Autophagy during early stages contributes to bovine viral diarrhea virus replication in MDBK cells

Non-cytopathic bovine viral diarrhea virus (BVDV) inhibits innate immune responses via induction of mitophagy

Bovine viral diarrhea virus (BVDV) belongs to the genus Pestivirus within the family Flaviviridae. Mitophagy plays important roles in virus-host interactions. Here, we provide evidence that non-cytopathic (NCP) BVDV shifts the balance of mitochondrial dynamics toward fission and induces mitophagy to inhibit innate immune responses. Mechanistically, NCP BVDV triggers the translocation of dynamin-related protein (Drp1) to mitochondria and stimulates its phosphorylation at Ser616, leading to mitochondrial fission. In parallel, NCP BVDV-induced complete mitophagy via Parkin-dependent pathway contributes to eliminating damaged mitochondria to inhibit MAVS- and mtDNA-cGAS-mediated innate immunity responses, mtROS-mediated inflammatory responses and apoptosis initiation. Importantly, we demonstrate that the LIR motif of ERNS is essential for mitophagy induction. In conclusion, this study is the first to show that NCP BVDV-induced mitophagy plays a central role in promoting cell survival and inhibiting innate immune responses in vitro.

Mitochondria-mediated ferroptosis contributes to the inflammatory responses of bovine viral diarrhea virus (BVDV) in vitro

Bovine viral diarrhea virus (BVDV) belongs to the family Flaviviridae and includes two biotypes in cell culture: cytopathic (CP) or non-cytopathic (NCP) effects. Ferroptosis is a non-apoptotic form of programmed cell death that contributes to inflammatory diseases. However, whether BVDV induces ferroptosis and the role of ferroptosis in viral infection remain unclear. Here, we provide evidence that both CP and NCP BVDV can induce ferroptosis in Madin-Darby bovine kidney cells at similar rate. Mechanistically, biotypes of BVDV infection downregulate cytoplasmic and mitochondrial GPX4 via Nrf2-GPX4 pathway, thereby resulting in lethal lipid peroxidation and promoting ferroptosis. In parallel, BVDV can degrade ferritin heavy chain and mitochondrial ferritin via NCOA4-mediated ferritinophagy to promote the accumulation of Fe2+ and initiate ferroptosis. Importantly, CP BVDV-induced ferroptosis is tightly associated with serious damage of mitochondria and hyperactivation of inflammatory responses. In contrast, mild or unapparent damage of mitochondria and slight inflammatory responses were detected in NCP BVDV-infected cells. More importantly, different mitophagy pathways in response to mitochondria damage by both biotypes of BVDV are involved in inflammatory responses. Overall, this study is the first to show that mitochondria may play key roles in mediating ferroptosis and inflammatory responses induced by biotypes of BVDV in vitro.IMPORTANCEBovine viral diarrhea virus (BVDV) threatens a wide range of domestic and wild cattle population worldwide. BVDV causes great economic loss in cattle industry through its immunosuppression and persistent infection. Despite extensive research, the mechanism underlying the pathogenesis of BVDV remains elusive. Our data provide the first direct evidence that mitochondria-mediated ferroptosis and mitophagy are involved in inflammatory responses in both biotypes of BVDV-infected cells. Importantly, we demonstrate that the different degrees of injury of mitochondria and inflammatory responses may attribute to different mitophagy pathways induced by biotypes of BVDV. Overall, our findings uncover the interaction between BVDV infection and mitochondria-mediated ferroptosis, which shed novel light on the physiological impacts of ferroptosis on the pathogenesis of BVDV infection, and provide a promising therapeutic strategy to treat this important infectious disease with a worldwide distribution.

