Regulation of Innate Immune Responses by Bovine Herpesvirus 1 and Infected Cell Protein 0 (bICP0)

A cell cycle regulator, E2F2, and glucocorticoid receptor cooperatively transactivate the bovine alphaherpesvirus 1 immediate early transcription unit 1 promoter

Bovine alphaherpesvirus 1 (BoHV-1) infection causes respiratory tract disorders and immune suppression and may induce bacterial pneumonia. BoHV-1 establishes lifelong latency in sensory neurons after acute infection. Reactivation from latency consistently occurs following stress or intravenous injection of the synthetic corticosteroid dexamethasone (DEX), which mimics stress. The immediate early transcription unit 1 (IEtu1) promoter drives expression of infected cell protein 0 (bICP0) and bICP4, two viral transcriptional regulators necessary for productive infection and reactivation from latency. The IEtu1 promoter contains two glucocorticoid receptor (GR) responsive elements (GREs) that are transactivated by activated GR. GC-rich motifs, including consensus binding sites for specificity protein 1 (Sp1), are in the IEtu1 promoter sequences. E2F family members bind a consensus sequence (TTTCCCGC) and certain specificity protein 1 (Sp1) sites. Consequently, we hypothesized that certain E2F family members activate IEtu1 promoter activity. DEX treatment of latently infected calves increased the number of E2F2+ TG neurons. GR and E2F2, but not E2F1, E2F3a, or E2F3b, cooperatively transactivate a 436-bp cis-regulatory module in the IEtu1 promoter that contains both GREs. A luciferase reporter construct containing a 222-bp fragment downstream of the GREs was transactivated by E2F2 unless two adjacent Sp1 binding sites were mutated. Chromatin immunoprecipitation studies revealed that E2F2 occupied IEtu1 promoter sequences when the BoHV-1 genome was transfected into mouse neuroblastoma (Neuro-2A) or monkey kidney (CV-1) cells. In summary, these findings revealed that GR and E2F2 cooperatively transactivate IEtu1 promoter activity, which is predicted to influence the early stages of BoHV-1 reactivation from latency.

IMPORTANCE

Bovine alpha-herpesvirus 1 (BoHV-1) acute infection in cattle leads to establishment of latency in sensory neurons in the trigeminal ganglia (TG). A synthetic corticosteroid dexamethasone consistently initiates BoHV-1 reactivation in latently infected calves. The BoHV-1 immediate early transcription unit 1 (IEtu1) promoter regulates expression of infected cell protein 0 (bICP0) and bICP4, two viral transcriptional regulators. Hence, the IEtu1 promoter must be activated for the reactivation to occur. The number of TG neurons expressing E2F2, a transcription factor and cell cycle regulator, increased during early stages of reactivation from latency. The glucocorticoid receptor (GR) and E2F2, but not E2F1, E2F3a, or E2F3b, cooperatively transactivated a 436-bp cis-regulatory module (CRM) in the IEtu1 promoter that contains two GR responsive elements. Chromatin immunoprecipitation studies revealed that E2F2 occupies IEtu1 promoter sequences in cultured cells. GR and E2F2 mediate cooperative transactivation of IEtu1 promoter activity, which is predicted to stimulate viral replication following stressful stimuli.

Tegument protein UL3 of bovine herpesvirus 1 suppresses antiviral IFN-I signaling by targeting STING for autophagic degradation

Bovine herpesvirus 1 (BoHV-1) is a highly contagious pathogen which causes infectious bovine rhinotracheitis in cattle worldwide. Although it has the ability to evade the host's antiviral innate immune response and establish persistent latent infections, the mechanisms are not fully understood, especially the function of the tegument protein to escape innate immunity and participate in viral replication. In this study, we showed that overexpression of tegument protein UL3 facilitates BoHV-1 replication and suppresses the expression of type-I interferon (IFN-I) and IFN-stimulated genes. Then, STING was identified as the target by which UL3 inhibits the IFN-I signaling pathway, and STING was degraded through the UL3-induced autophagy pathway. Furthermore, overexpression of UL3 promotes the expression of the autophagy-related protein ATG101, thereby inducing autophagy. Further study showed that UL3 enhances the interaction between ATG101 and STING, and then the degradation of STING was reversed following ATG101 silencing in UL3-overexpressing cells during BoHV-1 infection. Our research results demonstrate a novel function of UL3 in regulating host's antiviral response and provide a potential mechanism for BoHV-1 immune evasion.

The Cell-Mediated Immune Response against Bovine alphaherpesvirus 1 (BoHV-1) Infection and Vaccination

Bovine Alphaherpesvirus 1 (BoHV-1) is one of the major respiratory pathogens in cattle worldwide. Infection often leads to a compromised host immune response that contributes to the development of the polymicrobial disease known as “bovine respiratory disease”. After an initial transient phase of immunosuppression, cattle recover from the disease. This is due to the development of both innate and adaptive immune responses. With respect to adaptive immunity, both humoral and cell-mediated immunity are required to control infection. Thus, several BoHV-1 vaccines are designed to trigger both branches of the adaptive immune system. In this review, we summarize the current knowledge on cell-mediated immune responses directed against BoHV-1 infection and vaccination.

The Bovine Herpesvirus 1 Latency-Reactivation Cycle, a Chronic Problem in the Cattle Industry

Bovine alphaherpesvirus 1 (BoHV-1) is a persistent and recurring disease that affects cattle worldwide. It is a major contributor to bovine respiratory disease and reproductive failure in the US. A major complication of BoHV-1 arises from the lifelong latent infection established in the sensory ganglia of the peripheral nervous system following acute infection. Lifelong latency is marked by periodic reactivation from latency that leads to virus transmission and transient immunosuppression. Physiological and environmental stress, along with hormone fluctuations, can drive virus reactivation from latency, allowing the virus to spread rapidly. This review discusses the mechanisms of the latency/reactivation cycle, with particular emphasis on how different hormones directly regulate BoHV-1 gene expression and productive infection. Glucocorticoids, including the synthetic corticosteroid dexamethasone, are major effectors of the stress response. Stress directly regulates BoHV-1 gene expression through multiple pathways, including β-catenin dependent Wnt signaling, and the glucocorticoid receptor. Related type 1 nuclear hormone receptors, the androgen and progesterone receptors, also drive BoHV-1 gene expression and productive infection. These receptors form feed-forward transcription loops with the stress-induced Krüppel-like transcription factors KLF4 and KLF15. Understanding these molecular pathways is critical for developing novel therapeutics designed to block reactivation and reduce virus spread and disease.

Comparative transcriptome analysis of MDBK cells reveals that BoIFN-γ augmented host immune responses to bovine herpesvirus 1 infection

Bovine herpesvirus 1 (BoHV-1) is an alphaherpesvirus that causes infectious bovine rhinotracheitis and infectious pustular vulvovaginitis in cattle. Ιnterferon-gamma (IFN-γ) is a pleiotropic cytokine with antiviral activity that modulates the innate and adaptive immune responses. In this study, we prepared high-purity bovine interferon gamma (BoIFN-γ) dimer protein using prokaryotic expression system and affinity chromatography. We subsequently investigated the effect of BoIFN-γ on BoHV-1 infection in Madin-Darby bovine kidney (MDBK) cells. The results showed that BoIFN-γ pre-treament not only decreased the production of BoHV-1 but also reduced the cytopathic effect of the virus. Differential gene expression profiles of BoHV-1 infected MDBK cells were then analysed through high-throughput RNA sequencing. The data showed that BoIFN-γ pre-treatment reduced lipid metabolism disorder and DNA damage caused by BoHV-1 infection. Furthermore, BoIFN-γ treatment upregulated the transcription of interferon regulatory transcription factors (IRF1 and GBP5) and interferon-stimulated genes (ISGs) of MDBK cells. Additionally, BoIFN-γ promotes expression of cellular protein involved in complement activation and coagulation cascades response as well as antigen processing and presentation process, while BoHV-1 infection dramatically downregulates transcription of these immune components including C3, C1r, C1s, PLAT, ITGB2, PROCR, BoLA, CD74, B2M, PA28, BoLA-DRA, and TAPBP. Collectively, our findings revealed that BoIFN-γ pre-treatment can improve host resistance to BoHV-1 infection and regulate transcription or expression of host protein associated with cellular metabolism and innate immune response. This provides insights into the development of prophylactic agents for prevention and control of BoHV-1 infection.

