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

Glucose-regulated protein 94 (Grp94/gp96) in viral pathogenesis: Insights into its role and therapeutic potentials

Glucose-regulated protein 94 (Grp94/gp96) is endoplasmic reticulum (ER) resident form of the 90 kDa heat shock protein 90 (Hsp90) that is responsible for folding, maturation and stabilization of more than 400 client proteins. Grp94 has been implicated for various diseases including metastatic cancer, primary open-angle glaucoma, and infectious diseases. In fact, Grp94 plays critical roles in different stages of viral infection cycle. It chaperones receptor proteins and viral glycoproteins that are necessary for viral entry and replication. Beyond its role in protein homeostasis, Grp94 modulates host cellular processes such as apoptosis and immune responses, which are often exploited by viruses to sustain infection. This work provides an overview of the roles of Grp94 in viral pathogenesis across various viruses and its involvement in immune modulation with the development of Grp94-selective inhibitors and their potential as anti-viral therapeutics.

Pseudorabies virus inhibits the unfolded protein response for viral replication during the late stages of infection

Pseudorabies virus (PRV) poses a significant threat to the global swine breeding industry and public health, but how the virus transverses the host defense systems for efficient viral replication and pathogenesis remains unclear. Here, we report that PRV could inhibit the unfolded protein response (UPR), a critical component of host innate immunity against viral infection, to promote virus replication during the late infection stages. PERK was shown phosphorylated and active in PRV-infected cells, but the subsequent events were suppressed post virus infection, such as eIF2α phosphorylation, ATF4 expression, and the formation of stress granules (SGs). In the meantime, although IRE1α was also active, its activated effector XBP1s was suppressed through downregulation of XBP1 mRNA levels and cleavage of XBP1s protein. Our findings also indicate that the Golgi apparatus, where ATF6 activation occur, was severely damaged in PRV-infected cells. Meanwhile, the downstream regulatory genes associated with the three UPR sensors, such as ERp60, CHOP, and EDEM1, remained silent in PRV-infected cells. Enhanced viral replication was observed post knockdown of UPR effectors ATF4 or XBP1, while stimulation with UPR activators inhibits virus replication. In conclusion, our findings address the critical question of how PRV regulates cellular UPR in favor of viral replication, and expand understanding of viruses mediated UPR suppression in general.

Evasion of the Antiviral Innate Immunity by PRV

Pseudorabies virus (PRV) establishes persistent latent infections by effectively evading the host's antiviral innate immune response. PRV has developed sophisticated strategies to bypass immune surveillance through coevolution with its host. Currently, no effective vaccine exists to prevent or treat infections caused by emerging PRV variants, and the interactions between PRV and the host's innate immune defenses remain incompletely understood. Nevertheless, ongoing research is uncovering insights that may lead to novel treatments and preventive approaches for herpesvirus-related diseases. This review summarizes recent advances in understanding how PRV disrupts key adaptors in immune signaling pathways to evade antiviral immunity. Additionally, we explored the intrinsic cellular defenses that play crucial roles in combating viral invasion. A deeper understanding of the immune evasion strategies of PRV could inform the development of new therapeutic targets and vaccines.

The global landscapes of lysine crotonylation in pseudorabies virus infection

Lysine crotonylation is a common occurrence in eukaryotic cells, regulating various physiological functions, including chromatin remodeling, cellular growth, and development. However, its involvement in viral infections has rarely been documented. In this study, we reveal that pseudorabies virus (PRV) infection significantly alters the global lysine crotonylation levels in porcine kidney PK-15 cells. Specifically, we identified a few viral proteins, including UL54, gM, gD, UL19, UL37, and UL46, which undergo crotonylation modification. Our observations indicate that at 20 h post-infection (hpi), 551 crotonylation sites were reduced across 345 proteins, while 47 new sites emerged in 37 proteins compared to the control group. By 40 hpi, 263 sites had decreased in 190 proteins, while 389 new sites appeared in 240 proteins. Deeper analysis revealed that the proteins with altered crotonylation levels were primarily involved in binding, catalytic activity, biosynthetic processes, ribosome activity, and metabolic processes. Additionally, our findings underscored the significance of ribosomes and the endoplasmic reticulum (ER), which were enriched with proteins exhibiting altered crotonylation. Overall, our study for the first time offers new insights into the relationship between crotonylation and herpes virus infection, paving the way for future investigations into the role of crotonylation in viral infections.

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.

Comparative proteomic analysis of PK-15 cells infected with wild-type strain and its EP0 gene-deleted mutant strain of pseudorabies virus

IMPORTANCE

As one of the main etiologic agents of infectious diseases in pigs, pseudorabies virus (PRV) infections have caused enormous economic losses worldwide. EP0, one of the PRV early proteins (EP) plays a vital role in PRV infections, but the mechanisms are unclear.

OBJECTIVE

This study examined the function of EP0 to provide a direction for its in-depth analysis.

METHODS

In this study, the EP0-deleted PRV mutant was obtained, and Tandem Mass Tag-based proteomic analysis was used to screen the differentially expressed proteins (DEPs) quantitatively in EP0-deleted PRV- or wild-type PRV-infected porcine kidney 15 cells.

RESULTS

This study identified 7,391 DEPs, including 120 and 21 up-regulated and down-regulated DEPs, respectively. Western blot analysis confirmed the changes in the expression of the selected proteins, such as speckled protein 100. Comprehensive analysis revealed 141 DEPs involved in various biological processes and molecular functions, such as transcription regulator activity, biological regulation, and localization.

CONCLUSIONS AND RELEVANCE

These results holistically outlined the functions of EP0 during a PRV infection and might provide a direction for more detailed function studies of EP0 and the stimulation of lytic PRV infections.

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.

Glucose-regulated protein 94 facilitates the proliferation of the Bombyx mori nucleopolyhedrovirus via inhibiting apoptosis

Glucose regulatory protein 94 (GRP94) is an endoplasmic reticulum (ER)-resident member of the heat shock protein 90 (HSP90) family, that plays an important role in secreted protein folding. Bombyx mori nuclear polyhedrosis virus (BmNPV) is one of the main pathogens in sericulture, causing serious economic losses every year. Previous studies showed that HSP90 members promote BmNPV replication in silkworm, but the function of BmGRP94 in BmNPV infection and proliferation is still not understood. In this study, we investigated the interplay between BmGRP94 and BmNPV infection in silkworm. We first identified a single gene of BmGRP94 in the Bombyx mori genome, which encodes a polypeptide with 810 amino acids in length. Spatio-temporal expression profiles showed that BmGRP94 was highly expressed in hemocytes and midgut, and was significantly induced by BmNPV infection. Furthermore, overexpression of BmGRP94 facilitates viral proliferation, while BmGRP94 inhibition evidently decreased BmNPV proliferation in BmN cells and in silkworm midgut. Mechanistically, BmGRP94 inhibition triggers ER stress, as judged by increased expression of PERK/ATF4/ERO1, H2O2 production, and ER calcium efflux, which promotes cell apoptosis to restrict BmNPV replication in silkworm. These results suggest that BmGRP94 plays an important role in facilitating BmNPV proliferation, and provides a potential molecular target for BmNPV prevention.