Induction of the unfolded protein response (UPR) during pseudorabies virus infection

Tripartite motif 25 inhibits protein aggregate degradation during PRRSV infection by suppressing p62-mediated autophagy

Viral infection causes endoplasmic reticulum stress and protein metabolism disorder, influencing protein aggregates formation or degradation that originate from misfolded proteins. The mechanism by which host proteins are involved in the above process remains largely unknown. The present study found that porcine reproductive and respiratory syndrome virus (PRRSV) infection promoted the degradation of intracellular ubiquitinated protein aggregates via activating autophagy. The host cell E3 ligase tripartite motif-containing (TRIM)25 promoted the recruitment and aggregation of polyubiquitinated proteins and impeded their degradation caused by PRRSV. TRIM25 interacted with ubiquitinated aggregates and was part of the aggregates complex. Next, the present study investigated the mechanisms by which TRIM25 inhibited the degradation of protein aggregates, and it was found that TRIM25 interacted with both Kelch-like ECH-associated protein 1 (KEAP1) and nuclear factor E2-related factor 2 (Nrf2), facilitated the nuclear translocation of Nrf2 by targeting KEAP1 for K48-linked ubiquitination and proteasome degradation, and activated Nrf2-mediated p62 expression. Further studies indicated that TRIM25 interacted with p62 and promoted its K63-linked ubiquitination via its E3 ligase activity and thus caused impairment of its oligomerization, aggregation, and recruitment for the autophagic protein LC3, leading to the suppression of autophagy activation. Besides, TRIM25 also suppressed the p62-mediated recruitment of ubiquitinated aggregates. Activation of autophagy decreased the accumulation of protein aggregates caused by TRIM25 overexpression, and inhibition of autophagy decreased the degradation of protein aggregates caused by TRIM25 knockdown. The current results also showed that TRIM25 inhibited PRRSV replication by inhibiting the KEAP1-Nrf2-p62 axis-mediated autophagy. Taken together, the present findings showed that the PRRSV replication restriction factor TRIM25 inhibited the degradation of ubiquitinated protein aggregates during viral infection by suppressing p62-mediated autophagy.IMPORTANCESequestration of protein aggregates and their subsequent degradation prevents proteostasis imbalance and cytotoxicity. The mechanisms controlling the turnover of protein aggregates during viral infection are mostly unknown. The present study found that porcine reproductive and respiratory syndrome virus (PRRSV) infection promoted the autophagic degradation of ubiquitinated protein aggregates, whereas tripartite motif-containing (TRIM)25 reversed this process. It was also found that TRIM25 promoted the expression of p62 by activating the Kelch-like ECH-associated protein 1 (KEAP1) and nuclear factor E2-related factor 2 (Nrf2) pathway and simultaneously prevented the oligomerization of p62 by promoting its K63-linked ubiquitination, thus suppressing its recruitment of the autophagic adaptor protein LC3 and ubiquitinated aggregates, leading to the inhibition of PRRSV-induced autophagy activation and the autophagic degradation of protein aggregates. The present study identified a new mechanism of protein aggregate turnover during viral infection and provided new insights for understanding the pathogenic mechanism of PRRSV.

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

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

ATF4 Signaling in HIV-1 Infection: Viral Subversion of a Stress Response Transcription Factor

Cellular integrated stress response (ISR), the mitochondrial unfolded protein response (UPRmt), and IFN signaling are associated with viral infections. Activating transcription factor 4 (ATF4) plays a pivotal role in these pathways and controls the expression of many genes involved in redox processes, amino acid metabolism, protein misfolding, autophagy, and apoptosis. The precise role of ATF4 during viral infection is unclear and depends on cell hosts, viral agents, and models. Furthermore, ATF4 signaling can be hijacked by pathogens to favor viral infection and replication. In this review, we summarize the ATF4-mediated signaling pathways in response to viral infections, focusing on human immunodeficiency virus 1 (HIV-1). We examine the consequences of ATF4 activation for HIV-1 replication and reactivation. The role of ATF4 in autophagy and apoptosis is explored as in the context of HIV-1 infection programmed cell deaths contribute to the depletion of CD4 T cells. Furthermore, ATF4 can also participate in the establishment of innate and adaptive immunity that is essential for the host to control viral infections. We finally discuss the putative role of the ATF4 paralogue, named ATF5, in HIV-1 infection. This review underlines the role of ATF4 at the crossroads of multiple processes reflecting host-pathogen interactions.

