Singular PCV2a or PCV2b infection results in apoptosis of hepatocytes in clinically affected gnotobiotic pigs

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.

Matrine Targets Intestinal Lactobacillus acidophilus to Inhibit Porcine Circovirus Type 2 Infection in Mice

Porcine circovirus type 2 (PCV2) has caused huge economic losses to the pig industry across the world. Matrine is a natural compound that has been shown to regulate intestinal flora and has anti-PCV2 activity in mouse models. PCV2 infection can lead to changes in intestinal flora. The intestinal flora has proved to be one of the important pharmacological targets of the active components of Traditional Chinese Medicine. This study aimed to determine whether matrine exerts anti-PCV2 effects by regulating intestinal flora. In this study, fecal microbiota transplantation (FMT) was used to evaluate the effect of matrine on the intestinal flora of PCV2-infected Kunming (KM) mice. The expression of the Cap gene in the liver and the ileum, the relative expression of IL-1β mRNA, and the Lactobacillus acidophilus (L. acidophilus) gene in the ileum of mice were determined by real-time quantitative polymerase chain reaction (qPCR). ELISA was used to analyze the content of secretory immunoglobulin A (SIgA) in small intestinal fluid. L. acidophilus was isolated and identified from the feces of KM mice in order to study its anti-PCV2 effect in vivo. The expression of the Cap gene in the liver and the ileum and the relative expression of L. acidophilus and IL-1β mRNA in the ileum were determined by qPCR. The results showed that matrine could reduce the relative expression of IL-1β mRNA by regulating intestinal flora, and that its pharmacological anti-PCV2 and effect may be related to L. acidophilus. L. acidophilus was successfully isolated and identified from the feces of KM mice. The in vivo experiment revealed that administration of L. acidophilus also reduced the relative expression of IL-1β mRNA, and that it had anti-PCV2 effects in PCV2-infected mice. It was found that matrine could regulate the abundance of L. acidophilus in the gut of mice to exert an anti-PCV2 effect and inhibit PCV2-induced inflammatory response.

Matrine combined with Osthole inhibited the PERK apoptosis of splenic lymphocytes in PCV2-infected mice model

BACKGROUND

Porcine circovirus type 2 (PCV2) is one of the major pathogens commonly found in pigs, which causes immunosuppression and apoptosis. Vaccination and a single drug cannot totally prevent and treat PCV2 infection. Our previous in vitro study reported that the synergistic anti-PCV2 effect of Matrine and Osthole was better than that of Matrine or Osthole alone, This study was aimed to evaluate the synergistic anti-PCV2 effect as well as the underline molecular mechanism of Matrine and Osthole in Kunming (KM) mice model infected with PCV2. KM mice were randomly divided into 8 groups namely control group, PCV2 infected, Matrine combined with Osthole high dose treatment (40 mg/kg + 12 mg/kg), medium dose treatment (20 mg/kg + 6 mg/kg), low dose treatment (10 mg/kg + 3 mg/kg), Matrine treatment (40 mg/kg), Osthole treatment (12 mg/kg) and Ribavirin positive control (40 mg/kg) groups. PCV2 was intraperitoneally (i.p.) injected in all mice except the control group. 5 days of post-infection (dpi), mice in different treatment groups were injected i.p. with various doses of Matrine, Osthole and Ribavirin once daily for the next 5 consecutive days.

RESULTS

The synergistic inhibitory effect of Matrine and Osthole on PCV2 replication in mouse liver was significantly heigher than that of Matrine and Osthole alone. The expression of GRP78, p-PERK, p-eIF2α, ATF4, CHOP, cleaved caspase-3 and Bax proteins were significantly reduced, while that of Bcl-2 was significantly increased in Matrine combined with Osthole groups, which alleviated the pathological changes caused by PCV2, such as interstitial pneumonia, loss of spleen lymphocytes, infiltration of macrophages and eosinophils.

