Oxygen enhances antiviral innate immunity through maintenance of EGLN1-catalyzed proline hydroxylation of IRF3.

Oxygen is essential for aerobic organisms, but little is known about its role in antiviral immunity. Here, we report that during responses to viral infection, hypoxic conditions repress antiviral-responsive genes independently of HIF signaling. EGLN1 is identified as a key mediator of the oxygen enhancement of antiviral innate immune responses. Under sufficient oxygen conditions, EGLN1 retains its prolyl hydroxylase activity to catalyze the hydroxylation of IRF3 at proline 10. This modification enhances IRF3 phosphorylation, dimerization and nuclear translocation, leading to subsequent IRF3 activation. Furthermore, mice and zebrafish with Egln1 deletion, treatment with the EGLN inhibitor FG4592, or mice carrying an Irf3 P10A mutation are more susceptible to viral infections. These findings not only reveal a direct link between oxygen and antiviral responses, but also provide insight into the mechanisms by which oxygen regulates innate immunity.

Dual modifying of MAVS at lysine 7 by SIRT3-catalyzed deacetylation and SIRT5-catalyzed desuccinylation orchestrates antiviral innate immunity.

To effectively protect the host from viral infection while avoiding excessive immunopathology, the innate immune response must be tightly controlled. However, the precise regulation of antiviral innate immunity and the underlying mechanisms remain unclear. Here, we find that sirtuin3 (SIRT3) interacts with mitochondrial antiviral signaling protein (MAVS) to catalyze MAVS deacetylation at lysine residue 7 (K7), which promotes MAVS aggregation, as well as TANK-binding kinase I and IRF3 phosphorylation, resulting in increased MAVS activation and enhanced type I interferon signaling. Consistent with these findings, loss of Sirt3 in mice and zebrafish renders them more susceptible to viral infection compared to their wild-type (WT) siblings. However, Sirt3 and Sirt5 double-deficient mice exhibit the same viral susceptibility as their WT littermates, suggesting that loss of Sirt5 in Sirt3-deficient mice may counteract the increased viral susceptibility displayed in Sirt3-deficient mice. Thus, we not only demonstrate that SIRT3 positively regulates antiviral immunity in vitro and in vivo, likely via MAVS, but also uncover a previously unrecognized mechanism by which SIRT3 acts as an accelerator and SIRT5 as a brake to orchestrate antiviral innate immunity.

Opposing effects of deubiquitinase OTUD3 in innate immunity against RNA and DNA viruses.

Retinoic acid-inducible-I (RIG-I), melanoma differentiation-associated gene 5 (MDA5), and cyclic GMP-AMP synthase (cGAS) genes encode essential cytosolic receptors mediating antiviral immunity against viruses. Here, we show that OTUD3 has opposing role in response to RNA and DNA virus infection by removing distinct types of RIG-I/MDA5 and cGAS polyubiquitination. OTUD3 binds to RIG-I and MDA5 and removes K63-linked ubiquitination. This serves to reduce the binding of RIG-I and MDA5 to viral RNA and the downstream adaptor MAVS, leading to the suppression of the RNA virus-triggered innate antiviral responses. Meanwhile, OTUD3 associates with cGAS and targets at Lys279 to deubiquitinate K48-linked ubiquitination, resulting in the enhancement of cGAS protein stability and DNA-binding ability. As a result, Otud3-deficient mice and zebrafish are more resistant to RNA virus infection but are more susceptible to DNA virus infection. These findings demonstrate that OTUD3 limits RNA virus-triggered innate immunity but promotes DNA virus-triggered innate immunity.

EGLN1 prolyl hydroxylation of hypoxia-induced transcription factor HIF1α is repressed by SET7-catalyzed lysine methylation.

Egg-laying defective nine 1 (EGLN1) functions as an oxygen sensor to catalyze prolyl hydroxylation of the transcription factor hypoxia-inducible factor-1 α under normoxia conditions, leading to its proteasomal degradation. Thus, EGLN1 plays a central role in the hypoxia-inducible factor-mediated hypoxia signaling pathway; however, the posttranslational modifications that control EGLN1 function remain largely unknown. Here, we identified that a lysine monomethylase, SET7, catalyzes EGLN1 methylation on lysine 297, resulting in the repression of EGLN1 activity in catalyzing prolyl hydroxylation of hypoxia-inducible factor-1 α. Notably, we demonstrate that the methylation mimic mutant of EGLN1 loses the capability to suppress the hypoxia signaling pathway, leading to the enhancement of cell proliferation and the oxygen consumption rate. Collectively, our data identify a novel modification of EGLN1 that is critical for inhibiting its enzymatic activity and which may benefit cellular adaptation to conditions of hypoxia.