HIV-1 replication requires optimal activation of the unfolded protein response

Several human diseases including viral infections activate the unfolded protein response (UPR) due to abnormal accumulation of unfolded/misfolded proteins. However, UPR modulation and its functional relevance in HIV-1 infection lack comprehensive elucidation. This study reveals that HIV-1 activates IRE1, PERK, and ATF6 signaling pathways of UPR. The knockdown of PERK and ATF6 reduces HIV-1 long terminal repeat (LTR)-driven gene expression, whereas the endoplasmic reticulum (ER) chaperone HSPA5 prevents proteasomal degradation of HIV-1 p24 through its chaperone activity. Interestingly, overstimulation of UPR by a chemical inducer leads to anti-HIV activity through an enhanced type-1 interferon response. Also, treatment with a chemical ER stress inhibitor reduces HIV-1 replication. These findings suggest that an optimal UPR activation is crucial for effective viral replication, as either overstimulating UPR or inhibiting ER stress leads to viral suppression.

Protective effect of Platycodon grandiflorus polysaccharide on apoptosis and mitochondrial damage induced by pseudorabies virus in PK-15 cells

Previous studies have confirmed that Platycodon grandiflorus polysaccharide (PGPSt) has the effects of regulating immunity and anti-apoptosis, but its effect on mitochondrial damage and apoptosis caused by PRV infection is still unclear. In this research, the effects of PGPSt on the cell viability, mitochondria morphology, mitochondrial membrane potential and apoptosis caused by PRV based on PK-15 cells were respectively examined by CCK-F assay, Mito-Tracker Red CMXRos, JC-1 staining method and Western blot etc. CCK-F test results showed that PGPSt had a protective effect on the decrease of cell viability caused by PRV. The results of morphological observation found that PGPSt can improve mitochondrial morphology damage, mitochondrial swelling and thickening, and cristae fracture. Fluorescence staining test results showed that PGPSt alleviated the decrease of mitochondrial membrane potential and apoptosis in infected cells. The expression of apoptosis-related proteins showed that PGPSt down-regulated the expression of the pro-apoptotic protein Bax and up-regulated the expression of the anti-apoptotic protein Bcl-2 in infected cells. These results indicated that PGPSt protected against PRV-induced PK-15 cell apoptosis by inhibiting mitochondrial damage.

Replication of standard bovine viral diarrhea strain OregonC24Va induces endoplasmic reticulum stress-mediated apoptosis of bovine trophoblast cells

Bovine viral diarrhea (BVD) is a worldwide infectious disease caused by bovine viral diarrhea virus (BVDV) infection, which invades the placenta, causes abortion, produces immune tolerance and continuously infects calves, and causes huge economic losses to the cattle industry. The endoplasmic reticulum (ER) is an important organelle in cells, which is prone to ER stress after being stimulated by pathogens, thus activating the ER stress-related apoptosis. Studies have confirmed that BVDV can utilize the ER of its host to complete its own proliferation and stimulate ER stress to a certain extent. However, the role of ER stress in BVDV infecting bovine placental trophoblast cells (BTCs) and inducing apoptosis is still unclear. We are using the cytopathic strain of BVDV (OregonC24Va), which can cause apoptosis of BTCs, as a model system to determine how ER stress induced by BVDV affects placental toxicity. We show that OregonC24Va can infect BTCs and proliferate in it. With the proliferation of BVDV in BTCs, ER stress-related apoptosis is triggered. The ER stress inhibitor 4-PBA was used to inhibit the ER stress of BTCs, which not only inhibited the proliferation of BVDV, but also reduced the apoptosis of BTCs. The ER stress activator Tg can activate ER stress-related apoptosis, but the proliferation of BVDV does not change in BTCs. Therefore, BVDV utilizes the UPR of activated ER stress to promote the proliferation of BVDV in the early stage of infection, and activates the ER stress-related apoptosis of BTCs in the later stage with the virus proliferation to promote the cell apoptosis and further spread of the virus. Our research provides a new theoretical basis for exploring the placental infection and vertical transmission of BVDV.

