A point mutation in the putative ATP binding site of the pseudorabies virus US3 protein kinase prevents Bad phosphorylation and cell survival following apoptosis induction

Several Alphaherpesviruses Interact Similarly with the NF-κB Pathway and Suppress NF-κB-Dependent Gene Expression

Alphaherpesvirus infection is associated with attenuation of different aspects of the host innate immune response that is elicited to confine primary infections at the mucosal epithelia. Here, we report that infection of epithelial cells with several alphaherpesviruses of different species, including herpes simplex virus 1 and 2 (HSV-1 and HSV-2), feline alphaherpesvirus 1 (FHV-1), and bovine alphaherpesvirus 1 (BoHV-1) results in the inactivation of the responses driven by the nuclear factor kappa B (NF-κB) pathway, considered a pillar of the innate immune response. The mode to interact with and circumvent NF-κB-driven responses in infected epithelial cells is seemingly conserved in human, feline, and porcine alphaherpesviruses, consisting of a persistent activation of the NF-κB cascade but a potent repression of NF-κB-dependent transcription activity, which relies on replication of viral genomes. However, BoHV-1 apparently deviates from the other investigated members of the taxon in this respect, as BoHV-1-infected epithelial cells do not display the persistent NF-κB activation observed for the other alphaherpesviruses. In conclusion, this study suggests that inhibition of NF-κB transcription activity is a strategy used by several alphaherpesviruses to prevent NF-κB-driven responses in infected epithelial cells. IMPORTANCE The current study provides a side-by-side comparison of the interaction of different alphaherpesviruses with NF-κB, a key and central player in the (proinflammatory) innate host response, in infected nontransformed epithelial cell lines. We report that all studied viruses prevent expression of the hallmark NF-κB-dependent gene IκB, often but not always via similar strategies, pointing to suppression of NF-κB-dependent host gene expression in infected epithelial cells as a common and therefore likely important aspect of alphaherpesviruses.

Pseudorabies Virus Regulates the Extracellular Translocation of Annexin A2 To Promote Its Proliferation

Pseudorabies virus (PRV) infection causes enormous economic losses to the pork industry and severe health consequences in many hosts. Annexin A2 (ANXA2) is a membrane-associated protein with various intracellular functions associated with many viral infections. However, the role of ANXA2 in alphaherpesvirus replication is still not explored. In the present study, we identified the interaction between ANXA2 and PRV US3. The deficiency of ANXA2 significantly restricted PRV proliferation. PRV infection or US3 overexpression led to ANXA2 extracellular translocation. Furthermore, we confirmed that PRV or US3 could lead to the phosphorylation of the Tyr23 ANXA2 and Tyr419 Src kinase, which was associated with the ANXA2 cell surface transposition. US3 can also bind to Src in an ANXA2-independent manner and enhance the interaction between Src and ANXA2. Additionally, inhibitors targeting ANXA2 (A2ti-1) or Src (PP2) could remarkably inhibit PRV propagation in vitro and protect mice from PRV infection in vivo. Collectively, our findings broaden our understanding of the molecular mechanisms of ANXA2 in alphaherpesvirus pathogenicity and suggest that ANXA2 is a potential therapeutic target for treating alphaherpesvirus-induced infectious diseases. IMPORTANCE PRV belongs to the alphaherpesvirus and has recently re-emerged in China, causing severe economic losses. Recent studies also indicate that PRV may pose a potential public health challenge. ANXA2 is a multifunctional calcium- and lipid-binding protein implicated in immune function, multiple human diseases, and viral infection. Herein, we found that ANXA2 was essential to PRV efficient proliferation. PRV infection resulted in the extracellular translocation of ANXA2 through phosphorylation of ANXA2 and Src. ANXA2 and Src formed a complex with PRV US3. Importantly, inhibitors targeting ANXA2 or Src prevented PRV infection in vitro and in vivo. Therefore, our studies reveal a novel strategy by which alphaherpesvirus modifies ANXA2 to promote its replication and highlight ANXA2 as a target in developing novel promising antivirus agents in viral therapy.

The relationship between autophagy and apoptosis during pseudorabies virus infection

Both autophagy and apoptosis are mechanisms that maintain homeostasis in cells and that play essential roles in viral infections. Previous studies have demonstrated that autophagy and apoptosis pathways occurred with complex relationships in virus-infected cells. However, the regulation between these two processes in Pseudorabies virus (PRV) infection remains unclear. In the present study, we demonstrated that activated autophagy was induced at the early stage of PRV infection and that apoptosis was induced at the late stage of infection. Autophagy induction inhibited apoptosis and decreased viral replication, and autophagy inhibition promoted apoptosis and increased viral replication. We also found that viral infection resulted in an increase in the production of reactive oxygen species (ROS) and activation of apoptosis in autophagy-impaired cells, suggesting that ROS may participate in the cross-talk between autophagy and apoptosis in PRV-infected cells. Our studies provide possible molecular mechanisms for the cross-talk between apoptosis and autophagy induced by PRV infection in porcine cells. This suggests that these two cell death processes should be considered as the same continuum rather than as completely separate processes.

Alphaherpesvirus US3 protein-mediated inhibition of the m6A mRNA methyltransferase complex

Chemical modifications of mRNA, the so-called epitranscriptome, represent an additional layer of post-transcriptional regulation of gene expression. The most common epitranscriptomic modification, N6-methyladenosine (m6A), is generated by a multi-subunit methyltransferase complex. We show that alphaherpesvirus kinases trigger phosphorylation of several components of the m6A methyltransferase complex, including METTL3, METTL14, and WTAP, which correlates with inhibition of the complex and a near complete loss of m6A levels in mRNA of virus-infected cells. Expression of the viral US3 protein is necessary and sufficient for phosphorylation and inhibition of the m6A methyltransferase complex. Although m6A methyltransferase complex inactivation is not essential for virus replication in cell culture, the consensus m6A methylation motif is under-represented in alphaherpesvirus genomes, suggesting evolutionary pressure against methylation of viral transcripts. Together, these findings reveal that phosphorylation can be associated with inactivation of the m6A methyltransferase complex, in this case mediated by the viral US3 protein.

