Peroxisomes and Viruses: Overview on Current Knowledge and Experimental Approaches.

The general interest in the study of the interplay between peroxisomes and viruses has increased in recent years, with different reports demonstrating that distinct viruses modulate peroxisome-related mechanisms to either counteract the cellular antiviral response or support viral propagation. Nevertheless, mechanistical details are still scarce, and information is often incomplete. In this chapter, we present an overview of the current knowledge concerning the interplay between peroxisomes and different viruses. We furthermore present, compare, and discuss the most relevant experimental approaches and tools used in the different studies. Finally, we stress the importance of further, more detailed, and spatial-temporal analyses that encompass all the different phases of the viruses' infection cycles. These studies may lead to the discovery of novel peroxisome-related cellular mechanisms that can further be explored as targets for the development of novel antiviral therapies.

Cell-type specific distribution and activation of type I IFN pathway molecules at the placental maternal-fetal interface in response to COVID-19 infection.

Background and objective: COVID-19 infection in pregnancy significantly increases risks of adverse pregnancy outcomes. However, little is known how the innate immunity at the placental maternal-fetal interface responds to COVID-19 infection. Type I IFN cytokines are recognized as a key component of the innate immune response against viral infection. In this study, we specifically evaluated expression of IFN antiviral signaling molecules in placentas from women infected with COVID-19 during pregnancy. Methods: Expression of IFN activation signaling pathway molecules, including cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), interferon regulatory factor 3 (IRF3), Toll-like receptor 7 (TLR7), mitochondrial antiviral-signaling protein (MAVS), and IFNß were determined in formalin-fixed paraffin embedded (FFPE) placental tissue sections (villous and fetal membrane) by immunostaining. A total of 20 placentas were examined, 12 from COVID-19 patients and 8 from non-COVID-19 controls. Patient demographics, clinical data, and placental pathology report were acquired via EPIC medical record review. Results: Except BMI and placental weight, there was no statistical difference between COVID and non-COVID groups in maternal age, gestational age at delivery, gravity/parity, delivery mode, and newborn gender and weight. In COVID-exposed group, the main pathological characteristics in the placental disc are maternal and fetal vascular malperfusion and chronic inflammation. Compared to non-COVID controls, expression of IFN activation pathway molecules were all upregulated with distinct cell-type specific distribution in COVID-exposed placentas: STING in villous and decidual stromal cells; IRF3 in cytotrophoblasts (CTs) and extra-villous trophoblasts (EVTs); and TLR7 and MAVS in syncytiotrophoblasts (STs), CTs, and EVTs. Upregulation of STING, MAVS and TLR7 was also seen in fetal endothelial cells. Conclusions: STING, IRF3, TLR7, and MAVS are key viral sensing molecules that regulate type I IFN production. Type I IFNs are potent antiviral cytokines to impair and eradicate viral replication in infected cells. The finding of cell-type specific distribution and activation of these innate antiviral molecules at the placental maternal-fetal interface provide plausible evidence that type I IFN pathway molecules may play critical roles against SARS-CoV-2 infection in the placenta. Our findings also suggest that placental maternal-fetal interface has a well-defined antiviral defense system to protect the developing fetus from SARS-CoV-2 infection.

SARS-CoV-2-encoded small RNAs are able to repress the host expression of SERINC5 to facilitate viral replication.