Pseudorabies virus infection induces endoplasmic reticulum stress and unfolded protein response in suspension-cultured BHK-21 cells

Viral infections cause endoplasmic reticulum (ER) stress and subsequently unfolded protein response (UPR) which restores ER homeostasis. In this study, levels of proteins or transcription of three UPR pathways were examined in suspension-cultured BHK-21 cells to investigate Pseudorabies virus (PRV) infection-induced ER stress, in which glucose-related proteins 78 kD and 94 kD (GRP78 and GRP94) were upregulated. The downstream double-stranded RNA-activated protein kinase-like ER kinase (PERK) pathway was activated with upregulation of ATF4, CHOP, and GADD34, and the inositol requiring kinase 1 (IRE1) pathway was triggered by the splicing of X box-binding protein 1 (XBP1) mRNA and the enhanced expression of p58^IPK and EDEM. Furthermore, our results showed that the ER stress, induced by 0.005 µM thapsigargin, promoted PRV replication in suspension-cultured BHK-21 cells, and that PRV glycoprotein B (gB) overexpression triggered the PERK and IRE1 pathways.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

PCV2 and PRV Coinfection Induces Endoplasmic Reticulum Stress via PERK-eIF2α-ATF4-CHOP and IRE1-XBP1-EDEM Pathways

Porcine circovirus 2 (PCV2) and pseudorabies virus (PRV) are two important pathogens in the pig industry. PCV2 or PRV infection can induce endoplasmic reticulum stress (ERS) and unfolded protein response (UPR). However, the effect of PCV2 and PRV coinfection on the ERS and UPR pathways remains unclear. In this study, we found that PRV inhibited the proliferation of PCV2 mainly at 36 to 72 hpi, while PCV2 enhanced the proliferation of PRV in the middle stage of the infection. Notably, PRV is the main factor during coinfection. The results of the transcriptomic analysis showed that coinfection with PCV2 and PRV activated cellular ERS, and upregulated expressions of the ERS pathway-related proteins, including GRP78, eIF2α, and ATF4. Further research indicated that PRV played a dominant role in the sequential infection and coinfection of PCV2 and PRV. PCV2 and PRV coinfection induced the ERS activation via the PERK-eIF2α-ATF4-CHOP axis and IRE1-XBP1-EDEM pathway, and thus may enhance cell apoptosis and exacerbate the diseases.

A Tug of War: Pseudorabies Virus and Host Antiviral Innate Immunity

The non-specific innate immunity can initiate host antiviral innate immune responses within minutes to hours after the invasion of pathogenic microorganisms. Therefore, the natural immune response is the first line of defense for the host to resist the invaders, including viruses, bacteria, fungi. Host pattern recognition receptors (PRRs) in the infected cells or bystander cells recognize pathogen-associated molecular patterns (PAMPs) of invading pathogens and initiate a series of signal cascades, resulting in the expression of type I interferons (IFN-I) and inflammatory cytokines to antagonize the infection of microorganisms. In contrast, the invading pathogens take a variety of mechanisms to inhibit the induction of IFN-I production from avoiding being cleared. Pseudorabies virus (PRV) belongs to the family Herpesviridae, subfamily Alphaherpesvirinae, genus Varicellovirus. PRV is the causative agent of Aujeszky’s disease (AD, pseudorabies). Although the natural host of PRV is swine, it can infect a wide variety of mammals, such as cattle, sheep, cats, and dogs. The disease is usually fatal to these hosts. PRV mainly infects the peripheral nervous system (PNS) in swine. For other species, PRV mainly invades the PNS first and then progresses to the central nervous system (CNS), which leads to acute death of the host with serious clinical and neurological symptoms. In recent years, new PRV variant strains have appeared in some areas, and sporadic cases of PRV infection in humans have also been reported, suggesting that PRV is still an important emerging and re-emerging infectious disease. This review summarizes the strategies of PRV evading host innate immunity and new targets for inhibition of PRV replication, which will provide more information for the development of effective inactivated vaccines and drugs for PRV.

