Small cytosolic dsDNAs repress cGAS activation and induce autophagy

Abstract

Cyclic GMP-AMP (cGAMP) synthase (cGAS), a major cytosolic DNA sensor, activates innate immune responses by producing cGAMP, which activates stimulator of interferon genes (STING). Cytosolic DNA induces autophagy in a cGAS-dependent manner to avoid persistent immune stimulation. Although dsDNAs < 20 bp can bind to cGAS, robust cGAS activation requires dsDNAs > 45 bp. However, whether cytosolic dsDNAs < 45 bp exist in mammalian cells remains unclear. Here, we identified a class of small cytosolic DNAs (scDNAs) of ~20-40 bp in human and mouse cell lines. scDNAs competed with herring testis DNA (HT-DNA, ~200-1500 bp) for binding to cGAS, and repressing HT-DNA-induced cGAS activation and the associated interferon β (IFNβ) production. Moreover, scDNAs promoted cGAS and Beclin-1 interaction, triggering the release of Rubicon, a negative regulator of phosphatidylinositol 3-kinase class III (PI3KC3) from the Beclin-1-PI3KC3 complex, activating PI3KC3 and inducing autophagy. DNA damage decreased and autophagy inducers increased scDNA levels. scDNA transfection or autophagy induction attenuated DNA damage-induced cGAS-STING activation and IFNβ expression. Thus, scDNAs acted as molecular brakes of cGAS activation, preventing excessive inflammatory cytokine production following DNA damage. Our findings lay foundations for understanding the physiological and pathological functions of scDNAs.