Porcine circovirus type 2 explores the autophagic machinery for replication 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.

Detection and differentiation of seven porcine respiratory pathogens using a multiplex ligation-dependent probe amplification assay

Respiratory diseases due to viral or bacterial agents, either alone or in combination, cause substantial economic burdens to the swine industry worldwide. Rapid and reliable detection of causal pathogens is crucial for effective epidemiological surveillance and disease management. This research aimed to employ the multiplex ligation-dependent probe amplification (MLPA) assay for simultaneous detection of seven distinct pathogens causing respiratory problems in swine, porcine reproductive and respiratory syndrome virus (PRRSV), swine influenza virus (SIV), porcine respiratory coronavirus (PRCV), porcine circovirus type 2 (PCV2), Pasteurella multocida, Actinobacillus pleuropneumoniae, and Glässerella parasuis. The results indicated no probe cross-reactivity among the seven target agents with other swine pathogens. The detection limits ranged from 5 to 34 copies per assay for the target organisms. The MLPA assay was evaluated with 88 samples and compared to real-time or multiplex PCR for the target pathogens. The MLPA assay demonstrated high relative test sensitivities (100 %) and reasonable to good relative specificities at 62.5 %, 95.1 %, 86.8 %, and 97.6 % for PRRSV, P. multocida, G. parasuis, and PCV2, respectively, relative to comparator PCR assays. In 71 samples where MLPA and comparator PCR assays matched exactly, infections were detected in 64 samples (90.1 %), with PRRSV being the most commonly found virus and 50.7 % of the samples showing co-infection with two to five of the pathogens. This approach serves as a valuable tool for conducting differential diagnoses and epidemiological investigations of pathogen prevalence within swine populations.

Carboplatin restricts peste des petits ruminants virus replication by suppressing the STING-mediated autophagy

Peste des petits ruminants virus (PPRV) is a morbillivirus that causes the acute and highly pathogenic infectious disease peste des petits ruminants (PPR) in small ruminants and poses a major threat to the goat and sheep industries. Currently, there is no effective treatment for PPRV infection. Here, we propose Carboplatin, a platinum-based regimen designed to treat a range of malignancies, as a potential antiviral agent. We showed that Carboplatin exhibits significant antiviral activity against PPRV in a cell culture model. The mechanism of action of Carboplatin against PPRV is mainly attributed to its ability to block STING mediated autophagy. Together, our study supports the discovery of Carboplatin as an antiviral against PPRV and potentially other closely related viruses, sheds light on its mode of action, and establishes STING as a valid and attractive target to counteract viral infection.

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.

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.

miR-214-5p/C1QTNF1 axis enhances PCV2 replication through promoting autophagy by targeting AKT/mTOR signaling pathway

Porcine circovirus type 2 (PCV2) is the causative agent of PCV2-associated disease, which causes a relevant economic impact on the global swine industry. Accumulating data have indicated host microRNAs play essential roles in numerous virus replication of pigs, while their roles in PCV2 replication remain unclear. Herein, we demonstrated that PCV2 infection downregulated the expression of miR-214-5p in PK15 cells, and miR-214-5p promoted PCV2 replication. C1q/tumor necrosis factor-related protein 1 (C1QTNF1) was then identified as a target gene of miR-214-5p, and C1QTNF1 suppressed PCV2 replication. Interestingly, miR-214-5p/C1QTNF1 axis negatively regulated AKT/mTOR signaling, and then enhanced PCV2 replication through promoting autophagy in PK15 cells. Collectively, our findings provide insight into the mechanism of PCV2 replication and highlight miR-214-5p and C1QTNF1 as potential novel targets for the treatment of PCV2 infection.

Porcine Circovirus 2 Activates the PERK-Reactive Oxygen Species Axis To Induce p53 Phosphorylation with Subsequent Cell Cycle Arrest at S Phase in Favor of Its Replication

Coinfections or noninfectious triggers have long been considered to potentiate PCV2 infection, leading to manifestation of PCVAD. The triggering mechanisms remain largely unknown.

