West Nile virus differentially modulates the unfolded protein response to facilitate replication and immune evasion. J Virol. 2011;85:2723-2732. [PMID: 21191014 DOI: 10.1128/jvi.02050-10]

Zika virus non-coding RNAs antagonize antiviral responses by PKR-mediated translational arrest

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus that causes severe outbreaks in human populations. ZIKV infection leads to the accumulation of small non-coding viral RNAs (known as sfRNAs) that are crucial for evasion of antiviral responses and for viral pathogenesis. However, the mechanistic understanding of how sfRNAs function remains incomplete. Here, we use recombinant ZIKVs and ribosome profiling of infected human cells to show that sfRNAs block translation of antiviral genes. Mechanistically, we demonstrate that specific RNA structures present in sfRNAs trigger PKR activation, which instead of limiting viral replication, enhances viral particle production. Although ZIKV infection induces mRNA expression of antiviral genes, translation efficiency of type I interferon and interferon stimulated genes were significantly downregulated by PKR activation. Our results reveal a novel viral adaptation mechanism mediated by sfRNAs, where ZIKV increases its fitness by repurposing the antiviral role of PKR into a proviral factor.

The interaction of GRP78 and Zika virus E and NS1 proteins occurs in a chaperone-client manner

Glucose regulated protein 78 (GRP78) is a chaperone protein that is a central mediator of the unfolded protein response, a key cellular stress response pathway. GRP78 has been shown to be critically required for infection and replication of a number of flaviviruses, and to interact with both non-structural (NS) and structural flavivirus proteins. However, the nature of the specific interaction between GRP78 and viral proteins remains largely unknown. This study aimed to characterize the binding domain and critical amino acid residues that mediate the interaction of GRP78 to ZIKV E and NS1 proteins. Recombinant EGFP fused GRP78 and individual subdomains (the nucleotide binding domain (NBD) and the substrate binding domain (SBD)) were used as a bait protein and co-expressed with full length or truncated ZIKV E and NS1 proteins in HEK293T/17 cells. Protein-protein interactions were determined by a co-immunoprecipitation assay. From the results, both the NBD and the SBD of GRP78 were crucial for an effective interaction. Single amino acid substitutions in the SBD showed that R492E and T518A mutants significantly reduced the binding affinity of GRP78 to ZIKV E and NS1 proteins. Notably, the interaction of GRP78 with ZIKV E was stably maintained against various single amino acid substitutions on ZIKV E domain III and with all truncated ZIKV E and NS1 proteins. Collectively, the results suggest that the principal binding between GRP78 and viral proteins is mainly a classic canonical chaperone protein-client interaction. The blocking of GRP78 chaperone function effectively inhibited ZIKV infection and replication in neuronal progenitor cells. Our findings reveal that GRP78 is a potential host target for anti-ZIKV therapeutics.

Cellular stress is triggered by tick-borne encephalitis virus and limits the virus replication in PMJ2-R mouse macrophage cell line

Viral infection may represent a stress condition to the host cell. Cells react to it by triggering the defence programme to restore homeostasis and these events may in turn impact the viral replication. The knowledge about tick-borne encephalitis virus (TBEV) infection-associated stress is limited. Here we investigated the interplay between TBEV infection and stress pathways in PMJ2-R mouse macrophage cell line, as macrophages are the target cells in early phases of TBEV infection. First, to determine how stress influences TBEV replication, the effect of stress inducers H2O2 and tunicamycin (TM) was tested. Viral multiplication was decreased in the presence of both stress inducers suggesting that the stress and cellular stress responses restrict the virus replication. Second, we investigated the induction of oxidative stress and endoplasmic reticulum (ER) stress upon TBEV infection. The level of oxidative stress was interrogated by measuring the reactive oxygen species (ROS). ROS were intermittently increased in infected cells at 12 hpi and at 72 hpi. As mitochondrial dysfunction may result in increased ROS level, we evaluated the mitochondrial homeostasis by measuring the mitochondrial membrane potential (MMP) and found that TBEV infection induced the hyperpolarization of MMP. Moreover, a transient increase of gene expression of stress-induced antioxidative enzymes, like p62, Gclm and Hmox1, was detected. Next, we evaluated the ER stress upon TBEV infection by analysing unfolded protein responses (UPR). We found that infection induced gene expression of two general sensors BiP and CHOP and activated the IRE1 pathway of UPR. Finally, since the natural transmission route of TBEV from its tick vector to the host is mediated via tick saliva, the impact of tick saliva from Ixodes ricinus on stress pathways in TBEV-infected cells was tested. We observed only marginal potentiation of UPR pathway. In conclusion, we found that TBEV infection of PMJ2-R cells elicits the changes in redox balance and triggers cellular stress defences, including antioxidant responses and the IRE1 pathway of UPR. Importantly, our results revealed the negative effect of stress-evoked events on TBEV replication and only marginal impact of tick saliva on stress cellular pathways.

HIV-1 replication requires optimal activation of the unfolded protein response

Several human diseases including viral infections activate the unfolded protein response (UPR) due to abnormal accumulation of unfolded/misfolded proteins. However, UPR modulation and its functional relevance in HIV-1 infection lack comprehensive elucidation. This study reveals that HIV-1 activates IRE1, PERK, and ATF6 signaling pathways of UPR. The knockdown of PERK and ATF6 reduces HIV-1 long terminal repeat (LTR)-driven gene expression, whereas the endoplasmic reticulum (ER) chaperone HSPA5 prevents proteasomal degradation of HIV-1 p24 through its chaperone activity. Interestingly, overstimulation of UPR by a chemical inducer leads to anti-HIV activity through an enhanced type-1 interferon response. Also, treatment with a chemical ER stress inhibitor reduces HIV-1 replication. These findings suggest that an optimal UPR activation is crucial for effective viral replication, as either overstimulating UPR or inhibiting ER stress leads to viral suppression.

CXCR3 antagonist rescues ER stress and reduces inflammation and JEV infection in mice brain

The endoplasmic reticulum (ER) is crucial for maintaining cellular homeostasis, and synthesis and folding of proteins and lipids. The ER is sensitive to stresses including viral infection that perturb the intracellular energy level and redox state, and accumulating unfolded/misfolded proteins. Viruses including Japanese encephalitis virus (JEV) activates unfolded protein response (UPR) causing ER stress in host immune cells and promotes inflammation and apoptotic cell death. The chemokine receptor CXCR3 has been reported to play important role in the accumulation of inflammatory immune cells and neuronal cell death in several disease conditions. Recently we described the involvement of CXCR3 in regulating inflammation and JEV infection in mice brain. Supplementation with a CXCR3 antagonist AMG487 significantly reduced JEV infection in the mice brain in conjunction with the downregulation of UPR pathway via PERK:eIF2α:CHOP, and decreased mitochondrial ROS generation, inflammation and apoptotic cell death. Alongside, AMG487 treatment improved interferon (IFN)-α/β synthesis in JEV-infected mice brain. Thus, suggesting a potential therapeutic role of CXCR3 antagonist against JEV infection.

PKR-mediated stress response enhances dengue and Zika virus replication

IMPORTANCE

One of the fundamental features that make viruses intracellular parasites is the necessity to use cellular translational machinery. Hence, this is a crucial checkpoint for controlling infections. Here, we show that dengue and Zika viruses, responsible for nearly 400 million infections every year worldwide, explore such control for optimal replication. Using immunocompetent cells, we demonstrate that arrest of protein translations happens after sensing of dsRNA and that the information required to avoid this blocking is contained in viral 5'-UTR. Our work, therefore, suggests that the non-canonical translation described for these viruses is engaged when the intracellular stress response is activated.