Pseudorabies Virus Infection Results in a Broad Inhibition of Host Gene Transcription

Pseudorabies virus (PRV) is a porcine alphaherpesvirus that belongs to the Herpesviridae family. We showed earlier that infection of porcine epithelial cells with PRV triggers activation of the nuclear factor κB (NF-κB) pathway, a pivotal signaling axis in the early immune response. However, PRV-induced NF-κB activation does not lead to NF-κB-dependent gene expression. Here, using electrophoretic mobility shift assays (EMSAs), we show that PRV does not disrupt the ability of NF-κB to interact with its κB target sites. Assessing basal cellular transcriptional activity in PRV-infected cells by quantitation of prespliced transcripts of constitutively expressed genes uncovered a broad suppression of cellular transcription by PRV, which also affects the inducible expression of NF-κB target genes. Host cell transcription inhibition was rescued when viral genome replication was blocked using phosphonoacetic acid (PAA). Remarkably, we found that host gene expression shutoff in PRV-infected cells correlated with a substantial retention of the NF-κB subunit p65, the TATA box binding protein, and RNA polymerase II-essential factors required for (NF-κB-dependent) gene transcription-in expanding PRV replication centers in the nucleus and thereby away from the host chromatin. This study reveals a potent mechanism used by the alphaherpesvirus PRV to steer the protein production capacity of infected cells to viral proteins by preventing expression of host genes, including inducible genes involved in mounting antiviral responses. IMPORTANCE Herpesviruses are highly successful pathogens that cause lifelong persistent infections of their host. Modulation of the intracellular environment of infected cells is imperative for the success of virus infections. We reported earlier that a DNA damage response in epithelial cells infected with the alphaherpesvirus pseudorabies virus (PRV) results in activation of the hallmark proinflammatory NF-κB signaling axis but, remarkably, that this activation does not lead to NF-κB-induced (proinflammatory) gene expression. Here, we report that PRV-mediated inhibition of host gene expression stretches beyond NF-κB-dependent gene expression and in fact reflects a broad inhibition of host gene transcription, which correlates with a substantial recruitment of essential host transcription factors in viral replication compartments in the nucleus, away from the host chromatin. These data uncover a potent alphaherpesvirus mechanism to interfere with production of host proteins, including proteins involved in antiviral responses.

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.

Pseudorabies Virus Infection Triggers NF-κB Activation via the DNA Damage Response but Actively Inhibits NF-κB-Dependent Gene Expression

The nuclear factor kappa B (NF-κB) pathway is known to integrate signaling associated with very diverse intra- and extracellular stressors, including virus infections, and triggers a powerful (proinflammatory) response through the expression of NF-κB-regulated genes. Typically, the NF-κB pathway collects and transduces threatening signals at the cell surface or in the cytoplasm leading to nuclear import of activated NF-κB transcription factors. In the current work, we demonstrate that the swine alphaherpesvirus pseudorabies virus (PRV) induces a peculiar mode of NF-κB activation known as "inside-out" NF-κB activation. We show that PRV triggers the DNA damage response (DDR) and that this DDR response drives NF-κB activation since inhibition of the nuclear ataxia telangiectasia-mutated (ATM) kinase, a chief controller of DDR, abolished PRV-induced NF-κB activation. Initiation of the DDR-NF-κB signaling axis requires viral protein synthesis but occurs before active viral genome replication. In addition, the initiation of the DDR-NF-κB signaling axis is followed by a virus-induced complete shutoff of NF-κB-dependent gene expression that depends on viral DNA replication. In summary, the results presented in this study reveal that PRV infection triggers a noncanonical DDR-NF-κB activation signaling axis and that the virus actively inhibits the (potentially antiviral) consequences of this pathway, by inhibiting NF-κB-dependent gene expression. IMPORTANCE The NF-κB signaling pathway plays a critical role in coordination of innate immune responses that are of vital importance in the control of infections. The current report generates new insights into the interaction of the alphaherpesvirus pseudorabies virus (PRV) with the NF-κB pathway, as they reveal that (i) PRV infection leads to NF-κB activation via a peculiar "inside-out" nucleus-to-cytoplasm signal that is triggered via the DNA damage response (DDR), (ii) the DDR-NF-κB signaling axis requires expression of viral proteins but is initiated before active PRV replication, and (iii) late viral factor(s) allow PRV to actively and efficiently inhibit NF-κB-dependent (proinflammatory) gene expression. These data suggest that activation of the DDR-NF-κB during PRV infection is host driven and that its potential antiviral consequences are actively inhibited by the virus.

Pseudorabies Virus US3 Protein Inhibits IFN-β Production by Interacting With IRF3 to Block Its Activation

Pseudorabies virus is a typical swine alphaherpesvirus, which can cause obvious neurological disorders and reproductive failure in pigs. It is capable of evading host antiviral immune response. However, the mechanism by which many PRV proteins assist the virus to evade innate immunity is not fully understood. This study identified PRV US3 protein as a crucial antagonistic viral factor that represses interferon beta (IFN-β) expression. A in-depth study showed that US3 protein restricted type I IFN production by targeting interferon regulatory factor 3 (IRF3), a key molecule required for type I IFN induction. Additionally, US3 protein interacted with IRF3, degraded its protein expression to block the phosphorylation of IRF3. These findings suggested a novel strategy utilized by PRV to inhibit IFN-β production and escape the host innate immunity.

An Unbiased Approach to Mapping the Signaling Network of the Pseudorabies Virus US3 Protein

The US3 serine/threonine protein kinase is conserved among the alphaherpesvirus family and represents an important virulence factor. US3 plays a role in viral nuclear egress, induces dramatic alterations of the cytoskeleton, represses apoptosis, enhances gene expression and modulates the immune response. Although several substrates of US3 have been identified, an unbiased screen to identify US3 phosphorylation targets has not yet been described. Here, we perform a shotgun and phosphoproteomics analysis of cells expressing the US3 protein of pseudorabies virus (PRV) to identify US3 phosphorylation targets in an unbiased way. We identified several cellular proteins that are differentially phosphorylated upon US3 expression and validated the phosphorylation of lamin A/C at serine 404, both in US3-transfected and PRV-infected cells. These results provide new insights into the signaling network of the US3 protein kinase and may serve as a basis for future research into the role of the US3 protein in the viral replication cycle.

