Porcine parvovirus infection induces apoptosis in PK-15 cells through activation of p53 and mitochondria-mediated pathway

For better or worse: crosstalk of parvovirus and host DNA damage response

Parvoviruses are a group of non-enveloped DNA viruses that have a broad spectrum of natural infections, making them important in public health. NS1 is the largest and most complex non-structural protein in the parvovirus genome, which is indispensable in the life cycle of parvovirus and is closely related to viral replication, induction of host cell apoptosis, cycle arrest, DNA damage response (DDR), and other processes. Parvovirus activates and utilizes the DDR pathway to promote viral replication through NS1, thereby increasing pathogenicity to the host cells. Here, we review the latest progress of parvovirus in regulating host cell DDR during the parvovirus lifecycle and discuss the potential of cellular consequences of regulating the DDR pathway, targeting to provide the theoretical basis for further elucidation of the pathogenesis of parvovirus and development of new antiviral drugs.

Isolation and Identification of a Tibetan Pig Porcine Epidemic Diarrhoea Virus Strain and Its Biological Effects on IPEC-J2 Cells

Porcine epidemic diarrhoea virus (PEDV) is a coronavirus that can cause severe watery diarrhoea in piglets, with high morbidity and mortality rates, seriously hindering the healthy development of the global swine industry. In this study, we isolated a strain of PEDV from Tibetan pigs and named it CH/GS/2022. Subsequently, we screened the apoptosis signals of PEDV-infected IPEC-J2 cells and studied the correlation between apoptosis signals and cell apoptosis. The results showed that different infections of PEDV induced different degrees of apoptosis in cells, and PEDV-induced cell apoptosis was dose-dependent. We then detected the expression of the p53, p38, JNK, Bax, and Bcl-2 genes in the apoptosis signal pathway. The results showed that 24 h after PEDV infection, the expression of the p53, p38, JNK, and Bax genes in IPEC-J2 cells increased significantly, while the expression of the Bcl-2 gene decreased significantly (p < 0.05). Subsequently, we used Western blot to detect the protein levels of these five genes, and the results showed that PEDV infection upregulated the expression of p53, p38, JNK, and Bax proteins (p < 0.05) while downregulating the expression of Bcl-2 protein (p < 0.05). Thus, it was initially inferred that PEDV infection could regulate cell apoptosis by activating the p53, p38, and JNK signalling pathways. Finally, we further investigated the apoptosis of the cells through the use of inhibitors. The results indicated that the p53 inhibitor Pifithrin-α has a significant inhibitory effect on the expression of the p53 protein after PEDV infection and can reverse the expression levels of Bax and Bcl-2 proteins. This suggested that p53 is involved in PEDV-induced cell apoptosis. Similarly, the p38 MAPK inhibitor SB203580 has an inhibitory effect on the expression of the p38 protein and can reverse the expression levels of Bax and Bcl-2 proteins. This suggested that p38 is also involved in PEDV-induced cell apoptosis. On the other hand, the JNK inhibitor SP600125 has no inhibitory effect on the expression of the JNK protein after PEDV infection, but the expression levels of Bax and Bcl-2 proteins have changed. Furthermore, it is noteworthy that SP600125 can inhibit the activity of apoptotic proteins but not their levels, resulting in reduced cell apoptosis. These preliminary results indicated that JNK may be involved in PEDV-induced IPEC-J2 cell apoptosis.

Identification and in vitro characterization of a novel porcine parvovirus 6 in Russia

Porcine parvovirus 6 (PPV6) was first identified in aborted swine fetuses in China in 2014. Since its identification, an increased number of PPV6 cases have been reported in many countries with developed pig breeding. In this study, the first identification of porcine parvovirus 6 in Russia, its phylogenetic analysis, and its characterization in vitro are reported. During the investigation, 521 serum samples collected from pigs of different ages from seven regions of the Russian Federation were tested. In four regions, the DNA of the virus was detected. The overall prevalence of porcine parvovirus 6 in Russia was 9.4%. Fattening pigs were the group with the most frequent detection of the virus genome. Phylogenetic analysis of the Russian isolate detected in a domestic boar indicated high homology with strains from Spain. In vitro studies revealed that the most promising cell cultures for PPV6 isolation are SPEV and SK. Our results demonstrated that PPV6 induced typical apoptotic features in cells, including DNA fragmentation, chromatin margination, nuclear condensation, pyknosis of nuclei, symplast formation, and various pathological mitoses.

