VAMP8 Contributes to the TRIM6-Mediated Type I Interferon Antiviral Response during West Nile Virus Infection

Microscopic marvels: Decoding the role of micropeptides in innate immunity

The innate immune response is under selection pressures from changing environments and pathogens. While sequence evolution can be studied by comparing rates of amino acid mutations within and between species, how a gene's birth and death contribute to the evolution of immunity is less known. Short open reading frames, once regarded as untranslated or transcriptional noise, can often produce micropeptides of <100 amino acids with a wide array of biological functions. Some micropeptide sequences are well conserved, whereas others have no evolutionary conservation, potentially representing new functional compounds that arise from species-specific adaptations. To date, few reports have described the discovery of novel micropeptides of the innate immune system. The diversity of immune-related micropeptides is a blind spot for gene and functional annotation. Immune-related micropeptides represent a potential reservoir of untapped compounds for understanding and treating disease. This review consolidates what is currently known about the evolution and function of innate immune-related micropeptides to facilitate their investigation.

TRIMming down Flavivirus Infections

Flaviviruses comprise a large number of arthropod-borne viruses, some of which are associated with life-threatening diseases. Flavivirus infections are rising worldwide, mainly due to the proliferation and geographical expansion of their vectors. The main human pathogens are mosquito-borne flaviviruses, including dengue virus, Zika virus, and West Nile virus, but tick-borne flaviviruses are also emerging. As with any viral infection, the body's first line of defense against flavivirus infections is the innate immune defense, of which type I interferon is the armed wing. This cytokine exerts its antiviral activity by triggering the synthesis of hundreds of interferon-induced genes (ISGs), whose products can prevent infection. Among the ISGs that inhibit flavivirus replication, certain tripartite motif (TRIM) proteins have been identified. Although involved in other biological processes, TRIMs constitute a large family of antiviral proteins active on a wide range of viruses. Furthermore, whereas some TRIM proteins directly block viral replication, others are positive regulators of the IFN response. Therefore, viruses have developed strategies to evade or counteract TRIM proteins, and some even hijack certain TRIM proteins to their advantage. In this review, we summarize the current state of knowledge on the interactions between flaviviruses and TRIM proteins, covering both direct and indirect antiviral mechanisms.

Vesicle-associated membrane protein 8 knockdown exerts anti-proliferative, pro-apoptotic, anti-autophagic, and pro-ferroptotic effects on colorectal cancer cells by inhibition of the JAK/STAT3 pathway

Vesicle-associated membrane protein 8 (VAMP8), a soluble n-ethylmaleimide-sensitive factor receptor protein, acts as an oncogenic gene in the progression of several malignancies. Nevertheless, the roles and mechanisms of VAMP8 in colorectal cancer (CRC) progression remain unknown. The expression and prognostic significance of VAMP8 in CRC samples were analyzed through bioinformatics analyses. Cell proliferation was detected using CCK-8 and EdU incorporation assays and apoptosis was evaluated via flow cytometry. Western blot analysis was conducted to examine the protein expression. Ferroptosis was evaluated by measurement of iron metabolism, lipid peroxidation, and glutathione (GSH) content. VAMP8 was increased in CRC samples relative to normal samples on the basis of GEPIA and HPA databases. CRC patients with high level of VAMP8 had a worse overall survival. VAMP8 depletion led to a suppression of proliferation and promotion of apoptosis in CRC cells. Additionally, VAMP8 knockdown suppressed beclin1 expression and LC3-II/LC3-I ratio, elevated p62 expression, increased Fe2+, labile iron pool, lipid reactive oxygen species, and malondialdehyde levels, and repressed GSH content and glutathione peroxidase activity. Moreover, VAMP8 knockdown inhibited the activation of janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) pathway in CRC cells. Mechanistically, activation of the JAK/STAT3 pathway by JAK1 or JAK2 overexpression attenuated VAMP8 silencing-mediated anti-proliferative, pro-apoptotic, anti-autophagic, and pro-ferroptotic effects on CRC cells. In conclusion, VAMP8 knockdown affects the proliferation, apoptosis, autophagy, and ferroptosis by the JAK/STAT3 pathway in CRC cells.