Pseudorabies Virus Infection of Epithelial Cells Leads to Persistent but Aberrant Activation of the NF-κB Pathway, Inhibiting Hallmark NF-κB-Induced Proinflammatory Gene Expression

The nuclear factor kappa B (NF-κB) is a potent transcription factor, activation of which typically results in robust proinflammatory signaling and triggering of fast negative feedback modulators to avoid excessive inflammatory responses. Here, we report that infection of epithelial cells, including primary porcine respiratory epithelial cells, with the porcine alphaherpesvirus pseudorabies virus (PRV) results in the gradual and persistent activation of NF-κB, illustrated by proteasome-dependent degradation of the inhibitory NF-κB regulator IκB and nuclear translocation and phosphorylation of the NF-κB subunit p65. PRV-induced persistent activation of NF-κB does not result in expression of negative feedback loop genes, like the gene for IκBα or A20, and does not trigger expression of prototypical proinflammatory genes, like the gene for tumor necrosis factor alpha (TNF-α) or interleukin-6 (IL-6). In addition, PRV infection inhibits TNF-α-induced canonical NF-κB activation. Hence, PRV infection triggers persistent NF-κB activation in an unorthodox way and dramatically modulates the NF-κB signaling axis, preventing typical proinflammatory gene expression and the responsiveness of cells to canonical NF-κB signaling, which may aid the virus in modulating early proinflammatory responses in the infected host.IMPORTANCE The NF-κB transcription factor is activated via different key inflammatory pathways and typically results in the fast expression of several proinflammatory genes as well as negative feedback loop genes to prevent excessive inflammation. In the current report, we describe that infection of cells with the porcine alphaherpesvirus pseudorabies virus (PRV) triggers a gradual and persistent aberrant activation of NF-κB, which does not result in expression of hallmark proinflammatory or negative feedback loop genes. In addition, although PRV-induced NF-κB activation shares some mechanistic features with canonical NF-κB activation, it also shows remarkable differences; e.g., it is largely independent of the canonical IκB kinase (IKK) and even renders infected cells resistant to canonical NF-κB activation by the inflammatory cytokine TNF-α. Aberrant PRV-induced NF-κB activation may therefore paradoxically serve as a viral immune evasion strategy and may represent an important tool to unravel currently unknown mechanisms and consequences of NF-κB activation.

Evaluation of the anti-apoptotic activity of bovine alphaherpesvirus type 5 US3 protein kinase in insect cells using a recombinant baculovirus

Bovine alphaherpesvirus type 5 (BoHV-5) is one of the main agents responsible for meningoencephalitis in cattle in Brazil, causing significant economic losses. It is known that other viruses of the Herpesviridae family such as Bovine alphaherpesvirus type 1, Swine alphaherpesvirus type 1, and the Human alphaherpesvirus types 1 and 2 encode genes homologous to BoHV-5, with recognized action in the control of apoptosis. The objective of this work was to express the BoHV-5 US3 gene in a baculovirus-based expression system for the production of the serine/threonine kinase protein and to evaluate its activity in the control of apoptosis in vitro. A recombinant baculovirus derived from the Autographa californica multiple nucleopolyhedrovirus (AcMNPV) containing the US3 gene and a deletion in the baculovirus anti-apoptotic p35 gene was constructed using the Bac-to-Bac™ system. This recombinant baculovirus was used to evaluate the anti-apoptotic activity of the recombinant US3 protein in insect cells comparing with two other AcMNPV recombinants, one containing a functional copy of the AcMNPV anti-apoptotic p35 gene and an AcMNPV p35 knockout virus with the anti-apoptotic iap-3 gene from Anticarsia gemmatalis multiple nucleopolyhedrovirus (AgMNPV). We found that the caspase level was higher in insect cells infected with the US3-contanining recombinant virus than in cells infected with the AcMNPV recombinants containing the p35 and iap-3 genes. These results indicate that the BoHV-5 US3 protein kinase gene is not able to block apoptosis in insect cells induced by the infection of a p35 knockout AcMNPV.

Genomic analysis of a Chinese MDV strain derived from vaccine strain CVI988 through recombination

Disease caused by Marek's disease virus (MDV), a highly oncogenic alpha-herpesvirus, is controlled mainly by vaccination. Since 1990s, CVI988 has been widely used as vaccine strain. However, as an attenuated live vaccine, CVI988 has the potential of virulence enhancement and the risk of recombination that should be considered. In this study, we sequenced the whole genome of a Chinese strain HNLC503 and found the close relationship between HNLC503 and CVI988. Further study indicated that HNLC503 had undergone recombination in US region, the same position as that previously occurred in Eurasian strains isolated from 2010 to 2014. By comparing ORFs, it was found that non-synonymous mutations were introduced in US2, US3, SORF4 and gD genes by recombination, while natural mutations occurred in RLORF1, vIL-8, UL36, VP22 and gE, in HNLC503. In summary, our study revealed the phenomenon of MDV vaccine strain recombination, warning that vaccine strains have the potential to enhance virulence through recombination.

Inhibition of apoptosis in BHV-1-infected cells depends on Us3 serine/threonine kinase and its enzymatic activity

Us3 protein is a serine/threonine kinase conserved within the Alphaherpesvirinae subfamily of herpesviruses. The Us3 homologs of herpes simplex virus, pseudorabies virus, and bovine herpesvirus type 5 have been shown to block apoptosis triggered by viral infection or exogenous inducers. To determine whether these characteristics are shared by bovine herpesvirus type 1 Us3, we constructed two viral mutants: BHV-1 Us3 deletion mutant (BHV-1ΔUs3) and a kinase-dead mutant (BHV-1KD). Flow cytometry analysis and TUNEL assay clearly demonstrated, that only BHV-1 wild type virus suppressed infection-induced apoptosis and protected cells from apoptosis triggered by exogenous factors: sorbitol or staurosporine. Us3 of BHV-1 was directly capable of blocking apoptosis without the presence of other viral proteins. The presence of Us3 correlated with phosphorylation of BAD, a pro-apoptotic Bcl-2 family member. Our results clearly indicate that BHV-1 Us3 is necessary for efficient blocking of apoptosis triggered by viral infection and exogenous factors.