Serine incorporator protein 5 (SERINC5) is a key innate immunity factor that operates in the cell to restrict the infectivity of certain viruses. Different viruses have developed strategies to antagonize SERINC5 function but, how SERINC5 is controlled during viral infection is poorly understood. Here, we report that SERINC5 levels are reduced in COVID-19 patients during the infection by SARS-CoV-2 and, since no viral protein capable of repressing the expression of SERINC5 has been identified, we hypothesized that SARS-CoV-2 non-coding small viral RNAs (svRNAs) could be responsible for this repression. Two newly identified svRNAs with predicted binding sites in the 3'-untranslated region (3'-UTR) of the SERINC5 gene were characterized and we found that the expression of both svRNAs during the infection was not dependent on the miRNA pathway proteins Dicer and Argonaute-2. By using svRNAs mimic oligonucleotides, we demonstrated that both viral svRNAs can bind the 3'UTR of SERINC5 mRNA, reducing SERINC5 expression in vitro. Moreover, we found that an anti-svRNA treatment to Vero E6 cells before SARS-CoV-2 infection recovered the levels of SERINC5 and reduced the levels of N and S viral proteins. Finally, we showed that SERINC5 positively controls the levels of Mitochondrial Antiviral Signalling (MAVS) protein in Vero E6. These results highlight the therapeutic potential of targeting svRNAs based on their action on key proteins of the innate immune response during SARS-CoV-2 viral infection.

SARS-CoV-2 NSP7 inhibits type I and III IFN production by targeting the RIG-I/MDA5, TRIF, and STING signaling pathways.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a poor inducer of innate antiviral immunity, and the underlying mechanism still needs further investigation. Here, we reported that SARS-CoV-2 NSP7 inhibited the production of type I and III interferons (IFNs) by targeting the RIG-I/MDA5, Toll-like receptor (TLR3)-TRIF, and cGAS-STING signaling pathways. SARS-CoV-2 NSP7 suppressed the expression of IFNs and IFN-stimulated genes induced by poly (I:C) transfection and infection with Sendai virus or SARS-CoV-2 virus-like particles. NSP7 impaired type I and III IFN production activated by components of the cytosolic dsRNA-sensing pathway, including RIG-I, MDA5, and MAVS, but not TBK1, IKKε, and IRF3-5D, an active form of IRF3. In addition, NSP7 also suppressed TRIF- and STING-induced IFN responses. Mechanistically, NSP7 associated with RIG-I and MDA5 prevented the formation of the RIG-I/MDA5-MAVS signalosome and interacted with TRIF and STING to inhibit TRIF-TBK1 and STING-TBK1 complex formation, thus reducing the subsequent IRF3 phosphorylation and nuclear translocation that are essential for IFN induction. In addition, ectopic expression of NSP7 impeded innate immune activation and facilitated virus replication. Taken together, SARS-CoV-2 NSP7 dampens type I and III IFN responses via disruption of the signal transduction of the RIG-I/MDA5-MAVS, TLR3-TRIF, and cGAS-STING signaling pathways, thus providing novel insights into the interactions between SARS-CoV-2 and innate antiviral immunity.

TRIM18 is a critical regulator of viral myocarditis and organ inflammation.

BACKGROUND: Infections by viruses including severe acute respiratory syndrome coronavirus 2 could cause organ inflammations such as myocarditis, pneumonia and encephalitis. Innate immunity to viral nucleic acids mediates antiviral immunity as well as inflammatory organ injury. However, the innate immune mechanisms that control viral induced organ inflammations are unclear. METHODS: To understand the role of the E3 ligase TRIM18 in controlling viral myocarditis and organ inflammation, wild-type and Trim18 knockout mice were infected with coxsackievirus B3 for inducing viral myocarditis, influenza A virus PR8 strain and human adenovirus for inducing viral pneumonia, and herpes simplex virus type I for inducing herpes simplex encephalitis. Mice survivals were monitored, and heart, lung and brain were harvested for histology and immunohistochemistry analysis. Real-time PCR, co-immunoprecipitation, immunoblot, enzyme-linked immunosorbent assay, luciferase assay, flow cytometry, over-expression and knockdown techniques were used to understand the molecular mechanisms of TRIM18 in regulating type I interferon (IFN) production after virus infection in this study. RESULTS: We find that knockdown or deletion of TRIM18 in human or mouse macrophages enhances production of type I IFN in response to double strand (ds) RNA and dsDNA or RNA and DNA virus infection. Importantly, deletion of TRIM18 protects mice from viral myocarditis, viral pneumonia, and herpes simplex encephalitis due to enhanced type I IFN production in vivo. Mechanistically, we show that TRIM18 recruits protein phosphatase 1A (PPM1A) to dephosphorylate TANK binding kinase 1 (TBK1), which inactivates TBK1 to block TBK1 from interacting with its upstream adaptors, mitochondrial antiviral signaling (MAVS) and stimulator of interferon genes (STING), thereby dampening antiviral signaling during viral infections. Moreover, TRIM18 stabilizes PPM1A by inducing K63-linked ubiquitination of PPM1A. CONCLUSIONS: Our results indicate that TRIM18 serves as a negative regulator of viral myocarditis, lung inflammation and brain damage by downregulating innate immune activation induced by both RNA and DNA viruses. Our data reveal that TRIM18 is a critical regulator of innate immunity in viral induced diseases, thereby identifying a potential therapeutic target for treatment.