Early cellular and molecular signatures correlate with severity of West Nile virus infection

Infection with West Nile virus (WNV) drives a wide range of responses, from asymptomatic to flu-like symptoms/fever or severe cases of encephalitis and death. To identify cellular and molecular signatures distinguishing WNV severity, we employed systems profiling of peripheral blood from asymptomatic and severely ill individuals infected with WNV. We interrogated immune responses longitudinally from acute infection through convalescence employing single-cell protein and transcriptional profiling complemented with matched serum proteomics and metabolomics as well as multi-omics analysis. At the acute time point, we detected both elevation of pro-inflammatory markers in innate immune cell types and reduction of regulatory T cell activity in participants with severe infection, whereas asymptomatic donors had higher expression of genes associated with anti-inflammatory CD16+ monocytes. Therefore, we demonstrated the potential of systems immunology using multiple cell-type and cell-state-specific analyses to identify correlates of infection severity and host cellular activity contributing to an effective anti-viral response.

TRIM6 silencing for inhibiting growth and angiogenesis of gliomas by regulating VEGFA

Gliomas are the highest prevalent primary central nervous system (CNS) cancers with poor overall survival rate. There is an urgent need to conduct more research into molecular therapies targeting critical elements of gliomas. This study herein targeted to assess the impact of tripartite motif protein 6 (TRIM6) on gliomas. Using public databases, we found the increased TRIM6 expression in tissues of glioma which was linked with worst overall survival. Silencing TRIM6 promoted glioma cell proliferation, migration and angiogenesis, suggesting the promoting effects of TRIM6 on gliomas. Knockdown of TRIM6 expression downregulated the expression levels of Forkhead box M1 (FOXM1) and vascular endothelial growth factor A (VEGFA) in glioma cells. Afterwards, impact of TRIM6 on VEGFA expression was regulated by FOXM1. VEGFA overexpression reversed the decreased abilities of glioma cell proliferation, migration and angiogenesis caused by silencing TRIM6. Furthermore, we also found that TRIM6 promoted the growth of gliomas in the xenograft mouse model. In summary, the expression of TRIM6 was increased which was related to poor prognosis of glioma patients. TRIM6 promoted glioma cell proliferation, migration and angiogenesis through the FOXM1-VEGFA pathway. Therefore, TRIM6 carries capacity to be explored as a novel therapeutic target in clinical.

The Molecular and Function Characterization of Porcine MID2

Midline2 (MID2/TRIM1) is a member of the tripartite motif-containing (TRIM) family, which is involved in a wide range of cellular processes. However, fundamental studies on porcine MID2 (pMID2) are still lacking. In this study, we identified and characterized the full length MID2 gene of pig (Sus scrofa). The sequence alignment analysis results showed that pMID2 had an N-terminal RING zinc-finger domain, BBC domain, and C-terminal COS box, FN3 motif, and PRY-SPRY domain that were conserved and similar to those of other vertebrates. Furthermore, pMID2 had the highest expression levels in porcine lung and spleen. Serial deletion and site-directed mutagenesis showed that the putative nuclear factor-κB (NF-κB) binding site may be an essential transcription factor for regulating the transcription expression of pMID2. Furthermore, the immunofluorescence assay indicated that pMID2 presented in the cell membrane and cytoplasm. To further study the functions of pMID2, we identified and determined its potential ability to perceive poly (I:C) and IFN-α stimulation. Stimulation experiments showed pMID2 enhanced poly (I:C)-/IFN-α-induced JAK-STAT signaling pathway, indicating that pMID2 might participate in the immune responses. In conclusion, we systematically and comprehensively analyzed the characterizations and functions of pMID2, which provide valuable information to explore the pMID2 functions in innate immunity. Our findings not only enrich the current knowledge of MID2 in IFN signaling regulation but also offer the basis for future research of pig MID2 gene.

