Autophagy Activated by Bluetongue Virus Infection Plays a Positive Role in Its Replication

Grass Carp Reovirus triggers autophagy enhancing virus replication via the Akt/mTOR pathway

Autophagy impacts the replication cycle of many viruses. Grass Carp Reovirus (GCRV) is an agent that seriously affects the development of the grass carp aquaculture industry. The role of autophagy in GCRV infection is not clearly understood. In this study, we identified that GCRV infection triggered autophagy in CIK cells, which was demonstrated by transmission electron microscopy, the conversion of LC3B I to LC3B II and the level of autophagy substrate p62. Furthermore, we found that GCRV infection activated Akt-mTOR signaling pathway, and the conversion of LC3B I to LC3B II was increased by inhibiting mTOR with rapamycin (Rap) but decreased by activating Akt with insulin. We then assessed the effects of autophagy on GCRV replication. We found that inducing autophagy with Rap promoted GCRV proliferation but inhibiting autophagy with 3 MA or CQ inhibited GCRV replication in CIK cells. Moreover, it was found that enhancing Akt-mTOR activity by insulin, GCRV VP7 protein and viral titers of GCRV were decreased. Collectively, these results indicated that GCRV infection induced autophagy involved in GCRV replication via the Akt-mTOR signal pathway. Thus, new insights into GCRV pathogenesis and antiviral treatment strategies are provided.

The Multi-Faceted Role of Autophagy During Animal Virus Infection

Autophagy is a process of degradation to maintain cellular homeostatic by lysosomes, which ensures cellular survival under various stress conditions, including nutrient deficiency, hypoxia, high temperature, and pathogenic infection. Xenophagy, a form of selective autophagy, serves as a defense mechanism against multiple intracellular pathogen types, such as viruses, bacteria, and parasites. Recent years have seen a growing list of animal viruses with autophagy machinery. Although the relationship between autophagy and human viruses has been widely summarized, little attention has been paid to the role of this cellular function in the veterinary field, especially today, with the growth of serious zoonotic diseases. The mechanisms of the same virus inducing autophagy in different species, or different viruses inducing autophagy in the same species have not been clarified. In this review, we examine the role of autophagy in important animal viral infectious diseases and discuss the regulation mechanisms of different animal viruses to provide a potential theoretical basis for therapeutic strategies, such as targets of new vaccine development or drugs, to improve industrial production in farming.

ATG12 is involved in the antiviral immune response in large yellow croaker (Larimichthys crocea)

ATG12, a core autophagy protein, forms a conjugate with ATG5 to promote the formation of autophagosome membrane, and plays an important role in antiviral immunity. However, little is known about the function of ATG12 in fish. Here, we cloned the open reading frame (ORF) of large yellow croaker (Larimichthys crocea) ATG12 (LcATG12), which is 354 nucleotides long and encodes a protein of 117 amino acids. The deduced LcATG12 possesses a conserved APG12 domain (residues 31 to 117), and shares 91.45% identities with ATG12 in orange-spotted grouper (Epinephelus coioides). LcATG12 was constitutively expressed in all examined tissues, with the highest level in intestine. Its transcript was also detected in primary head kidney granulocytes (PKG), primary head kidney macrophages (PKM), primary head kidney lymphocytes (PKL), and large yellow croaker head kidney (LYCK) cell line, and was significantly up-regulated by poly(I:C). LcATG12 was regularly distributed in both cytoplasm and nucleus of LYCK and epithelioma papulosum cyprinid (EPC) cells. Overexpression of LcATG12 in EPC cells significantly inhibited the replication of spring viremia of carp virus (SVCV). Further studies reveled that LcATG12 could induce the occurrence of autophagy in LYCK cells. Furthermore, overexpression of LcATG12 in LYCK cells increased the expression levels of large yellow croaker type I interferons (IFNs, IFNc, IFNd, and IFNh), IFN regulatory factors (IRF3 and IRF7), and IFN-stimulated genes (PKR, Mx, and Viperin). All these data indicated that LcATG12 plays a role in the antiviral immunity possibly by inducing both autophagy and type I IFN response in large yellow croaker.