PDZK1 upregulates nitric oxide production through the PI3K/ERK2 pathway to inhibit porcine circovirus type 2 replication

Porcine circovirus type 2 (PCV2) is the causative agent of porcine circovirus-associated disease. Changes in host cell gene expression are induced by PCV2 infection. Here, we showed that porcine PDZ Domain-Containing 1 (PDZK1) expression was enhanced during PCV2 infection and that overexpression of PDZK1 inhibited the expression of PCV2 Cap protein. PCV2 genomic DNA copy number and viral titers were decreased in PDZK1-overexpressing PK-15B6 cells. PDZK1 knockdown enhanced the replication of PCV2. Overexpression of PDZK1 activated the phosphoinositide 3-kinase (PI3K)/ERK2 signaling pathway to enhance nitric oxide (NO) levels, while PDZK1 knockdown had the opposite effects. A PI3K inhibitor (LY294002) and a NO synthase inhibitor (L-NAME hydrochloride) decreased the activity of PDZK1 in restricting PCV2 replication. ERK2 knockdown enhanced the proliferation of PCV2 by decreasing levels of NO. Levels of interleukin (IL)- 4 mRNA were reduced in PDZK1 knockdown and ERK2 knockdown PK-15B6 cells. Increased IL-4 mRNA levels were unable to decrease NO production in PDZK1-overexpressing cells. Thus, we conclude that PDZK1 affected PCV2 replication by regulating NO production via PI3K/ERK2 signaling. PDZK1 affected IL-4 expression through the PI3K/ERK2 pathway, but PDZK1 modulation of PCV2 replication occurred independently of IL-4. Our results contribute to understanding the biological functions of PDZK1 and provide a theoretical basis for the pathogenic mechanisms of PCV2.

Low molecular weight sulfated chitosan efficiently reduces infection capacity of porcine circovirus type 2 (PCV2) in PK15 cells

Background

Porcine circovirus type 2 (PCV2)-associated diseases are a major problem for the swine industry worldwide. In addition to vaccines, the availability of antiviral polymers provides an efficient and safe option for reducing the impact of these diseases. By virtue of their molecular weight and repetitious structure, polymers possess properties not found in small-molecule drugs. In this perspective, we focus on chitosan, a ubiquitous biopolymer, that adjusts the molecular weight and sulfated-mediated functionality can act as an efficient antiviral polymer by mimicking PCV2-cell receptor interactions.

Methods

Sulfated chitosan (Chi-S) polymers of two molecular weights were synthesized and characterized by FTIR, SEM–EDS and elemental analysis. The Chi-S solutions were tested against PCV2 infection in PK15 cells in vitro and antiviral activity was evaluated by measuring the PCV2 DNA copy number, TCID50 and capsid protein expression, upon application of different molecular weights, sulfate functionalization, and concentrations of polymer. In addition, to explore the mode of action of the Chi-S against PCV2 infection, experiments were designed to elucidate whether the antiviral activity of the Chi-S would be influenced by when it was added to the cells, relative to the time and stage of viral infection.

Results

Chi-S significantly reduced genomic copies, TCID50 titers and capsid protein of PCV2, showing specific antiviral effects depending on its molecular weight, concentration, and chemical functionalization. Assays designed to explore the mode of action of the low molecular weight Chi-S revealed that it exerted antiviral activity through impeding viral attachment and penetration into cells.

Conclusions

These findings help better understanding the interactions of PCV2 and porcine cells and reinforce the idea that sulfated polymers, such as Chi-S, represent a promising candidates for use in antiviral therapies against PCV2-associated diseases. Further studies in swine are warranted.

The Antiviral Effect of Panax Notoginseng Polysaccharides by Inhibiting PRV Adsorption and Replication In Vitro

Porcine pseudorabies (PR) is an important infectious disease caused by pseudorabies virus (PRV), which poses a major threat to food safety and security. Vaccine immunization has become the main means to prevent and control the disease. However, since 2011, a new PRV variant has caused huge economic losses to the Chinese pig industry. Panax notoginseng polysaccharides have immunomodulatory activity and other functions, but the antiviral effect has not been reported. We studied the anti-PRV activity of Panax notoginseng polysaccharides in vitro. A less cytopathic effect was observed by increasing the concentration of Panax notoginseng polysaccharides. Western blot, TCID₅₀, plaque assay, and IFA revealed that Panax notoginseng polysaccharides could significantly inhibit the infectivity of PRV XJ5 on PK15 cells. In addition, we also found that Panax notoginseng polysaccharides blocked the adsorption and replication of PRV to PK15 cells in a dose-dependent manner. These results show that Panax notoginseng polysaccharides play an antiviral effect mainly by inhibiting virus adsorption and replication in vitro. Therefore, Panax notoginseng polysaccharides may be a potential anti-PRV agent.

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.