The activation of liver X receptors in Madin-Darby bovine kidney cells and mice restricts infection by bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) is prevalent worldwide and is an important pathogen that represents a serious threat to the development of the cattle industry by causing significant economic losses. Liver X receptors (LXRs) are members of the nuclear receptor superfamily and have become attractive therapeutic targets for cardiovascular disease. In the present study, we found that LXRs in both Madin-Darby bovine kidney (MDBK) cells and mice were associated with BVDV infection. GW3965, an agonist for LXRs, significantly inhibited BVDV RNA and protein levels in MDBK cells. In vivo studies in a mouse model also confirmed the inhibitory role of GW3965 in BVDV replication and the ameliorating effect of GW3965 on pathological injury to the duodenum. In vitro investigations of the potential mechanisms involved showed that GW3965 significantly inhibited BVDV-induced increases in cholesterol levels and viral internalization. Furthermore, the antiviral activity of GW3965 was significantly reduced following cholesterol replenishment, thus demonstrating that cholesterol was involved in the resistance of GW3965 to BVDV replication. Further studies indicated the role of ATP-binding cassette transporter A1 (ABCA1) and cholesterol-25-hydroxylase (CH25H) in the antiviral activity of GW3965. We also demonstrated the significant antiviral effect of 25hydroxycholesterol (25HC), a product of the catalysis of cholesterol by CH25H. In addition, the anti-BVDV effects of demethoxycurcumin (DMC), cyanidin-3-O-glucoside (C3G), and saikosaponin-A (SSA), three natural agonizts of LXRs, were also confirmed in both MDBK cells and mice. However, the antiviral activities of these agents were weakened by SR9243, a synthetic inhibitor of LXRs. For the first time, our research demonstrated that the activation of LXRs can exert significant anti-BVDV effects in MDBK cells and mice.

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.

Non-cytopathic bovine viral diarrhoea virus 2 induces autophagy to enhance its replication

BACKGROUND

Bovine viral diarrhoea virus (BVDV) is an important viral pathogen that has an economic impact on the livestock industry worldwide. Autophagy is one of the earliest cell-autonomous defence mechanisms against microbial invasion, and many types of viruses can induce autophagy by infecting host cells.

OBJECTIVES

The aim of this study was to identify the role of autophagy in the pathogenesis of non-cytopathic (ncp) BVDV2 infection.

METHODS

Madin-Darby bovine kidney (MDBK) cells were treated with ncp BVDV2, rapamycin, or 3-methyladenine (MA) and ncp BVDV2 and then incubated at 37°C for 24 h. Cells were harvested, and the effects of autophagy were determined by transmission electron microscopy (TEM), confocal laser microscopy, western blotting and qRT-PCR. Apoptotic analysis was also performed using western blotting and flow cytometry.

RESULTS

In ncp BVDV2-infected MDBK cells, more autophagosomes were observed by TEM, and the number of microtubule-associated protein 1 light chain 3B (LC3B) with green fluorescent protein puncta was also increased. The ncp BVDV2-infected cells showed significantly enhanced conversion of LC3-I to LC3-II, as well as upregulation of autophagy-related proteins, including ATG5 and Beclin 1, and substantial degradation of p62/SQSTM1. These results are similar to those induced by rapamycin, an autophagy inducer. E2 protein expression, which is associated with viral replication, increased over time in ncp BVDV2-infected cells. Inhibition of autophagy by 3-MA in ncp BVDV2-infected MDBK cells downregulated the expressions of LC3-II, ATG5 and Beclin 1 and prevented the degradation of p62/SQSTM1. Moreover, the expressions of phosphorylated Akt and procaspase-3 were significantly increased in ncp BVDV2-infected cells. In addition, the mRNA level of protein kinase R (PKR) was significantly reduced in ncp BVDV2-infected cells.

CONCLUSIONS

Our results demonstrate that ncp BVDV2 infection induced autophagy in MDBK cells via anti-apoptosis and PKR suppression. Therefore, autophagy may play a role in establishing persistent infection caused by ncp BVDV.

In Vivo and In Vitro Antiviral Activity of Phlorizin Against Bovine Viral Diarrhea Virus

Bovine viral diarrhea virus (BVDV) is one of the most serious pathogens affecting the cattle industry worldwide. Phlorizin, a kind of flavonoids extracted from apple tree roots, leaves, and fruits, has a variety of biological functions and has been widely used as a herbal supplement and food additive. Here, BALB/c mouse and Madin-Darby bovine kidney (MDBK) cells were used to explore the effect and mechanism of phlorizin against BVDV infection. The results showed that phlorizin significantly inhibited CP BVDV replication and improved the histopathological changes of duodenum and spleen in mice. In vitro studies also confirmed the activity of phlorizin against CP BVDV. Exploration on its potential mechanism suggested that phlorizin inhibited CP BVDV-induced beclin-1 level and the conversion rate of LC3B-I to LC3B-II. Interestingly, although phlorizin also showed a protective effect on MDBK cells, which were treated with 3-methyladenine A (3-MA), the effect was significantly weakened. Furthermore, phlorizin suppressed the stage of BVDV replication but showed no effect on stages of attachment and internalization. Our data further indicated that phlorizin promoted IFN-α and IFN-β levels, decreased IL-1β and IL-6 expression, and regulated RIG-I, MDA5, TLR3, and NLRP3 levels. Similar to CP BVDV results, in vivo and in vitro, phlorizin inhibited NCP BVDV (NY-1 and YNJG2020 strains) infection. These results were the first to be discovered that phlorizin might be used as a new dietary strategy for controlling BVDV infection.

