Innate sensing of picornavirus infection involves cGAS-STING-mediated antiviral responses triggered by mitochondrial DNA release

Cyclic GMP-AMP synthase (cGAS) plays a key role in the innate immune responses to both DNA and RNA virus infection. Here, we found that enterovirus 71 (EV-A71), Seneca Valley virus (SVV), and foot-and-mouth disease virus (FMDV) infection triggered mitochondria damage and mitochondrial DNA (mtDNA) release in vitro and vivo. These responses were mediated by picornavirus 2B proteins which induced mtDNA release during viral replication. SVV infection caused the opening of mitochondrial permeability transition pore (mPTP) and led to voltage-dependent anion channel 1 (VDAC1)- and BCL2 antagonist/killer 1 (Bak) and Bak/BCL2-associated X (Bax)-dependent mtDNA leakage into the cytoplasm, while EV-A71 and FMDV infection induced mPTP opening and resulted in VDAC1-dependent mtDNA release. The released mtDNA bound to cGAS and activated cGAS-mediated antiviral immune response. cGAS was essential for inhibiting EV-A71, SVV, and FMDV replication by regulation of IFN-β production. cGAS deficiency contributed to higher mortality of EV-A71- or FMDV-infected mice. In addition, we found that SVV 2C protein was responsible for decreasing cGAS expression through the autophagy pathway. The 9th and 153rd amino acid sites in 2C were critical for induction of cGAS degradation. Furthermore, we also show that EV-A71, CA16, and EMCV 2C antagonize the cGAS-stimulator of interferon genes (STING) pathway through interaction with STING, and highly conserved amino acids Y155 and S156 were critical for this inhibitory effect. In conclusion, these data reveal novel mechanisms of picornaviruses to block the antiviral effect mediated by the cGAS-STING signaling pathway, which will provide insights for developing antiviral strategies against picornaviruses.

Picornavirus infection enhances aspartate by the SLC38A8 transporter to promote viral replication

Foot-and-mouth disease, a class of animal diseases, is caused by foot-and-mouth disease virus (FMDV). The metabolic changes during FMDV infection remain unclear. Here, PK-15 cells, serum, and tonsils infected with FMDV were analyzed by metabolomics. A total of 284 metabolites in cells were significantly changed after FMDV infection, and most of them belong to amino acids and nucleotides. Further studies showed that FMDV infection significantly enhanced aspartate in vitro and in vivo. The amino acid transporter solute carrier family 38 member 8 (SLC38A8) was responsible for FMDV-upregulated aspartate. Enterovirus 71 (EV71) and Seneca Valley virus (SVV) infection also enhanced aspartate by SLC38A8. Aspartate aminotransferase activity was also elevated in FMDV-, EV71-, and SVV-infected cells, which may lead to reversible transition between the TCA cycle and amino acids synthesis. Aspartate and SLC38A8 were essential for FMDV, EV71, and SVV replication in cells. In addition, aspartate and SLC38A8 also promoted FMDV and EV71 replication in mice. Detailed analysis indicated that FMDV infection promoted the transfer of mTOR to lysosome to enhance interaction between mTOR and Rheb, and activated PI3K/AKT/TSC2/Rheb/mTOR/p70S6K1 pathway to promote viral replication. The mTORC1 signaling pathway was responsible for FMDV-induced SLC38A8 protein expression. For the first time, our data identified metabolic changes during FMDV infection. These data identified a novel mechanism used by FMDV to upregulate aspartate to promote viral replication and will provide new perspectives for developing new preventive strategies.