The endoplasmic reticulum stress sensor IRE1α protects cells from apoptosis induced by the coronavirus infectious bronchitis virus

Porcine parvovirus replication is suppressed by activation of the PERK signaling pathway and endoplasmic reticulum stress-mediated apoptosis

Endoplasmic reticulum (ER) stress is associated with numerous mammalian diseases, especially viral diseases. Porcine parvovirus (PPV) is the causative agent of reproductive failure in swine. Here, we observed that the PPV infection of porcine kidney 15 and porcine testis cells resulted in the activation of ER stress sensors mediated by protein kinase R-like ER kinase (PERK), but not inositol-requiring enzyme 1 and activating transcription factor 6 (ATF6). ER stress activation obviously blocked PPV replication. Depletion of proteins, such as PERK, eukaryotic initiation factor 2, and ATF4, by small interfering RNA significantly enhanced PPV replication. Moreover, the pro-apoptotic factor C/EBP homologous protein was identified a key factor in the inhibition of PPV replication. These data demonstrate that PPV infection activates ER stress through the PERK signaling pathway and that ER stress inhibits further PPV replication by promoting apoptosis.

Human Coronavirus: Host-Pathogen Interaction

Human coronavirus (HCoV) infection causes respiratory diseases with mild to severe outcomes. In the last 15 years, we have witnessed the emergence of two zoonotic, highly pathogenic HCoVs: severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV). Replication of HCoV is regulated by a diversity of host factors and induces drastic alterations in cellular structure and physiology. Activation of critical signaling pathways during HCoV infection modulates the induction of antiviral immune response and contributes to the pathogenesis of HCoV. Recent studies have begun to reveal some fundamental aspects of the intricate HCoV-host interaction in mechanistic detail. In this review, we summarize the current knowledge of host factors co-opted and signaling pathways activated during HCoV infection, with an emphasis on HCoV-infection-induced stress response, autophagy, apoptosis, and innate immunity. The cross talk among these pathways, as well as the modulatory strategies utilized by HCoV, is also discussed.

Sec62 Suppresses Foot-and-Mouth Disease Virus Proliferation by Promotion of IRE1α-RIG-I Antiviral Signaling

Foot-and-mouth disease virus (FMDV) is highly infectious and causes a major plague in animal farming. Unfolded protein response is one of the major cellular responses to pathogenic infections, which performs a crucial role in cell survival, apoptosis, and antiviral innate immune response. In this study, we showed that FMDV infection activated two unfolded protein response branches (PERK-eIF2α and ATF6 signaling) in both baby hamster kidney cells (BHK-21) and porcine kidney (PK-15) cells, whereas it suppressed the IRE1α-XBP1 signaling by decreasing IRE1α level. Further study revealed IRE1α signaling as an important antiviral innate immune mechanism against FMDV. Sec62, the transport protein, was greatly decreased at the late stages of FMDV infection. By overexpression and knockdown study, we also found that the expression of Sec62 was positively involved in the levels of IRE1α and RIG-I and subsequent activation of downstream antiviral signaling pathways in FMDV-infected PK-15 cells. Taken together, our study demonstrates that Sec62 is an important antiviral factor that upregulates IRE1α-RIG-I-dependent antiviral innate immune responses, and FMDV evades antiviral host defense mechanism by downregulating Sec62-IRE1α/RIG-I.

The ER stress sensor IRE1 and MAP kinase ERK modulate autophagy induction in cells infected with coronavirus infectious bronchitis virus

Coronavirus infection induces the generation of autophagosomes, and certain host proteins regulating cellular autophagy are hijacked by some coronaviruses to facilitate the formation of double membrane vesicles. However, mechanisms underlying coronavirus-induced autophagy remain largely unknown. In this study, we demonstrate that autophagosome formation and apparent autophagic flux are induced in cells infected with infectious bronchitis virus (IBV) - a gammacoronavirus. Notably, IBV-induced autophagy was dependent on autophagy related 5 (ATG5) but not beclin1 (BECN1), although both are essential proteins in the canonical autophagy pathway. Moreover, the ER stress sensor inositol requiring enzyme 1 (IRE1), but not its substrate X-box protein 1 (XBP1), was also essential for the induction of autophagy during IBV infection. Finally, the anti-apoptotic extracellular signal-regulated kinase 1/2 (ERK1/2) also contributed to IBV-induced autophagy. Our findings add new knowledge to the regulatory mechanisms governing coronavirus-induced autophagy, highlighting an extensive cross-talk among cellular signaling pathways during coronavirus infection.