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.

Orchestrated efforts on host network hijacking: Processes governing virus replication

With the high pervasiveness of viral diseases, the battle against viruses has never ceased. Here we discuss five cellular processes, namely "autophagy", "programmed cell death", "immune response", "cell cycle alteration", and "lipid metabolic reprogramming", that considerably guide viral replication after host infection in an orchestrated manner. On viral infection, "autophagy" and "programmed cell death" are two dynamically synchronized cell survival programs; "immune response" is a cell defense program typically suppressed by viruses; "cell cycle alteration" and "lipid metabolic reprogramming" are two altered cell housekeeping programs tunable in both directions. We emphasize on their functionalities in modulating viral replication, strategies viruses have evolved to tune these processes for their benefit, and how these processes orchestrate and govern cell fate upon viral infection. Understanding how viruses hijack host networks has both academic and industrial values in providing insights toward therapeutic strategy design for viral disease control, offering useful information in applications that aim to use viral vectors to improve human health such as gene therapy, and providing guidelines to maximize viral particle yield for improved vaccine production at a reduced cost.

A non-enveloped arbovirus released in lysosome-derived extracellular vesicles induces super-infection exclusion

Recent developments on extracellular vesicles (EVs) containing multiple virus particles challenge the rigid definition of non-enveloped viruses. However, how non-enveloped viruses hijack cell machinery to promote non-lytic release in EVs, and their functional roles, remain to be clarified. Here we used Bluetongue virus (BTV) as a model of a non-enveloped arthropod-borne virus and discovered that the majority of viruses are released in EVs. Based on the cellular proteins detected in these EVs, and use of inhibitors targeting the cellular degradation process, we demonstrated that these extracellular vesicles are derived from secretory lysosomes, in which the acidic pH is neutralized upon the infection. Moreover, we report that secreted EVs are more efficient than free-viruses for initiating infections, but that they trigger super-infection exclusion that only free-viruses can overcome.

Multiple Routes of Bluetongue Virus Egress

Bluetongue virus (BTV) is an arthropod-borne virus infecting livestock. Its frequent emergence in Europe and North America had caused significant agricultural and economic loss. BTV is also of scientific interest as a model to understand the mechanisms underlying non-enveloped virus release from mammalian and insect cells. The BTV particle, which is formed of a complex double-layered capsid, was first considered as a lytic virus that needs to lyse the infected cells for cell to cell transmission. In the last decade, however, a more in-depth focus on the role of the non-structural proteins has led to several examples where BTV particles are also released through different budding mechanisms at the plasma membrane. It is now clear that the non-structural protein NS3 is the main driver of BTV release, via different interactions with both viral and cellular proteins of the cell sorting and exocytosis pathway. In this review, we discuss the most recent advances in the molecular biology of BTV egress and compare the mechanisms that lead to lytic or non-lytic BTV release.

Nonstructural protein 6 of porcine epidemic diarrhea virus induces autophagy to promote viral replication via the PI3K/Akt/mTOR axis

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Autophagy is beneficial to PEDV replication.

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PEDV nonstructural protein 6 (nsp6) is a key protein involved in PEDV-induced autophagy.

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Nsp6 of PEDV induced autophagy via the PI3K/Akt/mTOR signaling pathway.