Impact of feed restriction, chloroquine and deoxynivalenol on viral haemorrhagic septicaemia virus IVb in fathead minnow Pimephales promelas Rafinesque

Autophagy can markedly alter host response to infectious disease, and several studies have demonstrated that a restricted diet or deoxynivalenol modulates autophagy and reduces mortality of fish due to bacterial disease. The picture is less clear for viral diseases of fish. Duplicate tanks of fathead minnow, Pimephales promelas Rafinesque, were fed a replete diet (control), 100 µM chloroquine, 5 µM deoxynivalenol, 10% (fasted) or 40% of a replete diet (pair-fed) for 2 weeks and then experimentally infected by intraperitoneal injection with 2 × 10⁵ viral haemorrhagic septicaemia virus IVb. Survival from highest to lowest for the different treatments was as follows: deoxynivalenol (average 43.3%); control (40.0%); pair-fed (35.0%); fasted (33.3%); and chloroquine (21.7%). No treatment significantly altered the survival rate of fathead minnow after VHSV IVb infection when compared to controls; however, the fish fed with chloroquine had significantly lower survival rate than the fish fed deoxynivalenol (p < .05).

VHSV IVb infection and autophagy modulation in the rainbow trout gill epithelial cell line RTgill-W1

Autophagy modulation influences the success of intracellular pathogens, and an understanding of the mechanisms involved might offer practical options to reduce the impact of infectious disease. Viral haemorrhagic septicaemia virus (VHSV) can cause high mortality and economic loss in some commercial fish species. VHSV IVb was used to infect a rainbow trout gill cell line, RTgill-W1, followed by the treatment of the cells with different autophagy-modulating reagents. LC3II protein using Western blot was significantly (p < .05) decreased for two days following VHSV infection, and immunofluorescence confirmed that LC3II-positive intracytoplasmic puncta were also decreased. Infection with VHSV resulted in significantly decreased expression of the autophagy-related (Atg) genes atg4, at12, atg13 and becn1 after one day using quantitative PCR. Both viral gene copy number and VHSV N protein were significantly decreased by treating the cells with autophagy-blocking (chloroquine) and autophagy-inhibiting reagents (deoxynivalenol and 3-methyladenine) after three days, while autophagy induction (restricted nutrition and rapamycin) had limited effect. Only treatment of RTgill-W1 with deoxynivalenol resulted in a significant increase in expression of type I interferon. Therefore, the suppression of autophagy initially occurs after VHSV IVb infection, but the modulation of autophagy can also inhibit VHSV IVb infection in RTgill-W1 after three days.

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 DNAJB6 promotes PCV2 replication via enhancing the formation of autophagy in host cells

Hsp40/DnaJ family proteins play important roles in the infection process of various viruses. Porcine DNAJB6 (pDNAJB6) is a major member of this family, but its role in modulating the replication of porcine circovirus type 2 (PCV2) is still unclear. In the present study, pDNAJB6 was found to be significantly upregulated by PCV2 infection, and confirmed to be interacted with PCV2 capsid (Cap) protein and co-localized at both cytoplasm and nucleus in the PCV2-infected cells. Knockout of pDNAJB6 significantly reduced the formation of autophagosomes in PCV2-infected cells or in the cells expressing Cap protein, whereas overexpression of pDNAJB6 showed an opposite effect. In addition, the domain mapping assay showed that the J domain of pDNAJB6 (amino acids (aa) 1–99) and the C terminus of Cap (162-234 aa) were required for the interaction of pDNAJB6 with Cap. Notably, the interaction of pDNAJB6 with Cap was very important to promoting the formation of autophagosomes induced by PCV2 infection or Cap expression and enhancing the replication of PCV2. Taken together, the results presented here show a novel function of pDNAJB6 in regulation of porcine circovirus replication that pDNAJB6 enhances the formation of autophagy to promote viral replication through interacting with viral capsid protein during PCV2 infection.

Simultaneous identification of 6 pathogens causing porcine reproductive failure by using multiplex ligation-dependent probe amplification

We developed a multiplex ligation-dependent probe amplification (MLPA) assay for the simultaneous detection of 6 clinically relevant viral pathogens causing porcine reproductive failure, that is porcine reproductive and respiratory syndrome virus (PRRSV), Japanese encephalitis virus (JEV), classical swine fever virus (CSFV), porcine circovirus type 2 (PCV2), pseudorabies virus (PRV) and porcine parvovirus (PPV). The limits of detection for the assay varied among the 6 target organisms from 1 to 8 copies per MLPA assay. The MLPA assay was evaluated with 346 heparinized porcine umbilical cord blood specimens, and the results of the assay were compared to those of real-time PCR. The MLPA assay showed specificities and sensitivities of 99.2% and 100%, respectively, for PRRSV; 100% and 100%, respectively, for CSFV, PCV2, PRV and PPV. No sample was found to be positive for JEV by either the MLPA assay or the real-time PCR. In conclusion, the MLPA assay has comparable clinical sensitivity to that of real-time PCR assay and provides a useful tool for fast screening porcine reproductive failure-associated viruses.