N-Linked glycosylation of the membrane protein ectodomain regulates infectious bronchitis virus-induced ER stress response, apoptosis and pathogenesis

Coronavirus membrane (M) protein is the most abundant structural protein playing a critical role in virion assembly. Previous studies show that the N-terminal ectodomain of M protein is modified by glycosylation, but its precise functions are yet to be thoroughly investigated. In this study, we confirm that N-linked glycosylation occurs at two predicted sites in the M protein ectodomain of infectious bronchitis coronavirus (IBV). Dual mutations at the two sites (N3D/N6D) did not affect particle assembly, virus-like particle formation and viral replication in culture cells. However, activation of the ER stress response was significantly reduced in cells infected with rN3D/N6D, correlated with a lower level of apoptosis and reduced production of pro-inflammatory cytokines. Taken together, this study demonstrates that although not essential for replication, glycosylation in the IBV M protein ectodomain plays important roles in activating ER stress, apoptosis and proinflammatory response, and may contribute to the pathogenesis of IBV.

The C/EBP Homologous Protein (CHOP) Transcription Factor Functions in Endoplasmic Reticulum Stress-Induced Apoptosis and Microbial Infection

Apoptosis is a form of cell death by which the body maintains the homeostasis of the internal environment. Apoptosis is an initiative cell death process that is controlled by genes and is mainly divided into endogenous pathways (mitochondrial pathway), exogenous pathways (death receptor pathway), and apoptotic pathways induced by endoplasmic reticulum (ER) stress. The homeostasis imbalance in ER results in ER stress. Under specific conditions, ER stress can be beneficial to the body; however, if ER protein homeostasis is not restored, the prolonged activation of the unfolded protein response may initiate apoptotic cell death via the up-regulation of the C/EBP homologous protein (CHOP). CHOP plays an important role in ER stress-induced apoptosis and this review focuses on its multifunctional roles in that process, as well as its role in apoptosis during microbial infection. We summarize the upstream and downstream pathways of CHOP in ER stress induced apoptosis. We also focus on the newest discoveries in the functions of CHOP-induced apoptosis during microbial infection, including DNA and RNA viruses and some species of bacteria. Understanding how CHOP functions during microbial infection will assist with the development of antimicrobial therapies.

BST2 inhibits infection of influenza A virus by promoting apoptosis of infected cells

BST2 is an antiviral factor that inhibits the release of enveloped virus at the plasma membrane via an unusual topology in which its N-terminal is in the cytosol while its C-terminal is anchored by glycophosphatidylinositol (GPI). BST2-deficient cells showed substantially higher release of virions than wild type cells. Influenza-infected BST2-deficient cells showed greatly reduced cytopathic effect (CPE) than wild type cells despite their generally robust virus production. This finding prompted us to determine whether BST2 was involved in the apoptotic process of virus-infected host cells. Our results revealed that BST2 might be involved in IRE1α-mediated ER stress pathway by increasing spliced form XBP-1. Consequently, levels of cytochrome C, caspase-3, caspase-9, and PARP as representative molecules of apoptosis were significantly increased in wild type cells than those in BST2-deficient cells. These results suggest that BST2 might participate in innate host defense by augmenting ER-stress-induced apoptotic signaling to inhibit the replication and spread of virus.

A low-protein diet eliminates the circadian rhythm of serum insulin and hepatic lipid metabolism in mice

Insulin is a key molecule that synchronizes peripheral clocks, such as that in the liver. Although we previously reported that mice fed a low-protein diet showed altered expression of lipid-related genes in the liver and induction of hepatic steatosis, it is unknown whether a low-protein diet impairs insulin secretion and modifies the hepatic circadian rhythm. Therefore, we investigated the effects of the intake of a low-protein diet on the circadian rhythm of insulin secretion and hepatic lipid metabolism in mice. Under 12-h light/12-h dark cycle, mice fed a low-protein diet for 7 days displayed enhanced food intake at the end of the light phase, although central and peripheral PER2 expression rhythm was maintained. Serum insulin levels in mice fed a low-protein diet remained low during the day, and the insulin secretion in OGTT was also markedly lower than in normal mice. In mice fed low-protein diet, hepatic TG accumulation was observed during the nighttime, with relatively high levels of ACC1 mRNA and total ACC proteins. Although there were no differences in the activity rhythm of hepatic mTOR between mice fed a normal or low-protein diet, hepatic IRS-2 expression in mice fed a low-protein diet remained low during the day, with no increase at the beginning of the light period. These results suggested that the low-protein diet eliminated the circadian rhythm of serum insulin and hepatic lipid metabolism in mice, providing insights into our understanding of the mechanisms of hepatic disorders of lipid metabolism.