DDIT3 antagonizes innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway

DNA damage-inducible transcript 3 (DDIT3), a transcription factor, is typically involved in virus replication control. We are the first to report that DDIT3 promotes the replication of bovine viral diarrhea virus, an RNA virus, by inhibiting innate immunity. However, whether the DDIT3 gene participates in DNA virus replication by regulating innate immunity remains unclear. This study reported that DDIT3 suppressed the innate immune response caused by DNA viruses to promote bovine herpesvirus 1 (BoHV-1) replication. After BoHV-1 infection of Madin-Darby bovine kidney (MDBK) cells, upregulated expression of DDIT3 induced SQSTM1-mediated autophagy and promoted STING degradation. Overexpression of the SQSTM1 protein effectively reduced STING protein levels, whereas SQSTM1 knockdown increased STING protein levels. Coimmunoprecipitation experiments and confocal laser scanning microscopy revealed that the SQSTM1 protein interacts with and colocalizes with STING. Knockdown of SQSTM1 expression in DDIT3-overexpressing cell lines restored STING protein levels. Moreover, a dual-luciferase reporter assay revealed that DDIT3 directly binds to the bovine SQSTM1 promoter and induces SQSTM1 transcription. Overexpression of SQSTM1 promoted BoHV-1 replication by inhibiting IFN-β and IFN-stimulated genes (ISGs) production; silencing of SQSTM1 promoted the expression of IFN-β and ISGs to inhibit BoHV-1 replication. In conclusion, DDIT3 targets STING via SQSTM1-mediated autophagy to promote BoHV-1 replication. These results suggest a novel mechanism by which DDIT3 regulates DNA virus replication by targeting innate immunity. DDIT3 antagonizes the innate immune response to promote bovine alphaherpesvirus 1 replication via the DDIT3-SQSTM1-STING pathway.

Progesterone Sporadically Induces Reactivation from Latency in Female Calves but Proficiently Stimulates Bovine Herpesvirus 1 Productive Infection

Acute infection of the ocular, oral, or nasal cavity by bovine herpesvirus 1 (BoHV-1) culminates in lifelong latency in sensory neurons within trigeminal ganglia. The BoHV-1 latency reactivation cycle, including calves latently infected with commercially available modified live vaccines, can lead to reproductive complications, including abortions. Recent studies demonstrated progesterone stimulated BoHV-1 productive infection and sporadically induced reactivation from latency in male rabbits. The progesterone receptor (PR) and progesterone transactivate the immediate early transcription unit 1 (IEtu1) promoter and the infected cell protein 0 (bICP0) early promoter. These viral promoters drive expression of two viral transcriptional regulatory proteins (bICP0 and bICP4) that are crucial for productive infection. Based on these observations, we hypothesize that progesterone induces reactivation in a subset of calves latently infected with BoHV-1. These studies demonstrated progesterone was less efficient than dexamethasone at initiating reactivation from latency in female calves. Notably, heat stress correlated with enhancing the ability of progesterone to induce reactivation from latency. Previous studies demonstrated that heat stress activates the glucocorticoid receptor (GR), which suggested GR activation augments progesterone-mediated reactivation from latency. Additional studies revealed GR and PR cooperatively stimulated productive infection and synergistically transactivated the IEtu1 promoter when cultures were treated with dexamethasone. Mutating one or both GR binding sites in the IEtu1 promoter blocked transactivation. Collectively, these studies indicated that progesterone intermittently triggered reactivation from latency, and heat stress augmented reactivation from reactivation. Finally, these studies suggest progesterone enhances virus spread in tissues and cells where PR is abundantly expressed. IMPORTANCE Steroid hormone fluctuations are predicted to enhance or initiate bovine herpesvirus 1 (BoHV-1) replication and virus spread in cattle. For example, stress increases the incidence of BoHV-1 reactivation from latency in cattle, and the synthetic corticosteroid dexamethasone consistently induces reactivation from latency. The glucocorticoid receptor (GR) and dexamethasone stimulate key viral regulatory promoters and productive infection, in part because the viral genome contains numerous consensus GR-responsive elements (GREs). The progesterone receptor (PR) and GR belong to the type I nuclear hormone receptor family. PR and progesterone specifically bind to and transactivate viral promoters that contain GREs and stimulate BoHV-1 productive infection. Although progesterone did not induce reactivation from latency in female calves as efficiently as dexamethasone, heat stress enhanced progesterone-mediated reactivation from latency. Consequently, we predict that low levels of stressful stimuli can cooperate with progesterone to induce reactivation from latency or promote virus spread.

Regulation of neurotropic herpesvirus productive infection and latency-reactivation cycle by glucocorticoid receptor and stress-induced transcription factors

Neurotropic α-herpesvirinae subfamily members, herpes simplex virus type 1 (HSV-1) and bovine herpesvirus 1 (BoHV-1), are important viral pathogens in their respective hosts. Following acute infection on mucosal surfaces, these viruses establish life-long latency in neurons within trigeminal ganglia (TG) and central nervous system. Chronic or acute stress (physiological or psychological) increases the frequency of reactivation from latency, which leads to virus shedding, virus transmission, and recurrent disease. While stress impairs immune responses and inflammatory signaling cascades, we predict stressful stimuli directly stimulate viral gene expression and productive infection during early stages of reactivation from latency. For example, BoHV-1 and HSV-1 productive infection is impaired by glucocorticoid receptor (GR) antagonists but is stimulated by the synthetic corticosteroid dexamethasone. Promoters that drive expression of key viral transcriptional regulatory proteins are cooperatively stimulated by GR and specific Krüppel like transcription factors (KLF) induced during stress induced reactivation from latency. The BoHV-1 immediate early transcription unit 1 promoter and contains two GR response elements (GRE) that are essential for cooperative transactivation by GR and KLF15. Conversely, the HSV-1 infected cell protein 0 (ICP0) and ICP4 promoter as well as the BoHV-1 ICP0 early promoter lack consensus GREs: however, these promoters are cooperatively transactivated by GR and KLF4 or KLF15. Hence, growing evidence suggests GR and stress-induced transcription factors directly stimulate viral gene expression and productive infection during early stages of reactivation from latency. We predict the immune inhibitory effects of stress enhance virus spread at late stages during reactivation from latency.

Cooperative activation of bovine herpesvirus 1 productive infection and viral regulatory promoters by androgen receptor and Krüppel-like transcription factors 4 and 15

Bovine herpesvirus 1 (BoHV-1), a significant viral pathogen, establishes latency in sensory neurons. The viral genome contains more than 100 consensus glucocorticoid receptor (GR) regulatory elements (GREs): consequently, stress stimulates viral replication and reactivation from latency. The immediate early transcription unit 1 (IEtu1) and bICP0 early promoters are transactivated by GR and synthetic corticosteroid dexamethasone. The androgen receptor (AR), like GR, is a Type 1 nuclear hormone receptor that binds and stimulates certain promoters containing GREs. Consequently, we hypothesized AR and 5α-Dihydrotestosterone (DHT) stimulate productive infection and key viral promoters. New studies demonstrated AR, DHT, and Krüppel like transcription factor 4 (KLF4) cooperatively stimulated productive infection and bICP0 E promoter activity in mouse neuroblastoma cells (Neuro-2A). KLF15 also cooperated with AR and DHT to stimulate IEtu1 promoter activity. We suggest AR and testosterone increase the prevalence of virus in semen by stimulating viral gene expression and replication.

Progesterone increases the incidence of bovine herpesvirus 1 reactivation from latency and stimulates productive infection

Bovine herpesvirus 1 (BoHV-1), including modified live vaccines, can cause abortions in pregnant cows. Progesterone maintains pregnancy and promotes spermiogenesis and testosterone biosynthesis in males: furthermore, progesterone is a neuro-steroid. Recent published studies demonstrated progesterone stimulated the BoHV-1 immediate early transcription unit 1 (IEtu1) promoter, and two glucocorticoid receptor response elements within the promoter were required for progesterone mediated transactivation. In this study, we tested whether progesterone induces reactivation from latency in rabbits. As expected, the synthetic corticosteroid dexamethasone consistently induced reactivation from latency in males and females. While progesterone induced reactivation from latency in approximately one-half of male rabbits, virus shedding was sporadic compared to dexamethasone and less efficient in female rabbits. Progesterone significantly increased productive infection in rabbit skin cells, which correlated with stimulating reactivation. These studies suggest progesterone promotes BoHV-1 spread in cattle, in part, by increasing the frequency of reactivation from latency.

Distinctive features of bovine alphaherpesvirus types 1 and 5 and the virus-host interactions that might influence clinical outcomes

Bovine herpesvirus types 1 (BoHV-1) and 5 (BoHV-5) are two closely related alphaherpesviruses. BoHV-1 causes several syndromes in cattle, including respiratory disease and sporadic cases of encephalitis, whereas BoHV-5 is responsible for meningoencephalitis in calves. Although both viruses are neurotropic, they differ in their neuropathogenic potential. This review summarizes the findings on the specific mechanisms and pathways known to modulate the pathogenesis of BoHV-1 and BoHV-5, particularly in relation to respiratory and neurological syndromes, which characterize BoHV-1 and BoHV-5 infections, respectively.

