Host restriction of emerging high-pathogenic bunyaviruses via MOV10 by targeting viral nucleoprotein and blocking ribonucleoprotein assembly

Pathogenesis and virulence of Heartland virus

Heartland virus (HRTV), an emerging tick-borne pathogenic bunyavirus, has been a concern since 2012, with an increasing incidence, expanding geographical distribution, and high pathogenicity in the United States. Infection from HRTV results in fever, thrombocytopenia, and leucopenia in humans, and in some cases, symptoms can progress to severe outcomes, including haemorrhagic disease, multi-organ failure, and even death. Currently, no vaccines or antiviral drugs are available for treatment of the HRTV disease. Moreover, little is known about HRTV-host interactions, viral replication mechanisms, pathogenesis and virulence, further hampering the development of vaccines and antiviral interventions. Here, we aimed to provide a brief review of HRTV epidemiology, molecular biology, pathogenesis and virulence on the basis of published article data to better understand this virus and provide clues for further study.

Current insights into human pathogenic phenuiviruses and the host immune system

Phenuiviruses are a class of segmented negative-sense single-stranded RNA viruses, typically consisting of three RNA segments that encode four distinct proteins. The emergence of pathogenic phenuivirus strains, such as Rift Valley fever phlebovirus (RVFV) in sub-Saharan Africa, Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) in East and Southeast Asia, and Heartland Virus (HRTV) in the United States has presented considerable challenges to global public health in recent years. The innate immune system plays a crucial role as the initial defense mechanism of the host against invading pathogens. In addition to continued research aimed at elucidating the epidemiological characteristics of phenuivirus, significant advancements have been made in investigating its viral virulence factors (glycoprotein, non-structural protein, and nucleoprotein) and potential host-pathogen interactions. Specifically, efforts have focused on understanding mechanisms of viral immune evasion, viral assembly and egress, and host immune networks involving immune cells, programmed cell death, inflammation, nucleic acid receptors, etc. Furthermore, a plethora of technological advancements, including metagenomics, metabolomics, single-cell transcriptomics, proteomics, gene editing, monoclonal antibodies, and vaccines, have been utilized to further our understanding of phenuivirus pathogenesis and host immune responses. Hence, this review aims to provide a comprehensive overview of the current understanding of the mechanisms of host recognition, viral immune evasion, and potential therapeutic approaches during human pathogenic phenuivirus infections focusing particularly on RVFV and SFTSV.

Innate immune response against vector-borne bunyavirus infection and viral countermeasures

Bunyaviruses are a large group of important viral pathogens that cause significant diseases in humans and animals worldwide. Bunyaviruses are enveloped, single-stranded, negative-sense RNA viruses that infect a wide range of hosts. Upon entry into host cells, the components of viruses are recognized by host innate immune system, leading to the activation of downstream signaling cascades to induce interferons (IFNs) and other proinflammatory cytokines. IFNs bind to their receptors and upregulate the expression of hundreds of interferon-stimulated genes (ISGs). Many ISGs have antiviral activities and confer an antiviral state to host cells. For efficient replication and spread, viruses have evolved different strategies to antagonize IFN-mediated restriction. Here, we discuss recent advances in our understanding of the interactions between bunyaviruses and host innate immune response.

Host factor MxA restricts Dabie bandavirus infection by targeting the viral NP protein to inhibit NP-RdRp interaction and ribonucleoprotein activity

