Heat shock protein 70 modulates influenza A virus polymerase activity

Chaperones Hsc70 and Hsp70 play distinct roles in the replication of bocaparvovirus minute virus of canines

Minute virus of canines (MVC) belongs to the genus Bocaparvovirus (formerly Bocavirus) within the Parvoviridae family and causes serious respiratory and gastrointestinal symptoms in neonatal canines worldwide. A productive viral infection relies on the successful recruitment of host factors for various stages of the viral life cycle. However, little is known about the MVC-host cell interactions. In this study, we identified that two cellular proteins (Hsc70 and Hsp70) interacted with NS1 and VP2 proteins of MVC, and both two domains of Hsc70/Hsp70 were mediated for their interactions. Functional studies revealed that Hsp70 was induced by MVC infection, knockdown of Hsc70 considerably suppressed MVC replication, whereas the replication was dramatically promoted by Hsp70 knockdown. It is interesting that low amounts of overexpressed Hsp70 enhanced viral protein expression and virus production, but high amounts of Hsp70 overexpression weakened them. Upon Hsp70 overexpressing, we observed that the ubiquitination of viral proteins changed with Hsp70 overexpression, and proteasome inhibitor (MG132) restored an accumulation of viral proteins. In addition, we verified that Hsp70 family inhibitors remarkably decreased MVC replication. Overall, we identified Hsc70 and Hsp70 as interactors of MVC NS1 and VP2 proteins and were involved in MVC replication, which may provide novel targets for anti-MVC approach.

Host proteins interact with viral elements and affect the life cycle of highly pathogenic avian influenza A virus H7N9

Host-virus interactions can significantly impact the viral life cycle and pathogenesis; however, our understanding of the specific host factors involved in highly pathogenic avian influenza A virus H7N9 (HPAI H7N9) infection is currently restricted. Herein, we designed and synthesized 65 small interfering RNAs targeting host genes potentially associated with various aspects of RNA virus life cycles. Afterward, HPAI H7N9 viruses were isolated and RNA interference was used to screen for host factors likely to be involved in the life cycle of HPAI H7N9. Moreover, the research entailed assessing the associations between host proteins and HPAI H7N9 proteins. Twelve key host proteins were identified: Annexin A (ANXA)2, ANXA5, adaptor related protein complex 2 subunit sigma 1 (AP2S1), adaptor related protein complex 3 subunit sigma 1 (AP3S1), ATP synthase F1 subunit alpha (ATP5A1), COPI coat complex subunit alpha (COP)A, COPG1, heat shock protein family A (Hsp70) member 1A (HSPA)1A, HSPA8, heat shock protein 90 alpha family class A member 1 (HSP90AA1), RAB11B, and RAB18. Co-immunoprecipitation revealed intricate interactions between viral proteins (hemagglutinin, matrix 1 protein, neuraminidase, nucleoprotein, polymerase basic 1, and polymerase basic 2) and these host proteins, presumably playing a crucial role in modulating the life cycle of HPAI H7N9. Notably, ANXA5, AP2S1, AP3S1, ATP5A1, HSP90A1, and RAB18, were identified as novel interactors with HPAI H7N9 proteins rather than other influenza A viruses (IAVs). These findings underscore the significance of host-viral protein interactions in shaping the dynamics of HPAI H7N9 infection, while highlighting subtle variations compared with other IAVs. Deeper understanding of these interactions holds promise to advance disease treatment and prevention strategies.

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.

HSPA13 modulates type I interferon antiviral pathway and NLRP3 inflammasome to restrict dengue virus infection in macrophages

Dengue virus (DENV) is a type of arthropod-borne Flavivirus, which leads to a series of serious diseases like dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS). DENV has a devastating health and economic impact worldwide. However, there are no suitable drugs to combat the virus. Here we reported that HSPA13, also known as stress chaperone (STCH), is a member of the HSP70 family and is a key regulator of type I interferon (IFN-I) and pro-inflammatory responses during DENV infection. HSPA13 expression was increased in macrophages infected with DENV or other Flaviviruses like Zika virus (ZIKV), Yellow fever virus (YFV) and Japanese encephalitis virus (JEV). Further, HSPA13 suppressed the replication of DENV and other Flaviviruses (ZIKV, JEV, YFV), which exhibited broad-spectrum antiviral effects. On the one hand, HSPA13 promoted production of IFN-β and interferon-stimulated genes (ISGs, such as ISG15, OAS and IFIT3) by interacting with RIG-I and up-regulating RIG-I expression during DENV infection. On the other hand, HSPA13 enhanced NLRP3 inflammasome activation and IL-1β secretion by interacting with ASC in DENV infection. We identified HSPA13 as a potential anti-DENV target. Our results provide clues for the development of antiviral drugs against DENV based on HSPA13 and reveal novel drug target against Flaviviruses.

CASC3 Biomolecular Condensates Restrict Turnip Crinkle Virus by Limiting Host Factor Availability

The exon-junction complex (EJC) plays a role in post-transcriptional gene regulation and exerts antiviral activity towards several positive-strand RNA viruses. However, the spectrum of RNA viruses that are targeted by the EJC or the underlying mechanisms are not well understood. EJC components from Arabidopsis thaliana were screened for antiviral activity towards Turnip crinkle virus (TCV, Tombusviridae). Overexpression of the accessory EJC component CASC3 inhibited TCV accumulation > 10-fold in Nicotiana benthamiana while knock-down of endogenous CASC3 resulted in a > 4-fold increase in TCV accumulation. CASC3 forms cytoplasmic condensates and deletion of the conserved SELOR domain reduced condensate size 7-fold and significantly decreased antiviral activity towards TCV. Mass spectrometry of CASC3 complexes did not identify endogenous stress granule or P-body markers and CASC3 failed to co-localize with an aggresome-specific dye suggesting that CASC3 condensates are distinct from well-established membraneless compartments. Mass spectrometry and bimolecular fluorescence complementation assays revealed that CASC3 sequesters Heat shock protein 70 (Hsp70-1) and Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), two host factors with roles in tombusvirus replication. Overexpression of Hsp70-1 or GAPDH reduced the antiviral activity of CASC3 2.1-fold and 2.8-fold, respectively, and suggests that CASC3 inhibits TCV by limiting host factor availability. Unrelated Tobacco mosaic virus (TMV) also depends on Hsp70-1 and CASC3 overexpression restricted TMV accumulation 4-fold and demonstrates that CASC3 antiviral activity is not TCV-specific. Like CASC3, Auxin response factor 19 (ARF19) forms poorly dynamic condensates but ARF19 overexpression failed to inhibit TCV accumulation and suggests that CASC3 has antiviral activities that are not ubiquitous among cytoplasmic condensates.

New Insights into the Role and Therapeutic Potential of Heat Shock Protein 70 in Bovine Viral Diarrhea Virus Infection

Bovine viral diarrhea virus (BVDV), a positive-strand RNA virus of the genus Pestivirus in the Flaviviridae family, is the causative agent of bovine viral diarrhea-mucosal disease (BVD-MD). BVDV's unique virion structure, genome, and replication mechanism in the Flaviviridae family render it a useful alternative model for evaluating the effectiveness of antiviral drugs used against the hepatitis C virus (HCV). As one of the most abundant and typical heat shock proteins, HSP70 plays an important role in viral infection caused by the family Flaviviridae and is considered a logical target of viral regulation in the context of immune escape. However, the mechanism of HSP70 in BVDV infection and the latest insights have not been reported in sufficient detail. In this review, we focus on the role and mechanisms of HSP70 in BVDV-infected animals/cells to further explore the possibility of targeting this protein for antiviral therapy during viral infection.