Bovine Herpesvirus 1 Counteracts Immune Responses and Immune-Surveillance to Enhance Pathogenesis and Virus Transmission

Infection of cattle by bovine herpesvirus 1 (BoHV-1) can culminate in upper respiratory tract disorders, conjunctivitis, or genital disorders. Infection also consistently leads to transient immune-suppression. BoHV-1 is the number one infectious agent in cattle that is associated with abortions in cattle. BoHV-1, as other α-herpesvirinae subfamily members, establishes latency in sensory neurons. Stressful stimuli, mimicked by the synthetic corticosteroid dexamethasone, consistently induce reactivation from latency in latently infected calves and rabbits. Increased corticosteroid levels due to stress have a two-pronged effect on reactivation from latency by: (1) directly stimulating viral gene expression and replication, and (2) impairing antiviral immune responses, thus enhancing virus spread and transmission. BoHV-1 encodes several proteins, bICP0, bICP27, gG, UL49.5, and VP8, which interfere with key antiviral innate immune responses in the absence of other viral genes. Furthermore, the ability of BoHV-1 to infect lymphocytes and induce apoptosis, in particular CD4+ T cells, has negative impacts on immune responses during acute infection. BoHV-1 induced immune-suppression can initiate the poly-microbial disorder known as bovine respiratory disease complex, which costs the US cattle industry more than one billion dollars annually. Furthermore, interfering with antiviral responses may promote viral spread to ovaries and the developing fetus, thus enhancing reproductive issues associated with BoHV-1 infection of cows or pregnant cows. The focus of this review is to describe the known mechanisms, direct and indirect, by which BoHV-1 interferes with antiviral immune responses during the course of infection.

The bovine herpesvirus 1 regulatory proteins, bICP4 and bICP22, are expressed during the escape from latency

Following acute infection of mucosal surfaces by bovine herpesvirus 1 (BoHV-1), sensory neurons are a primary site for lifelong latency. Stress, as mimicked by the synthetic corticosteroid dexamethasone, consistently induces reactivation from latency. Two viral regulatory proteins (VP16 and bICP0) are expressed within 1 h after calves latently infected with BoHV-1 are treated with dexamethasone. Since the immediate early transcription unit 1 (IEtu1) promoter regulates both BoHV-1 infected cell protein 0 (bICP0) and bICP4 expressions, we hypothesized that the bICP4 protein is also expressed during early stages of reactivation from latency. In this study, we tested whether bICP4 and bICP22, the only other BoHV-1 protein known to be encoded by an immediate early gene, were expressed during reactivation from latency by generating peptide-specific antiserum to each protein. bICP4 and bICP22 protein expression were detected in trigeminal ganglionic (TG) neurons during early phases of dexamethasone-induced reactivation from latency, operationally defined as the escape from latency. Conversely, bICP4 and bICP22 were not readily detected in TG neurons of latently infected calves. In summary, it seems clear that all proteins encoded by known BoHV-1 IE genes (bICP4, bICP22, and bICP0) were expressed during early stages of dexamethasone-induced reactivation from latency.

The Involvement of Histone H3 Acetylation in Bovine Herpesvirus 1 Replication in MDBK Cells

During bovine herpesvirus 1 (BoHV-1) productive infection in cell cultures, partial of intranuclear viral DNA is present in nucleosomes, and viral protein VP22 associates with histones and decreases histone H4 acetylation, indicating the involvement of histone H4 acetylation in virus replication. In this study, we demonstrated that BoHV-1 infection at the late stage (at 24 h after infection) dramatically decreased histone H3 acetylation [at residues K9 (H3K9ac) and K18 (H3K18ac)], which was supported by the pronounced depletion of histone acetyltransferases (HATs) including CBP/P300 (CREB binding protein and p300), GCN5L2 (general control of amino acid synthesis yeast homolog like 2) and PCAF (P300/CBP-associated factor). The depletion of GCN5L2 promoted by virus infection was partially mediated by ubiquitin-proteasome pathway. Interestingly, the viral replication was enhanced by HAT (histone acetyltransferase) activator CTPB [N-(4-Chloro-3-trifluoromethylphenyl)-2-ethoxy-6-pentadecylbenzamide], and vice versa, inhibited by HAT inhibitor Anacardic acid (AA), suggesting that BoHV-1 may take advantage of histone acetylation for efficient replication. Taken together, we proposed that the HAT-dependent histone H3 acetylation plays an important role in BoHV-1 replication in MDBK (Madin-Darby bovine kidney) cells.

MG-132 reduces virus release in Bovine herpesvirus-1 infection

Bovine herpesvirus 1 (BoHV-1) can provoke conjunctivitis, abortions and shipping fever. BoHV-1 infection can also cause immunosuppression and increased susceptibility to secondary bacterial infections, leading to pneumonia and occasionally to death. Herein, we investigated the influence of MG-132, a proteasome inhibitor, on BoHV-1 infection in bovine kidney (MDBK) cells. Infection of MDBK cells with BoHV-1 induces apoptotic cell death that enhances virus release. Whereas, MG-132 inhibited virus-induced apoptosis and stimulated autophagy. Protein expression of viral infected cell protein 0 (bICP0), which is constitutively expressed during infection and is able to stimulate Nuclear factor kappa B (NF-κB), was completely inhibited by MG-132. These results were accompanied by a significant delay in the NF-κB activation. Interestingly, the efficient virus release provoked by BoHV-1-induced apoptosis was significantly reduced by MG-132. Overall, this study suggests that MG-132, through the activation of autophagy, may limit BoHV-1 replication during productive infection, by providing an antiviral defense mechanism.

Combinatorial Effects of the Glucocorticoid Receptor and Krüppel-Like Transcription Factor 15 on Bovine Herpesvirus 1 Transcription and Productive Infection

Bovine herpesvirus 1 (BoHV-1), an important bovine pathogen, establishes lifelong latency in sensory neurons. Latently infected calves consistently reactivate from latency following a single intravenous injection of the synthetic corticosteroid dexamethasone. The immediate early transcription unit 1 (IEtu1) promoter, which drives bovine ICP0 (bICP0) and bICP4 expression, is stimulated by dexamethasone because it contains two glucocorticoid receptor (GR) response elements (GREs). Several Krüppel-like transcription factors (KLF), including KLF15, are induced during reactivation from latency, and they stimulate certain viral promoters and productive infection. In this study, we demonstrate that the GR and KLF15 were frequently expressed in the same trigeminal ganglion (TG) neuron during reactivation and cooperatively stimulated productive infection and IEtu1 GREs in mouse neuroblastoma cells (Neuro-2A). We further hypothesized that additional regions in the BoHV-1 genome are transactivated by the GR or stress-induced transcription factors. To test this hypothesis, BoHV-1 DNA fragments (less than 400 bp) containing potential GR and KLF binding sites were identified and examined for transcriptional activation by stress-induced transcription factors. Intergenic regions within the unique long 52 gene (UL52; a component of the DNA primase/helicase complex), bICP4, IEtu2, and the unique short region were stimulated by KLF15 and the GR. Chromatin immunoprecipitation studies revealed that the GR and KLF15 interacted with sequences within IEtu1 GREs and the UL52 fragment. Coimmunoprecipitation studies demonstrated that KLF15 and the GR were associated with each other in transfected cells. Since the GR stimulates KLF15 expression, we suggest that these two transcription factors form a feed-forward loop that stimulates viral gene expression and productive infection following stressful stimuli.IMPORTANCE Bovine herpesvirus 1 (BoHV-1) is an important viral pathogen that causes respiratory disease and suppresses immune responses in cattle; consequently, life-threatening bacterial pneumonia can occur. Following acute infection, BoHV-1 establishes lifelong latency in sensory neurons. Reactivation from latency is initiated by the synthetic corticosteroid dexamethasone. Dexamethasone stimulates lytic cycle viral gene expression in sensory neurons of calves latently infected with BoHV-1, culminating in virus shedding and transmission. Two stress-induced cellular transcription factors, Krüppel-like transcription factor 15 (KLF15) and the glucocorticoid receptor (GR), cooperate to stimulate productive infection and viral transcription. Additional studies demonstrated that KLF15 and the GR form a stable complex and that these stress-induced transcription factors bind to viral DNA sequences, which correlates with transcriptional activation. The ability of the GR and KLF15 to synergistically stimulate viral gene expression and productive infection may be critical for the ability of BoHV-1 to reactivate from latency following stressful stimuli.

The role of phospholipase C signaling in bovine herpesvirus 1 infection

Bovine herpesvirus 1 (BoHV-1) infection enhanced the generation of inflammatory mediator reactive oxidative species (ROS) and stimulated MAPK signaling that are highly possibly related to virus induced inflammation. In this study, for the first time we show that BoHV-1 infection manipulated phospholipase C (PLC) signaling, as demonstrated by the activation of PLCγ-1 at both early stages [at 0.5 h post-infection (hpi)] and late stages (4-12 hpi) during the virus infection of MDBK cells. Viral entry, and de novo protein expression and/or DNA replication were potentially responsible for the activation of PLCγ-1 signaling. PLC signaling inhibitors of both U73122 and edelfosine significantly inhibited BoHV-1 replication in both bovine kidney cells (MDBK) and rabbit skin cells (RS-1) in a dose-dependent manner by affecting the virus entry stage(s). In addition, the activation of Erk1/2 and p38MAPK signaling, and the enhanced generation of ROS by BoHV-1 infection were obviously ameliorated by chemical inhibition of PLC signaling, implying the requirement of PLC signaling in ROS production and these MAPK pathway activation. These results suggest that the activation of PLC signaling is a potential pathogenic mechanism for BoHV-1 infection.

