CSFV Infection Up-Regulates the Unfolded Protein Response to Promote Its Replication

AA147 Alleviates Symptoms in a Mouse Model of Multiple Sclerosis by Reducing Oligodendrocyte Loss

Inflammation-induced oligodendrocyte death and CNS demyelination are key features of multiple sclerosis (MS). Inflammation-triggered endoplasmic reticulum (ER) stress and oxidative stress promote tissue damage in MS and in its preclinical animal model, experimental autoimmune encephalitis (EAE). Compound AA147 is a potent activator of the ATF6 signaling arm of the unfolded protein response (UPR) that can also induce antioxidant signaling through activation of the NRF2 pathway in neuronal cells. Previous work showed that AA147 protects multiple tissues against ischemia/reperfusion damage through ATF6 and/or NRF2 activation; however, its therapeutic potential in neuroinflammatory disorders remains unexplored. Here, we demonstrate that AA147 ameliorated the clinical symptoms of EAE and reduced ER stress, oligodendrocyte loss, and demyelination. Additionally, AA147 suppressed T cells in the CNS without altering the peripheral immune response. Importantly, AA147 significantly increased the expressions of Grp78, an ATF6 target gene, in oligodendrocytes, while enhancing levels of Grp78 as well as Ho-1, an NRF2 target gene, in microglia. In cultured oligodendrocytes, AA147 promoted nuclear translocation of ATF6, but not NRF2. Intriguingly, AA147 altered the microglia activation profile, possibly by triggering the NRF2 pathway. AA147 was not therapeutically beneficial during the acute EAE stage in mice lacking ATF6 in oligodendrocytes, indicating that protection primarily involves ATF6 activation in these cells. Overall, our results suggest AA147 as a potential therapeutic opportunity for MS by promoting oligodendrocyte survival and regulating microglia status through distinct mechanisms.

Transcriptome analysis of 3D4/21 cells expressing CSFV NS4B

Classical swine fever (CSF) caused by classical swine fever virus (CSFV) has resulted in severe losses to the pig industry worldwide. CSFV non-structural protein 4B (NS4B) plays a crucial role in CSFV replication and pathogenicity. However, the function of NS4B is still limited during CSFV infection. In this study, the RNA-seq was used to investigate differentially expressed genes (DEGs) in 3D4/21 cells expressing CSFV NS4B. 4397 DEGs were identified in 3D4/21 cells expressing NS4B compared to cells expressing the empty vector (NC). Twelve DEGs were selected and further verified by RT-qPCR. Enrichment analyses of GO annotations and KEGG pathways revealed that these DEGs were associated with endocytosis, autophagy, cell adhesion, transport, immune response, apoptosis and so on. The expression of endocytosis-related genes, including CAV1/2, CAVIN2, Rab1B, CHMP2B/4C, VPS35, SNX2, Rab11B, CHMP6, MVB12B and VPS28, were found to be regulated. In addition, some genes associated with host immune defense, such as USP15, DHX29, DDX3, RIG-I and MDA5, were downregulated and the genes associated with host autophagy, such as WIPI2, ATG16L2, SMCR8, RPTOR and MLST8, were upregulated. Therefore, CSFV NS4B involved in virus invasion and intracellular trafficking, the induction of autophagy and the inhibition of antiviral response. Taken together, this study provides useful information for further understanding the function of NS4B during CSFV infection.