Identification of Host Proteins Interacting with IBV S1 Based on Tracheal Organ Culture

Infectious bronchitis virus (IBV) belongs to the gamma-coronavirus genus of Coronaviridae and causes serious infectious diseases in the poultry industry. However, only a few IBV strains can infect avian passage cell lines, seriously hindering the progress of basic research on IBV pathogenesis. Whereas IBV field strains can replicate in tracheal ring organ culture (TOC) without any previous adaptation in chicken embryos or primary cells. In this study, to investigate the potential use of TOC as an in vitro infection model for the study of IBV-host interaction, we first established a chicken embryo TOC culture system and carried out an investigation on the IBV replication kinetics in the system. We found that the selected strains of the IBV GI-1, GI-7, GI-13, GI-19, and GI-22 genotypes could successfully replicate in TOC and bring about damage to the infected trachea. Next, we identified host proteins of the chicken embryo trachea that interact with the IBV S1 protein by immunoprecipitation and protein mass spectrometry. A total of 127 candidate proteins were initially identified with major involvement in cell adhesion pathways and apoptosis- and autophagy-related pathways. The heat shock protein 70 (HSP70) was selected for further investigation in the interaction with IBV viral proteins. Our results showed that HSP70 interacted with IBV S1 in both TOC and CEK cells, whereas HSP70 overexpression inhibited viral replication. This study indicates that TOC is a good system for the elucidation of IBV-host interactions and HSP70 is a potential host antiviral factor.

Sex and molecular differences in cardiovascular parameters at peak influenza disease in mice

There is a growing interest in the detection of subtle changes in cardiovascular physiology in response to viral infection to develop better disease surveillance strategies. This is not only important for earlier diagnosis and better prognosis of symptomatic carriers but also useful to diagnose asymptomatic carriers of the virus. Previous studies provide strong evidence of an association between inflammatory biomarker levels and both blood pressure (BP) and heart rate (HR) during infection. The identification of novel biomarkers during an inflammatory event could significantly improve predictions for cardiovascular events. Thus, we evaluated changes in cardiovascular physiology induced in A/Puerto Rico/8/34 (PR8) influenza infections in female and male C57BL/6J mice and compared them with the traditional method of influenza disease detection using body weight (BW). Using radiotelemetry, changes in BP, HR, and activity were studied. Change in BW of infected females was significantly decreased from 5 to 13 days postinfection (dpi), yet alterations in normal physiology including loss of diurnal rhythm and reduced activity was observed starting at about 3 dpi for HR and 4 dpi for activity and BP; continuing until about 13 dpi. In contrast, males had significantly decreased BW 8 to 12 dpi and demonstrated altered physiological measurements for a shorter period compared with females with a reduction starting at 5 dpi for activity, 6 dpi for BP, and 7 dpi for HR until about 12 dpi, 10 dpi, and 9 dpi, respectively. Finally, females and males exhibited different patterns of inflammatory maker expression in lungs at peak disease by analyzing bulk RNA-sequencing data for lungs and Bio-plex cytokine assay for blood collected from influenza-infected and naïve C57BL/6J female and male mice at 7 dpi. In total, this study provides insight into cardiovascular changes and molecular markers to distinguish sex differences in peak disease caused by influenza virus infection.NEW & NOTEWORTHY This study performed longitudinal cardiovascular measurements of influenza viral infection and identified sex difference in both physiological and molecular markers at peak disease.

Host cell stress response as a predictor of COVID-19 infectivity and disease progression

The coronavirus disease (COVID-19) caused by a coronavirus identified in December 2019 has caused a global pandemic. COVID-19 was declared a pandemic in March 2020 and has led to more than 6.3 million deaths. The pandemic has disrupted world travel, economies, and lifestyles worldwide. Although vaccination has been an effective tool to reduce the severity and spread of the disease there is a need for more concerted approaches to fighting the disease. COVID-19 is characterised as a severe acute respiratory syndrome . The severity of the disease is associated with a battery of comorbidities such as cardiovascular diseases, cancer, chronic lung disease, and renal disease. These underlying diseases are associated with general cellular stress. Thus, COVID-19 exacerbates outcomes of the underlying conditions. Consequently, coronavirus infection and the various underlying conditions converge to present a combined strain on the cellular response. While the host response to the stress is primarily intended to be of benefit, the outcomes are occasionally unpredictable because the cellular stress response is a function of complex factors. This review discusses the role of the host stress response as a convergent point for COVID-19 and several non-communicable diseases. We further discuss the merits of targeting the host stress response to manage the clinical outcomes of COVID-19.

Genomic associations with poxvirus across divergent island populations in Berthelot's pipit

Understanding the mechanisms and genes that enable animal populations to adapt to pathogens is important from an evolutionary, health and conservation perspective. Berthelot's pipit (Anthus berthelotii) experiences extensive and consistent spatial heterogeneity in avian pox infection pressure across its range of island populations, thus providing an excellent system with which to examine how pathogen-mediated selection drives spatial variation in immunogenetic diversity. Here we test for evidence of genetic variation associated with avian pox at both an individual and population-level. At the individual level, we find no evidence that variation in MHC class I and TLR4 (both known to be important in recognising viral infection) was associated with pox infection within two separate populations. However, using genotype-environment association (Bayenv) in conjunction with genome-wide (ddRAD-seq) data, we detected strong associations between population-level avian pox prevalence and allele frequencies of single nucleotide polymorphisms (SNPs) at a number of sites across the genome. These sites were located within genes involved in cellular stress signalling and immune responses, many of which have previously been associated with responses to viral infection in humans and other animals. Consequently, our analyses indicates that pathogen-mediated selection may play a role in shaping genomic variation among relatively recently colonised island bird populations and highlights the utility of genotype-environment associations for identifying candidate genes potentially involved in host-pathogen interactions.

Heat Shock Protein A6 Is Especially Involved in Enterovirus 71 Infection

Hand foot and mouth disease (HFMD) caused by Enterovirus 71 (EV71) infection is still a major infectious disease threatening children’s life and health in the absence of effective antiviral drugs due to its high prevalence and neurovirulence. A study of EV71-specific host response might shed some light on the reason behind its unique epidemiologic features and help to find means to conquer EV71 infection. We reported that host heat shock protein A6 (HSPA6) was induced by EV71 infection and involved infection in both Rhabdomyosarcoma (RD) cells and neurogliocytes. Most importantly, we found that EV71 did not induce the expression of other heat shock proteins HSPA1, HSPA8, and HSPB1 under the same conditions, and other HFMD-associated viruses including CVA16, CVA6, CVA10, and CVB1-3 did not induce the upregulation of HSPA6. In addition, EV71 infection enhanced the cytoplasmic aggregation of HSPA6 and its colocalization with viral capsid protein VP1. These findings suggest that HSPA6 is a potential EV71-specific host factor worthy of further study.