The high mobility group AT-hook 1 protein stimulates bovine herpesvirus 1 productive infection

Bovine herpesvirus 1 (BoHV-1) is an important pathogen of cattle that causes clinical symptoms in the upper respiratory tract and conjunctivitis. Like most alpha-herpesvirinae subfamily members, BoHV-1 establishes latency in sensory neurons. Stress consistently induces reactivation from latency, which is essential for virus transmission. Recent studies demonstrated that a viral protein (ORF2) expressed in a subset of latently infected neurons is associated with β-catenin and the high mobility group AT-hook 1 protein (HMGA1), which correlates with increased expression of these proteins in latently infected neurons. Since HMGA1 is primarily expressed in actively growing cells, binds to the minor groove of A+T rich regions in double-stranded DNA, and mediates gene transcription, we hypothesized that HMGA1 regulates BoHV-1 productive infection. Studies in this report indicated that productive infection increased HMGA1 protein levels and re-localized the protein in the nucleus. Netropsin, a small molecule that binds to the minor groove of DNA and prevents HMGA1 from interacting with DNA inhibited viral replication and interfered with the ability of BoHV-1 to induce HMGA1 re-localization. Furthermore, netropsin reduced RNA and protein expression of two viral regulatory proteins (bICP0 and bICP22) during productive infection, but increased bICP4 levels. Small interfering RNAs (siRNAs) that specifically target HMGA1 reduced HMGA1 RNA levels and virus production confirming HMGA1 stimulates productive infection.

Protein Composition of the Bovine Herpesvirus 1.1 Virion

Bovine herpesvirus (BoHV) type 1 is an important agricultural pathogen that infects cattle and other ruminants worldwide. Acute infection of the oro-respiratory tract leads to immune suppression and allows commensal bacteria to infect an otherwise healthy lower respiratory tract. This condition is known as the Bovine Respiratory Disease (BRD). BoHV-1 latently infects the host for life and periodical stress events re-initiate BRD, translating into high morbidity and large economic losses. To gain a better understanding of the biology of BoHV-1 and the disease it causes, we elucidated the protein composition of extracellular virions using liquid chromatography-mass spectrometry analysis. We detected 33 viral proteins, including the expected proteins of the nucleocapsid and envelope as well as other regulatory proteins present in the viral tegument. In addition to viral proteins, we have also identified packaged proteins of host origin. This constitutes the first proteomic characterization of the BoHV virion.

The β-catenin signaling pathway stimulates bovine herpesvirus 1 productive infection

Bovine herpes virus 1 (BoHV-1), an important bovine pathogen, causes conjunctivitis and disorders in the upper respiratory tract. Following acute infection, BoHV1 establishes life-long latency in sensory neurons. Recent studies demonstrated that viral gene products expressed in trigeminal ganglionic neurons during latency stabilize β-catenin levels, an important signaling molecule that interacts with a family of DNA binding proteins (T-cell factors) and subsequently stimulates transcription. In this study, we provide new evidence demonstrating that BoHV-1 transiently increased β-catenin protein levels in bovine kidney (CRIB) cells, but not in rabbit skin cells. β-catenin dependent transcription was also stimulated by infection of CRIB cells. The β-catenin small molecule inhibitor (iCRT14) significantly reduced the levels of BoHV-1 virus during productive infection of CRIB cells and rabbit skin cells. In summary, these studies suggested the ability of β-catenin to stimulate cell survival and cell cycle regulatory factors enhances productive infection in non-neuronal cells.

Large-scale gene co-expression network as a source of functional annotation for cattle genes

Background

Genome sequencing and subsequent gene annotation of genomes has led to the elucidation of many genes, but in vertebrates the actual number of protein coding genes are very consistent across species (~20,000). Seven years after sequencing the cattle genome, there are still genes that have limited annotation and the function of many genes are still not understood, or partly understood at best. Based on the assumption that genes with similar patterns of expression across a vast array of tissues and experimental conditions are likely to encode proteins with related functions or participate within a given pathway, we constructed a genome-wide Cattle Gene Co-expression Network (CGCN) using 72 microarray datasets that contained a total of 1470 Affymetrix Genechip Bovine Genome Arrays that were retrieved from either NCBI GEO or EBI ArrayExpress.

Results

The total of 16,607 probe sets, which represented 11,397 genes, with unique Entrez ID were consolidated into 32 co-expression modules that contained between 29 and 2569 probe sets. All of the identified modules showed strong functional enrichment for gene ontology (GO) terms and Reactome pathways. For example, modules with important biological functions such as response to virus, response to bacteria, energy metabolism, cell signaling and cell cycle have been identified. Moreover, gene co-expression networks using “guilt-by-association” principle have been used to predict the potential function of 132 genes with no functional annotation. Four unknown Hub genes were identified in modules highly enriched for GO terms related to leukocyte activation (LOC509513), RNA processing (LOC100848208), nucleic acid metabolic process (LOC100850151) and organic-acid metabolic process (MGC137211). Such highly connected genes should be investigated more closely as they likely to have key regulatory roles.

Conclusions

We have demonstrated that the CGCN and its corresponding regulons provides rich information for experimental biologists to design experiments, interpret experimental results, and develop novel hypothesis on gene function in this poorly annotated genome. The network is publicly accessible at http://www.animalgenome.org/cgi-bin/host/reecylab/d.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3176-2) contains supplementary material, which is available to authorized users.

The serum and glucocorticoid-regulated protein kinases (SGK) stimulate bovine herpesvirus 1 and herpes simplex virus 1 productive infection

Serum and glucocorticoid-regulated protein kinases (SGK) are serine/threonine protein kinases that contain a catalytic domain resembling other protein kinases: AKT/protein kinase B, protein kinase A, and protein kinase C-Zeta for example. Unlike these constitutively expressed protein kinases, SGK1 RNA and protein levels are increased by growth factors and corticosteroids. Stress can directly stimulate SGK1 levels as well as stimulate bovine herpesvirus 1 (BoHV-1) and herpes simplex virus 1 (HSV-1) productive infection and reactivation from latency suggesting SGK1 can stimulate productive infection. For the first time, we provide evidence that a specific SGK inhibitor (GSK650394) significantly reduced BoHV-1 and HSV-1 replication in cultured cells. Proteins encoded by the three BoHV-1 immediate early genes (bICP0, bICP4, and bICP22) and two late proteins (VP16 and gE) were consistently reduced by GSK650394 during early stages of productive infection. In summary, these studies suggest SGK may stimulate viral replication following stressful stimuli.

BHV-1 induced oxidative stress contributes to mitochondrial dysfunction in MDBK cells

The levels of cellular reactive oxygen species (ROS) and ATP as well as the mitochondrial membrane potential (MMP) in response to bovine herpesvirus 1 (BHV-1) infection of MDBK cells were measured, respectively. BHV-1 infection increased ROS production which depended on viral entry, and de novo protein expression and/or DNA replication. Vice versa, excessive ROS was required for efficient viral replication. Levels of both ATP and MMP were significantly decreased after BHV-1 infection. Interestingly, the loss of MMP was ameliorated by ROS depression. Collectively, ROS dependent mitochondrial damage and ultimately disruption of energy metabolism (ATP depletion) are a potential pathogenic mechanism for BHV-1 infection.

Regulation of Notch-mediated transcription by a bovine herpesvirus 1 encoded protein (ORF2) that is expressed in latently infected sensory neurons

Bovine herpesvirus 1 (BoHV-1) is an Alphaherpesvirinae subfamily member that establishes life-long latency in sensory neurons. The latency-related RNA (LR-RNA) is abundantly expressed during latency. An LR mutant virus containing stop codons at the amino-terminus of open reading frame (ORF)2 does not reactivate from latency and replicates less efficiently in tonsils and trigeminal ganglia. ORF2 inhibits apoptosis, interacts with Notch family members, and interferes with Notch-dependent transcription suggesting ORF2 expression enhances survival of infected neurons. The Notch signaling pathway is crucial for neuronal differentiation and survival suggesting that interactions between ORF2 and Notch family members regulate certain aspects of latency. Consequently, for this study, we compared whether ORF2 interfered with the four mammalian Notch family members. ORF2 consistently interfered with Notch1-3-mediated transactivation of three cellular promoters. Conversely, Notch4-mediated transcription was not consistently inhibited by ORF2. Electrophoretic shift mobility assays using four copies of a consensus-DNA binding site for Notch/CSL (core binding factor (CBF)-1, Suppressor of Hairless, Lag-2) as a probe revealed ORF2 interfered with Notch1 and 3 interactions with a CSL family member bound to DNA. Additional studies demonstrated ORF2 enhances neurite sprouting in mouse neuroblastoma cells that express Notch1-3, but not Notch4. Collectively, these studies indicate that ORF2 inhibits Notch-mediated transcription and signaling by interfering with Notch interacting with CSL bound to DNA.

