Porcine circovirus type 2 induces autophagy via the AMPK/ERK/TSC2/mTOR signaling pathway in PK-15 cells

Matrine regulates autophagy in ileal epithelial cells in a porcine circovirus type 2-infected murine model

Introduction

Porcine circovirus type 2 (PCV2) is an important pathogen that causes diarrhea in nursery and fattening pigs, resulting in huge economic losses for commercial pig farms. Protective efficacy of vaccines is compromised by mutations in pathogens. There is an urgent need to articulate the mechanism by which PCV2 destroys the host's intestinal mucosal barrier and to find effective therapeutic drugs. Increasing attention has been paid to the natural antiviral compounds extracted from traditional Chinese medicines. In the present study, we investigated the role of Matrine in mitigating PCV2-induced intestinal damage and enhancing autophagy as a potential therapeutic strategy in mice.

Methods

A total of 40 female, specific-pathogen-free-grade Kunming mice were randomly divided into four groups with 10 mice in each group: control, PCV2 infection, Matrine treatment (40 mg/kg Matrine), and Ribavirin treatment (40 mg/kg Ribavirin). Except for the control group, all mice were injected intraperitoneally with 0.5 mL 105.4 50% tissue culture infectious dose (TCID50)/mL PCV2.

Results

While attenuating PCV2-induced downregulation of ZO-1 and occludin and restoring intestinal barrier function in a PCV2 Kunming mouse model, treatment with Matrine (40 mg/kg) attenuated ultrastructural damage and improved intestinal morphology. Mechanistically, Matrine reversed PCV2-induced autophagosome accumulation by inhibiting signal transducer and activator of transcription 3 (STAT3) phosphorylation and upregulating Beclin1 protein expression, thus resisting viral hijacking of enterocyte autophagy.

Discussion

Our findings demonstrate that Matrine may be a novel, potential antiviral agent against PCV2 by activating intestine cellular autophagy, which provides a new strategy for host-directed drug discovery.

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

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

The surface morphology of Atractylodes macrocephala polysaccharide and its inhibitory effect on PCV2 replication

BACKGROUND

Porcine infection with Porcine circovirus type 2 (PCV2) causes immunosuppression, which is easy to cause concurrent or secondary infection, making the disease complicated and difficult to treat, and causing huge economic losses to the pig industry. Total polysaccharide from the rhizoma of Atractylodes macrocephala Koidz. (PAMK) is outstanding in enhancing non-specific immunity and cellular immunity, and effectively improving the body's disease resistance, indicating its potential role in antiviral immunotherapy.

RESULTS

PAMK had the characteristics of compact, polyporous and agglomerated morphology, but does not have triple helix conformation. PCV2 infection led to the increase in LC3-II, degradation of p62 and the increase of viral Cap protein expression and viral copy number. PAMK treatment significantly alleviated PCV2-induced autophagy and inhibited PCV2 replication. Moreover, PAMK treatment significantly attenuated the increase of PINK1 protein expression and the decrease of TOMM20 protein expression caused by PCV2 infection, alleviated Parkin recruitment from cytoplasm to mitochondria and intracellular reactive oxygen species accumulation, restored mitochondrial membrane charge, alleviated viral Cap protein expression.

CONCLUSION

PAMK alleviates PCV2-induced mitophagy to suppress PCV2 replication by inhibiting the Pink 1/Parkin pathway. These findings may provide new insights into the prevention and treatment of PCV2. © 2023 Society of Chemical Industry.

Autophagy as a dual-faced host response to viral infections

Autophagy selectively degrades viral particles or cellular components, either facilitating or inhibiting viral replication. Conversely, most viruses have evolved strategies to escape or exploit autophagy. Moreover, autophagy collaborates with the pattern recognition receptor signaling, influencing the expression of adaptor molecules involved in the innate immune response and regulating the expression of interferons (IFNs). The intricate relationship between autophagy and IFNs plays a critical role in the host cell defense against microbial invasion. Therefore, it is important to summarize the interactions between viral infections, autophagy, and the host defense mechanisms against viruses. This review specifically focuses on the interactions between autophagy and IFN pathways during viral infections, providing a comprehensive summary of the molecular mechanisms utilized or evaded by different viruses.

Seneca Valley virus 3Cpro antagonizes host innate immune responses and programmed cell death

Seneca Valley virus (SVV), a member of the Picornaviridae family, may cause serious water blister diseases in pregnant sows and acute death in newborn piglets, which have resulted in economic losses in pig production. The 3C protease is a vital enzyme for SVV maturation and is capable of regulating protein cleavage and RNA replication of the virus. Additionally, this protease can impede the host's innate immune response by targeting the interferon pathway's principal factor and enhance virus replication by modulating the host's RNA metabolism while simultaneously triggering programmed cell death. This article reviews recent studies on SVV 3C functions, which include viral replication promotion, cell apoptosis modulation and host immune response evasion, and provides a theoretical basis for research on preventing and controlling SVV infection.

Revisiting Porcine Circovirus Infection: Recent Insights and Its Significance in the Piggery Sector

Porcine circovirus (PCV), a member of the Circoviridae family within the genus Circovirus, poses a significant economic risk to the global swine industry. PCV2, which has nine identified genotypes (a-i), has emerged as the predominant genotype worldwide, particularly PCV2d. PCV2 has been commonly found in both domestic pigs and wild boars, and sporadically in non-porcine animals. The virus spreads among swine populations through horizontal and vertical transmission routes. Despite the availability of commercial vaccines for controlling porcine circovirus infections and associated diseases, the continuous genotypic shifts from a to b, and subsequently from b to d, have maintained PCV2 as a significant pathogen with substantial economic implications. This review aims to provide an updated understanding of the biology, genetic variation, distribution, and preventive strategies concerning porcine circoviruses and their associated diseases in swine.

Porcine epidemic diarrhoea virus (PEDV) infection activates AMPK and JNK through TAK1 to induce autophagy and enhance virus replication

Autophagy plays an important role in defending against invading microbes. However, numerous viruses can subvert autophagy to benefit their replication. Porcine epidemic diarrhoea virus (PEDV) is an aetiological agent that causes severe porcine epidemic diarrhoea. How PEDV infection regulates autophagy and its role in PEDV replication are inadequately understood. Herein, we report that PEDV induced complete autophagy in Vero and IPEC-DQ cells, as evidenced by increased LC3 lipidation, p62 degradation, and the formation of autolysosomes. The lysosomal protease inhibitors chloroquine (CQ) or bafilomycin A and Beclin-1 or ATG5 knockdown blocked autophagic flux and inhibited PEDV replication. PEDV infection activated AMP-activated protein kinase (AMPK) and c-Jun terminal kinase (JNK) by activating TGF-beta-activated kinase 1 (TAK1). Compound C (CC), an AMPK inhibitor, and SP600125, a JNK inhibitor, inhibited PEDV-induced autophagy and virus replication. AMPK activation led to increased ULK1^S777 phosphorylation and activation. Inhibition of ULK1 activity by SBI-0206965 (SBI) and TAK1 activity by 5Z-7-Oxozeaenol (5Z) or by TAK1 siRNA led to the suppression of autophagy and virus replication. Our study provides mechanistic insights into PEDV-induced autophagy and how PEDV infection leads to JNK and AMPK activation.

Cellular communication network factor 1 interlinks autophagy and ERK signaling to promote osteogenesis of periodontal ligament stem cells

OBJECTIVES

To investigate the effects of cellular communication network factor 1 (CCN1), a critical matricellular protein, on alveolar bone regeneration, and to elucidate the underlying molecular mechanism.

BACKGROUND

In the process of orthodontic tooth movement, bone deposition on the tension side of human periodontal ligament stem cells (hPDLSCs) ensured high efficiency and long-term stability of the treatment. The matricellular protein CCN1 is responsive to mechanical stimulation, exhibiting important tasks in bone homoeostasis. However, the role and mechanism of CCN1 on alveolar bone remodeling of hPDLSCs remains unclear.

METHODS

The expression and distribution of CCN1 in rat periodontal ligament were detected by immunofluorescence staining and immunohistochemical staining. ELISA verified the secretion of CCN1 triggered by stretch loading. To examine the mineralization ability of hPDLSCs induced by CCN1, Western blotting, qRT-PCR, ARS, and ALP staining were performed. CCK-8 and cell migration assay were performed to detect the cell proliferation rate and the wound healing. PI3K/Akt, MAPK, and autophagy activation were examined via Western blotting and immunofluorescence.

