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The interferon-inducible protein TDRD7 inhibits AMP-activated protein kinase and thereby restricts autophagy-independent virus replication.

ABSTRACT: The interferon system is the first line of defense against virus infection. Recently, using a high-throughput genetic screen of a human interferon-stimulated gene short-hairpin RNA library, we identified a viral restriction factor, TDRD7 (Tudor domain-containing 7). TDRD7 inhibits the paramyxo-/pneumoviruses (e.g. Sendai virus and respiratory syncytial virus) by interfering with the virus-induced cellular autophagy pathway, which these viruses use for their replication. Here, we report that TDRD7 is a viral restriction factor against herpes simplex virus (HSV-1). Using knockdown, knockout, and ectopic expression systems, we demonstrate the anti-HSV-1 activity of TDRD7 in multiple human and mouse cell types. TDRD7 inhibited the virus-activated AMP-activated protein kinase (AMPK), which was essential for HSV-1 replication. Genetic ablation or chemical inhibition of AMPK activity suppressed HSV-1 replication in multiple human and mouse cells. Mechanistically, HSV-1 replication after viral entry depended on AMPK but not on its function in autophagy. The antiviral activity of TDRD7 depended on its ability to inhibit virus-activated AMPK. In summary, our results indicate that the newly identified viral restriction factor TDRD7 inhibits AMPK and thereby blocks HSV-1 replication independently of the autophagy pathway. These findings suggest that AMPK inhibition represents a potential strategy to manage HSV-1 infections.

SUBMITTER: Subramanian G

PROVIDER: S-EPMC7242695 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Host-directed therapeutics for human cytomegalovirus (HCMV) requires elucidation of cellular mechanisms that inhibit HCMV. We report a novel pathway used by cardiac glycosides to inhibit HCMV replication: induction of AMP-activated protein kinase (AMPK) activity and autophagy flux through the Na+,K+/ATPase ?1 subunit. Our data illustrate an intricate balance between the autophagy regulators AMPK, mammalian target of rapamycin (mTOR), and ULK1 during infection and treatment with the cardiac glycoside digitoxin. Both infection and digitoxin induced AMPK phosphorylation, but ULK1 was differentially phosphorylated at unique sites leading to opposing effects on autophagy. Suppression of autophagy during infection occurred via ULK1 phosphorylation at Ser757 by enhanced mTOR activity. Digitoxin continuously phosphorylated AMPK, leading to ULK1 phosphorylation at Ser317, and suppressed mTOR, resulting in increased autophagy flux and HCMV inhibition. In ATG5-deficient human fibroblasts, digitoxin did not inhibit HCMV, supporting autophagy induction as a mechanism for virus inhibition. Drug combination studies with digitoxin and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) further confirmed the role of autophagy activation in HCMV inhibition. Individually, each compound phosphorylated AMPK, but their combination reduced autophagy rather than inducing it and was antagonistic against HCMV, resulting in virus replication. The initial ULK1 activation by digitoxin was counteracted by AICAR, which prevented the downstream interaction of Beclin1 and phosphatidylinositol 3-kinase class III (PI3K-CIII), further supporting digitoxin-mediated HCMV inhibition through autophagy. Finally, the ?1 subunit was required for autophagy induction, since in ?1-deficient cells neither AMPK nor autophagy was activated and HCMV was not inhibited by digitoxin. In summary, induction of a novel pathway (?1-AMPK-ULK1) induces autophagy as a host-directed strategy for HCMV inhibition.IMPORTANCE Infection with human cytomegalovirus (HCMV) creates therapeutic challenges in congenitally infected children and transplant recipients. Side effects and selection of resistant mutants with the limited drugs available prompted evaluation of host-directed therapeutics. We report a novel mechanism of HCMV inhibition by the cardiac glycoside digitoxin. At low concentrations that inhibit HCMV, digitoxin induced signaling through the ?1 subunit of the Na+,K+/ATPase pump and the cellular kinase AMPK, resulting in binding and phosphorylation of ULK1 (Ser317) and autophagy activation. HCMV suppressed autophagy through ULK1 phosphorylation (Ser757) by activating the mTOR kinase. The pump-autophagy pathway was required for HCMV inhibition, since in ?1- or ATG5-deficient cells the virus was not inhibited. Furthermore, the AMPK activator AICAR antagonized digitoxin activity against HCMV, a phenomenon resulting from opposing effects downstream in the autophagy pathway, at the Beclin1 stage. In summary, autophagy may provide a strategy for harnessing HCMV replication.

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The interferon-inducible protein TDRD7 inhibits AMP-activated protein kinase and thereby restricts autophagy-independent virus replication.

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