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.

TLR2-mediated NF-κB signaling pathway is involved in PPV1-induced apoptosis in PK-15 cells

Porcine parvovirus 1 (PPV1) mainly induces severe reproductive failure in pregnant swine, and causes huge economic losses to the swine industry. Cell apoptosis induced by PPV1 infection has been identified the major cause of reproductive failure. However, the molecular mechanism was not fully elucidated. In this study, the potential mechanism of PPV1 induced apoptosis in PK-15 cells was investigated. Our results showed that PPV1 induced apoptosis in PK-15 cells. Further studies revealed toll-like receptor 2 (TLR2) was involved in the PPV1-mediated apoptosis. TLR2 siRNA significantly decreased the apoptosis. Finally, our study showed NF-κB was activated by TLR2 during PPV1-induced apoptosis. The activation of NF-κB signaling was demonstrated by the phosphorylation of p65, p65 nuclear translocation and degradation of inhibitor of kappa B α (IκBα). Together, these results provided evidence that the recognition between PPV1 and PK-15 cells was mainly through TLR2, and then induction of the NF-κB signaling pathway activation, which further induces apoptosis. Our study could provide information to understand the molecular mechanisms of PPV1 infection.

Biomimetic amphiphilic FAAP NPs nanoparticles: Synthesis, characterization and antivirus activity

Antiviral agents based on natural products have attracted substantial attention in clinical applications for their distinct biological activities,molecular structuralmultiformities, and low biotoxicities. Ferulic acid (FA) with apigenin propaneto form an esterified FA derivative (FAAP).Herein, we designed a CsPbBr3-modified chitosan oligosaccharide, a biomimetic nanoplatform that could load with FAAP. After self-assembly by combining FAAP with CsPbBr3-modified chitosan oligosaccharide (FAAP NPs), the resulting nanoparticles (FAAP NPs) showed high antioxidant and anti-inflammatory activities for enhancing the inhibition of porcineparvovirus.FAAP NPs exhibited no signs of acute toxicity in vitro or in vivo. DPPH and ABST are widely used for quantitative determination of antioxidant capacity. FAAP NPs exhibited excellent DPPH and ABTS radical scavenging abilities. In addition, we found that FAAP NPs inhibited PPV infection-induced PK-15 cell apoptosis, which was associated with regulating antioxidant and anti-inflammatory signaling pathways. Importantly, we showed that FAAP NPs blocked PPV infection-induced mitochondrial apoptosis in PK-15 cells via a p53/BH3 domain molecular-dependent mechanism.

Preparation of rabbit polyclonal antibody against porcine gasdermin D protein and determination of the expression of gasdermin D in cultured cells and porcine tissues

Gasdermin-D (GSDMD) is a member of the gasdermin (Gsdm) protein family, and its cleavage by inflammatory cysteine proteases (caspases, CASPs) is a critical event in cell pyroptosis. The role and functions of GSDMD on mice and humans are widely studied, but its expression, structure, and function in other species are less known. In the present work, rabbit anti-porcine GSDMD (pGSDMD) polyclonal antibody was prepared by immunizing New Zealand white rabbits with prokaryotic expressed recombinant pGSDMD (rpGSDMD). The prepared polyclonal antibody showed good specificity in Western blot and indirect immunofluorescence (IIF) assays. Western blot results showed that the polyclonal antibody could recognize overexpressed pGSDMD in human embryonic kidney cells (HEK293T) and endogenously expressed pGSDMD in cultured intestinal porcine enterocytes (IPEC-J2) and porcine kidney cells (PK-15). Western blot also revealed that pGSDMD was expressed in the heart, liver, lung, kidney, gallbladder, and jejunum of pigs. HEK293T cells overexpressing GSDMD showed green fluorescence in the IIF assay only after being treated with 0.3% Triton-X 100, which indicated that the full-length pGSDMD was located in the plasma but not on the cell membrane. This work provides a useful tool and basic information for further studies on pGSDMD.