Porcine epidemic diarrhea virus 3CLpro causes apoptosis and collapse of mitochondrial membrane potential requiring its protease activity and signaling through MAVS.

Porcine epidemic diarrhea (PED) is a highly contagious and virulent intestinal infectious disease characterized by diarrhea, vomiting and dehydration. Although PEDV-induced apoptosis has been characterized in vitro and vivo, the functional proteins related to this event and the mechanism still need further research. Here, we firstly demonstrated that PEDV epidemic strain JS2013 could trigger apoptosis in a dose- and time-dependent manner. Then, PEDV 3CLpro was further identified as a crucial inducer of PEDV-triggered apoptosis. In addition, using site-directed mutagenesis to disrupt the protease activity of 3CLpro by His41 and Cys144 mutations, we found that 3CLpro-induced apoptosis and mitochondrial damage significantly reduced, suggesting that the protease activity of 3CLpro was essential for apoptosis and mitochondrial damage. Furthermore, PEDV 3CLpro could synergistically promote MAVS-mediated apoptosis and MAVS was involved in the signaling pathway of 3CLpro-induced apoptosis, but no direct interaction between PEDV 3CLpro and MAVS was detected by immunoprecipitation assays. Our findings provide important insights into the role of 3CLpro in the pathogenicity of PEDV.

Porcine Sapelovirus 3Cpro Inhibits the Production of Type I Interferon.

Porcine sapelovirus (PSV) is the causative pathogen of reproductive obstacles, acute diarrhea, respiratory distress, or severe polioencephalomyelitis in swine. Nevertheless, the pathogenicity and pathogenic mechanism of PSV infection are not fully understood, which hinders disease prevention and control. In this study, we found that PSV was sensitive to type I interferon (IFN-ß). However, PSV could not activate the IFN-ß promoter and induce IFN-ß mRNA expression, indicating that PSV has evolved an effective mechanism to block IFN-ß production. Further study showed that PSV inhibited the production of IFN-ß by cleaving mitochondrial antiviral signaling (MAVS) and degrading melanoma differentiation-associated gene 5 (MDA5) and TANK-binding kinase 1 (TBK1) through viral 3Cpro. In addition, our study demonstrated that PSV 3Cpro degrades MDA5 and TBK1 through its protease activity and cleaves MAVS through the caspase pathway. Collectively, our results revealed that PSV inhibits the production of type I interferon to escape host antiviral immunity through cleaving and degrading the adaptor molecules.

Evolution of RNA sensing receptors in birds.

Birds are important hosts for many RNA viruses, including influenza A virus, Newcastle disease virus, West Nile virus and coronaviruses. Innate defense against RNA viruses in birds involves detection of viral RNA by pattern recognition receptors. Several receptors of different classes are involved, such as endosomal toll-like receptors and cytoplasmic retinoic acid-inducible gene I-like receptors, and their downstream adaptor proteins. The function of these receptors and their antagonism by viruses is well established in mammals; however, this has received less attention in birds. These receptors have been characterized in a few bird species, and the completion of avian genomes will permit study of their evolution. For each receptor, functional work has established ligand specificity and activation by viral infection. Engagement of adaptors, regulation by modulators and the supramolecular organization of proteins required for activation are incompletely understood in both mammals and birds. These receptors bind conserved nucleic acid agonists such as single- or double-stranded RNA and generally show purifying selection, particularly the ligand binding regions. However, in birds, these receptors and adaptors differ between species, and between individuals, suggesting that they are under selection for diversification over time. Avian receptors and signalling pathways, like their mammalian counterparts, are targets for antagonism by a variety of viruses, intent on escape from innate immune responses.