The Coronavirus Transmissible Gastroenteritis Virus Evades the Type I Interferon Response through IRE1α-Mediated Manipulation of the MicroRNA miR-30a-5p/SOCS1/3 Axis

In host innate immunity, type I interferons (IFN-I) are major antiviral molecules, and coronaviruses have evolved diverse strategies to counter the IFN-I response during infection. Transmissible gastroenteritis virus (TGEV), a member of the Alphacoronavirus family, induces endoplasmic reticulum (ER) stress and significant IFN-I production after infection. However, how TGEV evades the IFN-I antiviral response despite the marked induction of endogenous IFN-I has remained unclear. Inositol-requiring enzyme 1 α (IRE1α), a highly conserved ER stress sensor with both kinase and RNase activities, is involved in the IFN response. In this study, IRE1α facilitated TGEV replication via downmodulating the host microRNA (miR) miR-30a-5p abundance. miR-30a-5p normally enhances IFN-I antiviral activity by directly targeting the negative regulators of Janus family kinase (JAK)-signal transducer and activator of transcription (STAT), the suppressor of cytokine signaling protein 1 (SOCS1), and SOCS3. Furthermore, TGEV infection increased SOCS1 and SOCS3 expression, which dampened the IFN-I antiviral response and facilitated TGEV replication. Importantly, compared with mock infection, TGEV infection in vivo resulted in decreased miR-30a-5p levels and significantly elevated SOCS1 and SOCS3 expression in the piglet ileum. Taken together, our data reveal a new strategy used by TGEV to escape the IFN-I response by engaging the IRE1α-miR-30a-5p/SOCS1/3 axis, thus improving our understanding of how TGEV escapes host innate immune defenses.IMPORTANCE Type I interferons (IFN-I) play essential roles in restricting viral infections. Coronavirus infection induces ER stress and the interferon response, which reflects different adaptive cellular processes. An understanding of how coronavirus-elicited ER stress is actively involved in viral replication and manipulates the host IFN-I response has remained elusive. Here, TGEV inhibited host miR-30a-5p via the ER stress sensor IRE1α, which led to the increased expression of negative regulators of JAK-STAT signaling cascades, namely, SOCS1 and SOCS3. Increased SOCS1 or SOCS3 expression impaired the IFN-I antiviral response, promoting TGEV replication. These findings enhance our understanding of the strategies used by coronaviruses to antagonize IFN-I innate immunity via IRE1α-mediated manipulation of the miR-30a-5p/SOCS axis, highlighting the crucial role of IRE1α in innate antiviral resistance and the potential of IRE1α as a novel target against coronavirus infection.

Induction of the unfolded protein response (UPR) during Marek's disease virus (MDV) infection

Marek's disease (MD) is a pathology of chickens associated with paralysis, immune suppression, and the rapid formation of T-cell lymphomas. MD is caused by the herpesvirus, Marek's disease virus (MDV). We examined endoplasmic reticulum (ER) stress and the activation of unfolded protein response (UPR) pathways during MDV infection of cells in culture and lymphocytes in vivo. MDV strains activate the UPR as measured by increased mRNA expression of GRP78/BiP with concomitant XBP1 splicing and induction of its target gene, EDEM1. Cell culture replication of virulent, but not vaccine MDVs, activated the UPR at late in infection. Pathotype-associated UPR activation was induced to a greater level by a vv + MDV. Discrete UPR activation was observed during MDV in vivo infection, with the level of UPR modulation being affected by the MDV oncoprotein Meq. Finally, ATF6 was found to be activated in vv + MDV-induced primary lymphomas, suggesting a possible role in tumor progression.

Middle East respiratory syndrome coronavirus and bat coronavirus HKU9 both can utilize GRP78 for attachment onto host cells

Coronavirus tropism is predominantly determined by the interaction between coronavirus spikes and the host receptors. In this regard, coronaviruses have evolved a complicated receptor-recognition system through their spike proteins. Spikes from highly related coronaviruses can recognize distinct receptors, whereas spikes of distant coronaviruses can employ the same cell-surface molecule for entry. Moreover, coronavirus spikes can recognize a broad range of cell-surface molecules in addition to the receptors and thereby can augment coronavirus attachment or entry. The receptor of Middle East respiratory syndrome coronavirus (MERS-CoV) is dipeptidyl peptidase 4 (DPP4). In this study, we identified membrane-associated 78-kDa glucose-regulated protein (GRP78) as an additional binding target of the MERS-CoV spike. Further analyses indicated that GRP78 could not independently render nonpermissive cells susceptible to MERS-CoV infection but could facilitate MERS-CoV entry into permissive cells by augmenting virus attachment. More importantly, by exploring potential interactions between GRP78 and spikes of other coronaviruses, we discovered that the highly conserved human GRP78 could interact with the spike protein of bat coronavirus HKU9 (bCoV-HKU9) and facilitate its attachment to the host cell surface. Taken together, our study has identified GRP78 as a host factor that can interact with the spike proteins of two Betacoronaviruses, the lineage C MERS-CoV and the lineage D bCoV-HKU9. The capacity of GRP78 to facilitate surface attachment of both a human coronavirus and a phylogenetically related bat coronavirus exemplifies the need for continuous surveillance of the evolution of animal coronaviruses to monitor their potential for human adaptations.