Pseudorabies Virus US3-Induced Tunneling Nanotubes Contain Stabilized Microtubules, Interact with Neighboring Cells via Cadherins, and Allow Intercellular Molecular Communication

Tunneling nanotubes (TNTs) are long bridge-like structures that connect eukaryotic cells and mediate intercellular communication. We found earlier that the conserved alphaherpesvirus US3 protein kinase induces long cell projections that contact distant cells and promote intercellular virus spread. In this report, we show that the US3-induced cell projections constitute TNTs. In addition, we report that US3-induced TNTs mediate intercellular transport of information (e.g., green fluorescent protein [GFP]) in the absence of other viral proteins. US3-induced TNTs are remarkably stable compared to most TNTs described in the literature. In line with this, US3-induced TNTs were found to contain stabilized (acetylated and detyrosinated) microtubules. Transmission electron microscopy showed that virus particles are individually transported in membrane-bound vesicles in US3-induced TNTs and are released along the TNT and at the contact area between a TNT and the adjacent cell. Contact between US3-induced TNTs and acceptor cells is very stable, which correlated with a marked enrichment in adherens junction components beta-catenin and E-cadherin at the contact area. These data provide new structural insights into US3-induced TNTs and how they may contribute to intercellular communication and alphaherpesvirus spread.IMPORTANCE Tunneling nanotubes (TNT) represent an important and yet still poorly understood mode of long-distance intercellular communication. We and others reported earlier that the conserved alphaherpesvirus US3 protein kinase induces long cellular protrusions in infected and transfected cells. Here, we show that US3-induced cell projections constitute TNTs, based on structural properties and transport of biomolecules. In addition, we report on different particular characteristics of US3-induced TNTs that help to explain their remarkable stability compared to physiological TNTs. In addition, transmission electron microscopy assays indicate that, in infected cells, virions travel in the US3-induced TNTs in membranous transport vesicles and leave the TNT via exocytosis. These data generate new fundamental insights into the biology of (US3-induced) TNTs and into how they may contribute to intercellular virus spread and communication.

The suppression of apoptosis by α-herpesvirus

Apoptosis, an important innate immune mechanism that eliminates pathogen-infected cells, is primarily triggered by two signalling pathways: the death receptor pathway and the mitochondria-mediated pathway. However, many viruses have evolved various strategies to suppress apoptosis by encoding anti-apoptotic factors or regulating apoptotic signalling pathways, which promote viral propagation and evasion of the host defence. During its life cycle, α-herpesvirus utilizes an elegant multifarious anti-apoptotic strategy to suppress programmed cell death. This progress article primarily focuses on the current understanding of the apoptosis-inhibition mechanisms of α-herpesvirus anti-apoptotic genes and their expression products and discusses future directions, including how the anti-apoptotic function of herpesvirus could be targeted therapeutically.

The Pseudorabies Virus Glycoprotein gE/gI Complex Suppresses Type I Interferon Production by Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDC) play a central role in the antiviral immune response, both in the innate response and in shaping the adaptive response, mainly because of their ability to produce massive amounts of type I interferon (TI-IFN). Here, we report that cells infected with the live attenuated Bartha vaccine strain of porcine alphaherpesvirus pseudorabies virus (PRV) trigger a dramatically increased TI-IFN response by porcine primary pDC compared to cells infected with wild-type PRV strains (Becker and Kaplan). Since Bartha is one of the relatively few examples of a highly successful alphaherpesvirus vaccine, identification of factors that may contribute to its efficacy may provide insights for the rational design of other alphaherpesvirus vaccines. The Bartha vaccine genome displays several mutations compared to the genome of wild-type PRV strains, including a large deletion in the unique short (US) region, encompassing the glycoprotein E (gE), gI, US9, and US2 genes. Using recombinant PRV Becker strains harboring the entire Bartha US deletion or single mutations in the four affected US genes, we demonstrate that the absence of the viral gE/gI complex contributes to the observed increased IFN-α response. Furthermore, we show that the absence of gE leads to an enhanced extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in pDC, which correlates with a higher TI-IFN production by pDC. In conclusion, the PRV Bartha vaccine strain triggers strongly increased TI-IFN production by porcine pDC. Our data further indicate that the gE/gI glycoprotein complex suppresses TI-IFN production by pDC, which represents the first alphaherpesvirus factor that suppresses pDC activity.IMPORTANCE Several alphaherpesviruses, including herpes simpex virus, still lack effective vaccines. However, the highly successful Bartha vaccine has contributed substantially to eradication of the porcine alphaherpesvirus pseudorabies virus (PRV) in several countries. The impact of Bartha on the immune response is still poorly understood. Type I interferon (TI-IFN)-producing plasmacytoid dendritic cells (pDC) may play an important role in vaccine development. Here, we show that Bartha elicits a dramatically increased type I interferon (TI-IFN) response in primary porcine pDC compared to wild-type strains. In addition, we found that the gE/gI complex, which is absent in Bartha, inhibits the pDC TI-IFN response. This is the first description of an immune cell type that is differentially affected by Bartha versus wild-type PRV and is the first report describing an alphaherpesvirus protein that inhibits the TI-IFN response by pDC. These data may therefore contribute to the rational design of other alphaherpesvirus vaccines.

Pseudorabies Virus US3 Protein Kinase Protects Infected Cells from NK Cell-Mediated Lysis via Increased Binding of the Inhibitory NK Cell Receptor CD300a

Several reports have indicated that natural killer (NK) cells are of particular importance in the innate response against herpesvirus infections. As a consequence, herpesviruses have developed diverse mechanisms for evading NK cells, although few such mechanisms have been identified for the largest herpesvirus subfamily, the alphaherpesviruses. The antiviral activity of NK cells is regulated by a complex array of interactions between activating/inhibitory receptors on the NK cell surface and the corresponding ligands on the surfaces of virus-infected cells. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) displays previously uncharacterized immune evasion properties: it triggers the binding of the inhibitory NK cell receptor CD300a to the surface of the infected cell, thereby providing increased CD300a-mediated protection of infected cells against NK cell-mediated lysis. US3-mediated CD300a binding was found to depend on aminophospholipid ligands of CD300a and on group I p21-activated kinases. These data identify a novel alphaherpesvirus strategy for evading NK cells and demonstrate, for the first time, a role for CD300a in regulating NK cell activity upon contact with virus-infected target cells.