RESULTS

Mechanical stimuli induced the release of CCN1 into extracellular environment by hPDLSCs. Knockdown of CCN1 attenuated the osteogenesis of hPDLSCs while rhCCN1 enhanced the expression of Runx2, Col 1, ALPL, and promoted the mineralization nodule formation. CCN1 activated PI3K/Akt and ERK signaling, and blockage of PI3K/Akt signaling reversed the accelerated cell migration triggered by CCN1. The enhanced osteogenesis induced by CCN1 was abolished by ERK signaling inhibitor PD98059 or autophagy inhibitor 3-MA. Further investigation demonstrated PD98059 abrogated the activation of autophagy.

CONCLUSION

This study demonstrated that CCN1 promotes osteogenesis in hPDLSCs via autophagy and MAPK/ERK pathway.

Porcine circovirus type 2 induces CHOP-ERO1α-ROS-mediated apoptosis in PK-15 cells

Porcine circovirus type 2 (PCV2) infection induces endoplasmic reticulum (ER) stress and oxidative stress. These cellular responses could be connected with apoptosis. However, the mechanisms that link ER stress and oxidative stress in PCV2-induced apoptosis are poorly characterized. Here, we demonstrate that PCV2 infection increased expression of proapoptotic protein C/EBP homologous protein (CHOP) and ER oxidoreductase 1 alpha (ERO1α). Inhibition of CHOP by RNA silencing or inhibition of ERO1α by short hairpin RNA or EN460 repressed PCV2-induced reactive oxygen species (ROS) generation, cytosolic calcium level, and apoptotic rate in PK-15 cells. Overexpression of ERO1α enhanced PCV2-induced oxidative stress, caspase-3 cleavage, and apoptosis rate. Treatment of PCV2-infected cells with ROS scavenger N-acetyl-L-cysteine downregulated PCV2-induced ROS production, cytosolic calcium level, and apoptosis rate, but intriguingly decreased expression of CHOP and ERO1α. Thus, we propose that PCV2 induces apoptosis through ER Stress via CHOP-ERO1α-ROS signaling in host cells.

Zinc in Cognitive Impairment and Aging

Zinc, an essential micronutrient for life, was first discovered in 1869 and later found to be indispensable for the normal development of plants and for the normal growth of rats and birds. Zinc plays an important role in many physiological and pathological processes in normal mammalian brain development, especially in the development of the central nervous system. Zinc deficiency can lead to neurodegenerative diseases, mental abnormalities, sleep disorders, tumors, vascular diseases, and other pathological conditions, which can cause cognitive impairment and premature aging. This study aimed to review the important effects of zinc and zinc-associated proteins in cognitive impairment and aging, to reveal its molecular mechanism, and to highlight potential interventions for zinc-associated aging and cognitive impairments.

Mesenchymal Stem Cells and their Derived Exosomes Promote Malignant Phenotype of Polyploid Non-Small-Cell Lung Cancer Cells through AMPK Signaling Pathway

Chemotherapy is an important method for the treatment of non-small-cell lung cancer (NSCLC), but it can lead to side effects and polyploid cancer cells. The polyploid cancer cells can live and generate daughter cancer cells via budding. Mesenchymal stem cells (MSCs) are pluripotent stem cells with repair and regeneration functions and can resist tissue damage caused by tumor therapy. This study is aimed at investigating the effects of MSCs and their derived exosomes on the biological characteristics of polyploid NSCLC cells and the potential mechanisms. We found that MSC conditioned medium (CM), MSCs, and MSC-exosomes had no effect on cell proliferation of the polyploid A549 and H1299 cells. Compared with the control group, MSCs and MSC-exosomes significantly promoted epithelial mesenchymal transformation, cell migration, antiapoptosis, and autophagy in the polyploid A549 and H1299 by activating AMPK signaling pathway, but no significant changes were observed in MSC-CM treatment. These results revealed that MSCs and MSC-exosomes promoted malignant phenotype of polyploid NSCLC cells through the AMPK signaling pathway.

Non-Structural Protein 3 of Duck Tembusu Virus Induces Autophagy via the ERK and PI3K-AKT-mTOR Signaling Pathways

Despite autophagy’s pivotal role in the replication of viruses such as duck Tembusu virus (DTMUV), which has caused massive economic losses to the poultry industry in the world, the specific relationships between DTMUV and cellular autophagy remain largely unknown. In response, we investigated the interactions between autophagy and DTMUV, the effects of the structural and non-structural proteins of DTMUV on autophagy, and the autophagy-related signaling pathways induced by DTMUV. Among the results, DTMUV increased the autophagy flux in duck embryo fibroblasts (DEF) and BHK-21 cells, while autophagy facilitated viral replication. After we pharmacologically induced autophagy with rapamycin (RAPA), the replication of DTMUV increased by 15.23-fold compared with the control group of DEF cells. To identify which DTMUV protein primarily induced autophagy, all three structural proteins and seven non-structural proteins of DTMUV were transfected into cells, and the results showed that non-structural protein 3 (NS3) induced significant autophagy in DEF cells. By means of Western blot, immunofluorescence, and transmission electron microscopy, we confirmed that NS3 protein could significantly induce autophagy and autophagy flux. Furthermore, we showed that NS3 induced autophagy in DEF cells through extracellular signal-regulated kinase 2 (ERK2) and phosphatidylinositol-3-kinase (PI3K)/AKT and the mammalian target of rapamycin (mTOR) signaling pathways using specific inhibitors and RNA interference assays. Finally, autophagy induced by NS3 promoted DTMUV replication. These results provide novel insight into the relationship between DTMUV and autophagy, broadening the current understanding of the molecular pathogenesis of DTMUV.

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.

Synergetic contributions of viral VP1, VP3, and 3C to activation of the AKT-AMPK-MAPK-MTOR signaling pathway for Seneca Valley virus-induced autophagy

Seneca Valley virus (SVV), a member of the Picornaviridae family, can activate autophagy via the PERK and ATF6 unfolded protein response pathways and facilitate viral replication; however, the precise molecular mechanism that regulates SVV-induced autophagy remains unclear. Here, we revealed that SVV infection inhibited the phosphorylation of mechanistic target of rapamycin kinase (MTOR) and activated phosphorylation of the serine/threonine kinase AKT. We observed that activating adenosine monophosphate-activated protein kinase (AMPK), extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK), and p38 MAPK signaling by SVV infection promoted autophagy induction and viral replication; additionally, the SVV-induced autophagy was independent of the ULK1 complex. We further evaluated the role of viral protein(s) in the AKT-AMPK-MAPK-MTOR pathway during SVV-induced autophagy and found that VP1 induced autophagy, as evidenced by puncta colocalization with microtubule-associated protein 1 light chain 3 (LC3) in the cytoplasm and enhanced LC3-II levels. This might be associated with the interaction of VP1 with sequestosome 1 and promoting its degradation. In addition, the expression of VP1 enhanced AKT phosphorylation and AMPK phosphorylation, while MTOR phosphorylation was inhibited. These results indicate that VP1 induces autophagy by the AKT-AMPK-MTOR pathway. Additionally, expression of VP3 and 3C was found to activate autophagy induction via the ERK1/2 MAPK-MTOR and p38 MAPK-MTOR pathway. Taken together, our data suggest that SVV-induced autophagy has finely-tuned molecular mechanisms in which VP1, VP3, and 3C contribute synergistically to the AKT-AMPK-MAPK-MTOR pathway. IMPORTANCE Autophagy is an essential cellular catabolic process to sustain normal physiological processes that modulated by a variety of signaling pathways. Invading virus is a stimulus to induce autophagy that regulates viral replication. It has been demonstrated that Seneca Valley virus (SVV) induced autophagy via the PERK and ATF6 unfolded protein response pathways. However, the precise signaling pathway involved in autophagy is still poorly understood. In this study, our results demonstrated that viral proteins VP1, VP3, and 3C contribute synergistically to activation of the AKT-AMPK-MAPK-MTOR signaling pathway for SVV-induced autophagy. These findings reveal systemically the finely-tuned mocleular mechanism of SVV-induced autophagy, thereby facilitating to deeper insight into the development of potential control strategies against SVV infection.

Host immune response to infection with porcine circoviruses

Porcine circovirus type 2 (PCV2), which serves as a major causative agent of PCV2-associated diseases and causes severe loss to the pig industry worldwide, can dysregulate the immune response and induce immunosuppression in PCV2-infected pigs. Similar to PCV2, porcine circovirus type 3 (PCV3), a newly identified swine circovirus which might be closely associated with porcine dermatitis and nephropathy syndrome, reproductive disorder, and multisystemic inflammatory responses, also interferes with host immune defense. Interaction between host immune system and PCVs is considered to be a crucial determinant of pathogenicity in pigs. Here, we sought to briefly discuss the current knowledge regarding the interaction of porcine circovirus type 2 and/or 3 with host immune cells and immune responses to better depict the viral immunomodulatory capacity, pathogenic mechanisms, and the future research direction in host immune responses to infection with PCV2 and PCV3.