The roles and mechanisms of endoplasmic reticulum stress-mediated autophagy in animal viral infections

The endoplasmic reticulum (ER) is a unique organelle responsible for protein synthesis and processing, lipid synthesis in eukaryotic cells, and the replication of many animal viruses is closely related to ER. A considerable number of viral proteins are synthesised during viral infection, resulting in the accumulation of unfolded and misfolded proteins in ER, which in turn induces endoplasmic reticulum stress (ERS). ERS further drives three signalling pathways (PERK, IRE1, and ATF6) of the cellular unfolded protein response (UPR) to respond to the ERS. In numerous studies, ERS has been shown to mediate autophagy, a highly conserved cellular degradation mechanism to maintain cellular homeostasis in eukaryotic cells, through the UPR to restore ER homeostasis. ERS-mediated autophagy is closely linked to the occurrence and development of numerous viral diseases in animals. Host cells can inhibit viral replication by regulating ERS-mediated autophagy, restoring the ER's normal physiological process. Conversely, many viruses have evolved strategies to exploit ERS-mediated autophagy to achieve immune escape. These strategies include the regulation of PERK-eIF2α-Beclin1, PERK-eIF2α-ATF4-ATG12, IRE1α-JNK-Beclin1, and other signalling pathways, which provide favourable conditions for the replication of animal viruses in host cells. The ERS-mediated autophagy pathway has become a hot topic in animal virological research. This article reviews the most recent research regarding the regulatory functions of ERS-mediated autophagy pathways in animal viral infections, emphasising the underlying mechanisms in the context of different viral infections. Furthermore, it considers the future direction and challenges in the development of ERS-mediated autophagy targeting strategies for combating animal viral diseases, which will contribute to unveiling their pathogenic mechanism from a new perspective and provide a scientific reference for the discovery and development of new antiviral drugs and preventive strategies.

m6A modification associated with YTHDF1 is involved in Japanese encephalitis virus infection

N6-methyladenosine (m6A), the most common modification in mammalian mRNA and viral RNA, regulates mRNA structure, stability, translation, and nuclear export. The Japanese encephalitis virus (JEV) is a mosquito-borne flavivirus causing severe neurologic disease in humans. To date, the role of m6A modification in JEV infection remains unclear. Herein, we aimed to determine the impact of m6A methylation modification on JEV replication in vitro and in vivo. Our results demonstrated that the overexpression of the m6A reader protein YTHDF1 in vitro significantly inhibits JEV proliferation. Additionally, YTHDF1 negatively regulates JEV proliferation in YTHDF1 knockdown cells and YTHDF1 knockout mice. MeRIP-seq analysis indicated that YTHDF1 interacts with several interferon-stimulated genes (ISGs), especially in IFIT3. Overall, our data showed that YTHDF1 played a vital role in inhibiting JEV replication. These findings bring novel insights into the specific mechanisms involved in the innate immune response to infection with JEV. They can be used in the development of novel therapeutics for controlling JEV infection.

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.

Pseudorabies virus infection induces endoplasmic reticulum stress and unfolded protein response in suspension-cultured BHK-21 cells

Viral infections cause endoplasmic reticulum (ER) stress and subsequently unfolded protein response (UPR) which restores ER homeostasis. In this study, levels of proteins or transcription of three UPR pathways were examined in suspension-cultured BHK-21 cells to investigate Pseudorabies virus (PRV) infection-induced ER stress, in which glucose-related proteins 78 kD and 94 kD (GRP78 and GRP94) were upregulated. The downstream double-stranded RNA-activated protein kinase-like ER kinase (PERK) pathway was activated with upregulation of ATF4, CHOP, and GADD34, and the inositol requiring kinase 1 (IRE1) pathway was triggered by the splicing of X box-binding protein 1 (XBP1) mRNA and the enhanced expression of p58^IPK and EDEM. Furthermore, our results showed that the ER stress, induced by 0.005 µM thapsigargin, promoted PRV replication in suspension-cultured BHK-21 cells, and that PRV glycoprotein B (gB) overexpression triggered the PERK and IRE1 pathways.

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.