All hands on deck: SARS-CoV-2 proteins that block early anti-viral interferon responses.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is responsible for the current pandemic coronavirus disease of 2019 (COVID-19). Like other pathogens, SARS-CoV-2 infection can elicit production of the type I and III interferon (IFN) cytokines by the innate immune response. A rapid and robust type I and III IFN response can curb viral replication and improve clinical outcomes of SARS-CoV-2 infection. To effectively replicate in the host, SARS-CoV-2 has evolved mechanisms for evasion of this innate immune response, which could also modulate COVID-19 pathogenesis. In this review, we discuss studies that have reported the identification and characterization of SARS-CoV-2 proteins that inhibit type I IFNs. We focus especially on the mechanisms of nsp1 and ORF6, which are the two most potent and best studied SARS-CoV-2 type I IFN inhibitors. We also discuss naturally occurring mutations in these SARS-CoV-2 IFN antagonists and the impact of these mutations in vitro and on clinical presentation. As SARS-CoV-2 continues to spread and evolve, researchers will have the opportunity to study natural mutations in IFN antagonists and assess their role in disease. Additional studies that look more closely at previously identified antagonists and newly arising mutants may inform future therapeutic interventions for COVID-19.

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.

SARS-CoV-2 Nsp5 Activates NF-κB Pathway by Upregulating SUMOylation of MAVS.

The COVID-19 is an infectious disease caused by SARS-CoV-2 infection. A large number of clinical studies found high-level expression of pro-inflammatory cytokines in patients infected with SARS-CoV-2, which fuels the rapid development of the disease. However, the specific molecular mechanism is still unclear. In this study, we found that SARS-CoV-2 Nsp5 can induce the expression of cytokines IL-1ß, IL-6, TNF-α, and IL-2 in Calu-3 and THP1 cells. Further research found that Nsp5 enhances cytokine expression through activating the NF-κB signaling pathway. Subsequently, we investigated the upstream effectors of the NF-κB signal pathway on Nsp5 overexpression and discovered that Nsp5 increases the protein level of MAVS. Moreover, Nsp5 can promote the SUMOylation of MAVS to increase its stability and lead to increasing levels of MAVS protein, finally triggering activation of NF-κB signaling. The knockdown of MAVS and the inhibitor of SUMOylation treatment can attenuate Nsp5-mediated NF-κB activation and cytokine induction. We identified a novel role of SARS-CoV-2 Nsp5 to enhance cytokine production by activating the NF-κB signaling pathway.

SARS-CoV-2 ORF10 suppresses the antiviral innate immune response by degrading MAVS through mitophagy.

The global coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused severe morbidity and mortality in humans. It is urgent to understand the function of viral genes. However, the function of open reading frame 10 (ORF10), which is uniquely expressed by SARS-CoV-2, remains unclear. In this study, we showed that overexpression of ORF10 markedly suppressed the expression of type I interferon (IFN-I) genes and IFN-stimulated genes. Then, mitochondrial antiviral signaling protein (MAVS) was identified as the target via which ORF10 suppresses the IFN-I signaling pathway, and MAVS was found to be degraded through the ORF10-induced autophagy pathway. Furthermore, overexpression of ORF10 promoted the accumulation of LC3 in mitochondria and induced mitophagy. Mechanistically, ORF10 was translocated to mitochondria by interacting with the mitophagy receptor Nip3-like protein X (NIX) and induced mitophagy through its interaction with both NIX and LC3B. Moreover, knockdown of NIX expression blocked mitophagy activation, MAVS degradation, and IFN-I signaling pathway inhibition by ORF10. Consistent with our observations, in the context of SARS-CoV-2 infection, ORF10 inhibited MAVS expression and facilitated viral replication. In brief, our results reveal a novel mechanism by which SARS-CoV-2 inhibits the innate immune response; that is, ORF10 induces mitophagy-mediated MAVS degradation by binding to NIX.