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.

A radical form of nitric oxide inhibits porcine circovirus type 2 replication in vitro

Background

Porcine circovirus type 2 (PCV2) is the causal agent of postweaning multisystemic wasting syndrome (PMWS), causing large economical losses of the global swine industry. Nitric oxide (NO), as an important signaling molecule, has antiviral activity on some viruses. To date, there is little information on the role of NO during PCV2 infection.

Results

We used indirect fluorescence assay (IFA), TCID₅₀, real-time RT-qPCR and western blot assay to reveal the role of NO in restricting PCV2 replication. PCV2 replication was inhibited by a form of NO, NO^•, whereas PCV2 was not susceptible to another form of NO, NO⁺.

Conclusion

Our findings indicate that the form of NO^• has a potential role in the fight against PCV2 infection.

Evaluation of porcine circovirus type 2 infection in in vitro embryo production using naturally infected oocytes

In vitro fertilization (IVF) and somatic cell nuclear transfer (SCNT) are important breeding techniques for livestock. High-quality MII oocytes produced from in vitro maturation (IVM) are required for the two techniques listed above. The ovaries used for IVM operations are primarily acquired from commercial abattoirs, and the pathogen status of slaughtered animals becomes an unavoidable issue. Our previous monitoring data showed that porcine circovirus type 2 (PCV-2) is the main pathogen present in ovaries from abattoirs. However, the characteristics and effects of PCV-2 infection in oocyte maturation and in vitro production (IVP) of embryos are unclear, and currently there are no relevant studies. Therefore, the aim of this study was to determine the PCV-2 infection pattern and determine whether it affects oocyte in vitro maturation and IVP embryo development. More than five hundred ovaries and five thousand oocytes were utilized in the present study. Polymerase chain reaction (PCR) was used to detect PCV-2 DNA in ovaries, follicular fluid (FF), oocytes, cumulus cells and IVP embryos. The effects of viral infections on the rate of oocyte maturation and IVP embryo development were evaluated. We also analyzed the number of copies of the virus in the IVM and IVP process by absolute quantitative fluorescence PCR. Our study showed that the prevalent virus subgenotype in ovaries was PCV-2a. PCV-2a infection did not significantly affect ovarian/oocyte morphology and maturation. Moreover, virus infection did not have a significant effect on the development of the IVP embryos except for a reduction in IVF blastocyst cell numbers. Further tests showed that the viral copy numbers fluctuated at different stages between the IVP embryos and culture medium. For the first time, this study identified the infection pattern of naturally sourced PCV-2 in the course of oocyte maturation and embryo development.

Glutamine Deficiency Promotes PCV2 Infection through Induction of Autophagy via Activation of ROS-Mediated JAK2/STAT3 Signaling Pathway

Porcine circovirus type 2 (PCV2) is an important pathogen in swine herds. We previously reported that glutamine (Gln) deficiency promoted PCV2 infection in vitro. Here, we established a Gln deficiency model in vivo and further investigated the detailed molecular mechanisms. In vivo and in vitro, Gln deficiency promoted PCV2 infection, which was evident through increased viral yields and PCV2 Cap protein synthesis. It also induced autophagy, as demonstrated by the increases in LC3-II conversion, SQSTM1 degradation, and GFP-LC3 dot accumulation. Autophagy inhibition abolished the effects of Gln deficiency on PCV2 infection. Inhibition of ROS generation alleviated the Gln deficiency-activated JAK2/STAT3 signaling pathway, thereby inhibiting autophagy induction. In vitro, the inhibition of STAT3 by an inhibitor or RNA interference blocked autophagy, thus reversing the effects of Gln deficiency on PCV2 infection. These results indicate that Gln deficiency activates autophagy by upregulating ROS-medicated JAK2/STAT3 signaling and thereby promoting PCV2 infection.

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.