Hepatocyte CD36 modulates UBQLN1-mediated proteasomal degradation of autophagic SNARE proteins contributing to septic liver injury

Macroautophagy/autophagy plays a protective role in sepsis-induced liver injury. As a member of class B scavenger receptors, CD36 plays important roles in various disorders, such as atherosclerosis and fatty liver disease. Here we found that the expression of CD36 in hepatocytes was increased in patients and a mouse model with sepsis, accompanied by impaired autophagy flux. Furthermore, hepatocyte cd36 knockout (cd36-HKO) markedly improved liver injury and the impairment of autophagosome-lysosome fusion in lipopolysaccharide (LPS)-induced septic mice. Ubqln1 (ubiquilin 1) overexpression (OE) in hepatocyte blocked the protective effect of cd36-HKO on LPS-induced liver injury in mice. Mechanistically, with LPS stimulation, CD36 on the plasma membrane was depalmitoylated and distributed to the lysosome, where CD36 acted as a bridge molecule linking UBQLN1 to soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and hence promoting the proteasomal degradation of SNARE proteins, resulting in fusion impairment. Overall, our data reveal that CD36 is essential for modulating the proteasomal degradation of autophagic SNARE proteins in a UBQLN1-dependent manner. Targeting CD36 in hepatocytes is effective for improving autophagic flux in sepsis and therefore represents a promising therapeutic strategy for clinical treatment of septic liver injury.Abbreviations: AAV8: adeno-associated virus 8; AOSC: acute obstructive suppurative cholangitis; ATP1A1: ATPase, Na+/K+ transporting, alpha 1 polypeptide; CASP3: caspase 3; CASP8: caspase 8; CCL2: chemokine (C-C motif) ligand 2; cd36-HKO: hepatocyte-specific cd36 knockout; Co-IP: co-immunoprecipitation; CQ: chloroquine; Cys: cysteine; GOT1: glutamic-oxaloacetic transaminase 1, soluble; GPT: glutamic-pyruvic transaminase, soluble; IL1B: interleukin 1 beta; IL6: interleukin 6; KO: knockout; LAMP1: lysosomal associated membrane protein 1; LDH, lactate dehydrogenase; LPS: lipopolysaccharide; LYPLA1: lysophospholipase 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; OE: overexpression; qPCR: quantitative polymerase chain reaction; SNAP29: synaptosome associated protein 29; SNARE: soluble N-ethylmaleimide-sensitive factor attachment protein receptor; SQSTM1/p62: sequestosome 1; STX17: syntaxin 17; TNF: tumor necrosis factor; TRIM: tripartite motif-containing; UBA: ubiquitin-associated; UBL: ubiquitin-like; UBQLN: ubiquilin; VAMP8: vesicle associated membrane protein 8; WT: wild-type.

TRIM6: An Upregulated Biomarker with Prognostic Significance and Immune Correlations in Gliomas

This study investigates the expression and prognostic value of TRIM6 in gliomas, the most prevalent primary brain and spinal cord tumors. Our results show that TRIM6 is predominantly overexpressed in glioma tissues and is associated with reduced overall survival, disease-specific survival, and progression-free interval. Furthermore, TRIM6 expression is correlated with WHO grade and primary treatment outcomes. Functional analysis indicates that interactions between cytokines and their receptors play a critical role in the prognosis of glioma patients. A protein-protein interaction network reveals 10 hub genes closely linked to cytokine-cytokine receptor interaction. In vitro experiments demonstrate that silencing TRIM6 impairs the proliferation, invasion, and migration of glioma cells, while overexpressing TRIM6 enhances these abilities. Additionally, TRIM6 expression is positively associated with the abundance of innate immune cells and negatively associated with the abundance of adaptive immune cells. In summary, TRIM6 is significantly upregulated in gliomas and linked to poor prognosis, making it a potential diagnostic and prognostic biomarker. TRIM6 plays a crucial role in promoting cell viability, clonogenic potential, migration, and invasion in glioma cells. It may regulate glioma progression by modulating cytokine-cytokine receptor interaction, leading to an inflammatory response and an imbalance in immunomodulation, thereby representing a potential therapeutic target.