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease with high case mortality rates, which is caused by Dabie bandavirus (DBV), a novel pathogen also termed as SFTS virus (SFTSV). Currently, no specific therapeutic drugs or vaccines are available for SFTS. Myxovirus resistance protein A (MxA) has been shown to inhibit multiple viral pathogens; however, the role of MxA in DBV infection is unknown. Here, we demonstrated that DBV stimulates MxA expression which, in turn, restricts DBV infection. Mechanistic target analysis revealed that MxA specifically interacts with the viral nucleocapsid protein (NP) in a manner independent of RNA. Minigenome reporter assay showed that in agreement with its targeting of NP, MxA inhibits DBV ribonucleoprotein (RNP) activity. In detail, MxA interacts with the NP N-terminal and disrupts the interaction of NP with the viral RNA-dependent RNA polymerase (RdRp) but not NP multimerization, the critical activities of NP for RNP formation and function. Furthermore, MxA N-terminal domain was identified as the functional domain inhibiting DBV infection, and, consistently, then was shown to interact with NP and obstruct the NP-RdRp interaction. Additionally, threonine 103 within the N-terminal domain is important for MxA inhibition to DBV, and its mutation (T103A) attenuates MxA binding to NP and obstruction of the NP-RdRp interaction. This study uncovers MxA inhibition of DBV with a series of functional and mechanistical analyses, providing insights into the virus-host interactions and probably helping inform the development of antiviral agents in the future.IMPORTANCEDBV/SFTSV is an emerging high-pathogenic virus. Since its first identification in China in 2009, cases of DBV infection have been reported in many other countries, posing a significant threat to public health. Uncovering the mechanisms of DBV-host interactions is necessary to understand the viral pathogenesis and host response and may advance the development of antiviral therapeutics. Here, we found that host factor MxA whose expression is induced by DBV restricts the virus infection. Mechanistically, MxA specifically interacts with the viral NP and blocks the NP-RdRp interaction, inhibiting the viral RNP activity. Further studies identified the key domain and amino acid residue required for MxA inhibition to DBV. Consistently, they were then shown to be important for MxA targeting of NP and obstruction of the NP-RdRp association. These findings unravel the restrictive role of MxA in DBV infection and the underlying mechanism, expanding our knowledge of the virus-host interactions.

Human and animal exposure to newly discovered sand fly viruses, China

Introduction

The Hedi virus (HEDV) and Wuxiang virus (WUXV) are newly discovered Bunyaviruses transmitted by sandflies. The geographical distribution of isolation of these two viruses continues to expand and it has been reported that WUXV causes neurological symptoms and even death in suckling mice. However, little is known about the prevalence of the two viruses in mammalian infections.

Methods

In order to understand the infection status of HEDV and WUXV in humans and animals from regions where the viruses have been isolated, this study used Western blotting to detect the positive rates of HEDV and WUXV IgG antibodies in serum samples from febrile patients, dogs, and chickens in the forementioned regions.

Results

The results showed that of the 29 human serum samples, 17.24% (5/29) tested positive for HEDV, while 68.96% (20/29) were positive for WUXV. In the 31 dog serum samples, 87.10% (27/31) were positive for HEDV and 70.97% (22/31) were positive for WUXV, while in the 36 chicken serum samples, 47.22% (17/36) were positive for HEDV, and 52.78% (19/36) were positive for WUXV.

Discussion

These findings suggest there are widespread infections of HEDV and WUXV in mammals (dogs, chickens) and humans from the regions where these viruses have been isolated. Moreover, the positive rate of HEDV infections was higher in local animals compared to that measured in human specimens. This is the first seroepidemiological study of these two sandfly-transmitted viruses. The findings of the study have practical implications for vector-borne viral infections and related zoonotic infections in China, as well as providing an important reference for studies on the relationship between sandfly-transmitted viruses and zoonotic infections outside of China.

Interactome profiling of Crimean-Congo hemorrhagic fever virus glycoproteins

Crimean-Congo hemorrhagic fever virus (CCHFV) is a biosafety level-4 pathogen requiring urgent research and development efforts. The glycoproteins of CCHFV, Gn and Gc, are considered to play multiple roles in the viral life cycle by interactions with host cells; however, these interactions remain largely unclear to date. Here, we analyzed the cellular interactomes of CCHFV glycoproteins and identified 45 host proteins as high-confidence Gn/Gc interactors. These host molecules are involved in multiple cellular biological processes potentially associated with the physiological actions of the viral glycoproteins. Then, we elucidated the role of a representative cellular protein, HAX1. HAX1 interacts with Gn by its C-terminus, while its N-terminal region leads to mitochondrial localization. By the strong interaction, HAX1 sequestrates Gn to mitochondria, thus depriving Gn of its normal Golgi localization that is required for functional glycoprotein-mediated progeny virion packaging. Consistently, the inhibitory activity of HAX1 against viral packaging and hence propagation was further elucidated in the contexts of pseudotyped and authentic CCHFV infections in cellular and animal models. Together, the findings provide a systematic CCHFV Gn/Gc-cell protein-protein interaction map, but also unravel a HAX1/mitochondrion-associated host antiviral mechanism, which may facilitate further studies on CCHFV biology and therapeutic approaches.