Changes in Membrane Dielectric Properties of Porcine Kidney Cells Provide Insight into the Antiviral Activity of Glycine

The ability to monitor the status and progression of viral infections is important for development and screening of new antiviral drugs. Previous research illustrated that the osmolyte glycine (Gly) reduced porcine parvovirus (PPV) infection in porcine kidney (PK-13) cells by stabilizing the capsid protein and preventing virus capsid assembly into viable virus particles. Dielectrophoresis (DEP) was examined herein as a noninvasive, electric field- and frequency-dependent tool for real-time monitoring of PK-13 cell responses to obtain information about membrane barrier functionality and polarization. DEP responses of PK-13 cells were compared to those of PPV-infected cells in the absence and presence of the osmolyte glycine. With infection progression, PK-13 DEP spectra shifted toward lower frequencies, reducing crossover frequencies (f_CO). The spherical single-shell model was used to extract PK-13 cell dielectric properties. Upon PPV infection, specific membrane capacitance increased over the time progression of virus attachment, penetration, and capsid protein production and assembly. Following glycine treatment, the DEP spectra displayed attenuated f_CO and specific membrane capacitance values shifted back toward uninfected PK-13 cell values. These results suggest that DEP can be used to noninvasively monitor the viral infection cycle and screen antiviral compounds. DEP can augment traditional tools by elucidating membrane polarization changes related to drug mechanisms that interrupt the virus infection cycle.

Salmonella enterica serovar Typhimurium sseK3 induces apoptosis and enhances glycolysis in macrophages

Background

Salmonella enterica serovar Typhimurium (S. Typhimurium) is an important infectious disease pathogen that can survive and replicate in macrophages. Glycolysis is essential for immune responses against S. Typhimurium infection in macrophages, and is also associated with apoptosis. S. Typhimurium secreted effector K3 (SseK3) was recently identified as a novel translated and secreted protein. However, there is no study about the role of sseK3 in the relationship between apoptosis and glycolysis in cells infected with S. Typhimurium. It is unclear whether this protein exerts a significant role in the progress of apoptosis and glycolysis in S. Typhimurium-infected macrophages.

Results

Macrophages were infected with S. Typhimurium SL1344 wild-type (WT), ΔsseK3 mutant or sseK3-complemented strain, and the effects of sseK3 on apoptosis and glycolysis were determined. The adherence and invasion in the ΔsseK3 mutant group were similar to that in the WT and sseK3-complemented groups, indicating that SseK3 was not essential for the adherence and invasion of S. Typhimurium in macrophages. However, the percentage of apoptosis in the ΔsseK3 mutant group was much lower than that in the WT and sseK3-complemented groups. Caspase-3, caspase-8, and caspase-9 enzyme activity in the ΔsseK3 mutant group were significantly lower than in the WT group and sseK3-complemented groups, indicating that sseK3 could improve the caspase-3, caspase-8, and caspase-9 enzyme activity. We also found that there were no significant differences in pyruvic acid levels between the three groups, but the lactic acid level in the ΔsseK3 mutant group was much lower than that in the WT and sseK3-complemented groups. The ATP levels in the ΔsseK3 mutant group were remarkably higher than those in the WT and sseK3-complemented groups. These indicated that the sseK3 enhanced the level of glycolysis in macrophages infected by S. Typhimurium.

Conclusions

S. Typhimurium sseK3 is likely involved in promoting macrophage apoptosis and modulating glycolysis in macrophages. Our results could improve our understanding of the relationship between apoptosis and glycolysis in macrophages induced by S. Typhimurium sseK3.

Ferulic acid isolated from propolis inhibits porcine parvovirus replication potentially through Bid-mediate apoptosis

Propolis from honeybee hives, which is a traditional Chinese medicine, is widely used in veterinary clinics. Many compounds have been identified and isolated from propolis. Ferulic acid (FA), one of the propolis components, previous studies have proven that it has antiviral effects. To study the mechanism of FA antiviral effects, experiments such as immunofluorescence, quantitative real-time PCR and immunoblotting were introduced. In porcine kidney (PK-15) cells, PPV infection induced the expression of the proapoptotic genes Bid, Bad, Bim and Bak, disrupted mitochondrial membrane potential, promoted mitochondria-mediated, caspase-dependent apoptotic signaling and induced apoptosis. Furthermore, the infected PK-15 cells had increased intracellular reactive oxygen species (ROS) generation. FA treatment, however, reversed these effects and increased cell viability. FA treatment also significantly decreased the PPV-induced expression of Bid, Cyt-c and Apaf-1, suggesting that ROS were involved in the activation of the mitochondria-mediated apoptosis pathway. This in vitro study showed that the antiviral activity of FA was probably associated with inhibiting the replication of PPV by blocking proapoptotic factors such as Bid, Bcl-2 and Mcl-1, and attenuating the mitochondria-mediated response by inhibiting the activation of the Bid-related signaling pathway. Pharmacological inhibitors inhibited PPV-induced apoptosis by blocking Bid, and also suppressed the expression of Caspase family proteins in ppv-induced apoptosis. Taken together, our results suggested that PPV induced PK-15 cell apoptosis via activation of Bid and Bid-related signaling pathways and that the mitochondria act as the mediators of these pathways. FA effectively and extensively attenuated this PPV action, and thus is a potential antiviral agent against PPV.