The Role of Ubiquitin-Proteasome System in the Pathogenesis of Severe Acute Respiratory Syndrome Coronavirus-2 Disease

Different protein degradation pathways exist in cells. However, the bulk of cellular proteins are degraded by the ubiquitin-proteasome system (UPS), which is one of these pathways. The upkeep of cellular protein homeostasis is facilitated by the ubiquitin-proteasome system, which has a variety of important functions. With the emergence of eukaryotic organisms, the relationship between ubiquitylation and proteolysis by the proteasome became apparent. Severe acute respiratory syndrome coronavirus-2 (SARS-Coronavirus-2) hijacks the ubiquitin-proteasome system and causes their viral proteins to become ubiquitinated, facilitating assembly and budding. Ubiquitination of the enzyme keratin-38 (E-K38) residue gave the virion the ability to engage with at least one putative cellular receptor, T-cell immunoglobin-mucin (TIM-1), boosting virus entry, reproduction, and pathogenesis. A fraction of infectious viral particles produced during replication have been ubiquitinated. The ubiquitin system promotes viral replication. In order to replicate their viral genome after entering the host cell, viruses combine the resources of the host cell with recently generated viral proteins. Additionally, viruses have the ability to encode deubiquitinating (DUB)-active proteins that can boost viral replication through both direct and indirect means. The SARS-Coronavirus-2 papain-like protease (PLpro) protein is a DUB enzyme that is necessary for breaking down viral polyproteins to create a working replicase complex and promote viral propagation. The ubiquitin-like molecule interferon-stimulated gene 15 (ISG15), which is likewise a regulator of the innate immune response and has antiviral characteristics, can also be broken down by this enzyme. However, limiting the E1-activating enzyme's ability to suppress the ubiquitination pathway prevented virus infection but did not prevent viral RNA genome translation. Numerous investigations have revealed that the use of proteasome inhibitors has a detrimental effect on the replication of SARS-Coronavirus-2 and other viruses in the host cell. Studies have shown that the use of proteasome inhibitors is also known to deplete free cellular ubiquitin, which may have an impact on viral replication and other vital cellular functions.

Regulation of cGAS/STING signaling and corresponding immune escape strategies of viruses

Innate immunity is the first line of defense against invading external pathogens, and pattern recognition receptors (PRRs) are the key receptors that mediate the innate immune response. Nowadays, there are various PRRs in cells that can activate the innate immune response by recognizing pathogen-related molecular patterns (PAMPs). The DNA sensor cGAS, which belongs to the PRRs, plays a crucial role in innate immunity. cGAS detects both foreign and host DNA and generates a second-messenger cGAMP to mediate stimulator of interferon gene (STING)-dependent antiviral responses, thereby exerting an antiviral immune response. However, the process of cGAS/STING signaling is regulated by a wide range of factors. Multiple studies have shown that viruses directly target signal transduction proteins in the cGAS/STING signaling through viral surface proteins to impede innate immunity. It is noteworthy that the virus utilizes these cGAS/STING signaling regulators to evade immune surveillance. Thus, this paper mainly summarized the regulatory mechanism of the cGAS/STING signaling pathway and the immune escape mechanism of the corresponding virus, intending to provide targeted immunotherapy ideas for dealing with specific viral infections in the future.

Molecular pathogenesis of Japanese encephalitis and possible therapeutic strategies

Japanese encephalitis virus (JEV), a single-stranded, enveloped RNA virus, is a health concern across Asian countries, associated with severe neurological disorders, especially in children. Primarily, pigs, bats, and birds are the natural hosts for JEV, but humans are infected incidentally. JEV requires a few host proteins for its entry and replication inside the mammalian host cell. The endoplasmic reticulum (ER) plays a significant role in JEV genome replication and assembly. During this process, the ER undergoes stress due to its remodelling and accumulation of viral particles and unfolded proteins, leading to an unfolded protein response (UPR). Here, we review the overall strategy used by JEV to infect the host cell and various cytopathic effects caused by JEV infection. We also highlight the role of JEV structural proteins (SPs) and non-structural proteins (NSPs) at various stages of the JEV life cycle that are involved in up- and downregulation of different host proteins and are potentially relevant for developing efficient therapeutic drugs.