Genome-wide analysis of RNA-binding protein co-expression with alternative splicing events in acute respiratory distress syndrome following hematopoietic stem cell transplantation

Patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT) are at an increased risk of developing severe acute respiratory distress syndrome (ARDS), which is characterized by peripheral bilateral patchy lung involvement. The regulatory network of RNA-binding protein (RBP)-alternative splicing (AS) in ARDS following HSCT has not been investigated. We hypothesize that RBP-AS plays a regulatory role during HSCT-ARDS. The published ARDS transcriptome data after HSCT (GSE84439) were downloaded, and the transcriptome data of 13 mRNAs were obtained by sequencing the peripheral blood of 5 HSCT-ARDS patients and 8 ARDS patients through high-throughput sequencing technology. Systematic analysis of downloaded data was performed to obtain differentially expressed RBPs, and the differentially alternative spliced pre-mRNAs in HSCT-ARDS and control groups were used to explore the global gene RBP-AS regulatory network. A total of 1769 differentially expressed genes and 4714 regulated alternative splicing events were identified in peripheral blood from HSCT-ARDS, of which 254 genes had both differential expression and differential AS. In addition, 128 RBPs were identified, of which HDGF, PCBP2, RIOK3, CISD2, and TRIM21, DDX58, MOV10 showed significantly increased or decreased expression in the HSCT-ARDS. RBPs with decreased expression had antiviral activity, while those with increased expression were involved in ROS, fibrosis, and negative viral resistance. The RBP-RASE-RASG regulatory network is constructed. It is related to the dysregulation of antiviral immunomodulation, imbalance in ROS homeostasis and pro-pulmonary fibrosis, which are involved in the development of HSCT-ARDS.

A New Cellular Interactome of SARS-CoV-2 Nucleocapsid Protein and Its Biological Implications

There is still much to uncover regarding the molecular details of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. As the most abundant protein, coronavirus nucleocapsid (N) protein encapsidates viral RNAs, serving as the structural component of ribonucleoprotein and virion, and participates in transcription, replication, and host regulations. Virus-host interaction might give clues to better understand how the virus affects or is affected by its host during infection and identify promising therapeutic candidates. Considering the critical roles of N, we here established a new cellular interactome of SARS-CoV-2 N by using a high-specific affinity purification (S-pulldown) assay coupled with quantitative mass spectrometry and immunoblotting validations, uncovering many N-interacting host proteins unreported previously. Bioinformatics analysis revealed that these host factors are mainly involved in translation regulations, viral transcription, RNA processes, stress responses, protein folding and modification, and inflammatory/immune signaling pathways, in line with the supposed actions of N in viral infection. Existing pharmacological cellular targets and the directing drugs were then mined, generating a drug-host protein network. Accordingly, we experimentally identified several small-molecule compounds as novel inhibitors against SARS-CoV-2 replication. Furthermore, a newly identified host factor, DDX1, was verified to interact and colocalize with N mainly by binding to the N-terminal domain of the viral protein. Importantly, loss/gain/reconstitution-of-function experiments showed that DDX1 acts as a potent anti-SARS-CoV-2 host factor, inhibiting the viral replication and protein expression. The N-targeting and anti-SARS-CoV-2 abilities of DDX1 are consistently independent of its ATPase/helicase activity. Further mechanism studies revealed that DDX1 impedes multiple activities of N, including the N-N interaction, N oligomerization, and N-viral RNA binding, thus likely inhibiting viral propagation. These data provide new clues to better depiction of the N-cell interactions and SARS-CoV-2 infection and may help inform the development of new therapeutic candidates.

Molecular characterization, transcriptional regulation of sea perch Moloney leukemia virus 10 and its antiviral function against VHSV

Moloney leukemia virus 10 (MOV10) is a conserved RNA helicase and has multiple biological functions in mammals, but its role remains poorly understood in bony fish. Here, we cloned a MOV10 homolog from sea perch (Lateolabrax japonicus), which contained 23 exons and 22 introns, with an open reading frame of 3000 bp encoding 1000 amino acids. Tissue distribution analysis showed that MOV10 was high expressed in blood of sea perch. Promoter analysis revealed several putative multiple transcription factors binding sites, including upstream transcription factor 1, GATA-box, transcription initiation factor IIB, activator protein 1 and two interferon (IFN) stimulated response elements. Further analysis found that IFNc, IFNh, and IFNγ could not only activate IFN regulatory factor (IRF) 1 expression which in turn led to the induction of MOV10, but also prompted the expression of IRF10 to hinder excessive MOV10 expression. Moreover, IRF2 also suppressed MOV10 expression that was initiated by IRF1. Viral hemorrhagic septicemia virus (VHSV) infection upregulated MOV10 expression in vivo and in vitro, which in turn, enhanced IFNh expression and exhibited strong antiviral activity against VHSV proliferation. This study provides a basis to investigate the immune escape of VHSV by affecting the biological function of transcription factors in the signaling pathways associated with antiviral molecules.

