Akt Kinase-Mediated Checkpoint of cGAS DNA Sensing Pathway

Abstract

SUMMARY Upon DNA stimulation, cyclic GMP-AMP synthetase (cGAS) synthesizes the second messenger cyclic GMP-AMP (cGAMP) that binds to the STING, triggering antiviral interferon-β (IFN-β) production. However, it has remained undetermined how hosts regulate cGAS enzymatic activity after the resolution of DNA immunogen. Here, we show that Akt kinase plays a negative role in cGAS-mediated anti-viral immune response. Akt phosphorylated the S291 or S305 residue of the enzymatic domain of mouse or human cGAS, respectively, and this phosphorylation robustly suppressed its enzymatic activity. Consequently, expression of activated Akt led to the reduction of cGAMP and IFN-β production and the increase of herpes simplex virus 1 replication, whereas treatment with Akt inhibitor augmented cGAS-mediated IFN-β production. Furthermore, expression of the phosphorylation-resistant cGAS S291A mutant enhanced IFN-β production upon DNA stimulation, HSV-1 infection, and vaccinia virus infection. Our study identifies an Akt kinase-mediated checkpoint to fine-tune hosts’ immune responses to DNA stimulation.

Sumoylation Promotes the Stability of the DNA Sensor cGAS and the Adaptor STING to Regulate the Kinetics of Response to DNA Virus

“…In this context, we have demonstrated that phosphorylation of Sting at S365 is important for its recruitment of Senp2 at late phase of viral infection. Recently, cGas has been reported to be phosphorylated to turn off its activity (Seo et al, 2015). It is likely that phosphorylation of cGas also plays a role for its recruitment of Senp2 at late phase of viral infection.…”

Species-Specific Deamidation of cGAS by Herpes Simplex Virus UL37 Protein Facilitates Viral Replication

“…In this study, we show that deamidation of cGAS by UL37 specifically ablated the cGAMP-synthesizing activity of cGAS. Previous studies have shown that cGAS is subjected to diverse post-translational modifications, including sumoylation, glutamylation and phosphorylation (Cui et al, 2017; Hu et al, 2016; Seo et al, 2015; Xia et al, 2016), and direct antagonism by an array of microbial proteins (Aguirre et al, 2017; Wu et al, 2015; Zhang et al, 2016a). How deamidation, of RIG-I and cGAS, cross-talks with these regulatory mechanisms remain unclear and these potential cross-talks, if exist, will connect protein deamidation to other well-established signaling cascades and expand the functional repertoire of protein deamidation in high eukaryotes.…”

“…Mouse maltose-binding protein (MBP)-cGAS fusion protein (141–507) (Seo et al, 2015) was expressed in BL21 (DE3) and bacteria were grown at 37 °C to an OD600 of 0.6. Then, cultures were cooled to 18 °C and pro tein expression was induced by adding 0.1 mM Isopropyl b-D-1-thiogalactopyranoside (IPTG) for 16 h. Cells were collected by centrifugation and lysed with Triton X-100 buffer (20 mM Tris-HCl [pH 7.4], 200 mM NaCl, 10% glycerol, 0.5% Triton X-100, 0.2 mg/ml lysozyme supplemented with protease inhibitor cocktail).…”

“…Surprisingly, our understanding in cGAS regulation is in its infancy at best. Previous work has shown that post-translational modifications, including phosphorylation, sumolyation and glutamylation, modulate cGAS enzyme activity directly or indirectly (Cui et al, 2017; Hu et al, 2016; Seo et al, 2015; Xia et al, 2016). Investigating the evasion mechanism of herpes simplex virus 1 (HSV-1) conveyed by the UL37 protein deamidase, we discovered that human and mouse cGAS proteins, but not nonhuman primate cGAS, are targeted for deamidation by UL37.…”

Cellular Innate Immunity: An Old Game with New Players

“…Cells have adopted mechanisms to regulate the potent inflammatory IFN-I response, and 2 recent studies describe the inhibitory regulation of cGAS. Reversible glutamylation by tubulin tyrosine ligase-like (TTLL) glutamylases inhibits cGAS synthase and DNA-binding activity [194] , and the phosphorylation of cGAS by Akt 119 dampens its activity [195] . More regulatory mechanisms are likely to be discovered in the future.…”