Autophagy Promotes Porcine Parvovirus Replication and Induces Non-Apoptotic Cell Death in Porcine Placental Trophoblasts

Autophagy plays important roles in the infection and pathogenesis of many viruses, yet the regulatory roles of autophagy in the process of porcine parvovirus (PPV) infection remain unclear. Herein, we show that PPV infection induces autophagy in porcine placental trophoblasts (PTCs). Induction of autophagy by rapamycin (RAPA) inhibited the occurrence of apoptotic cell death, yet promoted viral replication in PPV-infected cells; inhibition of autophagy by 3-MA or ATG5 knockdown increased cellular apoptosis and reduced PPV replication. Interestingly, we found that in the presence of caspase-inhibitor zVAD-fmk, PPV induces non-apoptotic cell death that was characterized by lysosomal damage and associated with autophagy. Induction of complete autophagy flux by RAPA markedly promoted PPV replication compared with incomplete autophagy induced by RAPA plus bafilomycin (RAPA/BAF) in the early phase of PPV infection (24 h.p.i.). Meanwhile, induction of complete autophagy with RAPA increased lysosomal damage and non-apoptotic cell death in the later phase of PPV infection. Therefore, our data suggest that autophagy can enhance PPV replication and promote the occurrence of lysosomal-damage-associated non-apoptotic cell death in PPV-infected porcine placental trophoblasts.

A novel porcine parvovirus DNA-launched infectious clone carrying stable double labels as an effective genetic platform

Porcine parvovirus (PPV) is one of the major pathogens causing reproductive failure of swine. However, its specific pathogenesis has not been fully elucidated. Infectious clone is a powerful tool for further studying the pathogenic mechanism of PPV. In the present study, a PPV infectious clone was constructed, and the clone carries His-tag and Flag-tag double-genetic marker at the end of the ns1 gene 3' terminal and vp1 gene 5' terminal, respectively. The PPV DNA fragment F1 (1-182) in 5' end and the other PPV DNA fragment F2 (4788-5074) in 3' end were synthesized and assembled to the lower copy plasmid to construct pKQLL(F1 + F2), while the PPV DNA genome as a template to amplify carrying tags sequence PPV middle DNA fragment F3 and F4 by introducing Flag and His tags sequence in primers. Subsequently, the fused fragment F3/F4 were cloned into the Stu I/Sna B I sites of pKQLL(F1 + F2) plasmid to assemble the complete full-length PPV DNA recombinant plasmids, named as pD-PPV. The pD-PPV was transfected into PK-15 cells to gain rescued PPV virus, designed as D-PPV. Moreover, D-PPV showed similar replicate capability and pathogenicity comparing to the wild-type parental PPV through in vitro and in vivo studies, and the double labels can effectively indicate the expression and localization of viral proteins. Finally, the rescued D-PPV was found to be a convenient tool for antiviral drug screening. These data indicated that the newly established reverse genetic system for PPV would be a useful tool for further studying the pathogenesis mechanisms of PPV, developing labeled vaccine and screening antiviral drug.

Viral Nonstructural Protein 1 Induces Mitochondrion-Mediated Apoptosis in Mink Enteritis Virus Infection