Transcriptome profiling highlights regulated biological processes and type III interferon antiviral responses upon Crimean-Congo hemorrhagic fever virus infection

Crimean-Congo hemorrhagic fever virus (CCHFV) is a biosafety level-4 (BSL-4) pathogen that causes Crimean-Congo hemorrhagic fever (CCHF) characterized by hemorrhagic manifestation, multiple organ failure and high mortality rate, posing great threat to public health. Despite the recently increasing research efforts on CCHFV, host cell responses associated with CCHFV infection remain to be further characterized. Here, to better understand the cellular response to CCHFV infection, we performed a transcriptomic analysis in human kidney HEK293 ​cells by high-throughput RNA sequencing (RNA-seq) technology. In total, 496 differentially expressed genes (DEGs), including 361 up-regulated and 135 down-regulated genes, were identified in CCHFV-infected cells. These regulated genes were mainly involved in host processes including defense response to virus, response to stress, regulation of viral process, immune response, metabolism, stimulus, apoptosis and protein catabolic process. Therein, a significant up-regulation of type III interferon (IFN) signaling pathway as well as endoplasmic reticulum (ER) stress response was especially remarkable. Subsequently, representative DEGs from these processes were well validated by RT-qPCR, confirming the RNA-seq results and the typical regulation of IFN responses and ER stress by CCHFV. Furthermore, we demonstrate that not only type I but also type III IFNs (even at low dosages) have substantial anti-CCHFV activities. Collectively, the data may provide new and comprehensive insights into the virus-host interactions and particularly highlights the potential role of type III IFNs in restricting CCHFV, which may help inform further mechanistic delineation of the viral infection and development of anti-CCHFV strategies.

SFTS bunyavirus NSs protein sequestrates mTOR into inclusion bodies and deregulates mTOR-ULK1 signaling, provoking pro-viral autophagy

Autophagy is emerging as a critical player in host defense against diverse infections, in addition to its conserved function to maintain cellular homeostasis. Strikingly, some pathogens have evolved strategies to evade, subvert or exploit different steps of the autophagy pathway for their lifecycles. Here, we present a new viral mechanism of manipulating autophagy for its own benefit with severe fever with thrombocytopenia syndrome bunyavirus (SFTSV, an emerging high-pathogenic virus) as a model. SFTSV infection triggers autophagy, leading to complete autophagic flux. Mechanistically, we show that the nonstructural protein of SFTSV (NSs) interacts with mTOR, the pivotal regulator of autophagy, by targeting its kinase domain and captures mTOR into viral inclusion bodies (IBs) induced by NSs itself. Furthermore, NSsimpairs mTOR-mediated phosphorylation of unc-51-like kinase 1 (ULK1) at Ser757, disrupting the inhibitory effect of mTOR on ULK1 activity and thus contributing to autophagy induction. Pharmacologic treatment and Beclin-1 knockout experimental results establish that, in turn, autophagy enhances SFTSV infection and propagation. Moreover, the minigenome reporter system reveals that SFTSV ribonucleoprotein (the transcription and replication machinery) activity can be bolstered by autophagy. Additionally, we found that the NSs proteins of SFTSV-related bunyaviruses have a conserved function of targeting mTOR. Taken together, we unravel a viral strategy of inducing pro-viral autophagy by interacting with mTOR, sequestering mTOR into IBs and hence provoking the downstream ULK1 pathway, which presents a new paradigm for viral manipulation of autophagy and may help inform future development of specific antiviral therapies against SFTSV and related pathogens.