Selenizing astragalus polysaccharide attenuates PCV2 replication promotion caused by oxidative stress through autophagy inhibition via PI3K/AKT activation

Our previous studies have shown that oxidative stress could promote the porcine circovirus type 2 (PCV2) replication, and astragalus polysaccharide (APS)/selenium could suppress PCV2 replication. However, whether selenizing astragalus polysaccharide (sAPS) provides protection against oxidative stress-induced PCV2 replication promotion and the mechanism involved remain unclear. The present study aimed to explore the mechanism of the PCV2 replication promotion induced by oxidative stress and a novel pharmacotherapeutic approach involving the regulation of autophagy of sAPS. Our results showed that H₂O₂ promoted PCV2 replication via enhancing autophagy by using 3-methyladenine (3-MA) and autophagy-related gene 5 (ATG5) knockdown. Sodium selenite, APS, the mixture of sodium selenite and APS, and sAPS significantly inhibited H₂O₂-induced PCV2 replication promotion, respectively. Among these, sAPS exerted maximal inhibitory effect. sAPS could also significantly inhibit autophagy activated by H₂O₂ and increase the Akt and mTOR phosphorylation. Moreover, LY294002, the specific phosphoinositide 3-kinase/protein kinase B (PI3K/AKT) inhibitor, significantly alleviated the effects of sAPS on autophagy and PCV2 replication. Taken together, we conclude that H₂O₂ promotes PCV2 replication by inducing autophagy and sAPS attenuates the PCV2 replication promotion through autophagy inhibition via PI3K/AKT activation.

Porcine teschovirus 2 induces an incomplete autophagic response in PK-15 cells

Autophagy is a homeostatic process that has been shown to be vital in the innate immune defense against pathogens. However, little is known about the regulatory role of autophagy in porcine teschovirus 2 (PTV-2) replication. In this study, we found that PTV-2 infection induces a strong increase in GFP-LC3 punctae and endogenous LC3 lipidation. However, PTV-2 infection did not enhance autophagic protein degradation. When cellular autophagy was pharmacologically inhibited by wortmannin or 3-methyladenine, PTV-2 replication increased. The increase in virus yield via autophagy inhibition was further confirmed by silencing atg5, which is required for autophagy. Furthermore, PTV-2 replication was suppressed when autophagy was activated by rapamycin. Together, the results suggest that PTV-2 infection activates incomplete autophagy and that autophagy then inhibits further PTV-2 replication.

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.

Wild-type rabies virus induces autophagy in human and mouse neuroblastoma cell lines

Different rabies virus (RABV) strains have their own biological characteristics, but little is known about their respective impact on autophagy. Therefore, we evaluated whether attenuated RABV HEP-Flury and wild-type RABV GD-SH-01 strains triggered autophagy. We found that GD-SH-01 infection significantly increased the number of autophagy-like vesicles, the accumulation of enhanced green fluorescent protein (EGFP)-LC3 fluorescence puncta and the conversion of LC3-I to LC3-II, while HEP-Flury was not able to induce this phenomenon. When evaluating autophagic flux, we found that GD-SH-01 infection triggers a complete autophagic response in the human neuroblastoma cell line (SK), while autophagosome fusion with lysosomes was inhibited in a mouse neuroblastoma cell line (NA). In these cells, GD-SH-01 led to apoptosis and mitochondrial dysfunction while triggering autophagy, and apoptosis could be decreased by enhancing autophagy. To further identify the virus constituent causing autophagy, 5 chimeric recombinant viruses carrying single genes of HEP-Flury instead of those of GD-SH-01 were rescued. While the HEP-Flury virus carrying the wild-type matrix protein (M) gene of RABV triggered LC3-I to LC3-II conversion in SK and NA cells, replacement of genes of nucleoprotein (N), phosphoprotein (P) and glycoprotein (G) produced only minor autophagy. But no one single structural protein of GD-SH-01 induced autophagy. Moreover, the AMPK signaling pathway was activated by GD-SH-01 in SK. Therefore, our data provide strong evidence that autophagy is induced by GD-SH-01 and can decrease apoptosis in vitro. Furthermore, the M gene of GD-SH-01 may cooperatively induce autophagy.

Adapting the Stress Response: Viral Subversion of the mTOR Signaling Pathway

The mammalian target of rapamycin (mTOR) is a central regulator of gene expression, translation and various metabolic processes. Multiple extracellular (growth factors) and intracellular (energy status) molecular signals as well as a variety of stressors are integrated into the mTOR pathway. Viral infection is a significant stress that can activate, reduce or even suppress the mTOR signaling pathway. Consequently, viruses have evolved a plethora of different mechanisms to attack and co-opt the mTOR pathway in order to make the host cell a hospitable environment for replication. A more comprehensive knowledge of different viral interactions may provide fruitful targets for new antiviral drugs.