Targeting conserved co-opted host factors to block virus replication: Using allosteric inhibitors of the cytosolic Hsp70s to interfere with tomato bushy stunt virus replication

To further our understanding of the pro-viral roles of the host cytosolic heat shock protein 70 (Hsp70) family, we chose the conserved Arabidopsis thaliana Hsp70-2 and the unique Erd2 (early response to dehydration 2), which contain Hsp70 domains. Based on in vitro studies with purified components, we show that AtHsp70-2 and AtErd2 perform pro-viral functions equivalent to that of the yeast Ssa1 Hsp70. These functions include activation of the tombusvirus RdRp, and stimulation of replicase assembly. Yeast-based complementation studies demonstrate that AtHsp70-2 or AtErd2 are present in the purified tombusvirus replicase. RNA silencing and over-expression studies in Nicotiana benthamiana suggest that both Hsp70-2 and Erd2 are co-opted by tomato bushy stunt virus (TBSV). Moreover, we used allosteric inhibitors of Hsp70s to inhibit replication of TBSV and related plant viruses in plants. Altogether, interfering with the functions of the co-opted Hsp70s could be an effective antiviral approach against tombusviruses in plants.

Role of Heat Shock Proteins (HSP70 and HSP90) in Viral Infection

Heat shock proteins (HSPs) are a large group of chaperones found in most eukaryotes and bacteria. They are responsible for the correct protein folding, protection of the cell against stressors, presenting immune and inflammatory cytokines; furthermore, they are important factors in regulating cell differentiation, survival and death. Although the biological function of HSPs is to maintain cell homeostasis, some of them can be used by viruses both to fold their proteins and increase the chances of survival in unfavorable host conditions. Folding viral proteins as well as replicating many different viruses are carried out by, among others, proteins from the HSP70 and HSP90 families. In some cases, the HSP70 family proteins directly interact with viral polymerase to enhance viral replication or they can facilitate the formation of a viral replication complex and/or maintain the stability of complex proteins. It is known that HSP90 is important for the expression of viral genes at both the transcriptional and the translational levels. Both of these HSPs can form a complex with HSP90 and, consequently, facilitate the entry of the virus into the cell. Current studies have shown the biological significance of HSPs in the course of infection SARS-CoV-2. A comprehensive understanding of chaperone use during viral infection will provide new insight into viral replication mechanisms and therapeutic potential. The aim of this study is to describe the molecular basis of HSP70 and HSP90 participation in some viral infections and the potential use of these proteins in antiviral therapy.

Chicken Heat Shock Protein 70 Is an Essential Host Protein for Infectious Bursal Disease Virus Infection In Vitro

Infectious bursal disease virus (IBDV) infection causes pathogenicity and mortality in chickens, leading to huge economic losses in the poultry industry worldwide. Studies of host-virus interaction can help us to better understand the viral pathogenicity. As a highly conservative host factor, heat shock protein 70 (Hsp70) is observed to be involved in numerous viral infections. However, there is little information about the role of chicken Hsp70 (cHsp70) in IBDV infection. In the present study, the increased expression of cHsp70 was observed during IBDV-infected DF-1 cells. Further studies revealed that Hsp70 had similar locations with the viral double-stranded RNA (dsRNA), and the result of pull-down assay showed the direct interaction between cHsp70 with dsRNA, viral proteins (vp)2 and 3, indicating that maybe cHsp70 participates in the formation of the replication and transcription complex. Furthermore, overexpression of cHsp70 promoted IBDV production and knockdown of cHsp70 using small interfering RNAs (siRNA) and reducedviral production, implying the necessity of cHsp70 in IBDV infection. These results reveal that cHsp70 is essential for IBDV infection in DF-1 cells, suggesting that targeting cHsp70 may be applied as an antiviral strategy.

Generation of recombinant influenza virus bearing strep tagged PB2 and effective identification of interactional host factors

The genome of influenza A virus is negative-sense and segmented RNA, which is transcribed and replicated by the viral RNA-dependent RNA polymerase (RdRp) during the virus life cycle. The viral RdRp is thought to be an important host range and virulence determinant factor, and the 627 site of PB2 subunit is a highly acceptable key site of RdRp function. Besides, the function of RdRp is modulated by several host factors. Identification of the host factors interacting with RdRp is of great interest. Here, we tried to explore an effective method to study virus-host interaction by rescuing replication-competent recombinant influenza viruses carrying Strep tagged PB2. Subsequently, we tested several biological characteristics of recombinant viruses in cells and pathogenicity in mice. Then, we purified of protein complex of Strep tagged PB2 and host factors of interest from 293 T cells infected with recombinant viruses. After purification, we performed mass spectrometry to identify these proteins that interacting with PB2. We identified 57 host factors in total. Through Gene Ontology (GO) and Protein-Protein interaction (PPI) network analysis, we revealed the function and network of these proteins. In summary, we generated replication-competent recombinant influenza viruses by inserting a Strep-Tag into PB2 and purified host factors interacting with viral RdRp bearing a 627 K or 627E PB2. These proteins might function as host range and virulence determinants of influenza virus.

TGF-β1 signaling inhibit the in vitro apoptotic, infection and stimulatory cell response induced by influenza H1N1 virus infection on A549 cells

Influenza A virus (IAV) infection induces host cell responses that could derive in inflammatory and apoptotic response. In this respect, in multiple pathological situations, TGF-β1 has shown anti-inflammatory effect, but its role during IAV infection is poorly understood. Interestingly, recent profiling expression studies have suggested that the TGF-β1 pathway could be functionally related to the IAV infection's host response. To gain an understanding of the involvement of TGF-β1's signaling pathway during IAV infection, we compared different apoptotic proteins such as TNFR1, Fas ligand, XIAP, cIAP, among others proteins, and pro-inflammatory elements like IL-1β in the A549 cells during IAV infection (H1N1/NC/99), with and without 1 h of pre-treatment with TGF-β1. Pre-incubation with TGF-β1 significantly inhibited apoptosis and the presence of pro-apoptotic factors. Moreover, the relative abundance of immunodetected IAV M1 protein along 24 -h post-infection period was abridged, which correlated with a disminished infectious viral progeny Additionally, caspase 1 activation and increase of IL-1β induced by IAV infection was also reduced by TGF-β1 signaling activation. Whereas IAV infection increase of Smad-7 and, as consequence, partially inhibiting Smad2/3 phosphorylation, pre-treatment with TGF-β1 blocked IAV-dependent Smad7 induction and prevented Smad2/3 signaling shutdown. All these data suggest the role of TGF-β1 signaling pathway in the control of host cell response induced by the IAV infection and identify a potential clinical target to modulate acute cell death.

Molecular Elucidation and Therapeutic Targeting for combating COVID19: Current Scenario and Future Prospective

A corona virus disease 2019 (COVID-19) is a contagious disease which is caused by a novel corona virus. Human corona virus (HCoV) recognized as one of the most rapidly evolving viruses owing to its high genomic nucleotide substitution rates and recombination. Among the severe acute respiratory syndrome (SARS) and Middle-East respiratory syndrome (MERS), COVID-19 has spread more rapidly and increased the level of globalization and adaptation of the virus in every environmental condition due to their high rate of molecular diversity. The whole article highlights the general characteristics of corona virus, their molecular diversity, and molecular protein targeting against COVID-19 with their newer approaches. Through this review, an attempt has made to critically evaluate the recent advances and future aspects helpful to the treatment of COVID-19 based on the present understanding of SARS-CoV-2 infections, which may help offer new insights and potential therapeutic targets for the treatment of the COVID-19.