SARS-CoV-2 ORF3a Induces Incomplete Autophagy via the Unfolded Protein Response

In the past year and a half, SARS-CoV-2 has caused 240 million confirmed cases and 5 million deaths worldwide. Autophagy is a conserved process that either promotes or inhibits viral infections. Although coronaviruses are known to utilize the transport of autophagy-dependent vesicles for the viral life cycle, the underlying autophagy-inducing mechanisms remain largely unexplored. Using several autophagy-deficient cell lines and autophagy inhibitors, we demonstrated that SARS-CoV-2 ORF3a was able to induce incomplete autophagy in a FIP200/Beclin-1-dependent manner. Moreover, ORF3a was involved in the induction of the UPR (unfolded protein response), while the IRE1 and ATF6 pathways, but not the PERK pathway, were responsible for mediating the ORF3a-induced autophagy. These results identify the role of the UPR pathway in the ORF3a-induced classical autophagy process, which may provide us with a better understanding of SARS-CoV-2 and suggest new therapeutic modalities in the treatment of COVID-19.

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

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

Induction of the Unfolded Protein Response during Bovine Alphaherpesvirus 1 Infection

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes great economic losses in the cattle industry. Herpesvirus infection generally induces endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in infected cells. However, it is not clear whether ER stress and UPR can be induced by BoHV-1 infection. Here, we found that ER stress induced by BoHV-1 infection could activate all three UPR sensors (the activating transcription factor 6 (ATF6), the inositol-requiring enzyme 1 (IRE1), and the protein kinase RNA-like ER kinase (PERK)) in MDBK cells. During BoHV-1 infection, the ATF6 pathway of UPR did not affect viral replication. However, both knockdown and specific chemical inhibition of PERK attenuated the BoHV-1 proliferation, and chemical inhibition of PERK significantly reduced the viral replication at the post-entry step of the BoHV-1 life cycle. Furthermore, knockdown of IRE1 inhibits BoHV-1 replication, indicating that the IRE1 pathway may promote viral replication. Further study revealed that BoHV-1 replication was enhanced by IRE1 RNase activity inhibition at the stage of virus post-entry in MDBK cells. Furthermore, IRE1 kinase activity inhibition and RNase activity enhancement decrease BoHV1 replication via affecting the virus post-entry step. Our study revealed that BoHV-1 infection activated all three UPR signaling pathways in MDBK cells, and BoHV-1-induced PERK and IRE1 pathways may promote viral replication. This study provides a new perspective for the interactions of BoHV-1 and UPR, which is helpful to further elucidate the mechanism of BoHV-1 pathogenesis.

Pseudorabies virus infection inhibits stress granules formation via dephosphorylating eIF2α

Pseudorabies virus (PRV) is one of the most notorious pathogens in the global pig industry. During infection, viruses may evolve various strategies, such as modulating stress granules (SGs) formation, to create an optimal surroundings for viral replication. However, the interplay between PRV infection and SGs formation remains largely unknown. Here we showed that PRV infection markedly blocked SGs formation induced by sodium arsenate (AS) and DL-Dithiothreitol (DTT). Accordantly, the phosphorylation of eIF2α was markedly inhibited in PRV-infected cells, although two eIF2α kinases double-stranded RNA-activated protein kinase (PKR) and PKR-like ER kinase (PERK) were activated during PRV infection. Furthermore, we also found that the dephosphorylation of eIF2α occurred at the early stage of virus infection but without the elevated production of GADD34 and PP1. Moreover, inhibition of PP1 activity by salubrinal could counteract PRV-mediated eIF2α dephosphorylation partially and inhibit virus replication. Our results revealed that, on the one hand, PRV infection activated eIF2α kinases PKR (latter inhibited) and PERK, and on the other hand, PRV encoded-functions dephosphorylated eIF2α and inhibited SGs formation to facilitate virus replication.