Effects of Spatial Expression of Activating Transcription Factor 4 on the Pathogenicity of Two Phenotypes of Bovine Viral Diarrhea Virus by Regulating the Endoplasmic Reticulum-Mediated Autophagy Process

The endoplasmic reticulum (ER) stress response is a highly conserved stress-defense mechanism and activates the adaptive unfolded protein response (UPR) to mitigate imbalance. The ER stress-activated signaling pathways can also trigger autophagy to facilitate cellular repair. Bovine viral diarrhea virus (BVDV) utilizes the host cellular ER as the primary site of the life cycle. However, the interplay between cellular ER stress and BVDV replication remains unclear. This report reveals that cytopathic (cp) and noncytopathic (ncp) BVDV have distinct strategies to regulate UPR mechanisms and ER stress-mediated autophagy for their own benefit. Immunoblot analysis revealed that cp and ncp BVDV differentially regulated the abundance of ER chaperone GRP78 for viral replication, while the protein kinase RNA-like ER kinase (PERK)-eukaryotic translation initiation factor 2 subunit α (eIF2α)-activating transcription factor 4 (ATF4) pathway of the UPR was switched on at different stages of infection. Pretreatment with ER stress inducer promoted virion replication, but RNA interference (RNAi) knockdown of ATF4 in BVDV-infected cells significantly attenuated BVDV infectivity titers. More importantly, the effector ATF4 activated by cp BVDV infection translocated into the nucleus to mediate autophagy, but ATF4 was retained in the cytoplasm during ncp BVDV infection. In addition, we found that cp BVDV core protein was localized in the ER to induce ER stress-mediated autophagy. Overall, the potential therapeutic target ATF4 may contribute to the global eradication campaign of BVDV. IMPORTANCE The ER-tropic viruses hijack the host cellular ER as the replication platform of the life cycle, which can lead to strong ER stress. The UPR and related transcriptional cascades triggered by ER stress play a crucial role in viral replication and pathogenesis, but little is known about these underlying mechanisms. Here, we report that cytopathic and noncytopathic BVDV use different strategies to reprogram the cellular UPR and ER stress-mediated autophagy for their own advantage. The cytopathic BVDV unconventionally downregulated the expression level of GRP78, creating perfect conditions for self-replication via the UPR, and the noncytopathic BVDV retained ATF4 in the cytoplasm to provide an advantage for its persistent infection. Our findings provide new insights into exploring how BVDV and other ER-tropic viruses reprogram the UPR signaling pathway in the host cells for replication and reveal the attractive host target ATF4 for new antiviral agents.

Nonstructural proteins 2B and 4A of Tembusu virus induce complete autophagy to promote viral multiplication in vitro

Tembusu virus (TMUV) is an emerging flavivirus that has broken out in different regions of China. TMUV infection has been reported to induce autophagy in duck embryo fibroblast cells. However, the molecular mechanisms underlying this autophagy induction remain unclear. Here, we explored the interactions between autophagy and TMUV and the effects of the structural and nonstructural proteins of TMUV on autophagy in vitro. Among our results, TMUV infection enhanced autophagy to facilitate viral replication in HEK293T cells. After pharmacologically inducing autophagy with rapamycin (Rapa), the replication of TMUV increased by a maximum of 14-fold compared with the control group. To determine which TMUV protein primarily induced autophagy, cells were transfected with two structural proteins and seven nonstructural proteins of TMUV. Western blotting showed that nonstructural proteins 2B (NS2B) and 4 A (NS4A) of TMUV significantly induced the conversion of microtubule-associated protein 1 light chain 3 (LC3) from LC3-I to LC3-II in HEK293T cells. In addition, through immunofluorescence assays, we found that NS2B and NS4A significantly increased the punctate fluorescence of GFP-LC3-II. Furthermore, we found that both NS2B and NS4A interacted with polyubiquitin-binding protein sequestosome 1 (SQSTM1/p62) in a coimmunoprecipitation assay. Moreover, the autophagic degradation of p62 and LC3 mediated by NS2B or NS4A was inhibited by treatment with the autophagic flux inhibitor chloroquine (CQ). These results confirmed the vital effects of NS2B and NS4A in TMUV-induced complete autophagy and clarified the importance of complete autophagy for viral replication, providing novel insight into the relationship between TMUV and autophagy.

Viral Hemorrhagic Septicemia Virus Activates Integrated Stress Response Pathway and Induces Stress Granules to Regulate Virus Replication

Virus infection activates integrated stress response (ISR) and stress granule (SG) formation and viruses counteract by interfering with SG assembly, suggesting an important role in antiviral defense. The infection of fish cells by Viral Hemorrhagic Septicemia Virus (VHSV), activates the innate immune recognition pathway and the production of type I interferon (IFN). However, the mechanisms by which VHSV interacts with ISR pathway regulating SG formation is poorly understood. Here, we demonstrate that fish cells respond to heat shock, oxidative stress and VHSV infection by forming SG that localized key SG marker, Ras GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1). We show that PKR-like endoplasmic reticulum kinase (PERK), but not (dsRNA)-dependent protein kinase (PKR), is required for VHSV-induced SG formation. Furthermore, in VHSV Ia infected cells, PERK activity is required for IFN production, antiviral signaling and viral replication. SG formation required active virus replication as individual VHSV Ia proteins or inactive virus did not induce SG. Cells lacking G3BP1 produced increased IFN, antiviral genes and viral mRNA, however viral protein synthesis and viral titers were reduced. We show a critical role of the activation of ISR pathway and SG formation highlighting a novel role of G3BP1 in regulating VHSV protein translation and replication.

Therapeutics for flaviviral infections

Flaviviruses are vector-borne pathogens capable of causing devastating human diseases. The re-emergence of Zika in 2016 notoriously led to a widescale epidemic in the Americas. New daunting evidence suggests that a single mutation in Zika virus genome may increase transmission and pathogenesis, further highlighting the need to be prepared for flavivirus outbreaks. Dengue, in particular infects about 400 million people each year, leading to reoccurring local outbreaks. Public health efforts to mitigate flavivirus transmission is largely dependent on vector control strategies, as only a limited number of flavivirus vaccines have been developed thus far. There are currently no commercially available antivirals for flaviviruses, leaving supportive care as the primary treatment option. In this review, we will briefly paint a broad picture of the flavivirus landscape in terms of therapeutics, with particular focus on viral targets, promising novel compounds entering the drug discovery pipeline, as well as model systems for evaluating drug efficacy.

Pharmacological Manipulation of UPR: Potential Antiviral Strategy Against Chikungunya Virus

Abstract

Viruses invade the host cells and maneuver the cellular translation machinery to translate the viral proteins in substantial amounts, which may disturb Endoplasmic Reticulum homeostasis leading to induction of Unfolded Protein Response (UPR), a host response pathway involved in viral pathogenesis. Here, we investigated the effect of UPR pathways on the pathogenesis of chikungunya virus infection. We observed that chikungunya virus mediated the modulation of UPR. A positive modulation was observed in the activation of IRE1 and ATF6 branch while the PERK branch of UPR observed suppressed upon virus infection. We further investigated the effect of the inhibition of UPR pathways on chikungunya virus replication using inhibitors for each branch. Cells treated with 3-ethoxy-5,6-dibromosalicylaldehyde (IRE1 inhibitor) and AEBSF (ATF6 inhibitor) significantly inhibits the viral replication process. This study has provided a novel perspective in designing antivirals against chikungunya virus.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01046-5.

Characterization and subcellular localization of Alongshan virus proteins

Alongshan virus (ALSV) in the Jingmenvirus group within the family Flaviviridae is a newly discovered tick-borne virus associated with human disease, whose genome includes four segments and encodes four structural proteins (VP1a, VP1b, VP2, VP3, and VP4) and two non-structural proteins (NSP1 and NSP2). Here, we characterized the subcellular distribution and potential function of ALSV proteins in host cells. We found that viral proteins exhibited diverse subcellular distribution in multiple tissue-deriving cells and induced various morphological changes in the endoplasmic reticulum (ER), and NSP2, VP1b, VP2, and VP4 were all co-localized in the ER. The nuclear transfer and co-localization of VP4 and calnexin (a marker protein of ER), which were independent of their interaction, were unique to HepG2 cells. Expression of NSP1 could significantly reduce mitochondria quantity by inducing mitophagy. These findings would contribute to better understanding of the pathogenesis of emerging segmented flaviviruses.