Porcine epidemic diarrhea virus (PEDV) has caused, and continues to cause, severe economic losses to the swine industry worldwide. The pathogenic mechanism and immune regulatory interactions between PEDV and the host remain largely unknown. In this study, the interaction between autophagy and PEDV replication in intestinal porcine epithelial (IPEC-J2) cells was investigated. The effects of the structural and nonstructural proteins of PEDV on the autophagy process and the autophagy-related signaling pathways were also examined. The results shown that PEDV replication increased the autophagy flux in IPEC-J2 cells, and that autophagy was beneficial to PEDV replication, which may be one of the reasons for the rapid damage to intestinal epithelial cells and the enhanced virulence of PEDV in both newborn piglets and finishing pigs. When autophagy was pharmacologically induced by rapamycin, PEDV replication increased from 8.5 × 10⁵ TCID₅₀/mL to 8.8 × 10⁶ TCID₅₀/mL in IPEC-J2 cells. When autophagy was pharmacologically suppressed by hydroxychloroquine, PEDV replication decreased from 8.5 × 10⁵ TCID₅₀/mL to 7.9 × 10⁴ TCID₅₀/mL. To identify which PEDV proteins were the key inducers of autophagy, all 4 structural proteins and 17 nonstructural proteins of PEDV were eukaryotic expressed. It was found that the nonstructural protein 6 (nsp6) and ORF3 of PEDV were able to induce significant autophagy in IPEC-J2 cells, but the other proteins were unable to induce autophagy. It was indicated that nsp6-induced autophagy mainly occurred via the PI3K/Akt/mTOR signaling pathway. The results accelerate the understanding of the biology and pathogenesis of PEDV infection and provide new insights into the development of effective therapeutic strategies.

Grouper Atg12 negatively regulates the antiviral immune response against Singapore grouper iridovirus (SGIV) infection

Autophagy is an evolutionarily conserved, multi-step lysosomal degradation process used to maintain cell survival and homeostasis. A series of autophagy-related genes (Atgs) are involved in the autophagic pathway. In mammals, a growing number of studies have attributed functions to some Atgs that are distinct from their classical role in autophagosome biogenesis, such as resistance to pathogens. However, little is known about the functions of fish Atgs. In this study, we cloned and characterized an atg12 homolog from orange spotted grouper (Epinephelus coioides) (Ecatg12). Ecatg12 encodes a 117 amino acid protein that shares 94.0% and 76.8% identity with gourami (Anabas_testudineus) and humans (Homo sapiens), respectively. The transcription level of Ecatg12 was lower in cells infected with Singapore grouper iridovirus (SGIV) than in non-infected cells. Fluorescence microscopy revealed that EcAtg12 localized in the cytoplasm and nucleus in grouper spleen cells. Overexpression of EcAtg12 significantly increased the replication of SGIV, as evidenced by increased severity of the cytopathic effect, transcription levels of viral genes, levels of viral proteins, and progeny virus yield. Further studies showed that EcAtg12 overexpression decreased the expression levels of interferon (IFN) related molecules and pro-inflammatory factors and inhibited the promoter activity of IFN-3, interferon-stimulated response element, and nuclear factor-κB. Together, these results demonstrate that EcAtg12 plays crucial roles in SGIV replication by downregulating antiviral immune responses.

BmAtg13 promotes the replication and proliferation of Bombyx mori nucleopolyhedrovirus

Autophagy is a cell adaptive response that involves the process of microbial infections. Our previous study has indicated that Bombyx mori nucleopolyhedrovirus (BmNPV) infection triggers the complete autophagic process in BmN-SWU1 cells, which is beneficial to the viral infection. Autophagy-related (ATG) protein ATG13, as part of the ULK complex (a serine-threonine kinase complex composed of ULK1, ULK2, ATG13, ATG101, and FIP200), is the most upstream component of the autophagy pathway, and how it affects virus infections will improve our understanding of the interaction between the virus and the host. This study has determined that the overexpression of the BmAtg13 gene promotes the expression of viral genes and increases viral production in BmN-SWU1 cells, whereas knocking down the BmAtg13 gene suppresses BmNPV replication. Moreover, the BmAtg13 overexpression transgenic line contributed to viral replication and increased mortality rate of BmNPV infection. In contrast, the BmAtg13 knockout transgenic line reduced viral replication 96 h post-infection. Furthermore, BmATG13 directly interacted with viral protein BRO-B, forming a protein complex. Taken together, the findings of this study suggest that BmATG13 may be utilized by the BRO-B protein to promote BmNPV replication and proliferation, which, in turn, provides important insights into the mechanism that autophagy influences viral infection.