CONCLUSIONS

The synergistic anti-apoptotic effect of Matrine and Osthole was better than their alone effect, Both Matrine and Osthole had directly inhibited the expression of PCV2 Cap and the apoptosis of spleen cells induced by PCV2 Cap through the PERK pathway activated by endoplasmic reticulum (ER) GRP78. These results provided a new insight to control PCV2 infection and provide good component prescription candidate for the development of novel anti-PCV2 drugs.

Rift Valley Fever Virus Non-Structural Protein S Is Associated with Nuclear Translocation of Active Caspase-3 and Inclusion Body Formation

Rift Valley fever phlebovirus (RVFV) causes Rift Valley fever (RVF), an emerging zoonotic disease that causes abortion storms and high mortality rates in young ruminants as well as severe or even lethal complications in a subset of human patients. This study investigates the pathomechanism of intranuclear inclusion body formation in severe RVF in a mouse model. Liver samples from immunocompetent mice infected with virulent RVFV 35/74, and immunodeficient knockout mice that lack interferon type I receptor expression and were infected with attenuated RVFV MP12 were compared to livers from uninfected controls using histopathology and immunohistochemistry for RVFV nucleoprotein, non-structural protein S (NSs) and pro-apoptotic active caspase-3. Histopathology of the livers showed virus-induced, severe hepatic necrosis in both mouse strains. However, immunohistochemistry and immunofluorescence revealed eosinophilic, comma-shaped, intranuclear inclusions and an intranuclear (co-)localization of RVFV NSs and active caspase-3 only in 35/74-infected immunocompetent mice, but not in MP12-infected immunodeficient mice. These results suggest that intranuclear accumulation of RVFV 35/74 NSs is involved in nuclear translocation of active caspase-3, and that nuclear NSs and active caspase-3 are involved in the formation of the light microscopically visible inclusion bodies.

A Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Protects Pigs against PCV2b Challenge and Induces Serum Neutralizing Antibody Response against CSFV

Porcine circovirus type 2 (PCV2) is endemic worldwide. PCV2 causes immunosuppressive infection. Co-infection of pigs with other swine viruses, such as pseudorabies virus (PRV) and classical swine fever virus (CSFV), have fatal outcomes, causing the swine industry significant economic losses in many if not all pig-producing countries. Currently available inactivated/modified-live/vectored vaccines against PCV2/CSFV/PRV have safety and efficacy limitations. To address these shortcomings, we have constructed a triple gene (thymidine kinase, glycoprotein E [gE], and gG)-deleted (PRVtmv) vaccine vector expressing chimeric PCV2b-capsid, CSFV-E2, and chimeric E^rns-fused with bovine granulocytic monocyte-colony stimulating factor (E^rns-GM-CSF), designated as PRVtmv+, a trivalent vaccine. Here we compared this vaccine’s immunogenicity and protective efficacy in pigs against wild-type PCV2b challenge with that of the inactivated Zoetis Fostera Gold PCV commercial vaccine. The live PRVtmv+ prototype trivalent subunit vaccine is safe and highly attenuated in pigs. Based on PCV2b-specific neutralizing antibody titers, viremia, viral load in lymphoid tissues, fecal-virus shedding, and leukocyte/lymphocyte count, the PRVtmv+ yielded better protection for vaccinated pigs than the commercial vaccine after the PCV2b challenge. Additionally, the PRVtmv+ vaccinated pigs generated low to moderate levels of CSFV-specific neutralizing antibodies.

The combined usage of Matrine and Osthole inhibited endoplasmic reticulum apoptosis induced by PCV2

Background

Porcine circovirus type 2 (PCV2) is an important and common DNA virus that infect pig and can cause immunosuppression and induce apoptosis in the infected cells. To escape the host immune system, PCV2 constantly builds up complex mechanisms or mutates genes, and that is why it is difficult to eradicate complex PCV2 infection by relying on vaccines and single compound. At present, there is few literature reports on the effective prevention and treatment of PCV2 infection by a combination of two or more compounds. Previously, we have demonstrated the anti-PCV2 effect of Matrine in vitro, but its mechanism has not been further evaluated. Literatures have proven that Osthole has a variety of pharmacological activities, and we tested the ability of Osthole to inhibit PCV2 replication in cell culture. Therefore, this study explored the synergistic antiviral effect of Matrine combined with Osthole and their synergistic anti-apoptotic mechanism.