IMPORTANCE

Herpesviruses have developed fascinating mechanisms to evade elimination by key elements of the host immune system, contributing to their ability to cause lifelong infections with recurrent reactivation events. Natural killer (NK) cells are central in the innate antiviral response. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus displays a previously uncharacterized capacity for evasion of NK cells. Expression of US3 protects infected cells from NK cell-mediated lysis via increased binding of the inhibitory NK cell receptor CD300a. We show that this US3-mediated increase in CD300a binding depends on aminophospholipids and on cellular p21-activated kinases (PAKs). The identification of this novel NK cell evasion strategy may contribute to the design of improved herpesvirus vaccines and may also have significance for other PAK- and CD300a-modulating viruses and cancer cells.

Pseudorabies virus US3 triggers RhoA phosphorylation to reorganize the actin cytoskeleton

The conserved alphaherpesvirus serine/threonine kinase US3 causes dramatic changes in the actin cytoskeleton, consisting of actin stress fibre breakdown and protrusion formation, associated with increased virus spread. Here, we showed that US3 expression led to RhoA phosphorylation at serine 188 (S188), one of the hallmarks of suppressed RhoA signalling, and that expression of a non-phosphorylatable RhoA variant interfered with the ability of US3 to induce actin rearrangements. Furthermore, inhibition of cellular protein kinase A (PKA) eliminated the ability of US3 to induce S188 RhoA phosphorylation, pointing to a role for PKA in US3-induced RhoA phosphorylation. Hence, the US3 kinase leads to PKA-dependent S188 RhoA phosphorylation, which contributes to US3-mediated actin rearrangements. Our data suggest that US3 efficiently usurps the antagonistic RhoA and Cdc42/Rac1/p21-activated kinase signalling branches to rearrange the actin cytoskeleton.

Pseudorabies virus triggers glycoprotein gE-mediated ERK1/2 activation and ERK1/2-dependent migratory behavior in T cells

The interaction between viruses and immune cells of the host may lead to modulation of intracellular signaling pathways and to subsequent changes in cellular behavior that are of benefit for either virus or host. ERK1/2 (extracellular signal regulated kinase 1/2) signaling represents one of the key cellular signaling axes. Here, using wild-type and gE null virus, recombinant gE, and gE-transfected cells, we show that the gE glycoprotein of the porcine Varicellovirus pseudorabies virus (PRV) triggers ERK1/2 phosphorylation in Jurkat T cells and primary porcine T lymphocytes. PRV-induced ERK1/2 signaling resulted in homotypic T cell aggregation and increased motility of T lymphocytes. Our study reveals a new function of the gE glycoprotein of PRV and suggests that PRV, through activation of ERK1/2 signaling, has a substantial impact on T cell behavior.

IMPORTANCE

Herpesviruses are known to be highly successful in evading the immune system of their hosts, subverting signaling pathways of the host to their own advantage. The ERK1/2 signaling pathway, being involved in many cellular processes, represents a particularly attractive target for viral manipulation. Glycoprotein E (gE) is an important virulence factor of alphaherpesviruses, involved in viral spread. In this study, we show that gE has the previously uncharacterized ability to trigger ERK1/2 phosphorylation in T lymphocytes. We also show that virus-induced ERK1/2 signaling leads to increased migratory behavior of T cells and that migratory T cells can spread the infection to susceptible cells. In conclusion, our results point to a novel function for gE and suggest that virus-induced ERK1/2 activation may trigger PRV-carrying T lymphocytes to migrate and infect other cells susceptible to PRV replication.

Suppression of apoptosis by pseudorabies virus Us3 protein kinase through the activation of PI3-K/Akt and NF-κB pathways

The pseudorabies virus (PRV) is a major viral disease that causes huge economic loss in the pig industry globally. Most viruses have been found to generate anti-apoptotic factors that facilitate cell survival in the early stages of infection. This study aimed to investigate the anti-apoptotic effects of PRV and study the underlying mechanisms in the early stage of infection. We investigated and compared whether the two PRV Us3 isoforms, Us3a and Us3b, could block apoptosis induced by virus infection, and further identified molecules involved in the signaling pathways. Our results demonstrated that PRV elicits 3-phosphoinositide dependent protein kinase-1/phosphatidylinositide 3-kinases/Akt (PDK-1/PI3-K/Akt)- and nuclear factor-κB (NF-κB)-dependent signaling in the early stage of infection. Inhibition of the PI3-K/Akt or NF-κB pathway enhanced cell death but no effect was observed on virus replication or PRV gene expression. Transiently-expressed GFP- or His-tagged PRV Us3a and Us3b cDNA protect cells against PRV-, avian reovirus- or bovine ephemeral fever virus-induced apoptosis in the cell lines. Us3a and Us3b transient over-expression upregulated several anti-apopototic signaling events, and the anti-apoptosis activity of Us3a is greater than that of Us3b. Kinase activity-deficient point or double point mutated Us3a lost the kinase activity of Us3a, which showed that kinase activity is required for the anti-apoptosis effect of Us3. Akt and NF-κB activation still occurred in UV-inactivated PRV- and cycloheximide-treated cells. In vivo study showed that PRV-infected trigeminal ganglion increases the expression of anti-apoptosis signaling molecules, including Akt, PDK-1 and IκBα, which is a similar result to that seen in the in vitro experiments. Our study suggests that signaling mechanisms may play important roles in PRV pathogenesis.