Induction and modulation of the unfolded protein response during porcine deltacoronavirus infection

Porcine deltacoronavirus (PDCoV) is an emerging enteropathogenic coronavirus that has the potential for cross-species infection. Many viruses have been reported to induce endoplasmic reticulum stress (ERS) and activate the unfolded protein response (UPR). To date, little is known about whether and, if so, how the UPR is activated by PDCoV infection. Here, we investigated the activation state of UPR pathways and their effects on viral replication during PDCoV infection. We found that PDCoV infection induced ERS and activated all three known UPR pathways (inositol-requiring enzyme 1 [IRE1], activating transcription factor 6 [ATF6], and PKR-like ER kinase [PERK]), as demonstrated by IRE1-mediated XBP1 mRNA cleavage and increased mRNA expression of XBP1s, ATF4, CHOP, GADD34, GRP78, and GRP94, as well as phosphorylated eIF2α expression. Through pharmacologic treatment, RNA interference, and overexpression experiments, we confirmed the negative role of the PERK-eIF2α pathway and the positive regulatory role of the ATF6 pathway, but found no obvious effect of IRE1 pathway, on PDCoV replication. Taken together, our results characterize, for the first time, the state of the ERS response during PDCoV infection and identify the PERK and ATF6 pathways as potential antiviral targets.

The role and therapeutic implication of endoplasmic reticulum stress in inflammatory cancer transformation

Endoplasmic reticulum (ER) stress occurs when proteins are affected by various factors, fail to fold properly into higher structures and accumulate in the lumen of the ER, which activates the unfolded protein response (UPR) to restore normal cellular function or induce apoptosis as a self-protective mechanism. However, a growing number of studies have shown that the three branches of ER stress and the UPR can mediate inflammation and cancer development by interacting with inflammatory transformation-related signaling pathways. Targeting the UPR, especially the use of small molecules that target the active sites of the enzymes IRE1α and PERK and BIP/GRP78 inhibitors are potential strategies for treating tumors and have shown promising results in some tumor models. Therefore, in this review, we summarize the progress of ER stress/UPR research and the signaling pathways associated with inflammatory cancer transformation, provide an in-depth description of the mechanisms of these pathways, and outline strategies in the field of UPR biology in tumor therapy to provide new ideas for the mechanisms of inflammatory cancer transformation and tumor-related treatment.

The molecular regulation of autophagy in antimicrobial immunity

Autophagy is a catabolic process that can degrade worn-out organelles and invading pathogens. The activation of autophagy regulates innate and adaptive immunity, playing a key role in the response to microbial invasion. Microbial infection may cause different consequences such as the elimination of invaders through autophagy or xenophagy, host cell death, and symbiotic relationships. Pathogens adapt to the autophagy mechanism and further relieve intracellular stress, which is conducive to host cell survival and microbial growth. The regulation of autophagy forms a complex network through which host immunity is modulated, resulting in a variety of pathophysiological manifestations. Modification of the autophagic pathway is an essential target for the development of antimicrobial drugs.

Pharmacological Inhibition of IRE-1 Alpha Activity in Herpes Simplex Virus Type 1 and Type 2-Infected Dendritic Cells Enhances T Cell Activation

Herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2) infections are life-long and highly prevalent in the human population. These viruses persist in the host, eliciting either symptomatic or asymptomatic infections that may occur sporadically or in a recurrent manner through viral reactivations. Clinical manifestations due to symptomatic infection may be mild such as orofacial lesions, but may also translate into more severe diseases, such as ocular infections that may lead to blindness and life-threatening encephalitis. A key feature of herpes simplex viruses (HSVs) is that they have evolved molecular determinants that hamper numerous components of the host’s antiviral innate and adaptive immune system. Importantly, HSVs infect and negatively modulate the function of dendritic cells (DCs), by inhibiting their T cell-activating capacity and eliciting their apoptosis after infection. Previously, we reported that HSV-2 activates the splicing of the mRNA of XBP1, which is related to the activity of the unfolded protein response (UPR) factor Inositol-Requiring Enzyme 1 alpha (IRE-1α). Here, we sought to evaluate if the activation of the IRE-1α pathway in DCs upon HSV infection may be related to impaired DC function after infection with HSV-1 or HSV-2. Interestingly, the pharmacological inhibition of the endonuclease activity of IRE-1α in HSV-1- and HSV-2-infected DCs significantly reduced apoptosis in these cells and enhanced their capacity to migrate to lymph nodes and activate virus-specific CD4⁺ and CD8⁺ T cells. These findings suggest that the activation of the IRE-1α-dependent UPR pathway in HSV-infected DCs may play a significant role in the negative effects that these viruses exert over these cells and that the modulation of this signaling pathway may be relevant for enhancing the function of DCs upon infection with HSVs.

Albendazole reduces hepatic inflammation and endoplasmic reticulum-stress in a mouse model of chronic Echinococcus multilocularis infection

BACKGROUND

Echinococcus multilocularis causes alveolar echinococcosis (AE), a rising zoonotic disease in the northern hemisphere. Treatment of this fatal disease is limited to chemotherapy using benzimidazoles and surgical intervention, with frequent disease recurrence in cases without radical surgery. Elucidating the molecular mechanisms underlying E. multilocularis infections and host-parasite interactions ultimately aids developing novel therapeutic options. This study explored an involvement of unfolded protein response (UPR) and endoplasmic reticulum-stress (ERS) during E. multilocularis infection in mice.

METHODS

E. multilocularis- and mock-infected C57BL/6 mice were subdivided into vehicle, albendazole (ABZ) and anti-programmed death ligand 1 (αPD-L1) treated groups. To mimic a chronic infection, treatments of mice started six weeks post i.p. infection and continued for another eight weeks. Liver tissue was then collected to examine inflammatory cytokines and the expression of UPR- and ERS-related genes.

RESULTS

E. multilocularis infection led to an upregulation of UPR- and ERS-related proteins in the liver, including ATF6, CHOP, GRP78, ERp72, H6PD and calreticulin, whilst PERK and its target eIF2α were not affected, and IRE1α and ATF4 were downregulated. ABZ treatment in E. multilocularis infected mice reversed, or at least tended to reverse, these protein expression changes to levels seen in mock-infected mice. Furthermore, ABZ treatment reversed the elevated levels of interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α and interferon (IFN)-γ in the liver of infected mice. Similar to ABZ, αPD-L1 immune-treatment tended to reverse the increased CHOP and decreased ATF4 and IRE1α expression levels.

CONCLUSIONS AND SIGNIFICANCE

AE caused chronic inflammation, UPR activation and ERS in mice. The E. multilocularis-induced inflammation and consecutive ERS was ameliorated by ABZ and αPD-L1 treatment, indicating their effectiveness to inhibit parasite proliferation and downregulate its activity status. Neither ABZ nor αPD-L1 themselves affected UPR in control mice. Further research is needed to elucidate the link between inflammation, UPR and ERS, and if these pathways offer potential for improved therapies of patients with AE.

IRE1-Mediated Unfolded Protein Response Promotes the Replication of Tick-Borne Flaviviruses in a Virus and Cell-Type Dependent Manner

Tick-borne flaviviruses (TBFV) can cause severe neurological complications in humans, but differences in tissue tropism and pathogenicity have been described for individual virus strains. Viral protein synthesis leads to the induction of the unfolded protein response (UPR) within infected cells. The IRE1 pathway has been hypothesized to support flavivirus replication by increasing protein and lipid biogenesis. Here, we investigated the role of the UPR in TBFV infection in human astrocytes, neuronal and intestinal cell lines that had been infected with tick-borne encephalitis virus (TBEV) strains Neudoerfl and MucAr-HB-171/11 as well as Langat virus (LGTV). Both TBEV strains replicated better than LGTV in central nervous system (CNS) cells. TBEV strain MucAr-HB-171/11, which is associated with gastrointestinal symptoms, replicated best in intestinal cells. All three viruses activated the inositol-requiring enzyme 1 (IRE1) pathway via the X-box binding protein 1 (XBP1). Interestingly, the neurotropic TBEV strain Neudoerfl induced a strong upregulation of XBP1 in all cell types, but with faster kinetics in CNS cells. In contrast, TBEV strain MucAr-HB-171/11 failed to activate the IRE1 pathway in astrocytes. The low pathogenic LGTV led to a mild induction of IRE1 signaling in astrocytes and intestinal cells. When cells were treated with IRE1 inhibitors prior to infection, TBFV replication in astrocytes was significantly reduced. This confirms a supporting role of the IRE1 pathway for TBFV infection in relevant viral target cells and suggests a correlation between viral tissue tropism and the cell-type dependent induction of the unfolded protein response.