Mink enteritis virus (MEV), an autonomous parvovirus, causes acute hemorrhagic enteritis in minks. The molecular pathogenesis of MEV infection has not been fully understood. In this study, we observed significantly increased apoptosis in the esophagus, small intestine, mesenteric lymph nodes, and kidney in minks experimentally infected with strain MEVB. In vitro infection of feline F81 cells with MEVB decreased cell viability and induced cell cycle arrest at G₁ phase and apoptosis. By screening MEV nonstructural proteins (NS1 and NS2) and structural proteins (VP1 and VP2), we demonstrated that the MEV NS1 induced apoptosis in both F81 and human embryonic kidney 293T (HEK293T) cells, similar to that induced during MEV infection in minks. We found that the NS1 protein-induced apoptosis in HEK293T cells was mediated not by the death receptor but by the mitochondrial pathway, as demonstrated by mitochondrial depolarization, opening of mitochondrial transition pore, release of cytochrome c, and activation of caspase-9 and -3. Moreover, in NS1-transfected cells, we observed an increase of Bax expression and its translocation to the mitochondria, as well as an increased ratio of the Bax/Bcl-2, reactive oxygen species (ROS) production, and activated p38 mitogen-activated protein kinase (MAPK) and p53. Taken together, our results demonstrated that MEV induces apoptosis through activation of p38 MAPK and the p53-mediated mitochondrial apoptotic pathway induced by NS1 protein, which sheds light on the molecular pathogenesis of MEV infection.IMPORTANCE MEV causes fatal hemorrhagic enteritis in minks. Apoptosis is a cellular mechanism that effectively sacrifices virus-infected cells to maintain homeostasis between the virus and host. In this study, we demonstrated that MEV induces apoptosis both in vivo and in vitro Mechanistically, the viral large nonstructural protein NS1 activates p38 MAPK, which leads p53 phosphorylation to mediate the mitochondrial apoptotic pathway but not the death receptor-mediated apoptotic pathway. This is the first report to uncover the mechanism underlying MEV-induced apoptosis.

Construction and characterization of the infectious clone of porcine parvovirus carrying genetic marker

Porcine parvovirus (PPV) is one of the major pathogens that bring about reproductive failure of pregnant sows. However, the study of the pathogenesis mechanism is circumscribed due to the lack of efficient genetic manipulation method. Infectious clone is a powerful tool for further studying the genetic mechanisms of PPV. In the present study, the gene fragment (157-4812) of PPV was amplified by PPV China isolate strain as a template, and PPV DNA fragments (1-182) forming Y-structure within in 5' end and (4788-5074) forming U-structure in 3' end were synthesized. And then, the above three fragments were inserted into plasmid pKQLL to congregate a PPV full-length recombinant plasmid by means of In-Fusion cloning technology. After the successful sequencing identification of the recombinant plasmid, the EcoR I restriction site was brought out as a genetic marker by nonsense mutation (A3058 T) to produce plasmid Y-PPV, which was transfected into PK-15 cells for rescue of virus. The rescued viral particles were observed under transmission electron microscopy, and the sequencing analysis showed that Y-PPV could stably carry the genetic marker. It could be seen that Y-PPV has similar replicate capability and pathogenicity as the wild-type parental PPV strain by cellular and animal experiments. These results confirmed that Y-PPV maintain similar biological characteristics with wild-type parental PPV strain. Infectious clone could be a valuable tool for studying the individual genes of PPV and applications in gene deletion or live vector vaccines.

Porcine Parvovirus Infection Causes Pig Placenta Tissue Damage Involving Nonstructural Protein 1 (NS1)-Induced Intrinsic ROS/Mitochondria-Mediated Apoptosis

Porcine parvovirus (PPV) is an important pathogen causing reproductive failure in pigs. PPV-induced cell apoptosis has been recently identified as being involved in PPV-induced placental tissue damages resulting in reproductive failure. However, the molecular mechanism was not fully elucidated. Here we demonstrate that PPV nonstructural protein 1 (NS1) can induce host cell apoptosis and death, thereby indicating the NS1 may play a crucial role in PPV-induced placental tissue damages and reproductive failure. We have found that NS1-induced apoptosis was significantly inhibited by caspase 9 inhibitor, but not caspase 8 inhibitor, and transfection of NS1 gene into PK-15 cells significantly inhibited mitochondria-associated antiapoptotic molecules Bcl-2 and Mcl-1 expressions and enhanced proapoptotic molecules Bax, P21, and P53 expressions, suggesting that NS1-induced apoptosis is mainly through the mitochondria-mediated intrinsic apoptosis pathway. We also found that both PPV infection and NS1 vector transfection could cause host DNA damage resulting in cell cycle arrest at the G1 and G2 phases, trigger mitochondrial ROS accumulation resulting in mitochondria damage, and therefore, induce the host cell apoptosis. This study provides a molecular basis for elucidating PPV-induced cell apoptosis and reproductive failure.