PCV2 targets cGAS to inhibit type I interferon induction to promote other DNA virus infection

Viruses use diverse strategies to impair the antiviral immunity of host in order to promote infection and pathogenesis. Herein, we found that PCV2 infection promotes the infection of DNA viruses through inhibiting IFN-β induction in vivo and in vitro. In the early phase of infection, PCV2 promotes the phosphorylation of cGAS at S278 via activation of PI3K/Akt signaling, which directly silences the catalytic activity of cGAS. Subsequently, phosphorylation of cGAS at S278 can facilitate the K48-linked poly-ubiquitination of cGAS at K389, which can been served as a signal for recognizing by the ubiquitin-binding domain of histone deacetylase 6 (HDAC6), to promote the translocation of K48-ubiquitinated-cGAS from cytosol to autolysosome depending on the deacetylase activity of HDAC6, thereby eventually resulting in a markedly increased cGAS degradation in PCV2 infection-induced autophagic cells relative to Earle’s Balanced Salt Solution (EBSS)-induced autophagic cells (a typical starving autophagy). Importantly, we found that PCV2 Cap and its binding protein gC1qR act as predominant regulators to promote porcine cGAS phosphorylation and HDAC6 activation through mediating PI3K/AKT signaling and PKCδ signaling activation. Based on this finding, gC1qR-binding activity deficient PCV2 mutant (PCV2RmA) indeed shows a weakened inhibitory effect on IFN-β induction and a weaker boost effect for other DNA viruses infection compared to wild-type PCV2. Collectively, our findings illuminate a systematic regulation mechanism by which porcine circovirus counteracts the cGAS-STING signaling pathway to inhibit the type I interferon induction and promote DNA virus infection, and identify gC1qR as an important regulator for the immunosuppression induced by PCV2.

PCV2 is well known for its ability to induce immunosuppression in pigs. However, how PCV2 infection interferes cGAS-STING signaling is still poorly understood. Herein, we demonstrate that PCV2 infection can phosphorylate porcine cGAS via gC1qR-mediated PI3K/AKT signaling to silence the catalytic activity of cGAS, while activates PKCδ signaling to promote histone deacetylase 6 (HDAC6) activation depending on the assistance of gC1qR. Subsequently, phosphorylation of cGAS facilitates the poly-ubiquitination of cGAS, then ubiquitinated-cGAS proteins are recruited and transported to autolysosome by activated HDAC6 depending on its deacetylase activity and ubiquitin-binding function, thereby eventually resulting in the autophagic degradation of cGAS in PCV2-infected cells. This study reveals that PCV2 can inhibit the activation of cGAS signaling pathway through two different mechanisms at different stages of infection and clarifies the internal relationship and cooperation model between these two mechanisms.

Platycodon grandiflorus polysaccharides inhibit Pseudorabies virus replication via downregulating virus-induced autophagy

Pseudorabies virus (PRV) is one of the common pathogens in farms. Platycodon grandiflorus polysaccharide (PGPS) has been reported with a variety of biological activities. Autophagy is one of the vital mechanisms for cells to cope with virus infection, and it may also inhibit or promote virus replication. This study was conducted to investigate the antiviral activity of total PGPS(PGPS_t) against PRV and the role of virus-induced autophagy in the anti-PRV effect of PGPS_t in PK-15 cells. First, we established an infection model and detected the autophagy induced by PRV in PK-15 cells. Then, the protective effect of PGPS_t against PRV was evaluated, and the effect of PGPS_t on PRV replication and virus-induced autophagy were analysed by quantitative polymerase chain reaction, enzyme-linked immunosorbent assay, Western blot and confocal immunofluorescence. Results showed that PGPS_t can reduce the PRV replication. PRV infection resulted in the accumulation of autophagosomes, which were inhibited by PGPS_t. Moreover, PGPS_t upregulated the Akt/mammalian target of rapamycin (mTOR) signalling pathway repressed by PRV infection, whereas rapamycin attenuated the anti-PRV effect of PGPS_t. These findings suggest that PGPS_t possess a protective effect against PRV infection and can inhibit PRV replication through relieving PRV-induced autophagy. This article can provide ideas for the development of antiviral drugs.

Macrophage Polarization Modulated by Porcine Circovirus Type 2 Facilitates Bacterial Coinfection

Polarization of macrophages to different functional states is important for mounting responses against pathogen infections. Macrophages are the major target cells of porcine circovirus type 2 (PCV2), which is the primary causative agent of porcine circovirus-associated disease (PCVAD) leading to immense economic losses in the global swine industry. Clinically, PCV2 is often found to increase risk of other pathogenic infections yet the underlying mechanisms remain to be elusive. Here we found that PCV2 infection skewed macrophages toward a M1 status through reprogramming expression of a subset of M1-associated genes and M2-associated genes. Mechanistically, induction of M1-associated genes by PCV2 infection is dependent on activation of nuclear factor kappa B (NF-κB) and c-jun N-terminal kinase (JNK) signaling pathways whereas suppression of M2-associated genes by PCV2 is via inhibiting expression of jumonji domain containing-3 (JMJD3), a histone 3 Lys27 (H3K27) demethylase that regulates M2 activation of macrophages. Finally, we identified that PCV2 capsid protein (Cap) directly inhibits JMJD3 transcription to restrain expression of interferon regulatory factor (IRF4) that controls M2 macrophage polarization. Consequently, sustained infection of PCV2 facilitates bacterial infection in vitro. In summary, these findings showed that PCV2 infection functionally modulated M1 macrophage polarization via targeting canonical signals and epigenetic histone modification, which contributes to bacterial coinfection and virial pathogenesis.

Porcine circovirus 2 manipulates PERK-ERO1α axis of endoplasmic reticulum in favor of its replication by derepressing viral DNA from HMGB1 sequestration within nuclei

Porcine circovirus type 2 (PCV2) causes several disease syndromes in grower pigs. PCV2 infection triggers endoplasmic reticulum (ER) stress, autophagy and oxidative stress, all of which support PCV2 replication. We have recently reported that nuclear HMGB1 is an anti-PCV2 factor by binding to viral genomic DNA. However, how PCV2 manipulates host cell responses to favor its replication has not been explored. Here, we demonstrate that PCV2 infection increased expression of ERO1α, generation of ROS and nucleocytoplasmic migration of HMGB1 via PERK activation in PK-15 cells. Inhibition of PERK or ERO1α repressed ROS production in PCV2-infected cells and increased HMGB1 retention within nuclei. These findings indicate that PCV2-induced activation of the PERK-ERO1α axis would lead to enhanced generation of ROS sufficient to decrease HMGB1 retention in the nuclei, thus derepressing viral DNA from HMGB1 sequestration. The viral Rep and Cap proteins were able to induce PERK-ERO1α-mediated ROS accumulation. Cysteine residues 107 and 305 of Rep or 108 of Cap played important roles in PCV2-induced PERK activation and distribution of HMGB1. Of the mutant viruses, only the mutant PCV2 with substitution of all three cysteine residues failed to activate PERK with reduced ROS generation and decreased nucleocytoplasmic migration of HMGB1. Collectively, this study offers novel insight into the mechanism of enhanced viral replication in which PCV2 manipulates ER to perturb its redox homeostasis via the PERK-ERO1α axis and the ER-sourced ROS from oxidative folding is sufficient to reduce HMGB1 retention in the nuclei, hence the release of HMGB1-bound viral DNA for replication. IMPORTANCE Considering the fact that clinical PCVAD mostly results from activation of latent PCV2 infection by confounding factors such as co-infection or environmental stresses, we propose that such confounding factors might impose oxidative stress to the animals where PCV2 in infected cells might utilize the elevated ROS to promote HMGB1 migration out of nuclei in favor of its replication. An animal infection model with a particular stressor could be approached with or without antioxidant treatment to examine the relationship among the stressor, ROS level, HMGB1 distribution in target tissues, virus replication and severity of PCVAD. This will help decide the use of antioxidants in the feeding regime on pig farms that suffer from PCVAD. Further investigation could examine if similar strategies are employed by DNA viruses, such as PCV3 and BFDV and if there is cross-talk among ER stress, autophagy/mitophagy and mitochondria-sourced ROS in favor of PCV2 replication.