Isolation and adaptation of bovine herpes virus Type 1 in embryonated chicken eggs and in Madin-Darby bovine kidney cell line

AIM

Objective of the present study was to isolate bovine herpes virus Type 1 (BHV-1) from semen of infected bull and to adapt it onto embryonated eggs and Madin-Darby bovine kidney (MDBK) cell line. Further, the virus was identified by agar gel immunodiffusion (AGID) test.

MATERIALS AND METHODS

Semen samples were collected from five BHV-1 positive bulls previously confirmed for the presence of antibodies against BHV-1 using avidin-biotin enzyme linked immunosorbent assay test. The virus from semen samples was adapted in chorioallantoic membrane (CAM) of 11-day-old embryonated chickens eggs and in MDBK cell line. The presence of BHV-1 in infected CAM and cell culture fluid was confirmed by AGID test.

RESULTS

Virus infected CAM showed edema, congestion and thickening at first passage level. Small foci ranged from 1 to 2 mm in diameter, scattered all over the membrane were observed at first passage. More severe changes were observed in CAM after serial passaging. The large pock lesions, round in shape with opaque raised edge and depressed gray central area of necrosis ranged from 3 to 5 mm in diameter were developed at fourth passage. Blind passages in MDBK cell culture were made. The MDBK cell line at second passage level showed characteristic cytopathic effect viz. rounding of cells with shrinkage, followed by aggregation or clumping of cells which progressed rapidly and appeared as "bunch of grapes" at 72 h post inoculation. Few cells become elongated when compared with uninfected controls. A homogenate of CAM with distinct pock lesions and infected cell culture fluid developed precipitation line within 48 h against specific anti-BHV-1 immune serum by AGID test.

CONCLUSION

BHV-1 was easily adapted in CAM of chicken embryos and in MDBK cell line. Virus infected CAM and cell culture fluid showed precipitin band by AGID test.

β-Catenin, a Transcription Factor Activated by Canonical Wnt Signaling, Is Expressed in Sensory Neurons of Calves Latently Infected with Bovine Herpesvirus 1

Like many Alphaherpesvirinae subfamily members, bovine herpesvirus 1 (BoHV-1) expresses an abundant transcript in latently infected sensory neurons, the latency-related (LR)-RNA. LR-RNA encodes a protein (ORF2) that inhibits apoptosis, interacts with Notch family members, interferes with Notch-mediated transcription, and stimulates neurite formation in cells expressing Notch. An LR mutant virus containing stop codons at the amino terminus of ORF2 does not reactivate from latency or replicate efficiently in certain tissues, indicating that LR gene products are important. In this study, β-catenin, a transcription factor activated by the canonical Wnt signaling pathway, was frequently detected in ORF2-positive trigeminal ganglionic neurons of latently infected, but not mock-infected, calves. Conversely, the lytic cycle regulatory protein (BoHV-1 infected cell protein 0, or bICP0) was not frequently detected in β-catenin-positive neurons in latently infected calves. During dexamethasone-induced reactivation from latency, mRNA expression levels of two Wnt antagonists, Dickkopf-1 (DKK-1) and secreted Frizzled-related protein 2 (SFRP2), were induced in bovine trigeminal ganglia (TG), which correlated with reduced β-catenin protein expression in TG neurons 6 h after dexamethasone treatment. ORF2 and a coactivator of β-catenin, mastermind-like protein 1 (MAML1), stabilized β-catenin protein levels and stimulated β-catenin-dependent transcription in mouse neuroblastoma cells more effectively than MAML1 or ORF2 alone. Neuroblastoma cells expressing ORF2, MAML1, and β-catenin were highly resistant to cell death following serum withdrawal, whereas most cells transfected with only one of these genes died. The Wnt signaling pathway interferes with neurodegeneration but promotes neuronal differentiation, suggesting that stabilization of β-catenin expression by ORF2 promotes neuronal survival and differentiation.

IMPORTANCE

Bovine herpesvirus 1 (BoHV-1) is an important pathogen of cattle, and like many Alphaherpesvirinae subfamily members establishes latency in sensory neurons. Lifelong latency and the ability to reactivate from latency are crucial for virus transmission. Maintaining the survival and normal functions of terminally differentiated neurons is also crucial for lifelong latency. Our studies revealed that BoHV-1 gene products expressed during latency stabilize expression of the transcription factor β-catenin and perhaps its cofactor, mastermind-like protein 1 (MAML1). In contrast to expression during latency, β-catenin expression in sensory neurons is not detectable following treatment of latently infected calves with the synthetic corticosteroid dexamethasone to initiate reactivation from latency. A viral protein (ORF2) expressed in a subset of latently infected neurons stabilized β-catenin and MAML1 in transfected cells. ORF2, β-catenin, and MAML1 also enhanced cell survival when growth factors were withdrawn, suggesting that these genes enhance survival of latently infected neurons.

Bovine herpesvirus 1 productive infection and immediate early transcription unit 1 promoter are stimulated by the synthetic corticosteroid dexamethasone

The primary site for life-long latency of bovine herpesvirus 1 (BHV-1) is sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency; however the mechanism by which corticosteroids mediate reactivation is unclear. In this study, we demonstrate for the first time that dexamethasone stimulates productive infection, in part, because the BHV-1 genome contains more than 100 potential glucocorticoid receptor (GR) response elements (GREs). Immediate early transcription unit 1 (IEtu1) promoter activity, but not IEtu2 or VP16 promoter activity, was stimulated by dexamethasone. Two near perfect consensus GREs located within the IEtu1 promoter were necessary for dexamethasone-mediated stimulation. Electrophoretic mobility shift assays and chromatin immunoprecipitation studies demonstrated that the GR interacts with IEtu1 promoter sequences containing the GREs. Although we hypothesize that DEX-mediated stimulation of IEtu1 promoter activity is important during productive infection and perhaps reactivation from latency, stress likely has pleiotropic effects on virus-infected cells.

Bovine herpesvirus 1 regulatory proteins are detected in trigeminal ganglionic neurons during the early stages of stress-induced escape from latency

Bovine herpesvirus 1 (BHV-1) establishes latency in sensory neurons. The synthetic corticosteroid dexamethasone consistently induces reactivation from latency. Within 90 min after latently infected calves are treated with dexamethasone, two BHV-1 regulatory proteins, BHV-1-infected cell protein 0 (bICP0) and viral protein 16 (VP16), are expressed in the same neuron. In this study, we demonstrate that VP16 and bICP0 can be detected at 22 and 33 min after dexamethasone (DEX) treatment of latently infected calves. However, we were unable to discern whether VP16 or bICP0 was expressed at early times after reactivation. VP16+ neurons consistently express the glucocorticoid receptor suggesting corticosteroid-mediated activation of its receptor rapidly stimulates reactivation from latency.

A proteomic study of the differential protein expression in MDBK cells after bovine herpesvirus type 1 infection (BHV-1) strain treatment

Different BHV-1 strains, such as the virulent IBRV LN01/08 strains and the attenuated vaccine strain IBRV LNM, produces different clinical immune responses; however, the study of the differential protein expression in Madin-Darby bovine kidney (MDBK) cells after BHV-1-infection still remains unclear. Here, we applied a comparative proteomic strategy, based on 2D and MALDI-TOF/MS platforms, to examine the differential expression of proteins in MDBK cells that were treated and not treated with virulent IBRV LN01/08 and attenuated IBRV LNM strains. A total of eight differential proteins, including pyruvate kinase, heat shock protein (HSP) 90 (HSP90AA1 and HSP90AB1), annexin A, albumin (ALB), scinderin (SCIN), tubulin (alpha 1a) and vimentin (VIM), were identified. Among these proteins, pyruvate kinase, and HSP90 (HSP90AB1), tubulin and vimentin were identified in the virulent IBRV LN01/08 strain group, but were not identified in the attenuated IBRV LNM group. These results play an important role in tumor formation and development, cell migration, tumor cell line apoptosis, cell invasion and viral infection. The HSP90 (HSP90AA1) protein was identified in the control group and the attenuated IBRV LNM-infected group. Most studies have shown that HSP90 proteins were more of a cancer gene target, and inhibiting its function would result to oncogene degradation during cancer treatment. On the other hand, ALB is associated to cell differentiation, apoptosis, necrosis, cell death, viral infection, autophagy, interstitial tissue inflammation, and cell survival. These results provide a theoretical basis for the systematic understanding of BHV-1-infection mechanisms and BHV-1-induced immune responses.