Protection of catalpol against triptolide-induced hepatotoxicity by inhibiting excessive autophagy via the PERK-ATF4-CHOP pathway

Catalpol significantly reduces triptolide-induced hepatotoxicity, which is closely related to autophagy. The aim of this study was to explore the unclear protective mechanism of catalpol against triptolide. The detoxification effect of catalpol on triptolide was investigated in HepaRG cell line. The detoxification effects were assessed by measuring cell viability, autophagy, and apoptosis, as well as the endoplasmic reticulum stress protein and mRNA expression levels. We found that 5-20 µg/L triptolide treatments increased the levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), and lactate dehydrogenase (LDH), as well as the expression of autophagy proteins including LC3 and Beclin1. The expression of P62 was downregulated and the production of autophagosomes was increased, as determined by transmission electron microscope and monodansylcadaverine staining. In contrast, 40 µg/L catalpol reversed these triptolide-induced changes in the liver function index, autophagy level, and apoptotic protein expression, including Cleaved-caspase3 and Cleaved-caspase9 by inhibiting excessive autophagy. Simultaneously, catalpol reversed endoplasmic reticulum stress, including the expression of PERK, which regulates autophagy. Moreover, we used the PERK inhibitor GSK2656157 to prove that the PERK-ATF4-CHOP pathway of the unfolded protein response is an important pathway that could induce autophagy. Catalpol inhibited excessive autophagy by suppressing the PERK pathway. Altogether, catalpol protects against triptolide-induced hepatotoxicity by inhibiting excessive autophagy via the PERK-ATF4-CHOP pathway. The results of this study are beneficial to clarify the detoxification mechanism of catalpol against triptolide-induced hepatotoxicity and to promote the application of triptolide.

Anti-Classical Swine Fever Virus Strategies

Classical swine fever (CSF), caused by CSF virus (CSFV), is a highly contagious swine disease with high morbidity and mortality, which has caused significant economic losses to the pig industry worldwide. Biosecurity measures and vaccination are the main methods for prevention and control of CSF since no specific drug is available for the effective treatment of CSF. Although a series of biosecurity and vaccination strategies have been developed to curb the outbreak events, it is still difficult to eliminate CSF in CSF-endemic and re-emerging areas. Thus, in addition to implementing enhanced biosecurity measures and exploring more effective CSF vaccines, other strategies are also needed for effectively controlling CSF. Currently, more and more research about anti-CSFV strategies was carried out by scientists, because of the great prospects and value of anti-CSFV strategies in the prevention and control of CSF. Additionally, studies on anti-CSFV strategies could be used as a reference for other viruses in the Flaviviridae family, such as hepatitis C virus, dengue virus, and Zika virus. In this review, we aim to summarize the research on anti-CSFV strategies. In detail, host proteins affecting CSFV replication, drug candidates with anti-CSFV effects, and RNA interference (RNAi) targeting CSFV viral genes were mentioned and the possible mechanisms related to anti-CSFV effects were also summarized.

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.

Confirming whether novel rhein derivative 4a induces paraptosis-like cell death by endoplasmic reticulum stress in ovarian cancer cells

Ovarian cancer is the leading cause of death among gynecologic cancer patients. Although platinum-based chemotherapy as a frontline treatment for ovarian cancer has been widely used in clinical settings, its clinical efficacy is not satisfactory due to the resistance of ovarian cancer cells to apoptosis. Therefore, it is of great significance to induce non-apoptotic programed cell death patterns, such as paraptosis, in ovarian cancer. In this study, we aimed to explore the potential anticancer mechanisms of novel rhein derivative 4a, which was modified with rhein as a lead compound. The results showed that a wide range of vacuoles from the endoplasmic reticulum and mitochondria appeared in ovarian SKOV3, SKOV3-PM4, and A2780 cells treated with derivative 4a, and the cell death caused by derivative 4a is a type of non-apoptotic and non-autophagic death, which is caused by expansion and damage of the endoplasmic reticulum or mitochondria, showing the characteristics of para-apoptotic death. Furthermore, derivative 4a stimulated the unfolded protein reaction of ovarian cancer cells by upregulating the expression of Bip78 and activating the PERK-eIF2α-ATF4 pathways. Notably, rhein derivative 4a-induced cell death was positively correlated with activation of p38, ERK, and JNK, and negatively correlated with Alix, a known protein that inhibits paraptosis. In addition, derivative 4a treatment also induced G2/M phase arrest in ovarian cancer cells. Taken together, our study reveals that derivative 4a induces paraptosis, and this finding can serve as a basis in developing a new strategy for the treatment of antiapoptotic ovarian cancer.