Porcine circovirus 3 capsid protein induces autophagy in HEK293T cells by inhibiting phosphorylation of the mammalian target of rapamycin

Porcine circovirus 3 (PCV3) has been detected in major pig-producing countries around the world since its first report in the US in 2016. Most current studies have focused on epidemiological investigations and detection methods of PCV3 because of lack of live virus strains for research on its pathogenesis in porcine cells or even in pigs. We constructed a recombinant plasmid pCMV-Cap carrying the PCV3 orf2 gene to investigate the effects of capsid (Cap) protein expression on autophagic response in human embryonic kidney cell line 293T (HEK293T). We demonstrate that PCV3 Cap protein induced complete autophagy shown as formation of autophagosomes and autophagosome-like vesicles as well as LC3-II conversion from LC3-I via inhibiting phosphorylation of the mammalian target of rapamycin (mTOR) in HEK293T cells. The ubiquitin-proteasome pathway is also involved in the autophagy process. These findings provide insight for further exploration of PCV3 pathogenetic mechanisms in porcine cells.

Multifaceted Role of AMPK in Viral Infections

Viral pathogens often exploit host cell regulatory and signaling pathways to ensure an optimal environment for growth and survival. Several studies have suggested that 5'-adenosine monophosphate-activated protein kinase (AMPK), an intracellular serine/threonine kinase, plays a significant role in the modulation of infection. Traditionally, AMPK is a key energy regulator of cell growth and proliferation, host autophagy, stress responses, metabolic reprogramming, mitochondrial homeostasis, fatty acid β-oxidation and host immune function. In this review, we highlight the modulation of host AMPK by various viruses under physiological conditions. These intracellular pathogens trigger metabolic changes altering AMPK signaling activity that then facilitates or inhibits viral replication. Considering the COVID-19 pandemic, understanding the regulation of AMPK signaling following infection can shed light on the development of more effective therapeutic strategies against viral infectious diseases.

Induction of autophagy and suppression of type I IFN secretion by CSFV

Macroautophagy/autophagy plays an essential role in cellular responses to pathogens. However, the precise mechanisms and signaling pathways that modulate cellular autophagy in classical swine fever virus (CSFV)-infected host cells have not been confirmed. In this study, we showed that CSFV infection inhibits the phosphorylation of MTOR (mechanistic target of rapamycin kinase), subsequently leading to autophagy initiation. We also show that MAPK/ERK (mitogen-activated protein kinase) signaling is involved in CSFV-induced autophagy. The CSFV-induced inhibition of AKT/PKB (AKT serine/threonine kinase)-MTOR was observed to be partially responsible for the MTOR inactivation and subsequent autophagy initiation. Moreover, the CAMKK2/CaMKKβ (calcium/calmodulin dependent protein kinase kinase 2)-PRKAA/AMPK (protein kinase AMP-activated catalytic subunit alpha) axis was found to be involved in CSFV-induced autophagy. Meanwhile, CSFV non-structural protein NS5A induced autophagy via the CAMKK2-PRKAA-MTOR signaling pathway but not the AKT-MTOR or MAPK1/ERK2-MAPK3/ERK1-MTOR pathway. Although the AKT-MTOR pathway also plays an important role in the induction of autophagy by CSFV. We also found the interaction between HSP90AB1/HSPCB and NS5A by tandem affinity purification/liquid chromatography-mass spectrometry (LC-MS) and immunoprecipitation. Furthermore, the CSFV-induced [Ca2+]cyto increase potently induced autophagy through CAMKK2 and PRKAA. Moreover, we isolated and identified the BECN1/Beclin 1 protein complexes by tandem affinity purification/LC-MS and immunoprecipitation, the interaction between BECN1 and MAVS was confirmed by immunoprecipitation, laser scanning confocal microscope technology, and GST affinity-isolation experiments. Furthermore, CSFV-mediated autophagy suppressing type I IFN production is related to the interaction between MAVS and BECN1. Finally, the modulation of autophagy induction pathways by different autophagy regulatory factors significantly affected the replication of CSFV.Abbreviations: AKT: AKT serine/threonine kinase; AMPK: Adenosine monophosphate-activated protein kinase; CAMKK2: Calcium/calmodulin dependent protein kinase kinase 2; CSFV: Classical swine fever virus; HRP: Horseradish peroxidase; HSP90AB1: Heat shock protein 90 alpha family class B member 1; IFN: Interferon; ISGs: IFN-stimulated genes; LC-MS: Liquid chromatography-mass spectrometry; MAP1LC3/LC3: Microtubule associated protein 1 light chain 3; MAPK: Mitogen-activated protein kinase; MAVS: Mitochondrial antiviral signaling protein; MOI: Multiplicity of infection; MTOR: Mechanistic target of rapamycin kinase; PBS: Phosphate-buffered saline; PRKAA: Protein kinase AMP-activated catalytic subunit alpha; shRNA: short hairpin RNA.

Interplay Between Autophagy and Zinc

Autophagy is a conserved catabolic process that plays an important role in cellular homeostasis. The study of the interplay between autophagy and zinc has gained interest over the last years. Multiple studies have indicated that zinc stimulates autophagy and is critical for basal and induced autophagy in mammalian cells. Conversely, autophagy is induced by zinc starvation in yeast. There are no studies analyzing the role of zinc in either Microautophagy or Chaperone-Mediated-Autophagy. The mechanisms by which zinc modulates autophagy are still poorly understood. Studies examining loss of function of genes involved in cellular zinc homeostasis have provided novel insights into the role of zinc in autophagy. Autophagy may help cells adapt to changes in zinc availability in medium by controlling zinc mobilization, recycling, and secretion. Zinc is a key player in toxic and protective autophagy.

PCV2 Induces Reactive Oxygen Species To Promote Nucleocytoplasmic Translocation of the Viral DNA Binding Protein HMGB1 To Enhance Its Replication

Porcine circovirus type 2 (PCV2) is an important swine pathogen that causes significant economic losses to the pig industry. PCV2 interacts with host cellular factors to regulate its replication. High-mobility-group box 1 (HMGB1) protein, a major nonhistone protein in the nucleus, was recently discovered to participate in viral infections. Here, we demonstrate that nuclear HMGB1 negatively regulated PCV2 replication as shown by overexpression of HMGB1 or blockage of its nucleocytoplasmic translocation with ethyl pyruvate. The B box domain was essential in restricting PCV2 replication. Nuclear HMGB1 restricted PCV2 replication by sequestering the viral genome via binding to the Ori region. However, PCV2 infection induced translocation of HMGB1 from cell nuclei to the cytoplasmic compartment. Elevation of reactive oxygen species (ROS) induced by PCV2 infection was closely associated with cytosolic translocation of nuclear HMGB1. Treatment of PCV2-infected cells with ethyl pyruvate or N-acetylcysteine downregulated PCV2-induced ROS production, suppressed nucleocytoplasmic HMGB1 translocation, and decreased PCV2 replication. Collectively, these findings offer new insight into the mechanism of the PCV2 evasion strategy: PCV2 manages to escape restriction of its replication by nuclear HMGB1 by inducing ROS to trigger the nuclear-to-cytoplasmic translocation of HMGB1.IMPORTANCE Porcine circovirus type 2 (PCV2) is a small DNA virus that depends heavily on host cells for its infection. This study reports the close relationship between subcellular localization of host high-mobility-group box 1 (HMGB1) protein and viral replication during PCV2 infection. Restriction of PCV2 replication by nuclear HMGB1 is the early step of host defense at the host-pathogen interface. PCV2 then upregulates host reactive oxygen species (ROS) to prevent sequestration of its genome by expelling nuclear HMGB1 into the cytosol. It will be interesting to study if a similar evasion strategy is employed by other circoviruses such as beak and feather disease virus, recently discovered PCV3, and geminiviruses in plants. This study also provides insight into the justification and pharmacological basis of antioxidants as an adjunct therapy in PCV2 infection or possibly other diseases caused by the viruses that deploy the ROS-HMGB1 interaction favoring their replication.