Amino acid starvation accelerates replication of Ibaraki virus

Ibaraki virus (IBAV) is a strain of epizootic hemorrhagic disease virus 2 that belongs to the genus Orbivirus of the family Reoviridae. IBAV replication is suppressed by the inhibition of autophagy, and since mechanistic target of rapamycin complex 1 (mTORC1) is a key regulator of autophagy, we examined if mTORC1 inhibition by amino acid starvation or mTOR inhibitors (Torin 1 and rapamycin) affects IBAV replication. We found that IBAV replication is significantly enhanced after amino acid starvation of host cells, but not after treatment with mTOR inhibitors, during early stages of viral infection (0-1 hpi). Notably, inhibition of mTORC1 by amino acid starvation was reversible and thus restricted to 0-1 hpi, whereas mTOR inhibitors sustainably suppressed mTORC1 even after the 1-h treatment, suggesting that mTORC1 suppression itself does not affect IBAV replication. To investigate the mechanism of enhanced IBAV replication by amino acid starvation, we examined the endocytic pathway, since IBAV utilizes acidification of endosomes as a trigger for viral replication. Accordingly, we found that amino acid starvation, but not mTOR inhibitors, strongly induced acidification of endosomes/lysosomes and that inhibition of endosomal acidification by bafilomycin A1 effectively blocked enhancement of IBAV replication. Altogether, the inactivation of mTORC1 by amino acid starvation during early stages of infection enhances acidification of endosomes, which in turn enhances IBAV replication.

Respiratory Syncytial Virus Replication Is Promoted by Autophagy-Mediated Inhibition of Apoptosis

Respiratory syncytial virus (RSV) is the main cause of acute lower respiratory tract infection (ALRI) in children worldwide. Virus-host interactions affect the progression and prognosis of the infection. Autophagy plays important roles in virus-host interactions. Respiratory epithelial cells serve as the front line of host defense during RSV infection, However, it is still unclear how they interact with RSV. In this study, we found that RSV induced autophagy that favored RSV replication and exacerbated lung pathology in vivo Mechanistically, RSV induced complete autophagy flux through reactive oxygen species (ROS) generation and activation of the AMP-activated protein kinase/mammalian target of rapamycin (AMPK-MTOR) signaling pathway in HEp-2 cells. Furthermore, we evaluated the functions of autophagy in RSV replication and found that RSV replication was increased in HEp-2 cells treated with rapamycin but decreased remarkably in cells treated with 3-methylademine (3-MA) or wortmannin. Knockdown key molecules in the autophagy pathway with short hairpinp RNA (shRNA) against autophagy-related gene 5 (ATG5), autophagy-related gene 7 (ATG7), or BECN1/Beclin 1 or treatment with ROS scavenger N-acetyl-l-cysteine (NAC) and AMPK inhibitor (compound C) suppressed RSV replication. 3-MA or shATG5/BECN1 significantly decreased cell viability and increased cell apoptosis at 48 hours postinfection (hpi). Blocking apoptosis with Z-VAD-FMK partially restored virus replication at 48 hpi. Those results provide strong evidence that autophagy may function as a proviral mechanism in a cell-intrinsic manner during RSV infection.IMPORTANCE An understanding of the mechanisms that respiratory syncytial virus utilizes to interact with respiratory epithelial cells is critical to the development of novel antiviral strategies. In this study, we found that RSV induces autophagy through a ROS-AMPK signaling axis, which in turn promotes viral infection. Autophagy favors RSV replication through blocking cell apoptosis at 48 hpi. Mechanistically, RSV induces mitophagy, which maintains mitochondrial homeostasis and therefore decreases cytochrome c release and apoptosis induction. This study provides a novel insight into this virus-host interaction, which may help to exploit new antiviral treatments targeting autophagy processes.