Porcine Circovirus Type 2 Activates CaMMKβ to Initiate Autophagy in PK-15 Cells by Increasing Cytosolic Calcium

Porcine circovirus type 2 (PCV2) induces autophagy via the 5′ adenosine monophosphate-activated protein kinase (AMPK)/extracellular signal-regulated kinase (ERK)/tuberous sclerosis complex 2 (TSC2)/mammalian target of rapamycin (mTOR) pathway in pig kidney PK-15 cells. However, the underlying mechanisms of AMPK activation in autophagy induction remain unknown. With specific inhibitors and RNA interference (RNAi), we show that PCV2 infection upregulated calcium/calmodulin-dependent protein kinase kinase-beta (CaMKKβ) by increasing cytosolic Ca²⁺ via inositol 1,4,5-trisphosphate receptor (IP3R). Elevation of cytosolic calcium ion (Ca²⁺) did not seem to involve inositol 1,4,5-trisphosphate (IP3) release from phosphatidylinositol 4,5-bisphosphate (PIP2) by phosphoinositide phospholipase C-gamma (PLC-γ). CaMKKβ then activated both AMPK and calcium/calmodulin-dependent protein kinase I (CaMKI). PCV2 employed CaMKI and Trp-Asp (WD) repeat domain phosphoinositide-interacting protein 1 (WIPI1) as another pathway additional to AMPK signaling in autophagy initiation. Our findings could help better understanding of the signaling pathways of autophagy induction as part of PCV2 pathogenesis. Further research is warranted to study if PCV2 interacts directly with IP3R or indirectly with the molecules that antagonize IP3R activity responsible for increased cytosolic Ca²⁺ both in PK-15 cells and PCV2-targeted primary cells from pigs.

The ORF4 protein of porcine circovirus type 2 antagonizes apoptosis by stabilizing the concentration of ferritin heavy chain through physical interaction

Porcine circovirus type 2 (PCV2) is the primary aetiological agent of porcine circovirus-associated disease in swine. The mechanism of PCV2 pathogenesis remains largely unknown. A newly identified viral protein of PCV2, ORF4, has been suggested to be involved in virus-induced apoptosis. However, there is still no information regarding the molecular mechanism by which ORF4 regulates apoptosis. In this study, we reveal that a physical interaction between the PCV2 ORF4 protein and ferritin heavy chain (FHC) in the cytoplasm of host cells reduced the cellular concentration of FHC. The ORF4-mediated reduction of FHC inhibited reactive oxygen species accumulation in PCV2-infected cells. Consequently, the ORF4 protein inhibited apoptosis in host cells. This may be the first report to describe the mechanism of ORF4 cytoprotection against apoptosis during the early stages of PCV2 infection.

Binding of the pathogen receptor HSP90AA1 to avibirnavirus VP2 induces autophagy by inactivating the AKT-MTOR pathway

Autophagy is an essential component of host innate and adaptive immunity. Viruses have developed diverse strategies for evading or utilizing autophagy for survival. The response of the autophagy pathways to virus invasion is poorly documented. Here, we report on the induction of autophagy initiated by the pathogen receptor HSP90AA1 (heat shock protein 90 kDa α [cytosolic], class A member 1) via the AKT-MTOR (mechanistic target of rapamycin)-dependent pathway. Transmission electron microscopy and confocal microscopy revealed that intracellular autolysosomes packaged avibirnavirus particles. Autophagy detection showed that early avibirnavirus infection not only increased the amount of light chain 3 (LC3)-II, but also upregulated AKT-MTOR dephosphorylation. HSP90AA1-AKT-MTOR knockdown by RNA interference resulted in inhibition of autophagy during avibirnavirus infection. Virus titer assays further verified that autophagy inhibition, but not induction, enhanced avibirnavirus replication. Subsequently, we found that HSP90AA1 binding to the viral protein VP2 resulted in induction of autophagy and AKT-MTOR pathway inactivation. Collectively, our findings suggest that the cell surface protein HSP90AA1, an avibirnavirus-binding receptor, induces autophagy through the HSP90AA1-AKT-MTOR pathway in early infection. We reveal that upon viral recognition, a direct connection between HSP90AA1 and the AKT-MTOR pathway trigger autophagy, a critical step for controlling infection.

Porcine Circovirus 2 Deploys PERK Pathway and GRP78 for Its Enhanced Replication in PK-15 Cells

Porcine circovirus type 2 (PCV2) infection induces autophagy and apoptosis. These cellular responses could be connected with endoplasmic reticulum (ER) stress. It remains unknown if PCV2 induces ER stress and if autophagy or apoptosis is primary to PCV2 infection or secondary responses following ER stress. Here, we demonstrate that PCV2 triggered unfolded protein response (UPR) in PK-15 cells by activating the PERK/eIF2α pathway without concomitant activation of IRE1 or ATF6. Since ATF4 and CHOP were induced later than PERK/eIF2α, it is clear that persistent PCV2 infection could lead to selective activation of PERK via the PERK-eIF2α-ATF4-CHOP axis. Therefore, PERK activation could be part of the pro-apoptotic signaling via induced expression of CHOP by PCV2. Since PERK inhibition by GSK2606414 or RNA silencing or suppression of eIF2α dephosphorylation by salubrinal limited viral replication, we suppose that PCV2 deploys UPR to enhance its replication. Over-expression of GRP78 or treatment with tauroursodeoxycholic acid could enhance viral capsid expression and/or viral titers, indicating that these chaperones, endogenous or exogenous, could help correct folding of viral proteins. Our findings provide the first evidence that ER stress plays a role in the pathogenesis of PCV2 infection probably as part of autophagic and apoptotic responses.