Post-translational modifications of coronavirus proteins: roles and function

Post-translational modifications (PTMs) refer to the covalent modifications of polypeptides after they are synthesized, adding temporal and spatial regulation to modulate protein functions. Being obligate intracellular parasites, viruses rely on the protein synthesis machinery of host cells to support replication, and not surprisingly, many viral proteins are subjected to PTMs. Coronavirus (CoV) is a group of enveloped RNA viruses causing diseases in both human and animals. Many CoV proteins are modified by PTMs, including glycosylation and palmitoylation of the spike and envelope protein, N- or O-linked glycosylation of the membrane protein, phosphorylation and ADP-ribosylation of the nucleocapsid protein, and other PTMs on nonstructural and accessory proteins. In this review, we summarize the current knowledge on PTMs of CoV proteins, with an emphasis on their impact on viral replication and pathogenesis. The ability of some CoV proteins to interfere with PTMs of host proteins will also be discussed.

Apocynin protects endothelial cells from endoplasmic reticulum stress-induced apoptosis via IRE1α engagement

Endoplasmic reticulum (ER) stress-induced endothelial cell (EC) apoptosis has been implicated in a variety of human diseases. In addition to being regarded as an NADPH oxidase (NOX) inhibitor, apocynin (APO) exhibits an anti-apoptotic effect in various cells. The present study aimed to identify the protective role of apocynin in ER stress-mediated EC apoptosis and the underlying mechanisms. We found that ER stress resulted in a significant increase in c-Jun N-terminal kinase phosphorylation, and elicited caspase 3 cleavage and apoptosis. However, apocynin obviously attenuated EC apoptosis and this effect was partly dependent on ER stress sensor inositol-requiring enzyme 1α (IRE1α). Importantly, apocynin upregulated IRE1α expression in both protein and mRNA levels and promoted the pro-survival XBP1 splicing. Our results suggest that apocynin protects ECs against ER stress-induced apoptosis via IRE1α involvement. These findings may provide a novel mechanistic explanation for the anti-apoptotic effect of apocynin in ER stress.

miR-146a-5p promotes replication of infectious bronchitis virus by targeting IRAK2 and TNFRSF18

Avian infectious bronchitis virus (IBV) is a coronavirus which infects chickens (Gallus gallus) of all ages and causes significant economic losses to the poultry industry worldwide. The present study aims to analyze the miRNAs related to pathogenicity of nephropathogenic IBVs. It was found that four miRNAs (miR-1454, miR-3538, miR-146a-5p and miR-215-5p) were related to the infection of virulent nephropathogenic IBV with transcript per million (TPM) > 500 and more than a 2-fold alteration. In vitro study results showed that the alterations of these four miRNAs were consistent with in vivo data. In vitro, we found that high levels of miR-146a-5p could enhance the replication of IBV at the early stage of infection, and its down regulated level could slow down the replication of IBV. Finally, high levels of exogenous miR-146a-5p in HD11 cells led to down regulation of IL-1 receptor associated kinase-2 (IRAK2) and Tumor necrosis factor receptor superfamily member 18 (TNFRSF18) genes. Luciferase reporter assays revealed that miR-146a-5p could bind to the 3′-UTRs of IRAK2 and TNFRSF18. This is the first study demonstrating that IBV induced miR-146a-5p is related to virus pathogenesis by down regulating IRAK2 and TNFRSF18, which may serve as a therapeutic strategy for the prevention of IBV infections.

•

It is proved that miR-146a-5p regulates the replication of IBV Beaudette strain in HD11 cells.

•

The IRAK2 and TNFRSF18 genes in HD11 cells is the targeting inhibition by miR-146a-5p.

•

The alterations of miR-1454, miR-3538, miR-146a-5p and miR-215-5p in HD11 cells were consistent with in vivo data.

Activation of the c-Jun NH2-terminal kinase pathway by coronavirus infectious bronchitis virus promotes apoptosis independently of c-Jun

Mitogen-activated protein kinases (MAPKs) are conserved protein kinases that regulate a variety of important cellular signaling pathways. Among them, c-Jun N-terminal kinases (JNK) are known to be activated by various environmental stresses including virus infections. Previously, activation of the JNK pathway has been detected in cells infected with several coronaviruses. However, detailed characterization of the pathway as well as its implication in host-virus interactions has not been fully investigated. Here we report that the JNK pathway was activated in cells infected with the avian coronavirus infectious bronchitis virus (IBV). Of the two known upstream MAPK kinases (MKK), MKK7, but not MKK4, was shown to be responsible for IBV-induced JNK activation. Moreover, knockdown and overexpression experiments demonstrated that JNK served as a pro-apoptotic protein during IBV infection. Interestingly, pro-apoptotic activity of JNK was not mediated via c-Jun, but involved modulation of the anti-apoptotic protein B-cell lymphoma 2 (Bcl2). Taken together, JNK constitutes an important aspect of coronavirus-host interaction, along with other MAPKs.