Cytoplasmic ribonucleoprotein complexes, RNA helicases and coronavirus infection

RNA metabolism in the eukaryotic cell includes the formation of ribonucleoprotein complexes (RNPs) that, depending on their protein components, have a different function. Cytoplasmic RNPs, such as stress granules (SGs) or P-bodies (PBs) are quite relevant during infections modulating viral and cellular RNA expression and as key players in the host cell antiviral response. RNA helicases are abundant components of RNPs and could have a significant effect on viral infection. This review focuses in the role that RNPs and RNA helicases have during coronavirus (CoVs) infection. CoVs are emerging highly pathogenic viruses with a large single-stranded RNA genome. During CoV infection, a complex network of RNA-protein interactions in different RNP structures is established. In general, RNA helicases and RNPs have an antiviral function, but there is limited knowledge on whether the viral protein interactions with cell components are mediators of this antiviral effect or are part of the CoV antiviral counteraction mechanism. Additional data is needed to elucidate the role of these RNA-protein interactions during CoV infection and their potential contribution to viral replication or pathogenesis.

In Vitro Infection Dynamics of Wuxiang Virus in Different Cell Lines

Wuxiang virus (WUXV) is a newly discovered Bunyavirales transmitted by sandflies. It is found to infect humans and chickens and can cause neurological symptoms and even death in mice. However, the susceptibility of different hosts and tissue-derived cells to this virus is unclear. In this study, we examined cells derived from murine (BHK-21, N2A), human (HEK-293T, SH-SY5Y), dog (MDCK), pig (PK-15), monkey (Vero), and chicken (DF1), which were inoculated with WUXV at 0.05 MOI, and monitored for monolayer cytopathic effect (CPE). Culture supernatants and cells were collected from 0 to 96 h post-infection, cell viability was determined by trypan blue staining, numbers of infectious virus particles were quantified using plaque tests, and viral nucleic acid contents were determined by RT-qPCR. The presence of WUXV N antigen in infected cells was detected by Western blotting (WB). In response to virus infection, BHK-21, MDCK, and PK-15 cells were characterized by a clear CPE, and we observed reductions in the proportion of viable cells after 96 h. By contrast, no significant CPEs were observed in the other cell lines. We detected increases in viral titers, viral nucleic acid content, and N antigen expression in BHK-21, MDCK, PK-15, HEK-293T, N2A, SH-SY5Y, and DF1 cells post-infection. Vero cells showed no CPE, and the findings for other tests were negative. In conclusion, we tested the susceptibility of different cell lines to WUXV, enhanced our current understanding of WUXV biology at the cellular level, and laid the foundations for further investigation of the underlying virus infection mechanisms.

Characterization and functional interrogation of the SARS-CoV-2 RNA interactome

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the COVID-19 pandemic, which has led to a devastating global health crisis. The emergence of variants that escape neutralizing responses emphasizes the urgent need to deepen our understanding of SARS-CoV-2 biology. Using a comprehensive identification of RNA-binding proteins (RBPs) by mass spectrometry (ChIRP-MS) approach, we identify 107 high-confidence cellular factors that interact with the SARS-CoV-2 genome during infection. By systematically knocking down their expression in human lung epithelial cells, we find that the majority of the identified RBPs are SARS-CoV-2 proviral factors. In particular, we show that HNRNPA2B1, ILF3, QKI, and SFPQ interact with the SARS-CoV-2 genome and promote viral RNA amplification. Our study provides valuable resources for future investigations into the mechanisms of SARS-CoV-2 replication and the identification of host-centered antiviral therapies.

Host MOV10 is induced to restrict herpes simplex virus 1 lytic infection by promoting type I interferon response

Moloney leukemia virus 10 protein (MOV10) is an interferon (IFN)-inducible RNA helicase implicated in antiviral activity against RNA viruses, yet its role in herpesvirus infection has not been investigated. After corneal inoculation of mice with herpes simplex virus 1 (HSV-1), we observed strong upregulation of both MOV10 mRNA and protein in acutely infected mouse trigeminal ganglia. MOV10 suppressed HSV-1 replication in both neuronal and non-neuronal cells, and this suppression required the N-terminus, but not C-terminal helicase domain of MOV10. MOV10 repressed expression of the viral gene ICP0 in transfected cells, but suppressed HSV-1 replication independently of ICP0. MOV10 increased expression of type I IFN in HSV-1 infected cells with little effect on IFN downstream signaling. Treating the cells with IFN-α or an inhibitor of the IFN receptor eliminated MOV10 suppression of HSV-1 replication. MOV10 enhanced IFN production stimulated by cytoplasmic RNA rather than DNA. IKKε co-immunoprecipitated with MOV10 and was required for MOV10 restriction of HSV-1 replication. Mass spectrometry identified ICP27 as a viral protein interacting with MOV10. Co-immunoprecipitation results suggested that this interaction depended on the RGG box of ICP27 and both termini of MOV10. Overexpressed ICP27, but not its RGG-Box deletion mutant, rendered MOV10 unable to regulate HSV-1 replication and type I IFN production. In summary, MOV10 is induced to restrict HSV-1 lytic infection by promoting the type I IFN response through an IKKε-mediated RNA sensing pathway, and its activity is potentially antagonized by ICP27 in an RGG box dependent manner.