Porcine epidemic diarrhea virus infections induce apoptosis in Vero cells via a reactive oxygen species (ROS)/p53, but not p38 MAPK and SAPK/JNK signalling pathways

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p53 is activated, translocated to nucleus and involved in PEDV-induced apoptosis.

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ROS are accumulated during PEDV infection and involved in PEDV-induced apoptosis.

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ROS are the upper stream of p53 in PEDV infection.

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This is the first report that PEDV induce Vero cells apoptosis via ROS/p53 signal pathway.

Porcine epidemic diarrhea virus (PEDV) is a member of Coronavirus, which causes severe watery diarrhea in piglets with high morbidity and mortality. ROS and p53 play key roles in regulating many kinds of cell process during viral infection, however, the exact function in PEDV-induced apoptosis remains unclear. In this study, the pro-apoptotic effect of PEDV was examined in Vero cells and we observed that PEDV infection increased MDM2 and CBP, promoted p53 phosphorylation at serine 20 and, promoted p53 nuclear translocation, leading to p53 activation in Vero cells. Treatment with the p53 inhibitor PFT-α could significantly inhibit PEDV-induced apoptosis. We also observed PEDV infection induced time-dependent ROS accumulation. Treatment with antioxidants, such as pyrrolidine dithiocarbamate (PDTC) or N-acetylcysteine (NAC), significantly inhibited PEDV-induced apoptosis.

Moreover, further inhibition tests were established to prove that p53 was regulated by ROS in PEDV-induced apoptosis. In addition, we also found that p38 MAPK and SAPK/JNK were activated in PEDV-infected Vero cells. However, treatment with the p38 MAPK inhibitor SB203580, and the SAPK/JNK inhibitor SP600125 reversed PEDV-induced apoptosis. Taken together, the results of this study demonstrate that activated p53 and accumulated ROS participated in PEDV-induced apoptosis and p53 could be regulated by ROS during PEDV infection. Activated p38 MAPK and SAPK/JNK exerted no influence on PEDV-induced apoptosis. These findings provide new insights into the function of p53 and ROS in the interaction of PEDV with Vero cells.

cAMP Receptor Protein of Salmonella enterica Serovar Typhimurium Modulate Glycolysis in Macrophages to Induce Cell Apoptosis

We studied the role of glycolysis in the mechanism of cAMP receptor protein-induced macrophage cell death of Salmonella enterica serovar Typhimurium (S. Typhimurium). Cell apoptosis, caspase-3, -8, -9 enzyme activity, and pyruvic acid, lactic acid, ATP, and hexokinase (HK) contents were determined after infection of macrophages with S. Typhimurium SL1344 wild-type and a cAMP receptor protein mutant strain. While cell apoptosis, caspase-3, -8, -9 enzyme activity, lactic acid, hexokinase, and ATP levels significantly changed by infection with crp mutants compared to the wild-type strain (P < 0.05). Our data suggest that the cAMP receptor protein of S. Typhimurium can modulate macrophage death by effecting glycolysis levels. This finding may help to elucidate the mechanisms of S. Typhimurium pathogenesis.

Porcine deltacoronavirus induces caspase-dependent apoptosis through activation of the cytochrome c-mediated intrinsic mitochondrial pathway

Porcine deltacoronavirus (PDCoV), a newly discovered enteric coronavirus, is a causative agent of severe clinical diarrhea and intestinal pathological damage in piglets. As a first step toward understanding the effect of PDCoV on host cells, we elucidated mechanisms underlying the process of apoptotic cell death after PDCoV infection. The use of a pan-caspase inhibitor resulted in the inhibition of PDCoV-induced apoptosis and reduction of PDCoV replication, suggestive of the association of a caspase-dependent pathway. Furthermore, PDCoV infection necessitated the activation of the initiator caspase-9 responsible for the intrinsic mitochondrial apoptosis pathway. Experimental data indicated that PDCoV infection led to Bax-mediated mitochondrial outer membrane permeabilization (MOMP), resulting in specific relocation of the mitochondrial cytochrome c (cyt c) into the cytoplasm. Treatment with cyclosporin A (CsA), an inhibitor of mitochondrial permeability transition pore (MPTP) opening, significantly suppressed PDCoV-triggered apoptosis and viral replication. Moreover, cyt c release was completely abrogated in PDCoV-infected cells in the presence of CsA, suggesting the critical role of MPTP in intrinsic apoptosis in response to PDCoV infection. Altogether, our results indicate that PDCoV infection stimulates MOMP either via Bax recruitment or MPTP opening to permit the release of apoptogenic cyt c into the cytoplasm, thereby leading to execution of the caspase-dependent intrinsic apoptosis pathway to facilitate viral replication in vitro.