Comparative transcriptome analysis reveals induction of apoptosis in chicken kidney cells associated with the virulence of nephropathogenic infectious bronchitis virus

Avian infectious bronchitis virus (IBV) that causes respiratory and nephritic diseases in chicken is a major poultry pathogen leading to serious economic loss worldwide. The nephropathogenic IBV strains cause nephritis and kidney lesions intrinsically and the pathogenic mechanism is still unclear. In the present study, SPF chicks were infected with three nephropathogenic IBVs of different virulence and their gene expression profiles in chicken kidney were compared at transcriptome level. As a result, 1279 differentially expressed (DE) genes were found in very virulent SCDY2 inoculated group, 145 in virulent SCK2 group and 74 in non-virulent LDT3-A group when compared to mock infected group. Gene Ontology (GO) and KEGG pathway enrichment analysis on SCDY2 group displayed that the up-regulated DE genes were mainly involved in cell apoptosis, and the down-regulated genes were involved in metabolic processes and DNA replication. Protein-Protein Interaction (PPI) analysis showed that DE genes in SCDY2 group formed a network, and the core of the network was composed by cell apoptosis and immune response proteins. The clustering of gene expression profile among the three virus inoculated groups indicated that the majority of up-regulated DE genes on apoptosis in very virulent SCDY2 group were up-regulated more or less in virulent SCK2 group and those down-regulated on innate immune response in SCDY2 group were also down-regulated differently in SCK2 group. In addition, the number of apoptotic cells detected experimentally in kidney tissue were very different among the three virus inoculated groups and were positively accordant with the viral titer, kidney lesions and viral virulence of each group. Taken all together, the present study revealed that virulent nephropathogenic IBV infection modified a number of gene expression and induction of apoptosis in kidney cells may be a major pathogenic determinant for virulent nephropathogenic IBV.

•

Genes expression in chicken kidney cells post inoculation of three nephro IBVs was studied by transcriptome analysis.

•

DE genes post challenge mainly involved in the pathways of apoptosis, immune response, metabolic and DNA replication.

•

Activation of apoptosis and suppression of innate immune response were accordant with the virulence of inoculated IBVs.

•

Induction of apoptosis is triggered by suppression of immune response and productive replication of virus post infection.

Identification of N-linked glycosylation sites in the spike protein and their functional impact on the replication and infectivity of coronavirus infectious bronchitis virus in cell culture

Spike (S) glycoprotein on the viral envelope is the main determinant of infectivity. The S protein of coronavirus infectious bronchitis virus (IBV) contains 29 putative asparagine(N)-linked glycosylation sites. These post-translational modifications may assist in protein folding and play important roles in the functionality of S protein. In this study, we used bioinformatics tools to predict N-linked glycosylation sites and to analyze their distribution in IBV strains and variants. Among these sites, 8 sites were confirmed in the S protein extracted from partially purified virus particles by proteomics approaches. N-D and N-Q substitutions at 13 predicted sites were introduced into an infectious clone system. The impact on S protein-mediated cell-cell fusion, viral recovery and infectivity was assessed, leading to the identification of sites essential for the functions of IBV S protein. Further characterization of these and other uncharacterized sites may reveal novel aspects of N-linked glycosylation in coronavirus replication and pathogenesis.

Infectious Bronchitis Virus Infection Induces Apoptosis during Replication in Chicken Macrophage HD11 Cells

Avian infectious bronchitis has caused huge economic losses in the poultry industry. Previous studies have reported that infectious bronchitis virus (IBV) infection can produce cytopathic effects (CPE) and apoptosis in some mammalian cells and primary cells. However, there is little research on IBV-induced immune cell apoptosis. In this study, chicken macrophage HD11 cells were established as a cellular model that is permissive to IBV infection. Then, IBV-induced apoptosis was observed through a cell viability assay, morphological changes, and flow cytometry. The activity of caspases, the inhibitory efficacy of caspase-inhibitors and the expression of apoptotic genes further suggested the activation of apoptosis through both intrinsic and extrinsic pathways in IBV-infected HD11 cells. Additionally, ammonium chloride (NH₄Cl) pretreated HD11 cells blocked IBV from entering cells and inhibited IBV-induced apoptosis. UV-inactivated IBV also lost the ability of apoptosis induction. IBV replication was increased by blocking caspase activation. This study presents a chicken macrophage cell line that will enable further analysis of IBV infection and offers novel insights into the mechanisms of IBV-induced apoptosis in immune cells.

IRE1α promotes viral infection by conferring resistance to apoptosis

The unfolded protein response (UPR) is an ancient cellular pathway that detects and alleviates protein-folding stresses. The UPR components X-box binding protein 1 (XBP1) and inositol-requiring enzyme 1α (IRE1α) promote type I interferon (IFN) responses. We found that Xbp1-deficient mouse embryonic fibroblasts and macrophages had impaired antiviral resistance. However, this was not because of a defect in type I IFN responses but rather an inability of Xbp1-deficient cells to undergo viral-induced apoptosis. The ability to undergo apoptosis limited infection in wild-type cells. Xbp1-deficient cells were generally resistant to the intrinsic pathway of apoptosis through an indirect mechanism involving activation of the nuclease IRE1α. We observed an IRE1α-dependent reduction in the abundance of the proapoptotic microRNA miR-125a and a corresponding increase in the amounts of the members of the antiapoptotic Bcl-2 family. The activation of IRE1α by the hepatitis C virus (HCV) protein NS4B in XBP1-proficient cells also conferred apoptosis resistance and promoted viral replication. Furthermore, we found evidence of IRE1α activation and decreased miR-125a abundance in liver biopsies from patients infected with HCV compared to those in the livers of healthy controls. Our results reveal a prosurvival role for IRE1α in virally infected cells and suggest a possible target for IFN-independent antiviral therapy.