Herpes simplex virus 1 (HSV-1) is a ubiquitous DNA virus that can cause various human diseases. Upon HSV-1 invasion, the host elicited the type I interferon (IFN) response as the first line of defense, in which numerous host factors are induced to restrict viral infection, yet our knowledge about these restriction factors remains limited. Here we show that during HSV-1 acute infection Moloney leukemia virus 10 protein (MOV10) was induced to restrict HSV-1 productive infection. MOV10 restricted HSV-1 replication by promoting type I IFN production through an IKKε-mediated RNA sensing pathway. Moreover, we identified ICP27 as a viral protein that can interact with MOV10 and antagonize its antiviral activity. Thus we establish MOV10 as a host restriction factor against a herpesvirus for the first time and expand our knowledge about how viral and host proteins modulate the IFN response.

Animal Model of Severe Fever With Thrombocytopenia Syndrome Virus Infection

Severe fever with thrombocytopenia syndrome (SFTS), an emerging life-threatening infectious disease caused by SFTS bunyavirus (SFTSV; genus Bandavirus, family Phenuiviridae, order Bunyavirales), has been a significant medical problem. Currently, there are no licensed vaccines or specific therapeutic agents available and the viral pathogenesis remains largely unclear. Developing appropriate animal models capable of recapitulating SFTSV infection in humans is crucial for both the study of the viral pathogenic processes and the development of treatment and prevention strategies. Here, we review the current progress in animal models for SFTSV infection by summarizing susceptibility of various potential animal models to SFTSV challenge and the clinical manifestations and histopathological changes in these models. Together with exemplification of studies on SFTSV molecular mechanisms, vaccine candidates, and antiviral drugs, in which animal infection models are utilized, the strengths and limitations of the existing SFTSV animal models and some important directions for future research are also discussed. Further exploration and optimization of SFTSV animal models and the corresponding experimental methods will be undoubtedly valuable for elucidating the viral infection and pathogenesis and evaluating vaccines and antiviral therapies.

SAFA initiates innate immunity against cytoplasmic RNA virus SFTSV infection

Nuclear scaffold attachment factor A (SAFA) is a novel RNA sensor involved in sensing viral RNA in the nucleus and mediating antiviral immunity. Severe fever with thrombocytopenia syndrome virus (SFTSV) is a bunyavirus that causes SFTS with a high fatality rate of up to 30%. It remains elusive whether and how cytoplasmic SFTSV can be sensed by the RNA sensor SAFA. Here, we demonstrated that SAFA was able to detect SFTSV infection and mediate antiviral interferon and inflammatory responses. Transcription and expression levels of SAFA were strikingly upregulated under SFTSV infection. SAFA was retained in the cytoplasm by interaction with SFTSV nucleocapsid protein (NP). Importantly, SFTSV genomic RNA was recognized by cytoplasmic SAFA, which recruited and promoted activation of the STING-TBK1 signaling axis against SFTSV infection. Of note, the nuclear localization signal (NLS) domain of SAFA was important for interaction with SFTSV NP and recognition of SFTSV RNA in the cytoplasm. In conclusion, our study reveals a novel antiviral mechanism in which SAFA functions as a novel cytoplasmic RNA sensor that directly recognizes RNA virus SFTSV and mediates an antiviral response.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an RNA virus with a high fatality rate of up to 30%, which replicates exclusively in the cytoplasm. To date, many cytoplasmic RNA sensors were known to recognize SFTSV infection and trigger antiviral immune responses. Nuclear scaffold attachment factor A (SAFA) is a novel nuclear RNA sensor which can sense viral RNA in the nucleus and promote activation of antiviral immunity. However, there are no studies to investigate whether SAFA could detect cytoplasmic RNA virus infection. Here, we reported that SAFA was able to detect RNA virus SFTSV invasion. Under SFTSV infection, SAFA was retained in the cytoplasm and recognized SFTSV infection by interaction with SFTSV nucleocapsid protein (NP) and cytoplasmic SFTSV RNA directly. Importantly, SAFA recruited and promoted the activation of the STING-TBK1 signaling pathway-mediated antiviral immunity to suppress SFTSV infection. This study provides a further acquaintance in SAFA-mediated antiviral immune responses, illustrating the novel role of SAFA in sensing cytoplasmic SFTSV and mediates an antiviral response.

