AMBRA1 promotes apoptosis induced by dsRNA and virus through interacting with and stabilizing MAVS

Apoptosis is an important cellular response to viral infection. In current study, we identified activating molecule in Beclin1-regulated autophagy protein 1 (AMBRA1) as a positive regulator of apoptosis triggered by dsRNA. Depletion of AMBRA1 by gene editing significantly reduced dsRNA-induced apoptosis, which was largely restored by trans-complementation of AMBRA1. Mechanistically, AMBRA1 interacts with mitochondrial antiviral-signaling protein (MAVS), a key mitochondrial adaptor in the apoptosis pathway induced by dsRNA and viral infection. Further Co-IP analysis demonstrated that the mitochondrial localization of MAVS was essential for their interaction. The impact of AMBRA1 on dsRNA-induced apoptosis relied on the presence of MAVS and caspase-8. AMBRA1 was involved in the stabilization of MAVS through preventing its proteasomal degradation induced by dsRNA. Consistently, AMBRA1 upregulated the apoptosis induced by Semliki Forest virus infection. Taken together, our work illustrated a role of AMBRA1 in the virus-induced apoptosis through interacting with and stabilizing MAVS.

The Triangle Relationship Between Long Noncoding RNA, RIG-I-like Receptor Signaling Pathway, and Glycolysis

Long noncoding RNA (LncRNA), a noncoding RNA over 200nt in length, can regulate glycolysis through metabolic pathways, glucose metabolizing enzymes, and epigenetic reprogramming. Upon viral infection, increased aerobic glycolysis providzes material and energy for viral replication. Mitochondrial antiviral signaling protein (MAVS) is the only protein-specified downstream of retinoic acid-inducible gene I (RIG-I) that bridges the gap between antiviral immunity and glycolysis. MAVS binding to RIG-I inhibits MAVS binding to Hexokinase (HK2), thereby impairing glycolysis, while excess lactate production inhibits MAVS and the downstream antiviral immune response, facilitating viral replication. LncRNAs can also regulate antiviral innate immunity by interacting with RIG-I and downstream signaling pathways and by regulating the expression of interferons and interferon-stimulated genes (ISGs). Altogether, we summarize the relationship between glycolysis, antiviral immunity, and lncRNAs and propose that lncRNAs interact with glycolysis and antiviral pathways, providing a new perspective for the future treatment against virus infection, including SARS-CoV-2.

N4BP3 Regulates RIG-I-Like Receptor Antiviral Signaling Positively by Targeting Mitochondrial Antiviral Signaling Protein

Mitochondrial antiviral signaling protein (MAVS), an adaptor protein, is activated by RIG-I, which is critical for an effective innate immune response to infection by various RNA viruses. Viral infection causes the RIG-I-like receptor (RLR) to recognize pathogen-derived dsRNA and then becomes activated to promote prion-like aggregation and activation of MAVS. Subsequently, through the recruitment of TRAF proteins, MAVS activates two signaling pathways mediated by TBK1-IRF3 and IKK- NF-κb, respectively, and turns on type I interferon and proinflammatory cytokines. This study discovered that NEDD4 binding protein 3 (N4BP3) is a positive regulator of the RLR signaling pathway by targeting MAVS. Overexpression of N4BP3 promoted virus-induced activation of the interferon-β (IFN-β) promoter and interferon-stimulated response element (ISRE). Further experiments showed that knockdown or knockout N4BP3 impaired RIG-I-like receptor (RLR)-mediated innate immune response, induction of downstream antiviral genes, and cellular antiviral responses. We also detected that N4BP3 could accelerate the interaction between MAVS and TRAF2. Related experiments revealed that N4BP3 could facilitate the ubiquitination modification of MAVS. These findings suggest that N4BP3 is a critical component of the RIG-I-like receptor (RLR)-mediated innate immune response by targeting MAVS, which also provided insight into the mechanisms of innate antiviral responses.

Avian Metapneumovirus Subgroup C Induces Mitochondrial Antiviral Signaling Protein Degradation through the Ubiquitin-Proteasome Pathway

The mitochondrial antiviral signaling (MAVS) protein, a critical adapter, links the upstream recognition of viral RNA to downstream antiviral signal transduction. However, the interaction mechanism between avian metapneumovirus subgroup C (aMPV/C) infection and MAVS remains unclear. Here, we confirmed that aMPV/C infection induced a reduction in MAVS expression in Vero cells in a dose-dependent manner, and active aMPV/C replication was required for MAVS decrease. We also found that the reduction in MAVS occurred at the post-translational level rather than at the transcriptional level. Different inhibitors were used to examine the effect of proteasome or autophagy on the regulation of MAVS. Treatment with a proteasome inhibitor MG132 effectively blocked MAVS degradation. Moreover, we demonstrated that MAVS mainly underwent K48-linked ubiquitination in the presence of MG132 in aMPV/C-infected cells, with amino acids 363, 462, and 501 of MAVS being pivotal sites in the formation of polyubiquitin chains. Finally, E3 ubiquitin ligases for MAVS degradation were screened and identified and RNF5 targeting MAVS at Lysine 363 and 462 was shown to involve in MAVS degradation in aMPV/C-infected Vero cells. Overall, these results reveal the molecular mechanism underlying aMPV/C infection-induced MAVS degradation by the ubiquitin-proteasome pathway.