Mitophagy antagonism by ZIKV reveals Ajuba as a regulator of PINK1 signaling, PKR-dependent inflammation, and viral invasion of tissues

Dysregulated inflammation dominated by chemokine expression is a key feature of disease following infection with the globally important human pathogens Zika virus (ZIKV) and dengue virus, but a mechanistic understanding of how pro-inflammatory responses are initiated is lacking. Mitophagy is a quality-control mechanism that regulates innate immune signaling and cytokine production through selective degradation of damaged mitochondria. Here, we demonstrate that ZIKV nonstructural protein 5 (NS5) antagonizes mitophagy by binding to the host protein Ajuba and preventing its translocation to depolarized mitochondria where it is required for PINK1 activation and downstream signaling. Consequent mitophagy suppression amplifies the production of pro-inflammatory chemokines through protein kinase R (PKR) sensing of mitochondrial RNA. In Ajuba^-/- mice, ZIKV induces early expression of pro-inflammatory chemokines associated with significantly enhanced dissemination to tissues. This work identifies Ajuba as a critical regulator of mitophagy and demonstrates a role for mitophagy in limiting systemic inflammation following infection by globally important human viruses.

Opposite effects of apoptotic and necroptotic cellular pathways on rotavirus replication

Group A rotavirus (RVA), one of the leading pathogens causing severe acute gastroenteritis in children and a wide variety of young animals worldwide, induces apoptosis upon infecting cells. Though RVA-induced apoptosis mediated via the dual modulation of its NSP4 and NSP1 proteins is relatively well studied, the nature and signaling pathway(s) involved in RVA-induced necroptosis are yet to be fully elucidated. Here, we demonstrate the nature of RVA-induced necroptosis, the signaling cascade involved, and correlation with RVA-induced apoptosis. Infection with the bovine NCDV and human DS-1 RV strains was shown to activate receptor-interacting protein kinase 1 (RIPK1)/RIPK3/mixed lineage kinase domain-like protein (MLKL), the key necroptosis molecules in virus-infected cells. Using immunoprecipitation assay, RIPK1 was found to bind phosphorylated RIPK3 (pRIPK3) and pMLKL. pMLKL, the major executioner molecule in the necroptotic pathway, was translocated to the plasma membrane of RVA-infected cells to puncture the cell membrane. Interestingly, transfection of RVA NSP4 also induced necroptosis through the RIPK1/RIPK3/MLKL necroptosis pathway. Blockage of each key necroptosis molecule in the RVA-infected or NSP4-transfected cells resulted in decreased necroptosis but increased cell viability and apoptosis, thereby resulting in decreased viral yields in the RVA-infected cells. In contrast, suppression of RVA-induced apoptosis increased necroptosis and virus yields. Our findings suggest that RVA NSP4 also induces necroptosis via the RIPK1/RIPK3/MLKL necroptosis pathway. Moreover, necroptosis and apoptosis-which have proviral and antiviral effects, respectively-exhibited a crosstalk in RVA-infected cells. These findings significantly increase our understanding of the nature of RVA-induced necroptosis and the crosstalk between RVA-induced necroptosis and apoptosis. IMPORTANCE Viral infection usually culminates in cell death through apoptosis, necroptosis, and rarely, pyroptosis. Necroptosis is a form of programmed necrosis that is mediated by signaling complexes of the receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL). Although apoptosis induction by rotavirus and its NSP4 protein is well known, rotavirus-induced necroptosis is not fully understood. Here, we demonstrate that rotavirus and also its NSP4 protein can induce necroptosis in cultured cells through the activation of the RIPK1/RIPK3/MLKL necroptosis pathway. Moreover, rotavirus-induced necroptosis and apoptosis have opposite effects on viral yield, i.e., they function as proviral and antiviral processes, respectively, and counterbalance each other in rotavirus-infected cells. Our findings provide important insights for understanding the nature of rotavirus-induced necroptosis and the development of novel therapeutic strategies against infection with rotavirus and other RNA viruses.

Roles of Non-Structural Protein 4A in Flavivirus Infection

Infections with viruses in the genus Flavivirus are a worldwide public health problem. These enveloped, positive sense single stranded RNA viruses use a small complement of only 10 encoded proteins and the RNA genome itself to remodel host cells to achieve conditions favoring viral replication. A consequence of the limited viral armamentarium is that each protein exerts multiple cellular effects, in addition to any direct role in viral replication. The viruses encode four non-structural (NS) small transmembrane proteins (NS2A, NS2B, NS4A and NS4B) which collectively remain rather poorly characterized. NS4A is a 16kDa membrane associated protein and recent studies have shown that this protein plays multiple roles, including in membrane remodeling, antagonism of the host cell interferon response, and in the induction of autophagy, in addition to playing a role in viral replication. Perhaps most importantly, NS4A has been implicated as playing a critical role in fetal developmental defects seen as a consequence of Zika virus infection during pregnancy. This review provides a comprehensive overview of the multiple roles of this small but pivotal protein in mediating the pathobiology of flaviviral infections.

Inhibition of Calpain Alleviates Apoptosis in Coxsackievirus B3-induced Acute Virus Myocarditis Through Suppressing Endoplasmic Reticulum Stress

Virus myocarditis (VMC) is a common cardiovascular disease and a major cause of sudden death in young adults. However, there is still a lack of effective treatments. Our previous studies found that calpain activation was involved in VMC pathogenesis. This study aims to explore the underlying mechanisms further. Neonatal rat cardiomyocytes (NRCMs) and transgenic mice overexpressing calpastatin (Tg-CAST), the endogenous calpain inhibitor, were used to establish VMC model. Hematoxylin and eosin and Masson staining revealed inflammatory cell infiltration and fibrosis. An ELISA array detected myocardial injury. Cardiac function was measured using echocardiography. CVB3 replication was assessed by capsid protein VP1. Apoptosis was measured by TUNEL staining, flow cytometry, and western blot. The endoplasmic reticulum (ER) stress-related proteins were detected by western blot. Our data showed that CVB3 infection resulted in cardiac injury, as evidenced by increased inflammatory responses and fibrosis, which induced myocardial apoptosis. Inhibiting calpain, both by PD150606 and calpastatin overexpression, could attenuate these effects. Furthermore, ER stress was activated during CVB3 infection. However, calpain inhibition could downregulate some ER stress-associated protein levels such as GRP78, pancreatic ER kinase-like ER kinase (PERK), and inositol-requiring enzyme-1α (IRE-1α), and ER stress-related apoptotic factors, during CVB3 infection. In conclusion, calpain inhibition attenuated CVB3-induced myocarditis by suppressing ER stress, thereby inhibiting cardiomyocyte apoptosis.

Inhibition of eIF2α Phosphorylation by Peste des Petits Ruminant Virus Phosphoprotein Facilitates Viral Replication

Peste des petits ruminant virus (PPRV) causes a highly contagious disease in small ruminants. The molecular mechanism of PPRV replication and its interactions with hosts are poorly studied. In other paramyxoviruses, the viral phosphoprotein (P) has been associated with multiple functions for key biological processes such as the regulation of transcription, translation, and the control of cell cycle. Phosphorylation of the α subunit of eukaryotic initiation factor 2 (eIF2α) is an important process for gene regulation in host cells under stress, including viral infection. In the present study, molecular mechanisms associated with PPRV replication and viral interaction with host cells were investigated. We describe the ability of PPRV to dephosphorylate eIF2α and the potential of PPRV P protein to induce the host cellular growth arrest DNA damage protein (GADD34), which is known to be associated with eIF2α dephosphorylation. Furthermore, we observed that PPRV P protein alone could block PERK/eIF2α phosphorylation. We speculate that PPRV exploits eIF2α dephosphorylation to facilitate viral replication and that PPRV P protein is involved in this molecular mechanism. This work provides new insights into further understanding PPRV pathobiology and its viral/host interactions.

Protective effects of sodium butyrate on rotavirus inducing endoplasmic reticulum stress-mediated apoptosis via PERK-eIF2α signaling pathway in IPEC-J2 cells

BACKGROUND

Rotavirus (RV) is a major pathogen that causes severe gastroenteritis in infants and young animals. Endoplasmic reticulum (ER) stress and subsequent apoptosis play pivotal role in virus infection. However, the protective mechanisms of intestinal damage caused by RV are poorly defined, especially the molecular pathways related to enterocytes apoptosis. Thus, the aim of this study was to investigate the protective effect and mechanism of sodium butyrate (SB) on RV-induced apoptosis of IPEC-J2 cells.