Human Coronaviruses: A Review of Virus-Host Interactions

Human coronaviruses (HCoVs) are known respiratory pathogens associated with a range of respiratory outcomes. In the past 14 years, the onset of severe acute respiratory syndrome coronavirus (SARS-CoV) and Middle East respiratory syndrome coronavirus (MERS-CoV) have thrust HCoVs into spotlight of the research community due to their high pathogenicity in humans. The study of HCoV-host interactions has contributed extensively to our understanding of HCoV pathogenesis. In this review, we discuss some of the recent findings of host cell factors that might be exploited by HCoVs to facilitate their own replication cycle. We also discuss various cellular processes, such as apoptosis, innate immunity, ER stress response, mitogen-activated protein kinase (MAPK) pathway and nuclear factor kappa B (NF-κB) pathway that may be modulated by HCoVs.

Regulation of Stress Responses and Translational Control by Coronavirus

Similar to other viruses, coronavirus infection triggers cellular stress responses in infected host cells. The close association of coronavirus replication with the endoplasmic reticulum (ER) results in the ER stress responses, which impose a challenge to the viruses. Viruses, in turn, have come up with various mechanisms to block or subvert these responses. One of the ER stress responses is inhibition of the global protein synthesis to reduce the amount of unfolded proteins inside the ER lumen. Viruses have evolved the capacity to overcome the protein translation shutoff to ensure viral protein production. Here, we review the strategies exploited by coronavirus to modulate cellular stress response pathways. The involvement of coronavirus-induced stress responses and translational control in viral pathogenesis will also be briefly discussed.

Endoplasmic Reticulum Stress Interacts With Inflammation in Human Diseases

The endoplasmic reticulum (ER) is a critical organelle for normal cell function and homeostasis. Disturbance in the protein folding process in the ER, termed ER stress, leads to the activation of unfolded protein response (UPR) that encompasses a complex network of intracellular signaling pathways. The UPR can either restore ER homeostasis or activate pro-apoptotic pathways depending on the type of insults, intensity and duration of the stress, and cell types. ER stress and the UPR have recently been linked to inflammation in a variety of human pathologies including autoimmune, infectious, neurodegenerative, and metabolic disorders. In the cell, ER stress and inflammatory signaling share extensive regulators and effectors in a broad spectrum of biological processes. In spite of different etiologies, the two signaling pathways have been shown to form a vicious cycle in exacerbating cellular dysfunction and causing apoptosis in many cells and tissues. However, the interaction between ER stress and inflammation in many of these diseases remains poorly understood. Further understanding of the biochemistry, cell biology, and physiology may enable the development of novel therapies that spontaneously target these pathogenic pathways.

Bicyclol attenuates tetracycline-induced fatty liver associated with inhibition of hepatic ER stress and apoptosis in mice

Endoplasmic reticulum (ER) stress is known to be involved in the development of several metabolic disorders, including non-alcoholic fatty liver disease (NAFLD). Tetracycline can cause hepatic steatosis, and ER stress may be involved in tetracycline-induced fatty liver. Our previous study showed that bicyclol has been proven to protect against tetracycline-induced fatty liver in mice, and ER stress may also be involved in bicyclol’s hepatoprotective effect. Therefore, this study was performed to investigate the underlying mechanisms associated with ER stress and apoptosis, by which bicyclol attenuated tetracycline-induced fatty liver in mice. Bicyclol (300 mg/kg) was given to mice by gavage 3 times. Tetracycline (200 mg/kg, intraperitoneally) was injected at 1 h after the last dose of bicyclol. At 6 h and 24 h after single dose of tetracycline injection, serum ALT, AST, TG, CHO and hepatic histopathological examinations were performed to evaluate liver injuries. Hepatic steatosis was assessed by the accumulation of hepatic TG and CHO. Moreover, hepatic apoptosis and ER stress related markers were determined by TUNEL, real-time PCR, and western blot. As a result, bicyclol significantly protected against tetracycline-induced fatty liver as evidenced by the decrease of elevated serum transaminases and hepatic triglyceride, and the attenuation of histopathological changes in mice. In addition, bicyclol remarkably alleviated hepatic apoptosis and the gene expression of caspase–3, and increased the gene expression of XIAP. The gene expressions of ER stress-related markers, including CHOP, GRP78, IRE-1α, and ATF6, which were downregulated by bicyclol pretreatment in tetracycline-injected mice. These results suggested that bicyclol protected tetracycline-induced fatty liver partly due to its ability of anti-apoptosis associated with ER stress.