The RNA helicase DHX16 recognizes specific viral RNA to trigger RIG-I-dependent innate antiviral immunity

Type I interferons (IFN-I) are essential to establish antiviral innate immunity. Unanchored (or free) polyubiquitin (poly-Ub) has been shown to regulate IFN-I responses. However, few unanchored poly-Ub interactors are known. To identify factors regulated by unanchored poly-Ub in a physiological setting, we developed an approach to isolate unanchored poly-Ub from lung tissue. We identified the RNA helicase DHX16 as a potential pattern recognition receptor (PRR). Silencing of DHX16 in cells and in vivo diminished IFN-I responses against influenza virus. These effects extended to members of other virus families, including Zika and SARS-CoV-2. DHX16-dependent IFN-I production requires RIG-I and unanchored K48-poly-Ub synthesized by the E3-Ub ligase TRIM6. DHX16 recognizes a signal in influenza RNA segments that undergo splicing and requires its RNA helicase motif for direct, high-affinity interactions with specific viral RNAs. Our study establishes DHX16 as a PRR that partners with RIG-I for optimal activation of antiviral immunity requiring unanchored poly-Ub.

Tyrphostin AG1024 Suppresses Coronaviral Replication by Downregulating JAK1 via an IR/IGF-1R Independent Proteolysis Mediated by Ndfip1/2_NEDD4-like E3 Ligase Itch

JAK1 depletion or downregulation was previously reported to account for coronavirus inhibition. Here, we found that AG1024, an IR (insulin receptor) and IGF-1R (insulin-like growth factor 1 receptor) inhibitor, diminishes JAK1 protein levels and exerts anti-coronaviral activities with EC50 values of 5.2 ± 0.3 μM against transmissible gastroenteritis coronavirus (TGEV) and 4.3 ± 0.3 μM against human flu coronavirus OC43. However, although the IR and IGF-1R signaling pathways are activated by insulin or IGF-1 in swine testis cells, they are not triggered upon TGEV infection. AG1024, therefore, inhibits coronaviral replication and downregulates JAK1 protein levels independently of IR and IGF-1R. Moreover, JAK1 proteolysis caused by AG1024 was found through activation of upstream Ndfip1/2 and its effector NEDD4-like E3 ligase Itch. In addition, ouabain, which was reported to mediate JAK1 proteolysis causing anti-coronaviral activity by activation of Ndfip1/2 and NEDD4 E3 ligase, additively inhibited anti-coronaviral activity and JAK1 diminishment in combination with AG1024. This study provides novel insights into the pharmacological effects of AG1024 and Itch E3 ligase mediated JAK1 proteolysis and identified Ndfip1/2 as a cognate effector for JAK1 proteolysis via the diversified E3 ligases NEDD4 and NEDD4-like Itch. These findings are expected to provide valued information for the future development of anti-viral agents.

Flaviviruses: Innate Immunity, Inflammasome Activation, Inflammatory Cell Death, and Cytokines

The innate immune system is the host’s first line of defense against the invasion of pathogens including flavivirus. The programmed cell death controlled by genes plays an irreplaceable role in resisting pathogen invasion and preventing pathogen infection. However, the inflammatory cell death, which can trigger the overflow of a large number of pro-inflammatory cytokines and cell contents, will initiate a severe inflammatory response. In this review, we summarized the current understanding of the innate immune response, inflammatory cell death pathway and cytokine secretion regulation during Dengue virus, West Nile virus, Zika virus, Japanese encephalitis virus and other flavivirus infections. We also discussed the impact of these flavivirus and viral proteins on these biological processes. This not only provides a scientific basis for elucidating the pathogenesis of flavivirus, but also lays the foundation for the development of effective antiviral therapies.

Regulation of Tripartite Motif-Containing Proteins on Immune Response and Viral Evasion

Tripartite motif-containing proteins (TRIMs), exhibiting ubiquitin E3 ligase activity, are involved in regulation of not only autophagy and apoptosis but also pyrotosis and antiviral immune responses of host cells. TRIMs play important roles in modulating signaling pathways of antiviral immune responses via type I interferon, NF-κB, Janus kinase/signal transducer and activator of transcription (JAK/STAT), and Nrf2. However, viruses are able to antagonize TRIM activity or evenly utilize TRIMs for viral replication. This communication presents the current understanding of TRIMs exploited by viruses to evade host immune response.

