The role of O-GlcNAcylation in innate immunity and inflammation.

O-linked ß-N-acetylglucosaminylation (O-GlcNAcylation) is a highly dynamic and widespread post-translational modification (PTM) that regulates the activity, subcellular localization, and stability of target proteins. O-GlcNAcylation is a reversible PTM controlled by two cycling enzymes: O-linked N-acetylglucosamine transferase and O-GlcNAcase. Emerging evidence indicates that O-GlcNAcylation plays critical roles in innate immunity, inflammatory signaling, and cancer development. O-GlcNAcylation usually occurs on serine/threonine residues, where it interacts with other PTMs, such as phosphorylation. Thus, it likely has a broad regulatory scope. This review discusses the recent research advances regarding the regulatory roles of O-GlcNAcylation in innate immunity and inflammation. A more comprehensive understanding of O-GlcNAcylation could help to optimize therapeutic strategies regarding inflammatory diseases and cancer.

LPA maintains innate antiviral immunity in a pro-active state via STK38L-mediated IRF3 Ser303 phosphorylation.

Innate immunity is critical for the early detection and elimination of viral invasion. Extracellular signals are crucial for host resistance; however, how extracellular factors prepare the innate immunity for rapid antiviral response remains elusive. Here, we find that serum deprivation largely restricts the innate antiviral responses to RNA and DNA viruses. When serum is supplied, serine/threonine-protein kinase 38-like (STK38L), induced by serum response factor (SRF), phosphorylates IRF3 at Ser303, which prevents IRF3 from proteasome-mediated degradation in the rest state (non-infected), and ensures that enough IRF3 is called in the primed state (infected). STK38L-deficient mice exhibit compromised innate antiviral responses and elevated viral proliferation and mortality. Moreover, lysophosphatidic acid (LPA) or sphingosine 1-phosphate (S1P), the crucial activators of SRF, rescue immunosuppression caused by serum deprivation. These findings identify the SRF-STK38L-IRF3 axis as a novel mechanism that maintains the host in a pro-active state when not infected, which ensures the rapid immune response against virus.

Deactylation by SIRT1 enables liquid-liquid phase separation of IRF3/IRF7 in innate antiviral immunity.

Innate antiviral immunity deteriorates with aging but how this occurs is not entirely clear. Here we identified SIRT1-mediated DNA-binding domain (DBD) deacetylation as a critical step for IRF3/7 activation that is inhibited during aging. Viral-stimulated IRF3 underwent liquid-liquid phase separation (LLPS) with interferon (IFN)-stimulated response element DNA and compartmentalized IRF7 in the nucleus, thereby stimulating type I IFN (IFN-I) expression. SIRT1 deficiency resulted in IRF3/IRF7 hyperacetylation in the DBD, which inhibited LLPS and innate immunity, resulting in increased viral load and mortality in mice. By developing a genetic code expansion orthogonal system, we demonstrated the presence of an acetyl moiety at specific IRF3/IRF7 DBD site/s abolish IRF3/IRF7 LLPS and IFN-I induction. SIRT1 agonists rescued SIRT1 activity in aged mice, restored IFN signaling and thus antagonized viral replication. These findings not only identify a mechanism by which SIRT1 regulates IFN production by affecting IRF3/IRF7 LLPS, but also provide information on the drivers of innate immunosenescence.

Acetylation-Dependent Deubiquitinase OTUD3 Controls MAVS Activation in Innate Antiviral Immunity.

Accurate regulation of innate immunity is necessary for the host to efficiently respond to invading pathogens and avoid excessive harmful immune pathology. Here we identified OTUD3 as an acetylation-dependent deubiquitinase that restricts innate antiviral immune signaling. OTUD3 deficiency in mice results in enhanced innate immunity, a diminished viral load, and morbidity. OTUD3 directly hydrolyzes lysine 63 (Lys63)-linked polyubiquitination of MAVS and thus shuts off innate antiviral immune response. Notably, the catalytic activity of OTUD3 relies on acetylation of its Lys129 residue. In response to virus infection, the acetylated Lys129 is removed by SIRT1, which promptly inactivates OTUD3 and thus allows timely induction of innate antiviral immunity. Importantly, acetyl-OTUD3 levels are inversely correlated with IFN-ß expression in influenza patients. These findings establish OTUD3 as a repressor of MAVS and uncover a previously unknown regulatory mechanism by which the catalytic activity of OTUD3 is tightly controlled to ensure timely activation of antiviral defense.

FAF1 Regulates Antiviral Immunity by Inhibiting MAVS but Is Antagonized by Phosphorylation upon Viral Infection.

Mitochondrial antiviral signaling protein (MAVS) is an adaptor of the innate immune receptor retinoic acid-inducible gene 1 (RIG-I) that links recognition of viral RNA to antiviral signaling. Upon interacting with RIG-I, MAVS undergoes lysine 63-linked poly-ubiquitination by the E3 ligase TRIM31 and subsequently aggregates to activate downstream signaling effectors. We find that the scaffold protein FAF1 forms aggregates that negatively regulate MAVS. FAF1 antagonizes the poly-ubiquitination and aggregation of MAVS by competing with TRIM31 for MAVS association. FAF1 knockout mice are more resistant to RNA virus infection, and FAF1 deficiency in myeloid cells results in enhanced innate signaling and reduced viral load and morbidity in vivo. Upon virus infection, the kinase IKKɛ directly phosphorylates FAF1 at Ser556 and triggers FAF1 de-aggregation. Moreover, Ser556 phosphorylation promotes FAF1 lysosomal degradation, consequently relieving FAF1-dependent suppression of MAVS. These findings establish FAF1 as a modulator of MAVS and uncover mechanisms that regulate FAF1 to insure timely activation of antiviral defense.