RESULTS

The RV infection led to significant cell apoptosis, increased the expression levels of ER stress (ERS) markers, phosphorylated protein kinase-like ER kinase (PERK), eukaryotic initiation factor 2 alpha (eIF2α), caspase9, and caspase3. Blocking PERK pathway using specific inhibitor GSK subsequently reversed RV-induced cell apoptosis. The SB treatment significantly inhibited RV-induced ERS by decreasing the expression of glucose regulated protein 78 (GRP78), PERK, and eIF2α. In addition, SB treatment restrained the ERS-mediated apoptotic pathway, as indicated by downregulation of C/EBP homologous protein (CHOP) mRNA level, as well as decreased cleaved caspase9 and caspase3 protein levels. Furthermore, siRNA-induced GPR109a knockdown significantly suppressed the protective effect of SB on RV-induced cell apoptosis.

CONCLUSIONS

These results indicate that SB exerts protective effects against RV-induced cell apoptosis through inhibiting ERS mediated apoptosis by regulating PERK-eIF2α signaling pathway via GPR109a, which provide new ideas for the prevention and control of RV.

CHOP Pro-Apoptotic Transcriptional Program in Response to ER Stress Is Hacked by Zika Virus

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus considered as a threat to human health due to large epidemics and serious clinical outcomes such as microcephaly in new-borns. Like all flaviviruses, ZIKV relies on the cellular machinery to complete its viral cycle, with the endoplasmic reticulum (ER) being the critical site of viral replication factories. The sudden high protein load in the ER induces an ER stress to which the cell responds with an appropriate unfolded protein response (UPR) in an attempt to restore its disturbed homeostasis. When the restoration fails, the cell signalling leads to a programmed cell death by apoptosis with the upregulation of the UPR-induced C/EBP homologous protein (CHOP) which acts as the main trigger for this fatal outcome. Our previous studies have shown the ability of ZIKV to manipulate various cellular responses in order to optimize virus production. ZIKV is able to delay apoptosis to its benefit and although ER stress is induced, the UPR is not complete. Here we discovered that ZIKV impairs the expression of CHOP/DDIT3, the main factor responsible of ER-stress driven apoptosis. Surprisingly, the mechanism does not take place at the transcriptional level but at the translational level.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

Relevance of oxidative stress in inhibition of eIF2 alpha phosphorylation and stress granules formation during Usutu virus infection

Usutu virus (USUV) is an African mosquito-borne flavivirus closely related to West Nile, Japanese encephalitis, Zika, and dengue viruses. USUV emerged in 1996 in Europe, where quickly spread across the continent causing a considerable number of bird deaths and varied neurological disorders in humans, including encephalitis, meningoencephalitis, or facial paralysis, thus warning about USUV as a potential health threat. USUV replication takes place on the endoplasmic reticulum (ER) of infected cells, inducing ER stress and resulting in the activation of stress-related cellular pathways collectively known as the integrated stress response (ISR). The alpha subunit of the eukaryotic initiation factor eIF2 (eIF2α), the core factor in this pathway, is phosphorylated by stress activated kinases: protein kinase R (PKR), PKR-like endoplasmic reticulum kinase (PERK), heme-regulated inhibitor kinase (HRI), and general control non-repressed 2 kinase (GCN2). Its phosphorylation results, among others, in the downstream inhibition of translation with accumulation of discrete foci in the cytoplasm termed stress granules (SGs). Our results indicated that USUV infection evades cellular stress response impairing eIF2α phosphorylation and SGs assembly induced by treatment with the HRI activator ArsNa. This protective effect was related with oxidative stress responses in USUV-infected cells. Overall, these results provide new insights into the complex connections between the stress response and flavivirus infection in order to maintain an adequate cellular environment for viral replication.

Usutu virus (USUV) infection impairs eIF2α phosphorylation and SGs assembly, in an oxidative stress related manner, as a mechanism to evade cellular stress response. Our results provide new insights into the complex connections between the stress response and USUV infection to maintain a better cellular environment for viral replication.

A functional interaction between GRP78 and Zika virus E protein

Zika virus (ZIKV) is a mosquito-transmitted virus that has caused significant public health concerns around the world, partly because of an association with microcephaly in babies born to mothers who were infected with ZIKV during pregnancy. As a recently emerging virus, little is known as to how the virus interacts with the host cell machinery. A yeast-2-hybrid screen for proteins capable of interacting with the ZIKV E protein domain III, the domain responsible for receptor binding, identified 21 proteins, one of which was the predominantly ER resident chaperone protein GRP78. The interaction of GRP78 and ZIKV E was confirmed by co-immunoprecipitation and reciprocal co-immunoprecipitation, and indirect immunofluorescence staining showed intracellular and extracellular co-localization between GRP78 and ZIKV E. Antibodies directed against the N-terminus of GRP78 were able to inhibit ZIKV entry to host cells, resulting in significant reductions in the levels of ZIKV infection and viral production. Consistently, these reductions were also observed after down-regulation of GRP78 by siRNA. These results indicate that GRP78 can play a role mediating ZIKV binding, internalization and replication in cells. GRP78 is a main regulator of the unfolded protein response (UPR), and the study showed that expression of GRP78 was up-regulated, and the UPR was activated. Increases in CHOP expression, and activation of caspases 7 and 9 were also shown in response to ZIKV infection. Overall these results indicate that the interaction between GRP78 and ZIKV E protein plays an important role in ZIKV infection and replication, and may be a potential therapeutic target.

Classical swine fever virus employs the PERK- and IRE1-dependent autophagy for viral replication in cultured cells

Endoplasmic reticulum stress (ERS)-mediated autophagy is indispensable for modulation of replication and pathogenesis of numerous mammalian viruses. We have previously shown that classical swine fever virus (CSFV) infection induces ERS-mediated autophagy for maintaining viral replication both in vivo and in vitro, however, the underlying mechanism remains unclarified. Here we found that CSFV infection activates the PERK pathway-dependent complete autophagy to promote viral replication in cultured PK-15 and 3D4/2 cells. Likewise, our results also suggested the essential roles of the IRE1/GRP78-mediated complete autophagy in CSFV replication in vitro. Furthermore, we suggested that CSFV infection induces activation of the PERK and IRE1 pathway for potential immunoregulation via promoting transcription of proinflammatory cytokine (IFN-γ and TNF-α) genes in the CSFV-infected cells. Finally, pharmacological treatment of PERK- or IRE1-pathway regulators, and the corresponding SiRNAs interventions did not affect the viabilities of the cells, excluding the potential interference elicited by altered cell viabilities. Taken together, our results suggest that CSFV infection induces complete autophagy through activation of the PERK and IRE1 pathway to facilitate viral replication in cultured cells, and modulation of proinflammatory cytokines may be a potential mechanism involved in this event. Our findings will open new horizons for molecular mechanisms of sustainable replication and pathogenesis of CSFV, and lay a theoretical foundation for the development of ERS-autophagy-targeting therapeutic strategies for clinical control of CSF.

ER functions are exploited by viruses to support distinct stages of their life cycle

The endoplasmic reticulum (ER), with its expansive membranous system and a vast network of chaperones, enzymes, sensors, and ion channels, orchestrates diverse cellular functions, ranging from protein synthesis, folding, secretion, and degradation to lipid biogenesis and calcium homeostasis. Strikingly, some of the functions of the ER are exploited by viruses to promote their life cycles. During entry, viruses must penetrate a host membrane and reach an intracellular destination to express and replicate their genomes. These events lead to the assembly of new viral progenies that exit the host cell, thereby initiating further rounds of infection. In this review, we highlight how three distinct viruses - polyomavirus, flavivirus, and coronavirus - co-opt key functions of the ER to cause infection. We anticipate that illuminating this virus-ER interplay will provide rational therapeutic approaches to combat the virus-induced diseases.