Repression of p53 function by SIRT5-mediated desuccinylation at Lysine 120 in response to DNA damage.

p53 is a classic tumor suppressor that functions in maintaining genome stability by inducing either cell arrest for damage repair or cell apoptosis to eliminate damaged cells in response to different types of stress. Posttranslational modifications (PTMs) of p53 are thought to be the most effective way for modulating of p53 activation. Here, we show that SIRT5 interacts with p53 and suppresses its transcriptional activity. Using mass spectrometric analysis, we identify a previously unknown PTM of p53, namely, succinylation of p53 at Lysine 120 (K120). SIRT5 mediates desuccinylation of p53 at K120, resulting in the suppression of p53 activation. Moreover, using double knockout mice (p53-/-Sirt5-/-), we validate that the suppression of p53 target gene expression and cell apoptosis upon DNA damage is dependent on cellular p53. Our study identifies a novel PTM of p53 that regulates its activation as well as reveals a new target of SIRT5 acting as a desuccinylase.

Arginine monomethylation by PRMT7 controls MAVS-mediated antiviral innate immunity.

Accurate control of innate immune responses is required to eliminate invading pathogens and simultaneously avoid autoinflammation and autoimmune diseases. Here, we demonstrate that arginine monomethylation precisely regulates the mitochondrial antiviral-signaling protein (MAVS)-mediated antiviral response. Protein arginine methyltransferase 7 (PRMT7) forms aggregates to catalyze MAVS monomethylation at arginine residue 52 (R52), attenuating its binding to TRIM31 and RIG-I, which leads to the suppression of MAVS aggregation and subsequent activation. Upon virus infection, aggregated PRMT7 is disabled in a timely manner due to automethylation at arginine residue 32 (R32), and SMURF1 is recruited to PRMT7 by MAVS to induce proteasomal degradation of PRMT7, resulting in the relief of PRMT7 suppression of MAVS activation. Therefore, we not only reveal that arginine monomethylation by PRMT7 negatively regulates MAVS-mediated antiviral signaling in vitro and in vivo but also uncover a mechanism by which PRMT7 is tightly controlled to ensure the timely activation of antiviral defense.

Zebrafish prmt3 negatively regulates antiviral responses.

Arginine methylation catalyzed by protein arginine methyltransferases (PRMT) is a common post-translational modification in histone and nonhistone proteins, which regulates many cellular functions. Protein arginine methyltransferase 3 (prmt3), a type I arginine methyltransferase, has been shown to carry out the formation of stable monomethylarginine as an intermediate before the establishment of asymmetric dimethylarginine. To date, however, the role of PRMT3 in antiviral innate immunity has not been elucidated. This study showed that zebrafish prmt3 was upregulated by virus infection and that the overexpression of prmt3 suppressed cellular antiviral response. The PRMT3 inhibitor, SGC707, enhanced antiviral capability. Consistently, prmt3-null zebrafish were more resistant to Spring Viremia of Carp Virus (SVCV) and Grass Carp Reovirus (GCRV) infection. Further assays showed that the overexpression of prmt3 diminished the phosphorylation of irf3 and prmt3 interacted with rig-i. In addition, both zinc-finger domain and catalytic domain of prmt3 were required for the suppressive function of prmt3 on IFN activation. Our findings suggested that zebrafish prmt3 negatively regulated the antiviral responses, implicating the vital role of prmt3-or even arginine methylation-in antiviral innate immunity.

SIRT5 impairs aggregation and activation of the signaling adaptor MAVS through catalyzing lysine desuccinylation.

RLR-mediated type I IFN production plays a pivotal role in innate antiviral immune responses, where the signaling adaptor MAVS is a critical determinant. Here, we show that MAVS is a physiological substrate of SIRT5. Moreover, MAVS is succinylated upon viral challenge, and SIRT5 catalyzes desuccinylation of MAVS. Mass spectrometric analysis indicated that Lysine 7 of MAVS is succinylated. SIRT5-catalyzed desuccinylation of MAVS at Lysine 7 diminishes the formation of MAVS aggregation after viral infection, resulting in the inhibition of MAVS activation and leading to the impairment of type I IFN production and antiviral gene expression. However, the enzyme-deficient mutant of SIRT5 (SIRT5-H158Y) loses its suppressive role on MAVS activation. Furthermore, we show that Sirt5-deficient mice are resistant to viral infection. Our study reveals the critical role of SIRT5 in limiting RLR signaling through desuccinylating MAVS.

A pathogen-derived effector modulates host glucose metabolism by arginine GlcNAcylation of HIF-1α protein.

The essential role of pathogens in host metabolism is widely recognized, yet the mechanisms by which they affect host physiology remain to be fully defined. Here, we found that NleB, an enteropathogenic Escherichia coli (EPEC) type III secretion system effector known to possess N-acetylglucosamine (GlcNAc) transferase activity, GlcNAcylates HIF-1α, a master regulator of cellular O2 homeostasis. We determined that NleB-mediated GlcNAcylation at a conserved arginine 18 (Arg18) at the N-terminus of HIF-1α enhanced HIF-1α transcriptional activity, thereby inducing HIF-1α downstream gene expression to alter host glucose metabolism. The arginine transferase activity of NleB was required for its enhancement of HIF-1α transactivity and the subsequent effect on glucose metabolism in a mouse model of EPEC infection. In addition, HIF-1α acted as a mediator to transact NleB-mediated induction of glucose metabolism-associated gene expression under hypoxia. Thus, our results further show a causal link between pathogen infection and host glucose metabolism, and we propose a new mechanism by which this occurs.