Autophagy in farm animals: current knowledge and future challenges

Autophagy (a process of cellular self-eating) is a conserved cellular degradative process that plays important roles in maintaining homeostasis and preventing nutritional, metabolic, and infection-mediated stresses. Surprisingly, little attention has been paid to the role of this cellular function in species of agronomical interest, and the details of how autophagy functions in the development of phenotypes of agricultural interest remain largely unexplored. Here, we first provide a brief description of the main mechanisms involved in autophagy, then review our current knowledge regarding autophagy in species of agronomical interest, with particular attention to physiological functions supporting livestock animal production, and finally assess the potential of translating the acquired knowledge to improve animal development, growth and health in the context of growing social, economic and environmental challenges for agriculture.Abbreviations: AKT: AKT serine/threonine kinase; AMPK: AMP-activated protein kinase; ASC: adipose-derived stem cells; ATG: autophagy-related; BECN1: beclin 1; BNIP3: BCL2 interacting protein 3; BVDV: bovine viral diarrhea virus; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CMA: chaperone-mediated autophagy; CTSB: cathepsin B; CTSD: cathepsin D; DAP: Death-Associated Protein; ER: endoplasmic reticulum; GFP: green fluorescent protein; Gln: Glutamine; HSPA8/HSC70: heat shock protein family A (Hsp70) member 8; IF: immunofluorescence; IVP: in vitro produced; LAMP2A: lysosomal associated membrane protein 2A; LMS: lysosomal membrane stability; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MDBK: Madin-Darby bovine kidney; MSC: mesenchymal stem cells; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; NBR1: NBR1 autophagy cargo receptor; NDV: Newcastle disease virus; NECTIN4: nectin cell adhesion molecule 4; NOD1: nucleotide-binding oligomerization domain 1; OCD: osteochondritis dissecans; OEC: oviduct epithelial cells; OPTN: optineurin; PI3K: phosphoinositide-3-kinase; PPRV: peste des petits ruminants virus; RHDV: rabbit hemorrhagic disease virus; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy.

Cell death pathways and viruses: Role of microRNAs

Viral infections lead to the death of more than a million people each year around the world, both directly and indirectly. Viruses interfere with many cell functions, particularly critical pathways for cell death, by affecting various intracellular mediators. MicroRNAs (miRNAs) are a major example of these mediators because they are involved in many (if not most) cellular mechanisms. Virus-regulated miRNAs have been implicated in three cell death pathways, namely, apoptosis, autophagy, and anoikis. Several molecules (e.g., BECN1 and B cell lymphoma 2 [BCL2] family members) are involved in both apoptosis and autophagy, while activation of anoikis leads to cell death similar to apoptosis. These mechanistic similarities suggest that common regulators, including some miRNAs (e.g., miR-21 and miR-192), are involved in different cell death pathways. Because the balance between cell proliferation and cell death is pivotal to the homeostasis of the human body, miRNAs that regulate cell death pathways have drawn much attention from researchers. miR-21 is regulated by several viruses and can affect both apoptosis and anoikis via modulating various targets, such as PDCD4, PTEN, interleukin (IL)-12, Maspin, and Fas-L. miR-34 can be downregulated by viral infection and has different effects on apoptosis, depending on the type of virus and/or host cell. The present review summarizes the existing knowledge on virus-regulated miRNAs involved in the modulation of cell death pathways. Understanding the mechanisms for virus-mediated regulation of cell death pathways could provide valuable information to improve the diagnosis and treatment of many viral diseases.

DDIT3 Targets Innate Immunity via the DDIT3-OTUD1-MAVS Pathway To Promote Bovine Viral Diarrhea Virus Replication