Analysis of a bovine herpesvirus 1 protein encoded by an alternatively spliced latency related (LR) RNA that is abundantly expressed in latently infected neurons

The bovine herpes virus 1 (BoHV-1) encoded latency-related RNA (LR-RNA) is abundantly expressed in latently infected sensory neurons. A LR mutant virus with three stop codons at the amino-terminus of ORF2 does not reactivate from latency or replicate efficiently in certain tissues. ORF2 inhibits apoptosis, interacts with Notch1 or Notch3, and interferes with Notch mediated signaling. Alternative splicing of LR-RNA in trigeminal ganglia yields transcripts that have the potential to encode a protein containing most of ORF2 sequences and parts of other coding sequences located within the LR gene. In this study, we determined that an ORF2 protein fused with reading frame B (15d ORF) was more stable in transfected cells. ORF2 and the 15d ORF stimulated neurite formation in mouse neuroblastoma cells, interfered with Notch3 mediated trans-activation, and had similar DNA binding properties. Increased stability of the 15d ORF is predicted to enhance the establishment of latency.

Bovine herpesvirus 1 productive infection stimulates inflammasome formation and caspase 1 activity

Bovine herpesvirus 1 (BoHV-1), a significant viral pathogen of cattle, causes inflammation in affected tissue during acute infection. Consequently, we tested whether productively infected bovine cells stimulate inflammasome formation. Expression of two components required for inflammasome formation, the DNA sensor IFI16 (gamma-interferon-inducible protein 16) and NLRP3 (NOD-like receptor family, pyrin domain containing 3), were induced in bovine kidney cells by eight hours after infection. IFI16 was detected in punctate granules localized to the cytoplasm and nucleus. During productive infection, more than ten times more cells were caspase 1 positive, which is activated following inflammasome formation. Two caspase 1 inhibitors had no effect on productive infection. Conversely, another caspase 1 inhibitor, glyburide, significantly inhibited virus infection suggesting it had off-target effects on related enzymes or interfered with infection via non-enzymatic mechanisms. Collectively, these studies demonstrated that BoHV-1 infection stimulated inflammasome formation, which we predict is important for clinical symptoms in cattle.

Bovine herpesvirus 1 regulatory proteins bICP0 and VP16 are readily detected in trigeminal ganglionic neurons expressing the glucocorticoid receptor during the early stages of reactivation from latency

Bovine herpesvirus 1 (BHV-1) establishes a lifelong latent infection in sensory neurons following acute infection. Increased corticosteroid levels, due to stress, increases the incidence of reactivation from latency. Within minutes, corticosteroids activate the glucocorticoid receptor and transcription of promoters containing a glucocorticoid receptor element. A single intravenous injection of the synthetic corticosteroid dexamethasone consistently induces reactivation from latency in calves. Lytic cycle viral gene expression is detected within 6 h after dexamethasone treatment of calves latently infected with BHV-1. Cellular transcription factors are induced by dexamethasone in trigeminal ganglionic neurons within 1.5 h after dexamethasone treatment, suggesting they promote viral gene expression during the early phases of reactivation from latency, which we operationally defined as the escape from latency. In this study, immunohistochemistry was utilized to examine viral protein expression during the escape from latency. Within 1.5 h after dexamethasone treatment, bICP0 and a late protein (VP16) were consistently detected in a subset of trigeminal ganglionic neurons. Most neurons expressing bICP0 also expressed VP16. Additional studies revealed that neurons expressing the glucocorticoid receptor also expressed bICP0 or VP16 at 1.5 h after dexamethasone treatment. Two other late proteins, glycoprotein C and D, were not detected until 6 h after dexamethasone treatment and were detected in only a few neurons. These studies provide evidence that VP16 and the promiscuous viral trans-activator (bICP0) are expressed during the escape from latency, suggesting they promote the production of infectious virus in a small subset of latently infected neurons.

Immunity to Bovine Herpesvirus 1: I. Viral lifecycle and innate immunity

Bovine herpesvirus 1 (BHV-1) causes a variety of diseases and is globally distributed. It infects via mucosal epithelium, leading to rapid lytic replication and latent infection, primarily in sensory ganglia. Large amounts of virus can be excreted by the host on primary infection or upon recrudescence of latent infection, resulting in disease spread. The bovine immune response to BHV-1 is rapid, robust, balanced, and long-lasting. The innate immune system is the first to respond to the infection, with type I interferons (IFNs), inflammatory cytokines, killing of infected host cells, and priming of a balanced adaptive immune response. The virus possesses a variety of immune evasion strategies, including inhibition of type I IFN production, chemokine and complement binding, infection of macrophages and neutrophils, and latency. BHV-1 immune suppression contributes to the severity of its disease manifestations and to the bovine respiratory disease complex, the leading cause of cattle death loss in the USA.

Bovine Herpes Virus 1 (BHV-1) and Herpes Simplex Virus Type 1 (HSV-1) Promote Survival of Latently Infected Sensory Neurons, in Part by Inhibiting Apoptosis

α-Herpesvirinae subfamily members, including herpes simplex virus type 1 (HSV-1) and bovine herpes virus 1 (BHV-1), initiate infection in mucosal surfaces. BHV-1 and HSV-1 enter sensory neurons by cell-cell spread where a burst of viral gene expression occurs. When compared to non-neuronal cells, viral gene expression is quickly extinguished in sensory neurons resulting in neuronal survival and latency. The HSV-1 latency associated transcript (LAT), which is abundantly expressed in latently infected neurons, inhibits apoptosis, viral transcription, and productive infection, and directly or indirectly enhances reactivation from latency in small animal models. Three anti-apoptosis genes can be substituted for LAT, which will restore wild type levels of reactivation from latency to a LAT null mutant virus. Two small non-coding RNAs encoded by LAT possess anti-apoptosis functions in transfected cells. The BHV-1 latency related RNA (LR-RNA), like LAT, is abundantly expressed during latency. The LR-RNA encodes a protein (ORF2) and two microRNAs that are expressed in certain latently infected neurons. Wild-type expression of LR gene products is required for stress-induced reactivation from latency in cattle. ORF2 has anti-apoptosis functions and interacts with certain cellular transcription factors that stimulate viral transcription and productive infection. ORF2 is predicted to promote survival of infected neurons by inhibiting apoptosis and sequestering cellular transcription factors which stimulate productive infection. In addition, the LR encoded microRNAs inhibit viral transcription and apoptosis. In summary, the ability of BHV-1 and HSV-1 to interfere with apoptosis and productive infection in sensory neurons is crucial for the life-long latency-reactivation cycle in their respective hosts.

A protein (ORF2) encoded by the latency-related gene of bovine herpesvirus 1 interacts with DNA

Bovine herpesvirus 1 (BHV-1), like other members of the Alphaherpesvirinae subfamily, establishes latency in sensory neurons. The virally encoded latency-related RNA (LR-RNA) is expressed abundantly in latently infected sensory neurons and encodes several proteins, including ORF2. An LR mutant virus with stop codons at the amino terminus of ORF2 does not reactivate from latency after treatment with the synthetic corticosteroid dexamethasone, in part because it induces higher levels of apoptosis during the establishment of latency. ORF2 inhibits apoptosis, interacts with three cellular transcription factors (Notch1, Notch3, and C/EBP-α), and interferes with Notch-mediated signaling. Consequently, we predict that ORF2 expression is crucial for the latency reactivation cycle in cattle. In this study, we tested whether ORF2 interacts with nucleic acids, because it contains 18% basic amino acids and localizes to the nucleus. A subset of ORF2 proteins was associated with chromatin and preferentially associated with single-stranded DNA in transfected neuroblastoma cells (Neuro-2A). Alanine substitution of serine, threonine, and tyrosine residues in ORF2 increased the steady-state protein levels in Neuro-2A cells, and this protein preferentially interacted with double-stranded DNA. Certain in-frame transposon insertion mutants did not interact with DNA as efficiently as wild-type (wt) ORF2 did. ORF2 purified from bacteria under denaturing conditions preferentially interacted with double-stranded DNA, suggesting that the interaction between ORF2 and DNA was direct. In contrast, ORF2 purified under native conditions preferentially interacted with single-stranded DNA. We suggest that interactions between ORF2 and DNA mediate certain aspects of the latency reactivation cycle.

A bovine herpesvirus 1 protein expressed in latently infected neurons (ORF2) promotes neurite sprouting in the presence of activated Notch1 or Notch3

Bovine herpesvirus 1 (BHV-1) infection induces clinical symptoms in the upper respiratory tract, inhibits immune responses, and can lead to life-threatening secondary bacterial infections. Following acute infection, BHV-1 establishes latency in sensory neurons within trigeminal ganglia, but stress can induce reactivation from latency. The latency-related (LR) RNA is the only viral transcript abundantly expressed in latently infected sensory neurons. An LR mutant virus with stop codons at the amino terminus of the first open reading frame (ORF) in the LR gene (ORF2) is not reactivated from latency, in part because it induces higher levels of apoptosis in infected neurons. ORF2 inhibits apoptosis in transiently transfected cells, suggesting that it plays a crucial role in the latency-reactivation cycle. ORF2 also interacts with Notch1 or Notch3 and inhibits its ability to trans activate certain viral promoters. Notch3 RNA and protein levels are increased during reactivation from latency, suggesting that Notch may promote reactivation. Activated Notch signaling interferes with neuronal differentiation, in part because neurite and axon generation is blocked. In this study, we demonstrated that ORF2 promotes neurite formation in mouse neuroblastoma cells overexpressing Notch1 or Notch3. ORF2 also interfered with Notch-mediated trans activation of the promoter that regulates the expression of Hairy Enhancer of Split 5, an inhibitor of neurite formation. Additional studies provided evidence that ORF2 promotes the degradation of Notch3, but not that of Notch1, in a proteasome-dependent manner. In summary, these studies suggest that ORF2 promotes a mature neuronal phenotype that enhances the survival of infected neurons and consequently increases the pool of latently infected neurons.