Effect of a fever in viral infections — the ‘Goldilocks’ phenomenon?

Acute infections, including those due to Coronaviridae and other viruses, often stimulate a febrile response. A mild fever appears to improve outcome; it appears to diminish viral replication by several mechanisms, including virion entry into host cells and genome transcription, and improving host defence mechanisms against the pathogen. However, a fever may also damage host cellular and tissue function and increase metabolic demands. At temperatures at the lower end of the febrile range, the benefit of the fever appears to outweigh the detrimental effects. However, at higher temperatures, the outcome worsens, suggesting that the disadvantages of fever on the host predominate. A non-infective fever is associated with a worse outcome at lower temperatures, suggesting that hyperthermia carries less benefit in the absence of infection. This review discusses the risks and benefits of a fever on the host response, focusing on the effects of a fever on viral replication and host response, and the detrimental effect on the host.

Loss of the Nuclear Protein RTF2 Enhances Influenza Virus Replication

Viral infection triggers the secretion of type I interferons, which in turn induce the expression of hundreds of antiviral genes. However, the roles of these induced genes in controlling viral infections remain largely unknown, limiting our ability to develop host-based antiviral therapeutics against pathogenic viruses, such as influenza virus. Here, we performed a loss-of-function genetic CRISPR screen in cells prestimulated with type I interferon to identify antiviral genes that restrict influenza A virus replication. Besides finding key components of the interferon signaling pathway, we discovered a new restriction factor, RTF2, which acts in the nucleus, restricts influenza virus transcription, and contributes to the interferon-induced upregulation of known restriction factors. Our work contributes to the field of antiviral immunology by discovering and characterizing a novel restriction factor of influenza virus and may ultimately be useful for understanding how to control a virus that causes significant morbidity and mortality worldwide.

While hundreds of genes are induced by type I interferons, their roles in restricting the influenza virus life cycle remain mostly unknown. Using a loss-of-function CRISPR screen in cells prestimulated with interferon beta (IFN-β), we identified a small number of factors required for restricting influenza A virus replication. In addition to known components of the interferon signaling pathway, we found that replication termination factor 2 (RTF2) restricts influenza virus at the nuclear stage (and perhaps other stages) of the viral life cycle, based on several lines of evidence. First, a deficiency in RTF2 leads to higher levels of viral primary transcription, even in the presence of cycloheximide to block genome replication and secondary transcription. Second, cells that lack RTF2 have enhanced activity of a viral reporter that depends solely on four viral proteins that carry out replication and transcription in the nucleus. Third, when the RTF2 protein is mislocalized outside the nucleus, it is not able to restrict replication. Finally, the absence of RTF2 leads not only to enhanced viral transcription but also to reduced expression of antiviral factors in response to interferon. RTF2 thus inhibits primary influenza virus transcription, likely acts in the nucleus, and contributes to the upregulation of antiviral effectors in response to type I interferons.

IMPORTANCE Viral infection triggers the secretion of type I interferons, which in turn induce the expression of hundreds of antiviral genes. However, the roles of these induced genes in controlling viral infections remain largely unknown, limiting our ability to develop host-based antiviral therapeutics against pathogenic viruses, such as influenza virus. Here, we performed a loss-of-function genetic CRISPR screen in cells prestimulated with type I interferon to identify antiviral genes that restrict influenza A virus replication. Besides finding key components of the interferon signaling pathway, we discovered a new restriction factor, RTF2, which acts in the nucleus, restricts influenza virus transcription, and contributes to the interferon-induced upregulation of known restriction factors. Our work contributes to the field of antiviral immunology by discovering and characterizing a novel restriction factor of influenza virus and may ultimately be useful for understanding how to control a virus that causes significant morbidity and mortality worldwide.

Aquareovirus NS31 protein serves as a specific inducer for host heat shock 70-kDa protein

Elevation of heat-shock protein expression, known as cellular heat-shock responses, occurs during infection of many viruses, which is involved in viral replication through various mechanisms. Herein, transcriptome analysis revealed that over-expression of non-structural protein NS31 of grass carp reovirus (GCRV) in grass carp Ctenopharyngodon idellus kidney (CIK) cells specifically induced expression of heat-shock response (HSR) genes HSP30 and HSP70. We further found that, among the HSR genes, only HSP70 protein were shown to be efficiently induced in fish cells following NS31 over-expression or GCRV infection. Employing a luciferase assay, we were able to show that the promoter of the HSP70 gene can be specifically activated by NS31. In addition, over-expressing HSP70 in grass carp CIK cells resulted in enhanced replication efficiency of GCRV, and an inhibitor for HSP70 resulted in the inhibition of GCRV replication, indicating that HSP70 should serve as a pro-viral factor. We also found that NS31 could activate HSP70 expression in cells of other vertebrate animals. Similar inducing effect on HSP70 expression was demonstrated for NS31-homologue proteins of other aquareoviruses including American grass carp reovirus (AGCRV) and GRCV (green river chinook virus). All these results indicated NS31 proteins in the Aquareovirus genus should play a key role for up-regulating specific HSP70 gene during viral replication.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Host protein chaperones, RNA helicases and the ubiquitin network highlight the arms race for resources between tombusviruses and their hosts

Positive-strand RNA viruses need to arrogate many cellular resources to support their replication and infection cycles. These viruses co-opt host factors, lipids and subcellular membranes and exploit cellular metabolites to built viral replication organelles in infected cells. However, the host cells have their defensive arsenal of factors to protect themselves from easy exploitation by viruses. In this review, the author discusses an emerging arms race for cellular resources between viruses and hosts, which occur during the early events of virus-host interactions. Recent findings with tomato bushy stunt virus and its hosts revealed that the need of the virus to exploit and co-opt given members of protein families provides an opportunity for the host to deploy additional members of the same or associated protein family to interfere with virus replication. Three examples with well-established heat shock protein 70 and RNA helicase protein families and the ubiquitin network will be described to illustrate this model on the early arms race for cellular resources between tombusviruses and their hosts. We predict that arms race for resources with additional cellular protein families will be discovered with tombusviruses. These advances will fortify research on interactions among other plant and animal viruses and their hosts.

In Silico Discovery of Candidate Drugs against Covid-19

Previous studies reported that Angiotensin converting enzyme 2 (ACE2) is the main cell receptor of SARS-CoV and SARS-CoV-2. It plays a key role in the access of the virus into the cell to produce the final infection. In the present study we investigated in silico the basic mechanism of ACE2 in the lung and provided evidences for new potentially effective drugs for Covid-19. Specifically, we used the gene expression profiles from public datasets including The Cancer Genome Atlas, Gene Expression Omnibus and Genotype-Tissue Expression, Gene Ontology and pathway enrichment analysis to investigate the main functions of ACE2-correlated genes. We constructed a protein-protein interaction network containing the genes co-expressed with ACE2. Finally, we focused on the genes in the network that are already associated with known drugs and evaluated their role for a potential treatment of Covid-19. Our results demonstrate that the genes correlated with ACE2 are mainly enriched in the sterol biosynthetic process, Aryldialkylphosphatase activity, adenosylhomocysteinase activity, trialkylsulfonium hydrolase activity, acetate-CoA and CoA ligase activity. We identified a network of 193 genes, 222 interactions and 36 potential drugs that could have a crucial role. Among possible interesting drugs for Covid-19 treatment, we found Nimesulide, Fluticasone Propionate, Thiabendazole, Photofrin, Didanosine and Flutamide.

