Type-IInterferon-Inducible SERTAD3 Inhibits Influenza A Virus Replication by Blocking the Assembly of Viral RNA Polymerase Complex

SERTAD3 interacts with porcine reproductive and respiratory syndrome virus nonstructural protein 9 and inhibits virus replication

Porcine reproductive and respiratory syndrome virus (PRRSV)-encoded nonstructural protein 9 (Nsp9) has RNA-dependent RNA polymerase (RdRp) function and plays a key role in virus replication. Exploring the interaction of PRRSV Nsp9 with host proteins is important for understanding virus pathogenesis. Here, we found that the SERTA domain containing 3 (SERTAD3) is one of the host interacting factors of viral Nsp9 through yeast two-hybrid screening. The computational simulations and coimmunoprecipitation assay also confirmed their interaction. Further, SERTAD3 interacted with the nidovirus RdRp-associated nucleotidyltransferase (NiRAN) domain of Nsp9, while multiple domains of SERTAD3 interacted with Nsp9. PRRSV infection promoted SERTAD3 expression in vitro. SERTAD3 knockdown significantly facilitated PRRSV replication, while its overexpression led to a significant decrease in virus infection, demonstrating that SERTAD3 acts as a negative regulator during PRRSV replication. Mechanistically, we identified that the C-terminal activation domain of SERTAD3 was crucial for its inhibition of PRRSV. The deletion of the activation domain of SERTAD3 significantly impaired its binding ability to Nsp9, indicating that the potent interaction between its activation domain and Nsp9 is crucial for virus inhibition. This study reveals that SERTAD3 acts as a host limiting factor for PRRSV and provides a molecular basis for developing antivirals against PRRSV.

Restriction of influenza A virus replication by host DCAF7-CRL4B axis

The balance between cellular defense and viral escape determines the fate of influenza A virus (IAV) infection. Viral polymerase activity is critical for the replication and propagation of IAV. The antiviral strategies of host cells against IAV infection have not been fully elucidated. Here, we identified DCAF7 as an antiviral factor for IAV, which inhibits the replication of H1N1 and H3N2. Mechanistically, DCAF7 weakens the viral heterotrimer polymerase activity and restricts IAV replication and transcription. DCAF7 as a substrate recognition receptor forms a complete CRL4BDCAF7 E3 ligase with the CRL4B E3 complex to promote K48-linked polyubiquitination of the viral polymerase subunit PA at the K609 site and its degradation. We also showed that a specific cullin-RING E3 ligase (CRL) inhibitor MLN4924 upregulates the protein level of PA and promotes the replication of IAV in vivo. Moreover, activation of CUL4B by etoposide promotes the degradation of PA and inhibits IAV replication in vivo. Importantly, we found that viral NS1 protein decreases DCAF7 level to impair its antiviral efficacy. Taken together, these findings reveal a new mechanism of host resistance to IAV infection and suggest that regulation of the DCAF7-CRL4B axis is a potential target for antivirals.

IMPORTANCE

Until now, the key host factors that affect IAV polymerase have not been fully elucidated. In this study, we identified host DCAF7 as a novel restriction factor for IAV replication. Importantly, DCAF7 acts as a substrate recognition receptor to recruit CRL4B E3 ligase to mediate the degradation of PA through the ubiquitin-proteasome pathway. Further exploration demonstrated that a specific cullin-RING E3 ligase inhibitor MLN4924 promotes IAV replication in vivo, and activation of CUL4B by etoposide inhibits IAV replication in vivo. Notably, we found that the viral NS1 protein decreases DCAF7 level to impair its antiviral efficacy. These findings elucidate the critical function and mechanism of the DCAF7-CRL4B axis in IAV replication, reveal a novel host anti-IAV mechanism, and provide new anti-influenza drug development strategies.

