STIM1, an endoplasmic reticulum Ca 2+ sensor, upregulates cGAS activity in endothelial cells to induce type 1 interferon response

The stimulator of interferon genes (STING) is an endoplasmic reticulum (ER) adaptor protein responsible for inducing type I interferon response following bacterial and viral infections. Cyclic GMP‐AMP synthase (cGAS) activates STING by generating the second messenger, cyclic guanosine monophosphate–adenosine monophosphate (cGAMP). While it is known that double stranded (ds) DNA activates cGAS, however mechanisms regulating cGAS activity remains elusive. Here, we show that stromal interaction molecule 1 (STIM1), a Ca 2+ sensor in the endoplasmic reticulum (ER), is required for inducing cGAS activity and thereby type I interferon response. We show that activation of cGAS using Poly (dA:dT), bacterial dsDNA or herpes‐simplex virus DNA, and bacterial endotoxin‐induced STING and enhanced the expression of type I interferon in a cGAS dependent manner. Intriguingly, depletion of STIM1 completely blocked STING activity. STIM1 activates Ca 2+ entry from plasmalemma (PM) channels such as Orai1 and TRPC1 by forming ER‐PM junctions. We thus assessed if STIM1‐mediated Ca 2+ entry was involved in inducing cGAS activity. We show that phospho‐defective STIM1 (STIM1‐Y361F mutant), which inhibit Ca 2+ entry, also inhibited activation of STING signaling, indicating that STIM1‐dependent Ca 2+ entry mediates cGAS activity. Mechanistically, we show that dsDNA induced STIM1 puncta formation and Ca 2+ entry and phosphorylate cGAS at tyrosine residues. Next, we screened various tyrosine kinases and found that inhibition of c‐src blocked cGAS phosphorylation, indicating STIM1 activates cGAS via c‐src. Our finding suggests that STIM1 results in activation of cGAS enzymatic activity and thus results in cGAMP production and type1‐IFN production in the presence of pathogenic stimuli. Therefore, phosphorylation of cGAS at its tyrosine site represents the novel target for inhibiting STING activation and thereby suppressing tissue injury and inflammation.