Cold Exposure-Induced Up-Regulation of Hsp70 Positively Regulates PEDV mRNA Synthesis and Protein Expression In Vitro

Porcine epidemic diarrhea (PED) is a highly contagious, intestinal infectious disease caused by porcine epidemic diarrhea virus (PEDV). PEDV as an emerging and re-emerging epizootic virus of swine causes substantial economic losses to the pig industry in China and other countries. In China, the occurrence of PED shows significant seasonal variations, usually outbreak during the winter season. The epidemic characteristics of PED may be highly correlated with the changes of ambient temperature. However, molecular mechanism on the seasonal occurrence of PED still remains unclear. It has been widely observed that low ambient temperature up-regulates the expression of host heat shock protein 70 (Hsp70). Here, we showed that nucleotide and protein levels of Hsp70 were up-regulated in the intestinal of cold exposed pig and cold exposed Vero E6 cells. We found that overexpression of Hsp70 could increase PEDV mRNA synthesis and protein expression in Vero E6 and IPEC-J2 cells, while the siRNAs mediated knockdown of Hsp70 and VER155008 mediated inhibition of Hsp70 resulted in inhibition of viral mRNA synthesis and protein expression in Vero E6 cells. These data suggested that Hsp70 positively regulated PEDV mRNA synthesis and protein expression, which being helpful for understanding the seasonality of PED epidemics and development of novel antiviral therapies in the future.

Induction of pro-viral grass carp Ctenopharyngodon idella Hsp70 instead of Hsc70 during infection of grass carp reovirus

Grass carp Ctenopharyngodon idella Hsp70 has been identified to play a functional role in viral attachment of type III grass carp reovirus, GCRV-104. However, it remains to be clarified whether Hsc70, sharing 86% identity with Hsp70, plays a similar role during viral infection. In this study, grass carp Hsp70 was shown to be induced by GCRV-104 in different grass carp cell lines, whereas Hsc70 was expressed in a relatively constant level during the infection. The expression patterns of Hsc70 and Hsp70 were similar to their homologs in mammals. Notably, both inhibitor and over-expression assays indicated that Hsp70 was required for efficient viral replication. Thus, our study supported a novel pro-viral property of Hsp70 besides its reported role in the viral attachment. Results herein presented also suggested that the heat shock response of grass carp might be manipulated by aquareovirus to facilitate its replication in fish cells.

Comparison of transcriptomes of an orthotospovirus vector and non-vector thrips species

Thrips transmit one of the most devastating plant viruses worldwide–tomato spotted wilt tospovirus (TSWV). Tomato spotted wilt tospovirus is a type species in the genus Orthotospovirus and family Tospoviridae. Although there are more than 7,000 thrips species, only nine thrips species are known to transmit TSWV. In this study, we investigated the molecular factors that could affect thrips ability to transmit TSWV. We assembled transcriptomes of a vector, Frankliniella fusca [Hinds], and a non-vector, Frankliniella tritici [Fitch], and performed qualitative comparisons of contigs associated with virus reception, virus infection, and innate immunity. Annotations of F. fusca and F. tritici contigs revealed slight differences across biological process and molecular functional groups. Comparison of virus cell surface receptors revealed that homologs of integrin were present in both species. However, homologs of another receptor, heperan sulfate, were present in F. fusca alone. Contigs associated with virus replication were identified in both species, but a contig involved in inhibition of virus replication (radical s-adenosylmethionine) was only present in the non-vector, F. tritici. Additionally, some differences in immune signaling pathways were identified between vector and non-vector thrips. Detailed investigations are necessary to functionally characterize these differences between vector and non-vector thrips and assess their relevance in orthotospovirus transmission.

Identification of the Novel Host Protein Interacting With the Structural Protein VP1 of Chinese Sacbrood Virus by Yeast Two-Hybrid Screening

Chinese sacbrood virus (CSBV) is the major cause and lead to the collapse of Apis cerana colonies. VP1, the structural protein of CSBV, shows the highest variation in the amino acid sequences among proteins from different CSBV strains as well as exhibits excellent immunogenicity. However, its function with host protein still remains unclear. To clarify its function with host protein, we screened out host cellular proteins that interact with VP1 using the membrane protein yeast two-hybrid system. In addition, we verified interactions between heat shock protein 70 cognate 5 (Hsp70-c5) and VP1 using glutathione S-transferase (GST) pull-down and co-immunoprecipitation assays. VP1 and Hsp70-c5 were colocalized in the cytoplasm and nucleus. Using western blot and real-time polymerase chain reaction (PCR), Hsp70-c5 expression in CSBV-infected larvae was upregulated compared with that in healthy larvae. We observed that when we silenced Hsp70-c5, VP1 expression was significantly downregulated. These results demonstrate that Hsp70-c5 is involved in at least one stage(s) of the viral life cycle.

HspBP1 and anti-HspBP1 levels in the serum of HIV-infected individuals are associated to the disease progression

AIMS

The objective of this research was to quantify the levels of circulating HspBP1 and anti-HspBP1 IgG in HIV-infected individuals and to correlate them with CD4 T cell counts and viral load, as well as to determine the kinetics of those proteins during acute phase.

METHODS AND RESULTS

Sixty serum samples from HIV-positive outpatients, thirty with high viral load and thirty with low viral load were analysed. The HspBP1 and anti-HspBP1 were quantified by ELISA. To investigate the kinetic of HspBP1 and anti-HspBp1 during the acute phase, these proteins and antibodies were quantified in samples of a commercial seroconverting HIV panel. All dosages were compared with the CD4 and CD8 T cell counts and HIV viral load. The results indicated that HIV positive outpatients presented significant increase in HspBP1 and anti-HspBP1 serum levels, compared with uninfected healthy. HspBP1 and anti-HspBP1 were negatively correlated with CD4 counts and CD4:CD8 ratio. In the acute phase, HspBP1 became significantly elevated 15 days after HIV infection.

CONCLUSIONS

These results indicate that the quantification of HspBP1 can be associated to others well-established parameters of the HIV progression.

SIGNIFICANCE AND IMPACT OF THE STUDY

The discovery that HspBp1 and anti-HspBp1 are associated with progression of HIV infection is new and corroborates to validate the quantification of these proteins as an additional strategy in the management of the HIV infection.

Cellular Proteostasis During Influenza A Virus Infection-Friend or Foe?

In order to efficiently replicate, viruses require precise interactions with host components and often hijack the host cellular machinery for their own benefit. Several mechanisms involved in protein synthesis and processing are strongly affected and manipulated by viral infections. A better understanding of the interplay between viruses and their host-cell machinery will likely contribute to the development of novel antiviral strategies. Here, we discuss the current knowledge on the interactions between influenza A virus (IAV), the causative agent for most of the annual respiratory epidemics in humans, and the host cellular proteostasis machinery during infection. We focus on the manipulative capacity of this virus to usurp the cellular protein processing mechanisms and further review the protein quality control mechanisms in the cytosol and in the endoplasmic reticulum that are affected by this virus.