Results

Osthole alone had an anti-PCV2 effect, and then its synergistic anti-PCV2 effect of Osthole and Matrine was better than that of Matrine or Osthole alone as demonstrated by qRT-PCR, IFA and Western blotting results. The anti-apoptotic mechanism of these two compounds by inducing the PERK pathway by PCV2 was elucidated through Annexin V-FITC/PI, JC-1 and Western blotting. Matrine and Osthole combination could inhibit the expression of Cap in Cap-transfected PK-15 cells, thus inhibiting Cap-induced PERK apoptosis. Ribavirin was used as a positive control.

Conclusions

The combination of Osthole and Matrine had the synergistic effect of anti-PCV2 infection by directly inhibiting the expression of PCV2 Cap protein. The combination of these two compounds also inhibited PERK apoptosis induced by PCV2 Cap protein, possibly by regulating the level of GRP78. The results formed a base for further studies on the mechanism of anti-PCV2 in vivo using Matrine and Osthole combination and developing new anti-PCV2 compounds with Cap and GRP78 as therapeutic targets.

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.

Regulation of Apoptosis During Porcine Circovirus Type 2 Infection

Apoptosis, an indispensable innate immune mechanism, regulates cellular homeostasis by removing unnecessary or damaged cells. It contains three signaling pathways: the mitochondria-mediated pathway, the death receptor pathway and the endoplasmic reticulum pathway. The importance of apoptosis in host defenses is stressed by the observation that multiple viruses have evolved various strategies to inhibit apoptosis, thereby blunting the host immune responses and promoting viral propagation. Porcine Circovirus type 2 (PCV2) utilizes various strategies to induce or inhibit programmed cell death. In this article, we review the latest research progress of the apoptosis mechanisms during infection with PCV2, including several proteins of PCV2 regulate apoptosis via interacting with host proteins and multiple signaling pathways involved in PCV2-induced apoptosis, which provides scientific basis for the pathogenesis and prevention of PCV2.

Caspase-Dependent Apoptosis Induction via Viral Protein ORF4 of Porcine Circovirus 2 Binding to Mitochondrial Adenine Nucleotide Translocase 3

Apoptosis is an essential strategy of host defense responses and is used by viruses to maintain their life cycles. However, the apoptotic signals involved in virus replication are poorly known. In the present study, we report the molecular mechanism of apoptotic induction by the viral protein ORF4, a newly identified viral protein of porcine circovirus type 2 (PCV2). Apoptosis detection revealed not only that the activity of caspase-3 and -9 is increased in PCV2-infected and ORF4-transfected cells but also that cytochrome c release from the mitochondria to the cytosol is upregulated. Subsequently, ORF4 protein colocalization with adenine nucleotide translocase 3 (ANT3) was observed using structured illumination microscopy. Moreover, coimmunoprecipitation and pulldown analyses confirmed that the ORF4 protein interacts directly with mitochondrial ANT3 (mtANT3). Binding domain analysis further confirmed that N-terminal residues 1 to 30 of the ORF4 protein, comprising a mitochondrial targeting signal, are essential for the interaction with ANT3. Knockdown of ANT3 markedly inhibited the apoptotic induction of both ORF4 protein and PCV2, indicating that ANT3 plays an important role in ORF4 protein-induced apoptosis during PCV2 infection. Taken together, these data indicate that the ORF4 protein is a mitochondrial targeting protein that induces apoptosis by interacting with ANT3 through the mitochondrial pathway.IMPORTANCE The porcine circovirus type 2 (PCV2) protein ORF4 is a newly identified viral protein; however, little is known about its functions. Apoptosis is an essential strategy of the host defense response and is used by viruses to maintain their life cycles. In the present study, we report the molecular mechanism of the apoptosis induced by the ORF4 protein. The ORF4 protein contains a mitochondrial targeting signal and is an unstable protein that is degraded by the proteasome-dependent pathway. Viral protein ORF4 triggers caspase-3- and -9-dependent cellular apoptosis in mitochondria by directly binding to ANT3. We conclude that the ORF4 protein is a mitochondrial targeting protein and reveal a mechanism whereby circovirus recruits ANT3 to induce apoptosis.