DNA damage-inducible transcript 3 (DDIT3) plays important roles in endoplasmic reticulum (ER) stress-induced apoptosis and autophagy, but its role in innate immunity is not clear. Here, we report that DDIT3 inhibits the antiviral immune response during bovine viral diarrhea virus (BVDV) infection by targeting mitochondrial antiviral signaling (MAVS) in Madin-Darby bovine kidney (MDBK) cells and in mice. BVDV infection induced high DDIT3 mRNA and protein expression. DDIT3 overexpression inhibited type I interferon (IFN-I) and IFN-stimulated gene production, thereby promoting BVDV replication, while DDIT3 knockdown promoted the antiviral innate immune response to suppress viral replication. DDIT3 promoted NF-κB-dependent ovarian tumor (OTU) deubiquitinase 1 (OTUD1) expression. Furthermore, OTUD1 induced upregulation of the E3 ubiquitin ligase Smurf1 by deubiquitinating Smurf1, and Smurf1 degraded MAVS in MDBK cells in a ubiquitination-dependent manner, ultimately inhibiting IFN-I production. Moreover, knocking out DDIT3 promoted the antiviral innate immune response to reduce BVDV replication and pathological changes in mice. These findings provide direct insights into the molecular mechanisms by which DDIT3 inhibits IFN-I production by regulating MAVS degradation.IMPORTANCE Extensive studies have demonstrated roles of DDIT3 in apoptosis and autophagy during viral infection. However, the role of DDIT3 in innate immunity remains largely unknown. Here, we show that DDIT3 is positively regulated in bovine viral diarrhea virus (BVDV)-infected Madin-Darby bovine kidney (MDBK) cells and could significantly enhance BVDV replication. Importantly, DDIT3 induced OTU deubiquitinase 1 (OTUD1) expression by activating the NF-κB signaling pathway, thus increasing intracellular Smurf1 protein levels to degrade MAVS and inhibit IFN-I production during BVDV infection. Together, these results indicate that DDIT3 plays critical roles in host innate immunity repression and viral infection facilitation.

Comprehensive analysis of lncRNA expression profiles in cytopathic biotype BVDV-infected MDBK cells provides an insight into biological contexts of host-BVDV interactions

Bovine viral diarrhea virus (BVDV) is the causative agent of bovine viral diarrhea-mucosal disease, which significantly affects the production performance of cattle, causing serious economic losses to the cattle industries worldwide. Up to now, some mechanisms involved in host–BVDV interaction are still not fully understood. The discovery of long non-coding RNAs (lncRNAs) has provided a new perspective on gene regulation in diverse biological contexts, particularly in viral infection and host immune responses. However, little is known about the profiles and functions of lncRNAs in host cells in response to BVDV infection. Here, we utilized Illumina sequencing to explore lncRNAs profiles in cytopathic (CP) biotype BVDV-infected MDBK cells to further reveal the potential roles of lncRNAs in BVDV infection and host–BVDV interaction with integrated analysis of lncRNAs and mRNA expression profiles. A total of 1747 significantly differentially expressed genes, DEGs (156 lncRNAs and 1591 mRNAs) were obtained via RNA-seq in BVDV-infected MDBK cells compared to mock-infected cells. Next, these DE lncRNAs and mRNAs were subjected to construct lncRNAs-mRNAs co-expression network followed by the prediction of potential functions of the DE lncRNAs. Co-expression network analysis elucidated that DE lncRNAs were significant enrichment in NOD-like receptor, TNF, NF-ĸB, ErbB, Ras, apoptosis, and fatty acid biosynthesis pathways, indicating that DE lncRNAs play important roles in host–BVDV interactions. Our data give an overview of changes in transcriptome and potential roles of lncRNAs, providing molecular biology basis for further exploring the mechanisms of host–BVDV interaction.

Bovine viral diarrhea virus non-structural protein NS4B induces autophagosomes in bovine kidney cells

Bovine viral diarrhea virus (BVDV) is an important pathogen in cattle that causes economic losses in livestock industries. Autophagy is an essential cell system for the maintenance of homeostasis and is induced by various triggers, including infection by viruses. BVDV infection leads to autophagy in order to enhance its replication in cells. In this study, we investigated the effect of BVDV non-structural proteins on the induction of autophagosomes. We found that NS4B alone could induce autophagosomes, suggesting a novel and important function of NS4B in BVDV replication.

mTORC1 Negatively Regulates the Replication of Classical Swine Fever Virus Through Autophagy and IRES-Dependent Translation

Classical swine fever virus (CSFV) can utilize diverse host signaling pathways for its replication; however, the cross talk between mammalian target of rapamycin (mTOR) and CSFV remains unknown. Here, we describe the potential role of mTOR complex 1 (mTORC1) in promoting CSFV replication via virus-induced hypophosphorylation of the Akt/mTORC1/S6 pathway, especially at an early stage of viral infection. Conversely, activation of mTORC1 inhibited the replication of CSFV. Furthermore, we revealed the underlying mechanisms of mTORC1 pathway in mediating CSFV replication; in addition, our data also showed that CSFV-induced transient inhibition of mTORC1 elicited a negative feedback activation of PI3K/Akt/mTORC1pathway, likely contributing to maintain the dynamic balance between viral replication and host cell survival. This study has provided strong evidence showing how CSFV utilizes mTORC1 pathway for viral replication at an early stage in the viral replicative cycle and how the mTORC1 rescues itself by eliciting a feedback loop to limit viral replication and maintain cell survival.

