TRAF7 negatively regulates the RLR signaling pathway by facilitating the K48-linked ubiquitination of TBK1.

TANK-binding kinase 1 (TBK1) is a nodal protein involved in multiple signal transduction pathways. In RNA virus-mediated innate immunity, TBK1 is recruited to the prion-like platform formed by MAVS and subsequently activates the transcription factors IRF3/7 and NF-κB to produce type I interferon (IFN) and proinflammatory cytokines for the signaling cascade. In this study, TRAF7 was identified as a negative regulator of innate immune signaling. TRAF7 interacts with TBK1 and promotes K48-linked polyubiquitination and degradation of TBK1 through its RING domain, impairing the activation of IRF3 and the production of IFN-ß. In addition, we found that the conserved cysteine residues at position 131 of TRAF7 are necessary for its function toward TBK1. Knockout of TRAF7 could facilitate the activation of IRF3 and increase the transcript levels of downstream antiviral genes. These data suggest that TRAF7 negatively regulates innate antiviral immunity by promoting the K48-linked ubiquitination of TBK1.

HSPBP1 facilitates cellular RLR-mediated antiviral response by inhibiting the K48-linked ubiquitination of RIG-I.

Retinoic acid-inducible gene I (RIG-I) plays a critical role in the recognition of intracytoplasmic viral RNA. Upon binding to the RNA of invading viruses, the activated RIG-I translocates to mitochondria, where it recruits adapter protein MAVS, causing a series of signaling cascades. In this study, we demonstrated that Hsp70 binding protein 1 (HSPBP1) promotes RIG-I-mediated signal transduction. The overexpression of HSPBP1 can increase the stability of RIG-I protein by inhibiting its K48-linked ubiquitination, and promote the activation of IRF3 and the production of IFN-ß induced by Sendai virus. Knockdown and knockout of HSPBP1 leads to down-regulation of virus-induced RIG-I expression, inhibits IRF3 activation, and reduces the production of IFNB1. These results indicate that HSPBP1 positively regulates the antiviral signal pathway induced by inhibiting the K48-linked ubiquitination of RIG-I.

SNX5 inhibits RLR-mediated antiviral signaling by targeting RIG-I-VISA signalosome.

Upon invading the cell, the viral RNA is recognized by the RIG-I receptor located in the cytoplasm, causing the RIG-I receptor to be activated. The activated RIG-I receptor transmits downstream antiviral signals by interacting with the adaptor protein VISA located on the mitochondria, leading to the production of type Ⅰ interferons and crude inflammatory cytokine genes. Although there have been many studies on antiviral signal transduction of RIG-I receptors in recent years, the mechanism of RIG-I-VISA-mediated antiviral regulation is still not fully understood. In this study, we identified SNX5 as a negative regulator of RLR-mediated antiviral signaling. Our results show that overexpression of SNX5 inhibits viral-induced activation of the IFN-ß promoter, ISRE, NF-κB, and IRF3, whereas RNAi knockdown of SNX5 expression shows opposite results. We also found that overexpression of SNX5 enhanced RIG-I's K48 ubiquitination and attenuated its K63 ubiquitination, resulting in inhibition of virus-induced RIG-I expression. Besides, further studies show that SNX5 overexpression weakens the interaction between VISA and TRAF2/5. Our findings suggest that SNX5 negatively regulates RLR-mediated antiviral signaling by targeting the RIG-I-VISA signalosome and provide new evidence for the negative regulation of RIG-I-mediated innate immune response mechanisms.

CHID1 positively regulates RLR antiviral signaling by targeting the RIG-I/VISA signalosome.

Retinoic acid-inducible gene-I (RIG-I) belongs to the RIGI-like receptors (RLRs), a class of primary pattern recognition receptors. It senses viral double-strand RNA in the cytoplasm and delivers the activated signal to its adaptor virus-induced signaling adapter (VISA), which then recruits the downstream TNF receptor-associated factors and kinases, triggering a downstream signal cascade that leads to the production of proinflammatory cytokines and antiviral interferons (IFNs). However, the mechanism of RIG-I-mediated antiviral signaling is not fully understood. Here, we demonstrate that chitinase domain-containing 1 (CHID1), a member of the chitinase family, positively regulates the RLR antiviral signaling pathway by targeting the RIG-I/VISA signalosome. CHID1 overexpression enhances the activation of nuclear factor κB (NF-кB) and interferon regulatory factor 3 (IRF3) triggered by Sendai virus (SeV) by promoting the polyubiquitination of RIG-I and VISA, thereby potentiating IFN-ß production. CHID1 knockdown in human 239T cells inhibits SeV-induced activation of IRF3 and NF-κB and the induction of IFN-ß. These results indicate that CHID1 positively regulates RLR antiviral signal, revealing the novel mechanism of the RIG-I antiviral signaling pathway.

THO Complex Subunit 7 Homolog Negatively Regulates Cellular Antiviral Response against RNA Viruses by Targeting TBK1.

RNA virus invasion induces a cytosolic RIG-I-like receptor (RLR) signaling pathway by promoting assembly of the Mitochondrial antiviral-signaling protein (MAVS) signalosome and triggers the rapid production of type I interferons (IFNs) and proinflammatory cytokines. During this process, the pivotal kinase TANK binding kinase 1 (TBK1) is recruited to the MAVS signalosome to transduce a robust innate antiviral immune response by phosphorylating transcription factors interferon regulatory factor 3 (IRF3) and nuclear factor (NF)-κB and promoting their nuclear translocation. However, the molecular mechanisms underlying the negative regulation of TBK1 are largely unknown. In the present study, we found that THO complex subunit 7 homolog (THOC7) negatively regulated the cellular antiviral response by promoting the proteasomal degradation of TBK1. THOC7 overexpression potently inhibited Sendai virus- or polyI:C-induced IRF3 dimerization and phosphorylation and IFN-ß production. In contrast, THOC7 knockdown had the opposite effects. Moreover, we simulated a node-activated pathway to show that THOC7 regulated the RIG-I-like receptors (RLR)-/MAVS-dependent signaling cascade at the TBK1 level. Furthermore, THOC7 was involved in the MAVS signalosome and promoted TBK1 degradation by increasing its K48 ubiquitin-associated polyubiquitination. Together, these findings suggest that THOC7 negatively regulates type I IFN production by promoting TBK1 proteasomal degradation, thus improving our understanding of innate antiviral immune responses.