Porcine Circovirus 2 Induction of ROS Is Responsible for Mitophagy in PK-15 Cells via Activation of Drp1 Phosphorylation

Mitochondrial dynamics is essential for the maintenance of cell homeostasis. Previous studies have shown that porcine circovirus 2 (PCV2) infection decreases the mitochondrial membrane potential and causes the elevation of reactive oxygen species (ROS), which may ultimately lead to mitochondrial apoptosis. However, whether PCV2 induce mitophagy remains unknown. Here we show that PCV2-induced mitophagy in PK-15 cells via Drp1 phosphorylation and PINK1/Parkin activation. PCV2 infection enhanced the phosphorylation of Drp1 and its subsequent translocation to mitochondria. PCV2-induced Drp1 phosphorylation could be suppressed by specific CDK1 inhibitor RO-3306, suggesting CDK1 as its possible upstream molecule. PCV2 infection increased the amount of ROS, up-regulated PINK1 expression, and stimulated recruitment of Parkin to mitochondria. N-acetyl-L-cysteine (NAC) markedly decreased PCV2-induced ROS, down-regulated Drp1 phosphorylation, and lessened PINK1 expression and mitochondrial accumulation of Parkin. Inhibition of Drp1 by mitochondrial division inhibitor-1 Mdivi-1 or RNA silencing not only resulted in the reduction of ROS and PINK1, improved mitochondrial mass and mitochondrial membrane potential, and decreased mitochondrial translocation of Parkin, but also led to reduced apoptotic responses. Together, our study shows that ROS induction due to PCV2 infection is responsible for the activation of Drp1 and the subsequent mitophagic and mitochondrial apoptotic responses.

Porcine Circovirus Type 2 ORF5 Protein Induces Autophagy to Promote Viral Replication via the PERK-eIF2α-ATF4 and mTOR-ERK1/2-AMPK Signaling Pathways in PK-15 Cells

Porcine circovirus type 2 (PCV2) is the primary causative agent that causing porcine circovirus-associated disease (PCVAD). The open reading frame 5 (ORF5) protein is a newly discovered non-structural protein in PCV2, which the function in viral pathogenesis remains unknown. The aim of this study was to investigate the mechanism of PCV2 ORF5 protein on autophagy and viral replication. The pEGFP-tagged ORF5 gene was ectopic expressed in PK-15 cells and an ORF5-deficient PCV2 mutant strain (PCV2^ΔORF5) were used to infected PK-15 cells. This study demonstrated that the ORF5 is essential for the of PCV2-induced autophagy. The ORF5 protein triggers the phosphorylation of PERK, eIF2α and the expression of downstream transcription factor ATF4. In addition, ORF5 protein activated the AMPK-ERK1/2-mTOR signaling pathways. These findings suggest that ORF5 play essential roles in the induction of autophagy by PCV2. We further revealed that PCV2 ORF5 promotes viral replication through PERK-eIF2α-ATF4 and AMPK-ERK1/2-mTOR pathways. In conclusion, we showed that PCV2 ORF5 induces autophagy to promote virus replication in PK-15 cells.

Foot-and-Mouth Disease Virus Capsid Protein VP1 Interacts with Host Ribosomal Protein SA To Maintain Activation of the MAPK Signal Pathway and Promote Virus Replication

Foot-and-mouth disease virus (FMDV) is the causative agent of foot-and-mouth disease, a highly contagious, economically important viral disease. The structural protein VP1 plays significant roles during FMDV infection. Here, we identified that VP1 interacted with host ribosomal protein SA (RPSA). RPSA is a viral receptor for dengue virus and classical swine fever virus infections. However, the incubation of susceptible cells using the anti-RPSA antibodies did not block the infection of FMDV. Overexpression of porcine RPSA in the insusceptible cells could not trigger FMDV infection, suggesting that RPSA was not responsible for FMDV entry and infection. On the contrary, we found that overexpression of RPSA suppressed FMDV replication, and knockdown of RPSA enhanced FMDV replication. We further determined that FMDV infection activated the mitogen-activated protein kinase (MAPK) pathway and demonstrated that MAPK pathway activation was critically important for FMDV replication. RPSA negatively regulated MAPK pathway activation during FMDV infection and displayed an antiviral function. FMDV VP1 interacted with RPSA to abrogate the RPSA-mediated suppressive role in MAPK pathway activation. Together, our study indicated that MAPK pathway activation was required for FMDV replication and that host RPSA played a negatively regulatory role on MAPK pathway activation to suppress FMDV replication. FMDV VP1 bound to RPSA to promote viral replication by repressing RPSA-mediated function and maintaining the activation of MAPK signal pathway.IMPORTANCE Identification of virus-cell interactions is essential for making strategies to limit virus replication and refine the models of virus replication. This study demonstrated that FMDV utilized the MAPK pathway for viral replication. The host RPSA protein inhibited FMDV replication by suppressing the activation of the MAPK pathway during FMDV infection. FMDV VP1 bound to RPSA to repress the RPSA-mediated regulatory effect on MAPK pathway activation. This study revealed an important implication of the MAPK pathway for FMDV infection and identified a novel mechanism by which FMDV VP1 has evolved to interact with RPSA and maintain the activation of the MAPK pathway, elucidating new information regarding the signal reprogramming of host cells by FMDV.

The Upstream Pathway of mTOR-Mediated Autophagy in Liver Diseases

Autophagy, originally found in liver experiments, is a cellular process that degrades damaged organelle or protein aggregation. This process frees cells from various stress states is a cell survival mechanism under stress stimulation. It is now known that dysregulation of autophagy can cause many liver diseases. Therefore, how to properly regulate autophagy is the key to the treatment of liver injury. mechanistic target of rapamycin (mTOR)is the core hub regulating autophagy, which is subject to different upstream signaling pathways to regulate autophagy. This review summarizes three upstream pathways of mTOR: the phosphoinositide 3-kinase (PI3K)/protein kinase (AKT) signaling pathway, the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway, and the rat sarcoma (Ras)/rapidly accelerated fibrosarcoma (Raf)/mitogen-extracellular activated protein kinase kinase (MEK)/ extracellular-signal-regulated kinase (ERK) signaling pathway, specifically explored their role in liver fibrosis, hepatitis B, non-alcoholic fatty liver, liver cancer, hepatic ischemia reperfusion and other liver diseases through the regulation of mTOR-mediated autophagy. Moreover, we also analyzed the crosstalk between these three pathways, aiming to find new targets for the treatment of human liver disease based on autophagy.

MMP13 inhibition rescues cognitive decline in Alzheimer transgenic mice via BACE1 regulation

MMP13 (matrix metallopeptidase 13) plays a key role in bone metabolism and cancer development, but has no known functions in Alzheimer's disease. In this study, we used high-throughput small molecule screening in SH-SY5Y cells that stably expressed a luciferase reporter gene driven by the BACE1 (β-site amyloid precursor protein cleaving enzyme 1) promoter, which included a portion of the 5' untranslated region (5'UTR). We identified that CL82198, a selective inhibitor of MMP13, decreased BACE1 protein levels in cultured neuronal cells. This effect was dependent on PI3K (phosphatidylinositide 3-kinase) signalling, and was unrelated to BACE1 gene transcription and protein degradation. Further, we found that eukaryotic translation initiation factor 4B (eIF4B) played a key role, as the mutation of eIF4B at serine 422 (S422R) or deletion of the BACE1 5'UTR attenuated MMP13-mediated BACE1 regulation. In APPswe/PS1E9 mice, an animal model of Alzheimer's disease, hippocampal Mmp13 knockdown or intraperitoneal CL82198 administration reduced BACE1 protein levels and the related amyloid-β precursor protein processing, amyloid-β load and eIF4B phosphorylation, whereas spatial and associative learning and memory performances were improved. Collectively, MMP13 inhibition/CL82198 treatment exhibited therapeutic potential for Alzheimer's disease, via the translational regulation of BACE1.

Artemisinin Protects Human Retinal Pigmented Epithelial Cells Against Hydrogen Peroxide-induced Oxidative Damage by Enhancing the Activation of AMP-active Protein Kinase

Dry age-related macular degeneration (AMD), a leading cause of blindness in aged population, is directly associated with oxidative stress induced damage of the retinal pigmented epithelial (RPE) cells. In the current study, we investigated the role of AMPK in the protective effect of artemisinin, an FDA approved anti-malarial Chinese herbal drug, on RPE cell line D407, against H₂O₂ induced oxidative stress. Our results showed that artemisinin promoted the survival of D407 cells from H₂O₂. Artemisinin reduced intracellular ROS generation and oxidative stress, decreased LDH release and the loss of mitochondrial membrane potential in D407 cells treated with H₂O₂. Western blotting showed that artemisinin concentration- and time-dependently stimulated the phosphorylation of AMP-activated protein kinase (AMPK) in D407 cells while AMPK inhibitor Compound C or knock-down of AMPK by si-RNA, inhibited the survival protective effect of artemisinin. More importantly, artemisinin produced a similar protective effect in primary cultured retinal pigment cells which was also blocked by inhibitors of AMPK. Taken together, these results suggested that artemisinin promotes survival of human retinal pigment cells against H₂O₂-induced cell death at least in part through enhancing the activation of AMPK. Therefore, artemisinin may be a beneficial therapeutic candidate for the treatment of age-related diseases, including retinal disorders like AMD.