ICP27 protein encoded by bovine herpesvirus type 1 (bICP27) interferes with promoter activity of the bovine genes encoding beta interferon 1 (IFN-β1) and IFN-β3

Bovine herpes virus 1 (BHV-1) infection leads to upper respiratory tract infections, conjunctivitis, and the infection predisposes cattle to secondary bacterial infections. The infected cell protein 0 (bICP0) encoded by BHV-1 suppresses antiviral innate immune signaling by interfering with expression of interferon beta (IFN-β). In contrast to humans or mice, cattle contain three IFN-β genes that have distinct transcriptional promoters. We previously cloned and characterized all three bovine IFN-β promoters. In this study, we provide evidence that bICP27; a viral early protein that shuttles between the nucleus and cytoplasm inhibits transcriptional activity of two bovine IFN-β gene promoters (IFN-β1 and IFN-β3). Conversely, the BHV-1 infected cell protein 0 (bICP0) early promoter was not inhibited by bICP27. C-terminal mutants lacking the bICP27 zinc RING finger-like motif did not efficiently inhibit IFN-β3 promoter activity but inhibited IFN-β1 promoter activity as efficiently as wild type bICP27. An N-terminal mutant lacking the nuclear localization signal (NLS) and nucleolar localization signal (NoLS) was localized to the cytoplasm and this mutant had no effect on IFN-β promoter activity. In summary, these studies provided evidence that bICP27 inhibited IFN-β1 and IFN-β3 promoter activity in transiently transfected cells.

Cellular transcription factors induced in trigeminal ganglia during dexamethasone-induced reactivation from latency stimulate bovine herpesvirus 1 productive infection and certain viral promoters

Bovine herpesvirus 1 (BHV-1), an alphaherpesvirinae subfamily member, establishes latency in sensory neurons. Elevated corticosteroid levels, due to stress, reproducibly triggers reactivation from latency in the field. A single intravenous injection of the synthetic corticosteroid dexamethasone (DEX) to latently infected calves consistently induces reactivation from latency. Lytic cycle viral gene expression is detected in sensory neurons within 6 h after DEX treatment of latently infected calves. These observations suggested that DEX stimulated expression of cellular genes leads to lytic cycle viral gene expression and productive infection. In this study, a commercially available assay-Bovine Gene Chip-was used to compare cellular gene expression in the trigeminal ganglia (TG) of calves latently infected with BHV-1 versus DEX-treated animals. Relative to TG prepared from latently infected calves, 11 cellular genes were induced more than 10-fold 3 h after DEX treatment. Pentraxin three, a regulator of innate immunity and neurodegeneration, was stimulated 35- to 63-fold after 3 or 6 h of DEX treatment. Two transcription factors, promyelocytic leukemia zinc finger (PLZF) and Slug were induced more than 15-fold 3 h after DEX treatment. PLZF or Slug stimulated productive infection 20- or 5-fold, respectively, and Slug stimulated the late glycoprotein C promoter more than 10-fold. Additional DEX-induced transcription factors also stimulated productive infection and certain viral promoters. These studies suggest that DEX-inducible cellular transcription factors and/or signaling pathways stimulate lytic cycle viral gene expression, which subsequently leads to successful reactivation from latency in a small subset of latently infected neurons.

Regulation of the latency-reactivation cycle by products encoded by the bovine herpesvirus 1 (BHV-1) latency-related gene

Like other α-herpesvirinae subfamily members, the primary site for bovine herpesvirus 1 (BHV-1) latency is ganglionic sensory neurons. Periodically BHV-1 reactivates from latency, virus is shed, and consequently virus transmission occurs. Transcription from the latency-related (LR) gene is readily detected in neurons of trigeminal ganglia (TG) of calves or rabbits latently infected with BHV-1. Two micro-RNAs and a transcript encompassing a small open reading frame (ORF-E) located within the LR promoter can also be detected in TG of latently infected calves. A BHV-1 mutant that contains stop codons near the beginning of the first open reading frame (ORF2) within the major LR transcript (LR mutant virus) has been characterized. The LR mutant virus does not express ORF2, a reading frame that lacks an initiating ATG (reading frame B), and has reduced expression of ORF1 during productive infection. The LR mutant virus does not reactivate from latency following dexamethasone treatment suggesting that LR protein expression regulates the latency-reactivation cycle. Higher levels of apoptosis occur in TG neurons of calves infected with the LR mutant viruses when compared to wild-type BHV-1 indicating that the anti-apoptotic properties of the LR gene is necessary for the latency-reactivation cycle. ORF2 inhibits apoptosis and regulates certain viral promoters, in part, because it interacts with three cellular transcription factors (C/EBP-alpha, Notch1, and Notch3). Although ORF2 is important for the latency-reactivation cycle, we predict that other LR gene products play a supportive role during life-long latency in cattle.

Localization of sequences in a protein (ORF2) encoded by the latency-related gene of bovine herpesvirus 1 that inhibits apoptosis and interferes with Notch1-mediated trans-activation of the bICP0 promoter

Bovine herpesvirus 1 (BHV-1) infection induces clinical symptoms in the upper respiratory tract, inhibits immune responses, and can result in life-threatening secondary bacterial infections. Following acute infection, BHV-1 establishes latency in sensory neurons within trigeminal ganglia. Periodically, reactivation from latency occurs, resulting in virus transmission. The latency-related (LR) RNA is abundantly expressed in latently infected sensory neurons, suggesting that LR gene products regulate the latency-reactivation cycle. An LR mutant virus with stop codons at the amino terminus of the first open reading frame (ORF) in the LR gene (ORF2) does not reactivate from latency, in part because it induces higher levels of apoptosis in infected neurons. ORF2 inhibits apoptosis in transiently transfected cells, suggesting that it plays an important role in the latency-reactivation cycle. ORF2 also interacts with Notch1 or Notch3 and consequently inhibits their ability to trans-activate the bICP0 early and glycoprotein C promoters. In this study, we identified ORF2 sequences that were necessary for inhibiting cold shock-induced apoptosis or Notch1-mediated trans-activation of the bICP0 early promoter and stimulation of productive infection. Relative to ORF2 sequences necessary for inhibiting apoptosis, distinct domains in ORF2 were important for interfering with Notch1-mediated trans-activation. Five consensus protein kinase A and/or protein kinase C phosphorylation sites within ORF2 regulate the steady-state levels of ORF2 in transfected cells. A nuclear localization signal in ORF2 was necessary for inhibiting Notch1-mediated trans-activation but not apoptosis. In summary, ORF2 has more than one functional domain that regulates its stability and functional properties.

Two microRNAs encoded within the bovine herpesvirus 1 latency-related gene promote cell survival by interacting with RIG-I and stimulating NF-κB-dependent transcription and beta interferon signaling pathways

Sensory neurons latently infected with bovine herpesvirus 1 (BHV-1) abundantly express latency-related (LR) RNA (LR-RNA). Genetic evidence indicates that LR protein expression plays a role in the latency-reactivation cycle, because an LR mutant virus that contains three stop codons downstream of the first open reading frame (ORF2) does not reactivate from latency. The LR mutant virus induces higher levels of apoptotic neurons in trigeminal ganglia, and ORF2 interferes with apoptosis. Although ORF2 is important for the latency-reactivation cycle, other factors encoded by the LR gene are believed to play a supportive role. For example, two microRNAs (miRNAs) encoded within the LR gene are expressed in trigeminal ganglia of latently infected calves. These miRNAs interfere with bICP0 protein expression and productive infection in transient-transfection assays. In this report, we provide evidence that the two LR miRNAs cooperate with poly(I·C), interferon (IFN) regulatory factor 3 (IRF3), or IRF7 to stimulate beta interferon (IFN-β) promoter activity. Both miRNAs also stimulated IFN-β promoter activity and nuclear factor-kappa B (NF-κB)-dependent transcription when cotransfected with a plasmid expressing retinoic acid-inducible gene I (RIG-I). In the presence of RIG-I, the LR miRNAs enhanced survival of mouse neuroblastoma cells, which correlated with activation of the antiapoptosis cellular transcription factor, NF-κB. Immunoprecipitation assays demonstrated that both miRNAs stably interact with RIG-I, suggesting that this interaction directly stimulates the RIG-I signaling pathway. In summary, the results of these studies suggest that interactions between LR miRNAs and RIG-I promote the establishment and maintenance of latency by enhancing survival of infected neurons.