Bombyx mori Nuclear Polyhedrosis Virus (BmNPV) Induces Host Cell Autophagy to Benefit Infection

Bombyx mori nuclear polyhedrosis virus (BmNPV) is an important pathogen of silkworms. Despite extensive studies in recent decades, the interaction between BmNPV and host cells is still not clearly understood. Autophagy is an intrinsic innate immune mechanism and it controls infection autonomously in virus-infected cells. In this study, we found that BmNPV infection could trigger autophagy, as demonstrated by the formation of autophagosomes, fluorescent Autophagy-related gene 8-Green Fluorescent Protein (ATG8-GFP) punctate, and lipidated ATG8. Meanwhile, autophagic flux increased significantly when monitored by the ATG8-GFP-Red Fluorescent Protein (RFP) autophagy tandem sensor and protein degradation of p62. In addition, almost all of the identified autophagy-related genes (Atgs) had been up-regulated post infection in mRNA levels. Then, we screened Atgs with the greatest fold-change during virus infection. Interestingly, all of the screened Atgs positively regulated the expression of virus genes. Further studies showed that Atg7 and Atg9 could contribute to the level of autophagy caused by viral infection. Our results demonstrated that BmNPV induced host cell autophagy to benefit its infection. These results offer insight into the complex interactions between virus and host cell, and viral pathogenesis.

Autophagy pathway induced by a plant virus facilitates viral spread and transmission by its insect vector

Many viral pathogens are persistently transmitted by insect vectors and cause agricultural or health problems. Generally, an insect vector can use autophagy as an intrinsic antiviral defense mechanism against viral infection. Whether viruses can evolve to exploit autophagy to promote their transmission by insect vectors is still unknown. Here, we show that the autophagic process is triggered by the persistent replication of a plant reovirus, rice gall dwarf virus (RGDV) in cultured leafhopper vector cells and in intact insects, as demonstrated by the appearance of obvious virus-containing double-membrane autophagosomes, conversion of ATG8-I to ATG8-II and increased level of autophagic flux. Such virus-containing autophagosomes seem able to mediate nonlytic viral release from cultured cells or facilitate viral spread in the leafhopper intestine. Applying the autophagy inhibitor 3-methyladenine or silencing the expression of Atg5 significantly decrease viral spread in vitro and in vivo, whereas applying the autophagy inducer rapamycin or silencing the expression of Torc1 facilitate such viral spread. Furthermore, we find that activation of autophagy facilitates efficient viral transmission, whereas inhibiting autophagy blocks viral transmission by its insect vector. Together, these results indicate a plant virus can induce the formation of autophagosomes for carrying virions, thus facilitating viral spread and transmission by its insect vector. We believe that such a role for virus-induced autophagy is common for vector-borne persistent viruses during their transmission by insect vectors.

Oncolytic Reovirus Infection Is Facilitated by the Autophagic Machinery

Mammalian reovirus is a double-stranded RNA virus that selectively infects and lyses transformed cells, making it an attractive oncolytic agent. Despite clinical evidence for anti-tumor activity, its efficacy as a stand-alone therapy remains to be improved. The success of future trials can be greatly influenced by the identification and the regulation of the cellular pathways that are important for reovirus replication and oncolysis. Here, we demonstrate that reovirus induces autophagy in several cell lines, evident from the formation of Atg5-Atg12 complexes, microtubule-associated protein 1 light chain 3 (LC3) lipidation, p62 degradation, the appearance of acidic vesicular organelles, and LC3 puncta. Furthermore, in electron microscopic images of reovirus-infected cells, autophagosomes were observed without evident association with viral factories. Using UV-inactivated reovirus, we demonstrate that a productive reovirus infection facilitates the induction of autophagy. Importantly, knock-out cell lines for specific autophagy-related genes revealed that the expression of Atg3 and Atg5 but not Atg13 facilitates reovirus replication. These findings highlight a central and Atg13-independent role for the autophagy machinery in facilitating reovirus infection and contribute to a better understanding of reovirus-host interactions.