Herpes simplex virus US3 protein kinase regulates host responses and determines neurovirulence

The US3 of HSV encodes a serine/threonine protein kinase that is highly conserved among members of the alphaherpesviruses. It is an accessory gene that is not required for viral replication in cultured cells but appears essential for viral survival in humans. Although accumulating in vitro evidence suggested that the viral protein kinase is multifunctional, little information is available about its functions in vivo. Several reports point out that, upon invasion into the peripheral nervous system, HSV blocks virus-induced neuronal apoptosis, while presumably subverting host immune responses, largely through actions of the US3 protein kinase. In addition, the US3 protein kinase confers the viral neurovirulence. In the present article, functions of the HSV US3 protein kinase are briefly reviewed, with special attention given to its role in regulating host responses and neurovirulence.

The interaction between viral protein and host actin facilitates the virus infection to host

Although the virus-host interaction has attracted extensive studies, the host proteins essential for virus infection remain largely unknown. To address this issue, the shrimp Penaeus stylirostris densovirus (PstDNV), belonging to the family Parvoviridae, was characterized. PstDNV, a single-stranded DNA virus with a 3.9-kb genome, encoded only three open reading frames (ORFs). Among the three viral proteins, the PstDNV ORF2-encoded protein was discovered to interact with the shrimp actin, suggesting that the host actin played a very important role in virus infection. The RNAi assays revealed that the ORF2-encoded protein was required for the PstDNV infection. The confocal evidence demonstrated that the interaction between the ORF2-encoded protein and actin was essential for the virus infection. Therefore our study indicated that the manipulation of the host actin cytoskeleton was a necessary strategy for viral pathogens to invade host cells.

The IE180 protein of pseudorabies virus suppresses phosphorylation of translation initiation factor eIF2α

We have previously shown that the porcine alphaherpesvirus pseudorabies virus (PRV) efficiently interferes with phosphorylation of the eukaryotic translation initiation factor eIF2α. Inhibition of phosphorylation of eIF2α has been reported earlier for the closely related alphaherpesvirus herpes simplex virus 1 (HSV-1) through its ICP34.5 and US11 proteins. PRV, however, does not encode an ICP34.5 or US11 orthologue. Assays using cycloheximide, UV-inactivated PRV, or phosphonoacetic acid (PAA) showed that de novo expression of one or more (immediate) early viral protein(s) is required for interference with eIF2α phosphorylation. In line with this, a time course assay showed that eIF2α phosphorylation was abolished within 2 h after PRV inoculation. PRV encodes only one immediate-early protein, IE180, the orthologue of HSV-1 ICP4. As reported earlier, a combinational treatment of cells with cycloheximide and actinomycin D allowed expression of IE180 without detectable expression of the US3 early protein in PRV-infected cells. This led to a substantial reduction in eIF2α phosphorylation levels, indicative for an involvement of IE180. In support of this, transfection of IE180 also potently reduced eIF2α phosphorylation. IE180-mediated interference with eIF2α phosphorylation was not cell type dependent, as it occurred both in rat neuronal 50B11 cells and in swine testicle cells. Inhibition of the cellular phosphatase PP1 impaired PRV-mediated interference with eIF2α phosphorylation, indicating that PP1 is involved in this process. In conclusion, the immediate-early IE180 protein of PRV has the previously uncharacterized ability to suppress phosphorylation levels of the eukaryotic translation initiation factor eIF2α.

Effect of the US3 protein of bovine herpesvirus 5 on the actin cytoskeleton and apoptosis

The US3 protein is a unique protein kinase only present in the Alphaherpesvirinae subfamily of the herpesviruses. Studies performed with several alphaherpesviruses demonstrated that the US3 protein is involved in cytoskeleton modifications during viral infection and displays anti-apoptotic activity. However, the US3 protein of BoHV-5 has not been studied up to now. As reported for other alphaherpesviruses, our results showed that BoHV-5 US3 confers resistance against apoptosis and induces cytoskeletal reorganization leading to cell rounding, actin stress fiber breakdown and cell projections that interconnect cells. The expression of a kinase-dead version of BoHV-5 US3 showed that the anti-apoptotic activity and the induction of cell projections are kinase-dependent whereas kinase activity is not absolutely required for actin stress fiber breakdown. Besides, the kinase-dead version of US3, but not the wild type protein, was found excluded from the nucleus. These results constitute the first report on the BoHV-5 US3 functions, and highlight that there are functional differences and similarities among US3 proteins of different alphaherpesviruses.

Effects of interferon on immediate-early mRNA and protein levels in sensory neuronal cells infected with herpes simplex virus type 1 or pseudorabies virus

Most alphaherpesviruses are able to establish latency in sensory neurons and reactivate upon specific stimuli to cause recurrent symptoms. We have previously shown that interferon (IFN) is capable of inducing a quiescent HSV-1 and PRV infection that strongly resembles in vivo latency in primary cultures of TG neurons. This IFN-induced latency-like quiescence was found to correlate with suppression of the immediate-early protein ICP4 in HSV-1 and its ortholog IE180 in PRV. Here, we mechanistically investigated the IFN-mediated suppression of ICP4 and IE180 in sensory neuronal cells. RT-qPCR showed that mRNA levels of either HSV ICP4 or PRV IE180 at 4 hpi were mildly but not significantly different in IFN-treated samples versus control samples, whereas a strong reduction was observed at 8 hpi and 12 hpi. However, at 4 hpi, HSV ICP4 but not PRV IE180 protein expression was already markedly reduced in IFN-treated samples. In line with this difference in IFN-mediated suppression of HSV ICP4 versus PRV IE180 protein levels, we found that IFN resulted in an increase in phosphorylation of the translation initiation factor eIF2α in HSV-infected but not in PRV-infected cells. The latter finding indicates that PRV efficiently circumvents IFN-mediated translation inhibition by interfering with phosphorylation of eIF2α.