Leucine restores murine hepatic triglyceride accumulation induced by a low-protein diet by suppressing autophagy and excessive endoplasmic reticulum stress

Although it is known that a low-protein diet induces hepatic triglyceride (TG) accumulation in both rodents and humans, little is known about the underlying mechanism. In the present study, we modeled hepatic TG accumulation by inducing dietary protein deficiency in mice and aimed to determine whether certain amino acids could prevent low-protein diet-induced TG accumulation in the mouse liver. Mice fed a diet consisting of 3 % casein (3C diet) for 7 days showed hepatic TG accumulation with up-regulation of TG synthesis for the Acc gene and down-regulation of TG-rich lipoprotein secretion from hepatocytes for Mttp genes. Supplementing the 3 % casein diet with essential amino acids, branched-chain amino acids, or the single amino acid leucine rescued hepatic TG accumulation. In the livers of mice fed the 3 % casein diet, we observed a decrease in the levels of the autophagy substrate p62, an increase in the expression levels of the autophagy marker LC3-II, and an increase in the splicing of the endoplasmic reticulum (ER) stress-dependent Xbp1 gene. Leucine supplementation to the 3 % casein diet did not affect genes related to lipid metabolism, but inhibited the decrease in p62, the increase in LC3-II, and the increase in Xbp1 splicing levels in the liver. Our results suggest that ER stress responses and activated autophagy play critical roles in low-protein diet-induced hepatic TG accumulation in mice, and that leucine suppresses these two major protein degradation systems. This study contributes to understanding the mechanisms of hepatic disorders of lipid metabolism.

The integrated endoplasmic reticulum stress response in Leishmania amazonensis macrophage infection: the role of X-box binding protein 1 transcription factor

Endoplasmic reticulum (ER) stress triggers the integrated ER-stress response (IERSR) that ensures cellular survival of ER stress and represents a primordial form of innate immunity. We investigated the role of IERSR duringLeishmania amazonensisinfection. Treatment of RAW 264.7 infected macrophages with the ER stress-inducing agent thapsigargin (TG; 1 μM) increasedL. amazonensisinfectivity in an IFN1-α receptor (IFNAR)-dependent manner. In Western blot assays, we showed thatL. amazonensisactivates the inositol-requiring enzyme (IRE1)/ X-box binding protein (XBP)-1-splicing arms of the IERSR in host cells. In chromatin immunoprecipitation (ChIP) assays, we showed an increased occupancy of enhancer and promoter sequences for theIfnbgene by XBP1 in infected RAW 264.7 cells. Knocking down XBP1 expression by transducing RAW 264.7 cells with the short hairpin XBP1 lentiviral vector significantly reduced the parasite proliferation associated with impaired translocation of phosphorylated IFN regulatory transcription factor (IRF)-3 to the nucleus and a decrease in IFN1-β expression. Knocking down XBP1 expression also increased NO concentration, as determined by Griess reaction and reduced the expression of antioxidant genes, such as heme oxygenase (HO)-1, that protect parasites from oxidative stress. We conclude thatL. amazonensisactivation of XBP1 plays a critical role in infection by protecting the parasites from oxidative stress and increasing IFN1-β expression.-Dias-Teixeira, K. L., Calegari-Silva, T. C., Dos Santos, G. R. R. M., Vitorino dos Santos, J., Lima, C., Medina, J. M., Aktas, B. H., Lopes, U. G. The integrated endoplasmic reticulum stress response inLeishmania amazonensismacrophage infection: the role of X-box binding protein 1 transcription factor.

Fyn Activation of mTORC1 Stimulates the IRE1α-JNK Pathway, Leading to Cell Death

We previously reported that the skeletal muscle-specific overexpression of Fyn in mice resulted in a severe muscle wasting phenotype despite the activation of mTORC1 signaling. To investigate the bases for the loss of muscle fiber mass, we examined the relationship between Fyn activation of mTORC1, JNK, and endoplasmic reticulum stress. Overexpression of Fyn in skeletal muscle in vivo and in HEK293T cells in culture resulted in the activation of IRE1α and JNK, leading to increased cell death. Fyn synergized with the general endoplasmic reticulum stress inducer thapsigargin, resulting in the activation of IRE1α and further accelerated cell death. Moreover, inhibition of mTORC1 with rapamycin suppressed IRE1α activation and JNK phosphorylation, resulting in protecting cells against Fyn- and thapsigargin-induced cell death. Moreover, rapamycin treatment in vivo reduced the skeletal muscle IRE1α activation in the Fyn-overexpressing transgenic mice. Together, these data demonstrate the presence of a Fyn-induced endoplasmic reticulum stress that occurred, at least in part, through the activation of mTORC1, as well as subsequent activation of the IRE1α-JNK pathway driving cell death.