Ultraviolet-A light increases mitochondrial anti-viral signaling protein in confluent human tracheal cells via cell-cell signaling.

Mitochondrial antiviral signaling (MAVS) protein mediates innate antiviral responses, including responses to certain coronaviruses such as severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). We have previously shown that ultraviolet-A (UVA) therapy can prevent virus-induced cell death in human ciliated tracheal epithelial cells (HTEpC) infected with coronavirus-229E (CoV-229E), and results in increased intracellular levels of MAVS. In this study, we explored the mechanisms by which UVA light can activate MAVS, and whether local UVA light application can activate MAVS at locations distant from the light source (e.g. via cell-to-cell communication). MAVS levels were compared in HTEpC exposed to 2 mW/cm2 narrow band (NB)-UVA for 20 min and in unexposed controls at 30-40% and at 100% confluency, and in unexposed HTEpC treated with supernatants or lysates from UVA-exposed cells or from unexposed controls. MAVS was also assessed in different sections of confluent monolayer plates where only one section was exposed to NB-UVA. Our results showed that UVA increases the expression of MAVS protein. Further, cells in a confluent monolayer exposed to UVA conferred an elevation in MAVS in cells adjacent to the exposed section, and also in cells in the most distant sections which were not exposed to UVA. In this study, human ciliated tracheal epithelial cells exposed to UVA demonstrate increased MAVS protein, and also appear to transmit this influence to confluent cells not exposed to UVA, likely via cell-cell signaling.

SARS-CoV-2 Nsp5 Demonstrates Two Distinct Mechanisms Targeting RIG-I and MAVS To Evade the Innate Immune Response.

Newly emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) caused a global pandemic with astonishing mortality and morbidity. The high replication and transmission of SARS-CoV-2 are remarkably distinct from those of previous closely related coronaviruses, and the underlying molecular mechanisms remain unclear. The innate immune defense is a physical barrier that restricts viral replication. We report here that the SARS-CoV-2 Nsp5 main protease targets RIG-I and mitochondrial antiviral signaling (MAVS) protein via two distinct mechanisms for inhibition. Specifically, Nsp5 cleaves off the 10 most-N-terminal amino acids from RIG-I and deprives it of the ability to activate MAVS, whereas Nsp5 promotes the ubiquitination and proteosome-mediated degradation of MAVS. As such, Nsp5 potently inhibits interferon (IFN) induction by double-stranded RNA (dsRNA) in an enzyme-dependent manner. A synthetic small-molecule inhibitor blunts the Nsp5-mediated destruction of cellular RIG-I and MAVS and processing of SARS-CoV-2 nonstructural proteins, thus restoring the innate immune response and impeding SARS-CoV-2 replication. This work offers new insight into the immune evasion strategy of SARS-CoV-2 and provides a potential antiviral agent to treat CoV disease 2019 (COVID-19) patients. IMPORTANCE The ongoing COVID-19 pandemic is caused by SARS-CoV-2, which is rapidly evolving with better transmissibility. Understanding the molecular basis of the SARS-CoV-2 interaction with host cells is of paramount significance, and development of antiviral agents provides new avenues to prevent and treat COVID-19 diseases. This study describes a molecular characterization of innate immune evasion mediated by the SARS-CoV-2 Nsp5 main protease and subsequent development of a small-molecule inhibitor.

The Mitochondrial Outer Membrane Protein Tom70-Mediator in Protein Traffic, Membrane Contact Sites and Innate Immunity.

