compared with tumours that do not display CIN. Notably, this targetable vulnerability is conserved across cancer types that express high levels of IL-6 and/or IL-6R in vitro and in vivo. Together, our work demonstrates pro-tumorigenic traits of cGAS-STING signalling and explains why the cGAS-STING pathway is rarely inactivated in primary tumours. Repurposing tocilizumab could be a strategy to treat cancers with CIN that overexpress IL-6R. 

https://pure.rug.nl/ws/files/229114659/s41586_022_04847_2_1_.pdf

cGAS–STING drives the IL-6-dependent survival of chromosomally instable cancers

/pdf/cgas-sting-drives-the-il-6-dependent-survival-of-3u0wzgot.pdf

Liquid-liquid phase separation: Orchestrating cell signaling through time and space

The cyclic GMP-AMP synthase (cGAS) is a widely used DNA sensor, which detects cytosolic DNA species without a preference of self or non-self microbial DNA in interphase to initiate innate immune response. How cGAS is regulated to avoid self-DNA sensing upon nuclear envelope breakdown (NEBD) during mitosis remains enigmatic. Here we show that cGAS is mostly localized in the cytoplasm in interphase and rapidly translocated to chromosomes upon NEBD in mitosis. The major mitotic kinase CDK1-cyclin B complex phosphorylates human cGAS at S305 or mouse cGAS at S291, which inhibits its ability to synthesize cGAMP upon mitotic entry. The type 1 phosphatase PP1 dephosphorylates cGAS upon mitotic exit to enable its DNA sensing ability. Our findings reveal a mechanism on how the DNA sensor cGAS is post-translationally regulated by cell cycle-dependent enzymes to ensure its proper activation for host defense of cytosolic DNA in interphase and inert to self-DNA in mitosis.

/pdf/phosphorylation-of-cgas-by-cdk1-impairs-self-dna-sensing-in-1s9wwdjmc1.pdf

Qiang Chen

Papers

Phosphorylation of cGAS by CDK1 impairs self-DNA sensing in mitosis.