Glutamine starvation enhances PCV2 replication via the phosphorylation of p38 MAPK, as promoted by reducing glutathione levels

Glutamine has a positive effect on ameliorating reproductive failure caused by porcine circovirus type 2 (PCV2). However, the mechanism by which glutamine affects PCV2 replication remains unclear. This study was conducted to investigate the effects of glutamine on PCV2 replication and its underlying mechanisms in vitro. The results show that glutamine promoted PK-15 cell viability. Surprisingly, glutamine starvation significantly increased PCV2 replication. The promotion of PCV2 replication by glutamine starvation disappeared after fresh media with 4 mM glutamine was added. Likewise, promotion of PCV2 was observed after adding buthionine sulfoximine (BSO). Glutamine starvation or BSO treatment increased the level of p38 MAPK phosphorylation and PCV2 replication in PK-15 cells. Meanwhile, p38 MAPK phosphorylation and PCV2 replication significantly decreased in p38-knockdown PK-15 cells. Promotion of PCV2 replication caused by glutamine starvation could be blocked in p38-knockdown PK-15 cells. Therefore, glutamine starvation increased PCV2 replication by promoting p38 MAPK activation, which was associated with the down regulation of intracellular glutathione levels. Our findings may contribute toward interpreting the possible pathogenic mechanism of PCV2 and provide a theoretical reference for application of glutamine in controlling porcine circovirus-associated diseases.

VgrG2 of type VI secretion system 2 of Vibrio parahaemolyticus induces autophagy in macrophages

Type VI secretion system (T6SS) is a macromolecular transenvelope machine encoded within the genomes of several proteobacteria species. Vibrio parahaemolyticus contains two putative T6SS systems, VpT6SS1 and VpT6SS2, both contributing to adherence to Caco-2 and/or HeLa cells. However, it remains unknown if these systems are involved in cellular responses. In order to exclude the effects of other virulence factors known to induce cytotoxicity or autophagy, a triple deletion mutant dTTT (with deletion of tdh, and T3SS1 and T3SS2 structural protein genes) was used as the parent strain to construct deletion mutants of T6SS genes. The mutant dTTT-ΔicmF2, but not dTTT-ΔicmF1, reduced autophagic response upon 4 h of infection of the macrophage. Further attempt was made to search for the possible effector proteins that might be responsible for direct induction of autophagy by deletion of the genes encoding Hcp2 and VgrG2, two putative translocons of T6SS2 of V. parahaemolyticus. Deletion of either hcp2 or vgrG2 did reduce the autophagic response. However, increased LC3-II lipidation was seen only in the macrophage cells transfected with pVgrG2, but not with pHcp2. Chloroquinine treatment increased accumulation of LC3-II, suggesting that VgrG2 enhanced autophagic flux. The fact that vgrG2 deletion led to reduced level of intracellular cAMP suggests a possible role of cAMP signaling in autophagic responses to the bacterium. We conclude that VgrG2 of V. parahaemolyticus induces autophagy in macrophages.

Superoxide dismutase of Streptococcus suis serotype 2 plays a role in anti-autophagic response by scavenging reactive oxygen species in infected macrophages

Streptococcus suis serotype 2 (SS2) causes septic shock and meningitis. However, its pathogenesis is still not well-understood. We have recently shown that superoxide dismutase sodA of SS2 is a virulence factor probably by increasing resistance to oxidative stresses. Reactive oxygen species (ROS) are products of the respiratory burst of phagocytic cells and have been shown to activate autophagy. We wanted to know if and how SS2 explores its sodA to interfere with cell autophagic responses. A sodA deletion mutant (Δsod) was compared with its parent and complemented strain in autophagic response in the murine macrophage cell line RAW264.7. We found that the Δsod mutant induced significant autophagic responses in infected cells, shown as increased LC3 lipidation (LC3-II) and EGFP-LC3 punctae, than those infected by its parent or complemented strain at 1 or 2h post-infection. Co-localization of the autophagosomal EGFP-LC3 vesicles with lysosomes was seen in cells infected with Δsod mutant and its parent strain, indicating that SS2 infection induced complete autophagic responses. Reduced autophagic responses of cells infected with the wild-type strain might be related to decreased ROS by the scavenging effect of its sodA, as shown by increased superoxide anion or ROS level in cells infected with the Δsod mutant and in the cell free xanthine oxidase-hypoxanthine ROS-generating system, as compared with its parent or complemented strain. Taken together, SS2 makes use of its sodA for survival not only by scavenging ROS but also by alleviating the host autophagic responses due to ROS stimulation.