Phosphoproteomic Analysis Reveals the Importance of Kinase Regulation During Orbivirus Infection

Bluetongue virus (BTV) causes infections in wild and domesticated ruminants with high morbidity and mortality and is responsible for significant economic losses in both developing and developed countries. BTV serves as a model for the study of other members of the Orbivirus genus. Previously, the importance of casein kinase 2 for BTV replication was demonstrated. To identify intracellular signaling pathways and novel host-cell kinases involved during BTV infection, the phosphoproteome of BTV infected cells was analyzed. Over 1000 phosphosites were identified using mass spectrometry, which were then used to determine the corresponding kinases involved during BTV infection. This analysis yielded protein kinase A (PKA) as a novel kinase activated during BTV infection. Subsequently, the importance of PKA for BTV infection was validated using a PKA inhibitor and activator. Our data confirmed that PKA was essential for efficient viral growth. Further, we showed that PKA is also required for infection of equid cells by African horse sickness virus, another member of the Orbivirus genus. Thus, despite their preference in specific host species, orbiviruses may utilize the same host signaling pathways during their replication.

Autophagy promoted infectious kidney and spleen necrosis virus replication and decreased infectious virus yields in CPB cell line

Autophagy plays important functions in viral replication and pathogenesis. In this study, we investigated the role of autophagy in the replication of infectious kidney and spleen necrosis virus (ISKNV), an agent that has caused devastating losses in Chinese perch (Siniperca chuatsi) industry. We found that ISKNV infection triggered the complete autophagic process, as demonstrated by microtubule-associated protein 1 light chain 3B II (LC3B-II) conversion, an increased accumulation of punctate GFP-LC3-expressing cells, a higher number of autophagosome-double-membrane vesicles in the cytoplasm, and increased levels of autophagic flux in CPB cells. Then, we investigated the role of autophagy in the process of ISKNV replication. Results showed that inducing autophagy by rapamycin promoted ISKNV replication and proteins synthesis but decreased extracellular virus yields. While, blocking autophagosome-lysosome fusion by chloroquine (CQ) promoted infectious virus yields in culture supernatant. These results offer insight into the complex interactions between ISKNV and host cell, providing new insights into viral pathogenesis and antiviral treatment strategies.

Role of autophagy in cellular response to infection with Orf virus Jilin isolate

Autophagy is a conserved catabolic process of the cell, which has been described to be involved in the development of various viral diseases. However, the role of autophagy in Orf virus (ORFV) replication remains unknown. In this study, we provide the first evidence that ORFV infection triggered autophagy in primary ovine fetal turbinate cells (OFTu) based on the appearance of abundant double- and single-membrane vesicles, the accumulation of LC3 fluorescent puncta, the enhancement of LC3-I/-II conversion, and autophagic flux. Moreover, modulation of ORFV-induced autophagy by rapamycin (RAPA), Earle's balanced salts solution (EBSS), chloroquine (CQ) or 3-methyladenime (3-MA) does not affect virus production. In conclusion, these results suggest that autophagy can be induced in host cells by ORFV infection, but which maybe not essential for ORFV replication.

Rapamycin increases RSV RNA levels and survival of RSV-infected dendritic cell depending on T cell contact

The macrolide rapamycin inhibits mTOR (mechanist target of rapamycin) function and has been broadly used to unveil the role of mTOR in immune responses. Inhibition of mTOR on dendritic cells (DC) can influence cellular immune response and the survival of DC. RSV is the most common cause of hospitalization in infants and is a high priority candidate to vaccine development. In this study we showed that rapamycin treatment on RSV-infected murine bone marrow-derived DC (BMDC) decreases the frequency of CD8(+)CD44(high) T cells. However, inhibition of mTOR on RSV-infected BMDC did not modify the activation phenotype of these cells. RSV-RNA levels increase when infected BMDC were treated with rapamycin. Moreover, we observed that rapamycin diminishes apoptosis cell death of RSV-infected BMDC co-culture with T cells and this effect was abolished when the cells were co-cultured in a transwell system that prevents cell-to-cell contact or migration. Taken together, these data indicate that rapamycin treatment present a toxic effect on RSV-infected BMDC increasing RSV-RNA levels, affecting partially CD8 T cell differentiation and also increasing BMDC survival in a mechanism dependent on T cell contact.