The effects of autophagy on the replication of Nelson Bay orthoreovirus

BACKGROUND

Nelson Bay orthoreovirus (NBV) was first isolated over 40 years ago from a fruit bat in Australia. Normally, NBV does not cause human diseases, but recently several NBV strains have been associated with human respiratory tract infections, thus attracting clinical attention. Autophagy, an evolutionarily conserved process in eukaryotic cells, degrades intracellular substrates, participates in multiple physiological processes, and maintains cellular homeostasis. In addition, autophagy is intimately involved in viral infection.

METHODS

A new strain of NBV, isolated from a patient with a respiratory tract infection who returned to Japan from Bali, Indonesia, in 2007, was used in this study. NBV was rescued using a reverse genetics system involving cotransfection of BHK cells with 11 plasmids (pT7-L1 MB, pT7-L2 MB, pT7-L3 MB, pT7-M1 MB, pT7-M2 MB, pT7-M3 MB, pT7-S1 MB, pT7-S2 MB, pT7-S3 MB, pT7-S4 MB, and pcDNA3.1-T7), yielding NBV-MB. Recovered viruses were confirmed by immunofluorescence. The effect of NBV-MB on autophagy was evaluated by measuring the LC3-I/II proteins by immunoblot analysis after infection of BHK cells. Furthermore, after treatment with rapamycin (RAPA), 3-methyladenine (3-MA), chloroquine (CQ), or plasmid (GFP-LC3) transfection, the changes in expression of the LC3 gene and the amount of LC3-I/II protein were examined. In addition, variations in viral titer were assayed after treatment of BHK cells with drugs or after transfection with plasmids pCAGM3 and pCAGS3, which encode virus nonstructural proteins μNS and σNS, respectively.

RESULTS

NBV-MB infection induced autophagy in host cells; however, the level of induction was dependent on viral replication. Induction of autophagy increased viral replication. By contrast, inhibiting autophagy suppressed NBV replication, albeit not significantly. The NBV-MB nonstructural protein μNS was involved in the induction of autophagy with viral infection.

CONCLUSIONS

NBV-MB infection triggered autophagy. Also, the NBV nonstructural protein μNS may contribute to augmentation of autophagy upon viral infection.

The balance of apoptosis and autophagy via regulation of the AMPK signal pathway in aging rat striatum during regular aerobic exercise

The objective was to analyze the effects of aerobic exercise on aging striatum stress resistance, and the adaptive mechanisms related to neurodegenerative diseases, and the occurrence, and development of neural degeneration. The 10-weeks of regular moderate-intensity aerobic exercise intervention were carried out in the aerobic exercise runner Sprague-Dawley rats. Apoptotic nuclei appeared in the striatum of aged rats, showing a tendency to relate to aging. The apoptotic index of the striatum in young, middle-aged, and old-aged rats of the aerobic exercise groups increased by 205.56%, 57%, and 68.24%. Autophagy markers Beclin l and LC 3-II expression, AMPKα1 and pAMPKα1 expression increased significantly in all age-exercise groups. The ratio of AMPKα1/pAMPKα1 increased after exercise, and the tendency of exercise to alter autophagy and cell apoptosis increased with aging. Then SirT2 mRNA was significantly upregulated in the aerobic exercise runner groups. In conclusion, we showed that the balance of autophagy and apoptosis were closely regulated by regular aerobic exercise, which affected the development of aging, and via regulation of the AMPK/SirT2 signaling pathway.

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.

PCV2 replication promoted by oxidative stress is dependent on the regulation of autophagy on apoptosis

Porcine circovirus type 2 (PCV2) is an economically important swine pathogen but some extra trigger factors are required for the development of PCV2-associated diseases. By evaluating cap protein expression, viral DNA copies and the number of infected cells, the present study further confirmed that oxidative stress can promote PCV2 replication. The results showed that oxidative stress induced autophagy in PCV2-infected PK15 cells. Blocking autophagy with inhibitor 3-methyladenine or ATG5-specific siRNA significantly inhibited oxidative stress-promoted PCV2 replication. Importantly, autophagy inhibition significantly increased apoptosis in oxidative stress-treated PK15 cells. Suppression of apoptosis by benzyloxycarbonyl-Val-Ala-Asp fluoromethylketone in conditions of autophagy inhibition restored PCV2 replication. Taken together, autophagy protected host cells against potential apoptosis and then contributed to PCV2 replication promotion caused by oxidative stress. Our findings can partly explain the pathogenic mechanism of PCV2 related to the oxidative stress-induced autophagy.

Angiotensin II-Regulated Autophagy Is Required for Vascular Smooth Muscle Cell Hypertrophy

Hypertension is a disease associated to increased plasma levels of angiotensin II (Ang II). Ang II can regulate proliferation, migration, ROS production and hypertrophy of vascular smooth muscle cells (VSMCs). However, the mechanisms by which Ang II can affect VSMCs remain to be fully elucidated. In this context, autophagy, a process involved in self-digestion of proteins and organelles, has been described to regulate vascular remodeling. Therefore, we sought to investigate if Ang II regulates VSMC hypertrophy through an autophagy-dependent mechanism. To test this, we stimulated A7r5 cell line and primary rat aortic smooth muscle cells with Ang II 100 nM and measured autophagic markers at 24 h by Western blot. Autophagosomes were quantified by visualizing fluorescently labeled LC3 using confocal microscopy. The results showed that treatment with Ang II increases Beclin-1, Vps34, Atg-12-Atg5, Atg4 and Atg7 protein levels, Beclin-1 phosphorylation, as well as the number of autophagic vesicles, suggesting that this peptide induces autophagy by activating phagophore initiation and elongation. These findings were confirmed by the assessment of autophagic flux by co-administering Ang II together with chloroquine (30 μM). Pharmacological antagonism of the angiotensin type 1 receptor (AT1R) with losartan and RhoA/Rho Kinase inhibition prevented Ang II-induced autophagy. Moreover, Ang II-induced A7r5 hypertrophy, evaluated by α-SMA expression and cell size, was prevented upon autophagy inhibition. Taking together, our results suggest that the induction of autophagy by an AT1R/RhoA/Rho Kinase-dependent mechanism contributes to Ang II-induced hypertrophy in VSMC.

Autophagy Promotes Replication of Influenza A Virus In Vitro

Influenza A virus (IAV) infection could induce autophagosome accumulation. However, the impact of the autophagy machinery on IAV infection remains controversial. Here, we showed that induction of cellular autophagy by starvation or rapamycin treatment increases progeny virus production, while disruption of autophagy using a small interfering RNA (siRNA) and pharmacological inhibitor reduces progeny virus production. Further studies revealed that alteration of autophagy significantly affects the early stages of the virus life cycle or viral RNA synthesis. Importantly, we demonstrated that overexpression of both the IAV M2 and NP proteins alone leads to the lipidation of LC3 to LC3-II and a redistribution of LC3 from the cytosol to punctate vesicles indicative of authentic autophagosomes. Intriguingly, both M2 and NP colocalize and interact with LC3 puncta during M2 or NP transfection alone and IAV infection, leading to an increase in viral ribonucleoprotein (vRNP) export and infectious viral particle formation, which indicates that the IAV-host autophagy interaction plays a critical role in regulating IAV replication. We showed that NP and M2 induce the AKT-mTOR-dependent autophagy pathway and an increase in HSP90AA1 expression. Finally, our studies provided evidence that IAV replication needs an autophagy pathway to enhance viral RNA synthesis via the interaction of PB2 and HSP90AA1 by modulating HSP90AA1 expression and the AKT-mTOR signaling pathway in host cells. Collectively, our studies uncover a new mechanism that NP- and M2-mediated autophagy functions in different stages of virus replication in the pathogenicity of influenza A virus.IMPORTANCE Autophagy impacts the replication cycle of many viruses. However, the role of the autophagy machinery in IAV replication remains unclear. Therefore, we explored the detailed mechanisms utilized by IAV to promote its replication. We demonstrated that IAV NP- and M2-mediated autophagy promotes IAV replication by regulating the AKT-mTOR signaling pathway and HSP90AA1 expression. The interaction of PB2 and HSP90AA1 results in the increase of viral RNA synthesis first; subsequently the binding of NP to LC3 favors vRNP export, and later the interaction of M2 and LC3 leads to an increase in the production of infectious viral particles, thus accelerating viral progeny production. These findings improve our understanding of IAV pathogenicity in host cells.