Heat shock protein 70 as a supplementary receptor facilitates enterovirus 71 infections in vitro

As one of the major causative agents of hand, foot and mouth disease (HFMD), enterovirus 71 (EV71) is a small, non-enveloped positive stranded RNA virus. Children suffering EV71 infection may cause severe symptoms including neurological complications, pulmonary edema and aseptic meningitis. EV71 is a neurotropic virus and it can cause the damage of nervous cells, cytokine storm and toxic substance. Identifying the factors that mediate viral binding or entry to host cells is important to uncover the mechanisms which viruses utilize to cause diseases in human body. Heat shock protein 70 (HSP70) is induced during virus infection and facilitates proper protein folding during viral propagation. The role that HSP70 plays during EV71 infection is still unclear. In this study, siRNA interference technique and transgenic technique were used to investigate the interaction between HSP70 and EV71 virus. The result demonstrated that the cell surface HSP70 is not essential for EV71 infection but helps the initial binding of virus to host cells and that multiple receptors are involved during EV71 infection. In addition, HSP70 was upregulated in human neuroblastoma cells (SK-N-SH) infected with EV71.

The Pacific oyster (Crassostrea gigas) Hsp70 modulates the Ostreid herpes virus 1 infectivity

The Ostreid herpes virus type 1 (OsHV-1) is one of the most devastating pathogen in oyster cultures. Among several factors, as food limitation, oxygen depletion, salinity and temperature variations, episodes of "summer mortality" of the Pacific oyster Crassostrea gigas have also been associated with OsHV-1 infection. Mortalities of C. gigas spat and juveniles have increased significantly in Europe, and contemporary mortality records of this mollusk in México have been associated with the occurrence of OsHV-1. In the present study, the expression of the heat shock protein 70 gene from the Pacific oyster correlates with the abundance of DNA polymerase transcripts from the OsHV-1. This may suggest that the induction on the expression of the Pacific oyster hsp70 may potentially participate in the immune response against the virus. Furthermore, this study reports for the first time a TEM representative image of the OsHV-1 in aqueous solution, which possesses an icosahedral shape with a diameter of 70 nm × 100 nm. Finally, the examined sequence encoding the ORF4 of the OsHV-1 isolate from northwest Mexico showed specific sequence variations when compared with OsHV-1 isolates from distant geographical areas.

Influenza A Virus-Host Protein Interactions Control Viral Pathogenesis

The influenza A virus (IAV), a member of the Orthomyxoviridae family, is a highly transmissible respiratory pathogen and represents a continued threat to global health with considerable economic and social impact. IAV is a zoonotic virus that comprises a plethora of strains with different pathogenic profiles. The different outcomes of viral pathogenesis are dependent on the engagement between the virus and the host cellular protein interaction network. The interactions may facilitate virus hijacking of host molecular machinery to fulfill the viral life cycle or trigger host immune defense to eliminate the virus. In recent years, much effort has been made to discover the virus–host protein interactions and understand the underlying mechanisms. In this paper, we review the recent advances in our understanding of IAV–host interactions and how these interactions contribute to host defense and viral pathogenesis.

Delivery of parasite Cdg7_Flc_0990 RNA transcript into intestinal epithelial cells during Cryptosporidium parvum infection suppresses host cell gene transcription through epigenetic mechanisms

Cryptosporidial infection causes dysregulated transcription of host genes key to intestinal epithelial homeostasis, but the underlying mechanisms remain obscure. Previous studies demonstrate that several Cryptosporidium parvum (C. parvum) RNA transcripts are selectively delivered into epithelial cells during host cell invasion and may modulate gene transcription in infected cells. We report here that C. parvum infection suppresses the transcription of LRP5, SLC7A8, and IL33 genes in infected intestinal epithelium. Trans-suppression of these genes in infected host cells is associated with promoter enrichment of suppressive epigenetic markers (i.e., H3K9me3). Cdg7_FLc_0990, a C. parvum RNA that has previously demonstrated to be delivered into the nuclei of infected epithelial cells, is recruited to the promoter regions of LRP5, SLC7A8, and IL33 genes. Cdg7_FLc_0990 appears to be recruited to their promoter regions together with G9a, a histone methyltransferase for H3K9 methylation. The PR domain zinc finger protein 1, a G9a-interacting protein, is required for the assembly of Cdg7_FLc_0990 to the G9a complex and gene-specific enrichment of H3K9 methylation. Our data demonstrate that cryptosporidial infection induces epigenetic histone methylations in infected cells through nuclear transfer of parasite Cdg7_Flc_0990 RNA transcript, resulting in transcriptional suppression of the LRP5, SLC7A8, and IL33 genes.

Delivery of Parasite RNA Transcripts Into Infected Epithelial Cells During Cryptosporidium Infection and Its Potential Impact on Host Gene Transcription

Cryptosporidium parvum is an important opportunistic parasite pathogen for immunocompromised individuals and a common cause of diarrhea in young children. Previous studies have identified a panel of RNA transcripts of very low protein-coding potential in C. parvum. Using an in vitro model of human intestinal cryptosporidiosis, we report here that some of these C. parvum RNA transcripts were selectively delivered into the nuclei of host epithelial cells during C. parvum infection. Nuclear delivery of several such parasitic RNAs, including Cdg7_FLc_0990, involved heat-shock protein 70-mediated nuclear importing mechanism. Overexpression of Cdg7_FLc_0990 in intestinal epithelial cells resulted in significant changes in expression levels of specific genes, with significant overlapping with alterations in gene expression profile detected in host cells after C. parvum infection. Our data demonstrate that C. parvum transcripts of low protein-coding potential are selectively delivered into epithelial cells during infection and may modulate gene transcription in infected host cells.

Modulation the alternative splicing of GLA (IVS4+919G>A) in Fabry disease

While a base substitution in intron 4 of GLA (IVS4+919G>A) that causes aberrant alternative splicing resulting in Fabry disease has been reported, its molecular mechanism remains unclear. Here we reported that upon IVS4+919G>A transversion, H3K36me3 was enriched across the alternatively spliced region. PSIP1, an adapter of H3K36me3, together with Hsp70 and NONO were recruited and formed a complex with SF2/ASF and SRp20, which further promoted GLA splicing. Amiloride, a splicing regulator in cancer cells, could reverse aberrant histone modification patterns and disrupt the association of splicing complex with GLA. It could also reverse aberrant GLA splicing in a PP1-dependant manner. Our findings revealed the alternative splicing mechanism of GLA (IVS4+919G>A), and a potential treatment for this specific genetic type of Fabry disease by amiloride in the future.