Vimentin modulates infectious porcine circovirus type 2 in PK-15 cells

Porcine circovirus type 2 (PCV2) is the pathogen that causes postweaning multisystemic wasting syndrome, which leads to significant economic losses for swine farms worldwide. However, the infection mechanism of PCV2 is not completely understood yet. Vimentin is a part of the cytoskeleton network and plays an important role in several virus infections. It is not clear whether vimentin has a role in PCV2 infection nor how it affects PCV2 infection. In this study, the function of vimentin in PK-15 cells infected with PCV2 has been elucidated. We found that vimentin had a restrictive effect on the replication of PCV2 in PK-15 cells. Overexpression of vimentin by transferred pCAGGS-vimentin and down-regulation by the respective scrambled small interfering RNA showed that vimentin restricted the replication and virion production of PCV2. A special interaction between vimentin and PCV2 Cap protein was observed using laser confocal microscopy and immunoprecipitation assay. Moreover, overexpression of vimentin could decrease NF-κB activity and increase PCV2-induced caspase-3 activity in PK-15 cells. These data suggest that vimentin is involved in the replication of PCV2 and has a restrictive effect on it, which is helpful in the study of the replication mechanism of PCV2.

Porcine circovirus: a historical perspective

Porcine circoviruses (PCVs) belong to the genus Circovirus and the family Circoviridae, and they are the smallest known viruses that replicate autonomously in mammalian cells. They are nonenveloped, and they have characteristic single-stranded, negative-sense, circular DNA. Two types of divergent PCVs are recognized: PCV1 and PCV2. About 20 years ago, PCV2 began to emerge as a major pathogen of swine around the world, leading to burgeoning knowledge about the virus and porcine circovirus-associated diseases. However, much of the history of its discovery, including the controversy related to its importance, is not recorded. This review examines current issues related to the biology of PCV2 in the context of the original studies related to determining its causal association with disease and to the evolving understanding of the complex pathogenesis of PCV2 infections.

Regulatory role of ASK1 in porcine circovirus type 2-induced apoptosis

Porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome which is an emerging and important swine disease, can induce apoptotic responses in vitro and in vivo. However, the molecular mechanism of PCV2-induced apoptosis is not well understood. Apoptosis signal-regulating kinase 1 (ASK1), serves as an upstream enzyme that activates the JNK and p38 MAPK pathways, has been shown to play a target role in the regulation of apoptosis. Here, we showed that PCV2 infection induced ASK1 phosphorylation which preceded activation of JNK1/2 and p38 signaling in the cultured cells, and that the phosphatidylinostol 3-kinase (PI3K)/Akt signaling triggered by PCV2 infection limited the extent of JNK1/2 and p38 activation and thereby apoptotic cell death. Furthermore, inhibition of JNK and p38 activation is associated with PI3K-dependent negative-regulation of ASK1 in the PCV2-infected cells. These results indicate an important regulatory role of ASK1 in PCV2-induced apoptotic responses.