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.

The R614E mutation of mouse Mx1 protein contributes to the novel antiviral activity against classical swine fever virus

Mx proteins are interferon-induced GTPases that have broad antiviral activity against a wide range of RNA and DNA viruses. We previously demonstrated that porcine Mx1 protein (poMx1) inhibited the replication of classical swine fever virus (CSFV), an economically important Pestivirus, and that mouse Mx1 did so as well. It is unknown why the nucleus-localizing mouse Mx1 inhibits CSFV replication which occurs in the cytoplasm. To the end, we assessed the anti-CSFV actions of wild type mouse Mx1 and seven previously reported mutants (K49A, G83R, A222V, A516V, G540E, R614E and ΔL4) and identified the molecular mechanism of R614E action against CSFV replication. A series of experiments revealed that mmMx1 (R614E) mutant reposted to the cytoplasm and interacted with the CSFV nucleocapsid protein (Core), thereby inhibiting viral replication. These findings broaden our understanding of the function of Mx protein family members against CSFV and suggest that the relative conservation of Mx1 among species is the basis of broad-spectrum antiviral properties.

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.

Classical Swine Fever Virus Infection Induces Endoplasmic Reticulum Stress-Mediated Autophagy to Sustain Viral Replication in vivo and in vitro

Endoplasmic reticulum (ER) stress-mediated autophagy plays significant roles in replication and pathogenesis of many animal viruses. However, the relationship between ER stress, autophagy, and viral replication during in vivo and in vitro infection of classical swine fever virus (CSFV) remains unclear. In this study, we established a pig model for CSFV infection and found that viral loads of CSFV differed in 10 kinds of infected organs, and that the degree of tissue lesions was to some extent positively correlated with CSFV replication in vivo. Next, we found that CSFV infection not only induced ER stress and subsequently activated three unfolded protein responses (UPR) pathways including protein kinase R-like ER kinase (PERK), inositol requiring enzyme 1 (IRE1), and activating transcription factor-6 (ATF-6) pathways, but also triggered complete autophagy in main immune organs and partial nonimmune organs exhibiting severer pathological injuries and higher viral loads. However, only the IRE1 pathway and no autophagy were activated in some other nonimmune organs with slighter pathologies and lower viral loads. These results indicate a potential link between CSFV-induced ER stress and autophagy, both of which are associated with the CSFV replication in vivo. We further performed in vitro experiments and found that CSFV infection activates the PERK and IRE1 pathways and autophagy in cultured porcine kidney cell lines (PK-15) and macrophage cell lines (3D4/2), and pharmacological regulation of ER stress remarkably changed autophagic activities induced by CSFV, suggesting that CSFV-induced autophagy can be mediated by ER stress possibly via the PERK and IRE1 pathway. Furthermore, treatment with ER stress regulators significantly altered copy numbers of NS5B genes, expression of Npro proteins, and viral titers in CSFV-infected cells or in cells treated with autophagy regulators prior to CSFV infection, suggesting the requirement of ER stress-mediated autophagy for CSFV replication in vitro. Collectively, our data demonstrate that CSFV induces ER stress-mediated autophagy to sustain its replication in vivo and in vitro, which may be one of the potential strategies exploited by CSFV for immune evasion. This finding will provide new insights into mechanisms of replication and pathogenesis of CSFV, and development of new strategies for controlling CSF.