MOV10 Helicase Interacts with Coronavirus Nucleocapsid Protein and Has Antiviral Activity

Coronaviruses (CoVs) are emergent pathogens that may cause life-threatening respiratory diseases in humans. Understanding of CoV-host interactions may help to identify novel therapeutic targets. MOV10 is an RNA helicase involved in different steps of cellular RNA metabolism. Both MOV10 antiviral and proviral activities have been described in a limited number of viruses, but this protein has not been previously associated with CoVs. We found that during Middle East respiratory syndrome coronavirus (MERS-CoV) infection, MOV10 aggregated in cytoplasmic structures colocalizing with viral nucleocapsid (N) protein. MOV10-N interaction was confirmed by endogenous MOV10 coimmunoprecipitation, and the presence of other cellular proteins was also detected in MOV10 complexes. MOV10 silencing significantly increased both N protein accumulation and virus titer, with no changes in the accumulation of viral RNAs. Moreover, MOV10 overexpression caused a 10-fold decrease in viral titers. These data indicated that MOV10 has antiviral activity during MERS-CoV infection. We postulated that this activity could be mediated by viral RNA sequestration, and in fact, RNA immunoprecipitation data showed the presence of viral RNAs in the MOV10 cytoplasmic complexes. Expression of wild-type MOV10 or of a MOV10 mutant without helicase activity in MOV10 knockout cell lines, developed by CRISPR-Cas technology, indicated that the helicase activity of MOV10 was required for its antiviral effect. Interestingly MOV10-N interaction was conserved in other mildly or highly pathogenic human CoVs, including the recently emerged severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), although MOV10 antiviral activity was found only in highly pathogenic CoVs, suggesting a potential role of MOV10 in the modulation of human CoVs pathogenesis.

Non-structural Proteins of Severe Fever With Thrombocytopenia Syndrome Virus Suppress RNA Synthesis in a Transcriptionally Active cDNA-Derived Viral RNA Synthesis System

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging infectious disease caused by the tick-borne SFTS bunyavirus (SFTSV) resulting in a high fatality rate up to 30%. SFTSV is a negative-strand RNA virus containing three single-stranded RNA genome segments designated as L, M, and S, which respectively, encode the RNA-dependent RNA polymerase (RdRp), glycoproteins Gn and Gc, and nucleoprotein (N) and non-structural proteins (NSs). NSs can form inclusion bodies (IBs) in infected and transfected cells. A previous study has provided a clue that SFTSV NSs may be involved in virus-like or viral RNA synthesis; however, the details remain unclear. Our work described here reveals that SFTSV NSs can downregulate virus-like RNA synthesis in a dose-dependent manner within a cDNA-derived viral RNA synthesis system, i.e., minigenome (−) and minigenome (+) systems based on transfection, superinfection, and luciferase reporter activity determination; meanwhile, NSs also show a weak inhibitory effect on virus replication. By using co-immunoprecipitation (Co-IP) and RT-PCR combined with site-directed mutagenesis, we found that NSs suppress virus-like RNA or virus replication through interacting with N but not with RdRp, and the negative regulatory effect correlates closely with the IB structure it formed but is not associated with its role of antagonizing host innate immune responses. When the cytoplasmic structure of IB formed by SFTSV NSs was deprived, the inhibitory effect of NSs on virus-like RNA synthesis would weaken and even disappear. Similarly, we also evaluated other bandavirus NSs that cannot form IB in neither infected nor transfected cells, and the results showed that the NSs of Heartland bandavirus (HRTV) did not show a significant inhibitory effect on virus-like RNA synthesis within a minigenome system. Our findings provide experimental evidence that SFTSV NSs participate in regulating virus-like or viral RNA synthesis and the negative effect may be due to the NSs–N interaction.