Hsp70 positively regulates porcine circovirus type 2 replication in vitro

The Hsp70 chaperone plays a central role in multiple processes within cells. Porcine circovirus type 2 (PCV2) is the essential causal agent of post-weaning multisystemic wasting syndrome (PMWS), which has spread worldwide. But the mechanism of PCV2 replication remains poorly understood. In this study, we firstly found the positive effect of heat stress on the replication of PCV2 in the continuous porcine monocytic cell line 3D4/31. Downregulation of Hsp70 by the specific chaperone inhibitor Quercetin or RNA interference and upregulation of Hsp70 by expression from a recombinant adenovirus showed that Hsp70 enhanced PCV2 genome replication and virion production. A specific interaction between Hsp70 and PCV2 Cap was confirmed by colocalization by confocal microscopy and co-immunoprecipitation. Furthermore, the NF-κB pathway was activated and caspase-3 activity was reduced when Hsp70 was overexpressed in PCV2-infected 3D4/31 cells. These data suggested that Hsp70 positively regulated PCV2 replication, which being helpful for understanding the molecular mechanism of PCV2 infection.

Activation of the phosphatidylinositol 3-kinase/Akt signaling pathway during porcine circovirus type 2 infection facilitates cell survival and viral replication

Virus infection activates host cellular signaling pathways, including the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which regulates diverse cellular activities related to cell growth, survival, and apoptosis. The present study demonstrated for the first time that porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome, which is an emerging and important swine disease, can transiently induce the PI3K/Akt pathway in cultured cells at an early step during PCV2 infection. Activation of the PI3K/Akt signal was also induced by UV-irradiated PCV2, indicating that virus replication was not required for this induction. Inhibition of PI3K activation leads to reduced virus yield, which is associated with decreased viral DNA replication and lower virus protein expression. However, inhibition of PI3K activation greatly enhanced apoptotic responses as evidenced by the cleavage of poly-ADP ribose polymerase and caspase-3 as well as DNA fragmentation using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling staining during the early stage of PCV2 infection. Furthermore, the pancaspase inhibitor zVAD.fmk alleviated the reduction in Akt phosphorylation levels by inhibiting PI3K activation, indicating that the signaling promotes cell survival and thereby favors viral replication. These results reveal that an antiapoptotic role for the PI3K/Akt pathway induced by PCV2 infection to suppress premature apoptosis for improved virus growth after infection, extending our understanding of the molecular mechanism of PCV2 infection.

Current State of Knowledge on Porcine Circovirus Type 2-Associated Lesions

Porcine circovirus type 2 (PCV2), a small single-stranded DNA virus, was initially discovered in 1998 and is highly prevalent in the domestic pig population. Disease manifestations associated with PCV2 include postweaning multisystemic wasting disease (PMWS), enteric disease, respiratory disease, porcine dermatitis and nephropathy syndrome (PDNS), and reproductive failure. Although these clinical manifestations involve different organ systems, there is considerable overlap in clinical expression of disease and presence of lesions between pigs and within herds. It is now widely accepted that PCV2 can be further subdivided into different types, of which PCV2a and PCV2b are present worldwide and of greatest importance. This review will focus on PCV2-associated lesions in different organ systems.

Type 2 Porcine Reproductive and Respiratory Syndrome Virus infection mediated apoptosis in B- and T-cell areas in lymphoid organs of experimentally infected pigs

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection is characterized by persisting in lungs and lymphoid tissue, resulting in systemic lymphoid depletion. The aim of this study was to correlate the histological changes, viral antigen expression and apoptosis phenomena in tonsil, medial retropharyngeal and mediastinal lymph nodes of 12 pigs inoculated with a type 2 PRRSV isolate (Chilean strain 2402). Apoptosis phenomena were observed mainly in lymphocytes and secondly in macrophages of lymph nodes and tonsils of inoculated animals, showing a peak of both apoptotic cells and viral antigen expression at the end of the study (21 dpi). However, the number of apoptotic cells was higher than the number of PRRSV-positive cells at the end of the study. This finding together with the location of apoptotic cells and PRRSV-positive cells in different structures of lymphoid organs supports the hypothesis that PRRSV-positive macrophages might modulate the apoptosis phenomena in other cells, mainly lymphocytes, by means of an indirect mechanism. Furthermore, apoptotic cells were detected both in B- and T-cell areas of lymphoid organs, suggesting that apoptosis phenomena may play a role in the impairment of the host immune response during PRRS.