Quantitative Proteomic Analysis Reveals Unfolded-Protein Response Involved in Severe Fever with Thrombocytopenia Syndrome Virus Infection

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging, highly pathogenic, infectious disease caused by infection with a newly discovered tick-borne phlebovirus, SFTS virus (SFTSV). Limited information on the molecular mechanism of SFTSV infection and pathogenesis impedes the development of effective vaccines and drugs for SFTS prevention and treatment. In this study, an isobaric tag for relative and absolute quantification (iTRAQ)-based quantitative proteomic analysis of SFTSV-infected HEK 293 cells was performed to explore dynamic host cellular protein responses toward SFTSV infection. A total of 433 of 5,606 host proteins involved in different biological processes were differentially regulated by SFTSV infection. The proteomic results highlighted a potential role of endoplasmic reticular stress-triggered unfolded-protein response (UPR) in SFTSV infection. Further functional studies confirmed that all three major branches of the UPR, including the PKR-like endoplasmic reticulum kinase (PERK), the activating transcription factor-6 (ATF6), and the inositol-requiring protein-1 (IRE1)/X-box-binding protein 1 (XBP1) pathways, were activated by SFTSV. However, only the former two pathways play a crucial role in SFTSV infection. Furthermore, expression of SFTSV glycoprotein (GP) alone was sufficient to stimulate the UPR, whereas suppression of PERK and ATF6 notably decreased GP expression. Interestingly, two other newly discovered phleboviruses, Heartland virus and Guertu virus, also stimulated the UPR, suggesting a common mechanism shared by these genetically related phleboviruses. This study provides a global view to our knowledge on how host cells respond to SFTSV infection and highlights that host cell UPR plays an important role in phlebovirus infection.IMPORTANCE Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne bunyavirus that causes severe fever with thrombocytopenia syndrome in humans, with a mortality rate reaching up to 30% in some outbreaks. There are currently no U.S. Food and Drug Administration-approved vaccines or specific antivirals available against SFTSV. To comprehensively understand the molecular interactions occurring between SFTSV and the host cell, we exploit quantitative proteomic approach to investigate the dynamic host cellular responses to SFTSV infection. The results highlight multiple biological processes being regulated by SFTSV infection. Among these, we focused on exploration of the mechanism of how SFTSV infection stimulates the host cell's unfolded-protein response (UPR) and identified the UPR as a common feature shared by SFTSV-related new emerging phleboviruses. This study, for the first time to our knowledge, provides a global map for host cellular responses to SFTSV infection and highlighted potential host targets for further research.

The unfolded protein response induced by Tembusu virus infection

Background

Tembusu virus (TMUV), classified in the genus Flavivirus, causes reduced egg production and neurological problems in poultry. Flavivirus replication depends on the host endoplasmic reticulum (ER) and induces ER stress that leads to activation of the cellular unfolded protein response (UPR), an important signalling pathway that regulates many biological functions involved in viral pathogenesis and innate immunity. However, the mechanism of TMUV-induced UPR activation remains unclear.

Results

In this study, we systematically investigated the three UPR pathways in TMUV-infected BHK-21 cells. Our results showed that expression of glucose-related protein 78 (GRP78) and GRP94 was upregulated during the course of TMUV infection. We then demonstrated that TMUV activated the PERK pathway in the early stage of infection, resulting in upregulation of ATF4, GADD34 and CHOP, with CHOP induction leading to caspase-3 activation. We also found the IRE1 pathway to be activated, leading to splicing of X box binding protein 1 (XBP1) mRNA and enhanced expression of p58^IPK. Finally, we observed increased expression of ATF6 and activity of ER stress-response elements, suggesting stimulation of the ATF6 pathway. In addition, ATF6 pathway activation correlated with the induction of downstream chaperones calnexin, calreticulin, ERp57 and PDI. UPR activity was also observed by the marked elevation in GRP78 and sXBP1 levels in TMUV-infected DF-1 cells.

Conclusions

This is the first report that TMUV infection-induced ER stress activates three branches of the UPR, and these results lay the foundation for elucidating the pathogenesis of TMUV and understanding the inherent mechanism of TMUV infection as well as the host response.