Inositol-Requiring Enzyme 1α Promotes Zika Virus Infection through Regulation of Stearoyl Coenzyme A Desaturase 1-Mediated Lipid Metabolism

Zika virus (ZIKV) is an emerging mosquito-borne flavivirus which has become a global epidemic threat due to its rapid spread and association with serious consequences of infection, including neonatal microcephaly. Inositol-requiring enzyme 1α (IRE1α) is an endoplasmic reticulum (ER)-related transmembrane protein that mediates unfolded protein response (UPR) pathway and has been indicated to play an important role in flavivirus replication. However, the mechanism of how IRE1α affects ZIKV replication remains unknown. In this study, we explored the role of IRE1α in ZIKV infection in vitro and in vivo by using CRISPR/Cas9-based gene knockout and RNA interference-based gene knockdown techniques. Both knockout and knockdown of IRE1α dramatically reduced ZIKV replication levels, including viral RNA levels, protein expression, and titers in different human cell lines. Trans-complementation with IRE1α restored viral replication levels decreased by IRE1α depletion. Furthermore, the proviral effect of IRE1α was dependent on its kinase and RNase activities. Importantly, we found that IRE1α promoted the replication of ZIKV through upregulating the accumulation of monounsaturated fatty acid (MUFA) rate-limiting enzyme stearoyl coenzyme A (stearoyl-CoA) desaturase 1 (SCD1), which further affected the production of oleic acid (OA) and lipid droplet. Finally, our data demonstrated that in the brain tissues of ZIKV-infected mice, the replication levels of ZIKV and virus-related lesions were significantly suppressed by both the kinase and RNase inhibitors of IRE1α. Taken together, our results identified IRE1α as a ZIKV dependency factor which promotes viral replication through affecting SCD1-mediated lipid metabolism, potentially providing a novel molecular target for the development of anti-ZIKV agents.IMPORTANCE Zika virus (ZIKV) has been linked to serious neurologic disorders and causes widespread concern in the field of global public health. Inositol requiring enzyme 1α (IRE1α) is an ER-related transmembrane protein that mediates unfolded protein response (UPR) pathway. Here, we revealed that IRE1α is a proviral factor for ZIKV replication both in culture cells and mice model, which relies on its kinase and RNase activities. Importantly, we further provided evidence that upon ZIKV infection, IRE1α is activated and splices XBP1 mRNA which enhances the expression of monounsaturated fatty acids rate-limiting enzyme stearoyl coenzyme A (stearoyl-CoA) desaturase 1 (SCD1) and subsequent lipid droplet production. Our data uncover a novel mechanism of IRE1α proviral effect by modulating lipid metabolism, providing the first evidence of a close relationship between IRE1α-mediated UPR, lipid metabolism, and ZIKV replication and indicating IRE1α inhibitors as potentially effective anti-ZIKV agents.

Duck enteritis virus infection suppresses viability and induces apoptosis and endoplasmic reticulum stress in duck embryo fibroblast cells via the regulation of Ca2

Duck viral enteritis (DVE) is a lethal viral disease caused by duck enteritis virus (DEV) via an unknown mechanism. This study explores the relationship between Chinese standard challenge strain DEV (DEV-CSC)-induced apoptosis and endoplasmic reticulum stress (ERS) in duck embryo fibroblast (DEF) cells. Here we examined changes in Ca²⁺ concentration, cell proliferation, apoptosis, and the differential expression of C/EBP homologous protein (CHOP), glucose regulatory protein 78 (GRP78), and activating transcription factor 6 (ATF6) in infected cells. The results revealed that DEV-CSC infection significantly decreased Ca²⁺ concentration, suppressed cell viability, and induced apoptosis in DEF cells. Further experiments also demonstrated that DEV-CSC infection significantly upregulates CHOP, GRP78, and ATF6 expression. In addition, we show that the addition of ethylenediaminetetraacetic acid (EDTA) reverses the induction of apoptosis and the ERS mediated inhibition of cell viability in DEF cells associated with DEV-CSC infection. Therefore, we can conclude that infection with DEV-CSC induces apoptosis and ERS reducing the viability of DEF cells via the regulation of Ca²⁺. These findings may provide a new target for the treatment of DVE.

Dance with the Devil: Stress Granules and Signaling in Antiviral Responses

Cells have evolved highly specialized sentinels that detect viral infection and elicit an antiviral response. Among these, the stress-sensing protein kinase R, which is activated by double-stranded RNA, mediates suppression of the host translation machinery as a strategy to limit viral replication. Non-translating mRNAs rapidly condensate by phase separation into cytosolic stress granules, together with numerous RNA-binding proteins and components of signal transduction pathways. Growing evidence suggests that the integrated stress response, and stress granules in particular, contribute to antiviral defense. This review summarizes the current understanding of how stress and innate immune signaling act in concert to mount an effective response against virus infection, with a particular focus on the potential role of stress granules in the coordination of antiviral signaling cascades.

Induction of the Unfolded Protein Response during Bovine Alphaherpesvirus 1 Infection

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes great economic losses in the cattle industry. Herpesvirus infection generally induces endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in infected cells. However, it is not clear whether ER stress and UPR can be induced by BoHV-1 infection. Here, we found that ER stress induced by BoHV-1 infection could activate all three UPR sensors (the activating transcription factor 6 (ATF6), the inositol-requiring enzyme 1 (IRE1), and the protein kinase RNA-like ER kinase (PERK)) in MDBK cells. During BoHV-1 infection, the ATF6 pathway of UPR did not affect viral replication. However, both knockdown and specific chemical inhibition of PERK attenuated the BoHV-1 proliferation, and chemical inhibition of PERK significantly reduced the viral replication at the post-entry step of the BoHV-1 life cycle. Furthermore, knockdown of IRE1 inhibits BoHV-1 replication, indicating that the IRE1 pathway may promote viral replication. Further study revealed that BoHV-1 replication was enhanced by IRE1 RNase activity inhibition at the stage of virus post-entry in MDBK cells. Furthermore, IRE1 kinase activity inhibition and RNase activity enhancement decrease BoHV1 replication via affecting the virus post-entry step. Our study revealed that BoHV-1 infection activated all three UPR signaling pathways in MDBK cells, and BoHV-1-induced PERK and IRE1 pathways may promote viral replication. This study provides a new perspective for the interactions of BoHV-1 and UPR, which is helpful to further elucidate the mechanism of BoHV-1 pathogenesis.

Anti-Flavivirus Vaccines: Review of the Present Situation and Perspectives of Subunit Vaccines Produced in Escherichia coli

This article aims to review the present status of anti-flavivirus subunit vaccines, both those at the experimental stage and those already available for clinical use. Aspects regarding development of vaccines to Yellow Fever virus, (YFV), Dengue virus (DENV), West Nile virus (WNV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV) are highlighted, with particular emphasis on purified recombinant proteins generated in bacterial cells. Currently licensed anti-flavivirus vaccines are based on inactivated, attenuated, or virus-vector vaccines. However, technological advances in the generation of recombinant antigens with preserved structural and immunological determinants reveal new possibilities for the development of recombinant protein-based vaccine formulations for clinical testing. Furthermore, novel proposals for multi-epitope vaccines and the discovery of new adjuvants and delivery systems that enhance and/or modulate immune responses can pave the way for the development of successful subunit vaccines. Nonetheless, advances in this field require high investments that will probably not raise interest from private pharmaceutical companies and, therefore, will require support by international philanthropic organizations and governments of the countries more severely stricken by these viruses.

Porcine reproductive and respiratory syndrome virus infection induces endoplasmic reticulum stress, facilitates virus replication, and contributes to autophagy and apoptosis

During viral infection, the host cell synthesizes high amounts of viral proteins, which often causes stress to the endoplasmic reticulum (ER). To manage abnormal ER stress, mammalian cells trigger a response called the unfolded protein response (UPR). Previous studies have indicated that porcine reproductive and respiratory syndrome virus (PRRSV), an Arterivirus that has been devastating the swine industry worldwide, can induce ER stress and activate UPR, however, the activation pathways and the biological significance requires further investigation. In this study, we demonstrated that, among the three types of UPR pathways, PRRSV infection induced PERK and IRE1 pathways, but not the ATF6 pathway. Furthermore, the induction of UPR promoted PRRSV replication. We also found that PRRSV-induced UPR, particularly the PERK pathway, was involved in the induction of autophagy, a cellular degradation process that can alleviate cell stress. Besides, we also provided insights into the ER stress-mediated apoptosis in response to PRRSV infection. PRRSV infection induced the expression of the transcription factor CHOP, which activated caspase 3 and PARP led to ER stress-mediated apoptosis. Using 3-Methyladenine (3-MA) to inhibit autophagy, the increased ER stress and cell apoptosis were observed in the PRRSV infected cell. Taken together, our results revealed the associations of ER stress, autophagy, and apoptosis during PRRSV infection, helping us to further understand how PRRSV interacts with host cells.