Ataxia telangiectasia mutated kinase deficiency impairs the autophagic response early during myocardial infarction

Ataxia telangiectasia mutated kinase (ATM) is activated in response to DNA damage. We have previously shown that ATM plays a critical role in myocyte apoptosis and cardiac remodeling after myocardial infarction (MI). Here, we tested the hypothesis that ATM deficiency results in autophagic impairment in the heart early during MI. MI was induced in wild-type (WT) and ATM heterozygous knockout (hKO) mice by ligation of the left anterior descending artery. Structural and biochemical parameters of the heart were measured 4 h after left anterior descending artery ligation. M-mode echocardiography revealed that MI worsens heart function, as evidenced by reduced percent ejection fraction and fractional shortening in both groups. However, MI-induced increase in left ventricular end-diastolic and end-systolic diameters and volumes were significantly lower in hKO hearts. ATM deficiency resulted in autophagic impairment during MI, as evidenced by decreased microtubule-associated protein light chain 3-II increased p62, decreased cathepsin D protein levels, and increased aggresome accumulation. ERK1/2 activation was only observed in WT-MI hearts. Activation of Akt and AMP-activated protein kinase (AMPK) was lower, whereas activation of glycogen synthase kinase (GSK)-3β and mammalian target of rapamycin (mTOR) was higher in hKO-MI hearts. Inhibition of ATM using KU-55933 resulted in autophagic impairment in cardiac fibroblasts, as evidenced by decreased light chain 3-II protein levels and formation of acidic vesicular organelles. This impairment was associated with decreased activation of Akt and AMPK but enhanced activation of GSK-3β and mTOR in KU-55933-treated fibroblasts. Thus, ATM deficiency results in autophagic impairment in the heart during MI and cardiac fibroblasts. This autophagic impairment may occur via the activation of GSK-3β and mTOR and inactivation of Akt and AMPK. NEW & NOTEWORTHY Ataxia telangiectasia mutated kinase (ATM) plays a critical role in myocyte apoptosis and cardiac remodeling after myocardial infarction (MI). Here, we provide evidence that ATM deficiency results in autophagic impairment during MI. Further investigation of the role of ATM in autophagy post-MI may provide novel therapeutic targets for patients with ataxia telangiectasia suffering from heart disease.

PCV2 infection aggravates ochratoxin A-induced nephrotoxicity via autophagy involving p38 signaling pathway in vivo and in vitro

Ochratoxin A (OTA) is reported to induce nephrotoxicity in animals and humans. Porcine circovirus type 2 (PCV2) could induce porcine dermatitis and nephropathy syndrome. To date, little is known whether virus infection aggravates mycotoxin-induced toxicity. This work aimed to study the effects of PCV2 infection on OTA-induced nephrotoxicity and its mechanism in vivo and vitro. The results in vivo showed that PCV2 infection aggravated OTA-induced poor growth performance, nephrotoxicity, p38 phosphorylation and autophagy as demonstrated by Atg5, LC3 II and p62 protein expressions in kidney of pigs. The results in vitro indicated that PCV2 infection significantly aggravated OTA-induced nephrotoxicity as demonstrated by cell viabilities, annexin V/PI binding and caspase 3 activities, and induced p38 phosphorylation and autophagy in PK15 cells. p38 inhibitor decreased Atg5 and LC3 protein expression induced by PCV2 infection and OTA combined treatment. Adding autophagy inhibitor 3-MA or CQ alleviated the aggravating effects of PCV2 infection on OTA-induced nephrotoxicity. Atg5-specific siRNA eliminated the aggravating effects of PCV2 infection on OTA-induced nephrotoxicity. Taken together, these data indicate that in vivo and in vitro PCV2 infection aggravated OTA-induced nephrotoxicity via p38-mediated autophagy.

SeMet attenuates OTA-induced PCV2 replication promotion by inhibiting autophagy by activating the AKT/mTOR signaling pathway

Porcine circovirus type 2 (PCV2) is recognized as the causative agent of porcine circovirus-associated diseases. PCV2 replication could be promoted by low doses of ochratoxin A (OTA) as in our previous study and selenium has been shown to attenuate PCV2 replication. However, the underlying mechanism remains unclear. The aim of the study was to investigate the effects of selenomethionine (SeMet), the major component of organic selenium, on OTA-induced PCV2 replication promotion and its potential mechanism. The present study demonstrates that OTA could promote PCV2 replication as measured by cap protein expression, viral titer, viral DNA copies and the number of infected cells. In addition, OTA could activate autophagy as indicated by up-regulated light chain 3 (LC3)-II and autophagy-related protein 5 expressions and autophagosome formation. Further, OTA could down-regulate p-AKT and p-mTOR expressions and OTA-induced autophagy was inhibited when insulin was applied. SeMet at 2, 4 and 6 μM had significant inhibiting effects against OTA-induced PCV2 replication promotion. Furthermore, SeMet could attenuate OTA-induced autophagy and up-regulate OTA-induced p-AKT and p-mTOR expression inhibition. Rapamycin, an inhibitor of AKT/mTOR, could reverse the effects of SeMet on OTA-induced autophagy and the PCV2 replication promotion. In conclusion, SeMet could block OTA-induced PCV2 replication promotion by inhibiting autophagy by activating the AKT/mTOR pathway. Therefore, SeMet supplementation could be an effective prophylactic strategy against PCV2 infections and autophagy may be a potential marker to develop novel anti-PCV2 drugs.

Foot-and-mouth disease virus capsid protein VP2 activates the cellular EIF2S1-ATF4 pathway and induces autophagy via HSPB1

Foot-and-mouth disease virus (FMDV) can result in economical destruction of cloven-hoofed animals. FMDV infection has been reported to induce macroautophagy/autophagy; however, the precise molecular mechanisms of autophagy induction and effect of FMDV capsid protein on autophagy remain unknown. In the present study, we report that FMDV infection induced a complete autophagy process in the natural host cells of FMDV, and inhibition of autophagy significantly decreased FMDV production, suggesting that FMDV-induced autophagy facilitates viral replication. We found that the EIF2S1-ATF4 pathway was activated and the AKT-MTOR signaling pathway was inhibited by FMDV infection. We also observed that ultraviolet (UV)-inactivated FMDV can induce autophagy. Importantly, our work provides the first piece of evidence that expression of FMDV capsid protein VP2 can induce autophagy through the EIF2S1-ATF4-AKT-MTOR cascade, and we found that VP2 interacted with HSPB1 (heat shock protein family B [small] member 1) and activated the EIF2S1-ATF4 pathway, resulting in autophagy and enhanced FMDV replication. In addition, we show that VP2 induced autophagy in a variety of mammalian cell lines and decreased aggregates of a model mutant HTT (huntingtin) polyglutamine expansion protein (HTT103Q). Overall, our results demonstrate that FMDV capsid protein VP2 induces autophagy through interaction with HSPB1 and activation of the EIF2S1-ATF4 pathway.

Escherichia coli O157:H7 suppresses host autophagy and promotes epithelial adhesion via Tir-mediated and cAMP-independent activation of protein kinase A

Autophagy is a pivotal innate immune response that not only degrades cytosolic components, but also serves as one of the critical antimicrobial mechanisms eliminating intracellular pathogens. However, its role in host defense against extracellular pathogens is largely unknown. Here we showed that E. coli O157:H7 altered autophagy to evade host defense and facilitate adhesion. Enhancing host cell autophagy with tumor necrosis factor (TNF), host starvation or rapamycin reduced the adherence of E. coli O157:H7 to HT-29 cells. As a key regulator of autophagy, protein kinase A (PKA) was activated by E. coli O157:H7 infection. PKA inhibition by H89 abrogated E. coli O157:H7 inhibition of autophagy and prevented bacterial epithelial adhesion. Thus, PKA had a mediatory role in blocking autophagy and E. coli O157:H7 epithelial adhesion. Furthermore, deletion of translocated intimin receptor (tir) prevented PKA activation, whereas ectopic tir expression in a Δtir mutant strain restored its ability to activate PKA and inhibited autophagy in host cells. This indicated that Tir and PKA played pivotal roles in manipulating host autophagy during infection. Consistent with autophagy inhibition, E. coli O157:H7 infection inhibited endoplasmic reticulum (ER) stress in HT-29 cells, which was reversed by TNF, starvation, or H89 treatment. Additionally, E. coli O157:H7-induced PKA activation suppressed extracellular signal-regulated kinase 1/2 (ERK1/2) activation and enhanced phosphatidylinositol 3-kinase/Akt (PI3K/Akt) signaling, thereby repressing autophagic signaling. Conversely, PKA inhibition prevented downregulation of ERK1/2 signaling due to E. coli O157:H7 infection. In summary, E. coli O157:H7 inhibited host autophagy via Tir-mediated PKA activation that favored bacterial persistence on intestinal epithelial cell surfaces.