Immune Responses to Virulent and Vaccine Strains of Infectious Bronchitis Viruses in Chickens

Infectious bronchitis (IB) is an acute and highly contagious chicken viral disease, causing severe economic losses to poultry producers worldwide. In the last few decades, infectious bronchitis virus (IBV) has been extensively studied, but knowledge of immune responses to virulent or vaccine strains of IBVs remains limited. This review focuses on fundamental aspects of immune responses against IBV, including the role of pattern recognition receptors (PRRs) in identification of conserved viral structures and the role of different components of innate immunity (e.g., heterophils, macrophages, dendritic cells, acute phase protein, and cytokines). Studies on adaptive immune activation and the role of humoral and cellular immunity in IBV clearance are also reviewed. Multiple interlinking immune responses are essential for protection against virulent IBVs, including passive, innate, adaptive, and effector T cells active at mucosal surfaces. Although the development of approaches for chicken transcriptome and proteome analyses have greatly helped the understanding of the underlying genetic mechanisms for immunity, there are still major knowledge gaps, such as the role of mucosal and cellular responses to IBVs. In view of recent reports of emergent IBV variants in many countries, there is renewed interest in a more complete understanding of poultry immune responses to both virulent and vaccine strains of IBVs. This will be critical for developing new vaccine or vaccination strategies and other intervention programs.

The Emerging Roles of Viroporins in ER Stress Response and Autophagy Induction during Virus Infection

Viroporins are small hydrophobic viral proteins that oligomerize to form aqueous pores on cellular membranes. Studies in recent years have demonstrated that viroporins serve important functions during virus replication and contribute to viral pathogenicity. A number of viroporins have also been shown to localize to the endoplasmic reticulum (ER) and/or its associated membranous organelles. In fact, replication of most RNA viruses is closely linked to the ER, and has been found to cause ER stress in the infected cells. On the other hand, autophagy is an evolutionarily conserved "self-eating" mechanism that is also observed in cells infected with RNA viruses. Both ER stress and autophagy are also known to modulate a wide variety of signaling pathways including pro-inflammatory and innate immune response, thereby constituting a major aspect of host-virus interactions. In this review, the potential involvement of viroporins in virus-induced ER stress and autophagy will be discussed.

HIV infection and antiretroviral therapy lead to unfolded protein response activation

Background

The unfolded protein response (UPR) is one of the pathways triggered to ensure quality control of the proteins assembled in the endoplasmic reticulum (ER) when cell homeostasis is compromised. This mechanism is primarily composed of three transmembrane proteins serving as stress sensors: PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme 1 (IRE1). These three proteins’ synergic action elicits translation and transcriptional downstream pathways, leading to less protein production and activating genes that encode important proteins in folding processes, including chaperones. Previous reports showed that viruses have evolved mechanisms to curtail or customize this UPR signaling for their own benefit. However, HIV infection’s effect on the UPR has scarcely been investigated.

Methods

This work investigated UPR modulation by HIV infection by assessing UPR-related protein expression under in vitro and in vivo conditions via Western blotting. Antiretroviral (ARV) drugs’ influence on this stress response was also considered.

Results

In in vitro and in vivo analyses, our results confirm that HIV infection activates stress-response components and that ARV therapy contributes to changes in the UPR’s activation profile.

Conclusions

This is the first report showing UPR-related protein expression in HIV target cells derived directly from HIV-infected patients receiving different ARV therapies. Thus, two mechanisms may occur simultaneously: interference by HIV itself and the ARV drugs’ pharmacological effects as UPR activators. New evidence of how HIV modulates the UPR to enhance its own replication and secure infection success is also presented.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0298-0) contains supplementary material, which is available to authorized users.

Coronavirus-induced ER stress response and its involvement in regulation of coronavirus-host interactions

Coronavirus replication is structurally and functionally associated with the endoplasmic reticulum (ER), a major site of protein synthesis, folding, modification and sorting in the eukaryotic cells. Disturbance of ER homeostasis may occur under various physiological or pathological conditions. In response to the ER stress, signaling pathways of the unfolded protein response (UPR) are activated. UPR is mediated by three ER transmembrane sensors, namely the PKR-like ER protein kinase (PERK), the inositol-requiring protein 1 (IRE1) and the activating transcriptional factor 6 (ATF6). UPR facilitates adaptation to ER stress by reversible translation attenuation, enhancement of ER protein folding capacity and activation of ER-associated degradation (ERAD). In cells under prolonged and irremediable ER stress, UPR can also trigger apoptotic cell death. Accumulating evidence has shown that coronavirus infection causes ER stress and induces UPR in the infected cells. UPR is closely associated with a number of major signaling pathways, including autophagy, apoptosis, the mitogen-activated protein (MAP) kinase pathways, innate immunity and pro-inflammatory response. Therefore, studies on the UPR are pivotal in elucidating the complicated issue of coronavirus-host interaction. In this paper, we present the up-to-date knowledge on coronavirus-induced UPR and discuss its potential involvement in regulation of innate immunity and apoptosis.