Tom70 is a versatile adaptor protein of 70 kDa anchored in the outer membrane of mitochondria in metazoa, fungi and amoeba. The tertiary structure was resolved for the Tom70 of yeast, showing 26 α-helices, most of them participating in the formation of 11 tetratricopeptide repeat (TPR) motifs. Tom70 serves as a docking site for cytosolic chaperone proteins and co-chaperones and is thereby involved in the uptake of newly synthesized chaperone-bound proteins in mitochondrial biogenesis. In yeast, Tom70 additionally mediates ER-mitochondria contacts via binding to sterol transporter Lam6/Ltc1. In mammalian cells, TOM70 promotes endoplasmic reticulum (ER) to mitochondria Ca2+ transfer by association with the inositol-1,4,5-triphosphate receptor type 3 (IP3R3). TOM70 is specifically targeted by the Bcl-2-related protein MCL-1 that acts as an anti-apoptotic protein in macrophages infected by intracellular pathogens, but also in many cancer cells. By participating in the recruitment of PINK1 and the E3 ubiquitin ligase Parkin, TOM70 can be implicated in the development of Parkinson's disease. TOM70 acts as receptor of the mitochondrial antiviral-signaling protein (MAVS) and thereby participates in the corresponding system of innate immunity against viral infections. The protein encoded by Orf9b in the genome of SARS-CoV-2 binds to TOM70, probably compromising the synthesis of type I interferons.

Targeted Mitochondrial Therapy With Over-Expressed MAVS Protein From Mesenchymal Stem Cells: A New Therapeutic Approach for COVID-19.

The SARS-CoV-2, the virus that causes COVID-19, has infected millions of people worldwide. The symptoms of this disease are primarily due to pulmonary involvement, uncontrolled tissue inflammation, and inadequate immune response against the invader virus. Impaired interferon (IFN) production is one of the leading causes of the immune system's inability to control the replication of the SARS-CoV-2. Mitochondria play an essential role in developing and maintaining innate cellular immunity and IFN production. Mitochondrial function is impaired during cellular stress, affecting cell bioenergy and innate immune responses. The mitochondrial antiviral-signaling protein (MAVS), located in the outer membrane of mitochondria, is one of the key elements in engaging the innate immune system and interferon production. Transferring healthy mitochondria to the damaged cells by mesenchymal stem cells (MSCs) is a proposed option for regenerative medicine and a viable treatment approach to many diseases. In addition to mitochondrial transport, these cells can regulate inflammation, repair the damaged tissue, and control the pathogenesis of COVID-19. The immune regulatory nature of MSCs dramatically reduces the probability of an immune rejection. In order to induce an appropriate immune response against the SARS-CoV-2, we hypothesize to donate mitochondria to the host cells of the virus. We consider MSCs as an appropriate biological carrier for mitochondria. Besides, enhancing the expression of MAVS protein in MSCs and promoting the expression of SARS-CoV-2 viral spike protein as a specific ligand for ACE2+ cells will improve IFN production and innate immune responses in a targeted manner.

Role of Mitochondria in Viral Infections.

Viral diseases account for an increasing proportion of deaths worldwide. Viruses maneuver host cell machinery in an attempt to subvert the intracellular environment favorable for their replication. The mitochondrial network is highly susceptible to physiological and environmental insults, including viral infections. Viruses affect mitochondrial functions and impact mitochondrial metabolism, and innate immune signaling. Resurgence of host-virus interactions in recent literature emphasizes the key role of mitochondria and host metabolism on viral life processes. Mitochondrial dysfunction leads to damage of mitochondria that generate toxic compounds, importantly mitochondrial DNA, inducing systemic toxicity, leading to damage of multiple organs in the body. Mitochondrial dynamics and mitophagy are essential for the maintenance of mitochondrial quality control and homeostasis. Therefore, metabolic antagonists may be essential to gain a better understanding of viral diseases and develop effective antiviral therapeutics. This review briefly discusses how viruses exploit mitochondrial dynamics for virus proliferation and induce associated diseases.

Targeting Host Defense System and Rescuing Compromised Mitochondria to Increase Tolerance against Pathogens by Melatonin May Impact Outcome of Deadly Virus Infection Pertinent to COVID-19.