The Role of the Host Ubiquitin System in Promoting Replication of Emergent Viruses

Ubiquitination of proteins is a post-translational modification process with many different cellular functions, including protein stability, immune signaling, antiviral functions and virus replication. While ubiquitination of viral proteins can be used by the host as a defense mechanism by destroying the incoming pathogen, viruses have adapted to take advantage of this cellular process. The ubiquitin system can be hijacked by viruses to enhance various steps of the replication cycle and increase pathogenesis. Emerging viruses, including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), flaviviruses like Zika and dengue, as well as highly pathogenic viruses like Ebola and Nipah, have the ability to directly use the ubiquitination process to enhance their viral-replication cycle, and evade immune responses. Some of these mechanisms are conserved among different virus families, especially early during virus entry, providing an opportunity to develop broad-spectrum antivirals. Here, we discuss the mechanisms used by emergent viruses to exploit the host ubiquitin system, with the main focus on the role of ubiquitin in enhancing virus replication.

Type I Interferon Susceptibility Distinguishes SARS-CoV-2 from SARS-CoV

SARS-CoV-2, a novel coronavirus (CoV) that causes COVID-19, has recently emerged causing an ongoing outbreak of viral pneumonia around the world. While distinct from SARS-CoV, both group 2B CoVs share similar genome organization, origins to bat CoVs, and an arsenal of immune antagonists. In this report, we evaluate type I interferon (IFN-I) sensitivity of SARS-CoV-2 relative to the original SARS-CoV. Our results indicate that while SARS-CoV-2 maintains similar viral replication to SARS-CoV, the novel CoV is much more sensitive to IFN-I. In Vero E6 and in Calu3 cells, SARS-CoV-2 is substantially attenuated in the context of IFN-I pretreatment, whereas SARS-CoV is not. In line with these findings, SARS-CoV-2 fails to counteract phosphorylation of STAT1 and expression of ISG proteins, while SARS-CoV is able to suppress both. Comparing SARS-CoV-2 and influenza A virus in human airway epithelial cultures, we observe the absence of IFN-I stimulation by SARS-CoV-2 alone but detect the failure to counteract STAT1 phosphorylation upon IFN-I pretreatment, resulting in near ablation of SARS-CoV-2 infection. Next, we evaluated IFN-I treatment postinfection and found that SARS-CoV-2 was sensitive even after establishing infection. Finally, we examined homology between SARS-CoV and SARS-CoV-2 in viral proteins shown to be interferon antagonists. The absence of an equivalent open reading frame 3b (ORF3b) and genetic differences versus ORF6 suggest that the two key IFN-I antagonists may not maintain equivalent function in SARS-CoV-2. Together, the results identify key differences in susceptibility to IFN-I responses between SARS-CoV and SARS-CoV-2 that may help inform disease progression, treatment options, and animal model development.IMPORTANCE With the ongoing outbreak of COVID-19, differences between SARS-CoV-2 and the original SARS-CoV could be leveraged to inform disease progression and eventual treatment options. In addition, these findings could have key implications for animal model development as well as further research into how SARS-CoV-2 modulates the type I IFN response early during infection.

Post-fever retinitis - Newer concepts

Post-fever retinitis (PFR) is an infectious or para-infectious uveitic entity caused by bacterial or viral agents and seen mainly in tropical countries. Systemic symptoms such as joint pain, skin rash are common during the febrile stage. On the basis of only clinical presentation, it is difficult to pin-point the exact etiology for PFR. Serological investigations, polymerase chain reaction, and knowledge of concurrent epidemics in the community may help to identify the etiological organism. Bacterial causes of PFR such as rickettsia and typhoid are treated with systemic antibiotics, with or without systemic steroid therapy, whereas PFR of viral causes such as chikungunya, dengue, West Nile virus, and Zika virus have no specific treatment and are managed with steroids. Nevertheless, many authors have advocated mere observation and the uveitis resolved with its natural course of the disease. In this article, we have discussed the clinical features, pathogenesis, investigations, and management of PFR.