Unrestrained AMPylation targets cytosolic chaperones and activates the heat shock response

Protein AMPylation is a conserved posttranslational modification with emerging roles in endoplasmic reticulum homeostasis. However, the range of substrates and cell biological consequences of AMPylation remain poorly defined. We expressed human and Caenorhabditis elegans AMPylation enzymes-huntingtin yeast-interacting protein E (HYPE) and filamentation-induced by cyclic AMP (FIC)-1, respectively-in Saccharomyces cerevisiae, a eukaryote that lacks endogenous protein AMPylation. Expression of HYPE and FIC-1 in yeast induced a strong cytoplasmic Hsf1-mediated heat shock response, accompanied by attenuation of protein translation, massive protein aggregation, growth arrest, and lethality. Overexpression of Ssa2, a cytosolic heat shock protein (Hsp)70, was sufficient to partially rescue growth. In human cell lines, overexpression of active HYPE similarly induced protein aggregation and the HSF1-dependent heat shock response. Excessive AMPylation also abolished HSP70-dependent influenza virus replication. Our findings suggest a mode of Hsp70 inactivation by AMPylation and point toward a role for protein AMPylation in the regulation of cellular protein homeostasis beyond the endoplasmic reticulum.

Elucidation of the Cellular Interactome of Ebola Virus Nucleoprotein and Identification of Therapeutic Targets

Ebola virus (EBOV) infection results in severe disease and in some cases lethal hemorrhagic fever. The infection is directed by seven viral genes that encode nine viral proteins. By definition, viruses are obligate intracellular parasites and require aspects of host cell biology in order to replicate their genetic material, assemble new virus particles, and subvert host cell antiviral responses. Currently licensed antivirals are targeted against viral proteins to inhibit their function. However, experience with treating HIV and influenza virus demonstrates that resistant viruses are soon selected. An emerging area in virology is to transiently target host cell proteins that play critical proviral roles in virus biology, especially for acute infections. This has the advantage that the protein being targeted is evolutionary removed from the genome of the virus. Proteomics can aid in discovery biology and identify cellular proteins that may be utilized by the virus to facilitate infection. This work focused on defining the interactome of the EBOV nucleoprotein and identified that cellular chaperones, including HSP70, associate with this protein to promote stability. Utilization of a mini-genome replication system based on a recent Makona isolate demonstrated that disrupting the stability of NP had an adverse effect on viral RNA synthesis.

Human Metapneumovirus Impairs Apoptosis of Nasal Epithelial Cells in Asthma via HSP70

Asthmatics are highly susceptible to respiratory viral infections, possibly due to impaired innate immunity. However, the exact mechanisms of susceptibility are likely to differ amongst viruses. Therefore, we infected primary nasal epithelial cells (NECs) from adults with mild-to-moderate asthma, with respiratory syncytial virus (RSV) or human metapneumovirus (hMPV) in vitro and investigated the antiviral response. NECs from these asthmatics supported elevated hMPV but not RSV infection, compared to non-asthmatic controls. This correlated with reduced apoptosis and reduced activation of caspase-9 and caspase-3/7 in response to hMPV, but not RSV. The expression of heat shock protein 70 (HSP70), a known inhibitor of caspase activation and subsequent apoptosis, was amplified in response to hMPV infection. Chemical inhibition of HSP70 function restored caspase activation and reduced hMPV infection in NECs from asthmatic subjects. There was no impairment in the production of IFN by NECs from asthmatics in response to either hMPV or RSV, demonstrating that increased infection of asthmatic airway cells by hMPV is IFN-independent. This study demonstrates, for the first time, a mechanism for elevated hMPV infection in airway epithelial cells from adult asthmatics and identifies HSP70 as a potential target for antiviral and asthma therapies.

Heat Shock Protein 70 Family Members Interact with Crimean-Congo Hemorrhagic Fever Virus and Hazara Virus Nucleocapsid Proteins and Perform a Functional Role in the Nairovirus Replication Cycle

The Nairovirus genus of the Bunyaviridae family contains serious human and animal pathogens classified within multiple serogroups and species. Of these serogroups, the Crimean-Congo hemorrhagic fever virus (CCHFV) serogroup comprises sole members CCHFV and Hazara virus (HAZV). CCHFV is an emerging zoonotic virus that causes often-fatal hemorrhagic fever in infected humans for which preventative or therapeutic strategies are not available. In contrast, HAZV is nonpathogenic to humans and thus represents an excellent model to study aspects of CCHFV biology under conditions of more-accessible biological containment. The three RNA segments that form the nairovirus genome are encapsidated by the viral nucleocapsid protein (N) to form ribonucleoprotein (RNP) complexes that are substrates for RNA synthesis and packaging into virus particles. We used quantitative proteomics to identify cellular interaction partners of CCHFV N and identified robust interactions with cellular chaperones. These interactions were validated using immunological methods, and the specific interaction between native CCHFV N and cellular chaperones of the HSP70 family was confirmed during live CCHFV infection. Using infectious HAZV, we showed for the first time that the nairovirus N-HSP70 association was maintained within both infected cells and virus particles, where N is assembled as RNPs. Reduction of active HSP70 levels in cells by the use of small-molecule inhibitors significantly reduced HAZV titers, and a model for chaperone function in the context of high genetic variability is proposed. These results suggest that chaperones of the HSP70 family are required for nairovirus replication and thus represent a genetically stable cellular therapeutic target for preventing nairovirus-mediated disease.

IMPORTANCE Nairoviruses compose a group of human and animal viruses that are transmitted by ticks and associated with serious or fatal disease. One member is Crimean-Congo hemorrhagic fever virus (CCHFV), which is responsible for fatal human disease and is recognized as an emerging threat within Europe in response to climate change. No preventative or therapeutic strategies against nairovirus-mediated disease are currently available. Here we show that the N protein of CCHFV and the related Hazara virus interact with a cellular protein, HSP70, during both the intracellular and extracellular stages of the virus life cycle. The use of inhibitors that block HSP70 function reduces virus titers by up to 1,000-fold, suggesting that this interaction is important within the context of the nairovirus life cycle and may represent a potent target for antinairovirus therapies against which the virus cannot easily develop resistance.

A small stem-loop structure of the Ebola virus trailer is essential for replication and interacts with heat-shock protein A8

Ebola virus (EBOV) is a single-stranded negative-sense RNA virus belonging to the Filoviridae family. The leader and trailer non-coding regions of the EBOV genome likely regulate its transcription, replication, and progeny genome packaging. We investigated the cis-acting RNA signals involved in RNA–RNA and RNA–protein interactions that regulate replication of eGFP-encoding EBOV minigenomic RNA and identified heat shock cognate protein family A (HSC70) member 8 (HSPA8) as an EBOV trailer-interacting host protein. Mutational analysis of the trailer HSPA8 binding motif revealed that this interaction is essential for EBOV minigenome replication. Selective 2′-hydroxyl acylation analyzed by primer extension analysis of the secondary structure of the EBOV minigenomic RNA indicates formation of a small stem-loop composed of the HSPA8 motif, a 3′ stem-loop (nucleotides 1868–1890) that is similar to a previously identified structure in the replicative intermediate (RI) RNA and a panhandle domain involving a trailer-to-leader interaction. Results of minigenome assays and an EBOV reverse genetic system rescue support a role for both the panhandle domain and HSPA8 motif 1 in virus replication.