CRTC3 restricts SARS-CoV-2 replication and is antagonized by CREB

Virus-encoding RNA-dependent RNA polymerase (RdRp) is essential for genome replication and gene transcription of human coronaviruses (HCoVs), including severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). We previously identified the interaction between the catalytic subunit NSP12 of SARS-CoV-2 RdRp and the host protein CREB-regulated transcription coactivator 3 (CRTC3), a member of the CRTC family that regulates cyclic AMP response element-binding protein (CREB)-mediated transcriptional activation. Currently, the implication of CRTC3 in the pathogenesis of HCoVs is poorly understood. Herein, we demonstrated that CRTC3 attenuates RdRp activity and SARS-CoV-2 genome replication, therefore reducing the production of progeny viruses. The interaction of CRTC3 with NSP12 contributes to its inhibitory effect on RdRp activity. Furthermore, we expanded the suppressive effects of two other CRTC family members (CRTC1 and CRTC2) on the RdRp activities of lethal HCoVs, including SARS-CoV-2 and Middle East respiratory syndrome coronavirus (MERS-CoV), along with the CREB antagonization. Overall, our research suggests that CRTCs restrict the replication of HCoVs and are antagonized by CREB, which not only provides new insights into the replication regulation of HCoVs, but also offers important information for the development of anti-HCoV interventions.

Chicken UFL1 Restricts Avian Influenza Virus Replication by Disrupting the Viral Polymerase Complex and Facilitating Type I IFN Production

During avian influenza virus (AIV) infection, host defensive proteins promote antiviral innate immunity or antagonize viral components to limit viral replication. UFM1-specific ligase 1 (UFL1) is involved in regulating innate immunity and DNA virus replication in mammals, but the molecular mechanism by which chicken (ch)UFL1 regulates AIV replication is unclear. In this study, we first identified chUFL1 as a negative regulator of AIV replication by enhancing innate immunity and disrupting the assembly of the viral polymerase complex. Mechanistically, chUFL1 interacted with chicken stimulator of IFN genes (chSTING) and contributed to chSTING dimerization and the formation of the STING-TBK1-IRF7 complex. We further demonstrated that chUFL1 promoted K63-linked polyubiquitination of chSTING at K308 to facilitate chSTING-mediated type I IFN production independent of UFMylation. Additionally, chUFL1 expression was upregulated in response to AIV infection. Importantly, chUFL1 also interacted with the AIV PA protein to inhibit viral polymerase activity. Furthermore, chUFL1 impeded the nuclear import of the AIV PA protein and the assembly of the viral polymerase complex to suppress AIV replication. Collectively, these findings demonstrate that chUFL1 restricts AIV replication by disrupting the viral polymerase complex and facilitating type I IFN production, which provides new insights into the regulation of AIV replication in chickens.

Interferon-stimulated gene PVRL4 broadly suppresses viral entry by inhibiting viral-cellular membrane fusion

BACKGROUND

Viral infection elicits the type I interferon (IFN-I) response in host cells and subsequently inhibits viral infection through inducing hundreds of IFN-stimulated genes (ISGs) that counteract many steps in the virus life cycle. However, most of ISGs have unclear functions and mechanisms in viral infection. Thus, more work is required to elucidate the role and mechanisms of individual ISGs against different types of viruses.

RESULTS

Herein, we demonstrate that poliovirus receptor-like protein4 (PVRL4) is an ISG strongly induced by IFN-I stimulation and various viral infections. Overexpression of PVRL4 protein broadly restricts growth of enveloped RNA and DNA viruses, including vesicular stomatitis virus (VSV), herpes simplex virus 1 (HSV-1), influenza A virus (IAV) and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) whereas deletion of PVRL4 in host cells increases viral infections. Mechanistically, it suppresses viral entry by blocking viral-cellular membrane fusion through inhibiting endosomal acidification. The vivo studies demonstrate that Pvrl4-deficient mice were more susceptible to the infection of VSV and IAV.

CONCLUSION

Overall, our studies not only identify PVRL4 as an intrinsic broad-spectrum antiviral ISG, but also provide a candidate host-directed target for antiviral therapy against various viruses including SARS-CoV-2 and its variants in the future.