Role of group A p21-activated kinases in the anti-apoptotic activity of the pseudorabies virus US3 protein kinase

The alphaherpesvirus US3 kinase is a conserved multifunctional serine/threonine kinase that plays a role in several processes, including modulation of the actin cytoskeleton, egress of virus particles from the nucleus and inhibition of apoptosis. However, the mechanisms used by the US3 protein to exert its functions remain poorly understood. Recently, we identified the group A p21-activated kinases PAK1 and PAK2 as important effectors in the US3-mediated cytoskeletal rearrangements. Here, we investigated if group A PAKs are also involved in the anti-apoptotic properties of US3. Infection experiments using a group A PAK inhibitor pointed at a moderate role for group A PAKs in the anti-apoptotic properties of US3. Furthermore, infection assays using wild type and US3null PRV in wild type MEF, PAK1(-/-) MEF and PAK2(-/-) MEF indicated that PAK2 does not play a role in US3-mediated inhibition of apoptosis during infection, whereas PAK1 plays a significant, yet limited role. Experiments in US3-transfected MEF using staurosporine as apoptosis trigger confirmed these observations. These results show that PAK1 plays a significant, yet limited, role in the anti-apoptotic activity of US3.

Viral serine/threonine protein kinases

Phosphorylation represents one the most abundant and important posttranslational modifications of proteins, including viral proteins. Virus-encoded serine/threonine protein kinases appear to be a feature that is unique to large DNA viruses. Although the importance of these kinases for virus replication in cell culture is variable, they invariably play important roles in virus virulence. The current review provides an overview of the different viral serine/threonine protein kinases of several large DNA viruses and discusses their function, importance, and potential as antiviral drug targets.

Keep it in the subfamily: the conserved alphaherpesvirus US3 protein kinase

The US3 protein kinase is conserved over the alphaherpesvirus subfamily. Increasing evidence shows that, although the kinase is generally not required for virus replication in cell culture, it plays a pivotal and in some cases an essential role in virus virulence in vivo. The US3 protein is a multifunctional serine/threonine kinase that is involved in viral gene expression, virion morphogenesis, remodelling the actin cytoskeleton and the evasion of several antiviral host responses. In the current review, both the well conserved and virus-specific functions of alphaherpesvirus US3 protein kinase orthologues will be discussed.

Point mutations in BHV-1 Us3 gene abolish its ability to induce cytoskeletal changes in various cell types

The Us3 gene is conserved among alphaherpesviruses and codes for a protein kinase, a multifunctional protein involved in many phases of virus infection, like nuclear egress, modulation of apoptosis and modification of the cellular cytoskeleton. Bovine herpesvirus (BHV-1), a member of the Alphaherpesvirinae, contains an open reading frame homologous to Us3 of other herpesviruses, which has been identified as a serine/threonine kinase (Takashima, Y., Tamura, H., Xuan, X., Otsuka, H., 1999. Identification of the Us3 gene product of BHV-1 as a protein kinase and characterization of BHV-1 mutants of the Us3 gene. Virus Res. 59, 23-34). To study the activity of BHV-1 Us3, we have cloned its sequence under control of the human cytomegalovirus (HCMV) promoter/enhancer and introduced it into a recombinant baculovirus (Bac Us3). Confocal microscopy analysis showed profound cytoskeletal modifications in various BHV-1-permissive and non-permissive cells transduced with BacUs3. We observed that Us3 expression changed cellular shape and induced formation of long microtubule-containing cell projections, a phenomenon which had also been observed in cells expressing pseudorabies virus Us3. The intracellular localization of Us3 was mostly nuclear but when the protein accumulated it could be detected in the cytoplasm, cell membranes and projections. Mutated forms of BHV-1 Us3 with point mutations near or within the kinase catalytic domain did not affect cell morphology indicating that kinase activity of BHV-1 Us3 is required for its cytoskeleton remodelling function.

Pseudorabies virus US3-mediated inhibition of apoptosis does not affect infectious virus production

Preventing apoptosis during the early stages of infection of a host cell is generally thought to result in a higher yield of progeny virus. The US3 protein kinase of pseudorabies virus (PRV) and herpes simplex virus (HSV) is able to protect infected cells from apoptosis, which may be one of the reasons why both US3null PRV and US3null HSV replicate to lower virus titres in several cell types. However, such potential correlation between the higher amount of apoptosis in US3null virus-infected cells and the lower virus titres of US3null virus has not been investigated directly. In the current study, we found that a broad-spectrum caspase-inhibitor efficiently inhibited apoptosis in swine testicle and human laryngeal epidermoid carcinoma cells infected with US3null or wild-type (WT) PRV. However, inhibition of apoptosis did not affect US3null or WT PRV extracellular or cell-associated virus titres, nor did it restore the small plaque phenotype of US3null PRV.

Antiviral effect of diammonium glycyrrhizinate and lithium chloride on cell infection by pseudorabies herpesvirus

Diammonium glycyrrhizin (DG), a salt from glycyrrhizinate (GL) that is a major active component of licorice root extract with various pharmacological activities was investigated for its inhibitory effect on pseudorabies virus (PrV) infection. In parallel, lithium chloride (LiCl), a chemical reagent with potential antiviral activity was compared with DG for their inhibitory ability against PrV infection in vitro. Virus plaque-reduction assays, PCR and RT-PCR analysis indicated that both drugs inhibited cell infection by PrV. Moreover, addition of the drugs resulted in fewer apoptotic cells during PrV infection.

Intracellular localization of the pseudorabies virus large tegument protein pUL36

Homologs of the essential large tegument protein pUL36 of herpes simplex virus 1 are conserved throughout the Herpesviridae, complex with pUL37, and form part of the capsid-associated "inner" tegument. pUL36 is crucial for transport of the incoming capsid to and docking at the nuclear pore early after infection as well as for virion maturation in the cytoplasm. Its extreme C terminus is essential for pUL36 function interacting with pUL25 on nucleocapsids to start tegumentation (K. Coller, J. Lee, A. Ueda, and G. Smith, J. Virol. 81:11790-11797, 2007). However, controversy exists about the cellular compartment in which pUL36 is added to the nascent virus particle. We generated monospecific rabbit antisera against four different regions spanning most of pUL36 of the alphaherpesvirus pseudorabies virus (PrV). By immunofluorescence and immunoelectron microscopy, we then analyzed the intracellular location of pUL36 after transient expression and during PrV infection. While reactivities of all four sera were comparable, none of them showed specific intranuclear staining during PrV infection. In immunoelectron microscopy, neither of the sera stained primary enveloped virions in the perinuclear cleft, whereas extracellular mature virus particles were extensively labeled. However, transient expression of pUL36 alone resulted in partial localization to the nucleus, presumably mediated by nuclear localization signals (NLS) whose functionality was demonstrated by fusion of the putative NLS to green fluorescent protein (GFP) and GFP-tagged pUL25. Since PrV pUL36 can enter the nucleus when expressed in isolation, the NLS may be masked during infection. Thus, our studies show that during PrV infection pUL36 is not detectable in the nucleus or on primary enveloped virions, correlating with the notion that the tegument of mature virus particles, including pUL36, is acquired in the cytosol.