Biology of Porcine Parvovirus (Ungulate parvovirus 1)

Porcine parvovirus (PPV) is among the most important infectious agents causing infertility in pigs. Until recently, it was thought that the virus had low genetic variance, and that prevention of its harmful effect on pig fertility could be well-controlled by vaccination. However, at the beginning of the third millennium, field observations raised concerns about the effectiveness of the available vaccines against newly emerging strains. Subsequent investigations radically changed our view on the evolution and immunology of PPV, revealing that the virus is much more diverse than it was earlier anticipated, and that some of the “new” highly virulent isolates cannot be neutralized effectively by antisera raised against “old” PPV vaccine strains. These findings revitalized PPV research that led to significant advancements in the understanding of early and late viral processes during PPV infection. Our review summarizes the recent results of PPV research and aims to give a comprehensive update on the present understanding of PPV biology.

The SAT Protein of Porcine Parvovirus Accelerates Viral Spreading through Induction of Irreversible Endoplasmic Reticulum Stress

The SAT protein (SATp) of porcine parvovirus (PPV) accumulates in the endoplasmic reticulum (ER), and SAT deletion induces the slow-spreading phenotype. The in vitro comparison of the wild-type Kresse strain and its SAT knockout (SAT^-) mutant revealed that prolonged cell integrity and late viral release are responsible for the slower spreading of the SAT^- virus. During PPV infection, regardless of the presence or absence of SATp, the expression of downstream ER stress response proteins (Xbp1 and CHOP) was induced. However, in the absence of SATp, significant differences in the quantity and the localization of CHOP were detected, suggesting a role of SATp in the induction of irreversible ER stress in infected cells. The involvement of the induction of irreversible ER stress in porcine testis (PT) cell necrosis and viral egress was confirmed by treatment of infected cells by ER stress-inducing chemicals (MG132, dithiothreitol, and thapsigargin), which accelerated the egress and spreading of both the wild-type and the SAT^- viruses. UV stress induction had no beneficial effect on PPV infection, underscoring the specificity of ER stress pathways in the process. However, induction of CHOP and its nuclear translocation cannot alone be responsible for the biological effect of SAT, since nuclear CHOP could not complement the lack of SAT in a coexpression experiment.IMPORTANCE SATp is encoded by an alternative open reading frame of the PPV genome. Earlier we showed that SATp of the attenuated PPV NADL-2 strain accumulates in the ER and accelerates virus release and spreading. Our present work revealed that slow spreading is a general feature of SAT^- PPVs and is the consequence of prolonged cell integrity. PPV infection induced ER stress in infected cells regardless of the presence of SATp, as demonstrated by the morphological changes of the ER and expression of the stress response proteins Xbp1 and CHOP. However, the presence of SATp made the ER stress more severe and accelerated cell death during infection, as shown by the higher rate of expression of CHOP and alteration of the localization of CHOP. The beneficial effect of irreversible ER stress on PPV spread was confirmed by treatment of infected cells with ER stress-inducing chemicals.

Classical swine fever virus Shimen infection increases p53 signaling to promote cell cycle arrest in porcine alveolar macrophages

Classical swine fever virus (CSFV) replicates in macrophages and causes persistent infection. Despite its role in disastrous economic losses in swine industries, the molecular mechanisms underlying its pathogenesis are poorly understood. The virus evades the neutralizing immune response, subverting the immune system to ensure its own survival and persistence. Our genome-wide analysis of porcine alveolar macrophage transcriptional responses to CSFV Shimen infection using the Solexa/Illumina digital gene expression system revealed that p53 pathway components and cell cycle molecules were differentially regulated during infection compared to controls. Further, we investigated the molecular changes in macrophages infected with CSFV Shimen, focusing on the genes involved in the p53 pathway. CSFV Shimen infection led to phosphorylation and accumulation of p53 in a time-dependent manner. Furthermore, CSFV Shimen infection upregulated cyclin-dependent kinase inhibitor 1A (p21) mRNA and protein. In addition, CSFV Shimen infection induced cell cycle arrest at the G1 phase, as well as downregulation of cyclin E1 and cyclin-dependent kinase 2 (CDK2). The expression of genes in the p53 pathway did not change significantly after p53 knockdown by pifithrin-α during CSFV Shimen infection. Our data suggest that CSFV Shimen infection increases expression of host p53 and p21, and inhibits expression of cyclin E1 and CDK2, leading to cell cycle arrest at the G1 phase. CSFV may utilize this strategy to subvert the innate immune response and proliferate in host cells.