Furin Cleavage Site Is Key to SARS-CoV-2 Pathogenesis

SARS-CoV-2 has resulted in a global pandemic and shutdown economies around the world. Sequence analysis indicates that the novel coronavirus (CoV) has an insertion of a furin cleavage site (PRRAR) in its spike protein. Absent in other group 2B CoVs, the insertion may be a key factor in the replication and virulence of SARS-CoV-2. To explore this question, we generated a SARS-CoV-2 mutant lacking the furin cleavage site (ΔPRRA) in the spike protein. This mutant virus replicated with faster kinetics and improved fitness in Vero E6 cells. The mutant virus also had reduced spike protein processing as compared to wild-type SARS-CoV-2. In contrast, the ΔPRRA had reduced replication in Calu3 cells, a human respiratory cell line, and had attenuated disease in a hamster pathogenesis model. Despite the reduced disease, the ΔPRRA mutant offered robust protection from SARS-CoV-2 rechallenge. Importantly, plaque reduction neutralization tests (PRNT ₅₀ ) with COVID-19 patient sera and monoclonal antibodies against the receptor-binding domain found a shift, with the mutant virus resulting in consistently reduced PRNT ₅₀ titers. Together, these results demonstrate a critical role for the furin cleavage site insertion in SARS-CoV-2 replication and pathogenesis. In addition, these findings illustrate the importance of this insertion in evaluating neutralization and other downstream SARS-CoV-2 assays.

Importance

As COVID-19 has impacted the world, understanding how SARS-CoV-2 replicates and causes virulence offers potential pathways to disrupt its disease. By removing the furin cleavage site, we demonstrate the importance of this insertion to SARS-CoV-2 replication and pathogenesis. In addition, the findings with Vero cells indicate the likelihood of cell culture adaptations in virus stocks that can influence reagent generation and interpretation of a wide range of data including neutralization and drug efficacy. Overall, our work highlights the importance of this key motif in SARS-CoV-2 infection and pathogenesis.

Article Summary

A deletion of the furin cleavage site in SARS-CoV-2 amplifies replication in Vero cells, but attenuates replication in respiratory cells and pathogenesis in vivo. Loss of the furin site also reduces susceptibility to neutralization in vitro .

TRIM Proteins in Host Defense and Viral Pathogenesis

Purpose of Review

Tripartite motif (TRIM) proteins are a large group of E3 ubiquitin ligases involved in different cellular functions. Of special interest are their roles in innate immunity, inflammation, and virus replication. We discuss novel roles of TRIM proteins during virus infections that lead to increased pathogenicity.

Recent Findings

TRIM proteins regulate different antiviral and inflammatory signaling pathways, mostly by promoting ubiquitination of important factors including pattern recognition receptors, adaptor proteins, kinases, and transcription factors that are involved in type I interferon and NF-κB pathways. Therefore, viruses have developed mechanisms to target TRIMs for immune evasion. New evidence is emerging indicating that viruses have the ability to directly use TRIMs and the ubiquitination process to enhance the viral replication cycle and cause increased pathogenesis. A new report on TRIM7 also highlights the potential pro-viral role of TRIMs via ubiquitination of viral proteins and suggests a novel mechanism by which ubiquitination of virus envelope protein may provide determinants of tissue and species tropism.

Summary

TRIM proteins have important functions in promoting host defense against virus infection; however, viruses have adapted to evade TRIM-mediated immune responses and can hijack TRIMs to ultimately increase virus pathogenesis. Only by understanding specific TRIM-virus interactions and by using more in vivo approaches can we learn how to harness TRIM function to develop therapeutic approaches to reduce virus pathogenesis.

Type I interferon susceptibility distinguishes SARS-CoV-2 from SARS-CoV

SARS-CoV-2, a novel coronavirus (CoV) that causes COVID-19, has recently emerged causing an ongoing outbreak of viral pneumonia around the world. While distinct from SARS-CoV, both group 2B CoVs share similar genome organization, origins to bat CoVs, and an arsenal of immune antagonists. In this report, we evaluate type-I interferon (IFN-I) sensitivity of SARS-CoV-2 relative to the original SARS-CoV. Our results indicate that while SARS-CoV-2 maintains similar viral replication to SARS-CoV, the novel CoV is much more sensitive to IFN-I. In Vero and in Calu3 cells, SARS-CoV-2 is substantially attenuated in the context of IFN-I pretreatment, while SARS-CoV is not. In line with these findings, SARS-CoV-2 fails to counteract phosphorylation of STAT1 and expression of ISG proteins, while SARS-CoV is able to suppress both. Comparing SARS-CoV-2 and influenza A virus in human airway epithelial cultures (HAEC), we observe the absence of IFN-I stimulation by SARS-CoV-2 alone, but detect failure to counteract STAT1 phosphorylation upon IFN-I pretreatment resulting in near ablation of SARS-CoV-2 infection. Next, we evaluated IFN-I treatment post infection and found SARS-CoV-2 was sensitive even after establishing infection. Finally, we examined homology between SARS-CoV and SARS-CoV-2 in viral proteins shown to be interferon antagonists. The absence of an equivalent open reading frame (ORF) 3b and changes to ORF6 suggest the two key IFN-I antagonists may not maintain equivalent function in SARS-CoV-2. Together, the results identify key differences in susceptibility to IFN-I responses between SARS-CoV and SARS-CoV-2 that may help inform disease progression, treatment options, and animal model development.