AMP-activated Protein Kinase As a Target For Pathogens: Friends Or Foes?

Intracellular pathogens are known to manipulate host cell regulatory pathways to establish an optimal environment for their growth and survival. Pathogens employ active mechanisms to hijack host cell metabolism and acquire existing nutrient and energy store. The role of the cellular energy sensor AMP-activated protein kinase (AMPK) in the regulation of cellular energy homeostasis is well documented. Here, we highlight recent advances showing the importance of AMPK signaling in pathogen-host interactions. Pathogens interact with AMPK by a variety of mechanisms aimed at reprogramming host cell metabolism to their own benefit. Stimulation of AMPK activity provides an efficient process to rapidly adapt pathogen metabolism to the major nutritional changes often encountered during the different phases of infection. However, inhibition of AMPK is also used by pathogens to manipulate innate host response, indicating that AMPK appears relevant to restriction of pathogen infection. We also document the effects of pharmacological AMPK modulators on pathogen proliferation and survival. This review illustrates intricate pathogen-AMPK interactions that may be exploited to the development of novel anti-pathogen therapies.

Porcine circovirus type 2 capsid protein induces unfolded protein response with subsequent activation of apoptosis

Porcine circovirus type 2 (PCV2) has recently been reported to elicit the unfolded protein response (UPR) via activation of the PERK/eIF2α (RNA-activated protein kinase-like endoplasmic reticulum (ER) kinase/eukaryotic initiation factor 2α) pathway. This study attempted to examine which viral protein might be involved in inducing UPR and whether this cellular event would lead to apoptosis of the cells expressing the viral protein. By transient expression, we found that both replicase (Rep) and capsid (Cap) proteins of PCV2 could induce ER stress as shown by increased phosphorylation of PERK with subsequent activation of the eIF2α-ATF4 (activating transcription factor 4)-CHOP (CCAAT/enhancer-binding protein homologous protein) axis. Cap expression, but not Rep, significantly reduced anti-apoptotic B-cell lymphoma-2 (Bcl-2) and increased caspase-3 cleavage, possibly due to increased expression of CHOP. Since knockdown of PERK by RNA interference clearly reduced Cap-induced CHOP expression, caspase-3 cleavage, and apoptotic cell death possibly by partially rescuing Bcl-2 expression, we propose that there is connection between Cap-induced UPR and apoptosis via the PERK/eIF2α/ATF4/CHOP/Bcl-2 pathway. This study, together with our earlier studies, provides insight into the mechanisms underlying PCV2 pathogenesis.

Ochratoxin A-induced autophagy in vitro and in vivo promotes porcine circovirus type 2 replication

Ochratoxin A (OTA) is a mycotoxin produced by Aspergillus and Penicillium. Porcine circovirus type 2 (PCV2) is recognized as the causative agent of porcine circovirus-associated diseases. Recently, we reported that low doses of OTA promoted PCV2 replication in vitro and in vivo, but the underlying mechanism needed further investigation. The present studies further confirmed OTA-induced PCV2 replication promotion as measured by cap protein expression, viral titer, viral DNA copies and the number of infected cells. Our studies also showed that OTA induced autophagy in PK-15 cells, as assessed by the markedly increased expression of microtubule-associated protein 1 light chain 3 (LC3)-II, autophagy-related protein 5 (ATG5), and Beclin-1 and the accumulation of green fluorescent protein (GFP)-LC3 dots. OTA induced complete autophagic flux, which was detected by monitoring p62 degradation and LC3-II turnover using immunoblotting. Inhibition of autophagy by 3-methylademine (3-MA) and chloroquine (CQ) significantly attenuated OTA-induced PCV2 replication promotion. The observed phenomenon was further confirmed by the knock-down of ATG5 or Beclin-1 by specific siRNA. Further studies showed that N-acetyl-L-cysteine (NAC), an ROS scavenger could block autophagy induced by OTA, indicating that ROS may be involved in the regulation of OTA-induced autophagy. Furthermore, we observed significant increases in OTA concentrations in lung, spleen, kidney, liver and inguinal lymph nodes (ILN) and bronchial lymph nodes (BLN) of pigs fed 75 and 150 μg/kg OTA compared with controls in vivo. Administration of 75 μg/kg OTA significantly increased PCV2 replication and autophagy in the lung, spleen, kidney and BLN of pigs. Taken together, it could be concluded that OTA-induced autophagy in vitro and in vivo promotes PCV2 replication.

MicroRNA-30a-5p promotes replication of porcine circovirus type 2 through enhancing autophagy by targeting 14-3-3

Accumulating evidence demonstrates that autophagy and microRNAs (miRNAs) play key roles in regulating virus-host interactions and can restrict or facilitate viral replication. In the present study we examined whether a functional relationship exists between autophagy, miRNA and porcine circovirus type 2 (PCV2) infection, using several approaches. We demonstrated that there was a positive correlation between PCV2 infection and autophagy in 3D4/21 cells and autophagy induced by PCV2 infection triggered PCV2 replication. Four miRNA were selected by real-time PCR and further studied, but only miR-30a-5p mimic had a significant effect on PCV2 replication. Overexpression of miR-30a-5p significantly enhanced PCV2 infection and autophagy in a dose-dependent manner. Blockage of miR-30a-5p significantly decreased PCV2 replication. We provided further evidence that miR-30a-5p regulate the link between PCV2 infection and host immune system. Furthermore, miR-30a-5p targeted and regulated 14-3-3 gene, which is a regulator of autophagy. Flow cytometry data demonstrated that miR-30a-5p promotes cell cycle arrest at the G2 phase to regulate PCV2 replication and autophagy by interacting directly with 14-3-3, but not with the PCV2 genome. These data not only provide new insights into virus-host interactions during PCV2 infection but also suggest a potential new antiviral therapeutic strategy against PCV2 infection.

Muscovy duck reovirus σNS protein triggers autophagy enhancing virus replication

Background

Muscovy duck reovirus (MDRV) causes high morbidity and mortality in Muscovy ducklings at 10 days old and can persist in an infected flock until the ducklings of 6 weeks old. It shares common physicochemical properties with avian reovirus (ARV) and differs in coding assignment and pathogenicity. The ARV p17 protein has been shown to trigger autophagy via activation multiple signaling pathways, which benefits virus replication. Since MDRV lacks the p17 protein, whether and how MDRV induces autophagy remains unknown. The aim of this study was to explore whether MDRV induces autophagy and which viral proteins are involved in MDRV-induced autophagy.

Methods

The autophagosome-like structures in MDRV-infected cells was observed under transmission electron microscopy. MDRV-induced autophagy was examined by analyzing the LC3-II level and phosphorylated form of mammalian target of rapamycin (mTOR) by Western blot assays. The effects of 3-methyladenine, rapamycin, chloroquine on viral yields were measured with quantitative(q) real-time reverse transcription (RT)-polymerase chain reaction (PCR) and 50% tissue culture infective dose (TCID₅₀) assays, respectively. Additionally, to determine which viral protein is responsible for MDRV-induced autophagy, both p10.8- and σNS-encoding genes of MDRV were cloned into the pCI-neo-flag vector and transfected into DF-1 cells for detection of LC3-II.

Results

The typical double-membrane vesicles containing cytoplasmic inclusions were visible in MDRV-infected immortalized chicken embryo fibroblast (DF-1) cells under transmission electron microscopy. Both primary Muscovy duck embryo fibroblasts (MDEF) and DF-1 cells infected with MDRV exhibited a significant increased levels of LC3-II accompanied with downregulation of phosphorylated form of mTOR, further confirming that MDRV is capable of inducing autophagy. Autophagy could be suppressed by 3-methylademine and induced by rapamycin and chloroquine. Furthermore, we found that σNS induces an increased levels of LC3-II, suggesting that the MDRV σNS protein is one of viral proteins involved in induction of autophagy. Both qRT-PCR and TCID₅₀ assays showed that virus yield was increased in rapamycin treated DF-1 cells following MDRV infection. Conversely, when infected cells were pretreated with chloroquine, virus yield was decreased.

Conclusions

The MDRV σNS nonstructural protein is responsible for MDRV-induced autophagy and benefits virus replication.