Porcine parvovirus infection activates mitochondria-mediated apoptotic signaling pathway by inducing ROS accumulation

Background

Porcine parvovirus (PPV) infection primarily causes reproductive failure of pregnant swine and results in host cell death. Boars, as an important disseminator, shed PPV to sows via semen. PPV infects and numerously replicates in boar testicle, which results in damage of swine testicle in vivo. Reactive oxygen species (ROS), a mediator of cell apoptosis, play a crucial role in the mitochondria apoptotic pathway. However, whether PPV infection induces ST cells apoptosis and ROS accumulation is still unclear.

Methods

To determine the effects of PPV infection on the apoptosis, we detected morphological changes, DNA ladder, activities of caspases, and expression of PARP in PPV-infected ST cells. Moreover, aiming to investigate the effect of PPV infection on the mitochondrial apoptotic pathway and ROS accumulation, we detected the Δψm, apoptosis-related genes, and ROS. To investigate the role of ROS in the process of PPV-induced apoptosis, the ST cells were infected with PPV and treated with the ROS antioxidants. The ROS level was measured using Reactive Oxygen Species Assay Kit and the Δψm, expression level of Bcl-2, translocation of Bax, and redistribution of mitochondria cytochrome c were tested.

Results

In this study, we demonstrated that PPV infection could induce apoptosis that was characterized by morphological changes, DNA fragmentation and activation of caspases. Moreover, PPV infection suppressed Bcl-2 expression, enhanced Bax expression and translocation to mitochondria, decreased the mitochondrial transmembrane potential, and triggered the release of cytochrome c, which caused the subsequent activation of caspase-9 and caspase-3 and initiation of apoptosis. However, during the process of PPV-induced apoptosis, the protein levels of Fas and FasL were not affected. Further studies showed that PPV infection caused ROS accumulation. Inhibition of ROS could reduce mitochondrial transmembrane potential and could significantly block ST cells apoptosis via suppressing Bax translocation, cytochrome c and caspase-3 activation.

Conclusions

All these results suggest that PPV-induced ROS accumulation mediates apoptosis in ST cells, which provided theoretical basis for the molecular pathogenesis of PPV infection.

Immortalization of porcine placental trophoblast cells through reconstitution of telomerase activity

Placental trophoblast cells (PTCs) play a critical role in histotrophic nutrient absorption, gaseous exchange, endocrine activities, and barrier function between the maternal and fetal systems. Establishment of immortalized porcine PTCs will help us to investigate the potential effects of different viruses on porcine trophoblast. In the present study, primary porcine PTCs were isolated from healthy gilts at Day 30 to Day 50 of gestation through collagenase digestion, percoll gradient centrifugation, and anti-CD9 immunomagnetic negative selection. To provide stable and long lifespan cells, primary PTCs were transfected with human telomerase reverse transcriptase (hTERT) gene. One porcine placental trophoblast cell line, named as hTERT-PTCs, was chosen for characterization. Human telomerase reverse transcriptase-PTCs achieved an extended replicative lifespan without exhibiting any neoplastic transformation signs in vivo or in vitro. The morphologic and key physiological characteristics of the immortalized PTCs were similar to primary PTCs. The immortalized PTCs retained original cell polarity and normal karyotype, expressed trophoblast-specific marker cytokeratin 7 and E-cadherin but did not express vimentin and major histocompatibility complex class I antigens as well as primary PTCs. Human telomerase reverse transcriptase-PTCs secreted low levels of chorionic gonadotrophin β-subunit and placental lactogen that were coincident with primary PTCs. Taken together, our results demonstrated that the porcine PTCs could be immortalized through reconstitution of telomerase activity. The immortalized PTCs maintained its original characteristics and can be used as a model cells line to study the pathologic changes of porcine placental trophoblast in viruses infectious diseases.