The PB2 Subunit of the Influenza A Virus RNA Polymerase Is Imported into the Mitochondrial Matrix

The polymerase basic 2 (PB2) subunit of the RNA polymerase complex of seasonal human influenza A viruses has been shown to localize to the mitochondria. Various roles, including the regulation of apoptosis and innate immune responses to viral infection, have been proposed for mitochondrial PB2. In particular, PB2 has been shown to inhibit interferon expression by associating with the mitochondrial antiviral signaling (MAVS) protein, which acts downstream of RIG-I and MDA-5 in the interferon induction pathway. However, in spite of a growing body of literature on the potential roles of mitochondrial PB2, the exact location of PB2 in mitochondria has not been determined. Here, we used enhanced ascorbate peroxidase (APEX)-tagged PB2 proteins and electron microscopy to study the localization of PB2 in mitochondria. We found that PB2 is imported into mitochondria, where it localizes to the mitochondrial matrix. We also demonstrated that MAVS is not required for the import of PB2 into mitochondria by showing that PB2 associates with mitochondria in MAVS knockout mouse embryo fibroblasts. Instead, we found that amino acid residue 9 in the N-terminal mitochondrial targeting sequence is a determinant of the mitochondrial import of PB2, differentiating the localization of PB2 of human from that of avian influenza A virus strains. We also showed that a virus encoding nonmitochondrial PB2 is attenuated in mouse embryonic fibroblasts (MEFs) compared with an isogenic virus encoding mitochondrial PB2, in a MAVS-independent manner, suggesting a role for PB2 within the mitochondrial matrix. This work extends our understanding of the interplay between influenza virus and mitochondria.

IMPORTANCE The PB2 subunit of the influenza virus RNA polymerase is a major determinant of viral pathogenicity. However, the molecular mechanisms of how PB2 determines pathogenicity remain poorly understood. PB2 associates with mitochondria and inhibits the function of the mitochondrial antiviral signaling protein MAVS, implicating PB2 in the regulation of innate immune responses. We found that PB2 is imported into the mitochondrial matrix and showed that amino acid residue 9 is a determinant of mitochondrial import. The presence of asparagine or threonine in over 99% of all human seasonal influenza virus pre-2009 H1N1, H2N2, and H3N2 strains is compatible with mitochondrial import, whereas the presence of an aspartic acid in over 95% of all avian influenza viruses is not, resulting in a clear distinction between human-adapted and avian influenza viruses. These findings provide insights into the interplay between influenza virus and mitochondria and suggest mechanisms by which PB2 could affect pathogenicity.

Tombusvirus-Host Interactions: Co-Opted Evolutionarily Conserved Host Factors Take Center Court

Plant positive-strand (+)RNA viruses are intracellular infectious agents that reorganize subcellular membranes and rewire the cellular metabolism of host cells to achieve viral replication in elaborate replication compartments. This review describes the viral replication process based on tombusviruses, highlighting common strategies with other plant and animal viruses. Overall, the works on Tomato bushy stunt virus (TBSV) have revealed intriguing and complex functions of co-opted cellular translation factors, heat shock proteins, DEAD-box helicases, lipid transfer proteins, and membrane-deforming proteins in virus replication. The emerging picture is that many of the co-opted host factors are from highly expressed and conserved protein families. By hijacking host proteins, phospholipids, sterols, and the actin network, TBSV exerts supremacy over the host cell to support viral replication in large replication compartments. Altogether, these advances in our understanding of tombusvirus-host interactions are broadly applicable to many other viruses, which also usurp conserved host factors for various viral processes.

Integrating Transcriptomic and Proteomic Data Using Predictive Regulatory Network Models of Host Response to Pathogens

Mammalian host response to pathogenic infections is controlled by a complex regulatory network connecting regulatory proteins such as transcription factors and signaling proteins to target genes. An important challenge in infectious disease research is to understand molecular similarities and differences in mammalian host response to diverse sets of pathogens. Recently, systems biology studies have produced rich collections of omic profiles measuring host response to infectious agents such as influenza viruses at multiple levels. To gain a comprehensive understanding of the regulatory network driving host response to multiple infectious agents, we integrated host transcriptomes and proteomes using a network-based approach. Our approach combines expression-based regulatory network inference, structured-sparsity based regression, and network information flow to infer putative physical regulatory programs for expression modules. We applied our approach to identify regulatory networks, modules and subnetworks that drive host response to multiple influenza infections. The inferred regulatory network and modules are significantly enriched for known pathways of immune response and implicate apoptosis, splicing, and interferon signaling processes in the differential response of viral infections of different pathogenicities. We used the learned network to prioritize regulators and study virus and time-point specific networks. RNAi-based knockdown of predicted regulators had significant impact on viral replication and include several previously unknown regulators. Taken together, our integrated analysis identified novel module level patterns that capture strain and pathogenicity-specific patterns of expression and helped identify important regulators of host response to influenza infection.

An important challenge in infectious disease research is to understand how the human immune system responds to different types of pathogenic infections. An important component of mounting proper response is the transcriptional regulatory network that specifies the context-specific gene expression program in the host cell. However, our understanding of this regulatory network and how it drives context-specific transcriptional programs is incomplete. To address this gap, we performed a network-based analysis of host response to influenza viruses that integrated high-throughput mRNA- and protein measurements and protein-protein interaction networks to identify virus and pathogenicity-specific modules and their upstream physical regulatory programs. We inferred regulatory networks for human cell line and mouse host systems, which recapitulated several known regulators and pathways of the immune response and viral life cycle. We used the networks to study time point and strain-specific subnetworks and to prioritize important regulators of host response. We predicted several novel regulators, both at the mRNA and protein levels, and experimentally verified their role in the virus life cycle based on their ability to significantly impact viral replication.

Human Heat shock protein 40 (Hsp40/DnaJB1) promotes influenza A virus replication by assisting nuclear import of viral ribonucleoproteins

A unique feature of influenza A virus (IAV) life cycle is replication of the viral genome in the host cell nucleus. The nuclear import of IAV genome is an indispensable step in establishing virus infection. IAV nucleoprotein (NP) is known to mediate the nuclear import of viral genome via its nuclear localization signals. Here, we demonstrate that cellular heat shock protein 40 (Hsp40/DnaJB1) facilitates the nuclear import of incoming IAV viral ribonucleoproteins (vRNPs) and is important for efficient IAV replication. Hsp40 was found to interact with NP component of IAV RNPs during early stages of infection. This interaction is mediated by the J domain of Hsp40 and N-terminal region of NP. Drug or RNAi mediated inhibition of Hsp40 resulted in reduced nuclear import of IAV RNPs, diminished viral polymerase function and attenuates overall viral replication. Hsp40 was also found to be required for efficient association between NP and importin alpha, which is crucial for IAV RNP nuclear translocation. These studies demonstrate an important role for cellular chaperone Hsp40/DnaJB1 in influenza A virus life cycle by assisting nuclear trafficking of viral ribonucleoproteins.

At the centre: influenza A virus ribonucleoproteins

Influenza A viral ribonucleoprotein (vRNP) complexes comprise the eight genomic negative-sense RNAs, each of which is bound to multiple copies of the vRNP and a trimeric viral polymerase complex. The influenza virus life cycle centres on the vRNPs, which in turn rely on host cellular processes to carry out functions that are necessary for the successful completion of the virus life cycle. In this Review, we discuss our current knowledge about vRNP trafficking within host cells and the function of these complexes in the context of the virus life cycle, highlighting how structure contributes to function and the crucial interactions with host cell pathways, as well as on the information gaps that remain. An improved understanding of how vRNPs use host cell pathways is essential to identify mechanisms of virus pathogenicity, host adaptation and, ultimately, new targets for antiviral intervention.