Alphaherpesvirus US3-mediated reorganization of the actin cytoskeleton is mediated by group A p21-activated kinases

The US3 protein is a viral serine/threonine kinase that is conserved among all members of the Alphaherpesvirinae. The US3 protein of different alphaherpesviruses causes dramatic alterations in the actin cytoskeleton, such as the disassembly of actin stress fibers and formation of cell projections, which have been associated with increased intercellular virus spread. Here, we find that inhibiting group A p21-activated kinases (PAKs), which are key regulators in Cdc42/Rac1 Rho GTPase signaling pathways, impairs US3-mediated actin alterations. By using PAK1(-/-) and PAK2(-/-) mouse embryo fibroblasts (MEFs), we show that US3-mediated stress fiber disassembly requires PAK2, whereas US3-mediated cell projection formation mainly is mediated by PAK1, also indicating that PAK1 and PAK2 can have different biological effects on the organization of the actin cytoskeleton. In addition, US3 was found to bind and phosphorylate group A PAKs. Lack of group A PAKs in MEFs was correlated with inefficient virus spread. Thus, US3 induces its effect on the actin cytoskeleton via group A PAKs.

The kinase activity of pseudorabies virus US3 is required for modulation of the actin cytoskeleton

Different viruses exploit the host cytoskeleton to facilitate replication and spread. The conserved US3 protein of the alphaherpesvirus pseudorabies virus induces actin stress fiber disassembly and formation of actin-containing cell projections, which are associated with enhanced intercellular virus spread. Proteins of members of other virus families, notably vaccinia virus F11L protein and human immunodeficiency virus Nef protein, induce actin rearrangements that are very similar to those induced by US3. Interestingly, unlike F11L and Nef, the US3 protein displays serine/threonine kinase activity. Here, we report that the kinase activity of pseudorabies virus US3 is absolutely required for its actin modulating activity. These data show that different viruses have developed independent mechanisms to induce very similar actin rearrangements.

Varicella-zoster virus open reading frame 66 protein kinase is required for efficient viral growth in primary human corneal stromal fibroblast cells

Varicella-zoster virus (VZV) open reading frame 66 (ORF66) encodes a serine/threonine protein kinase that is not required for VZV growth in most cell types but is needed for efficient growth in T cells. The ORF66 kinase affects nuclear import and virion packaging of IE62, the major regulatory protein, and is known to regulate apoptosis in T cells. Here, we further examined the importance of ORF66 using VZV recombinants expressing green fluorescent protein (GFP)-tagged functional and kinase-negative ORF66 proteins. VZV virions with truncated or kinase-inactivated ORF66 protein were marginally reduced for growth and progeny yields in MRC-5 fibroblasts but were severely growth and replication impaired in low-passage primary human corneal stromal fibroblasts (PCF). To determine if the growth impairment was due to ORF66 kinase regulation of IE62 nuclear import, recombinant VZVs that expressed IE62 with alanine residues at S686, the suspected target by which ORF66 kinase blocks IE62 nuclear import, were made. IE62 S686A expressed by the VZV recombinant remained nuclear throughout infection and was not packaged into virions. However, the mutant virus still replicated efficiently in PCF cells. We also show that inactivation of the ORF66 kinase resulted in only marginally increased levels of apoptosis in PCF cells, which could not fully account for the cell-specific growth requirement of ORF66 kinase. Thus, the unique short region VZV kinase has important cell-type-specific functions that are separate from those affecting IE62 and apoptosis.

Enzymatically inactive U(S)3 protein kinase of Marek's disease virus (MDV) is capable of depolymerizing F-actin but results in accumulation of virions in perinuclear invaginations and reduced virus growth

Marek's disease (MD) is a highly contagious, lymphoproliferative disease of chickens caused by the cell-associated MD virus (MDV), a member of the alphaherpesvirus subfamily. In a previous study we showed that the absence of the serine/threonine protein kinase (pU(S)3) encoded in the MDV unique-short region resulted in accumulation of primarily enveloped virions in the perinuclear space and significant impairment of virus growth in vitro. It was also shown that pU(S)3 is involved in actin stress fiber breakdown [Schumacher, D., Tischer, B. K., Trapp, S., and Osterrieder, N. (2005). Here, we constructed a recombinant virus to test the importance of pU(S)3 kinase activity for MDV replication and its functions in actin rearrangement. Disruption of the kinase active site was achieved by substituting a lysine at position 220 with an alanine (K220A). Titers of a kinase-negative MDV mutant, 20U(S)3()K220A, were reduced when compared to parental virus similar to those of the U(S)3 deletion mutant. We were also able to demonstrate complete absence of phosphorylation of MDV-specific phosphoprotein pp38 in cells infected with the kinase-deficient virus, indicating that pp38 phosphorylation depends entirely on the kinase activity of pU(S)3. Enzymatically inactive pU(S)3()K220A was, however, still capable of mediating breakdown of the actin cytoskeleton in transfection studies, and this activity was indistinguishable from that of wild-type pU(S)3(). Furthermore, we demonstrated that pU(S)3 possesses anti-apoptotic activity, which is dependent on its kinase activity. Taken together, our results demonstrate that pU(S)3 and MDV-specific phosphoprotein pp38 represent a kinase-substrate pair and that growth impairment in the absence of pU(S)3 is caused by the absence of kinase activity. The unaltered disruption of F-actin by the K220A pU(S)3 mutant suggests that F-actin disassembly is unrelated to MDV growth restrictions in the absence of the unique-short protein kinase.