Porcine circovirus-2 capsid protein induces cell death in PK15 cells

Studies have shown that Porcine circovirus (PCV)-2 induces apoptosis in PK15 cells. Here we report that cell death is induced in PCV2b-infected PK15 cells that express Capsid (Cap) protein and this effect is enhanced in interferon gamma (IFN-γ)-treated cells. We further show that transient PCV2a and 2b-Cap protein expression induces cell death in PK15 cells at rate similar to PCV2 infection, regardless of Cap protein localization. These data suggest that Cap protein may have the capacity to trigger different signaling pathways involved in cell death. Although further investigation is needed to gain deeper insights into the nature of the pathways involved in Cap-induced cell death, this study provides evidence that PCV2-induced cell death in kidney epithelial PK15 cells can be mapped to the Cap protein and establishes the need for future research regarding the role of Cap-induced cell death in PCV2 pathogenesis.

Enterotoxigenic Escherichia coli infection induces intestinal epithelial cell autophagy

The morbidity and mortality in piglets caused by enterotoxigenic Escherichia coli (ETEC) results in large economic losses to the swine industry, but the precise pathogenesis of ETEC-associated diseases remains unknown. Intestinal epithelial cell autophagy serves as a host defense against pathogens. We found that ETEC induced autophagy, as measured by both the increased punctae distribution of GFP-LC3 and the enhanced conversion of LC3-I to LC3-II. Inhibiting autophagy resulted in decreased survival of IPEC-1 cells infected with ETEC. ETEC triggered autophagy in IPEC-1 cells through a pathway involving the mammalian target of rapamycin (mTOR), the extracellular signal-regulated kinases 1/2 (ERK1/2), and the AMP-activated protein kinase (AMPK).

Autophagy and viruses: adversaries or allies?

The autophagy pathway is an essential component of host defense against viral infection, orchestrating pathogen degradation (xenophagy), innate immune signaling, and certain aspects of adaptive immunity. Single autophagy proteins or cassettes of the core autophagy machinery can also function as antiviral factors independently of the canonical autophagy pathway. Moreover, to survive and propagate within the host, viruses have evolved a variety of strategies to evade autophagic attack and manipulate the autophagy machinery for their own benefit. This review summarizes recent advances in understanding the antiviral and proviral roles of autophagy and previously unappreciated autophagy-independent functions of autophagy-related genes.

Peste des petits ruminants virus exploits cellular autophagy machinery for replication

Peste des petits ruminants virus (PPRV) is an important pathogen that seriously influences the productivity of small ruminants worldwide. Although PPRV is known to induce apoptosis in infected cells, the interaction between PPRV and permissive cells requires further elucidation. Here, we provide the first evidence that PPRV infection triggered autophagy in Vero cells 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. We further demonstrated that induction of autophagy with rapamycin significantly increased PPRV progeny yield and nucleocapsid (N) protein expression, while inhibition of autophagy with siRNA targeting ATG7 resulted in diametrically opposite results. Our data indicate that PPRV exploits the autophagy machinery to facilitate its own replication in host cells, thus the production efficiency of live attenuated PPRV vaccines may be improved by targeting the autophagic pathway.

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

Porcine circovirus type 2 (PCV2) uses autophagy machinery to enhance its replication in PK-15 cells. However, the underlying mechanisms are unknown. By the use of specific inhibitors, RNA interference, and coimmunoprecipitation, we show that PCV2 induces autophagy in PK-15 cells through a pathway involving the kinases AMP-activated protein kinase (AMPK) and extracellular signal-regulated kinase 1/2 (ERK1/2), the tumor suppressor protein TSC2, and the mammalian target of rapamycin (mTOR). AMPK and ERK1/2 positively regulate autophagy through negative control of the mTOR pathway by phosphorylating TSC2 in PCV2-infected PK-15 cells. Thus, PCV2 might induce autophagy via the AMPK/ERK/TSC2/mTOR signaling pathway in the host cells, representing a pivotal mechanism for PCV2 pathogenesis.