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Akt/mTORC1 pathway negatively regulates the replication of CSFV

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CSFV induces autophagy for viral replication in an mTORC1/ULK1-dependent manner

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CSFV enhances the translation of viral proteins in an mTORC1/S6K1/eIF3-dependent manner

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Feedback activation of Akt/mTORC1 equilibrates viral replication and cell survival

Autophagy induced by bovine viral diarrhea virus infection counteracts apoptosis and innate immune activation

Bovine viral diarrhea virus (BVDV) is an important pathogen of cattle that plays a complex role in disease. There are two biotypes of BVDV: non-cytopathic (NCP) and cytopathic (CP). One strategy that has been used to treat or prevent virus-associated diseases is the modulation of autophagy, which is used by the innate immune system to defend against viral infection; however, at present, the interplay between autophagy and BVDV remains unclear. Madin-Darby bovine kidney cells stably expressing microtubule-associated protein 1 light chain 3B (LC3B) with green fluorescent protein (GFP) (GFP-LC3-MDBK cells) and autophagy-deficient MDBKs (shBCN1-MDBK cells) were constructed. Then MDBK, GFP-LC3-MDBK and shBCN1-MDBK cells were infected with CP or NCP BVDV strains. The LC3-II turnover rate was estimated by western blot, autophagosomes were visualized by confocal microscopy, and ultrastructural analysis was performed using electron microscopy. Autophagy flux was observed using chloroquine as an inhibitor of the autophagic process. The influence of autophagy on BVDV replication and release was investigated using virus titration, and its effect on cell viability was also studied. The effect of BVDV-induced autophagy on the survival of BVDV-infected host cell, cell apoptosis, and interferon (IFN) signalling was studied by flow cytometric analysis and quantitative RT-(q)PCR using shBCN1-MDBK cells. we found that infection with either CP or NCP BVDV strains induced steady-state autophagy in MDBK cells, as evident by the increased number of double- or single-membrane vesicles, the accumulation of GFP- microtubule-associated protein 1 light chain 3 (LC3) dots, and the conversion of LC3-I (cytosolic) to LC3-II (membrane-bound) forms. The complete autophagic process was verified by monitoring the LC3-II turnover ratio, lysosomal delivery, and proteolysis. In addition, we found that CP and NCP BVDV growth was inhibited in MDBK cells treated with high levels of an autophagy inducer or inhibitor, or in autophagy deficient-MDBK cells. Furthermore, our studies also suggested that CP and NCP BVDV infection in autophagy-knockdown MDBK cells increased apoptotic cell death and enhanced the expression of the mRNAs for IFN-α, Mx1, IFN-β, and OAS-1 as compared with control MDBK cells. Our study provides strong evidence that BVDV infection induces autophagy, which facilitates BVDV replication in MDBK cells and impairs the innate immune response. These findings might help to illustrate the pathogenesis of persistent infection caused by BVDV.

What can pestiviral endonucleases teach us about innate immunotolerance?

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In this review, we describe the identification of the PRRs involved in the recognition of pestiviruses, and the mechanisms of these viruses to prevent the activation of host’s innate immune response with special emphasis on viral RNases.

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Most importantly, we extend these data and present our model of innate immunotolerance requiring continuous prevention of detection of immunostimulatory self nucleic acids, in contrast to the well-known long-term tolerance of the adaptive immune system targeted predominantly against proteins.

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This hypothesis is very likely relevant beyond the bovine species and might answer more fundamental questions on the discrimination between “self” and “viral nonself RNA”, which are relevant also for the prevention and treatment of chronic IFN induction and autoimmunity induced by “self-RNAs”.