Influenza Virus Host Restriction Factors: The ISGs and Non-ISGs

Influenza virus has been one of the most prevalent and researched viruses globally. Consequently, there is ample information available about influenza virus lifecycle and pathogenesis. However, there is plenty yet to be known about the determinants of influenza virus pathogenesis and disease severity. Influenza virus exploits host factors to promote each step of its lifecycle. In turn, the host deploys antiviral or restriction factors that inhibit or restrict the influenza virus lifecycle at each of those steps. Two broad categories of host restriction factors can exist in virus-infected cells: (1) encoded by the interferon-stimulated genes (ISGs) and (2) encoded by the constitutively expressed genes that are not stimulated by interferons (non-ISGs). There are hundreds of ISGs known, and many, e.g., Mx, IFITMs, and TRIMs, have been characterized to restrict influenza virus infection at different stages of its lifecycle by (1) blocking viral entry or progeny release, (2) sequestering or degrading viral components and interfering with viral synthesis and assembly, or (3) bolstering host innate defenses. Also, many non-ISGs, e.g., cyclophilins, ncRNAs, and HDACs, have been identified and characterized to restrict influenza virus infection at different lifecycle stages by similar mechanisms. This review provides an overview of those ISGs and non-ISGs and how the influenza virus escapes the restriction imposed by them and aims to improve our understanding of the host restriction mechanisms of the influenza virus.

The aldehyde dehydrogenase ALDH1B1 exerts antiviral effects through the aggregation of the adaptor MAVS

Type I interferons (IFNs) are produced by almost all cell types and play a vital role in host defense against viral infection. Infection with an RNA virus activates receptors such as RIG-I, resulting in the recruitment of the adaptor protein MAVS to the RIG-I-like receptor (RLR) signalosome and the formation of prion-like functional aggregates of MAVS, which leads to IFN-β production. Here, we identified the aldehyde dehydrogenase 1B1 (ALDH1B1) as a previously uncharacterized IFN-stimulated gene (ISG) product with critical roles in the antiviral response. Knockout of ALDH1B1 increased, whereas overexpression of ALDH1B1 restricted, the replication of RNA viruses, such as vesicular stomatitis virus (VSV), Zika virus (ZIKV), dengue virus (DENV), and influenza A virus (IAV). We found that ALDH1B1 localized to mitochondria, where it interacted with the transmembrane domain of MAVS to promote MAVS aggregation. ALDH1B1 was recruited to MAVS aggregates. In addition, ALDH1B1 also enhanced the interaction between activated RIG-I and MAVS, thus increasing IFN-β production and the antiviral response. Furthermore, Aldh1b1-/- mice developed more severe symptoms than did wild-type mice upon IAV infection. Together, these data identify an aldehyde dehydrogenase in mitochondria that functionally regulates MAVS-mediated signaling and the antiviral response.

Inhibition of miR-200b-3p confers broad-spectrum resistance to viral infection by targeting TBK1

The host innate immune system's defense against viral infections depends heavily on type I interferon (IFN-I) production. Research into the mechanisms of virus-host interactions is essential for developing novel antiviral therapies. In this study, we compared the effect of the five members of the microRNA-200 (miR-200) family on IFN-I production during viral infection and found that miR-200b-3p displayed the most pronounced regulatory effect. During viral infection, we discovered that the transcriptional level of microRNA-200b-3p (miR-200b-3p) increased with the infection of influenza virus (IAV) and vesicular stomatitis virus (VSV), and miR-200b-3p production was modulated by the activation of the ERK and p38 pathways. We identified cAMP response element binding protein (CREB) as a novel transcription factor that binds to the miR-200b-3p promoter. MiR-200b-3p reduces NF-κB and IRF3-mediated IFN-I production by targeting the 3' untranslated region (3' UTR) of TBK1 mRNA. Applying miR-200b-3p inhibitor enhances IFN-I production in IAV and VSV-infected mouse models, thus inhibiting viral replication and improving mouse survival ratio. Importantly, in addition to IAV and VSV, miR-200b-3p inhibitors exhibited potent antiviral effects against multiple pathogenic viruses threatening human health worldwide. Overall, our study suggests that miR-200b-3p might be a potential therapeutic target for broad-spectrum antiviral therapy. IMPORTANCE The innate immune response mediated by type I interferon (IFN-I) is essential for controlling viral replication. MicroRNAs (miRNAs) have been found to regulate the IFN signaling pathway. In this study, we describe a novel function of miRNA-200b-3p in negatively regulating IFN-I production during viral infection. miRNA-200b-3p was upregulated by the MAPK pathway activated by IAV and VSV infection. The binding of miRNA-200b-3p to the 3' UTR of TBK1 mRNA reduced IFN-I activation mediated by IRF3 and NF-κB. Application of miR-200b-3p inhibitors exhibited potent antiviral effects against multiple RNA and DNA viruses. These results provide fresh insight into understanding the impact of miRNAs on host-virus interactions and reveal a potential therapeutic target for common antiviral intervention.