Fighting infectious diseases, particularly viral infections, is a demanding task for human health. Targeting the pathogens or targeting the host are different strategies, but with an identical purpose, i.e., to curb the pathogen's spreading and cure the illness. It appears that targeting a host to increase tolerance against pathogens can be of substantial advantage and is a strategy used in evolution. Practically, it has a broader protective spectrum than that of only targeting the specific pathogens, which differ in terms of susceptibility. Methods for host targeting applied in one pandemic can even be effective for upcoming pandemics with different pathogens. This is even more urgent if we consider the possible concomitance of two respiratory diseases with potential multi-organ afflictions such as Coronavirus disease 2019 (COVID-19) and seasonal flu. Melatonin is a molecule that can enhance the host's tolerance against pathogen invasions. Due to its antioxidant, anti-inflammatory, and immunoregulatory activities, melatonin has the capacity to reduce the severity and mortality of deadly virus infections including COVID-19. Melatonin is synthesized and functions in mitochondria, which play a critical role in viral infections. Not surprisingly, melatonin synthesis can become a target of viral strategies that manipulate the mitochondrial status. For example, a viral infection can switch energy metabolism from respiration to widely anaerobic glycolysis even if plenty of oxygen is available (the Warburg effect) when the host cell cannot generate acetyl-coenzyme A, a metabolite required for melatonin biosynthesis. Under some conditions, including aging, gender, predisposed health conditions, already compromised mitochondria, when exposed to further viral challenges, lose their capacity for producing sufficient amounts of melatonin. This leads to a reduced support of mitochondrial functions and makes these individuals more vulnerable to infectious diseases. Thus, the maintenance of mitochondrial function by melatonin supplementation can be expected to generate beneficial effects on the outcome of viral infectious diseases, particularly COVID-19.

SARS-CoV-2 ORF9b inhibits RIG-I-MAVS antiviral signaling by interrupting K63-linked ubiquitination of NEMO.

Coronavirus disease 2019 (COVID-19) is a current global health threat caused by the novel coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Emerging evidence indicates that SARS-CoV-2 elicits a dysregulated immune response and a delayed interferon (IFN) expression in patients, which contribute largely to the viral pathogenesis and development of COVID-19. However, underlying mechanisms remain to be elucidated. Here, we report the activation and repression of the innate immune response by SARS-CoV-2. We show that SARS-CoV-2 RNA activates the RIG-I-MAVS-dependent IFN signaling pathway. We further uncover that ORF9b immediately accumulates and antagonizes the antiviral type I IFN response during SARS-CoV-2 infection on primary human pulmonary alveolar epithelial cells. ORF9b targets the nuclear factor κB (NF-κB) essential modulator NEMO and interrupts its K63-linked polyubiquitination upon viral stimulation, thereby inhibiting the canonical IκB kinase alpha (IKKα)/ß/γ-NF-κB signaling and subsequent IFN production. Our findings thus unveil the innate immunosuppression by ORF9b and provide insights into the host-virus interplay during the early stage of SARS-CoV-2 infection.

SARS-CoV-2 membrane glycoprotein M antagonizes the MAVS-mediated innate antiviral response.

A novel SARS-related coronavirus (SARS-CoV-2) has recently emerged as a serious pathogen that causes high morbidity and substantial mortality. However, the mechanisms by which SARS-CoV-2 evades host immunity remain poorly understood. Here, we identified SARS-CoV-2 membrane glycoprotein M as a negative regulator of the innate immune response. We found that the M protein interacted with the central adaptor protein MAVS in the innate immune response pathways. This interaction impaired MAVS aggregation and its recruitment of downstream TRAF3, TBK1, and IRF3, leading to attenuation of the innate antiviral response. Our findings reveal a mechanism by which SARS-CoV-2 evades the innate immune response and suggest that the M protein of SARS-CoV-2 is a potential target for the development of SARS-CoV-2 interventions.