Pestiviruses including bovine viral diarrhea virus (BVDV), border disease virus (BDV) and classical swine fever virus (CSFV), occur worldwide and are important pathogens of livestock. A large part of their success can be attributed to the induction of central immunotolerance including B- and T-cells upon fetal infection leading to the generation of persistently infected (PI) animals. In the past few years, it became evident that evasion of innate immunity is a central element to induce and maintain persistent infection. Hence, the viral non-structural protease N^pro heads the transcription factor IRF-3 for proteasomal degradation, whereas an extracellularly secreted, soluble form of the envelope glycoprotein E^rns degrades immunostimulatory viral single- and double-stranded RNA, which makes this RNase unique among viral endoribonucleases. We propose that these pestiviral interferon (IFN) antagonists maintain a state of innate immunotolerance mainly pertaining its viral nucleic acids, in contrast to the well-established immunotolerance of the adaptive immune system, which is mainly targeted at proteins. In particular, the unique extension of ‘self’ to include the viral genome by degrading immunostimulatory viral RNA by E^rns is reminiscent of various host nucleases that are important to prevent inappropriate IFN activation by the host’s own nucleic acids in autoimmune diseases such as Aicardi-Goutières syndrome or systemic lupus erythematosus. This mechanism of “innate tolerance” might thus provide a new facet to the role of extracellular RNases in the sustained prevention of the body’s own immunostimulatory RNA to act as a danger-associated molecular pattern that is relevant across various species.

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Lentivirus-mediated Bos taurus bta-miR-29b overexpression interferes with bovine viral diarrhoea virus replication and viral infection-related autophagy by directly targeting ATG14 and ATG9A in Madin-Darby bovine kidney cells

MicroRNAs (miRNAs) are a class of short endogenous RNA molecules with the ability to control development, autophagy, apoptosis and the stress response in eukaryotes by pairing with partially complementary sites in the 3' UTRs of targeted genes. Recent studies have demonstrated that miRNAs serve as critical effectors in intricate networks of host-pathogen interactions. Notably, we found that Bos taurus bta-miR-29b (referred to as miR-29b herein) was significantly upregulated >2.3-fold in bovine viral diarrhoea virus (BVDV) strain NADL-infected Madin-Darby bovine kidney (MDBK) cells 6 h post-infection compared with normal MDBK cells. However, the roles of miR-29b in BVDV infection and pathogenesis remain unclear. Here, we report the inhibitory effects of miR-29b on BVDV NADL replication and viral infection-related autophagy. miR-29b overexpression mediated by miRNA precursor-expressing lentivirus resulted in the attenuation of BVDV NADL infection-related autophagy by directly downregulating the intracellular expression levels of two key autophagy-associated proteins, ATG14 and ATG9A. Moreover, ATG14 and ATG9A overexpression rescue not only reversed miR-29b-inhibited autophagy, but also increased BVDV NADL replication. In previous studies, we found that the early stages of autophagy contributed to BVDV NADL replication in MDBK cells and that the inhibition of autophagy repressed BVDV NADL replication, which was also proved in the present study. Collectively, our results establish a novel link between miR-29b and viral replication, and also provide a new pathway for the intimate interaction between host cells and pathogens.

bta-miR-29b attenuates apoptosis by directly targeting caspase-7 and NAIF1 and suppresses bovine viral diarrhea virus replication in MDBK cells

MicroRNAs (miRNAs) are small, endogenous, noncoding RNA molecules that serve as powerful regulators of multiple cellular processes, including apoptosis, differentiation, growth, and proliferation. Bovine viral diarrhea virus (BVDV) contributes significantly to health-related economic losses in the beef and dairy industries. Although BVDV-induced apoptosis correlates with increased intracellular viral RNA accumulation and with bta-miR-29b (miR-29b) expression upregulation in Madin-Darby bovine kidney (MDBK) cells infected with BVDV strain NADL, the role of miR-29b in regulating BVDV-infection-related apoptosis remains unexplored. Here, we report that miR-29b serves as a new miRNA regulating apoptosis. We showed that miR-29b target sequences were present in the 3' untranslated regions of 2 key apoptosis regulators mRNAs, cysteine aspartases-7 (caspase-7) and nuclear apoptosis-inducing factor 1 (NAIF1). Indeed, upon miRNA overexpression, both mRNA and protein levels of caspase-7 and NAIF1 were decreased. We further found that miR-29b attenuated apoptosis by directly regulating intracellular levels of caspase-7 and NAIF1. Moreover, apoptosis blockage by miR-29b was rescued upon co-infection of MDBK cells with lentiviruses expressing caspase-7 and NAIF1. Importantly, miR-29b decreased BVDV NADL envelope glycoprotein E1 mRNA levels and suppressed viral replication. These studies advance our understanding of the mechanisms of miRNAs in mediating the cells combating viral infections.