West Nile Virus Restriction in Mosquito and Human Cells: A Virus under Confinement

West Nile virus (WNV) is an emerging neurotropic flavivirus that naturally circulates between mosquitoes and birds. However, WNV has a broad host range and can be transmitted from mosquitoes to several mammalian species, including humans, through infected saliva during a blood meal. Although WNV infections are mostly asymptomatic, 20% to 30% of cases are symptomatic and can occasionally lead to severe symptoms, including fatal meningitis or encephalitis. Over the past decades, WNV-carrying mosquitoes have become increasingly widespread across new regions, including North America and Europe, which constitutes a public health concern. Nevertheless, mosquito and human innate immune defenses can detect WNV infection and induce the expression of antiviral effectors, so-called viral restriction factors, to control viral propagation. Conversely, WNV has developed countermeasures to escape these host defenses, thus establishing a constant arms race between the virus and its hosts. Our review intends to cover most of the current knowledge on viral restriction factors as well as WNV evasion strategies in mosquito and human cells in order to bring an updated overview on WNV-host interactions.

VAMP8 Contributes to the TRIM6-Mediated Type I Interferon Antiviral Response during West Nile Virus Infection

Several members of the tripartite motif (TRIM) family of E3 ubiquitin ligases regulate immune pathways, including the antiviral type I interferon (IFN-I) system. Previously, we demonstrated that TRIM6 is involved in IFN-I induction and signaling. In the absence of TRIM6, optimal IFN-I signaling is reduced, allowing increased replication of interferon-sensitive viruses. Despite having evolved numerous mechanisms to restrict the vertebrate host's IFN-I response, West Nile virus (WNV) replication is sensitive to pretreatment with IFN-I. However, the regulators and products of the IFN-I pathway that are important in regulating WNV replication are incompletely defined. Consistent with WNV's sensitivity to IFN-I, we found that in TRIM6 knockout (TRIM6-KO) A549 cells, WNV replication is significantly increased and IFN-I induction and signaling are impaired compared to wild-type (wt) cells. IFN-β pretreatment was more effective in protecting against subsequent WNV infection in wt cells than TRIM6-KO, indicating that TRIM6 contributes to the establishment of an IFN-induced antiviral response against WNV. Using next-generation sequencing, we identified VAMP8 as a potential factor involved in this TRIM6-mediated antiviral response. VAMP8 knockdown resulted in reduced JAK1 and STAT1 phosphorylation and impaired induction of several interferon-stimulated genes (ISGs) following WNV infection or IFN-β treatment. Furthermore, VAMP8-mediated STAT1 phosphorylation required the presence of TRIM6. Therefore, the VAMP8 protein is a novel regulator of IFN-I signaling, and its expression and function are dependent on TRIM6 activity. Overall, these results provide evidence that TRIM6 contributes to the antiviral response against WNV and identify VAMP8 as a novel regulator of the IFN-I system.IMPORTANCE WNV is a mosquito-borne flavivirus that poses a threat to human health across large discontinuous areas throughout the world. Infection with WNV results in febrile illness, which can progress to severe neurological disease. Currently, there are no approved treatment options to control WNV infection. Understanding the cellular immune responses that regulate viral replication is important in diversifying the resources available to control WNV. Here, we show that the elimination of TRIM6 in human cells results in an increase in WNV replication and alters the expression and function of other components of the IFN-I pathway through VAMP8. Dissecting the interactions between WNV and host defenses both informs basic molecular virology and promotes the development of host- and virus-targeted antiviral strategies.