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.

Chikungunya Virus: An Emergent Arbovirus to the South American Continent and a Continuous Threat to the World

Chikungunya virus (CHIKV) is an arthropod-borne virus (arbovirus) of epidemic concern, transmitted by Aedes ssp. mosquitoes, and is the etiologic agent of a febrile and incapacitating arthritogenic illness responsible for millions of human cases worldwide. After major outbreaks starting in 2004, CHIKV spread to subtropical areas and western hemisphere coming from sub-Saharan Africa, South East Asia, and the Indian subcontinent. Even though CHIKV disease is self-limiting and non-lethal, more than 30% of the infected individuals will develop chronic disease with persistent severe joint pain, tenosynovitis, and incapacitating polyarthralgia that can last for months to years, negatively impacting an individual's quality of life and socioeconomic productivity. The lack of specific drugs or licensed vaccines to treat or prevent CHIKV disease associated with the global presence of the mosquito vector in tropical and temperate areas, representing a possibility for CHIKV to continually spread to different territories, make this virus an agent of public health burden. In South America, where Dengue virus is endemic and Zika virus was recently introduced, the impact of the expansion of CHIKV infections, and co-infection with other arboviruses, still needs to be estimated. In Brazil, the recent spread of the East/Central/South Africa (ECSA) and Asian genotypes of CHIKV was accompanied by a high morbidity rate and acute cases of abnormal disease presentation and severe neuropathies, which is an atypical outcome for this infection. In this review, we will discuss what is currently known about CHIKV epidemics, clinical manifestations of the human disease, the basic concepts and recent findings in the mechanisms underlying virus-host interaction, and CHIKV-induced chronic disease for both in vitro and in vivo models of infection. We aim to stimulate scientific debate on how the characterization of replication, host-cell interactions, and the pathogenic potential of the new epidemic viral strains can contribute as potential developments in the virology field and shed light on strategies for disease control.

Amino acid 159 of the envelope protein affects viral replication and T-cell infiltration by West Nile virus in intracranial infection

West Nile virus (WNV) is an important cause of viral encephalitis in birds and animals, including humans. Amino acid 159 of the envelope (E) protein is reportedly implicated in the different levels of neurovirulence in mice infected with WNV NY99 or Eg101. We investigated the role of amino acid 159 of the E protein in the pathogenesis of WNV infection. We produced recombinant WNV with the structural proteins of the NY99 or Eg101 strain (NY-WT or EgCME-WT) and mutant viruses with substitutions of amino acid 159 of the E protein (NY-E-V159I or EgCME-E-I159V). The NY-WT and NY-E-V159I or EgCME-WT and EgCME-E-I159V titers in culture supernatant were similar. The mortality rate and viral titer in the brains of mice inoculated intraperitoneally with NY-WT or NY-E-V159I were also similar. In contrast, the mortality rate and viral titer in the brains of mice inoculated intracranially with EgCME-E-I159V were significantly higher than those of mice inoculated with EgCME-WT. The numbers of CD3-positive and CD8-positive T cells were greater in brains inoculated with EgCME-E-I159V than in those inoculated with EgCME-WT. Therefore, amino acid 159 of the E protein modulates the pathogenicity of WNV by affecting viral replication and T-cell infiltration in the brain.

Flavivirus Nonstructural Protein NS5 Dysregulates HSP90 to Broadly Inhibit JAK/STAT Signaling

Pathogenic flaviviruses antagonize host cell Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling downstream of interferons α/β. Here, we show that flaviviruses inhibit JAK/STAT signaling induced by a wide range of cytokines beyond interferon, including interleukins. This broad inhibition was mapped to viral nonstructural protein 5 (NS5) binding to cellular heat shock protein 90 (HSP90), resulting in reduced Janus kinase-HSP90 interaction and thus destabilization of unchaperoned JAKs (and other kinase clients) of HSP90 during infection by Zika virus, West Nile virus, and Japanese encephalitis virus. Our studies implicate viral dysregulation of HSP90 and the JAK/STAT pathway as a critical determinant of cytokine signaling control during flavivirus infection.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Role of autophagy during the replication and pathogenesis of common mosquito-borne flavi- and alphaviruses

Arboviruses that are transmitted to humans by mosquitoes represent one of the most important causes of febrile illness worldwide. In recent decades, we have witnessed a dramatic re-emergence of several mosquito-borne arboviruses, including dengue virus (DENV), West Nile virus (WNV), chikungunya virus (CHIKV) and Zika virus (ZIKV). DENV is currently the most common mosquito-borne arbovirus, with an estimated 390 million infections worldwide annually. Despite a global effort, no specific therapeutic strategies are available to combat the diseases caused by these viruses. Multiple cellular pathways modulate the outcome of infection by either promoting or hampering viral replication and/or pathogenesis, and autophagy appears to be one of them. Autophagy is a degradative pathway generally induced to counteract viral infection. Viruses, however, have evolved strategies to subvert this pathway and to hijack autophagy components for their own benefit. In this review, we will focus on the role of autophagy in mosquito-borne arboviruses with emphasis on DENV, CHIKV, WNV and ZIKV, due to their epidemiological importance and high disease burden.

PERK-Mediated Unfolded Protein Response Signaling Restricts Replication of the Tick-Borne Flavivirus Langat Virus

The unfolded protein response (UPR) maintains protein-folding homeostasis in the endoplasmic reticulum (ER) and has been implicated as both beneficial and detrimental to flavivirus infection. Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), a sensor of the UPR, is commonly associated with antiviral effects during mosquito-borne flavivirus (MBFV) infection, but its relation to tick-borne flavivirus (TBFV) infection remains largely unexplored. In this study, we identified changes in UPR and autophagic activity during Langat virus (LGTV) infection. LGTV robustly activated UPR and altered autophagic flux. Knockdown of endogenous PERK in human cells resulted in increased LGTV replication, but not that of closely related Powassan virus (POWV). Finally, on examining changes in protein levels of components associated with UPR and autophagy in the absence of PERK, we could show that LGTV-infected cells induced UPR but did not lead to expression of C/EBP homologous protein (CHOP), an important downstream transcription factor of multiple stress pathways. From these data, we hypothesize that LGTV can antagonize other kinases that target eukaryotic initiation factor 2α (eIF2α), but not PERK, implicating PERK as a potential mediator of intrinsic immunity. This effect was not apparent for POWV, a more pathogenic TBFV, suggesting it may be better equipped to mitigate the antiviral effects of PERK.

IRE1α Promotes Zika Virus Infection via XBP1

Zika virus (ZIKV) is an emergent member of the Flaviviridae family which causes severe congenital defects and other major sequelae, but the cellular processes that support ZIKV replication are incompletely understood. Related flaviviruses use the endoplasmic reticulum (ER) as a membranous platform for viral replication and induce ER stress during infection. Our data suggest that ZIKV activates IRE1α, a component of the cellular response to ER stress. IRE1α is an ER-resident transmembrane protein that possesses a cytosolic RNase domain. Upon activation, IRE1α initiates nonconventional cytoplasmic splicing of XBP1 mRNA. Spliced XBP1 encodes a transcription factor, which upregulates ER-related targets. We find that ZIKV infection induces XBP1 mRNA splicing and induction of XBP1 target genes. Small molecule inhibitors of IRE1α, including those specific for the nuclease function, prevent ZIKV-induced cytotoxicity, as does genetic disruption of IRE1α. Optimal ZIKV RNA replication requires both IRE1α and XBP1. Spliced XBP1 has been described to cause ER expansion and remodeling and we find that ER redistribution during ZIKV infection requires IRE1α nuclease activity. Finally, we demonstrate that inducible genetic disruption of IRE1α and XBP1 impairs ZIKV replication in a mouse model of infection. Together, our data indicate that the ER stress response component IRE1α promotes ZIKV infection via XBP1 and may represent a potential therapeutic target.