Cytoplasmic localized infected cell protein 0 (bICP0) encoded by bovine herpesvirus 1 inhibits β interferon promoter activity and reduces IRF3 (interferon response factor 3) protein levels

Bovine herpesvirus 1 (BHV-1), an alpha-herpesvirinae subfamily member, establishes a life-long latent infection in sensory neurons. Periodically, BHV-1 reactivates from latency, infectious virus is spread, and consequently virus transmission occurs. BHV-1 acute infection causes upper respiratory track infections and conjunctivitis in infected cattle. As a result of transient immune-suppression, BHV-1 infections can also lead to life-threatening secondary bacterial pneumonia that is referred to as bovine respiratory disease. The infected cell protein 0 (bICP0) encoded by BHV-1 reduces human β-interferon (IFN-β) promoter activity, in part, by inducing degradation of interferon response factor 3 (IRF3) and interacting with IRF7. In contrast to humans, cattle contain three IFN-β genes. All three bovine IFN-β proteins have anti-viral activity: but each IFN-β gene has a distinct transcriptional promoter. We have recently cloned and characterized the three bovine IFN-β promoters. Relative to the human IFN-β promoter, each of the three IFN-β promoters contain differences in the four positive regulatory domains that are required for virus-induced activity. In this study, we demonstrate that bICP0 effectively inhibits bovine IFN-β promoter activity following transfection of low passage bovine cells with interferon response factor 3 (IRF3) or IRF7. A bICP0 mutant that localizes to the cytoplasm inhibits bovine IFN-β promoter activity as efficiently as wt bICP0. The cytoplasmic localized bICP0 protein also induced IRF3 degradation with similar efficiency as wt bICP0. In summary, these studies suggested that cytoplasmic localized bICP0 plays a role in inhibiting the IFN-β response during productive infection.

Infection of cultured bovine cells with bovine herpesvirus 1 (BHV-1) or Sendai virus induces different beta interferon subtypes

In contrast to mice or humans, cattle contain three beta interferon (IFN-β) genes with distinct transcriptional promoters suggesting IFN-β gene expression is not stimulated the same by different viruses. To test this hypothesis, we compared expression of the three IFN-β subtypes after infection with a RNA virus, Sendai, versus a large DNA virus, bovine herpesvirus 1 (BHV-1). Infection of low passage bovine kidney (BK) or established bovine kidney cells (CRIB) with Sendai virus has consistently led to high levels of IFN-β1 RNA. Conversely, infection of CRIB cells, but not BK cells, with BHV-1 increased IFN-β3 RNA levels and to a lesser extent the other two IFN-β subtypes. Inhibition of de novo protein synthesis with cycloheximide resulted in higher levels of IFN-β1 and IFN-β2 RNA levels after BHV-1 infection. Further studies demonstrated that BHV-1 immediate early and/or early genes were primarily responsible for inhibiting the IFN response in BK cells. The three bovine IFN-β promoters were cloned upstream of a reporter gene construct, and their properties analyzed in transient transfection assays. Only the IFN-β3 promoter was trans-activated by IRF3 (interferon responsive factor 3). IRF7 and double stranded RNA (polyI:C) stimulated IFN-β1 and IFN-β3 promoter activity, but not IFN-β2. Relative to the human IFN-β promoter, the IFN-β3 promoter contained fewer nucleotide differences in the positive regulatory domain III (PRD III), PRD IV, and PRD I compared to the IFN-β1 and IFN-β2 promoter. Collectively, these studies provide evidence that virus infection differentially stimulates expression of the three bovine IFN-β genes.

A protein (ORF2) encoded by the latency-related gene of bovine herpesvirus 1 interacts with Notch1 and Notch3

Like other Alphaherpesvirinae subfamily members, bovine herpesvirus 1 (BHV-1) establishes latency in sensory neurons. The latency-related RNA (LR-RNA) is abundantly expressed in latently infected sensory neurons. An LR mutant virus with stop codons at the amino terminus of the first open reading frame (ORF) in the LR gene (ORF2) does not reactivate from latency, in part because it induces higher levels of apoptosis in infected neurons. ORF2 is not the only viral product expressed during latency, but it is important for the latency reactivation cycle because it inhibits apoptosis. In this study, a yeast 2-hybrid screen revealed that ORF2 interacted with two cellular transcription factors, Notch1 and Notch3. These interactions were confirmed in mouse neuroblastoma cells by confocal microscopy and in an in vitro "pulldown" assay. During reactivation from latency, Notch3 RNA levels in trigeminal ganglia were higher than those during latency, suggesting that Notch family members promote reactivation from latency or that reactivation promotes Notch expression. A plasmid expressing the Notch1 intercellular domain (ICD) stimulated productive infection and promoters that encode the viral transcription factor bICP0. The Notch3 ICD did not stimulate productive infection as efficiently as the Notch1 ICD and had no effect on bICP0 promoter activity. Plasmids expressing the Notch1 ICD or the Notch3 ICD trans-activated a late promoter encoding glycoprotein C. ORF2 reduced the trans-activation potential of Notch1 and Notch3, suggesting that ORF2 interfered with the trans-activation potential of Notch. These studies provide evidence that ORF2, in addition to inhibiting apoptosis, has the potential to promote establishment and maintenance of latency by sequestering cellular transcription factors.

Regulation of promyelocytic leukemia (PML) protein levels and cell morphology by bovine herpesvirus 1 infected cell protein 0 (bICP0) and mutant bICP0 proteins that do not localize to the nucleus

BHV-1 is an important pathogen of cattle. The infected cell protein 0 (bICP0) encoded by BHV-1 is an important regulatory protein because it is constitutively expressed and can activate all viral promoters. The mechanism by which bICP0 activates viral promoters is not well understood because bICP0 does not appear to be a sequence specific binding protein. A C(3)HC(4) zinc RING (really interesting novel gene) motif at the N-terminus of bICP0 has E3 ubiquitin ligase activity, which is important for activating viral gene expression and inhibiting interferon dependent transcription. Like other alpha-herpesvirinae ICP0 homologues, bICP0 is associated with promyelocytic leukemia (PML) protein-containing nuclear domains. During productive infection of cultured cells, BHV-1 induces degradation of the PML protein, which correlates with efficient productive infection. In this study, we demonstrated that a plasmid expressing bICP0 reduces steady state levels of the PML protein, and the C(3)HC(4) zinc RING finger is important for PML degradation. Surprisingly, bICP0 mutants with an intact C(3)HC(4) zinc RING finger that lack a nuclear localization signal also reduces steady PML protein levels. In addition, mutant bICP0 proteins that primarily localize to the cytoplasm induced morphological changes in transfected cells. During productive infection, bICP0 was detected in the cytoplasm of low-passage bovine kidney, but not established bovine kidney cells. These studies demonstrated that bICP0, even when not able to efficiently localize to the nucleus, was able to induce degradation of the PML protein and alter the morphology of transfected cells.

Small noncoding RNAs encoded within the bovine herpesvirus 1 latency-related gene can reduce steady-state levels of infected cell protein 0 (bICP0)

Following acute infection in mucosal epithelium, bovine herpes virus 1 (BHV-1) establishes lifelong latency in sensory neurons within trigeminal ganglia. The latency-related RNA (LR-RNA) is abundantly expressed in sensory neurons of latently infected calves. Expression of LR proteins is necessary for the latency reactivation cycle because a mutant virus that does not express LR proteins is unable to reactivate from latency after dexamethasone treatment. LR-RNA sequences also inhibit bICP0 expression, productive infection, and cell growth. However, it is unclear how LR-RNA mediates these functions. In this study, we identified a 463-bp region within the LR gene (the XbaI-PstI [XP] fragment) that inhibited bICP0 protein and RNA expression in transiently transfected mouse neuroblastoma cells. Small noncoding RNAs (sncRNAs) encoded within the XP fragment (20 to 90 nucleotides in length) were detected in transiently transfected mouse neuroblastoma cells. Two families of sncRNAs were cloned from this region, and each family was predicted to contain a mature microRNA (miRNA). Both miRNAs were predicted to base pair with bICP0 mRNA sequences, suggesting that they reduce bICP0 levels. To test this prediction, sequences encompassing the respective sncRNAs and mature miRNAs were synthesized and cloned into a small interfering RNA expression vector. Both sncRNA families and their respective miRNAs inhibited bICP0 protein expression in mouse neuroblastoma cells and productive infection in bovine cells. In trigeminal ganglia of latently infected calves, an sncRNA that migrated between nucleotides 20 and 25 hybridized to the XP fragment. During dexamethasone-induced reactivation from latency, XP-specific sncRNA levels were reduced, suggesting that these sncRNAs support the establishment and maintenance of lifelong latency in cattle.