Advances in deciphering the interactions between viral proteins of influenza A virus and host cellular proteins

Influenza A virus (IAV) poses a severe threat to the health of animals and humans. The genome of IAV consists of eight single-stranded negative-sense RNA segments, encoding ten essential proteins as well as certain accessory proteins. In the process of virus replication, amino acid substitutions continuously accumulate, and genetic reassortment between virus strains readily occurs. Due to this high genetic variability, new viruses that threaten animal and human health can emerge at any time. Therefore, the study on IAV has always been a focus of veterinary medicine and public health. The replication, pathogenesis, and transmission of IAV involve intricate interplay between the virus and host. On one hand, the entire replication cycle of IAV relies on numerous proviral host proteins that effectively allow the virus to adapt to its host and support its replication. On the other hand, some host proteins play restricting roles at different stages of the viral replication cycle. The mechanisms of interaction between viral proteins and host cellular proteins are currently receiving particular interest in IAV research. In this review, we briefly summarize the current advances in our understanding of the mechanisms by which host proteins affect virus replication, pathogenesis, or transmission by interacting with viral proteins. Such information about the interplay between IAV and host proteins could provide insights into how IAV causes disease and spreads, and might help support the development of antiviral drugs or therapeutic approaches.

Avian Influenza Virus Tropism in Humans

An influenza pandemic happens when a novel influenza A virus is able to infect and transmit efficiently to a new, distinct host species. Although the exact timing of pandemics is uncertain, it is known that both viral and host factors play a role in their emergence. Species-specific interactions between the virus and the host cell determine the virus tropism, including binding and entering cells, replicating the viral RNA genome within the host cell nucleus, assembling, maturing and releasing the virus to neighboring cells, tissues or organs before transmitting it between individuals. The influenza A virus has a vast and antigenically varied reservoir. In wild aquatic birds, the infection is typically asymptomatic. Avian influenza virus (AIV) can cross into new species, and occasionally it can acquire the ability to transmit from human to human. A pandemic might occur if a new influenza virus acquires enough adaptive mutations to maintain transmission between people. This review highlights the key determinants AIV must achieve to initiate a human pandemic and describes how AIV mutates to establish tropism and stable human adaptation. Understanding the tropism of AIV may be crucial in preventing virus transmission in humans and may help the design of vaccines, antivirals and therapeutic agents against the virus.

SERTAD3 induces proteasomal degradation of ZIKV capsid protein and represents a therapeutic target

Zika virus (ZIKV) is a mosquito-borne RNA virus that belongs to the Flaviviridae family. While flavivirus replication is known to occur in the cytoplasm, a significant portion of the viral capsid protein localizes to the nucleus during infection. However, the role of the nuclear capsid is less clear. Herein, we demonstrated SERTA domain containing 3 (SERTAD3) as an antiviral interferon stimulatory gene product had an antiviral ability to ZIKV but not JEV. Mechanistically, we found that SERTAD3 interacted with the capsid protein of ZIKV in the nucleolus and reduced capsid protein abundance through proteasomal degradation. Furthermore, an eight amino acid peptide of SERTAD3 was identified as the minimum motif that binds with ZIKV capsid protein. Remarkably, the eight amino acids synthetic peptide from SERTAD3 significantly prevented ZIKV infection in culture and pregnant mouse models. Taken together, these findings not only reveal the function of SERTAD3 in promoting proteasomal degradation of a specific viral protein but also provide a promising host-targeted therapeutic strategy against ZIKV infection.