Impaired cellular energy metabolism contributes to bluetongue-virus-induced autophagy

Bluetongue virus (BTV) has been found to trigger autophagy to favor its replication, but the underlying mechanisms have not been clarified. Here, we show that cellular energy metabolism is involved in BTV-induced autophagy. Cellular ATP synthesis was impaired by BTV1 infection, causing metabolic stress, which was responsible for activation of autophagy, since the conversion of LC3 and aggregation of GFP-LC3 (autophagy markers) were suppressed when infection-caused energy depletion was reversed via MP (metabolic substrate) treatment. The reduced virus yields with MP further supported this view. Overall, our findings suggest that BTV1-induced disruption of cellular energy metabolism contributes to autophagy, and this provides new insights into BTV-host interactions.

Dissection and integration of the autophagy signaling network initiated by bluetongue virus infection: crucial candidates ERK1/2, Akt and AMPK

Bluetongue virus (BTV), a complex double-stranded segmented RNA virus, has been found to initiate cellular autophagy for its own benefit. Here, with a view to understanding the underlying mechanisms, we first systematically dissected the exact signaling network in BTV-induced autophagy. We found that the activity of mTOR, a crucial pivot, was inhibited by BTV1 infection, subsequently leading to downstream p70S6K suppression and autophagy initiation. We then explored the upstream regulators of mTOR and analyzed their activities via a series of assays. We found BTV1-induced autophagy to be independent of the ERK1/2 signaling pathway. However, the BTV1-induced inhibition of PI3K/Akt was found to be partially responsible for mTOR inactivation and subsequent autophagy initiation. Furthermore, we found unexpectedly that AMPK seemed to play a more important role in BTV1-induced autophagy. Elevated [Ca²⁺]_cyto-mediated activation of CaMKKβ exactly managed the activation of AMPK, which then positively regulated autophagy through suppressing mTOR. We must emphasize that TSC2 is a fatal mediator between upstream Akt or AMPK and downstream mTOR through its phosphorylation. Taken together, our data suggested that the BTV1-induced inhibition of the Akt-TSC2-mTOR pathway and the upregulation of the AMPK-TSC2-mTOR pathway both contributed to autophagy initiation and further favored virus replication.

Endoplasmic reticulum stress-mediated autophagy contributes to bluetongue virus infection via the PERK-eIF2α pathway

Bluetongue virus (BTV) is an important pathogen of wild and domestic ruminants. We have previously reported that BTV1 infection induced autophagy for its own benefit, but how this occurs remains unclear. Here, the classical autophagy features including autophagsomes formation, GFP-LC3 dots and LC3-II conversation were shown in BTV1-infected cells, we also found the endoplasmic reticulum (ER) stress was triggered by BTV1 infection, which was demonstrated by the increased transcription level of the ER stress marker GRP78 and the expanded morphology of ER. During ER stress, PERK and eIF2α phosphorylation increased along with BTV1 infection, consistent with the elevated LC3 level, indicating that the PERK pathway of the unfolded protein response (UPR) was activated. In addition, both the blockage of PERK by GSK2656157 or knockdown of eIF2α by siRNA reduced the level of LC3, which suggested that the PERK-eIF2α pathway contributed to autophagy induced by BTV1. Furthermore, inactivation of PERK or silencing of eIF2α both significantly reduced the expression of VP2 protein and the viral yields in the supernatants. In sum, these data suggest that ER stress mediates autophagy via the PERK-eIF2α pathway and contributes to BTV1 replication, thus offering new insight into the molecular mechanisms of the BTV-host interaction.