Crosstalk between RNA Metabolism and Cellular Stress Responses during Zika Virus Replication

Zika virus (ZIKV) is a mosquito-borne virus associated with neurological disorders such as Guillain-Barré syndrome and microcephaly. In humans, ZIKV is able to replicate in cell types from different tissues including placental cells, neurons, and microglia. This intricate virus-cell interaction is accompanied by virally induced changes in the infected cell aimed to promote viral replication as well as cellular responses aimed to counteract or tolerate the virus. Early in the infection, the 11-kb positive-sense RNA genome recruit ribosomes in the cytoplasm and the complex is translocated to the endoplasmic reticulum (ER) for viral protein synthesis. In this process, ZIKV replication is known to induce cellular stress, which triggers both the expression of innate immune genes and the phosphorylation of eukaryotic translation initiation factor 2 (eIF2α), shutting-off host protein synthesis. Remodeling of the ER during ZIKV replication also triggers the unfolded protein response (UPR), which induces changes in the cellular transcriptional landscapes aimed to tolerate infection or trigger apoptosis. Alternatively, ZIKV replication induces changes in the adenosine methylation patterns of specific host mRNAs, which have different consequences in viral replication and cellular fate. In addition, the ZIKV RNA genome undergoes adenosine methylation by the host machinery, which results in the inhibition of viral replication. However, despite these relevant findings, the full scope of these processes to the outcome of infection remains poorly elucidated. This review summarizes relevant aspects of the complex crosstalk between RNA metabolism and cellular stress responses against ZIKV and discusses their possible impact on viral pathogenesis.

The E3 Ubiquitin Ligase SIAH1 Targets MyD88 for Proteasomal Degradation During Dengue Virus Infection

The dengue virus presents a serious threat to human health globally and can cause severe, even life-threatening, illness. Dengue virus (DENV) is endemic on all continents except Antarctica, and it is estimated that more than 100 million people are infected each year. Herein, we further mine the data from a previously described screen for microRNAs (miRNAs) that block flavivirus replication. We use miR-424, a member of the miR-15/16 family, as a tool to further dissect the role of host cell proteins during DENV infection. We observed that miR-424 suppresses expression of the E3 ubiquitin ligase SIAH1, which is normally induced during dengue virus 2 (DENV2) infection through activation of the unfolded protein response (UPR). Specific siRNA-mediated knockdown of SIAH1 also results in inhibition of DENV replication, demonstrating that this target is at least partly responsible for the antiviral activity of miR-424. We further show that SIAH1 binds to and ubiquitinates the innate immune adaptor protein MyD88 and that the antiviral effect of SIAH1 knockdown is reduced in cells in which MyD88 has been deleted by CRISPR/Cas9 gene editing. Additionally, MyD88-dependent signaling, triggered either by DENV2 infection or the Toll-like receptor 7 (TLR7) ligand imiquimod, is increased in cells in which SIAH1 has been knocked down by miR-424 or a SIAH1-specific siRNA. These observations suggest an additional pathway by which DENV2 harnesses aspects of the UPR to dampen the host innate immune response and promote viral replication.

Hydroxycarboxylic Acid Receptor 2 Is a Zika Virus Restriction Factor That Can Be Induced by Zika Virus Infection Through the IRE1-XBP1 Pathway

Zika virus (ZIKV) is an emerging arthropod-borne virus and belongs to the Flaviviridae family. The infection of ZIKV has become the global health crisis because of its rapid spread and association with severe neurological disorders, including congenital microcephaly and Guillain-Barre Syndrome. To identify host factors contributing to ZIKV pathogenesis, transcriptomic landscape in ZIKV-infected cells was examined with mRNA microarray analysis and we observed that the expression of hydroxycarboxylic acid receptor 2 (HCAR2) could be significantly induced by ZIKV infection. By utilizing two IRE1 inhibitors and XBP1-specific shRNAs, we revealed that the up-regulation of HCAR2 expression induced by ZIKV was dependent on the IRE1-XBP1 pathway. Through the CRISPR/Cas9 system, we generated HCAR2-deficient cell clones in two cell types (human lung carcinoma epithelial A549 cell and human hepatoma Huh7.5 cell). We found that the depletion of HCAR2 significantly increased the replication level of ZIKV, including RNA levels, protein expression levels, and viral titers. In addition, our data demonstrated that the antiviral effect of HCAR2 was not involved in viral entry process and was not dependent on its antilipolytic effect on nicotinic acid/HCAR2-mediated signaling pathway. Taken together, our results indicated that HCAR2 could function as a restriction factor in control of ZIKV replication, potentially providing a novel molecular target for anti-ZIKV therapeutics.

Marburg virus regulates the IRE1/XBP1-dependent unfolded protein response to ensure efficient viral replication

Viruses regulate cellular signalling pathways to ensure optimal viral replication. During Marburg virus (MARV) infection, large quantities of the viral glycoprotein GP are produced in the ER; this may result in the activation of the unfolded protein response (UPR). The most conserved pathway to trigger UPR is initiated by IRE1. Activation of IRE1 results in auto-phosphorylation, splicing of the XBP1 mRNA and translation of the XBP1s protein. XBP1s binds cis-acting UPR elements (UPRE) which leads to the enhanced expression of genes which should restore ER homeostasis. XBP1u protein is translated, if IRE1 is not activated. Here we show that ectopic expression of MARV GP activated the IRE1-XBP1 axis of UPR as monitored by UPRE luciferase assays. However, while at 24 h of infection with MARV IRE1 was phosphorylated, expression of XBP1s was only slightly enhanced and UPRE activity was not detected. The IRE1-XBP1 axis was not active at 48 h p.i. Co-expression studies of MARV proteins demonstrated that the MARV protein VP30 suppressed UPRE activation. Co-immunoprecipitation analyses revealed an RNA-dependent interaction of VP30 with XBP1u. Knock-out of IRE1 supported MARV infection at late time points. Taken together, these results suggest that efficient MARV propagation requires specific regulation of IRE1 activity.

CSFV protein NS5A activates the unfolded protein response to promote viral replication

Classical swine fever is a world organization for animal health listed disease and is caused by classical swine fever virus (CSFV). CSFV can induced unfolded protein response (UPR) and whether NS5A protein plays a role in this process remains unknown. Here, we demonstrate that CSFV induced all the three signal pathways ATF6, IRE1 and PERK of UPR. Furthermore, this phenomenon may be mediated by the NS5A protein since expression of NS5A alone can achieve the same effect. In the current study, we show that NS5A can interact with GRP78 as measured by using the CO-IP and GST pulldown assays. This interaction plays a positive role in the promotion of CSFV replication. Overexpression or knockdown of GRP78 mediated by lentivirus can enhance or decrease viral replication, respectively. Our findings provide the evidence that CSFV infection can activate the cellular UPRs, in which NS5A and GRP78 play key roles in the process.

Host Gene SEL1L Involved in Endoplasmic Reticulum-Associated Degradation Pathway Could Inhibit Hepatitis B Virus at RNA, DNA, and Protein Levels

Hepatitis B virus (HBV) belongs to the Hepadnaviridae family of enveloped DNA viruses. Recent studies have found that host factors can suppress HBV replication. HBV envelope proteins are reported to be degraded by the endoplasmic reticulum-associated degradation (ERAD) pathway. As a component of the ERAD pathway, suppressor of lin-12-like 1 (SEL1L) was earlier found to be upregulated in the inactive carrier phase of chronic HBV infection relative to that in the immune tolerant phase. However, the role of SEL1L in regulating HBV replication remains largely unknown. In this study, we found the levels of HBV RNA, DNA, and core and envelope proteins to be significantly downregulated by SEL1L overexpression and upregulated by SEL1L silencing in Huh7 cells transiently transfected with an overlength HBV genome. Similar upregulation was observed in HepG2.2.15 cells as well. SEL1L co-localized with HBV surface antigen (HBsAg), which changed its staining pattern. Treatment with an inhibitor of ERAD pathway remarkably increased intracellular S protein. Surprisingly, silencing SEL1L to block the ERAD pathway activated an alternative ER quality control (ERQC)-autophagy pathway, which might account for the increased HBV RNAs and core protein. Together, our results demonstrate that SEL1L is a host restriction factor that exerts anti-HBV effect through ERAD and alternative ERQC-autophagy pathway.