Application of the CRISPR/Cas9 System to Study Regulation Pathways of the Cellular Immune Response to Influenza Virus

Influenza A virus (IAV) causes a respiratory infection that affects millions of people of different age groups and can lead to acute respiratory distress syndrome. Currently, host genes, receptors, and other cellular components critical for IAV replication are actively studied. One of the most convenient and accessible genome-editing tools to facilitate these studies is the CRISPR/Cas9 system. This tool allows for regulating the expression of both viral and host cell genes to enhance or impair viral entry and replication. This review considers the effect of the genome editing system on specific target genes in cells (human and chicken) in terms of subsequent changes in the influenza virus life cycle and the efficiency of virus particle production.

Histone deacetylase 3 contributes to the antiviral innate immunity of macrophages by interacting with FOXK1 to regulate STAT1/2 transcription

It is well known that interferon (IFN)-α/-β activates the JAK/STAT signaling pathway and suppresses viral replication through the induction of IFN stimulated genes (ISGs). Here, we report that knockout of HDAC3 from macrophages results in the decreased expression of STAT1 and STAT2, leading to defective antiviral immunity in cells and mice. Further studies show that HDAC3 interacts with a conserved transcription factor Forkhead Box K1 (FOXK1), co-localizes with FOXK1 at the promoter of STAT1 and STAT2, and is required for protecting FOXK1 from lysosomal system-mediated degradation. FOXK1-deficient macrophages also show low STAT1 and STAT2 expression with defective responses to viruses. Thus, our studies uncover the biological importance of HDAC3 in regulating the antiviral immunity of macrophages through interacting with FOXK1 to regulate the expression of STAT1 and STAT2.

Host Non-Coding RNA Regulates Influenza A Virus Replication

Outbreaks of influenza, caused by the influenza A virus (IAV), occur almost every year in various regions worldwide, seriously endangering human health. Studies have shown that host non-coding RNA is an important regulator of host-virus interactions in the process of IAV infection. In this paper, we comprehensively analyzed the research progress on host non-coding RNAs with regard to the regulation of IAV replication. According to the regulation mode of host non-coding RNAs, the signal pathways involved, and the specific target genes, we found that a large number of host non-coding RNAs directly targeted the PB1 and PB2 proteins of IAV. Nonstructural protein 1 and other key genes regulate the replication of IAV and indirectly participate in the regulation of the retinoic acid-induced gene I-like receptor signaling pathway, toll-like receptor signaling pathway, Janus kinase signal transducer and activator of transcription signaling pathway, and other major intracellular viral response signaling pathways to regulate the replication of IAV. Based on the above findings, we mapped the regulatory network of host non-coding RNAs in the innate immune response to the influenza virus. These findings will provide a more comprehensive understanding of the function and mechanism of host non-coding RNAs in the cellular anti-virus response as well as clues to the mechanism of cell-virus interactions and the discovery of antiviral drug targets.

Mammalian and Avian Host Cell Influenza A Restriction Factors

The threat of a new influenza pandemic is real. With past pandemics claiming millions of lives, finding new ways to combat this virus is essential. Host cells have developed a multi-modular system to detect incoming pathogens, a phenomenon called sensing. The signaling cascade triggered by sensing subsequently induces protection for themselves and their surrounding neighbors, termed interferon (IFN) response. This response induces the upregulation of hundreds of interferon-stimulated genes (ISGs), including antiviral effectors, establishing an antiviral state. As well as the antiviral proteins induced through the IFN system, cells also possess a so-called intrinsic immunity, constituted of antiviral proteins that are constitutively expressed, creating a first barrier preceding the induction of the interferon system. All these combined antiviral effectors inhibit the virus at various stages of the viral lifecycle, using a wide array of mechanisms. Here, we provide a review of mammalian and avian influenza A restriction factors, detailing their mechanism of action and in vivo relevance, when known. Understanding their mode of action might help pave the way for the development of new influenza treatments, which are absolutely required if we want to be prepared to face a new pandemic.