Interaction between NS1 and Cellular MAVS Contributes to NS1 Mitochondria Targeting

Influenza A virus nonstructural protein 1 (NS1) plays an important role in evading host innate immunity. NS1 inhibits interferon (IFN) responses via multiple mechanisms, including sequestering dsRNA and suppressing retinoic acid-inducible gene I (RIG-I) signaling by interacting with RIG-I and tripartite motif-containing protein 25 (TRIM25). In the current study, we demonstrated the mitochondrial localization of NS1 at the early stage of influenza virus infection. Since NS1 does not contain mitochondria-targeting signals, we suspected that there is an association between the NS1 and mitochondrial proteins. This hypothesis was tested by demonstrating the interaction of NS1 with mitochondrial antiviral-signaling protein (MAVS) in a RIG-I-independent manner. Importantly, the association with MAVS facilitated the mitochondrial localization of NS1 and thereby significantly impeded MAVS-mediated Type I IFN production.

MAVS: A Two-Sided CARD Mediating Antiviral Innate Immune Signaling and Regulating Immune Homeostasis

Mitochondrial antiviral signaling protein (MAVS) functions as a “switch” in the immune signal transduction against most RNA viruses. Upon viral infection, MAVS forms prion-like aggregates by receiving the cytosolic RNA sensor retinoic acid-inducible gene I-activated signaling and further activates/switches on the type I interferon signaling. While under resting state, MAVS is prevented from spontaneously aggregating to switch off the signal transduction and maintain immune homeostasis. Due to the dual role in antiviral signal transduction and immune homeostasis, MAVS has emerged as the central regulation target by both viruses and hosts. Recently, researchers show increasing interest in viral evasion strategies and immune homeostasis regulations targeting MAVS, especially focusing on the post-translational modifications of MAVS, such as ubiquitination and phosphorylation. This review summarizes the regulations of MAVS in antiviral innate immune signaling transduction and immune homeostasis maintenance.

Differential roles of RIG-I like receptors in SARS-CoV-2 infection

Retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) sense viral RNA and activate antiviral immune responses. Herein we investigate their functions in human epithelial cells, the primary and initial target of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). A deficiency in MDA5, RIG-I or mitochondrial antiviral signaling protein (MAVS) enhanced viral replication. The expression of the type I/III interferon (IFN) during infection was impaired in MDA5−/− and MAVS−/−, but not in RIG-I−/−, when compared to wild type (WT) cells. The mRNA level of full-length angiotensin-converting enzyme 2 (ACE2), the cellular entry receptor for SARS-CoV-2, was ~ 2.5-fold higher in RIG-I−/− than WT cells. These data demonstrate MDA5 as the predominant SARS-CoV-2 sensor, IFN-independent induction of ACE2 and anti-SARS-CoV-2 role of RIG-I in epithelial cells.

Negative Regulation of RNF90 on RNA Virus-Triggered Antiviral Immune Responses Targeting MAVS

The antiviral innate immunity is the first line of host defense against viral infection. Mitochondrial antiviral signaling protein (MAVS, also named Cardif/IPS-1/VISA) is a critical protein in RNA virus-induced antiviral signaling pathways. Our previous research suggested that E3 ubiquitin-protein ligases RING-finger protein (RNF90) negatively regulate cellular antiviral responses by targeting STING for degradation, though its role in RNA virus infection remains unknown. This study demonstrated that RNF90 negatively regulated RNA virus-triggered antiviral innate immune responses in RNF90-silenced PMA-THP1 cells, RNF90-deficient cells (including HaCaTs, MEFs, and BMDMs), and RNF90-deficient mice. However, RNF90 regulated RNA virus-triggered antiviral innate immune responses independent of STING. RNF90 promoted K48-linked ubiquitination of MAVS and its proteasome-dependent degradation, leading to the inhibition of innate immune responses. Altogether, our findings suggested a novel function and mechanism of RNF90 in antiviral innate immunity.

Cell Mechanics and Signalization: SARS-CoV-2 Hijacks Membrane Liquid Crystals and Cytoskeletal Fractal Topology

We highlight changes to cell signaling under virus invasion (with the example of SARS-CoV-2), involving disturbance of membranes (plasma, mitochondrial, endothelial-alveolar) and of nanodomains, modulated by the cytoskeleton. Virus alters the mechanical properties of the membranes, impairing mesophase structures mediated by the fractal architecture initiated by actomyosin. It changes the topology of the membrane and its lipid composition distribution. Mechano-transduction, self-organization and topology far from equilibrium are omnipresent. We propose that the actomyosin contractility generates the cytoskeletons fractal organization. We focus on three membranar processus: The transition from lamellar configuration in cell and viral membranes to a bi-continuous organization in the presence of ethanolamine. (The energy for this transition is provided by change of the folding of the viral fusion protein from metastable to stable state). The action of mitochondrial antiviral signaling protein on the external mitochondrial envelope in contact with mitochondrial-associated membranes, modified by viral endoribonuclease, distorting innate immune response. The increased permeability of the epithelial-alveolar-pulmonary barrier involves the cytoskeleton membranes. The pulmonary surfactant is also perturbed in its liquid crystal state. Viral subversion disorganizes membrane structure and functions and thus the metabolism of the cell. We advocate systematic multidisciplinary exploration of membrane mesophases and their links with fractal dynamics, to enable novel therapies for SARS-CoV-2 infection.

Full-length MAVS Inhibits Hepatitis E Virus Replication Dependent of JAK Signaling

Hepatitis E virus (HEV) infection is the leading cause of acute hepatitis worldwide. Mitochondrial antiviral signaling protein (MAVS)-mediated interferon (IFN) response plays a pivotal role in the hepatic antiviral immunity. However, little is known about the effects of overexpression of MAVS on HEV infection. Here, we studied the effects of FL-MAVS on HEV. We found that overexpression of FL-MAVS profoundly inhibited HEV replication. The overexpression of FL-MAVS is accompanied by the secretion of functional IFNs and transcriptional induction of interferon stimulated genes (ISGs). Furthermore, we showed that the anti-HEV effect of FL-MAVS is largely dependent of the JAK signaling activation.