Unwinding the roles of RNA helicase MOV10

MOV10 is an RNA helicase that associates with the RNA‐induced silencing complex component Argonaute (AGO), likely resolving RNA secondary structures. MOV10 also binds the Fragile X mental retardation protein to block AGO2 binding at some sites and associates with UPF1, a principal component of the nonsense‐mediated RNA decay pathway. MOV10 is widely expressed and has a key role in the cellular response to viral infection and in suppressing retrotransposition. Posttranslational modifications of MOV10 include ubiquitination, which leads to stimulation‐dependent degradation, and phosphorylation, which has an unknown function. MOV10 localizes to the nucleus and/or cytoplasm in a cell type‐specific and developmental stage‐specific manner. Knockout of Mov10 leads to embryonic lethality, underscoring an important role in development where it is required for the completion of gastrulation. MOV10 is expressed throughout the organism; however, most studies have focused on germline cells and neurons. In the testes, the knockdown of Mov10 disrupts proliferation of spermatogonial progenitor cells. In brain, MOV10 is significantly elevated postnatally and binds mRNAs encoding cytoskeleton and neuron projection proteins, suggesting an important role in neuronal architecture. Heterozygous Mov10 mutant mice are hyperactive and anxious and their cultured hippocampal neurons have reduced dendritic arborization. Zygotic knockdown of Mov10 in Xenopus laevis causes abnormal head and eye development and mislocalization of neuronal precursors in the brain. Thus, MOV10 plays a vital role during development, defense against viral infection and in neuronal development and function: its many roles and regulation are only beginning to be unraveled.

This article is categorized under:

RNA Interactions with Proteins and Other Molecules > RNA‐Protein Complexes

RNA Interactions with Proteins and Other Molecules > Protein‐RNA Interactions: Functional Implications

CRL4-DCAF12 Ubiquitin Ligase Controls MOV10 RNA Helicase during Spermatogenesis and T Cell Activation

Multisubunit cullin-RING ubiquitin ligase 4 (CRL4)-DCAF12 recognizes the C-terminal degron containing acidic amino acid residues. However, its physiological roles and substrates are largely unknown. Purification of CRL4-DCAF12 complexes revealed a wide range of potential substrates, including MOV10, an "ancient" RNA-induced silencing complex (RISC) complex RNA helicase. We show that DCAF12 controls the MOV10 protein level via its C-terminal motif in a proteasome- and CRL-dependent manner. Next, we generated Dcaf12 knockout mice and demonstrated that the DCAF12-mediated degradation of MOV10 is conserved in mice and humans. Detailed analysis of Dcaf12-deficient mice revealed that their testes produce fewer mature sperms, phenotype accompanied by elevated MOV10 and imbalance in meiotic markers SCP3 and γ-H2AX. Additionally, the percentages of splenic CD4+ T and natural killer T (NKT) cell populations were significantly altered. In vitro, activated Dcaf12-deficient T cells displayed inappropriately stabilized MOV10 and increased levels of activated caspases. In summary, we identified MOV10 as a novel substrate of CRL4-DCAF12 and demonstrated the biological relevance of the DCAF12-MOV10 pathway in spermatogenesis and T cell activation.

Host Cell Restriction Factors of Bunyaviruses and Viral Countermeasures

The Bunyavirales order comprises more than 500 viruses (generally defined as bunyaviruses) classified into 12 families. Some of these are highly pathogenic viruses infecting different hosts, including humans, mammals, reptiles, arthropods, birds, and/or plants. Host cell sensing of infection activates the innate immune system that aims at inhibiting viral replication and propagation. Upon recognition of pathogen-associated molecular patterns (PAMPs) by cellular pattern recognition receptors (PRRs), numerous signaling cascades are activated, leading to the production of interferons (IFNs). IFNs act in an autocrine and paracrine manner to establish an antiviral state by inducing the expression of hundreds of IFN-stimulated genes (ISGs). Some of these ISGs are known to restrict bunyavirus infection. Along with other constitutively expressed host cellular factors with antiviral activity, these proteins (hereafter referred to as “restriction factors”) target different steps of the viral cycle, including viral entry, genome transcription and replication, and virion egress. In reaction to this, bunyaviruses have developed strategies to circumvent this antiviral response, by avoiding cellular recognition of PAMPs, inhibiting IFN production or interfering with the IFN-mediated response. Herein, we review the current knowledge on host cellular factors that were shown to restrict infections by bunyaviruses. Moreover, we focus on the strategies developed by bunyaviruses in order to escape the antiviral state developed by the infected cells.